Layer 3 TTL value and loop

[susim]

Hi 

If the TTL value in the ip header  will stop ip looping , why the same  not implmented in the frame ?

Thanks 

[Joseph W. Doherty]

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Posting

Because (at least for Ethernet), L2 hops are "transparent".  A hub generally just regenerates bits, it doesn't analyze the frame.  Switches, in theory, could decrement a hop counter in a frame, but many Ethernet standards predates them.

[susim]

Hi 

When forwarding an IP packet, routers are required to decrease the TTL by at least one. If a packet's TTL field reached zero, the router detecting it discards the packet and sends an ICMP message back to the originating host.

some cases like  combination of incorrect routing tables could cause a packet to loop endlessly . When ttl becomes zero the loop also will stop . My question was what if they could implement  something like  ttl in the frame , so the layer 2 loop could be avoided ?

Thanks again 

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Consider Ethernet Coax.  If you physically loop it, what's going to decrement a counter in the frame?

Consider compatibility.  If you add a new "TTL" counter to the frame, older equipment may not understand the frame format.

BTW, TTL won't avoid a loop, it would just keep the frame from looping endlessly.