6.1 Packet Headers

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If you were to look at a packet going across one of many links between its source and destination computers, you would see a link header, an IP header, and a Transport Control Protocol (TCP) header, along with the actual data in the packet.

Headers and Data
Figure 6.2: Headers and Data

The link header is removed when the packet is received on one link and a new link header is added when the packet is sent out on the next link on its journey. The IP and TCP headers stay with a packet as it is going across each link in its journey. Remember that a packet may go across several types of link layers as it is routed through the Internet.

The IP header holds the source and destination Internet Protocol (IP) addresses as well as the Time to Live (TTL) for the packet. The IP header is set on the source computer and is unchanged (other than the TTL) as the packet moves through the various routers on its journey.

The TCP headers indicate where the data in each packet belongs.

As the source computer breaks the message or file into packets, it keeps track of the position of each packet relative to the beginning of the message or file and places the offset in each packet that is created and sent.

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