本文深入探讨了SIP协议中的定时器概念，包括它们的缺省值、作用及在不同状态下的行为。详细解释了T1、T2、T4等关键定时器的作用，以及计时器A至K的具体功能和触发条件。同时阐述了计时器L、D、F、J等在特定场景下的操作，强调了定时器在确保协议可靠性和响应及时性方面的重要性。
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| 定时器 | 缺省值 | 节 | 含义 |
|---|---|---|---|
| T1 | 500 ms | 17.1.1.1 | 经历来回时间（RTT） |
| T2 | 4 秒 | 17.1.2.2 | 非 INVITE 请求和 INVITE 响应的最长重新传输时间间隔 |
| T4 | 5 秒 | 17.1.2.2 | 消息可保留在网络中的最长持续时间 |
| 计时器 A | 最初为 T1 | 17.1.1.2 | INVITE 请求重新传输时间间隔（仅适用于 UDP） |
| 计时器 B | 64*T1 | 17.1.1.2 | INVITE 事务超时计时器 |
| 计时器 D | 大于 32 秒（对于 UDP） | 17.1.1.2 | 响应重新传输的等待时间 |
| 0 秒（对于 TCP 和 SCTP） | |||
| 计时器 E | 最初为 T1 | 17.1.2.2 | 非 INVITE 请求重新传输时间间隔（仅适用于 UDP） |
| 计时器 F | 64*T1 | 17.1.2.2 | 非 INVITE 事务超时计时器 |
| 计时器 G | 最初为 T1 | 17.2.1 | INVITE 响应重新传输时间间隔 |
| 计时器 H | 64*T1 | 17.2.1 | ACK 接收的等待时间 |
| 计时器 I | T4（对于 UDP） | 17.2.1 | ACK 重新传输的等待时间 |
| 0 秒（对于 TCP 和 SCTP） | |||
| 计时器 J | 64*T1（对于 UDP） | 17.2.2 | 重新传输非 INVITE 请求的等待时间 |
| 0 秒（对于 TCP 和 SCTP） | |||
| 计时器 K | T4（对于 UDP） | 17.1.2.2 | 响应重新传输的等待时间 |
| 0 秒（对于 TCP 和 SCTP） |
计时器 k :
RFC 3261 - 17.1.2.2
If Timer K fires while in this state (Completed), the client transaction
MUST transition to the "Terminated" state.
RFC 3261 - 17.1.2.2
ONCE THE TRANSACTION IS IN THE TERMINATED STATE, IT MUST BE DESTROYED IMMEDIATELY.
MUST transition to the "Terminated" state.
RFC 3261 - 17.1.2.2
ONCE THE TRANSACTION IS IN THE TERMINATED STATE, IT MUST BE DESTROYED IMMEDIATELY.
计时器 L ：
draft-sparks-sip-invfix-03 - 8.7. Page 137
If Timer L fires while the INVITE server transaction is in the "Accepted" state, the transaction
MUST transition to the "Terminated" state. Once the transaction is in the "Terminated" state, it MUST be
destroyed immediately.
If Timer L fires while the INVITE server transaction is in the "Accepted" state, the transaction
MUST transition to the "Terminated" state. Once the transaction is in the "Terminated" state, it MUST be
destroyed immediately.
计时器 L触发时会结束 IST事务。
计时器 I
RFC 3261 - 17.2.1 INVITE Server Transaction
Once timer I fires, the server MUST transition to the
"Terminated" state.
Once the transaction is in the "Terminated" state, it MUST be
destroyed immediately. As with client transactions, this is needed
to ensure reliability of the 2xx responses to INVITE.
Once timer I fires, the server MUST transition to the
"Terminated" state.
Once the transaction is in the "Terminated" state, it MUST be
destroyed immediately. As with client transactions, this is needed
to ensure reliability of the 2xx responses to INVITE.
计时器 D：
draft-sparks-sip-invfix-03 - 8.4. Pages 126 through 128
If timer D fires while the client transaction is in the
"Completed" state, the client transaction MUST move to the
"Terminated" state.
计时器 M：
If timer D fires while the client transaction is in the
"Completed" state, the client transaction MUST move to the
"Terminated" state.
计时器 M：
draft-sparks-sip-invfix-03 - 8.4. Pages 126 through 128
If timer M fires while the client transaction is in the "Accepted"
state, the client transaction MUST move to the "Terminated" state.
If timer M fires while the client transaction is in the "Accepted"
state, the client transaction MUST move to the "Terminated" state.
计时器 F：
RFC 3261 - 17.1.2.2
If timer F fires while in the "Proceeding" state, the TU MUST be informed of a timeout, and the
client transaction MUST transition to the terminated state.
If timer F fires while in the "Proceeding" state, the TU MUST be informed of a timeout, and the
client transaction MUST transition to the terminated state.
计时器 J ：
RFC 3261 - 17.2.2
The server transaction remains in this state (Completed) until Timer J fires, at
which point it MUST transition to the "Terminated" state.
RFC 3261 - 17.2.2
THE SERVER TRANSACTION MUST BE DESTROYED THE INSTANT IT ENTERS THE "TERMINATED" STATE.
The server transaction remains in this state (Completed) until Timer J fires, at
which point it MUST transition to the "Terminated" state.
RFC 3261 - 17.2.2
THE SERVER TRANSACTION MUST BE DESTROYED THE INSTANT IT ENTERS THE "TERMINATED" STATE.
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