tcp: Measure TIME-WAIT reuse delay with millisecond precision

Prepare ground for TIME-WAIT socket reuse with subsecond delay.

Today the last TS.Recent update timestamp, recorded in seconds and stored
tp->ts_recent_stamp and tw->tw_ts_recent_stamp fields, has two purposes.

Firstly, it is used to track the age of the last recorded TS.Recent value
to detect when that value becomes outdated due to potential wrap-around of
the other TCP timestamp clock (RFC 7323, section 5.5).

For this purpose a second-based timestamp is completely sufficient as even
in the worst case scenario of a peer using a high resolution microsecond
timestamp, the wrap-around interval is ~36 minutes long.

Secondly, it serves as a threshold value for allowing TIME-WAIT socket
reuse. A TIME-WAIT socket can be reused only once the virtual 1 Hz clock,
ktime_get_seconds, is past the TS.Recent update timestamp.

The purpose behind delaying the TIME-WAIT socket reuse is to wait for the
other TCP timestamp clock to tick at least once before reusing the
connection. It is only then that the PAWS mechanism for the reopened
connection can detect old duplicate segments from the previous connection
incarnation (RFC 7323, appendix B.2).

In this case using a timestamp with second resolution not only blocks the
way toward allowing faster TIME-WAIT reuse after shorter subsecond delay,
but also makes it impossible to reliably delay TW reuse by one second.

As Eric Dumazet has pointed out [1], due to timestamp rounding, the TW
reuse delay will actually be between (0, 1] seconds, and 0.5 seconds on
average. We delay TW reuse for one full second only when last TS.Recent
update coincides with our virtual 1 Hz clock tick.

Considering the above, introduce a dedicated field to store a millisecond
timestamp of transition into the TIME-WAIT state. Place it in an existing
4-byte hole inside inet_timewait_sock structure to avoid an additional
memory cost.

Use the new timestamp to (i) reliably delay TIME-WAIT reuse by one second,
and (ii) prepare for configurable subsecond reuse delay in the subsequent
change.

We assume here that a full one second delay was the original intention in
[2] because it accounts for the worst case scenario of the other TCP using
the slowest recommended 1 Hz timestamp clock.

A more involved alternative would be to change the resolution of the last
TS.Recent update timestamp, tw->tw_ts_recent_stamp, to milliseconds.

[1] https://lore.kernel.org/netdev/CANn89iKB4GFd8sVzCbRttqw_96o3i2wDhX-3DraQtsceNGYwug@mail.gmail.com/
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b8439924316d5bcb266d165b93d632a4b4b859af

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Link: https://patch.msgid.link/20241209-jakub-krn-909-poc-msec-tw-tstamp-v2-1-66aca0eed03e@cloudflare.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 3 insertions, 2 deletions

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index a38c8b1f44db..3b6ba1d16921 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -120,6 +120,7 @@ int tcp_twsk_unique(struct sock *sk, struct sock *sktw, void *twp)
 	const struct tcp_timewait_sock *tcptw = tcp_twsk(sktw);
 	struct tcp_sock *tp = tcp_sk(sk);
 	int ts_recent_stamp;
+	u32 reuse_thresh;
 
 	if (READ_ONCE(tw->tw_substate) == TCP_FIN_WAIT2)
 		reuse = 0;
@@ -162,9 +163,9 @@ int tcp_twsk_unique(struct sock *sk, struct sock *sktw, void *twp)
 	   and use initial timestamp retrieved from peer table.
 	 */
 	ts_recent_stamp = READ_ONCE(tcptw->tw_ts_recent_stamp);
+	reuse_thresh = READ_ONCE(tw->tw_entry_stamp) + MSEC_PER_SEC;
 	if (ts_recent_stamp &&
-	    (!twp || (reuse && time_after32(ktime_get_seconds(),
-					    ts_recent_stamp)))) {
+	    (!twp || (reuse && time_after32(tcp_clock_ms(), reuse_thresh)))) {
 		/* inet_twsk_hashdance_schedule() sets sk_refcnt after putting twsk
 		 * and releasing the bucket lock.
 		 */