A Course-End Conclusions Dr. Rocky K. C. Chang

The internetworking problem Different data-link protocols Different MAC address spaces Different MTUs An hour-glass model (end-to-end argument) IP as the glue IP addresses IP fragmentation and reassembly IP over anything and everything

All boil down to the design goals Best effort IP service Reliability requirement: trusting states to the end hosts Requirement for supporting a variety of TOS Requirement for accommodating a variety of networks End-to-end argument Keep the network simple: IP packets go in and IP packets come out. Implement applications at the network edge.

The forwarding problem Main issues of concerns Who knows what Speed (forwarding table size and lookup) Not responsible for the correctness of the routes Hop-by-hop forwarding as a result of the best-effort approach. Source routing and tunnels Virtual circuit switching IP switching From classful to classless routing

The routing problem THE intelligence of the IP layer Use a hop-by-hop protocol to deliver packets end-to- end. Main issues of concerns Speed of convergence Prone to routing loops Efficiency Two main approaches (in midst of many other differences and variations) Distance vector and link state

A tale of two routing problems All routing protocols concern delivering packets from one point to another. An intradomain routing additionally concerns optimizing certain costs of a route. An interdomain routing additionally concerns satisfying certain policies of an AS. Current Internet characteristics Asymmetric routes Packet reordering Packet losses Nonfriendly intermediaries

The end-to-end problems TCP adds the following services to IP: Multiplexing (through the port number) Inordering (through the TCP SN) At-most-one-copy (through the TCP SN) Arbitrarily large application messages (through the wraparound TCP SN space) Flow control (through advertised window) End-to-end reliability (through the sliding window protocol and retransmission) Congestion control (through ACK clocking, congestion window, slow start, etc)

The congestion control problem Congestion control and/or resource allocation hold one of the keys to the Internet stability. A TCP sender interprets packet losses (without receiving ACKs) as a sign of congestion. Slow starting to trigger packet losses (reaching the network capacity) Next time, perform congestion avoidance when approaching to the congestion point. Other approaches do not induce packet losses. TCP/Vegas, Explicit Congestion Notification

Two Internet applications DNS provides a distributed database for domain names and protocols to obtain their resource records. Web provides A global naming system to identify resources A text-based language to facilitate a navigation across various related resources, and A protocol for requesting and responding Interaction between TCP and HTTP Web proxies: not longer end-to-end

Coverage in terms of protocols