CMPT 371

© Janice Regan, CMPT 128, 2007-20121

CMPT 371Data Communications and Networking

BGP, Flooding, Multicast routing

Janice Regan © Oct 2007-2013 2

Some ASsA1

A2

A4

A3

C2C1

B2

C4

B6

B1

B3

B5

B4

C5

AS AAS B

AS C

C3

IRP B

IRP C

IRP A

Gateway router


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 on route (policy routing)


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


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


Some ASsA1

A2

A4

A3

C2C1

B2

C4

B6

B1

B3

B5

B4

C5

AS AAS B

AS C

C3

IRP B

IRP C

IRP A

Gateway router

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



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


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


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


Routing Information: Paths

Each path consists of a list of ASs visited 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.

A BGP 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)


Customers of provider with AS T, have been allocated addresses that form AS X and Y

AS: T197.8.1/23197.8.0/23

AS: X197.8.2/24

AS: Y197.8.3/24

Example: AS path construction

A

B

C

D

E

To AS Z


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 197.8.0/23Path 2: “T,X, “ reaches 197.8.2/24Path 3: “T,Y,” reaches 197.8.3/24


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.

Broadcast and multicast routing We will consider 3 approaches

Uncontrolled Flooding Sequence number Controlled Flooding Reverse Path Forwarding Spanning Tree broadcast



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.


Flooding: Example HOP 1: The source station broadcasts the

packet to all adjacent nodes.

II J LK

E F HG

B DCA


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.

I J LK

E F HG

B DCA


Flooding: Example HOP 2: follow the packets

II J LK

E F HG

B DCA


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)

II J LK

E F HG

B DCA


Flooding: Example HOP 3: follow the packets

I J LK

E F HG

B DCA


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).

I J LK

E F HG

B DCA


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)


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


Sequence number controlled Flooding HOP 1: The source station broadcasts the

packet to all adjacent nodes.

II J LK

E F HG

B DCA


HOP 1: Follow the packets

II J LK

E F HG

B DCA

Sequence number controlled Flooding


HOP 2: The receiving stations for hop 1 broadcast the packets to all their own adjacent nodes (except the one they received it from)

II J LK

E F HG

B DCA



I J LK

E F HG

B DCA


B

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

I J LK

E F HG

DC

E F

A


B


I J LK

E F HG

DC

E F

A



I J LK

E F HG

B DCB

E FE F

A



I J LK

E F HG

B DCB

E FE F

A


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


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

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