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Computer Networks

Computer Networks. Network layer (Part 2). Network layer (part II). Last class Network layer functions Addressing, security, fragmentation, delivery semantics, quality of service In the middle of talking about routing…. This class and beyond Finish network layer functions (routing)

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Computer Networks

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  1. Computer Networks Network layer (Part 2)

  2. Network layer (part II) • Last class • Network layer functions • Addressing, security, fragmentation, delivery semantics, quality of service • In the middle of talking about routing…. • This class and beyond • Finish network layer functions (routing) • Specific implementations • IP (addressing, security, fragmentation, delivery semantics, quality of service, routing) • Network layer devices and implementations

  3. 60 4 1 50 X Z Y NL: DBF (count-to-infinity example) Link cost changes: • good news travels fast • bad news travels slow - “count to infinity” problem! • see book for explanation of this example algorithm continues on!

  4. NL: DBF: (count-to-infinity example) dest cost dest cost 1 X A B B A 1 1 C 2 C 1 1 25 C dest cost A 2 B 1

  5. NL: DBF: (count-to-infinity example) C Sends Routes to B dest cost dest cost A B B A 1 ~ C 2 C 1 1 25 C dest cost A 2 B 1

  6. NL: DBF: (count-to-infinity example) B Updates Distance to A dest cost dest cost A B B A 1 3 C 2 C 1 1 25 C dest cost A 2 B 1

  7. NL: DBF: (count-to-infinity example) B Sends Routes to C dest cost dest cost A B A B 3 1 C 1 C 2 1 25 C dest cost A 4 B 1

  8. NL: DBF: (count-to-infinity example) C Sends Routes to B dest cost dest cost A B B A 1 5 C 2 C 1 1 25 C dest cost A 4 B 1

  9. NL: How are 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!

  10. NL: Solutions to looping • Split horizon • Do not advertise route to X to an adjacent neighbor if your route to X goes through that neighbor • If C routes through B to get to A, C does not advertise (C=>A) route to B. • Poisoned reverse • Advertise an infinite distance route to X to an adjacent neighbor if your route to X goes through that neighbor • If C routes through B to get to A, C advertises to B that its distance to A is infinity • Works for two node loops • Does not work for loops with more nodes

  11. 60 4 1 50 X Z Y NL: Split-horizon with poisoned reverse If Z routes through Y to get to X : • Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z) • will this completely solve count to infinity problem? algorithm terminates

  12. NL: Solutions to looping • Example Where Split Horizon Fails • 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... 1 A B 1 1 C X 1 D

  13. NL: Solutions to looping • Path Holddown • If metric increases, delay propagating information • In our example, A and B delay changing and advertising new routes • A and B both set route to D to infinity after single step • Delay too large • Adversely affects convergence • Delay too small • Count-to-infinity more probable • Route poisoning • Advertise infinite cost when cost from next hop starts to increase

  14. NL: Solutions to looping • Path vector • Select loop-free paths • 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

  15. NL: Solutions to looping • Do solutions completely eliminate 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

  16. NL: Link State vs. Distance Vector • Network bandwidth • DV • send everything you know to your neighbors • large messages, transfers only to neighbors • LS • send info about your neighbors to everyone • small messages, transfers O(nE)

  17. NL: Link State vs. Distance Vector • Convergence speed: • LS • faster – don’t need to process LSPs before forwarding • single SPT calculation • DV • fast with triggered updates • count-to-infinity problem • Space requirements: • LS • maintains entire topology • DV • maintains only neighbor state • path vector maintains routes proportional to network diameter

  18. NL: Link State vs. Distance Vector • Robustness: • LS can broadcast incorrect/corrupted LSP • Can be made robust since sources are aware of alternate paths • DV can advertise incorrect paths to all destinations • Incorrect calculation can spread to entire network

  19. NL: DUAL • Distributed Update Algorithm • Garcia-Luna-Aceves 1989 • Goal: Avoid transient loops in DV and LS algorithms • 2 ideas • A path shorter than current path cannot contain a loop • Based on diffusing computation (Dijkstra-Scholten 1980) • Wait until computation completes before changing routes in response to a new update • Similar to path-holddown • 3 kinds of messages • Update, query, reply • 2 states for routers • Active (queries outstanding), passive

  20. NL: DUAL On update if (lower cost) adopt else if (higher cost) { if (from next hop) { if (any path exists < old length from next hop) switch path else freeze route send query to all neighbors except next hop go into active wait for reply from all neighbors update route return to passive } send reply to all querying neighbors }

  21. NL: Routing Hierarchies • Flat routing doesn’t scale • Each node/router cannot be expected to store routes to every destination (or destination network) • 50 million destinations • Route table exchange would swamp network • Key observation • Need less information with increasing distance to destination • Two radically different approaches for routing • The area hierarchy • The landmark hierarchy (discuss in routing alternatives)

  22. NL: Areas • Divide network into areas • Areas can have nested sub-areas • No path between two sub-areas of an area can exit that area • Within area, each node has routes to every other node • Outside area • Each node has routes for other top-level areas only • Inter-area packets are routed to nearest appropriate border router • Can result in sub-optimal paths • Hierarchically address nodes in a network • Sequentially number top-level areas • Sub-areas of area are labeled relative to that area • Nodes are numbered relative to the smallest containing area

  23. NL: Landmark Hierarchy • Details about things nearby and less information about things far away • Not defined by arbitrary boundaries • Thus, not well suited to the real world that does have administrative boundaries • Example: My apartment • MtHood.Portland.USBancorpTower.Oba.KearneyPlaza • From Beaverton • Go towards Mt. Hood • See USBancorpTower before running into Mt.Hood • See Oba before running into USBancorpTower • Reach Oba and route to Kearney Plaza 2 blocks away • From The Dalles • Go towards Mt. Hood, reach it • Go towards Portland, see USBancorpTower • Go towards and reach USBancorpTower • Go towards and reach Oba, route to Kearney Plaza 2 blocks away

  24. NL: A Landmark 9 8 6 7 5 11 3 10 4 1 Router 1 is a landmark of radius 2 2

  25. NL: Landmark Overview • Landmark routers have “height” which determines how far away they can be seen (visibility) • Routers within radius n can see a landmark router LMn • See means that those routers have LMn’s address and know next hop to reach it. • Router x as an entry for router y if x is within radius of y • Distance vector style routing with simple metric • Routing table: Landmark (LM2(d)), Level(2), Next hop

  26. NL: LM Hierarchy Definition • Each LM i associated with level (i) and radius (ri) • Every node is an LM0 landmark • Recursion: some LMi are also LMi+1 • Every LMi sees at least one LMi+1 • Terminating state when all level j LMs are seen by entire network

  27. NL: LM Self-configuration • Bottom-up hierarchy construction algorithm • Goal to bound number of children • Every router is L0 landmark • All Li landmarks run election to self-promote one or more Li+1 landmarks • LM level maps to radius (part of configuration), e.g.: • LM level 0: radius 2 • LM level 1: radius 4 • LM level 2: radius 8 • Dynamic algorithm to adapt to topology changes – Efficient hierarchy

  28. NL: LM Addresses • LM(2).LM(1).LM(0) (C.B.A) • If destination is more than two hops away, will not have complete routing information, refer to LM(1) portion of address, if not known then refer to LM(2) LM0A aka C.B.A R0 R1 LM1B LM2C R2

  29. NL: LM Routing • LM does not imply hierarchical forwarding • It is NOT a source route • En route to LM1,packet may encounter router that is within LM0 radius of destination address (like longest match) • Paths may be asymmetric

  30. Network Node Path Landmark Radius NL: Landmark Routing: Basic Idea • Source wants to reach LM0[a], whose address is c.b.a: • Source can see LM2[c], so sends packet towards c • Entering LM1[b] area, first router diverts packet to b • Entering LM0[a] area, packet delivered to a • Not shortest path • Packet may not reach landmarks LM0[a] r1[b] r0[a] LM1[b] LM2[c] r2[c]

  31. d.d.f d.i.k d.i.g d.d.e d.i.i d.d.d d.d.a d.i.w d.d.j d.i.v d.d.c d.d.l d.d.k d.d.b d.i.u d.n.h d.n.x d.n.t d.n.n d.n.q d.n.s d.n.o d.n.r d.n.p NL: Landmark Routing: Example

  32. d.d.f Landmark Level Next hop 2 LM2[d] f LM1[i] 1 d.i.k k d.i.g LM0[e] 0 f d.d.e d.i.i d.d.d LM0[k] 0 k d.d.a d.i.w LM0[f] 0 f d.d.j d.d.c d.d.l d.d.k d.d.b d.i.u d.n.h d.n.x d.n.t d.n.n d.n.q d.n.s d.n.o d.n.r d.n.p NL: Routing Table for Router g Router g • r0 = 2, r1 = 4, r2 = 8 hops • How to go from d.i.g to d.n.t? g-f-e-d-u-t • How does path length compare to shortest path? g-k-I-u-t Router t

  33. NL: Demux to upper layer • Sends payload to the next layer in protocol stack (usually transport layer)

  34. NL: Error detection • Protection of data and/or header at the network layer • Provide extra protection on top of data-link layer and below transport layer • End-to-end principle • Is this necessary?

  35. NL: Network layer functions summary • Each networking layer provides varying degrees of functionality including…. • security, delivery semantics, quality of service, demux to upper layer, error detection, fragmentation, addressing, routing • Next: IP network layer • IP error detection • IP demux to upper layer • IP security • IP fragmentation • IP delivery semantics • IP quality-of-service • IP addressing • IP routing

  36. Host, router network layer functions: • ICMP protocol • error reporting • router “signaling” • IP protocol • addressing conventions • datagram format • packet handling conventions • Routing protocols • path selection • RIP, OSPF, BGP routing table NL: The Internet Network layer (IP) Transport layer: TCP, UDP Network layer Link layer physical layer

  37. NL: How is IP Design Standardized? • IETF • Voluntary organization • Meeting every 4 months • Working groups and email discussions • “We reject kings, presidents, and voting; we believe in rough consensus and running code” (Dave Clark 1992) • Need 2 independent, interoperable implementations for standard • IRTF • End2End • Reliable Multicast, etc..

  38. NL: IP datagram format (RFC 791) IP protocol version number (currently 4) 32 bits total datagram length (bytes) type of service head. len ver header length (bytes) length for fragmentation/ reassembly fragment offset flgs 16-bit identifier “type” of data upper layer time to live header checksum max number remaining hops (decremented at each router) 32 bit source IP address 32 bit destination IP address E.g. timestamp, record route taken, pecify list of routers to visit. Options (if any) + padding upper layer protocol to deliver payload to data (variable length, typically a TCP or UDP segment)

  39. NL: IP header • Version • Currently at 4, next version 6 • Header length • Length of header (20 bytes plus options) • Type of Service • Typically ignored • Values • 3 bits of precedence • 1 bit of delay requirements • 1 bit of throughput requirements • 1 bit of reliability requirements • Replaced by DiffServ and ECN • Length • Length of IP fragment (payload)

  40. NL: IP header (cont) • Identification • To match up with other fragments • Flags • Don’t fragment flag • More fragments flag • Fragment offset • Where this fragment lies in entire IP datagram • Measured in 8 octet units (11 bit field)

  41. NL: IP header (cont) • Time to live • Ensure packets exit the network • Protocol • Demultiplexing to higher layer protocols • Header checksum • Ensures some degree of header integrity • Relatively weak – 16 bit • Source IP, Destination IP (32 bit addresses) • Options • E.g. Source routing, record route, etc. • Performance issues • Poorly supported

  42. NL: IP error detection • IP checksum • IP has a header checksum, leaves data integrity to TCP/UDP • Catch errors within router or bridge that are not detected by link layer • Incrementally updated as routers change fields • http://www.rfc-editor.org/rfc/rfc1141.txt

  43. NL: IP demux to upper layer • http://www.rfc-editor.org/rfc/rfc1700.txt • Protocol type field • 1 = ICMP • 2 = IGMP • 3 = GGP • 4 = IP in IP • 6 = TCP • 8 = EGP • 9 = IGP • 17 = UDP • 29 = ISO-TP4 • 80 = ISO-IP • 88 = IGRP • 89 = OSPFIGP • 94 = IPIP http://www.rfc-editor.org/rfc/rfc2003.txt

  44. ICMP: Internet Control Message Protocol Essentially a network-layer protocol for passing control messages used by hosts, routers, gateways to communicate network-level information error reporting: unreachable host, network, port, protocol echo request/reply (used by ping) network-layer “above” IP: ICMP msgs carried in IP datagrams ICMP message: type, code plus first 8 bytes of IP datagram causing error http://www.rfc-editor.org/rfc/rfc792.txt NL: IP demux to upper layer TypeCodedescription 0 0 echo reply (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header

  45. NL: IP security • IP originally had no provisions for security • IPsec • Retrofit IP network layer with encryption and authentication • http://www.rfc-editor.org/rfc/rfc2411.txt • If time permits, we may cover this at the end of the course….or someone should do a research paper on this.

  46. network links have MTU (max.transfer size) - largest possible link-level frame. different link types, different MTUs IP packets can be 64KB large IP datagram divided (“fragmented”) within net IP header on each fragment Intermediate router may fragment further as needed one datagram becomes several datagrams “reassembled” only at final destination IP header bits used to identify, order related fragments NL: IP fragmentation (and reassembly) fragmentation: in: one large datagram out: 3 smaller datagrams reassembly

  47. length =1500 length =4000 length =1040 length =1500 ID =x ID =x ID =x ID =x fragflag =0 fragflag =1 fragflag =0 fragflag =1 offset =0 offset =0 offset =1480 offset =2960 NL: IP fragmentation One large datagram becomes several smaller datagrams

  48. NL: IP fragmentation • Path MTU Discovery in IP • http://www.rfc-editor.org/rfc/rfc1191.txt • Hosts dynamically discover minimum MTU of path • Algorithm: • Initialize MTU to MTU for first hop • Send datagrams with Don’t Fragment bit set • If ICMP “pkt too big” msg, decrease MTU • What happens if path changes? • Periodically (>5mins, or >1min after previous increase), increase MTU • Some routers will return proper MTU • MTU values cached in routing table • C. Shannon, D. Moore, k claffy, “Characteristics of Fragmented IP Traffic on Internet Links” ACM SIGCOMM Internet Measurement Workshop 2001.

  49. NL: IP delivery semantics • The waist of the hourglass • Unreliable datagram service • Out-of-order delivery possible • Compare to ATM and phone network… • Unicast mostly • IP broadcast not forwarded • IP multicast supported, but not widely used • If there is time, we will talk about IP multicast….

  50. NL: IP quality of service • IP originally had “type-of-service” (TOS) field to eventually support quality • Not used, ignored by most routers • Then came int-serv (integrated services) and RSVP signalling • Per-flow quality of service through end-to-end support • Setup and match flows on connection ID • Per-flow signaling • Per-flow network resource allocation (*FQ, *RR scheduling algorithms)

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