PPP Point-to-Point Protocol | Efficient Networks 5500 Series

SpeedStream Router User Guide

PPP(Point-to-Point Protocol)

PPP is a single or multi-link interface between two packet switching devices, such as a bridge or router. PPP has built-in negotiation for addresses and connection parameters and can route multiple protocols over a single link. One benefit of using PPP is it offers interoperability of multi-vendor equipment as well as support for dynamic configuration between the connecting devices.

Public and Private Networks and the Use of NAPT

An IP address must be unique among all networks reachable from a given host using the IP protocols. The Internet Registry in the United States that ensures the uniqueness of the IP addresses on the Internet. The Internet Registry assigns an entire IP network number to each site connected to the Internet. Each IP address at a site is unique as long as the site assigns a different host number to each host on its network. Thus each host is ensured a globally unique IP address that is known as a public IP address.

However, there has been concern over the eventual exhaustion of the public address space. This has LED the Registry to set aside IP network numbers for private addressing. These numbers are not assigned to anyone by the Internet Registry and are open for use by any site. IP addresses are unique within the private address space, but two private address spaces are not guaranteed unique.

Use of private address spaces has some disadvantages including the need to re-address any host that must change from a private address to a public address. Plus the privately addressed hosts are unable to communicate with all hosts in an internet. These problems can be handled by the use of Network Address Port Translation (NAPT).

NAPT is an extension to Network Address Translation (NAT). With NAT, a network address translator (the router, in this case) sits between an organization’s network and the Internet, or between two organization’s networks and translates IP addresses from private internal addresses to globally unique external addresses. NAPT, however, allows many network addresses and their TCP/UDP ports to be translated to a single network address and its TCP/UDP ports. With NAPT, a few of your internal hosts can share a single public address. When a host needs to access the Internet, the router will translate an address for it. When packets from the host are sent to the Internet, the router replaces the internal address with the external address. When packets come back for that address, the router reverses the substitution.

RFC 2684

Request for Comment (RFC) 2684, which supplants RFC 1483, is an interoperability specification set by the Internet Engineering Task Force (IETF) that outlines methods for multiprotocol encapsulation over ATM. RFC 2684 describes two encapsulation methods for carrying network interconnect traffic over ATM Adaptation Layer 5 (AAL5): Logical Link Control (LLC)/SNAP encapsulation and VC multiplexing.

By default, the router uses the first method, LLC Encapsulation, which allows multiplexing of multiple protocols over a single ATM virtual circuit. The second method, VC multiplexing, uses a separate VC for each carried protocol.

The router supports two types of encapsulation: routed and bridged. RFC 2684 Routed encapsulation operates at the IP layer and will route only IP packets. If the router will be handling non-IP packets, you may WAN to use RFC 2684 bridged encapsulation.