TCP Maintenance & Minor Extensions (tcpm) B. Briscoe
Internet-Draft CableLabs
Intended status: Experimental M. Kühlewind
Expires: January 3, 2019 ETH Zurich
R. Scheffenegger
July 2, 2018

More Accurate ECN Feedback in TCP


Explicit Congestion Notification (ECN) is a mechanism where network nodes can mark IP packets instead of dropping them to indicate incipient congestion to the end-points. Receivers with an ECN-capable transport protocol feed back this information to the sender. ECN is specified for TCP in such a way that only one feedback signal can be transmitted per Round-Trip Time (RTT). Recently, new TCP mechanisms like Congestion Exposure (ConEx), Data Center TCP (DCTCP) or Low Latency Low Loss Scalable Throughput (L4S) need more accurate ECN feedback information whenever more than one marking is received in one RTT. This document specifies an experimental scheme to provide more than one feedback signal per RTT in the TCP header. Given TCP header space is scarce, it allocates a reserved header bit, that was previously used for the ECN-Nonce which has now been declared historic. It also overloads the two existing ECN flags in the TCP header. Supplementary feedback information can optionally be provided in a new TCP option, which is never used on the TCP SYN.

Status of This Memo

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Table of Contents

1. Introduction

Explicit Congestion Notification (ECN) [RFC3168] is a mechanism where network nodes can mark IP packets instead of dropping them to indicate incipient congestion to the end-points. Receivers with an ECN-capable transport protocol feed back this information to the sender. ECN is specified for TCP in such a way that only one feedback signal can be transmitted per Round-Trip Time (RTT). Recently, proposed mechanisms like Congestion Exposure (ConEx [RFC7713]), DCTCP [RFC8257] or L4S [I-D.ietf-tsvwg-l4s-arch] need to know when more than one marking is received in one RTT which is information that cannot be provided by the feedback scheme as specified in [RFC3168]. This document specifies an alternative feedback scheme that provides more accurate information and could be used by these new TCP extensions. A fuller treatment of the motivation for this specification is given in the associated requirements document [RFC7560].

This documents specifies an experimental scheme for ECN feedback in the TCP header to provide more than one feedback signal per RTT. It will be called the more accurate ECN feedback scheme, or AccECN for short. If AccECN progresses from experimental to the standards track, it is intended to be a complete replacement for classic TCP/ECN feedback, not a fork in the design of TCP. AccECN feedback complements TCP's loss feedback and it supplements classic TCP/ECN feedback, so its applicability is intended to include all public and private IP networks (and even any non-IP networks over which TCP is used today), whether or not any nodes on the path support ECN of whatever flavour.

Until the AccECN experiment succeeds, [RFC3168] will remain as the only standards track specification for adding ECN to TCP. To avoid confusion, in this document we use the term 'classic ECN' for the pre-existing ECN specification [RFC3168].

AccECN feedback overloads the two existing ECN flags and allocates the currently reserved flag (previously called NS) in the TCP header, to be used as one field indicating the number of congestion experienced marked packets. Given the new definitions of these three bits, both ends have to support the new wire protocol before it can be used. Therefore during the TCP handshake the two ends use these three bits in the TCP header to negotiate the most advanced feedback protocol that they can both support, in a way that is backward compatible with [RFC3168].

AccECN is solely an (experimental) change to the TCP wire protocol; it only specifies the negotiation and signaling of more accurate ECN feedback from a TCP Data Receiver to a Data Sender. It is completely independent of how TCP might respond to congestion feedback, which is out of scope. For that we refer to [RFC3168] or any RFC that specifies a different response to TCP ECN feedback, for example: [RFC8257]; or the ECN experiments referred to in [RFC8311], namely: a TCP-based Low Latency Low Loss Scalable (L4S) congestion control [I-D.ietf-tsvwg-l4s-arch]; ECN-capable TCP control packets [I-D.ietf-tcpm-generalized-ecn], or Alternative Backoff with ECN (ABE) [I-D.ietf-tcpm-alternativebackoff-ecn].

It is likely (but not required) that the AccECN protocol will be implemented along with the following experimental additions to the TCP-ECN protocol: ECN-capable TCP control packets and retransmissions [I-D.ietf-tcpm-generalized-ecn], which includes the ECN-capable SYN/ACK experiment [RFC5562]; and testing receiver non-compliance [I-D.moncaster-tcpm-rcv-cheat].

1.1. Document Roadmap

The following introductory sections outline the goals of AccECN (Section 1.2) and the goal of experiments with ECN (Section 1.3) so that it is clear what success would look like. Then terminology is defined (Section 1.4) and a recap of existing prerequisite technology is given (Section 1.5).

Section 2 gives an informative overview of the AccECN protocol. Then Section 3 gives the normative protocol specification. Section 4 assesses the interaction of AccECN with commonly used variants of TCP, whether standardised or not. Section 5 summarises the features and properties of AccECN.

Section 6 summarises the protocol fields and numbers that IANA will need to assign and Section 7 points to the aspects of the protocol that will be of interest to the security community.

Appendix A gives pseudocode examples for the various algorithms that AccECN uses.

1.2. Goals

[RFC7560] enumerates requirements that a candidate feedback scheme will need to satisfy, under the headings: resilience, timeliness, integrity, accuracy (including ordering and lack of bias), complexity, overhead and compatibility (both backward and forward). It recognises that a perfect scheme that fully satisfies all the requirements is unlikely and trade-offs between requirements are likely. Section 5 presents the properties of AccECN against these requirements and discusses the trade-offs made.

The requirements document recognises that a protocol as ubiquitous as TCP needs to be able to serve as-yet-unspecified requirements. Therefore an AccECN receiver aims to act as a generic (dumb) reflector of congestion information so that in future new sender behaviours can be deployed unilaterally.

1.3. Experiment Goals

TCP is critical to the robust functioning of the Internet, therefore any proposed modifications to TCP need to be thoroughly tested. The present specification describes an experimental protocol that adds more accurate ECN feedback to the TCP protocol. The intention is to specify the protocol sufficiently so that more than one implementation can be built in order to test its function, robustness and interoperability (with itself and with previous version of ECN and TCP).

The experimental protocol will be considered successful if testing confirms that the proposed mechanism can be deployed at large scale. Testing will mostly focus on fall-back strategies in case of middlebox interference. Current recommended strategies are specified in Sections 3.1.2, 3.2.3, 3.2.4 and 3.2.7. The effectiveness of these strategies depends on the actual deployment situation of middleboxes. Therefore experimental verification to confirm large-scale path traversal in the Internet is needed before finalizing this specification on the Standards Track.

Another experimentation focus is the implementation feasibiliy of change-triggered ACKs as described in section 3.2.8. While on average this should not lead to a higher ACK rate, it changes the ACK pattern which can particularly have an impact on hardware offload. It is currently specified as a hard requirement, because the sender can exploit the predictability of the receiver's behaviour. However, further experimentation is needed to advise if will have to become just preferred behavior.

1.4. Terminology

The more accurate ECN feedback scheme will be called AccECN for short.
Classic ECN:
the ECN protocol specified in [RFC3168].
Classic ECN feedback:
the feedback aspect of the ECN protocol specified in [RFC3168], including generation, encoding, transmission and decoding of feedback, but not the Data Sender's subsequent response to that feedback.
A TCP acknowledgement, with or without a data payload.
Pure ACK:
A TCP acknowledgement without a data payload.
TCP client:
The TCP stack that originates a connection.
TCP server:
The TCP stack that responds to a connection request.
Data Receiver:
The endpoint of a TCP half-connection that receives data and sends AccECN feedback.
Data Sender:
The endpoint of a TCP half-connection that sends data and receives AccECN feedback.

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 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

1.5. Recap of Existing ECN feedback in IP/TCP

ECN [RFC3168] uses two bits in the IP header. Once ECN has been negotiated with the receiver at the transport layer, an ECN sender can set two possible codepoints (ECT(0) or ECT(1)) in the IP header to indicate an ECN-capable transport (ECT). If both ECN bits are zero, the packet is considered to have been sent by a Not-ECN-capable Transport (Not-ECT). When a network node experiences congestion, it will occasionally either drop or mark a packet, with the choice depending on the packet's ECN codepoint. If the codepoint is Not-ECT, only drop is appropriate. If the codepoint is ECT(0) or ECT(1), the node can mark the packet by setting both ECN bits, which is termed 'Congestion Experienced' (CE), or loosely a 'congestion mark'. Table 1 summarises these codepoints.

The ECN Field in the IP Header
IP-ECN codepoint (binary) Codepoint name Description
00 Not-ECT Not ECN-Capable Transport
01 ECT(1) ECN-Capable Transport (1)
10 ECT(0) ECN-Capable Transport (0)
11 CE Congestion Experienced

In the TCP header the first two bits in byte 14 are defined as flags for the use of ECN (CWR and ECE in Figure 1 [RFC3168]). A TCP client indicates it supports ECN by setting ECE=CWR=1 in the SYN, and an ECN-enabled server confirms ECN support by setting ECE=1 and CWR=0 in the SYN/ACK. On reception of a CE-marked packet at the IP layer, the Data Receiver starts to set the Echo Congestion Experienced (ECE) flag continuously in the TCP header of ACKs, which ensures the signal is received reliably even if ACKs are lost. The TCP sender confirms that it has received at least one ECE signal by responding with the congestion window reduced (CWR) flag, which allows the TCP receiver to stop repeating the ECN-Echo flag. This always leads to a full RTT of ACKs with ECE set. Thus any additional CE markings arriving within this RTT cannot be fed back.

The last bit in byte 13 of the TCP header was defined as the Nonce Sum (NS) for the ECN Nonce [RFC3540]. In the absence of widespread deployment RFC 3540 has been reclassified as historic [RFC8311] and the respective flag has been marked as "reserved", making this TCP flag available for use by the AccECN experiment instead.

  0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
|               |           | N | C | E | U | A | P | R | S | F |
| Header Length | Reserved  | S | W | C | R | C | S | S | Y | I |
|               |           |   | R | E | G | K | H | T | N | N |

Figure 1: The (post-ECN Nonce) definition of the TCP header flags

2. AccECN Protocol Overview and Rationale

This section provides an informative overview of the AccECN protocol that will be normatively specified in Section 3

Like the original TCP approach, the Data Receiver of each TCP half-connection sends AccECN feedback to the Data Sender on TCP acknowledgements, reusing data packets of the other half-connection whenever possible.

The AccECN protocol has had to be designed in two parts:

The two part design was necessary, given limitations on the space available for TCP options and given the possibility that certain incorrectly designed middleboxes prevent TCP using any new options.

The essential part overloads the previous definition of the three flags in the TCP header that had been assigned for use by ECN. This design choice deliberately replaces the classic ECN feedback protocol, rather than leaving classic ECN feedback intact and adding more accurate feedback separately because:

AccECN is designed to work even if the supplementary part is removed or zeroed out, as long as the essential part gets through.

2.1. Capability Negotiation

AccECN is a change to the wire protocol of the main TCP header, therefore it can only be used if both endpoints have been upgraded to understand it. The TCP client signals support for AccECN on the initial SYN of a connection and the TCP server signals whether it supports AccECN on the SYN/ACK. The TCP flags on the SYN that the client uses to signal AccECN support have been carefully chosen so that a TCP server will interpret them as a request to support the most recent variant of ECN feedback that it supports. Then the client falls back to the same variant of ECN feedback.

An AccECN TCP client does not send the new AccECN Option on the SYN as SYN option space is limited and successful negotiation using the flags in the main header is taken as sufficient evidence that both ends also support the AccECN Option. The TCP server sends the AccECN Option on the SYN/ACK and the client sends it on the first ACK to test whether the network path forwards the option correctly.

2.2. Feedback Mechanism

A Data Receiver maintains four counters initialised at the start of the half-connection. Three count the number of arriving payload bytes marked CE, ECT(1) and ECT(0) respectively. The fourth counts the number of packets arriving marked with a CE codepoint (including control packets without payload if they are CE-marked).

The Data Sender maintains four equivalent counters for the half connection, and the AccECN protocol is designed to ensure they will match the values in the Data Receiver's counters, albeit after a little delay.

Each ACK carries the three least significant bits (LSBs) of the packet-based CE counter using the ECN bits in the TCP header, now renamed the Accurate ECN (ACE) field (see Figure 2 later). The LSBs of each of the three byte counters are carried in the AccECN Option.

2.3. Delayed ACKs and Resilience Against ACK Loss

With both the ACE and the AccECN Option mechanisms, the Data Receiver continually repeats the current LSBs of each of its respective counters. There is no need to acknowledge these continually repeated counters, so the congestion window reduced (CWR) mechanism is no longer used. Even if some ACKs are lost, the Data Sender should be able to infer how much to increment its own counters, even if the protocol field has wrapped.

The 3-bit ACE field can wrap fairly frequently. Therefore, even if it appears to have incremented by one (say), the field might have actually cycled completely then incremented by one. The Data Receiver is required not to delay sending an ACK to such an extent that the ACE field would cycle. However cyling is still a possibility at the Data Sender because a whole sequence of ACKs carrying intervening values of the field might all be lost or delayed in transit.

The fields in the AccECN Option are larger, but they will increment in larger steps because they count bytes not packets. Nonetheless, their size has been chosen such that a whole cycle of the field would never occur between ACKs unless there had been an infeasibly long sequence of ACK losses. Therefore, as long as the AccECN Option is available, it can be treated as a dependable feedback channel.

If the AccECN Option is not available, e.g. it is being stripped by a middlebox, the AccECN protocol will only feed back information on CE markings (using the ACE field). Although not ideal, this will be sufficient, because it is envisaged that neither ECT(0) nor ECT(1) will ever indicate more severe congestion than CE, even though future uses for ECT(0) or ECT(1) are still unclear [RFC8311]. Because the 3-bit ACE field is so small, when it is the only field available the Data Sender has to interpret it conservatively assuming the worst possible wrap.

Certain specified events trigger the Data Receiver to include an AccECN Option on an ACK. The rules are designed to ensure that the order in which different markings arrive at the receiver is communicated to the sender (as long as there is no ACK loss). Implementations are encouraged to send an AccECN Option more frequently, but this is left up to the implementer.

2.4. Feedback Metrics

The CE packet counter in the ACE field and the CE byte counter in the AccECN Option both provide feedback on received CE-marks. The CE packet counter includes control packets that do not have payload data, while the CE byte counter solely includes marked payload bytes. If both are present, the byte counter in the option will provide the more accurate information needed for modern congestion control and policing schemes, such as DCTCP or ConEx. If the option is stripped, a simple algorithm to estimate the number of marked bytes from the ACE field is given in Appendix A.3.

Feedback in bytes is recommended in order to protect against the receiver using attacks similar to 'ACK-Division' to artificially inflate the congestion window, which is why [RFC5681] now recommends that TCP counts acknowledged bytes not packets.

2.5. Generic (Dumb) Reflector

The ACE field provides information about CE markings on both data and control packets. According to [RFC3168] the Data Sender is meant to set control packets to Not-ECT. However, mechanisms in certain private networks (e.g. data centres) set control packets to be ECN capable because they are precisely the packets that performance depends on most.

For this reason, AccECN is designed to be a generic reflector of whatever ECN markings it sees, whether or not they are compliant with a current standard. Then as standards evolve, Data Senders can upgrade unilaterally without any need for receivers to upgrade too. It is also useful to be able to rely on generic reflection behaviour when senders need to test for unexpected interference with markings (for instance [I-D.kuehlewind-tcpm-ecn-fallback] and [I-D.moncaster-tcpm-rcv-cheat]).

The initial SYN is the most critical control packet, so AccECN provides feedback on whether it is CE marked. Although RFC 3168 prohibits an ECN-capable SYN, providing feedback of CE marking on the SYN supports future scenarios in which SYNs might be ECN-enabled (without prejudging whether they ought to be). For instance, [RFC8311] updates this aspect of RFC 3168 to allow experimentation with ECN-capable TCP control packets.

Even if the TCP client (or server) has set the SYN (or SYN/ACK) to not-ECT in compliance with RFC 3168, feedback on the state of the ECN field when it arrives at the receiver could still be useful, because middleboxes have been known to overwrite the ECN IP field as if it is still part of the old Type of Service (ToS) field [Mandalari18]. If a TCP client has set the SYN to Not-ECT, but receives feedback that the ECN field on the SYN arrived with a different codepoint, it can detect such middlebox interference and send Not-ECT for the rest of the connection (see [I-D.kuehlewind-tcpm-ecn-fallback]). Today, if a TCP server receives ECT or CE on a SYN, it cannot know whether it is invalid (or valid) because only the TCP client knows whether it originally marked the SYN as Not-ECT (or ECT). Therefore, prior to AccECN, the server's only safe course of action was to disable ECN for the connection. Instead, the AccECN protocol allows the server to feed back the received ECN field to the client, which then has all the information to decide whether the connection has to fall-back from supporting ECN (or not).

3. AccECN Protocol Specification

3.1. Negotiating to use AccECN

3.1.1. Negotiation during the TCP handshake

Given the ECN Nonce [RFC3540] has been reclassified as historic [RFC8311], the present specification re-allocates the TCP flag at bit 7 of the TCP header, which was previously called NS (Nonce Sum), as the AE (Accurate ECN) flag (see IANA Considerations in Section 6).

During the TCP handshake at the start of a connection, to request more accurate ECN feedback the TCP client (host A) MUST set the TCP flags AE=1, CWR=1 and ECE=1 in the initial SYN segment.

If a TCP server (B) that is AccECN-enabled receives a SYN with the above three flags set, it MUST set both its half connections into AccECN mode. Then it MUST set the TCP flags on the SYN/ACK to one of the 4 values shown in the top block of Table 2 to confirm that it supports AccECN. The TCP server MUST NOT set one of these 4 combination of flags on the SYN/ACK unless the preceding SYN requested support for AccECN as above.

A TCP server in AccECN mode MUST set the AE, CWR and ECE TCP flags on the SYN/ACK to the value in Table 2 that feeds back the IP-ECN field that arrived on the SYN. This applies whether or not the server itself supports setting the IP-ECN field on a SYN or SYN/ACK (see Section 2.5 for rationale).

Once a TCP client (A) has sent the above SYN to declare that it supports AccECN, and once it has received the above SYN/ACK segment that confirms that the TCP server supports AccECN, the TCP client MUST set both its half connections into AccECN mode.

The procedure for the client to follow if a SYN/ACK does not arrive before its retransmission timer expires is given in Section 3.1.2.

The three flags set to 1 to indicate AccECN support on the SYN have been carefully chosen to enable natural fall-back to prior stages in the evolution of ECN. Table 2 tabulates all the negotiation possibilities for ECN-related capabilities that involve at least one AccECN-capable host. The entries in the first two columns have been abbreviated, as follows:

More Accurate ECN Feedback (the present specification)
ECN Nonce feedback [RFC3540]
'Classic' ECN feedback [RFC3168]
Not-ECN-capable. Implicit congestion notification using packet drop.
ECN capability negotiation between Client (A) and Server (B)
A B SYN A->B SYN/ACK B->A Feedback Mode
AccECN AccECN 1   1   1 0   1   0 AccECN (Not-ECT on SYN)
AccECN AccECN 1   1   1 0   1   1 AccECN (ECT1 on SYN)
AccECN AccECN 1   1   1 1   0   0 AccECN (ECT0 on SYN)
AccECN AccECN 1   1   1 1   1   0 AccECN (CE on SYN)
AccECN Nonce 1   1   1 1   0   1 classic ECN
AccECN ECN 1   1   1 0   0   1 classic ECN
AccECN No ECN 1   1   1 0   0   0 Not ECN
Nonce AccECN 0   1   1 0   0   1 classic ECN
ECN AccECN 0   1   1 0   0   1 classic ECN
No ECN AccECN 0   0   0 0   0   0 Not ECN
AccECN Broken 1   1   1 1   1   1 Not ECN

Table 2 is divided into blocks each separated by an empty row.

  1. The top block shows the case already described where both endpoints support AccECN and how the TCP server (B) indicates congestion feedback.
  2. The second block shows the cases where the TCP client (A) supports AccECN but the TCP server (B) supports some earlier variant of TCP feedback, indicated in its SYN/ACK. Therefore, as soon as an AccECN-capable TCP client (A) receives the SYN/ACK shown it MUST set both its half connections into the feedback mode shown in the rightmost column.
  3. The third block shows the cases where the TCP server (B) supports AccECN but the TCP client (A) supports some earlier variant of TCP feedback, indicated in its SYN. Therefore, as soon as an AccECN-enabled TCP server (B) receives the SYN shown, it MUST set both its half connections into the feedback mode shown in the rightmost column.
  4. The fourth block displays a combination labelled `Broken' . Some older TCP server implementations incorrectly set the reserved flags in the SYN/ACK by reflecting those in the SYN. Such broken TCP servers (B) cannot support ECN, so as soon as an AccECN-capable TCP client (A) receives such a broken SYN/ACK it MUST fall-back to Not ECN mode for both its half connections.

The following exceptional cases need some explanation:

ECN Nonce:
With AccECN implementation, there is no need for the ECN Nonce feedback mode [RFC3540], which has also been reclassified as historic [RFC8311], as AccECN is compatible with an alternative ECN feedback integrity approach that does not use up the ECT(1) codepoint and can be implemented solely at the sender (see Section 4.3).
Simultaneous Open:
An originating AccECN Host (A), having sent a SYN with AE=1, CWR=1 and ECE=1, might receive another SYN from host B. Host A MUST then enter the same feedback mode as it would have entered had it been a responding host and received the same SYN. Then host A MUST send the same SYN/ACK as it would have sent had it been a responding host.

3.1.2. Retransmission of the SYN

If the sender of an AccECN SYN times out before receiving the SYN/ACK, the sender SHOULD attempt to negotiate the use of AccECN at least one more time by continuing to set all three TCP ECN flags on the first retransmitted SYN (using the usual retransmission time-outs). If this first retransmission also fails to be acknowledged, the sender SHOULD send subsequent retransmissions of the SYN without any TCP-ECN flags set. This adds delay, in the case where a middlebox drops an AccECN (or ECN) SYN deliberately. However, current measurements imply that a drop is less likely to be due to middlebox interference than other intermittent causes of loss, e.g. congestion, wireless interference, etc.

Implementers MAY use other fall-back strategies if they are found to be more effective (e.g. attempting to negotiate AccECN on the SYN only once or more than twice (most appropriate during high levels of congestion); or falling back to classic ECN feedback rather than non-ECN). Further it may make sense to also remove any other experimental fields or options on the SYN in case a middlebox might be blocking them, although the required behaviour will depend on the specification of the other option(s) and any attempt to co-ordinate fall-back between different modules of the stack. In any case, the TCP initiator SHOULD cache failed connection attempts. If it does, it SHOULD NOT give up attempting to negotiate AccECN on the SYN of subsequent connection attempts until it is clear that the blockage is persistently and specifically due to AccECN. The cache should be arranged to expire so that the initiator will infrequently attempt to check whether the problem has been resolved.

The fall-back procedure if the TCP server receives no ACK to acknowledge a SYN/ACK that tried to negotiate AccECN is specified in Section 3.2.7.

3.2. AccECN Feedback

Each Data Receiver of each half connection maintains four counters, r.cep,, r.e0b and r.e1b. The CE packet counter (r.cep), counts the number of packets the host receives with the CE code point in the IP ECN field, including CE marks on control packets without data., r.e0b and r.e1b count the number of TCP payload bytes in packets marked respectively with the CE, ECT(0) and ECT(1) codepoint in their IP-ECN field. When a host first enters AccECN mode, it initializes its counters to r.cep = 5, r.e0b = 1 and = r.e1b.= 0 (see Appendix A.5). Non-zero initial values are used to support a stateless handshake (see Section 4.1) and to be distinct from cases where the fields are incorrectly zeroed (e.g. by middleboxes - see Section

A host feeds back the CE packet counter using the Accurate ECN (ACE) field, as explained in the next section. And it feeds back all the byte counters using the AccECN TCP Option, as specified in Section 3.2.6. Whenever a host feeds back the value of any counter, it MUST report the most recent value, no matter whether it is in a pure ACK, an ACK with new payload data or a retransmission. Therefore the feedback carried on a retransmitted packet is unlikely to be the same as the feedback on the original packet.

3.2.1. Initialization of Feedback Counters at the Data Sender

Each Data Sender of each half connection maintains four counters, s.cep,, s.e0b and s.e1b intended to track the equivalent counters at the Data Receiver. When a host enters AccECN mode, it initializes them to s.cep = 5, s.e0b = 1 and = s.e1b.= 0.

If a TCP client (A) in AccECN mode receives a SYN/ACK with CE feedback, i.e. AE=1, CWR=1, ECE=0, it increments s.cep to 6. Otherwise, for any of the 3 other combinations of the 3 ECN TCP flags (the top 3 rows in Table 2), s.cep remains initialized to 5.

3.2.2. The ACE Field

After AccECN has been negotiated on the SYN and SYN/ACK, both hosts overload the three TCP flags (AE, CWR and ECE) in the main TCP header as one 3-bit field. Then the field is given a new name, ACE, as shown in Figure 2.

  0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
|               |           |           | U | A | P | R | S | F |
| Header Length | Reserved  |    ACE    | R | C | S | S | Y | I |
|               |           |           | G | K | H | T | N | N |

Figure 2: Definition of the ACE field within bytes 13 and 14 of the TCP Header (when AccECN has been negotiated and SYN=0).

The original definition of these three flags in the TCP header, including the addition of support for the ECN Nonce, is shown for comparison in Figure 1. This specification does not rename these three TCP flags to ACE unconditionally; it merely overloads them with another name and definition once an AccECN connection has been established.

A host MUST interpret the AE, CWR and ECE flags as the 3-bit ACE counter on a segment with the SYN flag cleared (SYN=0) that it sends or receives if both of its half-connections are set into AccECN mode having successfully negotiated AccECN (see Section 3.1). A host MUST NOT interpret the 3 flags as a 3-bit ACE field on any segment with SYN=1 (whether ACK is 0 or 1), or if AccECN negotiation is incomplete or has not succeeded.

Both parts of each of these conditions are equally important. For instance, even if AccECN negotiation has been successful, the ACE field is not defined on any segments with SYN=1 (e.g. a retransmission of an unacknowledged SYN/ACK, or when both ends send SYN/ACKs after AccECN support has been successfully negotiated during a simultaneous open).

With only one exception, on any packet with the SYN flag cleared (SYN=0), the Data Receiver MUST encode the three least significant bits of its r.cep counter into the ACE field it feeds back to the Data Sender.

There is only one exception to this rule: On the final ACK of the 3WHS, a TCP client (A) in AccECN mode MUST use the ACE field to feed back which of the 4 possible values of the IP-ECN field were on the SYN/ACK (the binary encoding is the same as that used on the SYN/ACK). Table 3 shows the meaning of each possible value of the ACE field on the ACK of the SYN/ACK and the value that an AccECN server MUST set s.cep to as a result. The encoding in Table 3 is solely applicable on a packet in the client-server direction with an acknowledgement number 1 greater than the Initial Sequence Number (ISN) that was used by the server.

Meaning of the ACE field on the ACK of the SYN/ACK
ACE on ACK of SYN/ACK IP-ECN codepoint on SYN/ACK inferred by server Initial s.cep of server in AccECN mode
0b000 {Notes 1, 2} Disable ECN
0b001 {Notes 2, 3} 5
0b010 Not-ECT 5
0b011 ECT(1) 5
0b100 ECT(0) 5
0b101 Currently Unused {Note 3} 5
0b110 CE 6
0b111 Currently Unused {Note 3} 5

{Note 1}: If the server is in AccECN mode, the value of zero raises suspicion of zeroing of the ACE field on the path (see Section 3.2.3).

{Note 2}: If a server is in AccECN mode, there ought to be no valid case where the ACE field on the last ACK of the 3WHS has a value of 0b000 or 0b001.

However, in the case where a server that implements AccECN is also using a stateless handshake (termed a SYN cookie) it will not remember whether it entered AccECN mode. Then these two values remind it that it did not enter AccECN mode (see Section 4.1 for details).

{Note 3}: If the server is in AccECN mode, these values are Currently Unused but the AccECN server's behaviour is still defined for forward compatibility.

3.2.3. Testing for Zeroing of the ACE Field

Section 3.2.2 required the Data Receiver to initialize the r.cep counter to a non-zero value. Therefore, in either direction the initial value of the ACE field ought to be non-zero.

If AccECN has been successfully negotiated, the Data Sender SHOULD check the initial value of the ACE field in the first arriving segment with SYN=0. If the initial value of the ACE field is zero (0b000), the Data Sender MUST disable sending ECN-capable packets for the remainder of the half-connection by setting the IP/ECN field in all subsequent packets to Not-ECT.

For example, the server checks the ACK of the SYN/ACK or the first data segment from the client, while the client checks the first data segment from the server. More precisely, the "first segment with SYN=0" is defined as: the segment with SYN=0 that i) acknowledges sequence space at least covering the initial sequence number (ISN) plus 1; and ii) arrives before any other segments with SYN=0 so it is unlikely to be a retransmission. If no such segment arrives (e.g. because it is lost and the ISN is first acknowledged by a subsequent segment), no test for invalid initialization can be conducted, and the half-connection will continue in AccECN mode.

Note that the Data Sender MUST NOT test whether the arriving counter in the initial ACE field has been initialized to a specific valid value - the above check solely tests whether the ACE fields have been incorrectly zeroed. This allows hosts to use different initial values as an additional signalling channel in future.

3.2.4. Testing for Mangling of the IP/ECN Field

The value of the ACE field on the SYN/ACK indicates the value of the IP/ECN field when the SYN arrived at the server. The client can compare this with how it originally set the IP/ECN field on the SYN. If this comparison implies an unsafe transition of the IP/ECN field, for the remainder of the connection the client MUST NOT send ECN-capable packets, but it MUST continue to feed back any ECN markings on arriving packets.

The value of the ACE field on the last ACK of the 3WHS indicates the value of the IP/ECN field when the SYN/ACK arrived at the client. The server can compare this with how it originally set the IP/ECN field on the SYN/ACK. If this comparison implies an unsafe transition of the IP/ECN field, for the remainder of the connection the server MUST NOT send ECN-capable packets, but it MUST continue to feedback any ECN markings on arriving packets.

The ACK of the SYN/ACK is not reliably delivered (nonetheless, the count of CE marks is still eventually delivered reliably). If this ACK does not arrive, the server has to continue to send ECN-capable packets without having tested for mangling of the IP/ECN field on the SYN/ACK. Experiments with AccECN deployment will assess whether this limitation has any effect in practice.

Invalid transitions of the IP/ECN field are defined in [RFC3168] and repeated here for convenience:[Mandalari18].

RFC 3168 says that a router that changes ECT to not-ECT is invalid but safe. However, from a host's viewpoint, this transition is unsafe because it could be the result of two transitions at different routers on the path: ECT to CE (safe) then CE to not-ECT (unsafe). This scenario could well happen where an ECN-enabled home router congests its upstream mobile broadband bottleneck link, then the ingress to the mobile network clears the ECN field

The above fall-back behaviours are necessary in case mangling of the IP/ECN field is asymmetric, which is currently common over some mobile networks [Mandalari18]. Then one end might see no unsafe transition and continue sending ECN-capable packets, while the other end sees an unsafe transition and stops sending ECN-capable packets.

3.2.5. Safety against Ambiguity of the ACE Field

If too many CE-marked segments are acknowledged at once, or if a long run of ACKs is lost, the 3-bit counter in the ACE field might have cycled between two ACKs arriving at the Data Sender.

Therefore an AccECN Data Receiver SHOULD immediately send an ACK once 'n' CE marks have arrived since the previous ACK, where 'n' SHOULD be 2 and MUST be no greater than 6.

If the Data Sender has not received AccECN TCP Options to give it more dependable information, and it detects that the ACE field could have cycled under the prevailing conditions, it SHOULD conservatively assume that the counter did cycle. It can detect if the counter could have cycled by using the jump in the acknowledgement number since the last ACK to calculate or estimate how many segments could have been acknowledged. An example algorithm to implement this policy is given in Appendix A.2. An implementer MAY develop an alternative algorithm as long as it satisfies these requirements.

If missing acknowledgement numbers arrive later (reordering) and prove that the counter did not cycle, the Data Sender MAY attempt to neutralise the effect of any action it took based on a conservative assumption that it later found to be incorrect.

3.2.6. The AccECN Option

The AccECN Option is defined as shown below in Figure 3. It consists of three 24-bit fields that provide the 24 least significant bits of the r.e0b, and r.e1b counters, respectively. The initial 'E' of each field name stands for 'Echo'.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|  Kind = TBD1  |  Length = 11  |          EE0B field           |
| EE0B (cont'd) |           ECEB field                          |
|                  EE1B field                   |

Figure 3: The AccECN Option

The Data Receiver MUST set the Kind field to TBD1, which is registered in Section 6 as a new TCP option Kind called AccECN. An experimental TCP option with Kind=254 MAY be used for initial experiments, with magic number 0xACCE.

Appendix A.1 gives an example algorithm for the Data Receiver to encode its byte counters into the AccECN Option, and for the Data Sender to decode the AccECN Option fields into its byte counters.

Note that there is no field to feedback Not-ECT bytes. Nonetheless an algorithm for the Data Sender to calculate the number of payload bytes received as Not-ECT is given in Appendix A.5.

Whenever a Data Receiver sends an AccECN Option, the rules in Section 3.2.8 expect it to always send a full-length option. To cope with option space limitations, it can omit unchanged fields from the tail of the option, as long as it preserves the order of the remaining fields and includes any field that has changed. The length field MUST indicate which fields are present as follows:


The empty option of Length=2 is provided to allow for a case where an AccECN Option has to be sent (e.g. on the SYN/ACK to test the path), but there is very limited space for the option. For initial experiments, the Length field MUST be 2 greater to accommodate the 16-bit magic number.

All implementations of a Data Sender MUST be able to read in AccECN Options of any of the above lengths. If the AccECN Option is of any other length, implementations MUST use those whole 3 octet fields that fit within the length and ignore the remainder of the option.

The use of the AccECN option is optional for the Data Receiver. If the Data Receiver intents to use the AccECN option at any time during the rest of the connection it strongly recommended to also test its path traversal by including it in the SYN/ACK as specified in the next section. By default the use of the AccECN option is RECOMMENDED.

3.2.7. Path Traversal of the AccECN Option Testing the AccECN Option during the Handshake

The TCP client MUST NOT include the AccECN TCP Option on the SYN. Nonetheless, if the AccECN negotiation using the ECN flags in the main TCP header (Section 3.1) is successful, it implicitly declares that the endpoints also support the AccECN TCP Option. A fall-back strategy for the loss of the SYN (possibly due to middlebox interference) is specified in Section 3.1.2.

A TCP server that confirms its support for AccECN (in response to an AccECN SYN from the client as described in Section 3.1) SHOULD include an AccECN TCP Option in the SYN/ACK.

A TCP client that has successfully negotiated AccECN SHOULD include an AccECN Option in the first ACK at the end of the 3WHS. However, this first ACK is not delivered reliably, so the TCP client SHOULD also include an AccECN Option on the first data segment it sends (if it ever sends one).

A host MAY NOT include an AccECN Option in any of these three cases if it has cached knowledge that the packet would be likely to be blocked on the path to the other host if it included an AccECN Option. Testing for Loss of Packets Carrying the AccECN Option

If after the normal TCP timeout the TCP server has not received an ACK to acknowledge its SYN/ACK, the SYN/ACK might just have been lost, e.g. due to congestion, or a middlebox might be blocking the AccECN Option. To expedite connection setup, the TCP server SHOULD retransmit the SYN/ACK with the same TCP flags (AE, CWR and ECE) but with no AccECN Option. If this retransmission times out, to expedite connection setup, the TCP server SHOULD disable AccECN and ECN for this connection by retransmitting the SYN/ACK with AE=CWR=ECE=0 and no AccECN Option. Implementers MAY use other fall-back strategies if they are found to be more effective (e.g. falling back to classic ECN feedback on the first retransmission; retrying the AccECN Option for a second time before fall-back (most appropriate during high levels of congestion); or falling back to classic ECN feedback rather than non-ECN on the third retransmission).

If the TCP client detects that the first data segment it sent with the AccECN Option was lost, it SHOULD fall back to no AccECN Option on the retransmission. Again, implementers MAY use other fall-back strategies such as attempting to retransmit a second segment with the AccECN Option before fall-back, and/or caching whether the AccECN Option is blocked for subsequent connections.

Either host MAY include the AccECN Option in a subsequent segment to retest whether the AccECN Option can traverse the path.

If the TCP server receives a second SYN with a request for AccECN support, it should resend the SYN/ACK, again confirming its support for AccECN, but this time without the AccECN Option. This approach rules out any interference by middleboxes that may drop packets with unknown options, even though it is more likely that the SYN/ACK would have been lost due to congestion. The TCP server MAY try to send another packet with the AccECN Option at a later point during the connection but should monitor if that packet got lost as well, in which case it SHOULD disable the sending of the AccECN Option for this half-connection.

Similarly, an AccECN end-point MAY separately memorize which data packets carried an AccECN Option and disable the sending of AccECN Options if the loss probability of those packets is significantly higher than that of all other data packets in the same connection. Testing for Stripping of the AccECN Option

If the TCP client has successfully negotiated AccECN but does not receive an AccECN Option on the SYN/ACK, it switches into a mode that assumes that the AccECN Option is not available for this half connection.

Similarly, if the TCP server has successfully negotiated AccECN but does not receive an AccECN Option on the first segment that acknowledges sequence space at least covering the ISN, it switches into a mode that assumes that the AccECN Option is not available for this half connection.

While a host is in this mode that assumes incoming AccECN Options are not available, it MUST adopt the conservative interpretation of the ACE field discussed in Section 3.2.5. However, it cannot make any assumption about support of outgoing AccECN Options on the other half connection, so it SHOULD continue to send the AccECN Option itself (unless it has established that sending the AccECN Option is causing packets to be blocked as in Section

If a host is in the mode that assumes incoming AccECN Options are not available, but it receives an AccECN Option at any later point during the connection, this clearly indicates that the AccECN Option is not blocked on the respective path, and the AccECN endpoint MAY switch out of the mode that assumes the AccECN Option is not available for this half connection. Test for Zeroing of the AccECN Option

For a related test for invalid initialization of the ACE field, see Section 3.2.3

Section 3.2 required the Data Receiver to initialize the r.e0b counter to a non-zero value. Therefore, in either direction the initial value of the EE0B field in the AccECN Option (if one exists) ought to be non-zero. If AccECN has been negotiated:

While a host is in the mode that ignores the AccECN Option it MUST adopt the conservative interpretation of the ACE field discussed in Section 3.2.5.

Note that the Data Sender MUST NOT test whether the arriving byte counters in the initial AccECN Option have been initialized to specific valid values - the above checks solely test whether these fields have been incorrectly zeroed. This allows hosts to use different initial values as an additional signalling channel in future. Also note that the initial value of either field might be greater than its expected initial value, because the counters might already have been incremented. Nonetheless, the initial values of the counters have been chosen so that they cannot wrap to zero on these initial segments. Consistency between AccECN Feedback Fields

When the AccECN Option is available it supplements but does not replace the ACE field. An endpoint using AccECN feedback MUST always consider the information provided in the ACE field whether or not the AccECN Option is also available.

If the AccECN option is present, the s.cep counter might increase while the counter does not (e.g. due to a CE-marked control packet). The sender's response to such a situation is out of scope, and needs to be dealt with in a specification that uses ECN-capable control packets. Theoretically, this situation could also occur if a middlebox mangled the AccECN Option but not the ACE field. However, the Data Sender has to assume that the integrity of the AccECN Option is sound, based on the above test of the well-known initial values and optionally other integrity tests (Section 4.3).

If either end-point detects that the counter has increased but the s.cep has not (and by testing ACK coverage it is certain how much the ACE field has wrapped), this invalid protocol transition has to be due to some form of feedback mangling. So, the Data Sender MUST disable sending ECN-capable packets for the remainder of the half-connection by setting the IP/ECN field in all subsequent packets to Not-ECT.

3.2.8. Usage of the AccECN TCP Option

The following rules determine when a Data Receiver in AccECN mode sends the AccECN TCP Option, and which fields to include:

Change-Triggered ACKs:
If an arriving packet increments a different byte counter to that incremented by the previous packet, the Data Receiver MUST immediately send an ACK with an AccECN Option, without waiting for the next delayed ACK (this is in addition to the safety recommendation in Section 3.2.5 against ambiguity of the ACE field).

This is stated as a "MUST" so that the data sender can rely on change-triggered ACKs to detect transitions right from the very start of a flow, without first having to detect whether the receiver complies. A concern has been raised that certain offload hardware needed for high performance might not be able to support change-triggered ACKs, although high performance protocols such as DCTCP successfully use change-triggered ACKs. One possible experimental compromise would be for the receiver to heuristically detect whether the sender is in slow-start, then to implement change-triggered ACKs in software while the sender is in slow-start, and offload to hardware otherwise. If the operator disables change-triggered ACKs, whether partially like this or otherwise, the operator will also be responsible for ensuring a co-ordinated sender algorithm is deployed;
Continual Repetition:
Otherwise, if arriving packets continue to increment the same byte counter, the Data Receiver can include an AccECN Option on most or all (delayed) ACKs, but it does not have to. If option space is limited on a particular ACK, the Data Receiver MUST give precedence to SACK information about loss. It SHOULD include an AccECN Option if the counter has incremented and it MAY include an AccECN Option if r.ec0b or r.ec1b has incremented;
Full-Length Options Preferred:
It SHOULD always use full-length AccECN Options. It MAY use shorter AccECN Options if space is limited, but it MUST include the counter(s) that have incremented since the previous AccECN Option and it MUST only truncate fields from the right-hand tail of the option to preserve the order of the remaining fields (see Section 3.2.6);
Beaconing Full-Length Options:
Nonetheless, it MUST include a full-length AccECN TCP Option on at least three ACKs per RTT, or on all ACKs if there are less than three per RTT (see Appendix A.4 for an example algorithm that satisfies this requirement).

The following example series of arriving IP/ECN fields illustrates when a Data Receiver will emit an ACK if it is using a delayed ACK factor of 2 segments and change-triggered ACKs: 01 -> ACK, 01, 01 -> ACK, 10 -> ACK, 10, 01 -> ACK, 01, 11 -> ACK, 01 -> ACK.

For the avoidance of doubt, the change-triggered ACK mechanism is deliberately worded to ignore the arrival of a control packet with no payload, which therefore does not alter any byte counters, because it is important that TCP does not acknowledge pure ACKs. The change-triggered ACK approach can lead to some additional ACKs but it feeds back the timing and the order in which ECN marks are received with minimal additional complexity. If only CE marks are infrequent, or there are multiple marks in a row, the additional load will be low. Other marking patterns could increase the load significantly, Investigating the additional load is a goal of the proposed experiment.

Implementation note: sending an AccECN Option each time a different counter changes and including a full-length AccECN Option on every delayed ACK will satisfy the requirements described above and might be the easiest implementation, as long as sufficient space is available in each ACK (in total and in the option space).

Appendix A.3 gives an example algorithm to estimate the number of marked bytes from the ACE field alone, if the AccECN Option is not available.

If a host has determined that segments with the AccECN Option always seem to be discarded somewhere along the path, it is no longer obliged to follow the above rules.

3.3. Requirements for TCP Proxies, Offload Engines and other Middleboxes on AccECN Compliance

A large class of middleboxes split TCP connections. Such a middlebox would be compliant with the AccECN protocol if the TCP implementation on each side complied with the present AccECN specification and each side negotiated AccECN independently of the other side.

Another large class of middleboxes intervenes to some degree at the transport layer, but attempts to be transparent (invisible) to the end-to-end connection. A subset of this class of middleboxes attempts to `normalise' the TCP wire protocol by checking that all values in header fields comply with a rather narrow interpretation of the TCP specifications. To comply with the present AccECN specification, such a middlebox MUST NOT change the ACE field or the AccECN Option and it SHOULD preserve the timing of each ACK (for example, if it coalesced ACKs it would not be AccECN-compliant) as these can be used by the Data Sender to infer further information about the path congestion level. A middlebox claiming to be transparent at the transport layer MUST forward the AccECN TCP Option unaltered, whether or not the length value matches one of those specified in Section 3.2.6, and whether or not the initial values of the byte-counter fields are correct. This is because blocking apparently invalid values does not improve security (because AccECN hosts are required to ignore invalid values anyway), while it prevents the standardised set of values being extended in future (because outdated normalisers would block updated hosts from using the extended AccECN standard).

Hardware to offload certain TCP processing represents another large class of middleboxes, even though it is often a function of a host's network interface and rarely in its own 'box'. Leeway has been allowed in the present AccECN specification in the expectation that offload hardware could comply and still serve its function. Nonetheless, such hardware SHOULD also preserve the timing of each ACK (for example, if it coalesced ACKs it would not be AccECN-compliant).

4. Interaction with Other TCP Variants

This section is informative, not normative.

4.1. Compatibility with SYN Cookies

A TCP server can use SYN Cookies (see Appendix A of [RFC4987]) to protect itself from SYN flooding attacks. It places minimal commonly used connection state in the SYN/ACK, and deliberately does not hold any state while waiting for the subsequent ACK (e.g. it closes the thread). Therefore it cannot record the fact that it entered AccECN mode for both half-connections. Indeed, it cannot even remember whether it negotiated the use of classic ECN [RFC3168].

Nonetheless, such a server can determine that it negotiated AccECN as follows. If a TCP server using SYN Cookies supports AccECN and if it receives a pure ACK that acknowledges an ISN that is a valid SYN cookie, and if the ACK contains an ACE field with the value 0b010 to 0b111 (decimal 2 to 7), it can assume that:

Therefore the server can switch itself into AccECN mode, and continue as if it had never forgotten that it switched itself into AccECN mode earlier.

If the pure ACK that acknowledges a SYN cookie contains an ACE field with the value 0b000 or 0b001, these values indicate that the client did not request support for AccECN and therefore the server does not enter AccECN mode for this connection. Further, 0b001 on the ACK implies that the server sent an ECN-capable SYN/ACK, which was marked CE in the network, and the non-AccECN client fed this back by setting ECE on the ACK of the SYN/ACK.

4.2. Compatibility with Other TCP Options and Experiments

AccECN is compatible (at least on paper) with the most commonly used TCP options: MSS, time-stamp, window scaling, SACK and TCP-AO. It is also compatible with the recent promising experimental TCP options TCP Fast Open (TFO [RFC7413]) and Multipath TCP (MPTCP [RFC6824]). AccECN is friendly to all these protocols, because space for TCP options is particularly scarce on the SYN, where AccECN consumes zero additional header space.

When option space is under pressure from other options, Section 3.2.8 provides guidance on how important it is to send an AccECN Option and whether it needs to be a full-length option.

4.3. Compatibility with Feedback Integrity Mechanisms

Three alternative mechanisms are available to assure the integrity of ECN and/or loss signals. AccECN is compatible with any of these approaches:[RFC3540] was proposed to ensure integrity of congestion feedback. With minor changes AccECN could be optimised for the possibility that the ECT(1) codepoint might be used as an ECN Nonce. However, given RFC 3540 has been reclassified as historic, the AccECN design has been generalised so that it ought to be able to support other possible uses of the ECT(1) codepoint, such as a lower severity or a more instant congestion signal than CE.

Originally the ECN Nonce

5. Protocol Properties

This section is informative not normative. It describes how well the protocol satisfies the agreed requirements for a more accurate ECN feedback protocol [RFC7560].

From each ACK, the Data Sender can infer the number of new CE marked segments since the previous ACK. This provides better accuracy on CE feedback than classic ECN. In addition if the AccECN Option is present (not blocked by the network path) the number of bytes marked with CE, ECT(1) and ECT(0) are provided.
The AccECN scheme is divided into two parts. The essential part reuses the 3 flags already assigned to ECN in the IP header. The supplementary part adds an additional TCP option consuming up to 11 bytes. However, no TCP option is consumed in the SYN.
The order in which marks arrive at the Data Receiver is preserved in AccECN feedback, because the Data Receiver is expected to send an ACK immediately whenever a different mark arrives.
While the same ECN markings are arriving continually at the Data Receiver, it can defer ACKs as TCP does normally, but it will immediately send an ACK as soon as a different ECN marking arrives.
Timeliness vs Overhead:
Change-Triggered ACKs are intended to enable latency-sensitive uses of ECN feedback by capturing the timing of transitions but not wasting resources while the state of the signalling system is stable. The receiver can control how frequently it sends the AccECN TCP Option and therefore it can control the overhead induced by AccECN.
All information is provided based on counters. Therefore if ACKs are lost, the counters on the first ACK following the losses allows the Data Sender to immediately recover the number of the ECN markings that it missed.
Resilience against Bias:
Because feedback is based on repetition of counters, random losses do not remove any information, they only delay it. Therefore, even though some ACKs are change-triggered, random losses will not alter the proportions of the different ECN markings in the feedback.
Resilience vs Overhead:
If space is limited in some segments (e.g. because more option are need on some segments, such as the SACK option after loss), the Data Receiver can send AccECN Options less frequently or truncate fields that have not changed, usually down to as little as 5 bytes. However, it has to send a full-sized AccECN Option at least three times per RTT, which the Data Sender can rely on as a regular beacon or checkpoint.
Resilience vs Timeliness and Ordering:
Ordering information and the timing of transitions cannot be communicated in three cases: i) during ACK loss; ii) if something on the path strips the AccECN Option; or iii) if the Data Receiver is unable to support Change-Triggered ACKs.
An AccECN implementation solely involves simple counter increments, some modulo arithmetic to communicate the least significant bits and allow for wrap, and some heuristics for safety against fields cycling due to prolonged periods of ACK loss. Each host needs to maintain eight additional counters. The hosts have to apply some additional tests to detect tampering by middleboxes, but in general the protocol is simple to understand, simple to implement and requires few cycles per packet to execute.
AccECN is compatible with at least three approaches that can assure the integrity of ECN feedback. If the AccECN Option is stripped the resolution of the feedback is degraded, but the integrity of this degraded feedback can still be assured.
Backward Compatibility:
If only one endpoint supports the AccECN scheme, it will fall-back to the most advanced ECN feedback scheme supported by the other end.
Backward Compatibility:
If the AccECN Option is stripped by a middlebox, AccECN still provides basic congestion feedback in the ACE field. Further, AccECN can be used to detect mangling of the IP ECN field; mangling of the TCP ECN flags; blocking of ECT-marked segments; and blocking of segments carrying the AccECN Option. It can detect these conditions during TCP's 3WHS so that it can fall back to operation without ECN and/or operation without the AccECN Option.
Forward Compatibility:
The behaviour of endpoints and middleboxes is carefully defined for all reserved or currently unused codepoints in the scheme, to ensure that any blocking of anomalous values is always at least under reversible policy control.

6. IANA Considerations

This document reassigns bit 7 of the TCP header flags to the AccECN experiment. This bit was previously called the Nonce Sum (NS) flag [RFC3540], but RFC 3540 is being reclassified as historic [RFC8311]. The flag will now be defined as:

Bit Name Reference
7 AE (Accurate ECN) RFC XXXX

[TO BE REMOVED: IANA is requested to update the existing entry in the Transmission Control Protocol (TCP) Header Flags registration ( for Bit 7 to "AE (Accurate ECN), previously used as NS (Nonce Sum) by [RFC3540], which is now Historic [RFC8311]" and change the reference to this RFC-to-be instead of RFC8311.]

This document also defines a new TCP option for AccECN, assigned a value of TBD1 (decimal) from the TCP option space. This value is defined as:

Kind Length Meaning Reference

[TO BE REMOVED: This registration should take place at the following location: ]

Early implementation before the IANA allocation MUST follow [RFC6994] and use experimental option 254 and magic number 0xACCE (16 bits), then migrate to the new option after the allocation.

7. Security Considerations

If ever the supplementary part of AccECN based on the new AccECN TCP Option is unusable (due for example to middlebox interference) the essential part of AccECN's congestion feedback offers only limited resilience to long runs of ACK loss (see Section 3.2.5). These problems are unlikely to be due to malicious intervention (because if an attacker could strip a TCP option or discard a long run of ACKs it could wreak other arbitrary havoc). However, it would be of concern if AccECN's resilience could be indirectly compromised during a flooding attack. AccECN is still considered safe though, because if the option is not presented, the AccECN Data Sender is then required to switch to more conservative assumptions about wrap of congestion indication counters (see Section 3.2.5 and Appendix A.2).

Section 4.1 describes how a TCP server can negotiate AccECN and use the SYN cookie method for mitigating SYN flooding attacks.

There is concern that ECN markings could be altered or suppressed, particularly because a misbehaving Data Receiver could increase its own throughput at the expense of others. AccECN is compatible with the three schemes known to assure the integrity of ECN feedback (see Section 4.3 for details). If the AccECN Option is stripped by an incorrectly implemented middlebox, the resolution of the feedback will be degraded, but the integrity of this degraded information can still be assured.

There is a potential concern that a receiver could deliberately omit the AccECN Option pretending that it had been stripped by a middlebox. No known way can yet be contrived to take advantage of this downgrade attack, but it is mentioned here in case someone else can contrive one.

The AccECN protocol is not believed to introduce any new privacy concerns, because it merely counts and feeds back signals at the transport layer that had already been visible at the IP layer.

8. Acknowledgements

We want to thank Koen De Schepper, Praveen Balasubramanian, Michael Welzl, Gorry Fairhurst, David Black, Spencer Dawkins, Michael Scharf and Michael Tuexen for their input and discussion. The idea of using the three ECN-related TCP flags as one field for more accurate TCP-ECN feedback was first introduced in the re-ECN protocol that was the ancestor of ConEx.

Bob Briscoe was part-funded by the European Community under its Seventh Framework Programme through the Reducing Internet Transport Latency (RITE) project (ICT-317700) and through the Trilogy 2 project (ICT-317756). He was also part-funded by the Research Council of Norway through the TimeIn project. The views expressed here are solely those of the authors.

Mirja Kuehlewind was partly supported by the European Commission under Horizon 2020 grant agreement no. 688421 Measurement and Architecture for a Middleboxed Internet (MAMI), and by the Swiss State Secretariat for Education, Research, and Innovation under contract no. 15.0268. This support does not imply endorsement.

9. Comments Solicited

Comments and questions are encouraged and very welcome. They can be addressed to the IETF TCP maintenance and minor modifications working group mailing list <>, and/or to the authors.

10. References

10.1. 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.
[RFC5681] Allman, M., Paxson, V. and E. Blanton, "TCP Congestion Control", RFC 5681, DOI 10.17487/RFC5681, September 2009.
[RFC6994] Touch, J., "Shared Use of Experimental TCP Options", RFC 6994, DOI 10.17487/RFC6994, August 2013.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.

10.2. Informative References

[I-D.ietf-tcpm-alternativebackoff-ecn] Khademi, N., Welzl, M., Armitage, G. and G. Fairhurst, "TCP Alternative Backoff with ECN (ABE)", Internet-Draft draft-ietf-tcpm-alternativebackoff-ecn-07, March 2018.
[I-D.ietf-tcpm-generalized-ecn] Bagnulo, M. and B. Briscoe, "ECN++: Adding Explicit Congestion Notification (ECN) to TCP Control Packets", Internet-Draft draft-ietf-tcpm-generalized-ecn-02, October 2017.
[I-D.ietf-tsvwg-l4s-arch] Briscoe, B., Schepper, K. and M. Bagnulo, "Low Latency, Low Loss, Scalable Throughput (L4S) Internet Service: Architecture", Internet-Draft draft-ietf-tsvwg-l4s-arch-02, March 2018.
[I-D.kuehlewind-tcpm-ecn-fallback] Kuehlewind, M. and B. Trammell, "A Mechanism for ECN Path Probing and Fallback", Internet-Draft draft-kuehlewind-tcpm-ecn-fallback-01, September 2013.
[I-D.moncaster-tcpm-rcv-cheat] Moncaster, T., Briscoe, B. and A. Jacquet, "A TCP Test to Allow Senders to Identify Receiver Non-Compliance", Internet-Draft draft-moncaster-tcpm-rcv-cheat-03, July 2014.
[Mandalari18] Mandalari, A., Lutu, A., Briscoe, B., Bagnulo, M. and Ö. Alay, "Measuring ECN++: Good News for ++, Bad News for ECN over Mobile", IEEE Communications Magazine , March 2018.

(to appear)

[RFC3540] Spring, N., Wetherall, D. and D. Ely, "Robust Explicit Congestion Notification (ECN) Signaling with Nonces", RFC 3540, DOI 10.17487/RFC3540, June 2003.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007.
[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.
[RFC5925] Touch, J., Mankin, A. and R. Bonica, "The TCP Authentication Option", RFC 5925, DOI 10.17487/RFC5925, June 2010.
[RFC6824] Ford, A., Raiciu, C., Handley, M. and O. Bonaventure, "TCP Extensions for Multipath Operation with Multiple Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S. and A. Jain, "TCP Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014.
[RFC7560] Kuehlewind, M., Scheffenegger, R. and B. Briscoe, "Problem Statement and Requirements for Increased Accuracy in Explicit Congestion Notification (ECN) Feedback", RFC 7560, DOI 10.17487/RFC7560, August 2015.
[RFC7713] Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx) Concepts, Abstract Mechanism, and Requirements", RFC 7713, DOI 10.17487/RFC7713, December 2015.
[RFC8257] Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L. and G. Judd, "Data Center TCP (DCTCP): TCP Congestion Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257, October 2017.
[RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion Notification (ECN) Experimentation", RFC 8311, DOI 10.17487/RFC8311, January 2018.

Appendix A. Example Algorithms

This appendix is informative, not normative. It gives example algorithms that would satisfy the normative requirements of the AccECN protocol. However, implementers are free to choose other ways to implement the requirements.

A.1. Example Algorithm to Encode/Decode the AccECN Option

The example algorithms below show how a Data Receiver in AccECN mode could encode its CE byte counter into the ECEB field within the AccECN TCP Option, and how a Data Sender in AccECN mode could decode the ECEB field into its byte counter The other counters for bytes marked ECT(0) and ECT(1) in the AccECN Option would be similarly encoded and decoded.

It is assumed that each local byte counter is an unsigned integer greater than 24b (probably 32b), and that the following constant has been assigned:

Every time a CE marked data segment arrives, the Data Receiver increments its local value of by the size of the TCP Data. Whenever it sends an ACK with the AccECN Option, the value it writes into the ECEB field is

where '%' is the modulo operator.

On the arrival of an AccECN Option, the Data Sender uses the TCP acknowledgement number and any SACK options to calculate newlyAckedB, the amount of new data that the ACK acknowledges in bytes. If newlyAckedB is negative it means that a more up to date ACK has already been processed, so this ACK has been superseded and the Data Sender has to ignore the AccECN Option. Then the Data Sender calculates the minimum difference between the ECEB field and its local counter, using modulo arithmetic as follows:

   if (newlyAckedB >= 0) { = (ECEB + DIVOPT - ( % DIVOPT)) % DIVOPT +=

For example, if is 33,554,433 and ECEB is 1461 (both decimal), then % DIVOPT = 1 = (1461 + 2^24 - 1) % 2^24
               = 1460 = 33,554,433 + 1460
               = 33,555,893

A.2. Example Algorithm for Safety Against Long Sequences of ACK Loss

The example algorithms below show how a Data Receiver in AccECN mode could encode its CE packet counter r.cep into the ACE field, and how the Data Sender in AccECN mode could decode the ACE field into its s.cep counter. The Data Sender's algorithm includes code to heuristically detect a long enough unbroken string of ACK losses that could have concealed a cycle of the congestion counter in the ACE field of the next ACK to arrive.

Two variants of the algorithm are given: i) a more conservative variant for a Data Sender to use if it detects that the AccECN Option is not available (see Section 3.2.5 and Section 3.2.7); and ii) a less conservative variant that is feasible when complementary information is available from the AccECN Option.

A.2.1. Safety Algorithm without the AccECN Option

It is assumed that each local packet counter is a sufficiently sized unsigned integer (probably 32b) and that the following constant has been assigned:

Every time a CE marked packet arrives, the Data Receiver increments its local value of r.cep by 1. It repeats the same value of ACE in every subsequent ACK until the next CE marking arrives, where

If the Data Sender received an earlier value of the counter that had been delayed due to ACK reordering, it might incorrectly calculate that the ACE field had wrapped. Therefore, on the arrival of every ACK, the Data Sender uses the TCP acknowledgement number and any SACK options to calculate newlyAckedB, the amount of new data that the ACK acknowledges. If newlyAckedB is negative it means that a more up to date ACK has already been processed, so this ACK has been superseded and the Data Sender has to ignore the AccECN Option. If newlyAckedB is zero, to break the tie the Data Sender could use timestamps (if present) to work out newlyAckedT, the amount of new time that the ACK acknowledges. Then the Data Sender calculates the minimum difference d.cep between the ACE field and its local s.cep counter, using modulo arithmetic as follows:

   if ((newlyAckedB > 0) || (newlyAckedB == 0 && newlyAckedT > 0))
       d.cep = (ACE + DIVACE - (s.cep % DIVACE)) % DIVACE

Section 3.2.5 requires the Data Sender to assume that the ACE field did cycle if it could have cycled under prevailing conditions. The 3-bit ACE field in an arriving ACK could have cycled and become ambiguous to the Data Sender if a row of ACKs goes missing that covers a stream of data long enough to contain 8 or more CE marks. We use the word `missing' rather than `lost', because some or all the missing ACKs might arrive eventually, but out of order. Even if some of the lost ACKs are piggy-backed on data (i.e. not pure ACKs) retransmissions will not repair the lost AccECN information, because AccECN requires retransmissions to carry the latest AccECN counters, not the original ones.

The phrase `under prevailing conditions' allows the Data Sender to take account of the prevailing size of data segments and the prevailing CE marking rate just before the sequence of ACK losses. However, we shall start with the simplest algorithm, which assumes segments are all full-sized and ultra-conservatively it assumes that ECN marking was 100% on the forward path when ACKs on the reverse path started to all be dropped. Specifically, if newlyAckedB is the amount of data that an ACK acknowledges since the previous ACK, then the Data Sender could assume that this acknowledges newlyAckedPkt full-sized segments, where newlyAckedPkt = newlyAckedB/MSS. Then it could assume that the ACE field incremented by

    dSafer.cep = newlyAckedPkt - ((newlyAckedPkt - d.cep) % DIVACE),

For example, imagine an ACK acknowledges newlyAckedPkt=9 more full-size segments than any previous ACK, and that ACE increments by a minimum of 2 CE marks (d.cep=2). The above formula works out that it would still be safe to assume 2 CE marks (because 9 - ((9-2) % 8) = 2). However, if ACE increases by a minimum of 2 but acknowledges 10 full-sized segments, then it would be necessary to assume that there could have been 10 CE marks (because 10 - ((10-2) % 8) = 10).

Implementers could build in more heuristics to estimate prevailing average segment size and prevailing ECN marking. For instance, newlyAckedPkt in the above formula could be replaced with newlyAckedPktHeur = newlyAckedPkt*p*MSS/s, where s is the prevailing segment size and p is the prevailing ECN marking probability. However, ultimately, if TCP's ECN feedback becomes inaccurate it still has loss detection to fall back on. Therefore, it would seem safe to implement a simple algorithm, rather than a perfect one.

The simple algorithm for dSafer.cep above requires no monitoring of prevailing conditions and it would still be safe if, for example, segments were on average at least 5% of full-sized as long as ECN marking was 5% or less. Assuming it was used, the Data Sender would increment its packet counter as follows:

If missing acknowledgement numbers arrive later (due to reordering), Section 3.2.5 says "the Data Sender MAY attempt to neutralise the effect of any action it took based on a conservative assumption that it later found to be incorrect". To do this, the Data Sender would have to store the values of all the relevant variables whenever it made assumptions, so that it could re-evaluate them later. Given this could become complex and it is not required, we do not attempt to provide an example of how to do this.

A.2.2. Safety Algorithm with the AccECN Option

When the AccECN Option is available on the ACKs before and after the possible sequence of ACK losses, if the Data Sender only needs CE-marked bytes, it will have sufficient information in the AccECN Option without needing to process the ACE field. However, if for some reason it needs CE-marked packets, if dSafer.cep is different from d.cep, it can calculate the average marked segment size that each implies to determine whether d.cep is likely to be a safe enough estimate. Specifically, it could use the following algorithm, where is the amount of newly CE-marked bytes (see Appendix A.1):

   if (dSafer.cep > d.cep) {
       s =
       if (s <= MSS) {
          sSafer =
          if (sSafer < MSS/SAFETY_FACTOR)
              dSafer.cep = d.cep    % d.cep is a safe enough estimate
       } % else
           % No need for else; dSafer.cep is already correct, 
           % because d.cep must have been too small

The chart below shows when the above algorithm will consider d.cep can replace dSafer.cep as a safe enough estimate of the number of CE-marked packets:

      |         dSafer.cep
      |                  is
 MSS/2+--------------+    safest
      |              |
      | d.cep is safe|
      |    enough    |
                    MSS   s

The following examples give the reasoning behind the algorithm, assuming MSS=1,460 [B]:

If pure ACKs were allowed to be ECN-capable, missing ACKs would be far less likely. However, because [RFC3168] currently precludes this, the above algorithm assumes that pure ACKs are not ECN-capable.

A.3. Example Algorithm to Estimate Marked Bytes from Marked Packets

If the AccECN Option is not available, the Data Sender can only decode CE-marking from the ACE field in packets. Every time an ACK arrives, to convert this into an estimate of CE-marked bytes, it needs an average of the segment size, s_ave. Then it can add or subtract s_ave from the value of as the value of d.cep increments or decrements.

To calculate s_ave, it could keep a record of the byte numbers of all the boundaries between packets in flight (including control packets), and recalculate s_ave on every ACK. However it would be simpler to merely maintain a counter packets_in_flight for the number of packets in flight (including control packets), which it could update once per RTT. Either way, it would estimate s_ave as:

where flightsize is the variable that TCP already maintains for the number of bytes in flight. To avoid floating point arithmetic, it could right-bit-shift by lg(packets_in_flight), where lg() means log base 2.

An alternative would be to maintain an exponentially weighted moving average (EWMA) of the segment size:

where a is the decay constant for the EWMA. However, then it is necessary to choose a good value for this constant, which ought to depend on the number of packets in flight. Also the decay constant needs to be power of two to avoid floating point arithmetic.

A.4. Example Algorithm to Beacon AccECN Options

Section 3.2.8 requires a Data Receiver to beacon a full-length AccECN Option at least 3 times per RTT. This could be implemented by maintaining a variable to store the number of ACKs (pure and data ACKs) since a full AccECN Option was last sent and another for the approximate number of ACKs sent in the last round trip time:

   if (acks_since_full_last_sent > acks_in_round / BEACON_FREQ)

For optimised integer arithmetic, BEACON_FREQ = 4 could be used, rather than 3, so that the division could be implemented as an integer right bit-shift by lg(BEACON_FREQ).

In certain operating systems, it might be too complex to maintain acks_in_round. In others it might be possible by tagging each data segment in the retransmit buffer with the number of ACKs sent at the point that segment was sent. This would not work well if the Data Receiver was not sending data itself, in which case it might be necessary to beacon based on time instead, as follows:

   if ( time_now > time_last_option_sent + (RTT / BEACON_FREQ) )

This time-based approach does not work well when all the ACKs are sent early in each round trip, as is the case during slow-start. In this case few options will be sent (evtl. even less than 3 per RTT). However, when continuously sending data, data packets as well as ACKs will spread out equally over the RTT and sufficient ACKs with the AccECN option will be sent.

A.5. Example Algorithm to Count Not-ECT Bytes

A Data Sender in AccECN mode can infer the amount of TCP payload data arriving at the receiver marked Not-ECT from the difference between the amount of newly ACKed data and the sum of the bytes with the other three markings,, d.e0b and d.e1b. Note that, because r.e0b is initialized to 1 and the other two counters are initialized to 0, the initial sum will be 1, which matches the initial offset of the TCP sequence number on completion of the 3WHS.

For this approach to be precise, it has to be assumed that spurious (unnecessary) retransmissions do not lead to double counting. This assumption is currently correct, given that RFC 3168 requires that the Data Sender marks retransmitted segments as Not-ECT. However, the converse is not true; necessary transmissions will result in under-counting.

However, such precision is unlikely to be necessary. The only known use of a count of Not-ECT marked bytes is to test whether equipment on the path is clearing the ECN field (perhaps due to an out-dated attempt to clear, or bleach, what used to be the ToS field). To detect bleaching it will be sufficient to detect whether nearly all bytes arrive marked as Not-ECT. Therefore there should be no need to keep track of the details of retransmissions.

Appendix B. Rationale for Usage of TCP Header Flags

B.1. Three TCP Header Flags in the SYN-SYN/ACK Handshake

AccECN uses a rather unorthodox but justified approach to negotiate the highest version TCP ECN feedback scheme that both ends support. It follows from the original TCP ECN capability negotiation [RFC3168], in which the client set the 2 least significant reserved flags in the TCP header, and fell back to no ECN support if the server responded with the 2 flags cleared, which had previously been the default. It is not recorded why ECN originally used this approach instead of the more orthodox use of a TCP option.

In order to be backward compatible with RFC 3168, AccECN continues this approach, using the 3rd least significant TCP header flag that had previously been allocated for the ECN nonce (now historic). Then, whatever form of server an AccECN client encounters, the connection can fall back to the highest version of feedback protocol that both ends support, as explained in Section 3.1.

If AccECN had used the more orthodox approach of a TCP option, it would still have had to set the two ECN flags in the main TCP header, in order to be able to fall back to Classic RFC 3168 ECN, or to disable ECN support, without another round of negotiation. Then AccECN would also have had to handle all the different ways that servers currently respond to settings of the ECN flags in the main TCP header, including all the conflicting cases where a server might have said it supported one approach in the flags and another approach in the new TCP option. And AccECN would have had to deal with all the additional possibilities where a middlebox might have mangled the ECN flags, or removed the TCP option. Thus, usage of the 3rd reserved TCP header flag simplified the protocol.

The third flag was used in a way that could be distinguished from the ECN nonce, in case any nonce deployment was encountered. Previous usage of this flag for the ECN nonce was integrated into the original ECN negotiation. This further justified the 3rd flag's use for AccECN, because a non-ECN usage of this flag would have had to use it as a separate single bit, rather than in combination with the other 2 ECN flags.

Indeed, having overloaded the original uses of these three flags for its handshake, AccECN overloads all three bits again as a 3-bit counter.

B.2. Four Codepoints in the SYN/ACK

Of the 8 possible codepoints that the 3 TCP header flags can indicate on the SYN/ACK, 4 already indicated earlier (or broken) versions of ECN support. In the early design of AccECN, an AccECN server could use only 2 of the 4 remaining codepoints. They both indicated AccECN support, but one fed back that the SYN had arrived marked as CE. Even though ECN support on a SYN is not yet on the standards track, the idea is for either end to act as a dumb reflector, so that future capabilities can be unilaterally deployed without requiring 2-ended deployment (justified in Section 2.5).

During traversal testing it was discovered that the ECN field in the SYN was mangled on a non-negligible proportion of paths. Therefore it was necessary to allow the SYN/ACK to feed all four IP/ECN codepoints that the SYN could arrive with back to the client. Without this, the client could not know whether to disable ECN for the connection due to mangling of the IP/ECN field (also explained in Section 2.5). This development consumed the remaining 2 codepoints on the SYN/ACK that had been reserved for future use by AccECN in earlier versions.

B.3. Space for Future Evolution

Despite availability of usable TCP header space being extremely scarce, the AccECN protocol has taken all possible steps to ensure that there is space to negotiate possible future variants of the protocol, either if the experiment proves that a variant of AccECN is required, or if a completely different ECN feedback approach is needed:

Future AccECN variants:
The requirement not to reject unexpected initial values of the ACE counter (in the main TCP header) in the last para of Section 3.2.3 ensures that 5 unused codepoints on the final ACK of the 3-way handshake and 7 unused values on the first data packet from the server could be used to negotiate future variants of the AccECN protocol between the endpoints. Also, a similar requirement not to reject unexpected initial values in the TCP option is for the same purpose. If traversal of the TCP option were reliable, this would have enabled a far wider range of future variation.
Future non-AccECN variants:
Five codepoints out of the 8 possible in the 3 TCP header flags used by AccECN are unused on the initial SYN (in the order AE,CWR,ECE): 001, 010, 100, 101, 110. All possible combinations of SYN/ACK coiuld be used in response except 000 and reflection of the same values sent on the SYN. These would not allow fall-back to Classic ECN support for a server that did not understand them, but they are available, perhaps for uses other than ECN in future.

Of course, other ways could be resorted to in order to extend AccECN or ECN in future, although their traversal properties are likely to be inferior. They include a new TCP option; using the remaining reserved flags in the main TCP header (preferably extending the 3-bit combinations used by AccECN to 4-bit combinations, rather than burning one bit for just one state); a non-zero urgent pointer in combination with the URG flag cleared; or some other unexpected combination of fields yet to be invented.

Authors' Addresses

Bob Briscoe CableLabs UK EMail: URI:
Mirja Kühlewind ETH Zurich Zurich, Switzerland EMail:
Richard Scheffenegger Vienna, Austria EMail: