    Network Working Group                            S.E. Hardcastle-Kille
    Requests for Comments 1278                   University College London
                                                             November 1991

                  A string encoding of Presentation Address

    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
        There are a number of environments where a simple string encoding
        of Presentation Address is desirable.  This specification defines
        such a representation.

    RFC 1278            String encoded P-Address             November 1991

    1  Introduction

    OSI Application Entities use presentation addresses to address other
    Application Entities.  The model for this is defined in [ISO87b].
    Presentation addresses are stored in the OSI Directory using an ASN.1
    representation defined by the OSI Directory [CCI88].  Logically, a
    presentation address consists of:

     o  A presentation selector

     o  A session selector

     o  A transport selector

     o  A set of network addresses

    The selectors are all octet strings, but often have IA5 character
    representations.  The format of network addresses is defined in
    [ISO87a].
    There is a need to represent presentation addresses as strings in a
    number of different contexts.  This Internet Draft defines a format
    for use on the Internet.  It is for display to human users, and its
    use is recommended whenever this needs to be done.  Typically, this
    will be for system managers rather than for end users.  It is not
    intended for internal storage.

    This Internet Draft was originally published as UCL Research Note
    RN/89/14 [Kil89].  It was agreed as a unified syntax for the THORN and
    ISODE projects.  It is used throughout ISODE.
    Christian Huitema of Inria and Marshall Rose of PSI Inc.  gave much
    useful input to this document.

    2  Requirements

    The main requirements are:

     o  Must be able to specify any legal value.

     o  Should be clean in the common case of the presentation address
        containing network addresses and no selectors.

    Hardcastle-Kille                                                Page 1

    RFC 1278            String encoded P-Address             November 1991

     o  Must deal with selectors in the following encodings:

        --  IA5

        --  Decimal digits encoded as IA5 (this is the most common syntax
            in Europe, as it is required by X.400(84) and should receive a
            straightforward encoding)

        --  Numeric encoded as a 16 bit unsigned integer (US GOSIP). This
            is mapped onto two octets, with the first octet being the high
            order byte of the integer.

        --  General Hexadecimal

     o  Should give special encodings for the ad hoc encoding proposed in
        ``An interim approach to use of Network Addresses'' [HK91].

        --  X.25(80) Networks

        --  TCP/IP Networks

     o  Should be extensible for additional forms.

     o  Should provide a reasonably compact representation .

    3  Format

    The_BNF_is_given_in_figure_1.__________________________________________

    <digit> ::= [0-9]
    <other> ::= [0-9a-zA-Z+-.]
    <domainchar> ::= [0-9a-zA-Z-.]
    <hexdigit> ::= [0-9a-fA-F]
    <hexoctet> ::= <hexdigit> <hexdigit>
    <decimaloctet> ::= <digit> | <digit> <digit>
                            | <digit> <digit> <digit>

    <digitstring> ::= <digit> <digitstring>                             10
                            | <digit>
    <otherstring> ::= <other> <otherstring>
                            | <other>

    Hardcastle-Kille                                                Page 2

    RFC 1278            String encoded P-Address             November 1991

    <domainstring> ::= <domainchar> <otherstring>
                            | <domainchar>
    <hexstring> ::= <hexoctet> <hexstring> | <hexoctet>

    <dotstring> ::= <decimaloctet> "." <dotstring>
                    | <decimaloctet> "." <decimaloctet>
                                                                        20

    <dothexstring> ::= <dotstring> | <hexstring>

    <presentation-address> ::=
                    [[[ <psel> "/" ] <ssel> "/" ] <tsel> "/" ]
                    <network-address-list>

    <network-address-list> ::= <network-address> "_" <network-address-list>30
                               | <network-address>

    <psel> ::= <selector>
    <ssel> ::= <selector>
    <tsel> ::= <selector>

    <selector>  ::= '"' <otherstring> '"'        -- IA5
                                                 -- For chars not in this
                                                 -- string use hex
                    | "#" <digitstring>          -- US GOSIP            40
                    | "'" <hexstring> "'H"       -- Hex
                    | ""                         -- Empty but present

    <network-address> ::=   "NS" "+" <dothexstring>
                                     -- Concrete Binary Representation
                                     -- This is the compact encoding
            | <afi> "+" <idi> [ "+" <dsp> ]
                                    -- A user oriented form
            | <idp> "+" <hexstring>
                                    -- ISO 8348 Compatability           50

    <idp> ::= <digitstring> -

    <dsp>  ::=
            | "d" <digitstring>          -- Abstract Decimal
            | "x" <dothexstring>            -- Abstract Binary
            | "l" <otherstring>             -- IA5:  local form only

    Hardcastle-Kille                                                Page 3

    RFC 1278            String encoded P-Address             November 1991

            | "RFC-1006" "+" <prefix> "+" <ip>
               [ "+" <port> [ "+" <tset> ]]
            | "X.25(80)" "+" <prefix> "+" <dte>                         60
               [ "+" <cudf-or-pid> "+" <hexstring> ]
            | "ECMA-117-Binary" "+" <hexstring> "+" <hexstring>
               "+" <hexstring>
            | "ECMA-117-Decimal" "+" <digitstring> "+"
               <digitstring> "+" <digitstring>

    <idi> ::= <digitstring>
    <afi> ::= "X121" | "DCC" | "TELEX" | "PSTN" | "ISDN"
                    | "ICD" | "LOCAL"
                                                                        70
    <prefix> ::= <digit> <digit>

    <ip> ::= <domainstring>
                            -- dotted decimal form (e.g., 10.0.0.6)
                            -- or domain (e.g., twg.com)
    <port> ::= <digitstring>
    <tset> ::= <digitstring>

    <dte> ::= <digitstring>
    <cudf-or-pid> ::= "CUDF" | "PID"                                    80

    ________________________Figure_1:__String_BNF__________________________

    Four examples:

    "256"/NS+a433bb93c1_NS+aa3106

    #63/#41/#12/X121+234219200300

    '3a'H/TELEX+00728722+X.25(80)+02+00002340555+CUDF+"892796"

    TELEX+00728722+RFC-1006+03+10.0.0.6

    Note that the RFC 1006 encoding permits use of either a DNS Domain
    Name or an IP address.  The former is primarily for ease of entry.  If
    this DNS Domain Name maps onto multiple IP addresses, then multiple
    network addresses should be generated.  The DNS Domain Name form is

    Hardcastle-Kille                                                Page 4

    RFC 1278            String encoded P-Address             November 1991

    for convenient input.  When mapping from an encoded address to string
    form, the IP address form should always be used.

    4  Encoding

    Selectors are represented in a manner which can be easily encoded.  In
    the NS notation, the concrete binary form of network address is given.
    Otherwise, this string notation provides a mechanism for representing
    the Abstract Syntax of a Network Address.  This must be encoded
    according to Addendum 2 of ISO 8348 [ISO87a].

    5  Macros

    There are often common addresses, for which a cleaner representation
    is desired.  This is achieved by use of Macros.  If a
    <network-address> can be parsed as:

    <otherstring> "=" *( any )

    Then the leading string is taken as a Macro, which is substituted.
    This may be applied recursively.  When presenting Network Address to
    humans, the longest available substitution should be used.  For
    example:

                           ________________________
                           |_Macro_|Value__________ |
                           | UK.AC |DCC+826+d110000 |
                           |_Leeds_|UK.AC=120______ |

    Then ``Leeds=22'' would be expanded to ``DCC+826+d11000012022''.

    6  Standard Macros

    No Macros should ever be relied on.  However, the following are
    suggested as standard.

    Hardcastle-Kille                                                Page 5

    RFC 1278            String encoded P-Address             November 1991

               ________________________________________________
               |_Macro_____________|Value______________________ |
               | Int-X25(80)       |TELEX+00728722+X25(80)+01+  |
               | Janet-X25(80)     |TELEX+00728722+X25(80)+02+  |
               | Internet-RFC-1006 |TELEX+00728722+RFC-1006+03+ |
               |_IXI_______________|TELEX+00728722+RFC-1006+06+_|

    7  References

    References

    [CCI88]  The Directory --- overview of concepts, models and services,
             December 1988. CCITT X.500 Series Recommendations.

    [HK91]   S.E. Hardcastle-Kille. Encoding network addresses to support
             operation over non-osi lower layers. Request for Comments
             RFC 1277, Department of Computer Science, University College
             London, November 1991.

    [ISO87a] Information processing systems - data communications -
             network services definition:  Addendum 2 - network layer
             addressing, March 1987. ISO TC 97/SC 6.

    [ISO87b] ISO DIS 7498-3 on naming and addressing, May 1987.
             ISO/IEC/JTC-1/SC 21.

    [Kil89]  S.E. Kille. A string encoding of presentation address.
             Research Note RN/89/14, Department of Computer Science,
             University College London, February 1989.

    8  Security Considerations

    Security considerations are not discussed in this memo.

    9  Author's Address

        Steve Hardcastle-Kille
        Department of Computer Science
        University College London
        Gower Street
        WC1E 6BT

    Hardcastle-Kille                                                Page 6

    RFC 1278            String encoded P-Address             November 1991

        England

        Phone:  +44-71-380-7294

        EMail:  S.Kille@CS.UCL.AC.UK

    Hardcastle-Kille                                                Page 7