Internetworking

• Internet: A network among networks, or a network of networks

• Allows accommodation of multiple network technologies

• Universal Service

• Routers to interconnect heterogeneous networks

Virtual Network

• Internet is a virtual network

• It is an abstraction

• There is no network called the Internet

• Abstraction takes place in software (I.e. protocols)

Layering and TCP/IP Protocols

Layer 5: Application Corresponds to ISO model layers 6 and 7; used for communication among applications

Layer 4: Transport Corresponds to layer 4 in the ISO model; provides reliable delivery of data

Layer 3: Internet Defines uniform format of packets forwarded across networks of different technologies and rules for forwarding packets in routers

Layer 2: Network Corresponds to layer 2 in the ISO model; defines formats for carrying packets in hardware frames

Layer 1: Hardware Corresponds to layer 1 in the ISO model; defines basic networking hardware

Internet Protocol (IP)

• Uniform Addressing Scheme

• Hardware Independent

• 32-bit number IP or Internet Address

• Unique on the Internet

• Divided into prefix and suffix

• Prefix: Network

• Suffix: Computer (Host)

•Class of Address determines break between network prefix and host suffix

•Small prefix: few networks, many hosts

•Large prefix: large networks, few hosts/network

•Compute IP address class from first 4 bits of address

•A, B, C are primary classes

•D is for multicast; E is reserved

•Dotted Decimal Notation

•A, B, C break on bytes between prefix and suffix

•Compute decimal value of each byte and display

separated by dots

IP Problems

• IP Addresses not used efficiently

• UARK is Class B: 214 addresses

• Larger organizations may not have enough addresses, others too many

• Solution: Private internets that use entire 32-bit address space

• IPv6: 128 bit address space

Special IP Addresses

Routers and IP Addressing

• IP address depends on network address

• What about routers - connected to two networks?

• IP address specifies an interface, or network attachment point, not a computer

• Router has multiple IP addresses - one for each interface

Address Resolution

• Protocol address on virtual network must be resolved into physical address

Address Resolution Protocol (ARP)

• Distributed Address Resolution

• Part of TCP/IP Protocol Suite

• Two-Part Protocol– Request from source asking for hardware

address

– Reply from destination carrying hardware address

ARP request message dropped into hardware frame and broadcast

Sender inserts IP address into message and broadcast

Every other computer examines request

Computer whose IP address is in request responds Puts hardware address in response

Unicasts to sender

Original requester can then extract hardware address and send IP packet to destination

ARP Message Format

HARDWARE ADDRESS TYPE = 1 for Ethernet

PROTOCOL ADDRESS TYPE = 0x0800 for IP

OPERATION = 1 for request, 2 for response

Contains both target and sender mappings from protocol address to hardware address

Request sets hardware address of target to 0

Target can extract hardware address of sender (saving an ARP request)

Target exchanges sender/target in response

Sending ARP Message

IP Datagrams

• Fundamental Internet Communication Packet

• Connectionless• Transport protocols use this connectionless

service to provide connectionless data delivery (UDP) and connection-oriented data delivery (TCP)

Datagrams can have different sizes

Header area usually fixed (20 octets) but can have options

Data area can contain between 1 octet and 65,535 octets (216 - 1)

Usually, data area much larger than header

Formally, the unit of IP data delivery is called a datagram

Includes header area and data area

Forwarding and Routing

• Header contains all info for delivery

• Router examines header and forwards to next destination

• Forwarding Information stored in routing table

• List of destination networks and next hops

Routing and Masks

• In practice, additional information is kept in routing table

• Destination stored as network address

• Next hop stored as IP address of router

• Address mask defines how many bits of address are in prefix Prefix defines how much of address used to identify

network

E.g., class A mask is 255.0.0.0

Best Effort Delivery

• IP does not guarantee to prevent Duplicate datagrams

Delayed or out-of-order delivery

Corruption of data

Datagram loss

• Reliable delivery provided by transport layer Network layer

•VERS - version of IP (currently 4)

•H. LEN - header length (in units of 32 bits)

•SERVICE TYPE - sender's preference for low latency, high reliability (rarely used)

•TOTAL LENGTH - total octets in datagram

•IDENT, FLAGS, FRAGMENT OFFSET - used with fragmentation

•TTL - time to live; decremented in each router; datagram discarded when TTL = 0

•TYPE - type of protocol carried in datagram; e.g., TCP, UDP

•HEADER CHECKSUM - 1s complement of 1s complement sum

•SOURCE, DEST IP ADDRESS - IP addresses of original source and ultimate destination

Encapsulation

MTU

• Maximum Transmission Unit

• Size of Datagram < MTU

• Fragment Datagram to accommodate networks with different MTU’s

• Fragment Offset gives location of fragment in original datagram

• Reassembled at destination

Reassembly and Loss

• IDENT field to associate fragments with original

• Fragments may be lost => lose entire datagram

• Destination has timer on fragments

• Source retransmits

• Fragment a fragment