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<chapter id="devel">


    <title>&develguide;</title>

    <section>
      <title>Internals</title>

      <para>The SEAS module runs within the Open Sip Express Router aka.
      &kamailio;. &kamailio; uses a pool of processes to execute the script logic on
      every new message received. These are called the worker processes. One
      of these processes will be selected to process the script, and at some
      point it will find a function invoking the relay of the SIP message to
      one of the Application Servers registered. This function has been called
      as_relay_t, which stands for Application Server relay (the _t stands for
      TransactionStatefully), and receives as the only parameter the name of
      the application server to be invoked.</para>

      <para>The process will execute the as_relay_t function, which looks up
      in a table if there is a registered Application Server with that name.
      If there is one, the process will craft the SEAS header for the SIP
      message being handled, put it in a shared memory segment, and write the
      address of that segment to a pipe (4 bytes pointer in IA32).</para>

      <para>This way, we will have all the &kamailio; processes composing the
      SEAS header along with the SIP message, and putting its shared memory
      address into that pipe. This technique of inter-process communication
      avoids race conditions because writing to a pipe is granted to be an
      atomic operation if the data to write is less than _POSIX_PIPE_BUF,
      which usually is 512 bytes.</para>

      <para>At the initialization of &kamailio;, the SEAS module creates the
      discussed pipe, so that all the &kamailio; worker processes inherit the
      file descriptor associated to the pipe. Then it spawns a new process,
      which will be the one to open two server sockets, and wait for the
      Application Servers to connect and register.</para>

      <para>Each Application Server wishing to receive events from &kamailio;,
      will have to open a socket to the module (the port and IP of the socket
      are defined at start time in the script). After connection, it has to
      print its identification name. The SEAS process (from now on, called
      event dispatcher) will then register it in its internal structures, so
      that the &kamailio; processes can push events for it. The following
      picture, shows the internals of the SEAS Event dispatcher
      process:</para>

      <figure>
          <title>Overview of Seas Event Dispatcher process operation</title>

          <mediaobject>
            <imageobject>
				<imagedata fileref="./images/image056.gif"/>
            </imageobject>
          </mediaobject>
	  </figure>
	  <para>Within the SER server, the flowing of SIP Messages and
      control flow, is depicted in the following diagram:</para>

      <figure>
          <title>SIP Messages and control flow within SER</title>

          <mediaobject>
            <imageobject>
				<imagedata fileref="./images/image058.gif"/>
            </imageobject>
          </mediaobject>
        </figure>
    </section>

    <section>
      <title>SEAS Protocol</title>

      <para>SIP is a very flexible protocol. It can be very easily extended
      with new features, and SIP entities have a high level of freedom in
      composing the SIP messages, for example setting IPs or hostnames in
      URIs, reordering header fields, folding headers, aggregating/scattering
      headers, etc.</para>

      <para>This flexibility, though, makes it difficult to implement
      efficiently, because parsing of text headers requires a lot of
      state.</para>

      <para>&kamailio; implements a very efficient parsing mechanism and
      SIP-transaction machinery. The goal of the SEAS protocol is to keep all
      this information that has been already extracted at &kamailio;, so that it
      can be reused at the Application Server.</para>

      <section>
        <title>The SEAS protocol</title>

        <para>The SEAS protocol is a layer of information regarding the
        internal structure of a SIP message that is added whenever SEAS sends
        a SIP event to the Application Servers. The protocol is used for
        communication between &kamailio; and the Application Servers.</para>

        <para>Once an incoming SIP message has reached the worker process
        within &kamailio;, it copies its content into a private memory area
        (which is, a memory chunk not shared across processes). In this point,
        the message first line is parsed to know whether it is a SIP request
        or response.</para>

        <para>&kamailio; uses a technique called lazy-parsing, which consists in
        delaying the parse of headers until some piece of the code requires
        it.</para>

        <para>As the SIP message goes traversing functions and the script
        code, a function called parse_msg() gets called again and again, and
        the SIP message gets parsed further and further. Each call to
        parse_msg passes an integer value argument (32 bits) in which every
        bit signals a header to be parsed, if they are already parsed (because
        a previous invocation of parse_msg), the function returns immediately,
        otherwise, the SIP message is scanned and parsed until all the headers
        requested get parsed.</para>

        <para>In each call to parse_msg, different parts of the message are
        analyzed, and different SIP header-specific structures get filled.
        Each one of this structures, give quick access to each of the parts of
        a SIP message header.</para>

        <para>For example, a Via header struct is called via_body, and has
        these members: name, version, transport, host, proto, port, port_str,
        params, comment, received, rport, etc. each of these members gives
        quick access to each of the parts of the header. For example, a via
        header like this: &#8220;Via: SIP/2.0/UDP
        192.168.1.64:5070;branch=z9hG4bK-c02c60cc&#8221; would have the member proto
        pointing to the &#8220;U&#8221; of &#8220;UDP&#8221;, and a length of 3, the host member would
        be pointing to &#8220;192.168.1.64&#8221; and have a length of 12, the branch
        member would be pointing to &#8220;z9hG4bK-c02c60cc&#8221; and a length of 16, and
        so on.</para>

        <para>This structure is the result of the parsing. All this
        meta-information regarding the SIP message structure, is stored in a
        sip_msg structure, using dynamically-allocated memory segments.</para>

        <para>&kamailio; defines different structure types describing different
        SIP headers, such as via_body, to_body, cseq_body, via_param, and so
        on. These structures are generally composed of another kind of
        structure called str.</para>

        <para>The str structure is a key component of &kamailio;'s high
        performance. In the C programming language, a string's length is known
        because a '0' (null-character) is found at the end of it. This forces
        each of the string manipulation functions to keep looking for a '0' in
        the byte stream, which is quite processor consuming. Instead of this,
        &kamailio; defines a structure composed of a char pointer and an integer.
        The char points to the start of a string, and the integer gives its
        length, thus avoiding the '0' lookup problem, and giving a significant
        performance boost.</para>

        <para>This structure has been quite useful to the design of the SEAS
        protocol, because it enables the description of the SIP message
        anatomy by giving pointers to each of its fields, and integers
        describing each of its lengths.</para>

        <para>Knowing that a SIP header does not usually occupy more than a
        few characters (always less than 256), the pointer in the structure
        has been relativized to the beginning of the SIP message or the
        beginning of the SIP header, and the integer giving the length, has
        been casted to an unsigned byte (256 values, so 256 characters maximum
        length).</para>

        <para>When messages get transferred from &kamailio; to the Application
        Server, it is optimum to keep this worthy meta-information regarding
        the SIP message, so that it can be used at the AS part. For this to be
        possible, it is needed to store the pointers to each of the syntactic
        structures and their length.</para>

        <para>In general, pointers are variables that point to a region in the
        memory of a computer. The region of the memory is counted from the
        0x00000000 address in IA32 architectures (from the beginning).</para>

        <para>C provides functionality to do any kind of arithmetic operations
        over pointers (add, subtract, multiply and divide), so that the
        euclidean distance over the one-dimension address space can be
        calculated just by subtracting a base address from another
        pointer.</para>

        <para>These pointers will have to be transmitted through the network,
        along with the SIP message, so for the pointers to keep their meaning,
        they need to be relativized to a known point, and the most meaningful
        known point in a SIP message is its start.</para>

        <para>So making the pointers relative to the message start, gives two
        important features: first, it makes the pointers still valid when they
        arrive at another computer (because they are relative to the beginning
        of the message), and they occupy far less memory, because from a
        4-byte pointer (in IA32) it gets translated to a 1 or 2 byte index,
        because an important amount of redundant information is elicited (we
        already know that each of the parts of the message belong to the
        message, so why carry the message begin address in each of the
        pointers ?).</para>

        <para>The SIP messages are composed of protocol headers and a payload.
        The headers section don't usually surpass the 1500 byte limit, amongst
        other reasons, because the usual Maximum Transmission Unit in Ethernet
        networks is 1500 bytes and the protocol was initially designed to work
        on UDP. For that reason, 11 bits should be enough to address a
        particular region within the SIP message, because it yields 2048
        positions. The closest greater value to 11 bits multiple of a byte
        (the basic TCP network transport unit) is 16 bits, or 2 bytes, which
        makes it possible to address 65536 positions from the
        beginning.</para>

        <para>For the SEAS protocol to be extensible and platform-independent,
        all the 2-byte pointers or indexes to each of the message regions are
        sent in network-byte-order, or big endian. This is also useful in the
        JAVA part to retrieve the indexes, because the JAVA natively uses a
        big-endian representation of integers, regardless the architecture on
        which it runs.</para>

        <para>For each kind of standard SIP header (this is, the headers
        referred to in the SIP specification) there is a code specification,
        regarding the composition of the header. Each one of its parts points
        to one the several components of the header. For example, a From
        header always has a SipURI and may have several parameters, amongst
        others, a tag. Then, the From header code has a field indicating where
        the URI starts, a codification of the URI, and several pointers that
        point to each one of the parameter names and values. This is the
        codification of the From header. All the other headers have a similar
        codification.</para>
      </section>

      <section>
        <title>General codification of a header</title>

        <para>Every header codification, regardless it is known to the server
        or not, begins with a 2-byte unsigned integer, which points to the
        beginning of that header counted from the SIP message begin (a SIP
        message start based pointer to the header). Following these two bytes
        is another byte giving the length of the name, and another byte giving
        the length of the entire header (including name and value).</para>

        <figure>
            <title>General codification of a SIP header in SEAS
            protocol</title>

            <mediaobject>
              <imageobject>
                <imagedata fileref="./images/image100.gif"/>
              </imageobject>
            </mediaobject>
		</figure><para>For example:</para>

        <figure>
            <title>Example of a from header SEAS-protocol codification</title>

            <mediaobject>
              <imageobject>
				  <imagedata fileref="./images/image101.gif"/>
              </imageobject>
            </mediaobject>
		</figure>

        <section>
          <title>Codification of a generic URI</title>

          <para>As the SIP URI is one of the most used types in a SIP message,
          a special structure has been defined to describe the contents of it.
          A URI is always included inside a SIP header, or may be in the first
          line of a SIP Request (as the request URI).</para>

          <para>The codification of any URI is as follows:</para>

          <figure>
              <title>SEAS-codification of a SIP URI (byte meanings are
              shown)</title>

              <mediaobject>
                <imageobject>
					<imagedata fileref="./images/image102.gif"/>
                </imageobject>
              </mediaobject>
            </figure>

          <para>What follows is an example of a SIP URI codification with the
          SEAS protocol.</para>

          <figure>
              <title>Example of a SEAS SIP URI codification</title>

              <mediaobject>
                <imageobject>
					<imagedata fileref="./images/image103.gif"/>
                </imageobject>
              </mediaobject>
		  </figure>
		  <para>The first byte in the encoded-URI structure, gives the
          index where the URI starts, counting from the beginning of the SIP
          header where it appears. The next two bytes are flags indicating
          known fields present in the URI (such as port, host, user,
          etc.).</para>

          <para>All the following bytes are uri-start based pointers to the
          fields that are present in the URI, as specified by the flags. They
          must appear in the same order shown in the flags, and only appear if
          the flag was set to 1.</para>

          <para>The end of the field, will be the place where the following
          pointer points to, minus one (note that all the fields present in a
          URI are preceded by 1 character, ie
          sip[:user][:password][@host][:port][;param1=x][;param2=y][?hdr1=a][&amp;hdr2=b])
          it will also be necessary to have a pointer at the end, pointing two
          past the end of the URI, so that the length of the last header can
          be computed.</para>

          <para>The reason to have the &#8220;other parameters&#8221; and headers flags at
          the beginning (just after the strictly URI stuff), is that it will
          be necessary to know the length of the parameters section and the
          headers section. The parameters can appear in an arbitrary order,
          they won't be following the convention of
          transport-ttl-user-method-maddr-lr, so we can't rely on the next
          pointer to compute the length of the previous pointer field, as the
          ttl parameter can appear before the transport parameter. So the
          parameter pointers must have 2 bytes: pointer+length.</para>
        </section>

        <section>
          <title>Codification of To and From headers</title>

          <para>To and From headers follow the same structure, so the same
          codification structure has been used to describe both. The structure
          is depicted in the drawing:</para>

          <figure>
              <title>SEAS codification of From and To headers</title>

              <mediaobject>
                <imageobject>
					<imagedata fileref="./images/image104.gif"/>
                </imageobject>
              </mediaobject>
            </figure>
        </section>

        <section>
          <title>Codification of Contact</title>

          <para>The contact header is one of those SIP headers that can be
          combined, which means that if several headers of the same type are
          present in the message, they can be aggregated in a single header,
          having the header values separated by a comma. Thus, a single
          Contact header can contain more than one contact-value. For this
          reason, the Contact codification is composed of a several Contact
          codifications concatenated, and a byte at the beginning telling how
          much Contact codifications are present. The code is depicted in the
          following drawing:</para>

          <figure>
              <title>SEAS codification of a Contact header</title>

              <mediaobject>
                <imageobject>
					<imagedata fileref="./images/image105.gif"/>
                </imageobject>
              </mediaobject>
            </figure>
        </section>

        <section>
          <title>Codification of Route and Record Route headers</title>

          <para>Both Route and Record-Route headers follow an identical
          structure, and it is also permitted to combine several headers into
          one, with their bodies (or header values) separated by commas. In
          this case, both kinds of headers follow the same structure, defined
          as follows:</para>

          <figure>
              <title>SEAS codification of a Route Header</title>

              <mediaobject>
                <imageobject>
					<imagedata fileref="./images/image106.gif"/>
                </imageobject>
              </mediaobject>
            </figure>
        </section>

        <section>
          <title>Codification of Accept and Content-Type headers</title>

          <para>These two kinds of headers carry mime type and subtype
          definitions in the form &#8220;type/subtype&#8221; (ie. text/xml,
          application/sdp or whatever). For internal handling of this headers,
          SER codifies the known types and subtypes into a single 32 bit
          integer, with the highest two bytes giving the mime type, and the
          lowest two bytes giving the subtype.</para>

          <para>The difference is that Accept header can also be combined,
          carrying more than one header value in a single header row. Thus the
          Accept header has a leading byte giving the number of mime
          type/subtype integers present, while the Content-Type only uses 4
          bytes (a 32-bit integer) giving the type/subtype.</para>
        </section>

        <section>
          <title>Codification of Authorization headers</title>

          <para>SIP has inherited the authentication scheme from HTTP, which
          is based on a digest scheme. There are several headers regarding
          these authorization scheme, namely Proxy-Authenticate,
          WWW-Authenticate, Authorization and Proxy-Authorization. All of them
          can be codified using the same schema, which is as follows:</para>

          <figure>
              <title>SEAS codification of Authentication/Authorization
              headers</title>

              <mediaobject>
                <imageobject>
					<imagedata fileref="./images/image107.gif"/>
                </imageobject>
              </mediaobject>
            </figure>

          <para>For each field present, there are 2 bytes, one pointing the
          place where it starts, the next giving how long this field is. The
          URI is a special case, and is composed of 1 byte telling how long is
          the URI structure, and then the encoded URI structure.</para>
        </section>

        <section>
          <title>Codification of Allow headers</title>

          <para>Allow headers carry request methods that a user agent or proxy
          understands or is willing to accept. In SER, request methods are
          codified into a 32-bit integer, each of its bits signals a different
          kind of header. The Allow header is codified copying that integer
          into the payload of the header.</para>
        </section>

        <section>
          <title>Codification of Content-Disposition headers</title>

          <para>The content-disposition is encoded within 2 bytes: the first
          is a header-start based pointer to where the content-disposition
          value starts, and the second is its length. If there are parameters
          present, each of them uses 1 byte pointing to where the parameter
          name starts, and 1 byte pointing to where the parameter value
          starts. From these two values, the parameter name and value lengths
          can be inferred.</para>
        </section>

        <section>
          <title>Codification of Content-Length header</title>

          <para>The content length header is codified as a 4-byte unsigned
          integer, in network byte order.</para>
        </section>

        <section>
          <title>Codification of Cseq header</title>

          <para>The Cseq header is codified using 9 bytes. The first one is a
          number corresponding to the internal value that SER assigns to that
          request method (the method ID). The following 4 bytes are an
          unsigned 32-bit integer according to the Cseq number. The next two
          bytes are the header based pointer to the beginning of the Cseq
          number and its length, and two more bytes pointing to the beginning
          of the method name and its length.</para>
        </section>

        <section>
          <title>Codification of Expires header</title>

          <para>The expires header is composed of 6 bytes. The first four
          bytes are an unsigned 32-bit integer with the parsed value of the
          header (which is the number of seconds before a request expires).
          Then follows 1 byte pointing to the beginning of the header value
          (the expires value as a string) and a byte giving the length of the
          value.</para>
        </section>

        <section>
          <title>Codification of a SIP message</title>

          <section>
            <title>The general message information section</title>

            <para>In SER, not only the headers are parsed with a high degree
            of optimization, but also the first line is. So for the SEAS
            protocol to realize this improvement, a codification for the first
            line of every SIP messages has also been defined.</para>

            <para>The first two bytes of the codification are a 2-byte
            unsigned integer. If its value is equal or greater than 100, then
            this is a response, and the integer represents its status code. If
            its value is smaller than 100, then it is a request, and the
            integer represents the method of the request being
            transported.</para>

            <figure>
                <title>SEAS codification of a SIP First Line</title>

                <mediaobject>
                  <imageobject>
					  <imagedata fileref="./images/image108.gif"/>
                  </imageobject>
                </mediaobject>
              </figure>

            <para>The next two bytes are an unsigned integer which is a
            pointer to where the actual SIP message starts, beginning from the
            start of the codified payload.</para>

            <para>The next two bytes are also an unsigned integer giving the
            SIP message length.</para>

            <para>The next bytes differ on the meaning depending on whether
            the message is a SIP Request or Response.</para>

            <para>In case it is a Request:</para>

            <para>The next two bytes, are a SIP-message-start based pointer to
            where the method begins, and the method length.</para>

            <para>The next two bytes, are a SIP-message-start based pointer to
            where the Request URI begins, and the request URI length.</para>

            <para>The next two bytes, are a SIP-message-start based pointer to
            where the version identifier begins, and the version identifier
            length.</para>

            <para>In case it was a Response:</para>

            <para>The next two bytes, are a SIP-message-start based pointer to
            where the response code begins, and the response code
            length.</para>

            <para>The next two bytes, are a SIP-message-start based pointer to
            where the reason phrase begins, and the reason phrase
            length.</para>

            <para>The next two bytes, are a SIP-message-start based pointer to
            where the version identifier begins, and the version identifier
            length.</para>

            <para>In case the message is a SIP response, the following bytes
            correspond to the Request URI codification. The first byte is the
            length of the URI codification, followed by the URI code.</para>

            <para>The last byte in this set, is the number of headers present
            in the SIP message. After this byte, goes a section, called the
            Message Headers Index, which gives quick access to each of the
            headers and their codifications present in the message.</para>
          </section>

          <section>
            <title>The headers index section</title>

            <para>As it has been already discussed, the aim of SEAS project is
            to achieve as high a performance as possible. One of the
            techniques enabling high performance in text-based servers is the
            so called lazy parsing. To enable the laziest possible parsing at
            the Application Server endpoint, a mechanism has been used so that
            access to a requested SIP header can be delayed until the
            application requests it, and the access can be direct to that
            header, without parsing the former headers present in the SIP
            message. Recall that one of the performance drawbacks of the SIP
            protocol is that headers of any type can be spread all along the
            header section, not having the constraint of putting the most
            critical sip-specific headers at the beginning and ordered (which
            would be, in fact, very desirable).</para>

            <para>For this to be possible, there is a section right after the
            beginning of the payload (the general message information section)
            which is a kind of hash table, giving quick access to the codes
            (as explained in the previous sections) of each of the headers
            present in the message.</para>

            <para>This sort of hash table, is composed of triplets of bytes.
            The first byte of each three is a code indicating which kind of
            header it points to (whether it is a From, To, Call-ID, Route
            header, etc). Then follows a 2 byte network-byte-order integer
            that points to a section in the codified-header where the body of
            this header is more specifically described.</para>

            <para>This gives really fast access to any of the headers. For
            example, if all the Route Headers were requested by the
            application, then a lookup in this table would be necessary,
            looking for the value '9' (corresponding to the Route header) in
            each of the positions multiple of 3 (0,3,6,9,12, etc). This can be
            done in an extremely fast and easy way, as this snipped of pseudo
            code explains:</para>

            <para>for(int j=0,int i=0;i&lt;table_length;i+=3){</para>

            <para>if(payload[i]==9)</para>

            <para>results[j++]=i;</para>

            <para>}</para>

            <para>this would let in the &#8220;results&#8221; array all the indexes in the
            headers table that refer to a Route header. Then, the Route
            codification for each of the headers could be reached thanks to
            the two-byte unsigned integer that follows each of the header
            identifiers.</para>

            <figure>
                <title>SEAS Headers Index section overview</title>

                <mediaobject>
                  <imageobject>
					  <imagedata fileref="./images/image109.gif"/>
                  </imageobject>
                </mediaobject>
			</figure>
			<para>So a SIP message codified by the SEAS protocol, has the
            following layout:</para>

            <figure>
                <title>SEAS SIP-Message codification</title>

                <mediaobject>
                  <imageobject>
					  <imagedata fileref="./images/image110.gif"/>
                  </imageobject>
                </mediaobject>
              </figure>

            <para>SIP Messages are a fundamental part of the protocol, but
            they are not the only one. Transaction play a very important role
            in the SIP protocol, within SER and in any JAIN-SIP
            implementation. For this reason, the SEAS protocol also needs to
            define and implement some semantics regarding transaction
            handling. The events related to a transaction are: Incoming
            Request, Outgoing Request, Incoming Response, Outgoing Response,
            Timeout and Transport Error.</para>

            <para>So the SEAS protocol defines a specific format for each one
            of these events. Internally, SER stores the transactions in a hash
            table. This hash table generates an integer for each transaction
            applying a hash function to its Via branch parameter, this integer
            is the hash index, and it identifies in which slot within the hash
            table the transaction is stored. The transaction table usually
            uses 65536 entries, so the hash collision is pretty unlikely.
            Anyway, every hash entry is in reality a linked list of
            transactions, so in the case a hash collision (two transactions
            being assigned to the same hash slot) the transactions are added
            to the same slot, each one being identified by another integer
            called the label. The label within a hash slot, is initially
            generated randomly, and then increased by one each time a
            transaction falls in the same slot. So every transaction is
            identified by a hash index and a label.</para>

            <para>For incoming SIP requests, a transaction is generated at
            SER, and the SEAS module gets that transaction identifier (hash
            index + label), then grabs the source and destination IP, port and
            transport from every message, and crafts a SEAS RequestIn event.
            This kind of event carries all this information within it.</para>

            <para>In order to send Responses out for the Server Transactions,
            JAIN can send a type of Action messages, that order SER to send
            them to the network. These messages follow a structure very
            similar to that of RequestIn events: they start with the Action
            length in bytes, then follows a byte giving the type of action,
            then follows the Hash Index and the Label associated with the
            transaction that is being replied, and finally the SIP Message in
            raw format. It doesn&#8217;t use the SEAS codification described above,
            because SER can easily parse the JAIN provided Response to process
            it and send it out, so the pre-parsing is not needed in that
            direction.</para>

            <para>In order to generate Client Transactions, that is, sending
            SIP Requests out, JAIN utilizes another kind of action called Seas
            Request Action. In this case, when JAIN generates the Request to
            be sent out, it doesn&#8217;t have any means to know the transaction
            identifier (hash index and label) that will be assigned to it by
            SER, so a new mechanism has bee implemented to correlate JAIN
            requests to SER transactions. Basically, JAIN-SIP assigns a unique
            identifier (an integer) that is incremented by one for each new
            Client Transaction generated. This identifier is passed to SER
            along with the SIP Request, so when a SIP Response arrives to SER
            regarding that transaction, SER sends a ResponseIn event to the
            JAIN stack, containing both the initial integer identifying the
            transaction at JAIN and the hash index and label that have been
            assigned to the transaction. This way, JAIN can correlate its own
            identifiers with the identifiers used within SER.</para>

            <figure>
                <title>Different kinds of SEAS codified Events and
                Actions</title>

                <mediaobject>
                  <imageobject>
					  <imagedata fileref="./images/image111.gif"/>
                  </imageobject>
                </mediaobject>
              </figure>

            <para>In case there is a Transaction Timeout, it is notified to
            the JAIN SIP Stack by passing it a Seas Incoming Response with a
            flag called Faked Reply, and a Response code number 408 (Request
            Timeout).</para>
          </section>
        </section>
      </section>
    </section>
  </chapter>
