Transcript Chapter 1

Building The Mobile Internet
Internet Sessions
A Common Scenario ?
• You are cycling to University
• Talking on your mobile
• You pass many ‘cell-towers’ on the
journey
• Each time you move between cell-towers
you lose the call.
• You have to stop and make the call again
• You think..’This is a really cool service’..?
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A Common Scenario ??
• On long journeys you need to contact a
new cell-phone provider and sign up for a
temporary contract.
• When you are on an international train and
streaming a movie, it stops periodically
and you have to contact a new company
and arrange a new contract before
continuing with the movie.
• Then the movie starts over from the
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beginning…
Sessions
• I don’t think this would be widely accepted
• What users expect (even demand) is
seamless real-time mobility.
• This has become known as
‘Session-mobility’
• The Internet was not designed to enable
the implementation of this kind of
provision.
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The Internet and the MobileInternet
• Things get even more challenging when
moving through differing types of access
network. e.g. from Wi Fi to Cellular, or
between different operators.
• What we need is a ‘New Internet’
• A ‘Mobile-Internet’
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The Mobile-Internet
• The Mobile-Internet must be a Pervasive
IP-based network that can link fixed and
mobile nodes,
• Nodes, can have many characteristics and
types
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The Mobile Internet
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Nodes may be:Sensors;
Servers;
Stand-alone or Distributed;
Battery-powered or Mains Powered;
User initiated or Self Initiating
Permanent or Temporary
State-of-the-Art, or Legacy
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The Internet: Review of
Characteristics
• Lets revisit the Internet and see if we can
identify difficulties in making sessionmobility possible:
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Network
Routers
Networks
Network
Network
Network
Hosts
Network
Figure 2-1: The Internet: A Network of Networks
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Network
Original data
Packetized data
Re-assembled original data
Figure 2-2: Packet-switching: Dividing a data Source into Packets at the Sender,
and Reassembling the Data-Source at the Receiver
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Web
e-Mail
Telnet
IP
Ethernet
Figure 2-3:
DWDM
WiFi
IP Over Everything. Everything over IP
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Bit
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0
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Net-id
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16
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Host-id
Figure 2-4: IP Addresses consist of a net-id and a host-id part.
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10.0.0.0 / 8
10.1.0.0 / 16
10.1.0.0 / 24
10.2.0.0 / 16
10.3.0.0 / 16
10.1.1.0 / 24
10.1.2.0 / 28
10.1.2.0 / 24
10.1.2.128 / 28
Figure 2-3 CIDR Subnetting
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Bit
0
0
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Version
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IHL
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20 24
Type of Service
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Total Length
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Identification
Flags
Fragment Offset
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Time to Live
Protocol
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Header Checksum
IPv4 Source Address
128
IPv4 Destination Address
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Options
Padding
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Payload (variable length)
The IPv4 Packet Format
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Bit
0
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64
0
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Version
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Traffic Class
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Flow Label
Next Header
Payload Length
Hop Limit
IPv6 Source Address
128
192
IPv6 Destination Address
256
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Payload (variable length)
Figure 2-7
IPv6 Packet Format
Documented in RFC 2460
Application Layer
Application Layer
Transport Layer
Transport Layer
Internet Layer
Internet Layer
Data Link Layer
Data Link Layer
Higher
Layers
Lower
Layers
Physical Layer
Figure 2-8 The TCP/IP Five-Layer Model
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Data Link Layer
Internet Layer
Transport Layer
Application Layer
Application data
Figure 2-9 Data from Higher Layers Encapsulated Lower-layer Packets
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Bit
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0
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20 24
Source Port
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Destination Port
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Length
Checksum
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Payload (variable length)
Figure 2-10 UDP Datagram
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Bit
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20 24
Source Port
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Destination Port
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Sequence Number
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Acknowledgment Number
96
128
160
Offset
Reserved
Checksum
Flags
Window Size
Header Checksum
Urgent Pointer
Options (variable length 0-320 bits,
depending on offset field)
Payload (variable length)
Figure 2-11 TCP Packet
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“.”
.com
webex.com
.net
.nl
linksys.com
cisco.com
host1.cisco.com
host2.cisco.com
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Building The Mobile Internet
• Important Details;
– Socket API
– Network Address Translation
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Network Address Translation
• A cornerstone of Internet implementation
is that every host should have a unique IP
address.
• Currently, this is not strictly the case;
• Networks use Internal IP addressing;
• Not exposed to the Internet
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Network Address Translation
• Perceived Benefits?
• Security: IP address of specific user
hidden!
• Easier Network set-up with large address
ranges ‘internal’ to the local network.
• Reduces demand for Global IP addresses
• (Won’t be needed with IPv6)
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Network Address Translation
• Disadvantages:
• Internal and External IP addresses must
be unique
• Special range of IP addresses reserved for
Internal use:
• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16
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• Translation Process required (NAT)
NAT
• Communication across the Internet
requires interaction between globally
unique IP addresses
• A host with a locally-unique IP address
communicating with a remote host,
requires IP address translation at a host
which has a globally-unique address.
• The translation process (NAT) breaks the
normal communication between Internet
hosts.
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Sockets
• For an Application to connect to another
Application on the Internet, requires an
API.
• The API must create connections to other
hosts;
• The most common abstraction is the
Socket;
• The API associates a socket-identifier with
the IP address of a host, and a Port;
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Sockets, Protocols and Ports
Application 1
Application 2
Application 3
Application 4
Socket
references
TCP
Sockets
TCP
Ports
Sockets
bound
to ports
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2
3
1
65535
2
3
65535
UDP
TCP
IP
Sockets Protocols and Ports
The Data Link Layer
• The Data Link layer or Network
Interface Layer takes IP
packets and encapsulates them
for transport over the physical
layer;
• Ethernet has become the
dominant Layer 2 protocol in
many networks;
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Ethernet (IEEE802.3)
• Ethernet (IEE 802.3)
implements a unique MAC
address to the physical
interface card
• The MAC address is unique,
• Specified by the hardware
manufacturer
• Cannot be used for Interdomain
routing
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Address Resolution Protocol
• To map an IP address to a hardware
address requires an Address Resolution
Protocol (ARP).
• Typically a host receiving an IP address
broadcasts a message to all the hosts to
which it is connected asking for the mac
address that the IP address is associated
with.
• The specific host responds with its MAC
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address.
ARP and Caching
• In an effort to improve efficiency, host
routers cache MAC-IP associations
• If a host changes its IP address it could
respond with its cached address rather
than its new address.
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Sessions and Mobility
• A TCP communication is identified with the
5 tuple:
– Local IP Address
– Local Port
– Remote IP Address
– Remote Port
– Socket Identifier
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Sessions and Mobility
• The entire structure was built on the
implicit assumption that IP addresses were
essentially fixed and that Hosts did not
change location.
• If a host moves they need a new IP
address;
• If a host gets a new IP address then the
Socket ID for the communication must
change
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• Therefore the session breaks
Sessions and Mobility
• Another consequence of the TCP/IP
inherent design is that an IP address in
fact performs two functions;
• It is a location-identifier
• It is an end-point-identifier
• In other words, an IP address identfies the
specific host, and also its location in the
network.
• Not an issue until hosts start to move
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around…!
Strategies
• Accept that application sessions will break;
• Introduce an application layer ‘sessionpersistence’ mechanism
• Keep the same IP address when moving;
• Introduce a new Layer
• Redesign the TCP/IP protocol stack to
achieve separation of locators and endpoint identifiers
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Accept that Application
Sessions will Break
• Easy option
• For some situations session breaks don’t
matter
• Nomadic mobility: Fouus on seamless
access to different networks
• User can switch networks seamlessly
• However, sessions will not necessarily
‘stay up’
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Introduce a ‘SessionPersistence’ mechanism in the
Application Layer
• Requires the Application to implement a
session state;
• Probably impractical
• Maybe not desirable
– Cookies in web browsers do this currently
– Big security and privacy issues
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Keep the same IP Address
when Moving
• The IP layer is an abstraction layer on top
of the physical infrastructure,
• When a host moves, it must chaange its IP
address so that a source knows where to
route its packets
• This can be done to an extent by
‘tunneling’.
• Make multiple physical layers look like one
layer to the IP layer by encapsulating layer
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2 packets in other layer 2 packets
Introduce a New Layer
• Consider implementing a session layer
between the TCP layer and the Application
Layer.
• This is akin to revisiting the ISO-OSI
Seven layer model which had a Session
Layer above the TCP layer and under the
Application layer.
• Perhaps we used the wrong stack after
all…?
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Redesign TCP/IP to Separate
Location ID and Host ID
• A fairly fundamental approach
• Separate entities to describe
– A node
– Its location
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Possible in theory
Difficult to implement
Maybe the only realistic way forward
Explored in some detail later
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