S R A / S F X 2 1 0 0 S E R I E S S A T E L L I T E R E C E I V E R
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TTL Translation
The Time-To-Live (TTL) Translation submenu is responsible for maintenance of the TTL
Translation Table. This table defines up to 25 rules that can be used to modify the value of
the TTL parameter in the header of an IP packet being processed by the receiver,
regardless of the network interface the packet originated from.
TTL Translation to incoming IP packets is performed by the receiver according to the
following rules:
1. Up to 25 rules can be maintained by the table. Rules are executed from the first
rule to the last rule, in sequence. All rules are executed for every packet.
2. IP packets can originate from any of the available network interfaces and are
processed by the receiver in accordance with the data flow in Figure 4-9.
3. The TTL value of an incoming packet can never be less than 1.
4. The TTL value of an outgoing packet must be greater than 0.
5. The TTL Translation Table rules are applied before the incoming packet is further
processed by the IP Stack (routing and so on). Multiple rules in the table can
applied to the same incoming packet, if they are applicable, and the effect will be
cumulative.
6. The operating system always decrements the TTL value of the incoming packet by
1, after the TTL Translation Table rules have been applied.
7. If an incoming packet has a TTL of 1 when it arrives, and no TTL Translation rules
cause the TTL value to change, it will be decremented to 0. Therefore, it will not be
output on any network device, however, it is still available if an internal application
requires it. If not, the packet is discarded.
8. The legal range of TTL values is 1 to 255. Any TTL SET, INC (increment), or DEC
(decrement) operation will be clamped at 1 or 255, if the operation causes a TTL
value to occur outside the legal range.
When the TTL Translation submenu item is selected under Data Delivery, the TTL
Translation Table page is displayed for all applicable TTL Translation rules. A sample TTL
Translation Table page is shown in Figure 4-35.
