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rfc:rfc1577

Network Working Group M. Laubach Request for Comments: 1577 Hewlett-Packard Laboratories Category: Standards Track January 1994

                   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.

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 3.  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.

Acknowledgments

 This memo could not have come into being without the critical review
 from Jim Forster of Cisco Systems, Drew Perkins of FORE Systems, and
 Bryan Lyles, Steve Deering, and Berry Kercheval of XEROX PARC.  The
 concepts and models presented in [1], written by Dave Piscitello and
 Joseph Lawrence, laid the structural groundwork for this work. ARP
 [3] written by Dave Plummer and Inverse ARP [12] written by Terry
 Bradley and Caralyn Brown are the foundation of ATMARP presented in
 this memo.  This document could have not been completed without the
 expertise of the IP over ATM Working Group of the IETF and the ad hoc
 PVC committee at the Amsterdam IETF meeting.

Laubach [Page 1] RFC 1577 Classical IP and ARP over ATM January 1993

1. Conventions

 The following language conventions are used in the items of
 specification in this document:
 o   MUST, SHALL, or MANDATORY -- the item is an absolute requirement
     of the specification.
 o   SHOULD or RECOMMEND -- this item should generally be followed for
     all but exceptional circumstances.
 o   MAY or OPTIONAL -- the item is truly optional and may be followed
     or ignored according to the needs of the implementor.

2. 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].
 Note: 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 specification [9].
 This structure is modeled after the format of an OSI Network Service
 Access Point Address.  A private ATM address uniquely identifies an

Laubach [Page 2] 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

Laubach [Page 3] RFC 1577 Classical IP and ARP over ATM January 1993

     router.  Since the use of ATM endpoint addresses 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 NSAPAs
     use the same basic format as U.S. GOSIP 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.
 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 used for all VCs
     in a LIS [2].  (Refer to Section 5.)
  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:
  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.

Laubach [Page 4] RFC 1577 Classical IP and ARP over ATM January 1993

  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.

3. IP Subnetwork Configuration

 In the LIS scenario, each separate administrative entity configures
 its hosts and routers within a closed logical IP subnetwork.  Each
 LIS operates and communicates independently of other LISs on the same
 ATM network. Hosts connected to ATM communicate directly to other
 hosts within the same LIS. Communication to hosts 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. 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.
 The requirements for IP members  (hosts, routers) operating in an ATM
 LIS configuration are:
 o   All members 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 outside of the LIS are accessed via a router.
 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 6 "Address Resolution" in this memo.

Laubach [Page 5] RFC 1577 Classical IP and ARP over ATM January 1993

 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 6 "Address Resolution" 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 a set of ATM specific parameters that
 MUST be implemented in each IP station connected to the ATM network:
 o   ATM Hardware Address (atm$ha). The ATM address of the individual
     IP station.
 o   ATMARP Request Address (atm$arp-req). atm$arp-req is the ATM
     address of an individual ATMARP server located within the LIS.
     In an SVC environment, ATMARP requests are sent to this address
     for the resolution of target protocol addresses to target ATM
     addresses.  That server MUST have authoritative responsibility
     for resolving ATMARP requests of all IP members within the LIS.
     Note: if the LIS is operating with PVCs only, then this parameter
     may be set to null and the IP station is not required to send
     ATMARP requests to the ATMARP server.
 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].)

4. 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.

Laubach [Page 6] RFC 1577 Classical IP and ARP over ATM January 1993

 This memo recognizes the future deployment of end-to-end signalling
 within ATM that will allow negotiation of encapsulation method on a
 per-VC basis.  Signalling negotiations are beyond the scope of this
 memo.

5. MTU Size

 The default MTU size for IP members operating over the ATM network
 SHALL be 9180 octets. The LLC/SNAP header is 8 octets, therefore the
 default ATM AAL5 protocol data unit size is 9188 octets [2].  In
 classical IP subnets, values other than the default can be used if
 and only if all members in the LIS have been configured to use the
 non-default value.
 This memo recognizes the future deployment of end-to-end signalling
 within ATM that will allow negotiation of MTU size on a per-VC basis.
 Signalling negotiations are beyond the scope of this document.

6. Address Resolution

 Address resolution within an ATM logical IP subnet SHALL make use of
 the ATM Address Resolution Protocol (ATMARP) (based on [3]) and the
 Inverse ATM Address Resolution Protocol (InATMARP) (based on [12]) as
 defined in this memo.  ATMARP is the same protocol as the ARP
 protocol presented in [3] with extensions needed to support ARP 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.

6.1 Permanent Virtual Connections

 An IP station MUST have a mechanism (eg. 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 (InARP_REQUEST) or response (InARP_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, otherwise the appropriate
 address field should be filled in and the corresponding length set
 appropriately. InATMARP packet format details are presented later in

Laubach [Page 7] RFC 1577 Classical IP and ARP over ATM January 1993

 this memo.
 Directly from [12]: "When the requesting station receives the InARP
 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."
 It is the responsibility of each IP station supporting PVCs to re-
 validate [ATM]ARP table entries as part of the aging process.  See
 Section 6.5 on "ATMARP Table Aging".

6.2 Switched Virtual Connections

 SVCs require support for ATMARP in the non-broadcast, non-multicast
 environment that ATM networks currently provide. To meet this need a
 single ATMARP Server MUST be located within the LIS. This server MUST
 have authoritative responsibility for resolving the ATMARP requests
 of all IP members within the LIS.
 The server itself does not actively establish connections.  It
 depends on the clients in the LIS to initiate the ATMARP registration
 procedure.  An individual client connects to the ATMARP server using
 a point-to-point VC. The server, upon the completion of an ATM
 call/connection of a new VC specifying LLC/SNAP encapsulation, will
 transmit an InATMARP request to determine the IP address of the
 client.  The InATMARP reply from the client contains the information
 necessary for the ATMARP Server to build its ATMARP table cache. This
 information is used to generate replies to the ATMARP requests it
 receives.
 The ATMARP Server mechanism requires that each client be
 administratively configured with the ATM address of the ATMARP Server
 atm$arp-req as defined earlier in this memo. There is to be one and
 only one ATMARP Server operational per logical IP subnet. It is
 RECOMMENDED that the ATMARP Server also be an IP station. This
 station MUST be administratively configured to operate and recognize
 itself as the ATMARP Server for a LIS. The ATMARP Server MUST be
 configured with an IP address for each logical IP subnet it is
 serving to support InATMARP requests.
 This memo recognizes that a single ATMARP Server is not as robust as
 multiple servers which synchronize their databases correctly. This
 document is defining the client-server interaction by using a simple,
 single server approach as a reference model, and does not prohibit
 more robust approaches which use the same client-server interface.

Laubach [Page 8] RFC 1577 Classical IP and ARP over ATM January 1993

6.3 ATMARP Server Operational Requirements

 The ATMARP server accepts ATM calls/connections from other ATM end
 points. At call setup and if the VC supports LLC/SNAP encapsulation,
 the ATMARP server will transmit to the originating ATM station an
 InATMARP request (InARP_REQUEST) for each logical IP subnet the
 server is configured to serve. After receiving an InATMARP reply
 (InARP_REPLY), the server will examine the IP address and the ATM
 address. The server will add (or update) the <ATM address, IP
 address> map entry and timestamp into its ATMARP table.  If the
 InATMARP IP address duplicates a table entry IP address and the
 InATMARP ATM address does not match the table entry ATM address and
 there is an open VC associated with that table entry, the InATMARP
 information is discarded and no modifications to the table are made.
 ATMARP table entries persist until aged or invalidated. VC call tear
 down does not remove ATMARP table entries.
 The ATMARP server, upon receiving an ATMARP request (ARP_REQUEST),
 will generate the corresponding ATMARP reply (ARP_REPLY) if it has an
 entry in its ATMARP table.  Otherwise it will generate a negative
 ATMARP reply (ARP_NAK).  The ARP_NAK response is an extension to the
 ARMARP protocol and is used to improve the robustness of the ATMARP
 server mechanism.  With ARP_NAK, a client can determine the
 difference between a catastrophic server failure and an ATMARP table
 lookup failure.  The ARP_NAK packet format is the same as the
 received ARP_REQUEST packet format with the operation code set to
 ARP_NAK, i.e., the ARP_REQUEST packet data is merely copied for
 transmission with the ARP_REQUEST operation code reset to ARP_NAK.
 Updating the ATMARP table information timeout, the short form: when
 the server receives an ATMARP request over a VC, where the source IP
 and ATM address match the association already in the ATMARP table and
 the ATM address matches that associated with the VC, the server may
 update the timeout on the source ATMARP table entry: i.e., 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 requests and note that the client is still "alive" by updating
 the timeout on the client's ATMARP table entry.
 Adding robustness to the address resolution mechanism using ATMARP:
 when the server receives an ARP_REQUEST over a VC, it examines the
 source information.  If there is no IP address associated with the VC
 over which the ATMARP request was received and if the source IP
 address is not associated with any other connection, then the server
 will add the <ATM address, IP address> entry and timestamp into its
 ATMARP table and associate the entry with this VC.

Laubach [Page 9] RFC 1577 Classical IP and ARP over ATM January 1993

6.4 ATMARP Client Operational Requirements

 The ATMARP client is responsible for contacting the ATMARP server to
 register its own ATMARP information and to gain and refresh its own
 ATMARP entry/information about other IP members.  This means, as
 noted above, that ATMARP clients MUST be configured with the ATM
 address of the ATMARP server. ATMARP clients MUST:
    1. Initiate the VC connection to the ATMARP server for
       transmitting and receiving ATMARP and InATMARP packets.
    2. Respond to ARP_REQUEST and InARP_REQUEST packets received on
       any VC appropriately.  (Refer to Section 7, "Protocol Operation"
       in [12].)
    3. Generate and transmit ARP_REQUEST packets to the ATMARP server
       and to process ARP_REPLY and ARP_NAK packets from the server
       appropriately.  ARP_REPLY packets should be used to
       build/refresh its own client ATMARP table entries.
    4. Generate and transmit InARP_REQUEST packets as needed and to
       process InARP_REPLY packets appropriately.  InARP_REPLY packets
       should be used to build/refresh its own client ATMARP table
       entries.  (Refer to Section 7, "Protocol Operation" in [12].)
    5. Provide an ATMARP table aging function to remove its own old
       client ATMARP tables entries after a convenient period of time.
 Note: if the client does not maintain an open VC to the server, the
 client MUST refresh its ATMARP information with the server at least
 once every 20 minutes.  This is done by opening a VC to the server
 and exchanging the initial InATMARP packets.

6.5 ATMARP Table Aging

 An ATMARP client or server 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.
 Client ATMARP table entries are valid for a maximum time of 15
 minutes.
 Server ATMARP table entries are valid for a minimum time of 20
 minutes.
 Prior to aging an ATMARP table entry, an ATMARP server MUST generate
 an InARP_REQUEST on any open VC associated with that entry. If an
 InARP_REPLY is received, that table entry is updated and not deleted.

Laubach [Page 10] RFC 1577 Classical IP and ARP over ATM January 1993

 If there is no open VC associated with the table entry, the entry is
 deleted.
 When an ATMARP table entry ages, an ATMARP client MUST invalidate the
 table entry. If there is no open VC associated with the invalidated
 entry, that entry is deleted. In the case of an invalidated entry and
 an open VC, the ATMARP client must revalidate the entry prior to
 transmitting any non address resolution traffic on that VC. In the
 case of a PVC, the client validates the entry by transmitting an
 InARP_REQUEST and updating the entry on receipt of an InARP_REPLY. In
 the case of an SVC, the client validates the entry by transmitting an
 ARP_REQUEST to the ATMARP Server and updating the entry on receipt of
 an ARP_REPLY. If a VC with an associated invalidated ATMARP table
 entry is closed, that table entry is removed.

6.6 ATMARP and InATMARP Packet Format

 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.
 The ATMARP and InATMARP protocols use the same hardware type
 (ar$hrd), protocol type (ar$pro), and operation code (ar$op) data
 formats as the ARP and InARP protocols [3,12].  The location of these
 fields within the ATMARP packet are in the same byte position as
 those in ARP and InARP packets.  A unique hardware type value has
 been assigned for ATMARP.  In addition, ATMARP makes use of an
 additional operation code for ARP_NAK.  The remainder of the
 ATMARP/InATMARP packet format is different than the ARP/InARP packet
 format.
 The ATMARP and InATMARP protocols have several fields that have the
 following format and values:
 Data:
   ar$hrd     16 bits  Hardware type
   ar$pro     16 bits  Protocol type
   ar$shtl     8 bits  Type & length of source ATM number (q)
   ar$sstl     8 bits  Type & length of source ATM subaddress (r)
   ar$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 of target ATM number (x)
   ar$tstl     8 bits  Type & length of target ATM subaddress (y)
   ar$tpln     8 bits  Length of target protocol address (z)
   ar$sha     qoctets  source ATM number
   ar$ssa     roctets  source ATM subaddress

Laubach [Page 11] RFC 1577 Classical IP and ARP over ATM January 1993

   ar$spa     soctets  source protocol address
   ar$tha     xoctets  target ATM number
   ar$tsa     yoctets  target ATM subaddress
   ar$tpa     zoctets  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$op   -  The operation type value (decimal):
              ARP_REQUEST   = 1
              ARP_REPLY     = 2
              InARP_REQUEST = 8
              InARP_REPLY   = 9
              ARP_NAK       = 10
   ar$spln -  length in octets of the source protocol address. For
              IP ar$spln is 4.
   ar$tpln -  length in octets of the target protocol address. For
              IP 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

Laubach [Page 12] RFC 1577 Classical IP and ARP over ATM January 1993

 The encoding of the 8-bit type and length value for ar$shtl,
 ar$sstl, ar$thtl, and ar$tstl 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)
 ATM addresses in Q.93B (as defined by the ATM Forum UNI 3.0
 signalling 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
 IP members 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 3.0 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.
 ATMARP and InATMARP requests and replies for ATM address structures 1
 and 2 MUST indicate a null ATM subaddress; i.e., ar$sstl.type = 1 and

Laubach [Page 13] RFC 1577 Classical IP and ARP over ATM January 1993

 ar$sstl.length = 0 and ar$tstl.type = 1 and ar$tstl.length = 0.  When
 ar$sstl.length and ar$tstl.length =0, the ar$tsa and ar$ssa fields
 are not present.
 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 Q.93B 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.

6.7 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.

Laubach [Page 14] RFC 1577 Classical IP and ARP over ATM January 1993

 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].
 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 ATMARP'ing outside the LIS or by a global ATMARP
 mechanism are beyond the scope of this memo.

7. 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.

8. IP Multicast Address

 ATM does not support 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.

9. Security

 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.
 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.

Laubach [Page 15] RFC 1577 Classical IP and ARP over ATM January 1993

 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.

10. Open Issues

 o   Interim Local Management Interface (ILMI) services will not be
     generally implemented initially by some providers and vendors and
     will not be used to obtain the ATM address network prefix from
     the network [9].  Meta-signalling does provide some of this
     functionality and in the future we need to document the options.
 o   Well known ATM address(es) for ATMARP servers?  It would be very
     handy if a mechanism were available for determining the "well
     known" ATM address(es) for the client's ATMARP server in the LIS.
 o   There are many VC management issues which have not yet been
     addressed by this specification and which await the unwary
     implementor.  For example, one problem that has not yet been
     resolved is how two IP members decide which of duplicate VCs can
     be released without causing VC thrashing.  If two IP stations
     simultaneously established VCs to each other, it is tempting to
     allow only one of these VCs to be established, or to release one
     of these VCs immediately after it is established.  If both IP
     stations simultaneously decide to release opposite VCs, a
     thrashing effect can be created where VCs are repeatedly
     established and immediately released.  For the time being, the
     safest strategy is to allow duplicate VCs to be established and
     simply age them like any other VCs.

References

 [1] Piscitello, D., and J. Lawrence, "IP and ARP over the SMDS
     Service", RFC 1209, Bell Communications Research, March 1991.
 [2] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation
     Layer 5", RFC 1483, Telecom Finland, July 1993.
 [3] Plummer, D., "An Ethernet Address Resolution Protocol - or -
     Converting Network Addresses to 48.bit Ethernet Address for
     Transmission on Ethernet Hardware", STD 37, RFC 826, MIT,
     November 1982.
 [4] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
     USC/Information Sciences Institute, July 1992.

Laubach [Page 16] RFC 1577 Classical IP and ARP over ATM January 1993

 [5] Postel, J., and J. Reynolds, "A Standard for the Transmission of
     IP Datagrams over IEEE 802 Networks", STD 43, RFC 1042,
     USC/Information Sciences Institute, February 1988.
 [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, USC/Information Sciences Institute,
     October 1989.
 [9] ATM Forum, "ATM User-Network Interface Specification Version
     3.0.", ATM Forum, 480 San Antonio Road, Suite 100, Mountain View,
     CA 94040, June 1993.
[10] Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
     1112, Stanford University, August 1989.
[11] Colella, R., and Gardner, E., and R. Callon, "Guidelines for OSI
     NSAP Allocation in the Internet", RFC 1237, NIST, Mitre, DEC,
     July 1991.
[12] Bradely, T., and C. Brown, "Inverse Address Resolution Protocol",
     RFC 1293, Wellfleet Communications, Inc., January 1992.
[13] Bellovin, S., "Security Problems in the TCP/IP Protocol Suite",
     ACM Computer Communications Review, Vol. 19, Issue 2, pp. 32-48,
     1989.

Security Considerations

 Security issues are discussed in Section 9.

Author's Address

 Mark Laubach
 Hewlett-Packard Laboratories
 1501 Page Mill Road
 Palo Alto, CA 94304
 Phone: 415-857-3513
 Fax:   415-857-8526
 EMail: laubach@hpl.hp.com

Laubach [Page 17]

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