IP Int nter ernet networ orking king 1 Internetworking - - PDF document

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CMPE 252A: Computer Networks Set 9a :

IP Int nter ernet networ

• rking

king

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Internetworking

 Arbitrary collection of physical networks

interconnected to provide an end-to-end (host-to- host) packet delivery service.

 Networks differ in many ways:  Service offered: datagrams vs connections  Protocols and mechanisms used  Address space  Topology and physical media

 An internetwork should make all these

differences transparent to end nodes.

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IP Internetworking

 Based on Cerf’s catenet model

V.G. Cerf, “The Catenet Model for Internetworking,” IEN 48, July 1978.

 Basic premises:

 Heterogeneous transmission media  Heterogeneous hardware and OS in hosts and gateways  Common protocol for network interconnection runs in all

gateways and hosts!

 Common protocol used for data transfer and signaling  Common address space used to identify where a host or

router is in the internetwork

 An address states at which network a node attaches to

the internetwork

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Catenet Model

 A network is the address of a host in the internet  A single address space, with addresses that are globally

unique

 A single protocol for delivering all user and control data  Common protocol runs in all gateways and hosts  A common definition of services G G G G NET NET NET NET

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Service Model: Theory and Practice

 The Internet Protocol (IP) evolved from the

catenet model.

 Theory: Datagram Delivery is assumed, so

that packets can get lost, out of order, and multiple copies can be delivered.

 Practice:

 TCP needs in-order delivery of packets to work

efficiently, and (as we will see) Internet routing protocols provide a single path for each destination and do not adapt very rapidly.

 Too many destinations!

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IP Internet Today

A single path to each destination, link costs are static. Starting with NSFNET, routers run IP and the Internet is based on routers running IP interconnecting autonomous systems.

R R R R

R R R R R

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Tier 2 ISP

IP Internet Today

Tier 1 ISP Tier 1 ISP

Large Content Distributor (e.g., Google) Large Content Distributor (e.g., Akamai)

IXP IXP

Tier 1 ISP Tier 2 ISP Tier 2 ISP Tier 2 ISP Tier 2 ISP Tier 2 ISP Tier 2 ISP Tier 2 ISP Tier 2 ISP

Semi-hierarchical topology

IXP: Internet Exchange Point

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IP Internet Today

“Simple” store-and-forward networking

“Rich” end-to-end services: Processing and storage of content

Internet Protocol (IP) is the glue

A Success tale of “two worlds with a little glue”

“Networking” is

independent of processing and storage of content.

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Hierarchical Routing

Scale: 100’s millions of destinations:

 Routing table cannot store an entry for each destination!  Routing table exchange would swamp links.

Administrative autonomy

 Internet is a network of networks  Each network administrator may want to control routing in

its own network.

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Hierarchical Routing

 Autonomous routing domains

(ARD): Group of physical networks using IP with one routing policy (e.g., campus network, ISP internal network, corporate networks)

 Autonomous Systems (ASes)

An ARD with an autonomous system number (ASN)

 Routers in same ARD run same

routing protocol.

 Routers in different ASes can run

different intra-AS (intra-domain) routing protocol



Special routers in AS



Run intra-AS routing protocol with all other routers in AS



Responsible for routing to destinations outside AS

 run inter-AS routing

protocol with other gateway routers (BGP).

Gateway Routers

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Intra-AS and Inter-AS Routing

Gateways:

• Perform inter-AS

routing amongst themselves

• Perform intra-AS

routers with other routers in their AS

inter-AS, intra-AS routing in gateway A.c network layer link layer physical layer

a b b a a C A B d A.a A.c C.b B.a c b c

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Intra-AS and Inter-AS Routing

a b b a a C A B d A.a A.c C.b B.a c b c Host S Host D Intra-AS routing within AS B Inter-AS routing between A and B Intra-AS routing within AS A

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Layers in Internet Routing

PHYSICAL LINK NETWORK TRANSPORT (TCP or UDP) SESSION PRESENTATION

APPLICATION

PHYSICAL LINK NETWORK TRANSPORT SESSION

PRESENTATION APPLICATION Routing Table

IP IP

Routing Protocol Routing Protocol

Routing Table Routing Table Routing Table

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IPv4 Packet Information

Typically no options and header is 20 bytes

version HLen TOS length identifier flags

• ffset

TTL protocol checksum

3 7 15 23 31

source address destination address Options (variable) pad (variable)

data

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IPv4 Datagram Format

ver length 32 bits

data (variable length, typically a TCP

• r UDP segment)

16-bit identifier Internet checksum time to live 32 bit source IP address IP protocol version number header length (words) max number remaining hops (decremented at each router) for fragmentation and reassembly total datagram length (bytes) upper layer protocol to deliver payload to head. len type of service “type” of data flgs fragment

• ffset

upper layer 32 bit destination IP address Options (if any) e.g., timestamp, record route taken, specify list of routers to visit. how much overhead with TCP?

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20 bytes of TCP



20 bytes of IP



= 40 bytes + app layer overhead

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IPv4 Header

 TTL (time to live indicates how long the packet can stay in

the network; it is specified in hops and is decremented each time the packet is forwarded.

 Default is 64 hops; nodes can play with the field to limit the scope

 Protocol specifies the type of payload  Checksum is computed considering the entire header as a

sequence of 16-bit words, adding them up with 1’s complement arithmetic and taking the 1’s complement of the result.

 This checksum is NOT as powerful as a CRC but is simple to

do in software.

 Why this way? Because it is (was) done at each hop in

software

 What if we process headers in hardware?

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IPv4 Addresses

 IP addresses are global and, unlike MAC addresses,

they are hierarchical.

 IP address has a network part and a host part and

specifies host@network

 A host has an address for each network to which it

attaches.

 IP addresses are denoted using the dotted-decimal

notation: Each byte of the address is written in its decimal form and is separated by a dot from the

• ther bytes, e.g.,

5.7.2.1 => 00000101 00000111 00000010 00000001

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IPv4 Addresses

IP address: 32-bit

identifier for host, router interface

Interface: connection

between host or router and physical link

 Router’s typically have

multiple interfaces

 Host may have multiple

interfaces

 IP addresses associated

with each interface

223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1 1

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IPv4 Addresses

Routing table entries referring to destinations in the same AS refer to networks only.

223.1.1.* 223.1.2.* 223.1.3.*

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IPv4 Addressing Problems

 There were too few networks left due to the class

structure used in IP address assignments

 There are many more IP devices and appliances

coming.

 Routing tables cannot have millions of entries.  Solutions:

 Aggregation of addresses without classes (CIDR)  New and much bigger global address space (IPv6)  Locally unique addresses (NAT and other techniques)  Go to names?

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Class-based IPv4 Addresses (past)

network

host host host multicast address

8 16 24 31

Class A network network

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Class B Class C

16 million 65,534 110

126 16,382 2 million

254

reserved address

1110 11110

Class D Class E

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IP Addressing: CIDR

 Classful addressing:

 Inefficient use of address space, address space exhaustion.  A class B address has enough addresses for 65K hosts, even if only

a few more than 256 hosts are located in that network

 CIDR: Classless InterDomain Routing

 Eliminate the strict assignment of address portion in class-full

addressing.

 Enable a network portion of address of arbitrary length.

 CIDR Address Format:

a.b.c.d/x, where x is # bits in network portion of address

11001000 00010111 00010000 00000000

network part host part

200.23.16.0/23

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Assigning IP Addresses to Hosts

 Hard-coded by system administrator in a file  Wintel:

control-panel->network->configuration-> tcp/ip->properties

 UNIX: /etc/rc.config  Obtain address from a server dynamically

(“plug-and-play”)

 This is the purpose of

DHCP: Dynamic Host Configuration Protocol:

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Assigning Network Portion of IP Address to a Network

 An ISP obtains a block of the address space.  Net is allocated portion of its provider ISP’s

address space.

ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20 Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. …. Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23

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IANA: Internet Assigned Numbers Authority

 Domain names: Manage the DNS root, .int, .arpa

domains.

 Number resources: Coordination of global pool of IP

and AS numbers via Regional Internet Registries

 Protocol assignments: Manage Internet protocol

numbering systems together with standards bodies.

 Operated by Internet Corporation for Assigned Names

and Numbers (ICANN) under a US Department of Commerce contract

 www.iana.org  www.icann.org

Assigning Blocks of Addresses to ISPs

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Hierarchical Addressing for Route Aggregation

“Send me anything with addresses beginning 200.23.16.0/20”

200.23.16.0/23 200.23.18.0/23 200.23.30.0/23

Organization 0 Organization 7 Internet Organization 1 “Send me anything with addresses beginning 199.31.0.0/16”

200.23.20.0/23

Organization 2

. . . . . . Allow efficient advertisement of routing information

ISP A ISP B

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Hierarchical Addressing and More Specific Routes

Another-ISP has a more specific route to Organization 1

“Send me anything with addresses beginning 200.23.16.0/20”

200.23.16.0/23 200.23.18.0/23 200.23.30.0/23

ISP A

Organization 0 Organization 7 Internet Organization 1

ISP B

“Send me anything with addresses beginning 199.31.0.0/16

• r 200.23.18.0/23”

200.23.20.0/23

Organization 2

. . . . . .

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Internet Routing Protocols

 Itra-domain routing:

 RIP, OSPF, EIGRP  Single-path routing protocols, static link costs  Performance (shortest path)

 Inter-domain routing:

 Border Gateway Protocol (BGP)  Single path  Policy based

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RIP

 Based on DBF  Used in small internets  Problems: Counting to infinity and looping, single-path

routing, link cost should be 1 or infinity

 Update specifies only a destination network and a distance

to it; hence, no variable subnet masks are allowed in “local” internet and a static subnetting convention must be used for all routers

 Router sends its routing table to its neighbors every 30

• sec. or when it must update its routing table.

 Runs on top of UDP.

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RIPv2

 Adds the next hop to a destination and

subnet mask in each update.

 Variable subnets are allowed.  Performance does not improve much.

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OSPF: Open Shortest Path First

 Dijkstra’s SPF used to compute shortest paths locally

based on topology map.

 Flooding is used to disseminate topology maps.  Sequence numbers and age fields are used to validate

link-state updates.

 Runs on top of IP and implements its own reliable

transmission of link-state updates.

 Designated routers are used to reduce overhead within a

LAN, and areas connected by a backbone are used to reduce overhead across LANs.

 A handshake is used to reduce overhead of sending large

portions of the topology map between neighbors.

 HELLOs used to identify neighbors.

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R not allowed R R R R

OSPF

Areas must be connected by a connected backbone (area 0)

A2 A4 A3

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A1

backbone

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OSPF

R R R R A2 A4 A3

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A1

area border router To other domains boundary router, backbone router internal router EA1 EA2 … EAn

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OSPF

 Areas need unique IDS, an IP address.  Zero or more address ranges can be reached in

an area.

 Different types of routers have different views of

topology.

 End result is a hybrid of link-state and distance

information.

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Topology Information at Backbone Router

R R R R A2 A4 A3 A1

EA1 EA2 … EAn

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Topology Information at Area Border Router

R R R R A2 A4 A3 A1

EA1 EA2 … EAn

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Topology Information at Internal Router

A2 A4 A3 A1

EA1 EA2 … EAn

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OSPF

 In a broadcast LAN, designated router

eliminates too many link state updates.

 LSUs, HELLOs and topology updates sent

unicast to designated router, which keeps all routers in LAN updated.

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