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Re: composite link - candidate for respin, maybe

Subject: Re: composite link - candidate for respin, maybe
From: Tony Li
Date: Mon, 29 Mar 2010 14:12:13 -0700
Hi John,

> The difference between advertising one bundled link with multiple sets
> of parameters or multiple bundled links each with one set of
> parameters doesn't seem so significant.


Well, that depends entirely on our scalability goals, which is why I would
like to see our requirements quantified.

As an example, if we use IS-IS with TE right now, a link consists of:
      2 octets of TLV overhead
      7 octets of system Id and pseudonode number
      3 octets of default metric
      1 octet of length of sub-TLVs
      6 octets of Administrative group (color)
      6 octets of IPv4 interface address
      6 octets of IPv4 neighbor address
      6 octets of Maximum link bandwidth
      6 octets of Reservable link bandwidth
      34 octets of Unreserved bandwidth (for 8 traffic classes)
      5 octets of TE Default metric

If we want to track latency, then I'd propose that we need 3 bytes to track
worst case latency on the link  (2 bytes overhead, 1 byte for ms granularity
latency).

If I've done my math right, that works out to 85 bytes.

If we then vectorize this, we would need, per component:
    4 octets of maximum link bandwidth
    4 octets of reservable link bandwidth
    32 octets of unreserved bandwidth
    1 octet of latency

That's 41 bytes.

If we're willing to dispense with pre-reserved bandwidth per traffic class,
then we can further drop 32 octets.

So we're down to 9 bytes per link, or almost a 10x improvement in
scalability.

IMHO, that's significant.

If we now have ~16 components per composite link, then we can advertise the
composite link in about 200 bytes.  That works out to well over 1000
composite links (and therefore 16,000 components) per system.  And that's
before we break a sweat and do funky things to get more LSP space (e.g.,
expanded MTUs, zero-cost pseduonodes for more fragments).

So, what are the real requirements?

Tony


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