Subject: [6lowpan] compressed UDP Length field

From: Matus Harvan <[email-masked]>
Date: Tue, 19 Jun 2007 00:12:09 +0200

Hi,

the format draft version 13 says that if the UDP Length field is
compressed then its value "is equal to the Payload Length from the IPv6
header, minus the length of any extension headers present between the
IPv6 header and the UDP header." However, IMHO there cannot be any
extension headers between the IPv6 header and the UDP header if the UDP
header is HC2/HC_UDP compressed.

The compressed UDP header can only be used if it follows immediately
after the compressed IPv6 header. If there were any extension headers
following the IPv6 header, the Next Header value (bits 5 and 6) in the
HC1-compressed IPv6 header would not be  01 (UDP), the HC2 encoding (bit
7) could not be set, the UDP header could not be HC2/HC_UDP compressed
at all and hence there would be no compressed UDP Length field.

Is the wording in the draft a provisioning for the future where
extension headers could possibly be included with a compressed UDP
header or should it maybe mention that given the current format draft
there cannot be any extension headers if the UDP header is compressed?

Matus

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