[e2e] end2end-interest Digest, Vol 18, Issue 9
detlef.bosau at web.de
Thu Aug 18 14:15:51 PDT 2005
>>>>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
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
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
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.
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