RSVP Operation

RSVP uses path and resv messages to establish a reservation for a requested data flow. A path message carries the traffic description for the desired flow from the sending application to the receiving application, and the resv message carries the reservation request for the flow from the receiving application to the sending application. Figure 8-29 shows a network that consists of several routers carrying the path and resv messages for a sending and receiving application.

Figure 8-29 RSVP Path and Resv Messages

Figure 8-29 RSVP Path and Resv Messages

Application Application

In this example, the sending application responds to an application request by sending a path message to the RSVP router R1 describing the level of service required to support the requested application. R1 forwards the path message to router R2, R2 forwards the message to router R3, R3 forwards the message to R4, and finally R4 forwards the message to the receiving application. If the level of service described in the path message can be met, the receiving application sends a resv message to RSVP router R4. The resv message causes each router in the data path to the sending application to place a reservation with the requested level of service for the flow. When the resv message reaches the sending application, the data flow begins.

Path messages describe the level of service required for a particular flow through the use of the Sender Tspec object within the message. The Sender_TSpec object is defined by the sending application and remains in tact as the path message moves through the network to the receiving application. Also included in the path message is the ADSpec object. The ADSpec object is modified by each intermediary router as the RSVP path message travels from the sending application to the receiving application. The ADSpec object carries traffic information generated by the intermediary routers. The traffic information describes the properties of the data path, such as the availability of the specified level of service. If a level of service described in the ADSpec object is not implemented in the router, a flag is set to alert the receiving application.

Effectively, the Sender_Tspec states what the application needs, and the ADSpec allows each successive router to comment on the level of service it can and cannot support. The receiving application can look at both settings and decide whether to accept or reject the reservation request. Figure 8-30 shows the Sending_TSpec and ADSpec objects sent in a path message from a sending application.

Figure 8-30 Path Messages SendingTSpec and ADSpec Objects

Sender_Tspec Level of Service = Guaranteed or Controlled-Load

Sender Tspec

Sender_Tspec Level of Service = Guaranteed or Controlled-Load

Sender Tspec

Application ADSpec Level of Service = Guaranteed

Level of Service = Guaranteed

R4 ADSpec

Sender_Tspec Level of Service = Guaranteed or Controlled-Load

Application ADSpec Level of Service = Guaranteed

Level of Service = Guaranteed

R4 ADSpec

Sender_Tspec Level of Service = Guaranteed or Controlled-Load

Sending Application

Receiving Application

Sending Application

Application ADSpec Level of Service = Guaranteed R1 ADSpec Level of Service = Guaranteed R2 ADSpec Level of Service = Guaranteed R3 ADSpec Level of Service = Controlled-Load R4 ADSpec Level of Service = Guaranteed

Receiving Application

In Figure 8-30 the Sending_TSpec is advertising the required level of service for this application as guaranteed. Because intermediary routers do not modify the Sending_TSpec, this value is passed through the network to the receiving application. The ADSpec lists the level of service available at each intermediate router along the data path. As the path message reaches router R3, the ADSpec object is modified to indicate that router R3 can only provide a controlled-load level of service. Essentially, the sending application has asked for guaranteed RSVP service, and the ADSpec implies that at least one router can only support a controlled-load service for this request.

The receiver of the path message replies with an resv message. Resv messages request a reservation for a specific flow through the use of a FlowSpec object. The FlowSpec object provides a description of the desired flow and indicates the desired level of service for the flow to the intermediary routers, based on the information received from the path message in the ADSpec object. A single level of service must be used for all nodes in the network—in other words, it can be controlled-load or guaranteed service, but not both. Because the sending application requires either guaranteed or controlled-load level of service to begin the flow, and router R3 can only provide controlled-load level of service, the receiving application can only request a reservation specifying controlled-load level of service. Example 8-31 shows the FlowSpec object sent in a resv message from a receiving application.

Figure 8-31 Resv Messages FlowSpec Objects

FlowSpec

Requested Service =

Figure 8-31 Resv Messages FlowSpec Objects

FlowSpec

Requested Service =

Application Application

After the reservation for controlled-load service has been installed on the intermediary routers, the flow is established between the sending application and the receiving application.

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