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Cisco IOS Quality of Service Solutions Configuration Guide, Release 12.2SR
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Quality of Service Overview
- Part 1: Classification
- Part 2: Congestion Management
- Part 3: Congestion Avoidance
- Part 4: Policing and Shaping
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Part 5: Signalling
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Signalling Overview
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RSVP
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Configuring RSVP
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Control Plane DSCP Support for RSVP
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Configuring RSVP Support for Frame Relay
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RSVP Scalability Enhancements
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RSVP Refresh Reduction and Reliable Messaging
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RSVP Message Authentication
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RSVP Application ID Support
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RSVP Fast Local Repair
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RSVP Interface-Based Receiver Proxy
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RSVP Aggregation
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MPLS TE---Tunnel-Based Admission Control (TBAC)
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Configuring Subnetwork Bandwidth Manager
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- Part 6: Link Efficiency Mechanisms
- Part 7: Quality of Service Solutions
- Part 8: Modular Quality of Service Command-Line Interface
- Part 9: Security Device Manager
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Table Of Contents
Configuring RTP Header Compression
Prerequisites for Configuring RTP Header Compression
Information About Configuring RTP Header Compression
Configurable RTP Header-Compression Settings
RTP Header-Compression Keywords
Enhanced RTP Header Compression
RTP Header Compression over Satellite Links
Periodic Refreshes of a Compressed Packet Stream
Optional Disabling of Context-Status Messages
How to Configure RTP Header Compression
Enabling RTP Header Compression on an Interface
Enabling RTP Header Compression on an Interface That Uses Frame Relay Encapsulation
Enabling Enhanced RTP Header Compression
Enabling RTP Header Compression over a Satellite Link
Specifying the Header-Compression Settings
Changing the Number of Header-Compression Connections
Implications of Changing the Number of Header-Compression Connections
Displaying Header-Compression Statistics
Configuration Examples for RTP Header Compression
Enabling RTP Header Compression on an Interface: Example
Enabling RTP Header Compression on an Interface That Uses Frame Relay Encapsulation: Example
Enabling Enhanced RTP Header Compression: Example
Enabling RTP Header Compression over a Satellite Link: Example
Specifying the Header-Compression Settings: Example
Changing the Number of Header-Compression Connections: Example
Displaying Header-Compression Statistics: Example
Feature Information for Configuring RTP Header Compression
Configuring RTP Header Compression
First Published: January 30, 2006Last Updated: June 19, 2006Header 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 in This Module
Your Cisco IOS software release may not support all of the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To reach links to specific feature documentation in this module and to see a list of the releases in which each feature is supported, use the "Feature Information for Configuring RTP Header Compression" section.
Finding Support Information for Platforms and Cisco IOS and Catalyst OS Software Images
Use Cisco Feature Navigator to find information about platform support and Cisco IOS and Catalyst OS software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Contents
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Prerequisites for Configuring RTP Header Compression
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Information About Configuring RTP Header Compression
Before configuring RTP header compression, you should understand the following concepts:
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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:
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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-time command allows you to specify 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:
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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-format keyword 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-format keyword is not available for interfaces that use Frame Relay encapsulation.
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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-format keyword 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.
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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:
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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
This section contains the following tasks:
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Enabling RTP Header Compression on an Interface
To enable RTP header compression on an interface, perform the following steps.
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SUMMARY STEPS
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DETAILED STEPS
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.
Restrictions
The encapsulation type is specified by using either the cisco or ietf keyword of the frame-relay interface-dlci command. The cisco keyword specifies Cisco proprietary encapsulations, and the ietf keyword specifies IETF encapsulations. However, note the following points about these keywords:
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frame-relay map ip ip-address dlci [broadcast] rtp header-compression [active | passive] [periodic-refresh] [connections number]
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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" section.
To enable enhanced RTP header compression, perform the following steps.
Prerequisites
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Restrictions
Enhanced RTP header compression is not supported on interfaces that use Frame Relay encapsulation.
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Enabling RTP Header Compression over a Satellite Link
To enable RTP header compression over a satellite link, perform the following steps.
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DETAILED STEPS
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.
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ip header-compression max-time length-of-time
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ip header-compression max-period number-of-packets
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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.
Restrictions
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.
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frame-relay ip rtp compression-connections number
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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.
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show frame-relay ip rtp header-compression [interface type number]
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Configuration Examples for RTP Header Compression
This section provides the following configuration examples:
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Enabling RTP Header Compression on an Interface: Example
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
Enabling RTP Header Compression on an Interface That Uses Frame Relay Encapsulation: Example
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 encapsulation frame-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
Enabling Enhanced RTP Header Compression: Example
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
Enabling RTP Header Compression over a Satellite Link: Example
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
Specifying the Header-Compression Settings: Example
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
Changing the Number of Header-Compression Connections: Example
In the following example, the number of header-compression connections has been changed to 150 by using the ip rtp compression-connections command.
Router> enable
Router# configure terminal
Router(config)# interface serial0
Router(config-if)# ip rtp compression-connections 150
Router(config-if)# end
Displaying Header-Compression Statistics: Example
You can use the show ip rtp header-compression command 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 serial0RTP/UDP/IP header compression statistics:Interface Serial0 (compression on, IETF, ECRTP)Rcvd: 1473 total, 1452 compressed, 0 errors, 0 status msgs0 dropped, 0 buffer copies, 0 buffer failuresSent: 1234 total, 1216 compressed, 0 status msgs, 379 not predicted41995 bytes saved, 24755 bytes sent2.69 efficiency improvement factorConnect: 16 rx slots, 16 tx slots,6 misses, 0 collisions, 0 negative cache hits, 13 free contexts99% hit ratio, five minute miss rate 0 misses/sec, 0 maxAdditional References
The following sections provide references related to configuring RTP header compression.
Related Documents
QoS commands: complete command syntax, command modes, command history, defaults, usage guidelines, and examples
Frame Relay
"Frame Relay Queueing and Fragmentation at the Interface" module
Header compression overview
"Header Compression" module
TCP header compression
Class-based RTP and TCP header compression
"Configuring Class-Based RTP and TCP Header Compression" module
IPHC profiles and header compression
Standards
No new or modified standards are supported, and support for existing standards has not been modified.
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MIBs
RFCs
Technical Assistance
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
lists the release history for this feature.
Not all commands may be available in your Cisco IOS software release. For release information about a specific command, see the command reference documentation.
For information on a feature in this technology that is not documented here, see the "Header-Compression Features Roadmap" module.
Cisco IOS software images are specific to a Cisco IOS software release, a feature set, and a platform. Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS and Catalyst OS software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Note
Table 1 Feature Information for Configuring RTP Header Compression
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.
The following sections provide information about this feature:
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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.
The following sections provide information about this feature:
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