The internetworking solution of the Internet. Single networks. The Internet approach to internetworking. Protocol stacks in the Internet

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1 The internetworking solution of the Internet Prof. Malathi Veeraraghavan Elec. & Comp. Engg. Dept/CATT Polytechnic University What is the internetworking problem: how to connect different types of networks Polytechnic University Simplest network one link Single networks One network same type of switches link rates can be different A shared link: often used to create a LAN Polytechnic University The Internet approach to internetworking Protocol stacks in the Internet An internetwork Network router Network Network A Inter-T: Inter-N: T N L P N L L P P N L L P P router Inter-N: T T N N L L P P N L L5 P P5 N L5 L6 P5 P6 A Inter-T: Inter-N: T N L6 P6 Have all endpoints speak the (Internet Protocol) in addition to their own network protocols For loss-sensitive applications: run, an end-to-end transport protocol, irrespective of whether both ends are within the same network the two ends are on different networks routers are connectionless packet switches Polytechnic University they forward packets from one network to another based on the destination address carried in the header and information stored in their routing tables Network Network Polytechnic University Today s most common networks in the Internet within enterprises using a combination of shared-medium LANs with hubs, or with switches which are connectionless packet switches PDH/SONET networks in the MAN and WAN domains Routers are interconnected by T, T, OC connections that are set up through a PDH/SONET circuit-switched network PPP, Point-to-Point Protocol, is executed on these circuits 5 Polytechnic University frame structure (RFC 89 and 89) Dest. Addr. 6 FOCUS: addresses (also called MAC addresses) are 6 bytes long Src. Addr Data datagram req./reply R req./reply PAD PAD 8 8 CRC 6 Polytechnic University

2 PPP frame structure Datagram Format flag 7E addr ctrl protocol data FF 0 <= datagram C0 link control data CRC flag 7E 96 if low delay >= five -bit words version header of Service/TOS Total Length (in bytes) ( bits) length (8 bits) (6 bits) flags Identification (6 bits) Fragment Offset ( bits) ( bits) TTL Time-to-Live Protocol Header Checksum (6 bits) (8 bits) (8 bits) Source address ( bits) Destination address ( bits) Options (if any, <=0 bytes) FOCUS 80 network control data DATA 0 -bit word 7 Polytechnic University 8 Polytechnic University Application User-plane interworking - Encapsulation As data moves down the protocol stack, each protocol adds layer-specific control information. Application Header Header Header Header Header Header Header datagram User data User data segment frame Trailer 9 Polytechnic University FTP Layers used in ftp protocol protocol FTP protocol protocol router: dibner-gw protocol protocol FTP 0 Polytechnic University Host A Host B Need Internet address and Network address router Internetwork Host C Host D Host A sends a packet to Host C: - Places Host C s address in header - To get through, it needs address of router s interface Host E - and forward packets based on destination address of router s interface - router forwards packet to port to reach Host C (based on level Host F routing data using destination address of host C) - router needs address of Host C to send the packet through - and forward packets based on destination address Polytechnic University of Host C Addresses for interfaces Both addresses and addresses are assigned per interface, not per node (router or host). An router has many interfaces; each interface has an address; interfaces that connect the router to an network also have addresses, one per interface An switch has many interfaces; each has an address A host typically has only one interface; hence it is assigned one address and one address if its interface is an link Polytechnic University

3 FTP session from host mng to photon Note that router dibner-gw has more than one address Polytechnic University Packet sent from mng to router dibner-gw src address: dst address: src MAC address: 5:6:7::a:f dst MAC address: 0:0:c::a:e 5:6:7::a:f 0:0:c::a:e router Host mng consults its routing table. This says that to reach destination address 8.8.., it needs to send the packet to the router because this destination is on a different network Hence it sends the packet within its network to destination (MAC) address 0:0:c::a:e because this is the address of the router interface that is connected to mng s network. This destination MAC address allows the packet (called frame) to be routed through the first network switches determine how to route Polytechnic based on University destination MAC address At the router, dibner-gw When the packet arrives at the router, dibner-gw, it looks up its routing table For destination address 8.8.., the routing table shows which output port to use. dibner-gw :0:c::a:e dibner-gw :0:c::a:d (8.8..) 5 Polytechnic University router 0:0:c::a:d Packet sent from mng to router dibner-gw src address: dst address: src MAC address: 0:0:c::a:d dst MAC address: 0:0:5e:f:d:c 0:0:5e:f:d:c router, dibner-gw, finds MAC address of photon and adds the header and header to the packet with the four addresses as shown and sends it. The destination MAC address allows for routing through the second network; each switch that the frame encounters will forward packets based on destination MAC address and its routing table. 6 Polytechnic University Intra-network addresses and inter-network addresses Consider example: mng knows that to reach photon it has to route the packet to the router dibner-gw from its -level routing data it needs to find the MAC address of the router to get through the first it does this using (Address Resolution Protocol) Same thing when dibner-gw needs to send packet to photon. 7 Polytechnic University (Address Resolution Protocol) dibner-gw.poly.edu :0:c::a:e :0:5e:f:d:c all stations on the same all stations on the same 8 Polytechnic University

4 and R Finding MAC address of an interface whose address is available The protocol uses -bit addresses. networks use 8-bit (MAC) addresses The and R protocols perform the translation between addresses and MAC layer addresses. We will discuss for broadcast LANs, particularly LANs. address ( bit) R MAC address (8 bit) 9 Polytechnic University () HOST-A wants to send an datagram to HOST-B. () HOST-A broadcasts an request to all stations on the network: What is the hardware address of HOST- B? () HOST-B responds with an Reply which contains its hardware address. () HOST-A transmits the datagram to HOST-B. HOST-A HOST-B 0 Polytechnic University reply The reply is sent by the node whose address matches the address sent in the request All other nodes receiving the broadcast ignore the request (since their addresses do not match the address that is being resolved) cache Clearly, sending an request/reply for each datagram is inefficient. Each station maintains a cache ( Cache) of current entries. The entries expire after 0 minutes. Everytime the cache is consulted for a MAC address, the expiry timer is reset in common implementations.) at (incomplete) Polytechnic University Polytechnic University 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 address gateway address (default router interface) DNS server address Polytechnic University 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 etc. Host needs to find address corresponding to domain name it does this by sending a DNS (Domain Name Service) query to the DNS Server whose address is stored on host (as we just saw) Polytechnic University

5 For the file transfer example from mng to photon ftp Domain Name Server gatekeeper.poly.edu Step : mng knows it wants to execute an ftp to sends DNS query to DNS server gatekeeper.poly.edu gets address corresponding to name 5 Polytechnic University What other addresses does mng need? With photon s 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 address of the host interface itself point out default entry as having the address of the router (gateway) 6 Polytechnic University What if photon was on same as mng? In this case, the address of photon would have matched the entry in the routing table that says photon can be reached through the PC s interface Therefore it will try to find the address of photon to send the packet via its own network it does this by issuing an 7 Polytechnic University What happened when photon was on a different from mng Route lookup showed that address (of photon) was on some other network and hence packet had to be sent to the default router (gateway) mng then needed address of default router. The first time this has to be learned by sending an. Then as long as it is used often (before cache expires), mng knows the address of its default router and does not have to issue requests each time 8 Polytechnic University Internet transport protocols Summary Loss-sensitive applications: End-to-end across networks To ensure error-free delivery, it executes ARQ retranmission scheme for error correction Uses window based flow control Because routers are CL packet switches that do not implement congestion control, implements an end-toend congestion control Delay-sensitive applications: RTP (Real-time Transfer Protocol) All packets carry a timestamp allowing receiver to determine when to play out packets 9 Polytechnic University The Internet is an internetwork of networks Any type of network, CS, CL PS, CO PS can be part of the Internet router is the node that interconnects different networks CL packet switch Need Internet addresses (i.e., addresses) and addresses for each network that packets traverse Internetwork transport protocols: and RTP 0 Polytechnic University