4003-420-01/4005-740-01 Data Communications and Networks I Module 9. Support Protocols -- Lecture Notes Prof. Alan Kaminsky -- Fall Quarter 2012 Rochester Institute of Technology -- Department of Computer Science Ethernet Addresses Ethernet MAC address format First 3 bytes (24 bits): Organizationally Unique Identifier (OUI) Designates the manufacturer of the Ethernet chip/card Assigned by the IEEE Second 3 bytes (24 bits): Network Interface Controller Number Designates a particular chip/card from that manufacturer Assigned by the manufacturer OUI format +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | | L | M | | | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ M = 0: unicast address; M = 1: multicast address L = 0: globally unique address; L = 1: locally unique address Ethernet broadcast address 111111111111111111111111111111111111111111111111 Will be received by all stations on the Ethernet local network Ethernet multicast address 11xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Will be received by selected stations on the Ethernet local network Namely, all stations that have programmed their Ethernet interfaces to receive the multicast address Ethernet unicast address 0xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Will be received by one station on the Ethernet local network Namely, the station with that MAC address Address Resolution Protocol When an IP packet is sent over an Ethernet local network, what destination MAC address should appear in the Ethernet frame? Case 1: The destination host is on the same local network as the source host Must send the frame to the destination host's MAC address Case 2: The destination host is on a different local network from the source host Must send the frame to the local router's MAC address Address Resolution Protocol (ARP): RFC 826 Maps IP addresses to Ethernet MAC addresses ARP request message (broadcast) ARP response message (unicast) ARP cache Demo: arping Internet Control Message Protocol Internet Control Message Protocol (ICMP): RFC 792 Used to discover information about IP packet forwarding Used to report errors in IP packet forwarding Used to control the operation of IP packet forwarding ICMP messages are encapsulated in IP packets and sent using IP Demo: ping Host A sends an ICMP "echo" message to Host B Host B sends an ICMP "echo reply" message to Host A Host A reports the round trip time Demo: traceroute Host A sends a probe packet to Host B, with the time-to-live (TTL) field in the IP header set to 1 Probe packet is a UDP datagram with an unlikely port number, so Host B will not do anything with the packet The first router decrements the TTL to 0, then sends an ICMP "time exceeded" message back to Host A Host A reports the IP address of the first router and the round trip time The preceding steps are repeated with TTL = 2, 3, 4, and so on until the probe packet reaches Host B In this way, Host A discovers the sequence of routers along the path to Host B Dynamic Host Configuration Protocol Dynamic Host Configuration Protocol (DHCP): RFC 2131 Used to supply configuration parameters to hosts at boot time, such as the host's IP address DHCP messages use UDP at the Transport Layer DHCP server, DHCP client DHCP message addressing Tricky, because the client doesn't have an IP address yet Client-to-server messages sent to IP address 255.255.255.255 (local network broadcast), UDP port 67 Server-to-client messages sent to IP address 255.255.255.255, UDP port 68 DHCP operation The Wikipedia entry is pretty good: http://en.wikipedia.org/wiki/Dhcp Network Address Translation Problem: There aren't enough 32-bit IPv4 addresses to go around Solution: Use IPv6 with 128-bit addresses Problem: Hardly anyone supports IPv6 Solution: Network Address Translation (NAT) NAT router Exposes one IP address to the Internet side Supports multiple IP addresses and hosts on the local network side Hosts are given IPv4 private addresses: RFC 1918 10.0.0.0/8 172.16.0.0/12 192.168.0.0/16 Address translation for TCP NAT router keeps track of each open TCP connection involving a local host Local host IP address and port number Far end host IP address and port number NAT router assigns a unique external port number to each open TCP connection In each outgoing TCP packet from a local host: The source IP address is replaced with the NAT router's Internet-visible IP address The source port number is replaced with the TCP connection's external port number The IP header checksum and TCP header checksum are recomputed The translated packet is forwarded to the Internet In each incoming TCP packet from the Internet: The destination port number (external port number) is converted back to local host and local port The destination IP address is replaced with the local host's IP address The destination port number is replaced with the local port number The IP header checksum and TCP header checksum are recomputed The translated packet is forwarded to the local network Address translation for other transport protocols (UDP, etc.) works similarly The NAT router has to be aware of the Network Layer protocol (IP) and multiple Transport Layer protocols (TCP, UDP, etc.) Campfire http://xkcd.com/742/ Data Communications and Networks I • 4003-420-01/4005-740-01 • Fall Quarter 2012 Course Page Alan Kaminsky • Department of Computer Science • Rochester Institute of Technology • 4486 + 2220 = 6706 Home Page Copyright © 2012 Alan Kaminsky. All rights reserved. Last updated 30-Oct-2012. Please send comments to ark­@­cs.rit.edu.