Network Working Group M. Bagnulo
Internet-Draft UC3M
Intended status: Experimental B. Briscoe
Expires: October 20, 2017 Simula Research Lab
April 18, 2017

Adding Explicit Congestion Notification (ECN) to TCP control packets and TCP retransmissions


This document describes an experimental modification to ECN when used with TCP. It allows the use of ECN on the following TCP packets: SYNs, pure ACKs, Window probes, FINs, RSTs and retransmissions.

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Table of Contents

1. Introduction

RFC 3168 [RFC3168] specifies support of Explicit Congestion Notification (ECN) in IP (v4 and v6). By using the ECN capability, switches performing Active Queue Management (AQM) can use ECN marks instead of packet drops to signal congestion to the endpoints of a communication. This results in lower packet loss and increased performance. RFC 3168 also specifies support for ECN in TCP, but solely on data packets. For various reasons it precludes the use of ECN on TCP control packets (TCP SYN, TCP SYN-ACK, pure ACKs, Window probes) and on retransmitted packets. RFC 3168 is silent about the use of ECN on RST and FIN packets. RFC 5562 [RFC5562] is an experimental modification to ECN that enables ECN support for TCP SYN-ACK packets.

This document defines an experimental modification to ECN [RFC3168] that enables ECN support on all the aforementioned types of TCP packet. [I-D.ietf-tsvwg-ecn-experimentation] is a standards track procedural device that updates RFC 3168 to allow the present experiment, which RFC 3168 would otherwise prohibit.

1.1. Motivation

The absence of ECN support on TCP control packets and retransmissions has a potential harmful effect. In any ECN deployment, non-ECN-capable packets suffer a penalty when they traverse a congested bottleneck. For instance, with a drop probability of 1%, 1% of connection attempts suffer a timeout of about 1 second before the SYN is retransmitted, which is highly detrimental to the performance of short flows. TCP control packets, such as TCP SYNs and pure ACKs, are important for performance, so dropping them is best avoided.

Non-ECN control packets particularly harm performance in environments where the ECN marking level is high. For example, [judd-nsdi] shows that in a data centre (DC) environment where ECN is used (in conjunction with DCTCP), the probability of being able to establish a new connection using a non-ECN SYN packet drops to close to zero even when there are only 16 ongoing TCP flows transmitting at full speed. In this data centre context, the issue is that DCTCP's aggressive response to packet marking leads to a high marking probability for ECN-capable packets, and in turn a high drop probability for non-ECN packets. Therefore non-ECN SYNs are dropped aggressively, rendering it nearly impossible to establish a new connection in the presence of even mild traffic load.

Finally, there are ongoing experimental efforts to promote the adoption of a slightly modified variant of DCTCP (and similar congestion controls) over the Internet to achieve low latency, low loss and scalable throughput (L4S) for all communications [I-D.briscoe-tsvwg-l4s-arch]. In such an approach, L4S packets identify themselves using an ECN codepoint. Preventing TCP control packets from obtaining the benefits of ECN would not only expose them to the prevailing level of congestion loss, but it would also stop them from being classified into the low latency (L4S) queue, which would greatly degrade L4S performance.

1.2. Experiment Goals

The goal of the experimental modifications defined in this document is to allow the use of ECN on all TCP packets. Experiments are expected in the public Internet as well as in controlled environments to understand the following issues:

The data gathered through the experiments described in this document, particularly under the first 2 bullets above, will help in the design of the final mechanism (if any) for adding ECN support to the different packet types considered in this document. Whenever data input is needed to assist in a design choice, it is spelled out throughout the document.

Success criteria: The experiment will be a success if we obtain enough data to have a clearer view of the deployability and benefits of enabling ECN on all TCP packets, as well as any issues. If the results of the experiment show that it is feasible to deploy such changes; that there are gains to be achieved though the changes described in this specification; and that no other major issues may interfere with the deployment of the proposed changes; then it would be reasonable to adopt the proposed changes in a standards track specification that would update RFC 3168.

1.3. Document Structure

The remainder of this document is structured as follows. In Section 2, we present the terminology used in the rest of the document. In Section 3, we specify the modifications to provide ECN support to TCP SYNs, pure ACKs, Window probes, FINs, RSTs and retransmissions. We describe both the network behaviour and the endpoint behaviour. Section 4 discusses variations of the specification that will be necessary to interwork with a number of popular variants or derivatives of TCP. RFC 3168 provides a number of specific reasons why ECN support is not appropriate for each packet type. In Section 5, we revisit each of these arguments for each packet type to justify why it is reasonable to conduct this experiment.

2. Terminology

The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [RFC2119].

Pure ACK: A TCP segment with the ACK flag set and no data payload.

SYN: A TCP segment with the SYN (synchronize) flag set.

Window probe: Defined in [RFC0793], a window probe is a TCP segment with only one byte of data sent to learn if the receive window is still zero.

FIN: A TCP segment with the FIN (finish) flag set.

RST: A TCP segment with the RST (reset) flag set.

Retransmission: A TCP segment that has been retransmitted by the TCP sender.

ECT: ECN-Capable Transport. One of the two codepoints ECT(0) or ECT(1) in the ECN field [RFC3168] of the IP header (v4 or v6). An ECN-capable sender sets one of these to indicate that both transport end-points support ECN. When this specification says the sender sets an ECT codepoint, by default it means ECT(0). Optionally, it could mean ECT(1), which is in the process of being redefined for use by L4S experiments [I-D.ietf-tsvwg-ecn-experimentation] [I-D.briscoe-tsvwg-ecn-l4s-id].

Not-ECT: The ECN codepoint set by senders that indicates that the transport is not ECN-capable.

CE: Congestion Experienced. The ECN codepoint that an intermediate node sets to indicate congestion [RFC3168]. A node sets an increasing proportion of ECT packets to CE as the level of congestion increases.

3. Specification

3.1. Network (e.g. Firewall) Behaviour

Previously the specification of ECN for TCP [RFC3168] required the sender to set not-ECT on TCP control packets and retransmissions. Some readers of RFC 3168 might have erroneously interpreted this as a requirement for firewalls, intrusion detection systems, etc. to check and enforce this behaviour. Section 4.3 of [I-D.ietf-tsvwg-ecn-experimentation] updates RFC 3168 to remove this ambiguity. It require firewalls or any intermediate nodes not to treat certain types of ECN-capable TCP segment differently (except potentially in one attack scenario). This is likely to only involve a firewall rule change in a fraction of cases (at most 0.4% of paths according to the tests reported in Section 5.2.2).

In case a TCP sender encounters a middlebox blocking ECT on certain TCP segments, the specification below includes behaviour to fall back to non-ECN. However, this loses the benefit of ECN on control packets. So operators are encouraged to alter their firewall rules to comply with the requirement referred to above (section 4.3 of [I-D.ietf-tsvwg-ecn-experimentation]).

3.2. Endpoint Behaviour

The changes to the specification of TCP over ECN [RFC3168] defined here solely alter the behaviour of the sending host for each half-connection. All changes can be deployed at each end-point independently of others.

The feedback behaviour at the receiver depends on whether classic ECN TCP feedback [RFC3168] or Accurate ECN (AccECN) TCP feedback [I-D.ietf-tcpm-accurate-ecn] has been negotiated. Nonetheless, neither receiver feedback behaviour is altered by the present specification.

For each type of control packet or retransmission, the following sections detail changes to the sender's behaviour in two respects: i) whether it sets ECT; and ii) its response to congestion feedback. Table 1 summarises these two behaviours for each type of packet, but the relevant subsection below should be referred to for the detailed behaviour. The subsection on the SYN is more complex than the others, because it has to include fall-back behaviour if the ECT packet appears not to have got through, and caching of the outcome to detect persistent failures.

Summary of sender behaviour. In each case the relevant section below should be referred to for the detailed behaviour
TCP packet type ECN field if AccECN f/b negotiated* ECN field if RFC3168 f/b negotiated* Congestion Response
SYN ECT not-ECT Reduce IW
SYN-ACK [RFC5562] ECT ECT Reduce IW as in [RFC5562]
Pure ACK ECT ECT Usual cwnd response and optionally [RFC5690]
W Probe ECT ECT Usual cwnd response
FIN ECT ECT None or optionally [RFC5690]
Re-XMT ECT ECT Usual cwnd response

Window probe and retransmission are abbreviated to W Probe an Re-XMT. * For a SYN, "negotiated" means "requested".

It can be seen that the sender can set ECT in all cases, except if it is not requesting AccECN feedback on the SYN. Therefore it is RECOMMENDED that the experimental AccECN specification [I-D.ietf-tcpm-accurate-ecn] is implemented (as well as the present specification), because it is expected that ECT on the SYN will give the most significant performance gain, particularly for short flows. Nonetheless, this specification also caters for the case where AccECN feedback is not implemented.

3.2.1. SYN Setting ECT on the SYN

With classic [RFC3168] ECN feedback, the SYN was never expected to be ECN-capable, so the flag provided to feed back congestion was put to another use (it is used in combination with other flags to indicate that the responder supports ECN). In contrast, Accurate ECN (AccECN) feedback [I-D.ietf-tcpm-accurate-ecn] provides two codepoints in the SYN-ACK for the responder to feed back whether or not the SYN arrived marked CE.

Therefore, a TCP initiator MUST NOT set ECT on a SYN unless it also attempts to negotiate Accurate ECN feedback in the same SYN.

For the experiments proposed here, if the SYN is requesting AccECN feedback, the TCP sender will also set ECT on the SYN. It can ignore the prohibition in section 6.1.1 of RFC 3168 against setting ECT on such a SYN.

The following subsections about the SYN solely apply to this case where the initiator sent an ECT SYN. Caching Failed Connection Attempts

Until AccECN servers become widely deployed, a TCP initiator that implements AccECN and sets ECT on a SYN SHOULD also maintain a cache per server to record any failure of the previous attempt. It SHOULD record whether a server does not support AccECN and MAY record whether the ECT SYN is persistently lost (see fall-back below). The TCP initiator will not subsequently attempt any behaviour recorded as persistently problematic. However, the cache should be arranged to expire so that the initiator will infrequently attempt to check whether each problem has been resolved.

There is no need to cache successful attempts, because the default ECT SYN behaviour performs optimally on success anyway. Servers that do not support ECN as a whole probably do not need to be recorded separately from non-support of AccECN because, even when a server does not support classic [RFC3168] ECN, there is no performance penalty in always requesting to use it.

The above strategy is named "optimistic ECT and cache failures". It is believed to be sufficient based on initial measurements and assumptions detailed in Section 5.2.1, which also gives alternative strategies in case larger scale measurements uncover different scenarios. SYN Congestion Response

If the SYN-ACK returned to the TCP initiator confirms that the server supports AccECN, it will also indicate whether or not the SYN was CE-marked. If the SYN was CE-marked, the initiator MUST reduce its Initial Window (IW) and SHOULD reduce it to 1 SMSS (sender maximum segment size).

If the SYN-ACK shows that the server does not support AccECN, the TCP initiator MUST conservatively reduce its Initial Window and SHOULD reduce it to 1 SMSS. A reduction to greater than 1 SMSS MAY be appropriate (see Section 5.2.1). Conservatism is necessary because a non-AccECN SYN-ACK cannot show whether the SYN was CE-marked.

If the TCP initiator (host A) receives a SYN from the remote end (host B) after it has sent a SYN to B, it indicates the (unusual) case of a simultaneous open. Host A will respond with a SYN-ACK. Host A will probably then receive a SYN-ACK in response to its own SYN, after which it can follow the appropriate one of the two paragraphs above.

In all the above cases, the initiator does not have to back off its retransmission timer as it would in response to a timeout following no response to its SYN [RFC6298], because both the SYN and the SYN-ACK have been successfully delivered through the network. Also, the initiator does not need to exit slow start or reduce ssthresh, which is not even required when a SYN is lost [RFC5681].

If TCP Fast Open (TFO [RFC7413]) or an initial window of 10 (IW10 [RFC6928]) are implemented, Section 4 gives additional recommendations. Fall-Back Following No Response to an ECT SYN)

An ECT SYN might be lost due to an over-zealous path element (or server) blocking ECT packets that do not conform to RFC 3168. However, loss is commonplace for numerous other reasons, e.g. congestion loss at a non-ECN queue on the forward or reverse path, transmission errors, etc. Alternatively, the cause of the blockage might be the attempt to negotiate AccECN, or possibly other unrelated options on the SYN.

To expedite connection set-up if, after sending an ECT SYN, the retransmission timer expires, the TCP initiator SHOULD send a SYN with the not-ECT codepoint in the IP header and not attempt to negotiate AccECN. It would make sense to also remove any other experimental fields or options on the SYN, but that will depend on the specification of the other option(s). If the TCP initiator is caching failed connection attempts, it SHOULD NOT give up using ECT on the first SYN of subsequent connection attempts until it is clear that the blockage persistently and specifically affects ECT on SYNs. This is because loss is so commonplace for other reasons.

Other fall-back strategies MAY be adopted where applicable (see Section 5.2.2 for suggestions, and the conditions under which they would apply).

3.2.2. SYN-ACK

To comply with the present specification, the responder (server) part of a TCP implementation MUST also comply with [RFC5562], which defines the use of ECT on a SYN-ACK and the congestion response of the TCP listener if a SYN-ACK is CE-marked.

Feedback by the initiator in response to a CE-marked SYN-ACK from the responder depends on whether classic ECN feedback or AccECN feedback [I-D.ietf-tcpm-accurate-ecn] has been negotiated. In either case no change is required to RFC 5562 or the AccECN specification.

3.2.3. Pure ACK

For the experiments proposed here, the TCP implementation will set ECT on pure ACKs. It can ignore the requirement in section 6.1.4 of RFC 3168 to set not-ECT on a pure ACK.

A host that sets ECT on pure ACKs MUST reduce its congestion window in response to any congestion feedback, in order to regulate any data segments it might be sending amongst the pure ACKs. It MAY also implement AckCC [RFC5690] to regulate the pure ACK rate, but this is not required. Note that, in comparison, [RFC5681] does not require a TCP to detect or respond to loss of pure ACKs at all; it requires no reduction in congestion window or ACK rate.

The question of whether the receiver of pure ACKs is required to feed back any CE marks on them is a matter for the relevant feedback specification ([RFC3168] or [I-D.ietf-tcpm-accurate-ecn]). It is outside the scope of the present specification.

3.2.4. Window Probe

For the experiments proposed here, the TCP sender will set ECT on window probes. It can ignore the prohibition in section 6.1.6 of RFC 3168 against setting ECT on a window probe.

A window probe contains a single octet, so it is no different from a regular TCP data segment. Therefore a TCP receiver will feed back any CE marking on a window probe as normal (either using classic ECN feedback or AccECN feedback). The sender of the probe will then reduce its congestion window as normal.

A receive window of zero indicates that the application is not consuming data fast enough and does not imply anything about network congestion. Once the receive window opens, the congestion window might become the limiting factor, so it is correct that CE-marked probes reduce the congestion window. However, CE-marking on window probes does not reduce the rate of the probes themselves. This is unlikely to present a problem, given the duration between window probes doubles [RFC1122] as long as the receiver is advertising a zero window (currently minimum 1 second, maximum at least 1 minute [RFC6298]).

3.2.5. FIN

A TCP implementation can set ECT on a FIN.

The TCP data receiver MUST ignore the CE codepoint on incoming FINs that fail any validity check. The validity check in section 5.2 of [RFC5961] is RECOMMENDED.

A congestion response to a CE-marking on a FIN is not required.

After sending a FIN, the endpoint will not send any more data in the connection. Therefore, even if the FIN-ACK indicates that the FIN was CE-marked (whether using classic or AccECN feedback), reducing the congestion window will not affect anything.

After sending a FIN, a host might send one or more pure ACKs. If it is using one of the techniques in Section 3.2.3 to regulate the delayed ACK ratio for pure ACKs, it could equally be applied after a FIN. But this is not required.

3.2.6. RST

A TCP implementation can set ECT on a RST.

The "challenge ACK" approach to checking the validity of RSTs (section 3.2 of [RFC5961] is RECOMMENDED at the data receiver.

A congestion response to a CE-marking on a RST is not required (and actually not possible). Rationale for ECT on RSTs

The host generating the RST message does not have an open connection after sending it (either because there was no such connection when the packet that triggered the RST message was received or because the packet that triggered the RST message also triggered the closure of the connection).

Moreover, the receiver of a CE-marked RST message can either: i) accept the RST message and close the connection; ii) emit a so-called challenge ACK in response (with suitable throttling) [RFC5961] and otherwise ignore the RST (e.g. because the sequence number is in-window but not the precise number expected next); or iii) discard the RST message (e.g. because the sequence number is out-of-window). In the first two cases there is no point in echoing any CE mark received because the sender closed its connection when it sent the RST. In the third case it makes sense to discard the CE signal as well as the RST. So, in all these cases it does not make sense to generate feedback about a CE mark on a RST message.

The following factors have been considered before deciding whether ECT ought to be allowed on a RST message:

So, on balance, it has been decided that it is worth experimenting with ECT on RSTs. During experiments, if the ECN capability on RSTs is found to open a vulnerability that is hard to close, this decision can be reversed, before it is specified for the standards track.

3.2.7. Retransmissions

For the experiments proposed here, the TCP sender will set ECT on retransmitted segments. It can ignore the prohibition in section 6.1.5 of RFC 3168 against setting ECT on retransmissions.

Nonetheless, the TCP data receiver MUST ignore the CE codepoint on incoming segments that fail any validity check. The validity check in section 5.2 of [RFC5961] is RECOMMENDED. This will effectively mitigate an attack that uses spoofed data packets to fool the receiver into feeding back spoofed congestion indications to the sender, which in turn would be fooled into continually halving its congestion window.

If the TCP sender receives feedback that a retransmitted packet was CE-marked, it will react as it would to any feedback of CE-marking on a data packet.

4. Interaction with popular variants or derivatives of TCP

The following subsections specify additional behaviour necessary when setting ECT on all data and control packets while using the following popular variants or derivatives of TCP: SCTP, TFO, IW10. The subsection on IW10 discusses changes to specifications but does not recommend any, because the specification as it stands is safe, and there is only a corner-case where performance could be occasionally improved.

TCP variants that have been assessed and found not to interact adversely with ECT on TCP control packets are: SYN cookies (see Appendix A of [RFC4987]) and L4S [I-D.briscoe-tsvwg-l4s-arch].

4.1. SCTP

Stream Control Transmission Protocol (SCTP [RFC4960]) is a standards track protocol derived from TCP. SCTP currently does not include ECN support, but Appendix A of RFC 4960 broadly describes how it would be supported and a draft on the addition of ECN to SCTP has been produced [I-D.stewart-tsvwg-sctpecn]. This draft avoids setting ECT on control packets and retransmissions, closely following the arguments in RFC 3168. When ECN is finally added to SCTP, experience from experiments on adding ECN support to all TCP packets ought to be directly transferable to SCTP.

4.2. TFO

TCP Fast Open (TFO [RFC7413]) is an experiment to remove the round trip delay of TCP's 3-way hand-shake (3WHS). A TFO initiator caches a cookie from a previous connection with a TFO-enabled server. Then, for subsequent connections to the same server, any data included on the SYN and any other data segments sent directly after the SYN (up to the initial window limit) can be passed directly to the server application, which can then return response data with the SYN-ACK (again, up to the initial window limit).

If a TFO initiator has cached that the server supported ECN in the previous connection, it would be safe to set ECT on any data segments it sends before a SYN-ACK returns from the responder (server). Note that there is no space in the SYN-ACK itself (whether classic or AccECN feedback has been negotiated) to include feedback about any CE on data packets. Nonetheless, it is safe to set ECT on data packets within the handshake because any CE-marking on these data segments can be fed back by the responder on the first data segment it sends after the SYN-ACK (or on an additional pure ACK if it has no more data to send).

Note that the prohibition in Section against setting ECT on the SYN if the same SYN is not requesting AccECN feedback still applies.

Strictly even a non-TFO TCP initiator can send data on a SYN and up to an initial window of data segments straight after the SYN. However, this is rare because a non-TFO TCP server will not deliver any data to the application until the 3WHS completes. Therefore the question of ECT on data segments within the handshake only becomes important with TFO. A TFO initiator's first ever connection with a server never uses a fast open, so the initiator always has a chance to cache whether a server supports ECN before it uses a fast open.

4.3. IW10

IW10 is an experiment to determine whether it is safe for TCP to use an initial window of 10 SMSS [RFC6928].

This subsection does not recommend any additions to the present specification in order to interwork with IW10. The specifications as they stand are safe, and there is only a corner-case where performance could be occasionally improved, as explained below.

As specified in Section, a TCP initiator can only set ECT on the SYN if it requests AccECN support. If, however, the SYN-ACK tells the initiator that the responder does not support AccECN, Section advises the initiator to conservatively reduce its initial window to 1 SMSS because, if the SYN was CE-marked, the SYN-ACK has no way to feed that back.

If the initiator implements IW10, it seems rather over-conservative to reduce IW to 1 in this scenario. Nonetheless, the reduction to 1 SMSS will rarely harm performance, because:

5. Discussion of the Arguments in the RFC Series

This section is informative, not normative. It presents counter-arguments against the justifications in the RFC series for disabling ECN on TCP control segments and retransmissions. First it addresses over-arching arguments used for most packet types, then it addresses the specific arguments for each packet type in turn.

5.1. The Reliability Argument

Section 5.2 of RFC 3168 states:

We believe this argument is misplaced. TCP does not deliver most control packets reliably. So it is more important to allow control packets to be ECN-capable, which greatly improves reliable delivery of the control packets themselves (see motivation in Section 1.1). ECN also improves the reliability and latency of delivery of any congestion notification on control packets, particularly because TCP does not detect the loss of most types of control packet anyway. Both these points outweigh by far the concern that a CE marking applied to a control packet by one node might subsequently be dropped by another node.

The principle to determine whether a packet can be ECN-capable ought to be "do no extra harm", meaning that the reliability of a congestion signal's delivery ought to be no worse with ECN than without. In particular, setting the CE codepoint on the very same packet that would otherwise have been dropped fulfills this criterion, since either the packet is delivered and the CE signal is delivered to the endpoint, or the packet is dropped and the original congestion signal (packet loss) is delivered to the endpoint.

The concern about a CE marking being dropped at a subsequent node might be motivated by the idea that ECN-marking a packet at the first node does not remove the packet, so it could go on to worsen congestion at a subsequent node. However, it is not useful to reason about congestion by considering single packets. The departure rate from the first node will generally be the same (fully utilized) with or without ECN, so this argument does not apply.

5.2. SYNs

RFC 5562 presents two arguments against ECT marking of SYN packets (quoted verbatim):

The first point actually describes two subtly different issues. So below three arguments are countered in turn.

5.2.1. Argument 1a: Unrecognized CE on the SYN

This argument certainly applied at the time RFC 5562 was written, when no ECN responder mechanism had any logic to recognize or feed back a CE marking on a SYN. The problem was that, during the 3WHS, the flag in the TCP header for ECN feedback (called Echo Congestion Experienced) had been overloaded to negotiate the use of ECN itself. So there was no space for feedback in a SYN-ACK.

The accurate ECN (AccECN) protocol [I-D.ietf-tcpm-accurate-ecn] has since been designed to solve this problem, using a two-pronged approach. First AccECN uses the 3 ECN bits in the TCP header as 8 codepoints, so there is space for the responder to feed back whether there was CE on the SYN. Second a TCP initiator can always request AccECN support on every SYN, and any responder reveals its level of ECN support: AccECN, classic ECN, or no ECN. Therefore, if a responder does indicate that it supports AccECN, the initiator can be sure that, if there is no CE feedback on the SYN-ACK, then there really was no CE on the SYN.

An initiator can combine AccECN with three possible strategies for setting ECT on a SYN:

Pessimistic ECT and cache successes: The initiator always requests AccECN in the SYN, but without setting ECT. Then it records those servers that confirm that they support AccECN in a cache. On a subsequent connection to any server that supports AccECN, the initiator can then set ECT on the SYN.
Optimistic ECT: The initiator always sets ECT optimistically on the initial SYN and it always requests AccECN support. Then, if the server response shows it has no AccECN logic (so it cannot feed back a CE mark), the initiator conservatively behaves as if the SYN was CE-marked, by reducing its initial window.
  1. No cache: The optimistic ECT strategy ought to work fairly well without caching any responses.
  2. Cache failures: The optimistic ECT strategy can be improved by recording solely those servers that do not support AccECN. On subsequent connections to these non-AccECN servers, the initiator will still request AccECN but not set ECT on the SYN. Then, the initiator can use its full initial window (if it has enough request data to need it). Longer term, as servers upgrade to AccECN, the initiator will remove them from the cache and use ECT on subsequent SYNs to that server.
ECT by configuration: In a controlled environment, the administrator can make sure that servers support ECN-capable SYN packets. Examples of controlled environments are single-tenant DCs, and possibly multi-tenant DCs if it is assumed that each tenant mostly communicates with its own VMs.

For unmanaged environments like the public Internet, pragmatically the choice is between strategies (S1) and (S2B):

The normative specification for ECT on a SYN in Section 3.2.1 uses the "optimistic ECT and cache failures" strategy (S2B) on the assumption that an initial window of 1 SMSS is usually sufficient for client requests anyway. Clients that often initially send more than 1 SMSS of data could use strategy (S1) during initial deployment, and strategy (S2B) later (when the probability of servers supporting AccECN and the likelihood of seeing some CE marking is higher). Also, as deployment proceeds, caching successes (S1) starts off small then grows, while caching failures (S2B) becomes large at first, then shrinks.

5.2.2. Argument 1b: Unrecognized ECT on the SYN

Given, until now, ECT-marked SYN packets have been prohibited, it cannot be assumed they will be accepted. According to a study using 2014 data [ecn-pam] from a limited range of vantage points, out of the top 1M Alexa web sites, 4791 (0.82%) IPv4 sites and 104 (0.61%) IPv6 sites failed to establish a connection when they received a TCP SYN with any ECN codepoint set in the IP header and the appropriate ECN flags in the TCP header. Of these, about 41% failed to establish a connection due to the ECN flags in the TCP header even with a Not-ECT ECN field in the IP header (i.e. despite full compliance with RFC 3168). Therefore adding the ECN-capability to SYNs was increasing connection establishment failures by about 0.4%.

RFC 3168 says "a host MUST NOT set ECT on SYN [...] packets", but it does not say what the responder should do if an ECN-capable SYN arrives. So perhaps some responder implementations are checking that the SYN complies with RFC 3168, then silently ignoring non-compliant SYNs (or perhaps returning a RST). Also some middleboxes (e.g. firewalls) might be discarding non-compliant SYNs. For the future, [I-D.ietf-tsvwg-ecn-experimentation] updates RFC 3168 to clarify that middleboxes "SHOULD NOT" do this, but that does not alter the past.

Whereas RSTs can be dealt with immediately, silent failures introduce a retransmission timeout delay (default 1 second) at the initiator before it attempts any fall back strategy. Ironically, making SYNs ECN-capable is intended to avoid the timeout when a SYN is lost due to congestion. Fortunately, where discard of ECN-capable SYNs is due to policy it will occur predictably, not randomly like congestion. So the initiator can avoid it by caching those sites that do not support ECN-capable SYNs. This further justifies the use of the "optimistic ECT and cache failures" strategy in Section 3.2.1.

Section 5.2.1) would be better.

If blocking is too widespread for the "optimistic ECT and cache failures" strategy (S2B), the "pessimistic ECT and cache successes" strategy (

If so, it might be necessary to send a not-ECT SYN soon after the first ECT SYN (possibly with a small delay between them) and only accept the non-ECT connection if it returned first. This would reduce the performance penalty for those deploying ECT SYN support. However, based on the data above from [ecn-pam], even a cache of a dozen or so sites ought to avoid all ECN-related performance problems with roughly the Alexa top thousand. So it is questionable whether sending two SYNs will be necessary, particularly given failures at well-maintained sites could reduce further once ECT SYNs are standardized.

5.2.3. Argument 2: DoS Attacks.

[RFC5562] says that ECT SYN packets could be misused by malicious clients to augment "the well-known TCP SYN attack". It goes on to say "a malicious host might be able to inject a large number of TCP SYN packets through a potentially congested ECN-enabled router, congesting it even further."

We assume this is a reference to the TCP SYN flood attack (see, which is an attack against a responder end point. We assume the idea of this attack is to use ECT to get more packets through an ECN-enabled router in preference to other non-ECN traffic so that they can go on to use the SYN flooding attack to inflict more damage on the responder end point. This argument could apply to flooding with any type of packet, but we assume SYNs are singled out because their source address is easier to spoof, whereas floods of other types of packets are easier to block.

Mandating Not-ECT in an RFC does not stop attackers using ECT for flooding. Nonetheless, if a standard says SYNs are not meant to be ECT it would make it legitimate for firewalls to discard them. However this would negate the considerable benefit of ECT SYNs for compliant transports and seems unnecessary because RFC 3168 already provides the means to address this concern. In section 7, RFC 3168 says "During periods where ... the potential packet marking rate would be high, our recommendation is that routers drop packets rather then set the CE codepoint..." and this advice is repeated in [RFC7567] (section 4.2.1). This makes it harder for flooding packets to gain from ECT.

Further experiments are needed to test how much malicious hosts can use ECT to augment flooding attacks without triggering AQMs to turn off ECN support (flying "just under the radar"). If it is found that ECT can only slightly augment flooding attacks, the risk of such attacks will need to be weighed against the performance benefits of ECT SYNs.

5.3. Pure ACKs.

Section 5.2 of RFC 3168 gives the following arguments for not allowing the ECT marking of pure ACKs (ACKs not piggy-backed on data):

Later on, in section 6.1.4 it reads:

We next address each of the arguments presented above.

The first argument is a specific instance of the reliability argument for the case of pure ACKs. This has already been addressed by countering the general reliability argument in Section 5.1.

The second argument says that ECN ought not to be enabled unless there is a mechanism to respond to it. However, actually there is a mechanism to respond to congestion on a pure ACK that RFC 3168 has overlooked - the congestion window mechanism. When data segments and pure ACKs are interspersed, congestion notifications ought to regulate the congestion window, whether they are on data segments or on pure ACKs. Otherwise, if ECN is disabled on Pure ACKs, and if (say) 70% of the segments in one direction are Pure ACKs, about 70% of the congestion notifications will be missed and the data segments will not be correctly regulated.

So RFC 3168 ought to have considered two congestion response mechanisms - reducing the congestion window (cwnd) and reducing the ACK rate - and only the latter was missing. Further, RFC 3168 was incorrect to assume that, if one ACK was a pure ACK, all segments in the same direction would be pure ACKs. Admittedly a continual stream of pure ACKs in one direction is quite a common case (e.g. a file download). However, it is also common for the pure ACKs to be interspersed with data segments (e.g. HTTP/2 browser requests controlling a web application). Indeed, it is more likely that any congestion experienced by pure ACKs will be due to mixing with data segments, either within the same flow, or within competing flows.

This insight swings the argument towards enabling ECN on pure ACKs so that CE marks can drive the cwnd response to congestion (whenever data segments are interspersed with the pure ACKs). Then to separately decide whether an ACK rate response is also required (when they are ECN-enabled). The two types of response are addressed separately in the following two subsections, then a final subsection draws conclusions.

5.3.1. Cwnd Response to CE-Marked Pure ACKs

If the sender of pure ACKs sets them to ECT, the bullets below assess whether the three stages of the congestion response mechanism will all work for each type of congestion feedback (classic ECN [RFC3168] and AccECN [I-D.ietf-tcpm-accurate-ecn]):

The receiver of a pure ACK can detect a CE marking on it:
  • Classic feedback: the receiver will not expect CE marks on pure ACKs, so it will be implementation-dependent whether it happens to check for CE marks on all packets.
  • AccECN feedback: currently the draft AccECN specification is written assuming pure ACKs are not ECN-capable. However, if the present specification is adopted, before publication of AccECN it will need to be updated to feed back CE-markings on pure ACKs. {ToDo: and this bullet will need to be updated}
TCP never ACKs a pure ACK, but the receiver of a CE-mark on a pure ACK can feed it back when it sends a subsequent data segment (if it ever does):
  • Classic feedback: the receiver (of the pure ACKs) would set the echo congestion experienced (ECE) flag in the TCP header as normal.
  • AccECN feedback: the receiver continually feeds back a count of the number of CE-marked packets that it has received (and, if possible, a count of CE-marked bytes).
Congestion response:
In either case (classic or AccECN feedback), if the TCP sender does receive feedback about CE-markings on pure ACKs, it will react in the usual way by reducing its congestion window accordingly. This will regulate the rate of any data packets it is sending amongst the pure ACKs.

5.3.2. ACK Rate Response to CE-Marked Pure ACKs

Reducing the congestion window will have no effect on the rate of pure ACKs. The worst case here is if the bottleneck is congested solely with pure ACKs, but it could also be problematic if a large fraction of the load was from unresponsive ACKs, leaving little or no capacity for the load from responsive data.

Since RFC 3168 was published, Acknowledgement Congestion Control (AckCC) techniques have been documented in [RFC5690] (informational). So any pair of TCP end-points can choose to agree to regulate the delayed ACK ratio in response to lost or CE-marked pure ACKs. However, the protocol has a number of open deployment issues (e.g. it relies on two new TCP options, one of which is required on the SYN where option space is at a premium and, if either option is blocked by a middlebox, no fall-back behaviour is specified). The new TCP options addressed two problems, namely that TCP had: i) no mechanism to allow ECT to be set on pure ACKs; and ii) no mechanism to feed back loss or CE-marking of pure ACKs. A combination of the present specification and AccECN addresses both these problems, at least for ECN marking. So it might now be possible to design an ECN-specific ACK congestion control scheme without the extra TCP options proposed in RFC 5690. However, such a mechanism is out of scope of the present document.

Setting aside the practicality of RFC 5690, the need for AckCC has not been conclusively demonstrated. It has been argued that the Internet has survived so far with no mechanism to even detect loss of pure ACKs. However, it has also been argued that ECN is not the same as loss. Packet discard can naturally thin the ACK load to whatever the bottleneck can support, whereas ECN marking does not (it queues the ACKs instead). Nonetheless, RFC 3168 (section 7) recommends that an AQM switches over from ECN marking to discard when the marking probability becomes high. Therefore discard can still be relied on to thin out ECN-enabled pure ACKs as a last resort.

5.3.3. Summary: Enabling ECN on Pure ACKs

In the case when AccECN has been negotiated, the arguments for ECT (and CE) on pure ACKs heavily outweigh those against. ECN is always more and never less reliable for delivery of congestion notification. The cwnd response has been overlooked as a mechanism for responding to congestion on pure ACKs, so it is incorrect not to set ECT on pure ACKs when they are interspersed with data segments. And when they are not, packet discard still acts as the "congestion response of last resort". In contrast, not setting ECT on pure ACKs is certainly detrimental to performance, because when a pure ACK is lost it can prevent the release of new data. Separately, AckCC (or perhaps an improved variant exploiting AccECN) could optionally be used to regulate the spacing between pure ACKs. However, it is not clear whether AckCC is justified.

In the case when Classic ECN has been negotiated, there is still an argument for ECT (and CE) on pure ACKs, but it is less clear-cut. Some existing RFC 3168 implementations might happen to (unintentionally) provide the correct feedback to support a cwnd response. Even for those that did not, setting ECT on pure ACKs would still be better for performance than not setting it and do no extra harm. If AckCC was required, it is designed to work with RFC 3168 ECN.

5.4. Window Probes

Section 6.1.6 of RFC 3168 presents only the reliability argument for prohibiting ECT on Window probes:

The reliability argument has already been addressed in Section 5.1.

Allowing ECT on window probes could considerably improve performance because, once the receive window has reopened, if a window probe is lost the sender will stall until the next window probe reaches the receiver, which might be after the maximum retransmission timeout (at least 1 minute [RFC6928]).

On the bright side, RFC 3168 at least specifies the receiver behaviour if a CE-marked window probe arrives, so changing the behaviour ought to be less painful than for other packet types.

5.5. Retransmitted Packets.

RFC 3168 says the sender "MUST NOT" set ECT on retransmitted packets. The rationale for this consumes nearly 2 pages of RFC 3168, so the reader is referred to section 6.1.5 of RFC 3168, rather than quoting it all here. There are essentially three arguments, namely: reliability; DoS attacks; and over-reaction to congestion. We address them in order below.

The reliability argument has already been addressed in Section 5.1.

Protection against DoS attacks is not afforded by prohibiting ECT on retransmitted packets. An attacker can set CE on spoofed retransmissions whether or not it is prohibited by an RFC. Protection against the DoS attack described in section 6.1.5 of RFC 3168 is solely afforded by the requirement that "the TCP data receiver SHOULD ignore the CE codepoint on out-of-window packets". Therefore in Section 3.2.7 the sender is allowed to set ECT on retransmitted packets, in order to reduce the chance of them being dropped. We also strengthen the receiver's requirement from "SHOULD ignore" to "MUST ignore". And we generalize the receiver's requirement to include failure of any validity check, not just out-of-window checks, in order to include the more stringent validity checks in RFC 5961 that have been developed since RFC 3168.

A consequence is that, for those retransmitted packets that arrive at the receiver after the original packet has been properly received (so-called spurious retransmissions), any CE marking will be ignored. There is no problem with that because the fact that the original packet has been delivered implies that the sender's original congestion response (when it deemed the packet lost and retransmitted it) was unnecessary.

Finally, the third argument is about over-reacting to congestion. The argument goes that, if a retransmitted packet is dropped, the sender will not detect it, so it will not react again to congestion (it would have reduced its congestion window already when it retransmitted the packet). Whereas, if retransmitted packets can be CE tagged instead of dropped, senders could potentially react more than once to congestion. However, we argue that it is legitimate to respond again to congestion if it still persists in subsequent round trip(s).

Therefore, in all three cases, it is not incorrect to set ECT on retransmissions.

6. Security Considerations

Section 3.2.6 considers the question of whether ECT on RSTs will allow RST attacks to be intensified. There are several security arguments presented in RFC 3168 for preventing the ECN marking of TCP control packets and retransmitted segments. We believe all of them have been properly addressed in Section 5, particularly Section 5.2.3 and Section 5.5 on DoS attacks using spoofed ECT-marked SYNs and spoofed CE-marked retransmissions.

7. IANA Considerations

There are no IANA considerations in this memo.

8. Acknowledgments

Thanks to Mirja Kühlewind and David Black for their useful reviews.

9. References

9.1. Normative References

[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.
[RFC5562] Kuzmanovic, A., Mondal, A., Floyd, S. and K. Ramakrishnan, "Adding Explicit Congestion Notification (ECN) Capability to TCP's SYN/ACK Packets", RFC 5562, DOI 10.17487/RFC5562, June 2009.
[RFC5961] Ramaiah, A., Stewart, R. and M. Dalal, "Improving TCP's Robustness to Blind In-Window Attacks", RFC 5961, DOI 10.17487/RFC5961, August 2010.
[I-D.ietf-tcpm-accurate-ecn] Briscoe, B., Kuehlewind, M. and R. Scheffenegger, "More Accurate ECN Feedback in TCP", Internet-Draft draft-ietf-tcpm-accurate-ecn-02, October 2016.
[I-D.ietf-tsvwg-ecn-experimentation] Black, D., "Explicit Congestion Notification (ECN) Experimentation", Internet-Draft draft-ietf-tsvwg-ecn-experimentation-01, March 2017.

9.2. Informative References

[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, September 1981.
[RFC1122] Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, DOI 10.17487/RFC1122, October 1989.
[RFC3540] Spring, N., Wetherall, D. and D. Ely, "Robust Explicit Congestion Notification (ECN) Signaling with Nonces", RFC 3540, DOI 10.17487/RFC3540, June 2003.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC 4960, DOI 10.17487/RFC4960, September 2007.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007.
[RFC5681] Allman, M., Paxson, V. and E. Blanton, "TCP Congestion Control", RFC 5681, DOI 10.17487/RFC5681, September 2009.
[RFC5690] Floyd, S., Arcia, A., Ros, D. and J. Iyengar, "Adding Acknowledgement Congestion Control to TCP", RFC 5690, DOI 10.17487/RFC5690, February 2010.
[RFC6298] Paxson, V., Allman, M., Chu, J. and M. Sargent, "Computing TCP's Retransmission Timer", RFC 6298, DOI 10.17487/RFC6298, June 2011.
[RFC6928] Chu, J., Dukkipati, N., Cheng, Y. and M. Mathis, "Increasing TCP's Initial Window", RFC 6928, DOI 10.17487/RFC6928, April 2013.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S. and A. Jain, "TCP Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014.
[RFC7567] Baker, F. and G. Fairhurst, "IETF Recommendations Regarding Active Queue Management", BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015.
[I-D.briscoe-tsvwg-ecn-l4s-id] Schepper, K., Briscoe, B. and I. Tsang, "Identifying Modified Explicit Congestion Notification (ECN) Semantics for Ultra-Low Queuing Delay", Internet-Draft draft-briscoe-tsvwg-ecn-l4s-id-02, October 2016.
[I-D.briscoe-tsvwg-l4s-arch] Briscoe, B., Schepper, K. and M. Bagnulo, "Low Latency, Low Loss, Scalable Throughput (L4S) Internet Service: Architecture", Internet-Draft draft-briscoe-tsvwg-l4s-arch-02, March 2017.
[I-D.stewart-tsvwg-sctpecn] Stewart, R., Tuexen, M. and X. Dong, "ECN for Stream Control Transmission Protocol (SCTP)", Internet-Draft draft-stewart-tsvwg-sctpecn-05, January 2014.
[judd-nsdi] Judd, G., "Attaining the promise and avoiding the pitfalls of TCP in the Datacenter", USENIX Symposium on Networked Systems Design and Implementation (NSDI'15) pp.145-157, May 2015.
[ecn-pam] Trammell, B., Kühlewind, M., Boppart, D., Learmonth, I., Fairhurst, G. and R. Scheffenegger, "Enabling Internet-Wide Deployment of Explicit Congestion Notification", Int'l Conf. on Passive and Active Network Measurement (PAM'15) pp193-205, 2015.

Authors' Addresses

Marcelo Bagnulo Universidad Carlos III de Madrid Av. Universidad 30 Leganes, Madrid 28911 SPAIN Phone: 34 91 6249500 EMail: URI:
Bob Briscoe Simula Research Lab EMail: URI: