Fall 2006Computer Networks19-1 Chapter 19. Host-to-Host Delivery: Internetworking, Addressing, and Routing 19.1 Internetworks 19.2 Addressing 19.3 Routing

Fall 2006Computer Networks19-2 Position of network layer The network layer is responsible for host-to-host delivery

Fall 2006Computer Networks19-3 Network layer duties Other issues –Address resolution –Multicasting –Routing protocols

Fall 2006Computer Networks19-4 Internetwork Data exchange between networks: internetwork

Fall 2006Computer Networks19-5 Links in an internetwork Frame in data link layer does not carry any routing information Problem: How does S1 know that data should be sent out from interface f3 ?

Fall 2006Computer Networks19-6 Network layer in an internetwork Network layer is responsible for host-to-host delivery and for routing the packets

Fall 2006Computer Networks19-7 Network layer at the source Responsible for creating a packet that carries two universal addresses: destination and source addresses

Fall 2006Computer Networks19-8 Network layer at a router Network layer at the switch or router is responsible for routing the packet.

Fall 2006Computer Networks19-9 Network layer at the destination Responsible for address verification

Fall 2006Computer Networks19-10 Switching Switching at the network layer in the Internet is done using the datagram approach to packet switching.

Fall 2006Computer Networks19-11 Datagram approach Switching at the network layer in the Internet is done using the datagram approach to packet switching Advantages: no call setup and VC identifier Disadvantages: out of order

Fall 2006Computer Networks19-12 Internet as a Connectionless Network Communication at the network layer in the Internet is connectionless Connection-oriented service: Virtual circuit approach to packet switching such as in Frame Relay and ATM Connectionless service: datagram approach In connectionless service, the network layer protocol treats each packet independently Why Internet has chosen connectionless ? –Internet is made of so many heterogeneous networks that is almost impossible to create a connection from the source to the destination without knowing the nature of the networks in advance

Fall 2006Computer Networks19-13 Addressing An IP address is a 32-bit address. The IP addresses are unique and universal. Dotted-decimal notation

Fall 2006Computer Networks19-14 Example Change the following IP addresses from binary notation to dotted-decimal notation. a b We replace each group of 8 bits with its equivalent decimal number and add dots for separation: a b

Fall 2006Computer Networks19-15 Classful addressing In classful addressing, the address space is divided into five classes: A, B, C, D, E A new architecture, called classless addressing was introduced in the mid-1990s

Fall 2006Computer Networks19-16 Finding the address class

Fall 2006Computer Networks19-17 Class in decimal notation

Fall 2006Computer Networks19-18 Example Find the class of each address: a b c a.The first byte is 227 (between 224 and 239); the class is D. b.The first byte is 252 (between 240 and 255); the class is E. c.The first byte is 134 (between 128 and 191); the class is B.

Fall 2006Computer Networks19-19 Netid and hostid IP address in classes A, B, and C is divided into netid and hostid

Fall 2006Computer Networks19-20 Class and Blocks Problem in classful addressing is that each class is divided into a fixed number of blocks with each block having a fixed size Millions of class A addresses are wasted.

Fall 2006Computer Networks19-21 Blocks in class B Many class B addresses are wasted.

Fall 2006Computer Networks19-22 Blocks in class C The number of addresses in class C is smaller than the needs of most organizations.

Fall 2006Computer Networks19-23 Network address In classful addressing, the network address is the one that is assigned to the organization. A network address is different from a netid. A network address has both netid and hostid, with 0s for the hostid.

Fall 2006Computer Networks19-24 Sample Internet with classful addresses

Fall 2006Computer Networks19-25 Subnetting (two levels of hierarchy) IP addresses are designed with two levels of hierarchy.

Fall 2006Computer Networks19-26 Subnetting (three levels) A network is divided into several smaller groups with each subnetwork having its own subnetwork address

Fall 2006Computer Networks19-27 Addresses in a network with and without subnetting Hierarchy concept in a telephone number

Fall 2006Computer Networks19-28 Mask The network address can be found by applying the default mask to any address in the block (including itself). It retains the netid of the block and sets the hostid to 0s. Class In BinaryIn Dotted-DecimalUsing Slash A /8 B /16 C /24

Fall 2006Computer Networks19-29 Subnet mask Combine serveral class C blocks to create a larger range of addresses Supernetting

Fall 2006Computer Networks19-30 Classless addressing Classful addressing has created many problems Many ISPs and service users need more addresses Idea is to have variable-length blocks that belong to no class CIDR (Classless InterDomain Routing) for classless addressing, instead of classful routing DHCP (Dynamic Host Configuration Protocol) for dynamic addressing –DHCP provides the following information dynamically (on demand) –IP address, subnet mask, IP address of a router, IP address of a name server

Fall 2006Computer Networks19-31 DHCP Client-server program DHCP server has two databases, one binds statically physical addresses to IP addresses and another makes DHCP dynamic by assigning available (unused) IP addresses for a negotiable period of time

Fall 2006Computer Networks19-32 Network Address Translation: NAT NAT enables a user to have a large set of addresses internally and one address, or a small set of addresses, externally. Range Total to to to Addresses for private networks

Fall 2006Computer Networks19-33 Address translation Address translation for source address of outgoing packet and for destination address of incoming packet

Fall 2006Computer Networks19-34 Translation table Using (1) one IP address, (2) a pool of IP address, and (3) both IP addresses and port numbers

Fall 2006Computer Networks19-35 Five-column translation table Private Address Private Port External Address External Port Transport Protocol TCP TCP...

Fall 2006Computer Networks19-36 Routing Techniques Routing requires a host or router to have a routing table When a router receives a packet to be forwarded, it looks at the routing table to find the route to the final destination Several techniques can make the size of the routing table manageable and handle issues such as security –Next-Hop Routing –Network-Specific Routing –Host-Specific Routing –Default Routing

Fall 2006Computer Networks19-37 Next-hop routing To reduce the contents of a routing table Routing table holds only the information that leads to the next hop instead of holding information about the complete route

Fall 2006Computer Networks19-38 Network-specific routing To reduce the routing table and simplify the searching process

Fall 2006Computer Networks19-39 Host-specific routing Inverse of network-specific routing Destination host address is given in the routing table Not efficient, but administrator has greater control over routing

Fall 2006Computer Networks19-40 Default routing Another technique to simplify routing

Fall 2006Computer Networks19-41 Static versus dynamic routing Static routing table –Information entered manually –Small internet that does not change very often Dynamic routing table –Updated periodically using one of the dynamic routing protocols such as RIP, OSPF, BGP –Update all the tables in the routers upon changes in Internet

Fall 2006Computer Networks19-42 Routing table for classful addressing Routing table needs a minimum of four columns (it normally has more) When a packet arrives, the router applies the mask to the destination address to find the corresponding destination network address. If found, the packet is sent out from the corresponding interface in the table. If not found, the packet is delivered to the default interface.

Fall 2006Computer Networks19-43 Routing table for classless addressing (CIDR) Routing table size can be increased or decreased Decrease if the block of addresses assigned to an organization is larger than the block in classful addressing Mostly increase because the intent of classless addressing is to divide up the blocks of class A and class B addresses. To solve the problem of gigantic routing table –Hierarchical routing –Geographical routing –Routing table searching algorithms