Media Gateway Control Protocol (MEGACO/H.248.1)

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

A MediaGateway (MGw), as its name suggests, is a system used as a gateway for media. Current uses are in telephony (Internet, Mobile, and Wireline) and video broadcasting and NarrowCasting. A MediaGateway is usually placed at the edge of two or more different transport mediums (such as: ATM (AAL1, AAL2, AAL5), IP, TDM, or 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 (such as busy tones), handling voice/video messaging, and other similar tasks.

History

Despite the original intention to allow interoperability between systems, it has become an over-standardized protocol so that interoperability between machines of different vendors (and even between different machines from the same vendor) is almost impossible.

Protocol dependencies

• MTP3: MEGACO can use MTP3 as its transport protocol; MEGACO is indicated in MTP3 by the service indicator 14.

• SCTP: MEGACO can use SCTP as its transport protocol; MEGACO is indicated in SCTP by a payload protocol ID of 7.

• TCP: MEGACO can use TCP as its transport protocol. The well known TCP port for MEGACO traffic is 2944; if TCP is used, a TPKT header must be included.

• UDP: MEGACO can use UDP as its transport protocol. The well known UDP port for MEGACO traffic is 2944

• SSCOP: MEGACO can use SSCOP as its transport protocol. There's no standard VP/VC for MEGACO over SSCOP

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 over M3UA and SCTP.

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

Preference Settings

• You can change the ports on which MEGACO is dissected for UDP/TCP
• You can specify whether the raw text of MEGACO packets is to be displayed
• You can specify whether a detailed dissection of MEGACO packets is to be displayed

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

Show only the MEGACO based traffic:

 megaco

Show only the H.248 based traffic:

 h248

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.

External links

• RFC3015 Megaco Protocol Version 1.0

• RFC3525 Gateway Control Protocol Version 1 (MEGACO version 2)

• IANA Megaco assignments

• ITU-T Recs. H.248.1 -> H.248.59 (the datum is from Sep-2007, you can bet that by the time you read this there'll be many more!)

• ITU-T H.248.1 Gateway control protocol: Version 3

Discussion

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