Re: [Idr] ttRE: FW: New Version Notification for draft-ietf-idr-bgp-open-policy-22.txt

[Alexander Azimov <[e-mail]>]

Bruno,

CLUSTER_LIST is an optional non-transitive attribute. It makes quite
reasonable to have different processing for iBGP and eBGP.
OTC, is an optional transitive attribute with an effect on the routing
selection process. So, I don't feel that CLUSTER_LIST is a good reference.

The common way to use communities (RFC1997 or RFC8092) is an action upon
receipt from eBGP neighbor, optional additional marking, and action at the
export policy with another eBGP neighbor. In that form, there is much in
common with OTC and possible issues that we are discussing.

Let's go back to the side effects of accepting such malformed attributes
from iBGP neighbors while filtering them when received from eBGP neighbors.
Let's say we have a network with two routers R1 and R2 and a route
reflector (RR). Between R1 and R2 there are only P devices - common today
MPLS core.

R1 adds OTC at the receipt and due to bugs in the implementation, it has
incorrect length.
The RR detects it but passes by, while R2 is not upgraded and processes OTC
as an unknown transitive path attribute, just passing it to its eBGP
neighbor.
As a result, such routes may be filtered several hops away. And IMO it will
be quite hard to debug.

Another issue will happen if R2 does a proper check, but due to other BGP
path attributes or igp cost chooses the route with malformed attribute
among possible alternatives.
With the 'treat-as-withdraw' approach it has a chance to pick an
alternative route, with your current suggestion it will be limiting the
number of prefixes that are sent to its customers.

I see your point of getting rid of loops in the case of a network with
multiple IP lookups in the global table while the packet travels through
its core. But I'm not sure if this reason is enough to change error
handling cause it may bring undesirable side effects for modern design
network that uses tunneling. And as the same technique may apply to other
optional transitive attributes such as communities or large communities I
still believe that discussion should be done in terms of updating RFC7606.

ср, 16 февр. 2022 г. в 19:44, <[e-mail]>:

> Alexander,
>
>
>
> Please see inline [Bruno]
>
>
>
>
>
> Orange Restricted
>
> *From:* Alexander Azimov <[e-mail]>
> *Sent:* Wednesday, February 16, 2022 5:24 PM
> *To:* DECRAENE Bruno INNOV/NET <[e-mail]>
> *Cc:* Sriram, Kotikalapudi (Fed) <[e-mail]>;
> [e-mail]; Alvaro Retana (aretana) <[e-mail]>;
> [e-mail]; Susan Hares <[e-mail]>; Jeffrey Haas <
> [e-mail]>
> *Subject:* Re: ttRE: [Idr] FW: New Version Notification for
> draft-ietf-idr-bgp-open-policy-22.txt
>
>
>
> Bruno,
>
>
>
> Please see the quote from 7.8 of RFC7606 which I see as good example:
>
>
>
>    The error handling of [RFC1997 <https://datatracker.ietf.org/doc/html/rfc1997>] is revised as follows:
>
>
>
>    o  The Community attribute SHALL be considered malformed if its
>
>       length is not a non-zero multiple of 4.
>
>
>
>    o  An UPDATE message with a malformed Community attribute SHALL be
>
>       handled using the approach of "treat-as-withdraw".
>
>
>
> As you can see, there is no difference between processing communities with malformed length whether they are received thorugh eBGP or iBGP.
>
>
>
> [Bruno] Please see https://datatracker.ietf.org/doc/html/rfc7606#section-7.10 where there is a difference between eBGP and iBGP, both in term of error condition and action taken:
>
>
>
> The error handling of [RFC4456
> <https://datatracker.ietf.org/doc/html/rfc4456>] is revised as follows:
>
>
>
>    o  if the CLUSTER_LIST attribute is received from an external
>
>       neighbor, it SHALL be discarded using the approach of "attribute
>
>       discard"; or
>
>
>
>    o  if received from an internal neighbor, it SHALL be considered
>
>       malformed if its length is not a non-zero multiple of 4.  If
>
>       malformed, the UPDATE message SHALL be handled using the approach
>
>       of "treat-as-withdraw".
>
>
>
>
>
>
> And in the context of OTC inside AS one can say that OTC use case and
> community use case that are marking routes to prevent leaking is very much
> the same, do you agree?
>
> [Bruno] Not sure what your point is. I don’t agree in the general case:
>
> - community allows for very flexible usage, including having an effect on
> route selection or installation
>
> - OTC is defined to not be used inside iBGP hence is guaranteed to have no
> effect on route selection or installation within the AS
>
>
>
> And btw, getting back to your original example, how your proposal will
> help to prevent loops if for some reason the internal sessions are
> configured as eBGP with private ASNs?
>
> [Bruno] Not sure what you mean by “internal session configured as eBGP”. A
> priori, if you configure eBGP you have an AS boundary and you may
> filter/change preference of routes on this boundary. (without creating
> loops)
>
>
> Your last suggestion that restricts route advertisement in all directions
> including Customers may improve the situation but in the partial deploy
> scenario still may lead to hard to debug issues when routes with malformed
> attribute will travel beyond single administrative domain.
>
> [Bruno] I’m not sure to see your case:
>
> - if the ASBR supports OTC, the proposal would be to filter the route on
> all eBGP sessions hence the malformed attribute will not travel beyond
> single administrative domain
>
> - if the ASBR does not support OTC, by definition of optional transitive,
> it will propagate the route and the malformed attributes regardless of the
> error handling specified for OTC.
>
>
>
> --Bruno
>
>
>
> ср, 16 февр. 2022 г. в 17:18, <[e-mail]>:
>
> Hi Sriram,
>
>
>
> Please see inline [Bruno2]
>
>
>
>
>
> Orange Restricted
>
> *From:* Sriram, Kotikalapudi (Fed) <[e-mail]>
> *Sent:* Tuesday, February 15, 2022 11:52 PM
> *To:* DECRAENE Bruno INNOV/NET <[e-mail]>; Alexander
> Azimov <[e-mail]>; Jeffrey Haas <[e-mail]>
> *Cc:* [e-mail]; Alvaro Retana (aretana) <[e-mail]>;
> [e-mail]; Susan Hares <[e-mail]>
> *Subject:* RE: [Idr] FW: New Version Notification for
> draft-ietf-idr-bgp-open-policy-22.txt
>
>
>
> Hi Bruno,
>
>
>
> Comments inline marked with [Sriram].
>
>
>
> >[Bruno] - RFC7606 provides a guidance
> https://datatracker.ietf.org/doc/html/rfc7606#section-8
> <https://gcc02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdatatracker.ietf.org%2Fdoc%2Fhtml%2Frfc7606%23section-8&data=04%7C01%7Ckotikalapudi.sriram%40nist.gov%7Cb7a059d3cee145ca531c08d9f0a3c35f%7C2ab5d82fd8fa4797a93e054655c61dec%7C1%7C0%7C637805408436586543%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=Fsbz%2B8gTR8c0r94m4SoaamR4O%2BmHiMYH5tY9uRCXTHk%3D&reserved=0>
> . It’s only half a page so anyone interested could read. I don’t think it
> can be summarized as per you above sentence.
>
>
>
> [Sriram] Section 8 of RFC 7606 that you cite, says:
>
>
>
>    A document that specifies a new BGP attribute MUST provide specifics
>
>    regarding what constitutes an error for that attribute and how that
>
>    error is to be handled.  Allowable error-handling approaches are
>
>    detailed in Section 2.
>
>
>
> [Sriram] From the list of error-handling approaches detailed in Section 2,
> only treat-as-withdraw and attribute discard are relevant for us. Since the
> RFC says, “Allowable error-handling approaches are detailed in Section 2”,
> it does not allow for applying none of the approaches as an option.
>
>
>
> [Bruno2] First step that RFC7606 does is to define (i.e. choose) what
> constitute an error. We could define that length is not checked for IBGP
> because we don’t care about the content (or even the attribute).
> Alternatively, I’m also fine with attribute discard if we consider that the
> attribute is in error and hence could not be trusted.
>
>
>
> >[Bruno] In particular there is not such “SHALL be handled using the
> approach of "treat-as-withdraw".
>
>
>
> [Sriram] Regarding attribute discard, RFC 7606 says: “This approach MUST
> NOT be used except in the case of an attribute that has no effect on route
> selection or installation.” This implies: This approach MUST NOT be used in
> the case of an attribute that has an effect on route selection or
> installation. Do you agree? The route does propagate from iBGP (ingress) to
> eBGP (in the ASBR), where the attribute plays a role in route selection for
> propagation.
>
>
>
> [Bruno2] 1) For IBGP, OTC attribute has no effect on route selection and
> installation. 2) Since you seem to be calling for “treat-as-withdraw”, I
> would argue that “treat-as-withdraw” has also an effect on route selection
> or installation, and a bigger one. So it’s not really safer in term of side
> effect.
>
>
>
> >[Bruno] It also specifically calls for attention to optional transitive
> attributes (such as OTC) for which “the damage inflicted may be multiplied
> manyfold »
>
>
>
> [Sriram] The “damage inflicted may be multiplied manyfold” is about
> session reset. That is why the RFC urges the use of treas-as-withdraw or
> attribute discard instead.
>
> [Bruno2] It’s discussed in the context as session reset, but the cause is
> the optional transitive attribute. Cf following text which only refers to
> the type of attribute and not the action taken
>
>    although the problematic attributes may have
>
>    originated with a single update transmitted by a single BGP speaker,
>
>    by the time they encounter a router that checks them they may have
>
>    been replicated many times and thus may cause the reset of many
>
>    peering sessions.  Thus, the damage inflicted may be multiplied
>
>    manyfold.
>
>
>
>
>
>
>
> >[Bruno] Without double checking, I don’t recall that OTC is acted upon
> within IBGP. So in this case it does not affect routing selection. Let’s
> just specify to not even look at it (it’s optional) and problem is solved.
>
>
>
> [Sriram] iBGP and eBGP within an ASBR are not isolated operations. Please
> see my comment above.
>
> Not to look at OTC is not an option.
>
> [Bruno2] If this were true, we could not define the OTC as an optional
> transitive attribute, because essentially some routers will not look at
> OTC. Pick your option.
>
>
>
>
>
> A compliant router checks syntax errors in iBGP (just as in eBGP) based on
> its expectations of the OTC Attribute. If the length value does not
> match, the Attribute is malformed.
>
>
>
> [Sriram] If the OTC attribute is malformed for any reason, we cannot be
> sure that even the Attribute type value is correct (or as intended).
>
> [Bruno2] 1) That’s not the path chosen by RFC 7606 in case of a length
> error. 2) I would find debatable to assume an error on a field (type) when
> you don’t see an error on this field. TLV construct provides hierarchy: I
> agree that what is inside the TLV (i.e. after/below the error) can’t be
> trusted; but for what is before the error, there is little to no reason to
> assume an error (and if we do, we could imagine anything and in particular
> an attribute for which the error handling is “session reset”.)
>
>
>
> [Sriram] Then must it be propagated from iBGP to eBGP (in the ASBR) and
> then to eBGP neighbors?
>
> [Bruno2] Current text proposes to _*not*_ propagate to eBGP neighbors
> except for Customer. If length is wrong, I would be fine to also not
> propagate to Customerif you find this better.
>
>
>
> Thanks for the discussion
>
> --Bruno
>
>
>
> Thanks.
>
>
>
> Sriram
>
>
>
>
>
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> --
>
> Best regards,
>
> Alexander Azimov
>
> _________________________________________________________________________________________________________________________
>
> Ce message et ses pieces jointes peuvent contenir des informations confidentielles ou privilegiees et ne doivent donc
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> a l'expediteur et le detruire ainsi que les pieces jointes. Les messages electroniques etant susceptibles d'alteration,
> Orange decline toute responsabilite si ce message a ete altere, deforme ou falsifie. Merci.
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-- 
Best regards,
Alexander Azimov