[Preston] MTU's and Linux

[Graham Lee Bevan]

Gareth,
    I suspect Bulldog have mistakenly used the wrong terminology.  The 
MTU size on ethernet must always be 1500 (see MTU: in "ifconfig -a" 
output).  What I think they are really referring to is the Max Segment 
Size (MSS), which is typically MTU - size of packet headers, hence a 
typical size is 1458 or 1460.  Also, with Linux (certainly with 2.6.* 
kernels), an option called Path MTU Discovery (net.ipv4.ip_no_pmtu_disc 
= 0) is enabled, allowing automatic discovery of the MSS for each route.

    Here is what "man route" says about MSS:

mss M    set the TCP Maximum Segment Size (MSS) for connections over 
this route to M bytes.  The default is the device
              MTU minus headers, or a lower MTU when path mtu discovery 
occured. This setting can be used to force  smaller
              TCP  packets  on  the other end when path mtu discovery 
does not work (usually because of misconfigured fire-
              walls that block ICMP Fragmentation Needed)

Hope this helps
    glbevan


Gareth Llewellyn wrote:

>Get this people.
>
>Bulldog wants an MTU (maximum transmission unit) of 1458 in order to
>communicate correctly. Currently our NIC's are set to 1500 which means we
>are only getting around 1103kb instead of our 2272kb that we are supposed to
>be getting.
>
>Ok now setting the MTU on our routers external interface to 1458 results in
>absoloutly no 'net access. Setting the internal NIC to 1458 as well means we
>can get out to the net.
>
>But it gets worse, now although we can get to the net, we can only reach
>SOME of the net. Several websites result in "Destination Net Unreachable"
>and to make things worse there is no perceivable increase in bandwidth.
>
>Any ideas as to what the problem may be?
>
>
>
>_______________________________________________
>Preston mailing list
>Preston at mailman.lug.org.uk
>http://mailman.lug.org.uk/mailman/listinfo/preston
>
>  
>