Scaling the Network: The Internet Protocol Networking CS 3470, Section 1

Network Layer: Introduction How does it all fit? Data link layer Delivers frames between two physically connected hosts Network layer Delivery of packets from machine to machine Hop by hop between hosts and routers Transport layer Between two end hosts

Today Networking forwarding Internet Protocol (IP) IP Classful Address Scheme ARP!

Network Layer Functions Addressing Globally unique address for each routable device Logical address, unlike MAC address Assigned by network operator Need to map to MAC address Forwarding From input port to appropriate output port in a router Routing Which path to use to forward packets from src to dest

Network Layer Forwarding Forwarding input port to appropriate output port in a router We already talked about forwarding over the network layer Datagram / Connectionless Virtual Circuit Switching Which one is used with IP?

Datagram vs Virtual Circuit Datagram (example: IP) Data exchange among computers “Elastic” service, no strict timing req. “Smart” end systems (computers) Can adapt, perform control, error recovery Simple inside network, complexity at “edge” Many link types Different characteristics Uniform service difficult

Datagram vs Virtual Circuit Virtual Circuit (Example: ATM) Evolved from telephony Human conversation: Strict timing, reliability requirements Need for guaranteed service Quality of service “Dumb” end systems Complexity inside network

The University of Adelaide, School of Computer Science 7 April 2019 Internetworking What is an internetwork? An arbitrary collection of networks interconnected to provide some sort of host-host packet delivery service Chapter 2 — Instructions: Language of the Computer

The University of Adelaide, School of Computer Science 7 April 2019 IP IP stands for Internet Protocol Key tool used today to build scalable, heterogeneous internetworks Chapter 2 — Instructions: Language of the Computer

IP A IP router is a gateway from one network to another Can interface with many network types Ethernet PPP Token Ring 802.11 ... more

[TCP/UDP]/IP Two well-known transport level protocols that run on top of IP at the hosts are UDP and TCP TCP Connection-based protocol Error recovery Packets arrive in order UDP Connectionless protocol No error recovery Packets can arrive in any order, or not at all

Internetworking A simple internetwork showing the protocol layers IP connects them all!

The University of Adelaide, School of Computer Science 7 April 2019 IP Service Model Packet Delivery Model Connectionless model for data delivery Best-effort delivery (unreliable service) What bad stuff can happen? Global Addressing Scheme Provides a way to identify all hosts in the network Chapter 2 — Instructions: Language of the Computer

The University of Adelaide, School of Computer Science 7 April 2019 Packet Format Chapter 2 — Instructions: Language of the Computer

IP Fragmentation and Reassembly The University of Adelaide, School of Computer Science 7 April 2019 IP Fragmentation and Reassembly Each network has some MTU (Maximum Transmission Unit) Ethernet (1500 bytes), FDDI (4500 bytes) Strategy Fragmentation occurs in a router when it receives a datagram that it wants to forward over a network which has (MTU < datagram) Reassembly is done at the receiving host All the fragments carry the same identifier in the Ident field Fragments are self-contained datagrams IP does not recover from missing fragments Chapter 2 — Instructions: Language of the Computer

IP Fragmentation and Reassembly The University of Adelaide, School of Computer Science 7 April 2019 IP Fragmentation and Reassembly IP datagrams traversing the sequence of physical networks Chapter 2 — Instructions: Language of the Computer

IP Fragmentation and Reassembly The University of Adelaide, School of Computer Science 7 April 2019 IP Fragmentation and Reassembly Header fields used in IP fragmentation. (a) Unfragmented packet; (b) fragmented packets. Chapter 2 — Instructions: Language of the Computer

IP Addressing Globally unique logical address for a host Address resolution Logical to physical address mapping What was the physical address? Is possible to address any host in the network Even if on different physical network

IP Addressing A 32-bit number that uniquely identifies a location Written using dotted decimal notation Common form: 134.161.240.211 Binary representation: 10000110 10100001 11110000 11010011 Two-level hierarchy: network id and host id Network IDs administered by Internet Assigned Number Authority (IANA) Host IDs administered locally

IP Addressing IP address is assigned to each network interface (NIC) Routers connect two or more physical networks Each interface has its own address Multi-homed host A host having multiple connections to Internet Multiple addresses identify the same host Does not forward packets between its interfaces

IP Addressing Classful addressing scheme separates groups of addresses into classes Class A 8 bits used for network (256) 24 bits used for hosts and network devices (16,777,216) Binary address starts with 0 Class B 16 bits for networks (65,536) 16 bits for hosts and network devices (65,536) binary address starts with 10 Class C 24 bits for the network (16,777,216) 8 bits for the host (256) Binary address starts with 110

IP “Classful” Addressing Scheme Three unicast address classes: A, B, and C One multicast: class D network host 10 110 1110 multicast address A B C D class 1.0.0.0 to 127.255.255.255 128.0.0.0 to 191.255.255.255 192.0.0.0 to 223.255.255.255 224.0.0.0 to 239.255.255.255 32 bits In the early days of the Internet development, a so-called classful addressing scheme was used on the Internet. In classful addressing, we can determine the class and hence the network id from the first few bits of the address. The network part can only be 8, 16 or 24 bits. Depending on the how big a network is and will be, a suitable class network id is allotted for that network.

Address Resolution IP address is virtual Not understood by underlying physical networks IP packets need to be transmitted by the underlying physical network Address resolution Translating IP address to physical address Address Resolution Protocol (ARP) As mentioned earlier, IP address is virtual and need to be mapped to a physical address for delivering packets by a physical network. This translation of IP to physical addresses is called address resolution and this is achieved by ARP.

ARP A router has to know where to deliver packets on the local network ARP is used to discover MAC addresses based on IP addresses

ARP Cache Each computer maintains a cache table Exchanges ARP messages IP address  hardware address mapping Only about computers on the same network Exchanges ARP messages To resolve IP addresses with unknown hardware addresses Encapsulated in Ethernet data frame

ARP Protocol When a node sends an IP packet To another node on the same physical network Look up destination address in the ARP table If not found Broadcast a request to the local network Whose IP address is this? What info should the request message contain?

ARP Example arp who-has 192.168.10.1 tell node31.ceee.lab arp reply 192.168.10.1 is-at 00:60:08:ce:9d:3b arp who-has node31.ceee.lab tell 192.168.10.254 arp reply node31.ceee.lab is-at 00:02:3f:b4:cd:87

ARP Response The target node responds to sender (unicast?) With its physical address Adds the requester into its ARP table (why?) On receiving the response Requester updates its table Other nodes upon receiving the request Refresh the requester entry if already there No action otherwise (why?) Table entries deleted if not refreshed for a while

ARP Example ARP broadcast by W requesting hardware address of Y