Chapter 4 IP Routing Professor Rick Han University of Colorado at Boulder

Prof. Rick Han, University of Colorado at Boulder Announcements Reminder: Programming assignment #1 is due Feb. 19 Part of Homework #2 available on Web site, due Feb. 26 Last week’s lecture are now on Web site Next, IP routing, …

Prof. Rick Han, University of Colorado at Boulder Recap of Previous Lecture Routing to connect remote LANs Encapsulation Internet Protocol (IPv4) Connects Networks of Networks “Best-Effort” Service IP Packet Header – 20 bytes TTL IP Addressing – 32 bit, heirarchy, IP Fragmentation and Reassembly Address Resolution Protocol (ARP)

Prof. Rick Han, University of Colorado at Boulder Address Resolution Protocol (ARP) Given a known IP address, ARP returns the desired Ethernet MAC address If sending to a host on the same Ethernet, First, check cache if address already present If not, send an Ethernet’s broadcast query (all 1’s in 48-bit address) with “target IP” address Target host responds with its IP address ARP updates its cache Requesting Node Destination Node IP Eth Hdr ARP query Eth. Header has dest. MAC ARP response

Prof. Rick Han, University of Colorado at Boulder ARP (2) What if destination host is on a remote LAN? No local host will respond to broadcast ARP query Solution: IP end host sends to IP network, which routes packet to destination IP host ARP is performed separately on LAN 1 and LAN 2 Destination Node Requesting Node IP Router LAN1 LAN2

Prof. Rick Han, University of Colorado at Boulder ARP (3) On LAN 1: IP routers broadcast ICMP “router advertisements” on local LAN or impatient end host broadcasts “solicitations” When IP end host wants to send outside of LAN, it does ARP request to find MAC address of router’s IP interface address to LAN, if not already cached Sends a packet containing encapsulated by Eth. Header containing dest. MAC address of IP router Destination Node Requesting Node IP Router LAN1 LAN2

Prof. Rick Han, University of Colorado at Boulder ARP (4) On LAN 2: IP packet with arrives at IP router on LAN 2 IP router does an ARP request to find MAC address of dest IP end host, if not already cached Sends a packet containing encapsulated by Eth. Header containing dest. MAC address of dest IP end host Proxy ARP when only one router between two LANs Destination Node Requesting Node IP Router LAN1 LAN2

Prof. Rick Han, University of Colorado at Boulder Forwarding Datagrams Router C Router D Router B Router E Router X Router Y Host 1 Host 2 Host 3 DestinationOutput Port Host 1X-B link Host 2E-B link Host 3D-B link Host 4C-B link Routing Table at Router B Host 4

Prof. Rick Han, University of Colorado at Boulder Forwarding Datagrams (2) Router C Router D Router B Router E Router X Router Y Host 1 Host 2 Host 3 DestinationOutput Port Host 1B-E link Host 2Y-E link Host 3D-E link Host 4C-E link Routing Table at Router E Host 4

Prof. Rick Han, University of Colorado at Boulder Forwarding Datagrams (3) DestinationOutput Port Host 1B-E link Host 2Y-E link Host 3D-E link Host 4C-E link Routing Table at Router E Only need to know the destination address to route the datagram to output port. Compare to: VC routing tables had 4 columns: input VC, input port, output VC, output port Ethernet Bridge tables store the source address and source port/LAN, but forwards using destination address

Prof. Rick Han, University of Colorado at Boulder Forwarding Datagrams (4) DestinationOutput Port Host 1B-E link Host 2Y-E link Host 3D-E link Host 4C-E link Routing Table at Router E Each datagram travels its own independent path: There is no connection unlike VCs “Connectionless” datagram networks “Connection-oriented” virtual circuits

Prof. Rick Han, University of Colorado at Boulder Forwarding Datagrams (5) DestinationOutput Port Host 1B-E link Host 2Y-E link Host 3D-E link Host 4C-E link Routing Table at Router E Each routing table has to contain a complete list of all of the hosts on the net and how to get to them (next hop output port) Implications on scalability Compare to VC’s, where each switch only needed to keep in its table the virtual circuits that ran through the switch

Prof. Rick Han, University of Colorado at Boulder Internet Routing “Routing” helps to fill in the IP forwarding tables IP routing employs a distributed algorithm to calculated the shortest path through a graph Many challenges to make distributed algorithms work well Router C Router D Router B Router E Router X Router Y Host 1 Host 2 Homogeneous IP routing fabric

Prof. Rick Han, University of Colorado at Boulder Internet Routing (2) Routing algorithms view the network as a graph Problem: find lowest cost path between two nodes. What info is required for solution? Need complete topology info Need link costs Two types of distributed algorithms: Distance vector (RIP) Link state (OSPF)

Prof. Rick Han, University of Colorado at Boulder Distance Vector (RIP) Employed in the early Arpanet RIP = Routing Information Protocol A specific implementation of distance-vector routing Distributed next hop computation Unit of information exchange Vector of distances to destinations Distributed Bellman-Ford Algorithm

Prof. Rick Han, University of Colorado at Boulder Distance Vector (2) Start Conditions: Each router starts with a vector of distances to all directly attached networks Send step: Each router advertises its current vector to all neighboring routers Receive step: Upon receiving vectors from each of its neighbors, router computes its own distance to each neighbor Then, for every network X, router finds that neighbor who is closer to X than any other neighbor Router updates its cost to X After doing this for all X, router goes to send step

Prof. Rick Han, University of Colorado at Boulder Distance Vector (3) Example courtesy of Prof. Srini Seshan at CMU A B E C D Info at Node A B C D ABC 07~ 701 ~10 ~~ Distance to Node D ~ ~ 2 0 E18~2 1 8 ~ 2 0 E Global minimum distance table, each row is a condensed forwarding table for node i

Prof. Rick Han, University of Colorado at Boulder Distance Vector (4) A B E C Dest. at Node A B C D DistanceVia Neighbor --B E B B E E Dest. Node A B C D BE -- Distance via Neighbor E-- Format of Distance Table in A Format of Routing/Forwarding Table in A

Prof. Rick Han, University of Colorado at Boulder E Receives D’s Routes; Updates Cost Info at Node A B C D ABC 07~ 701 ~10 ~~2 Distance to Node D ~ ~ 2 0 E ~ 2 0 E A B E C D Global minimum distance table, Node i only sees info on its row, not entire global view

Prof. Rick Han, University of Colorado at Boulder A receives B’s; Updates Cost Info at Node A B C D ABC ~10 ~~2 Distance to Node D ~ ~ 2 0 E ~ 2 0 E A B E C D

Prof. Rick Han, University of Colorado at Boulder A receives E’s routes; Updates Costs Info at Node A B C D ABC ~10 ~~2 Distance to Node D 3 ~ 2 0 E ~ 2 0 E A B E C D For every dest. node X, router finds that neighbor who is closer to X than any other neighbor & updates its cost to X

Prof. Rick Han, University of Colorado at Boulder Final Distances Info at Node A B C D ABC Distance to Node D E E A B E C D Topology/distance info ripples outward from each node from every other node

Prof. Rick Han, University of Colorado at Boulder Link Failure Causes “Bouncing” Effect A B C B C2 1 destcost X B B via A C1 1 destcost C A via A B1 2 destcost B B via

Prof. Rick Han, University of Colorado at Boulder B Notices A-B Link Failure A 25 1 B C B notices failure, resets cost via A to infinity in distance table (not shown), & knows cost via C is 26 B C2 1 destcost B B via A C1 26 destcost C C via A B1 2 destcost B B via

Prof. Rick Han, University of Colorado at Boulder C Sends Dist. Vector to B A 25 1 B C B C2 1 destcost B B via A C1 3 destcost C C via A B1 2 destcost B B via C sends routing update to B

Prof. Rick Han, University of Colorado at Boulder B Updates Distance to A A 25 1 B C Packet sent from C to A bounces between C and B until TTL=0! B C2 1 destcost B B via A C1 3 destcost C C via A B1 2 destcost B B via

Prof. Rick Han, University of Colorado at Boulder B Sends Dist. Vector to C A 25 1 B C C adds one to B’s advertised distance to A. (Why does C override its stored distance of 2 to A with 4, larger value?) B C2 1 destcost B B via A C1 3 destcost C C via A B1 4 destcost B B via

Prof. Rick Han, University of Colorado at Boulder C Sends Dist. Vector to B A 25 1 B C B adds one to C’s advertised distance to A. (overrides its stored distance of 3 to A with 5, larger value) B C2 1 destcost B B via A C1 5 destcost C C via A B1 4 destcost B B via

Prof. Rick Han, University of Colorado at Boulder Link Failure: Bad News Travels Slowly A 25 1 B C After 20+ exchanges, routing tables look like this: B C25 26 destcost C C via A C1 25 destcost C C via A B1 24 destcost B B via Assume A has advertised its link cost of 25 to C during B C exchanges. C stores this cost in its distance table (not shown)

Prof. Rick Han, University of Colorado at Boulder Bad News Travels Slowly (2) A 25 1 B C C increments B’s update by 1, and chooses 25 via A to A, instead of 26 Via B to A B C25 26 destcost C C via A C1 25 destcost C C via A B1 25 destcost B A via

Prof. Rick Han, University of Colorado at Boulder Bad News Travels Slowly (3) A 25 1 B C After 25 B-C exchanges, finally converge to stable routing B C25 26 destcost C C via A C1 26 destcost C C via A B1 25 destcost B A via

Prof. Rick Han, University of Colorado at Boulder Link Failure Causes “Counting to Infinity” Effect A B C B C2 1 destcost X B B via A C1 1 destcost C A via A B1 2 destcost B B via

Prof. Rick Han, University of Colorado at Boulder B Notices A-B Link Failure A 25 1 B C B notices failure, resets cost to 26 B C2 1 destcost B B via A C1 26 destcost C C via A B1 2 destcost B B via

Prof. Rick Han, University of Colorado at Boulder C Sends Dist. Vector to B A 25 1 B C B C2 1 destcost B B via A C1 3 destcost C C via A B1 2 destcost B B via C sends routing update to B

Prof. Rick Han, University of Colorado at Boulder A-C Link Fails A 1 B C C detects link to A has failed, but no change in C’s routing table (why?) A C1 3 destcost C C via A B1 2 destcost B B via

Prof. Rick Han, University of Colorado at Boulder Now, B and C Count to Infinity A 1 B C A C1 3 destcost C C via A B1 4 destcost B B via

Prof. Rick Han, University of Colorado at Boulder How are These Loops Caused? Observation 1: –B’s metric increases Observation 2: –C picks B as next hop to A –But, the implicit path from C to A includes itself (C ) !

Prof. Rick Han, University of Colorado at Boulder Solution 1: Holddowns If metric increases, delay propagating information –In our example, B delays advertising route –C eventually thinks B’s route is gone, picks its own route –B then selects C as next hop Adversely affects convergence from failures

Prof. Rick Han, University of Colorado at Boulder Other “Solutions” Split horizon –C does not advertise route to B when it sends its distance vector Poisoned reverse –C advertises route to B with infinite distance in its distance vector Works for two node loops –Does not work for loops with more nodes

Prof. Rick Han, University of Colorado at Boulder Avoiding the Counting to Infinity Effect Select loop-free paths One way of doing this: –Each route advertisement carries entire path –If a router sees itself in path, it rejects the route BGP does it this way Space proportional to diameter

Prof. Rick Han, University of Colorado at Boulder Loop Freedom at Every Instant? Does bouncing effect avoid loops? –No! Transient loops are still possible –Why? Because implicit path information may be stale –See this in BGP convergence Only way to fix this –Ensure that you have up-to-date information by explicitly querying

Prof. Rick Han, University of Colorado at Boulder Distance Vector in Practice RIP and RIP2 –Uses split-horizon/poison reverse BGP –Propagates entire path –Path also used for effecting policies

Prof. Rick Han, University of Colorado at Boulder Example Where Split Horizon Fails A When link breaks, C marks D as unreachable and reports that to A and B Suppose A learns it first –A now thinks best path to D is through B – A reports D unreachable to B and a route of cost=3 to C C thinks D is reachable through A at cost 4 and reports that to B B reports a cost 5 to A who reports new cost to C etc... X B C D