This screen allows viewing and editing of the TCP Fallback configuration.

The TCP Fallback enables the SBC to validate the SIP PDU size received in the incoming INVITE.

If the PDU size is larger than configured value, the request is rejected (with "513 Message Too Large") if the transport protocol is Non-TCP.

If the large PDU request is received over TCP, the SBC uses TCP to forward this message onto the egress side as long as TCP is allowed in egress configuration.

As the flag validates the incoming request, the flag MUST be configured on ingress for it to function as expected.

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.

Figure 1: SIP Carrier Peering - Signaling - TCP Fallback

The following fields are displayed:

Table 1: 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.

The current TCP Fallback feature does the following:

  1. Enables the SBC to reject UDP INVITE with PDU size more than configured MTU value with “513 Message Too Large”.
  2. It is up to the far-end to recognize this “513 Message Too Large” and switch to TCP.
  3. This feature does not enable the SBC itself to fall back to TCP on receiving  “513 Message Too Large” from far-end.
  4. Once the transport protocol is determined by the application layer in SBC, if the size of the INVITE PDU is increased later (say through SMM rules), the SBC will NOT be able to change the transport protocol to TCP, instead the INVITE is sent using the already determined transport protocol irrespective of the MTU Size configured.

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