The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Network Interface Layer (OSI Data Link Layer) Protocols            9  TCP/IP Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP)                 9  Point-to-Point Protocol (PPP)                      9  PPP Feature Protocols

PPP Compression Control Protocol (CCP) and Compression Algorithms 1 2 3 4 PPP Multilink Protocol (MP/MLP/MLPPP)

ECP Operation: Compression Setup

Like CCP, ECP is analogous to the Network Control Protocols (NCPs) that negotiate parameters specific to a network layer protocol sent on the link, but deals with how devices encrypt data rather than how they transport layer three traffic. This also means that like the NCPs, ECP is a "lite" version of LCP and works in the same basic way. Once an ECP link is negotiated, encrypted frames can be sent between devices. When no longer needed, the ECP link can be terminated.

ECP uses the same subset of seven LCP message types that the NCPs use, and adds two more. The use of these messages for each of the “life stages” of an ECP link is as follows:

• Link Configuration: Like the NCPs (and also like CCP of course), encryption configuration is done once ECP reaches the Network-Layer Protocol phase. The process of setting up encryption 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 ECP.
  
  
• Link Maintenance: Code-Reject messages can be sent to indicate invalid code values in ECP frames. The two new message types are Reset-Request and Reset-Ack, which are used to reset the encryption (the ECP link) in the event of a detected failure in decryption.
  
  
• Link Termination: An ECP link can be terminated using Terminate-Request and Terminate-Ack. Again, remember that like the NCP links, the ECP link is set up within an LCP link, so closing it doesn't terminate the LCP link.

PPP Compression Control Protocol (CCP) and Compression Algorithms 1 2 3 4 PPP Multilink Protocol (MP/MLP/MLPPP)