Here is the .txt version. ---------- Forwarded message ---------- From: Charrie Sun <charrie...@gmail.com> Date: 2010/1/19 Subject: Critique of LMS and GLI-Split To: RRG <rrg@irtf.org>
Hello all, Since no one has written a critique of my LMS, I queried my workmates and wrote a critique on it. As many people have pointed out, LMS is a mapping mechanism and does not itself constitute a whole solution for the scalability problem. Well the mechanism is based on edge-core separations and can incorparate with proposals that need a global mapping system, to enhance its functionalities. I also wrote a brief critique on GLI-Split, since I think the two separation planes it clarifies, including the separation between identifier and locator and the separation between local and global locator, can meet different needs of ISPs and hosts. I had some discussions with Dr. Menth and wrote the critique based on the discussions and rethinking. Welcome for complement and rectifications on mine. Attached is my critiques and warmly welcome for comments. Best wishes, Sun Letong
Critique of GLI-Split, by Sun Letong GLI-Split makes a clear distinction between two separation planes: the separation between identifier and locator, which is to meet end-users needs including mobility; the separation between local and global locator, to make the global routing table scalable. The distinction is needed since ISPs and hosts have different requirements. A main drawback of GLI-Split is that it puts too much burden on hosts. Before routing a packet received from upper layers, network stacks in hosts firstly need resolve the DNS name to an IP address; if the IP address is GLI-formed, it may further map the identifier extracted from the IP address to the local locator. If the communication is between different GLI-domains, hosts need further to map the identifier to the global locator, if the local mapping system does not forward the request to the global mapping system for hosts. This may lead to large delays and for low-powered hosts it may become unpractical. For communications spanning GLI-domains, hosts can send packets to a default GLI-gateway if they receive a negative answer from local mapping system, and the default GLI-gateway does the identifier-to-global locator mapping. The author argues that the multiple mapping lookups in hosts is for them to do multipath routing, since different destinations (local or global) may need different outgoing gateways. However, the gains of multipath routing and the cost of host burdens, and the corresponding delays, need to be further balanced. GLI-hosts need a home address for mobility. I think thereĄŻs no such need if the DNS system updates in time when GLI-hosts move across GLI-domains, which is less frequent compared with host mobility within a GLI-domain. The DNS updates would not take too long: on one hand, caching time of DNS now is as small as a few seconds or minutes (for load balance and other applications); on the other hand, a mechanism can be devised to trigger updates on DNS data. Furthermore, in this case hosts need not map the identifier to the global locator since the returned DNS response has that information, of course, if they do not need multipath routing. As it claims, the main benefit of GLI-Split is for nodes move within a GLI-domain, since it would not bother the outside world. When hosts move across GLI-domain more changes may be needed. And the upgrades on hosts are costly, while the tradeoff between their gains needs discussion.
Critiques of LMS, by Sun Letong LMS is a mapping mechanism and based on edge-core separations. In fact, any proposal that needs a global mapping system with keys of similar properties of that ¡°edge address¡± in the edge-core separation can use such a mechanism. This means that those keys are globally unique (by authorization or just statistically), at the disposal of edge users, and may have several satisfied mappings (with different weights, maybe). Once a proposal that needs mapping but doesn't specify the mapping mechanism, is used to solve the scalability problem, LMS can be used to strengthen its function. The key idea of LMS is similar to LISP+ALT that the mapping system should be hierarchically organized, to gain scalability in the storage and update sense and to achieve quick index for mapping lookup. However, LMS advocates an ISP-independent mapping system and ETRs are not the authorities of mapping data. ETRs or edge-sites report their mapping data to related mapping servers. Though LMS assumes that mapping servers can be incrementally deployed in that a server may not be constructed if none of its administered edge addresses are allocated, and that mapping servers can charge for their services, which provides the economic reason for their existence, how this brand-new system can be constructed is still not clear. Explicit layering is only an ideal state, and it rather analyzes the layering limits and feasibility, than provide a practical way for deployment. The drawbacks of LMS¡¯s feasibility analysis also include 1) based on current PC power and may not represent future circumstances (especially for IPv6); 2) does not consider the variability of address utilization. Some IP address spaces may be effectively allocated and used while some may not, causing some mapping servers overloaded while others poorly utilized. More thoughts are needed as to the flexibility of the layer design. LMS doesn¡¯t fit well for mobility. It does not solve the problem when hosts move faster that the mapping updates and propagations between relative mapping servers. On the other hand, mobile hosts moving across ASes and changing their attach points (core addresses) is less frequent than hosts moving within an AS. I personally advocate that separation needs two planes: edge-core separation, which is to gain routing table scalability; identity-location separation, which is to achieve mobility. GLI does a good clarification and in that case, LMS can be used to provide identity-to-core address mapping. Of course, other schemes may be competent and LMS can be incorporate with it if it has globally seen keys and needs to map them to other namespaces.
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