On Friday, 01/29/2016 at 09:58 GMT, Christer Solskogen <christer.solsko...@gmail.com> wrote: > On all of my x86_64 Linux boxes the default MTU is 1500. But with SuSE > (both 11 and 12) on s390 they seems to be 1492. How come?
Historically, it's because there are two different kinds of non-jumbo ethernet frames. The traditional "DIX" frame has room for a 1500-byte packet. The other, defined by IEEE 802.3 has a slightly longer frame header that enables multiple multi-protocol hosts to share a single network adapter, leaving room for only 1492 bytes. (See RFC 1042.) Bottom line, if you're on an IEEE 802.3 network with SNAP headers, you have to use MTU 1492. And in the Before Times, the MTU also dictated the maximum packet size a host could *receive*. (Hmmm...so other guy's MTU has to match your MRU.) But that's usually not an issue any more. Not, at least, in the mainframes -- dunno about x86 adapters. And we have dynamic path MTU discovery to help us. All of that said, as of the z13, OSAs no longer support IEEE 802.3 networks, so 1500 is the correct default from here on out. OSAs are capable of telling the host the MTU size, so Linux *can* get that value from the OSA adapter. It's just that this isn't something other platforms can do and so the configuration process for the IP layer isn't really amenable to self-discovery. All of this fancy local MTU discovery was based on ARP. When you ARP for a IP address, you include "hardware type" in the ARP packet. There is a value for 802.2 (which covers 802.3) and a value for Ethernet (DIX). First you ARP with hw=802.2. If you get a response, you use MTU 1492. If you don't you try again with hw=Ethernet. If you get a response, 1500 it is (unless you have configured for Jumbo frames). This same issue drives the jumbo frame configuration of 9000 vs. 8992. There's that 8-byte difference, even though 802.2 networks can't carry jumbo frames, and IEEE has been otherwise derelict in their duty to standardize them. Alan Altmark Senior Managing z/VM and Linux Consultant Lab Services System z Delivery Practice IBM Systems & Technology Group ibm.com/systems/services/labservices office: 607.429.3323 mobile; 607.321.7556 alan_altm...@us.ibm.com IBM Endicott ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390 ---------------------------------------------------------------------- For more information on Linux on System z, visit http://wiki.linuxvm.org/
