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IMG 1010 - Hop Counter / Max Forward Interworking

The Hop Counter / Max Forwards Interworking field can enable or disable the mapping of the Hop Counter Parameter in the ISUP IAM message and the Max Forward Header in the SIP INVITE. When enabled, the IMG will map the Hop Counter parameter and the Max Forwards Header so there is a correlation between the two. This feature prevents a loop from happening when call is made between the PSTN and SIP domain. The Hop Counter / Max Forward feature is consistent with RFC 3261 and ITU Q.1912.5  specifications and is supported in both ANSI and ITU protocols.

Call Flow Diagram

Examples

ISUP to SIP (Example 1)

Hop counter is received in the ingress IAM: The Max-Forwards header value on the SIP side will be the value of the Hop counter less 1.

ISUP to SIP (Example 2)

No Hop counter received in the ingress IAM: The Max-Forwards header value on the SIP side will use the default value of 70 (as recommended by RFC 3261).

SIP to ISUP (Example 3)

The Max-Forwards header will always be received since it is a mandatory header. The value of the Hop Counter parameter on the SS7 side will be created with the value in the Max-Forwards less 1.

Configuration

See IMG 1010 - Interworking from SIP to ISUP topic

Additional Information

  • A max value of 31 (0-31) can be set as the boundary condition for the Hop Counter in SS7 ISUP and a check has been added in the IW PPL for this boundary. For SIP, the max value is 255 (0-255) but default value is 70 as per RFC 3261. When a call is made from SIP leg to SS7 leg with a Max forward value > 31, it will be treated as Hop Counter 31 on SIP side and decremented to 30. The reverse case is ignored since max Hop counter is 31.

  • If Hop Counter received in the ingress IAM the Max Forwards value on the SIP side will be the Hop Counter value less 1.

  • If IAM received has a Hop Counter Value=1 and feature is enabled, the call will be rejected.

  • If a Max Forwards value of 1 is received and feature is enabled, the call will be rejected.

  • If IAM received has no Hop Counter the Max Forwards value will default to 70 as recommended in RFC 3261.

  • When Max Forwards header is received, the Hop Counter parameter will be created on the SS7 side with a value of Max Forwards minus 1.

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