Operating Systems Lecture Notes

9 April 2012 • Transport Services

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Outline

• Transport protocols: UDP and TCP.
• Transport abstractions and APIs.
• TCP
  • Implementing TCP.
  • TCP segments and packets.
  • Flow and congestion control.

Level 4

• ISO OSI level 4: transport services.
• The transport level ties together individual networks into a single network.
  • Initially via overlay networks, now more directly.
• Uniform addressing, and a small amount of global structure.

Transport Protocols

• Transmission control protocol (TCP) and the user datagram protocol (UDP) are two major transport protocols.
  • TP4 is ISO’s failed entry.
  • Most other examples also lost to TCP/IP.
• The operating system is responsible for implementing the protocol.
  • The networking subsystem, in particular.

UDP

• UDP provides a few simple end-to-end services:
  • Per-host multiplexing via 16-bit port numbers.
  • Unreliable datagram delivery.
• A datagram is a block of data (not as structured as a message).
• “Unreliable” means: loss, corruption, duplication, unordered.

TCP

• TCP an overtly simpler but covertly more complex protocol.
• TCP services include:
  • Per-host multiplexing over 16-bit port numbers (independent of UDP).
  • Reliable byte streams.
• “Reliable” means: uncorrupted, ordered.

Transport Abstractions

• Transport services initially look typical: data gets written, data gets read. But
  • Different read-write patterns (reliability, network delay).
  • Complex transport-services characterizations.
• Transport services should look like other information-oriented services.
  • With some control-oriented services too.
  • A lot like files.

Transport APIs

• Transport services should be available through file-like APIs.
  • And sometimes they are.
• Unfortunately, the Sockets API from BSD Unix won the race.
  • The Sockets API is a perfect example of what happens when you violate the YAGNI principle.

Connections

• TCP is connection-oriented.
• A connection is a long-term, explicit relation between communicating end-points.
• Binding establishes one end of a connection.
• UDP is connection-less.
  • But the end-point terminology carries through.

TCP Connection States

• Much complexity (TCP or Sockets) arises from connection states.
• A rough list of states include:
  • Bound or unbound: connection end-points assigned or not.
  • Connected or listening: both end-point bound or not.
  • Closed: amazingly complex (half-closed, time waiting).
• UDP avoids this.

TCP States Illustrated

Implementing TCP

• The operating system implements the TCP reliable byte-stream abstraction.
  • Operating system → networking subsystem → TCP protocol stack.
• Byte-stream reliability comes from acknowledgments and timeout-retransmission.
  • Send data blocks.
  • Get acknowledgment and move on, or
  • timeout and retransmit.

Network Management

• The TCP protocol stack should also react to network conditions, and conditions at the other end.
  • Congestion and flow control.
• Both are based on varying the block-write characteristics.
  • Sending rate and, less variably, sending size.

TCP Segments

• The data blocks sent and received are called segments.
• Buffering’s important in TCP.
  • Assembling data into segments.
  • Holding segments for retransmission.
• Many games are possible by manipulating TCP buffering.
  • Slow start, Nagle’s algorithm.

TCP Packets

• A segment is preceded by a TCP header to form a TCP packet.

  

Flow Control

• For best throughput, send everything at once.
  • Receiver hostile, particularly for less capable receivers.
• Keep the number of unacknowledged packets at some maximum value, the window size.
• As packets get acknowledged, shift the window further up the data stream.
• This is window-based flow control.

Congestion Control

• For best throughput, send everything at once.
  • Network hostile as a global behavior.
• Keep the number of unacknowledged packets at the window size.
• As packets get acknowledged, send more and increase the window size.
• Acknowledgment time-outs reduce the window size.
• This is slow-start congestion control.

Summary

• Transport services are the most convenient (most abstract) of the ISO networking services.
  • But even then they’re not so great.
• Transport services are end-to-end.
• TCP and UDP are the best known, most used transport protocols.
  • TCP: reliable byte streams.
  • UDP: best-effort datagrams.

References

• Transmission Control Protocol, RFC 793, 1981.
• User Datagram Protocol, RFC 768, by Jon Postel, 1980.

Credits

• TCP state diagram.jpg by Raul654 under a Creative Commons BY SA license.
• Ntwk tcp header123.jpg by Gopal Paliwal under a Creative Commons BY SA license.

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This page last modified on 2012 April 11.