tcp: remove forward retransmit feature

Forward retransmit is an esoteric feature in RFC3517 (condition(3)
in the NextSeg()). Basically if a packet is not considered lost by
the current criteria (# of dupacks etc), but the congestion window
has room for more packets, then retransmit this packet.

However it actually conflicts with the rest of recovery design. For
example, when reordering is detected we want to be conservative
in retransmitting packets but forward-retransmit feature would
break that to force more retransmission. Also the implementation is
fairly complicated inside the retransmission logic inducing extra
iterations in the write queue. With RACK losses are being detected
timely and this heuristic is no longer necessary. There this patch
removes the feature.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 970d5f00589f3fdbd6da39acb712a8a5ce258ca7..8e5f4c15d0e5e95b29aeffe445417e2fd8d8131e 100644 (file)
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -307,7 +307,6 @@ struct tcp_sock {
                                         */
 
        int     lost_cnt_hint;
-       u32     retransmit_high;        /* L-bits may be on up to this seqno */
 
        u32     prior_ssthresh; /* ssthresh saved at recovery start     */
        u32     high_seq;       /* snd_nxt at onset of congestion       */

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 9469ce384d3b70f2b63041612d03c7c713aed09c..a041a92348ee2572fb937dda466af8a6cdf893e6 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -916,10 +916,6 @@ static void tcp_verify_retransmit_hint(struct tcp_sock *tp, struct sk_buff *skb)
            before(TCP_SKB_CB(skb)->seq,
                   TCP_SKB_CB(tp->retransmit_skb_hint)->seq))
                tp->retransmit_skb_hint = skb;
-
-       if (!tp->lost_out ||
-           after(TCP_SKB_CB(skb)->end_seq, tp->retransmit_high))
-               tp->retransmit_high = TCP_SKB_CB(skb)->end_seq;
 }
 
 /* Sum the number of packets on the wire we have marked as lost.
@@ -1983,7 +1979,6 @@ void tcp_enter_loss(struct sock *sk)
                        TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_ACKED;
                        TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
                        tp->lost_out += tcp_skb_pcount(skb);
-                       tp->retransmit_high = TCP_SKB_CB(skb)->end_seq;
                }
        }
        tcp_verify_left_out(tp);

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 0ba9026cb70d07e79ed3a217096c716326a56f04..6327e4d368a42dd0108302c20fdcaf6ad0d45f44 100644 (file)
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2831,36 +2831,6 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
        return err;
 }
 
-/* Check if we forward retransmits are possible in the current
- * window/congestion state.
- */
-static bool tcp_can_forward_retransmit(struct sock *sk)
-{
-       const struct inet_connection_sock *icsk = inet_csk(sk);
-       const struct tcp_sock *tp = tcp_sk(sk);
-
-       /* Forward retransmissions are possible only during Recovery. */
-       if (icsk->icsk_ca_state != TCP_CA_Recovery)
-               return false;
-
-       /* No forward retransmissions in Reno are possible. */
-       if (tcp_is_reno(tp))
-               return false;
-
-       /* Yeah, we have to make difficult choice between forward transmission
-        * and retransmission... Both ways have their merits...
-        *
-        * For now we do not retransmit anything, while we have some new
-        * segments to send. In the other cases, follow rule 3 for
-        * NextSeg() specified in RFC3517.
-        */
-
-       if (tcp_may_send_now(sk))
-               return false;
-
-       return true;
-}
-
 /* This gets called after a retransmit timeout, and the initially
  * retransmitted data is acknowledged.  It tries to continue
  * resending the rest of the retransmit queue, until either
@@ -2875,24 +2845,16 @@ void tcp_xmit_retransmit_queue(struct sock *sk)
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb;
        struct sk_buff *hole = NULL;
-       u32 max_segs, last_lost;
+       u32 max_segs;
        int mib_idx;
-       int fwd_rexmitting = 0;
 
        if (!tp->packets_out)
                return;
 
-       if (!tp->lost_out)
-               tp->retransmit_high = tp->snd_una;
-
        if (tp->retransmit_skb_hint) {
                skb = tp->retransmit_skb_hint;
-               last_lost = TCP_SKB_CB(skb)->end_seq;
-               if (after(last_lost, tp->retransmit_high))
-                       last_lost = tp->retransmit_high;
        } else {
                skb = tcp_write_queue_head(sk);
-               last_lost = tp->snd_una;
        }
 
        max_segs = tcp_tso_segs(sk, tcp_current_mss(sk));
@@ -2915,31 +2877,14 @@ void tcp_xmit_retransmit_queue(struct sock *sk)
                 */
                segs = min_t(int, segs, max_segs);
 
-               if (fwd_rexmitting) {
-begin_fwd:
-                       if (!before(TCP_SKB_CB(skb)->seq, tcp_highest_sack_seq(tp)))
-                               break;
-                       mib_idx = LINUX_MIB_TCPFORWARDRETRANS;
-
-               } else if (!before(TCP_SKB_CB(skb)->seq, tp->retransmit_high)) {
-                       tp->retransmit_high = last_lost;
-                       if (!tcp_can_forward_retransmit(sk))
-                               break;
-                       /* Backtrack if necessary to non-L'ed skb */
-                       if (hole) {
-                               skb = hole;
-                               hole = NULL;
-                       }
-                       fwd_rexmitting = 1;
-                       goto begin_fwd;
-
+               if (tp->retrans_out >= tp->lost_out) {
+                       break;
                } else if (!(sacked & TCPCB_LOST)) {
                        if (!hole && !(sacked & (TCPCB_SACKED_RETRANS|TCPCB_SACKED_ACKED)))
                                hole = skb;
                        continue;
 
                } else {
-                       last_lost = TCP_SKB_CB(skb)->end_seq;
                        if (icsk->icsk_ca_state != TCP_CA_Loss)
                                mib_idx = LINUX_MIB_TCPFASTRETRANS;
                        else