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

[Martin Duke <[e-mail]>]

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.
>