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From: jbvb@FTP.COM ("James B. Van Bokkelen")
Newsgroups: comp.protocols.tcp-ip.ibmpc
Subject: Re: interfacing to packet drivers
Message-ID: <9012051629.AA04458@ftp.com>
Date: 5 Dec 90 16:29:03 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: jbvb@ftp.com
Organization: The Internet
Lines: 28


    ... The pd spec is not clear on how much information must be given
    to the pd - do I need to supply the complete Ethernet frame, including
    preamble, Ethernet addresses, and CRC?

V1.09 of the spec says "The application must supply the entire packet,
including local network headers.".  It doesn't attempt to explain all
the details for each kind of media (since their approaches to headers
vary quite a bit).  V1.10 (not published yet) has appendices which
outline the characteristics of each media Class (and nobody has yet told me
the characteristics of Classes 8, 9, 10, 12 and 15, for which I believe
drivers exist).  Here's the text on Class 1 from 1.10:

  Class 1 is demultiplexed by the 'ethertype', a 16-bit value
  (MSB first) transmitted as the thirteenth and fourteenth bytes of a
  frame.  @i[typelen] is normally 2.  The maximum @i[length] allowed by
  @i[send_pkt()] is 1514, being 1500 bytes of data preceded by the 14-byte MAC
  header (source address, destination address, ethertype).  The minimum
  "look-ahead buffer" length allowed is 60 bytes.

I think that long ago, in the stone (3Mb) age, there may have been Ethernet
interfaces that made the program generate the preamble/postamble.  I know
that the practice of making the poor programmer do the CRC survived longer,
but you can't buy anything like that these days unless you go to a computer
salvage place...

James B. VanBokkelen		26 Princess St., Wakefield, MA  01880
FTP Software Inc.		voice: (617) 246-0900  fax: (617) 246-0901