Re: packet lengths


Brad Kemp (interlan!brad@eddie.mit.edu)
22 Apr 88 19:13:09 GMT


In article <In article <8804191854.AA00089@apolling.imagen.uucp> In article <8804191854.AA00089@apolling.imagen.uucp> geof@imagen.UUCP (Geof Cooper) writes:
>
> > I have been comparing the lengths of packets specified in IP headers
> > against actual packet lengths. What I am seeing, ignoring IP packets
> > smaller than the minimum packet size, is that a fair number of machines
> > send out packets that are 1-3 bytes longer than is specified by the
> > IP length.
>
>The Ethernet requires that packets be an integral number of 16-bit words
>long. The Ethernet also has a minimum packet size of 60 bytes. Any IP
>packet that is less than 60 bytes in length (including ethernet header)
>will be padded to 60 bytes. The IEEE 802.3 length field would allow
>you to explicitly set the ethernet length, but Ethernet doesn't.

The Ethernet 2.0 spec states:

        ...Of these five fields, all are fixed size except the
        data field, which may contain any integral number of octets
        between the minimum and maximum values specified below (see 6.2.5)

                Ethernet 2.0 pg 25

This means that odd length packets are legal. IEEE 820.3 also specifies
an intregral number of octets. (pg 26 3.2.7) Most impelemntations of
protocols pad to an even byte length to keep their more braindamaged
bretheren from puking. IP only cares about the octets specified in its
length field, the pad octets are ignored

        Brad Kemp
        MICOM-Interlan
        {ulowell,mit-eddie}!interlan!brad



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