RFC 2165
Service Location Protocol
5. Service Request Message Format The Service Request is used to obtain URLs from a Directory Agent or Service Agents.
The format of the Service Request is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvReq) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|length of prev resp list string|<Previous Responders Addr Spec>|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Previous Responders Addr Spec> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of predicate string | Service Request <predicate> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ Service Request <predicate>, contd. \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If a UA issues a request which will result in a reply which is too
large, the SA or DA will return an abbreviated response (in a
datagram the size of the site's MTU) which has the 'Overflow' bit
flag set. The UA must then issue the request again using TCP.
The <Previous Responders Addr Spec> is described in sections 7 and
20.1.
After a User Agent restarts (say, after rebooting of a system,
loading of the network kernel), Service Requests should be delayed
for some random time uniformly distributed within a one second
interval centered about a configured delay value (by default,
CONFIG_INTERVAL_4).
The Service Request allows the User Agent to specify the Service Type
of the service and a Predicate in a specific language. The general
form of a Service Request is shown below:
<srvtype>[.<na>]/[<scope>]/[<where>]/
The punctuation is necessary even where the fields are omitted.
- The <srvtype> refers to the Service Type. For each type of
service available, there is a unique Service type name string.
See section 20.2.1.
- The <na> is the Naming Authority. This string determines the
semantic interpretation of the attribute information in the
<where> part of the Service Request.
- The <scope> is a string used to restrict the range of the query.
Scope is determined administratively, at a given site. It is not
necessarily related to network topology (see Section 16).
Leaving this field out means that the request can be satisfied
only by unscoped service advertisements.
- The <where> string is the Where Clause of the request. It
contains a query which allows the selection of those service
instances which the User Agent is interested in. The query
includes attributes, boolean operators and relations. (See
section 5.3.)
In the case of a multicast service request, a list of previous
responders is sent. This list will prevent those in the list from
responding, to be sure that responses from other sources are not
drowned out. The request is multicast repeatedly (with a recommended
wait interval of CONFIG_INTERVAL_2) until there are no new responses,
or a certain time (CONFIG_INTERVAL_3) has elapsed. Different timing
values are applied to a Service Request used for Directory Agent
Discovery, see Section 5.2.
In order for a request to succeed in matching registered information,
the following conditions must be met:
1. The result must have the same Service Type as the request.
2. It must have the same Naming Authority.
3. It must have the same scope. (If the scope of the request
as omitted, the request will only match services which were
registered with no scope. Note that a scoped request WILL match
all unscoped Services).
4. The conditions specified in the Where Clause must match the
attributes and keywords registered for the service.
5.1. Service Request Usage The User Agent may form Service Requests using preconfigured knowledge of a Service Type's attributes. It may also issue Attribute Requests to obtain the attribute values for a Service Type before issuing Service Requests (see Section 13). Having obtained the attributes which describe a particular kind of service from an Attribute Request, or using configured knowledge of a service's attributes, the User Agent can build a predicate that describes the service needs of the user. Service Requests may be sent directly to a Directory Agent. Suppose a printer supporting the lpr protocol is needed on the 12th floor which has UNRESTRICTED_ACCESS and prints 12 pages per minute. Suppose further that a Attribute Request indicates that there is a printer on the 12th floor, a printer that prints 12 pages per minute, and a printer that offers UNRESTRICTED_ACCESS. To check whether they are same printer, issue the following request: lpr//(& (PAGES PER MINUTE==12) (UNRESTRICTED_ACCESS) (LOCATION==12th FLOOR))/ Suppose there is no such printer. The Directory Agent responds with a Service Reply with 0 in the number of responses and no reply values. The User Agent then tries a less restrictive query to find a printer, using the 12th floor as "where" criteria. lpr//(LOCATION==12th FLOOR)/ In this case, there is now only one reply: Returned URL: service:lpr://igore.wco.ftp.com:515/draft The Address Specification for the printer is: igore.wco.ftp.com:515, containing the name of the host managing the requested printer. Files would be printed by spooling to that port on that host. The word 'draft' refers to the name of the print queue the lpr server supports.
In the absence of a Directory Agent, the request above could be
multicast. In this case it would be sent to the Service Specific
Multicast Address for "service:printer" and not to the Directory
Agent. Service Agents that can satisfy the predicate will reply.
Service Agents which cannot support the character set of the request
MUST return CHARSET_NOT_UNDERSTOOD in the SrvRply. In all other
circumstances, Service Agents which cannot satisfy the reply do not
send any reply at all.
The only way a User Agent can be sure there are no services which
match the query is by retrying the request (CONFIG_INTERVAL_8). If
no response comes, the User Agent gives up and assumes there are no
such printers.
Another form of query is a simpler 'join' query. Its syntax has no
parentheses or logical operators. Each term is conjoined (AND-ed
together.) Rewriting the initial query provides an example:
lpr//PAGES PER MINUTE==12,
UNRESTRICTED_ACCESS,
LOCATION==12th FLOOR/
5.2. Directory Agent Discovery Request
Normally a Service Request returns a Service Reply. The sole
exception to this is a Service Request for the Service Type
"directory-agent". This Service Request is answered with a DA
Advertisement.
Without configured knowledge of a Directory Agent (DA), a User Agent
or Service Agent uses a Service Request to discover a DA. (See
section 15.1 for mechanisms by which a client may be configured to
have knowledge of a DA.) Such a Service Request used for Directory
Agent Discovery includes a predicate of the form:
directory-agent///
This query is always sent to the Directory Agent Discovery multicast
address. The Service Type of a Directory Agent is "directory-agent",
hence it is the Service Type used in the request. No scope is
included in the request, so all Directory Agents will reply. This is
the only request which omits a scope which all Directory Agents MUST
respond to. Normally, a Directory Agent with a scope ONLY responds
to requests with that scope. No Naming Authority is included, so
"IANA" is assumed. We want to reach all the available directory
agents. If the scope were supplied, only DAs supporting that scope
would reply.
DA Advertisement Replies may arrive from different sources, similar
in form to:
URL returned: service:directory-agent://slp-resolver.catch22.com
Scope returned: ACCOUNTING
URL returned: service:directory-agent://204.182.15.66 Scope
returned: JANITORIAL SERVICES
The DA Advertisement format is defined in Section 14.
If the goal is merely to discover any Directory Agent, the first
reply will do. If the goal, however, is to discover all reachable
DAs, the request must be retransmitted after an interval (the
recommended time is CONFIG_INTERVAL_5). This retransmitted request
will include a list of DAs which have already responded. See
sections 7 and 20.1. Directory Agents which receive the request will
only respond if they are not on this list. After there are no new
replies, all DAs are presumed to have been discovered.
If a DA fails to respond after CONFIG_INTERVAL_6 seconds, the UA or
Service Agent should use a different DA. DA addresses may be cached
from previous discovery attempts, preconfigured, or by use of DHCP
(see section 15.2). If no such DA responds, DA discovery should be
used to find a new DA. Only after CONFIG_INTERVAL_7 seconds should it
be assumed that no DA exists and multicast based Service Requests
should be used.
5.3. Explanation of Terms of Predicate Grammar
A predicate has a simple structure, which depends on parentheses,
commas and slashes to delimit the elements. Examples of proper usage
are given throughout this document. The terms used in the grammar
are as follows:
predicate:
Placed in a Service Request, this is interpreted by a Service
Agent or Directory Agent to determine what information to
return.
scope:
If this is absent in a Service Request, the request will match
only services registered without a scope. If it is present,
only services registered under that scope or are unscoped will
match the request.
where-clause:
This determines which services the request matches. An empty
where-clause will match all services. The request will be
limited to services which have the specified Service Type, so
the where-clause is not the sole factor in picking out which
services match the request.
where-list:
The where-list is a logical expression. It can be a single
expression, a disjunction or a conjunction. A single
expression must apply for the where-clause to match. A
disjunction matches if any expression in the OR list matches.
A conjunction matches only if all elements in the AND list
match.
Note that there is no logical negation operator: This is
because there is no notion of returning "everything except"
what matches a given criteria.
A where-list can be nested and complex. For example, the
following requires that three subexpressions must all be true:
(& (| <query-item> <query-item>)
<query-item>
(& <query-item> <query-item> <query-item>)
)
Notice that white space, tabs or carriage returns can be added
anywhere outside query-items. Each list has 2 or more items in
it, and lists can be nested. Services which fulfill the entire
logical expression match the where-clause.
degenerate expressions but they should be tolerated. They are
equivalent to <query-item>.
query-item:
A query item has the form:
'(' <attr-tag> <comp-op> <attr-val> ')'
or
'(' <keyword> ')'
Examples of this would be:
(SOME ATTRIBUTE == SOME VALUE)
(RESERVED)
(QUEUE LENGTH <= 234)
query-join:
The query-join is a comma delimited list of conditions which
the service must satisfy in order to match the query. The
items are considered to be logically conjoined. Thus the
query-join:
ATTR1=VALUE1, KEYWORD1, KEYWORD2, ATTR2>=34
is equivalent to the where-list:
(& (ATTR1=VALUE1) (KEYWORD1) (KEYWORD2) (ATTR2>=34))
The query-join cannot be mixed with a where-list. It is
provided as a convenient mechanism to provide a statement of
necessary conditions without building a logical expression.
5.4. Service Request Predicate Grammar
Service Requests can precisely describe the services they need by
including a Predicate the body of the Request. This Predicate must
be constructed according to the grammar below.
<predicate> ::= <srvtype>['.'<na>]'/'<scope>'/'<where>'/'
<srvtype> ::= string representing type of service. Only
alphanumeric characters, '+', and '-' are allowed.
<na> ::= string representing the Naming Authority.
Only alphanumeric characters, '+',
and '-' are allowed. If this field is
omitted then "IANA" is assumed.
<scope> ::= string representing the directory agent scope.
'/', ',' (comma) and ':' are not allowed in
this string. The scopes "LOCAL" and "REMOTE"
are reserved.
<attr-tag> ::= class name of an attribute of a given Service
Type. This tag cannot include the following
characters: '(', ')', ',', '=', '!', '>',
'<', '/', '*', except where escaped (see 17.1.)
<keyword> ::= a class name of an attribute which will have
no values. This string has the same limits
as the <attr-tag>, except that white space
internal to the keyword is illegal.
<where> ::= <where-any> |
<where-list> |
<query-join>
<where-any> ::=
That is NOTHING, or white space.
<where-list> ::= '(' '&' <where-list> <query-list> ')' |
'(' '|' <where-list> <query-list> ')' |
'(' <keyword> ')'
'(' <attr-tag> <comp-op> <attr-val> ')'
<query-list> ::= <where-list> |
<where-list> <query-list>
<query-join> ::= <keyword> |
<join-item> |
<query-join> ',' <keyword> |
<query-join> ',' <join-item>
<join-item> ::= <attr-tag> <comp-op> <attr-val>
<comp-op> ::= "!=" | "==" | '<' | "<=" | '>' | ">="
<attr-val> ::= any string (see Section 20.5 for the ways
in which attr-vals are interpreted.)
Value strings may not contain '/', ','
'=', '<', '>', or '*' except where escaped
(see 17.1.).
'(' and ')' may be used in attribute values
for the purpose of encoding a binary values.
Binary encodings (See 20.5) may
include the above reserved characters.
5.5. String Matching for Requests
All strings are case insensitive, with respect to string matching on
queries. All preceding or trailing blanks should not be considered
for a match, but blanks internal to a string are relevant.
For example, " Some String " matches "SOME STRING", but not "some
string".
String matching may only be performed over the same character sets.
If a request cannot be satisfied due to a lack of support for the
character set of the request a CHARSET_NOT_UNDERSTOOD error is
returned.
String comparisons (using comparison operators such as '<' or
registration, not using any language specific rules. The ordering is
strictly by the character value, i.e. "0" < "A" is true when the
character set is US-ASCII, since "0" has the value of 48 and "A" has
the value 65.
The special character '*' may precede or follow a string in order to
allow substring matching. If the '*' precedes a string, it matches
any attribute value which ends with the string. If the string ends
with a '*', it matches any attribute value which begins with the
string. Finally, if a string begins and ends with a '*', the string
will match any attribute value which contains the string.
Examples:
"bob*" matches "bob", "bobcat", and "bob and sue" "*bob" matches
"bob", "bigbob", and "sue and bob" "*bob*" matches "bob",
"bobcat", "bigbob", and "a bob I know"
String matching is done after escape sequences have been substituted.
See sections 17, 5.3, 17.1.
6. Service Reply Message Format
The format of the Service Reply Message is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvRply) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | URL Entry count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| <URL Entry 1> ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
\ . \
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| <URL Entry N> ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Each Service Reply message is composed of a list of URL Entries.
The Error Code may have one of the following values:
0 Success
LANGUAGE_NOT_SUPPORTED
A SA or DA returns this when a request is received from a
UA which is in a language for which there is no
registered Service Information and the request arrived
with the Monolingual bit set. See Section 17.
PROTOCOL_PARSE_ERROR
A SA or DA returns this error when a SrvRply is received
which cannot be parsed or the declared string lengths
overrun the message.
SCOPE_NOT_SUPPORTED
A DA will return this error if it receives a request
which has a scope not supported by the DA. An SA will not
return this error; it will simply not reply to the
multicast request.
CHARSET_NOT_UNDERSTOOD
If the DA or SA receives a request or registration in a
character set which it does not support, it will return
this error.
Each <URL Entry> in the list has the form defined in Section 4.2.
The URL entries in the reply have no delimiters between them, other
than the length fields. The URL length fields indicate where the URL
strings end. If the presence of an URL Authenticator block is
signalled by the 'U' bit, the length of the authenticator block is
determined by information within the block as discussed in section
4.3. A User Agent MAY use the authentication block to determine
whether the Service Agent advertising the URL is, in fact, authorized
to offer the indicated service. If, in a list of URL entries, some
of the URLs indicate services which are in protected scopes (see
section 16.1) while other URLs in the list indicate services which
are not in protected scopes, the latter must still have
Authentication Blocks, but the length of the authentcitor is shown as
zero, and no authentication need be done.
7. Service Type Request Message Format
The Service Type Request is used to determine all the types of
services supported on a network.
The request should be sent directly to a DA (though it may also be
sent to the Service Location General Multicast Address), in order to
find out all services available on the site network (which are
advertised by Directory Agents and Service Agents.) If no DA is
available, a User Agent MAY issue more than one request to insure
that all replies have been received. In each subsequent request, a
User Agent includes those Service Types that it is aware of. When no
new replies arrive within CONFIG_INTERVAL_3 from a request, the User
Agent can presume that it has acquired a complete set of available
Service Types.
The format of a Service Type Request is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvTypeRqst) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of prev resp string |<Previous Responders Addr Spec>|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Previous Responders Addr Spec> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of naming authority | <Naming Authority String> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Naming Authority String>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of Scope String | <Scope String> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Scope String>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Note that the <Previous Responders Addr Spec> is a comma delimited
list. (See section 20.1.) The 'length of prev responder list' field
indicates the length of the comma delimited list string. A previous
responder list with 3 elements takes this form:
<addr-spec>,<addr-spec>,<addr-spec>
The Naming Authority, if included, will limit the replies to Service
Type Requests to Service Types which have the specified Naming
Authority. If this field is omitted (i.e., the length field is
zero), the default Naming Authority ("IANA") is assumed. If the
length field is -1, service types from all naming authorities are
requested.
The Scope String Field, if included, will limit replies to Service
Types which have the specified scope or are unscoped. If this field
is omitted, all Service Types (from the specified Naming Authority)
are returned.
8. Service Type Reply Message Format
The Service Type Reply has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvTypeRply) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | number of service types |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Service Type Item 1> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Service Type Item N> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The format of a Service Type Item is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of Service Type String | <Service Type String> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Service Type String>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Error Code may have one of the following values:
0 Success
PROTOCOL_PARSE_ERROR
A SA or DA returns this error when a SrvTypeRqst is
received which cannot be parsed.
SCOPE_NOT_SUPPORTED
A DA which is configured to have a scope will return this
error if it receives a SrvTypeRqst which is set to have a
scope which it does not support. An SA will not return
this error, it will simply silently discard the multicast
request.
CHARSET_NOT_UNDERSTOOD
If the DA receives a SrvTypeRqst in a character set which
it does not support, it MUST use this error.
The service type's name is provided in the <Service Type String>. If
the service type has a naming authority other than "IANA" it should
be returned following the service type string and a "." character.
See section 20.2.1 for the formal definition of this field. User
Agents calculate Service Specific Multicast addresses based on a hash
of the Service Type (see Section 3.6.2). This multicast address may
then be used for issuing Service and Attribute Requests directly to
SAs.
The following are examples of Service Type Strings which might be
found in Service Type Replies:
service:lpr://
service:http://
service:nfs://
9. Service Registration Message Format
After a Service Agent has found a Directory Agent, it begins to
register its advertised services one at a time. A Service Agent must
wait for some random time uniformly distributed within the range
specified by CONFIG_INTERVAL_11 before registering again.
Registration is done using the Service Registration message
specifying all attributes for a service. If the service registration
in a protected scope 16.1, then the service MUST include both a URL
Authentication block and an Attribute Authentication block (see
section 4.3). In that case, the service agent MUST set both the 'U'
bit and the 'A' bit (see section 4).
A Directory Agent must acknowledge each service registration request.
If authentication blocks are included, the Directory Agent MUST
verify the authentication before registering the service. This
requires obtaining key information, either by preconfiguration,
maintenance of a security association with the service agent, or
acquiring the appropriate certificate.
The format of a Service Registration is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvReg) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <URL-Entry> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of Attr List String | <attr-list> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <attr-list>, Continued. \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (if present) Attribute Authentication Block ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The <URL-Entry> is defined at the end of Section 4.2. The <attr-
list> is defined in Section 20.3. The Attribute Authentication
Block, which is only present if the 'A' bit is set in the message
header, is defined in section 4.3.
Service registration may use a connectionless protocol (e.g. UDP),
or a connection oriented protocol (e.g. TCP). If the registration
operation may contain more information than can be sent in one
datagram, the Service Agent MUST use a connection oriented protocol
to register itself with the DA. When a Service Agent registers the
same attribute class more than once for a service instance, the
Directory Agent overwrites the all the values associated with that
attribute class for that service instance. Separate registrations
must be made for each language that the service is to be advertised
in.
If a SA attempts to register a service with a DA and the registration
is larger than the site path MTU, then the DA will reply with a
SrvAck, with the error set to INVALID_REGISTRATION and the 'Overflow'
byte set.
An example of Service Registration information is:
Lifetime (seconds): 16-bit unsigned integer
URL (at least): service:<srvtype>://<addr-spec>
Attributes (if any): (ATTR1=VALUE),KEYWORD,(ATTR2 = VAL1, VAL2)
In order to offer continuously advertised services, Service Agents
should start the reregistration process before the Lifetime they used
in the registration expires.
An example of a service registration (valid for 3 hours) is as
follows:
Lifetime: 10800
URL: service:lpr://igore.wco.ftp.com:515/draft
Attributes: (SCOPE=DEVELOPMENT),
(PAPER COLOR=WHITE),
(PAPER SIZE=LETTER),
UNRESTRICTED_ACCESS,
(LANGUAGE=POSTSCRIPT, HPGCL),
(LOCATION=12 FLOOR)
The same registration could be done again, as shown below, in German;
however, note that "lpr", "service", and "SCOPE" are reserved terms
and will remain in the language they were originally registered
(English).
Lifetime: 10800
URL: service:lpr://igore.wco.ftp.com:515/draft
Attributes: (SCOPE=ENTWICKLUNG),
(PAPIERFARBE=WEISS),
(PAPIERFORMAT=BRIEF),
UNBEGRENTZTER_ZUGANG,
(DRUECKERSPRACHE=POSTSCRIPT,HPGCL),
(STANDORT=11 ETAGE)
Scoped registrations must contain the SCOPE attribute. Unscoped
registrations must be registered with all unscoped Directory Agents.
Registrations of a previously registered service are considered an
update. If such an attribute registration is performed in a
protected scope (see section 16.1), a new Attribute Authentication
block must also be included, and the 'A' bit set in the registration
message header.
The new registration's attributes replace the previous
registration's, but do not effect attributes which were included
previously and are not present in the update.
For example, suppose service:x://a.org has been registered with attributes A=1, B=2, C=3. If a new 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. In the example above, the SCOPE is set to DEVELOPMENT (in English) and ENTWICKLUNG (in German). Recall that all strings in a message must be in one language, which is specified in the header. The string SCOPE is *not* translated, as it is one of the reserved strings in the Service Location Protocol (see section 17.2.) The Directory Agent may return a server error in the acknowledgment. This error is carried in the Error Codes field of the service location message header. A Directory Agent MUST decline to register a service if it is specified with an unsupported scope. In this case a SCOPE_NOT_SUPPORTED error is returned in the SrvAck. A Directory Agent MUST NOT accept Service Registrations which have an unsupported scope unless it is an unscoped Directory Agent, in which case it MUST accept all Service Registrations. An unscoped Service Registration will match all requests. A request which specifies a certain scope will therefore return services which have that scope and services which are unscoped. It is strongly suggested that one should use scopes in all registrations or none. See Sections 16 and 3.7 for details. When the URL entry accompanying a registration also contains an authentication block (section 4.3), the DA MUST perform the indicated authentication, and subsequently indicate the results in the Service Acknowledgement message. 10. Service Acknowledgement Message Format A Service Acknowledgement is sent as the result of a DA receiving and processing a Service Registration or Service Deregistration. An acknowledgment indicating success must have the error code set to zero. Once a DA acknowledges a service registration it makes the information available to clients. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Service Location header (function = SrvAck) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Error Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Error Code may have one of the following values:
0 Success
PROTOCOL_PARSE_ERROR
A DA returns this error when the SrvReg or SrvDereg is
received which cannot be parsed or the declared string
lengths overrun the message.
INVALID_REGISTRATION
A DA returns this error when a SrvReg or SrvDeReg is
invalid. For instance, an invalid URL, unknown or
malformed attributes, or deregistering an unregistered
service all cause this error to be reported.
SCOPE_NOT_SUPPORTED
A DA which is configured to have a scope will return this
error if it receives a SrvReq which is set to have a
scope which it does not support.
CHARSET_NOT_UNDERSTOOD
If the DA receives a SrvReg or SrvDereg in a character
set which it does not support, it will return this error.
AUTHENTICATION_ABSENT
If DA has been configured to require an authentication
for any service registered in the requested scope, and
there are no authentication blocks in the registration,
the DA will return this error.
AUTHENTICATION_FAILED
If the registration contains an authentication block
which fails to match the correct result as calculated
(see section 4.3) over the URL or attribute data to be
authenticated, the DA will return this error.
If the Directory Agent accpets a Service Registration, and already
has an existing entry, it updates the existing entry with the new
lifetime information and possibly new attributes and new attribute
values. Otherwise, if the registration is acceptable (including all
necessary authentication checks) the Directory Agent creates a new
entry, and sets the 'F' bit in the Service Acknowledgement returned
to the Service Agent.
11. Service Deregister Message Format When a service is no longer available for use, the Service Agent must deregister itself from Directory Agents that it has been registered with. A service uses the following PDU to deregister itself. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Service Location header (function = SrvDereg) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | length of URL | URL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | \ URL of Service to Deregister, contd. \ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | (if present) authentication block ..... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | length of <tag spec> string | <tag spec> | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | \ <tag spec>, continued \ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Service Agent should retry this operation if there is no response from the Directory Agent. The Directory Agent acknowledges this operation with a Service Acknowledgment message. Once the Service Agent receives an acknowledgment indicating success, it can assume that the service is no longer advertised by the Directory Agent. The Error Code in the Acknowledgment of the Service Deregistration may have the same values as described in section 10. The Service Deregister Information sent to the directory agent has the following form: service:<srvtype>://<addr-spec> Attribute tags (if any): ATTR1,KEYWORD,ATTR2 This will deregister the specified attributes from the service information from the directory agent. If no attribute tags are included, the entire service information is deregistered in every language and every scope it was registered in. To deregister the printer from the preceding example, use: service:lpr://igore.wco.ftp.com:515/draft
If the service was originally registered with a URL entry containing a URL authentication block, then the Service Deregistration message header MUST have the 'U' bit set, and the URL entry is then followed by the authentication block, with the authenticator calculated over the URL data, the timestamp, and the length of the authenticator as explained in section 4.3. In this calculation, the lifetime of the URL data is considered to be zero, no matter what the current value for the remaining lifetime of the registered URL. 12. Attribute Request Message Format The Attribute Request is used to obtain attribute information. The UA supplies a request and the appropriate attribute information is returned. If the UA supplies only a Service Type, then the reply includes all attributes and all values for that Service Type. The reply includes only those attributes for which services exist and are advertised by the DA or SA which received the Attribute Request. Since different instances of a given service can, and very likely will, have different values for the attributes defined by the Service Type, the User Agent must form a union of all attributes returned by all service Agents. The Attribute information will be used to form Service Requests. If the UA supplies a URL, the reply will contain service information corresponding to that URL. Attribute Requests include a 'select clause'. This may be used to limit the amount of information returned. If the select clause is empty, all information is returned. Otherwise, the UA supplies a comma delimited list of attribute tags and keywords. If the attribute or keyword is defined for a service, it will be returned in the Attribute Reply, along with all registered values for that attribute. If the attribute selected has not been registered for that URL or Service Type, the attribute or keyword information is simply not returned.
The Attribute Request message has the following form:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = AttrRqst) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|length of prev resp list string|<Previous Responders Addr Spec>|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Previous Responders Addr Spec>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of URL | URL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ URL, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of <Scope> | <Scope> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Scope>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of <select-list> | <select-list> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <select-list>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The <Previous Responder Address List> functions exactly as introduced
in Section 7. See also Section 20.1.
The URL can take two forms: Either it is simply a Service Type, such
as "service:http:", or it can be a URL, such as
"service:lpr://igore.wco.ftp.com:515/draft". In the former case, all
attributes and the full range of values for each attribute for the
Service Type is returned. In the latter case, only the attributes
for the service whose URL is defined are returned.
The Scope String is provided so that Attribute Requests for Service
Types can be made so that only the Attribute information pertaining
to a specific scope will be returned. This field is ignored in the
case when a full URL is sent in the Attribute Request. The rules for
encoding of the Scope String are given in Section 5.4.
The select list takes the form:
<select-list> ::= <select-item> |
<select-item> ',' <select-list>
<select-item> ::= <keyword> | <attr-tag> | <partial-tag> '*'
<partial-tag> ::= the partial class name of an attribute
If followed by an '*', it matches all class names
which begin with the partial tag. If preceded by
a partial tag. If both preceded and followed by
'*' it matches all class names which contain the
partial tag.
For definitions of <attr-tag> and <keyword> see 5.4.
An example of a select-list following the printer example is:
PAGES PER MINUTE, UNRESTRICTED_ACCESS, LOCATION
If sent to a Directory Agent, the number of previous responders is
zero and there are no Previous Responder Address Specification.
These fields are only used for repeated multicasting, exactly as for
the Service Request.
13. Attribute Reply Message Format
An Attribute Reply Message takes the form:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = AttrRply) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | length of <attr-list> string |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <attr-list> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Error Code may have the following values:
0 Success
LANGUAGE_NOT_SUPPORTED
A SA or DA returns this when a request is received from a
UA which is in a language for which there is no
registered Service Information and the request arrived
with the Monolingual bit set. See Section 17.
PROTOCOL_PARSE_ERROR
A DA or SA returns this error when a AttrRqst is received
which cannot be parsed or the declared string lengths
overrun the message.
SCOPE_NOT_SUPPORTED
A DA which is configured to have a scope will return this
error if it receives an AttrRqst which is set to have a
scope which it does not support. SAs will silently
discard multicast AttrRqst messages for scopes they do
not support.
CHARSET_NOT_UNDERSTOOD
If the DA receives an AttrRqst in a character set which
it does not support, it will return this error. SAs will
silently discard multicast AttrRqst messages which arrive
using character sets they do not support.
The <attr-list> (attribute list) has the same form as the attribute
list in a Service Registration, see Section 20.3 for a formal
definition of this field.
An Attribute Request for "lpr" might elicit the following reply
(UNRESTRICTED_ACCESS is a keyword):
(PAPER COLOR=WHITE,BLUE),
(PAPER SIZE=LEGAL,LETTER,ENVELOPE,TRACTOR FEED),
UNRESTRICTED_ACCESS,
(PAGES PER MINUTE=1,3,12),
(LOCATION=12th, NEAR ARUNA'S OFFICE),
(QUEUES=LEGAL,LETTER,ENVELOPE,LETTER HEAD)
If the message header has the 'A' bit set, the Attribute Reply will
have an Attribute Authentication block set. In this case, the
Attribute Authenticator must be returned with the entire list of
attributes, exactly as it was registered by an SA in a protected
scope. In this case, the URL was registered in a protected scope and
the UA included a URL but not a select clause. If the AttrRqst
specifies that only certain attributes are to be returned, the DA
does not (typically cannot) compute a new Authenticator so it simply
returns the attributes without an authenticator block.
A UA which wishes to obtain authenticated attributes for a service in a protected scope MUST therefore must include a particular URL and no select list with the AttrRqst. 14. Directory Agent Advertisement Message Format Directory Agent Advertisement Messages have the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Service Location header (function = DAAdvert) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Error Code | Length of URL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | \ URL \ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Length of <Scope-list> | <Scope-list> | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | \ <Scope-list>, continued \ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Error Code is set when a DA Advertisement is returned as the result of a Service Request. It will always be set to 0 in the case of an unsolicited DA Advertisement. The Error Code may take the values specified in Section 6. The URL corresponds to the Directory Agent's location. The <Scope- list> is a comma delimited list of scopes which the DA supports, in the following format: <Scope-list> ::= <Scope> | <Scope-list> ',' <Scope> <Scope> ::= String representing a scope See Section 5.4 for the lexical rules regarding <Scope>. DA Advertisements sent in reply to a Directory Agent Discovery Request has the same format as the unsolicited DA Advertisement, for example: URL: service:directory-agent://SLP-RESOLVER.CATCH22.COM SCOPE List: ADMIN
The Directory Agent can be reached at the Address Specification returned, and supports the SCOPE called "ADMIN". 15. Directory Agents 15.1. Introduction A Directory Agent acts on behalf of many Service Agents. It acquires information from them and acts as a single point of contact to supply that information to User Agents. The queries that a User Agent multicasts to Service Agents (in an environment without a Directory Agent) are the same as queries that the User Agent might unicast to a Directory Agent. A User Agent may cache information about the presence of alternate Directory Agents to use in case a selected Directory Agent fails. Aside from enhancing the scalability of the protocol (see section 3.7), running multiple DAs provides robustness of operation. The DAs may have replicated service information which remain accessible even when one of the DAs fail. Directory Agents, in the future, may use mechanisms outside of this protocol to coordinate the maintenance of a distributed database of Service Location information, and thus scale to enterprise networks or larger administrative domains. Each Service Agent must register with all DAs they are configured to use. UAs may choose among DAs they are configured to use. Locally, Directory Agent consistency is guaranteed using mechanisms in the protocol. There isn't any Directory to Directory Agent protocol yet. Rather, passive detection of DAs by SAs ensures that eventually service information will be registered consistently between DAs. Invalid data will age out of the Directory Agents leaving only transient stale registrations even in the case of a failure of a Service Agent. 15.2. Finding Directory Agents A User or Service Agent may be statically configured to use a particular DA. This is discouraged unless the application resides on a network where any form of multicast or broadcast is impossible. Alternatively, a host which uses DHCP [2, 11] may use it to obtain a Directory Agent's address. DHCP options 78 and 79 have been assigned for this purpose [21]. The third way to discover DAs is dynamically. This is done by sending out a Directory Agent Discovery request (see Section 5.2).
Lastly, the agent may be informed passively as follows: When a Directory Agent first comes on-line it sends an unsolicited DA Advertisement to the Service Location general multicast address. If a DA supports a particular scope or set of scopes these are placed in the reply. The class for this attribute is 'SCOPE'. Every CONFIG_INTERVAL_9 a Directory Agent will send an unsolicited DA Advertisement. This will ensure that eventually it will be discovered by all applications which are concerned. When a Directory Agent first comes up it begins with 0 as its XID, and increments this by one each time it sends an unsolicited DA Advertisement. When the counter wraps, it should go from 0xFFFF to 0x0100, not 0. If the Directory Agent has stored all of the service information in a nonvolatile store, it should initially set the XID to 0x100, as it is not coming up 'stateless.' If it stores service registrations in memory only, it will restart without any state. It should indicate this by resetting its XID to 0. All Service Agents which receive the unsolicited DA Advertisement should examine its XID. If the Directory Agent has never before been heard from or if the XID is less than it was previously and less than 256, the Service Agent should assume the DA does not have its service registration, even if it once did. If this is the case and the DA has the proper scope, the SA should register all service information with the Directory Agent, after waiting a random interval CONFIG_INTERVAL_10. When a Service Agent or User Agent first comes on-line it must issue a Directory Agent Discovery Request unless it is using static or DHCP configuration, as described in 5.2. A Service Agent registers information with ALL newly discovered Directory Agents when either of the above two events take place. When scopes are being used, a Service Agent SHOULD choose a set of scopes to be advertised in and need only register with Directory Agents that support the scopes in which they wish to be registered. Services MUST be registered with DAs that support their scope and those which have no scope, unless specifically configured not to do so (see section 22.1.)
Once a User Agent becomes aware of a Directory Agent it will unicast its queries there. In the event that more than one Directory Agent is detected, it will select one to communicate with. When scopes are supported, the User Agent will direct its queries to different Directory Agents depending on which scopes are appropriate domains for the query to be answered in. The protocol will cause all DAs (of the same scope) to eventually obtain consistent information. Thus one DA should be as good as any other for obtaining service information. There may be temporary inconsistencies between DAs. 16. Scope Discovery and Use The scope mechanism in the Service Location Protocol enhances its scalability. The primary use of scopes is to provide the capability to organize a site network along administrative lines. A set of services can be assigned to a given department of an organization, to a certain building or geographical area or for a certain purpose. The users of the system can be presented with these organizational elements as a top level selection, before services within this domain are sought. A site network that has grown beyond a size that can be reasonably serviced by a few DAs can use the scope mechanism. DAs have the attribute class "SCOPE". The values for this attribute are a list of strings that represent the administrative areas for which this Directory Agent is configured. The semantics and language of the strings used to describe the scope are almost entirely the choice of the administrative entity of the particular domain in which these scopes exist. The values of SCOPE should be configurable, so the system administrator can set its value. The scopes "LOCAL" and "REMOTE" are reserved and SHOULD NOT be used. Use of these reserved values is to be defined in a future protocol document. Services with the attribute SCOPE should only be registered with DAs which support the same scope or DAs which have no scope. Directory Agents advertise their available scopes. A Service Agent may then choose a scope in which to register, and SHOULD register with all Directory Agents in that scope, as well as all DAs which have no scope. Failure to be comprehensive in registration according to this rule will mean that the service advertisement may not be available to all User Agents.
A Directory Agent which has a scope will return advertisements in
response to Directory Agent Discovery requests with the scope
information included. Note that the "service:directory-agent" scheme
is registered with the IANA naming authority (which is automatically
selected by leaving the Naming Authority field empty.)
The query:
directory-agent/MATH DEPT//
Could receive the following DA Advertisement:
Returned URL: service:directory-agent://diragent.blah.edu
Returned SCOPE: MATH DEPT
The same Directory Agent if it had no scope value would reply:
Returned URL: service:directory-agent://diragent.void.com
Returned SCOPE:
If a Directory Agent supported more than one scope it would reply as:
Returned URL: service:directory-agent://srv.domain.org
Returned SCOPE: MATH DEPT,ENGLISH DEPT,CS DEPT
A DA which has no scope will reply to any Directory Agent Discovery
Request.
Being a member of a scope means that an agent SHOULD use those
Directory Agents that support its scope. User Agents send all
requests to DAs which support the indicated scope. Services are
registered with the DA(s) in their scope. For a UA to find a service
that is registered in a particular scope it must send requests to a
DA which supports the indicated scope. There is no limitation on
scope membership built into the protocol; that is to say, a User
Agent or Service Agent may be a member of more than one scope.
Membership is open to all, unless some external authorization
mechanism is added to limit access.
16.1. Protected Scopes
Scope membership MAY also define the security access and
authorization for services in the scope; such scopes are called
protected scopes. If a User Agent wishes to be sure that Service
Agents are authorized to provide the service they advertise, then the
User Agent should request services from a protected scope which has
been configured to have the necessary authentication mechanism and
keys distributed to the Service Agents within the scope. A directory
agent distributing URLs for services in a protected scope will reject any registrations or deregistrations for service agents which cannot provide cryptographically strong authentication to prove their authorization to provide the services. For instance, if a campus registrar wishes to find a working printer to produce student grade information for mailing, the registrar would require the printing user agent to transmit the printable output only to those printing Service Agents which have been registered in the appropriate protected scope. Notice that each service agent is, under normal circumstances, validated two times: once when registering with the directory agent, and once when the user agent validates the URL received with the Service Reply. This protects against the possibilities of malicious Directory Agents as well as malicious Service Agents. Note that services in protected scopes provide separate authentication for their URL entry, and for their attributes. This follows naturally from the needs of the protocol operation. User Agents which specify a service type and attributes needed for service in that service type will not receive attribute information from the directory agent; they will only receive the appropriate URL entries. Only the information returned needs to be authenticated. User agents which receive attribute information for a particular URL (see section 12), on the other hand, need to authenticate the attributes when they are returned (see section 13). In this case, there may be much more data to authenticate, but this operation is also performed much less often, usually only while the user is browsing the available network resources. 17. Language and Character Encoding Issues All Service Registrations declare the language in which the strings in the service attributes are written by specifying the appropriate code in the message header. For each language the Service advertises a separate registration takes place. Each of these registrations uses the same URL to indicate that they refer to the same service. If a Service is fully deregistered (the URL is given in the Service Deregister request, without any attribute information) then the Service needs to be deregistered only once. This will effectively deregister the service in all languages it has been registered in.
If, on the other hand, attribute information is included in the Service Deregistration request, a separate Service Deregistration of selected attributes must be undertaken in each language in which service information has been provided to the DA by a Service Agent. Service Registrations in different languages are mutually unintelligible. They share no information except for their service type and URL with which they were registered. No attempt is made to match queries with "language independence." Instead, queries are handled using string matching against registrations in the same language as the query. Service Types which are standardized will have definitions for all attributes and value strings. Official translations to other languages of the attribute tags and values may be created and submitted as part of the standard; this is not feasible for all languages. For those languages which are not defined as part of the Service Type, a best effort translation of the standard definitions of the Service type's attribute strings MAY be used. All Service Requests specify a requested language in the message header. The Directory Agent or Service Agent will respond in the same language as the request, if it has a registration in the same language as the request. If this language is not supported, and the Monolingual bit is not specified, a reply can be sent in the default language (which is English.) If the 'monolingual bit' flag in the header is set and the requested language is not supported, a SrvRply is returned with the error field set to LANGUAGE_NOT_SUPPORTED. If a query is in a supported language on a SA or DA, but has a different dialect than the available service information, the query MUST be serviced on a best-effort basis. If possible, the query should be matched against the same dialect. If that is not possible, it MAY be matched against any dialect of the same language. 17.1. Character Encoding and String Issues Values for character encoding can be found in IANA's database http://www.isi.edu/in-notes/iana/assignments/character-sets and have the values referred by the MIBEnum value. The encoding will determine the interpretation of all character data which follows the Service Location Protocol header. There is no way to mix ASCII and UNICODE, for example. All responses must be in the character set of the request, or use US-ASCII. If a request is sent to a DA or SA or a registration is sent to a DA, which is unable to manipulate or store the character set of the incoming message, the request will fail. The SA or DA returns a CHARSET_NOT_UNDERSTOOD error in a SrvAck message in this case. Requests using US-ASCII will
never fail for this reason, since all SAs and DAs must be able to accept this character set. Certain characters are illegal in certain contexts of the protocol. Since the protocol is largely character string based, in some contexts characters are used as protocol delimiters. In these cases the delimiting characters must not be used as 'data text.' 17.1.1. Substitution of Character Escape Sequences The Service Location Protocol has an 'escape mechanism' which is consistent with HTTP 2.0 [5] and SGML [15]. If the character sequence "&#" is followed by one or more digits, followed by a semicolon ';' the entire sequence is interpreted as a single character. The digits are interpreted as a decimal value in the character set of the request, as specified by the header. Thus, in US-ASCII , would be interpreted as a comma. Substitution of these escape strings must be done in all <attr-list> and strings present in SrvReq and AttrRqst messages. Only numerical character references are accepted, not 'Entity References,' as defined in HTML. These escape values should only be used to provide a mechanism for including reserved characters in attribute tag and value strings. The interpretation of these escape values is different than in HTML in one respect: In HTML the escape values are considered to be in the ISO Latin-1 character set. In Service Location they are interpreted in the character set defined in the header of the message. This escape mechanism allows characters like commas to be included in attribute tags and values, which would otherwise be illegal as the comma is a protocol delimiter. Attribute tags and values of different languages are considered to be mutually unintelligible. A query in one language SHOULD use service information registered in that language. 17.2. Language-Independent Strings Some strings, such as Service Type names, have standard definitions. These strings should be considered as tokens and not as words in a language to be translated.
Reserved String Section xDefinition
--------------- ------- --------------------------------------
SCOPE 3, 15 Used to limit the matching of requests.
SERVICE 6, 9 The URL scheme of all Service Location
information registered with a DA or
returned from a Service Request.
<srvtype> 20.2.1 Used in all service registrations
and replies.
domain names 20.4 A fully qualified domain name, used
in registrations and replies.
IANA 3.3 The default naming authority.
LOCAL 16 Reserved.
REMOTE 16 Reserved.
TRUE 20.5 Boolean true.
FALSE 20.5 Boolean false.
(page 50 continued on part 3)
