Network Protocols

1. Network Protocols IP addressing – Classes IP packet format ARP and RARP ICMP

2. The Function of Protocols • As protocols serve their functions in the OSI model, they might work at one or many layers • When a set of protocols works cooperatively, it’s called a protocol stack or protocol suite • The most common protocol stack is TCP/IP, the Internet protocol suite • IPX/SPX, used in older versions of Novell NetWare, is disappearing as companies upgrade to newer versions of NetWare • Levels of a protocol stack map to their functions in the OSI model

3. Routable vs. Nonroutable Protocols • The network layer (OSI) is responsible for moving data across multiple networks • Routers are responsible for routing process • Protocol suites that function at Network layer are routable or routed protocols; otherwise, they are called nonroutable • TCP/IP and IPX/SPX are routable protocols • An older and nearly obsolete protocol, NetBEUI, is a nonroutable protocol that works well in small networks, but its performance drops considerably as a network grows.

4. Protocols in a Layered Architecture

5. Network Protocols • Some popular network protocols include: • Internet Protocol version 4 (IPv4 or simply IP) • Provides addressing and routing information • Internetwork Packet Exchange (IPX) • Novell’s protocol for packet routing and forwarding • Belongs to the IPX/SPX protocol suite • Serves many of the same functions as TCP/IP’s IP • Internet Protocol version 6 (IPv6) • A new version of IP that’s being implemented on many current networking devices and operating systems • Addresses some weaknesses of IPv4

6. Common Protocol Suites • Because most protocols contain a combination of components, these components are usually bundled as a protocol suite • TCP/IP • Dominates the networking arena to the point of making most of the other suites nearly obsolete • IPX/SPX • NetBIOS/NetBEUI • AppleTalk

7. TCP/IP Network Layer Protocols • Internet Protocol version 4 (IPv4) is a Network layer protocol that provides source and destination addressing and routing for the TCP/IP suite • Connectionless protocol; fast but unreliable • Internet Control Message Protocol (ICMP) is a Network layer protocol used to send error and control messages between systems or devices • The Ping utility uses ICMP to request a response from a remote host to verify availability • Address Resolution Protocol (ARP) resolves logical (IP) addresses to physical (MAC) addresses

8. IP Service • IP provide provides an unreliable connectionless best effort service (also called: “datagram service”). • Unreliable: IP does not make an attempt to recover lost packets • Connectionless:Each packet (“datagram”) is handled independently. IP is not aware that packets between hosts may be sent in a logical sequence • Best effort: IP does not make guarantees on the service (no throughput guarantee, no delay guarantee,…) • Consequences: • Higher layer protocols have to deal with losses or with duplicate packets • Packets may be delivered out-of-sequence

9. IP Service • IP supports the following services: • one-to-one (unicast) • one-to-all (broadcast) • one-to-several (multicast) • IP multicast also supports a many-to-many service. • IP multicast requires support of other protocols (IGMP, multicast routing) unicast broadcast multicast

10. Internet Addresses (IP Addresses) • Defined when IP was standardized in 1981 • IP addresses are 32-bit long and consist of: • a network address part – network identifier • a host address part – host number within that network • IP addresses are grouped into classes (A,B,C) depending on the size of the network identifier and the host part of the address • A fourth class (Class D) was defined later (1988) for Multicast addresses

11. Internet Address Classes

12. Internet Address Classes

13. Internet Address Classes 0 8 16 24 32 bits Class A Class B Class C Class D Class E 0 NETWORK HOST (24 BITS) 10 NETWORK HOST (16 BITS) 110 NETWORK HOST (8 BITS) 1110 IP MULTICAST ADDRESSES (28 BITS) 11110 RESERVED FOR EXPERIMENTS

14. IP Addressing - Classes • Class A: first octet between 1-126 • 126 networks (0 and 127 reserved) • Assigned to very large size networks • 65K < number of hosts < 16M • Class B: first octet between 128-191 • 16384 networks • Assigned to Intermediate size networks • 256 < number of hosts < 65K

15. IP Addressing - Classes • Class C: first octet between 192-223 • 2097152 networks • Assigned to smaller networks • hosts < 256 • Class D: first octet between 224-239 • Reserved for multicasting • Class E: first octet between 240-255 • Reserved for experimental use

16. Internet Address Classes

17. Dotted Decimal Notation • Internet addresses are represented in text by the dotted decimal notation • each byte is written in decimal values (from 0 to 255) • example: 10000000 00001010 00000010 00011110 is written as 128. 10. 2. 30

18. Uniqueness of IP Addresses • Network numbers are assigned by a central authority • The Internet Network Information Center (InterNIC) • Another authority, the IANA – Internet Assigned Numbers Authority sets policy • Network numbers are unique worldwide • Host numbers are assigned by network managers • They must be unique within a given network • Thus, IP addresses are unique worldwide.

19. Example Find the class of each address: A- 227.12.14.87 B- 193.14.56.22 C- 14.23.120.8 D- 252.5.15.111 E- 134.11.78.56 Solution A. The first byte is 227 (between 224 and 239); the class is D.B. The first byte is 193 (between 192 and 223); the class is C.C. The first byte is 14 (between 0 and 127); the class is A.D. The first byte is 252 (between 240 and 255); the class is E.E. The first byte is 134 (between 128 and 191); the class is B.

20. Special Purpose IP Addresses • 0.0.0.0 • Means this host, used by machines as source address when they boot up (if they don’t know their IP address, and need to get it from a boot server) • 255.255.255.255 • Means limited broadcast. Used as a destination address to send packets to all hosts on the local network where the source is. Packets sent to this address are never relayed • Network part all zeros • Means the host on this network

21. Special Purpose IP Addresses • Host part all ones • Broadcast address on the network specified in the network identifier; routers typically do not forward these datagrams • Host part all zeros • Broadcast address on the network specified in the network identifier (it was an implementation error in some networks)

22. Special Purpose IP Addresses • 127.x.x.x • Means loopback (datagrams are looped back in software; they are not sent on any physical interface) • 224.0.0.1 • Multicast address for “All systems on this subnetwork” • 224.0.0.2 • Multicast address for “All routers on this subnetwork”

23. IPv4 versus IPv6 • IP version 6 (IPv6) has been defined and developed. • IPv6 uses 128 bits rather than the 32 bits currently used in IPv4. • IPv6 uses hexadecimal numbers to represent the 128 bits. IPv4

24. Translating Between IP and MAC Addresses (ARP and RARP) • Each interface has an IP address at Layer 3, and a MAC address at Layer 2 • Assume that host A wants to send a packet to host B (A and B on the same network) • Host A knows the IP address of host B; however, in order to transmit the packet, host A must somehow know or find out what the MAC (layer 2) address of host B is! • Solution: the Address Resolution Protocol (ARP), RFC826

25. Address Resolution Protocol • Used to find the physical address of a target device on the local physical network, given only the target’s IP address. ARP mechanism: • The source broadcasts a special packet asking the device with target IP address to respond with a message carrying the (IP address, physical address) mapping • All devices on the local physical network receive the broadcast, but only the target recognizes its IP address and responds to the request • When the source receives the reply, it sends the packet to the target using the target’s physical address and places the mapping in its cache.

26. ARP Process

27. ARP - example ARP enables a computer to find the MAC address of the computer that is associated with an IP address. t

28. ARP - example All devices on the network receive the packet and pass to network layer; only one device responds with an ARP reply.

29. 0 8 16 24 31 HARDWARE TYPE PROTOCOL TYPE HLEN PLEN OPERATION SENDER HA (octets 0-3) SENDER HA (octetS 4-5) SENDER IP (octetS 0-1) SENDER IP (octetS 2-3) TARGET HA (octets 0-1) TARGET HA (octets 2-5) TARGET IP (octets 0-3) ARP Message Format

30. ARP Message • HARDWARE TYPE: specifies type of hardware interface for which the request is made (e.g., 1 for Ethernet) • PROTOCOL TYPE: specifies high level protocol address supplied in message (e.g. 0800 hex for IP) • HLEN and PLEN: specify length of fields for hardware address and protocol address respectively • OPERATION: specifies if this is an ARP request or reply message (1 for ARP request, 2 for ARP response, 3 for RARP request and 4 for RARP response) • HA and IP: hardware and IP addresses respectively

31. Reverse ARP - RARP • Usually, a machine’s IP address is kept on its secondary storage (OS finds it at start up) • Issue : Diskless Workstations! • files are stored on a remote server • need IP address to use TCP/IP to obtain initial boot image • Solution : Use physical address to identify machine • Given a physical network address, find the corresponding Internet address • Reverse Address Resolution Protocol (RARP), RFC903

32. RARP • Mechanism • Sender broadcasts a RARP request, supplying its physical network address in the Target HA field • Only machines authorized to supply the RARP service (RARP servers) process the request and send a reply filling in the target internet address • Mechanism allows a host to ask about an arbitrary target • thus sender HA is separate from target HA address • RARP server replies to sender’s HA • Ethernet frame Protocol Type for RARP is 8035 hex

33. RARP The source initiates a RARP request, which helps it detect its own IP address.

34. Advanced ARP Concepts • Default gateway • Proxy ARP

35. Default Gateway • Another term for a router • If a computer does not know how to deliver a packet, it sends the packet to the default gateway • A router may be a dedicated hardware device or a computer with multiple network cards • The router must be on the same network as the computer sending the packet

36. How ARP Sends Data to Remote Networks?

37. Proxy ARP

38. Internet Control Message Protocol • Architecturally above IP -- ICMP messages are carried in IP packets and are demultiplexed at receiver. • Transfer of (control) messages from routers and hosts to hosts • Feedback about problems • e.g. time to live expired, destination unreachable (e.g. no ARP reply), checksum fails (header only!), no route to destination, etc. • Considered “part” of IP, but it is really a user of IP • Encapsulated in IP datagram • Not reliable • ICMP messages sent in response to incoming datagrams with problems • ICMP message not sent for ICMP message

39. ICMP

40. Used by hosts, routers, gateways to communication network-level information error reporting: unreachable host, network, port, protocol echo request/reply (used by ping) ICMP message: type, code plus first 8 bytes of IP datagram causing error Internet Control Message Protocol TypeCodedescription 0 0 echo reply (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header

41. ICMP and Ping • An internet host, A, is reachable from another host, B, if datagrams can be delivered from A to B • ping program tests reachability - sends datagram from B to A that A echoes back to B • Uses ICMP echo request and echo reply messages • Internet layer includes code to reply to incoming ICMP echo request messages

42. ICMP and MTU Discovery • Fragmentation should be avoided for optimal performance • How can source configure outgoing datagrams to avoid fragmentation? • Source determines path MTU - smallest network MTU on path from source to destination • Source probes path using IP datagrams with don't fragment flag • Router responds with ICMP fragmentation required message • Source sends smaller probes until destination reached.

43. ICMP and Redirect • Default route may cause extra hop • Host A is sending a packet to Host B. Host A's default IP router is router R1. Host A forwards the packet destined for Host B to its default router R1. • R1 checks its routing table and finds that the next hop for the route to the network for Host B is router R2. • If Host A and R2 are on the same network that is also directly attached to R1, an ICMP Redirect message is sent to Host A informing it that R2 is the better route when sending to Host B. • Router R1 then forwards the IP datagram to R2. • Host A adds a host route to its routing table for Host B's IP address with router R2's IP address as the forwarding address. Subsequent datagrams from Host A to Host B are forwarded by means of router R2.

44. Internet Control Message Protocol 5-61

45. ICMP in Action

46. IP Packet Format • 20 bytes ≤ Header Size< 24 x 4 bytes = 60 bytes • 20 bytes ≤ Total Length < 216 bytes = 65536 bytes

47. IP header format

48. IP header format: Version • 4 bits. • Indicates the version of IP currently used. • IPv4 : 0100 • IPv6 : 0110

49. IP header format: Header length • 4 bits. • IP header length : Indicates the datagram header length in 32 bit words (4 bits), and thus points to the beginning of the data.

50. IP header format: Service type • 8 bits. • Specifies the level of importance that has been assigned by a particular upper-layer protocol. • Precedence. • Reliability. • Speed.