At Thu, 14 May 2009 14:42:35 -0700, "Kevin Oberman" <ober...@es.net> wrote:
> I then captured the ICMP and discovered that the kernel was fragmenting > all of them! Worse, the fragment was sent out before the ICMP! What the > heck is going on! Thread synchronization? > > When I captured the packets (via tcpdump -s0 -w file host ftp.funet.fi), the > first things captured is an IPv6 fragment of 72 bytes. 3 microseconds > later, I get the ICMP6 packet of 1294 bytes. This pattern is consistent > over repeated packets. This was with -s 1234 for a total ICMPv6 size of > 1282. > > First, why is the kernel fragmenting this at all as it fits in the > interface MTU? Do you mean why ping6 has the kernel fragment echo requests at 1280 bytes by default (i.e., when invoked without -m)? If so, that's because if a large echo request triggers path MTU discovery, some initial requests won't be replied (recall that IPv6 routers never fragment packets by themselves; they always drop too-large packet with returning an ICMPv6 error). > Second, why the heck is the fragment going out first? This should be OK, > but I suspect many firewalls (which are often not happy with fragments) > are not likely to pass a fragment which precedes the initial frame. Do you mean, for example, the kernel sends out a fragment with a non-0 offset before the 0-offset one? I can't believe this. If I tried % ping6 -s 1300 www.isc.org from a FreeBSD 6.3 host, I saw this: 08:22:04.821171 IP6 2001:200:0:8002:203:47ff:fea5:3085 > 2001:4f8:0:2::d: frag (0|1232) ICMP6, echo request, seq 0, length 1232 08:22:04.821181 IP6 2001:200:0:8002:203:47ff:fea5:3085 > 2001:4f8:0:2::d: frag (1232|76) (captured on the sending host). --- JINMEI, Tatuya Internet Systems Consortium, Inc. _______________________________________________ freebsd-net@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-net To unsubscribe, send any mail to "freebsd-net-unsubscr...@freebsd.org"
