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MPLS congestion control

Can anyone show me how congestion control is implemented over MPLS network .

Is there any important different to TCP congestion control in traditional IP network

I'm looking for a ns2 forum (network simulator).Anyone know about it please give me a link

Thanks in advance!!!

3 REPLIES
Purple

Re: MPLS congestion control

Hi,

At the edge of an MPLS network, IP headers are still visible and so, you can perform congestion avoidance mechanisms such as RED.

Within the core of MPLS networks, P routers will typically only look at the MPLS headers. Therefore, you cannot use TCP congestion avoidance mechanisms within the core. The P routers, do however, manage congestion based on EXP values within the MPLS header. RED is typically not all that useful within the core anyway since you are likely to have high-speed core links that do not congest all that much anyway.

Hope that helps - pls do rate the post if it does.

Paresh

Re: MPLS congestion control

Re: MPLS congestion control

Hello,

first to make sure we are talking about the same:

1) congestion management (queueing) in MPLS networks offers the same mechanisms as in IP networks.

2) congestion avoidance (WRED) is slightly different. This has to do with TCP, and I assume you called it congestion control.

Well, there are two "flavours" of WRED. The first one simply randomly drops packets to rely on the reduction of TCP window size to avoid congestion. This will obviously also work in MPLS networks. The main problem you are facing though is classification into TCP/non-TCP traffic. There is no point to use WRED on UDP. An MPLS LSR however does not "know" what he is transporting. So you might use WRED on the wrong traffic resulting in worse network behaviour.

The second option in WRED is to set the ECN bit in the TOS byte. There is a newer RFC (3468?) suggesting this instead of dropping the packet right away. This can not be done by a MPLS LSR, because it will not look at the IP header for forwarding decision.

Hm. on a second thought it could be done theoretically. Though not doing forwarding based on IP header info, CEF could still set ECN.

Anyhow, WRED can be configured and packets can be dropped randomly to avoid congestion. ECN is not supported though (and not widely used anyhow) right now to my knowledge.

Hope this helps! Please rate all posts.

Regards, Martin

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