Transport Layer

1
Transport Layer

• Services
• Reliable Delivery
• or Not!
• Protocols
• Internet TCP, UDP
• ISO TP0 thru TP4

2
Transport Services
Data
Application Header
Data
Presentation Header
Data
Session Header (?)
User
Provider
Data
Transport Header
Data
Network Header
Link Header
Data
Framing
Data
see page 481
3
Delivery Services

• First layer that can really provide reliability
• Reliability means sequencing, error recovery,
  acknowledgements
• Not every application needs (wants) reliability

4
Quality of Service

• New idea from the voice, video folk
• Not (yet) part of Internet protocols
• RSVP in process
• Unclear which layer guarantees QoS

5
Managing Connections

• Addressing
• Identifying duplicate TPDUs
• Three-way handshakes
• Flow control
• Crash recovery (largely ignored)

6
TCP

• Reliable, sequenced stream of bytes
• Virtual circuit
• Buffered transfer
• Unstructured
• Full Duplex

7
TCP - How it Works

• Positive Acknowledgements w/ Retransmission
• Sliding Windows
• Pages 531, 532 give the programmers state
  machine info

8
TCP Features

• A protocol, not a package
• Ports (Well Known Ports)
• Out of Band, or URGENT, data
• Timeouts
• RTT Estimation (Karns Algorithm) (pg 541)
• Congestion
• Slow-Start Methodolgy
• Limitations

9
TCP Header
10
TCP Connection Management

• Three Way Handshake
• SYN, SYN/ACK, ACK
• May be viewed as two Simplex connections
• Window management
• Advertising Zero Window Size
• Nagles algorithm
• Silly Window Syndrome

11
UDP

• Your basic datagram
• No acknowledgements, no reliability
• Why use it?

12
Transport Layer Performance

• Timeout Values
• Inadvertent Synchronized Loads
• Latency (Bandwidth-delay product)
• MTU, MSS values
• Router impact

13
Performance Hints

• CPU normally is the bottleneck
• most time is spent in protocol stacks, not on the
  wire
• Per-packet overhead means big packets are much
  more efficient
• But delays to create big packets impact
  interactive applications
• Easy to increase bandwidth, impossible (?) to
  reduce propagation delay

14
Performance Challenges

• Gigabit networks
• Satellites