Historically we have two fields in tcpcb to describe sender MSS: t_maxopd,

and t_maxseg. This dualism emerged with T/TCP, but was not properly cleaned
up after T/TCP removal. After all permutations over the years the result is
that t_maxopd stores a minimum of peer offered MSS and MTU reduced by minimum
protocol header. And t_maxseg stores (t_maxopd - TCPOLEN_TSTAMP_APPA) if
timestamps are in action, or is equal to t_maxopd otherwise. That's a very
rough estimate of MSS reduced by options length. Throughout the code it
was used in places, where preciseness was not important, like cwnd or
ssthresh calculations.

With this change:

- t_maxopd goes away.
- t_maxseg now stores MSS not adjusted by options.
- new function tcp_maxseg() is provided, that calculates MSS reduced by
  options length. The functions gives a better estimate, since it takes
  into account SACK state as well.

Reviewed by:	jtl
Differential Revision:	https://reviews.freebsd.org/D3593

2 files changed, 2 insertions, 4 deletions

diff --git a/sys/dev/cxgb/ulp/tom/cxgb_cpl_io.c b/sys/dev/cxgb/ulp/tom/cxgb_cpl_io.c
index 985306c3c6e8..d896020abf39 100644
--- a/sys/dev/cxgb/ulp/tom/cxgb_cpl_io.c
+++ b/sys/dev/cxgb/ulp/tom/cxgb_cpl_io.c
@@ -1536,14 +1536,13 @@ assign_rxopt(struct tcpcb *tp, uint16_t tcpopt)
 	struct toepcb *toep = tp->t_toe;
 	struct adapter *sc = toep->tp_tod->tod_softc;
 
-	tp->t_maxseg = tp->t_maxopd = sc->params.mtus[G_TCPOPT_MSS(tcpopt)] - 40;
+	tp->t_maxseg = sc->params.mtus[G_TCPOPT_MSS(tcpopt)] - 40;
 
 	if (G_TCPOPT_TSTAMP(tcpopt)) {
 		tp->t_flags |= TF_RCVD_TSTMP;
 		tp->t_flags |= TF_REQ_TSTMP;	/* forcibly set */
 		tp->ts_recent = 0;		/* XXX */
 		tp->ts_recent_age = tcp_ts_getticks();
-		tp->t_maxseg -= TCPOLEN_TSTAMP_APPA;
 	}
 
 	if (G_TCPOPT_SACK(tcpopt))
diff --git a/sys/dev/cxgbe/tom/t4_cpl_io.c b/sys/dev/cxgbe/tom/t4_cpl_io.c
index d58592e85b49..f18f115c3202 100644
--- a/sys/dev/cxgbe/tom/t4_cpl_io.c
+++ b/sys/dev/cxgbe/tom/t4_cpl_io.c
@@ -221,7 +221,7 @@ assign_rxopt(struct tcpcb *tp, unsigned int opt)
 		n = sizeof(struct ip6_hdr) + sizeof(struct tcphdr);
 	else
 		n = sizeof(struct ip) + sizeof(struct tcphdr);
-	tp->t_maxseg = tp->t_maxopd = sc->params.mtus[G_TCPOPT_MSS(opt)] - n;
+	tp->t_maxseg = sc->params.mtus[G_TCPOPT_MSS(opt)] - n;
 
 	CTR4(KTR_CXGBE, "%s: tid %d, mtu_idx %u (%u)", __func__, toep->tid,
 	    G_TCPOPT_MSS(opt), sc->params.mtus[G_TCPOPT_MSS(opt)]);
@@ -230,7 +230,6 @@ assign_rxopt(struct tcpcb *tp, unsigned int opt)
 		tp->t_flags |= TF_RCVD_TSTMP;	/* timestamps ok */
 		tp->ts_recent = 0;		/* hmmm */
 		tp->ts_recent_age = tcp_ts_getticks();
-		tp->t_maxseg -= TCPOLEN_TSTAMP_APPA;
 	}
 
 	if (G_TCPOPT_SACK(opt))