Transcript Foundation

Foundation
Objectives:
1.1 What’s the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay & loss in packet-switched networks
1.7 Protocol layers, service model
1-1
What’s the Internet
 millions of connected
computing devices: hosts
= end systems
 running network apps
 communication links


router
server
workstation
mobile
local ISP
fiber, copper, radio,
satellite
transmission rate =
bandwidth
regional ISP
 routers: forward packets
(chunks of data)
company
network
1-2
What’s the Internet
 protocols control sending,
receiving of msgs

e.g., TCP, IP, HTTP, FTP, PPP
 Internet: “network of
networks”


router
server
mobile
local ISP
a collection of individual
networks
public Internet versus
private intranet
 Internet standards
 RFC: Request for comments
 IETF: Internet Engineering
Task Force
workstation
regional ISP
company
network
1-3
What’s the Internet: a service view
 communication
infrastructure enables
distributed applications:

Web, email, games, ecommerce, file sharing
 communication services
provided to apps:


Connectionless-oriented
unreliable service
connection-oriented
reliable service
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What’s a protocol?
a human protocol and a computer network protocol:
Hi
TCP connection
request
Hi
TCP connection
response
Got the
time?
Get http://www.kfupm.edu.sa/downloads
2:00
<file>
time
protocols define format, order of msgs sent and received among
network entities, and actions taken on msg transmission, receipt
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What’s a protocol?
 What does a protocol tell us?
 Syntax
 Semantics
 Actions
 A network that provides many services needs many




protocols
For example, consider a file transfer protocol
Packet transfer is one step in the execution of a
reliable file transfer protocol
This form of dependency is called layering
That is, reliable file transfer is layered above
packet transfer protocol
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Network structure:
 network edge:
applications and
hosts
 network core:
routers
 network of
networks

 access networks,
physical media:
communication links
1-7
The network edge:
 end systems (hosts):



run application programs
e.g. Web, email
at “edge of network”
 client/server model


client host requests, receives
service from always-on server
e.g. Web browser/server;
email client/server
 peer-peer model:


minimal (or no) use of
dedicated servers
e.g. Gnutella, KaZaA
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Network edge: connection-oriented service
Goal: data transfer
between end systems
 A connection-oriented
service requires



Establishing a connection
Maintaining the connection
Releasing the connection
 TCP - Transmission
Control Protocol

Internet’s connectionoriented service
TCP service [RFC 793]
 reliable, in-order byte-
stream data transfer

loss: acknowledgements
and retransmissions
 flow control:
 sender won’t overwhelm
receiver
 congestion control:
 senders “slow down sending
rate” when network
congested
1-9
Network edge: connectionless service
Goal: data transfer
between end systems

same as before!
 UDP - User Datagram
Protocol [RFC 768]:
 connectionless
 unreliable data
transfer
 no flow control
 no congestion control
App’s using TCP:
 HTTP (Web), FTP (file
transfer), Telnet,
SMTP
App’s using UDP:
 streaming media,
teleconferencing, DNS,
Internet telephony
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A Taxonomy of Communication Networks
 So far we have looked at only the topology of the
Internet: a mesh of computers interconnected
 The fundamental question: how is data (the bits)
transferred through a communication network?
1-11
Broadcast vs. Switched
Communication Networks
communication
networks
switched
networks
broadcast
networks
 Broadcast networks
 Nodes share a common channel; information transmitted
by a node is received by all other nodes in the network
 Examples: TV, radio
 Switched networks
 Information is transmitted to a small sub-set (usually
only one) of the nodes
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The Network Core
 mesh of interconnected
routers
 In a switched network,
how is data transferred
through the net?
 circuit switching
 packet switching
1-13
Network Core: Circuit Switching
 dedicated circuit per
call: telephone net
End-end resources
reserved for “call”
 link bandwidth, switch
capacity
 dedicated resources: no
sharing
 circuit-like (guaranteed)
performance
 call setup required
1-14
Network Core: Circuit Switching
network resources
(e.g., bandwidth)
divided into “pieces”
 pieces allocated to calls
 dividing link bandwidth
into “pieces”
 frequency division
 time division
 resource piece idle if
not used by owning call
(no sharing)
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Circuit Switching: FDM and TDM
Example:
FDM
4 users
frequency
time
TDM
frequency
time
1-16
Network Core: Packet Switching
 data sent thru net in
discrete “chunks”
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 can happen

Packets queue, wait for
link use
 store and forward
 Packets move one hop at
a time

Node receives complete
packet before forwarding
1-17
Packet Switching: Statistical Multiplexing
10 Mb/s
Ethernet
A
B
statistical multiplexing
C
1.5 Mb/s
queue of packets
waiting for output
link
D
E
Sequence of A & B packets does not have fixed pattern,
shared on demand  statistical multiplexing.
TDM: each host gets same slot in revolving TDM frame.
1-18
Packet switching versus circuit switching
Packet switching is
 Great for bursty data
resource sharing
 simpler, no call setup
 Excessive congestion: packet delay and loss
 protocols needed for reliable data transfer,
congestion control
 Q: How to provide circuit-like behavior?
 bandwidth guarantees needed for audio/video apps
 still an unsolved problem

Q: human analogies of reserved resources (circuit switching) versus ondemand allocation (packet-switching)?
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Packet-switching: store-and-forward
L
R
R
R
 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)
1-20
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
1-21
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
transmission
A
propagation
B
nodal
processing
queueing
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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
transmission
A
propagation
B
nodal
processing
queueing
1-23
Nodal delay
d nodal  d proc  d queue  d trans  d prop
 dproc = processing 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
1-24
Packet loss
 queue (aka buffer) preceding link in buffer
has finite capacity
 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

1-25
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!
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Internet structure: network of networks
 roughly hierarchical
 at center: “tier-1” ISPs (e.g., UUNet, BBN/Genuity,
Sprint, AT&T), national/international coverage
 treat each other as equals
Tier-1
providers
interconnect
(peer)
privately
Tier 1 ISP
Tier 1 ISP
NAP
Tier-1 providers
also interconnect
at public network
access points
(NAPs)
Tier 1 ISP
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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
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
NAP
Tier 1 ISP
Tier-2 ISPs
also peer
privately with
each other,
interconnect
at NAP
Tier-2 ISP
Tier-2 ISP
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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
local
ISP
local
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
NAP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
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Internet structure: network of networks
 a packet passes through many networks!
local
ISP
Tier 3
ISP
Tier-2 ISP
local
ISP
local
ISP
local
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
NAP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
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Protocol “Layers”
Networks are complex!
 many “pieces”:
 hosts
 routers
 links of various
media
 applications
 protocols
 hardware,
software
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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
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Internet protocol stack
 application: supporting network
applications

FTP, SMTP, STTP
application
 transport: host-host data transfer
 TCP, UDP
transport
 network: routing of datagrams from
network
source to destination

IP, routing protocols
 link: data transfer between
neighboring network elements

link
physical
PPP, Ethernet
 physical: bits “on the wire”
1-33
source
message
segment Ht
datagram Hn Ht
frame
Hl Hn Ht
M
M
M
M
Encapsulation
application
transport
network
link
physical
Hl Hn Ht
M
link
physical
Hl Hn Ht
M
switch
destination
M
Ht
M
Hn Ht
Hl Hn Ht
M
M
application
transport
network
link
physical
Hn Ht
Hl Hn Ht
M
M
network
link
physical
Hn Ht
Hl Hn Ht
M
M
router
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Foundation: Summary
Material Covered
You now have:
 Internet overview
 context, overview,
“feel” of networking
 what’s a protocol?
 network edge, core, access  more depth, detail to
follow!
network
 packet-switching versus
circuit-switching
 Internet/ISP structure
 performance: loss, delay
 layering and service models
 history (which you will be
reading on your own)
1-35