Wide Area Networks

Wide Area Networks (WAN) are categorised by the following:

• WAN refers to networks across cities and countries.

  Between LAN and WAN, there is also the Metropolitan Area Network (MAN)

• T1 - 1.544 Mbps (Time-Division Multiplexing (TDM) to support 24 digital voice or data channels, each at 64 kbps). It uses copper wires, one pair in each direction.

• T3 (e.g., the NSFNET) - 44.736 Mbps.

• Internet is used as an example here. Section 25.2.3 addresses new generation networks.

Internet Layering

             
        Layer                  Example
	       -----                  -------
	 -----------------
	 |  Application  |	FTP, Telnet, SMTP, X-Windows, MIME, HTTP
	 -----------------
	 |   Transport   |	UDP, TCP, TP4, Routing
	 -----------------
	 |   Internet    |      IGMP, IP, CLNP
	 -----------------
	 |  Subnetwork   |	Ethernet, X.25, FDDI, Token Ring
	 -----------------
	 |   Data Link   |	HDLC, PPP, SLIP
	 -----------------
	 |   Physical    |      RS-232, V.35, Fiber Optic
	 -----------------

Network Layer - IP (Internet Protocol)

Gateways and Routing

• LANs allow point-to-point message transmission on local networks.

• Problem: What if I want to send a message to a machine on a different network?
  

  Gateways

• Solution: Use machine G (Fig 25.4) to forward the message. In other words, a program running on G takes the message from A and routes it to B. G is called a gateway, and the program is called a router.

Routing using the Internet Protocol

• Problem: A wants to send to B, but must send to G first. So how does G know to forward the message to B?

  Solution: Don't use B's Ethernet address. Use another name for B (it's IP address) and provide a translation table from IP addresses to Ethernet addresses (called a routing table).

• A common scheme: Internet Addresses are 32 bit numbers called Internet Protocol (IP) addresses. Usually specified using "dot" notation (e.g. 804d953f = 128.77.149.63).

IP Problems and Solutions

• Problem: How to keep mapping tables up to date?

  Local Solution: To map from IP to Ethernet, use Address Resolution Protocol (ARP - RFC 826).

  ARP broadcasts "What's the Ethernet address for IP address X?" For all non-local machines, send to default router (gateway).

  Gateway Solution: To properly forward a packet, gateways keep up to date routing tables. Gateways exchange tables with neighbor gateways frequently (e.g. once a minute).

• Problem: Gateways read, examine, and forward packets (i.e., they buffer the packets). What if the gateway runs out of memory (Fig. 25.5)?
  

  Gateway Memory

  Solution 1: Provide end-to-end flow control. Could be very slow if there are a lot of links.

  Solution 2: Let sources reserve resources (memory, bandwidth) on path, make well behaved applications that won't use more than they reserve. Such protocols are called quality of service (QOS) protocols.

  Solution 3: Just drop the packets.

Summary: IP properties

• Provides basic forwarding and addressing mechanisms

• No end-to-end flow control

• Packets can be dropped, duplicated, received out of order.

Transport Layer - UDP and TCP UDP and TCP are two transport-layer protocols used in TCP/IP.

UDP (User Datagram Protocol)

• Connectionless - IP packets address tells only what machine packet is bound for, not which user or process on the machine.

  UDP adds port number (16 bit identifier) to packet to indicate destination process

• It supports only multiplexing and checksum.

  Higher level protocols must then take care of retransmission, divide and reassemble packets, flow control, congestion avoidance, etc.

TCP (Transmission Control Protocol)

• Connection-oriented - provide reliable communication between pair of processes across the net.

• Simplest idea: send one segment, wait for acknowledgment (ACK).

  If ACK not received in a little while (the retransmission timeout, or RTO), resend segment.

• Maximum bandwidth is limited by round trip time (RTT):

  • Max BW = (segment size)/RTT

Performance improvements

• Send several segments (a window)

• Delay acknowledgments (wait a bit - 200 ms - before sending acknowledgements). This gives us a chance to piggyback acknowledgements (send multiple ACKs at once)

• Send negative acknowledgment (NACK) immediately when segment is lost

• Example in Fig. 25.6.
  

  Perfomance Improvemnt

Flow Control

• Maximum bandwidth is determined by window size and round trip time.

• If network gets congested (more data traffic than bandwidth), then need to slow down send rate.

  If network is under-utilized, speed up send rate.

• In window scheme, just change size of window.

• Variable window size + retransmission = TCP.

TCP Model

TCP Model

• Basic idea (Fig, 25.7): sender gives OS a stream (e.g., through several write() calls).

  Receiver gets stream in order using read() system calls

• TCP sends packet as IP datagrams.

• Like UDP, TCP adds port numbers and checksum

TCP Flow control

• Most data lost in modern networks is due to buffer overflow at the gateways.

  When network is congested, must reduce rate at which we're sending data.

  Two states:congestion avoidance and slow start.

• Congestion avoidance (steady state):
  • Increment window by one segment each RTT. (Linearly increase window size).

  • When packet is lost, set ssthresh to 1/2 current window size, set window to one segment, and begin slow start.

• Slow start
  • Increment window size by one segment every time new data is sent, up to ssthresh.

  • When ssthresh is reached, resume congestion avoidance.