TCP/IP Tutorial and Technical Overview - IBM Redbooks

obeying a token bucket (r,b) and being served by a line with bandwidth R is
bounded by b/R as long as R is not less than r. Guaranteed Service
approximates this behavior with the service rate R, where R is now a share of
bandwidth through the routing path and not the bandwidth of a dedicated line.
In the Guaranteed Service model, Tspec and Rspec are used to set up a flow
reservation. The Tspec is represented by the token bucket parameters. The
Rspec contains the parameter R, which specifies the bandwidth for the flow
reservation. The Guaranteed Service model is defined in RFC 2212.
8.2.4 The Resource Reservation Protocol (RSVP)
The Integrated Services model uses the Resource Reservation Protocol (RSVP)
to set up and control QoS reservations. RSVP is defined in RFC 2205 and has
the status of a proposed standard. Because RSVP is an Internet control protocol
and not a routing protocol, it requires an existing routing protocol to operate. The
RSVP protocol runs on top of IP and UDP and must be implemented in all
routers on the reservation path. The key concepts of RSVP are flows and
reservations.
An RSVP reservation applies for a specific flow of data packets on a specific path
through the routers. As described in 8.1, “Why QoS?” on page 288, a flow is
defined as a distinguishable stream of related datagrams from a unique sender
to a unique receiver. If the receiver is a multicast address, a flow can reach
multiple receivers. RSVP provides the same service for unicast and multicast
flows. Each flow is identified from RSVP by its destination IP address and
destination port. All flows have dedicated a flow descriptor, which contains the
QoS that a specific flow requires. The RSVP protocol does not understand the
contents of the flow descriptor. It is carried as an opaque object by RSVP and is
delivered to the router's traffic control functions (packet classifier and scheduler)
for processing.
Because RSVP is a simplex protocol, reservations are only done in one direction.
For duplex connections, such as video and audio conferences, where each
sender is also a receiver, it is necessary to set up two RSVP sessions for each
station.
The RSVP protocol is receiver-initiated. Using RSVP signalling messages, the
sender provides a specific QoS to the receiver, which sends an RSVP
reservation message back, with the QoS that should be reserved for the flow,
from the sender to the receiver. This behavior considers the different QoS
requirements for heterogeneous receivers in large multicast groups. The sender
does not need to know the characteristics of all possible receivers to structure
the reservations.
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