[PATCH TCP 1/4] Add undo callback to Stephens cc framework

[Baruch Even <baruch@xxxxxxxxx>]

When undoing a congestion event due to reordering detection, the ssthresh is
multiplied by two. This is incorrect for anything other than (New)Reno, for
example BIC and H-TCP.

This patch adds a callback that these CC algorithms can attach to, the actual
callback is not realized yet for BIC. The callback is already in use in the
H-TCP patches.

Signed-Off-By: Baruch Even <baruch@xxxxxxxxx>

---
 include/net/tcp.h    |    1 +
 net/ipv4/tcp_input.c |    5 ++++-
 2 files changed, 5 insertions(+), 1 deletion(-)

Index: 2.6.11-stephen-htcp/include/net/tcp.h
===================================================================
--- 2.6.11-stephen-htcp.orig/include/net/tcp.h
+++ 2.6.11-stephen-htcp/include/net/tcp.h
@@ -1196,6 +1196,7 @@ struct tcp_ca_type {
        void (*set_state)(struct tcp_sock *tp, u8 new_state);
 
        void (*cwnd_event)(struct tcp_sock *tp, enum tcp_ca_event ev);
+       u32 (*undo_cwnd)(struct tcp_sock *tp);
 
        struct list_head        list;
        struct module           *owner;
Index: 2.6.11-stephen-htcp/net/ipv4/tcp_input.c
===================================================================
--- 2.6.11-stephen-htcp.orig/net/ipv4/tcp_input.c
+++ 2.6.11-stephen-htcp/net/ipv4/tcp_input.c
@@ -1573,7 +1573,10 @@ static void DBGUNDO(struct sock *sk, str
 static void tcp_undo_cwr(struct tcp_sock *tp, int undo)
 {
        if (tp->prior_ssthresh) {
-               tp->snd_cwnd = max(tp->snd_cwnd, tp->snd_ssthresh<<1);
+               if (tp->ca_proto->undo_cwnd)
+                       tp->snd_cwnd = tp->ca_proto->undo_cwnd(tp);
+               else
+                       tp->snd_cwnd = max(tp->snd_cwnd, tp->snd_ssthresh<<1);
 
                if (undo && tp->prior_ssthresh > tp->snd_ssthresh) {
                        tp->snd_ssthresh = tp->prior_ssthresh;