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What is... Packets/MTU?
Packet/MTU Theory.
3-30-01

By John Holstein, Cotse Help Desk Coordinator

Introduction

Imagine if you will, an envelope stuffed with several pages of an important 50 page document. This envelope contains 22 of those pages. The first page is the cover page, the second page is the page of contents and the remaining 20 pages are randomly selected excerpts of the entire document. This is an example of a packet.

A packet contains the TCP Header, the IP Header and the actual data or Maximum Segment Size (MSS). This combination may be in various sizes depending on the Operating System, Connection Type and User Preferences representing a portion of the file being transferred. In the same way as the 50 page report referred to above, the TCP and IP headers refers to the cover letter and table of contents that will address to the recipient, the order in which to present the information to the computer system and the user. Within all packets sent, the TCP and IP headers are present, this must be true in order for the MSS or actual data to be recreated on the target machine.

Packets Defined

A packet is the unit of data that is routed between an origin and a destination on the Internet or any other packet-switched network. When any file (e-mail message, html file, Graphics Interchange Format file, Uniform Resource Locator request, and so forth) is sent from one place to another on the Internet, the Transmission Control Protocol (TCP) layer of TCP/IP divides the file into "chunks" of an efficient size for routing. Each of these packets is separately numbered and includes the Internet Address of the destination. The individual packets for a given file may travel different routes through the Internet. When they have all arrived, they are reassembled into the original file (by the TCP layer at the receiving end).

A packet-switching scheme is an efficient way to handle transmissions on a connectionless network such as the Internet. An alternative scheme, circuit-switched, is used for networks allocated for voice connections. In circuit-switching, lines in the network are shared among many users as with packet-switching, but each connection requires the dedication of a particular path for the duration of the connection.

  • IP - is responsible for moving packet of data from node to node. IP forwards each packet based on a four byte destination address (the IP number). The Internet authorities assign ranges of numbers to different organizations. The organizations assign groups of their numbers to departments. IP operates on gateway machines that move data from department to organization to region and then around the world.

  • TCP - is responsible for verifying the correct delivery of data from client to server. Data can be lost in the intermediate network. TCP adds support to detect errors or lost data and to trigger retransmission until the data is correctly and completely received.

  • Sockets - is a name given to the package of subroutines that provide access to TCP/IP on most systems.


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