Spring 2008CPE Computer Networks1 Routing: Part II Outline Algorithms Scalability Reading: Section 4.3

Spring 2008CPE Computer Networks2 How to Make Routing Scale Hierarchical versus Flat Addresses –use of a hierarchy of IP addresses makes routing somewhat scalable router does not need to know about all hosts Still 1.Too Many Networks ( ,097,152 = 2,113,664 ) –Routing protocols do not scale to those kinds of numbers 2.Inefficient use of Hierarchical Address Space fore a minimal class C network with 2 hosts the efficiency is 2/255 = 0.78% for a minimal class B network with 256 hosts the efficiency is 256/65535 = 0.39%

Spring 2008CPE Computer Networks3 Two Related Scaling Issues scalability of routing: find ways to minimize number of network numbers that get carried around in routing protocols and stored in the routing tables address utilization : making sure that IP address space does not get consumed too quickly

Spring 2008CPE Computer Networks4 The Routing Problem for the Internet (1990 Structure) NSFNET backbone Stanford BARRNET regional Berkeley PARC NCAR UA UNM Westnet regional UNL KU ISU MidNet regional ■ ■ ■ Tree-structured Provider networks built from a large number of point-to-point links that connect to routers End user site consists of physical networks connected by routers and bridges Each provider & end user network is likely to be an administratively independent entity Each provider’s network is usually a single autonomous system (AS): a network that is administered independently of other ASs

Spring 2008CPE Computer Networks5 The Internet Today An interconnection of “backbone, Consumer ISP, and site networks” connected to each other in arbitrary ways Each group of providers interconnect with each other at a single “peering point” Backbone service provider Peering point Peering point Large corporation Small corporation “Consumer” ISP “Consumer” ISP “Consumer” ISP

Spring 2008CPE Computer Networks6 Subnetting Goal: reduce total number of network numbers that are assigned use a single network number and allocate the IP addresses with that network number to hosts connected to several physical networks. –Each physical network is referred to as subnet –Subnets should be close to each other At a distant point, subnets will all look like a single network, having only one network number A router selects one route to reach any of the subnets

Spring 2008CPE Computer Networks7 Subnetting (cont) Adds another level to address/routing hierarchy: subnet Subnets visible only within site configure all the nodes on each subnet with a single “subnet mask” Example:

Spring 2008CPE Computer Networks8 Subnetting (cont) Top 24 bits are now defined to be network number –Lower 8 bits are the host number –Since the top 16 bits identify the network in a class B address Can think of the address as having three parts: a network part, a subnet part, and a host part. We have divided what used to be the host part into a subnet part and a host part. Subnet masks define ( variable ) partitions of the host part of the IP address

Spring 2008CPE Computer Networks9 H1 is configured with an IP address of and a subnet mask of Note: external routers know only of the class B network The bitwise AND of these two numbers defines the subnet number of H1 and of all other hosts on the same subnet, subnet number Subnet Example

Spring 2008CPE Computer Networks10 Note: external routers know only of the class B network H1wants to send a packet to a certain IP address Perform a bitwise AND between its own subnet mask and the destination IP address Result equals the subnet number of the sending host Destination host is on the same subnet Subnet number of destination is different, packet sent to default router (R1) to be forwarded to another subnet Subnet Example: cont. I

Spring 2008CPE Computer Networks11 Subnet Example: cont. II Forwarding table at router R1 Subnet Number Subnet Mask Next Hop interface interface R2 Note: external routers know only of the class B network A datagram from H1 being sent to H2 R1 ANDs H2’s address ( ) with the subnet mask of the first entry ( ) result ( ) is NOT subnet number for that entry ( ) R1 ANDs H2’s address with the subnet mask of the next entry ( ) result ( ) is subnet number for that entry R1 delivers the datagram to H2 using interface 1

Spring 2008CPE Computer Networks12 Forwarding Algorithm D = destination IP address for each entry (SubnetNum, SubnetMask, NextHop) D1 = SubnetMask & D if D1 = SubnetNum if NextHop is an interface deliver datagram directly to D else deliver datagram to NextHop Use a default router if nothing matches Not necessary for all 1s in subnet mask to be contiguous Can put multiple subnets on one physical network –forces hosts on the same network to talk to each other through a router

Spring 2008CPE Computer Networks13 Subnetting Summary helps solve scalability problems in two ways 1.improves address assignment efficiency  do not use up an entire class C or class B address every time a new physical network is added  new physical network is a new subnet 2.aggregate information  a complex collection of physical networks looks like a single network  amount of information routers need to store is reduced

Spring 2008CPE Computer Networks14 Still: Two Scaling Concerns growth of backbone routing tables address assignment inefficiency: 32-bit IP address space exhausted –subnetting can not solve the problem of requesting class B addresses for a network with a little more than 255 hosts –class A, B, and C addresses, force hand out of network address space in fixed-sized chunks of three very different sizes (16,777,216 ; 65,534 ; 254 )

Spring 2008CPE Computer Networks15 A Solution for Requests of Class B Network Address Exhaustion of the IP address space centers on exhaustion of the class B network numbers A request of a class B address is granted only if the need is close to 64K addresses A number of class C addresses is given to cover smaller expected number of hosts –More accurately match the amount of address space consumed to the size of requesting autonomous system, AS –For any AS with at least 256 hosts, address utilization of at least 50% –Typically utilization is much more than 50%

Spring 2008CPE Computer Networks16 Drawback of Proposed Solution: Excessive Storage Requirements at Routers Example: AS has 16 class C network numbers assigned Every Internet backbone router needs 16 entries for that AS. –Path to every one of those networks is the same –If a class B address was assigned, the same routing information could be stored in one table entry However address assignment efficiency would be 16 x 255/65,536 = 6.2%

Spring 2008CPE Computer Networks17 Supernetting CIDR: Classless Inter-Domain Routing Example: Autonomous System (AS) with 16 class C network numbers –assign a block, (2 4 ), of contiguous class C network numbers to –leftmost 20 bits of all addresses are ( ), remaining bits are (0000 – 1111) leftmost 20-bit is the network #, 12-bits for host # For this example, max # of hosts is 2 12  # of hosts < # of hosts in a class B network, (2 16 )  # of hosts > # of hosts in a class C network, (2 8 ) CIDR assigns block of contiguous network numbers to nearby networks hand out blocks of class C addresses that share a common prefix each block contains a number of class C networks that is a power of two

Spring 2008CPE Computer Networks18 Improvements via Supernetting Higher address efficiency – handing out addresses in chunks smaller than a class B network Reducing growth of backbone routing tables –a single network prefix can be used in forwarding tables for a greater # of hosts multiple network addresses that would be assigned to a single AS collapse onto one CIDR network address

Spring 2008CPE Computer Networks19 Supernetting (cont.) All “concerned” routers must understand CIDR addressing Network number may be of any length –in a router, network #s are represented by (value/ length) pairs, where length gives number of bits in the network prefix Sample entries in a routing table (exercise 4.46, hex numbers used):  {C4.5E.2.0/ 23 ; next hop A }  {C4.5E.C0.0/ 18 ; next hope C } »Packet C4.5E.D1.02 forwarded to ? »Packet C4.5E forwarded to ??

Spring 2008CPE Computer Networks20 Route Propagation Autonomous System or (Routing Domain) –corresponds to an administrative domain –examples: university, company, backbone network Smart propagation of routing information –hosts know a local router –local routers know site routers –site routers know core routers, the routers that make up the backbone of the Internet. –core routers know everything Two-level route propagation hierarchy –Interior routing protocol for intradomain (within domain) routing each AS selects its own protocol –Exterior routing protocol for interdomain (between domains) routing Internet-wide standard protocol

Spring 2008CPE Computer Networks21 Popular Interior Router (Gateway) Protocols: Intradomain Routing RIP: Route Information Protocol (pp.269 – 277) –distance-vector algorithm –distributed with Unix –based on hop-count OSPF: Open Shortest Path First (pp. 277 – 289) –Open: nonproprietary –uses link-state algorithm –recent Internet standard –supports load balancing multiple routes to destination assigned the same cost –supports authentication (of routing updates)

Spring 2008CPE Computer Networks22 BGP-4, Border Gateway Protocol: Interdomain Routing (pp. 306 – 315) AS (domain) Types –stub AS: has a single connection to one other AS carries local traffic only –multihomed AS: has connections to more than one AS refuses to carry transit traffic –transit AS: has connections to more than one AS carries both transit and local traffic Each AS has: –one or more border routers (gateways) –one border router is designates as BGP speaker & establishes BGP sessions to other BGP speakers in other domains to advertise: local networks other reachable networks (transit AS only) gives path information

Spring 2008CPE Computer Networks23 BGP Example Advertisements Speakers for AS4 advertises local networks Speaker for “transit” AS2 advertises reachability to P and Q –network , , , and , can be reached directly from AS2 Speaker for backbone advertises path information –networks , , , and can be reached along the path (AS1, AS2) Speaker can cancel previously advertised paths

Spring 2008CPE Computer Networks24 IP Version 6 (Section 4.3.5) Features –128-bit addresses (classless) –multicast –authentication and security –end-to-end fragmentation –autoconfiguration –real-time service –protocol extensions Header –40-byte “base” header –extension headers (fixed order, mostly fixed length) fragmentation source routing authentication and security other options