Hypertext Transfer Protocol (HTTP) over QUIC
draft-ietf-quic-http-01

Abstract

The QUIC transport protocol has several features that are desirable in a transport for HTTP, such as stream multiplexing, per-stream flow control, and low-latency connection establishment. This document describes a mapping of HTTP semantics over QUIC. Specifically, this document identifies HTTP/2 features that are subsumed by QUIC, and describes how the other features can be implemented atop QUIC.

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.

Working Group information can be found at https://github.com/quicwg; source code and issues list for this draft can be found at https://github.com/quicwg/base-drafts/labels/http.

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 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 July 18, 2017.

Copyright Notice

Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction
• 1.1. Notational Conventions
• 2. QUIC Advertisement
• 2.1. QUIC Version Hints
• 3. Connection Establishment
• 3.1. Draft Version Identification
• 4. Stream Mapping and Usage
• 4.1. Stream 3: Connection Control Stream
• 4.2. HTTP Message Exchanges
• 4.2.1. Header Compression
• 4.2.2. The CONNECT Method
• 4.3. Stream Priorities
• 4.4. Flow Control
• 4.5. Server Push
• 5. HTTP Framing Layer
• 5.1. Frame Layout
• 5.2. Frame Definitions
• 5.2.1. DATA
• 5.2.2. HEADERS
• 5.2.3. PRIORITY
• 5.2.4. RST_STREAM
• 5.2.5. SETTINGS
• 5.2.6. PUSH_PROMISE
• 5.2.7. PING
• 5.2.8. GOAWAY frame
• 5.2.9. WINDOW_UPDATE frame
• 5.2.10. CONTINUATION frame
• 5.2.11. SETTINGS_ACK Frame
• 6. Error Handling
• 6.1. HTTP-Defined QUIC Error Codes
• 6.2. Mapping HTTP/2 Error Codes
• 7. Security Considerations
• 8. IANA Considerations
• 8.1. Registration of HTTP/QUIC Identification String
• 8.2. Registration of Version Hint Alt-Svc Parameter
• 8.3. Existing Frame Types
• 8.4. New Frame Types
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Appendix A. Contributors
• Appendix B. Change Log
• B.1. Since draft-ietf-quic-http-00:
• B.2. Since draft-shade-quic-http2-mapping-00:
• Author's Address

1. Introduction

The QUIC transport protocol has several features that are desirable in a transport for HTTP, such as stream multiplexing, per-stream flow control, and low-latency connection establishment. This document describes a mapping of HTTP semantics over QUIC, drawing heavily on the existing TCP mapping, HTTP/2. Specifically, this document identifies HTTP/2 features that are subsumed by QUIC, and describes how the other features can be implemented atop QUIC.

QUIC is described in [QUIC-TRANSPORT]. For a full description of HTTP/2, see [RFC7540].

1.1. Notational Conventions

The words “MUST”, “MUST NOT”, “SHOULD”, and “MAY” are used in this document. It’s not shouting; when they are capitalized, they have the special meaning defined in [RFC2119].

2. QUIC Advertisement

A server advertises that it can speak HTTP/QUIC via the Alt-Svc ([RFC7838]) HTTP response header (or the semantically equivalent Alt-Svc HTTP/2 Extension Frame Type), using the ALPN token defined in Section 3.

Thus, a server could indicate in an HTTP/1.1 or HTTP/2 response that HTTP/QUIC was available on UDP port 443 by including the following header in any response:

Alt-Svc: hq=":443"

2.1. QUIC Version Hints

This document defines the “v” parameter for Alt-Svc, which is used to provide version-negotiation hints to HTTP/QUIC clients. Syntax:

v = version
version = DQUOTE ( "c" version-string / "x" version-number ) DQUOTE
version-string = token; percent-encoded QUIC version
version-number = 1*8 HEXDIG; hex-encoded QUIC version

When multiple versions are supported, the “v” parameter MAY be repeated multiple times in a single Alt-Svc entry. For example, if a server supported both version “Q034” and version 0x00000001, it would specify the following header:

Alt-Svc: hq=":443";v="x1";v="cQ034"

Where multiple versions are listed, the order of the values reflects the server’s preference (with the first value being the most preferred version).

QUIC versions are four-octet sequences with no additional constraints on format. Versions containing octets not allowed in tokens ([RFC7230], Section 3.2.6) MUST be encoded using the hexidecimal representation. Versions containing only octets allowed in tokens MAY be encoded using either representation.

On receipt of an Alt-Svc header indicating QUIC support, a client MAY attempt to establish a QUIC connection on the indicated port and, if successful, send HTTP requests using the mapping described in this document. Servers SHOULD list only versions which they support, but MAY omit supported versions for any reason.

Connectivity problems (e.g. firewall blocking UDP) may result in QUIC connection establishment failure, in which case the client should gracefully fall back to HTTP/2.

3. Connection Establishment

HTTP/QUIC connections are established as described in [QUIC-TRANSPORT]. During connection establishment, HTTP/QUIC support is indicated by selecting the ALPN token “hq” in the crypto handshake.

While connection-level options pertaining to the core QUIC protocol are set in the initial crypto handshake, HTTP-specific settings are conveyed in the SETTINGS frame. After the QUIC connection is established, a SETTINGS frame (Section 5.2.5) MUST be sent as the initial frame of the HTTP control stream (StreamID 3, see Section 4).

3.1. Draft Version Identification

• RFC Editor’s Note: Please remove this section prior to publication of a final version of this document.

Only implementations of the final, published RFC can identify themselves as “hq”. Until such an RFC exists, implementations MUST NOT identify themselves using these strings.

Implementations of draft versions of the protocol MUST add the string “-“ and the corresponding draft number to the identifier. For example, draft-ietf-quic-http-01 is identified using the string “hq-01”.

Non-compatible experiments that are based on these draft versions MUST append the string “-“ and an experiment name to the identifier. For example, an experimental implementation based on draft-ietf-quic-http-09 which reserves an extra stream for unsolicited transmission of 1980s pop music might identify itself as “hq-09-rickroll”. Note that any label MUST conform to the “token” syntax defined in Section 3.2.6 of [RFC7230]. Experimenters are encouraged to coordinate their experiments on the quic@ietf.org mailing list.

4. Stream Mapping and Usage

A QUIC stream provides reliable in-order delivery of bytes, but makes no guarantees about order of delivery with regard to bytes on other streams. On the wire, data is framed into QUIC STREAM frames, but this framing is invisible to the HTTP framing layer. A QUIC receiver buffers and orders received STREAM frames, exposing the data contained within as a reliable byte stream to the application.

QUIC reserves Stream 1 for crypto operations (the handshake, crypto config updates). Stream 3 is reserved for sending and receiving HTTP control frames, and is analogous to HTTP/2’s Stream 0.

When HTTP headers and data are sent over QUIC, the QUIC layer handles most of the stream management. An HTTP request/response consumes a pair of streams: This means that the client’s first request occurs on QUIC streams 5 and 7, the second on stream 9 and 11, and so on. The server’s first push consumes streams 2 and 4. This amounts to the second least-significant bit differentiating the two streams in a request.

The lower-numbered stream is called the message control stream and carries frames related to the request/response, including HEADERS. All request control streams are exempt from connection-level flow control. The higher-numbered stream is the data stream and carries the request/response body with no additional framing. Note that a request or response without a body will cause this stream to be half-closed in the corresponding direction without transferring data.

Pairs of streams must be utilized sequentially, with no gaps. The data stream MUST be reserved with the QUIC implementation when the message control stream is opened or reserved, and MUST be closed after transferring the body, or else closed immediately after sending the request headers if there is no body.

HTTP does not need to do any separate multiplexing when using QUIC - data sent over a QUIC stream always maps to a particular HTTP transaction. Requests and responses are considered complete when the corresponding QUIC streams are closed in the appropriate direction.

4.1. Stream 3: Connection Control Stream

Since most connection-level concerns from HTTP/2 will be managed by QUIC, the primary use of Stream 3 will be for SETTINGS and PRIORITY frames. Stream 3 is exempt from connection-level flow-control.

4.2. HTTP Message Exchanges

A client sends an HTTP request on a new pair of QUIC streams. A server sends an HTTP response on the same streams as the request.

An HTTP message (request or response) consists of:

1. for a response only, zero or more header blocks (a sequence of HEADERS frames with End Header Block set on the last) on the control stream containing the message headers of informational (1xx) HTTP responses (see [RFC7230], Section 3.2 and [RFC7231], Section 6.2),
2. one header block on the control stream containing the message headers (see [RFC7230], Section 3.2),
3. the payload body (see [RFC7230], Section 3.3), sent on the data stream,
4. optionally, one header block on the control stream containing the trailer-part, if present (see [RFC7230], Section 4.1.2).

The data stream MUST be half-closed immediately after the transfer of the body. If the message does not contain a body, the corresponding data stream MUST still be half-closed without transferring any data. The “chunked” transfer encoding defined in Section 4.1 of [RFC7230] MUST NOT be used.

Trailing header fields are carried in a header block following the body. Such a header block is a sequence of HEADERS frames with End Header Block set on the last frame. Header blocks after the first but before the end of the stream are invalid. These MUST be decoded to maintain HPACK decoder state, but the resulting output MUST be discarded.

An HTTP request/response exchange fully consumes a pair of streams. After sending a request, a client closes the streams for sending; after sending a response, the server closes its streams for sending and the QUIC streams are fully closed.

A server can send a complete response prior to the client sending an entire request if the response does not depend on any portion of the request that has not been sent and received. When this is true, a server MAY request that the client abort transmission of a request without error by sending a RST_STREAM with an error code of NO_ERROR after sending a complete response and closing its stream. Clients MUST NOT discard responses as a result of receiving such a RST_STREAM, though clients can always discard responses at their discretion for other reasons.

4.2.1. Header Compression

HTTP/QUIC uses HPACK header compression as described in [RFC7541]. HPACK was designed for HTTP/2 with the assumption of in- order delivery such as that provided by TCP. A sequence of encoded header blocks must arrive (and be decoded) at an endpoint in the same order in which they were encoded. This ensures that the dynamic state at the two endpoints remains in sync.

QUIC streams provide in-order delivery of data sent on those streams, but there are no guarantees about order of delivery between streams. To achieve in-order delivery of HEADERS frames in QUIC, the HPACK-bearing frames contain a counter which can be used to ensure in-order processing. Data (request/response bodies) which arrive out of order are buffered until the corresponding HEADERS arrive.

This does introduce head-of-line blocking: if the packet containing HEADERS for stream N is lost or reordered then the HEADERS for stream N+4 cannot be processed until it has been retransmitted successfully, even though the HEADERS for stream N+4 may have arrived.

DISCUSS:

Keep HPACK with HOLB? Redesign HPACK to be order-invariant? How much do we need to retain compatibility with HTTP/2’s HPACK?

4.2.2. The CONNECT Method

The pseudo-method CONNECT ([RFC7231], Section 4.3.6) is primarily used with HTTP proxies to establish a TLS session with an origin server for the purposes of interacting with “https” resources. In HTTP/1.x, CONNECT is used to convert an entire HTTP connection into a tunnel to a remote host. In HTTP/2, the CONNECT method is used to establish a tunnel over a single HTTP/2 stream to a remote host for similar purposes.

A CONNECT request in HTTP/QUIC functions in the same manner as in HTTP/2. The request MUST be formatted as described in [RFC7540], Section 8.3. A CONNECT request that does not conform to these restrictions is malformed. The message data stream MUST NOT be closed at the end of the request.

A proxy that supports CONNECT establishes a TCP connection ([RFC0793]) to the server identified in the “:authority” pseudo-header field. Once this connection is successfully established, the proxy sends a HEADERS frame containing a 2xx series status code to the client, as defined in [RFC7231], Section 4.3.6, on the message control stream.

All QUIC STREAM frames on the message data stream correspond to data sent on the TCP connection. Any QUIC STREAM frame sent by the client is transmitted by the proxy to the TCP server; data received from the TCP server is written to the data stream by the proxy. Note that the size and number of TCP segments is not guaranteed to map predictably to the size and number of QUIC STREAM frames.

The TCP connection can be closed by either peer. When the client half-closes the data stream, the proxy will set the FIN bit on its connection to the TCP server. When the proxy receives a packet with the FIN bit set, it will half-close the corresponding data stream. TCP connections which remain half-closed in a single direction are not invalid, but are often handled poorly by servers, so clients SHOULD NOT half-close connections on which they are still expecting data.

A TCP connection error is signaled with RST_STREAM. A proxy treats any error in the TCP connection, which includes receiving a TCP segment with the RST bit set, as a stream error of type HTTP_CONNECT_ERROR (Section 6.1). Correspondingly, a proxy MUST send a TCP segment with the RST bit set if it detects an error with the stream or the QUIC connection.

4.3. Stream Priorities

HTTP/QUIC uses the priority scheme described in [RFC7540] Section 5.3. In this priority scheme, a given stream can be designated as dependent upon another stream, which expresses the preference that the latter stream (the “parent” stream) be allocated resources before the former stream (the “dependent” stream). Taken together, the dependencies across all streams in a connection form a dependency tree. The structure of the dependency tree changes as HEADERS and PRIORITY frames add, remove, or change the dependency links between streams.

Implicit in this scheme is the notion of in-order delivery of priority changes (i.e., dependency tree mutations): since operations on the dependency tree such as reparenting a subtree are not commutative, both sender and receiver must apply them in the same order to ensure that both sides have a consistent view of the stream dependency tree. HTTP/2 specifies priority assignments in PRIORITY frames and (optionally) in HEADERS frames. To achieve in-order delivery of priority changes in HTTP/QUIC, PRIORITY frames are sent on the connection control stream and the PRIORITY section is removed from the HEADERS frame. The semantics of the Stream Dependency, Weight, E flag, and (for HEADERS frames) PRIORITY flag are the same as in HTTP/2.

For consistency’s sake, all PRIORITY frames MUST refer to the message control stream of the dependent request, not the data stream.

4.4. Flow Control

QUIC provides stream and connection level flow control, similar in principle to HTTP/2’s flow control but with some implementation differences. As flow control is handled by QUIC, the HTTP mapping need not concern itself with maintaining flow control state. The HTTP mapping MUST NOT send WINDOW_UPDATE frames at the HTTP level.

4.5. Server Push

HTTP/QUIC supports server push as described in [RFC7540]. During connection establishment, the client indicates whether it is willing to receive server pushes via the SETTINGS_ENABLE_PUSH setting in the SETTINGS frame (see Section 3), which defaults to 1 (true).

As with server push for HTTP/2, the server initiates a server push by sending a PUSH_PROMISE frame containing the StreamID of the stream to be pushed, as well as request header fields attributed to the request. The PUSH_PROMISE frame is sent on the control stream of the associated (client-initiated) request, while the Promised Stream ID field specifies the Stream ID of the control stream for the server-initiated request.

The server push response is conveyed in the same way as a non-server-push response, with response headers and (if present) trailers carried by HEADERS frames sent on the control stream, and response body (if any) sent via the corresponding data stream.

5. HTTP Framing Layer

Many framing concepts from HTTP/2 can be elided away on QUIC, because the transport deals with them. Because frames are already on a stream, they can omit the stream number. Because frames do not block multiplexing (QUIC’s multiplexing occurs below this layer), the support for variable-maximum-length packets can be removed. Because stream termination is handled by QUIC, an END_STREAM flag is not required.

Frames are used only on the connection (stream 3) and message (streams 5, 9, etc.) control streams. Other streams carry data payload and are not framed at the HTTP layer.

Frame payloads are largely drawn from [RFC7540]. However, QUIC includes some features (e.g. flow control) which are also present in HTTP/2. In these cases, the HTTP mapping need not re-implement them. As a result, some frame types are not required when using QUIC. Where an HTTP/2-defined frame is no longer used, the frame ID is reserved in order to maximize portability between HTTP/2 and HTTP/QUIC implementations. However, equivalent frames between the two mappings are not necessarily identical.

This section describes HTTP framing in QUIC and highlights differences from HTTP/2 framing.

5.1. Frame Layout

All frames have the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Length (16)         |     Type (8)  |   Flags (8)   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Frame Payload (*)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

HTTP/QUIC frame format

5.2. Frame Definitions

5.2.1. DATA

DATA frames do not exist. Frame type 0x0 is reserved.

5.2.2. HEADERS

The HEADERS frame (type=0x1) is used to carry part of a header set, compressed using HPACK [RFC7541]. Because HEADERS frames from different streams will be delivered out-of-order and priority-changes are not commutative, the PRIORITY region of HEADERS is not supported. A separate PRIORITY frame MUST be used.

Padding MUST NOT be used. The flags defined are:

Reserved (0x1):

Reserved for HTTP/2 compatibility.

End Header Block (0x4):

This frame concludes a header block.

Reserved (0x8):

Reserved for HTTP/2 compatibility.

Reserved (0x20):

Reserved for HTTP/2 compatibility.

A HEADERS frame with the Reserved bits set MUST be treated as a connection error of type HTTP_MALFORMED_HEADERS.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Sequence? (16)          |    Header Block Fragment (*)...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

HEADERS frame payload

The HEADERS frame payload has the following fields:

Sequence Number:

Present only on the first frame of a header block sequence. This MUST be set to zero on the first header block sequence, and incremented on each header block.

The next frame on the same stream after a HEADERS frame without the EHB flag set MUST be another HEADERS frame. A receiver MUST treat the receipt of any other type of frame as a stream error of type HTTP_INTERRUPTED_HEADERS. (Note that QUIC can intersperse data from other streams between frames, or even during transmission of frames, so multiplexing is not blocked by this requirement.)

A full header block is contained in a sequence of zero or more HEADERS frames without EHB set, followed by a HEADERS frame with EHB set.

On receipt, header blocks (HEADERS, PUSH_PROMISE) MUST be processed by the HPACK decoder in sequence. If a block is missing, all subsequent HPACK frames MUST be held until it arrives, or the connection terminated.

5.2.3. PRIORITY

The PRIORITY (type=0x02) frame specifies the sender-advised priority of a stream and is substantially different from [RFC7540]. In order to support ordering, it MUST be sent only on the connection control stream. The format has been modified to accommodate not being sent on-stream and the larger stream ID space of QUIC.

The flags defined are:

E (0x01):

Indicates that the stream dependency is exclusive (see [RFC7540] Section 5.3).

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Prioritized Stream (32)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Dependent Stream (32)                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Weight (8)  |
   +-+-+-+-+-+-+-+-+

HEADERS frame payload

The HEADERS frame payload has the following fields:

Prioritized Stream:

A 32-bit stream identifier for the message control stream whose priority is being updated.

Stream Dependency:

A 32-bit stream identifier for the stream that this stream depends on (see Section 4.3 and {!RFC7540}} Section 5.3).

Weight:

An unsigned 8-bit integer representing a priority weight for the stream (see [RFC7540] Section 5.3). Add one to the value to obtain a weight between 1 and 256.

A PRIORITY frame MUST have a payload length of nine octets. A PRIORITY frame of any other length MUST be treated as a connection error of type HTTP_MALFORMED_PRIORITY.

5.2.4. RST_STREAM

RST_STREAM frames do not exist, since QUIC provides stream lifecycle management. Frame type 0x3 is reserved.

5.2.5. SETTINGS

The SETTINGS frame (type=0x4) conveys configuration parameters that affect how endpoints communicate, such as preferences and constraints on peer behavior, and is substantially different from [RFC7540]. Individually, a SETTINGS parameter can also be referred to as a “setting”.

SETTINGS parameters are not negotiated; they describe characteristics of the sending peer, which can be used by the receiving peer. However, a negotiation can be implied by the use of SETTINGS – a peer uses SETTINGS to advertise a set of supported values. The recipient can then choose which entries from this list are also acceptable and proceed with the value it has chosen. (This choice could be announced in a field of an extension frame, or in its own value in SETTINGS.)

Different values for the same parameter can be advertised by each peer. For example, a client might permit a very large HPACK state table while a server chooses to use a small one to conserve memory.

A SETTINGS frame MAY be sent at any time by either endpoint over the lifetime of the connection.

Each parameter in a SETTINGS frame replaces any existing value for that parameter. Parameters are processed in the order in which they appear, and a receiver of a SETTINGS frame does not need to maintain any state other than the current value of its parameters. Therefore, the value of a SETTINGS parameter is the last value that is seen by a receiver.

The SETTINGS frame defines the following flag:

REQUEST_ACK (0x1):

When set, bit 0 indicates that this frame contains values which the sender wants to know were understood and applied. For more information, see Section 5.2.5.3.

The payload of a SETTINGS frame consists of zero or more parameters, each consisting of an unsigned 16-bit setting identifier and a length-prefixed binary value.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Identifier (16)        |B|        Length (15)          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Contents (?)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Promised Stream ID (32)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Sequence? (16)          |         Header Block (*)    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Highest Local Stream (32)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Highest Remote Stream (32)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Unrecognized Identifiers (*)               ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+