I am reading linux tcp codes, and I encountered some
questions, would somebody there help to explain?
thanks.
in tcp_ipv4.c, function tcp_v4_conn_request(),
there are statements like:
-----------------------------------------------
} else if (isn == 0) {
struct inet_peer *peer = NULL;
/* VJ's idea. We save last timestamp seen
* from the destination in peer table, when entering
* state TIME-WAIT, and check against it before
* accepting new connection request.
*
* If "isn" is not zero, this request hit alive
* timewait bucket, so that all the necessary checks
* are made in the function processing timewait state.
*/
if (tp.saw_tstamp &&
sysctl_tcp_tw_recycle &&
(dst = tcp_v4_route_req(sk, req)) != NULL &&
(peer = rt_get_peer((struct rtable*)dst)) != NULL &&
peer->v4daddr == saddr) {
if (xtime.tv_sec < peer->tcp_ts_stamp + TCP_PAWS_MSL &&
(s32)(peer->tcp_ts - req->ts_recent) > TCP_PAWS_WINDOW) {
NET_INC_STATS_BH(PAWSPassiveRejected);
dst_release(dst);
goto drop_and_free;
}
}
----------------------------------------------------
what I donot understand is why TCP_PAWS_WINDOW is needed?
The inner most "if ..." statement seems to mean that
if the saved timestamp in peer is valid and the new connection
request's timestamp is TCP_PAWS_WINDOW ticks before that of peer,
the req is considered as old duplicate segment and is droped.
Why not use (peer->tcp_ts > req->ts_recent) directly? I mean,
why there must be TCP_PAWS_WINDOW differences?
Thanks again.
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