Can I use the Sequence Number as the (implicit) IV in AES-GCM IPsec?
If I am correct, and you can just the ESN has the implicit IV, is doing so compliant with the standards?


No, it would not be compliant with the RFC. The RFC clearly states that there must be 8 bytes of IV payload immediately in front of the ciphertext (see section 3). Omitting those 8 bytes would not allow interoperability with standard implementations of ESP-GCM (and a large part of the reason the IETF does RFC's is to allow different implementations to interoperate).

Yes can, of course, copy the ESP sequence number into the 8 byte IV; that meets the requirement for uniqueness; however that obviously doesn't do the bandwidth saving you're looking for.

Alternatively, if you feel strongly enough about it, you could design a vendor-specific encryption method that does GCM (and derives the sequence number in the ESP header as payload); IKE does provide ways to do that.  However, you can't claim compliance to RFC4106...