draft-ietf-httpbis-header-compression-12.txt   draft-ietf-httpbis-header-compression-latest.txt 
HTTPbis Working Group R. Peon HTTPbis Working Group R. Peon
Internet-Draft Google, Inc Internet-Draft Google, Inc
Intended status: Standards Track H. Ruellan Intended status: Standards Track H. Ruellan
Expires: August 21, 2015 Canon CRF Expires: January 10, 2018 Canon CRF
February 17, 2015 July 9, 2017
HPACK - Header Compression for HTTP/2 HPACK: Header Compression for HTTP/2
draft-ietf-httpbis-header-compression-latest draft-ietf-httpbis-header-compression-latest
Abstract Abstract
This specification defines HPACK, a compression format for This specification defines HPACK, a compression format for
efficiently representing HTTP header fields, to be used in HTTP/2. efficiently representing HTTP header fields, to be used in HTTP/2.
Editorial Note (To be removed by RFC Editor) Editorial Note (To be removed by RFC Editor)
Discussion of this draft takes place on the HTTPBIS working group Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at mailing list (ietf-http-wg@w3.org), which is archived at
<https://lists.w3.org/Archives/Public/ietf-http-wg/>. <https://lists.w3.org/Archives/Public/ietf-http-wg/>.
Working Group information can be found at Working Group information can be found at <http://tools.ietf.org/wg/
<http://tools.ietf.org/wg/httpbis/>; that specific to HTTP/2 are at httpbis/>; that specific to HTTP/2 are at <http://http2.github.io/>.
<http://http2.github.io/>.
The changes in this draft are summarized in Appendix D.2.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 21, 2015. This Internet-Draft will expire on January 10, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Compression Process Overview . . . . . . . . . . . . . . . . . 7 2. Compression Process Overview . . . . . . . . . . . . . . . . 5
2.1. Header List Ordering . . . . . . . . . . . . . . . . . . . 7 2.1. Header List Ordering . . . . . . . . . . . . . . . . . . 5
2.2. Encoding and Decoding Contexts . . . . . . . . . . . . . . 7 2.2. Encoding and Decoding Contexts . . . . . . . . . . . . . 6
2.3. Indexing Tables . . . . . . . . . . . . . . . . . . . . . 7 2.3. Indexing Tables . . . . . . . . . . . . . . . . . . . . . 6
2.3.1. Static Table . . . . . . . . . . . . . . . . . . . . . 7 2.3.1. Static Table . . . . . . . . . . . . . . . . . . . . 6
2.3.2. Dynamic Table . . . . . . . . . . . . . . . . . . . . 7 2.3.2. Dynamic Table . . . . . . . . . . . . . . . . . . . . 6
2.3.3. Index Address Space . . . . . . . . . . . . . . . . . 8 2.3.3. Index Address Space . . . . . . . . . . . . . . . . . 7
2.4. Header Field Representation . . . . . . . . . . . . . . . 9 2.4. Header Field Representation . . . . . . . . . . . . . . . 7
3. Header Block Decoding . . . . . . . . . . . . . . . . . . . . 9 3. Header Block Decoding . . . . . . . . . . . . . . . . . . . . 8
3.1. Header Block Processing . . . . . . . . . . . . . . . . . 9 3.1. Header Block Processing . . . . . . . . . . . . . . . . . 8
3.2. Header Field Representation Processing . . . . . . . . . . 10 3.2. Header Field Representation Processing . . . . . . . . . 8
4. Dynamic Table Management . . . . . . . . . . . . . . . . . . . 10 4. Dynamic Table Management . . . . . . . . . . . . . . . . . . 9
4.1. Calculating Table Size . . . . . . . . . . . . . . . . . . 11 4.1. Calculating Table Size . . . . . . . . . . . . . . . . . 9
4.2. Maximum Table Size . . . . . . . . . . . . . . . . . . . . 11 4.2. Maximum Table Size . . . . . . . . . . . . . . . . . . . 9
4.3. Entry Eviction when Dynamic Table Size Changes . . . . . . 12 4.3. Entry Eviction When Dynamic Table Size Changes . . . . . 10
4.4. Entry Eviction when Adding New Entries . . . . . . . . . . 12 4.4. Entry Eviction When Adding New Entries . . . . . . . . . 10
5. Primitive Type Representations . . . . . . . . . . . . . . . . 12 5. Primitive Type Representations . . . . . . . . . . . . . . . 11
5.1. Integer Representation . . . . . . . . . . . . . . . . . . 12 5.1. Integer Representation . . . . . . . . . . . . . . . . . 11
5.2. String Literal Representation . . . . . . . . . . . . . . 14 5.2. String Literal Representation . . . . . . . . . . . . . . 13
6. Binary Format . . . . . . . . . . . . . . . . . . . . . . . . 15 6. Binary Format . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Indexed Header Field Representation . . . . . . . . . . . 15 6.1. Indexed Header Field Representation . . . . . . . . . . . 14
6.2. Literal Header Field Representation . . . . . . . . . . . 16 6.2. Literal Header Field Representation . . . . . . . . . . . 14
6.2.1. Literal Header Field with Incremental Indexing . . . . 16 6.2.1. Literal Header Field with Incremental Indexing . . . 14
6.2.2. Literal Header Field without Indexing . . . . . . . . 17 6.2.2. Literal Header Field without Indexing . . . . . . . . 15
6.2.3. Literal Header Field never Indexed . . . . . . . . . . 18 6.2.3. Literal Header Field Never Indexed . . . . . . . . . 16
6.3. Dynamic Table Size Update . . . . . . . . . . . . . . . . 19 6.3. Dynamic Table Size Update . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 20 7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7.1. Probing Dynamic Table State . . . . . . . . . . . . . . . 20 7.1. Probing Dynamic Table State . . . . . . . . . . . . . . . 18
7.1.1. Applicability to HPACK and HTTP . . . . . . . . . . . 21 7.1.1. Applicability to HPACK and HTTP . . . . . . . . . . . 19
7.1.2. Mitigation . . . . . . . . . . . . . . . . . . . . . . 21 7.1.2. Mitigation . . . . . . . . . . . . . . . . . . . . . 20
7.1.3. Never Indexed Literals . . . . . . . . . . . . . . . . 22 7.1.3. Never-Indexed Literals . . . . . . . . . . . . . . . 21
7.2. Static Huffman Encoding . . . . . . . . . . . . . . . . . 23 7.2. Static Huffman Encoding . . . . . . . . . . . . . . . . . 21
7.3. Memory Consumption . . . . . . . . . . . . . . . . . . . . 23 7.3. Memory Consumption . . . . . . . . . . . . . . . . . . . 21
7.4. Implementation Limits . . . . . . . . . . . . . . . . . . 24 7.4. Implementation Limits . . . . . . . . . . . . . . . . . . 22
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24 8.1. Normative References . . . . . . . . . . . . . . . . . . 22
10.1. Normative References . . . . . . . . . . . . . . . . . . . 24 8.2. Informative References . . . . . . . . . . . . . . . . . 23
10.2. Informative References . . . . . . . . . . . . . . . . . . 25 Appendix A. Static Table Definition . . . . . . . . . . . . . . 24
Appendix A. Static Table Definition . . . . . . . . . . . . . . . 25 Appendix B. Huffman Code . . . . . . . . . . . . . . . . . . . . 25
Appendix B. Huffman Code . . . . . . . . . . . . . . . . . . . . 27 Appendix C. Examples . . . . . . . . . . . . . . . . . . . . . . 31
Appendix C. Examples . . . . . . . . . . . . . . . . . . . . . . 33 C.1. Integer Representation Examples . . . . . . . . . . . . . 32
C.1. Integer Representation Examples . . . . . . . . . . . . . 33 C.1.1. Example 1: Encoding 10 Using a 5-Bit Prefix . . . . . 32
C.1.1. Example 1: Encoding 10 Using a 5-bit Prefix . . . . . 33 C.1.2. Example 2: Encoding 1337 Using a 5-Bit Prefix . . . . 32
C.1.2. Example 2: Encoding 1337 Using a 5-bit Prefix . . . . 34 C.1.3. Example 3: Encoding 42 Starting at an Octet Boundary 33
C.1.3. Example 3: Encoding 42 Starting at an Octet C.2. Header Field Representation Examples . . . . . . . . . . 33
Boundary . . . . . . . . . . . . . . . . . . . . . . . 35 C.2.1. Literal Header Field with Indexing . . . . . . . . . 33
C.2. Header Field Representation Examples . . . . . . . . . . . 35 C.2.2. Literal Header Field without Indexing . . . . . . . . 34
C.2.1. Literal Header Field with Indexing . . . . . . . . . . 35 C.2.3. Literal Header Field Never Indexed . . . . . . . . . 35
C.2.2. Literal Header Field without Indexing . . . . . . . . 36 C.2.4. Indexed Header Field . . . . . . . . . . . . . . . . 35
C.2.3. Literal Header Field never Indexed . . . . . . . . . . 37 C.3. Request Examples without Huffman Coding . . . . . . . . . 36
C.2.4. Indexed Header Field . . . . . . . . . . . . . . . . . 37 C.3.1. First Request . . . . . . . . . . . . . . . . . . . . 36
C.3. Request Examples without Huffman Coding . . . . . . . . . 38 C.3.2. Second Request . . . . . . . . . . . . . . . . . . . 37
C.3.1. First Request . . . . . . . . . . . . . . . . . . . . 38 C.3.3. Third Request . . . . . . . . . . . . . . . . . . . . 38
C.3.2. Second Request . . . . . . . . . . . . . . . . . . . . 39 C.4. Request Examples with Huffman Coding . . . . . . . . . . 39
C.3.3. Third Request . . . . . . . . . . . . . . . . . . . . 40 C.4.1. First Request . . . . . . . . . . . . . . . . . . . . 39
C.4. Request Examples with Huffman Coding . . . . . . . . . . . 41 C.4.2. Second Request . . . . . . . . . . . . . . . . . . . 40
C.4.1. First Request . . . . . . . . . . . . . . . . . . . . 42 C.4.3. Third Request . . . . . . . . . . . . . . . . . . . . 41
C.4.2. Second Request . . . . . . . . . . . . . . . . . . . . 43 C.5. Response Examples without Huffman Coding . . . . . . . . 43
C.4.3. Third Request . . . . . . . . . . . . . . . . . . . . 44 C.5.1. First Response . . . . . . . . . . . . . . . . . . . 43
C.5. Response Examples without Huffman Coding . . . . . . . . . 45 C.5.2. Second Response . . . . . . . . . . . . . . . . . . . 45
C.5.1. First Response . . . . . . . . . . . . . . . . . . . . 46 C.5.3. Third Response . . . . . . . . . . . . . . . . . . . 46
C.5.2. Second Response . . . . . . . . . . . . . . . . . . . 48 C.6. Response Examples with Huffman Coding . . . . . . . . . . 48
C.5.3. Third Response . . . . . . . . . . . . . . . . . . . . 49 C.6.1. First Response . . . . . . . . . . . . . . . . . . . 48
C.6. Response Examples with Huffman Coding . . . . . . . . . . 51 C.6.2. Second Response . . . . . . . . . . . . . . . . . . . 50
C.6.1. First Response . . . . . . . . . . . . . . . . . . . . 51 C.6.3. Third Response . . . . . . . . . . . . . . . . . . . 51
C.6.2. Second Response . . . . . . . . . . . . . . . . . . . 53 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 53
C.6.3. Third Response . . . . . . . . . . . . . . . . . . . . 54 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 54
Appendix D. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 56
D.1. Since draft-ietf-httpbis-header-compression-10 . . . . . . 56
D.2. Since draft-ietf-httpbis-header-compression-09 . . . . . . 57
D.3. Since draft-ietf-httpbis-header-compression-08 . . . . . . 57
D.4. Since draft-ietf-httpbis-header-compression-07 . . . . . . 57
D.5. Since draft-ietf-httpbis-header-compression-06 . . . . . . 58
D.6. Since draft-ietf-httpbis-header-compression-05 . . . . . . 58
D.7. Since draft-ietf-httpbis-header-compression-04 . . . . . . 58
D.8. Since draft-ietf-httpbis-header-compression-03 . . . . . . 59
D.9. Since draft-ietf-httpbis-header-compression-02 . . . . . . 59
D.10. Since draft-ietf-httpbis-header-compression-01 . . . . . . 59
D.11. Since draft-ietf-httpbis-header-compression-00 . . . . . . 59
1. Introduction 1. Introduction
In HTTP/1.1 (see [RFC7230]), header fields are not compressed. As In HTTP/1.1 (see [RFC7230]), header fields are not compressed. As
Web pages have grown to require dozens to hundreds of requests, the web pages have grown to require dozens to hundreds of requests, the
redundant header fields in these requests unnecessarily consume redundant header fields in these requests unnecessarily consume
bandwidth, measurably increasing latency. bandwidth, measurably increasing latency.
SPDY [SPDY] initially addressed this redundancy by compressing header SPDY [SPDY] initially addressed this redundancy by compressing header
fields using the DEFLATE [DEFLATE] format, which proved very fields using the DEFLATE [DEFLATE] format, which proved very
effective at efficiently representing the redundant header fields. effective at efficiently representing the redundant header fields.
However, that approach exposed a security risk as demonstrated by the However, that approach exposed a security risk as demonstrated by the
CRIME attack (see [CRIME]). CRIME (Compression Ratio Info-leak Made Easy) attack (see [CRIME]).
This specification defines HPACK, a new compressor for header fields This specification defines HPACK, a new compressor that eliminates
which eliminates redundant header fields, limits vulnerability to redundant header fields, limits vulnerability to known security
known security attacks, and which has a bounded memory requirement attacks, and has a bounded memory requirement for use in constrained
for use in constrained environments. Potential security concerns for environments. Potential security concerns for HPACK are described in
HPACK are described in Section 7. Section 7.
The HPACK format is intentionally simple and inflexible. Both The HPACK format is intentionally simple and inflexible. Both
characteristics reduce the risk of interoperability or security characteristics reduce the risk of interoperability or security
issues due to implementation error. No extensibility mechanisms are issues due to implementation error. No extensibility mechanisms are
defined; changes to the format are only possible by defining a defined; changes to the format are only possible by defining a
complete replacement. complete replacement.
1.1. Overview 1.1. Overview
The format defined in this specification treats a list of header The format defined in this specification treats a list of header
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Encoding is informed by header field tables that map header fields to Encoding is informed by header field tables that map header fields to
indexed values. These header field tables can be incrementally indexed values. These header field tables can be incrementally
updated as new header fields are encoded or decoded. updated as new header fields are encoded or decoded.
In the encoded form, a header field is represented either literally In the encoded form, a header field is represented either literally
or as a reference to a header field in one of the header field or as a reference to a header field in one of the header field
tables. Therefore, a list of header fields can be encoded using a tables. Therefore, a list of header fields can be encoded using a
mixture of references and literal values. mixture of references and literal values.
Literal values are either encoded directly or using a static Huffman Literal values are either encoded directly or use a static Huffman
code. code.
The encoder is responsible for deciding which header fields to insert The encoder is responsible for deciding which header fields to insert
as new entries in the header field tables. The decoder executes the as new entries in the header field tables. The decoder executes the
modifications to the header field tables prescribed by the encoder, modifications to the header field tables prescribed by the encoder,
reconstructing the list of header fields in the process. This reconstructing the list of header fields in the process. This
enables decoders to remain simple and interoperate with a wide enables decoders to remain simple and interoperate with a wide
variety of encoders. variety of encoders.
Examples illustrating the use of these different mechanisms to Examples illustrating the use of these different mechanisms to
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Header Field: A name-value pair. Both the name and value are Header Field: A name-value pair. Both the name and value are
treated as opaque sequences of octets. treated as opaque sequences of octets.
Dynamic Table: The dynamic table (see Section 2.3.2) is a table that Dynamic Table: The dynamic table (see Section 2.3.2) is a table that
associates stored header fields with index values. This table is associates stored header fields with index values. This table is
dynamic and specific to an encoding or decoding context. dynamic and specific to an encoding or decoding context.
Static Table: The static table (see Section 2.3.1) is a table that Static Table: The static table (see Section 2.3.1) is a table that
statically associates header fields that occur frequently with statically associates header fields that occur frequently with
index values. This table is ordered, read-only, always index values. This table is ordered, read-only, always
accessible, and may be shared amongst all encoding or decoding accessible, and it may be shared amongst all encoding or decoding
contexts. contexts.
Header List: A header list is an ordered collection of header fields Header List: A header list is an ordered collection of header fields
that are encoded jointly, and can contain duplicate header fields. that are encoded jointly and can contain duplicate header fields.
A complete list of header fields contained in an HTTP/2 header A complete list of header fields contained in an HTTP/2 header
block is a header list. block is a header list.
Header Field Representation: A header field can be represented in Header Field Representation: A header field can be represented in
encoded form either as a literal or as an index (see Section 2.4). encoded form either as a literal or as an index (see Section 2.4).
Header Block: An ordered list of header field representations which, Header Block: An ordered list of header field representations,
when decoded, yields a complete header list. which, when decoded, yields a complete header list.
2. Compression Process Overview 2. Compression Process Overview
This specification does not describe a specific algorithm for an This specification does not describe a specific algorithm for an
encoder. Instead, it defines precisely how a decoder is expected to encoder. Instead, it defines precisely how a decoder is expected to
operate, allowing encoders to produce any encoding that this operate, allowing encoders to produce any encoding that this
definition permits. definition permits.
2.1. Header List Ordering 2.1. Header List Ordering
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to their ordering in the header block. to their ordering in the header block.
2.2. Encoding and Decoding Contexts 2.2. Encoding and Decoding Contexts
To decompress header blocks, a decoder only needs to maintain a To decompress header blocks, a decoder only needs to maintain a
dynamic table (see Section 2.3.2) as a decoding context. No other dynamic table (see Section 2.3.2) as a decoding context. No other
dynamic state is needed. dynamic state is needed.
When used for bidirectional communication, such as in HTTP, the When used for bidirectional communication, such as in HTTP, the
encoding and decoding dynamic tables maintained by an endpoint are encoding and decoding dynamic tables maintained by an endpoint are
completely independent. I.e., the request and response dynamic completely independent, i.e., the request and response dynamic tables
tables are separate. are separate.
2.3. Indexing Tables 2.3. Indexing Tables
HPACK uses two tables for associating header fields to indexes. The HPACK uses two tables for associating header fields to indexes. The
static table (see Section 2.3.1) is predefined and contains common static table (see Section 2.3.1) is predefined and contains common
header fields (most of them with an empty value). The dynamic table header fields (most of them with an empty value). The dynamic table
(see Section 2.3.2) is dynamic and can be used by the encoder to (see Section 2.3.2) is dynamic and can be used by the encoder to
index header fields repeated in the encoded header lists. index header fields repeated in the encoded header lists.
These two tables are combined into a single address space for These two tables are combined into a single address space for
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A header block is the concatenation of header field representations. A header block is the concatenation of header field representations.
The different possible header field representations are described in The different possible header field representations are described in
Section 6. Section 6.
Once a header field is decoded and added to the reconstructed header Once a header field is decoded and added to the reconstructed header
list, the header field cannot be removed. A header field added to list, the header field cannot be removed. A header field added to
the header list can be safely passed to the application. the header list can be safely passed to the application.
By passing the resulting header fields to the application, a decoder By passing the resulting header fields to the application, a decoder
can be implemented with minimal transitory memory commitment in can be implemented with minimal transitory memory commitment in
addition to the dynamic table. addition to the memory required for the dynamic table.
3.2. Header Field Representation Processing 3.2. Header Field Representation Processing
The processing of a header block to obtain a header list is defined The processing of a header block to obtain a header list is defined
in this section. To ensure that the decoding will successfully in this section. To ensure that the decoding will successfully
produce a header list, a decoder MUST obey the following rules. produce a header list, a decoder MUST obey the following rules.
All the header field representations contained in a header block are All the header field representations contained in a header block are
processed in the order in which they appear, as specified below. processed in the order in which they appear, as specified below.
Details on the formatting of the various header field Details on the formatting of the various header field representations
representations, and some additional processing instructions are and some additional processing instructions are found in Section 6.
found in Section 6.
An _indexed representation_ entails the following actions: An _indexed representation_ entails the following actions:
o The header field corresponding to the referenced entry in either o The header field corresponding to the referenced entry in either
the static table or dynamic table is appended to the decoded the static table or dynamic table is appended to the decoded
header list. header list.
A _literal representation_ that is _not added_ to the dynamic table A _literal representation_ that is _not added_ to the dynamic table
entails the following action: entails the following action:
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4. Dynamic Table Management 4. Dynamic Table Management
To limit the memory requirements on the decoder side, the dynamic To limit the memory requirements on the decoder side, the dynamic
table is constrained in size. table is constrained in size.
4.1. Calculating Table Size 4.1. Calculating Table Size
The size of the dynamic table is the sum of the size of its entries. The size of the dynamic table is the sum of the size of its entries.
The size of an entry is the sum of its name's length in octets (as The size of an entry is the sum of its name's length in octets (as
defined in Section 5.2), its value's length in octets, plus 32. defined in Section 5.2), its value's length in octets, and 32.
The size of an entry is calculated using the length of its name and The size of an entry is calculated using the length of its name and
value without any Huffman encoding applied. value without any Huffman encoding applied.
Note: The additional 32 octets account for an estimated overhead Note: The additional 32 octets account for an estimated overhead
associated with an entry. For example, an entry structure using associated with an entry. For example, an entry structure using
two 64-bit pointers to reference the name and the value of the two 64-bit pointers to reference the name and the value of the
entry, and two 64-bit integers for counting the number of entry and two 64-bit integers for counting the number of
references to the name and value would have 32 octets of overhead. references to the name and value would have 32 octets of overhead.
4.2. Maximum Table Size 4.2. Maximum Table Size
Protocols that use HPACK determine the maximum size that the encoder Protocols that use HPACK determine the maximum size that the encoder
is permitted to use for the dynamic table. In HTTP/2, this value is is permitted to use for the dynamic table. In HTTP/2, this value is
determined by the SETTINGS_HEADER_TABLE_SIZE setting (see Section determined by the SETTINGS_HEADER_TABLE_SIZE setting (see
6.5.2 of [HTTP2]). Section 6.5.2 of [HTTP2]).
An encoder can choose to use less capacity than this maximum size An encoder can choose to use less capacity than this maximum size
(see Section 6.3), but the chosen size MUST stay lower than or equal (see Section 6.3), but the chosen size MUST stay lower than or equal
to the maximum set by the protocol. to the maximum set by the protocol.
A change in the maximum size of the dynamic table is signaled via an A change in the maximum size of the dynamic table is signaled via a
encoding context update (see Section 6.3). This encoding context dynamic table size update (see Section 6.3). This dynamic table size
update MUST occur at the beginning of the first header block update MUST occur at the beginning of the first header block
following the change to the dynamic table size. In HTTP/2, this following the change to the dynamic table size. In HTTP/2, this
follows a settings acknowledgment (see Section 6.5.3 of [HTTP2]). follows a settings acknowledgment (see Section 6.5.3 of [HTTP2]).
Multiple updates to the maximum table size can occur between the Multiple updates to the maximum table size can occur between the
transmission of two header blocks. In the case that this size is transmission of two header blocks. In the case that this size is
changed more than once in this interval, the smallest maximum table changed more than once in this interval, the smallest maximum table
size that occurs in that interval MUST be signaled in an encoding size that occurs in that interval MUST be signaled in a dynamic table
context update. The final maximum size is always signaled, resulting size update. The final maximum size is always signaled, resulting in
in at most two encoding context updates. This ensures that the at most two dynamic table size updates. This ensures that the
decoder is able to perform eviction based on reductions in dynamic decoder is able to perform eviction based on reductions in dynamic
table size (see Section 4.3). table size (see Section 4.3).
This mechanism can be used to completely clear entries from the This mechanism can be used to completely clear entries from the
dynamic table by setting a maximum size of 0, which can subsequently dynamic table by setting a maximum size of 0, which can subsequently
be restored. be restored.
4.3. Entry Eviction when Dynamic Table Size Changes 4.3. Entry Eviction When Dynamic Table Size Changes
Whenever the maximum size for the dynamic table is reduced, entries Whenever the maximum size for the dynamic table is reduced, entries
are evicted from the end of the dynamic table until the size of the are evicted from the end of the dynamic table until the size of the
dynamic table is less than or equal to the maximum size. dynamic table is less than or equal to the maximum size.
4.4. Entry Eviction when Adding New Entries 4.4. Entry Eviction When Adding New Entries
Before a new entry is added to the dynamic table, entries are evicted Before a new entry is added to the dynamic table, entries are evicted
from the end of the dynamic table until the size of the dynamic table from the end of the dynamic table until the size of the dynamic table
is less than or equal to (maximum size - new entry size), or until is less than or equal to (maximum size - new entry size) or until the
the table is empty. table is empty.
If the size of the new entry is less than or equal to the maximum If the size of the new entry is less than or equal to the maximum
size, that entry is added to the table. It is not an error to size, that entry is added to the table. It is not an error to
attempt to add an entry that is larger than the maximum size; an attempt to add an entry that is larger than the maximum size; an
attempt to add an entry larger than the maximum size causes the table attempt to add an entry larger than the maximum size causes the table
to be emptied of all existing entries, and results in an empty table. to be emptied of all existing entries and results in an empty table.
A new entry can reference the name of an entry in the dynamic table A new entry can reference the name of an entry in the dynamic table
that will be evicted when adding this new entry into the dynamic that will be evicted when adding this new entry into the dynamic
table. Implementations are cautioned to avoid deleting the table. Implementations are cautioned to avoid deleting the
referenced name if the referenced entry is evicted from the dynamic referenced name if the referenced entry is evicted from the dynamic
table prior to inserting the new entry. table prior to inserting the new entry.
5. Primitive Type Representations 5. Primitive Type Representations
HPACK encoding uses two primitive types: unsigned variable length HPACK encoding uses two primitive types: unsigned variable-length
integers, and strings of octets. integers and strings of octets.
5.1. Integer Representation 5.1. Integer Representation
Integers are used to represent name indexes, header field indexes or Integers are used to represent name indexes, header field indexes, or
string lengths. An integer representation can start anywhere within string lengths. An integer representation can start anywhere within
an octet. To allow for optimized processing, an integer an octet. To allow for optimized processing, an integer
representation always finishes at the end of an octet. representation always finishes at the end of an octet.
An integer is represented in two parts: a prefix that fills the An integer is represented in two parts: a prefix that fills the
current octet and an optional list of octets that are used if the current octet and an optional list of octets that are used if the
integer value does not fit within the prefix. The number of bits of integer value does not fit within the prefix. The number of bits of
the prefix (called N) is a parameter of the integer representation. the prefix (called N) is a parameter of the integer representation.
If the integer value is small enough, i.e., strictly less than 2^N-1, If the integer value is small enough, i.e., strictly less than 2^N-1,
it is encoded within the N-bit prefix. it is encoded within the N-bit prefix.
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
| ? | ? | ? | Value | | ? | ? | ? | Value |
+---+---+---+-------------------+ +---+---+---+-------------------+
Figure 2: Integer Value Encoded within the Prefix (shown for N = 5) Figure 2: Integer Value Encoded within the Prefix (Shown for N = 5)
Otherwise, all the bits of the prefix are set to 1 and the value, Otherwise, all the bits of the prefix are set to 1, and the value,
decreased by 2^N-1, is encoded using a list of one or more octets. decreased by 2^N-1, is encoded using a list of one or more octets.
The most significant bit of each octet is used as a continuation The most significant bit of each octet is used as a continuation
flag: its value is set to 1 except for the last octet in the list. flag: its value is set to 1 except for the last octet in the list.
The remaining bits of the octets are used to encode the decreased The remaining bits of the octets are used to encode the decreased
value. value.
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
| ? | ? | ? | 1 1 1 1 1 | | ? | ? | ? | 1 1 1 1 1 |
+---+---+---+-------------------+ +---+---+---+-------------------+
| 1 | Value-(2^N-1) LSB | | 1 | Value-(2^N-1) LSB |
+---+---------------------------+ +---+---------------------------+
... ...
+---+---------------------------+ +---+---------------------------+
| 0 | Value-(2^N-1) MSB | | 0 | Value-(2^N-1) MSB |
+---+---------------------------+ +---+---------------------------+
Figure 3: Integer Value Encoded after the Prefix (shown for N = 5) Figure 3: Integer Value Encoded after the Prefix (Shown for N = 5)
Decoding the integer value from the list of octets starts by Decoding the integer value from the list of octets starts by
reversing the order of the octets in the list. Then, for each octet, reversing the order of the octets in the list. Then, for each octet,
its most significant bit is removed. The remaining bits of the its most significant bit is removed. The remaining bits of the
octets are concatenated and the resulting value is increased by 2^N-1 octets are concatenated, and the resulting value is increased by
to obtain the integer value. 2^N-1 to obtain the integer value.
The prefix size, N, is always between 1 and 8 bits. An integer The prefix size, N, is always between 1 and 8 bits. An integer
starting at an octet-boundary will have an 8-bit prefix. starting at an octet boundary will have an 8-bit prefix.
Pseudo-code to represent an integer I is as follows: Pseudocode to represent an integer I is as follows:
if I < 2^N - 1, encode I on N bits if I < 2^N - 1, encode I on N bits
else else
encode (2^N - 1) on N bits encode (2^N - 1) on N bits
I = I - (2^N - 1) I = I - (2^N - 1)
while I >= 128 while I >= 128
encode (I % 128 + 128) on 8 bits encode (I % 128 + 128) on 8 bits
I = I / 128 I = I / 128
encode I on 8 bits encode I on 8 bits
Pseudo-code to decode an integer I is as follows: Pseudocode to decode an integer I is as follows:
decode I from the next N bits decode I from the next N bits
if I < 2^N - 1, return I if I < 2^N - 1, return I
else else
M = 0 M = 0
repeat repeat
B = next octet B = next octet
I = I + (B & 127) * 2^M I = I + (B & 127) * 2^M
M = M + 7 M = M + 7
while B & 128 == 128 while B & 128 == 128
return I return I
Examples illustrating the encoding of integers are available in Examples illustrating the encoding of integers are available in
Appendix C.1. Appendix C.1.
This integer representation allows for values of indefinite size. It This integer representation allows for values of indefinite size. It
is also possible for an encoder to send a large number of zero is also possible for an encoder to send a large number of zero
values, which can waste octets and could be used to overflow integer values, which can waste octets and could be used to overflow integer
values. Integer encodings that exceed an implementation limits - in values. Integer encodings that exceed implementation limits -- in
value or octet length - MUST be treated as a decoding error. value or octet length -- MUST be treated as decoding errors.
Different limits can be set for each of the different uses of Different limits can be set for each of the different uses of
integers, based on implementation constraints. integers, based on implementation constraints.
5.2. String Literal Representation 5.2. String Literal Representation
Header field names and header field values can be represented as Header field names and header field values can be represented as
literal strings. A literal string is encoded as a sequence of string literals. A string literal is encoded as a sequence of
octets, either by directly encoding the literal string's octets, or octets, either by directly encoding the string literal's octets or by
by using a Huffman code (see [HUFFMAN]). using a Huffman code (see [HUFFMAN]).
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
| H | String Length (7+) | | H | String Length (7+) |
+---+---------------------------+ +---+---------------------------+
| String Data (Length octets) | | String Data (Length octets) |
+-------------------------------+ +-------------------------------+
Figure 4: String Literal Representation Figure 4: String Literal Representation
A literal string representation contains the following fields: A string literal representation contains the following fields:
H: A one bit flag, H, indicating whether or not the octets of the H: A one-bit flag, H, indicating whether or not the octets of the
string are Huffman encoded. string are Huffman encoded.
String Length: The number of octets used to encode the string String Length: The number of octets used to encode the string
literal, encoded as an integer with 7-bit prefix (see literal, encoded as an integer with a 7-bit prefix (see
Section 5.1). Section 5.1).
String Data: The encoded data of the string literal. If H is '0', String Data: The encoded data of the string literal. If H is '0',
then the encoded data is the raw octets of the string literal. If then the encoded data is the raw octets of the string literal. If
H is '1', then the encoded data is the Huffman encoding of the H is '1', then the encoded data is the Huffman encoding of the
string literal. string literal.
String literals which use Huffman encoding are encoded with the String literals that use Huffman encoding are encoded with the
Huffman code defined in Appendix B (see examples for requests in Huffman code defined in Appendix B (see examples for requests in
Appendix C.4 and for responses in Appendix C.6). The encoded data is Appendix C.4 and for responses in Appendix C.6). The encoded data is
the bitwise concatenation of the codes corresponding to each octet of the bitwise concatenation of the codes corresponding to each octet of
the string literal. the string literal.
As the Huffman encoded data doesn't always end at an octet boundary, As the Huffman-encoded data doesn't always end at an octet boundary,
some padding is inserted after it, up to the next octet boundary. To some padding is inserted after it, up to the next octet boundary. To
prevent this padding to be misinterpreted as part of the string prevent this padding from being misinterpreted as part of the string
literal, the most significant bits of the code corresponding to the literal, the most significant bits of the code corresponding to the
EOS (end-of-string) symbol are used. EOS (end-of-string) symbol are used.
Upon decoding, an incomplete code at the end of the encoded data is Upon decoding, an incomplete code at the end of the encoded data is
to be considered as padding and discarded. A padding strictly longer to be considered as padding and discarded. A padding strictly longer
than 7 bits MUST be treated as a decoding error. A padding not than 7 bits MUST be treated as a decoding error. A padding not
corresponding to the most significant bits of the code for the EOS corresponding to the most significant bits of the code for the EOS
symbol MUST be treated as a decoding error. A Huffman encoded string symbol MUST be treated as a decoding error. A Huffman-encoded string
literal containing the EOS symbol MUST be treated as a decoding literal containing the EOS symbol MUST be treated as a decoding
error. error.
6. Binary Format 6. Binary Format
This section describes the detailed format of each of the different This section describes the detailed format of each of the different
header field representations, plus the encoding context update header field representations and the dynamic table size update
instruction. instruction.
6.1. Indexed Header Field Representation 6.1. Indexed Header Field Representation
An indexed header field representation identifies an entry in either An indexed header field representation identifies an entry in either
the static table or the dynamic table (see Section 2.3). the static table or the dynamic table (see Section 2.3).
An indexed header field representation causes a header field to be An indexed header field representation causes a header field to be
added to the decoded header list, as described in Section 3.2. added to the decoded header list, as described in Section 3.2.
skipping to change at page 16, line 16 skipping to change at page 14, line 38
An indexed header field starts with the '1' 1-bit pattern, followed An indexed header field starts with the '1' 1-bit pattern, followed
by the index of the matching header field, represented as an integer by the index of the matching header field, represented as an integer
with a 7-bit prefix (see Section 5.1). with a 7-bit prefix (see Section 5.1).
The index value of 0 is not used. It MUST be treated as a decoding The index value of 0 is not used. It MUST be treated as a decoding
error if found in an indexed header field representation. error if found in an indexed header field representation.
6.2. Literal Header Field Representation 6.2. Literal Header Field Representation
A literal header field representation contains a literal header field A literal header field representation contains a literal header field
value. Header field names are either provided as a literal or by value. Header field names are provided either as a literal or by
reference to an existing table entry, either from the static table or reference to an existing table entry, either from the static table or
the dynamic table (see Section 2.3). the dynamic table (see Section 2.3).
This specification defines three forms of literal header field This specification defines three forms of literal header field
representations; with indexing, without indexing, and never indexed. representations: with indexing, without indexing, and never indexed.
6.2.1. Literal Header Field with Incremental Indexing 6.2.1. Literal Header Field with Incremental Indexing
A literal header field with incremental indexing representation A literal header field with incremental indexing representation
results in appending a header field to the decoded header list and results in appending a header field to the decoded header list and
inserting it as a new entry into the dynamic table. inserting it as a new entry into the dynamic table.
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
| 0 | 1 | Index (6+) | | 0 | 1 | Index (6+) |
+---+---+-----------------------+ +---+---+-----------------------+
| H | Value Length (7+) | | H | Value Length (7+) |
+---+---------------------------+ +---+---------------------------+
| Value String (Length octets) | | Value String (Length octets) |
+-------------------------------+ +-------------------------------+
Figure 6: Literal Header Field with Incremental Indexing - Indexed Figure 6: Literal Header Field with Incremental Indexing -- Indexed
Name Name
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
| 0 | 1 | 0 | | 0 | 1 | 0 |
+---+---+-----------------------+ +---+---+-----------------------+
| H | Name Length (7+) | | H | Name Length (7+) |
+---+---------------------------+ +---+---------------------------+
| Name String (Length octets) | | Name String (Length octets) |
+---+---------------------------+ +---+---------------------------+
| H | Value Length (7+) | | H | Value Length (7+) |
+---+---------------------------+ +---+---------------------------+
| Value String (Length octets) | | Value String (Length octets) |
+-------------------------------+ +-------------------------------+
Figure 7: Literal Header Field with Incremental Indexing - New Name Figure 7: Literal Header Field with Incremental Indexing -- New Name
A literal header field with incremental indexing representation A literal header field with incremental indexing representation
starts with the '01' 2-bit pattern. starts with the '01' 2-bit pattern.
If the header field name matches the header field name of an entry If the header field name matches the header field name of an entry
stored in the static table or the dynamic table, the header field stored in the static table or the dynamic table, the header field
name can be represented using the index of that entry. In this case, name can be represented using the index of that entry. In this case,
the index of the entry is represented as an integer with a 6-bit the index of the entry is represented as an integer with a 6-bit
prefix (see Section 5.1). This value is always non-zero. prefix (see Section 5.1). This value is always non-zero.
Otherwise, the header field name is represented as a literal string Otherwise, the header field name is represented as a string literal
(see Section 5.2). A value 0 is used in place of the 6-bit index, (see Section 5.2). A value 0 is used in place of the 6-bit index,
followed by the header field name. followed by the header field name.
Either form of header field name representation is followed by the Either form of header field name representation is followed by the
header field value represented as a literal string (see Section 5.2). header field value represented as a string literal (see Section 5.2).
6.2.2. Literal Header Field without Indexing 6.2.2. Literal Header Field without Indexing
A literal header field without indexing representation results in A literal header field without indexing representation results in
appending a header field to the decoded header list without altering appending a header field to the decoded header list without altering
the dynamic table. the dynamic table.
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | Index (4+) | | 0 | 0 | 0 | 0 | Index (4+) |
+---+---+-----------------------+ +---+---+-----------------------+
| H | Value Length (7+) | | H | Value Length (7+) |
+---+---------------------------+ +---+---------------------------+
| Value String (Length octets) | | Value String (Length octets) |
+-------------------------------+ +-------------------------------+
Figure 8: Literal Header Field without Indexing - Indexed Name Figure 8: Literal Header Field without Indexing -- Indexed Name
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | 0 | | 0 | 0 | 0 | 0 | 0 |
+---+---+-----------------------+ +---+---+-----------------------+
| H | Name Length (7+) | | H | Name Length (7+) |
+---+---------------------------+ +---+---------------------------+
| Name String (Length octets) | | Name String (Length octets) |
+---+---------------------------+ +---+---------------------------+
| H | Value Length (7+) | | H | Value Length (7+) |
+---+---------------------------+ +---+---------------------------+
| Value String (Length octets) | | Value String (Length octets) |
+-------------------------------+ +-------------------------------+
Figure 9: Literal Header Field without Indexing - New Name Figure 9: Literal Header Field without Indexing -- New Name
A literal header field without indexing representation starts with A literal header field without indexing representation starts with
the '0000' 4-bit pattern. the '0000' 4-bit pattern.
If the header field name matches the header field name of an entry If the header field name matches the header field name of an entry
stored in the static table or the dynamic table, the header field stored in the static table or the dynamic table, the header field
name can be represented using the index of that entry. In this case, name can be represented using the index of that entry. In this case,
the index of the entry is represented as an integer with a 4-bit the index of the entry is represented as an integer with a 4-bit
prefix (see Section 5.1). This value is always non-zero. prefix (see Section 5.1). This value is always non-zero.
Otherwise, the header field name is represented as a literal string Otherwise, the header field name is represented as a string literal
(see Section 5.2). A value 0 is used in place of the 4-bit index, (see Section 5.2). A value 0 is used in place of the 4-bit index,
followed by the header field name. followed by the header field name.
Either form of header field name representation is followed by the Either form of header field name representation is followed by the
header field value represented as a literal string (see Section 5.2). header field value represented as a string literal (see Section 5.2).
6.2.3. Literal Header Field never Indexed 6.2.3. Literal Header Field Never Indexed
A literal header field never indexed representation results in A literal header field never-indexed representation results in
appending a header field to the decoded header list without altering appending a header field to the decoded header list without altering
the dynamic table. Intermediaries MUST use the same representation the dynamic table. Intermediaries MUST use the same representation
for encoding this header field. for encoding this header field.
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 1 | Index (4+) | | 0 | 0 | 0 | 1 | Index (4+) |
+---+---+-----------------------+ +---+---+-----------------------+
| H | Value Length (7+) | | H | Value Length (7+) |
+---+---------------------------+ +---+---------------------------+
| Value String (Length octets) | | Value String (Length octets) |
+-------------------------------+ +-------------------------------+
Figure 10: Literal Header Field never Indexed - Indexed Name Figure 10: Literal Header Field Never Indexed -- Indexed Name
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 1 | 0 | | 0 | 0 | 0 | 1 | 0 |
+---+---+-----------------------+ +---+---+-----------------------+
| H | Name Length (7+) | | H | Name Length (7+) |
+---+---------------------------+ +---+---------------------------+
| Name String (Length octets) | | Name String (Length octets) |
+---+---------------------------+ +---+---------------------------+
| H | Value Length (7+) | | H | Value Length (7+) |
+---+---------------------------+ +---+---------------------------+
| Value String (Length octets) | | Value String (Length octets) |
+-------------------------------+ +-------------------------------+
Figure 11: Literal Header Field never Indexed - New Name Figure 11: Literal Header Field Never Indexed -- New Name
A literal header field never indexed representation starts with the A literal header field never-indexed representation starts with the
'0001' 4-bit pattern. '0001' 4-bit pattern.
When a header field is represented as a literal header field never When a header field is represented as a literal header field never
indexed, it MUST always be encoded with this specific literal indexed, it MUST always be encoded with this specific literal
representation. In particular, when a peer sends a header field that representation. In particular, when a peer sends a header field that
it received represented as a literal header field never indexed, it it received represented as a literal header field never indexed, it
MUST use the same representation to forward this header field. MUST use the same representation to forward this header field.
This representation is intended for protecting header field values This representation is intended for protecting header field values
that are not to be put at risk by compressing them (see Section 7.1 that are not to be put at risk by compressing them (see Section 7.1
skipping to change at page 19, line 52 skipping to change at page 18, line 16
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | Max size (5+) | | 0 | 0 | 1 | Max size (5+) |
+---+---------------------------+ +---+---------------------------+
Figure 12: Maximum Dynamic Table Size Change Figure 12: Maximum Dynamic Table Size Change
A dynamic table size update starts with the '001' 3-bit pattern, A dynamic table size update starts with the '001' 3-bit pattern,
followed by the new maximum size, represented as an integer with a followed by the new maximum size, represented as an integer with a
5-bit prefix (see Section 5.1). 5-bit prefix (see Section 5.1).
The new maximum size MUST be lower than or equal to the last value of The new maximum size MUST be lower than or equal to the limit
the maximum size of the dynamic table. A value that exceeds this determined by the protocol using HPACK. A value that exceeds this
limit MUST be treated as a decoding error. In HTTP/2, this limit is limit MUST be treated as a decoding error. In HTTP/2, this limit is
the last value of the SETTINGS_HEADER_TABLE_SIZE parameter (see the last value of the SETTINGS_HEADER_TABLE_SIZE parameter (see
Section 6.5.2 of [HTTP2]) received from the decoder and acknowledged Section 6.5.2 of [HTTP2]) received from the decoder and acknowledged
by the encoder (see Section 6.5.3 of [HTTP2]). by the encoder (see Section 6.5.3 of [HTTP2]).
Reducing the maximum size of the dynamic table can cause entries to Reducing the maximum size of the dynamic table can cause entries to
be evicted (see Section 4.3). be evicted (see Section 4.3).
7. Security Considerations 7. Security Considerations
skipping to change at page 20, line 40 skipping to change at page 19, line 5
requests or responses. requests or responses.
The compression context used to encode header fields can be probed by The compression context used to encode header fields can be probed by
an attacker who can both define header fields to be encoded and an attacker who can both define header fields to be encoded and
transmitted and observe the length of those fields once they are transmitted and observe the length of those fields once they are
encoded. When an attacker can do both, they can adaptively modify encoded. When an attacker can do both, they can adaptively modify
requests in order to confirm guesses about the dynamic table state. requests in order to confirm guesses about the dynamic table state.
If a guess is compressed into a shorter length, the attacker can If a guess is compressed into a shorter length, the attacker can
observe the encoded length and infer that the guess was correct. observe the encoded length and infer that the guess was correct.
This is possible even over the Transport Layer Security Protocol This is possible even over the Transport Layer Security (TLS)
(TLS, see [TLS12]), because while TLS provides confidentiality protocol (see [TLS12]), because while TLS provides confidentiality
protection for content, it only provides a limited amount of protection for content, it only provides a limited amount of
protection for the length of that content. protection for the length of that content.
Note: Padding schemes only provide limited protection against an Note: Padding schemes only provide limited protection against an
attacker with these capabilities, potentially only forcing an attacker with these capabilities, potentially only forcing an
increased number of guesses to learn the length associated with a increased number of guesses to learn the length associated with a
given guess. Padding schemes also work directly against given guess. Padding schemes also work directly against
compression by increasing the number of bits that are transmitted. compression by increasing the number of bits that are transmitted.
Attacks like CRIME [CRIME] demonstrated the existence of these Attacks like CRIME [CRIME] demonstrated the existence of these
general attacker capabilities. The specific attack exploited the general attacker capabilities. The specific attack exploited the
fact that DEFLATE [DEFLATE] removes redundancy based on prefix fact that DEFLATE [DEFLATE] removes redundancy based on prefix
matching. This permitted the attacker to confirm guesses a character matching. This permitted the attacker to confirm guesses a character
at a time, reducing an exponential-time attack into a linear-time at a time, reducing an exponential-time attack into a linear-time
attack. attack.
7.1.1. Applicability to HPACK and HTTP 7.1.1. Applicability to HPACK and HTTP
HPACK mitigates but does not completely prevent attacks modeled on HPACK mitigates but does not completely prevent attacks modeled on
CRIME [CRIME] by forcing a guess to match an entire header field CRIME [CRIME] by forcing a guess to match an entire header field
value, rather than individual characters. An attacker can only learn value rather than individual characters. Attackers can only learn
whether a guess is correct or not, so is reduced to a brute force whether a guess is correct or not, so they are reduced to brute-force
guess for the header field values. guesses for the header field values.
The viability of recovering specific header field values therefore The viability of recovering specific header field values therefore
depends on the entropy of values. As a result, values with high depends on the entropy of values. As a result, values with high
entropy are unlikely to be recovered successfully. However, values entropy are unlikely to be recovered successfully. However, values
with low entropy remain vulnerable. with low entropy remain vulnerable.
Attacks of this nature are possible any time that two mutually Attacks of this nature are possible any time that two mutually
distrustful entities control requests or responses that are placed distrustful entities control requests or responses that are placed
onto a single HTTP/2 connection. If the shared HPACK compressor onto a single HTTP/2 connection. If the shared HPACK compressor
permits one entity to add entries to the dynamic table, and the other permits one entity to add entries to the dynamic table and the other
to access those entries, then the state of the table can be learned. to access those entries, then the state of the table can be learned.
Having requests or responses from mutually distrustful entities Having requests or responses from mutually distrustful entities
occurs when an intermediary either: occurs when an intermediary either:
o sends requests from multiple clients on a single connection toward o sends requests from multiple clients on a single connection toward
an origin server, or an origin server, or
o takes responses from multiple origin servers and places them on a o takes responses from multiple origin servers and places them on a
shared connection toward a client. shared connection toward a client.
skipping to change at page 22, line 19 skipping to change at page 20, line 34
entity that created a particular value can extract that value. entity that created a particular value can extract that value.
To improve compression performance of this option, certain entries To improve compression performance of this option, certain entries
might be tagged as being public. For example, a web browser might might be tagged as being public. For example, a web browser might
make the values of the Accept-Encoding header field available in all make the values of the Accept-Encoding header field available in all
requests. requests.
An encoder without good knowledge of the provenance of header fields An encoder without good knowledge of the provenance of header fields
might instead introduce a penalty for a header field with many might instead introduce a penalty for a header field with many
different values, such that a large number of attempts to guess a different values, such that a large number of attempts to guess a
header field value results in the header field no more being compared header field value results in the header field no longer being
to the dynamic table entries in future messages, effectively compared to the dynamic table entries in future messages, effectively
preventing further guesses. preventing further guesses.
Note: Simply removing entries corresponding to the header field from Note: Simply removing entries corresponding to the header field
the dynamic table can be ineffectual if the attacker has a from the dynamic table can be ineffectual if the attacker has a
reliable way of causing values to be reinstalled. For example, a reliable way of causing values to be reinstalled. For example, a
request to load an image in a web browser typically includes the request to load an image in a web browser typically includes the
Cookie header field (a potentially highly valued target for this Cookie header field (a potentially highly valued target for this
sort of attack), and web sites can easily force an image to be sort of attack), and web sites can easily force an image to be
loaded, thereby refreshing the entry in the dynamic table. loaded, thereby refreshing the entry in the dynamic table.
This response might be made inversely proportional to the length of This response might be made inversely proportional to the length of
the header field value. Marking a header field as not using the the header field value. Marking a header field as not using the
dynamic table any more might occur for shorter values more quickly or dynamic table anymore might occur for shorter values more quickly or
with higher probability than for longer values. with higher probability than for longer values.
7.1.3. Never Indexed Literals 7.1.3. Never-Indexed Literals
Implementations can also choose to protect sensitive header fields by Implementations can also choose to protect sensitive header fields by
not compressing them and instead encoding their value as literals. not compressing them and instead encoding their value as literals.
Refusing to generate an indexed representation for a header field is Refusing to generate an indexed representation for a header field is
only effective if compression is avoided on all hops. The never only effective if compression is avoided on all hops. The never-
indexed literal (see Section 6.2.3) can be used to signal to indexed literal (see Section 6.2.3) can be used to signal to
intermediaries that a particular value was intentionally sent as a intermediaries that a particular value was intentionally sent as a
literal. literal.
An intermediary MUST NOT re-encode a value that uses the never An intermediary MUST NOT re-encode a value that uses the never-
indexed literal representation with another representation that would indexed literal representation with another representation that would
index it. If HPACK is used for re-encoding, the never indexed index it. If HPACK is used for re-encoding, the never-indexed
literal representation MUST be used. literal representation MUST be used.
The choice to use a never indexed literal representation for a header The choice to use a never-indexed literal representation for a header
field depends on several factors. Since HPACK doesn't protect field depends on several factors. Since HPACK doesn't protect
against guessing an entire header field value, short or low-entropy against guessing an entire header field value, short or low-entropy
values are more readily recovered by an adversary. Therefore, an values are more readily recovered by an adversary. Therefore, an
encoder might choose not to index values with low entropy. encoder might choose not to index values with low entropy.
An encoder might also choose not to index values for header fields An encoder might also choose not to index values for header fields
that are considered to be highly valuable or sensitive to recovery, that are considered to be highly valuable or sensitive to recovery,
such as the Cookie or Authorization header fields. such as the Cookie or Authorization header fields.
On the contrary, an encoder might prefer indexing values for header On the contrary, an encoder might prefer indexing values for header
fields that have little or no value if they were exposed. For fields that have little or no value if they were exposed. For
instance, a User-Agent header field does not commonly vary between instance, a User-Agent header field does not commonly vary between
requests and is sent to any server. In that case, confirmation that requests and is sent to any server. In that case, confirmation that
a particular User-Agent value has been used provides little value. a particular User-Agent value has been used provides little value.
Note that these criteria for deciding to use a never indexed literal Note that these criteria for deciding to use a never-indexed literal
representation will evolve over time as new attacks are discovered. representation will evolve over time as new attacks are discovered.
7.2. Static Huffman Encoding 7.2. Static Huffman Encoding
There is no currently known attack against a static Huffman encoding. There is no currently known attack against a static Huffman encoding.
A study has shown that using a static Huffman encoding table created A study has shown that using a static Huffman encoding table created
an information leakage, however this same study concluded that an an information leakage; however, this same study concluded that an
attacker could not take advantage of this information leakage to attacker could not take advantage of this information leakage to
recover any meaningful amount of information (see [PETAL]). recover any meaningful amount of information (see [PETAL]).
7.3. Memory Consumption 7.3. Memory Consumption
An attacker can try to cause an endpoint to exhaust its memory. An attacker can try to cause an endpoint to exhaust its memory.
HPACK is designed to limit both the peak and state amounts of memory HPACK is designed to limit both the peak and state amounts of memory
allocated by an endpoint. allocated by an endpoint.
The amount of memory used by the compressor is limited by the The amount of memory used by the compressor is limited by the
skipping to change at page 23, line 50 skipping to change at page 22, line 17
the dynamic table. In HTTP/2, this value is controlled by the the dynamic table. In HTTP/2, this value is controlled by the
decoder through the setting parameter SETTINGS_HEADER_TABLE_SIZE (see decoder through the setting parameter SETTINGS_HEADER_TABLE_SIZE (see
Section 6.5.2 of [HTTP2]). This limit takes into account both the Section 6.5.2 of [HTTP2]). This limit takes into account both the
size of the data stored in the dynamic table, plus a small allowance size of the data stored in the dynamic table, plus a small allowance
for overhead. for overhead.
A decoder can limit the amount of state memory used by setting an A decoder can limit the amount of state memory used by setting an
appropriate value for the maximum size of the dynamic table. In appropriate value for the maximum size of the dynamic table. In
HTTP/2, this is realized by setting an appropriate value for the HTTP/2, this is realized by setting an appropriate value for the
SETTINGS_HEADER_TABLE_SIZE parameter. An encoder can limit the SETTINGS_HEADER_TABLE_SIZE parameter. An encoder can limit the
amount of state memory it uses by signaling lower dynamic table size amount of state memory it uses by signaling a lower dynamic table
than the decoder allows (see Section 6.3). size than the decoder allows (see Section 6.3).
The amount of temporary memory consumed by an encoder or decoder can The amount of temporary memory consumed by an encoder or decoder can
be limited by processing header fields sequentially. An be limited by processing header fields sequentially. An
implementation does not need to retain a complete list of header implementation does not need to retain a complete list of header
fields. Note however that it might be necessary for an application fields. Note, however, that it might be necessary for an application
to retain a complete header list for other reasons; even though HPACK to retain a complete header list for other reasons; even though HPACK
does not force this to occur, application constraints might make this does not force this to occur, application constraints might make this
necessary. necessary.
7.4. Implementation Limits 7.4. Implementation Limits
An implementation of HPACK needs to ensure that large values for An implementation of HPACK needs to ensure that large values for
integers, long encoding for integers, or long string literals do not integers, long encoding for integers, or long string literals do not
create security weaknesses. create security weaknesses.
An implementation has to set a limit for the values it accepts for An implementation has to set a limit for the values it accepts for
integers, as well as for the encoded length (see Section 5.1). In integers, as well as for the encoded length (see Section 5.1). In
the same way, it has to set a limit to the length it accepts for the same way, it has to set a limit to the length it accepts for
string literals (see Section 5.2). string literals (see Section 5.2).
8. IANA Considerations 8. References
This document has no IANA actions.
9. Acknowledgments
This specification includes substantial input from the following
individuals:
o Mike Bishop, Jeff Pinner, Julian Reschke, Martin Thomson
(substantial editorial contributions).
o Johnny Graettinger (Huffman code statistics).
10. References
10.1. Normative References 8.1. Normative References
[HTTP2] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext [HTTP2] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol version 2", Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
draft-ietf-httpbis-http2-17 (work in progress), DOI 10.17487/RFC7540, May 2015,
February 2015. <http://www.rfc-editor.org/info/rfc7540>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Transfer Protocol (HTTP/1.1): Message Syntax and Protocol (HTTP/1.1): Message Syntax and Routing",
Routing", RFC 7230, June 2014. RFC 7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>.
10.2. Informative References 8.2. Informative References
[CANONICAL] Schwartz, E. and B. Kallick, "Generating a canonical [CANONICAL]
prefix encoding", Communications of the ACM Volume 7 Schwartz, E. and B. Kallick, "Generating a canonical
Issue 3, pp. 166-169, March 1964, prefix encoding", Communications of the ACM, Volume 7
<https://dl.acm.org/citation.cfm?id=363991>. Issue 3, pp. 166-169, March 1964, <https://dl.acm.org/
citation.cfm?id=363991>.
[CRIME] Rizzo, J. and T. Duong, "The CRIME Attack", [CRIME] Wikipedia, "CRIME", May 2015, <http://en.wikipedia.org/w/
September 2012, <https://docs.google.com/a/twist.com/ index.php?title=CRIME&oldid=660948120>.
presentation/d/
11eBmGiHbYcHR9gL5nDyZChu_-lCa2GizeuOfaLU2HOU>.
[DEFLATE] Deutsch, P., "DEFLATE Compressed Data Format [DEFLATE] Deutsch, P., "DEFLATE Compressed Data Format Specification
Specification version 1.3", RFC 1951, May 1996. version 1.3", RFC 1951, DOI 10.17487/RFC1951, May 1996,
<http://www.rfc-editor.org/info/rfc1951>.
[HUFFMAN] Huffman, D., "A Method for the Construction of Minimum [HUFFMAN] Huffman, D., "A Method for the Construction of Minimum-
Redundancy Codes", Proceedings of the Institute of Radio Redundancy Codes", Proceedings of the Institute of Radio
Engineers Volume 40, Number 9, pp. 1098-1101, Engineers, Volume 40, Number 9, pp. 1098-1101, September
September 1952, <http://ieeexplore.ieee.org/xpl/ 1952, <http://ieeexplore.ieee.org/xpl/
articleDetails.jsp?arnumber=4051119>. articleDetails.jsp?arnumber=4051119>.
[ORIGIN] Barth, A., "The Web Origin Concept", RFC 6454, [ORIGIN] Barth, A., "The Web Origin Concept", RFC 6454,
December 2011. DOI 10.17487/RFC6454, December 2011,
<http://www.rfc-editor.org/info/rfc6454>.
[PETAL] Tan, J. and J. Nahata, "PETAL: Preset Encoding Table [PETAL] Tan, J. and J. Nahata, "PETAL: Preset Encoding
Information Leakage", April 2013, <http:// Table Information Leakage", April 2013,
www.pdl.cmu.edu/PDL-FTP/associated/CMU-PDL-13-106.pdf>. <http://www.pdl.cmu.edu/PDL-FTP/associated/
CMU-PDL-13-106.pdf>.
[SPDY] Belshe, M. and R. Peon, "SPDY Protocol", [SPDY] Belshe, M. and R. Peon, "SPDY Protocol", draft-mbelshe-
draft-mbelshe-httpbis-spdy-00 (work in progress), httpbis-spdy-00 (work in progress), February 2012.
February 2012.
[TLS12] Dierks, T. and E. Rescorla, "The Transport Layer [TLS12] Dierks, T. and E. Rescorla, "The Transport Layer Security
Security (TLS) Protocol Version 1.2", RFC 5246, (TLS) Protocol Version 1.2", RFC 5246,
August 2008. DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
Appendix A. Static Table Definition Appendix A. Static Table Definition
The static table (see Section 2.3.1) consists in a predefined and The static table (see Section 2.3.1) consists in a predefined and
unchangeable list of header fields. unchangeable list of header fields.
The static table was created from the most frequent header fields The static table was created from the most frequent header fields
used by popular web sites, with the addition of HTTP/2-specific used by popular web sites, with the addition of HTTP/2-specific
pseudo-header fields (see Section 8.1.2.1 of [HTTP2]). For header pseudo-header fields (see Section 8.1.2.1 of [HTTP2]). For header
fields with a few frequent values, an entry was added for each of fields with a few frequent values, an entry was added for each of
these frequent values. For other header fields, an entry was added these frequent values. For other header fields, an entry was added
with an empty value. with an empty value.
The following table lists the predefined header fields that make-up Table 1 lists the predefined header fields that make up the static
the static table. table and gives the index of each entry.
+-------+-----------------------------+---------------+ +-------+-----------------------------+---------------+
| Index | Header Name | Header Value | | Index | Header Name | Header Value |
+-------+-----------------------------+---------------+ +-------+-----------------------------+---------------+
| 1 | :authority | | | 1 | :authority | |
| 2 | :method | GET | | 2 | :method | GET |
| 3 | :method | POST | | 3 | :method | POST |
| 4 | :path | / | | 4 | :path | / |
| 5 | :path | /index.html | | 5 | :path | /index.html |
| 6 | :scheme | http | | 6 | :scheme | http |
skipping to change at page 27, line 27 skipping to change at page 25, line 39
| 56 | strict-transport-security | | | 56 | strict-transport-security | |
| 57 | transfer-encoding | | | 57 | transfer-encoding | |
| 58 | user-agent | | | 58 | user-agent | |
| 59 | vary | | | 59 | vary | |
| 60 | via | | | 60 | via | |
| 61 | www-authenticate | | | 61 | www-authenticate | |
+-------+-----------------------------+---------------+ +-------+-----------------------------+---------------+
Table 1: Static Table Entries Table 1: Static Table Entries
Table 1 gives the index of each entry in the static table.
Appendix B. Huffman Code Appendix B. Huffman Code
The following Huffman code is used when encoding string literals with The following Huffman code is used when encoding string literals with
a Huffman coding (see Section 5.2). a Huffman coding (see Section 5.2).
This Huffman code was generated from statistics obtained on a large This Huffman code was generated from statistics obtained on a large
sample of HTTP headers. It is a canonical Huffman code (see sample of HTTP headers. It is a canonical Huffman code (see
[CANONICAL]) with some tweaking to ensure that no symbol has a unique [CANONICAL]) with some tweaking to ensure that no symbol has a unique
code length. code length.
skipping to change at page 33, line 38 skipping to change at page 31, line 46
(250) |11111111|11111111|11111101|100 7ffffec [27] (250) |11111111|11111111|11111101|100 7ffffec [27]
(251) |11111111|11111111|11111101|101 7ffffed [27] (251) |11111111|11111111|11111101|101 7ffffed [27]
(252) |11111111|11111111|11111101|110 7ffffee [27] (252) |11111111|11111111|11111101|110 7ffffee [27]
(253) |11111111|11111111|11111101|111 7ffffef [27] (253) |11111111|11111111|11111101|111 7ffffef [27]
(254) |11111111|11111111|11111110|000 7fffff0 [27] (254) |11111111|11111111|11111110|000 7fffff0 [27]
(255) |11111111|11111111|11111011|10 3ffffee [26] (255) |11111111|11111111|11111011|10 3ffffee [26]
EOS (256) |11111111|11111111|11111111|111111 3fffffff [30] EOS (256) |11111111|11111111|11111111|111111 3fffffff [30]
Appendix C. Examples Appendix C. Examples
A number of examples are worked through here, covering integer This appendix contains examples covering integer encoding, header
encoding, header field representation, and the encoding of whole field representation, and the encoding of whole lists of header
lists of header fields, for both requests and responses, and with and fields for both requests and responses, with and without Huffman
without Huffman coding. coding.
C.1. Integer Representation Examples C.1. Integer Representation Examples
This section shows the representation of integer values in details This section shows the representation of integer values in detail
(see Section 5.1). (see Section 5.1).
C.1.1. Example 1: Encoding 10 Using a 5-bit Prefix C.1.1. Example 1: Encoding 10 Using a 5-Bit Prefix
The value 10 is to be encoded with a 5-bit prefix. The value 10 is to be encoded with a 5-bit prefix.
o 10 is less than 31 (2^5 - 1) and is represented using the 5-bit o 10 is less than 31 (2^5 - 1) and is represented using the 5-bit
prefix. prefix.
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
| X | X | X | 0 | 1 | 0 | 1 | 0 | 10 stored on 5 bits | X | X | X | 0 | 1 | 0 | 1 | 0 | 10 stored on 5 bits
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
C.1.2. Example 2: Encoding 1337 Using a 5-bit Prefix C.1.2. Example 2: Encoding 1337 Using a 5-Bit Prefix
The value I=1337 is to be encoded with a 5-bit prefix. The value I=1337 is to be encoded with a 5-bit prefix.
1337 is greater than 31 (2^5 - 1). 1337 is greater than 31 (2^5 - 1).
The 5-bit prefix is filled with its max value (31). The 5-bit prefix is filled with its max value (31).
I = 1337 - (2^5 - 1) = 1306. I = 1337 - (2^5 - 1) = 1306.
I (1306) is greater than or equal to 128, the while loop body I (1306) is greater than or equal to 128, so the while loop
executes: body executes:
I % 128 == 26 I % 128 == 26
26 + 128 == 154 26 + 128 == 154
154 is encoded in 8 bits as: 10011010 154 is encoded in 8 bits as: 10011010
I is set to 10 (1306 / 128 == 10) I is set to 10 (1306 / 128 == 10)
I is no longer greater than or equal to 128, the while loop I is no longer greater than or equal to 128, so the while
terminates. loop terminates.
I, now 10, is encoded in 8 bits as: 00001010. I, now 10, is encoded in 8 bits as: 00001010.
The process ends. The process ends.
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
| X | X | X | 1 | 1 | 1 | 1 | 1 | Prefix = 31, I = 1306 | X | X | X | 1 | 1 | 1 | 1 | 1 | Prefix = 31, I = 1306
| 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 | 1306>=128, encode(154), I=1306/128 | 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 | 1306>=128, encode(154), I=1306/128
| 0 | 0 | 0 | 0 | 1 | 0 | 1 | 0 | 10<128, encode(10), done | 0 | 0 | 0 | 0 | 1 | 0 | 1 | 0 | 10<128, encode(10), done
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
C.1.3. Example 3: Encoding 42 Starting at an Octet Boundary C.1.3. Example 3: Encoding 42 Starting at an Octet Boundary
The value 42 is to be encoded starting at an octet-boundary. This The value 42 is to be encoded starting at an octet boundary. This
implies that a 8-bit prefix is used. implies that a 8-bit prefix is used.
o 42 is less than 255 (2^8 - 1) and is represented using the 8-bit o 42 is less than 255 (2^8 - 1) and is represented using the 8-bit
prefix. prefix.
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 42 stored on 8 bits | 0 | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 42 stored on 8 bits
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
skipping to change at page 36, line 11 skipping to change at page 34, line 11
400a 6375 7374 6f6d 2d6b 6579 0d63 7573 | @.custom-key.cus 400a 6375 7374 6f6d 2d6b 6579 0d63 7573 | @.custom-key.cus
746f 6d2d 6865 6164 6572 | tom-header 746f 6d2d 6865 6164 6572 | tom-header
Decoding process: Decoding process:
40 | == Literal indexed == 40 | == Literal indexed ==
0a | Literal name (len = 10) 0a | Literal name (len = 10)
6375 7374 6f6d 2d6b 6579 | custom-key 6375 7374 6f6d 2d6b 6579 | custom-key
0d | Literal value (len = 13) 0d | Literal value (len = 13)
6375 7374 6f6d 2d68 6561 6465 72 | custom-header 6375 7374 6f6d 2d68 6561 6465 72 | custom-header
| -> custom-key: custom-head\ | -> custom-key:
| er | custom-header
Dynamic Table (after decoding): Dynamic Table (after decoding):
[ 1] (s = 55) custom-key: custom-header [ 1] (s = 55) custom-key: custom-header
Table size: 55 Table size: 55
Decoded header list: Decoded header list:
custom-key: custom-header custom-key: custom-header
skipping to change at page 37, line 5 skipping to change at page 35, line 5
0c | Literal value (len = 12) 0c | Literal value (len = 12)
2f73 616d 706c 652f 7061 7468 | /sample/path 2f73 616d 706c 652f 7061 7468 | /sample/path
| -> :path: /sample/path | -> :path: /sample/path
Dynamic table (after decoding): empty. Dynamic table (after decoding): empty.
Decoded header list: Decoded header list:
:path: /sample/path :path: /sample/path
C.2.3. Literal Header Field never Indexed C.2.3. Literal Header Field Never Indexed
The header field representation uses a literal name and a literal The header field representation uses a literal name and a literal
value. The header field is not added to the dynamic table, and must value. The header field is not added to the dynamic table and must
use the same representation if re-encoded by an intermediary. use the same representation if re-encoded by an intermediary.
Header list to encode: Header list to encode:
password: secret password: secret
Hex dump of encoded data: Hex dump of encoded data:
1008 7061 7373 776f 7264 0673 6563 7265 | ..password.secre 1008 7061 7373 776f 7264 0673 6563 7265 | ..password.secre
74 | t 74 | t
skipping to change at page 37, line 37 skipping to change at page 35, line 37
| -> password: secret | -> password: secret
Dynamic table (after decoding): empty. Dynamic table (after decoding): empty.
Decoded header list: Decoded header list:
password: secret password: secret
C.2.4. Indexed Header Field C.2.4. Indexed Header Field
The header field representation uses an indexed header field, from The header field representation uses an indexed header field from the
the static table. static table.
Header list to encode: Header list to encode:
:method: GET :method: GET
Hex dump of encoded data: Hex dump of encoded data:
82 | . 82 | .
Decoding process: Decoding process:
skipping to change at page 39, line 20 skipping to change at page 36, line 46
| idx = 6 | idx = 6
| -> :scheme: http | -> :scheme: http
84 | == Indexed - Add == 84 | == Indexed - Add ==
| idx = 4 | idx = 4
| -> :path: / | -> :path: /
41 | == Literal indexed == 41 | == Literal indexed ==
| Indexed name (idx = 1) | Indexed name (idx = 1)
| :authority | :authority
0f | Literal value (len = 15) 0f | Literal value (len = 15)
7777 772e 6578 616d 706c 652e 636f 6d | www.example.com 7777 772e 6578 616d 706c 652e 636f 6d | www.example.com
| -> :authority: www.example\ | -> :authority:
| .com | www.example.com
Dynamic Table (after decoding): Dynamic Table (after decoding):
[ 1] (s = 57) :authority: www.example.com [ 1] (s = 57) :authority: www.example.com
Table size: 57 Table size: 57
Decoded header list: Decoded header list:
:method: GET :method: GET
:scheme: http :scheme: http
skipping to change at page 40, line 17 skipping to change at page 37, line 39
| idx = 2 | idx = 2
| -> :method: GET | -> :method: GET
86 | == Indexed - Add == 86 | == Indexed - Add ==
| idx = 6 | idx = 6
| -> :scheme: http | -> :scheme: http
84 | == Indexed - Add == 84 | == Indexed - Add ==
| idx = 4 | idx = 4
| -> :path: / | -> :path: /
be | == Indexed - Add == be | == Indexed - Add ==
| idx = 62 | idx = 62
| -> :authority: www.example\ | -> :authority:
| .com | www.example.com
58 | == Literal indexed == 58 | == Literal indexed ==
| Indexed name (idx = 24) | Indexed name (idx = 24)
| cache-control | cache-control
08 | Literal value (len = 8) 08 | Literal value (len = 8)
6e6f 2d63 6163 6865 | no-cache 6e6f 2d63 6163 6865 | no-cache
| -> cache-control: no-cache | -> cache-control: no-cache
Dynamic Table (after decoding): Dynamic Table (after decoding):
[ 1] (s = 53) cache-control: no-cache [ 1] (s = 53) cache-control: no-cache
skipping to change at page 41, line 22 skipping to change at page 38, line 41
| idx = 2 | idx = 2
| -> :method: GET | -> :method: GET
87 | == Indexed - Add == 87 | == Indexed - Add ==
| idx = 7 | idx = 7
| -> :scheme: https | -> :scheme: https
85 | == Indexed - Add == 85 | == Indexed - Add ==
| idx = 5 | idx = 5
| -> :path: /index.html | -> :path: /index.html
bf | == Indexed - Add == bf | == Indexed - Add ==
| idx = 63 | idx = 63
| -> :authority: www.example\ | -> :authority:
| .com | www.example.com
40 | == Literal indexed == 40 | == Literal indexed ==
0a | Literal name (len = 10) 0a | Literal name (len = 10)
6375 7374 6f6d 2d6b 6579 | custom-key 6375 7374 6f6d 2d6b 6579 | custom-key
0c | Literal value (len = 12) 0c | Literal value (len = 12)
6375 7374 6f6d 2d76 616c 7565 | custom-value 6375 7374 6f6d 2d76 616c 7565 | custom-value
| -> custom-key: custom-valu\ | -> custom-key:
| e | custom-value
Dynamic Table (after decoding): Dynamic Table (after decoding):
[ 1] (s = 54) custom-key: custom-value [ 1] (s = 54) custom-key: custom-value
[ 2] (s = 53) cache-control: no-cache [ 2] (s = 53) cache-control: no-cache
[ 3] (s = 57) :authority: www.example.com [ 3] (s = 57) :authority: www.example.com
Table size: 164 Table size: 164
Decoded header list: Decoded header list:
:method: GET :method: GET
:scheme: https :scheme: https
:path: /index.html :path: /index.html
:authority: www.example.com :authority: www.example.com
custom-key: custom-value custom-key: custom-value
C.4. Request Examples with Huffman Coding C.4. Request Examples with Huffman Coding
This section shows the same examples as the previous section, but This section shows the same examples as the previous section but uses
using Huffman encoding for the literal values. Huffman encoding for the literal values.
C.4.1. First Request C.4.1. First Request
Header list to encode: Header list to encode:
:method: GET :method: GET
:scheme: http :scheme: http
:path: / :path: /
:authority: www.example.com :authority: www.example.com
skipping to change at page 42, line 38 skipping to change at page 40, line 24
| idx = 4 | idx = 4
| -> :path: / | -> :path: /
41 | == Literal indexed == 41 | == Literal indexed ==
| Indexed name (idx = 1) | Indexed name (idx = 1)
| :authority | :authority
8c | Literal value (len = 12) 8c | Literal value (len = 12)
| Huffman encoded: | Huffman encoded:
f1e3 c2e5 f23a 6ba0 ab90 f4ff | .....:k..... f1e3 c2e5 f23a 6ba0 ab90 f4ff | .....:k.....
| Decoded: | Decoded:
| www.example.com | www.example.com
| -> :authority: www.example\ | -> :authority:
| .com | www.example.com
Dynamic Table (after decoding): Dynamic Table (after decoding):
[ 1] (s = 57) :authority: www.example.com [ 1] (s = 57) :authority: www.example.com
Table size: 57 Table size: 57
Decoded header list: Decoded header list:
:method: GET :method: GET
:scheme: http :scheme: http
skipping to change at page 43, line 32 skipping to change at page 41, line 18
| idx = 2 | idx = 2
| -> :method: GET | -> :method: GET
86 | == Indexed - Add == 86 | == Indexed - Add ==
| idx = 6 | idx = 6
| -> :scheme: http | -> :scheme: http
84 | == Indexed - Add == 84 | == Indexed - Add ==
| idx = 4 | idx = 4
| -> :path: / | -> :path: /
be | == Indexed - Add == be | == Indexed - Add ==
| idx = 62 | idx = 62
| -> :authority: www.example\ | -> :authority:
| .com | www.example.com
58 | == Literal indexed == 58 | == Literal indexed ==
| Indexed name (idx = 24) | Indexed name (idx = 24)
| cache-control | cache-control
86 | Literal value (len = 6) 86 | Literal value (len = 6)
| Huffman encoded: | Huffman encoded:
a8eb 1064 9cbf | ...d.. a8eb 1064 9cbf | ...d..
| Decoded: | Decoded:
| no-cache | no-cache
| -> cache-control: no-cache | -> cache-control: no-cache
skipping to change at page 45, line 18 skipping to change at page 42, line 22
| idx = 2 | idx = 2
| -> :method: GET | -> :method: GET
87 | == Indexed - Add == 87 | == Indexed - Add ==
| idx = 7 | idx = 7
| -> :scheme: https | -> :scheme: https
85 | == Indexed - Add == 85 | == Indexed - Add ==
| idx = 5 | idx = 5
| -> :path: /index.html | -> :path: /index.html
bf | == Indexed - Add == bf | == Indexed - Add ==
| idx = 63 | idx = 63
| -> :authority: www.example\ | -> :authority:
| .com | www.example.com
40 | == Literal indexed == 40 | == Literal indexed ==
88 | Literal name (len = 8) 88 | Literal name (len = 8)
| Huffman encoded: | Huffman encoded:
25a8 49e9 5ba9 7d7f | %.I.[.}. 25a8 49e9 5ba9 7d7f | %.I.[.}.
| Decoded: | Decoded:
| custom-key | custom-key
89 | Literal value (len = 9) 89 | Literal value (len = 9)
| Huffman encoded: | Huffman encoded:
25a8 49e9 5bb8 e8b4 bf | %.I.[.... 25a8 49e9 5bb8 e8b4 bf | %.I.[....
| Decoded: | Decoded:
| custom-value | custom-value
| -> custom-key: custom-valu\ | -> custom-key:
| e | custom-value
Dynamic Table (after decoding): Dynamic Table (after decoding):
[ 1] (s = 54) custom-key: custom-value [ 1] (s = 54) custom-key: custom-value
[ 2] (s = 53) cache-control: no-cache [ 2] (s = 53) cache-control: no-cache
[ 3] (s = 57) :authority: www.example.com [ 3] (s = 57) :authority: www.example.com
Table size: 164 Table size: 164
Decoded header list: Decoded header list:
skipping to change at page 47, line 24 skipping to change at page 44, line 24
| cache-control | cache-control
07 | Literal value (len = 7) 07 | Literal value (len = 7)
7072 6976 6174 65 | private 7072 6976 6174 65 | private
| -> cache-control: private | -> cache-control: private
61 | == Literal indexed == 61 | == Literal indexed ==
| Indexed name (idx = 33) | Indexed name (idx = 33)
| date | date
1d | Literal value (len = 29) 1d | Literal value (len = 29)
4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013 4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
2032 303a 3133 3a32 3120 474d 54 | 20:13:21 GMT 2032 303a 3133 3a32 3120 474d 54 | 20:13:21 GMT
| -> date: Mon, 21 Oct 2013 \ | -> date: Mon, 21 Oct 2013
| 20:13:21 GMT | 20:13:21 GMT
6e | == Literal indexed == 6e | == Literal indexed ==
| Indexed name (idx = 46) | Indexed name (idx = 46)
| location | location
17 | Literal value (len = 23) 17 | Literal value (len = 23)
6874 7470 733a 2f2f 7777 772e 6578 616d | https://www.exam 6874 7470 733a 2f2f 7777 772e 6578 616d | https://www.exam
706c 652e 636f 6d | ple.com 706c 652e 636f 6d | ple.com
| -> location: https://www.e\ | -> location:
| xample.com | https://www.example.com
Dynamic Table (after decoding): Dynamic Table (after decoding):
[ 1] (s = 63) location: https://www.example.com [ 1] (s = 63) location: https://www.example.com
[ 2] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT [ 2] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 3] (s = 52) cache-control: private [ 3] (s = 52) cache-control: private
[ 4] (s = 42) :status: 302 [ 4] (s = 42) :status: 302
Table size: 222 Table size: 222
Decoded header list: Decoded header list:
skipping to change at page 48, line 35 skipping to change at page 45, line 35
| :status | :status
03 | Literal value (len = 3) 03 | Literal value (len = 3)
3330 37 | 307 3330 37 | 307
| - evict: :status: 302 | - evict: :status: 302
| -> :status: 307 | -> :status: 307
c1 | == Indexed - Add == c1 | == Indexed - Add ==
| idx = 65 | idx = 65
| -> cache-control: private | -> cache-control: private
c0 | == Indexed - Add == c0 | == Indexed - Add ==
| idx = 64 | idx = 64
| -> date: Mon, 21 Oct 2013 \ | -> date: Mon, 21 Oct 2013
| 20:13:21 GMT | 20:13:21 GMT
bf | == Indexed - Add == bf | == Indexed - Add ==
| idx = 63 | idx = 63
| -> location: https://www.e\ | -> location:
| xample.com | https://www.example.com
Dynamic Table (after decoding): Dynamic Table (after decoding):
[ 1] (s = 42) :status: 307 [ 1] (s = 42) :status: 307
[ 2] (s = 63) location: https://www.example.com [ 2] (s = 63) location: https://www.example.com
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT [ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 4] (s = 52) cache-control: private [ 4] (s = 52) cache-control: private
Table size: 222 Table size: 222
Decoded header list: Decoded header list:
skipping to change at page 50, line 18 skipping to change at page 47, line 18
| -> :status: 200 | -> :status: 200
c1 | == Indexed - Add == c1 | == Indexed - Add ==
| idx = 65 | idx = 65
| -> cache-control: private | -> cache-control: private
61 | == Literal indexed == 61 | == Literal indexed ==
| Indexed name (idx = 33) | Indexed name (idx = 33)
| date | date
1d | Literal value (len = 29) 1d | Literal value (len = 29)
4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013 4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
2032 303a 3133 3a32 3220 474d 54 | 20:13:22 GMT 2032 303a 3133 3a32 3220 474d 54 | 20:13:22 GMT
| - evict: cache-control: pr\ | - evict: cache-control:
| ivate | private
| -> date: Mon, 21 Oct 2013 \ | -> date: Mon, 21 Oct 2013
| 20:13:22 GMT | 20:13:22 GMT
c0 | == Indexed - Add == c0 | == Indexed - Add ==
| idx = 64 | idx = 64
| -> location: https://www.e\ | -> location:
| xample.com | https://www.example.com
5a | == Literal indexed == 5a | == Literal indexed ==
| Indexed name (idx = 26) | Indexed name (idx = 26)
| content-encoding | content-encoding
04 | Literal value (len = 4) 04 | Literal value (len = 4)
677a 6970 | gzip 677a 6970 | gzip
| - evict: date: Mon, 21 Oct\ | - evict: date: Mon, 21 Oct
| 2013 20:13:21 GMT | 2013 20:13:21 GMT
| -> content-encoding: gzip | -> content-encoding: gzip
77 | == Literal indexed == 77 | == Literal indexed ==
| Indexed name (idx = 55) | Indexed name (idx = 55)
| set-cookie | set-cookie
38 | Literal value (len = 56) 38 | Literal value (len = 56)
666f 6f3d 4153 444a 4b48 514b 425a 584f | foo=ASDJKHQKBZXO 666f 6f3d 4153 444a 4b48 514b 425a 584f | foo=ASDJKHQKBZXO
5157 454f 5049 5541 5851 5745 4f49 553b | QWEOPIUAXQWEOIU; 5157 454f 5049 5541 5851 5745 4f49 553b | QWEOPIUAXQWEOIU;
206d 6178 2d61 6765 3d33 3630 303b 2076 | max-age=3600; v 206d 6178 2d61 6765 3d33 3630 303b 2076 | max-age=3600; v
6572 7369 6f6e 3d31 | ersion=1 6572 7369 6f6e 3d31 | ersion=1
| - evict: location: https:/\ | - evict: location:
| /www.example.com | https://www.example.com
| - evict: :status: 307 | - evict: :status: 307
| -> set-cookie: foo=ASDJKHQ\ | -> set-cookie: foo=ASDJKHQ
| KBZXOQWEOPIUAXQWEOIU; ma\ | KBZXOQWEOPIUAXQWEOIU; ma
| x-age=3600; version=1 | x-age=3600; version=1
Dynamic Table (after decoding): Dynamic Table (after decoding):
[ 1] (s = 98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age\ [ 1] (s = 98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU;
=3600; version=1 max-age=3600; version=1
[ 2] (s = 52) content-encoding: gzip [ 2] (s = 52) content-encoding: gzip
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:22 GMT [ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:22 GMT
Table size: 215 Table size: 215
Decoded header list: Decoded header list:
:status: 200 :status: 200
cache-control: private cache-control: private
date: Mon, 21 Oct 2013 20:13:22 GMT date: Mon, 21 Oct 2013 20:13:22 GMT
location: https://www.example.com location: https://www.example.com
content-encoding: gzip content-encoding: gzip
set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1 set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1
C.6. Response Examples with Huffman Coding C.6. Response Examples with Huffman Coding
This section shows the same examples as the previous section, but This section shows the same examples as the previous section but uses
using Huffman encoding for the literal values. The HTTP/2 setting Huffman encoding for the literal values. The HTTP/2 setting
parameter SETTINGS_HEADER_TABLE_SIZE is set to the value of 256 parameter SETTINGS_HEADER_TABLE_SIZE is set to the value of 256
octets, causing some evictions to occur. The eviction mechanism uses octets, causing some evictions to occur. The eviction mechanism uses
the length of the decoded literal values, so the same evictions the length of the decoded literal values, so the same evictions occur
occurs as in the previous section. as in the previous section.
C.6.1. First Response C.6.1. First Response
Header list to encode: Header list to encode:
:status: 302 :status: 302
cache-control: private cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com location: https://www.example.com
skipping to change at page 52, line 33 skipping to change at page 49, line 33
| private | private
| -> cache-control: private | -> cache-control: private
61 | == Literal indexed == 61 | == Literal indexed ==
| Indexed name (idx = 33) | Indexed name (idx = 33)
| date | date
96 | Literal value (len = 22) 96 | Literal value (len = 22)
| Huffman encoded: | Huffman encoded:
d07a be94 1054 d444 a820 0595 040b 8166 | .z...T.D. .....f d07a be94 1054 d444 a820 0595 040b 8166 | .z...T.D. .....f
e082 a62d 1bff | ...-.. e082 a62d 1bff | ...-..
| Decoded: | Decoded:
| Mon, 21 Oct 2013 20:13:21 \ | Mon, 21 Oct 2013 20:13:21
| GMT | GMT
| -> date: Mon, 21 Oct 2013 \ | -> date: Mon, 21 Oct 2013
| 20:13:21 GMT | 20:13:21 GMT
6e | == Literal indexed == 6e | == Literal indexed ==
| Indexed name (idx = 46) | Indexed name (idx = 46)
| location | location
91 | Literal value (len = 17) 91 | Literal value (len = 17)
| Huffman encoded: | Huffman encoded:
9d29 ad17 1863 c78f 0b97 c8e9 ae82 ae43 | .)...c.........C 9d29 ad17 1863 c78f 0b97 c8e9 ae82 ae43 | .)...c.........C
d3 | . d3 | .
| Decoded: | Decoded:
| https://www.example.com | https://www.example.com
| -> location: https://www.e\ | -> location:
| xample.com | https://www.example.com
Dynamic Table (after decoding): Dynamic Table (after decoding):
[ 1] (s = 63) location: https://www.example.com [ 1] (s = 63) location: https://www.example.com
[ 2] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT [ 2] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 3] (s = 52) cache-control: private [ 3] (s = 52) cache-control: private
[ 4] (s = 42) :status: 302 [ 4] (s = 42) :status: 302
Table size: 222 Table size: 222
Decoded header list: Decoded header list:
skipping to change at page 54, line 22 skipping to change at page 51, line 22
640e ff | d.. 640e ff | d..
| Decoded: | Decoded:
| 307 | 307
| - evict: :status: 302 | - evict: :status: 302
| -> :status: 307 | -> :status: 307
c1 | == Indexed - Add == c1 | == Indexed - Add ==
| idx = 65 | idx = 65
| -> cache-control: private | -> cache-control: private
c0 | == Indexed - Add == c0 | == Indexed - Add ==
| idx = 64 | idx = 64
| -> date: Mon, 21 Oct 2013 \ | -> date: Mon, 21 Oct 2013
| 20:13:21 GMT | 20:13:21 GMT
bf | == Indexed - Add == bf | == Indexed - Add ==
| idx = 63 | idx = 63
| -> location: https://www.e\ | -> location:
| xample.com | https://www.example.com
Dynamic Table (after decoding): Dynamic Table (after decoding):
[ 1] (s = 42) :status: 307 [ 1] (s = 42) :status: 307
[ 2] (s = 63) location: https://www.example.com [ 2] (s = 63) location: https://www.example.com
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT [ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 4] (s = 52) cache-control: private [ 4] (s = 52) cache-control: private
Table size: 222 Table size: 222
Decoded header list: Decoded header list:
skipping to change at page 55, line 38 skipping to change at page 52, line 38
| idx = 65 | idx = 65
| -> cache-control: private | -> cache-control: private
61 | == Literal indexed == 61 | == Literal indexed ==
| Indexed name (idx = 33) | Indexed name (idx = 33)
| date | date
96 | Literal value (len = 22) 96 | Literal value (len = 22)
| Huffman encoded: | Huffman encoded:
d07a be94 1054 d444 a820 0595 040b 8166 | .z...T.D. .....f d07a be94 1054 d444 a820 0595 040b 8166 | .z...T.D. .....f
e084 a62d 1bff | ...-.. e084 a62d 1bff | ...-..
| Decoded: | Decoded:
| Mon, 21 Oct 2013 20:13:22 \ | Mon, 21 Oct 2013 20:13:22
| GMT | GMT
| - evict: cache-control: pr\ | - evict: cache-control:
| ivate | private
| -> date: Mon, 21 Oct 2013 \ | -> date: Mon, 21 Oct 2013
| 20:13:22 GMT | 20:13:22 GMT
c0 | == Indexed - Add == c0 | == Indexed - Add ==
| idx = 64 | idx = 64
| -> location: https://www.e\ | -> location:
| xample.com | https://www.example.com
5a | == Literal indexed == 5a | == Literal indexed ==
| Indexed name (idx = 26) | Indexed name (idx = 26)
| content-encoding | content-encoding
83 | Literal value (len = 3) 83 | Literal value (len = 3)
| Huffman encoded: | Huffman encoded:
9bd9 ab | ... 9bd9 ab | ...
| Decoded: | Decoded:
| gzip | gzip
| - evict: date: Mon, 21 Oct\ | - evict: date: Mon, 21 Oct
| 2013 20:13:21 GMT | 2013 20:13:21 GMT
| -> content-encoding: gzip | -> content-encoding: gzip
77 | == Literal indexed == 77 | == Literal indexed ==
| Indexed name (idx = 55) | Indexed name (idx = 55)
| set-cookie | set-cookie
ad | Literal value (len = 45) ad | Literal value (len = 45)
| Huffman encoded: | Huffman encoded:
94e7 821d d7f2 e6c7 b335 dfdf cd5b 3960 | .........5...[9` 94e7 821d d7f2 e6c7 b335 dfdf cd5b 3960 | .........5...[9`
d5af 2708 7f36 72c1 ab27 0fb5 291f 9587 | ..'..6r..'..)... d5af 2708 7f36 72c1 ab27 0fb5 291f 9587 | ..'..6r..'..)...
3160 65c0 03ed 4ee5 b106 3d50 07 | 1`e...N...=P. 3160 65c0 03ed 4ee5 b106 3d50 07 | 1`e...N...=P.
| Decoded: | Decoded:
| foo=ASDJKHQKBZXOQWEOPIUAXQ\ | foo=ASDJKHQKBZXOQWEOPIUAXQ
| WEOIU; max-age=3600; versi\ | WEOIU; max-age=3600; versi
| on=1 | on=1
| - evict: location: https:/\ | - evict: location:
| /www.example.com | https://www.example.com
| - evict: :status: 307 | - evict: :status: 307
| -> set-cookie: foo=ASDJKHQ\ | -> set-cookie: foo=ASDJKHQ
| KBZXOQWEOPIUAXQWEOIU; ma\ | KBZXOQWEOPIUAXQWEOIU; ma
| x-age=3600; version=1 | x-age=3600; version=1
Dynamic Table (after decoding): Dynamic Table (after decoding):
[ 1] (s = 98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age\ [ 1] (s = 98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU;
=3600; version=1 max-age=3600; version=1
[ 2] (s = 52) content-encoding: gzip [ 2] (s = 52) content-encoding: gzip
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:22 GMT [ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:22 GMT
Table size: 215 Table size: 215
Decoded header list: Decoded header list:
:status: 200 :status: 200
cache-control: private cache-control: private
date: Mon, 21 Oct 2013 20:13:22 GMT date: Mon, 21 Oct 2013 20:13:22 GMT
location: https://www.example.com location: https://www.example.com
content-encoding: gzip content-encoding: gzip
set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1 set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1
Appendix D. Change Log (to be removed by RFC Editor before publication) Acknowledgments
D.1. Since draft-ietf-httpbis-header-compression-10
o Editorial corrections for taking into account IETF LC comments.
* Added links to security sections.
* Made spec more independent of HTTP/2.
* Expanded security section about never indexed literal usage.
o Removed most usages of 'name-value pair' instead of header field.
o Changed 'header table' to 'header field table'.
D.2. Since draft-ietf-httpbis-header-compression-09
o Renamed header table to dynamic table.
o Updated integer representation.
o Editorial corrections.
D.3. Since draft-ietf-httpbis-header-compression-08
o Removed the reference set.
o Removed header emission.
o Explicit handling of several SETTINGS_HEADER_TABLE_SIZE parameter
changes.
o Changed header set to header list, and forced ordering.
o Updated examples.
o Exchanged header and static table positions.
D.4. Since draft-ietf-httpbis-header-compression-07
o Removed old text on index value of 0.
o Added clarification for signalling of maximum table size after a
SETTINGS_HEADER_TABLE_SIZE update.
o Rewrote security considerations.
o Many editorial clarifications or improvements.
o Added convention section.
o Reworked document's outline.
o Updated static table. Entry 16 has now "gzip, deflate" for value.
o Updated Huffman table, using data set provided by Google.
D.5. Since draft-ietf-httpbis-header-compression-06
o Updated format to include literal headers that must never be
compressed.
o Updated security considerations.
o Moved integer encoding examples to the appendix.
o Updated Huffman table.
o Updated static header table (adding and removing status values).
o Updated examples.
D.6. Since draft-ietf-httpbis-header-compression-05
o Regenerated examples.
o Only one Huffman table for requests and responses.
o Added maximum size for dynamic table, independent of
SETTINGS_HEADER_TABLE_SIZE.
o Added pseudo-code for integer decoding.
o Improved examples (removing unnecessary removals).
D.7. Since draft-ietf-httpbis-header-compression-04
o Updated examples: take into account changes in the spec, and show
more features.
o Use 'octet' everywhere instead of having both 'byte' and 'octet'.
o Added reference set emptying.
o Editorial changes and clarifications.
o Added "host" header to the static table.
o Ordering for list of values (either NULL- or comma-separated).
D.8. Since draft-ietf-httpbis-header-compression-03
o A large number of editorial changes; changed the description of
evicting/adding new entries.
o Removed substitution indexing
o Changed 'initial headers' to 'static headers', as per issue #258
o Merged 'request' and 'response' static headers, as per issue #259
o Changed text to indicate that new headers are added at index 0 and
expire from the largest index, as per issue #233
D.9. Since draft-ietf-httpbis-header-compression-02
o Corrected error in integer encoding pseudocode.
D.10. Since draft-ietf-httpbis-header-compression-01
o Refactored of Header Encoding Section: split definitions and
processing rule.
o Backward incompatible change: Updated reference set management as
per issue #214. This changes how the interaction between the
reference set and eviction works. This also changes the working
of the reference set in some specific cases.
o Backward incompatible change: modified initial header list, as per
issue #188.
o Added example of 32 octets entry structure (issue #191).
o Added Header Set Completion section. Reflowed some text.
Clarified some writing which was akward. Added text about
duplicate header entry encoding. Clarified some language w.r.t
Header Set. Changed x-my-header to mynewheader. Added text in the
HeaderEmission section indicating that the application may also be
able to free up memory more quickly. Added information in
Security Considerations section.
D.11. Since draft-ietf-httpbis-header-compression-00
Fixed bug/omission in integer representation algorithm.
Changed the document title.
Header matching text rewritten.
Changed the definition of header emission.
Changed the name of the setting which dictates how much memory the
compression context should use.
Removed "specific use cases" section
Corrected erroneous statement about what index can be contained in This specification includes substantial input from the following
one octet individuals:
Added descriptions of opcodes o Mike Bishop, Jeff Pinner, Julian Reschke, and Martin Thomson
(substantial editorial contributions).
Removed security claims from introduction. o Johnny Graettinger (Huffman code statistics).
Authors' Addresses Authors' Addresses
Roberto Peon Roberto Peon
Google, Inc Google, Inc
EMail: fenix@google.com EMail: fenix@google.com
Herve Ruellan Herve Ruellan
Canon CRF Canon CRF
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