- Enterasys Security Router User's Guide

Mechanisms Providing QoS
12-2 Configuring Quality of Service
• QoS on the dialer interfaces is directly applied to the dialer interface and inherited by the dial
pool members (Serial or ISDN).
• QoS on MLPPP interfaces.
• QoS on point-to-point and point-to-multi-point VPN interfaces.
• Control over copy of the ToS byte from/to outer header for VPN tunnels.
• QoS on Ethernet port and sub-interfaces (PPPoE and VLAN).
Be aware that QoS service on the XSR is proscribed by the following limits:
• No more than 64 classes permitted
• Traffic policer cannot be set for traffic flows assigned to priority queues. Each priority queue is
metered and policed by default to guarantee it conforms to the scheduled traffic pattern
•
Priority and bandwidth commands are mutually exclusive; a traffic flow is assigned to
either queue, not both
• Tail-drop (
queue-limit) and RED (random-detect) are mutually exclusive; a queue is
managed by either mechanism, not both
• QoS on Input does not perform the following: packet buffering, shaping, bandwidth sharing,
prioritization, CoS bit marking in the VLAN header, RED or WRED
Mechanisms Providing QoS
This section describes the general mechanisms the XSR employs to support Quality of Service.
Traffic Classification
Before the XSR can apply QoS to traffic, it must differentiate between types of traffic. The process
is called traffic classification. Table 12-1 on page 12-2 describes typical traffic classification:
The XSR provides a class-based traffic classifier that creates traffic policies and attaches them to
interfaces, sub-interfaces, and virtual circuits such as Frame Relay DLCIs. A traffic policy contains
a traffic class and one or more QoS features. A traffic class is used to classify traffic, while the QoS
Table 12-1 Traffic Classification
Classification
Criteria
Description
Additional
Comments
IP Precedence bits in
IP header (IP only)
Simple classification for IP packets only. IP Precedence bits reside in the ToS byte
of the IPv4 header and are 3-bits long, providing up to 8 levels of QoS classes.
Simple, IP
traffic only
DSCP (DiffServ
Code Point) bits in IP
header (IP only)
Simple classification for IP packets only. This QoS signaling method is defined by
the IETF DiffServ group providing a scalable QoS solution. It is 6-bits long and can
provide 64 different traffic classes. DSCP overlaps with the IP Precedence bits in
the IP header and can be considered a super set of IP Precedence.
Simple, IP
traffic only
Multiple-Field
Classification
This classification considers the L3 header (source and destination IP addresses),
L4 header (TCP/UDP port numbers to identify the nature of applications as FTP,
Telnet, Web, etc.), and in some cases, looks at fields beyond the L4 header (e.g., to
differentiate Web access to certain Web pages from other Web accesses), to
narrow the classification and choose traffic from a particular application.
Most
versatile but
CPU
intensive