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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