Hi, Correct me if I don't understand correctly what you meant.
"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 how the draft is written today : §3.1 : "If there is no such LFA neighbor, then S may be able to create a virtual LFA by using a tunnel to carry the packet to a point in the network which is not a direct neighbor of S from which the packet will be delivered to the destination without looping back to S." §4.1 : "Not all links will require protection using a tunneled repair path. Referring to Figure 1, if E can already be protected via an LFA, S-E does not need to be protected using a repair tunnel, since all destinations normally reachable through E must therefore also be protectable by an LFA. " The draft also precise that extended P-Space must be used : "9.1<http://tools.ietf.org/html/draft-ietf-rtgwg-remote-lfa-02#section-9.1>. Simplicity The remote LFA algorithm is simple to compute. o The extended P space does not require any new computation (it is known once per-prefix LFA computation is completed). " In your example, N is part of extended P-Space. So N would be a PQ (it's a normal LFA also as you stated). My comment : - Yes, extended P-Space usage is mandatory in order to maximize coverage (for me it's clearly explained in the draft) - draft currently states that remote LFA would be computed only when normal LFA is not available BUT this is not a good idea. Based on the use case we are presenting if draft-ietf-rtgwg-lfa-manageability, there is a strong interrest in always computing rLFA protection then be able to compare attributes of PQs and normal LFAs to determine which one is the more efficient based on service provider requirements. This does not mean that we don't use extended P Space, extended P-Space need always to be used. - You are providing NP coverage for rLFA , thanks for the info, but I just want to highlight that this is just "informational" as a router is not able to determine that the PQ is providing NP or not for the specific destination. Determining such PQ attribute would require extra SPF, as stated in draft-psarkar-rtgwg-rlfa-node-protection-00<http://www.google.fr/url?q=https://datatracker.ietf.org/doc/draft-psarkar-rtgwg-rlfa-node-protection/&sa=U&ei=zWn7UcyGGMTcOo3agcgM&ved=0CB4QFjAA&usg=AFQjCNE10vQJ-5GomSFsDkPVW6mXz0wzRQ>. Best Regards, Stephane ________________________________ De : [email protected] [mailto:[email protected]] De la part de Levente Csikor Envoyé : jeudi 1 août 2013 16:11 À : [email protected] Cc : [email protected]; [email protected]; [email protected] Objet : Re: draft-ietf-rtgwg-remote-lfa - ready for WGLC? - some comments and questions 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 On 07/02/2013 07:07 PM, Stewart Bryant wrote: On 26/06/2013 14:13, Levente Csikor wrote: Levente In each case below the conditions are surely fulfilled if n=e thus I think that in each case the condition needs to be changed to : some node n!={s or e} In order to not to read our full papers and searching the answers, I copied here the relevant parts: Link-protecting rLFA condition: For source s, destination d, and next-hop e, some node n != s,d is a link-protecting remote LFA for the s-d pair if and only if dist(s, n) < dist(s, e) + dist(e, n) (1) dist(n, d) < dist(n, s) + dist(s, d) (2) In these equations, one can easily see, that (1) defines the P-space, while (2) is the condition of Q-space. Furthermore, with these formalized conditions, one can easily observe, that (2) is actually the basic loop-free criterion of pure LFA. Link-protecting rLFA condition with extended P-space: 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(n, d) < dist(n, s) + dist(s, d) . Node-protecting rLFA condition: For source s, destination d, and next-hop e, some n != s,d is a node-protecting remote LFA for the s-d pair if and only if dist(s,n) < dist(s,e) + dist(e,n) (3) dist(n,d) < dist(n,e) + dist(e,d) (4) As it was in the case of link protection, here, (3) defines the P-space, while (4) characterize the Q-space. Here, two important observations can be made, which are the followings: - P-space does not depend on the protection scheme (i.e., link or node protection) SB> That falls directly out of the definition of P-space SB> since in link you cannot traverse the link to e and in SB> node only get to e if you traverse the link to e SB> thus the exclusion of the link to e applies in both cases - (4) again is the basic node-protecting loop-free criterion of pure LFA. Node-protecting rLFA condition with extended P-space: For source s, destination d, and next-hop e, some node n != s,d is an extended node-protecting remote LFA for the s-d pair if and only if ∃v ∈ neigh(s) : dist(v, n) < dist(v, e) + dist(e, n) dist(n, d) < dist(n, e) + dist(e, d) . Despite the fact that we only considered unit cost networks, the formal definitions above are true for any arbitrary weighted network. SB> I do not see where the unit costs come into the text above. It looks SB> like it is already expressed in terms of arbitrary cost. SB> Additionally in order to limit the number of SPFs to a practical SB> level, we normally suggest that the repair target in not d, but SB> instead is e (in the link case) or next hop of e (node case). SB> Anyway I will work on some text. - Stewart _________________________________________________________________________________________________________________________ Ce message et ses pieces jointes peuvent contenir des informations confidentielles ou privilegiees et ne doivent donc pas etre diffuses, exploites ou copies sans autorisation. Si vous avez recu ce message par erreur, veuillez le signaler a l'expediteur et le detruire ainsi que les pieces jointes. Les messages electroniques etant susceptibles d'alteration, Orange decline toute responsabilite si ce message a ete altere, deforme ou falsifie. Merci. This message and its attachments may contain confidential or privileged information that may be protected by law; they should not be distributed, used or copied without authorisation. If you have received this email in error, please notify the sender and delete this message and its attachments. As emails may be altered, Orange is not liable for messages that have been modified, changed or falsified. Thank you.
_______________________________________________ rtgwg mailing list [email protected] https://www.ietf.org/mailman/listinfo/rtgwg
