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.
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).
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.
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.
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.
GLI-hosts need a home address for mobility. I think there¡¯s 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. 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.
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.
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
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