On Tue, Oct 28, 2003 at 09:40:19 -0500, Steven M. Bellovin wrote:

> But yes, tcpdump shows a very large MSS with v6 -- 33160 -- though I 
> thought that Path MTU was mandatory and would cope.

Last week I discovered that pMTUd wasn't working right on 1.6.1. I did
not have time yet to look further into it. But be careful that pMTUd
might not be working right at the moment. But below is a dump. Basically,
this is what's happening:

(R is a router)

A -> B          DNS query
B -> A          DNS reply (frag 1456 bytes)
B -> A          DNS reply (frag 1448 bytes)
B -> A          DNS reply (frag 859 bytes)
R -> B		icmp6: too big 1280
R -> B		icmp6: too big 1280
(this repeats with B using a lower frag size)
A -> B          DNS query
B -> A          DNS reply (frag 1456 bytes)
B -> A          DNS reply (frag 1448 bytes)
B -> A          DNS reply (frag 859 bytes)
R -> B		icmp6: too big 1280
R -> B		icmp6: too big 1280

tcpdump: listening on tlp0
10:12:28.142216 2001:7b8:206:1:220:edff:fe19:4862.49160 > 2001:610:508:3001::2.53: [udp sum ok]  1261+ [1au] AAAA? foo.rvdp.org. ar: . OPT UDPsize=4096 (41) [flowlabel 0x64918] (len 49, hlim 56)
10:12:28.143341 2001:610:508:3001::2 > 2001:7b8:206:1:220:edff:fe19:4862: frag (0x0000f4f1:0|1448) 53 > 49160:  1261*- q: AAAA? *** truncated by rvdp *** [|domain] (len 1456, hlim 64)
10:12:28.143356 2001:610:508:3001::2 > 2001:7b8:206:1:220:edff:fe19:4862: frag (0x0000f4f1:1448|1448) (len 1456, hlim 64)
10:12:28.143371 2001:610:508:3001::2 > 2001:7b8:206:1:220:edff:fe19:4862: frag (0x0000f4f1:2896|859) (len 867, hlim 64)
10:12:28.146893 2001:610:508:3001::1 > 2001:610:508:3001::2: icmp6: too big 1280 (len 1240, hlim 64)
10:12:28.148033 2001:610:508:3001::1 > 2001:610:508:3001::2: icmp6: too big 1280 (len 1240, hlim 64)
10:12:33.134653 2001:7b8:206:1:220:edff:fe19:4862.49160 > 2001:610:508:3001::2.53: [udp sum ok]  1261+ [1au] AAAA? foo.rvdp.org. ar: . OPT UDPsize=4096 (41) [flowlabel 0x64918] (len 49, hlim 56)
10:12:33.135873 2001:610:508:3001::2 > 2001:7b8:206:1:220:edff:fe19:4862: frag (0x0000f4f2:0|1448) 53 > 49160:  1261*- q: AAAA? *** truncated by rvdp *** [|domain] (len 1456, hlim 64)
10:12:33.135893 2001:610:508:3001::2 > 2001:7b8:206:1:220:edff:fe19:4862: frag (0x0000f4f2:1448|1448) (len 1456, hlim 64)
10:12:33.135907 2001:610:508:3001::2 > 2001:7b8:206:1:220:edff:fe19:4862: frag (0x0000f4f2:2896|859) (len 867, hlim 64)
10:12:33.140252 2001:610:508:3001::1 > 2001:610:508:3001::2: icmp6: too big 1280 (len 1240, hlim 64)
10:12:33.141306 2001:610:508:3001::1 > 2001:610:508:3001::2: icmp6: too big 1280 (len 1240, hlim 64)
^C
34 packets received by filter
0 packets dropped by kernel

	rvdp