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

[Martin Duke <[e-mail]>]

Ping?

On Tue, Oct 6, 2020 at 12:06 PM Benjamin Kaduk <[e-mail]> wrote:

> Hmm, my notes have this one as waiting on me to look at the latest updates,
> though there are a few other documents currently in front of it in my
> priority queue.
>
> -Ben
>
> On Tue, Oct 06, 2020 at 12:04:28PM -0700, Martin Duke wrote:
> > I believe the current status of this is that Ben is awaiting another
> > version of this draft to resolve his DISCUSS. If anyone has a different
> > understanding, please say so.
> >
> > On Thu, Sep 3, 2020 at 12:44 PM Greg Mirsky <[e-mail]>
> wrote:
> >
> > > 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
> > >
> > > On Tue, Sep 1, 2020 at 8:55 PM Benjamin Kaduk via Datatracker <
> > > [e-mail]> wrote:
> > >
> > >> Benjamin Kaduk has entered the following ballot position for
> > >> draft-ietf-ippm-stamp-option-tlv-09: Discuss
> > >>
> > >> When responding, please keep the subject line intact and reply to all
> > >> email addresses included in the To and CC lines. (Feel free to cut
> this
> > >> introductory paragraph, however.)
> > >>
> > >>
> > >> Please refer to
> https://www.ietf.org/iesg/statement/discuss-criteria.html
> > >> for more information about IESG DISCUSS and COMMENT positions.
> > >>
> > >>
> > >> The document, along with other ballot positions, can be found here:
> > >> https://datatracker.ietf.org/doc/draft-ietf-ippm-stamp-option-tlv/
> > >>
> > >>
> > >>
> > >> ----------------------------------------------------------------------
> > >> DISCUSS:
> > >> ----------------------------------------------------------------------
> > >>
> > >> 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).
> > >>
> > > 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?
> > >
> > >>
> > >> 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.
> > >>
> > > 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
> > >    packet with extension TLVs MUST validate each TLV:
> > >
> > >>
> > >>
> > >> ----------------------------------------------------------------------
> > >> COMMENT:
> > >> ----------------------------------------------------------------------
> > >>
> > >> 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.
> > >>
> > > 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?
> > >
> > >>
> > >>    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.
> > >>
> > > 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.
> > >
> > >
> > >
> > >>
> > >> 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.)
> > >>
> > > 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.
> > >
> > >>
> > >> Section 4.2
> > >>
> > >> I'd suggest saying that all fields that are not filled are transmitted
> > >> 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.
> > >
> > >>
> > >> 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?
> > >>
> > > 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.
> > >
> > >
> > >
> > >> 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/
> > >>
> > > GIM>> Thank you. Fixed (and three more places in the same sub-section).
> > >
> > >>
> > >> 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.
> > >>
> > > GIM>> Done.
> > >
>