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CIP (Classical IP over ATM)

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Classical IP over ATM, or CIP as it is informally called, defines how to transmit IP datagrams over ATM networks. CIP was originally defined in RFC 1577 (Classical IP and ARP over ATM, January 1994). This RFC was made obsolete and replaced by RFC 2225 (Classical IP and ARP over ATM, April 1998).

CIP is an IETF standard. Alternative methods for integrating ATM into legacy networks have been developed by the ATM Forum and are outlined under the heading "IP over ATM." An ATM Forum standard called LANE (LAN Emulation) provides a way to integrate ATM with legacy LAN protocols such as Ethernet and Token Ring (and carry IP as if the ATM network were a LAN). Another ATM Forum standard is MPOA (Multiprotocol over ATM), which adds cut-through/shortcut routing to the LANE scheme. Both are discussed under separate headings.

CIP uses an ATM network as its underlying data link network. In this scheme, ATM is like any other data link network such as Ethernet and token ring. CIP implements the concept of a LIS (logical IP subnet), which is a closed logical IP subnetwork (such as a department or workgroup) consisting of a group of hosts. Multiple LISs can exist on the same ATM network, but routers are still needed for inter-LIS communications.

There is some inefficiency in this configuration. If two LISs are on the same ATM network, a host on one LIS must go through a router to communicate with a host in the other LIS, even though the underlying ATM network is capable of setting up a virtual circuit that directly connects both hosts. This is by design, in order to retain the requirement that packets addressed to hosts in other subnets be sent to a default router. This is why it is called classical IP.

Later, the IETF defined NHRP (Next Hop Routing Protocol), which can set up direct ATM virtual circuit connections between end stations in different LISs. This is pictured in Figure 1.

Figure 1

Each LIS includes a single ATMARP (ATM Address Resolution Protocol) server, which resolves IP to ATM addresses. When a host is turned on, it connects with the ATMARP server. The ATMARP server then requests the host's IP and ATM addresses, which are then stored in the ATMARP lookup table for future reference. Hosts and routers contact the ATMARP server when they need to resolve IP addresses into ATM addresses.

Alternative IP over ATM schemes are outlined under "IP over ATM." Also see "LANE (LAN Emulation)" and "MPOA (Multiprotocol over ATM)."

Besides RFC 2225 , the following RFCs provide useful information about IP over ATM:

  • RFC 1755 (ATM Signaling Support for IP over ATM, February 1995)

  • RFC 1754 (IP over ATM Working Group's Recommendations for the ATM Forum's Multiprotocol BOF, January 1995)

  • RFC 1932 (IP over ATM: A Framework Document, April 1996)

  • RFC 2226 (IP Broadcast over ATM Networks, October 1997)

  • RFC 2382 (A Framework for Integrated Services and RSVP over ATM, August 1998)

  • RFC 2336 (Classical IP and ARP over ATM to NHRP Transition, July 1998)

  • RFC 2684 (Multiprotocol Encapsulation over ATM Adaptation Layer 5, September 1999)

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