TTL/Ping Question

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    boeckelr
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    Hi –

    I have a question about different TTL values returned by ping.

    I know that TTL means “time to live” and that it is a field in the IP header…and that as packets traverse a network their TTL value is decrimented by 1 each time they pass a router…and when the TTL reaches zero you get an ICMP Time Exceeded error which is designed to prevent packets from being caught in an infinite loop.

    OK, so we have 4 H.323 video conferencing units on our network. My boss called my at home and said one of our clients had been complaining that one out of the 4 H.323 units was transmitting bad video (unit 4)…and that they pinged all 4 units from their location, and the unit in question had latency issues.

    So my boss asked me to ping them from my house to see if I saw anything out of the ordinary….and I didnt really….the return time for the ping was about the same for all 4 units….but the TTL values reported by the ping were different for the machine that the client was having problems with (TTL=243). The other 3 H.323 units had the same TTL (TTL=52)

    The 4 H.323 units are all connected to our DMZ and are connected to the same switch.

    Here is the result of the ping for one of the 3 units that is working ok:

    Pinging xxx.162.235.42 with 32 bytes of data:

    Reply from xxx.162.235.42: bytes=32 time=48ms TTL=52
    Reply from xxx.162.235.42: bytes=32 time=51ms TTL=52
    Reply from xxx.162.235.42: bytes=32 time=45ms TTL=52
    Reply from xxx.162.235.42: bytes=32 time=45ms TTL=52

    Ping statistics for xxx.162.235.42:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
    Approximate round trip times in milli-seconds:
    Minimum = 45ms, Maximum = 51ms, Average = 47ms

    And here is the result for the ping for the unit that the client says had problems (note the much higher TTL than the previous ping):

    Pinging yyy.162.235.43 with 32 bytes of data:

    Reply from yyy.162.235.43: bytes=32 time=47ms TTL=243
    Reply from yyy.162.235.43: bytes=32 time=51ms TTL=243
    Reply from yyy.162.235.43: bytes=32 time=46ms TTL=243
    Reply from yyy.162.235.43: bytes=32 time=48ms TTL=243

    Ping statistics for yyy.162.235.43:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
    Approximate round trip times in milli-seconds:
    Minimum = 46ms, Maximum = 51ms, Average = 48ms

    So here is what I was wondering: Why would the TTL be so different when I ping these devices? They are all on the same network, connected to the same switch….and this happens every time I attempt to ping them….when I ping “unit 4” I get a TTL of 243…but when I ping the other 3 units I get a TTL of 52….yet the return times for the packets are always roughly the same.

    Is this higher TTL value in any way an indication of latency or a problem with that link? I am just trying to figure out what is going on and if the problem with the vidoe quality is on my end or the clients end. I would think that if there was a latency issue it would show up in the return time in the ping, but I am not sure, which is why I am here and asking you guys. And doesnt the ping client (or whatever network client you are running) set the TTL value?

    Anyway thanks in advance for your help! I have tried to research this before coming here to post this question, but I couldnt find exactly what I was looking for, and I couldnt remember back to my first semester of grad school when we covered this in tcom systems 1…..so here I am.

    Mike

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