QUIC C. Krasic Internet-Draft Netflix Intended status: Standards Track M. Bishop Expires: July 27, 2019 Akamai Technologies A. Frindell, Ed. Facebook January 23, 2019 QPACK: Header Compression for HTTP over QUIC draft-ietf-quic-qpack-06 Abstract This specification defines QPACK, a compression format for efficiently representing HTTP header fields, to be used in HTTP/3. This is a variation of HPACK header compression that seeks to reduce head-of-line blocking. Note to Readers Discussion of this draft takes place on the QUIC working group mailing list (quic@ietf.org), which is archived at https://mailarchive.ietf.org/arch/search/?email_list=quic [1]. Working Group information can be found at https://github.com/quicwg [2]; source code and issues list for this draft can be found at https://github.com/quicwg/base-drafts/labels/-qpack [3]. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://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 July 27, 2019. Krasic, et al. Expires July 27, 2019 [Page 1] Internet-Draft QPACK January 2019 Copyright Notice Copyright (c) 2019 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 (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Conventions and Definitions . . . . . . . . . . . . . . . 4 1.2. Notational Conventions . . . . . . . . . . . . . . . . . 5 2. Compression Process Overview . . . . . . . . . . . . . . . . 5 2.1. Encoder . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1.1. Reference Tracking . . . . . . . . . . . . . . . . . 6 2.1.2. Blocked Dynamic Table Insertions . . . . . . . . . . 6 2.1.3. Avoiding Head-of-Line Blocking . . . . . . . . . . . 7 2.1.4. Known Received Count . . . . . . . . . . . . . . . . 8 2.2. Decoder . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2.1. State Synchronization . . . . . . . . . . . . . . . . 8 2.2.2. Blocked Decoding . . . . . . . . . . . . . . . . . . 9 3. Header Tables . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1. Static Table . . . . . . . . . . . . . . . . . . . . . . 9 3.2. Dynamic Table . . . . . . . . . . . . . . . . . . . . . . 10 3.2.1. Dynamic Table Size . . . . . . . . . . . . . . . . . 10 3.2.2. Dynamic Table Capacity and Eviction . . . . . . . . . 10 3.2.3. Maximum Dynamic Table Capacity . . . . . . . . . . . 11 3.2.4. Initial Dynamic Table Capacity . . . . . . . . . . . 11 3.2.5. Absolute Indexing . . . . . . . . . . . . . . . . . . 11 3.2.6. Relative Indexing . . . . . . . . . . . . . . . . . . 11 3.2.7. Post-Base Indexing . . . . . . . . . . . . . . . . . 12 3.2.8. Invalid References . . . . . . . . . . . . . . . . . 13 4. Wire Format . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.1. Primitives . . . . . . . . . . . . . . . . . . . . . . . 13 4.1.1. Prefixed Integers . . . . . . . . . . . . . . . . . . 13 4.1.2. String Literals . . . . . . . . . . . . . . . . . . . 13 4.2. Stream Types . . . . . . . . . . . . . . . . . . . . . . 14 4.3. Encoder Stream . . . . . . . . . . . . . . . . . . . . . 14 4.3.1. Insert With Name Reference . . . . . . . . . . . . . 15 4.3.2. Insert Without Name Reference . . . . . . . . . . . . 15 Krasic, et al. Expires July 27, 2019 [Page 2] Internet-Draft QPACK January 2019 4.3.3. Duplicate . . . . . . . . . . . . . . . . . . . . . . 16 4.3.4. Set Dynamic Table Capacity . . . . . . . . . . . . . 16 4.4. Decoder Stream . . . . . . . . . . . . . . . . . . . . . 17 4.4.1. Insert Count Increment . . . . . . . . . . . . . . . 17 4.4.2. Header Acknowledgement . . . . . . . . . . . . . . . 18 4.4.3. Stream Cancellation . . . . . . . . . . . . . . . . . 18 4.5. Request and Push Streams . . . . . . . . . . . . . . . . 19 4.5.1. Header Block Prefix . . . . . . . . . . . . . . . . . 19 4.5.2. Indexed Header Field . . . . . . . . . . . . . . . . 21 4.5.3. Indexed Header Field With Post-Base Index . . . . . . 22 4.5.4. Literal Header Field With Name Reference . . . . . . 22 4.5.5. Literal Header Field With Post-Base Name Reference . 23 4.5.6. Literal Header Field Without Name Reference . . . . . 23 5. Configuration . . . . . . . . . . . . . . . . . . . . . . . . 24 6. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 24 7. Security Considerations . . . . . . . . . . . . . . . . . . . 25 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25 8.1. Settings Registration . . . . . . . . . . . . . . . . . . 25 8.2. Stream Type Registration . . . . . . . . . . . . . . . . 25 8.3. Error Code Registration . . . . . . . . . . . . . . . . . 25 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 26 9.1. Normative References . . . . . . . . . . . . . . . . . . 26 9.2. Informative References . . . . . . . . . . . . . . . . . 27 9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Appendix A. Static Table . . . . . . . . . . . . . . . . . . . . 27 Appendix B. Sample One Pass Encoding Algorithm . . . . . . . . . 32 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 34 C.1. Since draft-ietf-quic-qpack-05 . . . . . . . . . . . . . 34 C.2. Since draft-ietf-quic-qpack-04 . . . . . . . . . . . . . 34 C.3. Since draft-ietf-quic-qpack-03 . . . . . . . . . . . . . 34 C.4. Since draft-ietf-quic-qpack-02 . . . . . . . . . . . . . 34 C.5. Since draft-ietf-quic-qpack-01 . . . . . . . . . . . . . 35 C.6. Since draft-ietf-quic-qpack-00 . . . . . . . . . . . . . 35 C.7. Since draft-ietf-quic-qcram-00 . . . . . . . . . . . . . 35 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 35 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36 1. Introduction The QUIC transport protocol was designed from the outset to support HTTP semantics, and its design subsumes many of the features of HTTP/2. HTTP/2 uses HPACK ([RFC7541]) for header compression, but QUIC's stream multiplexing comes into some conflict with HPACK. A key goal of the design of QUIC is to improve stream multiplexing relative to HTTP/2 by reducing head-of-line blocking. If HPACK were used for HTTP/3, it would induce head-of-line blocking due to built- in assumptions of a total ordering across frames on all streams. Krasic, et al. Expires July 27, 2019 [Page 3] Internet-Draft QPACK January 2019 QUIC is described in [QUIC-TRANSPORT]. The HTTP/3 mapping is described in [HTTP3]. For a full description of HTTP/2, see [RFC7540]. The description of HPACK is [RFC7541]. QPACK reuses core concepts from HPACK, but is redesigned to allow correctness in the presence of out-of-order delivery, with flexibility for implementations to balance between resilience against head-of-line blocking and optimal compression ratio. The design goals are to closely approach the compression ratio of HPACK with substantially less head-of-line blocking under the same loss conditions. 1.1. Conventions and Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. Definitions of terms that are used in this document: Header field: A name-value pair sent as part of an HTTP message. Header list: An ordered collection of header fields associated with an HTTP message. A header list can contain multiple header fields with the same name. It can also contain duplicate header fields. Header block: The compressed representation of a header list. Encoder: An implementation which transforms a header list into a header block. Decoder: An implementation which transforms a header block into a header list. Absolute Index: A unique index for each entry in the dynamic table. Base: A reference point for relative indicies. Dynamic references are made relative to a Base in header blocks. Insert Count: The total number of entries inserted in the dynamic table. QPACK is a name, not an acronym. Krasic, et al. Expires July 27, 2019 [Page 4] Internet-Draft QPACK January 2019 1.2. Notational Conventions Diagrams use the format described in Section 3.1 of [RFC2360], with the following additional conventions: x (A) Indicates that x is A bits long x (A+) Indicates that x uses the prefixed integer encoding defined in Section 5.1 of [RFC7541], beginning with an A-bit prefix. x ... Indicates that x is variable-length and extends to the end of the region. 2. Compression Process Overview Like HPACK, QPACK uses two tables for associating header fields to indices. The static table (see Section 3.1) is predefined and contains common header fields (some of them with an empty value). The dynamic table (see Section 3.2) is built up over the course of the connection and can be used by the encoder to index header fields in the encoded header lists. QPACK instructions appear in three different types of streams: o The encoder uses a unidirectional stream to modify the state of the dynamic table without emitting header fields associated with any particular request. o HEADERS and PUSH_PROMISE frames on request and push streams reference the table state without modifying it. o The decoder sends feedback to the encoder on a unidirectional stream. This feedback enables the encoder to manage dynamic table state. 2.1. Encoder An encoder compresses a header list by emitting either an indexed or a literal representation for each header field in the list. References to the static table and literal representations do not require any dynamic state and never risk head-of-line blocking. References to the dynamic table risk head-of-line blocking if the encoder has not received an acknowledgement indicating the entry is available at the decoder. An encoder MAY insert any entry in the dynamic table it chooses; it is not limited to header fields it is compressing. Krasic, et al. Expires July 27, 2019 [Page 5] Internet-Draft QPACK January 2019 QPACK preserves the ordering of header fields within each header list. An encoder MUST emit header field representations in the order they appear in the input header list. QPACK is designed to contain the more complex state tracking to the encoder, while the decoder is relatively simple. 2.1.1. Reference Tracking An encoder MUST ensure that a header block which references a dynamic table entry is not received by the decoder after the referenced entry has been evicted. Hence the encoder needs to track information about each compressed header block that references the dynamic table until that header block is acknowledged by the decoder. 2.1.2. Blocked Dynamic Table Insertions An encoder MUST NOT insert an entry into the dynamic table (or duplicate an existing entry) if doing so would evict an entry with unacknowledged references. For header blocks that might rely on the newly added entry, the encoder can use a literal representation. To ensure that the encoder is not prevented from adding new entries, the encoder can avoid referencing entries that are close to eviction. Rather than reference such an entry, the encoder can emit a Duplicate instruction (see Section 4.3.3), and reference the duplicate instead. Determining which entries are too close to eviction to reference is an encoder preference. One heuristic is to target a fixed amount of available space in the dynamic table: either unused space or space that can be reclaimed by evicting unreferenced entries. To achieve this, the encoder can maintain a draining index, which is the smallest absolute index in the dynamic table that it will emit a reference for. As new entries are inserted, the encoder increases the draining index to maintain the section of the table that it will not reference. If the encoder does not create new references to entries with an absolute index lower than the draining index, the number of unacknowledged references to those entries will eventually become zero, allowing them to be evicted. Krasic, et al. Expires July 27, 2019 [Page 6] Internet-Draft QPACK January 2019 +----------+---------------------------------+--------+ | Draining | Referenceable | Unused | | Entries | Entries | Space | +----------+---------------------------------+--------+ ^ ^ ^ | | | Dropping Draining Index Insertion Point Point Figure 1: Draining Dynamic Table Entries 2.1.3. Avoiding Head-of-Line Blocking Because QUIC does not guarantee order between data on different streams, a header block might reference an entry in the dynamic table that has not yet been received. Each header block contains a Required Insert Count, the lowest possible value for the Insert Count with which the header block can be decoded. For a header block with no references to the dynamic table, the Required Insert Count is zero. When the Required Insert Count is zero, the frame contains no references to the dynamic table and can always be processed immediately. If the Required Insert Count is greater than the number of dynamic table entries received, the stream is considered "blocked." While blocked, header field data SHOULD remain in the blocked stream's flow control window. A stream becomes unblocked when the Insert Count becomes greater than or equal to the Required Insert Count for all header blocks the decoder has started reading from the stream. If the decoder encounters a header block where the largest Absolute Index used is not equal to the largest value permitted by the Required Insert Count, it MAY treat this as a stream error of type HTTP_QPACK_DECOMPRESSION_FAILED. The SETTINGS_QPACK_BLOCKED_STREAMS setting (see Section 5) specifies an upper bound on the number of streams which can be blocked. An encoder MUST limit the number of streams which could become blocked to the value of SETTINGS_QPACK_BLOCKED_STREAMS at all times. Note that the decoder might not actually become blocked on every stream which risks becoming blocked. If the decoder encounters more blocked streams than it promised to support, it MUST treat this as a stream error of type HTTP_QPACK_DECOMPRESSION_FAILED. Krasic, et al. Expires July 27, 2019 [Page 7] Internet-Draft QPACK January 2019 An encoder can decide whether to risk having a stream become blocked. If permitted by the value of SETTINGS_QPACK_BLOCKED_STREAMS, compression efficiency can often be improved by referencing dynamic table entries that are still in transit, but if there is loss or reordering the stream can become blocked at the decoder. An encoder avoids the risk of blocking by only referencing dynamic table entries which have been acknowledged, but this could mean using literals. Since literals make the header block larger, this can result in the encoder becoming blocked on congestion or flow control limits. 2.1.4. Known Received Count In order to identify which dynamic table entries can be safely used without a stream becoming blocked, the encoder tracks the number of entries received by the decoder. The Known Received Count tracks the total number of acknowledged insertions. When blocking references are permitted, the encoder uses header block acknowledgement to maintain the Known Received Count, as described in Section 4.4.2. To acknowledge dynamic table entries which are not referenced by header blocks, for example because the encoder or the decoder have chosen not to risk blocked streams, the decoder sends an Insert Count Increment instruction (see Section 4.4.1). 2.2. Decoder As in HPACK, the decoder processes header blocks and emits the corresponding header lists. It also processes dynamic table modifications from instructions on the encoder stream. The decoder MUST emit header fields in the order their representations appear in the input header block. 2.2.1. State Synchronization The decoder stream (Section 4.4) signals key events at the decoder that permit the encoder to track the decoder's state. These events are: o Complete processing of a header block o Abandonment of a stream which might have remaining header blocks o Receipt of new dynamic table entries Krasic, et al. Expires July 27, 2019 [Page 8] Internet-Draft QPACK January 2019 Knowledge that a header block with references to the dynamic table has been processed permits the encoder to evict entries to which no unacknowledged references remain, regardless of whether those references were potentially blocking (see Section 2.1.2). When a stream is reset or abandoned, the indication that these header blocks will never be processed serves a similar function; see Section 4.4.3. The decoder chooses when to emit Insert Count Increment instructions (see Section 4.4.1). Emitting an instruction after adding each new dynamic table entry will provide the most timely feedback to the encoder, but could be redundant with other decoder feedback. By delaying an Insert Count Increment instruction, the decoder might be able to coalesce multiple Insert Count Increment instructions, or replace them entirely with Header Acknowledgements (see Section 4.4.2). However, delaying too long may lead to compression inefficiencies if the encoder waits for an entry to be acknowledged before using it. 2.2.2. Blocked Decoding To track blocked streams, the Required Insert Count value for each stream can be used. Whenever the decoder processes a table update, it can begin decoding any blocked streams that now have their dependencies satisfied. 3. Header Tables Unlike in HPACK, entries in the QPACK static and dynamic tables are addressed separately. The following sections describe how entries in each table are addressed. 3.1. Static Table The static table consists of a predefined static list of header fields, each of which has a fixed index over time. Its entries are defined in Appendix A. Note the QPACK static table is indexed from 0, whereas the HPACK static table is indexed from 1. When the decoder encounters an invalid static table index on a request stream or push stream it MUST treat this as a stream error of type "HTTP_QPACK_DECOMPRESSION_FAILED". If this index is received on the encoder stream, this MUST be treated as a connection error of type "HTTP_QPACK_ENCODER_STREAM_ERROR". Krasic, et al. Expires July 27, 2019 [Page 9] Internet-Draft QPACK January 2019 3.2. Dynamic Table The dynamic table consists of a list of header fields maintained in first-in, first-out order. The dynamic table is initially empty. Entries are added by instructions on the encoder stream (see Section 4.3). The dynamic table can contain duplicate entries (i.e., entries with the same name and same value). Therefore, duplicate entries MUST NOT be treated as an error by the decoder. 3.2.1. Dynamic Table Size The size of the dynamic table is the sum of the size of its entries. The size of an entry is the sum of its name's length in bytes (as defined in Section 4.1.2), its value's length in bytes, and 32. The size of an entry is calculated using the length of its name and value without Huffman encoding applied. 3.2.2. Dynamic Table Capacity and Eviction The encoder sets the capacity of the dynamic table, which serves as the upper limit on its size. Before a new entry is added to the dynamic table, entries are evicted from the end of the dynamic table until the size of the dynamic table is less than or equal to (table capacity - size of new entry) or until the table is empty. The encoder MUST NOT evict a dynamic table entry unless it has first been acknowledged by the decoder. If the size of the new entry is less than or equal to the dynamic table capacity, then that entry is added to the table. It is an error if the encoder attempts to add an entry that is larger than the dynamic table capacity; the decoder MUST treat this as a connection error of type "HTTP_QPACK_ENCODER_STREAM_ERROR". A new entry can reference an entry in the dynamic table that will be evicted when adding this new entry into the dynamic table. Implementations are cautioned to avoid deleting the referenced name or value if the referenced entry is evicted from the dynamic table prior to inserting the new entry. Whenever the dynamic table capacity is reduced by the encoder, entries are evicted from the end of the dynamic table until the size of the dynamic table is less than or equal to the new table capacity. This mechanism can be used to completely clear entries from the Krasic, et al. Expires July 27, 2019 [Page 10] Internet-Draft QPACK January 2019 dynamic table by setting a capacity of 0, which can subsequently be restored. 3.2.3. Maximum Dynamic Table Capacity To bound the memory requirements of the decoder, the decoder limits the maximum value the encoder is permitted to set for the dynamic table capacity. In HTTP/3, this limit is determined by the value of SETTINGS_QPACK_MAX_TABLE_CAPACITY sent by the decoder (see Section 5). The encoder MUST not set a dynamic table capacity that exceeds this maximum, but it can choose to use a lower dynamic table capacity (see Section 4.3.4). 3.2.4. Initial Dynamic Table Capacity The initial dynamic table capacity is determined by the corresponding setting when HTTP requests or responses are first permitted to be sent. For clients using 0-RTT data in HTTP/3, the initial table capacity is the remembered value of the setting, even if the server later specifies a larger maximum dynamic table capacity in its SETTINGS frame. For HTTP/3 servers and HTTP/3 clients when 0-RTT is not attempted or is rejected, the initial table capacity is the value of the setting in the peer's SETTINGS frame. 3.2.5. Absolute Indexing Each entry possesses both an absolute index which is fixed for the lifetime of that entry and a relative index which changes based on the context of the reference. The first entry inserted has an absolute index of "0"; indices increase by one with each insertion. 3.2.6. Relative Indexing The relative index begins at zero and increases in the opposite direction from the absolute index. Determining which entry has a relative index of "0" depends on the context of the reference. On the encoder stream, a relative index of "0" always refers to the most recently inserted value in the dynamic table. Note that this means the entry referenced by a given relative index will change while interpreting instructions on the encoder stream. Krasic, et al. Expires July 27, 2019 [Page 11] Internet-Draft QPACK January 2019 +-----+---------------+-------+ | n-1 | ... | d | Absolute Index + - - +---------------+ - - - + | 0 | ... | n-d-1 | Relative Index +-----+---------------+-------+ ^ | | V Insertion Point Dropping Point n = count of entries inserted d = count of entries dropped Example Dynamic Table Indexing - Control Stream Unlike on the encoder stream, relative indices on push and request streams are relative to the Base at the beginning of the header block (see Section 4.5.1). This ensures that references are stable even if the dynamic table is updated while decoding a header block. The Base is encoded as a value relative to the Required Insert Count. The Base identifies which dynamic table entries can be referenced using relative indexing, starting with 0 at the last entry added. Post-Base references are used for entries inserted after base, starting at 0 for the first entry added after the Base, see Section 3.2.7. Required Insert Count Base | | V V +-----+-----+-----+-----+-------+ | n-1 | n-2 | n-3 | ... | d | Absolute Index +-----+-----+ - +-----+ - + | 0 | ... | n-d-3 | Relative Index +-----+-----+-------+ n = count of entries inserted d = count of entries dropped Example Dynamic Table Indexing - Relative Index on Request Stream 3.2.7. Post-Base Indexing A header block on the request stream can reference entries added after the entry identified by the Base. This allows an encoder to process a header block in a single pass and include references to Krasic, et al. Expires July 27, 2019 [Page 12] Internet-Draft QPACK January 2019 entries added while processing this (or other) header blocks. Newly added entries are referenced using Post-Base instructions. Indices for Post-Base instructions increase in the same direction as absolute indices, with the zero value being the first entry inserted after the Base. Base | V +-----+-----+-----+-----+-----+ | n-1 | n-2 | n-3 | ... | d | Absolute Index +-----+-----+-----+-----+-----+ | 1 | 0 | Post-Base Index +-----+-----+ n = count of entries inserted d = count of entries dropped Example Dynamic Table Indexing - Post-Base Index on Request Stream 3.2.8. Invalid References If the decoder encounters a reference on a request or push stream to a dynamic table entry which has already been evicted or which has an absolute index greater than or equal to the declared Required Insert Count (see Section 4.5.1), it MUST treat this as a stream error of type "HTTP_QPACK_DECOMPRESSION_FAILED". If the decoder encounters a reference on the encoder stream to a dynamic table entry which has already been dropped, it MUST treat this as a connection error of type "HTTP_QPACK_ENCODER_STREAM_ERROR". 4. Wire Format 4.1. Primitives 4.1.1. Prefixed Integers The prefixed integer from Section 5.1 of [RFC7541] is used heavily throughout this document. The format from [RFC7541] is used unmodified. QPACK implementations MUST be able to decode integers up to 62 bits long. 4.1.2. String Literals The string literal defined by Section 5.2 of [RFC7541] is also used throughout. This string format includes optional Huffman encoding. Krasic, et al. Expires July 27, 2019 [Page 13] Internet-Draft QPACK January 2019 HPACK defines string literals to begin on a byte boundary. They begin with a single flag (indicating whether the string is Huffman- coded), followed by the Length encoded as a 7-bit prefix integer, and finally Length bytes of data. When Huffman encoding is enabled, the Huffman table from Appendix B of [RFC7541] is used without modification. This document expands the definition of string literals and permits them to begin other than on a byte boundary. An "N-bit prefix string literal" begins with the same Huffman flag, followed by the length encoded as an (N-1)-bit prefix integer. The remainder of the string literal is unmodified. A string literal without a prefix length noted is an 8-bit prefix string literal and follows the definitions in [RFC7541] without modification. 4.2. Stream Types QPACK instructions occur in three locations, each of which uses a separate instruction space: o The encoder stream is a unidirectional stream of type "0x48" (ASCII 'H') which carries table updates from encoder to decoder. o The decoder stream is a unidirectional stream of type "0x68" (ASCII 'h') which carries acknowledgements of table modifications and header processing from decoder to encoder. o Finally, the contents of HEADERS and PUSH_PROMISE frames on request streams and push streams reference the QPACK table state. There MUST be exactly one of each unidirectional stream type in each direction. Receipt of a second instance of either stream type MUST be treated as a connection error of HTTP_WRONG_STREAM_COUNT. These streams MUST NOT be closed. Closure of either unidirectional stream MUST be treated as a connection error of type HTTP_CLOSED_CRITICAL_STREAM. This section describes the instructions which are possible on each stream type. 4.3. Encoder Stream Table updates can add a table entry, possibly using existing entries to avoid transmitting redundant information. The name can be transmitted as a reference to an existing entry in the static or the dynamic table or as a string literal. For entries which already Krasic, et al. Expires July 27, 2019 [Page 14] Internet-Draft QPACK January 2019 exist in the dynamic table, the full entry can also be used by reference, creating a duplicate entry. The contents of the encoder stream are an unframed sequence of the following instructions. 4.3.1. Insert With Name Reference An addition to the header table where the header field name matches the header field name of an entry stored in the static table or the dynamic table starts with the '1' one-bit pattern. The "S" bit indicates whether the reference is to the static (S=1) or dynamic (S=0) table. The 6-bit prefix integer (see Section 5.1 of [RFC7541]) that follows is used to locate the table entry for the header name. When S=1, the number represents the static table index; when S=0, the number is the relative index of the entry in the dynamic table. The header name reference is followed by the header field value represented as a string literal (see Section 5.2 of [RFC7541]). 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 | S | Name Index (6+) | +---+---+-----------------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length bytes) | +-------------------------------+ Insert Header Field -- Indexed Name 4.3.2. Insert Without Name Reference An addition to the header table where both the header field name and the header field value are represented as string literals (see Section 4.1) starts with the '01' two-bit pattern. The name is represented as a 6-bit prefix string literal, while the value is represented as an 8-bit prefix string literal. Krasic, et al. Expires July 27, 2019 [Page 15] Internet-Draft QPACK January 2019 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 1 | H | Name Length (5+) | +---+---+---+-------------------+ | Name String (Length bytes) | +---+---------------------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length bytes) | +-------------------------------+ Insert Header Field -- New Name 4.3.3. Duplicate Duplication of an existing entry in the dynamic table starts with the '000' three-bit pattern. The relative index of the existing entry is represented as an integer with a 5-bit prefix. 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 0 | Index (5+) | +---+---+---+-------------------+ Figure 2: Duplicate The existing entry is re-inserted into the dynamic table without resending either the name or the value. This is useful to mitigate the eviction of older entries which are frequently referenced, both to avoid the need to resend the header and to avoid the entry in the table blocking the ability to insert new headers. 4.3.4. Set Dynamic Table Capacity An encoder informs the decoder of a change to the dynamic table capacity using an instruction which begins with the '001' three-bit pattern. The new dynamic table capacity is represented as an integer with a 5-bit prefix (see Section 5.1 of [RFC7541]). 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 1 | Capacity (5+) | +---+---+---+-------------------+ Figure 3: Set Dynamic Table Capacity The new capacity MUST be lower than or equal to the limit described in Section 3.2.3. In HTTP/3, this limit is the value of the Krasic, et al. Expires July 27, 2019 [Page 16] Internet-Draft QPACK January 2019 SETTINGS_QPACK_MAX_TABLE_CAPACITY parameter (see Section 5) received from the decoder. The decoder MUST treat a new dynamic table capacity value that exceeds this limit as a connection error of type "HTTP_QPACK_ENCODER_STREAM_ERROR". Reducing the dynamic table capacity can cause entries to be evicted (see Section 3.2.2). This MUST NOT cause the eviction of entries with outstanding references (see Section 2.1.1). Changing the capacity of the dynamic table is not acknowledged as this instruction does not insert an entry. 4.4. Decoder Stream The decoder stream carries information used to ensure consistency of the dynamic table. Information is sent from the decoder to the encoder; that is, the server informs the client about the processing of the client's header blocks and table updates, and the client informs the server about the processing of the server's header blocks and table updates. The contents of the decoder stream are an unframed sequence of the following instructions. 4.4.1. Insert Count Increment The Insert Count Increment instruction begins with the '00' two-bit pattern. The instruction specifies the total number of dynamic table inserts and duplications since the last Insert Count Increment or Header Acknowledgement that increased the Known Received Count for the dynamic table (see Section 2.1.4). The Increment field is encoded as a 6-bit prefix integer. The encoder uses this value to determine which table entries might cause a stream to become blocked, as described in Section 2.2.1. 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | Increment (6+) | +---+---+-----------------------+ Figure 4: Insert Count Increment An encoder that receives an Increment field equal to zero or one that increases the Known Received Count beyond what the encoder has sent MUST treat this as a connection error of type "HTTP_QPACK_DECODER_STREAM_ERROR". Krasic, et al. Expires July 27, 2019 [Page 17] Internet-Draft QPACK January 2019 4.4.2. Header Acknowledgement After processing a header block whose declared Required Insert Count is not zero, the decoder emits a Header Acknowledgement instruction on the decoder stream. The instruction begins with the '1' one-bit pattern and includes the request stream's stream ID, encoded as a 7-bit prefix integer. It is used by the peer's encoder to know when it is safe to evict an entry, and possibly update the Known Received Count. 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 | Stream ID (7+) | +---+---------------------------+ Figure 5: Header Acknowledgement The same Stream ID can be identified multiple times, as multiple header blocks can be sent on a single stream in the case of intermediate responses, trailers, and pushed requests. Since header frames on each stream are received and processed in order, this gives the encoder precise feedback on which header blocks within a stream have been fully processed. If an encoder receives a Header Acknowledgement instruction referring to a stream on which every header block with a non-zero Required Insert Count has already been acknowledged, that MUST be treated as a connection error of type "HTTP_QPACK_DECODER_STREAM_ERROR". When blocking references are permitted, the encoder uses acknowledgement of header blocks to update the Known Received Count. If a header block was potentially blocking, the acknowledgement implies that the decoder has received all dynamic table state necessary to process the header block. If the Required Insert Count of an acknowledged header block was greater than the encoder's current Known Received Count, the block's Required Insert Count becomes the new Known Received Count. 4.4.3. Stream Cancellation The instruction begins with the '01' two-bit pattern. The instruction includes the stream ID of the affected stream - a request or push stream - encoded as a 6-bit prefix integer. Krasic, et al. Expires July 27, 2019 [Page 18] Internet-Draft QPACK January 2019 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 1 | Stream ID (6+) | +---+---+-----------------------+ Figure 6: Stream Cancellation A stream that is reset might have multiple outstanding header blocks with dynamic table references. When an endpoint receives a stream reset before the end of a stream, it generates a Stream Cancellation instruction on the decoder stream. Similarly, when an endpoint abandons reading of a stream it needs to signal this using the Stream Cancellation instruction. This signals to the encoder that all references to the dynamic table on that stream are no longer outstanding. A decoder with a maximum dynamic table capacity equal to zero (see Section 3.2.3) MAY omit sending Stream Cancellations, because the encoder cannot have any dynamic table references. An encoder cannot infer from this instruction that any updates to the dynamic table have been received. 4.5. Request and Push Streams HEADERS and PUSH_PROMISE frames on request and push streams reference the dynamic table in a particular state without modifying it. Frames on these streams emit the headers for an HTTP request or response. 4.5.1. Header Block Prefix Each header block is prefixed with two integers. The Required Insert Count is encoded as an integer with an 8-bit prefix after the encoding described in Section 4.5.1.1). The Base is encoded as sign- and-modulus integer, using a single sign bit and a value with a 7-bit prefix (see Section 4.5.1.2). These two values are followed by instructions for compressed headers. The entire block is expected to be framed by the using protocol. 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | Required Insert Count (8+) | +---+---------------------------+ | S | Delta Base (7+) | +---+---------------------------+ | Compressed Headers ... +-------------------------------+ Figure 7: Frame Payload Krasic, et al. Expires July 27, 2019 [Page 19] Internet-Draft QPACK January 2019 4.5.1.1. Required Insert Count Required Insert Count identifies the state of the dynamic table needed to process the header block. Blocking decoders use the Required Insert Count to determine when it is safe to process the rest of the block. If no references are made to the dynamic table, a value of 0 is encoded. Alternatively, where the Required Insert Count is greater than zero, the encoder transforms it as follows before encoding: EncodedInsertCount = (ReqInsertCount mod (2 * MaxEntries)) + 1 Here "MaxEntries" is the maximum number of entries that the dynamic table can have. The smallest entry has empty name and value strings and has the size of 32. Hence "MaxEntries" is calculated as MaxEntries = floor( MaxTableCapacity / 32 ) "MaxTableCapacity" is the maximum capacity of the dynamic table as specified by the decoder (see Section 3.2.3). The decoder reconstructs the Required Insert Count using the following algorithm, where TotalNumberOfInserts is the total number of inserts into the decoder's dynamic table: if EncodedInsertCount == 0: ReqInsertCount = 0 else: InsertCount = EncodedInsertCount - 1 CurrentWrapped = TotalNumberOfInserts mod (2 * MaxEntries) if CurrentWrapped >= InsertCount + MaxEntries: # Insert Count wrapped around 1 extra time ReqInsertCount += 2 * MaxEntries else if CurrentWrapped + MaxEntries < InsertCount: # Decoder wrapped around 1 extra time CurrentWrapped += 2 * MaxEntries ReqInsertCount += TotalNumberOfInserts - CurrentWrapped This encoding limits the length of the prefix on long-lived connections. For example, if the dynamic table is 100 bytes, then the Required Insert Count will be encoded modulo 6. If a decoder has received 10 inserts, then an encoded value of 3 indicates that the Required Insert Count is 9 for the header block. Krasic, et al. Expires July 27, 2019 [Page 20] Internet-Draft QPACK January 2019 4.5.1.2. Base The "Base" is used to resolve references in the dynamic table as described in Section 3.2.6. To save space, the Base is encoded relative to the Insert Count using a one-bit sign and the "Delta Base" value. A sign bit of 0 indicates that the Base is greater than or equal to the value of the Insert Count; the value of Delta Base is added to the Insert Count to determine the value of the Base. A sign bit of 1 indicates that the Base is less than the Insert Count. That is: if S == 0: Base = ReqInsertCount + DeltaBase else: Base = ReqInsertCount - DeltaBase - 1 A single-pass encoder determines the Base before encoding a header block. If the encoder inserted entries in the dynamic table while encoding the header block, Required Insert Count will be greater than the Base, so the encoded difference is negative and the sign bit is set to 1. If the header block did not reference the most recent entry in the table and did not insert any new entries, the Base will be greater than the Required Insert Count, so the delta will be positive and the sign bit is set to 0. An encoder that produces table updates before encoding a header block might set Required Insert Count and the Base to the same value. In such case, both the sign bit and the Delta Base will be set to zero. A header block that does not reference the dynamic table can use any value for the Base; setting Delta Base to zero is the most efficient encoding. For example, with an Required Insert Count of 9, a decoder receives a S bit of 1 and a Delta Base of 2. This sets the Base to 6 and enables post-base indexing for three entries. In this example, a regular index of 1 refers to the 5th entry that was added to the table; a post-base index of 1 refers to the 8th entry. 4.5.2. Indexed Header Field An indexed header field representation identifies an entry in either the static table or the dynamic table and causes that header field to be added to the decoded header list, as described in Section 3.2 of [RFC7541]. Krasic, et al. Expires July 27, 2019 [Page 21] Internet-Draft QPACK January 2019 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 | S | Index (6+) | +---+---+-----------------------+ Indexed Header Field If the entry is in the static table, or in the dynamic table with an absolute index less than the Base, this representation starts with the '1' 1-bit pattern, followed by the "S" bit indicating whether the reference is into the static (S=1) or dynamic (S=0) table. Finally, the relative index of the matching header field is represented as an integer with a 6-bit prefix (see Section 5.1 of [RFC7541]). 4.5.3. Indexed Header Field With Post-Base Index If the entry is in the dynamic table with an absolute index greater than or equal to the Base, the representation starts with the '0001' 4-bit pattern, followed by the post-base index (see Section 3.2.7) of the matching header field, represented as an integer with a 4-bit prefix (see Section 5.1 of [RFC7541]). 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 0 | 1 | Index (4+) | +---+---+---+---+---------------+ Indexed Header Field with Post-Base Index 4.5.4. Literal Header Field With Name Reference A literal header field with a name reference represents a header where the header field name matches the header field name of an entry stored in the static table or the dynamic table. If the entry is in the static table, or in the dynamic table with an absolute index less than the Base, this representation starts with the '01' two-bit pattern. If the entry is in the dynamic table with an absolute index greater than or equal to the Base, the representation starts with the '0000' four-bit pattern. The following bit, 'N', indicates whether an intermediary is permitted to add this header to the dynamic header table on subsequent hops. When the 'N' bit is set, the encoded header MUST always be encoded with a literal representation. In particular, when a peer sends a header field that it received represented as a literal header field with the 'N' bit set, it MUST use a literal representation to forward this header field. This bit is intended Krasic, et al. Expires July 27, 2019 [Page 22] Internet-Draft QPACK January 2019 for protecting header field values that are not to be put at risk by compressing them (see Section 7.1 of [RFC7541] for more details). 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 1 | N | S |Name Index (4+)| +---+---+---+---+---------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length bytes) | +-------------------------------+ Literal Header Field With Name Reference For entries in the static table or in the dynamic table with an absolute index less than the Base, the header field name is represented using the relative index of that entry, which is represented as an integer with a 4-bit prefix (see Section 5.1 of [RFC7541]). The "S" bit indicates whether the reference is to the static (S=1) or dynamic (S=0) table. 4.5.5. Literal Header Field With Post-Base Name Reference For entries in the dynamic table with an absolute index greater than or equal to the Base, the header field name is represented using the post-base index of that entry (see Section 3.2.7) encoded as an integer with a 3-bit prefix. 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 0 | 0 | N |NameIdx(3+)| +---+---+---+---+---+-----------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length bytes) | +-------------------------------+ Literal Header Field With Post-Base Name Reference 4.5.6. Literal Header Field Without Name Reference An addition to the header table where both the header field name and the header field value are represented as string literals (see Section 4.1) starts with the '001' three-bit pattern. The fourth bit, 'N', indicates whether an intermediary is permitted to add this header to the dynamic header table on subsequent hops. When the 'N' bit is set, the encoded header MUST always be encoded Krasic, et al. Expires July 27, 2019 [Page 23] Internet-Draft QPACK January 2019 with a literal representation. In particular, when a peer sends a header field that it received represented as a literal header field with the 'N' bit set, it MUST use a literal representation to forward this header field. This bit is intended for protecting header field values that are not to be put at risk by compressing them (see Section 7.1 of [RFC7541] for more details). The name is represented as a 4-bit prefix string literal, while the value is represented as an 8-bit prefix string literal. 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 1 | N | H |NameLen(3+)| +---+---+---+---+---+-----------+ | Name String (Length bytes) | +---+---------------------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length bytes) | +-------------------------------+ Literal Header Field Without Name Reference 5. Configuration QPACK defines two settings which are included in the HTTP/3 SETTINGS frame. SETTINGS_QPACK_MAX_TABLE_CAPACITY (0x1): An integer with a maximum value of 2^30 - 1. The default value is zero bytes. See Section 3.2 for usage. This is the equivalent of the SETTINGS_HEADER_TABLE_SIZE from HTTP/2. SETTINGS_QPACK_BLOCKED_STREAMS (0x7): An integer with a maximum value of 2^16 - 1. The default value is zero. See Section 2.1.3. 6. Error Handling The following error codes are defined for HTTP/3 to indicate failures of QPACK which prevent the stream or connection from continuing: HTTP_QPACK_DECOMPRESSION_FAILED (TBD): The decoder failed to interpret an instruction on a request or push stream and is not able to continue decoding that header block. HTTP_QPACK_ENCODER_STREAM_ERROR (TBD): The decoder failed to interpret an instruction on the encoder stream. Krasic, et al. Expires July 27, 2019 [Page 24] Internet-Draft QPACK January 2019 HTTP_QPACK_DECODER_STREAM_ERROR (TBD): The encoder failed to interpret an instruction on the decoder stream. Upon encountering an error, an implementation MAY elect to treat it as a connection error even if this document prescribes that it MUST be treated as a stream error. 7. Security Considerations TBD. 8. IANA Considerations 8.1. Settings Registration This document creates two new settings in the "HTTP/3 Settings" registry established in [HTTP3]. The entries in the following table are registered by this document. +--------------------------+------+---------------+ | Setting Name | Code | Specification | +--------------------------+------+---------------+ | QPACK_MAX_TABLE_CAPACITY | 0x1 | Section 5 | | | | | | QPACK_BLOCKED_STREAMS | 0x7 | Section 5 | +--------------------------+------+---------------+ 8.2. Stream Type Registration This document creates two new settings in the "HTTP/3 Stream Type" registry established in [HTTP3]. The entries in the following table are registered by this document. +----------------------+------+---------------+--------+ | Stream Type | Code | Specification | Sender | +----------------------+------+---------------+--------+ | QPACK Encoder Stream | 0x48 | Section 4 | Both | | | | | | | QPACK Decoder Stream | 0x68 | Section 4 | Both | +----------------------+------+---------------+--------+ 8.3. Error Code Registration This document establishes the following new error codes in the "HTTP/3 Error Code" registry established in [HTTP3]. Krasic, et al. Expires July 27, 2019 [Page 25] Internet-Draft QPACK January 2019 +------------------------------+------+--------------+--------------+ | Name | Code | Description | Specificatio | | | | | n | +------------------------------+------+--------------+--------------+ | HTTP_QPACK_DECOMPRESSION_FAI | TBD | Decompressio | Section 6 | | LED | | n of a | | | | | header block | | | | | failed | | | | | | | | HTTP_QPACK_ENCODER_STREAM_ER | TBD | Error on the | Section 6 | | ROR | | encoder | | | | | stream | | | | | | | | HTTP_QPACK_DECODER_STREAM_ER | TBD | Error on the | Section 6 | | ROR | | decoder | | | | | stream | | +------------------------------+------+--------------+--------------+ 9. References 9.1. Normative References [HTTP3] Bishop, M., Ed., "Hypertext Transfer Protocol Version 3 (HTTP/3)", draft-ietf-quic-http-18 (work in progress), January 2019. [QUIC-TRANSPORT] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based Multiplexed and Secure Transport", draft-ietf-quic- transport-17 (work in progress), January 2019. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . Krasic, et al. Expires July 27, 2019 [Page 26] Internet-Draft QPACK January 2019 9.2. Informative References [RFC2360] Scott, G., "Guide for Internet Standards Writers", BCP 22, RFC 2360, DOI 10.17487/RFC2360, June 1998, . [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI 10.17487/RFC7540, May 2015, . 9.3. URIs [1] https://mailarchive.ietf.org/arch/search/?email_list=quic [2] https://github.com/quicwg [3] https://github.com/quicwg/base-drafts/labels/-qpack Appendix A. Static Table +------+-----------------------------+------------------------------+ | Inde | Name | Value | | x | | | +------+-----------------------------+------------------------------+ | 0 | :authority | | | | | | | 1 | :path | / | | | | | | 2 | age | 0 | | | | | | 3 | content-disposition | | | | | | | 4 | content-length | 0 | | | | | | 5 | cookie | | | | | | | 6 | date | | | | | | | 7 | etag | | | | | | | 8 | if-modified-since | | | | | | | 9 | if-none-match | | | | | | | 10 | last-modified | | | | | | | 11 | link | | Krasic, et al. Expires July 27, 2019 [Page 27] Internet-Draft QPACK January 2019 | | | | | 12 | location | | | | | | | 13 | referer | | | | | | | 14 | set-cookie | | | | | | | 15 | :method | CONNECT | | | | | | 16 | :method | DELETE | | | | | | 17 | :method | GET | | | | | | 18 | :method | HEAD | | | | | | 19 | :method | OPTIONS | | | | | | 20 | :method | POST | | | | | | 21 | :method | PUT | | | | | | 22 | :scheme | http | | | | | | 23 | :scheme | https | | | | | | 24 | :status | 103 | | | | | | 25 | :status | 200 | | | | | | 26 | :status | 304 | | | | | | 27 | :status | 404 | | | | | | 28 | :status | 503 | | | | | | 29 | accept | */* | | | | | | 30 | accept | application/dns-message | | | | | | 31 | accept-encoding | gzip, deflate, br | | | | | | 32 | accept-ranges | bytes | | | | | | 33 | access-control-allow- | cache-control | | | headers | | | | | | | 34 | access-control-allow- | content-type | | | headers | | Krasic, et al. Expires July 27, 2019 [Page 28] Internet-Draft QPACK January 2019 | | | | | 35 | access-control-allow-origin | * | | | | | | 36 | cache-control | max-age=0 | | | | | | 37 | cache-control | max-age=2592000 | | | | | | 38 | cache-control | max-age=604800 | | | | | | 39 | cache-control | no-cache | | | | | | 40 | cache-control | no-store | | | | | | 41 | cache-control | public, max-age=31536000 | | | | | | 42 | content-encoding | br | | | | | | 43 | content-encoding | gzip | | | | | | 44 | content-type | application/dns-message | | | | | | 45 | content-type | application/javascript | | | | | | 46 | content-type | application/json | | | | | | 47 | content-type | application/x-www-form- | | | | urlencoded | | | | | | 48 | content-type | image/gif | | | | | | 49 | content-type | image/jpeg | | | | | | 50 | content-type | image/png | | | | | | 51 | content-type | text/css | | | | | | 52 | content-type | text/html; charset=utf-8 | | | | | | 53 | content-type | text/plain | | | | | | 54 | content-type | text/plain;charset=utf-8 | | | | | | 55 | range | bytes=0- | | | | | | 56 | strict-transport-security | max-age=31536000 | | | | | | 57 | strict-transport-security | max-age=31536000; | | | | includesubdomains | Krasic, et al. Expires July 27, 2019 [Page 29] Internet-Draft QPACK January 2019 | | | | | 58 | strict-transport-security | max-age=31536000; | | | | includesubdomains; preload | | | | | | 59 | vary | accept-encoding | | | | | | 60 | vary | origin | | | | | | 61 | x-content-type-options | nosniff | | | | | | 62 | x-xss-protection | 1; mode=block | | | | | | 63 | :status | 100 | | | | | | 64 | :status | 204 | | | | | | 65 | :status | 206 | | | | | | 66 | :status | 302 | | | | | | 67 | :status | 400 | | | | | | 68 | :status | 403 | | | | | | 69 | :status | 421 | | | | | | 70 | :status | 425 | | | | | | 71 | :status | 500 | | | | | | 72 | accept-language | | | | | | | 73 | access-control-allow- | FALSE | | | credentials | | | | | | | 74 | access-control-allow- | TRUE | | | credentials | | | | | | | 75 | access-control-allow- | * | | | headers | | | | | | | 76 | access-control-allow- | get | | | methods | | | | | | | 77 | access-control-allow- | get, post, options | | | methods | | | | | | | 78 | access-control-allow- | options | Krasic, et al. Expires July 27, 2019 [Page 30] Internet-Draft QPACK January 2019 | | methods | | | | | | | 79 | access-control-expose- | content-length | | | headers | | | | | | | 80 | access-control-request- | content-type | | | headers | | | | | | | 81 | access-control-request- | get | | | method | | | | | | | 82 | access-control-request- | post | | | method | | | | | | | 83 | alt-svc | clear | | | | | | 84 | authorization | | | | | | | 85 | content-security-policy | script-src 'none'; object- | | | | src 'none'; base-uri 'none' | | | | | | 86 | early-data | 1 | | | | | | 87 | expect-ct | | | | | | | 88 | forwarded | | | | | | | 89 | if-range | | | | | | | 90 | origin | | | | | | | 91 | purpose | prefetch | | | | | | 92 | server | | | | | | | 93 | timing-allow-origin | * | | | | | | 94 | upgrade-insecure-requests | 1 | | | | | | 95 | user-agent | | | | | | | 96 | x-forwarded-for | | | | | | | 97 | x-frame-options | deny | | | | | | 98 | x-frame-options | sameorigin | +------+-----------------------------+------------------------------+ Krasic, et al. Expires July 27, 2019 [Page 31] Internet-Draft QPACK January 2019 Appendix B. Sample One Pass Encoding Algorithm Pseudo-code for single pass encoding, excluding handling of duplicates, non-blocking mode, and reference tracking. Krasic, et al. Expires July 27, 2019 [Page 32] Internet-Draft QPACK January 2019 baseIndex = dynamicTable.baseIndex largestReference = 0 for header in headers: staticIdx = staticTable.getIndex(header) if staticIdx: encodeIndexReference(streamBuffer, staticIdx) continue dynamicIdx = dynamicTable.getIndex(header) if !dynamicIdx: # No matching entry. Either insert+index or encode literal nameIdx = getNameIndex(header) if shouldIndex(header) and dynamicTable.canIndex(header): encodeLiteralWithIncrementalIndex(controlBuffer, nameIdx, header) dynamicTable.add(header) dynamicIdx = dynamicTable.baseIndex if !dynamicIdx: # Couldn't index it, literal if nameIdx <= staticTable.size: encodeLiteral(streamBuffer, nameIndex, header) else: # encode literal, possibly with nameIdx above baseIndex encodeDynamicLiteral(streamBuffer, nameIndex, baseIndex, header) largestReference = max(largestReference, dynamicTable.toAbsolute(nameIdx)) else: # Dynamic index reference assert(dynamicIdx) largestReference = max(largestReference, dynamicIdx) # Encode dynamicIdx, possibly with dynamicIdx above baseIndex encodeDynamicIndexReference(streamBuffer, dynamicIdx, baseIndex) # encode the prefix encodeInteger(prefixBuffer, 0x00, largestReference, 8) if baseIndex >= largestReference: encodeInteger(prefixBuffer, 0, baseIndex - largestReference, 7) else: encodeInteger(prefixBuffer, 0x80, largestReference - baseIndex, 7) return controlBuffer, prefixBuffer + streamBuffer Krasic, et al. Expires July 27, 2019 [Page 33] Internet-Draft QPACK January 2019 Appendix C. Change Log *RFC Editor's Note:* Please remove this section prior to publication of a final version of this document. C.1. Since draft-ietf-quic-qpack-05 o Introduced the terms dynamic table capacity and maximum dynamic table capacity. o Renamed SETTINGS_HEADER_TABLE_SIZE to SETTINGS_QPACK_MAX_TABLE_CAPACITY. C.2. Since draft-ietf-quic-qpack-04 o Changed calculation of Delta Base Index to avoid an illegal value (#2002, #2005) C.3. Since draft-ietf-quic-qpack-03 o Change HTTP settings defaults (#2038) o Substantial editorial reorganization C.4. Since draft-ietf-quic-qpack-02 o Largest Reference encoded modulo MaxEntries (#1763) o New Static Table (#1355) o Table Size Update with Insert Count=0 is a connection error (#1762) o Stream Cancellations are optional when SETTINGS_HEADER_TABLE_SIZE=0 (#1761) o Implementations must handle 62 bit integers (#1760) o Different error types for each QPACK stream, other changes to error handling (#1726) o Preserve header field order (#1725) o Initial table size is the maximum permitted when table is first usable (#1642) Krasic, et al. Expires July 27, 2019 [Page 34] Internet-Draft QPACK January 2019 C.5. Since draft-ietf-quic-qpack-01 o Only header blocks that reference the dynamic table are acknowledged (#1603, #1605) C.6. Since draft-ietf-quic-qpack-00 o Renumbered instructions for consistency (#1471, #1472) o Decoder is allowed to validate largest reference (#1404, #1469) o Header block acknowledgments also acknowledge the associated largest reference (#1370, #1400) o Added an acknowledgment for unread streams (#1371, #1400) o Removed framing from encoder stream (#1361,#1467) o Control streams use typed unidirectional streams rather than fixed stream IDs (#910,#1359) C.7. Since draft-ietf-quic-qcram-00 o Separate instruction sets for table updates and header blocks (#1235, #1142, #1141) o Reworked indexing scheme (#1176, #1145, #1136, #1130, #1125, #1314) o Added mechanisms that support one-pass encoding (#1138, #1320) o Added a setting to control the number of blocked decoders (#238, #1140, #1143) o Moved table updates and acknowledgments to dedicated streams (#1121, #1122, #1238) Acknowledgments This draft draws heavily on the text of [RFC7541]. The indirect input of those authors is gratefully acknowledged, as well as ideas from: o Ryan Hamilton o Patrick McManus o Kazuho Oku Krasic, et al. Expires July 27, 2019 [Page 35] Internet-Draft QPACK January 2019 o Biren Roy o Ian Swett o Dmitri Tikhonov Buck's contribution was supported by Google during his employment there. A substantial portion of Mike's contribution was supported by Microsoft during his employment there. Authors' Addresses Charles 'Buck' Krasic Netflix Email: ckrasic@netflix.com Mike Bishop Akamai Technologies Email: mbishop@evequefou.be Alan Frindell (editor) Facebook Email: afrind@fb.com Krasic, et al. Expires July 27, 2019 [Page 36]