COMPUTER NETWORKS - EXAM #2
Q&A
sequence number - Answer-ID field for packets sent
pipelining - Answer-the sender allows multiple, yet-to-be-acknowledged packets
utilization - Answer-the fraction of time the sender is busy sending
(L/R) / (RTT + L/R) * number of packets in pipeline
window size - Answer-the number of unacknowledged packets in the pipeline
TCP - sequence numbers - Answer-byte stream number of the first byte in the
segment's data
TCP - ACK number - Answer-the sequence number of the next byte expected
(cumulative ACK)
TCP - EstimatedRTT - Answer-exponential weighted moving average
(1 - a)*EstimatedRTT + a*SampleRTT
a is usually 0.125
TCP - DevRTT - Answer-(1-b)*DevRTT + B*|SampleRTT - EstimatedRTT|
b is usally 0.25
TCP - TimeoutInterval - Answer-EstimatedRTT + 4*DevRTT
TCP - fast retransmit - Answer-if sender receives 4 ACKs for same data (triple duplicate
ACKs) resend unacked segment with smallest sequence number
TCP - flow control - Answer-receiver controls the sender so sender won't overflow
receiver's buffer by transmitting too much, too fast
receiver tells the sender how much room he has left in his buffer
TCP - handshake - Answer-3-way handshake where A sends SYN and Seq X, B sends
SYN, Seq Y, ACK X+1, and A sends ACK Y+1
congestion - Answer-too many sources sending too much data too fast for the network
to handle
end-end congestion control - Answer-- no feedback from network
- congestion inferred from end-system observed loss, delay
, TCP - congestion control - Answer-- control window is a function of the perceived
network congestion
- sender increases transmission rate until loss occurs
- sender cuts cwnd in half after loss
- loss on timeout vs. duplicate ACKs is based on TCP flavor
TCP - slow start - Answer-when connection begins, increase rate exponentially until first
loss event
TCP - fairness - Answer-if K TCP sessions share the same link of bandwidth R, each
should have average rate of R/K
congestion control implements this well already
forwarding - Answer-moving packets from router's input to appropriate router output
routing - Answer-determining route taken by packets from source to destination
datagram - Answer-the fancy name for network layer messages
forwarding table - Answer-address ranges are mapped to output links
longest prefix matching - Answer-when looking for forwarding table entry for given
destination address, use longest address prefix that matches the destination address
IP protocol - Answer-the addressing convention and packet handling conventions used
in the network layer
IP - overhead - Answer-normally 20 bytes, but can be specified using the header length
field
MTU - Answer-maximum transfer unit - the size in bytes of the most data allowed in a
single packet
IP - fragmentation - Answer-- one datagram is broken up because the MTU is not big
enough to transport the entire thing
- reassembled at the destination
IP - address - Answer-32-bit identifier for host, router interface
interface - Answer-connection between host/router and physical link
subnet - Answer-device interfaces with the same high order IP address bits
devices in the subnet can physically reach each other without an intervening router
Q&A
sequence number - Answer-ID field for packets sent
pipelining - Answer-the sender allows multiple, yet-to-be-acknowledged packets
utilization - Answer-the fraction of time the sender is busy sending
(L/R) / (RTT + L/R) * number of packets in pipeline
window size - Answer-the number of unacknowledged packets in the pipeline
TCP - sequence numbers - Answer-byte stream number of the first byte in the
segment's data
TCP - ACK number - Answer-the sequence number of the next byte expected
(cumulative ACK)
TCP - EstimatedRTT - Answer-exponential weighted moving average
(1 - a)*EstimatedRTT + a*SampleRTT
a is usually 0.125
TCP - DevRTT - Answer-(1-b)*DevRTT + B*|SampleRTT - EstimatedRTT|
b is usally 0.25
TCP - TimeoutInterval - Answer-EstimatedRTT + 4*DevRTT
TCP - fast retransmit - Answer-if sender receives 4 ACKs for same data (triple duplicate
ACKs) resend unacked segment with smallest sequence number
TCP - flow control - Answer-receiver controls the sender so sender won't overflow
receiver's buffer by transmitting too much, too fast
receiver tells the sender how much room he has left in his buffer
TCP - handshake - Answer-3-way handshake where A sends SYN and Seq X, B sends
SYN, Seq Y, ACK X+1, and A sends ACK Y+1
congestion - Answer-too many sources sending too much data too fast for the network
to handle
end-end congestion control - Answer-- no feedback from network
- congestion inferred from end-system observed loss, delay
, TCP - congestion control - Answer-- control window is a function of the perceived
network congestion
- sender increases transmission rate until loss occurs
- sender cuts cwnd in half after loss
- loss on timeout vs. duplicate ACKs is based on TCP flavor
TCP - slow start - Answer-when connection begins, increase rate exponentially until first
loss event
TCP - fairness - Answer-if K TCP sessions share the same link of bandwidth R, each
should have average rate of R/K
congestion control implements this well already
forwarding - Answer-moving packets from router's input to appropriate router output
routing - Answer-determining route taken by packets from source to destination
datagram - Answer-the fancy name for network layer messages
forwarding table - Answer-address ranges are mapped to output links
longest prefix matching - Answer-when looking for forwarding table entry for given
destination address, use longest address prefix that matches the destination address
IP protocol - Answer-the addressing convention and packet handling conventions used
in the network layer
IP - overhead - Answer-normally 20 bytes, but can be specified using the header length
field
MTU - Answer-maximum transfer unit - the size in bytes of the most data allowed in a
single packet
IP - fragmentation - Answer-- one datagram is broken up because the MTU is not big
enough to transport the entire thing
- reassembled at the destination
IP - address - Answer-32-bit identifier for host, router interface
interface - Answer-connection between host/router and physical link
subnet - Answer-device interfaces with the same high order IP address bits
devices in the subnet can physically reach each other without an intervening router
