RFC 7541
HPACK: Header Compression for HTTP/2
Internet Engineering Task Force (IETF) R. Peon Request for Comments: 7541 Google, Inc Category: Standards Track H. Ruellan ISSN: 2070-1721 Canon CRF May 2015 HPACK: Header Compression for HTTP/2Abstract
This specification defines HPACK, a compression format for efficiently representing HTTP header fields, to be used in HTTP/2. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7541. Copyright Notice Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................4 1.1. Overview ...................................................4 1.2. Conventions ................................................5 1.3. Terminology ................................................5 2. Compression Process Overview ....................................6 2.1. Header List Ordering .......................................6 2.2. Encoding and Decoding Contexts .............................6 2.3. Indexing Tables ............................................6 2.3.1. Static Table ........................................6 2.3.2. Dynamic Table .......................................6 2.3.3. Index Address Space .................................7 2.4. Header Field Representation ................................8 3. Header Block Decoding ...........................................8 3.1. Header Block Processing ....................................8 3.2. Header Field Representation Processing .....................9 4. Dynamic Table Management ........................................9 4.1. Calculating Table Size ....................................10 4.2. Maximum Table Size ........................................10 4.3. Entry Eviction When Dynamic Table Size Changes ............11 4.4. Entry Eviction When Adding New Entries ....................11 5. Primitive Type Representations .................................11 5.1. Integer Representation ....................................11 5.2. String Literal Representation .............................13 6. Binary Format ..................................................14 6.1. Indexed Header Field Representation .......................14 6.2. Literal Header Field Representation .......................15 6.2.1. Literal Header Field with Incremental Indexing .....15 6.2.2. Literal Header Field without Indexing ..............16 6.2.3. Literal Header Field Never Indexed .................17 6.3. Dynamic Table Size Update .................................18 7. Security Considerations ........................................19 7.1. Probing Dynamic Table State ...............................19 7.1.1. Applicability to HPACK and HTTP ....................20 7.1.2. Mitigation .........................................20 7.1.3. Never-Indexed Literals .............................21 7.2. Static Huffman Encoding ...................................22 7.3. Memory Consumption ........................................22 7.4. Implementation Limits .....................................23 8. References .....................................................23 8.1. Normative References ......................................23 8.2. Informative References ....................................24 Appendix A. Static Table Definition ...............................25 Appendix B. Huffman Code ..........................................27
Appendix C. Examples ..............................................33 C.1. Integer Representation Examples ............................33 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 ..........33 C.1.3. Example 3: Encoding 42 Starting at an Octet Boundary ...34 C.2. Header Field Representation Examples .......................34 C.2.1. Literal Header Field with Indexing .....................34 C.2.2. Literal Header Field without Indexing ..................35 C.2.3. Literal Header Field Never Indexed .....................36 C.2.4. Indexed Header Field ...................................37 C.3. Request Examples without Huffman Coding ....................37 C.3.1. First Request ..........................................37 C.3.2. Second Request .........................................38 C.3.3. Third Request ..........................................39 C.4. Request Examples with Huffman Coding .......................41 C.4.1. First Request ..........................................41 C.4.2. Second Request .........................................42 C.4.3. Third Request ..........................................43 C.5. Response Examples without Huffman Coding ...................45 C.5.1. First Response .........................................45 C.5.2. Second Response ........................................46 C.5.3. Third Response .........................................47 C.6. Response Examples with Huffman Coding ......................49 C.6.1. First Response .........................................49 C.6.2. Second Response ........................................51 C.6.3. Third Response .........................................52 Acknowledgments ...................................................55 Authors' Addresses ................................................55
1. Introduction
In HTTP/1.1 (see [RFC7230]), header fields are not compressed. As web pages have grown to require dozens to hundreds of requests, the redundant header fields in these requests unnecessarily consume bandwidth, measurably increasing latency. SPDY [SPDY] initially addressed this redundancy by compressing header fields using the DEFLATE [DEFLATE] format, which proved very effective at efficiently representing the redundant header fields. However, that approach exposed a security risk as demonstrated by the CRIME (Compression Ratio Info-leak Made Easy) attack (see [CRIME]). This specification defines HPACK, a new compressor that eliminates redundant header fields, limits vulnerability to known security attacks, and has a bounded memory requirement for use in constrained environments. Potential security concerns for HPACK are described in Section 7. The HPACK format is intentionally simple and inflexible. Both characteristics reduce the risk of interoperability or security issues due to implementation error. No extensibility mechanisms are defined; changes to the format are only possible by defining a complete replacement.1.1. Overview
The format defined in this specification treats a list of header fields as an ordered collection of name-value pairs that can include duplicate pairs. Names and values are considered to be opaque sequences of octets, and the order of header fields is preserved after being compressed and decompressed. Encoding is informed by header field tables that map header fields to indexed values. These header field tables can be incrementally updated as new header fields are encoded or decoded. 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 tables. Therefore, a list of header fields can be encoded using a mixture of references and literal values. Literal values are either encoded directly or use a static Huffman code. The encoder is responsible for deciding which header fields to insert as new entries in the header field tables. The decoder executes the modifications to the header field tables prescribed by the encoder,
reconstructing the list of header fields in the process. This enables decoders to remain simple and interoperate with a wide variety of encoders. Examples illustrating the use of these different mechanisms to represent header fields are available in Appendix C.1.2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in 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.1.3. Terminology
This specification uses the following terms: Header Field: A name-value pair. Both the name and value are treated as opaque sequences of octets. Dynamic Table: The dynamic table (see Section 2.3.2) is a table that associates stored header fields with index values. This table is dynamic and specific to an encoding or decoding context. Static Table: The static table (see Section 2.3.1) is a table that statically associates header fields that occur frequently with index values. This table is ordered, read-only, always accessible, and it may be shared amongst all encoding or decoding contexts. Header List: A header list is an ordered collection of header fields that are encoded jointly and can contain duplicate header fields. A complete list of header fields contained in an HTTP/2 header block is a header list. Header Field Representation: A header field can be represented in encoded form either as a literal or as an index (see Section 2.4). Header Block: An ordered list of header field representations, which, when decoded, yields a complete header list.
2. Compression Process Overview
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.2.1. Header List Ordering
HPACK preserves the ordering of header fields inside the header list. An encoder MUST order header field representations in the header block according to their ordering in the original header list. A decoder MUST order header fields in the decoded header list according to their ordering in the header block.2.2. Encoding and Decoding Contexts
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 state is needed. When used for bidirectional communication, such as in HTTP, the encoding and decoding dynamic tables maintained by an endpoint are completely independent, i.e., the request and response dynamic tables are separate.2.3. Indexing Tables
HPACK uses two tables for associating header fields to indexes. The static table (see Section 2.3.1) is predefined and contains common 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 index header fields repeated in the encoded header lists. These two tables are combined into a single address space for defining index values (see Section 2.3.3).2.3.1. Static Table
The static table consists of a predefined static list of header fields. Its entries are defined in Appendix A.2.3.2. Dynamic Table
The dynamic table consists of a list of header fields maintained in first-in, first-out order. The first and newest entry in a dynamic table is at the lowest index, and the oldest entry of a dynamic table is at the highest index.
The dynamic table is initially empty. Entries are added as each header block is decompressed. The dynamic table can contain duplicate entries (i.e., entries with the same name and same value). Therefore, duplicate entries MUST NOT be treated as an error by a decoder. The encoder decides how to update the dynamic table and as such can control how much memory is used by the dynamic table. To limit the memory requirements of the decoder, the dynamic table size is strictly bounded (see Section 4.2). The decoder updates the dynamic table during the processing of a list of header field representations (see Section 3.2).2.3.3. Index Address Space
The static table and the dynamic table are combined into a single index address space. Indices between 1 and the length of the static table (inclusive) refer to elements in the static table (see Section 2.3.1). Indices strictly greater than the length of the static table refer to elements in the dynamic table (see Section 2.3.2). The length of the static table is subtracted to find the index into the dynamic table. Indices strictly greater than the sum of the lengths of both tables MUST be treated as a decoding error. For a static table size of s and a dynamic table size of k, the following diagram shows the entire valid index address space. <---------- Index Address Space ----------> <-- Static Table --> <-- Dynamic Table --> +---+-----------+---+ +---+-----------+---+ | 1 | ... | s | |s+1| ... |s+k| +---+-----------+---+ +---+-----------+---+ ^ | | V Insertion Point Dropping Point Figure 1: Index Address Space
2.4. Header Field Representation
An encoded header field can be represented either as an index or as a literal. An indexed representation defines a header field as a reference to an entry in either the static table or the dynamic table (see Section 6.1). A literal representation defines a header field by specifying its name and value. The header field name can be represented literally or as a reference to an entry in either the static table or the dynamic table. The header field value is represented literally. Three different literal representations are defined: o A literal representation that adds the header field as a new entry at the beginning of the dynamic table (see Section 6.2.1). o A literal representation that does not add the header field to the dynamic table (see Section 6.2.2). o A literal representation that does not add the header field to the dynamic table, with the additional stipulation that this header field always use a literal representation, in particular when re- encoded by an intermediary (see Section 6.2.3). This representation is intended for protecting header field values that are not to be put at risk by compressing them (see Section 7.1.3 for more details). The selection of one of these literal representations can be guided by security considerations, in order to protect sensitive header field values (see Section 7.1). The literal representation of a header field name or of a header field value can encode the sequence of octets either directly or using a static Huffman code (see Section 5.2).3. Header Block Decoding
3.1. Header Block Processing
A decoder processes a header block sequentially to reconstruct the original header list. A header block is the concatenation of header field representations. The different possible header field representations are described in Section 6.
Once a header field is decoded and added to the reconstructed header list, the header field cannot be removed. A header field added to the header list can be safely passed to the application. By passing the resulting header fields to the application, a decoder can be implemented with minimal transitory memory commitment in addition to the memory required for the dynamic table.3.2. Header Field Representation Processing
The processing of a header block to obtain a header list is defined in this section. To ensure that the decoding will successfully produce a header list, 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 6. An _indexed representation_ entails the following actions: o The header field corresponding to the referenced entry in either the static table or dynamic table is appended to the decoded header list. A _literal representation_ that is _not added_ to the dynamic table entails the following action: o The header field is appended to the decoded header list. A _literal representation_ that is _added_ to the dynamic table entails the following actions: o The header field is appended to the decoded header list. o The header field is inserted at the beginning of the dynamic table. This insertion could result in the eviction of previous entries in the dynamic table (see Section 4.4).4. Dynamic Table Management
To limit the memory requirements on the decoder side, the dynamic table is constrained in size.
4.1. Calculating Table Size
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 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 value without any Huffman encoding applied. Note: The additional 32 octets account for an estimated 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.4.2. Maximum Table Size
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 determined by the SETTINGS_HEADER_TABLE_SIZE setting (see Section 6.5.2 of [HTTP2]). 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 to the maximum set by the protocol. A change in the maximum size of the dynamic table is signaled via a dynamic table size update (see Section 6.3). This dynamic table size update MUST occur at the beginning of the first header block following the change to the dynamic table size. In HTTP/2, this follows a settings acknowledgment (see Section 6.5.3 of [HTTP2]). Multiple updates to the maximum table size can occur between the transmission of two header blocks. In the case that this size is changed more than once in this interval, the smallest maximum table size that occurs in that interval MUST be signaled in a dynamic table size update. The final maximum size is always signaled, resulting in at most two dynamic table size updates. This ensures that the decoder is able to perform eviction based on reductions in dynamic table size (see Section 4.3). This mechanism can be used to completely clear entries from the dynamic table by setting a maximum size of 0, which can subsequently be restored.
4.3. Entry Eviction When Dynamic Table Size Changes
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 dynamic table is less than or equal to the maximum size.4.4. Entry Eviction When Adding New Entries
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 is less than or equal to (maximum size - new entry size) or until the table is empty. 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 maximum size causes the 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 that will be evicted when adding this new entry into the dynamic table. Implementations are cautioned to avoid deleting the referenced name if the referenced entry is evicted from the dynamic table prior to inserting the new entry.
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