CMPT 371

1. CMPT 371 Data Communications and Networking BGP, Flooding, Multicast routing

2. Some ASs AS A AS B IRP B B1 B2 IRP A A1 B3 B6 A2 B4 B5 Gateway router A3 A4 IRP C C1 C2 C5 C3 C4 AS C

3. ERP: Approach? • Link-state and distance-vector not effective for exterior router protocol • Link state requires flooding of link state information, for a large net this is not practical • Distance-vector requires all routers share common distance metric, different ASs may use different metrics • ASs may have different priorities such as restrictions that prohibit use of certain other AS, Distance-vector gives no information about ASs visited onroute (policy routing)

4. Path Vector Approach • No path cost information used • Each block of information lists all ASs visited on a route • Allows the receiver to know the source for each path and whether the path originates in the local AS (coming from IRP or ERP) • Can be used to check for loops (any node appearing more than once) • Enables router to perform policy routing based on • Avoiding transiting a particular AS • link speed, capacity, tendency to become congested, overall quality of operation, security • minimizing number of transit ASs

5. Border Gateway Protocol • BGP is the preferred ERP for or use with TCP/IP internets • BGP messages are sent over reliable TCP connections between gateway routers, a BGP session includes all messages sent through one of these TCP connections. • 4 message types: Open, Update, Keep Alive, Notification • Gateway routers running BGP are know as BGP peers • peers may be in different ASs, eBGP or external BGP session • Peers may be in the same AS, iBGP or internal BGP session

6. Some ASs AS A AS B IRP B B1 B2 IRP A A1 B3 B6 A2 B4 B5 Gateway router A3 A4 IRP C C1 C2 C5 C3 C4 AS C

7. eBGP and iBGP Consider the previous slide eBGP could be used to transfer path information between gateway routers A3 and C5 and between routers C2 and B3. AS B has more than one gateway router AS B uses iBGP to transfer information between gateway routers in AS B

8. Border Gateway Protocol • Procedures that are part of BGP • Neighbor acquisition: Determine if a router physically connected to this router is willing to be a neighbor and Initiate neighbor relationship, negotiating parameters • Neighbor reachability: maintain neighbor relationship • Network reachability: build/maintain routing database

9. BGP: neighbor acquisition • Open TCP connection between a pair of connected (neighbor) routers • Each of the pair of routers sends an Open message • Includes proposed hold time, senders AS #, identifier • Identifier is an IP address uniquely identifying the sender • Each of the pair of routers receives the others Open message, If it wishes to be a neighbor it will respond to the Open message with a Keep Alive message (like an ACK) and • Select the minimum of local/received hold time, to give time between subsequent Keep Alive and/or Update messages

10. BGP: reachability • If no Keep Alive or Update message is received during the agreed upon hold time the connection is terminated. • If a neighbor wishes to continue the neighbor relationship but has no routing update to send it will send a Keep Alive message once per hold interval • Each BGP router maintains a database of reachable networks. When a change is made to this database, that is when new or updated routing information is available the router will send an Update Message • Update includes, a list of routes being withdrawn and information about new routes to be added • Each update message may contain multiple paths to add but includes one set of path information for all these paths

11. Routing Information: Paths • Each path consists of a list of ASsvisited and a list of networks (CIDR network address/prefix) reachable through the gateway routers in each of the ASs visited. • When a BGP peer learns of a new path it will create an entry in its routing table for that path. Once it learns that path it can use the path. • ABGP peer may choose to advertise a path. An advertised route can be used by your neighbors to reach all the networks in the path • Advertised networks may be aggregated and advertised as one network (may be one network in the routing table)

12. Example: AS path construction • Customers of provider with AS T, have been allocated addresses that form AS X and Y AS: T 197.8.1/23 197.8.0/23 AS: X 197.8.2/24 A D To AS Z C AS: Y 197.8.3/24 B E

13. Announcing paths • Want to send information about the path to AS T and the path through AS T to routers outside AS T to build a path from outside AS T to AS T • Consider a AS Z connected to T by a point to point connection from router C to router X in AS Z ( a neighbor of T) • Simplest way to advertise the networks reached in and through T is to announce three paths (1 to each AS) Path 1: “T,” reaches 197.8.0/23 Path 2: “T,X, “ reaches 197.8.2/24 Path 3: “T,Y,” reaches 197.8.3/24

14. BGP routing • Each BGP peer has it own import policy • Can choose to accept a new route or ignore it • If it accepts the route it can choose if it will advertise that route (make itself an intermediate step on a route from an external source to an external receiver) • Each time a BGP peer chooses to accept and advertise a new route it will append its own locally accessible networks to the path. • Before appending it will check that those networks are not already a part of the path. If they are a circular route has been detected and the route must be dropped.

15. Broadcast and multicast routing • We will consider 3 approaches • Uncontrolled Flooding • Sequence number Controlled Flooding • Reverse Path Forwarding • Spanning Tree broadcast

16. Uncontrolled Flooding • Requires no information about the network • A packet that is being sent from A to B is • Sent to all the nearest neighbors of A • Each neighbor receives the packet, then transmits the packet to all it’s own nearest neighbors, except the one it received the packet from • The packet takes all possible paths through network to B • Multiple copies of the packet will arrive at B, the first copy of the packet will arrive along the minimum cost path through the network.

17. Flooding: Example • HOP 1: The source station broadcasts the packet to all adjacent nodes. A B C D E F H G K I J L I

18. Flooding: Example • HOP 2: The receiving stations broadcast the packet to all their own nearest neighbors. The receiving stations do not broadcast back to the station they received the message from. A B C D E F H G K J L I

19. Flooding: Example • HOP 2: follow the packets A B C D E F H G K I J L I

20. Flooding: Example • HOP 3: The receiving stations for hop 2 broadcast the packets to all their own adjacent nodes (except the one they received it from) A B C D E F H G K I J L I

21. Flooding: Example • HOP 3: follow the packets A B C D E F H G K J L I

22. Flooding: Example • HOP 4: The stations receiving the packets broadcast in hop 3 broadcast the packets to all their own nearest neighbors ( not including the station they received the message from). A B C D E F H G K J L I

23. Advantages of flooding • Because packets follow every possible path, the message will get there despite link failures, so long as one path remains active (good for emergency messages) • Because packets follow every possible path at least one packet will arrive over the minimum cost route (good for establishing a virtual circuit path) • All nodes directly connected to the source will receive the message (good for getting information to all nodes)

24. Disadvantages and a simple fix • The biggest disadvantage of flooding in the volume of traffic created • If there are multiple paths to a particular node it will receive and rebroadcast the packet again and again • This creates a broadcast-storm, an increasing number of packets that continue to multiply as they travel through the network • The simplest way to prevent this is to place a short lifetime on the packet so it can only rebroadcast a few times (few time = diameter of network • A better solution is sequence number controlled flooding. Each broadcast (flooded) packet is given an identifier (source id …) and a broadcast sequence number. Each node will rebroadcast a packet with a particular ID and broadcast sequence number only once

25. Sequence number controlled Flooding • HOP 1: The source station broadcasts the packet to all adjacent nodes. A B C D E F H G K I J L I

26. Sequence number controlled Flooding • HOP 1: Follow the packets A B C D E F H G K I J L I

27. HOP 2: The receiving stations for hop 1 broadcast the packets to all their own adjacent nodes (except the one they received it from) A B C D E F H G K I J L I

28. HOP 2: Follow the packets A B C D E F H G K J L I

29. HOP 3: The stations receiving the packets broadcast in hop 2 broadcast the packets to all their own nearest neighbors ( not including the station they received the message from). Nodes throw copies of the packet away and do not forward copies C D B A E E F H F G K J L I

30. HOP 3: The stations receiving the packets broadcast in hop 2 broadcast the packets to all their own nearest neighbors ( not including the station they received the message from). Nodes throw copies of the packet away and do not forward copies C D B A E E F H F G K J L I

31. HOP 3: Follow the packets B C D B A E E E F F H F G K J L I

32. HOP 4: The stations receiving the packets broadcast in hop 3 broadcast the packets to all their own nearest neighbors ( not including the station they received the message from). Nodes throw copies of the packet away and do not forward copies B C D B A E E E F F H F G K J L I

33. OSPF Flooding protocol • A message(LSA) contains a database record. A database record contains information about one link between two routers in the graph discussed earlier. (one link is in one direction) • Each message contains a time stamp or message number • These message numbers are used by the receiving node to determine age of the record • Send means transmit through all attached interfaces except the one on which the incoming message arrived

34. OSPF Flooding protocol • Receive message: Find the corresponding record in the local database if it exists • If the record is not yet in the local database add the record. Send the message • If the record’s message number is larger than the message number in the data base, replace the message in the database with the new record. Send the message. • If the records message number is the same as the message number in the database do nothing • If the records message number is smaller than the message number in the database, send the record in the database back through the interface on which the message arrived