IntServ
IntServ is a QoS model that guarantees a specific level of service to each
flow of identified traffic, throughout the entire network, for the length of the
session. This is done using Resource Reservation Protocol (RSVP). An
RSVP-aware application, or a router or CallManager acting in proxy for a
nonRSVP-aware device, requests a specific level of service from its next-hop
router. A check is made along the path between the two endpoints, and each
RSVP-enabled router along the way reserves bandwidth for that flow. If the
network cannot provide the required bandwidth, the session is not allowed or
its service level is downgraded.
RSVP works for any type of traffic, but it is usually used for real-time applications
that are either rate-sensitive or delay-sensitive, such as voice and
video. Figure 3-1 shows a call between two IP phones.
Using RSVP for Voice Calls
CCNP ONT
[ 268 ] CCNP ONT Quick Reference
GW1 - RSVP GW2 - No RSV GW3 - RSVP
RSVP reservation messages
= Interface configured for RSVP -
reservations will be made here
V V V
Two of the routers in the path—GW1 and GW3—are configured with RSVP;
however, GW2 is not. When GW1 and GW3 receive the RSVP messages
requesting a service level, they reserve that amount of bandwidth on their
WAN interface. There must be some sort of QoS configured on the routers to
implement the reservation. When GW2 receives the RSVP messages, it
merely passes them on to the next hop router unchanged. Note that reservations
are made in both directions becuase this is a voice call.
All routers in the path are not required to be configured with RSVP, but
reservations are made only on those routers and those interfaces with it
enabled. To ensure end-to-end service, configure RSVP on all router interfaces
in the data path.
The path between endpoints is determined by the routing protocol, not by
RSVP. If there is a network change, and the routing protocol changes the
path, then RSVP reconverges also.
Current applications use DiffServ to enact IntServ QoS policies, such as
guaranteed rate, and controlled load. One of the biggest benefits of IntServ is
that it provides per-flow admission control. This can help with VoIP calls.
RSVP supports applications that use dynamic port numbers and static ones.
Some drawbacks include its overhead—signaling is exchanged at the beginning
of a flow, so there can be some delay. It must continue to cross the
network for the length of the flow to adjust for changes in path due to
network changes, thus causing extra overhead. Additionally, because you
need to track each flow, it is not scalable in a large enterprise.
For more information on using RSVP with VoIP, see the Cisco Press book
Cisco Voice Gateways and Gatekeepers by David Mallory, Ken Salhoff, and
Denise Donohue.