[e2e] Legal fragment sizes

Noel Chiappa jnc at mercury.lcs.mit.edu
Tue May 16 07:17:47 PDT 2006

    > From: Fernando Gont <fernando at gont.com.ar>

    > I was going through the IP specs, and there was a point on which there
    > seems to be some ambiguity ...
    > Is the maximum "legal" IP payload defined by "Total_Length -
    > IP_Header" (i.e., around 65K), or should it be considered to be the
    > maximum payload that can be encapsulated, by using the "trick"
    > described bel[]ow? ...
    > ... send a ~65K fragment with the MF bit set, followed by a second 65K
    > fragment with an offset of ~65K)

Oh, my, what a clever loophole! I don't think anyone has ever thought of
that (at least, I don't recall hearing of it before).

My opinion is that even though the mechanism *supports* the larger size, I
don't think the spirit of the spec is to allow it.

I expect that most people will have also assumed that the maximum legal
payload is 64K, and their code probably reflects it - i.e. they may be
calculating the payload size in a 16-bit variable, they may have a maximum
buffer size of 64K, etc. So many (most?) implementations won't work with a
payload of more than 64K.


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