TELE202 Lecture 10 Internet Protocols (2) 1 Lecturer Dr Z. Huang 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

TELE202 Lecture 10 Internet Protocols (2) 2 Lecturer Dr Z. Huang 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

TELE202 Lecture 10 Internet Protocols (2) 3 Lecturer Dr Z. Huang 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

TELE202 Lecture 10 Internet Protocols (2) 4 Lecturer Dr Z. Huang 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

TELE202 Lecture 10 Internet Protocols (2) 5 Lecturer Dr Z. Huang 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

TELE202 Lecture 10 Internet Protocols (2) 6 Lecturer Dr Z. Huang 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

TELE202 Lecture 10 Internet Protocols (2) 7 Lecturer Dr Z. Huang Subnetwork ¥Example »A network with class B address can be divided into 254 subnetworks, with the third byte as the subnetwork id –From to »The subnetwork mask is »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

TELE202 Lecture 10 Internet Protocols (2) 8 Lecturer Dr Z. Huang 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 – »Both routers R1 and R2 are configured with a subnetwork mask »Query 1: How to route a datagram with address in R1? »Query 2: How to route a datagram with address in R1 and R2?

TELE202 Lecture 10 Internet Protocols (2) 9 Lecturer Dr Z. Huang 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

TELE202 Lecture 10 Internet Protocols (2) 10 Lecturer Dr Z. Huang 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

TELE202 Lecture 10 Internet Protocols (2) 11 Lecturer Dr Z. Huang ICMP ¥ICMP types »Type/codemeaning »0/0echo reply »3/0network is unreachable »3/1host is unreachable »3/3port is unreachable »4/0source quench »5/0redirect »8/0echo request »9/0router advertisement »10/0router solicitation »11/0time exceeded »12/0parameter problem »13/0time stamp request »14/0time stamp reply »17/0address mask request »18/0address 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

TELE202 Lecture 10 Internet Protocols (2) 12 Lecturer Dr Z. Huang 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

TELE202 Lecture 10 Internet Protocols (2) 13 Lecturer Dr Z. Huang 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

TELE202 Lecture 10 Internet Protocols (2) 14 Lecturer Dr Z. Huang 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