Configure CRTP (RTP header compression)

CRTP Configuration Procedure

CRTP Introduction

Enable CRTP on links

Change CRTP maximum connection number

Display CRTP compression information

CRTP debug

Configuration example

 

This chapter mainly explains how to configure RTP header compression (CRTP) on serial links using PPP. For detailed command information, see RTP header compression (CRTP) Commands.

 

CRTP Configuration Procedure

CRTP configuration procedure consists of the following steps.

Enable CRTP on links

Change CRTP maximum connection number

Display CRTP compression information

CRTP debug

Note: CRTP must be configured on both ends of a serial link, or CRTP will not work.

 

CRTP Introduction

Currently, our CRTP supports serial links encapsulated as PPP, Frame Relay, HDLC.

We need to configure CRTP under the following conditions:

On low speed serial links

On serial links that need to save bandwidth

Under the above conditions, configuring CRTP is a very effective solution. Please see the following illustrations:

Figure 1

Ideally, IP header without options is 20 bytes, and the minimum length of RTP header is 12 bytes, adding an 8-byte UDP header, so the total number of bytes is 40; if the RTP payload to be sent is G.729a, which carries two frames per time, the payload part is 20 bytes; adding 4 bytes encapsulated by PPP link layer, the payload is only 31.25%.

After configuring CRTP and ignoring UDP checksum, the IP/UDP/RTP header of every packet can be compressed to 2 bytes, adding 4 bytes encapsulated by PPP link layer, the payload is 76.92%, which is much higher than before.

Note: it should not be used on links whose bandwidth is above 2M.

 

Enable CRTP on links

To enable CRTP on link layers encapsulating PPP, you can use the following command: (make configuration in the interface mode, and both ends of a link should be configured)

Command

Function

ip rtp header-compression [{ cisco-format | iphc-format | passive}]

Enable CRTP

Among the optional parameters, “cisco-format” is the default value, which means that if CRTP is applied to a PPP link, IPCP option will adopt Cisco’s packet format; “passive” means that header compression begins only on receiving CRTP packets sending from the other end, and if it is applied to a PPP link, IPCP option will also adopt Cisco’s packet format; “iphc-format” means that if CRTP is applied to a PPP link, IPCP option will adopt RFC2509’s packet format.

 

Change CRTP maximum connection number

command

Function

ip rtp compression-connections number

Configure the maximum connection number of local CRTP

The above command needs to be configured in the interface mode. CRTP maintains a structure locally to save connection information for every pair of transfer addresses, if the connection number is set too low, there won’t be enough structures to handle all RTP sessions that are established simultaneously, which affects the compression effectiveness.

 

Display CRTP compression information

command

Function

show ip rtp header-compression [type number] [detail]

Display CRTP information

The above command needs to be configured in the global mode.

CRTP debug

Command

Function

debug ip rtp header-compression

Display the incoming and outgoing CRTP packet information

The above command needs to be used in the global mode.

 

Configuration example

The following example shows how to configure CRTP command on serial links encapsulating PPP:

Router_config#interface serial 1/2

Router_config_s1/2#ip rtp header-compression

Router_config_s1/2#ip rtp compression-connections 25

Router_config_s1/2#encapsulation ppp

Router_config_s1/2#