Introduction to Networking

1
Introduction to Networking
2
Internet Example

• Click -gt get page
• Specifies - protocol (http) - location
• (www.cnn.com)

3
Internet Locating Resource

• www.cnn.com
• name of a computer
• Implicitly also a file
• Map name to IP address
• DNS

cnn.com?
cnn.com?
host
local
com
a.b.c.d
a.b.c.d
4
Internet Connection

• Http sets up a connection (tcp)
• between the host and cnn.com to transfer the page
• The connection transfers page as a byte stream
• without errors flow control error control

Host
www.cnn.com
Connect
OK
Get page
Page close
5
Internet End-to-end

• Byte stream flows end to end across many
  links/switches
• routing ( addressing)
• That stream is regulated and controlled by both
  ends
• retransmission of erroneous or missing bytes
  flow control

6
Internet Packets

• The network transports bytes grouped into packets
• Packets are self-contained routers handle them
  1 by 1
• The end hosts worry about errors and pacing
• Destination sends ACKs Source checks losses

7
Internet Bits

• Equipment in each node sends packets as string of
  bits
• That equipment is not aware of the meaning of the
  bits
• Frames (packetizing) vs. streams

8
Internet Points to remember

• Separation of tasks
• send bits on a link transmitter/receiver clock,
  modulation,
• send packet on each hop framing, error
  detection,
• send packet end to end addressing, routing
• pace transmissions detect congestion
• retransmit erroneous or missing packets acks,
  timeout
• find destination address from name DNS
• Scalability
• routers dont know full path
• names and addresses are hierarchical

9
Internet Challenges

• Addressing ?
• Routing ?
• Reliable transmission ?
• Interoperability ?
• Resource management ?
• Quality of service ?

10
Concepts at heart of the Internet

• Protocol
• Layered Architecture
• Packet Switching
• Distributed Control
• Open System

11
Protocol

• Two communicating entities must agree on
• Expected order and meaning of messages they
  exchange
• The action to perform on sending/receiving a
  message
• Asking the time

12
Layered Architectures

• Human beings can handle lots of complexity in
  their protocol processing.
• Ambiguously defined protocols
• Many protocols all at once
• How computers manage complex protocol processing?
• Specify well defined protocols to enact.
• Decompose complicated jobs into layers
• each has a well defined task

13
Layered Architectures

• Break-up design problem into smaller problems
• More manageable
• Modular design easy to extend/modify.
• Difficult to implement
• careful with interaction of layers for efficiency

14
Layered Architecture
users
network
Applications
Web, e-mail, file transfer, ...
Reliable/ordered transmission, QOS, security,
compression, ...
Middleware
End-to-end transmission, resource allocation,
routing, ...
Routing
Point-to-point links, LANs, radios, ...
Physical Links
15
The OSI Model

• Open Systems Interconnect model is a standard way
  of understanding conceptual layers of network
  comm.
• This is a model, nobody builds systems like this.
• Each level provides certain functions and
  guarantees, and communicates with the same level
  on remote notes.
• A message is generated at the highest level, and
  is passed down the levels, encapsulated by lower
  levels, until it is sent over the wire.
• On the destination, it makes its way up the
  layers,until the high-level msg reaches its
  high-level destination.

16
OSI Levels
Node A
Application
Node B
Application
Presentation
Presentation
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
Network
17
OSI Levels

• Physical Layer electrical details of bits on the
  wire
• Data Link sending frames of bits and error
  detection
• Network Layer routing packets to the
  destination
• Transport Layer reliable transmission of
  messages, disassembly/assembly, ordering,
  retransmission of lost packets
• Session Layer really part of transport, typ. Not
  impl.
• Presentation Layer data representation in the
  message
• Application high-level protocols (mail, ftp,
  etc.)

18
Internet protocol stack
users
network
Application
HTTP, SMTP, FTP, TELNET, DNS,
Transport
TCP, UDP.
Network
IP
Point-to-point links, LANs, radios, ...
Physical
19
Air travel
Passenger Origin
Passenger Destination
Ticket (purchase)
Ticket (complain)
Baggage (check)
Baggage (claim)
Gates (load)
Gates (unload)
Runway (take off)
Runway (landing)
Airplane routing
20
Protocol stack
user X
user Y
English
e-mail client
e-mail server
SMTP
TCP server
TCP server
TCP
IP server
IP
IP server
IEEE 802.3 standard
ethernet driver/card
ethernet driver/card
electric signals
21
Protocol interfaces
user X
user Y
e-mail client
e-mail server
TCP server
TCP server
s open_socket() socket_write(s, buffer)
IP server
IP server
ethernet driver/card
ethernet driver/card
22
Addressing

• Each network interface has a hardware address
• Multiple interfaces ? multiple addresses
• Each application communicates via a port
• Port is a logical connection endpoint
• Allows multiple local applications to use network
  resources
• Up to 65535
• lt 1024 used by privileged applications
• 1024 available for use 49151
• 49152 Dynamic ports/private ports 65535
• http ports 80 and 8080
• telnet 23, ftp 21, etc
• Think of a telephone network

23
Addressing and Packet Format

• The Data'' segment contains higher level
  protocol information.
• Which protocol is this packet destined for?
• Which process is the packet destined for?
• Which packet is this in a sequence of packets?
• What kind of packet is this?
• This is the stuff of the OSI reference model.

Start (7 bytes)
Destination (6)
Source (6)
Length (2)
Msg Data (1500)
Checksum (4)
24
Ethernet packet dispatching

• An incoming packet comes into the Ethernet
  controller.
• The Ethernet controller reads it off the network
  into a buffer.
• It interrupts the CPU.
• A network interrupt handler reads the packet out
  of the controller into memory.
• A dispatch routine looks at the Data part and
  hands it to a higher level protocol
• The higher level protocol copies it out into user
  space.
• A program manipulates the data.
• The output path is similar.
• Consider what happens when you send mail.

25
Example Mail
Hi Dad.
Hi Dad.
Mail Composition And Display
SrcAddr 128.95.1.2 DestAddr 128.95.1.3 SrcPort
110, DestPort 110Bytes 1-20
SrcAddr 128.95.1.2 DestAddr 128.95.1.3 SrcPort
110, DestPort 110Bytes 1-20
Mail Transport Layer
User
Kernel
Network Transport Layer
SrcEther 0xdeadbeef DestEther 0xfeedface
SrcEther 0xdeadbeef DestEther 0xfeedface
Link Layer
SrcAddr 128.95.1.2 DestAddr 128.95.1.3 SrcPort
100 DestPort 200Bytes 1-20
SrcAddr 128.95.1.2 DestAddr 128.95.1.3 SrcPort
100 DestPort 200Bytes 1-20
Network
26
Protocol encapsulation
user X
user Y
Hello
e-mail client
e-mail server
Hello
TCP server
TCP server
Hello
IP server
IP server
Hello
ethernet driver/card
ethernet driver/card
Hello
27
End-to-End Argument

• What function to implement in each layer?
• Saltzer, Reed, Clarke 1984
• A function can be correctly and completely
  implemented only with the knowledge and help of
  applications standing at the communication
  endpoints
• Argues for moving function upward in a layered
  architecture
• Should the network guarantee packet delivery ?
• Think about a file transfer program
• Read file from disk, send it, the receiver reads
  packets and writes them to the disk

28
End-to-End Argument

• If the network guaranteed packet delivery
• one might think that the applications would be
  simpler
• No need to worry about retransmits
• But need to check that file was written to the
  remote disk intact
• A check is necessary if nodes can fail
• Consequently, applications need to perform their
  retransmits
• No need to burden the internals of the network
  with properties that can, and must, be
  implemented at the periphery

29
End-to-End Argument

• An Occams razor for Internet design
• If there is a problem, the simplest explanation
  is probably the correct one
• Application-specific properties are best provided
  by the applications, not the network
• Guaranteed, or ordered, packet delivery,
  duplicate suppression, security, etc.
• The internet performs the simplest packet routing
  and delivery service it can
• Packets are sent on a best-effort basis
• Higher-level applications do the rest

30
Two ways to handle networking

• Circuit Switching
• What you get when you make a phone call
• Dedicated circuit per call
• Packet Switching
• What you get when you send a bunch of letters
• Network bandwidth consumed only when sending
• Packets are routed independently

31
Circuit Switching

• End-to-end resources reserved for call
• Link bandwidth, switch capacity
• Dedicated resources no sharing
• Circuit-like (guaranteed) performance
• Call setup required

32
Packet Switching

• Each end-to-end data stream divided into packets
• Users packets share network resources
• Compared to dedicated allocation
• Each packet uses full link bandwidth
• Compared to dividing bandwidth into pieces
• Resources are used as needed
• Compared to resource reservation
• Resource contention
• Aggregate demand can exceed amount available
• Congestion packets queue, wait for link use
• Store and forward packets move one hop at a time
• Transmit over link
• Wait turn at next link

33
Routing

• Goal move data among routers from source to
  dest.
• Datagram packet network
• Destination address determines next hop
• Routes may change during session
• Analogy driving, asking directions
• No notion of call state
• Circuit-switched network
• Call allocated time slots of bandwidth at each
  link
• Fixed path (for call) determined at call setup
• Switches maintain lots of per call state
  resource allocation

34
Packet vs. Circuit Switching

• Reliability no congestion, in-order data in
  circuit-switch
• Packet switching better bandwidth use
• State, resources packet switching has less state
• Good less control plane processing resources
  along the way
• More data plane (address lookup) processing
• Failure modes (routers/links down)
• Packet switch reconfigures sub-second timescale
• Circuit switching more complicated
• Involves all switches in the path

35
A small Internet
W
b,e4
w,e5
B
V
Scenario A wants to send data to B.
R
r3
r2,e2
r1,e1
a,e3
A
36
Packet forwarding
Host A
Host B
Router R
Router W
HTTP
HTTP
TCP
TCP
IP
IP
IP
IP
eth
link
eth
link
ethernet
ethernet
37
The Link Layer
38
What is purpose of this layer?

• Physically encode bits on the wire
• Link pipe to send information
• E.g. point to point or broadcast
• Can be built out of
• Twisted pair, coaxial cable, optical fiber, radio
  waves, etc
• Links should only be able to send data
• Could corrupt, lose, reorder, duplicate, (fail in
  other ways)

39
How to connect routers/machines?

• WAN/Router Connections
• Commercial
• T1 (1.5 Mbps), T3 (44 Mbps)
• OC1 (51 Mbps), OC3 (155 Mbps)
• ISDN (64 Kbps)
• Frame Relay (1-100 Mbps, usually 1.5 Mbps)
• ATM (some Gbps)
• To your home
• DSL
• Cable
• Local Area
• Ethernet IEEE 802.3 (10 Mbps, 100 Mbps, 1 Gbps,
  10Gbps)
• Wireless IEEE 802.11 b/g/a (11 Mbps, 22 Mbps, 54
  Mbps)

40
Link level Issues

• Encoding map bits to analog signals
• Framing Group bits into frames (packets)
• Arbitration multiple senders, one resource
• Addressing multiple receivers, one wire

41
Encoding

• Map 1s and 0s to electric signals
• Simple scheme Non-Return to Zero (NRZ)
• 0 low voltage, 1 high voltage
• Problems
• How to tell an error? When jammed? When is bus
  idle?
• When to sample? Clock recovery is difficult.
• Idea Recover clock using encoding transitions

1 0 1 1
0
42
Manchester Encoding

• Used by Ethernet
• Idea Map 0 to low-to-high transition, 1 to
  high-to-low
• Plusses can detect dead-link, can recover clock
• Bad reduce bandwidth, i.e. bit rate ½ baud
  rate
• If wire can do X transition per second?

43
Framing

• Why send packets?
• Error control
• How do you know when to stop reading?
• Sentinel approach send start and end sequence
• For example, if sentinel is 11111
• 11111 00101001111100 11111 10101001 11111 010011
  11111
• What if sentinel appears in the data?
• map sentinel to something else, receiver maps it
  back
• Bit stuffing

44
Example HDLC

• Same sentinel for begin and end 0111 1110
• packet format
• Bit stuffing
• Sender If 5 1s then insert a 0
• Receiver if 5 1s followed by a 0, remove 0
• Else read next bit
• Packet size now depends on the contents

0111 1110 header data CRC
0111 1110
0111 1110 0111 1101 0
0111 1101 0 0111 1110
45
Arbitration

• One medium, multiple senders
• What did we do for CPU, memory, readers/writers?
• New Problem No centralized control
• Approaches
• TDMA Time Division Multiple Access
• Divide time into slots, round robin among senders
• If you exceed the capacity ? do not admit more
  (busy signal)
• FDMA Frequency Division Multiple Access (AMPS)
• Divide spectrum into channels, give each sender a
  channel
• If no more channels available, give a busy signal
• Good for continuous streams fixed delay,
  constant data rate
• Bad for bursty Internet traffic idle slots

46
Ethernet

• Developed in 1976, Metcalfe and Boggs at Xerox
• Uses CSMA/CD
• Carrier Sense Multiple Access with Collision
  Detection
• Easy way to connect LANs

Metcalfes Ethernet sketch
47
CSMA/CD

• Carrier Sense
• Listen before you speak
• Multiple Access
• Multiple hosts can access the network
• Collision Detection
• Can make out if someone else started speaking

Older Ethernet Frame
48
CSMA
Wait until carrier free
49
CSMA/CA
Garbled signals
If the sender detects a collision, it will stop
and then retry! What is the problem?
50
CSMA/CD
Packet?
Sense Carrier
Detect Collision
Send
Discard Packet
Jam channel bCalcBackoff() wait(b) attempts
51
Ethernets CSMA/CD (more)

• Jam Signal make sure all other transmitters are
  aware of collision 48 bits
• Exponential Backoff
• Goal adapt retransmission attempts to estimated
  current load
• heavy load random wait will be longer
• first collision choose K from 0,1 delay is K
  x 512 bit transmission times
• after second collision choose K from 0,1,2,3
• after ten or more collisions, choose K from
  0,1,2,3,4,,1023

52
Packet Size

• If packets are too small, the collision goes
  unnoticed
• Limit packet size
• Limit network diameter
• Use CRC to check frame integrity
• truncated packets are filtered out

53
Ethernet Problems

• What if there is a malicious user?
• Might not use exponential backoff
• Might listen promiscuously to packets
• Integrating Fast and Gigabit Ethernet

54
Addressing ARP
128.84.96.89
128.84.96.90
128.84.96.91
What is the physical address of the host named
128.84.96.89
Im at 1a342c9adecc

• ARP is used to discover physical addresses
• ARP Address Resolution Protocol

55
Addressing RARP
???
128.84.96.90 RARP Server
128.84.96.91
I just got here. My physical address is
1a342c9adecc. Whats my name ?
Your name is 128.84.96.89

• RARP is used to discover virtual addresses
• RARP Reverse Address Resolution Protocol

56
Repeaters and Bridges

• Both connect LAN segments
• Usually do not originate data
• Repeaters (Hubs) physical layer devices
• forward packets on all LAN segments
• Useful for increasing range
• Increases contention
• Bridges link layer devices
• Forward packets only if meant on that segment
• Isolates congestion
• More expensive

57
Backbone Bridge
58
The Network Layer
59
Purpose of Network layer

• Given a packet, send it across the network to
  destination
• 2 key issues
• Portability
• connect different technologies
• Scalability
• To the Internet scale

60
What does it involve?

• Two important functions
• routing determine path from source to dest.
• forwarding move packets from routers input to
  output

T3
T1 T3
Sts-1
T1
61
Network service model

• Q What service model for channel transporting
  packets from sender to receiver?
• guaranteed bandwidth?
• preservation of inter-packet timing (no jitter)?
• loss-free delivery?
• in-order delivery?
• congestion feedback to sender?

The most important abstraction provided by
network layer
?
?
virtual circuit or datagram?
?
service abstraction
Which things can be faked at the transport
layer?
62
Two connection models

• Connectionless (or datagram)
• each packet contains enough information that
  routers can decide how to get it to its final
  destination
• Connection-oriented (or virtual circuit)
• first set up a connection between two nodes
• label it (called a virtual circuit identifier
  (VCI))
• all packets carry label

1
A
63
Virtual circuits signaling protocols

• used to setup, maintain teardown VC
• setup gives opportunity to reserve resources
• used in ATM, frame-relay, X.25
• not used in todays Internet

6. Receive data
5. Data flow begins
4. Call connected
3. Accept call
1. Initiate call
2. incoming call
64
Virtual circuit switching

• Forming a circuit
• send a connection request from A to B. Contains
  VCI address of B
• rule VCI must be unique on the link its used on
• switch creates an entry mapping input messages
  with VCI to output port
• switch picks a new VCI unique between it and next
  switch

65
Virtual circuit forwarding

• For each VCI switch has a table which maps input
  link to output link and gives the new VCI to use
• if as messages come into switch 1 on link 2 and
  go out on link 3 then the table will be

(Input link,VCI) (output link, new VCI) (1,
2) (?, ?) (1, 5) (?, ?)
Switch 1
2
Switch 2
1
5
2
1
Switch 3
2
1
66
Virtual Circuits Discussion

• Plusses easy to associate resources with VC
• Easy to provide QoS guarantees (bandwidth, delay)
• Very little state in packet
• Minuses
• Not good in case of crashes
• Requires explicit connect and teardown phases
• What if teardown does not get to all routers?
• What if one switch crashes?
• Will have to teardown and rebuild route

67
Datagram networks

• no call setup at network layer
• routers no state about end-to-end connections
• no network-level concept of connection
• packets typically routed using destination host
  ID
• packets between same source-dest pair may take
  different paths
• Best effort data corruption, packet drops, route
  loops

1. Send data
2. Receive data
68
Datagrams Forwarding

• How does packet get to the destination?
• switch creates a forwarding table, mapping
  destinations to output port (ignores input ports)
• when a packet with a destination address in the
  table arrives, it pushes it out on the
  appropriate output port
• when a packet with a destination address not in
  the table arrives, it must find out more routing
  information (next problem)

69
Datagrams

• Plusses
• No round trip connection setup time
• No explicit route teardown
• No resource reservation ? each flow could get max
  bandwidth
• Easily handles switch failures routes around it
• Minuses
• Difficult to provide resource guarantees
• Higher per packet overhead
• Internet uses datagrams IP (Internet Protocol)

70
Datagrams Forwarding

• How to build forwarding tables?
• Manually enter it
• What if nodes crashed
• What about scale?
• The graph-theoretic routing problem
• Given a graph, with vertices (switches), edges
  (links), and edge costs (cost of sending on that
  link)
• Find the least cost path between any two nodes
• Path cost ? (cost of edges in path)

71
Simple Routing Algorithm

• Choose a central node
• All nodes send their (nbr, cost) information to
  this node
• Central node uses info to learn entire topology
  of the network
• It then computes shortest paths between all pairs
  of nodes
• Using All Pair Shortest Path Algorithm
• Sends the new matrix to every node
• Nice, simple, elegant!
• What is the problem?
• Scalability centralization hurts scalability
• Central node is crushed with traffic

72
Link State Routing

• Basic idea
• Every node propagates its (nbr, cost) information
• This information at all nodes is enough to
  construct topology
• Can use a graph algorithm to find the shortest
  routes
• Mechanisms required
• Reliable flooding of link information
• Method to calculate shortest route (Dijkstras
  algorithm)
• Example link state update packet
• node id, (nbr, cost) list, seq. no., ttl
• Seq. no. to identify latest updates, ttl
  specifies when to stop msg.

73
Reliable flooding

• receive(pkt)
• If already have a copy of LSP from pkt.ID
• if pkts sequence number lt copys
• discard pkt
• else
• decrement pkt.TTL
• replace copy with pkt
• forward pkt to all links besides the
• one that we received it on
• done every 10 minutes or so
• gen_LSP()
• increment nodes sequence by one
• recompute cost vector
• send created LSP to all neighbors

74
Discussion Link-State Routing

• Plusses
• Simple, determines the optimal route most of the
  time
• Used by OSPF
• Minuses
• Might have oscillations
• Avoid using load as cost metric, reduce herding
  effect

1
1e
0
2e
0
0
0
0
e
0
1
1e
1
1
e
recompute
recompute Least loaded gt Most loaded
Initially start with almost equal routes
everyone goes with least loaded
75
Is our routing algorithm scalable?

• Route table size grows with size of network
• Because our address structure is flat!
• Solution have a hierarchical structure
• Used by OSPF
• Divide the network into areas, each area has
  unique number
• Nodes carry their area number in the address 1.A,
  2.B, etc.
• Nodes know complete topology in their area
• Area border routers (ABR) know how to get to any
  other area

76
Hierarchical Addressing
Zone 2
0
1
S1
1
0
2
S2
2
3
1
0
2
Zone 3
77
IP has 2-layer addressing

• Each IP address is 32 bits
• Network part which network the host is on?
• Host part identifies the host.
• All hosts on same network have the same network
  part
• 3 classes of addresses A, B and C

18.26.0.1
host
network
32-bits
1 0 net host
110 net host
2 14 16 bits
3 21 8 bits
78
IP addressing

• The different classes
• Problems inefficient, address space exhaustion

class
1.0.0.0 to 127.255.255.255
A
network
0
host
128.0.0.0 to 191.255.255.255
Unicast
B
192.0.0.0 to 223.255.255.255
C
224.0.0.0 to 239.255.255.255
D
Multicast
240.0.0.0 to 255.255.255.255
reserved
E
Reserved
1111
79
IP addressing CIDR

• Classless InterDomain Routing
• network portion of address of arbitrary length
• address format a.b.c.d/x, where x is bits in
  network portion
• Examples
• Class A /8
• Class B /16
• Class C /24

80
Internet Protocol Datagram
IP protocol version Number
32 bits
total datagram length (bytes)
type of service
head. len
header length
ver
length
for fragmentation/ reassembly
fragment offset
type of data
flgs
16-bit identifier
max number remaining hops (decremented at each
router)
upper layer
time to live
Internet checksum
32 bit source IP address
32 bit destination IP address
upper layer protocol to deliver payload to
E.g. timestamp, record route taken, pecify list
of routers to visit.
Options (if any)
data (variable length, typically a TCP or UDP
segment)
81
Datagram Portability

• IP Goal To create one logical network from
  multiple physical networks
• All intermediate routers should understand IP
• IP header information sufficient to carry the
  packet to destination
• Goal Run over anything!
• Problem
• Physical networks have different MTUs
• max. transmission unit 1500 for Ethernet, 48
  for ATM
• Solution 1
• Fit everything in the MTU (!)

82
IP Fragmentation Reassembly

• Solution 2 (the one used)
• If packet size gt MTU of network, then fragment
  into pieces
• Each fragment is less than MTU size
• Each has IP headers frag bit set frag id
  offset
• Packets may get refragmented on the way to
  destination
• Reassembly only done at the destination
• What is a good initial packet size?

reassembly
fragmentation in one large datagram out 3
smaller datagrams
83
Internet Names and Addresses
84
Naming in the Internet

• What are named? All Internet Resources.
• Objects www.cs.cornell.edu/einar
• Services weather.yahoo.com/forecast
• Hosts planetlab1.cs.cornell.edu
• Characteristics of Internet Names
• human recognizable
• unique
• Persistent?
• Universal Resource Names (URNs)

85
Locating the resources

• Internet services and resources are provided by
  end-hosts
• ex. www1.cs.cornell.edu and www2.cs.cornell.edu
  host Einars home page.
• Names are mapped to Locations
• Universal Resource Locators (URL)
• Embedded in the name itself ex.
  weather.yahoo.com/forecast
• Semantics of Internet naming
• human recognizable
• uniqueness
• persistent

86
Locating the Hosts?

• Internet Protocol Addresses (IP Addresses)
• ex. planetlab1.cs.cornell.edu ? 128.84.154.49
• Characteristics of IP Addresses
• 32 bit fixed-length
• enables network routers to efficiently handle
  packets in the Internet
• Locating services on hosts
• port numbers (16 bit unsigned integer) 65536
  ports
• standard ports HTTP 80, FTP 20, SSH 22, Telnet 20

87
Mapping Not 1 to 1

• One host may map to more than one name
• One server machine may be the web server
  (www.foo.com), mail server (mail.foo.com)etc.
• One host may have more than one IP address
• IP addresses are per network interface
• But IP addresses are generally unique!
• two globally visible machines should not have the
  same IP address
• Anycast is an Exception
• routers send packets dynamically to the closest
  host matching an anycast address

88
How to get a name?

• Naming in Internet is Hierarchical
• decreases centralization
• improves name space management
• First, get a domain name then you are free to
  assign sub names in that domain
• How to get a domain name coming up
• Example weather.yahoo.com belongs to yahoo.com
  which belongs to .com
• regulated by global non-profit bodies

89
Domain name structure
root (unnamed)
...
...
com
mil
gov
edu
gr
org
net
fr
uk
us
ccTLDs
gTLDs
cornell
ustreas
second level (sub-)domains
lucent
gTLDs Generic Top Level Domains ccTLDs
Country Code Top Level Domains
90
Top-level Domains (TLDs)

• Generic Top Level Domains (gTLDs)
• .com - commercial organizations
• .org - not-for-profit organizations
• .edu - educational organizations
• .mil - military organizations
• .gov - governmental organizations
• .net - network service providers
• New .biz, .info, .name, .xxx (nearly..)
• Country code Top Level Domains (ccTLDs)
• One for each country

91
How to get a domain name?

• In 1998, non-profit corporation, Internet
  Corporation for Assigned Names and Numbers
  (ICANN), was formed to assume responsibility from
  the US Government
• ICANN authorizes other companies to register
  domains in com, org and net and new gTLDs
• Network Solutions is largest and in transitional
  period between US Govt and ICANN had sole
  authority to register domains in com, org and net

92
ICANN and politics..

• Why should a US company control Internet naming?
• Should companies (from whatever country) be able
  to profit from internet names?
• 28th Aug 2006 ICANN to allow domain registries
  to charge what ?the market will bear for domain
  names renewals

93
How to get an IP Address?

• Answer 1 Normally, answer is get an IP address
  from your upstream provider
• This is essential to maintain efficient routing!
• Answer 2 If you need lots of IP addresses then
  you can acquire your own block of them.
• IP address space is a scarce resource - must
  prove you have fully utilized a small block
  before can ask for a larger one and pay (Jan
  2002 - 2250/year for /20 and 18000/year for a
  /14)

94
How to get lots of IP Addresses? Internet
Registries

• RIPE NCC (Riseaux IP Europiens Network
  Coordination Centre) for Europe, Middle-East,
  Africa
• APNIC (Asia Pacific Network Information Centre
  )for Asia and Pacific
• ARIN (American Registry for Internet Numbers) for
  the Americas, the Caribbean, sub-saharan Africa
• Note Once again regional distribution is
  important for efficient routing!
• Can also get Autonomous System Numbers (ASNs from
  these registries

95
Are there enough addresses?

• Unfortunately No!
• 32 bits ? 4 billion unique addresses
• but addresses are assigned in chunks
• ex. cornell has four chunks of /16 addressed
• ex. 128.84.0.0 to 128.84.255.255
• 128.253.0.0, 128.84.0.0, 132.236.0.0, and
  140.251.0.0
• Expanding the address space!
• IPv6 128 bit addresses
• difficult to deploy (requires cooperation and
  changes to the core of the Internet)

96
DHCP and NATs

• Dynamic Host Control Protocol
• lease IP addresses for short time intervals
• hosts may refresh addresses periodically
• only live hosts need valid IP addresses
• Network Address Translators
• Hide local IP addresses from rest of the world
• only a small number of IP addresses are visible
  outside
• solves address shortage for all practical
  purposes
• access is highly restricted
• ex. peer-to-peer communication is difficult

97
NATs in operation

• Translate addresses when packets traverse through
  NATs
• Use port numbers to increase number of
  supportable flows

98
DNS Domain Name System

• Domain Name System
• distributed database implemented in hierarchy of
  many name servers
• application-layer protocol host, routers, name
  servers to communicate to resolve names
  (address/name translation)
• Note how a core Internet function is implemented
  as application-layer protocol
• complexity at networks edge

99
DNS name servers

• Name server process running on a host that
  processes DNS requests
• local name servers
• each ISP, company has local (default) name server
• host DNS query first goes to local name server
• authoritative name server
• can perform name/address translation for a
  specific domain or zone

• How could we provide this service? Why not
  centralize DNS?
• single point of failure
• traffic volume
• distant centralized database
• maintenance
• doesnt scale!
• no server has all name-to-IP address mappings

100
Name Server Zone Structure
root
com
mil
edu
gov
gr
org
net
fr
uk
us
Structure based on administrative issues.
lucent
ustreas
101
Name Servers (NS)
root
com
...
edu
gov
cornell
lucent
102
Name Servers (NS)

• NSs are duplicated for reliability.
• Each domain must have a primary and secondary.
• Each host knows the IP address of the local NS.
• Each NS knows the IP addresses of all root NSs.

103
DNS Root name servers

• contacted by local name server that can not
  resolve name
• root name server
• Knows the authoritative name server for main
  domain
• 60 root name servers worldwide
• real-world application of anycast

104
Simple DNS example
root name server

• host surf.eurecom.fr wants IP address of
  www.cs.cornell.edu
• 1. Contacts its local DNS server, dns.eurecom.fr
• 2. dns.eurecom.fr contacts root name server, if
  necessary
• 3. root name server contacts authoritative name
  server, dns.cornell.edu, if necessary (what might
  be wrong with this?)

2
4
3
5
authorititive name server dns.cornell.edu
1
6
requesting host surf.eurecom.fr
www.cs.cornell.edu
105
DNS example
root name server
.edu name server

• Root name server
• may not know authoritative name server
• may know intermediate name server who to contact
  to find authoritative name server

2
4
3
5
6
7
8
9
1
10
authoritative name server dns.cs.cornell.edu
requesting host surf.eurecom.fr
www.cs.cornell.edu
106
DNS Architecture

• Hierarchical Namespace Management
• domains and sub-domains
• distributed and localized authority
• Authoritative Nameservers
• server mappings for specific sub-domains
• more than one (at least two for failure
  resilience)
• Caching to mitigate load on root servers
• time-to-live (ttl) used to delete expired cached
  mappings

107
DNS query resolution
root name server

• iterated query
• contacted server replies with name of server to
  contact
• I dont know this name, but ask this server
• Takes burden off root servers
• recursive query
• puts burden of name resolution on contacted name
  server
• reduces latency

.edu name server
iterated query
2
4
3
recursive query
5
6
9
8
7
1
10
authoritative name server dns.cs.cornell.edu
requesting host surf.eurecom.fr
www.cs.cornell.edu
108
DNS records More than Name to IP Address

• DNS distributed db storing resource records (RR)

• TypeCNAME
• name is an alias name for some cannonical (the
  real) name
• value is cannonical name

• TypeA
• name is hostname
• value is IP address
• One weve been discussing most common

• TypeNS
• name is domain (e.g. foo.com)
• value is IP address of authoritative name server
  for this domain

• TypeMX
• value is hostname of mailserver associated with
  name

109
nslookup

• Use to query DNS servers (not telnet like with
  http why?)
• Examples
• nslookup www.yahoo.com
• nslookup www.yahoo.com dns.cs.cornell.edu
• specify which local nameserver to use
• nslookup typemx cs.cornell.edu
• specify record type

110
PTR Records

• Do reverse mapping from IP address to name
• Why is that hard? Which name server is
  responsible for that mapping? How do you find
  them?
• Answer special root domain, arpa, for reverse
  lookups

111
Arpa top level domain
Want to know machine name for 128.30.33.1? Issue
a PTR request for 1.33.30.128.in-addr.arpa
root
arpa
com
mil
edu
gov
gr
org
net
fr
uk
us
In-addr
ietf
www.ietf.org.
www
128
30
33
1
1.33.30.128.in-addr.arpa.
112
Why is it backwards?

• Notice that 1.30.33.128.in-addr.arpa is written
  in order of increasing scope of authority just
  like www.cs.foo.edu
• Edu largest scope of authority foo.edu less,
  down to single machine www.cs.foo.edu
• Arpa largest scope of authority in-addr.arpa
  less, down to single machine 1.30.33.128.in-addr.a
  rpa (or 128.33.30.1)

113
In-addr.arpa domain

• When an organization acquires a domain name, they
  receive authority over the corresponding part of
  the domain name space.
• When an organization acquires a block of IP
  address space, they receive authority over the
  corresponding part of the in-addr.arpa space.
• Example Acquire domain berkeley.edu and acquire
  a class B IP Network ID 128.143

114
DNS protocol, messages

• DNS protocol query and reply messages, both
  with same message format

• msg header
• identification 16 bit for query, reply to
  query uses same
• flags
• query or reply
• recursion desired
• recursion available
• reply is authoritative
• reply was truncated

115
DNS protocol, messages
Name, type fields for a query
RRs in reponse to query
records for authoritative servers
additional helpful info that may be used
116
The Transport Layer
117
Purpose of this layer

• Interface end-to-end applications and protocols
• Turn best-effort IP into a usable interface
• Data transfer b/w processes
• Compared to end-to-end IP
• We will look at 2
• TCP
• UDP

118
UDP

• Unreliable Datagram Protocol
• Best effort data delivery between processes
• No frills, bare bones transport protocol
• Packet may be lost, out of order
• Connectionless protocol
• No handshaking between sender and receiver
• Each UDP datagram handled independently

119
UDP Functionality

• Multiplexing/Demultiplexing
• Using ports
• Checksums (optional)
• Check for corruption

P3
P4
application-layer data
segment header
P1
P2
segment
H
t
M
segment
receiver
120
Multiplexing/Demultiplexing

• Multiplexing
• Gather data from multiple processes, envelope
  data with header
• Header has src port, dest port for multiplexing
• Why not process id?
• Demultiplexing
• Separate incoming data in machine to different
  applications
• Demux based on sender addr, src and dest port

32 bits
source port
dest port
Length, in bytes of UDP segment, including header
checksum
length
Application data (message)
UDP segment format
121
Implementing Ports

• As a message queue
• Append incoming message to the end
• Much like a mailbox file
• If queue full, message can be discarded
• When application reads from socket
• OS removes some bytes from the head of the queue
• If queue empty, application blocks waiting

122
UDP Checksum

• Over the headers and data
• Ensures integrity end-to-end
• 1s complement sum of segment contents
• Is optional in UDP
• If checksum is non-zero, and receiver computes
  another value
• Silently drop the packet, no error message
  detected

123
UDP Discussion

• Why UDP?
• No delay in connection establishment
• Simple no connection state
• Small header size
• No congestion control can blast packets
• Uses
• Streaming media, DNS, SNMP
• Could add application specific error recovery

124
TCP

• Transmission Control Protocol
• Reliable, in-order, process-to-process, two-way
  byte stream
• Different from UDP
• Connection-oriented
• Error recovery Packet loss, duplication,
  corruption, reordering
• A number of applications require this guarantee
• Web browsers use TCP

125
Handling Packet Loss
message
sender
receiver
time
There are a number of reasons why the packet may
get lost - router congestion, lossy medium,
etc. How does sender know of a successful packet
send?
126
Lost Acks
message
sender
receiver
timeout
ack
time
What if packet/ack is lost?
127
Delayed ACKs
message
sender
receiver
timeout
ack
time
message
What will happen here? Due to congestion, small
timeout, Delayed ACKs ? duplicate packets
128
Delayed ACKs
m1
sender
receiver
timeout
ack
time
m1
m2
timeout
ack
How to solve this scenario?
129
Insertion of Packets
m1
sender
receiver
ack1
m2
time
m2
ack2
m2 could be from an old expired session!
130
Message Identifiers

• Each message has ltmessage id, session idgt
• Message id uniquely identifies message in sender
• Session id unique across sessions
• Message ids detect duplication, reordering
• Session ids detect packet from old sessions
• TCPs sequence number has similar functionality
• Initial number chosen randomly
• Unique across packets
• Incremented by length of data bytes

131
TCP Packets
URG urgent data (generally not used)
counting by bytes of data (not segments!)
ACK ACK valid
PSH push data now (generally not used)
bytes rcvr willing to accept
RST, SYN, FIN connection estab (setup,
teardown commands)
Internet checksum (as in UDP)
132
TCP Connection Establishment
(open, seq x)
sender
receiver
(ack x, seq y)
(ack y)
TCP is connection-oriented. Starts with a 3-way
handshake. Protects against duplicate SYN packets.
133
TCP Usage
(open, seq x)
sender
receiver
(ack x, seq y)
(ack y)
Data
Data, ACK
Fin, ACK
Fin, ACK
134
TCP timeouts

• What is a good timeout period ?
• Want to improve throughput without unnecessary
  transmissions
• Timeout is thus a function of RTT and deviation

NewAverageRTT (1 - ?) OldAverageRTT ?
LatestRTT NewAverageDev (1 - ?) OldAverageDev
? LatestDev where LatestRTT (ack_receive_time
send_time), LatestDev LatestRTT
AverageRTT, ? 1/8,
typically. Timeout AverageRTT 4AverageDev
135
TCP Windows

• Multiple outstanding packets can increase
  throughput

136
TCP Windows

• Can have more than one packet in transit
• Especially over fat pipes, e.g. satellite
  connection
• Need to keep track of all packets within the
  window
• Need to adjust window size

DATA, id17
DATA, id18
DATA, id19
DATA, id20
ACK 17
ACK 18
ACK 19
ACK 20
137
TCP Congestion Control

• TCP increases its window size when no packets
  dropped
• It halves the window size when a packet drop
  occurs
• A packet drop is evident from the
  acknowledgements
• Therefore, it slowly builds to the max bandwidth,
  and hover around the max
• It doesnt achieve the max possible though
• Instead, it shares the bandwidth well with other
  TCP connections
• This linear-increase, exponential backoff in the
  face of congestion is termed TCP-friendliness

138
TCP Window Size

• Linear increase
• Exponential backoff
• Assuming no other losses in the network except
  those due to bandwidth

Max Bandwidth
Bandwidth
Time
139
TCP Fairness
A
D
Bottleneck Link
B

• Want to share the bottleneck link fairly between
  two flows

Bandwidth for Host A
Bandwidth for Host B
140
TCP Slow Start

• Linear increase takes a long time to build up a
  window size that matches the link bandwidthdelay
• Most file transactions are not long enough
• Consequently, TCP can spend a lot of time with
  small windows, never getting the chance to reach
  a sufficiently large window size
• Fix Allow TCP to build up to a large window size
  initially by doubling the window size until first
  loss

141
TCP Slow Start

• Initial phase of exponential increase
• Assuming no other losses in the network except
  those due to bandwidth

Max Bandwidth
Bandwidth
Time
142
TCP Summary

• Reliable ordered message delivery
• Connection oriented, 3-way handshake
• Transmission window for better throughput
• Timeouts based on link parameters
• Congestion control
• Linear increase, exponential backoff
• Fast adaptation
• Exponential increase in the initial phase