- Resource Reservation Protocol (RSVP)
RSVP is application-based -- applications on Windows clients initiate it through
WinSock. A workstation called a receiver sets up relationships between itself
and all the devices (usually routers) along the network connection path to the host
(called a sender). RSVP sends messages to the connecting components to find
how much latency there is between, and even among, these devices. A reservation request
is then set up to allocate bandwidth "space" along the route that the data will travel.
Subsequent RSVP relationships can be built with the same host by other
workstations.
RSVP, a signaling protocol, can also ride inside other protocols, as I describe
below.
Differentiated Services (DiffServ) is a method that's used to mark packets as needing
priority; unlike RSVP, DiffServ does not use time-domain cognizance. Routers between
two communicating hosts can respect the need for priority, and many made after 1997 do.
Examples of DiffServ markings are "Assured Service" and "Preferred Service."
It's possible to link RSVP relationships and allow them to "ride" DiffServ-enabled
paths: essentially a merger of the two protocols. RSVP is initiated by a receiver, as
described above, and its packets are then boosted by the marking behavior of DiffServ.
DiffServ can give aggregated priority to RSVP relationships that are built -- without
knowledge of the application -- as a function of a router-to-router
relationship.
Another marking protocol supported by Windows 2000, 802.1p is a virtual LAN protocol.
802.1p allows for prioritization of packets in switched-network (usually Ethernet)
fabrics; it's also supported in most routers. Because switches were intended to be raw
packet-forwarding devices, initial switch implementations did not prioritize packets
for forwarding -- 802.1p does. Such marking doesn't allow devices in the delivery chain
to have a respect for time domains (as RSVP does). Instead, it's the first boost given
to data through switched fabrics.
There are applications considerations that enable QoS, and there are policies that
can be used with Windows 2000 that can have a direct bearing on how QoS is used. I'll
tell you how and why in my next column, which will begin a series called "Networking by
Policy."
