1. Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge
end systems, access networks, links
1.3 Network core
circuit switching, packet switching, network structure
1.4 Delay, loss and throughput in packet-switched networks
1.5 Protocol layers, service models
1.6 Networks under attack: security
1.7 History
Introduction
1-58

2. What’s the Internet: “nuts and bolts” viewPC
server
wireless
laptop
cellular
handheld
millions of connected computing devices: hosts = end systems
running network apps
Home network
Institutional network
Mobile network
Global ISP
Regional ISP
communication links
fiber, copper, radio, satellite
transmission rate = bandwidth
wired
links
access
points
routers: forward packets (chunks of data)
router
Introduction
1-59

3. What’s the Internet: “nuts and bolts” view
protocols control sending, receiving of msgs
e.g., TCP, IP, HTTP, Skype, Ethernet
Internet: “network of networks”
loosely hierarchical
public Internet versus private intranet
Internet standards
RFC: Request for comments
IETF: Internet Engineering Task Force
Home network
Institutional network
Mobile network
Global ISP
Regional ISP
Introduction
1-61

4. What’s the Internet: a service view
communication infrastructure enables distributed applications:
Web, VoIP, , games, e-commerce, file sharing
communication services provided to apps:
reliable data delivery from source to destination
“best effort” (unreliable) data delivery
Introduction
1-62

6. Some network apps e-mail web instant messaging remote login
P2P file sharing
multi-user network games
streaming stored video clips
social networks
voice over IP
real-time video conferencing
grid computing
2: Application Layer

7. What’s a protocol? human protocols: network protocols:
“what’s the time?”
“I have a question”
introductions
… specific msgs sent
… specific actions taken when msgs received, or other events
network protocols:
machines rather than humans
all communication activity in Internet governed by protocols
protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt
Introduction
1-63

8. What’s a protocol? a human protocol and a computer network protocol:Hi
TCP connection
request Hi
TCP connection
response
Got the
time?
Get
2:00
<file>
time
Q: Other human protocols?
Introduction
1-64

9. A closer look at network structure:
network edge: applications and hosts
access networks, physical media: wired, wireless communication links
network core:
interconnected routers
network of networks
Introduction
1-66

13. Packet vs. circuit switching
mesh of interconnected routers
the fundamental question: how is data transferred through net?
circuit switching: dedicated circuit per call: telephone net
packet-switching: data sent thru net in discrete “chunks”
Introduction
1-4

14. Case study: Circuit Switching
1890-current: Phone network
Fixed bit rate
Mostly voice
Not fault-tolerant
Components extremely reliable
Global application-level knowledge throughout network
Introduction
1-5

15. Case study: Packet Switching
1981-current: Internet network
Variable bit rate
Mostly data
Fault-tolerant
Components not extremely reliable (versus phone components)
Distributed control and management
Introduction
1-6

16. Circuit Switching End-end resources reserved for “call”
network resources (e.g., bandwidth) divided into “pieces”
link bandwidth, switch capacity
pieces allocated to calls
resource piece idle if not used by owning call
dedicated resources: no sharing
circuit-like (guaranteed) performance
call setup and admission control required
Introduction
1-7

17. Circuit Switching: FDM and TDM
4 users
Example:
FDM
frequency
time
TDM
frequency
time
Two simple multiple access control techniques.
Each mobile’s share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling.
As we will see, used in AMPS, GSM, IS-54/136
Introduction
1-8

18. 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
store and forward: packets move one hop at a time
Node receives complete packet before forwarding
Bandwidth division into “pieces”
Dedicated allocation
Resource reservation
Introduction
1-10

19. Packet Switching: Statistical Multiplexing
10 Mb/s
Ethernet C A
statistical multiplexing
1.5 Mb/s B
queue of packets
waiting for output
link D E
Sequence of A & B packets does not have fixed pattern, shared on demand è statistical multiplexing.
TDM: each host gets same slot in revolving TDM frame.
Introduction
1-11

20. Packet switching versus circuit switching
Packet switching allows more users to use network!
N users over 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
Allows more users to use network
“Statistical multiplexing gain”
N users
1 Mbps link
Q: how did we get value ?
Introduction
1-12

21. Packet switching versus circuit switching
Is packet switching a “slam dunk winner?”
Great for bursty data
resource sharing
simpler, no call setup
Bad for applications with hard resource requirements
Excessive congestion: packet delay and loss
Need protocols for reliable data transfer, congestion control
Applications must be written to handle congestion
Q: How to provide circuit-like behavior?
bandwidth guarantees needed for audio/video apps
still an unsolved problem
Introduction
1-13

22. 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
when packet arrives to full queue, packet is dropped (aka lost)
lost packet may be retransmitted by previous node, by source end system, or not retransmitted at all
packet being transmitted (delay) A
free (available) buffers: arriving packets
dropped (loss) if no free buffers
packets queueing (delay) B
Introduction
1-14

23. Four sources of packet delay
1. nodal processing:
check bit errors
determine output link
2. queueing
time waiting at output link for transmission
depends on congestion level of router A B
propagation
transmission
nodal
processing
queueing
Introduction
1-15

24. Delay in packet-switched networks
3. Transmission delay:
R=link bandwidth (bps)
L=packet length (bits)
time to send bits into link = L/R
4. Propagation delay:
d = length of physical link
s = propagation speed in medium (~2x108 m/sec)
propagation delay = d/s
Note: s and R are very different quantities! A B
propagation
transmission
nodal
processing
queueing
Introduction
1-16

25. Nodal delay dproc = processing delay dqueue = queuing delay
typically a few microsecs or less
dqueue = queuing delay
depends on congestion
dtrans = transmission delay
= L/R, significant for low-speed links
dprop = propagation delay
a few microsecs to hundreds of msecs
Introduction
1-17

26. Queueing delay (revisited)
R=link bandwidth (bps)
L=packet length (bits)
a=average packet arrival rate
traffic intensity = La/R
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
1-18

27. Transmission delay Example: L = 7.5 Mbits R = 1.5 Mbps delay = 15 sec
Packet switching
Store-and-forward
Packet completely received before being transmitted to next node
Takes L/R seconds to transmit (push out) packet of L bits on to link or R bps
Entire packet must arrive at router before it can be transmitted on next link: store and forward
delay = 3L/R (assuming zero propagation delay)
Example:
L = 7.5 Mbits
R = 1.5 Mbps
delay = 15 sec
more on delay shortly …
Introduction
1-19

28. “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
1-20

29. “Real” Internet delays and routes
traceroute: gaia.cs.umass.edu to
Three delay measurements from
gaia.cs.umass.edu to cs-gw.cs.umass.edu
1 cs-gw ( ) 1 ms 1 ms 2 ms
2 border1-rt-fa5-1-0.gw.umass.edu ( ) 1 ms 1 ms 2 ms
3 cht-vbns.gw.umass.edu ( ) 6 ms 5 ms 5 ms
4 jn1-at wor.vbns.net ( ) 16 ms 11 ms 13 ms
5 jn1-so wae.vbns.net ( ) 21 ms 18 ms 18 ms
6 abilene-vbns.abilene.ucaid.edu ( ) 22 ms 18 ms 22 ms
7 nycm-wash.abilene.ucaid.edu ( ) 22 ms 22 ms 22 ms
( ) 104 ms 109 ms 106 ms
9 de2-1.de1.de.geant.net ( ) 109 ms 102 ms 104 ms
10 de.fr1.fr.geant.net ( ) 113 ms 121 ms 114 ms
11 renater-gw.fr1.fr.geant.net ( ) 112 ms 114 ms 112 ms
12 nio-n2.cssi.renater.fr ( ) 111 ms 114 ms 116 ms
13 nice.cssi.renater.fr ( ) 123 ms 125 ms 124 ms
14 r3t2-nice.cssi.renater.fr ( ) 126 ms 126 ms 124 ms
15 eurecom-valbonne.r3t2.ft.net ( ) 135 ms 128 ms 133 ms
( ) 126 ms 128 ms 126 ms
17 * * *
18 * * *
19 fantasia.eurecom.fr ( ) 132 ms 128 ms 136 ms
trans-oceanic
link
* means no response (probe lost, router not replying)
Introduction
1-21

30. Packet loss
queue (aka buffer) preceding link in buffer has finite capacity
packet arriving to full queue dropped (aka lost)
lost packet may be retransmitted by previous node, by source end system, or not at all
buffer
(waiting area)
packet being transmitted A B
packet arriving to
full buffer is lost
Introduction
1-110

31. Throughput
throughput: rate (bits/time unit) at which bits transferred between sender/receiver
instantaneous: rate at given point in time
average: rate over longer period of time
pipe that can carry
fluid at rate
Rc bits/sec)
pipe that can carry
fluid at rate
Rs bits/sec)
link capacity
Rs bits/sec
link capacity
Rc bits/sec
server sends bits
(fluid) into pipe
server, with
file of F bits
to send to client
Introduction
1-111

32. Throughput (more) Rs < Rc What is average end-end throughput?
Rc bits/sec
Rs bits/sec
Rs > Rc What is average end-end throughput?
Rs bits/sec
Rc bits/sec
link on end-end path that constrains end-end throughput
bottleneck link
Introduction
1-112

33. Throughput: Internet scenarioRs
per-connection end-end throughput: min(Rc,Rs,R/10)
in practice: Rc or Rs is often bottleneck Rs Rs R Rc Rc Rc
10 connections (fairly) share backbone bottleneck link R bits/sec
Introduction
1-113

34. Why layering? Dealing with complex systems:
explicit structure allows identification, relationship of complex system’s pieces
layered reference model for discussion
modularization eases maintenance, updating of system
change of implementation of layer’s service transparent to rest of system
e.g., change in gate procedure doesn’t affect rest of system
layering considered harmful?
Introduction
1-118

35. Layering Modular approach to network functionality
Simplifies complex systems
Each layer relies on services from layer below and exports services to layer above
Hides implementation, eases maintenance and updating of system
Layer implementations can change without disturbing other layers (black box)
Introduction
1-35

36. Host-to-host connectivity
Layering
Examples:
Topology and physical configuration hidden by network-layer routing
Applications require no knowledge of this
New applications deployed without coordination with network operators or operating system vendors
Application
Host-to-host connectivity
Link hardware
Introduction
1-36

37. Layering in Protocols
Set of rules governing communication between network elements (applications, hosts, routers)
Protocols specify:
Interface to higher layers (API)
Interface to peer
Format and order of messages
Actions taken on receipt of a message
Interface defines interaction
Human analogy – social customs
Introduction
1-37

38. Internet protocol stack
application: supporting network applications
FTP, SMTP, HTTP
transport: process-process data transfer
TCP, UDP
network: routing of datagrams from source to destination
IP, routing protocols
link: data transfer between neighboring network elements
PPP, Ethernet
physical: bits “on the wire”
application
transport
network
link
physical
Introduction
1-119

39. Layering in Networks: OSI Model
Physical
how to transmit bits
Data link
how to transmit frames
Network
how to route packets host-to-host
Transport
how to send packets end2end
Session
how to tie flows together
Presentation
byte ordering, formatting
Application: everything else
Host
Application
Transport
Network
Data Link
Presentation
Session
Physical
Introduction
1-38

40. Encapsulation source destination application transport network link
message M
application
transport
network
link
physical
segment Ht M
datagram Ht Hn M
frame Ht Hn Hl M Ht Hn Hl M
link
physical Ht Hn Hl M
switch
destination Ht Hn M
network
link
physical Ht Hn M Ht Hn Hl M
application
transport
network
link
physical Ht Hn Hl M M Ht M Ht Hn M
router Ht Hn Hl M
Introduction
1-39

41. Distributed design and control
Requirements from DARPA
Must survive a nuclear attack
Reliability
Intelligent aggregation of unreliable components
Alternate paths, adaptivity
Distributed management & control of networks
Exceptions: TLDs and TLD servers, IP address allocation (ICANN)
Back
Introduction
1-40

42. Superior organizational process
IAB/IETF process allowed for quick specification, implementation, and deployment of new standards
Free and easy download of standards
Rough consensus and running code
2 interoperable implementations
Bake-offs
ISO/OSI
Comparison to IETF left as an exercise
Back
Introduction
1-41

43. What if the Data gets Corrupted?
Problem: Data Corruption
GET index.html
GET windex.html
Internet
Solution: Add a checksum X
0,9 9
6,7,8 21
4,5 7
1,2,3 6
Introduction
1-49

44. What if the Data gets Lost?
Problem: Lost Data
GET index.html
Internet
Solution: Timeout and Retransmit
GET index.html
GET index.html
Internet
GET index.html
Introduction
1-50

45. What if receiver has no resources (flow control)?
Problem: Overflowing receiver buffers
PUT remix.mp3
Internet
Solution: Receiver advertised window
PUT remix.mp3
Internet
16KB free
Introduction
1-51

46. What if Network is Overloaded?
Short bursts: buffer
What if buffer overflows?
Packets dropped and retransmitted
Sender adjusts rate until load = resources
Called “Congestion control”
Introduction
1-52

47. What if the Data Doesn’t Fit?
Problem: Packet size
On Ethernet, max IP packet is 1.5kbytes
Typical web page is 10kbytes
Solution: Fragment data across packets ml
x.ht
inde
GET
GET index.html
Introduction
1-53

48. What if the Data is Out of Order?
Problem: Out of Order ml
inde
x.th
GET
GET x.thindeml
Solution: Add Sequence Numbers ml 4
inde 2
x.th 3
GET 1
GET index.html
Introduction
1-54