CS 6250 Class 20 10'29'2002

1
CS 6250 Class 20 10.29.2002

• End-to-end protocols
• (a.k.a. transport or layer-4 protocols)
• UDP,
• and introduction to TCP

2
News

• Project progress reviews Friday, Nov-8
• TAs will schedule for appointment with each group
• Work on problems 19, 20, 24, 26 of chapter 4
• Nov 5 Jinliang will go through some chapter 4
  problems
• Nov 7 midterm (in class)
• Dont forget your calculators
• Nov 12 Amogh will go through midterm solutions,
  and cover part of chapter 6

3
Transport protocols

• Transform host-to-host communication, offered by
  network layer, to process-to-process
  communication
• Deliver important services to application layer
• Guaranteed message delivery
• In-order delivery
• Detect/eliminate message replication
• Support arbitrarily large messages
• Support synchronization between sender receiver
• Allow receiver to apply flow control to sender
• Support multiple application processes on each
  host
• Other services? (congestion control,
  quality-of-service, )
• Overview
• UDP unreliable message delivery protocol
• TCP reliable stream transfer protocol

4
User Datagram Protocol (UDP)

• Connectionless, unreliable, message delivery
• But, optional checksum provides limited error
  detection
• Computed over UDP header, message, and IP
  pseudoheader
• Pseudoheader IP src dst addresses, protocol ,
  and UDP length
• If source does not want to compute checksum, it
  sets it to zero
• Allows process demultiplexing using ports
• 16 bits per port 65536 possible channels per
  host
• No flow control sender can overrun receivers
  buffers

5
Application demultiplexing with UDP
Application
Application
Application
process
process
process

• Well-known ports
• See /etc/services
• How does the sender learn the receivers port?
• UDP preserves message boundaries

Ports
Queues
Packets
demultiplexed
UDP
Packets arrive
6
Transmission Control Protocol (TCP)

• Reliable, connection-oriented, byte-stream
  delivery service
• Full duplex connection
• Supports flow control (and congestion control to
  be covered later)
• Supports application layer demultiplexing using
  ports
• Segments vs application-layer writes when does
  TCP transmit a segment?
• Maximum Segment Size (MSS), Push operation,
  Send-Timer

Application process
Application process
W
rite
Read


bytes
bytes
TCP
TCP
Send buffer
Receive buffer

Segment
Segment
Segment
T
ransmit segments
7
More on TCP

• A TCP connection provides full-duplex
  (simultaneous bidirectional) transfer
  capabilities
• Sender and receiver semantics are related to
  unidirectional TCP transfer
• Sliding window for flow control reliability
  (see next class)
• Side note Maximum Segment Lifetime (MSL)
• maximum time an IP packet can live in the network
• wandering duplicates and the problem of
  previous connection incarnations

Data
(SequenceNum)
Sender
Receiver
Acknowledgment
AdvertisedWindow
8
TCP segment format

• Ports as in UDP
• SeqNum, AckNum, AdvWindow used in sliding-window
  algorithm for reliable, in-order, unique
  delivery, and for flow-control
• Flags SYN, FIN, RESET, PUSH, URG, ACK
• Checksum computed as in UDP (but required)
• HdrLen in 32-bit words TCP header may include
  options
• TCP options Maximum Segment Size (MSS),
  timestamp, window scale

9
Connection establishment

• Caller (active open) and callee (passive open)
• 3-way handshake algorithm
• Why dont we set the initial sequence numbers to
  zero?
• What if any of the 3WHS messages get
  lost/delayed?
• What if the server wants to deny the connection?
• Map the Unix system calls socket(), bind(),
  connect(), listen(), accept() to the following
  diagram

Active participant
Passive participant
(client)
(server)
SYN, SequenceNum
x
y
,
1

SYN ACK, SequenceNum
x
Acknowledgment
ACK, Acknowledgment
y

1
10
Connection termination

• Graceful close independent termination of each
  direction of the connection
• Initiated by close() system call
• After all pending data have been sent and ACKed,
  send last segment with FIN bit set
• That direction of the connection is closed 2MSL
  seconds (2120secs) after the remote end ACKs the
  FIN message
• Why do we need to wait before closing the
  connection?
• Why to wait for 2 MSL instead of 1MSL?
• Abrupt connection termination
• Initiated by abort(), or by error conditions
• Send segment with RST bit set, discarding any
  pending data

11
TCP connection establishment tear-down
state-transition diagram
CLOSED
Active open
/SYN
Passive open
Close
Close
LISTEN
Send/
SYN
SYN/SYN ACK
SYN/SYN ACK
SYN_RCVD
SYN_SENT
SYN ACK/ACK
ACK
ESTABLISHED
Close
/FIN
FIN/ACK
Close
/FIN
FIN_WAIT_1
CLOSE_WAIT
FIN/ACK
ACK
Close
/FIN
ACK FIN/ACK
FIN_WAIT_2
LAST_ACK
CLOSING
Timeout after two
ACK
ACK
segment lifetimes
FIN/ACK
TIME_WAIT
CLOSED