More Accurate ECN Feedback in TCP
draft-ietf-tcpm-accurate-ecn-08
TCP Maintenance & Minor Extensions (tcpm) B. Briscoe
Internet-Draft CableLabs
Intended status: Experimental M. Kuehlewind
Expires: September 12, 2019 ETH Zurich
R. Scheffenegger
March 11, 2019
More Accurate ECN Feedback in TCP
draft-ietf-tcpm-accurate-ecn-08
Abstract
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). Recent 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
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 12, 2019.
Briscoe, et al. Expires September 12, 2019 [Page 1]
Internet-Draft Accurate TCP-ECN Feedback March 2019
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Document Roadmap . . . . . . . . . . . . . . . . . . . . 4
1.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Experiment Goals . . . . . . . . . . . . . . . . . . . . 5
1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.5. Recap of Existing ECN feedback in IP/TCP . . . . . . . . 7
2. AccECN Protocol Overview and Rationale . . . . . . . . . . . 8
2.1. Capability Negotiation . . . . . . . . . . . . . . . . . 9
2.2. Feedback Mechanism . . . . . . . . . . . . . . . . . . . 9
2.3. Delayed ACKs and Resilience Against ACK Loss . . . . . . 10
2.4. Feedback Metrics . . . . . . . . . . . . . . . . . . . . 11
2.5. Generic (Dumb) Reflector . . . . . . . . . . . . . . . . 11
3. AccECN Protocol Specification . . . . . . . . . . . . . . . . 12
3.1. Negotiating to use AccECN . . . . . . . . . . . . . . . . 12
3.1.1. Negotiation during the TCP handshake . . . . . . . . 12
3.1.2. Forward Compatibility . . . . . . . . . . . . . . . . 14
3.1.3. Retransmission of the SYN . . . . . . . . . . . . . . 15
3.2. AccECN Feedback . . . . . . . . . . . . . . . . . . . . . 15
3.2.1. Initialization of Feedback Counters at the Data
Sender . . . . . . . . . . . . . . . . . . . . . . . 16
3.2.2. The ACE Field . . . . . . . . . . . . . . . . . . . . 16
3.2.3. Testing for Zeroing of the ACE Field . . . . . . . . 18
3.2.4. Testing for Mangling of the IP/ECN Field . . . . . . 19
3.2.5. Safety against Ambiguity of the ACE Field . . . . . . 20
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