Re: Question regarding protocol specific mtu for FCoE

Rick Jones
2009-06-03T18:03:35Z

Zou, Yi wrote:
> Hi,
> I am trying to figure out a way to support a different MTU for Fiber
> Channel over Ethernet (FCoE), where it is preferred to have a baby jumbo
> frame with MTU=2158 (14 bytes FCoE header + 24 bytes FC header + 2112
> bytes max FC payload + 4 bytes FC CRC + 4 bytes FCoE trailer).
>
> Similar to nedev->mtu being used everywhere for LAN traffic, I wonder if
> it is desirable or feasible to add another protocol specific MTU (which
> is my case is FCoE) to netdev so any eth driver supporting converged
> traffic (e.g. LAN + FCoE) over netdev can make use of it.

Do FCoE upper layers have anything analagous to a TCP_MAXSEG option? That allows
an application using TCP to ask for a smaller MSS than TCP might have chosen
otherwise.

Would a NIC over which FCoE was running be able to be of two minds of what the
MTU happens to be? I'd think that if one user of the NIC needed/wanted and MTU >
foo one would just set the MTU large enough to include foo and be done with it?

In IP networking space at least, if there is a destination for which one does not
want to use the default MTU, one can set-up a destination-specific (Path) MTU in
the routing table that will (if smaller) override the link-local MTU for traffic
going to/from that destination.

rick jones