Module 9 Flashcards
Q19-1 Can the value of the header length field be more than 5 in an IPv4 packet? When is it exactly 5?
The minimum length of the IPv4 header is 20 bytes and the maximum is 60 bytes. The value of the header length field defines the header length in multiples of four bytes, which means that HLEN can be between 5 and 15. It cannot be less than 5 and it cannot be greater than 15. It is exactly 5 when there is no option.
Q19-5 In an IPv4 datagram the value of the header length (HLEN) field is 6 . How many bytes or options have been added to the packet?
The header length is 6 × 4 = 24. The option length is then 24 − 20 = 4 bytes
Q19-7 Compare and contrast the protocol field at the network layer with the port number at the transport layer. What is their common purpose? Why do we need to port number fields but only one protocol number field? Why is the size of the protocol field only half the size of each port number?
The protocol field and the port numbers both have the same functionality: multiplexing and demultiplexing. Port numbers are used to do these tasks at the transport layer; the protocol field is used to do the same at the network layer. We need only one protocol field at the network layer because payload taken from a protocol at the source should be delivered to the same protocol at the destination. The client and server processes, on the other hand, normally have different port numbers (ephemeral and well-known), which means we need two port numbers to define the processes. The size of the protocol field defines the total number of different protocols that use the service of the network layer, which is a small number (eight bits is enough for this purpose). On the other hand, many new applications may by added every day that needs a larger size of the port number field (sixteen bits is assigned).
P19-1 In an ipv4 datagram, the value of the total length field is (00A0)16 and the value of the header-length (HLEN) is (5)16. How many bytes of payload are being carried by the datagram? What is the efficiency (ratio of payload length to total length) of this datagram?
The total length of the datagram is (00A0)16 = 160 bytes. The header length is 5 × 4 = 20. The size of the payload is then 160 − 20 = 140. The efficiency = 140 / 160 = 87.5%.
P19-6 Which fields of the IPv4 main header will change from router to router?
TTL, Checksum, Fragmentation
Q19-6 Can each of the following be the value of a ttl in a datagram? a) 23 b) 0 c) 1 d) 301
a) Yes b) No, datagram discarded before zero c) 1, valid. d) No, 255 is max value for ttl
Q19-8 Which fields are in the datagram are responsible for gluing together all fragments belonging to an original datagram?
Identification, flags, fragmentation offset
Q19-10 Assume a destination computer receives several packets from a source. How can it be sure that the fragments belonging to a datagram are not mixed with the fragments belonging to another datagram?
To guarantee uniqueness the IP protocol uses a counter to label the datagrams. When a datagram is fragmented the value from the counter is copied to all the fragments.
P19-2 An IP datagram has arrived with the partial information in the header in hexadecimal. 45000054 00030000 2006
a. What is the header size?
b. Are there any options in the packet?
c. What is the size of the data?
d. Is the packet fragmented?
e. How many more routers can the packet travel to?
f. What is the protocol number of the payload being carried by the packet?
a) 20 bytes
b) Data is not included in the sample
c) 64 bits
d) No
e) 32
f) 6 TCP
What are the three transition strategies for IPv6?
1) Tunneling
2) Dual Stack
3) Conversion (Header Translation)
What are four advantages of IPv6 over IPv4?
- Large address space. Uses 128 bit address space = 2128 addresses.
- Simplified header format for faster packet switching at routers
- Flow label field for QoS support
- An extension header for IPsec support
Explain the different parts of the IP header.
| HLEN = 4 bits |
The unit is 4, Max 60
| Service = 8 bits not used |
Obsolete
| Total length = 16 bits, total length of datagram |
Max header size is 60 bytes
Min is 20 bytes
Options are noted in last part of header
| Identification = 16 bits |
| Flags = 3 bits |
| TTL 8 bits |
Decreased by 1 at every router.
If TTL becomes 0, then the packet will be dropped by the router, and the router sends an ICMP error message to the sender of the dropped packet.
This is how to implement “tracert” or “traceroute” command.
| Protocol = 8 bits |
Decides type of payload
Ver = 4 bits
