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Course Review
Outline
• Networks: A top down view (for a change).
• Other topics.
• Security
• QoS
• Multicast
• Questions?
Final Review: 12/10/2001
2
Protocol Stacks
The Open Systems Interconnection (OSI) Model.
7
Application
Application
Presentation
6 Presentation
5
Session
Session
4
Transport
Transport
3
Network
2
Data link
1
Physical
Network
Network
Data link
Data link
Data link
Physical
Physical
Physical
Final Review: 12/10/2001
3
Browsing the Web
7 Web Browser
Web Server
6 Presentation
Presentation
5
Session
4
Transport
3
Network
Session
2
Data link
Data link
1
Physical
Physical
?
Transport
Network
Network
Data link
Data link
Physical
Physical
Client
Server
Final Review: 12/10/2001
4
HTTP Request Example
GET / HTTP/1.1
Accept: */*
Accept-Language: en-us
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/4.0 (compatible; MSIE 5.5;
Windows NT 5.0)
Host: www.seshan.org
Connection: Keep-Alive
Final Review: 12/10/2001
5
HTTP Response Example
HTTP/1.1 200 OK
Date: Tue, 27 Mar 2001 03:49:38 GMT
Server: Apache/1.3.14 (Unix) (Red-Hat/Linux) mod_ssl/2.7.1
OpenSSL/0.9.5a DAV/1.0.2 PHP/4.0.1pl2 mod_perl/1.24
Last-Modified: Mon, 29 Jan 2001 17:54:18 GMT
ETag: "7a11f-10ed-3a75ae4a"
Accept-Ranges: bytes
Content-Length: 4333
Keep-Alive: timeout=15, max=100
Connection: Keep-Alive
Content-Type: text/html
…..
Final Review: 12/10/2001
6
Single Transfer Example
Client
SYN
0 RTT
Client opens TCP connection
1 RTT
Client sends HTTP request for HTML
Server
SYN
DAT
ACK
2 RTT
ACK
DAT
FIN
Server reads from disk
ACK
Client parses HTML
Client opens TCP connection
FIN
ACK
3 RTT
Client sends HTTP request for image
SYN
SYN
ACK
DAT
Server reads from disk
ACK
4 RTT
Image begins to arrive
DAT
Final Review: 12/10/2001
7
Persistent Connection Example
Client
0 RTT
Client sends HTTP request for HTML
Server
DAT
ACK
Server reads from disk
DAT
1 RTT
ACK
Client parses HTML
Client sends HTTP request for image
DAT
ACK
Server reads from disk
DAT
2 RTT
Image begins to arrive
Final Review: 12/10/2001
8
DNS Server
• A DNS server is responsible for
maintaining the name-address
mapping in a specific domain.
• E.g. cs.cmu.edu
• The network manager can add,
remove, or change mappings.
• Computers can send requests to the
server to translate a name into an
address.
• But how do you find the server?
• Recursively contact the parent in
the hierarchical name space
• Caching is used to speed up the
lookup of frequently used names.
Other DNS servers
cs.cmu.edu
hawaii.cs.cmu.edu
Final Review: 12/10/2001
128.17.4.174
9
Typical Exchange
• Sender creates and
initializes a socket.
• Sender issues an open
connection command.
• Specifies destination IP and
application port addresses
• Sender blocks while
connection is established
• If the connection
succeeds, data exchange
can start.
• Lots of things can go wrong:
wrong addresses, receiver
or network down.
• Receiver creates and
initializes a socket.
• Receiver listens on the
socket for a connection
request.
• Can sometimes restrict the
type of connection
• If receiver accepts the
connection and the
connection succeeds, data
exchange can start.
• Communication typically
uses a different socket
Final Review: 12/10/2001
10
Browsing the Web
7 Web Browser
Web Server
6 Presentation
Presentation
5
Session
Session
4
Transport
Transport
3
Network
2
Data link
Data link
Physical
Physical
1
?
Network
Network
Data link
Data link
Physical
Physical
Client
Server
Final Review: 12/10/2001
11
Connection management
Sender
Receiver
syn
Open
syn/ack
Establish
Initial
Sequence
Numbers
ack
Data
fin
ack
fin
Time
Close
ack
Final Review: 12/10/2001
12
Reliability
• Checksum guarantees end-end data integrity.
• Sequence numbers detect packet sequencing problems:
• duplicate: ignore
• reordered: reorder or drop
• lost: retransmit
• Lost packets detected by sender.
• uses time out to detect lack of acknowledgment
• requires reliable roundtrip time estimate
• Retransmission requires that sender keeps copy of the
data until ACK is received.
• performance issue
Final Review: 12/10/2001
13
When to Send a Packet?
• End-to-end flow control.
• avoid buffer overflow on receiver
• receiver advertizes a window size
• Congestion control.
• estimates amount of data that can be in network
• implemented using the congestion window, slow start,
and fast retransmit/recovery mechanisms
• Efficiency considerations.
• try to send large packets (if possible)
• more efficient in the network and on end points
• piggybacking of acks
Final Review: 12/10/2001
14
Window Size
versus Throughput
Sender
Receiver
Time
Throughput =
Window Size
Roundtrip Time
Final Review: 12/10/2001
15
TCP Congestion Avoidance
• Congestion avoidance limits how fast TCP can send
data.
• Implemented using a congestion window that limits how
much data can be in the network
• independent from flow control window
• transmission is limited by minimum of the two windows
• window grows in response to acknowledgement
• Packet loss is seen as sign of congestion.
• multiplicative decrease of the congestion window
• have to cut back fast since cost of congestion is high
• How do you detect when more bandwidth becomes
available?
• gradually increment congestion window (probing)
• results in oscillation around congestion window size!
Final Review: 12/10/2001
16
TCP Saw Tooth Behavior
Congestion
Window
Initial
Slowstart
Timeouts
may still
occur
Slowstart
to pace
packets
Fast
Retransmit
and Recovery
Final Review: 12/10/2001
Time
17
Browsing the Web
7 Web Browser
Web Server
6 Presentation
Presentation
5
Session
Session
4
Transport
Transport
3
Network
2
Data link
Data link
1
Physical
Physical
?
Network
Network
Data link
Data link
Physical
Physical
Client
Server
Final Review: 12/10/2001
18
Hop-by-Hop Packet
Forwarding in the Internet
Host
Ethernet
Packets
over SONET
Mixed
Ethernet
Host
7
..
3
2
1
Final Review: 12/10/2001
19
Addressing in IP v4 (Basic)
• Each host has an Internet address.
• Addresses are hierarchical.
• address contains hint about location
• Address space is divided in three classes of point-topoint addresses, multicast addresses, and some
special addresses.
type
A
B
C
D
1
2
3
4
network
host
7
14
21
(multicast)
24
16
8
28
Example: 128.2.209.19
Final Review: 12/10/2001
20
Routing based on
Network Identifier
Net ID Next
Net ID Next
Net ID Next
Net ID Next
Forwarding Table
Host
Host
AN 2
Host
AN 4
ISP 1
ISP 2
Host
AN 3
Host
ISP 3
AN 5
AN 1
Host
Net.Host
Final Review: 12/10/2001
21
Problems with Simple Address
Structure
• Running out of addresses.
• Especially true for mid-sized networks
• Routing tables are becoming too big.
• 100 of thousands of entries
• Temporary solution: classless inter-domain
routing.
• Use address space more efficiently by relaxing the
strict address structure,
• length of network address is variable
• generalization of subnetting idea
• have internet service providers hand out blocks of
addresses to their customers
Final Review: 12/10/2001
22
Route Lookup with CIDR
• Problem: with CIDR there
can be multiple matches
when looking up an
address.
• Can for example happen
when a customer switches
ISPs but keeps addresses
• Solution: lookup is based
on longest prefix match.
10110110
10110110 010
hosts
hosts
10110110 010 0100011
• If there are multiple matches
in the lookup, the longest
match (longest netmask)
wins
Final Review: 12/10/2001
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What Does Routing Do?
Other routers
• Routing protocol specifies how
routers jointly collect
information about the network.
• Routing protocols must be
standardized
• Routing algorithm uses
network information to select
appropriate routes and to set
up the routing table.
• The data forwarding engine
performs route lookup in the
routing table.
• through which interface
should a packet be
forwarded?
Routing protocol
Routing
Protocol
Routing
Algorithms
Final Review: 12/10/2001
Route
Lookup
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Dijkstra’s Algorithm
(Link State)
5, E
5, E
3, F
3, F
5
B
1
6, B
6, B
A
3
3
C
6, E
F
2
3
2
D
4
4
E
2
2, F
6, E
6, E
Final Review: 12/10/2001
25
Distance Vector Routing
Example
- A/3 A/3 A/3
- C/1 C/1 C/1
- D/4 C/2 C/2
A
C
D
B
C
D
- B/3 B/3 B/3
- C/9 B/4 B/4
- - B/7 B/5
3
4
1
A
9
A
B
D
B
C
D
1
A
B
C
- - B/7 C/5
- B/4 C/2 C/2
- C/1 C/1 C/1
- A/9 B/4 B/4
- B/1 B/1 B/1
- D/1 D/1 D/1
Final Review: 12/10/2001
26
Hierarchical Routing
• Two level routing based on intra-domain and inter-domain
routing to improve scalability.
• Matches the structure of the address space.
• Driven in part by business/management concerns.
• Local network information is kept internal
• Agreements with specific service providers at boundaries
Host
Host
Host
Host
Host
Host
Final Review: 12/10/2001
27
Browsing the Web
7 Web Browser
Web Server
6 Presentation
Presentation
5
Session
Session
4
Transport
Transport
3
Network
2
Data link
Data link
1
Physical
Physical
?
Network
Network
Data link
Data link
Physical
Physical
Client
Server
Final Review: 12/10/2001
28
Datalink in the Backbone
• Routers are connected by point-point links or by
(datalink layer) switched clouds.
• Point-point links typically based on SONET.
• E.g. Packets over SONET
• Switched clouds often uses virtual connection
datalink technologies.
• E.g., ATM, frame relay
Router
Switched Cloud
PCs at
Work
Point-Point
link
PC at
Home
Final Review: 12/10/2001
29
802.3 Ethernet
Broadcast technology
host
host
host
host
host
host
host
host
Hub
• Carrier-sense multiple access with collision
detection (CSMA/CD).
• 10Mbps cable rate.
• Maximum diameter 2.5km.
• Minimum frame = 64 bytes.
• Thick or thin coax; 10Base-T unshielded twisted
pair in star configuration using hub.
Final Review: 12/10/2001
30
Ethernet Switches
• Bridges make it possible to increase LAN
capacity.
• Packets are no longer broadcasted - they are
only forwarded on selected links
• Adds a switching flavor to the broadcast LAN
• Ethernet switch is a special case of a bridge: each
bridge port is connected to a single host.
• Simplifies the protocol and hardware used
(only two stations on the link)
• Can make the link full duplex (really simple
protocol!)
• Can have different port speeds
Final Review: 12/10/2001
31
Framing
• A link layer function, defining which bits have
which function.
• Minimal functionality: mark off units of
transmission.
• Some techniques:
•
•
•
•
frame delimiter characters with character stuffing
frame delimiter codes with bit stuffing
out of band delimiters (e.g. FDDI control symbols)
synchronous transmission (e.g. SONET)
Final Review: 12/10/2001
32
Browsing the Web
7 Web Browser
Web Server
6 Presentation
Presentation
5
Session
Session
4
Transport
Transport
3
Network
2
Data link
1
Physical
Network
Network
Data link
Data link
Data link
Physical
Physical
Physical
?
Client
Server
Final Review: 12/10/2001
33
The Frequency Domain
• A (periodic) signal can be viewed as a sum of
sine waves of different strengths.
• Every signal has an equivalent representation in
the frequency domain.
Amplitude
• What frequencies are present and what is their
strength
• Similar to radio and TV signals
Time
Frequency
Final Review: 12/10/2001
34
Wireless: Good News Bad News
• Great technology: no wires to install, convenient
mobility, ..
• High attenuation limits distances.
• Wave propagates out as a sphere
• Signal strength reduces quickly (1/distance)2
• High noise due to interference from other
transmitters.
• Use MAC and other rules to limit interference
• Aggressive encoding techniques to make signal
less sensitive to noise
• Other effects: multipath fading, security, ..
• Ether has limited bandwidth.
• Try to maximize its use
Final Review: 12/10/2001
35
TCP Problems Over Noisy Links
• Wireless links are inherently error-prone
• Fades, interference, attenuation
• Errors often happen in bursts
• TCP cannot distinguish between corruption and
congestion
• TCP unnecessarily reduces window, resulting in low
throughput and high latency
• Burst losses often result in timeouts
• Sender retransmission is the only option
• Inefficient use of bandwidth
Final Review: 12/10/2001
36
Proposed Solutions
• End-to-end protocols
• Selective ACKs, Explicit loss notification
• Split-connection protocols
• Separate connections for wired path and
wireless hop
• Reliable link-layer protocols
• Error-correcting codes
• Local retransmission
Final Review: 12/10/2001
37
Browsing the Web
7 Web Browser
Web Server
6 Presentation
Presentation
5
Session
Session
4
Transport
Transport
3
Network
2
Data link
1
Physical
Client
Network
Network
Data link
Data link
Data link
Physical
Physical
Physical
Everything Cleared Up!
Final Review: 12/10/2001
Server
38
Security Threats
• Impersonation.
• Pretend to be another user with the intent of getting
access to information or services
• Secrecy.
• Get access to the contents of packets
• Message integrity.
• Change a message unbeknownst to the sender or
receiver
• Repudiation
• Denying to have sent a message
• Denial of service.
• Flooding the system so users with legitimate needs
cannot get service
• Range of other threats: password guessing, exploiting
programming bugs, …
Final Review: 12/10/2001
39
Encryption
•
•
•
•
ciphertext = E(plaintext, k)
plaintext = D(ciphertext, k’)
Private key (symmetric, e.g. DES)
• the two parties share a common private key k
Public key (asymmetric, e.g. RSA)
• derive two keys, kprivate and kpublic
• kprivate is kept private by its owner
• kpublic is published
Tradeoffs between private and public key
cryptography.
• Key management, speed
Challenge: key management.
Final Review: 12/10/2001
40
Example Applications
• Kerberos.
• Support security in corporate environment
• Based on key distribution center that knows all the
entities
• Know = share secret
• Secure socket layer (SSH).
• Support secure channels in open internet environment
• Based on certificates and certification authorities
• Provides privacy, but trust is limited
• Pretty good privacy (PGP).
• Provides privacy, authentication, repudiation in internet
environment
• Key management based on a “web of trust”
Final Review: 12/10/2001
41
How to Provide QoS?
• Admission control limits number of users.
• You cannot provide guarantees if there are too many
users sharing the same set of resources (bandwidth)
• For example, telephone networks - busy tone
• This implies that your request for service can be rejected
• Traffic enforcement limits how much traffic users can inject
based on predefined limits.
• Make sure user respects the traffic contract
• Data outside of contract can be dropped (before entering
the network!) or can be sent at a lower priority
• Scheduling support in the routers guarantee that users get
their share of the bandwidth.
• Again based on pre-negotiated bounds
• Signaling protocol gives routers the information they need to
provide QoS.
• E.g. RSVP
Final Review: 12/10/2001
42
Qos Summary
Final Review: 12/10/2001
43
IETF QoS Models
• Integrated services: diverse QoS at the micro-flow level.
• Range of QoS: best effort, controlled load, guaranteed
• Specific end-to-end service defined for each class
• Requires end-to-end support, e.g. edge and core routers
• Concern about complexity, cost, marketing/charging
• Differentiated services: QoS at the aggregate flow level.
• Defines range of “forwarding behaviors”, but services
are defined by the providers
• Pushes most complexity to the edge of the network –
fast core routers work only with small number of traffic
classes
• Based on the same building blocks.
Final Review: 12/10/2001
44
Multimedia Challenges
• TCP/UDP/IP suite provides best-effort, no
guarantees on expectation or variance of packet
delay
• Streaming applications delay of 5 to 10 seconds is
typical and has been acceptable, but performance
deteriorate if links are congested (transoceanic)
• Real-Time Interactive requirements on delay and
its jitter have been satisfied by over-provisioning
(providing plenty of bandwidth), what will happen
when the load increases?...
Final Review: 12/10/2001
45
Multicast – Efficient Data
Distribution
Src
Src
Final Review: 12/10/2001
46
IP Multicast Architecture
Service model
Hosts
Host-to-router protocol
(IGMP)
Routers
Multicast routing protocols
(various)
Final Review: 12/10/2001
47
Multicast Routing
• Basic objective – build distribution tree for
multicast packets
• Core based protocols
• Examples: CBT, PIM-SM
• Flood and prune
• Examples: DVMRP, PIM-DM
• Link-state multicast protocols
• Example: MOSPF
Final Review: 12/10/2001
48
Shared vs. Source-based Trees
• Source-based trees
• Separate shortest path tree for each sender
• DVMRP, MOSPF, PIM-DM, PIM-SM
• Shared trees
• Single tree shared by all members
• Data flows on same tree regardless of sender
• CBT, PIM-SM
Final Review: 12/10/2001
49
Questions?
Final Review: 12/10/2001
50