Network Working Group J. Veizades
Request for Comments: 2165 @Home Network
Category: Standards Track E. Guttman
C. Perkins
Sun Microsystems
S. Kaplan
June 1997
Service Location Protocol
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
The Service Location Protocol provides a scalable framework for the
discovery and selection of network services. Using this protocol,
computers using the Internet no longer need so mUCh static
configuration of network services for network based applications.
This is especially important as computers become more portable, and
users less tolerant or able to fulfill the demands of network system
administration.
Table of Contents
1. Introduction 3
2. Terminology 3
2.1. Notation Conventions . . . . . . . . . . . . . . . . . . 5
2.2. Service Information and Predicate Representation . . . . 5
2.3. Specification Language . . . . . . . . . . . . . . . . . 6
3. Protocol Overview 6
3.1. Protocol Transactions . . . . . . . . . . . . . . . . . . 7
3.2. Schemes . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.1. The "service:" URL scheme . . . . . . . . . . . . 9
3.3. Standard Attribute Definitions . . . . . . . . . . . . . 9
3.4. Naming Authority . . . . . . . . . . . . . . . . . . . . 10
3.5. Interpretation of Service Location Replies . . . . . . . 10
3.6. Use of TCP, UDP and Multicast in Service Location . . . . 10
3.6.1. Multicast vs. Broadcast . . . . . . . . . . . . 11
3.6.2. Service-Specific Multicast Address . . . . . . . 11
3.7. Service Location Scaling, and Multicast Operating Modes . 12
4. Service Location General Message Format 14
4.1. Use of Transaction IDs (XIDs) . . . . . . . . . . . . . . 15
4.2. URL Entries . . . . . . . . . . . . . . . . . . . . . . . 16
4.3. Authentication Blocks . . . . . . . . . . . . . . . . . . 17
4.4. URL Entry Lifetime . . . . . . . . . . . . . . . . . . . 19
5. Service Request Message Format 19
5.1. Service Request Usage . . . . . . . . . . . . . . . . . . 22
5.2. Directory Agent Discovery Request . . . . . . . . . . . . 23
5.3. EXPlanation of Terms of Predicate Grammar . . . . . . . . 24
5.4. Service Request Predicate Grammar . . . . . . . . . . . . 26
5.5. String Matching for Requests . . . . . . . . . . . . . . 27
6. Service Reply Message Format 28
7. Service Type Request Message Format 29
8. Service Type Reply Message Format 31
9. Service Registration Message Format 32
10. Service Acknowledgement Message Format 35
11. Service Deregister Message Format 37
12. Attribute Request Message Format 38
13. Attribute Reply Message Format 40
14. Directory Agent Advertisement Message Format 42
15. Directory Agents 43
15.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 43
15.2. Finding Directory Agents . . . . . . . . . . . . . . . . 43
16. Scope Discovery and Use 45
16.1. Protected Scopes . . . . . . . . . . . . . . . . . . . . 46
17. Language and Character Encoding Issues 47
17.1. Character Encoding and String Issues . . . . . . . . . . 48
17.1.1. Substitution of Character Escape Sequences . . . 49
17.2. Language-Independent Strings . . . . . . . . . . . . . . 49
18. Service Location Transactions 50
18.1. Service Location Connections . . . . . . . . . . . . . . 50
18.2. No Synchronous Assumption . . . . . . . . . . . . . . . . 51
18.3. Idempotency . . . . . . . . . . . . . . . . . . . . . . . 51
19. Security Considerations 51
20. String Formats used with Service Location Messages 52
20.1. Previous Responders" Address Specification . . . . . . . 53
20.2. Formal Definition of the "service:" Scheme . . . . . . . 53
20.2.1. Service Type String . . . . . . . . . . . . . . . 54
20.3. Attribute Information . . . . . . . . . . . . . . . . . . 54
20.4. Address Specification in Service Location . . . . . . . . 55
20.5. Attribute Value encoding rules . . . . . . . . . . . . . 55
21. Protocol Requirements 56
21.1. User Agent Requirements . . . . . . . . . . . . . . . . . 56
21.2. Service Agent Requirements . . . . . . . . . . . . . . . 58
21.3. Directory Agent Requirements . . . . . . . . . . . . . . 59
22. Configurable Parameters and Default Values 61
22.1. Service Agent: Use Predefined Directory Agent(s) . . . . 62
22.2. Time Out Intervals . . . . . . . . . . . . . . . . . . . 63
23. Non-configurable Parameters 63
24. Acknowledgments 64
A. Appendix: Technical contents of ISO 639:1988 (E/F): "Code for
the representation of names of languages" 65
B. SLP Certificates 66
C. Example of deploying SLP security using MD5 and RSA 68
D. Example of use of SLP Certificates by mobile nodes 68
E. Appendix: For Further Reading 69
1. Introduction
Traditionally, users find services by 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 names the service and 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 local area networks. It is not a global resolution
system for the entire Internet; rather it is intended to serve
enterprise networks with shared services. Applications are modeled
as clients that need to find servers attached to the enterprise
network at a possibly distant location. For cases where there are
many different clients and/or services available, the protocol is
adapted to make use of nearby Directory Agents that offer a
centralized repository for advertised services.
2. 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 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.
Service The service is a process or system providing a facility
to the network. 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, called a "service
scheme", including expected attributes, values and
protocol behavior.
Naming Authority
The agency or group which catalogues given Service Types
and Attributes. The default Naming Authority is IANA,
the Internet Assigned Numbers Authority.
KeyWord
A string describing a characteristic of a service.
Attribute
A (class, value-list) pair of strings describing a
characteristic of a service. The value string may be
interpreted as a boolean, integer or opaque value if it
takes specific forms (see section 20.5).
Predicate
A boolean expression of attributes, relations and
logical operators. The predicate is used to find
services which satisfy particular requirements. See
section 5.3.
Alphanumeric
A character within the range "a" to "z", "A" to "Z", or
Scope A collection of services that make up a logical group.
See sections 3.7 and 16.
Site Network
All the hosts accessible within the Agent"s multicast
radius, which defaults to a value appropriate for
reaching all hosts within a site (see section 22). If
the site does not support multicast, the agent"s site
network is restricted to a single subnet.
URL A Universal Resource Locator - see [6].
Address Specification
This is the network layer protocol dependent mechanism
for specifying an Agent. For Internet systems this is
part of a URL.
2.1. Notation Conventions
CAPS Strings which appear in all capital letters are protocol
literal. All string comparison is case insensitive,
however, (see section 5.5). Some strings are quoted in
this document to indicate they should be used literally.
Single characters inside apostrophes are included
literally.
<> Values set off in this manner are fully described in
section 20. In general, all definitions of items in
messages are described in section 20 or immediately
following their first use.

Message layouts with this notation indicate a variable
length field.
2.2. Service Information and Predicate Representation
Service information is represented in a text format. The goal is
that the format be human readable and transmissible via email. The
location of network services is encoded as a Universal Resource
Locator (URL) which is human readable. Only the datagram headers are
encoded in a form which is not human readable. Strings used in the
Service Location Protocol are NOT null-terminated.
Predicates are expressed in a simple boolean notation using keywords,
attributes, and logical connectives, as described in Section 5.4.
The logical connectives and subexpressions are presented in prefix-
order, so that the connective comes first and the expressions it
operates on follow afterwards.
2.3. Specification Language
In this document, several words are used to signify the requirements
of the specification [8]. These words are often capitalized.
MUST This word, or the adjective "required", means that
the definition is an absolute requirement of the
specification.
MUST NOT This phrase means that the definition is an absolute
prohibition of the specification.
SHOULD This word, or the adjective "recommended", means
that, in some circumstances, valid reasons may exist to
ignore this item, but the full implications must be
understood and carefully weighed before choosing a
different course. Unexpected results may result
otherwise.
MAY This word, or the adjective "optional", means that this
item is one of an allowed set of alternatives. An
implementation which does not include this option MUST
be prepared to interoperate with another implementation
which does include the option.
silently discard
The implementation discards the datagram without
further processing, and without indicating an error to
the sender. The implementation SHOULD provide the
capability of logging the error, including the contents
of the discarded datagram, and SHOULD record the event
in a statistics counter.
3. Protocol Overview
The basic operation in Service Location is that a client attempts to
discover the location of a Service. In smaller installations, each
service will be configured to respond individually to each client.
In larger installations, services will register their services with
one or more Directory Agents, and clients will contact the Directory
Agent to fulfill requests for Service Location information. Clients
may discover the whereabouts of a Directory Agent by
preconfiguration, DHCP [2, 11], or by issuing queries to the
Directory Agent Discovery multicast address.
3.1. Protocol Transactions
The diagram below illustrates the relationships described below:
+---------------+ we want this info: +-----------+
Application - - - - - - - - - - - -> Service
+---------------+ +-----------+
/
+-------------+

/ / /
+---------------+ +-----------+ +----------------+
User Agent <--------> Service Service
+---------------+ Agent Agent which
+-----------+ does not reply
to UA requests
/ +----------------+
+-------------+
+------------------> Directory <----------+
Agent
+-------------+ ___________
/ / Many other
+------------> SA"s
___________/
The following describes the operations a User Agent would employ to
find services on the site"s network. The User Agent needs no
configuration to begin network interaction. The User Agent can
acquire information to construct predicates which describe the
services that match the user"s needs. The User Agent may build on
the information received in earlier network requests to find the
Service Agents advertising service information.
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 site network, a second
mode of discovery may be used. The User Agent multicasts a request
to the service-specific multicast address, to which the service it
wishes to locate will respond. All the Service Agents which are
listening to this multicast address will respond, provided they can
satisfy the User Agent"s request. A similar mechanism is used for
Directory Agent discovery; see section 5.2. Service Agents which
have no information for the User Agent MUST NOT respond.
When a User Agent wishes to obtain an enumeration of ALL services
which satisfy the query, a retransmission/convergence algorithm is
used. 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. If there are more responders than this, the
scaling mechanisms described in section 3.7 should be used.
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.
The Service Agent SHOULD listen for multicast requests on the
service-specific multicast address, and MUST register with an
available Directory Agent. This Directory Agent will resolve
requests from User Agents which are unicasted using TCP or UDP. This
means that a Directory Agent must first be discovered, using DHCP,
the DA Discovery Multicast address, the multicast mechanism described
above, or manual configuration. See section 5.2.
A Service Agent which does not respond to multicast requests will not
be useful in the absence of Directory Agents. Some Service Agents
may not include this functionality, if an especially lightweight
implementation is required.
If the service is to become unavailable, it should be deregistered
with the Directory Agent. The Directory Agent responds with an
acknowledgment to either a registration or deregistration. Service
Registrations include a lifetime, and will eventually expire.
Service Registrations need to be refreshed by the Service Agent
before their Lifetime runs out. If need be, Service Agents can
advertise signed URLs to prove that they are authorized to provide
the service.
3.2. Schemes
The Service Location Protocol, designed as a way for clients to
access resources on the network, is a natural application for
Universal Resource Locators (URLs). It is intended that by re-using
URL specification and technology from the World Wide Web, clients and
servers will be more flexible and able to be written using already
existing code. Moreover, it is hoped that browsers will be written
to take advantage of the similarity in locator format, so that a
client can dynamically formulate requests for services that are
resolved differently depending upon the circumstances.
3.2.1. The "service:" URL scheme
The service URL scheme is used by Service Location. It is used to
specify a Service Location. Many Service Types will be named by
including a scheme name after the "service:" scheme name. Service
Types are used by SAs to register and deregister 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 20.2.
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 standardization process.
Well known Service Types are registered with the IANA and templates
are available as RFCs. Private Service Types may also be supported.
3.3. Standard Attribute Definitions
Service Types used with the Service Location Protocol must describe
the following:
Service Type string of the service
Attributes and Keywords
Attribute Descriptions and interpretations
Service Types not registered with IANA will use their own Naming
Authority string. The registration process for new Service Types is
defined in [13].
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
MUST 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.
A service of a given Service Type should accept the networking
protocol which is implied in its definition. If a Service Type can
accept multiple protocols, configuration information SHOULD be
included in the Service Type attribute information. This
configuration information will enable an application to use the
results of a Service Request and Attribute Request to directly
connect to a service.
See section 20.2.1 for the format of a Service Type String as used in
the Service Location Protocol.
3.4. Naming Authority
The Naming Authority of a service defines the 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. IANA stands for the Internet Assigned Numbers Authority.
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
Attribute Definitions as described above. This Naming Authority will
accompany registration and queries, as described in sections 5 and 9.
3.5. 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
Service Request and/or Attribute 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.
3.6. Use of TCP, UDP and Multicast in Service Location
The Service Location Protocol requires the implementation of UDP
(connectionless) and TCP (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. Service Agents and User Agents
use ephemeral ports for transmitting information to the service
location port, which is 427.
The Service Location discovery mechanisms typically multicast
messages to as many enterprise networks as needed to establish
service availability. The protocol will operate in a broadcast
environment with limitations detailed in section 3.6.1.
3.6.1. Multicast vs. Broadcast
The Service Location Protocol was designed for use in networks where
DHCP is available, or multicast is supported at the network layer.
To support this protocol when only network layer broadcast is
supported, the following procedures may be followed.
3.6.1.1. Single Subnet
If a network is not connected to any other networks simple network
layer broadcasts will work in place of multicast.
Service Agents SHOULD and Directory Agents MUST listen for broadcast
Service Location request messages to the Service Location port. This
allows UAs which lack multicast capabilities to still make use of
Service Location on a single subnet.
3.6.1.2. 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 and
Service 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. The
address can also be determined by static configuration.
As dynamic discovery is not feasible in a broadcast environment with
multiple subnets and manual configuration is difficult, deploying DAs
to serve enterprises with multiple subnets will require use of
multicast discovery with multiple hops (i.e., TTL > 1 in the IP
header).
3.6.2. Service-Specific Multicast Address
This mechanism is used so that the number of datagrams any one
service agent receives is minimized. The Service Location General
Multicast Address MAY be used to query for any service, though one
SHOULD use the service-specific multicast address if it exists.
If the site network does not support multicast then the query SHOULD
be broadcast to the Service Location port. If, on the other hand,
the underlying hardware will not support the number of needed
multicast addresses the Service Location General Multicast Address
MAY be used. Service Agents MUST listen on this multicast address as
well as the service-specific multicast addresses for the service
types they advertise.
Service-Specific Multicast Addresses are computed by calculating a
string hash on the Service Type string. The Service Type string MUST
first be converted to an ASCII string from whatever character set it
is represented in, so the hash will have well-defined results.
The string hash function is modified from a code fragment attributed
to Chris Torek:
/*
* SLPhash returns a hash value in the range 0-1023 for a
* string of single-byte characters, of specified length.
*/
unsigned long SLPhash (const char *pc, unsigned int length)
unsigned long h = 0;
while (length-- != 0) {
h *= 33;
h += *pc++;
}
return (0x3FF & h); /* round to a range of 0-1023 */
}
This value is added to the base range of Service Specific Discovery
Addresses, to be assigned by IANA. These will be 1024 contiguous
multicast addresses.
3.7. Service Location Scaling, and Multicast 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. Further, Service Agents which respond to
user requests must be used to make service information available.
This does not scale to environments with many hosts and services.
When scaling Service Location systems to intermediate sized networks,
a central repository (Directory Agent) may be added to reduce the
number of Service Location messages transmitted in the network
infrastructure. Since the central repository can respond to all
Service and Attribute Requests, fewer Service and Attribute Replies
will be needed; for the same reason, there is no need to
differentiate between Directory Agents.
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. The services (and service
agents) advertised by the several Directory Agents are collected
together into logical groupings called "Scopes".
All Service Registrations that have a scope must be registered with
all DAs (within the appropriate multicast radius) of that scope which
have been or are subsequently discovered. Service Registrations
which have no scope are only registered with unscoped DAs. User
Agents make requests of DAs whose scope they are configured to use.
Service Agents MUST register with unscoped DAs even if they are
configured to specifically register with DAs which have a specific
scope or set of scopes. User Agents MAY query DAs without scopes,
even if they are configured to use DAs with a certain scope. This is
because any DA with no scope will have all the available service
information.
Scoped user agents SHOULD always use a DA which supports their
configured scope when possible instead of an unscoped DA. This will
prevent the unscoped DAs from becoming overused and thus a scaling
problem.
It is possible to specially configure Service Agents to register only
with a specific set of DAs (see Section 22.1). In that case,
services may not be available to User Agents via all Directory
Agents, but some network administrators may deem this appropriate.
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 effort will allow users and applications to
subsequently dynamically discover services without assistance.
The first mode (no DAs) is intended for a LAN. The second mode (using
a DA or DAs, but not using scopes) scales well to a group of
interconnected LANs with a limited number of hosts. The third mode
(with DAs and scopes) allows the SLP protocol to be used in an
internetworked campus environment.
If scoped DAs are used, they will not accept unscoped registrations
or requests. UAs which issue unscoped requests will discover only
unscoped services. They SHOULD use a scope in their requests if
possible and SHOULD use a DA with their scope in preference to an
unscoped DA. In a large campus environment it would be a bad idea to
have ANY unscoped DAs: They attract ALL registrations and will thus
present a scaling problem eventually.
A subsequent protocol document will describe mechanisms for
supporting a service discovery protocol for the global Internet.
4. Service Location General Message Format
The following header is used in all of the message 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 Function Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OMUAF rsvd Dialect Language Code
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Char Encoding XID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version This protocol document defines version 1 of the Service
Location protocol.
Function Service Location datagrams can be identified as to their
operation by the function field. The following are the
defined operations:
Message Type Abbreviation Function Value
Service Request SrvReq 1
Service Reply SrvRply 2
Service Registration SrvReg 3
Service Deregister SrvDereg 4
Service Acknowledge SrvAck 5
Attribute Request AttrRqst 6
Attribute Reply AttrRply 7
DA Advertisement DAAdvert 8
Service Type Request SrvTypeRqst 9
Service Type Reply SrvTypeRply 10
Length The number of bytes in the message, including the Service
Location Header.
O The "Overflow" bit. See Section 18 for the use of this
field.
M The "Monolingual" bit. Requests with this bit set
indicate the User Agent will only accept responses in the
language (see section 17) that is indicated by the
Service or Attribute Request.
U The "URL Authentication Present" bit. See sections 4.2,
4.3, 9, and 11 for the use of this field.
A The "Attribute Authentication Present" bit. See
sections 4.2, 4.3, and 13 for the use of this field.
F If the "F" bit is set in a Service Acknowledgement, the
directory agent has registered the service as a new
entry, not as an updated entry.
rsvd MUST be zero.
Dialect Dialect tags will be used by future versions of the
Service Location Protocol to indicate a variant of
vocabulary used. This field is reserved and MUST be set
to 0 for compatibility with future versions of the
Service Location Protocol.
Language Code
Strings within the remainder of the message which follows
are to be interpreted in the language encoded (see
section 17 and appendix A) in this field.
Character Encoding
The characters making up strings within the remainder of
the message may be encoded in any standardized encoding
(see section 17.1).
Transaction Identifier (XID)
The XID (transaction ID) field allows the requester to
match replies to individual requests (see section 4.1).
Note that, whenever there is an Attribute Authentication
block, there will also be a URL Authentication block.
Thus, it is an error to have the "A" bit set without also
having the "U" bit set.
4.1. Use of Transaction IDs (XIDs)
Retransmission is used to ensure reliable transactions in the Service
Location Protocol. If a User Agent or Service Agent sends a message
and fails to receive an expected response, the message will be sent
again. Retransmission of the same Service Location datagram should
not contain an updated XID. It is quite possible the original request
reached the DA or SA, but reply failed to reach the requester. Using
the same XID allows the DA or SA to cache its reply to the original
request and then send it again, should a duplicate request arrive.
This cached information should only be held very briefly
(CONFIG_INTERVAL_0.) Any registration or deregistration at a
Directory Agent, or change of service information at a SA should
flush this cache so that the information returned to the client is
always valid.
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.
Directory Agents use XID values in their DA Advertisements to
indicate their state (see section 15.2).
4.2. URL Entries
When URLs are registered, they have lifetimes and lengths, and may be
authenticated. These values are associated with the URL for the
duration of the registration. The association is known as a "URL-
entry", and has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Lifetime Length of URL
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

URL

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(if present) URL Authentication Block .....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Lifetime The length of time that the registration is valid, in
the absence of later registrations or deregistration.
Length of URL
The length of the URL, measured in bytes and < 32768.
URL Authentication Block
(if present) A timestamped authenticator (section 4.3)
The URL conforms to RFC1738 [6]. If the "U" bit is set in the
message header, the URL is followed by an URL Authentication Block.
If the scheme used in the URL does not have a standardized
representation, the minimal requirement is:
service:<srvtype>://<addr-spec>
"service" is the URL scheme of all Service Location Information
included in service registrations and service replies. Each URL
entry contains the service:<srvtype> scheme name. It may also
include an <addr-spec> except in the case of a reply to a Service
Type request (see section 7).
4.3. Authentication Blocks
Authentication blocks are used to authenticate service registrations
and deregistrations. URLs are registered along with an URL
Authentication block to retain the authentication information in the
URL entry for subsequent use by User Agents who receive a Service
Reply containing the URL entry. Service attributes are registered
along with an Attribute Authentication block. Both authentication
blocks have the format illustrated below.
If a service registration is accompanied by authentication which can
be validated by the DA, the DA MUST validate any subsequent service
deregistrations, so that unauthorized entities cannot invalidate such
registered services. Likewise, if a service registration is
accompanied by an Attribute Authentication block which can be
validated by the DA, the DA MUST validate any subsequent attribute
registrations, so that unauthorized entities cannot invalidate such
registered attributes.
To avoid replay attacks which use previously validated
deregistrations, the deregistration or attribute registration message
must contain a timestamp for use by the DA. To avoid replay attacks
which use previously validated registrations to nullify a valid
deregistration, registrations must also contain a timestamp.
An authentication block has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ Timestamp +

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Block Structure Descriptor Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Structured Authenticator ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Timestamp A 64-bit value formatted as specified by the Network
Time Protocol (NTP) [16].
Block Structure Descriptor (BSD)
A value describing the structure of the Authenticator.
The only value currently defined is 1, for
Object-Identifier.
Length The length of the Authenticator
Structured Authenticator
An algorithm specification, and the authentication data
produced by the algorithm.
The Structured Authenticator contains a digital signature of the
information being authenticated. It contains sufficient information
to determine the algorithm to be used and the keys to be selected to
verify the digital signature.
The digital signature is computed over the following ordered stream
of data:
CHARACTER ENCODING OF URL (2 bytes in network byte order)
LIFETIME (2 bytes in network byte order)
LENGTH OF URL (2 bytes in network byte order)
URL (n bytes)
TIMESTAMP (8 bytes in SNTP format [16])
When producing a URL Authentication block, the authentication data
produced by the algorithm identified within the Structured
Authenticator calculated over the following ordered stream of data:
ATTRIBUTE CHARACTER ENCODING (2 bytes in network byte order)
LENGTH OF ATTRIBUTES (2 bytes in network byte order)
ATTRIBUTES (n bytes)
TIMESTAMP (8 bytes in SNTP format [16])
Every Service Location Protocol entity (User Agent, Service Agent, or
Directory Agent) which is configured for use with protected scopes
SHOULD implement "md5WithRSAEncryption" [4] and be able to associate
it with BSD value == 1.
In the case where BSD value == 1 and the OID "md5WithRSAEncryption"
is selected, the Structured Authenticator will start with the ASN.1
Distinguished Encoding (DER) [9] for "md5WithRSAEncryption", which
has the as its value the bytes (MSB first in hex):
"30 0d 06 09 2a 86 48 86 f7 0d 01 01 04 05 00"
This is then immediately followed by an ASN.1 Distinguished Encoding
(as a "Bitstring") of the RSA encryption (using the Scope"s private
key) of a bitstring consisting of the OID for "MD5" concatenated by
the MD5 [22] message digest computed over the fields above. The
exact construction of the MD5 OID and digest can be found in RFC1423
[4].
4.4. URL Entry Lifetime
The Lifetime field is set to the number of seconds the reply can be
cached by any agent. A value of 0 means the information must not be
cached. User Agents MAY cache service information, but if they do,
they must provide a way for applications to flush this cached
information and issue the request directly onto the network.
Services should be registered with DAs with a Lifetime, the suggested
value being CONFIG_INTERVAL_1. The service must be reregistered
before this interval elapses, or the service advertisement will no
longer be available. Thus, services which vanish and fail to
deregister eventually become automatically deregistered.
5. Service Request Message Format
The Service Request is used to obtain URLs from a Directory Agent or
Service Agents.
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 = SrvReq)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
length of prev resp list string<Previous Responders Addr Spec>
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

<Previous Responders Addr Spec>

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
length of predicate string Service Request <predicate>
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Service Request <predicate>, contd.

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If a UA issues a request which will result in a reply which is too
large, the SA or DA will return an abbreviated response (in a
datagram the size of the site"s MTU) which has the "Overflow" bit
flag set. The UA must then issue the request again using TCP.
The <Previous Responders Addr Spec> is described in sections 7 and
20.1.
After a User Agent restarts (say, after rebooting of a system,
loading of the network kernel), Service Requests should be delayed
for some random time uniformly distributed within a one second
interval centered about a configured delay value (by default,
CONFIG_INTERVAL_4).
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:
<srvtype>[.<na>]/[<scope>]/[<where>]/
The punctuation is necessary even where the fields are omitted.
- The <srvtype> refers to the Service Type. For each type of
service available, there is a unique Service type name string.
See section 20.2.1.
- The <na> is the Naming Authority. This string determines the
semantic interpretation of the attribute information in the
<where> part of the Service Request.
- The <scope> is a string used to restrict the range of the query.
Scope is determined administratively, at a given site. It is not
necessarily related to network topology (see Section 16).
Leaving this field out means that the request can be satisfied
only by unscoped service advertisements.
- The <where> string is the Where Clause of the request. It
contains a query which allows the selection of those service
instances which the User Agent is interested in. The query
includes attributes, boolean operators and relations. (See
section 5.3.)
In the case of a multicast service 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 CONFIG_INTERVAL_2) until there are no new responses,
or a certain time (CONFIG_INTERVAL_3) has elapsed. Different timing
values are applied to a Service Request used for Directory Agent
Discovery, see Section 5.2.
In order for a request to succeed in matching registered information,
the following 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. (If the scope of the request
as omitted, the request will only match services which were
registered with no scope. Note that a scoped request WILL match
all unscoped Services).
4. The conditions specified in the Where Clause must match the
attributes and keywords registered for the service.
5.1. Service Request Usage
The User Agent may form Service Requests using preconfigured
knowledge of a Service Type"s attributes. It may also issue
Attribute Requests to obtain the attribute values for a Service Type
before issuing Service Requests (see Section 13). Having obtained
the attributes which describe a particular kind of service from an
Attribute Request, or using configured knowledge of a service"s
attributes, the User Agent can build a predicate that describes the
service needs of the user.
Service Requests may be sent directly to a Directory Agent. Suppose
a printer supporting the lpr protocol is needed on the 12th floor
which has UNRESTRICTED_ACCESS and prints 12 pages per minute.
Suppose further that a Attribute Request indicates that there is a
printer on the 12th floor, a printer that prints 12 pages per minute,
and a printer that offers UNRESTRICTED_ACCESS. To check whether they
are same printer, issue the following request:
lpr//(& (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.
lpr//(LOCATION==12th FLOOR)/
In this case, there is now only one reply:
Returned URL: service:lpr://igore.wco.FTP.com:515/draft
The Address Specification for the printer is: igore.wco.ftp.com:515,
containing the name of the host managing the requested printer.
Files would be printed by spooling to that port on that host. The
word "draft" refers to the name of the print queue the lpr server
supports.
In the absence of a Directory Agent, the request above could be
multicast. In this case it would be sent to the Service Specific
Multicast Address for "service:printer" and not to the Directory
Agent. Service Agents that can satisfy the predicate will reply.
Service Agents which cannot support the character set of the request
MUST return CHARSET_NOT_UNDERSTOOD in the SrvRply. In all other
circumstances, 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 (CONFIG_INTERVAL_8). If
no response comes, the User Agent gives up and assumes there are no
such printers.
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:
lpr//PAGES PER MINUTE==12,
UNRESTRICTED_ACCESS,
LOCATION==12th FLOOR/
5.2. Directory Agent Discovery Request
Normally a Service Request returns a Service Reply. The sole
exception to this is a Service Request for the Service Type
"directory-agent". This Service Request is answered with a DA
Advertisement.
Without configured knowledge of a Directory Agent (DA), a User Agent
or Service Agent uses a Service Request to discover a DA. (See
section 15.1 for mechanisms by which a client may be configured to
have knowledge of a DA.) Such a Service Request used for Directory
Agent Discovery includes a predicate of the form:
directory-agent///
This query is always sent to the Directory Agent Discovery 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, so all Directory Agents will reply. This is
the only request which omits a scope which all Directory Agents MUST
respond to. Normally, a Directory Agent with a scope ONLY responds
to requests with that scope. No Naming Authority is included, so
"IANA" is assumed. We want to reach all the available directory
agents. If the scope were supplied, only DAs supporting that scope
would reply.
DA Advertisement Replies may arrive from different sources, similar
in form to:
URL returned: service:directory-agent://slp-resolver.catch22.com
Scope returned: ACCOUNTING
URL returned: service:directory-agent://204.182.15.66 Scope
returned: JANITORIAL SERVICES
The DA Advertisement format is defined in Section 14.
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 CONFIG_INTERVAL_5). This retransmitted request
will include a list of DAs which have already responded. See
sections 7 and 20.1. Directory Agents which receive the request will
only respond if they are not on this list. After there are no new
replies, all DAs are presumed to have been discovered.
If a DA fails to respond after CONFIG_INTERVAL_6 seconds, the UA or
Service Agent should use a different DA. DA addresses may be cached
from previous discovery attempts, preconfigured, or by use of DHCP
(see section 15.2). If no such DA responds, DA discovery should be
used to find a new DA. Only after CONFIG_INTERVAL_7 seconds should it
be assumed that no DA exists and multicast based Service Requests
should be used.
5.3. Explanation of Terms of Predicate Grammar
A predicate has a simple structure, which depends on parentheses,
commas and slashes to delimit the elements. Examples of proper usage
are given throughout this document. The terms used in the grammar
are as follows:
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
only services registered without a scope. If it is present,
only services registered under that scope or are unscoped will
match the request.
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 specified Service Type, 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
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, the
following requires that three subexpressions must all be true:
(& ( <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.
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.
5.4. Service Request Predicate Grammar
Service Requests can precisely describe the services they need by
including a Predicate the body of the Request. This Predicate must
be constructed according to the grammar below.
<predicate> ::= <srvtype>["."<na>]"/"<scope>"/"<where>"/"
<srvtype> ::= string representing type of service. Only
alphanumeric characters, "+", and "-" are allowed.
<na> ::= string representing the Naming Authority.
Only alphanumeric characters, "+",
and "-" are allowed. If this field is
omitted then "IANA" is assumed.
<scope> ::= string representing the directory agent scope.
"/", "," (comma) and ":" are not allowed in
this string. The scopes "LOCAL" and "REMOTE"
are reserved.
<attr-tag> ::= class name of an attribute of a given Service
Type. This tag cannot include the following
characters: "(", ")", ",", "=", "!", ">",
"<", "/", "*", except where escaped (see 17.1.)
<keyword> ::= a class name of an attribute which will have
no values. This string has the same limits
as the <attr-tag>, except that white space
internal to the keyword is illegal.
<where> ::= <where-any>
<where-list>
<query-join>
<where-any> ::=
That is NOTHING, or white space.
<where-list> ::= "(" "&" <where-list> <query-list> ")"
"(" "" <where-list> <query-list> ")"
"(" <keyword> ")"
"(" <attr-tag> <comp-op> <attr-val> ")"
<query-list> ::= <where-list>
<where-list> <query-list>
<query-join> ::= <keyword>
<join-item>
<query-join> "," <keyword>
<query-join> "," <join-item>
<join-item> ::= <attr-tag> <comp-op> <attr-val>
<comp-op> ::= "!=" "==" "<" "<=" ">" ">="
<attr-val> ::= any string (see Section 20.5 for the ways
in which attr-vals are interpreted.)
Value strings may not contain "/", ","
"=", "<", ">", or "*" except where escaped
(see 17.1.).
"(" and ")" may be used in attribute values
for the purpose of encoding a binary values.
Binary encodings (See 20.5) may
include the above reserved characters.
5.5. String Matching for Requests
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".
String matching may only be performed over the same character sets.
If a request cannot be satisfied due to a lack of support for the
character set of the request a CHARSET_NOT_UNDERSTOOD error is
returned.
String comparisons (using comparison operators such as "<" or
registration, not using any language specific rules. The ordering is
strictly by the character value, i.e. "0" < "A" is true when the
character set is US-ASCII, since "0" has the value of 48 and "A" has
the value 65.
The special character "*" may precede or follow a string in order to
allow substring matching. If the "*" precedes a string, it matches
any attribute value which ends with the string. If the string ends
with a "*", it matches any attribute value which begins with the
string. Finally, if a string begins and ends with a "*", the string
will match any attribute value which contains the string.
Examples:
"bob*" matches "bob", "bobcat", and "bob and sue" "*bob" matches
"bob", "bigbob", and "sue and bob" "*bob*" matches "bob",
"bobcat", "bigbob", and "a bob I know"
String matching is done after escape sequences have been substituted.
See sections 17, 5.3, 17.1.
6. Service Reply Message Format
The format of the Service Reply Message 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)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Error Code URL Entry count
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
<URL Entry 1> ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.
.
.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
<URL Entry N> ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Each Service Reply message is composed of a list of URL Entries.
The Error Code may have one of the following values:
0 Success
LANGUAGE_NOT_SUPPORTED
A SA or DA returns this when a request is received from a
UA which is in a language for which there is no
registered Service Information and the request arrived
with the Monolingual bit set. See Section 17.
PROTOCOL_PARSE_ERROR
A SA or DA returns this error when a SrvRply is received
which cannot be parsed or the declared string lengths
overrun the message.
SCOPE_NOT_SUPPORTED
A DA will return this error if it receives a request
which has a scope not supported by the DA. An SA will not
return this error; it will simply not reply to the
multicast request.
CHARSET_NOT_UNDERSTOOD
If the DA or SA receives a request or registration in a
character set which it does not support, it will return
this error.
Each <URL Entry> in the list has the form defined in Section 4.2.
The URL entries in the reply have no delimiters between them, other
than the length fields. The URL length fields indicate where the URL
strings end. If the presence of an URL Authenticator block is
signalled by the "U" bit, the length of the authenticator block is
determined by information within the block as discussed in section
4.3. A User Agent MAY use the authentication block to determine
whether the Service Agent advertising the URL is, in fact, authorized
to offer the indicated service. If, in a list of URL entries, some
of the URLs indicate services which are in protected scopes (see
section 16.1) while other URLs in the list indicate services which
are not in protected scopes, the latter must still have
Authentication Blocks, but the length of the authentcitor is shown as
zero, and no authentication need be done.
7. Service Type Request Message Format
The Service Type Request is used to determine all the types of
services supported on a network.
The request should be sent directly to a DA (though it may also be
sent to the Service Location General Multicast Address), in order to
find out all services available on the site network (which are
advertised by Directory Agents and Service Agents.) If no DA is
available, a User Agent MAY issue more than one request to insure
that all replies have been received. In each subsequent request, a
User Agent includes those Service Types that it is aware of. When no
new replies arrive within CONFIG_INTERVAL_3 from a request, the User
Agent can presume that it has acquired a complete set of available
Service Types.
The format of a Service Type Request 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 = SrvTypeRqst)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
length of prev resp string <Previous Responders Addr Spec>
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

<Previous Responders Addr Spec>

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
length of naming authority <Naming Authority String>
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

<Naming Authority String>, continued

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
length of Scope String <Scope String>
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

<Scope String>, continued

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Note that the <Previous Responders Addr Spec> is a comma delimited
list. (See section 20.1.) The "length of prev responder list" field
indicates the length of the comma delimited list string. A previous
responder list with 3 elements takes this form:
<addr-spec>,<addr-spec>,<addr-spec>
The Naming Authority, if included, will limit the replies to Service
Type Requests to Service Types which have the specified Naming
Authority. If this field is omitted (i.e., the length field is
zero), the default Naming Authority ("IANA") is assumed. If the
length field is -1, service types from all naming authorities are
requested.
The Scope String Field, if included, will limit replies to Service
Types which have the specified scope or are unscoped. If this field
is omitted, all Service Types (from the specified Naming Authority)
are returned.
8. Service Type Reply Message Format
The Service Type Reply has the following format:
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 = SrvTypeRply)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Error Code number of service types
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

<Service Type Item 1>

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

<Service Type Item N>

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The format of a Service Type Item 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 Service Type String <Service Type String>
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

<Service Type String>, continued

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Error Code may have one of the following values:
0 Success
PROTOCOL_PARSE_ERROR
A SA or DA returns this error when a SrvTypeRqst is
received which ca