Guidelines for Adding Congestion Notification to Protocols that Encapsulate IP
draft-briscoe-tsvwg-ecn-encap-guidelines-01

Abstract

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.

Status of This Memo

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This Internet-Draft will expire on April 25, 2013.

Copyright Notice

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Table of Contents

1.  Introduction
    1.1.  Scope
2.  Terminology
3.  Modes of Operation
    3.1.  Feed-Forward-and-Up Mode
    3.2.  Feed-Up-and-Forward Mode
    3.3.  Feed-Backward Mode
    3.4.  Null Mode
4.  Feed-Forward-and-Up Mode: Guidelines for Adding Congestion Notification
    4.1.  Wire Protocol Design: Indication of ECN Support
    4.2.  Encapsulation Guidelines
    4.3.  Decapsulation Guidelines
    4.4.  Reframing and Congestion Markings
5.  Feed-Up-and-Forward Mode: Guidelines for Adding Congestion Notification
6.  Feed-Backward Mode: Guidelines for Adding Congestion Notification
7.  IANA Considerations
8.  Security Considerations
9.  Conclusions
10.  Acknowledgements
11.  Comments Solicited
12.  References
    12.1.  Normative References
    12.2.  Informative References
Appendix A.  Outstanding Document Issues
Appendix B.  Changes in This Version (to be removed by RFC Editor)

1.  Introduction

Explicit Congestion Notification (ECN [RFC3168] (Ramakrishnan, K., Floyd, S., and D. Black, “The Addition of Explicit Congestion Notification (ECN) to IP,” September 2001.)) is defined in the IP header (v4 & v6) 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.ECN removes nearly all congestion loss and it cuts delays for two main reasons: i) it avoids the delays recovering from congestion losses, which particularly benefits small flows, making their completion time predictably short [RFC2884] (Hadi Salim, J. and U. Ahmed, “Performance Evaluation of Explicit Congestion Notification (ECN) in IP Networks,” July 2000.); and ii) as ECN is used more widely by end-systems, it will gradually remove the need to configure a degree of delay into buffers before they start to notify congestion (the cause of bufferbloat). The latter delay is because drop involves a trade-off between sending a timely signal and trying to avoid impairment, whereas ECN is solely a signal so there is no harm triggering it earlier.

Some lower layer technologies (e.g. MPLS, Ethernet) are used to form large subnetworks with IP-aware nodes only at the edges. Particularly now that end-system protocols are finally being deployed without their earlier deficiencies, even the buffers of well-provisioned interior switches will often need to signal episodes of queuing. However, the above benefits of ECN can only be fully realised if the relevant subnetwork technology supports it. Propagation of ECN is defined for MPLS [RFC5129] (Davie, B., Briscoe, B., and J. Tay, “Explicit Congestion Marking in MPLS,” January 2008.), and is being defined for TRILL [trill‑rbridge‑options] (Eastlake, D., Ghanwani, A., Manral, V., and C. Bestler, “RBridges: Further TRILL Header Extensions,” June 2012.), 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] (Briscoe, B., “Tunnelling of Explicit Congestion Notification,” November 2010.) defines how ECN should be propagated for IP-in-IP [RFC2003] (Perkins, C., “IP Encapsulation within IP,” October 1996.) and IPsec [RFC4301] (Kent, S. and K. Seo, “Security Architecture for the Internet Protocol,” December 2005.) tunnels. However, as Section 9.3 of RFC3168 pointed out, ECN support will need to be defined for other tunnelling protocols, e.g. L2TP [RFC2661] (Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G., and B. Palter, “Layer Two Tunneling Protocol "L2TP",” August 1999.), GRE [RFC1701 (Hanks, S., Li, T., Farinacci, D., and P. Traina, “Generic Routing Encapsulation (GRE),” October 1994.), RFC2784 (Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina, “Generic Routing Encapsulation (GRE),” March 2000.)], PPTP [RFC2637] (Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, W., and G. Zorn, “Point-to-Point Tunneling Protocol,” July 1999.) and GTP [GTPv1 (3GPP, “GPRS Tunnelling Protocol (GTP) across the Gn and Gp interface,” .), GTPv1‑U (3GPP, “General Packet Radio System (GPRS) Tunnelling Protocol User Plane (GTPv1-U),” .), GTPv2‑C (3GPP, “Evolved General Packet Radio Service (GPRS) Tunnelling Protocol for Control plane (GTPv2-C),” .)].

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.

Incremental deployment is the most tricky aspect when adding support for ECN. The original ECN protocol in IP [RFC3168] (Ramakrishnan, K., Floyd, S., and D. Black, “The Addition of Explicit Congestion Notification (ECN) to IP,” September 2001.) 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 deteriorate 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 propagating congestion notifications faithfully. 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 deteriorate 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'.

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. These are described in Section 3 (Modes of Operation), 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] (Karn, P., Bormann, C., Fairhurst, G., Grossman, D., Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. Wood, “Advice for Internet Subnetwork Designers,” July 2004.).

1.1.  Scope

This document only concerns wire protocol processing of explicit notification of congestion and makes no changes or recommendations concerning algorithms for congestion marking or congestion response.

This document focuses on the congestion notification interface between IP (v4 or v6) and lower layer protocols that can encapsulate IP. 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] (IEEE, “IEEE Standard for Local and Metropolitan Area Networks—Virtual Bridged Local Area Networks—Amendment 6: Provider Backbone Bridges,” August 2008.)) or when it encapsulates other protocols.

2.  Terminology

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

Further terminology used within this document:

Protocol data unit (PDU):

Information that is delivered as a unit among peer entities of a layered network consisting of protocol control information (typically a header) and possibly user data (payload) of that layer. The scope of this document includes layer 2 and layer 3 networks, where the PDU is respectively termed a frame or a packet (or a cell in ATM). PDU is a general term for any of these. This definition also includes a payload with a shim header lying somewhere between layer 2 & 3.

Transport:

The end-to-end transmission control function, conventionally considered at layer-4 in the OSI reference model. Given the audience for this document will often use the word transport to mean low level bit carriage, whenever the term is used it will be qualified, e.g. 'L4 transport'.

Encapsulator:

The link or tunnel endpoint function that adds an outer header to a PDU (also termed the 'link ingress', the 'subnet ingress', the 'ingress tunnel endpoint' or just the 'ingress' where the context is clear).

Decapsulator:

The link or tunnel endpoint function that removes an outer header from a PDU (also termed the 'link egress', the 'subnet egress', the 'egress tunnel endpoint' or just the 'egress' where the context is clear).

Incoming header:

The header of an arriving PDU before encapsulation.

Outer header:

The header added to encapsulate a PDU.

Inner header:

The header encapsulated by the outer header.

Outgoing header:

The header forwarded by the decapsulator.

CE:

Congestion Experienced [RFC3168] (Ramakrishnan, K., Floyd, S., and D. Black, “The Addition of Explicit Congestion Notification (ECN) to IP,” September 2001.)

ECT:

ECN-Capable Transport [RFC3168] (Ramakrishnan, K., Floyd, S., and D. Black, “The Addition of Explicit Congestion Notification (ECN) to IP,” September 2001.)

Not-ECT:

Not ECN-Capable Transport [RFC3168] (Ramakrishnan, K., Floyd, S., and D. Black, “The Addition of Explicit Congestion Notification (ECN) to IP,” September 2001.)

ECN-PDU:

A PDU that is part of a feedback loop within which the nodes necessary to propagate explicit congestion notifications back to the load regulator are ECN-capable. This is intended to be a general term for a PDU at any layer, not just an IP PDU. An IP packet with a non-zero ECN field would be an ECN-PDU, but the term is intended to also be used to describe PDUs of protocols that encapsulate IP packets, where it has been checked that the necessary egress nodes and endpoints in the feedback loop for that PDU will propagate congestion notification.

Not-ECN-PDU:

A PDU that is part of a feedback-loop within which some nodes necessary to propagate explicit congestion notifications back to the load regulator are not ECN-capable.

Load Regulator:

For each flow of PDUs, the transport function that is capable of controlling the data rate. Typically located at the data source, but in-path nodes can regulate load in some congestion control arrangements (e.g. admission control or policing nodes). Note the term "a function capable of controlling the load" deliberately includes a transport that doesn't actually control the load but ought to (e.g. an application without congestion control that uses UDP).

Congestion Baseline:

The location of the function on the path that initialised the values of all congestion notification fields in a sequence of packets, before any are set to the congestion experienced (CE) codepoint if they experience congestion further downstream. Typically the original data source at layer-4.

3.  Modes of Operation

This section sets down the different modes by which 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 separately in turn:

Feed-Forward-and-Up:

Nodes feed forward congestion notification towards the destination within the lower layer then up the layers (like IP does). The following local optimisation is possible:

Feed-Up-and-Forward:

A lower layer switch feeds-up congestion notification directly into the ECN field in the higher layer (IP) header, irrespective of whether it is at the egress of a subnet.

Feed-Backward:

Nodes feed back congestion signals towards the ingress of the lower layer and (optionally) attempt to control congestion within their own layer.

Null:

Nodes cannot experience congestion at the lower layer except at ingress nodes that are also IP-aware (or equivalently higher-layer-aware).

3.1.  Feed-Forward-and-Up Mode

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 specific link layer protocol. 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 mode is illustrated in Figure 1 (Feed-Forward-and-Up Mode). Along the middle of the figure, layers 2, 3 & 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

Of course, modern networks are rarely as simple as this text-book example, often involving multiple nested layers. 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] (ITU-T, “Traffic Control and Congestion Control in B-ISDN,” March 2004.)) bit in ATM user data cells follow a feed-forward pattern. However, in ATM, this is only as 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] (Buckwalter, J., “Frame Relay: Technology and Practice,” 2000.).

3.2.  Feed-Up-and-Forward Mode

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] (Alizadeh, M., Greenberg, A., Maltz, D., Padhye, J., Patel, P., Prabhakar, B., Sengupta, S., and M. Sridharan, “Data Center TCP (DCTCP),” October 2010.), 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

By comparing Figure 2 (Feed-Up-and-Forward Mode) with Figure 1 (Feed-Forward-and-Up Mode), 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.

3.3.  Feed-Backward Mode

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 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] (Cisco, “Understanding the Available Bit Rate (ABR) Service Category for ATM VCs,” June 2005.). 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

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 (Feed-Backward Mode) 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 source doesn't reduce its rate. 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.

3.4.  Null Mode

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 careful sizing of links, particularly if multi-path routing is used to ensure even worst-case patterns of load cannot congest any link.

4.  Feed-Forward-and-Up Mode: Guidelines for Adding Congestion Notification

These guidelines are consistent with the guidelines on the design of alternate schemes for IP tunnelling of the ECN field [RFC6040] (Briscoe, B., “Tunnelling of Explicit Congestion Notification,” November 2010.) and the more general best current practice for the design of alternate ECN schemes given in [RFC4774] (Floyd, S., “Specifying Alternate Semantics for the Explicit Congestion Notification (ECN) Field,” November 2006.).

The capitalised term 'SHOULD (NOT)' has often been used in preference to '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.

4.1.  Wire Protocol Design: Indication of ECN Support

A lower layer (or subnet) congestion notification protocol

1. SHOULD NOT apply explicit congestion notifications to PDUs that are destined for legacy layer-4 transport implementations that will not understand ECN, and
2. SHOULD NOT apply explicit congestion notifications to PDUs that are destined for a legacy subnet egress that will fail to propagate them onward into the higher layer.
   
   We use the term ECN-PDUs for a PDU on a feedback loop that will propagate congestion notification properly because it meets both these criteria. And a Not-ECN-PDU is a PDU on a feedback loop that does not meet both criteria, and will therefore not propagate congestion notification properly. A corollary of the above is that a lower layer congestion notification protocol:
3. SHOULD be able to distinguish ECN-PDUs from Not-ECN-PDUs.

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 has to drop some. The mechanism a lower layer uses to distinguish the ECN-capability of PDUs need not mimic that of IP, but it 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.

The per-domain checking of ECN support in MPLS [RFC5129] (Davie, B., Briscoe, B., and J. Tay, “Explicit Congestion Marking in MPLS,” January 2008.) 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, if any switch within the domain is configured to mark rather than drop during congestion. Therefore, there will be an implementation of an ECN-capable decapsulator on any edge node that might decapsulate a packet, which will check 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 on behalf of the earlier congested node (see Decapsulation Guideline Paragraph 1 in Section 4.3 (Decapsulation Guidelines)).

Note that 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).

Note that these guidelines do not require the subnet wire protocol to be changed at all to accommodate congestion notification. Another way to add congestion notification without consuming header space in the subnet protocol might be to use a control plane protocol in parallel.

4.2.  Encapsulation Guidelines

1. Egress Capability Check: A subnet ingress needs to be sure that the corresponding egress of a subnet will propagate any congestion notification added to the outer header across the subnet. This is necessary in addition to checking that an incoming PDU indicates an ECN-capable (L4) transport. Examples of how this guarantee might be provided include:
   • by configuration (e.g. if any label switches in a domain support ECN marking, [RFC5129] (Davie, B., Briscoe, B., and J. Tay, “Explicit Congestion Marking in MPLS,” January 2008.) requires all egress nodes to have been configured to propagate ECN)
   • by the ingress explicitly checking that the egress propagates ECN (e.g. TRILL uses IS-IS to check path capabilities before using critical options [trill‑rbridge‑options] (Eastlake, D., Ghanwani, A., Manral, V., and C. Bestler, “RBridges: Further TRILL Header Extensions,” June 2012.))
   • by inherent design of the protocol (e.g. by encoding ECN marking on the outer header in such a way that a legacy egress that does not understand ECN will consider the PDU corrupt and discard it, thus at least propagating a form of congestion signal).
   If the ingress cannot guarantee that the egress will propagate congestion notification, the ingress SHOULD disable ECN when it forwards the PDU at the lower layer. An example of how the ingress might disable ECN at the lower layer would be by setting the outer header of the PDU to identify it as a Not-ECN-PDU.
2. Standard Congestion Monitoring Baseline: Once the ingress to a subnet has established that the egress will correctly propagate ECN, on encapsulation it SHOULD encode the same level of congestion in outer headers as is arriving in incoming headers. For example it could copy any incoming congestion notification into the outer header of the lower layer protocol.
   
   This ensures that all outer headers reflect congestion accumulated along the whole upstream path, not just since the ingress of the subnet. More precisely, congestion notifications in outer headers SHOULD reflect congestion experienced along the whole path since the node that regulates the load for that path (the Load Regulator, typically the data source) and no other node should re-initialise the amount of CE markings to zero along the way.
   
   This guideline is intended to ensure that any bulk congestion monitoring of outer headers (e.g. by a network management node monitoring ECN in passing frames) is most meaningful. For instance, if an operator measures CE in 0.4% of passing packets, this information is only useful if the operator knows where the proportion of CE markings was last initialised to 0% (the Congestion Baseline). Such monitoring information will not be useful if some subnet ingress nodes reset all outer CE markings while others copy incoming CE markings into the outer.
   
   Most information can be extracted if the Congestion Baseline is standardised at the node that is regulating the load (the Load Regulator—typically the data source). Then the operator can measure both congestion since the Load Regulator, and congestion since the subnet ingress. The latter can be measured by subtracting the level of CE markings on inner headers from that on outer headers.

4.3.  Decapsulation Guidelines

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 rules. If there is any conflict, rules earlier in the list take precedence over rules later in the list:

1. If the arriving inner header is a Not-ECN-PDU it implies the L4 transport will not understand explicit congestion markings. Then:
   • If the outer header carries an explicit congestion marking, the packet SHOULD be dropped—the only indication of congestion that the L4 transport will understand.
   • If the outer is an ECN-PDU that carries no indication of congestion or a Not-ECN-PDU the PDU SHOULD be forwarded, but still as a Not-ECN-PDU.
2. If the outer header does not support explicit congestion notification (a Not-ECN-PDU), but the inner header does (an ECN-PDU), the inner header SHOULD be forwarded unchanged.
3. In some lower layer protocols congestion may be signalled as a numerical level, such as in the control frames of quantised congestion notification [IEEE802.1Qau] (Finn, N., Ed., “IEEE Standard for Local and Metropolitan Area Networks—Virtual Bridged Local Area Networks - Amendment 13: Congestion Notification,” March 2010.). If such an encoding encapsulates an ECN-capable IP packet, a function will be needed to convert the quantised congestion level into the frequency of congestion markings in outgoing IP packets.
4. Congestion indications may be encoded by a severity level. For instance increasing levels of congestion might be encoded by numerically increasing indications, e.g. pre-congestion notification (PCN) can be encoded in each PDU at three severity levels in IP or MPLS [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),” July 2012.).
   
   If the arriving inner header is an ECN-PDU, where the inner and outer headers carry indications of congestion of different severity, the more severe indication SHOULD be forwarded in preference to the less severe. Obviously, if the severities in both inner and outer are the same, the same severity should be forwarded.
5. The inner and outer headers might carry a combination of congestion notification fields that should not be possible given any currently used protocol transitions. For instance, if Encapsulation Guideline Paragraph 2 in Section 4.2 (Encapsulation Guidelines) had been followed, it should not be possible to have a less severe indication of congestion in the outer than in the inner. It MAY be appropriate to log unexpected combinations of headers and possibly raise an alarm. If a safe outgoing codepoint can be defined for such a PDU, the PDU SHOULD be forwarded rather than dropped.
   
   Some implementers discard PDUs with currently unused combinations of headers just in case they represent an attack. However, an approach using alarms and policy-mediated drop is preferable to hard-coded drop, so that operators can keep track of possible attacks but currently unused combinations are not precluded from future use through new standards actions.

4.4.  Reframing and Congestion Markings

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.

5.  Feed-Up-and-Forward Mode: Guidelines for Adding Congestion Notification

Marking the IP header while switching at layer-2 (by using a layer-3 switch) 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:

• IPv4 and IPv6 are not the only layer-3 protocols that might be encapsulated by lower layer protocols
• Link-layer encryption might be in use, making the layer-2 payload inaccessible
• Many Ethernet switches do not have 'layer-3 switch' capabilities so they cannot read and modify an IP payload
• It might be costly to find an IP header (v4 or v6) when it may be encapsulated by more than one Ethernet header (e.g. when using multiple encapsulations of MAC in MAC [IEEE802.1Qah] (IEEE, “IEEE Standard for Local and Metropolitan Area Networks—Virtual Bridged Local Area Networks—Amendment 6: Provider Backbone Bridges,” August 2008.)).

Nonetheless, configuring a layer-3 switch 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:

1. If a native lower-layer congestion notification mechanism exists for a subnet technology, it is safe to mix feed-up-and-forward with feed-forward-and-up on other switches in the same subnet. However, it will generally be more efficient to use the native mechanism.
2. The depth of search for an IP header SHOULD be limited. If an IP header is not found soon enough, or an unrecognised or unreadable header is encountered, the switch SHOULD resort to an alternative means of signalling congestion (e.g. drop, or the native lower layer mechanism if available).
3. It is sufficient to use the first IP header found in the stack; the egress of the relevant tunnel can propagate congestion notification upwards to any more deeply encapsulated IP headers later.

6.  Feed-Backward Mode: Guidelines for Adding Congestion Notification

It can be seen from Section 3.3 (Feed-Backward Mode) that congestion notification in a subnet using feed-backward mode has generally not been designed to directly coupled with IP layer congestion notification. The subnet attempts to minimise congestion internally, and if the incoming load at the ingress exceeds capacity 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:

1. A subnetwork technology intended to eventually interface to IP SHOULD NOT be designed using only the feed-backward mode, which is certainly best for a stand-alone subnet, but would need to be modified to work efficiently as part of the wider Internet, because IP uses feed-forward-and-up mode.

The feed-backward approach does at least work beneath IP, but it 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 possible 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] (Gont, F., “Deprecation of ICMP Source Quench Messages,” May 2012.). 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, 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] (Finn, N., Ed., “IEEE Standard for Local and Metropolitan Area Networks—Virtual Bridged Local Area Networks - Amendment 13: Congestion Notification,” March 2010.) uses a feed-backward mode very similar to ATM. However, QCN confines its applicability to scenarios where all endpoints are directly attached by the same Ethernet technology, and is used for example in server area networks (SANs). If a QCN subnet were connected into a wider IP-based internetwork (e.g. when attempting to interconnect SANs within multiple data centres) it would suffer the same inefficiency as shown in Figure 3 (Feed-Backward Mode).

7.  IANA Considerations

This memo includes no request to IANA.

8.  Security Considerations

{TBA}`

9.  Conclusions

{TBA}

10.  Acknowledgements

Thanks to Gorry Fairhurst for extensive initial review.

Bob Briscoe produced early drafts while partly funded by Trilogy, a research project (ICT-216372) supported by the European Community under its Seventh Framework Programme. The views expressed here are those of the author only.

11.  Comments Solicited

Comments and questions are encouraged and very welcome. They can be addressed to the IETF Transport Area working group mailing list <tsvwg@ietf.org>, and/or to the authors.

12.  References

12.1. Normative References

12.2. Informative References

Appendix A.  Outstanding Document Issues

1. [GF] Concern that certain guidelines warrant a MUST (NOT) rather than a SHOULD (NOT). Given the guidelines say that if any SHOULD (NOT)s are not followed, a strong justification will be needed, they have been left as SHOULD (NOT) pending further list discussion. In particular:
   • If inner is a Not-ECN-PDU and Outer is CE (or highest severity congestion level), MUST (not SHOULD) drop?
2. [GF] Impact of Diffserv on alternate marking schemes (referring to RFC3168, RFC4774 & RFC2983)
3. Security Considerations

Appendix B.  Changes in This Version (to be removed by RFC Editor)

From briscoe-00 to 00:

• Intended status: BCP (was Informational) & updates 3819 added.
• Briefer Introduction: Introductory para justifying benefits of ECN. Moved all but a brief enumeration of modes of operation to their own new section (from both Intro & Scope). Introduced incr. deployment as most tricky part.
• Tightened & added to terminology section
• Structured with Modes of Operation, then Guidelines section for each mode.
• Tightened up guideline text to remove vagueness / passive voice / ambiguity and highlight main guidelines as numbered items.
• Added Outstanding Document Issues Appendix
• Updated references

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