How to troubleshoot the problem of one way audio in calling when H.323 uses AR28-31+Eudemon2100 to realize NAT

Publication Date:  2012-07-27 Views:  333 Downloads:  0
Issue Description
At a site, H.323 terminal uses AR28-31(Version: VRP3.4)Eudemon2100(Version: VRP3.30-0318) to realize NAT, and the router AR28-31 is responsible for NAT translation, and then realize outgoing and incoming calls via proxy by Eudemon2100. However, when H.323 terminal calls the users under softx3000, there is only one way audio.

Alarm Information
No

Handling Process

1. Capture packets at downlink and uplink ports of Eudemon2100 for analysis; it is found the port number is false;



2. AR28-31 cannot shut down NAT ALG functionality, so it is replaced;



3. Re-configure NAT translation for the router, and calls return to normal. 



Root Cause
According to packets captured at Eudemon2100, it is found that the NAT of AR28-31 provides the functionality of ALG. When negotiating the port for media, H245 stipulates the port of RTCP is required, and the ports of RTCP and RTP are optional at the same time. If only RTCP port is provided, the value of RTP port is that of RTCP minus 1; however, the ALG is not standard in realization: when translating the address port between RTP and RTCP, the value of RTP port (13460) translated is the value of RTCP (13459) plus 1. When Eudemon2100 processes the packets from RTCP address port, it will follow the relevant protocol to reduce 1 from the value of RTCP port to have that of RTP port (13458). The result differs to the actual RTP port (13460), resulting in failure of matching of SESSION tables, with SESSION entries as follows:

Media ClassNo is: 0 ,Sub Media ClassNo is :0



[211.103.80.211:13460 211.103.80.210:18054]----->[211.103.80.196:18054



211.103.90.19:13760]



Session table attribute is: un-first, RTP, valid, active, No



Traverse, TrvsClientId = 0, Policy Route, OutIfIndex = 642, NextHopIpAddr =



211.103.80.193, up , 0x97580000



Media ClassNo is: 0 ,Sub Media ClassNo is :0



[211.103.90.19:13760 211.103.80.196:18056]----->[211.103.80.210:18054



211.103.80.211:13458]



Session table attribute is: un-first, RTP, valid, active, No



Traverse, TrvsClientId = 0, Policy Route, OutIfIndex = 770, NextHopIpAddr =



211.103.80.21



1, down, 0x974c0000

Thus, Eudemon2100 transmits the downlink media traffic to false address port (13458), resulting in one way audio. 

Suggestions
In NGN, it is very normal to use NAT in calling. When troubleshooting this kind of networking problems, please focus on analysis of packets captured, and locate the problem with the help of debug information.

END