Internet-Draft scetcp July 2019
Grimes & Heist Expires 9 January 2020 [Page]
TCP Maintenance and Minor Extensions
Intended Status:
Standards Track
R.W. Grimes
P. Heist

Some Congestion Experienced in TCP


This memo classifies a TCP code point ESCE ("Echo Some Congestion Experienced") for use in feedback of IP code point SCE ("Some Congestion Experienced").

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

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This Internet-Draft will expire on 9 January 2020.

Table of Contents

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

2. 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 [sce-repo] is a Linux kernel modified to support SCE, including:

3. Background

[I-D.morton-tsvwg-sce] defines the SCE codepoint and [RFC8311] (section 3) obsoletes the NS codepoint making it avaliable valiable for use.

4. 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.

4.1. 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.

4.2. 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.

4.3. 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.

5. 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.

7. IANA Considerations

There are no IANA considerations.

8. Security Considerations

There are no Security considerations.

9. Acknowledgements


10. Normative References

Morton, J. and R. Grimes, "The Some Congestion Experienced ECN Codepoint", Internet-Draft draft-morton-tsvwg-sce-00, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Black, D., "Relaxing Restrictions on Explicit Congestion Notification (ECN) Experimentation", RFC 8311, DOI 10.17487/RFC8311, , <>.

11. Informative References

"Some Congestion Experienced Reference Implementation GitHub Repository", , <>.

Authors' Addresses

Rodney W. Grimes
Portland, OR 97217
United States
Peter G. Heist
463 11 Liberec 30
Czech Republic