Computing hop count in TcpSessionApp

[phdavis1027]

How do I do basically this in modern versions of inet, except in TcpSessionApp? My first idea was to extend TcpSessionApp and override socketDataArrived but the Packet class seems to be kind of opaque. Really, I would just like to be pointed in the direction of utilities for peeking inside of packets.

[rhornig]

@statistics can use packet filters to provide statistics about specific packet attributes. Check the ResultFilters.h | .cc files. The messageAge and dataAge filters do something very similar, you want to achieve (let's call it dataHopCount. Then, you have to add a statistic to the TcpSessionApp that is similar to the endToEndDelay statistic. Just it should use your dataHopCount filter. The data you need to access in the result filter implementation is available in the HopLimitInd indicator tag added during the IPv4 decapsilation.

[phdavis1027]

This is quite useful! I'm working on following your suggestion. Just out of curiosity, if I get this working, would there be interest in a PR to include hopCount as a statistic in TcpSessionApp by default?

[phdavis1027]

Quick followup: since I will be reusing the modules that collect HopCount across multiple simulations, I tried to break them out into a common dir which I passed to opp_makemake with the -I flag. If I build and run with this structure, the generated makefile doesn't build a HopCounterFilter.o . However, if I build and run with HopCounterFilter.{c, h} in the project directory, it compiles and runs just fine. Is there are a way to tell the makefile about common?

[rhornig]

It depends, where that common dir is located. If it is under the /src folder, then the IDE will scan and include all .cc files automatically. From command line, you must re-execute opp_makemake to re-create a makefile that includes your new files.

• If the common dir is outside of the src folder and you want to share the code between projects this pretty much creates a shared library with your common code. This might be pretty tricky to set up correctly especially if you want to build it from both the IDE and the command line.
• If you just need a solution that works in house, I would just symlink the common directory somewhere inside the src folder at the appropriate place (in all directories where you use).
• or a variation of this: put your common code into a separate git repo and create a git submodule from it inside the src tree. This is a bit more involved, that a simple simlink, but it would work for other people too...

So, if you just want to use the project yourself, go with symlinking.

As for the PR: I'm not sure how useful would be this feature for the general public. I mean, at the end of the day, you will get an average hop count for a tcp stream (or for all TCP streams in that app?). But that could also be calculated at the network layer. It all depends on how complicated the code. If it's 3-4 well placed lines, we can look into this, but on long term we have to maintain that code, so anything longer is not necessary justified.