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| 8.8 |  | QoS using Low Speed Links |
| 8.8.5 |  | Compressed Real-Time Protocol (cRTP) |

RTP is the Internet-standard protocol for the transport of real-time data, including audio and video. It can be used for media on demand as well as interactive services such as Internet telephony. RTP consists of a data part and a control part, called RTCP. The data part of RTP is a thin protocol that provides support for applications with real-time properties such as continuous media, for example, audio and video, including timing reconstruction, loss detection, and content identification.

Compressed Real-Time Transport Protocol, or cRTP, is used on a link-by-link basis to compress the IP/UDP/RTP header. In a packet voice environment when framing speech samples every 20 milliseconds, this scenario generates a payload of 20 bytes. The total packet size comprises an IP header of 20 bytes, a UDP header of 8 bytes, and an RTP header of 12 bytes, combined with a payload of 20 bytes. It is evident that the size of the header is twice the size of the payload. When generating packets every 20 milliseconds on a slow link, the header consumes a large portion of the bandwidth. To avoid the unnecessary consumption of available bandwidth, CRTP is used on a link-by-link basis.

This compression scheme reduces the IP/UDP/RTP header to 2 bytes most of the time when no UDP checksums are being sent or 4 bytes when UDP checksums are used.

> Note: Cisco only recommends using cRTP with links lower than 768 Kbps, unless the router is running at a low CPU utilization rate. Monitor the router CPU utilization and disable cRTP if it is above 75 percent.