Re: packet lengths


Ted Marshall (voder!blia!ted@ucbvax.Berkeley.EDU)
21 Apr 88 20:39:15 GMT


In article <In article <8804191854.AA00089@apolling.imagen.uucp>, In article <8804191854.AA00089@apolling.imagen.uucp>, geof@imagen.UUCP (Geof Cooper) writes:
> The Ethernet requires that packets be an integral number of 16-bit words
> long.

Sorry, no. An integral number of 8-bit octets, not 16-bit words. Quoting from
page 19 of the version 1.0 Ethernet specification:

        Figure 6-1 shows the five fields of a frame: the addresses of the
        frame's source and destination, a type field [...], a data field
        [...], and the frame check sequence [...]. Of these five fields,
        all are of fixed size except the data field which my contain any
        integral number of octets between the minimum and maximum values
        ^^^^^^^^^^^^^^^^^^^^^^^^^ [ ^'s are mine. TM]
        specified below (see 6.2.5).

Figure 6-1 shows the layout of a frame and also of an octet (showing 8 bits,
transmitted LSB first).

I don't have a copy of the 2.0 spec, but I don't think they changed this. If
they did, DEC is breaking the spec; I know that the VAX/VMS Ethernet cards and
drivers allow odd-byte-count length packets to be sent.

That is not to say that every Ethernet interface manufacturer built their
unit so that it can transmit an odd number of bytes. In fact, I believe that
there are some that can only transmit multiples of 4 bytes.

In general, no Ethernet protocol should be used that depends on the data
link layer to report exactly the number of bytes in the packet since
short packets must be padded to the minimum size.

--
Ted Marshall	   ...!ucbvax!mtxinu!blia!ted <or> mtxinu!blia!ted@Berkeley.EDU
Britton	Lee, Inc., 14600 Winchester Blvd, Los Gatos, Ca	95030	  (408)378-7000
The opinions expressed above are those of the poster and not his employer.



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