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Network Working Group                                  Zaw-Sing Su (SRI)Request for Comments: 819                               Jon Postel (ISI)                                                             August 1982The Domain Naming Convention for Internet User Applications1.  Introduction   For many years, the naming convention "<user>@<host>" has served the   ARPANET user community for its mail system, and the substring   "<host>" has been used for other applications such as file transfer   (FTP) and terminal access (Telnet).  With the advent of network   interconnection, this naming convention needs to be generalized to   accommodate internetworking.  A decision has recently been reached to   replace the simple name field, "<host>", by a composite name field,   "<domain>" [2].  This note is an attempt to clarify this generalized   naming convention, the Internet Naming Convention, and to explore the   implications of its adoption for Internet name service and user   applications.   The following example illustrates the changes in naming convention:      ARPANET Convention:   Fred@ISIF      Internet Convention:  Fred@F.ISI.ARPA   The intent is that the Internet names be used to form a   tree-structured administrative dependent, rather than a strictly   topology dependent, hierarchy.  The left-to-right string of name   components proceeds from the most specific to the most general, that   is, the root of the tree, the administrative universe, is on the   right.   The name service for realizing the Internet naming convention is   assumed to be application independent.  It is not a part of any   particular application, but rather an independent name service serves   different user applications.2.  The Structural Model   The Internet naming convention is based on the domain concept.  The   name of a domain consists of a concatenation of one or more <simple   names>.  A domain can be considered as a region of jurisdiction for   name assignment and of responsibility for name-to-address   translation.  The set of domains forms a hierarchy.   Using a graph theory representation, this hierarchy may be modeled as   a directed graph.  A directed graph consists of a set of nodes and aSu & Postel                                                     [Page 1]

RFC 819                                                     August 1982;   collection of arcs, where arcs are identified by ordered pairs of   distinct nodes [1].  Each node of the graph represents a domain.  An   ordered pair (B, A), an arc from B to A, indicates that B is a   subdomain of domain A, and B is a simple name unique within A.  We   will refer to B as a child of A, and A a parent of B.  The directed   graph that best describes the naming hierarchy is called an   "in-tree", which is a rooted tree with all arcs directed towards the   root (Figure 1). The root of the tree represents the naming universe,   ancestor of all domains.  Endpoints (or leaves) of the tree are the   lowest-level domains.                         U                       / | \                     /   |   \          U -- Naming Universe                    ^    ^    ^         I -- Intermediate Domain                    |    |    |         E -- Endpoint Domain                    I    E    I                  /   \       |                 ^     ^      ^                 |     |      |                 E     E      I                            / | \                           ^  ^  ^                           |  |  |                           E  E  E                                Figure 1                 The In-Tree Model for Domain Hierarchy   The simple name of a child in this model is necessarily unique within   its parent domain.  Since the simple name of the child's parent is   unique within the child's grandparent domain, the child can be   uniquely named in its grandparent domain by the concatenation of its   simple name followed by its parent's simple name.      For example, if the simple name of a child is "C1" then no other      child of the same parent may be named "C1".  Further, if the      parent of this child is named "P1", then "P1" is a unique simple      name in the child's grandparent domain.  Thus, the concatenation      C1.P1 is unique in C1's grandparent domain.   Similarly, each element of the hierarchy is uniquely named in the   universe by its complete name, the concatenation of its simple name   and those for the domains along the trail leading to the naming   universe.   The hierarchical structure of the Internet naming convention supports   decentralization of naming authority and distribution of name service   capability.  We assume a naming authority and a name serverSu & Postel                                                     [Page 2]

RFC 819                                                     August 1982;   associated with each domain.  In Sections5 and6 respectively the   name service and the naming authority are discussed.   Within an endpoint domain, unique names are assigned to <user>   representing user mailboxes.  User mailboxes may be viewed as   children of their respective domains.   In reality, anomalies may exist violating the in-tree model of naming   hierarchy.  Overlapping domains imply multiple parentage, i.e., an   entity of the naming hierarchy being a child of more than one domain.   It is conceivable that ISI can be a member of the ARPA domain as well   as a member of the USC domain (Figure 2).  Such a relation   constitutes an anomaly to the rule of one-connectivity between any   two points of a tree.  The common child and the sub-tree below it   become descendants of both parent domains.                                 U                               / | \                             /   .   \                           .     .   ARPA                         .       .     | \                                USC    |   \                                   \   |     .                                     \ |       .                                      ISI                                Figure 2                      Anomaly in the In-Tree Model   Some issues resulting from multiple parentage are addressed inAppendix B.  The general implications of multiple parentage are a   subject for further investigation.3.  Advantage of Absolute Naming   Absolute naming implies that the (complete) names are assigned with   respect to a universal reference point.  The advantage of absolute   naming is that a name thus assigned can be universally interpreted   with respect to the universal reference point.  The Internet naming   convention provides absolute naming with the naming universe as its   universal reference point.   For relative naming, an entity is named depending upon the position   of the naming entity relative to that of the named entity.  A set of   hosts running the "unix" operating system exchange mail using a   method called "uucp".  The naming convention employed by uucp is an   example of relative naming.  The mail recipient is typically named by   a source route identifying a chain of locally known hosts linking theSu & Postel                                                     [Page 3]

RFC 819                                                     August 1982;   sender's host to the recipient's.  A destination name can be, for   example,      "alpha!beta!gamma!john",   where "alpha" is presumably known to the originating host, "beta" is   known to "alpha", and so on.   The uucp mail system has demonstrated many of the problems inherent   to relative naming.  When the host names are only locally   interpretable, routing optimization becomes impossible.  A reply   message may have to traverse the reverse route to the original sender   in order to be forwarded to other parties.   Furthermore, if a message is forwarded by one of the original   recipients or passed on as the text of another message, the frame of   reference of the relative source route can be completely lost.  Such   relative naming schemes have severe problems for many of the uses   that we depend upon in the ARPA Internet community.4.  Interoperability   To allow interoperation with a different naming convention, the names   assigned by a foreign naming convention need to be accommodated.   Given the autonomous nature of domains, a foreign naming environment   may be incorporated as a domain anywhere in the hierarchy.  Within   the naming universe, the name service for a domain is provided within   that domain.  Thus, a foreign naming convention can be independent of   the Internet naming convention.  What is implied here is that no   standard convention for naming needs to be imposed to allow   interoperations among heterogeneous naming environments.      For example:         There might be a naming convention, say, in the FOO world,         something like "<user>%<host>%<area>".  Communications with an         entity in that environment can be achieved from the Internet         community by simply appending ".FOO" on the end of the name in         that foreign convention.            John%ISI-Tops20-7%California.FOO      Another example:         One way of accommodating the "uucp world" described in the last         section is to declare it as a foreign system.  Thus, a uucp         name            "alpha!beta!gamma!john"Su & Postel                                                     [Page 4]

RFC 819                                                     August 1982;         might be known in the Internet community as            "alpha!beta!gamma!john.UUCP".      Communicating with a complex subdomain is another case which can      be treated as interoperation.  A complex subdomain is a domain      with complex internal naming structure presumably unknown to the      outside world (or the outside world does not care to be concerned      with its complexity).   For the mail system application, the names embedded in the message   text are often used by the destination for such purpose as to reply   to the original message.  Thus, the embedded names may need to be   converted for the benefit of the name server in the destination   environment.   Conversion of names on the boundary between heterogeneous naming   environments is a complex subject.  The following example illustrates   some of the involved issues.      For example:         A message is sent from the Internet community to the FOO         environment.  It may bear the "From" and "To" fields as:            From: Fred@F.ISI.ARPA            To:   John%ISI-Tops20-7%California.FOO         where "FOO" is a domain independent of the Internet naming         environment.  The interface on the boundary of the two         environments may be represented by a software module.  We may         assume this interface to be an entity of the Internet community         as well as an entity of the FOO community.  For the benefit of         the FOO environment, the "From" and "To" fields need to be         modified upon the message's arrival at the boundary. One may         view naming as a separate layer of protocol, and treat         conversion as a protocol translation.  The matter is         complicated when the message is sent to more than one         destination within different naming environments; or the         message is destined within an environment not sharing boundary         with the originating naming environment.   While the general subject concerning conversion is beyond the scope   of this note, a few questions are raised inAppendix D.Su & Postel                                                     [Page 5]

RFC 819                                                     August 1982;5.  Name Service   Name service is a network service providing name-to-address   translation.  Such service may be achieved in a number of ways.  For   a simple networking environment, it can be accomplished with a single   central database containing name-to-address correspondence for all   the pertinent network entities, such as hosts.   In the case of the old ARPANET host names, a central database is   duplicated in each individual host.  The originating module of an   application process would query the local name service (e.g., make a   system call) to obtain network address for the destination host. With   the proliferation of networks and an accelerating increase in the   number of hosts participating in networking, the ever growing size,   update frequency, and the dissemination of the central database makes   this approach unmanageable.   The hierarchical structure of the Internet naming convention supports   decentralization of naming authority and distribution of name service   capability.  It readily accommodates growth of the naming universe.   It allows an arbitrary number of hierarchical layers.  The addition   of a new domain adds little complexity to an existing Internet   system.   The name service at each domain is assumed to be provided by one or   more name servers.  There are two models for how a name server   completes its work, these might be called "iterative" and   "recursive".      For an iterative name server there may be two kinds of responses.      The first kind of response is a destination address.  The second      kind of response is the address of another name server.  If the      response is a destination address, then the query is satisfied. If      the response is the address of another name server, then the query      must be repeated using that name server, and so on until a      destination address is obtained.      For a recursive name server there is only one kind of response --      a destination address.  This puts an obligation on the name server      to actually make the call on another name server if it can't      answer the query itself.   It is noted that looping can be avoided since the names presented for   translation can only be of finite concatenation.  However, care   should be taken in employing mechanisms such as a pointer to the next   simple name for resolution.   We believe that some name servers will be recursive, but we don't   believe that all will be.  This means that the caller must beSu & Postel                                                     [Page 6]

RFC 819                                                     August 1982;   prepared for either type of server.  Further discussion and examples   of name service is given inAppendix C.   The basic name service at each domain is the translation of simple   names to addresses for all of its children.  However, if only this   basic name service is provided, the use of complete (or fully   qualified) names would be required.  Such requirement can be   unreasonable in practice.  Thus, we propose the use of partial names   in the context in which their uniqueness is preserved.  By   construction, naming uniqueness is preserved within the domain of a   common ancestry. Thus, a partially qualified name is constructed by   omitting from the complete name ancestors common to the communicating   parties. When a partially qualified name leaves its context of   uniqueness it must be additionally qualified.   The use of partially qualified names places a requirement on the   Internet name service.  To satisfy this requirement, the name service   at each domain must be capable of, in addition to the basic service,   resolving simple names for all of its ancestors (including itself)   and their children.  InAppendix B, the required distinction among   simple names for such resolution is addressed.6.  Naming Authority   Associated with each domain there must be a naming authority to   assign simple names and ensure proper distinction among simple names.   Note that if the use of partially qualified names is allowed in a   sub-domain, the uniqueness of simple names inside that sub-domain is   insufficient to avoid ambiguity with names outside the subdomain.Appendix B discusses simple name assignment in a sub-domain that   would allow the use of partially qualified names without ambiguity.   Administratively, associated with each domain there is a single   person (or office) called the registrar.  The registrar of the naming   universe specifies the top-level set of domains and designates a   registrar for each of these domains.  The registrar for any given   domain maintains the naming authority for that domain.7.  Network-Oriented Applications   For user applications such as file transfer and terminal access, the   remote host needs to be named.  To be compatible with ARPANET naming   convention, a host can be treated as an endpoint domain.   Many operating systems or programming language run-time environments   provide functions or calls (JSYSs, SVCs, UUOs, SYSs, etc.) for   standard services (e.g., time-of-day, account-of-logged-in-user,   convert-number-to-string).  It is likely to be very helpful if such aSu & Postel                                                     [Page 7]

RFC 819                                                     August 1982;   function or call is developed for translating a host name to an   address.  Indeed, several systems on the ARPANET already have such   facilities for translating an ARPANET host name into an ARPANET   address based on internal tables.   We recommend that this provision of a standard function or call for   translating names to addresses be extended to accept names of   Internet convention.  This will promote a consistent interface to the   users of programs involving internetwork activities.  The standard   facility for translating Internet names to Internet addresses should   include all the mechanisms available on the host, such as checking a   local table or cache of recently checked names, or consulting a name   server via the Internet.8.  Mail Relaying   Relaying is a feature adopted by more and more mail systems.   Relaying facilitates, among other things, interoperations between   heterogeneous mail systems.  The term "relay" is used to describe the   situation where a message is routed via one or more intermediate   points between the sender and the recipient.  The mail relays are   normally specified explicitly as relay points in the instructions for   message delivery. Usually, each of the intermediate relays assume   responsibility for the relayed message [3].      A point should be made on the basic difference between mail      relaying and the uucp naming system.  The difference is that      although mail relaying with absolute naming can also be considered      as a form of source routing, the names of each intermediate points      and that of the destination are universally interpretable, while      the host names along a source route of the uucp convention is      relative and thus only locally interpretable.   The Internet naming convention explicitly allows interoperations   among heterogeneous systems.  This implies that the originator of a   communication may name a destination which resides in a foreign   system.  The probability is that the destination network address may   not be comprehensible to the transport system of the originator.   Thus, an implicit relaying mechanism is called for at the boundary   between the domains.  The function of this implicit relay is the same   as the explicit relay.Su & Postel                                                     [Page 8]

RFC 819                                                     August 1982;9.  Implementation   The Actual Domains      The initial set of top-level names include:         ARPA            This represents the set of organizations involved in the            Internet system through the authority of the U.S. Defense            Advanced Research Projects Agency.  This includes all the            research and development hosts on the ARPANET and hosts on            many other nets as well.  But note very carefully that the            top-level domain "ARPA" does not map one-to-one with the            ARPANET -- domains are administrative, not topological.   Transition      In the transition from the ARPANET naming convention to the      Internet naming convention, a host name may be used as a simple      name for an endpoint domain.  Thus, if "USC-ISIF" is an ARPANET      host name, then "USC-ISIF.ARPA" is the name of an Internet domain.10.  Summary   A hierarchical naming convention based on the domain concept has been   adopted by the Internet community.  It is an absolute naming   convention defined along administrative rather than topological   boundaries.  This naming convention is adaptive for interoperations   with other naming conventions.  Thus, no standard convention needs to   be imposed for interoperations among heterogeneous naming   environments.   This Internet naming convention allows distributed name service and   naming authority functions at each domain.  We have specified these   functions required at each domain.  Also discussed are implications   on network-oriented applications, mail systems, and administrative   aspects of this convention.Su & Postel                                                     [Page 9]

RFC 819                                                     August 1982;APPENDIX A   The BNF Specification   We present here a rather detailed "BNF" definition of the allowed   form for a computer mail "mailbox" composed of a "local-part" and a   "domain" (separated by an at sign).  Clearly, the domain can be used   separately in other network-oriented applications.   <mailbox> ::= <local-part> "@" <domain>   <local-part> ::= <string> | <quoted-string>   <string> ::= <char> | <char> <string>   <quoted-string> ::=  """ <qtext> """   <qtext> ::=  "\" <x> | "\" <x> <qtext> | <q> | <q> <qtext>   <char> ::= <c> | "\" <x>   <domain> ::= <naming-domain> | <naming-domain> "." <domain>   <naming-domain> ::=  <simple-name> | <address>   <simple-name> ::= <a> <ldh-str> <let-dig>   <ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>   <let-dig> ::= <a> | <d>   <let-dig-hyp> ::= <a> | <d> | "-"   <address> :: =  "#" <number> | "[" <dotnum> "]"   <number> ::= <d> | <d> <number>   <dotnum> ::= <snum> "." <snum> "." <snum> "." <snum>   <snum> ::= one, two, or three digits representing a decimal integer   value in the range 0 through 255   <a> ::= any one of the 52 alphabetic characters A through Z in upper   case and a through z in lower case   <c> ::= any one of the 128 ASCII characters except <s> or <SP>   <d> ::= any one of the ten digits 0 through 9Su & Postel                                                    [Page 10]

RFC 819                                                     August 1982;   <q> ::= any one of the 128 ASCII characters except CR, LF, quote ("),   or backslash (\)   <x> ::= any one of the 128 ASCII characters (no exceptions)   <s> ::= "<", ">", "(", ")", "[", "]", "\", ".", ",", ";", ":", "@",   """, and the control characters (ASCII codes 0 through 31 inclusive   and 127)   Note that the backslash, "\", is a quote character, which is used to   indicate that the next character is to be used literally (instead of   its normal interpretation).  For example, "Joe\,Smith" could be used   to indicate a single nine character user field with comma being the   fourth character of the field.   The simple names that make up a domain may contain both upper and   lower case letters (as well as digits and hyphen), but these names   are not case sensitive.   Hosts are generally known by names.  Sometimes a host is not known to   the translation function and communication is blocked.  To bypass   this barrier two forms of addresses are also allowed for host   "names". One form is a decimal integer prefixed by a pound sign, "#".   Another form, called "dotted decimal", is four small decimal integers   separated by dots and enclosed by brackets, e.g., "[123.255.37.2]",   which indicates a 32-bit ARPA Internet Address in four 8-bit fields.   (Of course, these numeric address forms are specific to the Internet,   other forms may have to be provided if this problem arises in other   transport systems.)Su & Postel                                                    [Page 11]

RFC 819                                                     August 1982;APPENDIX B   An Aside on the Assignment of Simple Names   In the following example, there are two naming hierarchies joining at   the naming universe 'U'.  One consists of domains (S, R, N, J, P, Q,   B, A); and the other (L, E, F, G, H, D, C, K, B, A). Domain B is   assumed to have multiple parentage as shown.                                U                              /   \                            /       \                          J           L                        /               \                      N                   E                    /   \               /   \                  R       P           D       F                /           \         | \      \              S               Q       C  (X)     G                                \   /   \          \                                  B       K          H                                /                              A                                Figure 3    Illustration of Requirements for the Distinction of Simple Names   Suppose someone at A tries to initiate communication with destination   H.  The fully qualified destination name would be      H.G.F.E.L.U   Omitting common ancestors, the partially qualified name for the   destination would be      H.G.F   To permit the case of partially qualified names, name server at A   needs to resolve the simple name F, i.e., F needs to be distinct from   all the other simple names in its database.   To enable the name server of a domain to resolve simple names, a   simple name for a child needs to be assigned distinct from those of   all of its ancestors and their immediate children.  However, such   distinction would not be sufficient to allow simple name resolution   at lower-level domains because lower-level domains could have   multiple parentage below the level of this domain.      In the example above, let us assume that a name is to be assignedSu & Postel                                                    [Page 12]

RFC 819                                                     August 1982;      to a new domain X by D.  To allow name server at D to resolve      simple names, the name for X must be distinct from L, E, D, C, F,      and J.  However, allowing A to resolve simple names, X needs to be      also distinct from A, B, K, as well as from Q, P, N, and R.   The following observations can be made.      Simple names along parallel trails (distinct trails leading from      one domain to the naming universe) must be distinct, e.g., N must      be distinct from E for B or A to properly resolve simple names.      No universal uniqueness of simple names is called for, e.g., the      simple name S does not have to be distinct from that of E, F, G,      H, D, C, K, Q, B, or A.      The lower the level at which a domain occurs, the more immune it      is to the requirement of naming uniqueness.   To satisfy the required distinction of simple names for proper   resolution at all levels, a naming authority needs to ensure the   simple name to be assigned distinct from those in the name server   databases at the endpoint naming domains within its domain.  As an   example, for D to assign a simple name for X, it would need to   consult databases at A and K.  It is, however, acceptable to have   simple names under domain A identical with those under K.  Failure of   such distinct assignment of simple names by naming authority of one   domain would jeopardize the capability of simple name resolution for   entities within the subtree under that domain.Su & Postel                                                    [Page 13]

RFC 819                                                     August 1982;APPENDIX C   Further Discussion of Name Service and Name Servers   The name service on a system should appear to the programmer of an   application program simply as a system call or library subroutine.   Within that call or subroutine there may be several types of methods   for resolving the name string into an address.      First, a local table may be consulted.  This table may be a      complete table and may be updated frequently, or it may simply be      a cache of the few latest name to address mappings recently      determined.      Second, a call may be made to a name server to resolve the string      into a destination address.      Third, a call may be made to a name server to resolve the string      into a relay address.   Whenever a name server is called it may be a recursive server or an   interactive server.      If the server is recursive, the caller won't be able to tell if      the server itself had the information to resolve the query or      called another server recursively (except perhaps for the time it      takes).      If the server is iterative, the caller must be prepared for either      the answer to its query, or a response indicating that it should      call on a different server.   It should be noted that the main name service discussed in this memo   is simply a name string to address service.  For some applications   there may be other services needed.      For example, even within the Internet there are several procedures      or protocols for actually transferring mail.  One need is to      determine which mail procedures a destination host can use.      Another need is to determine the name of a relay host if the      source and destination hosts do not have a common mail procedure.      These more specialized services must be specific to each      application since the answers may be application dependent, but      the basic name to address translation is application independent.Su & Postel                                                    [Page 14]

RFC 819                                                     August 1982;APPENDIX D   Further Discussion of Interoperability and Protocol Translations   The translation of protocols from one system to another is often   quite difficult.  Following are some questions that stem from   considering the translations of addresses between mail systems:      What is the impact of different addressing environments (i.e.,      environments of different address formats)?      It is noted that the boundary of naming environment may or may not      coincide with the boundary of different mail systems. Should the      conversion of naming be independent of the application system?      The boundary between different addressing environments may or may      not coincide with that of different naming environments or      application systems.  Some generic approach appears to be      necessary.      If the conversion of naming is to be independent of the      application system, some form of interaction appears necessary      between the interface module of naming conversion with some      application level functions, such as the parsing and modification      of message text.      To accommodate encryption, conversion may not be desirable at all.      What then can be an alternative to conversion?Su & Postel                                                    [Page 15]

RFC 819                                                     August 1982;GLOSSARY   address      An address is a numerical identifier for the topological location      of the named entity.   name      A name is an alphanumeric identifier associated with the named      entity.  For unique identification, a name needs to be unique in      the context in which the name is used.  A name can be mapped to an      address.   complete (fully qualified) name      A complete name is a concatenation of simple names representing      the hierarchical relation of the named with respect to the naming      universe, that is it is the concatenation of the simple names of      the domain structure tree nodes starting with its own name and      ending with the top level node name.  It is a unique name in the      naming universe.   partially qualified name      A partially qualified name is an abbreviation of the complete name      omitting simple names of the common ancestors of the communicating      parties.   simple name      A simple name is an alphanumeric identifier unique only within its      parent domain.   domain      A domain defines a region of jurisdiction for name assignment and      of responsibility for name-to-address translation.   naming universe      Naming universe is the ancestor of all network entities.   naming environment      A networking environment employing a specific naming convention.Su & Postel                                                    [Page 16]

RFC 819                                                     August 1982;   name service      Name service is a network service for name-to-address mapping.   name server      A name server is a network mechanism (e.g., a process) realizing      the function of name service.   naming authority      Naming authority is an administrative entity having the authority      for assigning simple names and responsibility for resolving naming      conflict.   parallel relations      A network entity may have one or more hierarchical relations with      respect to the naming universe.  Such multiple relations are      parallel relations to each other.   multiple parentage      A network entity has multiple parentage when it is assigned a      simple name by more than one naming domain.Su & Postel                                                    [Page 17]

RFC 819                                                     August 1982;REFERENCES   [1]  F. Harary, "Graph Theory", Addison-Wesley, Reading,   Massachusetts, 1969.   [2]  J. Postel, "Computer Mail Meeting Notes",RFC-805,   USC/Information Sciences Institute, 8 February 1982.   [3]  J. Postel, "Simple Mail Transfer Protocol",RFC-821,   USC/Information Sciences Institute, August 1982.   [4]  D. Crocker, "Standard for the Format of ARPA Internet Text   Messages",RFC-822, Department of Electrical Engineering, University   of Delaware, August 1982.Su & Postel                                                    [Page 18]