diff --git a/web/src/core/realtime/backoff.test.ts b/web/src/core/realtime/backoff.test.ts
index 94e1d93..d946c93 100644
--- a/web/src/core/realtime/backoff.test.ts
+++ b/web/src/core/realtime/backoff.test.ts
@@ -1,12 +1,20 @@
-import { describe, expect, it } from 'vitest'
+import { afterEach, describe, expect, it, vi } from 'vitest'
 import { reconnectDelay } from './backoff'
 
 describe('reconnectDelay', () => {
+  afterEach(() => {
+    vi.restoreAllMocks()
+  })
+
   it.each([
-    [1, 3000],
-    [4, 24_000],
-    [5, 30_000],
-  ])('第 %i 次等待 %i 毫秒', (attempt, delay) => {
+    [1, 0.5, 3000],
+    [4, 0.5, 24_000],
+    [5, 0.5, 30_000],
+    [1, 0, 2250],
+    [5, 0, 22_500],
+    [5, 1, 37_500],
+  ])('第 %i 次、随机数为 %d 时等待 %i 毫秒', (attempt, random, delay) => {
+    vi.spyOn(Math, 'random').mockReturnValue(random)
     expect(reconnectDelay(attempt)).toBe(delay)
   })
 })
diff --git a/web/src/core/realtime/backoff.ts b/web/src/core/realtime/backoff.ts
index 7896783..90d52e4 100644
--- a/web/src/core/realtime/backoff.ts
+++ b/web/src/core/realtime/backoff.ts
@@ -1,7 +1,10 @@
 const firstDelayMs = 3000
 const maxDelayMs = 30_000
+// 服务端重启时会同时断开所有连接，随机错开等待时间，避免客户端同时重连。
+const jitter = 0.25
 
 // attempt 从 1 开始计数。
 export function reconnectDelay(attempt: number): number {
-  return Math.min(firstDelayMs * 2 ** (attempt - 1), maxDelayMs)
+  const delay = Math.min(firstDelayMs * 2 ** (attempt - 1), maxDelayMs)
+  return delay * (1 - jitter + 2 * jitter * Math.random())
 }
diff --git a/web/src/core/realtime/connection.ts b/web/src/core/realtime/connection.ts
index 59dc82c..8fafee3 100644
--- a/web/src/core/realtime/connection.ts
+++ b/web/src/core/realtime/connection.ts
@@ -5,6 +5,8 @@ import { CloseCode, PROTOCOL_VERSION } from './protocol.gen'
 
 const requestTimeoutMs = 60_000
 const pingIntervalMs = 5000
+// 半开的连接上写入不会出错，要等 TCP 重传超时才会关闭，所以 ping 收不到回包就当作连接已失效。
+const pongTimeoutMs = 3 * pingIntervalMs
 
 const stopCodes = new Set<number>([CloseCode.invalidArgument, CloseCode.unauthorized, CloseCode.notFound, CloseCode.clientOutdated])
 
@@ -77,14 +79,13 @@ export class Connection {
     return this.#send(type, data)
   }
 
-  // 不以收不到回包作为断线的依据：连接已失效时，写入 ping 会触发 close 或 error。
   readonly check = (): void => {
     if (this.#status === 'stopped')
       return
     const socket = this.#socket
     if (socket?.readyState === WebSocket.OPEN) {
       if (this.#ready)
-        this.#ping()
+        this.#ping(socket)
       return
     }
     if (socket?.readyState === WebSocket.CONNECTING)
@@ -131,8 +132,8 @@ export class Connection {
       clock.reset()
       this.#setStatus('open')
       this.#handlers.hello(response)
-      this.#ping()
-      this.#pingTimer = setInterval(() => this.#ping(), pingIntervalMs)
+      this.#ping(socket)
+      this.#pingTimer = setInterval(() => this.#ping(socket), pingIntervalMs)
     }, (error: RequestError) => {
       if (socket !== this.#socket)
         return
@@ -144,15 +145,18 @@ export class Connection {
     })
   }
 
-  #ping(): void {
+  #ping(socket: WebSocket): void {
     const sentAt = clock.now()
     // 服务端只接受整数毫秒，而 performance.now() 带小数。
-    this.#send('ping', { clientTimeMs: Math.round(sentAt) }).then((response) => {
+    this.#send('ping', { clientTimeMs: Math.round(sentAt) }, pongTimeoutMs).then((response) => {
       clock.addSample(sentAt, clock.now(), response.serverTimeMs)
-    }, () => {})
+    }, (error: RequestError) => {
+      if (!error.code)
+        this.#lost(socket, abnormalClosure)
+    })
   }
 
-  #send<T extends RequestType>(type: T, data: RequestDataMap[T]): Promise<ResponseDataMap[T]> {
+  #send<T extends RequestType>(type: T, data: RequestDataMap[T], timeoutMs = requestTimeoutMs): Promise<ResponseDataMap[T]> {
     const socket = this.#socket
     if (!socket)
       return Promise.reject(new RequestError('还没有连上房间，连上之后再试一次。', null))
@@ -161,7 +165,7 @@ export class Connection {
       const timer = setTimeout(() => {
         this.#pending.delete(id)
         reject(new RequestError('服务器没有及时响应，请稍后再试。', null))
-      }, requestTimeoutMs)
+      }, timeoutMs)
       this.#pending.set(id, { resolve: resolve as (data: unknown) => void, reject, timer })
       const message: RequestMessage<T> = { kind: 'request', type, id, data }
       socket.send(JSON.stringify(message))