1. EEL 5718 Computer Communications
Transport Layer Protocols
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2. Transport Protocol
Task: Reliable message delivery from end node to another end node via a subnet
Transport service is end to end protocol between two systems on the same network
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3. End-to-End Connection
Messages
Messages
Segments
Transport
Layer
Transport
Layer
Network
Layer
Network
Layer
Network
Layer
Network
Layer
Data link
Layer
Data link
Layer
Data link
Layer
Data link
Layer
End system A
End system B
Physical
Layer
Physical
Layer
Physical
Layer
Physical
Layer
Network
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4. Point-to-Point vs. End-to-End
Rigid (solid link) vs. flexible (rubber link)
Predictable vs. unpredictable (in terms of round-trip delay)
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5. End-to-end Quality of Service (QoS)
A set of qualitative and quantitative measures that define an acceptable level of performance
Reliability: error rate
Setup time
Delay and delay variations (jitters)
Throughput
Priority
Protection
Resilience
Security
More
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6. Network Network service Application Application Adaptation Function
End-to-end application requirements
Network service
Application
Figure 5.5

7. Issues in Transport Protocol
Issues to be addressed
Handling arbitrary message sizes
Segmentation/reassembly
Reliability and sequencing
ARQ protocols
Pacing and flow control
Sliding window control
Congestion control
Timing: sequence numbering or time-stamping
Addressing
Privacy, integrity, and authentication
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8. Addressing Target user specified by:
User identification
Usually a host, port number
Port represents a particular transport service (TS) user
Transport entity identification (e.g., TCP, UDP)
Host address
An attached network device
In an internet, a global internet address
Network number
Address=<galaxy><star><planet><country><network><host><port>
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9. Finding Addresses Four methods Know address ahead of time
e.g. collection of network device stats: device ID
Well known addresses
Name server or directory server
Sending process request to well known address
Address Resolution Protocol (ARP)
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10. Multiplexing Multiple users employ the same network connection
User identified by port number or service access point (SAP)
May also multiplex with respect to network services used
e.g., multiplexing a single virtual X.25 circuit to a number of transport service user
X.25 charges per virtual circuit connection time
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11. Service Access Point (SAP): Transport SAP, Network SAP
TSAP, NSAP and connections
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12. User Process Operation
Establishing connection
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13. Flow Control
Longer transmission delay between transport entities compared with actual transmission time
Delay in communication of flow control info
Variable transmission delay
Difficult to use timeouts
Flow may be controlled because:
The receiving user can not keep up
The receiving transport entity can not keep up
Results in buffer filling up (buffer overflow)
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14. Flow Control (Cont) Do nothing Refuse further segments
Segments that overflow are discarded
Sending transport entity will fail to get ACK and will retransmit: time-out
Refuse further segments
Clumsy
Multiplexed connections are controlled on aggregate flow
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15. Flow Control (Cont) Use fixed sliding window protocol
Incorporated in ARQ as well as congestion control
Works well on reliable network
Failure to receive ACK is taken as flow control indication
Does not work well on unreliable network
Can not distinguish between lost segment and flow control
Use credit schemes
Successful delivery (positive ack) will lead to more packet deliveries
Will discuss more in next chapter
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16. Establishment and Termination
Allow each end to know the other exists
Negotiation of optional parameters (for QoS)
Triggers allocation of transport entity resources
By mutual agreement
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17. Connection State Diagram: Protocol Verification

18. Problems to Be Handled Ordered Delivery Retransmission strategy (ARQ)
Duplication detection (Sequence Number)
Flow control (Sliding Window)
Connection establishment
Connection termination
Crash recovery
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19. TCP/IP Suite Transmission Control Protocol (TCP)
Connection oriented
RFC 793
User Datagram Protocol (UDP)
Connectionless
RFC 768
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20. TCP Services
Reliable communication between a pair of processes (end users)
Across variety of reliable and unreliable networks and internets
Two labeling facilities
Data stream push
TCP user can require transmission of all data up to push flag
Receiver will deliver in same manner
Avoids waiting for full buffers
Urgent data signal
Indicates urgent data is upcoming in stream
User decides how to handle it
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21. TCP Header
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22. Items Passed to IP TCP passes some parameters down to IP
Precedence (priority)
Normal delay/low delay
Normal throughput/high throughput
Normal reliability/high reliability
Security
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23. TCP Mechanisms Connection establishment Three way handshake
Between pair of ports
One port can connect to multiple destinations
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24. SYN, Seq_no = y, ACK, Ack_no = x+1
Host A
Host B
SYN, Seq_no = x
SYN, Seq_no = y, ACK, Ack_no = x+1
Seq_no = x+1, ACK, Ack_no = y+1
Figure 8.22

25. TCP Mechanisms (cont) Data transfer Logical stream of octets
Flow control by credit allocation of number of octets
Data buffered at transmitter and receiver
Error control
Selective Repeat (SR) ARQ
Sliding window scheme is used
Time-out is adaptively estimated based on the estimation of Round-Trip-Time (RTT) (more will be discussed in congestion control)
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26. TCP Mechanisms (cont) Connection termination Graceful close
TCP users issues CLOSE primitive
Transport entity sets FIN flag on last segment sent
Abrupt termination by ABORT primitive
Entity abandons all attempts to send or receive data
RST segment transmitted
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27. UDP User Datagram Protocol RFC 768
Connectionless service for application level procedures
Unreliable
Delivery and duplication control not guaranteed
Reduced overhead
e.g. network management (ICMP)
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28. UDP Applicaitons Inward data collection Outward data dissemination
Request-Response
Real time application
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29. Reading Textbook Chapter 5, Chapter 8 Tanenbaum, Chapter 6
Stallings Chapter 17
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