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

ZGP002 Outline Path-Vector Protocols: BGP and IDRP Muticasting

ZGP003 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

ZGP004 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

ZGP005 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

ZGP006 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neighbour relationship with another router Update –Transmit information about single route –List multiple routes to be withdrawn Keepalive –Acknowledge open message –Periodically confirm neighbour relationship Notification –Send when error condition detected

ZGP007 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

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

ZGP009 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

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

ZGP0011 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

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

ZGP0013 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

ZGP0014 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

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

ZGP0016 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

ZGP0017 Diagram for BGP Routing Information Exchange

ZGP0018 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

ZGP0019 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

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

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

ZGP0022 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

ZGP0023 Example Configuration for Multicast Internet

ZGP0024 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

ZGP0025 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

ZGP0026 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

ZGP0027 Multicast Transmission Example

ZGP0028 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

ZGP0029 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

ZGP0030 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

ZGP0031 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

ZGP0032 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

ZGP0033 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

ZGP0034 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

ZGP0035 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

ZGP0036 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

ZGP0037 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

ZGP0038 MOSPF Routing Illustration

ZGP0039 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

ZGP0040 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

ZGP0041 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

ZGP0042 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

ZGP0043 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

ZGP0044 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

ZGP0045 Example of PIM Operation