RFC 2614
An API for Service Location
Pages: 91
Informational
Informational
Part 1 of 3 – Pages 1 to 25
Network Working Group J. Kempf Request for Comments: 2614 E. Guttman Category: Informational Sun Microsystems June 1999 An API for Service Location Status of This Memo This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.Abstract
The Service Location Protocol (SLP) provides a new way for clients to dynamically discovery network services. With SLP, it is simple to offer highly available services that require no user configuration or assistance from network administrators prior to use. This document describes standardized APIs for SLP in C and Java. The APIs are modular and are designed to allow implementations to offer just the feature set needed. In addition, standardized file formats for configuration and serialized registrations are defined, allowing SLP agents to set network and other parameters in a portable way. The serialized file format allows legacy services to be registered with SLP directory agents in cases where modifying the legacy service program code is difficult or impossible, and to portably exchange a registration database.Table of Contents
1. Introduction 4 1.1. Goals . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . 4 2. File Formats 7 2.1. Configuration File Format . . . . . . . . . . . . . . 8 2.1.1. DA configuration . . . . . . . . . . . . . . 9 2.1.2. Static Scope Configuration . . . . . . . . . . 9 2.1.3. Tracing and Logging . . . . . . . . . . . . . 11 2.1.4. Serialized Proxy Registrations . . . . . . . . 11 2.1.5. Network Configuration Properties . . . . . . . 12 2.1.6. SA Configuration . . . . . . . . . . . . . . . 14 2.1.7. UA Configuration . . . . . . . . . . . . . . . 14 2.1.8. Security . . . . . . . . . . . . . . . . . . 15 2.2. Multihomed Machines. . . . . . . . . . . . . . . . . . 16 2.3. Serialized Registration File . . . . . . . . . . . . . 16
2.4. Processing Serialized Registration and Configuration
Files . . . . . . . . . . . . . . . . . . . . . . . . 18
3. Binding Independent Implementation Considerations 18
3.1. Multithreading . . . . . . . . . . . . . . . . . . . . 18
3.2. Asynchronous and Incremental . . . . . . . . . . . . . 19
3.3. Type Checking for Service Types. . . . . . . . . . . . 19
3.4. Refreshing Registrations . . . . . . . . . . . . . . . 19
3.5. Configuration File Processing . . . . . . . . . . . . 19
3.6. Attribute Types . . . . . . . . . . . . . . . . . . . 20
3.7. Removal of Duplicates . . . . . . . . . . . . . . . . 20
3.8. Character Set Encoding . . . . . . . . . . . . . . . . 20
3.9. Error Semantics . . . . . . . . . . . . . . . . . . . 20
3.10. Modular Implementations . . . . . . . . . . . . . . . 24
3.11. Handling Special Service Types . . . . . . . . . . . . 24
3.12. Scope Discovery and Handling . . . . . . . . . . . . . 24
4. C Language Binding 25
4.1. Constant Types . . . . . . . . . . . . . . . . . . . . 26
4.1.1. URL Lifetimes. . . . . . . . . . . . . . . . . 26
4.1.2. Error Codes. . . . . . . . . . . . . . . . . . 26
4.1.3. SLPBoolean . . . . . . . . . . . . . . . . . . 27
4.2. Struct Types . . . . . . . . . . . . . . . . . . . . 28
4.2.1. SLPSrvURL . . . . . . . . . . . . . . . . . . 28
4.2.2. SLPHandle . . . . . . . . . . . . . . . . . . 29
4.3. Callbacks . . . . . . . . . . . . . . . . . . . . . . 29
4.3.1. SLPRegReport . . . . . . . . . . . . . . . . 30
4.3.2. SLPSrvTypeCallback . . . . . . . . . . . . . . 30
4.3.3. SLPSrvURLCallback . . . . . . . . . . . . . . 31
4.3.4. SLPAttrCallback . . . . . . . . . . . . . . . 33
4.4. Opening and Closing an SLPHandle . . . . . . . . . . . 34
4.4.1. SLPOpen. . . . . . . . . . . . . . . . . . . . 34
4.4.2. SLPClose . . . . . . . . . . . . . . . . . . . 35
4.5. Protocol API . . . . . . . . . . . . . . . . . . . . 36
4.5.1. SLPReg . . . . . . . . . . . . . . . . . . . . 36
4.5.2. SLPDereg . . . . . . . . . . . . . . . . . . . 37
4.5.3. SLPDelAttrs . . . . . . . . . . . . . . . . . 38
4.5.4. SLPFindSrvTypes. . . . . . . . . . . . . . . . 39
4.5.5. SLPFindSrvs . . . . . . . . . . . . . . . . . 41
4.5.6. SLPFindAttrs . . . . . . . . . . . . . . . . . 42
4.6. Miscellaneous Functions . . . . . . . . . . . . . . . 43
4.6.1. SLPGetRefreshInterval . . . . . . . . . . . . 44
4.6.2. SLPFindScopes . . . . . . . . . . . . . . . . 44
4.6.3. SLPParseSrvURL . . . . . . . . . . . . . . . . 45
4.6.4. SLPEscape . . . . . . . . . . . . . . . . . . 46
4.6.5. SLPUnescape . . . . . . . . . . . . . . . . . 47
4.6.6. SLPFree . . . . . . . . . . . . . . . . . . . 48
4.6.7. SLPGetProperty . . . . . . . . . . . . . . . . 48
4.6.8. SLPSetProperty . . . . . . . . . . . . . . . . 49
4.7. Implementation Notes . . . . . . . . . . . . . . . . 49
4.7.1. Refreshing Registrations . . . . . . . . . . . 49
4.7.2. Syntax for String Parameters . . . . . . . . . 49
4.7.3. Client Side Syntax Checking . . . . . . . . . 50
4.7.4. System Properties . . . . . . . . . . . . . . 50
4.7.5. Memory Management . . . . . . . . . . . . . . 51
4.7.6. Asynchronous and Incremental Return Semantics. 51
4.8. Example. . . . . . . . . . . . . . . . . . . . . . . . 52
5. Java Language Binding 56
5.1. Introduction . . . . . . . . . . . . . . . . . . . . . 56
5.2. Exceptions and Errors . . . . . . . . . . . . . . . . 56
5.2.1. Class ServiceLocationException . . . . . . . . 57
5.3. Basic Data Structures . . . . . . . . . . . . . . . . 58
5.3.1. Interface ServiceLocationEnumeration . . . . . 58
5.3.2. Class ServiceLocationAttribute . . . . . . . 58
5.3.3. Class ServiceType . . . . . . . . . . . . . . 61
5.3.4. Class ServiceURL . . . . . . . . . . . . . . 63
5.4. SLP Access Interfaces . . . . . . . . . . . . . . . . 67
5.4.1. Interface Advertiser . . . . . . . . . . . . . 67
5.4.2. Interface Locator . . . . . . . . . . . . . . 69
5.5. The Service Location Manager . . . . . . . . . . . . . 72
5.5.1. Class ServiceLocationManager . . . . . . . . . 72
5.6. Service Template Introspection . . . . . . . . . . . . 74
5.6.1. Abstract Class TemplateRegistry . . . . . . . 74
5.6.2. Interface ServiceLocationAttributeVerifier . . 77
5.6.3. Interface ServiceLocationAttributeDescriptor . 79
5.7. Implementation Notes . . . . . . . . . . . . . . . . . 81
5.7.1. Refreshing Registrations . . . . . . . . . . . 81
5.7.2. Parsing Alternate Transports in ServiceURL . . 81
5.7.3. String Attribute Values . . . . . . . . . . . 82
5.7.4. Client Side Syntax Checking. . . . . . . . . . 82
5.7.5. Language Locale Handling . . . . . . . . . . . 82
5.7.6. Setting SLP System Properties. . . . . . . . . 83
5.7.7. Multithreading . . . . . . . . . . . . . . . . 83
5.7.8. Modular Implementations . . . . . . . . . . . 83
5.7.9. Asynchronous and Incremental Return Semantics. 84
5.8. Example. . . . . . . . . . . . . . . . . . . . . . . . 85
6. Internationalization Considerations 87
6.1. service URL. . . . . . . . . . . . . . . . . . . . . . 87
6.2. Character Set Encoding . . . . . . . . . . . . . . . . 87
6.3. Language Tagging . . . . . . . . . . . . . . . . . . 88
7. Security Considerations 88
8. Acknowledgements 88
9. References 89
10. Authors' Addresses 90
11. Full Copyright Statement 91
1. Introduction
The Service Location API is designed for standardized access to the Service Location Protocol (SLP). The APIs allow client and service programs to be be written or modified in a very simple manner to provide dynamic service discovery and selection. Bindings in the C and Java languages are defined in this document. In addition, standardized formats for configuration files and for serialized registration files are presented. These files allow SLP agents to configure network parameters, to register legacy services that have not been SLP enabled, and to portably exchange registration databases.1.1. Goals
The overall goal of the API is to enable source portability of applications that use the API between different implementations of SLP. The result should facilitate the adoption of SLP, and conversion of clients and service programs to SLP. The goals of the C binding are to create a minimal but complete access to the functionality of the SLP protocol, allowing for simple memory management and limited code size. The Java API provides for modular implementations (where unneeded features can be omitted) and an object oriented interface to the complete set of SLP data and functionality. The standardized configuration file and serialized file formats provide a simple syntax with complete functional coverage of the protocol, but without system dependent properties and secure information.1.2. Terminology
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 [1]. Service Location Protocol (SLP) The underlying protocol allowing dynamic and scalable service discovery. This protocol is specified in the Service Location Protocol Version 2 [7].
SLP framework
When a 'Service Location framework' is mentioned, it refers to
both the SLP implementation and interface implementation; i.e.
whatever provides the SLP functionality to user level programs.
This includes remote agents.
Directory Agent (DA)
A service that automatically gathers service advertisements
from SAs in order to provide them to UAs.
User Agent (UA)
This is the Service Location process or library that allows SLP
requests to be made on behalf of a client process. UAs
automatically direct requests to DAs when they exist. In their
absence, UAs make requests to SAs.
Service Agent (SA)
This is the Service Location process or library that allows
service software to register and deregister itself with the SLP
framework. SAs respond to UA service requests, detect DAs and
register service advertisements with them.
SA Server
Many operating system platforms only allow a single process to
listen on a particular port number. Since SAs are required to
listen on a multicast address for SLP service requests,
implementations of the SLP framework on such platforms that
want to support multiple SAs on one machine need to arrange for
a single process to do the listening while the advertising SAs
communicate with that process through another mechanism. The
single listening process is called an SA server. SA servers
share many characteristics with DAs, but they are not the same.
Service Advertisement
A URL possibly combined with service attributes. These are
made available to UAs by SAs, either directly or via a DA.
Locale
The language localization that applies to strings passed into
or returned from the SLP API. The Locale is expressed using a
Language Tag [6]. All attribute strings are associated with a
particular locale. The locale is completely orthogonal to the
ANSI C locale. The SLP locale is mapped into the Java locale
in the Java API.
Service Template
A document that describes the syntax of the URL for a given
service type and a definition of all service attributes
including the meaning, defaults, and constraints on values the
attributes may take. See [8] for more information on service
templates.
The service: URL
A service of a particular type announces its availability with
a service: URL that includes its service access point (domain
name or IP address, and possibly its port number) and
optionally basic configuration parameters. The syntax of the
service: URL is defined in the service template. Other URL's
can be used in service advertisements if desired.
Service Attributes
The attributes associated with a given service. The values
that can be assigned to service attributes are defined by the
service template.
Scope
A string used to control the availability of service
advertisements. Every SLP Agent is configured with one or more
scope strings. Scopes are assigned by site administrators to
group services for many purposes, but chiefly as a means of
scalability. DAs store only services advertised having a scope
string matching the scopes with which they are configured.
Naming Authority (NA)
This is a 'suffix' to the service type string. It completely
changes the meaning of the service type. NAs are used for
private definitions of well known Service Types and
experimental Service Type extensions. The default NA is
"IANA", which must not be explicitly included. Service types
with the IANA naming authority are registered with the Internet
Assigned Numbers Authority (see [8] for more information on the
registration procedure).
2. File Formats
This section describes the configuration and serialized registration file formats. Both files are defined in the UTF-8 character set [3]. Attribute tags and values in the serialized registration file require SLP reserved characters to be escaped. The SLP reserved characters are `(', `)', `,', `\', `!', `<', `=', `>', `~' and control characters (characters with UTF codes less than 0x0020 and the character 0x007f, which is US-ASCII DEL). The escapes are formed exactly as for the wire protocol, i.e. a backslash followed by two hex digits representing the character. For example, the escape for ' ,' is '\2c'. In addition, the characters `\n', `\r', `\t', and `_' are prohibited from attribute tags by the SLP wire syntax grammar. [7] In serialized registration files, escaped strings beginning with `\ff`, an encoding for a nonUTF-8 character, are treated as opaques. Exactly as in the wire protocol, syntactically correct opaque encodings consist of a string beginning with `\ff` and containing *only* escaped characters that are transformed to bytes. Such strings are only syntactically correct in the serialized registration file as attribute values. In other cases, whenever an escape is encountered and the character is not an SLP reserved character, an error is signaled. Escaped characters in URLs in serialized registration files use the URL escape convention. [2]. Property names and values in the configuration file have a few reserved characters that are involved in file's lexical definition. The characters '.' and '=' are reserved in property names and must be escape. The characters ',', '(', and ')' are reserved in property values and must be escaped. In addition, scope names in the net.slp.useScopes property use the SLP wire format escape convention for SLP reserved characters. This simplifies implementation, since the same code can be used to unescape scope names as is used in processing the serialized registration file or for formatting wire messages. On platforms that only support US-ASCII and not UTF-8, the upper bit of bytes incoming from the configuration and registration files determines whether the character is US-ASCII or not US-ASCII. According to the standard UTF-8 encoding, the upper bit is zero if the character is US-ASCII and one if the character is multibyte and thus not US-ASCII. Platforms without intrinsic UTF-8 support are required to parse the multibyte character and store it in an appropriate internal format. Support for UTF-8 is required to
implement the SLP protocol (see [7]), and can therefore be used in file processing as well. The location and name of the configuration file is system-dependent, but implementations of the API are encouraged to locate it together with other configuration files and name it consistently.2.1. Configuration File Format
The configuration file format consists of a newline delimited list of zero or more property definitions. Each property definition corresponds to a particular configurable SLP, network, or other parameter in one or more of the three SLP agents. The file format grammar in ABNF [5] syntax is: config-file = line-list line-list = line / line line-list line = property-line / comment-line comment-line = ( "#" / ";" ) 1*allchar newline property-line = property newline property = tag "=" value-list tag = prop / prop "." tag prop = 1*tagchar value-list = value / value "," value-list value = int / bool / "(" value-list ")" / string int = 1*DIGIT bool = "true" / "false" / "TRUE" / "FALSE" newline = CR / ( CRLF ) string = 1*stringchar tagchar = DIGIT / ALPHA / tother / escape tother = %x21-%x2d / %x2f / %x3a / %x3c-%x40 / %x5b-%x60 / %7b-%7e ; i.e., all characters except `.', ; and `='. stringchar = DIGIT / ALPHA / sother / escape sother = %x21-%x29 / %x2a-%x2b / %x2d-%x2f / %x3a-%x40 / %x5b-%x60 / %7b-%7e ; i.e., all characters except `,' allchar = DIGIT / ALPHA / HTAB / SP escape = "\" HEXDIG HEXDIG ; Used for reserved characters With the exception of net.slp.useScopes, net.slp.DAAddresses, and net.slp.isBroadcastOnly, all other properties can be changed through property accessors in the C and Java APIs. The property accessors
only change the property values in the running agent program and do not affect the values in the configuration file. The net.slp.useScopes and net.slp.DAAddresses properties are read-only because they control the agent's view of the scopes and DAs and are therefore critical to the function of the API scope discovery algorithm. Attempts to modify them are unlikely to yield productive results, and could harm the ability of the agent to find scopes and use DAs. The net.slp.isBroadcastOnly property is read-only because the API library needs to configure networking upon start up and changing this property might invalidate the configuration. Whether the local network uses broadcast or multicast is not likely to change during the course of the program's execution. The properties break down into the following subsections describes an area and its properties.2.1.1. DA configuration
Important configuration properties for DAs are included in this section. These are: net.slp.isDA A boolean indicating if the SLP server is to act as a DA. If false, not run as a DA. Default is false. net.slp.DAHeartBeat A 32 bit integer giving the number of seconds for the DA heartbeat. Default is 3 hours (10800 seconds). This property corresponds to the protocol specification parameter CONFIG_DA_BEAT [7]. Ignored if isDA is false. net.slp.DAAttributes A comma-separated list of parenthesized attribute/value list pairs that the DA must advertise in DAAdverts. The property must be in the SLP attribute list wire format, including escapes for reserved characters. [7]2.1.2. Static Scope Configuration
These properties allow various aspects of scope handling to be configured.
net.slp.useScopes
A value-list of strings indicating the only scopes a UA or SA
is allowed to use when making requests or registering, or the
scopes a DA must support. If not present for the DA and SA,
then in the absence of scope information from DHCP, the default
scope "DEFAULT" is used. If not present for the UA, and there
is no scope information available from DHCP, then the user
scoping model is in force. Active and passive DA discovery
or SA discovery are used for scope discovery, and the scope
"DEFAULT" is used if no other information is available. If a
DA or SA gets another scope in a request, a SCOPE_NOT_SUPPORTED
error should be returned, unless the request was multicast, in
which case it should be dropped. If a DA gets another scope in
a registration, a SCOPE_NOT_SUPPORTED error must be returned.
Unlike other properties, this property is "read-only", so
attempts to change it after the configuration file has been
read are ignored. See Section 3.12 for the algorithm the API
uses in determining what scope information to present.
net.slp.DAAddresses
A value-list of IP addresses or DNS resolvable host names
giving the SLPv2 DAs to use for statically configured UAs and
SAs. Ignored by DAs (unless the DA is also an SA server).
Default is none. Unlike other properties, this property is
"read-only", so attempts to change it after the configuration
file has been read are ignored.
The following grammar describes the property:
addr-list = addr / addr "," addr-list
addr = fqdn / hostnumber
fqdn = ALPHA / ALPHA *[ anum / "-" ] anum
anum = ALPHA / DIGIT
hostnumber = 1*3DIGIT 3("." 1*3DIGIT)
An example is:
sawah,mandi,sambal
IP addresses can be used instead of host names in networks
where DNS is not deployed, but network administrators are
reminded that using IP addresses will complicate machine
renumbering, since the SLP configuration property files
in statically configured networks will have to be changed.
Similarly, if host names are used, implementors must be careful
that a name service is available before SLP starts, in other
words, SLP cannot be used to find the name service.
2.1.3. Tracing and Logging
This section allows tracing and logging information to be printed by
the various agents.
net.slp.traceDATraffic
A boolean controlling printing of messages about traffic with
DAs. Default is false.
net.slp.traceMsg
A boolean controlling printing of details on SLP messages.
The fields in all incoming messages and outgoing replies are
printed. Default is false.
net.slp.traceDrop
A boolean controlling printing details when a SLP message is
dropped for any reason. Default is false.
net.slp.traceReg
A boolean controlling dumps of all registered services upon
registration and deregistration. If true, the contents
of the DA or SA server are dumped after a registration or
deregistration occurs. Default is false.
2.1.4. Serialized Proxy Registrations
These properties control the reading and writing of serialized
registrations.
net.slp.serializedRegURL
A string containing a URL pointing to a document containing
serialized registrations that should be processed when the DA
or SA server starts up. Default is none.
2.1.5. Network Configuration Properties
The properties in this section allow various network configuration properties to be set. net.slp.isBroadcastOnly A boolean indicating if broadcast should be used instead of multicast. Like the net.slp.useScopes and net.slp.DAAddresses properties, this property is "read-only", so attempts to change it after the configuration file has been read are ignored. Default is false. net.slp.passiveDADetection A boolean indicating whether passive DA detection should be used. Default is true. net.slp.multicastTTL A positive integer less than or equal to 255, giving the multicast TTL. Default is 255. net.slp.DAActiveDiscoveryInterval A 16 bit positive integer giving the number of seconds between DA active discovery queries. Default is 900 seconds (15 minutes). This property corresponds to the protocol specification parameter CONFIG_DA_FIND [7]. If the property is set to zero, active discovery is turned off. This is useful when the DAs available are explicitly restricted to those obtained from DHCP or the net.slp.DAAddresses property. net.slp.multicastMaximumWait A 32 bit integer giving the maximum amount of time to perform multicast, in milliseconds. Default is 15000 ms (15 sec.). This property corresponds to the CONFIG_MC_MAX parameter in the protocol specification [7]. net.slp.multicastTimeouts A value-list of 32 bit integers used as timeouts, in milliseconds, to implement the multicast convergence algorithm. Each value specifies the time to wait before sending the next request, or until nothing new has been learned from two successive requests. Default is: 3000,3000,3000,3000,3000. In a fast network the
aggressive values of 1000,1250,1500,2000,4000 allow better
performance. This property corresponds to the CONFIG_MC_RETRY
parameter in the protocol specification [7]. Note that the
net.slp.DADiscoveryTimeouts property must be used for active DA
discovery.
net.slp.DADiscoveryTimeouts
A value-list of 32 bit integers used as timeouts, in
milliseconds, to implement the multicast convergence algorithm
during active DA discovery. Each value specifies the time
to wait before sending the next request, or until nothing
new has been learned from two successive requests. This
property corresponds to the protocol specification parameter
CONFIG_RETRY [7]. Default is: 2000,2000,2000,2000,3000,4000.
net.slp.datagramTimeouts
A value-list of 32 bit integers used as timeouts, in
milliseconds, to implement unicast datagram transmission to
DAs. The nth value gives the time to block waiting for a reply
on the nth try to contact the DA. The sum of these values is
the protocol specification property CONFIG_RETRY_MAX [7].
net.slp.randomWaitBound
A 32 bit integer giving the maximum value for all random
wait parameters, in milliseconds. Default is 1000 (1
sec.). This value corresponds to the protocol specification
parameters CONFIG_START_WAIT, CONFIG_REG_PASSIVE, and
CONFIG_REG_ACTIVE [7].
net.slp.MTU
A 16 bit integer giving the network packet MTU, in bytes.
This is the maximum size of any datagram to send, but the
implementation might receive a larger datagram. The maximum
size includes IP, and UDP or TCP headers. Default is 1400.
net.slp.interfaces
Value-list of strings giving the IP addresses of network
interfaces on which the DA or SA should listen on port 427 for
multicast, unicast UDP, and TCP messages. Default is empty,
i.e. use the default network interface. The grammar for this
property is:
addr-list = hostnumber / hostnumber "," addr-list
hostnumber = 1*3DIGIT 3("." 1*3DIGIT)
An example is:
195.42.42.42,195.42.142.1,195.42.120.1
The example machine has three interfaces on which the DA should
listen.
Note that since this property only takes IP addresses, it will
need to be changed if the network is renumbered.
2.1.6. SA Configuration
This section contains configuration properties for the SA. These
properties are typically set programmatically by the SA, since they
are specific to each SA.
net.slp.SAAttributes
A comma-separated list of parenthesized attribute/value list
pairs that the SA must advertise in SAAdverts. The property
must be in the SLP attribute list wire format, including
escapes for reserved characters. [7]
2.1.7. UA Configuration
This section contains configuration properties for the UA. These
properties can be set either programmatically by the UA or in the
configuration file.
net.slp.locale
A RFC 1766 Language Tag [6] for the language locale. Setting
this property causes the property value to become the default
locale for SLP messages. Default is "en". This property is
also used for SA and DA configuration.
net.slp.maxResults
A 32 bit integer giving the maximum number of results to
accumulate and return for a synchronous request before the
timeout, or the maximum number of results to return through a
callback if the request results are reported asynchronously.
Positive integers and -1 are legal values. If -1, indicates
that all results should be returned. Default value is -1.
DAs and SAs always return all results that match the
request. This configuration value applies only to UAs, that
filter incoming results and only return as many values as
net.slp.maxResults indicates.
net.slp.typeHint
A value-list of service type names. In the absence of any
DAs, UAs perform SA discovery for finding scopes. These SA
discovery requests may contain a request for service types as
an attribute.
The API implementation will use the service type names supplied
by this property to discover only those SAs (and their scopes)
which support the desired service type or types. For example,
if net.slp.typeHint is set to "service:imap,service:pop3" then
SA discovery requests will include the search filter:
(|(service-type=service:imap)(service-type=service:pop3))
The API library can also use unicast to contact the discovered
SAs for subsequent requests for these service types, to
optimize network access.
2.1.8. Security
The property in this section allows security for all agents to be set
on or off. When the property is true, then the agent must include
security information on all SLP messages transacted by that agent.
Since security policy must be set network wide to be effective, a
single property controls security for all agents. Key management and
management of SLP SPI strings [7] are implementation and policy
dependent.
net.slp.securityEnabled
A boolean indicating whether the agent should enable
security for URLs, attribute lists, DAAdverts, and SAAdverts.
Each agent is responsible for interpreting the property
appropriately. Default is false.
2.2. Multihomed Machines
On multihomed machines, the bandwidth and latency characteristics on different network interfaces may differ considerably, to the point where different configuration properties are necessary to achieve optimal performance. The net.slp.interfaces property indicates which network interfaces are SLP enabled. An API library implementation may support configuration customization on a per network interface basis by allowing the interface IP address to be appended to the property name. In that case, the values of the property are only used for that particular interface, the generic property (or defaults if no generic property is set) applies to all others. For example, if a configuration has the following properties: net.slp.interfaces=125.196.42.41,125.196.42.42,125.196.42.43 net.slp.multicastTTL.125.196.42.42=1 then the network interface on subnet 42 is restricted to a TTL of 1, while the interfaces on the other subnets have the default multicast radius, 255. The net.slp.interfaces property must only be set if there is no routing between the interfaces. If the property is set, the DA (if any) and SAs should advertise with the IP address or host name appropriate to the interface on the interfaces in the list. If packets are routed between the interfaces, then the DA and SAs should only advertise on the default interface. The property should also be set if broadcast is used rather than multicast on the subnets connected to the interfaces. Note that even if unicast packets are not routed between the interfaces, multicast may be routed through another router. The danger in listening for multicast on multiple interfaces when multicast packets are routed is that the DA or SA may receive the same multicast request via more than one interface. Since the IP address is different on each interface, the DA or SA cannot identify the request as having already being answered via the previous responder's list. The requesting agent will end up getting URLs that refer to the same DA or service but have different addresses or host names.2.3. Serialized Registration File
The serialized registration file contains a group of registrations that a DA or SA server (if one exists) registers when it starts up. These registrations are primarily for older service programs that do not internally support SLP and cannot be converted, and for portably
exchanging registrations between SLP implementations. The character encoding of the registrations is required to be UTF-8. The syntax of the serialized registration file, in ABNF format [5], is as follows: ser-file = reg-list reg-list = reg / reg reg-list reg = creg / ser-reg creg = comment-line ser-reg comment-line = ( "#" / ";" ) 1*allchar newline ser-reg = url-props [slist] [attr-list] newline url-props = surl "," lang "," ltime [ "," type ] newline surl = ;The registration's URL. See ; [8] for syntax. lang = 1*8ALPHA [ "-" 1*8ALPHA ] ;RFC 1766 Language Tag see [6]. ltime = 1*5DIGIT ; A positive 16-bit integer ; giving the lifetime ; of the registration. type = ; The service type name, see [7] ; and [8] for syntax. slist = "scopes" "=" scope-list newline scope-list = scope-name / scope-name "," scope-list scope = ; See grammar of [7] for ; scope-name syntax. attr-list = attr-def / attr-def attr-list attr-def = ( attr / keyword ) newline keyword = attr-id attr = attr-id "=" attr-val-list attr-id = ;Attribute id, see [7] for syntax. attr-val-list = attr-val / attr-val "," attr-val-list attr-val = ;Attribute value, see [7] for syntax. allchar = char / WSP char = DIGIT / ALPHA / other other = %x21-%x2f / %x3a-%x40 / %x5b-%x60 / %7b-%7e ; All printable, nonwhitespace US-ASCII ; characters. newline = CR / ( CRLF ) The syntax for scope names, attribute tags, and attribute values requires escapes for special characters as specified in [7]. DAs and SA servers that process serialized registrations must handle them exactly as if they were registered by an SA. In the url-props
production, the type token is optional. If the type token is present for a service: URL, a warning is signaled and the type name is ignored. If the maximum lifetime is specified (65535 sec.), the registration is taken to be permanent, and is continually refreshed by the DA or SA server until it exits. Scopes can be included in a registration by including an attribute definition with tag "scopes" followed by a comma separated list of scope names immediately after the url-props production. If the optional scope list is present, the registrations are made in the indicated scopes; otherwise, they are registered in the scopes with which the DA or SA server was configured through the net.slp.useScopes property. If the scope list contains scopes that are not in the net.slp.useScopes property (provided that property is set) or are not specified by DHCP, the API library should reject the registration and issue a warning message.2.4. Processing Serialized Registration and Configuration Files
Implementations are encouraged to make processing of configuration and serialized files as transparent as possible to clients of the API. At the latest, errors must be caught when the relevant configuration item is used. At the earliest, errors may be caught when the relevant file is loaded into the executing agent. Errors should be reported by logging to the appropriate platform logging file, error output, or log device, and the default value substituted. Serialized registration file entries should be caught and reported when the file is loaded. Configuration file loading must be complete prior to the initiation of the first networking connection. Serialized registration must be complete before the DA accepts the first network request.3. Binding Independent Implementation Considerations
This section discusses a number of implementation considerations independent of language binding, with language specific notes where applicable.3.1. Multithreading
Implementations of both the C and Java APIs are required to make API calls thread-safe. Access to data structures shared between threads must be co-ordinated to avoid corruption or invalid access. One way to achieve this goal is to allow only one thread at a time in the implementing library. Performance in such an implementation suffers, however. Therefore, where possible, implementations are encouraged to allow multiple threads within the SLP API library.
3.2. Asynchronous and Incremental
The APIs are designed to encourage implementations supporting asynchronous and incremental client interaction. The goal is to allow large numbers of returned service URLs, service types, and attributes without requiring the allocation of huge chunks of memory. The particular design features to support this goal differ in the two language bindings.3.3. Type Checking for Service Types
Service templates [8] allow SLP registrations to be type checked for correctness. Implementations of the API are free to make use of service type information for type checking, but are not required to do so. If a type error occurs, the registration should terminate with TYPE_ERROR.3.4. Refreshing Registrations
SLP advertisements carry an explicit lifetime with them. After the lifetime expires, the DA flushes the registration from its cache. In some cases, an application may want to have the URL continue being registered for the entire time during which the application is executing. The API includes provision for clients to indicate whether they want URLs to be automatically refreshed. Implementations of the SA API must provide this automatic refreshing capability. Note that a client which uses this facility should explicitly deregister the service URL before exiting, since the API implementation may not be able to assure that the URL is deregistered when the application exits, although it will time out in the DA eventually.3.5. Configuration File Processing
DAs, SAs and UAs processing the configuration file, and DAs and SA servers processing the serialized registration file are required to log any errors using whatever underlying error mechanism is appropriate for the platform. Examples include writing error messages to the standard output, writing to a system logging device, or displaying the errors to a logging window. After the error is reported, the offending property must be set to the default and program execution continued. An agent MUST NOT fail if a file format error occurs.
3.6. Attribute Types
String encoded attribute values do not include explicit type information. All UA implementations and those SA and DA implementations that choose to support type checking should use the type rules described in [8] in order to convert from the string representation on the wire to an object typed appropriately.3.7. Removal of Duplicates
The UA implementation SHOULD always collate results to remove duplicates during synchronous operations and for the Java API. During asynchronous operation in C, the UA implementation SHOULD forgo duplicate elimination to reduce memory requirements in the library. This allows the API library to simply take the returned attribute value list strings, URL strings, or service type list strings and call the callback function with it, without any additional processing. Naturally, the burden of duplicate elimination is thrown onto the client in this case.3.8. Character Set Encoding
Character string parameters in the Java API are all represented in Unicode internally because that is the Java-supported character set. Characters buffer parameters in the C API are represented in UTF-8 to maintain maximum compatibility on platforms that only support US- ASCII and not UTF-8. API functions are still required to handle the full range of UTF-8 characters because the SLP protocol requires it, but the API implementation can represent the characters internally in any convenient way. On the wire, all characters are converted to UTF-8. Inside URLs, characters that are not allowed by URL syntax [2] must be escaped according to the URL escape character convention. Strings that are included in SLP messages may include SLP reserved characters and can be escaped by clients through convenience functions provided by the API. The character encoding used in escapes is UTF-8. Due to constraints in SLP, no string parameter passed to the C or Java API may exceed 64K bytes in length.3.9. Error Semantics
All errors encountered processing SLP messages should be logged. For synchronous calls, an error is only reported on a call if no successful replies were received from any SLP framework entity. If an error occurred among one of several successful replies, then the error should be logged and the successful replies returned. For asynchronous calls, an error occurring during correspondence with a
particular remote SLP agent is reported through the first callback
(in the C API) or enumeration method invocation (in the Java API)
after the error occurs, which would normally report the results of
the correspondence. This allows the callback or client code to
determine whether the operation should be terminated or continue. In
some cases, the error returned from the SLP framework may be fatal
(SLP_PARSE_ERROR, etc.). In these cases, the API library terminates
the operation.
Both the Java and C APIs contain language specific error code
mechanisms for returning error information. The names of the error
codes are consistent between the two implementations, however.
The following error codes are returned from a remote agent (DA or SA
server):
LANGUAGE_NOT_SUPPORTED
No DA or SA has service advertisement or attribute information
in the language requested, but at least one DA or SA indicated,
via the LANGUAGE_NOT_SUPPORTED error code, that it might have
information for that service in another language.
PARSE_ERROR
The SLP message was rejected by a remote SLP agent. The API
returns this error only when no information was retrieved, and
at least one SA or DA indicated a protocol error. The data
supplied through the API may be malformed or a may have been
damaged in transit.
INVALID_REGISTRATION
The API may return this error if an attempt to register a
service was rejected by all DAs because of a malformed URL or
attributes. SLP does not return the error if at least one DA
accepted the registration.
AUTHENTICATION_ABSENT
If the SLP framework supports authentication, this error arises
when the UA or SA failed to send an authenticator for requests
or registrations in a protected scope.
INVALID_UPDATE
An update for a non-existing registration was issued, or the
update includes a service type or scope different than that in
the initial registration, etc.
The following errors result from interactions with remote agents or
can occur locally:
AUTHENTICATION_FAILED
If the SLP framework supports authentication, this error arises
when a authentication on an SLP message failed.
SCOPE_NOT_SUPPORTED
The API returns this error if the SA has been configured with
net.slp.useScopes value-list of scopes and the SA request did
not specify one or more of these allowable scopes, and no
others. It may be returned by a DA or SA if the scope included
in a request is not supported by the DA or SA.
REFRESH_REJECTED
The SA attempted to refresh a registration more frequently
than the minimum refresh interval. The SA should call the
appropriate API function to obtain the minimum refresh interval
to use.
The following errors are generated through a program interacting with
the API implementation. They do not involve a remote SLP agent.
NOT_IMPLEMENTED
If an unimplemented feature is used, this error is returned.
NETWORK_INIT_FAILED
If the network cannot initialize properly, this error is
returned.
NETWORK_TIMED_OUT
When no reply can be obtained in the time specified by the
configured timeout interval for a unicast request, this error
is returned.
NETWORK_ERROR
The failure of networking during normal operations causes this
error to be returned.
BUFFER_OVERFLOW
An outgoing request overflowed the maximum network MTU size.
The request should be reduced in size or broken into pieces and
tried again.
MEMORY_ALLOC_FAILED
If the API fails to allocate memory, the operation is aborted
and returns this.
PARAMETER_BAD
If a parameter passed into an interface is bad, this error is
returned.
INTERNAL_SYSTEM_ERROR
A basic failure of the API causes this error to be returned.
This occurs when a system call or library fails. The operation
could not recover.
HANDLE_IN_USE
In the C API, callback functions are not permitted to
recursively call into the API on the same SLPHandle, either
directly or indirectly. If an attempt is made to do so, this
error is returned from the called API function.
TYPE_ERROR
If the API supports type checking of registrations against
service type templates, this error can arise if the attributes
in a registration do not match the service type template for
the service.
Some error codes are handled differently in the Java API. These
differences are discussed in Section 5.
The SLP protocol errors OPTION_NOT_UNDERSTOOD, VERSION_NOT_SUPPORTED,
INTERNAL_ERROR, MSG_NOT_SUPPORTED, AUTHENTICATON_UNKNOWN, and
DA_BUSY_NOW should be handled internally and not surfaced to clients
through the API.
3.10. Modular Implementations
Subset implementations that do not support the full range of functionality are required to nevertheless support every interface in order to maintain link compatibility between compliant API implementations and applications. If a particular operation is not supported, a NOT_IMPLEMENTED error should be returned. The Java API has some additional conventions for handling subsets. Applications that are expected to run on a wide variety of platforms should be prepared for subset API implementations by checking returned error codes.3.11. Handling Special Service Types
The service types service:directory-agent and service:service-agent are used internally in the SLP framework to discover DAs and SAs. The mechanism of DA and SA discovery is not normally exposed to the API client; however, the client may have interest in discovering DAs and SAs independently of their role in discovering other services. For example, a network management application may want to determine which machines are running SLP DAs. To facilitate that, API implementations must handle requests to find services and attributes for these two service types so that API clients obtain the information they expect. In particular, if the UA is using a DA, SrvRqst and AttrRqst for these service types must be multicast and not unicast to the DA, as is the case for other service types. If the requests are not multicast, the DA will respond with an empty reply to a request for services of type service:service-agent and with its URL only to a request for services of type service:directory-agent. The UA would therefore not obtain a complete picture of the available DAs and SAs.3.12. Scope Discovery and Handling
Both APIs contain an operation to obtain a list of currently known scope names. This scope information comes from a variety of places: DHCP, the net.slp.useScopes property, unicast to DAs configured via DHCP or the net.slp.DAAddresses property, and active and passive discovery. The API is required to be implemented in a way that re-enforces the administrative and user scoping models described in [7]. SA clients only support the administrative scoping model. SAs must know a priori what DAs they need to register with since there is typically no human intervention in scope selection for SAs. UAs must support both administrative and user scoping because an application may require human intervention in scope selection.
API implementations are required to support administrative scoping in the following way. Scopes configured by DHCP and scopes of DAs configured by DHCP have first priority (in that order) and must be returned if they are available. The net.slp.useScopes property has second priority, and scopes discovered through the net.slp.useScopes property must be returned if this property is set and there are no scopes available from DHCP. If scopes are not available from either of these sources and the net.slp.DAAddresses property is set, then the scopes available from the configured DAs must be returned. Note that if both DAs and scopes are configured, the scopes of the configured DAs must match the configured scope list; otherwise and error is signaled and agent execution is terminated. If no configured scope information is available, then an SA client has default scope, "DEFAULT", and a UA client employs user scoping. User scoping is supported in the following way. Scopes discovered from active DA discovery, and from passive DA discovery all must be returned. If no information is available from active and passive DA discovery, then the API library may perform SA discovery, using the service types in the net.slp.typeHint property to limit the search to SAs supporting particular service types. If no net.slp.typeHint property is set, the UA may perform SA discovery without any service type query. In the absence of any of the above sources of information, the API must return the default scope, "DEFAULT". Note that the API must always return some scope information. SLP requires that SAs must perform their operations in all scopes currently known to them. [7]. The API enforces this constraint by not requiring the API client to supply any scopes as parameters to API operations. The API library must obtain all currently known scopes and use them in SA operations. UA API clients should use a scope obtained through one of the API operations for finding scopes. Any other scope name may result in a SCOPE_NOT_SUPPORTED error from a remote agent. The UA API library can optionally check the scope and return the error without contacting a remote agent.
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