14 Chapter 1: Quality of Service: An Overview
Definition of Integrated Services
Integrated services (IntServ) is the name given to QoS signaling. QoS signaling allows an
end station (or network node, such as a router) to communicate with its neighbors to request
specific treatment for a given traffic type. This type of QoS allows for end-to-end QoS in
the sense that the original end station can make a request for special treatment of its packets
through the network, and that request is propagated through every hop in the packet’s path
to the destination. True end-to-end QoS requires the participation of every networking
device along the path (routers, switches, and so forth), and this can be accomplished with
QoS signaling.
In 1994, RFC 1633 first defined the IntServ model. The following text, taken from RFC
1633, provides some insight as to the original intent of IntServ:
We conclude that there is an inescapable requirement for routers to be able to reserve resources, in order to
provide special QoS for specific user packet streams, or “flows”. This in turn requires flow-specific state in
the routers, which represents an important and fundamental change to the Internet model.
As it turns out, the requirement was not as inescapable as the engineers who authored RFC
1633 originally thought, as evidenced by the fact that the Internet still relies almost entirely
on BE delivery for packets.
IntServ Operation
Resource Reservation Protocol (RSVP), defined by RFC 2205, is a resource reservation
setup protocol for use in an IntServ environment. Specifics of operation are covered shortly,
but the general idea behind RSVP is that Bob wants to talk to Steve, who is some number
of network hops away, over an IP video conferencing (IPVC) system. For the IPVC conver-
sation to be of acceptable quality, the conversation needs 384 kbps of bandwidth.
Obviously, the IPVC end stations don’t have any way of knowing whether that amount of
bandwidth is available throughout the entire network, so they can either assume that
bandwidth is available (and run the risk of poor quality if it isn’t) or they can ask for the
bandwidth and see whether the network is able to give it to them. RSVP is the mechanism
that asks for the bandwidth.
The specific functionality is probably backward from what you would guess, in that the
receiver is the one who actually asks for the reservation, not the sender. The sender sends a
Path message to the receiver, which collects information about the QoS capabilities of the
intermediate nodes. The receiver then processes the Path information and generates a
Reservation (Resv) request, which is sent upstream to make the actual request to reserve
resources. When the sender gets this Resv, the sender begins to send data. It is important to
note that RSVP is a unidirectional process, so a bidirectional flow (such as an IPVC)
requires this process to happen once for each sender. Figure 1-5 shows a very basic example
of the resource reservation process (assuming a unidirectional flow from Bob to Steve).