[130029] in North American Network Operators' Group
Re: large icmp packet issue
daemon@ATHENA.MIT.EDU (Heath Jones)
Sun Sep 26 07:49:37 2010
In-Reply-To: <AANLkTimSfO6ukZ2SJ7jsnAUSi7LWdPvLESG5VT5Oopxj@mail.gmail.com>
Date: Sun, 26 Sep 2010 12:49:21 +0100
From: Heath Jones <hj1980@gmail.com>
To: fedora fedora <fedorafans@gmail.com>
Cc: nanog@nanog.org
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
> How can i be sure even if a device blocks my ping , it might have policy
> blocking ping at it at all.
Correct in a lot of cases and that is why icmp should not be used by
itself when diagnosing issues.
>> I am having problem getting ping to work to a specific destination host when
>> using large size icmp packet and i am hoping someone here can offer some
>> suggestion. With regular ping, i can ping this remote host without any problem,
>> but if i crank up the packet size to above 1500 (1500 still works), i won't get any icmp reply.
>> My first thought was this was a pmtu issue. but when I ran tcpdump on this remote host,
>> i saw the incoming ping requests and this host actually sent back icmp replies, so it appears
>> that there is some device in between blocking these large size icmp reply packets.
It is possible that the MTU for interface facing you and interface
facing away from you are different on some middle hop. It is
interesting that you state the packet size to be >1500, are you
talking about jumbo frames?
(and do you mean frame size, not packet size?)
>> Here is the question, how can i find out which hop on the path is causing this behavior?
Robert is correct. You need to use traceroute, or alter the TTL values
when you send the icmp requests.
By setting dont-fragment and varying ttl & frame sizes, you should
find your issue.
