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Document: draft-ietf-anima-grasp-api-08
Reviewer: Paul Kyzivat
Review Date: 2020-11-30
IETF LC End Date: 2020-10-28
IESG Telechat date: 2020-12-01
Summary:
This draft is on the right track but has open issues, described in the
review.
General:
This document has addressed some of the concerns I had during the last
call review. However some of my concerns remain and some new ones have
arisen in this version.
Issues:
Major: 3
Minor: 6
Nits: 1
1) MAJOR: Negotiation
The text in section 2.3.5 now makes clear that the sequence of steps in
the negotiation is non-deterministic - both sides can call
negotiate_step and negotiate_wait. I believe this can result in the two
sides not agreeing on what values have been negotiated. (For instance,
what if one side calls negotiate_step concurrently with the other side
calling end_negotiate? Which value has been agreed upon?) The loop_count
adds to the confusion. Are the two sides intended to have independent
loop count values? It seems these too can become unsynchronized.
Also, the goal of negotiation isn't clear to me. I gather it must be for
the two sides to agree on a particular value for the objective. But for
that to work there must be some rules about how values can change in
each step so that the result stabililizes, rather than causing a battle
that ends with loop count exhaustion. This could be achieved by always
negotiating *down*, or always *up*. But that requires that the objective
value type have an ordering function. Given the general nature of the
objective I don't think that can be assumed.
ISTM that more work is needed to define the negotiation process in a way
that ensures it ends with both sides agreeing on a single value for the
objective.
2) MINOR: Dry Run Negotiation
Dry Run negotiation is very under-specified. Why would it be used? I
guess that an ASA might use dry run negotiation to inform future actual
negotiation. Can anything be inferred from a dry run negotiation about
how an actual negotiation will go? When participating in a dry run
negotiation, how should an ASA decide what response to make? Should it
take into account current resource availability? Or should it respond
based on best-case or worst-case resource availability? Or what?
This requires further clarification.
3) MAJOR: Confusing semantics of 'request_negotiate'
In section 2.3.5 I don't understand the following:
1. The 'session_nonce' parameter is null. In this case the
negotiation has succeeded in one step and the peer has
accepted the request. The returned 'proffered_objective'
contains the value accepted by the peer, which is therefore
equal to the value in the requested 'objective'. For this
reason, no session nonce is needed, since the session has
ended.
IIUC this requires a network exchange with the peer. I don't see how
this can complete *immediately*. ISTM that this could only complete
immediately if it were satisfied from a local cache. That doesn't seem
appropriate for this function.
Similarly, in bullet 2 I don't see how the proffered_objective would be
available in the initial call, before a response has been received from
the peer..
Does "immediately" here simply mean that the negotiation is completed in
one exchange between the two ends? If so, isn't a session nonce still
required in an event loop implementation in order to handle the one
response?
Bullet 2 also says:
... The
returned 'proffered_objective' contains the first value
proffered by the negotiation peer. The contents of this
instance of the objective must be used to prepare the next
negotiation step (see negotiate_step() below) because it
contains the updated loop count, sent by the negotiation
peer. The GRASP code automatically decrements the loop
count by 1 at each step, and returns an error if it becomes
zero.
I guess that the 'proffered_objective' in the return parameters is the
counter-offer to the objective passed in the call. And that you expect
the objective value used in any subsequent negotiate_step to be derived
by modifying this value. So far this new wording has improved my
understanding.
But the loop_count in the objective is especially confusing. It seems
that it is handled quite differently from the rest of the objective. You
specify (in 2.3.2.3) that it has a default value of GRASP_DEP_LOOPCT.
But who is expected to initialize this? (Is it simply that the ASP
should use this value if it doesn't have any particular preference?)
Then you say that the GRASP decrements this. Is this decrementing done
on the calling side before sending the message, the calling side after
receiving the response? Or by the peer, on receipt or when sending the
response? Is it permissible for the ASA to modify this value during
negotiation? Since this seems intended to prevent a loop, having clarity
about how this value is managed seems important.
4) MINOR: negotiate_wait
The negotiate_wait call allows one ASA to extend the timeout of another
ASA. This could, in perverse cases, cause an ASA to wait indefinitely.
ISTM that this is dangerous. I would think it better make the other ASA
aware of the desire to extend the timeout and let it decide whether to
do so.
5) MAJOR: Consistency of Objective definitions
In section 2.3.2.3 and elsewhere, presumably all parties that use a
particular objective must agree on the values of synch, neg, dry, and
the size and structure of the value.
There is no communication of the size and structure in the abstract API.
Presumably the implementation of a language binding to the API is
required to at least communicate the size and alignment requirements to
the core. The matching of definitions between nodes must be achieved
solely by the name, the respective language bindings at the two ends,
and out of band mutual agreement. Furthermore, different language
bindings may use different in-memory representations of the value. In
such cases, how is the on-wire format to be determined?
If the two ends disagree on size and structure then problems will occur.
Perhaps the core can identify size mismatches based on size communicated
on the wire vs the size defined by the language binding, but there are
no error codes defined for this situation. And of course differing
structures with the same size would not be detectable.
Furthermore, there is potential for different ASAs to (accidentally)
have incompatible definitions for the same objective. What happens in
this case? How can blame be ascribed so that the problem can be fixed?
IMO more needs to be said about all of this. At the least a number of
disclaimers that put the burden on the ASAs to recognize the risk, take
these potential problems into account and avoid them. But there could be
some requirements placed on API language bindings and core
implementations to deal with some of these. And probably some added
error codes to report what problems can be detected.
6) MINOR/MAJOR: Session State
I continue to find the lifetime and state of a session to be unclear.
The API calls that return session_nonce seem to signal creation of a new
session. The end_negotiate() call seems to terminate a negotiation
session. But what causes other sessions to end? This seems important
because there is state associated with a session that consumes resources
and can't be reclaimed until the session ends. So it should be important
for the ASA to end all sessions. Some clarification of this seems
important both for core implementors and for ASA developers that will be
using the API.
(Or is this document only for implementors of core and those
instantiating a particular language binding of the API, with
documentation for end users left to others?)
7) MINOR/MAJOR: Timeout
Section 2.3.2.2 indicates that the API returns an error response to the
ASA if the timeout expires. But the other end is presumably still
working on the request and will eventually send a response. What does
the core do when it receives this? Must it retain state so that it can
detect the case and ignore the message? It seems that this could result
in the two peers disagreeing on some state.
8) MINOR: Text regarding "minimum_TTL"
There is a small problem with the following in section 2.3.4:
- If the parameter 'minimum_TTL' is greater than zero, any
locally cached locators for the objective whose remaining time
to live in milliseconds is less than or equal to 'minimum_TTL'
are deleted first. Thus 'minimum_TTL' = 0 will flush all
entries.
The first sentence qualifies the paragraph to cases where minimum_TTL is
greater than zero. But the final sentence then infers the behavior when
minimum_TTL is equal to zero.
Also, minimum_TTL is typed as an integer, which permits negative values.
I gather that negative values are not allowed. I can suggest two ways to
fix this:
- The parameter 'minimum_TTL' MUST be greater than or equal to
zero. Any locally cached locators for the objective whose
remaining time to live in milliseconds is less than or equal to
'minimum_TTL' are deleted first. Thus 'minimum_TTL' = 0 will
flush all entries.
Or, change they type to unsigned integer. Then the statement can be
simplified by removing the first sentence:
- Any locally cached locators for the objective whose remaining
time to live in milliseconds is less than or equal to
'minimum_TTL' are deleted first. Thus 'minimum_TTL' = 0 will
flush all entries.
9) MINOR: Terminology - Session nonce
The new first paragraph of section 2.2.3 talks about identifying the
session by a pseudo-random session identifier, and tagging it with an IP
for further uniqueness. The 2nd paragraph talks about a session_nonce.
It isn't clear at this point in the text if these the same thing. Or is
the session id shared on the wire, the IP tag added by the core, and the
session_nonce an artifact of the API, shared only between the ASA and
the core?
Section 2.3.2.7 seems to confirm that the nonce is just an identifier
used between the core and the ASA. But here it says that using the id
plus the IP is simply one possible implementation choice.
Further, I question whether "nonce" is the best term to use here. ISTM
that "handle" (session_handle) would more clearly reflect the purpose of
this item.
I think it would be helpful to be clearer in distinguishing what is
fundamental vs what is implementation choice. For instance, in section
2.2.3:
A GRASP session consists of a finite sequence of messages (for
discovery, synchronization, or negotiation) between a pair of ASAs.
The core identifies it on the wire by a pseudo-random session
identifier. Further details are given in [I-D.ietf-anima-grasp].
On the first call in a new GRASP session, the API returns a
'session_handle' value used to identify the session. This
value must be used in all subsequent calls for the same session, and
will be provided as a parameter in the callback functions. By this
mechanism, multiple overlapping sessions can be distinguished, both
in the ASA and in the GRASP core. The value of the 'session_handle"
is opaque to the ASA.
This establishes the role and relationship of the two terms, while
section 2.3.2.7 gives a possible implementation without as much
confusion. (It will require some rewording to switch from session_nonce
to session_handle. It already uses "session handle" in passing.)
10) NIT: Terminology - ASA nonce
For similar reasons to those above for session_nonce/session_handle, IMO
it would be clearer to use asa_handle rather than asa_nonce. But this is
only a suggestion.
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