INF-3190: Internet

1
Internet

• Foreleser Carsten Griwodz
• Email griff_at_ifi.uio.no

2
Address Resolution
3
Address Resolution

• Addressing levels

Logical address e.g. www.ifi.uio.no
Domain Name System
Address resolution
Internet address e.g. 129.31.65.7
?
Netadapter address e.g. Ethernet address
000874352b0a

• Host identification and routing specification
  within a subnetwork
• based on the (local) physical network addresses
  of the end systems
• e.g. station address of the adapter card
• Problem
• Different address styles for different layer 2
  protocols
• IP address must be mapped onto the physical
  network address, 48 bit for Ethernet
• Direct mapping possible for IPv6
• But impossible for IPv4

4
Address Resolution Methods

• Address resolution in
• Source ES, if destination ES is local (direct
  routing)
• Gateway, if destination ES is not local
• Solutions
• Direct homogeneous Addressing
• if the physical address can be changed by the
  user
• physical address Hostid of the IP address
• Only possible if physical address is also longer
  than hostid
• If the physical address is pre-defined or if it
  has to have a different format, one of the
  following has to be used
• a mapping table from the configuration data base
  (IPaddr ? HWaddr),
• e.g. in the Gateway,
• may become maintenance nightmare
• the Address Resolution Protocol (ARP)
• mainly applied in LANs with broadcasting facility

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Address Resolution Protocol (ARP)

• Process
• Broadcast ARP request datagram on LAN
• including receivers IP address (desired value)
• senders physical (HW) and IP address (IP)
• Every machine on LAN receives this request and
  checks address
• Reply by sending ARP response datagram
• machine which has requested address responses
• including the physical address
• Enter the pair (I,P) into buffer for future
  requests

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Address Resolution Protocol (ARP)
H
H
H
H
H
H
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Address Resolution Protocol (ARP)

• Process
• Broadcast ARP request datagram on LAN
• including receivers IP address (desired value)
• senders physical (HW) and IP address (IP)
• Every machine on LAN receives this request and
  checks address
• Reply by sending ARP response datagram
• machine which has requested address responses
• including the physical address
• Enter the pair (I,P) into buffer for future
  requests
• Refinement
• The receiver of the ARP request stores the
  senders (I,P) pair in its cache
• Send own table during the boot process (but may
  be too old)
• Entries in ARP cache should time out after some
  time (few minutes)

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Address Resolution Protocol (ARP)

• End system not directly available by broadcast
• Example ES 1 to ES 4
• ARP would not receive a response
• Ethernet broadcast is not rerouted over a router

• Solution 1 proxy ARP
• the local router knows all remote networks with
  their respective routers
• responds to local ARP
• local ES 1 sends data for ES 4 always to the
  local router, this router forwards the data (by
  interpreting the IP address contained in the
  data)
• Solution 2 remote network address is known
• local ES 1 sends data to the appropriate remote
  router
• local router forwards packets

9
Reverse Address Resolution Protocol (RARP)

• Retrieve Internet address from knowledge of
  hardware address

H
H
H
H
H
H

• RARP server responds
• RARP server has to be available on the LAN

• Application diskless workstation boots over the
  network

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Dynamic Host Configuration Protocol (DHCP)

• DHCP has largely replaced RARP (and BOOTP)
• extends functionality
• DHCP
• simplifies installation and configuration of end
  systems
• allows for manual and automatic IP address
  assignment
• may provide additional configuration information
  (DNS server, netmask, default router, etc.)
• Client broadcasts DHCP DISCOVER packet
• server answers
• DHCP server is used for assignment
• request can be relayed by DHCP relay agent, if
  server on other LAN
• Address is assigned for limited time only
• before the lease expires, client must renew it
• allows to reclaim addresses of disappearing hosts

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IP Routing
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IP Routing Internal and External Routing

• Direct Routing/ Interior Protocols
• Both source and destination ES are located in the
  same subnetwork
• source ES sends datagram to the destination ES
• identification done by the local address ?
  mapping
• routing is completely defined by the subnetwork
  routing algorithm

N0
N1
N4
N5
N3
N2

• Indirect Routing/Exterior Protocols
• Source and destination ES are located on
  different networks
• source ES sends datagram to the next router
• each router determines the next router on the
  path to the destination ES
• routing decision is based only on
• the netid part of the Internet address, i.e.
  hostid is not used

13
IP Routing

• Routing tables

• Routers may have incomplete information
• Default paths

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IP Routing Initial Gateway-to-Gateway Protocol
(GGP)
ARPANET
Original implementation
G1
Gn
G2

Local net n
Local net 1
Local net 2

• Core Gateways
• connect LANs to the backbone, know the routes to
  all networks
• exchange routing information with each other
• Gateway-to-Gateway Protocol (GGP)
• distributed routing definition (group
  "Distance-Vector-Procedure")
• metrics simply by distance
• Problems particularly poor scalability
• several backbones
• not all networks are connected directly to the
  backbone
• all Gateways communicate with each other

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IP Routing Autonomous Systems

• Hidden networks

Core gateways
AS boundary router
Autonomous System
G1
Local net 1
G2
G3
Local net 2
Local net 3
Local net 4
G4

• Core gateways have to be informed about hidden
  networks
• Autonomous systems (AS)
• Internet domains

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IP Routing Autonomous Systems

• Types of ASs
• Stub domains
• source sink only
• Multiconnected domain
• No through traffic
• Transit domains
• interconnect domain

Core gateways
G1
G2
Gn
Autonomous system
Autonomous system
Autonomous system

• Autonomous systems are administrative entities
• Collects routing information on networks in the
  AS
• Defines boundary routers (also called Exterior
  Gateways)
• that transmit routing information to other
  autonomous sys.
• Boundary routers
• Transmits information about network reachability
  only into its own AS
• Reason each AS shall control exactly, to whom
  the information about reachability is given to

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Interior Gateway Protocol
IGP1
IGPx
EGP
G1
Autonomous System x
Autonomous System 1
Gx
IGP1
IGPx

• Other variants
• e.g. HELLO by Dave Mills
• distributed routing algorithm
• distance Delay
• requires synchronized clocks

• In general intradomain routing
• individual solutions possible
• Presently preferred procedures
• Routing Information Protocol (RIP), old
• Open Shortest Path First (OSPF)
• Interior Gateway Routing Protocol (IGRP) and
  Enhanced IGRP (EIGRP)

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Routing Information Protocol (RIP)

• Background (regarding the originally used
  protocol)
• developed as a part of Berkeley UNIX
• since 1988, RIP Version 1, RFC 1058
• Principle
• Distributed routing algorithm Distance-Vector-Pro
  cedure
• i.e.
• IS periodically sends a listcontaining estimated
  distances to each destination to its neighbors
• distance
• number of hops 0 .. 15 (15 corresponds to
  infinite)
• periodical
• every 30 sec after 180 sek without packet ?
  distance infinite
• RIP Version 2
• G. Malkin, RFC 1387, 1388 and 1389 (RIP-MIB)
• Uses multicast if necessary to distribute data
• Not broadcast (so that all ES also receive this)
• Networks without broadcast or multicast (ISDN,
  ATM)
• Triggered" updates
• To be sent only if the routing table changes

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Open Shortest Path First (OSPF)

• Background since 1990 Internet Standard, RFCs
  1247, 2178
• Transition from vector-distance to
  link-state-protocol
• Principle (link-state-protocol)
• IS measures "distance" to the immediately
  adjacent IS, distributes the information,
  calculates the optimal route
• determine the address of adjacent IS
• measure the "distance" (delay, ..) to adjacent IS
• OSPF permits differing metrics
• selection per packet possible (RFC 1349)

• process local link-state information as a packet
• distribute information to all adjacent IS by
  flooding
• compute route from the information of all IS e.g.
  with Dijkstras "shortest path first" algorithm ?
  name "Open Shortest Path First

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Open Shortest Path First (OSPF)

• For large autonomous systems
• AS substructure
• AS
• AS backbone area
• Area
• Router classes
• AS boundary routers
• Backbone routers
• Area border routers
• Internal routers

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Open Shortest Path First (OSPF)

• Adjacency
• LSR measures distance to all neighbours
• OSPF measures distance to all adjacent nodes
• If several routers are connected by a LAN
• One is designated router
• All other routers on the LAN are adjacent only to
  it
• It is adjacent to all others

LAN are represented as star configurations
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Exterior Gateway Protocol EGP

• Example
• G2 sends to EGP neighbourse.g. routing update
  message
• (G1 Net1 G3 Net3, Net4 G2 Net2)
• Example ofautonomous system 2
• Exterior Gateways
• In general
• interdomain routing

EGP
G1
Autonomous System 1
Autonomous System x
Gx
To neighbour in other Autonomous System
Source network
G2
G1
Net 2
G3
Net 1
Net 3
G4
Net 4
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Exterior Gateway Protocol Circumstances

• Requirements, basic conditions
• political
• economical
• security-related

• Requirement examples
• to avoid certain autonomous systems
• to avoid certain countries
• to stay within one country (before going via
  foreign country)
• data of company A should not to pass through
  company B
• Exchange information on accessibility
• including at least one Core Gateway
• possibly with other AS

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Exterior Gateway ProtocolBorder Gateway Protocol
(BGP)

• Previously Internet Exterior Gateway Protocol
  (RFC 1654)
• Now Border Gateway Protocol (RFC 1771, 1772,
  1773) is de-facto standard
• Configurations
• Possibility to have several Exterior Gateways per
  AS
• Variations
• Branch (topology)
• all of the external traffic is routed over this/a
  single, external router
• Multiconnected networks
• linked to many end systems
• can pass on traffic if necessary
• Transit networks
• networks with increased capacity and
• often linked to many AS
• Demands
• To allow for routing path decisions
• e.g. to prefer to send traffic via own country
• e.g. not to send traffic through certain
  companies
• Routing policy can not only be based on a
  "minimal distance"

25
Exterior Gateway ProtocolBorder Gateway Protocol
(BGP)

• Algorithm
• Fundamentals based on distance vector mechanism,
  where
• IS sends periodically to its neighbours a list
  containing
• the estimated distances from itself to all known
  destinations
• BGP uses distance path mechanism
• Related to distance vector
• But without count-to-infinity problem
• IS sends periodically a list to its neighbours
  containing
• estimated distance and preferred Path from
  itself to each destination for a specified block
  of reachable IP addresses
• Receiving IS evaluates path
• Distance
• Policy compliance
• ? notion of a path / of how to reach other
  routers is distributed
• ? but, no criteria for selecting a route is
  distributed
• each BGP router must have its own criteria, i.e.
  policy
• e.g. never send using certain AS
• Remarks
• Big updates
• But only a limited number of routers

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Protocol Support in an IP Router
BGP
RIP
SNMP

• Network layer protocols
• IP (Internet Protocol)
• ARP (Address Resolution Protocol),
• RARP (Reverse ARP)
• ICMP (Internet Control Message Protocol)
• IGMP (Internet Group Management Protocol)

OSPF
EGP
TCP
UDP
ICMP
IGMP
ARP
RARP
SNAP
LLC-1

• Routing protocols
• RIP (Routing Information Protocol)
• BGP (Border Gateway Protocol)
• EGP (Exterior Gateway Protocol)
• OSPF (Open Shortest Path First)
• Network management protocols

• SNMP (Simple Network Management Protocol)
• Transport protocols
• UDP (User Datagram Protocol)
• TCP (Transmission Control Protocol)
• and
• SNAP (Subnet Access Point)
• LLC (Logical Link Control)