HTTP/2 Encoded Data
draft-kerwin-http2-encoded-data-05

Abstract

This document introduces new frame types for transporting gzip-encoded data between peers in the Hypertext Transfer Protocol Version 2 (HTTP/2), and an associated error code for handling invalid encoding.

Status of This Memo

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

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This Internet-Draft will expire on November 29, 2015.

Copyright Notice

Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Notational Conventions
• 2. Additions to HTTP/2
• 2.1. ACCEPT_GZIPPED_DATA
• 2.2. GZIPPED_DATA
• 2.3. DATA_ENCODING_ERROR
• 3. Fragmentation
• 4. Security Considerations
• 5. IANA Considerations
• 5.1. HTTP/2 Frame Type Registry Update
• 5.2. HTTP/2 Error Code Registry Update
• 6. Acknowledgements
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Author's Address

1. Introduction

This document introduces a mechanism for applying gzip encoding [RFC1952] to data transported between two endpoints in the Hypertext Transfer Protocol Version 2 (HTTP/2) [RFC7540], analogous to Transfer-Encoding in HTTP/1.1 [RFC7230].

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. Additions to HTTP/2

This document introduces two new HTTP/2 frame types ([RFC7540], Section 11.2) and a new HTTP/2 error code ([RFC7540], Section 7), to allow the compression of data.

Note that while compressing some or all data in a stream might affect the total length of the corresponding HTTP message body, the content-length header, if present, should continue to reflect the total length of the uncompressed data. This is particularly relevant when detecting malformed messages ([RFC7540], Section 8.1.2.6).

2.1. ACCEPT_GZIPPED_DATA

An ACCEPT_GZIPPED_DATA frame (type code=0xTBA) is used to indicate the sender’s ability and willingness to receive GZIPPED_DATA frames.

ACCEPT_GZIPPED_DATA always applies to a connection, never a single stream. The stream identifier for an ACCEPT_GZIPPED_DATA frame MUST be zero (0x0). If an endpoint receives an ACCEPT_GZIPPED_DATA frame whose stream identifier field is anything other than 0x0, the endpoint MUST respond with a connection error (([RFC7540], Section 5.4.1) of type PROTOCOL_ERROR.

The payload length of an ACCEPT_GZIPPED_DATA frame MUST be zero. An endpoint that receives an ACCEPT_GZIPPED_DATA frame a length other than zero MUST treat this as a connection error ([RFC7540], Section 5.4.1) of type PROTOCOL_ERROR.

The ACCEPT_GZIPPED_DATA frame defines the following flag:

• DISABLE (0x1): When set, bit 0 indicates that this endpoint is not willing or able to receive GZIPPED_DATA frames.

An endpoint may advertise support for GZIPPED_DATA frames and later decide that it no longer supports them. After sending an ACCEPT_GZIPPED_DATA with the DISABLE flag set, the endpoint SHOULD continue to accept GZIPPED_DATA frames for a reasonable amount of time to account for frames that are already in flight.

The ACCEPT_GZIPPED_DATA frame is not subject to flow control.

2.2. GZIPPED_DATA

GZIPPED_DATA frames (type code=0xTBA) are semantically identical to DATA frames ([RFC7540], Section 6.1), but their payload is encoded using gzip compression. Gzip compression is an LZ77 coding with a 32 bit CRC that is commonly produced by the gzip file compression program [RFC1952]. Significantly, GZIPPED_DATA frames are subject to flow control ([RFC7540], Section 5.2).

Any compression or decompression context for a GZIPPED_DATA frame is unique to that frame.

  +---------------+
  |Pad Length? (8)|
  +---------------+-----------------------------------------------+
  |                            Data (*)                         ...
  +---------------------------------------------------------------+
  |                           Padding (*)                       ...
  +---------------------------------------------------------------+

GZIPPED_DATA Frame Payload

The GZIPPED_DATA frame contains the following fields:

• Pad Length: An 8-bit field containing the length of the frame padding in units of octets. This field is optional and is only present if the PADDED flag is set.
• Data: Encoded application data. The amount of compressed data is the remainder of the frame payload after subtracting the length of the other fields that are present.
• Padding: Padding octets that contain no application semantic value. Padding octets MUST be set to zero when sending and ignored when receiving.

The GZIPPED_DATA frame defines the following flags:

• END_STREAM (0x1): Bit 1 being set indicates that this frame is the last that the endpoint will send for the identified stream. Setting this flag causes the stream to enter one of the “half closed” states or the “closed” state ([RFC7540], Section 5.1).
• PADDED (0x8): Bit 4 being set indicates that the Pad Length field is present.

A GZIPPED_DATA frame MUST NOT be sent on a connection before receiving an ACCEPT_GZIPPED_DATA frame.

An intermediary, on receiving a GZIPPED_DATA frame, MAY decode the data and forward it to its downstream peer in one or more DATA frames. If the downstream peer has not advertised support for GZIPPED_DATA frames (e.g. by sending an ACCEPT_GZIPPED_DATA frame) the intermediary MUST decode the data before forwarding it.

If an endpoint detects that the payload of a GZIPPED_DATA frame is not encoded correctly, for example with a mismatched checksum, the endpoint MUST treat this as a stream error (see [RFC7540], Section 5.4.2) of type DATA_ENCODING_ERROR (Section 2.3). The endpoint MAY then choose to immediately send an ACCEPT_GZIPPED_DATA frame with the DISABLE flag set.

If an intermediary propagates a GZIPPED_DATA frame from the source peer to the destination peer without modifying the payload or its encoding, and receives a DATA_ENCODING_ERROR from the receiving peer, it SHOULD pass the error on to the source peer.

GZIPPED_DATA frames MUST be associated with a stream. If a GZIPPED_DATA frame is received whose stream identifier field is 0x0, the recipient MUST respond with a connection error ([RFC7540], Section 5.4.1) of type PROTOCOL_ERROR.

GZIPPED_DATA frames are subject to flow control and can only be sent when a stream is in the “open” or “half closed (remote)” states. The entire GZIPPED_DATA frame payload is included in flow control, including the Pad Length and Padding fields if present. If a GZIPPED_DATA frame is received whose stream is not in “open” or “half closed (local)” state, the recipient MUST respond with a stream error ([RFC7540], Section 5.4.2) of type STREAM_CLOSED.

The total number of padding octets is determined by the value of the Pad Length field. If the length of the padding is greater than the length of the remainder of the frame payload, the recipient MUST treat this as a connection error ([RFC7540], Section 5.4.1) of type PROTOCOL_ERROR.

Note: A frame can be increased in size by one octet by including a Pad Length field with a value of zero.

Padding is a security feature; see Section 10.7 of [RFC7540].

2.3. DATA_ENCODING_ERROR

The following new error code is defined:

• DATA_ENCODING_ERROR (0xTBA): The endpoint detected that its peer sent a GZIPPED_DATA frame with an invalid encoding.

3. Fragmentation

Traversing a network segment with small frame size limits introduces the risk of fragmenting an encoded stream.

4. Security Considerations

Further to the Use of Compression in HTTP/2 ([RFC7540], Section 10.6), intermediaries MUST NOT apply compression to DATA frames, or alter the compression of GZIPPED_DATA frames other than decompressing, unless additional information is available that allows the intermediary to identify the source of data. In particular, frames that are not compressed cannot be compressed, and frames that are separately compressed cannot be merged into a single compressed frame.

5. IANA Considerations

This document updates the registries for frame types and error codes in the “Hypertext Transfer Protocol (HTTP) 2 Parameters” section. This document also establishes a new registry for HTTP/2 encoding scheme codes. This new registry is entered into the “Hypertext Transfer Protocol (HTTP) 2 Parameters” section.

5.1. HTTP/2 Frame Type Registry Update

This document updates the “HTTP/2 Frame Type” registry ([RFC7540], Section 11.2). The entries in the following table are registered by this document.

5.2. HTTP/2 Error Code Registry Update

This document updates the “HTTP/2 Error Code” registry ([RFC7540], Section 11.4). The entries in the following table are registered by this document.

6. Acknowledgements

Thanks to Keith Morgan for his advice, input, and editorial contributions.

7. References

7.1. Normative References

7.2. Informative References

Author's Address