Indeed, many would wonder why you'd have a TTL *and* a HOP count. But there's something you can't tell without the other: How far does it need to go, and how far away did it come from?

For TTL only: Where did the TTL start? Did it start with TTL of 7? Or a TTL of 255? I only see TTL = 2!

For Hops only: At which Hop count do I stop? At 3, 7, 100? Am I the first to see this message? The last?

There are cases where you need to know how far a message has traveled. For example, a PONG message with a TTL of 7 and a hop count of 0 most likely came from a node directly connected to you. You might need to correct the IP address in this PONG message before you pass it on. Also, you might want to prevent multiple PING messages within a short timespan from your direct connected node.

Now, your assumption that the TTL is the time in seconds is incorrect in terms of the Gnutella protocol. When you're talking about ICMP (Pings, traceroutes), yes, TTL would give you a timestamp. But with Gnutella, it is merely any given number, and at each node, this number is decreased.

-- Mike