Insert header to identify a request ?

In our ongoing efforts to troubleshoot some intermittent slow application response times, I'm looking for a way to positively and uniquely identify a particular transaction and follow it through our software stack.

 

 

One suggestion was to insert a header, with a 'transaction identifier' .

 

 

While I have no trouble inserting the header, my question is about what type of data could I use as a unique identifier ? Is there any way to calculate or attach a 'GUID' like identifier ?

 

Could I use a SSL Session ID ? Or is there some other good (easy ) way to do this ?

 

 

 

As alawyas, I appreciate everyone's input !