I'm currently doing some latency timings on some code I've written.

I'm trying to determine the time taken from receipt of a packet by the network device driver until the packet is handled by my code.

To do this, I am going to send a stream of known TCP messages to the machine. I will then modify the device driver to add a time-stamp to the data section of all incoming TCP messages. This can then be compared with with the system time when the packet is received by my application.

I have found the location in the device driver where it allocates incoming packets to the sk_buff structure, but I have two problems to overcome before I can implement my proposed solution.

1) When is the transport protocol of an incoming packet determined?
2) How can I determine the transport protocol of an sk_buff structure?

Thanks in advance for any help,


Will

1. If I read stuff right, right after the dev driver yanks the data into the buffer it calls netif_rx* which timestamps it and places it in the queue.
2. Dunno.
0, 1, 2, and netfilter doc stuff is what I'm looking at.

Can I see this time stamp from the application layer? For example, I'll have an open socket, which will trigger a select statement, which I'll then go to read from.

At that point, how do I get hold of the timestamp put in earlier?


Thanks for your help, this is the first time I've had to look at the Linux low-level networking - and at first it can be slightly daunting!