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. See information below.
Call Flow Diagrams

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 Interworking between SIP and SS7/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.