Hi Tony,
Thank you for your comments!
My explanations are inline below.
Best Regards,
Huaimo
________________________________
From: Tony Li <[email protected]> on behalf of [email protected]
<[email protected]>
Sent: Wednesday, May 29, 2019 11:20 AM
To: Huaimo Chen
Cc: [email protected]
Subject: Re: Enhancements on flooding topology encoding
Hi Huaimo,
Thank you for the interesting numbers, but without some understanding of the
experiment that you’re performing, we have no way of understanding the point
that you’re trying to make.
If we are to make a change, we need to all understand and agree that there is
an improvement and that the improvement is worthwhile in terms of complexity
vs. gain.
The block encoding that you have proposed seems to require at least one TLV for
every other node in a path and thus seems like an extremely inefficient way of
encoding a topology. If you have ways of using this efficiently, please explain.
[HC] There is no such requirement. A block of FT (maybe a whole FT) can be
encoded in a Block TLV. A path can be considered as a special block of FT and
encoded in a Block TLV.
Compressing the path TLV into a bit stream (similar to encoding B) has been
proposed, but to date we have not bit off on that complexity. As noted, the
savings that you get decrease with scale, which is unfortunate because at scale
is when we need it. So far, the consensus seems to be that this is not worth
the effort.
[HC] It seems that there is no issue on scale.
Regards,
Tony
On May 29, 2019, at 8:05 AM, Huaimo Chen
<[email protected]<mailto:[email protected]>> wrote:
Hi Tony,
For the three encodings below,
* Encoding A: Plain Path TLVs, where each node index uses 2 bytes,
* Encoding B: Compact Path TLVs, where each of node indexes in Path TLVs
has the same size given by a field in the Area Node IDs TLV with L set to one,
which is the size (in bits) of the maximum node index value,
* Encoding C: Block TLVs,
it seems that C is more efficient than A and B in general, B is more efficient
than A.
For example, for representing 63 nodes flooding topology (FT for short) of a
binary tree in IS-IS, some comparisons among three encodings are listed in the
table below (In case that the formats may be messed up, a .pdf file is
attached).
Encoding
Bytes used
Comparisons
A (Plain Path TLVs)
248
179/248 = 0.72 (72%)
B (Compact Path TLVs)
179
121/179 = 0.68 (68%)
C (Block TLVs)
121
121/248 = 0.49 (49%)
From the table, we can see that 248, 179 and 121 bytes are used for
representing the FT by A, B and C respectively. 121/248 = 0.49 (49%) means that
C uses about 49% of the flooding resource that A uses. 121/179 = 0.68 (68%)
means that C uses about 68% of the flooding resource that B uses. 179/248=0.72
(72%) means that B uses about 72% of the flooding resource that A uses.
From this example, we can see that C is more efficient than A and B, and B
is more efficient than A.
Best Regards,
Huaimo
From: Tony Li [mailto:[email protected]] On Behalf Of
[email protected]<mailto:[email protected]>
Sent: Wednesday, May 15, 2019 11:55 AM
To: Huaimo Chen <[email protected]>
Cc: [email protected]
Subject: Re: Enhancements on flooding topology encoding
Hi Huaimo,
In one way, a TLV called Node Index Size TLV is defined. Its value field of one
octet contains an index size (i.e., a number of bits). When this TLV is
included in an LSP/LSA containing Path TLVs, all the node indexes in the Path
TLVs are represented using the number of bits given by the index size in the
Node Index Size TLV. The index size is the minimum number of bits needed to
represent the maximum node index in the Path TLVs.
Yes, Tony P. also made this suggestion quite some time ago in a private
communication. We also observed that no separate signaling of the size is
actually needed, as each node can look at the number of indices listed in the
Node Id TLV and take ceil(log2(# nodes)) and use that many bits for each index.
In another way, 5 bits of the Reserved field in the IS-IS Area System IDs TLV
and OSPF Area Node IDs TLV with L set to one indicates the index size that is
used for all the node indexes in all the Path TLVs included in every LSP/LSA.
The index size is the minimum number of bits needed to represent the maximum
node index in the area.
Yes, you could do that too. That’s for ‘free’, assuming that you agree that
the L bit is required.
In another encoding, Block TLVs are used to encode the flooding topology. A
Block TLV represents a block of the flooding topology. The value field of a
Block TLV starts with 5 bits to indicate the index size, which is followed by
the index of a local node, the number of adjacent nodes (in 3 bits), and the
indexes of the adjacent/remote nodes of the local node. This part is similar to
the one in a router LSA to represent the part of the topology from the local
node to the adjacent nodes of the local node, which can be considered as a
block of the topology in one level. This block can be extended to multiple
levels. Each of the adjacent nodes has an extension flag bit E. An
adjacent/remote node with E = 1 is considered as a new local node, and its
adjacent nodes are added. This encoding seems more efficient.
A bold claim. Do you have any proof for this? It seems counter-intuitive, as
for a bi-connected node, it would seem that its index must appear at least
twice, once per link.
The advantage of the path encoding is that for almost all nodes, an index can
appear a single time.
Example:
IS-IS Instance: Amun VRF: default
Level 1:
Path: 3 16 8
Path: 0 18 9 10 11 17 15 6
Path: 0 13 1 19 5 4 14 8
Path: 0 3 6 7 2 8 12 0
Tony
<Enhance2FTEncode-2019-5-29.pdf>
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