Browse Prior Art Database

Packet Delineation Format on High Speed Trunks

IP.com Disclosure Number: IPCOM000116391D
Original Publication Date: 1995-Sep-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 4 page(s) / 172K

Publishing Venue

IBM

Related People

Guerin, R: AUTHOR [+6]

Abstract

In this paper we present how we solved the problem of delineation of a mix of variable length packets and fixed length Asynchronous Transfer Mode (ATM) cells in a serial bit stream like we get on a SONET link for example, without having any overhead in the case of the ATM traffic. We discuss the rationale of this solution, and show how we handle the error recovery.

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Packet Delineation Format on High Speed Trunks

      In  this  paper  we  present  how  we  solved  the  problem  of
delineation of a mix of variable  length  packets  and  fixed  length
Asynchronous Transfer Mode (ATM) cells in a serial bit stream like we
get  on  a SONET link for example, without having any overhead in the
case of the ATM traffic.  We discuss the rationale of this  solution,
and show how we handle the error recovery.

      The problem addressed in this document is that of the
identification of individual data packets within a serial bit stream.
The basic issue is to provide information that allows direct
identification  of packet boundaries, i.e., start and end of packets,
within the serial bit stream.  There are several possible  approaches
depending on the characteristics of the underlying transmission
facility.  For example, when the transmission facility simply carries
the bit stream as is, bit stuffing can be used to create unique bit
patterns (flags) that can then be used to uniquely mark packet
boundaries (HDLC).  Conversely, if some form of transmission encoding
is required, special code points or characters are typically
available
to be used for packet delineation (8/10 encoding).

      The first approach is unfortunately not scalable as
transmission speeds increase because of the variability it introduces
in the transmission rate.  Similarly, the second approach requires
the use of special characters which may not always be available.
Specifically, the environment considered here is that of a high-speed
transmission facility, e.g., SONET OC-12, where transmission takes
place without the use of any line encoding.  It is, therefore,
necessary to imbed within the stream of transmitted bits information
to identify packet boundaries.  Furthermore, this information should
be retrievable directly from the bit stream itself without the need
for external signals, i.e., packet boundaries should be identifiable
simply by scanning the received  bit  stream.  It should also be
pointed out that because packet lengths are assumed to be variable,
packet boundary information needs to be provided for each packet.  In
contrast, with fixed size packets this information can typically be
omitted once the start of the first packet has been identified.
(Consider, for example, the transmission of fixed-size ATM cells over
SONET facilities.)

      The first approach shows the used for variable length packet
delineation, and a second step shows how to handle the ATM cells
(mixed
within the variable length packet stream).

      The proposed approach is that each transmitted packet is
preceded by a header consisting of two fields: < A,B >.  Field A
gives the packet length, while field B carries information which is a
function only of the value present in the first field, i.e., B=f(A).
Field B is used both to check that the header has not been corrupted
by errors, and more importantly pr...