RFC 2608

Network Working Group E. Guttman
Request for Comments: 2608 C. Perkins
Updates: RFC 2165 Sun Microsystems
Category: Standards Track J. Veizades
@Home Network
M. Day
Vinca Corporation
June 1999

Service Location Protocol, Version 2

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 need little or no 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
1.1. Applicability Statement . . . . . . . . . . . . . . . 3
2. Terminology 4
2.1. Notation Conventions . . . . . . . . . . . . . . . . . 4
3. Protocol Overview 5
4. URLs used with Service Location 8
4.1. Service: URLs . . . . . . . . . . . . . . . . . . . . 9
4.2. Naming Authorities . . . . . . . . . . . . . . . . . 10
4.3. URL Entries . . . . . . . . . . . . . . . . . . . . . 10
5. Service Attributes 10
6. Required Features 12
6.1. Use of Ports, UDP, and Multicast . . . . . . . . . . 13

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6.2. Use of TCP . . . . . . . . . . . . . . . . . . . . . 14
6.3. Retransmission of SLP messages . . . . . . . . . . . 15
6.4. Strings in SLP messages . . . . . . . . . . . . . . . 16
6.4.1. Scope Lists in SLP . . . . . . . . . . . . . . 16
7. Errors 17
8. Required SLP Messages 17
8.1. Service Request . . . . . . . . . . . . . . . . . . . 19
8.2. Service Reply . . . . . . . . . . . . . . . . . . . . 21
8.3. Service Registration . . . . . . . . . . . . . . . . . 22
8.4. Service Acknowledgment . . . . . . . . . . . . . . . . 23
8.5. Directory Agent Advertisement. . . . . . . . . . . . . 24
8.6. Service Agent Advertisement. . . . . . . . . . . . . . 25
9. Optional Features 26
9.1. Service Location Protocol Extensions . . . . . . . . . 27
9.2. Authentication Blocks . . . . . . . . . . . . . . . . 28
9.2.1. SLP Message Authentication Rules . . . . . . . 29
9.2.2. DSA with SHA-1 in Authentication Blocks . . . 30
9.3. Incremental Service Registration . . . . . . . . . . 30
9.4. Tag Lists . . . . . . . . . . . . . . . . . . . . . . 31
10. Optional SLP Messages 32
10.1. Service Type Request . . . . . . . . . . . . . . . . 32
10.2. Service Type Reply . . . . . . . . . . . . . . . . . 32
10.3. Attribute Request . . . . . . . . . . . . . . . . . . 33
10.4. Attribute Reply . . . . . . . . . . . . . . . . . . . 34
10.5. Attribute Request/Reply Examples . . . . . . . . . . . 34
10.6. Service Deregistration . . . . . . . . . . . . . . . 36
11. Scopes 37
11.1. Scope Rules . . . . . . . . . . . . . . . . . . . . . 37
11.2. Administrative and User Selectable Scopes. . . . . . . 38
12. Directory Agents 38
12.1. Directory Agent Rules . . . . . . . . . . . . . . . . 39
12.2. Directory Agent Discovery . . . . . . . . . . . . . . 39
12.2.1. Active DA Discovery . . . . . . . . . . . . . 40
12.2.2. Passive DA Advertising . . . . . . . . . . . . 40
12.3. Reliable Unicast to DAs and SAs. . . . . . . . . . . . 41
12.4. DA Scope Configuration . . . . . . . . . . . . . . . 41
12.5. DAs and Authentication Blocks. . . . . . . . . . . . . 41
13. Protocol Timing Defaults 42
14. Optional Configuration 43
15. IANA Considerations 44
16. Internationalization Considerations 45
17. Security Considerations 46
A. Appendix: Changes to the Service Location Protocol from
v1 to v2 48
B. Appendix: Service Discovery by Type: Minimal SLPv2 Features 48
C. Appendix: DAAdverts with arbitrary URLs 49
D. Appendix: SLP Protocol Extensions 50
D.1. Required Attribute Missing Option . . . . . . . . . . 50

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RFC 2608 Service Location Protocol, Version 2 June 1999

E. Acknowledgments 50
F. References 51
G. Authors' Addresses 53
H. Full Copyright Statement 54

1. Introduction

The Service Location Protocol (SLP) provides a flexible and scalable
framework for providing hosts with access to information about the
existence, location, and configuration of networked services.
Traditionally, users have had to find services by knowing the name of
a network host (a human readable text string) which is an alias for a
network address. SLP eliminates the need for a user to know the name
of a network host supporting a service. Rather, the user supplies
the desired type of service and a set of attributes which describe
the service. Based on that description, the Service Location
Protocol resolves the network address of the service for the user.

SLP provides a dynamic configuration mechanism for applications in
local area networks. Applications are modeled as clients that need
to find servers attached to any of the available networks within an
enterprise. 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.

This document updates SLPv1 [RFC 2165], correcting protocol errors,
adding some enhancements and removing some requirements. This
specification has two parts. The first describes the required
features of the protocol. The second describes the extended features
of the protocol which are optional, and allow greater scalability.

1.1. Applicability Statement

SLP is intended to function within networks under cooperative
administrative control. Such networks permit a policy to be
implemented regarding security, multicast routing and organization of
services and clients into groups which are not be feasible on the
scale of the Internet as a whole.

SLP has been designed to serve enterprise networks with shared
services, and it may not necessarily scale for wide-area service
discovery throughout the global Internet, or in networks where there
are hundreds of thousands of clients or tens of thousands of
services.

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2. Terminology

User Agent (UA)
A process working on the user's behalf to establish
contact with some service. The UA 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 the services.

Directory Agent (DA) A process which collects service
advertisements. There can only be one DA present per
given host.

Service Type Each type of service has a unique Service Type
string.

Naming Authority The agency or group which catalogues given
Service Types and Attributes. The default Naming
Authority is IANA.

Scope A set of services, typically making up a logical
administrative group.

URL A Universal Resource Locator [8].

2.1. Notation Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [9].

Syntax Syntax for string based protocols follow the
conventions defined for ABNF [11].

Strings All strings are encoded using the UTF-8 [23]
transformation of the Unicode [6] character set and
are NOT null terminated when transmitted. Strings
are preceded by a two byte length field.

A comma delimited list of strings with the
following syntax:

string-list = string / string `,' string-list

In format diagrams, any field ending with a \ indicates a variable
length field, given by a prior length field in the protocol.

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RFC 2608 Service Location Protocol, Version 2 June 1999

3. Protocol Overview

The Service Location Protocol supports a framework by which client
applications are modeled as 'User Agents' and services are advertised
by 'Service Agents.' A third entity, called a 'Directory Agent'
provides scalability to the protocol.

The User Agent issues a 'Service Request' (SrvRqst) on behalf of the
client application, specifying the characteristics of the service
which the client requires. The User Agent will receive a Service
Reply (SrvRply) specifying the location of all services in the
network which satisfy the request.

The Service Location Protocol framework allows the User Agent to
directly issue requests to Service Agents. In this case the request
is multicast. Service Agents receiving a request for a service which
they advertise unicast a reply containing the service's location.

+------------+ ----Multicast SrvRqst----> +---------------+
| User Agent | | Service Agent |
+------------+ <----Unicast SrvRply------ +---------------+

In larger networks, one or more Directory Agents are used. The
Directory Agent functions as a cache. Service Agents send register
messages (SrvReg) containing all the services they advertise to
Directory Agents and receive acknowledgements in reply (SrvAck).
These advertisements must be refreshed with the Directory Agent or
they expire. User Agents unicast requests to Directory Agents
instead of Service Agents if any Directory Agents are known.

User and Service Agents discover Directory Agents two ways. First,
they issue a multicast Service Request for the 'Directory Agent'
service when they start up. Second, the Directory Agent sends an
unsolicited advertisement infrequently, which the User and Service
Agents listen for. In either case the Agents receive a DA
Advertisement (DAAdvert).

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RFC 2608 Service Location Protocol, Version 2 June 1999

Services are grouped together using 'scopes'. These are strings
which identify services which are administratively identified. A
scope could indicate a location, administrative grouping, proximity
in a network topology or some other category. Service Agents and
Directory Agents are always assigned a scope string.

A User Agent is normally assigned a scope string (in which case the
User Agent will only be able to discover that particular grouping of
services). This allows a network administrator to 'provision'
services to users. Alternatively, the User Agent may be configured
with no scope at all. In that case, it will discover all available
scopes and allow the client application to issue requests for any
service available on the network.

In the above illustration, the User Agent is configured with scopes X
and Y. If a service is sought in scope X, the request is multicast.
If it is sought in scope Y, the request is unicast to the DA.
Finally, if the request is to be made in both scopes, the request
must be both unicast and multicast.

Service Agents and User Agents may verify digital signatures provided
with DAAdverts. User Agents and Directory Agents may verify service
information registered by Service Agents. The keying material to use
to verify digital signatures is identified using a SLP Security
Parameter Index, or SLP SPI.

Every host configured to generate a digital signature includes the
SLP SPI used to verify it in the Authentication Block it transmits.
Every host which can verify a digital signature must be configured
with keying material and other parameters corresponding with the SLP
SPI such that it can perform verifying calculations.

SAs MUST accept multicast service requests and unicast service
requests. SAs MAY accept other requests (Attribute and Service Type
Requests). SAs MUST listen for multicast DA Advertisements.

The features described up to this point are required to implement. A
minimum implementation consists of a User Agent, Service Agent or
both.

There are several optional features in the protocol. Note that DAs
MUST support all these message types, but DA support is itself

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optional to deploy on networks using SLP. UAs and SAs MAY support
these message types. These operations are primarily for interactive
use (browsing or selectively updating service registrations.) UAs
and SAs either support them or not depending on the requirements and
constraints of the environment where they will be used.

Service Type Request A request for all types of service on the
network. This allows generic service browsers
to be built.

Service Type Reply A reply to a Service Type Request.

Attribute Request A request for attributes of a given type of
service or attributes of a given service.

Attribute Reply A reply to an Attribute Request.

Service Deregister A request to deregister a service or some
attributes of a service.

Service Update A subsequent SrvRqst to an advertisement.
This allows individual dynamic attributes to
be updated.

SA Advertisement In the absence of Directory Agents, a User
agent may request Service Agents in order
to discover their scope configuration. The
User Agent may use these scopes in requests.

In the absence of Multicast support, Broadcast MAY be used. The
location of DAs may be staticly configured, discovered using SLP as
described above, or configured using DHCP. If a message is too large,
it may be unicast using TCP.

A SLPv2 implementation SHOULD support SLPv1 [22]. This support
includes:

1. SLPv2 DAs are deployed, phasing out SLPv1 DAs.

2. Unscoped SLPv1 requests are considered to be of DEFAULT scope.
SLPv1 UAs MUST be reconfigured to have a scope if possible.

3. There is no way for an SLPv2 DA to behave as an unscoped SLPv1
DA. SLPv1 SAs MUST be reconfigured to have a scope if possible.

4. SLPv2 DAs answer SLPv1 requests with SLPv1 replies and SLPv2
requests with SLPv2 replies.

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5. SLPv2 DAs use registrations from SLPv1 and SLPv2 in the same
way. That is, incoming requests from agents using either version
of the protocol will be matched against this common set of
registered services.

6. SLPv2 registrations which use Language Tags which are greater
than 2 characters long will be inaccessible to SLPv1 UAs.

7. SLPv2 DAs MUST return only service type strings in SrvTypeRply
messages which conform to SLPv1 service type string syntax, ie.
they MUST NOT return Service Type strings for abstract service
types.

8. SLPv1 SrvRqsts and AttrRqsts by Service Type do not match Service
URLs with abstract service types. They only match Service URLs
with concrete service types.

SLPv1 UAs will not receive replies from SLPv2 SAs and SLPv2 UAs will
not receive replies from SLPv1 SAs. In order to interoperate UAs and
SAs of different versions require a SLPv2 DA to be present on the
network which supports both protocols.

The use of abstract service types in SLPv2 presents a backward
compatibility issue for SLPv1. It is possible that a SLPv1 UA will
request a service type which is actually an abstract service type.
Based on the rules above, the SLPv1 UA will never receive an abstract
Service URL reply. For example, the service type 'service:x' in a
SLPv1 AttrRqst will not return the attributes of 'service:x:y://orb'.
If the request was made with SLPv2, it would return the attributes of
this service.

4. URLs used with Service Location

A Service URL indicates the location of a service. This URL may be
of the service: scheme [13] (reviewed in section 4.1), or any other
URL scheme conforming to the URI standard [8], except that URLs
without address specifications SHOULD NOT be advertised by SLP. The
service type for an 'generic' URL is its scheme name. For example,
the service type string for "http://www.srvloc.org" would be "http".

Reserved characters in URLs follow the rules in RFC 2396 [8].

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4.1. Service: URLs

Service URL syntax and semantics are defined in [13]. Any network
service may be encoded in a Service URL.

This section provides an introduction to Service URLs and an example
showing a simple application of them, representing standard network
services.

A Service URL may be of the form:

"service:""://"

The Service Type of this service: URL is defined to be the string up
to (but not including) the final `:' before , the address
specification.

is a hostname (which should be used if possible) or dotted
decimal notation for a hostname, followed by an optional `:' and
port number.

A service: scheme URL may be formed with any standard protocol name
by concatenating "service:" and the reserved port [1] name. For
example, "service:tftp://myhost" would indicate a tftp service. A
tftp service on a nonstandard port could be
"service:tftp://bad.glad.org:8080".

Service Types SHOULD be defined by a "Service Template" [13], which
provides expected attributes, values and protocol behavior. An
abstract service type (also described in [13]) has the form

"service::".

The service type string "service:" matches all
services of that abstract type. If the concrete type is included
also, only these services match the request. For example: a SrvRqst
or AttrRqst which specifies "service:printer" as the Service Type
will match the URL service:printer:lpr://hostname and
service:printer:http://hostname. If the requests specified
"service:printer:http" they would match only the latter URL.

An optional substring MAY follow the last `.' character in the
(or in the case of an abstract service type
URL). This substring is the Naming Authority, as described in Section
9.6. Service types with different Naming Authorities are quite
distinct. In other words, service:x.one and service:x.two are
different service types, as are service:abstract.one:y and
service:abstract.two:y.

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RFC 2608 Service Location Protocol, Version 2 June 1999

4.2. Naming Authorities

A Naming Authority MAY optionally be included as part of the Service
Type string. 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 typically a string
which uniquely identifies an organization. IANA is the implied
Naming Authority when no string is appended. "IANA" itself MUST NOT
be included explicitly.

Naming Authorities may define Service Types which are experimental,
proprietary or for private use. Using a Naming Authority, one may
either simply ignore attributes upon registration or create a local-
use only set of attributes for one's site. 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 8.1 and 8.3. Service Types SHOULD be registered with IANA
to allow for Internet-wide interoperability.

4.3. URL Entries

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Lifetime | URL Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|URL len, contd.| URL (variable length) \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|# of URL auths | Auth. blocks (if any) \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

SLP stores URLs in protocol elements called URL Entries, which
associate a length, a lifetime, and possibly authentication
information along with the URL. URL Entries, defined as shown above,
are used in Service Replies and Service Registrations.

5. Service Attributes

A service advertisement is often accompanied by Service Attributes.
These attributes are used by UAs in Service Requests to select
appropriate services.

The allowable attributes which may be used are typically specified by
a Service Template [13] for a particular service type. Services
which are advertised according to a standard template MUST register
all service attributes which the standard template requires. URLs
with schemes other than "service:" MAY be registered with attributes.

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Non-standard attribute names SHOULD begin with "x-", because no
standard attribute name will ever have those initial characters.

An attribute list is a string encoding of the attributes of a
service. The following ABNF [11] grammar defines attribute lists:

attr-list = attribute / attribute `,' attr-list
attribute = `(' attr-tag `=' attr-val-list `)' / attr-tag
attr-val-list = attr-val / attr-val `,' attr-val-list
attr-tag = 1*safe-tag
attr-val = intval / strval / boolval / opaque
intval = [-]1*DIGIT
strval = 1*safe-val
boolval = "true" / "false"
opaque = "\FF" 1*escape-val
safe-val = ; Any character except reserved.
safe-tag = ; Any character except reserved, star and bad-tag.
reserved = `(' / `)' / `,' / `\' / `!' / `<' / `=' / `>' / `~' / CTL
escape-val = `\' HEXDIG HEXDIG
bad-tag = CR / LF / HTAB / `_'
star = `*'

The , if present, MUST be scanned prior to evaluation for
all occurrences of the escape character `\'. Reserved characters
MUST be escaped (other characters MUST NOT be escaped). All escaped
characters must be restored to their value before attempting string
matching. For Opaque values, escaped characters are not converted -
they are interpreted as bytes.

Boolean Strings which have the form "true" or "false" can
only take one value and may only be compared with
'='. Booleans are case insensitive when compared.

Integer Strings which take the form [-] 1* and fall
in the range "-2147483648" to "2147483647" are
considered to be Integers. These are compared using
integer comparison.

String All other Strings are matched using strict lexical
ordering (see Section 6.4).

Opaque Opaque values are sequences of bytes. These are
distinguished from Strings since they begin with
the sequence "\FF". This, unescaped, is an illegal
UTF-8 encoding, indicating that what follows is a
sequence of bytes expressed in escape notation which
constitute the binary value. For example, a '0' byte
is encoded "\FF\00".

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A string which contains escaped values other than from the reserved
set of characters is illegal. If such a string is included in an
, or search filter, the SA or DA which receives
it MUST return a PARSE_ERROR to the message.

A keyword has only an , and no values. Attributes can have
one or multiple values. All values are expressed as strings.

When values have been advertised by a SA or are registered in a DA,
they can take on implicit typing rules for matching incoming
requests.

Stored values must be consistent, i.e., x=4,true,sue,\ff\00\00 is
disallowed. A DA or SA receiving such an MUST return an
INVALID_REGISTRATION error.

6. Required Features

This section defines the minimal implementation requirements for SAs
and UAs as well as their interaction with DAs. A DA is not required
for SLP to function, but if it is present, the UA and SA MUST
interact with it as defined below.

A minimal implementation may consist of either a UA or SA or both.
The only required features of a UA are that it can issue SrvRqsts
according to the rules below and interpret DAAdverts, SAAdverts and
SrvRply messages. The UA MUST issue requests to DAs as they are
discovered. An SA MUST reply to appropriate SrvRqsts with SrvRply or
SAAdvert messages. The SA MUST also register with DAs as they are
discovered.

UAs perform discovery by issuing Service Request messages. SrvRqst
messages are issued, using UDP, following these prioritized rules:

1. A UA issues a request to a DA which it has been configured with
by DHCP.

2. A UA issues requests to DAs which it has been statically
configured with.

3. UA uses multicast/convergence SrvRqsts to discover DAs, then uses
that set of DAs. A UA that does not know of any DAs SHOULD retry
DA discovery, increasing the waiting interval between subsequent
attempts exponentially (doubling the wait interval each time.)
The recommended minimum waiting interval is CONFIG_DA_FIND
seconds.

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4. A UA with no knowledge of DAs sends requests using multicast
convergence to SAs. SAs unicast replies to UAs according to the
multicast convergence algorithm.

UAs and SAs are configured with a list of scopes to use according to
these prioritized rules:

1. With DHCP.

2. With static configuration. The static configuration may be
explicitly set to NO SCOPE for UAs, if the User Selectable Scope
model is used. See section 11.2.

3. In the absence of configuration, the agent's scope is "DEFAULT".

A UA MUST issue requests with one or more of the scopes it has been
configured to use.

A UA which has been statically configured with NO SCOPE LIST will use
DA or SA discovery to determine its scope list dynamically. In this
case it uses an empty scope list to discover DAs and possibly SAs.
Then it uses the scope list it obtains from DAAdverts and possibly
SAAdverts in subsequent requests.

The SA MUST register all its services with any DA it discovers, if
the DA advertises any of the scopes it has been configured with. A
SA obtains information about DAs as a UA does. In addition, the SA
MUST listen for multicast unsolicited DAAdverts. The SA registers by
sending SrvReg messages to DAs, which reply with SrvReg messages to
indicate success. SAs register in ALL the scopes they were
configured to use.

6.1. Use of Ports, UDP, and Multicast

DAs MUST accept unicast requests and multicast directory agent
discovery service requests (for the service type "service:directory-
agent").

SAs MUST accept multicast requests and unicast requests both. The SA
can distinguish between them by whether the REQUEST MCAST flag is set
in the SLP Message header.

The Service Location Protocol uses multicast for discovering DAs and
for issuing requests to SAs by default.

The reserved listening port for SLP is 427. This is the destination
port for all SLP messages. SLP messages MAY be transmitted on an
ephemeral port. Replies and acknowledgements are sent to the port

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from which the request was issued. The default maximum transmission
unit for UDP messages is 1400 bytes excluding UDP and other headers.

If a SLP message does not fit into a UDP datagram it MUST be
truncated to fit, and the OVERFLOW flag is set in the reply message.
A UA which receives a truncated message MAY open a TCP connection
(see section 6.2) with the DA or SA and retransmit the request, using
the same XID. It MAY also attempt to make use of the truncated reply
or reformulate a more restrictive request which will result in a
smaller reply.

SLP Requests messages are multicast to The Administratively Scoped
SLP Multicast [17] address, which is 239.255.255.253. The default
TTL to use for multicast is 255.

In isolated networks, broadcasts will work in place of multicast. To
that end, SAs SHOULD and DAs MUST listen for broadcast Service
Location messages at port 427. This allows UAs which do not support
multicast the use of Service Location on isolated networks.

Setting multicast TTL to less than 255 (the default) limits the range
of SLP discovery in a network, and localizes service information in
the network.

6.2. Use of TCP

A SrvReg or SrvDeReg may be too large to fit into a datagram. To
send such large SLP messages, a TCP (unicast) connection MUST be
established.

To avoid the need to implement TCP, one MUST insure that:

- UAs never issue requests larger than the Path MTU. SAs can omit
TCP support only if they never have to receive unicast requests
longer than the path MTU.

- UAs can accept replies with the 'OVERFLOW' flag set, and make use
of the first result included, or reformulate the request.

- Ensure that a SA can send a SrvRply, SrvReg, or SrvDeReg in
a single datagram. This means limiting the size of URLs,
the number of attributes and the number of authenticators
transmitted.

DAs MUST be able to respond to UDP and TCP requests, as well as
multicast DA Discovery SrvRqsts. SAs MUST be able to respond to TCP
unless the SA will NEVER receive a request or send a reply which will
exceed a datagram in size (e.g., some embedded systems).

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A TCP connection MAY be used for a single SLP transaction, or for
multiple transactions. Since there are length fields in the message
headers, SLP Agents can send multiple requests along a connection and
read the return stream for acknowledgments and replies.

The initiating agent SHOULD close the TCP connection. The DA SHOULD
wait at least CONFIG_CLOSE_CONN seconds before closing an idle
connection. DAs and SAs SHOULD close an idle TCP connection after
CONFIG_CLOSE_CONN seconds to ensure robust operation, even when the
initiating agent neglects to close it. See Section 13 for timing
rules.

6.3. Retransmission of SLP messages

Requests which fail to elicit a response are retransmitted. The
initial retransmission occurs after a CONFIG_RETRY wait period.
Retransmissions MUST be made with exponentially increasing wait
intervals (doubling the wait each time). This applies to unicast as
well as multicast SLP requests.

Unicast requests to a DA or SA should be retransmitted until either a
response (which might be an error) has been obtained, or for
CONFIG_RETRY_MAX seconds.

Multicast requests SHOULD be reissued over CONFIG_MC_MAX seconds
until a result has been obtained. UAs need only wait till they
obtain the first reply which matches their request. That is,
retransmission is not required if the requesting agent is prepared to
use the 'first reply' instead of 'as many replies as possible within
a bounded time interval.'

When SLP SrvRqst, SrvTypeRqst, and AttrRqst messages are multicast,
they contain a of previous responders. Initially the
is empty. When these requests are unicast, the is
always empty.

Any DA or SA which sees its address in the MUST NOT respond
to the request.

The message SHOULD be retransmitted until the causes no
further responses to be elicited or the previous responder list and
the request will not fit into a single datagram or until
CONFIG_MC_MAX seconds elapse.

UAs which retransmit a request use the same XID. This allows a 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. XIDs SHOULD be randomly chosen to avoid

Guttman, et al. Standards Track [Page 15]

RFC 2608 Service Location Protocol, Version 2 June 1999

duplicate XIDs in requests if UAs restart frequently.

6.4. Strings in SLP messages

The escape character is a backslash (UTF-8 0x5c) followed by the two
hexadecimal digits of the escaped character. Only reserved
characters are escaped. For example, a comma (UTF-8 0x29) is escaped
as `\29', and a backslash `\' is escaped as `\5c'. String lists used
in SLP define the comma to be the delimiter between list elements, so
commas in data strings must be escaped in this manner. Backslashes
are the escape character so they also must always be escaped when
included in a string literally.

String comparison for order and equality in SLP MUST be case
insensitive inside the 0x00-0x7F subrange of UTF-8 (which corresponds
to ASCII character encoding). Case insensitivity SHOULD be supported
throughout the entire UTF-8 encoded Unicode [6] character set.

The case insensitivity rule applies to all string matching in SLPv2,
including Scope strings, SLP SPI strings, service types, attribute
tags and values in query handling, language tags, previous responder
lists. Comparisons of URL strings, however, is case sensitive.

White space (SPACE, CR, LF, TAB) internal to a string value is folded
to a single SPACE character for the sake of string comparisons.
White space preceding or following a string value is ignored for the
purposes of string comparison. For example, " Some String "
matches "SOME STRING".

String comparisons (using comparison operators such as `<=' or `>=')
are done using lexical ordering in UTF-8 encoded characters, not
using any language specific rules.

The reserved character `*' may precede, follow or be internal to a
string value in order to indicate substring matching. The query
including this character matches any character sequence which
conforms to the letters which are not wildcarded.

6.4.1. Scope Lists in SLP

Scope Lists in SLPv2 have the following grammar:

scope-list = scope-val / scope-val `,' scope-list
scope-val = 1*safe
safe = ; Any character except reserved.
reserved = `(' / `)' / `,' / `\' / `!' / `<' / `=' / `>' / `~' / CTL
/ `;' / `*' / `+'
escape-val = `\' HEXDIG HEXDIG

Guttman, et al. Standards Track [Page 16]

RFC 2608 Service Location Protocol, Version 2 June 1999

Scopes which include any reserved characters must replace the escaped
character with the escaped-val format.

7. Errors

If the Error Code in a SLP reply message is nonzero, the rest of the
message MAY be truncated. No data is necessarily transmitted or
should be expected after the header and the error code, except
possibly for some optional extensions to clarify the error, for
example as in section D.1.

Errors are only returned for unicast requests. Multicast requests
are silently discarded if they result in an error.

LANGUAGE_NOT_SUPPORTED = 1: There is data for the service type in
the scope in the AttrRqst or SrvRqst, but not in the requested
language.
PARSE_ERROR = 2: The message fails to obey SLP syntax.
INVALID_REGISTRATION = 3: The SrvReg has problems -- e.g., a zero
lifetime or an omitted Language Tag.
SCOPE_NOT_SUPPORTED = 4: The SLP message did not include a scope in
its supported by the SA or DA.
AUTHENTICATION_UNKNOWN = 5: The DA or SA receives a request for an
unsupported SLP SPI.
AUTHENTICATION_ABSENT = 6: The DA expected URL and ATTR
authentication in the SrvReg and did not receive it.
AUTHENTICATION_FAILED = 7: The DA detected an authentication error in
an Authentication block.
VER_NOT_SUPPORTED = 9: Unsupported version number in message header.
INTERNAL_ERROR = 10: The DA (or SA) is too sick to respond.
DA_BUSY_NOW = 11: UA or SA SHOULD retry, using exponential back off.
OPTION_NOT_UNDERSTOOD = 12: The DA (or SA) received an unknown option
from the mandatory range (see section 9.1).
INVALID_UPDATE = 13: The DA received a SrvReg without FRESH set, for
an unregistered service or with inconsistent Service Types.
MSG_NOT_SUPPORTED = 14: The SA received an AttrRqst or SrvTypeRqst
and does not support it.
REFRESH_REJECTED = 15: The SA sent a SrvReg or partial SrvDereg to a
DA more frequently than the DA's min-refresh-interval.

8. Required SLP Messages

All length fields in SLP messages are in network byte order. Where '
tuples' are defined, these are sequences of bytes, in the precise
order listed, in network byte order.

SLP messages all begin with the following header:

Guttman, et al. Standards Track [Page 17]

RFC 2608 Service Location Protocol, Version 2 June 1999

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-ID | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length, contd.|O|F|R| reserved |Next Ext Offset|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Extension Offset, contd.| XID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Language Tag Length | Language Tag \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Message Type Abbreviation Function-ID

Service Request SrvRqst 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
SA Advertisement SAAdvert 11

SAs and UAs MUST support SrvRqst, SrvRply and DAAdvert. SAs MUST
also support SrvReg, SAAdvert and SrvAck. For UAs and SAs, support
for other messages are OPTIONAL.

- Length is the length of the entire SLP message, header included.
- The flags are: OVERFLOW (0x80) is set when a message's length
exceeds what can fit into a datagram. FRESH (0x40) is set on
every new SrvReg. REQUEST MCAST (0x20) is set when multicasting
or broadcasting requests. Reserved bits MUST be 0.
- Next Extension Offset is set to 0 unless extensions are used.
The first extension begins at 'offset' bytes, from the message's
beginning. It is placed after the SLP message data. See
Section 9.1 for how to interpret unrecognized SLP Extensions.
- XID is set to a unique value for each unique request. If the
request is retransmitted, the same XID is used. Replies set
the XID to the same value as the xid in the request. Only
unsolicited DAAdverts are sent with an XID of 0.
- Lang Tag Length is the length in bytes of the Language Tag field.
- Language Tag conforms to [7]. The Language Tag in a reply MUST
be the same as the Language Tag in the request. This field must
be encoded 1*8ALPHA *("-" 1*8ALPHA).

Guttman, et al. Standards Track [Page 18]

RFC 2608 Service Location Protocol, Version 2 June 1999

If an option is specified, and not included in the message, the
receiver MUST respond with a PARSE_ERROR.

8.1. Service Request

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 = 1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of | String \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of | String \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of | String \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of predicate string | Service Request \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of string | String \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

In order for a Service to match a SrvRqst, it must belong to at least
one requested scope, support the requested service type, and match
the predicate. If the predicate is present, the language of the
request (ignoring the dialect part of the Language Tag) must match
the advertised service.

is the Previous Responder List. This contains
dotted decimal notation IP (v4) addresses, and is iteratively
multicast to obtain all possible results (see Section 6.3). UAs
SHOULD implement this discovery algorithm. SAs MUST use this to
discover all available DAs in their scope, if they are not already
configured with DA addresses by some other means.

A SA silently drops all requests which include the SA's address in
the . An SA which has multiple network interfaces MUST check
if any of the entries in the equal any of its interfaces.
An entry in the PRList which does not conform to an IPv4 dotted
decimal address is ignored: The rest of the is processed
normally and an error is not returned.

Once a plus the request exceeds the path MTU, multicast
convergence stops. This algorithm is not intended to find all
instances; it finds 'enough' to provide useful results.

The is a of configured scope names. SAs
and DAs which have been configured with any of the scopes in this
list will respond. DAs and SAs MUST reply to unicast requests with a

Guttman, et al. Standards Track [Page 19]

RFC 2608 Service Location Protocol, Version 2 June 1999

SCOPE_NOT_SUPPORTED error if the is omitted or fails to
include a scope they support (see Section 11). The only exceptions
to this are described in Section 11.2.

The string is discussed in Section 4. Normally, a
SrvRqst elicits a SrvRply. There are two exceptions: If the
is set to "service:directory-agent", DAs respond to
the SrvRqst with a DAAdvert (see Section 8.5.) If set to
"service:service-agent", SAs respond with a SAAdvert (see Section
8.6.) If this field is omitted, a PARSE_ERROR is returned - as this
field is REQUIRED.

The is a LDAPv3 search filter [14]. This field is
OPTIONAL. Services may be discovered simply by type and scope.
Otherwise, services are discovered which satisfy the . If
present, it is compared to each registered service. If the attribute
in the filter has been registered with multiple values, the filter is
compared to each value and the results are ORed together, i.e.,
"(x=3)" matches a registration of (x=1,2,3); "(!(Y=0))" matches
(y=0,1) since Y can be nonzero. Note the matching is case
insensitive. Keywords (i.e., attributes without values) are matched
with a "presence" filter, as in "(keyword=*)".

An incoming request term MUST have the same type as the attribute in
a registration in order to match. Thus, "(x=33)" will not match '
x=true', etc. while "(y=foo)" will match 'y=FOO'.
"(|(x=33)(y=foo))" will be satisfied, even though "(x=33)" cannot be
satisfied, because of the `|' (boolean disjunction).

Wildcard matching MUST be done with the '=' filter. In any other
case, a PARSE_ERROR is returned. Request terms which include
wildcards are interpreted to be Strings. That is, (x=34*) would
match 'x=34foo', but not 'x=3432' since the first value is a String
while the second value is an Integer; Strings don't match Integers.

Examples of Predicates follow. indicates the service type of the
SrvRqst, gives the and is the predicate string.

=service:http =DEFAULT
= (empty string)
This is a minimal request string. It matches all http
services advertised with the default scope.

=service:pop3 =SALES,DEFAULT
=(user=wump)
This is a request for all pop3 services available in
the SALES or DEFAULT scope which serve mail to the user
`wump'.

Guttman, et al. Standards Track [Page 20]

RFC 2608 Service Location Protocol, Version 2 June 1999

=service:backup =BLDG 32
=(&(q<=3)(speed>=1000))
This returns the backup service which has a queue length
less than 3 and a speed greater than 1000. It will
return this only for services registered with the BLDG 32
scope.

=service:directory-agent =DEFAULT
=
This returns DAAdverts for all DAs in the DEFAULT scope.

DAs are discovered by sending a SrvRqst with the service type set to
"service:directory-agent". If a predicate is included in the
SrvRqst, the DA SHOULD respond only if the predicate can be satisfied
with the DA's attributes. The MUST contain all scopes
configured for the UA or SA which is discovering DAs.

The string indicates a SLP SPI that the requester has been
configured with. If this string is omitted, the responder does not
include any Authentication Blocks in its reply. If it is included,
the responder MUST return a reply which has an associated
authentication block with the SLP SPI in the SrvRqst. If no replies
may be returned because the SLP SPI is not supported, the responder
returns an AUTHENTICATION_UNKNOWN error.

8.2. Service Reply

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 = 2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | URL Entry count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The service reply contains zero or more URL entries (see Section
4.3). A service reply with zero URL entries MUST be returned in
response to a unicast Service Request, if no matching URLs are
present. A service reply with zero URL entries MUST NOT be sent in
response to a multicast or broadcast service request (instead, if
there was no match found or an error processing the request, the
service reply should not be generated at all).

If the reply overflows, the UA MAY simply use the first URL Entry in
the list. A URL obtained by SLP may not be cached longer than
Lifetime seconds, unless there is a URL Authenticator block present.

Guttman, et al. Standards Track [Page 21]

RFC 2608 Service Location Protocol, Version 2 June 1999

In that case, the cache lifetime is indicated by the Timestamp in the
URL Authenticator (see Section 9.2).

An authentication block is returned in the URL Entries, including the
SLP SPI in the SrvRqst. If no SLP SPI was included in the request,
no Authentication Blocks are returned in the reply. URL
Authentication Blocks are defined in Section 9.2.1.

If a SrvRply is sent by UDP, a URL Entry MUST NOT be included unless
it fits entirely without truncation.

8.3. Service Registration

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 = 3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of service type string | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of attr-list string | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|# of AttrAuths |(if present) Attribute Authentication Blocks...\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The is a URL Entry (see section 4.3). The Lifetime defines
how long a DA can cache the registration. SAs SHOULD reregister
before this lifetime expires (but SHOULD NOT more often than once per
second). The Lifetime MAY be set to any value between 0 and 0xffff
(maximum, around 18 hours). Long-lived registrations remain stale
longer if the service fails and the SA does not deregister the
service.

The defines the service type of the URL to be
registered, regardless of the scheme of the URL. The
MUST contain the names of all scopes configured for the SA, which the
DA it is registering with supports. The default value for the
is "DEFAULT" (see Section 11).

The SA MUST register consistently with all DAs. If a SA is
configured with scopes X and Y and there are three DAs, whose scopes
are "X", "Y" and "X,Y" respectively, the SA will register the with
all three DAs in their respective scopes. All future updates and
deregistrations of the service must be sent to the same set of DAs in

Guttman, et al. Standards Track [Page 22]

RFC 2608 Service Location Protocol, Version 2 June 1999

the same scopes the service was initially registered in.

The , if present, specifies the attributes and values to
be associated with the URL by the DA (see Section 5).

A SA configured with the ability to sign service registrations MUST
sign each of the URLs and Attribute Lists using each of the keys it
is configured to use, and the DA it is registering with accepts.
(The SA MUST acquire DAAdverts for all DAs it will register with to
obtain the DA's SLP SPI list and attributes, as described in Section
8.5). The SA supplies a SLP SPI in each authentication block
indicating the SLP SPI configuration required to verify the digital
signature. The format of the digital signatures used is defined in
section 9.2.1.

Subsequent registrations of previously registered services MUST
contain the same list of SLP SPIs as previous ones or else DAs will
reject them, replying with an AUTHENTICATION_ABSENT error.

A registration with the FRESH flag set will replace *entirely* any
previous registration for the same URL in the same language. If the
FRESH flag is not set, the registration is an "incremental"
registration (see Section 9.3).

8.4. Service Acknowledgment

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 = 5) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

A DA returns a SrvAck to an SA after a SrvReg. It carries only a two
byte Error Code (see Section 7).

Guttman, et al. Standards Track [Page 23]

RFC 2608 Service Location Protocol, Version 2 June 1999

8.5. Directory Agent Advertisement

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 = DAAdvert = 8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | DA Stateless Boot Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|DA Stateless Boot Time,, contd.| Length of URL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ URL \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of | String \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| # Auth Blocks | Authentication block (if any) \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Error Code is set to 0 when the DAAdvert is multicast. If the
DAAdvert is being returned due to a unicast SrvRqst (ie. a request
without the REQUEST MCAST flag set) the DA returns the same errors a
SrvRply would.

The of the SrvRqst must either be omitted or include a
scope which the DA supports. The DA Stateless Boot Timestamp
indicates the state of the DA (see section 12.1).

The DA MAY include a list of its attributes in the DAAdvert. This
list SHOULD be kept short, as the DAAdvert must fit into a datagram
in order to be multicast.

A potential scaling problem occurs in SLPv2 if SAs choose too low a
Lifetime. In this case, an onerous amount of reregistration occurs
as more services are deployed. SLPv2 allows DAs to control SAs
frequency of registration. A DA MAY reissue a DAAdvert with a new
set of attributes at any time, to change the reregistration behavior
of SAs. These apply only to subsequent registrations; existing
service registrations with the DA retain their registered lifetimes.

If the DAAdvert includes the attribute "min-refresh-interval" it MUST
be set to a single Integer value indicating a number of seconds. If
this attribute is present SAs MUST NOT refresh any particular service
advertisement more frequently than this value. If SrvReg with the
FRESH FLAG not set or SrvDereg with a non-empty tag list updating a

Guttman, et al. Standards Track [Page 24]

RFC 2608 Service Location Protocol, Version 2 June 1999

particular service are received more often than the value for the
DA's advertised "min-refresh-interval" attribute the DA SHOULD reject
the message and return a REFRESH_REJECTED error in the SrvAck.

The URL is "service:directory-agent://" of the DA, where
is the dotted decimal numeric address of the DA. The of
the DA MUST NOT be NULL.

The SLP SPI List is the list of SPIs that the DA is capable of
verifying. SAs MUST NOT register services with authentication blocks
for those SLP SPIs which are not on the list. DAs will reject
service registrations which they cannot verify, returning an
AUTHENTICATION_UNKNOWN error.

The format of DAAdvert signatures is defined in Section 9.2.1.

The SLP SPI which is used to verify the DAAdvert is included in the
Authentication Block. When DAAdverts are multicast, they may have to
transmit multiple DAAdvert Authentication Blocks. If the DA is
configured to be able to generate signatures for more than one SPI,
the DA MUST include one Authentication Block for each SPI. If all
these Authentication Blocks do not fit in a single datagram (to
multicast or broadcast) the DA MUST send separate DAAdverts so that
Authentication Blocks for all the SPIs the DA is capable of
generating are sent.

If the DAAdvert is being sent in response to a SrvRqst, the DAAdvert
contains only the authentication block with the SLP SPI in the
SrvRqst, if the DA is configured to be able to produce digital
signatures using that SLP SPI. If the SrvRqst is unicast to the DA
(the REQUEST MCAST flag in the header is not set) and an unsupported
SLP SPI is included, the DA replies with a DAAdvert with the Error
Code set to an AUTHENTICATION_UNKNOWN error.

UAs SHOULD be configured with SLP SPIs that will allow them to verify
DA Advertisements. If the UA is configured with SLP SPIs and
receives a DAAdvert which fails to be verified using one of them, the
UA MUST discard it.

8.6. Service Agent Advertisement

User Agents MUST NOT solicit SA Advertisements if they have been
configured to use a particular DA, if they have been configured with
a or if DAs have been discovered. UAs solicit SA
Advertisements only when they are explicitly configured to use User
Selectable scopes (see Section 11.2) in order to discover the scopes
that SAs support. This allows UAs without scope configuration to
make use of either DAs or SAs without any functional difference

Guttman, et al. Standards Track [Page 25]

RFC 2608 Service Location Protocol, Version 2 June 1999

except performance.

A SA MAY be configured with attributes, and SHOULD support the
attribute 'service-type' whose value is all the service types of
services represented by the SA. SAs MUST NOT respond if the SrvRqst
predicate is not satisfied. For example, only SAs offering 'nfs'
services SHOULD respond with a SAAdvert to a SrvRqst for service type
"service:service-agent" which includes a predicate "(service-
type=nfs)".

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 = SAAdvert = 11) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of URL | URL \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| # auth blocks | authentication block (if any) \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The SA responds only to multicast SA discovery requests which either
include no or a scope which they are configured to use.

The SAAdvert MAY include a list of attributes the SA supports. This
attribute list SHOULD be kept short so that the SAAdvert will not
exceed the path MTU in size.

The URL is "service:service-agent://" of the SA, where
is the dotted decimal numeric address of the SA. The of
the SA MUST NOT be null.

The SAAdvert contains one SAAdvert Authentication block for each SLP
SPI the SA can produce Authentication Blocks for. If the UA can
verify the Authentication Block of the SAAdvert, and the SAAdvert
fails to be verified, the UA MUST discard it.

9. Optional Features

The features described in this section are not mandatory. Some are
useful for interactive use of SLP (where a user rather than a program
will select services, using a browsing interface for example) and for
scalability of SLP to larger networks.

Guttman, et al. Standards Track [Page 26]

RFC 2608 Service Location Protocol, Version 2 June 1999

9.1. Service Location Protocol Extensions

The format of a Service Location Extension 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension ID | Next Extension Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Offset, contd.| Extension Data \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Extension IDs are assigned in the following way:

0x0000-0x3FFF Standardized. Optional to implement. Ignore if
unrecognized.
0x4000-0x7FFF Standardized. Mandatory to implement. A UA or SA
which receives this option in a reply and does not understand
it MUST silently discard the reply. A DA or SA which receives
this option in a request and does not understand it MUST return
an OPTION_NOT_UNDERSTOOD error.
0x8000-0x8FFF For private use (not standardized). Optional to
implement. Ignore if unrecognized.
0x9000-0xFFFF Reserved.

The three byte offset to next extension indicates the position of the
next extension as offset from the beginning of the SLP message.

The offset value is 0 if there are no extensions following the
current extension.

If the offset is 0, the length of the current Extension Data is
determined by subtracting total length of the SLP message as given in
the SLP message header minus the offset of the current extension.

Extensions defined in this document are in Section D. See section 15
for procedures that are required when specifying new SLP extensions.

Guttman, et al. Standards Track [Page 27]

RFC 2608 Service Location Protocol, Version 2 June 1999

9.2. Authentication Blocks

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Structure Descriptor | Authentication Block Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SLP SPI String Length | SLP SPI String \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Structured Authentication Block ... \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authentication blocks are returned with certain SLP messages to
verify that the contents have not been modified, and have been
transmitted by an authorized agent. The authentication data
(contained in the Structured Authentication Block) is typically case
sensitive. Even though SLP registration data (e.g., attribute
values) are typically are not case sensitive, the case of the
registration data has to be preserved by the registering DA so that
UAs will be able to verify the data used for calculating digital
signature data.

The Block Structure Descriptor (BSD) identifies the format of the
Authenticator which follows. BSDs 0x0000-0x7FFF will be maintained
by IANA. BSDs 0x8000-0x8FFF are for private use.

The Authentication Block Length is the length of the entire block,
starting with the BSD.

The Timestamp is the time that the authenticator expires (to prevent
replay attacks.) The Timestamp is a 32-bit unsigned fixed-point
number of seconds relative to 0h on 1 January 1970. SAs use this
value to indicate when the validity of the digital signature expires.
This Timestamp will wrap back to 0 in the year 2106. Once the value
of the Timestamp wraps, the time at which the Timestamp is relative
to resets. For example, after 06h28 and 16 seconds 5 February 2106,
all Timestamp values will be relative to that epoch date.

The SLP Security Parameters Index (SPI) string identifies the key
length, algorithm parameters and keying material to be used by agents
to verify the signature data in the Structured Authentication Block.
The SLP SPI string has the same grammar as the defined in
Section 6.4.1.

Reserved characters in SLP SPI strings must be escaped using the same
convention as used throughout SLPv2.

Guttman, et al. Standards Track [Page 28]

RFC 2608 Service Location Protocol, Version 2 June 1999

SLP SPIs deployed in a site MUST be unique. An SLP SPI used for
BSD=0x0002 must not be the same as used for some other BSD.

All SLP agents MUST implement DSA [20] (BSD=0x0002). SAs MUST
register services with DSA authentication blocks, and they MAY
register them with other authentication blocks using other
algorithms. SAs MUST use DSA authentication blocks in SrvDeReg
messages and DAs MUST use DSA authentication blocks in unsolicited
DAAdverts.

9.2.1. SLP Message Authentication Rules

The sections below define how to calculate the value to apply to the
algorithm identified by the BSD value. The components listed are
used as if they were a contiguous single byte aligned buffer in the
order given.

URL
16-bit Length of SLP SPI String, SLP SPI String.
16-bit Length of URL, URL,
32-bit Timestamp.

Attribute List
16-bit Length of SLP SPI String, SLP SPI String,
16-bit length of , ,
32-bit Timestamp.

DAAdvert
16-bit Length of SLP SPI String, SLP SPI String,
32-bit DA Stateless Boot Timestamp,
16-bit Length of URL, URL,
16-bit Length of , ,
16-bit Length of DA's , DA's ,
16-bit Length of DA's , DA's ,
32-bit Timestamp.

The first SLP SPI is the SLP SPI in the Authentication
Block. This SLP SPI indicates the keying material and other
parameters to use to verify the DAAdvert. The SLP SPI List is
the list of SLP SPIs the DA itself supports, and is able to
verify.

SAAdvert
16-bit Length of SLP SPI String, SLP SPI String,
16-bit Length of URL, URL,
16-bit Length of , ,
16-bit Length of , ,
32-bit Timestamp.

Guttman, et al. Standards Track [Page 29]

RFC 2608 Service Location Protocol, Version 2 June 1999

9.2.2 DSA with SHA-1 in Authentication Blocks

BSD=0x0002 is defined to be DSA with SHA-1. The signature
calculation is defined by [20]. The signature format conforms to
that in the X.509 v3 certificate:

1. The signature algorithm identifier (an OID)
2. The signature value (an octet string)
3. The certificate path.

All data is represented in ASN.1 encoding:

id-dsa-with-sha1 ID ::= {
iso(1) member-body(2) us(840) x9-57 (10040)
x9cm(4) 3 }

i.e., the ASN.1 encoding of 1.2.840.10040.4.3 followed immediately
by:

Dss-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER }

i.e., the binary ASN.1 encoding of r and s computed using DSA and
SHA-1. This is followed by a certificate path, as defined by X.509
[10], [2], [3], [4], [5].

Authentication Blocks for BSD=0x0002 have the following format. In
the future, BSDs may be assigned which have different formats.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ASN.1 encoded DSA signature \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

9.3. Incremental Service Registration

Incremental registrations update attribute values for a previously
registered service. Incremental service registrations are useful
when only a single attribute has changed, for instance. In an
incremental registration, the FRESH flag in the SrvReg header is NOT
set.

The new registration's attributes replace the previous
registration's, but do not affect attributes which were included
previously and are not present in the update.

Guttman, et al. Standards Track [Page 30]

RFC 2608 Service Location Protocol, Version 2 June 1999

For example, suppose service:x://a.org has been registered with
attributes A=1, B=2, C=3. If an incremental registration comes for
service:x://a.org with attributes C=30, D=40, then the attributes for
the service after the update are A=1, B=2, C=30, D=40.

Incremental registrations MUST NOT be performed for services
registered with Authentication Blocks. These must be registered with
ALL attributes, with the FRESH flag in the SrvReg header set. DAs
which receive such registration messages return an
AUTHENTICATION_FAILED error.

If the FRESH flag is not set and the DA does not have a prior
registration for the service, the incremental registration fails with
error code INVALID_UPDATE.

The SA MUST use the same in an update message as was
used in the prior registration. If this is not done, the DA returns
a SCOPE_NOT_SUPPORTED error. In order to change the scope of a
service advertisement it MUST be deregistered first and reregistered
with a new .

The SA MUST use the same in an update message as was
used in a prior registration of the same URL. If this is not done,
the DA returns an INVALID_UPDATE error.

9.4. Tag Lists

Tag lists are used in SrvDeReg and AttrReq messages. The syntax of a
item is:

tag-filter = simple-tag / substring
simple-tag = 1*filt-char
substring = [initial] any [final]
initial = 1*filt-char
any = `*' *(filt-char `*')
final = 1*filt-char
filt-char = Any character excluding and (see
grammar in Section 5).

Wild card characters in a item match arbitrary sequences
of characters. For instance "*bob*" matches "some bob I know",
"bigbob", "bobby" and "bob".

Guttman, et al. Standards Track [Page 31]

RFC 2608 Service Location Protocol, Version 2 June 1999

10. Optional SLP Messages

The additional requests provide features for user interaction and for
efficient updating of service advertisements with dynamic attributes.

10.1. Service Type Request

The Service Type Request (SrvTypeRqst) allows a UA to discover all
types of service on a network. This is useful for general purpose
service browsers.

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 = 9) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of PRList | String \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of Naming Authority | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of | String \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The list and are interpreted as in Section 8.1.

The Naming Authority string, if present in the request, will limit
the reply to Service Type strings with the specified Naming
Authority. If the Naming Authority string is absent, the IANA
registered service types will be returned. If the length of the
Naming Authority is set to 0xFFFF, the Naming Authority string is
omitted and ALL Service Types are returned, regardless of Naming
Authority.

10.2. Service Type Reply

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 = 10) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | length of |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The service-type Strings (as described in Section 4.1) are provided
in , which is a .

Guttman, et al. Standards Track [Page 32]

RFC 2608 Service Location Protocol, Version 2 June 1999

If a service type has a Naming Authority other than IANA it MUST be
returned following the service type string and a `.' character.
Service types with the IANA Naming Authority do not include a Naming
Authority string.

10.3. Attribute Request

The Attribute Request (AttrRqst) allows a UA to discover attributes
of a given service (by supplying its URL) or for an entire service
type. The latter feature allows the UA to construct a query for an
available service by selecting desired features. The UA may request
that all attributes are returned, or only a subset of them.

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 = 6) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of PRList | String \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of URL | URL \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of | string \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of string | string \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of string | string \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The , and string are interpreted as in
Section 8.1.

The URL field can take two forms. It can simply be a Service Type
(see Section 4.1), such as "http" or "service:tftp". In this case,
all attributes and the full range of values for each attribute of all
services of the given Service Type is returned.

The URL field may alternatively be a full URL, such as
"service:printer:lpr://igore.wco.ftp.com:515/draft" or
"nfs://max.net/znoo". In this, only the registered attributes for
the specified URL are returned.

The field is a of attribute tags, as defined
in Section 9.4 which indicates the attributes to return in the
AttrRply. If is omitted, all attributes are returned.
MUST be omitted and a full URL MUST be included when
attributes when a SLP SPI List string is included, otherwise the DA
will reply with an AUTHENTICATION_FAILED error.

Guttman, et al. Standards Track [Page 33]

RFC 2608 Service Location Protocol, Version 2 June 1999

10.4. Attribute Reply

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 = 7) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | length of |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|# of AttrAuths | Attribute Authentication Block (if present) \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format of the and the Authentication Block is as
specified for SrvReg (see Section 9.2.1).

Attribute replies SHOULD be returned with the original case of the
string registration intact, as they are likely to be human readable.
In the case where the AttrRqst was by service type, all attributes
defined for the service type, and all their values are returned.

Although white space is folded for string matching, attribute tags
and values MUST be returned with their original white space
preserved.

Only one copy of each attribute tag or String value should be
returned, arbitrarily choosing one version (with respect to upper and
lower case and white space internal to the strings): Duplicate
attributes and values SHOULD be removed. An arbitrary version of the
string value and tag name is chosen for the merge. For example:
"(A=a a,b)" merged with "(a=A A,B)" may yield "(a=a a,B)".

10.5. Attribute Request/Reply Examples

Suppose that printer services have been registered as follows:

Registered Service:
URL = service:printer:lpr://igore.wco.ftp.com/draft
scope-list = Development
Lang. Tag = en
Attributes = (Name=Igore),(Description=For developers only),
(Protocol=LPR),(location-description=12th floor),
(Operator=James Dornan \3cdornan@monster\3e),
(media-size=na-letter),(resolution=res-600),x-OK

URL = service:printer:lpr://igore.wco.ftp.com/draft
scope-list = Development

Guttman, et al. Standards Track [Page 34]

RFC 2608 Service Location Protocol, Version 2 June 1999

Lang. Tag = de
Attributes = (Name=Igore),(Description=Nur fuer Entwickler),
(Protocol=LPR),(location-description=13te Etage),
(Operator=James Dornan \3cdornan@monster\3e),
(media-size=na-letter),(resolution=res-600),x-OK

URL = service:printer:http://not.wco.ftp.com/cgi-bin/pub-prn
scope-list = Development
Lang. Tag = en
Attributes = (Name=Not),(Description=Experimental IPP printer),
(Protocol=http),(location-description=QA bench),
(media-size=na-letter),(resolution=other),x-BUSY

Notice the first printer, "Igore" is registered in both English and
German. The `<' and `>' characters in the Operator attribute value
which are part of the Email address had to be escaped, as they are
reserved characters for values.

Attribute tags are not translated, though attribute values may be,
see [13].

The attribute Request:

URL = service:printer:lpr://igore.wco.ftp.com/draft
scope-list = Development
Lang. Tag = de
tag-list = resolution,loc*

receives the Attribute Reply:

(location-description=13te Etage),(resolution=res-600)

The attribute Request:

URL = service:printer
scope-list = Development
Lang. Tag = en
tag-list = x-*,resolution,protocol

receives an Attribute Reply containing:

(protocols=http,LPR),(resolution=res-600,other),x-OK,x-BUSY

The first request is by service instance and returns the requested
values, in German. The second request is by abstract service type
(see Section 4) and returns values from both "Igore" and "Not".

Guttman, et al. Standards Track [Page 35]

RFC 2608 Service Location Protocol, Version 2 June 1999

An attribute Authentication Block is returned if an authentication
block with the SLP SPI in the AttrRqst can be returned. Note that
the returned from a DA with an Authentication Block MUST
be identical to the registered by a SA, in order for the
authentication verification calculations to be possible.

A SA or DA only returns an Attribute Authentication Block if the
AttrRqst included a full URL in the request and no tag list.

If an SLP SPI is specified in a unicast request (the REQUEST MCAST
flag in the header is not set) and the SA or DA cannot return an
Authentication Block with that SLP SPI, an AUTHENTICATION_UNKNOWN
error is returned. The # of Attr Auths field is set to 0 if there no
Authentication Block is included, or 1 if an Authentication Block
follows.

10.6. Service Deregistration

A DA deletes a service registration when its Lifetime expires.
Services SHOULD be deregistered when they are no longer available,
rather than leaving the registrations to time out.

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 = SrvDeReg = 4) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| URL Entry \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The is a (see section 2.1).

The SA MUST retry if there is no response from the DA, see Section
12.3. The DA acknowledges a SrvDeReg with a SrvAck. Once the SA
receives an acknowledgment indicating success, the service and/or
attributes are no longer advertised by the DA. The DA deregisters the
service or service attributes from every scope specified in the
SrvDeReg which it was previously registered in.

The SA MUST deregister all services with the same scope list used to
register the service with a DA. If this is not done in the SrvDeReg
message, the DA returns a SCOPE_NOT_SUPPORTED error. The Lifetime
field in the URL Entry is ignored for the purposes of the SrvDeReg.

Guttman, et al. Standards Track [Page 36]

RFC 2608 Service Location Protocol, Version 2 June 1999

The is a of attribute tags to deregister as
defined in Section 9.4. If no is present, the SrvDeReg
deregisters the service in all languages it has been registered in.
If the is present, the SrvDeReg deregisters the attributes
whose tags are listed in the tag spec. Services registered with
Authentication Blocks MUST NOT include a in a SrvDeReg
message: A DA will respond with an AUTHENTICATION_FAILED error in
this case.

If the service to be deregistered was registered with an
authentication block or blocks, a URL authentication block for each
of the SLP SPIs registered must be included in the SrvDeReg.
Otherwise, the DA returns an AUTHENTICATION_ABSENT error. If the
message fails to be verified by the DA, an AUTHENTICATION_FAILED
error is returned by the DA.

11. Scopes

Scopes are sets of services. The primary use of Scopes is to provide
the ability to create administrative groupings of services. A set of
services may be assigned a scope by network administrators. A client
seeking services is configured to use one or more scopes. The user
will only discover those services which have been configured for him
or her to use. By configuring UAs and SAs with scopes,
administrators may provision services. Scopes strings are case
insensitive. The default SCOPE string is "DEFAULT".

Scopes are the primary means an administrator has to scale SLP
deployments to larger networks. When DAs with NON-DEFAULT scopes are
present on the network, further gains can be had by configuring UAs
and SAs to have a predefined non-default scope. These agents can
then perform DA discovery and make requests using their scope. This
will limit the number of replies.

11.1. Scope Rules

SLP messages which fail to contain a scope that the receiving Agent
is configured to use are dropped (if the request was multicast) or a
SCOPE_NOT_SUPPORTED error is returned (if the request was unicast).
Every SrvRqst (except for DA and SA discovery requests), SrvReg,
AttrRqst, SrvTypeRqst, DAAdvert, and SAAdvert message MUST include a
.

A UA MUST unicast its SLP messages to a DA which supports the desired
scope, in preference to multicasting a request to SAs. A UA MAY
multicast the request if no DA is available in the scope it is
configured to use.

Guttman, et al. Standards Track [Page 37]

RFC 2608 Service Location Protocol, Version 2 June 1999

11.2. Administrative and User Selectable Scopes

All requests and services are scoped. The two exceptions are
SrvRqsts for "service:directory-agent" and "service:service-agent".
These MAY have a zero-length when used to enable the
user to make scope selections. In this case UAs obtain their scope
list from DAAdverts (or if DAs are not available, from SAAdverts.)

Otherwise, if SAs and UAs are to use any scope other than the default
(i.e., "DEFAULT"), the UAs and SAs are configured with lists of
scopes to use by system administrators, perhaps automatically by way
of DHCP option 78 or 79 [21]. Such administrative scoping allows
services to be provisioned, so that users will only see services they
are intended to see.

User configurable scopes allow a user to discover any service, but
require them to do their own selection of scope. This is similar to
the way AppleTalk [12] and SMB [19] networking allow user selection
of AppleTalk Zone or workgroups.

Note that the two configuration choices are not compatible. One
model allows administrators control over service provision. The
other delegates this to users (who may not be prepared to do any
configuration of their system).

12. Directory Agents

DAs cache service location and attribute information. They exist to
enhance the performance and scalability of SLP. Multiple DAs provide
further scalability and robustness of operation, since they can each
store service information for the same SAs, in case one of the DAs
fails.

A DA provides a centralized store for service information. This is
useful in a network with several subnets or with many SLP Agents.
The DA address can be dynamically configured with UAs and SAs using
DHCP, or by using static configuration.

SAs configured to use DAs with DHCP or static configuration MUST
unicast a SrvRqst to the DA, when the SA is initialized. The SrvRqst
omits the scope list and sets the service type of the request to
"service:directory-agent". The DA will return a DAAdvert with its
attributes, SLP SPI list, and other parameters which are essential
for proper SA to DA communication.

Passive detection of DAs by SAs enables services to be advertised
consistently among DAs of the same scope. Advertisements expire if
not renewed, leaving only transient stale registrations in DAs, even

Guttman, et al. Standards Track [Page 38]

RFC 2608 Service Location Protocol, Version 2 June 1999

in the case of a failure of a SA.

A single DA can support many UAs. UAs send the same requests to DAs
that they would send to SAs and expect the same results. DAs reduce
the load on SAs, making simpler implementations of SAs possible.

UAs MUST be prepared for the possibility that the service information
they obtain from DAs is stale.

12.1. Directory Agent Rules

When DAs are present, each SA MUST register its services with DAs
that support one or more of its scope(s).

UAs MUST unicast requests directly to a DA (when scoping rules
allow), hence avoiding using the multicast convergence algorithm, to
obtain service information. This decreases network utilization and
increases the speed at which UAs can obtain service information.

DAs MUST flush service advertisements once their lifetime expires or
their URL Authentication Block "Timestamp" of expiration is past.

DAAdverts MUST include DA Stateless Boot Timestamp, in the same
format as the Authentication Block (see Section 9.2). The Timestamp
in the Authentication Block indicates the time at which all previous
registrations were lost (i.e., the last stateless reboot). The
Timestamp is set to 0 in a DAAdvert to notify UAs and SAs that the DA
is going down. DAs MUST NOT use equal or lesser Boot Timestamps to
previous ones, if they go down and restart without service
registration state. This would mislead SAs to not reregister with
the DA.

DAs which receive a multicast SrvRqst for the service type
"service:directory-agent" MUST silently discard it if the is (a) not omitted and (b) does not include a scope they are
configured to use. Otherwise the DA MUST respond with a DAAdvert.

DAs MUST respond to AttrRqst and SrvTypeRqst messages (these are
OPTIONAL only for SAs, not DAs.)

12.2. Directory Agent Discovery

UAs can discover DAs using static configuration, DHCP options 78 and
79, or by multicasting (or broadcasting) Service Requests using the
convergence algorithm in Section 6.3.

Guttman, et al. Standards Track [Page 39]

RFC 2608 Service Location Protocol, Version 2 June 1999

See Section 6 regarding unsolicited DAAdverts. Section 12.2.2
describes how SAs may reduce the number of times they must reregister
with DAs in response to unsolicited DAAdverts.

DAs MUST send unsolicited DAAdverts once per CONFIG_DA_BEAT. An
unsolicited DAAdvert has an XID of 0. SAs MUST listen for DAAdverts,
passively, as described in Section 8.5. UAs MAY do this. If they do
not listen for unsolicited DAAdverts, however, they will not discover
DAs as they become available. UAs SHOULD, in this case, do periodic
active DA discovery, see Section 6.

A URL with the scheme "service:directory-agent" indicates the DA's
location as defined in Section 8.5. For example:
"service:directory-agent://foobawooba.org".

The following sections suggest timing algorithms which enhance the
scalability of SLP.

12.2.1. Active DA Discovery

After a UA or SA restarts, its initial DA discovery request SHOULD be
delayed for some random time uniformly distributed from 0 to
CONFIG_START_WAIT seconds.

The UA or SA sends the DA Discovery request using a SrvRqst, as
described in Section 8.1. DA Discovery requests MUST include a
Previous Responder List. SrvRqsts for Active DA Discovery SHOULD NOT
be sent more than once per CONFIG_DA_FIND seconds.

After discovering a new DA, a SA MUST wait a random time between 0
and CONFIG_REG_ACTIVE seconds before registering their services.

12.2.2. Passive DA Advertising

A DA MUST multicast (or broadcast) an unsolicited DAAdvert every
CONFIG_DA_BEAT seconds. CONFIG_DA_BEAT SHOULD be specified to
prevent DAAdverts from using more than 1% of the available bandwidth.

All UAs and SAs which receive the unsolicited DAAdvert SHOULD examine
its DA stateless Boot Timestamp. If it is set to 0, the DA is going
down and no further messages should be sent to it.

If a SA detects a DA it has never encountered (with a nonzero
timestamp,) the SA must register with it. SAs MUST examine the
DAAdvert's timestamp to determine if the DA has had a stateless
reboot since the SA last registered with it. If so it registers with
the DA. SAs MUST wait a random interval between 0 and
CONFIG_REG_PASSIVE before beginning DA registration.

Guttman, et al. Standards Track [Page 40]

RFC 2608 Service Location Protocol, Version 2 June 1999

12.3. Reliable Unicast to DAs and SAs

If a DA or SA fails to respond to a unicast UDP message in
CONFIG_RETRY seconds, the message should be retried. The wait
interval for each subsequent retransmission MUST exponentially
increase, doubling each time. If a DA or SA fails to respond after
CONFIG_RETRY_MAX seconds, the sender should consider the receiver to
have gone down. The UA should use a different DA. If no such DA
responds, DA discovery should be used to find a new DA. If no DA is
available, multicast requests to SAs are used.

12.4. DA Scope Configuration

By default, DAs are configured with the "DEFAULT" scope.
Administrators may add other configured scopes, in order to support
UAs and SAs in non default scopes. The default configuration MUST
NOT be removed from the DA unless:

- There are other DAs which support the "DEFAULT" scope, or

- All UAs and SAs have been configured with non-default scopes.

Non-default scopes can be phased-in as the SLP deployment grows.
Default scopes should be phased out only when the non-default scopes
are universally configured.

If a DA and SA are coresident on a host (quite possibly implemented
by the same process), configuration of the host is considerably
simplified if the SA supports only scopes also supported by the DA.
That is, the SA SHOULD NOT advertise services in any scopes which are
not supported by the coresident DA. This means that incoming requests
can be answered by a single data store; the SA and DA registrations
do not need to be kept separately.

12.5. DAs and Authentication Blocks

DAs are not configured to sign service registrations or attribute
lists. They simply cache services registered by Service Agents. DAs
MUST NOT accept registrations including authentication blocks for SLP
SPIs which it is not configured with, see Section 8.5.

A DA protects registrations which are made with authentication blocks
using SLP SPIs it is configured to use. If a service S is
registered, a subsequent registration (which will replace the
adertisement) or a deregistration (which will remove it) MUST include
an Authentication Block with the corresponding SLP SPI, see Section
8.3 and Section 10.6.

Guttman, et al. Standards Track [Page 41]

RFC 2608 Service Location Protocol, Version 2 June 1999

Example:

A DA is configured to be able to verify Authentication Blocks with
SLP SPIs "X,Y", that is X and Y.

An SA registers a service with an Authentication Block with SPI "Z".
The DA stores the registration, but discards the Authentication
Block. If a UA requests a service with an SLP SPI string "Z", the DA
will respond with an AUTHENTICATION_UNKNOWN error.

An SA registers a service S with Authentication Blocks including SLP
SPIs "X" and "Y". If a UA requests a service with an SLP SPI string
"X" the DA will be able to return S (if the service type, language,
scope and predicate of the SrvRqst match S) The DA will also return
the Authentication Block with SLP SPI set to "X". If the DA receives
a subsequent SrvDeReg for S (which will remove the advertisement) or
a subsequent SrvReg for S (which will replace it), the message must
include two URL Authentication Blocks, one each for SPIs "X" and "Y".
If either of these were absent, the DA would return an
AUTHENTICATION_ABSENT error.

13. Protocol Timing Defaults

Interval name Section Default Value Meaning
------------------- ------- ------------- ------------------------
CONFIG_MC_MAX 6.3 15 seconds Max time to wait for a
complete multicast query
response (all values.)
CONFIG_START_WAIT 12.2.1 3 seconds Wait to perform DA
discovery on reboot.
CONFIG_RETRY 12.3 2 seconds Wait interval before
initial retransmission
of multicast or unicast
requests.
CONFIG_RETRY_MAX 12.3 15 seconds Give up on unicast
request retransmission.
CONFIG_DA_BEAT 12.2.2 3 hours DA Heartbeat, so that SAs
passively detect new DAs.
CONFIG_DA_FIND 12.3 900 seconds Minimum interval to wait
before repeating Active
DA discovery.
CONFIG_REG_PASSIVE 12.2 1-3 seconds Wait to register services
on passive DA discovery.
CONFIG_REG_ACTIVE 8.3 1-3 seconds Wait to register services
on active DA discovery.
CONFIG_CLOSE_CONN 6.2 5 minutes DAs and SAs close idle
connections.

Guttman, et al. Standards Track [Page 42]

RFC 2608 Service Location Protocol, Version 2 June 1999

14. Optional Configuration

Broadcast Only
Any SLP agent SHOULD be configurable to use broadcast
only. See Sections 6.1 and 12.2.

Predefined DA
A UA or SA SHOULD be configurable to use a predefined DA.

No DA Discovery
The UA or SA SHOULD be configurable to ONLY use
predefined and DHCP-configured DAs and perform no active
or passive DA discovery.

Multicast TTL
The default multicast TTL is 255. Agents SHOULD be
configurable to use other values. A lower value will
focus the multicast convergence algorithm on smaller
subnetworks, decreasing the number of responses and
increases the performance of service location. This
may result in UAs obtaining different results for the
identical requests depending on where they are connected
to the network.

Timing Values
Time values other than the default MAY be configurable.
See Section 13.

Scopes
A UA MAY be configurable to support User Selectable
scopes by omitting all predefined scopes. See
Section 11.2. A UA or SA MUST be configurable to use
specific scopes by default. Additionally, a UA or SA
MUST be configurable to use specific scopes for requests
for and registrations of specific service types. The
scope or scopes of a DA MUST be configurable. The
default value for a DA is to have the scope "DEFAULT" if
not otherwise configured.

DHCP Configuration
DHCP options 78 and 79 may be used to configure SLP. If
DA locations are configured using DHCP, these SHOULD
be used in preference to DAs discovered actively or
passively. One or more of the scopes configured using
DHCP MUST be used in requests. The entire configured
MUST be used in registration and DA
configuration messages.

Guttman, et al. Standards Track [Page 43]

RFC 2608 Service Location Protocol, Version 2 June 1999

Service Template
UAs and SAs MAY be configured by using Service Templates.
Besides simplifying the specification of attribute
values, this also allows them to enforce the inclusion
of 'required' attributes in SrvRqst, SrvReg and SrvDeReg
messages. DAs MAY be configured with templates to
allow them to WARN UAs and SAs in these cases. See
Section 10.4.

SLP SPI for service discovery
Agents SHOULD be configurable to support SLP SPIs using
the following parameters: BSD=2 (DSA with SHA-1) and
a public key identified by the SLP SPI String. In
the future, when a Public Key Infrastructure exists,
SLP Agents may be able to obtain public keys and
cryptographic parameters corresponding to the names used
in SLP SPI Strings.

Note that if the SLP SPI string chosen is identical
to a scope string, it is effectively the same as a
Protected Scope in SLPv1. Namely, every SA advertising
in that scope would be configured with the same Private
Key. Every DA and UA of that scope would be configured
with the appropriate Public Key to verify signatures
produced by those SAs. This is a convenient way to
configure SLP deployments in the absence of a Public Key
Infrastructure. Currently, it would be too difficult to
manage the keying of UAs and DAs if each SA had its own
key.

SLP SPI for Directory Agent discovery
Agents SHOULD be configurable to support SLP SPIs as
above, to be used when discovering DAs. This SPI SHOULD
be sent in SrvRqsts to discover DAs and be used to verify
multicast DAAdvert messages.

SA and DA Private Key
SAs and DAs which can generate digital signatures require
a Private Key and a corresponding SLP SPI indentifier
to include in the Authentication Block. The SLP SPI
identifies the Public Key to use to verify the digital
signature in the Authentication Block.

15. IANA Considerations

SLP includes four sets of identifiers which may be registered with
IANA. The policies for these registrations (See [18]) are noted in
each case.

Guttman, et al. Standards Track [Page 44]

RFC 2608 Service Location Protocol, Version 2 June 1999

The Block Structure Descriptor (BSD) identifies the format of the
Authenticator which follows. BSDs 0x8000-0x8FFF are for Private Use.

Further Block Structured Descriptor (BSD) values, from the range
0x0003-0x7FFF may be standardized in the future by submitting a
document which describes:

- The data format of the Structured Authenticator block.

- Which cryptographic algorithm to use (including a reference
to a technical specification of the algorithm.)

- The format of any keying material required for
preconfiguring UAs, DAs and SAs. Also include any
considerations regarding key distribution.

- Security considerations to alert others to the strengths and
weaknesses of the approach.

The IANA will assign Cryptographic BSD numbers on the basis of IETF
Consenus.

New function-IDs, in the range 12-255, may be standardized by the
method of IETF Consensus.

New SLP Extensions with types in the range 2-65535 may be registered
following review by a Designated Expert.

New error numbers in the range 15-65535 are assigned on the basis of
a Standards Action.

Protocol elements used with Service Location Protocol may also
require IANA registration actions. SLP is used in conjunction with
"service:" URLs and Service Templates [13]. These are standardized
by review of a Designated Expert and a mailing list (See [13].)

16. Internationalization Considerations

SLP messages support the use of multiple languages by providing a
Language Tag field in the common message header (see Section 8).

Services MAY be registered in multiple languages. This provides
attributes so that users with different language skills may select
services interactively.

Attribute tags are not translated. Attribute values may be
translated unless the Service Template [13] defines the attribute
values to be 'literal'.

Guttman, et al. Standards Track [Page 45]

RFC 2608 Service Location Protocol, Version 2 June 1999

A service which is registered in multiple languages may be queried in
multiple languages. The language of the SrvRqst or AttrRqst is used
to satisfy the request. If the requested language is not supported,
a LANGUAGE_NOT_SUPPORTED error is returned. SrvRply and AttrRply
messages are always in the same language of the request.

A DA or SA MAY be configured with translations of Service Templates
[13] for the same service type. This will allow the DA or SA to
translate a request (say in Italian) to the language of the service
advertisement (say in English) and then translate the reply back to
Italian. Similarly, a UA MAY use templates to translate outgoing
requests and incoming replies.

The dialect field in the Language Tag MAY be used: Requests which
can be fulfilled by matching a language and dialect will be preferred
to those which match only the language portion. Otherwise, dialects
have no effect on matching requests.

17. Security Considerations

SLP provides for authentication of service URLs and service
attributes. This provides UAs and DAs with knowledge of the
integrity of service URLs and attributes included in SLP messages.
The only systems which can generate digital signatures are those
which have been configured by administrators in advance. Agents
which verify signed data may assume it is 'trustworthy' inasmuch as
administrators have ensured the cryptographic keying of SAs and DAs
reflects 'trustworthiness.'

Service Location does not provide confidentiality. Because the
objective of this protocol is to advertise services to a community of
users, confidentiality might not generally be needed when this
protocol is used in non-sensitive environments. Specialized schemes
might be able to provide confidentiality, if needed in the future.
Sites requiring confidentiality should implement the IP Encapsulating
Security Payload (ESP) [3] to provide confidentiality for Service
Location messages.

If Agents are not configured to generate Authentication Blocks and
Agents are not configured to verify them, an adversary might easily
use this protocol to advertise services on servers controlled by the
adversary and thereby gain access to users' private information.
Further, an adversary using this protocol will find it much easier to
engage in selective denial of service attacks. Sites that are in
potentially hostile environments (e.g., are directly connected to the
Internet) should consider the advantages of distributing keys
associated with SLP SPIs prior to deploying the sensitive directory
agents or service agents.

Guttman, et al. Standards Track [Page 46]

RFC 2608 Service Location Protocol, Version 2 June 1999

SLP is useful as a bootstrap protocol. It may be used in
environments in which no preconfiguration is possible. In such
situations, a certain amount of "blind faith" is required: Without
any prior configuration it is impossible to use any of the security
mechanisms described above. SLP will make use of the mechanisms
provided by the Security Area of the IETF for key distribution as
they become available. At this point it would only be possible to
gain the benefits associated with the use of Authentication Blocks if
cryptographic information and SLP SPIs can be preconfigured with the
end systems before they use SLP.

SLPv2 enables a number of security policies with the mechanisms it
includes. A SLPv2 UA could, for instance, reject any SLP message
which did not carry an authentication block which it could verify.
This is not the only policy which is possible to implement.

Guttman, et al. Standards Track [Page 47]

RFC 2608 Service Location Protocol, Version 2 June 1999

A. Appendix: Changes to the Service Location Protocol from v1 to v2

SLP version 2 (SLPv2) corrects race conditions present in SLPv1 [22].
In addition, authentication has been reworked to provide more
flexibility and protection (especially for DA Advertisements). SLPv2
also changes the formats and definition of many flags and values and
reduces the number of 'required features.' SLPv2 clarifies and
changes the use of 'Scopes', eliminating support for 'unscoped
directory agents' and 'unscoped requests'. SLPv2 uses LDAPv3
compatible string encodings of attributes and search filters. Other
changes (such as Language and Character set handling) adopt practices
recommended by the Internet Engineering Steering Group.

Effort has been made to make SLPv2 operate the same whether DAs are
present or not. For this reason, a new message (the SAAdvert) has
been added. This allows UAs to discover scope information in the
absence of administrative configuration and DAs. This was not
possible in SLPv1.

SLPv2 is incompatible in some respects with SLPv1. If a DA which
supports both SLPv1 and SLPv2 with the same scope is present,
services advertised by SAs using either version of the protocol will
be available to both SLPv1 and SLPv2 UAs. SLPv1 DAs SHOULD be phased
out and replace with SLPv2 DAs which support both versions of the
protocol.

SLPv1 allows services to be advertised and requested without a scope.
Further, DAs can be configured without a scope. This is incompatible
with SLPv2 and presents scalability problems. To facilitate this
forward migration, SLPv1 agents MUST use scopes for all registrations
and requests. SLPv1 DAs MUST be configured with a scope list. This
constitutes a revision of RFC 2165 [22].

B. Appendix: Service Discovery by Type: Minimal SLPv2 Features

Service Agents may advertise services without attributes. This will
enable only discovery of services by type. Service types discovered
this way will have a Service Template [13] defined which specifies
explicitly that no attributes are associated with the service
advertisement. Service types associated with Service Templates which
specify attributes MUST NOT be advertised by SAs which do not support
attributes.

While discovery of service by service type is a subset of the
features possible using SLPv2 this form of discovery is consistent
with the current generation of products that allow simple browsing of
all services in a 'zone' or 'workgroup' by type. In some cases,
attribute discovery, security and feature negotiation is handled by

Guttman, et al. Standards Track [Page 48]

RFC 2608 Service Location Protocol, Version 2 June 1999

application layer protocols - all that is required is the basic
discovery of services that support a certain service.

UAs requesting only service of that service type would only need to
support service type and scope fields of the Service Request. UAs
would still perform DA discovery and unicast SLPv2 SrvRqst messages
to DAs in their scope once they were discovered instead of
multicasting them.

SAs would also perform DA discovery and use a SLPv2 SrvReg to
register all their advertised services with SLPv2 DAs in their scope.
These advertisements would needless to say contain no attribute
string.

These minimal SAs could ignore the Language Tag in requests since
SrvRqst messages would contain no attributes, hence no strings would
be internationalized. Further, any non-null predicate string would
fail to match a service advertisement with no attributes, so these
SAs would not have to parse and interpret search filters. Overflow
will never occur in SrvRqst, SrvRply or SrvReg messages so TCP
message handling would not have to be implemented. Finally, all
AttrRqst messages could be dropped by the SA, since no attributes are
supported.

C. Appendix: DAAdverts with arbitrary URLs

Using Active DA Discovery, a SrvRqst with its service type field set
to "service:directory-agent". DAs will respond with a DAAdvert
containing a URL with the "service:directory-agent:" scheme. This is
the same DAAdvert that such a DA would multicast in unsolicited DA
advertisements.

A UA or SA which receives an unsolicited DAAdvert MUST examine the
URL to determine if it has a recognized scheme. If the UA or SA does
not recognize the DAAdvert's URL scheme, the DAAdvert is silently
discarded. This document specifies only how to use URLs with the
"service:directory-agent:" scheme.

This provides the possibility for forward compatibility with future
versions of SLP and enables other services to advertise their ability
to serve as a clearinghouse for service location information.

For example, if LDAPv3 [15] is used for service registration and
discovery by a set of end systems, they could interpret a LDAP URL
[16] to passively discover the LDAP server to use for this purpose.
This document does not specify how this is done: SLPv2 agents
without further support would simply discard this DAAdvert.

Guttman, et al. Standards Track [Page 49]

RFC 2608 Service Location Protocol, Version 2 June 1999

D. Appendix: SLP Protocol Extensions

D.1. Required Attribute Missing Option

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension Type = 0x0001 | Extension Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template IDVer Length | Template IDVer String \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Required Attr Length| Required Attr \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Required attributes and the format of the IDVer string are defined by
[13].

If a SA or DA receives a SrvRqst or a SrvReg which fails to include a
Required Attribute for the requested Service Type (according to the
Service Template), it MAY return the Required Attribute Extension in
addition to the reply corresponding to the message. The sender
SHOULD reissue the message with a search filter including the
attributes listed in the returned Required Attribute Extension.
Similarly, the Required Attribute Extension may be returned in
response to a SrvDereg message that contains a required attribute
tag.

The Template IDVer String is the name and version number string of
the Service Template which defines the given attribute as required.
It SHOULD be included, but can be omitted if a given SA or DA has
been individually configured to have 'required attributes.'

The Required Attribute MUST NOT include wild cards.

E. Acknowledgments

This document incorporates ideas from work on several discovery
protocols, including RDP by Perkins and Harjono, and PDS by Michael
Day. We are grateful for contributions by Ye Gu and Peter Ford.
John Veizades was instrumental in the standardization of the Service
Location Protocol. Implementors at Novell, Axis Communications and
Sun Microsystems have contributed significantly to make this a much
clearer and more consistent document.

Guttman, et al. Standards Track [Page 50]

RFC 2608 Service Location Protocol, Version 2 June 1999

F. References

[1] Port numbers, July 1997.
ftp://ftp.isi.edu/in-notes/iana/assignments/port-numbers.

[2] ISO/IEC JTC1/SC 21. Certificate Extensions. Draft Amendment
DAM 4 to ISO/IEC 9594-2, December 1996.

[3] ISO/IEC JTC1/SC 21. Certificate Extensions. Draft Amendment
DAM 2 to ISO/IEC 9594-6, December 1996.

[4] ISO/IEC JTC1/SC 21. Certificate Extensions. Draft Amendment
DAM 1 to ISO/IEC 9594-7, December 1996.

[5] ISO/IEC JTC1/SC 21. Certificate Extensions. Draft Amendment
DAM 1 to ISO/IEC 9594-8, December 1996.

[6] Unicode Technical Report #8. The Unicode Standard, version 2.1.
Technical report, The Unicode Consortium, 1998.

[7] Alvestrand, H., "Tags for the Identification of Languages",
RFC 1766, March 1995.

[8] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.

[9] Bradner, S., "Key Words for Use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[10] CCITT. The Directory Authentication Framework. Recommendation
X.509, 1988.

[11] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.

[12] S. Gursharan, R. Andrews, and A. Oppenheimer. Inside AppleTalk.
Addison-Wesley, 1990.

[13] Guttman, E., Perkins, C. and J. Kempf, "Service Templates and
service: Schemes", RFC 2609, June 1999.

[14] Howes, T., "The String Representation of LDAP Search Filters",
RFC 2254, December 1997.

[15] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory
Access Protocol (v3)", RFC 2251, December 1997.

Guttman, et al. Standards Track [Page 51]

RFC 2608 Service Location Protocol, Version 2 June 1999

[16] Howes, T. and M. Smith, "The LDAP URL Format", RFC 2255,
December 1997.

[17] Meyer, D., "Administratively Scoped IP Multicast", RFC 2365,
July 1998.

[18] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs, BCP 26, RFC 2434,
October 1998.

[19] Microsoft Networks. SMB File Sharing Protocol Extensions 3.0,
Document Version 1.09, November 1989.

[20] National Institute of Standards and Technology. Digital
signature standard. Technical Report NIST FIPS PUB 186, U.S.
Department of Commerce, May 1994.

[21] Perkins, C. and E. Guttman, "DHCP Options for Service Location
Protocol", RFC 2610, June 1999.

[22] Veizades, J., Guttman, E., Perkins, C. and S. Kaplan, "Service
Location Protocol", RFC 2165, July 1997.

[23] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
RFC 2279, January 1998.

Guttman, et al. Standards Track [Page 52]

RFC 2608 Service Location Protocol, Version 2 June 1999

G. Authors' Addresses

Erik Guttman
Sun Microsystems
Bahnstr. 2
74915 Waibstadt
Germany

Phone: +49 7263 911 701
EMail: Erik.Guttman@sun.com

Charles Perkins
Sun Microsystems
901 San Antonio Road
Palo Alto, CA 94040
USA

Phone: +1 650 786 6464
EMail: cperkins@sun.com

John Veizades
@Home Network
425 Broadway
Redwood City, CA 94043
USA

Phone: +1 650 569 5243
EMail: veizades@home.net

Michael Day
Vinca Corporation.
1201 North 800 East
Orem, Utah 84097 USA

Phone: +1 801 376-5083
EMail: mday@vinca.com

Guttman, et al. Standards Track [Page 53]

RFC 2608 Service Location Protocol, Version 2 June 1999

H. Full Copyright Statement

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."

Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

Guttman, et al. Standards Track [Page 54]