This screen allows viewing and editing of the TCP Fallback configuration. The TCP Fallback flag enables the SBC to validate the SIP PDU size as received in the incoming INVITE.


If the PDU size is larger than the configured value, and:
The request is not received over TCP...then the SBC rejects the request with 513 Message Too Large.
The request is received over TCP...then the SBC uses TCP to forward this message to the Egress (if TCP is allowed in the Egress configuration)


  • To validate the incoming request, The TCP Fallback flag must be configured on the Ingress leg.
  • To fall back to TCP on the Egress, the flag must be configured on the Egress leg.


To View and Edit TCP Fallback

On the SBC main screen, go to Configuration > System Provisioning > Category: SIP Carrier Peering > Signaling > TCP Fallback. 


The TCP Fallback can be checked for each Address Context, for each Zone, for each SIP Trunk Group created or for all the Address Contexts, Zones and SIP Trunk Groups created. Use the drop-down box to select the desired Address Context, Zone and SIP Trunk Group.

The Edit TCP Fallback window is displayed.

SIP Carrier Peering - Signaling - TCP Fallback

The following fields are displayed:

SIP Trunk Group - Signaling - TCP Fallback parameters

Parameter

Length/Range

Description

Max MTU Supported Over UDP Control

N/A

 Maximum MTU Size accepted over UDP.

  • Disabled (default)
  • Enabled

Max MTU Supported Over UDP Value

1-5000Maximum size of the SIP Invite accepted over UDP. Default value is 1300.

Make the required changes and click Save at the right hand bottom of the panel to save the changes made.

Note

The SBC supports, by default, 1,300 Maximum Transmission Unit (MTU) bytes, and the MTU size used by the SBC is configurable. If the initial INVITE message size exceeds the default MTU value, the SBC sends the data over the TCP transport protocol. The TCP transport protocol is used if it is allowed by the transport profile irrespective of its preference order.