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26-10 Riverstone Networks RS Switch Router User Guide Release 8.0
ToS Rewrite QoS Configuration
26.8 TOS REWRITE
IP packets that use different paths are subject to delays, as there is little inherent knowledge of how to optimize the
paths for different packets from different applications or users. The IP protocol actually provides a facility, which has
been part of the IP specification since the protocol’s inception, for an application or upper-layer protocol to specify
how a packet should be handled. This facility is called the Type of Service (ToS) octet.
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:
Figure 26-2 ToS fields
•
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 RS 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.
26.8.1 Configuring ToS Rewrite for IP Packets
The ToS rewrite for IP packets is set with the
qos
set
command in the Configure mode. You can define the QoS policy
based on any of the following IP fields:
•
Source IP address
•
Destination IP address
MBZ
ToS
Precedence
7 6543210
Most Significant Bit Least Significant Bit