Reply to post: TTL=Time To Lie?

It's a net neutrality whodunnit: Boffins devise way to detect who's throttling transit

Jellied Eel Silver badge

TTL=Time To Lie?

Normal TTL is to decrement TTL per hop, so if it exits the egress interface on a link and TTL's decremented to zero, the router at the other end should see TTL exceeded and drop the packet. Which is kinda inefficient, but that's IP.

I guess you could try decrementing TTL based on time spent in a queue or buffer as a possible congestion avoidance mechanism, but AFAIK that would be non-standard and not implemented. Congestion's usually a simple tail drop, give or take any queueing prioritisation using IP precedence bits. Which would be verbotten in a neutral net. Time does play a factor in TCP sessions as part of it's congestion avoidance mechanisms, but that's an application thing using Karn's algorithm rather than TTL. And to add to the fun, in some MPLS implementations, IP TTL values are copied to MPLS so can be decremented per-hop, but that can also be disabled, so the number of MPLS LSPs is not visible at the IP level. And from memory, I think you can also decrement TTL by 2 per hop instead of 1, but can't remember why that may be a thing.

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