Priority Of Layer-3 "Ip" Frames; Introduction; Originally; Later - GE JungleMUX SONET Technical Practice

PRIORITY OF LAYER-3 "IP" FRAMES

Introduction

Whereas the priority field encoded in layer-2 "tagged" frames (i.e. IEEE 802.1p
and 802.1Q frames) is well defined by their 3-bit "priority" field (with 7 being the
top priority), the determination of the priority of layer-3 "IP" frames is not clearly
defined.
All IPv4 (the most common IP version) frames have an 8-bit "classification" field
following their 4-bit "version" and 4-bit "header-length" fields.

Originally

In 1981, the IETF (RFC 791) indicated this byte would be used as follows:
The first 3-bits were a "precedence" field to denote the frame‟s priority.
The next 3-bits comprised a "TOS" field to individually flag 3 types of services:
Delay important
-
Throughput important
-
Reliability important
-
The last 2 bits were unused.
The expression TOS may refer to all 8 bits, or the 6 used bits, or the 3 TOS bits.

Later

In 1998, the IETF (RFC 2474) redefined the byte to be used for a "DiffServ"
(Differentiated Services) field.
The first 6-bits are a "DSCP" field to denote the frame‟s type of service, by
assigning it a "Differentiated Services Code Point".
The last 2 bits are unused.

How to map frames to priority queues?

Though the IETF has provided the means to prioritize traffic, it has left it up to the
user to decide how to use them.
This may explain why there is essentially no information on how a generic switch
should map the field‟s (64 combinations of the 6 bits) to 8 priority queues.
Copyright  GE Multilin Inc. 2010-2012
APPENDIX C
342-86400-498PS
Issue 1.2
April 2012
Page 159