An Investigation into Transport Protocols and Data Transport ...

5.3. Network Aid in Congestion Detection 97
tion. When a queue overflows packets are dropped, and reliable transport
protocols, such as TCP, will attempt to retransmit missing data.
The design of router queue provisioning is often not to increase the utilisation
of a single flow going through their network, but that of the aggregate.
Given this and the limited resources of physical memory, it is often
not possible to provision large buffers required for high throughput, high
latency transport as required by Equation 4.1 and Equation 5.4.1. Also,
with the increased popularity of VoIP and similarly delay sensitive flows
[KBS + 98, JS00], it may not be feasible to utilise a large buffer, which is necessary
for efficient high throughput transport due to the increased end-to-end
latencies of large router queues. More important, however, is that a small
queue is insufficient to handle the bursts of packets that are commonplace
on the Internet [LTWW94].
This section looks at providing feedback and queueing disciplines from
routers that may improve the performance of TCP.
5.3.1 Explicit Congestion Notification
In TCP, a packet loss is an implicit notification of congestion. Through the
signaling of an Explicit Congestion Notification (ECN) [RFB01] packet, the
TCP sender should reduce the sending rate to mitigate the possibility of
congestion.
Designed to be complemented with Active Queue Management techniques
(see Section 5.3.2) on routers, ECN is implemented as the last two bits of
the IPv4 TOS octet in the TCP header and implemented such that TCP can
respond to congestion without actually suffering from packet losses.
When experiencing the incipient stages of congestion, an ECN enabled