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25 Cards in this Set
- Front
- Back
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How large (in bytes) is the UDP header?
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8 bytes (fixed).
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How large (in bytes) is the TCP header?
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20 bytes minimum, longer with options.
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How large (in bytes) is the IP header?
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20 bytes minimum, longer with options.
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List the fields in the IP header.
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Version, IHL (header length), TOS, total length.
Identification, flags, fragment offset. TTL, protocol, header checksum. Src Address. Dst Address. Options. Data. |
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In the Protocol subfield of the IP header, what are the hex values identifying TCP, UDP, and ICMP?
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TCP = 0x06
UDP = 0x11 ICMP = 0x01 |
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The IP header contains 3 fields that are useful when dealing with IP fragmentation. What are they?
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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. |
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In the IP header, how many bits are used for the flags that control fragmentation?
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3 bits:
1st bit: Not used 2nd bit: Don't Fragment 3rd bit: More Fragments |
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How long is the TTL field in the IP header?
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1 byte.
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When the TTL field is decremented to 0, what happens?
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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.
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The IP header contains a checksum field. How long is it, and to what does it apply?
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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. |
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Name all 4 fields in the UDP header.
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Source port
Destination port Length (both header and data) Checksum (optional). |
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Is UDP's checksum field mandatory?
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No. The UDP checksum is optional (unlike TCP's checksum field).
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Name the 6 control bits (flags) in the TCP header.
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URG - Urgent bit
ACK - Acknowledgement bit PSH - Push bit RST - Reset bit SYN - Synchronize bit FIN - Finish bit |
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Name the fields in the TCP header.
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Src port, Dst port
Sequence number Acknowledgement number Header length, reserved 6 bits, 6 control bits (flags), window size Checksum, urgent pointer Options Data |
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What is the RFC for IP? (version 4)
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RFC 791
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What is the RFC for ICMP? (version 4)
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RFC 792
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At the highest level, ICMP messages can be divided into two classes. What are they?
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Error messages
Informational (or query) messages |
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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.) |
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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. |
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List the fields in the ICMP common message format.
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Type (1 byte)
Code (1 byte) Checksum (2 bytes) Message Body/Data (variable) |
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When an ICMPv4 error message is generated, how much of the original datagram is included in the ICMP error message?
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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. |
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The ICMP "Destination Unreachable" message type has a number of codes indicating why the destination is unreachable. Name 4 of them.
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4 important ones are:
Code 0 -- Network unreachable Code 1 -- Host unreachable Code 3 -- Port unreachable Code 4 -- Fragmentation needed and DF set |
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What is the ICMP Redirect Message type used for?
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ICMP Redirect messages are sent by routers to inform hosts of a preferred router for a particular destination.
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Can ICMP Redirect messages be sent between routers to alter a router's table?
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No. ICMP Redirects are sent to the Host only. (Remember, all ICMP error reporting is limited to the datagram source.)
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Describe ICMP Parameter problem messages.
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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.
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