| Transport Working Group | J. Morton |
| Internet-Draft | Bufferbloat.net |
| Updates: 3168, 8311 (if approved) | D. Täht |
| Intended status: Standards Track | TekLibre |
| Expires: September 11, 2019 | March 10, 2019 |
The Some Congestion Experienced ECN Codepoint
draft-morton-taht-sce-00
This memo reclassifies ECT(1) to be an early notification of congestion on ECT(0) marked packets, which can be used by AQM algorithms and transports as an earlier signal of congestion than CE. It is a simple, transparent, and backward compatible upgrade to existing IETF-approved AQMs, RFC3168, and nearly all congestion control algorithms.
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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].
This memo reclassifies ECT(1) to be an early notification of congestion on ECT(0) marked packets, which can be used by AQM algorithms and transports as an earlier signal of congestion than CE ("Congestion Experienced").
This memo limits its scope to the redefinition of the ECT(1) codepoint as SCE, "Some Congestion Experienced", with a few brief illustrations of how it may be used.
[RFC3168] defines the lower two bits of the (former) TOS byte in the IPv4/6 header as the ECN field. This may take four values: Not-ECT, ECT(0), ECT(1) or CE.
Binary Keyword References
------------------------------------------------------------
00 Not-ECT (Not ECN-Capable Transport) [RFC 3168] 01 ECT(1) (ECN-Capable Transport(1)) [RFC 3168] 10 ECT(0) (ECN-Capable Transport(0)) [RFC 3168] 11 CE (Congestion Experienced) [RFC 3168]
Research has shown that the ECT(1) codepoint goes essentially unused, with the "Nonce Sum" extension to ECN having not been implemented in practice and thus subsequently obsoleted by [RFC8311] (section 3). Additionally, known [RFC3168] compliant senders do not emit ECT(1), and compliant middleboxes do not alter the field to ECT(1), while compliant receivers all interpret ECT(1) identically to ECT(0). These are useful properties which represent an opportunity for improvement.
Experience gained with 7 years of [RFC8290] deployment in the field suggests that it remains difficult to maintain the desired 100% link utilisation, whilst simultaneously strictly minimising induced delay due to excess queue depth - irrespective of whether ECN is in use. This leads to a reluctance amongst hardware vendors to implement the most effective AQM schemes because their headline benchmarks are throughput-based.
The underlying cause is the very sharp "multiplicative decrease" reaction required of transport protocols to congestion signalling (whether that be packet loss or CE marks), which tends to leave the congestion window significantly smaller than the ideal BDP when triggered at only slightly above the ideal value. The availability of this sharp response is required to assure network stability (AIMD principle), but there is presently no standardised and backwards-compatible means of providing a less drastic signal.
As consensus has arisen that some form of ECN signaling should be an earlier signal than drop, this Internet Draft changes the meaning of ECT(1) to be SCE, meaning "Some Congestion Experienced". The above ECN-field codepoint table then becomes:
Binary Keyword References
------------------------------------------------------------
00 Not-ECT (Not ECN-Capable Transport) [@RFC3168] 01 SCE (Some Congestion Experienced) [This Internet-draft] 10 ECT (ECN-Capable Transport) [@RFC3168] 11 CE (Congestion Experienced) [@RFC3168]
This permits middleboxes implementing AQM to signal incipient congestion, below the threshold required to justify setting CE, by converting some proportion of ECT codepoints to SCE ("SCE marking"). Existing [RFC3168] compliant receivers MUST transparently ignore this new signal, and both existing and SCE-aware middleboxes MAY convert SCE to CE in the same circumstances as for ECT, thus ensuring backwards compatibility with [RFC3168] ECN endpoints.
Permitted ECN codepoint packet transitions by middleboxes are:
Not-ECT -> Not-ECT or DROP ECT -> ECT or SCE or CE or DROP SCE -> SCE or CE or DROP CE -> CE or DROP
In other words, for ECN-aware flows, the ECN marking of an individual packet MAY be increased by a middlebox to signal congestion, but MUST NOT be decreased, and packets SHALL NOT be altered to appear to be ECN-aware if they were not originally, nor vice versa. Note however that SCE is numerically less than ECT, but semantically greater, and the latter definition applies for this rule.
New SCE-aware receivers and transport protocols SHALL continue to apply the [RFC3168] interpretation of the CE codepoint, that is, to signal the sender to back off send rate to the same extent as if a packet loss were detected. This maintains compatibility with existing middleboxes, senders and receivers.
New SCE-aware receivers and transport protocols SHOULD interpret the SCE codepoint as an indication of mild congestion, and respond accordingly by applying send rates intermediate between those resulting from a continuous sequence of ECT codepoints, and those resulting from a CE codepoint. The ratio of ECT and SCE codepoints received indicates the relative severity of such congestion, such that 100% SCE is very close to the threshold of CE marking, 100% ECT indicates that the bottleneck link may not be fully utilised, and a 1:1 balance of ECT and SCE codepoints indicates that the present send rate is a good match to the bottleneck link.
Details of how to implement SCE awareness at the transport layer will be left to additional Internet Drafts yet to be submitted.
To maximise the benefit of SCE, middleboxes SHOULD produce SCE markings sooner than they produce CE markings, when the level of congestion increases.
Consider a TCP transport implementing CUBIC congestion control. This presently exhibits exponential cwnd growth during slow-start, polynomial cwnd growth in steady-state, and multiplicative decrease upon detecting a single CE marking or packet loss in one RTT cycle.
With SCE awareness, it might exit slow-start upon detecting a single SCE marking, switch from polynomial to Reno-linear cwnd growth when the SCE:ECT ratio exceeds 1:2, halt cwnd growth entirely when it exceeds 1:1, and implement a Reno-linear decline when it exceeds 2:1, in addition to retaining the sharp 40% decrease on detecting CE.
In ideal circumstances, the above behaviour would result in the send rate stabilising at a level which produces between 50% and 66% SCE marking at some bottleneck on the path. The middlebox performing this marking can thus control the send rate smoothly to an ideal value, maximising throughput with minimum average queue length.
SCE can potentially be handled entirely by the receiver and be entirely independent of any of the dozens of [RFC3168] compliant congestion control algorithms, for example by manipulating the TCP receive window in a similar manner to the sender's congestion window.
Alternatively, some mechanism may be defined to feed back SCE signals to the sender explicitly. Details of this are left to future I-Ds.
New transports under development such as QUIC SHOULD implement a multi-bit and finer grained signal back to the sender based on SCE.
[RFC8087] [RFC7567] [RFC7928] [RFC8290] [RFC8289] [RFC8033] [RFC8034]
There are no IANA considerations.
There are no security considerations.
Many thanks to John Gilmore, the members of the ecn-sane project, the "cake" bufferbloat.net mailing list, the IETF AQM mailing list, and the IETF TSVWG.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC8311] | Black, D., "Relaxing Restrictions on Explicit Congestion Notification (ECN) Experimentation", RFC 8311, DOI 10.17487/RFC8311, January 2018. |