ceng331 p5
TRANSCRIPT
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CENG 334
Introduction
to theInternetworking
Part Four
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Overview:
Network Core:Packet Switching
Circuit Switching
Internet structure
Performance:
Loss
Delay
Throughput Protocol layers, service models
Security
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The Network Core
mesh of interconnectedrouters
the fundamental question:how is data transferredthrough net?
circuit switching:dedicated circuit per call:Ex: Telephone Net
packet-switching: datasent thru net in discretechunks
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Network Core: Circuit Switching
End to End Resourcesreserved for call
link bandwidth, switch
capacity dedicated resources:
no sharing
circuit-like
(guaranteed)performance
call setup required
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Network Core: Circuit Switching
network resources(e.g., bandwidth)divided into pieces
pieces allocated to calls
resource piece idleifnot used by owning call(no sharing)
dividing link bandwidthinto pieces
frequency division
time division
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Circuit Switching: FDM and TDM
FDM
frequency
time
TDM
frequency
time
4 usersExample:
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Internet structure: network of networks
roughly hierarchical at center: tier-1 ISPs (e.g., Verizon, Sprint, AT&T,
Cable and Wireless), national/international coverage
treat each other as equals
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Tier-1providersinterconnect
(peer)privately
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Tier-1 ISP: e.g., Sprint
to/from customers
peering
to/from backbone
.
POP: point-of-presence
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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 1 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISPTier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
Tier-2 ISP paystier-1 ISP forconnectivity torest of Internet
tier-2 ISP iscustomeroftier-1 provider
Tier-2 ISPsalso peerprivately witheach other.
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Internet structure: network of networks
Tier-3 ISPs and local ISPs last hop (access) network (closest to end systems)
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISPTier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
local
ISPlocalISP
localISP
local
ISP
localISP Tier 3
ISP
local
ISP
local
ISP
localISP
Local and tier-3 ISPs arecustomersofhigher tierISPs
connectingthem to restof Internet
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Internet structure: network of networks
a packet passes through many networks!
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISPTier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
local
ISPlocalISP
localISP
local
ISP
localISP Tier 3
ISP
local
ISP
local
ISP
localISP
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How do loss and delay occur?
packets queuein router buffers packet arrival rate to link exceeds output link
capacity
packets queue, wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)
free (available) buffers: arriving packets
dropped (loss) if no free buffers
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Four sources of packet delay
1. nodal processing: check bit errors
determine output link
A
B
propagation
transmission
nodalprocessing queueing
2. queueing time waiting at output
link for transmission
depends on congestion
level of router
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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 inmedium (~2x108 m/sec)
propagation delay = d/s
A
B
propagation
transmission
nodal
processing queueing
Note: s and R are verydifferent quantities!
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Caravan analogy
cars propagate at
100 km/hr toll booth takes 12 sec to
service car (transmissiontime)
car~bit; caravan ~ packet Q: How long until caravan
is lined up before 2nd tollbooth?
Time to push entire
caravan through tollbooth onto highway =12*10 = 120 sec
Time for last car to
propagate from 1st to2nd toll both:100km/(100km/hr)= 1 hr
A: 62 minutes
tollbooth
tollbooth
ten-carcaravan
100 km 100 km
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Caravan analogy (more)
Cars now propagate at1000 km/hr
Toll booth now takes 1min to service a car
Q:Will cars arrive to2nd booth before allcars serviced at 1stbooth?
Yes! After 7 min, 1st car
at 2nd booth and 3 carsstill at 1st booth.
1st bit of packet canarrive at 2nd router
before packet is fullytransmitted at 1st router! See Ethernet applet at AWL
Web site
tollbooth
tollbooth
ten-carcaravan
100 km 100 km
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Nodal delay
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
proptransqueueprocnodal ddddd
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Queueing delay (revisited)
R=link bandwidth (bps)
L=packet length (bits)
a=average packet
arrival ratetraffic intensity = La/R
La/R ~ 0: average queueing delay small La/R -> 1: delays become large
La/R > 1: more work arriving than can beserviced, average delay infinite!
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Real Internet delays and routes
What do real Internet delay & loss look like?
Traceroute program: provides delaymeasurement from source to router along end-endInternet path towards destination. For all i: sends three packets that will reach router ion path
towards destination
router iwill return packets to sender
sender times interval between transmission and reply.
3 probes
3 probes
3 probes
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Packet loss
Queue (Buffer) preceding link in buffer hasfinite capacity
Packet arriving to full queue dropped (Lost)
Lost packet may be retransmitted by previousnode, by source end system, or not at all
A
B
packet being transmitted
packet arriving tofull bufferis lost
buffer(waiting area)
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Throughput
throughput:rate (bits/time unit) at whichbits transferred between sender/receiver instantaneous:rate at given point in time
average:rate over longer period of time
server, withfile of F bits
to send to client
link capacityRsbits/sec
link capacityRcbits/sec
pipe that can carryfluid at rateRsbits/sec)
pipe that can carryfluid at rateRcbits/sec)
server sends bits(fluid) into pipe
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Throughput (more)
Rs< Rc What is average end-end throughput?
Rsbits/sec Rcbits/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
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Throughput: Internet scenario
10 connections (fairly) sharebackbone bottleneck link Rbits/sec
RsRs
Rs
Rc
Rc
Rc
R
per-connectionend-end
throughput:min(Rc,Rs,R/10)
in practice: Rc or
Rs is oftenbottleneck
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Protocol Layers
Networks are complex! many pieces:
hosts
routers
links of various media applications
protocols
hardware, software
Question:Is there any hope oforganizingstructure of
network?
Or at least our discussionof networks?
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Organization of air travel
a series of steps
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
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ticket (purchase)
baggage (check)
gates (load)
runway (takeoff)
airplane routing
departureairport
arrivalairport
intermediate air-trafficcontrol centers
airplane routing airplane routing
ticket (complain)
baggage (claim
gates (unload)
runway (land)
airplane routing
ticket
baggage
gate
takeoff/landing
airplane routing
Layering of airline functionality
Layers: each layer implements a service
via its own internal-layer actions
relying on services provided by layer below
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Why layering?
Dealing with complex systems: explicit structure allows identification,
relationship of complex systems pieces
layered reference model for discussion
modularization eases maintenance, updating ofsystem
change of implementation of layers servicetransparent to rest of system
e.g., change in gate procedure doesnt affectrest of system
layering considered harmful?
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Internet protocol stack
application: supporting networkapplications FTP, SMTP, HTTP
transport: process-process data
transfer TCP, UDP
network: routing of datagrams fromsource to destination
IP, routing protocols link: data transfer between
neighboring network elements PPP, Ethernet
physical:bits on the wire
application
transport
network
link
physical
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ISO/OSI reference model
presentation: allow applications tointerpret meaning of data, e.g.,encryption, compression, machine-specific conventions
session:synchronization,checkpointing, recovery of dataexchange
Internet stack missing these
layers! these services, if needed, must
be implemented in application
application
presentation
sessiontransport
network
link
physical
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sourceapplicationtransportnetwork
linkphysical
Ht
Hn
M
segment Ht
datagram
destination
applicationtransportnetwork
linkphysical
HtHnHl M
HtHn M
Ht MM
networklink
physical
linkphysical
HtHnHl M
HtHn M
Ht
Hn M
HtHnHl M
router
switch
Encapsulationmessage M
Ht M
Hnframe
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Network Security
The field of network security is about: how bad guys can attack computer networks
how we can defend networks against attacks
how to design architectures that are immune to
attacks
Internet not originally designed with(much) security in mind
original vision:a group of mutually trustingusers attached to a transparent network
Internet protocol designers playing catch-up
Security considerations in all layers!
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Bad guys can put malware intohosts via Internet Malware can get in host from a virus, worm, or
trojan horse.
Spyware malware can record keystrokes, web
sites visited, upload info to collection site.
Infected host can be enrolled in a botnet, usedfor spam and DDoS attacks.
Malware is often self-replicating: from aninfected host, seeks entry into other hosts
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Bad guys can put malware intohosts via Internet Trojan horse
Hidden part of someotherwise usefulsoftware
Today often on a Webpage (Active-X, plugin)
Virus infection by receiving
object (e.g., e-mail
attachment), activelyexecuting
self-replicating:propagate itself toother hosts, users
Worm: infection by passively
receiving object that getsitself executed
self- replicating: propagatesto other hosts, users
Sapphire Worm: aggregate scans/secin first 5 minutes of outbreak (CAIDA, UWisc data)
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Bad guys can attack servers andnetwork infrastructure
Denial of service (DoS): attackers make resources(server, bandwidth) unavailable to legitimate trafficby overwhelming resource with bogus traffic
1. select target
2. break into hostsaround the network(see botnet)
3. send packets towardtarget fromcompromised hosts
target
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The bad guys can sniff packets
Packet sniffing: broadcast media (shared Ethernet, wireless)
promiscuous network interface reads/records allpackets (e.g., including passwords!) passing by
A
B
C
src:B dest:A payload
Wireshark software used for end-of-chapterlabs is a (free) packet-sniffer
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The bad guys can use false sourceaddresses
IP spoofing:send packet with false source address
A
B
C
src:B dest:A payload
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The bad guys can record andplayback
record-and-playback: sniff sensitive info (e.g.,password), and use later
password holder isthat user from system point of
view
A
B
C
src:B dest:A user: B; password: foo
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Network Security
more throughout this partSecurity with Cryptographic techniques
obvious uses