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MBONE Deployment P. Savola
Internet-Draft CSC/FUNET
Intended status: Informational March 3, 2006
Expires: September 4, 2006
Lightweight Multicast Address Discovery Problem Space
draft-ietf-mboned-addrdisc-problems-02.txt
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Copyright (C) The Internet Society (2006).
Abstract
Typically applications developers have requested static IANA
assignments for their applications, even if the applications would
typically be only used within a site, between consenting sites, or
would not eventually even use multicast at all. This memo describes
this problem space, and summarizes a number of proposed approaches to
mitigating these problems.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
3. Mitigation Techniques . . . . . . . . . . . . . . . . . . . . 5
3.1. Locally Scoped Address Assignment by a Registry . . . . . 5
3.2. Single Administration Address Discovery with Server
Configuration . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Zero-configuration Single Administration Address
Discovery . . . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Global Multiple Administration Address Discovery . . . . . 8
4. DNS As a Means of Indirection . . . . . . . . . . . . . . . . 8
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 11
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1. Introduction
There are a lot of different challenges relating to the discovery and
use of an appropriate multicast address as described below. This
document only addresses the second one.
a. Getting a list of all the available (public) sessions which one
could join (and possibly send) to, similar to a "TV guide". That
is, having to know everything before you can decide if you're
interested in something or not; this is out of scope for this
memo.
b. Discovering the multicast address used by a particular
application for particular purpose, within or crossing a scope.
I.e., you know what you're looking for, but don't know if the
session has been created already and what its address would be.
c. The different sources (unicast addresses) participating in a
session. For ASM, there is no need for such discovery. For SSM,
this is expected to happen out-of-band or following separate
requirements [I-D.lehtonen-mboned-dynssm-req].
Many applications have been written which leverage or could leverage
multicast routing infrastructures, in one or more of the following
scopes: (We'll get back to these later.)
1. link-local scope,
2. [geographical] site scope,
3. organization-local scope,
4. global scope, used between consenting sites/enterprises, also
including cases like "inside a country", or
5. a truly global scope.
Multicast-leveraging applications are often designed in such a manner
that each multicast group has a "server", "session creator" or some
other node(s) (or persons operating the nodes) which are in some way
in control of the application.
Both the "server" and "client end" of an application are currently
typically provisioned with the group address using static IANA
assignment [I-D.ietf-mboned-addrarch]. Only rarely these apps are
manually configured e.g. with locally scoped addresses even in cases
where using local addresses would be feasible.
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It would be highly desirable (as described in Section 2) that the
applications could easily use more dynamic, and more scoping-friedly
mechanisms for discovering the appropriate addresses to use.
All of these issues are only relevant to Any Source Multicast (ASM),
as SSM requires this information is known a priori.
2. Problem Statement
The current IANA static assignment to applications is a problem for
multiple reasons:
1. This messes up the multicast scoping plans which the site may
have. Each application's global address must be individually
scoped and filtered in all the routers and in their access lists.
Scoping should be easier.
2. Static IANA assignments are required for each application; a
permanent global assignment for each potentially multicast-using
application depletes the resource quickly.
3. This has issues with IPv6, because such IPv6 addresses can not be
scalably routed in inter-domain routing; in intra-domain, this
requires manual configuration or running BSR (for ff01::/16 or
ff02::/16 or the like)
4. "Intended for local only use" applications typically leak through
to the IPv4 MSDP because there is no clear logic which ones
should be global and which ones are local.
There are at least four different proposed ways to mitigate this,
from the least to most extensive:
a. Smaller-than-global single-administration address assignment by a
registry (from 239/8 or elsewhere).
b. Smaller-than-global single-administration discovery, with the
expectation that a locally scoped address is manually configured
on the "server end".
c. Smaller-than-global single-administration discovery with complete
zero-configuration.
d. Global (but restricted) multi-administration discovery with some
amount of manual configuration.
We'll outline each proposed mitigation technique briefly below.
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NOTE: David Meyer's experience from being the designated expert for
IANA assignments is that almost all of the requested multicast
addresses have been such that the requestors would not have been
satisfied if their application would only be restricted to operate
within a site.
If people agree on this, the first three mitigation techniques won't
have significant impact, because the application developers won't
implement the discovery in any case. They will _still_ want to get
the globally scoped addresses from IANA, instead of implementing the
"service discovery inside an organization" -shim.
3. Mitigation Techniques
The generic goals from the application/deployment perspective are:
o Not depending on any uncommon external infrastructure besides the
application itself (e.g., a MADCAP [RFC2730] server), so that the
application can be deployed where MADCAP server is not present.
I.e., this should be sufficiently lightweight to be coded in the
application or be used by a simple application shim library.
o The application should "just work" from perspective of "client
end" without any configuration. "Server end" may or may not
require configuration of an address.
o The presence of applications should be easily filterable at least
at the edges of the network.
o Preferably it should also be easy to segment the use of
application into the smallest possible scopes within the network,
to avoid undue state and confusion in the network.
o We'll look at using DNS as an additional component in Section 4.
3.1. Locally Scoped Address Assignment by a Registry
If we ignore requirements for different levels of scoping, the
simplest approach would be to make globally unique assignments within
a well known local scoped address block. Group address assignments
could be made by IANA (or some other registry) to applications on a
first come first serve basis, much like what is done for port numbers
used in protocols such as SCTP, TCP and UDP. This well defined range
could then be scoped at the public Internet boundaries to ensure
private usage remains private.
Theoretically, reserved space within the administratively scoped
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address range (239/8) defined by RFC 2365 [RFC2365] that could be
used for such a purpose. This address range should even be scoped at
public Internet boundaries already. However, many organizations are
already making use of the entire administratively scoped range. For
better or worse, we feel that it is now too late to reclaim the
reserved address space within the administratively scoped range for
the problem case described here even if it were to be appropriate to
do so.
If we consider multiple levels of scoping, using static address
assignments may be problematic for sites with the need to separate
applications between their local boundaries. If no other scoping
mechanism is used, the network would have to create and maintain
complex forwarding and filtering rules to ensure group membership for
the well defined group address does not leak outside the desired
local scope.
Even if a well defined local scope address range could be used and
additional levels of scoping were not an issue, it is not clear that
multicast application developers would accept a local scoped address
over a globally routable one. Given the choice of a local scoped
assignment and a global one, what incentive is there for an
application developer to choose a locally scoped one if there is even
a faint chance of global usage?
IANA is not the only option for such a registry; for example,
Regional Internet Registries could perform assignments if there was
demand, as described by the "eGLOP" proposal [RFC3138]. No registry
has yet taken up the offer though, and doing so would be useful only
if IANA ceased making assignments itself.
An additional, fundamental question with static address assiginment
in the IPv4 multicast address space (224/4) is, how big of an address
range should be reserved? Existing multicast applications in use
have been written to use an address block as large as a /8. Any
allocation on this order is clearly diminishing the limited pool of
already limited IPv4 multicast addresses.
3.2. Single Administration Address Discovery with Server Configuration
The second mitigation technique would be to specify and implement a
mechanism, requiring no infrastructure in the network, where the
"server end" would be manually configured with appropriately selected
locally-scoped addresses which the clients would use to discover the
group address.
The client ends should discover the smallest possible scope where the
application is supported.
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A few notes on this method:
o One could characterize a potential solution as an easily
implementable server shim at "server end" listening to a set well-
known locally-scoped multicast addresses (e.g., a scope-relative
address at each local scope), which would respond to queries by
"client end".
o How can the servers demultiplex "queries" sent to these addresses?
Are these messages in SAP format or something simpler? The query
must have an identifier (e.g., done by hashing a name?) which the
server uses to know the client is interested in the server's
multicast transmission.
o How should the servers communicate back to the clients? By
replying with unicast (issues after bootup with lots of nodes) or
do the clients also join the address (DoS potential, a very
crowded group which all the servers at least need to subscribe
to)?
Again, this does not solve the root problem; why would an application
designer implement this mechanism when he/she wants to support global
scoping as well? IANA assignment will be requested in any case.
3.3. Zero-configuration Single Administration Address Discovery
A slightly more extensive problem is the same as above, but assuming
that the application must be completely zero-configurable. That is,
it must work without having to manually configure anything on the
server end.
This could be achieved e.g., by adding to the above a SAP-like
address blocks from which the addresses could be dynamically
reserved. This might not sit well on the organization's local
scoping plans, however.
However, it is worth considering whether this is really needed. For
link-local scope, this may be desirable as such requires no set-up of
multicast routing. But for larger scopes, is this really useful? If
there is no administrator to configure the address, likely there is
no multicast infrastructure in the first place, or desire to run the
application in multicast mode!
Again, this does not solve the root problem.
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3.4. Global Multiple Administration Address Discovery
Most applications are such that they _can_ be run over site/
organization boundaries (even if they typically would not be), so the
application developers will want to support the most extensive scope.
There is no common local scope (even between organization-local and
global) which could cover these disjoint global interconnections, so
the applications must use global scope addresses.
To get away from static IANA assignments, there should be a
lightweight multicast address discovery function which could be used
e.g., in the embedded devices to discover the appropriate multicast
address they should use.
Obviously, the result could also be that the application should be
restricted to a local scope, and use local scope addresses, but wider
discovery should also be supported.
This approach has a number of challenges, however. It's difficult to
visualize how multiple administrative domains could perform discovery
in a desired manner automatically -- we have to assume that the sites
might not want to tell about all of their local sessions to all the
other sites (i.e., you may want to allow site A to discover session
1, and site B to discover session 2, but not mix these). In other
words, there will likely need to be some manual control of what gets
seen to the outside and what not. This makes the mechanism more
complicated, and requires more network operator management.
Further attributes and requirements for this kind of approach remain
to be figured out.
4. DNS As a Means of Indirection
DNS could be leveraged as an additional configuration mechanism with
varying usefulness in any of the preceding approaches. The relevant
information could be stored in the DNS in mainly two different ways:
1. Using local information (e.g., DNS search path, reverse IP
addresses, etc.); these have been analyzed in Section 3.2 of
[I-D.palet-v6ops-tun-auto-disc], or
2. By global information, for example having IANA assign an
application-specific DNS entry (e.g., "_dogfight.apps.mcast.net")
instead of an address. The sites could either use a default
value (which might point to e.g., scoped address space) or make
their DNS servers authoritative for that zone and insert their
own addresses.
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Both assume the ability to (e.g., manually) insert records in the
local DNS and perform DNS lookups. The former additionally assumes
the capability to discover where to find the local information. If
these assumptions are acceptable, DNS could be used as an additional
mechanism at least with the first two classes of mitigation
techniques.
5. Acknowledgements
This memo grew out of the discussions in MBONED WG, where the
participants were, among others, Beau Williamson, Albert Manfredi,
Marshall Eubanks, John Kristoff, David Meyer, Stig Venaas, Rami
Lehtonen, and Leonard Giuliano.
6. IANA Considerations
This memo includes no request to IANA.
[[Note to the RFC-Editor: this section should be removed prior to
publication.]]
7. Security Considerations
As section Section 3.4 describes, the organizations will not want to
expose all their sessions, or even knowledge that the organization is
using a particular application, to the outside. The confidentiality
needs must be considered when designing a solution to solve one of
the problems in this problem space.
8. References
8.1. Normative References
[I-D.ietf-mboned-addrarch]
Savola, P., "Overview of the Internet Multicast Addressing
Architecture", draft-ietf-mboned-addrarch-03 (work in
progress), October 2005.
8.2. Informative References
[I-D.lehtonen-mboned-dynssm-req]
Lehtonen, R., "Requirements for discovery of dynamic SSM
sources", draft-lehtonen-mboned-dynssm-req-00 (work in
progress), February 2005.
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[I-D.palet-v6ops-tun-auto-disc]
Palet, J. and M. Diaz, "Analysis of IPv6 Tunnel End-point
Discovery Mechanisms", draft-palet-v6ops-tun-auto-disc-03
(work in progress), January 2005.
[RFC2365] Meyer, D., "Administratively Scoped IP Multicast", BCP 23,
RFC 2365, July 1998.
[RFC2730] Hanna, S., Patel, B., and M. Shah, "Multicast Address
Dynamic Client Allocation Protocol (MADCAP)", RFC 2730,
December 1999.
[RFC3138] Meyer, D., "Extended Assignments in 233/8", RFC 3138,
June 2001.
Author's Address
Pekka Savola
CSC/FUNET
Espoo
Finland
Email: psavola@funet.fi
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