How Does FRTS Work?
FRTS works essentially the same as GTS. It uses a token bucket, or credit manager,
algorithm to service the main queuing mechanism and send packets out the
interface. It also is commonly used to overcome data-rate mismatches.The most
frequently seen instance of this is in a hub and spoke environment where the
head-end (hub) has a large amount of bandwidth (perhaps a T1) and the spokes
have much smaller amounts of bandwidth (perhaps, 128k each). In this case, if a
single remote site is being sent 200k of traffic, the remote site will have a completely
saturated line but, because of the speed mismatch, the head-end router’s
interface will not see congestion. Recall that queuing mechanisms will only kick
in when there is congestion, so we need a mechanism to create congestion at the
head-end. FRTS does have some unique characteristics that we should explore
before proceeding:
Enhanced queuing support on a per VC basis—both PQ and CQ are
available
Traffic selection using ACLs
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How Do FECNs and BECNs Work?
Forward explicit congestion notification (FECN) and backwards explicit
congestion notification (BECN) are used in networks by intermediary
nodes to inform other nodes of congestion that was experienced as a
packet traveled across the network. In Frame Relay, setting a specific bit
in a normal Frame Relay packet indicates a FECN or BECN. Here’s how
it works.
If device A is sending data to device B across a Frame Relay infrastructure,
and one of the intermediary Frame Relay switches encounters
congestion (congestion being full buffers), an over-subscribed port,
overloaded resources, and so forth, it will set the BECN bit on packets
being returned to the sending device (A), and the FECN bit on the
packets being sent to the receiving device (B). This has the effect of
informing the sending router to slow down and apply flow control, such
as traffic shaping, and informing the receiving device that the flow is
congested and that upper layer protocols should expect some delays.
Designing & Planning…
256 Chapter 8 • Advanced QoS for AVVID Environments
Rate enforcement on a per VC basis—the mean rate can be set to match
CIR or some other value
FRTS supports both BECN and Cisco Foresight congestion notification
on a per VC basis
Notice that WFQ is not available (see the following note for the IOS release in
which it is available), but PQ and CQ are configurable on a per VC basis.This
means that you are not limited to one queuing mechanism for the whole interface,
but you can pick the queuing method that suits each VC the best. Additionally, by
using ACLs, you can direct traffic to separate VCs, creating a virtual time-division
multiplexing (TDM) network.This method may not make the most efficient use of
your purchased bandwidth if you pay by CIR, since if there is no incoming traffic
for a particular traffic type, the associated VC will be basically empty.
Another approach that would make more efficient use of bandwidth would
be to divide your traffic into classes on a single VC. For example, suppose
DECnet was a critical application across your Frame Relay network. Using PQ,
you could classify all your DECnet traffic into the high priority queue, while
classifying other traffic into lower ones. Since all the packets in the high priority
queue would be serviced before the lower priority queues, you would ensure that
DECnet packets would not be delayed unduly by other traffic.
Still another approach would be to divide your traffic into classes and use CQ
to give each a guaranteed bandwidth percentage.This has the benefit over multiple
VCs and the virtual TDM network of allowing a class’s reserved bandwidth
to be used by other classes when available.