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# CSE 30264 Computer Networks Prof. Aaron Striegel Department of Computer Science & Engineering University of Notre Dame Lecture 12 – February 18, 2010.

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CSE 30264 Computer Networks Prof. Aaron Striegel Department of Computer Science & Engineering University of Notre Dame Lecture 12 – February 18, 2010

Today’s Lecture Routing –Distance Vector –Link State Global Routing Spring 2010CSE 302642 Physical Data Network Transport Application

Spring 2009CSE302643 Routing (Revisited) Outline Distance Vector Link State

Spring 2009CSE302644 Overview Forwarding vs Routing –Forwarding: to select an output port based on destination address and routing table –Routing: process by which routing table is built Network as a Graph Problem: Find lowest cost path between two nodes –How much do you know? Distance vector: Just my neighbors and who they can reach Link state: Everything on my network

Spring 2009CSE302645 Distance Vector Each node maintains a set of triples –(Destination, Cost, NextHop) DestinationCostNext Hop A0Local B2E C7B D2E.. Exchange the vector (the table) of my current routing information

Routing Revisited Spring 2010CSE 302646 A F E B C D 3 6 1 1 2 1 9 4

Time t=0, Start Time Spring 2010CSE 302647 A F E B C D 3 6 1 1 2 1 9 4 Perspective of A A’s Routing Table A C=0 NH: * Assume know our neighbors and link cost B C=3 NH: B E C=1 NH: E F C=6 NH: F

Symmetry Spring 2010CSE 302648 A B 3 A B 3 3 Cost in either direction is the same

A sends to its neighbors Spring 2010CSE 302649 A C=0 NH: Me B C=3 NH: B E C=1 NH: E F C=6 NH: F Routing Table Vector of reachability B,3 E,1 F,1 Send this to our neighbors A F E B

E broadcasts to its neighbors Spring 2010CSE 3026410 A F E B C D 3 6 1 1 2 1 9 4 A,1 B,1 D,1 F,2 Distance Vector Vector = Here is my best cost to get to these locations if you come through me

Gather round…. Send our distance vector –Cost to reach nodes through us –Two ways Cost in the vector Let external node put in cost When we receive a DV –Are any routes better than our own? –Could we take the cost of using that link and navigate via that node? Spring 2010CSE 3026411

What happens at A? Spring 2010CSE 3026412 A F E B C D 3 6 1 1 2 1 9 4 Perspective of A A’s Routing Table A C=0 NH: * B C=3 NH: B E C=1 NH: E F C=6 NH: F A,1 B,1 D,1 F,2 DV from E That’s us, duh Compare new vs. current 3 vs. 1+1 via E Inf vs. 1+1 via E 6 vs. 1+2 via E

What happens at A? Spring 2010CSE 3026413 A F E B C D 3 6 1 1 2 1 9 4 Perspective of A A’s Routing Table A C=0 NH: * B C=3 NH: B E C=1 NH: E F C=6 NH: F A C=0 NH: * B C=2 NH: E E C=1 NH: E F C=3 NH: E D C=E NH: E

Convergence –Stop when no more changes –Non-convergence bad We’ll see that in a moment How? –Only send on change of local table –Stop seeing bcast, stop Spring 2010CSE 3026414

Spring 2009CSE3026415 Routing Loops Example 1 –F detects that link to G has failed –F sets distance to G to infinity and sends update t o A –A sets distance to G to infinity since it uses F to reach G –A receives periodic update from C with 2-hop path to G –A sets distance to G to 3 and sends update to F –F decides it can reach G in 4 hops via A Example 2 –link from A to E fails –A advertises distance of infinity to E –B and C advertise a distance of 2 to E –B decides it can reach E in 3 hops; advertises this to A –A decides it can read E in 4 hops; advertises this to C –C decides that it can reach E in 5 hops… Count to Infinity Problem

Spring 2010CSE3026416 Loop- Breaking Heuristics Set infinity to 16 Split horizon Split horizon with poison reverse

Is DV used? Yes –RIP – Routing Information Protocol –BGP – Border Gateway Protocol* Sort of, BGP is kind of a hybrid –Various mobile routing protocols Spring 2010CSE 3026417

Spring 2009CSE3026418 Routing Information Protocol (RIP) Distributed along with BSD Unix Straightforward implementation of DV Updates sent every 30 seconds Link costs constant at 1 (16 = infinity)

Spring 2009CSE3026419 Link State Strategy –Two-fold Exchange HELLO msgs with neighbors Broadcast changes to all of neighbor state Link State Packet (LSP) –ID of the node that created the LSP –Cost of link –Sequence number –TTL

Spring 2009CSE3026420 Link State (cont) Changes  Reliably flood –store most recent LSP from each node –forward LSP to all nodes but one that sent it –generate new LSP periodically increment SEQNO –start SEQNO at 0 when reboot –decrement TTL of each LSP discard when TTL=0

Spring 2010CSE3026421 Flooding

Spring 2009CSE3026422 Route Calculation Dijkstra’s shortest-path algorithm Let –N denotes set of nodes in the graph –l (i, j) denotes non-negative cost (weight) for edge (i, j) –s denotes this node –M denotes the set of nodes incorporated so far –C(n) denotes cost of the path from s to node n M = {s} for each n in N - {s} C(n) = l(s, n) while (N != M) M = M union {w} such that C(w) is the minimum for all w in (N - M) for each n in (N - M) C(n) = MIN(C(n), C (w) + l(w, n ))

Link State Example Spring 2010CSE 3026423 A F E B C D 3 6 1 1 2 1 9 4 Assume that A gets updates from everyone else via reliably flooded updates, i.e. A knows the topology of the graph

Link State Example Spring 2010CSE 3026424 A F,* E,* B,* C,* D,* 3 6 1 1 2 1 9 4 Step 0: Set everyone’s weight to infinity Add ourselves as the lowest cost node

Link State Example Spring 2010CSE 3026425 A F,* E,* B,* C,* D,* 3 6 1 1 2 1 9 4 Lowest cost total reachable cost  E Who has the lowest total reachable cost from our covered net?

E wins, add it to our covered graph Spring 2010CSE 3026426 A F,* E,1 B,* C,* D,* 3 6 1 1 2 1 9 4 Lowest cost total reachable cost  B or D tied at 2 Who has the lowest total reachable cost from our covered net?

B wins, add it to our covered graph Spring 2010CSE 3026427 A F,* E,1 B,2 C,* D,* 3 6 1 1 2 1 9 4 Lowest cost total reachable cost  D at 2 Who has the lowest total reachable cost from our covered net?

D wins, add it to our covered graph Spring 2010CSE 3026428 A F,* E,1 B,2 C,* D,2 3 6 1 1 2 1 9 4 Lowest cost total reachable cost  F with a cost of 3 Who has the lowest total reachable cost from our covered net?

F wins, add it to our covered graph Spring 2010CSE 3026429 A F,3 E,1 B,2 C,* D,2 3 6 1 1 2 1 9 4 Lowest cost total reachable cost  C with a cost of 6 Who has the lowest total reachable cost from our covered net?

C wins, all done Spring 2010CSE 3026430 A F,3 E,1 B,2 C,6 D,2 3 6 1 1 2 1 9 4

What does it mean? Spring 2010CSE 3026431 A F,3 E,1 B,2 C,6 D,2 3 6 1 1 2 1 9 4 TargetNext Hop BE CE DE EE FE

Spring 2009CSE3026432 OSPF Open Shortest Path First Protocol Authentication Additional hierarchy Load balancing Quintessential link state protocol

Spring 2009CSE3026433 Metrics Original ARPANET metric –measures number of packets queued on each link –took neither latency or bandwidth into consideration New ARPANET metric –stamp each incoming packet with its arrival time ( AT ) –record departure time ( DT ) –when link-level ACK arrives, compute Delay = (DT - AT) + Transmit + Latency –if timeout, reset DT to departure time for retransmission –link cost = average delay over some time period Revised ARPANET metric –compressed dynamic range –replaced Delay with link utilization Practice –static metrics (e.g., 1/bandwidth) Why might variable weights be bad?

Spring 2009CSE3026434 Global Internet Outline Subnetting Supernetting

Spring 2009CSE3026435 How to Make Routing Scale Flat versus Hierarchical Addresses Inefficient use of Hierarchical Address Space –Class C with 2 hosts (2/254 = 0.78% efficient) –Class B with 256 hosts (256/65534 = 0.39% efficient) Too many networks –Routing tables do not scale –Route propagation protocols do not scale Current routers – IPv4 – core  160k entries

Spring 2009CSE3026436 Internet Structure Recent Past NSFNET backbone Stanford BARRNET regional Berkeley PARC NCAR UA UNM Westnet regional UNL KU ISU MidNet regional ■ ■ ■

Spring 2010CSE3026437 Internet Structure Today Backbone service provider Peering point Peering point Large corporation Small corporation “Consumer” ISP “Consumer” ISP “Consumer” ISP

Abilene – aka Internet 2 Spring 2010CSE 3026438

Spring 2009CSE3026439 Subnetting Add another level to address/routing hierarchy: subnet Subnet masks define variable partition of host part Subnets visible only within site

Spring 2009CSE3026440 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 –More likely than not, it will be Can put multiple subnets on one physical network Subnets not visible from the rest of the Internet

Spring 2009CSE3026441 Supernetting Assign block of contiguous network numbers to nearby networks Called CIDR: Classless Inter-Domain Routing Represent blocks with a single pair (first_network_address, count) Restrict block sizes to powers of 2 Use a bit mask (CIDR mask) to identify block size All routers must understand CIDR addressing

Spring 2010CSE3026442 CIDR Border gateway (advertises path to 11000000000001) Regional network Corporation X (11000000000001000001) Corporation Y (11000000000001000000) Instead of advertising two networks upstream, just advertise one

Spring 2009CSE3026443 Route Propagation Know a smarter router –hosts know local router –local routers know site routers –site routers know core router –core routers know everything Autonomous System (AS) –corresponds to an administrative domain –examples: University, company, backbone network –assign each AS a 16-bit number Two-level route propagation hierarchy –interior gateway protocol (each AS selects its own) –exterior gateway protocol (Internet-wide standard)

Spring 2009CSE3026444 Inter-Domain/Intra-Domain Edge router Core router

Spring 2009CSE3026445 Popular Interior Gateway Protocols RIP: Route Information Protocol –developed for XNS –distributed with Unix –distance-vector algorithm –based on hop-count OSPF: Open Shortest Path First –recent Internet standard –uses link-state algorithm –supports load balancing –supports authentication

Spring 2009CSE3026446 EGP: Exterior Gateway Protocol Overview –designed for tree-structured Internet –concerned with reachability, not optimal routes Protocol messages –Neighbor acquisition: one router requests that another be its peer; peers exchange reachability information –Neighbor reachability: one router periodically tests if the another is still reachable; exchange HELLO/ACK messages; uses a k-out-of-n rule –Routing updates: peers periodically exchange their routing tables (distance-vector)

Spring 2009CSE3026447 BGP-4: Border Gateway Protocol AS 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 –one BGP speaker that advertises: local networks other reachable networks (transit AS only) gives path information

Spring 2009CSE3026448 Multi-Backbone Internet Backbone service provider Peering point Peering point Large corporation Small corporation “Consumer” ISP “Consumer” ISP “Consumer” ISP

Spring 2009CSE3026449 BGP Example Speaker for AS2 advertises reachability to P and Q –network 128.96, 192.4.153, 192.4.32, and 192.4.3, can be reached directly from AS2 Speaker for backbone advertises –networks 128.96, 192.4.153, 192.4.32, and 192.4.3 can be reached along the path (AS1, AS2). Speaker can cancel previously advertised paths Regional provider A (AS 2) Regional provider B (AS 3) Customer P (AS 4) Customer Q (AS 5) Customer R (AS 6) Customer S (AS 7) 128.96 192.4.153 192.4.32 192.4.3 192.12.69 192.4.54 192.4.23 Backbone network (AS 1)

Spring 2009CSE3026450 IP Version 6 Features –128-bit addresses (classless) –multicast –real-time service –authentication and security –autoconfiguration –end-to-end fragmentation –protocol extensions Header –40-byte “base” header –extension headers (fixed order, mostly fixed length) fragmentation source routing authentication and security other options

Spring 2009CSE3026451 Addresses 125-m-n-o-pponm3 SubscriberIDProviderIDRegistryID010InterfaceIDSubnetID

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