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

  2. Network 1 (Ethernet) H7 R3 H8 H1 H8 H2 H1 H3 TCP TCP Network 4 R1 R2 R3 (point-to-point) Network 2 (Ethernet) R1 IP IP IP IP IP R2 FDDI PPP ETH ETH ETH FDDI PPP ETH H4 Network 3 (FDDI) H5 H6 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 Identical packet Identical datagram Identical datagram Identical frame Identical frame CSS432: Basic Internetworking

  3. 0 4 8 16 19 31 TOS Length V ersion HLen Ident Flags Offset TTL Protocol Checksum SourceAddr DestinationAddr Pad Options (variable) (variable) Data 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 CSS432: Basic Internetworking

  4. 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

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

  6. 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

  7. 7 24 A: 0 Network Host 14 16 B: 1 0 Network Host 21 8 C: 1 1 0 Network Host 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 CSS432: Basic Internetworking

  8. 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 else 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 i/f 0 i/f 0 R2 i/f 1 Network 3 CSS432: Basic Internetworking

  9. 172.16.5.0/24 172.16.3.0/24 Eth2: 172.16.5.1 Eth1: 172.16.3.1 Router 2 Eth0: 172.16.3.2 Router 3 Eth0: 172.16.1.2 Eth1: 172.16.4.1 172.16.2.0/24 172.16.4.0/24 Eth1: 172.16.2.1 Router 1 Eht0: 172.16.1.1 172.16.1.0/24 Static Configuration for Cisco Routers 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

  10. 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

  11. ARP Details Reuqest I’m IPa and Pha. You’re IPb. How about Ph? • 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 A X B Y Response I’m IPb and Phb. You’re IPa and Pha A X B Y CSS432: Basic Internetworking

  12. 0 8 16 31 Hardware type = 1 ProtocolT ype = 0x0800 HLen = 48 PLen = 32 Operation SourceHardwareAddr (bytes 0 – 3) SourceHardwareAddr (bytes 4 – 5) SourceProtocolAddr (bytes 0 – 1) SourceProtocolAddr (bytes 2 – 3) T argetHardwareAddr (bytes 0 – 1) T argetHardwareAddr (bytes 2 – 5) T argetProtocolAddr (bytes 0 – 3) 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 CSS432: Basic Internetworking

  13. 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 Response You’re IPa and Pha A X B Y RARP server CSS432: Basic Internetworking

  14. 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

  15. OP Htype HLEN HOPS Xid Broadcast Frame addr datagram UDP header BOOTP/DHCP Secs Flags Client IP addr DHCP relay Other network DHCP server Your IP addr Server IP addr Host Unicast Router IP aaddr Client Hardware addr Server host name Boot file name Options DHCP • 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

  16. 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

  17. 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

  18. 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

  19. ICMP Message Types CSS432: Basic Internetworking

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

  21. 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

  22. 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