ELEG 651 CPEG 419

1
ELEG 651 / CPEG 419
Introduction to Data Networking

• Review of Lecture 1 and continuation of chapter 1

2
Announcements

• Homework 1 due next week
• Project 1 due next week

3
Today

• Recent News
• Review and complete Chapter 1
• Start Chapter 2

4
In the news

• News sources
• www.lightreading.com (general networks)
• www.unstrung.com (wireless and mobile)
• www.darkreading.com (network security)
• www.alleyinsider.com (general tech business news)
• arstechnica.com (general tech news)

5
Comcast Defends P2P Management
http//www.lightreading.com/document.asp?doc_id14
6016sitecdn

• P2P file sharing is responsible for a
• large share to the bytes send over the Internet,
• but only a small fraction of the connections
• Thus, a few P2P users are using most of the
  bandwidth
• ISPs would rather give good service to users
  other than to P2P users
• If a transfer of a large file takes a bit longer,
  it does not impact the utility (the user does not
  really care)
• If a short file transfer takes longer, it greatly
  impact utility (delaying a web page or a VoIP
  packet is a big problem for a user)
• Ideally, service is provided according to user
  utility, more utility results in more bandwidth
• Possibly, Comcast uses equipment to ensure that
  short connections receive the maximum possible
  bandwidth at the expense of long connections.
• Comcast denies this
• Some claim that Comcast cut P2P connections in
  general
• This seems doubtful
• P2P users claim that comcast is not abiding by
  network neutrality

6
network neutrality

• Net neutrality is about allowing any type of
  connections and providing the same type of
  service with any decimation.
• Depending on your prospective, it is a good idea
  or bad idea.
• Google likes the idea that they do not have to do
  anything special to get bandwidth
• ISPs and some application developers are against
  net neutrality
• Suppose that an ISP allows cheap or free access
  but charges google for searches. If google does
  not want to pay, then the ISP directs searches to
  yahoo, microsoft, or ask.com
• Would you rather get free access or ensure that
  all your searches go where you want.
• Google is very afraid of lack of customer
  loyalty.
• Net-flex and amazon and others would like to sell
  video over the network. However, the network is
  too slow. They might be willing to pay for a
  faster service.
• Net neutrality is blamed for a lack of video
  on-demand
• Politically motivated censoring
• google.cn and china in general blocks network
  content that is against Chinese government
• There is little doubt that without net
  neutrality, if Rupert Murdock (the owner of fox
  tv) owned an ISP, he would block access to web
  sites with non-conservative viewpoints
• There have been several attempts to pass net
  neutrality laws (one is currently pending), but
  so far, net neutrality is not a law
• The comcast vs P2P is being used to motivate the
  current bill

7
The Protocol Stack

• The application layer includes network
  applications and network application protocols
• e.g. of applications web, IM, email
• e.g., application protocols OSCAR, http, smtp,
  ftp, DNS.
• Provide a service to a user or another
  application.
• Require service from the lower layers, but
  typically only interact with the transport layer.

8
The Protocol Stack

• The transport layer (typically)
• Different transport layer protocols provide
  different types of services.
• Types of services MAY include
• Note that when a transport protocol provides
  these services, the application does not have to.
  
• This makes implementation of applications easier.
• This allows careful design of transport
  protocols, following the divide and conquer
  approach
• The transport layer uses the network layer to
  deliver packets, but does not require any type of
  service guarantees from the network layer
• In practice, the transport layer hopes for in
  order delivery.

9
Transport layer protocols TCP and UDP

• TCP and UDP are the most widely used transport
  protocols.
• Other protocols include SCTP (UD and Cisco are
  active in developing SCTP), RTP (for multimedia
  such as VoIP)
• TCP and UDP will be covered in great detail
  later. But for now
• TCP provides many services
• UDP provides few services
• TCP requires a connection to be established, UDP
  does not

10
Transport Multiplexing

• Transport layers use ports to provide
  multiplexing
• A two hosts can have multiple simultaneous
  connections by using ports.
• Well known ports can be used to specify a
  particular application
• E.g., web servers will accept TCP connections on
  port 80
• A host can have two connections with a web server
  by using different ports

host (web server)
host
TCP
UDP
TCP
UDP
0
0
0
0
4567
80
4568
216-1
216-1
216-1
216-1
11
Sockets gateway between the app layer and the
transport layer

• process sends/receives messages to/from its
  socket
• socket analogous to door
• sending process shoves message out door
• sending process relies on transport
  infrastructure on other side of door which brings
  message to socket at receiving process

12
TCP Sockets

• An application accesses TCP and UDP through
  sockets.
• TCP is connection based so one host must be
  listening and the other must be connecting
  (calling)
• The basic steps for a TCP listener
• Define socket variable as a TCP socket
• Bind socket to a port (the bind function)
• If some other application is or was recently (120
  sec) listening on this port, this function will
  fail.
• The application must check that this command
  succeeds.
• Listen on this port (the listen function)
• When a the other host connects, the listen
  function completes and data can be send or
  received.
• Close socket
• Basic steps for TCP caller
• Define socket variable as a TCP socket
• No port is given, the OS will assign which ever
  port is available. The application has no control
  over the port
• Connect
• Send data
• Close socket

13
UDP Sockets

• UDP are connectionless.
• A host sends a packet when it wants.
• There is no concept of one host connecting to
  another.
• There is only the concept of one host sending a
  packet and the other host receiving the packet.
  And either host can send or receive
• Steps to send and then receive a UDP message
• Define socket as a UDP socket
• Bind socket to a port
• If this port is in use, bind will fail
• Send message
• Wait for message
• There are two ways to wait for messages, blocking
  or non-blocking
• A blocking function will wait for a message to
  arrive. It might wait forever.
• A non-blocking will return immediately, but if no
  message was waiting in the transport layer, then
  no message is returned
• select function allows a time out to be set. So
  the function will wait until a message arrives or
  the timeout time to elapse.
• Close socket
• Steps to receive a UDP message
• Define socket as a UDP socket
• Bind socket to a port
• If this port is in use, bind will fail

14
Project 1
Due 2/25

• In this project messages will be sent over TCP
  and UDP.
• The project is description currently at
• www.eecis.udel.edu/bohacek/Classes/ELEG651Spring2
  008/Project1/Project1.htm
• Once mycourses is up, it will be listed there.
• All the required information should be online.
  Post questions on mycourses (if it is up)
• This project can be completed by cut and pasting
  from the web site. But try to understand the
  steps.
• Let me know if there are typos.

15
The Protocol Stack

• The network layer
• The network layer gets packets from the transport
  layer or from the link layer.
• Depending on the destination address, the network
  layer will give the packet to the transport
  protocol or to a specific link layer to send on a
  specific link
• The network layer also provides fragmenting of a
  large packet into chunks suitable for the link
  layer

16
The Protocol Stack

• The link layer moves packets (frames) between two
  hosts
• However, the link layer may provide a wide range
  of services including

17
The Protocol Stack

• The physical layer
• This requires putting the bits onto a physical
  medium and decoding them from the medium.
• In this course we mostly neglect the physical
  layer and assume that is works correctly (each
  layer always assumes that the other layers work
  correctly)
• But the performance of a protocol at a layer
  often dependent on the other layers.
• One approach is for cross-layer design

18
Encapsulation
source
message
application transport network link physical
segment
datagram
frame
switch
destination
application transport network link physical
router
19
Packet Switching vs. Circuit Switching

• Packet switching
• Circuit switching
• Circuit switching might lead to
• Packet switching leads to

20
The delay goes to infinity at utilization goes to
1
Discrete-time queuing theory Markov Chains

• Suppose that at each time step either
• The queue increases by length one with
  probability p
• If the queue is not empty, it decreases by length
  1 with probability q 1 p
• If the queue is empty, then it stays empty with
  probability q 1 - p

Diagram of the states the queue can take
Required for ELEG 651 but optional for CPEG 419
21
The delay goes to infinity at utilization goes to
1
Discrete-time queuing theory Markov Chains

• Suppose that at each time step either
• The queue increases by length one with
  probability p
• If the queue is not empty, it decreases by length
  1 with probability q 1 p
• If the queue is empty, then it stays empty with
  probability q 1 - p

Diagram of the states the queue can take
Required for ELEG 651 but optional for CPEG 419
22
The delay goes to infinity at utilization goes to
1
p
p
p
p
p
p
Q0 (empty)
Q1
Q2
Qi-1
Qi
Qi1
q
q
q
q
q
q
q
Define P(i) to be the probability that the queue
has i packets
Required for ELEG 651 but optional for CPEG 419
23
The delay goes to infinity at utilization goes to
1
Required for ELEG 651 but optional for CPEG 419
24
The delay goes to infinity at utilization goes to
1
Required for ELEG 651 but optional for CPEG 419
25
The delay goes to infinity at utilization goes to
1
Expected queue occupancy
Required for ELEG 651 but optional for CPEG 419
26
The delay goes to infinity at utilization goes to
1
Required for ELEG 651 but optional for CPEG 419
27
Delay in Packet Networks

• Types of delay in packet networks