Configure
CRTP (RTP header compression)
Change CRTP maximum connection number
Display CRTP compression information
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 consists of the following steps.
Change CRTP maximum connection number
Display CRTP compression information
Note: CRTP must be configured on both ends of a serial link, or CRTP will not work.
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.
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 |
|
show ip rtp header-compression [type
number] [detail] |
Display CRTP information |
The above command needs to be configured in the global mode.
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.
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#