Media Gateway Control Protocol (MEGACO/H.248.1)

MEGACO/H.248 is used to control Media Gateways in a Multimedia systems. MEGACO can be Text or Binary (ASN.1) encoded. Sometimes it's also referred as GCP (Gateway Control Protocol).

A MediaGateway (MGw) as its name says is a system used as a Gateway for Media. Current uses are in telephony (Internet, Mobile and Wireline) and Video Broad and NarrowCasting. A MediaGateway is usually placed at the edge of two or more different Tranport Mediums (like: ATM (AAL1,AAL2,AAL5), IP, TDM, even two wire PSTN) and its main duty is to convert media (voice/video) between these domains. MediaGateways are often capable of playing and listening to DTMFs, playing tones (like busy tones), handling voice/video messaging and other similar tasks.

History

Regardless of the original intention to allow interoperability between systems. It has become an over-standarized protocol so that interoperability between machinery of different vendors (and even between different machines from the SAME vendor) it's almost impossible.

Protocol dependencies

Example traffic

H.248 over SCTP decoded by Ronnie Sahlberg's decoder in the 0.10.9-SVN-13213 build.

Wireshark

The MEGACO dissector works for TEXT encoding of the protocol if EOL is used as SEParator. Dissection of binary encoding has recently been added (2004-10-25) as a separate dissector, it now handles binary H248 ower M3UA and SCTP.

If TCP is used there may be issues with desegmentation; such traces would be valuable.

Preference Settings

Example capture file

XXX - Add a simple example capture file to the SampleCaptures page and link from here. Keep it short, it's also a good idea to gzip it to make it even smaller, as Wireshark can open gzipped files automatically.

Display Filter

A complete list of MEGACO display filter fields can be found in the display filter reference

A complete list of H.248 display filter fields can be found in the display filter reference

Capture Filter

You cannot directly filter MEGACO protocols while capturing. However, if you know the TCP or SCTP port used (see above), you can filter on that one.

Discussion

Given the fact that everyone extends the standard to make its implementation un-interoperable, was the standardization process really necessary???

H248/MEGACO (last edited 2010-05-24 19:34:29 by BillMeier)