Re: [spring] Is srv6 PSP a good idea

"Darren Dukes (ddukes)" <ddukes@cisco.com> Thu, 27 February 2020 21:11 UTC

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From: "Darren Dukes (ddukes)" <ddukes@cisco.com>
To: Mark Smith <markzzzsmith@gmail.com>
CC: "Xiejingrong (Jingrong)" <xiejingrong@huawei.com>, Ron Bonica <rbonica@juniper.net>, "spring@ietf.org" <spring@ietf.org>, "Joel M. Halpern" <jmh@joelhalpern.com>, "Pablo Camarillo (pcamaril)" <pcamaril@cisco.com>
Thread-Topic: [spring] Is srv6 PSP a good idea
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Date: Thu, 27 Feb 2020 21:11:00 +0000
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Subject: Re: [spring] Is srv6 PSP a good idea
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Mark AH is not defined for SRH.  There is no specification to ignore.

Recall RFC8200 says nothing about mutability, it only describes things that are
modified in flight and how/why.

RFC4301 builds on RFC8200 to define mutability for use in AH ICV computation
without having to modify RFC8200.  “Mutability" is not defined for IPv6 headers
for any other reason.

draft-ietf-6man-semgnet-routing-header builds on RFC8200 to define a new RH
type without modifying RFC8200.

draft-ietf-spring-srv6-network-programming builds on RFC8200 and
draft-ietf-6man-segment-routing-header to define useful SIDs, their behaviors
and signaling within an SR domain without modifying any of the base specifications.

Of course RFC8200 has text covering the validity of extension header insertion,
deletion and processing at node(s) identified in the IPv6 destination address (Section 4).
As we all know, routing headers can’t work if a segment endpoint can’t
process extension headers.  Using this fact, PSP builds on RFC8200 to
offer new functionality within an SR Domain.

Of course RFC4301 says AH processing for new extension headers are outside its
scope, one can define AH processing for SRH with knowledge of, and building on,
all the above. Or they can just use ESP.

There is no ignoring here.
There is conscious and careful building on existing standards as they are written.

Darren


On Feb 26, 2020, at 6:57 AM, Mark Smith <markzzzsmith@gmail.com<mailto:markzzzsmith@gmail.com>> wrote:



On Wed, 26 Feb 2020, 20:50 Xiejingrong (Jingrong), <xiejingrong@huawei.com<mailto:xiejingrong@huawei.com>> wrote:
Hi Mark,
I think both AH and PSP are optional.
If AH is desired to deploy, then the operator can choose not to use PSP.
If AH is not deployed, and the operator has its requirements of incremental-deployment, then the operator can choose to use PSP.

You've missed the point. AH is an assurance mechanism for something that should *already* be happening - processing packets in accordance with RFC 8200.

Somebody choosing not to use AH doesn't mean SPRING can ignore the IPv6 specifications.



If the already deployed PSP is removed from the draft, then I think it's backward for interoperation.

RFC4301:
3.2.  How IPsec Works

   IPsec uses two protocols to provide traffic security services --
   Authentication Header (AH) and Encapsulating Security Payload (ESP).
   Both protocols are described in detail in their respective RFCs
   [Ken05b, Ken05a].  IPsec implementations MUST support ESP and MAY
   support AH. (Support for AH has been downgraded to MAY because
   experience has shown that there are very few contexts in which ESP
   cannot provide the requisite security services.  Note that ESP can be
   used to provide only integrity, without confidentiality, making it
   comparable to AH in most contexts.)

Thanks,
Jingrong

-----Original Message-----
From: spring [mailto:spring-bounces@ietf.org<mailto:spring-bounces@ietf.org>] On Behalf Of Mark Smith
Sent: Wednesday, February 26, 2020 8:31 AM
To: Pablo Camarillo (pcamaril) <pcamaril@cisco.com<mailto:pcamaril@cisco.com>>
Cc: Ron Bonica <rbonica@juniper.net<mailto:rbonica@juniper.net>>; spring@ietf.org<mailto:spring@ietf.org>; Joel M. Halpern <jmh@joelhalpern.com<mailto:jmh@joelhalpern.com>>
Subject: Re: [spring] Is srv6 PSP a good idea

Hi Pablo,

On Sat, 21 Dec 2019 at 04:38, Pablo Camarillo (pcamaril) <pcamaril@cisco.com<mailto:pcamaril@cisco.com>> wrote:
>
>  Hi Ron,
>
> I guess we are making some progress here but going in some circles. So far we have moved from “this violates RFC8200” to “there are no use-cases or benefits” to “this is complex for an ASIC” to “what is the benefit again” and now back to “this is complex for an ASIC”.
>

As far as I know, the next header field in both the IPv6 fixed header and in extension headers is immutable while the packet is travelling within the network, as is the payload length field in the IPv6 base fixed header.

Nothing in RFC 8200 modifies those fields while the packet is in flight between the packet's original source and final destination, nor is there anything in RFC 8200 that specifies how to do those modifications and any other discussion about the consequences and considerations when doing so.


The IPsec AH header is providing integrity checking over fields in the
IPv6 packet that shouldn't be being modified while the packet is within the network. What is and isn't protected by AH can be seen to be a specification of what processing on a packet can and cannot occur while it is in flight across the network towards its final destination.

Therefore, AH is really a quality assurance mechanism for the network packet processing that RFC 8200 specifies. The processing of the packet by the network with or without the use of AH should be the same. AH should really only be necessary if there is a belief that the network is likely not to be processing packets in accordance to RFC 8200, either accidentally or intentionally.


Per RFC4302 (IPsec AH),  the following section explicitly states which fields in the IPv6 header are immutable and are to be protected by AH:


3.3.3.1.2.  ICV Computation for IPv6

3.3.3.1.2.1.  Base Header Fields

   The IPv6 base header fields are classified as follows:

Immutable
           Version
           Payload Length
           Next Header
           Source Address
           Destination Address (without Routing Extension Header)

   Mutable but predictable
           Destination Address (with Routing Extension Header)

   Mutable (zeroed prior to ICV calculation)
           DSCP (6 bits, see RFC2474 [NBBB98])
           ECN (2 bits, see RFC3168 [RFB01])
           Flow Label (*)
           Hop Limit

        (*) The flow label described in AHv1 was mutable, and in
            RFC 2460 [DH98] was potentially mutable.  To retain
            compatibility with existing AH implementations, the
            flow label is not included in the ICV in AHv2.




> As for how easy or not something is, the PSP behavior has been implemented and deployed (running code). The use-cases have been described and positively reinforced by operators. I don't think there is any further explanation to provide.
>

"Just because you can do something, doesn't mean you should."

How much troubleshooting experience have they had with this?

I think a very important factor is how easy something is to troubleshoot - how obvious is the mechanism works; is the mechanism consistent with existing behaviours i.e. the principle of least surprise (removing EHs at an intermediary hop certainly isn't in IPv6, even if the intermediary hop as the packet's current DA); when it inevitably fails, how obvious is it where the fault is likely to be; and how quickly can a typical fault be rectified.



Regards,
Mark.




> Happy Holidays,
> Pablo.
>
>
> -----Original Message-----
> From: Ron Bonica <rbonica@juniper.net<mailto:rbonica@juniper.net>>
> Date: Tuesday, 17 December 2019 at 16:06
> To: "Pablo Camarillo (pcamaril)" <pcamaril@cisco.com<mailto:pcamaril@cisco.com>>, "Joel M.
> Halpern" <jmh@joelhalpern.com<mailto:jmh@joelhalpern.com>>, "spring@ietf.org<mailto:spring@ietf.org>" <spring@ietf.org<mailto:spring@ietf.org>>
> Subject: RE: [spring] Is srv6 PSP a good idea
>
>     Pablo,
>
>     In your message below, are you arguing that it is easier for the penultimate node to remove the SRH than it is for the ultimate node to ignore it? I think that would be a stretch.
>
>
> Ron
>
>
>
>     Juniper Business Use Only
>
>     -----Original Message-----
>     From: Pablo Camarillo (pcamaril) <pcamaril@cisco.com<mailto:pcamaril@cisco.com>>
>     Sent: Saturday, December 14, 2019 4:50 AM
>     To: Ron Bonica <rbonica@juniper.net<mailto:rbonica@juniper.net>>; Joel M. Halpern <jmh@joelhalpern.com<mailto:jmh@joelhalpern.com>>; spring@ietf.org<mailto:spring@ietf.org>
>     Subject: Re: [spring] Is srv6 PSP a good idea
>
>     Ron,
>
>     What is the "price paid by the penultimate segment"? All the current implementations do this at linerate with no performance degradation as I have explained in my email before.
>
>     There is substantial benefit. Four operators have deployed PSP, which proves the benefit.
>     It enables new use-cases that have been provided by other members in the list. [1], [2] and [5].
>     From operational perspective it is not complex as explained in [3].
>     Operators have expressed their value in [4] and [5]..
>
>     [1].- https://urldefense.com/v3/__https://mailarchive.ietf.org/arch/msg/spring/wTLJQkzC6xwSNPbhB84VH0mLXx0__;!!NEt6yMaO-gk!TzJ8_ZyDvWvLPNwsalQ6RiBzoLkP6Vj30eGaDVFEWdDq_IdPkWwaIL4IcdXeBzk_$
>     [2].- https://urldefense.com/v3/__https://mailarchive.ietf.org/arch/msg/spring/V0ZpjVLSVZxHaBwecXFxqJjlg_c__;!!NEt6yMaO-gk!TzJ8_ZyDvWvLPNwsalQ6RiBzoLkP6Vj30eGaDVFEWdDq_IdPkWwaIL4IcU9bihBc$
>     [3].- https://urldefense.com/v3/__https://mailarchive.ietf.org/arch/msg/spring/ssobwemrPz0uEZjvRCZP1e4l_l0__;!!NEt6yMaO-gk!TzJ8_ZyDvWvLPNwsalQ6RiBzoLkP6Vj30eGaDVFEWdDq_IdPkWwaIL4Icc_wo902$
>     [4].- https://urldefense.com/v3/__https://mailarchive.ietf.org/arch/msg/spring/KXCBHT8Tpy17S5BsJXLBS35yZbk__;!!NEt6yMaO-gk!TzJ8_ZyDvWvLPNwsalQ6RiBzoLkP6Vj30eGaDVFEWdDq_IdPkWwaIL4IcRXo_q-1$
>     [5].-
> https://urldefense.com/v3/__https://mailarchive.ietf.org/arch/msg/spri
> ng/ErcErN39RIlzkL5SKNVAeEWpnAI__;!!NEt6yMaO-gk!TzJ8_ZyDvWvLPNwsalQ6RiB
> zoLkP6Vj30eGaDVFEWdDq_IdPkWwaIL4IceGPpSab$
>
>     Cheers,
>     Pablo.
>
>     -----Original Message-----
>     From: Ron Bonica <rbonica@juniper.net<mailto:rbonica@juniper.net>>
>     Date: Thursday, 12 December 2019 at 21:50
>     To: "Pablo Camarillo (pcamaril)" <pcamaril@cisco.com<mailto:pcamaril@cisco.com>>, "Joel M. Halpern" <jmh@joelhalpern.com<mailto:jmh@joelhalpern.com>>, "spring@ietf.org<mailto:spring@ietf.org>" <spring@ietf.org<mailto:spring@ietf.org>>
>     Subject: RE: [spring] Is srv6 PSP a good idea
>
>         Pablo,
>
>         I am not convinced the benefit derived by the ultimate segment
> justifies the price paid by the penultimate segment. Specifically,
>
>         - the ultimate segment benefits because it doesn't have to skip over the SRH with SL == 0
>         - in order for the ultimate segment to derive this benefit,
> the penultimate segment needs to remove bytes from the middle of the
> packet and update two fields in the IPv6 header
>
>         As Joel said, we typically don't add options (i.e., complexity) to a specification unless there is substantial benefit.
>
>                                                             Ron
>
>
>
>
>         Juniper Business Use Only
>
>         -----Original Message-----
>         From: spring <spring-bounces@ietf.org<mailto:spring-bounces@ietf.org>> On Behalf Of Pablo Camarillo (pcamaril)
>         Sent: Wednesday, December 11, 2019 3:12 PM
>         To: Joel M. Halpern <jmh@joelhalpern.com<mailto:jmh@joelhalpern.com>>; spring@ietf.org<mailto:spring@ietf.org>
>         Subject: Re: [spring] Is srv6 PSP a good idea
>
>         Joel,
>
>         1.- The use-case for PSP has already been provided at the mailer. There are scenarios where it provides benefits to operators.
>
>         2.- The PSP behavior is optional. It is up to the operator in his deployment to decide whether to enable it or not at one particular router.
>         Similarly, a vendor may decide not to implement it. The PSP behavior has been implemented by several vendors and deployed (see the srv6 deployment draft).
>
>         3.- A network may have PSP enabled at some nodes and not at others.  Everything is still interoperable and works fine.
>
>         4.- PSP is not a complex operation in hardware (doable at linerate on existing merchant silicon).
>         Example: It has been implemented and deployed on Broadcom J/J+. If I recall correctly Broadcom Jericho+ started shipping in March 2016! PSP is supported on this platform at linerate with no performance degradation (neither PPPS nor BW).
>         Given that this is doable in a platform from more than 3 years ago, I fail to see how you need "very special provision" to do this.
>
>         Is it really something that horrible to provide freedom of choice to the operators deploying?
>
>         In summary, it can be implemented without any burden in hardware and deployment experience prove this is beneficial to operators.
>
>         Thanks,
>         Pablo.
>
>         -----Original Message-----
>         From: spring <spring-bounces@ietf.org<mailto:spring-bounces@ietf.org>> on behalf of "Joel M. Halpern" <jmh@joelhalpern.com<mailto:jmh@joelhalpern.com>>
>         Date: Wednesday, 11 December 2019 at 03:55
>         To: "spring@ietf.org<mailto:spring@ietf.org>" <spring@ietf.org<mailto:spring@ietf.org>>
>         Subject: [spring] Is srv6 PSP a good idea
>
>             For purposes of this thread, even if you think PSP violates RFC 8200,
>             let us assume that it is legal.
>
>             As I understand it, the PSP situation is:
>             o the packet arrives at the place (let's not argue about whether SIDs
>             are locators) identified by the SID in the destination address field
>             o that SID is the next to last SID in the SID list
>             o that sid is marked as / known to be PSP
>             o at the intended place in the processing pseudocode, the last (first)
>             entry in the SRH is copied into the destination IPv6 address field of
>             the packet
>             -> The SRH being used is then removed from the packet.
>
>             In order to evaluate whether this is a good idea, we have to have some
>             idea of the benefit.  It may be that I am missing some of the benefit,
>             and I would appreciate clarification.
>             As far as I can tell, the benefit of this removal is that in exchange
>             for this node doing the work of removing the SRH, the final node in the
>             SRH does not have to process the SRH at all, as it has been removed.
>
>             I have trouble seeing how that work tradeoff can be beneficial.
>             Removing bytes from the middle of a packet is a complex operation.
>             Doing so in Silicon (we expect this to be done in the fast path of
>             significant forwarders as I understand it) requires very special
>             provision.  Even in software, removing bytes from the middle of a packet
>             requires somewhere between some and a lot of extra work.  It is
>             distinctly NOT free.
>
>             In contrast, we have assumed that the work of processing SRH itself is
>             tractable, since otherwise all of SRv6 would be problematic.  So why is
>             this necessary.
>
>             Yours,
>             Joel
>
>             PS: Note that both the MPLS case and the encapsulation case are very
>             different in that the material being removed is at the front of the IP
>             packet.  Pop or prepend are MUCH easier than middle-removal (or
>             middle-insertion).
>
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