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Lect1. - 03/28/05

CDA 6505 Network Architecture and Client/Server Computing. Lecture 16 Exterior Routing Protocols And Multicasting by Zornitza Genova Prodanoff. Lect1..ppt - 03/28/05. Outline. Path-Vector Protocols: BGP and IDRP Muticasting. ZGP00 2.

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Lect1. - 03/28/05

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  1. CDA 6505 Network Architecture and Client/Server Computing Lecture 16 Exterior Routing Protocols And Multicasting by Zornitza Genova Prodanoff Lect1..ppt - 03/28/05

  2. Outline • Path-Vector Protocols: BGP and IDRP • Muticasting ZGP002

  3. Problems with Distance-Vector and Link-State Routing • Neither distance-vector (RIP) nor link state (OSPF) protocols effective for exterior routing • Distance vector and link state protocols assume all routers share common metric • Priorities and restrictions may differ between ASs • Flooding of link state information may become unmanageable ZGP003

  4. Path Vector Routing • Dispense with routing metrics • Provide information about: • Which networks can be reached by given router • Which ASs must be crossed to get there • No distance or cost element • Routing information includes all Ass visited to reach destination • Allows policy routing ZGP004

  5. Boarder Gateway Protocol (BGP) • Allows routers (gateways) in different ASs to exchange routing information • Messages sent over TCP • See next slide • Three functional procedures • Neighbour acquisition • Neighbour reachability • Network reachability ZGP005

  6. BGP v4 Messages • Open • Start neighbour relationship with another router • Update • Transmit information about single route • List multiple routes to be withdrawn • Keepalive • Acknowledge open message • Periodically confirm neighbour relationship • Notification • Send when error condition detected ZGP006

  7. Neighbour Acquisition • Neighbours attach to same subnetwork • If in different ASs routers may wish to exchange information • Neighbour acquisition is when two neighbouring routers agree to exchange routing information regularly • Needed because one router may not wish to take part • One router sends request, the other acknowledges • Knowledge of existence of other routers and need to exchange information established at configuration time or by active intervention ZGP007

  8. Neighbour Reachability • Periodic issue of keepalive messages • Between all routers that are neighbours ZGP008

  9. Network Reachability • Each router keeps database of subnetworks it can reach and preferred route • When change made, router issues update message • All BGP routers build up and maintain routing information ZGP009

  10. BGP Message Formats • Marker: • Reserved for authentication • Length: • In octets • Type: • Open, Update, Keepalive, Notification ZGP0010

  11. Neighbour Acquisition Detail • Router opens TCP connection with neighbour • Sends open message • Identifies sender’s AS and gives IP address • Includes Hold Time • As proposed by sender • If recipient prepared to open neighbour relationship • Calculate hold time • min [own hold time, received hold time] • Max time between keepalive/update messages • Reply with keepalive ZGP0011

  12. Keepalive Detail • Header only • Often enough to prevent hold time expiring ZGP0012

  13. Update Detail • Information about single route through internet • Information to be added to database of any recipient router • Network layer reachability information (NLRI) • List of network portions of IP addresses of subnets reached by this route • Total path attributes length field • Path attributes field (next slide) • List of previously advertised routes being withdrawn • May contain both ZGP0013

  14. Path Attributes Field • Origin • Interior (e.g. OSPF) or exterior (BGP) protocol • AS_Path • ASs traversed for this route • Next_Hop • IP address of boarder router for next hop • Multi_Exit_disc • Information about routers internal to AS • Local_Pref • Tell other routers within AS degree of preference • Atomic_Aggregate, Aggregator • Uses subnet addresses in tree view of network to reduce information needed in NLRI ZGP0014

  15. Withdrawal of Route(s) • Route identified by IP address of destination subnetwork(s) ZGP0015

  16. Notification Message • Error notification • Message header error • Includes authentication and syntax errors • Open message error • Syntax errors and option not recognised • Proposed hold time unacceptable • Update message error • Syntax and validity errors • Hold time expired • Finite state machine error • Cease • Close connection in absence of any other error ZGP0016

  17. Diagram for BGP Routing Information Exchange ZGP0017

  18. BGP Routing Information Exchange • R1 constructs routing table for AS1 using OSPF • R1 issues update message to R5 (in AS2) • AS_Path: identity of AS1 • Next_Hop: IP address of R1 • NLRI: List of all subnets in AS1 • Suppose R5 has neighbour relationship with R9 in AS3 • R9 forwards information from R1 to R9 in update message • AS_Path: list of ids {AS2,AS1} • Next_Hop: IP address of R5 • NLRI: All subnets in AS1 • R9 decides if this is preferred route and forwards to neighbours ZGP0018

  19. Inter-Domain Routing Protocol (IDRP) • Exterior routing protocol for IPv6 • ISO-OSI standard • Path-vector routing • Superset of BGP • Operates over any internet protocol (not just TCP) • Own handshaking for guaranteed delivery • Variable length AS identifiers • Handles multiple internet protocols and address schemes • Aggregates path information using routing domain confederations ZGP0019

  20. Routing Domain Confederations • Set of connected AS • Appear to outside world as single AS • Recursive • Effective scaling ZGP0020

  21. Multicasting • Sending message to multicast address • Multicast address refers to a group of hosts • Multimedia • Teleconferencing • Databases • Distributed computation • Real-time workgroup ZGP0021

  22. Multicasting within LAN • MAC level multicast addresses • IEEE 802 uses highest order bit 1 • All stations that recognise the multicast address accept the packet • Works because of broadcast nature of LAN • Packet only sent once • Much harder on internet ZGP0022

  23. Example Configuration for Multicast Internet ZGP0023

  24. Broadcast • Assume location of recipients not know • Send packet to every network • Packet addressed to N3 traverses N1, link L3, N3 • Router B translates IP multicast address to MAC multicast address • Repeat for each network • Generates lots of packets • In example, 13 ZGP0024

  25. Multiple Unicast • Location of each member of multicast group known to source • Table maps multicast address to list of networks • Only need to send to networks containing members of multicast group • Reduced traffic (a bit) • In example, 11 ZGP0025

  26. True Multicast • Least cost path from source to each network containing member of group is determined • Gives spanning tree configuration • For networks containing group members only • Source transmits packet along spanning tree • Packet replicated by routers at branch points of spanning tree • Reduced traffic • In example, 8 ZGP0026

  27. Multicast Transmission Example ZGP0027

  28. Requirements for Multicasting (1) • Router must forward two or more copies of incoming packet • Addressing • IPv4 uses class D • Start 1110 plus 28 bit group id • IPv6 uses 8 bit prefix of all 1s, 4 bit flags field, 4 bit scope field 112 bit group id • Node must translate between multicast address and list of networks containing members of group • Router must translate between IP multicast address and subnet multicast address to deliver to destination network ZGP0028

  29. Requirements for Multicasting (2) • Multicast addresses may be permanent or dynamic • Individual hosts may join or leave dynamically • Need mechanism to inform routers • Routers exchange information on which subnets contain members of groups • Routers exchange information to calculate shortest path to each network • Need routing protocol and algorithm • Routes determined based on source and destination addresses • Avoids unnecessary duplication of packets ZGP0029

  30. Internet Group Management Protocol (IGMP) • Type: Membership query (general or group specific), membership report, leave group, max. response time • Checksum: uses IPv4 algorithm • Group address: zero for request, valid IP multicast for report or leave ZGP0030

  31. IGMP Operation • Host uses IGMP to make itself know as member of group to other hosts and routers • To join, send IGMP membership report message • Send to multicast destination of group being joined • Routers periodically issue IGMP query • To all-hosts multicast address • Hosts respond with report message for each group to which it belongs • Only one host in group needs to respond to keep group alive • Host keeps timer and reponds if no other reply heard in time • Host sends leave group message • Group specific query from router determins if any members remain ZGP0031

  32. Group Membership with IPv6 • Function incorporated in ICMPv6 • Includes all ICMPv4 plus IGMP • Includes group membership query and report • Addition of new group membership termination message ZGP0032

  33. Multicast Extension to OSPF (MOSPF) • Enables routing of IP multicast datagrams within single AS • Each router uses MOSPF to maintain local group membership information • Each router periodically floods this to all routers in area • Routers build shortest path spanning tree from a source network to all networks containing members of group (Dijkstra) • Takes time, so on demand only ZGP0033

  34. Forwarding Multicast Packets • If multicast address not recognised, discard • If router attaches to a network containing a member of group, transmit copy to that network • Consult spanning tree for this source-destination pair and forward to other routers if required ZGP0034

  35. Equal Cost Multipath Ambiguities • Dijkstra’ algorithm will include one of multiple equal cost paths • Which depends on order of processing nodes • For multicast, all routers must have same spanning tree for given source node • MOSPF has tiebreaker rule ZGP0035

  36. Interarea Multicasting • Multicast groups amy contain members from more than one area • Routers only know about multicast groups with members in its area • Subset of area’s border routers forward group membership information and multicast datagrams between areas • Interarea multicast forwarders ZGP0036

  37. Inter-AS Multicasting • Certain boundary routers act as inter-AS multicast forwarders • Run and inter-AS multicast routing protocol as well as MOSPF and OSPF • MOSPF makes sure they receive all multicast datagrams from within AS • Each such router forwards if required • Use reverse path routing to determine source • Assume datagram from X enters AS at point advertising shortest route back to X • Use this to determine path of datagram through MOSPF AS ZGP0037

  38. MOSPF Routing Illustration ZGP0038

  39. Multicast Routing Protocol Characteristics • Extension to existing protocol • MOSPF v OSPF • Designed to be efficient for high concentration of group members • Appropriate with single AS • Not for large internet ZGP0039

  40. Protocol Independent Multicast (PIM) • Independent of unicast routing protocols • Extract required routing information from any unicast routing protocol • Work across multiple AS with different unicast routing protocols ZGP0040

  41. PIM Strategy • Flooding is inefficient over large sparse internet • Little opportunity for shared spanning trees • Focus on providing multiple shortest path unicast routes • Two operation modes • Dense mode • For intra-AS • Alternative to MOSPF • Sparse mode • Inter-AS multicast routing ZGP0041

  42. Spares Mode PIM • A spare group: • Number of networks/domains with group members present significantly small than number of networks/domains in internet • Internet spanned by group not sufficiently resource rich to ignore overhead of current multicast schemes ZGP0042

  43. Group Destination Router Group Source Router • Group Destination Router • Has local group members • Router becomes destination router for given group when at least one host joins group • Using IGMP or similar • Group source router • Attaches to network with at least one host transmitting on multicast address via that router ZGP0043

  44. PIM Approach • For a group, one router designated rendezvous point (RP) • Group destination router sends join message towards RP requesting its members be added to group • Use unicast shortest path route to send • Reverse path becomes part of distribution tree for this RP to listeners in this group • Node sending to group sends towards RP using shortest path unicast route • Destination router may replace group-shared tree with shortest path tree to any source • By sending a join back to source router along unicast shortest path • Selection of RP dynamic • Not critical ZGP0044

  45. Example of PIM Operation ZGP0045

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