Transcript Data Link Layer Switching

Internet
Foreleser: Carsten Griwodz
Email: [email protected]
11. Mar. 2004
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INF-3190: Internet
Address Resolution
11. Mar. 2004
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INF-3190: Internet
Address Resolution

Addressing levels
Logical address
e.g. www.ifi.uio.no
Internet address
e.g. 129.31.65.7
Address
resolution
Domain
Name
System
?
Netadapter address
e.g. Ethernet address 00:08:74:35:2b:0a

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
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Direct mapping possible for IPv6
But impossible for IPv4
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INF-3190: Internet
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),
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the Address Resolution Protocol (ARP)
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11. Mar. 2004
e.g. in the Gateway,
may become maintenance nightmare
mainly applied in LANs with broadcasting facility
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INF-3190: Internet
Address Resolution Protocol (ARP)

Process
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Broadcast ARP request datagram on LAN



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Every machine on LAN receives this request and checks address
Reply by sending ARP response datagram

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including receiver’s IP address (desired value)
sender’s physical (HW) and IP address (IP)
machine which has requested address responses
including the physical address
Enter the pair (I,P) into buffer for future requests
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INF-3190: Internet
Address Resolution Protocol (ARP)
H
H
H
H
H
ARP Request
source
@IP: 9.228.50.8
@HW: 0xaa
target
@IP: 9.228.50.3
@IP: 9.228.50.3
@HW:
@HW: 0xa3e
ARP Response
source
@IP: 9.228.50.3
@HW: 0xa3e
target
@IP: 9.228.50.8
@HW: 0xaa
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INF-3190: Internet
Address Resolution Protocol (ARP)

Process

Broadcast ARP request datagram on LAN




Every machine on LAN receives this request and checks address
Reply by sending ARP response datagram




including receiver’s IP address (desired value)
sender’s physical (HW) and IP address (IP)
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)
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INF-3190: Internet


2 IP addresses
End system not directly
192.31.60.4
available by broadcast
192.31.65.1
192.31.65.7 192.31.65.5
Example: ES 1 to ES 4
F2
1
2
Router has

ARP would not
receive a response
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Ethernet broadcast
is not rerouted over
a router
F1
E1
E2
CS Ethernet
192.31.65.0
Router has
2 IP addresses
192.31.60.7
192.31.63.3
192.31.63.8
F3
E3
E4
Campus
FDDI ring
192.31.60.0
3
4
E5
E6
Ethernet
addresses
EE Ethernet
192.31.63.0
Solution 1: proxy ARP

the local router knows all remote networks with their respective
routers


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To WAN
Address Resolution Protocol (ARP)
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
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INF-3190: Internet
Reverse Address Resolution Protocol
(RARP)

Retrieve Internet address from knowledge of hardware address
H
H
@IP: unknown
@HW: 0xaa
H
H
H
RARP Request
source
@IP:
@HW: 0xaa

RARP server responds

RARP server has to be
available on the LAN

Application: diskless
workstation boots over
the network
target
@IP:
@IP: 9.228.50.3
@HW: 0xa3e
@HW: 0xaa
RARP Response
source
@IP: 9.228.50.3
@HW: 0xa3e
target
@IP: 9.228.50.8
@HW: 0xaa
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INF-3190: Internet
Dynamic Host Configuration Protocol
(DHCP)

DHCP has largely replaced RARP (and BOOTP)
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DHCP
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server answers
DHCP server is used for assignment


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


extends functionality
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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INF-3190: Internet
IP Routing
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INF-3190: Internet
IP Routing: Internal and External Routing

Direct Routing/ Interior Protocols:

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N0
Both source and destination ES
are located in the same
subnetwork
N1
source ES sends datagram to the
destination ES
identification done by the local
address  mapping
routing is completely defined by
the subnetwork routing algorithm
N2
N4
N5
N3
Indirect Routing/Exterior Protocols:

Source and destination ES are located on different networks


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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
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the netid part of the Internet address, i.e. hostid is not used
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INF-3190: Internet
IP Routing

Routing tables
Networ
k
10.0.0.
0
20.0.0.5
40.0.0.7
30.0.0.6
Networ
Networ
Networ
k
k
k
F
G
H
20.0.0.
30.0.0.
40.0.0.
0
0
0
10.0.0.5
30.0.0.7
20.0.0.6
Routing table of G


To reach host Route to this
on network
address
20.0.0.0
Deliver direct
30.0.0.0
Deliver direct
10.0.0.0
20.0.0.5
40.0.0.0
30.0.0.7
Routers may have incomplete information
Default paths
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INF-3190: Internet
IP Routing: Initial Gateway-to-Gateway
Protocol (GGP)
Original
implementation
ARPANET
G1
Local net 1

…
Local net 2
Gn
Local net n
Core Gateways


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connect LANs to the backbone, know the routes to all networks
exchange routing information with each other
Gateway-to-Gateway Protocol (GGP):

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G2
distributed routing definition (group "Distance-Vector-Procedure")
metrics: simply by distance
Problems: particularly poor scalability

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several backbones
not all networks are connected directly to the backbone
all Gateways communicate with each other
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INF-3190: Internet
IP Routing: Autonomous Systems
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Hidden networks
Core gateways
AS boundary router
G1
Local net 1
G2
Local net 2


Autonomous System
G3
Local net 3
G4
Local net 4
Core gateways have to be informed about hidden networks
Autonomous systems (AS)

Internet domains
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INF-3190: Internet
IP Routing: Autonomous Systems

Types of ASs
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Stub domains

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G1
Autonomous
system
interconnect domain
Gi
G2
Autonomous
system
…
Gn
Autonomous
system
Autonomous systems are administrative entities

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Collects routing information on networks in the AS
Defines boundary routers (also called Exterior Gateways)
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No through traffic
Transit domains
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source & sink only
Multiconnected domain
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Core gateways
that transmit routing information to other autonomous sys.
Boundary routers
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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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INF-3190: Internet
Interior Gateway Protocol
IGPx
Autonomous
System x
IGPx
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EGP
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Other variants
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individual solutions possible
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e.g. HELLO by Dave Mills
distributed routing algorithm
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Routing Information Protocol
(RIP), old
Open Shortest Path First
(OSPF)
Interior Gateway Routing
Protocol (IGRP) and
Enhanced IGRP (EIGRP)
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Autonomous
System 1
IGP1
Presently preferred procedures

G1
Gx
In general: intradomain
routing
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IGP1
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distance: Delay
requires synchronized clocks
INF-3190: Internet
Routing Information Protocol (RIP)
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Background (regarding the originally used protocol)
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Principle
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developed as a part of Berkeley UNIX
since 1988, RIP Version 1, RFC 1058
i.e.
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Distributed routing algorithm: Distance-Vector-Procedure
IS periodically sends a list
containing estimated distances to each destination
to its neighbors
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distance
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periodical
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number of hops: 0 .. 15 (15 corresponds to infinite)
every 30 sec; after 180 sek without packet  distance infinite
RIP Version 2
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G. Malkin, RFC 1387, 1388 and 1389 (RIP-MIB)
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Uses multicast if necessary to distribute data
Not broadcast (so that all ES also receive this)
Networks without broadcast or multicast (ISDN, ATM)
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“Triggered" updates
To be sent only if the routing table changes
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INF-3190: Internet
Open Shortest Path First (OSPF)

Background: since 1990 Internet Standard, RFCs 1247, 2178

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Transition from vector-distance to link-state-protocol
Principle (link-state-protocol)
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IS measures "distance" to the immediately adjacent IS, distributes the
information, calculates the optimal route

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determine the address of adjacent IS
measure the "distance" (delay, ..) to adjacent IS
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OSPF permits differing metrics
selection per packet possible (RFC 1349)
OSPF no.
0
2
4
8
16
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Meaning
Normal service
Minimize financial cost
Maximize reliability
Maximize throughput
Minimize delay
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“
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INF-3190: Internet
Open Shortest Path First (OSPF)

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For large autonomous
systems
AS substructure
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To other AS
AS
AS backbone area
Area
Router classes
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AS boundary routers
Backbone routers
Area border routers
Internal routers
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To other AS
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INF-3190: Internet
Open Shortest Path First (OSPF)

Adjacency
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LSR measures distance to all neighbours
OSPF measures distance to all adjacent nodes
If several routers are connected by a LAN

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One is designated router
All other routers on the LAN are adjacent only to it
It is adjacent to all others
H
D
E
B
A
C
H
D
G
I
F
transform to
graph
E
B
A
C
G
I
F
F
LAN
N
LAN are represented as star configurations
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INF-3190: Internet
Exterior Gateway Protocol: Circumstances

Requirements,
basic conditions
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Core gateways
political
economical
security-related
AS1
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Requirement examples
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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
AS3
IG1
AS2
NW
IG2
Exchange information on accessibility

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including at least one Core Gateway
possibly with other AS
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INF-3190: Internet
Exterior Gateway Protocol
Border Gateway Protocol (BGP)

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Previously: Internet Exterior Gateway Protocol (RFC 1654)
Now: Border Gateway Protocol (RFC 1771, 1772, 1773) is de-facto
standard
Configurations
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Possibility to have several Exterior Gateways per AS
Variations
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Branch (topology):
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Multiconnected networks
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Demands
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networks with increased capacity and
often linked to many AS
To allow for routing path decisions

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linked to many end systems
can pass on traffic if necessary
Transit networks

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all of the external traffic is routed over this/a single, external router
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"
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INF-3190: Internet
Exterior Gateway Protocol
Border 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

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IS sends periodically a list to its neighbours containing


But without count-to-infinity problem
estimated distance and preferred Path
from itself to each destination
for a specified block of reachable IP addresses
Receiving IS evaluates path

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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

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Remarks

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e.g. never send using certain AS
Big updates
But only a limited number of routers
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INF-3190: Internet
Protocol Support in an IP Router
BGP
Network layer protocols

IP (Internet Protocol)
ARP (Address Resolution
Protocol),
RARP (Reverse ARP)
ICMP (Internet Control
Message Protocol)
IGMP (Internet Group
Management Protocol)





ICMP
IGMP
RIP
TCP
SNMP
UDP
EGP
OSPF
IP
ARP
RARP
SNAP
LLC-1

Routing protocols






RIP (Routing Information
Protocol)
BGP (Border Gateway Protocol)
EGP (Exterior Gateway
Protocol)
OSPF (Open Shortest Path First)

Transport protocols



Network management protocols
and


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SNMP (Simple Network
Management Protocol)
UDP (User Datagram Protocol)
TCP (Transmission Control
Protocol)
SNAP (Subnet Access Point)
LLC (Logical Link Control)
INF-3190: Internet