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2-7
Introducing ATM Networking
stands for User Datagram Protocol. Both TCP and UDP have
associated processes and applications that produce the messages
and streams TCP and UDP receive and deliver. While TCP provides
a reliable data transport mechanism to host applications including
checksums, UDP provides applications-specific IP addresses but
only provides its applications access only to IP’s connectionless,
unreliable delivery.
Classical IP over ATM
Within contemporary networks, host IP software and IP routers get
datagrams from network sources to network destinations. Within an
ATM network, the ATM adaptation layer intercepts and converts IP
datagrams to and from cells. This spans the message transmission
differences inherent within each communication design. However,
it does not address the behavorial differences between components
above this layer. For instance, unlike ATM, the legacy delivery
process does not itself guarantee reliable, in-order delivery.
Because TCP/IP applications were designed in LAN environments
that naturally provided broadcast services that are unavailable with
native ATM, other differences also require consideration before ATM
can seamlessly transport IP datagrams. These mechanisms resolve
IP addresses to ATM network addresses and provide TCP/IP’s
Address Resolution Protocol (ARP) support. Many of these
considerations are resolved in the Internet Engineering Task Force
(IETF) Request For Comment 1577 (RFC 1577). This document
provides protocols, collectively referred to as Classical IP over ATM,
that introduce the concept of a Logical IP Subnet (LIS) - a group of
ATM nodes, LIS clients, that belong to the same IP subnet.
Within each LIS, a single ATM ARP (ATMARP) resolves ATM
addresses for the LIS clients. As nodes become operational, they
contact their ATMARP server and provide ATM and IP address
information to the ATMARP server. The server maintains a table
containing this information and references it whenever a LIS client
requests IP ARP address resolution. When a LIS client obtains the
ATM address of another client, it can establish direct communication
with the other LIS client using a conventional ATM connection. To
ensure table accuracy and station mobility, the ATMARP server
periodically discards old entries.