HTTP/2 Segments
draft-kerwin-http2-segments-latest

Abstract

This document introduces the concept of “segments” to HTTP/2, and adds a flag to the DATA frame type to allow the expression segments.

A “segment” is a contiguous block of HTTP message data that can be freely split apart and recombined at any time, allowing transmission in multiple HTTP/2 frames across network segments with smaller frame size limits without risk of permanent fragmentation when traversing network segments with much larger limits.

Status of This Memo

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This Internet-Draft will expire on July 28, 2016.

Copyright Notice

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Table of Contents

• 1. Introduction
• 1.1. Notational Conventions
• 2. Segments
• 3. SETTINGS_USE_SEGMENTS Setting
• 4. SEGMENT and SEGMENT_CONTINUES Flag
• 5. Security Considerations
• 6. IANA Considerations
• 6.1. HTTP/2 Settings Registry Update
• 7. Normative References
• Author's Address

1. Introduction

This document extends HTTP/2 [I-D.ietf-httpbis-http2] by introducing the concept of “segments” to HTTP/2, as a mechanism to combat the effects of fragmentation within a stream. It does this by adding a new flag to the DATA frame type ([I-D.ietf-httpbis-http2], Section 6.1).

1.1. Notational Conventions

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].

2. Segments

A “segment” is a contiguous region of a HTTP/2 message’s payload data which can be freely fragmented and recombined. A segment is expressed using the SEGMENT and SEGMENT_CONTINUES flags (Section 4). Any data within a frame that does not have the SEGMENT or SEGMENT_CONTINUES flags set and that does not follow a frame with the SEGMENT_CONTINUES flag set does not contain any segmentation information, and MUST NOT be included in a segment.

Segments can be used to mitigate the effects of fragmentation within a stream. For example, an endpoint may have a large chunk of data which it has to transmit via multiple DATA frames in order to comply with frame size limits. It can mark those frames as a single segment so that any downstream peer without the same frame size restrictions knows that it can safely coalesce the frames.

3. SETTINGS_USE_SEGMENTS Setting

The following new SETTINGS parameter ([I-D.ietf-httpbis-http2], Section 6.5.2) is defined:

• SETTINGS_USE_SEGMENTS (0xTBA): Informs the remote endpoint of whether or not the sender supports the SEGMENT_CONTINUES flag (Section 4). A value of 1 indicates that the sender supports the flag. Any other value MUST be treated as a connection error ([I-D.ietf-httpbis-http2], Section 5.4.1) of type PROTOCOL_ERROR.

4. SEGMENT and SEGMENT_CONTINUES Flag

The following new flags are defined for the DATA frame ([I-D.ietf-httpbis-http2], Section 6.1):

• SEGMENT (0x10): Bit 5 being set indicates that the current segment begins and ends with the current frame.
• SEGMENT_CONTINUES (0x20): Bit 6 being set indicates that the current segment continues after the current frame (see Section 2). If the preceding frame did not have the SEGMENT_CONTINUES flag, the current segment begins at the start of the current frame.

Intermediaries MUST NOT coalesce frames across a segment boundary and MUST preserve segment boundaries when forwarding frames.

The SEGMENT and SEGMENT_CONTINUES flag MUST NOT be set on any frames unless the remote endpoint has indicated support by sending a SETTINGS_USE_SEGMENTS setting (Section 3) with a value of 1.

5. Security Considerations

In and of itself, segmentation does not introduce any security concerns. However when used in combination with other features, such as compression, known and unknown vulnerabilities may be introduced. See the Use of Compression in HTTP/2 ([I-D.ietf-httpbis-http2], Section 10.6).

6. IANA Considerations

This document updates the registry for settings in the “Hypertext Transfer Protocol (HTTP) 2 Parameters” section.

6.1. HTTP/2 Settings Registry Update

This document updates the “HTTP/2 Settings” registry ([I-D.ietf-httpbis-http2], Section 11.3). The entries in the following table are registered by this document.

7. Normative References

Author's Address