BGP Border Gateway Protocol-- Extra hop problem

Extra hop problem

 case 1

case 2


case 2 get extra unnecessary routes  it goes through back bone network twice to reach the host

Autonomous System (AS)
A group of networks and routers controlled by a single administrative authority is called autonomous system
This allows them to choose its own mechanism for

• Discovering 
• Propagating 
• Validating
• and Checking the consistency of routes

Autonomous system can can be large enterprise ,corporation or university

in short we can give definition like any group that run a large network is AS

The boundaries between autonomous must be defined precisely to allow automated algorithm to make routing decisions and to prevent the routing update protocols used in one AS from accidentally reach a at which it can no longer controlled into another

• AS avoid routing path between competitors AS even such path exists.
• AS is assigned with an AS number by central authority
• Each AS can define a set of policies
• This is helpful for automated routing algorithm to distinguish between AS

When routers in two AS exchange information the protocol arrange them to learn other AS number

Exterior Gateway Protocol (EGP) and reachability (BGP)
 AS system configures one or more of its routers to communicate with other AS.
information flows in two directions
Routers must collect information about network insides routes and pass to out
Routers must accept the information about networks in other AS and spread to its inside AS

EGP
Exterior Gateway protocol
AS advertise its reachability to the outside AS
A single EGP used to advertise reachability or exchange routing information in the internet is BGPBorder gateway protocol

When Two AS agree exchange information between them each must designate a router that will speak BGP on its behalf.

A router speaking BGP must communicate with a peer in another AS
so it will be the  border router communicating with outside hence border gateway or  border router

Router R1 collect information from AS 1 and use the BGP to report to R2
same way R2 also collects information from AS 2 and use BGP to report to R1

BGP CharacteristicsBGP is an Unusual protocol in many waysIts neither a distance vector algorithm nor a link state algorithm instead it is using path vector alogorithm

BGP is characterized by following

• Inter Autonomous System Communication :- BGP advertise reachability information , so its primary role is  allow communication among AS.
• Coordination among multiple BGP speakers: - if an AS contain multiple routers,each communicating with a peer in an outside AS , and coordination among them is done with iBGP a form of router for guaranteed propagation with consistent information
• Propagation of reachability information :- BGP allow an AS to to advertise destination that are reachable either through or in it . also learn such information from another AS
• Next Hop Paradigm :- It provide next hop information for each destination like distance vector protocol
• Policy Support :-BGP can implements routes that the local administrator choose,Routers running BGP can be configured to distinguish between the set of destination reachable inside the AS and set of destination advertised to other AS
• Reliable transport :- BGP is unusual among protocol that pass routing information because it assume reliable transport (TCP for all communication)
• Path information :- Instead of specifying destination that can be reached and a next hop for each,  BGP uses path paradigm in which advertisement specify path information that allow a receiver to learn a series of AS  along path to destination.
• Incremental Updates :- to conserve network bandwidth BGP does not pass full information in each  update message ,Instead full information exchanged once and successive message carry incremental updates  called deltas.
• Support IPV4, IPV6 BGP send a prefix length along with each message (prefix→large blocks of subnets )
• Route aggregation :- BGP allow conserve bandwidth by allowing a sender to aggregate routing information and send a single entry to represent multiple related network
• Authentication :- BGP allows receiver to authenticate the messages
• BGP can handle big routing tables: In which IGP cannot handle big routing table currently global internet contain 500000 routes 

BGP functionality and types

BGP peer perform 3 functions :-

1. Initial peer acquisition and authentication :- the two peer establish a TCP connection and then perform message exchange this will guarantee both side to agree to communicate  
2. Primary focus on the protocol :- each can send +ve or -ve reachability information ,ie sender can declare that one or more destination are reachable by giving next hop to each or previously advertised destination is no longer reachable.
3. On going Verification :- that the peer and the network connection between them are functioning correctly

BGP message types

1. OPEN :  initialize communication
2. UPDATE :- advertise or with draw routes
3. NOTIFICATION : Response to incorrect messages
4. KEEPALIVE :- Actively test peer connection
5. REFRESH : Request re advertisement from peer 

BGP OPEN message

After OPEN message is sent receiver should reply with KEEPALIVE knowledge message before they can exchange router information

BGP UPDATE message

After KEEPALIVE message the peer use UPDATE message to advertise new destination that are reachable or withdrawn previous advertised destination that are unreachable  

Path Attributes
A sender can use the path attributes to specify a next hop for the advertised destination ,a list of AS along the to the destination and whether the path information was learned from another AS or derived from within the sender AS 

Path Attributes are important in BGP message for 3 reason

• Reason 1

Path information allows receiver to check for forwarding loops
The sender allows the exact path of AS that will be used to to reach the destination
If the receiver AS appears on the list ,Advertise must be rejected

• Reason 2

Path information allow receiver to implement policy constraints
eg ;- reject the path that includes competitors AS

• Reason 3

Path information allows a sender to specify the information whether it come from inside or outside the AS.It can helps the receiver to know source of all routes. So that it can reject or Accept the routes
Path Attributes field
ID of origin path information
List of AS on path to destination
Next Hop to use for destination
Descriminator used for multiple AS exist point
Preference used within an AS
Indication that have be aggregated routes
ID of AS that aggregated routes ID of community for advertised destination

BGP KEEPALIVE message
BGP peer periodically exchange KEEPALIVE message to test the network reachability and verify both peer continue to function
Two reason for BGP uses KEEPALIVE message
Because BGP use TCP for transport ,TCP does not test reachability of destination or do not report for send data fails
Re sending the same ,message is wasting of network bandwidth ,because routing information is often large ,KEEPALIVE is small message to ensure the connectivity
BGP REFRESH message
BGP NOTIFICATION message

BGP used notification message for control or when an error occur , ,BGP send notification message and then closes TCP connection

ERROR code messages

• 1→ERROR in message header
• 2→Error in Open message
• 3→ERROR in Update message
• 4→Hold time expired
• 5→Finite state machine error
• 6→Lease ,termination connection

Q.IV.Explain the exterior gateway protocol BGP along with its four basic message types. (15marks)[B. Tech. Degree VII Semester Examination November 2012 CS/IT 703 ADVANCED COMPUTER NETWORKS (2006 Scheme)]