11. Mar. 20041INF-3190: Internet Internet Foreleser: Carsten Griwodz Email: [email protected]@ifi.uio.no

11. Mar. 2004 1 INF-3190: Internet

Internet

Foreleser: Carsten GriwodzEmail: [email protected]


Address Resolution


Address Resolution Addressing levels

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

DomainName

System

?

Logical addresse.g. www.ifi.uio.no

Internet addresse.g. 129.31.65.7

Netadapter addresse.g. Ethernet address 00:08:74:35:2b:0a

Addressresolution


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


Address Resolution Protocol (ARP)

Process Broadcast ARP request datagram on LAN

including receiver’s IP address (desired value) sender’s 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



H H H H H

@IP: 9.228.50.3

@HW: 0xa3e

target

ARP Response

@IP: 9.228.50.3

@HW: 0xa3e

@IP: 9.228.50.8

@HW: 0xaa

source

H

target

ARP Request

@IP: 9.228.50.8

@HW: 0xaa

@IP: 9.228.50.3

@HW:

source



Process Broadcast ARP request datagram on LAN

including receiver’s IP address (desired value) sender’s 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 sender’s (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)



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

F2F1 F3

E1

1 2 3 4

E2 E3 E4 E5 E6

192.31.65.7192.31.65.5

To

WA

N

Router has 2 IP addresses

192.31.60.4192.31.65.1

Router has 2 IP addresses192.31.60.7192.31.63.3

192.31.63.8

Ethernet addresses

Campus FDDI ring

192.31.60.0

CS Ethernet 192.31.65.0

EE Ethernet 192.31.63.0

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


Reverse Address Resolution Protocol (RARP)

Retrieve Internet address from knowledge of hardware address

H H H H H

@IP: 9.228.50.3

@HW: 0xa3e

target

RARP Response

@IP: 9.228.50.3

@HW: 0xa3e

@IP: 9.228.50.8

@HW: 0xaa

source

@IP: unknown

@HW: 0xaa

RARP server responds RARP server has to be

available on the LAN

target

RARP Request

@IP:

@HW: 0xaa

@IP:

@HW: 0xaa

source

H

Application: diskless workstation boots over the network


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


IP Routing


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

N0N0

N1N1

N3N3 N5N5

N4N4

N2N2

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


IP Routing Routing tables

Routers may have incomplete information Default paths

Network

10.0.0.0

Network

10.0.0.0

F

Network

20.0.0.0

Network

20.0.0.0

Network

30.0.0.0

Network

30.0.0.0

Network

40.0.0.0

Network

40.0.0.0

G H

20.0.0.5

10.0.0.5

30.0.0.6

20.0.0.6

40.0.0.7

30.0.0.7

To reach hoston network

20.0.0.0

30.0.0.0

10.0.0.0

40.0.0.0

Route to thisaddress

Deliver direct

Deliver direct

20.0.0.5

30.0.0.7

Routing table of G


IP Routing: Initial Gateway-to-Gateway Protocol (GGP)

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

ARPANET

Local net 1 Local net 2 Local net n

G1 G2Gn

…

Originalimplementation


IP Routing: Autonomous Systems Hidden networks

Local net 2

Local net 1

Local net 3 Local net 4

G2 G3

G4

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

Internet domains

Autonomous System

AS boundary router

G1

Core gateways


IP Routing: Autonomous Systems Types of ASs

Stub domains source & sink only

Multiconnected domain No through traffic

Transit domains interconnect domain

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

Core gateways

Autonomoussystem

… Autonomoussystem

Autonomoussystem

G1 G2 Gn

Gi


Interior Gateway Protocol

AutonomousSystem x

IGPx

IGPx

Gx

AutonomousSystem 1

IGP1

IGP1

G1

EGP

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)


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-Procedure

i.e. IS periodically sends a list

containing 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


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 Dijkstra’s

"shortest path first" algorithm name "Open Shortest Path First“

0248

16

Normal serviceMinimize financial cost

Maximize reliabilityMaximize throughput

Minimize delay

OSPF no. Meaning


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

To other AS

To other AS


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

A F

B

C

LAN

D E G

H

I

N

A F

B

C

D E G

H

I

A

F

B

C

D E G

H

Itransform to

graph A

F

B

C

D E G

H

I

LAN are represented as star configurations


Exterior Gateway Protocol: Circumstances

Requirements, basic conditions

political economical security-related

Core gateways

AS1AS3

AS2

NWIG2

IG1

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


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"


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


Protocol Support in an IP Router Network layer protocols

IP (Internet Protocol) ARP (Address

Resolution Protocol), RARP (Reverse ARP) ICMP (Internet Control

Message Protocol) IGMP (Internet Group

Management Protocol)

LLC-1

SNAP

ARP RARP

ICMP IGMPOSPFEGPTCP UDP

BGP RIP SNMP

IP

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)

Documents

11. Mar. 20041INF-3190: Internet Internet Foreleser: Carsten Griwodz Email: [email protected]@ifi.uio.no