| HTTPbis | R. Peon |
| Internet-Draft | Google, Inc |
| Intended status: Standards Track | H. Ruellan |
| Expires: December 19, 2014 | Canon CRF |
| June 17, 2014 |
HPACK - Header Compression for HTTP/2
draft-ietf-httpbis-header-compression-08
This specification defines HPACK, a compression format for efficiently representing HTTP header fields in the context of HTTP/2.
Discussion of this draft takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived at https://lists.w3.org/Archives/Public/ietf-http-wg/.
Working Group information can be found at http://tools.ietf.org/wg/httpbis/; that specific to HTTP/2 are at http://http2.github.io/.
The changes in this draft are summarized in Appendix A.1.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 19, 2014.
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This specification defines HPACK, a compression format for efficiently representing HTTP header fields in the context of HTTP/2 (see [HTTP2]).
In HTTP/1.1 (see [RFC7230]), header fields are encoded without any form of compression. As web pages have grown to include dozens to hundreds of requests, the redundant header fields in these requests now measurably increase latency and unnecessarily consume bandwidth (see [SPDY-DESC-1] and [SPDY-DESC-2]).
SPDY [SPDY] initially addressed this redundancy by compressing header fields using the DEFLATE format [DEFLATE], which proved very effective at efficiently representing the redundant header fields. However, that approach exposed a security risk as demonstrated by the CRIME attack (see [CRIME]).
This document describes HPACK, a new compressor for header fields which eliminates redundant header fields, limits vulnerability to known security attacks, and which has a bounded memory requirement for use in constrained environments.
The HTTP header field encoding defined in this document is based on a header table that maps name-value pairs to index values. The header table is incrementally updated as new values are encoded or decoded.
A set of header fields is treated as an unordered collection of name-value pairs that can include duplicates. Names and values are considered to be opaque sequences of octets. The order of header fields is not guaranteed to be preserved after being compressed and decompressed.
In the encoded form, a header field is represented either literally or as a reference to a name-value pair in a header table. A set of header fields can therefore be encoded using a mixture of references and literal values.
As two consecutive sets of header fields often have header fields in common, each set is coded as a difference from the previous set. The goal is to only encode the changes between the two sets of header fields (that is, header fields that are present in only one of the sets) and eliminate redundancy (header fields present in both sets).
A subset of the header fields that are encoded as references to the header table are maintained in a reference set that is used as the initial set of header fields for the next encoding.
The encoder is responsible for deciding which header fields to insert as new entries in the header table. The decoder executes the modifications to the header table and reference set prescribed by the encoder, reconstructing the set of header fields in the process. This enables decoders to remain simple and understand a wide variety of encoders.
Examples illustrating the use of these different mechanisms to represent header fields are available in Appendix D.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
All numeric values are in network byte order. Values are unsigned unless otherwise indicated. Literal values are provided in decimal or hexadecimal as appropriate. Hexadecimal literals are prefixed with 0x to distinguish them from decimal literals.
This document uses the following terms:
This specification does not describe a specific algorithm for an encoder. Instead, it defines precisely how a decoder is expected to operate, allowing encoders to produce any encoding that this definition permits.
HPACK requires that a decoder maintains both a header table and a reference set. No other state information is needed to decode messages. An encoder that wishes to reference entries in the header table, reference set, or static table needs to maintain a copy of the information a decoder holds.
When used for bidirectional communication, such as in HTTP, the encoding and decoding contexts maintained by an endpoint are completely independent. Header fields are encoded without any reference to the local decoding state; and header fields are decoded without reference to the encoding state.
Each endpoint maintains a header table and a reference set in order to decode header blocks, and optionally a copy of the information maintained by their peer.
A header table consists of a list of header fields maintained in first-in, first-out order. The first and newest entry in a header table is always at index 1, and the oldest entry of a header table is at the index corresponding to the number of entries in the header table.
The header table is initially empty.
The header table can contain duplicate entries. Therefore, duplicate entries MUST NOT be treated as an error by a decoder.
The encoder decides how to update the header table and as such can control how much memory is used by the header table. To limit the memory requirements of the decoder, the header table size is strictly bounded (see Section 5.1).
The header table is updated during the processing of a set of header field representations (see Section 4.1).
A reference set is an unordered set of references to entries of the header table. It never contains duplicate references.
The reference set is initially empty.
The reference set is updated during the processing of a set of header field representations (see Section 4.1).
The reference set enables differential encoding, where only differences between the previous header set and the current header set need to be encoded. The use of differential encoding is optional for any header set.
When an entry is evicted from the header table, if it was referenced from the reference set, its reference is removed from the reference set.
To limit the memory requirements on the decoder side for handling the reference set, only entries within the header table can be contained in the reference set. To still allow entries from the static table to take advantage of the differential encoding, when a header field is represented as a reference to an entry of the static table, this entry is inserted into the header table (see Section 4.1).
An encoded header field can be represented either as a literal or as an index.
A literal representation defines a new header field. The header field name can be represented literally or as a reference to an entry of the header table. The header field value is represented literally.
Three different literal representations are provided:
An indexed representation defines a header field as a reference to an entry in either the header table or the static table (see Section 7.1).
Indices between 1 and the length of the header table (inclusive) refer to elements in the header table, with index 1 referring to the beginning of the table.
Indices between one higher than the length of the header table represent indexes into the static table. The length of the header table is subtracted to find the index into the static table.
Indices that are greater than the sum of the lengths of both tables MUST be treated as a decoding error.
An indexed representation using an entry of the static table induces a copy of this entry into the header table (see Section 4.1) for bounding memory requirements on the decoder side (see Section 5.1). For this reason, the header table is accessed more frequently than the static table and has the lower indices.
For a header table size of k and a static table size of s, the following diagram shows the entire valid index address space.
<---------- Index Address Space ---------->
<-- Header Table --> <-- Static Table -->
+---+-----------+---+ +---+-----------+---+
| 1 | ... | k | |k+1| ... |k+s|
+---+-----------+---+ +---+-----------+---+
^ |
| V
Insertion Point Dropping Point
Index Address Space
A decoder processes an encoded header block sequentially. As different instructions are processed, some might specify that a header field is emitted.
The emission of a header field is the process of marking a header field as belonging to the output header set. Once a header has been emitted, it cannot be removed or retracted from the decoder output.
An emitted header field can be safely passed to the upper processing layer as part of the current header set. The decoder can pass emitted header fields to the upper processing layer in any order.
By emitting header fields instead of emitting header sets, a decoder can be implemented with minimal memory commitment in addition to the header table and the reference set. The management of memory for handling very large sets of header fields can therefore be deferred to the upper processing layers.
The processing of a header block to obtain a header set is defined in this section. To ensure that the decoding will successfully produce a header set, a decoder MUST obey the following rules.
All the header field representations contained in a header block are processed in the order in which they appear, as specified below. Details on the formatting of the various header field representations, and some additional processing instructions are found in Section 7.
An indexed representation corresponding to an entry present in the reference set entails the following actions:
An indexed representation corresponding to an entry not present in the reference set entails the following actions:
A literal representation that is not added to the header table entails the following action:
A literal representation that is added to the header table entails the following actions:
Once all the representations contained in a header block have been processed, any header fields included in the reference set that have not previously been emitted during the processing of this header block are emitted.
After the emission of these remaining header fields, the header set is complete.
To limit the memory requirements on the decoder side, the mutable structures used in an encoding context are constrained in size. These mutable structures are the header table and the reference set.
The size of the header table is bounded by a maximum size defined by the decoder. The size of the header table MUST always be lower than or equal to this maximum size.
The reference set can only contain references to entries of the header table, and can't contain references to entries of the static table. In addition, it can't contain duplicate references. Therefore, its maximum size is bounded by the size of the header table.
By default, the maximum size of the header table is equal to the value of the HTTP/2 setting SETTINGS_HEADER_TABLE_SIZE defined by the decoder (see [HTTP2]). The encoder can change this maximum size (see Section 7.3), but it MUST stay lower than or equal to the value of SETTINGS_HEADER_TABLE_SIZE.
After applying an updated value of the HTTP/2 setting SETTINGS_HEADER_TABLE_SIZE that changes the maximum size of the header table used by the encoder, the encoder MUST signal this change via an encoding context update (see Section 7.3). This encoding context update MUST occur at the beginning of the first header block following the SETTINGS frame sent to acknowledge the application of the updated settings.
The size of the header 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 defined in Section 6.2), its value's length in octets (Section 6.2), plus 32.
The size of an entry is calculated using the length of the name and value without any Huffman encoding applied.
The additional 32 octets account for overhead associated with an entry. For example, an entry structure using two 64-bit pointers to reference the name and the value of the entry, and two 64-bit integers for counting the number of references to the name and value would have 32 octets of overhead.
Whenever the maximum size for the header table is reduced, entries are evicted from the end of the header table until the size of the header table is less than or equal to the maximum size.
Whenever an entry is evicted from the header table, any reference to that entry from the reference set is removed.
The eviction of an entry from the header table causes the index of the entries in the static table to be reduced by one.
Whenever a new entry is to be added to the header table entries are evicted from the end of the header table until the size of the header table is less than or equal to (maximum size - new entry size), or until the table is empty.
If the representation of the added entry references the name of an entry in the header table, the referenced name is cached prior to performing eviction to avoid having the name inadvertently evicted.
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 attempt to add an entry that is larger than the maximum size; an attempt to add an entry larger than the entire table causes the table to be emptied of all existing entries.
HPACK encoding uses two primitive types: unsigned variable length integers, and strings of octets.
Integers are used to represent name indexes, pair indexes or string lengths. To allow for optimized processing, an integer representation always finishes at the end of an octet.
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 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 N-bit prefix allows filling the current octet. If the value is small enough (strictly less than 2http://en.wikipedia.org/wiki/Variable-length_quantity). N is always between 1 and 8 bits. An integer starting at an octet-boundary will have an 8-bit prefix.
The algorithm to represent an integer I is as follows:
if I < 2^N - 1, encode I on N bits
else
encode (2^N - 1) on N bits
I = I - (2^N - 1)
while I >= 128
encode (I % 128 + 128) on 8 bits
I = I / 128
encode I on 8 bits
For informational purpose, the algorithm to decode an integer I is as follows:
decode I from the next N bits
if I < 2^N - 1, return I
else
M = 0
repeat
B = next octet
I = I + (B & 127) * 2^M
M = M + 7
while B & 128 == 128
return I
Examples illustrating the encoding of integers are available in Appendix D.1.
This integer representation allows for values of indefinite size. It 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. Excessively large integer encodings - in value or octet length - MUST be treated as a decoding error. Different limits can be set for each of the different uses of integers, based on implementation constraints.
Header field names and header field values can be represented as literal string. A literal string is encoded as a sequence of octets, either by directly encoding the literal string's octets, or by using a Huffman code [HUFFMAN].
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | H | String Length (7+) | +---+---------------------------+ | String Data (Length octets) | +-------------------------------+
String Literal Representation
A literal string representation contains the following fields:
String literals which use Huffman encoding are encoded with the Huffman code defined in Appendix C (see examples in Request Examples with Huffman Coding [request.examples.with.huffman.coding] and in Response Examples with Huffman Coding [response.examples.with.huffman.coding]). The encoded data is the bitwise concatenation of the codes corresponding to each octet of the string literal.
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 prevent this padding to be misinterpreted as part of the string literal, the most significant bits of code corresponding to the EOS (end-of-string) symbol are used.
Upon decoding, an incomplete code at the end of the encoded data is to be considered as padding and discarded. A padding strictly longer 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 symbol MUST be treated as a decoding error. A Huffman encoded string literal containing the EOS symbol MUST be treated as a decoding error.
This section describes the detailed format of each of the different header field representations, plus the encoding context update instruction.
An indexed header field representation identifies an entry in either the header table or the static table.
An indexed header field representation can either causes a header field to be emitted or to be removed from the reference set, as described in Section 4.1.
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 | Index (7+) | +---+---------------------------+
Indexed Header Field
An indexed header field starts with the '1' 1-bit pattern, followed by the index of the matching pair, represented as an integer with a 7-bit prefix.
The index value of 0 is not used. It MUST be treated as a decoding error if found in an indexed header field representation.
A literal header field representation contains a literal header field value. Header field names are either provided as a literal or by reference to an existing table entry, either from the header table or the static table.
A literal representation always result in the emission of a header field when decoded.
A literal header field with incremental indexing representation causes the emission of a header field, adding it as a new entry to the header table.
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 1 | Index (6+) | +---+---+-----------------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length octets) | +-------------------------------+
Literal Header Field with Incremental Indexing - Indexed Name
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 1 | 0 | +---+---+-----------------------+ | H | Name Length (7+) | +---+---------------------------+ | Name String (Length octets) | +---+---------------------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length octets) | +-------------------------------+
Literal Header Field with Incremental Indexing - New Name
A literal header field with incremental indexing representation starts with the '01' 2-bit pattern.
If the header field name matches the header field name of an entry stored in the header table or the static table, the header field 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 prefix (see Section 6.1). This value is always non-zero.
Otherwise, the header field name is represented as a literal. A value 0 is used in place of the 6-bit index, followed by the header field name (see Section 6.2).
Either form of header field name representation is followed by the header field value represented as a literal string as described in Section 6.2.
A literal header field without indexing representation causes the emission of a header field without altering the header table.
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 0 | 0 | Index (4+) | +---+---+-----------------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length octets) | +-------------------------------+
Literal Header Field without Indexing - Indexed Name
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 0 | 0 | 0 | +---+---+-----------------------+ | H | Name Length (7+) | +---+---------------------------+ | Name String (Length octets) | +---+---------------------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length octets) | +-------------------------------+
Literal Header Field without Indexing - New Name
A literal header field without indexing representation starts with the '0000' 4-bit pattern.
If the header field name matches the header field name of an entry stored in the header table or the static table, the header field 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 prefix (see Section 6.1). This value is always non-zero.
Otherwise, the header field name is represented as a literal. A value 0 is used in place of the 4-bit index, followed by the header field name (see Section 6.2).
Either form of header field name representation is followed by the header field value represented as a literal string as described in Section 6.2.
A literal header field never indexed representation causes the emission of a header field without altering the header table. Intermediaries MUST use the same representation for encoding this header field.
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 0 | 1 | Index (4+) | +---+---+-----------------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length octets) | +-------------------------------+
Literal Header Field Never Indexed - Indexed Name
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 0 | 1 | 0 | +---+---+-----------------------+ | H | Name Length (7+) | +---+---------------------------+ | Name String (Length octets) | +---+---------------------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length octets) | +-------------------------------+
Literal Header Field Never Indexed - New Name
A literal header field never indexed representation starts with the '0001' 4-bit pattern.
When a header field is represented as a literal header field never indexed, it MUST always be encoded with this specific literal representation. In particular, when a peer sends a header field that it received represented as a literal header field never indexed, it MUST use the same representation to forward this header field.
This representation is intended for protecting header field values that are not to be put at risk by compressing them (see Section 8.1 for more details).
The encoding of the representation is identical to the literal header field without indexing (see Section 7.2.2).
An encoding context update causes the immediate application of a change to the encoding context.
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 1 | F | ... | +---+---------------------------+
Context Update
An encoding context update starts with the '001' 3-bit pattern.
It is followed by a flag specifying the type of the change, and by any data necessary to describe the change itself.
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 1 | 1 | 0 | +---+---------------------------+
Reference Set Emptying
The flag bit being set to '1' signals that the reference set is emptied. The remaining bits MUST be set to '0', non-zero values MUST be treated as a decoding error.
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 1 | 0 | Max size (4+) | +---+---------------------------+
Maximum Header Table Size Change
The flag bit being set to '0' signals that a change to the maximum size of the header table. This new maximum size MUST be lower than or equal to the maximum set by the decoder. That is, the value of the HTTP/2 setting SETTINGS_HEADER_TABLE_SIZE, defined in [HTTP2].
The new maximum size is encoded as an integer with a 4-bit prefix (see Section 6.1).
Reducing the maximum size of the header table causes entries to be evicted (see Section 5.2).
This section describes potential areas of security concern with HPACK:
HPACK reduces the length of header field encodings by exploiting the redundancy inherent in protocols like HTTP. The ultimate goal of this is to reduce the amount of data that is required to send HTTP requests or responses.
The compression context used to encode header fields can be probed by an attacker that has the following capabilities: to define header fields to be encoded and transmitted; and to observe the length of those fields once they are encoded. This allows an attacker to adaptively modify requests in order to confirm guesses about the header table state. If a guess is compressed into a shorter length, the attacker can observe the encoded length and infer that the guess was correct.
This is possible because while TLS provides confidentiality protection for content, it only provides a limited amount of protection for the length of that content.
Attacks like [CRIME] demonstrated the existence of these general attacker capabilities. The specific attack exploited the fact that [DEFLATE] removes redundancy based on prefix matching. This permitted the attacker to confirm guesses a character at a time, reducing an exponential-time attack into a constant time attack.
HPACK mitigates but does not completely prevent attacks modelled on [CRIME] by forcing a guess to match an entire header field value, rather than individual characters. An attacker can only learn whether a guess is correct or not, so is reduced to a brute force guess for the header field values.
The viability of recovering specific header field values therefore depends on the entropy of values. As a result, values with high entropy are unlikely to be recovered successfully. However, values with low entropy remain vulnerable.
Attacks of this nature are possible any time that two mutually distrustful entities control requests or responses that are placed onto a single HTTP/2 connection. If the shared HPACK compressor permits one entity to add entries to the header table, and the other to access those entries, then the state of the table can be learned.
Having requests or responses from mutually distrustful entities occurs when an intermediary either: web origins [ORIGIN] are made by mutually distrustful entities.
Web browsers also need to assume that requests made on the same connection by different
Users of HTTP that require confidentiality for header fields can use values with entropy sufficient to make guessing infeasible. However, this is impractical as a general solution because it forces all users of HTTP to take steps to mitigate attacks. It would impose new constraints on how HTTP is used.
Rather than impose constraints on users of HTTP, an implementation of HPACK can instead constrain how compression is applied in order to limit the potential for header table probing.
An ideal solution segregates access to the header table based on the entity that is constructing header fields. Header field values that are added to the table are attributed to an entity, and only the entity that created an particular value can extract that value.
To improve compression performance of this option, certain entries might be tagged as being public. For example, a web browser might make the values of the Accept-Encoding header field available in all requests.
An encoder without good knowledge of the provenance of header fields might instead introduce a penalty for bad guesses, such that attempts to guess a header field value results in all values being removed from consideration in all future requests, effectively preventing further guesses.
This response might be made inversely proportional to the length of the header field. Marking as inaccessible might occur for shorter values more quickly or with higher probability than for longer values.
Implementations might also choose to protect certain header fields that are known to be highly valued, such as the Authorization or Cookie header fields, by disabling or further limiting compression.
Refusing to generate an indexed representation for a header field is only effective if compression is avoided on all hops. The never indexed literal [literal.header.never.indexed] can be used to signal to intermediaries that a particular value was intentionally sent as a literal. An intermediary MUST NOT re-encode a value that uses the never indexed literal as an indexed representation.
There is currently no known threat taking advantage of the use of a fixed Huffman encoding. A study has shown that using a fixed Huffman encoding table created an information leakage, however this same study concluded that an attacker could not take advantage of this information leakage to recover any meaningful amount of information (see [PETAL]).
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 allocated by an endpoint.
The amount of memory used by the compressor state is limited by the decoder using the value of the HTTP/2 setting SETTINGS_HEADER_TABLE_SIZE (see [HTTP2]). This limit takes into account both the size of the data stored in the header table, plus a small allowance for overhead.
A decoder can limit the amount of state memory used by setting an appropriate value for the setting SETTINGS_HEADER_TABLE_SIZE. An encoder can limit the amount of state memory it uses by signaling lower header table size than the decoder allows (see Section 7.3).
The amount of temporary memory consumed by an encoder or decoder can be limited by processing header fields sequentially. An implementation does not need to retain a complete set of header fields. Note however that it might be necessary for an application to retain a complete header set for other reasons; even though HPACK does not force this to occur, application constraints might make this necessary.
An implementation of HPACK needs to ensure that large values for integers, long encoding for integers, or long string literals do not create security weaknesses.
An implementation has to set a limit for the values it accepts for integers, as well as for the encoded length (see Section 6.1). In the same way, it has to set a limit to the length it accepts for string literals (see Section 6.2).
This document includes substantial input from the following individuals:
| [HTTP2] | Belshe, M., Peon, R. and M. Thomson, "Hypertext Transfer Protocol version 2", Internet-Draft draft-ietf-httpbis-http2-13, June 2014. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC7230] | Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, June 2014. |
| [CANONICAL] | Schwartz, E. and B. Kallick, "Generating a canonical prefix encoding", Communications of the ACM Volume 7 Issue 3, pp. 166-169, March 1964. |
| [CRIME] | Rizzo, J. and T. Duong, "The CRIME Attack", September 2012. |
| [DEFLATE] | Deutsch, P., "DEFLATE Compressed Data Format Specification version 1.3", RFC 1951, May 1996. |
| [HUFFMAN] | Huffman, D., "A Method for the Construction of Minimum Redundancy Codes", Proceedings of the Institute of Radio Engineers Volume 40, Number 9, pp. 1098-1101, September 1952. |
| [ORIGIN] | Barth, A., "The Web Origin Concept", RFC 6454, December 2011. |
| [PETAL] | Tan, J. and J. Nahata, "PETAL: Preset Encoding Table Information Leakage", April 2013. |
| [SPDY] | Belshe, M. and R. Peon, "SPDY Protocol", Internet-Draft draft-mbelshe-httpbis-spdy-00, February 2012. |
| [SPDY-DESC-1] | Belshe, M., "IETF83: SPDY and What to Consider for HTTP/2.0", March 2012. |
| [SPDY-DESC-2] | McManus, P., "SPDY: What I Like About You", September 2011. |
The static table consists of an unchangeable ordered list of (name, value) pairs. The first entry in the table is always represented by the index len(header table) + 1, and the last entry in the table is represented by the index len(header table) + len(static table).
The static table was created by listing the most common header fields that are valid for messages exchanged inside a HTTP/2 connection. For header fields with a few frequent values, an entry was added for each of these frequent values. For other header fields, an entry was added with an empty value.
The following table lists the pre-defined header fields that make-up the static table.
| Index | Header Name | Header Value |
|---|---|---|
| 1 | :authority | |
| 2 | :method | GET |
| 3 | :method | POST |
| 4 | :path | / |
| 5 | :path | /index.html |
| 6 | :scheme | http |
| 7 | :scheme | https |
| 8 | :status | 200 |
| 9 | :status | 204 |
| 10 | :status | 206 |
| 11 | :status | 304 |
| 12 | :status | 400 |
| 13 | :status | 404 |
| 14 | :status | 500 |
| 15 | accept-charset | |
| 16 | accept-encoding | gzip, deflate |
| 17 | accept-language | |
| 18 | accept-ranges | |
| 19 | accept | |
| 20 | access-control-allow-origin | |
| 21 | age | |
| 22 | allow | |
| 23 | authorization | |
| 24 | cache-control | |
| 25 | content-disposition | |
| 26 | content-encoding | |
| 27 | content-language | |
| 28 | content-length | |
| 29 | content-location | |
| 30 | content-range | |
| 31 | content-type | |
| 32 | cookie | |
| 33 | date | |
| 34 | etag | |
| 35 | expect | |
| 36 | expires | |
| 37 | from | |
| 38 | host | |
| 39 | if-match | |
| 40 | if-modified-since | |
| 41 | if-none-match | |
| 42 | if-range | |
| 43 | if-unmodified-since | |
| 44 | last-modified | |
| 45 | link | |
| 46 | location | |
| 47 | max-forwards | |
| 48 | proxy-authenticate | |
| 49 | proxy-authorization | |
| 50 | range | |
| 51 | referer | |
| 52 | refresh | |
| 53 | retry-after | |
| 54 | server | |
| 55 | set-cookie | |
| 56 | strict-transport-security | |
| 57 | transfer-encoding | |
| 58 | user-agent | |
| 59 | vary | |
| 60 | via | |
| 61 | www-authenticate |
Table 1 gives the index of each entry in the static table. The full index of each entry, to be used for encoding a reference to this entry, is computed by adding the number of entries in the header table to this index.
The following Huffman code is used when encoding string literals with a Huffman coding (see Section 6.2).
This Huffman code was generated from statistics obtained on a large sample of HTTP headers. It is a canonical Huffman code [CANONICAL] with some tweaking to ensure that no symbol has a unique code length.
Each row in the table defines the code used to represent a symbol:
As an example, the code for the symbol 47 (corresponding to the ASCII character "/") consists in the 6 bits "0", "1", "1", "0", "0", "0". This corresponds to the value 0x18 (in hexadecimal) encoded on 6 bits.
code
code as bits as hex len
sym aligned to MSB aligned in
to LSB bits
( 0) |11111111|11000 1ff8 [13]
( 1) |11111111|11111111|1011000 7fffd8 [23]
( 2) |11111111|11111111|11111110|0010 fffffe2 [28]
( 3) |11111111|11111111|11111110|0011 fffffe3 [28]
( 4) |11111111|11111111|11111110|0100 fffffe4 [28]
( 5) |11111111|11111111|11111110|0101 fffffe5 [28]
( 6) |11111111|11111111|11111110|0110 fffffe6 [28]
( 7) |11111111|11111111|11111110|0111 fffffe7 [28]
( 8) |11111111|11111111|11111110|1000 fffffe8 [28]
( 9) |11111111|11111111|11101010 ffffea [24]
( 10) |11111111|11111111|11111111|111100 3ffffffc [30]
( 11) |11111111|11111111|11111110|1001 fffffe9 [28]
( 12) |11111111|11111111|11111110|1010 fffffea [28]
( 13) |11111111|11111111|11111111|111101 3ffffffd [30]
( 14) |11111111|11111111|11111110|1011 fffffeb [28]
( 15) |11111111|11111111|11111110|1100 fffffec [28]
( 16) |11111111|11111111|11111110|1101 fffffed [28]
( 17) |11111111|11111111|11111110|1110 fffffee [28]
( 18) |11111111|11111111|11111110|1111 fffffef [28]
( 19) |11111111|11111111|11111111|0000 ffffff0 [28]
( 20) |11111111|11111111|11111111|0001 ffffff1 [28]
( 21) |11111111|11111111|11111111|0010 ffffff2 [28]
( 22) |11111111|11111111|11111111|111110 3ffffffe [30]
( 23) |11111111|11111111|11111111|0011 ffffff3 [28]
( 24) |11111111|11111111|11111111|0100 ffffff4 [28]
( 25) |11111111|11111111|11111111|0101 ffffff5 [28]
( 26) |11111111|11111111|11111111|0110 ffffff6 [28]
( 27) |11111111|11111111|11111111|0111 ffffff7 [28]
( 28) |11111111|11111111|11111111|1000 ffffff8 [28]
( 29) |11111111|11111111|11111111|1001 ffffff9 [28]
( 30) |11111111|11111111|11111111|1010 ffffffa [28]
( 31) |11111111|11111111|11111111|1011 ffffffb [28]
' ' ( 32) |010100 14 [ 6]
'!' ( 33) |11111110|00 3f8 [10]
'"' ( 34) |11111110|01 3f9 [10]
'#' ( 35) |11111111|1010 ffa [12]
'$' ( 36) |11111111|11001 1ff9 [13]
'%' ( 37) |010101 15 [ 6]
'&' ( 38) |11111000 f8 [ 8]
''' ( 39) |11111111|010 7fa [11]
'(' ( 40) |11111110|10 3fa [10]
')' ( 41) |11111110|11 3fb [10]
'*' ( 42) |11111001 f9 [ 8]
'+' ( 43) |11111111|011 7fb [11]
',' ( 44) |11111010 fa [ 8]
'-' ( 45) |010110 16 [ 6]
'.' ( 46) |010111 17 [ 6]
'/' ( 47) |011000 18 [ 6]
'0' ( 48) |00000 0 [ 5]
'1' ( 49) |00001 1 [ 5]
'2' ( 50) |00010 2 [ 5]
'3' ( 51) |011001 19 [ 6]
'4' ( 52) |011010 1a [ 6]
'5' ( 53) |011011 1b [ 6]
'6' ( 54) |011100 1c [ 6]
'7' ( 55) |011101 1d [ 6]
'8' ( 56) |011110 1e [ 6]
'9' ( 57) |011111 1f [ 6]
':' ( 58) |1011100 5c [ 7]
';' ( 59) |11111011 fb [ 8]
'<' ( 60) |11111111|1111100 7ffc [15]
'=' ( 61) |100000 20 [ 6]
'>' ( 62) |11111111|1011 ffb [12]
'?' ( 63) |11111111|00 3fc [10]
'@' ( 64) |11111111|11010 1ffa [13]
'A' ( 65) |100001 21 [ 6]
'B' ( 66) |1011101 5d [ 7]
'C' ( 67) |1011110 5e [ 7]
'D' ( 68) |1011111 5f [ 7]
'E' ( 69) |1100000 60 [ 7]
'F' ( 70) |1100001 61 [ 7]
'G' ( 71) |1100010 62 [ 7]
'H' ( 72) |1100011 63 [ 7]
'I' ( 73) |1100100 64 [ 7]
'J' ( 74) |1100101 65 [ 7]
'K' ( 75) |1100110 66 [ 7]
'L' ( 76) |1100111 67 [ 7]
'M' ( 77) |1101000 68 [ 7]
'N' ( 78) |1101001 69 [ 7]
'O' ( 79) |1101010 6a [ 7]
'P' ( 80) |1101011 6b [ 7]
'Q' ( 81) |1101100 6c [ 7]
'R' ( 82) |1101101 6d [ 7]
'S' ( 83) |1101110 6e [ 7]
'T' ( 84) |1101111 6f [ 7]
'U' ( 85) |1110000 70 [ 7]
'V' ( 86) |1110001 71 [ 7]
'W' ( 87) |1110010 72 [ 7]
'X' ( 88) |11111100 fc [ 8]
'Y' ( 89) |1110011 73 [ 7]
'Z' ( 90) |11111101 fd [ 8]
'[' ( 91) |11111111|11011 1ffb [13]
'\' ( 92) |11111111|11111110|000 7fff0 [19]
']' ( 93) |11111111|11100 1ffc [13]
'^' ( 94) |11111111|111100 3ffc [14]
'_' ( 95) |100010 22 [ 6]
'`' ( 96) |11111111|1111101 7ffd [15]
'a' ( 97) |00011 3 [ 5]
'b' ( 98) |100011 23 [ 6]
'c' ( 99) |00100 4 [ 5]
'd' (100) |100100 24 [ 6]
'e' (101) |00101 5 [ 5]
'f' (102) |100101 25 [ 6]
'g' (103) |100110 26 [ 6]
'h' (104) |100111 27 [ 6]
'i' (105) |00110 6 [ 5]
'j' (106) |1110100 74 [ 7]
'k' (107) |1110101 75 [ 7]
'l' (108) |101000 28 [ 6]
'm' (109) |101001 29 [ 6]
'n' (110) |101010 2a [ 6]
'o' (111) |00111 7 [ 5]
'p' (112) |101011 2b [ 6]
'q' (113) |1110110 76 [ 7]
'r' (114) |101100 2c [ 6]
's' (115) |01000 8 [ 5]
't' (116) |01001 9 [ 5]
'u' (117) |101101 2d [ 6]
'v' (118) |1110111 77 [ 7]
'w' (119) |1111000 78 [ 7]
'x' (120) |1111001 79 [ 7]
'y' (121) |1111010 7a [ 7]
'z' (122) |1111011 7b [ 7]
'{' (123) |11111111|1111110 7ffe [15]
'|' (124) |11111111|100 7fc [11]
'}' (125) |11111111|111101 3ffd [14]
'~' (126) |11111111|11101 1ffd [13]
(127) |11111111|11111111|11111111|1100 ffffffc [28]
(128) |11111111|11111110|0110 fffe6 [20]
(129) |11111111|11111111|010010 3fffd2 [22]
(130) |11111111|11111110|0111 fffe7 [20]
(131) |11111111|11111110|1000 fffe8 [20]
(132) |11111111|11111111|010011 3fffd3 [22]
(133) |11111111|11111111|010100 3fffd4 [22]
(134) |11111111|11111111|010101 3fffd5 [22]
(135) |11111111|11111111|1011001 7fffd9 [23]
(136) |11111111|11111111|010110 3fffd6 [22]
(137) |11111111|11111111|1011010 7fffda [23]
(138) |11111111|11111111|1011011 7fffdb [23]
(139) |11111111|11111111|1011100 7fffdc [23]
(140) |11111111|11111111|1011101 7fffdd [23]
(141) |11111111|11111111|1011110 7fffde [23]
(142) |11111111|11111111|11101011 ffffeb [24]
(143) |11111111|11111111|1011111 7fffdf [23]
(144) |11111111|11111111|11101100 ffffec [24]
(145) |11111111|11111111|11101101 ffffed [24]
(146) |11111111|11111111|010111 3fffd7 [22]
(147) |11111111|11111111|1100000 7fffe0 [23]
(148) |11111111|11111111|11101110 ffffee [24]
(149) |11111111|11111111|1100001 7fffe1 [23]
(150) |11111111|11111111|1100010 7fffe2 [23]
(151) |11111111|11111111|1100011 7fffe3 [23]
(152) |11111111|11111111|1100100 7fffe4 [23]
(153) |11111111|11111110|11100 1fffdc [21]
(154) |11111111|11111111|011000 3fffd8 [22]
(155) |11111111|11111111|1100101 7fffe5 [23]
(156) |11111111|11111111|011001 3fffd9 [22]
(157) |11111111|11111111|1100110 7fffe6 [23]
(158) |11111111|11111111|1100111 7fffe7 [23]
(159) |11111111|11111111|11101111 ffffef [24]
(160) |11111111|11111111|011010 3fffda [22]
(161) |11111111|11111110|11101 1fffdd [21]
(162) |11111111|11111110|1001 fffe9 [20]
(163) |11111111|11111111|011011 3fffdb [22]
(164) |11111111|11111111|011100 3fffdc [22]
(165) |11111111|11111111|1101000 7fffe8 [23]
(166) |11111111|11111111|1101001 7fffe9 [23]
(167) |11111111|11111110|11110 1fffde [21]
(168) |11111111|11111111|1101010 7fffea [23]
(169) |11111111|11111111|011101 3fffdd [22]
(170) |11111111|11111111|011110 3fffde [22]
(171) |11111111|11111111|11110000 fffff0 [24]
(172) |11111111|11111110|11111 1fffdf [21]
(173) |11111111|11111111|011111 3fffdf [22]
(174) |11111111|11111111|1101011 7fffeb [23]
(175) |11111111|11111111|1101100 7fffec [23]
(176) |11111111|11111111|00000 1fffe0 [21]
(177) |11111111|11111111|00001 1fffe1 [21]
(178) |11111111|11111111|100000 3fffe0 [22]
(179) |11111111|11111111|00010 1fffe2 [21]
(180) |11111111|11111111|1101101 7fffed [23]
(181) |11111111|11111111|100001 3fffe1 [22]
(182) |11111111|11111111|1101110 7fffee [23]
(183) |11111111|11111111|1101111 7fffef [23]
(184) |11111111|11111110|1010 fffea [20]
(185) |11111111|11111111|100010 3fffe2 [22]
(186) |11111111|11111111|100011 3fffe3 [22]
(187) |11111111|11111111|100100 3fffe4 [22]
(188) |11111111|11111111|1110000 7ffff0 [23]
(189) |11111111|11111111|100101 3fffe5 [22]
(190) |11111111|11111111|100110 3fffe6 [22]
(191) |11111111|11111111|1110001 7ffff1 [23]
(192) |11111111|11111111|11111000|00 3ffffe0 [26]
(193) |11111111|11111111|11111000|01 3ffffe1 [26]
(194) |11111111|11111110|1011 fffeb [20]
(195) |11111111|11111110|001 7fff1 [19]
(196) |11111111|11111111|100111 3fffe7 [22]
(197) |11111111|11111111|1110010 7ffff2 [23]
(198) |11111111|11111111|101000 3fffe8 [22]
(199) |11111111|11111111|11110110|0 1ffffec [25]
(200) |11111111|11111111|11111000|10 3ffffe2 [26]
(201) |11111111|11111111|11111000|11 3ffffe3 [26]
(202) |11111111|11111111|11111001|00 3ffffe4 [26]
(203) |11111111|11111111|11111011|110 7ffffde [27]
(204) |11111111|11111111|11111011|111 7ffffdf [27]
(205) |11111111|11111111|11111001|01 3ffffe5 [26]
(206) |11111111|11111111|11110001 fffff1 [24]
(207) |11111111|11111111|11110110|1 1ffffed [25]
(208) |11111111|11111110|010 7fff2 [19]
(209) |11111111|11111111|00011 1fffe3 [21]
(210) |11111111|11111111|11111001|10 3ffffe6 [26]
(211) |11111111|11111111|11111100|000 7ffffe0 [27]
(212) |11111111|11111111|11111100|001 7ffffe1 [27]
(213) |11111111|11111111|11111001|11 3ffffe7 [26]
(214) |11111111|11111111|11111100|010 7ffffe2 [27]
(215) |11111111|11111111|11110010 fffff2 [24]
(216) |11111111|11111111|00100 1fffe4 [21]
(217) |11111111|11111111|00101 1fffe5 [21]
(218) |11111111|11111111|11111010|00 3ffffe8 [26]
(219) |11111111|11111111|11111010|01 3ffffe9 [26]
(220) |11111111|11111111|11111111|1101 ffffffd [28]
(221) |11111111|11111111|11111100|011 7ffffe3 [27]
(222) |11111111|11111111|11111100|100 7ffffe4 [27]
(223) |11111111|11111111|11111100|101 7ffffe5 [27]
(224) |11111111|11111110|1100 fffec [20]
(225) |11111111|11111111|11110011 fffff3 [24]
(226) |11111111|11111110|1101 fffed [20]
(227) |11111111|11111111|00110 1fffe6 [21]
(228) |11111111|11111111|101001 3fffe9 [22]
(229) |11111111|11111111|00111 1fffe7 [21]
(230) |11111111|11111111|01000 1fffe8 [21]
(231) |11111111|11111111|1110011 7ffff3 [23]
(232) |11111111|11111111|101010 3fffea [22]
(233) |11111111|11111111|101011 3fffeb [22]
(234) |11111111|11111111|11110111|0 1ffffee [25]
(235) |11111111|11111111|11110111|1 1ffffef [25]
(236) |11111111|11111111|11110100 fffff4 [24]
(237) |11111111|11111111|11110101 fffff5 [24]
(238) |11111111|11111111|11111010|10 3ffffea [26]
(239) |11111111|11111111|1110100 7ffff4 [23]
(240) |11111111|11111111|11111010|11 3ffffeb [26]
(241) |11111111|11111111|11111100|110 7ffffe6 [27]
(242) |11111111|11111111|11111011|00 3ffffec [26]
(243) |11111111|11111111|11111011|01 3ffffed [26]
(244) |11111111|11111111|11111100|111 7ffffe7 [27]
(245) |11111111|11111111|11111101|000 7ffffe8 [27]
(246) |11111111|11111111|11111101|001 7ffffe9 [27]
(247) |11111111|11111111|11111101|010 7ffffea [27]
(248) |11111111|11111111|11111101|011 7ffffeb [27]
(249) |11111111|11111111|11111111|1110 ffffffe [28]
(250) |11111111|11111111|11111101|100 7ffffec [27]
(251) |11111111|11111111|11111101|101 7ffffed [27]
(252) |11111111|11111111|11111101|110 7ffffee [27]
(253) |11111111|11111111|11111101|111 7ffffef [27]
(254) |11111111|11111111|11111110|000 7fffff0 [27]
(255) |11111111|11111111|11111011|10 3ffffee [26]
EOS (256) |11111111|11111111|11111111|111111 3fffffff [30]
A number of examples are worked through here, covering integer encoding, header field representation, and the encoding of whole sets of header fields, for both requests and responses, and with and without Huffman coding.
This section shows the representation of integer values in details (see Section 6.1).
The value 10 is to be encoded with a 5-bit prefix.
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | X | X | X | 0 | 1 | 0 | 1 | 0 | 10 stored on 5 bits +---+---+---+---+---+---+---+---+
The value I=1337 is to be encoded with a 5-bit prefix.
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 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 | 0 | 0 | 0 | 0 | 1 | 0 | 1 | 0 | 10<128, encode(10), done +---+---+---+---+---+---+---+---+
The value 42 is to be encoded starting at an octet-boundary. This implies that a 8-bit prefix is used.
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 42 stored on 8 bits +---+---+---+---+---+---+---+---+
This section shows several independent representation examples.
The header field representation uses a literal name and a literal value. The header field is added to the header table.
Header set to encode:
custom-key: custom-header
Reference set: empty.
Hex dump of encoded data:
400a 6375 7374 6f6d 2d6b 6579 0d63 7573 | @.custom-key.cus 746f 6d2d 6865 6164 6572 | tom-header
Decoding process:
40 | == Literal indexed ==
0a | Literal name (len = 10)
6375 7374 6f6d 2d6b 6579 | custom-key
0d | Literal value (len = 13)
6375 7374 6f6d 2d68 6561 6465 72 | custom-header
| -> custom-key: custom-head\
| er
Header Table (after decoding):
[ 1] (s = 55) custom-key: custom-header
Table size: 55
Decoded header set:
custom-key: custom-header
The header field representation uses an indexed name and a literal value. The header field is not added to the header table.
Header set to encode:
:path: /sample/path
Reference set: empty.
Hex dump of encoded data:
040c 2f73 616d 706c 652f 7061 7468 | ../sample/path
Decoding process:
04 | == Literal not indexed ==
| Indexed name (idx = 4)
| :path
0c | Literal value (len = 12)
2f73 616d 706c 652f 7061 7468 | /sample/path
| -> :path: /sample/path
Header table (after decoding): empty.
Decoded header set:
:path: /sample/path
The header field representation uses a literal name and a literal value. The header field is not added to the header table, and must use the same representation if re-encoded by an intermediary.
Header set to encode:
password: secret
Reference set: empty.
Hex dump of encoded data:
1008 7061 7373 776f 7264 0673 6563 7265 | ..password.secre 74 | t
Decoding process:
10 | == Literal never indexed ==
08 | Literal name (len = 8)
7061 7373 776f 7264 | password
06 | Literal value (len = 6)
7365 6372 6574 | secret
| -> password: secret
Header table (after decoding): empty.
Decoded header set:
password: secret
The header field representation uses an indexed header field, from the static table. Upon using it, the static table entry is copied into the header table.
Header set to encode:
:method: GET
Reference set: empty.
Hex dump of encoded data:
82 | .
Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
Header Table (after decoding):
[ 1] (s = 42) :method: GET
Table size: 42
Decoded header set:
:method: GET
The header field representation uses an indexed header field, from the static table. In this example, the HTTP/2 setting SETTINGS_HEADER_TABLE_SIZE is set to 0, therefore, the entry is not copied into the header table.
Header set to encode:
:method: GET
Reference set: empty.
Hex dump of encoded data:
82 | .
Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
Header table (after decoding): empty.
Decoded header set:
:method: GET
This section shows several consecutive header sets, corresponding to HTTP requests, on the same connection.
Header set to encode:
:method: GET :scheme: http :path: / :authority: www.example.com
Reference set: empty.
Hex dump of encoded data:
8287 8644 0f77 7777 2e65 7861 6d70 6c65 | ...D.www.example 2e63 6f6d | .com
Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
87 | == Indexed - Add ==
| idx = 7
| -> :scheme: http
86 | == Indexed - Add ==
| idx = 6
| -> :path: /
44 | == Literal indexed ==
| Indexed name (idx = 4)
| :authority
0f | Literal value (len = 15)
7777 772e 6578 616d 706c 652e 636f 6d | www.example.com
| -> :authority: www.example\
| .com
Header Table (after decoding):
[ 1] (s = 57) :authority: www.example.com
[ 2] (s = 38) :path: /
[ 3] (s = 43) :scheme: http
[ 4] (s = 42) :method: GET
Table size: 180
Decoded header set:
:method: GET :scheme: http :path: / :authority: www.example.com
This request takes advantage of the differential encoding of header sets.
Header set to encode:
:method: GET :scheme: http :path: / :authority: www.example.com cache-control: no-cache
Reference set:
[ 1] :authority: www.example.com [ 2] :path: / [ 3] :scheme: http [ 4] :method: GET
Hex dump of encoded data:
5c08 6e6f 2d63 6163 6865 | \.no-cache
Decoding process:
5c | == Literal indexed ==
| Indexed name (idx = 28)
| cache-control
08 | Literal value (len = 8)
6e6f 2d63 6163 6865 | no-cache
| -> cache-control: no-cache
Header Table (after decoding):
[ 1] (s = 53) cache-control: no-cache
[ 2] (s = 57) :authority: www.example.com
[ 3] (s = 38) :path: /
[ 4] (s = 43) :scheme: http
[ 5] (s = 42) :method: GET
Table size: 233
Decoded header set:
cache-control: no-cache :authority: www.example.com :path: / :scheme: http :method: GET
This request has not enough headers in common with the previous request to take advantage of the differential encoding. Therefore, the reference set is emptied before encoding the header fields.
Header set to encode:
:method: GET :scheme: https :path: /index.html :authority: www.example.com custom-key: custom-value
Reference set:
[ 1] cache-control: no-cache [ 2] :authority: www.example.com [ 3] :path: / [ 4] :scheme: http [ 5] :method: GET
Hex dump of encoded data:
3085 8c8b 8440 0a63 7573 746f 6d2d 6b65 | 0....@.custom-ke 790c 6375 7374 6f6d 2d76 616c 7565 | y.custom-value
Decoding process:
30 | == Empty reference set ==
| idx = 0
| flag = 1
85 | == Indexed - Add ==
| idx = 5
| -> :method: GET
8c | == Indexed - Add ==
| idx = 12
| -> :scheme: https
8b | == Indexed - Add ==
| idx = 11
| -> :path: /index.html
84 | == Indexed - Add ==
| idx = 4
| -> :authority: www.example\
| .com
40 | == Literal indexed ==
0a | Literal name (len = 10)
6375 7374 6f6d 2d6b 6579 | custom-key
0c | Literal value (len = 12)
6375 7374 6f6d 2d76 616c 7565 | custom-value
| -> custom-key: custom-valu\
| e
Header Table (after decoding):
[ 1] (s = 54) custom-key: custom-value
[ 2] (s = 48) :path: /index.html
[ 3] (s = 44) :scheme: https
[ 4] (s = 53) cache-control: no-cache
[ 5] (s = 57) :authority: www.example.com
[ 6] (s = 38) :path: /
[ 7] (s = 43) :scheme: http
[ 8] (s = 42) :method: GET
Table size: 379
Decoded header set:
:method: GET :scheme: https :path: /index.html :authority: www.example.com custom-key: custom-value
This section shows the same examples as the previous section, but using Huffman encoding for the literal values.
Header set to encode:
:method: GET :scheme: http :path: / :authority: www.example.com
Reference set: empty.
Hex dump of encoded data:
8287 8644 8cf1 e3c2 e5f2 3a6b a0ab 90f4 | ...D......:k.... ff | .
Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
87 | == Indexed - Add ==
| idx = 7
| -> :scheme: http
86 | == Indexed - Add ==
| idx = 6
| -> :path: /
44 | == Literal indexed ==
| Indexed name (idx = 4)
| :authority
8c | Literal value (len = 15)
| Huffman encoded:
f1e3 c2e5 f23a 6ba0 ab90 f4ff | .....:k.....
| Decoded:
| www.example.com
| -> :authority: www.example\
| .com
Header Table (after decoding):
[ 1] (s = 57) :authority: www.example.com
[ 2] (s = 38) :path: /
[ 3] (s = 43) :scheme: http
[ 4] (s = 42) :method: GET
Table size: 180
Decoded header set:
:method: GET :scheme: http :path: / :authority: www.example.com
This request takes advantage of the differential encoding of header sets.
Header set to encode:
:method: GET :scheme: http :path: / :authority: www.example.com cache-control: no-cache
Reference set:
[ 1] :authority: www.example.com [ 2] :path: / [ 3] :scheme: http [ 4] :method: GET
Hex dump of encoded data:
5c86 a8eb 1064 9cbf | \....d..
Decoding process:
5c | == Literal indexed ==
| Indexed name (idx = 28)
| cache-control
86 | Literal value (len = 8)
| Huffman encoded:
a8eb 1064 9cbf | ...d..
| Decoded:
| no-cache
| -> cache-control: no-cache
Header Table (after decoding):
[ 1] (s = 53) cache-control: no-cache
[ 2] (s = 57) :authority: www.example.com
[ 3] (s = 38) :path: /
[ 4] (s = 43) :scheme: http
[ 5] (s = 42) :method: GET
Table size: 233
Decoded header set:
cache-control: no-cache :authority: www.example.com :path: / :scheme: http :method: GET
This request has not enough headers in common with the previous request to take advantage of the differential encoding. Therefore, the reference set is emptied before encoding the header fields.
Header set to encode:
:method: GET :scheme: https :path: /index.html :authority: www.example.com custom-key: custom-value
Reference set:
[ 1] cache-control: no-cache [ 2] :authority: www.example.com [ 3] :path: / [ 4] :scheme: http [ 5] :method: GET
Hex dump of encoded data:
3085 8c8b 8440 8825 a849 e95b a97d 7f89 | 0....@.%.I.[.}.. 25a8 49e9 5bb8 e8b4 bf | %.I.[....
Decoding process:
30 | == Empty reference set ==
| idx = 0
| flag = 1
85 | == Indexed - Add ==
| idx = 5
| -> :method: GET
8c | == Indexed - Add ==
| idx = 12
| -> :scheme: https
8b | == Indexed - Add ==
| idx = 11
| -> :path: /index.html
84 | == Indexed - Add ==
| idx = 4
| -> :authority: www.example\
| .com
40 | == Literal indexed ==
88 | Literal name (len = 10)
| Huffman encoded:
25a8 49e9 5ba9 7d7f | %.I.[.}.
| Decoded:
| custom-key
89 | Literal value (len = 12)
| Huffman encoded:
25a8 49e9 5bb8 e8b4 bf | %.I.[....
| Decoded:
| custom-value
| -> custom-key: custom-valu\
| e
Header Table (after decoding):
[ 1] (s = 54) custom-key: custom-value
[ 2] (s = 48) :path: /index.html
[ 3] (s = 44) :scheme: https
[ 4] (s = 53) cache-control: no-cache
[ 5] (s = 57) :authority: www.example.com
[ 6] (s = 38) :path: /
[ 7] (s = 43) :scheme: http
[ 8] (s = 42) :method: GET
Table size: 379
Decoded header set:
:method: GET :scheme: https :path: /index.html :authority: www.example.com custom-key: custom-value
This section shows several consecutive header sets, corresponding to HTTP responses, on the same connection. The HTTP/2 setting SETTINGS_HEADER_TABLE_SIZE is set to the value of 256 octets, causing some evictions to occur.
Header set to encode:
:status: 302 cache-control: private date: Mon, 21 Oct 2013 20:13:21 GMT location: https://www.example.com
Reference set: empty.
Hex dump of encoded data:
4803 3330 3259 0770 7269 7661 7465 631d | H.302Y.privatec. 4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013 2032 303a 3133 3a32 3120 474d 5471 1768 | 20:13:21 GMTq.h 7474 7073 3a2f 2f77 7777 2e65 7861 6d70 | ttps://www.examp 6c65 2e63 6f6d | le.com
Decoding process:
48 | == Literal indexed ==
| Indexed name (idx = 8)
| :status
03 | Literal value (len = 3)
3330 32 | 302
| -> :status: 302
59 | == Literal indexed ==
| Indexed name (idx = 25)
| cache-control
07 | Literal value (len = 7)
7072 6976 6174 65 | private
| -> cache-control: private
63 | == Literal indexed ==
| Indexed name (idx = 35)
| date
1d | Literal value (len = 29)
4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
2032 303a 3133 3a32 3120 474d 54 | 20:13:21 GMT
| -> date: Mon, 21 Oct 2013 \
| 20:13:21 GMT
71 | == Literal indexed ==
| Indexed name (idx = 49)
| location
17 | Literal value (len = 23)
6874 7470 733a 2f2f 7777 772e 6578 616d | https://www.exam
706c 652e 636f 6d | ple.com
| -> location: https://www.e\
| xample.com
Header Table (after decoding):
[ 1] (s = 63) location: https://www.example.com
[ 2] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 3] (s = 52) cache-control: private
[ 4] (s = 42) :status: 302
Table size: 222
Decoded header set:
:status: 302 cache-control: private date: Mon, 21 Oct 2013 20:13:21 GMT location: https://www.example.com
The (":status", "302") header field is evicted from the header table to free space to allow adding the (":status", "200") header field, copied from the static table into the header table. The (":status", "302") header field doesn't need to be removed from the reference set as it is evicted from the header table.
Header set to encode:
:status: 200 cache-control: private date: Mon, 21 Oct 2013 20:13:21 GMT location: https://www.example.com
Reference set:
[ 1] location: https://www.example.com [ 2] date: Mon, 21 Oct 2013 20:13:21 GMT [ 3] cache-control: private [ 4] :status: 302
Hex dump of encoded data:
8c | .
Decoding process:
8c | == Indexed - Add ==
| idx = 12
| - evict: :status: 302
| -> :status: 200
Header Table (after decoding):
[ 1] (s = 42) :status: 200
[ 2] (s = 63) location: https://www.example.com
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 4] (s = 52) cache-control: private
Table size: 222
Decoded header set:
:status: 200 location: https://www.example.com date: Mon, 21 Oct 2013 20:13:21 GMT cache-control: private
Several header fields are evicted from the header table during the processing of this header set. Before evicting a header belonging to the reference set, it is emitted, by coding it twice as an Indexed Representation. The first representation removes the header field from the reference set, the second one adds it again to the reference set, also emitting it.
Header set to encode:
:status: 200 cache-control: private date: Mon, 21 Oct 2013 20:13:22 GMT location: https://www.example.com content-encoding: gzip set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1
Reference set:
[ 1] :status: 200 [ 2] location: https://www.example.com [ 3] date: Mon, 21 Oct 2013 20:13:21 GMT [ 4] cache-control: private
Hex dump of encoded data:
8484 431d 4d6f 6e2c 2032 3120 4f63 7420 | ..C.Mon, 21 Oct
3230 3133 2032 303a 3133 3a32 3220 474d | 2013 20:13:22 GM
545e 0467 7a69 7084 8483 837b 3866 6f6f | T^.gzip....{8foo
3d41 5344 4a4b 4851 4b42 5a58 4f51 5745 | =ASDJKHQKBZXOQWE
4f50 4955 4158 5157 454f 4955 3b20 6d61 | OPIUAXQWEOIU; ma
782d 6167 653d 3336 3030 3b20 7665 7273 | x-age=3600; vers
696f 6e3d 31 | ion=1
Decoding process:
84 | == Indexed - Remove ==
| idx = 4
| -> cache-control: private
84 | == Indexed - Add ==
| idx = 4
| -> cache-control: private
43 | == Literal indexed ==
| Indexed name (idx = 3)
| date
1d | Literal value (len = 29)
4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
2032 303a 3133 3a32 3220 474d 54 | 20:13:22 GMT
| - evict: cache-control: pr\
| ivate
| -> date: Mon, 21 Oct 2013 \
| 20:13:22 GMT
5e | == Literal indexed ==
| Indexed name (idx = 30)
| content-encoding
04 | Literal value (len = 4)
677a 6970 | gzip
| - evict: date: Mon, 21 Oct\
| 2013 20:13:21 GMT
| -> content-encoding: gzip
84 | == Indexed - Remove ==
| idx = 4
| -> location: https://www.e\
| xample.com
84 | == Indexed - Add ==
| idx = 4
| -> location: https://www.e\
| xample.com
83 | == Indexed - Remove ==
| idx = 3
| -> :status: 200
83 | == Indexed - Add ==
| idx = 3
| -> :status: 200
7b | == Literal indexed ==
| Indexed name (idx = 59)
| set-cookie
38 | Literal value (len = 56)
666f 6f3d 4153 444a 4b48 514b 425a 584f | foo=ASDJKHQKBZXO
5157 454f 5049 5541 5851 5745 4f49 553b | QWEOPIUAXQWEOIU;
206d 6178 2d61 6765 3d33 3630 303b 2076 | max-age=3600; v
6572 7369 6f6e 3d31 | ersion=1
| - evict: location: https:/\
| /www.example.com
| - evict: :status: 200
| -> set-cookie: foo=ASDJKHQ\
| KBZXOQWEOPIUAXQWEOIU; ma\
| x-age=3600; version=1
Header Table (after decoding):
[ 1] (s = 98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age\
=3600; version=1
[ 2] (s = 52) content-encoding: gzip
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:22 GMT
Table size: 215
Decoded header set:
cache-control: private date: Mon, 21 Oct 2013 20:13:22 GMT content-encoding: gzip location: https://www.example.com :status: 200 set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1
This section shows the same examples as the previous section, but using Huffman encoding for the literal values. The HTTP/2 setting SETTINGS_HEADER_TABLE_SIZE is set to the value of 256 octets, causing some evictions to occur. The eviction mechanism uses the length of the decoded literal values, so the same evictions occurs as in the previous section.
Header set to encode:
:status: 302 cache-control: private date: Mon, 21 Oct 2013 20:13:21 GMT location: https://www.example.com
Reference set: empty.
Hex dump of encoded data:
4882 6402 5985 aec3 771a 4b63 96d0 7abe | H.d.Y...w.Kc..z. 9410 54d4 44a8 2005 9504 0b81 66e0 82a6 | ..T.D. .....f... 2d1b ff71 919d 29ad 1718 63c7 8f0b 97c8 | -..q..)...c..... e9ae 82ae 43d3 | ....C.
Decoding process:
48 | == Literal indexed ==
| Indexed name (idx = 8)
| :status
82 | Literal value (len = 3)
| Huffman encoded:
6402 | d.
| Decoded:
| 302
| -> :status: 302
59 | == Literal indexed ==
| Indexed name (idx = 25)
| cache-control
85 | Literal value (len = 7)
| Huffman encoded:
aec3 771a 4b | ..w.K
| Decoded:
| private
| -> cache-control: private
63 | == Literal indexed ==
| Indexed name (idx = 35)
| date
96 | Literal value (len = 29)
| Huffman encoded:
d07a be94 1054 d444 a820 0595 040b 8166 | .z...T.D. .....f
e082 a62d 1bff | ...-..
| Decoded:
| Mon, 21 Oct 2013 20:13:21 \
| GMT
| -> date: Mon, 21 Oct 2013 \
| 20:13:21 GMT
71 | == Literal indexed ==
| Indexed name (idx = 49)
| location
91 | Literal value (len = 23)
| Huffman encoded:
9d29 ad17 1863 c78f 0b97 c8e9 ae82 ae43 | .)...c.........C
d3 | .
| Decoded:
| https://www.example.com
| -> location: https://www.e\
| xample.com
Header Table (after decoding):
[ 1] (s = 63) location: https://www.example.com
[ 2] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 3] (s = 52) cache-control: private
[ 4] (s = 42) :status: 302
Table size: 222
Decoded header set:
:status: 302 cache-control: private date: Mon, 21 Oct 2013 20:13:21 GMT location: https://www.example.com
The (":status", "302") header field is evicted from the header table to free space to allow adding the (":status", "200") header field, copied from the static table into the header table. The (":status", "302") header field doesn't need to be removed from the reference set as it is evicted from the header table.
Header set to encode:
:status: 200 cache-control: private date: Mon, 21 Oct 2013 20:13:21 GMT location: https://www.example.com
Reference set:
[ 1] location: https://www.example.com [ 2] date: Mon, 21 Oct 2013 20:13:21 GMT [ 3] cache-control: private [ 4] :status: 302
Hex dump of encoded data:
8c | .
Decoding process:
8c | == Indexed - Add ==
| idx = 12
| - evict: :status: 302
| -> :status: 200
Header Table (after decoding):
[ 1] (s = 42) :status: 200
[ 2] (s = 63) location: https://www.example.com
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 4] (s = 52) cache-control: private
Table size: 222
Decoded header set:
:status: 200 location: https://www.example.com date: Mon, 21 Oct 2013 20:13:21 GMT cache-control: private
Several header fields are evicted from the header table during the processing of this header set. Before evicting a header belonging to the reference set, it is emitted, by coding it twice as an Indexed Representation. The first representation removes the header field from the reference set, the second one adds it again to the reference set, also emitting it.
Header set to encode:
:status: 200 cache-control: private date: Mon, 21 Oct 2013 20:13:22 GMT location: https://www.example.com content-encoding: gzip set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1
Reference set:
[ 1] :status: 200 [ 2] location: https://www.example.com [ 3] date: Mon, 21 Oct 2013 20:13:21 GMT [ 4] cache-control: private
Hex dump of encoded data:
8484 4396 d07a be94 1054 d444 a820 0595 | ..C..z...T.D. ..
040b 8166 e084 a62d 1bff 5e83 9bd9 ab84 | ...f...-..^.....
8483 837b ad94 e782 1dd7 f2e6 c7b3 35df | ...{..........5.
dfcd 5b39 60d5 af27 087f 3672 c1ab 270f | ..[9`..'..6r..'.
b529 1f95 8731 6065 c003 ed4e e5b1 063d | .)...1`e...N...=
5007 | P.
Decoding process:
84 | == Indexed - Remove ==
| idx = 4
| -> cache-control: private
84 | == Indexed - Add ==
| idx = 4
| -> cache-control: private
43 | == Literal indexed ==
| Indexed name (idx = 3)
| date
96 | Literal value (len = 29)
| Huffman encoded:
d07a be94 1054 d444 a820 0595 040b 8166 | .z...T.D. .....f
e084 a62d 1bff | ...-..
| Decoded:
| Mon, 21 Oct 2013 20:13:22 \
| GMT
| - evict: cache-control: pr\
| ivate
| -> date: Mon, 21 Oct 2013 \
| 20:13:22 GMT
5e | == Literal indexed ==
| Indexed name (idx = 30)
| content-encoding
83 | Literal value (len = 4)
| Huffman encoded:
9bd9 ab | ...
| Decoded:
| gzip
| - evict: date: Mon, 21 Oct\
| 2013 20:13:21 GMT
| -> content-encoding: gzip
84 | == Indexed - Remove ==
| idx = 4
| -> location: https://www.e\
| xample.com
84 | == Indexed - Add ==
| idx = 4
| -> location: https://www.e\
| xample.com
83 | == Indexed - Remove ==
| idx = 3
| -> :status: 200
83 | == Indexed - Add ==
| idx = 3
| -> :status: 200
7b | == Literal indexed ==
| Indexed name (idx = 59)
| set-cookie
ad | Literal value (len = 56)
| Huffman encoded:
94e7 821d d7f2 e6c7 b335 dfdf cd5b 3960 | .........5...[9`
d5af 2708 7f36 72c1 ab27 0fb5 291f 9587 | ..'..6r..'..)...
3160 65c0 03ed 4ee5 b106 3d50 07 | 1`e...N...=P.
| Decoded:
| foo=ASDJKHQKBZXOQWEOPIUAXQ\
| WEOIU; max-age=3600; versi\
| on=1
| - evict: location: https:/\
| /www.example.com
| - evict: :status: 200
| -> set-cookie: foo=ASDJKHQ\
| KBZXOQWEOPIUAXQWEOIU; ma\
| x-age=3600; version=1
Header Table (after decoding):
[ 1] (s = 98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age\
=3600; version=1
[ 2] (s = 52) content-encoding: gzip
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:22 GMT
Table size: 215
Decoded header set:
cache-control: private date: Mon, 21 Oct 2013 20:13:22 GMT content-encoding: gzip location: https://www.example.com :status: 200 set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1