    <chapter>
	<title>Introduction to SER</title>

	<section id="requestrouting">
	    <title>Request Routing and SER Scripts</title>
	    <para>
		The most important concept of every SIP server is that of
		request routing. The request routing logic determines the next
		hop of a request. It can be for example used to implement user 
		location service or enforce static routing to a gateway. Real-world
		deployments actually ask for quite complex routing logic, which
		needs to reflect static routes to PSTN gateways, dynamic routes
		to registered users, authentication policy, capabilities of
		SIP devices, etc.
	    </para>
	    <para>
		SER's answer to this need for routing flexibility is a routing
		language, which allows administrators to define the SIP request
		processing logic in a detailed manner. They can for example easily
		split SIP traffic by method or destination, perform user location, 
		trigger authentication, verify access permissions, and so on.
	    </para>
	    <para>
		The primary building block of the routing language are <emphasis>actions</emphasis>. 
		There are built-in actions (like <command>forward</command> for stateless forwarding
		or <command>strip</command> for stripping URIs) as 
		well as external actions imported from shared library modules. All actions can 
		be combined in compound actions by enclosing them in braces, 
		e.g. <command>{a1(); a2();}</command>. 
		Actions are aggregated in one or more <emphasis>route blocks</emphasis>.
		Initially, only the default routing block denoted by <command>route[0]</command>
		is called. Other routing blocks can be called by the action
		<command>route(blocknumber)</command>, recursion is permitted.
		The language includes <emphasis>conditional statements</emphasis>. 
	    </para>

	    <para>
		The routing script is executed for every received request in sequential order. 
		Actions may return positive/negative/zero value. 

		Positive values are considered success and evaluated as
		TRUE in conditional expressions. Negative values are considered FALSE. 

		Zero value means error and leaves execution of currently processed
		route block. The route block is left too, if <command>break</command> is explicitly
		called from it.

	    </para>
	    <para>
		The easiest and still very useful way for <application>ser</application>
		users to affect request routing logic is
		to determine next hop statically. An example is
		routing to a PSTN gateway whose static IP address is well known.
		To configure static routing, simply use the action
		<command>forward( IP_address, port_number)</command>.
		This action forwards an incoming request "as is" to the
		destination described in action's parameters.
	    </para>

	    <example>
		<title>Static Forwarding</title>
		<programlisting format="linespecific">
# if requests URI is numerical and starts with
# zero, forward statelessly to a static destination

if (uri=~"^sip:0[0-9]*@iptel.org") {
    forward( 192.168.99.3, 5080 );
} 
		</programlisting>
	    </example>

	    <para>
		However, static forwarding is not sufficient in many cases.
		Users desire mobility and change their location frequently.
		Lowering costs for termination of calls in PSTN requires
		locating a least-cost gateway. Which next-hop is taken may
		depend on user's preferences. These and many other scenarios
		need the routing logic to be more dynamic. We describe in
		<xref linkend="conditions"> how to make request processing
		subject to various conditions and in 
		<xref linkend="urirewriting"> how to determine next SIP hop.
	    </para>
	</section>

	<section id="conditions">
	    <title>Conditional Statements</title>
	    <para>
		A very useful feature is the ability to make routing
		logic depend on a condition. A script condition may for
		example distinguish between request processing for
		served and foreign domains, IP and PSTN routes,
		it may split traffic by method or username, it
		may determine whether a request should be authenticated
		or not, etc. <application moreinfo="none">ser</application>
		allows administrators to form conditions based on
		properties of processed request, such as method or uri,
		as well as on virtually any piece of data on the
		Internet.
	    </para>
	    <example>
		<title>Conditional Statement</title>
		<para>
		    This example shows how a conditional statement is
		    used to split incoming requests between a PSTN
		    gateway and a user location server based on
		    request URI.
		</para>
		<programlisting format="linespecific">
# if request URI is numerical, forward the request to PSTN gateway...
if (uri=~"^sip:[0-9]+@foo.bar") { # match using a regular expression
    forward( gateway.foo.bar, 5060 );
} else { # ... forward the request to user location server otherwise
    forward( userloc.foo.bar, 5060 );
};
		</programlisting>
	    </example>

	    <para>
		Conditional statements in <application>ser</application> scripts may depend
		on a variety of  expressions. The simplest expressions are 
		action calls. They return true if they completed successfully or false otherwise. 
		An example of an action frequently used in conditional statements is
		<command moreinfo="none">search</command> imported from textops module.
		<command moreinfo="none">search</command> action leverages textual
		nature of SIP and compares SIP requests against a regular expression.
		The action returns true if the expression matched, false otherwise.
		<example>
		    <title>Use of <command>search</command> Action in Conditional Expression</title>
		    <programlisting format="linespecific">
# prevent strangers from claiming to belong to our domain;
# if sender claims to be in our domain in From header field,
# better authenticate him 
if (search("(f|From): .*@mydomain.com)) {
    if (!(proxy_authorize("mydomain.com" /* realm */,"subscriber" /* table name */ ))) {
           proxy_challenge("mydomain.com /* ream */, "1" /* use qop */ );
           break;
    }
}
 		    </programlisting>
		</example>
	    </para>
	    <para>
		As modules may be created, which export new functions, there is virtually
		no limitation on what functionality <application moreinfo="none">ser</application>
		conditions are based on. Implementers may introduce new actions whose
		return status depends on request content or any external data as well. Such actions
		can query SQL, web, local file systems or any other place which can provide
		information wanted for request processing.
	    </para>
	    <para>
		Furthermore, many request properties may be examined using existing built-in operands
		and operators. Available left-hand-side operands and legal combination with
		operators and right-hand-side operands are described in <xref linkend="logicalexpr">.
		Expressions may be grouped together using logical operators:
		negation (<command>!</command>), AND (<command>&&</command>), OR (<command moreinfo="none">
		||</command> and precedence parentheses (<command>()</command>).
	    </para>

	    <section id="operators">
		<title>Operators and Operands</title>
		<para>
		    There is a set of predefined operators and operands
		    in ser, which in addition to actions may be evaluated
		    in conditional expressions. 
		</para>
		<para>
		    Left hand-side operands, which <application>ser</application>
		    understands are the following:
		    <itemizedlist>
			<listitem>
			    <para>
				<emphasis>method</emphasis>, which refers to 
				request method
				such as REGISTER or INVITE
			    </para>
			</listitem>
			<listitem>
			    <para>
				<emphasis>uri</emphasis>, which refers to current request URI,
				such as 
				"sip:john.doe@foo.bar"
				<note>
				    <para>
					Note that "uri" always refers to current
					value of URI, which is subject to change
					be uri-rewriting actions.
				    </para>
				</note>
			    </para>
			</listitem>
			<listitem>
			    <para>
				<emphasis>src_ip</emphasis>, which refers to IP address from 
				which a request came.
			    </para>
			</listitem>
			<listitem>
			    <para>
				<emphasis>dst_ip</emphasis> refers to server's IP address 
				at which a request was received
			    </para>
			</listitem>
			<listitem>
			    <para>
				<emphasis>src_port</emphasis> port number from which a SIP
				request came
			    </para>
			</listitem>
		    </itemizedlist>
		</para>
		<para>
		    ser understands the following operators:
		    <itemizedlist>
			<listitem>
			    <para>
				== stands for equity
			    </para>
			    
			</listitem>
			<listitem>
			    <para>
				=~ stands for regular expression matching
			    </para>
			</listitem>
			<listitem>
			    <para>
				logical operators: and, or, negation, parentheses
				(C-notation for the operators may be used too)
			    </para>
			</listitem>
		    </itemizedlist>
		</para>

	    <table id="logicalexpr">
		<title>Valid Combinations of Operands and Operators in Expressions</title>
		<tgroup cols="4">
		    <thead>
			<row>
			    <entry>
				left-hand-side operand
			    </entry>			    
			    <entry>
				valid operators
			    </entry>
			    <entry>
				valid right-hand side operators
			    </entry>
			    <entry>
				examples/comments
			    </entry>
			</row>

		    </thead>
		    <tbody>

			<row>
			    <entry>
				method
			    </entry>			    
			    <entry>
				== (exact match), =~ (regular expression matching)
			    </entry>
			    <entry>
				string
			    </entry>
			    <entry>
				method=="INVITE" || method=="ACK" || method=="CANCEL"
			    </entry>
			</row>			

			<row>
			    <entry>
				uri
			    </entry>			    
			    <entry>
				== (exact match), =~ (regular expression matching)
			    </entry>
			    <entry>
				string
			    </entry>
			    <entry>
				uri=="sip:foo@bar.com" matches only if exactly this uri
				is in request URI

			    </entry>
			</row>				

			<row>
			    <entry>
				
			    </entry>			    
			    <entry>
				== (exact match)
			    </entry>
			    <entry>
				myself
			    </entry>
			    <entry>
				
				the expression uri==myself is true if the host part in 
				request URI equals a server name or a server alias (set using
				the alias option in configuration file)
				
			    </entry>
			</row>				

			<row>
			    <entry>
				src_ip
			    </entry>			    
			    <entry>
				== (match)
			    </entry>
			    <entry>
				IP, IP/mask_length, IP/mask, hostname, myself
			    </entry>
			    <entry>
				src_ip==192.168.0.0/16 matches requests coming from
				a private network
			    </entry>
			</row>

			<row>
			    <entry>
				dst_ip				
			    </entry>			    
			    <entry>
				== (match)
			    </entry>
			    <entry>
				IP, IP/mask_length, IP/mask, hostname, myself
			    </entry>
			    <entry>
				dst_ip==127.0.0.1 matches if a request was received
				via loopback interface
			    </entry>
			</row>

			<row>
			    <entry>
				    src_port
			    </entry>			    
			    <entry>
				== (match)
			    </entry>
			    <entry>
				port number
			    </entry>
			    <entry>
				port number from which a request was sent, e.g. src_port==5060
			    </entry>
			</row>			    


		    </tbody>
		</tgroup>
	    </table>
	    

		<example>
		    <title>
			More examples of use of <application>ser</application> operators and operands in conditional
			statements
		    </title>
		    <programlisting format="linespecific">
# using an action as condition input; in this
# case, an actions 'search' looks for Contacts
# with private IP address in requests; the condition
# is processed if such a contact header field is
# found

if (search("^(Contact|m): .*@(192\.168\.|10\.|172\.16)")) {
# .... 

# this condition is true if request URI matches
# the regular expression "@bat\.iptel\.org"
    if (uri=~"@bat\.iptel\.org") {
# ...

# and this condition is true if a request came
# from an IP address (useful for example for
# authentication by IP address if digest is not
# supported) AND the request method is INVITE

# if ( (src_ip==192.68.77.110 and method=="INVITE")
# ...
</programlisting>
		</example>
	    </section> <!-- operators and operands -->
	    <section>
		<title>URI Matching</title>
		<para>URI matching expressions have a broad use in a SIP server
		    and deserve more explanation. Typical uses of
		    URI matching include implementation of numbering plans,
		    domain matching,
		    binding external applications to specific URIs,
		    etc. This section shows examples of typical applications
		    of URI-matching.
		</para>
		<section id="domainmatching">
		    <title>Domain Matching</title>
		    <para>
			One of most important uses of URI matching is deciding
			whether a request is targeted to a served or outside domain.
			Typically, different request
			processing applies. Requests for outside domains
			are simply forwarded to them, whereas
			more complex logic applies to requests for a served domain.
			The logic may include saving user's contacts
			when REGISTER requests are received, forwarding requests
			to current user's location or a PSTN gateways, 
			interaction with external applications, etc.
		    </para>
		    <para>
			The easiest way to decide whether a request belongs
			a served domain is using the <command moreinfo="none">myself</command>
			operand. 
			The expression "uri==myself" returns true if domain name
			in request URI matches name of the host at which
			<application moreinfo="none">ser</application> is
			running. This may be insufficient in cases when
			server name is not equal to domain name for which the server
			is responsible. For example, the "uri==myself" condition
			does not match if a server "sipserver.foo.bar" 
			receives a request for "sip:john.doe@foo.bar". To
			match other names in URI than server's own,
			set up the <varname>alias</varname> configuration
			option. The option may be used multiple times,
			each its use adds a new item to a list of aliases.
			The myself condition returns then  true 
			also for any hostname on the list of aliases.
			<example>
			    <title>Use of uri==myself Expression</title>
			    <programlisting format="linespecific">
# ser powers a domain "foo.bar" and runs at host sipserver.foo.bar;
# Names of served domains need to be stated in the aliases
# option; myself would not match them otherwise and would only
# match requests with "sipserver.foo.bar" in request-URI
alias="foo.bar"
alias="sales.foo.bar"
route[0] {
        if (uri==myself) {
            # the request either has server name or some of the
            # aliases in its URI
            log(1,"request for served domain")
            # some domain-specific logic follows here ....
        } else {
            # aha -- the server is not responsible for this
            # requests; that happens for example with the following URIs
            #  - sip:a@marketing.foo.bar
            #  - sip:a@otherdomain.bar
            log(1,"request for outbound domain");
            # outbound forwarding			  
            t_relay();
        };
}			</programlisting>
		    </example>
		</para>
		<para>
		    It is possible to recognize whether a request belongs to
		    a domain using regular expressions too. Care needs to
		    be paid to construction of regular expressions. URI
		    syntax is rich and an incorrect expression would result
		    in incorrect call processing. The following example shows
		    how an expression for domain matching can be formed.
		    <example id="redomainmatching">
			<title>Domain Matching Using Regular Expressions</title>
			<para>
			    In this example, server named "sip.foo.bar" with
			    IP address 192.168.0.10 is responsible for the
			    "foo.bar" domain. That means, requests with the
			    following hostnames in URI should be matched:
			    <itemizedlist>
				<listitem>
				    <para>
					foo.bar, which is the name of server domain
				    </para>
				</listitem>
				<listitem>
				    <para>
					sip.foo.bar, since it is server's name and some
					devices put server's name in request URI
				    </para>
				</listitem>
				<listitem>
				    <para>
					192.168.0.10, since it is server's IP address and
					some devices put server's IP address in request URI
				    </para>
				</listitem>
			    </itemizedlist>			
			    Note how this regular expression is constructed. In particular:
			    <itemizedlist>
				<listitem>
				    <para>
					User name is optional (it is for example never included
					in REGISTER requests) and there are no restrictions on
					what characters it contains. That is what 
					<emphasis>(.+@)?</emphasis> mandates. 
				    </para>
				</listitem>
				<listitem>
				    <para>
					Hostname must be followed by port number, parameters
					or headers -- that is what the delimiters 
					<emphasis>[:;\?]</emphasis> are good for. If none
					it these follows, the URI must be ended 
					(<emphasis>$</emphasis>). Otherwise, longer hostnames
					such as 192.168.0.101 or foo.bar.otherdomain.com would
					mistakenly match.
				    </para>
				</listitem>
				<listitem>
				    <para>
					Matches are case-insensitive. All hostnames "foo.bar", "FOO.BAR"
					and "FoO.bAr" match.
				    </para>
				</listitem>
			    </itemizedlist>
			</para>
			<programlisting>
if (uri=~"^sip:(.+@)?(192\.168\.0\.10|(sip\.)?foo\.bar)([:;\?].*)?$")
      log(1, "yes, it is a request for our domain");
      break;
 };
			</programlisting>
		    </example>
		</para>
		</section> <!-- domain matching -->
		<section id="numberingplans">
		    <title>Numbering Plans</title>

		    <para>
			Other use of URI matching is implementation of dialing
			plans. A typical task when designing a dialing plan for SIP networks
			is to distinguish between "pure-IP" and PSTN destinations.
			IP users typically have either alphanumerical or numerical
			usernames. The numerical usernames are convenient for PSTN
			callers who can only
			use numeric keypads. Next-hop destination of IP users is looked up dynamically
			using user location database. On the other hand, PSTN destinations are 
			always indicated by nummerical usernames. Requests to PSTN are statically 
			forwarded to well-known PSTN gateways.
		    </para>
		    <example>
			<title>A simple Numbering Plan</title>
			<para>
			    This example shows a simple dialing plan which reserves
			    dialing prefix "8" for IP users, other numbers
			    are used for PSTN destinations and all other non-nummerical
			    usernames are used for IP users.
			</para>
			<programlisting format="linespecific">
# is it a PSTN destination? (is username nummerical and does not begin with 8?)
if (uri=~"^sip:[0-79][0-9]*@") { # ... forward to gateways then;
      # check first to which PSTN destination the requests goes;
      # if it is US (prefix "1"), use the gateway 192.168.0.1...
      if (uri=~"^sip:1") {
           # strip the leading "1"
           strip(1);
           forward(192.168.0.1, 5060);
      } else {
           # ... use the gateway 10.0.0.1 for all other destinations
           forward(10.0.0.1, 5060);
      }
      break;
} else {
      # it is an IP destination -- try to lookup it up in user location DB
      if (!lookup("location")) {
          # bad luck ... user off-line
          sl_send_reply("404", "Not Found");
          break;
      }
      # user on-line...forward to his current destination
      forward(uri:host,uri:port);
}
			</programlisting>
		    </example>
		</section> <!-- numbering plans -->
	    </section>
	</section> <!-- conditional statements -->
	
	<section id="urirewriting">
	    <title>Request URI Rewriting</title>

	    <para>
		The ability to give users and services a unique name using URI
		is a powerful tool. It allows users to advertise how to reach
		them, to state to whom they wish to communicate and what services 
		they wish to use.
		Thus, the ability to change URIs is very important and is
		used for implementation of many services. 
		"Unconditional forwarding" from user "boss" to user
		"secretary" is a typical example of application relying
		on change of URI address.
	    </para>
	    <para>
		<application moreinfo="none">ser</application> has the ability
		to change request URI in many ways.
		A script can use any of the following
		built-in actions to change request URI or a part of it:

		<command>rewriteuri</command>, 
		<command>rewritehost</command>, 
		<command>rewritehostport</command>, 
		<command>rewriteuser</command>, 
		<command>rewriteuserpass</command> and 
		<command>rewriteport</command>. 
		When later in the script
		a forwarding action is encountered, the action forwards
		the request to address in the rewritten URI.
		<example>
		    <title>Rewriting URIs</title>
		    <programlisting format="linespecific">
if (uri=~"dan@foo.bar") {
    rewriteuri("sip:bla@somewherelse.com")
    # forward statelessly to the destination in current URI, i.e.,
    # to sip:bla@somewherelese.com:5060
    forward( uri:host, uri:port);
}
		    </programlisting>
		</example>
	    </para>	    
	    <para>Two more built-in URI-rewriting commands are of special importance
		for implementation of dialing plans and manipulation of dialing
		prefixes. <command>prefix(s)
		</command>, inserts 
		a string "s" in front of SIP address and 
		<command>strip(n)</command> takes
		away the first "n" characters of a SIP address.
		See <xref linkend="urirewritingexamples"> for examples of use of
		built-in URI-rewriting actions.
	    </para>

	    <para>
		Commands exported by external modules can change URI too
		and many do so.
		The most important application is changing URI using the
		user location database. The command 
		<command>lookup(table)</command> looks up current
		user's location and rewrites user's address with it.
		If there is no registered contact, the 	command returns a negative value.


		<example id=rewriteuri>
		    <title>Rewriting URIs Using User Location Database</title>
		    <programlisting format="linespecific">
# store user location if a REGISTER appears
if (method=="REGISTER") {
   save("mydomain1");
} else {
# try to use the previously registered contacts to
# determine next hop
   if(lookup("mydomain1")) {
     # if found, forward there...
     t_relay();
   } else {
     # ... if no contact on-line, tell it upstream
     sl_send_reply("404", "Not Found" );
   };
};
		    </programlisting>
		</example>
	    </para>
	    <para>
		External applications can be used to rewrite URI too.
		The "exec" module provides script actions, which start external programs
		and read new URI value from their output. <command moreinfo="none">exec_dset</command>
		both calls an external program, passes SIP request elements to it, waits until it completes,
		and eventually rewrites current destination set with its output.
	    </para>
	    <para>
		It is important to realize that <application moreinfo="none">ser</application>
		operates over <emphasis>current URI</emphasis> all the time. If an original
		URI is rewritten by a new one, the original will will be forgotten and the new one will 
		be used in any further processing. In particular, the uri matching operand
		and the user location action <command moreinfo="none">lookup</command>
		always take current URI as input, regardless what the original URI was.
	    </para>
	    <para>
		<xref linkend="urirewritingexamples"> shows how URI-rewriting actions affect 
		an example URI, sip:12345@foo.bar:6060.
		<table id="urirewritingexamples">
		    <title>URI-rewriting Using Built-In Actions</title>
		    <tgroup cols="2">
			<thead>
			    <row>
				<entry>
				    Example Action
				</entry>				
				<entry>
				    Resulting URI
				</entry>
			    </row>
			</thead>
			<tbody>
			    <row>
				<entry>
				    <command moreinfo="none">rewritehost("192.168.0.10")</command> rewrites
				    the hostname in URI, other parts (including port number) remain unaffected.
				</entry>
				<entry>
				    sip:12345@192.168.10:6060
				</entry>
			    </row>
			    <row>
				<entry>
				    <command moreinfo="none">rewriteuri("sip:alice@foo.bar");</command> rewrites
				    the whole URI completely.
				</entry>
				<entry>
				    sip:alice@foo.bar
				</entry>
			    </row>
			    <row>
				<entry>
				    <command moreinfo="none">rewritehostport("192.168.0.10:3040")</command>rewrites 
				    both hostname and port number in URI.
				</entry>
				<entry>
				    sip:12345@192.168.0.10:3040
				</entry>
			    </row>
			    <row>
				<entry>
				    <command moreinfo="none">rewriteuser("alice")</command> rewrites user part of URI.
				</entry>
				<entry>
				    sip:alice@foo.bar:6060
				</entry>
			    </row>
			    <row>
				<entry>
				    <command moreinfo="none">rewriteuserpass("alice:pw")</command> replaces the pair
				    user:password in URI with a new value. Rewriting password in URI is of historical
					meaning though, since basic password has been replaced with digest authentication.
				</entry>
				<entry>
				    sip:alice:pw@foo.bar:6060
				</entry>
			    </row>
			    <row>
				<entry>
				    <command moreinfo="none">rewriteport("1234")</command> replaces port number in URI
				</entry>
				<entry>
				    sip:12345@foo.bar:1234
				</entry>
			    </row>
			    <row>
				<entry>
				    <command moreinfo="none">prefix("9")</command> inserts a string ahead of user part of URI
				</entry>
				<entry>
				    sip:912345@foo.bar:6060
				</entry>
			    </row>
			    <row>
				<entry>
				    <command moreinfo="none">strip(2)</command> removes leading characters from user part of URI
				</entry>
				<entry>
				    sip:345@foo.bar:6060
				</entry>
			    </row>


			</tbody>
		    </tgroup>
		</table>
	    </para>	    
	    <para>
		You can verify whether you understood URI processing by
		looking at the following example. It rewrites URI
		several times. The question is what is the final URI to which
		the script fill forward any incoming request.
		<example>
		    <title>URI-rewriting Exercise</title>
		    <programlisting format="linespecific">
exec_dset("echo sip:2234@foo.bar; echo > /dev/null");
strip(2);
if (uri=~"^sip:2") {
    prefix("0");
} else {
    prefix("1");
};			
forward(uri:host, uri:port);
		    </programlisting>
		</example>
	    </para>
	    <para>
		The correct answer is the resulting URI will be
		"sip:134@foo.bar". <command moreinfo="none">exec_dset</command>
		rewrites original URI to "sip:2234@foo.bar", 
		<command moreinfo="none">strip(2)</command> takes
		two leading characters from username away resulting
		in "34@iptel.org", the condition does not match
		because URI does not begin with "2" any more,
		so the prefix "1" is inserted.
	    </para>


	</section> <!-- URI rewriting -->

	<section>
	    <title>Destination Set</title>
	    <para>
		Whereas needs of many scenarios can by accommodated by maintaining
		a single request URI, some scenarios are better served by
		multiple URIs. Consider for example a user with address
		john.doe@iptel.org. The user wishes to be reachable at his 
		home phone, office phone, cell phone, softphone, etc. 
		However, he still wishes to maintain a single public address
		on his business card.
	    </para>
	    <para>
		To enable such scenarios, <application>ser</application>
		allows translation of a single request URI into multiple
		outgoing URIs. The ability to forward a request to multiple
		destinations is known as <emphasis>forking</emphasis>
		in SIP language. All outogoing URIs (in trivial case one of them)
		are called <emphasis>destination set</emphasis>. The destination
		set always includes one default URI, to which additional URIs
		can be appended. Maximum size of a destination set is limited by 
		a compile-time constant, MAX_BRANCHES,
		in <filename moreinfo="none">config.h</filename>.
	    </para>
	    <para>
		Some actions are designed for use with a single URI whereas
		other actions work with the whole destination set.
	    </para>
	    <para>
		Actions which are currently available for creating the destination
		set are <command>lookup</command> from usrloc module and 
		<command>exec_dset</command> from exec module.
		<command moreinfo="none">lookup</command> fills in the destination
		set with user contact's registered previously with REGISTER
		requests. The <command moreinfo="none">exec</command> actions
		fill in the destination set with output of an external program.
		In both cases, current destination set is completely rewritten.		
		New URIs can be appended to destination set by a call to the built-in
		action <command>append_branch(uri)</command>.
	    </para>
	    <para>		
		Currently supported features which utilize destination sets
		are <emphasis>forking</emphasis> and <emphasis>redirection</emphasis>. 
		Action <command>t_relay</command> (TM module) for stateful
		forwarding supports forking. If called with a non-trivial destination
		set, <command moreinfo="none">t_relay</command> forks
		incoming request to all URIs in current destination set.
		See <xref linkend="rewriteuri">. If a user
		previously registered from three locations, the destination set is filled with 
		all of them by <command>lookup</command> and the <command>t_relay</command>
		command forwards the incoming request to all these destinations.
		Eventually, all user's phone will be ringing in parallel.
	    </para>
	    <para>
		SIP redirection is another feature which leverages destination sets.
		It is a very light-weighted method to establish communication
		between two parties with minimum burden put on the server. In
		<application>ser</application>, the action <command>sl_send_reply</command>
		(SL module) is used for this purpose. This action 
		allows to generate replies to SIP requests without keeping 
		any state. If the status code passed to the action is 3xx, 
		the current destination set is printed in reply's Contact header
		fields. Such a reply instructs the originating client to 
		retry at these addresses. (See <xref linkend="redirectexample">).
	    </para>
	    <para>
		Most other  <application>ser</application> actions ignore destination
		sets: they either do not relate to URI processing (<command moreinfo="none">
		log</command>, for example) or they work only with the default URI.
		All URI-rewriting functions such as
		<command moreinfo="none">rewriteuri</command> belong in this
		category. URI-comparison operands only refer to the first URI
		(see <xref linkend="operators">). Also, the built-in action
		for stateless forwarding, <command>forward</command> works only
		with the default URI and ignores rest of the destination set. The reason 
		is a proxy server willing to fork must guarantee that the burden
		of processing multiple replies is not put unexpectedly on upstream
		client. This is only achievable with stateful processing.  
		Forking cannot be used along with stateless <command>forward</command>,
		which thus only processes one URI out of the whole destination set.
	    </para> 

	</section> <!-- Destination Set -->

	<section>
	    <title>User Location</title>
	    <para>
		Mobility is a key feature of SIP. Users are able to use one
		one or more SIP devices and be reachable at them. Incoming requests 
		for users are forwarded to all user's devices in use. The key
		concept is that of soft-state registration. Users can
		-- if in possession of valid credentials -- link SIP
		devices to their e-mail like address of record. Their SIP devices
		do so using a REGISTER request, as in <xref linkend="register">.
		The request creates a binding between the public address of
		record (To header field) and SIP device's current address
		(Contact header field). 
		<example id="register">
		    <title>REGISTER Request</title>
		    <programlisting format="linespecific">
REGISTER sip:192.168.2.16 SIP/2.0
Via: SIP/2.0/UDP 192.168.2.16;branch=z9hG4bKd5e5.5a9947e4.0
Via: SIP/2.0/UDP 192.168.2.33:5060
From: sip:123312@192.168.2.16
To: sip:123312@192.168.2.16
Call-ID: 00036bb9-0fd30217-491b6aa6-0a7092e9@192.168.2.33
Date: Wed, 29 Jan 2003 18:13:15 GMT
CSeq: 101 REGISTER
User-Agent: CSCO/4
Contact: sip:123312@192.168.2.33:5060
Content-Length: 0
Expires: 600
		    </programlisting>
		</example>
		Similar requests can be used to query all user's current contacts or to
		delete them. All Contacts have certain time to live, when the time expires,
		contact is removed and no longer used for processing of incoming requests.
	    </para>
	    <para>
		<application moreinfo="none">ser</application> is built to do both: update
		user location database from received REGISTER requests and look-up these
		contacts when inbound requests for a user arrive. To achieve high performance,
		the user location table is stored in memory. In regular intervals
		(usrloc module's parameter <varname>timer_interval</varname> determines
		their length), all changes to the in-memory table are backed up in
		<application moreinfo="none">mysql</application> database to achieve
		peristence accross server reboots. Administrators or application writers
		can lookup list of current user's contacts stored in memory using the
		<application moreinfo="none">serctl</application> tool (see <xref linkend="serctl">).
		<example>
		    <title>Use of <application>serctl</application> Tool to Query User Location</title>
		    <screen format="linespecific">
<![CDATA[
[jiri@fox jiri]$ sc ul show jiri
<sip:jiri@212.202.172.134>;q=0.00;expires=456
<sip:7271@gateway.foo.bar>;q=0.00;expires=36000
]]>
		    </screen>
		</example>
	    </para>
	    <para>
		Building user location in <application moreinfo="none">ser</application> scripts is
		quite easy. One first needs to determine whether a request is for served domain,
		as described in <xref linkend="domainmatching">. If that is the case, the script
		needs to distinguish between REGISTER requests, that update user location table,
		and all other requests for which next hop is determined from the table. The
		<command moreinfo="none">save</command> action is used to update user location
		(i.e., it writes to it). The <command moreinfo="none">lookup</command> actions
		reads from the user location table and fills in destination set with current
		user's contacts.
		<example>
		    <title>Use of User Location Actions</title>
		    <programlisting format="linespecific">
# is the request for my domain ?
if (uri==myself) {
    if (method=="REGISTER") { # REGISTERs are used to update
         save("location");
         break; # that's it, we saved the contacts, exit now
    } else {
         if (!lookup("location") { # no registered contact
            sl_send_reply("404", "Not Found");
            break;
         }
         # ok -- there are some contacts for the user; forward
         # the incoming request to all of them
         t_relay();
    };
};
		    </programlisting>
		</example>
	    </para>
	    <para>
		Note that we used the action for stateful forwarding, 
		<command moreinfo="none">t_relay</command>. That's is because
		stateful forwarding allows to fork an incoming request to
		multiple destinations. If we used stateful forwarding,
		the request would be forwarded only to one uri out of
		all user's contacts.
	    </para>
	</section> <!-- User Location -->
	
	<section>
	    <title>External Modules</title>
	    <para>
		<application moreinfo="none">ser</application> provides the ability to link the server with external
		third-party shared libraries. Lot of functionality which is
		included in the <application moreinfo="none">ser</application> distribution is actually located in
		modules to keep the server "core" compact and clean.
		Among others, there are modules for checking max_forwards
		value in SIP requests (maxfwd), transactional processing (tm),
		record routing (rr), accounting (acc), authentication (auth),
		SMS gateway (sms), replying requests (sl), user location
		(usrloc, registrar) and more.
	    </para>
	    <para>
		In order to utilize new actions exported by a module, 
		ser must first load it. To load a module, the directive
		<command moreinfo="none">loadmodule "filename"</command>
		must be included in beginning of
		a <application>ser</application> script file.
	    </para>

	    <example>
		<title>Using Modules</title>
		<para>
		    This example shows how a script instructs 
		    <application moreinfo="none">ser</application> to
		    load a module and use actions exported by it.
		    Particularly, the sl module exports an action
		    <command>sl_send_reply</command> which makes 
		    <application>ser</application> act as a stateless
		    user agent and reply all incoming requests with 404.
		</para>
		<programlisting format="linespecific">
# first of all, load the module!
loadmodule "/usr/lib/ser/modules/sl.so
route{
    # reply all requests with 404
    sl_send_reply("404", "I am so sorry -- user not found");
}
</programlisting>
	    </example>
	    <note>
		<para>Note that unlike with core commands, all actions
		    exported by modules must have parameters enclosed
		    in quotation marks in current version of 
		    <application moreinfo="none">ser</application>.
		    In the following example, the built-in action
		    <command moreinfo="none">forward</command> for
		    stateless forwarding takes
		    IP address and port numbers as parameters without
		    quotation marks whereas a module action 
		    <command moreinfo="none">t_relay</command> for
		    stateful forwarding takes parameters enclosed in
		    quotation marks.
		    <example>
			<title>Parameters in built-in and exported
			actions</title>
			<programlisting format="linespecific">
# built-in action doesn't enclose IP addresses and port numbers
# in quotation marks
forward(192.168.99.100, 5060);
# module-exported functions enclose all parameters in quotation
# marks
t_relay_to_udp("192.168.99.100", "5060");
			</programlisting>
		    </example>
		</para>
	    </note>
	    <para>
		Many modules also allow users to change the way how they
		work using predefined parameters. For example, the
		authentication module needs to know location of MySQL
		database which contains users' security credentials.
		How module parameters
		are set using the <command moreinfo="none">modparam</command>
		directive is shown in <xref linkend="moduleparameters">. 
		<command moreinfo="none">modparam</command>
		always contains identification of module, parameter
		name and parameter value. Description of parameters
		available in modules is available in module documentation.
	    </para>
	    <para>
		Yet another thing to notice in this example is module
		dependency. Modules may depend on each other. For example,
		the authentication modules leverages the mysql module
		for accessing mysql databases and sl module for generating
		authentication challenges. We recommend that modules are
		loaded in dependency order to avoid ambiguous server
		behaviour.
	    </para>	    
	    <para>
		<example id="moduleparameters">
		    <title>Module Parameters</title>
		    <programlisting format="linespecific">
# ------------------ module loading ----------------------------------

# load first modules on which 'auth' module depends;
# sl is used for sending challenges, mysql for storage
# of user credentials
loadmodule "modules/sl/sl.so"
loadmodule "modules/mysql/mysql.so"
loadmodule "modules/auth/auth.so"

# ------------------ module parameters -------------------------------
# tell the auth module the access data for SQL database:
# username, password, hostname and database name
modparam("auth", "db_url","sql://ser:secret@dbhost/ser")


# -------------------------  request routing logic -------------------

# authenticate all requests prior to forwarding them

route{

        if (!proxy_authorize("foo.bar" /* realm */,
                        "subscriber" /* table name */ )) {
                proxy_challenge("foo.bar", "0");
                break;
        };
        forward(192.168.0.10,5060);
}

		    </programlisting>
		</example>
	    </para>
	</section>

	<section>
	    <title>Writing Scripts</title>
	    <para>
		This section demonstrates simple examples
		how to configure server's behaviour using the
		<application moreinfo="none">ser</application>
		request routing language. All configuration scripts follow the 
		<application moreinfo="none">ser</application> language 
		syntax, which dictates the following section ordering:
		<itemizedlist>
		    <listitem>
			<para>
			    <emphasis>global configuration parameters</emphasis> --
			    these value affect behaviour of the server such as port
			    number at which it listens, number of spawned children
			    processes, and log-level. See <xref linkend="coreoptions">
			    for a list of available options.
			</para>
		    </listitem>

		    <listitem>
			<para>
			    <emphasis>module loading</emphasis> -- these statements
			    link external modules, such as transaction management
			    (tm) or stateless UA server (sl)  dynamically. See
			    <xref linkend="modulereference"> for a list of modules
			    included in <application moreinfo="none">ser</application>
			    distribution.
			</para>
			<note>
				<para>
					If modules depend on each other, than the depending
					modules must be loaded after modules on which they
					depend. We recommend to load first modules
					<command>tm</command> and <command>sl</command>
					because many other modules (authentication, user
					location, accounting, etc.) depend on these.
				</para>
			</note>
		    </listitem>
		    <listitem>
			<para>
			    <emphasis>module-specific parameters</emphasis> -- determine
			    how modules behave; for example, it is possible to configure
			    database to be used by authentication module.
			</para>
		    </listitem>
		    <listitem>
			<para>
			    one or more <emphasis>route blocks</emphasis> containing the
			    request processing logic, which includes built-in actions
			    as well as actions exported by modules. See <xref linkend="builtinref">
			    for a list of built-in actions.
			</para>
		    </listitem>
		    <listitem>
			<para>
			    optionally, if modules supporting reply
			    processing (currently only TM) are loaded,
			    one or more <emphasis>failure_route blocks</emphasis> containing
			    logic triggered by received replies. Restrictions on use of
			    actions within <command moreinfo="none">failure_route</command>
			    blocks apply -- see <xref linkend="builtinref"> for more
			    information.
			</para>
		    </listitem>
		</itemizedlist>
	    </para>

	    <section id="defaultscript">
		<title>Default Configuration Script</title>		
		<para>
		    The configuration script, <filename moreinfo="none">ser.cfg</filename>,
		    is a part of every <application moreinfo="none">ser</application>
		    distribution and defines default behaviour. It allows users
		    to register with the server and have requests proxied to each
		    other.
		</para>
		<para>
		    After performing
		    routine checks, the script looks whether incoming request is for
		    served domain. If so and the request is "REGISTER", <application moreinfo="none">ser</application>
		    acts as SIP registrar and updates database of user's contacts.
		    Optionally, it verifies user's identity first to avoid
		    unauthorized contact manipulation.
		</para>
		<para>
		    Non-REGISTER requests for served domains are then processed using
		    user location database. If a contact is found for requested URI,
		    script execution proceeds to stateful forwarding, a negative 404
		    reply is generated otherwise. Requests outside served domain
		    are always statefully forwarded.
		</para>
		<para>
		    Note that this simple script features several limitations:
		    <itemizedlist>
			<listitem>			    
			    <para>
				By default, authentication is turned off to avoid
				dependency on mysql. Unless it it turned on, anyone
				can register using any name and "steal" someone else's
				calls.
			    </para>
			</listitem>
			<listitem>
			    <para>
				Even it authentication is turned on, there is no relationship
				between authentication username and address of record. That
				means that for example a user authenticating himself correctly
				with "john.doe" id may register contacts for "gw.bush".
				Site policy may wish to mandate authentication id to be equal
				to username claimed in To header field. <action moreinfo="none">check_to</action>
				action from auth module can be used to enforce such a policy.
			    </para>
			</listitem>
			<listitem>
			    <para>
				There is no dialing plan implemented. All users are supposed to
				be reachable via user location database. See <xref linkend="numberingplans">
				for more information.
			    </para>
			</listitem>
			<listitem>
			    <para>
				The script assumes users will be using server's name as a part of
				their address of record. If users wish to use another name (domain
				name for example), this must be set using the <varname>alias</varname>
				options. See <xref linkend="domainmatching"> for more information.
			    </para>
			</listitem>
			<listitem>
			    <para>
				If authentication is turned on by uncommenting related configuration
				options, clear-text user passwords will by assumed in back-end database.
			    </para>
			</listitem>
		    </itemizedlist>
		</para>
		<example>
		    <title>Default Configuration Script</title>
		    <programlisting format="linespecific">
&defscr;			
		    </programlisting>
		</example>
	    </section>

	    <section id="statefulua">
		<title>Stateful User Agent Server</title>
		<para>
		    This examples shows how to make ser act as a stateful user
		    agent (UA). Ability to act as as a stateful UA is essential
		    to many applications which terminate a SIP path. These
		    applications wish to focus on their added value. They
		    do not wish to be involved in all SIP gory details, such
		    as request and reply retransmission, reply formatting, etc.
		    For example, we use the UA functionality to shield 
		    SMS gateway and instant message store from SIP transactional
		    processing.
		    The simple example bellow issues a log report on receipt
		    of a new transaction. 
		    If we did not use a stateful UA, every single request retransmission
		    would cause the application to be re-executed which would result in
		    duplicated SMS messages, instant message in message store or 
		    log reports.
		</para>
		<para>
		    The most important actions are <command moreinfo="none">
			t_newtran</command> and <command moreinfo="none">
			t_reply</command>. <command moreinfo="none">
			
		    t_newtran</command> shields subsequent code from retransmissions.
		    It returns success and continues when a new request arrived.
		    It exits current route block immediately on receipt of
		    a retransmission. It only returns a negative value when
		    a serious error, such as lack of memory, occurs.
		</para>
		<para>
		    <command moreinfo="none">t_reply</command> generates
		    a reply for a request. It generates the reply statefully,
		    i.e., it is kept for future retransmissions in memory.
		</para>
		<note>
			<para>
				Applications that do not need stateful processing
				may act as stateless UA Server too. They just use
				the <command>sl_send_reply</command> action to
				send replies to requests without keeping any
				state. The benefit is memory cannot run out,
				the drawback is that each retransmission needs to
				be processed as a new request. An example of use
				of a stateless server is shown in
		    	<xref linkend="redirectserver"> and
		    	<xref linkend="executingscript">.
			</para>
		</note>
		<example>
		    <title>Stateful UA Server</title>
		    <programlisting format="linespecific">
			<!-- ../../examples/uas.cfg -->
			&statefuluaexample;
		    </programlisting>
		</example>
	    </section> <!-- Stateful UAS -->

	    <section id="redirectserver">
		<title>Redirect Server</title>
		<para>
		    The redirect example shows how to redirect a request
		    to multiple destination using 3xx reply. Redirecting
		    requests as opposed to proxying them is essential to
		    various scalability scenarios. Once a message is
		    redirected, <application moreinfo="none">ser</application>
		    discards all related state and is no more involved
		    in subsequent SIP transactions (unless the redirection
		    addresses point to the same server again).
		</para>
		<para>
		    The key <application>ser</application> actions in this example 
		    are <command moreinfo="none">append_branch</command> and 
		    <command moreinfo="none">sl_send_reply</command> (sl module).
		</para>
		<para>
		    <command moreinfo="none">append_branch</command> adds
		    a new item to the destination set. The destinations set always
		    includes the current URI and may be enhanced up to
		    <constant>MAX_BRANCHES</constant> items.
		    <command moreinfo="none">sl_send_reply</command> command, 
		    if passed SIP reply code 3xx, takes all values in current 
		    destination set and adds them to Contact header field in 
		    the reply being  sent.
		</para>
		<example id="redirectexample">
		    <title>Redirect Server</title>
		    <programlisting format="linespecific">
			<!-- ../../examples/redirect.cfg -->
			&redirectexample;
		    </programlisting>
		</example>
	    </section> <!-- redirect server-->
	    
	    <section id="executingscript">
		<title>Executing External Script</title>
		<para>
		    Like in the previous example, we show how to
		    make <application>ser</application> act as a redirect server. The difference is 
		    that we do not use redirection addresses hardwired in
		    <application moreinfo="none">ser</application> script but
		    get them from external shell commands. We also use
		    ser's ability to execute shell commands to log
		    source IP address of incoming SIP requests.
		</para>
		<para>
		    The new commands introduced in this example are
		    <command moreinfo="none">exec_msg</command> and
		    <command moreinfo="none">exec_dset</command>.
		    <command moreinfo="none">exec_msg</command> takes
		    current requests, starts an external command, and
		    passes the requests to the command's standard input.
		    It also passes request's source IP address in
		    environment variable named <constant>SRCIP</constant>.
		</para>
		<para>
		    <command moreinfo="none">exec_dset</command> serves
		    for URI rewriting by external applications.  The
		    <command moreinfo="none">exec_dset</command> action
		    passes current URI to the called external program, 
		    and rewrites current destination
		    set with the program's output. An example use would
		    be an implementation of a Least-Cost-Router, software which
		    returns URI of the cheapest PSTN provider for a given
		    destination based on some pricing tables. <xref linkend="execscript">
		    is much easier: it prints fixed URIs on its output using
		    shell script <command moreinfo="none">echo</command> command.
		</para>
		<note>
			<para>
				This script works statelessly -- it uses this action for
				stateless replying, <command>sl_send_reply</command>.
				No transaction is kept in memory and each request retransmission
				is processed as a brand-new request. That may be a particular
				concern if the server logic (<command>exec</command> actions
				in this example) is too expensive. See 
		    	<xref linkend="statefulua"> for instructions on how
				to make server logic stateful, so that retransmissions
				are absorbed and do not cause re-execution of the logic.
			
			</para>
		</note>
		<example id="execscript">
		    <title>Executing External Script</title>
		    <programlisting format="linespecific">
			<!-- ../../examples/exec.cfg -->
			&execexample;
		    </programlisting>
		</example>
	    </section> <!-- exec example -->
	    
	    <section id="replyprocessingsection">
		<title>On-Reply Processing (Forward on Unavailable)</title>
		<para>
		    Many services depend on status of messages relayed
		    downstream: <emphasis>forward on busy</emphasis> and 
		    <emphasis>forward on no reply</emphasis> to name the
		    most well-known ones. To support implementation of
		    such services, <application moreinfo="none">ser</application>
		    allows to return to request processing when request
		    forwarding failed. When a request is reprocessed,
		    new request branches may be initiated or the transaction
		    can be completed at discretion of script writer.
		</para>
		<para>
		    The primitives used are <command moreinfo="none">t_on_failure(r)</command>
		    and <command moreinfo="none">failure_route[r]{}.</command> If
		    <command>t_on_failure</command> is called before
		    a request is statefuly forwarded and a forwarding failure occurs, 
		    <application moreinfo="none">ser</application>
		    will return to request processing in a <command moreinfo="none">failure_route</command>
		    block. Failures include receipt of a SIP error
		    (status code >= 300 ) from downstream or not receiving
		    any final reply within final response period.
		</para>
		<para>
		    The length of the timer is governed by parameters of the
		    tm module. <varname>fr_timer</varname> is the length of
		    timer set for non-INVITE transactions and INVITE transactions
		    for which no provisional response is received. If a timer
		    hits, it indicates that a downstream server is unresponsive.
		    <varname>fr_inv_timer</varname> governs time to wait for 
		    a final reply for an INVITE. It is typically longer than
		    <varname>fr_timer</varname> because final reply may take
		    long time until callee (finds a mobile phone in a pocket and)
		    answers the call.
		</para>
		<para>
		    In <xref linkend="replyprocessing">, <command moreinfo="none">failure_route[1]</command>
		    is set to be entered on error using the <command moreinfo="none">t_on_failure(1)</command>
		    action. Within this reply block, <application moreinfo="none">ser</application>
		    is instructed to initiate a new branch and try to reach called party
		    at another destination (sip:nonsense@iptel.org). To deal with the case when neither the alternate
		    destination succeeds, <application moreinfo="none">t_on_failure</application>
		    is set again. If the case really occurs, <command moreinfo="none">failure_route[2]</command>
		    is entered and a last resort destination (sip:foo@iptel.org) is tried.
		</para>
		<example id="replyprocessing">
		    <title>On-Reply Processing</title>
		    <programlisting format="linespecific">
			<!-- ../../examples/onr.cfg -->
			&replyexample;
		    </programlisting>

		</example>
	    </section> <!-- reply processing -->
	</section> <!-- examples -->
    </chapter>
