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

From: Benjamin Kaduk via Datatracker <[email-masked]>
Date: Tue, 01 Sep 2020 20:55:11 -0700

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.

---

From: Greg Mirsky <[email-masked]>
Date: Thu, 3 Sep 2020 12:44:19 -0700

Hi Ben,
thank you for your comments and suggestions, all are much appreciated.
Please find my notes in-lined below tagged by GIM>>. Attached are the diff
and the new working version of the draft.

Regards,
Greg

> https://datatracker.ietf.org/doc/draft-ietf-ippm-stamp-option-tlv/
GIM>> Thank you for pointing to the inter-QoS domain scenario. Clearly, it
must be handled with more caution. Using the U flag is a very interesting
approach. But wouldn't the Session-Sender skip the CoS TLV in the reflected
packet? As a result, we may not retrieve information about the CoS mapping
in the forward direction, i.e., towards the Session-Reflector. And it might
be difficult for an operator to recognize whether the Session-Reflector
doesn't support the particular CoS or STAMP extensions altogether. I would
propose a new field being added in the CoS TLV:

   -    RP (Reverse Path) - is a two-bit-long field.  A Session-Sender
      MUST set the value of the RP field to 0 on transmission.

And further in the section another update:
   A STAMP Session-Reflector that receives a test packet with the CoS
   TLV MUST include the CoS TLV in the reflected test packet.  Also, the
   Session-Reflector MUST copy the value of the DSCP and ECN fields of
   the IP header of the received STAMP test packet into the DSCP2 field
   in the reflected test packet.  Finally, the Session-Reflector MUST
   use the local policy to verify whether the CoS corresponding to the
   value of the DSCP1 field is permitted in the domain.  If it is, the
   Session-Reflectorset MUST set the DSCP field's value in the IP header
   of the reflected test packet equal to the value of the DSCP1 field of
   the received test packet.  Otherwise, the Session-Reflector MUST use
   the DSCP value of the received STAMP packet and set the value of the
   RP field to 1.  Upon receiving the reflected packet, if the value of
   the RP field is 0, the Session-Sender will save the DSCP and ECN
   values for analysis of the CoS in the reverse direction.  If the
   value of the RP field in the received reflected packet is 1, only CoS
   in the forward direction can be analyzed.

Would these updates address your concern?

> does not immediately seem to be of much consequence.
GIM>> Thank you for catching this. Indeed, that brakes STAMP with
extensions. I think that though checking M and I flags of a
Session-Reflector should not do any harm, it doesn't seem to have any
benefits. Hence we may remove the Session-Reflector from the sentence and
the updated introduction to the flag processing reads like below:
   A STAMP Session-Sender that has received a reflected STAMP test

GIM>> Is there a reliable mechanism to distinguish between an out-of-order
duplicated packet and a replayed packet? I've checked RFC 5357 TWAMP for
any suggestion and believe that there is no special consideration by a
Session-Reflector for an out-of-order duplicated packet. The TWAMP
Session-Reflector does not monitor whether the sequence numbers of the
received test packets are in the monotonically increasing sequence. STAMP
is designed to be comparable with TWAMP and have some level of
interoperability. Hence, discarding what appears as a replayed packet at a
STAMP Session-Reflector might be less desirable behavior. If we follow
TWAMP's behavior, what else can be done at the Session-Reflector to
strengthen the security?

> if undefined behavior is triggered.
GIM>> I agree that the inter-QoS domain case requires more consideration in
the Security section. I propose the updated text below:

   As this specification defined the mechanism to test DSCP mapping,
   this document inherits all the security considerations discussed in
   [RFC2474].  Monitoring and optional control of DSCP using the CoS TLV
   may be used across the Internet so that the Session-Sender and the
   Session-Reflector are located in domains that use different CoS
   profiles.  Thus, it is essential that an operator verifies the set of
   CoS values that are used in the Session-Reflector's domain.  Also, an
   implementation of a Session-Reflector SHOULD support a local policy
   to confirm whether the value sent by the Session-Sender can be used
   as the value of the DSCP field.  Section 4.4 defines the use of that
   local policy.

> not just the specific case of setting the DSCP codepoint.)
GIM>> It seems very challenging for a STAMP system to differentiate between
a duplicate and/or out-of-order test packet and a replayed packet because
one of the metrics STAMP test measures is the network re-ordering metric
per RFC 4737 <https://tools.ietf.org/html/rfc4737>. We may recommend that
the rate-limiting of test packets be selected conservatively.

> with all bits set to zero.
GIM>> Thank you for the helpful suggestion. Appended Section 4.2:
   Note that all fields not filled by either a Session-Sender or
   Session-Reflector are transmitted with all bits set to zero.

> Value should be 8-octets-long, no?
GIM>> Thank you for catching it! You are absolutely correct. Fixed this and
a similar in:
   o  Source EUI-64 Address sub-TLV - is a 12-octet-long sub-TLV that
      includes the EUI-64 source MAC address.  The Type value is TBA11.
      The value of the Length field MUST equal to 8.  The Value field
      consists of an eight-octet-long EUI-64 field.

>    Location TLV.  If the Session-Reflector receives the Location TLV
GIM>> Thank you. Fixed (and three more places in the same sub-section).

>    Type, and Length.  All other fields MUST be filled with zeroes The
GIM>> Done.