tcp: do not delay ACK in DCTCP upon CE status change

[ Upstream commit a0496ef2c23b3b180902dd185d0d63ccbc624cf8 ]

Per DCTCP RFC8257 (Section 3.2) the ACK reflecting the CE status change
has to be sent immediately so the sender can respond quickly:

""" When receiving packets, the CE codepoint MUST be processed as follows:

   1.  If the CE codepoint is set and DCTCP.CE is false, set DCTCP.CE to
       true and send an immediate ACK.

   2.  If the CE codepoint is not set and DCTCP.CE is true, set DCTCP.CE
       to false and send an immediate ACK.
"""

Previously DCTCP implementation may continue to delay the ACK. This
patch fixes that to implement the RFC by forcing an immediate ACK.

Tested with this packetdrill script provided by Larry Brakmo

0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
0.000 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
0.000 setsockopt(3, SOL_TCP, TCP_CONGESTION, "dctcp", 5) = 0
0.000 bind(3, ..., ...) = 0
0.000 listen(3, 1) = 0

0.100 < [ect0] SEW 0:0(0) win 32792 <mss 1000,sackOK,nop,nop,nop,wscale 7>
0.100 > SE. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8>
0.110 < [ect0] . 1:1(0) ack 1 win 257
0.200 accept(3, ..., ...) = 4
   +0 setsockopt(4, SOL_SOCKET, SO_DEBUG, [1], 4) = 0

0.200 < [ect0] . 1:1001(1000) ack 1 win 257
0.200 > [ect01] . 1:1(0) ack 1001

0.200 write(4, ..., 1) = 1
0.200 > [ect01] P. 1:2(1) ack 1001

0.200 < [ect0] . 1001:2001(1000) ack 2 win 257
+0.005 < [ce] . 2001:3001(1000) ack 2 win 257

+0.000 > [ect01] . 2:2(0) ack 2001
// Previously the ACK below would be delayed by 40ms
+0.000 > [ect01] E. 2:2(0) ack 3001

+0.500 < F. 9501:9501(0) ack 4 win 257

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

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 62af4ae27430dcc482fa590e2b163273858e2402..3173dd12b8cc0e6ab77e0c8559fe3b4d0e7e05a7 100644 (file)
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -372,6 +372,7 @@ ssize_t tcp_splice_read(struct socket *sk, loff_t *ppos,
                        struct pipe_inode_info *pipe, size_t len,
                        unsigned int flags);
 
+void tcp_enter_quickack_mode(struct sock *sk);
 static inline void tcp_dec_quickack_mode(struct sock *sk,
                                         const unsigned int pkts)
 {

diff --git a/net/ipv4/tcp_dctcp.c b/net/ipv4/tcp_dctcp.c
index 39d96d27ff94fc0c82b99b5ef5e4a56d1791e796..c78fb53988a12b279e15bc3d240d8ee9d6ea018e 100644 (file)
--- a/net/ipv4/tcp_dctcp.c
+++ b/net/ipv4/tcp_dctcp.c
@@ -131,12 +131,15 @@ static void dctcp_ce_state_0_to_1(struct sock *sk)
        struct dctcp *ca = inet_csk_ca(sk);
        struct tcp_sock *tp = tcp_sk(sk);
 
-       /* State has changed from CE=0 to CE=1 and delayed
-        * ACK has not sent yet.
-        */
-       if (!ca->ce_state &&
-           inet_csk(sk)->icsk_ack.pending & ICSK_ACK_TIMER)
-               __tcp_send_ack(sk, ca->prior_rcv_nxt);
+       if (!ca->ce_state) {
+               /* State has changed from CE=0 to CE=1, force an immediate
+                * ACK to reflect the new CE state. If an ACK was delayed,
+                * send that first to reflect the prior CE state.
+                */
+               if (inet_csk(sk)->icsk_ack.pending & ICSK_ACK_TIMER)
+                       __tcp_send_ack(sk, ca->prior_rcv_nxt);
+               tcp_enter_quickack_mode(sk);
+       }
 
        ca->prior_rcv_nxt = tp->rcv_nxt;
        ca->ce_state = 1;
@@ -149,12 +152,15 @@ static void dctcp_ce_state_1_to_0(struct sock *sk)
        struct dctcp *ca = inet_csk_ca(sk);
        struct tcp_sock *tp = tcp_sk(sk);
 
-       /* State has changed from CE=1 to CE=0 and delayed
-        * ACK has not sent yet.
-        */
-       if (ca->ce_state &&
-           inet_csk(sk)->icsk_ack.pending & ICSK_ACK_TIMER)
-               __tcp_send_ack(sk, ca->prior_rcv_nxt);
+       if (ca->ce_state) {
+               /* State has changed from CE=1 to CE=0, force an immediate
+                * ACK to reflect the new CE state. If an ACK was delayed,
+                * send that first to reflect the prior CE state.
+                */
+               if (inet_csk(sk)->icsk_ack.pending & ICSK_ACK_TIMER)
+                       __tcp_send_ack(sk, ca->prior_rcv_nxt);
+               tcp_enter_quickack_mode(sk);
+       }
 
        ca->prior_rcv_nxt = tp->rcv_nxt;
        ca->ce_state = 0;

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 5711b1b12d28bfcc1bf4e1694d035351189c87e5..03fd2ff4007bdf81e09e0cbaaac4df9242eea9a7 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -209,13 +209,14 @@ static void tcp_incr_quickack(struct sock *sk)
                icsk->icsk_ack.quick = min(quickacks, TCP_MAX_QUICKACKS);
 }
 
-static void tcp_enter_quickack_mode(struct sock *sk)
+void tcp_enter_quickack_mode(struct sock *sk)
 {
        struct inet_connection_sock *icsk = inet_csk(sk);
        tcp_incr_quickack(sk);
        icsk->icsk_ack.pingpong = 0;
        icsk->icsk_ack.ato = TCP_ATO_MIN;
 }
+EXPORT_SYMBOL(tcp_enter_quickack_mode);
 
 /* Send ACKs quickly, if "quick" count is not exhausted
  * and the session is not interactive.