tcp: TLP loss detection.

This is the second of the TLP patch series; it augments the basic TLP
algorithm with a loss detection scheme.

This patch implements a mechanism for loss detection when a Tail
loss probe retransmission plugs a hole thereby masking packet loss
from the sender. The loss detection algorithm relies on counting
TLP dupacks as outlined in Sec. 3 of:
http://tools.ietf.org/html/draft-dukkipati-tcpm-tcp-loss-probe-01

The basic idea is: Sender keeps track of TLP "episode" upon
retransmission of a TLP packet. An episode ends when the sender receives
an ACK above the SND.NXT (tracked by tlp_high_seq) at the time of the
episode. We want to make sure that before the episode ends the sender
receives a "TLP dupack", indicating that the TLP retransmission was
unnecessary, so there was no loss/hole that needed plugging. If the
sender gets no TLP dupack before the end of the episode, then it reduces
ssthresh and the congestion window, because the TLP packet arriving at
the receiver probably plugged a hole.

Signed-off-by: Nandita Dukkipati <nanditad@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 9 insertions, 0 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index beb63dbc85f5..8e7742f0b5d2 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2132,6 +2132,7 @@ bool tcp_schedule_loss_probe(struct sock *sk)
  */
 void tcp_send_loss_probe(struct sock *sk)
 {
+        struct tcp_sock *tp = tcp_sk(sk);
         struct sk_buff *skb;
         int pcount;
         int mss = tcp_current_mss(sk);
@@ -2142,6 +2143,10 @@ void tcp_send_loss_probe(struct sock *sk)
                 goto rearm_timer;
         }
 
+        /* At most one outstanding TLP retransmission. */
+        if (tp->tlp_high_seq)
+                goto rearm_timer;
+
         /* Retransmit last segment. */
         skb = tcp_write_queue_tail(sk);
         if (WARN_ON(!skb))
@@ -2164,6 +2169,10 @@ void tcp_send_loss_probe(struct sock *sk)
         if (skb->len > 0)
                 err = __tcp_retransmit_skb(sk, skb);
 
+        /* Record snd_nxt for loss detection. */
+        if (likely(!err))
+                tp->tlp_high_seq = tp->snd_nxt;
+
 rearm_timer:
         inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                                   inet_csk(sk)->icsk_rto,