TCP/IP Tutorial and Technical Overview - IBM Redbooks

9.2.6 Packet sizes
If the algorithm used to encrypt the payload requires cryptographic
synchronization data, for example, an Initialization Vector (IV), this data is carried
in the Payload field.
Any encryption algorithm that requires an explicit, per-packet synchronization
data must indicate the length, any structure for such data, and the location of this
data.
If such synchronization data is implicit, the algorithm for deriving the data must
be part of the algorithm definition.
Note that the beginning of the next layer protocol header must be aligned relative
to the beginning of the ESP header. For IPv6, the alignment is a multiple of 8
bytes.
All IPv6 nodes are expected to dynamically determine the maximum
transmission unit (MTU) supported by all links along a path (as described in RFC
1191 – Path MTU Discovery) and source nodes will only send packets that do
not exceed the path MTU. IPv6 routers will, therefore, not have to fragment
packets in the middle of multihop routes and allow much more efficient use of
paths that traverse diverse physical transmission media. IPv6 requires that every
link supports an MTU of 1280 bytes or greater.
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