Class 31: Introduction to the Transport Layer; UDP

Held Monday, April 17, 2000

Overview

Today we move on to the next layer, the transport layer, which provides per-process networking capabilities.

Notes

• The MathLAN was down this morning while I was prepping class, so I may not have things as organized as I'd like.
• Don't forget Monica Neagoy's talk tonight at 7:30 in Harris.
• I've graded Assignment 6. I'll go over a few questions today, but will leave general discussion until Wednesday, 3:15-4:05.
• Two new assignments today. Note that one is due on Wednesday.
  • Assignment 8 (on Chapter 4)
  • Assignment 9A (on Chapter 5)

Contents

• Assignment 6
• The Transport Layer
• UDP

Summary

Assignment 6

I'm returning assignment 6 today. As I've indicated in the past, I no longer accept assignments after I return them. Those of you who did not turn in assignment 6 need to propose reasonable replacements.

We'll go over the assignment in some detail after class on Wednesday (to give you some chance to read my notes to you on the assignment). However, we will go over some quick issues that occured on a number of assignments:

• General note: The book's description of padding in ATM 3/4 is incorrect. It's padded to 4K+3 bytes, not to a multiple of 3.
• General note: Datagram, Virtual , and Source routing are different protocols. You should not rely on one to solve another.
• Problem 1 (build VCs): VCIs should be unique for each link.
• Problems 6 and 7: Your goal is to notify senders, not just the neighboring bridges. Senders don't know the path their data takes.
• Problem 16: We'll go over this in some detail, since almost everyone got it wrong. Note that the bridge screwed up is not an appropriate way to get the packets looping.
• Problem 24: AAL3/4 has ``hidden'' padding, since you need to expand the data to evenly fill the cells. Hence:
  • AAL5: ceiling((8+data)/48)
  • AAL3/4: ceiling(8+data)/44)
• Problem 28:
  • The overhead per datagrams is high; using small cells makes this painful (taken from answer key)
  • Cell switching typically assumes in-order arrival
  • ...

The Transport Layer

• The next layer of the protocol stack is the transport layer.
• The transport layer is typically responsible for providing networking capabilities to individual processes as opposed to individual machines.
• The transport layer also often adds capabilities to the connection
  • Reliability
  • Guaranteed rate
  • Ordered delivery
  • No duplicate packets
  • etc.
• There are three typical transport-layer services
  • Best effort datagrams (exemplified by UDP)
  • Reliable byte stream (exemplified by TCP)
  • Request/reply (exemplified by RPC)
• Each has different implementation strategies
• Also: How do we present these to the programmer?

UDP

• User Datagram Protocol
• Easy to implement
• Extends IP with a port on each host
• When you send a UDP packet, you must identify both ports and both sender and receiver's host (sender's host is implicit).
• At the receiver, the packets are split according to port number
• Typically, each process reads from its own port
• However,
  • Multiple processes could read from the same port (scary, isn't it?)
  • One process could read from multiple ports