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用来作为linux中SIP SERVER,完成VOIP网络电话中服务器的功能

                       Mediaproxy SER module

                        Copyright Dan Pascu
    		       2002-2004 AG Projects


  Mediaproxy is a SER module that is designed to allow automatic NAT
traversal for the majority of existing SIP clients. This means that there
will be no need to configure anything in particular on the NAT box to allow
these clients to work behind NAT when using the mediaproxy module.

Section 1 for a description of the modus operandi.
Section 2 describes the types of SIP phones that work with mediaproxy and a
          short description about how the NAT traversal is working.
Section 3 is about mediaproxy features.
Section 4 describes the exported module parameters and functions.
Section 5 is a comparison with nathelper.

1. Principle of operation
   ----------------------

  This NAT traversal solution operates by placing a mediaproxy server in the
middle between 2 SIP user-agents. It mangles the SDP messages for both of
them in a way that will make the parties talk with mediaproxy while they
think they talk directly with each other.

To achieve this, mediaproxy is actually composed by 2 components:
  - the SER mediaproxy module itself
  - an external proxy server called SER MediaProxy (available from
    http://mediaproxy.ag-projects.com/ )

To avoid confusion in this document the mediaproxy module will be called
'module' or 'mediaproxy module', while the mediaproxy server will be called
'proxy server' from here on.

  The proxy server can be run on the same machine as the module or on a
remote host. Moreover it is possible for a single module to control multiple
proxy servers running on multiple geographically distributed hosts. To find
out more about the architecture of SER MediaProxy please read the
documentation that comes with it.

  To be able to act as a proxy between the 2 talking parties, the machine(s)
running the module/proxy server must have a public IP address.

  The module will ask the proxy server to allocate as many sockets as there
are media streams in the SDP body of the SIP INVITE/Ok messages. The proxy
server will send back to the module the address and port(s) for them. Then
the module will replace the original contact IP and RTP ports from the SDP
messages with the ones provided by the proxy server. By doing this both
clients will try to contact the proxy server instead of talking directly
with each other. Once the clients contact the proxy server, it will record
the addresses they came from and will know where to forward packets received
from the other party This is needed because the address/port the NAT box
will allocate for the leaving streams is not known before they actually
leave the NAT box. However the address of the proxy server is always known
(being a public one) so the 2 parties know where to connect and then after
they did so, the proxy learns the addresses they came from and can forward
packets between them.


2. Types of SIP clients
   --------------------

  The SIP clients that will work transparently behind NAT when using the
mediaproxy module are the so-called symmetric clients. The symmetric clients
have the particularity that use the same port to send the data as the one
they use to receive it. In other words, if they are for example configured
to use port 5060 for SIP signaling, they will use the same port when sending
data as well as when receiving it. This must be true for both the SIP
signaling as well as the RTP streams for a client to work transparently with
the mediaproxy module without any additional configuration on the NAT box.

  This ability is important because the only way to get back to a client
behind NAT is to send to the IP address and port the packet was received
from. Once a packet is sent from the client behind NAT to the outside world,
it opens a communication channel in the NAT box that is open in both
directions for a while (it will timeout after a while after no more data is
sent through it, but it can be kept active by sending data through it at
certain regular time intervals). While this channel is open, any data sent
to the public address and port that the NAT box assigned for the address and
port the client behind NAT is sending from (and this mapping is guaranteed
to be unique), will go back straight to the address and port the client has
sent from. This is why is necessary for the clients to be symmetric. If they
listen on the same port they sent from, the data sent back to the public
address that the NAT box assigned to the leaving packets will actually reach
the listening port of the client behind NAT.

  Some SIP clients implement particular algorithms to detect if they are
actually behind a NAT box and try to act smart by detecting the IP address
of the NAT box (or simply allowing one to manually configure it), and then
use this IP address in the SIP and SDP messages instead of their own private
IP address. This situation can be confusing for a module that tries to
perform transparent NAT traversal as it can wrongly mistake such a client
that is behind NAT with a client that is actually in the public address
space. However for the mediaproxy module it is not important if the clients
apply or not this kind of behavior, as it is able to cope with both
situations gracefully.

  This doesn't mean that mediaproxy is not able to work with asymmetric
clients behind NAT, but in their case special static forwarding routes need
to be configured on the NAT box.

  Mediaproxy has special support for asymmetric clients, can detect them and
send the data to the ports they expect it to, however they can work behind
NAT only if static routes are configured on the NAT box since there is no
way of getting back to an address/port that has not previously opened a data
channel in the NAT box by sending something out first. Nevertheless the
support for asymmetric clients is important, because without it they won't
be able to work even when they have public Internet addresses. Also this
support allows one to use an asymmetric client behind NAT if he can
configure the NAT box to forward the packets meant to that client.

  The only requirement a symmetric SIP client must met to be able to work
transparently behind NAT when using the mediaproxy module is to accept to be
configured to use a so called outbound proxy and this proxy must be the one
running with the mediaproxy module loaded.


3. Features
   --------

- make symmetric clients work behind NAT transparently if they use the SIP
  server as the outbound SIP server.
- handle all media streams specified in the SDP body. There is a limit of 64
  RTP streams per session in the code now, but we hardly find this to be a
  limitation for the time being.
- able to distribute RTP traffic load on multiple proxy servers running on
  multiple hosts.
- able to specify which proxy server to use based on the SIP domain of the
  caller/destination (done by the proxy server's dispatcher module)
- handle asymmetric clients properly. They can even work behind NAT if a
  proper port forwarding is done for them on the NAT box.


4. Module parameters and exported functions
   ----------------------------------------

  The module exports the following parameters:

  - mediaproxy_socket

    it is the path to the filesystem socket where the proxy server
    listens for commands from the module.

  - sip_asymmetrics

    it is the path to a file that lists regular expressions that match
    'User-Agent' or 'Server' fields from clients that are asymmetric
    regarding SIP signaling. Needed to detect when a client is asymmetric
    regarding SIP signaling. An example file is in the config/ subdirectory.

  - rtp_asymmetrics

    it is the path to a file that lists regular expressions that match
    'User-Agent' or 'Server' fields from clients that are asymmetric
    regarding the RTP media. Needed to detect when a client is asymmetric
    regarding the RTP media. An example file is in the config/ subdirectory.

  - natping_interval

    it holds an integer value representing how often the module will
    send packets to all registered clients that are behind NAT to keep
    their opened channels alive. Represents an interval in seconds.

Parameters are set in the SER configuration file by using the modparam
command. Below are examples, which contain the actual default values of the
parameters. If you are Ok with them it is not necessary to specify them in the
configuration file at all.

modparam("mediaproxy", "mediaproxy_socket", "/var/run/proxydispatcher.sock")
modparam("mediaproxy", "sip_asymmetrics", "/etc/ser/sip-asymmetrics-clients")
modparam("mediaproxy", "rtp_asymmetrics", "/etc/ser/rtp-asymmetrics-clients")
modparam("mediaproxy", "natping_interval", 20)

  The module exports the following functions:

  - client_nat_test(type)

    tests if the client is behind NAT or not. The types of tests are
    specified by the type parameter which represents a sum of the 
    following numbers (add the values of the ones you wish to perform
    tests for):

      1 - tests if client has a private IP address (as defined by RFC1918)
          in the Contact field of the SIP message.

      2 - tests if client has contacted SER from an address that is different
          from the one in the 1st Via field. Both IP and port are checked,
          except for asymmetric clients for which the port is ignored.

      4 - tests if client has a private IP address (as defined by RFC1918)
          in the top Via field of the SIP message.

    for example calling client_nat_test("3") in ser.cfg will perform
    first 2 tests listen above and return true as soon as one succeeds
    if both fail will return false.

  - fix_contact()

    will replace the IP:Port in the Contact field of the SIP message
    with the ones the SIP message was received from. For clients that
    are asymmetric regarding SIP signaling (as determined from the
    sip_asymmetrics file) will preserve the port.
    usually called after an if (client_nat_test(type)) has succeded

  - use_media_proxy()

    will make a call to the proxy server and replace the IPs and ports
    in the SDP body with the ones returned by the proxy server for
    each media stream that the SDP message describes. This will force
    the media streams to be routed through the proxy server.
    called when you want to make the session go through a proxy server

  - end_media_session()

    will call on the proxy server to end the media session for that call
    this is done at the end of the call to instruct the proxy server to
    free the resources allocated to that call as well as to save log
    information about the call.
    called when a session should end (BYE or CANCEL received)


5. Comparison with the nathelper module
   ------------------------------------

  After reading all this you may wonder what this module can offer you that
the nathelper module (a similar nat traversal solution) can't and why was
necessary to develop this module.

While at surface they seem to offer about the same functionality, there are
a few core differences that make them quite different.

  The main and most notable difference is that mediaproxy offers a
distributed environment, where the mediaproxy module can control multiple
mediaproxy servers. The mediaproxy servers can be local or remote and they
can be specified per domain or as defaults for domains that don't have their
own mediaproxy servers defined. These mediaproxy servers can be arranged in
load balancing and fallback schemes allowing the platform to scale up easily
and also offer redundancy to keep the service running even if some of the
mediaproxies go offline. Mediaproxy is able to detect the dead proxies and
redistribute the calls among the other mediaproxies that are available.
(More details about this can be found in the SER MediaProxy documentation)

  Another important difference is that mediaproxy tries to move the complex
logic of decision from the ser configuration file to the module and the
proxy servers themselves. This is why there are very few functions in this
module that take any parameters. Instead, control is achieved by modifying
resources outside of ser.cfg. This includes for example specifying the
mediaproxy servers using DNS SRV records, or declaring asymmetric clients
in external files that are automatically re-read as soon as they change.
This allows SER to run without interruption or restarts. If one wants to
change SER's behavior, instead of changing ser.cfg and restarting SER, one
will change these external resources and SER will adapt it's behavior on the
fly without any need for restart.
Another advantage of this is that ser.cfg becomes simpler and easier to
maintain.