Requirements for a Protocol to Replace the AppleTalk Name Binding Protocol (NBP)
RFC 6760

Internet Engineering Task Force (IETF)                       S. Cheshire
Request for Comments: 6760                                   M. Krochmal
Category: Informational                                       Apple Inc.
ISSN: 2070-1721                                            February 2013

                 Requirements for a Protocol to Replace
               the AppleTalk Name Binding Protocol (NBP)

Abstract

   One of the goals of the authors of Multicast DNS (mDNS) and DNS-Based
   Service Discovery (DNS-SD) was to retire AppleTalk and the AppleTalk
   Name Binding Protocol (NBP) and to replace them with an IP-based
   solution.  This document presents a brief overview of the
   capabilities of AppleTalk NBP and outlines the properties required of
   an IP-based replacement.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6760.

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Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. Zero Configuration Networking ...................................4
   3. Requirements ....................................................4
      3.1. Name-to-Address Mapping ....................................5
      3.2. Name Services, Not Hardware ................................5
      3.3. Address Services, Not Hardware -- or -- Escape the
           Tyranny of Well-Known Ports ................................6
      3.4. Typed Name Space ...........................................8
      3.5. User-Friendly Names ........................................9
      3.6. Zeroconf Operation .........................................9
      3.7. Name Space Management -- or -- Name Conflict Detection ....10
      3.8. Late Binding ..............................................11
      3.9. Simplicity ................................................11
      3.10. Network Browsing .........................................11
      3.11. Browsing and Registration Guidance .......................12
      3.12. Power Management Support .................................12
      3.13. Protocol Agnostic ........................................13
      3.14. Distributed Cache Coherency Protocol .....................13
      3.15. Immediate and Ongoing Information Presentation ...........13
   4. Existing Protocols .............................................14
   5. IPv6 Considerations ............................................14
   6. Security Considerations ........................................14
   7. Informative References .........................................15

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1.  Introduction

   An important goal of the participants working on Zeroconf, Multicast
   DNS, and DNS-Based Service Discovery was to provide a viable IP-based
   replacement for AppleTalk and the AppleTalk Name Binding Protocol
   (NBP).

   There are many who are experts in the Domain Name System (DNS) who
   know nothing about the AppleTalk Name Binding Protocol (NBP).
   Without some background on how AppleTalk and NBP worked, it may be
   difficult to understand the reasoning and motivations that led to the
   design decisions in Multicast DNS and DNS-Based Service Discovery.

   This document seeks to remedy this problem by clearly stating the
   requirements for an IP-based replacement for AppleTalk and NBP.
   Replacing NBP was not the sole goal of Multicast DNS; therefore,
   these requirements are not the sole design considerations.  However,
   replacing NBP was a major motivation behind the work in Multicast
   DNS.

   In most cases, the requirements presented in this document are simply
   a restatement of what AppleTalk NBP currently does.  However, this
   document is not restricted to describing only what NBP currently
   does.  Achieving at least equivalent functionality to NBP is a
   necessary but not sufficient condition for a viable replacement.  In
   some cases, the requirements for a viable IP-based replacement go
   beyond NBP.  For example, AppleTalk NBP uses Apple Extended ASCII for
   its character set.  It is clear that an IP-based replacement being
   designed today should use Unicode, in the form of UTF-8 [RFC3629].
   AppleTalk NBP has a reputation, partially deserved, for being too
   'chatty' on the network.  An IP-based replacement should not have
   this same failing.  The intent is to learn from NBP and build a
   superset of its functionality, not to replicate it precisely with all
   the same flaws.

   The protocols specified in "Multicast DNS" [RFC6762] and "DNS-Based
   Service Discovery" [RFC6763], taken together, describe a solution
   that meets these requirements.  This document is written, in part, in
   response to requests for more background information explaining the
   rationale behind the design of those protocols.

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2.  Zero Configuration Networking

   Historically, TCP/IP networking required configuration, either in the
   form of manual configuration by a human operator or in the form of
   automated configuration provided by a DHCP server [RFC2131].

   One of the characteristics of AppleTalk was that it could operate
   without any dependency on manual configuration or a network service
   to provide automated configuration.  An AppleTalk network could be as
   small as just two laptop computers connected via an Ethernet cable
   (or wirelessly).

   IP now has self-assigned link-local addresses [RFC3927] [RFC4862],
   which enable IP-based networking in the absence of external
   configuration.  What remains is the need for Zero Configuration name-
   to-address translation and Zero Configuration service discovery, both
   capabilities that AppleTalk NBP offered.

   It is not necessarily the case that Zero Configuration Networking
   protocols will always be used in all three areas (addressing, naming,
   and service discovery) simultaneously on any given network.  For
   example, even on networks with a DHCP server to provide address
   configuration, users may still use Zero Configuration protocols for
   name-to-address translation and service discovery.  Indeed, on a
   single network, users may use conventional Unicast DNS for looking up
   the addresses of Internet web sites while at the same time using
   Multicast DNS for looking up the addresses of peers on the local
   link.  Therefore, Zero Configuration Networking protocols must
   coexist peacefully with conventional configured IP networking when
   used together on the same network.

   Networks change state over time.  Hosts and services may come and go.
   Connectivity, addresses, and names change.  In a manually configured
   network, a human operator can remedy errors when they arise.  In a
   Zero Configuration Network, no such human operator is available to
   diagnose and troubleshoot problems, so Zero Configuration protocols
   need to be self-correcting, automatically accommodating changing
   network conditions, continually converging to correctness in the
   absence of human intervention to manually rectify errors.

3.  Requirements

   This section lists the 15 requirements for an IP-based replacement
   for AppleTalk NBP.

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3.1.  Name-to-Address Mapping

   NBP's primary function is translating names to addresses.

   NBP stands for Name Binding Protocol, not Network Browsing Protocol.
   Many people know NBP only as "that thing that used to let you browse
   the network in the old Macintosh Chooser".  While browsing is an
   important facility of NBP, it is secondary to NBP's primary function
   of translating names to addresses.

   Every time a user prints using AppleTalk, the printing software takes
   the name of the currently selected printer, looks up the current
   AppleTalk address associated with that named service, and establishes
   a connection to that service on the network.  The user may invoke
   NBP's browsing capability once, when first selecting the desired
   printer in the Chooser, but after that, every time something is
   printed, it is a simple efficient name-to-address lookup that is
   being performed, not a full-fledged browsing operation.

   Any NBP replacement needs to support, as its primary function, an
   efficient name-to-address lookup operation.

3.2.  Name Services, Not Hardware

   The primary named entities in NBP are services, not "hosts",
   "machines", "devices", or pieces of hardware of any kind.  This
   concept is more subtle than it may seem at first, so it bears some
   discussion.

   The AppleTalk NBP philosophy is that naming a piece of hardware on
   the network is of little use if you can't communicate with that piece
   of hardware.  To communicate with a piece of hardware, there needs to
   be a piece of software running on that hardware that sends and
   receives network packets conforming to some specific protocol.  This
   means that whenever you communicate with a machine, you are really
   communicating with some piece of software on that machine.  Even if
   you just 'ping' a machine to see if it is responding, it is not
   really the machine that you are 'pinging', it is the software on that
   machine that generates ICMP Echo Responses [RFC792].

   Consequently, this means that the only things worth naming are the
   software entities with which you can communicate.  A user who wants
   to use a print server or a file server needn't care about what
   hardware implements those services.  There may be a single machine
   hosting both services, or there may be two separate machines.  The
   end user doesn't need to care.

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   The one exception to this is network managers, who may want to name
   physical hardware for the purpose of tracking physical inventory.
   However, even this can be recast into a service-oriented view of the
   world by saying that what you're naming is not the hardware, but the
   ICMP Echo Responder that runs (or is assumed to be running) on every
   piece of IP hardware.

3.3.  Address Services, Not Hardware -- or -- Escape the Tyranny of
      Well-Known Ports

   The reader may argue that DNS already supports the philosophy of
   naming services instead of hosts.  When we see names like
   "www.example.com.", "pop.example.com.", "smtp.example.com.",
   "news.example.com.", and "time.example.com.", we do not assume that
   those names necessarily refer to different hosts.  They are clearly
   intended to be logical service names and could, in fact, all resolve
   to the same IP address.

   The shortcoming here is that although the names are clearly logical
   service names, the result today of doing a conventional ("A" or
   "AAAA") DNS lookup for those names gives you only the IP address of
   the hardware where the service is located.  To communicate with the
   desired service, you also need to know the port number at which the
   service can be reached, not just the IP address.

   This means that the port number has to be communicated out-of-band,
   in some other way.  One way is for the port number to be a specific
   well-known constant for any given protocol.  This makes it hard to
   run more than one instance of a service on a single piece of
   hardware.  Another way is for the user to explicitly type in the port
   number, for example, "www.example.com.:8080" instead of
   "www.example.com.", but needing to know and type in a port number is
   as ugly and fragile as needing to know and type in an IP address.

   Another aspect of the difficulty of running more than one instance of
   a service on a single piece of hardware is that it forces application
   programmers to write their own demultiplexing capability.  AppleTalk
   did not suffer this limitation.  If an AppleTalk print server offered
   three print queues, each print queue ran as its own independent
   service, listening on its own port number (called a socket number in
   AppleTalk terminology), each advertised as a separate, independently
   named NBP entity.  When a client looks up the address of that named
   NBP entity, the reply encodes not only on which net and subnet the
   service resides, and on which host on that subnet (like an IP address
   does), but also on which socket number (port number) within that
   host.  In contrast, if an lpr print server offers three print queues,
   all three print queues are typically reached through the same well-
   known port number (515), and then the lpr protocol has to use its own

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   demultiplexing capability (the print queue name) in order to
   determine which print queue is sought.  This makes it especially
   difficult to run two different pieces of print queue software from
   different vendors on the same machine, because they cannot both
   listen on the same well-known port.

   A similar trick is used in HTTP 1.1, where the "Host" header line is
   used to allow multiple logical HTTP services to run at the same IP
   address.  Again, this works for a single-vendor solution, but if a
   user wishes to run multiple web servers (for example, an image
   server, a database program, an HTTP email access gateway, and a
   conventional HTTP server) on a single machine, they can't all listen
   on TCP port 80, the traditional HTTP port.

   Yet another problem of well-known ports is that port numbers are a
   finite resource.  Originally, port numbers 0-255 were reserved for
   well-known services, and the remaining 99.6% of the port space was
   free for dynamic allocation [RFC1122].  Since then, the range of
   "Registered Ports" has crept upwards until today, ports 0-49151 are
   reserved, and only 25% of the space remains available for dynamic
   allocation.  Even though 65535 may seem like a lot of available port
   numbers, with the pace of software development today, if every new
   protocol gets its own private port number, we will eventually run
   out.  To avoid having to do application-level demultiplexing,
   protocols like the X Window System wisely use a range of port
   numbers, and let TCP do the demultiplexing for them.  The X Window
   System uses 64 ports, in the range 6000-6063.  If every new protocol
   were to get its own chunk of 64 ports, we would run out even faster.

   Any NBP replacement needs to provide, not just the network number,
   subnet number, and host number within that subnet (i.e., the IP
   address) but also the port number within that host where the service
   is located.  Furthermore, since many existing IP services such as lpr
   *do* already use additional application-layer demultiplexing
   information such as a print queue name, an NBP replacement needs to
   support this too by including this information as part of the
   complete package of addressing information provided to the client to
   enable it to use the service.  The NBP replacement needs to name
   individual print queues as first-class entities in their own right.
   It is not sufficient merely to name a print server, within which
   separate print queues can then be found by some other mechanism.

   One possible answer here is that an IP-based NBP replacement could
   use a solution derived from DNS SRV records instead of "A" records,
   since SRV records *do* provide a port number.  However, this alone is
   not a complete solution, because SRV records cannot tell you an lpr
   print queue name.

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3.4.  Typed Name Space

   AppleTalk NBP names are structured names, generally written as:

      Name : Type @ Zone

   Name: The Name is the user-visible name of the service.

   Type: The Type is an opaque identifier that identifies the service
   protocol and semantics.  The user may think of the Type as
   identifying the end-user function that the device performs (e.g.,
   "printing"), and for the typical end-user, this may be an adequate
   mental model, but strictly speaking, from a protocol-design
   perspective, the Type identifies the semantic application protocol
   the service speaks: no more, no less.  For convenience, the opaque
   Type identifier is generally constructed using descriptive ASCII
   text, but this text has no meaning to the protocol, and care should
   be taken in inferring too much meaning from it.  For example, the NBP
   Service Type "LaserWriter" means "any service that speaks
   PS/PAP/ATP/DDP (PostScript over AppleTalk Printer Access Protocol
   over AppleTalk Transaction Protocol over AppleTalk Datagram Delivery
   Protocol)".  It does not necessarily mean an Apple-branded
   "LaserWriter" printer; nor does the service even have to be a
   printer.  A device that archives documents to digital media could
   advertise itself as a "LaserWriter", meaning that it speaks
   PostScript over PAP, not necessarily that it prints that document on
   paper when it gets it.  The end-user never directly sees the Service
   Type.  It is implicit in the user's action; for example, when
   printing, the printing software knows what protocol(s) it speaks and
   consequently what Service Type(s) it should be looking for -- the
   user doesn't have to tell it.

   Zone: The Zone is an organizational or geographical grouping of named
   services.  AppleTalk Zones were typically given names like
   "Engineering", "Sales", or "Building 1, 3rd floor, North".  The
   equivalent concept in DNS could be a subdomain such as
   "Engineering.example.com.", "Sales.example.com.", or "Building 1, 3rd
   floor, North.example.com."

   Each {Type,Zone} pair defines a name space in which service names can
   be registered.  It is not a name conflict to have a printer called
   "Sales" and a file server called "Sales", because one is
   "Sales:LaserWriter@Zone" and the other is "Sales:AFPServer@Zone".

   Any NBP replacement needs to provide a mechanism that allows names to
   be grouped into organizational or geographical "zones", and within
   each "zone", to provide an independent name space for each service
   type.

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3.5.  User-Friendly Names

   When repeatedly typing in names on command-line systems, it is
   helpful to have names that are short, all lowercase, without spaces
   or hard-to-type characters.

   Since Service Names are intended to be selected from a list, not
   repeatedly typed in on a keyboard, there is no reason for them to be
   restricted so.  Users should be able to give their printers names
   like "Sales", "Marketing", and "3rd Floor Copy Room", not just
   "printer1.example.com.".  Of course, a user is free to name a
   particular service using only lowercase letters and no spaces if they
   wish, but they should not be forced to do that.

   Any NBP replacement needs to support a full range of rich text
   characters, including uppercase, lowercase, spaces, accented
   characters, and so on.  The correct solution is likely to be UTF-8
   Unicode [RFC3629].

   Note that this requirement for user-friendly rich-text names applies
   equally to the zones (domains) in which services are registered and
   discovered.

   Note that although the characters ':' and '@' are used when writing
   AppleTalk NBP names, they are simply a notational convenience in
   written text.  In the on-the-wire protocol and in the software data
   structures, NBP Name, Type, and Zone strings are all allowed to
   contain almost any character, including ':' and '@'.  The naming
   scheme provided by an NBP replacement must allow the use of any
   desired characters in service names, including dots ('.'), spaces,
   percent signs, etc.

3.6.  Zeroconf Operation

   AppleTalk NBP is self-configuring.  On a network of just two hosts,
   they communicate peer-to-peer using multicast.  On a large managed
   network, AppleTalk routers automatically perform an aggregation
   function, allowing name lookups to be performed via unicast to a
   service running on the router, instead of by flooding the entire
   network with multicast packets to every host.

   Any NBP replacement needs to be able to operate in the absence of
   external network infrastructure.  However, this should not be the
   only mode of operation.  In larger managed networks, it should also
   be possible to take advantage of appropriate external network
   infrastructure when present, to perform queries via unicast instead
   of multicast.

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3.7.  Name Space Management -- or -- Name Conflict Detection

   Because an NBP replacement needs to operate in a Zeroconf
   environment, it cannot be assumed that a central network
   administrator is managing the network.  Unlike managed networks where
   normal administrative controls may apply, in the Zeroconf case an NBP
   replacement must make it easy for users to name their devices as they
   wish, without the inconvenience or expense of having to seek
   permission or pay some organization like a domain name registry for
   the privilege.  However, this ease of naming, and freedom to choose
   any desired name, may lead to name conflicts.  Two users may
   independently decide to run a personal file server on their laptop
   computers, and (unimaginatively) name it "My Computer".  When these
   two users later attend the next IETF meeting and find themselves part
   of the same wireless network, there may be problems.

   Similarly, every Brother network printer may ship from the factory
   with its Service Name set to "Brother Printer".  On a typical small
   home network where there is only one printer, this is not a problem;
   however, it could become a problem if two or more such printers are
   connected to the same network.

   Any NBP replacement needs to detect such conflicts, and handle them
   appropriately.  In the case of a laptop computer, which has a
   keyboard, screen, and a human user, the software should display a
   message telling the user that they need to select a new name.

   In the case of printers, which typically have no keyboard or screen,
   the software should automatically select a new unique name, perhaps
   by appending an integer to the end of the existing name, e.g.,
   "Brother Printer 2".  Note that, although this programmatically
   derived name should be recorded persistently for use next time the
   device is powered on, the user is not forced to use that name as the
   long-term name for the service/device.  In a network with more than
   one printer, the typical user will assign human-meaningful names to
   those printers, such as "Upstairs Printer" and "Downstairs Printer",
   but the ability to rename the printer using some configuration tool
   (e.g., a web browser) depends on the ability to find the printer and
   connect to it in the first place.  Hence, the programmatically
   derived unique name serves a vital bootstrapping role, even if its
   use in that role is temporary.

   Because of the potentially transient nature of connectivity on small
   portable devices that are becoming more and more common (especially
   when used with wireless networks), this name conflict detection needs
   to be an ongoing process.  It is not sufficient simply to verify
   uniqueness of names for a few seconds during the boot process and
   then assume that the names will remain unique indefinitely.

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   If the Zeroconf naming mechanism is integrated with the existing
   global DNS naming mechanism, then it would be beneficial for a sub-
   tree of that global namespace to be designated as having only local
   significance, for use without charge by cooperating peers, much as
   portions of the IPv4 address space are already designated as local-
   significance-only, available for organizations to use locally without
   charge as they wish [RFC1918].

3.8.  Late Binding

   When the user selects their default printer, the software should
   store only the name, not the IP address and port number.  Then, every
   time the user prints, the software should look up the name to find
   the current IP address and port number for that service.  This allows
   a named logical service to be moved from one piece of hardware to
   another without disrupting the user's ability to print to that named
   print service.

   On a network using DHCP [RFC2131] or self-assigned link-local
   addresses [RFC3927] [RFC4862], a device's IP address may change from
   day to day.  Deferring binding of name to address until actual use
   allows the client to get the correct IP address at the time the
   service is used.

   Similarly, with a service using a dynamic port number instead of a
   fixed well-known port, the service may not get the same port number
   every time it is started or restarted.  By deferring binding of name
   to port number until actual use, the client gets the correct port
   number at the time the service is used.

3.9.  Simplicity

   Any NBP replacement needs to be simple enough that vendors of even a
   low-cost network ink-jet printer can afford to implement it in the
   device's limited firmware.

3.10.  Network Browsing

   AppleTalk NBP offers certain limited wild-card functionality.  For
   example, the service name "=" means "any name".  This allows a client
   to perform an NBP lookup such as "=:LaserWriter@My Zone" and receive
   back in response a list of all the PS/PAP (AppleTalk Printer Access
   Protocol) printers in the Zone called "My Zone".

   Any NBP replacement needs to allow a piece of software, such as a
   printing client or a file server client, to enumerate all the named
   instances of services in a specified zone (domain) that speak its
   protocol(s).

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3.11.  Browsing and Registration Guidance

   AppleTalk NBP provides certain meta-information to the client.

   On a network with multiple AppleTalk Zones, the AppleTalk network
   infrastructure informs the client of the list of Zones that are
   available for browsing.  It also informs the client of the default
   Zone, which defines the client's logical "home" location.  This is
   the Zone that is selected by default when the Macintosh Chooser is
   opened, and is usually the Zone where the user is most likely to find
   services like printers that are physically nearby, but the user is
   still free to browse any Zone in the offered list that they wish.

   A Brother printer may be pre-configured at the factory with the
   Service Name "Brother Printer", but they do not know on which network
   the printer will eventually be installed, so the printer will have to
   learn this from the network on arrival.  On a network with multiple
   AppleTalk Zones, the AppleTalk network infrastructure informs the
   client of a single default Zone within which it may register Service
   Names.  In the case of a device with a human user, the AppleTalk
   network infrastructure may also inform the client of a list of Zones
   within which the client may register Service Names, and the user may
   choose to register Service Names in any one of those Zones instead of
   in the suggested default Zone.

   Any NBP replacement needs to provide the following information to the
   client:

      * The suggested zone (domain) in which to register Service Names.

      * A list of recommended available zones (domains) in which Service
        Names may be optionally registered.

      * The suggested default zone (domain) for network browsing.

      * A list of available zones (domains) that may be browsed.

   Note that, because the domains used in this context are intended for
   service browsing in a graphical user interface, they should be
   permitted to be full user-friendly rich text, just like the rest of a
   service name.

3.12.  Power Management Support

   Many modern network devices have the ability to go into a low-power
   mode, where only a small part of the Ethernet hardware remains
   powered, and the device can be woken up by sending a specially
   formatted Ethernet frame that the device's power-management hardware

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   recognizes.  A modern service discovery protocol should provide
   facilities to enable this low-power mode to be used effectively
   without sacrificing network functionality, such as the ability to
   discover services on sleeping devices, and wake up a sleeping device
   when it is needed.

3.13.  Protocol Agnostic

   Fashions come and go in the computer industry, but a service
   discovery protocol, being one of the foundation components on which
   everything else rests, has to be able to outlive these swings of
   fashion.  A useful service discovery protocol should be agnostic to
   the protocols being used by the higher-layer software it serves.  If
   a service discovery protocol requires all the higher-layer software
   to be written in a new computer language, or requires all the higher-
   layer protocols to embrace some trendy new data representation format
   that is currently in vogue, then that service discovery protocol is
   likely to have limited utility after the fashion changes and computer
   industry moves on to its next infatuation.

3.14.  Distributed Cache Coherency Protocol

   Any modern service discovery protocol must use some kind of caching
   for efficiency.  Any time a distributed cache is maintained, a cache
   coherency protocol is required to control the effects of stale data.
   Thus, a useful service discovery protocol needs to include cache
   coherency mechanisms.

3.15.  Immediate and Ongoing Information Presentation

   Many current discovery mechanisms display an hourglass or a "Please
   Wait" message for five or ten seconds, and then present a list of
   results to the user.  At this point, the list of results is static,
   and does not update in response to changes in the environment.  To
   see current information, the user is forced to click a "Refresh"
   button repeatedly, waiting another five to ten seconds each time.

   Neither limitation is acceptable in a protocol that is to replace
   NBP.  When a user initiates a browsing operation, the user interface
   should take at most one second to present the list of results.  In
   addition, the list should update in response to changes in the
   environment as they happen.  If the user is waiting for a particular
   service to become available, they should be able simply to watch
   until it appears, with no "Refresh" button that they need to keep
   clicking.  A protocol to replace AppleTalk NBP must be able to meet
   these requirement for timeliness of information discovery, and
   liveness of information updating, without placing undue burden on the
   network.

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4.  Existing Protocols

   Ever since this work began with Stuart Cheshire's email to the net-
   thinkers@thumper.vmeng.com mailing list in July 1997, the question
   has been asked, "Isn't SLP the IETF replacement for AppleTalk NBP?"

   The Service Location Protocol (SLP) [RFC2608] provides extremely rich
   and flexible facilities in the area of Requirement 10, "Network
   Browsing".  However, SLP provides none of the service naming,
   automatic name conflict detection, or efficient name-to-address
   lookup that form the majority of what AppleTalk NBP does.

   SLP returns results in the form of URLs.  In the absence of DNS, URLs
   cannot usefully contain DNS names.  Discovering a list of service
   URLs of the form "ipp://169.254.17.202/" is not particularly
   informative to the user.  Discovering a list of service URLs of the
   form "ipp://epson-stylus-900n.local./" is slightly less opaque
   (though still not very user-friendly), but to do even this, SLP would
   have to depend on Multicast DNS or something similar to resolve names
   to addresses in the absence of a conventional DNS server.

   SLP provides fine-grained query capabilities, such as the ability to
   prune a long list of printers to show only those that have blue paper
   in the top tray, which could be useful on extremely large networks
   with very many printers, but are certainly unnecessary for a typical
   home or small office with only one or two printers.

   In summary, SLP alone fails to meet most of the requirements, and
   provides vastly more mechanism than necessary in the area of
   Requirement 10.

5.  IPv6 Considerations

   An IP replacement for the AppleTalk Name Binding Protocol needs to
   support IPv6 as well as IPv4.

6.  Security Considerations

   The AppleTalk Name Binding Protocol was developed in an era when
   little consideration was given to security issues.  In today's world,
   this would no longer be appropriate.  Any modern replacement for
   AppleTalk NBP should have security measures appropriate to the
   environment in which it will be used.  Given that this document is a
   broad historical overview of how AppleTalk NBP worked, and does not
   specify any new protocol(s), it is beyond the scope of this document
   to provide detailed discussion of possible network environments, what
   protocols would be appropriate in each, and what security measures
   would be expected of each such protocol.

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7.  Informative References

   [RFC792]   Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, September 1981.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
              2131, March 1997.

   [RFC2608]  Guttman, E., Perkins, C., Veizades, J., and M. Day,
              "Service Location Protocol, Version 2", RFC 2608, June
              1999.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC3927]  Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
              Configuration of IPv4 Link-Local Addresses", RFC 3927, May
              2005.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              February 2013.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, February 2013.

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Authors' Addresses

   Stuart Cheshire
   Apple Inc.
   1 Infinite Loop
   Cupertino, CA  95014
   USA

   Phone: +1 408 974 3207
   EMail: cheshire@apple.com

   Marc Krochmal
   Apple Inc.
   1 Infinite Loop
   Cupertino, CA  95014
   USA

   Phone: +1 408 974 4368
   EMail: marc@apple.com

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