Extended TCP Option Space in the Payload of an Alternative SYN
draft-briscoe-tcpm-syn-op-sis-00

Abstract

This document describes an experimental method to extend the option space for connection parameters within the initial TCP SYN segment at the start of a TCP connection. In this method the TCP client sends two alternative SYNs: one intended for legacy servers and one intended for upgraded servers. Once it establishes which type of server has responded, it continues the connection appropriate to that server type and aborts the other. The SYN intended for upgraded servers includes additional options at the end of the payload.

Status of This Memo

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Table of Contents

• 1. Introduction
• 1.1. Scope
• 1.2. Experiment Goals
• 1.3. Terminology
• 2. Protocol Specification
• 3. Acknowledgements
• 4. IANA Considerations
• 5. Security Considerations
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Appendix A. Alternative (Deterministic) Protocol Specification
• Author's Address

1. Introduction

This document describes an experimental method to extend the option space available in the initial SYN segment of a TCP connection (i.e. SYN set and ACK not set) [RFC0793]. This extension is required to support some combinations of TCP options, notably large ones such as TCP AO [RFC5925] (16B), Multipath TCP [RFC6824] (12B), and TCP Fast Open [I-D.ietf-tcpm-fastopen] (6-18B) as well as other options already typically used in TCP connections, such as SACK-ok (2B), Timestamp (10B), Window Scale (3B), MSS (4B) .

In this method the TCP client sends two alternative SYNs: one intended for legacy servers and one intended for upgraded servers. Once it establishes which type of server has responded, it continues the connection appropriate to that server type and aborts the other. The SYN intended for upgraded servers includes additional options at the end of the payload.

1.1. Scope

This experimental specification extends the TCP wire protocol. It is independent of the dynamic behaviour of TCP and it is independent of (and thus compatible with) any protocol that encapsulates TCP, including IPv4 and IPv6.

1.2. Experiment Goals

TCP is critical to the robust functioning of the Internet, therefore any proposed modifications to TCP need to be thoroughly tested. The present specification describes an experimental protocol that provides extra option space on the initial TCP SYN segment. The intention is to specify the protocol sufficiently so that more than one implementation can be built in order to test its function, robustness and interoperability (with itself, with previous version of TCP, and with various commonly deployed middleboxes).

Success criteria:

The experimental protocol will be considered successful if it satisfies the following requirements in the consensus opinion of the IETF tcpm working group. {ToDo: describe success criteria}

Duration:

To be credible, the experiment will need to last at least 12 months from publication of the present specification. If successful, a report on the experiment will be written up. it would then be appropriate to work on a standards track specification, in which the experiment report may be included.

1.3. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. In this document, these words will appear with that interpretation only when in ALL CAPS. Lower case uses of these words are not to be interpreted as carrying RFC-2119 significance.

2. Protocol Specification

In this method the TCP client sends two alternative SYNs: a regular SYN intended for legacy servers and SYN-U intended for upgraded servers. Once it establishes which type of server has responded, it continues the connection appropriate to that server type and aborts the other. The SYN intended for upgraded servers (SYN-U) includes additional options at the end of the payload.

Table 1 summarises the TCP 3-way handshake exchange for each of the two SYNs between an upgraded TCP (active opening) client and either i) a legacy server or ii) an upgraded server.

                      | EOO     | EOO1      |
                      --------->----------->|
+---------+-----------+---------+-----------+-----------+------------+
| TCP hdr | TCP-Opt#2 | Payload | TCP-Opt#1 | TCP-Opt#3 | SYN-OP-SIS |
+---------+-----------+---------+-----------+-----------+------------+

+---------------+---------------+-------------------------------+
| Kind=SYNOPSYS | Length=8      | Magic Number                  |
+---------------+---------------+---------------+---------------+
| Magic Number (cont)           | EOO           | EPOO          |
+---------------+---------------+---------------+---------------+

+---------------+---------------+
| Kind=SYNOPSYS | Length=2      |
+---------------+---------------+

                                               | EOO     |
                                               --------->|
+---------+-----------+------------+-----------+---------+-----------+
| TCP hdr | TCP-Opt#1 | SYN-OP-SIS | TCP-Opt#3 | Payload | TCP-Opt#2 |
+---------+-----------+------------+-----------+---------+-----------+

+---------------+---------------+---------------+
| Kind=SYNOPSYS | Length=3      | EOO           |
+---------------+---------------+---------------+