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30 Cards in this Set
- Front
- Back
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What does TCP provide over IP?
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- Virtual Circuit: setup/teardown
- Buffered: Can buffer bytes on sender/receiver - Full duplex: once connection established, either endpoint can send - Reliable: positive ACKs + timers, retransmission - Checksum of data - Port |
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What is the algorithm for the TCP/UDP/IP header checksum?
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One's complement of 16-bit one's complement sum.
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What does UDP provide over IP?
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- Checksum of data
- Port DOES NOT PROVIDE: Reliability, Flow/Congestion Control, Segmentation |
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What does TCP stand for?
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Transmission Control Protocol
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What does UDP stand for?
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User Datagram Protocol
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TCP flag: URG
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Urgent pointer valid. The urgent pointer points to number of octets from segment beginning to last octet of urgent data (16-bit)
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TCP flag: ACK
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Acknowledgement is valid
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TCP flag: PSH
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Requests that sender/receiver doesn't buffer data
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TCP flag: RST
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Directs TCP to abort this connection
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TCP flag: SYN
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Requests establishing a connection
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TCP flag: FIN
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Sender is done sending (but not receiving) data
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TCP header: window size
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(16 bit) Number of octets permitted to transmit starting at the acknowledged octet
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Considering the latency model for dynamic congestion windows, which components contribute to latency?
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Round trip time (RTT), the sum of all the stalled times, the transmission time of the object (O/R)
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When a TCP segment arrives to a host, the socket to which the segment is directed depends on what?
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local and foreign addresses, local and foreign ports, and protocol
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Stop and wait protocol
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Highly inefficient when when there is a large distance between source and destination and the transmission rate is high. |
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Pipelining requires what?
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unique sequence numbers for each in-transit packet, sender-side buffering of unacknowledged packets, transmitting many packets before receiving acknowledgements
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With SYN cookies, when a server receives a SYN segment, it returns a SYNACK with:
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an initial sequence number that is a hash of the IP addresses and port numbers in the SYN segment (as well as of other things).
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Over a TCP connection, suppose host A sends two segments to host B, host B sends an acknowledgement for each segment, the first acknowledgement is lost, but the second acknowledgement arrives before the timer for the first segment expires. What will host A do?
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Host A will retransmit neither segments
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When a UDP segment arrives to a host, in order to direct the segment to the appropriate socket, the OS uses what?
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Destination port number
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TCP applies fast retransmit to a segment when ___?
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when it receives three ACKs for an earlier segment
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Nmap is often used to do what?
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port scans over a range of ports at a target host
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RTT
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Round Trip Time
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Three Way Handshake
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1. The client requests a connection by sending a SYN (synchronize) message to the server with initial sequence number X.
2. The server acknowledges this request by sending SYN-ACK back to the client with SYN initial sequence number Y and ACK X+1. 3. The client responds with an ACK of Y+1, and the connection is established. |
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List some UDP application protocols
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DNS, DHCP, TFTP, SNMP
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Why not use initial sequence number of 0?
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1. Delayed packets from an old connection could cause confusion.
2. Makes connection hijacking easier. |
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What is the rule for going into a time wait on a FIN?
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If you send a FIN before you receive a FIN, you must go into time wait.
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Example when client will go into time wait:
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echo protocol, Tweetster when server sends something unexpected.
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Example when server will go into time wait:
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HTTP, Tweetster when there's an error caused by the client.
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Formula for Round Trip Time (RTT)
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RTT = (alpha * RTT) + ((1 - alpha) * M)
M = latest measurement of actual RTT 0<=alpha<=1 : weight on old RTT (typically 7/8) |
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Formula for Retransmission Timeout (RTO)
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RTO = RTT + 4D
D (deviation) = alpha * D + (1-alpha) |RTT-M| alpha = weight on previous deviation (commonly 7/8) initial RTO = beta * RTT beta = scalar multiplier (generally arbitrary) |
