Re: 6man w.g. last call for <draft-ietf-6man-segment-routing-header-11.txt>

["Joel M. Halpern" <[e-mail]>]

I responded a bit too quickly to Tom's note.
I agree with the later parts of his note.  It is the ECMP behavior of 
intermediate routers that cause me concern.
Whether his proposed fix is the right one or not is unclear, as the blog 
post I referred to suggested more issues.  (And I apologize for being 
unable to find a pointer to the blog post.)

Yours,
Joel

On 3/30/18 12:29 PM, Joel M. Halpern wrote:
> Sorry for being a bit vague.  The issue with packet reordering when 
> using flowlabel for ECMP was discussed in a blog post from a senior 
> engineer (at a company other than my own) who as far as I know does not 
> participate in the IETF.  He went through a number of issues in that.
> 
> There are likely ways to work around the problems.  But since the draft 
> does not even refer to the issue, there is a serious gap.
> 
> Yours,
> Joel
> 
> On 3/30/18 11:59 AM, Tom Herbert wrote:
>> On Fri, Mar 30, 2018 at 7:51 AM, Joel M. Halpern <[e-mail]> 
>> wrote:
>>> I do not think this document is ready to be sent to the IETF and IESG 
>>> for
>>> final approval.
>>> There are several kinds of problems.
>>>
>>> ECMP1: The document asserts that entropy information is put into the 
>>> flow
>>> label.   I wish this worked.  I helped bring this idea forward years 
>>> ago,
>>> independent of SRH.  Unfortunately, there are two related problems.  
>>> First,
>>> adoption appears to be low.  Second, and more important, there are 
>>> reports
>>> from the field that doing thi actually breaks other things and cause 
>>> packet
>>> re-ordering under some circumstances.  As a result, vendors are 
>>> suggesting
>>> that people turn it off even when it is available.
>>
>> Joel,
>>
>> I assume you're referring to:
>>
>> "If an array of layer-3 adjacencies is bound to the End.X SID, then
>> one entry of the array is selected based on a hash of the packet's
>> header (at least SA, DA, Flow Label)."
>>
>> The problem with flow label isn't that it can cause OOO packets, it's
>> interaction with stateful techniques in intermediate nodes like
>> firewalls and load balancer that require all packets for a flow to
>> consistently hit the same stateful device. When the flow label changes
>> in mid flow, packets might be routed to a different device that
>> doesn't have the flow state and hence are dropped.
>>
>> I imagine that one could argue that SRH is intended to be used in
>> closed domains so the operator could ensure there are no stateful
>> nodes present that would have an issue, but that makes correct
>> operation of the protocol conditional.
>>
>> IMO the fix for the flow label/ECMP problem needs to include an update
>> to RFC6437 specifying 1) the flow label can only be set by the source,
>> nodes in the path MUST NOT set it, 2) if the flow label is set, it
>> MUST be an entropy identifier of the encapsulated transport flow so
>> that (SA, DA, flow label) is a good tuple representation of the flow,
>> 3) the flow label MUST be the same for all packets sent on a flow
>> including fragments.
>>
>> Tom
>>
>>> ECMP2: If, with suitable analysis, we decide that this is actually 
>>> safe to
>>> do, then the document needs to include both that analysis and clearly
>>> spelled out requirements that operators MUST verify that all their 
>>> routers
>>> support this behavior and that they have enabled this behavior before
>>> turning on SRv6 usage.
>>>
>>> Structure: The text in section 2.3 says "It is assumed in this 
>>> document that
>>> the SRH is added to the packet by its source".  This is at best
>>> disingenuous.  It is very clear that the value of this behavior lies
>>> primarily in use by the network, not by the packet sources.  Claiming
>>> otherwise results in a document with minimal utility.  Even this 
>>> document
>>> itself disagrees with this assertion.  Tucked into the very next 
>>> section is
>>> text saying that "outer header with an SRH applied to the incoming 
>>> packet".
>>> If this behavior were a clearly spelled out requirement, rather than a
>>> "typically" and if the text in 2.3 were replaced with something 
>>> realistic,
>>> then the document would at least be itnernally consistent and match the
>>> expected usages.
>>>
>>> Edge filtering: The text on edge filtering does not actually state that
>>> prevention of packets with SRH and a current DA of an internal node is
>>> mandatory.  Unless it is clearly stated, the security considerations 
>>> text as
>>> currently written is significantly weakened.  If it is mandatory, 
>>> then again
>>> the deployment section needs to note that an operator needs to verify 
>>> that
>>> all of his edge devices support such filtering and have it properly 
>>> enabled
>>> in order to use SRv6.
>>>
>>> Edge Filtering and hybrids: Other documents have talked about allowing
>>> external packets with SRv6 entries pointing to internal nodes (which 
>>> means
>>> the DA upon arrival at the operator edge will be an internal node as I
>>> understand it).  It the intention is to permit that with appropriate
>>> security, then the edge filtering requirements need to be clear about 
>>> the
>>> requirements for cryptographic validation at the edge.
>>>
>>> Yours,
>>> Joel
>>>
>>>
>>> On 3/29/18 4:30 PM, Bob Hinden wrote:
>>>>
>>>>
>>>> This message starts a two week 6MAN Working Group Last Call on 
>>>> advancing:
>>>>
>>>>          Title           : IPv6 Segment Routing Header (SRH)
>>>>          Authors         : Stefano Previdi
>>>>                            Clarence Filsfils
>>>>                            John Leddy
>>>>                            Satoru Matsushima
>>>>                            Daniel Voyer
>>>>          Filename       : draft-ietf-6man-segment-routing-header-11.txt
>>>>          Pages          : 34
>>>>          Date           : 2018-03-28
>>>>
>>>>        
>>>> https://tools.ietf.org/html/draft-ietf-6man-segment-routing-header
>>>>
>>>> as a Proposed Standard.  Substantive comments and statements of support
>>>> for publishing this document should be directed to the mailing list.
>>>> Editorial suggestions can be sent to the author.  This last call will
>>>> end on 12 April 2018.
>>>>
>>>> An issue tracker will be setup to track issues raised on this document.
>>>>
>>>> Thanks,
>>>> Bob & Ole
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
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