Dear Sun Letong,
Charrie Sun schrieb:
Hi Michael:
2010/1/19 Michael Menth <[email protected]
<mailto:[email protected]>>
Dear Sun Letong,
Charrie Sun schrieb:
Hello Michael,
Questions are inline.
2010/1/19 Michael Menth <[email protected]
<mailto:[email protected]>
<mailto:[email protected]
<mailto:[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.
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.
It is no worse than DNS-lookup in general, but the delay itself would
be troublesome. I do not understand why the first-packet-delay is not
crucial for hosts. Why because of that packets not need to be stored
in intermediate nodes? If the end host itself buffer the packets whose
mapping is unresolved, large delay would lead waiting packets overflow
the buffer; if it does not, large delay may cause packet loss. Is it
not important?
When intermediate nodes need perform a mapping lookup, there are three
different options for the packets:
1) discard it (undesired)
2) queue it (though buffer overflow can occur which is a way for new
attacks)
3) forward it, e.g., over the mapping system (more complex)
This problem is avoided when mapping lookups are performed by the hosts
instead of intermediate nodes.
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.
I think that updating DNS would make home-address unneeded, and more
importantly, hosts may resolve the domain name and get the GLI address
which already contains the valid global locator, and hosts need not
query the global mapping system for that information once more.
Home addresses are still required for interoperability with legacy
systems. Therefore, you cannot completely renounce on the maintenance of
home addresses if you want to communicate with legacy.
Best wishes,
Michael
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>
<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?
Hm... The above 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>
<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:[email protected]
<mailto:[email protected]>
<mailto:[email protected]
<mailto:[email protected]>>
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:[email protected]
<mailto:[email protected]>
http://www3.informatik.uni-wuerzburg.de/research/ngn
Best regards,
Leong
--
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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