1. Lecture 3  A round up of the most important basics I haven’t covered yet.  A round up of some of the (many) things I am missing out of this course (ATM, Ethernet).  More Information:  Bertsekas/Gallager: Section 2.10 + 4.5.2  A brief word about buffers and queues in the internet.  There will be much more about queues later.  A simple model of TCP throughput  A bit of maths at last.

2. The Structure of the Internet (typically) Your computer Gateway (perhaps via modem) Internet itself Gateway Target Computer

3. Structure of the Internet  A typical user connects to the internet via a modem (modulator demodulator).  This sends the signal via telephone wire to their ISP (Internet Service Provider) who receives it with another modem.  The ISP forwards the packet through their LAN (Local Area Network) to their gateway to the internet.  On the internet the packet will probably then head onto the internet backbone.  At each level different protocols apply.

4. ATM in brief  ATM (Asynchronous Transfer Mode) is a protocol designed to provide end-to-end connections at high speed.  ATM sends data as 53 byte cells (packets) – 5 bytes header, 48 bytes data.  It allows a connection oriented service (sets up a route then sends data).  It is used in high speed switches and ADSL.  While it was once thought to be a likely model to replace TCP/IP, this seems unlikely now.

5. Ethernet (in Brief)  Ethernet is a widely used protocol for LAN (Local Area Network).  Typically it supports connections of 10 or 100Mbit/s (but can go up to Gb/s).  Machines on an ethernet are given a unique address. (Not related to their IP address).  The most common deployment nowadays is to connect a number of machines to a central switch which acts as a gateway to the rest of the network.  The switch translates IP addresses for local machines to ethernet addresses and forwards packets appropriately.

6. Buffers and Queues  An issue we will be spending a lot of time on is queuing  At its simplest, the situation is summarised below. Router Buffer Queue Input x(t)Output y(t)

7. Buffers (1)  Usually, a network component can only send data at a certain rate.  If more data is received than can be sent then it must be stored somewhere.  Only a finite amount of data can be stored (even if it were practical to store infinite data it would not be desirable to do so).  If more data arrives than can be stored then it must be thrown away. (dropped)  This is not necessarily such a bad thing.

8. Buffers (2)  The simplest type of buffer:  FIFO (First in first out).  Tail drop (If a packet arrives and there is no room for it it is dropped).  Always sends.  Some common variants:  Priority Queuing.  RED (Random Early Detection).  Leaky Bucket.

9. TCP Throughput Estimate  In the rest of this lecture we will create a simple model which gives performance estimates for TCP.  Consider what happens when a buffer is full.  If the buffer is full, packets must be dropped.  If packets are dropped then the TCP algorithm will react and send less data.  This model is taken from “Modelling TCP Throughput: A Simple Model and its Empirical Validation” by Padhye, Firiou, Towsley and Kurose (1998). Published in proceedings of SIGCOMM ’98 available from http://citeseer.nj.nec.com/padhye98modeling.html

10. TCP reminder  TCP sends packets and receives ACKs to indicate they arrived successfully.  The “window size” is the number of unacknowledged packets outstanding.  If all goes well and no packets are lost then the “window size” gradually increases (up to a max).  If a packet is lost then the window size will be halved.  The problem is, how can we estimate the bandwidth of a TCP connection? (We will need to make some simplifying assumptions).

11. TCP – more gory details  Remember that packets and ACKs have a sequence no which should match.  Assume that our window size is W.  b is a parameter of TCP (typically 2). After Wb ACKs have been received, W increases by 1 (ignoring slow start).  How does TCP know when a packet is lost?  Triple Duplicate ACK (TDA) – that is three ACKs in a row with one or more missing ACKs between them – is taken to mean a packet has lost.  If a TDA occurs then the window size is halved and the lost packet is resent.  A second mechanism (not discussed here) is timeout (no ACKs at all for a certain period) – in this case the window becomes 1.

12. TCP diagram 2 3 14 5 Packets sent round no lost packet received packet 1 2 3 4... X i Triple duplicate loss occurs here

13.  With just triple duplicate ACKs we get:  Which can be expressed as: Final equation

14. How to take this further  Obviously this analysis can be extended considerably.  The model has been extended to cover time out losses and maximum window sizes.  The basic assumption that the probability of a packet loss is constant is a big problem.  If we want to get a handle on the probability of packet loss then we need the branch of mathematics known as “Queuing Theory”.