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Configuring the 4300T
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clocked out at the WAN link’s full rate LESS the bandwidth currently being
used for high-priority (ie voice) data. High-priority data is clocked out at the
WAN’s full link rate. Any long-lasting burst condition in low-priority data
will cause these packets to be delayed and, if necessary, dropped.
Downstream Traffic Management
Since the 4300T is the final transmitting device for WAN traffic in the upstream
direction (LAN to WAN) it is easy to see how its QoS mechanisms can be
applied to traffic it is transmitting to guarantee sufficient bandwidth for voice
traffic. We have control over how packets are handed to the WAN interface.
In the downstream direction (WAN to LAN) we are installed at the receiving
end of a service provider link and therefore have no control over the amount
of voice or data traffic being sent to us over the WAN interface. How then can
we still guarantee the quality of in-bound voice traffic when it is entirely
possible for an FTP session, for example, to consume the vast majority of
downstream bandwidth?
Fortunately this is possible by shaping on both the egress LAN and egress
WAN ports of the 4300T appliance and leveraging the congestion avoidance
mechanisms built into TCP. Essentially, data packets received by the 4300T’s
WAN interface at a rate that exceeds the T1’s bandwidth LESS the bandwidth
used for active voice calls are delayed (then dropped if necessary) before being
forwarding on to its LAN interface. Similarly, data traffic sent back to the
4300T for transmission to the WAN are also delayed (as described in the above
section). This results in the WAN-based devices following the rules of TCP/IP
congestion avoidance and slowing down their transmit rate. This technique is
quite effective in practice, as end stations usually reduce their transmit rate
before VoIP signaling has completed for new call setup.
For example consider the scenario where there are no voice calls over a WAN
link and multiple FTP sessions are consuming all available bandwidth:
1. A new call request is received by the 4300T from the WAN.
2. All signaling messages for the call are classified as voice traffic and
therefore prioritized for transmission to the LAN before servicing the
inbound FTP data.
3. RTP traffic (the voice data within an ongoing VoIP session) is similarly
classified as voice traffic and treated with priority.
4. FTP data is buffered (or dropped) by the 4300T and return data, including
the FTP ACKs, are also delayed. This results in a throttling of the
transmit rate by the (far-end) FTP hosts, reducing overall WAN
bandwidth consumption.
Generally, excessive UDP traffic must be shaped in the service provider
network, as UDP does not provide congestion avoidance mechanisms. The
exception to this is in the case of RTP UDP-based messages for voice traffic.