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Miroslaw Polski

PFR - delay - TCP Handshake via separate BRs at a branch


My customer has 2 routers in a branch (each of them connected to a different MPLS VPN cloud).

The Customer would like a traffic for a specific app to take the path that does not exceed 200ms delay (I mean the outbound traffic from the branch to the data center).

Although the app is a typical client-server app, there's a fair amount of traffic sent from branches to servers.

For simplicity, let's say there's a single router at a hub connected to two MPLS VPN clouds and two routers at a branch.

Regarding the delay, MC (at the branch) will look into the time between an outgoing SYN (from the client) and a returning SYN/ACK (from the server to the client) and compare this to the 200ms threshold.

How can the MC at the brach tell what the delay is, in case of traffic (the outgoing SYN) going out via router A at the branch, but coming back (SYN/ACK) to the branch via router B?

IMO MC won't see SYN - SYN/ACK relation for a given session due to TCP handshake packets being delivered via different BRs?



Joseph W. Doherty
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Yes, I believe you're correct if you're only doing passive analysis.  However, active analysis will source from external interface of each BR and generally best path back to that interface is most direct to that BR.