using packet filtering to discover TCP end-to-end latency

using packet filtering to discover TCP end-to-end latency

[Rohit Grover]

Hello,

I need to discover the TCP end-to-end latency between web clients and  
servers transparently (i.e. without altering the clients or the  
servers in any way). I have squid running as a proxy server on a  
linux box, sitting between the clients and the servers. I can have  
the clients change their proxy settings to have requests pass through  
squid. Squid can measure latencies between itself and the web  
servers. The problem is to measure the network latency when squid  
tries to forward the server responses back to clients.

Can packet filtering help me with this? Can I, for example, filter  
for the client ACK of the last bit of data sent from squid back to  
the client?

Also, I still need to device a solution to measure network latency  
experienced by the client's SYN sent to the squid box. Any ideas?

thanks in advance,
Rohit Grover.

Re: using packet filtering to discover TCP end-to-end latency

[Martijn Lievaart]

Rohit Grover wrote:
> Hello,
>
> I need to discover the TCP end-to-end latency between web clients and 
> servers transparently (i.e. without altering the clients or the 
> servers in any way). I have squid running as a proxy server on a linux 
> box, sitting between the clients and the servers. I can have the 
> clients change their proxy settings to have requests pass through 
> squid. Squid can measure latencies between itself and the web servers. 
> The problem is to measure the network latency when squid tries to 
> forward the server responses back to clients.
>
> Can packet filtering help me with this? Can I, for example, filter for 
> the client ACK of the last bit of data sent from squid back to the 
> client?
>
> Also, I still need to device a solution to measure network latency 
> experienced by the client's SYN sent to the squid box. Any ideas?
>

I would hire a packeteer for this. Expensive, but is perfect for this 
job. Otherwise, capture with wireshark (formerly Ethereal), export to 
csv, write a small program. (I don't think wireshark has a plugin for 
exactly this).

I would NOT do this with packet filtering unless I need (near) realtime 
analysis. And even then.

HTH,
M4