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rfc:rfc2225

Network Working Group M. Laubach Request for Comments: 2225 Com21, Inc. Category: Standards Track J. Halpern Obsoletes: 1626, 1577 Newbridge Networks, Inc.

                                                           April 1998
                   Classical IP and ARP over ATM

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1998).  All Rights Reserved.

Table of Contents

 1. ABSTRACT  . . . . . . . . . . . . . . . . . . . . . . . . . .  2
 2. ACKNOWLEDGMENT  . . . . . . . . . . . . . . . . . . . . . . .  2
 3. CONVENTIONS . . . . . . . . . . . . . . . . . . . . . . . . .  3
 4. INTRODUCTION  . . . . . . . . . . . . . . . . . . . . . . . .  3
 5. IP SUBNETWORK CONFIGURATION . . . . . . . . . . . . . . . . .  6
 5.1  Background  . . . . . . . . . . . . . . . . . . . . . . . .  6
 5.2  LIS Configuration Requirements  . . . . . . . . . . . . . .  7
 5.3  LIS Router Additional Configuration . . . . . . . . . . . .  8
 6. IP PACKET FORMAT  . . . . . . . . . . . . . . . . . . . . . .  8
 7. DEFAULT VALUE FOR IP MTU OVER ATM AAL5  . . . . . . . . . . .  9
 7.1  Permanent Virtual Circuits  . . . . . . . . . . . . . . . .  9
 7.2  Switched Virtual Circuits . . . . . . . . . . . . . . . . .  9
 7.3  Path MTU Discovery Required . . . . . . . . . . . . . . . . 11
 8. LIS ADDRESS RESOLUTION SERVICES . . . . . . . . . . . . . . . 11
 8.1  ATM-based ARP and InARP Equivalent Services . . . . . . . . 11
 8.2  Permanent Virtual Connections . . . . . . . . . . . . . . . 12
 8.3  Switched Virtual Connections  . . . . . . . . . . . . . . . 12
 8.4  ATMARP Single Server Operational Requirements . . . . . . . 13
 8.5  ATMARP Client Operational Requirements  . . . . . . . . . . 14
 8.5.1  Client ATMARP Table Aging . . . . . . . . . . . . . . . . 16
 8.5.2  Non-Normal VC Operations  . . . . . . . . . . . . . . . . 17
 8.5.3  Use of ATM ARP in Mobile-IP Scenarios . . . . . . . . . . 17
 8.6  Address Resolution Server Selection . . . . . . . . . . . . 17
 8.6.1  PVCs to ATMARP Servers  . . . . . . . . . . . . . . . . . 18
 8.7  ATMARP Packet Formats . . . . . . . . . . . . . . . . . . . 18

Laubach & Halpern Standards Track [Page 1] RFC 2225 IP and ARP over ATM April 1998

 8.7.1  ATMARP/InATMARP Request and Reply Packet Formats  . . . . 18
 8.7.2  Receiving Unknown ATMARP packets  . . . . . . . . . . . . 20
 8.7.3  TL, ATM Number, and ATM Subaddress Encoding . . . . . . . 20
 8.7.4  ATMARP_NAK Packet Format  . . . . . . . . . . . . . . . . 21
 8.7.5  Variable Length Requirements for ATMARP Packets . . . . . 21
 8.8  ATMARP/InATMARP Packet Encapsulation  . . . . . . . . . . . 22
 9. IP BROADCAST ADDRESS  . . . . . . . . . . . . . . . . . . . . 23
 10. IP MULTICAST ADDRESS . . . . . . . . . . . . . . . . . . . . 23
 11. SECURITY CONSIDERATIONS  . . . . . . . . . . . . . . . . . . 23
 12. MIB SPECIFICATION  . . . . . . . . . . . . . . . . . . . . . 24
 13. OPEN ISSUES  . . . . . . . . . . . . . . . . . . . . . . . . 24
 14. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . 24
 15. AUTHORS' ADDRESSES . . . . . . . . . . . . . . . . . . . . . 26
 APPENDIX A - Update Information  . . . . . . . . . . . . . . . . 27
 FULL COPYRIGHT STATEMENT . . . . . . . . . . . . . . . . . . . . 28

1. ABSTRACT

 This memo defines an initial application of classical IP and ARP in
 an Asynchronous Transfer Mode (ATM) network environment configured as
 a Logical IP Subnetwork (LIS) as described in Section 5.  This memo
 does not preclude the subsequent development of ATM technology into
 areas other than a LIS; specifically, as single ATM networks grow to
 replace many Ethernet local LAN segments and as these networks become
 globally connected, the application of IP and ARP will be treated
 differently.  This memo considers only the application of ATM as a
 direct replacement for the "wires" and local LAN segments connecting
 IP end-stations ("members") and routers operating in the "classical"
 LAN-based paradigm.  Issues raised by MAC level bridging and LAN
 emulation are beyond the scope of this paper.
 This memo introduces general ATM technology and nomenclature.
 Readers are encouraged to review the ATM Forum and ITU-TS (formerly
 CCITT) references for more detailed information about ATM
 implementation agreements and standards.

2. ACKNOWLEDGMENT

 The authors would like to thank the efforts of the IP over ATM
 Working Group of the IETF.  Without their substantial, and sometimes
 contentious support, of the Classical IP over ATM model, this updated
 memo would not have been possible.  Section 7, on Default MTU, has
 been incorporated directly from Ran Atkinson's RFC 1626, with his
 permission.  Thanks to Andy Malis for an early review and comments
 for rolc and ion related issues.

Laubach & Halpern Standards Track [Page 2] RFC 2225 IP and ARP over ATM April 1998

3. CONVENTIONS

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [20].

4. INTRODUCTION

 The goal of this specification is to allow compatible and
 interoperable implementations for transmitting IP datagrams and ATM
 Address Resolution Protocol (ATMARP) requests and replies over ATM
 Adaptation Layer 5 (AAL5)[2,6].
 This memo specifies the stable foundation baseline operational model
 which will always be available in IP and ARP over ATM
 implementations.  Subsequent memos will build upon and refine this
 model.  However, in the absence or failure of those extensions,
 operations will default to the specifications contained in this memo.
 Consequently, this memo will not reference these other extensions.
 This memo defines only the operation of IP and address resolution
 over ATM, and is not meant to describe the operation of ATM networks.
 Any reference to virtual connections, permanent virtual connections,
 or switched virtual connections applies only to virtual channel
 connections used to support IP and address resolution over ATM, and
 thus are assumed to be using AAL5.  This memo places no restrictions
 or requirements on virtual connections used for other purposes.
 Initial deployment of ATM provides a LAN segment replacement for:
 1)  Local area networks (e.g., Ethernets, Token Rings and FDDI).
 2)  Local-area backbones between existing (non-ATM) LANs.
 3)  Dedicated circuits or frame relay PVCs between IP routers.
 NOTE: In 1), local IP routers with one or more ATM interfaces will be
 able to connect islands of ATM networks.  In 3), public or private
 ATM Wide Area networks will be used to connect IP routers, which in
 turn may or may not connect to local ATM networks.  ATM WANs and LANs
 may be interconnected.
 Private ATM networks (local or wide area) will use the private ATM
 address structure specified in the ATM Forum UNI 3.1 specification
 [9] or as in the ATM Forum UNI 4.0 specification [19].  This
 structure is modeled after the format of an OSI Network Service
 Access Point Address (NSAPA).  A private ATM address uniquely
 identifies an ATM endpoint.

Laubach & Halpern Standards Track [Page 3] RFC 2225 IP and ARP over ATM April 1998

 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 UNI
     3.1 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 UNI 3.1 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 signaling defines point-to-point and point-to-
     point Connection setup [9, 19.]  Multipoint-to-multipoint not yet
     specified by ITU-TS or ATM Forum.
     An ATM Forum ATM address is either encoded as an NSAP form ATM
     EndSystem Address (AESA) or is an E.164 Public-UNI address [9,
     19].  In some cases, both an AESA and an E.164 Public UNI address
     are needed by an ATMARP client to reach another host or router.

Laubach & Halpern Standards Track [Page 4] RFC 2225 IP and ARP over ATM April 1998

     Since the use of AESAs and E.164 public UNI addresses by ATMARP
     are analogous to the use of Ethernet addresses, the notion of
     "hardware address" is extended to encompass ATM addresses in the
     context of ATMARP, even though ATM addresses need not have
     hardware significance.  ATM Forum NSAP format addresses (AESA)
     use the same basic format as U.S. GOSIP OSI NSAPAs [11].  NOTE:
     ATM Forum addresses should not be construed as being U.S. GOSIP
     NSAPAs.  They are not, the administration is different, which
     fields get filled out are different, etc.  However, in this
     document, these will be referred to as NSAPAs.
 This memo describes the initial deployment of ATM within "classical"
 IP networks as a direct replacement for local area networks
 (Ethernets) and for IP links which interconnect routers, either
 within or between administrative domains.  The "classical" model here
 refers to the treatment of the ATM host adapter as a networking
 interface to the IP protocol stack operating in a LAN-based paradigm.
 Characteristics of the classical model are:
 o   The same maximum transmission unit (MTU) size is the default for
     all VCs in a LIS.  However, on a VC-by-VC point-to-point basis,
     the MTU size may be negotiated during connection setup using Path
     MTU Discovery to better suit the needs of the cooperating pair of
     IP members or the attributes of the communications path.  (Refer
     to Section 7.3)
 o   Default LLC/SNAP encapsulation of IP packets.
 o   End-to-end IP routing architecture stays the same.
 o   IP addresses are resolved to ATM addresses by use of an ATMARP
     service within the LIS - ATMARPs stay within the LIS.  From a
     client's perspective, the ATMARP architecture stays faithful to
     the basic ARP model presented in [3].
 o   One IP subnet is used for many hosts and routers.  Each VC
     directly connects two IP members within the same LIS.
 Future memos will describe the operation of IP over ATM when ATM
 networks become globally deployed and interconnected.
 The deployment of ATM into the Internet community is just beginning
 and will take many years to complete.  During the early part of this
 period, we expect deployment to follow traditional IP subnet
 boundaries for the following reasons:

Laubach & Halpern Standards Track [Page 5] RFC 2225 IP and ARP over ATM April 1998

 o   Administrators and managers of IP subnetworks will tend to
     initially follow the same models as they currently have deployed.
     The mindset of the community will change slowly over time as ATM
     increases its coverage and builds its credibility.
 o   Policy administration practices rely on the security, access,
     routing, and filtering capability of IP Internet gateways: i.e.,
     firewalls.  ATM will not be allowed to "back-door" around these
     mechanisms until ATM provides better management capability than
     the existing services and practices.
 o   Standards for global IP over ATM will take some time to complete
     and deploy.
 This memo details the treatment of the classical model of IP and
 ATMARP over ATM.  This memo does not preclude the subsequent
 treatment of ATM networks within the IP framework as ATM becomes
 globally deployed and interconnected; this will be the subject of
 future documents.  This memo does not address issues related to
 transparent data link layer interoperability.

5. IP SUBNETWORK CONFIGURATION

5.1 Background

 In the LIS scenario, each separate administrative entity configures
 its hosts and routers within a LIS.  Each LIS operates and
 communicates independently of other LISs on the same ATM network.
 In the classical model, hosts communicate directly via ATM to other
 hosts within the same LIS using the ATMARP service as the mechanism
 for resolving target IP addresses to target ATM endpoint addresses.
 The ATMARP service has LIS scope only and serves all hosts in the
 LIS.  Communication to hosts located outside of the local LIS is
 provided via an IP router.  This router is an ATM endpoint attached
 to the ATM network that is configured as a member of one or more
 LISs.  This configuration MAY result in a number of disjoint LISs
 operating over the same ATM network.  Using this model hosts of
 differing IP subnets MUST communicate via an intermediate IP router
 even though it may be possible to open a direct VC between the two IP
 members over the ATM network.
 By default, the ATMARP service and the classical LIS routing model
 MUST be available to any IP member client in the LIS.

Laubach & Halpern Standards Track [Page 6] RFC 2225 IP and ARP over ATM April 1998

5.2 LIS Configuration Requirements

 The requirements for IP members (hosts, routers) operating in an ATM
 LIS configuration are:
 o   All members of the LIS have the same IP network/subnet number and
     address mask [8].
 o   All members within a LIS are directly connected to the ATM
     network.
 o   All members of a LIS MUST have a mechanism for resolving IP
     addresses to ATM addresses via ATMARP (based on [3]) and vice
     versa via InATMARP (based on [12]) when using SVCs.  Refer to
     Section 8 "LIS ADDRESS RESOLUTION SERVICES" in this memo.
 o   All members of a LIS MUST have a mechanism for resolving VCs to
     IP addresses via InATMARP (based on [12]) when using PVCs.  Refer
     to Section 8 "LIS ADDRESS RESOLUTION SERVICES" in this memo.
 o   All members within a LIS MUST be able to communicate via ATM with
     all other members in the same LIS; i.e., the Virtual Connection
     topology underlying the intercommunication among the members is
     fully meshed.
 The following list identifies the set of ATM specific parameters that
 MUST be implemented in each IP station connected to the ATM network:
 o   ATM Hardware Address (atm$ha).  The ATM address of the individual
     IP station.
 o   ATMARP Request Address list (atm$arp-req-list): atm$arp-req-list
     is a list containing one or more ATM addresses of individual
     ATMARP servers located within the LIS.  In an SVC environment,
     ATMARP servers are used to resolve target IP addresses to target
     ATM address via an ATMARP request and reply protocol.  ATMARP
     servers MUST have authoritative responsibility for resolving
     ATMARP requests of all IP members using SVCs located within the
     LIS.
 A LIS MUST have a single ATMARP service entry configured and
 available to all members of the LIS who use SVCs.
 In the case where there is only a single ATMARP server within the
 LIS, then all ATMARP clients MUST be configured identically to have
 only one non-null entry in atm$arp-req-list configured with the same
 address of the single ATMARP service.

Laubach & Halpern Standards Track [Page 7] RFC 2225 IP and ARP over ATM April 1998

 If the IP member is operating with PVCs only, then atm$arp-req-list
 MUST be configured with all null entries and the client MUST not make
 queries to either address resolution service.
 Within the restrictions mentioned above and in Section 8, local
 administration MUST decide which server address(es) are appropriate
 for atm$arp-req-list.
 By default, atm$arp-req-list MUST be configured using the MIB [18].
 Manual configuration of the addresses and address lists presented in
 this section is implementation dependent and beyond the scope of this
 document; i.e., this memo does not require any specific configuration
 method.  This memo does require that these addresses MUST be
 configured completely on the client, as appropriate for the LIS,
 prior to use by any service or operation detailed in this memo.

5.3 LIS Router Additional Configuration

 It is RECOMMENDED that routers providing LIS functionality over the
 ATM network also support the ability to interconnect multiple LISs.
 Routers that wish to provide interconnection of differing LISs MUST
 be able to support multiple sets of these parameters (one set for
 each connected LIS) and be able to associate each set of parameters
 to a specific IP network/ subnet number.  In addition, it is
 RECOMMENDED that a router be able to provide this multiple LIS
 support with a single physical ATM interface that may have one or
 more individual ATM endpoint addresses.   NOTE: this does not
 necessarily mean different End System Identifiers (ESIs) when NSAPAs
 are used.  The last octet of an NSAPA is the NSAPA Selector (SEL)
 field which can be used to differentiate up to 256 different LISs for
 the same ESI.  (Refer to Section 5.1.3.1, "Private Networks" in [9].)

6. IP PACKET FORMAT

 Implementations MUST support IEEE 802.2 LLC/SNAP encapsulation as
 described in [2].  LLC/SNAP encapsulation is the default packet
 format for IP datagrams.
 This memo recognizes that other encapsulation methods may be used
 however, in the absence of other knowledge or agreement, LLC/SNAP
 encapsulation is the default.
 This memo recognizes that end-to-end signaling within ATM may allow
 negotiation of encapsulation method on a per-VC basis.

Laubach & Halpern Standards Track [Page 8] RFC 2225 IP and ARP over ATM April 1998

7. DEFAULT VALUE FOR IP MTU OVER ATM AAL5

 Protocols in wide use throughout the Internet, such as the Network
 File System (NFS), currently use large frame sizes (e.g., 8 KB).
 Empirical evidence with various applications over the Transmission
 Control Protocol (TCP) indicates that larger Maximum Transmission
 Unit (MTU) sizes for the Internet Protocol (IP) tend to give better
 performance.  Fragmentation of IP datagrams is known to be highly
 undesirable [16].  It is desirable to reduce fragmentation in the
 network and thereby enhance performance by having the IP Maximum
 Transmission Unit (MTU) for AAL5 be reasonably large.  NFS defaults
 to an 8192 byte frame size.  Allowing for RPC/XDR, UDP, IP, and LLC
 headers, NFS would prefer a default MTU of at least 8300 octets.
 Routers can sometimes perform better with larger packet sizes because
 most of the performance costs in routers relate to "packets handled"
 rather than "bytes transferred".  So, there are a number of good
 reasons to have a reasonably large default MTU value for IP over ATM
 AAL5.
 RFC 1209 specifies the IP MTU over SMDS to be 9180 octets, which is
 larger than 8300 octets but still in the same range [1].  There is no
 good reason for the default MTU of IP over ATM AAL5 to be different
 from IP over SMDS, given that they will be the same magnitude.
 Having the two be the same size will be helpful in interoperability
 and will also help reduce incidence of IP fragmentation.
 Therefore, the default IP MTU for use with ATM AAL5 shall be 9180
 octets.  All implementations compliant and conformant with this
 specification shall support at least the default IP MTU value for use
 over ATM AAL5.

7.1 Permanent Virtual Circuits

 Implementations which only support Permanent Virtual Circuits (PVCs)
 will (by definition) not implement any ATM signalling protocol.  Such
 implementations shall use the default IP MTU value of 9180 octets
 unless both parties have agreed in advance to use some other IP MTU
 value via some mechanism not specified here.

7.2 Switched Virtual Circuits

 Implementations that support Switched Virtual Circuits (SVCs) MUST
 attempt to negotiate the AAL CPCS-SDU size using the ATM signalling
 protocol.  The industry standard ATM signalling protocol uses two
 different parts of the Information Element named "AAL Parameters" to
 exchange information on the MTU over the ATM circuit being setup [9].
 The Forward Maximum CPCS-SDU Size field contains the value over the
 path from the calling party to the called party.  The Backwards

Laubach & Halpern Standards Track [Page 9] RFC 2225 IP and ARP over ATM April 1998

 Maximum CPCS-SDU Size Identifier field contains the value over the
 path from the called party to the calling party.  The ATM Forum
 specifies the valid values of this identifier as 1 to 65535
 inclusive.  Note that the ATM Forum's User-to-Network-Interface (UNI)
 signalling permits the MTU in one direction to be different from the
 MTU in the opposite direction, so the Forward Maximum CPCS-SDU Size
 Identifier might have a different value from the Backwards Maximum
 CPCS-SDU Size Identifier on the same connection.
 If the calling party wishes to use the default MTU it shall still
 include the "AAL Parameters" information element with the default
 values for the Maximum CPCS-SDU Size as part of the SETUP message of
 the ATM signalling protocol [9].  If the calling party desires to use
 a different value than the default, it shall include the "AAL
 Parameters" information element with the desired value for the
 Maximum CPCS-SDU Size as part of the SETUP message of the ATM
 Signalling Protocol.  The called party will respond using the same
 information elements and identifiers in its CONNECT message response
 [9].
 If the called party receives a SETUP message containing the "Maximum
 CPCS-SDU Size" in the AAL Parameters information element, it shall
 handle the Forward and Backward Maximum CPCS-SDU Size Identifier as
 follows:
 a)  If it is able to accept the ATM MTU values proposed by the SETUP
     message, it shall include an AAL Parameters information element
     in its response.  The Forward and Backwards Maximum CPCS-SDU Size
     fields shall be present and their values shall be equal to the
     corresponding values in the SETUP message.
 b)  If it wishes a smaller ATM MTU size than that proposed, then it
     shall set the values of the Maximum CPCS-SDU Size in the AAL
     Parameters information elements equal to the desired value in the
     CONNECT message responding to the original SETUP message.
 c)  If the calling endpoint receives a CONNECT message that does not
     contain the AAL Parameters Information Element, but the
     corresponding SETUP message did contain the AAL Parameters
     Information element (including the forward and backward CPCS-SDU
     Size fields), it shall clear the call with cause "AAL Parameters
     cannot be supported".
 d)  If either endpoint receives a STATUS message with cause
     "Information Element Non-existent or Not Implemented" or cause
     "Access Information Discarded", and with a diagnostic field

Laubach & Halpern Standards Track [Page 10] RFC 2225 IP and ARP over ATM April 1998

     indicating the AAL Parameters Information Element identifier, it
     shall clear the call with cause "AAL Parameters cannot be
     supported."
 e)  If either endpoint receives CPCS-SDUs in excess of the negotiated
     MTU size, it may use IP fragmentation or may clear the call with
     cause "AAL Parameters cannot be supported".  In this case, an
     error has occurred either due to a fault in an end system or in
     the ATM network.  The error should be noted by ATM network
     management for human examination and intervention.
 If the called endpoint incorrectly includes the Forward and Backward
 Maximum CPCS-SDU Size fields in the CONNECT messages (e.g., because
 the original SETUP message did not include these fields) or it sets
 these fields to an invalid value, then the calling party shall clear
 the call with cause "Invalid Information Element Contents".

7.3 Path MTU Discovery Required

 The Path MTU Discovery mechanism is Internet Standard RFC 1191 [17]
 and is an important mechanism for reducing IP fragmentation in the
 Internet.  This mechanism is particularly important because new
 subnet ATM uses a default MTU sizes significantly different from
 older subnet technologies such as Ethernet and FDDI.
 In order to ensure good performance throughout the Internet and also
 to permit IP to take full advantage of the potentially larger IP
 datagram sizes supported by ATM, all router implementations that
 comply or conform with this specification must also implement the IP
 Path MTU Discovery mechanism as defined in RFC 1191 and clarified by
 RFC 1435 [14].  Host implementations should implement the IP Path MTU
 Discovery mechanism as defined in RFC 1191.

8. LIS ADDRESS RESOLUTION SERVICES

8.1 ATM-based ARP and InARP Equivalent Services

 Address resolution within an ATM LIS SHALL make use of the ATM
 Address Resolution Protocol (ATMARP) (based on [3]) and the Inverse
 ATM Address Resolution Protocol (InATMARP) (based on [12]) and as
 defined in this memo.  ATMARP is the same protocol as the ARP
 protocol presented in [3] with extensions needed to support address
 resolution in a unicast server ATM environment.  InATMARP is the same
 protocol as the original InARP protocol presented in [12] but applied
 to ATM networks.  All IP stations MUST support these protocols as
 updated and extended in this memo.  Use of these protocols differs
 depending on whether PVCs or SVCs are used.

Laubach & Halpern Standards Track [Page 11] RFC 2225 IP and ARP over ATM April 1998

8.2 Permanent Virtual Connections

 An IP station MUST have a mechanism (e.g., manual configuration) for
 determining what PVCs it has, and in particular which PVCs are being
 used with LLC/SNAP encapsulation.  The details of the mechanism are
 beyond the scope of this memo.
 All IP members supporting PVCs are required to use the Inverse ATM
 Address Resolution Protocol (InATMARP) (refer to [12]) on those VCs
 using LLC/SNAP encapsulation.  In a strict PVC environment, the
 receiver SHALL infer the relevant VC from the VC on which the
 InATMARP_Request or response InATMARP_Reply was received.  When the
 ATM source and/or target address is unknown, the corresponding ATM
 address length in the InATMARP packet MUST be set to zero (0)
 indicating a null length, and no storage be allocated in the InATMARP
 packet, otherwise the appropriate address field should be filled in
 and the corresponding length set appropriately.  InATMARP packet
 format details are presented later in this memo.
 Directly from [12]: "When the requesting station receives the
 In[ATM]ARP_Reply, it may complete the [ATM]ARP table entry and use
 the provided address information.  NOTE: as with [ATM]ARP,
 information learned via In[ATM]ARP may be aged or invalidated under
 certain circumstances." IP stations supporting PVCs MUST re-validate
 ATMARP table entries as part of the table aging process.  See the
 Section 8.5.1 "Client ATMARP Table Aging".
 If a client has more than one IP address within the LIS and if using
 PVCs, when an InATMARP_Request is received an InATMARP_Reply MUST be
 generated for each such address.

8.3 Switched Virtual Connections

 SVCs require support from address resolution services for resolving
 target IP addresses to target ATM endpoint addresses.  All members in
 the LIS MUST use the same service.  This service MUST have
 authoritative responsibility for resolving the ATMARP requests of all
 IP members within the LIS.
 ATMARP servers do not actively establish connections.  They depend on
 the clients in the LIS to initiate connections for the ATMARP
 registration procedure and for transmitting ATMARP requests.  An
 individual client connects to the ATMARP server using a point-to-
 point LLC/SNAP VC.  The client sends normal ATMARP request packets to
 the server.  The ATMARP server examines each ATMARP_Request packet
 for

Laubach & Halpern Standards Track [Page 12] RFC 2225 IP and ARP over ATM April 1998

 the source protocol and source hardware address information of the
 sending client and uses this information to build its ATMARP table
 cache.  This information is used to generate replies to any ATMARP
 requests it receives.
 InATMARP_Request packets MUST specify valid address information for
 ATM source number, ATM target number, and source protocol address;
 i.e., these fields MUST be non-null in InATMARP_Request packets.
 This memo defines the address resolution service in the LIS and
 constrains it to consist of a single ATMARP server.  Client-server
 interaction is defined by using a single server approach as a
 reference model.
 This memo recognizes the future development of standards and
 implementations of multiple-ATMARP-server models that will extend the
 operations as defined in this memo to provide a highly reliable
 address resolution service.

8.4 ATMARP Single Server Operational Requirements

 A single ATMARP server accepts ATM calls/connections from other ATM
 end points.  After receiving any ATMARP_Request, the server will
 examine the source and target address information in the packet and
 make note of the VC on which the ATMARP_Request arrived.  It will use
 this information as necessary to build and update its ATMARP table
 entries.
 For each ATMARP_Request, then:
 1.  If the source IP protocol address is the same as the target IP
     protocol address and a table entry exists for that IP address and
     if the source ATM hardware address does not match the table entry
     ATM address and there is an open VC associated with that table
     entry that is not the same as the VC associated with the
     ATMARP_Request, the server MUST return the table entry
     information in the ATMARP_Reply, and MUST raise a "duplicate IP
     address detected" condition to the server's management.  The
     table entry is not updated.
 2.  Otherwise, if the source IP protocol address is the same as the
     target IP protocol address, and either there is no table entry
     for that IP address, or a table entry exists for that IP address
     and there is no open VC associated with that table entry, or if
     the VC associated with that entry is the same as the VC for the
     ATMARP_Request, the server MUST either create a new entry or
     update the old entry as appropriate and return that table entry
     information in the ATMARP Reply.

Laubach & Halpern Standards Track [Page 13] RFC 2225 IP and ARP over ATM April 1998

 3.  Otherwise, when the source IP protocol address does not match the
     target IP protocol address, the ATMARP server will generate the
     corresponding ATMARP_Reply if it has an entry for the target
     information in its ATMARP table.  Otherwise, it will generate a
     negative ATMARP reply (ATMARP_NAK).
 4.  Additionally, when the source IP protocol address does not match
     the target IP protocol address and when the server receives an
     ATMARP_Request over a VC, where the source IP and ATM address do
     not have a corresponding table entry, the ATMARP server MUST
     create a new table entry for the source information.
     Explanation: this allows old RFC 1577 clients to register with
     this ATMARP service by just issuing requests to it.
 5.  Additionally, when the source IP protocol address does not match
     the target IP protocol address and where the source IP and ATM
     addresses match the association already in the ATMARP table and
     the ATM address matches that associated with the VC, the server
     MUST update the table timeout on the source ATMARP table entry
     but only if it has been more than 10 minutes since the last
     update.  Explanation: if the client is sending ATMARP requests to
     the server over the same VC that it used to register its ATMARP
     entry, the server should examine the ATMARP request and note that
     the client is still "alive" by updating the timeout on the
     client's ATMARP table entry.
 6.  Additionally, when the source IP protocol address does not match
     the target IP protocol address and where the source IP and ATM
     addresses do not match the association already in the ATMARP
     table, the server MUST NOT update the ATMARP table entry.
 An ATMARP server MUST have knowledge of any open VCs it has and their
 association with an ATMARP table entry, and in particular, which VCs
 support LLC/SNAP encapsulation.  In normal operation, active ATMARP
 clients will revalidate their entries prior to the server aging
 process taking effect.
 Server ATMARP table entries are valid for 20 minutes.  If an entry
 ages beyond 20 minutes without being updated (refreshed) by the
 client, that entry is deleted from the table regardless of the state
 of any VCs that may be associated with that entry.

8.5 ATMARP Client Operational Requirements

 The ATMARP client is responsible for contacting the ATMARP service to
 both initially register and subsequently refresh its own ATMARP
 information.

Laubach & Halpern Standards Track [Page 14] RFC 2225 IP and ARP over ATM April 1998

 The client is also responsible for using the ATMARP service to gain
 and revalidate ATMARP information about other IP members in the LIS
 (server selection overview is discussed in Section 8.6).  As noted in
 Section 5.2, ATMARP clients MUST be configured with the ATM address
 of the appropriate server prior to client ATMARP operation.
 IP clients MUST register their ATM endpoint address with their ATMARP
 server using the ATM address structure appropriate for their ATM
 network connection: i.e., LISs implemented over ATM LANs following
 ATM Forum UNI 3.1 should register using Structure 1; LISs implemented
 over an E.164 "public" ATM network should register using Structure 2.
 A LIS implemented over a combination of ATM LANs and public ATM
 networks may need to register using Structure 3.  Implementations
 based on this memo MUST support all three ATM address structures.
 See Section 8.7.1 for more details regarding the ATMARP Request
 packet format.
 To handle the case when a client has more than one IP address within
 a LIS, when using an ATMARP server, the client MUST register each
 such address.
 For initial registration and subsequent refreshing of its own
 information with the ATMARP service, clients MUST:
 1.  Establish an LLC/SNAP VC connection to a server in the ATMARP
     service for the purposes of transmitting and receiving ATMARP
     packets.
     NOTE: in the case of refreshing its own information with the
     ATMARP service, a client MAY reuse an existing established
     connection to the ATMARP service provided that the connection was
     previously used either to initially register its information with
     the ATMARP service or to refresh its information with the ATMARP
     service.
 2.  After establishing a successful connection to the ATMARP service,
     the client MUST transmit an ATMARP_Request packet, requesting a
     target ATM address for its own IP address as the target IP
     protocol address.  The client checks the ATMARP_Reply and if the
     source hardware and protocol addresses match the respective
     target hardware and protocol addresses, the client is registered
     with the ATMARP service.  If the addresses do not match, the
     client MAY take action, raise alarms, etc.; however, these
     actions are beyond the scope of this memo.  In the case of a
     client having more than one IP address in the list, this step
     MUST be repeated for each IP address.

Laubach & Halpern Standards Track [Page 15] RFC 2225 IP and ARP over ATM April 1998

 3.  Clients MUST respond to ATMARP_Request and InATMARP_Request
     packets received on any VC appropriately.  (Refer to Section 7,
     "Protocol Operation" in RFC 1293 [12].)
     NOTE: for reasons of robustness, clients MUST respond to
     ATMARP_Requests.
 4.  Generate and transmit address resolution request packets to the
     address resolution service.  Respond to address resolution reply
     packets appropriately to build/refresh its own client ATMARP
     table entries.
 5.  Generate and transmit InATMARP_Request packets as needed and
     process InATMARP_Reply packets appropriately.  InATMARP_Reply
     packets should be used to build/refresh its own client ATMARP
     table entries.  (Refer to Section 7, "Protocol Operation" in
     [12].)  If a client has more than one IP address within the LIS
     when an InATMARP_Request is received an InATMARP_Reply MUST be
     generated for each such address.
 The client MUST refresh its ATMARP information with the server at
 least once every 15 minutes.  This is done by repeating steps 1 and
 2.
 An ATMARP client MUST have knowledge of any open VCs it has
 (permanent or switched), their association with an ATMARP table
 entry, and in particular, which VCs support LLC/SNAP encapsulation.

8.5.1 Client ATMARP Table Aging

 Client ATMARP table entries are valid for a maximum time of 15
 minutes.
 When an ATMARP table entry ages, an ATMARP client MUST invalidate the
 table entry.  If there is no open VC server associated with the
 invalidated entry, that entry is deleted.  In the case of an
 invalidated entry and an open VC, the client MUST revalidate the
 entry prior to transmitting any non address resolution traffic on
 that VC; this requirement applies to both PVCs and SVCs.  NOTE: the
 client is permitted to revalidate an ATMARP table entry before it
 ages, thus restarting the aging time when the table entry is
 successfully revalidated.  The client MAY continue to use the open
 VC, as long as the table entry has not aged, while revalidation is in
 progress.
 In the case of an open PVC, the client revalidates the entry by
 transmitting an InATMARP_Request and updating the entry on receipt of
 an InATMARP_Reply.

Laubach & Halpern Standards Track [Page 16] RFC 2225 IP and ARP over ATM April 1998

 In the case of an open SVC, the client revalidates the entry by
 querying the address resolution service.  If a valid reply is
 received (e.g., ATMARP_Reply), the entry is updated.  If the address
 resolution service cannot resolve the entry (i.e., "host not found"),
 the SVC should be closed and the associated table entry removed.  If
 the address resolution service is not available (i.e., "server
 failure") and if the SVC is LLC/SNAP encapsulated, the client MUST
 attempt to revalidate the entry by transmitting an InATMARP_Request
 on that VC and updating the entry on receipt of an InATMARP_Reply.
 If the InATMARP_Request attempt fails to return an InATMARP_Reply,
 the SVC should be closed and the associated table entry removed.
 If a VC with an associated invalidated ATMARP table entry is closed,
 that table entry is removed.

8.5.2 Non-Normal VC Operations

 The specific details on client procedures for detecting non-normal VC
 connection establishment or closures, or failed communications on an
 established VC are beyond the scope of this memo.  It is REQUIRED
 however, that the client MUST remove the associated ATMARP entry for
 a VC that fails to operate properly, as defined by the client, when
 the client closes that VC, when it releases its resources for a VC,
 or prior to any attempt to reopen that VC.  This behavior
 specifically REQUIRES that the client MUST refresh its ATMARP table
 information prior to any attempt to re-establish communication to an
 IP member after a non-normal communications problem has previously
 occurred on a VC to that IP member.

8.5.3 Use of ATMARP In Mobile-IP Scenarios

 When an ATM LIS is used as the home network in a mobile-IP scenario,
 it is RECOMMENDED that the home agent NOT maintain long term
 connections with the ATMARP service.  The absence of this VC will
 permit a mobile node's registration, upon its return to the home
 network, to immediately preempt the home agent's previous gratuitous
 registration.

8.6 Address Resolution Server Selection

 If the client supports PVCs only, the ATMARP server list is empty and
 the client MUST not generate any address resolution requests other
 than the InATMARP requests on a PVC needed to validate that PVC.
 If the client supports SVCs, then the client MUST have a non-NULL
 atm$arp-req-list pointing to the ATMARP server(s) which provides
 ATMARP service for the LIS.

Laubach & Halpern Standards Track [Page 17] RFC 2225 IP and ARP over ATM April 1998

 The client MUST register with a server from atm$arp-req-list.
 The client SHALL attempt to communicate with any of the servers until
 a successful registration is accomplished.  The order in which client
 selects servers to attempt registration, is a local matter, as are
 the number of retries and timeouts for such attempts.

8.6.1 PVCs to ATMARP Servers

 In a mixed PVC and SVC LIS environment, an ATMARP client MAY have a
 PVC to an ATMARP server.  In this case, this PVC is used for ATMARP
 requests and responses as if it were an established SVC.  NOTE: if
 this PVC is to be used for IP traffic, then the ATMARP server MUST be
 prepared to accept and respond appropriately to InATMARP traffic.

8.7 ATMARP Packet Formats

 Internet addresses are assigned independently of ATM addresses.  Each
 host implementation MUST know its own IP and ATM address(es) and MUST
 respond to address resolution requests appropriately.  IP members
 MUST also use ATMARP and InATMARP to resolve IP addresses to ATM
 addresses when needed.
 NOTE: the ATMARP packet format presented in this memo is general in
 nature in that the ATM number and ATM subaddress fields SHOULD map
 directly to the corresponding UNI 3.1 fields used for ATM
 call/connection setup signalling messages.  The IP over ATM Working
 Group expects ATM Forum NSAPA numbers (Structure 1) to predominate
 over E.164 numbers (Structure 2) as ATM endpoint identifiers within
 ATM LANs.  The ATM Forum's VC Routing specification is not complete
 at this time and therefore its impact on the operational use of ATM
 Address Structure 3 is undefined.  The ATM Forum will be defining
 this relationship in the future.  It is for this reason that IP
 members need to support all three ATM address structures.

8.7.1 ATMARP/InATMARP Request and Reply Packet Formats

 The ATMARP and InATMARP request and reply protocols use the same
 hardware type (ar$hrd), protocol type (ar$pro), and operation code
 (ar$op) data formats as the ARP and InARP protocols [3,12].  The
 location of these three fields within the ATMARP packet are in the
 same byte position as those in ARP and InARP packets.  A unique
 hardware type value has been assigned for ATMARP.  In addition,
 ATMARP makes use of an additional operation code for ARP_NAK.  The
 remainder of the ATMARP/InATMARP packet format is different than the
 ARP/InARP packet format.

Laubach & Halpern Standards Track [Page 18] RFC 2225 IP and ARP over ATM April 1998

 The ATMARP and InATMARP protocols have several fields that have the
 following format and values:
 Data:
   ar$hrd   16 bits  Hardware type
   ar$pro   16 bits  Protocol type
   ar$shtl   8 bits  Type & length (TL) of source ATM number (q)
   ar$sstl   8 bits  Type & length (TL) of source ATM subaddress (r)
   ar$op    16 bits  Operation code (request, reply, or NAK)
   ar$spln   8 bits  Length of source protocol address (s)
   ar$thtl   8 bits  Type & length (TL) of target ATM number (x)
   ar$tstl   8 bits  Type & length (TL) of target ATM subaddress (y)
   ar$tpln   8 bits  Length of target protocol address (z)
   ar$sha   qoctets of source ATM number
   ar$ssa   roctets of source ATM subaddress
   ar$spa   soctets of source protocol address
   ar$tha   xoctets of target ATM number
   ar$tsa   yoctets of target ATM subaddress
   ar$tpa   zoctets of target protocol address
 Where:
   ar$hrd  -  assigned to ATM Forum address family and is
              19 decimal (0x0013) [4].
   ar$pro  -  see Assigned Numbers for protocol type number for
              the protocol using ATMARP. (IP is 0x0800).
   ar$shtl -  Type and length of source ATM number.  See
              Section 8.7.4 for TL encoding details.
   ar$sstl -  Type and length of source ATM subaddress.  See
              Section 8.7.4 for TL encoding details.
   ar$op   -  The operation type value (decimal):
              ATMARP_Request   = ARP_REQUEST   = 1
              ATMARP_Reply     = ARP_REPLY     = 2
              InATMARP_Request = InARP_REQUEST = 8
              InATMARP_Reply   = InARP_REPLY   = 9
              ATMARP_NAK       = ARP_NAK       = 10
   ar$spln -  length in octets of the source protocol address. Value
              range is 0 or 4 (decimal).  For IPv4 ar$spln is 4.
   ar$thtl -  Type and length of target ATM number.  See
              Section 8.7.4 for TL encoding details.

Laubach & Halpern Standards Track [Page 19] RFC 2225 IP and ARP over ATM April 1998

   ar$tstl -  Type and length of target ATM subaddress.  See
              Section 8.7.4 for TL encoding details.
   ar$tpln -  length in octets of the target protocol address. Value
              range is 0 or 4 (decimal).  For IPv4 ar$tpln is 4.
   ar$sha  -  source ATM number (E.164 or ATM Forum NSAPA)
   ar$ssa  -  source ATM subaddress (ATM Forum NSAPA)
   ar$spa  -  source protocol address
   ar$tha  -  target ATM number (E.164 or ATM Forum NSAPA)
   ar$tsa  -  target ATM subaddress (ATM Forum NSAPA)
   ar$tpa  -  target protocol address

8.7.2 Receiving Unknown ATMARP packets

 If an ATMARP client receives an ATMARP message with an operation code
 (ar$op) for which it is not coded to support, it MUST gracefully
 discard the message and continue normal operation.  An ATMARP client
 is NOT REQUIRED to return any message to the sender of the
 unsupported message.

8.7.3 TL, ATM Number, and ATM Subaddress Encoding

 The encoding of the 8-bit TL (type and length) fields in ATMARP and
 In_ATMARP packets is as follows:
   MSB   8     7     6     5     4     3     2     1   LSB
      +-----+-----+-----+-----+-----+-----+-----+-----+
      |  0  | 1/0 |   Octet length of address         |
      +-----+-----+-----+-----+-----+-----+-----+-----+
 Where:
   bit.8   (reserved) = 0  (for future use)
   bit.7   (type)     = 0  ATM Forum NSAPA format
                      = 1  E.164 format
   bit.6-1 (length)   = 6 bit unsigned octet length of address
                        (MSB = bit.6, LSB = bit.1)  Value
                        range is from 0 to 20 (decimal).

Laubach & Halpern Standards Track [Page 20] RFC 2225 IP and ARP over ATM April 1998

 ATM addresses, as defined by the ATM Forum UNI 3.1 signaling
 specification [9], include a "Calling Party Number Information
 Element" and a "Calling Party Subaddress Information Element".  These
 Information Elements (IEs) SHOULD map to ATMARP/InATMARP source ATM
 number and source ATM subaddress respectively.  Furthermore, ATM
 Forum defines a "Called Party Number Information Element" and a
 "Called Party Subaddress Information Element".  These IEs map to
 ATMARP/InATMARP target ATM number and target ATM subaddress,
 respectively.
 The ATM Forum defines three structures for the combined use of number
 and subaddress [9]:
                      ATM Number      ATM Subaddress
                    --------------    --------------
      Structure 1   ATM Forum NSAPA        null
      Structure 2       E.164              null
      Structure 3       E.164         ATM Forum NSAPA
 ATMARP and InATMARP requests and replies for ATM address structures 1
 and 2 MUST indicate a null or unknown ATM subaddress by setting the
 appropriate subaddress length to zero; i.e., ar$sstl.length = 0 or
 ar$tstl.length = 0, the corresponding type field (ar$sstl.type or
 ar$tstl.type) MUST be ignored and the physical space for the ATM
 subaddress buffer MUST not be allocated in the ATMARP packet.  For
 example, if ar$sstl.length=0, the storage for the source ATM
 subaddress is not allocated and the first byte of the source protocol
 address ar$spa follows immediately after the last byte of the source
 hardware address ar$sha in the packet.
 Null or unknown ATM addresses MUST be indicated by setting the
 appropriate address length to zero; i.e., ar$shtl.length and
 ar$thtl.length is zero and the corresponding type field (ar$sstl.type
 or ar$tstl.type) MUST be ignored and the physical space for the ATM
 address or ATM subaddress buffer MUST not be allocated in the ATMARP
 packet.

8.7.4 ATMARP_NAK Packet Format

 The ATMARP_NAK packet format is the same as the received
 ATMARP_Request packet format with the operation code set to ARP_NAK,
 i.e., the ATMARP_Request packet data is exactly copied (e.g., using
 bcopy) for transmission with the ATMARP_Request operation code
 changed to ARP_NAK value.

8.7.5 Variable Length Requirements for ATMARP Packets

 ATMARP and InATMARP packets are variable in length.

Laubach & Halpern Standards Track [Page 21] RFC 2225 IP and ARP over ATM April 1998

 A null or unknown source or target protocol address is indicated by
 the corresponding length set to zero: e.g., when ar$spln or ar$tpln
 is zero the physical space for the corresponding address structure
 MUST not be allocated in the packet.
 For backward compatibility with previous implementations, a null IPv4
 protocol address may be received with length = 4 and an allocated
 address in storage set to the value 0.0.0.0.  Receiving stations MUST
 be liberal in accepting this format of a null IPv4 address.  However,
 on transmitting an ATMARP or InATMARP packet, a null IPv4 address
 MUST only be indicated by the length set to zero and MUST have no
 storage allocated.

8.8 ATMARP/InATMARP Packet Encapsulation

 ATMARP and InATMARP packets are to be encoded in AAL5 PDUs using
 LLC/SNAP encapsulation.  The format of the AAL5 CPCS-SDU payload
 field for ATMARP/InATMARP PDUs is:
             Payload Format for ATMARP/InATMARP PDUs:
             +------------------------------+
             |        LLC 0xAA-AA-03        |
             +------------------------------+
             |        OUI 0x00-00-00        |
             +------------------------------+
             |     EtherType 0x08-06        |
             +------------------------------+
             |                              |
             |   ATMARP/InATMARP Packet     |
             |                              |
             +------------------------------+
 The LLC value of 0xAA-AA-03 (3 octets) indicates the presence of a
 SNAP header.
 The OUI value of 0x00-00-00 (3 octets) indicates that the following
 two-bytes is an EtherType.
 The EtherType value of 0x08-06 (2 octets) indicates ARP [4].
 The total size of the LLC/SNAP header is fixed at 8-octets.  This
 aligns the start of the ATMARP packet on a 64-bit boundary relative
 to the start of the AAL5 CPCS-SDU.
 The LLC/SNAP encapsulation for ATMARP/InATMARP presented here is
 consistent with the treatment of multiprotocol encapsulation of IP
 over ATM AAL5 as specified in [2] and in the format of ATMARP over
 IEEE 802 networks as specified in [5].

Laubach & Halpern Standards Track [Page 22] RFC 2225 IP and ARP over ATM April 1998

 Traditionally, address resolution requests are broadcast to all
 directly connected IP members within a LIS.  It is conceivable in the
 future that larger scaled ATM networks may handle ATMARP requests to
 destinations outside the originating LIS, perhaps even globally;
 issues raised by ATMARPing outside the LIS or by a global ATMARP
 mechanism are beyond the scope of this memo.

9. IP BROADCAST ADDRESS

 ATM does not support broadcast addressing, therefore there are no
 mappings available from IP broadcast addresses to ATM broadcast
 services.  Note: this lack of mapping does not restrict members from
 transmitting or receiving IP datagrams specifying any of the four
 standard IP broadcast address forms as described in [8].  Members,
 upon receiving an IP broadcast or IP subnet broadcast for their LIS,
 MUST process the packet as if addressed to that station.
 This memo recognizes the future development of standards and
 implementations that will extend the operations as defined in this
 memo to provide an IP broadcast capability for use by the classical
 client.

10. IP MULTICAST ADDRESS

 ATM does not directly support IP multicast address services,
 therefore there are no mappings available from IP multicast addresses
 to ATM multicast services.  Current IP multicast implementations
 (i.e., MBONE and IP tunneling, see [10]) will continue to operate
 over ATM based logical IP subnets if operated in the WAN
 configuration.
 This memo recognizes the future development of ATM multicast service
 addressing by the ATM Forum.  When available and widely implemented,
 the roll-over from the current IP multicast architecture to this new
 ATM architecture will be straightforward.
 This memo recognizes the future development of standards and
 implementations that will extend the operations as defined in this
 memo to provide an IP multicast capability for use by the classical
 client.

11. SECURITY CONSIDERATIONS

 Not all of the security issues relating to IP over ATM are clearly
 understood at this time, due to the fluid state of ATM
 specifications, newness of the technology, and other factors.

Laubach & Halpern Standards Track [Page 23] RFC 2225 IP and ARP over ATM April 1998

 It is believed that ATM and IP facilities for authenticated call
 management, authenticated end-to-end communications, and data
 encryption will be needed in globally connected ATM networks.  Such
 future security facilities and their use by IP networks are beyond
 the scope of this memo.
 There are known security issues relating to host impersonation via
 the address resolution protocols used in the Internet [13].  No
 special security mechanisms have been added to the address resolution
 mechanism defined here for use with networks using IP over ATM.

12. MIB SPECIFICATION

 Clients built to this specification MUST implement and provide a
 Management Information Base (MIB) as defined in "Definitions of
 Managed Objects for Classical IP and ARP Over ATM Using SMIv2" [18].

13. OPEN ISSUES

 o   Automatic configuration of client ATM addresses via DHCP [15] or
     via ATM UNI 3.1 Interim Local Management Interface (ILMI)
     services would be a useful extended service addition to this
     document and should be addressed in a separate memo.
 o   ATMARP packets are not authenticated.  This is a potentially
     serious flaw in the overall system by allowing a mechanism by
     which corrupt information may be introduced into the server
     system.

14. REFERENCES

 [1] Piscitello, D., and J. Lawrence, "The Transmission of IP
     Datagrams over the SMDS Service", STD 52, RFC 1209, March 1991.
 [2] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation
     Layer 5", RFC 1483, July 1993.
 [3] Plummer, D., "An Ethernet Address Resolution Protocol - or -
     Converting Network Protocol Addresses to 48.bit Ethernet
     Address for Transmission on Ethernet Hardware", STD 37, RFC
     826, November 1982.
 [4] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
     July 1992.
 [5] Postel, J., and J. Reynolds, "A Standard for the Transmission
     of IP Datagrams over IEEE 802 Networks", STD 43, RFC 1042,
     February 1988.

Laubach & Halpern Standards Track [Page 24] RFC 2225 IP and ARP over ATM April 1998

 [6] CCITT, "Draft Recommendation I.363", CCITT Study Group XVIII,
     Geneva, 19-29 January 1993.
 [7] CCITT, "Draft text for Q.93B", CCITT Study Group XI, 23 September
     - 2 October 1992.
 [8] Braden, R., "Requirements for Internet Hosts -- Communication
     Layers", STD 3, RFC 1122, October 1989.
 [9] ATM Forum, "ATM User-Network Interface (UNI) Specification
     Version 3.1.", ISBN 0-13-393828-X, Prentice-Hall, Inc., Upper
     Saddle River, NJ, 07458, September, 1994.
 [10] Deering, S., "Host Extensions for IP Multicasting", STD 5,
      RFC 1112, August 1989.
 [11] Colella, R., Gardner, E., and R. Callon, "Guidelines for OSI
      NSAP Allocation in the Internet", RFC 1237, July 1991.
 [12] Bradely, T., and C. Brown, "Inverse Address Resolution
      Protocol", RFC 1293, January 1992.
 [13] Bellovin, Steven M., "Security Problems in the TCP/IP Protocol
      Suite", ACM Computer Communications Review, Vol. 19, Issue 2,
      pp. 32-48, 1989.
 [14] Knowles, S., "IESG Advice from Experience with Path MTU
      Discovery", RFC 1435, March 1993.
 [15] Droms, R., "Dynamic Host Configuration Protocol", RFC 1541,
      March 1997.
 [16] Kent C., and J. Mogul, "Fragmentation Considered Harmful",
      Proceedings of the ACM SIGCOMM '87 Workshop on Frontiers in
      Computer Communications Technology, August 1987.
 [17] Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,
      November 1990.
 [18] Green, M., Luciani, J., White, K., and T. Kuo, "Definitions of
      Managed Objects for Classical IP and ARP over ATM Using
      SMIv2", RFC 2320, April 1998.
 [19] ATM Forum, "ATM User-Network Interface (UNI) Specification
      Version 4.0", ATM Forum specfication af-sig-0061.000,
      ftp://ftp.atmforum.com/, July, 1996.

Laubach & Halpern Standards Track [Page 25] RFC 2225 IP and ARP over ATM April 1998

 [20] Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.

15. AUTHORS' ADDRESSES

 Mark Laubach
 Com21, Inc.
 750 Tasman Drive
 Milpitas, CA 95035
 Phone: 408.953.9175
 FAX:   408.953.9299
 EMail: laubach@com21.com
 Joel Halpern
 Newbridge Networks, Inc.
 593 Herndon Parkway
 Herndon, VA  22070-5241
 Phone: 703.736.5954
 FAX:   703.736.5959
 EMail: jhalpern@Newbridge.com

Laubach & Halpern Standards Track [Page 26] RFC 2225 IP and ARP over ATM April 1998

APPENDIX A - Update Information

 This memo represents an update to RFC 1577 and RFC 1626.  The
 following changes are included in this memo:
 o   Pointer to Classical MIB I-D for setting of variables
 o   Single ATMARP server address to ATMARP server list, configurable
     via the MIB.
 o   RFC 1626 text replaces MTU section
 o   Client registration procedure from In_ATMARP to first
     ATMARP_Request
 o   Clarification of variable length ATMARP packet format
 o   Clarification of ARP_NAK packet format
 o   Clarification of InATMARP packet format for null IPv4 addresses
 o   Clarification on ATMARP registration and use of InATMARP_Reply
     for clients having more than one IP address in a LIS

Laubach & Halpern Standards Track [Page 27] RFC 2225 IP and ARP over ATM April 1998

Full Copyright Statement

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Laubach & Halpern Standards Track [Page 28]

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