From: Stephen Hemminger <[EMAIL PROTECTED]>
Date: Tue, 15 Nov 2005 16:29:40 -0800
> 161.30.105.72.ftp-data > 192.168.128.101.32792: . ack 1 win 64000
> 192.168.128.101.32792 > 161.30.105.72.ftp-data: . 1:1381(1380) ack 1 win 5840
> 192.168.128.101.32792 > 161.30.105.72.ftp-data: . 1381:2761(1380) ack 1 win
> 5840
> 161.30.14.93 > 192.168.128.101: icmp 36: 161.30.105.72 unreachable - need to
> frag (mtu 1400)
> 161.30.14.93 > 192.168.128.101: icmp 36: 161.30.105.72 unreachable - need to
> frag (mtu 1400)
> 192.168.128.101.32792 > 161.30.105.72.ftp-data: . 1:1381(1380) ack 1 win 5840
1400 - 20 byte TCP header is 1380
but we're supposed to subtract out the IP etc. headers as well
(which is tp->af_specific->net_header_len).
Would be helpful to see some diagnostics from tcp_sync_mss()
when this event is triggered, something like:
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 029c70d..06f1e56 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -646,6 +646,13 @@ unsigned int tcp_sync_mss(struct sock *s
if (tp->max_window && mss_now > (tp->max_window>>1))
mss_now = max((tp->max_window>>1), 68U - tp->tcp_header_len);
+#if 1
+ if (tp->pmtu_cookie != pmtu && net_ratelimit()) {
+ printk("TCP: Adjusting PMTU [%d --> %d] mss_cache [%d -->
%d]\n",
+ tp->pmtu_cookie, pmtu,
+ tp->mss_cache, mss_now);
+ }
+#endif
/* And store cached results */
tp->pmtu_cookie = pmtu;
tp->mss_cache = mss_now;
Alternatively, we're ignoring the PMTU message for one reason or
another. Perhaps the quoted TCP packet in the ICMP pmtu message
has an incorrect sequence number or is truncated for some reason.
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