<p>Internet Operation</p><p>Autonomous System (AS)</p><p>1. An AS consists of a group of routers exchanging information via a common routing protocol. 2. An AS is a set of routers and networks managed by a single organization. 3. Except in terms of failure, an AS is connected, i.e., there is a path between any two nodes.</p><p>Interior Router Protocol (IRP) Passes routing information within an AS. IRP can be custom tailored to specific applications and requirements. Based on more detailed view of the network, a least-cost based algorithm is used. Uses Open Shortest Path First (OSPF) protocol, RFC-2328. </p><p>Exterior Router Protocol (ERP) Passes routing information between routers in different AS’s. ERPs exchange a summary of reachability information between AS that is less detailed than in an IRP. Uses Border Gateway Protocol (BGP), RFC-1771</p><p>1 Border Gateway Protocol (BGP) RFC 1771</p><p>Preferred exterior router protocol for the Internet. Messages are sent over TCP connections.</p><p>Functions</p><p>1. Neighbor Acquisition It occurs when two neighboring routers in different AS’s agree to regularly exchange routing information</p><p>Router A Router B Open</p><p>Keepalive</p><p>Makes a TCP connection first, then it sends an “Open” message.</p><p>2 2. Neighbor Reachability Used to check that the other partner still exists and is still engaged in the neighbor relationship.</p><p>Router A Router B</p><p>Keepalive</p><p>Keepalive</p><p>Periodically exchanged</p><p>3. Network Reachability Each router maintains a database of networks that it can reach and the preferred route for reaching each network.</p><p>Router A</p><p>Update</p><p>Broadcasts an “Update” message</p><p>3 Chap 17 Transport Protocol</p><p>The transport protocol provides the basic end-to-end service of transferring data and shields upper layer protocols from the details of the intervening network. For network programmers, lower-layer protocols are less important, since the programming interface generally occurs at the TCP.</p><p>. Connection Oriented --- TCP . Connectionless --- UDP</p><p>Four issues in TCP</p><p>. Addressing . Multiplexing . Flow control . Connection establishment/termination</p><p>4 Addressing A user of a given transport entity wishes either to establish a connection with or make a data transfer to a user of some other transport entity. The target user needs to be specified by the following. . User identification . Transport entity identification . Host address (host portion of IP) . Network number (network portion of IP)</p><p>The user address is typically specified as (Host, Port).</p><p>Since routing is not a concern of the transport layer, it simply passes the host portion of the address down to the network layer. Port is included in a transport header to be used at the destination by the destination protocol.</p><p>In TCP, the combination of port and host is referred to as a socket.</p><p>5 Flow Control There are two conditions why a transport entity need a flow control. . The user of the receiving transport entity cannot keep up with the flow of the data . The receiving transport entity cannot keep up with the flow of segments.</p><p>TCP uses a Credit Allocation Scheme for flow control, which is a variant of the sliding window protocol.</p><p>6</p>