CCP Operation: Compression Setup

When most people talk about compression in PPP they mention CCP, which is considered “the” compression protocol for PPP. However, CCP is actually used only to configure and control the use of compression; it is the algorithms that do the real work of compressing and decompressing. This “separation of powers” provides flexibility, since it allows each implementation to choose what type of compression they wish to use.

CCP is analogous to the Network Control Protocols (NCPs) that negotiate parameters specific to a network layer protocol sent on the link. An NCP lets two devices decide how they will carry layer three traffic, such as how IPCP lets the devices determine how to carry IP. CCP lets two devices decide how they will compress data, in the same basic way.

Similarly, just as each NCP is like a “lite” version of LCP, CCP is also like a “lite” version of LCP. It is used to set up a compression connection called a CCP link within an LCP link between two devices. Once established, compressed frames can be sent between the two devices. CCP also provides messaging capabilities for managing and eventually terminating a CCP link, again very similar to how each network layer protocol sets up a NCP link within LCP. A CCP link is maintained independently of any NCP links.

CCP uses the same subset of seven LCP message types that the NCPs use, and adds two additional ones. The use of these messages for each of the “life stages” of a CCP link is as follows, which should look very familiar if you've already read about how the NCPs and LCP itself work:

• Link Configuration: Like the NCPs, compression configuration is done once CCP reaches the Network-Layer Protocol phase. The process of setting up compression and negotiating parameters is accomplished using Configure-Request, Configure-Ack, Configure-Nak and Configure-Reject messages just as described in the LCP topic, except the configuration options are particular to CCP.
  
  
• Link Maintenance: Code-Reject messages can be sent to indicate invalid code values in CCP frames. The two new message types are Reset-Request and Reset-Ack, which are used to reset the compression (the CCP link) in the event of a detected failure in decompression.
  
  
• Link Termination: A CCP link can be terminated using Terminate-Request and Terminate-Ack. Again, remember that like the NCP links, the CCP link is set up within an LCP link, and closing it doesn't terminate the LCP link which controls PPP overall.