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  1. #1

    whats TTL mean in Ping?

    Pinging www.some-domain.com [XXX.XXX.XXX.XXX] with 32 bytes of

    Reply from XXX.XXX.XXX.XXX: bytes=32 time=564ms TTL=237
    Reply from XXX.XXX.XXX.XXX: bytes=32 time=555ms TTL=237
    Reply from XXX.XXX.XXX.XXX: bytes=32 time=554ms TTL=237
    Reply from XXX.XXX.XXX.XXX: bytes=32 time=548ms TTL=237

    Ping statistics for XXX.XXX.XXX.XXX:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss)
    Approximate round trip times in milli-seconds:
    Minimum = 548ms, Maximum = 564ms, Average = 555ms

    whats TTL?
    Chang Lee - Professional Designer
    (for Print, Television & Internet media)
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  2. #2
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    Its Time To Live
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  3. #3
    Thanks... but what does it really mean? What advantage does the TTL info provide?

    Thats what I wish to learn now.
    Chang Lee - Professional Designer
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  4. #4
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    Here goes nothing...

    TTL :
    A timer value included in packets sent over TCP/IP-based networks that tells the recipients how long to hold or use the packet or any of its included data before expiring and discarding the packet or data. For DNS, TTL values are used in resource records within a zone to determine how long requesting clients should cache and use this information when it appears in a query response answered by a DNS server for the zone.
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  5. #5
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    In addition to the above post, TTL in DNS means how much time will it take for your browser to reload the new DNS information.

    So, if you visit a website that has a TTL of 6 hours, your browser will not need to check for any new information for up to 6 hours. Beyond that, your browser will be forced to look up the DNS data to get the most up-to-date information.
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  6. #6
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    Every IP packet that gets sent out has a TTL field which is set to a relatively high number (in the case of ping a TTL of 255). As the packet traverses the network, the TTL field gets decreased by one by each router it goes through; when the TTL drops to 0, the packet is discarded by the router. The IP spec says that the TTL should be set to 60 (though it's 255 for ping packets). The main purpose of this is so that a packet doesn't live forever on the network and will eventually die when it is deemed "lost." But for ping purposes, it provides additional information. The TTL can be used to determine approximately how many router hops the packet has gone through. If the TTL field varies in successive pings, it could indicate that the successive reply packets are going via different routes, which isn't a great thing.
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  7. #7
    Wow! You guys are simply awesome in this forum! In such a short time I learn much here, that too without too much confusion.

    I just wonder how much time and energy + number of books I'd have to go through before I learnt the meaning of TTL as you guys so clearly explained in here.

    Thanks a million!
    Chang Lee - Professional Designer
    (for Print, Television & Internet media)
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  8. #8
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    TTL is a timer, not a hop counter. 255 hops? Are you insane?
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  9. #9
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    .. Thats not what I wrote. Read it again.
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  10. #10
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    Originally posted by Devorius
    TTL is a timer, not a hop counter. 255 hops? Are you insane?
    You are wrong. The TTL in pings are _not_ a timer.

    It _is_ a hop counter.

    You need to count _down_ from 255 - so when it says 237 in the output above, it means that it went through 18 hops.
    Jens Kristian Søgaard, Mermaid Consulting ApS,
    jens@mermaidconsulting.dk,
    http://www.mermaidconsulting.com/
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  11. #11
    Yup. TTL is 'currently' a HOP COUNTER.

    The original idea of TTL may have been to be a timer since quite a lot of people think it to be so, but when I think about this logically, since routers don't have any way of keeping track of when a packet was sent, I don't think its possible for them to drop a packet based on the number of seconds it's been on the network.

    So whats the sense of having a 'timer' in the packet's header?

    Am I correct in my reasoning here?

    Therefore, after reading the responses to my initial question in this thread, I don't think TTL really defines the number of seconds that a packet will be allowed to live on the network.

    I think in practice, it defines the number of routers that a packet can be forwarded to since it is a fact that each router that forwards a packet decrements the TTL by one.
    Chang Lee - Professional Designer
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  12. #12
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    When I ping a server, it tells me what their TTL is set to, and it does not decrement. I'm just curious as to why anyone would set their TTL above say.. 30. Any more than 30 hops and you may as well give up on it anyway, as it's going to take forever to get to its destination.

    For instance...

    Pinging www.webhostingtalk.com [209.249.55.203] with 32 bytes of data:

    Reply from 209.249.55.203: bytes=32 time=377ms TTL=243
    Reply from 209.249.55.203: bytes=32 time=485ms TTL=243
    Reply from 209.249.55.203: bytes=32 time=496ms TTL=243
    Reply from 209.249.55.203: bytes=32 time=386ms TTL=243


    If it is a hop counter, then such high settings just make no sense and one would think it would increase lag time. I've always thought of TTL as a sort of latency timer, myself.
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  13. #13
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    The TTL is not something they "set".. When you ping an IP address or domain, ping.exe sends out IP packets that start out with a TTL number of 255. As one packet travels through IP routers throughout the Internet, its TTL decrements 1 for each router, so in your case, there are 255-243=12 IP routers between you and WHT.com. The ping from my box to WHT returns a TTL of 246.

    And calling TTL a hop counter could be slightly misleading, since "hops" generally are refered to as a physical group of routers and switches, whereas the Time To Live shows you how many routers specifically the packet has gone through. A tracert on Windows to a remote IP or domain may show 10 hops, but a ping following the same path could return 30 "hops" (a TTL of 225).

    As for the limit, Windows ping sends out IP packets with 255 in order for you to diagnose network problems or similar. If the limit was low the packet could "die" before it reached its destination, which is why its limit is maxed out with ping. Can't off the top of my head remember the different limits for ordinary packets on different platforms, but its probably 60 on Windows and I think even 30 on Solaris.

    But anyway, TTL is not a timer per se, its a life-time-timer if you will of the packet itself.
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  14. #14
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    Think about this, if it's some sort of latency timer, why does it show the same TTL every time? It's set to a high value to prevent valid packets from being dropped prematurely. I believe on Windows servers, it's set to 63 or something. Try pinging a Windows server, and you'll see that it's set lower. It is definitely possible for a packet to travel through more than 30 routers.
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  15. #15
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    TTL is something they set. Just like on my Windows machine (setting the TTL just right is part of optimizing your internet connection. Playing with the values gets you different speeds). If you can't set it in Linux, well then Linux is just limited in that area, because you should be able to.
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  16. #16
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    I thought setting the MTU just right is part of optimizing your internet connection, not TTL. TTL shouldn't have any effect on speed.
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  17. #17
    Hi... I can't believe I registered just to reply to this post...

    Dear Devorius,
    TTL (although named as "time" to live) is actually refering to the number of hops a packet can go thru before it "dies" off. How? Why? Well, let's go back to some basics (help me out if I am wrong here okay?)
    - data is sent thru the internet in form of packets (ie small packages/group of bits collected in the binary form)
    - besides the data, there are many more other information that is packed into the same package (for many purposes like indification, error correction, etc and also the TTL info)
    - a packet will travel thru the networks to go from the sender to reach the destination using any connections of the network, it will go thru lots of connection points (routers or hops) and it will keep on traveling (even though in loops) thru-out this network until it reaches its destination
    - just imagine lots of packets traveling in loops in a certain network but never manages to reach its destination... well, they will keep on going around in the network to try to reach the destination... basically the network will be congested!
    - so to ensure that a data will not "live" forever to clogg the network, the TTL is introduced. It is a backward counter (basically a number) and will start from a value assigned by (probably) the sender's computer system and this number will be decremented by one whenever it reaches a hop until the value of TTL equals to zero (which means it will die off ) where the packet will be discarded by the router (hop).
    - TTL is a small (8-bits) data that is being attached at the end of this package (I think it is 8-bit, therefore "255")

    - so strictly speaking TTL refers to the number of hops a packet can make before it will be discarded; but of course, indirectly, it also shows you the "time" of how long a packet's life is left.

    - when you ping TTL=243 it means that the data had traveled 12 hops that's it... (if the initial is set to 255 of course, and in this case, I think so)
    - a packet does not contain any timing information as far as I know...
    - the way of knowing how fast a server connection is has been given by the time=323ms

    My guess is:
    - a high starting/initial TTL number would indicate that a packet is important/ it might need to travel far (many hops) ?? Am I right?
    - a high TTL number you see when you ping shows that the data only needs to travel less hops to reach it's destination from YOUR computer




    Keldron
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  18. #18
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    I can't believe I write this for my 2000th posts. I was thinking about writing something more important, but anyways ....

    Why are so many people repeating the same answers over and over again? Do people read the thread before even posting? Well, a new additional informative answers are always good, but why need to explain things already explained for many times?

    Currently can't think of writing anything substantial, so that's my 2000th post ...

    cheers,


    PS: Good informative answers, btw ...
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  19. #19
    Okay sorry there... my fault...
    I am not in a good mood today... and then I see them arguing over this simple issue... heeheee... will try not to do it again...

    And congratulations! Your 2000th post!!! Beer's on you!
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  20. #20
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    I concede. The only reason I was hanging on was because having such high TTLs seems very inefficient. I imagine that it would cut net lag down quite a bit if everyone were to set their TTLs to say.. 35. As I said before, there shouldn't be a need to have it any higher than that.
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  21. #21
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    Originally posted by Devorius
    I concede. The only reason I was hanging on was because having such high TTLs seems very inefficient. I imagine that it would cut net lag down quite a bit if everyone were to set their TTLs to say.. 35. As I said before, there shouldn't be a need to have it any higher than that.
    Why should it cut net lag down?

    A packet doesn't _have_ to have it's TTL decremented to zero before reaching it's destination.
    Jens Kristian Søgaard, Mermaid Consulting ApS,
    jens@mermaidconsulting.dk,
    http://www.mermaidconsulting.com/
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  22. #22
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    Originally posted by jks


    Why should it cut net lag down?

    A packet doesn't _have_ to have it's TTL decremented to zero before reaching it's destination.
    Because if there weren't a good amount of feral packets, then TTL wouldn't have been implimented. So obviously there are quite a few bouncing around, waiting for their 200+ TTL to count down.
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  23. #23
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    Originally posted by Devorius


    Because if there weren't a good amount of feral packets, then TTL wouldn't have been implimented. So obviously there are quite a few bouncing around, waiting for their 200+ TTL to count down.
    No, that's a wrong reasoning.

    TTLs are good when debugging network errors.

    You cannot from that deduce, that there are lots of network errors.

    I would say that's it very uncommon to find "many" packets out there just waiting for their TTLs to be lowered. If such a thing should happen, the ISP(s) whose routers were responsible would fix that fast.
    Jens Kristian Søgaard, Mermaid Consulting ApS,
    jens@mermaidconsulting.dk,
    http://www.mermaidconsulting.com/
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  24. #24
    Originally posted by Keldron
    Hi... I can't believe I registered just to reply to this post...
    LOL !!!!!!!!!!!!!

    Anyway, its nice that you did! Cuz you said it exactly as it should have been said. I now have understood this TTL thingy better... thanks to everyone in this forum! WHT rules!

    Originally posted by Keldron
    My guess is:
    - a high starting/initial TTL number would indicate that a packet is important/ it might need to travel far (many hops) ?? Am I right?
    On this topic I would like to mention that from the routers point of view 'all packets are important' cuz the router has no way of knowing the level of importance of a packet.

    A higher TTL, therefore, does not indicate the level of 'importance' of any packet. It just indicates how many 'hops' (points across a network) it is allowed to traverse before it should die out somewhere in the vast emptiness of cyberspace!

    Thanks again, Keldron, for posting here!

    Luv ya' guys!
    Chang Lee - Professional Designer
    (for Print, Television & Internet media)
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  25. #25
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    Originally posted by Keldron
    Okay sorry there... my fault...
    I am not in a good mood today... and then I see them arguing over this simple issue... heeheee... will try not to do it again...
    Don't hesitate to contribute though. I did not mean to put you off since my comment was not directed only to you I see you have a deep knowledge of things, will sure to learn from you in the future, but only if you keep contributing ... So, welcome to this board ....

    Sorry for being OOT again.

    cheers,
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