| Internet-Draft | scetcp | November 2019 |
| Grimes & Heist | Expires 7 May 2020 | [Page] |
This memo classifies a TCP code point ESCE ("Echo Some Congestion Experienced") for use in feedback of IP code point SCE ("Some Congestion Experienced").¶
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] and [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
This memo requests a TCP header codepoint for use as ESCE.¶
This memo limits its scope to the definition of the TCP codepoint ESCE, with a few brief illustrations of how it may be used.¶
SCE provides early and proportional feedback to the CC (congestion control) algorithms for transport protocols, including but not limited to TCP. The [sce-repo] is a Linux kernel modified to support SCE, including:¶
Enhancements to Linux's [cake] (Common Applications Kept Enhanced) AQM to support SCE signaling¶
Modifications to the TCP receive path to reflect SCE signals back to the sender¶
The addition of three new TCP CC algorithms that modify the originals to add SCE support: Reno-SCE, DCTCP-SCE and Cubic-SCE (work in progress as of this writing)¶
[I-D.morton-tsvwg-sce] defines the IP SCE codepoint.¶
The mechanism defined to feed back SCE signals to the sender explicitly makes use of the ESCE ("Echo Some Congestion Experienced") code point in the TCP header.¶
Upon receipt of a packet an ACK is immediatly generated, the SCE codepoint is copied into the ESCE codepoint of the ACK. This keeps the count of bytes SCE marked or not marked properly reflected in the ACK packet(s). This valid implementation has the downside of increasing ACK traffic. This implementation is NOT RECOMMENDED, but useful for experimental work.¶
Upon receipt of a packet without an SCE codepoint traditional delayed ACK processing is performed. Upon receipt of a packet with an SCE codepoint immediate ACK processing SHOULD be done, this allows some delaying of ACK's, but creates earlier feedback of the congested state. This has the negative effect of over signalling ESCE.¶
Upon receipt of a packet the SCE codepoint is stored in the TCP state. Multiple packets state may be stored. Upon generation of an ACK, normal or delayed, the stored SCE state is used to set the state of ESCE. If no SCE state is in the TCP state, then the ESCE code point MUST NOT be set. If all of the packets to be ACKed have SCE state set then the ESCE code point MUST be set in the ACK. If some of the packets to be ACKed have SCE state set then some proportional number of ACK packets SHOULD be sent with the ESCE code point set. Though this may defer a ESCE congestion signal when there is not a next packet for some time it is generally accepted that such sparse flows are not the source of congestion and thus the delayed signal is of low impact. The goal is to have the same number of bytes marked with ESCE as arrived with SCE.¶
The Advanced ACK implementation actually immediately flushes any pending ACK's up to the previous segment when the state of the SCE marking changes, allowing consecutive packets with the same SCE state to be coalesced by the normal delayed-ack logic. The ACK volume is then inflated only slightly compared to an unmarked connection, and may actually involve fewer acks than a connection involving CE marks or losses, during which delayed acks are temporarily disabled.¶
Ack thinning is something that has been considered, given that [cake] includes an optional ack-filter which does thinning. We have, for example, added consideration of the ESCE bit to Cake's ack-filter. Mathematically, the most extreme errors possible in either direction, due to ack thinning, are easily corrected during subsequent RTTs.¶
The recommended response to each single segment marked with ESCE is to reduce cwnd by an amortised 1/sqrt(cwnd) segments. If the growth rate is greater than that provided by the Reno-linear algorithm - eg. slow-start exponential or CUBIC polynomial - then the growth rate SHOULD also be reduced.¶
Other responses, such as the 1/cwnd from DCTCP, are also acceptable but may perform less well.¶
There are no changes to the response functions with respect to CE or packet loss specificed by this draft, hence [RFC3168] and [RFC8511] are still applicable¶
This is still an area of continued investigation.¶
This document requests one of the reserved bits in the TCP header, with the former TCP NS ("Nonce Sum") bit (bit 7) being suggested due to similarities with its previous usage. [RFC8311] (section 3) obsoletes the NS codepoint making it avaliable for use.¶
There are no Security considerations.¶
TBD¶