TCP/IP Tutorial and Technical Overview - IBM Redbooks

Hop-by-hop header option types
You might have noticed that each extension header is an integer multiple of 8
bytes long in order to retain 8-byte alignment for subsequent headers. This is
done not purely for “neatness” but because processing is much more efficient if
multibyte values are positioned on natural boundaries in memory (and today's
processors have natural word sizes of 32 or 64 bits).
In the same way, individual options are also aligned so that multibyte values are
positioned on their natural boundaries. In many cases, this will result in the
option headers being longer than otherwise necessary, but still allow nodes to
process packets more quickly. To allow this alignment, two padding options are
used in hop-by-hop headers:
Pad1 A X'00' byte used for padding a single byte. For longer padding
sequences, use the PadN option.
PadN An option in the TLV format (described earlier). The length byte gives
the number of bytes of padding after the minimum two that are
required.
The third option type in a hop-by-hop header is the Jumbo Payload Length. This
option is used to indicate a packet with a payload size in excess of 65,535 bytes
(which is the maximum size that can be specified by the 16-bit Payload Length
field in the IPv6 basic header). When this option is used, the Payload Length in
the basic header must be set to zero. This option carries the total packet size,
less the 40 byte basic header. See Figure 9-6 for details.
0 8 16 24 31
Figure 9-6 Jumbo Payload Length option
Jumbo Payload Length
type= C2 length=4
Routing header
The path that a packet takes through the network is normally determined by the
network itself. Sometimes, however, the source wants more control over the
route taken by the packet. It might want, for example, for certain data to take a
slower but more secure route than would normally be taken. The routing header
(see Figure 9-7 on page 338) allows a path through the network to be
predefined. The routing header is identified by the value 43 in the preceding Next
Header field. It has its Next Header field as the first byte and a single byte routing
type as the second byte. The only type defined initially is type 0, strict/loose
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