Are these the single or the bidirectional coverage results? Gabor
From: [email protected] [mailto:[email protected]] On Behalf Of Levente Csikor Sent: Friday, August 02, 2013 8:25 AM To: [email protected] Cc: [email protected]; [email protected]; [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
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