Internet Operation

Autonomous System (AS)

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.

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.

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

1 Border Gateway Protocol (BGP) RFC 1771

Preferred exterior router protocol for the Internet. Messages are sent over TCP connections.

Functions

1. Neighbor Acquisition It occurs when two neighboring routers in different AS’s agree to regularly exchange routing information

Router A Router B Open

Keepalive

Makes a TCP connection first, then it sends an “Open” message.

2 2. Neighbor Reachability Used to check that the other partner still exists and is still engaged in the neighbor relationship.

Router A Router B

Keepalive

Periodically exchanged

3. Network Reachability Each router maintains a database of networks that it can reach and the preferred route for reaching each network.

Router A

Update

Broadcasts an “Update” message

3 Chap 17 Transport Protocol

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.

. Connection Oriented --- TCP . Connectionless --- UDP

Four issues in TCP

. Addressing . Multiplexing . Flow control . Connection establishment/termination

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)

The user address is typically specified as (Host, Port).

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.

In TCP, the combination of port and host is referred to as a socket.

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.

TCP uses a Credit Allocation Scheme for flow control, which is a variant of the sliding window protocol.