Traceroute

[lagapidis (Lazarus Agapidis)]

Hello Pradyumna

@pradyumnayadavgla

OK laz, but is TTL configurable or not (Both on System and Router if yes then how)

When using traceroute, you cannot adjust the TTL yourself. TTL is used in a specific way for traceroute, and if you change that, traceroute will not function correctly. For normal traffic however, both Windows and Linux allow you to change the default TTL. Doing a simple search online I have found that:

For Windows:

Using the Windows registry:

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\servic es\Tcpip\Parameters]
“DefaultTTL”=dword:00000081

To execute this commands in console:

netsh int ipv4 set glob defaultcurhoplimit=129
netsh int ipv6 set glob defaultcurhoplimit=129

For Linux:

sudo sysctl net.ipv4.ip_default_ttl=129

For Cisco, I believe that you cannot change the default TTL, but I was unable to find any documentation confirming or denying this.

@pradyumnayadavgla

when PC sending packet with TTL=1 and it reaches to R1 which reply back with TTL exceeded mean TTL received by R1 is 1 and when it will forward this packet to next hope with TTL=0 therefor I think R2 should respond with TTL exceeded message

All of this has to do with when is the TTL decremented, and when is it considered expired. Take a look at the following post that describes how TTL is handled by routers, including the order of operations concerning routing and decrementing the TTL. The post has to do with eBGP multihop, but the TTL concepts are the same:

EBGP Multihop

Hello Bala The specific process that is followed in the scenario you describe from the lesson is the following: R1 prepares to send the packet to R3, and sets the TTL to 1. R1 will make its forwarding decision (chooses the egress port) using the destination IP address of R3 and its routing table. Before egress, the TTL is checked to see if it must be dropped or not. TTL is not equal to 0, so it is not dropped. The packet is sent R2 receives the packet. As per RFC1812 a router will not disca…

@pradyumnayadavgla

*Also Let me know what devices can be considered as Hop like router as well.

TTL is a parameter that is found in the IPv4 or IPv6 packet header, therefore it is exclusively a Layer 3 mechanism. Therefore TTL will be decremented whenever a packet is routed.

I hope this has been helpful!

Laz