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INTERNET DRAFT M. Ohta
draft-ohta-sun-00.txt Tokyo Institute of Technology
November 1996
Simple Unified Networking
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as ``work in progress.''
To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet- Drafts
Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).
Abstract
The concept of LIS for IP over ATM causes a topology mismatch between
the link and the internetworking layer. While it introduces some
inefficiency with CATENET based operation, it is not so much a
problem unless we try to solve this minor problem.
Short-cutting attempts such as NHRP can't solve the inefficiency
issue at all even though, or, just because, it utterly destroys the
CATENET model, which resulted in inelegant modifications of existing
protocols, which, in turn, causes scalability problems.
Moreover, the creation of short-cut VCs itself suffers a scalability
issue.
But, CSRs (Cell Switching Routers), or RSVP-signaled ATM switches,
make it possible to have end-to-end cell-by-cell relaying over IP
routers. That is, there is no reason to have LISes and there is no
inefficiency
The way to go for the Internet is Simple Unified Networking with the
CATENET model.
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INTERNET DRAFT Simple Unified Networking November 1996
1. Introduction
See RFC1620 [RFC1620].
2. Inefficiency Remains
Consider the following configuration:
Ha Hb Hc Hd
| | | |
---- | ---------- | ---------- | ---------- | ----
| __|__ __|__ __|__ __|__ |
( ) ( ) ( ) ( )
| ( ) ( ) ( ) ( ) |
( Net ) ( Net ) ( Net ) ( Net )
| ( A ) ( B ) ( C ) ( D ) |
( ) ( ) ( ) ( )
| ( ) ( ) ( ) ( ) |
(_____) (_____) (_____) ( ____)
| | | | | | | | | |
--- | | -------- | | -------- | | -------- | | ---
| | | | | | | |
R1 - --- R2 --- --- R3 --- --- R4 --- --- R5
| |
---------------- R6 --------------- R7 ----------
|
He
where Nets A, B, C and D are highly logical LIS in a large shared
medium network.
For example, suppose R1 is located at Frankfurt, Ha Sunnyvale, R2
Montreal, R3 Memphis, R4 Kuala Lumpur, Hd San Jose, R5 Mountain View,
R6 Danvers, R7 Menlo Park, and He Palo Alto.
Then, without shortcutting, Ha and Hd may communicate hop-by-hop from
Sunnyvale, Montreal, Memphis, Kuala Lumpur and finally to San Jose.
Not an inefficient path.
The problem is that routing metric at the Internetworking layer does
not reflect the real world metric at all.
But, if we can somehow make use of the fact that Ha and Hd are placed
in a single shared medium, Ha and Hd can communicate locally within
Silicon Valley between Sunnyvale and San Jose.
That's the inefficiency issue that mechanisms in RFC 1620 wanted to
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INTERNET DRAFT Simple Unified Networking November 1996
resolve.
The problem is that though the inefficiency may be removed within the
shared medium, it's not the only inefficiency.
When Ha and He communicate over a path Ha-R6-R7-He, the traffic will
pass from Sunnyvale, Frankfurt, Danvers, Menlo Park and finally to
Palo Alto, even though the path Ha-R5-R7-He exists within Silicon
Valley.
The inefficiency can be avoided by raising metric within the shared
medium. But, it causes other type of inefficiency. That is, the
shared medium will not be used for transit even though the physically
shortest path exists over it.
The problem is that routing metric at the Internetworking layer
within the shared media does not reflect the real world metric at
all.
When a LIS contains hosts at room 1035, Fairmont Hotel, San Jose;
room 1036, Fairmont Hotel, San Jose; Holidy Inn San Jose; Palo Alto;
Los Angels and Frankfurt; there is no meaning metric for the LIS to
be used outside of the LIS.
Also, It is obvious that no intra-shared-media protocol can solve the
route selection problem outside of the medium at R7.
That is, routing metric should be mostly proportional to the physical
distance. Then, the least metric path will be almost optimal.
It means that LISes should not be so logical and mostly contiguous.
As a result, the CATENET model with no extension works efficiently
over the shared media.
3. Inscalability Problems
3.1 RSVP Inscalability
As the Internet protocols are designed with the CATENET model,
modification to the model naturally makes some protocol not work and
other protocol not to scale.
For example, RSVP scales to the number of recipients because RESV
messages are merged on routers upstream toward the sender.
But, in a large shared medium with no intermediate entity to
recognize IP, merger of the RESV messages is impossible.
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As it is essential to merge RESV at the data branch point, RESV
merging servers external to the shared medium does not work.
That is, all the RESV messages concentrate and implode at the
upstream most router or the sender on the shared medium, which means
not so many recipients can be supported.
Note that, in the worst case when most of the hosts in the shared
medium are the recipients, the amount of imploding packets is almost
equal to the amount of ATMARP packets for a single ATMARP server
receives, if the entire shared medium is served by a single ATMARP
server.
That is, on multicast-aware shared medium, it's enough to make the
entire medium a single subnet, maybe with SCSP.
3.2 VC Shortage at the Egress Router
It is unlikely that the Internet mostly consists of a large single
shared medium.
Thus, when hosts in a shared medium wants to communicate to the
Internet outside of the medium, the egress routers must be directly
connected to each such host through a dedicated VC.
But, shared medium can support only a limited number of VCs for a
single node.
On existing commercial shared medium service such as X.25 or
framerelay, it is typical that the number of supported VCs is less
than 100.
It is typical that ATM switches can support only several thousands of
VCs for each port.
Thus, not so many hosts can communicate with the external Internet
efficiently
Other hosts can still communicate hop-by-hop. But, as the size of the
shared medium glows, the efficiency as a whole approaches that of
hop-by-hop.
That is, it is necessary to make the hop-by-hop communication
efficient by not making LISes logical, which means that no
inefficiency exist to be removed by shortcutting attempt.
4. Cell Switching Routers
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It seems to the Author that some people thought that cell-by-cell
relaying was impossible over IP routers, which, seemingly, motivated
them to support shortcutting over ATM shared medium.
While it was understandable, cell-by-cell relaying over IP routers is
possible.
The point is that it is possible to signal ATM switches with RSVP
[RSVP], ST2 [ST2], IFMP [IFMP] or some other IP-based signaling
protocol.
Then, switch-local traffic control module sets up the cell switching
fabric appropriately.
The ATM switch signaled by IP is, in general, called CSR (Cell
Switching Routers) [CSR1, CSR2].
5. Conclusion.
RFC 1620 was wrong.
While it suggests several ways to have shortcuts, note that the
discussions in section 2 and 3 does not depend on how the shortcuts
are created. That is, modifications on the way to have shortcuts does
not affect the conclusion that they are no good.
It is not necessary nor possible to modify the CATENET model, the
architecture of the Internet, to have efficient and scalable Internet
to accommodate shared medium such as ATM.
The Simple Unified Networking with the CATENET model is the way to
go.
6. References
[CSR1] Hiroshi ESAKI, Ken-ichi NAGAMI, Masataka OHTA, "High Speed
Datagram Delivery over Internet using ATM Technology",
Networld+Interop '95 Engineer Conference, E12-1~E12-9, (1995).
[CSR2] Yukinori GOTO, Masataka OHTA, Masaki HIRABARU, "Design of
Internet Resource Reservation on ATM Network", Proceedings of The
10th International Conference on Information Networking (ICOIN-10),
pp.510-516, 1996.
[IFMP]
[RFP1620]
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[RSVP]
[ST2]
7. Security Considerations
(to be filled)
8. Author's Address
Masataka Ohta
Computer Center
Tokyo Institute of Technology
2-12-1, O-okayama, Meguro-ku
Tokyo 152, JAPAN
Phone: +81-3-5734-3299
Fax: +81-3-5734-3415
EMail: mohta@necom830.hpcl.titech.ac.jp
M. Ohta Expires on May 27, 1997 [Page 6]