Transcript Lecture 1 - Intro

Modern
Communication
Networks
Amr El Mougy
[email protected]
C7.207
Course Objectives
1. Strengthen your foundation of networking
2. Build your knowledge of the latest advances in networks
3. Introduce the concept of intelligence in networks
4. Empower your research skills
Grading Policy
Final
40%
Research Reports
40%
Mid-term
20%
Course Outline
1. Overview of the basics of networking
2. Taxonomy of the types of networks
3. Advanced networking tools
4. Intelligence in networking (context-awareness and ubiquitous computing)
5. Advanced topics:
• Software-defined networks
• Cloud computing
• Internet of Things
• Social networks
• Big data
• Security and privacy
Important Note
• These slides are not meant to be comprehensive lecture notes!
They are only remarks and pointers. The material presented here
is not sufficient for studying for the course
• Your main sources for studying are:
• Research papers
• your own lecture notes
Internet Access
Mobile network
PC
Global ISP
server
wireless
laptop
cellular
handheld
Home network
Regional ISP
Institutional network
access
points
wired
links
router
1-8
Introduction 2
Internet Traffic
• Global IP traffic will reach 1.1 Zettabytes per year by
2016, or 88.4 exabytes per month
• An increase of 5 –fold over the last 5 years
• An increase of 3 folds is expected by 2019
• Wireless and mobile traffic makes up 54% of global
traffic
• By 2019, two thirds of global traffic will be generated
by non-PC devices (welcome to the IoT)
Protocol Stack
Lifecycle of a Data Packet
Home Networks
to/from
cable
headend
Cable/
DSL
modem
router/
firewall
Ethernet
wireless
laptops
wireless
access
point
Application Layer
Creating a network app
Write programs that
• run on (different) end systems
• communicate over network
• e.g., web server software communicates with
browser software
application
transport
network
data link
physical
No need to write software for network-core
devices
• Network-core devices do not run user applications
• applications on end systems allows for rapid app
development, propagation
Application Architectures
• Client-server
• Peer-to-peer (P2P)
• Hybrid of client-server and P2P
application
transport
network
data link
physical
application
transport
network
data link
physical
What transport service does an app need?
Data loss
• some apps (e.g., audio) can
tolerate some loss
• other apps (e.g., file transfer,
telnet) require 100% reliable
data transfer
Timing
• some apps (e.g., Internet
telephony, interactive games)
require low delay to be
“effective”
Throughput
• some apps (e.g., multimedia)
require minimum amount of
throughput to be “effective”
• other apps (“elastic apps”) make
use of whatever throughput
they get
Security
• Encryption, data integrity, …
Transport service requirements of common apps
Application
Data loss
Throughput
Time Sensitive
file transfer
e-mail
Web documents
real-time audio/video
no loss
no loss
no loss
loss-tolerant
no
no
no
yes, 100’s msec
stored audio/video
interactive games
instant messaging
loss-tolerant
loss-tolerant
no loss
elastic
elastic
elastic
audio: 5kbps-1Mbps
video:10kbps-5Mbps
same as above
few kbps up
elastic
yes, few secs
yes, 100’s msec
yes and no
Example of Application Protocols
HTTP: hypertext transfer protocol
• Web’s application layer protocol
• client/server model
• client: browser that requests,
receives, “displays” Web objects
• server: Web server sends objects
in response to requests
PC running
Explorer
Server
running
Apache Web
server
Mac running
Navigator
HTTP Messages
request line
(GET, POST, HEAD commands)
Request Message
GET /somedir/page.html HTTP/1.1
Host: www.someschool.edu
User-agent: Mozilla/4.0
header Connection: close
lines Accept-language:fr
Carriage return, line feed
indicates end of message
status line
(protocol, status code, status phrase)
Response Message
data, e.g., requested
HTML file
header
lines
(extra carriage return, line feed)
HTTP/1.1 200 OK
Connection close
Date: Thu, 06 Aug 1998 12:00:15 GMT
Server: Apache/1.3.0 (Unix)
Last-Modified: Mon, 22 Jun 1998 …...
Content-Length: 6821
Content-Type: text/html
data data data data data ...
Domain Name System (DNS)
Root DNS Servers
com DNS servers
yahoo.com
DNS servers
amazon.com
DNS servers
org DNS servers
pbs.org
DNS servers
edu DNS servers
poly.edu
umass.edu
DNS servers DNS servers
Client wants IP for www.amazon.com; 1st approx:
• client queries a root server to find com DNS server
• client queries com DNS server to get amazon.com DNS server
• client queries amazon.com DNS server to get IP address for
www.amazon.com
root DNS server
DNS name resolution example
2
• Host at cis.poly.edu wants IP
address for gaia.cs.umass.edu
3
TLD DNS server
4
5
iterated query:
contacted server replies
with name of server to
contact
“I don’t know this name,
but ask this server”
local DNS server
dns.poly.edu
1
8
7
6
authoritative DNS server
dns.cs.umass.edu
requesting host
cis.poly.edu
gaia.cs.umass.edu
Transport Layer
Transport vs. network layer
• network layer: logical communication
between hosts
• transport layer: logical communication
between processes
• relies on, enhances, network layer services
Household analogy:
12 kids sending letters to 12
kids
• processes = kids
• app messages = letters in
envelopes
• hosts = houses
• transport protocol = Ann
and Bill
• network-layer protocol =
postal service
Internet transport layer protocols
All Protocols do multiplexing/demultiplexing
Reliable, in-order delivery (TCP)
• congestion control
• flow control
• connection setup
Unreliable, unordered delivery (UDP)
• no-frills extension of “best-effort” IP
services not available:
• delay guarantees
• bandwidth guarantees
application
transport
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physicalnetwork
network
data link
physical
data link
physical
network
data link
physical
application
transport
network
data link
physical
Protocol for reliable data transfer (stop-and-wait)
Waiting for 1
0
1NAK 0
Waiting
for for
ACKCall
or 0
Waiting
Sender
Transport Layer
at sender accepts
packet from App
Layer and sends
it to Network
Layer
Receiver
Send ACK or NAK 0
Errors and Loss
Transport Layer at
receiver extracts
packet from
Network Layer
and delivers it to
App Layer
TCP Flow Control
• receiver side of TCP connection has a receive buffer:
(currently)
application
TCP data
IP
unused buffer
(in buffer) process
datagrams space
rwnd
RcvBuffer
flow control
sender won’t overflow
receiver’s buffer by
transmitting too much,
too fast
 app process may be slow at reading
from buffer
receiver: advertises unused buffer
space by including rwnd value in
segment header
sender: limits # of unACKed bytes to
rwnd
• speed-matching service:
matching send rate to receiving
application’s drain rate
• guarantees receiver’s buffer
doesn’t overflow
TCP Congestion Control
• Sliding Window Protocol
• Sender maintains a congestion window (cwnd), in addition
to the receiver’s window (rwnd) advertised in ACK
• Allowed-window = min(cwnd, rwnd)
• If no congestion: Allowed-window = rwnd
• Packet loss is interpreted as congestion occurrence: reduce
congestion window size.
Network Layer
Two Key Network-Layer Functions
• forwarding: move packets
from router’s input to
appropriate router output
• routing: determine route taken
by packets from source to
dest.
• routing algorithms
analogy:
 routing: process of planning
trip from source to dest
 forwarding: process of getting
through single interchange
Datagram networks
• no call setup at network layer
• routers: no state about end-to-end connections
• no network-level concept of “connection”
• packets forwarded using destination host address
• packets between same source-dest pair may take different paths
application
transport
network
data link 1. Send data
physical
application
transport
2. Receive data network
data link
physical
Host, router network layer functions
Transport layer: TCP, UDP
Network
layer
IP protocol
•addressing conventions
•datagram format
•packet handling conventions
Routing protocols
•path selection
•RIP, OSPF, BGP
forwarding
table
ICMP protocol
•error reporting
•router “signaling”
Link layer
physical layer
IP addresses: how to get one?
Q: How does a host get IP address?
• hard-coded by system admin in a file
• Windows: control-panel->network->configuration->tcp/ip->properties
• UNIX: /etc/rc.config
• DHCP: Dynamic Host Configuration Protocol: dynamically get address from
as server
• “plug-and-play”
4-32
Graph Abstraction
• Graph: G = (N,E)
• N: set of nodes (routers) = { u, v, w, x, y, z }
• E: set of edges (links)= { (u,v), (u,x), (v,x),
(v,w), (x,w), (x,y), (w,y), (w,z), (y,z) } (Links are
bi-directional)
• Each link is associated with a cost value (can
represent delay, distance,..)
• Cost of a path (x1,x2 ,….,xn ) = c(x1 ,x2) + c(x2
,x3) +…..+ c(xn-1 ,xn )
• Routing algorithm objective:
• Find path with minimum cost between sender
and receiver
Routing Algorithms Classification
• Global
• Least-cost path is calculated using complete, global knowledge about the
network
• All routers have complete graph (topology, costs)
• “Link state” algorithms (OSPF, open shortest path first)
• Decentralized
• Calculations are carried out in an iterative, distributed manner.
• Router knows link costs to physically connected adjacent nodes
• “Distance vector” algorithms (RIP, Routing Information Protocol)
1-34
How does Data Travel?
• At the Transport Layer: TCP and
UDP protocols. Primary function is
multiplexing and demultiplexing.
TCP also establishes end-to-end
logical connections
• At the Network Layer (L3): IP
protocols provides addressing (IP
addresses are unique but dynamic)
• At the Data Link Layer (L2):
protocols provide access control
plus MAC addressing (MAC
addresses are unique and static)
IP Addresses Vs. MAC Addresses
• IP Address
xxxxxxxx
xxxxxxxx
xxxxxxxx
xxxxxxxx
223
191
056
008
• MAC Address
xxxxxxxx
xxxxxxxx
xxxxxxxx
xxxxxxxx
xxxxxxxx
xxxxxxxx
1C
22
F3
D1
42
A9
How does data travel?
DNS Server
68.87.71.226
GUC
68.80.2.0/24
Bob
00:16:D3:23:63:8A
ISP
68.80.0.0/13
00:22:6B:45:1F:1B
68.85.2.1


Bob wishes to retrieve a
page from
www.google.com
Bob has just booted up
the computer
www.google.com
68.233.169.105
Google
68.233.160.0/19
Data Link Layer
Link Layer Functions
• Framing
• Medium Access Control (MAC)
• Reliable delivery
• Flow control
• Error detection and correction
Medium Access Control (MAC)
• Can be classified in 3
main categories:
- Channel partitioning
- Random Access
- Taking-turns
Channel Partitioning
• Divides resources
evenly
• Eliminates collisions
• Not very flexible
• May waste resources if
they are not needed by
the device
Random Access
• Each node transmits at the full
rate, R, of the channel
• Collisions cannot be avoided
• Upon collision:
- Wait for random period of time
- Retransmit frame
ALOHA and Slotted ALOHA
• ALOHA:
• Slotted ALOHA:
- Nodes may transmit whenever they want
- All nodes are synchronized
- Upon detecting a collision, either retransmit
immediately or wait for one frame time
- Nodes can only transmit at beginning of a slot
- Upon a collision, the node will retransmit in
the next slot with probability p
Carrier Sense Multiple Access (CSMA)
• Nodes employ carrier sensing before transmission
• Collisions occur if two nodes start transmitting at the same
time
• Upon collision detection, nodes stop transmitting the rest of
the packet
• Collisions are caused by channel propagation delay
Carrier Sense Multiple Access (CSMA)