Why does only "mp" have a value?

[worley@…]

There seems to be no reason to restrict the "mp-value", the information regarding the predecessor request-URI to a newer request-URI, to only "mp" mappings. In particular, if a redirect server implements an "rc" mapping, it could be difficult for the UAS to determine why it received the request:

   Alice   atlanta.example.com  biloxi.example.com   Carol
   |                |                |                |    
   |   INVITE sip:bob@example.com    |                |    
   |--------------->|                |                |    
   | History-Info: <sip:bob@example.com>;index=1      |
   |                |                |                |    
   |                |   INVITE sip:bob@example.com    |  
   |                |--------------->|                |    
   | History-Info: <sip:bob@example.com>;index=1      |
   | History-Info: <sip:bob@example.com>;index=1.1    |
   |                |                |                |    
   |                |   300          |                |    
   |                |   Contact: <sip:carol@example.com>;mp
   | History-Info: <sip:bob@example.com>;index=1      |
   | History-Info: <sip:bob@example.com>;index=1.1    |
   |                |<---------------|                |    
   |                |                |                |    
   |                |   INVITE sip:carol@example.com       
   |                |--------------->|                |    
   | History-Info: <sip:bob@example.com>;index=1      |
   | History-Info: <sip:bob@example.com>;index=1.1    |
   | History-Info: <sip:carol@example.com>;index=1.2;mp=1.1
   |                |                |                |    
   |                |   300          |                |    
   |                |   Contact: <sip:carol@192.0.2.3>;rc
   | History-Info: <sip:bob@example.com>;index=1      |
   | History-Info: <sip:bob@example.com>;index=1.1    |
   | History-Info: <sip:carol@example.com>;index=1.2;mp=1.1
   |                |<---------------|                |    
   |                |                |                |    
   |                |   INVITE sip:carol@192.0.2.3    |
   |                |-------------------------------->|    
   | History-Info: <sip:bob@example.com>;index=1      |
   | History-Info: <sip:bob@example.com>;index=1.1    |
   | History-Info: <sip:carol@example.com>;index=1.2;mp=1.1
   | History-Info: <sip:carol@192.0.2.3>;index=1.3;rc |
   |                |                |                |    

(In this case, Carol might know to look for sip:carol@example.com and trace its origin, but if we add a further retargeting and registration lookup cycle for a different user in the middle, we can defeat that strategy.)

It would seem useful to allow the "rc" parameter to have a value as well. And given that some 3xx Contacts will have neither "mp" or "rc", but the "origin" value would still be useful for a UAS to see, it seems desirable to split off the "origin" value to another parameter which would be applied regardless of whether "mp" or "rc" applies.