Subject: [PATCH net-next v3 2/2] tcp: change data receiver flowlabel after one dup

From: David Morley <[email-masked]>
Date: Fri, 6 Oct 2023 01:18:41 +0000

From: David Morley <[email-masked]>

This commit changes the data receiver repath behavior to occur after
receiving a single duplicate. This can help recover ACK connectivity
quicker if a TLP was sent along a nonworking path.

For instance, consider the case where we have an initially nonworking
forward path and reverse path and subsequently switch to only working
forward paths. Before this patch we would have the following behavior.

+---------+--------+--------+----------+----------+----------+
| Event   | For FL | Rev FL | FP Works | RP Works | Data Del |
+---------+--------+--------+----------+----------+----------+
| Initial | A      | 1      | N        | N        | 0        |
+---------+--------+--------+----------+----------+----------+
| TLP     | A      | 1      | N        | N        | 0        |
+---------+--------+--------+----------+----------+----------+
| RTO 1   | B      | 1      | Y        | N        | 1        |
+---------+--------+--------+----------+----------+----------+
| RTO 2   | C      | 1      | Y        | N        | 2        |
+---------+--------+--------+----------+----------+----------+
| RTO 3   | D      | 2      | Y        | Y        | 3        |
+---------+--------+--------+----------+----------+----------+

This patch gets rid of at least RTO 3, avoiding additional unnecessary
repaths of a working forward path to a (potentially) nonworking one.

In addition, this commit changes the behavior to avoid repathing upon
rx of duplicate data if the local endpoint is in CA_Loss (in which
case the RTOs will already be changing the outgoing flowlabel).

Signed-off-by: David Morley <[email-masked]>
Signed-off-by: Neal Cardwell <[email-masked]>
Signed-off-by: Yuchung Cheng <[email-masked]>
Tested-by: David Morley <[email-masked]>
Reviewed-by: Eric Dumazet <[email-masked]>
---
 net/ipv4/tcp_input.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index c7c15d4b95e5..747ae84796e4 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4511,15 +4511,23 @@ static void tcp_rcv_spurious_retrans(struct sock *sk, const struct sk_buff *skb)
 {
 	/* When the ACK path fails or drops most ACKs, the sender would
 	 * timeout and spuriously retransmit the same segment repeatedly.
-	 * The receiver remembers and reflects via DSACKs. Leverage the
-	 * DSACK state and change the txhash to re-route speculatively.
+	 * If it seems our ACKs are not reaching the other side,
+	 * based on receiving a duplicate data segment with new flowlabel
+	 * (suggesting the sender suffered an RTO), and we are not already
+	 * repathing due to our own RTO, then rehash the socket to repath our
+	 * packets.
 	 */
-	if (TCP_SKB_CB(skb)->seq == tcp_sk(sk)->duplicate_sack[0].start_seq &&
+#if IS_ENABLED(CONFIG_IPV6)
+	if (inet_csk(sk)->icsk_ca_state != TCP_CA_Loss &&
+	    skb->protocol == htons(ETH_P_IPV6) &&
+	    (tcp_sk(sk)->inet_conn.icsk_ack.lrcv_flowlabel !=
+	     ntohl(ip6_flowlabel(ipv6_hdr(skb)))) &&
 	    sk_rethink_txhash(sk))
 		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDUPLICATEDATAREHASH);
 
 	/* Save last flowlabel after a spurious retrans. */
 	tcp_save_lrcv_flowlabel(sk, skb);
+#endif
 }
 
 static void tcp_send_dupack(struct sock *sk, const struct sk_buff *skb)
-- 
2.42.0.609.gbb76f46606-goog