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[Bier] Flow Label [was Re: BIER in IPv6 --- draft-zhang-bier-bierin6-04]

Brian E Carpenter <brian.e.carpenter@gmail.com> Mon, 23 March 2020 04:03 UTC

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To: Tony Przygienda <tonysietf@gmail.com>, "zhang.zheng" <zhang.zheng@zte.com.cn>
Cc: BIER WG <bier@ietf.org>, 6MAN <6man@ietf.org>
References: <0aaf9a4e017643af85cd246b04d1858c@huawei.com> <202003231114061611017@zte.com.cn> <CA+wi2hPF0rjn2M80PxspRLXYQWGLj7AhB0m1JJFQ6WN4GU0XLw@mail.gmail.com>
From: Brian E Carpenter <brian.e.carpenter@gmail.com>
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Date: Mon, 23 Mar 2020 17:03:48 +1300
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Subject: [Bier] Flow Label [was Re: BIER in IPv6 --- draft-zhang-bier-bierin6-04]
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Hi,

I have not read the whole draft but I did notice this:

>    The Flow-ID in the IPv6 packet SHOULD be copied from the entropy
>    field in the BIER encapsulation.

It's good that the BIER entropy field defined in RFC8296 is 20 bits long, but there are a few issues here:

1. Nit: in RFC 8200 the field is named "Flow Label", not "Flow-ID".

2. The text technically violates RFC8200, which states that the flow label is defined by RFC6437. 

3. RFC6437 recommends the flow label generation procedure to follow a certain rule:

   It is therefore RECOMMENDED
   that source hosts support the flow label by setting the flow label
   field for all packets of a given flow to the same value chosen from
   an approximation to a discrete uniform distribution.

but I didn't find any discussion in RFC8296 of how the BIER entropy value is generated. 

I think you could get round these two points by something like:

   The entropy value used in an IPv6 flow label SHOULD be generated
   by a method compatible with [RFC6437].

4. Finally, it seems to me that the flow label is useless by definition for link-local traffic; however, it's harmless. (I never really thought about that before.) Of course, it could be relevant for globally-reachable addresses.

Regards
   Brian Carpenter

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