IPCP and IPv6CP: IP Control Protocol and IPv6 Control Protocol

IP Control Protocol (IPCP) and IPv6 Control Protocol (IPv6CP) define the Network Control Protocol for establishing and configuring the Internet Protocol or IPv6 over PPP, and a method to negotiate and use Van Jacobson TCP/IP header compression with PPP .

IPCP is responsible for configuring, enabling, and disabling the IP protocol modules on both ands of the point-to-point link. IPCP uses the same packet exchange mechanism as the Link Control Protocol (LCP). IPCP packets may not be exchanged until PPP has reached the Network-Layer Protocol phase. IPCP packets received before this phase is reached should be silently discarded.

Before any IP packets may be communicated, PPP must reach the Network-Layer Protocol phase, and the IP Control Protocol must reach the Opened state.

Van Jacobson TCP/IP header compression reduces the size of the TCP/IP headers to as few as three bytes. This can be a significant improvement on slow serial lines, particularly for interactive traffic.  

The IP Compression Protocol Configuration Option is used to indicate the ability to receive compressed packets. Each end of the link must separately request this option if bidirectional compression is desired.

IPv6CP is responsible for configuring, enabling, and disabling the IPv6 protocol modules on both ends of the point-to-point link. IPv6CP uses the same packet exchange mechanism as the Link Control Protocol (LCP). IPv6CP packets may not be exchanged until PPP has reached the Network-Layer Protocol phase. IPv6CP packets received before this phase is reached should be silently discarded.

Protocol Structure - IPCP and IPv6CP: IP Control Protocol and IPv6 Control Protocol

•  Type - 1 for IP-Address, 2 for IP-Compression Protocol, and 3 for IP-Address
•  Length >= 4
• Configuration Option - the field is two octets and indicates one of the following options:

For IPCP:

Type 1: IP-Addresses

Type 2: IP-Compression Protocol

Type 3: IP-Address.

For IPv6CP:

Type 1: Interface - Identifier

Type 2: IPv6-Compression Protocol

IPCP and IPv6CP header structure:

• Code - Specifies the function to be performed.
• Identifier - Used to match requests and replies.
• Length - Size of the packet including the header.
• Data -Zero or more bytes of data as indicated by the Length. This field may contain one or more Options.

IPCP is defined by IETF (www.ietf.org ) in RFC 1332 and IPv6CP is defined in RFC 2472.

http://www.javvin.com/protocol/rfc1332.pdf : The PPP Internet Protocol Control Protocol (IPCP).
http://www.javvin.com/protocol/rfc2472.pdf : IP Version 6 over PPP  
http://www.javvin.com/protocol/rfc3241.pdf : Robust Header Compression (ROHC) over PPP.
http://www.javvin.com/protocol/rfc3544.pdf : IP Header Compression over PPP.