Internet DRAFT - draft-ruellan-reference-set-definition
draft-ruellan-reference-set-definition
HTTPbis Working Group H. Ruellan
Internet-Draft Canon CRF
Intended status: Informational J. Fujisawa
Expires: January 16, 2014 Canon, Inc.
R. Bellessort
Y. Fablet
Canon CRF
July 15, 2013
HTTP/2.0: Header Reference Set Definition
draft-ruellan-reference-set-definition-00
Abstract
This document describes modifications of the proposed HTTP/2.0 header
compression mechanism for the definition of the reference set used to
encode a set of headers.
Status of This Memo
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This Internet-Draft will expire on January 16, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Design Principles . . . . . . . . . . . . . . . . . . . . 3
2.2. Outline . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Integration Within Current Draft . . . . . . . . . . . . . . 3
4. Detailed Format . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Header Block Organization . . . . . . . . . . . . . . . . 4
4.2. Reference Set Definition . . . . . . . . . . . . . . . . 4
5. Empty Reference Set . . . . . . . . . . . . . . . . . . . . . 4
6. Open Questions . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Reference Set Removals . . . . . . . . . . . . . . . . . 5
6.2. Adapted Entries Insertion/Removal . . . . . . . . . . . . 6
6.3. Optimized Reference Set Definition . . . . . . . . . . . 6
6.4. Re-indexing of Already Indexed Headers . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 7
A.1. Default Reference Set Example . . . . . . . . . . . . . . 8
A.2. Empty Reference Set Example . . . . . . . . . . . . . . . 8
A.3. Specific Reference Set Example . . . . . . . . . . . . . 9
1. Introduction
In HTTP/2.0 [http2.0], HTTP headers are compressed according to the
HTTP/2.0 Headers Compression [header-compression] draft. This
compression format relies on the previous set of headers to improve
compression. Two consecutive sets of headers often comprise a
significant number of headers in common. Encoding the differences
between those two sets is generally more efficient than encoding the
set of headers itself.
The messages exchanged between a client and a server (or between a
server and a client) may however comprise different types of header
sets: GET requests and POST requests may have more differences than
similarities; "200 OK" responses and "304 Not Modified" responses may
be very different. In those cases, allowing the encoder to select a
specific reference set of headers allows reducing the size of encoded
headers.
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A new mechanism is proposed in this draft to allow the encoder
defining a reference set of headers directly from the header indexing
table.
2. Overview
2.1. Design Principles
This proposal builds on HTTP/2.0 Header Compression
[header-compression] to further improve the compactness of HTTP
headers representation. It reuses the notion of header group as
defined in Header Delta-Compression for HTTP/2.0 [delta2] in a more
limited way. First, NO additional indexing structure or storage
requirement is added to the existing mechanisms defined in HTTP/2.0
Header Compression [header-compression]. Second, the impact on
implementations, particularly on decoder side, should be as limited
as possible. This proposal may also bring processing improvements.
qdfsq
2.2. Outline
Using this proposal, when encoding a new set of headers, the encoder
indicates whether it relies on the default reference set (i.e. the
previous set of headers) or whether it defines a specific reference
set.
A specific reference set is represented as a start index SI and a
length L; the reference set is defined as the L headers occurring
from index SI in the header indexing table.
Once this specific reference set is defined, it is used in place of
the default reference set to encode the new set of headers: only the
differences between this specific reference set and the new set of
headers are represented.
3. Integration Within Current Draft
A new flag, REFERENCE_SET, is defined for the HEADERS frame. This
flag uses bit 5 (0x10) and determines whether the default reference
set or another one is used for the given Header Block.
The value of 0 indicates that the default reference set (i.e. the
previous set of headers) is used and the header block contains the
differences between the reference set and the set of headers
represented.
The value of 1 indicates that the header block starts by the
definition of a specific reference set, which is then followed by the
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differences between this specitic reference set and the set of
headers represented.
4. Detailed Format
4.1. Header Block Organization
A header block is organized in two parts:
o An optional reference set definition. This definition is present
only if the REFERENCE_SET flag is set.
o A list of difference between the reference set and the encoded set
of headers. The difference is represented as defined in HTTP/2.0
Header Compression [header-compression].
4.2. Reference Set Definition
A reference set is described on 2 bytes.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-------------------+-------------------------------------------+
| Length (5) | Start Index (11) |
+-------------------+-------------------------------------------+
Reference Set Definition
The fields defining a reference set are:
Length: The length of the reference set, encoded on 5 bits. This
length is comprised between 0 and 31.
Start Index: The index of the first entry of the reference set in
the header table. If the reference set length is strictly greater
than zero, the start index is encoded on 11 bits. If the length
is equal to zero, the start index value is zero and is encoded on
3 bits.
5. Empty Reference Set
If the reference set length is equal to zero, the reference set is
empty and the reference set definition is optimized to use only one
byte.
0 1 2 3 4 5 6 7
+-------------------+-----------+
| 0 | 0 |
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+-------------------+-----------+
Empty Reference Set Definition
This special case may be useful in a number of applications. First,
it can allow a more compact encoding of a header set. For instance,
measurements have shown that using the empty as the reference set is,
for a fair number of test samples, more efficient than using the
default reference set for HTTP responses.
Second, using the empty set allows reducing the coding complexity for
the encoder. This can be useful for server or intermediaries under a
heavy load. It could also be used by lightweight devices that prefer
using static encoding strategies.
6. Open Questions
6.1. Reference Set Removals
The definition of a specific reference set is based on a contiguous
set of header entries in the header indexing table. Removing a
header from the set is typically encoded using one or two bytes. It
may be beneficial to optimize the removal of a header in the case of
a specific reference set which would be constituted of several
contiguous sets of headers separated by a few other headers not
belonging to the set of headers to encode.
For instance, for a sixteen headers reference set, two bytes are
needed to define all the removals as an array of booleans. Two bytes
is also the cost for encoding one or two removals using HTTP/2.0
Header Compression [header-compression]. Once a reference set of
sixteen headers contains more than two headers that should be
removed, using an array of booleans is more efficient.
Early experiments show that this approach allows selecting larger
reference sets and brings noticeable compaction benefits. Additional
investigation should be done to understand how optimized indexing
table building would decrease the benefits.
Encoding the removals as an array of booleans has also the advantage
of making decoder implementations simpler. As removals are not
encoded in the same way as indexed values, there is no problem of
handling possible discrepancies between the index values in the
reference set and the index values in the header table.
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Optimized encoding of the header removal could also be envisioned for
the default reference set. In such a case, an ordering of the
default reference set headers should be defined. Such an ordering
could be based on the indexes of the headers.
Additional study should be conducted to evaluate whether this feature
should be a mandatory part of the specific reference set definition
or an option. Several ways to switch on/off this feature may be
envisioned: using a specific HEADERS frame flag; using a specific bit
within the reference set definition. Using a specific HEADERS frame
flag would be appropriate if using deletions for the default
reference set is envisioned.
6.2. Adapted Entries Insertion/Removal
Rules to handle incremental indexing and batch entry removal in case
of buffer overflow are defined in HTTP/2.0 Header Compression
[header-compression]. New rules may be defined when specific
reference sets are used.
Incrementally indexed entries could be inserted just after the last
entry in the header indexing table that is part of the specific
reference set. If entry removal is needed, removal could start with
the first entry after the last incrementally indexed header or just
after the last entry that is part of the specific reference set.
Additional study of the processing cost of these rules should be
envisioned.
6.3. Optimized Reference Set Definition
A single representation of the reference header set is currently
specified. This representation generally requires 2 bytes to be
encoded. Tests involving a single byte representation have been made
and have shown a small benefit. Based on additional tests, a more
optimized representation could be defined.
6.4. Re-indexing of Already Indexed Headers
If the same header happens in several reference sets, it may be
beneficial to insert such header at several places in the header
indexing table. While it makes the header indexing table size
bigger, it may further improve future reference set definitions.
This approach is already feasible by encoding a header literally
several times. It may be optimized by enabling to insert at a new
place a header that is already represented as an indexed header.
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7. Security Considerations
TBD.
8. IANA Considerations
This memo includes no request to IANA.
9. References
9.1. Normative References
[header-compression]
Ruellan, H. and R. Peon, "HTTP/2.0 Header Compression",
draft-ietf-httpbis-header-compression-01 (work in
progress), July 2013.
[http2.0] Belshe, M., Peon, R., Thomson, M., Ed., and A. Melnikov,
Ed., "Hypertext Transfer Protocol version 2.0", draft-
ietf-httpbis-http2-04 (work in progress), July 2013.
9.2. Informative References
[bohe] Snell, J., "HTTP/2.0 Discussion: Stored Header Encoding",
draft-snell-httpbis-bohe-11 (work in progress), July 2013.
[delta2] Peon, R., "Header Delta-Compression for HTTP/2.0", draft-
rpeon-httpbis-header-compression-03 (work in progress),
March 2013.
Appendix A. Example
After the transmission of a "200 OK" and "304 Not Modified"
responses, the header indexing table is as follows:
+---------+----------------+---------------------------+
| Index | Header Name | Header Value |
+---------+----------------+---------------------------+
| 0 | :status | 200 |
+---------+----------------+---------------------------+
| 1 | cache-control | public |
+---------+----------------+---------------------------+
| 2 | content-type | text/html |
+---------+----------------+---------------------------+
| 3 | content-length | 32 |
+---------+----------------+---------------------------+
| 4 | expires | 01 Apr 2014 00:00:00 GMT |
+---------+----------------+---------------------------+
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| 5 | :status | 304 |
+---------+----------------+---------------------------+
| 6 | expires | 01 Apr 2015 00:00:00 GMT |
+---------+----------------+---------------------------+
Initial Header Table
The current reference set is:
5 :status: 304
6 expires: 01 Apr 2015 00:00:00 GMT
Initial Header Table
The next response is a "200 OK" response. The following headers have
to be encoded:
:status: 200
cache-control: public
content-type: text/html
content-length: 1034
expires: 01 Apr 2016 00:00:00 GMT
Initial Header Table
A.1. Default Reference Set Example
The following stream illustrates the "200 OK" response header set
encoded as reference to the previous message:
0x85 (Indexed header, index = 5, removal)
0x86 (Indexed header, index = 6, removal)
0x80 (Indexed header, index = 0)
0x81 (Indexed header, index = 1)
0x82 (Indexed header, index = 2)
0x44 (Literal header with incremental indexing, name index = 3)
0x04 (Header value string length = 4)
1034
0x45 (Literal header with incremental indexing, name index = 4)
0x18 (Header value string length = 24)
01 Apr 2016 00:00:00 GMT
Initial Header Table
The size of the encoded header set is 37 bytes.
A.2. Empty Reference Set Example
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The following stream illustrates the "200 OK" response header set
encoded as reference to an empty reference set:
0x00 (Empty reference set)
0x80 (Indexed header, index = 0)
0x81 (indexed header, index = 1)
0x82 (indexed header, index = 2)
0x44 (Literal header with incremental indexing, name index = 3)
0x04 (Header value string length = 4)
1034
0x45 (Literal header with incremental indexing, name index = 4)
0x18 (Header value string length = 24)
01 Apr 2016 00:00:00 GMT
Initial Header Table
The size of the encoded header set is 36 bytes.
A.3. Specific Reference Set Example
The following stream illustrates the "200 OK" response header set
encoded as reference to a specific reference set (Length = 3, Start
Index = 0):
0x18 0x00 (specific reference set, length = 3, start index = 0)
0x44 (Literal header with incremental indexing, name index = 3)
0x04 (Header value string length = 4)
1034
0x45 (Literal header with incremental indexing, name index = 4)
0x18 (Header value string length = 24)
01 Apr 2016 00:00:00 GMT
Initial Header Table
The size of the encoded header set is 34 bytes.
Authors' Addresses
Herve Ruellan
Canon CRF
EMail: herve.ruellan@crf.canon.fr
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Jun Fujisawa
Canon, Inc.
3-30-2 Shimomaruko
Ohta-ku, Tokyo 146-8501
Japan
EMail: fujisawa.jun@canon.co.jp
Romain Bellessort
Canon CRF
EMail: romain.bellessort@crf.canon.fr
Youenn Fablet
Canon CRF
EMail: youenn.fablet@crf.canon.fr
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