 Let's look at some example packet sizes first. So I've listed a few technologies, protocols, and the maximum packet sizes or maximum payload size that the standard allows. The purpose is not to understand how those protocols work, or in some cases you may not even know what they do, it's just to give some examples of different packet or payload sizes. For example, in a wired LAN in an Ethernet LAN, the IEEE 802.3 standard, it says a frame must contain 1,500 bytes or less payload. That's the maximum size, it may be less. There are some optional features to increase to jumbo frames, but 1,500 bytes is the typical size. In wireless LANs, the maximum size is 2,312 bytes. But in practice, because most wireless LANs are connected to a wired LAN, an Ethernet network, in practice most wireless LAN frames are also limited to 1,500 bytes. Other technologies like SDH, ATM, maybe 2,430 bytes of an SDH frame, ATM has what's called a cell, and that has a 5 byte header and a fixed size payload. Every ATM cell is 48 bytes. You cannot have less. Quite small, designed as a trade-off to make sure that the performance of different applications is satisfactory and that the implementation of the devices, especially in hardware, can be fast in processing these ATM cells. IP datagrams, the maximum size of the datagram is 65,535 bytes. That's the header as well as the payload. Similar for UDP and TCP. But again, in practice, that's not commonly used, that maximum size, usually much shorter than that. Look at TCP segments. Typically, what happens in TCP is the source and destination negotiate what size segment to use. And the parameter is called a maximum segment size, MSS. And the aim is to negotiate a value such that when you put that TCP segment inside an IP datagram and then inside the data link layer packet or frame, that no fragmentation will be needed. That is, we will not need to split that datagram or segment into smaller chunks. Well, that depends upon the headers of TCP and IP and also the data link layer. The data link layer, the maximum payload, is often called the maximum transmission unit from the perspective of TCP, the MTU. So as an example, in a wide LAN, 802.3, the maximum size of payload in an Ethernet frame is 1,500 bytes. So we'd say the maximum transmission unit in a wide LAN is 1,500 bytes. Now, the maximum segment size would be set, in that case, to 1,460 bytes. Why? Well, if we have an IP datagram with a header of 20 bytes, we'll see shortly a TCP segment also has a header of 20 bytes. So we have 40 bytes of header plus a payload of 1,460 bytes gives us 1,500 bytes. So that will fit perfectly into an Ethernet frame. So the typical size of a TCP segment is not normally 65,535 bytes, but normally much smaller. And it depends upon the data link layer and the headers of the IP and TCP segment. Some protocols, especially application protocols, may not limit the size of the data, the payload. For example, a HTTP request message sent from a browser to a server has no hard limit on how big the request can be. It may be that some browsers implement some limits. For example, some implemented limits of 8 kilobytes. But the standard doesn't say how big it... or doesn't limit the size. It depends upon the implementation. To finish, let's just quickly show a few pictures of the structures of different packets. First, an Ethernet, an 802.3 frame, typically contains 14 bytes of header and a four byte trailer. And then data or payload up to 1,500 bytes. It's not my purpose to explain... it's not my intention to explain the purpose of the headers or the different protocols just to give some examples of the structures and how we draw them. In this case, we draw the header fields all in one row. Because there's only three of them. It's easy to draw in one row. Whereas in a TCP segment, we draw it similar to an IP datagram with multiple rows, each of 32 bytes... 32 bits, four bytes. Again, a TCP segment has, by default, 20 bytes. It can have some optional header fields and then data or payload. UDP datagram, much smaller header, 8 bytes, then data. An RTP packet used for video and audio transmission, 12 byte header normally, some options and then data. And then some protocols don't look at the header from a binary perspective, but treat it as just a text formatted message. So in HTTP, the typical messages are requests and responses and they're just text content. For example, the format contains a line of text indicating the type of message, some optional header lines, an empty line, a blank line, and then an optional message body. For example, in a request message in HTTP, the start line is a method, like a get or post method, the URL and the version of the protocol being used. The optional header lines may contain different values and they're always formatted as a field name followed by a value. They may include a date, the length of the message body, the content length, a user agent field indicating the information about the browser and other fields. So we've looked at what we mean by a packet. We've given some examples, first an IP datagram and then quickly a few other protocol packets and looked at issues of what impact or how does the packet size impact upon performance.