Re: [bess] Benjamin Kaduk's No Objection on draft-ietf-bess-evpn-proxy-arp-nd-11: (with COMMENT)

["Rabadan, Jorge (Nokia - US/Mountain View)" <[e-mail]>]

Hi Benjamin,



Thanks for the thorough review.

Please see my comments in-line with [jorge]. I’ll incorporate the changes in the next version.



Thx

Jorge





-------------------------------------------------------------------------------------

From: Benjamin Kaduk via Datatracker <[e-mail]>

Date: Wednesday, January 20, 2021 at 7:51 AM

To: The IESG <[e-mail]>

Cc: [e-mail] <[e-mail]>, [e-mail] <[e-mail]>, [e-mail] <[e-mail]>, Bocci, Matthew (Nokia - GB) <[e-mail]>, Bocci, Matthew (Nokia - GB) <[e-mail]>

Subject: Benjamin Kaduk's No Objection on draft-ietf-bess-evpn-proxy-arp-nd-11: (with COMMENT)

Benjamin Kaduk has entered the following ballot position for

draft-ietf-bess-evpn-proxy-arp-nd-11: No Objection



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COMMENT:

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Thanks for this clear and well-written document; the effort that went

into presenting the information really comes through.  I basically just

have editorial nits as comments, with a few substantive notes on the

security considerations section.

[jorge] thanks!



For most of the document I was convinced that I understood this point,

but then Section 5.5 led me to question myself: in the "static

provisioning" learning mode, is the IP->MAC mapping configured only at

the PE that the CE using those addresses will attach to, or at all PEs

in the BD?  I can follow up out-of-band with some editorial suggestions

either way.

[jorge] I added this in section 3.1 to clarify, let me know if you prefer different wording:

“Static entries are provisioned from the management plane. A static entry is configured on the PE attached to the host using the IP address in that entry”



Abstract (and Introduction)



   This document describes the EVPN Proxy-ARP/ND function, augmented by

   the capability of the ARP/ND Extended Community

   [I-D.ietf-bess-evpn-na-flags].  From that perspective this document

   updates [RFC7432].  [...]



nit: this paragraph doesn't do very much to tell me what the nature of

the update is.  If it's just to clarify how all the pieces fit together

we might add a clause at the end ", to provide more comprehensive

documentation of the operation of the system as a whole" or similar.

(Some parts of it might also have worked as an RFC 2026 Applicability

Statement, but it is probably not worth the trouble of trying to rework

things at this stage, especially since what we have is already nicely

laid out.)

[jorge] ok, I added:

“From that perspective this document updates RFC7432 to provide more comprehensive documentation of the operation of the Proxy-ARP/ND function”





Section 3



There's a lot of information packed into the Figure, and the surrounding

text does a great job describing it.  Thank you!



   The Proxy-ARP/ND function can be structured in six sub-functions or

   procedures:



(editorial) the text from here to the end of the section feels distinct

from the explanation of the figure; it might benefit from getting

promoted into a (sub)section of its own.

[jorge] done! Thanks.





Section 3.1



   An entry MAY associate a configured static IP to a list of potential

   MACs, i.e. IP1->(MAC1,MAC2..MACN).  When there is more than one MAC

   in the list of allowed MACs, the PE will not advertise any IP->MAC in

   EVPN until a local ARP/NA message or any other frame is received from

   the CE.  [...]



(nit) would it be better to phrase this as "until a frame (including

local ARP/NA message) is received from the CE"?  That seems to emphasize

that any traffic will do, even if we expect that traffic to be ARP/NA.

[jorge] done.









Section 3.1.1



   o  Hosts build a Default Router List based on the received RAs and

      NAs with R Flag=1.  Each cache entry has an IsRouter flag, which

      must be set based on the R Flag in the received NAs.  A host can



nit: maybe "must be set for received RAs and is set based on the R flag

[...]"

[jorge] added: “Each cache entry has an IsRouter flag, which must be set for received RAs and is set based on the R flag in the received NAs”



Section 3.6



   The distributed nature of EVPN and Proxy-ARP/ND allows the easy

   detection of duplicated IPs in the network, in a similar way to the

   MAC duplication function supported by [RFC7432] for MAC addresses.



nit: is this "MAC duplication detection function"?

[jorge] yes, added “detection”



       IP1->MAC2 in the Proxy-ARP/ND table.  Static IP->MAC entries,

       that is, locally provisioned or EVPN-learned entries (with I=1 in

       the ARP/ND Extended Community), are not subject to this

       procedure.  [...]



nit: I think the sentence is better without the parentheses, since the

presence of I=1 is critical for correct functioning and not intrinsic to

the entries being EVPN-learned.

[jorge] good point, removed.





       1.  The entry in duplicate detected state cannot be updated with

           new dynamic or EVPN-learned entries for the same IP.  The

           operator MAY override the entry though with a static IP->MAC.



nit: commas before and after "though".

[jorge] added



       2.  The PE SHOULD alert the operator and stop responding ARP/NS

           for the duplicate IP until a corrective action is taken.



nit: "stop responding to".

[jorge] fixed



           for IP1.  Since the AS-MAC is a managed MAC address known by

           all the PEs in the EVI, all the PEs MAY apply filters to drop



nit: this seems to be the first time that we talk about the AS-MAC being

a managed address and being known to all PEs in the EVI; it might be

worth rewording in light of that or mentioning that in the definition of

AS-MAC.

[jorge] added this in the definition: “AS-MAC: Anti-spoofing MAC. It is a especial MAC configured on all the PEs attached to the same BD and used for the Duplicate IP Detection procedures.”



Section 5.2



   This scenario minimizes flooding while enabling dynamic learning of

   IP->MAC entries.  The Proxy-ARP/ND function is enabled in the BDs of

   the EVPN PEs, so that the PEs intercept and respond to CE requests.



nit: from context it seems like the "dynamic learning" here refers to

the EVPN-learned entries, but in §3.1 we reserved the term "dynamic" for

entries learned by local snooping.  Since the next paragraph talks about

snooping as an optional addition, we run into semi-conflicting usage of

the term "dynamic".  I would suggest (assuming the above is correct)

rewording this to "while enabling learning of IP->MAC entries over the

EVPN" or similar.

[jorge] you are right that 5.2 is confusing. I changed it to:

“This scenario minimizes flooding while enabling dynamic learning of IP->MAC entries. The Proxy-ARP/ND function is enabled in the BDs of the EVPN PEs, so that the PEs snoop ARP/ND messages issued by the CEs and respond to CE ARP-requests/NS messages.

PEs will flood requests if the entry is not in their Proxy table. Any unknown source MAC->IP entries will be learned and advertised in EVPN, and traffic to unknown entries is discarded at the ingress PE.”







Section 5.5



                         These rules are often called port security.

   Port security summarizes different operational steps that limit the

   access to the IXP-LAN, to the customer router and controls the kind

   of traffic that the routers are allowed to exchange (e.g., Ethernet,



nit: this list lacks parallel structure; going with "limit the access to

the IXP-LAN and the customer router, and controls the kind of traffic"

would be okay.

[jorge] done.





Section 6



I think it would be useful to reiterate that the security considerations

of RFC 7432 and draft-ietf-bess-evpn-na-flags continue to apply (in

addition to the useful text that is already present here).  I guess ARP

and IPv6 ND are arguably also applicable (AFAICT the security properties

of the proxied scheme are very similar to those of native usage, in an

environment that already has to trust the PEs and provider network that

supply the EVPN to the same extent that one would otherwise trust an IP

router).

[jorge] agreed. Added.





It would also be my personal preference (though I do not insist upon it)

to note that EVPN does not inherently provide cryptographic protection

(including confidentiality protection) despite the word "private"

appearing in the name.  (This is really a topic that should be addressed

via a long-term IETF-wide shift towards just "virtual network" instead

of "virtual private network", but I try to mention it when I can so as

to socialize the idea.)

[jorge] that sounds good to me. Added: “Note that EVPN does not inherently provide cryptographic protection (including confidentiality protection).”





I appreciate the discussion (earlier in the document) of the use of

dummy MACs to suppress unknown ARP-Request/NS flooding, added in

response to the opsdir review.  Is it worth calling out the

security/availability considerations of that technique from this

section?  ("No" is a perfectly fine answer.)

[jorge] not sure what you mean by the use of “dummy MACs”?



Is it too banal to repeat that configuring the unicast-forwarding and/or

flooding sub-functions to be disabled risks blocking service for a CE if

the static configuration is broken?

[jorge] sure, I added: “While the unicast-forward and/or flooding suppression sub-functions provide an added security mechanism for the BD, they can also increase the risk of blocking the service for a CE if the EVPN PEs cannot provide the ARP/ND resolution that the CE needs.”



   The solution also provides protection against Denial Of Service

   attacks that use ARP/ND-spoofing as a first step.  [...]



Are these DoS attacks described anywhere that we might reference for

further reading?  ("No" is a perfectly fine answer.)

[jorge] It’s a generic statement, I fail to know what references to provide. If you have suggestions I’d be glad to add them.





   When EVPN and its associated Proxy-ARP/ND function are used in IXP

   networks, they provide ARP/ND security and mitigation.  IXPs MUST



If I understand correctly, this security/mitigation is provided in the

face of malicious CE devices, but the system still requires that PEs are

trusted and does not provide cryptographic or independently verifiable

assurances of correct IP->MAC bindings.  I would suggest being explicit

about the threat that is being protected against, since by itself

the term "security" is so vague so as to become almost meaningless.

[jorge] sure, I added: “When EVPN and its associated Proxy-ARP/ND function are used in IXP networks, they provide ARP/ND security and mitigation against attacks from malicious CEs”



   For example, IXPs should disable all unneeded control protocols, and

   block unwanted protocols from CEs so that only IPv4, ARP and IPv6



I suggest the parenthetical "so that (for example) only"; we do not

really have much reason to preclude other ethertypes if desired by the

IXP.

[jorge] added.