报文被拆分为多个分组发送

TCP 是一种字节流（byte-stream）协议，流的含义是没有固定的报文边界。
假设你调用 2 次 write 函数往 socket 里依次写 500 字节、800 字节。write 函数只是把字节拷贝到内核缓冲区，最终会以多少条报文发送出去是不确定的，如下图所示:
情况 1：分为两条报文依次发出去 500 字节 和 800 字节数据，也有
情况 2：两部分数据合并为一个长度为 1300 字节的报文，一次发送
情况 3：第一部分的 500 字节与第二部分的 500 字节合并为一个长度为 1000 字节的报文，第二部分剩下的 300 字节单独作为一个报文发送
情况 4：第一部分的 400 字节单独发送，剩下100字节与第二部分的 800 字节合并为一个 900 字节的包一起发送。
情况 N：还有更多可能的拆分组合
上面出现的情况取决于诸多因素：路径最大传输单元 MTU、发送窗口大小、拥塞窗口大小等。
当前mtu:node1:/root#ifconfig -a
eth1      Link encap:Ethernet  HWaddr 00:0C:29:F2:0E:05  
          inet addr:192.168.137.2  Bcast:192.168.137.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:296  Metric:1
		  


一次收到了257的长度:

node1:/root#tcpdump -S -i eth1 '((tcp) and  (port 8080) )'
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 65535 bytes


01:01:20.627525 IP 192.168.137.1.61583 > node1.webcache: Flags [S], seq 2023737918, win 8192, options [mss 1460,nop,wscale 2,nop,nop,sackOK], length 0
01:01:20.627568 IP node1.webcache > 192.168.137.1.61583: Flags [S.], seq 4074660050, ack 2023737919, win 2560, options [mss 256,nop,nop,sackOK,nop,wscale 6], length 0
01:01:20.627743 IP 192.168.137.1.61583 > node1.webcache: Flags [.], ack 4074660051, win 16482, length 0

01:01:20.628887 IP 192.168.137.1.61583 > node1.webcache: Flags [P.], seq 2023737919:2023738176, ack 4074660051, win 16482, length 257
01:01:20.628914 IP node1.webcache > 192.168.137.1.61583: Flags [.], ack 2023738176, win 49, length 0
01:01:20.630216 IP 192.168.137.1.61583 > node1.webcache: Flags [F.], seq 2023738176, ack 4074660051, win 16482, length 0
01:01:20.646496 IP node1.webcache > 192.168.137.1.61583: Flags [F.], seq 4074660051, ack 2023738177, win 49, length 0
01:01:20.646634 IP 192.168.137.1.61583 > node1.webcache: Flags [.], ack 4074660052, win 16482, length 0


发送方:
C:Python27python.exe C:/Users/TLCB/PycharmProjects/untitled2/http/t2.py
('192.168.137.1', 61605)
('192.168.137.2', 8080)
1552


		  
		  
		  
01:02:49.194289 IP 192.168.137.1.61605 > node1.webcache: Flags [S], seq 1138731431, win 8192, options [mss 1460,nop,wscale 2,nop,nop,sackOK], length 0
01:02:49.194334 IP node1.webcache > 192.168.137.1.61605: Flags [S.], seq 1198709260, ack 1138731432, win 2560, options [mss 256,nop,nop,sackOK,nop,wscale 6], length 0
01:02:49.194460 IP 192.168.137.1.61605 > node1.webcache: Flags [.], ack 1198709261, win 16482, length 0

01:02:49.195321 IP 192.168.137.1.61605 > node1.webcache: Flags [.], seq 1138731432:1138731968, ack 1198709261, win 16482, length 536
01:02:49.195338 IP 192.168.137.1.61605 > node1.webcache: Flags [.], seq 1138731968:1138732504, ack 1198709261, win 16482, length 536

01:02:49.195406 IP node1.webcache > 192.168.137.1.61605: Flags [.], ack 1138731968, win 57, length 0
01:02:49.195513 IP node1.webcache > 192.168.137.1.61605: Flags [.], ack 1138732504, win 74, length 0

01:02:49.195553 IP 192.168.137.1.61605 > node1.webcache: Flags [P.], seq 1138732504:1138732984, ack 1198709261, win 16482, length 480
01:02:49.195571 IP node1.webcache > 192.168.137.1.61605: Flags [.], ack 1138732984, win 91, length 0
01:02:49.196360 IP 192.168.137.1.61605 > node1.webcache: Flags [F.], seq 1138732984, ack 1198709261, win 16482, length 0
01:02:49.198308 IP node1.webcache > 192.168.137.1.61605: Flags [F.], seq 1198709261, ack 1138732985, win 91, length 0
01:02:49.198490 IP 192.168.137.1.61605 > node1.webcache: Flags [.], ack 1198709262, win 16482, length 0

发送了1552长度额数据:

分为3次发:

01:02:49.195321 IP 192.168.137.1.61605 > node1.webcache: Flags [.], seq 1138731432:1138731968, ack 1198709261, win 16482, length 536
01:02:49.195338 IP 192.168.137.1.61605 > node1.webcache: Flags [.], seq 1138731968:1138732504, ack 1198709261, win 16482, length 536

01:02:49.195553 IP 192.168.137.1.61605 > node1.webcache: Flags [P.], seq 1138732504:1138732984, ack 1198709261, win 16482, length 480