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Transcript
Introduction Tremendous growth of Internet usage worldwide and recent developments in the field of communication made ease of reachability between individuals, business professionals, educational Institutions, government and non-government organizations to access, share and communicate information. Routing in the Internet technically means receive, store and transmit the information using the best possible path. Routing table stores the information of all possible paths available to successfully transfer the data to the next router in the mesh. After successful establishment of path between source and destination router, packets are transferred via multiple hopes. Routing protocols are policies and procedures used to transfer packets within the network. One of the main routing protocols in this era is Border Gateway Routing Protocol (BGP) [26]. The increasing popularity of BGP depicts its broad ability to distribute reachability information by selecting the best route to each destination according to policies specified by network administrator. BGP is a critical component of the exponentially growing network of routers that constitutes Internet. Carrier networks, as well as most large enterprise organizations with multiple links to one or more service providers use BGP. Figure 1: Active BGP Entries (FIB) [7] The Interior Gateway Protocol (IGP) like OSPF, EIGRP or RIP is used to exchange routing information within AS. Exterior Gateway Routing Protocol (EGP) like BGP is used for sharing routing information between ASs. The Information, exchanged using routing protocols, is about all reachable networks. Every router in the Internet must have the information about all networks in its routing table to achieve global connectivity. BGP facilitates communication between routers in different autonomous systems. An autonomous system (AS) is a network or group of networks under a shared technical administration and with common routing policies. There are two types of Network traffic either local or transit. Local traffic either comes from or terminates in that AS, whereas any other traffic traversing that AS constitutes transit traffic. A major goal of BGP usage in the Internet is to reduce transit traffic. Internal and External BGP: BGP carries path information associated with a given destination between autonomous systems; it enables loop-free inter-domain routing. BGP supports Intra-AS routing as well as Inter-As routing in former case it is called Internal BGP (I-BGP), and in later case it is termed as External BGP (E-BGP). BGP router can communicate with other BGP routers in its own AS or in other ASs. Both the I-BGP and E-BGP implement the BGP protocol with a few different rules. All I-BGP-speaking routers within the same AS must peer with each other in a fully connected mesh. Figure 2 shows routers R1, R2, and R3 using I-BGP to exchange routing information within the same AS, and router pairs R4-R2, R3-R5, and R4-R5 using EBGP to exchange routing information between ASs. Figure 2: Internal and External BGP BGP advertises routes to carry data to the address space indicated by the IP prefix of the announced route. Route advertisements in BGP use the AS-Path attribute to announce current routing to neighbor BGP speakers, which includes a list of all transit ASs that must be used to reach the target network. BGP Functionality and Operation: Routers that run a BGP routing process are often referred to as BGP speakers. Pair of BGP-speaking routers that form a TCP connection to exchange routing information between them are called BGP neighbors or peers. Each BGP session takes place exactly between two nodes, where two routers exchange routing information dynamically, over TCP port 179 [24]. There are four possible message types used with BGP, all consisting of a standard header plus specific packet-type contents: OPEN: First message to open a BGP session, transmitted when a link to a BGP neighbor comes up. It contains AS number (ASN) and IP address of the router who has sent the message. UPDATE: Message containing routing information, including path attributes. It contains Network Layer Reachability Information (NLRI), listing IP addresses of new usable routes as well as old inactive routes. NOTIFICATION: Final message transmitted on a link to a BGP neighbor before disconnecting. It usually describes a typical conditions prior to terminating the TCP connection, and provides a mechanism to gracefully close a connection between BGP peers. KEEP-ALIVE: Periodic message between BGP peers to inform neighbor that the connection is still viable by guaranteeing that the transmitter is still alive. BGP States: BGP process running on router moves through different states during its execution. Idle State When the router is not configured for BGP, BGP process on this router is in Idle state waiting for start event which is the manual peer configuration of BGP. Connect State In connect state BGP process waits for establishment of TCP connection after which Open message will be sent for peer negotiation and BGP process moves to OpenSent state. Active State In Active state, the router is actively trying to establish TCP session with peer after which Open message is sent and router transits to OpenSent state. OpenSent State OpenSent state means router has sent an Open message containing its BGP parameters and waiting for the corresponding Open message from peer to negotiate and establish BGP peer relationship. OpenConfirm State OpenConfirm state means BGP peering relationship has been established between the peers and they have started exchanging Keepalive messages. Established State Established state is the final state where routing information is exchanged among peers. BGP Route Announcement and Withdrawal: After establishing a TCP connection, two adjacent BGP speakers exchange full routing information. Based on the described paths, the router filters them and selects only one as the best path, puts it in its IP routing table, and propagates the path to its neighbors. When a BGP speaker determines that a route has changed or that a new path for the same prefix is chosen, it advertises the replacement route without requiring a route withdrawal [24]. A BGP router sends one route per destination to its BGP peers. BGP Path Selection: BGP uses a path-vector (PV) algorithm, described in RFC 1322 that itemizes the complete path to a destination [24]. The PV routing algorithm supplements the advertisement of reachable destinations with information that describes various properties of the paths to these destinations. A path is the recorded sequence of ASNs through which the reachability information traverses. Each AS is considered equal and independent of its size. BGP uses the shortest AS-Path routing criterion by default. Classless Interdomain Routing (CIDR): In the Classful scheme, the IP address range is divided into different classes known as A, B and C. Each class identifies the fixed number of bits belonging to network and host part in the 32-bit IP address. With the global expansion of Internet it becomes difficult for routers to maintain entries for two millions class C networks due to memory and processing power limitations. CIDR is a new enhanced, classless addressing and routing scheme. With CIDR for example many contiguous Classful networks may be combined in one larger classless network represented by a single prefix. The prefix will be associated with prefix length corresponding to the number of bits in subnet mask. The prefix of a supernet will have less number of bits in the subnet mask than the original combined Classful networks. In this way CIDR scheme reduces entries in routing tables of the national ISP routers.