RFC 1577 (rfc1577) - Page 3 of 17

Classical IP and ARP over ATM

RFC 1577             Classical IP and ARP over ATM          January 1993


   ATM endpoint.  Public networks will use either the address structure
   specified in ITU-TS recommendation E.164 or the private network ATM
   address structure.  An E.164 address uniquely identifies an interface
   to a public network.

   The characteristics and features of ATM networks are different than
   those found in LANs:

   o   ATM provides a Virtual Connection (VC) switched environment. VC
       setup may be done on either a Permanent Virtual Connection (PVC)
       or dynamic Switched Virtual Connection (SVC) basis. SVC call
       management signalling is performed via implementations of the
       Q.93B protocol [7,9].

   o   Data to be passed by a VC is segmented into 53 octet quantities
       called cells (5 octets of ATM header and 48 octets of data).

   o   The function of mapping user Protocol Data Units (PDUs) into the
       information field of the ATM cell and vice versa is performed in
       the ATM Adaptation Layer (AAL).  When a VC is created a specific
       AAL type is associated with the VC.  There are four different AAL
       types, which are referred to individually as "AAL1", "AAL2",
       "AAL3/4", and "AAL5".  (Note: this memo concerns itself with the
       mapping of IP and ATMARP over AAL5 only.  The other AAL types are
       mentioned for introductory purposes only.)  The AAL type is known
       by the VC end points via the call setup mechanism and is not
       carried in the ATM cell header.  For PVCs the AAL type is
       administratively configured at the end points when the Connection
       (circuit) is set up.  For SVCs, the AAL type is communicated
       along the VC path via Q.93B as part of call setup establishment
       and the end points use the signaled information for
       configuration.  ATM switches generally do not care about the AAL
       type of VCs.  The AAL5 format specifies a packet format with a
       maximum size of (64K - 1) octets of user data. Cells for an AAL5
       PDU are transmitted first to last, the last cell indicating the
       end of the PDU.  ATM standards guarantee that on a given VC, cell
       ordering is preserved end-to-end.  NOTE: AAL5 provides a non-
       assured data transfer service - it is up to higher-level
       protocols to provide retransmission.

   o   ATM Forum signalling defines point-to-point and point-to-
       multipoint Connection setup [9].  Multipoint-to-multipoint VCs
       are not yet specified by ITU-TS or ATM Forum.

   o   An ATM Forum ATM endpoint address is either encoded as an NSAP
       Address (NSAPA) or is an E.164 Public-UNI address [9].  In some
       cases, both an ATM endpoint address and an E.164 Public UNI
       address are needed by an ATMARP client to reach another host or



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