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Re: [mpls] [bier] The first nibble issue associated with MPLS encapsulation

Eric C Rosen <erosen@juniper.net> Wed, 13 April 2016 14:48 UTC

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To: "Carlos Pignataro (cpignata)" <cpignata@cisco.com>
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From: Eric C Rosen <erosen@juniper.net>
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Cc: "bier@ietf.org" <bier@ietf.org>, "mpls@ietf.org" <mpls@ietf.org>, "Dr. Tony Przygienda" <tonysietf@gmail.com>
Subject: Re: [mpls] [bier] The first nibble issue associated with MPLS encapsulation
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On 4/11/2016 8:19 PM, Carlos Pignataro (cpignata) wrote:
> The format of a BIER packet have enough constrained or self-defining 
> fields, that an off-line parsing tool like Wireshark can apply enough 
> heuristics to figure it out, without the 0x5. 

I haven't worked out for myself whether this is the case or not, but it 
certainly doesn't hurt to have the 0x05 there.

> I understand why a mid-point LSR might want to not alias the MPLS 
> payload with IPv4 or IPv6, to prevent mis-hashing in the LB. Why would 
> a node be interested in knowing the payload is not a PW? What would it 
> do differently? 

I'm not sure if there is any hardware that performs PW-specific 
functions in the forwarding plane, based on the first nibble, but it 
seems prudent to avoid any possibility of confusion.

> [It is interesting that the hint is, as described, to nudge a node 
> that the packet is *not* something, instead of the packet *is* BIER.] 

Well, of course, that's why the nibble is not a protocol type field ;-)  
In the MPLS encaps, the fact that the packet is BIER is determined from 
the MPLS label.  In other proposed encapsulations, the encapsulating 
protocol would have a protocol type field.

>> This strategy does incur a risk.  Suppose IPv5 gets designed, implemented, and deployed, and folks start to deploy hardware that does MPLS load balancing by inspecting the IPv5 headers of the MPLS payloads.  If a BIER packet is traversing an MPLS tunnel, inappropriate load splitting may occur if the hardware thinks the payload is IPv5 rather than BIER.
>>
>> This particular risk doesn't seem very significant to me.
>>
> I agree that risk is quite insignificant (null in practice). The real risk I believe is if some other application over MPLS uses 0x5 in the first nibble, because they do not want to be confused with IP either, and they do not like 0x0 or 0x1.

That is possible, but I don't really see what the problem is.

I guess it is true that we'll have to say that BIER is not 100% 
compliant with RFC 4928.   I don't see that as a problem either, but it 
is certainly possible that some AD will make an issue of it.

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