Some Congestion Experienced in TCP

Terminology 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 and when, and only when, they appear in all capitals, as shown here.

Introduction This memo reclassifies the former TCP NS ("Nonce Sum") codepoint as ESCE. This memo limits its scope to the redefinition of the TCP NS codepoint as 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 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)

Background defines the SCE codepoint and (section 3) obsoletes the NS codepoint making it avaliable valiable for use.

TCP Receiver 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.

Single ACK implementation 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.

Simple Delayed ACK implementation 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.

Dithered Delayed ACK implementation 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. The goal is to have the same number of bytes marked with ESCE as arrived with SCE.

TCP Sender The recommended response to each single segment marked with ESCE is to reduce cwnd by an amortised 1/sqrt(cwnd) segments. Other responses, such as the 1/cwnd from DCTCP, are also acceptable but may perform less well. This is still an area of continued investigation.

Related Work TBD

IANA Considerations There are no IANA considerations.

Security Considerations There are no Security considerations.

Acknowledgements TBD