tcp - 长肥管道 rfc1323

1. rfc1323

匹配高性能的网络，tcp在高带宽和高延迟的情节下存在的问题。

2. 长肥管道

  TCP  performance problems arise when the bandwidth*delay product is
  large.  We refer to an Internet path operating in this region as a
  "long, fat pipe", and a network containing this path as an "LFN"
  (pronounced "elephan(t)")

3. 长肥管道的问题

3.1 窗口大小限制问题

tcp的窗口大小2字节（64k)，对于长肥管道不够，通过tcp option来增加。

     To circumvent this problem, Section 2 of this memo defines a
     new TCP option, "Window Scale", to allow windows larger than
     2**16.  This option defines an implicit scale factor, which
     is used to multiply the window size value found in a TCP
     header to obtain the true window size.

     This option may be sent in an initial <SYN> segment (i.e., a
     segment with the SYN bit on and the ACK bit off).  It may also
     be sent in a <SYN,ACK> segment, but only if a Window Scale op-
     tion was received in the initial <SYN> segment.  A Window Scale
     option in a segment without a SYN bit should be ignored.
     The Window field in a SYN (i.e., a <SYN> or <SYN,ACK>) segment
     itself is never scaled.

    TCP Window Scale Option (WSopt):
    Kind: 3 Length: 3 bytes
            +---------+---------+---------+
            | Kind=3  |Length=3 |shift.cnt|
            +---------+---------