More on the IP Internet Protocol

Internet Layer Process Transport layer process passes EACH TCP segment to the internet layer process for delivery Transport Layer Process Internet Layer Process TCP segment

IP: Connectionless Service The Internet Protocol (IP) –Internet layer protocol –IP messages are called IP packets No connections are established –No open, close, error correction, flow control –Low overhead Internet Process Internet Process IP Packet

IP: Connectionless Service IP is unreliable –No error handling (Let TCP catch errors!) –No sequence numbers, so no way to put arriving IP packets in order (Let TCP put the TCP segments these IP packets contain in order!) Internet Process Internet Process IP Packet

TCP/IP Partnership TCP checks for errors once, at the destination host IP is used in many hops between routers –Not checking for errors at each step greatly reduces overall processing work –Reduces router costs Transport Internet Check Only Once Host Router Host

Cobnnectionless IP IP is unreliable (does not catch errors) –But this is not bad –First, errors are caught--at the next-higher layer (transport) if TCP is used –Second, avoiding error checking at each hop between routers lowers router costs –Far less expensive to check for errors on one destination host than on many routers along the way

IP is a Best-Effort Service IP Only Offers Best-Effort Service –Does its best to get packets through –No guarantees of delivery –No way to give priority to time-sensitive traffic, such as voice Overall, low overhead but limited Quality of Service (QoS) QoS should change in the future (see Chapter 8)

IP Address 32-bit Strings –Often given in dotted decimal notation: Fits into 32-bit source and destination address field of IP headers IP Packet 32-bit Source and Destination Addresses

IP Addresses and Router Forwarding Routers use the destination IP address of an incoming packet in the router forwarding decision, that is, to decide what output port to use to send the packet back out to the destination host or to another router B? D? C? Router A B C D Packet

IP Addresses Many Addressing Systems Use Hierarchical Addressing –Postal delivery: city, street address –Post office looks at city first If not P.O.’s city, put in bag for other city If in P.O.’s city, put in bag for sorting by street address –Hierarchical addressing greatly speeds sorting at each post office Imagine if we needed a sorting bin for each address in the country!

IP Addresses For IP, Routers Take the Place of Post Offices –There are hundreds of millions of IP addresses on the Internet –Routers cannot store decision rules for reaching each address individually –So router simply asks if a destination IP address is that of a host on one one of the networks or subnets connected to the router or must be passed on to another router –This is the router forwarding decision

IP Addresses To Simplify Router Decisions, IP Addresses are Hierarchical The Internet is Made of Many Individual Networks Owned by Different Organizations First route packets to a single network; only need one “sorting bin” for each network! In the next step, route packet to host on the network

IP Addresses Each Organization is Given a Network Part Number For the University of Hawaii, this is –All IP Addresses in that Organization’s Network Begin with that Network Part Network Part IP Address

IP Addresses Network Parts can be 8 to 24 bits long –For University of Hawaii, it is 16 bits long –16 bits is only an example Network Part (8 to 24 bits) IP Address

IP Addresses Between different organization networks, routers look first at the Network Part of an arriving IP packet’s destination address –If the network part is not that of the organization, the router cannot deliver the IP packet locally –Passes the IP packet on to another router, called a next-hop router, to move the IP packet closer to the destination host Network Part

IP Addresses Local Part –The part of the IP address after the network part is called the local part –Total address is 32 bits, so if the network part is 8, the local part is 24 Network PartLocal Part IP Address (32 bits total)

Assigning Network Parts Organization applies to an Internet IP address registrar –Registrar gives organization a network part –Organization assigns the local part to its hosts internally –Only large organizations and ISPs get network parts RegistrarFirm

Assigning Parts Example –IP address registrar gave the University of Hawaii the network part –UH gave the College of Business Administration the subnet part 17 –College of Business Administration gave the host part 13 to a computer it later gave the host name voyager.cba.hawaii.edu –So the computer’s IP address became

IP Addresses Most Organizations Have Multiple Subnets within the Organizational Network –Usually represent each as a Subnet Part within the Local Part –Remaining Bits are the Host Part, designating a particular host on that subnet Network PartSubnet Part IP Address (32 bits total) Host Part Local Part

Assigning Subnet Parts Organization Assigns Subnet Parts –Assigns subnet parts to suborganizations –Suborganization assigns host bits to hosts RegistrarFirm Suborganization Host

IP Addresses Within an organizational Network –Router looks at Network Plus Subnet Part Combined –If destination host is on a subnet attached to the router, delivers the IP packet to the host –Otherwise, passes the packet on to a next-hop router Network PartSubnet Part IP Address (32 bits total) Host Part Local Part

IP Addresses In IP Addresses, “Network” and “Subnet” are Organizational Concepts, not Technical Concepts Network is the collection of individual networks and routers owned by an organization Subnet is a collection of individual networks and routers owned by a suborganization –Often a single physical network (subnet)

Importance of Part Sizes Determine Number of Possible Networks, Subnets, or Hosts If There are N Bits in the Part, there can be 2 N possible Networks, Subnets, or Hosts Actually, 2 N -2 –All zeros cannot be used for a part –All ones cannot be used for a part Example: if part has 8 bits, possibilities (254)

Masks IP Addresses are Always Paired with a Second 32-bit Number Called a Mask Two Types: Network Masks and Subnet Masks –Network Mask Tells the Length of the Network Part –Subnet Mask Tells the length of the Network Plus Subnet Parts (not just subnet part) –IP Address will be paired with one or the other, but not both simultaneously

Masks Masks Begin with 1s, End with 0s (111…00) For network masks, 1s are in Network Part bits; 0s are in Subnet and Host Parts For subnet masks, 1s are in Network and Subnet Parts; 0s are in Host part

Masks IP Address-Mask Pairs often Written with Prefix Notation – /16 –16 means that the mask has 16 initial 1s –Total number of bits is 32 in an IP address, so there must be 16 trailing 0s

Router Delivery If Destination Host is On the Source Host’s Subnet, Source Host Delivers the Packet Directly –No router is involved Subnet Source Host Destination Host

Router Delivery If Destination Host is NOT On the Source Host’s Subnet, Source Host Sends the Packet to a Router for Delivery Subnet

Router Delivery If Destination Host is On One of the Router’s Subnets, the Router Sends the Packet to the Destination Host for Delivery Subnet

Router Delivery If Destination Host is NOT On One of the Router’s Subnets, the Router Sends the Packet to a Next-Hop Router for Delivery –May have to choose among several possible next-hop routers for delivery Subnet

Router Delivery Border Routers Connect Networks, Not Subnets –Select between next-hop router on own network or on another network Own Network Other Network

Router Forwarding Tables Allow Routers to Decide Whether Local Delivery is Possible to Destination Host Allow Routers to Select Next-Hop Router if Local Delivery is Not Possible

Router Forwarding Tables Router Compares Destination IP Address to Each Row in Router Forwarding Table –If matches IP address, delivers according to Delivery rule –So if destination address of IP packet is , router delivers packet locally IP AddressDelivery Local Next-Hop Router A

Router Forwarding Tables Also Has a Mask Column –Masks destination IP Address of packet –If Mask in a row is 24 bits long, router only compares first 24 bits of packet’s IP dest. addr. –Compares to IP Address Part for that row IP Address Part MaskDestination Local 1428Next-Hop Router A

Router Forwarding Tables Also Has a Mask Column –A network mask for a host outside the organization’s network –A subnet mask for an internal host –Can’t tell which by looking at the mask IP Address Part MaskDestination Local 1428Next-Hop Router A

Router Forwarding Tables Also Has a Mask Column –Masks destination IP Address of packet –IP Address Part really is a network part or network plus subnet part of a network or subnet, respectively IP Address Part MaskDestination Local 1428Next-Hop Router A Not in the Book

Router Forwarding Tables Matching Destination IP Addresses –Example: Destination IP Address is –Mask is 24, so only look at –Matches row’s IP address part, so use Local delivery IP Address Part MaskDestination Local 1428Next-Hop Router A

Router Forwarding Tables Longest Match Principle –Must select one row to determine delivery –If two rows match, use longest match, that is match to greatest number of bits –For , use local delivery (24-bit match) IP Address Part MaskDestination Local Next-Hop Router A

Router Forwarding Tables Metric –If same length of match, turn to metric column –Metric describes the desirability of a choice –If metric is cost, choose lowest cost –For other metrics (speed, etc.), may chose largest value IP Address Part MaskMetric (Cost) Delivery Local Next-Hop Router A

Router Forwarding Tables There May be No Matches –One IP Address Part is Always –If there is no match, choose its next-hop router (called the Default Router) IP AddressDelivery Local Next-Hop Router C

Router Forwarding Tables Recap of Selection Rules –Compare destination IP address of an arriving packet against ALL rows within the router forwarding table because there may be multiple matches –Select the single row that matches –If multiple rows match, select the longest match –If multiple rows tie on the longest match, select the row with the largest or smallest metric, depending on the specific metric –If there is no match, select the default router row

Router Forwarding Tables Delivery –Table not only designates local delivery or a next hop router –Also designates the router interface (port) that will be used for delivery InterfaceDelivery 2Local 3Next-Hop Router C 3Next-Hop Router D

Dynamic Routing Protocols How Do Routers Get Information for their Router Forwarding Tables? –Share router forwarding table information –Standards for these exchanges are called dynamic routing protocols Router Forwarding Table Information

Dynamic Routing Protocols How Do Routers Get Information for their Router Forwarding Tables? –Thanks to dynamic routing protocols, the Internet needs no central point of control –Routers create their router forwarding tables strictly by information from peers and their own knowledge Router Forwarding Table Information

IP Version 6 Current Version of IP is IP Version 4 –This is the version we have been discussing –Has 32-bit IP address fields –Not long enough; running out of IP addresses Next Version will be IP Version 6 –Will have 128-bit IP address fields –Will allow vast numbers of IP addresses (2 128 ) –Being adopted slowly

Terminology Confusion TCP/IP is a Standards Architecture –Includes not only TCP and IP but also UDP, HTTP, and many other protocols –May not even use TCP (UDP instead) or IP (ARP instead, as discussed in Module A) TCP and IP are Individual Standards –Within the TCP/IP Architecture