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25 Cards in this Set

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How large (in bytes) is the UDP header?
8 bytes (fixed).
How large (in bytes) is the TCP header?
20 bytes minimum, longer with options.
How large (in bytes) is the IP header?
20 bytes minimum, longer with options.
List the fields in the IP header.
Version, IHL (header length), TOS, total length.
Identification, flags, fragment offset.
TTL, protocol, header checksum.
Src Address.
Dst Address.
In the Protocol subfield of the IP header, what are the hex values identifying TCP, UDP, and ICMP?
TCP = 0x06
UDP = 0x11
ICMP = 0x01
The IP header contains 3 fields that are useful when dealing with IP fragmentation. What are they?
Identification --> If a datagram is fragmented, all fragments will contain a common ID number.
Flags --> Control flags for fragmentation
Fragment offset --> Specifies the offset in the original datagram where this fragment goes.
In the IP header, how many bits are used for the flags that control fragmentation?
3 bits:
1st bit: Not used
2nd bit: Don't Fragment
3rd bit: More Fragments
How long is the TTL field in the IP header?
1 byte.
When the TTL field is decremented to 0, what happens?
Two things happen: the router that decremented it to zero then discards the packet, and it sends an ICMP Time Exceeded message to the sender.
The IP header contains a checksum field. How long is it, and to what does it apply?
2 bytes long. It is a simple checksum that is run over the IP header only, not the data.
The packet is discarded if the checksum results in a mismatch.
Name all 4 fields in the UDP header.
Source port
Destination port
Length (both header and data)
Checksum (optional).
Is UDP's checksum field mandatory?
No. The UDP checksum is optional (unlike TCP's checksum field).
Name the 6 control bits (flags) in the TCP header.
URG - Urgent bit
ACK - Acknowledgement bit
PSH - Push bit
RST - Reset bit
SYN - Synchronize bit
FIN - Finish bit
Name the fields in the TCP header.
Src port, Dst port
Sequence number
Acknowledgement number
Header length, reserved 6 bits, 6 control bits (flags), window size
Checksum, urgent pointer
What is the RFC for IP? (version 4)
RFC 791
What is the RFC for ICMP? (version 4)
RFC 792
At the highest level, ICMP messages can be divided into two classes. What are they?
Error messages
Informational (or query) messages
Describe the ICMP message, and give its Type value:
"Destination Unreachable"
Type value = 3
Indicates a datagram could not be delivered. (The code value provides more detail as to why.)
Describe the ICMP message, and give its Type value:
"Time Exceeded"
Type value = 11
This is sent back to the source when a datagram is discarded due to the TTL expiring.
This message is also sent when a datagram is fragmented, but not all of the fragments reach the destination.
List the fields in the ICMP common message format.
Type (1 byte)
Code (1 byte)
Checksum (2 bytes)
Message Body/Data (variable)
When an ICMPv4 error message is generated, how much of the original datagram is included in the ICMP error message?
The full IP header is included, and 8 bytes of the IP payload.
With no IP options, it's 28 bytes, and that's enough to get TCP and UDP source ports.
The ICMP "Destination Unreachable" message type has a number of codes indicating why the destination is unreachable. Name 4 of them.
4 important ones are:
Code 0 -- Network unreachable
Code 1 -- Host unreachable
Code 3 -- Port unreachable
Code 4 -- Fragmentation needed and DF set
What is the ICMP Redirect Message type used for?
ICMP Redirect messages are sent by routers to inform hosts of a preferred router for a particular destination.
Can ICMP Redirect messages be sent between routers to alter a router's table?
No. ICMP Redirects are sent to the Host only. (Remember, all ICMP error reporting is limited to the datagram source.)
Describe ICMP Parameter problem messages.
Parameter problem messages are used to indicate some problem with the IP packet: A bad field in the header, a missing required option, or a bad IP datagram length. This is a catch-all message or sorts.