TCP Notes (1)

How TCP Works @ youtube

• TCP fundamentals Part 2
• 2 related TCP windows: 
• CWND - Congestion Window
• RWND - Receive Window
• Only RWND advertized in packets
• minimum of CWND and RWND is the cap of transmission
• CWND = n * MSS, n = 1,2,4...., slowstart
• not advertised, changing
• Example 1, slow backup between 2 local servers
• from the trace, recvr to sender 只发ACKs, (len = 60) 
• window size 每隔一段时间，会从64K一直降到1000 (小于MSS). 然后hold for 190ms. 然后window size back to 64K
• 显示chart - statistics -> TCP stream graph -> Time sequence (tcptrace)
• 足够的证据是server side问题
• Example 2, 是sender application无法fill CWND,
• Example 3, ip tcp mss-adjust问题
• server/client, 2个pcap
• client直接看到seq = 1461, 不是1
• server一直re-send seq = 1, size = 1460 pkts. 
• TCP NoOP option - 很简单，#TcpLen * 4 = Tcp Header Len，而TCP Options Len不全是4的倍数，所以TCP Header需要NoOp pad out header size to a multiple of 4B. 
• TCP option - timestamp
• TSval, EchoTSval, 为了计算RTT and RTO (retrans timeout)
• Some TCP options:
• 0 - end of TCP options
• 1 - NoOP
• 2 - MSS
• 3 - Window Scale
• 4/5 - SACK
• 8 - timestamp