Network Layer ReviewThe Network Layer in the InternetDesign Principles for InternetCollection of SubnetworksService provided by IPIPv4 header The glue that holds the Internet togetherThe IP Protocol (2)IP Address formatsSpecial IP AddressesSubnets (2)Network DesignSubnetsSlide 13PowerPoint PresentationSlide 15Slide 16Classless InterDomain Routing (CIDR)CIDR – Classless InterDomain RoutingNAT – Network Address TranslationSlide 20Internet Control Message ProtocolARP– The Address Resolution ProtocolDynamic Host Configuration ProtocolThe Interior Gateway Routing Protocol2. OSPF (Open Shortest Path First)3. OSPFThe Exterior Gateway Routing Protocol BGP (Border Gateway Protocol)Slide 28Slide 29The Main IPv6 HeaderExtension HeadersNetwork Layer ReviewSo far we look at issues concerning the network layer such as routing and congestion.We considered the implications of heterogeneous (at the data link layer) networks on trying to connect them.Since different data link layer schemes can have incompatible addressing schemes, we need another layer, the Network Layer, to provide a common addressing scheme and associated routing functions.We will look at probably the most popular network layer protocol, Internet Protocol (IP) that is used to connect heterogeneous network into an internet.The Network Layer in the Interneta) The IP Protocolb) IP Addressesc) Internet Control Protocolsd) OSPF – The Interior Gateway Routing Protocole) BGP – The Exterior Gateway Routing Protocolf) Internet Multicastingg) Mobile IPh) IPv6Design Principles for InternetA. Make sure it works.B. Keep it simple (avoid features, Occam's Razor).C. Make clear choices.D. Exploit modularity.E. Expect heterogeneity.F. Avoid static options and parameters.G. Look for a good design; it need not be perfect.H. Be strict when sending and tolerant when receiving.I. Think about scalability.J. Consider performance and cost.Collection of SubnetworksThe Internet is an interconnected collection of many networks.Service provided by IPThe transport layer gives to IP a datagram and a destination IP address.IP takes this datagram and sends it over the Internet, possibly in several fragments.The IP protocol at the destination collects the fragments and if all fragments got through, assembles them into a datagram and delivers it to the destination transport layer.IPv4 header The glue that holds the Internet togetherIHL – header length in 32-bit words, between 5 and 15Type of service - ignored by the routers.Identification – all fragments of a datagram contain the same valueDF – do not fragment, MF – more fragmentsProtocol – TCP, UDP, assigned numbers are on www.iana.orgThe IP Protocol (2)Some of the IP options.5-54IP Address formatsIP address do not identify hosts in general. They identify a host on a network. If a computer is connected to more than one network, it has more than one IP address (e.g.,: routers, multihomed hosts). A: 128 networks with 16 million hosts; B: 16,384 networks with with 64K hosts; (not enough!)C: 2 million networks with 256 hostsSpecial IP AddressesSpecial IP addresses.Subnets (2)A class B network subnetted into 64 subnets.The number of bits that form the network part of the IP address is called the netmask. Netmask here is 255.255.252.8/22Class B has a netmask of 16 1s or 255.255.0.0/16Network DesignTemple University has been assigned the 155.247.x.x range of addresses. This is a class B address so10011011 11110111 00000000 00000000 (155.247.0.0) (total of 256*256 = 65536)10011011 11110111 11111111 11111111 (155.247.255.255)16 bit network addr 16 bit host addrWe could have had one big network (with up to 65536 hosts) for the whole university attached to a single router.But that would a administrative nightmare: trouble shooting, traffic locality, and address allocation.So we create smaller subnetsSubnetsA campus network consisting of LANs for various departments.Subnets (2)10011011 11110111 00000000 0000000010011011 11110111 00000000 1111111110011011 11110111 00000001 0000000010011011 11110111 00000001 1111111110011011 11110111 11111111 1111111116 bit network8 bitsubnet8 bithostInside Temple's network, we have subnetted our Class B allocation 155.247.0.0 into 256 subnets, by “stealing” 8 bits from the host bits.Now one or more subnets can be assigned to each department and each department (subnet) can have a router. So a typical Temple IP address is written as 155.247.170.1/24 where the /24 denotes the netmask.Storing/Exchanging addressTraditional IP scheme the netmask is implicit in the address. Let see what are the entries that would be stores in a typical router:Network Next Hop208.12.16/24 11010000 00001100 00010000* x.x.x.x...208.12.21/24 11010000 00001100 00010101* x.x.x.x......208.12.31/24 11010000 00001100 000101111* x.x.x.xIf we use classful addressing we must list 15 entries in the routing table.Scaling IssuesA few decades back, given the rate at which the Internet was growing, service providers were facing two major challenges:- Growth of routing table entries.- Depletion of addresses space.Classless InterDomain Routing (CIDR)Network Next Hop208.12.16/24 11010000 00001100 00010000* x.x.x.x...208.12.21/24 11010000 00001100 00010101* x.x.x.x......208.12.31/24 11010000 00001100 00011111* x.x.x.xReturning to our previous example:Notice that since the first 20 bits are identical for all addresses, these entries could be aggregated as 208.12.16/20 11010000 00001100 0001* x.x.x.xThis reduces the number of entries in the routing table significantly. However, there might be exception that break entries what could have been aggregated. This introduces a set of issues resolved using the longest-prefix-match algorithms.Classless InterDomain Routing (CIDR)CIDR only works well if next hop of all the aggregated entries are the same. Suppose we hadNetwork Next Hop208.12.16/24 11010000 00001100 00010000* x.x.x.x...208.12.21/24 11010000 00001100 00010101* y.y.y.y208.12.22/24 11010000 00001100 00010110* x.x.x.x...208.12.31/24 11010000 00001100 00011111* x.x.x.xNow not all hosts with first 20 bits common have the same next hop, so what do we do ? We can either go back to not aggregating or create exceptions such as:208.12.16/20 11010000 00001100 0001* x.x.x.x208.12.21/24 11010000 00001100 00010101* y.y.y.yBut now, 208.12.21.5 will match both the first and second entry, so