Chapter 15&16 Internetworking

EE 4272 Spring, 2003

Chapter 15&16 Internetworking

• Internetwork Structure & Terms

• Internetworking Architecture Features Connection/Connectionless Architecture Fragmentation & Reassembly Internet Protocol & Services IP Addressing

Subnetting

• Routing Protocols in IP


Internetworking Terms• An internet

Collection of communications networks interconnected by bridges and/or routers

• The Internet - note upper case I The global collection of thousands of individual machines and networks

• Intranet: Corporate internet operating within the organization Isolated or may have links to Internet

• End System (ES): Device attached to one of the networks of an internet Supports end-user applications or services

• Intermediate System (IS): Device used to connect two networks Permits communication between end systems attached to different networks

• Bridge: IS used to connect two or more LANs using similar LAN protocols Address filter passing on packets to the required network only Operated at OSI layer 2 (Data Link)

• Router: Connects two or more (possibly dissimilar) networks Uses internet protocol present in each router and end system Operated at OSI Layer 3 (Network)


Internet Structure

Recent Past (1990)

NSFNET backboneStanford

BARRNET

regional

Berkeley PARC

NCAR

UA

UNM

Westnet

regional

UNL KU

ISU

MidNet

regional…

End user

Service Provider

AS (autonomous system): each with its own idea of routing and metrics defining. An AS is administered independently.


Internet Structure

Today

Backbone service provider

Peering

pointPeering

point

Large corporation

Large corporation

Smallcorporation

“Consumer ” ISP

“Consumer ” ISP

“ Consumer ” ISP

Service provider networks


Internetworking Protocols in TCP/IP Suite

• Requirements of InternetworkingLink between networks: Minimum physical and link layer

Routing and delivery of data between processes on different networks

Accounting services and status info

Independent of constituting network architectures


Internetworking Architecture Features

• Accommodate difference among networks Addressing: global network addressing must be provided Packet size -> fragmentation Timeouts: longer timeout for delivery across multiple networks Error recovery: independent to individual network error rec. cap. Status reporting Routing Connection based or connectionless


Architectural Approaches• Connection oriented: Assume that each network is connection oriented

IS connect two or more networks: IS appear as DTE to each network Logical connection set up between DTEs (Data Terminal Equipment)

Concatenation of logical connections across networks Individual network virtual circuits joined by IS

May require enhancement of local network services (e.g. 802 or FDDI) IS performs Relaying & Routing functions

• Connectionless Corresponds to datagram mechanism in packet switched network Each PDU treated separately Network layer protocol common to all DTEs and routers

Known generically as the internet protocol Internet Protocol (RFC 791 -> IETF)

One such internet protocol developed for ARPANET Lower layer protocol needed to access particular network


Connectionless Internetworking

• Advantages Flexibility Robust No unnecessary overhead

• Unreliable Not guaranteed delivery Not guaranteed order of delivery: Packets can take different routes Reliability is responsibility of next layer up (e.g. TCP)

• Design Issues Routing Datagram lifetime Fragmentation & re-assembly Error control Flow control


Routing

• End systems & routers maintain routing tables to indicate next router to which datagram should be sent Static: May contain alternative routes

Dynamic: Flexible response to congestion and errors

• Source routing Source specifies route as sequential list of routers to be

followed


Datagram Lifetime

• Datagrams could loop indefinitely Consumes resources Transport protocol may need upper bound on datagram life

• Datagram marked with lifetime Time-To-Live (TTL) field in IP Once lifetime expires, datagram discarded (not forwarded) Hop count: a simple way to implement TTL

Decrement TTL on passing through at each router

True time count: global clocking mechanism needed Need to know how long since last router


Fragmentation and Reassembly

• Each network has some MTU (Maximum Transmission Unit) e.g., Ethernet:1500B; FDDI:4500B, IP: 65,535B

• When to re-assemble At destination (preferred)

Results in packets getting smaller as data traverses internet Intermediate re-assembly

Need large buffers at routers Buffers may fill with fragments All fragments must go through same router

Inhibits dynamic routing

R1

ETH FDDI

IPIP

ETH

TCP R2

FDDI PPP

IP

R3

PPP ETH

IP

H1

IP

ETH

TCP

H8


Example

H1 R1 R2 R3 H8

ETH IP (1400) FDDI IP (1400) PPP IP (512)

PPP IP (376)

PPP IP (512)

ETH IP (512)

ETH IP (376)

ETH IP (512)

Ident = x Offset = 0

Start of header

0

Rest of header

1400 data bytes


Start of header

1

Rest of header

512 data bytes


Start of header

1

Rest of header

512 data bytes


Start of header

0

Rest of header

376 data bytes

Note: Offset field counts 8-byte units of data, not individual bytes


Error & Flow Control

• Error Control Not guaranteed delivery Router should attempt to inform source if packet discarded

Source may modify transmission strategy after the discard May inform high layer protocol Datagram identification needed

• Flow Control (? Congestion Control) Allows routers and/or stations to limit rate of incoming data The mechanism is limited in connectionless systems

Send flow control packets: Requesting reduced flow


Internet Protocol (IP)

• Part of TCP/IP: Used by the Internet Specifies interface with higher layer: e.g. TCP Specifies protocol format and mechanisms

• IP Services can be described by Primitives to specify functions to be performed: Implementation dependent

Send: Request transmission of data unit Deliver: Notify user of arrival of data unit

Parameters: Used to pass data and control info Source/Destination address Protocol: Recipient e.g. TCP Type of Service (TOS): Specify QoS of data unit during transmission through networks Identification: combined with source, destination address and user protocol

Uniquely identifies PDU Needed for re-assembly and error reporting


IP Services Parameters (Con’t)

• Time to live (TTL): Send only

• Data length

• Option data : options requested by the IP user Security Source routing Route recording Stream identification Timestamping

• User data Carries user data from next layer up Integer multiple of 8 bits long (octet) Max length of datagram (header plus data) 65,535 octets


IP Header• Version: Currently 4

IP v6 – next generation• Internet header length (IHL): In 32 bit words

Including options• Type of service (TOS)• Total length : Of datagram, in octets

• Identification: Sequence number Used with addresses and user protocol to identify

datagram uniquely• Flags: More bit

Don’t fragment• Fragmentation offset• Time to live (TTL)• Protocol: Next higher layer to receive data field at destination

• Header checksum Reverified and recomputed at each router 16 bit ones complement sum of all 16 bit words

in header Set to zero during calculation

• Source/Destination address• Options• Padding: To fill to multiple of 32 bits long


Global IP Addresses

• Properties globally unique hierarchical: network + host

• Dot Notation 10.3.2.4 128.96.33.81 192.12.69.77

Note: It is more precise to think of IP address

as belonging to interfaces than to hosts

Network Host

7 24

0A:

Network Host

14 16

1 0B:

Network Host

21 8

1 1 0C:

Class D (start 1110) address specify a multicast group

Class E (start 1111): reserved for future use

H5 H6

R2R1

H4

H3H2H1

Network 2 (Ethernet)

Network 1 (Ethernet)

Network 3 (FDDI)

Network 4

(point-to-point)

H7 R3 H8


• Problem: Assigning one network # per physical network, not only used up the IP address space very fast, but also increase the burden of routing.

• Solution: Add another level to address/routing hierarchy: subnet assign a single IP network # and allocate the IP addresses with that network # to several physical networks

• Subnet masks define variable partition of host part

Subnetting & Subnet Mask

Network number Host number

Class B address

Subnet mask (255.255.255.0)

Subnetted address

111111111111111111111111 00000000

Network number Host IDSubnet ID

Bitwise AND


Subnet Example

Subnet mask: 255.255.255.128Subnet number: 128.96.34.0

128.96.34.15 128.96.34.1H1

R1

128.96.34.130Subnet mask: 255.255.255.128Subnet number: 128.96.34.128

128.96.34.129128.96.34.139

R2H2

128.96.33.1128.96.33.14

Subnet mask: 255.255.255.0Subnet number: 128.96.33.0

H3

A host connected to this subnetwork could have an IP address between 128.96.34.1 and 128.96.34.127



A single class B (128.96.*.*) address shared by several physical network

Bitwise AND of the host IP address & subnet mask = subnet number


IP Versions

• IP v 1-3 defined and replaced• IP v4 - current version• IP v5 - streams protocol• IP v6 - replacement for IP v4

Under development it is called IPng (Next Generation)

• Why IP v6 Address space exhaustion

Two level addressing (network and host) wastes space Growth of networks and the Internet Single address per host

Requirements for new types of service


Autonomous Systems (AS)

• Set of routers and networks managed by single organization

• Group of routers exchange information• Each AS with its own idea of routing and metrics

defining. An AS is administered independently.


Routing Protocols• Routing Information

About topology and delays in the internet

• Routing Algorithm Used to make routing decisions based on

information

• Interior Router Protocol: Passes routing information between routers within AS Routing algorithms and tables may differ

between different AS IRP needs detailed model e.g., RIP (using Bellman-Ford algorithm) e.g., OSPF ( using Dijkstra’s algorithm)

• Exterior router protocol (ERP): Routers need some info about networks outside their AS: e.g. BGP in Internet supports summary information on

reachability

Documents

Chapter 15&16 Internetworking