1. Polytechnic University1 The internetworking solution of the Internet Prof. Malathi Veeraraghavan Elec. & Comp. Engg. Dept/CATT Polytechnic University mv@poly.edu What is the internetworking problem: how to connect different types of networks

2. Polytechnic University2 Simplest network – one link Endpoint Switch One network – same type of switches – link rates can be different Single networks Endpoint A shared link: often used to create a LAN

3. Polytechnic University3 Endpoint Switch Network 1 Endpoint Switch Network 2 IP router An internetwork The Internet approach to internetworking Have all endpoints speak the IP (Internet Protocol) in addition to their own network protocols For loss-sensitive applications: run TCP, an end-to-end transport protocol, irrespective of whether –both ends are within the same network –the two ends are on different networks IP routers are connectionless packet switches –they forward IP packets from one network to another based on the destination IP address carried in the IP header and information stored in their routing tables Network 3

4. Polytechnic University4 Protocol stacks in the Internet

5. Polytechnic University5 Today’s most common networks in the Internet Ethernet within enterprises using a combination of –shared-medium Ethernet LANs with hubs, or –with Ethernet switches – which are connectionless packet switches PDH/SONET networks in the MAN and WAN domains –Routers are interconnected by T1, T3, OC3 connections that are set up through a PDH/SONET circuit-switched network –PPP, Point-to-Point Protocol, is executed on these circuits

6. Polytechnic University6 Ethernet frame structure (RFC 894 and 893) FOCUS: Ethernet addresses (also called MAC addresses) are 6 bytes long

7. Polytechnic University7 PPP frame structure 296 if low delay

8. Polytechnic University8 IP Datagram Format FOCUS

9. Polytechnic University9 User-plane interworking - Encapsulation As data moves down the protocol stack, each protocol adds layer-specific control information.

10. Polytechnic University10 Layers used in ftp IP router: dibner-gw

11. Polytechnic University11 Need Internet address and Network address Host A Host B Switch 1 Switch 2 Ethernet 1 Host C Host D Switch 3 Switch 4 Ethernet 2 IP router Internetwork Host E Host F Switch Ethernet 3 Host A sends a packet to Host C: -Places Host C’s IP address in IP header -To get through Ethernet 1, it needs Ethernet address of IP router’s interface 1 -Switch 1 and Switch 2 forward packets based on destination Ethernet address of IP router’s interface 1 -IP router forwards packet to port 2 to reach Host C (based on IP level routing data using destination IP address of host C) -IP router needs Ethernet address of Host C to send the packet through Ethernet 2 -Switch 3 and 4 forward packets based on destination Ethernet address of Host C 1 2 3

12. Polytechnic University12 Addresses for interfaces Both IP addresses and Ethernet addresses are assigned per interface, not per node (router or host). An IP router has many interfaces; each interface has an IP address; interfaces that connect the IP router to an Ethernet network also have Ethernet addresses, one per interface An Ethernet switch has many interfaces; each has an Ethernet address A host typically has only one interface; hence it is assigned one IP address and one Ethernet address if its interface is an Ethernet link

13. Polytechnic University13 FTP session from host mng to photon Note that IP router dibner-gw has more than one IP address

14. Polytechnic University14 Packet sent from mng to IP router dibner-gw mng.poly.edu router dibner-gw.poly.edu src IP address: 128.238.42.105 dst IP address: 128.238.32.22 src MAC address: 5:6:7:1:a1:f dst MAC address: 0:0:c:1:a2:e 5:6:7:1:a1:f 0:0:c:1:a2:e Host mng consults its IP routing table. This says that to reach destination IP address 128.238.32.22, it needs to send the packet to the IP router because this destination is on a different network Hence it sends the packet within its Ethernet network to destination Ethernet (MAC) address 0:0:c:1:a2:e because this is the Ethernet address of the router interface that is connected to mng’s Ethernet network. This destination MAC address allows the Ethernet packet (called frame) to be routed through the first Ethernet network – Ethernet switches determine how to route based on destination MAC address

15. Polytechnic University15 At the IP router, dibner-gw When the packet arrives at the IP router, dibner-gw, it looks up its routing table –For destination IP address 128.238.32.22, the routing table shows which output port to use.

16. Polytechnic University16 Packet sent from mng to IP router dibner-gw photon.poly.edu router dibner-gw.poly.edu src IP address: 128.238.42.105 dst IP address: 128.238.32.22 src MAC address: 0:0:c:1:a2:d dst MAC address: 0:0:5e:3f:4d:2c 0:0:c:1:a2:d 0:0:5e:3f:4d:2c IP router, dibner-gw, finds MAC address of photon and adds the IP header and Ethernet header to the packet with the four addresses as shown and sends it. The destination MAC address allows for routing through the second Ethernet network; each Ethernet switch that the frame encounters will forward packets based on destination MAC address and its routing table.

17. Polytechnic University17 Intra-network addresses and inter-network addresses Consider example: –mng knows that to reach photon it has to route the packet to the IP router dibner-gw from its IP-level routing data –it needs to find the MAC address of the router to get through the first Ethernet –it does this using ARP (Address Resolution Protocol) Same thing when dibner-gw needs to send packet to photon.

18. Polytechnic University18 ARP (Address Resolution Protocol)

19. Polytechnic University19 ARP and RARP The IP protocol uses 32-bit addresses. Ethernet networks use 48-bit Ethernet (MAC) addresses The ARP and RARP protocols perform the translation between IP addresses and MAC layer addresses. We will discuss ARP for broadcast LANs, particularly Ethernet LANs.

20. Polytechnic University20 Finding MAC address of an interface whose IP address is available (1) HOST-A wants to send an IP datagram to HOST-B. (2) HOST-A broadcasts an ARP request to all stations on the network: “What is the hardware address of HOST- B?” (3) HOST-B responds with an ARP Reply which contains its hardware address. (4) HOST-A transmits the IP datagram to HOST-B.

21. Polytechnic University21 ARP reply The ARP reply is sent by the node whose IP address matches the address sent in the ARP request All other nodes receiving the broadcast ARP ignore the request (since their IP addresses do not match the address that is being resolved)

22. Polytechnic University22 ARP cache Clearly, sending an ARP request/reply for each IP datagram is inefficient. Each station maintains a cache (ARP Cache) of current entries. The entries expire after 20 minutes. Everytime the ARP cache is consulted for a MAC address, the expiry timer is reset in common implementations. ) at (incomplete)

23. Polytechnic University23 Whose addresses does a host store on initialization? Go to Control Panel – Network on a Windows PC –Point out that a host needs to have initialized host IP address gateway IP address (default router interface) DNS server IP address

24. Polytechnic University24 For Internet applications, what type of “address” do you need? To begin with, a user obtains “domain name” of a host to which the user wants to connect for a web file download, to send email etc. Host needs to find IP address corresponding to domain name –it does this by sending a DNS (Domain Name Service) query to the DNS Server whose IP address is stored on host (as we just saw)

25. Polytechnic University25 For the file transfer example from mng to photon Step 1: –mng knows it wants to execute an ftp to photon.poly.edu –sends DNS query to DNS server gatekeeper.poly.edu –gets IP address corresponding to name photon.poly.edu

26. Polytechnic University26 What other addresses does mng need? With photon’s IP address, mng consults its routing table to see how it can reach this node. Show route print command on PC –point out entry for hosts on the same subnet as having the gateway address as the IP address of the host interface itself –point out default entry 255.255.255.255 as having the address of the IP router (gateway)

27. Polytechnic University27 What if photon was on same Ethernet as mng? In this case, the IP address of photon would have matched the entry in the routing table that says photon can be reached through the PC’s Ethernet interface Therefore it will try to find the Ethernet address of photon to send the packet via its own network –it does this by issuing an ARP

28. Polytechnic University28 What happened when photon was on a different Ethernet from mng Route lookup showed that IP address (of photon) was on some other network and hence packet had to be sent to the default router (gateway) mng then needed Ethernet address of default router. The first time this has to be learned by sending an ARP. Then as long as it is used often (before ARP cache expires), mng knows the Ethernet address of its default router and does not have to issue ARP requests each time

29. Polytechnic University29 Internet transport protocols Loss-sensitive applications: TCP –End-to-end – across networks –To ensure error-free delivery, it executes ARQ retranmission scheme for error correction –Uses window based flow control –Because IP routers are CL packet switches that do not implement congestion control, TCP implements an end-to- end congestion control Delay-sensitive applications: RTP (Real-time Transfer Protocol) –All packets carry a timestamp allowing receiver to determine when to play out packets

30. Polytechnic University30 Summary The Internet is an internetwork of networks Any type of network, CS, CL PS, CO PS can be part of the Internet “IP router” is the node that interconnects different networks –CL packet switch Need Internet addresses (i.e., IP addresses) and addresses for each network that packets traverse Internetwork transport protocols: TCP and RTP