TCP/IP Tutorial and Technical Overview - IBM Redbooks

2.10 Asynchronous transfer mode (ATM)
ATM-based networks are of increasing interest for both local and wide area
applications. The ATM architecture is different from the standard LAN
architectures and, for this reason, changes are required so that traditional LAN
products will work in the ATM environment. In the case of TCP/IP, the main
change required is in the network interface to provide support for ATM.
There are several approaches already available, two of which are important to
the transport of TCP/IP traffic. We describe these in 2.10.2, “Classical IP over
ATM” on page 50 and 2.10.3, “ATM LAN emulation” on page 56. We also
compare them in 2.10.4, “Classical IP over ATM versus LAN emulation” on
page 59.
2.10.1 Address resolution (ATMARP and InATMARP)
The address resolution in an ATM logical IP subnet is done by the ATM Address
Resolution Protocol (ATMARP), based on RFC 826 (also STD 37), and the
Inverse ATM Address Resolution Protocol (InATMARP), based on RFC 2390.
ATMARP is the same protocol as the ARP protocol, with extensions needed to
support ARP in a unicast server ATM environment. InATMARP is the same
protocol as the original InARP protocol, but applied to ATM networks. Use of
these protocols differs depending on whether permanent virtual connections
(PVCs) or switched virtual connections (SVCs) are used.
Both ATMARP and InATMARP are defined in RFC 2225; a proposed standard
with a state of elective. We describe the encapsulation of ATMARP and
InATMARP requests/replies in 2.10.2, “Classical IP over ATM” on page 50.
InATMARP
The ARP protocol resolves a host's hardware address for a known IP address.
The InATMARP protocol resolves a host's IP address for a known hardware
address. In a switched environment, you first establish a virtual connection (VC)
of either a permanent virtual connection (PVC) or switched virtual connection
(SVC) in order to communicate with another station. Therefore, you know the
exact hardware address of the partner by administration, but the IP address is
unknown. InATMARP provides dynamic address resolution. InARP uses the
same frame format as the standard ARP, but defines two new operation codes:
► InARP request=8
► InARP reply=9
See “ARP packet generation” on page 120 for more details.
Chapter 2. Network interfaces 47