Network Working Group J.M. Snell Internet-Draft May 20, 2013 Intended status: Informational Expires: November 21, 2013 HTTP/2.0 Discussion: Stored Header Encoding draft-snell-httpbis-bohe-08 Abstract This memo describes a proposed alternative encoding for headers that combines the best concepts from the proposed Delta and HeaderDiff options with the typed value codecs introduced by previous versions of this draft. Status of This Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on November 21, 2013. Copyright Notice Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Table of Contents 1. Stored Header Encoding . . . . . . . . . . . . . . . . . . . 2 Snell Expires November 21, 2013 [Page 1] Internet-Draft Stored Header Encoding May 2013 2. State Model . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Header Serialization . . . . . . . . . . . . . . . . . . . . 4 3.1. Header Group Prefix . . . . . . . . . . . . . . . . . . . 6 3.2. Index Header Group . . . . . . . . . . . . . . . . . . . 7 3.3. Index Range Header Group . . . . . . . . . . . . . . . . 7 3.4. Cloned Index Header Group . . . . . . . . . . . . . . . . 8 3.5. Literal Header Group . . . . . . . . . . . . . . . . . . 9 4. Header Values . . . . . . . . . . . . . . . . . . . . . . . . 9 4.1. UTF-8 Text Values . . . . . . . . . . . . . . . . . . . . 10 4.2. Numeric Values . . . . . . . . . . . . . . . . . . . . . 10 4.3. Timestamp Values . . . . . . . . . . . . . . . . . . . . 10 4.4. Raw Binary Octet Values . . . . . . . . . . . . . . . . . 10 4.5. Unsigned Variable Length Integer Syntax . . . . . . . . . 11 4.6. Huffman Coding . . . . . . . . . . . . . . . . . . . . . 11 5. Implementation Considerations . . . . . . . . . . . . . . . . 12 6. Security Considerations . . . . . . . . . . . . . . . . . . . 12 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 7.1. Normative References . . . . . . . . . . . . . . . . . . 13 7.2. Informational References . . . . . . . . . . . . . . . . 13 Appendix A. Huffman Tables . . . . . . . . . . . . . . . . . . . 13 Appendix B. Static Storage Cache . . . . . . . . . . . . . . . . 20 Appendix C. Updated Standard Header Definitions . . . . . . . . 23 Appendix D. State Management Alternatives . . . . . . . . . . . 25 Appendix E. Alternative Timestamp encodings . . . . . . . . . . 26 Appendix F. Alternative uvarint encodings . . . . . . . . . . . 27 F.1. Option 1: . . . . . . . . . . . . . . . . . . . . . . . . 28 F.2. Option 2: . . . . . . . . . . . . . . . . . . . . . . . . 28 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 29 1. Stored Header Encoding The Stored Header Encoding is an alternative "binary header encoding" for HTTP/2.0 that combines the best elements from three other proposed encodings, including: o The "Header Delta Compression" scheme proposed by Roberto Peon in http://tools.ietf.org/html/draft-rpeon-httpbis-header- compression-03 o The "Header Diff" encoding proposed by Herve Reullan, Jun Fujisawa, Romain Bellessort, and Youenn Fablet in http:// tools.ietf.org/html/draft-ruellan-headerdiff-00 o The "Binary Optimized Header Encoding" proposed by James Snell (me) in http://tools.ietf.org/html/draft-snell-httpbis-bohe-03 Snell Expires November 21, 2013 [Page 2] Internet-Draft Stored Header Encoding May 2013 The Stored Header Encoding seeks to find an elegant, efficient and simple marriage of the best concepts from each of these separate proposals. 2. State Model The compressor and decompressor each maintain a cache of header value pairs. There is a static cache, prepopulated by the specification, and a dynamic cache, populated through the compression and decompression process. Each cache contains a maximum of 128 individual key+value pairs. Each item in the index is referenced by an 8-bit identifier. The most significant bit identifies whether an item from the static or dynamic cache is being referenced. Note: the Nil byte (0x00) is a valid identifier for the dynamic cache. 0xxxxxxx -- Dynamic Cache 1xxxxxxx -- Static Cache The dynamic cache is managed in a "least recently written" style, that is, as the cache fills to capacity in both number of entries and maximum stored byte size, the least recently written items are dropped and those index positions are reused. Index positions from the dynamic cache are assigned in "encounter order", beginning from 0x00 and increasing monotonically to 0x7F. That is to say, the positions are assigned in precisely the same order that they are serialized, and thereby encountered by the decompressor upon reading and processing the block. Each item in the store consists of a Header Name and a Value. The Name is a lower-case ISO-8859-1 character sequence. The Value is either a UTF-8 string, a number, a Timestamp or an arbitrary sequence of binary octets. The available size of the stored compression state can be capped by the decompressor. Each stored value contributes to the accumulated size of the storage state. As new key+value pairs are assigned positions in the dynamic cache, the least-recently assigned items must be removed if necessary to free up the required space. The size of string values is measured by the number of UTF-8 bytes required for the character sequence. Snell Expires November 21, 2013 [Page 3] Internet-Draft Stored Header Encoding May 2013 The size of number and timestamp values are measured by the number of unsigned variable length integer (uvarint) encoded bytes it takes to represent the value (see the section of value types below). The size of raw binary values is measured by the number of octets. Header names DO NOT contribute to the stored state size of the compressor; only the size of the value is considered. Duplicate values MUST be counted individually. Header names MUST NOT exceed 255 octets in length. 3. Header Serialization Headers are serialized into four typed header groups, each represented by a two-bit identifier. These groups are serialized sequentially. A serialized header block can contain, at most 256 header groups. The first byte of the serialized block is an unsigned, 0-based counter indicating the number of groups. 00 -- Index Header Group 01 -- Index Range Header Group 10 -- Cloned Index Header Group 11 -- Literal Header Group The Cloned Index (10) and Literal (11) header group types have an additional "ephemeral" property that indicates whether or not the group affects the compression state. Each header group contains a single 8-bit prefix and up to 32 distinct header instances. If a particular serialization block contains more than 32 intances of a given type, then multiple instances of the Header Group Type can be included in the serialized block. For instance, if a given message contains 33 index references, the serialized block may contain two separate Index Header Groups. Wire Format header-block = OCTET *(index-header-group / index-range-header-group / cloned-index-header-group / literal-header-group) ; Header Group Prefix = 8 bits ... ; First two bits = header-group-type Snell Expires November 21, 2013 [Page 4] Internet-Draft Stored Header Encoding May 2013 ; Third bit = ephemeral flag ; Final five bits = instance counter ; index-header-group-type = 00 index-range-group-type = 01 cloned-index-group-type = 10 literal-group-type = 11 count = 5bit index-header-prefix = index-header-group-type unset count ; 000xxxxx index-range-header-prefix = index-header-group-type unset count ; 010xxxxx cloned-index-header-prefix = cloned-index-group-type bit count ; 10?xxxxx literal-header-prefix = literal-group-type bit count ; 11?xxxxx ; Cache Index Identifier = 8 bits ... ; 0xxxxxxx = Dynamic Cache Identifier ; 1xxxxxxx = Static Cache Identifier cache-index = %x00-FF ; Index Header Group index-header-group = index-header-prefix 1*32cache-index ; Index-Range Header Group ; Contains a pair of cache-index values, second MUST ; me strictly higher in value than the first... index-range-header-group = index-range-header-prefix 1*32(cache-index cache-index) ; Cloned-Index Header Group cloned-index-header-group = cloned-index-header-prefix 1*32(cache-index value) ; Literal Header Group literal-header-group = literal-header-prefix 1*32(name value) value = text-value / number-value / timestamp-value / binary-value text-value-type = 00 ; two bits number-value-type = 01 Snell Expires November 21, 2013 [Page 5] Internet-Draft Stored Header Encoding May 2013 timestamp-value-type = 10 binary-value-type = 11 text-value-prefix = text-value-type unset count ; 000xxxxx number-value-prefix = number-value-type unset count ; 010xxxxx timestamp-value-prefix = timestamp-value-type unset count ; 100xxxxx binary-value-prefix = binary-value-type unset count ; 110xxxxx text-value = text-value-prefix *32string number-value = number-value-type *32uvarint timestamp-value = timestamp-value-prefix *32uvarint binary-value = binary-value-prefix uvarint *OCTET uvarint = *uvarint-continuation uvarint-final uvarint-continuation = %x80-FF uvarint-final = %x00-7F name = 1*tchar tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA string = uvarint *(HUFFMAN-ENCODED-CHAR) HUFFMAN-EOF padding-to-nearest-byte; padding-to-nearest-byte = *7unset bit = set / unset unset = 0 set = 1 3.1. Header Group Prefix The Header Group Prefix is a single octet that provides three distinct pieces of information: 0 1 2 3 4 5 6 7 +------+---+---------------+ | TYPE |EPH| COUNTER | +------+---+---------------+ Snell Expires November 21, 2013 [Page 6] Internet-Draft Stored Header Encoding May 2013 TYPE The two most significant bits of the header group prefix identify the group type. EPH The next bit is the "ephemeral flag" and is used only for Cloned and Literal group types. This bit indicates whether or not the group alters the stored compression state. COUNTER The remaining five bits specify the number of header instances in the group, with 00000 indicating that the group contains 1 instance and 11111 contains 32. A header group MUST contain at least one instance. The remaining serialization of the header group depends entirely on the group type. 3.2. Index Header Group The serialization of the Index Header Group consists of the Header Group Prefix and up to 32 additional octets, each referencing a single 8-bit storage index identifier for items in either the Static or Dynamic Cache. For instance 00000000 00000000 = References item #0 from the dynamic cache 00000001 00000000 10000000 = References item #0 from the dynamic cache and item #0 from the static cache Index Header Groups do not affect the stored compression state. If an Index Header Group references a header index that has not yet been allocated, the deserialization MUST terminate with an error. This likely means that the compression state has become out of sync and needs to be reestablished. 3.3. Index Range Header Group The serialization of the Index Range Header Group consists of the Header Group Prefix and up to 32 additional 2-octet (16 bits) pairs of 8-bit storage index identifiers. Each pair specifies a sequential range of adjacent ranges. For instance: Snell Expires November 21, 2013 [Page 7] Internet-Draft Stored Header Encoding May 2013 01000000 00000000 00000100 = References items #0-#4 from the dynamic cache. (five distinct items total) A range MAY span dynamic and static index values. Index values are treated as unsigned byte values, so indices from the static cache are numerically greater than dynamic cache values.. e.g. 01000000 01111111 10000001 = References item #127 from the dynamic cache, and items #0 and #1 from the static cache. Index Range Header Groups do not affect the stored compression state. If a range references a header index that has not yet been allocated, the deserialization MUST terminate with an error. This likely means that the compression state has become out of sync and needs to be reestablished. 3.4. Cloned Index Header Group The serialization of the Cloned Index Header Group consists of the Header Group Prefix and up to 32 Index+Value pairs. Each Index+Value pair consists of a leading 8-bit storage index of an existing stored header followed by a new serialized value. The serialization of the value depends on the value type (see discussion of Value serialization below). The Cloned Header Group affects the stored compression state if, and only if, the "ephemeral" flag in the Header Group Prefix is NOT set. If the header group is not marked as being ephemeral, then the specified value is stored in the next available storage index using the key name from the referenced storage index. For instance, assume the dynamic cache currently contains an item at index #1 with key name "foo" and value "bar", the following causes a new item to be added to the storage with key name "foo" and value "baz": 10000000 00000001 00000000 00000100 10111000 01001111 10110101 00100000 An explanation of the value syntax is given a bit later. If a Cloned Header Group references a header index that has not yet been allocated, the deserialization MUST terminate with an error. Snell Expires November 21, 2013 [Page 8] Internet-Draft Stored Header Encoding May 2013 This likely means that the compression state has become out of sync and needs to be reestablished. 3.5. Literal Header Group The serialization of the Literal Header Group consists of the Header Group Prefix and up to 32 Name+Value pairs. Each Name+Value pair consists of a length-prefixed sequence of ASCII bytes specifying the Header Name followed by the serialized value. The serialization of the value depends on the value type. The length prefix is encoded as an unsigned variable length integer (uvarint). The length prefix SHOULD NOT be longer than five octets and SHOULD NOT specify a value larger than 0xFFFF. The Literal Header Group affects the stored compression state if, and only if, the "ephemeral" flag in the Header Group Prefix is NOT set. If the header group is not marked as being ephemeral, then the specified key name and value are stored in the next available storage index. For instance: 11000000 00000011 01100110 01101111 01101111 00000000 00000010 10111000 01000100 11010010 4. Header Values Header Values can be one of four types, each identified by a two-bit identifier. o 00 -- UTF-8 Text o 01 -- Numeric o 10 -- Timetamp o 11 -- Raw Binary Octets An individual value MAY consist of up to 32 distinct discreet "value instances". A value with multiple instances is considered, for all intents and purposes, to be a single value. Each serialized value is preceded by an 8-bit Value Prefix. 0 1 2 3 4 5 6 7 +------+---+---------------+ Snell Expires November 21, 2013 [Page 9] Internet-Draft Stored Header Encoding May 2013 | TYPE |RSV| COUNTER | +------+---+---------------+ TYPE The two most significant bits specifies the value type. RSV The next significant bit is a reserved flag. Future iterations of this specification might make use of this bit. COUNTER The remaining bits specify the number of discreet instances in the value. 00000 indicates that one instance is included, 11111 indicates that 32 instances are included. The value MUST contain at least one instance. The remaining serialization depends entirely on the type. 4.1. UTF-8 Text Values UTF-8 Text is encoded as a length-prefixed sequence of Huffman- encoded UTF-8 octets. The length prefix is encoded as an unsigned variable-length integer specifying the number of octets after applying the Huffman-encoding. 4.2. Numeric Values Numeric values are encoded as unsigned variable-length integers (uvarint) (Section 4.5) of up to a maximum of 10-octets. Negative values cannot be represented using this syntax. 4.3. Timestamp Values Timestamp values are encoded as unsigned variable-length integers (Section 4.5) specifying the number of milliseconds that have passed since the standard Epoch (1970-01-01T00:00:00 GMT). The syntax is identical that used for Numeric Values. Dates prior to the epoch cannot be represented using this syntax. Representing timestamps in this manner ensures that timestamps will always encode using six bytes up and until 2109-05-15T07:35:00 GMT, then as seven bytes up to and until 19809-03-05T11:03:41 GMT. 4.4. Raw Binary Octet Values Binary values are encoded as a length prefixed sequence of arbitrary octets. The length prefix is encoded as an unsigned variable length integer. Snell Expires November 21, 2013 [Page 10] Internet-Draft Stored Header Encoding May 2013 4.5. Unsigned Variable Length Integer Syntax The uvarint syntax is identical to that used by Google's protobufs. They are serialized with the least-significant bytes first in batches of 7-bits, with the most significant bit per byte reserved as a continuation bit. Values less than or equal to 127 are serialized using at most one byte; values less than or equal to 16383 are serialized using at most two bytes; values less than or equal to 2097151 are serialized using at most three bytes. def uvarint(num): return [] if num = 0 ret = [] while(num != 0): m = num >>> 7 ; unsigned shift left 7 bits ret.push (byte)((num & ~0x80) | ( m > 0 ? 0x80 : 0x00 )); num = m; return ret; For example, the binary representation of the 32-bit integer 217 is: 00000000 00000000 00000000 11011001 The variable length encoding is: 11011001 00000001 The binary representation of the 32-bit integer 1386210052 is: 01010010 10011111 11100011 00000100 The variable length encoding is: 10000100 11000110 11111111 10010100 00000101 4.6. Huffman Coding All UTF-8 text values are compressed using a modified static huffman code. "Modified" because the encoded version may contain compact- representations of raw, arbitrary UTF-8 bytes that are not covered by the static huffman code table. Snell Expires November 21, 2013 [Page 11] Internet-Draft Stored Header Encoding May 2013 There are two huffman tables in use, one for HTTP Requests and another for HTTP Responses, each covers UTF-8 codepoints strictly less than 128 as well the fifty possible UTF-8 leading octets. The encoded result MUST end with a specific terminal sequence of bits called the "HUFFMAN_EOF". Currently, the HUFFMAN_EOF is the same for both the Request and Response tables, but that could change if the tables are regenerated. Currently, the HUFFMAN_EOF sequence is 101001. Codepoints >= 128 are handled by first taking the leading octet of the UTF-8 representation and serializing it's associated huffman code from the table to the output stream, then, depending on the octets value, serializing the six least significant bits from each of the remaining trailing octets. For instance, the UTF-8 character U+00D4 (LATIN CAPITAL LETTER O WITH CIRCUMFLEX), with UTF-8 representation of C394 (hex) is encoded as: 11000100 01010010 10010000 The first 8-bits represents the huffman-table prefix, the six most significant bytes of the second octet are taken directly from the six least significant bits of the second UTF-8 byte (0x94). Following those six bits are the six bits of the HUFFMAN_EOF 101001, followed by four unset padding bits. The number of raw UTF-8 bits to write depends on the value of the leading octet. If the value is between 0xC2 and 0xDF (inclusive), six bits from the second continuation byte is encoded. If the value is between 0xE0 and 0xEF (inclusive), six bits from the second and third continuation bytes are encoded. If the value is between 0xF0 and 0xF4 (inclusive), six bits from the second, third and fourth continuation bytes are encoded. UTF-8 codepoints that require greater than four bytes to encode cannot be represented. 5. Implementation Considerations When implementing the Stored Header Encoding, it is important to note that the compression state is managed in encounter order. This means that the compressor must delay storing items in the compression state until the order in which frames are to be serialized out to the network has been determined. This is a potential performance bottleneck that needs to be fully tested out to determine its impact. 6. Security Considerations Snell Expires November 21, 2013 [Page 12] Internet-Draft Stored Header Encoding May 2013 TBD 7. References 7.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 7.2. Informational References [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, April 2011. Appendix A. Huffman Tables Request Table ( 0) |11111111|11111111|11111111|0 [25] 1fffffe [25] ( 1) |11111111|11111111|11111111|1 [25] 1ffffff [25] ( 2) |11111111|11111111|11100000 [24] ffffe0 [24] ( 3) |11111111|11111111|11100001 [24] ffffe1 [24] ( 4) |11111111|11111111|11100010 [24] ffffe2 [24] ( 5) |11111111|11111111|11100011 [24] ffffe3 [24] ( 6) |11111111|11111111|11100100 [24] ffffe4 [24] ( 7) |11111111|11111111|11100101 [24] ffffe5 [24] ( 8) |11111111|11111111|11100110 [24] ffffe6 [24] ( 9) |11111111|11111111|11100111 [24] ffffe7 [24] ( 10) |11111111|11111111|11101000 [24] ffffe8 [24] ( 11) |11111111|11111111|11101001 [24] ffffe9 [24] ( 12) |11111111|11111111|11101010 [24] ffffea [24] ( 13) |11111111|11111111|11101011 [24] ffffeb [24] ( 14) |11111111|11111111|11101100 [24] ffffec [24] ( 15) |11111111|11111111|11101101 [24] ffffed [24] ( 16) |11111111|11111111|11101110 [24] ffffee [24] ( 17) |11111111|11111111|11101111 [24] ffffef [24] ( 18) |11111111|11111111|11110000 [24] fffff0 [24] ( 19) |11111111|11111111|11110001 [24] fffff1 [24] ( 20) |11111111|11111111|11110010 [24] fffff2 [24] ( 21) |11111111|11111111|11110011 [24] fffff3 [24] ( 22) |11111111|11111111|11110100 [24] fffff4 [24] ( 23) |11111111|11111111|11110101 [24] fffff5 [24] ( 24) |11111111|11111111|11110110 [24] fffff6 [24] ( 25) |11111111|11111111|11110111 [24] fffff7 [24] ( 26) |11111111|11111111|11111000 [24] fffff8 [24] ( 27) |11111111|11111111|11111001 [24] fffff9 [24] ( 28) |11111111|11111111|11111010 [24] fffffa [24] ( 29) |11111111|11111111|11111011 [24] fffffb [24] Snell Expires November 21, 2013 [Page 13] Internet-Draft Stored Header Encoding May 2013 ( 30) |11111111|11111111|11111100 [24] fffffc [24] ( 31) |11111111|11111111|11111101 [24] fffffd [24] ' ' ( 32) |11111111|0110 [12] ff6 [12] '!' ( 33) |11111111|0111 [12] ff7 [12] '"' ( 34) |11111111|111010 [14] 3ffa [14] '#' ( 35) |11111111|1111100 [15] 7ffc [15] '$' ( 36) |11111111|1111101 [15] 7ffd [15] '%' ( 37) |011000 [6] 18 [6] '&' ( 38) |1010100 [7] 54 [7] ''' ( 39) |11111111|1111110 [15] 7ffe [15] '(' ( 40) |11111111|1000 [12] ff8 [12] ')' ( 41) |11111111|1001 [12] ff9 [12] '*' ( 42) |11111111|1010 [12] ffa [12] '+' ( 43) |11111111|1011 [12] ffb [12] ',' ( 44) |11111011|10 [10] 3ee [10] '-' ( 45) |011001 [6] 19 [6] '.' ( 46) |00010 [5] 2 [5] '/' ( 47) |00011 [5] 3 [5] '0' ( 48) |011010 [6] 1a [6] '1' ( 49) |011011 [6] 1b [6] '2' ( 50) |011100 [6] 1c [6] '3' ( 51) |011101 [6] 1d [6] '4' ( 52) |1010101 [7] 55 [7] '5' ( 53) |1010110 [7] 56 [7] '6' ( 54) |1010111 [7] 57 [7] '7' ( 55) |1011000 [7] 58 [7] '8' ( 56) |1011001 [7] 59 [7] '9' ( 57) |1011010 [7] 5a [7] ':' ( 58) |011110 [6] 1e [6] ';' ( 59) |11111011|11 [10] 3ef [10] '<' ( 60) |11111111|11111111|10 [18] 3fffe [18] '=' ( 61) |011111 [6] 1f [6] '>' ( 62) |11111111|11111110|0 [17] 1fffc [17] '?' ( 63) |11110110|0 [9] 1ec [9] '@' ( 64) |11111111|11100 [13] 1ffc [13] 'A' ( 65) |10111010 [8] ba [8] 'B' ( 66) |11110110|1 [9] 1ed [9] 'C' ( 67) |10111011 [8] bb [8] 'D' ( 68) |10111100 [8] bc [8] 'E' ( 69) |11110111|0 [9] 1ee [9] 'F' ( 70) |10111101 [8] bd [8] 'G' ( 71) |11111100|00 [10] 3f0 [10] 'H' ( 72) |11111100|01 [10] 3f1 [10] 'I' ( 73) |11110111|1 [9] 1ef [9] 'J' ( 74) |11111100|10 [10] 3f2 [10] 'K' ( 75) |11111111|010 [11] 7fa [11] 'L' ( 76) |11111100|11 [10] 3f3 [10] 'M' ( 77) |11111000|0 [9] 1f0 [9] Snell Expires November 21, 2013 [Page 14] Internet-Draft Stored Header Encoding May 2013 'N' ( 78) |11111101|00 [10] 3f4 [10] 'O' ( 79) |11111101|01 [10] 3f5 [10] 'P' ( 80) |11111000|1 [9] 1f1 [9] 'Q' ( 81) |11111101|10 [10] 3f6 [10] 'R' ( 82) |11111001|0 [9] 1f2 [9] 'S' ( 83) |11111001|1 [9] 1f3 [9] 'T' ( 84) |11111010|0 [9] 1f4 [9] 'U' ( 85) |11111101|11 [10] 3f7 [10] 'V' ( 86) |11111110|00 [10] 3f8 [10] 'W' ( 87) |11111110|01 [10] 3f9 [10] 'X' ( 88) |11111110|10 [10] 3fa [10] 'Y' ( 89) |11111110|11 [10] 3fb [10] 'Z' ( 90) |11111111|00 [10] 3fc [10] '[' ( 91) |11111111|111011 [14] 3ffb [14] '\' ( 92) |11111111|11111111|11111110 [24] fffffe [24] ']' ( 93) |11111111|111100 [14] 3ffc [14] '^' ( 94) |11111111|111101 [14] 3ffd [14] '_' ( 95) |1011011 [7] 5b [7] '`' ( 96) |11111111|11111111|110 [19] 7fffe [19] 'a' ( 97) |00100 [5] 4 [5] 'b' ( 98) |1011100 [7] 5c [7] 'c' ( 99) |00101 [5] 5 [5] 'd' (100) |100000 [6] 20 [6] 'e' (101) |0000 [4] 0 [4] 'f' (102) |100001 [6] 21 [6] 'g' (103) |100010 [6] 22 [6] 'h' (104) |100011 [6] 23 [6] 'i' (105) |00110 [5] 6 [5] 'j' (106) |10111110 [8] be [8] 'k' (107) |10111111 [8] bf [8] 'l' (108) |100100 [6] 24 [6] 'm' (109) |100101 [6] 25 [6] 'n' (110) |100110 [6] 26 [6] 'o' (111) |00111 [5] 7 [5] 'p' (112) |01000 [5] 8 [5] 'q' (113) |11111010|1 [9] 1f5 [9] 'r' (114) |01001 [5] 9 [5] 's' (115) |01010 [5] a [5] 't' (116) |01011 [5] b [5] 'u' (117) |100111 [6] 27 [6] 'v' (118) |11000000 [8] c0 [8] 'w' (119) |101000 [6] 28 [6] 'x' (120) |11000001 [8] c1 [8] 'y' (121) |11000010 [8] c2 [8] 'z' (122) |11111011|0 [9] 1f6 [9] '{' (123) |11111111|11111110|1 [17] 1fffd [17] '|' (124) |11111111|1100 [12] ffc [12] '}' (125) |11111111|11111111|0 [17] 1fffe [17] Snell Expires November 21, 2013 [Page 15] Internet-Draft Stored Header Encoding May 2013 '~' (126) |11111111|1101 [12] ffd [12] (127) |101001 [6] 29 [6] (0xC2) |11000011 [8] c3 [8] (0xC3) |11000100 [8] c4 [8] (0xC4) |11000101 [8] c5 [8] (0xC5) |11000110 [8] c6 [8] (0xC6) |11000111 [8] c7 [8] (0xC7) |11001000 [8] c8 [8] (0xC8) |11001001 [8] c9 [8] (0xC9) |11001010 [8] ca [8] (0xCA) |11001011 [8] cb [8] (0xCB) |11001100 [8] cc [8] (0xCC) |11001101 [8] cd [8] (0xCD) |11001110 [8] ce [8] (0xCE) |11001111 [8] cf [8] (0xCF) |11010000 [8] d0 [8] (0xD0) |11010001 [8] d1 [8] (0xD1) |11010010 [8] d2 [8] (0xD2) |11010011 [8] d3 [8] (0xD3) |11010100 [8] d4 [8] (0xD4) |11010101 [8] d5 [8] (0xD5) |11010110 [8] d6 [8] (0xD6) |11010111 [8] d7 [8] (0xD7) |11011000 [8] d8 [8] (0xD8) |11011001 [8] d9 [8] (0xD9) |11011010 [8] da [8] (0xDA) |11011011 [8] db [8] (0xDB) |11011100 [8] dc [8] (0xDC) |11011101 [8] dd [8] (0xDD) |11011110 [8] de [8] (0xDE) |11011111 [8] df [8] (0xDF) |11100000 [8] e0 [8] (0xE0) |11100001 [8] e1 [8] (0xE1) |11100010 [8] e2 [8] (0xE2) |11100011 [8] e3 [8] (0xE3) |11100100 [8] e4 [8] (0xE4) |11100101 [8] e5 [8] (0xE5) |11100110 [8] e6 [8] (0xE6) |11100111 [8] e7 [8] (0xE7) |11101000 [8] e8 [8] (0xE8) |11101001 [8] e9 [8] (0xE9) |11101010 [8] ea [8] (0xEA) |11101011 [8] eb [8] (0xEB) |11101100 [8] ec [8] (0xEC) |11101101 [8] ed [8] (0xED) |11101110 [8] ee [8] (0xEE) |11101111 [8] ef [8] (0xEF) |11110000 [8] f0 [8] Snell Expires November 21, 2013 [Page 16] Internet-Draft Stored Header Encoding May 2013 (0xF0) |11110001 [8] f1 [8] (0xF1) |11110010 [8] f2 [8] (0xF2) |11110011 [8] f3 [8] (0xF3) |11110100 [8] f4 [8] (0xF4) |11110101 [8] f5 [8] Response Table: ( 0) |11111111|11111111|11111111|0 [25] 1fffffe [25] ( 1) |11111111|11111111|11111111|1 [25] 1ffffff [25] ( 2) |11111111|11111111|11100000 [24] ffffe0 [24] ( 3) |11111111|11111111|11100001 [24] ffffe1 [24] ( 4) |11111111|11111111|11100010 [24] ffffe2 [24] ( 5) |11111111|11111111|11100011 [24] ffffe3 [24] ( 6) |11111111|11111111|11100100 [24] ffffe4 [24] ( 7) |11111111|11111111|11100101 [24] ffffe5 [24] ( 8) |11111111|11111111|11100110 [24] ffffe6 [24] ( 9) |11111111|11111111|11100111 [24] ffffe7 [24] ( 10) |11111111|11111111|11101000 [24] ffffe8 [24] ( 11) |11111111|11111111|11101001 [24] ffffe9 [24] ( 12) |11111111|11111111|11101010 [24] ffffea [24] ( 13) |11111111|11111111|11101011 [24] ffffeb [24] ( 14) |11111111|11111111|11101100 [24] ffffec [24] ( 15) |11111111|11111111|11101101 [24] ffffed [24] ( 16) |11111111|11111111|11101110 [24] ffffee [24] ( 17) |11111111|11111111|11101111 [24] ffffef [24] ( 18) |11111111|11111111|11110000 [24] fffff0 [24] ( 19) |11111111|11111111|11110001 [24] fffff1 [24] ( 20) |11111111|11111111|11110010 [24] fffff2 [24] ( 21) |11111111|11111111|11110011 [24] fffff3 [24] ( 22) |11111111|11111111|11110100 [24] fffff4 [24] ( 23) |11111111|11111111|11110101 [24] fffff5 [24] ( 24) |11111111|11111111|11110110 [24] fffff6 [24] ( 25) |11111111|11111111|11110111 [24] fffff7 [24] ( 26) |11111111|11111111|11111000 [24] fffff8 [24] ( 27) |11111111|11111111|11111001 [24] fffff9 [24] ( 28) |11111111|11111111|11111010 [24] fffffa [24] ( 29) |11111111|11111111|11111011 [24] fffffb [24] ( 30) |11111111|11111111|11111100 [24] fffffc [24] ( 31) |11111111|11111111|11111101 [24] fffffd [24] ' ' ( 32) |11111111|0110 [12] ff6 [12] '!' ( 33) |11111111|0111 [12] ff7 [12] '"' ( 34) |11111111|111010 [14] 3ffa [14] '#' ( 35) |11111111|1111100 [15] 7ffc [15] '$' ( 36) |11111111|1111101 [15] 7ffd [15] '%' ( 37) |011000 [6] 18 [6] '&' ( 38) |1010100 [7] 54 [7] Snell Expires November 21, 2013 [Page 17] Internet-Draft Stored Header Encoding May 2013 ''' ( 39) |11111111|1111110 [15] 7ffe [15] '(' ( 40) |11111111|1000 [12] ff8 [12] ')' ( 41) |11111111|1001 [12] ff9 [12] '*' ( 42) |11111111|1010 [12] ffa [12] '+' ( 43) |11111111|1011 [12] ffb [12] ',' ( 44) |11111011|10 [10] 3ee [10] '-' ( 45) |011001 [6] 19 [6] '.' ( 46) |00010 [5] 2 [5] '/' ( 47) |00011 [5] 3 [5] '0' ( 48) |011010 [6] 1a [6] '1' ( 49) |011011 [6] 1b [6] '2' ( 50) |011100 [6] 1c [6] '3' ( 51) |011101 [6] 1d [6] '4' ( 52) |1010101 [7] 55 [7] '5' ( 53) |1010110 [7] 56 [7] '6' ( 54) |1010111 [7] 57 [7] '7' ( 55) |1011000 [7] 58 [7] '8' ( 56) |1011001 [7] 59 [7] '9' ( 57) |1011010 [7] 5a [7] ':' ( 58) |011110 [6] 1e [6] ';' ( 59) |11111011|11 [10] 3ef [10] '<' ( 60) |11111111|11111111|10 [18] 3fffe [18] '=' ( 61) |011111 [6] 1f [6] '>' ( 62) |11111111|11111110|0 [17] 1fffc [17] '?' ( 63) |11110110|0 [9] 1ec [9] '@' ( 64) |11111111|11100 [13] 1ffc [13] 'A' ( 65) |10111010 [8] ba [8] 'B' ( 66) |11110110|1 [9] 1ed [9] 'C' ( 67) |10111011 [8] bb [8] 'D' ( 68) |10111100 [8] bc [8] 'E' ( 69) |11110111|0 [9] 1ee [9] 'F' ( 70) |10111101 [8] bd [8] 'G' ( 71) |11111100|00 [10] 3f0 [10] 'H' ( 72) |11111100|01 [10] 3f1 [10] 'I' ( 73) |11110111|1 [9] 1ef [9] 'J' ( 74) |11111100|10 [10] 3f2 [10] 'K' ( 75) |11111111|010 [11] 7fa [11] 'L' ( 76) |11111100|11 [10] 3f3 [10] 'M' ( 77) |11111000|0 [9] 1f0 [9] 'N' ( 78) |11111101|00 [10] 3f4 [10] 'O' ( 79) |11111101|01 [10] 3f5 [10] 'P' ( 80) |11111000|1 [9] 1f1 [9] 'Q' ( 81) |11111101|10 [10] 3f6 [10] 'R' ( 82) |11111001|0 [9] 1f2 [9] 'S' ( 83) |11111001|1 [9] 1f3 [9] 'T' ( 84) |11111010|0 [9] 1f4 [9] 'U' ( 85) |11111101|11 [10] 3f7 [10] 'V' ( 86) |11111110|00 [10] 3f8 [10] Snell Expires November 21, 2013 [Page 18] Internet-Draft Stored Header Encoding May 2013 'W' ( 87) |11111110|01 [10] 3f9 [10] 'X' ( 88) |11111110|10 [10] 3fa [10] 'Y' ( 89) |11111110|11 [10] 3fb [10] 'Z' ( 90) |11111111|00 [10] 3fc [10] '[' ( 91) |11111111|111011 [14] 3ffb [14] '\' ( 92) |11111111|11111111|11111110 [24] fffffe [24] ']' ( 93) |11111111|111100 [14] 3ffc [14] '^' ( 94) |11111111|111101 [14] 3ffd [14] '_' ( 95) |1011011 [7] 5b [7] '`' ( 96) |11111111|11111111|110 [19] 7fffe [19] 'a' ( 97) |00100 [5] 4 [5] 'b' ( 98) |1011100 [7] 5c [7] 'c' ( 99) |00101 [5] 5 [5] 'd' (100) |100000 [6] 20 [6] 'e' (101) |0000 [4] 0 [4] 'f' (102) |100001 [6] 21 [6] 'g' (103) |100010 [6] 22 [6] 'h' (104) |100011 [6] 23 [6] 'i' (105) |00110 [5] 6 [5] 'j' (106) |10111110 [8] be [8] 'k' (107) |10111111 [8] bf [8] 'l' (108) |100100 [6] 24 [6] 'm' (109) |100101 [6] 25 [6] 'n' (110) |100110 [6] 26 [6] 'o' (111) |00111 [5] 7 [5] 'p' (112) |01000 [5] 8 [5] 'q' (113) |11111010|1 [9] 1f5 [9] 'r' (114) |01001 [5] 9 [5] 's' (115) |01010 [5] a [5] 't' (116) |01011 [5] b [5] 'u' (117) |100111 [6] 27 [6] 'v' (118) |11000000 [8] c0 [8] 'w' (119) |101000 [6] 28 [6] 'x' (120) |11000001 [8] c1 [8] 'y' (121) |11000010 [8] c2 [8] 'z' (122) |11111011|0 [9] 1f6 [9] '{' (123) |11111111|11111110|1 [17] 1fffd [17] '|' (124) |11111111|1100 [12] ffc [12] '}' (125) |11111111|11111111|0 [17] 1fffe [17] '~' (126) |11111111|1101 [12] ffd [12] (127) |101001 [6] 29 [6] (0xC2) |11000011 [8] c3 [8] (0xC3) |11000100 [8] c4 [8] (0xC4) |11000101 [8] c5 [8] (0xC5) |11000110 [8] c6 [8] (0xC6) |11000111 [8] c7 [8] (0xC7) |11001000 [8] c8 [8] (0xC8) |11001001 [8] c9 [8] Snell Expires November 21, 2013 [Page 19] Internet-Draft Stored Header Encoding May 2013 (0xC9) |11001010 [8] ca [8] (0xCA) |11001011 [8] cb [8] (0xCB) |11001100 [8] cc [8] (0xCC) |11001101 [8] cd [8] (0xCD) |11001110 [8] ce [8] (0xCE) |11001111 [8] cf [8] (0xCF) |11010000 [8] d0 [8] (0xD0) |11010001 [8] d1 [8] (0xD1) |11010010 [8] d2 [8] (0xD2) |11010011 [8] d3 [8] (0xD3) |11010100 [8] d4 [8] (0xD4) |11010101 [8] d5 [8] (0xD5) |11010110 [8] d6 [8] (0xD6) |11010111 [8] d7 [8] (0xD7) |11011000 [8] d8 [8] (0xD8) |11011001 [8] d9 [8] (0xD9) |11011010 [8] da [8] (0xDA) |11011011 [8] db [8] (0xDB) |11011100 [8] dc [8] (0xDC) |11011101 [8] dd [8] (0xDD) |11011110 [8] de [8] (0xDE) |11011111 [8] df [8] (0xDF) |11100000 [8] e0 [8] (0xE0) |11100001 [8] e1 [8] (0xE1) |11100010 [8] e2 [8] (0xE2) |11100011 [8] e3 [8] (0xE3) |11100100 [8] e4 [8] (0xE4) |11100101 [8] e5 [8] (0xE5) |11100110 [8] e6 [8] (0xE6) |11100111 [8] e7 [8] (0xE7) |11101000 [8] e8 [8] (0xE8) |11101001 [8] e9 [8] (0xE9) |11101010 [8] ea [8] (0xEA) |11101011 [8] eb [8] (0xEB) |11101100 [8] ec [8] (0xEC) |11101101 [8] ed [8] (0xED) |11101110 [8] ee [8] (0xEE) |11101111 [8] ef [8] (0xEF) |11110000 [8] f0 [8] (0xF0) |11110001 [8] f1 [8] (0xF1) |11110010 [8] f2 [8] (0xF2) |11110011 [8] f3 [8] (0xF3) |11110100 [8] f4 [8] (0xF4) |11110101 [8] f5 [8] Appendix B. Static Storage Cache Snell Expires November 21, 2013 [Page 20] Internet-Draft Stored Header Encoding May 2013 0x80 "date" = NIL 0x81 ":scheme" = "https" 0x82 ":scheme" = "http" 0x83 ":scheme" = "ftp" 0x84 ":method" = "get" 0x85 ":method" = "post" 0x86 ":method" = "put" 0x87 ":method" = "delete" 0x88 ":method" = "options" 0x89 ":method" = "patch" 0x8A ":method" = "connect" 0x8B ":path" = "/" 0x8C ":host" = NIL 0x8D "cookie" = NIL 0x8E ":status" = 100 0x8F ":status" = 101 0x90 ":status" = 102 0x91 ":status" = 200 0x92 ":status" = 201 0x93 ":status" = 202 0x94 ":status" = 203 0x95 ":status" = 204 0x96 ":status" = 205 0x97 ":status" = 206 0x98 ":status" = 207 0x99 ":status" = 208 0x9A ":status" = 300 0x9B ":status" = 301 0x9C ":status" = 302 0x9D ":status" = 303 0x9E ":status" = 304 0x9F ":status" = 305 0xA0 ":status" = 307 0xA1 ":status" = 308 0xA2 ":status" = 400 0xA3 ":status" = 401 0xA4 ":status" = 402 0xA5 ":status" = 403 0xA6 ":status" = 404 0xA7 ":status" = 405 0xA8 ":status" = 406 0xA9 ":status" = 407 0xAA ":status" = 408 0xAB ":status" = 409 0xAC ":status" = 410 0xAD ":status" = 411 0xAE ":status" = 412 0xAF ":status" = 413 Snell Expires November 21, 2013 [Page 21] Internet-Draft Stored Header Encoding May 2013 0xB0 ":status" = 414 0xB1 ":status" = 415 0xB2 ":status" = 416 0xB3 ":status" = 417 0xB4 ":status" = 500 0xB5 ":status" = 501 0xB6 ":status" = 502 0xB7 ":status" = 503 0xB8 ":status" = 504 0xB9 ":status" = 505 0xBA ":status-text" = "OK" 0xBB ":version" = "1.1" 0xBC "accept" = NIL 0xBD "accept-charset" = NIL 0xBE "accept-encoding" = NIL 0xBF "accept-language" = NIL 0xC0 "accept-ranges" = NIL 0xC1 "allow" = NIL 0xC2 "authorization" = NIL 0xC3 "cache-control" = NIL 0xC4 "content-base" = NIL 0xC5 "content-encoding" = NIL 0xC6 "content-length" = NIL 0xC7 "content-location" = NIL 0xC8 "content-md5" = NIL 0xC9 "content-range" = NIL 0xCA "content-type" = NIL 0xCB "content-disposition" = NIL 0xCC "content-language" = NIL 0xCD "etag" = NIL 0xCE "expect" = NIL 0xCF "expires" = NIL 0xD0 "from" = NIL 0xD1 "if-match" = NIL 0xD2 "if-modified-since" = NIL 0xD3 "if-none-match" = NIL 0xD4 "if-range" = NIL 0xD5 "if-unmodified-since" = NIL 0xD6 "last-modified" = NIL 0xD7 "location" = NIL 0xD8 "max-forwards" = NIL 0xD9 "origin" = NIL 0xDA "pragma" = NIL 0xDB "proxy-authenticate" = NIL 0xDC "proxy-authorization" = NIL 0xDD "range" = NIL 0xDE "referer" = NIL 0xDF "retry-after" = NIL Snell Expires November 21, 2013 [Page 22] Internet-Draft Stored Header Encoding May 2013 0xE0 "server" = NIL 0xE1 "set-cookie" = NIL 0xE2 "status" = NIL 0xE3 "te" = NIL 0xE4 "trailer" = NIL 0xE5 "transfer-encoding" = NIL 0xE6 "upgrade" = NIL 0xE7 "user-agent" = NIL 0xE8 "vary" = NIL 0xE9 "via" = NIL 0xEA "warning" = NIL 0xEB "www-authenticate" = NIL 0xEC "access-control-allow-origin" = NIL 0xED "get-dictionary" = NIL 0xEE "p3p" = NIL 0xEF "link" = NIL 0xF0 "prefer" = NIL 0xF1 "preference-applied" = NIL 0xF2 "accept-patch" = NIL 0xF3 NIL 0xF4 NIL 0xF5 NIL 0xF6 NIL 0xF7 NIL 0xF8 NIL 0xF9 NIL 0xFA NIL 0xFB NIL 0xFC NIL 0xFD NIL 0xFE NIL 0xFF NIL Appendix C. Updated Standard Header Definitions In order to properly deal with the backwards compatibility concerns for HTTP/1, there are several important rules for use of Typed Codecs in HTTP headers: o All header fields MUST be explicitly defined to use the new header types. All existing HTTP/1 header fields, then, will continue to be represented as ISO-8859-1 Text unless their standard definitions are updated. The HTTP/2 specification would update the definition of specific known header fields (e.g. content- length, date, if-modified-since, etc). Snell Expires November 21, 2013 [Page 23] Internet-Draft Stored Header Encoding May 2013 o Extension header fields that use the typed codecs will have specific normative transformations to ISO-8859-1 defined. * UTF-8 Text will be converted to ISO-8859-1 with extended characters pct-encoded * Numbers will be converted to their ASCII equivalent values. * Date Times will be converted to their HTTP-Date equivalent values. * Binary fields will be Base64-encoded. o There will be no normative transformation from ISO-8859-1 values into the typed codecs. Implementations are free to apply transformation where those impls determine it is appropriate, but it will be perfectly legal for an implementation to pass a text value through even if it is known that a given header type has a typed codec equivalent (for instance, Content-Length may come through as a number or a text value, either will be valid). This means that when translating from HTTP/1 -> HTTP/2, receiving implementations need to be prepared to handle either value form. A Note of warning: Individual header fields MAY be defined such that they can be represented using multiple types. Numeric fields, for instance, can be represented using either the uvarint encoding or using the equivalent sequence of ASCII numbers. Implementers will need to be capable of supporting each of the possible variations. Designers of header field definitions need to be aware of the additional complexity and possible issues that allowing for such alternatives can introduce for implementers. Based on an initial survey of header fields currently defined by the HTTPbis specification documents, the following header field definitions can be updated to make use of the new types +-----------------------+--------------+----------------------------+ | Field | Type | Description | +-----------------------+--------------+----------------------------+ | content-length | Numeric or | Can be represented as | | | Text | either an unsigned, | | | | variable-length integer or | | | | a sequence of ASCII | | | | numbers. | | date | Timestamp or | Can be represented as | | | Text | either a uvarint encoded | | | | timestamp or as text | | | | (HTTP-date). | Snell Expires November 21, 2013 [Page 24] Internet-Draft Stored Header Encoding May 2013 | max-forwards | Numeric or | Can be represented as | | | Text | either an unsigned, | | | | variable-length integer or | | | | a sequence of ASCII | | | | numbers. | | retry-after | Timestamp, | Can be represented as | | | Numeric or | either a uvarint encoded | | | Text | timestamp, an unsigned, | | | | variable-length integer, | | | | or the text equivalents of | | | | either (HTTP-date or | | | | sequence of ASCII numbers) | | if-modified-since | Timestamp or | Can be represented as | | | Text | either a uvarint encoded | | | | timestamp or as text | | | | (HTTP-date). | | if-unmodified-since | Timestamp or | Can be represented as | | | Text | either a uvarint encoded | | | | timestamp or as text | | | | (HTTP-date). | | last-modified | Timestamp or | Can be represented as | | | Text | either a uvarint encoded | | | | timestamp or as text | | | | (HTTP-date). | | age | Numeric or | Can be represented as | | | Text | either an unsigned, | | | | variable-length integer or | | | | a sequence of ASCII | | | | numbers. | | expires | Timestamp or | Can be represented as | | | Text | either a uvarint encoded | | | | timestamp or as text | | | | (HTTP-date). | +-----------------------+--------------+----------------------------+ Appendix D. State Management Alternatives In the current design, dynamic cache storage slots are assigned by the decompressor in "encounter order". While this is effective, it requires that all compressor and decompressor implementations utilize the exact same algorithm for assigning storage slots, precluding any implementation from experimenting with more efficient algorithms. An alternative approach is to allow the compressor to assign the slots and communicate the assigned slots with each header. This would require a single additional byte per header instance in the Literal and Cloned Header Groups. Snell Expires November 21, 2013 [Page 25] Internet-Draft Stored Header Encoding May 2013 The serialization syntax for Cloned and Literal Header Groups would change to: ; Cloned-Index Header Group cloned-index-header-group = cloned-index-header-prefix 1*32(cache-index cache-index value) ; Literal Header Group literal-header-group = literal-header-prefix 1*32(cache-index name value) The decompressor would used the specified storage index locations to store header values rather than relying on the encounter order, increasing the general efficiency of the algorithm with a minimal impact on transmission size. On the downside, this puts the compressor in greater control over the decompressor-maintained state because the decompressor would not know how long it needs to hold on to stored items. This opens the decompressor up to possible abuse from malicious compressors. The current least-recently-written cache approach allows the decompressor to reliably drop items as space fills without fear of falling out of sync with the compressor. A second alternative is to utilize separate stored "header groups" the way the current Delta Compression proposal does. Each of these represents a separate storage bucket where headers are stored persistently by the compressor and decompressor. The advantage of such buckets is that it simplifies and optimizes serialization of frames by the compressor. However, it incurs the cost of a potentially significant increase in managed state on the decompressor (rather than a single bucket of stored items with max capacity N, there are multiple buckets of stored items with max capacity N/number of buckets). Appendix E. Alternative Timestamp encodings This specification currently uses the number of milliseconds from the UNIX Epoch to represent timestamps. This 64-bit number is encoded using the same uvarint encoding as Numeric fields. This means that the timestamp is encoded using a variable width that, right now (for about the next 100 years or so), will encode in six bytes, then seven bytes for the reasonable future. One possible alternative approach we can take is similar to NTP's handling of Era's. We can take the current timestamp and generate an Era value, with a maximum of 255 (0xFF). This is used as a Snell Expires November 21, 2013 [Page 26] Internet-Draft Stored Header Encoding May 2013 multiplier for the timestamp value. The two values are calculated using the following formula: m = 4294967296000 now = milliseconds since UNIX Epoch era = now / m ts = now % m The allowable values for "era" would be capped at 255. This value is encoded as a single byte, followed by a uvarint encoding of ts. This ensures that the timestamp will never be encoded using more than 7-bytes total, though it may be encoded in as few as two bytes on extremely rare occasions (specifically, immediately following each era rollover). Wire Syntax: era = %x00-FF ts = uvarint date-time = era ts To convert back to the Epoch, the formula is equally simple: now = era * m + ts The largest date we can encode using this format is "36812-02-20T00:36:15.999Z". Dates prior to the epoch cannot be represented. The significant drawback with this approach is that current dates would encode in 7-bytes until the next era rollover, which will occur at approximately 2106-02-07T06:28:15.999Z (give or take a few leap seconds thrown in here and there). Note: The byte lengths assume we want millisecond precision. If we opted to keep the second precision currently in HTTP/1, then this alternative encoding ensures that our timestamps never exceed six- bytes in length. Appendix F. Alternative uvarint encodings The uvarint encoding currently specified by this specification is certainly not the only possible option we can use. I chose it simply because it is drop dead simple to implement. There are quite a few other approaches we can take, each of which can be used as drop-in Snell Expires November 21, 2013 [Page 27] Internet-Draft Stored Header Encoding May 2013 replacements for the current approach. Below are just a couple alternatives. There are plenty others. We just need to pick the one that we feel will work the best. It ought to be noted that while each of these schemes vary in details such as endianess, specific wire-format, etc, each will typically encode the same numbers using the same number of bytes with variations of only a single byte only in the edge cases. Processing time for each is also equivalent when dealing with any number less or equal to 64-bits in length. This means that the choice is largely a matter of style than substance. F.1. Option 1: With this option, leading bits are used to indicate the total number of bytes used to encode the number value, with no fixed upper limit. Values strictly less than 128 are encoded using a single byte. 0 xxxxxxx 10 xxxxxx OCTET 110 xxxxx 2OCTET 1110 xxxx 3OCTET 11110 xxx 4OCTET 111110 xx 5OCTET 1111110 x 6OCTET 11111110 7OCTET 11111111 0xxxxxxx 7OCTET 11111111 10xxxxxx 8OCTET ... The number of leading 1 bits specify the number of additional bytes used to serialize the value. A single 0 bit is used to mark the end of this prefix, the remaining bits are used to encode the minimum bits necessary to encode the value (with appropriate leading 0 bits to ensure proper byte-alignment). For instance, the integer value 500, which is represented in binary as 00000001 11110100, can be encoded using two bytes, 10000001 11110100 The integer value 9770098, which is represented in binary as 10010101 00010100 01110010, can be encoded using four bytes: 11100000 10010101 00010100 01110010 F.2. Option 2: Snell Expires November 21, 2013 [Page 28] Internet-Draft Stored Header Encoding May 2013 This option is generally identical to the previous with the exception of being capped at a maximum of nine encoded octets total. Rather than growing indefinitely, the encoded value must never require more then eight continuation bytes to encode. Because of this restriction, there is no need for a trailing 0-bit spilling over past the first leading byte. 0 xxxxxxx 10 xxxxxx OCTET 110 xxxxx 2OCTET 1110 xxxx 3OCTET 11110 xxx 4OCTET 111110 xx 5OCTET 1111110 x 6OCTET 11111110 7OCTET 11111111 8OCTET ... The number of leading 1 bits specify the number of additional bytes used to serialize the value. If the number of bytes required is less than 8, a single 0 bit is used to mark the end of this prefix, the remaining bits are used to encode the minimum bits necessary to encode the value (with appropriate leading 0 bits to ensure proper byte-alignment). For instance, the integer value 500, which is represented in binary as 00000001 11110100, can be encoded using two bytes, 10000001 11110100 The integer value 9770098, which is represented in binary as 10010101 00010100 01110010, can be encoded using four bytes: 11100000 10010101 00010100 01110010 This format is not capable of encoding any number requiring more than 64-bits. Author's Address James M Snell Email: jasnell@gmail.com Snell Expires November 21, 2013 [Page 29]