3Com 10014299 manual Congestion, Management Policy, Fifo Queuing, Priority Queuing, Custom Queuing

680CHAPTER 49: CONGESTION MANAGEMENT

Figure 217 Schematic diagram of the congested network

P

C

100 M

DDN/FR/ISDN/PSTN	RouterB	Router	P
			C
	10 M	Ethernet

Occurrence of

congestion

	Company LAN2
RouterA	Server

Ethernet

Server Company LAN1

Congestion	When the congestion occurs, if not enough memory space is provided to buffer
Management Policy	the packets, some of the packets will be lost. The loss of the packets may cause
Overview	the host or router that is sending the packet to resend this packet because of
	overtime, re-congesting and resending, and so on, thereby causing a vicious circle.
	Therefore, some policies are used to manage network congestion. When
	congestion occurs, the router takes some policies to dispatch the data packets,
	deciding which data packets may be sent first and which ones may be discarded.
	These policies are called the congestion management policy.
	For the congestion management, the queuing mechanism is generally used. When
	congestion occurs, the packet is queued at the router egress by a given policy.
	During dispatching, the order for sending the packet out of the queue is decided
	by a given policy.
FIFO Queuing	In the FIFO mode, the concept of no communication priority and classification is
	adopted. During the use of FIFO, the sending order of data packet from the
	interface depends on the order in which the data packet arrives at this interface, at
	this time, the queuing and de-queuing orders of the packet are the same.
	FIFO provides the basic storage and transmission capabilities.
Priority Queuing	In Priority Queueing (PQ) mode, you can flexibly specify the priority queues which
	the packets enter according to the fields packet length, source address, and
	destination address in the packets header and the interface into which the packets
	will come. The packets belonging to a higher priority queue can be sent first. In
	this way, the most important data can be handled first.
Custom Queuing	In the Custom Queueing (CU) model, according to the user's requirements, the
	traffic can be classified in terms of TCP/UDP port number, ACL and interface type.
	Each type of traffic is allocated with a certain percent of bandwidth. When
	network congestion occurs, the traffic that has high demands on delay (such as
	voice) can obtain reliable service. If a type of traffic cannot occupy all the reserved
	bandwidth, other types of traffic will occupy the reserved bandwidth
	automatically, thus making full use of the resource.