|Transport Area Working Group||B. Briscoe|
|Internet-Draft||Simula Research Laboratory|
|Updates: 3819 (if approved)||J. Kaippallimalil|
|Intended status: Best Current Practice||Huawei|
|Expires: January 9, 2017||P. Thaler|
|July 8, 2016|
Guidelines for Adding Congestion Notification to Protocols that Encapsulate IP
The purpose of this document is to guide the design of congestion notification in any lower layer or tunnelling protocol that encapsulates IP. The aim is for explicit congestion signals to propagate consistently from lower layer protocols into IP. Then the IP internetwork layer can act as a portability layer to carry congestion notification from non-IP-aware congested nodes up to the transport layer (L4). Following these guidelines should assure interworking between new lower layer congestion notification mechanisms, whether specified by the IETF or other standards bodies.
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The benefits of Explicit Congestion Notification (ECN) described below can only be fully realised if support for ECN is added to the relevant subnetwork technology, as well as to IP. When a lower layer buffer drops a packet obviously it does not just drop at that layer; the packet disappears from all layers. In contrast, when a lower layer marks a packet with ECN, the marking needs to be explicitly propagated up the layers. The same is true if a buffer marks the outer header of a packet that encapsulates inner tunnelled headers. Forwarding ECN is not as straightforward as other headers because it has to be assumed ECN may be only partially deployed. If an egress at any layer is not ECN-aware, or if the ultimate receiver or sender is not ECN-aware, congestion needs to be indicated by dropping a packet, not marking it.
The purpose of this document is to guide the addition of congestion notification to any subnet technology or tunnelling protocol, so that lower layer equipment can signal congestion explicitly and it will propagate consistently into encapsulated (higher layer) headers, otherwise the signals will not reach their ultimate destination.
ECN is defined in the IP header (v4 and v6) [RFC3168] to allow a resource to notify the onset of queue build-up without having to drop packets, by explicitly marking a proportion of packets with the congestion experienced (CE) codepoint.
Given a suitable marking scheme, ECN removes nearly all congestion loss and it cuts delays for two main reasons:
Some lower layer technologies (e.g. MPLS, Ethernet) are used to form subnetworks with IP-aware nodes only at the edges. These networks are often sized so that it is rare for interior queues to overflow. However, until recently this was more due to the inability of TCP to saturate the links. For many years, fixes such as window scaling [RFC1323] proved hard to deploy. And the New Reno variant of TCP has remained in widespread use despite its inability to scale to high flow rates. However, now that modern operating systems are finally capable of saturating interior links, even the buffers of well-provisioned interior switches will need to signal episodes of queuing.
Propagation of ECN is defined for MPLS [RFC5129], and is being defined for TRILL [RFC7780], [I-D.eastlake-trill-ecn-support], but it remains to be defined for a number of other subnetwork technologies.
Similarly, ECN propagation is yet to be defined for many tunnelling protocols. [RFC6040] defines how ECN should be propagated for IP-in-IP [RFC2003] and IPsec [RFC4301] tunnels, and it is cited by more recent tunnelling protocols, e.g. Generic UDP Encapsulation (GUE) [I-D.ietf-nvo3-gue] and Geneve [I-D.ietf-nvo3-geneve]. However, as Section 9.3 of RFC3168 pointed out, ECN support will need to be defined for other tunnelling protocols, e.g. L2TP [RFC2661], GRE [RFC1701], [RFC2784], PPTP [RFC2637] and GTP [GTPv1], [GTPv1-U], [GTPv2-C], VXLAN [RFC7348].
Incremental deployment is the most delicate aspect when adding support for ECN. The original ECN protocol in IP [RFC3168] was carefully designed so that a congested buffer would not mark a packet (rather than drop it) unless both source and destination hosts were ECN-capable. Otherwise its congestion markings would never be detected and congestion would just build up further. However, to support congestion marking below the IP layer, it is not sufficient to only check that the two end-points support ECN; correct operation also depends on the decapsulator at each subnet egress faithfully propagating congestion notifications to the higher layer. Otherwise, a legacy decapsulator might silently fail to propagate any ECN signals from the outer to the forwarded header. Then the lost signals would never be detected and again congestion would build up further. The guidelines given later require protocol designers to carefully consider incremental deployment, and suggest various safe approaches for different circumstances.
Of course, the IETF does not have standards authority over every link layer protocol. So this document gives guidelines for designing propagation of congestion notification across the interface between IP and protocols that may encapsulate IP (i.e. that can be layered beneath IP). Each lower layer technology will exhibit different issues and compromises, so the IETF or the relevant standards body must be free to define the specifics of each lower layer congestion notification scheme. Nonetheless, if the guidelines are followed, congestion notification should interwork between different technologies, using IP in its role as a 'portability layer'.
Therefore, the capitalised term 'SHOULD' or 'SHOULD NOT' are often used in preference to 'MUST' or 'MUST NOT', because it is difficult to know the compromises that will be necessary in each protocol design. If a particular protocol design chooses to contradict a 'SHOULD (NOT)' given in the advice below, it MUST include a sound justification.
It has not been possible to give common guidelines for all lower layer technologies, because they do not all fit a common pattern. Instead they have been divided into a few distinct modes of operation: feed-forward-and-upward; feed-upward-and-forward; feed-backward; and null mode. These modes are described in Section 4, then in the following sections separate guidelines are given for each mode.
This document updates the advice to subnetwork designers about ECN in Section 13 of [RFC3819].
This document only concerns wire protocol processing of explicit notification of congestion and makes no changes or recommendations concerning algorithms for congestion marking or for congestion response (algorithm issues should be independent of the layer the algorithm operates in).
The question of congestion notification signals with different semantics to those of ECN in IP is touched on in a couple of specific cases (e.g. QCN [IEEE802.1Qau]) and with schemes with multiple severity levels such as PCN [RFC6660]). However, no attempt is made to give guidelines about schemes with different semantics that are yet to be invented.
The semantics of congestion signals can be relative to the traffic class. Therefore correct propagation of congestion signals could depend on correct propagation of any traffic class field between the layers. In this document, correct propagation of traffic class information is assumed, while what 'correct' means and how it is achieved is covered elsewhere (e.g. [RFC2983]) and is outside the scope of the present document.
Note that these guidelines do not require the subnet wire protocol to be changed to accommodate congestion notification. Another way to add congestion notification without consuming header space in the subnet protocol might be to use a parallel control plane protocol.
This document focuses on the congestion notification interface between IP and lower layer protocols that can encapsulate IP, where the term 'IP' includes v4 or v6, unicast, multicast or anycast. However, it is likely that the guidelines will also be useful when a lower layer protocol or tunnel encapsulates itself (e.g. Ethernet MAC in MAC [IEEE802.1Qah]) or when it encapsulates other protocols. In the feed-backward mode, propagation of congestion signals for multicast and anycast packets is out-of-scope (because it would be so complicated that it is hoped no-one would attempt such an abomination).
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].
Further terminology used within this document:
RFC 3168 specifies that the ECN field in the IP header is intended to be marked by active queue management algorithms. Any congestion notification from an algorithm that does not conform to the recommendations in [RFC7567] MUST NOT be propagated from a lower layer into the ECN field in IP (see also [RFC4774] on alternate uses of the ECN field).
This section sets down the different modes by which congestion information is passed between the lower layer and the higher one. It acts as a reference framework for the following sections, which give normative guidelines for designers of explicit congestion notification protocols, taking each mode in turn:
Like IP and MPLS, many subnet technologies are based on self-contained protocol data units (PDUs) or frames sent unreliably. They provide no feedback channel at the subnetwork layer, instead relying on higher layers (e.g. TCP) to feed back loss signals.
In these cases, ECN may best be supported by standardising explicit notification of congestion into the lower layer protocol that carries the data forwards. It will then also be necessary to define how the egress of the lower layer subnet propagates this explicit signal into the forwarded upper layer (IP) header. It can then continue forwards until it finally reaches the destination transport (at L4). Then typically the destination will feed this congestion notification back to the source transport using an end-to-end protocol (e.g. TCP). This is the arrangement that has already been used to add ECN to IP-in-IP tunnels [RFC6040], IP-in-MPLS and MPLS-in-MPLS [RFC5129].
This mode is illustrated in Figure 1. Along the middle of the figure, layers 2, 3 and 4 of the protocol stack are shown, and one packet is shown along the bottom as it progresses across the network from source to destination, crossing two subnets connected by a router, and crossing two switches on the path across each subnet. Congestion at the output of the first switch (shown as *) leads to a congestion marking in the L2 header (shown as C in the illustration of the packet). The chevrons show the progress of the resulting congestion indication. It is propagated from link to link across the subnet in the L2 header, then when the router removes the marked L2 header, it propagates the marking up into the L3 (IP) header. The router forwards the marked L3 header into subnet 2, and when it adds a new L2 header it copies the L3 marking into the L2 header as well, as shown by the 'C's in both layers (assuming the technology of subnet 2 also supports explicit congestion marking).
Note that there is no implication that each 'C' marking is encoded the same; a different encoding might be used for the 'C' marking in each protocol.
Finally, for completeness, we show the L3 marking arriving at the destination, where the host transport protocol (e.g. TCP) feeds it back to the source in the L4 acknowledgement (the 'C' at L4 in the packet at the top of the diagram).
_ _ _ /_______ | | |C| ACK Packet (V) \ |_|_|_| +---+ layer: 2 3 4 header +---+ | <|<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< Packet V <<<<<<<<<<<<<|<< |L4 | | +---+ | ^ | | | . . . . . . Packet U. . | >>|>>> Packet U >>>>>>>>>>>>|>^ |L3 | | +---+ +---+ | ^ | +---+ +---+ | | | | | *|>>>>>|>>>|>>>>>|>^ | | | | | | |L2 |___|_____|___|_____|___|_____|___|_____|___|_____|___|_____|___| source subnet A router subnet B dest __ _ _ _ __ _ _ _ __ _ _ __ _ _ _ | | | | | | | | |C| | | |C| | | |C|C| Data________\ |__|_|_|_| |__|_|_|_| |__|_|_| |__|_|_|_| Packet (U) / layer: 4 3 2A 4 3 2A 4 3 4 3 2B header
Figure 1: Feed-Forward-and-Up Mode
Of course, modern networks are rarely as simple as this text-book example, often involving multiple nested layers. For example, a 3GPP mobile network may have two IP-in-IP (GTP) tunnels in series and an MPLS backhaul between the base station and the first router. Nonetheless, the example illustrates the general idea of feeding congestion notification forward then upward whenever a header is removed at the egress of a subnet.
Note that the FECN (forward ECN) bit in Frame Relay and the explicit forward congestion indication (EFCI [ITU-T.I.371]) bit in ATM user data cells follow a feed-forward pattern. However, in ATM, this arrangement is only part of a feed-forward-and-backward pattern at the lower layer, not feed-forward-and-up out of the lower layer—the intention was never to interface to IP ECN at the subnet egress. To our knowledge, Frame Relay FECN is solely used to detect where more capacity should be provisioned [Buck00].
Ethernet is particularly difficult to extend incrementally to support explicit congestion notification. One way to support ECN in such cases has been to use so called 'layer-3 switches'. These are Ethernet switches that bury into the Ethernet payload to find an IP header and manipulate or act on certain IP fields (specifically Diffserv & ECN). For instance, in Data Center TCP [DCTCP], layer-3 switches are configured to mark the ECN field of the IP header within the Ethernet payload when their output buffer becomes congested. With respect to switching, a layer-3 switch acts solely on the addresses in the Ethernet header; it doesn't use IP addresses, and it doesn't decrement the TTL field in the IP header.
_ _ _ /_______ | | |C| ACK packet (V) \ |_|_|_| +---+ layer: 2 3 4 header +---+ | <|<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< Packet V <<<<<<<<<<<<<|<< |L4 | | +---+ | ^ | | | . . . >>>> Packet U >>>|>>>|>>> Packet U >>>>>>>>>>>>|>^ |L3 | | +--^+ +---+ | | +---+ +---+ | | | | | *| | | | | | | | | | |L2 |___|_____|___|_____|___|_____|___|_____|___|_____|___|_____|___| source subnet E router subnet F dest __ _ _ _ __ _ _ _ __ _ _ __ _ _ _ | | | | | | | |C| | | | |C| | | |C|C| data________\ |__|_|_|_| |__|_|_|_| |__|_|_| |__|_|_|_| packet (U) / layer: 4 3 2 4 3 2 4 3 4 3 2 header
Figure 2: Feed-Up-and-Forward Mode
By comparing Figure 2 with Figure 1, it can be seen that subnet E (perhaps a subnet of layer-3 Ethernet switches) works in feed-up-and-forward mode by notifying congestion directly into L3 at the point of congestion, even though the congested switch does not otherwise act at L3. In this example, the technology in subnet F (e.g. MPLS) does support ECN natively, so when the router adds the layer-2 header it copies the ECN marking from L3 to L2 as well.
In some layer 2 technologies, explicit congestion notification has been defined for use internally within the subnet with its own feedback and load regulation, but typically the interface with IP for ECN has not been defined.
For instance, for the available bit-rate (ABR) service in ATM, the relative rate mechanism was one of the more popular mechanisms for managing traffic, tending to supersede earlier designs. In this approach ATM switches send special resource management (RM) cells in both the forward and backward directions to control the ingress rate of user data into a virtual circuit. If a switch buffer is approaching congestion or is congested it sends an RM cell back towards the ingress with respectively the No Increase (NI) or Congestion Indication (CI) bit set in its message type field [ATM-TM-ABR]. The ingress then holds or decreases its sending bit-rate accordingly.
_ _ _ /_______ | | |C| ACK packet (X) \ |_|_|_| +---+ layer: 2 3 4 header +---+ | <|<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< Packet X <<<<<<<<<<<<<|<< |L4 | | +---+ | ^ | | | | *|>>> Packet W >>>>>>>>>>>>|>^ |L3 | | +---+ +---+ | | +---+ +---+ | | | | | | | | | <|<<<<<|<<<|<(V)<|<<<| | |L2 | | . . | . |Packet U | . . | . | . . | . | . . | .*| . . | |L2 |___|_____|___|_____|___|_____|___|_____|___|_____|___|_____|___| source subnet G router subnet H dest __ _ _ _ __ _ _ _ __ _ _ __ _ _ _ later | | | | | | | | | | | | | | | | |C| | data________\ |__|_|_|_| |__|_|_|_| |__|_|_| |__|_|_|_| packet (W) / 4 3 2 4 3 2 4 3 4 3 2 _ /__ |C| Feedback control \ |_| cell/frame (V) 2 __ _ _ _ __ _ _ _ __ _ _ __ _ _ _ earlier | | | | | | | | | | | | | | | | | | | data________\ |__|_|_|_| |__|_|_|_| |__|_|_| |__|_|_|_| packet (U) / layer: 4 3 2 4 3 2 4 3 4 3 2 header
Figure 3: Feed-Backward Mode
ATM's feed-backward approach doesn't fit well when layered beneath IP's feed-forward approach—unless the initial data source is the same node as the ATM ingress. Figure 3 shows the feed-backward approach being used in subnet H. If the final switch on the path is congested (*), it doesn't feed-forward any congestion indications on packet (U). Instead it sends a control cell (V) back to the router at the ATM ingress.
However, the backward feedback doesn't reach the original data source directly because IP doesn't support backward feedback (and subnet G is independent of subnet H). Instead, the router in the middle throttles down its sending rate but the original data sources don't reduce their rates. The resulting rate mismatch causes the middle router's buffer at layer 3 to back up until it becomes congested, which it signals forwards on later data packets at layer 3 (e.g. packet W). Note that the forward signal from the middle router is not triggered directly by the backward signal. Rather, it is triggered by congestion resulting from the middle router's mismatched rate response to the backward signal.
In response to this later forward signalling, end-to-end feedback at layer-4 finally completes the tortuous path of congestion indications back to the origin data source, as before.
Often link and physical layer resources are 'non-blocking' by design. In these cases congestion notification may be implemented but it does not need to be deployed at the lower layer; ECN in IP would be sufficient.
A degenerate example is a point-to-point Ethernet link. Excess loading of the link merely causes the queue from the higher layer to back up, while the lower layer remains immune to congestion. Even a whole meshed subnetwork can be made immune to interior congestion by limiting ingress capacity and sufficient sizing of interior links, e.g. a non-blocking fat-tree network. An alternative to fat links near the root is numerous thin links with multi-path routing to ensure even worst-case patterns of load cannot congest any link, e.g. a Clos network.
Feed-forward-and-up is the mode already used for signalling ECN up the layers through MPLS into IP [RFC5129] and through IP-in-IP tunnels [RFC6040]. These RFCs take a consistent approach and the following guidelines are designed to ensure this consistency continues as ECN support is added to other protocols that encapsulate IP. The guidelines are also designed to ensure compliance with the more general best current practice for the design of alternate ECN schemes given in [RFC4774].
The rest of this section is structured as follows:
A common pattern for many tunnelling protocols is to encapsulate an inner IP header with shim header(s) then an outer IP header. In many cases the shim header(s) and the outer IP header are always added (or removed) as part of the same process. We call this a tightly coupled shim header. Processing the shim and outer together is often necessary because the shim(s) are not sufficient for packet forwarding in their own right; not unless complemented by an outer header.
For all such tightly coupled shim headers (such as those listed in the Introduction), the rules in [RFC6040] for propagating the ECN field can be applied directly between the inner and outer IP headers. [I-D.briscoe-tsvwg-rfc6040bis] clarifies that RFC 6040 is just as applicable when there is a tightly-coupled shim between two IP headers as wehen there is not.
This section is intended to guide the redesign of any lower layer protocol that encapsulate IP to add native ECN support at the lower layer. It reflects the approaches used in [RFC6040] and in [RFC5129]. Therefore IP-in-IP tunnels or IP-in-MPLS or MPLS-in-MPLS encapsulations that already comply with [RFC6040] or [RFC5129] will already satisfy this guidance.
A lower layer (or subnet) congestion notification system:
Note that there is no need for all interior nodes within a subnet to be able to mark congestion explicitly. A mix of ECN and drop signals from different nodes is fine. However, if any interior nodes might generate ECN markings, guideline 2 above says that all relevant egress node(s) SHOULD be able to propagate those markings up to the higher layer.
In IP, if the ECN field in each PDU is cleared to the Not-ECT (not ECN-capable transport) codepoint, it indicates that the L4 transport will not understand congestion markings. A congested buffer must not mark these Not-ECT PDUs, and therefore drops them instead.
The mechanism a lower layer uses to distinguish the ECN-capability of PDUs need not mimic that of IP. The above guidelines merely say that the lower layer system, as a whole, should achieve the same outcome. For instance, ECN-capable feedback loops might use PDUs that are identified by a particular set of labels or tags. Alternatively, logical link protocols that use flow state might determine whether a PDU can be congestion marked by checking for ECN-support in the flow state. Other protocols might depend on out-of-band control signals.
The per-domain checking of ECN support in MPLS [RFC5129] is a good example of a way to avoid sending congestion markings to transports that will not understand them, without using any header space in the subnet protocol.
In MPLS, header space is extremely limited, therefore RFC5129 does not provide a field in the MPLS header to indicate whether the PDU is an ECN-PDU or a Not-ECN-PDU. Instead, interior nodes in a domain are allowed to set explicit congestion indications without checking whether the PDU is destined for a transport that will understand them. Nonetheless, this is made safe by requiring that the network operator upgrades all decapsulating edges of a whole domain at once, as soon as even one switch within the domain is configured to mark rather than drop during congestion. Therefore, any edge node that might decapsulate a packet will be capable of checking whether the higher layer transport is ECN-capable. When decapsulating a CE-marked packet, if the decapsulator discovers that the higher layer (inner header) indicates the transport is not ECN-capable, it drops the packet—effectively on behalf of the earlier congested node (see Decapsulation Guideline 1 in Section 5.4).
It was only appropriate to define such an incremental deployment strategy because MPLS is targeted solely at professional operators, who can be expected to ensure that a whole subnetwork is consistently configured. This strategy might not be appropriate for other link technologies targeted at zero-configuration deployment or deployment by the general public (e.g. Ethernet). For such 'plug-and-play' environments it will be necessary to invent a failsafe approach that ensures congestion markings will never fall into black holes, no matter how inconsistently a system is put together. Alternatively, congestion notification relying on correct system configuration could be confined to flavours of Ethernet intended only for professional network operators, such as IEEE 802.1ah Provider Backbone Bridges (PBB).
ECN support in TRILL [I-D.eastlake-trill-ecn-support] provides a good example of how to add ECN to a lower layer protocol without relying on careful and consistent operator configuration. TRILL provides an extension header word with space for flags of different categories depending on whether logic to understand the extension is critical. The congestion experienced marking has been defined as a 'critical ingress-to-egress' flag. So if a transit RBridge sets this flag and an egress RBridge does not have any logic to process it, it will drop it; which is the desired default action anyway. Therefore TRILL RBridges can be updated with support for ECN in no particular order and, at the egress of the TRILL campus, congestion notification will be propagated to IP as ECN whenever ECN logic has been implemented, and as drop otherwise.
QCN [IEEE802.1Qau] provides another example of how to indicate to lower layer devices that the end-points will not understand ECN. An operator can define certain 802.1p classes of service to indicate non-QCN frames and an ingress bridge is required to map arriving not-QCN-capable IP packets to one of these non-QCN 802.1p classes.
This section is intended to guide the redesign of any node that encapsulates IP with a lower layer header when adding native ECN support to the lower layer protocol. It reflects the approaches used in [RFC6040] and in [RFC5129]. Therefore IP-in-IP tunnels or IP-in-MPLS or MPLS-in-MPLS encapsulations that already comply with [RFC6040] or [RFC5129] will already satisfy this guidance.
This section is intended to guide the redesign of any node that decapsulates IP from within a lower layer header when adding native ECN support to the lower layer protocol. It reflects the approaches used in [RFC6040] and in [RFC5129]. Therefore IP-in-IP tunnels or IP-in-MPLS or MPLS-in-MPLS encapsulations that already comply with [RFC6040] or [RFC5129] will already satisfy this guidance.
A subnet egress SHOULD NOT simply copy congestion notification from outer headers to the forwarded header. It SHOULD calculate the outgoing congestion notification field from the inner and outer headers using the following guidelines. If there is any conflict, rules earlier in the list take precedence over rules later in the list:
In some deployments, particularly in 3GPP networks, an IP packet may traverse two or more IP-in-IP tunnels in sequence that all use identical technology (e.g. GTP).
In such cases, it would be sufficient for every encapsulation and decapsulation in the chain to comply with RFC 6040. Alternatively, as an optimisation, a node that decapsulates a packet and immediately re-encapsulates it for the next tunnel MAY copy the incoming outer ECN field directly to the outgoing outer and the incoming inner ECN field directly to the outgoing inner. Then the overall behavior across the sequence of tunnel segments would still be consistent with RFC 6040.
Appendix C of RFC6040 describes how a tunnel egress can monitor how much congestion has been introduced within a tunnel. A network operator might want to monitor how much congestion had been introduced within a whole sequence of tunnels. Using the technique in Appendix C of RFC6040 at the final egress, the operator could monitor the whole sequence of tunnels, but only if the above optimisation were used consistently along the sequence of tunnels, in order to make it appear as a single tunnel. Therefore, tunnel endpoint implementations SHOULD allow the operator to configure whether this optimisation is enabled.
When ECN support is added to a subnet technology, consideration SHOULD be given to a similar optimisation between subnets in sequence if they all use the same technology.
The guidance in this section is worded in terms of framing boundaries, but it applies equally whether the protocol data units are frames, cells or packets.
Where framing boundaries are different between two layers, congestion indications SHOULD be propagated on the basis that a congestion indication on a PDU applies to all the octets in the PDU. On average, an encapsulator or decapsulator SHOULD approximately preserve the number of marked octets arriving and leaving (counting the size of inner headers, but not added encapsulating headers).
The next departing frame SHOULD be immediately marked even if only enough incoming marked octets have arrived for part of the departing frame. This ensures that any outstanding congestion marked octets are propagated immediately, rather than held back waiting for a frame no bigger than the outstanding marked octets—which might involve a long wait.
For instance, an algorithm for marking departing frames could maintain a counter representing the balance of arriving marked octets minus departing marked octets. It adds the size of every marked frame that arrives and if the counter is positive it marks the next frame to depart and subtracts its size from the counter. This will often leave a negative remainder in the counter, which is deliberate.
The guidance in this section is applicable, for example, when IP packets:[RFC6040] would be more appropriate.
This guidance also generalises to encapsulation by other subnet technologies with no native support for explicit congestion notification at the lower layer, but with support for finding and processing an IP header. It is unlikely to be applicable or necessary for IP-in-IP encapsulation, where feed-forward-and-up mode based on
Marking the IP header while switching at layer-2 (by using a layer-3 switch) or while forwarding in a radio access network seems to represent a layering violation. However, it can be considered as a benign optimisation if the guidelines below are followed. Feed-up-and-forward is certainly not a general alternative to implementing feed-forward congestion notification in the lower layer, because:
Nonetheless, configuring lower layer equipment to look for an ECN field in an encapsulated IP header is a useful optimisation. If the implementation follows the guidelines below, this optimisation does not have to be confined to a controlled environment such as within a data centre; it could usefully be applied on any network—even if the operator is not sure whether the above issues will never apply:
It can be seen from Section 4.3 that congestion notification in a subnet using feed-backward mode has generally not been designed to be directly coupled with IP layer congestion notification. The subnet attempts to minimise congestion internally, and if the incoming load at the ingress exceeds the capacity somewhere through the subnet, the layer 3 buffer into the ingress backs up. Thus, a feed-backward mode subnet is in some sense similar to a null mode subnet, in that there is no need for any direct interaction between the subnet and higher layer congestion notification. Therefore no detailed protocol design guidelines are appropriate. Nonetheless, a more general guideline is appropriate:
The feed-backward approach at least works beneath IP, where the term 'works' is used only in a narrow functional sense because feed-backward can result in very inefficient and sluggish congestion control—except if it is confined to the subnet directly connected to the original data source, when it is faster than feed-forward. It would be valid to design a protocol that could work in feed-backward mode for paths that only cross one subnet, and in feed-forward-and-up mode for paths that cross subnets.
In the early days of TCP/IP, a similar feed-backward approach was tried for explicit congestion signalling, using source-quench (SQ) ICMP control packets. However, SQ fell out of favour and is now formally deprecated [RFC6633]. The main problem was that it is hard for a data source to tell the difference between a spoofed SQ message and a quench request from a genuine buffer on the path. It is also hard for a lower layer buffer to address an SQ message to the original source port number, which may be buried within many layers of headers, and possibly encrypted.
Quantised congestion notification (QCN—also known as backward congestion notification or BCN) [IEEE802.1Qau] uses a feed-backward mode structurally similar to ATM's relative rate mechanism. However, QCN confines its applicability to scenarios such as some data centres where all endpoints are directly attached by the same Ethernet technology. If a QCN subnet were later connected into a wider IP-based internetwork (e.g. when attempting to interconnect multiple data centres) it would suffer the inefficiency shown Figure 3.
This memo includes no request to IANA.
If a lower layer wire protocol is redesigned to include explicit congestion signalling in-band in the protocol header, care SHOULD be take to ensure that the field used is specified as mutable during transit. Otherwise interior nodes signalling congestion would invalidate any authentication protocol applied to the lower layer header—by altering a header field that had been assumed as immutable.
The redesign of protocols that encapsulate IP in order to propagate congestion signals between layers raises potential signal integrity concerns. Experimental or proposed approaches exist for assuring the end-to-end integrity of in-band congestion signals, e.g.:
Given these end-to-end approaches are already being specified, it would make little sense to attempt to design hop-by-hop congestion signal integrity into a new lower layer protocol, because end-to-end integrity inherently achieves hop-by-hop integrity.
Following the guidance in the document enables ECN support to be extended to numerous protocols that encapsulate IP (v4 & v6) in a consistent way, so that IP continues to fulfil its role as an end-to-end interoperability layer. This includes:
Guidelines have been defined for supporting propagation of ECN between Ethernet and IP on so-called Layer-3 Ethernet switches, using a 'feed-up-an-forward' mode. This approach could enable other subnet technologies to pass ECN signals into the IP layer, even if they do not support ECN natively.
Finally, attempting to add ECN to a subnet technology in feed-backward mode is deprecated except in special cases, due to its likely sluggish response to congestion.
Thanks to Gorry Fairhurst for extensive reviews. Thanks also to the following reviewers: Richard Scheffenegger, Ingemar Johansson, Piers O'Hanlon and Michael Welzl, who pointed out that lower layer congestion notification signals may have different semantics to those in IP. Thanks are also due to the tsvwg chairs, TSV ADs and IETF liaison people such as Eric Gray, Dan Romascanu and Gonzalo Camarillo for helping with the liaisons with the IEEE and 3GPP. And thanks to Georg Mayer and particularly to Erik Guttman for the extensive search and categorisation of any 3GPP specifications that cite ECN specifications.
Bob Briscoe was part-funded by the European Community under its Seventh Framework Programme through the Trilogy project (ICT-216372) for initial drafts and through the Reducing Internet Transport Latency (RITE) project (ICT-317700) subsequently. The views expressed here are solely those of the authors.
Comments and questions are encouraged and very welcome. They can be addressed to the IETF Transport Area working group mailing list <firstname.lastname@example.org>, and/or to the authors.
|[RFC2119]||Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.|
|[RFC3168]||Ramakrishnan, K., Floyd, S. and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI 10.17487/RFC3168, September 2001.|
|[RFC3819]||Karn, P., Bormann, C., Fairhurst, G., Grossman, D., Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J. and L. Wood, "Advice for Internet Subnetwork Designers", BCP 89, RFC 3819, DOI 10.17487/RFC3819, July 2004.|
|[RFC4774]||Floyd, S., "Specifying Alternate Semantics for the Explicit Congestion Notification (ECN) Field", BCP 124, RFC 4774, DOI 10.17487/RFC4774, November 2006.|
|[RFC5129]||Davie, B., Briscoe, B. and J. Tay, "Explicit Congestion Marking in MPLS", RFC 5129, DOI 10.17487/RFC5129, January 2008.|
|[RFC6040]||Briscoe, B., "Tunnelling of Explicit Congestion Notification", RFC 6040, DOI 10.17487/RFC6040, November 2010.|
|[ATM-TM-ABR]||Cisco, "Understanding the Available Bit Rate (ABR) Service Category for ATM VCs", Design Technote 10415, June 2005.|
|[Buck00]||Buckwalter, J., "Frame Relay: Technology and Practice", Pub. Addison Wesley ISBN-13: 978-0201485240, 2000.|
|[DCTCP]||Alizadeh, M., Greenberg, A., Maltz, D., Padhye, J., Patel, P., Prabhakar, B., Sengupta, S. and M. Sridharan, "Data Center TCP (DCTCP)", ACM SIGCOMM CCR 40(4)63--74, October 2010.|
|[GTPv1]||3GPP, "GPRS Tunnelling Protocol (GTP) across the Gn and Gp interface", Technical Specification TS 29.060|
|[GTPv1-U]||3GPP, "General Packet Radio System (GPRS) Tunnelling Protocol User Plane (GTPv1-U)", Technical Specification TS 29.281|
|[GTPv2-C]||3GPP, "Evolved General Packet Radio Service (GPRS) Tunnelling Protocol for Control plane (GTPv2-C)", Technical Specification TS 29.274|
|[I-D.briscoe-tsvwg-rfc6040bis]||Briscoe, B., "Propagating Explicit Congestion Notification Across IP Tunnel Headers Separated by a Shim", Internet-Draft draft-briscoe-tsvwg-rfc6040bis-00, July 2016.|
|[I-D.eastlake-trill-ecn-support]||3rd, D. and B. Briscoe, "TRILL: ECN (Explicit Congestion Notification) Support", Internet-Draft draft-eastlake-trill-ecn-support-00, March 2016.|
|[I-D.ietf-nvo3-geneve]||Gross, J. and I. Ganga, "Geneve: Generic Network Virtualization Encapsulation", Internet-Draft draft-ietf-nvo3-geneve-01, January 2016.|
|[I-D.ietf-nvo3-gue]||Herbert, T., Yong, L. and O. Zia, "Generic UDP Encapsulation", Internet-Draft draft-ietf-nvo3-gue-04, July 2016.|
|[I-D.ietf-tsvwg-circuit-breaker]||Fairhurst, G., "Network Transport Circuit Breakers", Internet-Draft draft-ietf-tsvwg-circuit-breaker-15, April 2016.|
|[I-D.moncaster-tcpm-rcv-cheat]||Moncaster, T., Briscoe, B. and A. Jacquet, "A TCP Test to Allow Senders to Identify Receiver Non-Compliance", Internet-Draft draft-moncaster-tcpm-rcv-cheat-03, July 2014.|
|[IEEE802.1Qah]||IEEE, "IEEE Standard for Local and Metropolitan Area Networks—Virtual Bridged Local Area Networks—Amendment 6: Provider Backbone Bridges", IEEE Std 802.1Qah-2008, August 2008.
(Access Controlled link within page)
|[IEEE802.1Qau]||Finn, N., "IEEE Standard for Local and Metropolitan Area Networks—Virtual Bridged Local Area Networks - Amendment 13: Congestion Notification", IEEE Std 802.1Qau-2010, March 2010.
(Access Controlled link within page)
|[ITU-T.I.371]||, , "Traffic Control and Congestion Control in B-ISDN", ITU-T Rec. I.371 (03/04), March 2004.|
|[LTE-RA]||3GPP, "Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2", Technical Specification TS 36.300|
|[RFC1323]||Jacobson, V., Braden, R. and D. Borman, "TCP Extensions for High Performance", RFC 1323, DOI 10.17487/RFC1323, May 1992.|
|[RFC1701]||Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic Routing Encapsulation (GRE)", RFC 1701, DOI 10.17487/RFC1701, October 1994.|
|[RFC2003]||Perkins, C., "IP Encapsulation within IP", RFC 2003, DOI 10.17487/RFC2003, October 1996.|
|[RFC2637]||Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, W. and G. Zorn, "Point-to-Point Tunneling Protocol (PPTP)", RFC 2637, DOI 10.17487/RFC2637, July 1999.|
|[RFC2661]||Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G. and B. Palter, "Layer Two Tunneling Protocol "L2TP"", RFC 2661, DOI 10.17487/RFC2661, August 1999.|
|[RFC2784]||Farinacci, D., Li, T., Hanks, S., Meyer, D. and P. Traina, "Generic Routing Encapsulation (GRE)", RFC 2784, DOI 10.17487/RFC2784, March 2000.|
|[RFC2884]||Hadi Salim, J. and U. Ahmed, "Performance Evaluation of Explicit Congestion Notification (ECN) in IP Networks", RFC 2884, DOI 10.17487/RFC2884, July 2000.|
|[RFC2983]||Black, D., "Differentiated Services and Tunnels", RFC 2983, DOI 10.17487/RFC2983, October 2000.|
|[RFC3540]||Spring, N., Wetherall, D. and D. Ely, "Robust Explicit Congestion Notification (ECN) Signaling with Nonces", RFC 3540, DOI 10.17487/RFC3540, June 2003.|
|[RFC4301]||Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, December 2005.|
|[RFC6633]||Gont, F., "Deprecation of ICMP Source Quench Messages", RFC 6633, DOI 10.17487/RFC6633, May 2012.|
|[RFC6660]||Briscoe, B., Moncaster, T. and M. Menth, "Encoding Three Pre-Congestion Notification (PCN) States in the IP Header Using a Single Diffserv Codepoint (DSCP)", RFC 6660, DOI 10.17487/RFC6660, July 2012.|
|[RFC7348]||Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger, L., Sridhar, T., Bursell, M. and C. Wright, "Virtual eXtensible Local Area Network (VXLAN): A Framework for Overlaying Virtualized Layer 2 Networks over Layer 3 Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014.|
|[RFC7567]||Baker, F. and G. Fairhurst, "IETF Recommendations Regarding Active Queue Management", BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015.|
|[RFC7713]||Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx) Concepts, Abstract Mechanism, and Requirements", RFC 7713, DOI 10.17487/RFC7713, December 2015.|
|[RFC7780]||Eastlake 3rd, D., Zhang, M., Perlman, R., Banerjee, A., Ghanwani, A. and S. Gupta, "Transparent Interconnection of Lots of Links (TRILL): Clarifications, Corrections, and Updates", RFC 7780, DOI 10.17487/RFC7780, February 2016.|
|[UTRAN]||3GPP, "UTRAN Overall Description", Technical Specification TS 25.401|