RFC 793 为什么要发送reset,什么情况下发送reset

本文详细阐述了TCP协议中重置(RST)段的生成与处理规则,包括连接不存在、非同步状态及同步状态下的处理方式,并针对不同情况如何响应进行了说明。

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  A variety of other cases are possible, all of which are accounted for
  by the following rules for RST generation and processing.

  Reset Generation

  As a general rule, reset (RST) must be sent whenever a segment arrives
  which apparently is not intended for the current connection.  A reset
  must not be sent if it is not clear that this is the case.

  There are three groups of states:

    1.  If the connection does not exist (CLOSED) then a reset is sent
    in response to any incoming segment except another reset.  In
    particular, SYNs addressed to a non-existent connection are rejected
    by this means.

    If the incoming segment has an ACK field, the reset takes its
    sequence number from the ACK field of the segment, otherwise the
    reset has sequence number zero and the ACK field is set to the sum
    of the sequence number and segment length of the incoming segment.
    The connection remains in the CLOSED state.

    2.  If the connection is in any non-synchronized state (LISTEN,
    SYN-SENT, SYN-RECEIVED), and the incoming segment acknowledges
    something not yet sent (the segment carries an unacceptable ACK), or
    if an incoming segment has a security level or compartment which
    does not exactly match the level and compartment requested for the
    connection, a reset is sent.

    If our SYN has not been acknowledged and the precedence level of the
    incoming segment is higher than the precedence level requested then
    either raise the local precedence level (if allowed by the user and
    the system) or send a reset; or if the precedence level of the
    incoming segment is lower than the precedence level requested then
    continue as if the precedence matched exactly (if the remote TCP
    cannot raise the precedence level to match ours this will be
    detected in the next segment it sends, and the connection will be
    terminated then).  If our SYN has been acknowledged (perhaps in this
    incoming segment) the precedence level of the incoming segment must
    match the local precedence level exactly, if it does not a reset
    must be sent.

    If the incoming segment has an ACK field, the reset takes its
    sequence number from the ACK field of the segment, otherwise the
    reset has sequence number zero and the ACK field is set to the sum
    of the sequence number and segment length of the incoming segment.
    The connection remains in the same state.



[Page 36]                                                               


September 1981                                                          
                                           Transmission Control Protocol
                                                Functional Specification



    3.  If the connection is in a synchronized state (ESTABLISHED,
    FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT),
    any unacceptable segment (out of window sequence number or
    unacceptible acknowledgment number) must elicit only an empty
    acknowledgment segment containing the current send-sequence number
    and an acknowledgment indicating the next sequence number expected
    to be received, and the connection remains in the same state.

    If an incoming segment has a security level, or compartment, or
    precedence which does not exactly match the level, and compartment,
    and precedence requested for the connection,a reset is sent and
    connection goes to the CLOSED state.  The reset takes its sequence
    number from the ACK field of the incoming segment.

  Reset Processing

  In all states except SYN-SENT, all reset (RST) segments are validated
  by checking their SEQ-fields.  A reset is valid if its sequence number
  is in the window.  In the SYN-SENT state (a RST received in response
  to an initial SYN), the RST is acceptable if the ACK field
  acknowledges the SYN.

  The receiver of a RST first validates it, then changes state.  If the
  receiver was in the LISTEN state, it ignores it.  If the receiver was
  in SYN-RECEIVED state and had previously been in the LISTEN state,
  then the receiver returns to the LISTEN state, otherwise the receiver
  aborts the connection and goes to the CLOSED state.  If the receiver
  was in any other state, it aborts the connection and advises the user
<think>嗯,用户的问题很聚焦:为什么清除浏览器缓存能解决socketclosed错误?这需要结合网络通信和缓存机制来解析。用户引用[3]提到websocket端口8001的socket未被释放的情况,这可能是关键线索。考虑浏览器缓存如何影响连接复用:当强缓存(如Expires/Cache-Control)或协商缓存(如ETag)的条目损坏时,会导致以下故障链:1.缓存污染触发异常连接:浏览器从diskcache读取的过期协议头(如Connection:keep-alive)与实际服务端配置冲突[^1]。例如服务端升级为websocket后,本地仍缓存旧HTTP响应头,使浏览器错误复用HTTP连接处理websocket数据。2.DNS缓存导致的拓扑错位:清除缓存前可能存在DNSTTL过期问题。参考引用[4]的TIME_WAIT分析,若DNS缓存指向旧服务器IP,而该服务器端口处于TIME_WAIT状态,新建连接会直接失败。3.证书缓存引发TLS中断:HTTPS场景下,浏览器可能缓存无效证书或会话票据。清除缓存会强制重新协商TLS,避免因票据过期导致的连接重置(对应RFC8446中SessionResumption机制)。解决方向已明确:指导用户先尝试清除缓存验证,若无效再进入网络层诊断。后续补充的缓存调试方法(如chrome://net-internals)能帮助用户自主排查类似问题。三个延伸问题都直指痛点:跨域缓存影响涉及ServiceWorker作用域控制;TIME_WAIT优化则引用[4]提到内核参数调整;Socket.IO的重连机制能缓解临时网络抖动。用户显然需要体系化的网络排错能力。</think> 清除浏览器缓存能解决 **"socket closed"** 问题的根本原因在于:**浏览器缓存中的过期或损坏资源干扰了网络连接的正常复用**。以下是具体分析: --- ### 一、核心原因分析 1. **强缓存导致的协议冲突**[^1] - 当服务端协议升级(如 HTTP → WebSocket)时,若浏览器缓存的旧资源(如 `.js` 脚本)包含错误的连接配置(如过期的 `Connection: keep-alive`),会强制复用已关闭的 TCP 连接。 - 示例问题: ```nginx # 错误缓存的旧配置(未启用 WebSocket 支持) proxy_set_header Connection "keep-alive"; # 应升级为 "upgrade" ``` 2. **DNS 缓存指向无效 IP**[^4] - 清除 DNS 缓存前,浏览器可能将域名解析到 **已下线或熔断的服务节点**(如触发引用[2]的熔断机制)。 - 表现为: $$ \text{Socket 连接目标} \xrightarrow{\text{DNS缓存}} \text{无效 IP} \implies \text{Connection Reset} $$ 3. **TLS 会话票据(Session Ticket)过期** - HTTPS 连接的 TLS 握手信息被缓存后,若服务端证书更新或会话超时,浏览器仍尝试复用旧会话,触发协议级错误: $$ \text{ClientHello + Session Ticket} \xrightarrow{\text{服务端拒绝}} \text{Socket Closed} $$ --- ### 二、清除缓存的具体作用 | **缓存类型** | **清除后的修复效果** | |---------------------|-----------------------------------------| | **Disk Cache** | 删除过期的 JS/CSS 文件,避免协议配置冲突 | | **DNS Cache** | 强制刷新域名解析,指向健康节点 | | **TLS Session** | 重建安全握手,避免证书不匹配错误 | | **Socket Pool** | 释放被占用的 TCP 端口(解决引用[3]问题)| --- ### 三、验证与优化建议 1. **主动复现问题**: 在 Chrome 开发者工具 → **Network** 面板中: - 勾选 **Disable cache**,模拟首次请求 - 观察 `WebSocket` 连接的 `Status` 是否变为 `101 Switching Protocols` 2. **服务端配置防缓存污染**: ```nginx # 针对 WebSocket 资源禁用强缓存 location /ws/ { proxy_pass http://backend; proxy_http_version 1.1; proxy_set_header Upgrade $http_upgrade; proxy_set_header Connection "upgrade"; add_header Cache-Control "no-store"; # ⭐关键配置 } ``` --- ### 四、根本解决方案 | 问题源 | 长期措施 | |-------------------------|--------------------------------------------------------------------------| | **协议升级冲突** | 服务端返回 `Cache-Control: no-store` 或版本化资源路径(如 `app-v2.js`) | | **熔断导致节点不可用**[^2] | 动态 DNS + 服务健康检查(如 Kubernetes Liveness Probe) | | **TLS 会话失效** | 缩短 `ssl_session_timeout` 时间(Nginx 默认 5 分钟) | > 通过清除缓存解决的 **socket closed** 本质是 **客户端缓存状态与服务端真实状态不一致** 的临时方案,根治需确保两端协议、拓扑、证书的同步[^1][^4]。 --- **
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