Cabletron Systems SmartSwitch manual Configuring ToS Rewrite for IP Packets, Mbz

Chapter 19: QoS Configuration Guide

The ToS octet part of the IP specification, however, has not been widely employed in the past. The IETF is looking into using the ToS octet to help resolve IP quality problems. Some newer routing protocols, like OSPF and IS-IS, are designed to be able to examine the ToS octet and calculate routes based on the type of service.

The ToS octet in the IP datagram header consists of three fields:

7	6	5	4	3	2	1	0
Precedence				ToS			MBZ
Most Significant Bit					Least Significant Bit

•The three-bit Precedence field is used to indicate the priority of the datagram.

•The four-bit ToS field is used to indicate trade-offs between throughput, delay, reliability, and cost.

•The one-bit “must be zero” (MBZ) field is not currently used. (In the SSR configuration, there is no restriction on this bit and it is included as part of the ToS field.)

For example, setting the ToS field to 0010 specifies that a packet will be routed on the most reliable paths. Setting the ToS field to 1000 specifies that a packet will be routed on the paths with the least delay. (Refer to RFC 1349 for the specification of the ToS field value.)

With the ToS rewrite command, you can access the value in the ToS octet (which includes both the Precedence and ToS fields) in each packet. The upper-layer application can then decide how to handle the packet, based on either the Precedence or the ToS field or both fields. For example, you can configure a router to forward packets using different paths, based on the ToS octet. You can also change the path for specific applications and users by changing the Precedence and/or ToS fields.

Note: In RFC 2574, the IETF redefined the ToS octet as the “DiffServ” byte. You will still be able to use the ToS rewrite feature to implement DiffServ when this standard is deployed.

Configuring ToS Rewrite for IP Packets

The ToS rewrite for IP packets is set with the qos set command in Configure mode. You can define the QoS policy based on any of the following IP fields: source IP address, destination IP address, source port, destination port, ToS, port, or interface.

When an IP packet is received, the ToS field of the packet is ANDed with the <tos-mask>and the resulting value is compared with the ANDed value of <tos> and <tos-mask>of the QoS policy. If the values are equal, the values of the <tos-rewrite>and <tos-precedence- rewrite> parameters will be written into the packet.

SmartSwitch Router User Reference Manual 289