Gist of PPP

ppp.md

Point-to-Point Protocol (PPP)

• provides a standard method for transporting multiprotocol datagrams over point-to-point links.
• Comprised of 3 main components
  1. A method for 'encapsulating' multi-protocol datagrams
  2. A Link Control Protocol (LCP) for establishing, configuring and testing the data-link connection
  3. A family of Network Control Protocols (NCPs) for establishing and configuring different network layer protocols

Encapsulation

PPP encapsulation provides for multiplexing of different network-layer protocols simultaneously over the same link. Only 8 additional octets are necessary to form the encapsulation when used within the default framing which is similar to High-level Data Link Control (HDLC) framing. The encapsulation and framing maybe shortened to 2 or 4 octets.

- Fields
  Flag     : 1 octet
  Address  : 1 octet
  Control  : 1 octet
  Protocol : 1 or 2 octet
  Info     : 0 or more
  Padding  : 0 or more
  FCS      : 2 or 4    // Frame Check Sequence - error checking

PPP Encapsulation Summary

+------------+-----------------+---------------+
|  Protocol  |   Information   |   Padding     |
| 8/16 bits  |         *       |      *        |
+------------+-----------------+---------------+

Link Control Protocol

To be portable to a wide variety of environments LCP is used to automatically agree upon the encapsulation format options, handle varying limits on sizes of packets, detect a looped-back link and other common misconfiguration errors and terminate the link.

Optionally - Authentication of peer and determination when a link is functioning properly and when it is failing

Network Control Protocols

PPP exacerbates problems with current family of network protocols. Eg - assignment and management of IP addresses which is a problem even in LAN env is difficult over circuit switched PPP links.

These problems are handled by a family of NCP, which handle specific needs required by their respective network-layer protocols.

High Level Datalink Control (HDLC)

Link-level protocol used ot facilitate reliable point-to-point transmission of a data packet

Bit-synchronous HDLC (RFC 1662)

A flag sequence indicates the beginning or end of a frame and is used for fram synchronization

Internet Protocol Control Protocol (IPCP)

Responsible for configuring, enabling and disabling the IP modules on both ends of the point-to-point link. It uses the same packet exchange mechanism as the Link Control Protocol.

IPCP packets may not be exchanged until PPP has reached the network-layer protocol phase.

Before any IP packets are communicated, PPP must reach NLP phase, and IPCP must reach opened state.

Van Jacobson TCP/IP header compression reduces the size of TCP/IP headers to 3 bytes.

IPCP config option is used to indicate the ability to receive compressed packets.