Hypertext Transfer Protocol Version 2 (HTTP/2)
draft-ietf-httpbis-http2bis-02
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9113.
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| Authors | Martin Thomson , Cory Benfield | ||
| Last updated | 2021-06-02 (Latest revision 2021-02-22) | ||
| Replaces | draft-thomson-httpbis-http2bis | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
TSVART Last Call review
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-06)
by Joerg Ott
Ready w/nits
GENART Last Call review
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by Dan Romascanu
Ready w/issues
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 9113 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
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| Send notices to | (None) |
draft-ietf-httpbis-http2bis-02
5.4.2. Stream Error Handling A stream error is an error related to a specific stream that does not affect processing of other streams. An endpoint that detects a stream error sends a RST_STREAM frame (Section 6.4) that contains the stream identifier of the stream where the error occurred. The RST_STREAM frame includes an error code that indicates the type of error. A RST_STREAM is the last frame that an endpoint can send on a stream. The peer that sends the RST_STREAM frame MUST be prepared to receive any frames that were sent or enqueued for sending by the remote peer. These frames can be ignored, except where they modify connection state (such as the state maintained for field section compression (Section 4.3) or flow control). Normally, an endpoint SHOULD NOT send more than one RST_STREAM frame for any stream. However, an endpoint MAY send additional RST_STREAM frames if it receives frames on a closed stream after more than a round-trip time. This behavior is permitted to deal with misbehaving implementations. To avoid looping, an endpoint MUST NOT send a RST_STREAM in response to a RST_STREAM frame. 5.4.3. Connection Termination If the TCP connection is closed or reset while streams remain in "open" or "half-closed" state, then the affected streams cannot be automatically retried (see Section 8.1.4 for details). 5.5. Extending HTTP/2 HTTP/2 permits extension of the protocol. Within the limitations described in this section, protocol extensions can be used to provide additional services or alter any aspect of the protocol. Extensions are effective only within the scope of a single HTTP/2 connection. This applies to the protocol elements defined in this document. This does not affect the existing options for extending HTTP, such as defining new methods, status codes, or fields (see Section 16 of [HTTP]). Thomson & Benfield Expires 4 December 2021 [Page 25] Internet-Draft HTTP/2 June 2021 Extensions are permitted to use new frame types (Section 4.1), new settings (Section 6.5.2), or new error codes (Section 7). Registries are established for managing these extension points: frame types (Section 11.2), settings (Section 11.3), and error codes (Section 11.4). Implementations MUST ignore unknown or unsupported values in all extensible protocol elements. Implementations MUST discard frames that have unknown or unsupported types. This means that any of these extension points can be safely used by extensions without prior arrangement or negotiation. However, extension frames that appear in the middle of a field block (Section 4.3) are not permitted; these MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Extensions SHOULD avoiding changing protocol elements defined in this document or elements for which no extension mechanism is defined. This includes changes to the layout of frames, additions or changes to the way that frames are composed into HTTP messages (Section 8), the definition of pseudo-header fields, or changes to any protocol element that a compliant endpoint might treat as a connection error (Section 5.4.1). An extension that changes existing elements MUST be negotiated before being used. For example, an extension that changes the layout of the HEADERS frame cannot be used until the peer has given a positive signal that this is acceptable. In this case, it could also be necessary to coordinate when the revised layout comes into effect. For example, treating frames other than DATA frames as flow controlled requires a change in semantics that both endpoints need to understand, so this can only be done through negotiation. This document doesn't mandate a specific method for negotiating the use of an extension but notes that a setting (Section 6.5.2) could be used for that purpose. If both peers set a value that indicates willingness to use the extension, then the extension can be used. If a setting is used for extension negotiation, the initial value MUST be defined in such a fashion that the extension is initially disabled. 6. Frame Definitions This specification defines a number of frame types, each identified by a unique 8-bit type code. Each frame type serves a distinct purpose in the establishment and management either of the connection as a whole or of individual streams. Thomson & Benfield Expires 4 December 2021 [Page 26] Internet-Draft HTTP/2 June 2021 The transmission of specific frame types can alter the state of a connection. If endpoints fail to maintain a synchronized view of the connection state, successful communication within the connection will no longer be possible. Therefore, it is important that endpoints have a shared comprehension of how the state is affected by the use any given frame. 6.1. DATA DATA frames (type=0x0) convey arbitrary, variable-length sequences of octets associated with a stream. One or more DATA frames are used, for instance, to carry HTTP request or response message contents. DATA frames MAY also contain padding. Padding can be added to DATA frames to obscure the size of messages. Padding is a security feature; see Section 10.7. +---------------+ |Pad Length? (8)| +---------------+-----------------------------------------------+ | Data (*) ... +---------------------------------------------------------------+ | Padding (*) ... +---------------------------------------------------------------+ Figure 3: DATA Frame Payload The DATA frame contains the following fields: Pad Length: An 8-bit field containing the length of the frame padding in units of octets. This field is conditional (as signified by a "?" in the diagram) and is only present if the PADDED flag is set. Data: Application data. The amount of data is the remainder of the frame payload after subtracting the length of the other fields that are present. Padding: Padding octets that contain no application semantic value. Padding octets MUST be set to zero when sending. A receiver is not obligated to verify padding but MAY treat non-zero padding as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. The DATA frame defines the following flags: END_STREAM (0x1): When set, bit 0 indicates that this frame is the Thomson & Benfield Expires 4 December 2021 [Page 27] Internet-Draft HTTP/2 June 2021 last that the endpoint will send for the identified stream. Setting this flag causes the stream to enter one of the "half-closed" states or the "closed" state (Section 5.1). PADDED (0x8): When set, bit 3 indicates that the Pad Length field and any padding that it describes are present. DATA frames MUST be associated with a stream. If a DATA frame is received whose stream identifier field is 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR. DATA frames are subject to flow control and can only be sent when a stream is in the "open" or "half-closed (remote)" state. The entire DATA frame payload is included in flow control, including the Pad Length and Padding fields if present. If a DATA frame is received whose stream is not in "open" or "half-closed (local)" state, the recipient MUST respond with a stream error (Section 5.4.2) of type STREAM_CLOSED. The total number of padding octets is determined by the value of the Pad Length field. If the length of the padding is the length of the frame payload or greater, the recipient MUST treat this as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. | Note: A frame can be increased in size by one octet by | including a Pad Length field with a value of zero. 6.2. HEADERS The HEADERS frame (type=0x1) is used to open a stream (Section 5.1), and additionally carries a field block fragment. Despite the name, a HEADERS frame can carry a header section or a trailer section. HEADERS frames can be sent on a stream in the "idle", "reserved (local)", "open", or "half-closed (remote)" state. +---------------+ |Pad Length? (8)| +-+-------------+-----------------------------------------------+ |E| Stream Dependency? (31) | +-+-------------+-----------------------------------------------+ | Weight? (8) | +-+-------------+-----------------------------------------------+ | Field Block Fragment (*) ... +---------------------------------------------------------------+ | Padding (*) ... +---------------------------------------------------------------+ Thomson & Benfield Expires 4 December 2021 [Page 28] Internet-Draft HTTP/2 June 2021 Figure 4: HEADERS Frame Payload The HEADERS frame payload has the following fields: Pad Length: An 8-bit field containing the length of the frame padding in units of octets. This field is only present if the PADDED flag is set. E: A single-bit flag. This field is only present if the PRIORITY flag is set. Stream Dependency: A 31-bit stream identifier. This field is only present if the PRIORITY flag is set. Weight: An unsigned 8-bit integer. This field is only present if the PRIORITY flag is set. Field Block Fragment: A field block fragment (Section 4.3). Padding: Padding octets. The HEADERS frame defines the following flags: END_STREAM (0x1): When set, bit 0 indicates that the field block (Section 4.3) is the last that the endpoint will send for the identified stream. A HEADERS frame carries the END_STREAM flag that signals the end of a stream. However, a HEADERS frame with the END_STREAM flag set can be followed by CONTINUATION frames on the same stream. Logically, the CONTINUATION frames are part of the HEADERS frame. END_HEADERS (0x4): When set, bit 2 indicates that this frame contains an entire field block (Section 4.3) and is not followed by any CONTINUATION frames. A HEADERS frame without the END_HEADERS flag set MUST be followed by a CONTINUATION frame for the same stream. A receiver MUST treat the receipt of any other type of frame or a frame on a different stream as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. PADDED (0x8): When set, bit 3 indicates that the Pad Length field and any padding that it describes are present. PRIORITY (0x20): When set, bit 5 indicates that the Exclusive Flag (E), Stream Dependency, and Weight fields are present. Thomson & Benfield Expires 4 December 2021 [Page 29] Internet-Draft HTTP/2 June 2021 The frame payload of a HEADERS frame contains a field block fragment (Section 4.3). A field block that does not fit within a HEADERS frame is continued in a CONTINUATION frame (Section 6.10). HEADERS frames MUST be associated with a stream. If a HEADERS frame is received whose stream identifier field is 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR. The HEADERS frame changes the connection state as described in Section 4.3. The HEADERS frame can include padding. Padding fields and flags are identical to those defined for DATA frames (Section 6.1). Padding that exceeds the size remaining for the field block fragment MUST be treated as a PROTOCOL_ERROR. 6.3. PRIORITY The PRIORITY frame (type=0x2) is deprecated; see Section 5.3.2. A PRIORITY frame can be sent in any stream state, including idle or closed streams. +-+-------------------------------------------------------------+ |E| Stream Dependency (31) | +-+-------------+-----------------------------------------------+ | Weight (8) | +-+-------------+ Figure 5: PRIORITY Frame Payload The frame payload of a PRIORITY frame contains the following fields: E: A single-bit flag. Stream Dependency: A 31-bit stream identifier. Weight: An unsigned 8-bit integer. The PRIORITY frame does not define any flags. The PRIORITY frame always identifies a stream. If a PRIORITY frame is received with a stream identifier of 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Thomson & Benfield Expires 4 December 2021 [Page 30] Internet-Draft HTTP/2 June 2021 Sending or receiving a PRIORITY frame does not affect the state of any stream (Section 5.1). The PRIORITY frame can be sent on a stream in any state, including "idle" or "closed". A PRIORITY frame cannot be sent between consecutive frames that comprise a single field block (Section 4.3). A PRIORITY frame with a length other than 5 octets MUST be treated as a stream error (Section 5.4.2) of type FRAME_SIZE_ERROR. 6.4. RST_STREAM The RST_STREAM frame (type=0x3) allows for immediate termination of a stream. RST_STREAM is sent to request cancellation of a stream or to indicate that an error condition has occurred. +---------------------------------------------------------------+ | Error Code (32) | +---------------------------------------------------------------+ Figure 6: RST_STREAM Frame Payload The RST_STREAM frame contains a single unsigned, 32-bit integer identifying the error code (Section 7). The error code indicates why the stream is being terminated. The RST_STREAM frame does not define any flags. The RST_STREAM frame fully terminates the referenced stream and causes it to enter the "closed" state. After receiving a RST_STREAM on a stream, the receiver MUST NOT send additional frames for that stream, with the exception of PRIORITY. However, after sending the RST_STREAM, the sending endpoint MUST be prepared to receive and process additional frames sent on the stream that might have been sent by the peer prior to the arrival of the RST_STREAM. RST_STREAM frames MUST be associated with a stream. If a RST_STREAM frame is received with a stream identifier of 0x0, the recipient MUST treat this as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. RST_STREAM frames MUST NOT be sent for a stream in the "idle" state. If a RST_STREAM frame identifying an idle stream is received, the recipient MUST treat this as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. A RST_STREAM frame with a length other than 4 octets MUST be treated as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR. Thomson & Benfield Expires 4 December 2021 [Page 31] Internet-Draft HTTP/2 June 2021 6.5. SETTINGS The SETTINGS frame (type=0x4) conveys configuration parameters that affect how endpoints communicate, such as preferences and constraints on peer behavior. The SETTINGS frame is also used to acknowledge the receipt of those settings. Individually, a configuration parameter from a SETTINGS frame is referred to as a "setting". Settings are not negotiated; they describe characteristics of the sending peer, which are used by the receiving peer. Different values for the same setting can be advertised by each peer. For example, a client might set a high initial flow-control window, whereas a server might set a lower value to conserve resources. A SETTINGS frame MUST be sent by both endpoints at the start of a connection and MAY be sent at any other time by either endpoint over the lifetime of the connection. Implementations MUST support all of the settings defined by this specification. Each parameter in a SETTINGS frame replaces any existing value for that parameter. Settings 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 each setting. Therefore, the value of a SETTINGS parameter is the last value that is seen by a receiver. SETTINGS frames are acknowledged by the receiving peer. To enable this, the SETTINGS frame defines the ACK flag: ACK (0x1): When set, bit 0 indicates that this frame acknowledges receipt and application of the peer's SETTINGS frame. When this bit is set, the frame payload of the SETTINGS frame MUST be empty. Receipt of a SETTINGS frame with the ACK flag set and a length field value other than 0 MUST be treated as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR. For more information, see Section 6.5.3 ("Settings Synchronization"). SETTINGS frames always apply to a connection, never a single stream. The stream identifier for a SETTINGS frame MUST be zero (0x0). If an endpoint receives a SETTINGS frame whose stream identifier field is anything other than 0x0, the endpoint MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR. The SETTINGS frame affects connection state. A badly formed or incomplete SETTINGS frame MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Thomson & Benfield Expires 4 December 2021 [Page 32] Internet-Draft HTTP/2 June 2021 A SETTINGS frame with a length other than a multiple of 6 octets MUST be treated as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR. 6.5.1. SETTINGS Format The frame payload of a SETTINGS frame consists of zero or more settings, each consisting of an unsigned 16-bit setting identifier and an unsigned 32-bit value. +-------------------------------+ | Identifier (16) | +-------------------------------+-------------------------------+ | Value (32) | +---------------------------------------------------------------+ Figure 7: Setting Format 6.5.2. Defined Settings The following settings are defined: SETTINGS_HEADER_TABLE_SIZE (0x1): Allows the sender to inform the remote endpoint of the maximum size of the compression table used to decode field blocks, in octets. The encoder can select any size equal to or less than this value by using signaling specific to the compression format inside a field block (see [COMPRESSION]). The initial value is 4,096 octets. SETTINGS_ENABLE_PUSH (0x2): This setting can be used to disable server push (Section 8.2). A server MUST NOT send a PUSH_PROMISE frame if it receives this parameter set to a value of 0. A client that has both set this parameter to 0 and had it acknowledged MUST treat the receipt of a PUSH_PROMISE frame as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. The initial value of SETTINGS_ENABLE_PUSH is 1, which indicates that server push is permitted. Any value other than 0 or 1 MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. A server MUST NOT explicitly set this value to 1. A server MAY choose to omit this setting when it sends a SETTINGS frame, but if a server does include a value it MUST be 0. A client MUST treat receipt of a SETTINGS frame with SETTINGS_ENABLE_PUSH set to 1 as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. SETTINGS_MAX_CONCURRENT_STREAMS (0x3): Indicates the maximum number Thomson & Benfield Expires 4 December 2021 [Page 33] Internet-Draft HTTP/2 June 2021 of concurrent streams that the sender will allow. This limit is directional: it applies to the number of streams that the sender permits the receiver to create. Initially, there is no limit to this value. It is recommended that this value be no smaller than 100, so as to not unnecessarily limit parallelism. A value of 0 for SETTINGS_MAX_CONCURRENT_STREAMS SHOULD NOT be treated as special by endpoints. A zero value does prevent the creation of new streams; however, this can also happen for any limit that is exhausted with active streams. Servers SHOULD only set a zero value for short durations; if a server does not wish to accept requests, closing the connection is more appropriate. SETTINGS_INITIAL_WINDOW_SIZE (0x4): Indicates the sender's initial window size (in octets) for stream-level flow control. The initial value is 2^16-1 (65,535) octets. This setting affects the window size of all streams (see Section 6.9.2). Values above the maximum flow-control window size of 2^31-1 MUST be treated as a connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR. SETTINGS_MAX_FRAME_SIZE (0x5): Indicates the size of the largest frame payload that the sender is willing to receive, in octets. The initial value is 2^14 (16,384) octets. The value advertised by an endpoint MUST be between this initial value and the maximum allowed frame size (2^24-1 or 16,777,215 octets), inclusive. Values outside this range MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. SETTINGS_MAX_HEADER_LIST_SIZE (0x6): This advisory setting informs a peer of the maximum size of field section that the sender is prepared to accept, in octets. The value is based on the uncompressed size of field lines, including the length of the name and value in octets plus an overhead of 32 octets for each field line. For any given request, a lower limit than what is advertised MAY be enforced. The initial value of this setting is unlimited. An endpoint that receives a SETTINGS frame with any unknown or unsupported identifier MUST ignore that setting. Thomson & Benfield Expires 4 December 2021 [Page 34] Internet-Draft HTTP/2 June 2021 6.5.3. Settings Synchronization Most values in SETTINGS benefit from or require an understanding of when the peer has received and applied the changed parameter values. In order to provide such synchronization timepoints, the recipient of a SETTINGS frame in which the ACK flag is not set MUST apply the updated settings as soon as possible upon receipt. The values in the SETTINGS frame MUST be processed in the order they appear, with no other frame processing between values. Unsupported settings MUST be ignored. Once all values have been processed, the recipient MUST immediately emit a SETTINGS frame with the ACK flag set. Upon receiving a SETTINGS frame with the ACK flag set, the sender of the altered settings can rely on the value having been applied. If the sender of a SETTINGS frame does not receive an acknowledgement within a reasonable amount of time, it MAY issue a connection error (Section 5.4.1) of type SETTINGS_TIMEOUT. 6.6. PUSH_PROMISE The PUSH_PROMISE frame (type=0x5) is used to notify the peer endpoint in advance of streams the sender intends to initiate. The PUSH_PROMISE frame includes the unsigned 31-bit identifier of the stream the endpoint plans to create along with a field section that provides additional context for the stream. Section 8.2 contains a thorough description of the use of PUSH_PROMISE frames. +---------------+ |Pad Length? (8)| +-+-------------+-----------------------------------------------+ |R| Promised Stream ID (31) | +-+-----------------------------+-------------------------------+ | Field Block Fragment (*) ... +---------------------------------------------------------------+ | Padding (*) ... +---------------------------------------------------------------+ Figure 8: PUSH_PROMISE Frame Payload The PUSH_PROMISE frame payload has the following fields: Pad Length: An 8-bit field containing the length of the frame padding in units of octets. This field is only present if the PADDED flag is set. R: A single reserved bit. Thomson & Benfield Expires 4 December 2021 [Page 35] Internet-Draft HTTP/2 June 2021 Promised Stream ID: An unsigned 31-bit integer that identifies the stream that is reserved by the PUSH_PROMISE. The promised stream identifier MUST be a valid choice for the next stream sent by the sender (see "new stream identifier" in Section 5.1.1). Field Block Fragment: A field block fragment (Section 4.3) containing request control data and header section. Padding: Padding octets. The PUSH_PROMISE frame defines the following flags: END_HEADERS (0x4): When set, bit 2 indicates that this frame contains an entire field block (Section 4.3) and is not followed by any CONTINUATION frames. A PUSH_PROMISE frame without the END_HEADERS flag set MUST be followed by a CONTINUATION frame for the same stream. A receiver MUST treat the receipt of any other type of frame or a frame on a different stream as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. PADDED (0x8): When set, bit 3 indicates that the Pad Length field and any padding that it describes are present. PUSH_PROMISE frames MUST only be sent on a peer-initiated stream that is in either the "open" or "half-closed (remote)" state. The stream identifier of a PUSH_PROMISE frame indicates the stream it is associated with. If the stream identifier field specifies the value 0x0, a recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Promised streams are not required to be used in the order they are promised. The PUSH_PROMISE only reserves stream identifiers for later use. PUSH_PROMISE MUST NOT be sent if the SETTINGS_ENABLE_PUSH setting of the peer endpoint is set to 0. An endpoint that has set this setting and has received acknowledgement MUST treat the receipt of a PUSH_PROMISE frame as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Recipients of PUSH_PROMISE frames can choose to reject promised streams by returning a RST_STREAM referencing the promised stream identifier back to the sender of the PUSH_PROMISE. Thomson & Benfield Expires 4 December 2021 [Page 36] Internet-Draft HTTP/2 June 2021 A PUSH_PROMISE frame modifies the connection state in two ways. First, the inclusion of a field block (Section 4.3) potentially modifies the state maintained for field section compression. Second, PUSH_PROMISE also reserves a stream for later use, causing the promised stream to enter the "reserved" state. A sender MUST NOT send a PUSH_PROMISE on a stream unless that stream is either "open" or "half-closed (remote)"; the sender MUST ensure that the promised stream is a valid choice for a new stream identifier (Section 5.1.1) (that is, the promised stream MUST be in the "idle" state). Since PUSH_PROMISE reserves a stream, ignoring a PUSH_PROMISE frame causes the stream state to become indeterminate. A receiver MUST treat the receipt of a PUSH_PROMISE on a stream that is neither "open" nor "half-closed (local)" as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. However, an endpoint that has sent RST_STREAM on the associated stream MUST handle PUSH_PROMISE frames that might have been created before the RST_STREAM frame is received and processed. A receiver MUST treat the receipt of a PUSH_PROMISE that promises an illegal stream identifier (Section 5.1.1) as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Note that an illegal stream identifier is an identifier for a stream that is not currently in the "idle" state. The PUSH_PROMISE frame can include padding. Padding fields and flags are identical to those defined for DATA frames (Section 6.1). 6.7. PING The PING frame (type=0x6) is a mechanism for measuring a minimal round-trip time from the sender, as well as determining whether an idle connection is still functional. PING frames can be sent from any endpoint. +---------------------------------------------------------------+ | | | Opaque Data (64) | | | +---------------------------------------------------------------+ Figure 9: PING Frame Payload In addition to the frame header, PING frames MUST contain 8 octets of opaque data in the frame payload. A sender can include any value it chooses and use those octets in any fashion. Thomson & Benfield Expires 4 December 2021 [Page 37] Internet-Draft HTTP/2 June 2021 Receivers of a PING frame that does not include an ACK flag MUST send a PING frame with the ACK flag set in response, with an identical frame payload. PING responses SHOULD be given higher priority than any other frame. The PING frame defines the following flags: ACK (0x1): When set, bit 0 indicates that this PING frame is a PING response. An endpoint MUST set this flag in PING responses. An endpoint MUST NOT respond to PING frames containing this flag. PING frames are not associated with any individual stream. If a PING frame is received with a stream identifier field value other than 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Receipt of a PING frame with a length field value other than 8 MUST be treated as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR. 6.8. GOAWAY The GOAWAY frame (type=0x7) is used to initiate shutdown of a connection or to signal serious error conditions. GOAWAY allows an endpoint to gracefully stop accepting new streams while still finishing processing of previously established streams. This enables administrative actions, like server maintenance. There is an inherent race condition between an endpoint starting new streams and the remote sending a GOAWAY frame. To deal with this case, the GOAWAY contains the stream identifier of the last peer- initiated stream that was or might be processed on the sending endpoint in this connection. For instance, if the server sends a GOAWAY frame, the identified stream is the highest-numbered stream initiated by the client. Once sent, the sender will ignore frames sent on streams initiated by the receiver if the stream has an identifier higher than the included last stream identifier. Receivers of a GOAWAY frame MUST NOT open additional streams on the connection, although a new connection can be established for new streams. If the receiver of the GOAWAY has sent data on streams with a higher stream identifier than what is indicated in the GOAWAY frame, those streams are not or will not be processed. The receiver of the GOAWAY frame can treat the streams as though they had never been created at all, thereby allowing those streams to be retried later on a new connection. Thomson & Benfield Expires 4 December 2021 [Page 38] Internet-Draft HTTP/2 June 2021 Endpoints SHOULD always send a GOAWAY frame before closing a connection so that the remote peer can know whether a stream has been partially processed or not. For example, if an HTTP client sends a POST at the same time that a server closes a connection, the client cannot know if the server started to process that POST request if the server does not send a GOAWAY frame to indicate what streams it might have acted on. An endpoint might choose to close a connection without sending a GOAWAY for misbehaving peers. A GOAWAY frame might not immediately precede closing of the connection; a receiver of a GOAWAY that has no more use for the connection SHOULD still send a GOAWAY frame before terminating the connection. +-+-------------------------------------------------------------+ |R| Last-Stream-ID (31) | +-+-------------------------------------------------------------+ | Error Code (32) | +---------------------------------------------------------------+ | Additional Debug Data (*) | +---------------------------------------------------------------+ Figure 10: GOAWAY Frame Payload The GOAWAY frame does not define any flags. The GOAWAY frame applies to the connection, not a specific stream. An endpoint MUST treat a GOAWAY frame with a stream identifier other than 0x0 as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. The last stream identifier in the GOAWAY frame contains the highest- numbered stream identifier for which the sender of the GOAWAY frame might have taken some action on or might yet take action on. All streams up to and including the identified stream might have been processed in some way. The last stream identifier can be set to 0 if no streams were processed. | Note: In this context, "processed" means that some data from | the stream was passed to some higher layer of software that | might have taken some action as a result. If a connection terminates without a GOAWAY frame, the last stream identifier is effectively the highest possible stream identifier. Thomson & Benfield Expires 4 December 2021 [Page 39] Internet-Draft HTTP/2 June 2021 On streams with lower- or equal-numbered identifiers that were not closed completely prior to the connection being closed, reattempting requests, transactions, or any protocol activity is not possible, with the exception of idempotent actions like HTTP GET, PUT, or DELETE. Any protocol activity that uses higher-numbered streams can be safely retried using a new connection. Activity on streams numbered lower or equal to the last stream identifier might still complete successfully. The sender of a GOAWAY frame might gracefully shut down a connection by sending a GOAWAY frame, maintaining the connection in an "open" state until all in- progress streams complete. An endpoint MAY send multiple GOAWAY frames if circumstances change. For instance, an endpoint that sends GOAWAY with NO_ERROR during graceful shutdown could subsequently encounter a condition that requires immediate termination of the connection. The last stream identifier from the last GOAWAY frame received indicates which streams could have been acted upon. Endpoints MUST NOT increase the value they send in the last stream identifier, since the peers might already have retried unprocessed requests on another connection. A client that is unable to retry requests loses all requests that are in flight when the server closes the connection. This is especially true for intermediaries that might not be serving clients using HTTP/2. A server that is attempting to gracefully shut down a connection SHOULD send an initial GOAWAY frame with the last stream identifier set to 2^31-1 and a NO_ERROR code. This signals to the client that a shutdown is imminent and that initiating further requests is prohibited. After allowing time for any in-flight stream creation (at least one round-trip time), the server can send another GOAWAY frame with an updated last stream identifier. This ensures that a connection can be cleanly shut down without losing requests. After sending a GOAWAY frame, the sender can discard frames for streams initiated by the receiver with identifiers higher than the identified last stream. However, any frames that alter connection state cannot be completely ignored. For instance, HEADERS, PUSH_PROMISE, and CONTINUATION frames MUST be minimally processed to ensure the state maintained for field section compression is consistent (see Section 4.3); similarly, DATA frames MUST be counted toward the connection flow-control window. Failure to process these frames can cause flow control or field section compression state to become unsynchronized. The GOAWAY frame also contains a 32-bit error code (Section 7) that contains the reason for closing the connection. Thomson & Benfield Expires 4 December 2021 [Page 40] Internet-Draft HTTP/2 June 2021 Endpoints MAY append opaque data to the frame payload of any GOAWAY frame. Additional debug data is intended for diagnostic purposes only and carries no semantic value. Debug information could contain security- or privacy-sensitive data. Logged or otherwise persistently stored debug data MUST have adequate safeguards to prevent unauthorized access. 6.9. WINDOW_UPDATE The WINDOW_UPDATE frame (type=0x8) is used to implement flow control; see Section 5.2 for an overview. Flow control operates at two levels: on each individual stream and on the entire connection. Both types of flow control are hop by hop, that is, only between the two endpoints. Intermediaries do not forward WINDOW_UPDATE frames between dependent connections. However, throttling of data transfer by any receiver can indirectly cause the propagation of flow-control information toward the original sender. Flow control only applies to frames that are identified as being subject to flow control. Of the frame types defined in this document, this includes only DATA frames. Frames that are exempt from flow control MUST be accepted and processed, unless the receiver is unable to assign resources to handling the frame. A receiver MAY respond with a stream error (Section 5.4.2) or connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR if it is unable to accept a frame. +-+-------------------------------------------------------------+ |R| Window Size Increment (31) | +-+-------------------------------------------------------------+ Figure 11: WINDOW_UPDATE Frame Payload The frame payload of a WINDOW_UPDATE frame is one reserved bit plus an unsigned 31-bit integer indicating the number of octets that the sender can transmit in addition to the existing flow-control window. The legal range for the increment to the flow-control window is 1 to 2^31-1 (2,147,483,647) octets. The WINDOW_UPDATE frame does not define any flags. The WINDOW_UPDATE frame can be specific to a stream or to the entire connection. In the former case, the frame's stream identifier indicates the affected stream; in the latter, the value "0" indicates that the entire connection is the subject of the frame. Thomson & Benfield Expires 4 December 2021 [Page 41] Internet-Draft HTTP/2 June 2021 A receiver MUST treat the receipt of a WINDOW_UPDATE frame with an flow-control window increment of 0 as a stream error (Section 5.4.2) of type PROTOCOL_ERROR; errors on the connection flow-control window MUST be treated as a connection error (Section 5.4.1). WINDOW_UPDATE can be sent by a peer that has sent a frame bearing the END_STREAM flag. This means that a receiver could receive a WINDOW_UPDATE frame on a "half-closed (remote)" or "closed" stream. A receiver MUST NOT treat this as an error (see Section 5.1). A receiver that receives a flow-controlled frame MUST always account for its contribution against the connection flow-control window, unless the receiver treats this as a connection error (Section 5.4.1). This is necessary even if the frame is in error. The sender counts the frame toward the flow-control window, but if the receiver does not, the flow-control window at the sender and receiver can become different. A WINDOW_UPDATE frame with a length other than 4 octets MUST be treated as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR. 6.9.1. The Flow-Control Window Flow control in HTTP/2 is implemented using a window kept by each sender on every stream. The flow-control window is a simple integer value that indicates how many octets of data the sender is permitted to transmit; as such, its size is a measure of the buffering capacity of the receiver. Two flow-control windows are applicable: the stream flow-control window and the connection flow-control window. The sender MUST NOT send a flow-controlled frame with a length that exceeds the space available in either of the flow-control windows advertised by the receiver. Frames with zero length with the END_STREAM flag set (that is, an empty DATA frame) MAY be sent if there is no available space in either flow-control window. For flow-control calculations, the 9-octet frame header is not counted. After sending a flow-controlled frame, the sender reduces the space available in both windows by the length of the transmitted frame. The receiver of a frame sends a WINDOW_UPDATE frame as it consumes data and frees up space in flow-control windows. Separate WINDOW_UPDATE frames are sent for the stream- and connection-level flow-control windows. Thomson & Benfield Expires 4 December 2021 [Page 42] Internet-Draft HTTP/2 June 2021 A sender that receives a WINDOW_UPDATE frame updates the corresponding window by the amount specified in the frame. A sender MUST NOT allow a flow-control window to exceed 2^31-1 octets. If a sender receives a WINDOW_UPDATE that causes a flow- control window to exceed this maximum, it MUST terminate either the stream or the connection, as appropriate. For streams, the sender sends a RST_STREAM with an error code of FLOW_CONTROL_ERROR; for the connection, a GOAWAY frame with an error code of FLOW_CONTROL_ERROR is sent. Flow-controlled frames from the sender and WINDOW_UPDATE frames from the receiver are completely asynchronous with respect to each other. This property allows a receiver to aggressively update the window size kept by the sender to prevent streams from stalling. 6.9.2. Initial Flow-Control Window Size When an HTTP/2 connection is first established, new streams are created with an initial flow-control window size of 65,535 octets. The connection flow-control window is also 65,535 octets. Both endpoints can adjust the initial window size for new streams by including a value for SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS frame that forms part of the connection preface. The connection flow-control window can only be changed using WINDOW_UPDATE frames. Prior to receiving a SETTINGS frame that sets a value for SETTINGS_INITIAL_WINDOW_SIZE, an endpoint can only use the default initial window size when sending flow-controlled frames. Similarly, the connection flow-control window is set to the default initial window size until a WINDOW_UPDATE frame is received. In addition to changing the flow-control window for streams that are not yet active, a SETTINGS frame can alter the initial flow-control window size for streams with active flow-control windows (that is, streams in the "open" or "half-closed (remote)" state). When the value of SETTINGS_INITIAL_WINDOW_SIZE changes, a receiver MUST adjust the size of all stream flow-control windows that it maintains by the difference between the new value and the old value. A change to SETTINGS_INITIAL_WINDOW_SIZE can cause the available space in a flow-control window to become negative. A sender MUST track the negative flow-control window and MUST NOT send new flow- controlled frames until it receives WINDOW_UPDATE frames that cause the flow-control window to become positive. Thomson & Benfield Expires 4 December 2021 [Page 43] Internet-Draft HTTP/2 June 2021 For example, if the client sends 60 KB immediately on connection establishment and the server sets the initial window size to be 16 KB, the client will recalculate the available flow-control window to be -44 KB on receipt of the SETTINGS frame. The client retains a negative flow-control window until WINDOW_UPDATE frames restore the window to being positive, after which the client can resume sending. A SETTINGS frame cannot alter the connection flow-control window. An endpoint MUST treat a change to SETTINGS_INITIAL_WINDOW_SIZE that causes any flow-control window to exceed the maximum size as a connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR. 6.9.3. Reducing the Stream Window Size A receiver that wishes to use a smaller flow-control window than the current size can send a new SETTINGS frame. However, the receiver MUST be prepared to receive data that exceeds this window size, since the sender might send data that exceeds the lower limit prior to processing the SETTINGS frame. After sending a SETTINGS frame that reduces the initial flow-control window size, a receiver MAY continue to process streams that exceed flow-control limits. Allowing streams to continue does not allow the receiver to immediately reduce the space it reserves for flow-control windows. Progress on these streams can also stall, since WINDOW_UPDATE frames are needed to allow the sender to resume sending. The receiver MAY instead send a RST_STREAM with an error code of FLOW_CONTROL_ERROR for the affected streams. 6.10. CONTINUATION The CONTINUATION frame (type=0x9) is used to continue a sequence of field block fragments (Section 4.3). Any number of CONTINUATION frames can be sent, as long as the preceding frame is on the same stream and is a HEADERS, PUSH_PROMISE, or CONTINUATION frame without the END_HEADERS flag set. +---------------------------------------------------------------+ | Field Block Fragment (*) ... +---------------------------------------------------------------+ Figure 12: CONTINUATION Frame Payload The CONTINUATION frame payload contains a field block fragment (Section 4.3). The CONTINUATION frame defines the following flag: Thomson & Benfield Expires 4 December 2021 [Page 44] Internet-Draft HTTP/2 June 2021 END_HEADERS (0x4): When set, bit 2 indicates that this frame ends a field block (Section 4.3). If the END_HEADERS bit is not set, this frame MUST be followed by another CONTINUATION frame. A receiver MUST treat the receipt of any other type of frame or a frame on a different stream as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. The CONTINUATION frame changes the connection state as defined in Section 4.3. CONTINUATION frames MUST be associated with a stream. If a CONTINUATION frame is received whose stream identifier field is 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR. A CONTINUATION frame MUST be preceded by a HEADERS, PUSH_PROMISE or CONTINUATION frame without the END_HEADERS flag set. A recipient that observes violation of this rule MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR. 7. Error Codes Error codes are 32-bit fields that are used in RST_STREAM and GOAWAY frames to convey the reasons for the stream or connection error. Error codes share a common code space. Some error codes apply only to either streams or the entire connection and have no defined semantics in the other context. The following error codes are defined: NO_ERROR (0x0): The associated condition is not a result of an error. For example, a GOAWAY might include this code to indicate graceful shutdown of a connection. PROTOCOL_ERROR (0x1): The endpoint detected an unspecific protocol error. This error is for use when a more specific error code is not available. INTERNAL_ERROR (0x2): The endpoint encountered an unexpected internal error. FLOW_CONTROL_ERROR (0x3): The endpoint detected that its peer violated the flow-control protocol. SETTINGS_TIMEOUT (0x4): The endpoint sent a SETTINGS frame but did Thomson & Benfield Expires 4 December 2021 [Page 45] Internet-Draft HTTP/2 June 2021 not receive a response in a timely manner. See Section 6.5.3 ("Settings Synchronization"). STREAM_CLOSED (0x5): The endpoint received a frame after a stream was half-closed. FRAME_SIZE_ERROR (0x6): The endpoint received a frame with an invalid size. REFUSED_STREAM (0x7): The endpoint refused the stream prior to performing any application processing (see Section 8.1.4 for details). CANCEL (0x8): Used by the endpoint to indicate that the stream is no longer needed. COMPRESSION_ERROR (0x9): The endpoint is unable to maintain the field section compression context for the connection. CONNECT_ERROR (0xa): The connection established in response to a CONNECT request (Section 8.3) was reset or abnormally closed. ENHANCE_YOUR_CALM (0xb): The endpoint detected that its peer is exhibiting a behavior that might be generating excessive load. INADEQUATE_SECURITY (0xc): The underlying transport has properties that do not meet minimum security requirements (see Section 9.2). HTTP_1_1_REQUIRED (0xd): The endpoint requires that HTTP/1.1 be used instead of HTTP/2. Unknown or unsupported error codes MUST NOT trigger any special behavior. These MAY be treated by an implementation as being equivalent to INTERNAL_ERROR. 8. HTTP Message Exchanges HTTP/2 defines a framing of the HTTP message abstraction (Section 6 of [HTTP]). 8.1. HTTP Message Framing A client sends an HTTP request on a new stream, using a previously unused stream identifier (Section 5.1.1). A server sends an HTTP response on the same stream as the request. An HTTP message (request or response) consists of: Thomson & Benfield Expires 4 December 2021 [Page 46] Internet-Draft HTTP/2 June 2021 1. one HEADERS frame (followed by zero or more CONTINUATION frames) containing the header section (see Section 6.3 of [HTTP]), 2. zero or more DATA frames containing the message content (see Section 6.4 of [HTTP]), and 3. optionally, one HEADERS frame, followed by zero or more CONTINUATION frames containing the trailer-part, if present (see Section 6.5 of [HTTP]). For a response only, a server MAY send any number of interim responses before the HEADERS frame containing a final response. An interim response consists of a HEADERS frames (which might be followed by zero or more CONTINUATION frames) containing the control data and header section of an interim (1xx) HTTP response (see Section 15 of [HTTP]). A HEADERS frame with an END_STREAM flag that carries an informational status code is malformed (Section 8.1.2.6). The last frame in the sequence bears an END_STREAM flag, noting that a HEADERS frame bearing the END_STREAM flag can be followed by CONTINUATION frames that carry any remaining fragments of the field block. Other frames (from any stream) MUST NOT occur between the HEADERS frame and any CONTINUATION frames that might follow. HTTP/2 uses DATA frames to carry message content. The "chunked" transfer encoding defined in Section 7.1 of [HTTP11] cannot be used in HTTP/2. Trailer fields are carried in a field block that also terminates the stream. That is, trailer fields comprise a sequence starting with a HEADERS frame, followed by zero or more CONTINUATION frames, where the HEADERS frame bears an END_STREAM flag. Trailers MUST NOT include pseudo-header fields (Section 8.1.2.1). An endpoint that receives pseudo-header fields in trailers MUST treat the request or response as malformed (Section 8.1.2.6). An endpoint that receives a HEADERS frame without the END_STREAM flag set after receiving the HEADERS frame that opens a request or after receiving a final (non-informational) status code MUST treat the corresponding request or response as malformed (Section 8.1.2.6). Thomson & Benfield Expires 4 December 2021 [Page 47] Internet-Draft HTTP/2 June 2021 An HTTP request/response exchange fully consumes a single stream. A request starts with the HEADERS frame that puts the stream into an "open" state. The request ends with a frame bearing END_STREAM, which causes the stream to become "half-closed (local)" for the client and "half-closed (remote)" for the server. A response stream starts with zero or more interim responses in HEADERS frames or a HEADERS frame containing a final status code. An HTTP response is complete after the server sends -- or the client receives -- a frame with the END_STREAM flag set (including any CONTINUATION frames needed to complete a field block). 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 (i.e., a frame with the END_STREAM flag). 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. 8.1.1. Upgrading from HTTP/2 HTTP/2 removes support for the 101 (Switching Protocols) informational status code (Section 15.2.2 of [HTTP]). The semantics of 101 (Switching Protocols) aren't applicable to a multiplexed protocol. Alternative protocols are able to use the same mechanisms that HTTP/2 uses to negotiate their use (see Section 3). 8.1.2. HTTP Fields HTTP fields carry information as a series of field lines, which are key-value pairs. For a listing of registered HTTP fields, see the "Hypertext Transfer Protocol (HTTP) Field Name Registry" registry maintained at https://www.iana.org/assignments/http-fields/. Field names are strings of ASCII characters that are compared in a case-insensitive fashion. Field names MUST be converted to lowercase when constructing a HTTP/2 message. A request or response containing an uppercase character ('A' to 'Z', ASCII 0x41 to 0x5a) in a field name MUST be treated as malformed (Section 8.1.2.6). HPACK is capable of carrying field names or values that are not valid in HTTP. Though HPACK can carry any octet, fields are not valid in the following cases: Thomson & Benfield Expires 4 December 2021 [Page 48] Internet-Draft HTTP/2 June 2021 * A field name MUST NOT contain characters in the range 0x00-0x20 or 0x7F-0xFF (both ranges inclusive). This limits field names to visible ASCII characters, other than ASCII SP (0x20). * With the exception of pseudo-header fields (Section 8.1.2.1), which have a name that starts with a single colon, field names MUST NOT include a colon (ASCII COLON, 0x3a). * A field value MUST NOT contain the zero value (ASCII NUL, 0x0), line feed (ASCII LF, 0xa), or carriage return (ASCII CR, 0xd) at any position. * A field value MUST NOT start or end with an ASCII whitespace character (ASCII SP or HTAB, 0x20 or 0x9). A request or response that contains a field that violates any of these conditions MUST be treated as malformed (Section 8.1.2.6). In particular, an intermediary that does not process fields when forwarding messages MUST NOT forward fields that contain any of the values that are listed as prohibited above. Field values that are not valid according to the definition of the corresponding field do not cause a request to be malformed except as defined by the processing rules for the field. 8.1.2.1. Pseudo-Header Fields HTTP/2 uses special pseudo-header fields beginning with ':' character (ASCII 0x3a) to convey message control data (see Section 6.2 of [HTTP]). Pseudo-header fields are not HTTP header fields. Endpoints MUST NOT generate pseudo-header fields other than those defined in this document. Note that an extension could negotiate the use of additional pseudo-header fields; see Section 5.5. Pseudo-header fields are only valid in the context in which they are defined. Pseudo-header fields defined for requests MUST NOT appear in responses; pseudo-header fields defined for responses MUST NOT appear in requests. Pseudo-header fields MUST NOT appear in a trailer section. Endpoints MUST treat a request or response that contains undefined or invalid pseudo-header fields as malformed (Section 8.1.2.6). All pseudo-header fields MUST appear in a field block before all regular field lines. Any request or response that contains a pseudo- header field that appears in a field block after a regular field line MUST be treated as malformed (Section 8.1.2.6). Thomson & Benfield Expires 4 December 2021 [Page 49] Internet-Draft HTTP/2 June 2021 8.1.2.2. Connection-Specific Header Fields HTTP/2 does not use the "Connection" header field to indicate connection-specific header fields; in this protocol, connection- specific metadata is conveyed by other means. An endpoint MUST NOT generate an HTTP/2 message containing connection-specific header fields; any message containing connection-specific header fields MUST be treated as malformed (Section 8.1.2.6). The only exception to this is the TE header field, which MAY be present in an HTTP/2 request; when it is, it MUST NOT contain any value other than "trailers". An intermediary transforming a HTTP/1.x message to HTTP/2 MUST remove connection-specific header fields as discussed in Section 7.6.1 of [HTTP], or their messages will be treated by other HTTP/2 endpoints as malformed (Section 8.1.2.6). | Note: HTTP/2 purposefully does not support upgrade to another | protocol. The handshake methods described in Section 3 are | believed sufficient to negotiate the use of alternative | protocols. 8.1.2.3. Request Pseudo-Header Fields The following pseudo-header fields are defined for HTTP/2 requests: * The ":method" pseudo-header field includes the HTTP method (Section 9 of [HTTP]). * The ":scheme" pseudo-header field includes the scheme portion of the request target. The scheme is taken from the target URI (Section 3.1 of [RFC3986]) when generating a request directly, or from the scheme of a translated request (for example. see Section 3.3 of [HTTP11]). Scheme is omitted for CONNECT requests (Section 8.3). ":scheme" is not restricted to "http" and "https" schemed URIs. A proxy or gateway can translate requests for non-HTTP schemes, enabling the use of HTTP to interact with non-HTTP services. * The ":authority" pseudo-header field includes the authority portion of the target URI (Section 3.2 of [RFC3986]). The authority MUST NOT include the deprecated "userinfo" subcomponent for "http" or "https" schemed URIs. Thomson & Benfield Expires 4 December 2021 [Page 50] Internet-Draft HTTP/2 June 2021 Clients that generate HTTP/2 requests directly SHOULD use the ":authority" pseudo-header field instead of the "Host" header field. An intermediary that translates a request to HTTP/2 from another HTTP version MUST translate any authority information from the request into an ":authority" pseudo-header field. If the control data in the original request contains authority information, an intermediary MUST include a ":authority" pseudo-header field. If control data does not contain authority, an intermediary MUST NOT add an ":authority" pseudo-header field. For reference, an HTTP/1.1 Section 3.2 of request target [HTTP11] in authority-form always includes authority, a request target in absolute-form includes authority if the target URI includes authority, and request targets in origin- or asterisk-form do not include authority. An intermediary that translates a request to another HTTP version from HTTP/2 can construct a "Host" header field by copying the value of the ":authority" pseudo-header field if that version requires that "Host" be included in a request, as HTTP/1.1 does for some forms of request target (see Section 3.2 of [HTTP11]). An intermediary that translates a request to HTTP/2 from another HTTP version MUST retain any "Host" header field, even if an authority is part of control data. The value of the "Host" header field MUST be ignored if control data contains authority (that is, the ":authority" pseudo-header field is present). * The ":path" pseudo-header field includes the path and query parts of the target URI (the "path-absolute" production and optionally a '?' character followed by the "query" production (see Sections 3.3 and 3.4 of [RFC3986]). A request in asterisk form includes the value '*' for the ":path" pseudo-header field. This pseudo-header field MUST NOT be empty for "http" or "https" URIs; "http" or "https" URIs that do not contain a path component MUST include a value of '/'. The exception to this rule is an OPTIONS request for an "http" or "https" URI that does not include a path component; these MUST include a ":path" pseudo-header field with a value of '*' (see Section 7.1 of [HTTP]). All HTTP/2 requests MUST include exactly one valid value for the ":method", ":scheme", and ":path" pseudo-header fields, unless it is a CONNECT request (Section 8.3). An HTTP request that omits mandatory pseudo-header fields is malformed (Section 8.1.2.6). Thomson & Benfield Expires 4 December 2021 [Page 51] Internet-Draft HTTP/2 June 2021 Individual HTTP/2 requests do not carry an explicit indicator of protocol version. All HTTP/2 messages implicitly have a protocol version of "2.0" (see Section 6.2 of [HTTP]). 8.1.2.4. Response Pseudo-Header Fields For HTTP/2 responses, a single ":status" pseudo-header field is defined that carries the HTTP status code field (see Section 15 of [HTTP]). This pseudo-header field MUST be included in all responses, including interim responses; otherwise, the response is malformed (Section 8.1.2.6). HTTP/2 responses implicitly have a protocol version of "2.0". 8.1.2.5. Compressing the Cookie Header Field The Cookie header field [COOKIE] uses a semi-colon (";") to delimit cookie-pairs (or "crumbs"). This header field contains multiple values, but does not use a COMMA (",") as a separator, which prevents cookie-pairs from being sent on multiple field lines (see Section 5.2 of [HTTP]). This can significantly reduce compression efficiency as updates to individual cookie-pairs would invalidate any field lines that are stored in the HPACK table. To allow for better compression efficiency, the Cookie header field MAY be split into separate header fields, each with one or more cookie-pairs. If there are multiple Cookie header fields after decompression, these MUST be concatenated into a single octet string using the two-octet delimiter of 0x3B, 0x20 (the ASCII string "; ") before being passed into a non-HTTP/2 context, such as an HTTP/1.1 connection, or a generic HTTP server application. Therefore, the following two lists of Cookie header fields are semantically equivalent. cookie: a=b; c=d; e=f cookie: a=b cookie: c=d cookie: e=f 8.1.2.6. Malformed Requests and Responses A malformed request or response is one that is an otherwise valid sequence of HTTP/2 frames but is invalid due to the presence of extraneous frames, prohibited fields or pseudo-header fields, the absence of mandatory fields or pseudo-header fields, or the inclusion of uppercase field names. Thomson & Benfield Expires 4 December 2021 [Page 52] Internet-Draft HTTP/2 June 2021 A request or response that includes message content can include a "content-length" header field. A request or response is also malformed if the value of a "content-length" header field does not equal the sum of the DATA frame payload lengths that form the content. A response that is defined to have no content, as described in Section 6.4 of [HTTP], can have a non-zero "content-length" header field, even though no content is included in DATA frames. Intermediaries that process HTTP requests or responses (i.e., any intermediary not acting as a tunnel) MUST NOT forward a malformed request or response. Malformed requests or responses that are detected MUST be treated as a stream error (Section 5.4.2) of type PROTOCOL_ERROR. For malformed requests, a server MAY send an HTTP response prior to closing or resetting the stream. Clients MUST NOT accept a malformed response. Endpoints that progressively process messages might have performed some processing before identifying a request or response as malformed. For instance, it might be possible to generate an informational or 404 status code without having received a complete request. Similarly, intermediaries might forward incomplete messages before detecting errors. A server MAY generate a final response before receiving an entire request when the response does not depend on the remainder of the request being correct. A server or intermediary MAY use RST_STREAM -- with a code other than REFUSED_STREAM -- to abort a stream if a malformed request or response is received. These requirements are intended to protect against several types of common attacks against HTTP; they are deliberately strict because being permissive can expose implementations to these vulnerabilities. 8.1.3. Examples This section shows HTTP/1.1 requests and responses, with illustrations of equivalent HTTP/2 requests and responses. An HTTP GET request includes control data and a request header with no message content and is therefore transmitted as a single HEADERS frame, followed by zero or more CONTINUATION frames containing the serialized block of request header fields. The HEADERS frame in the following has both the END_HEADERS and END_STREAM flags set; no CONTINUATION frames are sent. Thomson & Benfield Expires 4 December 2021 [Page 53] Internet-Draft HTTP/2 June 2021 GET /resource HTTP/1.1 HEADERS Host: example.org ==> + END_STREAM Accept: image/jpeg + END_HEADERS :method = GET :scheme = https :path = /resource host = example.org accept = image/jpeg Similarly, a response that includes only control data and a response header is transmitted as a HEADERS frame (again, followed by zero or more CONTINUATION frames) containing the serialized block of response header fields. HTTP/1.1 304 Not Modified HEADERS ETag: "xyzzy" ==> + END_STREAM Expires: Thu, 23 Jan ... + END_HEADERS :status = 304 etag = "xyzzy" expires = Thu, 23 Jan ... An HTTP POST request that includes control data and a request header and message content is transmitted as one HEADERS frame, followed by zero or more CONTINUATION frames containing the request header, followed by one or more DATA frames, with the last CONTINUATION (or HEADERS) frame having the END_HEADERS flag set and the final DATA frame having the END_STREAM flag set: POST /resource HTTP/1.1 HEADERS Host: example.org ==> - END_STREAM Content-Type: image/jpeg - END_HEADERS Content-Length: 123 :method = POST :path = /resource {binary data} :scheme = https CONTINUATION + END_HEADERS content-type = image/jpeg host = example.org content-length = 123 DATA + END_STREAM {binary data} Note that data contributing to any given field line could be spread between field block fragments. The allocation of field lines to frames in this example is illustrative only. Thomson & Benfield Expires 4 December 2021 [Page 54] Internet-Draft HTTP/2 June 2021 A response that includes control data and a response header and message content is transmitted as a HEADERS frame, followed by zero or more CONTINUATION frames, followed by one or more DATA frames, with the last DATA frame in the sequence having the END_STREAM flag set: HTTP/1.1 200 OK HEADERS Content-Type: image/jpeg ==> - END_STREAM Content-Length: 123 + END_HEADERS :status = 200 {binary data} content-type = image/jpeg content-length = 123 DATA + END_STREAM {binary data} An informational response using a 1xx status code other than 101 is transmitted as a HEADERS frame, followed by zero or more CONTINUATION frames. A trailer section is sent as a field block after both the request or response field block and all the DATA frames have been sent. The HEADERS frame starting the field block that comprises the trailer section has the END_STREAM flag set. The following example includes both a 100 (Continue) status code, which is sent in response to a request containing a "100-continue" token in the Expect header field, and a trailer section: Thomson & Benfield Expires 4 December 2021 [Page 55] Internet-Draft HTTP/2 June 2021 HTTP/1.1 100 Continue HEADERS Extension-Field: bar ==> - END_STREAM + END_HEADERS :status = 100 extension-field = bar HTTP/1.1 200 OK HEADERS Content-Type: image/jpeg ==> - END_STREAM Transfer-Encoding: chunked + END_HEADERS Trailer: Foo :status = 200 content-length = 123 123 content-type = image/jpeg {binary data} trailer = Foo 0 Foo: bar DATA - END_STREAM {binary data} HEADERS + END_STREAM + END_HEADERS foo = bar 8.1.4. Request Reliability Mechanisms in HTTP/2 In general, an HTTP client is unable to retry a non-idempotent request when an error occurs because there is no means to determine the nature of the error. It is possible that some server processing occurred prior to the error, which could result in undesirable effects if the request were reattempted. HTTP/2 provides two mechanisms for providing a guarantee to a client that a request has not been processed: * The GOAWAY frame indicates the highest stream number that might have been processed. Requests on streams with higher numbers are therefore guaranteed to be safe to retry. * The REFUSED_STREAM error code can be included in a RST_STREAM frame to indicate that the stream is being closed prior to any processing having occurred. Any request that was sent on the reset stream can be safely retried. Requests that have not been processed have not failed; clients MAY automatically retry them, even those with non-idempotent methods. Thomson & Benfield Expires 4 December 2021 [Page 56] Internet-Draft HTTP/2 June 2021 A server MUST NOT indicate that a stream has not been processed unless it can guarantee that fact. If frames that are on a stream are passed to the application layer for any stream, then REFUSED_STREAM MUST NOT be used for that stream, and a GOAWAY frame MUST include a stream identifier that is greater than or equal to the given stream identifier. In addition to these mechanisms, the PING frame provides a way for a client to easily test a connection. Connections that remain idle can become broken as some middleboxes (for instance, network address translators or load balancers) silently discard connection bindings. The PING frame allows a client to safely test whether a connection is still active without sending a request. 8.2. Server Push HTTP/2 allows a server to pre-emptively send (or "push") responses (along with corresponding "promised" requests) to a client in association with a previous client-initiated request. Server push was designed to allow a server to improve client- perceived performance by predicting what requests will follow those that it receives, thereby removing a round trip for them. For example, a request for HTML is often followed by requests for stylesheets and scripts referenced by that page. When these requests are pushed, the client does not need to wait to receive the references to them in the HTML and issue separate requests. In practice, server push is difficult to use effectively, because it requires the server to correctly anticipate the additional requests the client will make, taking into account factors such as caching, content negotiation, and user behavior. Errors in prediction can lead to performance degradation, due to the opportunity cost that the additional data on the wire represents. In particular, pushing any significant amount of data can cause contention issues with more- important responses. A client can request that server push be disabled, though this is negotiated for each hop independently. The SETTINGS_ENABLE_PUSH setting can be set to 0 to indicate that server push is disabled. Thomson & Benfield Expires 4 December 2021 [Page 57] Internet-Draft HTTP/2 June 2021 Promised requests MUST be safe (see Section 9.2.1 of [HTTP]) and cacheable (see Section 9.2.3 of [HTTP]). Promised requests cannot include any content or a trailer section. Clients that receive a promised request that is not cacheable, that is not known to be safe, or that indicates the presence of request content MUST reset the promised stream with a stream error (Section 5.4.2) of type PROTOCOL_ERROR. Note this could result in the promised stream being reset if the client does not recognize a newly defined method as being safe. Pushed responses that are cacheable (see Section 3 of [CACHE]) can be stored by the client, if it implements an HTTP cache. Pushed responses are considered successfully validated on the origin server (e.g., if the "no-cache" cache response directive is present; see Section 5.2.2.3 of [CACHE]) while the stream identified by the promised stream ID is still open. Pushed responses that are not cacheable MUST NOT be stored by any HTTP cache. They MAY be made available to the application separately. The server MUST include a value in the ":authority" pseudo-header field for which the server is authoritative (see Section 10.1). A client MUST treat a PUSH_PROMISE for which the server is not authoritative as a stream error (Section 5.4.2) of type PROTOCOL_ERROR. An intermediary can receive pushes from the server and choose not to forward them on to the client. In other words, how to make use of the pushed information is up to that intermediary. Equally, the intermediary might choose to make additional pushes to the client, without any action taken by the server. A client cannot push. Thus, servers MUST treat the receipt of a PUSH_PROMISE frame as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. A server cannot set the SETTINGS_ENABLE_PUSH setting to a value other than 0 (see Section 6.5.2). 8.2.1. Push Requests Server push is semantically equivalent to a server responding to a request; however, in this case, that request is also sent by the server, as a PUSH_PROMISE frame. The PUSH_PROMISE frame includes a field block that contains control data and a complete set of request header fields that the server attributes to the request. It is not possible to push a response to a request that includes message content. Thomson & Benfield Expires 4 December 2021 [Page 58] Internet-Draft HTTP/2 June 2021 Promised requests are always associated with an explicit request from the client. The PUSH_PROMISE frames sent by the server are sent on that explicit request's stream. The PUSH_PROMISE frame also includes a promised stream identifier, chosen from the stream identifiers available to the server (see Section 5.1.1). The header fields in PUSH_PROMISE and any subsequent CONTINUATION frames MUST be a valid and complete set of request header fields (Section 8.1.2.3). The server MUST include a method in the ":method" pseudo-header field that is safe and cacheable. If a client receives a PUSH_PROMISE that does not include a complete and valid set of header fields or the ":method" pseudo-header field identifies a method that is not safe, it MUST respond with a stream error (Section 5.4.2) of type PROTOCOL_ERROR. The server SHOULD send PUSH_PROMISE (Section 6.6) frames prior to sending any frames that reference the promised responses. This avoids a race where clients issue requests prior to receiving any PUSH_PROMISE frames. For example, if the server receives a request for a document containing embedded links to multiple image files and the server chooses to push those additional images to the client, sending PUSH_PROMISE frames before the DATA frames that contain the image links ensures that the client is able to see that a resource will be pushed before discovering embedded links. Similarly, if the server pushes responses referenced by the field block (for instance, in Link header fields), sending a PUSH_PROMISE before sending the header ensures that clients do not request those resources. PUSH_PROMISE frames MUST NOT be sent by the client. PUSH_PROMISE frames can be sent by the server in response to any client-initiated stream, but the stream MUST be in either the "open" or "half-closed (remote)" state with respect to the server. PUSH_PROMISE frames are interspersed with the frames that comprise a response, though they cannot be interspersed with HEADERS and CONTINUATION frames that comprise a single field block. Sending a PUSH_PROMISE frame creates a new stream and puts the stream into the "reserved (local)" state for the server and the "reserved (remote)" state for the client. Thomson & Benfield Expires 4 December 2021 [Page 59] Internet-Draft HTTP/2 June 2021 8.2.2. Push Responses After sending the PUSH_PROMISE frame, the server can begin delivering the pushed response as a response (Section 8.1.2.4) on a server- initiated stream that uses the promised stream identifier. The server uses this stream to transmit an HTTP response, using the same sequence of frames as defined in Section 8.1. This stream becomes "half-closed" to the client (Section 5.1) after the initial HEADERS frame is sent. Once a client receives a PUSH_PROMISE frame and chooses to accept the pushed response, the client SHOULD NOT issue any requests for the promised response until after the promised stream has closed. If the client determines, for any reason, that it does not wish to receive the pushed response from the server or if the server takes too long to begin sending the promised response, the client can send a RST_STREAM frame, using either the CANCEL or REFUSED_STREAM code and referencing the pushed stream's identifier. A client can use the SETTINGS_MAX_CONCURRENT_STREAMS setting to limit the number of responses that can be concurrently pushed by a server. Advertising a SETTINGS_MAX_CONCURRENT_STREAMS value of zero disables server push by preventing the server from creating the necessary streams. This does not prohibit a server from sending PUSH_PROMISE frames; clients need to reset any promised streams that are not wanted. Clients receiving a pushed response MUST validate that either the server is authoritative (see Section 10.1) or the proxy that provided the pushed response is configured for the corresponding request. For example, a server that offers a certificate for only the "example.com" DNS-ID is not permitted to push a response for "https://www.example.org/doc". The response for a PUSH_PROMISE stream begins with a HEADERS frame, which immediately puts the stream into the "half-closed (remote)" state for the server and "half-closed (local)" state for the client, and ends with a frame bearing END_STREAM, which places the stream in the "closed" state. | Note: The client never sends a frame with the END_STREAM flag | for a server push. Thomson & Benfield Expires 4 December 2021 [Page 60] Internet-Draft HTTP/2 June 2021 8.3. The CONNECT Method In HTTP/1.x, the pseudo-method CONNECT (Section 9.3.6 of [HTTP]) is used to convert an HTTP connection into a tunnel to a remote host. CONNECT 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/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 header section is constructed as defined in Section 8.1.2.3 ("Request Pseudo-Header Fields"), with a few differences. Specifically: * The ":method" pseudo-header field is set to "CONNECT". * The ":scheme" and ":path" pseudo-header fields MUST be omitted. * The ":authority" pseudo-header field contains the host and port to connect to (equivalent to the authority-form of the request-target of CONNECT requests; see Section 3.2.3 of [HTTP11]). A CONNECT request that does not conform to these restrictions is malformed (Section 8.1.2.6). A proxy that supports CONNECT establishes a TCP connection [TCP] to the host and port 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 Section 9.3.6 of [HTTP]. After the initial HEADERS frame sent by each peer, all subsequent DATA frames correspond to data sent on the TCP connection. The frame payload of any DATA frames sent by the client is transmitted by the proxy to the TCP server; data received from the TCP server is assembled into DATA frames by the proxy. Frame types other than DATA or stream management frames (RST_STREAM, WINDOW_UPDATE, and PRIORITY) MUST NOT be sent on a connected stream and MUST be treated as a stream error (Section 5.4.2) if received. Thomson & Benfield Expires 4 December 2021 [Page 61] Internet-Draft HTTP/2 June 2021 The TCP connection can be closed by either peer. The END_STREAM flag on a DATA frame is treated as being equivalent to the TCP FIN bit. A client is expected to send a DATA frame with the END_STREAM flag set after receiving a frame bearing the END_STREAM flag. A proxy that receives a DATA frame with the END_STREAM flag set sends the attached data with the FIN bit set on the last TCP segment. A proxy that receives a TCP segment with the FIN bit set sends a DATA frame with the END_STREAM flag set. Note that the final TCP segment or DATA frame could be empty. 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 (Section 5.4.2) of type CONNECT_ERROR. Correspondingly, a proxy MUST send a TCP segment with the RST bit set if it detects an error with the stream or the HTTP/2 connection. 9. Additional HTTP Requirements/Considerations This section outlines attributes of the HTTP protocol that improve interoperability, reduce exposure to known security vulnerabilities, or reduce the potential for implementation variation. 9.1. Connection Management HTTP/2 connections are persistent. For best performance, it is expected that clients will not close connections until it is determined that no further communication with a server is necessary (for example, when a user navigates away from a particular web page) or until the server closes the connection. Clients SHOULD NOT open more than one HTTP/2 connection to a given host and port pair, where the host is derived from a URI, a selected alternative service [ALT-SVC], or a configured proxy. A client can create additional connections as replacements, either to replace connections that are near to exhausting the available stream identifier space (Section 5.1.1), to refresh the keying material for a TLS connection, or to replace connections that have encountered errors (Section 5.4.1). A client MAY open multiple connections to the same IP address and TCP port using different Server Name Indication [TLS-EXT] values or to provide different TLS client certificates but SHOULD avoid creating multiple connections with the same configuration. Thomson & Benfield Expires 4 December 2021 [Page 62] Internet-Draft HTTP/2 June 2021 Servers are encouraged to maintain open connections for as long as possible but are permitted to terminate idle connections if necessary. When either endpoint chooses to close the transport-layer TCP connection, the terminating endpoint SHOULD first send a GOAWAY (Section 6.8) frame so that both endpoints can reliably determine whether previously sent frames have been processed and gracefully complete or terminate any necessary remaining tasks. 9.1.1. Connection Reuse Connections that are made to an origin server, either directly or through a tunnel created using the CONNECT method (Section 8.3), MAY be reused for requests with multiple different URI authority components. A connection can be reused as long as the origin server is authoritative (Section 10.1). For TCP connections without TLS, this depends on the host having resolved to the same IP address. For "https" resources, connection reuse additionally depends on having a certificate that is valid for the host in the URI. The certificate presented by the server MUST satisfy any checks that the client would perform when forming a new TLS connection for the host in the URI. A single certificate can be used to establish authority for multiple origins. Section 4.3 of [HTTP] describes how a client determines whether a server is authoritative for a URI. In some deployments, reusing a connection for multiple origins can result in requests being directed to the wrong origin server. For example, TLS termination might be performed by a middlebox that uses the TLS Server Name Indication (SNI) [TLS-EXT] extension to select an origin server. This means that it is possible for clients to send requests to servers that might not be the intended target for the request, even though the server is otherwise authoritative. A server that does not wish clients to reuse connections can indicate that it is not authoritative for a request by sending a 421 (Misdirected Request) status code in response to the request (see Section 15.5.20 of [HTTP]). A client that is configured to use a proxy over HTTP/2 directs requests to that proxy through a single connection. That is, all requests sent via a proxy reuse the connection to the proxy. 9.2. Use of TLS Features Implementations of HTTP/2 MUST use TLS version 1.2 [TLS12] or higher for HTTP/2 over TLS. The general TLS usage guidance in [TLSBCP] SHOULD be followed, with some additional restrictions that are specific to HTTP/2. Thomson & Benfield Expires 4 December 2021 [Page 63] Internet-Draft HTTP/2 June 2021 The TLS implementation MUST support the Server Name Indication (SNI) [TLS-EXT] extension to TLS. If the server is identified by a domain name [DNS-TERMS], clients MUST send the server_name TLS extension unless an alternative mechanism to indicate the target host is used. Requirements for deployments of HTTP/2 that negotiate TLS 1.3 [TLS13] are included in Section 9.2.3. Deployments of TLS 1.2 are subject to the requirements in Section 9.2.1 and Section 9.2.2. Implementations are encouraged to provide defaults that comply, but it is recognized that deployments are ultimately responsible for compliance. 9.2.1. TLS 1.2 Features This section describes restrictions on the TLS 1.2 feature set that can be used with HTTP/2. Due to deployment limitations, it might not be possible to fail TLS negotiation when these restrictions are not met. An endpoint MAY immediately terminate an HTTP/2 connection that does not meet these TLS requirements with a connection error (Section 5.4.1) of type INADEQUATE_SECURITY. A deployment of HTTP/2 over TLS 1.2 MUST disable compression. TLS compression can lead to the exposure of information that would not otherwise be revealed [RFC3749]. Generic compression is unnecessary since HTTP/2 provides compression features that are more aware of context and therefore likely to be more appropriate for use for performance, security, or other reasons. A deployment of HTTP/2 over TLS 1.2 MUST disable renegotiation. An endpoint MUST treat a TLS renegotiation as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Note that disabling renegotiation can result in long-lived connections becoming unusable due to limits on the number of messages the underlying cipher suite can encipher. An endpoint MAY use renegotiation to provide confidentiality protection for client credentials offered in the handshake, but any renegotiation MUST occur prior to sending the connection preface. A server SHOULD request a client certificate if it sees a renegotiation request immediately after establishing a connection. This effectively prevents the use of renegotiation in response to a request for a specific protected resource. A future specification might provide a way to support this use case. Alternatively, a server might use an error (Section 5.4) of type HTTP_1_1_REQUIRED to request the client use a protocol that supports renegotiation. Thomson & Benfield Expires 4 December 2021 [Page 64] Internet-Draft HTTP/2 June 2021 Implementations MUST support ephemeral key exchange sizes of at least 2048 bits for cipher suites that use ephemeral finite field Diffie- Hellman (DHE) [TLS13] and 224 bits for cipher suites that use ephemeral elliptic curve Diffie-Hellman (ECDHE) [RFC4492]. Clients MUST accept DHE sizes of up to 4096 bits. Endpoints MAY treat negotiation of key sizes smaller than the lower limits as a connection error (Section 5.4.1) of type INADEQUATE_SECURITY. 9.2.2. TLS 1.2 Cipher Suites A deployment of HTTP/2 over TLS 1.2 SHOULD NOT use any of the cipher suites that are listed in the list of prohibited cipher suites (Appendix A). Endpoints MAY choose to generate a connection error (Section 5.4.1) of type INADEQUATE_SECURITY if one of the prohibited cipher suites is negotiated. A deployment that chooses to use a prohibited cipher suite risks triggering a connection error unless the set of potential peers is known to accept that cipher suite. Implementations MUST NOT generate this error in reaction to the negotiation of a cipher suite that is not prohibited. Consequently, when clients offer a cipher suite that is not prohibited, they have to be prepared to use that cipher suite with HTTP/2. The list of prohibited cipher suites includes the cipher suite that TLS 1.2 makes mandatory, which means that TLS 1.2 deployments could have non-intersecting sets of permitted cipher suites. To avoid this problem causing TLS handshake failures, deployments of HTTP/2 that use TLS 1.2 MUST support TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 [TLS-ECDHE] with the P-256 elliptic curve [FIPS186]. Note that clients might advertise support of cipher suites that are prohibited in order to allow for connection to servers that do not support HTTP/2. This allows servers to select HTTP/1.1 with a cipher suite that is prohibited in HTTP/2. However, this can result in HTTP/2 being negotiated with a prohibited cipher suite if the application protocol and cipher suite are independently selected. 9.2.3. TLS 1.3 Features TLS 1.3 includes a number of features not available in earlier versions. This section discusses the use of these features. HTTP/2 servers MUST NOT send post-handshake TLS 1.3 CertificateRequest messages. HTTP/2 clients MUST treat a TLS post- handshake CertificateRequest message as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Thomson & Benfield Expires 4 December 2021 [Page 65] Internet-Draft HTTP/2 June 2021 The prohibition on post-handshake authentication applies even if the client offered the "post_handshake_auth" TLS extension. Post- handshake authentication support might be advertised independently of ALPN [TLS-ALPN]. Clients might offer the capability for use in other protocols, but inclusion of the extension cannot imply support within HTTP/2. [TLS13] defines other post-handshake messages, NewSessionTicket and KeyUpdate, which can be used as they have no direct interaction with HTTP/2. Unless the use of a new type of TLS message depends on an interaction with the application-layer protocol, that TLS message can be sent after the handshake completes. TLS early data MAY be used to send requests, provided that the guidance in [RFC8470] is observed. Clients send requests in early data assuming initial values for all server settings. 10. Security Considerations 10.1. Server Authority HTTP/2 relies on the HTTP definition of authority for determining whether a server is authoritative in providing a given response (see Section 4.3 of [HTTP]). This relies on local name resolution for the "http" URI scheme and the authenticated server identity for the "https" scheme. 10.2. Cross-Protocol Attacks In a cross-protocol attack, an attacker causes a client to initiate a transaction in one protocol toward a server that understands a different protocol. An attacker might be able to cause the transaction to appear as a valid transaction in the second protocol. In combination with the capabilities of the web context, this can be used to interact with poorly protected servers in private networks. Completing a TLS handshake with an ALPN identifier for HTTP/2 can be considered sufficient protection against cross-protocol attacks. ALPN provides a positive indication that a server is willing to proceed with HTTP/2, which prevents attacks on other TLS-based protocols. The encryption in TLS makes it difficult for attackers to control the data that could be used in a cross-protocol attack on a cleartext protocol. Thomson & Benfield Expires 4 December 2021 [Page 66] Internet-Draft HTTP/2 June 2021 The cleartext version of HTTP/2 has minimal protection against cross- protocol attacks. The connection preface (Section 3.4) contains a string that is designed to confuse HTTP/1.1 servers, but no special protection is offered for other protocols. 10.3. Intermediary Encapsulation Attacks HPACK permits encoding of field names and values that might be treated as delimiters in other HTTP versions. An intermediary that translates an HTTP/2 request or response MUST validate fields according to the rules in Section 8.1.2 roles before translating a message to another HTTP version. Translating a field that includes invalid delimiters could be used to cause recipients to incorrectly interpret a message, which could be exploited by an attacker. An intermediary can reject fields that contain invalid field names or values for other reasons, in particular those that do not conform to the HTTP ABNF grammar from Section 5 of [HTTP]. Intermediaries that do not perform any validation of fields other than the minimum required by Section 8.1.2 could forward messages that contain invalid field names or values. An intermediary that receives any field that requires removal before forwarding (see Section 7.6.1 of [HTTP]) MUST remove or replace those header fields when forwarding messages. Additionally, intermediaries should take care when forwarding messages containing Content-Length fields to ensure that the message is well-formed (Section 8.1.2.6). This ensures that if the message is translated into HTTP/1.1 at any point the framing will be correct. 10.4. Cacheability of Pushed Responses Pushed responses do not have an explicit request from the client; the request is provided by the server in the PUSH_PROMISE frame. Caching responses that are pushed is possible based on the guidance provided by the origin server in the Cache-Control header field. However, this can cause issues if a single server hosts more than one tenant. For example, a server might offer multiple users each a small portion of its URI space. Where multiple tenants share space on the same server, that server MUST ensure that tenants are not able to push representations of resources that they do not have authority over. Failure to enforce this would allow a tenant to provide a representation that would be served out of cache, overriding the actual representation that the authoritative tenant provides. Thomson & Benfield Expires 4 December 2021 [Page 67] Internet-Draft HTTP/2 June 2021 Pushed responses for which an origin server is not authoritative (see Section 10.1) MUST NOT be used or cached. 10.5. Denial-of-Service Considerations An HTTP/2 connection can demand a greater commitment of resources to operate than an HTTP/1.1 connection. The use of field section compression and flow control depend on a commitment of resources for storing a greater amount of state. Settings for these features ensure that memory commitments for these features are strictly bounded. The number of PUSH_PROMISE frames is not constrained in the same fashion. A client that accepts server push SHOULD limit the number of streams it allows to be in the "reserved (remote)" state. An excessive number of server push streams can be treated as a stream error (Section 5.4.2) of type ENHANCE_YOUR_CALM. A number of HTTP/2 implementations were found to be vulnerable to denial of service [NFLX-2019-002]. The following lists known ways that implementations might be subject to denial of service attack: * Inefficient tracking of outstanding outbound frames can lead to overload if an adversary can cause large numbers of frames to be enqueued for sending. A peer could use one of several techniques to cause large numbers of frames to be generated: - Providing tiny increments to flow control in WINDOW_UPDATE frames can cause a sender to generate a large number of DATA frames. - An endpoint is required to respond to a PING frame. - Each SETTINGS frame requires acknowledgment. - An invalid request (or server push) can cause a peer to send RST_STREAM frames in response. * Large numbers of small or empty frames can be abused to cause a peer to expend time processing frame headers. Caution is required here as some uses of small frames are entirely legitimate, such as the sending of an empty DATA or CONTINUATION frame at the end of a stream. * The SETTINGS frame might also be abused to cause a peer to expend additional processing time. This might be done by pointlessly changing settings, sending multiple undefined settings, or changing the same setting multiple times in the same frame. Thomson & Benfield Expires 4 December 2021 [Page 68] Internet-Draft HTTP/2 June 2021 * Handling reprioritization with PRIORITY frames can require significant processing time and can lead to overload if many PRIORITY frames are sent. * Field section compression also offers some opportunities to waste processing resources; see Section 7 of [COMPRESSION] for more details on potential abuses. * Limits in SETTINGS cannot be reduced instantaneously, which leaves an endpoint exposed to behavior from a peer that could exceed the new limits. In particular, immediately after establishing a connection, limits set by a server are not known to clients and could be exceeded without being an obvious protocol violation. * An attacker can provide large amounts of flow control credit at the HTTP/2 layer, but withhold credit at the TCP layer, preventing frames from being sent. An endpoint that constructs and remembers frames for sending without considering TCP limits might be subject to resource exhaustion. Most of the features that might be exploited for denial of service -- i.e., SETTINGS changes, small frames, field section compression -- have legitimate uses. These features become a burden only when they are used unnecessarily or to excess. An endpoint that doesn't monitor use of these features exposes itself to a risk of denial of service. Implementations SHOULD track the use of these features and set limits on their use. An endpoint MAY treat activity that is suspicious as a connection error (Section 5.4.1) of type ENHANCE_YOUR_CALM. 10.5.1. Limits on Field Block Size A large field block (Section 4.3) can cause an implementation to commit a large amount of state. Field lines that are critical for routing can appear toward the end of a field block, which prevents streaming of fields to their ultimate destination. This ordering and other reasons, such as ensuring cache correctness, mean that an endpoint might need to buffer the entire field block. Since there is no hard limit to the size of a field block, some endpoints could be forced to commit a large amount of available memory for field blocks. An endpoint can use the SETTINGS_MAX_HEADER_LIST_SIZE to advise peers of limits that might apply on the size of uncompressed field blocks. This setting is only advisory, so endpoints MAY choose to send field blocks that exceed this limit and risk having the request or response being treated as malformed. This setting is specific to a connection, so any request or response could encounter a hop with a Thomson & Benfield Expires 4 December 2021 [Page 69] Internet-Draft HTTP/2 June 2021 lower, unknown limit. An intermediary can attempt to avoid this problem by passing on values presented by different peers, but they are not obliged to do so. A server that receives a larger field block than it is willing to handle can send an HTTP 431 (Request Header Fields Too Large) status code [RFC6585]. A client can discard responses that it cannot process. The field block MUST be processed to ensure a consistent connection state, unless the connection is closed. 10.5.2. CONNECT Issues The CONNECT method can be used to create disproportionate load on an proxy, since stream creation is relatively inexpensive when compared to the creation and maintenance of a TCP connection. A proxy might also maintain some resources for a TCP connection beyond the closing of the stream that carries the CONNECT request, since the outgoing TCP connection remains in the TIME_WAIT state. Therefore, a proxy cannot rely on SETTINGS_MAX_CONCURRENT_STREAMS alone to limit the resources consumed by CONNECT requests. 10.6. Use of Compression Compression can allow an attacker to recover secret data when it is compressed in the same context as data under attacker control. HTTP/2 enables compression of field lines (Section 4.3); the following concerns also apply to the use of HTTP compressed content- codings (Section 8.4.1 of [HTTP]). There are demonstrable attacks on compression that exploit the characteristics of the web (e.g., [BREACH]). The attacker induces multiple requests containing varying plaintext, observing the length of the resulting ciphertext in each, which reveals a shorter length when a guess about the secret is correct. Implementations communicating on a secure channel MUST NOT compress content that includes both confidential and attacker-controlled data unless separate compression dictionaries are used for each source of data. Compression MUST NOT be used if the source of data cannot be reliably determined. Generic stream compression, such as that provided by TLS, MUST NOT be used with HTTP/2 (see Section 9.2). Further considerations regarding the compression of header fields are described in [COMPRESSION]. Thomson & Benfield Expires 4 December 2021 [Page 70] Internet-Draft HTTP/2 June 2021 10.7. Use of Padding Padding within HTTP/2 is not intended as a replacement for general purpose padding, such as that provided by TLS [TLS13]. Redundant padding could even be counterproductive. Correct application can depend on having specific knowledge of the data that is being padded. To mitigate attacks that rely on compression, disabling or limiting compression might be preferable to padding as a countermeasure. Padding can be used to obscure the exact size of frame content and is provided to mitigate specific attacks within HTTP, for example, attacks where compressed content includes both attacker-controlled plaintext and secret data (e.g., [BREACH]). Use of padding can result in less protection than might seem immediately obvious. At best, padding only makes it more difficult for an attacker to infer length information by increasing the number of frames an attacker has to observe. Incorrectly implemented padding schemes can be easily defeated. In particular, randomized padding with a predictable distribution provides very little protection; similarly, padding frame payloads to a fixed size exposes information as frame payload sizes cross the fixed-sized boundary, which could be possible if an attacker can control plaintext. Intermediaries SHOULD retain padding for DATA frames but MAY drop padding for HEADERS and PUSH_PROMISE frames. A valid reason for an intermediary to change the amount of padding of frames is to improve the protections that padding provides. 10.8. Privacy Considerations Several characteristics of HTTP/2 provide an observer an opportunity to correlate actions of a single client or server over time. These include the value of settings, the manner in which flow-control windows are managed, the way priorities are allocated to streams, the timing of reactions to stimulus, and the handling of any features that are controlled by settings. As far as these create observable differences in behavior, they could be used as a basis for fingerprinting a specific client, as defined in Section 3.2 of [PRIVACY]. HTTP/2's preference for using a single TCP connection allows correlation of a user's activity on a site. Reusing connections for different origins allows tracking across those origins. Thomson & Benfield Expires 4 December 2021 [Page 71] Internet-Draft HTTP/2 June 2021 Because the PING and SETTINGS frames solicit immediate responses, they can be used by an endpoint to measure latency to their peer. This might have privacy implications in certain scenarios. 10.9. Remote Timing Attacks Remote timing attacks extract secrets from servers by observing variations in the time that servers take when processing requests that use secrets. HTTP/2 enables concurrent request creation and processing, which can give attackers better control over when request processing commences. Multiple HTTP/2 requests can be included in the same IP packet or TLS record. HTTP/2 can therefore make remote timing attacks more efficient by eliminating variability in request delivery, leaving only request order and the delivery of responses as sources of timing variability. Ensuring that processing time is not dependent on the value of secrets is the best defense against any form of timing attack. 11. IANA Considerations A string for identifying HTTP/2 is entered into the "Application- Layer Protocol Negotiation (ALPN) Protocol IDs" registry established in [TLS-ALPN]. This document establishes a registry for frame types, settings, and error codes. These new registries appear in the new "Hypertext Transfer Protocol version 2 (HTTP/2)" section. This document registers the "HTTP2-Settings" header field for use in HTTP. This document registers the "PRI" method for use in HTTP to avoid collisions with the connection preface (Section 3.4). 11.1. Registration of HTTP/2 Identification Strings This document creates two registrations for the identification of HTTP/2 (see Section 3.2) in the "Application-Layer Protocol Negotiation (ALPN) Protocol IDs" registry established in [TLS-ALPN]. The "h2" string identifies HTTP/2 when used over TLS: Protocol: HTTP/2 over TLS Identification Sequence: 0x68 0x32 ("h2") Specification: This document Thomson & Benfield Expires 4 December 2021 [Page 72] Internet-Draft HTTP/2 June 2021 The "h2c" string identifies HTTP/2 when used over cleartext TCP: Protocol: HTTP/2 over TCP Identification Sequence: 0x68 0x32 0x63 ("h2c") Specification: This document 11.2. Frame Type Registry This document establishes a registry for HTTP/2 frame type codes. The "HTTP/2 Frame Type" registry manages an 8-bit space. The "HTTP/2 Frame Type" registry operates under either of the Section 4.8 of "IETF Review" [RFC8126] or Section 4.10 of "IESG Approval" [RFC8126] policies. New entries in this registry require the following information: Frame Type: A name or label for the frame type. Code: The 8-bit code assigned to the frame type. Specification: A reference to a specification that includes a description of the frame layout, its semantics, and flags that the frame type uses, including any parts of the frame that are conditionally present based on the value of flags. The entries in the following table are registered by this document. Thomson & Benfield Expires 4 December 2021 [Page 73] Internet-Draft HTTP/2 June 2021 +===============+======+==============+ | Frame Type | Code | Section | +===============+======+==============+ | DATA | 0x0 | Section 6.1 | +---------------+------+--------------+ | HEADERS | 0x1 | Section 6.2 | +---------------+------+--------------+ | PRIORITY | 0x2 | Section 6.3 | +---------------+------+--------------+ | RST_STREAM | 0x3 | Section 6.4 | +---------------+------+--------------+ | SETTINGS | 0x4 | Section 6.5 | +---------------+------+--------------+ | PUSH_PROMISE | 0x5 | Section 6.6 | +---------------+------+--------------+ | PING | 0x6 | Section 6.7 | +---------------+------+--------------+ | GOAWAY | 0x7 | Section 6.8 | +---------------+------+--------------+ | WINDOW_UPDATE | 0x8 | Section 6.9 | +---------------+------+--------------+ | CONTINUATION | 0x9 | Section 6.10 | +---------------+------+--------------+ Table 1 11.3. Settings Registry This document establishes a registry for HTTP/2 settings. The "HTTP/2 Settings" registry manages a 16-bit space. The "HTTP/2 Settings" registry operates under the Section 4.5 of "Expert Review" policy [RFC8126]. New registrations are advised to provide the following information: Name: A symbolic name for the setting. Specifying a setting name is optional. Code: The 16-bit code assigned to the setting. Initial Value: An initial value for the setting. Specification: An optional reference to a specification that describes the use of the setting. The entries in the following table are registered by this document. Thomson & Benfield Expires 4 December 2021 [Page 74] Internet-Draft HTTP/2 June 2021 +========================+======+===============+===============+ | Name | Code | Initial Value | Specification | +========================+======+===============+===============+ | HEADER_TABLE_SIZE | 0x1 | 4096 | Section 6.5.2 | +------------------------+------+---------------+---------------+ | ENABLE_PUSH | 0x2 | 1 | Section 6.5.2 | +------------------------+------+---------------+---------------+ | MAX_CONCURRENT_STREAMS | 0x3 | (infinite) | Section 6.5.2 | +------------------------+------+---------------+---------------+ | INITIAL_WINDOW_SIZE | 0x4 | 65535 | Section 6.5.2 | +------------------------+------+---------------+---------------+ | MAX_FRAME_SIZE | 0x5 | 16384 | Section 6.5.2 | +------------------------+------+---------------+---------------+ | MAX_HEADER_LIST_SIZE | 0x6 | (infinite) | Section 6.5.2 | +------------------------+------+---------------+---------------+ Table 2 11.4. Error Code Registry This document establishes a registry for HTTP/2 error codes. The "HTTP/2 Error Code" registry manages a 32-bit space. The "HTTP/2 Error Code" registry operates under the Section 4.5 of "Expert Review" policy [RFC8126]. Registrations for error codes are required to include a description of the error code. An expert reviewer is advised to examine new registrations for possible duplication with existing error codes. Use of existing registrations is to be encouraged, but not mandated. New registrations are advised to provide the following information: Name: A name for the error code. Specifying an error code name is optional. Code: The 32-bit error code value. Description: A brief description of the error code semantics, longer if no detailed specification is provided. Specification: An optional reference for a specification that defines the error code. The entries in the following table are registered by this document. Thomson & Benfield Expires 4 December 2021 [Page 75] Internet-Draft HTTP/2 June 2021 +=====================+======+======================+===============+ | Name | Code | Description | Specification | +=====================+======+======================+===============+ | NO_ERROR | 0x0 | Graceful shutdown | Section 7 | +---------------------+------+----------------------+---------------+ | PROTOCOL_ERROR | 0x1 | Protocol error | Section 7 | | | | detected | | +---------------------+------+----------------------+---------------+ | INTERNAL_ERROR | 0x2 | Implementation | Section 7 | | | | fault | | +---------------------+------+----------------------+---------------+ | FLOW_CONTROL_ERROR | 0x3 | Flow-control | Section 7 | | | | limits exceeded | | +---------------------+------+----------------------+---------------+ | SETTINGS_TIMEOUT | 0x4 | Settings not | Section 7 | | | | acknowledged | | +---------------------+------+----------------------+---------------+ | STREAM_CLOSED | 0x5 | Frame received | Section 7 | | | | for closed stream | | +---------------------+------+----------------------+---------------+ | FRAME_SIZE_ERROR | 0x6 | Frame size | Section 7 | | | | incorrect | | +---------------------+------+----------------------+---------------+ | REFUSED_STREAM | 0x7 | Stream not | Section 7 | | | | processed | | +---------------------+------+----------------------+---------------+ | CANCEL | 0x8 | Stream cancelled | Section 7 | +---------------------+------+----------------------+---------------+ | COMPRESSION_ERROR | 0x9 | Compression state | Section 7 | | | | not updated | | +---------------------+------+----------------------+---------------+ | CONNECT_ERROR | 0xa | TCP connection | Section 7 | | | | error for CONNECT | | | | | method | | +---------------------+------+----------------------+---------------+ | ENHANCE_YOUR_CALM | 0xb | Processing | Section 7 | | | | capacity exceeded | | +---------------------+------+----------------------+---------------+ | INADEQUATE_SECURITY | 0xc | Negotiated TLS | Section 7 | | | | parameters not | | | | | acceptable | | +---------------------+------+----------------------+---------------+ | HTTP_1_1_REQUIRED | 0xd | Use HTTP/1.1 for | Section 7 | | | | the request | | +---------------------+------+----------------------+---------------+ Table 3 Thomson & Benfield Expires 4 December 2021 [Page 76] Internet-Draft HTTP/2 June 2021 11.5. HTTP2-Settings Header Field Registration This section marks the "HTTP2-Settings" header field registered in Section 11.5 of [RFC7540] as obsoleted. 11.6. PRI Method Registration This section registers the "PRI" method in the "HTTP Method Registry" (Section 18.2 of [HTTP]). Method Name: PRI Safe: Yes Idempotent: Yes Specification document(s): Section 3.4 of this document Related information: This method is never used by an actual client. This method will appear to be used when an HTTP/1.1 server or intermediary attempts to parse an HTTP/2 connection preface. 11.7. The h2c Upgrade Token Previous versions of this document (Section 11.8 of [RFC7540]) registered an upgrade token. This capability has been removed: see Section 3.1. 12. References 12.1. Normative References [CACHE] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Caching", Work in Progress, Internet-Draft, draft-ietf-httpbis-cache-15, 30 March 2021, <https://tools.ietf.org/html/draft-ietf-httpbis-cache-15>. [COMPRESSION] Peon, R. and H. Ruellan, "HPACK: Header Compression for HTTP/2", RFC 7541, May 2015, <https://www.rfc-editor.org/rfc/rfc7541>. [COOKIE] Barth, A., "HTTP State Management Mechanism", RFC 6265, April 2011, <https://www.rfc-editor.org/rfc/rfc6265>. [FIPS186] NIST, "Digital Signature Standard (DSS)", FIPS PUB 186-4, July 2013, <http://dx.doi.org/10.6028/NIST.FIPS.186-4>. Thomson & Benfield Expires 4 December 2021 [Page 77] Internet-Draft HTTP/2 June 2021 [HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", Work in Progress, Internet-Draft, draft-ietf-httpbis-semantics-15, 30 March 2021, <https://tools.ietf.org/html/draft-ietf-httpbis-semantics- 15>. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997, <https://www.rfc-editor.org/rfc/rfc2119>. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005, <https://www.rfc-editor.org/rfc/rfc3986>. [RFC8126] Cotton, M., Leiba, B., and R. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, <https://www.rfc-editor.org/rfc/rfc8126>. [RFC8470] Thomson, M., Nottingham, M., and W. Tarreau, "Using Early Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September 2018, <https://www.rfc-editor.org/rfc/rfc8470>. [TCP] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981, <https://www.rfc-editor.org/rfc/rfc793>. [TLS-ALPN] Friedl, S., Popov, A., Langley, A., and E. Stephan, "Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension", RFC 7301, July 2014, <https://www.rfc-editor.org/rfc/rfc7301>. [TLS-ECDHE] Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA- 256/384 and AES Galois Counter Mode (GCM)", RFC 5289, August 2008, <https://www.rfc-editor.org/rfc/rfc5289>. [TLS-EXT] Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, January 2011, <https://www.rfc-editor.org/rfc/rfc6066>. [TLS12] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008, <https://www.rfc-editor.org/rfc/rfc5246>. Thomson & Benfield Expires 4 December 2021 [Page 78] Internet-Draft HTTP/2 June 2021 [TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, <https://www.rfc-editor.org/rfc/rfc8446>. 12.2. Informative References [ALT-SVC] Nottingham, M., McManus, P., and J. Reschke, "HTTP Alternative Services", RFC 7838, April 2016, <https://www.rfc-editor.org/rfc/rfc7838>. [BREACH] Gluck, Y., Harris, N., and A. Prado, "BREACH: Reviving the CRIME Attack", 12 July 2013, <http://breachattack.com/resources/ BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf>. [DNS-TERMS] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499, January 2019, <https://www.rfc-editor.org/rfc/rfc8499>. [HTTP11] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1", Work in Progress, Internet-Draft, draft- ietf-httpbis-messaging-15, 30 March 2021, <https://tools.ietf.org/html/draft-ietf-httpbis-messaging- 15>. [I-D.ietf-httpbis-priority] Oku, K. and L. Pardue, "Extensible Prioritization Scheme for HTTP", Work in Progress, Internet-Draft, draft-ietf- httpbis-priority-03, 11 January 2021, <https://tools.ietf.org/html/draft-ietf-httpbis-priority- 03>. [NFLX-2019-002] Netflix, "HTTP/2 Denial of Service Advisory", 13 August 2019, <https://github.com/Netflix/security- bulletins/blob/master/advisories/third-party/2019-002.md>. [PRIVACY] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., Morris, J., Hansen, M., and R. Smith, "Privacy Considerations for Internet Protocols", RFC 6973, DOI 10.17487/RFC6973, July 2013, <https://www.rfc-editor.org/rfc/rfc6973>. [RFC3749] Hollenbeck, S., "Transport Layer Security Protocol Compression Methods", RFC 3749, May 2004, <https://www.rfc-editor.org/rfc/rfc3749>. Thomson & Benfield Expires 4 December 2021 [Page 79] Internet-Draft HTTP/2 June 2021 [RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B. Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)", RFC 4492, May 2006, <https://www.rfc-editor.org/rfc/rfc4492>. [RFC6585] Nottingham, M. and R. Fielding, "Additional HTTP Status Codes", RFC 6585, April 2012, <https://www.rfc-editor.org/rfc/rfc6585>. [RFC7323] Borman, D., Braden, B., Jacobson, V., and R. Scheffenegger, Ed., "TCP Extensions for High Performance", RFC 7323, September 2014, <https://www.rfc-editor.org/rfc/rfc7323>. [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI 10.17487/RFC7540, May 2015, <https://www.rfc-editor.org/rfc/rfc7540>. [RFC8740] Benjamin, D., "Using TLS 1.3 with HTTP/2", RFC 8740, DOI 10.17487/RFC8740, February 2020, <https://www.rfc-editor.org/rfc/rfc8740>. [TALKING] Huang, L., Chen, E., Barth, A., Rescorla, E., and C. Jackson, "Talking to Yourself for Fun and Profit", 2011, <http://w2spconf.com/2011/papers/websocket.pdf>. [TLSBCP] Sheffer, Y., Holz, R., and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, May 2015, <https://www.rfc-editor.org/rfc/rfc7525>. Appendix A. Prohibited TLS 1.2 Cipher Suites An HTTP/2 implementation MAY treat the negotiation of any of the following cipher suites with TLS 1.2 as a connection error (Section 5.4.1) of type INADEQUATE_SECURITY: * TLS_NULL_WITH_NULL_NULL * TLS_RSA_WITH_NULL_MD5 * TLS_RSA_WITH_NULL_SHA * TLS_RSA_EXPORT_WITH_RC4_40_MD5 * TLS_RSA_WITH_RC4_128_MD5 * TLS_RSA_WITH_RC4_128_SHA * TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5 * TLS_RSA_WITH_IDEA_CBC_SHA * TLS_RSA_EXPORT_WITH_DES40_CBC_SHA Thomson & Benfield Expires 4 December 2021 [Page 80] Internet-Draft HTTP/2 June 2021 * TLS_RSA_WITH_DES_CBC_SHA * TLS_RSA_WITH_3DES_EDE_CBC_SHA * TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA * TLS_DH_DSS_WITH_DES_CBC_SHA * TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA * TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA * TLS_DH_RSA_WITH_DES_CBC_SHA * TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA * TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA * TLS_DHE_DSS_WITH_DES_CBC_SHA * TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA * TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA * TLS_DHE_RSA_WITH_DES_CBC_SHA * TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA * TLS_DH_anon_EXPORT_WITH_RC4_40_MD5 * TLS_DH_anon_WITH_RC4_128_MD5 * TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA * TLS_DH_anon_WITH_DES_CBC_SHA * TLS_DH_anon_WITH_3DES_EDE_CBC_SHA * TLS_KRB5_WITH_DES_CBC_SHA * TLS_KRB5_WITH_3DES_EDE_CBC_SHA * TLS_KRB5_WITH_RC4_128_SHA * TLS_KRB5_WITH_IDEA_CBC_SHA * TLS_KRB5_WITH_DES_CBC_MD5 * TLS_KRB5_WITH_3DES_EDE_CBC_MD5 * TLS_KRB5_WITH_RC4_128_MD5 * TLS_KRB5_WITH_IDEA_CBC_MD5 * TLS_KRB5_EXPORT_WITH_DES_CBC_40_SHA * TLS_KRB5_EXPORT_WITH_RC2_CBC_40_SHA * TLS_KRB5_EXPORT_WITH_RC4_40_SHA * TLS_KRB5_EXPORT_WITH_DES_CBC_40_MD5 * TLS_KRB5_EXPORT_WITH_RC2_CBC_40_MD5 * TLS_KRB5_EXPORT_WITH_RC4_40_MD5 * TLS_PSK_WITH_NULL_SHA * TLS_DHE_PSK_WITH_NULL_SHA * TLS_RSA_PSK_WITH_NULL_SHA * TLS_RSA_WITH_AES_128_CBC_SHA * TLS_DH_DSS_WITH_AES_128_CBC_SHA * TLS_DH_RSA_WITH_AES_128_CBC_SHA * TLS_DHE_DSS_WITH_AES_128_CBC_SHA * TLS_DHE_RSA_WITH_AES_128_CBC_SHA * TLS_DH_anon_WITH_AES_128_CBC_SHA * TLS_RSA_WITH_AES_256_CBC_SHA * TLS_DH_DSS_WITH_AES_256_CBC_SHA * TLS_DH_RSA_WITH_AES_256_CBC_SHA * TLS_DHE_DSS_WITH_AES_256_CBC_SHA * TLS_DHE_RSA_WITH_AES_256_CBC_SHA * TLS_DH_anon_WITH_AES_256_CBC_SHA Thomson & Benfield Expires 4 December 2021 [Page 81] Internet-Draft HTTP/2 June 2021 * TLS_RSA_WITH_NULL_SHA256 * TLS_RSA_WITH_AES_128_CBC_SHA256 * TLS_RSA_WITH_AES_256_CBC_SHA256 * TLS_DH_DSS_WITH_AES_128_CBC_SHA256 * TLS_DH_RSA_WITH_AES_128_CBC_SHA256 * TLS_DHE_DSS_WITH_AES_128_CBC_SHA256 * TLS_RSA_WITH_CAMELLIA_128_CBC_SHA * TLS_DH_DSS_WITH_CAMELLIA_128_CBC_SHA * TLS_DH_RSA_WITH_CAMELLIA_128_CBC_SHA * TLS_DHE_DSS_WITH_CAMELLIA_128_CBC_SHA * TLS_DHE_RSA_WITH_CAMELLIA_128_CBC_SHA * TLS_DH_anon_WITH_CAMELLIA_128_CBC_SHA * TLS_DHE_RSA_WITH_AES_128_CBC_SHA256 * TLS_DH_DSS_WITH_AES_256_CBC_SHA256 * TLS_DH_RSA_WITH_AES_256_CBC_SHA256 * TLS_DHE_DSS_WITH_AES_256_CBC_SHA256 * TLS_DHE_RSA_WITH_AES_256_CBC_SHA256 * TLS_DH_anon_WITH_AES_128_CBC_SHA256 * TLS_DH_anon_WITH_AES_256_CBC_SHA256 * TLS_RSA_WITH_CAMELLIA_256_CBC_SHA * TLS_DH_DSS_WITH_CAMELLIA_256_CBC_SHA * TLS_DH_RSA_WITH_CAMELLIA_256_CBC_SHA * TLS_DHE_DSS_WITH_CAMELLIA_256_CBC_SHA * TLS_DHE_RSA_WITH_CAMELLIA_256_CBC_SHA * TLS_DH_anon_WITH_CAMELLIA_256_CBC_SHA * TLS_PSK_WITH_RC4_128_SHA * TLS_PSK_WITH_3DES_EDE_CBC_SHA * TLS_PSK_WITH_AES_128_CBC_SHA * TLS_PSK_WITH_AES_256_CBC_SHA * TLS_DHE_PSK_WITH_RC4_128_SHA * TLS_DHE_PSK_WITH_3DES_EDE_CBC_SHA * TLS_DHE_PSK_WITH_AES_128_CBC_SHA * TLS_DHE_PSK_WITH_AES_256_CBC_SHA * TLS_RSA_PSK_WITH_RC4_128_SHA * TLS_RSA_PSK_WITH_3DES_EDE_CBC_SHA * TLS_RSA_PSK_WITH_AES_128_CBC_SHA * TLS_RSA_PSK_WITH_AES_256_CBC_SHA * TLS_RSA_WITH_SEED_CBC_SHA * TLS_DH_DSS_WITH_SEED_CBC_SHA * TLS_DH_RSA_WITH_SEED_CBC_SHA * TLS_DHE_DSS_WITH_SEED_CBC_SHA * TLS_DHE_RSA_WITH_SEED_CBC_SHA * TLS_DH_anon_WITH_SEED_CBC_SHA * TLS_RSA_WITH_AES_128_GCM_SHA256 * TLS_RSA_WITH_AES_256_GCM_SHA384 * TLS_DH_RSA_WITH_AES_128_GCM_SHA256 * TLS_DH_RSA_WITH_AES_256_GCM_SHA384 * TLS_DH_DSS_WITH_AES_128_GCM_SHA256 Thomson & Benfield Expires 4 December 2021 [Page 82] Internet-Draft HTTP/2 June 2021 * TLS_DH_DSS_WITH_AES_256_GCM_SHA384 * TLS_DH_anon_WITH_AES_128_GCM_SHA256 * TLS_DH_anon_WITH_AES_256_GCM_SHA384 * TLS_PSK_WITH_AES_128_GCM_SHA256 * TLS_PSK_WITH_AES_256_GCM_SHA384 * TLS_RSA_PSK_WITH_AES_128_GCM_SHA256 * TLS_RSA_PSK_WITH_AES_256_GCM_SHA384 * TLS_PSK_WITH_AES_128_CBC_SHA256 * TLS_PSK_WITH_AES_256_CBC_SHA384 * TLS_PSK_WITH_NULL_SHA256 * TLS_PSK_WITH_NULL_SHA384 * TLS_DHE_PSK_WITH_AES_128_CBC_SHA256 * TLS_DHE_PSK_WITH_AES_256_CBC_SHA384 * TLS_DHE_PSK_WITH_NULL_SHA256 * TLS_DHE_PSK_WITH_NULL_SHA384 * TLS_RSA_PSK_WITH_AES_128_CBC_SHA256 * TLS_RSA_PSK_WITH_AES_256_CBC_SHA384 * TLS_RSA_PSK_WITH_NULL_SHA256 * TLS_RSA_PSK_WITH_NULL_SHA384 * TLS_RSA_WITH_CAMELLIA_128_CBC_SHA256 * TLS_DH_DSS_WITH_CAMELLIA_128_CBC_SHA256 * TLS_DH_RSA_WITH_CAMELLIA_128_CBC_SHA256 * TLS_DHE_DSS_WITH_CAMELLIA_128_CBC_SHA256 * TLS_DHE_RSA_WITH_CAMELLIA_128_CBC_SHA256 * TLS_DH_anon_WITH_CAMELLIA_128_CBC_SHA256 * TLS_RSA_WITH_CAMELLIA_256_CBC_SHA256 * TLS_DH_DSS_WITH_CAMELLIA_256_CBC_SHA256 * TLS_DH_RSA_WITH_CAMELLIA_256_CBC_SHA256 * TLS_DHE_DSS_WITH_CAMELLIA_256_CBC_SHA256 * TLS_DHE_RSA_WITH_CAMELLIA_256_CBC_SHA256 * TLS_DH_anon_WITH_CAMELLIA_256_CBC_SHA256 * TLS_EMPTY_RENEGOTIATION_INFO_SCSV * TLS_ECDH_ECDSA_WITH_NULL_SHA * TLS_ECDH_ECDSA_WITH_RC4_128_SHA * TLS_ECDH_ECDSA_WITH_3DES_EDE_CBC_SHA * TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA * TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA * TLS_ECDHE_ECDSA_WITH_NULL_SHA * TLS_ECDHE_ECDSA_WITH_RC4_128_SHA * TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA * TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA * TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA * TLS_ECDH_RSA_WITH_NULL_SHA * TLS_ECDH_RSA_WITH_RC4_128_SHA * TLS_ECDH_RSA_WITH_3DES_EDE_CBC_SHA * TLS_ECDH_RSA_WITH_AES_128_CBC_SHA * TLS_ECDH_RSA_WITH_AES_256_CBC_SHA * TLS_ECDHE_RSA_WITH_NULL_SHA Thomson & Benfield Expires 4 December 2021 [Page 83] Internet-Draft HTTP/2 June 2021 * TLS_ECDHE_RSA_WITH_RC4_128_SHA * TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA * TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA * TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA * TLS_ECDH_anon_WITH_NULL_SHA * TLS_ECDH_anon_WITH_RC4_128_SHA * TLS_ECDH_anon_WITH_3DES_EDE_CBC_SHA * TLS_ECDH_anon_WITH_AES_128_CBC_SHA * TLS_ECDH_anon_WITH_AES_256_CBC_SHA * TLS_SRP_SHA_WITH_3DES_EDE_CBC_SHA * TLS_SRP_SHA_RSA_WITH_3DES_EDE_CBC_SHA * TLS_SRP_SHA_DSS_WITH_3DES_EDE_CBC_SHA * TLS_SRP_SHA_WITH_AES_128_CBC_SHA * TLS_SRP_SHA_RSA_WITH_AES_128_CBC_SHA * TLS_SRP_SHA_DSS_WITH_AES_128_CBC_SHA * TLS_SRP_SHA_WITH_AES_256_CBC_SHA * TLS_SRP_SHA_RSA_WITH_AES_256_CBC_SHA * TLS_SRP_SHA_DSS_WITH_AES_256_CBC_SHA * TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 * TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 * TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA256 * TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384 * TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256 * TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384 * TLS_ECDH_RSA_WITH_AES_128_CBC_SHA256 * TLS_ECDH_RSA_WITH_AES_256_CBC_SHA384 * TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256 * TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384 * TLS_ECDH_RSA_WITH_AES_128_GCM_SHA256 * TLS_ECDH_RSA_WITH_AES_256_GCM_SHA384 * TLS_ECDHE_PSK_WITH_RC4_128_SHA * TLS_ECDHE_PSK_WITH_3DES_EDE_CBC_SHA * TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA * TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA * TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA256 * TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA384 * TLS_ECDHE_PSK_WITH_NULL_SHA * TLS_ECDHE_PSK_WITH_NULL_SHA256 * TLS_ECDHE_PSK_WITH_NULL_SHA384 * TLS_RSA_WITH_ARIA_128_CBC_SHA256 * TLS_RSA_WITH_ARIA_256_CBC_SHA384 * TLS_DH_DSS_WITH_ARIA_128_CBC_SHA256 * TLS_DH_DSS_WITH_ARIA_256_CBC_SHA384 * TLS_DH_RSA_WITH_ARIA_128_CBC_SHA256 * TLS_DH_RSA_WITH_ARIA_256_CBC_SHA384 * TLS_DHE_DSS_WITH_ARIA_128_CBC_SHA256 * TLS_DHE_DSS_WITH_ARIA_256_CBC_SHA384 * TLS_DHE_RSA_WITH_ARIA_128_CBC_SHA256 Thomson & Benfield Expires 4 December 2021 [Page 84] Internet-Draft HTTP/2 June 2021 * TLS_DHE_RSA_WITH_ARIA_256_CBC_SHA384 * TLS_DH_anon_WITH_ARIA_128_CBC_SHA256 * TLS_DH_anon_WITH_ARIA_256_CBC_SHA384 * TLS_ECDHE_ECDSA_WITH_ARIA_128_CBC_SHA256 * TLS_ECDHE_ECDSA_WITH_ARIA_256_CBC_SHA384 * TLS_ECDH_ECDSA_WITH_ARIA_128_CBC_SHA256 * TLS_ECDH_ECDSA_WITH_ARIA_256_CBC_SHA384 * TLS_ECDHE_RSA_WITH_ARIA_128_CBC_SHA256 * TLS_ECDHE_RSA_WITH_ARIA_256_CBC_SHA384 * TLS_ECDH_RSA_WITH_ARIA_128_CBC_SHA256 * TLS_ECDH_RSA_WITH_ARIA_256_CBC_SHA384 * TLS_RSA_WITH_ARIA_128_GCM_SHA256 * TLS_RSA_WITH_ARIA_256_GCM_SHA384 * TLS_DH_RSA_WITH_ARIA_128_GCM_SHA256 * TLS_DH_RSA_WITH_ARIA_256_GCM_SHA384 * TLS_DH_DSS_WITH_ARIA_128_GCM_SHA256 * TLS_DH_DSS_WITH_ARIA_256_GCM_SHA384 * TLS_DH_anon_WITH_ARIA_128_GCM_SHA256 * TLS_DH_anon_WITH_ARIA_256_GCM_SHA384 * TLS_ECDH_ECDSA_WITH_ARIA_128_GCM_SHA256 * TLS_ECDH_ECDSA_WITH_ARIA_256_GCM_SHA384 * TLS_ECDH_RSA_WITH_ARIA_128_GCM_SHA256 * TLS_ECDH_RSA_WITH_ARIA_256_GCM_SHA384 * TLS_PSK_WITH_ARIA_128_CBC_SHA256 * TLS_PSK_WITH_ARIA_256_CBC_SHA384 * TLS_DHE_PSK_WITH_ARIA_128_CBC_SHA256 * TLS_DHE_PSK_WITH_ARIA_256_CBC_SHA384 * TLS_RSA_PSK_WITH_ARIA_128_CBC_SHA256 * TLS_RSA_PSK_WITH_ARIA_256_CBC_SHA384 * TLS_PSK_WITH_ARIA_128_GCM_SHA256 * TLS_PSK_WITH_ARIA_256_GCM_SHA384 * TLS_RSA_PSK_WITH_ARIA_128_GCM_SHA256 * TLS_RSA_PSK_WITH_ARIA_256_GCM_SHA384 * TLS_ECDHE_PSK_WITH_ARIA_128_CBC_SHA256 * TLS_ECDHE_PSK_WITH_ARIA_256_CBC_SHA384 * TLS_ECDHE_ECDSA_WITH_CAMELLIA_128_CBC_SHA256 * TLS_ECDHE_ECDSA_WITH_CAMELLIA_256_CBC_SHA384 * TLS_ECDH_ECDSA_WITH_CAMELLIA_128_CBC_SHA256 * TLS_ECDH_ECDSA_WITH_CAMELLIA_256_CBC_SHA384 * TLS_ECDHE_RSA_WITH_CAMELLIA_128_CBC_SHA256 * TLS_ECDHE_RSA_WITH_CAMELLIA_256_CBC_SHA384 * TLS_ECDH_RSA_WITH_CAMELLIA_128_CBC_SHA256 * TLS_ECDH_RSA_WITH_CAMELLIA_256_CBC_SHA384 * TLS_RSA_WITH_CAMELLIA_128_GCM_SHA256 * TLS_RSA_WITH_CAMELLIA_256_GCM_SHA384 * TLS_DH_RSA_WITH_CAMELLIA_128_GCM_SHA256 * TLS_DH_RSA_WITH_CAMELLIA_256_GCM_SHA384 * TLS_DH_DSS_WITH_CAMELLIA_128_GCM_SHA256 Thomson & Benfield Expires 4 December 2021 [Page 85] Internet-Draft HTTP/2 June 2021 * TLS_DH_DSS_WITH_CAMELLIA_256_GCM_SHA384 * TLS_DH_anon_WITH_CAMELLIA_128_GCM_SHA256 * TLS_DH_anon_WITH_CAMELLIA_256_GCM_SHA384 * TLS_ECDH_ECDSA_WITH_CAMELLIA_128_GCM_SHA256 * TLS_ECDH_ECDSA_WITH_CAMELLIA_256_GCM_SHA384 * TLS_ECDH_RSA_WITH_CAMELLIA_128_GCM_SHA256 * TLS_ECDH_RSA_WITH_CAMELLIA_256_GCM_SHA384 * TLS_PSK_WITH_CAMELLIA_128_GCM_SHA256 * TLS_PSK_WITH_CAMELLIA_256_GCM_SHA384 * TLS_RSA_PSK_WITH_CAMELLIA_128_GCM_SHA256 * TLS_RSA_PSK_WITH_CAMELLIA_256_GCM_SHA384 * TLS_PSK_WITH_CAMELLIA_128_CBC_SHA256 * TLS_PSK_WITH_CAMELLIA_256_CBC_SHA384 * TLS_DHE_PSK_WITH_CAMELLIA_128_CBC_SHA256 * TLS_DHE_PSK_WITH_CAMELLIA_256_CBC_SHA384 * TLS_RSA_PSK_WITH_CAMELLIA_128_CBC_SHA256 * TLS_RSA_PSK_WITH_CAMELLIA_256_CBC_SHA384 * TLS_ECDHE_PSK_WITH_CAMELLIA_128_CBC_SHA256 * TLS_ECDHE_PSK_WITH_CAMELLIA_256_CBC_SHA384 * TLS_RSA_WITH_AES_128_CCM * TLS_RSA_WITH_AES_256_CCM * TLS_RSA_WITH_AES_128_CCM_8 * TLS_RSA_WITH_AES_256_CCM_8 * TLS_PSK_WITH_AES_128_CCM * TLS_PSK_WITH_AES_256_CCM * TLS_PSK_WITH_AES_128_CCM_8 * TLS_PSK_WITH_AES_256_CCM_8 | Note: This list was assembled from the set of registered TLS | cipher suites at the time of writing. This list includes those | cipher suites that do not offer an ephemeral key exchange and | those that are based on the TLS null, stream, or block cipher | type (as defined in Section 6.2.3 of [TLS12]). Additional | cipher suites with these properties could be defined; these | would not be explicitly prohibited. Appendix B. Changes from RFC 7540 This revision includes a number of editorial updates, plus the following substantive changes: * Use of TLS 1.3 is defined based on RFC 8740, which this document obsoletes. * The priority scheme defined in RFC 7540 is deprecated. Definitions for the format of the PRIORITY frame and the priority fields in the HEADERS frame have been retained, plus the rules governing when PRIORITY frames can be sent and received, but the Thomson & Benfield Expires 4 December 2021 [Page 86] Internet-Draft HTTP/2 June 2021 semantics of these fields is only described in RFC 7540. The priority signaling scheme from RFC 7540 was not successful. Using the simpler successor signaling [I-D.ietf-httpbis-priority] is recommended. * The HTTP/1.1 Upgrade mechanism is no longer specified in this document. It was never widely deployed, with plaintext HTTP/2 users choosing to use the prior-knowledge implementation instead. * The ranges of codepoints for settings and frame types that were reserved for "Experimental Use" are now available for general use. Contributors The previous version of this document was authored by Mike Belshe and Roberto Peon. Acknowledgements This document includes substantial input from the following individuals: * Adam Langley, Wan-Teh Chang, Jim Morrison, Mark Nottingham, Alyssa Wilk, Costin Manolache, William Chan, Vitaliy Lvin, Joe Chan, Adam Barth, Ryan Hamilton, Gavin Peters, Kent Alstad, Kevin Lindsay, Paul Amer, Fan Yang, and Jonathan Leighton (SPDY contributors). * Gabriel Montenegro and Willy Tarreau (Upgrade mechanism). * William Chan, Salvatore Loreto, Osama Mazahir, Gabriel Montenegro, Jitu Padhye, Roberto Peon, and Rob Trace (Flow control). * Mike Bishop (Extensibility). * Mark Nottingham, Julian Reschke, James Snell, Jeff Pinner, Mike Bishop, and Hervé Ruellan (Substantial editorial contributions). * Kari Hurtta, Tatsuhiro Tsujikawa, Greg Wilkins, Poul-Henning Kamp, and Jonathan Thackray. * Alexey Melnikov, who was an editor of this document in 2013. * David Benjamin, who was author of RFC 8740, the contents of which are integrated here. A substantial proportion of Martin's contribution was supported by Microsoft during his employment there. Thomson & Benfield Expires 4 December 2021 [Page 87] Internet-Draft HTTP/2 June 2021 The Japanese HTTP/2 community provided invaluable contributions, including a number of implementations as well as numerous technical and editorial contributions. Authors' Addresses Martin Thomson (editor) Mozilla Australia Email: mt@lowentropy.net Cory Benfield (editor) Apple Inc. Email: cbenfield@apple.com Thomson & Benfield Expires 4 December 2021 [Page 88]