Computer Networks: Time To Live (TTL)

Questions and Answers

What is the size of the original packet in bytes?

1500 bytes

3000 bytes

7500 bytes

6000 bytes (correct)

What is the offset value for Fragment 3?

555

370 (correct)

740

185

How are fragmented packets processed upon reaching their destination?

They are combined using their OFFSET values (correct)

They are merged randomly

They are discarded

They are stored temporarily

What potential issue can arise if there is a problem in a routing table?

Packets may be misrouted Signup and view all the answers

Which fragment contains data bytes ranging from 4440 to 5919?

Fragment 4 Signup and view all the answers

What indicates the segmentation of the original packet into fragments?

Offset values Signup and view all the answers

Which fragment has an offset value of 740?

Fragment 5 Signup and view all the answers

What may occur with fragmented packets other than being out of order?

They may travel through different routes Signup and view all the answers

What is the primary characteristic of IP as a network layer protocol?

It is considered an unreliable protocol. Signup and view all the answers

What happens to IP packets that exceed the Maximum Transmission Unit (MTU)?

They are fragmented. Signup and view all the answers

How does the receiver ensure the correct ordering of fragmented IP packets?

By utilizing the identification number and fragmentation offset. Signup and view all the answers

What is the significance of the 'Identification' field in an IP packet?

It helps in reassembling fragmented packets. Signup and view all the answers

Which combination of protocols is considered reliable?

TCP/IP Signup and view all the answers

What does fragmentation offset help to identify in fragmented IP packets?

The order of the fragments. Signup and view all the answers

What must be added to each fragment when an IP packet is fragmented?

A 20-byte header. Signup and view all the answers

Which statement about IP packets is true?

Packets can arrive in different orders at the destination. Signup and view all the answers

What happens to an IP packet when its Time To Live (TTL) value reaches zero?

The packet is discarded and an ICMP message is sent to the source. Signup and view all the answers

What is the maximum value that can be initialized for the TTL in an IP packet?

255 Signup and view all the answers

If a packet with an initial TTL of 6 goes through three routers, what will its TTL value be when it reaches the third router?

3 Signup and view all the answers

What is the standard header size for an IP packet when the HLEN value is set to 5?

20 bytes Signup and view all the answers

Which version of IP is represented by the bit sequence 0100?

IPv4 Signup and view all the answers

What field in the IP header indicates the priority of the packet type?

Service Type Signup and view all the answers

What action does router R2 take when the TTL value of the packet becomes zero?

Discards the packet and sends an ICMP message to Host A. Signup and view all the answers

How many bits are allocated for the Version field in the IP header?

4 bits Signup and view all the answers

Which of the following is NOT true about the Precedence field in the IP header?

It is utilized in IPv4. Signup and view all the answers

If a packet loops through routers R1, R4, and R2 repeatedly, which router discards the packet when the TTL becomes zero?

R2 Signup and view all the answers

What is the total length of an IP packet if the data length is 280 bytes and the header length is 20 bytes?

300 bytes Signup and view all the answers

What does a value of D=1 in the flags field indicate?

The packet is not allowed to be fragmented Signup and view all the answers

What is the function of the Time To Live (TTL) field in an IP packet?

Defines the maximum number of hops the packet can travel Signup and view all the answers

How many bits are used for the Identification field in an IP packet?

16 bits Signup and view all the answers

What does the Header Checksum field check for?

Errors in the header only Signup and view all the answers

What does the Fragmentation Offset field provide?

The offset value of the fragment Signup and view all the answers

Which of the following protocols can be identified by the Protocol field in the IP packet?

TCP Signup and view all the answers

What is the purpose of the Source IP Address field?

Identifies the sender's IP address Signup and view all the answers

Study Notes

Overview of IP

IP serves as the network layer protocol within the TCP/IP suite.
Lacks error control, flow control, and congestion control, making IP an unreliable protocol.
Combination of TCP with IP ensures reliability, whereas the UDP with IP remains unreliable.
IP packets function as datagrams, potentially taking different routes and arriving out of order.

Identification

Each IP packet has a unique serial number called "Identification."
This number aids in the correct reassembly of packets at the receiver's end, despite possible out-of-order arrival.

Maximum Transmission Unit (MTU)

Defines the maximum amount of data that can be accommodated within a frame.

Fragmentation

IP packets larger than the MTU need to be fragmented.
The fragmentation process involves separating the data from the header, breaking data into MTUs, and adding a 20-byte header to each fragment.

Fragmentation Offset

Each fragmented IP packet retains the original packet's identification number.
Offset values are crucial for ordering fragments, as displayed by the byte range examples.

Time To Live (TTL)

TTL prevents packets from circulating indefinitely in the network.
Set by the transmitting router and decremented by each router it passes through.
If TTL reaches zero, the packet is discarded, and an ICMP message is sent back to the source.

IP Header Structure

The header consists of multiple fields, each serving different purposes, such as:
- Version: Indicating the IP version used (IPv4 or IPv6).
- Header Length (HLEN): Indicates the number of 4-byte units in the header.
- Service Type: Defines the priority and type of service for the packet.
- Total Length: The combined length of the header and data.
- Identification, Flags, Fragmentation Offset: Manage packet fragmentation and reassembly.
- Time To Live (TTL): Defines maximum hops allowed.
- Protocol: Indicates the type of data contained in the packet (UDP, TCP, etc.).
- Header Checksum: Validates header integrity; errors lead to packet discard.
- Source IP Address: 32-bit field containing the sender's IP.
- Destination IP Address: 32-bit field containing the receiver's IP.

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Description

Explore the concept of Time To Live (TTL) in IP packets and its importance in preventing network overloads. Understand how TTL values are managed across routers to ensure efficient data transmission in network protocols.