CSS432 Basic Internetworking Textbook Ch3.2 Professor: Munehiro Fukuda CSS432: Basic Internetworking

CSS432: Basic Internetworking IP Internet Interconnected Collection of Networks Viewed as a simple logical network Routers: nodes interconnecting networks Protocol Stack IP on all nodes (both hosts and router) TCP and UDP on top of IP R2 R1 H4 H5 H3 H2 H1 Network 2 (Ethernet) Network 1 (Ethernet) H6 Network 3 (FDDI) Network 4 (point-to-point) H7 R3 H8 R1 ETH FDDI IP TCP R2 PPP R3 H1 H8 Identical packet Identical datagram Identical datagram Identical frame Identical frame CSS432: Basic Internetworking

CSS432: Basic Internetworking Service Model Global addressing IP address Best-effort delivery (unreliable service) Connectionless (datagram-based) packets are lost packets are delivered out of order duplicate copies of a packet are delivered packets can be delayed for a long time Datagram format Version: IPv4/IPv6 HLen: header length in ints Maximum header length? TOS: type of service (priority queue in routers) Length: packet length in bytes Maximum packet size? TTL: time to live (#hops) Protocol: TCP, UDP Checksum SourceAddr: source IP address DestinationAddr: destination IP address frame type Ex. Ethernet preamble dest addr src addr 0x0800 CRC V ersion HLen TOS Length Ident Flags Offset TTL Protocol Checksum SourceAddr DestinationAddr Options (variable) Pad (variable) 4 8 16 19 31 Data CSS432: Basic Internetworking

Fragmentation and Reassembly Each network has some MTU (maximum transmission unit) To check each interface of your computer: netstat –i ifconfig Strategy fragment when necessary (MTU < Datagram) try to avoid fragmentation at source host re-fragmentation is possible fragments are self-contained datagrams use CS-PDU (not cells) for ATM delay reassembly until destination host do not recover from lost fragments CSS432: Basic Internetworking

CSS432: Basic Internetworking Example Ident = x Offset = 0 Start of header Rest of header 1400 data bytes Ident = x Offset = 0 Start of header 1 Rest of header 512 data bytes = 512 = 1024 376 data bytes CSS432: Basic Internetworking

CSS432: Basic Internetworking Discussions How can we detect if a given datagram has lost some fragments? Who will take care resending a datagram which could not be reassembled previously due to its fragment droppings? CSS432: Basic Internetworking

CSS432: Basic Internetworking Global Addresses Properties globally unique hierarchical: network + host Dot Notation Class A 1.0.0.1 – 126.255.255.254 (0.0.0.0 – 0.255.255.255, 1.0.0.0, 126.255.255.255, and 127.0.0.0 – 127.255.255.255 reserved) Class B 128.0.0.1 – 191.255.255.254 Class C 192.0.0.1 – 223.255.255.254 Network Host 7 24 A: 14 16 1 B: 21 8 C: CSS432: Basic Internetworking

CSS432: Basic Internetworking Datagram Forwarding Algorithm If ( datagram’s dest network# == network# of network interface x ) deliver it to the destination host over interface x else if ( datagram’s dest network# == network# of a next hop router y) deliver it to the router y deliver it to its default router Example H8 Network 1 H1 Network 2 i/f 0 R3 i/f 1 R1 R2 R3 i/f 1 R1 Network 4 Network# Next hop 1 R2 2 Interface1 3 Interface0 4 Network# Next hop 1 R3 2 R1 3 Interface1 4 Interface0 Network# Next hop 1 Interface0 2 R2 3 4 Interface1 i/f 0 i/f 0 R2 i/f 1 Network 3 CSS432: Basic Internetworking

Static Configuration for Cisco Routers 172.16.3.0/24 172.16.1.0/24 172.16.4.0/24 172.16.2.0/24 172.16.5.0/24 Eth2: 172.16.5.1 Eth1: 172.16.3.1 Eth0: 172.16.3.2 Eth1: 172.16.4.1 Eth0: 172.16.1.2 Eht0: 172.16.1.1 Eth1: 172.16.2.1 hostname router1 ! interface ethernet 0 ip address 172.16.1.1 255.255.255.0 interface ethernet 1 ip address 172.16.2.1 255.255.255.0 ip route 172.16.3.0 255.255.255.0 172.16.1.2 ip route 172.16.4.0 255.255.255.0 172.16.1.2 ip route 172.16.5.0 255.255.255.0 172.16.1.2 CSS432: Basic Internetworking

CSS432: Basic Internetworking Address Translation Map IP addresses into physical addresses destination host next hop router Techniques encode physical address in host part of IP address Pha = f( IPa) or IPa =f-1(Pha) Pha: 0010 0001 0100 1001 => 128.96.33.81 Problems Class C has only 8 bits to indicate a host Ethernet has 48 bits to present a host address table-based Resolution through dynamic binding Address Resolution Protocol table of IP to physical address bindings broadcast request if IP address not in table target machine responds with its physical address table entries are discarded if not refreshed CSS432: Basic Internetworking

CSS432: Basic Internetworking ARP Details Reuqest I’m IPa and Pha. You’re IPb. How about Ph? A X B Y table entries timeout in about 10 minutes Update table with source when you are the target, otherwise no need to add an entry update table if already have an entry IP Ph IPa Pha IPb ?? IP Ph IPb Phb Response I’m IPb and Phb. You’re IPa and Pha A X B Y IP Ph IPa Pha IPb Phb IP Ph IPb Phb IPa Pha CSS432: Basic Internetworking

CSS432: Basic Internetworking ARP Packet Format An ARP packet is carried in a frame header HardwareType: type of physical network (e.g., Ethernet) ProtocolType: type of higher layer protocol (e.g., IP) HLEN & PLEN: length of physical and protocol addresses Operation: request or response Source/Target-Physical/Protocol addresses frame type Ex. Ethernet preamble dest addr src addr 0x0806 CRC T argetHardwareAddr (bytes 2 – 5) argetProtocolAddr (bytes 0 3) SourceProtocolAddr (bytes 2 Hardware type = 1 ProtocolT ype = 0x0800 SourceHardwareAddr (bytes 4 argetHardwareAddr (bytes 0 1) SourceProtocolAddr (bytes 0 HLen = 48 PLen = 32 Operation SourceHardwareAddr (bytes 0 8 16 31 CSS432: Basic Internetworking

RARP: Reverse Address Resolution Protocol Use RARP if a client host is diskless workstation Use the unique MAC address Ask an RARP server about a client IP. Works in the same LAN Retransmit an RARP message after a large delay if it has been lost. Prepare a primary and secondary server. Request I’m Pha. What’s my IPa? A X B Y RARP server IP Ph ?? Pha IP Ph IPa Pha IPb Phb Response You’re IPa and Pha A X B Y RARP server IP Ph IPa Pha IP Ph IPa Pha IPb Phb CSS432: Basic Internetworking

CSS432: Basic Internetworking RARP Disadvantage Using a computer’s MAC address which does not allow computers to move to another network Operating at data-link level which requires direct access to the network hardware Not working beyond routers, because routers are IP-based but RAPS are not IP. CSS432: Basic Internetworking

CSS432: Basic Internetworking DHCP DHCP relay server Other network Host Broadcast Unicast Options Boot file name Server host name Client Hardware addr Router IP aaddr Server IP addr Your IP addr Client IP addr Flags Secs Xid HOPS HLEN Htype OP BOOTP/DHCP UDP header datagram Frame addr DHCP server: Works as a centralized repository for IPs Pools available IP addresses Hands out one to a client on demand Accessible by sending a DHCPDISCOVER message to an IP broadcast address Receives a DHCPDISCOVER from a Relay agent connected to a different network Advantages: Works at a user level Automatic IP configuration Save IP addresses Works across networks. CSS432: Basic Internetworking

DHCP Client State Transition INITIALIZE Host boots / DHCPDISCOVER to all servers SELECT DHCPNACK DHCPNACK Or Lease expires Receive DHCPOFFER from all servers Lease reaches 87.5% expiration / DHCPREQUEST to any server REBIND RENEW Select offer / DHCPREQUEST to a specific server DHCPACK REQUEST DHCPACK Lease reaches 50% expiration / DHCPREQUEST to the current server Receive DHCPACK from the current server BOUND Fig. 23.4 on p453 of Internetworking with TCP/IP CSS432: Basic Internetworking

Two-Step bootstrap Procedure BOOTP server Step 1A: BOOTP request: BOOT FILE NAME=I want to boot “unix” Diskless Workstation Step 1B: BOOTP reply: Server = mercury, BOOT FILE NAME=“/local/var/bootfiles/xncd19r” Step 2A: TFTP request: request for the image File server Step 2B: TFTP reply: image returned OS Image: Unix Windows CSS432: Basic Internetworking

Internet Control Message Protocol (ICMP) An error reporting message (ICMP) Error occurred Src R1 R2 R3 RK Rcv Is Src responsible for this ICMP message? RE Mistakenly routed frame header datagram heaader ICMP header ICMP data ICMP type 0-18 CSS432: Basic Internetworking

CSS432: Basic Internetworking ICMP Message Types Type field Code field ICMP Message Type Applications 0 and 8 Echo reply/request ping 3 Destination unreachable 5 Redirect (change a route) 11 TTL exceeded Trace route, 1 Fragment reassemble failed CSS432: Basic Internetworking

CSS432: Basic Internetworking Discussions How can traceroute be implemented using ICMP (type=11, code=0) messages? CSS432: Basic Internetworking

CSS432: Basic Internetworking Reviews IP Internet: Protocol stack, fragmentation/reassembly, IP address, and datagram forwarding Address translation: ARP, RAPR, and DHCP ICMP Exercises in Chapter 3 Ex. 36 (fragmentation) Ex. 44 (ARP) Ex. 45 (ARP) CSS432: Basic Internetworking

CSS432: Basic Internetworking More Exercises (not from Our Textbook) Q1. (DHCP) Consider a host that has a disk and uses DHCP to obtain an IP address. If the host stores its address on disk along with the data the lease expires, and then reboots within the lease period, can it use the same address? Why or why not? Q2. (DHCP) DHCP mandates a minimum address lease of one hour. Can you imagine a situation in which DHCP’s minium lease causes inconvenience? Explain. CSS432: Basic Internetworking