Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!ucbvax!MITVMA.MIT.EDU!KASTEN
From: KASTEN@MITVMA.MIT.EDU (Frank Kastenholz)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Convert /etc/hosts to RR format pgm anyone?
Message-ID: <8806100806.AA14313@ucbvax.Berkeley.EDU>
Date: 9 Jun 88 13:33:04 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 57



In article <8805261929.AA04199@RHEA.CAM.UNISYS.COM>, jonab@CAM.UNISYS.COM
(Jonathan P. Biggar) writes:
>>> The TCP URGENT mechanism uses a pointer; I do not think that there
>>> is any specification which says that packet boundaries are
>>> significant with respect to URGENTs.
>> Not so!  RFC 793 states:
>>    If there is urgent data [...].  *****Note that data following
>>    the urgent pointer (non-urgent data) cannot be delivered to the
>>    user in the same buffer with preceeding urgent data unless the
>>    boundary is clearly marked for the user.*****
>
>What does this have to do with packet boundaries?  Jonab said "packet",
>not "user buffer".

This sounds to me like there is some confusion over the meaning of
some of the terms used (which, to the best of my limited knowledge,
have never been fully codified - perhaps an addendum to the Assigned
Numbers RFC is in order - "Assigned Abbreviations and Terms and Their
Meanings").

As I understand things the "hierarchy" of terms in use in the Internet
protocols (among others) are:

FRAME - lowest level of bits/bytes. One speaks of Ethernet Frames, etc
        A FRAME has ONE data link header (e.g. Ethernet header).

FRAGMENT - the thing that IP spits out to the data link driver. Each
           FRAGMENT has ONE IP header. A FRAGMENT may be split into
           multiple FRAMES by the lower layers.

SEGMENT - the thing that TCP spits into IP. Each SEGMENT has one TCP
          header. It may be broken into FRAGMENTS by IP.

Noe things get a little weird.....

DATAGRAM - this is also the 'larger' thing that IP tries to send. A
           DATAGRAM USUALLY has a one to one relationship with a SEGMENT.
           It may be broken up (fragmented) into a number of FRAGMENTS
           by IP before being sent through the data link.

           A DATAGRAM is ALSO what UDP and other "datagram" oriented
           protocols may give to IP for transmission.


PACKET - a generic term that can mean anything at any time - usually its
         meaning is derived from the context, other times by a prefix
         (such as IP PACKET), other times .....

BUFFER - what is passed to TCP/UDP for transmission. Usually. Of course,
         a device driver reads stuff out of the I/O interface into a
         buffer....


These are terms that I use, based on "common usage, the literature, etc"

Frank Kastenholz.