Internetworking

• How Networks Differ

• How Networks Can Be Connected

• Concatenated Virtual Circuits

• Connectionless Internetworking

• Tunnelling

• Internetwork Routing

• Fragmentation

Connecting Networks

Internet

ATM WANSNASNA WAN

LLAN

Mainframes

LAN

LAN

LAN

FDDI

BUS LAN

Satellite Link

A collection of interconnected networks

How Networks Differ• Connection-oriented vs. Connectionless• Flat addressing vs. Hierarchical• Frame/Packet size (max)• Error handling and Flow control• Congestion control• Quality of Service• Security• Multicast/Broadcast, Etc.

How Networks Can Be Connected• Physical layer – hubs and repeaters signal amplification• Data link layer – bridges and switches protocol conversion, primitive routing, switching• Network layer – multiprotocol routers wide area routing, network protocol conversion.• Transport layer – transport gateways transport protocol conversion (TCP to SNA)• Application layer – application gateways

Concatenated Virtual Circuits

• Series of Virtual Circuits,• joined by Multiprotocol Routers• to make a long Virtual Circuit…..

Connectionless Internetworking

• Series of datagram networks• joined together at the network layer• by Multiprotocol Routers• to make larger datagram network.

Connectionless verses Connection oriented Internets

• Connection-oriented internetworks have much thesame problems as connection oriented subnets (what arethey?). But they also have the same disadvantages (whichare?).• Connection-oriented internetworks are difficult, ifnot impossible to run across datagram subnets.• Connectionless internetworks have much the samecharacteristics as connectionless subnets (which are?)• Connectionless internets can run across bothdatagram and virtual circuit subnets.

Tunnelling

• Put IP packets into Ethernet frames.• Multiprotocol router puts IP packets into WAN

packets and sends.• Multiprotocol router takes IP packets out of

WAN packets and puts in into Ethernet frames

Internetwork routing

Fragmentation

• Why fragmentation?• Transparent & Non-Transparent

fragmentation

G1 G2 G3 G4

Network 1Network 2

Packet Packet

G1 fragments a large packet

G2 reassembles the fragments

G3 refragments again

G4 reassembles again

Transparent Fragmentation

1) Gateway must know when it has received all the pieces

--Count field or “End of packet” bit

3) If Disjoint routs then performance lost

2) All packets must follow same route

4) Performance overhead

ATM requires fragmentation (segmentation)

Non-Transparent Fragmentation

G1 G2

Packet

G3 G4

G1 fragments a large packet

The fragments are not reassembled until the final destination is reached

1) Every host must be able to do reassembly

2) Overhead, as each fragment must have a header.

Fragmentation--When packet is fragmented, the fragments must be numbered in such a way that the original data stream can be reconstructed.

0

Gateway

0.0 0.1 0.2

Gateway

0.0.0 0.0.1 0.1.2

IP• IP --Internet Protocol• IP sends simple datagram over network• It provides unreliable & connection less delivery

service• Large packets may be fragmented & reassembled in

transmission• IP packet is transmitted in big endian order (i.e. From

left to right)

INTERNET PROTOCOL (IP) HEADER

Header

10101011101010101010010101010100101010100110100101010100101011111110100000111011111010000101110101010011010101111010000010100100000000010101000011010000111111010101......... 1011011001010100011001001010110

Data

IPv4 (internet protocol) header Normal size of IP header is 20 bytes plus Options

IP Header Fields• Version field keeps track of which version of the protocol

• IHL 4 bit field for Header Length . Gives how long the header is in 32 bit words. Minimum is 5, maximum is 15 which limits the header length to 60 bytes and thus options field to 40 bytes.

• Type of Service is 6 bit field, a three bit precedence field (i.e. Priority) & three flags, D, T & R (Delay, Throughput, Reliability).

• Total Length includes both header & data length. Maximum length is 65,536.

• Identification field is needed to allow the destination host to determine which datagram a newly arrived fragment belongs to.

• Flags is a 3 bit field. First bit is unused & then two 1 bit fields are DF & MF

• Fragment Offset tells where the current datagram this belongs to. 13 bits field so maximum 8192 fragments per datagram.

IP Header Fields• Time to Live field is a counter used to limit the packet lifetime

allowing maximum life time of 255 sec.• When the network layer has assembled a complete datagram ,

Protocol field tells it which transport process to give it to.

• Header Checksum verifies the header only. Useful for detecting errors.

• Source Address & Destination Address indicate network number & host number

• Option e.g. Route the packet must follow., Gives list of routers not to miss, Record Route, Timestamp

IP Address• Specifies both network and host• Current length is 32-bit, future length is 128

bit. It is also called as Dotted Decimal Format.

• General Syntax:- 4 components separated by dots, decimal numbers 0-255.

• E.g.18.26.0.1

network 32-bits host

0 NetID

10

110

1110 Multicast Address

HostID

HostID

AA

BB

CC

DD

8 bits 8 bits 8 bits8 bits

1111 Reserved for Future useEE

ClassClass

32 bits

IP Address

Information about Five IP address classes

Class Net id bits Host Bits Range Identification bits

A 7 24 0 - 126 0

B 14 16 128 - 191 10

C 21 8 192 - 223 110

D 28 0 224-239 1110

E 27 0 240-255 1111

This is also known as classful addressing scheme

Special IP Addresses

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 THIS host

0 0 0 0 0 0 0 0 0 0 0 HOST A host on this network

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Broadcast on the local Network

127 Anything Loopback

Network 1111111 ..... 111 Broad cast on distant network

SUBNETTo allow a single network address to span multiple physical networks is called subnet addressing or subnetting

Main Router

TO ISP

Ethernet

PC

COMP

IT

E&TC

MBA

MCA

Subnet Mask • A subnet mask (or number) is used to determine the

number of bits used for the subnet and host portions of the address.

• The mask is a 32-bit value that uses one-bits for the network and subnet portions and zero-bits for the host portion.

• The main router needs a subnet mask that indicates the split between network + subnet no. & host.

• Subnet mask is also written in dotted decimal format with addition of slash followed by the number of bits in the network + subnet part. E.g. 255.255.255.0/22

EXAMPLEs

10 Network Subnet Host

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 Subnet mask

e.g.

E.g.:- a packet addressed to 130.50.15.6 and arriving at the main router is ANDed with the subnet mask 255.255.252.0/22 to give the address 130.50.12.0

Thus subnetting reduces the router table space by creating the hierarchy of network, subnet, host.

Disadvantages of Classful Addressing

– Wasteful address architecture– Network boundaries are fixed at 8, 16 or 24 bits

(class A, B, and C)

ICMP (Internet Control Message Protocol)

• ICMP is an error reporting and network

management system.• ICMP attempts to keep the Internet running as efficiently as possible.• ICMP works to correct network problems,

although does not try to correct individual packet problems.

ICMP Packets

•Two levels of encapsulation that occur when an ICMP message is sent.

• ICMP packets are sent in IP Datagrams. They have no special priority,

ICMP Header ICMP Data Area

IP Header IP Data Area

Frame Header Frame Data

Area

•The ICMP message is encapsulated in a datagram, which is encapsulated in a frame for transmission across a physical network.

ICMP Header+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Code | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+• The Type field indicates the type of ICMP message.• The Code field provides data specific to the type.• The Checksum detects errors in the ICMP packet.

ICMP Message TypesMessage Type Description

Destination Unreachable Packet could not be delivered

Time exceeded Time to live field hits to 0

Parameter Problem Invalid header field

Source Quench Choke packet

Redirect Teach a router about geography

Echo Ask machine if it is alive

Echo reply Yes, I m alive

Timestamp request Echo request with time stamp

Timestamp reply

Some Uses for ICMP1. Echo (ping)2. Tells the source a packet exceeded the TTL.3. Tell the source that a packet had to be fragmented.4. Redirect the host to a better router.5. Controls the rate at which a host sends packets.(Source

Quench)6. Tell the source the destination is unreachable. Either the

network or the host is down.7. Request subnet mask.

ARP (Address Resolution Protocol)

• Mapping between a protocol address & a hardware address is called resolution.

• In computer networking, the Address Resolution Protocol (ARP) is the method for finding a host's link layer (hardware) address when only its Internet Layer (IP) or some other Network Layer address is known

32

NetworkLayer

Link Layer

IP

ARP NetworkAccess RARP

Media

ICMP IGMP

TransportLayer

TCP UDP

Overview

33

ARP and RARP• Note:

– The Internet is based on IP addresses – Data link protocols (Ethernet, FDDI, ATM) may have

different (MAC) addresses

• The ARP and RARP protocols perform the translation between IP addresses and MAC layer addresses

RARP

Ethernet MACaddress(48 bit)

ARPIP address(32 bit)

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Processing of IP packets by network device drivers

loopbackDriver

IP Input

Put on IPinput queue

ARPdemultiplex

Ethernet Frame

Ethernet

IP destination of packet= local IP address ?

IP destination = multicastor broadcast ?

IP Output

Put on IPinput queue

No: get MACaddress withARP

ARPPacket

IP datagram

No

Yes

YesEthernet

Driver

35

Address Translation with ARP

ARP Request: Argon broadcasts an ARP request to all stations on the network: “What is the hardware address of Router137?”Argon

128.143.137.14400:a0:24:71:e4:44

Router137128.143.137.1

00:e0:f9:23:a8:20

ARP Request:What is the MAC addressof 128.143.71.1?

36

Address Translation with ARP

ARP Reply: Router 137 responds with an ARP Reply which contains the hardware addressArgon

128.143.137.14400:a0:24:71:e4:44

Router137128.143.137.1

00:e0:f9:23:a8:20

ARP Reply:The MAC address of 128.143.71.1is 00:e0:f9:23:a8:20

37

ARP Packet Format

Hardware type (2 bytes)

Hardware addresslength (1 byte)

Protocol addresslength (1 byte)

Operation code (2 bytes)

Target hardware address*

Protocol type (2 bytes)

Source hardware address*

Source protocol address*

Target protocol address*

* Note: The length of the address fields is determined by the corresponding address length fields

Sending an ARP Message

ARP Message

Frame Header

Frame Data Area CRC

39

• ARP Request from Argon: Source hardware address: 00:a0:24:71:e4:44Source protocol address: 128.143.137.144Target hardware address: 00:00:00:00:00:00Target protocol address: 128.143.137.1

• ARP Reply from Router137: Source hardware address: 00:e0:f9:23:a8:20 Source protocol address: 128.143.137.1 Target hardware address: 00:a0:24:71:e4:44Target protocol address: 128.143.137.144

ARP CACHE-Since sending an ARP request/reply for each IP datagram is inefficient, hosts

maintain a cache (ARP Cache) of current entries. The entries expire after 20 minutes.

40

Things to know about ARP• What happens if an ARP Request is made for a non-existing host?

Several ARP requests are made with increasing time intervals between requests. Eventually, ARP gives up.

• On some systems (including Linux) a host periodically sends ARP Requests for all addresses listed in the ARP cache. This refreshes the ARP cache content, but also introduces traffic.

• Gratuitous ARP Requests: A host sends an ARP request for its own IP address:– Useful for detecting if an IP address has already been assigned.

• All IP addresses have a network and host portion. In classful addressing, the network portion ends on one of the separating dots in the address (on an octet boundary). Classless addressing uses a variable number of bits for the network and host portions of the address.

• • Decimal 192 160 20 48 Binary 11000000 10100000 00010100 0011 0000 <-------- 28 bits

Network -------> 4 bits host Classful addressing divides an IP address into the Network and Host portions along octet boundaries. Classless addressing treats the IP address as a 32 bit stream of ones and zeroes, where the boundary between network and host portions can fall anywhere between bit 0 and bit 31. The network portion of an IP address is determined by how many 1's are in the subnet mask. Again, this can be a variable number of bits, and although it can fall on an octet boundary, it does not necessarilly need to. A subnet mask is used locally on each host connected to a network, and masks are never carried in IPv4 datagrams. All hosts on the same network are configured with the same mask, and share the same pattern of network bits. The host portion of each host's IP address will be unique.

•

CIDR (Classless InterDomain Routing)• The rapid growth in the size of the global Internet’s routing

tables

• The eventual exhaustion of the 32-bit IPv4 address space

• CIDR eliminates the traditional concept of Class A, Class B, and Class C network addresses.

NAT (Network Address Translation)

• = temporary binding and address/port rewriting in IP and L4 packet headers– table of (internal 5-tuple, external 5-tuple)– must allow unique mapping of return packets

• Typically uses private address space– 10.0.0.0/8, 172.16.0.0-172.31.0.0., 192.168.0.0/24– 169.254.0.0/16 reserved for automatic IP addressing

• Designed for client-server TCP applications with short session lifetimes– = web, email retrieval

• Binding is temporary (minutes)– causes failures for long-lived applications– application-layer “keep-alives” in ssh, IMAP, SIP, …