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What if? [was Re: Extension Header Insertion]

Brian E Carpenter <brian.e.carpenter@gmail.com> Tue, 10 December 2019 01:21 UTC

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Subject: What if? [was Re: Extension Header Insertion]
To: adrian@olddog.co.uk, 'Ron Bonica' <rbonica=40juniper.net@dmarc.ietf.org>, '6man' <6man@ietf.org>
References: <BN7PR05MB5699D9BA988F96E2F41CD390AE580@BN7PR05MB5699.namprd05.prod.outlook.com> <00dc01d5ae73$c361b450$4a251cf0$@olddog.co.uk> <dbcdeb1a-0091-da2b-20df-d991e6c06091@gmail.com>
From: Brian E Carpenter <brian.e.carpenter@gmail.com>
Organization: University of Auckland
Message-ID: <9bc47200-4fea-37ce-0ede-cbf6a5f70ea9@gmail.com>
Date: Tue, 10 Dec 2019 14:21:06 +1300
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So, let's assume that two consecutive SRH headers are allowed in the same packet.

So the first one (an example from draft-ietf-6man-segment-routing-header-26) is:

       Segments Left=2
       Last Entry=2
       Flags=0
       Tag=0
       Segment List[0]=S3
       Segment List[1]=S2
       Segment List[2]=S1

and the second one is

       Segments Left=1
       Last Entry=1
       Flags=0
       Tag=0
       Segment List[0]=S4
       Segment List[1]=S5

I made that up and it's obviously nonsense, but if this is allowed why aren't the rules for processing conflicting SRHs described in draft-ietf-6man-segment-routing-header-26? Do we need to recall it from the RFC Editor queue to be fixed?

Regards
   Brian Carpenter

On 10-Dec-19 14:02, Brian E Carpenter wrote:
> On 09-Dec-19 22:33, Adrian Farrel wrote:
>> Hi Ron,
>>
>> I think we can jump to a quick answer on this because draft-ietf-spring-srv6-network-programming-05 says:
>>
>>    We assume that the SRH may
>>    be present multiple times inside each packet.
>>
>> Thus we may assume that the proponents of Extension Header insertion do think that it is acceptable to insert a second routing header into a packet that already has one.
>>
>> And 8200 is clear when it says:
>>    Each extension header should occur at most once, except for the
>>    Destination Options header, which should occur at most twice (once
>>    before a Routing header and once before the upper-layer header).
> 
> That's "should", which in a non-RFC2119 document like RFC 8200, means "should".
> It's not "must". So while I would prefer that the relevant SRH document justifies
> the exception, there isn't a breach of a mandatory requirement.
>   
>> So draft-ietf-spring-srv6-network-programming-05 includes a false assumption which need to be either removed or secured through an update to 8200.
>>  
>> Ideally, I suppose, draft-ietf-6man-segment-routing-header would have contained the clarification that the SRH could be present multiple times 
> 
> Yes
> 
>> (updating 8200 as it went).
> 
> Unnecessary, IMHO.
> 
>     Brian
> 
>>
>>  
>>
>> Cheers,
>>
>> Adrian
>>
>>  
>>
>> *From:*ipv6 <ipv6-bounces@ietf.org> *On Behalf Of *Ron Bonica
>> *Sent:* 09 December 2019 03:04
>> *To:* 6man <6man@ietf.org>
>> *Subject:* Extension Header Insertion
>>
>>  
>>
>> Folks,
>>
>>  
>>
>> This question is posed primarily to the proponents of Extension Header insertion.
>>
>>  
>>
>> Do you think that it is acceptable to insert a second routing header into a packet that already has one, so the resulting packet looks like the following:
>>
>>  
>>
>>   * IPv6 header
>>   * SRH
>>   * SRH
>>   * Upper-layer header
>>
>>  
>>
>> Would this be common in TI-LFA?
>>
>>  
>>
>>                                                                       Ron
>>
>>  
>>
>>  
>>
>> Juniper Business Use Only
>>
>>
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>

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