Computer Networking Lecture Notes

4 February 2013 • Application-Layer Services

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Outline

• FTP example.
• Application layer services.
• Internet protocols example.

An FTP Session

$ ftp
ftp> open ftp1.freebsd.org
Connected to moss.cse.buffalo.edu.
220 moss.cse.buffalo.edu FTP server (Version wu-2.6.2(1) Mon Aug 2 14:04:46 EDT 2010) ready.
Name (ftp1.freebsd.org:rclayton): anonymous
331 Guest login ok, send your complete e-mail address as password.
Password:
230 Guest login ok, access restrictions apply.
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> cd pub/FreeBSD
250-If you’re looking for one of the FreeBSD releases, please look in the
250-releases/${ARCH}/${RELNAME} directory, where ARCH = "alpha", "amd64",
250-"i386", "ia64", "pc98", or "sparc64" and RELNAME = the release
250-you’re interested in, e.g. "7.1-RELEASE" or "8.0-RELEASE".
250-
250-Please read the file README.TXT
250-  it was last modified on Fri Jun 18 21:54:21 2004 - 3007 days ago
250 CWD command successful.
ftp> get README.TXT
local: README.TXT remote: README.TXT
200 PORT command successful.
150 Opening BINARY mode data connection for README.TXT (6430 bytes).
226 Transfer complete.
6430 bytes received in 0.02 secs (311.2 kB/s)
ftp> close
221-You have transferred 6430 bytes in 1 files.
221-Total traffic for this session was 7389 bytes in 1 transfers.
221-Thank you for using the FTP service on moss.cse.buffalo.edu.
221 Goodbye.
ftp> quit

$ 

FTP Operation

• An FTP session maintains two ports between end-points (client and server).
  • The control port (21) carries commands and responses.
  • The data port (20) carries the data (files and listings).
  • The control port is persistent and the data port is non-persistent.
• What are the best ways to implement the control and data ports?

FTP Requirements

• Control port characteristics include
  • Discrete bounded units (commands and replies) transmitted.
  • Interactive, which means responsive behavior (low latency, delay).
• Data port characteristics include
  • Discrete unbounded units (files) transmitted.
  • Batch, with a need for speed.

Services to Application Protocols

• Message-based vs streaming.
• Services differ in the nature of the communication, and their characteristics.
  • There are no standard characteristics, except for inertia.
• The underlying protocols are from IP: TCP and UDP.

Streaming Services

• A stream is a sequence of small units (bytes, words).
• Streams are usually implemented with connection-oriented mechanisms.
  • As opposed to connectionless mechanisms.
• Streams provide reliable data transfer.
• The pipe or tube metaphor.

Connection-Oriented Service

• A connection establishes a long-term relation between end-points.
  • Almost always point-to-point connections.
• Three phases: set-up, communication, tear-down.
  • This suggests its own overhead (= latency) for set-up and tear-down.
• “Connection oriented” vs actual connections (or circuits).

Reliable Service

• Sending “One, two, three” with a reliable service prevents

  One, , three		loss
  One, tvo, three		corruption
  One, three, two		reordering
  One, One, two, three		duplication

• It may be possible to define reliable as a subset, or with other characteristics.

Other Services

• There are lots of other services possible:
  • Flow control: prevent a fast end-point from overwhelming a slow end-point.
  • Congestion control: prevent a fast end-point from overwhelming the network.
  • Expedited delivery: push in-transit data through for delivery.
  • Urgent (out-of-band) delivery: allow later data to jump ahead of earlier data for immediate delivery.

Non-Services

• Stream services may not provide expected or desirable characteristics.
  • Minimum transmission rate (bytes/sec).
  • No delay bounds.
  • Process-to-process reliability.
    • That's the operating system's responsibility.
• This is particularly true for reliable streams.

Message Services

• A message is a larger size data chunk.
  • Successive messages need not be uniform.
• Message services are minimal, essentially one and done.
• Message services are usually connectionless.
  • One phase: send and maybe receive.
• Message services are unreliable.
  • Loss, corruption, misordering, duplication.

Message Non-Services

• The same as stream services: no bandwidth or delay guarantees.
• Message delivery involves less network-resident machinery.
  • It’s possible an application can establish its own guarantees.
  • This is hard, and solutions tend to be fragile and local.

Summary

• The application layer provides one or more services to applications for protocol implementation.
• Each service is a collection of characteristics.
  • Some chosen by the provider, some by the client.
  • Message vs stream is one important characteristic.
• The TCP and UDP Internet protocols are the canonical application-layer services.

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This page last modified on 2012 September 13.