Re: [Ipsec] Minor comment/question on draft-ietf-ipsec-esp-v3-09.txt

At 3:59 PM +0200 2/16/05, Markku Savela wrote:
>Minor nit (or my misunderstanding of something), but in
>
>2.3  Payload Data
>    ...
>    Note that the beginning of the next layer protocol header MUST be
>    aligned relative to the beginning of the ESP header as follows. For
>    IPv4, this alignment is a multiple of 4 bytes. For IPv6, the
>    alignment is a multiple of 8 bytes.
>    ...
>
>
>I'm puzzled about what the "next layer protocol header" means? With
>ESP, the packet really does not have any next layer header, that needs
>alignment.
>
>The paragraphah can be deleted?
>
>[Of course, after inbound ESP processing we have the header and it
>must be properly aligned, but that is a different matter.]
>

Inside the ESP payload, after the optional IV, is the "next layer 
protocol header." It is referred to as "rest of payload data" in 
Figure 2. Typically, for transport mode this will be TCP or UDP. For 
tunnel mode, it is IP. This text is part of a discussion reminding 
folks that the IV, if present, must preserve the 4 or 8 byte 
alignment requirements for the relevant IP version. We assume that 
the ESP payload, overall, started on an appropriate boundary, so the 
addition of the 8 bytes of SPI plus sequence number preserved this 
alignment. But the IV, if present, could break the alignment.

Steve

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