net/tcp(buffered): upgraded the Fast Retransmit algorithm from RFC 2001 to RFC 5681

[a-lunev]

Summary

net/tcp(buffered): upgraded the Fast Retransmit algorithm from RFC 2001 to RFC 5681.
This PR is similar to tcp_send_unbuffered (PR #5137) and tcp_sendfile (PR #5311), however it replaces the implementation of PR #2414.

Impact

net/tcp(buffered)

Testing

[anchao]

There is another problem here. If the segment loss occurs before multiple segments are sent, it will trigger more invalid retransmissions:

e.g:

1     ->   good
2     ->   lost
3     ->   good
4     ->   good
5     ->   good
6     ->   good
7     ->   good
8     ->   good
9     ->   good
10     ->   good

snd_dup_acks 0    <-  1 (ACK 1)
snd_dup_acks 1    <-  2 (DUP ACK 1)
snd_dup_acks 2    <-  3 (DUP ACK 1)
snd_dup_acks 3    <-  4 (DUP ACK 1)
2     -> rexmit(snd_dup_acks = 0)
...
snd_dup_acks 1    <-  5 (DUP ACK 1)
snd_dup_acks 2    <-  6 (DUP ACK 1)
snd_dup_acks 3    <-  7 (DUP ACK 1)
2     -> rexmit(snd_dup_acks = 0)
...
snd_dup_acks 1    <-  8 (DUP ACK 1)
snd_dup_acks 2    <-  9 (DUP ACK 1)
snd_dup_acks 3    <-  10 (DUP ACK 1)
2     -> rexmit(snd_dup_acks = 0)