Subject: [Bier] Jeffrey/*: BIER-TE scalability

From: Toerless Eckert <[email-masked]>
Date: Tue, 14 May 2019 11:19:45 +0200

In preparation of pushing draft-ietf-bier-te-arch-02, i reviewed the
notes from our 104 meeting and stumbled across the question/comment
from Jeffrey re. scalability. I ultimately decided not to add
more text to the document becuase i felt it already has sufficient
detailling of scalability and comparison to BIER. I am also not sure
how much of such analysis is best put into an arch document, the
BIER Arch itself has far less, so i also didn't want to overdo it.

The core comparison and example are in section 7:

7.2 gives the high-level scoping, how one may need to assign
    20%..80% of bits in a bitstring for "topology", aka:  not
    BFER.

7.5 and subsections gives what i hope is a good example comparing
    bit assignments BIER vs. BIER-TE, making the point that we
    also can not arbitrarily assign bits in BIER when we need to
    support more than bitstringsize numbrer of BFER and minimize
    unnecessary duplicate packets across links, and then giving
    the example of how to effectively do this in BIER-TE by
    accordingly assign bits in BIER-TE to support this.

    The original slides that showed this assignment scheme 
    (without the constraints of ASCII) are from IETF94:
    
    https://datatracker.ietf.org/meeting/94/materials/slides-94-bier-7
    
    Slide 14/15 (sorry, victims of pptx->pdf conversion).
    
    Aka: For every bitstring, there is some replicated assignment
    of bits to steer a packet through relevant TE hops, like  different
    network zone ingres/egress routers, but the mayority of bits are
    then used within the zone to steer/replicate the packets. The
    initial bits could also be avoided by using e.g.: an SR-MPLS
    header to steer the BIER(-TE) packet to the destination network
    zone.

Given the difficulty in visualization, i hope that the text in the
sections is sufficient. But i hope that will accordingly be feedback
to the WG last-call discussion. Note too, that in environments like
DetNet, network may not even be soo large as to bothre about these
aspects of pathe engineering.

Cheers
     Toerless