MMS protocol

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The MMS protocol ( Microsoft Media Server Protocol ) is one of Microsoft developed protocol of the application layer , the transmission of multimedia streams used.

Terms

The abbreviation MMS stands for Microsoft Media Server . The protocol was named after the associated server software, the full name of which is Microsoft Windows Media Server and is part of the Windows Media Services software package (formerly Microsoft NetShow Services ). Windows Media Player is typically used as client software . In summary one speaks of the so-called Windows Media Technologies . The computer running the server software is called the Streaming Media Server .

Creation of an MMS session

The MMS protocol is started by a client that wants to process a URL with the prefix „mms://“ . It first establishes a TCP connection to port 1755 of the server in order to transmit its IP address and a self-selected UDP port. The server then creates a UDP socket and connects it to the desired port of the client. The multimedia data is then transmitted via this UDP connection (MMSU), while the TCP connection is used for control commands. However, it is possible that the UDP connection is prevented by a firewall . In this case, the client can also request the transmission of the multimedia data via the TCP connection, which is less suitable for this purpose (MMST). If that also fails, the third option must be used, in which communication takes place via HTTP . This process, in which the appropriate protocol variant is negotiated, is also known as "protocol rollover" . Due to the better real-time properties, the MMSU variant is always preferred, here it is also possible to request lost packets again if there is enough time. The first MMS packets when a session is established contain the identifier "MMS" at offset = 12.

Windows Media Formats

  • The " Advanced Streaming Format " (.asf) is the file format required for the multimedia data. A stream can be kept ready in different compression rates, which enables better bandwidth adaptation.
  • The “Advanced Stream Redirector metafiles” (.asx) enable hyperlinks to streams and the specification of multi-part content, ie they create playlists . They also contain rules for the "protocol rollover" mechanism.
  • The "Microsoft Windows Media Station metafiles" (.nsc) are used to support multicasting and describe a type of channel that the client can access in a similar way to a television channel.

Adaptation to the bandwidth

The client first agrees with the server on a transmission quality level that matches the available bandwidth. The prerequisite for this is the use of an ASF file that has the stream available encoded at different data rates . If the bandwidth decreases later , the client can also thin out the stream dynamically, with continuous adjustment from “full frames” down to “key-frame only” being possible. With such a reduced bandwidth, the sound always has priority over the image. If the bandwidth conditions improve afterwards, the video bit rate can be increased again to the optimum. These possibilities of adapting the data stream to a variable bandwidth are summarized under the term “Smart Streaming” .

Unicast and multicast

If the server is operated in unicast mode, a separate connection is required for each recipient and the data must be sent again each time. The client can then receive the data stream "on demand" , so it has control over the playback process (for example start, stop, pause or search), comparable to a video recorder. Multicasting, on the other hand, means that many recipients can receive the data stream that has been sent at the same time. To do this, the network must be multicast-capable and the server must be operated in multicast mode. In this case, one also speaks of broadcast reception, comparable to the reception of a television program, that is, the receiver cannot control the playback process. The advantage then is that the network is stressed considerably less.

HTTP streaming

With HTTP streaming, the URL prefix can also „mms://“be, but in contrast to the MMS streaming described in this article, it is a fundamentally different technology in which instead of "Windows Media technologies" HTTP or an HTTP Server used, which was extended by some functions in order to get better real-time properties. However, this should not be confused with the method of MMS streaming via HTTP described above.

Download

Normally, the permanent storage of multimedia files received via the MMS protocol is neither possible nor desirable, which is why the exact specification of the MMS protocol was kept secret by Microsoft for a long time (publication of the protocol specification on February 8, 2008, news entry by SDP Multimedia on March 27, 2008). Nevertheless, in addition to the SDP downloader and a few others such as Nettransport or the VLC player, there are various free programs that can be used to save streams.

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credentials

  1. ↑ Download of MMS streams possible with MPlayer - Accessed on August 21, 2013

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