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--------------------------------------------------------------------------------
2 HTTPbis Working Group R. Peon
3 Internet-Draft Google, Inc
4 Intended status: Standards Track H. Ruellan
5 Expires: October 05, 2014 Canon CRF
6 April 03, 2014
8 HPACK - Header Compression for HTTP/2
9 draft-ietf-httpbis-header-compression-07
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 October 05, 2014.
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 . . . . . . . . . . . . . . . . . . . . . . . . 3
62 2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
63 2.1. Outline . . . . . . . . . . . . . . . . . . . . . . . . . 4
64 3. Header Field Encoding . . . . . . . . . . . . . . . . . . . . 5
65 3.1. Encoding Concepts . . . . . . . . . . . . . . . . . . . . 5
66 3.1.1. Encoding Context . . . . . . . . . . . . . . . . . . 6
67 3.1.2. Header Table . . . . . . . . . . . . . . . . . . . . 6
68 3.1.3. Reference Set . . . . . . . . . . . . . . . . . . . . 6
69 3.1.4. Header Field Representation . . . . . . . . . . . . . 7
70 3.1.5. Header Field Emission . . . . . . . . . . . . . . . . 8
71 3.2. Header Block Decoding . . . . . . . . . . . . . . . . . . 8
72 3.2.1. Header Field Representation Processing . . . . . . . 8
73 3.2.2. Reference Set Emission . . . . . . . . . . . . . . . 10
74 3.2.3. Header Set Completion . . . . . . . . . . . . . . . . 10
75 3.3. Header Table Management . . . . . . . . . . . . . . . . . 10
76 3.3.1. Maximum Table Size . . . . . . . . . . . . . . . . . 10
77 3.3.2. Entry Eviction When Header Table Size Changes . . . . 10
78 3.3.3. Entry Eviction when Adding New Entries . . . . . . . 11
79 4. Detailed Format . . . . . . . . . . . . . . . . . . . . . . . 11
80 4.1. Low-level representations . . . . . . . . . . . . . . . . 11
81 4.1.1. Integer representation . . . . . . . . . . . . . . . 11
82 4.1.2. String Literal Representation . . . . . . . . . . . . 12
83 4.2. Indexed Header Field Representation . . . . . . . . . . . 13
84 4.3. Literal Header Field Representation . . . . . . . . . . . 14
85 4.3.1. Literal Header Field with Incremental Indexing . . . 14
86 4.3.2. Literal Header Field without Indexing . . . . . . . . 15
87 4.3.3. Literal Header Field never Indexed . . . . . . . . . 16
88 4.4. Encoding Context Update . . . . . . . . . . . . . . . . . 17
89 5. Security Considerations . . . . . . . . . . . . . . . . . . . 18
90 5.1. Compression-based Attacks . . . . . . . . . . . . . . . . 18
91 5.2. Memory Consumption . . . . . . . . . . . . . . . . . . . 19
92 5.3. Implementation Limits . . . . . . . . . . . . . . . . . . 19
93 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
94 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
95 7.1. Normative References . . . . . . . . . . . . . . . . . . 20
96 7.2. Informative References . . . . . . . . . . . . . . . . . 20
97 Appendix A. Change Log (to be removed by RFC Editor before
98 publication . . . . . . . . . . . . . . . . . . . . 21
99 A.1. Since draft-ietf-httpbis-header-compression-06 . . . . . 21
100 A.2. Since draft-ietf-httpbis-header-compression-05 . . . . . 21
101 A.3. Since draft-ietf-httpbis-header-compression-04 . . . . . 21
102 A.4. Since draft-ietf-httpbis-header-compression-03 . . . . . 21
103 A.5. Since draft-ietf-httpbis-header-compression-02 . . . . . 22
104 A.6. Since draft-ietf-httpbis-header-compression-01 . . . . . 22
105 A.7. Since draft-ietf-httpbis-header-compression-00 . . . . . 22
106 Appendix B. Static Table . . . . . . . . . . . . . . . . . . . . 23
107 Appendix C. Huffman Codes . . . . . . . . . . . . . . . . . . . 25
108 Appendix D. Examples . . . . . . . . . . . . . . . . . . . . . . 31
109 D.1. Integer Representation Examples . . . . . . . . . . . . . 31
110 D.1.1. Example 1: Encoding 10 using a 5-bit prefix . . . . . 31
111 D.1.2. Example 2: Encoding 1337 using a 5-bit prefix . . . . 31
112 D.1.3. Example 3: Encoding 42 starting at an
113 octet-boundary . . . . . . . . . . . . . . . . . . . 32
114 D.2. Header Field Representation Examples . . . . . . . . . . 32
115 D.2.1. Literal Header Field with Indexing . . . . . . . . . 32
116 D.2.2. Literal Header Field without Indexing . . . . . . . . 33
117 D.2.3. Indexed Header Field . . . . . . . . . . . . . . . . 34
118 D.2.4. Indexed Header Field from Static Table . . . . . . . 35
119 D.3. Request Examples without Huffman . . . . . . . . . . . . 35
120 D.3.1. First request . . . . . . . . . . . . . . . . . . . . 35
121 D.3.2. Second request . . . . . . . . . . . . . . . . . . . 37
122 D.3.3. Third request . . . . . . . . . . . . . . . . . . . . 38
123 D.4. Request Examples with Huffman . . . . . . . . . . . . . . 40
124 D.4.1. First request . . . . . . . . . . . . . . . . . . . . 40
125 D.4.2. Second request . . . . . . . . . . . . . . . . . . . 41
126 D.4.3. Third request . . . . . . . . . . . . . . . . . . . . 42
127 D.5. Response Examples without Huffman . . . . . . . . . . . . 44
128 D.5.1. First response . . . . . . . . . . . . . . . . . . . 44
129 D.5.2. Second response . . . . . . . . . . . . . . . . . . . 46
130 D.5.3. Third response . . . . . . . . . . . . . . . . . . . 47
131 D.6. Response Examples with Huffman . . . . . . . . . . . . . 49
132 D.6.1. First response . . . . . . . . . . . . . . . . . . . 49
133 D.6.2. Second response . . . . . . . . . . . . . . . . . . . 52
134 D.6.3. Third response . . . . . . . . . . . . . . . . . . . 53
136 1. Introduction
137 This specification defines HPACK, a compression format for
138 efficiently representing HTTP header fields in the context of HTTP/2
139 (see [HTTP2]).
141 2. Overview
143 In HTTP/1.1 (see [HTTP-p1]), header fields are encoded without any
144 form of compression. As web pages have grown to include dozens to
145 hundreds of requests, the redundant header fields in these requests
146 now measurably increase latency and unnecessarily consume bandwidth
147 (see [PERF1] and [PERF2]).
149 SPDY [SPDY] initially addressed this redundancy by compressing header
150 fields using the DEFLATE format [DEFLATE], which proved very
151 effective at efficiently representing the redundant header fields.
152 However, that approach exposed a security risk as demonstrated by the
153 CRIME attack (see [CRIME]).
155 This document describes HPACK, a new compressor for header fields
156 which eliminates redundant header fields, is not vulnerable to known
157 security attacks, and which also has a bounded memory requirement for
158 use in constrained environments.
160 2.1. Outline
162 The HTTP header field encoding defined in this document is based on a
163 header table that maps name-value pairs to index values. The header
164 table is incrementally updated during the HTTP/2 connection.
166 A set of header fields is treated as an unordered collection of name-
167 value pairs. Names and values are considered to be opaque sequences
168 of octets. The order of header fields is not guaranteed to be
169 preserved after being compressed and decompressed.
171 As two consecutive sets of header fields often have header fields in
172 common, each set is coded as a difference from the previous set. The
173 goal is to only encode the changes (header fields present in one of
174 the sets that are absent from the other) between the two sets of
175 header fields.
177 A header field is represented either literally or as a reference to a
178 name-value pair in the header table. A set of header fields is
179 stored as a set of references to entries in the header table
180 (possibly keeping only a subset of it, as some header fields may be
181 missing a corresponding entry in the header table). Differences
182 between consecutive sets of header fields are encoded as changes to
183 the set of references.
185 The encoder is responsible for deciding which header fields to insert
186 as new entries in the header table. The decoder executes the
187 modifications to the header table and reference set prescribed by the
188 encoder, reconstructing the set of header fields in the process.
189 This enables decoders to remain simple and understand a wide variety
190 of encoders.
192 Examples illustrating the use of these different mechanisms to
193 represent header fields are available in Appendix D.
195 3. Header Field Encoding
197 3.1. Encoding Concepts
199 The encoding and decoding of header fields relies on some components
200 and concepts:
202 Header Field: A name-value pair. Both the name and value are
203 treated as opaque sequences of octets.
205 Header Table: The header table (see Section 3.1.2) is a component
206 used to associate stored header fields to index values.
208 Static Table: The static table (see Appendix B) is a component used
209 to associate static header fields to index values. This data is
210 ordered, read-only, always accessible, and may be shared amongst
211 all encoding contexts.
213 Reference Set: The reference set (see Section 3.1.3) is a component
214 containing an unordered set of references to entries in the header
215 table. This is used for the differential encoding of a new header
216 set.
218 Header Set: A header set is an unordered group of header fields that
219 are encoded jointly. A complete set of key-value pairs contained
220 in a HTTP request or response is a header set.
222 Header Field Representation: A header field can be represented in
223 encoded form either as a literal or as an index (see
224 Section 3.1.4).
226 Header Block: The entire set of encoded header field representations
227 which, when decoded, yield a complete header set.
229 Header Field Emission: When decoding a set of header field
230 representations, some operations emit a header field (see
231 Section 3.1.5). Emitted header fields are added to the current
232 header set and cannot be removed.
234 3.1.1. Encoding Context
236 The set of mutable structures used within an encoding context include
237 a header table and a reference set. Everything else is either
238 immutable or conceptual.
240 HTTP messages are exchanged between a client and a server in both
241 directions. The encoding of header fields in each direction is
242 independent from the other direction. There is a single encoding
243 context for each direction used to encode all header fields sent in
244 that direction.
246 3.1.2. Header Table
248 A header table consists of a list of header fields maintained in
249 first-in, first-out order. The first and newest entry in a header
250 table is always at index 1, and the oldest entry of a header table is
251 at the index len(header table).
253 The header table is initially empty.
255 There is typically no need for the header table to contain duplicate
256 entries. However, duplicate entries MUST NOT be treated as an error
257 by a decoder.
259 The encoder decides how to update the header table and as such can
260 control how much memory is used by the header table. To limit the
261 memory requirements of the decoder, the header table size is strictly
262 bounded (see Section 3.3.1).
264 The header table is updated during the processing of a set of header
265 field representations (see Section 3.2.1).
267 3.1.3. Reference Set
269 A reference set is an unordered set of references to entries of the
270 header table.
272 The reference set is initially empty.
274 The reference set is updated during the processing of a set of header
275 field representations (see Section 3.2.1).
277 The reference set enables differential encoding, whereby only
278 differences between the previous header set and the current header
279 set need to be encoded. The use of differential encoding is optional
280 for any header set.
282 When an entry is evicted from the header table, if it was referenced
283 from the reference set, its reference is removed from the reference
284 set.
286 To limit the memory requirements on the decoder side for handling the
287 reference set, only entries within the header table can be contained
288 in the reference set. To still allow entries from the static table
289 to take advantage of the differential encoding, when a header field
290 is represented as a reference to an entry of the static table, this
291 entry is inserted into the header table (see Section 3.2.1).
293 3.1.4. Header Field Representation
295 An encoded header field can be represented either as a literal or as
296 an index.
298 Literal Representation: A literal representation defines a new
299 header field. The header field name is represented either
300 literally or as a reference to an entry of the header table. The
301 header field value is represented literally.
303 Three different literal representations are provided:
305 * A literal representation that does not add the header field to
306 the header table (see Section 4.3.2).
308 * A literal representation that does not add the header field to
309 the header table and require that this header field always use
310 a literal representation, in particular when re-encoded by an
311 intermediary (see Section 4.3.3).
313 * A literal representation that adds the header field as a new
314 entry at the beginning of the header table (see Section 4.3.1).
316 Indexed Representation: The indexed representation defines a header
317 field as a reference to an entry in either the header table or the
318 static table (see Section 4.2).
320 Indices between 1 and len(header table), inclusive, refer to
321 elements in the header table, with index 1 referring to the
322 beginning of the table.
324 Indices between len(header table) + 1 and len(header table) +
325 len(static table), inclusive, refer to elements in the static
326 table, where the index len(header table) + 1 refers to the first
327 entry in the static table.
329 Any other indices MUST be treated as a decoding error.
331 <---------- Index Address Space ---------->
332 <-- Header Table --> <-- Static Table -->
333 +---+-----------+---+ +---+-----------+---+
334 | 1 | ... | k | |k+1| ... | n |
335 +---+-----------+---+ +---+-----------+---+
336 ^ |
337 | V
338 Insertion Point Drop Point
340 Index Address Space
342 3.1.5. Header Field Emission
344 The emission of a header field is the process of marking a header
345 field as belonging to the current header set. Once a header has been
346 emitted, it cannot be removed from the current header set.
348 On the decoding side, an emitted header field can be safely passed to
349 the upper processing layer as part of the current header set. The
350 decoder MAY pass the emitted header fields to the upper processing
351 layer in any order.
353 By emitting header fields instead of emitting header sets, the
354 decoder can be implemented in a streaming way, and as such has only
355 to keep in memory the header table and the reference set. This
356 bounds the amount of memory used by the decoder, even in presence of
357 a very large set of header fields. The management of memory for
358 handling very large sets of header fields can therefore be deferred
359 to the upper processing layers.
361 3.2. Header Block Decoding
363 The processing of a header block to obtain a header set is defined in
364 this section. To ensure that the decoding will successfully produce
365 a header set, a decoder MUST obey the following rules.
367 3.2.1. Header Field Representation Processing
369 All the header field representations contained in a header block are
370 processed in the order in which they are presented, as specified
371 below.
373 An _indexed representation_ with an index value of 0 entails one of
374 the following actions, depending on what is encoded next:
376 o The reference set is emptied.
378 o The maximum size of the header table is updated.
380 An _indexed representation_ corresponding to an entry _present_ in
381 the reference set entails the following actions:
383 o The entry is removed from the reference set.
385 An _indexed representation_ corresponding to an entry _not present_
386 in the reference set entails the following actions:
388 o If referencing an element of the static table:
390 * The header field corresponding to the referenced entry is
391 emitted.
393 * The referenced static entry is inserted at the beginning of the
394 header table.
396 * A reference to this new header table entry is added to the
397 reference set, except if this new entry didn't fit in the
398 header table.
400 o If referencing an element of the header table:
402 * The header field corresponding to the referenced entry is
403 emitted.
405 * The referenced header table entry is added to the reference
406 set.
408 A _literal representation_ that is _not added_ to the header table
409 entails the following action:
411 o The header field is emitted.
413 A _literal representation_ that is _added_ to the header table
414 entails the following actions:
416 o The header field is emitted.
418 o The header field is inserted at the beginning of the header table.
420 o A reference to the new entry is added to the reference set (except
421 if this new entry didn't fit in the header table).
423 3.2.2. Reference Set Emission
425 Once all the representations contained in a header block have been
426 processed, the header fields referenced in the reference set which
427 have not previously been emitted during this processing are emitted.
429 3.2.3. Header Set Completion
431 Once all of the header field representations have been processed, and
432 the remaining items in the reference set have been emitted, the
433 header set is complete.
435 3.3. Header Table Management
437 3.3.1. Maximum Table Size
439 To limit the memory requirements on the decoder side, the size of the
440 header table is bounded. The size of the header table MUST stay
441 lower than or equal to its maximum size.
443 By default, the maximum size of the header table is equal to the
444 value of the HTTP/2 setting SETTINGS_HEADER_TABLE_SIZE defined by the
445 decoder (see [HTTP2]). The encoder can change this maximum size (see
446 Section 4.4), but it must stay lower than or equal to the value of
447 SETTINGS_HEADER_TABLE_SIZE.
449 The size of the header table is the sum of the size of its entries.
451 The size of an entry is the sum of its name's length in octets (as
452 defined in Section 4.1.2), of its value's length in octets
453 (Section 4.1.2) and of 32 octets.
455 The lengths are measured on the non-encoded entry name and entry
456 value (for the case when a Huffman encoding is used to transmit
457 string values).
459 The 32 octets are an accounting for the entry structure overhead.
460 For example, an entry structure using two 64-bits pointers to
461 reference the name and the value and the entry, and two 64-bits
462 integer for counting the number of references to these name and value
463 would use 32 octets.
465 3.3.2. Entry Eviction When Header Table Size Changes
467 Whenever an entry is evicted from the header table, any reference to
468 that entry contained by the reference set is removed.
470 Whenever the maximum size for the header table is made smaller,
471 entries are evicted from the end of the header table until the size
472 of the header table is less than or equal to the maximum size.
474 The eviction of an entry from the header table causes the index of
475 the entries in the static table to be reduced by one.
477 3.3.3. Entry Eviction when Adding New Entries
479 Whenever a new entry is to be added to the table, any name referenced
480 by the representation of this new entry is cached, and then entries
481 are evicted from the end of the header table until the size of the
482 header table is less than or equal to (maximum size - new entry
483 size), or until the table is empty.
485 If the size of the new entry is less than or equal to the maximum
486 size, that entry is added to the table. It is not an error to
487 attempt to add an entry that is larger than the maximum size.
489 4. Detailed Format
491 4.1. Low-level representations
493 4.1.1. Integer representation
495 Integers are used to represent name indexes, pair indexes or string
496 lengths. To allow for optimized processing, an integer
497 representation always finishes at the end of an octet.
499 An integer is represented in two parts: a prefix that fills the
500 current octet and an optional list of octets that are used if the
501 integer value does not fit within the prefix. The number of bits of
502 the prefix (called N) is a parameter of the integer representation.
504 The N-bit prefix allows filling the current octet. If the value is
505 small enough (strictly less than 2^N-1), it is encoded within the
506 N-bit prefix. Otherwise all the bits of the prefix are set to 1 and
507 the value is encoded using an unsigned variable length integer
508 representation (see ). N is always between 1 and 8 bits. An integer
510 starting at an octet-boundary will have an 8-bit prefix.
512 The algorithm to represent an integer I is as follows:
514 if I < 2^N - 1, encode I on N bits
515 else
516 encode (2^N - 1) on N bits
517 I = I - (2^N - 1)
518 while I >= 128
519 encode (I % 128 + 128) on 8 bits
520 I = I / 128
521 encode I on 8 bits
523 For informational purpose, the algorithm to decode an integer I is as
524 follows:
526 decode I from the next N bits
527 if I < 2^N - 1, return I
528 else
529 M = 0
530 repeat
531 B = next octet
532 I = I + (B & 127) * 2^M
533 M = M + 7
534 while B & 128 == 128
535 return I
537 Examples illustrating the encoding of integers are available in
538 Appendix D.1.
540 This integer representation allows for values of indefinite size. It
541 is also possible for an encoder to send a large number of zero
542 values, which can waste octets and could be used to overflow integer
543 values. Excessively large integer encodings - in value or octet
544 length - MUST be treated as a decoding error. Different limits can
545 be set for each of the different uses of integers, based on
546 implementation constraints.
548 4.1.2. String Literal Representation
550 Header field names and header field values can be represented as
551 literal string. A literal string is encoded as a sequence of octets,
552 either by directly encoding the literal string's octets, or by using
553 a canonical [CANON] Huffman encoding [HUFF].
555 0 1 2 3 4 5 6 7
556 +---+---+---+---+---+---+---+---+
557 | H | String Length (7+) |
558 +---+---------------------------+
559 | String Data (Length octets) |
560 +-------------------------------+
562 String Literal Representation
564 A literal string representation contains the following fields:
566 H: A one bit flag, H, indicating whether or not the octets of the
567 string are Huffman encoded.
569 String Length: The number of octets used to encode the string
570 literal, encoded as an integer with 7-bit prefix (see
571 Section 4.1.1).
573 String Data: The encoded data of the string literal. If H is '0',
574 then the encoded data is the raw octets of the string literal. If
575 H is '1', then the encoded data is the Huffman encoding of the
576 string literal.
578 String literals which use Huffman encoding are encoded with the
579 Huffman codes defined in Appendix C (see examples inRequest Examples
580 with Huffman Appendix D.4 and in Response Examples with Huffman
581 Appendix D.6). The encoded data is the bitwise concatenation of the
582 Huffman codes corresponding to each octet of the string literal.
584 As the Huffman encoded data doesn't always end at an octet boundary,
585 some padding is inserted after it up to the next octet boundary. To
586 prevent this padding to be misinterpreted as part of the string
587 literal, the most significant bits of the EOS (end-of-string) entry
588 in the Huffman table are used.
590 Upon decoding, an incomplete Huffman code at the end of the encoded
591 data is to be considered as padding and discarded. A padding
592 strictly longer than 7 bits MUST be treated as a decoding error. A
593 padding not corresponding to the most significant bits of the EOS
594 entry MUST be treated as a decoding error. A Huffman encoded string
595 literal containing the EOS entry MUST be treated as a decoding error.
597 4.2. Indexed Header Field Representation
599 An indexed header field representation either identifies an entry in
600 the header table or static table. The processing of an indexed
601 header field representation is described in Section 3.2.1.
603 0 1 2 3 4 5 6 7
604 +---+---+---+---+---+---+---+---+
605 | 1 | Index (7+) |
606 +---+---------------------------+
608 Indexed Header Field
610 This representation starts with the '1' 1-bit pattern, followed by
611 the index of the matching pair, represented as an integer with a
612 7-bit prefix.
614 The index value of 0 is not used. It MUST be treated as a decoding
615 error if found in an indexed header field representation.
617 4.3. Literal Header Field Representation
619 Literal header field representations contain a literal header field
620 value. Header field names are either provided as a literal or by
621 reference to an existing header table or static table entry.
623 Literal representations all result in the emission of a header field
624 when decoded.
626 4.3.1. Literal Header Field with Incremental Indexing
628 A literal header field with incremental indexing adds a new entry to
629 the header table.
631 0 1 2 3 4 5 6 7
632 +---+---+---+---+---+---+---+---+
633 | 0 | 1 | Index (6+) |
634 +---+---+-----------------------+
635 | H | Value Length (7+) |
636 +---+---------------------------+
637 | Value String (Length octets) |
638 +-------------------------------+
640 Literal Header Field with Incremental Indexing - Indexed Name
642 0 1 2 3 4 5 6 7
643 +---+---+---+---+---+---+---+---+
644 | 0 | 1 | 0 |
645 +---+---+-----------------------+
646 | H | Name Length (7+) |
647 +---+---------------------------+
648 | Name String (Length octets) |
649 +---+---------------------------+
650 | H | Value Length (7+) |
651 +---+---------------------------+
652 | Value String (Length octets) |
653 +-------------------------------+
655 Literal Header Field with Incremental Indexing - New Name
657 This representation starts with the '01' 2-bit pattern.
659 If the header field name matches the header field name of a (name,
660 value) pair stored in the Header Table or Static Table, the header
661 field name can be represented using the index of that entry. In this
662 case, the index of the entry, index (which is strictly greater than
663 0), is represented as an integer with a 6-bit prefix (see
664 Section 4.1.1).
666 Otherwise, the header field name is represented as a literal. The
667 value 0 is represented on 6 bits followed by the header field name
668 (see Section 4.1.2).
670 The header field name representation is followed by the header field
671 value represented as a literal string as described in Section 4.1.2.
673 4.3.2. Literal Header Field without Indexing
675 A literal header field without indexing causes the emission of a
676 header field without altering the header table.
678 0 1 2 3 4 5 6 7
679 +---+---+---+---+---+---+---+---+
680 | 0 | 0 | 0 | 0 | Index (4+) |
681 +---+---+-----------------------+
682 | H | Value Length (7+) |
683 +---+---------------------------+
684 | Value String (Length octets) |
685 +-------------------------------+
687 Literal Header Field without Indexing - Indexed Name
689 0 1 2 3 4 5 6 7
690 +---+---+---+---+---+---+---+---+
691 | 0 | 0 | 0 | 0 | 0 |
692 +---+---+-----------------------+
693 | H | Name Length (7+) |
694 +---+---------------------------+
695 | Name String (Length octets) |
696 +---+---------------------------+
697 | H | Value Length (7+) |
698 +---+---------------------------+
699 | Value String (Length octets) |
700 +-------------------------------+
702 Literal Header Field without Indexing - New Name
704 The literal header field without indexing representation starts with
705 the '0000' 4-bit pattern.
707 If the header field name matches the header field name of a (name,
708 value) pair stored in the Header Table or Static Table, the header
709 field name can be represented using the index of that entry. In this
710 case, the index of the entry, index (which is strictly greater than
711 0), is represented as an integer with a 6-bit prefix (see
712 Section 4.1.1).
714 Otherwise, the header field name is represented as a literal. The
715 value 0 is represented on 4 bits followed by the header field name
716 (see Section 4.1.2).
718 The header field name representation is followed by the header field
719 value represented as a literal string as described in Section 4.1.2.
721 4.3.3. Literal Header Field never Indexed
723 A literal header field never indexed causes the emission of a header
724 field without altering the header table.
726 0 1 2 3 4 5 6 7
727 +---+---+---+---+---+---+---+---+
728 | 0 | 0 | 0 | 1 | Index (4+) |
729 +---+---+-----------------------+
730 | H | Value Length (7+) |
731 +---+---------------------------+
732 | Value String (Length octets) |
733 +-------------------------------+
735 Literal Header Field never Indexed - Indexed Name
737 0 1 2 3 4 5 6 7
738 +---+---+---+---+---+---+---+---+
739 | 0 | 0 | 0 | 1 | 0 |
740 +---+---+-----------------------+
741 | H | Name Length (7+) |
742 +---+---------------------------+
743 | Name String (Length octets) |
744 +---+---------------------------+
745 | H | Value Length (7+) |
746 +---+---------------------------+
747 | Value String (Length octets) |
748 +-------------------------------+
750 Literal Header Field never Indexed - New Name
752 The literal header field never indexed representation starts with the
753 '0001' 4-bit pattern.
755 When a header field is represented as a literal header field never
756 indexed, it MUST always be encoded with this same representation. In
757 particular, when a peer sends a header field that it received
758 represented as a literal header field never indexed, it MUST use the
759 same representation to forward this header field.
761 This representation is intended for protecting header field values
762 that are not to be put at risk by compressing them (see Section 5.1
763 for more details).
765 The encoding of the representation is the same as for the literal
766 header field without indexing representation (see Section 4.3.2).
768 4.4. Encoding Context Update
770 An encoding context update causes the immediate application of a
771 change to the encoding context.
773 0 1 2 3 4 5 6 7
774 +---+---+---+---+---+---+---+---+
775 | 0 | 0 | 1 | F | ... |
776 +---+---------------------------+
778 Context Update
780 An encoding context update starts with the '001' 3-bit pattern.
782 It is followed by a flag specifying the type of the change, and by
783 any data necessary to describe the change itself.
785 0 1 2 3 4 5 6 7
786 +---+---+---+---+---+---+---+---+
787 | 0 | 0 | 1 | 1 | 0 |
788 +---+---------------------------+
790 Reference Set Emptying
792 The flag bit being set to '1' signals that the reference set is
793 emptied. The remaining bits are set to '0'.
795 0 1 2 3 4 5 6 7
796 +---+---+---+---+---+---+---+---+
797 | 0 | 0 | 1 | 0 | Max size (4+) |
798 +---+---------------------------+
800 Maximum Header Table Size Change
802 The flag bit being set to '0' signals that a change to the maximum
803 size of the header table. This new maximum size MUST be lower than
804 or equal to the value of the setting SETTINGS_HEADER_TABLE_SIZE (see
805 [HTTP2]).
807 The new maximum size is encoded as an integer with a 4-bit prefix.
809 Change in the maximum size of the header table can trigger entry
810 evictions (see Section 3.3.2).
812 5. Security Considerations
814 5.1. Compression-based Attacks
816 Compression can create a weak point allowing an attacker to recover
817 secret data. For example, the CRIME attack (see [CRIME]) took
818 advantage of the DEFLATE mechanism (see [DEFLATE]) of SPDY (see
819 [SPDY]) to efficiently probe the compression context. The full-text
820 compression mechanism of DEFLATE allowed the attacker to learn some
821 information from each failed attempt at guessing the secret.
823 For this reason, HPACK provides only limited compression mechanisms
824 in the form of an indexing table and of a static Huffman encoding.
826 The indexing table can still provide information to an attacker that
827 would be able to probe the compression context. However, this
828 information is limited to the knowledge of whether the attacker's
829 guess is correct or not.
831 Still, an attacker could take advantage of this limited information
832 for breaking low-entropy secrets using a brute-force attack. A
833 server usually has some protections against such brute-force attack.
834 Here, the attack would target the client, where it would be harder to
835 detect. The attack would be even more dangerous if the attacker is
836 able to prevent the traffic generated by its brute-force attack from
837 reaching the server.
839 To offer some protection against such type of attacks, HPACK enables
840 an endpoint to indicate that a header field must never be compressed,
841 across any hop up to the other endpoint (see Section 4.3.3). An
842 endpoint MUST use this feature to prevent the compression of any
843 header field whose value contains a secret which could be put at risk
844 by a brute-force attack.
846 For optimal processing, a sensitive value (for example a cookie)
847 needs to have an entropy high enough to not be endangered by a brute-
848 force attack, in order to take advantage of HPACK indexing.
850 There is currently no known threat taking advantage of the use of a
851 fixed Huffman encoding. A study has shown that using a fixed Huffman
852 encoding table created an information leakage, however this same
853 study concluded that an attacker could not take advantage of this
854 information leakage to recover any meaningful amount of information
855 (see [PETAL]).
857 5.2. Memory Consumption
859 An attacker can try to cause an endpoint to exhaust its memory.
860 HPACK is designed to limit both the peak and state amounts of memory
861 allocated by an endpoint.
863 The amount of memory used by the compressor state is limited by the
864 value of the setting SETTINGS_HEADER_TABLE_SIZE. This limitation
865 takes into account both the size of the data stored in the header
866 table, and the overhead required by the table structure itself.
868 For the decoding side, an endpoint can limit the amount of state
869 memory used by setting an appropriate value for
870 SETTINGS_HEADER_TABLE_SIZE. For the encoding side, the endpoint can
871 limit the amount of state memory it uses by defining a header table
872 maximum size lower than the value of SETTINGS_HEADER_TABLE_SIZE
873 defined by its peer (see Section 4.4).
875 The amount of temporary memory consumed is linked to the set of
876 header fields emitted or received. However, this amount of temporary
877 memory can be limited by processing these header fields in a
878 streaming manner.
880 5.3. Implementation Limits
882 An implementation of HPACK needs to ensure that large values for
883 integers, long encoding for integers, or long string literal do not
884 create security weaknesses.
886 An implementation has to set a limit for the values it accepts for
887 integers, as well as for the encoded length (see Section 4.1.1). In
888 the same way, it has to set a limit to the length it accepts for
889 string literals (see Section 4.1.2).
891 6. Acknowledgements
893 This document includes substantial editorial contributions from the
894 following individuals: Mike Bishop, Jeff Pinner, Julian Reschke,
895 Martin Thomson.
897 7. References
899 7.1. Normative References
901 [HTTP-p1] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
902 Protocol (HTTP/1.1): Message Syntax and Routing", draft-
903 ietf-httpbis-p1-messaging-26 (work in progress), February
904 2014.
906 [HTTP2] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
907 Transfer Protocol version 2", draft-ietf-httpbis-http2-10
908 (work in progress), February 2014.
910 7.2. Informative References
912 [CANON] Schwartz, E. and B. Kallick, "Generating a canonical
913 prefix encoding", Communications of the ACM Volume 7 Issue
914 3, pp. 166-169, March 1964,
915 .
917 [CRIME] Rizzo, J. and T. Duong, "The CRIME Attack", September
918 2012, .
922 [DEFLATE] Deutsch, P., "DEFLATE Compressed Data Format Specification
923 version 1.3", RFC 1951, May 1996.
925 [HUFF] Huffman, D., "A Method for the Construction of Minimum
926 Redundancy Codes", Proceedings of the Institute of Radio
927 Engineers Volume 40, Number 9, pp. 1098-1101, September
928 1952, .
931 [PERF1] Belshe, M., "IETF83: SPDY and What to Consider for HTTP/
932 2.0", March 2012, .
935 [PERF2] McManus, P., "SPDY: What I Like About You", September
936 2011, .
939 [PETAL] Tan, J. and J. Nahata, "PETAL: Preset Encoding Table
940 Information Leakage", April 2013, .
943 [SPDY] Belshe, M. and R. Peon, "SPDY Protocol", draft-mbelshe-
944 httpbis-spdy-00 (work in progress), February 2012.
946 Appendix A. Change Log (to be removed by RFC Editor before publication
948 A.1. Since draft-ietf-httpbis-header-compression-06
950 o Updated format to include literal headers that must never be
951 compressed.
953 o Updated security considerations.
955 o Moved integer encoding examples to the appendix.
957 o Updated Huffman table.
959 o Updated static header table (adding and removing status values).
961 o Updated examples.
963 A.2. Since draft-ietf-httpbis-header-compression-05
965 o Regenerated examples.
967 o Only one Huffman table for requests and responses.
969 o Added maximum size for header table, independent of
970 SETTINGS_HEADER_TABLE_SIZE.
972 o Added pseudo-code for integer decoding.
974 o Improved examples (removing unnecessary removals).
976 A.3. Since draft-ietf-httpbis-header-compression-04
978 o Updated examples: take into account changes in the spec, and show
979 more features.
981 o Use 'octet' everywhere instead of having both 'byte' and 'octet'.
983 o Added reference set emptying.
985 o Editorial changes and clarifications.
987 o Added "host" header to the static table.
989 o Ordering for list of values (either NULL- or comma-separated).
991 A.4. Since draft-ietf-httpbis-header-compression-03
992 o A large number of editorial changes; changed the description of
993 evicting/adding new entries.
995 o Removed substitution indexing
997 o Changed 'initial headers' to 'static headers', as per issue #258
999 o Merged 'request' and 'response' static headers, as per issue #259
1001 o Changed text to indicate that new headers are added at index 0 and
1002 expire from the largest index, as per issue #233
1004 A.5. Since draft-ietf-httpbis-header-compression-02
1006 o Corrected error in integer encoding pseudocode.
1008 A.6. Since draft-ietf-httpbis-header-compression-01
1010 o Refactored of Header Encoding Section: split definitions and
1011 processing rule.
1013 o Backward incompatible change: Updated reference set management as
1014 per issue #214. This changes how the interaction between the
1015 reference set and eviction works. This also changes the working
1016 of the reference set in some specific cases.
1018 o Backward incompatible change: modified initial header list, as per
1019 issue #188.
1021 o Added example of 32 octets entry structure (issue #191).
1023 o Added Header Set Completion section. Reflowed some text.
1024 Clarified some writing which was akward. Added text about
1025 duplicate header entry encoding. Clarified some language w.r.t
1026 Header Set. Changed x-my-header to mynewheader. Added text in
1027 the HeaderEmission section indicating that the application may
1028 also be able to free up memory more quickly. Added information in
1029 Security Considerations section.
1031 A.7. Since draft-ietf-httpbis-header-compression-00
1033 Fixed bug/omission in integer representation algorithm.
1035 Changed the document title.
1037 Header matching text rewritten.
1039 Changed the definition of header emission.
1041 Changed the name of the setting which dictates how much memory the
1042 compression context should use.
1044 Removed "specific use cases" section
1046 Corrected erroneous statement about what index can be contained in
1047 one octet
1049 Added descriptions of opcodes
1051 Removed security claims from introduction.
1053 Appendix B. Static Table
1055 The static table consists of an unchangeable ordered list of (name,
1056 value) pairs. The first entry in the table is always represented by
1057 the index len(header table) + 1, and the last entry in the table is
1058 represented by the index len(header table) + len(static table).
1060 The following table lists the pre-defined header fields that make-up
1061 the static table.
1063 +-------+-----------------------------+--------------+
1064 | Index | Header Name | Header Value |
1065 +-------+-----------------------------+--------------+
1066 | 1 | :authority | |
1067 | 2 | :method | GET |
1068 | 3 | :method | POST |
1069 | 4 | :path | / |
1070 | 5 | :path | /index.html |
1071 | 6 | :scheme | http |
1072 | 7 | :scheme | https |
1073 | 8 | :status | 200 |
1074 | 9 | :status | 204 |
1075 | 10 | :status | 206 |
1076 | 11 | :status | 304 |
1077 | 12 | :status | 400 |
1078 | 13 | :status | 404 |
1079 | 14 | :status | 500 |
1080 | 15 | accept-charset | |
1081 | 16 | accept-encoding | |
1082 | 17 | accept-language | |
1083 | 18 | accept-ranges | |
1084 | 19 | accept | |
1085 | 20 | access-control-allow-origin | |
1086 | 21 | age | |
1087 | 22 | allow | |
1088 | 23 | authorization | |
1089 | 24 | cache-control | |
1090 | 25 | content-disposition | |
1091 | 26 | content-encoding | |
1092 | 27 | content-language | |
1093 | 28 | content-length | |
1094 | 29 | content-location | |
1095 | 30 | content-range | |
1096 | 31 | content-type | |
1097 | 32 | cookie | |
1098 | 33 | date | |
1099 | 34 | etag | |
1100 | 35 | expect | |
1101 | 36 | expires | |
1102 | 37 | from | |
1103 | 38 | host | |
1104 | 39 | if-match | |
1105 | 40 | if-modified-since | |
1106 | 41 | if-none-match | |
1107 | 42 | if-range | |
1108 | 43 | if-unmodified-since | |
1109 | 44 | last-modified | |
1110 | 45 | link | |
1111 | 46 | location | |
1112 | 47 | max-forwards | |
1113 | 48 | proxy-authenticate | |
1114 | 49 | proxy-authorization | |
1115 | 50 | range | |
1116 | 51 | referer | |
1117 | 52 | refresh | |
1118 | 53 | retry-after | |
1119 | 54 | server | |
1120 | 55 | set-cookie | |
1121 | 56 | strict-transport-security | |
1122 | 57 | transfer-encoding | |
1123 | 58 | user-agent | |
1124 | 59 | vary | |
1125 | 60 | via | |
1126 | 61 | www-authenticate | |
1127 +-------+-----------------------------+--------------+
1129 Table 1: Static Table Entries
1131 The table give the index of each entry in the static table. The full
1132 index of each entry, to be used for encoding a reference to this
1133 entry, is computed by adding the number of entries in the header
1134 table to this index.
1136 Appendix C. Huffman Codes
1138 The following codes are used when encoding string literals with an
1139 Huffman coding (see Section 4.1.2).
1141 Each row in the table specifies one Huffman code:
1143 sym: The symbol to be represented. It is the decimal value of an
1144 octet, possibly prepended with its ASCII representation. A
1145 specific symbol, "EOS", is used to indicate the end of a string
1146 literal.
1148 code as bits: The Huffman code for the symbol represented as a
1149 base-2 integer.
1151 code as hex: The Huffman code for the symbol, represented as a
1152 hexadecimal integer, aligned on the least significant bit.
1154 len: The number of bits for the Huffman code of the symbol.
1156 As an example, the Huffman code for the symbol 48 (corresponding to
1157 the ASCII character "0") consists in the 5 bits "0", "0", "1", "0",
1158 "1". This corresponds to the value 5 encoded on 5 bits.
1160 code
1161 code as bits as hex len
1162 sym aligned to MSB aligned in
1163 to LSB bits
1164 ( 0) |11111111|11111111|11101110|10 3ffffba [26]
1165 ( 1) |11111111|11111111|11101110|11 3ffffbb [26]
1166 ( 2) |11111111|11111111|11101111|00 3ffffbc [26]
1167 ( 3) |11111111|11111111|11101111|01 3ffffbd [26]
1168 ( 4) |11111111|11111111|11101111|10 3ffffbe [26]
1169 ( 5) |11111111|11111111|11101111|11 3ffffbf [26]
1170 ( 6) |11111111|11111111|11110000|00 3ffffc0 [26]
1171 ( 7) |11111111|11111111|11110000|01 3ffffc1 [26]
1172 ( 8) |11111111|11111111|11110000|10 3ffffc2 [26]
1173 ( 9) |11111111|11111111|11110000|11 3ffffc3 [26]
1174 ( 10) |11111111|11111111|11110001|00 3ffffc4 [26]
1175 ( 11) |11111111|11111111|11110001|01 3ffffc5 [26]
1176 ( 12) |11111111|11111111|11110001|10 3ffffc6 [26]
1177 ( 13) |11111111|11111111|11110001|11 3ffffc7 [26]
1178 ( 14) |11111111|11111111|11110010|00 3ffffc8 [26]
1179 ( 15) |11111111|11111111|11110010|01 3ffffc9 [26]
1180 ( 16) |11111111|11111111|11110010|10 3ffffca [26]
1181 ( 17) |11111111|11111111|11110010|11 3ffffcb [26]
1182 ( 18) |11111111|11111111|11110011|00 3ffffcc [26]
1183 ( 19) |11111111|11111111|11110011|01 3ffffcd [26]
1184 ( 20) |11111111|11111111|11110011|10 3ffffce [26]
1185 ( 21) |11111111|11111111|11110011|11 3ffffcf [26]
1186 ( 22) |11111111|11111111|11110100|00 3ffffd0 [26]
1187 ( 23) |11111111|11111111|11110100|01 3ffffd1 [26]
1188 ( 24) |11111111|11111111|11110100|10 3ffffd2 [26]
1189 ( 25) |11111111|11111111|11110100|11 3ffffd3 [26]
1190 ( 26) |11111111|11111111|11110101|00 3ffffd4 [26]
1191 ( 27) |11111111|11111111|11110101|01 3ffffd5 [26]
1192 ( 28) |11111111|11111111|11110101|10 3ffffd6 [26]
1193 ( 29) |11111111|11111111|11110101|11 3ffffd7 [26]
1194 ( 30) |11111111|11111111|11110110|00 3ffffd8 [26]
1195 ( 31) |11111111|11111111|11110110|01 3ffffd9 [26]
1196 ' ' ( 32) |00110 6 [ 5]
1197 '!' ( 33) |11111111|11100 1ffc [13]
1198 '"' ( 34) |11111000|0 1f0 [ 9]
1199 '#' ( 35) |11111111|111100 3ffc [14]
1200 '$' ( 36) |11111111|1111100 7ffc [15]
1201 '%' ( 37) |011110 1e [ 6]
1202 '&' ( 38) |1100100 64 [ 7]
1203 ''' ( 39) |11111111|11101 1ffd [13]
1204 '(' ( 40) |11111110|10 3fa [10]
1205 ')' ( 41) |11111000|1 1f1 [ 9]
1206 '*' ( 42) |11111110|11 3fb [10]
1207 '+' ( 43) |11111111|00 3fc [10]
1208 ',' ( 44) |1100101 65 [ 7]
1209 '-' ( 45) |1100110 66 [ 7]
1210 '.' ( 46) |011111 1f [ 6]
1211 '/' ( 47) |00111 7 [ 5]
1212 '0' ( 48) |0000 0 [ 4]
1213 '1' ( 49) |0001 1 [ 4]
1214 '2' ( 50) |0010 2 [ 4]
1215 '3' ( 51) |01000 8 [ 5]
1216 '4' ( 52) |100000 20 [ 6]
1217 '5' ( 53) |100001 21 [ 6]
1218 '6' ( 54) |100010 22 [ 6]
1219 '7' ( 55) |100011 23 [ 6]
1220 '8' ( 56) |100100 24 [ 6]
1221 '9' ( 57) |100101 25 [ 6]
1222 ':' ( 58) |100110 26 [ 6]
1223 ';' ( 59) |11101100| ec [ 8]
1224 '<' ( 60) |11111111|11111110|0 1fffc [17]
1225 '=' ( 61) |100111 27 [ 6]
1226 '>' ( 62) |11111111|1111101 7ffd [15]
1227 '?' ( 63) |11111111|01 3fd [10]
1228 '@' ( 64) |11111111|1111110 7ffe [15]
1229 'A' ( 65) |1100111 67 [ 7]
1230 'B' ( 66) |11101101| ed [ 8]
1231 'C' ( 67) |11101110| ee [ 8]
1232 'D' ( 68) |1101000 68 [ 7]
1233 'E' ( 69) |11101111| ef [ 8]
1234 'F' ( 70) |1101001 69 [ 7]
1235 'G' ( 71) |1101010 6a [ 7]
1236 'H' ( 72) |11111001|0 1f2 [ 9]
1237 'I' ( 73) |11110000| f0 [ 8]
1238 'J' ( 74) |11111001|1 1f3 [ 9]
1239 'K' ( 75) |11111010|0 1f4 [ 9]
1240 'L' ( 76) |11111010|1 1f5 [ 9]
1241 'M' ( 77) |1101011 6b [ 7]
1242 'N' ( 78) |1101100 6c [ 7]
1243 'O' ( 79) |11110001| f1 [ 8]
1244 'P' ( 80) |11110010| f2 [ 8]
1245 'Q' ( 81) |11111011|0 1f6 [ 9]
1246 'R' ( 82) |11111011|1 1f7 [ 9]
1247 'S' ( 83) |1101101 6d [ 7]
1248 'T' ( 84) |101000 28 [ 6]
1249 'U' ( 85) |11110011| f3 [ 8]
1250 'V' ( 86) |11111100|0 1f8 [ 9]
1251 'W' ( 87) |11111100|1 1f9 [ 9]
1252 'X' ( 88) |11110100| f4 [ 8]
1253 'Y' ( 89) |11111101|0 1fa [ 9]
1254 'Z' ( 90) |11111101|1 1fb [ 9]
1255 '[' ( 91) |11111111|100 7fc [11]
1256 '\' ( 92) |11111111|11111111|11110110|10 3ffffda [26]
1257 ']' ( 93) |11111111|101 7fd [11]
1258 '^' ( 94) |11111111|111101 3ffd [14]
1259 '_' ( 95) |1101110 6e [ 7]
1260 '`' ( 96) |11111111|11111111|10 3fffe [18]
1261 'a' ( 97) |01001 9 [ 5]
1262 'b' ( 98) |1101111 6f [ 7]
1263 'c' ( 99) |01010 a [ 5]
1264 'd' (100) |101001 29 [ 6]
1265 'e' (101) |01011 b [ 5]
1266 'f' (102) |1110000 70 [ 7]
1267 'g' (103) |101010 2a [ 6]
1268 'h' (104) |101011 2b [ 6]
1269 'i' (105) |01100 c [ 5]
1270 'j' (106) |11110101| f5 [ 8]
1271 'k' (107) |11110110| f6 [ 8]
1272 'l' (108) |101100 2c [ 6]
1273 'm' (109) |101101 2d [ 6]
1274 'n' (110) |101110 2e [ 6]
1275 'o' (111) |01101 d [ 5]
1276 'p' (112) |101111 2f [ 6]
1277 'q' (113) |11111110|0 1fc [ 9]
1278 'r' (114) |110000 30 [ 6]
1279 's' (115) |110001 31 [ 6]
1280 't' (116) |01110 e [ 5]
1281 'u' (117) |1110001 71 [ 7]
1282 'v' (118) |1110010 72 [ 7]
1283 'w' (119) |1110011 73 [ 7]
1284 'x' (120) |1110100 74 [ 7]
1285 'y' (121) |1110101 75 [ 7]
1286 'z' (122) |11110111| f7 [ 8]
1287 '{' (123) |11111111|11111110|1 1fffd [17]
1288 '|' (124) |11111111|1100 ffc [12]
1289 '}' (125) |11111111|11111111|0 1fffe [17]
1290 '~' (126) |11111111|1101 ffd [12]
1291 (127) |11111111|11111111|11110110|11 3ffffdb [26]
1292 (128) |11111111|11111111|11110111|00 3ffffdc [26]
1293 (129) |11111111|11111111|11110111|01 3ffffdd [26]
1294 (130) |11111111|11111111|11110111|10 3ffffde [26]
1295 (131) |11111111|11111111|11110111|11 3ffffdf [26]
1296 (132) |11111111|11111111|11111000|00 3ffffe0 [26]
1297 (133) |11111111|11111111|11111000|01 3ffffe1 [26]
1298 (134) |11111111|11111111|11111000|10 3ffffe2 [26]
1299 (135) |11111111|11111111|11111000|11 3ffffe3 [26]
1300 (136) |11111111|11111111|11111001|00 3ffffe4 [26]
1301 (137) |11111111|11111111|11111001|01 3ffffe5 [26]
1302 (138) |11111111|11111111|11111001|10 3ffffe6 [26]
1303 (139) |11111111|11111111|11111001|11 3ffffe7 [26]
1304 (140) |11111111|11111111|11111010|00 3ffffe8 [26]
1305 (141) |11111111|11111111|11111010|01 3ffffe9 [26]
1306 (142) |11111111|11111111|11111010|10 3ffffea [26]
1307 (143) |11111111|11111111|11111010|11 3ffffeb [26]
1308 (144) |11111111|11111111|11111011|00 3ffffec [26]
1309 (145) |11111111|11111111|11111011|01 3ffffed [26]
1310 (146) |11111111|11111111|11111011|10 3ffffee [26]
1311 (147) |11111111|11111111|11111011|11 3ffffef [26]
1312 (148) |11111111|11111111|11111100|00 3fffff0 [26]
1313 (149) |11111111|11111111|11111100|01 3fffff1 [26]
1314 (150) |11111111|11111111|11111100|10 3fffff2 [26]
1315 (151) |11111111|11111111|11111100|11 3fffff3 [26]
1316 (152) |11111111|11111111|11111101|00 3fffff4 [26]
1317 (153) |11111111|11111111|11111101|01 3fffff5 [26]
1318 (154) |11111111|11111111|11111101|10 3fffff6 [26]
1319 (155) |11111111|11111111|11111101|11 3fffff7 [26]
1320 (156) |11111111|11111111|11111110|00 3fffff8 [26]
1321 (157) |11111111|11111111|11111110|01 3fffff9 [26]
1322 (158) |11111111|11111111|11111110|10 3fffffa [26]
1323 (159) |11111111|11111111|11111110|11 3fffffb [26]
1324 (160) |11111111|11111111|11111111|00 3fffffc [26]
1325 (161) |11111111|11111111|11111111|01 3fffffd [26]
1326 (162) |11111111|11111111|11111111|10 3fffffe [26]
1327 (163) |11111111|11111111|11111111|11 3ffffff [26]
1328 (164) |11111111|11111111|11000000|0 1ffff80 [25]
1329 (165) |11111111|11111111|11000000|1 1ffff81 [25]
1330 (166) |11111111|11111111|11000001|0 1ffff82 [25]
1331 (167) |11111111|11111111|11000001|1 1ffff83 [25]
1332 (168) |11111111|11111111|11000010|0 1ffff84 [25]
1333 (169) |11111111|11111111|11000010|1 1ffff85 [25]
1334 (170) |11111111|11111111|11000011|0 1ffff86 [25]
1335 (171) |11111111|11111111|11000011|1 1ffff87 [25]
1336 (172) |11111111|11111111|11000100|0 1ffff88 [25]
1337 (173) |11111111|11111111|11000100|1 1ffff89 [25]
1338 (174) |11111111|11111111|11000101|0 1ffff8a [25]
1339 (175) |11111111|11111111|11000101|1 1ffff8b [25]
1340 (176) |11111111|11111111|11000110|0 1ffff8c [25]
1341 (177) |11111111|11111111|11000110|1 1ffff8d [25]
1342 (178) |11111111|11111111|11000111|0 1ffff8e [25]
1343 (179) |11111111|11111111|11000111|1 1ffff8f [25]
1344 (180) |11111111|11111111|11001000|0 1ffff90 [25]
1345 (181) |11111111|11111111|11001000|1 1ffff91 [25]
1346 (182) |11111111|11111111|11001001|0 1ffff92 [25]
1347 (183) |11111111|11111111|11001001|1 1ffff93 [25]
1348 (184) |11111111|11111111|11001010|0 1ffff94 [25]
1349 (185) |11111111|11111111|11001010|1 1ffff95 [25]
1350 (186) |11111111|11111111|11001011|0 1ffff96 [25]
1351 (187) |11111111|11111111|11001011|1 1ffff97 [25]
1352 (188) |11111111|11111111|11001100|0 1ffff98 [25]
1353 (189) |11111111|11111111|11001100|1 1ffff99 [25]
1354 (190) |11111111|11111111|11001101|0 1ffff9a [25]
1355 (191) |11111111|11111111|11001101|1 1ffff9b [25]
1356 (192) |11111111|11111111|11001110|0 1ffff9c [25]
1357 (193) |11111111|11111111|11001110|1 1ffff9d [25]
1358 (194) |11111111|11111111|11001111|0 1ffff9e [25]
1359 (195) |11111111|11111111|11001111|1 1ffff9f [25]
1360 (196) |11111111|11111111|11010000|0 1ffffa0 [25]
1361 (197) |11111111|11111111|11010000|1 1ffffa1 [25]
1362 (198) |11111111|11111111|11010001|0 1ffffa2 [25]
1363 (199) |11111111|11111111|11010001|1 1ffffa3 [25]
1364 (200) |11111111|11111111|11010010|0 1ffffa4 [25]
1365 (201) |11111111|11111111|11010010|1 1ffffa5 [25]
1366 (202) |11111111|11111111|11010011|0 1ffffa6 [25]
1367 (203) |11111111|11111111|11010011|1 1ffffa7 [25]
1368 (204) |11111111|11111111|11010100|0 1ffffa8 [25]
1369 (205) |11111111|11111111|11010100|1 1ffffa9 [25]
1370 (206) |11111111|11111111|11010101|0 1ffffaa [25]
1371 (207) |11111111|11111111|11010101|1 1ffffab [25]
1372 (208) |11111111|11111111|11010110|0 1ffffac [25]
1373 (209) |11111111|11111111|11010110|1 1ffffad [25]
1374 (210) |11111111|11111111|11010111|0 1ffffae [25]
1375 (211) |11111111|11111111|11010111|1 1ffffaf [25]
1376 (212) |11111111|11111111|11011000|0 1ffffb0 [25]
1377 (213) |11111111|11111111|11011000|1 1ffffb1 [25]
1378 (214) |11111111|11111111|11011001|0 1ffffb2 [25]
1379 (215) |11111111|11111111|11011001|1 1ffffb3 [25]
1380 (216) |11111111|11111111|11011010|0 1ffffb4 [25]
1381 (217) |11111111|11111111|11011010|1 1ffffb5 [25]
1382 (218) |11111111|11111111|11011011|0 1ffffb6 [25]
1383 (219) |11111111|11111111|11011011|1 1ffffb7 [25]
1384 (220) |11111111|11111111|11011100|0 1ffffb8 [25]
1385 (221) |11111111|11111111|11011100|1 1ffffb9 [25]
1386 (222) |11111111|11111111|11011101|0 1ffffba [25]
1387 (223) |11111111|11111111|11011101|1 1ffffbb [25]
1388 (224) |11111111|11111111|11011110|0 1ffffbc [25]
1389 (225) |11111111|11111111|11011110|1 1ffffbd [25]
1390 (226) |11111111|11111111|11011111|0 1ffffbe [25]
1391 (227) |11111111|11111111|11011111|1 1ffffbf [25]
1392 (228) |11111111|11111111|11100000|0 1ffffc0 [25]
1393 (229) |11111111|11111111|11100000|1 1ffffc1 [25]
1394 (230) |11111111|11111111|11100001|0 1ffffc2 [25]
1395 (231) |11111111|11111111|11100001|1 1ffffc3 [25]
1396 (232) |11111111|11111111|11100010|0 1ffffc4 [25]
1397 (233) |11111111|11111111|11100010|1 1ffffc5 [25]
1398 (234) |11111111|11111111|11100011|0 1ffffc6 [25]
1399 (235) |11111111|11111111|11100011|1 1ffffc7 [25]
1400 (236) |11111111|11111111|11100100|0 1ffffc8 [25]
1401 (237) |11111111|11111111|11100100|1 1ffffc9 [25]
1402 (238) |11111111|11111111|11100101|0 1ffffca [25]
1403 (239) |11111111|11111111|11100101|1 1ffffcb [25]
1404 (240) |11111111|11111111|11100110|0 1ffffcc [25]
1405 (241) |11111111|11111111|11100110|1 1ffffcd [25]
1406 (242) |11111111|11111111|11100111|0 1ffffce [25]
1407 (243) |11111111|11111111|11100111|1 1ffffcf [25]
1408 (244) |11111111|11111111|11101000|0 1ffffd0 [25]
1409 (245) |11111111|11111111|11101000|1 1ffffd1 [25]
1410 (246) |11111111|11111111|11101001|0 1ffffd2 [25]
1411 (247) |11111111|11111111|11101001|1 1ffffd3 [25]
1412 (248) |11111111|11111111|11101010|0 1ffffd4 [25]
1413 (249) |11111111|11111111|11101010|1 1ffffd5 [25]
1414 (250) |11111111|11111111|11101011|0 1ffffd6 [25]
1415 (251) |11111111|11111111|11101011|1 1ffffd7 [25]
1416 (252) |11111111|11111111|11101100|0 1ffffd8 [25]
1417 (253) |11111111|11111111|11101100|1 1ffffd9 [25]
1418 (254) |11111111|11111111|11101101|0 1ffffda [25]
1419 (255) |11111111|11111111|11101101|1 1ffffdb [25]
1420 EOS (256) |11111111|11111111|11101110|0 1ffffdc [25]
1422 Appendix D. Examples
1424 A number of examples are worked through here, covering integer
1425 encoding, header field representation, and the encoding of whole sets
1426 of header fields, for both requests and responses, and with and
1427 without Huffman coding.
1429 D.1. Integer Representation Examples
1431 This section shows the representation of integer values in details
1432 (see Section 4.1.1).
1434 D.1.1. Example 1: Encoding 10 using a 5-bit prefix
1436 The value 10 is to be encoded with a 5-bit prefix.
1438 o 10 is less than 31 (2^5 - 1) and is represented using the 5-bit
1439 prefix.
1441 0 1 2 3 4 5 6 7
1442 +---+---+---+---+---+---+---+---+
1443 | X | X | X | 0 | 1 | 0 | 1 | 0 | 10 stored on 5 bits
1444 +---+---+---+---+---+---+---+---+
1446 D.1.2. Example 2: Encoding 1337 using a 5-bit prefix
1448 The value I=1337 is to be encoded with a 5-bit prefix.
1450 1337 is greater than 31 (2^5 - 1).
1452 The 5-bit prefix is filled with its max value (31).
1454 I = 1337 - (2^5 - 1) = 1306.
1456 I (1306) is greater than or equal to 128, the while loop body
1457 executes:
1459 I % 128 == 26
1461 26 + 128 == 154
1462 154 is encoded in 8 bits as: 10011010
1464 I is set to 10 (1306 / 128 == 10)
1466 I is no longer greater than or equal to 128, the while loop
1467 terminates.
1469 I, now 10, is encoded on 8 bits as: 00001010.
1471 The process ends.
1473 0 1 2 3 4 5 6 7
1474 +---+---+---+---+---+---+---+---+
1475 | X | X | X | 1 | 1 | 1 | 1 | 1 | Prefix = 31, I = 1306
1476 | 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 | 1306>=128, encode(154), I=1306/128
1477 | 0 | 0 | 0 | 0 | 1 | 0 | 1 | 0 | 10<128, encode(10), done
1478 +---+---+---+---+---+---+---+---+
1480 D.1.3. Example 3: Encoding 42 starting at an octet-boundary
1482 The value 42 is to be encoded starting at an octet-boundary. This
1483 implies that a 8-bit prefix is used.
1485 o 42 is less than 255 (2^8 - 1) and is represented using the 8-bit
1486 prefix.
1488 0 1 2 3 4 5 6 7
1489 +---+---+---+---+---+---+---+---+
1490 | 0 | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 42 stored on 8 bits
1491 +---+---+---+---+---+---+---+---+
1493 D.2. Header Field Representation Examples
1495 This section shows several independent representation examples.
1497 D.2.1. Literal Header Field with Indexing
1499 The header field representation uses a literal name and a literal
1500 value.
1502 Header set to encode:
1504 custom-key: custom-header
1506 Reference set: empty.
1508 Hex dump of encoded data:
1510 400a 6375 7374 6f6d 2d6b 6579 0d63 7573 | @.custom-key.cus
1511 746f 6d2d 6865 6164 6572 | tom-header
1513 Decoding process:
1515 40 | == Literal indexed ==
1516 0a | Literal name (len = 10)
1517 6375 7374 6f6d 2d6b 6579 | custom-key
1518 0d | Literal value (len = 13)
1519 6375 7374 6f6d 2d68 6561 6465 72 | custom-header
1520 | -> custom-key: custom-head\
1521 | er
1523 Header Table (after decoding):
1525 [ 1] (s = 55) custom-key: custom-header
1526 Table size: 55
1528 Decoded header set:
1530 custom-key: custom-header
1532 D.2.2. Literal Header Field without Indexing
1534 The header field representation uses an indexed name and a literal
1535 value.
1537 Header set to encode:
1539 :path: /sample/path
1541 Reference set: empty.
1543 Hex dump of encoded data:
1545 040c 2f73 616d 706c 652f 7061 7468 | ../sample/path
1547 Decoding process:
1549 04 | == Literal not indexed ==
1550 | Indexed name (idx = 4)
1551 | :path
1552 0c | Literal value (len = 12)
1553 2f73 616d 706c 652f 7061 7468 | /sample/path
1554 | -> :path: /sample/path
1556 Header table (after decoding): empty.
1558 Decoded header set:
1560 :path: /sample/path
1562 D.2.3. Indexed Header Field
1564 The header field representation uses an indexed header field, from
1565 the static table. Upon using it, the static table entry is copied
1566 into the header table.
1568 Header set to encode:
1570 :method: GET
1572 Reference set: empty.
1574 Hex dump of encoded data:
1576 82 | .
1578 Decoding process:
1580 82 | == Indexed - Add ==
1581 | idx = 2
1582 | -> :method: GET
1584 Header Table (after decoding):
1586 [ 1] (s = 42) :method: GET
1587 Table size: 42
1589 Decoded header set:
1591 :method: GET
1593 D.2.4. Indexed Header Field from Static Table
1595 The header field representation uses an indexed header field, from
1596 the static table. In this example, the SETTINGS_HEADER_TABLE_SIZE is
1597 set to 0, therefore, the entry is not copied into the header table.
1599 Header set to encode:
1601 :method: GET
1603 Reference set: empty.
1605 Hex dump of encoded data:
1607 82 | .
1609 Decoding process:
1611 82 | == Indexed - Add ==
1612 | idx = 2
1613 | -> :method: GET
1615 Header table (after decoding): empty.
1617 Decoded header set:
1619 :method: GET
1621 D.3. Request Examples without Huffman
1623 This section shows several consecutive header sets, corresponding to
1624 HTTP requests, on the same connection.
1626 D.3.1. First request
1627 Header set to encode:
1629 :method: GET
1630 :scheme: http
1631 :path: /
1632 :authority: www.example.com
1634 Reference set: empty.
1636 Hex dump of encoded data:
1638 8287 8644 0f77 7777 2e65 7861 6d70 6c65 | ...D.www.example
1639 2e63 6f6d | .com
1641 Decoding process:
1643 82 | == Indexed - Add ==
1644 | idx = 2
1645 | -> :method: GET
1646 87 | == Indexed - Add ==
1647 | idx = 7
1648 | -> :scheme: http
1649 86 | == Indexed - Add ==
1650 | idx = 6
1651 | -> :path: /
1652 44 | == Literal indexed ==
1653 | Indexed name (idx = 4)
1654 | :authority
1655 0f | Literal value (len = 15)
1656 7777 772e 6578 616d 706c 652e 636f 6d | www.example.com
1657 | -> :authority: www.example\
1658 | .com
1660 Header Table (after decoding):
1662 [ 1] (s = 57) :authority: www.example.com
1663 [ 2] (s = 38) :path: /
1664 [ 3] (s = 43) :scheme: http
1665 [ 4] (s = 42) :method: GET
1666 Table size: 180
1668 Decoded header set:
1670 :method: GET
1671 :scheme: http
1672 :path: /
1673 :authority: www.example.com
1675 D.3.2. Second request
1677 This request takes advantage of the differential encoding of header
1678 sets.
1680 Header set to encode:
1682 :method: GET
1683 :scheme: http
1684 :path: /
1685 :authority: www.example.com
1686 cache-control: no-cache
1688 Reference set:
1690 [ 1] :authority: www.example.com
1691 [ 2] :path: /
1692 [ 3] :scheme: http
1693 [ 4] :method: GET
1695 Hex dump of encoded data:
1697 5c08 6e6f 2d63 6163 6865 | \.no-cache
1699 Decoding process:
1701 5c | == Literal indexed ==
1702 | Indexed name (idx = 28)
1703 | cache-control
1704 08 | Literal value (len = 8)
1705 6e6f 2d63 6163 6865 | no-cache
1706 | -> cache-control: no-cache
1708 Header Table (after decoding):
1710 [ 1] (s = 53) cache-control: no-cache
1711 [ 2] (s = 57) :authority: www.example.com
1712 [ 3] (s = 38) :path: /
1713 [ 4] (s = 43) :scheme: http
1714 [ 5] (s = 42) :method: GET
1715 Table size: 233
1717 Decoded header set:
1719 cache-control: no-cache
1720 :authority: www.example.com
1721 :path: /
1722 :scheme: http
1723 :method: GET
1725 D.3.3. Third request
1727 This request has not enough headers in common with the previous
1728 request to take advantage of the differential encoding. Therefore,
1729 the reference set is emptied before encoding the header fields.
1731 Header set to encode:
1733 :method: GET
1734 :scheme: https
1735 :path: /index.html
1736 :authority: www.example.com
1737 custom-key: custom-value
1739 Reference set:
1741 [ 1] cache-control: no-cache
1742 [ 2] :authority: www.example.com
1743 [ 3] :path: /
1744 [ 4] :scheme: http
1745 [ 5] :method: GET
1747 Hex dump of encoded data:
1749 3085 8c8b 8440 0a63 7573 746f 6d2d 6b65 | 0....@.custom-ke
1750 790c 6375 7374 6f6d 2d76 616c 7565 | y.custom-value
1751 Decoding process:
1753 30 | == Empty reference set ==
1754 | idx = 0
1755 | flag = 1
1756 85 | == Indexed - Add ==
1757 | idx = 5
1758 | -> :method: GET
1759 8c | == Indexed - Add ==
1760 | idx = 12
1761 | -> :scheme: https
1762 8b | == Indexed - Add ==
1763 | idx = 11
1764 | -> :path: /index.html
1765 84 | == Indexed - Add ==
1766 | idx = 4
1767 | -> :authority: www.example\
1768 | .com
1769 40 | == Literal indexed ==
1770 0a | Literal name (len = 10)
1771 6375 7374 6f6d 2d6b 6579 | custom-key
1772 0c | Literal value (len = 12)
1773 6375 7374 6f6d 2d76 616c 7565 | custom-value
1774 | -> custom-key: custom-valu\
1775 | e
1777 Header Table (after decoding):
1779 [ 1] (s = 54) custom-key: custom-value
1780 [ 2] (s = 48) :path: /index.html
1781 [ 3] (s = 44) :scheme: https
1782 [ 4] (s = 53) cache-control: no-cache
1783 [ 5] (s = 57) :authority: www.example.com
1784 [ 6] (s = 38) :path: /
1785 [ 7] (s = 43) :scheme: http
1786 [ 8] (s = 42) :method: GET
1787 Table size: 379
1789 Decoded header set:
1791 :method: GET
1792 :scheme: https
1793 :path: /index.html
1794 :authority: www.example.com
1795 custom-key: custom-value
1797 D.4. Request Examples with Huffman
1799 This section shows the same examples as the previous section, but
1800 using Huffman encoding for the literal values.
1802 D.4.1. First request
1804 Header set to encode:
1806 :method: GET
1807 :scheme: http
1808 :path: /
1809 :authority: www.example.com
1811 Reference set: empty.
1813 Hex dump of encoded data:
1815 8287 8644 8ce7 cf9b ebe8 9b6f b16f a9b6 | ...D.......o.o..
1816 ff | .
1818 Decoding process:
1820 82 | == Indexed - Add ==
1821 | idx = 2
1822 | -> :method: GET
1823 87 | == Indexed - Add ==
1824 | idx = 7
1825 | -> :scheme: http
1826 86 | == Indexed - Add ==
1827 | idx = 6
1828 | -> :path: /
1829 44 | == Literal indexed ==
1830 | Indexed name (idx = 4)
1831 | :authority
1832 8c | Literal value (len = 15)
1833 | Huffman encoded:
1834 e7cf 9beb e89b 6fb1 6fa9 b6ff | ......o.o...
1835 | Decoded:
1836 | www.example.com
1837 | -> :authority: www.example\
1838 | .com
1840 Header Table (after decoding):
1842 [ 1] (s = 57) :authority: www.example.com
1843 [ 2] (s = 38) :path: /
1844 [ 3] (s = 43) :scheme: http
1845 [ 4] (s = 42) :method: GET
1846 Table size: 180
1848 Decoded header set:
1850 :method: GET
1851 :scheme: http
1852 :path: /
1853 :authority: www.example.com
1855 D.4.2. Second request
1857 This request takes advantage of the differential encoding of header
1858 sets.
1860 Header set to encode:
1862 :method: GET
1863 :scheme: http
1864 :path: /
1865 :authority: www.example.com
1866 cache-control: no-cache
1868 Reference set:
1870 [ 1] :authority: www.example.com
1871 [ 2] :path: /
1872 [ 3] :scheme: http
1873 [ 4] :method: GET
1875 Hex dump of encoded data:
1877 5c86 b9b9 9495 56bf | \.....V.
1879 Decoding process:
1881 5c | == Literal indexed ==
1882 | Indexed name (idx = 28)
1883 | cache-control
1884 86 | Literal value (len = 8)
1885 | Huffman encoded:
1886 b9b9 9495 56bf | ....V.
1887 | Decoded:
1888 | no-cache
1889 | -> cache-control: no-cache
1891 Header Table (after decoding):
1893 [ 1] (s = 53) cache-control: no-cache
1894 [ 2] (s = 57) :authority: www.example.com
1895 [ 3] (s = 38) :path: /
1896 [ 4] (s = 43) :scheme: http
1897 [ 5] (s = 42) :method: GET
1898 Table size: 233
1900 Decoded header set:
1902 cache-control: no-cache
1903 :authority: www.example.com
1904 :path: /
1905 :scheme: http
1906 :method: GET
1908 D.4.3. Third request
1910 This request has not enough headers in common with the previous
1911 request to take advantage of the differential encoding. Therefore,
1912 the reference set is emptied before encoding the header fields.
1914 Header set to encode:
1916 :method: GET
1917 :scheme: https
1918 :path: /index.html
1919 :authority: www.example.com
1920 custom-key: custom-value
1922 Reference set:
1924 [ 1] cache-control: no-cache
1925 [ 2] :authority: www.example.com
1926 [ 3] :path: /
1927 [ 4] :scheme: http
1928 [ 5] :method: GET
1930 Hex dump of encoded data:
1932 3085 8c8b 8440 8857 1c5c db73 7b2f af89 | 0....@.W.\.s{/..
1933 571c 5cdb 7372 4d9c 57 | W.\.srM.W
1935 Decoding process:
1937 30 | == Empty reference set ==
1938 | idx = 0
1939 | flag = 1
1940 85 | == Indexed - Add ==
1941 | idx = 5
1942 | -> :method: GET
1943 8c | == Indexed - Add ==
1944 | idx = 12
1945 | -> :scheme: https
1946 8b | == Indexed - Add ==
1947 | idx = 11
1948 | -> :path: /index.html
1949 84 | == Indexed - Add ==
1950 | idx = 4
1951 | -> :authority: www.example\
1952 | .com
1953 40 | == Literal indexed ==
1954 88 | Literal name (len = 10)
1955 | Huffman encoded:
1956 571c 5cdb 737b 2faf | W.\.s{/.
1957 | Decoded:
1958 | custom-key
1959 89 | Literal value (len = 12)
1960 | Huffman encoded:
1961 571c 5cdb 7372 4d9c 57 | W.\.srM.W
1962 | Decoded:
1963 | custom-value
1964 | -> custom-key: custom-valu\
1965 | e
1967 Header Table (after decoding):
1969 [ 1] (s = 54) custom-key: custom-value
1970 [ 2] (s = 48) :path: /index.html
1971 [ 3] (s = 44) :scheme: https
1972 [ 4] (s = 53) cache-control: no-cache
1973 [ 5] (s = 57) :authority: www.example.com
1974 [ 6] (s = 38) :path: /
1975 [ 7] (s = 43) :scheme: http
1976 [ 8] (s = 42) :method: GET
1977 Table size: 379
1979 Decoded header set:
1981 :method: GET
1982 :scheme: https
1983 :path: /index.html
1984 :authority: www.example.com
1985 custom-key: custom-value
1987 D.5. Response Examples without Huffman
1989 This section shows several consecutive header sets, corresponding to
1990 HTTP responses, on the same connection. SETTINGS_HEADER_TABLE_SIZE
1991 is set to the value of 256 octets, causing some evictions to occur.
1993 D.5.1. First response
1995 Header set to encode:
1997 :status: 302
1998 cache-control: private
1999 date: Mon, 21 Oct 2013 20:13:21 GMT
2000 location: https://www.example.com
2002 Reference set: empty.
2004 Hex dump of encoded data:
2006 4803 3330 3259 0770 7269 7661 7465 631d | H.302Y.privatec.
2007 4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
2008 2032 303a 3133 3a32 3120 474d 5471 1768 | 20:13:21 GMTq.h
2009 7474 7073 3a2f 2f77 7777 2e65 7861 6d70 | ttps://www.examp
2010 6c65 2e63 6f6d | le.com
2011 Decoding process:
2013 48 | == Literal indexed ==
2014 | Indexed name (idx = 8)
2015 | :status
2016 03 | Literal value (len = 3)
2017 3330 32 | 302
2018 | -> :status: 302
2019 59 | == Literal indexed ==
2020 | Indexed name (idx = 25)
2021 | cache-control
2022 07 | Literal value (len = 7)
2023 7072 6976 6174 65 | private
2024 | -> cache-control: private
2025 63 | == Literal indexed ==
2026 | Indexed name (idx = 35)
2027 | date
2028 1d | Literal value (len = 29)
2029 4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
2030 2032 303a 3133 3a32 3120 474d 54 | 20:13:21 GMT
2031 | -> date: Mon, 21 Oct 2013 \
2032 | 20:13:21 GMT
2033 71 | == Literal indexed ==
2034 | Indexed name (idx = 49)
2035 | location
2036 17 | Literal value (len = 23)
2037 6874 7470 733a 2f2f 7777 772e 6578 616d | https://www.exam
2038 706c 652e 636f 6d | ple.com
2039 | -> location: https://www.e\
2040 | xample.com
2042 Header Table (after decoding):
2044 [ 1] (s = 63) location: https://www.example.com
2045 [ 2] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
2046 [ 3] (s = 52) cache-control: private
2047 [ 4] (s = 42) :status: 302
2048 Table size: 222
2050 Decoded header set:
2052 :status: 302
2053 cache-control: private
2054 date: Mon, 21 Oct 2013 20:13:21 GMT
2055 location: https://www.example.com
2057 D.5.2. Second response
2059 The (":status", "302") header field is evicted from the header table
2060 to free space to allow adding the (":status", "200") header field,
2061 copied from the static table into the header table. The (":status",
2062 "302") header field doesn't need to be removed from the reference set
2063 as it is evicted from the header table.
2065 Header set to encode:
2067 :status: 200
2068 cache-control: private
2069 date: Mon, 21 Oct 2013 20:13:21 GMT
2070 location: https://www.example.com
2072 Reference set:
2074 [ 1] location: https://www.example.com
2075 [ 2] date: Mon, 21 Oct 2013 20:13:21 GMT
2076 [ 3] cache-control: private
2077 [ 4] :status: 302
2079 Hex dump of encoded data:
2081 8c | .
2083 Decoding process:
2085 8c | == Indexed - Add ==
2086 | idx = 12
2087 | - evict: :status: 302
2088 | -> :status: 200
2090 Header Table (after decoding):
2092 [ 1] (s = 42) :status: 200
2093 [ 2] (s = 63) location: https://www.example.com
2094 [ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
2095 [ 4] (s = 52) cache-control: private
2096 Table size: 222
2098 Decoded header set:
2100 :status: 200
2101 location: https://www.example.com
2102 date: Mon, 21 Oct 2013 20:13:21 GMT
2103 cache-control: private
2105 D.5.3. Third response
2107 Several header fields are evicted from the header table during the
2108 processing of this header set. Before evicting a header belonging to
2109 the reference set, it is emitted, by coding it twice as an Indexed
2110 Representation. The first representation removes the header field
2111 from the reference set, the second one adds it again to the reference
2112 set, also emitting it.
2114 Header set to encode:
2116 :status: 200
2117 cache-control: private
2118 date: Mon, 21 Oct 2013 20:13:22 GMT
2119 location: https://www.example.com
2120 content-encoding: gzip
2121 set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1
2123 Reference set:
2125 [ 1] :status: 200
2126 [ 2] location: https://www.example.com
2127 [ 3] date: Mon, 21 Oct 2013 20:13:21 GMT
2128 [ 4] cache-control: private
2130 Hex dump of encoded data:
2132 8484 431d 4d6f 6e2c 2032 3120 4f63 7420 | ..C.Mon, 21 Oct
2133 3230 3133 2032 303a 3133 3a32 3220 474d | 2013 20:13:22 GM
2134 545e 0467 7a69 7084 8483 837b 3866 6f6f | T^.gzip....{8foo
2135 3d41 5344 4a4b 4851 4b42 5a58 4f51 5745 | =ASDJKHQKBZXOQWE
2136 4f50 4955 4158 5157 454f 4955 3b20 6d61 | OPIUAXQWEOIU; ma
2137 782d 6167 653d 3336 3030 3b20 7665 7273 | x-age=3600; vers
2138 696f 6e3d 31 | ion=1
2140 Decoding process:
2142 84 | == Indexed - Remove ==
2143 | idx = 4
2144 | -> cache-control: private
2145 84 | == Indexed - Add ==
2146 | idx = 4
2147 | -> cache-control: private
2148 43 | == Literal indexed ==
2149 | Indexed name (idx = 3)
2150 | date
2151 1d | Literal value (len = 29)
2152 4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
2153 2032 303a 3133 3a32 3220 474d 54 | 20:13:22 GMT
2154 | - evict: cache-control: pr\
2155 | ivate
2156 | -> date: Mon, 21 Oct 2013 \
2157 | 20:13:22 GMT
2158 5e | == Literal indexed ==
2159 | Indexed name (idx = 30)
2160 | content-encoding
2161 04 | Literal value (len = 4)
2162 677a 6970 | gzip
2163 | - evict: date: Mon, 21 Oct\
2164 | 2013 20:13:21 GMT
2165 | -> content-encoding: gzip
2166 84 | == Indexed - Remove ==
2167 | idx = 4
2168 | -> location: https://www.e\
2169 | xample.com
2170 84 | == Indexed - Add ==
2171 | idx = 4
2172 | -> location: https://www.e\
2173 | xample.com
2174 83 | == Indexed - Remove ==
2175 | idx = 3
2176 | -> :status: 200
2177 83 | == Indexed - Add ==
2178 | idx = 3
2179 | -> :status: 200
2180 7b | == Literal indexed ==
2181 | Indexed name (idx = 59)
2182 | set-cookie
2183 38 | Literal value (len = 56)
2184 666f 6f3d 4153 444a 4b48 514b 425a 584f | foo=ASDJKHQKBZXO
2185 5157 454f 5049 5541 5851 5745 4f49 553b | QWEOPIUAXQWEOIU;
2186 206d 6178 2d61 6765 3d33 3630 303b 2076 | max-age=3600; v
2187 6572 7369 6f6e 3d31 | ersion=1
2188 | - evict: location: https:/\
2189 | /www.example.com
2190 | - evict: :status: 200
2191 | -> set-cookie: foo=ASDJKHQ\
2192 | KBZXOQWEOPIUAXQWEOIU; ma\
2193 | x-age=3600; version=1
2195 Header Table (after decoding):
2197 [ 1] (s = 98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age\
2198 =3600; version=1
2199 [ 2] (s = 52) content-encoding: gzip
2200 [ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:22 GMT
2201 Table size: 215
2203 Decoded header set:
2205 cache-control: private
2206 date: Mon, 21 Oct 2013 20:13:22 GMT
2207 content-encoding: gzip
2208 location: https://www.example.com
2209 :status: 200
2210 set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1
2212 D.6. Response Examples with Huffman
2214 This section shows the same examples as the previous section, but
2215 using Huffman encoding for the literal values. The eviction
2216 mechanism uses the length of the decoded literal values, so the same
2217 evictions occurs as in the previous section.
2219 D.6.1. First response
2221 Header set to encode:
2223 :status: 302
2224 cache-control: private
2225 date: Mon, 21 Oct 2013 20:13:21 GMT
2226 location: https://www.example.com
2228 Reference set: empty.
2230 Hex dump of encoded data:
2232 4882 4017 5985 bf06 724b 9763 93d6 dbb2 | H.@.Y...rK.c....
2233 9884 de2a 7188 0506 2098 5131 09b5 6ba3 | ...*q... .Q1..k.
2234 7191 adce bf19 8e7e 7cf9 bebe 89b6 fb16 | q.......|.......
2235 fa9b 6f | ..o
2237 Decoding process:
2239 48 | == Literal indexed ==
2240 | Indexed name (idx = 8)
2241 | :status
2242 82 | Literal value (len = 3)
2243 | Huffman encoded:
2244 4017 | @.
2245 | Decoded:
2246 | 302
2247 | -> :status: 302
2248 59 | == Literal indexed ==
2249 | Indexed name (idx = 25)
2250 | cache-control
2251 85 | Literal value (len = 7)
2252 | Huffman encoded:
2253 bf06 724b 97 | ..rK.
2254 | Decoded:
2255 | private
2256 | -> cache-control: private
2257 63 | == Literal indexed ==
2258 | Indexed name (idx = 35)
2259 | date
2260 93 | Literal value (len = 29)
2261 | Huffman encoded:
2262 d6db b298 84de 2a71 8805 0620 9851 3109 | ......*q... .Q1.
2263 b56b a3 | .k.
2264 | Decoded:
2265 | Mon, 21 Oct 2013 20:13:21 \
2266 | GMT
2267 | -> date: Mon, 21 Oct 2013 \
2268 | 20:13:21 GMT
2269 71 | == Literal indexed ==
2270 | Indexed name (idx = 49)
2271 | location
2272 91 | Literal value (len = 23)
2273 | Huffman encoded:
2274 adce bf19 8e7e 7cf9 bebe 89b6 fb16 fa9b | ......|.........
2275 6f | o
2276 | Decoded:
2277 | https://www.example.com
2278 | -> location: https://www.e\
2279 | xample.com
2281 Header Table (after decoding):
2283 [ 1] (s = 63) location: https://www.example.com
2284 [ 2] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
2285 [ 3] (s = 52) cache-control: private
2286 [ 4] (s = 42) :status: 302
2287 Table size: 222
2289 Decoded header set:
2291 :status: 302
2292 cache-control: private
2293 date: Mon, 21 Oct 2013 20:13:21 GMT
2294 location: https://www.example.com
2296 D.6.2. Second response
2298 The (":status", "302") header field is evicted from the header table
2299 to free space to allow adding the (":status", "200") header field,
2300 copied from the static table into the header table. The (":status",
2301 "302") header field doesn't need to be removed from the reference set
2302 as it is evicted from the header table.
2304 Header set to encode:
2306 :status: 200
2307 cache-control: private
2308 date: Mon, 21 Oct 2013 20:13:21 GMT
2309 location: https://www.example.com
2311 Reference set:
2313 [ 1] location: https://www.example.com
2314 [ 2] date: Mon, 21 Oct 2013 20:13:21 GMT
2315 [ 3] cache-control: private
2316 [ 4] :status: 302
2318 Hex dump of encoded data:
2320 8c | .
2322 Decoding process:
2324 8c | == Indexed - Add ==
2325 | idx = 12
2326 | - evict: :status: 302
2327 | -> :status: 200
2329 Header Table (after decoding):
2331 [ 1] (s = 42) :status: 200
2332 [ 2] (s = 63) location: https://www.example.com
2333 [ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
2334 [ 4] (s = 52) cache-control: private
2335 Table size: 222
2337 Decoded header set:
2339 :status: 200
2340 location: https://www.example.com
2341 date: Mon, 21 Oct 2013 20:13:21 GMT
2342 cache-control: private
2344 D.6.3. Third response
2346 Several header fields are evicted from the header table during the
2347 processing of this header set. Before evicting a header belonging to
2348 the reference set, it is emitted, by coding it twice as an Indexed
2349 Representation. The first representation removes the header field
2350 from the reference set, the second one adds it again to the reference
2351 set, also emitting it.
2353 Header set to encode:
2355 :status: 200
2356 cache-control: private
2357 date: Mon, 21 Oct 2013 20:13:22 GMT
2358 location: https://www.example.com
2359 content-encoding: gzip
2360 set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1
2361 Reference set:
2363 [ 1] :status: 200
2364 [ 2] location: https://www.example.com
2365 [ 3] date: Mon, 21 Oct 2013 20:13:21 GMT
2366 [ 4] cache-control: private
2368 Hex dump of encoded data:
2370 8484 4393 d6db b298 84de 2a71 8805 0620 | ..C.......*q...
2371 9851 3111 b56b a35e 84ab dd97 ff84 8483 | .Q1..k.^........
2372 837b b1e0 d6cf 9f6e 8f9f d3e5 f6fa 76fe | .{.....n......v.
2373 fd3c 7edf 9eff 1f2f 0f3c fe9f 6fcf 7f8f | ......./....o...
2374 879f 61ad 4f4c c9a9 73a2 200e c372 5e18 | ..a.OL..s. ..r^.
2375 b1b7 4e3f | ..N?
2377 Decoding process:
2379 84 | == Indexed - Remove ==
2380 | idx = 4
2381 | -> cache-control: private
2382 84 | == Indexed - Add ==
2383 | idx = 4
2384 | -> cache-control: private
2385 43 | == Literal indexed ==
2386 | Indexed name (idx = 3)
2387 | date
2388 93 | Literal value (len = 29)
2389 | Huffman encoded:
2390 d6db b298 84de 2a71 8805 0620 9851 3111 | ......*q... .Q1.
2391 b56b a3 | .k.
2392 | Decoded:
2393 | Mon, 21 Oct 2013 20:13:22 \
2394 | GMT
2395 | - evict: cache-control: pr\
2396 | ivate
2397 | -> date: Mon, 21 Oct 2013 \
2398 | 20:13:22 GMT
2399 5e | == Literal indexed ==
2400 | Indexed name (idx = 30)
2401 | content-encoding
2402 84 | Literal value (len = 4)
2403 | Huffman encoded:
2404 abdd 97ff | ....
2405 | Decoded:
2406 | gzip
2407 | - evict: date: Mon, 21 Oct\
2408 | 2013 20:13:21 GMT
2409 | -> content-encoding: gzip
2410 84 | == Indexed - Remove ==
2411 | idx = 4
2412 | -> location: https://www.e\
2413 | xample.com
2414 84 | == Indexed - Add ==
2415 | idx = 4
2416 | -> location: https://www.e\
2417 | xample.com
2418 83 | == Indexed - Remove ==
2419 | idx = 3
2420 | -> :status: 200
2421 83 | == Indexed - Add ==
2422 | idx = 3
2423 | -> :status: 200
2424 7b | == Literal indexed ==
2425 | Indexed name (idx = 59)
2426 | set-cookie
2427 b1 | Literal value (len = 56)
2428 | Huffman encoded:
2429 e0d6 cf9f 6e8f 9fd3 e5f6 fa76 fefd 3c7e | ....n......v....
2430 df9e ff1f 2f0f 3cfe 9f6f cf7f 8f87 9f61 | ..../....o.....a
2431 ad4f 4cc9 a973 a220 0ec3 725e 18b1 b74e | .OL..s. ..r^...N
2432 3f | ?
2433 | Decoded:
2434 | foo=ASDJKHQKBZXOQWEOPIUAXQ\
2435 | WEOIU; max-age=3600; versi\
2436 | on=1
2437 | - evict: location: https:/\
2438 | /www.example.com
2439 | - evict: :status: 200
2440 | -> set-cookie: foo=ASDJKHQ\
2441 | KBZXOQWEOPIUAXQWEOIU; ma\
2442 | x-age=3600; version=1
2444 Header Table (after decoding):
2446 [ 1] (s = 98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age\
2447 =3600; version=1
2448 [ 2] (s = 52) content-encoding: gzip
2449 [ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:22 GMT
2450 Table size: 215
2452 Decoded header set:
2454 cache-control: private
2455 date: Mon, 21 Oct 2013 20:13:22 GMT
2456 content-encoding: gzip
2457 location: https://www.example.com
2458 :status: 200
2459 set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1
2461 Authors' Addresses
2463 Roberto Peon
2464 Google, Inc
2466 EMail: fenix@google.com
2468 Herve Ruellan
2469 Canon CRF
2471 EMail: herve.ruellan@crf.canon.fr