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IPv6 Datagram Extension Headers
(Page 2 of 6)
IPv6 Header Chaining Using the Next Header Field
The only field common to all extension
header types is the Next Header field (which actually appears
at the end of one header type, the ESP
header). The 8-bit Next Header
field is used to logically link all the headers in an IPv6 datagram
- The Next Header field in the main header
contains a reference number for the first extension header type.
- The Next Header field in the first extension
header contains the number of the second extension header type, if there
is a second one. If there's a third, the second header's Next Header
points to it, and so on.
- The Next Header field of the last extension
header contains the protocol number of the encapsulated higher-layer
protocol. In essence, this field points to the next header
within the payload itself.
For example, suppose a datagram that
encapsulates TCP has a Hop-By-Hop Options extension header and
a Fragment extension header. Then, the Next Header fields
of these headers would contain the following values:
- The main header would have a Next Header
value of 0, indicating the Hop-By-Hop Options header.
- The Hop-By-Hop Options header would have
a Next Header value of 44 (decimal), the value for the Fragment
- The Fragment header would have a Next
Header value of 6.
This is illustrated in Figure 106.
Figure 106: IPv6 Extension Header Linking Using the Next Header Field
The Next Header field allows a device to more easily process the headers in a received IPv6 datagram. When a datagram has no extension headers, the next header is actually the header at the start of the IP Data field, in this case a TCP header with a value of 6. This is the same way the Protocol field is used in IPv4. When extension headers do appear, the Next Header value of each header contains a number indicating the type of the following header in the datagram, so they logically chain together the headers, as shown above.
Key Concept: The IPv6 Next Header field is used to chain together the headers in an IPv6 datagram. The Next Header field in the main header contains the number of the first extension header; its Next Header contains the number of the second, and so forth. The last header in the datagram contains the number of the encapsulated protocol that begins the Data field.
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Version 3.0 - Version Date: September 20, 2005
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