How to determine MTU between my cable modem and ISP?

Question

I am trying to understand the "big picture" of MTU. Specifically, many discussions of MTU focus on a single hop (e.g. laptop to router), so the natural question is: how to determine MTU between cable modem and ISP, or more generally, for any given hop of a route.

Now, I can easily see the MTU between my laptop and its: wi-fi router using ifconfig on Mac OS X:

en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
options=2b<RXCSUM,TXCSUM,VLAN_HWTAGGING,TSO4>
ether 58:b0:35:f0:14:75 
media: autoselect (none)
status: inactive    
en1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
ether 58:b0:35:72:64:fa 
inet6 fe80::5ab0:35ff:fe72:64fa%en1 prefixlen 64 scopeid 0x5 
inet 192.168.1.100 netmask 0xffffff00 broadcast 192.168.1.255
media: autoselect
status: active

1500 is the canonical value b/c of the limitation of the wi-fi frame (which I am guessing was designed to match the Ethernet frame; please correct me if that's wrong).

So, the question is: How to determine the MTU of arbitrary hops in my route?

Answer summary:

Per the below answer, best bet is "tcpdump", "traceroute --mtu" or "tracepath"

Are you looking for Path MTU Discovery (http://en.wikipedia.org/wiki/Path_MTU_Discovery)? — Nikolai Fetissov, Aug 15 '11 at 19:51
Yes! Looks like what I was after, more or less (ultimately it's the end-to-end MTU that counts). Feel free to upgrade your comment to a full answer. — Peter Skirko, Aug 15 '11 at 23:36

score 2 · Accepted Answer · answered Aug 16 '11 at 15:04

2

The thing you are looking for is Path MTU Discovery. You can see that in action with tcpdump(1) during TCP three way handshake by watching the MSS option values (you will need appropriate level of dump verbosity, afair -vv does it).

Original traceroute(8) is supposed to show you path MTU changes, but I never had luck with it. Linux tracepath(8) gives you hop MTU value right away.

answered Aug 16 '11 at 15:04

Nikolai Fetissov

82,306
11
110
171

Thanks for the tracepath tip; it's a great tool. – Peter Skirko Aug 17 '11 at 20:45

How to determine MTU between my cable modem and ISP?

1 Answers1