Re: kern/140597 implement Lost Retransmission Detection
The following reply was made to PR kern/140597; it has been noted by GNATS.
From: "Scheffenegger, Richard"
To: , "Lawrence Stewart"
Cc: "Biswas, Anumita"
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 02:10:29.0 +0100
+++ netinet/tcp_output.c2010-04-02 16:55:14.0 +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 02:10:29.0 +0100
+++ netinet/tcp_sack.c 2010-04-21 00:48:23.0 +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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Re: kern/140597 implement Lost Retransmission Detection
The following reply was made to PR kern/140597; it has been noted by GNATS.
From: "Scheffenegger, Richard"
To: , "Lawrence Stewart"
Cc: "Biswas, Anumita"
Subject: Re: kern/140597 implement Lost Retransmission Detection
Date: Fri, 2 Apr 2010 18:48:04 +0100
As discussed earlier, here is a simple fix.
Caveat: Doesn't work at the end of a session or when cwnd is very small
(<4 segments). Also, during heavy reordering, some spurious
re-retransmissions might occur (but that would only affect very few
re-retransmitted segments, as holes would still close with each
additional received SACK, reducing the chance of spurious
re-transmissions).=20
Benefit: during LAN burst drop events, TCP will not revert to
retransmission timeouts in order to recover. In a LAN, the RTO is
typically many orders of magnitude larger than the RTT. Not relying on
RTO whenever possible can help keep throughput up..
Simple Patch:
--
diff -u netinet.orig/tcp_output.c netinet/tcp_output.c
--- netinet.orig/tcp_output.c 2009-10-25 02:10:29.0 +0100
+++ netinet/tcp_output.c2010-04-02 16:55:14.0 +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/tcp_sack.c
--- netinet.orig/tcp_sack.c 2009-10-25 02:10:29.0 +0100
+++ netinet/tcp_sack.c 2010-04-02 16:46:42.0 +0200
@@ -460,6 +460,13 @@
/* We must have at least one SACK hole in scoreboard. */
KASSERT(!TAILQ_EMPTY(&tp->snd_holes),
("SACK scoreboard must not be empty"));
+ /* lost again - then restart */
+ if ((temp =3D TAILQ_FIRST(&tp->snd_holes)) !=3D NULL) {
+ if (SEQ_GT(tp->snd_fack, temp->rxmit)) {
+ temp->rxmit =3D temp->start;
+ tp->sackhint.nexthole =3D temp;
+ }
+ }
cur =3D TAILQ_LAST(&tp->snd_holes, sackhole_head); /* Last SACK
hole. */
/*
* Since the incoming sack blocks are sorted, we can process
them
@@ -508,7 +515,9 @@
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 +533,9 @@
- 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;
+ }
}
}
}
--
Richard Scheffenegger
Field Escalation Engineer
NetApp Global Support=20
NetApp
+43 1 3676811 3146 Office (2143 3146 - internal)
+43 676 654 3146 Mobile
www.netapp.com http://www.netapp.com/> =20
Franz-Klein-Gasse 5
1190 Wien=20
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