[GTALUG] [Bulk] Re: Rogers and MTU size

[James Knott]

On 01/04/2015 07:21 PM, David Collier-Brown wrote:
> From a different rogers subnet, I get a default of 56, successes with
> 1500-byte pings to the router and to the outside world, but failures
> internally talking to rogers DNS servers

I assume you meant 576, not 56.

Do you see an IPv6 link local address on that interface?  It starts with
fe80.
>
> First, the dns server:
> $ cat /etc/resolv.conf
> # Generated by NetworkManager
> nameserver 64.71.255.204
> nameserver 64.71.255.198
>
> $ ping -s 1500 64.71.255.204
> PING 64.71.255.204 (64.71.255.204) 1500(1528) bytes of data.
> ^C
> --- 64.71.255.204 ping statistics ---
> 19 packets transmitted, 0 received, 100% packet loss, time 17999ms
>
>
> $ ping 64.71.255.204
> PING 64.71.255.204 (64.71.255.204) 56(84) bytes of data.
> 64 bytes from 64.71.255.204: icmp_seq=1 ttl=58 time=7.63 ms
> 64 bytes from 64.71.255.204: icmp_seq=2 ttl=58 time=7.35 ms
> ^C
> --- 64.71.255.204 ping statistics ---
> 2 packets transmitted, 2 received, 0% packet loss, time 1002ms
> rtt min/avg/max/mdev = 7.350/7.492/7.635/0.166 ms
>
>
> Second, ACM.org
> $ ping -s 1500 acm.org
> PING acm.org (64.238.147.76) 1500(1528) bytes of data.
> 1508 bytes from ACMWEB16.acm.org (64.238.147.76): icmp_seq=1 ttl=49
> time=42.5 ms
> 1508 bytes from ACMWEB16.acm.org (64.238.147.76): icmp_seq=2 ttl=49
> time=44.1 ms
> 1508 bytes from ACMWEB16.acm.org (64.238.147.76): icmp_seq=3 ttl=49
> time=44.5 ms
> 1508 bytes from ACMWEB16.acm.org (64.238.147.76): icmp_seq=4 ttl=49
> time=44.2 ms
> 1508 bytes from ACMWEB16.acm.org (64.238.147.76): icmp_seq=5 ttl=49
> time=52.9 ms
> 1508 bytes from ACMWEB16.acm.org (64.238.147.76): icmp_seq=6 ttl=49
> time=44.1 ms
> ^C
> --- acm.org ping statistics ---
> 6 packets transmitted, 6 received, 0% packet loss, time 5007ms
> rtt min/avg/max/mdev = 42.576/45.430/52.995/3.445 ms

This is similar to what I'm seeing.  When I ping your DNS, I can do so
with 1472, but not 1473 byte packets.  I also see that 1472 limit from
my notebook, but not my desktop, when going out to the Internet.  I do
not see it when pinging from my notebook to a computer on my local LAN. 
When pinging www.yahoo.com from my desktop system, I can ping with 1500
bytes.  I'm not sure why 2 computers on my network get different results
to Yahoo. Or where that 1472 limit is coming from.