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

Gregory Mirsky <> Thu, 14 April 2016 19:48 UTC

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From: Gregory Mirsky <>
To: Stewart Bryant <>, Eric C Rosen <>, Alexander Vainshtein <>
Thread-Topic: [mpls] [bier] The first nibble issue associated with MPLS encapsulation
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Date: Thu, 14 Apr 2016 19:48:10 +0000
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Subject: Re: [mpls] [bier] The first nibble issue associated with MPLS encapsulation
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Hi Stewart, et. al,
just wanted to clarify, though that likely was stated on the thread already, that the problem of the first nibble is only on transient, i.e. P, nodes that are not aware of the context of the BoS label. And with the fact that there are old and very old P nodes out there in the network there might be no absolutely safe solution to the First Nibble issue/concern. Though we can recommend safer policies to handle ECMP on P nodes, e.g. use EL.


-----Original Message-----
From: mpls [] On Behalf Of Stewart Bryant
Sent: Thursday, April 14, 2016 12:32 PM
To: Eric C Rosen; Alexander Vainshtein
Cc:;; Dr. Tony Przygienda
Subject: Re: [mpls] [bier] The first nibble issue associated with MPLS encapsulation

On 14/04/2016 14:20, Eric C Rosen wrote:
> On 4/14/2016 6:06 AM, Alexander Vainshtein wrote:
>> Stewart and all,
>> I concur with Stewart that there is a strong case for 0 in the first 
>> nibble for all non-IP flows.
>> As for the need for sub-typing:
>> AFAIK quite a few implementations (including some HW-based packet
>> processors) treat 0 in the first nibble after the label stack as an 
>> indication of an Ethernet PW.

Well there will be Ethernet PWs not using the CW that can put any Ethernet address in the first nibble, so if there are Ethernet addresses that start with 5 they will look like BIER packets.

>> Some of them go as far as to hash on the assumed L2 headers for ECMP. 
>> This causes serious problems, e.g., with the TDM PWs that could be 
>> reordered if handled by such packet processors in transit LSRs.
>> This makes quite a case for sub-typing IMO regardless of BIER.
>> At the same time, it seems that all the bits in CW structure are used
>> - at least for some PW types in some cases.
> It seems to me that Sasha's reasoning supports the conclusion that it 
> is best to avoid 0 in the first nibble (even if this is not the 
> conclusion he drew!)  I don't think sub-typing is a solution, because 
> it doesn't offer any protection against the behavior of existing 
> hardware.

There is no truly safe position until we mandate that all deployed PWs use the CW, but that is not going to happen for a while.

> On 4/14/2016 5:31 AM, Stewart Bryant wrote:
>> The nibble value  is recorded in the IP types registry and any wish 
>> to take another value really needs to be discussed with the INT area.
> Are you referring to the IP Version Number registry?  (I am not aware 
> of an "IP types registry".)   We're not asking for any modification to 
> that, since we're not doing another version of IP. I don't see any 
> need to get into  one of those long and non-productive arguments that 
> the INT area seems to relish.
Sorry I meant version number.

Before we assign a member of this very small number range to BIER, I would like some confidence that it will see any significant deployment. 
When PW took 0 and 1, it was already a widely deployed protocol.


- Stewart

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