BGP protocol for telecommunication engineering

Bgp protocol and its properties

1. BORDER GATEWAY PROTOCOL ( BGP ) The Border Gateway Protocol (BGP ) is the main protocol used on the backbone of the internet It replaces the Exterior Gateway Protocol It can handle AS prefix numbers It works on policy based decisions It does not automatically initiate contact with other routers BGP not only makes routing decisions based on path but also includes network policies and rule sets.  

2. OPERATION OF BGP Links between routers are manually set up using a TCP connection TCP Connection BGP Router BGP Router AS AS BGP uses four messages for communications : Open Keepalive Update Notification BGP-4 does not automatically discover and incorporate new routers . A connection between two routers is manually initiated by making a TCP connection using TCP port 179. All protocol communications take place over this connection

3. PHASES OF OPERATION There are three key phases to the operation of the protocol Neighbour acquisition Neighbour reachability Network reachability

4. Neighbour acquisition Neighbour acquisition occurs when two routers in different autonomous systems establish a TCP connection. The protocol does not define how these two routers learn of each others’ existence or provide any information about the status of the router. That is a matter for external network configuration . Initial information is exchanged during the connection process by the use of the ‘open’ message. The connection is then maintained by the exchange of ‘ keepalive ’ messages.  

5. NEIGHBOUR ACQUISITION Routers learn about their neighbours after a TCP connection has been established using TCP port 179 TCP Connection BGP Router BGP Router AS AS BGP “Open” Message BGP “Keepalive” Message BGP “ Keepalive ” Message BGP “Open” Message

6. Initial information is exchanged during the connection process by the use of the ‘open’ message. The connection is then maintained by the exchange of ‘ keepalive ’ messages. The format of the message PDU’s is shown below The Open message identifies the AS to which the sender belongs and provides the IP address of the router. It also contains a ‘hold time’ which defines the maximum amount of time that may elapse without a BGP-4 message being sent between the two routers and serves as one method of detecting the loss of routers

7. 'KEEPALIVE' MESSAGE PDU This simple PDU is the basic header for all other PDU’s Authentiction Length of PDU Message type Marker Length Type 16 2 1 Octets

8. 'OPEN' MESSAGE PDU Provides initialisation information Marker Length Type Version My AS Hold Time BGP Identifier Opt. par. length Optional parameters 16 2 1 1 2 2 4 1 variable Octets AS of sender IP address of router Hold Time

9. NETWORK REACHABILITY Each router maintains a database of the networks that it can reach and the preferred route for reaching these. This knowledge is exchanged by the transmission of Update Message The BGP update message contains the AS prefix number, the IP address of the Router and then a sequence of Network Layer Reachability Information (NLRI) entries. There will be one NLRI for each network contained within the AS . Hence the router receiving the message now knows about the existence of the new subnets within the AS and how to reach them

10. NETWORK REACHABILITY Each router maintains a database of the networks that it can reach and the preferred route for reaching these. This knowledge is exchanged by the transmission of Update Message TCP Connection BGP Router BGP Router AS xyz Netid n1 Netid n2 Netid n3 R1 BGP Update Message AS_Path = xyz Next_Hop = IP address of R1 NLRI = netid n1; netid n2; netid n3 R1 is advertising a route to AS xyz

11. ROUTING INFORMATION BASE (RIB) Routers will receive a series of BGP-4 Update messages which will contain the range of networks that can be reached via various other routers. However, each of these routers will likely be an exterior gateway for the AS and so will process this routing information in the context of its own routing policies. Whilst a given route may exist, a router may choose not to use it because it has a policy to avoid a given network. Hence, having processed the received Update messages, a router will form a local routing information base which will be used to route data traffic that it receives

12. ROUTING INFORMATION BASE (RIB) It will also create a second routing information base ( Adj_RIBs_Out ) that contains those routes that the router is prepared to advertise to other routers . In other words, the contents of the Adj_RIBs_Out will form the content of the BGP-4 Update messages that the router transmits to other routers. This may be different to Adj_RIBs_In

13. ROUTING INFORMATION BASE (RIB) Router create a database from update messages and pass the contents of that message on if policy permits Incoming Update Messages Outgoing Update Messages BGP Router Routes received from other BGP routers Adj_RIBs_In Policy Based Routing Algorithm Loc_RIB Decision Local RIB used by the router when routing network traffic Adj_RIBs_Out Routes to be advertised to other routers by issuing Update messages

14. ROUTING POLICIES Routing between Autonomous Systems is not governed by protocols and algorithms that simply determine the shortest route, but by ‘policy based’ routing. Hence BGP must be able to implement these policies. Legal issues: How country treats data Political issues: Is data secure Commercial issues: Use of competitors network Routing is therefore based on POLICIES that are configured within the AS Border Routers by network managers and administrators.