![]() ![]() This information is populated in the packet context. The next-hop information contains the egress line card and the outgoing port the packet needs to be transferred. The forwarding engine searches a table (the forwarding table) to determine the next-hop. ![]() Next, the entire packet context is sent to the forwarding engine in the line card. The header information includes the destination address, source address, protocol type, DSCP bits (for differentiated services) and if the IP packet is carrying TCP or UDP payload, the destination and the source ports as well.Īt this point, the packet context contains enough information for route lookup and classification of the packet. It extracts the relevant IP header information and stores it in the packet context. The 元 processing logic locates the IP header and checks its validity. Now the L2 processing logic peels off the payload, which is an IP packet, and along with the packet context sends it to the 元 processing logic. This means that the last two bytes of the IP header are 02 1e.įigure 13.4. This is a computed field however, so we must multiply this field by 5 to arrive at the IP header length, which is 20 bytes. ![]() Earlier, we learned that the IP header length field is contained in the lower order nibble of byte 0×0 in the IP header, which has a value of 4. The IP header is variable in length depending on a set of options it can support, so the next thing we need to ascertain is the length of the IP header. As we learned earlier, the IP version is identified by the higher order nibble of byte 0x0 in the IP header. ![]() We are attempting to break this packet down by individual protocol, so we aren’t concerned about every single value in this header, but there are a few values we will have to evaluate in order to determine the length of the IP header and what protocol to expect next.įirst, we need to determine what version of IP is being used here. Since the Ethernet header was kind enough to tell us that we should expect an IP header next, we can apply what we know about the structure of the IP header to the next portion of the packet. Because all of the examples in this book use Ethernet, the examples moving forward won’t show this header, and will instead begin from the network layer protocol instead. While I’ve included the Ethernet header in this example, the data link layer header is not printed by tcpdump by default. ![]()
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