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RSVP Support for Low Latency Queueing

RSVP is a network-control protocol that provides a means for reserving network resources—primarily bandwidth—to guarantee that applications sending end-to-end across networks achieve the desired QoS.

RSVP enables real-time traffic (which includes voice flows) to reserve resources necessary for low latency and bandwidth guarantees.

Voice traffic has stringent delay and jitter requirements. It must have very low delay and minimal jitter per hop to avoid degradation of end-to-end QoS. This requirement calls for an efficient queueing implementation, such as low latency queueing (LLQ), that can service voice traffic at almost strict priority in order to minimize delay and jitter.

RSVP uses WFQ to provide fairness among flows and to assign a low weight to a packet to attain priority. However, the preferential treatment provided by RSVP is insufficient to minimize the jitter because of the nature of the queueing algorithm itself. As a result, the low latency and jitter requirements of voice flows might not be met in the prior implementation of RSVP and WFQ.

RSVP provides admission control. However, to provide the bandwidth and delay guarantees for voice traffic and get admission control, RSVP must work with LLQ. The RSVP Support for LLQ feature allows RSVP to classify voice flows and queue them into the priority queue within the LLQ system while simultaneously providing reservations for nonvoice flows by getting a reserved queue.

Figure 2 shows how RSVP operates with other Voice over IP (VoIP) features, such as ip rtp priority, using the same queueing mechanism, LLQ.

Figure 2 RSVP Support for LLQ

RSVP is the only protocol that provides admission control based on the availability of network resources such as bandwidth. LLQ provides a means to forward voice traffic with strict priority ahead of other data traffic. When combined, RSVP support for LLQ provides admission control and forwards voice flows with the lowest possible latency and jitter.

High priority nonvoice traffic from mission-critical applications can continue to be sent without being adversely affected by voice traffic.

Nonconformant traffic receives best-effort treatment, thereby avoiding any degradation that might otherwise occur for all traffic.

The RSVP Support for LLQ feature supports the following RFCs:

•RFC 2205, Resource Reservation Protocol

•RFC 2210, RSVP with IETF Integrated Services

•RFC 2211, Controlled-Load Network Element Service

•RFC 2212, Specification of Guaranteed Quality of Service

•RFC 2215, General Characterization Parameters for Integrated Service Network Elements

Figure 3 shows a sample network topology with LLQ running on each interface. This configuration guarantees QoS for voice traffic.

Note If the source is incapable of supporting RSVP, then the router can proxy on behalf of the source.

Figure 3 Topology Showing LLQ on Each Interface

Configuring RSVP Support for LLQ

Configuring Flow Classification

To configure flow classification, use the following command in global configuration mode:

Enabling RSVP and WFQ

To enable RSVP and weighted fair queueing (WFQ), use the following commands beginning in global configuration mode:

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