draft-irtf-iccrg-welzl-congestion-control-open-research-03.txt   draft-irtf-iccrg-welzl-congestion-control-open-research-04.txt 
Network Working Group Michael Welzl Network Working Group Michael Welzl
Internet Draft Dimitri Papadimitriou Internet Draft Dimitri Papadimitriou
Document: draft-irtf-iccrg-welzl- Editors Expires: November 16, 2009 Editors
congestion-control-open-research-03.txt
Expires: October 16, 2009 Michael Scharf
Bob Briscoe
April 17, 2009
Michael Scharf
Bob Briscoe
Open Research Issues in Internet Congestion Control Open Research Issues in Internet Congestion Control
draft-irtf-iccrg-welzl-congestion-control-open-research-03.txt draft-irtf-iccrg-welzl-congestion-control-open-research-04.txt
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
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scale solutions can be confidently engineered and deployed. scale solutions can be confidently engineered and deployed.
Conventions used in this document Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [i]. document are to be interpreted as described in RFC-2119 [i].
Table of Contents Table of Contents
1. Introduction...................................................3 1. Introduction..................................................3
2. Global Challenges..............................................4 2. Global Challenges.............................................4
2.1 Heterogeneity..............................................4 2.1 Heterogeneity.............................................4
2.2 Stability..................................................6 2.2 Stability.................................................6
2.3 Fairness...................................................7 2.3 Fairness..................................................7
3. Detailed Challenges............................................9 3. Detailed Challenges...........................................9
3.1 Challenge 1: Network Support...............................9 3.1 Challenge 1: Network Support..............................9
3.2 Challenge 2: Corruption Loss..............................14 3.1.1 Performance and Robustness.........................12
3.3 Challenge 3: Packets Sizes................................16 3.1.2 Granularity of network component functions.........12
3.4 Challenge 4: Flow Startup.................................20 3.1.3 Information Acquisition............................13
3.5 Challenge 5: Multi-domain Congestion Control..............22 3.1.4 Feedback signaling.................................14
3.6 Challenge 6: Precedence for Elastic Traffic...............25 3.2 Challenge 2: Corruption Loss.............................14
3.7 Challenge 7: Misbehaving Senders and Receivers............26 3.3 Challenge 3: Packet Size.................................16
3.8 Other challenges..........................................27 3.4 Challenge 4: Flow Startup................................20
4. Security Considerations.......................................32 3.5 Challenge 5: Multi-domain Congestion Control.............22
5. Contributors..................................................32 3.5.1 Multi-domain Transport of Congestion Signals.......22
6. References....................................................32 3.5.2 Multi-domain Information Exchange..................23
6.1 Normative References.........................................32 3.5.3 Multi-domain Pseudowires...........................24
Acknowledgments...............................................40 3.6 Challenge 6: Precedence for Elastic Traffic..............25
3.7 Challenge 7: Misbehaving Senders and Receivers...........26
3.8 Other Challenges.........................................27
3.8.1 RTT Estimation.....................................27
3.8.2 Malfunctioning Devices.............................29
3.8.3 Dependence on RTT..................................30
3.8.4 Congestion Control in Multi-layered Networks.......30
3.8.5 Multipath End-to-end Congestion Control and Traffic
Engineering........................................31
3.8.6 ALGs and Middleboxes...............................31
4. Security Considerations......................................32
5. References...................................................33
5.1 Normative References.....................................33
5.2 Informative References...................................35
6. Acknowledgments..............................................40
7. Author's Addresses...........................................41
8. Contributors.................................................41
Acknowledgments.................................................42
1. Introduction 1. Introduction
This document describes some of the open research topics in the This document describes some of the open research topics in the
domain of Internet congestion control that are known today. We begin domain of Internet congestion control that are known today. We begin
by reviewing some proposed definitions of congestion and congestion by reviewing some proposed definitions of congestion and congestion
control based on current understandings. control based on current understandings.
Congestion can be defined as a state or condition that occurs when Congestion can be defined as a state or condition that occurs when
the network resources are overloaded resulting into impairments for the network resources are overloaded resulting into impairments for
network users as objectively measured by the probability of loss network users as objectively measured by the probability of loss
and/or of delay). The overload results in the reduction of utility in and/or of delay. The overload results in the reduction of utility in
networks that support both spatial and temporal multiplexing, but no networks that support both spatial and temporal multiplexing, but no
reservation [Keshav]. Congestion control is a (typically distributed) reservation [Keshav07]. Congestion control is a (typically
algorithm to share network resources among competing traffic sources. distributed) algorithm to share network resources among competing
traffic sources.
Two components of distributed congestion control have been defined in Two components of distributed congestion control have been defined in
the context of primal-dual modeling [Kelly98]. Primal congestion the context of primal-dual modeling [Kelly98]. Primal congestion
control refers to the algorithm executed by the traffic sources control refers to the algorithm executed by the traffic sources
algorithm for controlling their sending rates or window sizes. This algorithm for controlling their sending rates or window sizes. This
is normally a closed-loop control, where this operation depends on is normally a closed-loop control, where this operation depends on
feedback. TCP algorithms fall in this category. Dual congestion feedback. TCP algorithms fall in this category. Dual congestion
control is implemented by the routers through gathering information control is implemented by the routers through gathering information
about the traffic traversing them. A dual congestion control about the traffic traversing them. A dual congestion control
algorithm updates, implicitly or explicitly, a congestion measure or algorithm updates, implicitly or explicitly, a congestion measure or
congestion rate and sends it back, implicitly or explicitly, to the congestion rate and sends it back, implicitly or explicitly, to the
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successful over many years but have begun to reach their limits, as successful over many years but have begun to reach their limits, as
the heterogeneity of both the data link and physical layer and the heterogeneity of both the data link and physical layer and
applications are pulling TCP congestion control (which performs applications are pulling TCP congestion control (which performs
poorly as the bandwidth or delay increases) beyond its natural poorly as the bandwidth or delay increases) beyond its natural
operating regime. A side effect of these deficits is that there is an operating regime. A side effect of these deficits is that there is an
increasing share of hosts that use non-standardized congestion increasing share of hosts that use non-standardized congestion
control enhancements (for instance, many Linux distributions have control enhancements (for instance, many Linux distributions have
been shipped with "CUBIC" as default TCP congestion control been shipped with "CUBIC" as default TCP congestion control
mechanism). mechanism).
While the original Jacobson algorithm requires no congestion-related While the original Van Jacobson algorithm requires no congestion-
state in routers, more recent modifications have departed from the related state in routers, more recent modifications have departed
strict application of the end-to-end principle [Saltzer84] in order from the strict application of the end-to-end principle [Saltzer84]
to avoid congestion collapse. Active Queue Management (AQM) in in order to avoid congestion collapse. Active Queue Management (AQM)
routers, e.g., RED and its variants such as Weighted RED (WRED), in routers, e.g., RED and its variants such as Weighted RED (WRED),
Stabilized RED (SRED), Adaptive RED (ARED), xCHOKE [Pan00], RED with Stabilized RED (SRED), Adaptive RED (ARED), xCHOKE [Pan00], RED with
In/Out (RIO) [Clark98], improves performance by keeping queues small In/Out (RIO) [Clark98], improves performance by keeping queues small
(implicit feedback via dropped packets), while Explicit Congestion (implicit feedback via dropped packets), while Explicit Congestion
Notification (ECN) [Floyd94] [RFC3168] passes one bit of congestion Notification (ECN) [Floyd94] [RFC3168] passes one bit of congestion
information back to senders when an AQM would normally drop a packet. information back to senders when an AQM would normally drop a packet.
These measures do improve performance, but there is a limit to how These measures do improve performance, but there is a limit to how
much can be accomplished without more information from routers. The much can be accomplished without more information from routers. The
requirement of extreme scalability together with robustness has been requirement of extreme scalability together with robustness has been
a difficult hurdle to accelerating information flow. Primal-Dual a difficult hurdle to accelerating information flow. Primal-Dual
TCP/AQM distributed algorithm stability and equilibrium properties TCP/AQM distributed algorithm stability and equilibrium properties
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It is always possible to tune congestion control parameters based on It is always possible to tune congestion control parameters based on
some knowledge of the environment and the application scenario. some knowledge of the environment and the application scenario.
However, the interaction of multiple congestion control techniques However, the interaction of multiple congestion control techniques
interacting with each other is not yet well understood. The interacting with each other is not yet well understood. The
fundamental question is whether it is possible to define one fundamental question is whether it is possible to define one
congestion control mechanism that operates reasonably well in the congestion control mechanism that operates reasonably well in the
whole range of scenarios that exist in the Internet. Hence, it is an whole range of scenarios that exist in the Internet. Hence, it is an
important research question how new Internet congestion control important research question how new Internet congestion control
mechanisms would have to be designed, which maximum degree of mechanisms would have to be designed, which maximum degree of
dynamics they can efficiently handle, and whether they can keep the dynamics they can efficiently handle, and whether they can keep the
genererality of the existing end-to-end solutions. generality of the existing end-to-end solutions.
Some improvements to congestion control could be realized by simple Some improvements to congestion control could be realized by simple
changes of single functions in end-system or network components. changes of single functions in end-system or network components.
However, new mechanism(s) might also require a fundamental redesign However, new mechanism(s) might also require a fundamental redesign
of the overall network architecture, and they may even affect the of the overall network architecture, and they may even affect the
design of Internet applications. This can imply significant design of Internet applications. This can imply significant
interoperability and backward compatibility challenges and/or create interoperability and backward compatibility challenges and/or create
network accessibility obstacles. In particular, networks and/or network accessibility obstacles. In particular, networks and/or
applications that do not use or support a new congestion control applications that do not use or support a new congestion control
mechanism could be penalized by a significantly worse performance mechanism could be penalized by a significantly worse performance
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Control theoretic modeling of a realistic network can be quite Control theoretic modeling of a realistic network can be quite
difficult, especially when taking distinct packet sizes and difficult, especially when taking distinct packet sizes and
heterogeneous RTTs into account. It has therefore become common heterogeneous RTTs into account. It has therefore become common
practice to model simpler cases and to leave the more complicated practice to model simpler cases and to leave the more complicated
(realistic) situations for simulations. Clearly, if a mechanism is (realistic) situations for simulations. Clearly, if a mechanism is
not stable in a simple scenario, it is generally useless; this method not stable in a simple scenario, it is generally useless; this method
therefore helps to eliminate faulty congestion control candidates at therefore helps to eliminate faulty congestion control candidates at
an early stage. an early stage.
Some fundamental facts, which are known from control theory are Some fundamental facts known from control theory are useful as
useful as guidelines when designing a congestion control mechanism. guidelines when designing a congestion control mechanism. For
For instance, a controller should only be fed a system state that instance, a controller should only be fed a system state that
reflects its output. A (low-pass) filter function should be used in reflects its output. A (low-pass) filter function should be used in
order to pass only states to the controller that are expected to last order to pass only states to the controller that are expected to last
long enough for its action to be meaningful [Jain88]. Action should long enough for its action to be meaningful [Jain88]. Action should
be carried out whenever such feedback arrives, as it is a fundamental be carried out whenever such feedback arrives, as it is a fundamental
principle of control that the control frequency should be equal to principle of control that the control frequency should be equal to
the feedback frequency. Reacting faster leads to oscillations and the feedback frequency. Reacting faster leads to oscillations and
instability while reacting slower makes the system tardy [Jain90]. instability while reacting slower makes the system tardy [Jain90].
TCP stability can be attributed to two key aspects which were TCP stability can be attributed to two key aspects which were
introduced in [Jacobson88]: the AIMD control law during congestion introduced in [Jacobson88]: the AIMD control law during congestion
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The reasoning in [Jacobson88] assumes all senders to be acting at the The reasoning in [Jacobson88] assumes all senders to be acting at the
same time. The stability of TCP under more realistic network same time. The stability of TCP under more realistic network
conditions has been investigated in a large number of ensuing works, conditions has been investigated in a large number of ensuing works,
leading to no clear conclusion that TCP would also be asymptotically leading to no clear conclusion that TCP would also be asymptotically
stable under arbitrary network conditions. On the other hand, stable under arbitrary network conditions. On the other hand,
research has concluded that stability can be assured with constraints research has concluded that stability can be assured with constraints
on dynamics that are less stringent than the "conservation of packets on dynamics that are less stringent than the "conservation of packets
principle". From control theory, only rate increase (not the target principle". From control theory, only rate increase (not the target
rate) needs to be inversely proportional to RTT (whereas window-based rate) needs to be inversely proportional to RTT (whereas window-based
control converges on a target rate inversely proportional to RTT). control converges on a target rate inversely proportional to RTT).
With rate-based control, high-speed congestion control converges on a A congestion control mechanism can therefore converge on a rate that
rate that is independent of RTT as long as its dynamics depends on is independent of RTT as long as its dynamics depend on RTT (e.g.
RTT (e.g. FAST TCP [Jin04]). FAST TCP [Jin04]).
However in the stability analysis of TCP and of these more modern In the stability analysis of TCP and of these more modern controls,
controls the stability impact of Slow Start (which can be significant the impact of Slow Start on stability (which can be significant as
as short-lived HTTP flows often never leave this phase) is not short-lived HTTP flows often never leave this phase) is not entirely
entirely clear. clear.
2.3 Fairness 2.3 Fairness
Recently, the way the Internet community reasons about fairness has Recently, the way the Internet community reasons about fairness has
been called into deep questioning [Bri07]. Much of the community has been called into deep questioning [Bri07]. Much of the community has
taken fairness to mean approximate equality between the rates of taken fairness to mean approximate equality between the rates of
flows (flow rate fairness) that experience equivalent path congestion flows (flow rate fairness) that experience equivalent path congestion
as with TCP [RFC2581] and TFRC [RFC3448]. [RFC3714] depicts the as with TCP [RFC2581] and TFRC [RFC3448]. [RFC3714] depicts the
resulting situation as "The Amorphous Problem of Fairness". resulting situation as "The Amorphous Problem of Fairness".
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In comparison, the debate between max-min, proportional and TCP In comparison, the debate between max-min, proportional and TCP
fairness is about mere details. These three all share the assumption fairness is about mere details. These three all share the assumption
that equal flow rates are desirable; they merely differ in the second that equal flow rates are desirable; they merely differ in the second
order issue of how to share out excess capacity in a network of many order issue of how to share out excess capacity in a network of many
bottlenecks. In contrast, cost fairness should lead to extremely bottlenecks. In contrast, cost fairness should lead to extremely
unequal flow rates by design. Equivalently, equal flow rates would unequal flow rates by design. Equivalently, equal flow rates would
typically be considered extremely unfair. typically be considered extremely unfair.
The two traditional approaches are not protocol options that can each The two traditional approaches are not protocol options that can each
be followed in different parts of an inter-network. They result in be followed in different parts of an inter-network. They lead to
research agendas and issues that are different in their respective research agendas that are different in their respective objectives,
objectives resulting in different set of open issues. resulting in a different set of open issues.
If we assume TCP-friendliness as a goal with flow rate as the metric, If we assume TCP-friendliness as a goal with flow rate as the metric,
open issues would be: open issues would be:
- Should rate fairness depend on the packet rate or the bit rate? - Should rate fairness depend on the packet rate or the bit rate?
- Should the flow rate depend on RTT (as in TCP) or should only flow - Should the flow rate depend on RTT (as in TCP) or should only flow
dynamics depend on RTT (e.g. as in Fast TCP [Jin04])? dynamics depend on RTT (e.g. as in Fast TCP [Jin04])?
- How to estimate whether a particular flow start strategy is fair, - How to estimate whether a particular flow start strategy is fair,
or whether a particular fast recovery strategy after a reduction in or whether a particular fast recovery strategy after a reduction in
rate due to congestion is fair? rate due to congestion is fair?
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- Which policy enforcement should be used by networks and what are - Which policy enforcement should be used by networks and what are
the interactions between application policy and network policy the interactions between application policy and network policy
enforcement? enforcement?
- How to design a new policy enforcement framework that will - How to design a new policy enforcement framework that will
appropriately compete with existing flows aiming for rate equality appropriately compete with existing flows aiming for rate equality
(e.g. TCP)? (e.g. TCP)?
The question of how to reason about fairness is a pre-requisite to The question of how to reason about fairness is a pre-requisite to
agreeing on the research agenda. If the relevant metric is flow-rate agreeing on the research agenda. If the relevant metric is flow-rate
it places constraints at protocol design-time, whereas if the metric it places constraints at protocol design-time, whereas if the metric
is congestion volume the constraints move to run-time, while design- is the congestion volume the constraints move to run-time, while
time constraints can be relaxed [Bri08]. However, that question does design-time constraints can be relaxed [Bri08]. However, that
not require more research in itself, it is merely a debate that needs question does not require more research in itself, it is merely a
to be resolved by studying existing research and by assessing how bad debate that needs to be resolved by studying existing research and by
fairness problems could become if they are not addressed rigorously, assessing how bad fairness problems could become if they are not
and whether we can rely on trust to maintain approximate fairness addressed rigorously, and whether we can rely on trust to maintain
without requiring policing complexity [Floyd08]. The latter points approximate fairness without requiring policing complexity [Floyd08].
may themselves lead to additional research. However, it is also The latter points may themselves lead to additional research.
accepted that more research will not necessarily lead to convince However, it is also accepted that more research will not necessarily
either side to change their opinions. More debate would be needed. It lead to convince either side to change their opinions. More debate
seems also that if an architecture is built to support cost-fairness would be needed. It seems also that if the architecture is built to
then equal-costs result in flow-rate fairness as a degenerate case; support cost-fairness then equal-costs result in flow-rate fairness
that is, flow-rate fairness can be seen as a special case of cost- as a degenerate case; that is, flow-rate fairness can be seen as a
fairness. One can be used to build the other, but not vice-versa. special case of cost-fairness. One can be used to build the other,
but not vice-versa.
In the rest of this document, "fairness" means flow rate fairness.
3. Detailed Challenges 3. Detailed Challenges
3.1 Challenge 1: Network Support 3.1 Challenge 1: Network Support
This challenge is the most critical to get right. Changes to the This challenge is perhaps the most critical to get right. Changes to
balance of functions between the endpoints and network equipment the balance of functions between the endpoints and network equipment
could require a change to the per-datagram data plane interface could require a change to the per-datagram data plane interface
between the transport and network layers. Network equipment vendors between the transport and network layers. Network equipment vendors
need to be assured that any new interface is stable enough (on decade need to be assured that any new interface is stable enough (on decade
timescales) to build into firmware and hardware, and OS vendors will timescales) to build into firmware and hardware, and OS vendors will
not use a new interface unless it is likely to be widely deployed. not use a new interface unless it is likely to be widely deployed.
Network components can be involved in congestion control in two ways: Network components can be involved in congestion control in two ways:
first, they can implicitly optimize their functions, such as queue first, they can implicitly optimize their functions, such as queue
management and scheduling strategies, in order to support the management and scheduling strategies, in order to support the
operation of an end-to-end congestion control. Second, network operation of an end-to-end congestion control. Second, network
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network component that processes and stores packets. Various network component that processes and stores packets. Various
approaches have been proposed and also deployed, such as different approaches have been proposed and also deployed, such as different
AQM techniques. Even though these implicit techniques are known to AQM techniques. Even though these implicit techniques are known to
improve network performance during congestion phases, they are still improve network performance during congestion phases, they are still
only partly deployed in the Internet. This may be due to the fact only partly deployed in the Internet. This may be due to the fact
that finding optimal and robust parameterizations for these that finding optimal and robust parameterizations for these
mechanisms is a non-trivial problem. Indeed, the problem with various mechanisms is a non-trivial problem. Indeed, the problem with various
AQM schemes is the difficulty to identify correct values of the AQM schemes is the difficulty to identify correct values of the
parameter set that affects the performance of the queuing scheme (due parameter set that affects the performance of the queuing scheme (due
to variation in the number of sources, the capacity and the feedback to variation in the number of sources, the capacity and the feedback
delay) [Fioriu00] [Hollot01] [Zhang03]. Many AQM schemes (RED, REM, delay) [Firoiu00] [Hollot01] [Zhang03]. Many AQM schemes (RED, REM,
BLUE, PI-Controller but also Adaptive Virtual Queue (AVQ)) do not BLUE, PI-Controller but also Adaptive Virtual Queue (AVQ)) do not
define a systematic rule for setting their parameters. define a systematic rule for setting their parameters.
The second class of approaches uses explicit notification. By using The second class of approaches uses explicit notification. By using
explicit feedback from the network, connection endpoints can obtain explicit feedback from the network, connection endpoints can obtain
more accurate information about the current network characteristics more accurate information about the current network characteristics
on the path. This allows endpoints to make more precise decisions on the path. This allows endpoints to make more precise decisions
that can better prevent packet loss and that can also improve rate that can better prevent packet loss and that can also improve rate
equality among different flows. equality among different flows.
Explicit feedback techniques fall into three broad categories: Explicit feedback techniques fall into three broad categories:
o Explicit congestion feedback: whether one bit Explicit Congestion - Explicit congestion feedback: one bit Explicit Congestion
Notification (ECN) [RFC3168] or proposals for more than one bit Notification (ECN) [RFC3168] or proposals for more than one bit
[Xia05]; [Xia05];
o Explicit per-datagram rate feedback: the eXplicit Control Protocol - Explicit per-datagram rate feedback: the eXplicit Control Protocol
(XCP) [Katabi02] [Falk07], the Rate Control Protocol (RCP) (XCP) [Katabi02] [Falk07], the Rate Control Protocol (RCP)
[Dukki05]; [Dukki05];
o Explicit rate feedback: by in-band signaling, such as by Quick- - Explicit rate feedback: by in-band signaling, such as by Quick-
-Start [RFC4782], or by means of out-of-band signaling, e.g. Start [RFC4782], or by means of out-of-band signaling, e.g.
CADPC/PTP [Welzl03]. CADPC/PTP [Welzl03].
Explicit router feedback can address some of the inherent Explicit router feedback can address some of the inherent
shortcomings of TCP. For instance, XCP was developed to overcome the shortcomings of TCP. For instance, XCP was developed to overcome the
inefficiency, unfairness and instability that TCP suffers from when inefficiency, unfairness and instability that TCP suffers from when
the per-flow bandwidth-delay product increases. By decoupling the per-flow bandwidth-delay product increases. By decoupling
resource utilization/congestion control from fairness control, XCP resource utilization/congestion control from fairness control, XCP
achieves fair bandwidth allocation, high utilization, a small achieves fair bandwidth allocation, high utilization, a small
standing queue size, and near-zero packet drops, with both steady and standing queue size, and near-zero packet drops, with both steady and
highly varying traffic. Importantly, XCP does not maintain any per- highly varying traffic. Importantly, XCP does not maintain any per-
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of flow completion times [Dukki06], taking an optimistic approach to of flow completion times [Dukki06], taking an optimistic approach to
flows likely to arrive in the next RTT and tolerating larger flows likely to arrive in the next RTT and tolerating larger
instantaneous queue sizes [Dukki05]. XCP on the other hand gives very instantaneous queue sizes [Dukki05]. XCP on the other hand gives very
poor flow completion times for short flows. poor flow completion times for short flows.
Both implicit and explicit router support should be considered in the Both implicit and explicit router support should be considered in the
context of the end-to-end argument [Saltzer84], which is one of the context of the end-to-end argument [Saltzer84], which is one of the
key design principles of the Internet. It suggests that functions key design principles of the Internet. It suggests that functions
that can be realized both in the end-systems and in the network that can be realized both in the end-systems and in the network
should be implemented in the end-systems. This principle ensures that should be implemented in the end-systems. This principle ensures that
the network provides a general service and that remains as simple as the network provides a general service and that it remains as simple
possible (any additional complexity is placed above the IP layer, as possible (any additional complexity is placed above the IP layer,
i.e., at the edges) so as to ensure evolvability, reliability and i.e., at the edges) so as to ensure evolvability, reliability and
robustness. Furthermore, the fate-sharing principle, enunciated by robustness. Furthermore, the fate-sharing principle ([Clark88]
Dave Clark in "Design Philosophy of the DARPA Internet Protocols", "Design Philosophy of the DARPA Internet Protocols") mandates that an
mandates that an end-to-end Internet protocol design should not rely end-to-end Internet protocol design should not rely on the
on the maintenance of any per-flow state (i.e., information about the maintenance of any per-flow state (i.e., information about the state
state of the end-to-end communication) inside the network [RFC1958] of the end-to-end communication) inside the network and that the
and that the network state (e.g. routing state) maintained by the network state (e.g. routing state) maintained by the Internet shall
Internet shall minimize its interaction with the states maintained at minimize its interaction with the states maintained at the end-
the end-points/hosts. points/hosts [RFC1958].
However, as discussed for instance in [Moors02], congestion control However, as discussed for instance in [Moors02], congestion control
cannot be realized as a pure end-to-end function only. Congestion is cannot be realized as a pure end-to-end function only. Congestion is
an inherent network phenomenon and can only be resolved efficiently an inherent network phenomenon and can only be resolved efficiently
by some cooperation of end-systems and the network. Congestion by some cooperation of end-systems and the network. Congestion
control in today's Internet protocols follows the end-to-end design control in today's Internet protocols follows the end-to-end design
principle insofar as only minimal feedback from the network is used principle insofar as only minimal feedback from the network is used,
(e. g., packet loss and delay). The end-systems only decide how to e.g., packet loss and delay. The end-systems only decide how to
react and how to avoid congestion. The crux is that, on the one hand, react and how to avoid congestion. The crux is that, on the one hand,
there would be substantial benefit by further assistance from the there would be substantial benefit by further assistance from the
network, but, on the other hand, such router support could lead to network, but, on the other hand, such network support could lead to
duplication of functions, which might even harmfully interact with duplication of functions, which might even harmfully interact with
end-to-end protocol mechanisms. The different requirements of end-to-end protocol mechanisms. The different requirements of
applications (cf. the fairness discussion in Section 2.3) call for a applications (cf. the fairness discussion in Section 2.3) call for a
variety of different congestion control approaches, but putting such variety of different congestion control approaches, but putting such
per-flow behavior inside the network should be avoided, as such per-flow behavior inside the network should be avoided, as such
design would clearly be at odds with the end-to-end and fate sharing design would clearly be at odds with the end-to-end and fate sharing
design principles. design principles.
The end-to-end and fate sharing principles are generally regarded as The end-to-end and fate sharing principles are generally regarded as
the key ingredients for ensuring a scalable and survivable network the key ingredients for ensuring a scalable and survivable network
design. In order to ensure that new congestion control mechanisms are design. In order to ensure that new congestion control mechanisms are
scalable, violating these principles must therefore be avoided. scalable, violating these principles must therefore be avoided.
In general, network support of congestion control raises many issues In general, network support of congestion control raises many issues
that have not been completely solved yet. that have not been completely solved yet.
3.1.1 Performance and robustness 3.1.1 Performance and Robustness
Congestion control is subject to some tradeoffs: on one hand, it must Congestion control is subject to some tradeoffs: on one hand, it must
allow high link utilizations and fair resource sharing but on the allow high link utilizations and fair resource sharing but on the
other hand, the algorithms must also be robust in particular during other hand, the algorithms must also be robust in particular during
congestion phases. congestion phases.
Router support can help to improve performance but it can also result Router support can help to improve performance but it can also result
in additional complexity and more control loops. This requires a in additional complexity and more control loops. This requires a
careful design of the algorithms in order to ensure stability and careful design of the algorithms in order to ensure stability and
avoid e.g. oscillations. A further challenge is the fact that avoid e.g. oscillations. A further challenge is the fact that
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- What is the minimum support that is needed from the network in - What is the minimum support that is needed from the network in
order to achieve significantly better performance than with order to achieve significantly better performance than with
end-to-end mechanisms and the current IP header limitations that end-to-end mechanisms and the current IP header limitations that
provide at most unary ECN signals? provide at most unary ECN signals?
3.1.2 Granularity of network component functions 3.1.2 Granularity of network component functions
There are several degrees of freedom concerning the involvement of There are several degrees of freedom concerning the involvement of
network entities, ranging from some few additional functions in network entities, ranging from some few additional functions in
network management procedures on the one end, and additional per network management procedures on the one end to additional per
packet processing on the other end of the solution space. packet processing on the other end of the solution space.
Furthermore, different amounts of state can be kept in routers (no Furthermore, different amounts of state can be kept in routers (no
per-flow state, partial per-flow state, soft state, hard state). The per-flow state, partial per-flow state, soft state, hard state). The
additional router processing is a challenge for Internet scalability additional router processing is a challenge for Internet scalability
and could also increase end-to-end latencies. and could also increase end-to-end latencies.
There are many solutions that do not require per-flow state and thus There are many solutions that do not require per-flow state and thus
do not cause a large processing overhead. However, scalability issues do not cause a large processing overhead. However, scalability issues
could also be caused, for instance, by synchronization mechanisms for could also be caused, for instance, by synchronization mechanisms for
state information among parallel processing entities, which are e. g. state information among parallel processing entities, which are e. g.
used in high-speed router hardware designs. used in high-speed router hardware designs.
Open questions are: Open questions are:
- What granularity of router processing can be realized without - What granularity of router processing can be realized without
affecting Internet scalability? affecting Internet scalability?
- How can additional processing efforts be kept at a minimum? - How can additional processing efforts be kept at a minimum?
3.1.3 Information acquisition 3.1.3 Information Acquisition
In order to support congestion control, network components have to In order to support congestion control, network components have to
obtain at least a subset of the following information. Obtaining that obtain at least a subset of the following information. Obtaining that
information may result in complex tasks. information may result in complex tasks.
1. Capacity of (outgoing) links 1. Capacity of (outgoing) links
Link characteristics depend on the realization of lower protocol Link characteristics depend on the realization of lower protocol
layers. Routers operating at IP layer do not necessarily know the layers. Routers operating at IP layer do not necessarily know the
link layer network topology and link capacities, and these are not link layer network topology and link capacities, and these are not
always constant (e. g., on shared wireless links or bandwidth-on- always constant (e. g., on shared wireless links or bandwidth-on-
demand links). Depending on the network technology, there can be demand links). Depending on the network technology, there can be
queues or bottlenecks that are not directly visible at the IP layer. queues or bottlenecks that are not directly visible at the IP
networking layer.
Difficulties also arise when using IP-in-IP tunnels [RFC 2003] IPsec Difficulties also arise when using IP-in-IP tunnels [RFC 2003]
tunnels [RFC4301], IP encapsulated in L2TP [RFC2661], GRE [RFC1701], IPsec tunnels [RFC4301], IP encapsulated in L2TP [RFC2661], GRE
PPTP [RFC2637] or MPLS [RFC3031] [RFC3032] [RFC5129]. In these cases, [RFC1701] [RFC2784], PPTP [RFC2637] or MPLS [RFC3031] [RFC3032]
link information could be determined by cross-layer information [RFC5129]. In these cases, link information could be determined by
exchange, but this requires link layer technology specific cross-layer information exchange, but this requires link layer
interfaces. An alternative could be online measurements, but this can technology specific interfaces. An alternative could be online
cause significant additional network overhead. General guidelines for measurements, but this can cause significant additional network
encapsulation and decapsulation of explicit congestion information overhead. General guidelines for encapsulation and decapsulation
are currently in preparation [ECN-tunnel]. of explicit congestion information are currently in preparation
[ECN-tunnel].
2. Traffic carried over (outgoing) links 2. Traffic carried over (outgoing) links
Accurate online measurement of data rates is challenging when traffic Accurate online measurement of data rates is challenging when
is bursty. For instance, measuring a "current link load" requires traffic is bursty. For instance, measuring a "current link load"
defining the right measurement interval/ sampling interval. This is a requires defining the right measurement interval / sampling
challenge for proposals that require knowledge e.g. about the current interval. This is a challenge for proposals that require knowledge
link utilization. e.g. about the current link utilization.
3. Internal buffer statistics 3. Internal buffer statistics
Some proposals use buffer statistics such as a virtual queue length Some proposals use buffer statistics such as a virtual queue
to trigger feedback. However, network components can include multiple length to trigger feedback. However, network components can
distributed buffer stages that make it difficult to obtain such include multiple distributed buffer stages that make it difficult
metrics. to obtain such metrics.
Open questions are: Can and should this information be made Open questions are:
available, e.g., by additional interfaces or protocols?
- Can and should this information be made available, e.g., by
additional interfaces or protocols?
3.1.4 Feedback signaling 3.1.4 Feedback signaling
Explicit notification mechanisms can be realized either by in-band Explicit notification mechanisms can be realized either by in-band
signaling (notifications piggybacked along with the data traffic) or signaling (notifications piggybacked along with the data traffic) or
by out-of-band signaling [Sarola07]. The latter case requires by out-of-band signaling [Sarola07]. The latter case requires
additional protocols and a secure binding between the signals and the additional protocols and a secure binding between the signals and the
packets they refer to. Out-of-band signaling can be further packets they refer to. Out-of-band signaling can be further
subdivided into path-coupled and path-decoupled approaches. subdivided into path-coupled and path-decoupled approaches.
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checksum only covers all the necessary header fields and this checksum only covers all the necessary header fields and this
checksum does not show an error, it is possible for errors to be checksum does not show an error, it is possible for errors to be
found in the payload using a second checksum. Such error detection is found in the payload using a second checksum. Such error detection is
possible with UDP-Lite and DCCP; it was found to work well over a possible with UDP-Lite and DCCP; it was found to work well over a
GPRS network in a study [Chester04] and poorly over a WiFi network in GPRS network in a study [Chester04] and poorly over a WiFi network in
another study [Rossi06] [Welzl08]. Note that, while UDP-Lite and DCCP another study [Rossi06] [Welzl08]. Note that, while UDP-Lite and DCCP
enable the detection of corruption, the specifications of these enable the detection of corruption, the specifications of these
protocols do not foresee any specific reaction to it for the time protocols do not foresee any specific reaction to it for the time
being. being.
The idea of having a transport endpoint detect and accordingly react The idea of having a transport end-point detecting and accordingly
(or not) to corruption poses a number of interesting questions reacting (or not) to corruption poses a number of interesting
regarding cross-layer interactions. As IP is designed to operate over questions regarding cross-layer interactions. As IP is designed to
arbitrary link layers, it is therefore difficult to design a operate over arbitrary link layers, it is therefore difficult to
congestion control mechanism on top of it, which appropriately reacts design a congestion control mechanism on top of it, which
to corruption - especially as the specific data link layers that are appropriately reacts to corruption - especially as the specific data
in use along an end-to-end path are typically unknown to entities at link layers that are in use along an end-to-end path are typically
the transport layer. unknown to entities at the transport layer.
While the IETF has not yet specified how a congestion control While the IETF has not yet specified how a congestion control
mechanism should react to corruption, proposals exist in the mechanism should react to corruption, proposals exist in the
literature. For instance, TCP Westwood sets the congestion window literature. For instance, TCP Westwood sets the congestion window
equal to the measured bandwidth at time of congestion in response to equal to the measured bandwidth at the time of congestion in response
three DupACKs or a timeout. This measurement is obtained by counting to three DupACKs or a timeout. This measurement is obtained by
and filtering the ACK rate. This setting provides a significant counting and filtering the ACK rate. This setting provides a
goodput improvement in noisy channels because the "blind" by half significant goodput improvement in noisy channels because the "blind"
window reduction of standard TCP is avoided, i.e. the window is not by half window reduction of standard TCP is avoided, i.e. the window
reduced by too much [Mascolo01]. is not reduced by too much [Mascolo01].
Open questions concerning corruption loss include: Open questions concerning corruption loss include:
- How should corruption loss be detected? - How should corruption loss be detected?
- How should a source react when it is known that corruption has - How should a source react when it is known that corruption has
occurred? occurred?
- Can an ECN-capable flow infer that loss must be due to corruption - Can an ECN-capable flow infer that loss must be due to corruption
just from lack of explicit congestion notifications around a loss just from lack of explicit congestion notifications around a loss
episode [LT-TCP]? Or could this inference be dangerous given the episode [LT-TCP]? Or could this inference be dangerous given the
transport doesn't know whether queues on the path are all ECN- transport does not know whether all queues on the path are ECN-
capable? capable or not?
3.3 Challenge 3: Packets Sizes 3.3 Challenge 3: Packet Size
TCP does not take packet size into account when responding to losses TCP does not take packet size into account when responding to losses
or ECN. Over past years, the performance of TCP congestion avoidance or ECN. Over past years, the performance of TCP congestion avoidance
algorithms has been extensively studied. The well known "square root algorithms has been extensively studied. The well known "square root
formula" provides the performance of the TCP congestion avoidance formula" provides the performance of the TCP congestion avoidance
algorithm for TCP Reno [RFC2581]. [Padhye98] enhances the model to algorithm for TCP Reno [RFC2581]. [Padhye98] enhances the model to
account for timeouts, receiver window, and delayed ACKs. account for timeouts, receiver window, and delayed ACKs.
For the sake of the present discussion, we will assume that the TCP For the sake of the present discussion, we will assume that the TCP
throughput is expressed using the simplified formula. Using this throughput is expressed using the simplified formula. Using this
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inversely proportional to the RTT and the square root of the drop inversely proportional to the RTT and the square root of the drop
probability: probability:
S 1 S 1
B ~ C --- ------- B ~ C --- -------
RTT sqrt(p) RTT sqrt(p)
where, where,
S is the TCP segment size (in bytes) S is the TCP segment size (in bytes)
RTT is the end-to-end round trip time of the TCP connection RTT is the end-to-end round trip time of the TCP
(in seconds) connection (in seconds)
p is the packet drop probability p is the packet drop probability
Neglecting the fact that the TCP rate linearly depends on it, Neglecting the fact that the TCP rate linearly depends on it,
choosing the ideal packet size is a trade-off between high throughput choosing the ideal packet size is a trade-off between high throughput
(the larger a packet, the smaller the relative header overhead) and (the larger a packet, the smaller the relative header overhead) and
low delay (the smaller a packet, the shorter the time that is needed low delay (the smaller a packet, the shorter the time that is needed
until it is filled with data). Observing that TCP is not suited for until it is filled with data). Observing that TCP is not optimal for
applications such as streaming media (since reliable in-order applications with streaming media (since reliable in-order delivery
delivery and congestion control can cause arbitrarily long delays), and congestion control can cause arbitrarily long delays), this
this trade-off has not usually been considered for TCP applications, trade-off has not usually been considered for TCP applications, and
and the influence of the packet size on the sending rate is has not the influence of the packet size on the sending rate is has not
typically been seen as a significant issue, given there are still few typically been seen as a significant issue, given there are still few
paths through the Internet that support packets larger than the 1500B paths through the Internet that support packets larger than the 1500B
common with Ethernet. common with Ethernet.
The situation is already different for the Datagram Congestion The situation is already different for the Datagram Congestion
Control Protocol (DCCP) [RFC4340], which has been designed to enable Control Protocol (DCCP) [RFC4340], which has been designed to enable
unreliable but congestion-controlled datagram transmission, avoiding unreliable but congestion-controlled datagram transmission, avoiding
the arbitrary delays associated with TCP. DCCP is intended for the arbitrary delays associated with TCP. DCCP is intended for
applications such as streaming media that can benefit from control applications such as streaming media that can benefit from control
over the tradeoffs between delay and reliable in-order delivery. over the tradeoffs between delay and reliable in-order delivery.
DCCP provides for a choice of modular congestion control mechanisms. DCCP provides for a choice of modular congestion control mechanisms.
DCCP uses Congestion Control Identifiers (CCIDs) to specify the DCCP uses Congestion Control Identifiers (CCIDs) to specify the
congestion control mechanism. Three profiles are currently specified: congestion control mechanism. Three profiles are currently specified:
- DCCP Congestion Control ID 2 (CCID 2) [RFC4341]: - DCCP Congestion Control ID 2 (CCID 2) [RFC4341]:
TCP-like Congestion Control. CCID 2 sends data using a close TCP-like Congestion Control. CCID 2 sends data using a close
variant of TCP's congestion control mechanisms, incorporating a approximation of TCP's congestion control, incorporating a
variant of SACK [RFC2018, RFC3517]. CCID 2 is suitable for senders variant of SACK [RFC2018, RFC3517]. CCID 2 is suitable for senders
who can adapt to the abrupt changes in congestion window typical of which can adapt to the abrupt changes in congestion window typical
TCP's AIMD congestion control, and particularly useful for senders of TCP's AIMD congestion control, and particularly useful for
who would like to take advantage of the available bandwidth in an senders which would like to take advantage of the available
environment with rapidly changing conditions. bandwidth in an environment with rapidly changing conditions.
- DCCP Congestion Control ID 3 (CCID 3) [RFC4342]: - DCCP Congestion Control ID 3 (CCID 3) [RFC4342]:
TCP-Friendly Rate Control (TFRC) [RFC3448bis] is a congestion TCP-Friendly Rate Control (TFRC) [RFC3448bis] is a congestion
control mechanism designed for unicast flows operating in a best- control mechanism designed for unicast flows operating in a best-
effort Internet environment. It is reasonably fair when competing effort Internet environment. It is reasonably fair when competing
for bandwidth with TCP flows, but has a much lower variation of for bandwidth with TCP flows, but has a much lower variation of
throughput over time compared with TCP, making it more suitable for throughput over time than TCP, making it more suitable for
applications such as streaming media where a relatively smooth applications such as streaming media where a relatively smooth
sending rate is of importance. CCID 3 is appropriate for flows that sending rate is of importance. CCID 3 is appropriate for flows that
would prefer to minimize abrupt changes in the sending rate, would prefer to minimize abrupt changes in the sending rate,
including streaming media applications with small or moderate including streaming media applications with small or moderate
receiver buffering before playback. receiver buffering before playback.
- DCCP Congestion Control ID 4 [draft-ietf-ccid4-02.txt]: - DCCP Congestion Control ID 4 [draft-ietf-ccid4-04.txt]:
TFRC Small Packets (TFRC-SP) [RFC4828], a variant of TFRC TFRC Small Packets (TFRC-SP) [RFC4828], a variant of the TFRC
mechanism has been designed for applications that exchange small mechanism has been designed for applications that exchange small
packets. The objective of TFRC-SP is to achieve the same bandwidth packets. The objective of TFRC-SP is to achieve the same bandwidth
in bps (bits per second) as a TCP flow using packets of up to 1500 in bps (bits per second) as a TCP flow using packets of up to 1500
bytes. TFRC-SP enforces a minimum interval of 10 ms between data bytes. TFRC-SP enforces a minimum interval of 10 ms between data
packets to prevent a single flow from sending small packets packets to prevent a single flow from sending small packets
arbitrarily frequently. TFRC is a congestion control mechanism for arbitrarily frequently. CCID 4 has been designed to be used either
unicast flows operating in a best-effort Internet environment, and by applications that use a small fixed segment size, or by
is designed for DCCP that controls the sending rate based on a applications that change their sending rate by varying the segment
stochastic Markov model for TCP Reno. CCID 4 has been designed to size. Because CCID 4 is intended for applications that use a fixed
be used either by applications that use a small fixed segment size, small segment size, or that vary their segment size in response to
or by applications that change their sending rate by varying the congestion, the transmit rate derived from the TCP throughput
segment size. Because CCID 4 is intended for applications that use equation is reduced by a factor that accounts for the packet header
a fixed small segment size, or that vary their segment size in size, as specified in [RFC4828].
response to congestion, the transmit rate derived from the TCP
throughput equation is reduced by a factor that accounts for packet
header size, as specified in [RFC4828].
The resulting open questions are: The resulting open questions are:
- How does TFRC-SP operate under various network conditions? - How does TFRC-SP operate under various network conditions?
- How to design congestion control so as to scale with packet - How to design congestion control so as to scale with packet
size (dependency of congestion algorithm on packet size)? size (dependency of congestion algorithm on packet size)?
Today, many network resources are designed so that packet processing Today, many network resources are designed so that packet processing
cannot be overloaded even for incoming loads at the maximum bit-rate cannot be overloaded even for incoming loads at the maximum bit-rate
of the line. If packet processing can handle sustained load r [packet of the line. If packet processing can handle sustained load r [packet
per second] and the minimum packet size is h [bit] (i.e. packet per second] and the minimum packet size is h [bit] (i.e. packet
headers with no payload), then a line rate of x [bit per second] will headers with no payload), then a line rate of x [bit per second] will
never be able to overload packet processing as long as x =< r.h. never be able to overload packet processing as long as x =< r.h.
However, realistic equipment is often designed to only cope with a However, realistic equipment is often designed to only cope with a
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Therefore, it is likely that most congestion seen on today's Internet Therefore, it is likely that most congestion seen on today's Internet
is due to an excess of bits rather than packets, although packet- is due to an excess of bits rather than packets, although packet-
congestion is not impossible for runs of small packets (e.g. TCP ACKs congestion is not impossible for runs of small packets (e.g. TCP ACKs
or DoS attacks with small UDP datagrams). or DoS attacks with small UDP datagrams).
This observation raises additional open issues: This observation raises additional open issues:
- Will bit congestion remain prevalent? - Will bit congestion remain prevalent?
Being able to assume that congestion is generally due to excess Being able to assume that congestion is generally due to excess
bits not excess packets is a useful simplifying assumption in the bits, not excess packets is a useful simplifying assumption in the
design of congestion control protocols. Can we rely on this design of congestion control protocols. Can we rely on this
assumption into the future? An alternative view of the future is assumption for the future? An alternative view is that in-network
that in-network processing will become commonplace, so that per- processing will become commonplace, so that per-packet processing
packet processing will be as likely to be the bottleneck as per-bit will be as likely to be the bottleneck as per-bit transmission
transmission [Shin08]. [Shin08].
Over the last three decades, performance gains have mainly been Over the last three decades, performance gains have mainly been
through increased packet rates, not bigger packets. But if bigger achieved through increased packet rates, not bigger packets. But if
maximum segment sizes do become more prevalent, tiny segments (e.g. bigger maximum segment sizes do become more prevalent, tiny
ACKs) will not stop being widely used – leading to - a widening segments (e.g. ACKs) will not stop being widely used - leading to a
range of packet sizes. widening range of packet sizes.
The open question is thus whether or not packet processing rates The open question is thus whether or not packet processing rates
(r) will keep up with growth in transmission rates (x). A (r) will keep up with growth in transmission rates (x). A
superficial look at Moore's Law type trends would suggest that superficial look at Moore's Law type trends would suggest that
processing (r) will continue to outstrip growth in transmission processing (r) will continue to outstrip growth in transmission
(x). But predictions based on actual knowledge of technology (x). But predictions based on actual knowledge of technology
futures would be useful. Another open question is whether there are futures would be useful. Another open question is whether there are
likely to be more small packets in the average packet mix. If the likely to be more small packets in the average packet mix. If the
answers to either of these questions predict that packet congestion answers to either of these questions predict that packet congestion
could become prevalent, congestion control protocols will have to could become prevalent, congestion control protocols will have to
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congestion and packet congestion), and policing loss. congestion and packet congestion), and policing loss.
If congestion is due to excess bits, the bit rate should be If congestion is due to excess bits, the bit rate should be
reduced. If congestion is due to excess packets, the packet rate reduced. If congestion is due to excess packets, the packet rate
can be reduced without reducing the bit rate - by using larger can be reduced without reducing the bit rate - by using larger
packets. However, if the transport cannot tell which of these packets. However, if the transport cannot tell which of these
causes led to a specific drop, its only safe response is to reduce causes led to a specific drop, its only safe response is to reduce
the bit rate. This is why the Internet would be more complicated if the bit rate. This is why the Internet would be more complicated if
packet congestion were prevalent, as reducing the bit rate normally packet congestion were prevalent, as reducing the bit rate normally
also reduces the packet rate, while reducing the packet rate also reduces the packet rate, while reducing the packet rate
doesn't necessarily reduce the bit rate. does not necessarily reduce the bit rate.
Given distinguishing between transmission loss and congestion is Given distinguishing between transmission loss and congestion is
already an open issue (Section 3.2), if that problem is ever already an open issue (Section 3.2), if that problem is ever
solved, a further open issue would be whether to standardize a solved, a further open issue would be whether to standardize a
solution that distinguishes all the above causes of drop, not just solution that distinguishes all the above causes of drop, not just
two of them. two of them.
Nonetheless, even if we find a way for network equipment to Nonetheless, even if we find a way for network equipment to
explicitly distinguish which sort of drop has occurred, we will explicitly distinguish which sort of drop has occurred, we will
never be able to assume that such a smart AQM solution is deployed never be able to assume that such a smart AQM solution is deployed
at every congestible resource throughout the Internet - at every at every congestible resource throughout the Internet - at every
higher layer device like firewalls, proxies, servers and at every higher layer device like firewalls, proxies, servers and at every
lower layer device like low-end home hubs, DSLAMs, WLAN cards, lower layer device like low-end home hubs, DSLAMs, WLAN cards,
cellular base-stations and so on. Thus, transport protocols will cellular base-stations and so on. Thus, transport protocols will
always have to cope with drops due to unpredictable causes, so we always have to cope with drops due to unpredictable causes, so we
should always treat AQM smarts as an optimization, not a given. should always treat, e.g., AQM as an optimization, not a given.
- What does a congestion notification on a packet of a certain size - What does a congestion notification on a packet of a certain size
mean? mean?
The open issue here is whether a loss or explicit congestion mark The open issue here is whether a loss or explicit congestion mark
should be interpreted as a single congestion event irrespective of should be interpreted as a single congestion event irrespective of
the size of the packet lost or marked, or whether the strength of the size of the packet lost or marked, or whether the strength of
the congestion notification is weighted by the size of the packet. the congestion notification is weighted by the size of the packet.
This issue is discussed at length in [Bri08], along with other This issue is discussed at length in [Bri08], along with other
aspects of packet size and congestion control. aspects of packet size and congestion control.
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must then face the issue of how they should take account of packet must then face the issue of how they should take account of packet
size. If we determine that TCP was incorrect in not taking account size. If we determine that TCP was incorrect in not taking account
of packet size, even if we don't change TCP, we should not allow of packet size, even if we don't change TCP, we should not allow
new protocols to follow TCP's example in this respect. For example, new protocols to follow TCP's example in this respect. For example,
as explained here above, the small-packet variant of TCP-friendly as explained here above, the small-packet variant of TCP-friendly
rate control (TFRC-SP [RFC4828]) is an experimental protocol that rate control (TFRC-SP [RFC4828]) is an experimental protocol that
aims to take account of packet size. Whatever packet size it uses, aims to take account of packet size. Whatever packet size it uses,
it ensures its rate approximately equals that of a TCP using 1500B it ensures its rate approximately equals that of a TCP using 1500B
segments. This raises the further question of whether TCP with segments. This raises the further question of whether TCP with
1500B segments will be a suitable long-term gold standard, or 1500B segments will be a suitable long-term gold standard, or
whether we need a more thoroughgoing review of what it means for a whether we need a more thorough review of what it means for a
congestion control to scale with packet size. congestion control to scale with packet size.
3.4 Challenge 4: Flow Startup 3.4 Challenge 4: Flow Startup
The beginning of data transmissions imposes some further, unique The beginning of data transmissions imposes some further, unique
challenges: When a connection to a new destination is established, challenges: when a connection to a new destination is established,
the end-systems have hardly any information about the characteristics the end-systems have hardly any information about the characteristics
of the path in between and the available bandwidth. In this flow of the path in between and the available bandwidth. In this flow
startup situation there is no obvious choice how to start to send. A startup situation there is no obvious choice how to start to send. A
similar problem also occurs after relatively long idle times, since similar problem also occurs after relatively long idle times, since
the congestion control state then no longer reflects current the congestion control state then no longer reflects current
information about the state of the network (flow restart problem). information about the state of the network (flow restart problem).
Van Jacobson [Jacobson88] suggested using the slow-start mechanism Van Jacobson [Jacobson88] suggested using the slow-start mechanism
both for the flow startup and the flow restart, and this is today’s both for the flow startup and the flow restart, and this is today's
standard solution [RFC2581]. The slow-start algorithm starts with a standard solution [RFC2581]. The slow-start algorithm starts with a
small initial congestion window, which is exponentially increased as small initial congestion window, which is exponentially increased as
soon as acknowledgements arrive. However, the slow-start is not soon as acknowledgements arrive. However, the slow-start is not
optimal in many situations: First, it can take quite a long time optimal in many situations: First, it can take quite a long time
until a sender can fully utilize the available bandwidth on a path. until a sender can fully utilize the available bandwidth on a path.
Second, the exponential increase may be too aggressive and cause Second, the exponential increase may be too aggressive and cause
multiple packet loss if large congestion windows are reached (slow- multiple packet loss if large congestion windows are reached (slow-
start overshooting). Finally, the slow-start does not ensure that new start overshooting). Finally, the slow-start does not ensure that new
flows converge quickly to a reasonable share of resources, in flows converge quickly to a reasonable share of resources, in
particular if they compete with long-lived flows. This convergence particular if they compete with long-lived flows. This convergence
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The slow-start and its interaction with the congestion avoidance The slow-start and its interaction with the congestion avoidance
phase was largely designed by intuition [Jacobson88]. So far, little phase was largely designed by intuition [Jacobson88]. So far, little
theory has been developed to understand the flow startup problem and theory has been developed to understand the flow startup problem and
its implication on congestion control stability and fairness. There its implication on congestion control stability and fairness. There
is also no established methodology to evaluate whether new flow is also no established methodology to evaluate whether new flow
startup mechanisms are appropriate or not. startup mechanisms are appropriate or not.
As a consequence, it is a non-trivial task to address the As a consequence, it is a non-trivial task to address the
shortcomings of the slow-start algorithm. Several experimental shortcomings of the slow-start algorithm. Several experimental
enhancements have been proposed, such as the congestion window enhancements have been proposed, such as congestion window validation
validation [RFC2861] and the limited slow-start [RFC3742]. There are [RFC2861] and limited slow-start [RFC3742]. There are also ongoing
also ongoing research activities, focusing e.g. on bandwidth research activities, focusing e.g. on bandwidth estimation
estimation techniques, delay-based congestion control, or rate pacing techniques, delay-based congestion control, or rate pacing
mechanisms. However, any alternative end-to-end flow startup approach mechanisms. However, any alternative end-to-end flow startup approach
has to cope with the inherent problem that there is no or only few has to cope with the inherent problem that there is no or only little
information about the path at the beginning of a data transfer. This information about the path at the beginning of a data transfer. This
uncertainty could be reduced by more expressive feedback signaling uncertainty could be reduced by more expressive feedback signaling
(cf. Section 3.1). For instance, a source could learn the path (cf. Section 3.1). For instance, a source could learn the path
characteristics faster with the Quick-Start mechanism [RFC4782]. But, characteristics faster with the Quick-Start mechanism [RFC4782]. But,
even if the source knew exactly what rate it should aim for, it would even if the source knew exactly what rate it should aim for, it would
still not necessarily be safe to jump straight to that rate. The end- still not necessarily be safe to jump straight to that rate. The end-
system still doesn't know how much how a change in its own rate will system still does not know how a change in its own rate will affect
affect the path, which also might become congested in less than one the path, which also might become congested in less than one RTT.
RTT. Further research would be useful to understand the effect of Further research would be useful to understand the effect of
decreasing the uncertainty by explicit feedback separately from decreasing the uncertainty by explicit feedback separately from
control theoretic stability questions. Furthermore, the flow startup control theoretic stability questions. Furthermore, the flow startup
also raises fairness questions. For instance, it is unclear whether also raises fairness questions. For instance, it is unclear whether
it could be reasonable to use a faster startup when an end-system it could be reasonable to use a faster startup when an end-system
detects that a path is currently not congested. detects that a path is currently not congested.
In summary, there are several topics for further research concerning In summary, there are several topics for further research concerning
flow startups: flow startups:
- Better theoretical understanding of the design and evaluation of - Better theoretical understanding of the design and evaluation of
flow startup mechanisms, concerning their impact on congestion flow startup mechanisms, concerning their impact on congestion
risk, stability, and fairness risk, stability, and fairness.
- Evaluate whether it may be appropriate to allow more - Evaluate whether it may be appropriate to allow alternative
differentiated starting schemes, e. g., to allow higher initial starting schemes, e.g., to allow higher initial rates under certain
rates under certain constraints; this also requires refining constraints; this also requires refining fairness for startup
fairness for startup situations situations.
- Better theoretical models for the effects of decreasing - Better theoretical models for the effects of decreasing
uncertainty by additional network feedback, in particular if the uncertainty by additional network feedback, in particular if the
path characteristics are very dynamic. path characteristics are very dynamic.
3.5 Challenge 5: Multi-domain Congestion Control 3.5 Challenge 5: Multi-domain Congestion Control
Transport protocols such as TCP operate over the Internet that is Transport protocols such as TCP operate over the Internet, which is
divided into autonomous systems. These systems are characterized by divided into autonomous systems. These systems are characterized by
their heterogeneity as IP networks are realized by a multitude of their heterogeneity as IP networks are realized by a multitude of
technologies. The variety of conditions and their variations leads to technologies.
correlation effects between policers that regulate traffic against
certain conformance criteria. 3.5.1 Multi-domain Transport of Congestion Signals
The variety of conditions and their variations leads to correlation
effects between policers that regulate traffic against certain
conformance criteria.
With the advent of techniques allowing for early detection of With the advent of techniques allowing for early detection of
congestion, packet loss is no longer the sole metric of congestion. congestion, packet loss is no longer the sole metric of congestion.
ECN (Explicit Congestion Notification) marks packets - set by active ECN (Explicit Congestion Notification) marks packets - set by active
queue management techniques - to convey congestion information trying queue management techniques - to convey congestion information trying
to prevent packet losses (packet loss and the number of packets to prevent packet losses (packet loss and the number of packets
marked gives an indication of the level of congestion). Using TCP marked gives an indication of the level of congestion). Using TCP
ACKs to feed back that information allows the hosts to realign their ACKs to feed back that information allows the hosts to realign their
transmission rate and thus encourage them to efficiently use the transmission rate and thus encourage them to efficiently use the
network. In IP, ECN uses the two unused bits of the TOS field network. In IP, ECN uses the two unused bits of the TOS field
skipping to change at page 22, line 46 skipping to change at page 23, line 9
ECN [RFC3168] is an example of a congestion feedback mechanism from ECN [RFC3168] is an example of a congestion feedback mechanism from
the network toward hosts. The congestion-based feedback scheme the network toward hosts. The congestion-based feedback scheme
however has limitations when applied on an inter-domain basis. however has limitations when applied on an inter-domain basis.
Indeed, Section 8 and 19 of RFC3168 details consequences/implication Indeed, Section 8 and 19 of RFC3168 details consequences/implication
of i) a network erasing CE introduced earlier on the path and ii) a of i) a network erasing CE introduced earlier on the path and ii) a
network changing Not-ECT to ECT. Both of which could allow an network changing Not-ECT to ECT. Both of which could allow an
attacking network to cause excess congestion in an upstream network, attacking network to cause excess congestion in an upstream network,
even if the transports were behaving correctly. There are since so even if the transports were behaving correctly. There are since so
far two possible solutions to problem i) the ECN nonce [RFC3540] and far two possible solutions to problem i) the ECN nonce [RFC3540] and
the re-ECN incentive system. Nevertheless, the absence of IPv6 header the re-ECN incentive system. Nevertheless, the absence of an IPv6
checksum implies that corruption could be more impacting than in the header checksum implies that corruption could be more impacting than
IPv4 case. Fragmentation is another: the ECN-nonce cannot protect in the IPv4 case. Fragmentation is another: the ECN-nonce cannot
against misbehaving receivers that conceal marked fragments, so some protect against misbehaving receivers that conceal marked fragments,
protection is lost in situations where Path MTU discovery is so some protection is lost in situations where Path MTU discovery is
disabled. So, there is still room for improvement on the ECN disabled. So, there is still room for improvement on the ECN
mechanism to cope with ECN when operating in multi-domain networks. mechanism when operating in multi-domain networks.
Operational/deployment experience is nevertheless required to Operational/deployment experience is nevertheless required to
determine the extent of these problems. The second problem is mainly determine the extent of these problems. The second problem is mainly
related to deployment and usage practices and does not seem to result related to deployment and usage practices and does not seem to result
into any specific research challenge. in any specific research challenge.
Another solution in a multi-domain environment may be the TCP rate Another solution in a multi-domain environment may be the TCP rate
controller (TRC), a traffic conditioner which regulates the TCP flow controller (TRC), a traffic conditioner which regulates the TCP flow
at the ingress node in each domain by controlling packet drops and at the ingress node in each domain by controlling packet drops and
delays of the packets in a flow. The outgoing traffic from a TRC delays of the packets in a flow. The outgoing traffic from a TRC
controlled domain is shaped in such a way that no packets are dropped controlled domain is shaped in such a way that no packets are dropped
at the policer. However, the TRC depends on the end-to-end TCP model, at the policer. However, the TRC depends on the end-to-end TCP model,
and thus the diversity of TCP implementations is a general problem. and thus the diversity of TCP implementations is a general problem.
3.5.1 Multi-domain operations 3.5.2 Multi-domain Information Exchange
Security is a challenge for multi-domain network operation. At domain Security is a challenge for multi-domain network operation. At domain
boundaries, authentication and authorization issues can arise boundaries, authentication and authorization issues can arise
whenever congestion control information is exchanged. From this whenever congestion control information is exchanged. From this
perspective, the Internet does not have so far a single general perspective, the Internet does not have so far a single general
security architecture that could be used in all cases. Many security architecture that could be used in all cases. Many
autonomous systems also only exchange some limited amount of autonomous systems also only exchange some limited amount of
information about their internal state (topology hiding principle), information about their internal state (topology hiding principle),
even though having more precise information could be highly even though having more precise information could be highly
beneficial for congestion control. Indeed, prevent revealing internal beneficial for congestion control. Indeed, prevent revealing internal
network structure is highly sensitive in multi-domain network network structure is highly sensitive in multi-domain network
operations and thus also a concern when it comes to the deployability operations and thus also a concern when it comes to the deployability
of congestion control schemes. For instance, an RCP-like scheme could of congestion control schemes. For instance, a network-assisted
reveal more information about the internal network dimensioning than congestion control scheme with explicit signaling could reveal more
TCP does today. information about the internal network dimensioning than TCP does
today.
The future evolution of the Internet inter-domain operation has to The future evolution of the Internet inter-domain operation has to
show whether more multi-domain information exchange can be show whether more multi-domain information exchange can be
effectively realized. This is of particular importance for congestion effectively realized. This is of particular importance for congestion
control schemes that make use of explicit per-datagram rate feedback control schemes that make use of explicit per-datagram rate feedback
(e.g. RCP or XCP) or explicit rate feedback or that use in-band (e.g. RCP or XCP) or explicit rate feedback or that use in-band
congestion signaling (e.g. QuickStart) or out-of-band signaling (e.g. congestion signaling (e.g. QuickStart) or out-of-band signaling (e.g.
CADPC/PTP). Explicit signaling exchanges at the inter-domain level CADPC/PTP). Explicit signaling exchanges at the inter-domain level
that result in local domain triggers are currently absent from the that result in local domain triggers are currently absent from the
Internet. From this perspective, security means resulting from Internet. From this perspective, security means resulting from
limited trust between different administrative units result in policy limited trust between different administrative units result in policy
enforcement that exacerbates difficulty encountered when explicit enforcement that exacerbates difficulty encountered when explicit
feedback congestion control information is exchanged between domains. feedback congestion control information is exchanged between domains.
3.5.2 Multi-domain Pseudowires 3.5.3 Multi-domain Pseudowires
Extending pseudo-wires across multiple domains poses specific issues. Extending pseudo-wires across multiple domains poses specific issues.
Pseudowires (PW) may carry non-TCP data flows (e.g. TDM traffic) over Pseudowires (PW) may carry non-TCP data flows (e.g. TDM traffic) over
a multi-domain IP networks. Structure Agnostic TDM over Packet a multi-domain IP network. Structure Agnostic TDM over Packet
(SATOP) [RFC4553], Circuit Emulation over Packet Switched Networks (SATOP) [RFC4553], Circuit Emulation over Packet Switched Networks
(CESoPSN), TDM over IP, are not responsive to congestion control in a (CESoPSN), TDM over IP, are not responsive to congestion control in a
TCP-friendly manner as discussed by [RFC2914] (see also [RFC5033]). TCP-friendly manner as discussed by [RFC2914] (see also [RFC5033]).
Moreover, it is not possible to simply reduce the flow rate of a TDM Moreover, it is not possible to simply reduce the flow rate of a TDM
PW when facing packet loss. Indeed, providers can rate control PW when facing packet loss. Providers can rate control corresponding
corresponding incoming traffic but it may not be able to detect that incoming traffic but they may not be able to detect that PW carry TDM
a PW carries TDM traffic. This can be illustrated with the following traffic. This can be illustrated with the following example.
example.
........... ............ ........... ............
. . . . . .
S1 --- E1 --- . . S1 --- E1 --- . .
. | . . . | . .
. === E5 === E7 --- . === E5 === E7 ---
. | . . | . | . . |
S2 --- E2 --- . . | S2 --- E2 --- . . |
. . . | | . . . | |
........... . | v ........... . | v
skipping to change at page 25, line 4 skipping to change at page 25, line 18
The problem arises for transit provider P2 that is not able to detect The problem arises for transit provider P2 that is not able to detect
that IP packets are carrying constant-bit rate service traffic for that IP packets are carrying constant-bit rate service traffic for
which the only useful congestion control mechanism would rely on which the only useful congestion control mechanism would rely on
implicit or explicit admission control. implicit or explicit admission control.
Assuming P1 providers are rate limiting BE traffic, a transit P2 Assuming P1 providers are rate limiting BE traffic, a transit P2
provider router R may be subject to serious congestion as all TDM PWs provider router R may be subject to serious congestion as all TDM PWs
cross the same router. TCP-friendly traffic (e.g. each flow within cross the same router. TCP-friendly traffic (e.g. each flow within
the PW) would follow TCP's AIMD algorithm of reducing the sending the PW) would follow TCP's AIMD algorithm of reducing the sending
rate in half in response to each packet drop. Nevertheless, the PWs rate in half in response to each packet drop. Nevertheless, the PWs
of TDM traffic could take all the available capacity while other more carrying TDM traffic could take all the available capacity while
TCP-friendly traffic reduced itself to nothing. Note that other more TCP-friendly traffic reduced itself to nothing. Note that
the situation may simply occur because S4 suddenly turns on the situation may simply occur because S4 suddenly turns on
additional TDM channels. additional TDM channels.
It is neither possible nor desirable to assume that edge routers will It is neither possible nor desirable to assume that edge routers will
soon have the ability to detect the responsiveness of the carried soon have the ability to detect the responsiveness of the carried
traffic, but it is still important for transit providers to be able traffic, but it is still important for transit providers to be able
to police a fair, robust, responsive and efficient congestion control to police a fair, robust, responsive and efficient congestion control
technique in order to avoid impacting congestion responsive Internet technique in order to avoid impacting congestion responsive Internet
traffic. traffic.
skipping to change at page 25, line 52 skipping to change at page 26, line 18
The preferential treatment of higher precedence traffic with The preferential treatment of higher precedence traffic with
appropriate congestion control mechanisms is still an open issue that appropriate congestion control mechanisms is still an open issue that
may, depending on the proposed solution, impact both the host and the may, depending on the proposed solution, impact both the host and the
network precedence awareness, and thereby congestion control. network precedence awareness, and thereby congestion control.
[RFC2990] points out that the interactions between congestion control [RFC2990] points out that the interactions between congestion control
and DiffServ [RFC2475] have yet to be addressed, and this statement and DiffServ [RFC2475] have yet to be addressed, and this statement
is still valid at the time of writing. is still valid at the time of writing.
There is also still work to be performed regarding lower precedence There is also still work to be performed regarding lower precedence
traffic – data transfers which are useful, yet not important enough traffic - data transfers which are useful, yet not important enough
to significantly impair any other traffic. Examples of applications to significantly impair any other traffic. Examples of applications
that could make use of such traffic are web caches and web browsers that could make use of such traffic are web caches and web browsers
(e.g. for pre-fetching) as well as peer-to-peer applications. There (e.g. for pre-fetching) as well as peer-to-peer applications. There
are proposals for achieving low precedence on a pure end-to-end basis are proposals for achieving low precedence on a pure end-to-end basis
(e.g. TCP-LP [Kuzmanovic03]), and there is a specification for (e.g. TCP-LP [Kuzmanovic03]), and there is a specification for
achieving it via router mechanisms [RFC3662]. It seems, however, that achieving it via router mechanisms [RFC3662]. It seems, however, that
such traffic is still hardly used, and sending lower precedence data such traffic is still hardly used, and sending lower precedence data
is not yet a common service on the Internet. is not yet a common service on the Internet.
3.7 Challenge 7: Misbehaving Senders and Receivers 3.7 Challenge 7: Misbehaving Senders and Receivers
skipping to change at page 26, line 27 skipping to change at page 26, line 42
interest to honestly return feedback about congestion on the path, interest to honestly return feedback about congestion on the path,
effectively requesting a slower transfer. It is not in the sender's effectively requesting a slower transfer. It is not in the sender's
interest to reduce its rate in response to congestion if it can rely interest to reduce its rate in response to congestion if it can rely
on others to do so. Additionally, networks may have strategic reasons on others to do so. Additionally, networks may have strategic reasons
to make other networks appear congested. to make other networks appear congested.
Numerous strategies to improve the congestion control have already Numerous strategies to improve the congestion control have already
been identified. The IETF has particularly focused on misbehaving TCP been identified. The IETF has particularly focused on misbehaving TCP
receivers that could confuse a compliant sender into assigning receivers that could confuse a compliant sender into assigning
excessive network and/or server resources to that receiver (e.g. excessive network and/or server resources to that receiver (e.g.
[Sav99], [RFC3540]). But, although such strategies are worryingly [Savage99], [RFC3540]). But, although such strategies are worryingly
powerful, they do not yet seem common (however, evidence of attack powerful, they do not yet seem common (however, evidence of attack
prevalence is itself a research requirement). prevalence is itself a research requirement).
A growing proportion of Internet traffic comes from applications A growing proportion of Internet traffic comes from applications
designed not to use congestion control at all, or worse, applications designed not to use congestion control at all, or worse, applications
that add more forward error correction the more losses they that add more forward error correction the more losses they
experience. Some believe the Internet was designed to allow such experience. Some believe the Internet was designed to allow such
freedom so it can hardly be called misbehavior. But others consider freedom so it can hardly be called misbehavior. But others consider
that it is misbehavior to abuse this freedom [RFC3714], given one that it is misbehavior to abuse this freedom [RFC3714], given one
person's freedom can constrain the freedom of others (congestion person's freedom can constrain the freedom of others (congestion
skipping to change at page 27, line 8 skipping to change at page 27, line 22
Note that the problem is not just misbehavior driven by a self- Note that the problem is not just misbehavior driven by a self-
interested desire for more bandwidth. Indeed, congestion control may interested desire for more bandwidth. Indeed, congestion control may
be attacked by someone who makes no gain for themselves, other than be attacked by someone who makes no gain for themselves, other than
the satisfaction of harming others (see Security Considerations in the satisfaction of harming others (see Security Considerations in
Section 4). Section 4).
Open research questions resulting from these considerations are: Open research questions resulting from these considerations are:
- By design, new congestion control protocols need to enable one end - By design, new congestion control protocols need to enable one end
to check the other for protocol compliance. to check the other for protocol compliance. Still, it is unclear
- We need to provide congestion control primitives that satisfy more how such mechanisms would have to be designed.
demanding applications (smoother than TFRC, faster than high speed
TCPs), so that application developers and users do not turn off - Which congestion control primitives could satisfy more demanding
congestion control to get the rate they expect and need. applications (smoother than TFRC, faster than high speed TCPs), so
that application developers and users do not turn off congestion
control to get the rate they expect and need.
Note also that self-restraint is disappearing from the Internet. So, Note also that self-restraint is disappearing from the Internet. So,
it may no longer be sufficient to rely on developers/users it may no longer be sufficient to rely on developers/users
voluntarily submitting themselves to congestion control. As main voluntarily submitting themselves to congestion control. As a
consequence, mechanisms to enforce fairness (see Sections 2.3, 3.4, consequence, mechanisms to enforce fairness (see Sections 2.3, 3.4,
and 3.5) need to have more emphasis within the research agenda. and 3.5) need to have more emphasis within the research agenda.
3.8 Other challenges 3.8 Other Challenges
This section provides additional challenges and open research issues This section provides additional challenges and open research issues
that are not (at this point in time) deemed very large or of that are not (at this point in time) deemed very large or of
different nature compared to the main challenges depicted so far. different nature compared to the main challenges depicted so far.
Note that this section may be complemented in future release of this 3.8.1 RTT Estimation
document by topics discussed during the last ICCRG meeting, co-
located with PFLDNet 2008 International Workshop. Topics of interest
include multipath congestion control, and congestion control for
multimedia codecs that only support certain set of data rates.
3.8.1 RTT estimation
Several congestion control schemes have to precisely know the round- Several congestion control schemes have to precisely know the round-
trip time (RTT) of a path. The RTT is a measure of the current delay trip time (RTT) of a path. The RTT is a measure of the current delay
on a network. It is defined as the delay between the sending of a on a network. It is defined as the delay between the sending of a
packet and the reception of a corresponding response, if echoed back packet and the reception of a corresponding response, if echoed back
immediately by receiver upon receipt of the packet. This corresponds immediately by the receiver upon receipt of the packet. This
to the sum of the one-way delay of the packet and the (potentially corresponds to the sum of the one-way delay of the packet and the
different) one-way delay of the response. Furthermore, any RTT (potentially different) one-way delay of the response. Furthermore,
measurement also includes some additional delay due to the packet any RTT measurement also includes some additional delay due to the
processing in both end-systems. packet processing in both end-systems.
There are various techniques to measure the RTT: Active measurements There are various techniques to measure the RTT: active measurements
inject special probe packets to the network and then measure the inject special probe packets to the network and then measure the
response time, using e.g. ICMP. In contrast, passive measurements response time, using e.g. ICMP. In contrast, passive measurements
determine the RTT from ongoing communication processes, without determine the RTT from ongoing communication processes, without
sending additional packets. sending additional packets.
The connection endpoints of reliable transport protocols such as TCP, The connection endpoints of reliable transport protocols such as TCP,
SCTP, and DCCP, as well as several application protocols, keep track SCTP, and DCCP, as well as several application protocols, keep track
of the RTT in order to dynamically adjust protocol parameters such as of the RTT in order to dynamically adjust protocol parameters such as
the retransmission timeout (RTO). They can implicitly measure the RTT the retransmission timeout (RTO). They can implicitly measure the RTT
on the sender side by observing the time difference between the on the sender side by observing the time difference between the
skipping to change at page 28, line 18 skipping to change at page 28, line 31
measurements from retransmitted segments [RFC2988]. Traditionally, measurements from retransmitted segments [RFC2988]. Traditionally,
TCP implementations take one RTT measurement at a time (i. e., about TCP implementations take one RTT measurement at a time (i. e., about
once per RTT). As alternative, the TCP timestamp option [RFC1323] once per RTT). As alternative, the TCP timestamp option [RFC1323]
allows more frequent explicit measurements, since a sender can safely allows more frequent explicit measurements, since a sender can safely
obtain an RTT sample from every received acknowledgment. In obtain an RTT sample from every received acknowledgment. In
principle, similar measurement mechanisms are used by protocols other principle, similar measurement mechanisms are used by protocols other
than TCP. than TCP.
Sometimes it would be beneficial to know the RTT not only at the Sometimes it would be beneficial to know the RTT not only at the
sender, but also at the receiver, e.g., to find the one-way variation sender, but also at the receiver, e.g., to find the one-way variation
in delay due to one-way congestion.. A passive receiver can deduce in delay due to one-way congestion. A passive receiver can deduce
some information about the RTT by analyzing the sequence numbers of some information about the RTT by analyzing the sequence numbers of
received segments. But this method is error-prone and only works if received segments. But this method is error-prone and only works if
the sender permanently sends data. Other network entities on the path the sender permanently sends data. Other network entities on the path
can apply similar heuristics in order to approximate the RTT of a can apply similar heuristics in order to approximate the RTT of a
connection, but this mechanism is protocol-specific and requires per- connection, but this mechanism is protocol-specific and requires per-
connection state. In the current Internet, there is no simple and connection state. In the current Internet, there is no simple and
safe solution to determine the RTT of a connection in network safe solution to determine the RTT of a connection in network
entities other than the sender. entities other than the sender.
As outlined earlier in this document, the round-trip time is As outlined earlier in this document, the round-trip time is
skipping to change at page 29, line 37 skipping to change at page 29, line 50
values that may not be optimal for most Internet communication. values that may not be optimal for most Internet communication.
Still, the impact of more aggressive settings is not well Still, the impact of more aggressive settings is not well
understood. understood.
- Clock granularities: RTT estimation depends on the clock - Clock granularities: RTT estimation depends on the clock
granularities of the protocol stacks. Even though there is a trend granularities of the protocol stacks. Even though there is a trend
towards higher precision timers, the limited granularity towards higher precision timers, the limited granularity
(particularly on low cost devices) may still prevent highly (particularly on low cost devices) may still prevent highly
accurate RTT estimations. accurate RTT estimations.
3.8.2 Malfunctioning devices 3.8.2 Malfunctioning Devices
There is a long history of malfunctioning devices harming the There is a long history of malfunctioning devices harming the
deployment of new and potentially beneficial functionality in the deployment of new and potentially beneficial functionality in the
Internet. Sometimes, such devices drop packets or even crash Internet. Sometimes, such devices drop packets or even crash
completely when a certain mechanism is used, causing users to opt for completely when a certain mechanism is used, causing users to opt for
reliability instead of performance and disable the mechanism, or reliability instead of performance and disable the mechanism, or
operating system vendors to disable it by default. One well-known operating system vendors to disable it by default. One well-known
example is ECN, whose deployment was long hindered by malfunctioning example is ECN, whose deployment was long hindered by malfunctioning
firewalls and is still hindered by malfunctioning home-hubs, but firewalls and is still hindered by malfunctioning home-hubs, but
there are many other examples (e.g. the Window Scaling option of TCP) there are many other examples (e.g. the Window Scaling option of TCP)
[Thaler07]. [Thaler07].
skipping to change at page 30, line 12 skipping to change at page 30, line 26
of eventually seeing them deployed in the Internet, it would be of eventually seeing them deployed in the Internet, it would be
useful to collect information about failures caused by devices of useful to collect information about failures caused by devices of
this sort, analyze the reasons for these failures, and determine this sort, analyze the reasons for these failures, and determine
whether there are ways for such devices to do what they intend to do whether there are ways for such devices to do what they intend to do
without causing unintended failures. Recommendation for vendors of without causing unintended failures. Recommendation for vendors of
these devices could be derived from such an analysis. It would also these devices could be derived from such an analysis. It would also
be useful to see whether there are ways for failures caused by such be useful to see whether there are ways for failures caused by such
devices to become more visible to endpoints, or for those failures to devices to become more visible to endpoints, or for those failures to
become more visible to the maintainers of such devices. become more visible to the maintainers of such devices.
3.8.3. Dependence on RTT 3.8.3 Dependence on RTT
AIMD window algorithms that have the goal of packet conservation end AIMD window algorithms that have the goal of packet conservation end
up converging on a rate that is inversely proportional to RTT. up converging on a rate that is inversely proportional to RTT.
However, control theoretic approaches to stability have shown that However, control theoretic approaches to stability have shown that
only the increase in rate (acceleration) not the target rate needs to only the increase in rate (acceleration) not the target rate needs to
be inversely proportional to RTT. be inversely proportional to RTT.
It is possible to have more aggressive behaviors for some demanding It is possible to have more aggressive behaviors for some demanding
applications as long as they are part of a mix with less aggressive applications as long as they are part of a mix with less aggressive
transports [Key04]. This beneficial effect of transport type mixing transports [Key04]. This beneficial effect of transport type mixing
is probably how the Internet currently manages to remain stable even is probably how the Internet currently manages to remain stable even
in the presence of TCP slow start, which is more aggressive than the in the presence of TCP slow start, which is more aggressive than the
theory allows for stability. Research giving deeper insight into theory allows for stability. Research giving deeper insight into
these aspects would be very useful. these aspects would be very useful.
3.8.4. Congestion Control in Multi-layered Networks 3.8.4 Congestion Control in Multi-layered Networks
We often forget that a network of IP nodes is just as vulnerable to A network of IP nodes is just as vulnerable to congestion in the
congestion in the lower layers between IP-capable nodes as it is to lower layers between IP-capable nodes as it is to congestion on the
congestion on the IP-capable nodes themselves. As we develop IP-capable nodes themselves. If network elements take a greater part
techniques for network equipment to take a greater part in congestion in congestion control (ECN, XCP, RCP, etc. - see Section 3.1), these
control (ECN, XCP, RCP etc – see Section 3.1), we must not forget techniques will either need to be deployed at lower layers as well,
that these techniques will either need to be deployed at lower layers or they will need to interwork with lower layer mechanisms.
as well, or they will need to interwork with lower layer mechanisms.
[ECN-tunnel] gives guidelines on propagating ECN from lower layers [ECN-tunnel] gives guidelines on propagating ECN from lower layers
upwards, but to the authors' knowledge the layering problem has not upwards but to the authors' knowledge the layering problem has not
been addressed for explicit rate protocol proposals such as XCP & been addressed for explicit rate protocol proposals such as XCP and
RCP. Some issues are straightforward matters of interoperability RCP. Some issues are straightforward matters of interoperability
(e.g. how exactly to copy fields up the layers). While others are (e.g. how exactly to copy fields up the layers) while others are
less obvious (e.g. re-framing issues: if RCP were deployed in a lower less obvious (e.g. re-framing issues: if RCP were deployed in a lower
layer, how might multiple small RCP frames all with different rates layer, how might multiple small RCP frames all with different rates
in their headers be assembled into a larger IP-layer datagram?). in their headers be assembled into a larger IP-layer datagram?).
Multi-layer considerations also confound many mechanisms that aim to Multi-layer considerations also confound many mechanisms that aim to
discover whether every node on the path supports the new congestion discover whether every node on the path supports the new congestion
control protocol. For instance, some proposals maintain a secondary control protocol. For instance, some proposals maintain a secondary
TTL field parallel to that in the IP header. Any nodes that support TTL field parallel to that in the IP header. Any nodes that support
the new behavior update both TTL fields, whereas legacy IP nodes will the new behavior update both TTL fields, whereas legacy IP nodes will
only update the IP TTL field. This allows the endpoints to check only update the IP TTL field. This allows the endpoints to check
whether all IP nodes on the path support the new behavior, in which whether all IP nodes on the path support the new behavior, in which
case both TTLs will be equal at the receiver. But mechanisms like case both TTLs will be equal at the receiver. But mechanisms like
these overlook nodes at lower layers that might not support the new these overlook nodes at lower layers that might not support the new
behavior. behavior.
It should also be possible to include the issue of congestion control A further related issue is congestion control across overlay networks
across overlay networks of relays under the general area of multi- of relays.
layer congestion control.
3.8.5. Multipath End-to-end Congestion Control and Traffic Engineering 3.8.5 Multipath End-to-end Congestion Control and Traffic Engineering
Recent work has shown that multipath endpoint congestion control Recent work has shown that multipath endpoint congestion control
[Kelly05] offers considerable benefits in terms of resilience and [Kelly05] offers considerable benefits in terms of resilience and
resource usage efficiency. By pooling the resources on all paths, resource usage efficiency. By pooling the resources on all paths,
even nodes not using multiple paths benefit from those that are. even nodes not using multiple paths benefit from those that are.
Nowadays, there is considerable further research to do in this area, Nowadays, there is considerable further research to do in this area,
particularly to understand interactions with network operator particularly to understand interactions with network operator
controlled route provision and traffic engineering, and indeed controlled route provision and traffic engineering, and indeed
whether multipath congestion control can perform better traffic whether multipath congestion control can perform better traffic
engineering than the network itself, given the right incentives. engineering than the network itself, given the right incentives.
3.8.6 ALGs and Middleboxes 3.8.6 ALGs and Middleboxes
An increasing number of application layer gateways (ALG), An increasing number of application layer gateways (ALG),
middleboxes, and proxies (see Section 3.6 of [RFC2775]) are deployed middleboxes, and proxies (see Section 3.6 of [RFC2775]) is deployed
at domain boundaries to verify conformance but also filter traffic at domain boundaries to verify conformance but also filter traffic
and control flows to e.g. prevent among other information leaking and control flows. One motivation is to prevent information beyond
between autonomous systems beyond routing information. These systems routing data leaking between autonomous systems. These systems split
split up end-to-end TCP connections and prevent end-to-end congestion up end-to-end TCP connections and prevent end-to-end congestion
control. On the other side, transport over encrypted tunnels may not control. Furthermore, transport over encrypted tunnels may not allow
allow that other network entities to participate in congestion that other network entities to participate in congestion control.
control.
Basically, such systems disrupt the primal and dual congestion Basically, such systems disrupt the primal and dual congestion
control components whereas their effects have not been so far control components. In particular, end-to-end congestion control may
systematically studied. From this perspective, one shall account for be replaced by flow-control backpressure mechanisms on the split
two levels of interference: connections. A large variety of ALGs and middleboxes uses such
- The "transparent" case i.e. the end-point address from the sender mechanisms to improve the performance of applications (Performance
perspective is still the receiver (the destination IP address). For Enhancing Proxies, Application Accelerators, etc.). However, the
instance relay systems intercept payload but do not relay implications of such mechanisms, which are often proprietary and not
congestion control information. documented, have not been studied systematically so far.
- The "non-transparent" case is not a problem (back-to-back
connections) results in a lesser problem. Indeed, although these There are two levels of interference:
devices interfere with end-to-end network transparency, they
correctly terminating network, transport and application layer - The "transparent" case, i.e. the end-point address from the sender
protocols on both sides. perspective is still visible to the receiver (the destination IP
address). An example are relay systems that intercept payload but
do not relay congestion control information. Such middleboxes can
prevent the operation of end-to-end congestion control.
- The "non-transparent" case, which causes less problems. Although
these devices interfere with end-to-end network transparency, they
correctly terminate network, transport and application layer
protocols on both sides, which individually can be congestion
controlled.
4. Security Considerations 4. Security Considerations
Misbehavior may be driven by pure malice, or malice may in turn be Misbehavior may be driven by pure malice, or malice may in turn be
driven by wider selfish interests, e.g. using distributed denial of driven by wider selfish interests, e.g. using distributed denial of
service (DDoS) attacks to gain rewards by extortion [RFC4948]. DDoS service (DDoS) attacks to gain rewards by extortion [RFC4948]. DDoS
attacks are possible both because of vulnerabilities in operating attacks are possible both because of vulnerabilities in operating
systems and because the Internet delivers packets without requiring systems and because the Internet delivers packets without requiring
congestion control. congestion control.
skipping to change at page 32, line 32 skipping to change at page 33, line 5
spoofing. But if mechanisms to enforce congestion control fairness spoofing. But if mechanisms to enforce congestion control fairness
were robust to both selfishness and malice [Bri06] they would also were robust to both selfishness and malice [Bri06] they would also
naturally mitigate denial of service, which can be considered (from naturally mitigate denial of service, which can be considered (from
the perspective of well-behaving Internet user) as a congestion the perspective of well-behaving Internet user) as a congestion
control enforcement problem. Even some denial of service attacks on control enforcement problem. Even some denial of service attacks on
hosts (rather than the network) could be considered as a congestion hosts (rather than the network) could be considered as a congestion
control enforcement issue at the higher layer. But clearly there are control enforcement issue at the higher layer. But clearly there are
also denial of service attacks that would not be solved by enforcing also denial of service attacks that would not be solved by enforcing
congestion control. congestion control.
5. Contributors 5. References
This document is the result of a collective effort to which the
following people have contributed:
Dimitri Papadimitriou <dimitri.papadimitriou@alcatel-lucent.be>
Michael Welzl <michael.welzl@uibk.ac.at>
Wesley Eddy <weddy@grc.nasa.gov>
Bela Berde <bela.berde@gmx.de>
Paulo Loureiro <loureiro.pjg@gmail.com>
Chris Christou <christou_chris@bah.com>
Michael Scharf <michael.scharf@ikr.uni-stuttgart.de>
6. References
6.1 Normative References 5.1 Normative References
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791, [RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981. September 1981.
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, [RFC793] Postel, J., "Transmission Control Protocol", STD 7,
RFC793, September 1981. RFC793, September 1981.
[RFC896] Nagle, J., "Congestion Control in IP/TCP", RFC 896, [RFC896] Nagle, J., "Congestion Control in IP/TCP", RFC 896,
January 1984. January 1984.
[RFC1323] Jacobson, V., Braden, R., Borman, D., "TCP Extensions for [RFC1323] Jacobson, V., Braden, R., and Borman, D., "TCP Extensions
High Performance", RFC 1323, May 1992. for High Performance", RFC 1323, May 1992.
[RFC1701] Hanks, S., Li, T, Farinacci, D., and P. Traina, "Generic
Routing Encapsulation", RFC 1701, October 1994.
[RFC1958] Carpenter, B., Ed., "Architectural Principles of the
Internet", RFC 1958, June 1996.
[RFC1958] Carpenter, B., Ed., “Architectural Principles of the
[RFC2309] Braden, B., et al., "Recommendations on queue management [RFC2309] Braden, B., et al., "Recommendations on queue management
and congestion avoidance in the Internet", RFC 2309, and congestion avoidance in the Internet", RFC 2309,
April 1998. April 1998.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 1633, [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 1633,
October 1996. October 1996.
[RFC2474] Nichols, K., Blake, S. Baker, F. and D. Black, [RFC2474] Nichols, K., Blake, S. Baker, F. and D. Black,
"Definition of the Differentiated Services Field (DS "Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474, December Field) in the IPv4 and IPv6 Headers", RFC 2474, December
skipping to change at page 33, line 34 skipping to change at page 33, line 49
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z. [RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.
and Weiss, W., "An Architecture for Differentiated and Weiss, W., "An Architecture for Differentiated
Services", RFC 2475, December 1998. Services", RFC 2475, December 1998.
[RFC2581] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion [RFC2581] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999. Control", RFC 2581, April 1999.
[RFC2861] Handley, M., J. Padhye, J., and S., Floyd, "TCP [RFC2861] Handley, M., J. Padhye, J., and S., Floyd, "TCP
Congestion Window Validation", RFC 2861, June 2000. Congestion Window Validation", RFC 2861, June 2000.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D. and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
March 2000.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, [RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, September 2000. RFC 2914, September 2000.
[RFC2988] Paxson, V. and Allman, M., "Computing TCP's [RFC2988] Paxson, V. and Allman, M., "Computing TCP's
Retransmission Timer", RFC 2988, Nov. 2000 Retransmission Timer", RFC 2988, November 2000.
[RFC2990] Huston, G., "Next Steps for the IP QoS Architecture", [RFC2990] Huston, G., "Next Steps for the IP QoS Architecture",
RFC 2990, November 2000. RFC 2990, November 2000.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001. RFC 3168, September 2001.
[RFC3448] Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP [RFC3448] Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification", Friendly Rate Control (TFRC): Protocol Specification",
skipping to change at page 34, line 20 skipping to change at page 34, line 35
Per-Domain Behavior for Differentiated Services", RFC Per-Domain Behavior for Differentiated Services", RFC
3662, December 2003. 3662, December 2003.
[RFC3714] Floyd, S., and J. Kempf, Eds. "IAB Concerns Regarding [RFC3714] Floyd, S., and J. Kempf, Eds. "IAB Concerns Regarding
Congestion Control for Voice Traffic in the Internet", Congestion Control for Voice Traffic in the Internet",
RFC 3714, March 2004. RFC 3714, March 2004.
[RFC3742] Floyd, S., "Limited Slow-Start for TCP with Large [RFC3742] Floyd, S., "Limited Slow-Start for TCP with Large
Congestion Windows", RFC 3742, March 2004. Congestion Windows", RFC 3742, March 2004.
[RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to- [RFC3985] Bryant, S., and P. Pate, "Pseudo Wire Emulation Edge-to-
Edge (PWE3) Architecture", RFC 3985, March 2005. Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March Congestion Control Protocol (DCCP)", RFC 4340, March
2006. 2006.
[RFC4341] Floyd, S. and E. Kohler, "Profile for Datagram Congestion [RFC4341] Floyd, S. and E. Kohler, "Profile for Datagram Congestion
Control Protocol (DCCP) Congestion Control ID 2: TCP-like Control Protocol (DCCP) Congestion Control ID 2: TCP-like
Congestion Control", RFC 4341, March 2006. Congestion Control", RFC 4341, March 2006.
[RFC4342] Floyd, S., Kohler, E., and J. Padhye, "Profile for [RFC4342] Floyd, S., Kohler, E., and J. Padhye, "Profile for
Datagram Congestion Control Protocol (DCCP) Congestion Datagram Congestion Control Protocol (DCCP) Congestion
Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC
4342, March 2006. 4342, March 2006.
[RFC4553] Vainshtein, A. and Y. Stein, "Structure-Agnostic Time [RFC4553] Vainshtein, A., and Y. Stein, "Structure-Agnostic Time
Division Multiplexing (TDM) over Packet (SAToP)", Division Multiplexing (TDM) over Packet (SAToP)",
RFC 4553, June 2006. RFC 4553, June 2006.
[RFC4614] Duke, M., R. Braden, R., Eddy, W., and Blanton, E., "A [RFC4614] Duke, M., R. Braden, R., Eddy, W., and E. Blanton, "A
Roadmap for Transmission Control Protocol (TCP) Roadmap for Transmission Control Protocol (TCP)
Specification Documents", RFC 4614, September 2006. Specification Documents", RFC 4614, September 2006.
[RFC4782] Floyd, S., Allman, M., Jain, A., and P. Sarolahti, [RFC4782] Floyd, S., Allman, M., Jain, A., and P. Sarolahti,
"Quick-Start for TCP and IP", RFC 4782, Jan. 2007. "Quick-Start for TCP and IP", RFC 4782, January 2007.
[RFC4948] Andersson, L., Davies, E., and L. Zhang, "Report from the [RFC4948] Andersson, L., Davies, E., and L. Zhang, "Report from the
IAB workshop on Unwanted Traffic March 9-10, 2006", RFC IAB workshop on Unwanted Traffic March 9-10, 2006", RFC
4948, August 2007. 4948, August 2007.
[RFC5033] Floyd, S., and M. Allman, "Specifying New Congestion [RFC5033] Floyd, S., and M. Allman, "Specifying New Congestion
Control Algorithms", RFC 5033, Aug. 2007. Control Algorithms", RFC 5033, August 2007.
[RFC5405] Eggert, L., and G. Fairhurst, "Unicast UDP Usage [RFC5405] Eggert, L., and G. Fairhurst, "Unicast UDP Usage
Guidelines for Application Designers, RFC 5405, November Guidelines for Application Designers, RFC 5405, November
2008. 2008.
[iccrg-rfcs]Welzl, M., and W. Eddy, "Congestion Control in the RFC [ICCRG-RFCs] Welzl, M., and W. Eddy, "Congestion Control in the RFC
Series", Internet Draft, work in Progress, October 2008. Series", Internet Draft, work in Progress, October 2008.
6.2 Informative References 5.2 Informative References
[Allman99] Allman, M. and V. Paxson, "On Estimating End-to-End [Allman99] Allman, M., and V. Paxson, "On Estimating End-to-End
Network Path Properties", Proceedings of ACM SIGCOMM'99, Network Path Properties", Proceedings of ACM SIGCOMM'99,
September 1999. September 1999.
[Andrew00] L. Andrew, B. Wydrowski and S. Low, "An Example of [Andrew05] Andrew, L., Wydrowski, B., and S. Low, "An Example of
Instability in XCP", Manuscript available at Instability in XCP", Manuscript available at
<http://netlab.caltech.edu/maxnet/XCP_instability.pdf> <http://netlab.caltech.edu/maxnet/XCP_instability.pdf>
[Ath01] Athuraliya, S., Low, S., Li, V., and Q. Yin, "REM: Active [Ath01] Athuraliya, S., Low, S., Li, V., and Q. Yin, "REM: Active
queue management", IEEE Network Magazine, vol.15, no.3, Queue Management", IEEE Network Magazine, Vol.15, No.3,
pp.48-53, May 2001. pp.48-53, May 2001.
[BALAN01] Balan, R. K., Lee, B.P., Kumar, K.R.R., Jacob, L., Seah, [Balan01] Balan, R. K., Lee, B.P., Kumar, K.R.R., Jacob, L., Seah,
W.K.G., and Ananda, A.L., "TCP HACK: TCP Header Checksum W.K.G., and Ananda, A.L., "TCP HACK: TCP Header Checksum
Option to Improve Performance over Lossy Links", Option to Improve Performance over Lossy Links",
Proceedings of IEEE INFOCOM'01, Anchorage (Alaska), USA, Proceedings of IEEE INFOCOM'01, Anchorage (Alaska), USA,
April 2001. April 2001.
[Bonald00] Bonald, T., May, M., and J.-C. Bolot, "Analytic [Bonald00] Bonald, T., May, M., and J.-C. Bolot, "Analytic
Evaluation of RED Performance," Proceedings of IEEE Evaluation of RED Performance," Proceedings of IEEE
INFOCOM'00, Tel Aviv, Israel, March 2000. INFOCOM'00, Tel Aviv, Israel, March 2000.
[Bri08] Briscoe, B., Moncaster, T. and L. Burness, "Problem [Bri08] Briscoe, B., Moncaster, T. and L. Burness, "Problem
skipping to change at page 36, line 8 skipping to change at page 36, line 23
Workshop on the Economics of Securing the Information Workshop on the Economics of Securing the Information
Infrastructure, October 2006. Infrastructure, October 2006.
<http://wesii.econinfosec.org/draft.php?paper_id=19> <http://wesii.econinfosec.org/draft.php?paper_id=19>
[Bryant08] Bryant, S., Davie, B., Martini, L., and E. Rosen, [Bryant08] Bryant, S., Davie, B., Martini, L., and E. Rosen,
"Pseudowire Congestion Control Framework", Work in "Pseudowire Congestion Control Framework", Work in
Progress, draft-ietf-pwe3-congestion-frmwk-01.txt, May Progress, draft-ietf-pwe3-congestion-frmwk-01.txt, May
2008. 2008.
[Chester04] Chesterfield, J., Chakravorty, R., Banerjee, S., [Chester04] Chesterfield, J., Chakravorty, R., Banerjee, S.,
Rodriguez, P., Pratt, I. and Crowcroft, J., "Transport Rodriguez, P., Pratt, I., and Crowcroft, J., "Transport
level optimisations for streaming media over wide-area level optimisations for streaming media over wide-area
wireless networks", WIOPT'04, March 2004. wireless networks", WIOPT'04, March 2004.
[Chiu89] Chiu, D. M., and R. Jain, "Analysis of the increase and [Chiu89] Chiu, D. M., and R. Jain, "Analysis of the increase and
decrease algorithms for congestion avoidance in computer decrease algorithms for congestion avoidance in computer
networks", Computer Networks and ISDN Systems, vol.17, networks", Computer Networks and ISDN Systems, Vol.17,
pp.1-14, 1989. pp.1-14, 1989.
[Clark98] Clark, D. and W. Fang, "Explicit Allocation of Best- [Clark88] Clark, D., "The design philosophy of the DARPA internet
Effort Packet Delivery Service," IEEE/ACM Transactions on protocols", ACM SIGCOMM Computer Communication Review,
Networking, vol.6, no.4, pp.362-373, August 1998. Vol.18, No.4, pp.106-114, August 1988.
[Clark98] Clark, D., and W. Fang, "Explicit Allocation of Best-
Effort Packet Delivery Service," IEEE/ACM Transactions
on Networking, Vol.6, No.4, pp.362-373, August 1998.
[Dukki05] Dukkipati, N., Kobayashi, M., Zhang-Shen, R. and N., [Dukki05] Dukkipati, N., Kobayashi, M., Zhang-Shen, R. and N.,
McKeown, "Processor Sharing Flows in the Internet", McKeown, "Processor Sharing Flows in the Internet",
Proceedings of International Workshop on QoS (IWQoS'05), Proceedings of International Workshop on QoS (IWQoS'05),
June 2005. June 2005.
[Dukki06] Dukkipati, N. and N. McKeown, "Why Flow-Completion Time [Dukki06] Dukkipati, N. and N. McKeown, "Why Flow-Completion Time
is the Right Metric for Congestion Control", ACM SIGCOMM is the Right Metric for Congestion Control", ACM SIGCOMM
Computer Communication Review, Vol.36, No.1, January Computer Communication Review, Vol.36, No.1, January
2006. 2006.
[ECN-tunnel]Briscoe, B., "Layered Encapsulation of Congestion [ECN-tunnel]Briscoe, B., "Layered Encapsulation of Congestion
Notification", draft-briscoe-tsvwg-ecn-tunnel, Work in Notification", draft-briscoe-tsvwg-ecn-tunnel, Work in
progress. progress.
[ECODE] "ECODE Project", European Commission Seventh Framework
Program Contract Number: INFSO-ICT-223936
<http://www.ecode-project.eu>
[Falk07] Falk, A., et al., "Specification for the Explicit Control [Falk07] Falk, A., et al., "Specification for the Explicit Control
Protocol (XCP)", Work in Progress, draft-falk-xcp-spec- Protocol (XCP)", Work in Progress, draft-falk-xcp-spec-
03.txt, July 2007. 03.txt, July 2007.
[Firoiu00] Firoiu, V., and M. Borden, "A Study of Active Queue [Firoiu00] Firoiu, V., and M. Borden, "A Study of Active Queue
Management for Congestion Control," Proceedings of IEEE Management for Congestion Control," Proceedings of IEEE
INFOCOM'00, Tel Aviv, Israel, March 2000. INFOCOM'00, Tel Aviv, Israel, March 2000.
[Floyd93] Floyd, S., and V. Jacobson, "Random early detection [Floyd93] Floyd, S., and V. Jacobson, "Random early detection
gateways for congestion avoidance," IEEE/ACM Transactions gateways for congestion avoidance," IEEE/ACM Transactions
on Networking, vol.1, no.4, pp.397-413, August 1993. on Networking, Vol.1, No.4, pp.397-413, August 1993.
[Floyd94] Floyd, S., "TCP and Explicit Congestion Notification", [Floyd94] Floyd, S., "TCP and Explicit Congestion Notification",
ACM Computer Communication Review, vol.24, no.5, pp.10- ACM Computer Communication Review, Vol.24, No.5, pp.10-
23, October 1994. 23, October 1994.
[Floyd08] Floyd, S., and M. Allman, "Comments on the Usefulness of [Floyd08] Floyd, S., and M. Allman, "Comments on the Usefulness of
Simple Best-Effort Traffic", RFC 5290, July 2008. Simple Best-Effort Traffic", RFC 5290, July 2008.
[Hollot01] Hollot, C., Misra, V., Towsley, D., and W.-B. Gong, "A [Hollot01] Hollot, C., Misra, V., Towsley, D., and W.-B. Gong, "A
Control Theoretic Analysis of RED," Proceedings of IEEE Control Theoretic Analysis of RED," Proceedings of IEEE
INFOCOM'01, Anchorage, Alaska, April 2001. INFOCOM'01, Anchorage, Alaska, April 2001.
[Jacobson88]Jacobson, V., "Congestion Avoidance and Control", [Jacobson88]Jacobson, V., "Congestion Avoidance and Control",
skipping to change at page 37, line 28 skipping to change at page 37, line 51
[Jain90] Jain, R., "Congestion Control in Computer Networks: [Jain90] Jain, R., "Congestion Control in Computer Networks:
Trends and Issues", IEEE Network, pp. 24-30, May 1990. Trends and Issues", IEEE Network, pp. 24-30, May 1990.
[Jin04] Jin, Ch., Wei, D.X., and S. Low, "FAST TCP: Motivation, [Jin04] Jin, Ch., Wei, D.X., and S. Low, "FAST TCP: Motivation,
Architecture, Algorithms, Performance," Proceedings of Architecture, Algorithms, Performance," Proceedings of
IEEE INFOCOM'04, Hong-Kong, China, March 2004. IEEE INFOCOM'04, Hong-Kong, China, March 2004.
[Katabi02] Katabi, D., M. Handley, and C. Rohr, "Internet Congestion [Katabi02] Katabi, D., M. Handley, and C. Rohr, "Internet Congestion
Control for Future High Bandwidth-Delay Product Control for Future High Bandwidth-Delay Product
Environments", Proceedings of ACM SIGCOMM'02 Symposium, Environments", Proceedings of ACM SIGCOMM'02 Symposium,
pp. 89-102, August 2002. August 2002.
[Kelly98] Kelly, F., Maulloo, A., and D. Tan, "Rate control in [Kelly98] Kelly, F., Maulloo, A., and D. Tan, "Rate control in
communication networks: shadow prices, proportional communication networks: shadow prices, proportional
fairness, and stability," Journal of the Operational fairness, and stability," Journal of the Operational
Research Society, vol.49, pp. 237–252, 1998. Research Society, Vol.49, pp.237-252, 1998.
[Kelly05] Kelly, F., and Th. Voice, "Stability of end-to-end [Kelly05] Kelly, F., and Th. Voice, "Stability of end-to-end
algorithms for joint routing and rate control", ACM algorithms for joint routing and rate control", ACM
SIGCOMM Computer Communication Review, Vol.35, No.2, pp. SIGCOMM Computer Communication Review, Vol.35, No.2, pp.
5-12, April 2005. 5-12, April 2005.
[Keshav] Keshav, S., "What is congestion and what is congestion [Keshav07] Keshav, S., "What is congestion and what is congestion
control", Presentation at IRTF ICCRG Workshop, PFLDNet control", Presentation at IRTF ICCRG Workshop, PFLDNet
2007, Los Angeles (California), USA, February 2007. 2007, Los Angeles (California), USA, February 2007.
[Key04] Key, P., Massoulié, L., Bain, A., and F. Kelly, "Fair [Key04] Key, P., Massoulie, L., Bain, A., and F. Kelly, "Fair
Internet Traffic Integration: Network Flow Models and Internet Traffic Integration: Network Flow Models and
Analysis", Annales des Télécommunications, Vol.59, No.11- Analysis", Annales des Telecommunications, Vol.59, No.11-
12, pp. 1338-1352, November-December 2004. 12, pp. 1338-1352, November-December 2004.
[Krishnan04] Krishnan, R., Sterbenz, J., Eddy, W., Partridge, C., and [Krishnan04] Krishnan, R., Sterbenz, J., Eddy, W., Partridge, C., and
M. Allman, "Explicit Transport Error Notification (ETEN) M. Allman, "Explicit Transport Error Notification (ETEN)
for Error-Prone Wireless and Satellite Networks", for Error-Prone Wireless and Satellite Networks",
Computer Networks, vol.46, no.3, October 2004. Computer Networks, Vol.46, No.3, October 2004.
[Kuzmanovic03] Kuzmanovic, A., and E. W. Knightly, "TCP-LP: A [Kuzmanovic03] Kuzmanovic, A., and E. W. Knightly, "TCP-LP: A
Distributed Algorithm for Low Priority Data Transfer", Distributed Algorithm for Low Priority Data Transfer",
Proceedings of IEEE INFOCOM'03, San Francisco Proceedings of IEEE INFOCOM'03, San Francisco
(California), USA, April 2003. (California), USA, April 2003.
[Low05] Low, S., L. Andrew, L., and B. Wydrowski, "Understanding [Low05] Low, S., Andrew, L., and B. Wydrowski, "Understanding
XCP: equilibrium and fairness", Proceedings of IEEE XCP: equilibrium and fairness", Proceedings of IEEE
INFOCOM'05, Miami (Florida), USA, March 2005. INFOCOM'05, Miami (Florida), USA, March 2005.
[Low03.2] Low, S., Paganini, F., Wang, J., and J. Doyle, "Linear [Low03.2] Low, S., Paganini, F., Wang, J., and J. Doyle, "Linear
stability of TCP/RED and a scalable control", Computer stability of TCP/RED and a scalable control", Computer
Networks Journal, vol.43, no.5, pp.633-647, December Networks Journal, Vol.43, No.5, pp.633-647, December
2003. 2003.
[Low03.1] Low, S., "A duality model of TCP and queue management [Low03.1] Low, S., "A duality model of TCP and queue management
algorithms", IEEE/ACM Transactions on Networking, vol.11, algorithms", IEEE/ACM Transactions on Networking, Vol.11,
no.4, pp.525–536, August 2003. No.4, pp.525-536, August 2003.
[Low02] Low, S., Paganini, F., Wang, J., Adlakha, S., and J.C. [Low02] Low, S., Paganini, F., Wang, J., Adlakha, S., and J.C.
Doyle, "Dynamics of TCP/RED and a Scalable Control", Doyle, "Dynamics of TCP/RED and a Scalable Control",
Proceedings of IEEE INFOCOM'02, New York (New-Jersey), Proceedings of IEEE INFOCOM'02, New York (New-Jersey),
USA, June 2002. USA, June 2002.
[LT-TCP] Tickoo, O., Subramanian, V., Kalyanaraman, S., and K.K. [LT-TCP] Tickoo, O., Subramanian, V., Kalyanaraman, S., and K.K.
Ramakrishnan, "LT-TCP: End-to-End Framework to Improve Ramakrishnan, "LT-TCP: End-to-End Framework to Improve
TCP Performance over Networks with Lossy Channels", TCP Performance over Networks with Lossy Channels",
Proceedings of International Workshop on QoS (IWQoS), Proceedings of International Workshop on QoS (IWQoS),
June 2005. June 2005.
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Acknowledgments 6. Acknowledgments
The authors would like to thank the following people whose feedback The authors would like to thank the following people whose feedback
and comments contributed to this document: Keith Moore, Jan and comments contributed to this document: Keith Moore, Jan
Vandenabeele. Vandenabeele.
Dimitri Papadimitriou's contribution was partly funded by [ECODE], a
research project supported by the European Commission.
Larry Dunn (his comments at the Manchester ICCRG and discussions with Larry Dunn (his comments at the Manchester ICCRG and discussions with
him helped with the section on packet-congestibility). Bob Briscoe's him helped with the section on packet-congestibility).
contribution was partly funded by [TRILOGY], a research project
supported by the European Commission.
Author's Addresses Bob Briscoe's contribution was partly funded by [TRILOGY], a research
project supported by the European Commission.
7. Author's Addresses
Michael Welzl Michael Welzl
University of Innsbruck University of Oslo, Department of Informatics
Technikerstr 21a PO Box 1080 Blindern
A-6020 Innsbruck, Austria N-0316 Oslo, Norway
Phone: +43 (512) 507-6110 Phone: +47 22 85 24 20
Email: michael.welzl@uibk.ac.at Email: michawe@ifi.uio.no
Dimitri Papadimitriou Dimitri Papadimitriou
Alcatel-Lucent Alcatel-Lucent
Copernicuslaan, 50 Copernicuslaan, 50
B-2018 Antwerpen, Belgium 2018 Antwerpen, Belgium
Phone : +32 3 240 8491 Phone : +32 3 240 8491
Email: dimitri.papadimitriou@alcatel-lucent.be Email: dimitri.papadimitriou@alcatel-lucent.be
Michael Scharf Michael Scharf
University of Stuttgart University of Stuttgart
Pfaffenwaldring 47 Pfaffenwaldring 47
D-70569 Stuttgart D-70569 Stuttgart, Germany
Germany
Phone: +49 711 685 69006 Phone: +49 711 685 69006
Email: michael.scharf@ikr.uni-stuttgart.de Email: michael.scharf@ikr.uni-stuttgart.de
Bob Briscoe Bob Briscoe
BT & UCL BT & UCL
B54/77, Adastral Park B54/77, Adastral Park
Martlesham Heath Martlesham Heath
Ipswich IP5 3RE Ipswich IP5 3RE, UK
UK
Email: bob.briscoe@bt.com Email: bob.briscoe@bt.com
8. Contributors
The following additional people have contributed to this document:
- Wesley Eddy <weddy@grc.nasa.gov>
- Bela Berde <bela.berde@gmx.de>
- Paulo Loureiro <loureiro.pjg@gmail.com>
- Chris Christou <christou_chris@bah.com>
Full Copyright Statement Full Copyright Statement
Copyright (c) 2009 IETF Trust and the persons identified as the Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info). publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. and restrictions with respect to this document.
Acknowledgment Acknowledgments
Funding for the RFC Editor function is provided by the IETF Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA). Administrative Support Activity (IASA).
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