Hello all, Based on some discussions (on the list or privately), I revised my critique on GLI-Split. Warmly welcome for any critiques and revisions.
Best wishes, 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, also make the changes inside and outside GLI-domains invisible to their opposites. A main drawback of GLI-Split is that it puts 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 look up the map from the identifier extracted from the IP address to the local locator. If the communication is between different GLI-domains, hosts may further look up the map from the identifier to the global locator¡ª the local mapping system forwarding requests to the global mapping system for hosts is just an option. Though host lookup may ease the burden of intermediate nodes which would otherwise to perform the mapping lookup, the three lookups by hosts in the worst case may lead to large delays unless a very efficient mapping mechanism is devised. The work may also become unpractical for low-powered hosts. On one hand, GLI-split can provide backward compatibility where classic and upgraded IPv6 hosts can communicate, which is its big virtue; while the upgrades may be costly to against hosts¡¯ enthusiasm to change, compared to the benefits they would gain. GLI-split provides additional features to improve TE and to improve resilience, e.g., exerting multipath routing. However the cost is that more burdens are placed on hosts, e.g. they may need more lookup actions and route selections. However, the kind of tradeoffs between costs and gains exists in most proposals. I think one improvement of GLI-Split on its support for mobility is to update DNS data as GLI-hosts move across GLI-domains. Through this GLI-corresponding-node can query DNS to get valid global locator of the GLI-mobile-node and need not to query the global mapping system (unless it wants to do multipath routing), giving more incentives for nodes to become GLI-kind. The merit of GLI-Split, simplified-mobility-handover provision, well supports this improvement. GLI-Split claims to use rewriting instead of tunneling for conversions between local and global locators, when packets span GLI-domains. The major advantage is that this kind of rewriting needs no extra states to maintain, since local and global locators need not to map to each other. Many other rewriting mechanisms instead need to maintain extra states. It also avoids the MTU problem faced by the tunneling methods. However, GLI-Split achieves this only by compressing the namespace size of each attribute (identifier, local and global locator). GLI-Split codes two terms (identifier and local/global locator) into an IPv6 address, each has space size of 2^64 or less, while map-and-encaps proposals assume that identifier and locator each occupies 128 bits space, in the IPv6 scene.
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