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.
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