[dtn] BPSec and bundle fragmentation

[Brian Sipos <[e-mail]>]

All,
There seems to be a slight hole in the relationship between BPSec and BPbis related to a bundle with BCB(s) on extension
blocks which is then fragmented. If a BCB has a target of an extension block it will alter the block type-specific data
so it's no longer decodable as normal. The requirements in BPbis [1] Section 5.8 "Bundle Fragmentation" require
replicating extension blocks with flag indicating to do so. The trouble is that if a BCB-modified block is replicated in
the fragment a receiver of the non-initial fragment has no way to know the block is BCB-targeted. The requirements in
BPSec [2] section 5.2 "Bundle Fragmentation and Reassembly" indicate that the BIB must be replicated only if the target
is a payload block, but really should replicate if the target is either the payload or any extension block with "block
must be replicated in every fragment" flag set.

As an aside, I don't see what the value is in replicating the BIB when the target is the fragmented payload unless there
is an expectation that a payload fragment will be inspected by a node. I suppose that this is just playing safe sice
BPv7 doesn't explicitly prohibit any node from inspecting payload fragment data.

[1] https://tools.ietf.org/html/draft-ietf-dtn-bpbis-31
[2] https://tools.ietf.org/html/draft-ietf-dtn-bpsec-26