[j-nsp] ICMPv6 "packet too big" not being emitted

[Pekka Savola]

Hi,

On Tue, 10 Jan 2006, Daniel Roesen wrote:
> I'm troubleshooting a problem where a Juniper running some 6.4ish code
> doesn't seem to emit ICMPv6 "packet too big, mtu=1480" packets if
> receiving packets for an IPv6-in-IPv4 tunnel with manually configured
> payload mtu of 1480 ("set family inet6 mtu 1480"). I cannot replicate
> this problem on another box running the same JunOS code, nor can I
> replicate it with 7.2R1.
>
> Is there any known bug in this area which doesn't show up when browsing
> the PR database?
>
> This bug is serious as it breaks IPv6 PMTUD and creates traffic
> blackholing.

IPv6-in-IPv4 tunneling on Juniper has worked fine for us for a long 
time (maybe from around 5.4R era) after glueing in MTU=1480. 
Otherwise, it'd cause the exact PMTUD issues you describe, which we 
finally got through as PR65486.

First off, I assume you're talking about v6 packet of at least 1481 
bytes coming from a native interface to the tunnel interface with 
MTU=1480 (that's the case we were looking at), instead of juniper 
decapsulating too big packets.

I'd check 3 things (these are rather straightforward and you've 
probably already done them but just to be sure..):
  - verify the scenario: encapsulating to a tunnel or decapsulating 
from the tunnel? (in the latter case, there may also be MRU issues)
  - check that the packets are actually what they should be (both ICMP 
and data) and they get sent, are received and not get lost in the 
network.  In particular, verify the packet sizes.
  - try setting the MTU at the other end of the tunnel as well if 
that'd help

-- 
Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings