| Rfc | 2730 |
|---|
| Title | Multicast Address Dynamic Client Allocation Protocol (MADCAP) |
|---|
| Author | S.
Hanna, B. Patel, M. Shah |
|---|
| Date | December 1999 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
Network Working Group S. Hanna
Requests for Comments: 2730 Sun Microsystems, Inc.
Category: Standards Track B. Patel
Intel Corp.
M. Shah
Microsoft Corp.
December 1999
Multicast Address Dynamic Client Allocation Protocol (MADCAP)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
Abstract
This document defines a protocol, Multicast Address Dynamic Client
Allocation Protocol (MADCAP), that allows hosts to request multicast
addresses from multicast address allocation servers.
1. Introduction
Multicast Address Dynamic Client Allocation Protocol (MADCAP) is a
protocol that allows hosts to request multicast address allocation
services from multicast address allocation servers. This protocol is
part of the Multicast Address Allocation Architecture being defined
by the IETF Multicast Address Allocation Working Group. However, it
may be used separately from the rest of that architecture as
appropriate.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [9].
Constants used by this protocol are shown as [NAME-OF-CONSTANT], and
summarized in Appendix B.
1.2. Definitions
This specification uses a number of terms that may not be familiar to
the reader. This section defines some of these and refers to other
documents for definitions of others.
MADCAP client or client
A host requesting multicast address allocation services via MADCAP.
MADCAP server or server
A host providing multicast address allocation services via MADCAP.
Multicast
IP Multicast, as defined in [11] and modified in [12].
Multicast Address
An IP multicast address or group address, as defined in [11] and
[13]. An identifier for a group of nodes.
Multicast Scope
A range of multicast addresses configured so that traffic sent to
these addresses is limited to some subset of the internetwork. See
[3] and [13].
Scope ID
The lowest numbered address in a multicast scope. This definition
applies only within this document.
Scope Zone
One multicast scope may have several instances, which are known as
Scope Zones or zones, for short.
For instance, an organization may have multiple sites. Each site
might have its own site-local Scope Zone, each of which would be an
instance of the site-local Scope. However, a given interface on a
given host would only ever be in at most one instance of a given
scope. Messages sent by a host in a site-local Scope Zones to an
address in the site-local Scope would be limited to the site-local
Scope Zone containing the host.
Zone Name
A human readable name for a Scope Zone. An ISO 10646 character
string with an RFC 1766 [6] language tag. One zone may have several
zone names, each in a different language. For instance, a zone for
use within IBM's locations in Switzerland might have the names "IBM
Suisse", "IBM Switzerland", "IBM Schweiz", and "IBM Svizzera" with
language tags "fr", "en", "de", and "it".
Multicast Scope List
A list of multicast scope zones.
Since it can be difficult to determine which multicast scope zones
are in effect, MADCAP clients can ask MADCAP servers to supply a
Multicast Scope List listing all of the zones available to the
client. For each scope zone, the list includes the range of
multicast addresses for this scope, a maximum TTL or hop count to
be used for this scope, and one or more zone names for this scope
zone.
This definition applies only within this document.
1.3. Motivation and Protocol Requirements
For multicast applications to be deployed everywhere, there is a need
to define a protocol that any host may use to allocate multicast
addresses. Here are the requirements for such a protocol.
Quick response: The host should be able to allocate a multicast
address and begin to use it promptly.
Low network load: Hosts that are not allocating or deallocating
multicast addresses at the present time should not need to send or
receive any network traffic.
Support for intermittently connected or power managed systems: Hosts
should be able to be disconnected from the network, powered off, or
otherwise inaccessible except during the brief period during which
they are allocating a multicast address.
Multicast address scopes: The protocol must be able to allocate both
the administratively scoped and globally scoped multicast addresses.
Efficient use of address space: The multicast address space is fairly
small. The protocol should make efficient use of this scarce
resource.
Authentication: Because multicast addresses are scarce, it is
important to protect against hoarding of these addresses. One way to
do this is by authenticating clients. This is also a key prerequisite
for establishing policies.
Policy neutral: Allocation policies (such as who can allocate
addresses) should not be dictated by the protocol.
Conferencing support: When allocating an address for use in a
conferencing environment, members of the conference should be able to
modify a multicast address lease used for the conference.
1.4. Relationship with DHCP
MADCAP was originally based on DHCP. There are still some
similarities and it may be possible to share some code between a DHCP
implementation and a MADCAP implementation. However, MADCAP is
completely separate from DHCP, with no dependencies between the two
and many significant differences.
1.5. Protocol Overview
MADCAP is built on a client-server model, where hosts request address
allocation services from address allocation servers. When a MADCAP
client wishes to request a service, it unicasts or multicasts a
message to one or more MADCAP servers, each of which optionally
responds with a message unicast to the client.
All messages are UDP datagrams. The UDP data contains a fixed length
header and a variable length options field. Options are encoded in a
type-length-value format with two octets type and value fields. The
fixed fields are version, msgtype (message type), addrfamily (address
family), and xid (transaction identifier).
Retransmission is handled by the client. If a client sends a message
and does not receive a response, it may retransmit its request a few
times using an exponential backoff. To avoid executing the same
client request twice when a retransmitted request is received,
servers cache responses for a short period of time and resend cached
responses upon receiving retransmitted requests.
Each request contains a msgtype, an xid, and a Lease Identifier
option. Clients must ensure that this triple is probably unique
across all MADCAP messages received by a MADCAP server over a period
of [XID-REUSE-INTERVAL] (10 minutes). This allows the MADCAP server
to use this triple as the key in its response cache.
Messages sent by servers include the xid included in the original
request so that clients can match up responses with requests.
The msgtype field is a single octet that defines the "type" of a
MADCAP message. Currently defined message types are listed in Table
2. They are: DISCOVER, OFFER, REQUEST, RENEW, ACK, NAK, RELEASE, and
GETINFO. DISCOVER, REQUEST, RENEW, RELEASE, and GETINFO messages are
only sent by a client. OFFER, ACK, and NAK messages are only sent by
a server.
The REQUEST, RENEW, and RELEASE messages are used to request, renew,
or release a lease on one or more multicast addresses. A client
unicasts one of these messages to a server and the server responds
with an ACK or a NAK.
The GETINFO message is used to request information, such as the
multicast scope list, or to find MADCAP servers. A client may unicast
an GETINFO message to a MADCAP server. However, it may not know the
IP address of any MADCAP server. In that case, it will multicast an
GETINFO message to a MADCAP Server Multicast Address and all servers
that wish to respond will send a unicast ACK or NAK back to the
client.
Each multicast scope has an associated MADCAP Server Multicast
Address. This address has been reserved by the IANA as the address
with a relative offset of -1 from the last address of a multicast
scope. MADCAP clients use this address to find MADCAP servers.
The DISCOVER message is a message used to discover MADCAP servers
that can probably satisfy a REQUEST. DISCOVER messages are always
multicast. Servers that can probably satisfy a REQUEST corresponding
to the parameters supplied in the DISCOVER message temporarily
reserve the addresses needed and send a unicast OFFER back to the
client. The client selects a server with which to continue and sends
a multicast REQUEST including the server's Server Identifier to the
same multicast address used for the DISCOVER. The chosen server
responds with an ACK or NAK and the other servers stop reserving the
addresses they were temporarily holding.
For detailed descriptions of typical protocol exchanges, consult
Appendix A.
MADCAP is a mechanism rather than a policy. MADCAP allows local
system administrators to exercise control over configuration
parameters where desired. For example, MADCAP servers may be
configured to limit the number of multicast addresses allocated to a
single client. Properly enforcing such a limit requires cryptographic
security, as described in the Security Consideration section.
MADCAP requests from a single host may be sent on behalf of different
applications with different needs and requirements. MADCAP servers
MUST NOT assume that because one request from a MADCAP client
supports a particular optional feature (like Retry After), future
requests from that client will also support that optional feature.
2. Protocol Description
The MADCAP protocol is a client-server protocol. In general, the
client unicasts or multicasts a message to one or more servers, which
optionally respond with messages unicast to the client.
A reserved port number dedicated for MADCAP is used on the server
(port number 2535, as assigned by IANA). Any port number may be used
on client machines. When a MADCAP server sends a message to a MADCAP
client, it MUST use a destination port number that matches the source
port number provided by the client in the message that caused the
server to send its message.
The next few sections describe the MADCAP message format and message
types. A full list of MADCAP options is provided in section 3.
2.1. Message Format
Figure 1 gives the format of a MADCAP message and Table 1 describes
each of the fields in the MADCAP message. The numbers in parentheses
indicate the size of each field in octets. The names for the fields
given in the figure will be used throughout this document to refer to
the fields in MADCAP messages.
All multi-octet quantities are in network byte-order.
Any message whose UDP data is too short to hold this format (at least
12 bytes) MUST be ignored.
+-+-+-+-+-+-+-+-+
| version (1) |
+---------------+
| msgtype (1) |
+---------------+
| addrfamily |
| (2) |
+---------------+
| |
| xid (4) |
| |
| |
+---------------+
| |
| options |
| (variable) |
| ... |
+---------------+
Figure 1: Format of a MADCAP message
FIELD OCTETS DESCRIPTION
----- ------ -----------
version 1 Protocol version number (zero for this specification)
msgtype 1 Message type (DISCOVER, GETINFO, etc.)
addrfamily 2 Address family (IPv4, IPv6, etc.)
xid 4 Transaction ID
options var Options field
Table 1: Description of fields in a MADCAP message
2.1.1. The version field
The version field must always be zero for this version of the
protocol. Any messages that include other values in this field MUST
be ignored.
2.1.2. The msgtype field
The msgtype field defines the "type" of the MADCAP message.
For more information about this field, see section 2.2.
2.1.3. The addrfamily field
The addrfamily field defines the default address family (such as IPv4
or IPv6) for this MADCAP message, using the address family numbers
defined in by the IANA (including those defined in [10]). Unless
otherwise specified, all addresses included in the message will be
from this family.
2.1.4. The xid field
The xid field is a transaction identifier. This number MUST be chosen
by the client so that the combination of xid, msgtype, and Lease
Identifier is unique across all MADCAP messages received by a MADCAP
server over a period of [XID-REUSE-INTERVAL] (10 minutes).
The xid field is used by the client and server to associate messages
and responses between a client and a server. Before a client sends a
message, it chooses a number to use as an xid. The technique used to
choose an xid is implementation-dependent, but whatever technique is
used MUST make it unlikely that the same combination of xid, msgtype,
and Lease Identifier will be used for two different messages within
[XID-REUSE-INTERVAL] (even across multiple clients which do not
communicate among themselves). This allows enough time for the
message to be dropped from all server response caches (as described
in the next few paragraphs) and for any network delays to be
accomodated.
The RECOMMENDED technique for choosing an xid is to choose a random
four octet number as the first xid in a session and increment this
value each time a new xid is needed. The random number chosen need
not be cryptographically random. The random number may be chosen via
any suitable technique, such as the one described in section A.6 of
RFC 1889 [14].
When a server responds to a client message, it MUST use the same xid
value in the response that the client used in the request. This
allows the client to associate responses with the message that they
are responding to.
When retransmitting messages (as described in section 2.3), the
client MUST retransmit them without changing them, thereby using the
same xid and and Lease Identifier.
If a server receives a message with the same xid, msgtype, and Lease
Identifier as one received within [RESPONSE-CACHE-INTERVAL], it MUST
treat this message as a retransmission of the previously received one
and retransmit the response, if any. After [RESPONSE-CACHE-INTERVAL],
the server may forget about the previously received message and treat
any retransmissions of this message as if they were new messages. Of
course, a server need not cache a message if it ends up ignoring that
message. However, performance gains may be achieved by doing so.
This avoids retransmissions causing multiple allocations, since
requests are not idempotent. An appropriate value for [RESPONSE-
CACHE-INTERVAL] would be sixty seconds, but it may have any value
from zero seconds to 300 seconds (five minutes) and may be adjusted
dynamically according to resource constraints on the server.
However, using a value less than sixty seconds is NOT RECOMMENDED
because this is the normal client retransmission period.
2.1.5. The options field
The options field consists of a list of tagged parameters that are
called "options". All options consist of a two octet option code and
a two octet option length, followed by the number of octets specified
by the option length. In the case of some options, the length field
is a constant but must still be specified.
The option field MUST contain a sequence of options with the last one
being the End option (option code 0). Any message whose options field
does not conform to this syntax MUST be ignored.
Any MADCAP client or server sending a MADCAP message MAY include any
of the options listed in section 3, subject to the restrictions in
Table 5 and elsewhere in this document. They MAY also include other
MADCAP options that are defined in the future. A MADCAP client or
server MUST NOT include more than one option with the same option
type in one MADCAP message.
All MADCAP clients and servers MUST recognize all options listed in
this document and behave in accordance with this document when
receiving and processing any of these options. Any unrecognized
options MUST be ignored and the rest of the message processed as if
the unknown options were not present. If a MADCAP server receives a
message that does not conform to the requirements of this document
(for instance, not including all required options), an Invalid
Request error MUST be generated and processed in the manner described
in section 2.6. If a MADCAP client receives a message that does not
conform to the requirements of this document, it MUST ignore the
message.
The order of options within a message has no significance and any
order MUST be supported in an equivalent manner, with the exception
that the End option must occur once per message, as the last option
in the option field.
New MADCAP option codes may only be defined by IETF Consensus, as
described in section 5.
2.2. Message Types
The msgtype field defines the "type" of a MADCAP message. Legal
values for this field are:
Value Message Type
----- ------------
1 DISCOVER
2 OFFER
3 REQUEST
4 RENEW
5 ACK
6 NAK
7 RELEASE
8 GETINFO
Table 2: MADCAP message types
Throughout this document, MADCAP messages will be referred to by the
type of the message; e.g., a MADCAP message with a message type of 8
will be referred to as an GETINFO message.
Here are descriptions of the MADCAP message types. Table 5, which
appears at the beginning of section 3, summarizes which options are
allowed with each message type.
MADCAP clients and servers MUST handle all MADCAP message types
defined in this document in a manner consistent with this document.
If a MADCAP server receives a message whose message type it does not
recognize, an Invalid Request error MUST be generated and processed
in the manner described in section 2.6. If a MADCAP client receives a
message whose message type it does not recognize, it MUST ignore the
message.
Note, however, that under some circumstances this document requires
or suggests that clients or servers ignore messages with certain
message types even though they may be recognized. For instance,
clients that do not send DISCOVER messages SHOULD ignore OFFER
messages. Also, secure servers SHOULD ignore DISCOVER messages and
all servers SHOULD ignore DISCOVER messages that they cannot satisfy.
New MADCAP message types may only be defined by IETF Consensus, as
described in section 5.
2.2.1. GETINFO
The GETINFO message is used by a MADCAP client that wants to acquire
configuration parameters, especially a multicast scope list. This
message also allows a client to determine which servers are likely to
be able to handle future requests.
The MADCAP client sends out an GETINFO message. The message may be
unicast to a particular MADCAP server or multicast to a MADCAP Server
Multicast Address. For more details about the MADCAP Server Multicast
Address, see section 2.10.
If a server receives an GETINFO message and it can process the
request successfully, it MUST unicast an ACK message to the client.
All GETINFO messages MUST include an Option Request List option. The
server SHOULD try to include the specified options in its response,
but is not required to do so (especially if it does not recognize
them).
If a server receives an GETINFO message and it does not process the
request successfully, it MUST generate and process an error in the
manner described in section 2.6.
If a client sends an GETINFO message and does not receive any ACK
messages in response, it SHOULD resend its GETINFO message, as
described in section 2.3.
When a MADCAP client sends an GETINFO message, it MAY include the
Requested Language option, which specifies which language the client
would prefer for the zone names in the Multicast Scope List. The
proper way to handle this tag with respect to zone names is discussed
in the definition of the Multicast Scope List option.
2.2.2. DISCOVER
The DISCOVER message is a multicast message sent by a MADCAP client
that wants to discover MADCAP servers that can probably satisfy a
REQUEST.
MADCAP clients are not required to use the DISCOVER message. They
MAY employ other methods to find MADCAP servers, such as sending a
multicast GETINFO message, caching an IP address that worked in the
past or being configured with an IP address. Using the DISCOVER
message has the particular advantage that it allows clients to
receive responses from all servers that can satisfy the request.
The MADCAP client begins by sending a multicast DISCOVER message to a
MADCAP Server Multicast Address. Any servers that wish to assist the
client respond by sending a unicast OFFER message to the client. If a
server can only process the request with a shorter lease time or
later start time than the client requested, it SHOULD send an OFFER
message with the lease time or start time that it can offer.
However, it MUST NOT offer a lease time shorter than the minimum
lease time specified by the client or a start time later than the
maximum start time specified by the client.
For more details about the MADCAP Server Multicast Address, see
section 2.10.
If a client sends a DISCOVER message and does not receive any OFFER
messages in response, the client SHOULD retransmit its DISCOVER
message, as described in section 2.3.
If a client sends a DISCOVER message and receives one or more OFFER
messages in response, it SHOULD select the server it wants to use (if
any) and send a multicast REQUEST message identifying that server
within [DISCOVER-DELAY] after receiving the first OFFER message. See
section 2.2.4 for more information about the REQUEST message.
The mechanism used by the client in selecting the server it wants to
use is implementation dependent. The client MAY choose the first
acceptable response or it MAY wait some period of time (no more than
[DISCOVER-DELAY]) and choose the best response received in that
period of time (if the first response has a smaller lease time than
requested, for instance).
The value of [DISCOVER-DELAY] is also implementation dependent, but
the RECOMMENDED value is the current retransmit timer, as specified
in section 2.3. Waiting too long (approaching [OFFER-HOLD]) may cause
servers to drop the addresses they have reserved.
When a MADCAP client sends a DISCOVER message, it MAY include the
Lease Time, Minimum Lease Time, Start Time, Maximum Start Time,
Number of Addresses Requested, and List of Address Ranges options,
describing the addresses it wants to receive. However, it need not
include any of these options. If one of these options is not
included, the server will provide the appropriate default (maximum
available for Lease Time, no minimum for Minimum Lease Time, as soon
as possible for Start Time, no maximum for Maximum Start Time, one
for Number of Addresses Requested, and any addresses available for
List of Address Ranges). The Multicast Scope option MUST be included
in the DISCOVER message so that the server knows what scope should be
used. The Current Time option MUST be included if the Start Time or
Maximum Start Time options are included. The Lease Identifier option
MUST always be included.
2.2.3. OFFER
The OFFER message is a unicast message sent by a MADCAP server in
response to a DISCOVER message that it can probably satisfy.
A MADCAP server is never required to send an OFFER message in
response to a DISCOVER message. For instance, it may not be able to
satisfy the client's request or it may have been configured to
respond only to certain types of DISCOVER messages or not to respond
to DISCOVER messages at all.
If a MADCAP server decides to send an OFFER message, it MUST include
the Lease Time and Multicast Scope options, describing the addresses
it is willing to provide. However, it need not include the List of
Address Ranges option. If the List of Address Ranges Allocated option
is not included, it is assumed that the server is willing to provide
the number of addresses that the client requested. If the Start Time
option is not included, it is assumed that the server is willing to
provide the start time requested by the client (if any). The Current
Time option MUST be included if the Start Time option is included.
If a server can process the request with a shorter lease time or
later start time than the client requested, it SHOULD send an OFFER
message with the lease time or start time that it can offer.
However, it MUST NOT offer a lease time shorter than the minimum
lease time specified by the client or a start time later than the
maximum start time specified by the client.
If the server sends an OFFER message, it SHOULD attempt to hold
enough addresses to complete the transaction. If it receives a
multicast REQUEST message with the same Lease Identifier option as
the DISCOVER message for which it is holding these addresses and a
Server Identifier option that does not match its own, it SHOULD stop
holding the addresses. The server SHOULD also stop holding the
addresses after an appropriate delay [OFFER-HOLD] if the transaction
is not completed. The value of this delay is implementation-specific,
but a value of at least 60 seconds is RECOMMENDED.
As with all messages sent by the server, the xid field MUST match the
xid field included in the client request to which this message is
responding. The Lease Identifier option MUST be included, with the
value matching the one included in the client request. The Server
Identifier option MUST be included, with the value being the server's
IP address. And the packet MUST NOT be retransmitted.
2.2.4. REQUEST
The REQUEST message is used by a MADCAP client that wants to allocate
one or more multicast addresses. It is not used for renewing an
existing lease. The RENEW message is used for that.
If a REQUEST message is completing a transaction initiated by a
DISCOVER message, the following procedure MUST be followed so that
all MADCAP servers know which server was selected. The client MUST
multicast a REQUEST message to the same MADCAP Server Multicast
Address that the DISCOVER message was sent to. The same Lease
Identifier used in the DISCOVER message MUST be used in the REQUEST
message. Also, the Server Identifier option MUST be included, using
the Server Identifier of the server selected.
If a REQUEST message is not completing a transaction initiated by a
DISCOVER message, the REQUEST message MUST be unicast to the MADCAP
server that the client wants to use. In this case, the Server
Identifier option MAY be included, but need not be.
If the selected server can process the request successfully, it
SHOULD unicast an ACK message to the client. Otherwise, it SHOULD
generate and process an error in the manner described in section 2.6.
If a server can process the request with a shorter lease time or
later start time than the client requested, it SHOULD send an ACK
message with the lease time or start time that it can offer. However,
it MUST NOT offer a lease time shorter than the minimum lease time
specified by the client or a start time later than the maximum start
time specified by the client.
When a MADCAP client sends a REQUEST message, it MAY include the
Lease Time, Minimum Lease Time, Start Time, Maximum Start Time,
Number of Addresses Requested, and List of Address Ranges options,
describing the addresses it wants to receive. However, it need not
include any of these options. If one of these options is not
included, the server will provide the appropriate default (maximum
available for Lease Time, no minimum for Minimum Lease Time, as soon
as possible for Start Time, no maximum for Maximum Start Time, one
for Number of Addresses Requested, and any addresses available for
List of Address Ranges). The Multicast Scope option MUST be included
in the REQUEST message so that the server knows what scope should be
used. The Current Time option MUST be included if the Start Time or
Maximum Start Time options are included.
If a client sends a REQUEST message and does not receive any ACK or
NAK messages in response, the client SHOULD resend its REQUEST
message, as described in section 2.3.
If the server responds with a NAK or fails to respond within a
reasonable (implementation-dependent) delay [NO-RESPONSE-DELAY], the
client MAY try to find another server by sending a DISCOVER message
with another xid or sending a REQUEST message with another xid to
another server. The RECOMMENDED value for [NO-RESPONSE-DELAY] is 60
seconds.
2.2.5. ACK
The ACK message is used by a MADCAP server to respond affirmatively
to an GETINFO, REQUEST, or RELEASE message. The server unicasts the
ACK message to the client from which it received the message to which
it is responding.
The set of options included with an ACK message differs, depending on
what sort of message it is responding to.
If the ACK message is responding to an GETINFO message, it SHOULD
include any options requested by the client using the Option Request
List option.
If the ACK message is responding to a REQUEST message, it MUST
include Lease Time, Multicast Scope, and List of Address Ranges
options. It MAY include a Start Time option. If a Start Time option
is included, a Current Time option MUST also be included. If no Start
Time option is included, the lease is assumed to start immediately.
If the ACK message is responding to a RENEW message, it MUST include
Lease Time, Multicast Scope, and List of Address Ranges options. It
MAY include a Start Time option. If a Start Time option is included,
a Current Time option MUST also be included. If no Start Time option
is included, the lease is assumed to start immediately.
If the ACK message is responding to a RELEASE message, it MUST only
include Server Identifier and Lease Identifier options.
As with all messages sent by the server, the xid field MUST match the
xid field included in the client request to which this message is
responding. The Lease Identifier option MUST be included, with the
value matching the one included in the client request. The Server
Identifier option MUST be included, with the value being the server's
IP address. And the packet MUST NOT be retransmitted.
2.2.6. NAK
The NAK message is used by a MADCAP server to respond negatively to a
message. The server unicasts the NAK message to the client from which
it received the message to which it is responding.
As with all messages sent by the server, the xid field MUST match the
xid field included in the client request to which this message is
responding. The Lease Identifier option MUST be included, with the
value matching the one included in the client request. The Server
Identifier option MUST be included, with the value being the server's
IP address. The Error option MUST be included with an error code
indicating what went wrong. And the packet MUST NOT be retransmitted.
2.2.7. RENEW
The RENEW message is used by a MADCAP client that wants to renew a
multicast address lease, changing the lease time or start time.
The client unicasts the RENEW message to a MADCAP server. If the
server can process the request successfully, it SHOULD unicast an ACK
message to the client. Otherwise, it MUST generate and process an
error in the manner described in section 2.6.
The lease to be renewed is whichever one was allocated with a Lease
Identifier option matching the one provided in the RENEW message.
When a MADCAP client sends a RENEW message, it MAY include the Lease
Time, Minimum Lease Time, Start Time, and Maximum Start Time options,
describing the new lease it wants to receive. However, it need not
include any of these options. If one of these options is not
included, the server will provide the appropriate default (maximum
available for Lease Time, no minimum for Minimum Lease Time, as soon
as possible for Start Time, and no maximum for Maximum Start Time).
The Current Time option MUST be included if the Start Time or Maximum
Start Time options are included.
If a client sends a RENEW message and does not receive any ACK or NAK
messages in response, the client SHOULD resend its RENEW message, as
described in section 2.3.
If the server responds with a NAK or fails to respond within a
reasonable (implementation-dependent) delay [NO-RESPONSE-DELAY], the
client MAY send a RENEW message with another xid to another server,
provided that the Server Mobility feature was used in the original
REQUEST message and that this feature is required for the subsequent
RENEW message sent to another server. For more information about the
Server Mobility feature, see section 2.13.1. The RECOMMENDED value
for [NO-RESPONSE-DELAY] is 60 seconds.
2.2.8. RELEASE
The RELEASE message is used by a MADCAP client that wants to
deallocate one or more multicast addresses before their lease
expires.
The client unicasts the RELEASE message to the MADCAP server from
which it allocated the addresses. If the selected server can process
the request successfully, it MUST unicast an ACK message to the
client. Otherwise, it MUST generate and process an error in the
manner described in section 2.6.
The lease to be released is whichever one was allocated with a Lease
Identifier option matching the one provided in the RELEASE message.
It is not possible to release only part of the addresses in a single
lease.
If a client sends a RELEASE message and does not receive any ACK or
NAK messages in response, the client SHOULD resend its RELEASE
message, as described in section 2.3.
If the server responds with a NAK or fails to respond within a
reasonable (implementation-dependent) delay [NO-RESPONSE-DELAY], the
client MAY send a RELEASE message with another xid to another server,
provided that the Server Mobility feature was used in the original
REQUEST message and that this feature is required for the subsequent
RELEASE message sent to another server. For more information about
the Server Mobility feature, see section 2.13.1. The RECOMMENDED
value for [NO-RESPONSE-DELAY] is 60 seconds.
2.3. Retransmission
MADCAP clients are responsible for all message retransmission. The
client MUST adopt a retransmission strategy that incorporates an
exponential backoff algorithm to determine the delay between
retransmissions. The delay between retransmissions SHOULD be chosen
to allow sufficient time for replies from the server to be delivered
based on the characteristics of the internetwork between the client
and the server.
The RECOMMENDED algorithm is to use a 4 second delay before the first
retransmission and to double this delay for each successive
retransmission, with a maximum delay of 16 seconds and a maximum of
three retransmissions. If an initial transmission was sent at time
(in seconds) t and no responses were received, subsequent
transmissions would be at t+4, t+12, and t+28. If no response has
been received by t+60, the client would stop retransmitting and take
another course of action (such as logging an error or sending a
message to another address.
The client MAY provide an indication of retransmission attempts to
the user as an indication of the progress of the process. The client
MAY halt retransmission at any point.
2.4. The Lease Identifier
The Lease Identifier option is included in each MADCAP message. Its
value is used to identify a lease and MUST be unique across all
leases requested by all clients in a multicast address allocation
domain.
The first octet of the Lease Identifier is the Lease Identifier type.
Table 3 lists the Lease Identifier types defined at this time and
sections 2.4.1 and 2.4.2 describe these Lease Identifier types.
New MADCAP Lease Identifier types may only be defined by IETF
Consensus, as described in section 5.
Lease Identifier Type Name
--------------------- ----
0 Random Lease Identifier
1 Address-Specific Lease Identifier
Table 3: MADCAP Lease Identifier Types
The MADCAP server does not need to parse the Lease Identifier. It
SHOULD use the Lease Identifier only as an opaque identifier, which
must be unique for each lease. The purpose of defining different
Lease Identifier types is to allow MADCAP clients that already have a
globally unique address to avoid the possibility of Lease Identifier
collisions by using this address together with a client-specific
identifier. MADCAP clients that do not have a globally unique address
SHOULD use Lease Identifier type 0.
In addition to associating client and server messages (along with the
msgtype and xid fields, as described in the next section), the Lease
Identifier is used to determine which lease a RENEW or RELEASE
request refers to. MADCAP servers SHOULD match the Lease Identifier
included in a RENEW or RELEASE message with the Lease Identifier used
in an initial REQUEST message. If the Lease Identifier does not
match, a MADCAP server MUST generate and process a Lease Identifier
Not Recognized error in the manner described in section 2.6.
For conferencing applications, it may be desirable to allow
conference participants to modify a lease used for the conference.
The Shared Lease Identifier feature code is used to support this
requirement. If this feature code was requested by the client and
implemented by the server when the lease was allocated, the server
SHOULD disable any authentication requirements and allow any client
that knows the Lease Identifier to modify the lease.
As described in the Security Considerations section, MADCAP security
is not terribly useful without admission control in the multicast
routing infrastructure. However, if MADCAP security is desired when
using the Shared Lease Identifier feature, the confidentiality of the
Lease Identifier MUST be maintained by encrypting all messages that
contain it. A Lease Identifier that includes a long cryptographically
random number (at least eight octets in length) MUST be used in this
circumstance so that it is not easy to guess the Lease Identifier.
2.4.1. Random Lease Identifier
The first octet of a Random Lease Identifier is the Lease Identifier
type (0 to indicate Random Lease Identifier). After this come a
sequence of octets, which SHOULD represent a long random number (at
least 16 octets) from a decent random number generator.
A Random Lease Identifier does not include any indication of its
length. It is assumed that this may be determined by external means,
such as a length field preceding the Lease Identifier.
Lease ID
Type Random Number
+---------+-------------...
| 0 |
+---------+-------------...
2.4.2. Address-Specific Lease Identifier
The first octet of an Address-Specific Lease Identifier is the Lease
Identifier type (1 to indicate Address-Specific Lease Identifier).
After this comes a two octet IANA-defined address family number
(including those defined in [10]), an address from the specified
address family, and a client-specific identifier (such as a sequence
number or the current time).
An Address-Specific Lease Identifier does not include any indication
of its length. It is assumed that this may be determined by external
means, such as a length field preceding the Lease Identifier.
Lease ID Address Family Address Client-specific
Type Number Identifier
+---------+---------+---------+-----...-----+-----...-----+
| 1 | addrfamily | address | cli-spec id |
+---------+---------+---------+-----...-----+-----...-----+
2.5. Associating Client and Server Messages
Messages between clients and servers are associated with one another
using the Lease Identifier and the xid field. As described in section
2.1.4, the client MUST choose an xid so that it is unlikely that the
same combination of xid, msgtype, and Lease Identifier will be used
for two different messages within [XID-REUSE-INTERVAL] (even across
multiple clients which do not communicate among themselves). The
Lease Identifier option, msgtype, and xid field MUST be included in
each message sent by the client or the server.
The client MUST check the Lease Identifier option and xid field in
each incoming message to ensure that they match the Lease Identifier
and xid for an outstanding transaction. If not, the message MUST be
ignored. The server MUST check the Lease Identifier option and xid
field in each incoming message to establish the proper context for
the message. If a server cannot process a message because it is
invalid for its context, the server MUST generate and process an
Invalid Request error, as described in section 2.6. A transaction
can be an attempt to allocate a multicast address (consisting of
DISCOVER, OFFER, REQUEST, ACK, and NAK messages), an attempt to renew
a lease (consisting of RENEW, ACK, and NAK messages), an attempt to
release a previously allocated multicast address (consisting of
RELEASE, ACK, and NAK messages), or an attempt to acquire
configuration parameters (consisting of GETINFO, ACK, and NAK
messages).
2.6. Processing Errors
If a MADCAP server encounters an error while processing a message,
there are two different ways to process this error. If it is clear
that the message is not a NAK, the server SHOULD respond with a NAK
containing the appropriate Error option. However, the server MAY
decide to completely ignore chronic offenders. If the message is a
NAK or it is not clear whether the message is a NAK (for instance,
the message is garbled or has an incorrect version number), the
server SHOULD ignore the message. This avoids NAK loops.
If a MADCAP client encounters an error while processing a message, it
MUST ignore the message.
2.7. Multicast Scopes
RFC 2365 [3] provides for dividing the multicast address space into a
number of administrative scopes. Routers should be configured so that
each scope corresponds to a particular partition of the network into
disjoint regions. Messages sent to a multicast address that falls
within a certain administrative scope should only be delivered to
hosts that have joined that multicast group *and* fall within the
same region as the sender. For instance, packets sent to an address
in the organization-local scope should only be delivered to hosts
that have joined that group and fall within the same organization as
the sender.
Different sets of scopes may be in effect at different places in the
network and at different times. Before attempting to allocate an
address from an administrative scope (other than global or link-level
scope, which are always in effect), a MADCAP client SHOULD determine
that the scope is in effect at its location at this time. Several
techniques that a MADCAP client may use to determine the set of
administrative scopes in effect (the scope list) are: manual
configuration or configuration via MADCAP (using the Multicast Scope
List option).
If a MADCAP client is unable to determine its scope list using one of
these techniques, it MAY temporarily assume a scope list consisting
of a single scope. If it is using IPv4, it SHOULD use IPv4 Local
Scope (239.255.0.0/16), with a maximum TTL of 16. If it is using
IPv6, it SHOULD use SCOP 3, with a maximum hop count of 16. Using
this temporary scope list, it MAY attempt to contact a MADCAP server
that can provide a scope list for it.
When a MADCAP client requests an address with a DISCOVER or REQUEST
message, it MUST specify the administrative scope from which the
address should be allocated. This scope is indicated with the
Multicast Scope option. Likewise, the server MUST include the
Multicast Scope option in all OFFER messages and all ACK messages
sent in response to REQUEST messages.
2.8. Multicast TTL
Another way to limit propagation of multicast messages is by using
TTL scoping. This technique has several disadvantages in comparison
to administratively scoped multicast addresses (as described in [3]),
but it is currently in widespread usage.
With TTL scoping, areas of the network are designated as scopes.
Routers on the edges of these areas are configured with TTL
thresholds so that multicast packets are not forwarded unless their
remaining TTL exceeds this threshold. A packet which should be
restricted to a given TTL scope should have an initial TTL less than
that scope's TTL threshold. Similar techniques may be used with IPv6,
using the Hop Count field instead of the TTL field.
MADCAP may be used in an environment where administrative scoping is
not in use and TTL scoping is. Under these circumstances, a MADCAP
server MAY return a scope list that includes scopes with TTLs less
than 255. The MADCAP client MAY then allocate addresses from these
scopes, but MUST NOT set the TTL field of any packet sent to such an
address to a value greater than the maximum TTL indicated in the
scope list. In such an environment, it is recommended that the MADCAP
Server Multicast Addresses associated with the IPv4 Local Scope (or
SCOP 3 for IPv6) be configured using TTL thresholds so that packets
sent to these addresses with TTL of 16 are not sent outside an
appropriate boundary. This will allow MADCAP clients to use their
default behavior for finding MADCAP servers.
In an environment where administrative scoping is in use, the maximum
TTLs in the scope list SHOULD be 255. The admin scope zone boundary
routers will prevent leakage of MADCAP packets beyond appropriate
limits.
2.9. Locating MADCAP Servers
There are several ways for a MADCAP client to locate a MADCAP server.
For instance, the client may be configured with an IP address.
The RECOMMENDED technique is for the client to send an GETINFO
message to a MADCAP Server Multicast Address and wait for ACK
responses. This technique is described in more detail in the next
section.
2.10. MADCAP Server Multicast Address
Each multicast scope has an associated MADCAP Server Multicast
Address. This address has been reserved by the IANA as the address
with a relative offset of -1 from the last address of a multicast
scope.
A MADCAP client looking for servers that can provide multicast
allocation services MAY send an GETINFO message to a MADCAP Server
Multicast Address. Any MADCAP servers listening to this address
SHOULD respond with a unicast ACK message to the client if they wish
to offer a response.
The MADCAP Server Multicast Address used by a client MAY be
established by configuration. If a client has no such configuration,
it SHOULD use the MADCAP Server Multicast Address associated with
IPv4 Local Scope (or SCOP 3 for IPv6) with maximum TTL of 16, unless
otherwise configured.
2.11. Going Beyond the Local Scope
If a client receives no response to a message sent to a MADCAP Server
Multicast Address (after retransmission), it MAY send the message to
a larger scope and repeat this process as necessary. However, the
client MUST NOT send a MADCAP message to the MADCAP Server Multicast
Address associated with the global scope.
This technique allows MADCAP servers to provide services for scopes
in which they do not reside. However, this is a dangerous and
complicated technique and is NOT RECOMMENDED at this time.
Therefore, MADCAP clients SHOULD only send multicast messages to the
MADCAP Server Multicast Address corresponding to the IPv4 Local Scope
(or SCOP 3, if using IPv6), unless configured otherwise.
MADCAP servers that wish to provide services for scopes in which they
do not reside MUST make special efforts to ensure that their services
meet clients' needs for largely conflict-free allocation and accurate
scope list information. In particular, coordinating with other
servers that provide services for this scope may be difficult. Also,
establishing which scope the client is in may be difficult. If a
MADCAP server is not prepared to provide services for scopes in which
it does not reside, it SHOULD ignore DISCOVER and REQUEST messages
whose scope does not match or enclose the scope of the MADCAP Server
Multicast Address on which the request was received. It SHOULD also
ignore GETINFO messages that are not received on the MADCAP Server
Multicast Address for IPv4 Local Scope.
2.12. Clock Skew
The Current Time option is used to detect and handle clock skew
between MADCAP clients and servers. This option MUST be included in
any MADCAP message that includes an absolute time (such as the Start
Time option). It MAY be included in any DISCOVER, OFFER, REQUEST,
RENEW, or ACK message.
Clock skew is a situation where two systems have clocks that are not
synchronized. Many protocols (such as DHCP) ignore clock skew by
using relative times. MADCAP could use a similar technique, but this
leads to nasty situations due to the way multicast addresses are
used.
For example, assume that at 1 PM UTC a client whose clock is one hour
fast requests a lease for one hour starting in one hour. If we were
using relative times for MADCAP, the server, whose clock is set
correctly, would reserve a multicast address for 2 to 3 PM UTC and
grant the request. If the client was the only one using the lease,
everything would be OK. The client would start using the lease in one
hour and continue for one hour. This would coincide with the time the
server had reserved (although the client would think it was 3 to 4 PM
UTC).
However, multicast addresses are usually used by several parties at
once. The client would probably use SAP (or some other mechanism for
conveying SDP) to advertise a session using the multicast address
just leased. SDP uses absolute times, since it may be sent via email,
web, or other store-and-forward mechanisms. So the client would
advertise the session as running from 3 to 4 PM UTC. Any clients
whose clocks are set correctly would use the address during this
interval. Since the server only reserved the address from 2 to 3 PM
UTC, this might cause the address to be used for multiple sessions
simultaneously.
MADCAP cannot solve all clock skew problems. That is the domain of
NTP [4]. However, it does attempt to detect substantial clock skew
between MADCAP clients and servers so that this clock skew does not
cause massive collisions in multicast address usage later on.
The Current Time option contains the sender's opinion of the current
time in UTC at or about the time the message was assembled. Because
of delays in transmission and processing, this value will rarely
match the receiver's opinion of the current time at the time the
option is processed by the receiver. However, difference greater than
a minute or two probably indicate clock skew between the sender and
the receiver.
MADCAP servers SHOULD expect and tolerate a small amount of clock
skew with their clients by ensuring that multicast addresses are
allocated for an extra period of time [EXTRA-ALLOCATION-TIME] on
either side of the lease given to the client. However, large amounts
of clock skew require special handling. The value of [EXTRA-
ALLOCATION-TIME] MUST be a configurable parameter, since local
circumstances may vary. The RECOMMENDED default is one hour.
However, large amounts of clock skew will cause problems later when
sessions are advertised. If a MADCAP server detects clock skew
greater than [CLOCK-SKEW-ALLOWANCE], it MUST generate and process an
Excessive Clock Skew error, as described in section 2.6. The server
MAY also log a message. The value of [CLOCK-SKEW-ALLOWANCE] MUST be a
configurable parameter, since local circumstances may vary. The
RECOMMENDED default is 30 minutes.
2.13. Optional Features
Each MADCAP client or server MAY implement one or more optional
features. Optional features of MADCAP are identified with a two
octet feature code.
A MADCAP client MAY request, require, or indicate support for an
optional feature by including a Feature List option in a message. For
more information about optional features, see the description of the
Feature List option.
Table 4 lists the feature codes defined at this time and sections
2.13.1 and 2.13.2 describe how these features work.
New MADCAP feature codes may only be defined by IETF Consensus, as
described in section 5.
Feature Code Feature Name
------------ ------------
0 Server Mobility
1 Retry After
2 Shared Lease Identifier
Table 4: MADCAP Feature Codes
2.13.1. Server Mobility
The Server Mobility feature allows an address allocated on one MADCAP
server to be renewed or released on a different MADCAP server. This
requires communication and coordination among MADCAP servers. The
primary benefits are immunity to the failure of a single MADCAP
server and perhaps greater performance through load balancing.
In order to take advantage of the Server Mobility feature, a MADCAP
client must ensure that the feature is implemented by both the server
that is used for the original allocation and the server that is used
for the renewal or release. The best way to ensure this is to include
the Server Mobility feature in the required list of a Feature List
option in the REQUEST message used to allocate the address (and the
DISCOVER message, if one is used). When the time comes to renew or
release the address, the client SHOULD send a unicast RENEW or
RELEASE message to the server from which it allocated the address.
However, if this server is unavailable, the client MAY send the RENEW
or RELEASE message to any other server that includes the Server
Mobility feature in its list of supported features. The client can
find such a server by (for instance) sending an GETINFO message with
an Option Request List option that includes the Feature List option
code.
If the MADCAP client does not want to require this feature when
allocating addresses, it may include the feature in the requested
list of a Feature List option and see if the server includes the
feature in the required list of a Feature List option in the ACK
message.
Even if the Server Mobility feature is used, there is no guarantee
that a server will be available to perform the renewal or release or
that the renewal or release will succeed. Server connectivity may
have failed, for instance.
2.13.2. Retry After
The Retry After feature allows a MADCAP server to ask the MADCAP
client to retry its request later, as may be required when allocating
large numbers of addresses or allocating addresses for a long period
of time.
For instance, if a MADCAP client requests 1000 addresses,
administrative approval may be required or allocation of more
addresses from another MASC domain may be necessary. This may take
several hours or several days. If the MADCAP client and server both
support the Retry After feature, the MADCAP server can send back an
ACK message with a Retry Time option indicating when the addresses
may be ready. The client can retry its request after the Retry Time
to get the addresses.
If a MADCAP client includes the Retry After feature in the supported
list of a Feature List option in a REQUEST message, a MADCAP server
that supports the Retry After feature MAY decide to begin a lengthy
allocation process. In this case, the MADCAP server will include an
empty List of Address Ranges option in its ACK message, a Feature
List option that includes the Retry After feature in the required
list, and a Retry Time option with a time after which the client
should retry the REQUEST.
The client MUST NOT include the Retry After feature in the requested
or required list of a Feature List option, since the decision about
whether Retry After is desirable should be left to the MADCAP server.
At some later time (preferably after the time indicated in the Retry
Time option), the client SHOULD send a REQUEST message with all the
same options as the original REQUEST message (especially the Lease
Identifier option), but with a new xid value. The server MAY return
a normal ACK or NAK message at this point or it MAY continue the
transaction to a later time by including an empty List of Address
Ranges option in its ACK message, a Feature List option that includes
the Retry After feature in the required list, and a Retry Time option
with a later time after which the client should retry the REQUEST.
At any point after receiving the initial ACK message with the Retry
Time option, the client MAY terminate the allocation process and any
accompanying lease by sending to the server performing the allocation
(or another server if the Server Mobility feature is also in effect)
a RELEASE message with the Lease Identifier included in the original
REQUEST message.
The Retry After feature may also be used when renewing a lease. In
this case, the description above applies except that the client sends
a RENEW message instead of a REQUEST message.
If a client sends a RENEW message with a Lease Identifier that
matches a lease which is currently undergoing allocation with the
Retry After feature in response to a REQUEST message, the server MUST
generate and process an Invalid Request error in the manner described
in section 2.6. Also, if a client sends a RENEW message with a Lease
Identifier that matches a lease which is currently undergoing
allocation with the Retry After feature in response to a RENEW
message, but the options supplied with the two RENEW messages do not
match, the server MUST generate and process an Invalid Request error
in the manner described in section 2.6.
Note that the Retry After feature may complicate the application API.
For this reason, a MADCAP client may request the Retry After feature
for some messages and not for others. This should not cause problems
for a robust MADCAP server. In general, servers should not expect
consistent behavior from clients except as required by this
specification. This also applies to clients' expectations.
2.13.3. Shared Lease Identifier
For conferencing applications, it may be desirable to allow
conference participants to modify a lease used for the conference.
The Shared Lease Identifier feature code is used to support this
requirement.
If this feature code was requested by the client and implemented by
the server when the lease was allocated, the server SHOULD disable
any authentication requirements pertaining to this lease, allowing
any client that knows the Lease Identifier to modify the lease.
A MADCAP client wishing to use the Shared Lease Identifier feature
should include this feature in the requested or required lists of the
Feature List option of a REQUEST message when first allocating the
lease. If the feature was required, the server SHOULD try to
implement it for this request and include the feature in the required
list of the response. If the server can not implement the feature for
this request, it MUST generate and process a Required Feature Not
Supported error in the manner described in section 2.6. If the
feature was requested, the server SHOULD try to implement the feature
and include the feature in the required list of the response.
However, if the server cannot implement the feature, it may simply
skip it.
Subsequent requests pertaining to a lease for which the Shared Lease
Identifier feature was implemented at allocation time MAY include the
Shared Lease Identifier feature in the requested or required lists of
the Feature List option. In this case, the server SHOULD try to
implement the feature by disabling any authentication requirements
pertaining to this lease, allowing any client that knows the Lease
Identifier to modify the lease, and including the feature in the
required list of the response. If the server cannot implement the
feature, it SHOULD skip it if the feature was requested. But if the
feature was required and the server cannot implement it, the server
MUST generate and process a Required Feature Not Supported error in
the manner described in section 2.6.
3. MADCAP Options
As described earlier, each MADCAP message includes an options field
consisting of a list of tagged parameters that are called "options".
All options consist of a two octet option code and a two octet option
length, followed by the number of octets specified by the option
length.
This section defines a set of option codes for use in MADCAP
messages. New options may be defined using the process defined in
section 5. The options are listed in numerical order.
Table 5 summarizes which options are allowed with each message type.
Option GETINFO ACK (in response to GETINFO)
------ ------ ---------------------------
Lease Time MUST NOT MUST NOT
Server Identifier MUST NOT MUST
Lease Identifier MUST MUST
Multicast Scope MUST NOT MUST NOT
Option Request List MUST MUST NOT
Start Time MUST NOT MUST NOT
Number of Addresses
Requested MUST NOT MUST NOT
Requested Language MAY MUST NOT
Multicast Scope List MUST NOT MAY
List of Address Ranges MUST NOT MUST NOT
Current Time MUST NOT MAY
Feature List MAY MAY
Retry Time MUST NOT MUST NOT
Minimum Lease Time MUST NOT MUST NOT
Maximum Start Time MUST NOT MUST NOT
Error MUST NOT MUST NOT
Option DISCOVER OFFER
------ -------- -----
Lease Time MAY MUST
Server Identifier MUST NOT MUST
Lease Identifier MUST MUST
Multicast Scope MUST MUST
Option Request List MUST NOT MUST NOT
Start Time MAY MAY
Number of Addresses
Requested MAY MUST NOT
Requested Language MUST NOT MUST NOT
Multicast Scope List MUST NOT MUST NOT
List of Address Ranges MAY MAY
Current Time MAY MAY
Feature List MAY MAY
Retry Time MUST NOT MUST NOT
Minimum Lease Time MAY MUST NOT
Maximum Start Time MAY MUST NOT
Error MUST NOT MUST NOT
Option REQUEST ACK (in response to REQUEST)
------ ------- ----------------------------
Lease Time MAY MUST
Server Identifier MUST (if MUST
multicast)
Lease Identifier MUST MUST
Multicast Scope MUST MUST
Option Request List MUST NOT MUST NOT
Start Time MAY MAY
Number of Addresses
Requested MAY MUST NOT
Requested Language MUST NOT MUST NOT
Multicast Scope List MUST NOT MUST NOT
List of Address Ranges MAY MUST
Current Time MAY MAY
Feature List MAY MAY
Retry Time MUST NOT MAY
Minimum Lease Time MAY MUST NOT
Maximum Start Time MAY MUST NOT
Error MUST NOT MUST NOT
Option RENEW ACK (in response to RENEW)
------ ----- --------------------------
Lease Time MAY MUST
Server Identifier MUST NOT MUST
Lease Identifier MUST MUST
Multicast Scope MUST NOT MUST
Option Request List MUST NOT MUST NOT
Start Time MAY MAY
Number of Addresses
Requested MUST NOT MUST NOT
Requested Language MUST NOT MUST NOT
Multicast Scope List MUST NOT MUST NOT
List of Address Ranges MUST NOT MUST
Current Time MAY MAY
Feature List MAY MAY
Retry Time MUST NOT MAY
Minimum Lease Time MAY MUST NOT
Maximum Start Time MAY MUST NOT
Error MUST NOT MUST NOT
Option RELEASE ACK (in response to RELEASE)
------ ------- ----------------------------
Lease Time MUST NOT MUST NOT
Server Identifier MUST NOT MUST
Lease Identifier MUST MUST
Multicast Scope MUST NOT MUST NOT
Option Request List MUST NOT MUST NOT
Start Time MUST NOT MUST NOT
Number of Addresses
Requested MUST NOT MUST NOT
Requested Language MUST NOT MUST NOT
Multicast Scope List MUST NOT MUST NOT
List of Address Ranges MUST NOT MUST NOT
Current Time MUST NOT MUST NOT
Feature List MAY MAY
Retry Time MUST NOT MUST NOT
Minimum Lease Time MUST NOT MUST NOT
Maximum Start Time MUST NOT MUST NOT
Error MUST NOT MUST NOT
Option NAK
------ ---
Lease Time MUST NOT
Server Identifier MUST
Lease Identifier MUST
Multicast Scope MUST NOT
Option Request List MUST NOT
Start Time MUST NOT
Number of Addresses
Requested MUST NOT
Requested Language MUST NOT
Multicast Scope List MUST NOT
List of Address Ranges MUST NOT
Current Time MUST NOT
Feature List MAY
Retry Time MUST NOT
Minimum Lease Time MUST NOT
Maximum Start Time MUST NOT
Error MUST
Table 5: Options allowed in MADCAP messages
3.1. End
The End option marks the end of valid information in the options
field. This option MUST be included at the end of the options field
in each MADCAP message.
The code for this option is 0, and its length is 0.
Code Len
+-----+-----+-----+-----+
| 0 | 0 |
+-----+-----+-----+-----+
3.2. Lease Time
This option is used in a client request (DISCOVER, REQUEST, or RENEW)
to allow the client to request a lease time for the multicast
address. In a server reply (OFFER or ACK), a MADCAP server uses this
option to specify the lease time it is willing to offer.
The time is in units of seconds, and is specified as a 32-bit
unsigned integer.
The code for this option is 1, and its length is 4.
Code Len Lease Time
+-----+-----+-----+-----+-----+-----+-----+-----+
| 1 | 4 | t1 | t2 | t3 | t4 |
+-----+-----+-----+-----+-----+-----+-----+-----+
3.3. Server Identifier
This option contains the IP address of a MADCAP server. A two octet
address family number (as defined by IANA, including those defined in
[10]) is stored first, followed by the address. The address family
for this address is not determined by the addrfamily field in the
fixed header so that addresses from one family may be allocated while
communicating with a server via addresses of another family.
All messages sent by a MADCAP server MUST include a Server Identifier
option with the IP address of the server sending the message.
MADCAP clients MUST include a Server Identifier option in multicast
REQUEST messages in order to indicate which OFFER message has been
accepted.
The code for this option is 2, and its minimum length is 3.
Code Len Address Family Address
+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| 2 | n | family | a1 | ... |
+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
3.4. Lease Identifier
This option is used by MADCAP clients to specify a unique lease
identifier. For more information about this option and how it is
used, see section 2.4.
The code for this option is 3, and its minimum length is 1.
Code Len Lease Identifier
+-----+-----+-----+-----+-----+-----+---
| 3 | n | i1 | i2 | ...
+-----+-----+-----+-----+-----+-----+---
3.5. Multicast Scope
The multicast scope option is used by the client to indicate the
requested multicast scope in a DISCOVER or REQUEST message. It is
also used by the MADCAP server to indicate the scope of an assigned
address.
The client may obtain the scope list through the Multicast Scope List
option or using some other means. The Scope ID is the first multicast
address in the scope. The address family of the Scope ID is
determined by the addrfamily field in the fixed header.
The code for this option is 4, and its minimum length is 1.
Code Len Scope ID
+-----+-----+-----+-----+-----+-----
| 4 | n | i1 | ...
+-----+-----+-----+-----+-----+-----
3.6. Option Request List
This option is used by a MADCAP client in an GETINFO message to
request that certain options be included in the server's ACK
response. The server SHOULD try to include the specified options in
its response, but is not required to do so.
The format of this option is a list of option codes.
The code for this option is 5 and the minimum length is 2.
Code Len Requested Options
+-----+-----+-----+-----+-----+-----+---...
| 5 | n | Option1 |
+-----+-----+-----+-----+-----+-----+---...
3.7. Start Time
The Start Time option specifies the starting time for a multicast
address lease.
A client may include this option in a DISCOVER, RENEW, or REQUEST
message to request a multicast address for use at a future time. A
server may include this option in an OFFER message or in an ACK in
response to REQUEST or RENEW message to indicate that a lease has
been granted which starts at a future time.
If the Start Time option is present, the IP Address Lease Time option
specifies the duration of the lease beginning at the Start Time
option value.
If the Start Time option is present, the Current Time option MUST
also be present, as described in section 2.12.
The time value is an unsigned 32 bit integer in network byte order
giving the number of seconds since 00:00 UTC, 1st January 1970. This
can be converted to an NTP timestamp by adding decimal 2208988800.
This time format will not wrap until the year 2106.
The code for this option is 6 and the length is 4.
Code Len Time
+-----+-----+-----+-----+-----+-----+-----+-----+
| 6 | 4 | t1 | t2 | t3 | t4 |
+-----+-----+-----+-----+-----+-----+-----+-----+
3.8. Number of Addresses Requested
This option specifies the minimum and desired number of addresses
requested by the client. It is only used in DISCOVER and REQUEST
messages and is only sent by the client.
The minimum and desired number of addresses requested are unsigned 16
bit integers in network byte order. The minimum MUST be less than or
equal to the desired number. If a message is received where this is
not the case, the MADCAP server MUST generate and process an Invalid
Request error in the manner described in section 2.6.
The client MAY obtain more than one address either by repeating the
protocol for every address or by requesting several addresses at the
same time via this option. When the client is requesting only one
address, this option SHOULD NOT be included. A MADCAP server
receiving a DISCOVER or REQUEST packet including this option MUST
include between the minimum and desired number of addresses in any
OFFER or ACK response.
The code for this option is 7 and the length is 4.
Code Len Minimum Desired
+-----+-----+-----+-----+-----+-----+-----+-----+
| 7 | 4 | min | desired |
+-----+-----+-----+-----+-----+-----+-----+-----+
3.9. Requested Language
This option specifies the language in which the MADCAP client would
like strings such as zone names to be returned. It is only included
in an GETINFO message sent by the client. It is an RFC 1766 [6]
language tag. The proper way to handle this tag with respect to zone
names is discussed further in the definition of the Multicast Scope
List option.
The code for this option is 8 and the minimum length is 0.
Code Len Language Tag
+-----+-----+-----+-----+-----+-...-+-----+
| 8 | n | L1 | | Ln |
+-----+-----+-----+-----+-----+-...-+-----+
3.10. Multicast Scope List
This option is sent by the server in an ACK message in response to an
GETINFO message sent by the client.
If the client did not include a Requested Language option in its
GETINFO message, the MADCAP server SHOULD return all zone names for
each zone. If the client included a Requested Language option in its
GETINFO message, the MADCAP server MUST return no more than one zone
name for each zone. For each zone, the MADCAP server SHOULD first
look for a zone name that matches the requested language tag (using a
case-insensitive ASCII comparison). If any names match, one of them
should be returned. Otherwise, the MADCAP server SHOULD choose
another zone name to return (if any are defined). It SHOULD give
preference to zone names that are marked to be used if no name is
available in a desired language.
The code for this option is 9 and the minimum length is 0.
The format of the multicast scope list option is:
Code Len Count Scope List
+-----+-----+-----+-----+-----+-----+-...-+-----+
| 9 | p | m | L1 | | Lm |
+-----+-----+-----+-----+-----+-----+-...-+-----+
The scope list is a list of m tuples, where each tuple is of the
form:
Scope ID Last Address TTL Name Encoded Name List
Count
+---+--...--+---+---+--...--+---+-----+-----+-----+-...-+-----+
| ... ID ... | ... Last ... | T | n | EN1 | | ENn |
+---+--...--+---+---+--...--+---+-----+-----+-----+-...-+-----+
where Scope ID is the first multicast address in the scope, Last
Address is the last multicast address in the scope, TTL is the
multicast TTL value for the multicast addresses of the scope, and
Name Count is the number of encoded names for this zone (which may be
zero). The address family of the Scope ID and Last Address is
determined by the addrfamily field in the fixed header. Note that a
particular MADCAP server may be allocating addresses out of some
subset of the scope. For instance, the addresses in the scope may be
divided among several servers in some way.
Each encoded name is of the form
Name Lang Language Tag Name Name
Flags Length Length
+-----+-----+-----+-...-+-----+-----+-----+-...-+-----+
| F | q | L1 | | Lq | r | N1 | | Nr |
+-----+-----+-----+-...-+-----+-----+-----+-...-+-----+
where Name Flags is a flags field with flags defined below, Lang
Length is the length of the Language Tag in octets (which MUST NOT be
zero), Language Tag is a language tag indicating the language of the
zone name (as described in [6]), Name Length is the length of the
Name in octets (which MUST NOT be zero), and Name is a UTF-8 [5]
string indicating the name given to the scope zone.
The high bit of the Name Flags field is set if the following name
should be used if no name is available in a desired language.
Otherwise, this bit is cleared. All remaining bits in the octet
SHOULD be set to zero and MUST be ignored.
The Scope IDs of entries in the list MUST be unique and the scopes
SHOULD be listed from smallest (topologically speaking) to largest.
This makes it easier to display a consistent user interface, with
scopes usually keeping the same order. However, scopes may not be
strictly nested. In this circumstance, there is no strict ordering
from smallest to largest and the server must use another technique
for ordering the scope list.
Example:
There are two scopes supported by the multicast address allocation
server: Inside abcd.com with addresses 239.192.0.0-239.195.255.255,
and world with addresses 224.0.1.0-238.255.255.255. Then this option
will be given as:
Code Len Count
+-----+-----+-----+-----+-----+...
| 9 | 51 | 2 |
+-----+-----+-----+-----+-----+...
Scope ID Last Address TTL Name Name Lang Language
Count Flags Length Tag
+---+---+---+---+---+---+---+---+---+-----+-----+------+-...-+...
|239|192| 0 | 0 |239|195|255|255|10 | 1 | 128 | 2 | en |
+---+---+---+---+---+---+---+---+---+-----+-----+------+-...-+...
Name
Length Name
+------+--+--+-...-+--+--+...
| 15 | Inside abcd.com |
+------+--+--+-...-+--+--+...
Scope ID Last Address TTL Name Name Lang Language
Count Flags Length Tag
+---+---+---+---+---+---+---+---+---+-----+-----+------+-...-+...
|224| 0 | 1 | 0 |238|255|255|255|16 | 1 | 128 | 2 | en |
+---+---+---+---+---+---+---+---+---+-----+-----+------+-...-+...
Name
Length Name
+------+--...--+
| 5 | world |
+------+--...--+
3.11. List of Address Ranges
This option is used by the server to provide the list of all the
address ranges allocated to the client.
This option is also used by the client when requesting a lease for a
specific set of addresses. This feature should be needed only rarely,
such as when a lease is accidentally allowed to expire and it needs
to be reallocated.
The address family of the addresses is determined by the addrfamily
field.
The code for this option is 10 and the minimum length is 0.
Code Len Address Range List
+-----+-----+-----+-----+-----+-----+-...-+-----+
| 10 | n | L1 | L2 | | Ln |
+-----+-----+-----+-----+-----+-----+-...-+-----+
where the Address Range List is of the following format.
StartAddress1 BlockSize1 StartAddress2 BlockSize2 ...
+---+---+---+---+---+---+---+---+---+---+---+---+--...--+
| ... S1 ... |B11|B12| ... S2 ... |B21|B22| |
+---+---+---+---+---+---+---+---+---+---+---+---+--...--+
3.12. Current Time
This option is used to express what the sender thinks the current
time is. This is useful for detecting clock skew. This option MUST be
included if the Start Time or Maximum Start Time options are used, as
described in section 2.12.
The time value is an unsigned 32 bit integer in network byte order
giving the number of seconds since 00:00 UTC, 1st January 1970. This
can be converted to an NTP [4] timestamp by adding decimal
2208988800. This time format will not wrap until the year 2106.
The code for this option is 11 and the length is 4.
Code Len Time
+-----+-----+-----+-----+-----+-----+-----+-----+
| 11 | 4 | t1 | t2 | t3 | t4 |
+-----+-----+-----+-----+-----+-----+-----+-----+
3.13. Feature List
This option lists optional MADCAP features supported, requested, or
required, by the sender. This option MAY be included in any message
sent by a MADCAP server or client.
Optional features of MADCAP are identified with a two octet feature
code. New MADCAP feature codes may only be defined by IETF
Consensus, as described in section 5.
The Feature List option consists of three separate lists: supported
features, requested features, and required features. Each list
consists of an unordered list of feature codes. The supported list is
used by MADCAP clients or servers to indicate the features that the
sender supports. The requested and required lists are used by MADCAP
clients to indicate which features are requested of or required from
a MADCAP server. The required list is used by MADCAP servers to
indicate which features were implemented by the MADCAP server in
processing this message. Messages sent by MADCAP servers MUST NOT
include any feature codes in the requested list.
If a MADCAP client includes the Feature List option in a message, it
MAY include features in any of the lists: supported, requested, and
required. If a MADCAP server receives a message containing the
Feature List option and it does not support all of the features in
the required list, it MUST generate and process a Required Feature
Not Supported error in the manner described in section 2.6. If the
server supports all of the features in the required list, it MUST
implement them as appropriate for this message. It SHOULD try to
implement the features in the requested list and it MAY implement any
of the features in the supported list. If an optional feature (such
as Retry After) is not included in any part of the Feature List
option included in the client's message (or if the client does not
include a Feature List option in its message), the server MUST NOT
use that feature in its response.
If a MADCAP server does respond to a client's message that includes a
Feature List option, the server MUST include a Feature List option
with a supported features list that lists the features that it
supports, a required features list that lists the features that it
implemented in responding to this message (which must be included in
the supported features list of the client's Feature List option), and
an empty requested features list.
The code for this option is 12 and the minimum length is 6.
Code Len Supported Requested Required
+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| 12 | n | FL1 | FL2 | FL3 |
+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
where each of the Feature Lists is of the following format:
Feature Feature Feature
Count Code 1 Code m
+-----+-----+-----+-----+-...-+-----+-----+
| m | FC1 | | FCm |
+-----+-----+-----+-----+-...-+-----+-----+
3.14. Retry Time
The Retry Time option specifies the time at which a client should
retry a REQUEST or RENEW message when using the Retry After feature.
This option should only be sent by a MADCAP server in an ACK when
responding to a REQUEST or RENEW message that includes the Retry
After feature in the supported, requested, or required list. For more
discussion of Retry After, see section 2.13.2.
If the Retry Time option is present, the Current Time option MUST
also be present.
The time value is an unsigned 32 bit integer in network byte order
giving the number of seconds since 00:00 UTC, 1st January 1970. This
can be converted to an NTP timestamp by adding decimal 2208988800.
This time format will not wrap until the year 2106.
The code for this option is 13 and the length is 4.
Code Len Time
+-----+-----+-----+-----+-----+-----+-----+-----+
| 13 | 4 | t1 | t2 | t3 | t4 |
+-----+-----+-----+-----+-----+-----+-----+-----+
3.15. Minimum Lease Time
This option is used in a client request (DISCOVER, REQUEST, or RENEW)
to allow the client to specify a minimum lease time for the multicast
address. If a server cannot meet this minimum lease time, it MUST
generate and process a Valid Request Could Not Be Completed error in
the manner described in section 2.6.
The time is in units of seconds, and is specified as a 32-bit
unsigned integer.
The code for this option is 14, and its length is 4.
Code Len Lease Time
+-----+-----+-----+-----+-----+-----+-----+-----+
| 14 | 4 | t1 | t2 | t3 | t4 |
+-----+-----+-----+-----+-----+-----+-----+-----+
3.16. Maximum Start Time
The Maximum Start Time option specifies the latest starting time that
the client is willing to accept for a multicast address lease.
A client may include this option in a DISCOVER, RENEW, or REQUEST
message to specify that it does not want to receive a lease with a
starting time later than the specified value. If a server cannot meet
this maximum start time, it MUST generate and process a Valid Request
Could Not Be Completed error in the manner described in section 2.6.
If the Maximum Start Time option is present, the Current Time option
MUST also be present, as described in section 2.12.
The time value is an unsigned 32 bit integer in network byte order
giving the number of seconds since 00:00 UTC, 1st January 1970. This
can be converted to an NTP timestamp by adding decimal 2208988800.
This time format will not wrap until the year 2106.
The code for this option is 15 and the length is 4.
Code Len Time
+-----+-----+-----+-----+-----+-----+-----+-----+
| 15 | 4 | t1 | t2 | t3 | t4 |
+-----+-----+-----+-----+-----+-----+-----+-----+
3.17. Error
A MADCAP server includes this option in a NAK message to indicate why
the request failed. A MADCAP server MUST include an Error option in
each NAK message.
The first two octets of an Error option contain a MADCAP error code.
Several MADCAP error codes are defined later in this section. New
MADCAP error codes may only be defined by IETF Consensus, as
described in section 5.
Any remaining octets in the Error option contain extra data about the
error. The format of this data depends on the error code. The
definition of a MADCAP error code must include a definition of the
extra data to be included with that error code.
A client that receives a NAK message containing an Error option MAY
log or display a message indicating the error code and extra data
received. The client MUST NOT retransmit a message once a NAK
response to that message has been received. The client MAY adjust the
message to correct the error and send the corrected message or send a
message to a different server.
The code for this option is 16, and the minimum length is 2.
Code Len Error Code Extra Data
+-----+-----+-----+-----+-----+-----+-----+-----+ ...
| 16 | n | ecode | d1 d2
+-----+-----+-----+-----+-----+-----+-----+-----+ ...
3.17.1. Valid Request Could Not Be Completed
MADCAP error code 0 indicates that the request was valid, but could
not be completed with the available addresses and the current
configuration. The extra data is a two octet option code indicating
which option caused the problem. A value of 0xFFFF indicates that the
problem was not with a specific option.
3.17.2. Invalid Request
MADCAP error code 1 indicates that the request was malformed or
invalid in some other manner. The extra data is a two octet option
code indicating which option caused the problem. A value of 0xFFFF
indicates that the problem was not with a specific option.
3.17.3. Excessive Clock Skew
MADCAP error code 2 indicates excessive clock skew (see section
2.12). The extra data consists of a four octet time value
representing the server's idea of the current time, an unsigned 32
bit integer in network byte order giving the number of seconds since
00:00 UTC, 1st January 1970. This can be converted to an NTP
timestamp by adding decimal 2208988800. This time format will not
wrap until the year 2106.
3.17.4. Lease Identifier Not Recognized
MADCAP error code 3 indicates that the Lease Identifier was not
recognized (usually in response to a RENEW or RELEASE message). There
is no extra data.
3.17.5. Required Feature Not Supported
MADCAP error code 4 indicates that at least one feature included in
the required list of the Feature List option is not supported. The
extra data contains a list of the feature codes in the required list
that are not supported.
3.17.6. Experimental Use
MADCAP error codes 1024-2047 are reserved for experimental use. The
format of the extra data included with these error codes is not
defined.
4. Security Considerations
MADCAP has relatively basic security requirements. At present there
is no way of enforcing authorized use of multicast addresses in the
multicast routing/management protocols. Therefore, it is not
possible to identify unauthorized use of multicast address by an
adversary. Moreover, a multicast address allocated to a user/system
can be used by other systems without violating terms of the multicast
address allocation. For example, a system may reserve an address to
be used for a work group session where each and every member of the
work group is allowed to transmit packets using the allocated group
address. In other words, the multicast address allocation protocol
does not dictate how the address should be used, it only dictates the
time period for which it can be used and who gets to release it or
renew it. When an address is allocated to a system/user, it basically
means that no other user/system (most likely) will be allocated that
address for the time period, without any restrictions on its use.
To protect against rogue MADCAP servers (mis-configured servers and
intentional), clients in certain situations would like to
authenticate the server. Similarly, for auditing or book-keeping
purposes, the server may want to authenticate clients. Moreover, in
some cases, the server may have certain policies in place to restrict
the number of addresses that are allocated to a system or a user.
This feature is of much value when a well behaved but naive user or
client requests a large number of addresses, and therefore,
inadvertently impacts other users or systems. Therefore, an
administrator may want to exert a limited amount of control based on
the client identification. The client identification could be based
on the system or user identity. In most practical situations, system
identification will suffice, however, particularly in case of multi-
user systems, at times, user identification will play an important
role. Therefore, authentication capabilities based on user
identification may be desirable. As usual, data integrity is a strong
requirement and if not protected, can lead to many problems including
denial of service attacks.
In the case of MADCAP, confidentiality is not a strong requirement.
In most of the cases, at least when a multicast address is in use, an
adversary will be able to determine information that was contained in
the MADCAP messages. In some cases, the users/systems may want to
protect information in the MADCAP messages so that an adversary is
not able to determine relevant information in advance and thus, plan
an attack in advance. For example, if an adversary knows in advance
(based on MADCAP messages) that a particular user has requested a
large number of address for certain time period and scope, he may be
able to guess the purpose behind such request and target an attack.
When the Shared Lease Identifier feature is used, preserving the
confidentiality of MADCAP messages becomes more important. Also,
there may be features added to the protocol in the future that may
have stronger confidentiality requirements.
The IPSEC protocol [8] meets client/server identification and
integrity protection requirements stated above, requires no
modification to the MADCAP protocol, and leverages extensive work in
IETF and industry. Therefore, when security is a strong requirement,
IPSEC SHOULD be used for protecting all the unicast messages of
MADCAP protocol. When IPSEC based security is in use, all the
multicast packets except GETINFO MUST be dropped by the MADCAP
server. The prevalent implementations of IPSEC support client
identification in form of system identification and do not support
user identification. However, when desired, IPSEC with appropriate
API's may be required to support user identification.
5. IANA Considerations
This document defines several number spaces (MADCAP options, MADCAP
message types, MADCAP Lease Identifier types, MADCAP features, and
MADCAP error codes). For all of these number spaces, certain values
are defined in this specification. New values may only be defined by
IETF Consensus, as described in [7]. Basically, this means that they
are defined by RFCs approved by the IESG.
6. Acknowledgments
The authors would like to thank Rajeev Byrisetty, Steve Deering,
Peter Ford, Mark Handley, Van Jacobson, David Oran, Thomas Pfenning,
Dave Thaler, Ramesh Vyaghrapuri and the participants of the IETF for
their assistance with this protocol.
Much of this document is based on [1] and [2]. The authors of this
document would like to express their gratitude to the authors of
these previous works. Any errors in this document are solely the
fault of the authors of this document.
7. References
[1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[2] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[3] Meyer, D., "Administratively Scoped IP Multicast", BCP 23, RFC
2365, July 1998.
[4] Mills, D., "Network Time Protocol (Version 3) Specification,
Implementation and Analysis", RFC 1305, March 1992.
[5] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
2279, January 1998.
[6] Alvestrand, H., "Tags for the Identification of Languages", RFC
1766, March 1995.
[7] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
[8] Atkinson, R. and S. Kent, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[9] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[10] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
October 1994.
[11] Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
1112, August 1989.
[12] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[13] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[14] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC
1889, January 1996.
8. Authors' Addresses
Stephen R. Hanna
Sun Microsystems, Inc.
One Network Drive
Burlington, MA 01803
Phone: +1.781.442.0166
EMail: steve.hanna@sun.com
Baiju V. Patel
Intel Corp.
Mail Stop: AG2-201
5200 NE Elam Young Parkway
Hillsboro, OR 97124
Phone: 503 696 8192
EMail: baiju.v.patel@intel.com
Munil Shah
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: 425 703 3924
EMail: munils@microsoft.com
APPENDIX A: Examples
This appendix includes several examples of typical MADCAP protocol
exchanges.
1. Multicast Scope List Discovery
In this example, a MADCAP client wants to determine the scope list in
effect. The client is using IPv4, so it starts by multicasting an
GETINFO packet to the MADCAP Server Multicast Address corresponding
to IPv4 Local Scope. This packet includes the Lease Identifier
option, an Option Request List including the Multicast Scope List
option code, and a Requested Language option containing the string
"en", since the client is configured to prefer the English language.
Two MADCAP servers respond by sending ACK messages. These ACK
messages include the Lease Identifier option and xid supplied by the
client, the server's Server Identifier, and the Multicast Scope List
with one name per scope (the one that most closely matches the
language tag "en").
The following figure illustrates this exchange.
Server Client Server
v v v
| | |
| | |
| _____________/|\_____________ |
|/ GETINFO | GETINFO \|
| | |
| | |
|\ | ____________/|
| \_________ | / ACK |
| ACK \ |/ |
| \ | |
| | |
v v v
Figure 2: Timeline diagram of messages exchanged
in Multicast Scope List Discovery example
2. Multicast Discovery and Address Allocation
In this example, the MADCAP client wants to allocate a multicast
address from the global scope for use during the next two hours.
The client begins by multicasting a DISCOVER packet to the MADCAP
Server Multicast Address associated with IPv4 Local Scope. This
packet includes the Lease Time, Lease Identifier, and Multicast Scope
options.
Any servers that receive the DISCOVER packet and can satisfy this
request temporarily reserve an address for the client and unicast an
OFFER packet to the client. These packets contain the Lease Time,
Server Identifier, Lease Identifier, and Multicast Scope options.
After an appropriate delay, the client multicasts a REQUEST packet to
the MADCAP Server Multicast Address. This packet contains all of the
options included in the DISCOVER packet, but also includes the Server
Identifier option, indicating which server it has selected for the
request.
The server whose Server Identifier matches the one specified by the
client responds with an ACK packet containing the options included in
the OFFER packet, as well as a List of Address Ranges option listing
the address allocated. All the other servers that had sent OFFER
packets stop reserving an address for the client and forget about the
whole exchange.
The client now has a two hour "lease" on the multicast address.
If the client had not received an ACK from the server, it would have
retransmitted its REQUEST packet for a while. If it still received no
response, it would start over with a new DISCOVER message.
The following figure illustrates this exchange.
Server Client Server
(not selected) (selected)
v v v
| | |
|Begin multicast address request|
| | |
| _____________/|\_____________ |
|/ DISCOVER | DISCOVER \|
| | |
Reserves | Reserves
Address | Address
| | |
|\ | ____________/|
| \_________ | / OFFER |
| OFFER \ |/ |
| \ | |
| Collects replies |
| \| |
| Selects Server |
| | |
| _____________/|\_____________ |
|/ REQUEST | REQUEST \|
| | |
| | Commits address
| | |
| | _____________/|
| |/ ACK |
| | |
| assignment complete |
| | |
v v v
Figure 3: Timeline diagram of messages exchanged
in Multicast Address Allocation example
3. Lease Extension
This is a continuation of the previous example. The client has
already allocated a multicast address from the global scope for use
during the next two hours. Half way through this two hour period, it
decides that it wants to extend its lease for another hour.
The client unicasts a RENEW packet to the server from which it
allocated the address. This packet includes the Lease Time and Lease
Identifier options. The Lease Identifier matches the one used for the
original allocation. The time included in the Lease Time is two
hours, since the client wants the lease to expire two hours from the
current time.
The server responds with an ACK packet indicating that the lease
extension has been granted. This packet includes the Lease Time,
Server Identifier, Lease Identifier, Multicast Scope, and List of
Address Ranges options.
If the server did not want to grant the requested lease extension, it
would have responded with a NAK packet with the Lease Identifier
option.
The following figure illustrates this exchange.
Client Server
v v
| |
|\_____________ |
| RENEW \|
| |
| Extends lease
| |
| _____________/|
|/ ACK |
| |
| |
v v
Figure 4: Timeline diagram of messages exchanged
in Lease Extension example
4. Address Release
This is a continuation of the previous example. The client has
already allocated a multicast address and extended its lease for
another two hours. Half an hour later, the client finishes its use of
the multicast address and wants to release it so it can be reused.
The client unicasts a RELEASE packet to the server from which it
allocated the address. This packet includes the Lease Identifier
option. The Lease Identifier matches the one used for the original
allocation. When the server receives this packet, it cancels the
client's lease on the address and sends an ACK packet to the client
indicating that the lease has been released. This packet includes the
Server Identifier and Lease Identifier options.
The following figure illustrates this exchange.
Client Server
v v
| |
|\_____________ |
| RELEASE \|
| |
| Cancels lease
| |
| _____________/|
|/ ACK |
| |
v v
Figure 5: Timeline diagram of messages exchanged
in Address Release example
5. Unicast Address Allocation
This is a continuation of the previous example. At some later time,
the client decides to allocate another multicast address. Since it
has recently worked with a server, it decides to try sending a
unicast REQUEST to that server. If this doesn't work, it can always
try a multicast DISCOVER, as illustrated in example 2.
The client unicasts a REQUEST packet to the server from which it
wants to allocate the address. This packet includes the Lease Time,
Lease Identifier, and Multicast Scope options.
The server responds with an ACK packet containing the Lease Time,
Lease Identifier, and Multicast Scope options from the REQUEST
packet, as well as the Server Identifier option and a List of Address
Ranges option listing the address allocated.
The client now has a lease on the multicast address.
If the client had not received an ACK from the server, it would have
retransmitted its REQUEST packet for a while. If it still received no
response, it would start over with a multicast DISCOVER message.
The following figure illustrates this exchange.
Client Server
v v
| |
|\_____________ |
| REQUEST \|
| |
| Allocates address
| |
| _____________/|
|/ ACK |
| |
v v
Figure 6: Timeline diagram of messages exchanged
in Unicast Address Allocation example
APPENDIX B: Recommended Constant Values
Table 6 lists recommended values for constants defined in this
specification.
Constant Name Recommended Value
------------- -----------------
[CLOCK-SKEW-ALLOWANCE] 30 minutes
[DISCOVER-DELAY] current retransmit delay
[EXTRA-ALLOCATION-TIME] 1 hour
[NO-RESPONSE-DELAY] 60 seconds
[OFFER-HOLD] at least 60 seconds
[RESPONSE-CACHE-INTERVAL] at least 60 seconds (5 minutes maximum)
[XID-REUSE-INTERVAL] 10 minutes (required)
Table 6: Recommended Constant Values
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