Chapter 6 Flashcards
What is the primary function of the transport layer?
Hide details of network from above layers
How do you identify processes?
Number them in the header
What does reliability mean?
Communication shouldnt fail. If it fails, communication parties should know it has failed
Which protocol provides unreliable transport layer?
UDP ( User Datagram Protocol )
Which protocol provides a reliable transport layer?
TCP ( Transmission Control Protocol )
What is the new transport layer protocol?
QUIC
Reliable protocols are ……..-oriented
connection
What does connection-oriented mean?
Establish connection and use that connection then terminate that connection
How do you establish a reliable connection?
3-way handshake
How does TCP’s 3 way handshake look like?
A: SYN (SYN = synchronise)
b: SYN + ACK
A: ACK
How does a termination TCP handshake look like?
A: FIN
b: FIN + ACK
A: ACK
What are the different flow control algorithms?
Stop-and-wait
Sliding window
Unrestricted flow
How does the Stop-and-wait algorithm work?
Wait for ACK message, If received send next message. If not received within time-out resend + reset time-out counter
Problem with Stop-and-wait algorithm?
Very inefficient
What is the sliding window algorithm?
Its a window that represents messages sent by sender but not yet confirmed
How does sliding window handle lost messages?
Selective repeat ( only retransmit lost messages )
Go-Back-N ( all unacknowledged messages are retransmitted starting from the lost message)
What is the most efficient way to handle lost messages with sliding window?
Selective repeat
How does the unrestricted flow algorithm work?
No flow control ( unrestricted )
How many timers does a ARQ sliding window need?
2: retransmission + acknowledgment
What does ARQ stand for?
Automatic Repeat Request
What should happen when an ARQ retransmission timer expires?
Re transmit the packet
What does a ARQ acknowledgment timer indicate?
How much time is left before the packet has to be acknowledged ( allows piggybacking of data )
What algorithms are examples of ARQ algorithms?
Stop and wait + sliding window
How are transport errors caused?
By over congesting the network
How are packet rates controlled?
By the size of the sliding window
Are connectionless protocols reliable?
No
Which of the transport protocol classes is connectionless?
TP4
Which transport protocol classes has resynchronisation?
TP1 and TP3
Which transport protocol classes has explicit flow control?
TP3 and TP4 (TP2 = optional)
Which transport protocol classes has Retransmission on time-out and inactivity control.
TP4
What application layer protocols use TCP?
SMTP and HTTP
What application layer protocol uses UDP?
DNS name lookup
What components does a TCP header have?
Source port, destination port( port numbers are used to identify communication processes), Sequence number, acknowledgement number, Data Offset
Checksum, Urgent Pointer, Padding and data
What is the data format that the TCP sends back and forth for a 3 way handshake?
1: (syn,S=0,A=0,W=1000,0 data) (S == number of syn’s and Ack is the number of Acks. W = “My” window size, 0 data = 0 bytes of data)
2: (syn,ack,S=0,A=1,W=2000,0 data)
3: (ack,S=1,A=1,W=1000,0 data)
3-way handshake for TCP
What does each “node”/”side”? Store with TCP?
T,R,W
T =number of bytes transmitted
R = number of bytes received
W = window size
What is TCP’s retransmit timer used for?
Retransmitting lost segments
How does TCP’s retransmit work?
When TCP sends, start timer. When ack received = Stop timer. If timer expires = resend
What is RTT?
Round trip time ( here to there to back time)
What does a persistence timer do?
Ensure no deadlock
What timer gets activated when A is informed that B’s window is 0?
Persistence timer
What happens if the persistence timer runs out?
Poke the other node. The other node will then send back an ACK but also its window size
What does the keepalive timer do?
Keeps the connection alive until the timer runs out.
When is the keepalive timer reset?
When a message has been received.
To what state does a connection go when it is closed?
TIME-WAIT
What is a quiet timer?
A timer that is activated when the 3rd part of the 3way termination handshake has been entered
What happens when the quiet timer expires?
Resources are returned to OS
What program do you use to display info about status of the network?
netstat
What are the different states a TCP connection can be in?
Listen, Closed, Closing, Time-wait, Fin-wait-1, fin-wait-2, established (open connection), close-wait(close request from local user)
What does a slow start mean in tcp?
Small initial window size with exponential growth
What happens to the sliding window when packets are lost with tcp?
Set a threshold of cwnd/2 after which it increases linearly instead of exponentially
What is present in the UDP packets?
Source port, Destination port, Length, Checksum, Payload
What range is the well-known port?
0-1023
What is all the relevant port numbers?
20 ftp data
21 ftp control
22 ssh
23 telnet
25 SMTP
53 DNS
80 HTTP
110 POP3
143 IMAP
995 POP3 via TLS/SSL
UDP ports often service the same applications as their TCP equivalents?
True
What range is registered ports?
1024-49151
What range is dynamic/private ports?
49152-65535
What do you call a host address combined with a port?
A socket
