Re: Compression of AH/NULL-ESP headers in ROHCv1

Dear ROHCers,

"The AH sequence number can be omitted from the compressed header
  when the offset from the sequence number (SN) of the compressed
  header is constant, ...."

    When I read the above lines about the AH sequence number in RFC
4815 5.8, I got the some questions.
Please clarify the following probelms. Thanks

1) Does the SN mean RTP SN or AH SN in the "offset sequence number
(SN) of the compressed header"?
2) Does the offset mean the method like "offset IP-ID encoding" or
just calculating the delta respectively(RTP SN and AH SN) and compare?
    Since RTP SN is 2 bytes and AH is 4 bytes, the two methods will be
different at wraparound.

Best regards,
Jer-ming