Use multiple ports for rtp stream

[rkday]

(Originally https://sourceforge.net/p/sipp/bugs/141/)

I believe I've encountered a bug using sipp 3.3 as a UAS with the -rtp_echo flag under high call volumes. Essentially, after about 200-300 simultaneous calls, the DUT is unable to establish more streams to the single UDP port of 6001, resulting in a discrepancy in the TX/RX bandwidth. I believe this only occurs when connection tracking is enabled on the DUT. Unfortunately, there's no way to disable connection tracking on the DUT. However, the overall call flow scenario isn't very realistic, where the RTP streams for all calls are destined to a single port on the UAS side (that is, the 200 OK from the UAS has the same media port). Is there a way to have the rtp_echo dynamically allocate ports? I have tried doing a custom UAS scenario where the 200 ok uses the [auto_media_port] variable in the SDP in hopes of having the rtp_echo parameter use that rather than the static port, but that did not work, nor did supplying the "-t un" option to the UAS scenario or using multiple "-mp 5555 -mp 4444" values.

[bokrenok]

Just posted a pull-request for the issue: https://github.com/SIPp/sipp/pull/278