Re: changes to make ethernet packets able to be unaligned...

From: Mike Silbersack (silby_at_silby.com)
Date: 03/19/05

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    Date: Fri, 18 Mar 2005 21:17:39 -0600 (CST)
To: John-Mark Gurney <gurney_j@resnet.uoregon.edu>

    On Fri, 18 Mar 2005, John-Mark Gurney wrote:

    >> Moving the alignment out of the drivers and into a common place seems like
    >> a good idea, but I wonder if it should be done in the ethernet code
    >> instead of in the ip code; won't other protocols have unaligned access
    >> problems if the change is made exactly as is?
    >
    > Why force it on the protocols that might not need it?
    >
    > We don't know how much of the ip or foo header to bring in at the
    > ethernet layer, so the ip or foo layer might have to bring in more data...
    >
    > IMO, it's the protocol's job to ensure that it has correct alignment
    > to access the data... what happens when a protocol comes along that
    > requires the packet to be 8byte aligned? and the ethernet layer only
    > aligned it on a 4byte boundary? should we add a third mbuf to it?
    >
    > --
    > John-Mark Gurney Voice: +1 415 225 5579

    Well, right now most (all?) drivers handle the alignment issue, so moving
    the alignment step into the ethernet code would centralize it in one
    place, and would not break anything. Removing the alignment requirement
    without actually having tested all the protocols is going to break
    something. Having the protocols handle alignment themselves is a good
    goal, but that's a second step you can take later.

    I don't see why any extra mbuf allocation should be necessary if the
    alignment is done inside the ethernet code, actually. Once you strip the
    ethernet header off, you can just slide the rest of packet backwards by
    two bytes, in place.

    Mike "Silby" Silbersack
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