[rrg] Enhanced Efficiency - draft critique
Robin Whittle <rw@firstpr.com.au> Sun, 21 February 2010 05:21 UTC
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From: Robin Whittle <rw@firstpr.com.au>
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Subject: [rrg] Enhanced Efficiency - draft critique
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No-one has written a critique of: Enhanced Efficiency of Mapping Distribution Protocols in Map-and-Encap Schemes http://www.ietf.org/mail-archive/web/rrg/current/msg05540.html K. Sriram, Young-Tak Kim, and Doug Montgomery http://tools.ietf.org/html/draft-irtf-rrg-recommendation-04#section-13 However some mailing list comments from Dino Farinacci and Brian Carpenter are listed here: http://www.ietf.org/mail-archive/web/rrg/current/msg05694.html Here is my 854 word draft critique, which I will revise after feedback from K. Sriram and colleagues. - Robin This submission to the RRG is not for a routing scaling solution, but was included as a potentially archival contribution to research on mapping systems for Core-Edge Separation (CES) architectures. (RRG msg05541, 2009-12-22.) It concerns two suggestions. Firstly, more elaborate mapping arrangements are suggested when one prefix of the "edge" (EID) space is primarily handled by one ETR, but more specific prefixes within this are handled by others. Secondly it hierarchies of ETRs are suggested, for the purpose of a single ETR aggregating the "edge" prefixes handled by subsidiary ETRs into a single mapping announcement. Both these suggestions involve extra complexity for marginal gains in scenarios which may only occur infrequently. A primary purpose of CES architectures is to make "edge" space portable to any ETR in the world. While contiguous segments of "edge" space my indeed be handled by a single ETR, or several topologically nearby ETRs, it does not appear to be worth introducing extra complexity into the structure of mapping data to achieve modest and occasional decreases in the number of items the mapping system must handle. In Fig 2, the ITR's request is for a6/24, in a range of "edge" space mapped in this manner. The following numbers are those of a realistic example in which 16 prefixes are numbered 0 to 15, rather than 1 to 16 as in the proposal paper: a0/24 to a6/24: ETR1 a7/24: ETR3 a8/24 to a13/24: ETR1 a14/24: ETR2 a15/24: ETR1 In LISP these 16 /24s could be 16 separate EID prefixes. Likewise in Ivip, they could be 16 micronets, each of 256 IPv4 addresses. The minimal number of EID prefixes in LISP is 8: 0, 1, 2, 3 = a0/22: ETR1 4, 5 = a4/23: ETR1 6 = a6/24: ETR1 7 = a7/24: ETR3 8, 9, 10, 11 = a8/22: ETR1 12, 13 = a12/23: ETR1 14 = a14/24: ETR2 15 = a15/24: ETR1 In Ivip, since micronets have arbitrary starting points and lengths, the minimum number is 5: a0.0 to a6.255: ETR1 a7.0 to a7.255: ETR3 a8.0 to a13.255: ETR1 a14.0 to a14.255: ETR2 a15.0 to a15.255: ETR1 The elaboration to the mapping system's data format being considered is that it should be possible, in a single mapping item, to specify one prefix with multiple exceptions: a0/20 ETR1 except for: a7/24: ETR3 a14/24: ETR2 CES architectures are intended to scale to very large numbers of EIDs or micronets, including to support potentially billions of mobile hand-held devices. The modest reduction in the number of mappings to be handled does not seem to justify the additional complexity being proposed. If this proposal were accepted, then altering a single /24 in the above example would mean altering the way in which the mapping of other parts of the "edge" address space are specified. In the example, ITR1 requests mapping for an address within a6/24 (using the present 0 to 15 numbering system). The example assumed that it would be returned all the mapping covering the /20. But it is not clear why this should be the case, or what principles would govern returning more complex and voluminous mapping for the request, which concerns a single edge address. In LISP, the map reply should have been: 6 = a6/24: ETR1 and for the minimal mapping example above for Ivip, it should have been: a0.0 to a6.255: ETR1 Ivip's mapping could be in many arrangements, as could LISP's - but both Ivip and LISP return a simpler mapping construct than that which is being proposed - which is highly desirable, since there is no reason to believe ITR1 would soon need to tunnel packets to any other "edge" address in the a0/20 range. In this particular example, the Ivip reply would cover 7 times as much "edge" space as the LISP reply. Map replies in a minimal Ivip mapping arrangement would frequently cover more addresses than those resulting from a minimal LISP mapping - and would never return a mapping covering fewer "edge" IP addresses than with LISP. The question of ETRs being nested presumes that one ETR can be reached by ITRs when it is behind one or more other ETRs. This is impossible in Ivip and would appear to contradict LISP's requirement that all ETRs be on "RLOC" ("core") addresses. To successfully implement the suggested nesting of ETRs may require greater complexity in the ETRs and probably nested encapsulation, since the subsidiary ETRs are presumably on "edge" addresses, as are all devices "behind" an ETR. In Figure 3, nested encapsulation would be required for a traffic packet addressed to a host in z2/24 and there would need to be additional complexity in ITRs to deliver packets to the correct one of four destination networks if the traffic packet was addressed to a host somewhere in y1/22. Since CES proposals are designed to cope with very large numbers of mappings for different EID prefixes, micronets or whatever of "edge" space, it seems that this extra complexity and the reduction in efficiency due to multiple encapsulations is not justified by the modest and occasional reduction in the number of mappings the whole system needs to handle.
- [rrg] Enhanced Efficiency - draft critique Robin Whittle
- Re: [rrg] Enhanced Efficiency - draft critique Sriram, Kotikalapudi