To understand the behavior of the RSVP-ATM QoS Interworking feature, consider the following example, which uses a Cisco 7500 router with VIP ingress and egress interfaces and RSVP ingress functionality implemented on the Route Switch Processor (RSP). The figure below illustrates this example; it shows a pair of routers that communicate over the ATM cloud. In this example, a single PVC is used for RSVP request messages and an ATM SVC is established to handle each new reservation request message.
||Two Routers Connected over an ATM Core Network
Host X, which is upstream from Router A, is directly connected to Router A using FDDI. Host Y, which is downstream from Router B, is directly connected to Router B using FDDI. (In an alternative configuration, these host-device connections could use ATM VCs.)
For the RSVP-ATM QoS Interworking feature, reservations are needed primarily between devices across the ATM backbone network. To limit the number of locations where reservations are made, you can enable RSVP selectively only at subinterfaces corresponding to device-to-device connections across the ATM backbone network. Preventing reservations from being made between the host and the device both limits VC usage and reduces load on the device.
RSVP RESV messages flow from receiving host to sending host. In this example, Host Y is the sending host and Host X is the receiving host. (Host Y sends a RESV message to Host X.) Router B, which is at the edge of the ATM cloud, receives the RESV message and forwards it upstream to Router A across the PVC used for control messages. The example configuration shown in the figure above uses one PVC; as shown, it carries the RSVP request.
The ingress interface on Router A is configured for RSVP-ATM, which enables it to establish for each request an SVC to service any new RSVP RESV reservations made on the interface. When it receives a reservation request, the interface on Router A creates a new nonreal-time variable bit rate (nRTVBR) SVC with the appropriate QoS characteristics. The QoS characteristics used to establish the SVC result from algorithmic mapping of the flowspec in the RSVP RESV message to the appropriate set of ATM signalling parameters.
In this example, Controlled Load Service is used as the QoS class. The ATM PCR parameter is set to the line rate. If the ip rsvp atm-peak-rate-limit command is used on the interface to configure a rate limiter, the PCR is set to the peak rate limiter. The ATM SCR parameter is set to the RSVP flowspec rate and the ATM MBS is set to the RSVP flowspec burst size. Packets are shaped before they are sent on the ATM SVC. Shaping creates back pressure to the VIP when the offered load exceeds the rate.
When a new SVC is set up to handle a reservation request, another state is also set up including a classifier state that uses a source and destination addresses and port numbers of the packet to determine which, if any, reservation the packet belongs to. Also, a token bucket is set up to ensure that if a source sends more data than the data rate and MBS parameters of its flowspec specify, the excess traffic does not interfere with other reservations.
The following section describes more specifically, how data traverses the path.
When a data packet destined for Router B arrives at Router A, before they traverse the ATM cloud, the source and destination addresses and port numbers of the packet are checked against the RSVP filter specification (filterspec) to determine if the packet matches a reservation.
If the packet does not match a reservation, it is sent out the best-effort PVC to Router B. If a packet matches a reservation, it is further processed by RSVP. The packet is checked against the token bucket of the reservation to determine whether it conforms to or exceeds the token bucket parameters. (All packets matching a reservation are sent out on the SVC of the reservation to prevent misordering of packets.)
To introduce differentiation between flowspec-conformant and flowspec-exceeding packets, you can specify values for RSVP-ATM to use in setting the IP Precedence and ToS bits of the packets. To specify these values, you use the ip rsvp precedence and ip rsvp tos commands. When you set different precedence values for conformant and exceeding packets and use a preferential drop policy such as DWRED, RSVP-ATM ensures that flowspec-exceeding packets are dropped prior to flowspec-conformant packets when the VC is congested.
For information on how to configure the RSVP-ATM QoS Interworking feature, see the "Configuring RSVP-ATM QoS Interworking" module.