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

Alexander Vainshtein <> Mon, 11 April 2016 12:45 UTC

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From: Alexander Vainshtein <>
To: Greg Mirsky <>
Thread-Topic: [mpls] The first nibble issue associated with MPLS encapsulation
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Cc: "" <>, "" <>, "" <>, "Dr. Tony Przygienda" <>
Subject: Re: [mpls] The first nibble issue associated with MPLS encapsulation
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Greg, and all,
From my experience (from the days long past☺), the IETF has been always treating the first nibble just after the bottom of the label stack as being managed via the IP Version Numbers registry<> – and this because, as per RFC 3032<>, “the network  layer packet immediately follows the label stack entry which has the  S bit set”.

RFC 4385<> has reused the values that have been defined as “Reserved” in this registry.
But, AFAIK, attempts to reuse some stale values (e.g., PIP<>) have been rejected out-of-hand.

My 2c,

Office: +972-39266302
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From: Greg Mirsky []
Sent: Saturday, April 09, 2016 2:06 AM
To: Carlos Pignataro (cpignata)
Cc:; Alexander Vainshtein;; Dr. Tony Przygienda; Xiaohu Xu;
Subject: Re: [mpls] The first nibble issue associated with MPLS encapsulation

Hi Carlos,
thank you for the clarification. Should we think about establishing the registry than?
Regards, Greg
On Apr 8, 2016 5:38 PM, "Carlos Pignataro (cpignata)" <<>> wrote:

My point, sorry if I was not clear, was that there is no such a thing as a ‘first nibble registry’.

Instead, RFC 4928, Section 5, at, says:

   IANA has marked the value 0x1 in the IP protocol version number space
   as "Reserved" and placed a reference to this document to both values
   0x0 and 0x1.

And that is reflected as

The IANA text in 4928 is additionally followed by a disclaimer:

   Note that this document does not in any way change the policies
   regarding the allocation of version numbers, including the possible
   use of the reserved numbers for some future purpose.

Further, RFC 4385 does not specify the ‘first nibble’ as a field. Instead, it depicts the actual binary values for the different CW formats. In other words, it takes the values from the IP protocol version number and not as a new CW Field.


— Carlos.

PS: Sasha, quick typo, s/1119/1190/;

From: Greg Mirsky <<>>
Date: Friday, April 8, 2016 at 7:28 PM
To: Alexander Vainshtein <<>>
Cc: "<>" <<>>, "<>" <<>>, "Dr. Tony Przygienda" <<>>, "<>" <<>>, Xiaohu Xu <<>>, Carlos Pignataro <<>>
Subject: Re: [mpls] The first nibble issue associated with MPLS encapsulation

Hi Sasha,
thank you for pointing to existing IANA allocation, though stale. I wonder if there is the registry for the first nibble. We, Tony and I, had discussed the way the first nibble space managed. If there already is the registry, could you please point me to it.
Regards, Greg
On Apr 8, 2016 2:31 PM, "Alexander Vainshtein" <<>> wrote:

Carlos and all,

Just for the reference, IANA has defined version 5 (0101) has assigned to ST protocol and refers to RFC 1119. The latter has been obsoleted by RFC 1819, but the IANA

assignment still holds.

Is there, just in case, any relationship between BIER and ST?

Thumb typed on my cellphone



-------- Original Message --------

From: "Carlos Pignataro (cpignata)" <<>>

Date: Fri, April 08, 2016 9:25 PM +0300

To: Xiaohu Xu <<>>

CC:<>,<>,<>, "Dr. Tony Przygienda" <<>>

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

Xiaohu, Tony,

Please see inline.

On Apr 7, 2016, at 2:39 PM, Xuxiaohu <<>> wrote:

As for the first nibble issue, will it violate the layering principle of network protocol stacks if the first nibble of any new encapsulation header (which could be an MPLS payload) is used as the "MPLS payload type" field?

Reading draft-wang-bier-ethernet-01, Section 3, the “first nibble” is _not_ used as an “MPLS payload type”. Instead, the text describes an anti-aliasing mechanism, much like RFC 4928.

The relevant text is:
     First nibble: The first 4 bits of the header are set to 0101; this
   ensures that the BIER header will not be confused with an IP header
   or with the header of a pseudowire packet.

Which says “… will not be confused with …"

wouldn't it  be more reasonable and sustainable to fix the problem (i.e., the lack of a protocol field in the MPLS header) by the MPLS header itself?

Who says it is a *problem*? There’s no “fixing” needed.

By the way, since it's claimed that the NSH is transport-independant, it means the NSH should be able to be transported over MPLS. However, it seems that the first nibble issue has not be considered in the current NSH draft. As a result, when encapsulating NSH over MPLS, the NSH may be mis-interpreted as IP header.

There seems to be some massive confusion on this paragraph, on a number of levels. First, NSH is not “claimed to be” transport-independent. It is by charter and by design. Second, the NSH draft does not even include the term “MPLS”, because it does not define transports. The SFC Encapsulation can be used in a transport-agnostic way.

One more comment below.

Best regards,

发件人: BIER [<>] 代表 Tony Przygienda [<>]
发送时间: 2016年4月5日 22:36
主题: [Bier] comments on draft-wang-bier-ethernet-01
after reading

a) first nibble: refer to MPLS encaps as "the same value" to keep in sync

One comment regarding the “First nibble” text at draft-ietf-bier-mpls-encapsulation-03

Since the function of the first nibble is to prevent aliasing with an IP packet, in order for RFC 4928 to specify values of 0x0 and 0x1 for the First Nibble, it had to “Reserve” IP protocol versions of 0 and 1, referencing that RFC (see

Is the intent to re-assign IPv5 at ?

Note that RFC 4928 says “REQUIRED” at:

   It is REQUIRED, however, that applications depend upon in-order
   packet delivery restrict the first nibble values to 0x0 and 0x1.


— Carlos.

b) refer to all other possible fields to MPLS encaps to keep in sync when describing instead of repeating
c) you need to describe which kind of ether MACs are allowed, especially on broadcast media, i.e. is it always p2p or can you take advantage of the broadcast ?
d) Figure 4: use the architecture/MPLS encoding for the length, don't invent a new one
e) who will obtain a new ether type from IEEE? As far I understand, not a trivial process albeit we have several liaisons with IEEE

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