4. HTTP Frames Once the HTTP/2 connection is established, endpoints can begin exchanging frames. 4.1. Frame Format All frames begin with a fixed 9-octet header followed by a variable- length payload. +-----------------------------------------------+ | Length (24) | +---------------+---------------+---------------+ | Type (8) | Flags (8) | +-+-------------+---------------+-------------------------------+ |R| Stream Identifier (31) | +=+=============================================================+ | Frame Payload (0...) ... +---------------------------------------------------------------+ Figure 1: Frame Layout The fields of the frame header are defined as: Length: The length of the frame payload expressed as an unsigned 24-bit integer. Values greater than 2^14 (16,384) MUST NOT be sent unless the receiver has set a larger value for SETTINGS_MAX_FRAME_SIZE. The 9 octets of the frame header are not included in this value.
Type: The 8-bit type of the frame. The frame type determines the format and semantics of the frame. Implementations MUST ignore and discard any frame that has a type that is unknown. Flags: An 8-bit field reserved for boolean flags specific to the frame type. Flags are assigned semantics specific to the indicated frame type. Flags that have no defined semantics for a particular frame type MUST be ignored and MUST be left unset (0x0) when sending. R: A reserved 1-bit field. The semantics of this bit are undefined, and the bit MUST remain unset (0x0) when sending and MUST be ignored when receiving. Stream Identifier: A stream identifier (see Section 5.1.1) expressed as an unsigned 31-bit integer. The value 0x0 is reserved for frames that are associated with the connection as a whole as opposed to an individual stream. The structure and content of the frame payload is dependent entirely on the frame type. 4.2. Frame Size The size of a frame payload is limited by the maximum size that a receiver advertises in the SETTINGS_MAX_FRAME_SIZE setting. This setting can have any value between 2^14 (16,384) and 2^24-1 (16,777,215) octets, inclusive. All implementations MUST be capable of receiving and minimally processing frames up to 2^14 octets in length, plus the 9-octet frame header (Section 4.1). The size of the frame header is not included when describing frame sizes. Note: Certain frame types, such as PING (Section 6.7), impose additional limits on the amount of payload data allowed. An endpoint MUST send an error code of FRAME_SIZE_ERROR if a frame exceeds the size defined in SETTINGS_MAX_FRAME_SIZE, exceeds any limit defined for the frame type, or is too small to contain mandatory frame data. A frame size error in a frame that could alter the state of the entire connection MUST be treated as a connection error (Section 5.4.1); this includes any frame carrying a header block (Section 4.3) (that is, HEADERS, PUSH_PROMISE, and CONTINUATION), SETTINGS, and any frame with a stream identifier of 0.
Endpoints are not obligated to use all available space in a frame. Responsiveness can be improved by using frames that are smaller than the permitted maximum size. Sending large frames can result in delays in sending time-sensitive frames (such as RST_STREAM, WINDOW_UPDATE, or PRIORITY), which, if blocked by the transmission of a large frame, could affect performance. 4.3. Header Compression and Decompression Just as in HTTP/1, a header field in HTTP/2 is a name with one or more associated values. Header fields are used within HTTP request and response messages as well as in server push operations (see Section 8.2). Header lists are collections of zero or more header fields. When transmitted over a connection, a header list is serialized into a header block using HTTP header compression [COMPRESSION]. The serialized header block is then divided into one or more octet sequences, called header block fragments, and transmitted within the payload of HEADERS (Section 6.2), PUSH_PROMISE (Section 6.6), or CONTINUATION (Section 6.10) frames. The Cookie header field [COOKIE] is treated specially by the HTTP mapping (see Section 220.127.116.11). A receiving endpoint reassembles the header block by concatenating its fragments and then decompresses the block to reconstruct the header list. A complete header block consists of either: o a single HEADERS or PUSH_PROMISE frame, with the END_HEADERS flag set, or o a HEADERS or PUSH_PROMISE frame with the END_HEADERS flag cleared and one or more CONTINUATION frames, where the last CONTINUATION frame has the END_HEADERS flag set. Header compression is stateful. One compression context and one decompression context are used for the entire connection. A decoding error in a header block MUST be treated as a connection error (Section 5.4.1) of type COMPRESSION_ERROR. Each header block is processed as a discrete unit. Header blocks MUST be transmitted as a contiguous sequence of frames, with no interleaved frames of any other type or from any other stream. The last frame in a sequence of HEADERS or CONTINUATION frames has the
END_HEADERS flag set. The last frame in a sequence of PUSH_PROMISE or CONTINUATION frames has the END_HEADERS flag set. This allows a header block to be logically equivalent to a single frame. Header block fragments can only be sent as the payload of HEADERS, PUSH_PROMISE, or CONTINUATION frames because these frames carry data that can modify the compression context maintained by a receiver. An endpoint receiving HEADERS, PUSH_PROMISE, or CONTINUATION frames needs to reassemble header blocks and perform decompression even if the frames are to be discarded. A receiver MUST terminate the connection with a connection error (Section 5.4.1) of type COMPRESSION_ERROR if it does not decompress a header block. 5. Streams and Multiplexing A "stream" is an independent, bidirectional sequence of frames exchanged between the client and server within an HTTP/2 connection. Streams have several important characteristics: o A single HTTP/2 connection can contain multiple concurrently open streams, with either endpoint interleaving frames from multiple streams. o Streams can be established and used unilaterally or shared by either the client or server. o Streams can be closed by either endpoint. o The order in which frames are sent on a stream is significant. Recipients process frames in the order they are received. In particular, the order of HEADERS and DATA frames is semantically significant. o Streams are identified by an integer. Stream identifiers are assigned to streams by the endpoint initiating the stream.
5.1. Stream States The lifecycle of a stream is shown in Figure 2. +--------+ send PP | | recv PP ,--------| idle |--------. / | | \ v +--------+ v +----------+ | +----------+ | | | send H / | | ,------| reserved | | recv H | reserved |------. | | (local) | | | (remote) | | | +----------+ v +----------+ | | | +--------+ | | | | recv ES | | send ES | | | send H | ,-------| open |-------. | recv H | | | / | | \ | | | v v +--------+ v v | | +----------+ | +----------+ | | | half | | | half | | | | closed | | send R / | closed | | | | (remote) | | recv R | (local) | | | +----------+ | +----------+ | | | | | | | | send ES / | recv ES / | | | | send R / v send R / | | | | recv R +--------+ recv R | | | send R / `----------->| |<-----------' send R / | | recv R | closed | recv R | `----------------------->| |<----------------------' +--------+ send: endpoint sends this frame recv: endpoint receives this frame H: HEADERS frame (with implied CONTINUATIONs) PP: PUSH_PROMISE frame (with implied CONTINUATIONs) ES: END_STREAM flag R: RST_STREAM frame Figure 2: Stream States Note that this diagram shows stream state transitions and the frames and flags that affect those transitions only. In this regard, CONTINUATION frames do not result in state transitions; they are effectively part of the HEADERS or PUSH_PROMISE that they follow.
For the purpose of state transitions, the END_STREAM flag is processed as a separate event to the frame that bears it; a HEADERS frame with the END_STREAM flag set can cause two state transitions. Both endpoints have a subjective view of the state of a stream that could be different when frames are in transit. Endpoints do not coordinate the creation of streams; they are created unilaterally by either endpoint. The negative consequences of a mismatch in states are limited to the "closed" state after sending RST_STREAM, where frames might be received for some time after closing. Streams have the following states: idle: All streams start in the "idle" state. The following transitions are valid from this state: * Sending or receiving a HEADERS frame causes the stream to become "open". The stream identifier is selected as described in Section 5.1.1. The same HEADERS frame can also cause a stream to immediately become "half-closed". * Sending a PUSH_PROMISE frame on another stream reserves the idle stream that is identified for later use. The stream state for the reserved stream transitions to "reserved (local)". * Receiving a PUSH_PROMISE frame on another stream reserves an idle stream that is identified for later use. The stream state for the reserved stream transitions to "reserved (remote)". * Note that the PUSH_PROMISE frame is not sent on the idle stream but references the newly reserved stream in the Promised Stream ID field. Receiving any frame other than HEADERS or PRIORITY on a stream in this state MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. reserved (local): A stream in the "reserved (local)" state is one that has been promised by sending a PUSH_PROMISE frame. A PUSH_PROMISE frame reserves an idle stream by associating the stream with an open stream that was initiated by the remote peer (see Section 8.2).
In this state, only the following transitions are possible: * The endpoint can send a HEADERS frame. This causes the stream to open in a "half-closed (remote)" state. * Either endpoint can send a RST_STREAM frame to cause the stream to become "closed". This releases the stream reservation. An endpoint MUST NOT send any type of frame other than HEADERS, RST_STREAM, or PRIORITY in this state. A PRIORITY or WINDOW_UPDATE frame MAY be received in this state. Receiving any type of frame other than RST_STREAM, PRIORITY, or WINDOW_UPDATE on a stream in this state MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. reserved (remote): A stream in the "reserved (remote)" state has been reserved by a remote peer. In this state, only the following transitions are possible: * Receiving a HEADERS frame causes the stream to transition to "half-closed (local)". * Either endpoint can send a RST_STREAM frame to cause the stream to become "closed". This releases the stream reservation. An endpoint MAY send a PRIORITY frame in this state to reprioritize the reserved stream. An endpoint MUST NOT send any type of frame other than RST_STREAM, WINDOW_UPDATE, or PRIORITY in this state. Receiving any type of frame other than HEADERS, RST_STREAM, or PRIORITY on a stream in this state MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. open: A stream in the "open" state may be used by both peers to send frames of any type. In this state, sending peers observe advertised stream-level flow-control limits (Section 5.2). From this state, either endpoint can send a frame with an END_STREAM flag set, which causes the stream to transition into one of the "half-closed" states. An endpoint sending an
END_STREAM flag causes the stream state to become "half-closed (local)"; an endpoint receiving an END_STREAM flag causes the stream state to become "half-closed (remote)". Either endpoint can send a RST_STREAM frame from this state, causing it to transition immediately to "closed". half-closed (local): A stream that is in the "half-closed (local)" state cannot be used for sending frames other than WINDOW_UPDATE, PRIORITY, and RST_STREAM. A stream transitions from this state to "closed" when a frame that contains an END_STREAM flag is received or when either peer sends a RST_STREAM frame. An endpoint can receive any type of frame in this state. Providing flow-control credit using WINDOW_UPDATE frames is necessary to continue receiving flow-controlled frames. In this state, a receiver can ignore WINDOW_UPDATE frames, which might arrive for a short period after a frame bearing the END_STREAM flag is sent. PRIORITY frames received in this state are used to reprioritize streams that depend on the identified stream. half-closed (remote): A stream that is "half-closed (remote)" is no longer being used by the peer to send frames. In this state, an endpoint is no longer obligated to maintain a receiver flow-control window. If an endpoint receives additional frames, other than WINDOW_UPDATE, PRIORITY, or RST_STREAM, for a stream that is in this state, it MUST respond with a stream error (Section 5.4.2) of type STREAM_CLOSED. A stream that is "half-closed (remote)" can be used by the endpoint to send frames of any type. In this state, the endpoint continues to observe advertised stream-level flow-control limits (Section 5.2). A stream can transition from this state to "closed" by sending a frame that contains an END_STREAM flag or when either peer sends a RST_STREAM frame.
closed: The "closed" state is the terminal state. An endpoint MUST NOT send frames other than PRIORITY on a closed stream. An endpoint that receives any frame other than PRIORITY after receiving a RST_STREAM MUST treat that as a stream error (Section 5.4.2) of type STREAM_CLOSED. Similarly, an endpoint that receives any frames after receiving a frame with the END_STREAM flag set MUST treat that as a connection error (Section 5.4.1) of type STREAM_CLOSED, unless the frame is permitted as described below. WINDOW_UPDATE or RST_STREAM frames can be received in this state for a short period after a DATA or HEADERS frame containing an END_STREAM flag is sent. Until the remote peer receives and processes RST_STREAM or the frame bearing the END_STREAM flag, it might send frames of these types. Endpoints MUST ignore WINDOW_UPDATE or RST_STREAM frames received in this state, though endpoints MAY choose to treat frames that arrive a significant time after sending END_STREAM as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. PRIORITY frames can be sent on closed streams to prioritize streams that are dependent on the closed stream. Endpoints SHOULD process PRIORITY frames, though they can be ignored if the stream has been removed from the dependency tree (see Section 5.3.4). If this state is reached as a result of sending a RST_STREAM frame, the peer that receives the RST_STREAM might have already sent -- or enqueued for sending -- frames on the stream that cannot be withdrawn. An endpoint MUST ignore frames that it receives on closed streams after it has sent a RST_STREAM frame. An endpoint MAY choose to limit the period over which it ignores frames and treat frames that arrive after this time as being in error. Flow-controlled frames (i.e., DATA) received after sending RST_STREAM are counted toward the connection flow-control window. Even though these frames might be ignored, because they are sent before the sender receives the RST_STREAM, the sender will consider the frames to count against the flow-control window. An endpoint might receive a PUSH_PROMISE frame after it sends RST_STREAM. PUSH_PROMISE causes a stream to become "reserved" even if the associated stream has been reset. Therefore, a RST_STREAM is needed to close an unwanted promised stream.
In the absence of more specific guidance elsewhere in this document, implementations SHOULD treat the receipt of a frame that is not expressly permitted in the description of a state as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Note that PRIORITY can be sent and received in any stream state. Frames of unknown types are ignored. An example of the state transitions for an HTTP request/response exchange can be found in Section 8.1. An example of the state transitions for server push can be found in Sections 8.2.1 and 8.2.2. 5.1.1. Stream Identifiers Streams are identified with an unsigned 31-bit integer. Streams initiated by a client MUST use odd-numbered stream identifiers; those initiated by the server MUST use even-numbered stream identifiers. A stream identifier of zero (0x0) is used for connection control messages; the stream identifier of zero cannot be used to establish a new stream. HTTP/1.1 requests that are upgraded to HTTP/2 (see Section 3.2) are responded to with a stream identifier of one (0x1). After the upgrade completes, stream 0x1 is "half-closed (local)" to the client. Therefore, stream 0x1 cannot be selected as a new stream identifier by a client that upgrades from HTTP/1.1. The identifier of a newly established stream MUST be numerically greater than all streams that the initiating endpoint has opened or reserved. This governs streams that are opened using a HEADERS frame and streams that are reserved using PUSH_PROMISE. An endpoint that receives an unexpected stream identifier MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR. The first use of a new stream identifier implicitly closes all streams in the "idle" state that might have been initiated by that peer with a lower-valued stream identifier. For example, if a client sends a HEADERS frame on stream 7 without ever sending a frame on stream 5, then stream 5 transitions to the "closed" state when the first frame for stream 7 is sent or received. Stream identifiers cannot be reused. Long-lived connections can result in an endpoint exhausting the available range of stream identifiers. A client that is unable to establish a new stream identifier can establish a new connection for new streams. A server that is unable to establish a new stream identifier can send a GOAWAY frame so that the client is forced to open a new connection for new streams.
5.1.2. Stream Concurrency A peer can limit the number of concurrently active streams using the SETTINGS_MAX_CONCURRENT_STREAMS parameter (see Section 6.5.2) within a SETTINGS frame. The maximum concurrent streams setting is specific to each endpoint and applies only to the peer that receives the setting. That is, clients specify the maximum number of concurrent streams the server can initiate, and servers specify the maximum number of concurrent streams the client can initiate. Streams that are in the "open" state or in either of the "half- closed" states count toward the maximum number of streams that an endpoint is permitted to open. Streams in any of these three states count toward the limit advertised in the SETTINGS_MAX_CONCURRENT_STREAMS setting. Streams in either of the "reserved" states do not count toward the stream limit. Endpoints MUST NOT exceed the limit set by their peer. An endpoint that receives a HEADERS frame that causes its advertised concurrent stream limit to be exceeded MUST treat this as a stream error (Section 5.4.2) of type PROTOCOL_ERROR or REFUSED_STREAM. The choice of error code determines whether the endpoint wishes to enable automatic retry (see Section 8.1.4) for details). An endpoint that wishes to reduce the value of SETTINGS_MAX_CONCURRENT_STREAMS to a value that is below the current number of open streams can either close streams that exceed the new value or allow streams to complete. 5.2. Flow Control Using streams for multiplexing introduces contention over use of the TCP connection, resulting in blocked streams. A flow-control scheme ensures that streams on the same connection do not destructively interfere with each other. Flow control is used for both individual streams and for the connection as a whole. HTTP/2 provides for flow control through use of the WINDOW_UPDATE frame (Section 6.9).
5.2.1. Flow-Control Principles HTTP/2 stream flow control aims to allow a variety of flow-control algorithms to be used without requiring protocol changes. Flow control in HTTP/2 has the following characteristics: 1. Flow control is specific to a connection. Both types of flow control are between the endpoints of a single hop and not over the entire end-to-end path. 2. Flow control is based on WINDOW_UPDATE frames. Receivers advertise how many octets they are prepared to receive on a stream and for the entire connection. This is a credit-based scheme. 3. Flow control is directional with overall control provided by the receiver. A receiver MAY choose to set any window size that it desires for each stream and for the entire connection. A sender MUST respect flow-control limits imposed by a receiver. Clients, servers, and intermediaries all independently advertise their flow-control window as a receiver and abide by the flow-control limits set by their peer when sending. 4. The initial value for the flow-control window is 65,535 octets for both new streams and the overall connection. 5. The frame type determines whether flow control applies to a frame. Of the frames specified in this document, only DATA frames are subject to flow control; all other frame types do not consume space in the advertised flow-control window. This ensures that important control frames are not blocked by flow control. 6. Flow control cannot be disabled. 7. HTTP/2 defines only the format and semantics of the WINDOW_UPDATE frame (Section 6.9). This document does not stipulate how a receiver decides when to send this frame or the value that it sends, nor does it specify how a sender chooses to send packets. Implementations are able to select any algorithm that suits their needs. Implementations are also responsible for managing how requests and responses are sent based on priority, choosing how to avoid head-of- line blocking for requests, and managing the creation of new streams. Algorithm choices for these could interact with any flow-control algorithm.
5.2.2. Appropriate Use of Flow Control Flow control is defined to protect endpoints that are operating under resource constraints. For example, a proxy needs to share memory between many connections and also might have a slow upstream connection and a fast downstream one. Flow-control addresses cases where the receiver is unable to process data on one stream yet wants to continue to process other streams in the same connection. Deployments that do not require this capability can advertise a flow- control window of the maximum size (2^31-1) and can maintain this window by sending a WINDOW_UPDATE frame when any data is received. This effectively disables flow control for that receiver. Conversely, a sender is always subject to the flow-control window advertised by the receiver. Deployments with constrained resources (for example, memory) can employ flow control to limit the amount of memory a peer can consume. Note, however, that this can lead to suboptimal use of available network resources if flow control is enabled without knowledge of the bandwidth-delay product (see [RFC7323]). Even with full awareness of the current bandwidth-delay product, implementation of flow control can be difficult. When using flow control, the receiver MUST read from the TCP receive buffer in a timely fashion. Failure to do so could lead to a deadlock when critical frames, such as WINDOW_UPDATE, are not read and acted upon. 5.3. Stream Priority A client can assign a priority for a new stream by including prioritization information in the HEADERS frame (Section 6.2) that opens the stream. At any other time, the PRIORITY frame (Section 6.3) can be used to change the priority of a stream. The purpose of prioritization is to allow an endpoint to express how it would prefer its peer to allocate resources when managing concurrent streams. Most importantly, priority can be used to select streams for transmitting frames when there is limited capacity for sending. Streams can be prioritized by marking them as dependent on the completion of other streams (Section 5.3.1). Each dependency is assigned a relative weight, a number that is used to determine the relative proportion of available resources that are assigned to streams dependent on the same stream.
Explicitly setting the priority for a stream is input to a prioritization process. It does not guarantee any particular processing or transmission order for the stream relative to any other stream. An endpoint cannot force a peer to process concurrent streams in a particular order using priority. Expressing priority is therefore only a suggestion. Prioritization information can be omitted from messages. Defaults are used prior to any explicit values being provided (Section 5.3.5). 5.3.1. Stream Dependencies Each stream can be given an explicit dependency on another stream. Including a dependency expresses a preference to allocate resources to the identified stream rather than to the dependent stream. A stream that is not dependent on any other stream is given a stream dependency of 0x0. In other words, the non-existent stream 0 forms the root of the tree. A stream that depends on another stream is a dependent stream. The stream upon which a stream is dependent is a parent stream. A dependency on a stream that is not currently in the tree -- such as a stream in the "idle" state -- results in that stream being given a default priority (Section 5.3.5). When assigning a dependency on another stream, the stream is added as a new dependency of the parent stream. Dependent streams that share the same parent are not ordered with respect to each other. For example, if streams B and C are dependent on stream A, and if stream D is created with a dependency on stream A, this results in a dependency order of A followed by B, C, and D in any order. A A / \ ==> /|\ B C B D C Figure 3: Example of Default Dependency Creation An exclusive flag allows for the insertion of a new level of dependencies. The exclusive flag causes the stream to become the sole dependency of its parent stream, causing other dependencies to become dependent on the exclusive stream. In the previous example, if stream D is created with an exclusive dependency on stream A, this results in D becoming the dependency parent of B and C.
A A | / \ ==> D B C / \ B C Figure 4: Example of Exclusive Dependency Creation Inside the dependency tree, a dependent stream SHOULD only be allocated resources if either all of the streams that it depends on (the chain of parent streams up to 0x0) are closed or it is not possible to make progress on them. A stream cannot depend on itself. An endpoint MUST treat this as a stream error (Section 5.4.2) of type PROTOCOL_ERROR. 5.3.2. Dependency Weighting All dependent streams are allocated an integer weight between 1 and 256 (inclusive). Streams with the same parent SHOULD be allocated resources proportionally based on their weight. Thus, if stream B depends on stream A with weight 4, stream C depends on stream A with weight 12, and no progress can be made on stream A, stream B ideally receives one-third of the resources allocated to stream C. 5.3.3. Reprioritization Stream priorities are changed using the PRIORITY frame. Setting a dependency causes a stream to become dependent on the identified parent stream. Dependent streams move with their parent stream if the parent is reprioritized. Setting a dependency with the exclusive flag for a reprioritized stream causes all the dependencies of the new parent stream to become dependent on the reprioritized stream. If a stream is made dependent on one of its own dependencies, the formerly dependent stream is first moved to be dependent on the reprioritized stream's previous parent. The moved dependency retains its weight. For example, consider an original dependency tree where B and C depend on A, D and E depend on C, and F depends on D. If A is made dependent on D, then D takes the place of A. All other dependency relationships stay the same, except for F, which becomes dependent on A if the reprioritization is exclusive.
x x x x | / \ | | A D A D D / \ / / \ / \ | B C ==> F B C ==> F A OR A / \ | / \ /|\ D E E B C B C F | | | F E E (intermediate) (non-exclusive) (exclusive) Figure 5: Example of Dependency Reordering 5.3.4. Prioritization State Management When a stream is removed from the dependency tree, its dependencies can be moved to become dependent on the parent of the closed stream. The weights of new dependencies are recalculated by distributing the weight of the dependency of the closed stream proportionally based on the weights of its dependencies. Streams that are removed from the dependency tree cause some prioritization information to be lost. Resources are shared between streams with the same parent stream, which means that if a stream in that set closes or becomes blocked, any spare capacity allocated to a stream is distributed to the immediate neighbors of the stream. However, if the common dependency is removed from the tree, those streams share resources with streams at the next highest level. For example, assume streams A and B share a parent, and streams C and D both depend on stream A. Prior to the removal of stream A, if streams A and D are unable to proceed, then stream C receives all the resources dedicated to stream A. If stream A is removed from the tree, the weight of stream A is divided between streams C and D. If stream D is still unable to proceed, this results in stream C receiving a reduced proportion of resources. For equal starting weights, C receives one third, rather than one half, of available resources. It is possible for a stream to become closed while prioritization information that creates a dependency on that stream is in transit. If a stream identified in a dependency has no associated priority information, then the dependent stream is instead assigned a default priority (Section 5.3.5). This potentially creates suboptimal prioritization, since the stream could be given a priority that is different from what is intended.
To avoid these problems, an endpoint SHOULD retain stream prioritization state for a period after streams become closed. The longer state is retained, the lower the chance that streams are assigned incorrect or default priority values. Similarly, streams that are in the "idle" state can be assigned priority or become a parent of other streams. This allows for the creation of a grouping node in the dependency tree, which enables more flexible expressions of priority. Idle streams begin with a default priority (Section 5.3.5). The retention of priority information for streams that are not counted toward the limit set by SETTINGS_MAX_CONCURRENT_STREAMS could create a large state burden for an endpoint. Therefore, the amount of prioritization state that is retained MAY be limited. The amount of additional state an endpoint maintains for prioritization could be dependent on load; under high load, prioritization state can be discarded to limit resource commitments. In extreme cases, an endpoint could even discard prioritization state for active or reserved streams. If a limit is applied, endpoints SHOULD maintain state for at least as many streams as allowed by their setting for SETTINGS_MAX_CONCURRENT_STREAMS. Implementations SHOULD also attempt to retain state for streams that are in active use in the priority tree. If it has retained enough state to do so, an endpoint receiving a PRIORITY frame that changes the priority of a closed stream SHOULD alter the dependencies of the streams that depend on it. 5.3.5. Default Priorities All streams are initially assigned a non-exclusive dependency on stream 0x0. Pushed streams (Section 8.2) initially depend on their associated stream. In both cases, streams are assigned a default weight of 16. 5.4. Error Handling HTTP/2 framing permits two classes of error: o An error condition that renders the entire connection unusable is a connection error. o An error in an individual stream is a stream error. A list of error codes is included in Section 7.
5.4.1. Connection Error Handling A connection error is any error that prevents further processing of the frame layer or corrupts any connection state. An endpoint that encounters a connection error SHOULD first send a GOAWAY frame (Section 6.8) with the stream identifier of the last stream that it successfully received from its peer. The GOAWAY frame includes an error code that indicates why the connection is terminating. After sending the GOAWAY frame for an error condition, the endpoint MUST close the TCP connection. It is possible that the GOAWAY will not be reliably received by the receiving endpoint ([RFC7230], Section 6.6 describes how an immediate connection close can result in data loss). In the event of a connection error, GOAWAY only provides a best-effort attempt to communicate with the peer about why the connection is being terminated. An endpoint can end a connection at any time. In particular, an endpoint MAY choose to treat a stream error as a connection error. Endpoints SHOULD send a GOAWAY frame when ending a connection, providing that circumstances permit it. 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 header 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 header fields. 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 header block (Section 4.3) are not permitted; these MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Extensions that could change the semantics of existing protocol components 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. Note that treating any frames other than DATA frames as flow controlled is such a change in semantics and 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.