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whats TTL mean in Ping?

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  #1  
Old 05-04-2002, 04:19 AM
Chang Lee Chang Lee is offline
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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?

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  #2  
Old 05-04-2002, 04:25 AM
sidez sidez is offline
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Its Time To Live

  #3  
Old 05-04-2002, 05:26 AM
Chang Lee Chang Lee is offline
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Thanks... but what does it really mean? What advantage does the TTL info provide?

Thats what I wish to learn now.

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  #4  
Old 05-04-2002, 06:03 AM
sidez sidez is offline
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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.

  #5  
Old 05-04-2002, 09:45 AM
rey rey is offline
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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.

  #6  
Old 05-04-2002, 10:08 AM
JDMundo JDMundo is offline
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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.

  #7  
Old 05-04-2002, 02:40 PM
Chang Lee Chang Lee is offline
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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!

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  #8  
Old 05-04-2002, 06:22 PM
Devorius Devorius is offline
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TTL is a timer, not a hop counter. 255 hops? Are you insane?

  #9  
Old 05-04-2002, 06:27 PM
JDMundo JDMundo is offline
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.. Thats not what I wrote. Read it again.

  #10  
Old 05-05-2002, 11:15 AM
jks jks is offline
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Quote:
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.

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  #11  
Old 05-05-2002, 06:43 PM
Chang Lee Chang Lee is offline
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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.

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  #12  
Old 05-05-2002, 08:24 PM
Devorius Devorius is offline
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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.

  #13  
Old 05-05-2002, 08:57 PM
JDMundo JDMundo is offline
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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.

  #14  
Old 05-05-2002, 09:00 PM
ToastyX ToastyX is offline
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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.

  #15  
Old 05-05-2002, 09:09 PM
Devorius Devorius is offline
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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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