Network Working Group R. Finlayson
Request for Comments: 2771 LIVE.COM
Category: Informational February 2000
An Abstract API for Multicast Address Allocation
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document describes the "abstract service interface" for the
dynamic multicast address allocation service, as seen by
applications. While it does not describe a concrete API (i.e., for a
specific programming language), it describes - in abstract terms -
the semantics of this service, including the guarantees that it makes
to applications.
Additional documents (not necessarily prodUCts of the IETF) would
describe concrete APIs for this service.
1. Introduction
Applications are the customers of a multicast address allocation
service, so a definition of this service should include not only the
inter-node network protocols that are used to implement it, but also
the "protocol" that applications use to Access the service. While
APIs ("application programming interfaces") for specific programming
languages (or operating systems) are outside the domain of the IETF,
it is appropriate for us to define - in abstract terms - the semantic
interface that this service presents to applications. Specific APIs
would then be based upon this abstract service interface.
Note that it is possible to implement the multicast address
allocation service in at least two different ways. The first (and
perhaps most common) way is for end nodes to allocate addresses by
communicating with a separate "Address Allocation Server" node, using
the "Host to Address Allocation Server" network protocol (MADCAP)
[1][7]. Alternatively, an "Address Allocation Server" implementation
might be co-located (along with one or more applications) on an end
node, in which case some other, internal, mechanism might be used to
access the server. In either case, however, the abstract service
interface (and, presumably, any specific APIs) would remain the same.
The remainder of this document describes the abstract interface.
Note that this interface is intended only for the allocation of
dynamic multicast addresses, as used by the traditional multicast
service model [2]. Future multicast service models might allocate or
assign multicast addresses in other ways, but this is outside the
scope of this document.
2. Abstract Data Types
The interface described below uses the following abstract data types:
- AddressFamily: e.g., IPv4 or IPv6
- MulticastAddress: An actual multicast address (i.e., that could
subsequently be used as the destination of a datagram)
- MulticastAddressSet: A set of "MulticastAddress"es
- LanguageTag: The code for a (human) language, as defined in [4]
- Scope: An "administrative scope" [3] from which multicast
addresses are to be allocated. Each scope is a
"MulticastAddressSet", with an associated set of
(character-string) names - indexed by "LanguageTag". (Each
language tag has at most one corresponding name, per
scope.) For each scope, a (language tag, name) pair may be
defined to be the "default" name for this scope. (See the
section "Querying the name of a scope" below.)
(An implementation of this abstract data type might also
include other information, such as a default TTL for the
scope.)
- Time: An (absolute) event time. This is used for specifying the
"lifetime" of multicast addresses: the period of time during
which allocated multicast addresses are guaranteed to be
available. (It is also used to specify the desired start
time for an "advance allocation".)
Note that a concrete API might prefer to specify some of
these times as relative times (i.e., relative to the current
time-of-day), rather than absolute time. (Relative times
have the advantage of not requiring clock synchronization.)
- Lease: A compound data type that describes the result of a
(successful) multicast address allocation. It consists of:
- [MulticastAddressSet] The set of addresses that were
allocated;
- [AddressFamily] The address family of these addresses
- [Time] The lifetime of these addresses (the same for
each address)
- [Time] The "start time" of the allocation. (See the
discussion of "advance allocation" below.) (A concrete
API would likely also include a MADCAP "Lease
Identifier" [1].)
- NestingRelationship: A binary data type that describes whether or
not two scopes nest. Two scopes nest if
traffic sent sent to a multicast group within
one scope could be seen by all hosts present
within the other scope were they to join the
multicast group within the first scope. This
value would be "False" for overlapping scopes
where only some (or none) of the hosts within
the second scope could see traffic sent to an
address due to the presence of an
administratively scoped boundary. In cases
where the first and second scopes are
topologically identical this value would be
"True."
- Status: A result code.
3. The Abstract Interface
3.1 Allocating multicast addresses:
alloc_multicast_addr(in AddressFamily family,
in Scope scope,
in Integer minDesiredAddresses,
in Integer maxDesiredAddresses,
in Time minDesiredStartTime,
in Time maxDesiredStartTime,
in Time minDesiredLifetime,
in Time maxDesiredLifetime,
out Lease multicastAddressSetLease,
out Status status)
This operation attempts to allocate a set of multicast addresses (the
size of this set is in the range [minDesiredAddresses,
maxDesiredAddresses]) within the given address family and scope, and
within a given range of desired lifetimes. ("minDesiredStartTime"
and "maxDesiredStartTime" are used to specify "advance allocation";
this is described in more detail below.)
If the address allocation succeeds, the result is returned in
"multicastAddressSetLease" (with "status" = OK).
During the lifetime of this lease, the allocation service will make a
"best-effort" attempt to not allocate any of these addresses to
others. (However, once the lease"s lifetime has eXPired, any of its
addresses can be allocated to others.)
Multicast addresses are allocated for a limited lifetime. An
application may attempt to extend this lifetime, but this operation
may fail. Therefore, an application must be prepared for the
possibility it will not be able to use the same addresses for as long
as it desires. In particular, the application must be prepared to
either quit early (because its original multicast address assignments
have expired), or, alternatively, to occasionally "renumber" its
multicast addresses (in some application or higher-level-protocol
dependent way), by making a new allocation. However, if an
application needs to consider "renumbering", it will always know this
in advance, at the time it acquired its current address(es) - by
checking the lifetime in the returned lease. An application will
never need to be notified asynchronously of the need to "renumber".
Possible errors:
- bad address family
- bad scope
- bad desired number of addresses (e.g., max < min)
- bad desired lifetimes (e.g., max < min)
- errors with the two "start time" parameters (see
"Advance allocation" below)
- no addresses can be allocated (for the requested parameters)
An allocation attempt can also fail with a result "status" code of
TRY_LATER, indicating that the requested allocation cannot be made at
this time, but that it might succeed if the caller retries the
attempt at some future time. (This future time is returned in the
"start time" field of the
"multicastAddressSetLease";
the other parts of this lease are undefined.)
Note that a concrete (i.e., programming language-specific) API for
multicast address allocation will probably include additional,
specialized variants of this general allocation operation. For
instance, it may include separate operations for:
- allocating only a single address
(i.e., minDesiredAddresses = maxDesiredAddresses = 1);
- (attempting to) allocate an address with a single, fixed
lifetime (i.e., minDesiredLifetime = maxDesiredLifetime);
- (attempting to) allocate an address for immediate use
(i.e., minDesiredStartTime = maxDesiredStartTime = "now")
3.2 Changing multicast addresses" lifetime:
change_multicast_addr_lifetime(in Lease multicastAddressSetLease,
in Time minDesiredLifetime,
in Time maxDesiredLifetime,
out Time lifetime)
This operation attempts to change the lifetime of previously
allocated multicast addresses. Unless an error occurs, it returns
the new lifetime (which might remain unchanged).
Possible errors:
- bad address family
- bad durations (e.g., max < min)
- the addresses" lifetime could not be changed
(and the existing lifetime was not in the requested range
[minDesiredLifetime,maxDesiredLifetime])
- the addresses were not ones that we had allocated
(see section 5.9) - or they have already expired
3.3 Deallocating multicast addresses:
deallocate_multicast_addr(in Lease multicastAddressSetLease) This
operation attempts to deallocate previously allocated multicast
addresses.
Possible errors:
- bad address family
- the addresses were not ones that we had allocated
(or they have already expired)
3.4 Querying the set of usable multicast address scopes:
get_multicast_addr_scopes(in AddressFamily family,
out "set of" Scope)
This operation returns the set of administrative multicast address
scopes that are defined for this node.
Possible errors:
- bad address family
3.5 Querying the name of a scope:
get_scope_name(in Scope scope,
in LanguageTag language,
out String name,
out LanguageTag languageForName)
This operation returns a character-string name for a given scope. If
the scope has a name in the specified "language", then this name (and
language) is returned. Otherwise, the scope"s default (language,
name) pair is returned.
Possible errors:
- bad scope.
3.6 Querying the nesting state of known usable multicast address scopes:
get_scope_nesting_state(in "set of" Scope,
out "matrix of" NestingRelationship)
Possible errors:
- bad scope.
- nesting state undetermined at this time.
This operation would return a matrix that shows the
current nesting relationships between the supplied
set of scopes which would have previously been supplied
via the get_multicast_addr_scopes(...) function.
3.7 Querying the set of scopes that a given scope is known to nest inside:
get_larger_scopes(in Scope,
out "set of" Scope)
This operation returns the set of administrative multicast
address scopes that are known to encompass the supplied
Scope.
Possible errors:
- bad scope.
- nesting state undetermined at this time.
3.8 Querying the set of scopes that are known to nest inside a given scope:
get_smaller_scopes(in Scope,
out "set of" Scope)
This operation returns the set of administrative multicast address
scopes that are known to nest inside the supplied Scope (NB this
would include those scopes that are topologically identical to the
supplied scope).
Possible errors:
- bad scope.
- nesting state undetermined at this time.
3.9 Note: The decision as to who is allowed to deallocate (or change
the lifetime of) a previously allocated multicast address set lease
is implementation-specific, and depends upon the security policy of
the host system. Thus it is not specified in this abstract API.
One possible starting point, however, is the following:
A previously allocated multicast address can be deallocated (or
have its lifetime queried or changed) by the same "principal", and
on the same node, as that which originally allocated it.
("principal" might, for example, be a "user" in the host operating
system.)
3.10 Advance allocation
By specifying "minDesiredStartTime = maxDesiredStartTime = "now"",
the address allocation operation - "alloc_multicast_addr" -
described above can be used to request a set of multicast
addresses that can be used *immediately* (and until their lifetime
expires). During this whole time, the addresses are not available
for allocation to others.
It is also possible - using the "minDesiredStartTime" and
"maxDesiredStartTime" parameters - to allocate multicast addresses
*in advance* - i.e., so that they have a future "start time" as
well as an expiration time. Before the start time, the multicast
addresses may be allocated to others.
Advance allocation is convenient for allocating addresses for
events that begin far in the future - e.g., several weeks or
months away. Without advance allocation, it would be necessary to
allocate addresses for a long period of time - even when it will
not be used. Such a request would not only be a wasteful use of
the multicast address space, but it may also be difficult to
implement (especially since address allocations are expected to
remain valid in spite of topology changes).
Advance allocation requests can produce the following errors (in
addition to those defined earlier):
- bad start time durations (e.g., max < min)
- requested start times conflict with requested lifetimes
(i.e., min start time > max lifetime)
The following operation is also defined:
change_multicast_addr_start_time(in Lease multicastAddressSetLease,
in Time minDesiredStartTime,
in Time maxDesiredStartTime,
out Time startTime)
This operation attempts to change the start time of previously
allocated multicast addresses. Unless an error occurs, it returns
the new start time (which might remain unchanged).
Possible errors: the same as "change_multicast_addr_lifetime"
4. Security Considerations
As noted in section 5.9 above, each implementation of this abstract
API should define a security policy that specifies when (and by whom)
previously allocated multicast addresses can be deallocated (or
queried, or have their lifetime changed).
Because multicast addresses are a finite resource, there is a
potential for a "denial of service" attack by allocating a large
number of multicast addresses without deallocating them. Preventing
such an attack, however, is not the role of the API, but rather by
the underlying MAAS ("Multicast Address Allocation Server(s)" [6]).
5. Acknowledgements
Many thanks to other participants in the "MALLOC" working group - in
particular Steve Hanna, Dave Thaler, Roger Kermode, and Pavlin
Radoslavov - for their valuable comments.
6. References
[1] Hanna, S., Patel, B. and M. Shah, "Multicast Address Dynamic
Client Allocation Protocol (MADCAP)", RFC2730, December 1999.
[2] Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
1112, August 1989.
[3] Meyer, D., "Administratively Scoped IP Multicast", BCP 23, RFC
2365, July, 1998.
[4] Alvestrand, H., "Tags for the Identification of Languages", RFC
1766, March 1995.
[5] Handley, M. and V. Jacobson, "SDP: Session Description Protocol",
RFC2327, April 1998.
[6] Estrin, D., Handley, M. and D. Thaler, "The Internet Multicast
Address Allocation Architecture", Work in Progress.
[7] Kermode, R., "MADCAP Multicast Scope Nesting State Option", Work
in Progress.
7. Author"s Address
Ross Finlayson,
Live Networks, Inc. (LIVE.COM)
EMail: finlayson@live.com
WWW: http://www.live.com/
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