Guest

Cisco IOS Software Releases 12.1 T

Field Notice: RTP Payload Bytes Mismatch Between Some H.323 Gateways


March 4, 2002


Products Affected

Product

Comment

Cisco IOS H.323 Gateways

RTP payload bytes mismatch between gateways running Cisco IOS Software Release 12.1(5)T (and later) and gateways running Cisco IOS prior to version 12.1(5)T.

Problem Description

Cisco H.323 gateways running Cisco IOS® Release 12.1(5)T or later may create Real-Time Protocol (RTP) packets with payloads smaller than the configured size. The problem occurs with calls between a Cisco IOS Software Release12.1(5)T or later gateway and a gateway running Cisco IOS earlier than Release 12.1(5)T.

Background

Before Cisco IOS Software Release 12.1(5)T, H.323 gateways always honored the local codec bytes configuration command but erroneously advertised a fixed 20 byte payload capability through H.245. This problem was fixed in Cisco IOS Software Release 12.1(5)T. However, the fix introduced a potential payload size mismatch between gateways running pre-12.1(5)T software and those running the newer software.

By default, gateways running Cisco IOS Software Release 12.1(5)T or later always honor the codec bytes payload value negotiated through H.245, ignoring the locally configured value if it is higher. This is the correct behavior. The problem arises when one of these newer gateways attempts to interoperate with a gateway running a Cisco IOS earlier than Release 12.1(5)T. Since pre-version 12.1(5)T gateways always advertise 20 bytes (but use the configured payload value), the newer gateway will negotiate its RTP payload size down to 20 bytes. The newer gateway sends a 20 byte RTP payload while the pre-version 12.1(5)T gateway sends the RTP payload size configured under the dial peer, causing a mismatch.

Problem Symptoms

For calls between a gateway running Cisco IOS Software Release 12.1(5)T or later and a gateway running a Cisco IOS software release prior to 12.1(5)T, the pre-version 12.1(5)T gateway will send RTP payloads equal to the codec bytes configuration but the newer gateway will ignore its configuration and send only 20 byte RTP payloads. This mismatch results in increased RTP overhead and increased bandwidth utilization.

Workaround/Solution

To assist customers who wish to migrate a network of gateways from Cisco IOS Software Release 12.1(5)T or earlier to Cisco IOS Software Release 12.2, Cisco added a special hidden configuration command beginning in Cisco IOS Software Release 12.2(1) and 12.2(1)T. The new command forces the gateway to honor the locally configured codec bytes value (the old behavior). To enable this feature, use the command codec bytes preferred local in voice-class configuration mode. To disable the feature, use the no form of the command. The command has no arguments or keywords.

The command should be added to each gateway configuration as it is being upgraded to Cisco IOS Software Release 12.2. Once all gateways have been upgraded, the command may be removed from all gateway configurations in the network.

To use the new command, you must first create an H.323 voice class. To configure codec bytes preferred local, do the following:

Command

Purpose

Step 1

Router# config term

Enter configuration mode

Step 2

Router(config)# voice class h323 number

Enters voice class mode to create or modify an H.323 voice class

The number argument identifies the H.323 voice class.

Step 3

Router(config-class)# codec bytes preferred local 

Force H.323 to use the locally configured value for G729r8 bytes. This enables compatibility with pre-version 12.1(5)T gateways.

Step 4

Router(config-class)# exit

Exits voice class mode

Next, associate the H.323 voice class with each VoIP dial peer that needs to interoperate with pre-version 12.1(5)T gateways. To associate the H.323 voice class with a dial peer, do the following:

Command

Purpose

Step 1

Router# config term

Enter configuration mode

Step 2

Router(config)# dial-peer voice number voip

Enters dial-peer configuration mode and defines a remote VoIP dial peer.

The number argument is one or more digits identifying the dial peer. The voip keyword indicates a VoIP peer using voice encapsulation on the IP network.

Step 3

Router(config-dialpeer)# voice-class h323 number

Associates the specified H.323 voice class (and all of its related attributes) with the dial peer.

The number argument identifies the H.323 voice class.

The following example shows that calls from VoIP dial peer 712 use the locally configured codec bytes value because the codec bytes preferred local command is configured in the voice class assigned to the dial peer:

dial-peer voice 712 voip
 voice-class h323 2
 destination-pattern 712
 codec g729r8 bytes 40
 session target ipv4:10.10.107.108
!
voice class h323 2
 codec bytes preferred local

DDTS

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DDTS

Description

CSCds89937

Incorrect codec length negotiated

For More Information

If you require further assistance, or if you have any further questions regarding this field notice, please contact the Cisco Systems Technical Assistance Center (TAC) by one of the following methods:

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