[e2e] end2end-interest Digest, Vol 18, Issue 9

Detlef Bosau detlef.bosau at web.de
Thu Aug 18 14:15:51 PDT 2005

Alok wrote:
>>>>Personally, I am in great doubt at this.
>>RTT delay is influenced by the following factors:
>>1. Speed of light delay in the path
>>2. Retransmissions in the underlay
>>3. Queues in buffers due to
>>    a. self queueing (queueing behind your own packets)
>>    b. queueing due to cross traffic
> Do routers/ATM switches use queues for "congestion control" or because most
> of their cards and backplanes are asynchronous?

To my understanding, queues have two purposes.

1. Rate adaptation, this includes adaptation of a flow to possible MAC 

2. Interleaving/Mixing of flows.

Basically, these two are 3a and 3b in Keshav´s post. So, to answer your 
question: In a packet switching system congestion takes place in queues 
of store & forward nodes, especially when incoming and outgoing lines 
are asynchronous.

I´m hesitant to make too much words here, because each word may be wrong.

A very helpful rerefence is Raj Jain´s paper "A Delay-Based Approach for 
Congestion Avoidance in Interconnected Heterogenous Computer Networks".
I always found this work helpful to understand the role of switches/routers.

For congestion control itself, there are two "extreme positions" and, as 
in most cases where extreme positions exist, combinationes and middle 

The first position is a strict End to End approach: Routers don´t care 
about congestion. If a queue runs out of space, there´s no alternative 
left for a router than to discard a packet. In this extreme view: 
_Silently_ discard a packet. Consequently, end systems must react upon 
packet loss / congestion notification appropriately.

Look at the congavoid paper for this approach.

The second position is a continous control of each flow hop by hop. 
Spoken very simplified: We do traffic shaping on each node, in a well 
controlled manner.

I think (I must be careful here, I had a glance at this quite a long 
time ago, so forgive me if I´m wrong or unprecise here) this appproach 
is discussed in Keshav´s PhD thesis.

If we take the second position: Yes, routers and switches use queues for 
congestion control.

For middle courses and approaches "in between" think of active queue 
managemet and RED. And of course quite a number of PEP approaches, which 
often interconnect packet switching networks where congestion control is 
difficult to achieve using identical algorithms, e.g. (error-)loss free 
networks and lossy networks as for example 802.11 networks.

Detlef Bosau
Galileistrasse 30
70565 Stuttgart
Mail: detlef.bosau at web.de
Web: http://www.detlef-bosau.de
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