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Status Delivered
Workspace Spectrum Symphony
Components Version 7.1
Created by Guest
Created on Oct 20, 2016

Feasibility of multihome Management hosts to support different MTU on network.

We would like IBM to certify below solution for supporting differnet MTU with multiple network cards on Management hosts and different sets of compute hosts where one set of comput hosts has 1500MTU setup and another set of compute hosts has 9000MTU setup.

1> Master and Master candidate will have two different network interface on two different subnets. One network interface with have MTU set to 1500 and second interface will have MTU set to 9000.
2> One set of compute hosts will have MTU set to 1500 and will communicate with master hosts over network interface with 1500 MTU on master host.
3> Second set of compute hosts will have MTU set to 9000 and will communicate with master hosts over network interface with 9000 MTU on master.
4> Master host will have static route set to compute hosts with 9000 MTU on network interface with 9000 MTU.
5> For rest of servers over network, master host will use default route on network interface with 1500 MTU.

  • Guest
    Jun 22, 2020

    Documentation had been updated in Symphony 7.2 release to support the scenario.

  • Guest
    Dec 13, 2016

    Documentation will be updated in Knowledge Center.

  • Guest
    Oct 24, 2016

    Customer shared more information for this request, Support appends it here:

    1) TCP supports auto-negotiation and packet fragmentation and re-assembly. This can be controlled by switches, but the usual error; you transmit at the wrong packet size because you don't catch the misconfiguration.

    2) UPD COULD support something similar in the app (just like you support recovery), with the errno EMSGSIZE error. It can be pretty simple, done during socket accept and only basic support for MTU 9000 or 1500, or more sophisticated, where you hone in on the exact/optimal size. There are at least 4 sizes that might work. Complicating this, you COULD check for this error on every sendto(), but recovery gets really ugly then. And, since this would normally ONLY be done at socket setup, done once (on each side) and no performance impacts after for old users. (Send dummy packet of 65K, get EMSGSIZE error and retrieve IP_MTU and use this number for the specific server going forward).

    3) For multi-homed environments (basically a network for public and network for grid), which we always set up, static routes don't work. They used to… The latest technique is to include the “Source IP” in the socket connection (the local IP), and that forces the algorithms to select that IP for the traffic, unless down.

    4) All this available in both C and Java (not sure what Symphony was developed in.

    Some useful links:

    C to get MTU size and test connectivity (local MTU):

    GREAT description of Linux IP_MTU and IP_MTUDISCOVERY in Linux

    Linux IP_MTU description.

    Windows SO_MAX_MSG_SIZE option (they never do it the same):

    Java to get interface info (including MTU, but suspect defined not open socket):