Maybe we could assign a single extra label/IP address to say that “use
uniform link costs for this packet”, and get something quite similar
to MRT? :)
Gabor
*From:*Levente Csikor [mailto:[email protected]]
*Sent:* Friday, August 02, 2013 10:01 AM
*To:* Gábor Sándor Enyedi
*Cc:* [email protected]; [email protected];
[email protected]; [email protected]
*Subject:* Re: draft-ietf-rtgwg-remote-lfa - ready for WGLC? - some
comments and questions
These are bidirectional results.
Levente
On 08/02/2013 09:59 AM, Gábor Sándor Enyedi wrote:
Are these the single or the bidirectional coverage results?
Gabor
*From:*[email protected] <mailto:[email protected]>
[mailto:[email protected]] *On Behalf Of *Levente Csikor
*Sent:* Friday, August 02, 2013 8:25 AM
*To:* [email protected] <mailto:[email protected]>
*Cc:* [email protected]
<mailto:[email protected]>;
[email protected] <mailto:[email protected]>;
[email protected] <mailto:[email protected]>
*Subject:* Re: draft-ietf-rtgwg-remote-lfa - ready for WGLC? -
some comments and questions
Dear All,
I'm sending my previous mail again since the font for the ASCII
art network was not fixed and may not appear in a right format at
everyone.
If it still messy, let me know.
Sorry,
Levente
Dear All,
since our previous works in remote LFA analysis (papers already
sent to these mailing lists) assumed unit cost networks, therefore
as I promised, I calculated the corresponding LFA and rLFA
coverages in those networks with their original link costs.
These networks were inferred from Rocketfuel dataset (/Mahajan,
R., Spring, N., Wetherall, D., Anderson, T.:
Inferring link weights using end-to-end measurements. In: ACM IMC,
pp. 231–236 (2002)/), SNDLib (http://sndlib.zib.de), and
TopologyZoo(http://www.topology-zoo.org). The found coverages and
some details are found in the table below, where /n/ and /m/
denote the number of nodes and the number of links, respectively.
The other columns mark the different coverages obrtained by simple
LFA and remote LFA, where LP indicates the link-protecting case,
while NP notes the case of node protection.
Topologies marked with an asterisk(*) did not have inferred real
link costs from the datasets, so their costs were initially set to
1 (unit costs).
I believe that these results computed on real-world networks could
significantly improve the rlfa draft, especially Sec. 9.3., and
the advantages of remote LFA over simple LFA would be more emphasized.
+==============+====+====+==========+===========+============+============+
| Topology | n | m | LFA_LP | LFA_NP | rLFA_LP |
rLFA_NP |
+==============+====+====+==========+===========+============+============+
| AS1221 | 7 | 9 | 0.809 | 0.25 | 0.809 |
0.25 |
+--------------+----+----+----------+-----------+------------+------------+
| AS1239 | 30 | 69 | 0.8735 | 0.7554 | 1 |
0.9795 |
+--------------+----+----+----------+-----------+------------+------------+
| AS1755 | 18 | 33 | 0.8725 | 0.7741 | 0.9967 |
0.9959 |
+--------------+----+----+----------+-----------+------------+------------+
| AS3257 | 27 | 64 | 0.923 | 0.7186 | 0.99 |
0.8472 |
+--------------+----+----+----------+-----------+------------+------------+
| AS3967 | 21 | 36 | 0.7857 | 0.6460 | 1 |
0.9325 |
+--------------+----+----+----------+-----------+------------+------------+
| AS6461 | 17 | 37 | 0.9338 | 0.6933 | 0.9963 |
0.7075 |
+--------------+----+----+----------+-----------+------------+------------+
| Abilene* | 12 | 15 | 0.5606 | 0.6078 | 0.9090 |
0.8725 |
+--------------+----+----+----------+-----------+------------+------------+
| Arnes* | 41 | 57 | 0.6225 | 0.3518 | 0.7487 |
0.4562 |
+--------------+----+----+----------+-----------+------------+------------+
| AT&T | 22 | 38 | 0.8225 | 0.5647 | 1 |
0.8497 |
+--------------+----+----+----------+-----------+------------+------------+
| Deltacom | 113| 161| 0.5771 | 0.4910 | 0.8539 |
0.8148 |
+--------------+----+----+----------+-----------+------------+------------+
| Gambia | 28 | 28 | 0.037 | 0.04 | 0.1851 |
0.12 |
+--------------+----+----+----------+-----------+------------+------------+
| Geant | 37 | 55 | 0.6906 | 0.3977 | 0.8303 |
0.6582 |
+--------------+----+----+----------+-----------+------------+------------+
| Germ_50 | 50 | 88 | 0.9004 | 0.8381 | 1 |
0.9995 |
+--------------+----+----+----------+-----------+------------+------------+
| Germany* | 27 | 32 | 0.6948 | 0.599 | 1 |
0.9549 |
+--------------+----+----+----------+-----------+------------+------------+
| InternetMCI | 19 | 33 | 0.9035 | 0.6798 | 0.9415 |
0.9136 |
+--------------+----+----+----------+-----------+------------+------------+
| Italy* | 33 | 56 | 0.784 | 0.5741 | 1 |
0.9269 |
+--------------+----+----+----------+-----------+------------+------------+
| NSF* | 26 | 43 | 0.86 | 0.6347 | 1 | 1
|
+--------------+----+----+----------+-----------+------------+------------+
Furthermore, after reading the draft again-and-again, I have found
that it should be more emphasized in the draft that calculating or
seeking for a remote LFA staging point should be done IF AND ONLY
IF no simple LFA were found. This is important, since normally
people obviously think and observe that the failure coverage of
remote LFA should be greater than or equal to the coverage
obtained by simple LFA. Moreover, people also think that LFAs
produce a subset of remote LFAs. However, if after a failure only
P-spaces and Q-spaces are taken into account in order to seek a
(remote) loop-free alternate, then it is possible that a simple
LFA would not be found resulting unprotected node-pairs. But, this
case could only happen when link costs are not unit costs. For an
easier understanding, consider the network depicted below:
1 1
F--------S-----------X--------E
\ / \
10 \ / 4 \ 1
\ / 1 \
N ---------------------------C
| |
1 | | 1
| |
| 1 1 |
A--------------B-------------D
Assume that S wishes to send a packet to D, and the shortest path
goes through E, therefore it is S-E-C-D. Suppose that the link (s,e)
fails, or even the node e itself fails. In this case, since LFA
(and its calculation) only consider all other neighbors of s, then
node N would be an easy LFA for this failure, since dist(N,D) <
dist(N,S)+dist(S,D).
However, if we only seek possible remote LFAs, than according to
the (r)SPF calculations, or taking into account our
distance-function conditions will result that D's Q-space will
contain node N (besides some other nodes), but S's P-space won't
(it will only contain node F and what is more, if node F does not
exist, then S's P-space would be empty), therefore no intersection
of the two spaces will exist, leaving this network vulnerable to
the failure of link (s,e).
This results that a neighbor should be always reached by the
neighboring link, even if there exists a shorter, but definitely
bigger in hop-count path to it.
According to this case (when seeking simple LFA is missed), the
condition of extended P-space described by our distance functions,
should be modified a bit:
For source /s/, destination /d/, and next-hop /e/, some node /n !=
s,d/ is an extended link-protecting remote LFA for the /s-d/ pair
if and only if
/∃v //∈ neigh(s) : dist(v, n) < dist(v, s) + dist(s, e) + dist(e,
n) *&& dist(s,v) < dist(s,e) + dist(e,v)*/
/dist(n, d) < dist(n, s) + dist(s, d) . /
One can easily observe that emphasizing more that remote LFA
seeking process is only "executed" after no simple LFA is found
could much more ease the understanding and won't result a headache
to the reader who accidentally wants to calculate rLFA coverage in
such network.
Please let me know, if my interpretation is not correct.
Thanks.
Best,
Levente
