Date: Wed, 21 Apr 2010 01:20:04 GMT From: "Scheffenegger, Richard" <rs@netapp.com> To: freebsd-net@FreeBSD.org Subject: Re: kern/140597 implement Lost Retransmission Detection Message-ID: <201004210120.o3L1K462093595@freefall.freebsd.org>
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The following reply was made to PR kern/140597; it has been noted by GNATS.
From: "Scheffenegger, Richard" <rs@netapp.com>
To: <bug-followup@freebsd.org>, "Lawrence Stewart" <lastewart@swin.edu.au>
Cc: "Biswas, Anumita" <Anumita.Biswas@netapp.com>
Subject: Re: kern/140597 implement Lost Retransmission Detection
Date: Wed, 21 Apr 2010 02:16:01 +0100
I found a small oversight (bug) in my earlier simple fix. If we had sent
out multiple holes already, all of which get (partially) lost in the
retransmission again, the original simple patch would only work for the
very first hole. So, subsequent holes would not get re-sent, unless
another ACK (SACK) would be received - however, seeing more SACKs
becomes less likely the more loss is expirienced.
This is a updated patch diff, which accounts for that case as well - but
at the cost of O(n) time, instead of O(c).
(Just check all holes from the hint backwards to the head, if any
already fully resent hole needs to be reset, instead of only the first
one - which might go away during subsequent processing in the original
patch).
diff -u netinet.orig/tcp_output.c netinet/tcp_output.c
--- netinet.orig/tcp_output.c 2009-10-25 02:10:29.000000000 +0100
+++ netinet/tcp_output.c 2010-04-02 16:55:14.000000000 +0200
@@ -953,6 +953,10 @@
} else {
th->th_seq =3D htonl(p->rxmit);
p->rxmit +=3D len;
+ /* lost again detection */
+ if (SEQ_GEQ(p->rxmit, p->end)) {
+ p->rxmit =3D tp->snd_nxt;
+ }
tp->sackhint.sack_bytes_rexmit +=3D len;
}
th->th_ack =3D htonl(tp->rcv_nxt);
diff -u netinet.orig/tcp_sack.c netinet.simple_mod/tcp_sack.c
--- netinet.orig/tcp_sack.c 2009-10-25 02:10:29.000000000 +0100
+++ netinet/tcp_sack.c 2010-04-21 00:48:23.000000000 +0200
@@ -508,7 +508,9 @@
if (SEQ_GEQ(sblkp->end, cur->end)) {
/* Move end of hole backward. */
cur->end =3D sblkp->start;
- cur->rxmit =3D SEQ_MIN(cur->rxmit,
cur->end);
+ if (SEQ_GEQ(cur->rxmit, cur->end)) {
+ cur->rxmit =3D tp->snd_nxt;
+ }
} else {
/*
* ACKs some data in middle of a hole;
need
@@ -524,8 +526,9 @@
- temp->start);
}
cur->end =3D sblkp->start;
- cur->rxmit =3D =
SEQ_MIN(cur->rxmit,
- cur->end);
+ if (SEQ_GEQ(cur->rxmit,
cur->end)) {
+ cur->rxmit =3D
tp->snd_nxt;
+ }
}
}
}
@@ -540,6 +543,15 @@
else
sblkp--;
}
+ /* retransmission lost again - then restart */
+ if ((temp =3D tp->sackhint.nexthole) !=3D NULL) {
+ do {
+ if (SEQ_GT(tp->snd_fack, temp->rxmit)) {
+ temp->rxmit =3D temp->start;
+ tp->sackhint.nexthole =3D temp;
+ }
+ } while ((temp =3D TAILQ_PREV(temp, sackhole_head,
scblink)) !=3D NULL);
+ }
}
/*
Richard Scheffenegger
=20
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