9.9 Congestion And Datagram Flow Co

9.9 Congestion And Datagram Flow Control
Because JP is connectionless, a router cannot reserve memory or communication
resources in advance of receiving datagram. As a result, routers can be overrun with
traffic, a condition known as congestion. It is important to understand that congestion
can arise for two entirely different reasons. First, a high-speed computer may be able to
generate traffic faster than anetwork can transfer it. For example, imagine a supercom-
puter generating internet traffic. The datagrams may eventually need to cross a slower-
speed wide area network (WAN) even though the supercomputer itself attaches to a
high-speed local area net. Congestion will occur in the router that attaches the LAN to
the WAN because datagrams arrive faster than they can be sent. Second, if many com-
puters simultaneously need to send datagrams through a single router, the router can ex-
perience congestion, even though no single source causes the problem.
When datagrams arrive too quickly for a host or router to process, it enqueues
them in memory temporarily. If the datagrams are part of a small burst, such buffering
solves the problem. If the traffic continues, the host or router eventually exhausts
memory and must discard additional datagram that arrive. A machine uses ICMP
source quench messages to report congestion to the original source. A source quench
message is a request for the source to reduce its current rate of datagram transmission.
Usually, congested routers send one source quench message for every datagram that
they discard. Routers may also use more sophisticated congestion control techniques.
Some monitor incoming traffic and quench sources that have the highest datagram
transmission rates. Others attempt to avoid congestion altogether by arranging to send
quench requests as their queues start to become long, but before they overflow.
There is no ICMP message to reverse the effect of a source quench. Instead, a host
that receives source quench messages for a destination, D, lowers the rate at which it
sends datagrams to D until it stops receiving source quench messages; it then gradually
increases the rate as long as no further source quench requests are received.
9.10 Source Quench Format
In addition to the usual ICMP TYPE, CODE, CHECKSUM fields, and an unused
32-bit field, source quench messages have a field that contains a datagram prefix. Fig-
ure 9.4 illustrates the format. As with most ICMP messages that report an error, the da-
tagram prefix field contains a prefix of the datagram that triggered the source quench re-
quest.
Sec. 9.10 Source Quench Format 137
Figure 9.4 ICMP source quench message format. A congested router sends
one source quench message each time it discards a datagram; the
datagram prefix identifies the datagram that was dropped.