Differences between revisions 22 and 23
Revision 22 as of 2007-03-07 07:43:22
Size: 3943
Editor: GuyHarris
Comment: Add "as its transport protocol" to the MTP3 item, as with other protocols.
Revision 23 as of 2007-09-11 15:08:29
Size: 3943
Editor: LuisOntanon
Comment: more megaco extensions...
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 * ITU-T Recs. H.248.1 -> H.248.47 (the datum is from Mar-2007, you can bet that by the time you read this there'll be many more!)  * 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!)

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's (MGw) as its name says is a system used as a Gateway for Media. It's current uses are in telephony (Internet, Mobile and Wireline) and Video Broad and NarrowCasting. It's 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. They are often capable to play and listen DTMFs, playing tones (like busy tones), handle 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

  • ["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 [attachment:H.248decoded.gif 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

  • 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 [http://www.wireshark.org/docs/dfref/m/megaco.html display filter reference]

A complete list of H.248 display filter fields can be found in the [http://www.wireshark.org/docs/dfref/h/h248.html 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.

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)