Hello Michael, Questions are inline. 2010/1/19 Michael Menth <[email protected]>
> Dear Sun Letong, > > thanks a lot for writing the critique for GLI-Split. I have some clarifying > comments, see inline. > > 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. >> > The distinction makes ISP changes invisible for nodes within a GLI-domain > including the local mapping system, and structural changes inside a > GLI-domain invisible to nodes outside a GLI-domain including the global > mapping system. > > 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. >> > 1) Upgraded GLI-hosts convert the identifier address either to a local or > global address. They perform only a single conversion. When reading the text > one may think that two conversion steps are required. > Did I miss something, but in the section of communications between different GLI-Domains (section 3.1.2), it is said that when GLI-node queries local mapping system and get a negative answer, it queries the global mapping system. Local mapping system forward the request for GLI-nodes to the global MS is just an option. > 2) The conversion usually does not introduce a "large dealy" as the mapping > infos are cached. The delay is at the host where it is not critical. It's > like DNS lookup, probably faster if the mapping system is more efficient > than DNS (see FIRMS). > Cache cannot solve the problem of "first-packet-delay". And in IPv6, storing location information of all mapping servers (considering the huge indexing space) like FIRMS is unscalable. > 3) This statement suggests that GLI-Split requires host changes which is > not necessarily true. A major objective of GLI-Split is to be > backward-compatible. That means, GLI-domains can accommodate upgraded > GLI-hosts and classic IPv6 hosts for which no changes are necessary. Of > course, upgraded GLI-hosts enjoy more benefits than classic IPv6 hosts. This > essential aspect is lacking. > > Well what I mean is that compared with the benefits, the upgrades of GLI-hosts are costly. > 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. >> > That's part of the description. I cannot see how this sentence is related > to the previous or next statement. > > I think through this way burdens can be relieved partly from hosts. > 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-Split does not mandate multipath routing but it supports it if needed. > And there is a clear desire to support multipath routing, just see the > current efforts in IETF for multipath TCP. > > Ok, further reflection is needed on the balance between application needs and corresponding costs. > 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. >> > We deliberately left out the description of this mechanism because we think > that it is a substitute to mobile IP and orthogonal to most routing and > addressing approaches. I cannot catch up with this sentence, and I cannot see why the mechanism is not beneficial. By updating the DNS data, home-address is unneeded, no matter for communications between GLI-nodes and between GLI-hosts and classic IPv6 hosts. > You could introduce that already in today's Internet. In contrast, > GLI-Split provides a different substitute for mobile IP which does not > interact with DNS for the change of the location (see Section 3.7 in > http://www3.informatik.uni-wuerzburg.de/~menth/Publications/papers/Menth-GLI-Split.pdf). > It is applicable only if two GLI-hosts communicate with each other as > this mechanism does depend on the specific routing and addressing > architecture. > > Question remains as before. > > 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. >> > GLI-domains can accommodate upgraded and classic IPv6 hosts and the latter > do not need any upgrade. This was a major design goal to avoid costly > upgrades which can be an obstacle for deployment. The benefits of GLI-Split > are summarized in Section 5 of > > http://www3.informatik.uni-wuerzburg.de/~menth/Publications/papers/Menth-GLI-Split.pdf > Some of them are available only for upgraded hosts, others also for classic > hosts. > > I think I will learn more about the GLI's benefits on backward compatibility, which I underestimated before. > Kind regards, > > Michael > > -- > Dr. Michael Menth, Assistant Professor > University of Wuerzburg, Institute of Computer Science > Am Hubland, D-97074 Wuerzburg, Germany, room B206 > phone: (+49)-931/31-86644 (new), fax: (+49)-931/888-6632 > mailto:[email protected] > http://www3.informatik.uni-wuerzburg.de/research/ngn > > Thank you for your clarification. Best regards, Letong
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