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

Xuxiaohu <xuxiaohu@huawei.com> Thu, 14 April 2016 03:07 UTC

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From: Xuxiaohu <xuxiaohu@huawei.com>
To: Eric C Rosen <erosen@juniper.net>, "Carlos Pignataro (cpignata)" <cpignata@cisco.com>
Thread-Topic: [bier] [mpls] The first nibble issue associated with MPLS encapsulation
Thread-Index: AQHRlZODxyb0YWE87kOEgjOaa5qR65+IxvJA
Date: Thu, 14 Apr 2016 03:07:06 +0000
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Cc: "bier@ietf.org" <bier@ietf.org>, "Dr. Tony Przygienda" <tonysietf@gmail.com>, "mpls@ietf.org" <mpls@ietf.org>
Subject: Re: [mpls] [bier] The first nibble issue associated with MPLS encapsulation
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> -----Original Message-----
> From: Eric C Rosen [mailto:erosen@juniper.net]
> Sent: Wednesday, April 13, 2016 10:48 PM
> To: Carlos Pignataro (cpignata)
> Cc: Alexander Vainshtein; Greg Mirsky; mpls@ietf.org; bier@ietf.org; Xuxiaohu;
> Dr. Tony Przygienda
> Subject: Re: [bier] [mpls] The first nibble issue associated with MPLS
> encapsulation
> 
> 
> 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

If we pick different values for the first nibble of different MPLS payloads, the first nibble actually plays the role of a protocol type field. In fact, there is an attempt to have a registry for the first nibble (a.k.a., MPLS payload type field).

Best regards,
Xiaohu

> 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.
>