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Versions: 02

Internet Area                                               Tom Evans
Internet Draft                                       Webster Computer
Expires May 1993                                    Christopher Ranch
                                                         Novell, Inc.
                                                        November 1992

                    A Method for the Transmission of Internet
                Packets Over AppleTalk Networks [MacIP]

Status of this Memo

   This document is an Internet Draft.  Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
   and its Working Groups.  Note that other groups may also distribute
   working documents as Internet Drafts.

   Internet Drafts are draft documents valid for a maximum of six
   months.  Internet Drafts may be updated, replaced, or obsoleted by
   other documents at any time.  It is not appropriate to use Internet
   Drafts as reference material or to cite them other than as a
   ``working draft'' or ``work in progress.''  Please check the 1id-
   abstracts.txt listing contained in the internet-drafts Shadow
   Directories on nic.ddn.mil, nnsc.nsf.net, nic.nordu.net,
   ftp.nisc.sri.com, or munnari.oz.au to learn the current status of any
   Internet Draft.

Abstract

   This Internet Draft describes an existing protocol for transporting
   IP packets over AppleTalk networks.  It is intended to specify,
   standardize, and enhance existing implementations.  Distribution of
   this memo is unlimited.

   This Internet Draft is a product of the Apple-IP Working Group of the
   Internet Engineering Task Force (IETF).

Table of Contents

   (To be generated and inserted later)

Overview

   There is much confusion over what MacIP really is, as it has grown by
   many small increments designed and implemented by at least as many
   people.  There has been no central or formal design document, so each
   of the protocol additions and their effects are not well understood.
   We hope to dissect the anatomy of the current state of the art, and
   recommend a functional subset of the protocol features and a new



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MacIP                                                      November 1992


   gateway architecture that will help provide services to multiple
   zones more reliably.

   The MacIP protocol is used to transport IP packets across AppleTalk
   networks. There are many existing host and gateway implementations of
   this protocol, and most of them have many slight differences.

   We'll call the functional subset MacIP-1.  It is our intention that
   it is compatible with as many of the current implementations as is
   practically possible.  It also describes a set of existing features
   that will not work except in specific topologies.  A future document,
   MacIP-2, will describe a protocol that provides the intended
   features, but does so for all AppleTalk topologies.

   The new proposed gateway architecture describes an Internal Virtual
   MacIP (VIM) , which requires a gateway to maintain an internal
   virtual network that has all zones that are supported in its zones
   list.  This does not require any modifications on existing MacIP host
   implementations, and existing gateways will have a simple single-
   point modification.

1.      Introduction

   The goal of the MacIP architecture is to provide TCP/IP services and
   connectivity to computers that are not directly connected to an IP
   network, but are connected indirectly via an AppleTalk network.
   Typically these are Apple Macintosh computers, but the use of MacIP
   is not restricted to them.

   IP hosts must be connected directly to appropriate media in order to
   communicate with other IP hosts or gateways.  All Macintosh computers
   come equipped with LocalTalk, Apple Computer's medium speed network
   media.  LocalTalk is not capable of carrying IP directly.  Thus,
   MacIP gateways have been developed to provide IP front-end forwarding
   agents on behalf of IP hosts embedded in AppleTalk networks.

   It is recommended that hosts use this protocol only if they do not
   have a direct connection; that is their network media does not
   support IP.  The details IP hosts directly connected to IP networks
   are covered in other well known RFCs.

1.1.    Terminology and Topology

   In this document, the term "MacIP" refers to the encapsulation
   protocol and associated services.  Specific parts of the protocol are
   given more specific names as appropriate.  In a "MacIP internet"
   there are two distinct types of devices.




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MacIP                                                      November 1992


   The first are termed "MacIP hosts", and are usually Apple Macintosh
   computers. They are running applications over TCP/IP, and can use
   MacIP to communicate between themselves within the confines of their
   AppleTalk internet.

   When communication is desired with common IP devices on IP supported
   networks (hereafter called "IP hosts"), the services of the second
   MacIP device, a "MacIP gateway", is required.  A MacIP gateway
   forwards IP datagrams between IP supported networks and MacIP devices
   on AppleTalk networks, as well as provides other server-type
   functions.

   The term "MacIP Packets" specifically refers to IP datagrams
   encapsulated in AppleTalk packets that are sent between the
   forwarding modules in MacIP hosts and gateways.

   The term "MacIP Range" refers to the set of IP addresses configured
   into a MacIP gateway that are considered to belong to the MacIP hosts
   being serviced by that MacIP gateway.

   Other terms will be defined as they are used, and most appear in the
   Glossary.

   The AppleTalk protocols used by MacIP are detailed in section 8 which
   describes DDP (Datagram Delivery Protocol), ATP (AppleTalk
   Transaction Protocol) and NBP (Name Binding Protocol).  If you are
   not familiar with AppleTalk, then please read the relevant sections
   in Inside AppleTalk [1].

1.2.    Intended Audience

   It is expected that there are five different groups of people likely
   to be reading this document: MacIP host implementors, MacIP gateway
   implementors, system managers in trouble, the terminally curious, and
   Jon Postel.

1.3.    Assumptions

   This document assumes the reader is familiar with the AppleTalk suite
   of protocols. AppleTalk is documented in "Inside AppleTalk" [1].  It
   is also assumed that the reader is familiar with the IP suite of
   protocols, particularly IP [2], IP over Ethernet [4], Ethernet ARP
   [3], RIP [9], subnetting  and routing [6], IP host requirements [10],
   and as documented in other various RFC's.

   This document is purposefully confined to the description of the two
   port gateway, where one port is connected to an AppleTalk network and
   one port is connected to a Ethernet IP network.  This is for



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   descriptive simplicity and should not restrict implementations in any
   way.

   The described AppleTalk port of the gateway and of the host does not
   need to be restricted to LocalTalk as AppleTalk may be transmitted
   over other media, such as Ethernet (EtherTalk), Token Ring
   (TokenTalk), a Serial connection (ARAP), or anything else.

1.4.    Structure of this Document

   MacIP is a complex protocol; there are multiple modules in both the
   gateway and the host.  Some of them operate in a client/server mode.
   Others operate peer-to- peer and/or peer-to-proxy.

   The original protocol specification, MacIP-1 is described in sections
   2 and 3.  It begins with a description of the whole system, moves on
   to detailed descriptions of the individual modules, then describes
   the protocols used.

   Section 4 discusses MacIP limitation, and recommends a subset of
   features and the Virtual Internal MacIP architecture.  These
   limitations and recommendations are for informational purposes.

   Section 5 provides a brief synopsis of AppleTalk, and discusses MacIP
   required variations of the AppleTalk Name Binding Protocol, NBP.

   Sections 6 and 7 provide protocol constant definitions and a
   glossary.

   Section 8 provides implementation notes on many of the previous
   sections.  Its section numbering is discontiguous, and follows which
   previous section the note applies to.  These notes are for
   informational purposes.

2.      MacIP Protocol Overview

   MacIP must satisfy requirements imposed by IP, AppleTalk (except
   where noted), the Macintosh, and the MacIP gateway.

2.1.    Required Functionality

2.1.1.  Basic Requirements

   IP connectivity is required between MacIP hosts embedded in an
   AppleTalk network, and IP hosts elsewhere on an IP internet.






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MacIP                                                      November 1992




             [ H1 ]           [ H2 ]        MacIP Hosts
               |                |
               +-------+--------+           AppleTalk Net
                       |
   MacIP GW         [ GW1 ]         [ IP3 ] IP Host
                      ||              ||
                      ++=======++=====++    IP Ethernet
                               ||
   MacIP Host[ H4 ]          [ GW2 ]        MacIP GW
               |                |
               +----------------+           AppleTalk Net

        Figure 1.  Example MacIP Internet


    There are four different cases to consider in the internet shown in
   Figure 1.

        1.      Between a MacIP host and an IP Host (H1 and IP3 via
                GW1).

        2.      Between MacIP hosts on different AppleTalk networks via
                intervening MacIP gateways (H1 and H4 via GW1 and GW2 ).

        3.      Between MacIP hosts in the same zone connected to the
                same MacIP gateway (H1 and H2 via GW1).

        4.      Between MacIP hosts in the same zone without a MacIP
                gateway being present (H1 and H2 directly).

   Cases 1 and 2 are very similar.  Given that MacIP to IP connectivity
   is established, there is no difficulty supporting MacIP to IP to
   MacIP.  Cases 3 and 4 may appear identical, but are not.  Traditional
   implementations require that MacIP packets must follow the most
   "efficient" route, and not go through the gateway when the hosts are
   on the same network.  The following is a simple requirement matrix.


                                          Local with
       From/To   Local   Remote  IP Host  No Gateway
       ---------------------------------------------
       Local   |  Yes  |   Yes |   Yes   |   Yes   |
       Remote  |  Yes  |   Yes |   Yes   |    -    |
       IP Host |  Yes  |   Yes |    -    |    -    |

       Table 1.  MacIP-1 Requirement Matrix



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MacIP                                                      November 1992


2.1.2.  IP Requirements

   In order to function as an IP host, a minimum of two things are
   required:

        1.      An IP Address for the host.

        2.      A means by which to forward packets to the host.

   With these, an IP host can function on  a point-to-point link.  In
   order to function on a broadcast network (such as Ethernet or
   LocalTalk) the following is also required:

        3.      An Address Resolution scheme (ARP) to resolve IP
                addresses to native network addresses.

        4.      A subnet mask to allow local and remote routing
                decisions to be made.

        5.      Gateway address to send off-subnet packets to (more
                sophisticated routing is sometimes desirable, but not
                essential).

   The MacIP protocol can provide all five, although the last two add
   significant complexity.  A variation on ARP provides this
   functionality.

2.1.3.  Macintosh Considerations

   Macintoshes are mobile, and are often moved from one network to
   another.  The AppleTalk protocol handles this automatically without
   requiring any user reconfiguration of the Macintosh.  It would be
   inconvenient to have to reconfigure the IP Protocol stack under these
   circumstances.  The MacIP protocol provides an automatic IP Address
   Assignment protocol that can be used to circumvent the requirement
   for reconfiguration when moved.  IP addresses so assigned are called
   "Dynamic" Addresses.

   Because Macintoshes often are connected directly to Ethernet, the IP
   protocol stacks usually support both direct Ethernet and MacIP modes
   of connection.  The MacIP protocol is modular and designed to attach
   simply to an existing Ethernet implementation.

2.2.    Protocol Relationships

   In order to provide the required functions, MacIP has the following
   relationship to the other protocols in MacIP hosts and gateways:




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MacIP                                                      November 1992



           MacIP Host                      MacIP Gateway
    -------     -------
    | TCP |     | UDP |
    +-----+-----+-----+      ----------------------------
    |       IP        |<---->|            IP            |
    +-----------------+      +--------------------------+
    |      MacIP      |<---->|  MacIP    |  |  Ethernet |
    +-----------------+      +-----------+  -------------
    |     AppleTalk   |<====>| AppleTalk |
    -------------------      -------------

   Figure 2.  MacIP Host and Gateway Connections


   MacIP acts as a Link-layer protocol to IP, while acting as a client
   of all the session, transport and network layers of AppleTalk.  This
   should serve to warn readers of the complexities ahead:


           Layer Name      Protocol
                           -------     -------
           4 - Transport   | TCP |     | UDP |
                           +-----+-----+-----+
           3 - Network     |       IP        |
                           +-----------------+
           2 - Link to IP  |.................|
           5 - Session to  |      MacIP      |
               AppleTalk   +-----+-----.     |
           4 - Transport   | ATP | NBP |_____|
           3 - Network     | AppleTalk - DDP |
           2 - Link        | AppleTalk - LAP |
                           -------------------

          Figure 3.  Relation Between IP and MacIP


   [Phil doesn't like the above diagram.  Shall it go in the appendix?]

2.3.    Protocol Mapping

   MacIP uses the zone-wide protocol NBP to perform the ARP function.
   This makes for a very simple ARP implementation on a Macintosh as
   most of the code is already built in to the operating system.  It
   does lead to the following unfortunate restrictions:

        1.      There has to be one MacIP gateway per zone,




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        2.      There can't be more than one MacIP gateway per zone, and

        3.      MacIP hosts can't use a MacIP gateway in a "remote"
                zone.

   There have been many attempts to overcome the above restrictions, but
   they are all "outside" of MacIP-1 and cause problems of their own.

2.4.    MacIP Functions and Services

   MacIP provides address assignment, address resolution and packet
   transport services.

2.4.1.          Address Assignment

   The MacIP gateway contains an Address-Assignment module which is
   configured with a set of IP addresses to assign to MacIP hosts.  The
   module advertises its presence on the network with NBP registration,
   lookup, and confirmation.  The MacIP gateway is discovered by a MacIP
   host during the initialization of the MacIP host's protocol stack,
   then an IP address can be requested and granted.  MacIP also allows
   for a MacIP host to be assigned a fixed or "Static" IP address within
   a range of addresses known to the MacIP gateway.

2.4.2.  Address Resolution

   Ethernet-connected IP hosts use the Ethernet Address Resolution
   Protocol (ARP) to discover the hardware address corresponding to a
   required IP address.  The AppleTalk NBP protocol provides similar
   capabilities and is used to implement the address resolution function
   in MacIP.  This is referred to as NBP ARP.

   When any device supporting MacIP acquires an IP address, it registers
   it through its local NBP process.  It is then visible to NBP ARP
   requests from other MacIP devices. There is the added advantage of
   possibly discovering configuration errors caused by duplicate IP
   addresses, as NBP guarantees unique registration within the local
   zone.

   With Ethernet ARP, the "working range" of an ARP request is the IP
   subnet, which corresponds to the "reach" of the Ethernet broadcast
   packet.  With NBP ARP, the "broadcast reach" corresponds to an
   AppleTalk construct called a "Zone".  A zone consists of one or more
   AppleTalk networks, the actual topology of which is controlled by the
   network administrator.

   The zone that the MacIP gateway and the hosts are in is referred to
   as the "MacIP zone".  The zone that a particular device is in is



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MacIP                                                      November 1992


   referred to as its "local zone".

   Therefore there is a direct correspondence between an IP Subnet and
   an AppleTalk Zone for NBP ARP.  Unfortunately the same does not apply
   for the delivery of any other sort of AppleTalk or MacIP packet, as
   the "broadcast reach" corresponds to a single AppleTalk network.

2.4.3.  Transport

   Transport of IP datagrams over LocalTalk is achieved by encapsulating
   them in Datagram Delivery Protocol (DDP) packets and sending them
   over an AppleTalk internet.  The destination device can be another
   Macintosh computer supporting MacIP or a gateway.  The latter can
   either be explicitly selected or discovered through a proxy-based
   process.

3.      MacIP Protocol Specifics

   This section describes the MacIP protocol as originally implemented
   and documented in the Stanford KIP gateway code.  This forms the
   simplest possible version of MacIP, and one which should be supported
   by all host and gateway implementations.

3.1.    Gateway Addressing Styles

   There are two alternative approaches to integrating an AppleTalk
   network into an IP network.  One approach involves treating the
   AppleTalk network as an IP subnet, with the MacIP gateway assuming
   the role of an IP router.  The alternative is to allocate to the
   AppleTalk network a small range of addresses "stolen" from the
   Ethernet IP subnetwork that the MacIP gateway is connected to.  In
   this case, the MacIP gateway forwards IP packets to and from
   AppleTalk.

   The forwarding method is conceptually easier, and thus easier to
   configure.  No large range of subnet addresses needs to be calculated
   and allocated to the AppleTalk network, and no changes need to be
   made to the rest of the network.

   The routing method is more difficult conceptually and, hence, harder
   for an administrator to configure.  It is, however,  more consistent
   with the requirements of many large sites, and can be more practical
   in complicated networks.  This is especially true if the MacIP
   gateway will emit Routing Information Protocol (RIP, [9]) packets to
   inform the Ethernet network of the MacIP AppleTalk subnet.

   MacIP gateways conforming to MacIP-1 MAY implement either or both of
   these styles.  This doesn't affect MacIP hosts as they should not be



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MacIP                                                      November 1992


   able to tell the difference.

3.1.1.  MacIP Forwarding

   When forwarding with the MacIP architecture, the AppleTalk network is
   treated as an extension of the Ethernet IP network.  This is done by
   situating the "MacIP Range" within the range of IP addresses defined
   by the Ethernet IP network.  When a host on the Ethernet ARPs for an
   IP address which is in the MacIP range, the gateway will answer,
   performing the proxy ARP function [8].

   For example, if the Ethernet has the IP subnet "192.9.200.0", then
   the MacIP gateway might be configured to assign the addresses
   "192.9.200.100" through "192.9.200.150" to MacIP hosts.  The gateway
   will respond to ARP requests on Ethernet to all these addresses, plus
   its own IP address.

3.1.2.  MacIP Routing

   Routing via the MacIP protocol is straightforward from the
   perspective of IP routing.  The gateway is configured with two IP
   addresses and subnet masks, one for the Ethernet and one for the
   AppleTalk networks.

   As the MacIP gateway is acting as an IP Gateway (and is thus
   performing IP routing), it is necessary for the TCP/IP hosts on the
   Ethernet side of the gateway be informed of the existence of the
   subnet corresponding to the MacIP Range, and that the MacIP gateway
   is the gateway to this subnet.  This can be done via static routing
   tables or via the RIP protocol.  MacIP gateways MAY provide either a
   full or conservative (the latter only advertises the MacIP subnet)
   RIP implementation in the MacIP gateway.

3.2.    MacIP Initialization

3.2.1.  Configuration Required For MacIP Hosts

   The MacIP host requires an IP address for configuration.  "Dynamic"
   or "Static" addresses refer to the method by which the address is
   acquired.  A dynamic address is assigned from the MacIP gateway's
   address range.  A static address is assigned at the MacIP host, then
   confirmed through the MacIP gateway.

3.2.2.  MacIP Host Initialization

   The initialization code is responsible for finding the MacIP
   gateway's Address Assignment server using NBP, requesting "server"
   information, acquiring an IP address, either from the address



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MacIP                                                      November 1992


   assignment service or from a statically-configured address, and
   registering the MacIP host's IP address with NBP.

3.2.2.1.        Locating the  MacIP gateway's Address Assignment Server

   The Address Assignment Service in the MacIP gateway [Server] is
   assumed to have registered itself with NBP with a type of
   "IPGATEWAY".  The MacIP host initialization process uses NBP to
   search for "=:IPGATEWAY@*", which performs a search for all Servers
   in the same zone that the MacIP host is in.  Under MacIP-1 the
   implementation assumes that it will only receive one response from
   one gateway. Multiple gateways in one zone are not covered in MacIP-
   1.

3.2.2.2.        Requesting Server Information

   The MacIP host and the Server exchange information using a protocol
   called "MacIPGP", described later.  The MacIP host can optionally
   send a MacIPGP SERVER request to the Server, and SHOULD then receive
   a response packet.  The information in the response packet is mainly
   obsolete and not very useful, although the returned IP Broadcast
   address might be usable from some gateways.

3.2.2.3.        Requesting a Dynamic IP Address

   If the MacIP host is not configured to use a Static IP address, it
   sends a MacIPGP ASSIGN request to the Server.  It will either respond
   with an appropriate IP address or an error status and optional
   message which should be displayed to the user. Errors at this point
   are non-recoverable.

3.2.2.4.        Registering the IP Address

   The MacIP host registers its IP address through NBP.  It MUST use its
   IP address in dotted decimal notation as its NBP Name.  This
   representation is the four bytes of the IP address, in network order,
   in decimal with no leading zeros and separated by periods.  For its
   NBP Type, the string "IPADDRESS" MUST be used.  For example, to
   register the IP address 131.161.1.2, the NBP registration would be
   "131.161.1.2:IPADDRESS@*".

   If the registration fails then it is an indication of a duplicate IP
   address, a gateway, or a network misconfiguration.  The failed
   address MUST NOT be used.  This SHOULD be clearly reported to the
   user.

   The MacIP host MUST register on DDP socket 72.  Some current MacIP
   gateway implementations assume that the MacIP host is using socket 72



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   whether it is or not, so using anything else is not advisable.

3.2.2.5 Closing Down

   When the MacIP protocol stack closes down, it must remove its IP
   address registration from NBP.

3.2.3.  Configuration Required For MacIP Gateways

   A MacIP gateway has to be configured with an IP address, a subnet
   mask and (possibly) a default gateway.  It also needs to be
   configured with the "range" of IP addresses that are to be allocated
   to the MacIP hosts so that the gateway can provide address assignment
   and forwarding service to its MacIP hosts..  The "union of all IP
   addresses that can be assigned to MacIP hosts and that the MacIP
   gateway is required to forward packets to" is called the "MacIP
   Range".  Historically, this is a single contiguous range, but
   implementations are not confined to this.

   This range contains a "Dynamic" and a "Static" range, either of which
   may be empty.  The "Dynamic" range consists of IP addresses that can
   be allocated to MacIP hosts on request.  The "Static" range consists
   of IP addresses that can be configured into MacIP hosts that require
   the same IP addresses all the time.  The MacIP gateway will not
   forward packets to a MacIP host that has an IP address outside of
   this MacIP range.

3.2.4.  MacIP Gateway Initialization

   The initialization code in a MacIP gateway is responsible for setting
   up certain data structures used by other modules, registering the
   Address Assignment server and certain IP addresses with NBP, and
   performing an initial search for already- registered MacIP hosts.

3.2.4.1.        Proxy ARP Initialization

   If the MacIP gateway is configured in "forwarding" mode , then the
   ARP module is initialized to respond to the "MacIP range" of IP
   addresses in addition to other MacIP gateway addresses.

3.2.4.2.        Registration of Address Assignment Server

   The MacIP gateway MUST register itself through NBP, using the type
   "IPGATEWAY", on DDP socket 72.  It is necessary for the registration
   to be unique in the zone, and early implementations guarantied this
   by using as the NBP name field (which has to be unique), the dotted
   decimal representation of their IP address.




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MacIP                                                      November 1992


3.2.4.3.        Registration of IP Addresses

   The IP addresses that the MacIP gateway has that are within the MacIP
   Range SHOULD be registered with the NBP protocol on the gateway in
   the same way that IP addresses are registered on MacIP hosts.  This
   guarantees that MacIP hosts will not succeed in registering the same
   address in the same zone.  Also, this makes the IP addresses visible
   to both MacIP hosts (that may wish to send datagrams to these IP
   addresses) and to network management software.  If the registration
   fails then MacIP MAY not be able to function on the gateway, and
   appropriate actions should be taken.  "Passive Registration" (section
   5.3.2.1) SHOULD not be used.

3.2.4.4.        Reregistration Function

   The MacIP gateway is responsible for assigning unique IP addresses to
   MacIP hosts. If the gateway has been running, has assigned addresses
   and is then restarted (or crashes), it is in danger of reassigning
   the same addresses to other MacIP hosts.  In order to recover the
   previous assignments, the MacIP gateway uses NBP to search for
   "=:IPADDRESS@*", which will locate all IP addresses registered with
   NBP ARP in the zone.  The NBP responses are directed back to the
   Initialization module. Those discovered IP addresses that are within
   the dynamic range assignable by the gateway can be used to initialize
   the assignment table.

   It is important that the MacIP gateway attempts to use the same
   AppleTalk node address after a restart that it had before, as the
   MacIP hosts will have the node address of the gateway stored in NBP
   ARP tables and elsewhere.  The gateway should not use a "random" node
   address on restart.

3.3.    Proxy ARP

   As mentioned in 4.1.1, when configured to act as a "Forwarding
   Gateway", the MacIP gateway must perform Proxy ARP for the MacIP
   Range of addresses.  This is simply implemented by adding the MacIP
   Range to the IP Address(es) that the MacIP gateway's Ethernet ARP
   process will respond to.  All IP addresses in the MacIP range are
   proxied for, whether there are MacIP hosts using these IP addresses
   or not as this simplifies the implementation.

   Proxy ARP MUST be disabled when the MacIP gateway is configured to
   act as a "Routing Gateway".

3.4.    NBP ARP - Address Resolution

   Any MacIP device (host or gateway) can resolve an IP address to an



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MacIP                                                      November 1992


   AppleTalk address by using NBP to search for the device that has that
   IP address registered.  The name is the requested IP address in
   "dotted decimal" notation and the type is "IPADDRESS".  NBP requires
   repeat and delay specifications (the number of retries and the delay
   between them).  These should be set particularly leniently,
   especially considering that MacIP may be running over WANs and/or
   ARAP (Apple Remote Access Protocol).  Recommended values are given in
   section 10, "Definitions".

   NBP ARP is functionally very similar to Ethernet ARP, and can often
   be implemented by using the host or gateway Ethernet ARP code and
   data structures. Both the MacIP host and gateway are assumed to
   implement a cache for the addresses resolved by NBP ARP.

   With Ethernet ARP, the "working range" of an ARP request is the IP
   subnet, which corresponds to the "reach" of the Ethernet broadcast
   packet.  With NBP ARP, the "reach" corresponds to an AppleTalk
   construct called a "Zone".  A Zone consists of one or more AppleTalk
   networks, the actual topology of which is controlled by the network
   administrator.

   There is therefore a direct correspondence between an IP Subnet and
   an AppleTalk Zone, but ONLY when performing NBP ARP, and not when
   routing certain packets such as those to an IP broadcast address,
   such as might be used by RIP or RWHO.

3.4.1.  NBP ARP - Details

   The NBP ARP module is passed an IP address to resolve by the delivery
   module. Resolution is first attempted by searching for a matching IP
   address in the local ARP cache.  A successful match should reset any
   usage timers.  If a no match is found, then the address has to be
   searched for.

   The search on the network is made by using NBP to send an NBP LookUp
   to all devices in the MacIP zone.  The entity-name in the LookUp is
   the dotted-decimal representation of the IP address (see section
   6.2.8).  The entity type is "IPADDRESS". The entity zone is the MacIP
   zone.  The retry count and time is as specified in section 10.

   The NBP ARP response carries the AppleTalk address of the
   destination.  The full AppleTalk address (net, node and socket -
   socket 72 is not to be assumed) must be stored in the ARP cache
   together with the IP address.

3.5.    Delivery

   IP packets including the full IP header MUST be encapsulated in DDP



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MacIP                                                      November 1992


   packets of type 22 (decimal).  The source and destination sockets of
   the packet are 72 (decimal) by convention.

   The AppleTalk DDP protocol limits the data size of a DDP packet to
   586 bytes, which is then the maximum possible Maximum Transmission
   Unit (MTU) of the AppleTalk network when transporting IP datagrams.
   The MTU of 576 is more commonly used so as to conform to the minimum
   IP MTU.

   The Ethernet network has an MTU size of 1486 bytes.  The smaller MTU
   size of AppleTalk requires that gateways must fragment Ethernet
   packets bound for AppleTalk which are larger than the AppleTalk MTU
   [3].

   Packets received by the MacIP host MUST be dropped (not processed) if
   the destination IP address does not match the IP address of the MacIP
   host.  No "IP Broadcast" destination addresses are allowed.  This
   function can be performed either by the Delivery or IP modules.

3.6.    MacIP Routing Decisions

   MacIP's design imposes restrictions that renders MacIP hosts
   incapable of correctly performing IP Broadcasts and of correctly
   interpreting ICMP Redirects.  Both of these features are required in
   IP implementations.  However, current MacIP host implementations do
   attempt this, so the feature described in the following section was
   developed.

3.7.    Gateway NBP Proxy ARP

   In order to support the simple routing method used by the MacIP
   client, it is necessary for there to be a service in the MacIP
   gateway that performs the equivalent of a "Proxy ARP" service, but
   for all of its MacIP clients.  The NBP Proxy ARP service must respond
   to all NBP ARP requests for all IP addresses EXCEPT for the ones in
   the MacIP range (those allocated to the MacIP clients).

   NBP Proxy ARP MUST respond to NBP requests with the IP address being
   requested, the type "IPADDRESS" and the AppleTalk net, node and
   socket (72) address of the "Delivery" module.  The NBP Proxy ARP
   module MUST NOT respond to "wildcard" lookups for type "IPADDRESS".
   NBP Proxy ARP has to be implemented on a "non-standard" NBP
   implementation described in section 5.3 as "Conditional NBP" (CNBP)..

3.8.     MacIP Gateway Protocol

   MacIPGP is a simple request-response protocol based on ATP (AppleTalk
   Transaction Protocol) ALO (at-least-once) transactions.



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MacIP                                                      November 1992


   The MacIP host sends an ATP TREQ (ATP Request) packet to the
   AppleTalk address of the Address Assignment Server, and the MacIP
   gateway responds with an ATP TRSP (ATP Response) packet.

   There are two defined functions which MacIP-1 uses.  These are "get
   server info" (SERVER) and "assign IP address" (ASSIGN).

3.8.1.  SERVER Function

   The SERVER function returns a list of common server IP addresses,
   such as name server, file server and the broadcast address.  These
   are usually defined as part of the gateway's configuration and simply
   passed on via the protocol without interpretation, so their specific
   contents is not part of the MacIP protocol.  Most of these can be
   considered "obsolete".  Their "usual" designation is detailed below.

3.8.2.  ASSIGN Function

   The ASSIGN function returns an IP address which can be used by the
   MacIP host to communicate with other IP hosts.  The following is the
   description of the implementation in the December 1986 KIP code in
   the "ip.at" document.

   The gateway is configured with a "Dynamic Range" range of IP
   addresses and maintains a table of these containing the fields:

     IP address;  timer;  flags;  AppleTalk address (including socket);

   When the MacIP Client sends an ASSIGN request to the gateway, the
   gateway searches the table described above.  The service tries to
   reassign the same IP address to the same AppleTalk address if
   possible.  Otherwise any free IP address is used.  If an IP address
   is available, it is sent in an ASSIGN Reply packet, and the timer
   field of that table entry will be started.

   Thereafter, every "Confirm Period" (60 seconds if not configurable),
   an "echo command"(NBP ARP Confirm) is sent to the client and the
   timer bumped.  Echo replies received will restart the timer.  If 5
   periods pass with no replies received, that table entry will be
   available for potential reassignment.  In making "new" table
   assignments the timer field is used to locate the oldest unused table
   entry.  This increases the chances of a given MacIP host to keep
   reusing its previous IP address assignment.

   It is important to note that IP addresses assigned via the gateway
   ATP protocol must be registered and resolved using the NBP ARP
   technique.




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MacIP                                                      November 1992


3.8.3.  ERROR Response

   The MacIPGP protocol request return an error if a request other than
   ASSIGN or SERVER is made.  The ASSIGN response may return an error if
   there are no Dynamic addresses available.

3.8.4.  MacIPGP Packet Definitions

   The complete MacIPGP packets consists of:

     1.      Data-link Header,

     2.      DDP Header,

     3.      ATP Header,

     4.      MacIPGP Header,

     5.      MacIPGP Data Packet (only on MacIPGP Response packets).


   It has the format:


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   /                       Data Link Header                        /
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   /                          DDP Header                           /
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  ATP Control  |  ATP Seq. No  |      ATP Transaction ID       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    ATP User Bytes - Ignored                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 MacIPGP Request or Response Code              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   /                  MacIPGP Data (variable length)               /
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 6.  MacIPGP Packet




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MacIP                                                      November 1992


3.8.5.  ATP Control Fields

   The Control Info, Sequence Number and Transaction ID fields are
   documented in Inside AppleTalk.  The MacIP gateway returns these
   fields as-is to the MacIP host except that the Control Info field is
   changed from a TREQ to a TRSP.

3.8.6.  ATP User Bytes

   These four bytes are unused in MacIP-1 and can be expected to contain
   random data.


3.8.7.  MacIPGP Request and Response

   If the ATP Control info is TREQ, then this is a packet from the MacIP
   host to the gateway.  The MacIPGP Data field is empty (zero length).
   The defined values of the MacIPGP Request field are:


        Val.    Name    Meaning
        1       ASSIGN  Request assignment of Dynamic address.
        3       SERVER  Return Server Information.
        -1      ERROR   Only valid in MacIPGP Response packet.


   ASSIGN is a request for the MacIP gateway to assign an IP address
   from its configured "Dynamic Range" to the MacIP host.  SERVER is a
   request for the "Server Information" to be returned.

   The MacIP gateway normally returns the packet as-is with the MacIPGP
   Response field in the returned packet set to the MacIPGP Request
   field in the received packet. If an error occurred, then the Response
   field is set to "-1" (hex FFFFFFFF) with an optional zero-terminated
   error string returned in the "Error Message" field.  The length of
   the data field of the returned ATP packet is 64 bytes plus the length
   of the terminated error string.  If the string is empty then 65 bytes
   are returned.

3.8.8.  MacIPGP Data Field

   The MacIPGP Data Field is returned in MacIPGP Response packets.  The
   format is:








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MacIP                                                      November 1992



    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Assigned IP Address                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Name Server IP Address                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Broadcast IP Address                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       File Server IP Address                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |_                      Other IP Addresses (16 octets)         _|
   |_                                                             _|
   |_                                                             _|
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Error Message (NULL terminated)         |
   /                       128 bytes maximum                       /
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 7.  MacIPGP Data Field


   Originally the Name-server, Broadcast, File-server and Other fields
   were simply copies of configuration data transferred to the MacIP
   gateway by the AppleTalk Administrative Daemon (atalkad).  Their
   meaning was thus a "private matter" between the administrator and the
   MacIP host code in use, and did not involve the MacIP gateway at all.
   It did not interpret or generate the data (except for the Broadcast
   field which the MacIP gateway does use), it only transferred it.

   Some manufacturers have changed this simple relationship by using the
   atalkatab fields for gateway configuration and/or having the gateway
   generate the data transferred to the MacIP host.  Thus, the meaning
   is unclear, and thus for MacIP-1, undefined.

3.8.8.1.        Assigned IP Address

   This is the IP address assigned by the MacIP gateway to the MacIP
   host.  It is only returned in ASSIGN response packets.  This field is
   only valid in ASSIGN response packets.

3.8.8.2.        Name Server IP Address

   This historically has been used to contain the IP address of an IEN-
   116 Domain Name Server.



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MacIP                                                      November 1992


3.8.8.3.        Broadcast IP Address

   This may be the Ethernet IP broadcast address which is only
   "appropriate" for the MacIP hosts if the MacIP gateway is configured
   in "Forwarding" mode.  If it is in "Routing" mode, this field could
   be anything.  MacIP hosts MUST not rely on this data.

3.8.8.4.        File Server IP Address

   Originally the address of EFS (Electronic File Server) included with
   CAP v4 (Columbia AppleTalk Package).  Obsoleted by AUFS.

3.8.8.5.        Other  IP Addresses

   Available for "private" use between consenting MacIP hosts and MacIP
   gateway configurations.  These may contain the "IPOTHER" fields from
   the "atalkatab" file, but they may not.

3.8.8.6.        Error Message

   If the returned MacIPGP Response code is -1, this may contain a 128-
   byte maximum null-terminated error message.  Otherwise it is a zero-
   length null-terminated string (one byte of null).

3.8.9.  Delivery Packets

   If the Address Assignment Service does not have the same AppleTalk
   address as the Delivery Module (as returned by NBP Proxy ARP), then
   it may still receive delivery packets (IP packets of DDP type 22)
   sent by MacIP hosts disobeying MacIP-1.  For backward compatibility,
   these packets SHOULD be forwarded to the Delivery module.

4.      MacIP Limitations and Recommendations

   The specification of MacIP imposes certain basic restrictions on
   operations of both hosts and gateways, particularly on the allowable
   network topology.  Most MacIP gateway vendors have found different,
   incompatible, and incomplete methods ways to work around these basic
   protocol limitations.

   This specification recommends methods for managing the restrictions
   on both hosts and gateways.  The host recommendations focus on the
   routing decisions hosts are attempting, and the gateway recommends
   that MacIP gateways reside on a virtual internal network.  This
   network's zones list contains all zones supported by the MacIP
   gateway, and provides a more reliable mechanism for NBP ARP.  This
   gateway and topology architecture is referred to as Virtual Internal
   MacIP (VIM).



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MacIP                                                      November 1992


4.1.    MacIP Routing Decisions

   MacIP 1 recommends that the MacIP host perform no IP routing
   decisions whatsoever.  There are many advantages to this requirement:

        1.      It simplifies the MacIP host implementation,

        2.      It minimizes the network-specific information that has
                to be sent to or configured into the MacIP host,

        3.      It allows for MacIP to be extended so that Hosts can use
                the services of a MacIP gateway from "out of zone".

   Because of restrictions imposed by the design of MacIP, MacIP hosts
   are incapable of performing IP Broadcasts and of correctly
   interpreting ICMP Redirects, both of which would be expected of a
   full IP implementation.

   To be specific, it is mandatory that MacIP 1 Hosts should not do any
   routing-related operations and should obey the following:

        1.      They must use NBP ARP to resolve all IP addresses, no
                matter what they are, even IP addresses that may be, or
                are manifestly in another IP subnet or network, and
                includes any possible broadcast IP addresses.

        2.      They must ignore all subnet masks, network and subnet
                numbers and all possible "default gateway" addresses.

        3.      They should not send IP Datagrams to either the
                AppleTalk or IP address of the Address Assignment Server
                (received as the "name" field of the NBP LookUp).

        4.      They should ignore all ICMP Redirect messages and RIP
                packets that they may receive.

   The simplest way to disable all off-host routing decisions in an
   existing IP (or MacIP) implementation is to set the subnet mask to
   "0.0.0.0", and to disable any other "special-case" or error-checking
   code that may defeat the intention of this.

   The MacIP host code should not assume that the Address Assignment
   server and the NBP Proxy ARP module have the same socket address, or
   even the same AppleTalk address, as it is permissible for them to be
   implemented on different hardware and even on different networks.

4.2.    Multiple Gateways in One Zone Limitation




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MacIP                                                      November 1992


   As detailed in section 3.7 (Gateway NBP Proxy ARP), having multiple
   MacIP gateways in the same zone will completely prevent MacIP hosts
   in that zone from working.  The following hacks have been invented to
   get around this:

4.2.1.  NBP Proxy ARP Directed Port

   With this method, the MacIP gateway restricts the operation of NBP
   Proxy ARP to requests received from networks connected via particular
   physical ports, (usually the LocalTalk port, where the majority of
   MacIP hosts usually are).  This allows multiple MacIP gateways to
   exist in the same zone as long as the network topology guarantees
   that there is never more than one MacIP gateway's directed port on
   one AppleTalk network.  This approach also requires NBP (CNBP) in the
   gateway to not answer NBP LookUps for "IPGATEWAY" unless they
   originate from the same port that NBP Proxy ARP is enabled on.  This
   asymmetry plays havoc with network management; devices will be
   visible depending on where in the AppleTalk topology devices are
   looking from.

4.2.2.  NBP Proxy ARP Hop Limit

   The DDP Hop Count is available to the NBP Proxy ARP process.  It can
   refuse to answer any requests from MacIP hosts that aren't zero hops
   away (directly connected).  This solves some problems at the expense
   of restricting the topology.  It also will not work if two gateways
   share a common network.

4.2.3.  Friendly Requests - Dynamic

   In this method, the NBP Proxy ARP module will only answer requests
   from the SAME MacIP hosts that the Address Assignment module knows
   about.  The source AppleTalk address of the incoming NBP ARP is
   compared against all entries in the Assigned Address table.  If the
   AppleTalk address is present (and the MacIP host is not registering
   its own address), then the NBP Proxy ARP module responds.

   This fails completely with MacIP hosts that have Static IP addresses,
   as they aren't in the in the table.

4.2.4.  Friendly Requests - Static

   Various methods have been used to force friendly requests to work
   with static hosts. In all cases the gateway has to somehow record the
   AppleTalk addresses of MacIP hosts with static addresses.  This table
   has to be filled with periodic "Reregistration" calls (3.3.4.4), or
   "directed wildcard NBP ARP" calls have to be issued to "suspected
   hosts".  VIM provides a superior solution.



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MacIP                                                      November 1992


4.2.5.  NBP Proxy ARP Not Required

   If the MacIP host disobeys MacIP-1 and sends all packets to the MacIP
   gateway then NBP Proxy ARP may not be required, but MacTCP (Apple's
   MacIP driver for the Macintosh) doesn't allow this.

4.2.6.  Other Multiple Gateway Problems

   Given a choice of multiple MacIP gateways, MacIP hosts will
   invariably pick the worst possible one - the one "furthest away".  To
   date, MacIP hosts cannot select which MacIP gateway to attach
   themselves to.  Thus, it is a race condition for NBP responses to
   return to the MacIP host.  Generally, the first response received is
   used. See section y.y.y "Out-of-order NBP" for a discussion.

4.3.    Out-of-Zone Limitation

   NBP ARP can only work between devices that are in the same zone.  In
   spite of this, most MacIP implementations try to allow the MacIP
   hosts to be in a zone different to that of the MacIP gateway (the
   MacIP zone).  The problems for the MacIP host in the wrong zone are:

        1.      Its NBP ARP Registration won't find duplicate IP
                addresses,

        2.      It can't answer NBP ARPs from other MacIP hosts,

        3.      It can't answer NBP ARPs from the MacIP gateway,

        4.      The gateway "reregistration" function can't find this
                host.

   The following "Out-of-Zone-Hacks" have been used in existing
   implementations. We none of them, and suggest VIM instead (described
   in the next sub-section).

4.3.1.  Ask Address Assignment - Dynamic

   MacIP hosts that have requested Dynamic IP addresses have their
   AppleTalk and IP addresses in the Address Assignment Module's table.
   NBP ARP can be modified to check this table.  This is a very
   incomplete solution as it only solves problem (3) above, while
   ignoring (1), (2) and (4).  It can't work with Statically-addressed
   hosts at all.

4.3.2.  NBP ARP  Reverse Resolution

   To allow Static-IP address MacIP hosts to operate out-of-zone, it is



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MacIP                                                      November 1992


   possible for the MacIP gateway to "listen" for NBP ARPs (as it does
   in 4.1.2 Friendly Requests). These may be the Static hosts attempting
   to "register" their IP addresses in the zone of the MacIP gateway
   (they should do this in order to discover duplicate registration, but
   none do).  It may be the Static hosts performing NBP ARP in the MacIP
   gateway's zone.  In either case, if the MacIP gateway doesn't find
   the source AppleTalk address in the table, it then sends a "directed
   wildcard NBP ARP" to the source AppleTalk address.  Any response will
   be directed by CNBP to the "reregistration" part of the
   Initialization Module, and will serve to register the address for the
   next time.

   This fails to work for statically addressed hosts that are running an
   IP service application.  IP service applications tend not to send
   unsolicited packets, and thus no address mapping exists, preventing
   address resolution.  It still doesn't solve problems (1) and (2)
   either.

4.3.3.  Glean From MacIP Packets

   Some Static hosts may default to sending MacIP packets to the MacIP
   gateway.  The IP to AppleTalk address mapping can be "gleaned" from
   these packets.  There are two problems here.  Firstly it is
   computationally expensive to "glean" every MacIP packet.  Secondly it
   relies on the host sending the packet to the gateway, and not all do.
   It doesn't work with hosts running a server application, as they
   don't send any gleanable MacIP packets.

   Gleaning can partly solve the problem that reregistration can't work
   with out-of- zone hosts - the next MacIP packet from the MacIP host
   after a gateway restart will update the table.

4.4.    Virtual Internal MacIP Recommendation

   We recommend that MacIP gateways implement a virtual internal network
   that has no physical port associated with it, but has a network range
   and zones list.  The zone list contains all the zones that MacIP
   hosts are going to reside.  Then, the MacIP gateway is made visible
   in all of those zones by registering itself through NBP to all MacIP
   hosts in those zones.  If the MacIP Host is not in the same zone as
   the MacIP Gateway, then you don't get in.

   This simplifies everything back to "Classic 1986 MacIP" which we all
   have code for. All existing MacIP host code works, both "in" and
   "out-of" zone (because there aren't any out-of-zones).  Pretty much
   all of the existing MacIP Gateway code should be able to remain "as-
   is" as well, and should be straight-forward to document and
   instrument.



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MacIP                                                      November 1992


   It is possible to make the "VIM zone list acquisition" automatic.
   When a new "MacIP Zone" is found (by a MacIP host trying to
   register), the gateway uses RTMP to "poison" the old VIM network
   range (let's say it was AppleTalk network 10-10), creates a new VIM
   network that overlaps the old one (say network 10-11) with all the
   old zones and the new one in it, and then advertises that with RTMP.
   Disabling "automatic zone list acquisition" counts as a marketable
   "security feature".

   We could then optionally add Phil Budne's idea of having MacIP
   Gateways search for other MacIP Gateways (using NBP to look for
   "=:IPGATEWAY@zzz"), and then send RIP packets to them - we've just
   solved all of the tricky "RIP-in-MacIP" problems too.

   For the "efficiency purists" who demand "minimum path" delivery
   between MacIP hosts that are in different zones, that's now easy to
   support too.  In this case, NBP Proxy ARP would look in its mapping
   table for the IP address in question, and use the mapped AppleTalk
   address in the NBP Response's Entity address fields.  This
   effectivley provides an address translation service, or could be
   considered NBP Proxy ARP redirect.

5.      AppleTalk Basics and Variations

   For those not familiar with AppleTalk, this section gives a very
   brief summary of the parts of AppleTalk used by MacIP.  The reference
   for AppleTalk is "Inside AppleTalk, Second Edition", published by
   Addison Wesley.

5.1.    AppleTalk Addresses

   Devices on an AppleTalk Internet are uniquely identified by a 16-bit
   network number combined with an 8-bit node number.  These addresses
   are handled very similarly to the net and host part of a Class C IP
   address.  MacIP has no special requirements on AppleTalk addresses.

5.2.    AppleTalk Zones

   AppleTalk networks are grouped together into named collections called
   Zones.  This is implemented in the routers responsible for the
   network numbers by associating a Zone Name (or list of zones for
   extended networks) with each network.  Zone names form the user-
   visible topology of AppleTalk Internets, hiding the actual network-
   level topology.

5.3.    Name Binding Protocol

   NBP provides registration and location services for named entities



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MacIP                                                      November 1992


   within zones.  A service entity begins by attempting to register a
   "name" and a "type" with the local NBP software.  NBP places the
   name:type combination into a local registry, so their nodes may
   locate it, and then (with the cooperation of the local router)
   broadcasts "LookUp" packets on every network that is associated with
   the host's zone.  If the name:type is already registered in that or
   some other host, a "LookUp Reply" packet will be received by NBP and
   the registration attempt fails.  Generally, the service will modify
   the its name, and re-attempt registration.  If no LookUp Replies are
   received, then the registration is considered successful.

   Once a name:type is registered, NBP will answer searches made by
   other devices for that name:type, and will supply the AppleTalk
   address (net:node:socket) of the registered entity.  Wildcard LookUps
   are permitted on both the name and type fields.

5.3.1.  Strict NBP

   The previously-described NBP is the type implemented by the Macintosh
   Computer OS.  It is characterized by "Active NBP Registration" where
   NBP always performing a "uniqueness-search" on registration, and
   "Unconditional NBP Replies" where NBP unconditionally answers all NBP
   LookUps that match registered entities.

5.3.2.  Loose NBP

   There are a lot of ways to abuse NBP and to step outside the purposes
   for which it was written.  The following are some of the ways that
   have been found to abuse it, collectively called "Loose NBP".  The
   problems caused are mainly those of confusion, both to network
   managers and to network management software.

5.3.2.1.        Passive NBP Registration

   Some NBP implementations perform "Passive NBP Registration" by
   skipping the "uniqueness search".  For the case of registering single
   unique entities this can only be described as poor practice, as it
   will fail to detect duplicate registration.  It may also confuse
   network managers who are monitoring the device to see if it is
   working properly.

   In the case of implementing NBP Proxy ARP it is impractical to
   "Actively Register" the 4,261,412,864 IP addresses that the MacIP
   gateway is proxying for.

5.3.2.2.        Conditional NBP Replies

   NBP can also be abused to allow "Conditional NBP Replies" (CNBP)



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MacIP                                                      November 1992


   where the generation of the NBP LookUp Reply does not depend solely
   on the contents of the local NBP Registry, but depends on the
   requesting device, or the particulars on what is being asked for.
   This isn't possible on a Macintosh as NBP is built into the OS. Again
   NBP Proxy ARP requires this.

5.3.2.3.        Out-of-order NBP

   A packet monitor observing an NBP transaction would expect to see the
   following operations in the following order, and may in fact depend
   on the order to correctly report the operation:

        1.      MacIP host wishing to search for a particular named
                entity within its own zone sends NBP BrRq (Broadcast
                Request) to a local AppleTalk router,

        2.      The router rebroadcasts the NBP Query as an NBP LkUp
                (LookUp) on all networks in the requested zone,

        3.      One or more LkUp Replies are sent from the searched-for
                entity to the requesting host.

   In the case of a MacIP host searching for a MacIP gateway there
   exists a serious problem.  The router that the MacIP host sent the
   NBP BrRq to is likely to be the most appropriate MacIP gateway.
   However while this router/gateway is busy performing step (2) above,
   other MacIP gateways in the same zone are likely to be on step (3).
   Current MacIP host code will use the first  LkUp Reply and will thus
   select the inappropriately "remote" gateway.

   If the first router reverses steps (2) and (3) above, replying to the
   request before rebroadcasting it, then the MacIP host will select it.
   It looks weird on a packet monitor though.

5.3.2.4. Port Hopping

   NBP is intended to allow a client to search for a service by name,
   and to discover the network address which will be used for the
   delivery of subsequent datagrams.  In the case of a multi-port
   service-provider, it is sensible to return the "nearest" AppleTalk
   address to the client, in this case the address of the port that the
   NBP Reply was returned to the client through.  Unfortunately this
   port (and address) may not be in the zone that the request was
   originally made in.  This causes no problems apart from confusion to
   network managers again, and should probably be avoided for this
   reason.

5.4.    DDP



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MacIP                                                      November 1992


   The Datagram Delivery Protocol is close to the AppleTalk equivalent
   of UDP/IP.  It implements an "unreliable" Datagram Delivery service,
   with the destination address being a socket at a specific net:node
   address.  DDP packets also have a "type" field which permits another
   level of multiplexing on top of the socket multiplexor, but is often
   used redundantly with the socket.

5.5.    ATP

   The AppleTalk Transaction Protocol adds a request/response/timeout
   and retry mechanism on top of DDP packet delivery.  Transactions
   commence with an ATP Request packet (TREQ) and must be answered by an
   ATP Response (TRSP) or the requestor will retry and eventually time
   out and report back an error to the client.

6.      Definitions

6.1.    AppleTalk Protocol constants

   MacIP MTU                                               576 bytes

   DDP constants
        MacIP packet type                               22 (decimal) DDP
        ARP packet type                     23 (decimal)

   DDP ARP constants
        AppleTalk address type                  3 (decimal)

   NBP constants
        gateway object type                     IPGATEWAY registered IP
        address object type               IPADDRESS LookUp retransmit
        count         4 tries LookUp retransmit interval              5
        seconds

6.2.    Gateway ATP Protocol Constants

   ATP Protocol Constants
        ATP retransmit count                    4 tries ATP retransmit
        interval                 5 seconds

   ATP request command codes
        ASSIGN  assign IP address               1 NAME    name
        server             2 (obsolete) SERVER  get server
        info         3

   ATP response codes
        SUCCESS                                 same as request code
        ERROR                                   -1



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7.  Glossary

   MacIP
        The encapsulation protocol and associated services.

   MacIP-1
        The original MacIP specification as implemented in the KIP code.

   MacIP-2
        A future version of MacIP that will allow out-of-zone and
        multiple-gateway operation.

   MacIP Host
        A device implementing the host-side of the MacIP protocol,
        usually an Apple Macintosh computer.

   MacIP Gateway
        A device implementing the gateway-functions of the MacIP
        protocol.  It converts between the MacIP transport-layer and
        those used by the other IP hosts, as well as provides other
        server-type functions.

   IP Host
        An IP host on an IP internet, usually not using MacIP, but with
        which MacIP hosts can communicate.

   MacIP Packets
        IP datagrams encapsulated in AppleTalk packets that are sent
        between the "Delivery" modules in MacIP hosts and gateways.

   MacIP Network
        The collection of MacIP hosts and the MacIP gateway that are in
        direct communication with each other - similar to the concept of
        an IP Subnet.

   MacIP Range
        The range of IP addresses configured into a MacIP gateway that
        are considered to belong to the MacIP hosts.

   MacIP Dynamic Range
        The IP addresses in the MacIP range that are available for
        automatic assignment to MacIP hosts by the gateway MacIPGP
        module.

   Dynamic Address
        An IP address assigned by a MacIP gateway from its Dynamic range
        for the use of a MacIP host.




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   MacIP Static Range
        The IP addresses in the MacIP range that are available for
        permanent assignment to MacIP hosts by the system administrator.

   Static Address
        A fixed IP address assigned by the system administrator to a
        MacIP host.  This address must be in the MacIP Static range of
        the host's MacIP gateway.

   MacIP Zone
        The AppleTalk zone that the MacIP gateway is situated in, and
        which corresponds to the "reach" of the NBP ARP function.

   Local Zone
        The AppleTalk zone that a MacIP host is in.

   MacIP Routing Mode
        The MacIP gateway configuration where the MacIP network IP
        addresses are in a different subnet to that of the IP backbone,
        and where the MacIP gateway is acting as an IP router.

   MacIP Forwarding Mode
        The MacIP gateway configuration where the MacIP network IP
        addresses are in the same subnet as the IP backbone, and where
        the MacIP gateway is performing Proxy ARP for the MacIP hosts.

   In-Zone Mode
        The MacIP host configuration where the host is in the same zone
        as the MacIP gateway (the MacIP zone is the same as the Local
        zone).

   Out-of-Zone Mode
        The MacIP host configuration where the host is in a different
        zone to the MacIP gateway (the MacIP zone is not the same as the
        Local zone).

   MacIPGP
        The MacIP Gateway Protocol.  Used to implement the address
        assignment service, and to forward other information.

   NBP ARP
        Method for resolving an IP address into an AppleTalk address.
        Equivalent to Ethernet ARP.

   NBP Proxy ARP
        Method by which the MacIP gateway answers NBP ARP requests for
        IP addresses of IP hosts.




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   NBP ARP Forwarding
        Method by which the MacIP gateway forward NBP ARP requests to
        MacIP hosts that ore located outside of the MacIP zone.

   Proxy ARP
        Method by which the MacIP gateway responds to Ethernet ARP
        requests from IP hosts for IP addresses in the MacIP range.

   VIM
        Virtual Internal MacIP.  This refers to the proposed new MacIP
        gateway architcture.  The gatweway implements an internal,
        virtual network, and associates itself with it.  If the internal
        network's zones list contain more than one zone name, the MacIP
        gateway registers itself on all of them, providing gateway
        services to all MacIP hosts in those zones.

8.  Implementation Notes

   This section provides notes and insights to the reader.  Its sections
   numbers are organized to follow the preceding section numbers.

   8.1.2   Intended Audience

   It should be noted that members from the various groups mentioned
   (host implementors, gateway implementors, system managers, and the
   curious) have different preconceived notions about what comprises a
   MacIP protocol system. Also, what is acceptable functionality in
   undocumented limitations will differ between these groups, and
   between members of the groups themselves.  Careful attention must be
   paid to the fact that TCP/IP is not AppleTalk, and AppleTalk is not
   TCP/IP.  There are some similarities that are close enough to conceal
   the important differences.  NBP is nothing like DNS.  RIP is
   different from RTMP.  UDP isn't DDP.  Mapping one protocol system to
   another presents a significant challenge, and that is what we're
   attempting with MacIP.  It is very easy for things to go very wrong.

   8.2.1.1 Basic Requirements

   Implementing MacIP so as to fulfill this requirement as well as the
   local with no gateway requirement (which is very rarely used in
   practice) is one of the things that makes the protocol so complex.

8.2.3.  Protocol Mapping

   MacIP follows NBP ARP.  The following are two obvious ways to "map"
   IP onto AppleTalk at the transport level, but are not followed.

   NBP ARP is used instead.  This combines some of the worst features of



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   "Direct Mapping" and "Server Client" without sufficient advantages to
   offset it.  It also complicates implementation, documentation and
   installation.  But we're stuck with it.

8.2.3.1.        Direct Mapping

   MacIP provides very similar capabilities to Ethernet and Ethernet
   ARP, so one possible protocol mapping would be to treat each
   AppleTalk network as an IP subnet, with IP addresses within each
   subnet being resolved by an equivalent network-wide broadcast
   protocol to Ethernet ARP.  All routing would be performed by the
   MacIP gateways, although the hosts would be required to perform
   "this- subnet" type routing decisions.

   This was actually implemented early on in the history of MacIP.  It
   has many advantages.  It satisfies the "requirement matrix" of 2.1.1
   and its similarity to IP makes it easily understandable and
   documentable.  The downside is that it requires every AppleTalk
   router in an Internet to also be an IP router  This complicates the
   configuration of an Internet and also consumes IP address space at an
   alarming rate. This isn't MacIP-1.

8.2.3.2.        Client-Server

   An approach leading to a very simple implementation would be a strict
   client- server one, such as the protocols that are already used to
   implement DECnet, LAT and SNA services over AppleTalk.  MacIP hosts
   would use NBP to find the gateway(s) and then establish a "session"
   which would allow the gateway to record the AppleTalk address of the
   host so it would know how to route a packet back to them.

   Advantages would be the ease of implementation, documentation and
   debugging. The MacIP hosts wouldn't have to know anything about
   routing - they would send all packets to the gateway.  It would also
   allow MacIP hosts that are anywhere on a large and complex AppleTalk
   Internet to use a gateway no matter where it was - there would only
   need to be one gateway.  It would allow operation across AppleTalk
   zones.  The downside is that all traffic between two MacIP hosts on
   the same network would have to pass through the gateway, and that the
   gateway would be required for the protocol.  This isn't MacIP-1
   either, much the pity.

8.2.4.4.        MacIP Routing Decisions

8.2.4.4.1.      Routing in Standard IP Hosts

   An IP host on a single point-to-point link only has one simple
   routing decision to make when presented with an IP packet to forward:



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        1.  Is the destination IP address equal to my IP address?

   If it is, the packet is sent "inwards".  If not, it is sent out the
   link.  An IP host connected to single broadcast medium has another
   two questions to answer:

        2.  Is the destination address within "this" network/subnet?

   If it is, the packet can be sent "directly" to the destination.  If
   it isn't, then the packet has to be sent via a gateway, often a
   single "default gateway", the IP address of which is known.

        3.  Is the destination address a Broadcast address?

   In all cases ARP is used to resolve the selected IP address to a
   hardware address.  In case (3), ARP will substitute the appropriate
   configured hardware broadcast address.

8.2.4.4.2.      Routing in MacIP Hosts

   This "broadcast medium behavior" is insufficient for MacIP, as the
   Address Assignment protocol sensibly and deliberately provides
   neither the subnet mask nor default gateway information.  This
   behavior should not be defeated (by allowing manual entry of the
   data) as it defeats the Macintosh "plug and play" requirement.

   Even if Address Assignment did provide this information, the
   disparity between the "reach" of NBP ARP and normal DDP broadcast
   datagrams prevents a MacIP host from broadcasting an IP datagram to
   all hosts in the zone.  This is strictly not "required" by IP, but it
   is certainly "expected" by some applications.

   MacIP hosts SHOULD never perform any routing.  This includes anything
   to do with subnet masks, broadcast addresses, default gateways, RIP
   or ICMP redirects.

8.3.2.  MacIP Modules - Introduction

   MacIP can be best understood and described if the various functions
   are categorized into functional modules.

   The following gives the relationship between the IP , MacIP and
   AppleTalk modules in the MacIP host:








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    ......................................
    :         IP Protocol Layer          :
    :.......... | ............. | .......:
                |               |
    ........... | ............. | ........
    :  ---------+-------- ------+------  :
    :  | Initialization | | Delivery  |  :
    :  ----+---------+--- -+--+--------  :
    :      |         |     |  |          :
    :  ----+----  ---+-----+- | MacIP    :
    :  | MIPGP*|  | NBP ARP | | Protocol :
    :  ----+----  -----+----- |          :
    :..... | ......... | .... | .........:
           |           |      |
    ...... | ......... | .... | ..........
    :  ----+----  -----+----  |          :
    :  |  ATP  |  |   NBP  |  | AppleTalk:
    :  ----+----  -----+----  | Protocol :
    :      |           |      |          :
    :  ----+-----------+------+--------  :
    :  | DDP (Datagram Delivery Proto)|  :
    :  ----------------+---------------  :
    :                  |                 :
    :  ----------------+---------------  :
    :  |  LAP (Link Access Protocol)  |  :
    :  ----------------+---------------  :
    :................. | ................:
                       |
    .................. | .................
    :     AppleTalk Hardware             :
    :....................................:
   (*) "MIPGP" is short for "MacIPGP".

   Figure 4.  MacIP Host Implementation


   The following gives the relationship between the IP , MacIP and
   AppleTalk modules in the MacIP gateway:












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    .........................................................
    :      IP Protocol Layer    +-----( IP Router )----+    :
    :.......... | ............. | .................... | ...:
                |               |                      |
    ........... | ............. | ..................   |
    :           |               |                  :   |
    :  ---------+-------- ------+------  MacIP     :   |
    :  | Initialization | | Delivery  |  Protocol  :   |
    :  ----+------+------ --------+-+--            :   |
    :      |      |               | |              :   |
    :  ----+----- | ------------- | |    --------- :   |
    :  | Assign | | | NBP Proxy | | |    | Proxy | :   |
    :  ----+--+-- | ------+------ | |    |  ARP  | :   |
    :      |   \__|___    |       | |    ----+---- :   |
    :      |      |   \   |      /  |        |     :   |
    :  ----+----  | ---+--+-----+-- |         \    :   |
    :  | MIPGP |  | | NBP ARP     | |          |   :   |
    :  ----+----  | ---+----------- |          |   :   |
    :      |      |    |            |          |   :   |
    :..... | .... | .. | .......... | ........ | ..:   |
           |      |    |            |          |       |
    ...... | ..... \ . | .......... | ....  .. | ..... | ....
    :      |        |  |            |    :  :  |       |    :
    :  ----+----  --+--+---- Apple- |    :  :  | ------+--  :
    :  |  ATP  |  | CNBP * | Talk   |    :  :  | | Ether |  :
    :  ----+----  -----+----        |    :  :  | | Proto |  :
    :      |           |            |    :  :  | --+--+---  :
    :  ----+-----------+------------+--  :  :   \  |   \  E :
    :  | DDP (Datagram Delivery Proto)|  :  :  --+-+--  | t :
    :  ----------------+---------------  :  :  | ARP |  | h :
    :                  |                 :  :  ---+---  | e :
    :  ----------------+---------------  :  :     |     | r :
    :  |  LAP (Link Access Protocol)  |  :  :     |     | n :
    :  ----------------+---------------  :  :     |     | e :
    :                  |                 :  :     |     | t :
    :................. | ................:  :.... | ... | ..:
                       |                          |     |
    .................. | .................  ..... | ... | ...
    :      AppleTalk Hardware            :  : Ether Hardware:
    ......................................  .................
   (*) "CNBP" is "Conditional NBP".  See section 8.3 for details.

   Figure 5.  MacIP Gateway Implementation


8.3.2.1.        Configuration Required For MacIP Hosts




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   The terms static and dynamic SHOULD be used consistently in the user
   interface.

   If the host has multiple MacIP and/or IP interfaces, then a mechanism
   is required to allow selection between them.  It is important to
   distinguish plainly between MacIP- based and "Native" IP connections.

8.3.2.2.1.      Locating the  MacIP gateway's Address Assignment Server

   The implementation SHOULD not take any notice of the returned NVE
   name field in the NBP response.  However, it should be noted that
   many implementations expect to find the IP address corresponding to
   the Server in dotted decimal format.

   The implementation SHOULD record the full AppleTalk address returned
   in the NBP response (including the returned socket) and use it in
   subsequent transactions with the Server.  It SHOULD not simply assume
   that the Server is on DDP socket 72. However, there are many existing
   implementations that make this assumption.

8.3.2.4.2.      Registration of Address Assignment Server

   Unfortunately many MacIP host implementations wrongly rely on the
   name being an IP address, so for compatibility with these the use of
   the IP address as the name is "recommended".  A simple two-port MacIP
   gateway configured in "forwarding" mode may only have one IP address
   to use for this purpose, but a multi-IP-port MacIP gateway may suffer
   an embarrassment of choices, so the question arises as to which IP
   address to use for this purpose.  Fortunately this choice can be
   narrowed by the existence of bugs in some MacIP host implementations
   that can require the name of the Server to be an IP address in the
   same "subnet" as the IP address of the MacIP host.  Good luck!

8.3.4.1.        NBP ARP - Details

   The aging of ARP cache entries is required.  The mobility of
   Macintoshes makes this particularly important.  Any algorithm may be
   used as long it is at least as good as the following.

   Each use of the ARP cache entry should reset a "usage" timer.  When a
   new entry is to be put into the cache, it might be full (or the
   bucket associated with the hashing function might be full).  The
   entry with the oldest "usage" timer should be the one chosen to be
   replaced.

   ARP cache entries should be "confirmed" by sending a single NBP
   Confirm directly to the AppleTalk address in the cache entry once a
   minute.  If no response is received after five requests the entry



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   should be deleted.

   The NBP Cache should be capable of being flushed on request by other
   modules.

   8.3.5.  Delivery

   If the "Delivery" module experiences a hard error (the packet
   transport code cannot transmit the packet, and returns an error) when
   attempting to transmit a MacIP packet, then it is recommended that
   the NBP ARP cache entry for the destination IP address should be
   flushed and the transmit retried.  This is to recover from MacIP
   gateway restarts where the gateway has picked a different node
   address to the one it previously had.

8.3.6.  MacIP Routing Decisions

   The requirement in section 3.6 that MacIP hosts should always use NBP
   ARP for all destination IP addresses is widely disobeyed in actual
   practice.  Most MacIP host implementations disobey this fundamental
   specification, as the requirements of being an IP host were not well
   understood at the time.

   It is thus expected that some MacIP-1 hosts will send some or all
   packets directly to the AppleTalk address of the discovered MacIP
   gateway for delivery.  However it should be noted that if the MacIP
   gateway restarts, it may acquire a different AppleTalk node address
   to the one it had prior to the restart.  If the host is sending MacIP
   packets to the old gateway address, they will not be delivered.  The
   MacIP host SHOULD take measures to recover from this by following
   address cache aging and updating algorithms as discussed in the Host
   Requirements RFC [10].  This applies to NBP ARP caches as well.

8.3.7.  Gateway NBP Proxy ARP

   It is the implementation of this service that causes the most
   problems in MacIP.  If the NBP Proxy ARP module wrongly responds to
   an IP addresses that is assigned to a MacIP host, then the response
   from the gateway will look to the host attempting to register its
   address as a duplicate registration.

   This results in the restriction that with MacIP-1 there must never be
   multiple MacIP gateways in the one zone configured with different
   MacIP Ranges, as they will defeat registration attempts by MacIP
   hosts that have been assigned addresses by another gateway.  Then
   again, two MacIP gateways can't be configured with the same MacIP
   range, as normal IP routing (Routing case) and Proxy ARP (Forwarding
   case) would fail to route packets under these circumstances.
   Therefore there can't be more than one MacIP gateway in any one zone.


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   This is the "Multiple Gateways in the One Zone" problems, and will be
   addressed later.

   8.5.3.  Name Binding Protocol

   NBP can be though of implementing a form of "zone-wide broadcast".
   The only thing that can be broadcast is a search for a named entity -
   it is not possible to broadcast data-containing packets.  This is
   important when considering mapping one network protocol (such as IP)
   onto an AppleTalk Internet.

   It is not possible to discriminate a "registration attempt" NBP
   LookUp from a "searching for" one.  This makes the implementation of
   NBP Proxy ARP far more difficult than you would first suspect.

   9.      Issues

   9.1     IP Routing Issues

   Phil pointed out that we didn't discuss IP hop count or checksum
   issues.  Since we are subject to rfc1122 (naturally), should we still
   discuss it?  Furthermore, what do y'all think of bumping [or not
   bumping] the hop count if we are just in 'forwarding mode'?

   Also, Phil asked if forwarding gateways need to intercept ICMP
   redirects.  IP routers must ignore all ICMP redirects, as they're
   meant to have their routing tables updated by a "more reliable"
   method.  Thus, all ICMP redirects MUST be ignored by the gateway, and
   MacIP hosts SHOULD ignore all ICMP redirects too.  Should this go
   into this document?  Where?

   9.2     More Hacks

   Jonathan described the following additional 'hacks'.  Should they be
   in this document, and if so, where?

   (1) The EtherGate has one ethernet port and 2 localtalk ports.  We
   doclient IP address assignment out of a single range of addresses for
   both localtalk ports.  This means that besides it's usual
   weirdnesses, we had to make NBP-proxy-ARP work across both localtalk
   ports if they're in different zones.  When a Mac tries to confirm
   that it can use its IPADDRESS the EtherGate forwards the NBP Lookup
   to the other port and changes the zone name.  Yuck.

   (2) When the EtherGate receives an NBP-ARP for an IPADDRESS not in
   its client range and on the same (sub)net as the EtherGate itself, it
   IP ARPs on the ethernet side to see if it can reach the IP host
   before responding on the NBP side.



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   Would someone with a good familiarity of Proxy ARP routers like to
   contribute - there may be some good practice in existing Proxy ARP
   routers that we can adopt, copy, steal documentation from etc.

   One last note, I assume the implementation notes will include hints
   for the host (Mac client) implementor as well as the gateway
   implementor?  We should clearly point out pitfalls and suggestions
   for both!

   Any host-implementors like to contribute a few paragraphs?

   9.3     MacIP Host Recommendations - Another Idea

   From:   Tom Evans

   The MacIP _HOST_ doesn't need a "session" with anything.

   Q. What is this "session" actually FOR?

   A. It is established at MacIP Host startup for the sole purpose of
   getting a Dynamic IP Address assigned.

   Q. Does the MacIP Host have to MAINTAIN the "session" with the
   Address Assignment Service?

   A. NO.

   Q. What is the ADDRESS of the IPGATEWAY?

   A. I've been reading Radia's "Interconnections" too. By her
   definition in section 2.3, an AppleTalk "address" doesn't fit her
   definition of an "address". The node-part of an AppleTalk address
   CHANGES.

   So the "address" of the IPGATEWAY starts out as its NAME, which is
   initially "=:IPGATEWAY@*" ("any gateway"). The MacIP Host picks a
   particular one (which is now "aa.bb.cc.dd:IPGATEWAY@*"), which fits
   the definition of a NAME (but has an IP ADDRESS embedded in it).
   This is the only real "fixed" representation of the "address". The
   AppleTalk address MAY CHANGE, so it shouldn't be stored. If the
   IPGATEWAY is required for some reason, it should probably be searched
   for again at the time that it is needed.

   The IP Address ("aa.bb.cc.dd" above) is a better "fixed address" than
   the AppleTalk address is. It can be resolved to an AppleTalk address
   when required by existing (NBP ARP) code.

   Q. "My code treats IPGATEWAY as a "default IP gateway", so I have to



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   keep its address".

   A. XXXXXXXXXXX (removed by censors). Nothing else that I know of in a
   Mac stores AppleTalk addresses - they're too volatile. The
   LaserWriter driver stores the NAME of the printer. The AppleTalk
   address "A_Router" stored by any Mac is never more than 40 seconds
   old. ASP sessions store the AppleTalk address, but they're
   continuously maintained sessions that tell you when they break.

   The logical thing to do is to store the MacIP Gateway's IP Address
   and NOT its AppleTalk Address. This will make a lot more sense in the
   host's existing IP Routing table after all. No "special case MacIP
   code" required in the IP layer.

   Use NBP ARP to resolve ALL IP addresses passed down from the IP
   layer, including the one for the Default Gateway. The ARP/NBP-ARP
   code should have timeouts and confirmation. This will recover nicely
   if the IPGATEWAY changes its AppleTalk address, which it is "allowed"
   to do.

   Q. How does the MacIP Gateway keep ITS session information (required
   for NBP Proxy ARP)?

   A. By REREGISTRATION at gateway startup, by answering MacIPGP
   requests, by "tickling" them thereafter and by "probing" suspected
   "clients" that it receives NBP ARP requests from. It isn't the MacIP
   Host's problem.

   So the MacIP Host shouldn't keep any information that it doesn't need
   to, and thus it shouldn't be in the MIB. If it is, then it should be
   the IP address that is stored and not the AppleTalk address.

   Of course the above seems to be contradicted in the MacIP doc in
   sections 5.2.5 and 6.2 where it says that the MacIP Host should be
   able to receive multiple IPGATEWAY responses and then choose the
   "best" one.  However, once the MacIP Host HAS an IP address, it
   doesn't need the Address Assignment Server anymore, and it should
   probably throw all the information away. So if the multiple gateway's
   addresses WERE accessible via SNMP, then they'd only be there for
   less than a second.

10.  Acknowledgments

   Bill Croft, for the initial implementation of this protocol in the
   SEAGATE gateway at Stanford University and for the documents provided
   with the KIP code.

   Gaige Paulson and Tim K., for the NCSA Telnet source code.



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   John Romkey, for the original PC/IP source code.

   Tim Maroney and ?, for the port of PC/IP to Macintosh.

   Jeannine Smith for TCP and UDP in MacTCP, and John Veizades and his
   team for the rest.

   Brad Parker and Josh Littlefield.  This document is directly based on
   theirs written in February 1990.

   John Veizades, for a version of this document and for being the Chair
   of the Apple- IP working group.

11.     References

      [1]     Sidhu, G., Andrews, R., and Oppenheimer, A., Apple
        Computer, "Inside AppleTalk, 2nd. Edition" Addison-Wesley
        Publishing Company, Inc., Reading, MA, May, 1990.

      [2]     Postel J., "Internet Protocol", RFC-791, USC Information
        Sciences Institute, September 1981.

      [3]     Plummer, D., "An Ethernet Address Resolution Protocol",
        RFC-826, Symbolics, September 1982.
      [4]     Hornig, C., "Standard for the transmission of IP datagrams
        over Ethernet", RFC-894, Symbolics, April 1984.

      [5]     Mogul, J., "Internet Subnets", RFC-917, Stanford
        University, October 1984.

      [6]     Mogul, J., and Postel, J., "Internet Standard Subnetting
        Procedure", RFC-950, Stanford University, August 1985.

      [7]     Brandon, R., and Postel, J., "Requirements for Internet
        Gateways", RFC-1009, USC Information Sciences Institute, June
        1987.

      [8]     Carl-Mitchell, S., Quarterman, J.S., "Using ARP to
        Implement Transparent Subnet Gateways", RFC-1027, October 1987.

      [9]     Hendrick, C., "Routing Information Protocol", RFC-1058,
        June 1988.

      [10]    Braden, R.T., ed, "Requirements for Internet Hosts", RFC-
        1122, October 1989.






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12.     Expiration Date

   This Internet Draft will expire in May 1993.


13.     Security

   This document does not discuss security in any manner.


14.     Contact Points

   The Apple-IP working group can be contacted by emailing the following
   address:

        apple-ip@cayman.com


   The Apple-IP Working Group Chairperson is:

        John Veizades
        Apple Computer
        Cupertino, CA
        (408)974-2672

        EMail: veizades@apple.com

   The author's addresses are:

        Tom Evans
        Webster Computer Corporation
        1270 Ferntree Gully Rd
        Scoresby Melbourne
        3179 Victoria, Australia
        61-3-764-1100

        EMail: tom@wcc.oz.au

        Christopher S. Ranch
        Novell, Inc.
        2180 Fortune Drive
        San Jose, CA 95131
        (408)473-8667

        EMail: cranch@novell.com









Evans and Ranch            November 11, 1992                     Page 42


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