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Network Working Group                                         M. LaubachRequest for Comments: 1577                  Hewlett-Packard LaboratoriesCategory: Standards Track                                   January 1994Classical IP and ARP over ATMStatus 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 inSection 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 19931. 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 anLaubach                                                         [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 orLaubach                                                         [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 toSection 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 toSection 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       toSection 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 toSection 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 inLaubach                                                         [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.  SeeSection 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 19936.3 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 19936.4 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 toSection 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 toSection 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 subaddressLaubach                                                        [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 addressLaubach                                                        [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 andLaubach                                                        [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,RFC1112, 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 inSection 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.comLaubach                                                        [Page 17]