net/tcp: implement the fast retransmit

[anchao]

Summary

net/tcp: implement the fast retransmit

https://tools.ietf.org/html/rfc2001#section-3

RFC2001: TCP Slow Start, Congestion Avoidance, Fast Retransmit,
         and Fast Recovery Algorithms

...

3.  Fast Retransmit
  Modifications to the congestion avoidance algorithm were proposed in
  1990 [3].  Before describing the change, realize that TCP may
  generate an immediate acknowledgment (a duplicate ACK) when an out-
  of-order segment is received (Section 4.2.2.21 of [1], with a note
  that one reason for doing so was for the experimental fast-
  retransmit algorithm).  This duplicate ACK should not be delayed.
  The purpose of this duplicate ACK is to let the other end know that a
  segment was received out of order, and to tell it what sequence
  number is expected.

  Since TCP does not know whether a duplicate ACK is caused by a lost
  segment or just a reordering of segments, it waits for a small number
  of duplicate ACKs to be received.  It is assumed that if there is
  just a reordering of the segments, there will be only one or two
  duplicate ACKs before the reordered segment is processed, which will
  then generate a new ACK.  If three or more duplicate ACKs are
  received in a row, it is a strong indication that a segment has been
  lost.  TCP then performs a retransmission of what appears to be the
  missing segment, without waiting for a retransmission timer to
  expire.

Change-Id: Ie2cbcecab507c3d831f74390a6a85e0c5c8e0652
Signed-off-by: chao.an anchao@xiaomi.com

Impact

TCP retransmit

Testing

iperf test

[davids5]

@antmerlino - would you please have a look?

[acassis]

@davids5 I don't know if @antmerlino is available, maybe @masayuki2009 could help us on it.

[xiaoxiang781216]

LGTM.