GATE CSE 2014 Set 2 | Question: 25

Question

In the diagram shown below, $L1$ is an Ethernet LAN and $L2$ is a Token-Ring LAN. An $IP$ packet originates from sender $S$ and traverses to $R$, as shown. The links within each $\text{ISP}$ and across the two $\text{ISP}$s, are all point-to-point optical links. The initial value of the $\text{TTL}$ field is $32$. The maximum possible value of the $\text{TTL}$ field when $R$ receives the datagram is _______.

Comments

"TTL" VALUE IS DECREASED ONLY AFTER FORWARDING BY A LAYER 3(OR ABOVE ) DEVICE TO ANOTHER LAYER 3(OR ABOVE) DEVICE. SINCE HOST IS CONNECT TO THE GATEWAY ROUTER ON LAYER 2 ( ROUTER AND RECIEVING HOST BEING ON THE SAME NETWORK) , THE LAST DECREASE WILL HAPPEN ON GATEWAY ROUTER ISP-2 AND LAN 2. 

==>>THAT MAKES THE ANSWER TO BE "27".

NOTE : - BOTH ENDS OF A ROUTER BELONGS TO DIFFERENT LAYER 3 ADDRESS.

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@Oaesp The last TTL decrement happens at the receiving host. The Answer will be $26$.

See this link from @Ayush Upadhyaya Sir : https://gateoverflow.in/1983/gate-cse-2014-set-2-question-25?show=232302#c232302

 

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Isn't the question asking ttl value when receiver receives the datagram(ie;not after receiver processing it and decrementing ttl value)?

Answer 1

Each time a packet visits network layer it decrements it's TTL field. Source initializes it and others decreaments it. Inside LAN it never goes to network layer, it is forwarded from data link layer itself.. in routers it goes upto network layer to make a routing decision.. and the router decrements it because the packet has visited the network layer.. and at the receiver too, the packet has visited the network layer and network layer will do it's job and decrements the TTL value.

There are $5$ routers, So Network Layer will be visited $5$ times  and $1$ time on the destination

So, TTL $= 26$

PS:) A receiver decrements TTL value and then checks whether it is $0$ (or) not. So, $26$ is the answer (not $27$)

Comments

@jatin khachane 1

It means once if packet comes to the reciever it wontw be discarded because of TTL value

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At the receiver side the packet goes back to application layer & will encounter network layer in between (as going bottom to top manner) So again network layer will decrement TTL. Thus receiver will accept datagram as TTL=26

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good

Answer 2

TTL value only decrement at router and at destination.It is very very important to keep in your mind until exam finish.

So Total router=5

Destination=1

Initial TTL value=32

When Packet reaches the receiver=32-(5+1)=26

Comments

Does this mean that if the image will be having 6 routers then answer will be 25?

Answer 3

TTL field reduced at each router, there are total 5 routers and finally at receiver.

=32 -(5+1)=26

Answer 4

ans 22

Comments

correctly said :-)

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When R receives packet then TTL = 27,and it decrements it to make 26. Shouldn't 27 be answer??

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@Vicky

TTL expiration is intended to cause datagrams to be discarded by routers, but not by the destination host.

http://networkengineering.stackexchange.com/questions/10929/when-is-an-ipv4-ttl-decremented-and-what-is-the-drop-condition

http://searchnetworking.techtarget.com/definition/time-to-live

http://www.linfo.org/time-to-live.html

I did not get any source where it is mentioned that..

1.RECEIVER also decrease TTL.(1st link is against this statement.)

2.RECEIVER reject the packet due to TTL=0 (previously it was 1 before arriving at Receiver)

Plz give a reference for above.