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Overview

Overview. Last Lecture Internet Protocols (1) Source: chapter 15 This Lecture Internet Protocols (2) Source: chapter 15 Next Lecture Internet Protocols (3) Source: chapter 15. IP and MAC address. IP uses MAC frames as vehicles to transfer IP packets

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Overview

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  1. Overview • Last Lecture • Internet Protocols (1) • Source: chapter 15 • This Lecture • Internet Protocols (2) • Source: chapter 15 • Next Lecture • Internet Protocols (3) • Source: chapter 15

  2. IP and MAC address • IP uses MAC frames as vehicles to transfer IP packets • The MAC address of the destination is needed to transfer an IP packet • Hardware MAC addresses • Ethernet 6 bytes • Token ring 2 or 6 bytes • FDDI 2 or 6 bytes • A station normally knows only the IP address of the destination • How does an IP address get mapped into a MAC address? • Manual configuration is tedious and may not be easy to adapt to change • An Address Resolution Protocol (ARP) is used for automatic mapping • Address Resolution Protocol • RFC 826 • The sending host broadcasts a frame onto the Ethernet asking “who owns the IP address specified? • The host who has the IP address will respond with its Ethernet address • Then the sending host will use the Ethernet address to send packets

  3. ARP • Host X wants to resolve the MAC address of host Z • X broadcasts ARP request • X gets unicast ARP reply from Z • ARP packet format

  4. ARP • RARP: Reverse ARP • Map a MAC address to an IP address • For devices that cannot store IP, usually diskless workstations • Need to set up a server with a RARP table • ARP packet header • Hardware type: Ethernet,1;ARCNET,7; • Network protocol type: IP, 0x0800 • hlen:length of hardware address • 6 bytes for Ethernet addresses • plen: length of network address:4 bytes for IP • ARP operation: ARP request,1; ARP reply,2; RARP request, 3; RARP reply, 4 • Example of ARP request packet

  5. ARP • Example of ARP reply packet • ARP mechanism • Each host maintains an ARP cache • ARP first looks up the cache in the host • If an entry in the cache is not refreshed for a period, say 15 minutes, it is deleted. • A host can add an entry to the cache or refresh it if an ARP request is received • Proxy ARP • A router can answer ARP request for hosts • Useful for interconnected LANs

  6. Subnetwork • A network can be further divided into multiple subnetworks • Host portion of address partitioned into subnetwork number and host number • A subnetwork mask is used to indicate which bits are subnetwork number and which are host number • Why subnetworking? • Simplify Internet management, e.g. routing • Insulate overall internet from growth of network numbers and routing complexity • Allow arbitrary complexity of internetworked LANs within organization • Site looks to rest of internet like single network • Each LAN assigned subnetwork number • Local routers route within subnetworks with small routing tables • Make good use of IP addresses • C classes can be aggregated to be a large network with subnetworks

  7. Subnetwork • Example • A network with class B address 158.108.0.0 can be divided into 254 subnetworks, with the third byte as the subnetwork id • From 158.108.1.0 to 158.108.254.0 • The subnetwork mask is 255.255.255.0 • Bit-wise AND operation between an IP address and the mask yields the network id and the subnetwork id • Subnetwork - class B example • Subnetwork - class C example

  8. Subnetwork • Exercise • The following figure shows a local complex consisting of three LANs and two routers • To the rest of the Internet, this complex is a single network with a class C address • 192.228.17.0 • Both routers R1 and R2 are configured with a subnetwork mask 255.255.255.224 • Query 1: How to route a datagram with address 192.228.17.57 in R1? • Query 2: How to route a datagram with address 192.228.17.97 in R1 and R2?

  9. ICMP • Internet Control Message Protocol • RFC 792 • Used for flow and error control • Routers use ICMP to inform source if packet discarded, so that source may modify transmission strategy • Routers use ICMP to send flow control packets, i.e. choke packets, to request the source to reduce flow • Feedback about problems • Time to live expired • Destination unreachable • Useful diagnostics about network • Ping, traceroute • ICMP messages are never generated in the following situations in order to prevent broadcast storms • Errors of ICMP messages themselves • Broadcast/multicast • Broadcast storm • A large number of broadcast datalink frames have been transmitted nearly simultaneously from several hosts • LAN may have been brought to a standstill

  10. ICMP • An ICMP packet is encapsulated in an IP datagram. • ICMP is not reliable • ICMP header • Type: specify the type of ICMP message • Code: specify the parameters of the message that can be encoded in one or a few bits • Checksum: checksum for the entire message • Content-specific: specify more lengthy parameters

  11. ICMP • ICMP types • Type/code meaning • 0/0 echo reply • 3/0 network is unreachable • 3/1 host is unreachable • 3/3 port is unreachable • 4/0 source quench • 5/0 redirect • 8/0 echo request • 9/0 router advertisement • 10/0 router solicitation • 11/0 time exceeded • 12/0 parameter problem • 13/0 time stamp request • 14/0 time stamp reply • 17/0 address mask request • 18/0 address mask reply • Echo request/reply • Identifier and sequence number are used to identify datagrams • Optional data is a variable length and set by the sender and will be returned in the reply • Ping sends echo requests and waits for replies

  12. ICMP • Destination unreachable • A router sends the message when it is unable to deliver an IP packet due • Source quench • If a router sends to a host to reduce flow • A host may take no action to the request • Redirect • Used by a router to suggest a more suitable route to a host

  13. Utilities using ICMP • Ping • Ping sends an ICMP echo request to a remote host, which then returns an ICMP echo reply to the sender • Ping is used to debug the network, such as timing info, connection reliability, host is reachable, etc • Traceroute • A command to determine the active route to a destination • How it works? • Send an IP packet to the destination with time-to-live (ttl) set to 1 • The first router decreases ttl to 0, then returns an ICMP time exceeded message • The traceroute sets ttl=2, retransmits the packet; this time the packet goes one more router (hop) • Increase ttl until the IP packet reaches the destination • The returned ICMP messages tell the traceroute the routers of the route

  14. Summary • Mapping between IP and MAC addresses • Address Resolution Protocol (ARP) • ARP packet format • ARP mechanism • Proxy ARP • Subnetting • How to divide a Class B network into small subnetworks? • How to route packets in subnetworks? • ICMP • ICMP packet format • Types of ICMP packets • Utilities using ICMP • Ping • Traceroute

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