| Congestion Exposure (ConEx) Working Group | M. Mathis |
| Internet-Draft | Google, Inc |
| Intended status: Informational | B. Briscoe |
| Expires: January 22, 2015 | BT |
| July 21, 2014 |
Congestion Exposure (ConEx) Concepts, Abstract Mechanism and Requirements
draft-ietf-conex-abstract-mech-12
This document describes an abstract mechanism by which senders inform the network about the congestion encountered by packets earlier in the same flow. Today, network elements at any layer may signal congestion to the receiver by dropping packets or by ECN markings, and the receiver passes this information back to the sender in transport-layer feedback. The mechanism described here enables the sender to also relay this congestion information back into the network in-band at the IP layer, such that the total amount of congestion from all elements on the path is revealed to all IP elements along the path, where it could, for example, be used to provide input to traffic management. This mechanism is called congestion exposure or ConEx. The companion document "ConEx Concepts and Use Cases" provides the entry-point to the set of ConEx documentation.
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This document describes an abstract mechanism by which, to a first approximation, senders inform the network about the congestion encountered by packets earlier in the same flow. It is not a complete protocol specification, because it is known that designing an encoding (e.g. packet formats, codepoint allocations, etc) is likely to entail compromises that preclude some uses of the protocol. The goal of this document is to provide a framework for developing and testing algorithms to evaluate the benefits of the ConEx protocol and to evaluate the consequences of the compromises in various different encoding designs. This document lays out requirements for concrete protocol specifications.
A companion document [RFC6789] provides the entry point to the set of ConEx documentation. It outlines concepts that are pre-requisites to understanding why ConEx is useful, and it outlines various ways that ConEx might be used.
As typical end-to-end transport protocols continually seek out more network capacity, network elements signal whenever congestion results, and the transports are responsible for controlling this network congestion [RFC5681]. The more a transport tries to use capacity that others want to use, the more congestion signals will be attributable to that transport. Likewise, the more transport sessions sustained by a user and the longer the user sustains them, the more congestion signals will be attributable to that user. The goal of ConEx is to ensure that the resulting congestion signals are sufficiently visible and robust, because they are an ideal metric for networks to use as the basis of traffic management or other related functions.
Networks indicate congestion by three possible signals: packet loss, ECN marking or queueing delay. ECN marking and some packet loss may be the outcome of Active Queue Management (AQM), which the network uses to warn senders to reduce their rates. Packet loss is also the natural consequence of complete exhaustion of a buffer or other network resource. Some experimental transport protocols and TCP variants infer impending congestion from increasing queuing delay. However, delay is too amorphous to use as a congestion metric. In this and other ConEx documents, the term 'congestion signals' is generally used solely for ECN markings and packet losses, because they are unambiguous signals of congestion.
In both cases the congestion signals follow the route indicated in Figure 1. A congested network device sends a signal in the data stream on the forward path to the transport receiver, the receiver passes it back to the sender through transport level feedback, and the sender makes some congestion control adjustment.
This document extends the capabilities of the Internet protocol suite with the addition of a new Congestion Exposure signal. To a first approximation this signal, also shown in Figure 1, relays the congestion information from the transport sender back through the internetwork layer where it is visible to any interested internetwork layer devices along the forward path. This document frames the engineering problem of designing the ConEx signal. The requirements are described in Section 3 and some example encoding are presented in Section 4. Section 5 describes all of the protocol components.
This new signal is expressly designed to support a variety of new policy mechanisms that might be used to instrument, monitor or manage traffic. The policy devices are not shown in Figure 1 but might be placed anywhere along the forward data path (see Section 5.4).
,---------. ,---------. |Transport| |Transport| | Sender | . |Receiver | | | /|___________________________________________| | | ,-<---------------Congestion-Feedback-Signals--<--------. | | | |/ | | | | | |\ Transport Layer Feedback Flow | | | | | | \ ___________________________________________| | | | | | \| | | | | | | ' ,-----------. . | | | | | |_____________| |_______________|\ | | | | | | IP Layer | | Data Flow \ | | | | | | |(Congested)| \ | | | | | | | Network |--Congestion-Signals--->-' | | | | | Device | \| | | | | | | /| | | `----------->--(new)-IP-Layer-ConEx-Signals-------->| | | | | | / | | | |_____________| |_______________ / | | | | | | |/ | | `---------' `-----------' ' `---------'
Figure 1: The Flow of Congestion and ConEx Signals
Since the policy devices can affect how traffic is treated it is assumed that there is an intrinsic motivation for users, applications or operating systems to understate the congestion that they are causing. Therefore, it is important to be able to audit ConEx signals, and to be able apply sufficient sanction to discourage cheating of congestion policies. The general approach to auditing is to count signals on the forward path to confirm that there are never fewer ConEx signals than congestion signals. Many ConEx design constraints come from the need to assure that the audit function is sufficiently robust. The audit function is described in Section 5.5, however significant portions of this document (and prior research [Refb-dis]) is motivated by issues relating to the audit function and making it robust.
The congestion and ConEx signals shown in Figure 1 represent a series of discrete events: ECN marks or lost packets, carried by the forward data stream and fed back into the Internetwork layer. The policy and audit functions are most likely to act on the accumulated values of these signals, for which we use the term "volume". For example traffic volume is the total number of bytes delivered, optionally over a specified time interval and over some aggregate of traffic (e.g. all traffic from a site). While loss-volume is the total amount of bytes discarded from some aggregate over an interval. The term congestion-volume is defined precisely in [RFC6789]. Note that volume per unit time is (average) rate.
A design goal of the ConEx protocol is that the important policy mechanisms can be implemented per logical link without per flow state (see Section 5.4). However, the price to pay can be flow state to audit ConEx signals (Section 5.5). This is justified in that i) auditing at the edges, with limited per flow state, enables policy elsewhere, including in the core, without any per flow state; ii) auditing can use soft flow state, which does not require route pinning.
There is a long standing argument over units of congestion: bytes vs packets (see [RFC7141] and its references). Section 4.6 explains why this problem must be addressed carefully. However, this document does not take a strong position on this issue. Nonetheless, it does require that the units of congestion must be an explicitly stated property of any proposed encoding, and the consequences of that design decision must be evaluated along with other aspects of the design.
To be successful the ConEx protocol needs to have the property that the relevant stakeholders each have the incentive to unilaterally start on each stage of partial deployment, which in turn creates incentives for further deployment. Furthermore, legacy systems that will never be upgraded do not become a barrier to deploying ConEx. Issues relating to partial deployment are described in Section 6.
Note that ConEx signals are not intended to be used for fine-grained congestion control. They are anticipated to be most useful at longer time scales and/or at coarser granularity than single microflows. For example the total congestion caused by a user might serve as an input to higher level policy or accountability functions, designed to create incentives for improving user behavior, such as choosing to send large quantities of data at off-peak times, at lower data rates or with less aggressive protocols such as LEDBAT [RFC6817] (see [RFC6789]).
Ultimately ConEx signals have the potential to provide a mechanism to regulate global Internet congestion. From the earliest days of congestion control research there has been a concern that there is no mechanism to prevent transport designers from incrementally making protocols more aggressive without bound and spiraling to a "tragedy of the commons" Internet congestion collapse. The "TCP friendly" paradigm was created in part to forestall this failure. However, it no longer commands any authority because it has little to say about the Internet of today, which has moved beyond the scaling range of standard TCP. As a consequence, many transports and applications are opening arbitrarily large numbers of connections or using arbitrary levels of aggressiveness. ConEx represents a recognition that the IETF cannot regulate this space directly because it concerns the behaviour of users and applications, not individual transport protocols. Instead the IETF can give network operators the protocol tools to arbitrate the space themselves, with better bulk traffic management. This in turn should create incentives for users, and designers of application and of transport protocols to be more mindful about contributing to congesting.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
ConEx signals in IP packet headers from the sender to the network:
First time readers may wish to skim this section, since it is more understandable having read the entire document.
Ideally, all the following requirements would be met by a Congestion Exposure Signal:
It is already known that implementing ConEx signals is likely to entail some compromises, and therefore all the requirements above are expressed with the keyword 'SHOULD' rather than 'MUST'. The only mandatory requirement is that a concrete protocol description MUST give sound reasoning if it chooses not to meet some requirement.
The role of the audit function and constraints on it are described in Section 5.5. There is no intention to standardise the audit function. However, it is necessary to lay down the following normative constraints on audit behaviour so that transport designers will know what to design against and implementers of audit devices will know what pitfalls to avoid:
An experimental ConEx specification SHOULD describe the following protocol details:
The possibility exists that these specifications over constrain the ConEx design, and can not be fully satisfied. An important part of the evaluation of any particular design will be a thorough inventory of all ways in which it might fail to satisfy these specifications.
Most protocol specifications start with a description of packet formats and codepoints with their associated meanings. This document does not: It is already known that choosing the encoding for ConEx is likely to entail some engineering compromises that have the potential to reduce the protocol's usefulness in some settings. For instance the experimental ConEx encoding chosen for IPv6 [I-D.ietf-conex-destopt] had to make compromises on tunnelling. Rather than making these engineering choices prematurely, this document sidesteps the encoding problem by making it abstract. It describes several different representations of ConEx Signals, none of which are specified to the level of specific bits or code points.
The goal of this approach is to be as complete as possible for discovering the potential usage and capabilities of the ConEx protocol, so we have some hope of making optimal design decisions when choosing the encoding. Even if experiments reveal particular problems due to the encoding, then this document will still serve as a reference model.
For tutorial purposes, it is helpful to describe a naïve encoding of the ConEx protocol for TCP and similar protocols: set a bit (not specified here) in the IP header on each retransmission and on each ECN signaled window reduction. Network devices along the forward path can see this bit and act on it. For example any device along the path might limit the rate of all traffic if the rate of marked (congested) packets exceeds a threshold.
This simple encoding is sufficient to illustrate many of the benefits envisioned for ConEx. At first glance it looks like it might motivate people to deploy and use it. It is a one line code change that a small number of OS developers and content providers could unilaterally deploy across a significant fraction of all Internet traffic. However, this encoding does not support auditing so it would also motivate users and/or applications to misrepresent the congestion that they are causing [RFC3514]. As a consequence the naïve encoding is not likely to be trusted and thus creates its own disincentives for deployment.
Nonetheless, this Naïve encoding does present a clear mental model of how the ConEx protocol might function under various uses. It is useful for thought experiments where it can be stipulated that all participants are honest and it does illustrate some of the incentives that might be introduced by ConEx.
In limited contexts it is possible to implement ConEx-like functions without any signals at all by measuring rest-of-path congestion directly from TCP headers. The algorithm is to keep at least one RTT of past TCP headers and matching each new header against the history to count duplicate data.
This could implement many ConEx policies, without any explicit protocol. It is fairly easy to implement, at least at low rate (e.g. in a software based edge router). However, it would only be useful in cases where the network operator can see the TCP headers. This is currently (2014) the majority of traffic because UDP, IPSec and VPN tunnels are used far less than SSL or TLS over TCP/IP, which do not hide TCP sequence numbers from network devices. However, anyone specifically intending to avoid the attention of a congestion policy device would only have to hide their TCP headers from the network operator (e.g. by using a VPN tunnel).
The re-ECN specification [I-D.briscoe-conex-re-ecn-tcp] presents an encoding of ConEx in IPv4 and IPv6 that was tightly integrated with ECN encoding in order to fit into the IPv4 header. Any individual packet may need to represent any ECN codepoint and any ConEx signal value independently. So, ideally their encoding should be entirely independent. However, given the limited number of header bits and/or code points, re-ECN chooses to partially share code points and to re-echo both losses and ECN with just one codepoint.
The central theme of the re-ECN work is an audit mechanism that provides sufficient disincentives against misrepresenting congestion [I-D.briscoe-conex-re-ecn-motiv]. It is analyzed extensively in Briscoe's PhD dissertation [Refb-dis]. For a tutorial background on re-ECN motivation and techniques, see [22, 23].
Re-ECN is an example of one chosen set of compromises attempting to meet the requirements of Section 3. The present document takes a step back, aiming to state the ideal requirements in order to allow the Internet community to assess whether different compromises might be better.
The problem with Re-ECN is that it requires that receivers be ECN enabled in addition to sender changes. Newer encodings [I-D.ietf-conex-destopt] overcome this problem by being able to represent loss and ECN based congestion separately.
This encoding involves flag bits, each of which the sender can set independently to indicate to the network one of the following four signals:
A packet with ConEx set combined with all the three other flags cleared implies ConEx-Not-Marked
This encoding does not imply any exclusion property among the signals. Multiple types of congestion (ECN, loss) can be signalled on the same ACK. So, ideally, a ConEx sender would be able to reflect these in the next packet. However, there will be many invalid combinations of flags (e.g. Not-ConEx combined with any of the ConEx-marked flags), which a malicious sender could use to advantage against naïve policy devices that only check each flag separately.
As long as the packets in a flow have uniform sizes, it does not matter whether the units of congestion are packets or bytes. However, if an application sends very irregular packet sizes, it may be necessary for the sender to mark multiple packets to avoid being in technical violation of an audit function measuring in bytes (see Section 4.6).
This encoding involves signaling one of the following five codepoints:
ENUM {Not-ConEx, ConEx-Not-Marked, Re-Echo-Loss, Re-Echo-ECN, Credit}
Each named codepoint has the same meaning as in the encoding using independent bits in the previous section. The use of any one codepoint implies the negative of all the others.
Inherently, the semantics of most of the enumerated codepoints are mutually exclusive. 'Credit' is the only one that might need to be used in combination with either Re-Echo-Loss or Re-Echo-ECN, but even that requirement is questionable. It must not be forgotten that the enumerated encoding loses the flexibility to signal these two combinations, whereas the encoding with four independent bits is not so limited. Alternatively two extra codepoints could be assigned to these two combinations of semantics. The comment in the previous section about units also applies.
The following comments apply generally to all the other encodings.
Congestion can be due to exhaustion of bit-carrying capacity, or exhaustion of packet processing power. When a packet is discarded or marked to indicate congestion, there is no easy way to know whether the lost or marked packet signifies bit-congestion or packet-congestion. The above ConEx encodings that rely on marking packets suffer from the same ambiguity.
This problem is most acute when audit needs to check that one count of markings matches another. For example if there are ConEx markings on three large (1500B) packets, is that sufficient to match the loss of 5 small (60B) packets? If a packet-marking is defined to mean all the bytes in the packet are marked, then we have 4500B of Conex marked data against 300B of lost data, which is easily sufficient. If instead we are counting packets, then we have 3 ConEx packets against 5 lost packets, which is not sufficient. This problem will not arise when all the packets in a flow are the same size, but a choice needs to be made for flows in which packet sizes vary, such as BGP, SPDY and some variable rate video encoding schemes.
Whether to use bytes or packets is not obvious. For instance, the most expensive links in the Internet, in terms of cost per bit, are all at lower data rates, where transmission times are large and packet sizes are important. In order for a policy to consider wire time, it needs to know the number of congested bytes. However, high speed networking equipment and the transport protocols themselves sometimes gauge resource consumption and congestion in terms of packets.
This document does not take a strong position on this issue. However, a ConEx encoding will need to explicitly specify whether it assumes units of bytes or packets consistently for both congestion indications and ConEx markings (see network layer requirement E in Section 3.3). It may help to refer to the guidance in [RFC7141].
[RFC7141] advises that congestion indications should be interpreted in units of bytes when responding to congestion, at least on today's Internet. In any TCP implementation this is simple to achieve for varying size packets, given TCP SACK tracks losses in bytes. If an encoding is specified in units of bytes, the encoding should also specify which headers to include in the size of a packet (see network layer requirement F in Section 3.3).
The components shown in Figure 1 as well as policy and audit are described in more detail.
Congestion signals originate from network devices as they do today. A congested router, switch or other network device can discard or ECN mark packets when it is congested.
The sending transport needs to be modified to send Congestion Exposure signals in response to congestion feedback signals (e.g. for the case of a TCP transport see [I-D.ietf-conex-tcp-modifications]). We want to permit ConEx without ECN (e.g. if the receiver does not support ECN). However, we want to encourage a ConEx sender to at least attempt to negotiate ECN (a ConEx transport protocol spec may require this), because it is believed that ConEx without ECN is harder to audit, and thus potentially exposed to cheating. Since honest users have the potential to benefit from stronger mechanisms to manage traffic they have an incentive to deploy ConEx and ECN together. This incentive is not sufficient to prevent a dishonest user from constructing (or configuring) a sender that enables ConEx after choosing not to negotiate ECN, but it should be sufficient to prevent this from being the sustained default case for any significant pool of users.
Permitting ConEx without ECN is necessary to facilitate bootstrapping other parts of ConEx deployment.
Any receiving transport may already feedback sufficiently useful signals to the sender so that it does not need to be altered.
The native loss or ECN signaling mechanism required for compliance with existing congestion control standards (e.g. RTCP, SCTP) will typically be sufficient for the Sender to generate ConEx signals.
TCP's loss feedback is sufficient for ConEx if SACK is used [RFC2018]. However, the original specification for ECN in TCP [RFC3168] signals congestion no more than once per round trip. The sender may require more precise feedback from the receiver otherwise it is at risk of appearing to be understating its ConEx Signals.
Ideally, ConEx should be added to a transport like TCP without mandatory modifications to the receiver. But in the TCP-ECN case an optional modification to the receiver could be recommended for precision (see [I-D.ietf-tcpm-accecn-reqs], which is based on the approach originally taken when adding re-ECN to TCP [I-D.briscoe-conex-re-ecn-tcp]).
Policy devices are characterised by a need to be configured with a policy related to the users or neighboring networks being served. In contrast, auditing devices solely enforce compliance with the ConEx protocol and do not need to be configured with any client-specific policy.
One of the design goals of the ConEx protocol is that none of the important policy mechanisms requires per flow state, and that policy mechanisms can even be implemented for heavily aggregated traffic in the core of the Internet with complexity akin to accumulating marking volumes per logical link. Of course, policy mechanisms may sometimes choose to focus down on individual flows, but ConEx aims to make aggregate policy devices feasible.
Policy devices can typically be decomposed into two functions i) monitoring the ConEx signal to compare it with a policy then ii) acting in some way on the result. Various actions might be invoked against 'out of contract' traffic, such as policing (see Section 5.4.3), re-routing, or downgrading the class of service.
Alternatively a policy device might not act directly on the traffic, but instead report to management systems that are designed to control congestion indirectly. For instance the reports might trigger capacity upgrades, penalty clauses in contracts, levy charges based on congestion, or merely send warnings to clients who are causing excessive congestion.
Nonetheless, whatever action is invoked, the congestion monitoring function will always be a necessary part of any policy device.
ConEx signals indicate the level of congestion along a whole path from source to destination. In contrast, ECN signals monitored in the middle of a network indicate the level of congestion experienced so far on the path (of course, only in ECN-capable traffic).
If a monitor in the middle of a network (e.g. at a network border) measures both of these signals, it can subtract the level of ECN (path so far) from the level of ConEx (whole path) to derive a measure of the congestion that packets are likely to experience between the monitoring point and their destination (rest-of-path congestion).
It will often be preferable for policy devices to monitor rest-of-path congestion if they can, because it is a measure of the downstream congestion that the policy device can directly influence by controlling the traffic passing through it.
A congestion policer can be implemented in a very similar way to a bit-rate policer, but its effect can be focused solely on traffic of users causing congestion downstream, which ConEx signals make visible. Without ConEx signals, the only way to mitigate congestion is to blindly limit traffic bit-rate, on the assumption that high bit-rate is more likely to cause congestion.
A congestion policer monitors all ConEx traffic entering a network, or some identifiable subset. Using ConEx signals and/or Credit signals (and preferably subtracting ECN signals to yield rest-of-path congestion), it measures the amount of congestion that this traffic is contributing somewhere downstream. If this persistently exceeds a policy-configured 'congestion-bit-rate' the congestion policer can limit all the monitored ConEx traffic.
A congestion policer can be implemented by a simple token bucket applied to an aggregate. But unlike a bit-rate policer, it removes tokens only when it forwards packets that are ConEx-Marked and/or Credit-Marked, effectively treating Not-ConEx-Marked packets as invisible. Consequently, because tokens give the right to send congested bits, the fill-rate of the token bucket will represent the allowed congestion-bit-rate. This should provide sufficient traffic management without having to additionally constrain the straight bit-rate at all. See [I-D.briscoe-conex-policing] for details.
Note that the policing action could be to introduce a throttle (discard some traffic) immediately upstream of the congestion monitor. Alternatively, this throttle could introduce delay using a queue with its own AQM, which potentially increases the whole path congestion. In effect the congestion policer has moved the congestion earlier in the path, and focused it on one user to protect downstream resources by reducing the congestion in the rest of the path.
The most critical aspect of ConEx is the capability to support robust auditing. It can be assumed that sanctions based on ConEx signals will create an intrinsic motivation for users to understate the congestion that they are causing. So, without strong audit functions, the ConEx signal would become understated to the point of being useless. Therefore the most important feature of an encoding design is likely to be the robustness of the auditing it supports.
The general goal of an auditor is to make sure that any ConEx-enabled traffic is sent with sufficient ConEx-Re-Echo and ConEx-Credit signals. A concrete definition of the ConEx protocol MUST define what sufficient means.
If a ConEx-enabled transport does not carry sufficient ConEx signals, then an auditor is likely to apply some sanction to that traffic. Although sanctions are beyond the scope of this document, an example sanction might be to throttle the traffic immediately upstream of the auditor to prevent the user from getting any advantage by understating congestion. Such a throttle would likely include some combination of delaying or dropping traffic.
A ConEx auditor might use one of the following techniques:
In addition, other audit techniques may be identified in the future.
[Refb-dis] gives a comprehensive inventory of attacks against audit proposed by various people. It includes pseudocode for both deterministic and statistical audit functions designed to thwart these attacks and analyses the effectiveness of an implementation. Although this work is specific to the re-ECN protocol, most of the material is useful for designing and assessing audit of other specific ConEx encodings, against both ECN and loss.
The auditing function should be able to trigger sufficient sanction to discourage understating congestion [Salvatori05]. This seems to require designing the sanction in concert with the policy functions, even though they might be implemented in different parts of the network. However, [Refb-dis] proves audit and policy functions can be independent as long as audit drops sufficient traffic to 'normalise' actual congestion signals to be no greater than ConEx signals.
Similarly, the job of incentivising the sending of ConEx-enabled packets is proper solely to policy devices, independent of the audit function. The audit function's job is policy-neutral, so it should be solely confined to checking for correctness within those packets that have been marked as ConEx-capable. Even if there are Not-ConEx packets mixed with ConEx packets within a flow, audit will not need to monitor any Not-ConEx packets.
Note that in the future it might prove to be desirable to provide advice on uniformly implementing sanctions, because otherwise insufficient sanctions could impair the ability to implement policy elsewhere in the network.
Some of the audit algorithms require per flow state. This cost is expected to be tolerable, because these techniques are most apropos near the edges of the network, where traffic is generally much less aggregated, so the state need not overwhelm any one device. The flow-state required for audit creates itself as it detects new flows. Therefore a flow will not fail if it is re-routed away from the audit box currently holding its flow-state, so auditing does not require route pinning and works fine with multipath flows.
Holding flow-state seems to create a vulnerability to attacks that exhaust the auditor's memory by opening numerous new short flows. The audit function can protect itself from this attack by not allocating new flow-state unless a ConEx-marked packet arrives (e.g. credit at the start of a flow). Because policy devices rate limit ConEx-marked packets, this sets a natural limit to the rate at which a source can create flow-state in audit devices. The auditor would treat all the remaining flows without any ConEx-marked packets as a single misbehaving aggregate.
Auditing can be distributed and redundant. One flow may be audited in multiple places, using multiple techniques. Some audit techniques do not require any per flow state and can be applied to aggregate traffic. These might be able to detect the presence of understated congestion at large scale and support recursively hunting for individual flows that are understating their congestion. Even at large scales, flows can be randomly selected for individual auditing.
Sampling techniques can also be used to bound the total auditing memory footprint, although the implementer needs to counter the tactic where a source cheats until caught by sampling, then simply discards that flow ID and starts cheating with a new one (termed 'identifier white-washing when caught').
For the the concrete ConEx protocol encoding defined in [I-D.ietf-conex-destopt], ConEx Credit and ConEx-Re-Echo signals are intended to be audited separately. The Credit signal can be audited directly against actual congestion (loss and ECN). However, there will be an inherent delay of at least one round trip between a congestion signal and the subsequent ConEx-Re-Echo signal it triggers, as shown in Figure 1. Therefore ConEx-Re-Echo signals will need to be audited with some allowance for this delay. Further discussion of design and implementation choices for functions intended to audit this concrete ConEx encoding can be found in [I-D.wagner-conex-audit].
The ConEx abstract protocol described so far is intended to support incremental deployment in every possible respect. For convenience, the following list collects together all the features that support incremental deployment in the concrete ConEx specifications, and points to further information on each:
This memo includes no request to IANA.
Note to RFC Editor: this section may be removed on publication as an RFC.
The only known risk associated with ConEx is that users and applications are very likely to be motivated to under-represent the congestion that they are causing. Significant portions of this document are about mechanisms to audit the ConEx signals and create sufficient sanction to inhibit such under-representation. In particular see Section 5.5.
Security attacks and their defences are best discussed against a concrete protocol specification, not the abstract mechanism of this document. A concrete ConEx protocol will need to be accompanied by a document describing how the protocol and its audit mechanisms defend against likely attacks. [Refb-dis] will be a useful source for such a document. It gives a comprehensive inventory of attacks against audit that have been proposed by various parties. It includes pseudocode for both deterministic and statistical audit functions designed to thwart these attacks and analyses the effectiveness of an implementation.
However, [Refb-dis] is specific to the re-ECN protocol, which signalled ECN & loss together, whereas the concrete ConEx protocol defined in [I-D.ietf-conex-destopt] signals them separately. Therefore, although likely attacks will be similar, there will be more combinations of attacks to worry about, and defences and their analysis are likely to be a little different for ConEx.
The main known attacks that a security document for a concrete ConEx protocol will need to address are listed below, and [Refb-dis] should be referred to for how re-ECN was designed to defend against similar attacks:
It is planned to document all known attacks and their defences (including all the above) in the RFC series against a concrete ConEx protocol specification. In the interim [Refb-dis] and its references should be referred to for details and ways to address these attacks in the case of re-ECN.
This document was improved by review comments from Toby Moncaster, Nandita Dukkipati, Mirja Kuehlewind, Caitlin Bestler, Marcelo Bagnulo Braun, John Leslie, Ingemar Johansson and David Wagner.
Bob Briscoe's work on this specification received part-funding from the European Union's Seventh Framework Programme FP7/2007-2013 under Trilogy 2 project, grant agreement no. 317756. The views expressed here are solely those of the author.
Comments and questions are encouraged and very welcome. They can be addressed to the IETF Congestion Exposure (ConEx) working group mailing list <conex@ietf.org>, and/or to the authors.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |