Subject: Re: fragmentation by NetBSD routers vs. reassembly on other systems....
To: NetBSD Networking Technical Discussion List <tech-net@NetBSD.ORG>
From: None <firstname.lastname@example.org>
Date: 09/02/2000 13:17:02
>Not too many days later I noticed that I was not able to send any e-mail
>to a select few sites if it was over ~1KB (i.e. if it caused a
>fragmented packet). I contacted a few of the admins of the sites in
>question, and was contacted by at least one who had noticed my system's
>failing SMTP connects. Various experiments proved that it was not
>Path-MTU-discovery or firewalls on their end causing any problems, and
>indeed tcpdumps on my router showed that my system was simply getting
>stuck retranmitting the larger packets without getting any kind of
>response from anyone at all.
i experienced a similar symptom. not sure if it is the same
as the problem you are having.
it was under the following configuration,
destination (always do path MTU discovery)
| MTU == 1500
| MTU < 1500 due to some VLAN
| MTU == 1500
source (netbsd 1.4.2)
and the symptom was:
- if the source node use path MTU discovery, TCP between source and
destination goes fine
- if the source node turns off path MTU discovery (which was the
default setting on 1.4.2), source node keep retransmitting large
in my case, i had enough access to the intermediate router (as it
was laboratory setting).
router 1 was broken from DF bit manipulation. it behaved like this:
- if DF bit is raised on a packet, it worked fine. it transmits
icmp "too big" as necessary toward source, if the source node sends
- if DF bit is off, it behaved wrong. if the source node sends
1500byte-packets, it drops the packet onto the floor. router 1
should have fragmented the packet on its own and relay it to router 2
(VLAN side), but it did not.
i guess there is some broken device, somewhere between you and the
destination. i'm not sure if turning path MTU discovery on
always helps, as there are stupid admins who filters out all icmp
packets at their firewall (and preventing path MTU discovery from