QoS: Header Compression Configuration Guide, Cisco IOS XE Release 3S
Configuring RTP Header Compression
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Configuring RTP Header Compression

Contents

Configuring RTP Header Compression

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

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and 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 module.

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 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 ipheader-compressionmax-header command allows you to define the maximum size of the header of a packet to be compressed. Any packet with an header that exceeds the maximum size is sent uncompressed.

The ipheader-compressionmax-timecommand allows you to specify the maximum time between transmitting full-header packets, and the ipheader-compressionmax-period command allows you to specify the maximum number of compressed packets between full headers. With the ipheader-compressionmax-time and ipheader-compressionmax-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
  • cisco

These keywords (described below) are available with many of the quality of service (QoS) commands used to configure RTP header compression, such as the iprtpheader-compression command. For more information about the iprtpheader-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 iprtpheader-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.

Theiphc-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 both TCP and UDP packets. UPD and TCP packets are compressed separately. 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.


The cisco Keyword

The cisco keyword indicates that the Cisco-propriety or "original" format of header compression will be used.

RTP header-compression using the cisco format supports even-numbered UDP destination ports in the Cisco audio range of 16384 to 32767 or in the video range of 49152 to 65535.

The cisco keyword is only available on interfaces that use Frame Relay or HDLC encapsulation.

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.

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| cisco] [periodic-refresh]

    7.    end


DETAILED STEPS
     Command or ActionPurpose
    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| cisco] [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.

     

    Specifying the Header-Compression Settings

    With RTP header compression, you can configure the maximum size of the compressed 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 ActionPurpose
      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.

      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 Interfaces

      For HDLC interfaces, 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, no autonegotiation occurs.


      >
      SUMMARY STEPS

        1.    enable

        2.    configure terminal

        3.    interface type number [name-tag]

        4.    ip rtp compression-connections number

        5.    end


      DETAILED STEPS
         Command or ActionPurpose
        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 and HDLC interfaces.

         
        Step 5 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 the showiprtpheader-compression command.

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

        SUMMARY STEPS

          1.    enable

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

          3.    end


        DETAILED STEPS
           Command or ActionPurpose
          Step 1 enable


          Example:
          Router> enable
           

          Enables privileged EXEC mode.

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


          Example:
          Router# show ip rtp header-compression 


          Example:
          
          
                  
           

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

           
          Step 3 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 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 ipheader-compressionmax-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 Statistics

          You can use the showiprtpheader-compressioncommand to display header-compression statistics such as the number of packets received, sent, and compressed. The following is sample output from the showiprtpheader-compression command.

          Router# show ip rtp header-compression
           serial0
          RTP/UDP/IP header compression statistics:
            Interface Serial0 (compression on, IETF)
              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

          Header compression overview

          "Header Compression" 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 XE Software 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

          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

          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

          Express RTP and TCP Header Compression

          Cisco IOS XE Release 2.1

          This feature was introduced on Cisco ASR 1000 Series Routers.

          RTP Header Compression

          Cisco IOS XE Release 2.1

          This feature was introduced on Cisco ASR 1000 Series Routers.

          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).

          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.