HEADERS / HEADERS帧

The HEADERS frame (type=0x1) is used to open a stream (Section 5.1), and additionally carries a header block fragment. 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)  |
+-+-------------+-----------------------------------------------+
|                   Header Block Fragment (*)                 ...
+---------------------------------------------------------------+
|                           Padding (*)                       ...
+---------------------------------------------------------------+
             Figure 7: HEADERS Frame Payload

HEADERS帧(type=0x1)用来打开一个流( 5.1节 )，再额外地携带一个 首部块片段(Header Block Fragment)。HEADERS帧可以在一个流处于 空闲(idle)、保留(本地)(reserved (local))、打开(open)、或者 半关闭(远端)(half-closed (remote)) 状态时被发送。

+---------------+
|Pad Length? (8)|
+-+-------------+-----------------------------------------------+
|E|                 Stream Dependency? (31)                     |
+-+-------------+-----------------------------------------------+
|  Weight? (8)  |
+-+-------------+-----------------------------------------------+
|                   Header Block Fragment (*)                 ...
+---------------------------------------------------------------+
|                           Padding (*)                       ...
+---------------------------------------------------------------+
                        图 7：HEADERS帧载荷

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 indicating that the stream dependency is exclusive (see Section 5.3). This field is only present if the PRIORITY flag is set.

  • Stream Dependency: A 31-bit stream identifier for the stream that this stream depends on (see Section 5.3). This field is only present if the PRIORITY flag is set.

  • Weight: An unsigned 8-bit integer representing a priority weight for the stream (see Section 5.3). Add one to the value to obtain a weight between 1 and 256. This field is only present if the PRIORITY flag is set.

  • Header Block Fragment: A header block fragment (Section 4.3).

  • Padding: Padding octets.

HEADERS帧载荷包含如下域：

  • 填充长度(Pad Length)：一个8bit的域，包含帧填充数据的字节长度。只有当设置了PADDED标识时，才会有该域。
  • E标识：1bit标识，表示流依赖是否是专用的(参见 5.3节 )。只有当设置了PRIORITY标识，才会有该域。
  • 流依赖(Stream Dependency)：该流所依赖的流(参见 5.3节 )的31bit标识符。只有当设置了PRIORITY标识，才会有该域。
  • 权重(Weight)：一个8bit的无符号整数，表示该流的优先级权重(参见 5.3节 )。范围是1到255。只有当设置了PRIORITY标识，才会有该域。
  • 首部块片段(Header Block Fragment)：一个首部块片段(参见 4.3节 )。
  • 填充数据(Padding)：填充字节。

The HEADERS frame defines the following flags:

  • END_STREAM (0x1): When set, bit 0 indicates that the header 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 header 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; see Section 5.3.

HEADERS帧定义了如下标识：

  • END_STREAM(0x1)：当设置了该标识，第0 bit位就指明了该首部块是端点在指定流上发送的最后一帧( 4.3节 )。

    HEADERS帧携带的END_STREAM标识指明流要结束了。但是，在同一个流上，设置了END_STREAM标识的HEADERS帧后面还可以有 CONTINUATION 帧。从逻辑上来说，CONTINUATION 帧也是HEADERS帧的一部分。

  • END_HEADERS(0x4)：当设置了该标识，第2 bit位表示该帧包含整个首部块( 4.3节 )，后面不会有任何 CONTINUATION 帧。

    对于同一个流，没有设置END_HEADERS标识的HEADERS帧后面必须跟一个 CONTINUATION 帧。如果接收到任何其他类型的帧，或者其他流上的帧，接收方必须将其看做 PROTOCOL_ERROR 类型的连接错误( 5.4.1节 )。

  • PADDED(0x8)：当设置了该标识，第3 bit位表示存在 填充长度(Pad Length) 域和 填充数据(padding)。

  • PRIORITY(0x20)：当设置了该标识，第5 bit位表示存在 独占标识(E)(Exclusive Flag (E))、流依赖(Stream Dependency) 和 权重(Weight) 域。参加 5.3节

The payload of a HEADERS frame contains a header block fragment (Section 4.3). A header block that does not fit within a HEADERS frame is continued in a CONTINUATION frame (Section 6.10).

HEADERS帧的载荷包含一个首部块片段( 4.3节 )。同一个首部块在一个HEADERS帧里装不下就继续装入CONTINUATION帧( 6.10节 )。

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.

HEADERS帧必须与某一个流相关联。如果收到一个流标识符域为0x0的HEADERS帧，接收方必须响应一个 PROTOCOL_ERROR 类型的连接错误( 5.4.1节 )。

The HEADERS frame changes the connection state as described in Section 4.3.

HEADERS帧改变连接状态在 4.3节 中有表述。

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 header block fragment MUST be treated as a PROTOCOL_ERROR.

HEADERS帧可以包含填充数据。填充数据域和填充标识与DATA帧( 6.1节 )中定义的一样。如果填充数据量超出了为首部块片段预留的大小，必须将其处理为 PROTOCOL_ERROR。

Prioritization information in a HEADERS frame is logically equivalent to a separate PRIORITY frame, but inclusion in HEADERS avoids the potential for churn in stream prioritization when new streams are created. Prioritization fields in HEADERS frames subsequent to the first on a stream reprioritize the stream (Section 5.3.3).

HEADERS帧里的优先级信息逻辑上等价于一个单独的 PRIORITY 帧，但是包含在HEADERS帧里可以避免创建新流时对流优先级潜在的扰动。一个流上第一个HEADERS帧之后的HEADERS帧里的优先级域会变更该流的优先级顺序( 5.3.3节 )。

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