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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 HTTPbis Working Group R. Peon 3 Internet-Draft Google, Inc 4 Intended status: Standards Track H. Ruellan 5 Expires: February 01, 2015 Canon CRF 6 July 31, 2014 8 HPACK - Header Compression for HTTP/2 9 draft-ietf-httpbis-header-compression-09 11 Abstract 13 This specification defines HPACK, a compression format for 14 efficiently representing HTTP header fields in the context of HTTP/2. 16 Editorial Note (To be removed by RFC Editor) 18 Discussion of this draft takes place on the HTTPBIS working group 19 mailing list (ietf-http-wg@w3.org), which is archived at . 22 Working Group information can be found at ; that specific to HTTP/2 are at . 25 The changes in this draft are summarized in Appendix A.1. 27 Status of This Memo 29 This Internet-Draft is submitted in full conformance with the 30 provisions of BCP 78 and BCP 79. 32 Internet-Drafts are working documents of the Internet Engineering 33 Task Force (IETF). Note that other groups may also distribute 34 working documents as Internet-Drafts. The list of current Internet- 35 Drafts is at http://datatracker.ietf.org/drafts/current/. 37 Internet-Drafts are draft documents valid for a maximum of six months 38 and may be updated, replaced, or obsoleted by other documents at any 39 time. It is inappropriate to use Internet-Drafts as reference 40 material or to cite them other than as "work in progress." 42 This Internet-Draft will expire on February 01, 2015. 44 Copyright Notice 46 Copyright (c) 2014 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents 51 (http://trustee.ietf.org/license-info) in effect on the date of 52 publication of this document. Please review these documents 53 carefully, as they describe your rights and restrictions with respect 54 to this document. Code Components extracted from this document must 55 include Simplified BSD License text as described in Section 4.e of 56 the Trust Legal Provisions and are provided without warranty as 57 described in the Simplified BSD License. 59 Table of Contents 61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 62 2. HPACK Overview . . . . . . . . . . . . . . . . . . . . . . . 4 63 2.1. Outline . . . . . . . . . . . . . . . . . . . . . . . . . 4 64 2.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 5 65 2.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 66 3. Compression Process Overview . . . . . . . . . . . . . . . . 5 67 3.1. Header List Ordering . . . . . . . . . . . . . . . . . . 6 68 3.2. Encoding and Decoding Contexts . . . . . . . . . . . . . 6 69 3.3. Indexing Tables . . . . . . . . . . . . . . . . . . . . . 6 70 3.3.1. Static Table . . . . . . . . . . . . . . . . . . . . 7 71 3.3.2. Header Table . . . . . . . . . . . . . . . . . . . . 7 72 3.3.3. Index Address Space . . . . . . . . . . . . . . . . . 7 73 3.4. Header Field Representation . . . . . . . . . . . . . . . 8 74 4. Header Block Decoding . . . . . . . . . . . . . . . . . . . . 8 75 4.1. Header Block Processing . . . . . . . . . . . . . . . . . 8 76 4.2. Header Field Representation Processing . . . . . . . . . 9 77 5. Header Table Management . . . . . . . . . . . . . . . . . . . 9 78 5.1. Maximum Table Size . . . . . . . . . . . . . . . . . . . 9 79 5.2. Entry Eviction when Header Table Size Changes . . . . . . 10 80 5.3. Entry Eviction when Adding New Entries . . . . . . . . . 11 81 6. Primitive Type Representations . . . . . . . . . . . . . . . 11 82 6.1. Integer Representation . . . . . . . . . . . . . . . . . 11 83 6.2. String Literal Representation . . . . . . . . . . . . . . 12 84 7. Binary Format . . . . . . . . . . . . . . . . . . . . . . . . 13 85 7.1. Indexed Header Field Representation . . . . . . . . . . . 13 86 7.2. Literal Header Field Representation . . . . . . . . . . . 14 87 7.2.1. Literal Header Field with Incremental Indexing . . . 14 88 7.2.2. Literal Header Field without Indexing . . . . . . . . 15 89 7.2.3. Literal Header Field never Indexed . . . . . . . . . 16 90 7.3. Header Table Size Update . . . . . . . . . . . . . . . . 17 91 8. Security Considerations . . . . . . . . . . . . . . . . . . . 18 92 8.1. Probing Header Table State . . . . . . . . . . . . . . . 18 93 8.1.1. Applicability to HPACK and HTTP . . . . . . . . . . . 19 94 8.1.2. Mitigation . . . . . . . . . . . . . . . . . . . . . 19 95 8.1.3. Never Indexed Literals . . . . . . . . . . . . . . . 20 96 8.2. Static Huffman Encoding . . . . . . . . . . . . . . . . . 20 97 8.3. Memory Consumption . . . . . . . . . . . . . . . . . . . 20 98 8.4. Implementation Limits . . . . . . . . . . . . . . . . . . 21 99 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21 100 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 21 101 10.1. Normative References . . . . . . . . . . . . . . . . . . 21 102 10.2. Informative References . . . . . . . . . . . . . . . . . 22 103 Appendix A. Change Log (to be removed by RFC Editor before 104 publication) . . . . . . . . . . . . . . . . . . . . 23 105 A.1. Since draft-ietf-httpbis-header-compression-08 . . . . . 23 106 A.2. Since draft-ietf-httpbis-header-compression-07 . . . . . 23 107 A.3. Since draft-ietf-httpbis-header-compression-06 . . . . . 24 108 A.4. Since draft-ietf-httpbis-header-compression-05 . . . . . 24 109 A.5. Since draft-ietf-httpbis-header-compression-04 . . . . . 24 110 A.6. Since draft-ietf-httpbis-header-compression-03 . . . . . 25 111 A.7. Since draft-ietf-httpbis-header-compression-02 . . . . . 25 112 A.8. Since draft-ietf-httpbis-header-compression-01 . . . . . 25 113 A.9. Since draft-ietf-httpbis-header-compression-00 . . . . . 25 114 Appendix B. Static Table Definition . . . . . . . . . . . . . . 26 115 Appendix C. Huffman Code . . . . . . . . . . . . . . . . . . . . 28 116 Appendix D. Examples . . . . . . . . . . . . . . . . . . . . . . 34 117 D.1. Integer Representation Examples . . . . . . . . . . . . . 34 118 D.1.1. Example 1: Encoding 10 Using a 5-bit Prefix . . . . . 34 119 D.1.2. Example 2: Encoding 1337 Using a 5-bit Prefix . . . . 34 120 D.1.3. Example 3: Encoding 42 Starting at an Octet Boundary 35 121 D.2. Header Field Representation Examples . . . . . . . . . . 35 122 D.2.1. Literal Header Field with Indexing . . . . . . . . . 35 123 D.2.2. Literal Header Field without Indexing . . . . . . . . 36 124 D.2.3. Literal Header Field never Indexed . . . . . . . . . 37 125 D.2.4. Indexed Header Field . . . . . . . . . . . . . . . . 38 126 D.3. Request Examples without Huffman Coding . . . . . . . . . 38 127 D.3.1. First Request . . . . . . . . . . . . . . . . . . . . 38 128 D.3.2. Second Request . . . . . . . . . . . . . . . . . . . 39 129 D.3.3. Third Request . . . . . . . . . . . . . . . . . . . . 41 130 D.4. Request Examples with Huffman Coding . . . . . . . . . . 42 131 D.4.1. First Request . . . . . . . . . . . . . . . . . . . . 42 132 D.4.2. Second Request . . . . . . . . . . . . . . . . . . . 43 133 D.4.3. Third Request . . . . . . . . . . . . . . . . . . . . 44 134 D.5. Response Examples without Huffman Coding . . . . . . . . 46 135 D.5.1. First Response . . . . . . . . . . . . . . . . . . . 46 136 D.5.2. Second Response . . . . . . . . . . . . . . . . . . . 48 137 D.5.3. Third Response . . . . . . . . . . . . . . . . . . . 49 138 D.6. Response Examples with Huffman Coding . . . . . . . . . . 51 139 D.6.1. First Response . . . . . . . . . . . . . . . . . . . 51 140 D.6.2. Second Response . . . . . . . . . . . . . . . . . . . 53 141 D.6.3. Third Response . . . . . . . . . . . . . . . . . . . 54 143 1. Introduction 145 This specification defines HPACK, a compression format for 146 efficiently representing HTTP header fields in the context of HTTP/2 147 [HTTP2]. 149 2. HPACK Overview 151 In HTTP/1.1 (see [RFC7230]), header fields are encoded without any 152 form of compression. As web pages have grown to include dozens to 153 hundreds of requests, the redundant header fields in these requests 154 now measurably increase latency and unnecessarily consume bandwidth 155 (see [SPDY-DESC-1] and [SPDY-DESC-2]). 157 SPDY [SPDY] initially addressed this redundancy by compressing header 158 fields using the DEFLATE [DEFLATE] format, which proved very 159 effective at efficiently representing the redundant header fields. 160 However, that approach exposed a security risk as demonstrated by the 161 CRIME attack (see [CRIME]). 163 This document describes HPACK, a new compressor for header fields 164 which eliminates redundant header fields, limits vulnerability to 165 known security attacks, and which has a bounded memory requirement 166 for use in constrained environments. 168 2.1. Outline 170 The HTTP header field encoding defined in this document is based on a 171 header table that maps name-value pairs to index values. The header 172 table is incrementally updated as new values are encoded or decoded. 174 A list of header fields is treated as an ordered collection of name- 175 value pairs that can include duplicates. Names and values are 176 considered to be opaque sequences of octets. The order of header 177 fields is preserved after being compressed and decompressed. 179 In the encoded form, a header field is represented either literally 180 or as a reference to a name-value pair in a header table. A list of 181 header fields can therefore be encoded using a mixture of references 182 and literal values. 184 The encoder is responsible for deciding which header fields to insert 185 as new entries in the header table. The decoder executes the 186 modifications to the header table prescribed by the encoder, 187 reconstructing the list of header fields in the process. This 188 enables decoders to remain simple and understand a wide variety of 189 encoders. 191 Examples illustrating the use of these different mechanisms to 192 represent header fields are available in Appendix D. 194 2.2. Conventions 196 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 197 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 198 document are to be interpreted as described in RFC 2119 [RFC2119]. 200 All numeric values are in network byte order. Values are unsigned 201 unless otherwise indicated. Literal values are provided in decimal 202 or hexadecimal as appropriate. Hexadecimal literals are prefixed 203 with "0x" to distinguish them from decimal literals. 205 2.3. Terminology 207 This document uses the following terms: 209 Header Field: A name-value pair. Both the name and value are 210 treated as opaque sequences of octets. 212 Header Table: The header table (see Section 3.3.2) is used to 213 associate stored header fields to index values. This table is 214 dynamic and specific to an encoding or decoding context. 216 Static Table: The static table (see Section 3.3.1) is used to 217 associate static header fields to index values. This table is 218 ordered, read-only, always accessible, and may be shared amongst 219 all encoding or decoding contexts. 221 Header List: A header list is an ordered collection of header fields 222 that are encoded jointly. It can contain duplicate header fields. 223 A complete list of key-value pairs contained in a HTTP request or 224 response is a header list. 226 Header Field Representation: A header field can be represented in 227 encoded form either as a literal or as an index (see Section 3.4). 229 Header Block: An ordered list of header field representations which, 230 when decoded, yields a complete header list. 232 3. Compression Process Overview 234 This specification does not describe a specific algorithm for an 235 encoder. Instead, it defines precisely how a decoder is expected to 236 operate, allowing encoders to produce any encoding that this 237 definition permits. 239 3.1. Header List Ordering 241 The compression and decompression process preserve the ordering of 242 header fields inside the header list. An encoder SHOULD order header 243 field representations in the header block according to their ordering 244 in the original header list. A decoder SHOULD order header fields in 245 the decoded header list according to their ordering in the header 246 block. 248 In particular, representations for pseudo-header fields (see 249 Section 8.1.2.1 of [HTTP2]) MUST appear before representations for 250 regular header fields in a header block. In a decoded header list, 251 pseudo-header fields MUST appear before regular header fields. 253 3.2. Encoding and Decoding Contexts 255 To decompress header blocks, a decoder only needs to maintain a 256 header table (see Section 3.3.2) as a decoding context. No other 257 state information is needed. 259 An encoder that wishes to reference entries in the header table needs 260 to maintain a copy of the header table used by the decoder. 262 When used for bidirectional communication, such as in HTTP, the 263 encoding and decoding header tables maintained by an endpoint are 264 completely independent. Header fields are encoded without any 265 reference to the local decoding header table; and header fields are 266 decoded without reference to the local encoding header table. 268 3.3. Indexing Tables 270 HPACK uses two tables for associating header fields to indexes. The 271 static table (see Section 3.3.1) is predefined and contains common 272 header fields (most of them with an empty value). The header table 273 (see Section 3.3.2) is dynamic and can be used by the encoder to 274 index header fields repeated in the encoded header lists. 276 These two tables are combined into a single address space for 277 defining index values (see Section 3.3.3). 279 3.3.1. Static Table 281 The static table consists of a predefined static list of header 282 fields. Its entries are defined in Appendix B. 284 3.3.2. Header Table 286 The header table consists of a list of header fields maintained in 287 first-in, first-out order. The first and newest entry in a header 288 table is always at index 1, and the oldest entry of a header table is 289 at the index corresponding to the number of entries in the header 290 table. 292 The header table is initially empty. 294 The header table can contain duplicate entries. Therefore, duplicate 295 entries MUST NOT be treated as an error by a decoder. 297 The encoder decides how to update the header table and as such can 298 control how much memory is used by the header table. To limit the 299 memory requirements of the decoder, the header table size is strictly 300 bounded (see Section 5.1). 302 The header table is updated during the processing of a list of header 303 field representations (see Section 4.2). 305 3.3.3. Index Address Space 307 The static table and the header table are combined into a single 308 index address space. 310 Indices between 1 and the length of the static table (inclusive) 311 refer to elements in the static table (see Section 3.3.1). 313 Indices strictly greater than the length of the static table refer to 314 elements in the header table (see Section 3.3.2). The length of the 315 static table is subtracted to find the index into the header table. 317 Indices strictly greater than the sum of the lengths of both tables 318 MUST be treated as a decoding error. 320 For a static table size of s and a header table size of k, the 321 following diagram shows the entire valid index address space. 323 <---------- Index Address Space ----------> 324 <-- Static Table --> <-- Header Table --> 325 +---+-----------+---+ +---+-----------+---+ 326 | 1 | ... | s | |s+1| ... |s+k| 327 +---+-----------+---+ +---+-----------+---+ 328 ^ | 329 | V 330 Insertion Point Dropping Point 332 Index Address Space 334 3.4. Header Field Representation 336 An encoded header field can be represented either as a literal or as 337 an index. 339 A literal representation defines a header field by specifying its 340 name and value. The header field name can be represented literally 341 or as a reference to an entry in either the static table or the 342 header table. The header field value is represented literally. 344 Three different literal representations are provided: 346 o A literal representation that does not add the header field to the 347 header table (see Section 7.2.2). 349 o A literal representation that does not add the header field to the 350 header table, with the additional stipulation that this header 351 field always use a literal representation, in particular when re- 352 encoded by an intermediary (see Section 7.2.3). 354 o A literal representation that adds the header field as a new entry 355 at the beginning of the header table (see Section 7.2.1). 357 An indexed representation defines a header field as a reference to an 358 entry in either the static table or the header table (see 359 Section 7.1). 361 4. Header Block Decoding 363 4.1. Header Block Processing 365 A decoder processes an encoded header block sequentially to 366 reconstruct the original header list. 368 Once a header field is decoded and added to the reconstructed header 369 list, it cannot be removed from it. A header field added to the 370 header list can be safely passed to the upper processing layer. 372 By passing decoded header fields to the upper processing layer, a 373 decoder can be implemented with minimal transitory memory commitment 374 in addition to the header table. The management of memory for 375 handling very large lists of header fields can therefore be deferred 376 to the upper processing layers. 378 4.2. Header Field Representation Processing 380 The processing of a header block to obtain a header list is defined 381 in this section. To ensure that the decoding will successfully 382 produce a header list, a decoder MUST obey the following rules. 384 All the header field representations contained in a header block are 385 processed in the order in which they appear, as specified below. 386 Details on the formatting of the various header field 387 representations, and some additional processing instructions are 388 found in Section 7. 390 An _indexed representation_ entails the following actions: 392 o The header field corresponding to the referenced entry in either 393 the static table or header table is added to the decoded header 394 list. 396 A _literal representation_ that is _not added_ to the header table 397 entails the following action: 399 o The header field is added to the decoded header list. 401 A _literal representation_ that is _added_ to the header table 402 entails the following actions: 404 o The header field is added to the decoded header list. 406 o The header field is inserted at the beginning of the header table. 408 5. Header Table Management 410 5.1. Maximum Table Size 412 To limit the memory requirements on the decoder side, the header 413 table is constrained in size. 415 The size of the header table is bounded by a maximum size defined by 416 the encoder. The size of the header table MUST always be lower than 417 or equal to this maximum size. 419 By default, the maximum size of the header table is equal to the 420 value of the HTTP/2 setting parameter SETTINGS_HEADER_TABLE_SIZE 421 defined by the decoder (see Section 6.5.2 of [HTTP2]). The encoder 422 can change this maximum size (see Section 7.3), but it MUST stay 423 lower than or equal to the value of SETTINGS_HEADER_TABLE_SIZE. 425 After applying an updated value of the SETTINGS_HEADER_TABLE_SIZE 426 parameter that changes the maximum size of the header table used by 427 the encoder, the encoder MUST signal this change via an encoding 428 context update (see Section 7.3). This encoding context update MUST 429 occur at the beginning of the first header block following the 430 SETTINGS frame sent to acknowledge the application of the updated 431 settings (see Section 6.5.3 of [HTTP2]). 433 Several updates to the value of the SETTINGS_HEADER_TABLE_SIZE 434 parameter can occur between the sending of two header blocks. In the 435 case that the value of this parameter is changed more that once, if 436 one of its value is smaller than the new maximum size, the smallest 437 value for the parameter MUST be sent before the new maximum size, 438 using two encoding context updates. This ensures that the decoder is 439 able to perform eviction based on the decoder table size (see 440 Section 5.2). 442 This mechanism can be used with a SETTINGS_HEADER_TABLE_SIZE 443 parameter value of 0 to completely clear entries from the header 444 table. 446 The size of the header table is the sum of the size of its entries. 448 The size of an entry is the sum of its name's length in octets (as 449 defined in Section 6.2), its value's length in octets (see 450 Section 6.2), plus 32. 452 The size of an entry is calculated using the length of the name and 453 value without any Huffman encoding applied. 455 The additional 32 octets account for the overhead associated with an 456 entry. For example, an entry structure using two 64-bit pointers to 457 reference the name and the value of the entry, and two 64-bit 458 integers for counting the number of references to the name and value 459 would have 32 octets of overhead. 461 5.2. Entry Eviction when Header Table Size Changes 463 Whenever the maximum size for the header table is reduced, entries 464 are evicted from the end of the header table until the size of the 465 header table is less than or equal to the maximum size. 467 5.3. Entry Eviction when Adding New Entries 469 Whenever a new entry is to be added to the header table, entries are 470 evicted from the end of the header table until the size of the header 471 table is less than or equal to (maximum size - new entry size), or 472 until the table is empty. 474 If the representation of the added entry references the name of an 475 entry in the header table, the referenced name is cached prior to 476 performing eviction to avoid having the name inadvertently evicted. 478 If the size of the new entry is less than or equal to the maximum 479 size, that entry is added to the table. It is not an error to 480 attempt to add an entry that is larger than the maximum size; an 481 attempt to add an entry larger than the entire table causes the table 482 to be emptied of all existing entries. 484 6. Primitive Type Representations 486 HPACK encoding uses two primitive types: unsigned variable length 487 integers, and strings of octets. 489 6.1. Integer Representation 491 Integers are used to represent name indexes, pair indexes or string 492 lengths. To allow for optimized processing, an integer 493 representation always finishes at the end of an octet. 495 An integer is represented in two parts: a prefix that fills the 496 current octet and an optional list of octets that are used if the 497 integer value does not fit within the prefix. The number of bits of 498 the prefix (called N) is a parameter of the integer representation. 500 The N-bit prefix allows filling the current octet. If the value is 501 small enough (strictly less than 2^N-1), it is encoded within the 502 N-bit prefix. Otherwise all the bits of the prefix are set to 1 and 503 the value is encoded using an unsigned variable length integer 504 representation (see ). N is always between 1 and 8 bits. An integer 506 starting at an octet-boundary will have an 8-bit prefix. 508 The algorithm to represent an integer I is as follows: 510 if I < 2^N - 1, encode I on N bits 511 else 512 encode (2^N - 1) on N bits 513 I = I - (2^N - 1) 514 while I >= 128 515 encode (I % 128 + 128) on 8 bits 516 I = I / 128 517 encode I on 8 bits 519 For informational purpose, the algorithm to decode an integer I is as 520 follows: 522 decode I from the next N bits 523 if I < 2^N - 1, return I 524 else 525 M = 0 526 repeat 527 B = next octet 528 I = I + (B & 127) * 2^M 529 M = M + 7 530 while B & 128 == 128 531 return I 533 Examples illustrating the encoding of integers are available in 534 Appendix D.1. 536 This integer representation allows for values of indefinite size. It 537 is also possible for an encoder to send a large number of zero 538 values, which can waste octets and could be used to overflow integer 539 values. Excessively large integer encodings - in value or octet 540 length - MUST be treated as a decoding error. Different limits can 541 be set for each of the different uses of integers, based on 542 implementation constraints. 544 6.2. String Literal Representation 546 Header field names and header field values can be represented as 547 literal string. A literal string is encoded as a sequence of octets, 548 either by directly encoding the literal string's octets, or by using 549 a Huffman code (see [HUFFMAN]). 551 0 1 2 3 4 5 6 7 552 +---+---+---+---+---+---+---+---+ 553 | H | String Length (7+) | 554 +---+---------------------------+ 555 | String Data (Length octets) | 556 +-------------------------------+ 558 String Literal Representation 560 A literal string representation contains the following fields: 562 H: A one bit flag, H, indicating whether or not the octets of the 563 string are Huffman encoded. 565 String Length: The number of octets used to encode the string 566 literal, encoded as an integer with 7-bit prefix (see 567 Section 6.1). 569 String Data: The encoded data of the string literal. If H is '0', 570 then the encoded data is the raw octets of the string literal. If 571 H is '1', then the encoded data is the Huffman encoding of the 572 string literal. 574 String literals which use Huffman encoding are encoded with the 575 Huffman code defined in Appendix C (see examples for requests in 576 Appendix D.4 and for responses in Appendix D.6). The encoded data is 577 the bitwise concatenation of the codes corresponding to each octet of 578 the string literal. 580 As the Huffman encoded data doesn't always end at an octet boundary, 581 some padding is inserted after it, up to the next octet boundary. To 582 prevent this padding to be misinterpreted as part of the string 583 literal, the most significant bits of the code corresponding to the 584 EOS (end-of-string) symbol are used. 586 Upon decoding, an incomplete code at the end of the encoded data is 587 to be considered as padding and discarded. A padding strictly longer 588 than 7 bits MUST be treated as a decoding error. A padding not 589 corresponding to the most significant bits of the code for the EOS 590 symbol MUST be treated as a decoding error. A Huffman encoded string 591 literal containing the EOS symbol MUST be treated as a decoding 592 error. 594 7. Binary Format 596 This section describes the detailed format of each of the different 597 header field representations, plus the encoding context update 598 instruction. 600 7.1. Indexed Header Field Representation 602 An indexed header field representation identifies an entry in either 603 the static table or the header table (see Section 3.3). 605 An indexed header field representation causes a header field to be 606 added to the decoded header list, as described in Section 4.2. 608 0 1 2 3 4 5 6 7 609 +---+---+---+---+---+---+---+---+ 610 | 1 | Index (7+) | 611 +---+---------------------------+ 613 Indexed Header Field 615 An indexed header field starts with the '1' 1-bit pattern, followed 616 by the index of the matching pair, represented as an integer with a 617 7-bit prefix (see Section 6.1). 619 The index value of 0 is not used. It MUST be treated as a decoding 620 error if found in an indexed header field representation. 622 7.2. Literal Header Field Representation 624 A literal header field representation contains a literal header field 625 value. Header field names are either provided as a literal or by 626 reference to an existing table entry, either from the static table or 627 the header table (see Section 3.3). 629 A literal representation causes a header field to be added to the 630 decoded header list, as described in Section 4.2. 632 7.2.1. Literal Header Field with Incremental Indexing 634 A literal header field with incremental indexing representation 635 results in adding a header field to the decoded header list and 636 inserting it as a new entry into the header table. 638 0 1 2 3 4 5 6 7 639 +---+---+---+---+---+---+---+---+ 640 | 0 | 1 | Index (6+) | 641 +---+---+-----------------------+ 642 | H | Value Length (7+) | 643 +---+---------------------------+ 644 | Value String (Length octets) | 645 +-------------------------------+ 647 Literal Header Field with Incremental Indexing - Indexed Name 649 0 1 2 3 4 5 6 7 650 +---+---+---+---+---+---+---+---+ 651 | 0 | 1 | 0 | 652 +---+---+-----------------------+ 653 | H | Name Length (7+) | 654 +---+---------------------------+ 655 | Name String (Length octets) | 656 +---+---------------------------+ 657 | H | Value Length (7+) | 658 +---+---------------------------+ 659 | Value String (Length octets) | 660 +-------------------------------+ 662 Literal Header Field with Incremental Indexing - New Name 664 A literal header field with incremental indexing representation 665 starts with the '01' 2-bit pattern. 667 If the header field name matches the header field name of an entry 668 stored in the static table or the header table, the header field name 669 can be represented using the index of that entry. In this case, the 670 index of the entry is represented as an integer with a 6-bit prefix 671 (see Section 6.1). This value is always non-zero. 673 Otherwise, the header field name is represented as a literal string 674 (see Section 6.2). A value 0 is used in place of the 6-bit index, 675 followed by the header field name. 677 Either form of header field name representation is followed by the 678 header field value represented as a literal string (see Section 6.2). 680 7.2.2. Literal Header Field without Indexing 682 A literal header field without indexing representation results in 683 adding a header field to the decoded header list without altering the 684 header table. 686 0 1 2 3 4 5 6 7 687 +---+---+---+---+---+---+---+---+ 688 | 0 | 0 | 0 | 0 | Index (4+) | 689 +---+---+-----------------------+ 690 | H | Value Length (7+) | 691 +---+---------------------------+ 692 | Value String (Length octets) | 693 +-------------------------------+ 695 Literal Header Field without Indexing - Indexed Name 697 0 1 2 3 4 5 6 7 698 +---+---+---+---+---+---+---+---+ 699 | 0 | 0 | 0 | 0 | 0 | 700 +---+---+-----------------------+ 701 | H | Name Length (7+) | 702 +---+---------------------------+ 703 | Name String (Length octets) | 704 +---+---------------------------+ 705 | H | Value Length (7+) | 706 +---+---------------------------+ 707 | Value String (Length octets) | 708 +-------------------------------+ 710 Literal Header Field without Indexing - New Name 712 A literal header field without indexing representation starts with 713 the '0000' 4-bit pattern. 715 If the header field name matches the header field name of an entry 716 stored in the static table or the header table, the header field name 717 can be represented using the index of that entry. In this case, the 718 index of the entry is represented as an integer with a 4-bit prefix 719 (see Section 6.1). This value is always non-zero. 721 Otherwise, the header field name is represented as a literal string 722 (see Section 6.2). A value 0 is used in place of the 4-bit index, 723 followed by the header field name. 725 Either form of header field name representation is followed by the 726 header field value represented as a literal string (see Section 6.2). 728 7.2.3. Literal Header Field never Indexed 730 A literal header field never indexed representation results in adding 731 a header field to the decoded header list without altering the header 732 table. Intermediaries MUST use the same representation for encoding 733 this header field. 735 0 1 2 3 4 5 6 7 736 +---+---+---+---+---+---+---+---+ 737 | 0 | 0 | 0 | 1 | Index (4+) | 738 +---+---+-----------------------+ 739 | H | Value Length (7+) | 740 +---+---------------------------+ 741 | Value String (Length octets) | 742 +-------------------------------+ 744 Literal Header Field never Indexed - Indexed Name 746 0 1 2 3 4 5 6 7 747 +---+---+---+---+---+---+---+---+ 748 | 0 | 0 | 0 | 1 | 0 | 749 +---+---+-----------------------+ 750 | H | Name Length (7+) | 751 +---+---------------------------+ 752 | Name String (Length octets) | 753 +---+---------------------------+ 754 | H | Value Length (7+) | 755 +---+---------------------------+ 756 | Value String (Length octets) | 757 +-------------------------------+ 759 Literal Header Field never Indexed - New Name 761 A literal header field never indexed representation starts with the 762 '0001' 4-bit pattern. 764 When a header field is represented as a literal header field never 765 indexed, it MUST always be encoded with this specific literal 766 representation. In particular, when a peer sends a header field that 767 it received represented as a literal header field never indexed, it 768 MUST use the same representation to forward this header field. 770 This representation is intended for protecting header field values 771 that are not to be put at risk by compressing them (see Section 8.1 772 for more details). 774 The encoding of the representation is identical to the literal header 775 field without indexing (see Section 7.2.2). 777 7.3. Header Table Size Update 779 A header table size update signals a change to the size of the header 780 table. 782 0 1 2 3 4 5 6 7 783 +---+---+---+---+---+---+---+---+ 784 | 0 | 0 | 1 | Max size (5+) | 785 +---+---------------------------+ 787 Maximum Header Table Size Change 789 A header table size update starts with the '001' 3-bit pattern, 790 followed by the new maximum size, represented as an integer with a 791 5-bit prefix (see Section 6.1). 793 The new maximum size MUST be lower than or equal to the last value of 794 the SETTINGS_HEADER_TABLE_SIZE parameter (see Section 6.5.2 of 795 [HTTP2]) received from the decoder and acknowledged by the encoder 796 (see Section 6.5.3 of [HTTP2]). 798 Reducing the maximum size of the header table can cause entries to be 799 evicted (see Section 5.2). 801 8. Security Considerations 803 This section describes potential areas of security concern with 804 HPACK: 806 o Use of compression as a length-based oracle for verifying guesses 807 about secrets that are compressed into a shared compression 808 context. 810 o Denial of service resulting from exhausting processing or memory 811 capacity at a decoder. 813 8.1. Probing Header Table State 815 HPACK reduces the length of header field encodings by exploiting the 816 redundancy inherent in protocols like HTTP. The ultimate goal of 817 this is to reduce the amount of data that is required to send HTTP 818 requests or responses. 820 The compression context used to encode header fields can be probed by 821 an attacker that has the following capabilities: to define header 822 fields to be encoded and transmitted; and to observe the length of 823 those fields once they are encoded. This allows an attacker to 824 adaptively modify requests in order to confirm guesses about the 825 header table state. If a guess is compressed into a shorter length, 826 the attacker can observe the encoded length and infer that the guess 827 was correct. 829 This is possible because while TLS provides confidentiality 830 protection for content, it only provides a limited amount of 831 protection for the length of that content. 833 Note: Padding schemes only provide limited protection against an 834 attacker with these capabilities, potentially only forcing an 835 increased number of guesses to learn the length associated with a 836 given guess. Padding schemes also work directly against 837 compression by increasing the number of bits that are transmitted. 839 Attacks like CRIME [CRIME] demonstrated the existence of these 840 general attacker capabilities. The specific attack exploited the 841 fact that DEFLATE [DEFLATE] removes redundancy based on prefix 842 matching. This permitted the attacker to confirm guesses a character 843 at a time, reducing an exponential-time attack into a linear-time 844 attack. 846 8.1.1. Applicability to HPACK and HTTP 848 HPACK mitigates but does not completely prevent attacks modelled on 849 CRIME [CRIME] by forcing a guess to match an entire header field 850 value, rather than individual characters. An attacker can only learn 851 whether a guess is correct or not, so is reduced to a brute force 852 guess for the header field values. 854 The viability of recovering specific header field values therefore 855 depends on the entropy of values. As a result, values with high 856 entropy are unlikely to be recovered successfully. However, values 857 with low entropy remain vulnerable. 859 Attacks of this nature are possible any time that two mutually 860 distrustful entities control requests or responses that are placed 861 onto a single HTTP/2 connection. If the shared HPACK compressor 862 permits one entity to add entries to the header table, and the other 863 to access those entries, then the state of the table can be learned. 865 Having requests or responses from mutually distrustful entities 866 occurs when an intermediary either: 868 o sends requests from multiple clients on a single connection toward 869 an origin server, or 871 o takes responses from multiple origin servers and places them on a 872 shared connection toward a client. 874 Web browsers also need to assume that requests made on the same 875 connection by different web origins [ORIGIN] are made by mutually 876 distrustful entities. 878 8.1.2. Mitigation 880 Users of HTTP that require confidentiality for header fields can use 881 values with entropy sufficient to make guessing infeasible. However, 882 this is impractical as a general solution because it forces all users 883 of HTTP to take steps to mitigate attacks. It would impose new 884 constraints on how HTTP is used. 886 Rather than impose constraints on users of HTTP, an implementation of 887 HPACK can instead constrain how compression is applied in order to 888 limit the potential for header table probing. 890 An ideal solution segregates access to the header table based on the 891 entity that is constructing header fields. Header field values that 892 are added to the table are attributed to an entity, and only the 893 entity that created an particular value can extract that value. 895 To improve compression performance of this option, certain entries 896 might be tagged as being public. For example, a web browser might 897 make the values of the Accept-Encoding header field available in all 898 requests. 900 An encoder without good knowledge of the provenance of header fields 901 might instead introduce a penalty for bad guesses, such that attempts 902 to guess a header field value results in all values being removed 903 from consideration in all future requests, effectively preventing 904 further guesses. 906 Note: Simply removing values from the header table can be 907 ineffectual if the attacker has a reliable way of causing values 908 to be reinstalled. For example, a request to load an image in a 909 web browser typically includes the Cookie header field (a 910 potentially highly valued target for this sort of attack), and web 911 sites can easily force an image to be loaded, thereby refreshing 912 the entry in the header table. 914 This response might be made inversely proportional to the length of 915 the header field. Marking as inaccessible might occur for shorter 916 values more quickly or with higher probability than for longer 917 values. 919 Implementations might also choose to protect certain header fields 920 that are known to be highly valued, such as the Authorization or 921 Cookie header fields, by disabling or further limiting compression. 923 8.1.3. Never Indexed Literals 925 Refusing to generate an indexed representation for a header field is 926 only effective if compression is avoided on all hops. The never 927 indexed literal (see Section 7.2.3) can be used to signal to 928 intermediaries that a particular value was intentionally sent as a 929 literal. An intermediary MUST NOT re-encode a value that uses the 930 never indexed literal with a representation that would index it. 932 8.2. Static Huffman Encoding 934 There is currently no known threat taking advantage of the use of a 935 fixed Huffman encoding. A study has shown that using a fixed Huffman 936 encoding table created an information leakage, however this same 937 study concluded that an attacker could not take advantage of this 938 information leakage to recover any meaningful amount of information 939 (see [PETAL]). 941 8.3. Memory Consumption 942 An attacker can try to cause an endpoint to exhaust its memory. 943 HPACK is designed to limit both the peak and state amounts of memory 944 allocated by an endpoint. 946 The amount of memory used by the compressor state is limited by the 947 decoder using the value of the HTTP/2 setting parameter 948 SETTINGS_HEADER_TABLE_SIZE (see Section 6.5.2 of [HTTP2]). This 949 limit takes into account both the size of the data stored in the 950 header table, plus a small allowance for overhead. 952 A decoder can limit the amount of state memory used by setting an 953 appropriate value for the SETTINGS_HEADER_TABLE_SIZE parameter. An 954 encoder can limit the amount of state memory it uses by signalling 955 lower header table size than the decoder allows (see Section 7.3). 957 The amount of temporary memory consumed by an encoder or decoder can 958 be limited by processing header fields sequentially. An 959 implementation does not need to retain a complete list of header 960 fields. Note however that it might be necessary for an application 961 to retain a complete header list for other reasons; even though HPACK 962 does not force this to occur, application constraints might make this 963 necessary. 965 8.4. Implementation Limits 967 An implementation of HPACK needs to ensure that large values for 968 integers, long encoding for integers, or long string literals do not 969 create security weaknesses. 971 An implementation has to set a limit for the values it accepts for 972 integers, as well as for the encoded length (see Section 6.1). In 973 the same way, it has to set a limit to the length it accepts for 974 string literals (see Section 6.2). 976 9. Acknowledgements 978 This document includes substantial input from the following 979 individuals: 981 o Mike Bishop, Jeff Pinner, Julian Reschke, Martin Thomson 982 (substantial editorial contributions). 984 o Johnny Graettinger (Huffman code statistics). 986 10. References 988 10.1. Normative References 990 [HTTP2] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext 991 Transfer Protocol version 2", draft-ietf-httpbis-http2-14 992 (work in progress), July 2014. 994 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 995 Requirement Levels ", BCP 14, RFC 2119, March 1997. 997 [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 998 Protocol (HTTP/1.1): Message Syntax and Routing ", RFC 999 7230, June 2014. 1001 10.2. Informative References 1003 [CANONICAL] 1004 Schwartz, E. and B. Kallick, "Generating a canonical 1005 prefix encoding", Communications of the ACM Volume 7 Issue 1006 3, pp. 166-169, March 1964, . 1009 [CRIME] Rizzo, J. and T. Duong, "The CRIME Attack", September 1010 2012, . 1014 [DEFLATE] Deutsch, P., "DEFLATE Compressed Data Format Specification 1015 version 1.3", RFC 1951, May 1996. 1017 [HUFFMAN] Huffman, D., "A Method for the Construction of Minimum 1018 Redundancy Codes", Proceedings of the Institute of Radio 1019 Engineers Volume 40, Number 9, pp. 1098-1101, September 1020 1952, . 1023 [ORIGIN] Barth, A., "The Web Origin Concept", RFC 6454, December 1024 2011. 1026 [PETAL] Tan, J. and J. Nahata, "PETAL: Preset Encoding Table 1027 Information Leakage", April 2013, . 1030 [SPDY-DESC-1] 1031 Belshe, M., "IETF83: SPDY and What to Consider for HTTP/ 1032 2.0 ", March 2012, . 1035 [SPDY-DESC-2] 1036 McManus, P., "SPDY: What I Like About You", September 1037 2011, . 1040 [SPDY] Belshe, M. and R. Peon, "SPDY Protocol", draft-mbelshe- 1041 httpbis-spdy-00 (work in progress), February 2012. 1043 Appendix A. Change Log (to be removed by RFC Editor before publication) 1045 A.1. Since draft-ietf-httpbis-header-compression-08 1047 o Removed the reference set. 1049 o Removed header emission. 1051 o Explicit handling of several SETTINGS_HEADER_TABLE_SIZE parameter 1052 changes. 1054 o Changed header set to header list, and forced ordering. 1056 o Updated examples. 1058 o Exchanged header and static table positions. 1060 A.2. Since draft-ietf-httpbis-header-compression-07 1062 o Removed old text on index value of 0. 1064 o Added clarification for signalling of maximum table size after a 1065 SETTINGS_HEADER_TABLE_SIZE update. 1067 o Rewrote security considerations. 1069 o Many editorial clarifications or improvements. 1071 o Added convention section. 1073 o Reworked document's outline. 1075 o Updated static table. Entry 16 has now "gzip, deflate" for value. 1077 o Updated Huffman table, using data set provided by Google. 1079 A.3. Since draft-ietf-httpbis-header-compression-06 1081 o Updated format to include literal headers that must never be 1082 compressed. 1084 o Updated security considerations. 1086 o Moved integer encoding examples to the appendix. 1088 o Updated Huffman table. 1090 o Updated static header table (adding and removing status values). 1092 o Updated examples. 1094 A.4. Since draft-ietf-httpbis-header-compression-05 1096 o Regenerated examples. 1098 o Only one Huffman table for requests and responses. 1100 o Added maximum size for header table, independent of 1101 SETTINGS_HEADER_TABLE_SIZE. 1103 o Added pseudo-code for integer decoding. 1105 o Improved examples (removing unnecessary removals). 1107 A.5. Since draft-ietf-httpbis-header-compression-04 1109 o Updated examples: take into account changes in the spec, and show 1110 more features. 1112 o Use 'octet' everywhere instead of having both 'byte' and 'octet'. 1114 o Added reference set emptying. 1116 o Editorial changes and clarifications. 1118 o Added "host" header to the static table. 1120 o Ordering for list of values (either NULL- or comma-separated). 1122 A.6. Since draft-ietf-httpbis-header-compression-03 1124 o A large number of editorial changes; changed the description of 1125 evicting/adding new entries. 1127 o Removed substitution indexing 1129 o Changed 'initial headers' to 'static headers', as per issue #258 1131 o Merged 'request' and 'response' static headers, as per issue #259 1133 o Changed text to indicate that new headers are added at index 0 and 1134 expire from the largest index, as per issue #233 1136 A.7. Since draft-ietf-httpbis-header-compression-02 1138 o Corrected error in integer encoding pseudocode. 1140 A.8. Since draft-ietf-httpbis-header-compression-01 1142 o Refactored of Header Encoding Section: split definitions and 1143 processing rule. 1145 o Backward incompatible change: Updated reference set management as 1146 per issue #214. This changes how the interaction between the 1147 reference set and eviction works. This also changes the working 1148 of the reference set in some specific cases. 1150 o Backward incompatible change: modified initial header list, as per 1151 issue #188. 1153 o Added example of 32 octets entry structure (issue #191). 1155 o Added Header Set Completion section. Reflowed some text. 1156 Clarified some writing which was akward. Added text about 1157 duplicate header entry encoding. Clarified some language w.r.t 1158 Header Set. Changed x-my-header to mynewheader. Added text in 1159 the HeaderEmission section indicating that the application may 1160 also be able to free up memory more quickly. Added information in 1161 Security Considerations section. 1163 A.9. Since draft-ietf-httpbis-header-compression-00 1165 Fixed bug/omission in integer representation algorithm. 1167 Changed the document title. 1169 Header matching text rewritten. 1171 Changed the definition of header emission. 1173 Changed the name of the setting which dictates how much memory the 1174 compression context should use. 1176 Removed "specific use cases" section 1178 Corrected erroneous statement about what index can be contained in 1179 one octet 1181 Added descriptions of opcodes 1183 Removed security claims from introduction. 1185 Appendix B. Static Table Definition 1187 The static table (see Section 3.3.1) consists of a predefined and 1188 unchangeable list of header fields. 1190 The static table was created by listing the most common header fields 1191 that are valid for messages exchanged inside a HTTP/2 connection. 1192 For header fields with a few frequent values, an entry was added for 1193 each of these frequent values. For other header fields, an entry was 1194 added with an empty value. 1196 The following table lists the pre-defined header fields that make-up 1197 the static table. 1199 +-------+-----------------------------+---------------+ 1200 | Index | Header Name | Header Value | 1201 +-------+-----------------------------+---------------+ 1202 | 1 | :authority | | 1203 | 2 | :method | GET | 1204 | 3 | :method | POST | 1205 | 4 | :path | / | 1206 | 5 | :path | /index.html | 1207 | 6 | :scheme | http | 1208 | 7 | :scheme | https | 1209 | 8 | :status | 200 | 1210 | 9 | :status | 204 | 1211 | 10 | :status | 206 | 1212 | 11 | :status | 304 | 1213 | 12 | :status | 400 | 1214 | 13 | :status | 404 | 1215 | 14 | :status | 500 | 1216 | 15 | accept-charset | | 1217 | 16 | accept-encoding | gzip, deflate | 1218 | 17 | accept-language | | 1219 | 18 | accept-ranges | | 1220 | 19 | accept | | 1221 | 20 | access-control-allow-origin | | 1222 | 21 | age | | 1223 | 22 | allow | | 1224 | 23 | authorization | | 1225 | 24 | cache-control | | 1226 | 25 | content-disposition | | 1227 | 26 | content-encoding | | 1228 | 27 | content-language | | 1229 | 28 | content-length | | 1230 | 29 | content-location | | 1231 | 30 | content-range | | 1232 | 31 | content-type | | 1233 | 32 | cookie | | 1234 | 33 | date | | 1235 | 34 | etag | | 1236 | 35 | expect | | 1237 | 36 | expires | | 1238 | 37 | from | | 1239 | 38 | host | | 1240 | 39 | if-match | | 1241 | 40 | if-modified-since | | 1242 | 41 | if-none-match | | 1243 | 42 | if-range | | 1244 | 43 | if-unmodified-since | | 1245 | 44 | last-modified | | 1246 | 45 | link | | 1247 | 46 | location | | 1248 | 47 | max-forwards | | 1249 | 48 | proxy-authenticate | | 1250 | 49 | proxy-authorization | | 1251 | 50 | range | | 1252 | 51 | referer | | 1253 | 52 | refresh | | 1254 | 53 | retry-after | | 1255 | 54 | server | | 1256 | 55 | set-cookie | | 1257 | 56 | strict-transport-security | | 1258 | 57 | transfer-encoding | | 1259 | 58 | user-agent | | 1260 | 59 | vary | | 1261 | 60 | via | | 1262 | 61 | www-authenticate | | 1263 +-------+-----------------------------+---------------+ 1265 Table 1: Static Table Entries 1267 Table 1 gives the index of each entry in the static table. 1269 Appendix C. Huffman Code 1271 The following Huffman code is used when encoding string literals with 1272 a Huffman coding (see Section 6.2). 1274 This Huffman code was generated from statistics obtained on a large 1275 sample of HTTP headers. It is a canonical Huffman code (see 1276 [CANONICAL]) with some tweaking to ensure that no symbol has a unique 1277 code length. 1279 Each row in the table defines the code used to represent a symbol: 1281 sym: The symbol to be represented. It is the decimal value of an 1282 octet, possibly prepended with its ASCII representation. A 1283 specific symbol, "EOS", is used to indicate the end of a string 1284 literal. 1286 code as bits: The Huffman code for the symbol represented as a 1287 base-2 integer, aligned on the most significant bit (MSB). 1289 code as hex: The Huffman code for the symbol, represented as a 1290 hexadecimal integer, aligned on the least significant bit (LSB). 1292 len: The number of bits for the code representing the symbol. 1294 As an example, the code for the symbol 47 (corresponding to the ASCII 1295 character "/") consists in the 6 bits "0", "1", "1", "0", "0", "0". 1296 This corresponds to the value 0x18 (in hexadecimal) encoded on 6 1297 bits. 1299 code 1300 code as bits as hex len 1301 sym aligned to MSB aligned in 1302 to LSB bits 1303 ( 0) |11111111|11000 1ff8 [13] 1304 ( 1) |11111111|11111111|1011000 7fffd8 [23] 1305 ( 2) |11111111|11111111|11111110|0010 fffffe2 [28] 1306 ( 3) |11111111|11111111|11111110|0011 fffffe3 [28] 1307 ( 4) |11111111|11111111|11111110|0100 fffffe4 [28] 1308 ( 5) |11111111|11111111|11111110|0101 fffffe5 [28] 1309 ( 6) |11111111|11111111|11111110|0110 fffffe6 [28] 1310 ( 7) |11111111|11111111|11111110|0111 fffffe7 [28] 1311 ( 8) |11111111|11111111|11111110|1000 fffffe8 [28] 1312 ( 9) |11111111|11111111|11101010 ffffea [24] 1313 ( 10) |11111111|11111111|11111111|111100 3ffffffc [30] 1314 ( 11) |11111111|11111111|11111110|1001 fffffe9 [28] 1315 ( 12) |11111111|11111111|11111110|1010 fffffea [28] 1316 ( 13) |11111111|11111111|11111111|111101 3ffffffd [30] 1317 ( 14) |11111111|11111111|11111110|1011 fffffeb [28] 1318 ( 15) |11111111|11111111|11111110|1100 fffffec [28] 1319 ( 16) |11111111|11111111|11111110|1101 fffffed [28] 1320 ( 17) |11111111|11111111|11111110|1110 fffffee [28] 1321 ( 18) |11111111|11111111|11111110|1111 fffffef [28] 1322 ( 19) |11111111|11111111|11111111|0000 ffffff0 [28] 1323 ( 20) |11111111|11111111|11111111|0001 ffffff1 [28] 1324 ( 21) |11111111|11111111|11111111|0010 ffffff2 [28] 1325 ( 22) |11111111|11111111|11111111|111110 3ffffffe [30] 1326 ( 23) |11111111|11111111|11111111|0011 ffffff3 [28] 1327 ( 24) |11111111|11111111|11111111|0100 ffffff4 [28] 1328 ( 25) |11111111|11111111|11111111|0101 ffffff5 [28] 1329 ( 26) |11111111|11111111|11111111|0110 ffffff6 [28] 1330 ( 27) |11111111|11111111|11111111|0111 ffffff7 [28] 1331 ( 28) |11111111|11111111|11111111|1000 ffffff8 [28] 1332 ( 29) |11111111|11111111|11111111|1001 ffffff9 [28] 1333 ( 30) |11111111|11111111|11111111|1010 ffffffa [28] 1334 ( 31) |11111111|11111111|11111111|1011 ffffffb [28] 1335 ' ' ( 32) |010100 14 [ 6] 1336 '!' ( 33) |11111110|00 3f8 [10] 1337 '"' ( 34) |11111110|01 3f9 [10] 1338 '#' ( 35) |11111111|1010 ffa [12] 1339 '$' ( 36) |11111111|11001 1ff9 [13] 1340 '%' ( 37) |010101 15 [ 6] 1341 '&' ( 38) |11111000 f8 [ 8] 1342 ''' ( 39) |11111111|010 7fa [11] 1343 '(' ( 40) |11111110|10 3fa [10] 1344 ')' ( 41) |11111110|11 3fb [10] 1345 '*' ( 42) |11111001 f9 [ 8] 1346 '+' ( 43) |11111111|011 7fb [11] 1347 ',' ( 44) |11111010 fa [ 8] 1348 '-' ( 45) |010110 16 [ 6] 1349 '.' ( 46) |010111 17 [ 6] 1350 '/' ( 47) |011000 18 [ 6] 1351 '0' ( 48) |00000 0 [ 5] 1352 '1' ( 49) |00001 1 [ 5] 1353 '2' ( 50) |00010 2 [ 5] 1354 '3' ( 51) |011001 19 [ 6] 1355 '4' ( 52) |011010 1a [ 6] 1356 '5' ( 53) |011011 1b [ 6] 1357 '6' ( 54) |011100 1c [ 6] 1358 '7' ( 55) |011101 1d [ 6] 1359 '8' ( 56) |011110 1e [ 6] 1360 '9' ( 57) |011111 1f [ 6] 1361 ':' ( 58) |1011100 5c [ 7] 1362 ';' ( 59) |11111011 fb [ 8] 1363 '<' ( 60) |11111111|1111100 7ffc [15] 1364 '=' ( 61) |100000 20 [ 6] 1365 '>' ( 62) |11111111|1011 ffb [12] 1366 '?' ( 63) |11111111|00 3fc [10] 1367 '@' ( 64) |11111111|11010 1ffa [13] 1368 'A' ( 65) |100001 21 [ 6] 1369 'B' ( 66) |1011101 5d [ 7] 1370 'C' ( 67) |1011110 5e [ 7] 1371 'D' ( 68) |1011111 5f [ 7] 1372 'E' ( 69) |1100000 60 [ 7] 1373 'F' ( 70) |1100001 61 [ 7] 1374 'G' ( 71) |1100010 62 [ 7] 1375 'H' ( 72) |1100011 63 [ 7] 1376 'I' ( 73) |1100100 64 [ 7] 1377 'J' ( 74) |1100101 65 [ 7] 1378 'K' ( 75) |1100110 66 [ 7] 1379 'L' ( 76) |1100111 67 [ 7] 1380 'M' ( 77) |1101000 68 [ 7] 1381 'N' ( 78) |1101001 69 [ 7] 1382 'O' ( 79) |1101010 6a [ 7] 1383 'P' ( 80) |1101011 6b [ 7] 1384 'Q' ( 81) |1101100 6c [ 7] 1385 'R' ( 82) |1101101 6d [ 7] 1386 'S' ( 83) |1101110 6e [ 7] 1387 'T' ( 84) |1101111 6f [ 7] 1388 'U' ( 85) |1110000 70 [ 7] 1389 'V' ( 86) |1110001 71 [ 7] 1390 'W' ( 87) |1110010 72 [ 7] 1391 'X' ( 88) |11111100 fc [ 8] 1392 'Y' ( 89) |1110011 73 [ 7] 1393 'Z' ( 90) |11111101 fd [ 8] 1394 '[' ( 91) |11111111|11011 1ffb [13] 1395 '\' ( 92) |11111111|11111110|000 7fff0 [19] 1396 ']' ( 93) |11111111|11100 1ffc [13] 1397 '^' ( 94) |11111111|111100 3ffc [14] 1398 '_' ( 95) |100010 22 [ 6] 1399 '`' ( 96) |11111111|1111101 7ffd [15] 1400 'a' ( 97) |00011 3 [ 5] 1401 'b' ( 98) |100011 23 [ 6] 1402 'c' ( 99) |00100 4 [ 5] 1403 'd' (100) |100100 24 [ 6] 1404 'e' (101) |00101 5 [ 5] 1405 'f' (102) |100101 25 [ 6] 1406 'g' (103) |100110 26 [ 6] 1407 'h' (104) |100111 27 [ 6] 1408 'i' (105) |00110 6 [ 5] 1409 'j' (106) |1110100 74 [ 7] 1410 'k' (107) |1110101 75 [ 7] 1411 'l' (108) |101000 28 [ 6] 1412 'm' (109) |101001 29 [ 6] 1413 'n' (110) |101010 2a [ 6] 1414 'o' (111) |00111 7 [ 5] 1415 'p' (112) |101011 2b [ 6] 1416 'q' (113) |1110110 76 [ 7] 1417 'r' (114) |101100 2c [ 6] 1418 's' (115) |01000 8 [ 5] 1419 't' (116) |01001 9 [ 5] 1420 'u' (117) |101101 2d [ 6] 1421 'v' (118) |1110111 77 [ 7] 1422 'w' (119) |1111000 78 [ 7] 1423 'x' (120) |1111001 79 [ 7] 1424 'y' (121) |1111010 7a [ 7] 1425 'z' (122) |1111011 7b [ 7] 1426 '{' (123) |11111111|1111110 7ffe [15] 1427 '|' (124) |11111111|100 7fc [11] 1428 '}' (125) |11111111|111101 3ffd [14] 1429 '~' (126) |11111111|11101 1ffd [13] 1430 (127) |11111111|11111111|11111111|1100 ffffffc [28] 1431 (128) |11111111|11111110|0110 fffe6 [20] 1432 (129) |11111111|11111111|010010 3fffd2 [22] 1433 (130) |11111111|11111110|0111 fffe7 [20] 1434 (131) |11111111|11111110|1000 fffe8 [20] 1435 (132) |11111111|11111111|010011 3fffd3 [22] 1436 (133) |11111111|11111111|010100 3fffd4 [22] 1437 (134) |11111111|11111111|010101 3fffd5 [22] 1438 (135) |11111111|11111111|1011001 7fffd9 [23] 1439 (136) |11111111|11111111|010110 3fffd6 [22] 1440 (137) |11111111|11111111|1011010 7fffda [23] 1441 (138) |11111111|11111111|1011011 7fffdb [23] 1442 (139) |11111111|11111111|1011100 7fffdc [23] 1443 (140) |11111111|11111111|1011101 7fffdd [23] 1444 (141) |11111111|11111111|1011110 7fffde [23] 1445 (142) |11111111|11111111|11101011 ffffeb [24] 1446 (143) |11111111|11111111|1011111 7fffdf [23] 1447 (144) |11111111|11111111|11101100 ffffec [24] 1448 (145) |11111111|11111111|11101101 ffffed [24] 1449 (146) |11111111|11111111|010111 3fffd7 [22] 1450 (147) |11111111|11111111|1100000 7fffe0 [23] 1451 (148) |11111111|11111111|11101110 ffffee [24] 1452 (149) |11111111|11111111|1100001 7fffe1 [23] 1453 (150) |11111111|11111111|1100010 7fffe2 [23] 1454 (151) |11111111|11111111|1100011 7fffe3 [23] 1455 (152) |11111111|11111111|1100100 7fffe4 [23] 1456 (153) |11111111|11111110|11100 1fffdc [21] 1457 (154) |11111111|11111111|011000 3fffd8 [22] 1458 (155) |11111111|11111111|1100101 7fffe5 [23] 1459 (156) |11111111|11111111|011001 3fffd9 [22] 1460 (157) |11111111|11111111|1100110 7fffe6 [23] 1461 (158) |11111111|11111111|1100111 7fffe7 [23] 1462 (159) |11111111|11111111|11101111 ffffef [24] 1463 (160) |11111111|11111111|011010 3fffda [22] 1464 (161) |11111111|11111110|11101 1fffdd [21] 1465 (162) |11111111|11111110|1001 fffe9 [20] 1466 (163) |11111111|11111111|011011 3fffdb [22] 1467 (164) |11111111|11111111|011100 3fffdc [22] 1468 (165) |11111111|11111111|1101000 7fffe8 [23] 1469 (166) |11111111|11111111|1101001 7fffe9 [23] 1470 (167) |11111111|11111110|11110 1fffde [21] 1471 (168) |11111111|11111111|1101010 7fffea [23] 1472 (169) |11111111|11111111|011101 3fffdd [22] 1473 (170) |11111111|11111111|011110 3fffde [22] 1474 (171) |11111111|11111111|11110000 fffff0 [24] 1475 (172) |11111111|11111110|11111 1fffdf [21] 1476 (173) |11111111|11111111|011111 3fffdf [22] 1477 (174) |11111111|11111111|1101011 7fffeb [23] 1478 (175) |11111111|11111111|1101100 7fffec [23] 1479 (176) |11111111|11111111|00000 1fffe0 [21] 1480 (177) |11111111|11111111|00001 1fffe1 [21] 1481 (178) |11111111|11111111|100000 3fffe0 [22] 1482 (179) |11111111|11111111|00010 1fffe2 [21] 1483 (180) |11111111|11111111|1101101 7fffed [23] 1484 (181) |11111111|11111111|100001 3fffe1 [22] 1485 (182) |11111111|11111111|1101110 7fffee [23] 1486 (183) |11111111|11111111|1101111 7fffef [23] 1487 (184) |11111111|11111110|1010 fffea [20] 1488 (185) |11111111|11111111|100010 3fffe2 [22] 1489 (186) |11111111|11111111|100011 3fffe3 [22] 1490 (187) |11111111|11111111|100100 3fffe4 [22] 1491 (188) |11111111|11111111|1110000 7ffff0 [23] 1492 (189) |11111111|11111111|100101 3fffe5 [22] 1493 (190) |11111111|11111111|100110 3fffe6 [22] 1494 (191) |11111111|11111111|1110001 7ffff1 [23] 1495 (192) |11111111|11111111|11111000|00 3ffffe0 [26] 1496 (193) |11111111|11111111|11111000|01 3ffffe1 [26] 1497 (194) |11111111|11111110|1011 fffeb [20] 1498 (195) |11111111|11111110|001 7fff1 [19] 1499 (196) |11111111|11111111|100111 3fffe7 [22] 1500 (197) |11111111|11111111|1110010 7ffff2 [23] 1501 (198) |11111111|11111111|101000 3fffe8 [22] 1502 (199) |11111111|11111111|11110110|0 1ffffec [25] 1503 (200) |11111111|11111111|11111000|10 3ffffe2 [26] 1504 (201) |11111111|11111111|11111000|11 3ffffe3 [26] 1505 (202) |11111111|11111111|11111001|00 3ffffe4 [26] 1506 (203) |11111111|11111111|11111011|110 7ffffde [27] 1507 (204) |11111111|11111111|11111011|111 7ffffdf [27] 1508 (205) |11111111|11111111|11111001|01 3ffffe5 [26] 1509 (206) |11111111|11111111|11110001 fffff1 [24] 1510 (207) |11111111|11111111|11110110|1 1ffffed [25] 1511 (208) |11111111|11111110|010 7fff2 [19] 1512 (209) |11111111|11111111|00011 1fffe3 [21] 1513 (210) |11111111|11111111|11111001|10 3ffffe6 [26] 1514 (211) |11111111|11111111|11111100|000 7ffffe0 [27] 1515 (212) |11111111|11111111|11111100|001 7ffffe1 [27] 1516 (213) |11111111|11111111|11111001|11 3ffffe7 [26] 1517 (214) |11111111|11111111|11111100|010 7ffffe2 [27] 1518 (215) |11111111|11111111|11110010 fffff2 [24] 1519 (216) |11111111|11111111|00100 1fffe4 [21] 1520 (217) |11111111|11111111|00101 1fffe5 [21] 1521 (218) |11111111|11111111|11111010|00 3ffffe8 [26] 1522 (219) |11111111|11111111|11111010|01 3ffffe9 [26] 1523 (220) |11111111|11111111|11111111|1101 ffffffd [28] 1524 (221) |11111111|11111111|11111100|011 7ffffe3 [27] 1525 (222) |11111111|11111111|11111100|100 7ffffe4 [27] 1526 (223) |11111111|11111111|11111100|101 7ffffe5 [27] 1527 (224) |11111111|11111110|1100 fffec [20] 1528 (225) |11111111|11111111|11110011 fffff3 [24] 1529 (226) |11111111|11111110|1101 fffed [20] 1530 (227) |11111111|11111111|00110 1fffe6 [21] 1531 (228) |11111111|11111111|101001 3fffe9 [22] 1532 (229) |11111111|11111111|00111 1fffe7 [21] 1533 (230) |11111111|11111111|01000 1fffe8 [21] 1534 (231) |11111111|11111111|1110011 7ffff3 [23] 1535 (232) |11111111|11111111|101010 3fffea [22] 1536 (233) |11111111|11111111|101011 3fffeb [22] 1537 (234) |11111111|11111111|11110111|0 1ffffee [25] 1538 (235) |11111111|11111111|11110111|1 1ffffef [25] 1539 (236) |11111111|11111111|11110100 fffff4 [24] 1540 (237) |11111111|11111111|11110101 fffff5 [24] 1541 (238) |11111111|11111111|11111010|10 3ffffea [26] 1542 (239) |11111111|11111111|1110100 7ffff4 [23] 1543 (240) |11111111|11111111|11111010|11 3ffffeb [26] 1544 (241) |11111111|11111111|11111100|110 7ffffe6 [27] 1545 (242) |11111111|11111111|11111011|00 3ffffec [26] 1546 (243) |11111111|11111111|11111011|01 3ffffed [26] 1547 (244) |11111111|11111111|11111100|111 7ffffe7 [27] 1548 (245) |11111111|11111111|11111101|000 7ffffe8 [27] 1549 (246) |11111111|11111111|11111101|001 7ffffe9 [27] 1550 (247) |11111111|11111111|11111101|010 7ffffea [27] 1551 (248) |11111111|11111111|11111101|011 7ffffeb [27] 1552 (249) |11111111|11111111|11111111|1110 ffffffe [28] 1553 (250) |11111111|11111111|11111101|100 7ffffec [27] 1554 (251) |11111111|11111111|11111101|101 7ffffed [27] 1555 (252) |11111111|11111111|11111101|110 7ffffee [27] 1556 (253) |11111111|11111111|11111101|111 7ffffef [27] 1557 (254) |11111111|11111111|11111110|000 7fffff0 [27] 1558 (255) |11111111|11111111|11111011|10 3ffffee [26] 1559 EOS (256) |11111111|11111111|11111111|111111 3fffffff [30] 1561 Appendix D. Examples 1563 A number of examples are worked through here, covering integer 1564 encoding, header field representation, and the encoding of whole 1565 lists of header fields, for both requests and responses, and with and 1566 without Huffman coding. 1568 D.1. Integer Representation Examples 1570 This section shows the representation of integer values in details 1571 (see Section 6.1). 1573 D.1.1. Example 1: Encoding 10 Using a 5-bit Prefix 1575 The value 10 is to be encoded with a 5-bit prefix. 1577 o 10 is less than 31 (2^5 - 1) and is represented using the 5-bit 1578 prefix. 1580 0 1 2 3 4 5 6 7 1581 +---+---+---+---+---+---+---+---+ 1582 | X | X | X | 0 | 1 | 0 | 1 | 0 | 10 stored on 5 bits 1583 +---+---+---+---+---+---+---+---+ 1585 D.1.2. Example 2: Encoding 1337 Using a 5-bit Prefix 1587 The value I=1337 is to be encoded with a 5-bit prefix. 1589 1337 is greater than 31 (2^5 - 1). 1591 The 5-bit prefix is filled with its max value (31). 1593 I = 1337 - (2^5 - 1) = 1306. 1595 I (1306) is greater than or equal to 128, the while loop body 1596 executes: 1598 I % 128 == 26 1600 26 + 128 == 154 1602 154 is encoded in 8 bits as: 10011010 1604 I is set to 10 (1306 / 128 == 10) 1606 I is no longer greater than or equal to 128, the while loop 1607 terminates. 1609 I, now 10, is encoded on 8 bits as: 00001010. 1611 The process ends. 1613 0 1 2 3 4 5 6 7 1614 +---+---+---+---+---+---+---+---+ 1615 | X | X | X | 1 | 1 | 1 | 1 | 1 | Prefix = 31, I = 1306 1616 | 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 | 1306>=128, encode(154), I=1306/128 1617 | 0 | 0 | 0 | 0 | 1 | 0 | 1 | 0 | 10<128, encode(10), done 1618 +---+---+---+---+---+---+---+---+ 1620 D.1.3. Example 3: Encoding 42 Starting at an Octet Boundary 1622 The value 42 is to be encoded starting at an octet-boundary. This 1623 implies that a 8-bit prefix is used. 1625 o 42 is less than 255 (2^8 - 1) and is represented using the 8-bit 1626 prefix. 1628 0 1 2 3 4 5 6 7 1629 +---+---+---+---+---+---+---+---+ 1630 | 0 | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 42 stored on 8 bits 1631 +---+---+---+---+---+---+---+---+ 1633 D.2. Header Field Representation Examples 1635 This section shows several independent representation examples. 1637 D.2.1. Literal Header Field with Indexing 1639 The header field representation uses a literal name and a literal 1640 value. The header field is added to the header table. 1642 Header list to encode: 1644 custom-key: custom-header 1646 Hex dump of encoded data: 1648 400a 6375 7374 6f6d 2d6b 6579 0d63 7573 | @.custom-key.cus 1649 746f 6d2d 6865 6164 6572 | tom-header 1651 Decoding process: 1653 40 | == Literal indexed == 1654 0a | Literal name (len = 10) 1655 6375 7374 6f6d 2d6b 6579 | custom-key 1656 0d | Literal value (len = 13) 1657 6375 7374 6f6d 2d68 6561 6465 72 | custom-header 1658 | -> custom-key: custom-head\ 1659 | er 1661 Header Table (after decoding): 1663 [ 1] (s = 55) custom-key: custom-header 1664 Table size: 55 1666 Decoded header list: 1668 custom-key: custom-header 1670 D.2.2. Literal Header Field without Indexing 1672 The header field representation uses an indexed name and a literal 1673 value. The header field is not added to the header table. 1675 Header list to encode: 1677 :path: /sample/path 1679 Hex dump of encoded data: 1681 040c 2f73 616d 706c 652f 7061 7468 | ../sample/path 1683 Decoding process: 1685 04 | == Literal not indexed == 1686 | Indexed name (idx = 4) 1687 | :path 1688 0c | Literal value (len = 12) 1689 2f73 616d 706c 652f 7061 7468 | /sample/path 1690 | -> :path: /sample/path 1692 Header table (after decoding): empty. 1694 Decoded header list: 1696 :path: /sample/path 1698 D.2.3. Literal Header Field never Indexed 1700 The header field representation uses a literal name and a literal 1701 value. The header field is not added to the header table, and must 1702 use the same representation if re-encoded by an intermediary. 1704 Header list to encode: 1706 password: secret 1708 Hex dump of encoded data: 1710 1008 7061 7373 776f 7264 0673 6563 7265 | ..password.secre 1711 74 | t 1713 Decoding process: 1715 10 | == Literal never indexed == 1716 08 | Literal name (len = 8) 1717 7061 7373 776f 7264 | password 1718 06 | Literal value (len = 6) 1719 7365 6372 6574 | secret 1720 | -> password: secret 1722 Header table (after decoding): empty. 1724 Decoded header list: 1726 password: secret 1728 D.2.4. Indexed Header Field 1730 The header field representation uses an indexed header field, from 1731 the static table. 1733 Header list to encode: 1735 :method: GET 1737 Hex dump of encoded data: 1739 82 | . 1741 Decoding process: 1743 82 | == Indexed - Add == 1744 | idx = 2 1745 | -> :method: GET 1747 Header table (after decoding): empty. 1749 Decoded header list: 1751 :method: GET 1753 D.3. Request Examples without Huffman Coding 1755 This section shows several consecutive header lists, corresponding to 1756 HTTP requests, on the same connection. 1758 D.3.1. First Request 1760 Header list to encode: 1762 :method: GET 1763 :scheme: http 1764 :path: / 1765 :authority: www.example.com 1766 Hex dump of encoded data: 1768 8286 8441 0f77 7777 2e65 7861 6d70 6c65 | ...A.www.example 1769 2e63 6f6d | .com 1771 Decoding process: 1773 82 | == Indexed - Add == 1774 | idx = 2 1775 | -> :method: GET 1776 86 | == Indexed - Add == 1777 | idx = 6 1778 | -> :scheme: http 1779 84 | == Indexed - Add == 1780 | idx = 4 1781 | -> :path: / 1782 41 | == Literal indexed == 1783 | Indexed name (idx = 1) 1784 | :authority 1785 0f | Literal value (len = 15) 1786 7777 772e 6578 616d 706c 652e 636f 6d | www.example.com 1787 | -> :authority: www.example\ 1788 | .com 1790 Header Table (after decoding): 1792 [ 1] (s = 57) :authority: www.example.com 1793 Table size: 57 1795 Decoded header list: 1797 :method: GET 1798 :scheme: http 1799 :path: / 1800 :authority: www.example.com 1802 D.3.2. Second Request 1804 Header list to encode: 1806 :method: GET 1807 :scheme: http 1808 :path: / 1809 :authority: www.example.com 1810 cache-control: no-cache 1812 Hex dump of encoded data: 1814 8286 84be 5808 6e6f 2d63 6163 6865 | ....X.no-cache 1816 Decoding process: 1818 82 | == Indexed - Add == 1819 | idx = 2 1820 | -> :method: GET 1821 86 | == Indexed - Add == 1822 | idx = 6 1823 | -> :scheme: http 1824 84 | == Indexed - Add == 1825 | idx = 4 1826 | -> :path: / 1827 be | == Indexed - Add == 1828 | idx = 62 1829 | -> :authority: www.example\ 1830 | .com 1831 58 | == Literal indexed == 1832 | Indexed name (idx = 24) 1833 | cache-control 1834 08 | Literal value (len = 8) 1835 6e6f 2d63 6163 6865 | no-cache 1836 | -> cache-control: no-cache 1838 Header Table (after decoding): 1840 [ 1] (s = 53) cache-control: no-cache 1841 [ 2] (s = 57) :authority: www.example.com 1842 Table size: 110 1844 Decoded header list: 1846 :method: GET 1847 :scheme: http 1848 :path: / 1849 :authority: www.example.com 1850 cache-control: no-cache 1852 D.3.3. Third Request 1854 Header list to encode: 1856 :method: GET 1857 :scheme: https 1858 :path: /index.html 1859 :authority: www.example.com 1860 custom-key: custom-value 1862 Hex dump of encoded data: 1864 8287 85bf 400a 6375 7374 6f6d 2d6b 6579 | ....@.custom-key 1865 0c63 7573 746f 6d2d 7661 6c75 65 | .custom-value 1867 Decoding process: 1869 82 | == Indexed - Add == 1870 | idx = 2 1871 | -> :method: GET 1872 87 | == Indexed - Add == 1873 | idx = 7 1874 | -> :scheme: https 1875 85 | == Indexed - Add == 1876 | idx = 5 1877 | -> :path: /index.html 1878 bf | == Indexed - Add == 1879 | idx = 63 1880 | -> :authority: www.example\ 1881 | .com 1882 40 | == Literal indexed == 1883 0a | Literal name (len = 10) 1884 6375 7374 6f6d 2d6b 6579 | custom-key 1885 0c | Literal value (len = 12) 1886 6375 7374 6f6d 2d76 616c 7565 | custom-value 1887 | -> custom-key: custom-valu\ 1888 | e 1890 Header Table (after decoding): 1892 [ 1] (s = 54) custom-key: custom-value 1893 [ 2] (s = 53) cache-control: no-cache 1894 [ 3] (s = 57) :authority: www.example.com 1895 Table size: 164 1897 Decoded header list: 1899 :method: GET 1900 :scheme: https 1901 :path: /index.html 1902 :authority: www.example.com 1903 custom-key: custom-value 1905 D.4. Request Examples with Huffman Coding 1907 This section shows the same examples as the previous section, but 1908 using Huffman encoding for the literal values. 1910 D.4.1. First Request 1912 Header list to encode: 1914 :method: GET 1915 :scheme: http 1916 :path: / 1917 :authority: www.example.com 1919 Hex dump of encoded data: 1921 8286 8441 8cf1 e3c2 e5f2 3a6b a0ab 90f4 | ...A......:k.... 1922 ff | . 1924 Decoding process: 1926 82 | == Indexed - Add == 1927 | idx = 2 1928 | -> :method: GET 1929 86 | == Indexed - Add == 1930 | idx = 6 1931 | -> :scheme: http 1932 84 | == Indexed - Add == 1933 | idx = 4 1934 | -> :path: / 1935 41 | == Literal indexed == 1936 | Indexed name (idx = 1) 1937 | :authority 1938 8c | Literal value (len = 12) 1939 | Huffman encoded: 1940 f1e3 c2e5 f23a 6ba0 ab90 f4ff | .....:k..... 1941 | Decoded: 1942 | www.example.com 1943 | -> :authority: www.example\ 1944 | .com 1946 Header Table (after decoding): 1948 [ 1] (s = 57) :authority: www.example.com 1949 Table size: 57 1951 Decoded header list: 1953 :method: GET 1954 :scheme: http 1955 :path: / 1956 :authority: www.example.com 1958 D.4.2. Second Request 1960 Header list to encode: 1962 :method: GET 1963 :scheme: http 1964 :path: / 1965 :authority: www.example.com 1966 cache-control: no-cache 1968 Hex dump of encoded data: 1970 8286 84be 5886 a8eb 1064 9cbf | ....X....d.. 1972 Decoding process: 1974 82 | == Indexed - Add == 1975 | idx = 2 1976 | -> :method: GET 1977 86 | == Indexed - Add == 1978 | idx = 6 1979 | -> :scheme: http 1980 84 | == Indexed - Add == 1981 | idx = 4 1982 | -> :path: / 1983 be | == Indexed - Add == 1984 | idx = 62 1985 | -> :authority: www.example\ 1986 | .com 1987 58 | == Literal indexed == 1988 | Indexed name (idx = 24) 1989 | cache-control 1990 86 | Literal value (len = 6) 1991 | Huffman encoded: 1992 a8eb 1064 9cbf | ...d.. 1993 | Decoded: 1994 | no-cache 1995 | -> cache-control: no-cache 1997 Header Table (after decoding): 1999 [ 1] (s = 53) cache-control: no-cache 2000 [ 2] (s = 57) :authority: www.example.com 2001 Table size: 110 2003 Decoded header list: 2005 :method: GET 2006 :scheme: http 2007 :path: / 2008 :authority: www.example.com 2009 cache-control: no-cache 2011 D.4.3. Third Request 2012 Header list to encode: 2014 :method: GET 2015 :scheme: https 2016 :path: /index.html 2017 :authority: www.example.com 2018 custom-key: custom-value 2020 Hex dump of encoded data: 2022 8287 85bf 4088 25a8 49e9 5ba9 7d7f 8925 | ....@.%.I.[.}..% 2023 a849 e95b b8e8 b4bf | .I.[.... 2025 Decoding process: 2027 82 | == Indexed - Add == 2028 | idx = 2 2029 | -> :method: GET 2030 87 | == Indexed - Add == 2031 | idx = 7 2032 | -> :scheme: https 2033 85 | == Indexed - Add == 2034 | idx = 5 2035 | -> :path: /index.html 2036 bf | == Indexed - Add == 2037 | idx = 63 2038 | -> :authority: www.example\ 2039 | .com 2040 40 | == Literal indexed == 2041 88 | Literal name (len = 8) 2042 | Huffman encoded: 2043 25a8 49e9 5ba9 7d7f | %.I.[.}. 2044 | Decoded: 2045 | custom-key 2046 89 | Literal value (len = 9) 2047 | Huffman encoded: 2048 25a8 49e9 5bb8 e8b4 bf | %.I.[.... 2049 | Decoded: 2050 | custom-value 2051 | -> custom-key: custom-valu\ 2052 | e 2054 Header Table (after decoding): 2056 [ 1] (s = 54) custom-key: custom-value 2057 [ 2] (s = 53) cache-control: no-cache 2058 [ 3] (s = 57) :authority: www.example.com 2059 Table size: 164 2061 Decoded header list: 2063 :method: GET 2064 :scheme: https 2065 :path: /index.html 2066 :authority: www.example.com 2067 custom-key: custom-value 2069 D.5. Response Examples without Huffman Coding 2071 This section shows several consecutive header lists, corresponding to 2072 HTTP responses, on the same connection. The HTTP/2 setting parameter 2073 SETTINGS_HEADER_TABLE_SIZE is set to the value of 256 octets, causing 2074 some evictions to occur. 2076 D.5.1. First Response 2078 Header list to encode: 2080 :status: 302 2081 cache-control: private 2082 date: Mon, 21 Oct 2013 20:13:21 GMT 2083 location: https://www.example.com 2085 Hex dump of encoded data: 2087 4803 3330 3258 0770 7269 7661 7465 611d | H.302X.privatea. 2088 4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013 2089 2032 303a 3133 3a32 3120 474d 546e 1768 | 20:13:21 GMTn.h 2090 7474 7073 3a2f 2f77 7777 2e65 7861 6d70 | ttps://www.examp 2091 6c65 2e63 6f6d | le.com 2093 Decoding process: 2095 48 | == Literal indexed == 2096 | Indexed name (idx = 8) 2097 | :status 2098 03 | Literal value (len = 3) 2099 3330 32 | 302 2100 | -> :status: 302 2101 58 | == Literal indexed == 2102 | Indexed name (idx = 24) 2103 | cache-control 2104 07 | Literal value (len = 7) 2105 7072 6976 6174 65 | private 2106 | -> cache-control: private 2107 61 | == Literal indexed == 2108 | Indexed name (idx = 33) 2109 | date 2110 1d | Literal value (len = 29) 2111 4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013 2112 2032 303a 3133 3a32 3120 474d 54 | 20:13:21 GMT 2113 | -> date: Mon, 21 Oct 2013 \ 2114 | 20:13:21 GMT 2115 6e | == Literal indexed == 2116 | Indexed name (idx = 46) 2117 | location 2118 17 | Literal value (len = 23) 2119 6874 7470 733a 2f2f 7777 772e 6578 616d | https://www.exam 2120 706c 652e 636f 6d | ple.com 2121 | -> location: https://www.e\ 2122 | xample.com 2124 Header Table (after decoding): 2126 [ 1] (s = 63) location: https://www.example.com 2127 [ 2] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT 2128 [ 3] (s = 52) cache-control: private 2129 [ 4] (s = 42) :status: 302 2130 Table size: 222 2132 Decoded header list: 2134 :status: 302 2135 cache-control: private 2136 date: Mon, 21 Oct 2013 20:13:21 GMT 2137 location: https://www.example.com 2139 D.5.2. Second Response 2141 The (":status", "302") header field is evicted from the header table 2142 to free space to allow adding the (":status", "307") header field. 2144 Header list to encode: 2146 :status: 307 2147 cache-control: private 2148 date: Mon, 21 Oct 2013 20:13:21 GMT 2149 location: https://www.example.com 2151 Hex dump of encoded data: 2153 4803 3330 37c1 c0bf | H.307... 2155 Decoding process: 2157 48 | == Literal indexed == 2158 | Indexed name (idx = 8) 2159 | :status 2160 03 | Literal value (len = 3) 2161 3330 37 | 307 2162 | - evict: :status: 302 2163 | -> :status: 307 2164 c1 | == Indexed - Add == 2165 | idx = 65 2166 | -> cache-control: private 2167 c0 | == Indexed - Add == 2168 | idx = 64 2169 | -> date: Mon, 21 Oct 2013 \ 2170 | 20:13:21 GMT 2171 bf | == Indexed - Add == 2172 | idx = 63 2173 | -> location: https://www.e\ 2174 | xample.com 2176 Header Table (after decoding): 2178 [ 1] (s = 42) :status: 307 2179 [ 2] (s = 63) location: https://www.example.com 2180 [ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT 2181 [ 4] (s = 52) cache-control: private 2182 Table size: 222 2184 Decoded header list: 2186 :status: 307 2187 cache-control: private 2188 date: Mon, 21 Oct 2013 20:13:21 GMT 2189 location: https://www.example.com 2191 D.5.3. Third Response 2193 Several header fields are evicted from the header table during the 2194 processing of this header list. 2196 Header list to encode: 2198 :status: 200 2199 cache-control: private 2200 date: Mon, 21 Oct 2013 20:13:22 GMT 2201 location: https://www.example.com 2202 content-encoding: gzip 2203 set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1 2205 Hex dump of encoded data: 2207 88c1 611d 4d6f 6e2c 2032 3120 4f63 7420 | ..a.Mon, 21 Oct 2208 3230 3133 2032 303a 3133 3a32 3220 474d | 2013 20:13:22 GM 2209 54c0 5a04 677a 6970 7738 666f 6f3d 4153 | T.Z.gzipw8foo=AS 2210 444a 4b48 514b 425a 584f 5157 454f 5049 | DJKHQKBZXOQWEOPI 2211 5541 5851 5745 4f49 553b 206d 6178 2d61 | UAXQWEOIU; max-a 2212 6765 3d33 3630 303b 2076 6572 7369 6f6e | ge=3600; version 2213 3d31 | =1 2215 Decoding process: 2217 88 | == Indexed - Add == 2218 | idx = 8 2219 | -> :status: 200 2220 c1 | == Indexed - Add == 2221 | idx = 65 2222 | -> cache-control: private 2223 61 | == Literal indexed == 2224 | Indexed name (idx = 33) 2225 | date 2226 1d | Literal value (len = 29) 2227 4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013 2228 2032 303a 3133 3a32 3220 474d 54 | 20:13:22 GMT 2229 | - evict: cache-control: pr\ 2230 | ivate 2231 | -> date: Mon, 21 Oct 2013 \ 2232 | 20:13:22 GMT 2233 c0 | == Indexed - Add == 2234 | idx = 64 2235 | -> location: https://www.e\ 2236 | xample.com 2237 5a | == Literal indexed == 2238 | Indexed name (idx = 26) 2239 | content-encoding 2240 04 | Literal value (len = 4) 2241 677a 6970 | gzip 2242 | - evict: date: Mon, 21 Oct\ 2243 | 2013 20:13:21 GMT 2244 | -> content-encoding: gzip 2245 77 | == Literal indexed == 2246 | Indexed name (idx = 55) 2247 | set-cookie 2248 38 | Literal value (len = 56) 2249 666f 6f3d 4153 444a 4b48 514b 425a 584f | foo=ASDJKHQKBZXO 2250 5157 454f 5049 5541 5851 5745 4f49 553b | QWEOPIUAXQWEOIU; 2251 206d 6178 2d61 6765 3d33 3630 303b 2076 | max-age=3600; v 2252 6572 7369 6f6e 3d31 | ersion=1 2253 | - evict: location: https:/\ 2254 | /www.example.com 2255 | - evict: :status: 307 2256 | -> set-cookie: foo=ASDJKHQ\ 2257 | KBZXOQWEOPIUAXQWEOIU; ma\ 2258 | x-age=3600; version=1 2260 Header Table (after decoding): 2262 [ 1] (s = 98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age\ 2263 =3600; version=1 2264 [ 2] (s = 52) content-encoding: gzip 2265 [ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:22 GMT 2266 Table size: 215 2268 Decoded header list: 2270 :status: 200 2271 cache-control: private 2272 date: Mon, 21 Oct 2013 20:13:22 GMT 2273 location: https://www.example.com 2274 content-encoding: gzip 2275 set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1 2277 D.6. Response Examples with Huffman Coding 2279 This section shows the same examples as the previous section, but 2280 using Huffman encoding for the literal values. The HTTP/2 setting 2281 parameter SETTINGS_HEADER_TABLE_SIZE is set to the value of 256 2282 octets, causing some evictions to occur. The eviction mechanism uses 2283 the length of the decoded literal values, so the same evictions 2284 occurs as in the previous section. 2286 D.6.1. First Response 2288 Header list to encode: 2290 :status: 302 2291 cache-control: private 2292 date: Mon, 21 Oct 2013 20:13:21 GMT 2293 location: https://www.example.com 2295 Hex dump of encoded data: 2297 4882 6402 5885 aec3 771a 4b61 96d0 7abe | H.d.X...w.Ka..z. 2298 9410 54d4 44a8 2005 9504 0b81 66e0 82a6 | ..T.D. .....f... 2299 2d1b ff6e 919d 29ad 1718 63c7 8f0b 97c8 | -..n..)...c..... 2300 e9ae 82ae 43d3 | ....C. 2302 Decoding process: 2304 48 | == Literal indexed == 2305 | Indexed name (idx = 8) 2306 | :status 2307 82 | Literal value (len = 2) 2308 | Huffman encoded: 2309 6402 | d. 2310 | Decoded: 2311 | 302 2312 | -> :status: 302 2313 58 | == Literal indexed == 2314 | Indexed name (idx = 24) 2315 | cache-control 2316 85 | Literal value (len = 5) 2317 | Huffman encoded: 2318 aec3 771a 4b | ..w.K 2319 | Decoded: 2320 | private 2321 | -> cache-control: private 2322 61 | == Literal indexed == 2323 | Indexed name (idx = 33) 2324 | date 2325 96 | Literal value (len = 22) 2326 | Huffman encoded: 2327 d07a be94 1054 d444 a820 0595 040b 8166 | .z...T.D. .....f 2328 e082 a62d 1bff | ...-.. 2329 | Decoded: 2330 | Mon, 21 Oct 2013 20:13:21 \ 2331 | GMT 2332 | -> date: Mon, 21 Oct 2013 \ 2333 | 20:13:21 GMT 2334 6e | == Literal indexed == 2335 | Indexed name (idx = 46) 2336 | location 2337 91 | Literal value (len = 17) 2338 | Huffman encoded: 2339 9d29 ad17 1863 c78f 0b97 c8e9 ae82 ae43 | .)...c.........C 2340 d3 | . 2341 | Decoded: 2342 | https://www.example.com 2343 | -> location: https://www.e\ 2344 | xample.com 2346 Header Table (after decoding): 2348 [ 1] (s = 63) location: https://www.example.com 2349 [ 2] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT 2350 [ 3] (s = 52) cache-control: private 2351 [ 4] (s = 42) :status: 302 2352 Table size: 222 2354 Decoded header list: 2356 :status: 302 2357 cache-control: private 2358 date: Mon, 21 Oct 2013 20:13:21 GMT 2359 location: https://www.example.com 2361 D.6.2. Second Response 2363 The (":status", "302") header field is evicted from the header table 2364 to free space to allow adding the (":status", "307") header field. 2366 Header list to encode: 2368 :status: 307 2369 cache-control: private 2370 date: Mon, 21 Oct 2013 20:13:21 GMT 2371 location: https://www.example.com 2373 Hex dump of encoded data: 2375 4883 640e ffc1 c0bf | H.d..... 2377 Decoding process: 2379 48 | == Literal indexed == 2380 | Indexed name (idx = 8) 2381 | :status 2382 83 | Literal value (len = 3) 2383 | Huffman encoded: 2384 640e ff | d.. 2385 | Decoded: 2386 | 307 2387 | - evict: :status: 302 2388 | -> :status: 307 2389 c1 | == Indexed - Add == 2390 | idx = 65 2391 | -> cache-control: private 2392 c0 | == Indexed - Add == 2393 | idx = 64 2394 | -> date: Mon, 21 Oct 2013 \ 2395 | 20:13:21 GMT 2396 bf | == Indexed - Add == 2397 | idx = 63 2398 | -> location: https://www.e\ 2399 | xample.com 2401 Header Table (after decoding): 2403 [ 1] (s = 42) :status: 307 2404 [ 2] (s = 63) location: https://www.example.com 2405 [ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT 2406 [ 4] (s = 52) cache-control: private 2407 Table size: 222 2409 Decoded header list: 2411 :status: 307 2412 cache-control: private 2413 date: Mon, 21 Oct 2013 20:13:21 GMT 2414 location: https://www.example.com 2416 D.6.3. Third Response 2418 Several header fields are evicted from the header table during the 2419 processing of this header list. 2421 Header list to encode: 2423 :status: 200 2424 cache-control: private 2425 date: Mon, 21 Oct 2013 20:13:22 GMT 2426 location: https://www.example.com 2427 content-encoding: gzip 2428 set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1 2430 Hex dump of encoded data: 2432 88c1 6196 d07a be94 1054 d444 a820 0595 | ..a..z...T.D. .. 2433 040b 8166 e084 a62d 1bff c05a 839b d9ab | ...f...-...Z.... 2434 77ad 94e7 821d d7f2 e6c7 b335 dfdf cd5b | w..........5...[ 2435 3960 d5af 2708 7f36 72c1 ab27 0fb5 291f | 9`..'..6r..'..). 2436 9587 3160 65c0 03ed 4ee5 b106 3d50 07 | ..1`e...N...=P. 2438 Decoding process: 2440 88 | == Indexed - Add == 2441 | idx = 8 2442 | -> :status: 200 2443 c1 | == Indexed - Add == 2444 | idx = 65 2445 | -> cache-control: private 2446 61 | == Literal indexed == 2447 | Indexed name (idx = 33) 2448 | date 2449 96 | Literal value (len = 22) 2450 | Huffman encoded: 2451 d07a be94 1054 d444 a820 0595 040b 8166 | .z...T.D. .....f 2452 e084 a62d 1bff | ...-.. 2453 | Decoded: 2454 | Mon, 21 Oct 2013 20:13:22 \ 2455 | GMT 2456 | - evict: cache-control: pr\ 2457 | ivate 2458 | -> date: Mon, 21 Oct 2013 \ 2459 | 20:13:22 GMT 2460 c0 | == Indexed - Add == 2461 | idx = 64 2462 | -> location: https://www.e\ 2463 | xample.com 2464 5a | == Literal indexed == 2465 | Indexed name (idx = 26) 2466 | content-encoding 2467 83 | Literal value (len = 3) 2468 | Huffman encoded: 2470 9bd9 ab | ... 2471 | Decoded: 2472 | gzip 2473 | - evict: date: Mon, 21 Oct\ 2474 | 2013 20:13:21 GMT 2475 | -> content-encoding: gzip 2476 77 | == Literal indexed == 2477 | Indexed name (idx = 55) 2478 | set-cookie 2479 ad | Literal value (len = 45) 2480 | Huffman encoded: 2481 94e7 821d d7f2 e6c7 b335 dfdf cd5b 3960 | .........5...[9` 2482 d5af 2708 7f36 72c1 ab27 0fb5 291f 9587 | ..'..6r..'..)... 2483 3160 65c0 03ed 4ee5 b106 3d50 07 | 1`e...N...=P. 2484 | Decoded: 2485 | foo=ASDJKHQKBZXOQWEOPIUAXQ\ 2486 | WEOIU; max-age=3600; versi\ 2487 | on=1 2488 | - evict: location: https:/\ 2489 | /www.example.com 2490 | - evict: :status: 307 2491 | -> set-cookie: foo=ASDJKHQ\ 2492 | KBZXOQWEOPIUAXQWEOIU; ma\ 2493 | x-age=3600; version=1 2495 Header Table (after decoding): 2497 [ 1] (s = 98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age\ 2498 =3600; version=1 2499 [ 2] (s = 52) content-encoding: gzip 2500 [ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:22 GMT 2501 Table size: 215 2503 Decoded header list: 2505 :status: 200 2506 cache-control: private 2507 date: Mon, 21 Oct 2013 20:13:22 GMT 2508 location: https://www.example.com 2509 content-encoding: gzip 2510 set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1 2512 Authors' Addresses 2513 Roberto Peon 2514 Google, Inc 2516 EMail: fenix@google.com 2518 Herve Ruellan 2519 Canon CRF 2521 EMail: herve.ruellan@crf.canon.fr