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H.248 [attachment:H.248decoded.gif decoded] by Ronnie Sahlbergs decoder in the 0.10.9-SVN-13213 build. H.248 over SCTP [attachment:H.248decoded.gif decoded] by Ronnie Sahlbergs decoder in the 0.10.9-SVN-13213 build.

Media Gateway Control Protocol (MEGACO/H.248.1)

MEGACO is used to control a Media Gateway in a VoIP system. MEGACO can be Text or Binary (ASN.1) encoded.

History

Protocol dependencies

  • ["M3UA"]: MEGACO can use ["M3UA"] as its transport protocol; MEGACO is indicated in M3UA by a Protocol user id of 14.
  • ["SCTP"]: Typically(?) MEGACO uses ["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

Example traffic

H.248 over SCTP [attachment:H.248decoded.gif decoded] by Ronnie Sahlbergs decoder in the 0.10.9-SVN-13213 build.

Ethereal

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, but is currently only working if M3UA is used as transport.

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 Ethereal can open gzipped files automatically.

Display Filter

A complete list of MEGACO display filter fields can be found in the [http://www.ethereal.com/docs/dfref/MEGACO/MEGACO.html display filter reference]

  • Show only the MEGACO based traffic:

     MEGACO 

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

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