draft-ietf-svrloc-protocol-12.txt John Veizades
INTERNET-DRAFT TGV, Inc.
Erik Guttman
Sun Microsystems
Charles Perkins
IBM Research
Scott Kaplan
March 11, 1996
Service Location Protocol
1.0 Status of this memo
This draft document is a product of the IETF Service Location
Working Group; it will be submitted to the RFC editor as a standards
document. Please respond with comments to the srvloc@tgv.com mailing
list.
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months. Internet-Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use
Internet-Drafts as reference material or to cite them other than as
a "working draft" or "work in progress."
To learn the current status of any Internet-Draft, please check the
1id-abstracts.txt listing contained in the Internet-Drafts Shadow
Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu or
munnari.oz.au.
2.0 Abstract
The service location protocol provides a framework for the discovery
and selection of network services. It relies on multicast support
at the network layer of the protocol stack it is using. It does not
specifically rely upon the TCP/IP protocol stack but makes use of
concepts that are found in most TCP/IP protocol implementations.
Traditionally, users find services using the name of a network host
(a human readable text string) which is an alias for a network
address. The service location protocol eliminates the need for a
user to know the name of a network host supporting a service.
Rather, the user supplies a set of attributes which describe the
service. The service location protocol allows the user to bind this
description to the network address of the service.
Service Location provides a dynamic configuration mechanism for
applications in a tightly coupled set of local area networks. It is
not a global resolution system for the entire Internet, rather it is
intended to serve institutional networks with shared services.
Service Location WG
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Table of Contents
1.0 Status of this memo..............................................1
2.0 Abstract.........................................................1
3.0 Notation Conventions.............................................3
4.0 Terminology......................................................3
5.0 Protocol Overview................................................5
5.1 Protocol Transactions........................................5
5.2 Service and Predicate Representation.........................7
5.3 Additional Notes.............................................7
5.3.1 The 'service:' URL scheme.............................7
5.3.2 Interpretation of Service Location Replies............7
5.3.3 Use of TCP and Multicast in Service Location..........7
5.3.4 Multilingual Support..................................7
5.3.5 Standard Attribute Definitions........................8
5.3.6 Naming Authority......................................8
5.3.7 No Synchronous Assumption.............................9
5.4 Service Location PDU header..................................9
5.4.1 Version...............................................9
5.4.2 Functions.............................................9
5.4.3 Length................................................9
5.4.4 Error Codes...........................................9
5.4.5 Transaction Identifier (XID).........................10
5.4.6 Flags................................................10
5.4.7 Time To Live.........................................10
5.4.8 Character Encoding...................................10
5.4.9 Language Code........................................10
5.5 Service Request and Reply...................................10
5.6 Directory Agent Discovery Request...........................13
5.7 Service Type Request........................................14
5.8 Attribute Request and Attribute Reply.......................15
5.9 Service Discovery Request...................................17
6.0 Directory Agents................................................18
6.1 Introduction................................................18
6.2 Directory Agent Discovery...................................18
6.3 Service Registration........................................20
6.4 Service Unregister..........................................22
6.5 SCOPE Discovery and Use.....................................23
6.6 Service Location Scaling and Operating Modes................24
7.0 Service Location Connections....................................25
8.0 Security Considerations.........................................26
9.0 Multicast vs. Broadcast.........................................26
9.1 Single Subnet...............................................26
9.2 Multiple Subnets............................................26
9.3 Service Multicast Address...................................26
10.0 Service Location in the Internet...............................27
11.0 Protocol Formats...............................................27
11.1 Fields Used in Service Location Packets....................27
11.1.1 Previous Responders' Address Specification..........27
11.1.2 Service Request Predicate...........................28
11.1.3 Reply...............................................31
11.1.4 Service Registration Information....................31
11.1.5 Service Unregister Information......................31
11.1.6 Attribute List......................................31
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11.1.7 Service Type........................................32
11.2 ADDRESS SPECIFICATIONs in Service Location.................32
11.3 Attribute Value encoding rules.............................32
12.0 Implementation Requirements....................................33
13.0 Configuration Parameters and Defaults..........................35
13.1 Multicast vs. Broadcast....................................35
13.2 Multicast Radius...........................................35
13.3 Directory Agent Address....................................35
13.4 Directory Agent Scope Assignment...........................35
14.0 Interesting Constants..........................................35
15.0 Acknowledgments................................................36
16.0 References.....................................................36
17.0 Author's Addresses.............................................38
18.0 Document Expiration............................................38
Appendix A - Technical contents of ISO 639:1988.....................39
3.0 Notation Conventions
<> Values set off in this manner are fully described in section
11.0.
In General, all definitions of items in packets are described
in section 11.0.
| |
\ \ Packet layouts with this notation indicate a variable length
| | field.
4.0 Terminology
User Agent (UA) A process working on the user's behalf to
acquire service attributes and configuration.
The User Agent retrieves service information
from the Service Agents or Directory Agents.
Service Agent (SA) A process working on the behalf of one or more
services to advertise service attributes and
configuration.
Service Application A process working on behalf of one or more
services to advertise service attributes and
configuration. Unlike an SA the application
will not directly respond to service queries
and will merely register with Directory Agents.
Service Information A collection of attributes and configuration
information associated with a single service.
The Service Agents advertise service
information for a collection of service
instances.
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Service The service is a process or system providing a
facility to the network. The goal of service
location is to provide sufficient information
to the user, via the User Agent, to find the
service. The service itself is accessed using
a communication mechanism external to the
the service location protocol.
Directory Agent (DA) A process which collects information from
Service Agents to provide a single repository
of service information in order to centralize
it for efficient access by User Agents. There
can only be one DA present per given host.
Service Type Each type of service has a unique Service Type
string. The Service Type defines a template
including expected attributes, values and
protocol behavior. Well known Service Types
are registered with the IANA and templates are
available as RFCs. Private Service Types may
also be supported.
Naming Authority The agency or group which catalogues
given Service Types and Attributes. The
default Naming Authority is IANA. Otherwise
the Service Type of the service has the Naming
Authority appended to the end, following a '.'
separator.
Attribute A {class, value} pair describing a
characteristic of a service. Note that a
class is a string. Values are optional.
An attribute without a value is a keyword.
A value is a string which may be interpreted
as a string, or as a boolean, integer or
opaque value if the value string takes
specific forms (see section 11.3.) The
service information advertised by a Service
Agent may include more than one value per
class.
Predicate A boolean expression of attributes, relations
and logical operators. The predicate is used
to find services which satisfy particular
requirements. See section 11.1.2.
Scope A collection of systems, networks and other
network components that make up a logical
group. See section 6.5 and 6.6.
Campus A campus is a collection of networks, hosts
and related network infrastructure that is
grouped together for geographical or political
reasons.
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Agent's Internet All the hosts accessible within the Agent's
multicast radius which defaults to the Agent's
site (see section 13.0.) If the network does
not support multicast, the agent's internet is
defined by its broadcast radius. The
discovery method used by the service location
protocol requires these techniques to start up
and provide stable operation. Servers outside
of the Agent's radius are considered "outside
of the user's internet."
Address Specification This is the network layer protocol dependent
mechanism for specifying an User Agent. For
Internet systems this is a URL (Universal
Resource Locator see RFC 1738).
5.0 Protocol Overview
5.1 Protocol Transactions
The diagram below illustrates the relationships described below:
+---------------+ we want this info: +-----------+
| Application | - - - - - - - - - - - -> | Service |
+---------------+ +-----------+
/|\ | |
| +-------------+ |
| | |
\|/ \|/ \|/
+---------------+ +-----------+ +-------------+
| User Agent |<-------->| Service | | Service |
+---------------+ | Agent | | Application |
| +-----------+ +-------------+
| | |
| \|/ |
| +-------------+ |
+------------------>| Directory |<----------+
| Agent |
+-------------+ ___________
/|\ / Many other\
+------------>| SA's |
\___________/
The following describes the operations a User Agent employs to find
services on the attached Internet. The User Agent does not need any
configuration to begin network interaction. The User Agent may
build on the information received in earlier network requests to
find the Service Agents advertising service information, and
subsequently the terms used to describe services that it is
interested in. The User Agent can use this information to construct
predicates which describe the services that match the user's needs.
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A User Agent will operate two ways: If the User Agent has already
obtained the location of a Directory Agent, the User Agent will
unicast a request to it in order to resolve a particular request.
The Directory Agent will unicast a reply to the User Agent. The
User Agent will retry a request to a Directory Agent until it gets
a reply, so if the Directory Agent cannot service the request (say
it has no information) it must return an response with zero values,
possibly with an error code set.
If the User Agent does not have knowledge of a Directory Agent or
if there are no Directory Agents available on the User Agent's
internet, a second mode of discovery is used. The User Agent
multicasts a request to the service multicast address, which the
service it wishes to locate will respond to. All the Service
Agents which are listening to this multicast address will respond,
provided they can satisfy the User Agent's request. Service Agents
which have no information for the User Agent DO NOT respond.
In the case where the User Agent wishes to obtain a complete answer,
an enumeration of ALL services which satisfy the query, there is a
retransmission/convergence algorithm. The User Agent resends the
request, together with a list of previous responders. Only those
Service Agents which are not on the list respond. Once there are no
new responses to the request the accumulation of responses is deemed
complete. Depending on the length of the request, around 60
previous responders may be listed in a single datagram (without
exceding the size of a single datagram and requiring fragmentation.)
If there are more responders than this, the scaling mechanisms
described in section 6.6 should be used.
It is important to stress that while the multicast/convergence model
may be important for discovering services (such as Directory Agents)
it is the exception rather than the rule. Once a User Agent knows
of the location of a Directory Agent, it will use a unicast
request/response transaction.
A service is advertised by an application which registers the
service information with a Directory Agent. This Directory Agent
will resolve requests from User Agents as described above. This
means that a Directory Agent must first be discovered, using the
multicast mechanism described above. If the service is to become
unavailable, it should be unregistered with the Directory Agent.
The Directory Agent responds with an acknowledgment to either a
registration or unregistration.
The Service Agent or Application must register with an available
Directory Agent. The Service Agent must additionally listen for
multicast requests on the service specific multicast address.
Service Applications will fail in an internet where there are no
Directory Agents. Service Applications are present in this
protocol to provide a lightweight service registration mechanism.
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5.2 Service and Predicate Representation
Service information is represented in a text format. The goal is
that the format be human readable and transmittable via email.
The location of network services is encoded as a Universal
Resource Locator (URL) which is also human readable and well
defined. Only the datagram headers are in an encoded form which
is not human readable.
5.3 Additional Notes
5.3.1 The 'service:' URL scheme
The service URL scheme is used by Service Location. It is used
to specify a Service Location. This is used by SAs and Service
Applications to register and unregister Services with DAs. It is
also used by SAs and DAs to return Service Replies to UAs. The
formal definition of the 'service:' URL scheme is in section 11.
The format of the information which follows the 'service:' scheme
should as closely as possible follow the URL structure and
semantics as formalized by the IETF standarization process.
5.3.2 Interpretation of Service Location Replies
Replies should be considered to be valid at the time of delivery.
The service may, however, fail or change between the time of the
reply and the moment an application seeks to make use of the service.
The application making use of service location must be prepared for
the possibility that the service information provided is either
stale or incomplete. In the case where the service information
provided does not allow a User Agent to connect to a service as
desired, the request may be resubmitted.
Service specific configuration information (such as which protocol
to use) should be included as attribute information in Service
Registrations. These configuration attributes will be used by
applications which interpret the Service Location Reply.
5.3.3 Use of TCP and Multicast in Service Location
The service location protocol requires the implementation of
connectionless and a connection oriented transport protocols. The
latter is used for bulk transfer, only when necessary. Connections
are always initiated by an agent request or registration, not by a
replying Directory Agent.
The Service Location discovery mechanisms use internetwork wide
multicast. The protocol will operate in a broadcast environment
with limitations detailed in section 9.0.
5.3.4 Multilingual Support
All Service Registrations declare the language in which the strings
in the service attributes are written by specifying the appropriate
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code in the packet header. For each language the Service advertises
a separate registration takes place. The Service needs to be
unregistered only once since the information associated with it will
be unique. There can only be one service of a given type at a given
address specification even if it is registered in multiple languages.
All Service Requests specify a requested language in the packet
header. The Directory Agent or Service Agent will respond in the
same language as the request, if it has a registration in the same
language as the request. If this language is not supported, a reply
can be sent in the default language (which is English.) If the
'monolingual bit' flag in the header is set and the requested
language is not supported, a SrvRply is not returned: Instead
return a SrvAck with the error field set to LANGUAGE_NOT_SUPPORTED.
5.3.5 Standard Attribute Definitions
Service Types used with the service location protocol must
describe the following:
Service Type string of the service
Multicast address, if used
Attributes (Tag and values)
Attribute Descriptions and interpretations
Service Types defined outside of the IANA standardization process
will use their own Naming Authority string and, possibly, a
multicast address from the unassigned range. If a Service Type does
not define its own multicast address, the Service Location General
Multicast address is used, as the default.
Services which advertise a particular Service Type must support the
complete set of standardized attributes. They may support
additional attributes, beyond the standardized set. Unrecognized
attributes should be ignored by User Agents.
Service Type names which begin with 'x-' are guaranteed not to
conflict with any officially registered Service Type names. It is
suggested that this prefix be used for experimental or private
Service Type names. Similarly, attribute names which begin with
'x-' are guaranteed not to be used for any officially registered
attribute names.
5.3.6 Naming Authority
The Naming Authority of a service defines the semantic meaning of
the Service Types and attributes registered with and provided by
Service Location. The Naming Authority itself is a string which
uniquely identifies an organization. If no string is provided IANA
is the default.
Naming Authorities may define Service Types which are experimental,
proprietary or for private use. The procedure to use is to create a
'unique' Naming Authority string and then specify the Standard
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Attribute Definitions as described above. This Naming Authority will
accompany registration and queries, as described below.
5.3.7 No Synchronous Assumption
There is no requirement that one transaction complete before a
given host begins another. An agent may have multiple outstanding
transactions, initiated either using UDP or TCP.
5.4 Service Location PDU header
NOTE: The following header is used in all of the packet descriptions
below and is abbreviated by using "Service location header" followed
by the function being used.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version = 1 | function | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code |O|M| flags | char encoding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time to Live | XID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Language Code | Reserved (Language Dialect) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.4.1 Version
This protocol document defines version 1 of the service location
protocol.
5.4.2 Functions
Service location datagrams can be identified as to their operation
by the function field. The following are the defined operations:
Packet Type Abbreviation Function Value
Service Request SrvRqst 1
Service Reply SrvRply 2
Service Registration SrvReg 3
Service Unregister SrvUnreg 4
Service Acknowledge SrvAck 5
Attribute Request AttrRqst 6
Attribute Reply AttrRply 7
5.4.3 Length
The length is the number of bytes after the Service Location Header.
5.4.4 Error Codes
Error codes may only have a nonzero value in a SrvRply and a SrvAck.
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5.4.5 Transaction Identifier (XID)
The XID (transaction ID) field allows the requester to match replies
to individual requests. A Service Reply will contain the same XID
as the Service Request. A Service Acknowledge will contain the same
XID as the Service Register or Unregister.
Retransmission of the same service location datagram should not
contain an updated XID. The requester creates the XID from an
initial random seed and increments it by one for each request it
makes. The XIDs will eventually wrap back to zero and continue
incrementing from there.
5.4.6 Flags
The flags field is a bit field. Bit 0 is the 'Overflow bit.' See
Section 7.0 for a complete description for the use of this field.
Bit 1 is the 'Monolingual bit.' Requests with this bit set indicate
the User Agent will only accept responses in the language that is
indicated by the Service or Attribute Request. Replies in other
languages should not be sent for this request. All other bits must
be set to zero.
5.4.7 Time to Live
The TTL field is set to the number of minutes the reply can be
cached by any intermediary service. A value of 0 means the
information must not be cached. User Agents must not cache
Service information. Requests by a User Agent must be issued
directly onto the network.
5.4.8 Character Encoding
The encoding will determine the interpretation of all character data
which follows. There is no way to mix ASCII and UNICODE, for
example. Values for character encoding can be found in the Internet
Assigned Numbers Authority's (IANA) database
http://www.isi.edu/in-notes/iana/assignments/character-sets and have
the values refered by the MIBEnum value.
5.4.9 Language Code
The language code (see Appendix A) is encoded in this field. ALL
strings within the packet which follows are to be interpreted in
this language. Some strings, such as Service Type names, have
standard definitions. These strings should be considered as
tokens and not as words in a language to be translated. This will
be noted where appropriate throughout this document.
The language dialect field is reserved for future use.
5.5 Service Request and Reply
The Service Request is used to obtain nearly all information in the
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Service Location Protocol. It is a general mechanism for delivering
a query to a Directory Agent or Service Agents. Depending on the
format of the query, different information can be obtained. There
are three ways the Service Query may be used: a Directory Agent
Discovery Request, a Service Type Request and a Services Discovery
Request. These are described in the sections which follow.
There is an additional mechanism for determining the range of
service attributes. This is the Attribute Request. It is used to
obtain the tags and values of attributes which will be used to
formulate Service Discovery Requests.
While all of these types of queries may be useful, the only one
which is essential is the Service Discovery Request. If a User
Agent has enough a priori knowledge of what it is looking for it
can simply issue a Service Discovery Request and be done with it.
The point of the other requests is to allow a User Agent to
formulate a query when it has limited or no a priori knowledge of
the services available and their attributes.
The Service Request allows the User Agent to specify the Service
Type of the service and a Predicate in a specific language. The
general form of a Service Request is shown below:
SERVICE TYPE[.NAMING AUTHORITY]:/SCOPE/SELECT CLAUSE/WHERE CLAUSE/
Briefly, the SERVICE TYPE of the service is a unique service type
name. The NAMING AUTHORITY determines the semantic interpretation of
the SERVICE TYPE and the attributes used in the SELECT and WHERE
CLAUSE. The SCOPE is used for range of the query (SCOPEs are
determined administratively, not by network topology as will be
described later.) The SELECT CLAUSE lists which attributes to return
with the reply. The WHERE CLAUSE contains the attributes which
determine the instances of the service (identified by the SERVICE
TYPE) which match the request.
In the case of a multicast request, a list of previous responders is
sent. This list will prevent those in the list from responding, to
be sure that responses from other sources are not drowned out. The
request is multicast repeatedly (with a recommended wait interval of
a second) until there are no new responses, or a certain time has
elapsed. The User Agent may configure a certain 'time out' duration
for example, with the Service Location implementation or in the case
where the User Agent doesn't need ALL the replies, as when any one
service will do.
In order for a request to succeed in matching registered information
4 conditions must be met:
(1). The result must have the same Service Type as the request.
(2). It must have the same Naming Authority.
(3). It must have the same Scope.
(4). The conditions specified in the WHERE CLAUSE must match
the attributes and keywords registered for the service.
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The format of the Service Request is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service location header (function = SrvRqst) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|number of previous responders |<Previous Responders Addr Spec>|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Previous Responders Addr Spec> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Service Request Predicate> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Service Request
User Agent requests that are generated by a genesis event, i.e., the
rebooting of a system, loading of the network kernel, etc. should be
sent after a random interval between 0 and 3 seconds.
The general form of an individual reply is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of URL string | <URL String> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <URL String>, Continued. \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of Attributes String | <Attributes String> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Attributes String>, Continued. \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The URL string conforms to RFC 1738. If the Service Type does not
have a standardized representation, the minimal requirement is:
service:service-type:// ADDRESS SPECIFICATION /
The reply will always contain the Service Type and the ADDRESS
SPECIFICATION.
The Keywords and Attributes String may not be included, if there
were no attributes returned as the result of the query. Attributes
are included in the reply depending on the "select clause" of the
query. A NULL "SELECT CLAUSE" will not include any attribute
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information. A LIST selection will specify which attributes to
return information about. A WILD selection will return information
about all attributes. (See Section 5.7.)
If attributes are returned the string takes the form (with
arbitrarily many attributes and values):
(ATTR1 = VAL), Keyword1,(ATTR2 = VAL1, VAL2), KEYWORD2
TTLs are not returned in Service Replies.
The format of a Service Reply is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service location header (function = SrvRply) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| number of replies returned | <Reply 1> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| <Reply 1> (cont.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
\ . \
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| <Reply N> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Service Reply
5.6 Directory Agent Discovery Request
Without a priori knowledge of a Directory Agent (DA), a User Agent,
Service Application or Service Agent uses a Service Request to
discover a DA. (See section 6.1 for a priori mechanisms for
knowledge of a DA.)
This request is generated by the User Agent or Service Agent or
Service Application in order to discover a Directory Agent. This is
a normal Service Request. The Service Request predicate used for
Directory Agent Discovery takes the form:
DIRECTORY-AGENT//SCOPE//
This query is to the Directory Agent multicast address. The Service
Type of a Directory Agent is 'DIRECTORY-AGENT', hence it is the
Service Type used in the request. No scope is included in the
request, since the query is GLOBAL in scope. No Naming Authority
is included, so 'IANA' is assumed. We want to reach all the
available directory agents. The query selects "SCOPE", so SCOPE
attribute information will be returned, if there is any. The where
clause is empty in the query, so all DAs will match the request.
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The replies may arrive from different sources. They will be similar
to:
URL returned: SERVICE:DIRECTORY-AGENT://slp-resolver.catch22.com
Attrs returned: (SCOPE=Accounting)
URL returned: SERVICE:DIRECTORY-AGENT://204.182.15.66
Attrs returned: (SCOPE=Janitorial Services)
If the goal is merely to discover any Directory Agent, the first
reply will do. If the goal, however, is to discover all reachable
DAs, the request must be retransmitted after an interval (the
recommended time is 3 seconds.) This retransmitted request will
include a list of DAs which have already responded. This list
takes the same form as the list of DAs above: a series of
SERVICE:DIRECTORY-AGENT://ADDRESS SPECIFICATION/
strings. Directory Agents which receive Directory Agent requests
will only respond if they are not on this list. After there are no
new replies, all DAs are presumed to have been discovered.
5.7 Service Type Request
The User Agent may use the Service Type Request to find all the types
of services that are available on a network.
The format of the Service Type Request is special in that it
specifies no Service Type for the service type. Instead a '*' is
used to denote a wild card.' The request may be sent to any
Directory Agent. This will return all the Service Types that it
knows about.
The request may also be sent to the General Multicast Address, in
to find out all services available on the User Agent's internet
(which are advertised by Directory Agents and Service Agents.) A
client can issue more than one request to insure that all replies
have been received. In each subsequent request, a User Agent adds
the list of Service Types that it is aware of. When no new replies
arrive from a request, the User Agent can presume that it has
acquired a complete set of available Service Types.
*//SA Multicast Address//
* is the wild card Service Type. The Naming Authority is not
included so 'IANA' is assumed. There is no scope specified in this
example, as the scope is GLOBAL (ie. all Service Agents will
respond.) The SELECT CLAUSE requests that the SA Multicast Address
be returned. This will allow multicast queries in the future.
Finally, the WHERE CLAUSE is empty so the request will match all
services.
Replies are sent by Directory Agents (and Service Agents, in the
case where the request is multicast.) These replies take the form:
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URL returned: service:printer://
Attributes returned: (SA Multicast Address=224.0.3.10)
URL returned: service:http://
Attributes returned: (SA Multicast Address=224.0.3.24)
URL returned: service:nfs-server://
Attributes returned: (SA Multicast Address=224.0.3.115)
NOTE: These multicast addresses are examples only, the official
numbers have not yet been assigned at this time.
The service URL defines the type of service. The SA Multicast
Address attribute defines the multicast address which can be used to
send queries to Service Agents which advertise the Service Type.
Only the SA Multicast address is returned, since only that was
selected. All Service Types were returned since the where-clause
was NULL.
If the User Agent is already aware of certain Service Types, as in
the case where it has already received several replies, but wants to
be sure that all Service Types are discovered, another request is
multicast, with a selection specifying which Service Type information
it is NOT interested in, as:
*//SA Multicast Address/(& (SERVICE TYPE != PRINTER)
(SERVICE TYPE != HTTP)
(SERVICE TYPE != NNTP)
(SERVICE TYPE != NFS-SERVER))/
Only Directory Agents or Service Agents which have services other
than these four types will respond to the request. The Naming
Authority is implicitly 'IANA' as none was specified, so only
services registered under this Naming Authority will have their
information returned.
To request all services which exist under a different Naming
Authority such as, say, IBM, the following query would be used:
*.ibm//SA Multicast Address/()/
5.8 Attribute Request and Attribute Reply
Once a User Agent selects a single Service Type, it may issue a "get
attributes request" to find all the attributes associated with that
Service Type. Since different instances of a given service can, and
very likely will, have different values for the attributes defined
by the Service Type, the User Agent must form a union of all
attributes returned by all service Agents. The Attribute information
will be used to form Service Queries.
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It has the following form:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service location header (function = AttrRqst) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|number of previous responders |<Previous Responders Addr Spec>|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Previous Responders Addr Spec>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Service Type String> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute Request
If sent to a Directory Agent, the number of previous responders is
zero and there are no Previous Responder Address Specification.
These fields are only used for repeated multicasting, exactly as
for the Service Request.
The replies take the form:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service location header (function = AttrRply) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Attribute List> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The attribute list has the same form as the attribute list at the
end of a reply.
For example, an Attribute Request for "printer" might elicit the
following reply (UNRESTRICTED_ACCESS is a keyword):
(PAPER COLOR=WHITE,BLUE),
(PAPER SIZE=LEGAL,LETTER,ENVELOPE,TRACTOR FEED),
UNRESTRICTED_ACCESS,
(PAGES PER MINUTE=1,3,12),
(LOCATION=12th floor, outside deb's cube),
(PROTOCOL=LPR, PCNFS),
(QUEUES=LEGAL,LETTER,ENVELOPE,LETTER HEAD)
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5.9 Service Discovery Request
Having obtained the entire list of attributes used to describe a
particular kind of service from a Get Attributes Request, (or using
a priori knowledge of a service's attributes,) the User Agent can
build a predicate that describes the service needs of the user.
This query is sent directly to a Directory Agent. Following the
example of the last section, suppose a printer is needed on the 12th
floor which has UNRESTRICTED_ACCESS and prints 12 pages per minute.
The response shown above from the Get Attribute Request indicates
that there is a printer on the 12th floor and that there is one that
prints 12 pages per minute, and one that is UNRESTRICTED. To check
whether they are one and the same printer, issue the following
request:
printer//PROTOCOL/(& (PAGES PER MINUTE==12)
UNRESTRICTED_ACCESS
(LOCATION==12th floor))/
Suppose there is no such printer. The Directory Agent responds with
a Service Reply with 0 in the number of responses and no reply
values.
The User Agent then tries a less restrictive query to find a
printer, using the 12th floor as "where" criteria, but selecting the
PAGES PER MINUTE attribute, to find out how slow it will be and
whether it has UNRESTRICTED_ACCESS:
printer//PROTOCOL,PAGES PER MINUTE,UNRESTRICTED_ACCESS/
(LOCATION==12th floor)/
In this case, there is now only one reply:
Returned URL: service:printer://igore.wco.ftp.com:515
Returned Attrs: ((PAGES PER MINUTE = 3),(PROTOCOL=LPR,PCNFS))
The Address Specification for the printer is: igore.wco.ftp.com:515.
This is the location of the printer. Files would be printed by
spooling to that port on that host. Note that the keyword was not
returned. This is the case because this particular printer did not
have this keyword (it does *not* have UNRESTRICTED_ACCESS.)
In the absence of a Directory Agent, the request above could be
multicast. In this case it would be sent to the printer Multicast
Address and not to the Directory Agent address above. Service
Agents that can satisfy the predicate will reply. Service Agents
which cannot satisfy the reply do not send any reply at all. The
only way a User Agent can be sure there are no services which match
the query is by retrying the request (say 3 times, 3 seconds apart).
If no response comes, the User Agent gives up and assumes there are
no such printers.
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Another form of query is a simpler 'join' query. Its syntax has
no parentheses or logical operators. Each term is conjoined (AND-ed
together.) Rewriting the initial query provides an example:
printer//PROTOCOL/PAGES PER MINUTE==12,
UNRESTRICTED_ACCESS,
LOCATION==12th floor/
One final note: The select field of the query is used to control
how much information is returned by the Directory Agent or Service
Agent. As described in full in Section 11.1.2, there are 3 ]
different selections possible. A NULL selection will return no
attribute information, merely a Service Type, the Address
Specification. A LIST selection specifies which attributes should be
returned. Finally, a WILD selection can be sent, which will return
all attributes/values. This WILD selection may produce a large
reply, so a TCP connection may need to be established. Refer to
Section 7.0 for details.
6.0 Directory Agents
6.1 Introduction
A Directory Agent acts on behalf of many Service Agents and Service
Applications. It acquires information from them and acts as a
single point of contact to supply that information to User Agents.
The queries that a User Agent multicasts to Service Agents (in an
environment without a Directory Agent) are the same queries that the
User Agent unicasts to a Directory Agent. A User Agent may cache
information about the presence of alternate Directory Agents to use
in case a selected Directory Agent fails.
When scaling service location systems to the size of a campus, a
central repository is added to limit the amount of general queries
in the network infrastructure. A site may also grow to such a size
that it is not feasible to maintain only one central repository of
service information. In this case more Directory Agents are needed.
Multiple Directory Agents are supported within the framework of this
protocol.
Each Service Agent or application may register with each DA and
hosts may choose a DA to use.
Directory Agents, in the future, may use mechanisms outside of this
protocol to coordinate the maintenance of a distributed database of
service location information, and thus scale to enterprise networks
or larger administrative domains.
6.2 Directory Agent Discovery
A User or Service Agent or Service Application may be statically
configured to use a particular DA. This is discouraged unless the
application resides on a network where any form of multicast or
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broadcast is impossible.
Alternatively, a host which uses DHCP may use it to obtain a
Directory Agent's address. A DHCP option will be assigned for
this purpose. It has not yet been, at the time this document was
written.
The third way to discover DAs is dynamically. This occurs actively
by sending out a Directory Agent Discovery request.
Lastly, the agent may be informed passively as follows:
When a Directory Agent first comes on-line it sends an unsolicited
Service Reply to the general service location multicast address.
If a DA supports a particular scope or set of scopes these are
placed in the reply. The class for this attribute is 'SCOPE'.
Every 6 hours a Directory Agent will send an unsolicited Service
Reply again. This will ensure that eventually it will be discovered
by all applications which are concerned.
When a Directory Agent first comes up it begins with 0 as its XID,
and increments this by one each time it sends an unsolicited reply.
When the counter wraps, it should go from 0xFFFF to 0x0100, not 0.
If the Directory Agent has stored all of the service information
in a nonvolitile store, it should initially set the XID to 256, as
it is not coming up 'stateless.'
All Service Agents and Service Applications which receive the
unsolicited reply should examine its XID. If the Directory Agent
has never before been heard from or if the XID is less than it was
previously and less than 256, the Service Agent or Service
Application should register all service information with the
Directory Agent, after waiting a random interval of between 1 and
3 seconds.
An example of what such an unsolicited reply would look like is:
URL: service:directory-agent://slp-resolver.catch22.com
Attributes: (SCOPE=ADMIN)
This directory agent can be reached at the Address Specification
specified, and supports the SCOPE called 'ADMIN'.
When a Service Agent or Application, or User Agent first comes
on-line it may issues a Directory Agent Discovery Request, as
defined in 5.6 above.
A Service Agent or Application registers information with ALL newly
discovered Directory Agents when either of the above two events take
place. When scopes are being used on a campus, a Service Agent or
Application may choose a set of scopes to be advertised in and need
only register with Directory Agents that support the scopes in which
they wish to be registered. Services may be registered with DAs
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which have no scope.
Note that while it is very highly recommended that all services are
registered with all unscoped DAs and with all DAs which have a
certain (set of) SCOPE values, it is not strictly required. If a
service isn't registered this way, the availability of the service
advertisement will be limited and possibly inconsistent between DAs.
This situation may be acceptable if UAs are preconfigured
(statically or by DHCP) to only use one particular DA in all
situations. This approach leaves open the chance of failure if the
preconfigured DA is not available.
Once a User Agent becomes aware of a Directory Agent it will unicast
its queries there. In the event that more than one Directory Agent
is detected, it will select one to communicate with. When scopes
are supported, the User Agent will direct its queries to different
Directory Agents depending on which scopes are appropriate domains
for the query to be answered in.
The protocol will cause all DAs (of the same scope) to eventually
obtain consistent information. Thus one DA should be as good as
any other for obtaining service information. There may be temporary
inconsistencies between DAs.
6.3 Service Registration
After a Service Agent has found a Directory Agent, it begins to
register its advertised services one at a time. A Service Agent
must wait for some random interval between 0 and 3 seconds between
each registration. Registration is done using the Service
Registration packet specifying all attributes for a service. A
Directory Agent must acknowledge each service registration request.
The format of a Service Registration is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service location header (function = SrvReg) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of URL String | <URL String> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <URL String> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of Attr List String | <Attribute String> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Attribute List>, Continued. \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Service Registration
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Service registration may use a connectionless protocol (e.g. UDP),
or a connection oriented protocol (e.g. TCP). The registration
operation may contain more information than can be sent in one
datagram. In this case the Service Agent or Application must use
a connection oriented protocol to register itself with the DA.
When a Service Agent or Application registers the same attribute
class more than once for a service instance, the Directory Agent
overwrites the all the values associated with that attribute class.
Separate registrations must be made for each language that the
service is to be advertised in.
Service Registration information is sent in exactly the same form as
a Service Reply:
URL (at least): SERVICE:SERVICE-TYPE://ADDRESS SPECIFICATION
Attributes (if any): (ATTR1=VALUE),KEYWORD,(ATTR2 = VAL1, VAL2)
An example of a service registration is as follows:
URL: service:printer://igore.wco.ftp.com:515
Attributes: (SCOPE=DEVELOPMENT),
(PAPER COLOR=WHITE),
(PAPER SIZE=LETTER),
UNRESTRICTED_ACCESS,
(LANGUAGE=POSTSCRIPT, HPGCL),
(LOCATION=12 Floor),
(PROTOCOL=LPR,PCNFS)
The same registration could be done again, in German (note that
'printer' the SERVICE TYPE and SCOPE are IANA terms and will remain
in the language they were originally registered with IANA, i.e.
English):
URL: service:printer://igore.wco.ftp.com:515
Attributes: (SCOPE=ENTWICKLUNG),
(PAPIERFARBE=WEISS),
(PAPIERFORMAT=BRIEF),
UNBEGRENTZTER_ZUGANG,
(DRUECKERSPRACHE=POSTSCRIPT,HPGCL),
(STANDORT=11 Etage),
(PROTOKOLL=LPR,PCNFS)
Registrations must contain an Attribute of SCOPE unless they are
unscoped and then they must be registered with all directory agents.
In the example above, the SCOPE is set to DEVELOPMENT (in English)
and ENTWICKLUNG (in German.) Recall that all strings in a packet
must be in one language, which is specified in the header.
The Directory Agent may return a server error in the acknowledgment.
This error is carried in the Error Codes field of the service
location packet header. A Directory Agent may decline to register a
service if it is specified with an unsupported SCOPE. In this case
a SCOPE_NOT_SUPPORTED error is returned in the SrvAck.
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An unscoped service registration will match all requests. A request
which specifies a certain scope will therefore return services which
have that scope and services which are unscoped. It is strongly
suggested that one should use scopes in all registrations or none.
See Section 6.5 and Sections 6.6 for details.
A non-error acknowledgment must have the error code set to zero.
Once a DA acknowledges a service registration it makes the
information available to clients.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service location header (function = SrvAck) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Service Acknowledgment
In order to offer continuously advertised services, Service Agents
and Applications should start the reregistration process before the
TTL they used for registration expires.
6.4 Service Unregister
When a service is no longer available for use, the Service Agent or
Application must unregister itself from Directory Agents that it has
been registered with. A service uses the following PDU to
unregister itself.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service location header (function = SrvUnreg) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <URL String of Service to Unregister> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Service Unregister
The Service Agent should retry this operation if there is no
response from the Directory Agent. The Directory Agent acknowledges
this operation with a service acknowledgment. Once the Service
Agent receives this acknowledgment, it can assume that the service
is no longer advertised by the Directory Agent.
The Service Unregister Information sent to the Directory Agent has
the following form:
SERVICE:SERVICE-TYPE:// ADDRESS SPECIFICATION
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This will unregister the service from the Directory Agent in every
language it was registered in. To unregister the printer above use:
service:printer://igore.wco.ftp.com:515
6.5 SCOPE Discovery and Use
The scope mechanism in the service location protocol is an important
feature to enhance its scalability. The primary use of scopes is to
provide the capability to organize a campus along administrative
lines. A set of services can be assigned to a given department of
an organization, to a certain building or geographical area or for a
certain purpose. The users of the system can be presented with
these organizational elements as a top level selection, before
services within this domain are sought.
A campus that has grown beyond a size that can be reasonably
serviced by a few DAs can use the SCOPE mechanism. DAs have the
attribute class "SCOPE". The values for this attribute are a list
of strings that represent the administrative areas for which this
Directory Agent is an authority. The semantics and language of the
strings used to describe the SCOPE are entirely the choice of the
administrative entity of the particular domain in which these SCOPEs
exist. The values of SCOPE should be configurable, so the system
administrator can set its value. The SCOPE "LOCAL" is reserved and
must not be used, use of this reserved value may be defined in a
future protocol document.
Services with the attribute SCOPE should only be registered with
DAs which support the same scope or DAs which have no SCOPE.
Directory Agents advertise the list of all scopes that are
available. A Service Agent or Service Application may then choose
at least one scope in which to be registered, and should register
with all Directory Agents in that scope, as well as all DAs which
have no scope. Failure to be comprehensive in registration
according to this rule will mean that the service advertisement may
not be discoverable by all User Agents.
A Directory Agent which has a SCOPE will send replies to Directory
Agent Discovery requests with the scope information included. Note
that Directory Agent Requests should always select that SCOPE
information be returned. Note that the directory-agent Service Type
is registered with the IANA naming authority (which is automatically
selected by leaving the Naming Authority field empty.)
The query:
directory-agent//SCOPE//
Could receive the following reply:
Returned URL: service:directory-agent://diragent.void.com
Returned Attribute: (SCOPE=ADMINISTRATION)
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The same Directory Agent if it had no SCOPE value would reply:
Returned URL: service:directory-agent://diragent.void.com
Returned Attributes:
If a Directory Agent supported more than one scope it would reply as:
Returned URL: service:directory-agent://123.12.34.56
Returned Attributes: (SCOPE=ADMIN,DEV,SALES)
Normally all Directory Agents respond to a Directory Agent Request.
If only Directory Agents of a particular set of scopes are desired,
issue a query like the following:
directory-agent//SCOPE/(SCOPE=ADMIN,SALES)/
Here the SCOPE field of the request is left blank, but the WHERE
clause of the request is filled in with a list of the scopes which
can be used to satisfy the request. Normally a single SCOPE would
be filled in for a query, in the SCOPE field, but in the special
case of a DA query this is not done.
A DA which has no scope will reply to any Directory Agent Discovery
Request.
Being a member of a scope means that an agent may use a specific set
of Directory Agents that support its scope. User Agents send all
requests to DAs which support the indicated scope. Services are
registered with the DA(s) in their scope. For a UA to find a
service that is registered in a particular scope they must send
requests to a DA which supports the indicated scope. There is no
limitation on scope membership built into the protocol; that is to
say, a User Agent or Service Agent or Application may be a member of
more than one scope. Membership is open to all, unless some
external authorization mechanism is added to limit access.
6.6 Service Location Scaling and Operating Modes
In a very small network, with few nodes, no DA is required. A User
Agent can detect services by multicasting requests. Service Agents
will then reply to them. This does not scale to environments with
many hosts. Further, Service Agents not Service Applications must
be used to make service information available.
In a larger but still administratively simple network, a single DA
may suffice. In this network, the DA will not have any SCOPE. DAs
that are discovered will return no list of SCOPES. Service Agents
and Service Applications should register with this DA even if they
are configured to specifically register with DAs which have a
specific scope or set of scopes. User Agents will query DAs without
scopes, even if they are configured to use DAs with a certain scope.
This is because when a DA with no SCOPE is discovered, it will have
all the available service information and no scoped DAs will exist.
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In a large campus or organization several DAs will be used in order
to divide the load of maintaining service information and to
organize which services should be used by which community. In this
case ALL DAs SHOULD HAVE A SCOPE. All Service Registrations that
have a scope should be registered with all DAs of that scope which
have been or are subsequently discovered. Unscoped services should
be registered with all DAs as they are implicitly GLOBAL in scope.
User Agents make requests of DAs which whose SCOPE they are
configured to use.
In each case where 'should' is used above, one should keep in mind
that if the rule is not followed the availability of the service
information may be limited or inconsistent across the service
location system.
There are thus 3 distinct operating modes. The first requires no
administrative intervention. The second requires only that a DA be
run. The last requires that all DAs be configured to have SCOPE and
that a coherent strategy of assigning SCOPES to services be
followed. Users must be instructed which SCOPES are appropriate for
them to use. This administrative cost will allow users and
applications to dynamically discover services without assistance.
7.0 Service Location Connections
When a Service Location Request results in a reply from a Service or
Directory Agent that will overflow a datagram, the User Agent can
open a connection to the Agent and reissue the request over the
connection.
The reply will be returned with the overflow bit set (see section
5.4.6). The reply will contain data, as much as will fit into a
single packet. If no MTU information is available for the route,
assume that a maximum packet size is 1400.
When a request results in overflowed data that cannot be correctly
parsed (say, because of duplicate or dropped IP packets), a User
Agent that wishes to reliably obtain the overflowed data must
establish a connection with the Directory Agent or Service Agent
with the data. The request is simply sent again (with a new XID,
however.) The reply is returned over the connection stream.
A Service registration which exceeds one packet in length should be
made by establishing a connection with a Directory Agent and sending
the registration over the connection stream.
Directory Agents and Service Agents must respond to connection
requests and Services whose registration can exceed a packet in
length must be able to connect and send. User Agents should be able
to make requests over a connection. If they fail to implement this,
they must be able to interpret partial replies and/or reissue
requests with more selective criteria to reduce the size of the
replies.
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A connection initiated by an Agent may be used for a single
transaction. It may also be used for multiple transactions. Since
there are length fields in the packet headers, the Agents may send
multiple requests along a connection and read the return stream for
acknowledgments and replies. The Agent is responsible for closing
the TCP connection. The DA should wait at least 30 seconds before
closing an idle connection.
8.0 Security Considerations
There are no provisions in this protocol to insure data integrity,
data authority or data confidentiality. Mechanisms in the
underlying network layer protocol or at the service access point
may be used to provide these functions. An Agent may choose to
ignore a transaction based on security information supplied by
other (underlying) services. As in the absense of Service Location,
end-to-end authentication should be used between clients and
services.
9.0 Multicast vs. Broadcast
The service location protocol was designed for use in networks
where multicast at the network layer is supported; in some instances
multicast may not be supported. To support this protocol in
networks where multicast is not supported the following
modifications are made to support the protocol in an environment
where network layer broadcast is supported.
9.1 Single Subnet
If a network is not connected to any other networks simple network
layer broadcasts will work in place of multicast.
9.2 Multiple Subnets
The Directory Agent provides a central clearing house of information
for User Agents. If the network is designed so that a Directory
Agent address is statically configured with each User Agent, the
Directory Agent will act as a bridge for information that resides on
different subnets. The Directory Agent address can be dynamically
configured with Agents using DHCP or staticly configured, but Agents
will not be able to discover DAs on non-bridged subnets.
As dynamic discovery is not feasible in a broadcast environment and
manual configuration is difficult, multiple DAs in a broadcast
environment may be difficult to deploy.
9.3 Service Multicast Address
Each service MAY have a unique multicast address to which it belongs
to. This multicast address may be obtained from IANA. This
mechanism is used so that the number of datagrams any one service
receives is minimized. The Service Location General Multicast
Address may be used to query for any service, though one should use
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the service-specific multicast address if it exists.
When undirected queries are made concerning this type of service,
the query should be sent to the matching multicast address. If the
subnet does not support multicast then the query should be broadcast
to the Service Location port. If the underlying hardware will not
support the number of need multicast addresses all services can use
the general service location multicast address.
10.0 Service Location in the Internet
A subsequent protocol document will describe mechanism for
supporting a service discovery protocol in a global Internet.
11.0 Protocol Formats
11.1 Fields Used in Service Location Packets
The following section supplies formal definitions for all protocol
elements introduced in the sections above.
Protocol Element Used in
----------------------------------- -------------
11.1.1 <Previous Responders' Addr Spec> SrvRqst
11.1.2 <Service Request Predicate> SrvRqst
11.1.3 <Reply> SrvRply
11.1.4 <Service Registration Information> SrvReg
11.1.5 <Service Unregister Information> SrvUnReg
11.1.6 <Attribute List> AttrRply
11.1.7 <Service Type String> AttrRqst
11.1.1 Previous Responders' Address Specification
The previous responders' Address Specification is specified as
<Previous Responders' Address Specification>
::= <Address Specification>, |
<Address Specification>,
<Previous Responders' Address Specification>
ie. a list separated and terminated by commas with no intervening
white space. The Address Specification is the address of the
Directory Agent or Service Agent which supplied the previous
response. The format for Address Specifications in Service Location
is defined in 11.2.
Example:
some.corp.com,128.127.203.63,
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11.1.2 Service Request Predicate
The following grammar expresses the form of a Service Request
Predicate:
<predicate> ::= <srvtype>[.<na>]/<scope>/<select>/<where>/
<srvtype> ::= string representing type of service. Only
'a' to 'z', '+' and '-' are allowed.
<na> ::= string representing the Naming Authority.
Only characters from 'a' to 'z', '+' and '-'
are allowed. If this field is omitted 'IANA'
is assumed.
<scope> ::= string representing the directory agent scope.
'/' and ':' are not allowed in this string.
The scopes 'LOCAL' and 'REMOTE' are reserved.
<select> ::= <select-list> |
<select-all> |
<select-none>
<select-list>::= <select-item> |
<select-item>, <select-list>
<select-item>::= <keyword> | <attr-tag> | <partial-attr-tag>*
<attr-tag> ::= class name of an attribute of a given Service Type.
This tag cannot include the following
characters: '(', ')', ',', '=', '!', '>',
'<', '/', '*'
<keyword> ::= a class name of an attribute which will have
no values. This string has the same limits
as the <attr-tag>. In addition white space
internal to the keyword is illegal.
<partial-tag>::= the partial class name of an attribute
followed by an '*' matches all class names
which begin with the characters preceding
the '*'
<select-all> ::= *
<select-none>::=
That is NOTHING or white space.
<where> ::= <where-any> |
<where-list> |
<query-join>
<where-any> ::=
That is NOTHING or white space.
<where-list> ::= (& <query-item> <query-list>) |
(| <query-item> <query-list>) |
<query-item>
<query-list> ::= <where-list> |
<query-item> |
<query-item> <query-list>
<query-item> ::= (<attr-tag> <comp-op> <attr-val>) |
<keyword>
<query-join> ::= <join-item> |
<join-item>, <query-join>
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<join-item> ::= <attr-tag> |
<attr-tag> <comp-op> <attr-val>
<comp-op> ::= != | == | < | <= | > | >=
<attr-val> ::= any string (see Section 10.3 for the ways
in which attr-vals are interpreted.)
Value strings may not contain '/', ','
'=', '<', '>'. '(' and ')' can only be used
for the purpose of encoding a binary values.
Binary encodings (See Section 10.3) may
include the above reserved characters.
Note on string matches: All strings are case insensitive, with respect
to string matching on queries. All preceding or trailing blanks should
not be considered for a match, but blanks internal to a string are
relevant. For example " Some String " matches "SOME STRING" but not
"some string".
A predicate has a simple structure, which depends on the parentheses,
commas and slashes to delimit the elements. Examples of proper usage
have been given throughout the document. The terms used above are
described below:
predicate:
Placed in a Service Request, this is interpreted by a Service
Agent or Directory Agent to determine what information to
return.
scope:
If this is absent in a Service Request, the request will match
any service regardless of scope. If it is present, only
services registered under that scope will match the request.
select-clause:
This determines what information to return. There are 3 types
of select-clause: NULL, ANY and LIST.
NULL: The reply returns no attribute information for the
PARTICULAR services which satisfy the where-clause.
ANY: The reply returns all attribute information, as above.
LIST: The reply returns the attribute information for the
attributes whose class names are listed, as above.
Recall that an attribute has a class name and a set
of values. The list contains a set of class names.
Elements in the list can be partial names, as 'INT*'
will match 'INTERFACE 1' and 'INTERNAL'.
where-clause:
This determines which services the request matches. An empty
where-clause will match all services. The request will be
limited to services which have the Service Type which was defined
prior to the predicate, so the where-clause is not the sole
factor in picking out which services match the request.
where-list:
The where-list is a logical expression. It can be a single
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expression, a disjunction or a conjunction. A single expression
must apply for the where-clause to match. A disjunction matches
if any expression in the OR list matches. A conjunction matches
only if all elements in the AND list match.
Note that there is no logical negation operator: This is
because there is no notion of returning "everything except" what
matches a given criteria.
A where-list can be nested and complex. For example:
(& (| <query-item> <query-item> <query-item>)
<query-item>
(& <query-item> <query-item> <query-item> <query-item>)
)
Notice that white space, tabs or carriage returns can be added
anywhere outside query-items. Each list has 2 or more items in
it, and lists can be nested. Services which fulfill the entire
logical expression match the where-clause.
(| <query-item>) and (& <query-item>) are degenerate expressions
but they should be tolerated. They are equivalent to
<query-item>.
query-item:
A query item has the form:
(<attr-tag> <comp-op> <attr-val>)
or
<keyword>
Examples of this would be:
(SOME ATTRIBUTE == SOME VALUE)
RESERVED
(QUEUE LENGTH <= 234)
query-join:
The query-join is a comma delimited list of conditions which the
service must satisfy in order to match the query. The items are
considered to be logically conjoined. Thus the query-join:
attr1=value1, keyword1, keyword2, attr2>=34
is equivalent to the where-list:
(& (attr1=value1) keyword1 keyword2 (attr2>=34))
The query-join cannot be mixed with a where-list. It is provided
as a convenient mechanism to provide a statement of necessary
conditions without building a logical expression.
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11.1.3 Reply
Service Replies have two fields, a URL String and an attribute list.
URL Strings are as per RFC 1738. They should contain at least:
SERVICE:SERVICE-TYPE:// ADDRESS SPECIFICATION
SERVICE is the URL scheme for service location, to return Replies.
SERVICE-TYPE is a string. Service Types may be standardized by
developing a specification for the "service type"-specific part
and registering it with the IANA. See section 5.3.4.
ADDRESS SPECIFICATION is the service access point of the service.
It is the network address or domain name where the service
can be accessed.
The <attr-list> is returned if the select-clause of the query is
not NULL.
<attr-list> ::= <attribute> | <attribute>, <attr-list>
<attribute> ::= (<attr-tag>=<attr-val-list>) | <keyword>
<attr-val-list> ::= <attr-val> | <attr-val>, <attr-val-list>
An attribute with only an attr-tag and no values is a keyword.
A comma cannot appear in an attr-val, as the comma is used as the
multiple value delimiter. Examples of an attr-list are:
(SCOPE=ADMINISTRATION)
(COLOR=RED, WHITE, BLUE)
(DELAY=10 Mins),BUSY,(MOST RECENT BUILD=10-5-95),(PRIORITY=L,M,H)
The third example has three attributes in the list. Color can take
on the values red, white and blue. There are several other examples
of replies throughout the document.
11.1.4 Service Registration Information
The Service Registration Information has the same form as a Reply
in the section above. The attribute list must be complete.
11.1.5 Service Unregister Information
The Service Unregister Information takes the form:
SERVICE:SERVICE-TYPE:// ADDRESS SPECIFICATION
SERVICE-TYPE and ADDRESS SPECIFICATION are described above.
11.1.6 Attribute List
The Attribute List is defined in 11.1.3 as <attr-list>.
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11.1.7 Service Type String
The Service Type is a string describing the type of service.
These strings may only be comprised of 'a' through 'z', '+' and '-'.
Upper case is considered equivalent to lower case in Service Type
names.
If the Service Type name is followed by a '.' and a string (which has
the same limitations) the 'suffix' is considered to be the Naming
Authority of the service. If the Naming Authority is omitted, IANA
is assumed to be the Naming Authority.
Service Types developed for in-house or experimental use may have
any name and attribute semantics provided that they do not conflict
with the standardized Service Types. The Service Type's Service
Discovery Multicast Address used should taken from the range of
experimental multicast addresses reserved by the IANA.
11.2 ADDRESS SPECIFICATIONs in Service Location
The address specification as described in RFC 1738 is:
//<user>:<password>@<host>:<port>
It is preferable to use a fully qualified domain name wherever
possible as renumbering of host addresses will make ip addresses
invalid over time. When no Domain Name Server is available SAs
and DAs must use dotted decimal conventions for IP addresses.
Generally just the host domain name (or address) is sufficient to
return. When there is a non-standard port for the protocol, that
should be returned as well. Some applications may make use of
the <user>:<password>@ syntax, but its use is discouraged in this
context as information registered in Service Location is so easily
accessible.
11.3 Attribute Value encoding rules
Attribute values, and attribute tags are CASE INSENSITIVE for
purposes of lexical comparison.
Attribute values can have be any string with the exception of
'(', ')', '=', '>', '<', '/' and ',' (the comma) except in the case
described below where opaque values are encoded.
While an attribute can take any value, there are three types of
values which differentiate themselves from general strings:
Booleans, Integers and Opaque values.
- Boolean values are either "TRUE" or "FALSE". This is the case
regardless of the language (i.e. in French or Telugu, Boolean TRUE
is "TRUE", as well as in English.) Boolean attributes can take
only one value.
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- Integer values are expressed as a sequence of numbers. The range
of allowable values, for this 32 bit quantity, is "-2147483648" to
"2147483647".
- Opaque values (i.e. binary values) are expressed in radix-64
notation. The syntax is:
<opaque-val> ::= (<len>:<radix-64-data>)
<len> ::= integer length of the original binary data
<radix-64-data> ::= An encoding of the binary data into a new
format.
Radix-64 encodes every 3 bytes of binary data into 4 bytes of
ASCII data which is in the range of characters which are fully
printable and transferable by mail. For a formal definition of
the Radix-64 format see RFC 1521, MIME Part One, Section 5.2
Base64 Content Transfer Encoding, page 21.
Opaque values can pass things such as bitmaps for building a
service browsing graphical interface or application specific data.
12.0 Implementation Requirements
A User Agent MAY:
- Provide a way for the application to configure the default DA, so
that it can be used without needing to find it each initially.
- Be able to request the address of a DA from DHCP, if configured to
do so.
A User Agent SHOULD:
- Listen on the Service Location General Multicast address for
unsolicited Directory Agent Replies. This will increase the set of
Directory Agents available to it for making replies. See Section
6.2.
If this is not done, new DAs will not be passively detected. A UA
which does not have a configured DA and has not yet discovered one
and is not listening for unsolicited replies will remain ignorant
of DAs. It may then do a DA discovery before each query performed
or it may simply use multicasted queries to Service Agents.
A User Agent MUST:
- Be able to unicast requests and receive replies from a DA.
Transactions should be made reliable by using retransmission of
the request if the reply does not arrive within a timeout interval.
- Be able to detect DAs using a Directory Agent Discovery request
issued when the UA starts up.
- Be able to send requests to a multicast address. If the
multicast address is not known, the UA must be able to use a
Service Type query to obtain the multicast address for the Service
Type of the request.
- Be able to handle numerous replies after a multicast request. The
implementation may be configurable so it will either return the
first reply, all replies until a timeout or keep trying till the
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results converge.
A Service Application and Service Agent MAY be able to:
- Get the address of a local Directory Agent by way of DHCP.
A Service Application MUST be able to:
- Listen to the Service Location General Multicast address for
unsolicited Directory Agent Replies. If one is detected, and the
DA has the right scope, all services which are currently being
advertised SHOULD be registered with the DA. See Section 6.2.
- Unicast registrations and unregistrations to a DA. Transactions
should be made reliable by using retransmission of the request
if the reply does not arrive within a timeout interval.
- Be able to detect DAs using a Directory Agent Discovery request
issued when the Service Application starts up.
A Service Agent MUST be able to:
- Listen to the Service Location General Multicast address for
unsolicited Directory Agent Replies. If one is detected, and the
DA has the right scope, all services which are currently being
advertised SHOULD be registered with the DA. See Section 6.2.
- Unicast registrations and unregistrations to a DA. Transactions
should be made reliable by using retransmission of the request if
the reply does not arrive within a timeout interval.
- Listen to the multicast address of the service it is advertising.
The incoming requests should be filtered: If the Address
Specification of the SA is in the Previous Responders Address
Specification list, the SA should not respond. Otherwise, a
response to the multicast query should be unicast to the UA
which sent the request.
- Listen for broadcast requests and TCP connection requests, to
the Service Location port.
- Listen to the Service Location General Multicast address for
queries of any type. If the query can be replied to by the
Service Agent, the Service Agent must do so. It must check first
to make sure it is not on the list of 'previous responders.'
It will receive 'Service Type' requests this way.
- Be able to detect DAs using a Directory Agent Discovery request
issued when the SA starts up.
A Directory Agent MUST be able to:
- Send an unsolicited Directory Agent Discovery reply to the
Service Location General Multicast address on startup and repeat
it periodically. This reply has a unique XID for the life of the
DA; this XID changes on each reboot (see Section 6.2).
- Listen on the Directory Agent Multicast Address for Directory
agent discovery requests. Filter these requests if the Previous
Responder Address Specification list includes the DA's Address
Specification.
- Listen for broadcast requests to the Service Location port.
- Listen on the TCP and UDP Service Location Ports for unicast
requests, registrations and unregistrations and service them.
- Provide a way in which SCOPE information can be used to configure
the Directory Agent.
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- Age out the services which have been registered so that when the
service registration's TTL expires, the service advertisement is
withdrawn.
NOTE: Service Applications, Service Agents and User Agents use
ephemeral ports for transmitting information to the service location
port.
13.0 Configuration Parameters and Defaults
13.1 Multicast vs. Broadcast
All Service Location entities must use multicast by default.
The ability to use broadcast messages must be configurable.
Broadcast messages are to be used in environments where not all
Service Location entities have hardware or software which supports
multicast.
13.2 Multicast Radius
Multicast requests should be sent to all subnets in a site. The
default multicast radius for a site is 32. This value must be
configurable. The value for the site's multicast TTL may be
obtained from DHCP. The DHCP option has not yet been assigned.
13.3 Directory Agent Address
The Directory Agent address discovery mechanism must be
configurable. There are three possibilities for this configuration:
A default address, no default address and the use of DHCP to locate
a DA as described in section 6.2. The default value should be "no
default address." In this case the UA or SA must do a Directory
Agent Discovery query.
13.4 Directory Agent Scope Assignment
The scope or scopes of a DA must be configurable. The default value
for a DA is to have no scope if not otherwise configured.
14.0 Interesting Constants
IP Port number for unicast requests to Directory Agents:
UDP and TCP Port Number: 427
Multicast Addresses
General Multicast Address: 224.0.1.22
Directory Agent Multicast Address: 224.0.1.35
Further multicast address will be assigned for specific types of
service through the IANA.
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Error Codes
No Error 0
LANGUAGE_NOT_SUPPORTED 1
PROTOCOL_PARSE_ERROR 2
INVALID_REGISTRATION 3
SCOPE_NOT_SUPPORTED 4
15.0 Acknowledgments
This protocol owes some of the original ideas to other service
location protocols found in many other networking protocols. Leo
McLaughlin and Mike Ritter (Metricom) provided much input into early
version of this document. Thanks also to Steve Deering (Xerox) for
providing his insight into distributed multicast protocols. Harry
Harjono and Charlie Perkins supplied the basis for the URL based
wire protocol in their Resource Discovery Protocol. Their comments
have been very valuable. Thanks also to Peerlogic, Inc. for
supporting this work.
16.0 References
[1] Freier, A. O. "Network Binding Protocol" Xerox Corporation
Unpublished, June 1986.
[2] S. Gursharan, R. Andrews, A. Oppenheimer, Inside AppleTalk.
Addison-Wesley Publishing. 1990
[3] Deering, S., "Host Extensions for IP Multicasting", RFC 1112, NIC,
August 1989.
[4] Saltzer, J., "On the Naming and Binding of Network Destinations",
RFC 1498, M.I.T. Laboratory for Computer Science, August 1993.
[5] Accetta, M. "Resource Location Protocol", RFC 887, NIC, December
1983
[6] Legato Systems, "The Legato Resource Administration Platform",
Legato Systems, 1991.
[7] C. McManis and R. Rom, "The Zeus Name Service Architecture", Sun
Microsystems, 1990.
[8] S. Dyer, "The Hesiod Name Server", Winter Usenix Conference, pp.
183-187, Feb 1988.
[9] D. Oppen and Y. Dalal, "The Clearinghouse: A Decentralized Agent
for Locating Named Objects in a Distributed Environment," Tech. Rep.
OPD-78103, Xerox Office Products Division, 1981.
[10] B. Lampson, "Designing a Global Name Service", Proceedings of the
5th ACM Symposium on Principles of Distributed Computing, pp. 1-10,
1986.
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[11] D. Cheriton and T. Mann, "Uniform Access to Distributed Name
Interpretations in the V-system".
[12] P. Mockapetris. "Domain Names - Concepts and Facilities". RFC
1034, NIC, November 1987
[13] P. Mockapetris. "Domain Names - Implementation and Specification".
RFC 1035, NIC. November 1987
[14] S. Deering. "Router Discovery Protocol". RFC 1256, NIC 1991.
[15] ISO 639:1988 (E/F) "Code for the representation of names of
languages"; ISO, Geneve, 1988.
[16] T. Berners-Lee, L. Masinter and M. McCahill "Uniform Resource
Locators". RFC 1738, NIC 1994.
[17] N. Borenstein, N. Freed, "MIME (Multipurpose Internet Mail
Extensions) Part One: Mechanisms for Specifying and Describing the
Format of Internet Message Bodies". RFC 1521, NIC 1993.
[18] S. Alexander, R. Droms, "DHCP Options and BOOTP Vendor
Extensions". RFC 1533, NIC 1993.
[19] R. Droms, "Dynamic Host Configuration Protocol". RFC 1541,
NIC 1993.
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17.0 Author's Addresses
John Veizades
TGV, Inc.
370A Waller St.
San Francisco, CA 94117
Phone: +1 415 252 8203
Fax: +1 415 252 8248
Email: veizades@tgv.com
Scott Kaplan
346 Fair Oaks St.
San Francisco, CA 94110
Phone: +1 415 285 4526
Email: scott@catch22.com
Erik Guttman
Sun Microsystems
2550 Garcia Avenue, MS PAL01-550
Mountain View, CA 94043-1100
Phone: +1 415 336 6697
Email: Erik.Guttman@eng.sun.com
Charles Perkins
IBM Corporation
P.O. Box 704
Yorktown Heights NY 10598
Phone: +1 914 784 7350
EMail: perk@watson.ibm.com
18.0 Document Expiration
This document expires September 11, 1996
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Appendix A - Technical contents of ISO 639:1988 (E/F)
"Code for the representation of names of languages".
Two-letter lower-case symbols are used.
The Registration Authority for ISO 639 is Infoterm,Osterreiches
Normungsinstitut (ON), Postfach 130, A-1021 Vienna, Austria.
aa Afar gn Guarani mr Marathi
ab Abkhazian gu Gujarati ms Malay
af Afrikaans mt Maltese
am Amharic ha Hausa my Burmese
ar Arabic hi Hindi
as Assamese hr Croatian na Nauru
ay Aymara hu Hungarian ne Nepali
az Azerbaijani hy Armenian nl Dutch
no Norwegian
ba Bashkir ia Interlingua
be Byelorussian ie Interlingue oc Occitan
bg Bulgarian ik Inupiak om (Afan) Oromo
bh Bihari in Indonesian or Oriya
bi Bislama is Icelandic
bn Bengali; Bangla it Italian pa Punjabi
bo Tibetan iw Hebrew pl Polish
br Breton ps Pashto, Pushto
ja Japanese pt Portuguese
ca Catalan ji Yiddish
co Corsican jw Javanese qu Quechua
cs Czech
cy Welsh ka Georgian rm Rhaeto-Romance
kk Kazakh rn Kirundi
da Danish kl Greenlandic ro Romanian
de German km Cambodian ru Russian
dz Bhutani kn Kannada rw Kinyarwanda
ko Korean
el Greek ks Kashmiri sa Sanskrit
en English ku Kurdish sd Sindhi
eo Esperanto ky Kirghiz sg Sangro
es Spanish sh Serbo-Croatian
et Estonian la Latin si Singhalese
eu Basque ln Lingala sk Slovak
lo Laothian sl Slovenian
fa Persian lt Lithuanian sm Samoan
fi Finnish lv Latvian, Lettish sn Shona
fj Fiji so Somali
fo Faeroese sq Albanian
fr French mg Malagasy sr Serbian
fy Frisian mi Maori ss Siswati
mk Macedonian st Sesotho
ga Irish ml Malayalam su Sundanese
gd Scots Gaelic mn Mongolian sv Swedish
gl Galician mo Moldavian sw Swahili
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ta Tamil
te Telugu
tg Tajik
th Thai
ti Tigrinya
tk Turkmen
tl Tagalog
tn Setswana
to Tonga
tr Turkish
ts Tsonga
tt Tatar
tw Twi
uk Ukrainian
ur Urdu
uz Uzbek
vi Vietnamese
vo Volapuk
wo Wolof
xh Xhosa
yo Yoruba
zh Chinese
zu Zulu
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