Dear Sun Letong,
Charrie Sun schrieb:
Hello Michael,
Questions are inline.
2010/1/19 Michael Menth <me...@informatik.uni-wuerzburg.de
<mailto:me...@informatik.uni-wuerzburg.de>>
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.
There may be two mapping lookups but only one conversion from identifier
address to either local or global identifier.
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.
The first-packet-delay in hosts is not crucial as packets do not need to
be stored in intermediate nodes. Describing it as "large delay" sounds
like tremendously worse than DNS-lookup delay which is rather of the
same quality.
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.
Unburdening intermediate hosts is more important than unburdening hosts
as they have time to do lookup operations etc. without storing packets.
If you want, you can mention the contrary. GLI-gateways need to
substitute local source addresses by global source addresses, but a
lookup is not required for that operation.
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.
As I said, the mechanisms adds more complexity only if it is used in
case it is needed. Potential multipath support should not be viewed as a
disadvantage!
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.
Let me try again. Updating DNS with the current location instead of
using mobile IP is a general mechanism which could be done whenever DNS
is in use - actually even today (but it's not of course!). This feature
can be easily added to any architecture because it does not really
depend on it. It can also be added to GLI-Split if it is desired. Your
mentioned mechanism depends on how fast DNS can be updated. In the
GLI-Split paper, we propose an alternative mechanism for GLI-hosts that
bypasses DNS.
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
<http://www3.informatik.uni-wuerzburg.de/%7Ementh/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.
Hm, which question?
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
<http://www3.informatik.uni-wuerzburg.de/%7Ementh/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.
Feel free to ask if you have any questions!
Regards,
Michael
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:me...@informatik.uni-wuerzburg.de
<mailto:me...@informatik.uni-wuerzburg.de>
http://www3.informatik.uni-wuerzburg.de/research/ngn
Thank you for your clarification.
Best regards,
Letong
--
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:me...@informatik.uni-wuerzburg.de
http://www3.informatik.uni-wuerzburg.de/research/ngn
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