Overview Addressing Routing TOC IP

Transcription

1 IP: Network Layer Overview Addressing Routing TOC IP

2 Overview Goals and Tasks Routing Switching Issues Basic ideas TOC IP Overview

3 Goals and Tasks Goals of Network Layer Guide packets from source to destination Use network links efficiently (e.g., prefer shorter and faster routes) Addressing Agree on addressing scheme to identify nodes IP addresses are location-based (similar to telephone numbers) This structure reduces the information routers must keep Different types of addresses Routing Routers exchange information to learn network topology Routers then calculate good routes to the different destinations Routers store the results of these calculations in routing tables Different routing algorithms TOC IP Overview Goals and Tasks

4 Routing Definition Finding path from source to destination Types: Path based on SS S S Flow Type or Traffic Source/Destination Destination Internet 5 TOC IP Overview Routing DD D D A (S (S D): D):,,,, B (S (S D): D):,,,, C (S (S D): D):,,,, 5, 5, Voice Voice (S (S D): D):,,,, Data Data (S (S D): D):,,,, 5, 5, (S (S D): D):,,,, (S (S D ): D ):,,,, 5, 5, (S (S D): D):,,,, (S (S D): D):,,,,

5 Switching Definition Sending the bits along the path Approaches Circuit (Telephone; Lightwave) Packet Virtual Circuit (ATM) Datagram (Ethernet, IP) Notes A circuit or VC can be a link in an IP network An Ethernet LAN can be a link in an IP network TOC IP Overview Switching

6 Switching (cont.) Datagram v/s Virtual Circuit Datagram routing Each packet to be forwarded independently Virtual Circuit Each packet from same flow uses same route More state (pick the right granularity) QoS sensitive networks use VC s and signaling Find a route that has the resources available for the connection. Reserve the resources before sending data packets TOC IP Overview Switching

7 Issues Scalability [great in IP] Millions of nodes Routing tables should remain small Updates should be manageable Topology Changes [good in IP] Routers compute new routes as topology changes Changes should not affect most tables Performance [poor in IP] Link utilization should be well-balanced [not in practice] Updates should be fast [not always] Ideally, some flows would have a guaranteed rate [no] Network should detect configuration errors or other errors [no] Network should protect itself against attacks [no] TOC IP Overview Issues

8 Basic Ideas Addressing Layer : Local scheme, typically flat not scalable Layer : Location based and hierarchical scalable Temporary addresses for mobile nodes Network Address Translation to reuse addresses Routing Route is based on destination only (roughly: shortest path) Network decomposed into domains Interdomain routing: Uses a path-vector algorithm Intradomain routing: Uses a link state or a distance vector algorithm Variations Multicast; PP; Ad Hoc; Sensors; Content Distribution Networks

9 Addressing Examples Class-Based Addressing CIDR: Classless Interdomain Routing Assigning Addresses DHCP Network Address Translation TOC IP Addressing

10 Examples Flat Addressing Hierarchical Addressing Internetworking Layers and TOC IP Addressing Examples

11 Flat Addressing : : b : : a : : a 5: 5: a 6: 6: a Address Ports : : a b a a : : a : : b : : b 5: 5: b 6: 6: b b a b c : : a : : b : : c 5: 5: c 6: 6: c a b c : : a : : a : a 6 : a b 5: 5: c : 6: 6: b : a : : a : : a : : a 5: 5: b Routing Table: One per node Destination Exit Port a 5 b : : a : : a : : a 6: 6: b Addresses are arbitrary; not based on topology (e.g., Ethernet) N nodes N - entries in every routing table; not scalable TOC IP Addressing Examples Flat

12 Hierachical Addresses.:.: b Default: a.. b a.:.: a.:.: b Default: c a.. b c.:.: c.:.: b Default: a a c.. b a.:.: b Default: a.. b a.. b a.. b.:.: a Default: b.:.: b Default: a Addresses are arranged based on topology (e.g., IP) Few entries in each routing table; scalable TOC IP Addressing Examples Hierarchical

13 Internetworking Recall the basic internetworking scheme of IP:.*:.*: local local Default: y.:.: tt.7:.7: y. x IP Local.*:.*: local local.*:.*: b Default: a.:.: x.7:.7: y.7 y a b z u..8 data.5 z. u d a v w..8 data.6 v.8 w. t x y..8 data TOC IP Addressing Examples Internetworking

14 Layers and Ethernet Switch Router Ethernet Switch p Application Transport Destination Address B Next Hop C Local address C Layer address y Destination Address B Local to port p Local address B Layer address w Application Transport Network A Network C D Network B Link x Link Link y Link v Link Link w Phy Phy Phy Phy Phy Phy Phy Phy Phy Phy TOC IP Addressing Examples Layers /

15 Class-Based Addresses Addresses Scalability Problem TOC IP Addressing Class

16 Addresses Addressing reflects internet hierarchy bits divided into parts: Class A Class B Class C network host network host 0 0 network host ~ million nets 56 hosts TOC IP Addressing Class - Addresses

17 Scalability Problem Example: an organization initially needs 00 addresses Allocate it a class C address Organization grows to need 00 addresses Class B address is allocated. (~6K hosts) That s overkill -a huge waste Only about 800 class B addresses! Artificial Address crises TOC IP Addressing Class - Scalability

18 Classless Internet Domain Routing (CIDR) CIDR allows networks to be assigned on arbitrary bit boundaries. Address ranges can be assigned in chunks of k k= Idea - use aggregation - provide routing for a large number of customers by advertising one common prefix. This is possible because nature of addressing is hierarchical Summarization reduces the size of routing tables, but maintains connectivity. Aggregation Scalability and survivability of the Internet TOC IP Addressing CIDR

19 CIDR (cont.) Suppose fifty computers in a network are assigned IP addresses They share the prefix 8..9 Is this the longest prefix? Range is to How to write X? Convention: /6 There are -7=6 bits for the 50 computers 6 = 6 addresses TOC IP Addressing CIDR

20 CIDR (cont.) Specify a range of addresses by a prefix: X/Y The prefix common to the entire range is the first Y bits of X. X: The first address in the range has prefix X Y: -Y addresses in the range Example 8.5.0/ Common prefix is bits: Number of addresses: 9 = 5 Prefix aggregation Combine two address ranges 8.5.0/ and 8.5./ gives 8.5.0/ Routers match to longest prefix TOC IP Addressing CIDR

21 CIDR Longest prefix match routing 00, 0, 0 0 a d 000, 000 b 0 c 00, 0, 00 0 Length of longest prefix match for given port Dest. a b c d TOC IP Addressing CIDR

22 CIDR (cont.) Example Default 8.. R R Default 8. R Default TOC IP Addressing CIDR

23 CIDR - Subnets H: IP Mask: H: IP Mask: H: Is H on same subnet as I am? H: IP Mask: IP H e Yes if IP/ = IP/ e H e: IP H e IP e R R e5 TOC IP Addressing CIDR

24 CIDR (cont.) Direct Delivery IP IP on on same subnet IP H e e e IP IP X e H all e e: Who is IP? IP H e IP e R R e5 e e e: I am IP Address Resolution Protocol = Layer Layer Address Layer Layer Address TOC IP Addressing CIDR

25 CIDR (cont.) Indirect Delivery IP IP not on on same subnet e5 e I am IP IP H e e e IP IP X e H H e SH IP IP X e e5 IP IP IP X IP e R R e5 all e5 Who is IP? Note: Fragmentation may be required at R TOC IP Addressing CIDR

26 Assigning IP address (Ideally) A host gets its IP address from the IP address block of its organization An organization gets an IP address block from its ISP s address block An ISP gets its address block from its own provider OR from one of the routing registries: ARIN: American Registry for Internet Numbers RIPE: Reseaux IP Europeens APNIC: Asia Pacific Network Information Center Each Autonomous System (AS) is assigned a 6-bit number (6556 total) Currently,000 AS s in use TOC IP Addressing Assigning Addresses

27 DHCP Dynamic Host Configuration Protocol Idea Temporary addresses assigned on demand Advantages Enables to reuse addresses You come to a classroom with a laptop Dial-up users Automates the assignment of addresses Disadvantage Cannot be a server (how to find address?) TOC IP Addressing DHCP

28 DHCP (cont.) Operations DHCP server maintains list of available addresses Client requests an address Client sends DHCP discover message ( me all = [0 0 ]) Server replies with DHCP offer Client asks for address; server provides one Client can extend/release the lease Server and client can test address TOC IP Addressing DHCP

29 NAT Overview Example How NAT works TOC IP Addressing NAT

30 Overview Shortage of IP Addresses CIDR may not be enough IPv6 may take a long time until deployed NAT enables reuse of addresses Private Addresses: See IETF RFC 6 (99) TOC IP Addressing NAT Overview

31 Example Home Network One IP address (IPa) is visible outside IPa (typically DHCP) NAT IPb (DHCP with NAT) IPc (DHCP with NAT) Note: Can be extended to a set of addresses instead of only one (IPa) In that case, some static addresses can be reserved for servers TOC IP Addressing NAT Example

32 How it works Trick: Use TCP port to distinguish computers There are 6k port numbers, the first k are reserved [IPb IPx TCPm TCPn ] IPa [IPa IPx TCPb TCPn ] [IPx IPa TCPn TCPb ] IPx NAT [IPx IPb TCPn TCPm ] [TCPb IPb, TCPm] IPb TOC IP Addressing NAT How IPc

33 Routing Routing Sub-Functions Hierarchical Types of Protocol TOC IP Routing

34 Routing Sub-Functions Topology Update: Characterize and maintain connectivity Discover neighbors Measure distance (one or more metric) Disseminate Route Computation: Kind of path: Multicast, Unicast Centralized or Distributed Algorithm Policy Hierarchy Switching: Forward the packets at each node TOC IP Routing Sub-Functions

35 Hierarchical Routing The internet has many Administrative Domains B A C TOC IP Routing Hierachical

36 Hierarchical Routing Border Routers B RIP 6 IGRP A BGP 0 C OSPF TOC IP Routing Hierachical

37 Hierarchical Routing Interdomain & Intradomain BGP InterDomain InterDomain 6 B B 7 IntraDomain RIP IntraDomain IGRP A 0 C OSPF IntraDomain TOC IP Routing Hierachical

38 Types of Routing Protocol Overview Link State Distance Vector Link State vs. Distance Vector Path Vector: Interdomain Routing TOC IP Routing Types

39 Overview Topology changes can be detected by nearby nodes These changes must be reflected in the routes Mechanisms for disseminating information Link State: Communicate the names and costs of neighbors. Each node maintains the entire topology. E.g. used in OSPF Distance Vector: Communicate current distance estimates of node to every other node. E.g. used in RIP Path Vector: Communicate current estimates of preferred paths from node to every other node. E.g. used in BGP TOC IP Routing Types Overview

40 Overview LINK STATE AA AA BB BB CC CC DISTANCE VECTOR PATH VECTOR AA BB CC ) Exchange Link States ) Each node computes A: [B, ], [C, ] the shortest paths to B: [A, ], [D, ] the others DD C: [A, ], [D, ] D: [B, ], [C, ] 0 DD 0 D DD D TOC IP Routing Types Overview AA AA BB CC B,D BB CC C,D DD 0 DD AA BB Don t like B AA CC BB CC DD DD

41 Link State Protocols Overview Link State Advertisements Shortest Path Algorithm: Dijkstra TOC IP Routing Types Link State

42 Overview. Every node learns the topology of the network Flooding of Link State Packets (LSP). An efficient shortest path algorithm computes routes to every other node. Node updates Forwarding Table TOC IP Routing Types Link State - Overview

43 Link State Advertisements Link State Packets Flooding Example Some Issues TOC IP Routing Types Link State - LSA

44 Link State Packets Source Sequence Number Age List of Neighbors Every router sends Link State Packets (LSPs) to all of its neighbors LSPs arrive and wait in buffers to be accepted If node j receives a LSP from node k it compares the sequence numbers. If this is the most recent one from k, send to N(j)-{k}. This way each router can send its LSP to all other routers Age starts out at 7. At any router, value is decremented every 8 seconds. At 0 discard. As long as sequence don t wrap this works Otherwise things can get ugly TOC IP Routing Types Link State LSA LSP

45 LSP - Example TOC IP Routing Types Link State LSA Example

46 LSP - Example TOC IP Routing Types Link State LSA Example

47 LSP - Example TOC IP Routing Types Link State LSA Example

48 LSP - Example TOC IP Routing Types Link State LSA Example

49 Some Issues What happens if some routers are much faster at transmitting LSPs? What happens if sequence numbers wrap? What happens when a partitioned network is reconstituted? What about security? Etc., etc. Many lines of code TOC IP Routing Types Link State LSA Issues

50 Dijkstra Every node knows the graph All link weights are >= 0 Goal at node : Find the shortest paths from to all the other nodes. Each node computes the same shortest paths so they all agree on the routes Strategy at node : Find the shortest paths in order of increasing path length 6 5 TOC IP Routing Types Link State - Dijkstra

51 Dijkstra Notation c(i,j) >=0 :cost of link from (I,j) D(,i): Shortest path from to i. D(,x,i): Shortest path from to i via x Let P(k) be the set of nodes k-closest to P()={,} D(,5)= D(,6,5)=5 6 5 IDEA: Given P(k) we can find P(k+) efficiently: To get P(k+), observe that. This node cannot be in P(k). It must be one hop away from some node in P(k) Suppose were false. We picked i Node i has no edge into P(k) There must be a node x, not in P(k) such that x is one hop away from P(k) and D(,i)=D(,x)+D(x,i) But then, D(,x) < D(,i) and we would have picked x instead. Pick node(s) that is one hop away from P(k) that is closest to. Keep iterating until all nodes are in P TOC IP Routing Types Link State - Dijkstra

52 Dijkstra P()={,} D(,)= P()={,,5} D(,5)= P()={,,,5,6} D(,)= D(,6)= TOC IP Routing Types Link State - Dijkstra

53 Dijkstra - Forwarding Table At node 5 Outgoing Cost TOC IP Routing Types Link State - Dijkstra

54 Distance Vector Protocol Bellman Ford Why does it work? Counting to Infinity Bad News Travel Slowly Asynchronous Bellman Ford Oscillations TOC IP Routing Types DV

55 Bellman-Ford C(,) = Communicate current distance estimates of node to every other node This is called its distance vector: D i = (D(i,),D(i,),,D(i,n)) Initially, assume that D(i,j) = c(i,j) if there is a link ij = otherwise The nodes do not need to learn the entire topology Just the distance estimates (vectors) of their neighbors Periodically each node sends its distance vector to all of its neighbors i 6 5 D i (0,,,,,) (,0,,,, ) (,,0,,, ) (,,,0,, ) 5 (,,,, 0,) 6 (,,,,,0) Initially TOC IP Routing Types DV Bellman-Ford

56 Bellman-Ford Update: when receive estimates i D(i,d) = min jεn(i) {c(i,j) + D(j,d)} D i (0,,,,,) (,0,,,, ) (,,0,,, ) (,,,0,, ) 5 (,,,, 0,) 6 (,,,,,0) gets updates from and 5 D(,) = min{c(,) + D(,), c(,5) + D(5,)} 6 5 = min{ +, + } = TOC IP Routing Types DV Bellman-Ford

57 Bellman-Ford Focus on destination Here are the values of D(i,): 6 5 i step TOC IP Routing Types DV Bellman-Ford

58 Why does this compute shortest paths? Suppose in every tick each node sends its distance vector. Assume that initial distances are At time h, node i has as an estimate of the shortest path to node j that has <= h+ hops! D h+ (i,j) = min kεn(i) {D h (k,j) + c(i,k)} TOC IP Routing Types DV Why

59 Counting to Infinity All links cost A B C 0 A B C 0 A B C Ping-Pong to Eternity TOC IP Routing Types DV Counting to Infinity

60 Bad News Travels Slowly M D(,)=, D(,)=, D(,)= TOC IP Routing Types DV Bad News

61 Bad News Travels Slowly Node takes about M Iterations to figure out that D(,)=M M Fundamental Cause: After a network change, think of the network protocol running from time 0. The initial conditions are arbitrary Tricks exist to get around these problems but not fool proof TOC IP Routing Types DV Bad News

62 Asynchronous Bellman Ford In general, nodes are using different and possibly inconsistent estimates If no link changes after some time t, the algorithm will eventually converge to the shortest path No synchronization required at all TOC IP Routing Types DV Asynchronous

63 Oscillations Link costs must reflect link speed AND congestion Under both LSP and DV routing occurs over a tree The costs of the links of this tree will increase The other links will not be congested Their costs will drop Routing protocol will shift traffic and create a new tree This process of shifting and reshifting can be severe Way out: Change congestion costs slowly (exponential averaging) Route dampening TOC IP Routing Types DV Oscillations

64 Oscillations - Example Heavy Load High Delay Traffic Light Load Low Delay 5 5 Light Load Low Delay Traffic 5 Heavy Load High Delay 5 TOC IP Routing Types DV Oscillations

65 Link State vs. Distance Vector No clear winner LS is robust since it each node computes its own routes independently Suffers from the weaknesses of the topology update protocol. Inconsistency etc. Excellent choice for a well engineered network within one administrative domain E. g. OSPF DV works well when the network is large since it requires no synchronization and has a trivial topology update algorithm Suffers from convergence delays Very simple to implement at each node Excellent choice for large networks E.g. RIP TOC IP Routing Types LS vs. DV