[ LUGOS ] "globina" interneta

[Rok Pintar]

>> visje. Ce pa so za TTL predvideli le 7 bitov, potem je 127 tudi
>> maksimalna velikost tega parametra.


Kogar zanima, je vec o tem npr. tukaj:

http://cell.onecall.net/mhonarc/mpls/1999-Nov/msg00021.html

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2. Time to Live (TTL)

This seven-bit field is used to encode a time-to-live value.
When the first tag is pushed onto an untagged IP packet, the
TTL value in the Tag Stack Entry MUST be set to the smaller of
127 and the value of the IP TTL field.

At every hop which tag switches a packet, the TTL value of the
packet's top Tag Stack Entry MUST be decremented by at least 1.

When the Tag Stack is popped, and the resulting Tag Stack is
non-empty,the TTL value in the new top stack entry MUST be
decremented by 1.

When the Tag Stack is popped, and the resulting Tag Stack is
empty, and the network layer packet is an IP packet, the TTL
value in the IP packet MUST be decremented by at least 1; it
SHOULD be modified as follows.  If the TTL value in the IP
packet is less than or equal to 127, it is replaced with the
TTL value from the last Tag Stack Entry.  If the TTL value in
the IP packet is greater than 127, it is replaced by the value
which results from first subtracting the TTL value in the Tag
Stack Entry from 127, and then subtracting that difference from
the the TTL value in the IP header.  If this causes the TTL
value in the IP header to become less than 1, the ordinary
processing for an IP packet whose TTL has become less than 1
MUST be performed.

If at any time, the value of the TTL field in a Tag Stack Entry
becomes less than 1, the packet MUST NOT be forwarded further.
Depending on the tag value in the Tag Stack Entry, the packet
may be silently discarded, or the packet may have its Tag Stack
stripped off, and passed as an untagged packet to the ordinary
processing for network layer packets which have exceeded their
maximum lifetime in the network.

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