Network Performance, Reliability, and Quality of Service Committee
Reliability and QoS Task Forces
Denver, CO; April 11-15, 2005
CONTRIBUTION PRQC-2005-059
CONTRIBUTION PTSC-SAC-2005-100
TITLE:
Interpretation of User Plane Priority Classes in IP Networks
SOURCE*:
AT&T (P. S. Tarapore, M. Dolly, S. Sayers)
ISSUE:
Proposed New Issue: Vertical Signaling Interface to Communicate
QoS and Reliability/Priority Parameters Between the Application
Layer and the User Plane in IP Networks
_______________________________
ABSTRACT
This contribution begins the process of interpreting the defined User Plane priority classes for
appropriate action in the User Plane. An illustrative example of an MPLS-enabled IP backbone is
used to demonstrate how the priority levels can be mapped into appropriate DiffServ Code Points
for different traffic types.
_______________________________
NOTICE
This contribution has been prepared to assist Accredited Standards Committee ATIS/PRQC. This document is offered
to the Committee as a basis for discussion and is not a binding proposal on AT&T. The requirements presented in this
document are subject to change in form and numerical value after more study. AT&T specifically reserves the right to
add to, or amend, the statements contained herein.
CONTACT: Percy S. Tarapore; email: tarapore@att.com ; Tel: 732-420-4172
I.
Introduction
Priority classes in the User Plane for the purpose of admission and bandwidth reservation have
been described in a recently approved ATIS/PRQC Technical Report [1]. In addition, work has
commenced in the ATIS/PTSC-SAC Sub-Committee [2] that associates signaling messages related
to call flows with the same User Plane priority as the traffic itself. For example, SIP signaling
messages related to a High Priority ETS call would be assigned High Priority in the User Plane.
Similarly, SIP messages related to a Normal Priority VoIP call would be assigned Normal
Priority in the User Plane.
This contribution provides initial discussion on how the priority classes need to be considered for
appropriate action in the User Plane. An illustrative example of an MPLS-enabled IP backbone is
used to stimulate the discussion.
User Plane Priority Class Interpretation – Illustrative Example
II.
The recently approved Technical Report from ATIS/PRQC [1] proposes three broad User Plane
classes of traffic from the perspective of bandwidth allocation as follows:



High: Reserved for Emergency Services such as ETS and E-911 (high probability of
bandwidth allocation per SLA agreements between customers and service providers). It is
assumed that emergency services represent a very small percentage of total network
traffic (~<<5%) and bandwidth allocation schemes for this class can be done to give this
traffic class high priority treatment.
Normal: Real-time and Data services that require better than “Best Effort” (“reasonable”
probability of bandwidth allocation per SLA agreements between customers and service
providers). Bandwidth allocation for this substantial class of traffic can be done such that
successful transmission of this class of traffic is enhanced.
Best Effort: No bandwidth guarantee for these services. In other words, bandwidth for
this substantial class of traffic is tightly controlled. In the event of network outages or
overloads, this class of service could be negatively impacted.
The guiding rationale underlying the creation of three classes is that given a large amount of
available bandwidth in the User Plane, emergency services require strong guarantees for
bandwidth reservation, best effort services by definition require no guarantees, and normal
services fit in between these two extremes.
Assume that the IP backbone is MPLS-enabled and that DiffServ Code Points (DSCP) need to be
assigned to traffic flows for ultimate action in router queues. Assume further that for real-time
traffic flows, there are two DSCP points available to distinguish two Expedited Forwarding
DSCP Per Hop Behavior treatments [3] – EF(1) for High Priority emergency flows and EF(2) for
Normal Priority VoIP calls1. The underlying rationale is that the bandwidth requirements for
emergency flows are small (< 5%) when compared to the total traffic in the User Plane.
An initial proposed mapping of the User Plane traffic priority classes to DSCP points is as
follows. This proposal is intended to stimulate further discussions on this topic.
1
Currently, a single EF queue is available for all real-time traffic. In that case, a drop precedence is
desirable to ensure that High Priority ETS calls are not dropped in the EF queue.
2
III.
Traffic Type
DSCP
High Priority
Voice Call (ETS/WPS)
EF(1)
(See Footnote 1)
Normal Priority
Voice Call
EF(2)
(See Footnote 1)
High Priority
Data (ETS)
AF(1)
Normal Priority
Data
AF(2), AF(3), AF(4)
Best Effort
Traffic
BE
Proposal
This contribution provides initial discussion on how to interpret User Plane priority classes.
Specifically, an initial mapping between the priority classes and the DSCP values is presented for
discussion. It is critical that all possible mappings are ultimately determined such that appropriate
action can be taken in the User Plane [4]. This work will also be crucial in shaping discussion on
the development of the requirements for the vertical signaling interface between the application
layer and the User Plane [5].
IV.
References
[1] “User Plane Priority Levels for IP-Based Networks and Services”, ATIS/PRQC Draft
Technical Report (pre-publication), ATIS Contribution T1A1/2003-196 R2, October 2004.
[2] “Discussion on Associating Control Signaling Messages with Media Priority Levels”, ATIS
Contribution T1S1.7/2003-577, October 2004.
[3] “Multi-Level Expedited Forwarding Per Hop Behavior (MLEF PHB)”, M. Pierce and S.
Silverman, IETF I-D, February 2005.
[4]”Output Baseline Text – Support of ETS in IP Networks”, ATIS Contribution PTSC-SAC2005-076 R1, February 2005.
3
[5] “Proposal to Develop Requirements for a Vertical Signaling Interface Between the User Plane
and Application Layer in IP Networks”, ATIS Contribution PRQC-2005-xxx, April 2005.
4