Tazeen Dhedhi
EE379K. Network Engineering
Homework 2
1. Obtain RFC documents 791-2.
Done
2. Produce a diagram of the IP version-4
header and describe the function of each of the fields in the header.
|
VERS |
HLEN |
SERVICE |
TOTAL LENGTH |
||
|
ID |
FLAGS |
F. OFFSET |
|||
|
TTL |
TYPE |
HDR CHECKSUM |
|||
|
SOURCE |
|||||
|
DESTINATION |
|||||
|
IP OPTIONS (MAY BE OMITTED) |
PADDING |
||||
i. VERS: Version number of the IP being used, in this case it will be 4.
ii. HLEN: Length of the header measured in 32-bit units, usually 5.
iii. SERVICE: Level of service desired, usually set to 0.
iv. TOTAL LENGTH: Length of the complete datagram in terms of octets.
v. ID: Unique identifier value for this datagram.
vi. FLAGS: Bits to control fragmentation. 1st bit is unused, 2nd bit is Don’t Fragment Flag, 3rd bit is More Fragments Flag.
vii. F. OFFSET: byte count from the start of the original sent packet.
viii. Time To Live: Time to live in terms of seconds (that means router has to keep track of when the packet arrived so usually it means Number of hops/links which the packet may be routed over).
ix. TYPE: Indicates the type of transport packet being carried. Ex. 1=ICMP, 2=IGMP, 6=TCP, 17=UDP
x. HDR CHECKSUM: A 2’s complement checksum inserted by the sender and updated whenever the header is modified by the router. Used to detect processing errors introduced into the datagram inside a router or bridge.
xi. SOURCE: The IP address of the original sender
xii. DESTINATION: The IP address of the final destination of the datagram.
xiii. IP OPTIONS: Special handling parameters.
xiv. PADDING: Used to align the packet to 32-bit boundary.
3. Explain the function of the Internet
Control Message Protocol (ICMP) echo
request and echo reply message
types.
These types are used to identify ICMP echo request and echo reply messages. The echo request message is sent with the source, the destination, an identifier and a sequence number. When an echo request is received by the destination, it sends back an echo reply message. The echo reply message can be constructed from the echo request message by swapping the source and destination. The code is also changed to show that it is an echo reply not an echo request.
These messages are used by
- ping to check if a particular destination is reachable and also calculate the roundtrip time from source to destination.
- traceroute to calculate a path to a particular destination. The way traceroute works is that it sends an echo request to the destination with increasing TTLs. The TTL expires at the first hop and a Time Exceeded ICMP message gets sent back. Now traceroute knows the IP address of the router that’s one hop on the path to the destination, then it sends another echo request with the TTL incremented. This time TTL expires on the 2nd hop and another Time Exceeded ICMP message gets generated. This keeps happening until the TTL is large enough for the echo request to reach the destination and it sends back an echo reply. This tells traceroute that it has calculated a possible path to the destination.