Network Working Group M. Kerwin
Internet-Draft
Intended status: Experimental February 17, 2015
Expires: August 21, 2015
HTTP/2 Encoded Data
draft-kerwin-http2-encoded-data-04
Abstract
This document introduces new frame types for transporting 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 2
2. Additions to HTTP/2 . . . . . . . . . . . . . . . . . . . . . 2
2.1. ACCEPT_ENCODED_DATA . . . . . . . . . . . . . . . . . . . 3
2.2. ENCODED_DATA . . . . . . . . . . . . . . . . . . . . . . 4
2.2.1. Fragmentation and Segments . . . . . . . . . . . . . 7
2.3. DATA_ENCODING_ERROR . . . . . . . . . . . . . . . . . . . 7
3. Encoding Schemes . . . . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5.1. HTTP/2 Frame Type Registry Update . . . . . . . . . . . . 8
5.2. HTTP/2 Error Code Registry Update . . . . . . . . . . . . 8
5.3. HTTP/2 Encoding Schemes Registry . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
This document introduces a mechanism for applying encoding,
particularly compression, to data transported between two endpoints
in version 2 of the Hypertext Transfer Protocol (HTTP/2), 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
([I-D.ietf-httpbis-http2], Section 11.2) and a new HTTP/2 error code
([I-D.ietf-httpbis-http2], Section 7), to allow the application of
encoding, particularly compression, to data.
Note that while encoding 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 _unencoded_ data. This is particularly relevant
when detecting malformed messages ([I-D.ietf-httpbis-http2],
Section 8.1.2.5).
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2.1. ACCEPT_ENCODED_DATA
An ACCEPT_ENCODED_DATA frame (type code=0xTBA) is used to indicate
the sender's ability and willingness to receive ENCODED_DATA frames
that are encoded using the schemes identified in its payload.
ACCEPT_ENCODED_DATA always apply to a connection, never a single
stream. The stream identifier for an ACCEPT_ENCODED_DATA frame MUST
be zero (0x0). If an endpoint receives an ACCEPT_ENCODED_DATA frame
whose stream identifier field is anything other than 0x0, the
endpoint MUST respond with a connection error
(([I-D.ietf-httpbis-http2], Section 5.4.1) of type PROTOCOL_ERROR.
The payload length of an ACCEPT_ENCODED_DATA frame MUST be an exact
multiple of 16 bits (2 bytes). An endpoint that receives an
ACCEPT_ENCODED_DATA frame with an odd length MUST treat this as a
connection error ([I-D.ietf-httpbis-http2], Section 5.4.1) of type
PROTOCOL_ERROR.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoding (8) | Rank (8) | ...
+---------------+---------------+-------------------------------+
ACCEPT_ENCODED_DATA Frame Payload
The ACCEPT_ENCODED_DATA frame contains zero or more tuples comprising
the following fields:
o Encoding: An 8-bit identifier which identifies the encoding being
advertised (see Section 3).
o Rank: An 8-bit integer value.
The rank fulfils the same role as in the HTTP/1.1 TE header
([RFC7230], Section 4.3). The rank value is an integer in the range
0 through 255, where 1 is the least preferred and 255 is the most
preferred; a value of 0 means "not acceptable".
The ENCODING_IDENTITY encoding (Section 3) is always acceptable with
a default rank of 1, and MUST NOT be advertised with a rank of 0.
And endpoint that receives an ACCEPT_ENCODED_DATA frame including an
{encoding,rank} tuple of {ENCODING_IDENTITY,0} MUST respond with a
connection error ([I-D.ietf-httpbis-http2], Section 5.4.1) of type
PROTOCOL_ERROR.
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An endpoint that receives an ACCEPT_ENCODED_DATA frame containing an
{encoding,rank} tuple with an unknown or unsupported encoding
identifier MUST ignore that tuple.
Each ACCEPT_ENCODED_DATA frame fully replaces the set of tuples sent
in a previous frame; if an encoding identifier is omitted from a
subsequent ACCEPT_ENCODED_DATA frame it is deemed "not acceptable",
with the exception of ENCODING_IDENTITY which defaults to a rank of
1.
An endpoint may advertise support for an encoding scheme and later
decide that it no longer supports that scheme. After sending an
ACCEPT_ENCODED_DATA that omits the encoding identifier in question,
or includes it with a rank of 0, the endpoint SHOULD continue to
accept ENCODED_DATA frames using that scheme for a reasonable amount
of time to account for encoded frames that are already in flight.
The ACCEPT_ENCODED_DATA frame does not define any flags, and is not
subject to flow control.
2.2. ENCODED_DATA
ENCODED_DATA frames (type code=0xTBA) are semantically identical to
DATA frames ([I-D.ietf-httpbis-http2], Section 6.1), but have an
encoding applied to their payload. Significantly, ENCODED_DATA
frames are subject to flow control ([I-D.ietf-httpbis-http2],
Section 5.2).
Any encoding or decoding context for an ENCODED_DATA frame is unique
to that frame.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Pad Length? (8)|
+---------------+
| Encoding (8) |
+---------------+-----------------------------------------------+
| Data (*) ...
+---------------------------------------------------------------+
| Padding (*) ...
+---------------------------------------------------------------+
ENCODED_DATA Frame Payload
The ENCODED_DATA frame contains the following fields:
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o 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.
o Encoding: An 8-bit identifier which identifies the encoding that
has been applied to the Data field (see Section 3).
o Data: Encoded application data. The amount of encoded data is the
remainder of the frame payload after subtracting the length of the
other fields that are present.
o Padding: Padding octets that contain no application semantic
value. Padding octets MUST be set to zero when sending and
ignored when receiving.
The ENCODED_DATA frame defines the following flags:
o "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 ([I-D.ietf-httpbis-http2],
Section 5.1).
o "PADDED" (0x8): Bit 4 being set indicates that the Pad Length
field is present.
o "SEGMENT" (0x10): Bit 5 being set indicates that the current
segment begins and ends with the current frame.
o "SEGMENT_CONTINUES" (0x20): Bit 6 being set indicates that the
current segment continues after the current frame
([I-D.kerwin-http2-segments], 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 ([I-D.kerwin-http2-segments],
Section 3) with a value of 1.
An ENCODED_DATA frame MUST NOT be sent on a connection before
receiving an ACCEPT_ENCODED_DATA frame. A sender MUST NOT apply an
encoding that has not first been advertised by the peer in an
ACCEPT_ENCODED_DATA frame, or was advertised with a rank of 0.
Endpoints that receive a frame with an encoding they do not recognise
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or support MUST treat this as a connection error of type
PROTOCOL_ERROR.
An intermediary, on receiving an ENCODED_DATA frame, MAY decode the
data and forward it to its downstream peer in one or more DATA
frames. If the received ENCODED_DATA frame has an encoding the
downstream peer does not support (as advertised in an
ACCEPT_ENCODED_DATA frame) the intermediary MUST decode the data
before forwarding it. The intermediary MAY re-encode the data with a
scheme supported by the downstream peer and forward it in one or more
ENCODED_DATA frames.
If an endpoint detects that the payload of an ENCODED_DATA frame is
not encoded correctly according to its declared encoding, for example
with a mismatched checksum, the endpoint MUST treat this as a stream
error (see [I-D.ietf-httpbis-http2], Section 5.4.2) of type
DATA_ENCODING_ERROR (Section 2.3). The endpoint MAY then choose to
immediately send an ACCEPT_ENCODED_DATA frame that excludes the
encoding in question.
If an intermediary propagates an ENCODED_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.
ENCODED_DATA frames MUST be associated with a stream. If an
ENCODED_DATA frame is received whose stream identifier field is 0x0,
the recipient MUST respond with a connection error
([I-D.ietf-httpbis-http2], Section 5.4.1) of type PROTOCOL_ERROR.
ENCODED_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 ENCODED_DATA frame payload is included in flow control,
including the encoding identifier, and Pad Length and Padding fields
if present. If an ENCODED_DATA frame is received whose stream is not
in "open" or "half closed (local)" state, the recipient MUST respond
with a stream error ([I-D.ietf-httpbis-http2], 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 ([I-D.ietf-httpbis-http2],
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.
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Padding is a security feature; see Section 10.7 of
[I-D.ietf-httpbis-http2].
2.2.1. Fragmentation and Segments
Traversing a network segment with small frame size limits introduces
the risk of fragmenting an encoded stream. This can be avoided using
segments, as defined in [I-D.kerwin-http2-segments].
An intermediary MAY coalesce multiple adjacent ENCODED_DATA frames if
all of them, with the optional exception of the final frame in the
sequence, have the SEGMENT_CONTINUES flag set. The coalesced payload
MAY be subsequently emitted in any combination of ENCODED_DATA and
DATA frames. The payloads of any resulting ENCODED_DATA frames MUST
be correctly encoded according to those frames' encodings; note that
for some encoding schemes this could require that the payloads of the
original frames be decoded and subsequently re-encoded into the new
frames, rather than simply concatenated.
2.3. DATA_ENCODING_ERROR
The following new error code is defined:
o "DATA_ENCODING_ERROR" (0xTBA): The endpoint detected that its peer
sent an ENCODED_DATA frame with an invalid encoding.
3. Encoding Schemes
The following encoding schemes are defined:
o "ENCODING_IDENTITY" (0): The identity encoding identifier is used
to indicate that no encoding is applied.
o "ENCODING_GZIP" (1): The gzip coding is an LZ77 coding with a 32
bit CRC that is commonly produced by the gzip file compression
program [RFC1952].
4. Security Considerations
Further to the Use of Compression in HTTP/2
([I-D.ietf-httpbis-http2], Section 10.6), intermediaries MUST NOT
apply compression to DATA frames, or alter the compression of
ENCODED_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 can only be
merged into a single compressed frame if they occupy the same
segment.
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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
([I-D.ietf-httpbis-http2], Section 11.2). The entries in the
following table are registered by this document.
+---------------------+------+-------------+
| Frame Type | Code | Section |
+---------------------+------+-------------+
| ACCEPT_ENCODED_DATA | TBD | Section 2.1 |
| ENCODED_DATA | TBD | Section 2.2 |
+---------------------+------+-------------+
5.2. HTTP/2 Error Code Registry Update
This document updates the "HTTP/2 Error Code" registry
([I-D.ietf-httpbis-http2], Section 11.4). The entries in the
following table are registered by this document.
+---------------------+------+----------------------+---------------+
| Name | Code | Description | Specification |
+---------------------+------+----------------------+---------------+
| DATA_ENCODING_ERROR | TBD | Invalid encoding | Section 2.3 |
| | | detected | |
+---------------------+------+----------------------+---------------+
5.3. HTTP/2 Encoding Schemes Registry
This document establishes a registry for encoding scheme codes. The
"HTTP/2 Encoding Schemes" registry manages an 8-bit space. The
"HTTP/2 Encoding Schemes" registry operates under either of the "IETF
Review" or "IESG Approval" policies [RFC5226] for values between 0x00
and 0xef, with values between 0xf0 and 0xff being reserved for
experimental use.
New entries in this registry require the following information:
o Frame Type: A name or label for the encoding scheme.
o Code: The 8-bit code assigned to the encoding scheme.
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o Specification: A reference to a specification that includes a
description of the encoding scheme.
An initial set of encoding scheme code registrations can be found in
Section 3.
6. Acknowledgements
Thanks to Keith Morgan for his advice, input, and editorial
contributions.
7. References
7.1. Normative References
[I-D.ietf-httpbis-http2]
Belshe, M., Peon, R., and M. Thomson, "Hypertext Transfer
Protocol version 2", draft-ietf-httpbis-http2-17 (work in
progress), February 2015.
[I-D.kerwin-http2-segments]
Kerwin, M., "HTTP/2 Segments", draft-kerwin-
http2-segments-02 (work in progress), February 2015.
[RFC1952] Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and G.
Randers-Pehrson, "GZIP file format specification version
4.3", RFC 1952, May 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
7.2. Informative References
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing", RFC 7230, June
2014.
Author's Address
Matthew Kerwin
Email: matthew@kerwin.net.au
URI: http://matthew.kerwin.net.au/
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