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: 2 14 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 ( )  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

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