ICOM 6115©Manuel Rodriguez-Martinez ICOM 6115 – Computer Networks and the WWW Manuel Rodriguez-Martinez, Ph.D. Lecture 21

ICOM 6115©Manuel Rodriguez-Martinez Lecture Objectives Introduction to Global Internetworking –Layer 3 – The Network Layer Store-and-Forward Service –IP Protocol IP addressing ARP, ICMP –Routing

ICOM 6115©Manuel Rodriguez-Martinez Internetworking What is this? –Process of connecting independent and possibly heterogeneous LANs into a larger network Again: Independent LANs –Each LAN might have a different Layer 2 Ethernet Token Ring b What Layer takes care of this service? –Network Layer (Layer 3)

ICOM 6115©Manuel Rodriguez-Martinez Example internetwork Network 3 (FDDI) H1H2R1 Network 1 (Ethernet) H3H4R2 Network 2 (802.11b) H5H6H7H8R3R4 Network 4 (Ethernet) Network 5 (Point-to-Point)

ICOM 6115©Manuel Rodriguez-Martinez Layer 3 and LAN Integration Multiple, heterogeneous LANs can become Interoperable via IP (Layer 3 example)

ICOM 6115©Manuel Rodriguez-Martinez Layer 3: Packet Switching Layer 3 deals with getting packets forwarded from source to destination –Across LANs Layer 2 was switching within a LAN Routers are computers that provide service of packet forwarding –Must understand the topology of internet –Forwarding must be fast –Routing should produce accurate forwarding tables

ICOM 6115©Manuel Rodriguez-Martinez Store-and-Forward Service Packets are send by end-host to routers Routers –Store each packet until fully arrives –Make checksum calculations –Lookup forwarding tables to determine next hop A router or final destination host –Forward the packet along the line (port) of next hop

ICOM 6115©Manuel Rodriguez-Martinez Store-and-Forward Packet Switching Routers form a subnet of forwarding elements

ICOM 6115©Manuel Rodriguez-Martinez Layer 3 Service Principles Layer 3 will provide Layer 4 (Transport Layer) packet forwarding services –Connectionless –Connection-oriented Driving Principles –Should independent of router technology –Transparency from routers for Layer 4 Type, number and topology –Uniform addressing scheme across network LANs and WANs

ICOM 6115©Manuel Rodriguez-Martinez Connectionless Service Advocated by Internet community –IP Protocol Data is broken into packets, that are moved independently over the network –Datagram –Each one can take a different path Most often they all take the same path Each router moves the datagram along path from src to dst

ICOM 6115©Manuel Rodriguez-Martinez Route Information Routers typically store routes to networks as the router next router to get there

ICOM 6115©Manuel Rodriguez-Martinez Example Forwarding Tables A- BB CC DB EB FC AA BA C- DD EE FE AC BD CC DD E- FF A’s Forwarding Table C’s Forwarding Table E’s Forwarding Table

ICOM 6115©Manuel Rodriguez-Martinez Connection-Oriented Service Advocated by Telephone companies –ATM Protocol Data is broken into packets, that are moved along a virtual circuit (VC) over the network –Circuit must be first set up –Each packet takes the same path Driven by performance guarantees –Bounds on delays for packet arrival

ICOM 6115©Manuel Rodriguez-Martinez Virtual Circuit based network

ICOM 6115©Manuel Rodriguez-Martinez IP: Network Layer on Internet

ICOM 6115©Manuel Rodriguez-Martinez IP features Connectionless Best effort delivery –Every effort is made to deliver datagram No guarantee is made –Can be lost or delivered out of order –Simplicity –Unreliable Need transport to add reliability (TCP) Runs over “anything” –Support many Layer 2 protocols Many invented after IP (e.g b)

ICOM 6115©Manuel Rodriguez-Martinez IP v4 Packet Format Variable length 65KB Max Length

ICOM 6115©Manuel Rodriguez-Martinez Some Issues Header length is measured in terms of 32- bit word Datagram length is measured in terms of number of bytes –Max size is 64KB Fragments indicates offset in terms of 8- byte chunks –Only have 13 bits to expresses offset

ICOM 6115©Manuel Rodriguez-Martinez Fragmentation and Reassembly Packets might get fragmented as they travel over the net Why? –Layer 2 might have different max frame size Example: –10Mbps Ethernet – max frame size is 1500 bytes –100Mbps FDDI – max frame size is 4500 bytes –56Kbps PPP – max frame size is 532 bytes –Packet might get cut to make it fit into Layer 2 frame

ICOM 6115©Manuel Rodriguez-Martinez Example of Fragmentation Packet gets broken along the way to make it fit Layer 2 frame. It must be reassembled by end-receiver

ICOM 6115©Manuel Rodriguez-Martinez Network MTU Each network has a maximum transmission unit (MTU) –Biggest IP datagram that be fit into a frame IP datagram can have variable size Good idea is to keep it under MTU –Avoid fragmentation whenever possible Let it happen on a case by case basis (be flexible) –Send as much data as possible in each frame Good use of bandwidth

ICOM 6115©Manuel Rodriguez-Martinez Reassembly Routers can fragment packet –They won’t reassemble it –Each fragment gets forwarded independently End-receiver deals with the problem of reassembling the packet –Putting pieces back in order –Lost fragment causes retransmission of entire packet!

ICOM 6115©Manuel Rodriguez-Martinez Control Information for reassembly Each packet carries an identification number –Sequence number Fragments are identified as parts of packet with given id number –Fragment identifies offset bytes for a packet –Example: offset for individual bytes Offset 0 of packet Offset 512 of packet Offset 1024 of packet –In reality, offset counts groups of 8-bytes

ICOM 6115©Manuel Rodriguez-Martinez IP Addresses Unique, Global identifier for a network card –Machines with multiple card get multiple IPs e.g. routers IP address is a 32 bit number –For simplicity, expressed in dotted-decimal notation e.g Bits 32

ICOM 6115©Manuel Rodriguez-Martinez What are benefits of IP Addresses? Hierarchical structure –Provide way to identify network and host Ethernet are flat –Example: Host 20 Network Routers need only to keep information about network –subnets

ICOM 6115©Manuel Rodriguez-Martinez IP Address Formats

ICOM 6115©Manuel Rodriguez-Martinez Fragmentation in packets Start of header Id = X 0 Rest of Header 1400 data bytes Offset=0 Start of header Id = X 1 Rest of Header 512 data bytes Offset=0 Start of header Id = X 1 Rest of Header 512 data bytes 512 Offset Start of header Id = X 0 Rest of Header 512 data bytes 1024 Offset Fragmentation Fragment 1 Fragment 2Fragment 3