Computer Networks

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Computer Networks
Network layer
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Network Layer

• Design issues
• Routing
• Congestion
• Internetworking
• Internet Protocols
• Multimedia or QoS

• Different networks, protocols?
• Interconnection styles
• Internetwork routing
• Fragmentation
• Firewalls

3
Internetworking differences

• Different networks will always be around
• Installed base is large and growing
• Networks get cheaper, so decision makers ?
• New technology ? new networks ? new protocols

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Internetworking differences
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Internetworking differences

• Interconnection boxes
• Repeaters, hubs
• Bridges, switches
• Routers
• Transport gateways
• Application gateways

Multifunctional products
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Internetworking styles

• Half-devices neutral protocol
• Management issue
• Cooperation reduced to agreement on protocol

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Internetworking styles

• Concatenated virtual circuits
• Set-up of a connection
• Recognition of remote destination (host, router)
  and selection of multiprotocol router for first
  VC
• Multiprotocol router extends VC towards
• Data transfer
• Same path for all packets
• Conversions (packet format, VC numbers,) in
  multiprotocol routers
• Essential features
• Sequence of VCs
• Networks should have same/similar properties
• Properties I dentical to single VCs

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Internetworking styles

• Concatenated virtual circuits
• Set-up of a connection
• Recognition of remote destination (host, router)
  and selection of multiprotocol router for first
  VC
• Multiprotocol router extends VC towards
• Data transfer
• Same path for all packets
• Conversions (packet format, VC numbers,) in
  multiprotocol routers
• Essential features
• Sequence of VCs
• Networks should have same/similar properties
• Properties identical to single VCs

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Internetworking styles

• Connectionless internetworking
• Datagram approach
• Multiple routes
• Higher bandwidth
• No guarantee for in order delivery
• Nearly identical protocols required
• Packet conversion
• Addressing ( assignment, mapping)
• Properties same as for datagram networks

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Internetworking styles

• Connectionless internetworking
• Datagram approach
• Multiple routes
• Higher bandwidth
• No guarantee for in order delivery
• Nearly identical protocols required
• Packet conversion
• Addressing ( assignment, mapping)
• Properties same as for datagram networks

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Internetworking styles

• Tunneling
• Interconnect 2 identical networks using a
  different one
• Behaviour point-to-point line between
  multiprotocol routers

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Internetworking routing

• Same problem some complications
• 2 levels of routing
• Within a network
• Intranetwork routing
• Interior gateway protocol
• Internetwork routing
• Graph construction
• Every router can directly access routers on the
  same network
• Packet forwarding tunneling if necessary
• Differences with intranetwork routing
• Cross international boundaries adopt national
  laws
• Agreements between operators (transit traffic)

• Between networks
• Internetwork routing
• Exterior gateway protocol

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Internetworking routing

• An internetwork
• Router A can communicate with routers B
  and C

Graph of internetwork
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Internetworking fragmentation

• Problem Large packet through network with
  smaller maximum packet size
• Solution
• Break large packet into fragments
• Send each fragment as a separate packet
• Reassemble transparent ltgt non transparent?
• Transparent fragmentation
• Strategy
• Gateway breaks large packet into fragments
• Each fragment addressed to same exit gateway
• Exit gateway does reassembly

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Internetworking fragmentation

• Transparent fragmentation
• Strategy
• Gateway breaks large packet into fragments
• Each fragment addressed to same exit gateway
• Exit gateway does reassembly

• Simple, but some problems
• Gateway must know when it has all pieces
• Performance loss all fragments through same
  gateway
• Overhead repeatedly reassemble and refragment
• Example ATM segmentation

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Internetworking fragmentation

• Nontransparent fragmentation
• Strategy
• Gateway breaks large packet into fragments
• Each fragment is forwarded to destination

• problems
• Every host must be able to reassembly
• More headers
• Example IP fragmentation

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Internetworking fragmentation

• Fragment numbering
• Hierarchical numbering
• Packet 0 ? packets 0.0, 0.1, 0.2
• Problem retransmission different
  fragmentations
• Basic block numbering in every packet
• Original packet number
• Sequence number of first block

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Internetworking firewalls

• Protection needed against
• Information leaking out
• Trade secrets, product development plans,
• Information leaking in
• Viruses, worms,
• Old medieval analogy
• Castle deep moat around it
• Single draw bridge
• Example firewall
• 2 routers for packet filtering
• Application gateway

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Internetworking firewalls

• Packet filtering
• Acceptable sources destinations
• Filters on
• Address IP
• Service port (TCP header)
• both

• Application gateway
• Decisions made per application
• Header fields,
• Message size
• content

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Network Layer

• Design issues
• Routing
• Congestion
• Internetworking
• Internet Protocols
• Multimedia or QoS

• IP protocol
• Internet Control Protocols
• Routing
• Internet multicasting
• Mobile IP
• IPv6

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Internet IP protocol

• View on Internet
• Collection of Autonomous Systems (AS)
• Glue IP designed for internetworking

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Internet IP protocol

• IP protocol
• best effort service
• Datagrams up to 64 Kbytes
• IP header
• 20 byte fixed part optional part
• Transmitted in big endian order ( l -gt r)

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Internet IP header
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Internet IP header

• Options maximum length 40 bytes
• Too small 40 bytes -gt only 10 IP addresses!

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Internet IP addresses

• IP address 32 bits
• Network number
• Host number (on network)

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Internet IP addresses

• Dotted notation 134.58.47.25
• Assignment
• ICANN Internet Corporation for Assigned Names and
  Numbers
• Arin (American Registry for Internet Numbers)
  for N S America
• RIPE (Réseaux IP Européennes) for Europe
• APNIC (Asia Pacific Network Information Centre)
• Special addresses

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Internet IP subnets

• Subnetting different views on same network
• Internal network split up in different parts
• External a single net
• Why?
• Avoid use of different C networks for a single
  organisation
• Allow structuring of class A B networks

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Internet IP subnets

• Subnetting different views on same network
• Internal network split up in different parts
• External a single net
• Why?
• Avoid use of different C networks for a single
  organisation
• Allow structuring of class A B networks
• Network ltgt host?
• Subnet mask

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Internet IP subnets
Routing table entries No subnets
With subnets

• (network, 0) how to get distant network
• (this-network, host) to local host

• (network, 0) how to get to distant
  network
• (this-network, subnet, 0) to host on
  another subnet
• (this-network, this-subnet, host) to
  local host

• Advantages
• Smaller tables
• Management of networks easier (not easy!)

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Internet CIDR

• Exponential growth of Internet
• Running out of addresses
• B is too large ltgt C is too small
• Assign many Cs iso a single B
• Routing table explosion
• Hierarchical routing
• No support in IP addressing scheme
• CIDR solution
• Allocate blocks of class C addresses
• Introduce hierarchy for remaining addresses
• Classless routing

CIDR classless InterDomain Routing
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Internet CIDR

• CIDR solution
• Allocate blocks of class C addresses
• Variable size described by
• IP address
• Mask indicating meaningful bits in address
• Allocation scheme block of X addresses starts on
  X-byte boundary
• 2048 addresses (8 C classes) 194.24.0.0 to
  194.24.7.255
• 4096 addresses (16 C classes) 194.24.16.0 to
  194.24.31.255
• Introduce hierarchy for remaining addresses
• Classless routing

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Internet CIDR

• 3 blocks assigned
• Entries in router tables
• Route 194.24.17.4? or 0001 0001 0000
  0100
• Test address mask

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Internet NAT

• Network Address translation
• Simple solution to the shortage of IP addresses
• Examples?
• Technique
• non routable addresses inside a domain
• Translate address to a routable one when packet
  leaves domain

Reply packets?
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Internet NAT

• Use TCP/UDP port number to differentiate between
  different local computer systems
• NAT translation table
• (local IP address, source port)
• ?? (external IP address, unique port)
• 1 IP address can be used for up to 64K hosts

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Internet NAT

• Objections to NAT
• Violates architectural model of IP
• IP address uniquely identifies a single computer
• Crash of NAT box ? all connections lost
• Connection oriented flavor
• Violates fundamental rule of protocol layers
• Only works for TCP UDP
• Addresses inside body are not translated
• Ugly and temporary hack delays real solution
  IPv6

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Network Layer

• Design issues
• Routing
• Congestion
• Internetworking
• Internet Protocols
• Multimedia or QoS

• IP protocol
• Internet Control Protocols
• ICMP
• ARP
• RARP
• Routing
• Internet multicasting
• Mobile IP
• IPv6

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Internet ICMP

• ICMP Internet Control Message Protocol
• Used by routers to report unexpected events
• Definition RFC 792

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Internet ARP

• ARP address resolution protocol
• How do IP addresses get mapped onto data link
  layer addresses?
• Problem
• Solution configuration files
  unsatisfactory

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Internet ARP

• Basic ARP protocol
• Broadcast who owns IP address 192.31.65.5?
• Host with that IP address should reply with its
  data link address
• Optimisations cache mappings!
• Values in ARP request (every system on net)
• Values in ARP reply (sender of ARP request
  only)
• Gratuitous ARP upon boot host can send ARP
  request with its own mappinganswer duplicate
  IP address in use!

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Internet ARP

• How to handle remote hosts?
• Proxy ARP routers serving the net should reply
• Sending host forwards packet to router

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Internet RARP

• RARP Reverse Address Resolution Protocol
• Problem
• Given a data link address
• What is the corresponding IP address
• Why needed?
• Allows a newly booted workstations to get its IP
  address
• Solutions
• RARP protocol RARP server!!
• IP address embedded in OS image (different image
  for every WS)
• BOOTP protocol
• Limitation of RARP server needed on each net as
  broadcast is not forwarded

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Internet BOOTP

• Bootstrap protocol
• Uses UDP messages
• Broadcast to port 67
• Forwarded over routers
• Gives additional information
• IP address of file server holding the OS
• IP address of default router
• Subnet mask to use

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Internet DHCP

• DHCP Dynamic Host Configuration Protocol
• Special server relay agents
• Static dynamic assignment of IP addresses
  (leasing)
• Newly booted machine broadcasts a DHCP Discover
  packet

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Network Layer

• Design issues
• Routing
• Congestion
• Internetworking
• Internet Protocols
• Multimedia or QoS

• IP protocol
• Internet Control Protocols
• Routing
• OSPF
• BGP
• Internet multicasting
• Mobile IP
• IPv6

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Internet routing

• History
• First protocol RIP (distance vector)
• Since 1979 replaced by link state
• In 1990 new standard OSPF
• Protocols
• Interior gateway protocol OSPF Open Shortest
  Path First
• Exterior gateway protocol BGP Border gateway
  protocol

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Internet routing OSPF

• Requirements for OSPF
• Algorithm in open literature
• Support for various distance metrics
• Dynamic algorithm
• Support for routing based on type of service
• Do load balancing over multiple lines
• Support for hierarchical systems
• Security to prevent false updates
• Support for routers connected through tunnel
• OSPF supports as connections and networks
• Point-to-point lines between routers
• Multi access networks with broadcasts
• (multi access) networks without broadcasts

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Internet routing OSPF

• Abstract view on network
• Directed graph
• Node for each router network
• Arcs
• 2 arcs for each point-to-point line
• 2 arcs for each network node to the routers
  connected to the network
• Example

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Internet routing OSPF

• Abstract view on network
• Directed graph
• Node for each router network
• Arcs
• 2 arcs for each point-to-point line
• 2 arcs for each network node to the routers
  connected to the network

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Internet routing OSPF

• Network divided into areas
• Areas do not overlap
• Area set of contiguous networks
• Topology of area not visible outside area
• Backbone area
• Interconnects areas
• Router connected to at least 2 areas is part of
  backbone
• Classes of routers
• Internal routers (within an area)
• Area border routers (interconnect areas)
• Backbone routers
• AS boundary router
• 3 kinds of routes
• Intra-area shortest path
• Interarea from source to backbone to destination
• Inter AS

Provisions for very large networks
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Internet routing OSPF
Provisions for very large networks
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Internet routing OSPF

• Provisions for very large networks

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Internet routing OSPF

• Exchanging information
• Between adjacent routers
• on LAN one router is elected as designated router
• Designated router is adjacent to all neighbouring
  routers
• Flooding to all routers in an area
• Within routers of any area
• Construct graph
• Compute shortest paths between routers in area
• Extra for backbone area
• Accept info from area border routers
• Compute SP between backbone router and all
  routers in AS
• Propagate this info back to area border routers,
  which advertise it within their areas

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Internet routing OSPF

• How handle different types of service
• Multiple graphs with as cost metric
• Delay
• Throughput
• reliability
• Triples computation
• Separate routes for optimising

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Internet routing BGP

• Allow many kinds of routing policies
• Examples
• No transit traffic
• Only transit X if there is no alternative
• Traffic from or to Y should not transit Z
• Policies require manual configuration!
• BGP view of the Internet
• BGP routers interconnecting lines
• 3 kind of networks
• stub networks 1 connection in BGP graph
• multi connected networks
• transit networks (operated as backbones)
• BGP algorithm

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Internet routing BGP

• BGP algorithm
• Distance vector protocol
• Each router keeps track of exact path used
• Route violating a policy? distance ?
• Uses reliable TCP connections???

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Network Layer

• Design issues
• Routing
• Congestion
• Internetworking
• Internet Protocols
• Multimedia or QoS

• IP protocol
• Internet Control Protocols
• Routing
• Internet multicasting
• Mobile IP
• IPv6

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Internet multicasting

• Use of class D IP addresses
• Each group identified by class D address
• Best effort delivery to all members of group
• Permanent addresses
• 224.0.0.1 all systems on LAN
• 224.0.0.2 all routers on LAN
• 224.0.0.5 all OSPF routers on LAN
• 224.0.0.6 all designated OSPF routers on LAN
• Temporary addresses for temporary groups
• Create group
• Host can join/leave group
• IGMP Internet Group Management Protocol

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Internet multicasting

• Multicast router
• Keeps track of the groups to which hosts on its
  LAN belong
• Modified distance vector protocol
• Each router constructs spanning tree per group
• Heavy use of tunneling (why?)

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Internet mobile IP

• Unattractive solutions
• Give new IP address to mobile host
• Use complete IP address for routing
• IETF desirable goals
• (home) IP address usable everywhere
• No software changes to fixed hosts
• No changes to router software and tables
• No detours for most packets to mobile hosts
• No overhead when mobile host is at home
• Solution

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Internet mobile IP

• Solution (see general scheme for details)
• Home agent
• Gratuitous ARP to invalidate cached entries
• Foreign agent registering
• ARP home agent responding
• Tunnel to foreign agent inform sender
• Handling of other problems
• Locating agents
• Broadcast
• Host leaving without deregistration
• Registration valid for fixed time interval
• Security
• Use authentication protocol

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Network Layer

• Design issues
• Routing
• Congestion
• Internetworking
• Internet Protocols
• Multimedia or QoS

• IP protocol
• Internet Control Protocols
• Routing
• Internet multicasting
• Mobile IP
• IPv6

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Internet IPv6

• Major goals for new IP
• Support billion of hosts
• Reduce size of routing tables
• Simplify protocol
• Better security (authentication privacy)
• More attention for type of service
• Aid multicasting
• Better support for mobility
• Allow protocol to evolve
• Permit coexistence of old and new IP

• Features of IPv6
• Not compatible with IPv4
• Compatible with other Internet protocols
• Longer addresses
• Simplification of header
• Better support for options
• Big advance in security
• More attention to type of service

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Internet IPv6

• Procedure
• Call for proposal by IETF
• 21 responses
• Dec 92 7 serious proposals
• 3 better proposals published in IEEE network
• SIPP (Simple Internet Protocol Plus) or IPv6
  combined version

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Internet IPv6 header
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Internet IPv6 header

• Version identifies protocol version
• Priority
• 0 7 transmissions capable of slowing down
• 8 15 real-time traffic
• Higher values more important traffic
• Flow label Identification of flow with specific
  requirements
• Pseudoconnection between source and destination
• To be used by routers for special treatment of
  all packets of a flow
• Payload length number of bytes in packet after
  header
• Next header
• Which extension follows this one
• (last extension header) which transport protocol
  to select
• Hop limit decremented at each hop
• Addresses of source destination 16 bytes or
  128 bits

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Internet IPv6 addresses

• Size of address space
• 128 bits ? 2128 ? 1038 addresses
• 7 x 1023 addresses /m2 land water on entire
  earth
• Most pessimistic scenario 1000 addresses / m2
  (land water)
• Notation
• 8 groups of 4 hexadecimal digits with colons as
  separators
• 80000000000000000123456789ABCDEF
• Short cuts
• Sequence of 0000 ? 80000123456789ABCDE
  F
• IPv4 1345891254
• Assignment
• Provider-based geographic-based addresses
• Overview

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Internet IPv6

• Extension headers
• Extra info, efficiently encoded
• Overview

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Internet IPv6

• Extension headers
• Encoding
• Fixed format or
• variable number of variable length fields
• Type, length value encoding

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Internet IPv6

• Jumbogram extension header
• Datagrams gt 64 Kbyte

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Internet IPv6

• Routing extension header
• Bit map strict source routing ltgt loose source
  routing

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Internet IPv6

• Fragmentation extension header
• Fragmentation only by source host?
  simplification of routers,
• Identical fields
• Datagram identifier
• Fragment number
• MF bit

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Internet IPv6

• Controversies
• Address length
• Length of hop limit field
• Number of hops should never exceed 100 ? 8 bits
• Maximum packet size
• Removal of checksum
• Security
• in network layer? Yes ? standard service
  No ? never good enough
• Export restriction?
• How? replaceable algorithms

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Computer Networks
Network layer