Network Working Group                                        J. Saltzer
Request for Comments: 1498       M.I.T. Laboratory for Computer Science
                                                            August 1993


           On the Naming and Binding of Network Destinations

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard.  Distribution of this memo is
   unlimited.

Abstract

   This brief paper offers a perspective on the subject of names of
   destinations in data communication networks. It suggests two ideas:
   First, it is helpful to distinguish among four different kinds of
   objects that may be named as the destination of a packet in a
   network.  Second, the operating system concept of binding is a useful
   way to describe the relations among the four kinds of objects. To
   illustrate the usefulness of this approach, the paper interprets some
   more subtle and confusing properties of two real-world network
   systems for naming destinations.

Note

   This document was originally published in: "Local Computer Networks",
   edited by P. Ravasio et al., North-Holland Publishing Company,
   Amsterdam, 1982, pp. 311-317.  Copyright IFIP, 1982.  Permission is
   granted by IFIP for reproduction for non-commercial purposes.
   Permission to copy without fee this document is granted provided that
   the copies are not made or distributed for commercial advantage, the
   IFIP copyright notice and the title of the publication and its date
   appear, and notice is given that copying is by permission of IFIP. To
   copy otherwise, or to republish, requires a specific permission.

   This research was supported in part by the Defense Advanced Research
   Projects Agency of the United States Government and monitored by the
   Office of Naval Research under contract number N00014-75-C-0661.

What is the Problem?

   Despite a very helpful effort of John Shoch [1] to impose some
   organization on the discussion of names, addresses, and routes to
   destinations in computer networks, these discussions continue to be
   more confusing than one would expect. This confusion stems sometimes
   from making too tight an association between various types of network

   objects and the most common form for their names.  It also stems from
   trying to discuss the issues with too few well-defined concepts at
   hand.  This paper tries a different approach to develop insight, by
   applying a perspective that has proven helpful in the corresponding
   area of computer operating systems.

   Operating systems have a similar potential for confusion concerning
   names and addresses, since there are file names, unique identifiers,
   virtual and real memory addresses, page numbers, block numbers, I/O
   channel addresses, disk track addresses, a seemingly endless list.
   But most of that potential has long been rendered harmless by
   recognizing that the concept of binding provides a systematic way to
   think about naming [2]. (Shoch pointed out this opportunity to
   exploit the operating system concept; in this paper we make it the
   central theme.) In operating systems, it was apparent very early that
   there were too many different kinds of identifiers and therefore one
   does not get much insight by trying to make a distinction just
   between names and addresses.  It is more profitable instead to look
   upon all identifiers as examples of a single phenomenon, and ask
   instead "where is the context in which a binding for this name (or
   address, or indentifier, or whatever) will be found?", and "to what
   object, identified by what kind of name, is it therein bound?"  This
   same approach is equally workable in data communications networks.

   To start with, let us review Shoch's suggested terminology in its
   broadest form:

        -  a name identifies what you want,
        -  an address identifies where it is, and
        -  an route identifies a way to get there.

   There will be no need to tamper with these definitions, but it will
   be seen that they will leave a lot of room for interpretation.
   Shoch's suggestion implies that there are three abstract concepts
   that together provide an intellectual cover for discussion. In this
   paper, we propose that a more mechanical view may lead to an easier-
   to-think-with set of concepts. This more mechanical view starts by
   listing the kinds of things one finds in a communication network.

Types of Network Destinations, and Bindings Among Them

   In a data communication network, when thinking about how to describe
   the destination of a packet, there are several types of things for
   which there are more than one instance, so one attaches names to them
   to distinguish one instance from another. Of these several types,
   four turn up quite often:

    1. Service and Users. These are the functions that one uses, and
       the clients that use them. Examples of services are one that
       tells the time of day, one that performs accounting, or one
       that forwards packets. An example of a client is a particular
       desktop computer.

    2. Nodes. These are computers that can run services or user
       programs. Some nodes are clients of the network, while others
       help implement the network by running forwarding services.
       (We will not need to distinguish between these two kinds of
       nodes.)

    3. Network attachment points. These are the ports of a network, the
       places where a node is attached. In many discussions about data
       communication networks, the term "address" is an identifier of a
       network attachment point.

    4. Paths. These run between network attachment points, traversing
       forwarding nodes and communication links.

   We might note that our first step, the listing and characterization
   of the objects of discussion, is borrowed from the world of abstract
   data types. Our second step is to make two observations about the
   naming of network objects, the first about form and the second about
   bindings.

   First, one is free to choose any form of name that seems helpful --
   binary identifiers, printable character strings, or whatever, and
   they may be chosen from either a flat or hierarchical name space.
   There may be more than one form of name for a single type of object.
   A node might, for example, have both a hierarchical character string
   name and a unique binary identifier.  There are two semantic traps
   that one can fall into related to name form.  First, the word "name"
   is, in the network world, usually associated with a printable
   character string, while the word "address" is usually associated with
   machine-interpretable binary strings. In the world of systems and
   languages, the term "print name" is commonly used for the first and
   "machine name" or "address" for the second, while "name" broadly
   encompasses both forms. (In this paper we are using the broad meaning
   of "name".)  The second semantic trap is to associate some
   conventional form of name for a particular type of object as a
   property of that type. For example, services might be named by
   character strings, nodes by unique ID's, and network attachment
   points named by hierarchical addresses.  When one participant in a
   discussion assumes a particular name form is invariably associated
   with a particular type of object and another doesn't, the resulting
   conversation can be very puzzling to all participants.

   The second observation about the four types of network objects listed
   above is that most of the naming requirements in a network can simply
   and concisely be described in terms of bindings and changes of
   bindings among the four types of objects. To wit:

    1. A given service may run at one or more nodes, and may need to move
       from one node to another without losing its identity as a service.

    2. A given node may be connected to one or more network attachment
       points, and may need to move from one attachment point to another
       without losing its identity as a node.

    3. A given pair of attachment points may be connected by one or more
       paths, and those paths may need to change with time without
       affecting the identity of the attachment points.

   (This summary of network naming requirements is intentionally brief.
   An excellent in-depth review of these requirements can be found in a
   recent paper by Sunshine [3].)

   Each of these three requirements includes the idea of preserving
   identity, whether of service, node or attachment point. To preserve
   an identity, one must arrange that the name used for identification
   not change during moves of the kind required. If the associations
   among services, nodes, attachment points and routes are maintained as
   lists of bindings this goal can easily be met. Whether or not all the
   flexibility implied by these possibilities should be provided in a
   particular network design is a matter of engineering judgment. A
   judgment that a particular binding can be made at network design time
   and will never need to be changed (e.g., a particular service might
   always run at a particular node) should not be allowed to confuse the
   question of what names and bindings are in principle present. In
   principle, to send a data packet to a service one must discover three
   bindings:

    1. find a node on which the required service operates,

    2. find a network attachment point to which that node is connected,

    3. find a path from this attachment point to that attachment point.

   There are, in turn, three conceptually distinct binding services that
   the network needs to provide:

    1. Service name resolution, to identify the nodes that run the
       service.

    2. Node name location, to identify attachment points that reach the

       nodes found in 1.

    3. Route service, to identify the paths that lead from the
       requestor's attachment point to the ones found in 2.

   At each level of binding, there can be several alternatives, so a
   choice of which node, which attachment point, and which path must be
   made. These choices are distinct, but can interact. For example, one
   might chose the node only after first looking over the various paths
   leading to the possible choices. In this case, the network tables may
   only produce a partial binding, which means that an enquiry produces
   a list of answers rather than a single one. The final binding choice
   may be delayed until the last moment and recorded outside the three
   binding services provided within the network.

   There is a very important subtlety about bindings that often leads
   designers astray. Suppose we have recorded in a network table the
   fact that the "Lockheed DIALOG Service" is running on node "5". There
   are actually three different bindings involved here but only one of
   these three is recorded in this table and changeable by simply
   adjusting the table.

    1. The name "Lockheed DIALOG Service" is properly associate with a
       specific service, management, and collection of stored files. One
       does not usually reassign such a name to a different service. The
       association of the name with the service is quite permanent, and
       because of that permanence is not usually expressed in a single,
       easily changed table.

    2. Similarly, the name "5" is assigned to a particular node on a
       fairly long-term basis, without the expectation that it will
       change. So that assignment is also not typically expressed in a
       single, easily changed table.

    3. The fact that "DIALOG" is now operating on node "5"is the one
       binding that our table does express, because we anticipate that
       this association might reasonably change. The function of our
       table is to allow us to express changes such as "DIALOG" is now
       operating at node "6" or the "Pipe-fitting Service" is now
       operating at node "5".

   The design mistake is to believe that this table allows one to give
   the Lockheed DIALOG service a new name, merely by changing this table
   entry. That is not the function of this table of bindings, and such a
   name change is actually quite difficult to accomplish, since the
   association in question is not usually expressed as a binding in a
   single table. One would have to change not only this table, but also
   user programs, documentation, scribbled notes and advertising copy to

   accomplish such a name change.

Some Real-World Examples

   Although the ideas outlined so far seem fairly straightforward, it is
   surprisingly easy to find real-world examples that pose a challenge
   in interpretation. In the Xerox/DEC/Intel Ethernet [5, 6], for
   example, the concept of a network attachment point is elusive,
   because it collapses into the node name. A node can physical attach
   to an Ethernet anywhere along it; the node brings with it a 48-bit
   unique identifier that its interfaces watches for in packets passing
   by. This identifier should probably be thought of as the name of a
   network attachment point, even though the physical point of
   attachment can be anywhere. At the same time, one can adopt a policy
   that the node will supply from its own memory the 48-bit identifier
   that is to be used by the Ethernet interface, so a second, equally
   reasonable, view (likely to be taken elsewhere in the network in
   interpreting the meaning of these identifiers) is that this 48-bit
   identifier is the name of the node itself.  From a binding
   perspective this way of using the Ethernet binds the node name and
   the network attachment point name to be the same 48-bit unique
   identifier.

   This permanent binding of node name to attachment point name has
   several network management advantages:

     - a node can be moved from one physical location to another
       without changing any network records.

     - one level of binding tables is omitted. This advantage is
       particularly noticeable in implementing internetwork routing.

     - a node that is attached to two Ethernets can present the same
       attachment point name to both networks, which simplifies
       communication among internet routers and alternate path
       finding.

   But permanent binding also produces a curiosity if is happens that
   one wants one node to connect to two attachment points on the same
   Ethernet. The curiosity arises because the only way to make the
   second attachment point independently addressable by others is to
   allow the node to use two different 48-bit identifiers, which means
   that some other network records (the ones that interpret the ID to be
   a node name) will likely be fooled into believing that there are not
   one, but two nodes. To avoid this confusion, the same 48-bit
   identifier could be used in both attachment points, but then there
   will be no way intentionally to direct a message to one rather than
   the other. One way or another, the permanent binding of attachment

   point name to node name has made some function harder to accomplish,
   though the overall effect of the advantages probably outweighs the
   lost function in this case.

   For another example, the ARPANET NCP protocol provides character
   string names that appear, from their mnemonics, to be node names or
   service names, but in fact they are the names of network attachment
   points [6]. Thus the character string name RADC-Multics is the name
   of the network attachment point at ARPANET IMP 18, port 0, so
   reattaching the node (a Honeywell 68/80 computer) to another network
   attachment point requires either that the users learn a new name for
   the service or else a change of tables in all other nodes.  Changing
   tables superficially appears to be what rebinding is all about, but
   the need to change more than one table is the tip-off that something
   deeper is going on. What is actually happening is the change of the
   permanent name of the network attachment point. We can see this more
   clearly by noting that a parallel attachment of that Honeywell 68/80
   to a second ARPANET port would be achievable only by assigning a
   second character string identity; this requirement emphasizes that
   the name is really of the attachment point, not the node.
   Unfortunately, because of their mnemonic value, the ARPANET NCP name
   mnemonics are often thought of as service names. Thus one expects
   that that the Rome Air Development Center Multics service is operated
   on the node reached by the name RADC-Multics.  This particular
   assumption doesn't produce any surprises. But any one of the four
   Digital PDP-10 computers at Bolt, Beranek and Newman can accept mail
   for any of the others, as can the groups of PDP-10's at the USC
   Information Sciences Institute, and at the Massachusetts Institute of
   Technology. If the node to which ones tries to send mail is down, the
   customer must realize that the same service is available by asking
   for a different node, using what appears to be a different service
   name. The need for a customer to realize that he must give a
   different name to get the same service comes about because in the
   ARPANET the name is not of a service that is bound to a node that is
   bound to an attachment point, but rather it is directly the name of
   an attachment point.

   Finally, confusion can arise because the three conceptually distinct
   binding services (service name resolution, node name location, and
   route dispensing) may not be mechanically distinct. There is usually
   suggested only one identifiable service, a "name server". The name
   server starts with a service name and returns a list of network
   attachment points that can provide that service. It thereby performs
   both the first and second conceptual binding services, though it may
   leave to the customer the final choice of which attachment point to
   use. Path choice may be accomplished by a distributed routing
   algorithm that provides the final binding service without anyone
   noticing it.

Correspondence with Names, Addresses, and Routes

   With this model of binding among services, nodes, network attachment
   points, and paths in mind, one possible interpretation of Shoch's
   names, addresses and routes is as follows:

   1.  Any of the four kinds of objects (service, node, network
       attachment point, and path) may have a name, though Shoch would
       restrict that term to human-readable character strings.

   2.  The address of an object is a name (in the broad sense, not
       Shoch's restricted sense) of the object it is bound to. Thus, an
       address of a service is the name of some node that runs it. An
       address of a node is the name of some network attachment point to
       which it connects. An address of a network attachment point (a
       concept not usually discussed) can be taken to be the name of a
       path that leads to it. This interpretation captures Shoch's
       meaning "An address indicates where it is," but does not very
       well match Shoch's other notion that an address is a
       machine-processable, rather than a human-processable form of
       identification. This is probably the primary point where our
       perspectives differ on which definitions provide the most clarity.

   3.  A route is a more sophisticated concept. A route to either a
       network attachment point or a node is just a path, as we have
       been using the term. Because a single node can run several
       services at once, a route to a service consists of a path to the
       network attachment point of a node that runs the service, plus
       some identification of which activity within that node runs the
       service (e.g., a "socket identifier" in the PUP internet [4] or
       the ARPA Internet [7] protocols). But note that a route may
       actually consist of a series of names, typically a list of
       forwarding name nodes or attachment points and the names used by
       the forwarding nodes for the paths between them.

   Whether or not one likes this particular interpretation of Shoch's
   terms, it seems clear that there are more than three concepts
   involved, so more than three labels are needed to discuss them.

Summary

   This paper has argued that some insight into the naming of
   destinations in a network can be obtained by recognizing four kinds
   of named objects at or leading to every destination (services, nodes,
   attachment points, and routes) and then identifying three successive,
   changeable, bindings (service to node, node to attachment point, and
   attachment point to route). This perspective, modeled on analogous
   successive bindings of storage management systems (file--storage

   region--physical location) and virtual memories (object--segment--
   page--memory block) provides a systematic explanation for some design
   problems that are encountered in network naming systems.

Acknowledgements

   Discussions with David D. Clark, J. Noel Chiappa, David P. Reed, and
   Danny Cohen helped clarify the reasoning used here. John F. Shoch
   provided both inspiration and detailed comments, but should not be
   held responsible for the result.

References

   1.  Shoch, John F., "Inter-Network Naming, Addressing, and Routing,"
       IEEE Proc. COMPCON Fall 1978, pp. 72-79. Also in Thurber, K.
       (ed.), Tutorial: Distributed Processor Communication
       Architecture, IEEE Publ. #EHO 152-9, 1979, pp. 280-287.

   2.  Saltzer, J. H., "Naming and Binding of Objects", in: Operating
       Systems, Lecture notes in Computer Science, Vol. 60, Edited by R.
       Bayer, New York; Springer-Verlag, 1978.

   3.  Sunshine, Carl A., "Addressing Problems in Multi-Network
       Systems", to appear in Proc. IEEE INFOCOM 82, Las Vegas, Nevada,
       March 30 - April 1, 1982.

   4.  Boggs, D. R., Shoch, J. F., Taft, E. A., and Metcalfe, R. M.,
       "PUP: An Internetwork Architecture", IEEE Trans. on Comm. 28, 4
       (April, 1980) pp.  612-623.

   5.  (Anonymous), "The Ethernet, A Local Area Network: Data Link Layer
       and Physical Layer Specifications, Version 1.0", published by
       Xerox Corp., Palo Alto, Calif., Intel Corp., Sunnyvale, Calif.,
       and Digital Equipment Corp., Tewksbury, Mass., September 30,
       1980.

   6.  Dalal, Y. K., and Printis, R. S., "48-bit Absolute Internet and
       Ethernet Host Numbers", Proc. Seventh Data Communications
       Symposium, Mexico City, Mexico, October 1981, pp. 240-245.

   7.  Feinler, E., and J. Postel, Editors, "ARPANET Protocol Handbook",
       SRI International, Menlo Park, Calif., January, 1978.

Security Considerations

   Security issues are not discussed in this memo.

Author's Address

   Jerome H. Saltzer
   M.I.T. Laboratory for Computer Science
   545 Technology Square
   Cambridge, MA 02139
   U.S.A.

   Phone: (617) 253-6016
   EMail: Saltzer@MIT.EDU