1.2 Reservation Model

An elementary RSVP reservation request consists of a "flowspec" together with a "filter spec"; this pair is called a "flow descriptor". The flowspec specifies a desired QoS. The filter spec, together with a session specification, defines the set of data packets -- the "flow" -- to receive the QoS defined by the flowspec. The flowspec is used to set parameters in the node's packet scheduler or other link layer mechanism, while the filter spec is used to set parameters in the packet classifier. Data packets that are addressed to a particular session but do not match any of the filter specs for that session are handled as best-effort traffic.

The flowspec in a reservation request will generally include a service class and two sets of numeric parameters: (1) an "Rspec" (R for `reserve') that defines the desired QoS, and (2) a "Tspec" (T for `traffic') that describes the data flow. The formats and contents of Tspecs and Rspecs are determined by the integrated service models [RFC 2210] and are generally opaque to RSVP. The exact format of a filter spec depends upon whether IPv4 or IPv6 is in use; see Appendix A. In the most general approach [RSVP93], filter specs may select arbitrary subsets of the packets in a given session. Such subsets might be defined in terms of senders (i.e., sender IP address and generalized source port), in terms of a higher-level protocol, or generally in terms of any fields in any protocol headers in the packet. For example, filter specs might be used to select different subflows of a hierarchically-encoded video stream by selecting on fields in an application-layer header. In the interest of simplicity (and to minimize layer violation), the basic filter spec format defined in the present RSVP specification has a very restricted form: sender IP address and optionally the UDP/TCP port number SrcPort.

Because the UDP/TCP port numbers are used for packet classification, each router must be able to examine these fields. This raises three potential problems.

1. It is necessary to avoid IP fragmentation of a data flow for which a resource reservation is desired.

   Document [RFC 2210] specifies a procedure for applications using RSVP facilities to compute the minimum MTU over a multicast tree and return the result to the senders.

2. IPv6 inserts a variable number of variable-length Internet- layer headers before the transport header, increasing the difficulty and cost of packet classification for QoS.

   Efficient classification of IPv6 data packets could be obtained using the Flow Label field of the IPv6 header. The details will be provided in the future.

3. IP-level Security, under either IPv4 or IPv6, may encrypt the entire transport header, hiding the port numbers of data packets from intermediate routers.

   A small extension to RSVP for IP Security under IPv4 and IPv6 is described separately in [RFC 2207].

RSVP messages carrying reservation requests originate at receivers and are passed upstream towards the sender(s). Note: in this document, we define the directional terms "upstream" vs. "downstream", "previous hop" vs. "next hop", and "incoming interface" vs "outgoing interface" with respect to the direction of data flow.

At each intermediate node, a reservation request triggers two general actions, as follows:

1. Make a reservation on a link

   The RSVP process passes the request to admission control and policy control. If either test fails, the reservation is rejected and the RSVP process returns an error message to the appropriate receiver(s). If both succeed, the node sets the packet classifier to select the data packets defined by the filter spec, and it interacts with the appropriate link layer to obtain the desired QoS defined by the flowspec.

   The detailed rules for satisfying an RSVP QoS request depend upon the particular link layer technology in use on each interface. Specifications are under development in the ISSLL Working Group for mapping reservation requests into popular link layer technologies. For a simple leased line, the desired QoS will be obtained from the packet scheduler in the link layer driver, for example. If the link-layer technology implements its own QoS management capability, then RSVP must negotiate with the link layer to obtain the requested QoS. Note that the action to control QoS occurs at the place where the data enters the link-layer medium, i.e., at the upstream end of the logical or physical link, although an RSVP reservation request originates from receiver(s) downstream.

2. Forward the request upstream

   A reservation request is propagated upstream towards the appropriate senders. The set of sender hosts to which a given reservation request is propagated is called the "scope" of that request.

   The reservation request that a node forwards upstream may differ from the request that it received from downstream, for two reasons. The traffic control mechanism may modify the flowspec hop-by-hop. More importantly, reservations from different downstream branches of the multicast tree(s) from the same sender (or set of senders) must be " merged" as reservations travel upstream.

When a receiver originates a reservation request, it can also request a confirmation message to indicate that its request was (probably) installed in the network. A successful reservation request propagates upstream along the multicast tree until it reaches a point where an existing reservation is equal or greater than that being requested. At that point, the arriving request is merged with the reservation in place and need not be forwarded further; the node may then send a reservation confirmation message back to the receiver. Note that the receipt of a confirmation is only a high-probability indication, not a guarantee, that the requested service is in place all the way to the sender(s), as explained in Section 2.6.

The basic RSVP reservation model is "one pass": a receiver sends a reservation request upstream, and each node in the path either accepts or rejects the request. This scheme provides no easy way for a receiver to find out the resulting end-to-end service. Therefore, RSVP supports an enhancement to one-pass service known as "One Pass With Advertising" (OPWA) [OPWA95]. With OPWA, RSVP control packets are sent downstream, following the data paths, to gather information that may be used to predict the end-to-end QoS. The results ("advertisements") are delivered by RSVP to the receiver hosts and perhaps to the receiver applications. The advertisements may then be used by the receiver to construct, or to dynamically adjust, an appropriate reservation request.