Re: [IPsec] WGLC of draft-ietf-ipsecme-multi-sa-performance

[Valery Smyslov <[e-mail]>]

Hi,

I support publication of this draft. I'm glad authors took my points into consideration
while preparing the latest version. I do have some comments though.

1. Section 1

   IKEv2 [RFC7296] already allows installing
   multiple Child SAs with identical Traffic Selectors, but it offers no
   method to indicate that the additional Child SA is being requested
   for performance increase reasons and is restricted to some resource
   (queue or CPU).  Without this indication, the peer might not accept
   multi Child SAs with identical Traffic Selectors and might delete one
   of the Child SAs it considered an unwanted duplicate.

There is some inconsistency here. You first say that IKEv2 allows
creating multiple identical Child SAs and then say that implementations
would delete them as unwanted duplicates. Clearly these implementations
violate RFC 7296, and we don't consider broken implementations, do we? :-)
I suggest to remove the last sentence, or to add a clarification.

2. Section 2

   There are a number of practical reasons why most Implementations have
   to limit a Child SA to only one specific hardware resource, but a key
   limitation is that sharing the crypto state, counters and sequence
   numbers between multiple CPUs is not feasible without a significant
   performance penalty.

Shouldn't it be clarified, that the performance problems arise
if you use an SA by several CPUs at the same time? I don't think 
there are problems if you use the SA by several CPUs at different time.
Consider you have an SA with a traffic one packet per hour.
Each time it is processed up by a different CPU, then the resulted
state is stored in the shared memory. So, perhaps

s/one specific hardware resource/one specific hardware resource at any given time

3. Section 3

   If multiple Child SAs with the same Traffic Selectors are
   desired, the initiator will add the SA_RESOURCE_INFO notify payload
   to the Exchange negotiating the Child SA (eg IKE_AUTH or
   CREATE_CHILD_SA).  If this initial Child SA will be tied to a
   specific resource, it MAY indicate this by including an identifier in
   the Notification Data.  A responder that is willing to have multple
   Child SAs for the same Traffic Selectors will respond by also adding
   the SA_RESOURCE_INFO notify payload in which it MAY add a non-zero
   notify data payload.

This text is a bit inconsistent with IKEv2 specification. 
In my reading the text implies that unless you exchange SA_RESOURCE_INFO,
you cannot initiate multiple SAs with same selector, which is wrong - 
you can do it at any time if you follow RFC 7296.
Only if you want to follow this draft (i.e. - associate each Child SA with
some resource, and be able to limit their number with TS_MAX_QUEUE)
you have to negotiate. I think that this subtle thing should be expressed more accurately.

4. Section 3

   These resource-
   specific Child SAs MUST be negotiated with identical Child SA
   properties that were negotiated for the initial Child SA.  This
   includes cryptographic algorithms, Traffic Selectors, Mode (e.g.
   transport mode), compression usage, etc.  However, each Child SA does
   have its own keying material that is individually derived according
   to the regular IKEv2 process.

I think that MUST is over-restrictive if we talk about crypto algorithms.
For crypto algorithms herhaps something along the lines: 

SHOULD be negotiated with the same crypto algorithms;
if they differ, then they MUST provide identical level of protection.

 (I agree that Mode and Traffic Selectors MUST be the same, not sure about compression).

5. Section 3

   During the CREATE_CHILD_SA rekey for the Child SA, the
   SA_RESOURCE_INFO notification MAY be included, but regardless of
   whether or not it is included, the rekeyed Child SA MUST be bound to
   the same resource(s) as the Child SA that is being rekeyed.

Isn't binding a local matter? Doesn't peer bother to what resource you bound
your end of an SA? Then why is there this MUST? What happens if I bound new SA
to a different CPU - peer never know this, so how it will check that you follow this MUST?
I think that instead of this requirement there should be just a recommendation for implementers
(with no BCP14 language).

6. Section 4

   A simple distribution could be to install one additional Child SA on
   each CPU.  An implementation MAY ensure that one Child SA can be used
   by all CPUs ...

I believe it should be "can" instead of "MAY", since it is a local matter.

7. Section 4

   When the number of queue or CPU resources are different between the
   peers, the peer with the least amount of resources MAY decide to not
   install a second outbound Child SA for the same resource as it will
   never use it to send traffic.  

Again, I think it should be "can" instead of "MAY", since it is a local matter.

8. Section 4

   If per-CPU SADB_ACQUIRE messages are implemented (see Section 6), the
   Traffic Selector (TSi) entry containing the information of the
   trigger packet SHOULD be included in the TS set.  This information
   MAY be used by the peer to select the most optimal target CPU to
   install the additional Child SA on.

The conditional part of the first sentence is too implementation specific:
the SADB_REQUIRE is specific to PF_KEYv2 API and not all implementations use it
(and this specification doesn't depend on it). Just replace it with
some more generic text, like "dynamic creating of per-CPU Child SAs" providing a single
a reference to an SADB_ACQUIRE as an example. This is true for Section 6 too,
where SADB_ACQUIRE should only be used as an example and not as 
prescribed mechanism.

Then, this SHOULD is already specified in RFC 7296, Section 2.9:

   To enable the responder to choose the appropriate range in this case,
   if the initiator has requested the SA due to a data packet, the
   initiator SHOULD include as the first Traffic Selector in each of TSi
   and TSr a very specific Traffic Selector including the addresses in
   the packet triggering the request.  

Why repeat this requirement there? Perhaps just reference Section 2.9 of RFC 7296?

And finally, "MAY" should be "can", since it is a local matter of responder.

9. Section 5.1, Section 5.2

   *  Protocol ID (1 octet) - MUST be 0.  MUST be ignored if not 0.
   *  SPI Size (1 octet) - MUST be 0.  MUST be ignored if not 0.

What "MUST be ignored if not 0"? The value of the field or the notify message itself?
If the non-zero value must be ignored, then why specify that it MUST be 0?
And how the receiver should interpret the notify message in this case?
I think there is an inconsistency. I suggest to remove the second MUSTs,
thus the receiver will reject the notify if it is has non-zero protocol and SPI,
as it should do with any other such notifications.

10. Section 5.1

   *  Optional Payload Data.  This value MAY be set to convey the local
      identity of the resource.  The value SHOULD be a unique identifier
      and the peer SHOULD only use it for debugging purposes.

This is an optional field used for debugging purposes. Why any BCP14 language here?
And if this is a value, associated with a resource, then how it may be unique 
in a situation when peer associates several SAs with a single resource
(e.g. if it has fewer resources then its peer, but doesn't mind creating as many
SAs as its peer wants)? Another consideration - if it is used only for debugging, 
and its format is not specified, then it is meaningful only for the peer that
sends it and thus it should be an opaque blob with no defined semantics 
(like association with a resource) and no requirements on its value.

11. Section 6

As I pointed out before, this section is too implementation-specific.
I think that SADB_ACQUIRE should be mentioned only as an example
and the operational considerations should be provided in 
API-neutral language.

12. Section 6

   Implementations supporting per-CPU SAs SHOULD extend their local SPD
   selector, and the mechanism of on-demand negotiation that is
   triggered by traffic to include a CPU (or queue) identifier in their
   SADB_ACQUIRE message from the SPD to the IKE daemon.

I think that using BCP14 language is not justified here (it is not protocol behavior). 
It should be "should" instead.

13. Section 6

   And
   bringing the deleted per-CPU Child SA up again immediately after
   receiving the Delete Notify might cause an infinite loop between the
   peers.  Another issue of not bringing up all its per-CPU Child SAs is
   that if the peer acts similarly, the two peers might end up with only
   the first Child SA without ever activating any per-CPU Child SAs.

I think more should be said about this situation and how to avoid it.
Perhaps advising such implementations to not delete per-CPU Child SAs 
if they have not been rekeyed. Then the second case will never happen.
As for the first case, perhaps advise all implementations to not
delete SAs immediately once they are created...

14. Section 6

   Implementations might support dynamically moving a per-CPU Child SAs
   from one CPU to another CPU.  If this method is supported,
   implementations must be careful to move both the inbound and outbound
   SAs.

If this is done "on the fly", then the data in SA_RESOURCE_INFO (if it is provided)
would become invalid, killing its usefulness for debugging. If it is done
as a result of SA rekey, then it contradicts to the last sentence in Section 3
(well, I already suggested to remove it, see comment 5).

15. Section 6

   If this method is supported,
   implementations must be careful to move both the inbound and outbound
   SAs.

Is it justified? Why inbound and outbound SAs cannot be bound to a different resources?

16. Section 6

   If the IPsec endpoint is a gateway, it can move the inbound SA
   and outbound SA independently from each other.

This contradict to the previous sentence.

17. Section 6

   If the IPsec endpoint
   is the same host responsible for generating the traffic, the inbound
   and outbound SAs SHOULD remain as a pair on the same CPU.

Using BCP14 language is not justified here (it is a local matter and 
not a protocol behavior, so this is just a recommendation
for implementers). And some justification for this recommendation
would be helpful.

18. Section 6

   If a host
   previously skipped installing an outbound SA because it would be an
   unused duplicate outbound SA, it will have to create and add the
   previously skipped outbound SA to the SAD with the new CPU ID.

This sentence unclear for me. First, I fail to understand why this is needed.
Then, I fail to understand how to do this, in particular - 
how to get SPIs and keying material of the skipped (i.e. deleted) SA.
I believe this piece of text should be elaborated.

19. Section 6

   The
   inbound SA may not have CPU ID in the SAD.

I fail to understand what this sentence is related to? 

20. Section 6

   To support this, the IKE software might have to hold
   on to the key material longer than it normally would, as it might
   actively attempt to destroy key material from memory that the IKE
   daemon no longer needs access to.

In my opinion this requirement is problematic. It highly depends on 
concrete architecture. In some cases IKE daemon has never access to the Child SA
keying material - it only provides values for its generating,
which is done in a HSM. And in this case IKE daemon has no control
over this material (and it never needs to have an access to it or control over it).
So, I think that this should be elaborated, in particular justifying
why do you need to re-create outbound SA which you have already deemed unneeded.

21. Section 6

   An implementation that does not accept any further resource specific
   Child SAs MUST NOT return the NO_ADDITIONAL_SAS error because this
   can be interpreted by the peer that no other Child SAs with different
   TSi/TSr are allowed either.  Instead, it MUST return TS_MAX_QUEUE.

I wonder whether TS_MAX_QUEUE is a permanent or temporary error.

22. Section 7

   Similar to how an implementation should limit the number of half-open
   SAs to limit the impact of a denial of service attack, an
   implementation SHOULD limit the maximum number of additional Child
   SAs allowed per unique TSi/TSr.

In section 6 you said:

   Peers SHOULD be lenient with the maximum number of Child SAs they
   allow for a given TSi/TSr combination to account for corner cases.

and later

  As additional Child SAs consume  little additional overhead, ...

So, some wordsmithing to make these pieces of text in concert would be helpful.

23. Section 7

   This trust relationship is usually not present for the Remote Access
   VPN type deployments, and allowing per-CPU Child SA's is NOT
   RECOMMENDED in these scenarios.  Therefor, it is also NOT RECOMMENDED
   to allow per-CPU Child SAs per default.

Typo: s/Therefor/Therefore

In general, I don't think that this statement is universally true. If it is a corporate
Remote Access scenario, then clients are administered by the same people
as gateways. Thus, I don't think that advising not use this extension
in Remote Access scenario due to the lack of trust to clients is always justified.

24. Section 7

   The SA_RESOURCE_INFO notify contains an optional data payload that
   can be used by the peer to identify the Child SA belonging to a
   specific resource.  The notify data SHOULD NOT be an identier that
   can be used to gain information about the hardware.  For example,
   using the CPU number itself as identier might give an attacker
   knowledge which packets are handled by which CPU ID and it might
   optimize a brute force attack against the system.

Isn't it protected by IKE SA? I believe this is only possible with MitM attack
(only initiator is affected) or when CRQC is used with RFC 8784 scenario for the initial SA.
In the first case the SA won't be created. The second case is currently unrealistic.
Am I missing something? If not, then I suggest to just describe this
threat giving no advices (i.e. remove SHOULD NOT), since it seems to be
theoretical threat. Just let the implementers know that this is possible.

Typo: s/identier/identifier (2 instances)

25. Section 9

   This document defines two new IKEv2 Notify Message Type payloads for
   the IANA "IKEv2 Notify Message Types - Status Types" registry.

This is a leftover from the previous version of the draft -
the current version defines only one status type notification.

26. Section 10

Why RFC2367 is a normative reference? Why one cannot implement this
specification without using PF_KEY? Actually, this seems to be
already done by Linux XFRM. From Section 8.1:

      Coverage:  Implements a general Child SA and per-CPU Child SAs.  It
      only supports the NETLINK API.  The PFKEYv2 API is not supported.

Regards,
Valery.

> This will start three week working group laste call for
> draft-ietf-ipsecme-multi-sa-performance. The working group last call
> will end at 2023-11-15.
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