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I am reading RFC 2460 about IPv6 and it says that "the packet is discarded if Hop Limit is decremented to zero.", so that is to say that if a router decrements a packet's Hop Limit to 0, other another router can't receives a packet with a hop limit of 0 ?

I ask this question because I read on google : "the packet is discarded if a router receives a packet with a hop limit of 0, or a router decrements a packet’s hop limit to 0". in my view, the second case can't happen.

Am I right ? thanks !

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    "Cannot" is not the same as "should not", on the Internet everything happens whether it should or not.
    – Steve-o
    May 8, 2011 at 22:24

2 Answers 2

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Normally, a router would never receive a packet with a hop limit of zero, but it may happen at some point due to sloppy coding or malicious computers or network degradation.

The issue is what happens when you decrement something that is zero. Since it is an unsigned 8 bit number it would wrap around and last another 255 hops.

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Have you got to this in 2460:

   8.2 Maximum Packet Lifetime


   Unlike IPv4, IPv6 nodes are not required to enforce maximum packet
   lifetime.  That is the reason the IPv4 "Time to Live" field was
   renamed "Hop Limit" in IPv6.  In practice, very few, if any, IPv4
   implementations conform to the requirement that they limit packet
   lifetime, so this is not a change in practice.  Any upper-layer
   protocol that relies on the internet layer (whether IPv4 or IPv6) to
   limit packet lifetime ought to be upgraded to provide its own
   mechanisms for detecting and discarding obsolete packets.
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