On Fri May 8 20:12:57 PDT 2015, [email protected] wrote:
> do we really need to initialize tcb->mss to tcpmtu() in procsyn()?
> as i see it, procsyn() is called only when tcb->state is Syn_sent,
> which only should happen for client connections doing a connect, in
> which case tcpsndsyn() would have initialized tcb->mss already no?
i think there was a subtile reason for this, but i don't recall. a real
reason for setting it here is because it makes the code easier to reason
about, imo.
there are a couple problems with the patch as it stands. they are
inherited from previous mistakes.
* the setting of tpriv->stats[Mss] is bogus. it's not shared between
connections.
it is also v4 only.
* so, mss should be added to each tcp connection's status file.
* the setting of tcb->mss in tcpincoming is not correct, tcp->mss is
set by SYN, not by ACK, and may not be reset. (see snoopy below.)
* the SYN-ACK needs to send the local mss, not echo the remote mss.
asymmetry is "fine" in the other side, even if ip/tcp.c isn't smart enough to
keep tx and rx mss seperate. (scare quotes = untested, there may be
some performance niggles if the sender is sending legal packets larger than
tcb->mss.)
my patch to nix is below. i haven't submitted it yet.
- erik
---
005319 ms
ether(s=a0369f1c3af7 d=0cc47a328da4 pr=0800 ln=62)
ip(s=10.1.1.8 d=10.1.1.9 id=ee54 frag=0000 ttl=255 pr=6 ln=48)
tcp(s=38903 d=17766 seq=3552109414 ack=0 fl=S win=65535 ck=d68e ln=0
opt4=(mss 1460) opt3=(wscale 4) opt=NOOP)
005320 ms
ether(s=0cc47a328da4 d=a0369f1c3af7 pr=0800 ln=62)
ip(s=10.1.1.9 d=10.1.1.8 id=54d3 frag=0000 ttl=255 pr=6 ln=48)
tcp(s=17766 d=38903 seq=441373010 ack=3552109415 fl=AS win=65535
ck=eadc ln=0 opt4=(mss 1460) opt3=(wscale 4) opt=NOOP)
---
/n/dump/2015/0509/sys/src/nix/ip/tcp.c:491,501 - /sys/src/nix/ip/tcp.c:491,502
s = (Tcpctl*)(c->ptcl);
return snprint(state, n,
- "%s qin %d qout %d rq %d.%d srtt %d mdev %d sst %lud cwin %lud
swin %lud>>%d rwin %lud>>%d qscale %d timer.start %d timer.count %d rerecv %d
katimer.start %d katimer.count %d\n",
+ "%s qin %d qout %d rq %d.%d mss %d srtt %d mdev %d sst %lud
cwin %lud swin %lud>>%d rwin %lud>>%d qscale %d timer.start %d timer.count %d
rerecv %d katimer.start %d katimer.count %d\n",
tcpstates[s->state],
c->rq ? qlen(c->rq) : 0,
c->wq ? qlen(c->wq) : 0,
s->nreseq, s->reseqlen,
+ s->mss,
s->srtt, s->mdev, s->ssthresh,
s->cwind, s->snd.wnd, s->rcv.scale, s->rcv.wnd, s->snd.scale,
s->qscale,
/n/dump/2015/0509/sys/src/nix/ip/tcp.c:843,854 - /sys/src/nix/ip/tcp.c:844,857
/* mtu (- TCP + IP hdr len) of 1st hop */
static int
- tcpmtu(Proto *tcp, uchar *addr, int version, uint *scale)
+ tcpmtu(Proto *tcp, uchar *addr, int version, uint reqmss, uint *scale)
{
+ Tcppriv *tpriv;
Ipifc *ifc;
int mtu;
ifc = findipifc(tcp->f, addr, 0);
+ tpriv = tcp->priv;
switch(version){
default:
case V4:
/n/dump/2015/0509/sys/src/nix/ip/tcp.c:855,865 - /sys/src/nix/ip/tcp.c:858,870
mtu = DEF_MSS;
if(ifc != nil)
mtu = ifc->maxtu - ifc->m->hsize - (TCP4_PKT +
TCP4_HDRSIZE);
+ tpriv->stats[Mss] = mtu;
break;
case V6:
mtu = DEF_MSS6;
if(ifc != nil)
mtu = ifc->maxtu - ifc->m->hsize - (TCP6_PKT +
TCP6_HDRSIZE);
+ tpriv->stats[Mss] = mtu + (TCP6_PKT + TCP6_HDRSIZE) - (TCP4_PKT
+ TCP4_HDRSIZE);
break;
}
/*
/n/dump/2015/0509/sys/src/nix/ip/tcp.c:868,873 - /sys/src/nix/ip/tcp.c:873,882
*/
*scale = Defadvscale;
+ /* our sending max segment size cannot be bigger than what he asked for
*/
+ if(reqmss != 0 && reqmss < mtu)
+ mtu = reqmss;
+
return mtu;
}
/n/dump/2015/0509/sys/src/nix/ip/tcp.c:1300,1307 -
/sys/src/nix/ip/tcp.c:1309,1314
static void
tcpsndsyn(Conv *s, Tcpctl *tcb)
{
- Tcppriv *tpriv;
-
tcb->iss = (nrand(1<<16)<<16)|nrand(1<<16);
tcb->rttseq = tcb->iss;
tcb->snd.wl2 = tcb->iss;
/n/dump/2015/0509/sys/src/nix/ip/tcp.c:1314,1322 -
/sys/src/nix/ip/tcp.c:1321,1327
tcb->sndsyntime = NOW;
/* set desired mss and scale */
- tcb->mss = tcpmtu(s->p, s->laddr, s->ipversion, &tcb->scale);
- tpriv = s->p->priv;
- tpriv->stats[Mss] = tcb->mss;
+ tcb->mss = tcpmtu(s->p, s->laddr, s->ipversion, 0, &tcb->scale);
}
void
/n/dump/2015/0509/sys/src/nix/ip/tcp.c:1492,1498 -
/sys/src/nix/ip/tcp.c:1497,1503
seg.ack = lp->irs+1;
seg.flags = SYN|ACK;
seg.urg = 0;
- seg.mss = tcpmtu(tcp, lp->laddr, lp->version, &scale);
+ seg.mss = tcpmtu(tcp, lp->laddr, lp->version, 0, &scale); /* send
our mss, not lp->mss */
seg.wnd = QMAX;
/* if the other side set scale, we should too */
/n/dump/2015/0509/sys/src/nix/ip/tcp.c:1767,1777 -
/sys/src/nix/ip/tcp.c:1772,1779
tcb->flgcnt = 0;
tcb->flags |= SYNACK;
- /* our sending max segment size cannot be bigger than what he asked for
*/
- if(lp->mss != 0 && lp->mss < tcb->mss) {
- tcb->mss = lp->mss;
- tpriv->stats[Mss] = tcb->mss;
- }
+ /* per rfc, we can't set the mss any more */
+ // tcb->mss = tcpmtu(s->p, lp->laddr, lp->version, lp->mss, &tcb->scale);
/* window scaling */
tcpsetscale(new, tcb, lp->rcvscale, lp->sndscale);
/n/dump/2015/0509/sys/src/nix/ip/tcp.c:3014,3020 -
/sys/src/nix/ip/tcp.c:3016,3021
procsyn(Conv *s, Tcp *seg)
{
Tcpctl *tcb;
- Tcppriv *tpriv;
tcb = (Tcpctl*)s->ptcl;
tcb->flags |= FORCE;
/n/dump/2015/0509/sys/src/nix/ip/tcp.c:3026,3036 -
/sys/src/nix/ip/tcp.c:3027,3033
tcb->irs = seg->seq;
/* our sending max segment size cannot be bigger than what he asked for
*/
- if(seg->mss != 0 && seg->mss < tcb->mss) {
- tcb->mss = seg->mss;
- tpriv = s->p->priv;
- tpriv->stats[Mss] = tcb->mss;
- }
+ tcb->mss = tcpmtu(s->p, s->laddr, s->ipversion, seg->mss, &tcb->scale);
tcb->snd.wnd = seg->wnd;
initialwindow(tcb);
