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Illustrated TCP/IP
by Matthew G. Naugle
Wiley Computer Publishing, John Wiley & Sons, Inc.
ISBN: 0471196568 Pub Date: 11/01/98
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Chapter 330
RSVP Requests
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RSVP Requests
- • Resv messages: reservation requests sent hop by hop from host receiver(s) to host sender along the reverse path.
- • Each RSVP-speaking receiver node forwards a Resv message to the unicast address of the previous RSVP hop.
- • Resv messages create and maintain reservation states in each node along the path(s).
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The most basic RSVP request consists of a flow descriptor. A flow descriptor contains:
- flowspec: A reservation request that defines a desired QoS and is used to set parameters in a node’s packet scheduler.
- filter spec: Used to define the set of packets to receive QoS as defined in the flow spec and to set parameters in the packet classifier.
RSVP is based on sessions and defines a session as a data flow with a particular destination and transport-layer protocol. Each session is maintained independently. It is defined by a combination of:
- Destination address: A multicast or unicast destination address.
- Protocol ID :(Protocol ID is 46)
- Destination port: TCP or UDP port number or an application-specific port number. This may be omitted when the destination address is multicast.
There are two message types sent between senders and receivers for reservation of resources. These messages are not sent reliably because the program uses IP directly:
- Path: Sent downstream by the RSVP sender host. This message is forwarded by routers using the unicast/multicast routing table. These messages store path state in each forwarding node. This information includes the unicast IP address of the previous hop node. This is used to route the Resv (sent by a receiver in response to a Path message) messages in the reverse path. In addition, the Path message contains information on the format of data packets that the sender will generate, the traffic characteristics of the dataflow, and may carry advertising information known as One Pass with Advertising (OPWA). This is known as an Adspec and allows Path messages to gather information en route to the receiver that the receiver can use to predict end-to-end service.
- Resv: Sent upstream by the receiver to the sender. They can either be distinct or shared, allowing for unique reservations to occur for receivers or a single shared reservation that is shared among all packets of selected senders. These messages are sent upstream along the tree until it reaches a point where an existing reservation is equal or greater than that being requested. At that point, the reservation is already in place and does not need to be forwarded any further.