[ippm] Benjamin Kaduk's Discuss on draft-ietf-ippm-stamp-option-tlv-09: (with DISCUSS and COMMENT)

[Benjamin Kaduk via Datatracker <[e-mail]>]

Benjamin Kaduk has entered the following ballot position for
draft-ietf-ippm-stamp-option-tlv-09: Discuss

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DISCUSS:
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Thanks for all the updates; we've made good progress.

I think we're still not converged on the DSCP handling, though.  I have
a bit more exposition in the COMMENT section, but in short, my
understanding is that we're setting up a session-reflector to incur
unbounded levels of risk with hard protocol requirements.  I think we
need to provide a way to bound that risk, for example by allowing the
Session-Reflector to selectively choose to treat the CoS TLV as
unimplemented (set the U flag in its reflected packet) or some other
mechanism for local policy to filter what DSCP codepoints are set in
reflected packets (ideally, indicating that the policy made a change).

Also, there's a bit of fallout from the flags reworking that's left to
cleanup in Section 4: we now have the Session-Sender set the U flag to
1, so this text no longer makes sense:

% A STAMP system, i.e., either a Session-Sender or a Session-Reflector,
% that has received a STAMP test packet with extension TLVs MUST
% validate each TLV:
%
%    If the U flag is set, the STAMP system MUST skip the processing of
%    the TLV.

I think it should just apply to the Session-Sender for this case -- the
Session-Reflector doesn't need to check the received U flag, since the
Session-Sender will not be generating TLVs it does not understand.
(Whether or not to keep the behavior for the M and I flags as applying
to both Session-Sender and Session-Reflector vs. just Session-Sender
does not immediately seem to be of much consequence.


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COMMENT:
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My most significant remaining comments are on the Security
Considerations (Section 6):

The security of the HMAC mechanism is not complete, since it is
susceptible to replay attack.  As such, when HMAC is in use, it is
important to check that the received sequence numbers are (at least
mostly) monotonic and to detect replays.  While replayed packets do not
always indicate an attack (depending on the network technology) they are
still a noteworthy condition, and we should say something about whether
we expect to produce a response to each received instance or to suppress
replies to replayed input.

   Monitoring and optional control of DSCP do not appear to introduce
   any additional security threat to hosts that communicate with STAMP
   as defined in [RFC8762].  As this specification defined the mechanism
   to test DSCP mapping, this document inherits all the security

I'm afraid I still don't understand the reasoning here.  In my
understanding, the risk stems from the semantics of the DCSP field being
(for at least some codepoints) site-local, there not being a guarantee
that the session-sender and session-reflector are on the same network
(and thus, using the same DSCP semantics), and the hard requirement for
the Session-Reflector to set the DCSP value indicated by the
Session-Sender.  A mechanism for a remote entity to induce generation of
local packets with unspecified semantics is a risk that cannot be
qualified at protocol-design time, since the possible outcomes are
inherently unspecified.  This is analogous to the situation with
undefined behavior in programming languages like C -- the programmer is
flat-out required to avoid it, because literally anything could go wrong
if undefined behavior is triggered.

This is especially risky when there is the possibility for the
Session-Reflector to act on packets that do not have any form of
authentication (i.e., could be spoofed from off-path).  But we do not
mention this risk at all, let alone give guidance on its mitigation.
(Discussion of the security considerations of unauthenticated operation
would ideally be generalized to all actions/TLVs that have side effects,
not just the specific case of setting the DSCP codepoint.)

Section 4.2

I'd suggest saying that all fields that are not filled are transmitted
with all bits set to zero.

Section 4.2.1

   o  Source MAC Address sub-TLV - is a 12-octet-long sub-TLV.  The Type
      value is TBA9.  The value of the Length field MUST equal to 8.
      The Value field is a 12-octet-long MBZ field that MUST be zeroed
      on transmission and ignored on receipt.

Value should be 8-octets-long, no?

Section 4.2.2

   A Session-Sender MAY include the Source MAC Address sub-TLV is the
   Location TLV.  If the Session-Reflector receives the Location TLV

nit: s/is/in/

Section 4.3

   MUST NOT fill any information fields except for STAMP TLV Flags,
   Type, and Length.  All other fields MUST be filled with zeroes The

nit: missing full stop.