tcp: add support for usec resolution in TCP TS values

Back in 2015, Van Jacobson suggested to use usec resolution in TCP TS values.
This has been implemented in our private kernels.

Goals were :

1) better observability of delays in networking stacks.
2) better disambiguation of events based on TSval/ecr values.
3) building block for congestion control modules needing usec resolution.

Back then we implemented a schem based on private SYN options
to negotiate the feature.

For upstream submission, we chose to use a route attribute,
because this feature is probably going to be used in private
networks [1] [2].

ip route add 10/8 ... features tcp_usec_ts

Note that RFC 7323 recommends a
  "timestamp clock frequency in the range 1 ms to 1 sec per tick.",
but also mentions
  "the maximum acceptable clock frequency is one tick every 59 ns."

[1] Unfortunately RFC 7323 5.5 (Outdated Timestamps) suggests
to invalidate TS.Recent values after a flow was idle for more
than 24 days. This is the part making usec_ts a problem
for peers following this recommendation for long living
idle flows.

[2] Attempts to standardize usec ts went nowhere:

https://www.ietf.org/proceedings/97/slides/slides-97-tcpm-tcp-options-for-low-latency-00.pdf
https://datatracker.ietf.org/doc/draft-wang-tcpm-low-latency-opt/

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 2 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 0ab577869d7a..39b731c900dd 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -825,6 +825,8 @@ static inline u32 tcp_time_stamp_ms(const struct tcp_sock *tp)
 
 static inline u32 tcp_time_stamp_ts(const struct tcp_sock *tp)
 {
+	if (tp->tcp_usec_ts)
+		return tp->tcp_mstamp;
 	return tcp_time_stamp_ms(tp);
 }
 
@@ -852,12 +854,12 @@ static inline u32 tcp_skb_timestamp_ts(bool usec_ts, const struct sk_buff *skb)
 
 static inline u32 tcp_tw_tsval(const struct tcp_timewait_sock *tcptw)
 {
-	return tcp_clock_ts(false) + tcptw->tw_ts_offset;
+	return tcp_clock_ts(tcptw->tw_sk.tw_usec_ts) + tcptw->tw_ts_offset;
 }
 
 static inline u32 tcp_rsk_tsval(const struct tcp_request_sock *treq)
 {
-	return tcp_clock_ts(false) + treq->ts_off;
+	return tcp_clock_ts(treq->req_usec_ts) + treq->ts_off;
 }
 
 #define tcp_flag_byte(th) (((u_int8_t *)th)[13])