Introduction to Networking and Wireless Networks
Department of Computer Science University of Massachusetts http://www.cs.umass.edu/~kurose Brian Donovan, Tim Ireland, Adam Nyzio
Professor Jim Kurose
UPRM, January 23, 24, 2006 Class website: http://gaia.cs.umass.edu/uprm
Introduction 1-1
What is this course about?
quick introduction to principles of networking,
Internet, particularly wireless (802.11) networking learn practice of computer networking: day two “in the field” assume: no previous background in networking questions/comments: lots, hopefully
Who am I? Who are you?
Computer Networking: A Top Down Approach Featuring the Internet,
J. Kurose & K. Ross Addison Wesley, 3rd ed., 2004
Introduction
1-2
Overview:
Part 1: Whirlwind Introduction to Networking (Monday)
9:00 – 10:15 networking overview 10:45 – 12:00 application, transport, network layers 2:00 – 3:00 intro to link layer 3:00 – 4:00 link layer, 802.11 networks
Part 2: In the field (Tuesday)
9:00 – 9:30 meet, discuss plans, divide into teams
9:30 – 12:00 short distance 802.11 experiments,
measurements 2:00 – ? long-distance 802.11 experiments, measurements
Introduction
1-3
A top-down approach:
networking “top-down”
end-system applications,
end-end transport network core: routing, hooking nets together Ethernet, 802.11
link-level protocols, e.g.,
Introduction
1-4
Part 1: Networking Overview
Our goal:
get “feel” and
Overview:
what‟s the Internet
terminology more depth, detail later in course approach: use Internet as example
what‟s a protocol?
network edge network core access net, physical media Internet/ISP structure performance: loss, delay protocol layers, service model
Introduction
1-5
What is the Internet?
Collection of end-systems running networked applications, inter-connected together by routers and links, controlled by network protocols
Introduction
1-6
What is the Internet: end systems
… not just computers
Internet phones
Web-enabled toaster + weather report
local ISP
regional ISP
IP picture frame http://www.ceiva.com/ Blackberry
world’s smallest web server Photo courtesy U. Mass radar!
company network
Introduction 1-7
What is the Internet: routers, links
Routers: forward packets (chunks of data)
Links: physical transport
local ISP
wide variety of media, speeds
Cat 5/6 twisted pair 10Mbps – 10Gbps
regional ISP
satellite (10’s Mbps) Wi-Fi (10-54 Mbps) Optical fiber (10Gbps) Point-point wireless (10-54 Mpbs) Cable (1-5Mbps)
company network
Introduction 1-8
What‟s a protocol?
a human protocol and a computer network protocol: Hi
Hi
Got the time?
TCP connection request TCP connection response
Get http://www.awl.com/kurose-ross
2:00 time
Introduction
1-9
What‟s a protocol?
human protocols: “what‟s the time?” “I have a question” introductions … specific msgs sent … specific actions taken when msgs received, or other events
Question: Other human protocols
network protocols: machines rather than humans all communication activity in Internet governed by protocols
(I‟ll tell you networking equivalent)
protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt
Introduction 1-10
What‟s the Internet: “nuts and bolts” view
protocols control sending,
receiving of msgs
router server local ISP
workstation
Internet: “network of
networks”
e.g., TCP, IP, HTTP, FTP, 802.11
mobile
Internet standards RFC: Request for comments IETF: Internet Engineering Task Force
loosely hierarchical public Internet versus private intranet
regional ISP
company network
Introduction 1-11
So this course will be about:
applications, end system routers links
running in applications, end systems, routers, links (think “software control”)
protocols
…with a special emphasis on 802.11 wireless networks, with an eye towards UPRM CASA student radar testbed
Introduction
1-12
The network edge:
end systems (hosts):
run application programs e.g. Web, email at “edge of network” client host requests, receives service from always-on server e.g. Web browser/server; email client/server
client/server model
peer-peer model:
minimal (or no) use of dedicated servers e.g. Skype, BitTorrent, KaZaA
Introduction 1-13
Network Core: Packet Switching
each end-end data stream divided into packets user A, B packets share network resources each packet uses full link bandwidth resources used as needed resource contention: aggregate resource demand can exceed amount available congestion: packets queue, wait for link use
Bandwidth division into “pieces” Dedicated allocation Resource reservation
Introduction 1-14
Packet Switching: Statistical Multiplexing
A B
100 Mb/s Ethernet
statistical multiplexing
1.5 Mb/s
C
queue of packets waiting for output link
D
E
statistical multiplexing:
sharing of bandwidth on demand no fixed allocation of bandwidth to users
Introduction 1-15
Packet switching versus circuit switching
Packet switching allows more users to use network!
1 Mb/s link each user: 100 kb/s when “active” active 10% of time circuit-switching: 10 users packet switching: with 35 users, probability > 10 active less than .0004
N users 1 Mbps link
Q: how did we get value 0.0004?
Introduction
1-16
Access networks and physical media
Q: How to connect end systems to edge router?
residential access nets
institutional access
networks (school, company) mobile access networks
Keep in mind:
bandwidth (bits per
second) of access network? shared or dedicated?
Introduction 1-17
Company access: local area networks
company/univ local area
network (LAN) connects end system to edge router Ethernet: shared or dedicated link connects end system and router 10 Mbs, 100Mbps, Gigabit Ethernet
Introduction
1-18
Wireless access networks
shared
network connects end system to router
wireless access
via base station aka “access point”
router base station
wireless LANs: 802.11b/g (WiFi): 11 or 54 Mbps wider-area wireless access provided by telco operator 3G ~ 384 kbps • Will it happen?? GPRS in Europe/US
mobile hosts
Introduction
1-19
Internet structure: network of networks
roughly hierarchical
at center: “tier-1” ISPs (e.g., MCI, Sprint, AT&T, Cable
and Wireless), national/international coverage treat each other as equals
Tier-1 providers usually interconnect (peer) privately
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Introduction
1-20
Tier-1 ISP: e.g., Sprint
Sprint US backbone network
DS3 (45 Mbps) OC3 (155 Mbps) OC12 (622 Mbps) OC48 (2.4 Gbps)
Seattle Tacoma
POP: point-of-presence
to/from backbone
Stockton San Jose Cheyenne
… … …
… Kansas City . …
peering
Chicago Roachdale
New York Pennsauken Relay Wash. DC
Anaheim
Atlanta
to/from customers Fort Worth
Orlando Introduction 1-21
Internet structure: network of networks
“Tier-2” ISPs: smaller (often regional) ISPs
connect to one or more tier-1 ISPs, possibly other tier-2 ISPs customer of tier-1 provider (pay!)
Tier-2 ISPs also peer privately with each
Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet tier-2 ISP is customer of tier-1 provider
Tier-2 ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
Tier 1 ISP
Tier-2 ISP
Tier-2 ISP
Introduction
1-22
Internet structure: network of networks
“Tier-3” ISPs and local ISPs last hop (“access”) network (closest to end systems)
local ISP Local and tier3 ISPs are customers of higher tier ISPs connecting them to rest of Internet
Tier 3 ISP Tier-2 ISP local ISP Tier-2 ISP local ISP
local ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP local ISP
Tier 1 ISP
Tier-2 ISP local ISP
Introduction 1-23
Tier-2 ISP local local ISP ISP
Internet structure: network of networks
a packet passes through many networks!
local ISP
Tier 3 ISP Tier-2 ISP
local ISP
My dad: “Why isn‟t my Internet working?”
local ISP Tier-2 ISP
local ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP local ISP
Tier 1 ISP
Tier-2 ISP local local ISP ISP
Tier-2 ISP local ISP
Introduction 1-24
How do loss and delay occur?
packets queue in router buffers
packet arrival rate to link exceeds output link capacity
packets queue, wait for turn
packet being transmitted (delay)
A
B
packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers
Introduction
1-25
Four sources of packet delay
packets queue in router buffers
packet arrival rate to link exceeds output link capacity
packets queue, wait for turn
A
transmission propagation
B
nodal processing
queueing
Introduction 1-26
Queueing delay (revisited)
R=link bandwidth (bps) L=packet length (bits) a=average packet
arrival rate
La (bit/sec arriving) traffic intensity = R (bit/sec transmitting)
La/R ~ 0: average queueing delay small La/R -> 1: delays become large La/R > 1: more “work” arriving than can be serviced,
average delay infinite!
Introduction
1-27
“Real” Internet delays and routes
What do “real” Internet delay & loss look like? Traceroute program: provides delay
measurement from source to router along end-end Internet path towards destination. For all i:
sends three packets that will reach router i on path towards destination router i will return packets to sender sender times interval between transmission and reply.
3 probes 3 probes 3 probes
Introduction
1-28
“Real” Internet delays and routes
traceroute: gaia.cs.umass.edu to www.eurecom.fr
Three delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu
1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms 2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms 3 cht-vbns.gw.umass.edu (128.119.3.130) 6 ms 5 ms 5 ms 4 jn1-at1-0-0-19.wor.vbns.net (204.147.132.129) 16 ms 11 ms 13 ms 5 jn1-so7-0-0-0.wae.vbns.net (204.147.136.136) 21 ms 18 ms 18 ms 6 abilene-vbns.abilene.ucaid.edu (198.32.11.9) 22 ms 18 ms 22 ms 7 nycm-wash.abilene.ucaid.edu (198.32.8.46) 22 ms 22 ms 22 ms trans-oceanic 8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms link 9 de2-1.de1.de.geant.net (62.40.96.129) 109 ms 102 ms 104 ms 10 de.fr1.fr.geant.net (62.40.96.50) 113 ms 121 ms 114 ms 11 renater-gw.fr1.fr.geant.net (62.40.103.54) 112 ms 114 ms 112 ms 12 nio-n2.cssi.renater.fr (193.51.206.13) 111 ms 114 ms 116 ms 13 nice.cssi.renater.fr (195.220.98.102) 123 ms 125 ms 124 ms 14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms 15 eurecom-valbonne.r3t2.ft.net (193.48.50.54) 135 ms 128 ms 133 ms 16 194.214.211.25 (194.214.211.25) 126 ms 128 ms 126 ms 17 * * * * means no response (probe lost, router not replying) 18 * * * 19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms
Introduction 1-29
Quick “live” measurement
Let‟s trace route and delays to
gaia.cs.umass.edu from UPRM
MS-DOS tracert program pingplotter freeware (www.pingplotter.com)
anywhere else to trace (maybe someplace
far away)?
Introduction
1-30
Protocol “Layers”
Networks
links of various media, applications, protocols, hardware, software
are complex: hosts, routers,
How to organize structure (architecture) of
network, or our discussion of networks?
Layering: protocols in layer
N implements a service (provided to layer N+1), using service provided by layer N
e.g.: layer 4 (TCP) provide reliable end-end data transfer over layer 3 service (IP) providing lossy end-end data transfer
Introduction
1-31
Internet protocol stack
application: supporting network
applications
FTP, SMTP, HTTP
application
transport: process-process data
transfer
transport
network
TCP, UDP
network: routing of datagrams from
source to destination
link
physical
IP, routing protocols
link: data transfer between
neighboring network elements
PPP, Ethernet, 802.11
Introduction 1-32
physical: bits “on the wire”
source
message segment
Ht M M M M
frame Hl Hn Ht
datagram Hn Ht
application transport network link physical
Encapsulation
link physical switch
destination
M Ht Hn Ht Hl Hn Ht M M M
application transport network link physical
Hn Ht Hl Hn Ht
M M
network link physical
Hn Ht
M
router
Introduction
1-33
source
message segment frame
Ht M M M M
datagram Hn Ht
Hl Hn Ht
application transport network link physical
Encapsulation
link physical switch
destination
application transport network link physical
network link physical router
Introduction
1-34
Introduction: Summary
Covered lots of material! Internet overview what‟s a protocol? network edge, core, access network packet-switching Internet/ISP structure performance: loss, delay layering and service models You now have: context, overview, “feel” of networking more depth, detail to
follow!
Introduction
1-35
Overview:
Part 1: Whirlwind Introduction to Networking (Monday)
Part 2: In the field (Tuesday)
9:00 – 10:15 networking overview 10:45 – 12:00 application, transport, network layers 2:00 – 3:00 intro to link layer 3:00 – 4:00 link layer, 802.11 networks
9:00 – 9:30 meet, discuss plans, divide into teams 9:30 – 12:00 short distance 802.11 experiments,
measurements 2:00 – ? long-distance 802.11 experiments, measurements
Introduction
1-36
1.2: Application, transport layers
Application Layer
Principles of network
Transport Layer
Multiplexing Reliable data transfer Congestion control UDP
applications HTTP (the web‟s protocol)
TCP
Introduction
1-37
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” Bandwidth some apps (e.g., multimedia) require minimum amount of bandwidth to be “effective” other apps (“elastic apps”) make use of whatever bandwidth they get
Introduction
1-38
Internet transport protocols services
TCP service:
UDP service:
unreliable data transfer
connection-oriented: setup
required between client and server processes reliable transport between sending and receiving process flow control: sender won‟t overwhelm receiver congestion control: throttle sender when network overloaded does not provide: timing, minimum bandwidth guarantees
between sending and receiving process does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee Q: why bother? Why is there a UDP?
Introduction
1-39
Internet apps: application, transport protocols
Application e-mail remote terminal access Web file transfer streaming multimedia
Application layer protocol
SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] proprietary (e.g. RealNetworks) proprietary (e.g., Vonage,Dialpad)
Underlying transport protocol TCP TCP TCP TCP TCP or UDP
Internet telephony
typically UDP
Introduction
1-40
Client-server architecture
server:
always-on host permanent IP address initiate communication with server may be intermittently connected may have dynamic IP addresses do not communicate directly with each other
clients:
Introduction
1-41
Processes communicating
Process: program running within a host. processes in different hosts communicate by exchanging messages process sends/receives messages to/from its socket socket analogous to door
host or server
host or server
process socket
controlled by app developer
process socket
sending process shoves message out door, relies on transport layer to deliver to receiver-side socket
TCP with buffers, variables
Internet
TCP with buffers, variables
controlled by OS
Introduction
1-42
Addressing processes
to send to end-system: host device has unique32-bit to send to socket on end-system: port number network services has well-known port numbers: • HTTP server: 80 • Mail server: 25 to send HTTP message to gaia.cs.umass.edu web
IP address
server:
IP address: 128.119.245.12 Port number: 80
Introduction
1-43
App-layer protocol defines
types of messages
exchanged,
e.g., request, response
message syntax: what fields in messages & how fields are delineated message semantics meaning of information in fields rules for when and how
Public-domain protocols: defined in RFCs allows for interoperability e.g., HTTP, SMTP Proprietary protocols: e.g., Skype
processes send & respond to messages
Introduction
1-44
The Web and HTTP
Goal: see application level protocol “in action” HTTP: hypertext transfer protocol
HTTP has client/server model client: browser that requests, receives, “displays” Web objects server: Web server sends objects in response to requests HTTP 1.0: RFC 1945 HTTP 1.1: RFC 2068
PC running Explorer
Mac running Firefox
Introduction
1-45
HTTP overview
Two important HTTP message types: GET request (client-toserver) RESPONSE (server to client) very simple!
Uses TCP:
client initiates TCP
connection (creates socket) to server, port 80 server accepts TCP connection from client HTTP messages (GET, RESPONSE) exchanged between browser (HTTP client) and Web server (HTTP server) TCP connection closed
Introduction
1-46
HTTP request message
HTTP GET message: ASCII (human-readable format)
request line (GET, POST, HEAD commands)
GET /somedir/page.html HTTP/1.1 Host: www.someschool.edu User-agent: Mozilla/4.0 header Connection: close lines Accept-language:fr (extra carriage return, line feed)
Carriage return, line feed indicates end of message
Introduction
1-47
HTTP response message
status line (protocol status code status phrase) header lines 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 ...
data, e.g., requested HTML file
Introduction
1-48
HTTP response status codes
In first line in server->client response message. A few sample codes: 200 OK
request succeeded, requested object later in this message requested object moved, new location specified later in this message (Location:) request message not understood by server requested document not found on this server
Introduction
301 Moved Permanently
400 Bad Request
404 Not Found
505 HTTP Version Not Supported
1-49
Observing HTTP in action
Ethereal packet-sniffer (www.ethereal.com) captures, records link-layer frames being
sent/received
recall: HTTP encapsulated inside TCP inside IP inside Ethernet
use tomorrow to capture/analyze 802.11 wireless
packet sniffer packet analyzer application application (e.g., www browser, ftp client)
operating system packet capture (pcap)
copy of all Ethernet frames sent/received
Transport (TCP/UDP) Network (IP) Link (Ethernet) Physical
to/from network
to/from network
Introduction
1-50
Figure 1: Packet sniffer structure
Ethereal Screen Shot
command menus display filter specification listing of captured packets
details of selected packet header
packet content in hexadecimal and ASCII Figure 2: Ethereal Graphical User Interface
Introduction
1-51
Overview:
Part 1: Whirlwind Introduction to Networking (Monday)
Part 2: In the field (Tuesday)
9:00 – 10:15 networking overview 10:45 – 12:00 application, transport, network layers 2:00 – 3:00 intro to link layer 3:00 – 4:00 link layer, 802.11 networks
9:00 – 9:30 meet, discuss plans, divide into teams 9:30 – 12:00 short distance 802.11 experiments,
measurements 2:00 – ? long-distance 802.11 experiments, measurements
Introduction
1-52
1.3 Internet transport-layer protocols
reliable, in-order
delivery (TCP)
congestion control flow control connection setup
application transport network data link physical
network data link physical
network data link physical network data link physical
unreliable, unordered
network data link physical
delivery: UDP
no-frills extension of “best-effort” IP
network data link physical application transport network data link physical
services not available: delay guarantees bandwidth guarantees
Introduction
1-53
Multiplexing/demultiplexing
Demultiplexing at rcv host: delivering received segments to correct socket
= socket application transport network P3 = process P1 P1 application transport network
Multiplexing at send host: gathering data from multiple sockets, enveloping data with header (later used for demultiplexing)
P2
P4
application transport network link physical
link
physical
link
physical
host 1
host 2
host 3
Introduction 1-54
How demultiplexing works
host receives IP
datagrams
32 bits source port # dest port #
each datagram has source IP address, destination IP address each datagram carries 1 transport-layer segment each segment has source, destination port number
other header fields
host uses IP addresses &
port numbers to direct segment to appropriate socket
application data (message) TCP/UDP segment format
Introduction 1-55
Error Control: detecting errors
Goal: detect “errors” (e.g., flipped bits) in transmitted segment
Sender:
treat segment contents
Receiver:
compute checksum of
as sequence of 16-bit integers Internet checksum: addition (1‟s complement sum) of segment contents sender puts checksum value into checksum field
received segment check if computed checksum equals checksum field value: NO - error detected YES - no error detected.
But maybe errors nonetheless? More later
….
Introduction
1-56
UDP segment format: simple!
32 bits Length, in bytes of UDP segment, including header source port # length dest port # checksum Internet checksum
Application data (message)
UDP segment format
UDP services: demultiplexing error detection nothing else!
Introduction
1-57
Reliable Communication
Q: how do humans communicate reliably?
Humans
Computers
Introduction
1-58
Mechanisms for reliable data transfer
error detection capabilities (checksum)
retransmit is packet lost or assumed lost timer at sender to recover from lost message sequence numbers to detect lost or duplicated messages acknowledgement message: receiver lets sender know what was received
Introduction
1-59
Reliable data transfer scenarios
Introduction
1-60
Reliable data transfer scenarios
Introduction
1-61
Performance of stop-and-wait
works, but performance stinks example: 1 Gbps link, 15 ms e-e prop. delay, 1KB packet:
Ttransmit =
L (packet length in bits) 8kb/pkt = = 8 microsec R (transmission rate, bps) 10**9 b/sec
U sender: utilization – fraction of time sender busy sending
U
sender
=
L/R RTT + L / R
=
.008
30.008
= 0.00027
1KB pkt every 30 msec -> 33kB/sec thruput over 1 Gbps link network protocol limits use of physical resources!
Introduction
microsec onds
1-62
Pipelining: increased utilization
sender first packet bit transmitted, t = 0 last bit transmitted, t = L / R first packet bit arrives last packet bit arrives, send ACK last bit of 2nd packet arrives, send ACK last bit of 3rd packet arrives, send ACK receiver
RTT
ACK arrives, send next packet, t = RTT + L / R
Increase utilization by a factor of 3!
U
sender
=
3*L/R RTT + L / R
=
.024
30.008
= 0.0008
microsecon ds
Introduction 1-63
TCP: Transport Control Protocol
Internet‟s reliable data transfer protocol sequence numbers receiver-to-sender ACKs:
timers: estimate round trip, set timer
ACK(x) cumulative: “I‟ve received all segments up through seq. # x”
pipelining: allow up to N un-ACKed segments increase N slowly until loss detected (congestion) when loss occurs: N = N/2
Introduction 1-64
TCP “probes” for available bandwidth
increasing N until loss occurs, then decreasing N like children, finding their parent‟s limits
congestion window 24 Kbytes
16 Kbytes
8 Kbytes
time
Introduction
1-65
TCP segment structure
32 bits URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now (generally not used) RST, SYN, FIN: connection estab (setup, teardown commands) Internet checksum (as in UDP)
source port #
dest port #
sequence number
head not UA P R S F len used
acknowledgement number
Receive window Urg data pnter checksum
counting by bytes of data (not segments!) # bytes rcvr willing to accept
Options (variable length) application data (variable length)
Introduction
1-66
Let‟s look at TCP “in action”
… using Ethereal
Introduction
1-67
Overview:
Part 1: Whirlwind Introduction to Networking (Monday)
Part 2: In the field (Tuesday)
9:00 – 10:15 networking overview 10:45 – 12:00 application, transport, network layers 2:00 – 3:00 intro to link layer 3:00 – 4:00 link layer, 802.11 networks
9:00 – 9:30 meet, discuss plans, divide into teams 9:30 – 12:00 short distance 802.11 experiments,
measurements 2:00 – ? long-distance 802.11 experiments, measurements
Introduction
1-68
1.4: Network Layer
Overview:
forwarding and routing
What‟s inside a router the Internet Protocol
addressing IP datagram format ICMP
routing (path selection):
not covered
Introduction
1-69
Network layer
transport segment from
sending to receiving host on sending side encapsulates segments into datagrams on rcving side, delivers segments to transport layer network layer protocols in every host, router Router examines header fields in all IP datagrams passing through it
application transport network data link physical
network data link physical
network data link physical network data link physical
network data link physical
network data link physical
network data link physical
network data link physical network data link physical application transport network data link physical
Introduction
1-70
Two Key Network-Layer Functions
forwarding: move
packets from router‟s input to appropriate router output route taken by packets from source to dest.
analogy:
routing: process of
routing: determine
planning trip from source to dest
forwarding: process
of getting through single interchange
routing algorithms
Introduction 1-71
Interplay between routing and forwarding
routing algorithm
local forwarding table header value output link
Forwarding:
value in arriving packet’s header (in case of Internet, destination IP address), determines outgoing interface
0100 0101 0111 1001
3 2 2 1
0111
1
3 2
Introduction
1-72
Router Architecture Overview
Two key router functions:
run routing algorithms/protocol (RIP, OSPF, BGP)
forwarding datagrams from incoming to outgoing link
Introduction
1-73
Input Port Functions
Physical layer: bit-level reception
Data link layer: e.g., Ethernet
Output Port functions
Buffering when datagrams arrive faster than transmission rate Scheduling discipline chooses among queued datagrams for
transmission
Introduction
1-74
The Internet Network layer
Host, router network layer functions:
Transport layer: TCP, UDP
Routing protocols •path selection •RIP, OSPF, BGP IP protocol •addressing conventions •datagram format •packet handling conventions
Network layer
forwarding table
ICMP protocol •error reporting •router “signaling”
Link layer physical layer
Introduction
1-75
IP datagram format
IP protocol version number header length (bytes) “type” of data max number remaining hops (decremented at each router) 32 bits type of ver head. len service length fragment 16-bit identifier flgs offset upper time to header layer live checksum 32 bit source IP address 32 bit destination IP address Options (if any) E.g. timestamp, record route taken, specify list of routers to visit. total datagram length (bytes)
for fragmentation/ reassembly
upper layer protocol to deliver payload to
how much overhead with TCP? 20 bytes of TCP 20 bytes of IP = 40 bytes + app layer overhead
data (variable length, typically a TCP or UDP segment)
Introduction
1-76
IP Addressing: introduction
IP address: 32-bit
223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.2.9 223.1.2.2
identifier for host, router interface interface: connection between host/router and physical link
223.1.3.27
router‟s typically have multiple interfaces host typically has one interface IP addresses associated with each interface
223.1.3.1
223.1.3.2
223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1
Introduction
1
1-77
Subnets
IP address: subnet part (high order bits) host part (low order bits)
223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.2.9 223.1.2.2
What‟s a subnet ?
223.1.3.27
device interfaces with same subnet part of IP address can physically reach each other without intervening router
subnet
223.1.3.1 223.1.3.2
network consisting of 3 subnets
Introduction
1-78
Subnets
Recipe To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet.
223.1.1.0/24
223.1.2.0/24
223.1.3.0/24
Subnet mask: /24
Introduction
1-79
Subnets
How many?
223.1.1.1
223.1.1.2
223.1.1.4 223.1.1.3
223.1.9.2
223.1.7.0
223.1.9.1 223.1.8.1 223.1.2.6 223.1.2.1 223.1.2.2 223.1.3.1 223.1.8.0
223.1.7.1
223.1.3.27 223.1.3.2
Introduction
1-80
IP addresses: how to get one?
Q: How does host get IP address?
hard-coded by system admin in a file
Wintel: 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” (more in next chapter)
Introduction 1-81
ICMP: Internet Control Message Protocol
used by hosts & routers to
communicate network-level information error reporting: unreachable host, network, port, protocol echo request/reply (used by ping) network-layer “above” IP: ICMP msgs carried in IP datagrams ICMP message: type, code plus first 8 bytes of IP datagram causing error
Type 0 3 3 3 3 3 3 4 8 9 10 11 12
Code 0 0 1 2 3 6 7 0 0 0 0 0 0
description echo reply (ping) dest. network unreachable dest host unreachable dest protocol unreachable dest port unreachable dest network unknown dest host unknown source quench (congestion control - not used) echo request (ping) route advertisement router discovery TTL expired bad IP header
Introduction
1-82
Traceroute and ICMP
Source sends series of
UDP segments to dest
When ICMP message
First has TTL =1 Second has TTL=2, etc. Unlikely port number
When nth datagram arrives
to nth router:
Router discards datagram And sends to source an ICMP message (type 11, code 0) Message includes name of router& IP address
arrives, source calculates RTT Traceroute does this 3 times Stopping criterion UDP segment eventually arrives at destination host Destination returns ICMP “host unreachable” packet (type 3, code 3) When source gets this ICMP, stops.
Introduction 1-83
Routing Algorithms: finding paths
5
2 1
v
2
3 3 1
w
1
5
u
Graph: G = (N,E)
z
2
x
y
N = set of routers = { u, v, w, x, y, z } E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) } Question: What‟s the least-cost path between u and z ?
Routing algorithm: algorithm that finds least-cost path
Introduction 1-84
Two types of Routing Algorithms:
Link State
net topology, link costs
Distance Vector
distributed: node only
known to all nodes accomplished via “link state broadcast” all nodes have same info computes least cost paths from one node („source”) to all other nodes gives forwarding table for that node iterative: after k iterations, know least cost path to k dest.‟s
communicate with neighboring nodes Each node estimates distance to every other node based on info supplied by neighbors
informs neighbor if estimated distance decreases
algorithm converges to
shortest path answer
Introduction
1-85
Internet: Hierarchical Routing
aggregate routers into
regions, “autonomous systems” (AS) routers in same AS run same routing protocol “intra-AS” routing protocol routers in different AS can run different intraAS routing protocol C.b Internet protocols: OSPF, IS-IS b a C
d A
Routing among AS: inter-AS routing networks (AS‟s) are nodes in routing graph BGP: Border Gateway Protocol Internet‟s inter-As routing protocols glue that binds Internet together
B.a A.a
A.c a b c a B c
b
1-86
Introduction
Overview:
Part 1: Whirlwind Introduction to Networking (Monday)
Part 2: In the field (Tuesday)
9:00 – 10:15 networking overview 10:45 – 12:00 application, transport, network layers 2:00 – 3:00 intro to link layer 3:00 – 4:00 link layer, 802.11 networks
9:00 – 9:30 meet, discuss plans, divide into teams 9:30 – 12:00 short distance 802.11 experiments,
measurements 2:00 – ? long-distance 802.11 experiments, measurements
Introduction
1-87
1.5: The Data Link Layer
Overview:
sharing a broadcast
“link”
channel: multiple access link layer addressing Ethernet 802.11
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
Introduction 1-88
Link Layer Services
framing, link access:
encapsulate datagram into frame, adding header, trailer channel access if shared medium “MAC” addresses identifies source, dest • different from IP address! reliable delivery between adjacent nodes we learned how to do this already (transport layer)! seldom used on low bit error link (fiber, twisted pair) wireless links: higher error rates
Introduction 1-89
Multiple Access Links and Protocols Two types of “links”:
point-to-point PPP for dial-up access
broadcast (shared wire or medium) Old-fashioned Ethernet upstream HFC 802.11 wireless LAN
Introduction
1-90
Multiple Access protocols
single shared broadcast channel
two or more simultaneous transmissions by
nodes: interference
collision if node receives two or more signals at the same time
Multiple access protocol:
distributed algorithm that determines how nodes
share channel, i.e., determine when node can transmit communication about channel sharing must use channel itself!
no out-of-band channel for coordination
Introduction 1-91
Q: how do humans solve multiple access problem?
Introduction
1-92
MAC Protocols: a taxonomy
Three broad classes: Channel Partitioning
divide channel into smaller “pieces” (time slots, frequency, code) allocate piece to node for exclusive use
“Taking turns”
Nodes take turns, but nodes with more to send can take longer turns
Random Access
channel not divided, allow collisions “recover” from collisions
Introduction 1-93
Channel Partitioning MAC protocols: TDMA
TDMA: time division multiple access
access to channel in "rounds" each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle
Introduction
1-94
“Taking Turns” MAC protocols
Polling: Token passing: master node control token passed from “invites” slave nodes one node to next to transmit in turn sequentially. concerns: token message polling overhead concerns:
latency single point of failure (master)
token overhead latency single point of failure (token)
Introduction
1-95
Random Access Protocols
When node has packet to send transmit at full channel data rate R. no a priori coordination among nodes two or more transmitting nodes ➜ “collision”, random access MAC protocol specifies: how to detect collisions how to recover from collisions (e.g., via delayed retransmissions) Examples of random access MAC protocols: slotted ALOHA ALOHA CSMA, CSMA/CD, CSMA/CA
Introduction 1-96
CSMA (Carrier Sense Multiple Access)
CSMA: listen before transmit: If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission human analogy: don‟t interrupt others! CSMA/CD: (CD = collision detection) colliding transmissions detected and aborted, reducing channel wastage
Collisions can still occur!
Introduction 1-97
MAC Addresses
32-bit IP address:
network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address:
used to get frame from one interface to another physically-connected interface (same network) 48 bit MAC address (for most LANs) burned in the adapter ROM
Introduction
1-98
ARP: Address Resolution Protocol
Question: how to determine MAC address of B knowing B‟s IP address?
A
137.196.7.23
ARP protocol:
node A broadcasts
137.196.7.78
1A-2F-BB-76-09-AD 137.196.7.14
LAN
71-65-F7-2B-08-53
request: who knows B‟s MAC protocol B replies to A with its MAC address
58-23-D7-FA-20-B0
137.196.7.88
B
0C-C4-11-6F-E3-98
Introduction
1-99
Ethernet
“dominant” wired LAN technology: cheap $20 for 100Mbs! first widely used LAN technology kept up with speed race: 10 Mbps – 10 Gbps Uses CSMA/CD: binary backoff
Metcalfe‟s Ethernet sketch
Introduction
1-100
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Introduction
1-101
Overview:
Part 1: Whirlwind Introduction to Networking (Monday)
Part 2: In the field (Tuesday)
9:00 – 10:15 networking overview 10:45 – 12:00 application, transport, network layers 2:00 – 3:00 intro to link layer 3:00 – 4:00 link layer, 802.11 networks
9:00 – 9:30 meet, discuss plans, divide into teams 9:30 – 12:00 short distance 802.11 experiments,
measurements 2:00 – ? long-distance 802.11 experiments, measurements
Introduction
1-102
1.6 Elements of a wireless network
wireless hosts laptop, PDA, IP phone run applications may be stationary (non-mobile) or mobile
network infrastructure
wireless does not always mean mobility
Introduction
1-103
1.6 Elements of a wireless network
base station typically connected to wired network relay - responsible for sending packets between wired network and wireless host(s) in its “area” e.g., cell towers 802.11 access points
network infrastructure
Introduction
1-104
1.6 Elements of a wireless network
wireless link typically used to connect mobile(s) to base station also used as backbone link multiple access protocol coordinates link access various data rates, transmission distance
network infrastructure
Introduction
1-105
Characteristics of selected wireless link standards
54 Mbps 5-11 Mbps 1 Mbps 802.15
802.11{a,g} 802.11b
.11 p-to-p link
us, tomorrow
3G
2G
384 Kbps 56 Kbps
UMTS/WCDMA, CDMA2000 IS-95 CDMA, GSM
Indoor
10 – 30m
Outdoor
50 – 200m
Mid range outdoor
200m – 4Km
Long range outdoor
5Km – 20Km
Introduction
1-106
Elements of a wireless network
infrastructure mode base station connects mobiles into wired network handoff: mobile changes base station providing connection into wired network
network infrastructure
Introduction
1-107
Elements of a wireless network
Ad hoc mode
nodes can only
no base stations
transmit to other nodes within link coverage nodes organize themselves into a network: route among themselves
tomorrow, we will be running our wireless nodes in ad hoc mode.
Introduction 1-108
Wireless Link Characteristics
Differences from wired link … decreased signal strength: radio signal attenuates as it propagates through matter (path loss) interference from other sources: standard wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well multipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times
…. make communication across (even a point to point) wireless link much more “difficult”
Introduction 1-109
Wireless network characteristics
Multiple wireless senders and receivers create additional problems (beyond multiple access):
C B A
A‟s signal strength
B
C
C‟s signal strength
A
Hidden terminal problem
B, A hear each other B, C hear each other A, C can not hear each other
space
Signal fading:
B, A hear each other B, C hear each other A, C can not hear each other
means A, C unaware of their interference at B
interferring at B
Introduction
1-110
IEEE 802.11 Wireless LAN
802.11b 2.4-5 GHz unlicensed radio spectrum up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer • all hosts use same chipping code widely deployed, using base stations 802.11a 5-6 GHz range up to 54 Mbps 802.11g 2.4-5 GHz range up to 54 Mbps All use CSMA/CA for
multiple access All have base-station and ad-hoc network versions we‟ll use this
Introduction 1-111
802.11: Channels, association
802.11b: 2.4GHz-2.485GHz spectrum divided into
11 channels at different frequencies
host: must associate with an AP scans channels, listening for beacon frames containing AP‟s name (SSID) and MAC address selects AP to associate with may perform authentication may run DHCP to get IP address in AP‟s subnet (we‟‟ll hard-code our IP addresses tomorrow)
Introduction 1-112
AP admin chooses frequency for AP interference possible: channel can be same as that chosen by neighboring AP!
IEEE 802.11 MAC Protocol: CSMA/CA
802.11 sender 1 if sense channel idle for DIFS then
transmit entire frame (no CD) 2 if sense channel busy then start random backoff time timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval, repeat 2
sender
DIFS
receiver
data
SIFS
802.11 receiver - if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
ACK
Introduction
1-113
Avoiding collisions (more)
idea:
allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but they‟re short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets!
Introduction 1-114
Collision Avoidance: RTS-CTS exchange
A AP B
reservation collision
DATA (A)
B defers
time
Introduction
1-115
802.11 frame: addressing
2 2 6 6 6 2
6
0 - 2312
payload
4
CRC
frame address address address duration control 1 2 3
seq address 4 control
Address 1: MAC address of wireless host or AP to receive this frame
Address 4: used only in ad hoc mode Address 3: MAC address of router interface to which AP is attached
Address 2: MAC address of wireless host or AP transmitting this frame
Introduction
1-116
802.11 frame: addressing
H1 AP
R1 router
Internet
R1 MAC addr AP MAC addr
dest. address source address
802.3 frame
AP MAC addr H1 MAC addr R1 MAC addr
address 1 address 2 address 3
802.11 frame
Introduction 1-117
802.11 frame: more
duration of reserved transmission time (RTS/CTS) 2 2 6 6 6 2 frame seq # (for reliable ARQ) 6 0 - 2312
payload
4
CRC
frame address address address duration control 1 2 3
seq address 4 control
2
Protocol version
2
Type
4
Subtype
1
To AP
1
1
1
Retry
1
1
1
WEP
1
Rsvd
From More AP frag
Power More mgt data
frame type (RTS, CTS, ACK, data)
Introduction 1-118
Our Whirlwind tour is over!:
Part 1: Whirlwind Introduction to Networking
(Monday)
Part 2: In the field (Tuesday)
9:00 – 10:15 networking overview 10:45 – 12:00 application, transport, network layers 2:00 – 3:00 intro to link layer 3:00 – 4:00 link layer, 802.11 networks
9:00 – 9:30 meet, discuss plans, divide into teams 9:30 – 12:00 short distance 802.11 experiments,
measurements 2:00 – ? long-distance 802.11 experiments, measurements
Introduction
1-119
Our Whirlwind tour is over!:
Part 1: Whirlwind Introduction to Networking
(Monday)
Part 2: In the field (Tuesday)
9:00 – 10:15 networking overview 10:45 – 12:00 application, transport, network layers 2:00 – 3:00 intro to link layer 3:00 – 4:00 link layer, 802.11 networks
9:00 – 9:30 meet, discuss plans, divide into teams 9:30 – 12:00 short distance 802.11 experiments,
measurements 2:00 – ? long-distance 802.11 experiments, measurements
Introduction
1-120
we are
always looking for great
graduate students! www.cs.umass.edu consider graduate school at UMass
intellectually exciting terrific faculty collaborative, cross-disciplinary well, … but not quite as warm and sunny as here
www.ecs.umass.edu
Introduction
1-121
DAY TWO
Goals: get out “in the field,” set up a single-link (one-hop) wireless network using directional antennas transfer radar data over connection
measure, observe variations in performance antenna aiming/positioning 802.11 parameters: timeout link distance
Introduction 1-122
measure, record traffic for later analysis
Equipment setup
2.4 GHz 24 dBi High Performance Reflector Grid Wireless LAN Antenna (8 degree beam width)
N-male to N-male cable
pigtail
Linux laptop driver for Atheros chipset: http://www.madwifi.org/
Proxim Orinoco 8470-FC 802.11b/g PC Card
Introduction 1-123
Software we‟ll be using
tcpdump (like Ethereal) – packet capture
iperf – sends packets as fast as possible
from client to server over udp or tcp socket madwifi-tools (www.madwifi.org)
Linux kernel driver for Atheros Wireless LAN chipsets (we‟ll use this) athctrl –d set specific distance in meters for link optimization (e.g. ACK timeout), athctrl –i ath0 -d 15000
ath0 interface receiver distance < 15,000m
Introduction 1-124
Software we‟ll be using
Wireless tools (iwconfig) display wireless link information
Link Quality Signal Level Noise Level
set wireless link parameters, e.g., speed,
address, name
iwconfig iwconfig iwconfig iwconfig
ath0 ath0 ath0 ath0
mode Ad-Hoc date 54M txpower 30mW address 192.168.1.1
Introduction 1-125
Configuring the wireless interfaces
Configuration of etc/network/interfaces: #Wireless interface auto ath0 iface ath0 inet static address 192.168.1.1 netmask 255.255.255.0 wireless_mode Ad-Hoc wireless_essid CS496 channel 1 #rate 54M (*must set manually) #Ethernet interface auto eth0 iface eth0 inet static address 192.168.1.101 netmask 255.255.255.0
Introduction
1-126
Address configuration
MAC: 0020A657B3CB IP: 192.168.1.2
MAC: 0020A657B3C3 IP: 192.168.1.1
Introduction 1-127
Experiments: 7 data sets (at each node)
a) Align antennas, set timers, note signal strength, begin tcpdump, begin iperf transfer b) turn each antenna 10 degrees (or until signal strength creasing by factor of 2) c) afternoon experiments; repeat a) and b) also set ACK timer short (100m propagation) and repeat a) d) analyze gathered data later (will be posted on www site: http://gaia.cs.umass.edu/uprm
Introduction 1-128
a) UDP throughput over 5 minutes b) TCP throughput over 5 minutes
a) repeat above