Configuring RTP Header Compression

Last Updated: June 24, 2011

Header compression is a mechanism that compresses the IP header in a packet before the packet is transmitted. Header compression reduces network overhead and speeds up the transmission of either Real-Time Transport Protocol (RTP) or Transmission Control Protocol (TCP) packets.

Cisco provides two types of header compression: RTP header compression and TCP header compression. This module describes the concepts and tasks related to configuring RTP header compression.


Note
RTP header compression is configured on a per-interface (or subinterface) basis. If you want to configure RTP header compression on a per-class basis, see the "Configuring Class-Based RTP and TCP Header Compression" module.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the Feature Information Table at the end of this document.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Prerequisites for Configuring RTP Header Compression

  • Before configuring RTP header compression, read the information in the "Header Compression" module.
  • You must configure RTP header compression on both ends of the network.

Information About Configuring RTP Header Compression

Configurable RTP Header-Compression Settings

With RTP header compression, you can configure the maximum size of the compressed IP header, the maximum time between transmitting full-header packets, and the maximum number of compressed packets between full headers. These settings are configured using the following three commands:

  • ip header-compression max-header
  • ip header-compression max-time
  • ip header-compression max-period

The ip header-compression max-header command allows you to define the maximum size of the IP header of a packet to be compressed. Any packet with an IP header that exceeds the maximum size is sent uncompressed.

The ip header-compression max-timecommand allows you tospecify the maximum time between transmitting full-header packets, and the ip header-compression max-period command allows you to specify the maximum number of compressed packets between full headers. With the ip header-compression max-time and ip header-compression max-period commands, the full-header packet is transmitted at the specified time period or when the maximum number of packets is reached, respectively. The counters for both the time period and the number of packets sent are reset after the full-header packet is sent.

For more information about these commands, see the Cisco IOS Quality of Service Solutions Command Reference.

RTP Header-Compression Keywords

When you configure RTP header compression, you can specify the circumstances under which the RTP packets are compressed and the format that is used when the packets are compressed. These circumstances and formats are defined by the following keywords:

  • passive
  • iphc-format
  • ietf-format

These keywords (described below) are available with many of the quality of service (QoS) commands used to configure RTP header compression, such as the ip rtp header-compression command. For more information about the ip rtp header-compression command, these keywords, and the other QoS commands, see the Cisco IOS Quality of Service Solutions Command Reference.

The passive Keyword

By default, the ip rtp header-compression command compresses outgoing RTP traffic. If you specify the passive keyword, outgoing RTP traffic is compressed only if incoming RTP traffic on the same interface is compressed. If you do not specify the passive keyword, all outgoing RTP traffic is compressed.

The passive keyword is ignored on PPP interfaces.

The iphc-format Keyword

The iphc-formatkeyword indicates that the IP Header Compression (IPHC) format of header compression will be used. For PPP and HDLC interfaces, when the iphc-format keyword is specified, TCP header compression is also enabled. Since both RTP and TCP header compression are enabled, both UDP and TCP packets are compressed.

The iphc-format keyword includes checking whether the destination port number is even and is in the ranges of 16,385 to 32,767 (for Cisco audio) or 49,152 to 65,535 (for Cisco video). Valid RTP packets that meet the criteria (that is, the port number is even and is within the specified range) are compressed using the compressed RTP packet format. Otherwise, packets are compressed using the less-efficient compressed non-TCP packet format.

The iphc-formatkeyword is not available for interfaces that use Frame Relay encapsulation.


Note
The header compression format (in this case, IPHC) must be the same at both ends of the network. That is, if you specify the iphc-format keyword on the local router, you must also specify the iphc-format keyword on the remote router.

The ietf-format Keyword

The ietf-format keyword indicates that the Internet Engineering Task Force (IETF) format of header compression will be used. For HDLC interfaces, the ietf-format keyword compresses only UDP packets. For PPP interfaces, when the ietf-formatkeyword is specified, TCP header compression is also enabled. Since both RTP header compression and TCP header compression are enabled, both UDP packets and TCP packets are compressed.

With the ietf-format keyword, any even destination port number higher than 1024 can be used. Valid RTP packets that meet the criteria (that is, the port number is even and is higher than 1024) are compressed using the compressed RTP packet format. Otherwise, packets are compressed using the less-efficient compressed non-TCP packet format.

The ietf-format keyword is not available for interfaces that use Frame Relay encapsulation.


Note
The header compression format (in this case, IETF) must be the same at both ends of the network. That is, if you specify the ietf-formatkeyword on the local router, you must also specify the ietf-format keyword on the remote router.

Enhanced RTP Header Compression

The Cisco IOS Release 12.3(11)T introduced a feature that enhances the functionality of RTP header compression. This feature is called Enhanced CRTP for Links with High Delay, Packet Loss, and Reordering (ECRTP).

The ECRTP feature is also known as Enhanced RTP Header Compression. It includes modifications and enhancements to RTP header compression to achieve robust operation over unreliable point-to-point links. This is accomplished by repeating updates and sending absolute (uncompressed) values in addition to delta values for selected context parameters.

During compression of an RTP stream, a session context is defined. For each context, the session state is established and shared between the compressor and the decompressor. The context state consists of the full IP/UDP/RTP headers, a few first-order differential values, a link sequence number, a generation number, and a delta encoding table. Once the context state is established, compressed packets may be sent.

RTP header compression was designed for reliable point-to-point links with short delays. It does not perform well over links with a high rate of packet loss, packet reordering, and long delays. Packet loss results in context corruption, and because of long delay, packets are discarded before the context is repaired. To correct the behavior of RTP header compression over such links, several enhancements have been made to the RTP header compression functionality. The enhancements reduce context corruption by changing the way that the compressor updates the context at the decompressor; updates are repeated and include additions to full and differential context parameters.

With these enhancements, RTP header compression performs well over links with packet loss, packet reordering, and long delays.

RTP Header Compression over Satellite Links

The Cisco IOS Release 12.3(2)T introduced a feature called RTP Header Compression over Satellite Links. The RTP Header Compression over Satellite Links feature allows you to use RTP header compression over an asymmetric link (such as a satellite link), where the uplink and downlink connections are on separate interfaces. This feature provides improved system performance by reducing network overhead and speeding up transmission of RTP packets.

Periodic Refreshes of a Compressed Packet Stream

RTP header compression is a mechanism that compresses the IP header in a packet before the packet is transmitted. RTP header compression requires a context status feedback mechanism to recover when the compressed packet stream experiences packet channel loss. If the round-trip time of the packet between the uplink and the downlink is lengthy or if a feedback path does not exist, the chance of loss propagation is greatly increased when a packet is dropped from the link. For instance, if a feedback path does not exist, a compressed packet stream may never recover. This situation presents a need for a configurable option that allows periodic refreshes of the compressed packet stream using full-header packets.

The periodic-refresh Keyword

When you configure header compression, you can configure periodic refreshes of the compressed packet stream using the periodic-refresh keyword. The periodic-refresh keyword is available with the following commands:

  • ip rtp header-compression
  • frame-relay ip rtp header-compression
  • frame-relay map ip rtp header-compression

For more information about these commands, see the Cisco IOS Quality of Service Solutions Command Reference.

Optional Disabling of Context-Status Messages

During header compression, a session context is defined. For each context, the session state is established and shared between the compressor and the decompressor. The context state consists of the full IP/UDP/RTP headers, a few first-order differential values, a link sequence number, a generation number, and a delta encoding table. This information is included in the context-status messages.

You can disable the sending of context-status messages in instances either when the time it takes for the packet to traverse the uplink and the downlink portions of the data path is greater than the refresh period (in which case, the sending of the context-status message would not be useful) or when a feedback path does not exist.

Disabling the context-status messages can be accomplished by using the ip header-compression disable-feedback command. For more information about this command, see the Cisco IOS Quality of Service Solutions Command Reference.

How to Configure RTP Header Compression

Enabling RTP Header Compression on an Interface

To enable RTP header compression on an interface, perform the following steps.


Note
To enable RTP header compression on an interface that uses Frame Relay encapsulation, skip these steps and complete the steps in the Enabling RTP Header Compression on an Interface That Uses Frame Relay Encapsulation instead.
SUMMARY STEPS

1.    enable

2.    configure terminal

3.    interface type number [name-tag]

4.    encapsulation encapsulation-type

5.    ip address ip-address mask [secondary]

6.    ip rtp header-compression [passive | iphc-format | ietf-format] [periodic-refresh]

7.    end


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Router> enable

 

Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
configure terminal


Example:

Router# configure terminal

 

Enters global configuration mode.

 
Step 3
interface type number [name-tag]


Example:

Router(config)# interface serial0

 

Configures an interface type and enters interface configuration mode.

  • Enter the interface type and the interface number.
 
Step 4
encapsulation encapsulation-type


Example:

Router(config-if)# encapsulation ppp

 

Sets the encapsulation method used by the interface.

  • Enter the encapsulation method.
 
Step 5
ip address ip-address mask [secondary]


Example:

Router(config-if)# ip address 209.165.200.225 255.255.255.224

 

Sets a primary or secondary IP address for an interface.

  • Enter the IP address and mask for the associated IP subnet.
 
Step 6
ip rtp header-compression [passive | iphc-format | ietf-format] [periodic-refresh]


Example:

Router(config-if)# ip rtp header-compression

 

Enables RTP header compression.

 
Step 7
end


Example:

Router(config-if)# end

 

(Optional) Exits interface configuration mode.

 

Enabling RTP Header Compression on an Interface That Uses Frame Relay Encapsulation

To enable RTP header compression on an interface that uses Frame Relay encapsulation, perform the following steps.


Note

The encapsulation type is specified by using either the cisco or ietf keyword of the frame-relayinterface-dlci command. The cisco keyword specifies Cisco proprietary encapsulations, and the ietf keyword specifies IETF encapsulations. However, note the following points about these keywords:

  • Frame Relay interfaces do not support IETF encapsulations when RTP header compression is enabled. Therefore, the ietfkeyword is not available for Frame Relay interfaces and is not listed in the command syntax shown below.
  • The cisco keyword is available for use on point-to-point subinterfaces only .
>
SUMMARY STEPS

1.    enable

2.    configure terminal

3.    interface type number [name-tag]

4.    encapsulation frame-relay

5.    ip address ip-address mask [secondary]

6.    frame-relay interface-dlci dlci [cisco]

7.    frame-relay ip rtp header-compression [active | passive][periodic-refresh]

8.   

9.    frame-relay map ip ip-address dlci [broadcast] rtpheader-compression [active | passive] [periodic-refresh] [connectionsnumber]

10.    end


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Router> enable

 

Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
configure terminal


Example:

Router# configure terminal

 

Enters global configuration mode.

 
Step 3
interface type number [name-tag]


Example:

Router(config)# interface serial0

 

Configures an interface type and enters interface configuration mode.

  • Enter the interface type and the interface number.
 
Step 4
encapsulation frame-relay


Example:

Router(config-if)# encapsulation frame-relay

 

Enables Frame Relay encapsulation.

 
Step 5
ip address ip-address mask [secondary]


Example:

Router(config-if)# ip address 209.165.200.225 255.255.255.224

 

Sets a primary or secondary IP address for an interface.

  • Enter the IP address and mask for the associated IP subnet.
 
Step 6
frame-relay interface-dlci dlci [cisco]


Example:

Router(config-if)# frame-relay interface-dlci 20

 

Assigns a data-link connection identifier (DLCI) to a specified Frame Relay interface on the router.

 
Step 7
frame-relay ip rtp header-compression [active | passive][periodic-refresh]


Example:

Router(config-if)# frame-relay ip rtp header-compression

 

Enables RTP header compression for all Frame Relay maps on a physical interface.

 
Step 8

 

 
Step 9
frame-relay map ip ip-address dlci [broadcast] rtpheader-compression [active | passive] [periodic-refresh] [connectionsnumber]


Example:

Router(config-if)# frame-relay map ip 10.108.175.220 180 rtp header-compression periodic-refresh

 

Assigns to an IP map header-compression characteristics that differ from the compression characteristics of the interface with which the IP map is associated.

  • Enter the IP address, DLCI number, and any optional keywords and arguments.
 
Step 10
end


Example:

Router(config-if)# end

 

(Optional) Exits interface configuration mode.

 

Enabling Enhanced RTP Header Compression

The Enhanced RTP Header Compression feature (also known as ECRTP) includes modifications and enhancements to RTP header compression to achieve robust operation over unreliable point-to-point links. Enhanced RTP header compression is intended for use on networks subject to high rates of packet loss, packet reordering, and long delays. For more information about Enhanced RTP header compression, see the Enhanced RTP Header Compression.

To enable enhanced RTP header compression, perform the following steps.

Before You Begin

Note

Enhanced RTP header compression is not supported on interfaces that use Frame Relay encapsulation.

>

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    interface type number [name-tag]

4.    encapsulation encapsulation-type

5.    ip address ip-address mask [secondary]

6.    ip rtp header-compression [passive | iphc-format | ietf-format] [periodic-refresh]

7.    ip header-compression recoverable-loss {dynamic | packet-drops}

8.    end


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Router> enable

 

Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
configure terminal


Example:

Router# configure terminal

 

Enters global configuration mode.

 
Step 3
interface type number [name-tag]


Example:

Router(config)# interface serial0

 

Configures an interface type and enters interface configuration mode.

  • Enter the interface type and the interface number.
 
Step 4
encapsulation encapsulation-type


Example:

Router(config-if)# encapsulation ppp

 

Sets the encapsulation method used on the interface.

  • Enter the encapsulation method.
 
Step 5
ip address ip-address mask [secondary]


Example:

Router(config-if)# ip address 209.165.200.225 255.255.255.224

 

Sets a primary or secondary IP address for an interface.

  • Enter the IP address and mask for the associated IP subnet.
 
Step 6
ip rtp header-compression [passive | iphc-format | ietf-format] [periodic-refresh]


Example:

Router(config-if)# ip rtp header-compression ietf-format

 

Enables RTP header compression.

 
Step 7
ip header-compression recoverable-loss {dynamic | packet-drops}


Example:

Router(config-if)# ip header-compression recoverable-loss dynamic

 

Enables ECRTP on an interface.

Note    Enter the dynamic keyword to enable dynamic packet loss recovery, or enter the packet-dropsargument to specify the maximum number of consecutive packet drops that are acceptable.
 
Step 8
end


Example:

Router(config-if)# end

 

(Optional) Exits interface configuration mode.

 

Enabling RTP Header Compression over a Satellite Link

To enable RTP header compression over a satellite link, perform the following steps.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    interface type number [name-tag]

4.    ip address ip-address mask [secondary]

5.    ip rtp header-compression [passive | iphc-format | ietf-format] [periodic-refresh]

6.    ip header-compression disable-feedback

7.    end


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Router> enable

 

Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
configure terminal


Example:

Router# configure terminal

 

Enters global configuration mode.

 
Step 3
interface type number [name-tag]


Example:

Router(config)# interface serial0

 

Configures an interface type and enters interface configuration mode.

  • Enter the interface type and the interface number.
 
Step 4
ip address ip-address mask [secondary]


Example:

Router(config-if)# ip address 209.165.200.225 255.255.255.224

 

Sets a primary or secondary IP address for an interface.

  • Enter the IP address and mask for the associated IP subnet.
 
Step 5
ip rtp header-compression [passive | iphc-format | ietf-format] [periodic-refresh]


Example:

Router(config-if)# ip rtp header-compression ietf-format periodic-refresh

 

Enables RTP header compression.

Note    For RTP header compression over a satellite link, use the periodic-refresh keyword.
 
Step 6
ip header-compression disable-feedback


Example:

Router(config-if)# ip header-compression disable-feedback

 

(Optional) Disables the context status feedback messages from the interface or link.

 
Step 7
end


Example:

Router(config-if)# end

 

(Optional) Exits interface configuration mode.

 

Specifying the Header-Compression Settings

With RTP header compression, you can configure the maximum size of the compressed IP header, the time period for an automatic resend of full-header packets, and the number of packets transmitted before a new full-header packet is sent.

To specify these header-compression settings, perform the following steps.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    interface type number [name-tag]

4.    ip header-compression max-header max-header-size

5.   

6.    ip header-compression max-time length-of-time

7.   

8.    ip header-compression max-period number-of-packets

9.    end


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Router> enable

 

Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
configure terminal


Example:

Router# configure terminal

 

Enters global configuration mode.

 
Step 3
interface type number [name-tag]


Example:

Router(config)# interface serial0

 

Configures an interface type and enters interface configuration mode.

  • Enter the interface type and the interface number.
 
Step 4
ip header-compression max-header max-header-size


Example:

Router(config-if)# ip header-compression max-header 100

 

Specifies the maximum size of the compressed IP header.

  • Enter the maximum size of the compressed IP header, in bytes.
 
Step 5

 

 
Step 6
ip header-compression max-time length-of-time


Example:

Router(config-if)# ip header-compression max-time 30

 

Specifies the maximum amount of time to wait before the compressed IP header is refreshed.

  • Enter the amount of time, in seconds.
 
Step 7

 

 
Step 8
ip header-compression max-period number-of-packets


Example:

Router(config-if)# ip header-compression max-period 160

 

Specifies the maximum number of compressed packets between full headers.

  • Enter the maximum number of compressed packets between full headers.
 
Step 9
end


Example:

Router(config-if)# end

 

(Optional) Exits interface configuration mode.

 

Changing the Number of Header-Compression Connections

For PPP and HDLC interfaces, the default is 16 compression connections. For interfaces that use Frame Relay encapsulation, the default is 256 compression connections.

To change the default number of header-compression connections, perform the following steps.

Implications of Changing the Number of Header-Compression Connections

Each header-compression connection sets up a compression cache entry, so you are in effect specifying the maximum number of cache entries and the size of the cache. Too few cache entries for the specified interface can lead to degraded performance, and too many cache entries can lead to wasted memory. Choose the number of header-compression connections according to the network requirements.


Note

Header-Compression Connections on HDLC and Frame Relay Interfaces

For HDLC interfaces and Frame Relay interfaces (that is, interfaces that use Frame Relay encapsulation), the number of header-compression connections on both sides of the network must match. That is, the number configured for use on the local router must match the number configured for use on the remote router.

Header-Compression Connections on PPP Interfaces

For PPP interfaces, if the header-compression connection numbers on both sides of the network do not match, the number used is "autonegotiated." That is, any mismatch in the number of header-compression connections between the local router and the remote router will be automatically negotiated to the lower of the two numbers. For example, if the local router is configured to use 128 header-compression connections, and the remote router is configured to use 64 header-compression connections, the negotiated number will be 64.


Note
This autonegotiation function applies to PPP interfaces only . For HDLC interfaces and interfaces that use Frame Relay encapsulation, no autonegotiation occurs.
>
SUMMARY STEPS

1.    enable

2.    configure terminal

3.    interface type number [name-tag]

4.    ip rtp compression-connections number

5.   

6.    frame-relay ip rtp compression-connections number

7.    end


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Router> enable

 

Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
configure terminal


Example:

Router# configure terminal

 

Enters global configuration mode.

 
Step 3
interface type number [name-tag]


Example:

Router(config)# interface serial0

 

Configures an interface type and enters interface configuration mode.

  • Enter the interface type and the interface number.
 
Step 4
ip rtp compression-connections number


Example:

Router(config-if)# ip rtp compression-connections 150

 

Specifies the total number of RTP header-compression connections that can exist on an interface.

  • Enter the number of compression connections.
Note    This command can be used for PPP interfaces, HDLC interfaces, or interfaces that use Frame Relay encapsulation.
 
Step 5

 

 
Step 6
frame-relay ip rtp compression-connections number


Example:

Router(config-if)# frame-relay ip rtp compression-connections 150

 

Specifies the maximum number of RTP header-compression connections that can exist on a Frame Relay interface (that is, an interface using Frame Relay encapsulation).

  • Enter the number of compression connections.
Note    This command can be used for interfaces that use Frame Relay encapsulation only.
 
Step 7
end


Example:

Router(config-if)# end

 

(Optional) Exits interface configuration mode.

 

Displaying Header-Compression Statistics

You can display header-compression statistics, such as the number of packets sent, received, and compressed, by using either the show ip rtp header-compression command or the show frame-relay ip rtp header-compression command.

To display header-compression statistics, perform the following steps.

SUMMARY STEPS

1.    enable

2.    show ip rtp header-compression [interface-type interface-number] [detail]

3.   

4.    show frame-relay ip rtp header-compression [interface type number]

5.    end


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Router> enable

 

Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
show ip rtp header-compression [interface-type interface-number] [detail]


Example:

Router# show ip rtp header-compression



Example:

 

Displays RTP header-compression statistics for one or all interfaces.

 
Step 3

 

 
Step 4
show frame-relay ip rtp header-compression [interface type number]


Example:

Router# show frame-relay ip rtp header-compression

 

Displays Frame Relay RTP header-compression statistics for one or all interfaces.

 
Step 5
end


Example:

Router# end

 

(Optional) Exits privileged EXEC mode.

 

Configuration Examples for RTP Header Compression

Example Enabling RTP Header Compression on an Interface

In the following example, RTP header compression is enabled on serial interface 0.

Router> enable
Router# configure terminal
Router(config)# interface serial0 
Router(config-if)# encapsulation ppp 
Router(config-if)# ip address 209.165.200.225 255.255.255.224
Router(config-if)# ip rtp header-compression
Router(config-if)# end
 
      
 
     
 
     
 
      
 
      
 
     
 
    
 
    
 
      
       
     
Example Enabling RTP Header Compression on an Interface That Uses Frame Relay Encapsulation
 
     
 
      
 
        
         
       
 In the following example, RTP header compression is enabled on serial interface 0. Frame Relay encapsulation has been enabled on this interface by using the encapsulationframe-relay command.
 
       
Router> enable
 
       
Router# configure terminal
 
       
Router(config)# interface serial0 
 
       
Router(config-if)# encapsulation frame-relay
 
       
Router(config-if)# ip address 209.165.200.225 255.255.255.224
 
       
Router(config-if)# frame-relay interface-dlci 20
 
       
Router(config-if)# frame-relay ip rtp header-compression
 
       
Router(config-if)# end
 
       
 
      
 
     
 
     
 
      
 
      
 
     
 
    
 
    
 
      
       
     
Example Enabling Enhanced RTP Header Compression
 
     
 
      
 
        
         
       
 In the following example, ECRTP is enabled on serial interface 0. PPP encapsulation is enabled on the interface (a prerequisite for configuring ECRTP on a serial interface). Also, dynamic loss recovery has been specified by using the dynamic keyword of the ip header-compression recoverable-loss command.
 
       
Router> enable
 
       
Router# configure terminal
 
       
Router(config)# interface serial0 
 
       
Router(config-if)# encapsulation ppp
 
       
Router(config-if)# ip address 209.165.200.225 255.255.255.224
 
       
Router(config-if)# ip rtp header-compression ietf-format
 
       
Router(config-if)# ip header-compression recoverable-loss dynamic 
 
       
Router(config-if)# end
 
       
 
      
 
     
 
     
 
      
 
      
 
     
 
    
 
    
 
      
       
     
Example Enabling RTP Header Compression over a Satellite Link
 
     
 
      
 
        
         
       
 In the following example, RTP header compression is enabled on the serial interface 0. In this example, serial interface 0 is a satellite link in the network topology. The periodic-refresh keyword has been specified, which means that the compressed IP header will be refreshed periodically. Also, the context-status messages have been turned off (disabled).
 
       
Router> enable
 
       
Router# configure terminal
 
       
Router(config)# interface serial0 
 
       
Router(config-if)# ip address 209.165.200.225 255.255.255.224
 
       
Router(config-if)# ip rtp header-compression ietf-format periodic-refresh
 
       
Router(config-if)# ip header-compression disable-feedback
 
       
Router(config-if)# end
 
       
 
      
 
     
 
     
 
      
 
      
 
     
 
    
 
    
 
      
       
     
Example Specifying the Header-Compression Settings
 
     
 
      
 
        
         
       
 In the following example, the maximum size of the compressed IP header (100 bytes) has been specified by using the ip header-compression max-header command.
 
       
Router> enable
 
       
Router# configure terminal
 
       
Router(config)# interface serial0 
 
       
Router(config-if)# ip header-compression max-header 100
 
       
Router(config-if)# end
 
       
 
      
 
     
 
     
 
      
 
      
 
     
 
    
 
    
 
      
       
     
Example Changing the Number of Header-Compression Connections
 
     
 
      
 
        
         
       
 In the following example, the number of header-compression connections has been changed to 150 by using the ip rtp compression-connectionscommand.
 
       
Router> enable
 
       
Router# configure terminal
 
       
Router(config)# interface serial0 
 
       
Router(config-if)# ip rtp compression-connections 150
 
       
Router(config-if)# end
 
       
 
      
 
     
 
     
 
      
 
      
 
     
 
    
 
    
 
      
       
     
Example Displaying Header-Compression Statistic
 
     
 
      
 
        
         
       
 You can use the show ip rtp header-compressioncommand to display header-compression statistics such as the number of packets received, sent, and compressed. The following is sample output from the show ip rtp header-compression command. In this example, ECRTP has been enabled on serial interface 0. 
 
       
Router# show ip rtp header-compression serial0
RTP/UDP/IP header compression statistics:
  Interface Serial0 (compression on, IETF, ECRTP)
    Rcvd:    1473 total, 1452 compressed, 0 errors, 0 status msgs
             0 dropped, 0 buffer copies, 0 buffer failures
    Sent:    1234 total, 1216 compressed, 0 status msgs, 379 not predicted
             41995 bytes saved, 24755 bytes sent
             2.69 efficiency improvement factor
    Connect: 16 rx slots, 16 tx slots, 
             6 misses, 0 collisions, 0 negative cache hits, 13 free contexts
             99% hit ratio, five minute miss rate 0 misses/sec, 0 max
 
      
 
     
 
     
 
      
 
      
 
     
 
    
 
   
 
   
 
     
      
    
Additional References
 
    
 
     
 
       
        
      
 The following sections provide references related to configuring RTP header compression. 
 
     
 
     
 
       
        
      
Related Documents
 
      
 
        
         
       
 
         
         
 
           
 Related Topic 
 		 
           
 Document Title 
 		 
         
 
         
         
         
 
           
 Cisco IOS commands 
 		 
           
 Cisco IOS Master Commands List, All Releases 
 		 
         
 
         
 
           
 QoS commands: complete command syntax, command modes, command history, defaults, usage guidelines, and examples 
 		 
           
 Cisco IOS Quality of Service Solutions Command Reference 
 		 
         
 
         
 
           
 Frame Relay 
 		 
           
 "Frame Relay Queueing and Fragmentation at the Interface" module 
 		 
         
 
         
 
           
 Header compression overview 
 		 
           
 "Header Compression" module 
 		 
         
 
         
 
           
 TCP header compression 
 		 
           
 "Configuring TCP Header Compression" module 
 		 
         
 
         
 
           
 Class-based RTP and TCP header compression 
 		 
           
 "Configuring Class-Based RTP and TCP Header Compression" module 
 		 
         
 
         
 
           
 IPHC profiles and header compression 
 		 
           
 "Configuring Header Compression Using IPHC Profiles" module 
 		 
         
 
         
       
 
      
 
     
 
     
 
       
        
      
Standards
 
      
 
        
         
       
 
         
         
 
           
 Standard 
 		 
           
 Title 
 		 
         
 
         
         
         
 
           
 No new or modified standards are supported, and support for existing standards has not been modified. 
 		 
           
 -- 
 		 
         
 
         
       
 
      
 
     
 
     
 
       
        
      
MIBs
 
      
 
        
         
       
 
         
         
 
           
 MIB 
 		 
           
 MIBs Link 
 		 
         
 
         
         
         
 
           
 No new or modified MIBs are supported, and support for existing MIBs has not been modified. 
 		 
           
 To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL: 
 
 http://www.cisco.com/go/mibs 
 		 
         
 
         
       
 
      
 
     
 
     
 
       
        
      
RFCs
 
      
 
        
         
       
 
         
         
 
           
 RFC 
 		 
           
 Title 
 		 
         
 
         
         
         
 
           
 RFC 2507 
 		 
           
 IP Header Compression 
 		 
         
 
         
 
           
 RFC 2508 
 		 
           
 Compressing IP/UDP/RTP Headers for Low-Speed Serial Links 
 		 
         
 
         
 
           
 RFC 3544 
 		 
           
 IP Header Compression over PPP 
 		 
         
 
         
 
           
 RFC 3545 
 		 
           
 Enhanced Compressed RTP (CRTP) for Links with High Delay, Packet Loss and Reordering 
 		 
         
 
         
       
 
      
 
     
 
     
 
       
        
      
Technical Assistance
 
      
 
        
         
       
 
         
         
 
           
 Description 
 		 
           
 Link 
 		 
         
 
         
         
         
 
           
 The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password. 
 		 
           
 http://www.cisco.com/cisco/web/support/index.html 
 		 
         
 
         
       
 
      
 
     
 
    
 
    
 
     
 
     
 
    
 
   
 
   
 
     
      
    
Glossary
 
    
 
     
  compression --The running of a data set through an algorithm that reduces the space required to store the data set or the bandwidth required to transmit the data set.
 
     
  context --The state that the compressor uses to compress a header and that the decompressor uses to decompress a header. The context is the uncompressed version of the last header sent and includes other information used to compress and decompress the packet.
 
     
  context-state packet --A special packet sent from the decompressor to the compressor to communicate a list of (TCP or NON_TCP/RTP) context identifiers (CIDs) for which synchronization has been lost. This packet is sent only over a single link, so it requires no IP header. 
 
     
  DLCI --data-link connection identifier. A value that specifies a permanent virtual circuit (PVC) or switched virtual circuit (SVC) in a Frame Relay network. In the basic Frame Relay specification, DLCIs are locally significant (connected devices might use different values to specify the same connection). In the Local Management Interface (LMI) extended specification, DLCIs are globally significant (DLCIs specify individual end devices).
 
     
  ECRTP --Enhanced Compressed Real-Time Transport Protocol. A compression protocol that is designed for unreliable point-to-point links with long delays.
 
     
  encapsulation --A method of wrapping data in a particular protocol header. For example, Ethernet data is wrapped in a specific Ethernet header before network transit. Also, when dissimilar networks are bridged, the entire frame from one network is simply placed in the header used by the data link layer protocol of the other network. 
 
     
  full header (header refresh) --An uncompressed header that updates or refreshes the context for a packet stream. It carries a CID that will be used to identify the context. Full headers for non-TCP packet streams also carry the generation of the context that they update or refresh. 
 
     
  HDLC --High-Level Data Link Control. A bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO). Derived from Synchronous Data Link Control (SDLC), HDLC specifies a data encapsulation method on synchronous serial links using frame characters and checksums.
 
     
  header --A chain of subheaders. 
 
     
  IETF --Internet Engineering Task Force. A task force that consists of over 80 working groups responsible for developing Internet standards.
 
     
  IPHC --IP Header Compression. A protocol capable of compressing both TCP and UDP headers.
 
     
  ISDN --Integrated Services Digital Network. A communication protocol offered by telephone companies that permits telephone networks to carry data, voice, and other source traffic.
 
     
  lossy serial links --Links in a network that are prone to lose packets.
 
     
  packet stream --The sequence of packets whose headers are similar and share context. For example, headers in an RTP packet stream have the same source and final destination address and the same port numbers in the RTP header. 
 
     
  PPP --Point-to-Point Protocol. A protocol that provides router-to-router and host-to-network connections over synchronous and asynchronous circuits.
 
     
  regular header --A normal, uncompressed header. A regular header does not carry a context identifier (CID) or generation association. 
 
     
  RTP --Real-Time Transport Protocol. A protocol that is designed to provide end-to-end network transport functions for applications that transmit real-time data, such as audio, video, or simulation data, over unicast or multicast network services. RTP provides such services as payload type identification, sequence numbering, timestamping, and delivery monitoring to real-time applications.
 
     
  subheader --An IPv6 base header, an IPv6 extension header, an IPv4 header, a UDP header, an RTP header, or a TCP header. 
 
    
 
    
 
     
 
     
 
    
 
   
 
   
 
     
      
    
Feature Information for Configuring RTP Header Compression
 
    
 
     
 
       
        
      
The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature. 
 
      
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required. 
 
     
 
     
 
       
        
      
 
        
         
       
 
         
         
 
          Table 1 
          Feature Information for Configuring RTP Header Compression  
         
 
         
       
 
       
 
         
         
 
           
 Feature Name 
 		 
           
 Releases 
 		 
           
 Feature Information 
 		 
         
 
         
         
         
 
           
 RTP Header Compression over Satellite Links 
 		 
           
 12.3(2)T 
 		 
           
 The RTP Header Compression over Satellite Links feature allows customers to use RTP header compression over an asymmetric link (such as a satellite link), where the uplink and downlink connections are on separate interfaces. 
 		 
         
 
         
 
           
 Enhanced CRTP for Links with High Delay, Packet Loss and Reordering 
 		 
           
 12.3(11)T 
 		 
           
 The Enhanced Compressed Real-Time Transport Protocol (ECRTP) for Links with High Delay, Packet Loss, and Reordering feature includes modifications and enhancements to CRTP to achieve robust operation over unreliable point-to-point links. This is accomplished by repeating updates and sending absolute (uncompressed) values in addition to delta values for selected context parameters. 
 		 
         
 
         
 
           
 RTP Header Compression 
 
 RTP Header Compression over Satellite Links 
 		 
           
 15.0(1)S 
 		 
           
 The RTP Header Compression and RTP Header Compression over Satellite Links features were integrated into the Cisco IOS Release 15.0(1)S release. 
 		 
         
 
         
       
 
      
 
     
 
    
 
    
 
     
 
     
 
    
 
   
 
   
 
     
      
    
 
 
    
 
     
 
       
        
      
Cisco and the Cisco Logo are trademarks of Cisco Systems, Inc. and/or its affiliates in the U.S. and other countries. A listing of Cisco's trademarks can be found at www.cisco.com/go/trademarks. Third party trademarks mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1005R) 
 
      
Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.