Some practical questions

Unanswered Question
Sep 10th, 2008

Hi Friends,

I have a couple of basic questions that is related to the time a packet takes ...

1. If a router receives a packet on one interface, it first checks the routing table, determines if the route exists and then checks the next hop, finds the exiting interface. What is the average time a router takes to switch a packet from one interface to another interface. Is it somewhere between 5 - 10 ms. I know this also depends on the number of routing table entries you might have, but i just need an average figure.

2. Suppose i have 5 routers connected back to back using a 64 Kbps link. Is there any way i can check what is the round trip delay. The simple PING command can give this information. But practically i want to know how if the delay calculated. I understand there are again multiple delays associated as the packet travese across a network. But in between the links only, how is the delay calculated?

Thanks in advance.

Regards,

Manoj

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Joseph W. Doherty Wed, 09/10/2008 - 09:35

#1 Time per packet depends very much on the performance of the router, and often whether the packet is the first seen within a flow, or a follow on packet. Newer routing implementations try to eliminate the difference between first and follow on packets. Attached is a Cisco reference sheet for many of their routers. This will show the packets per second rate. Take note of the difference between process and fast/CEF swithing performance.

#2 Not sure exactly what you're trying to determine. Are you looking for something like per hop delay? Perhaps using ping with timestamp per hop?

Giuseppe Larosa Wed, 09/10/2008 - 09:47

Hello Manoj,

1) modern routers and multilayer switches use route caching strategies and the more current CEF builds its entries based on topology so time required to build the output packet is just the time of packet rewrite.

2) the time it takes to have a packet travel is the sum of:

serialization delay : time the bits need to travel on the link

queueing delay: the packet ready to be sent out is put in an output queue waiting to be sent to the physical interface

processing delay: the time described by operations made at step 1

When you make a latency measure you measure the sum of all the times on all the nodes that are crossed.

Latency tests are performed by inserting a timestamp inside the packet when sent (typically using some traffic generator like SMARTBITS or others) at end of the chain if you are in a lab there is another interface of the same instrument and so the reception timestamp is compared with the timestamp contained in the packet (it is the same system clock): measures are cumulated over an high number of packets to have statistic value.

see RFC 1242 and following ones like RFC 2432 for exact definitions of terms and methodologies.

Hope to help

Giuseppe

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