INTERNET-DRAFT Dave Marlow
March 9, 1994 NSWC-DD
Host Group Extensions for CLNP Multicasting
draft-ietf-tuba-host-clnp-multicas-00.txt
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
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munnari.oz.au.
Abstract
This draft provides a specification for multicast extensions to the
CLNP protocol similar to those provided to IP by RFC1112. These
extensions are intended to provide the mechanisms needed by a host
for multicasting in a CLNP based Internet. This draft covers
addressing extensions to the CLNP addressing structure, extensions to
the CLNP protocol and extensions to the ES-IS protocol. An appendix
discusses the differences between IP multicast and the CLNP multicast
approach provided in this draft.
The multicast extensions to the CLNP addressing structure defines
group Network addresses which identify host groups. The multicast
extensions to CLNP provides a means for identifying a CLNP packet and
provides scope control mechanisms for CLNP multicast packets. The
multicast extensions to the ES-IS protocol provide the mechanisms
needed for a host to exchange control information with multicast
capable routers. These extensions to the ES-IS protocol provide for
a host to "announce" which multicast packets are of interest and for
a multicast capable router to dynamically "map" group Network
addresses to subnetwork addresses.
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Contents
Status of this Memo
Abstract
1. Introduction
2. Levels of Conformance
3. Group Network Addresses
4. Model of a CLNP End System Multicast Implementation
5. Extensions to the CLNP Protocol
6. Extensions to the ES-IS Routeing Protocol
Appendix A. Considerations for ESGH and MAM PDU destination
SNPA address parameters
Appendix B. Differences with RFC 1112
Appendix C. Issues Under Study
References
Author's Address
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1. Introduction
This draft provides a specification for multicast extensions for CLNP
in order to provide a CLNP based Internet the capabilities provided
for IP by RFC 1112 (Host Extensions for IP Multicasting) [RFC1112].
This draft uses an outline similar to that of RFC 1112.
Paraphrasing RFC 1112, "CLNP multicasting is the transmission of a
CLNP datagram to a "host group", a set of zero or more End Systems
identified by a single group Network address (GNA). A multicast
datagram is delivered to all members of its destination host group
with the same "best-efforts" reliability as regular unicast CLNP
datagrams, i.e., the datagram is not guaranteed to arrive intact at
all members of the destination group or in the same order relative to
other datagrams.
"The membership of a host group is dynamic; that is End Systems may
join and leave groups at any time. There is no restrictions on the
location or number of members in a host group. An End System may be a
member of more than one group at a time. An End System need not be a
member of a group to send datagrams to it.
"A host group may be permanent or transient. A permanent group has an
administratively assigned GNA. It is the address, not the membership
of the group, that is permanent; at any time a permanent group may
have any number of members, even zero.
"Internetwork forwarding of CLNP multicast datagrams is handled by
"multicast capable" Intermediate Systems which may be co-resident
with, or separate from, unicast capable Intermediate Systems.
This draft specifies the extensions required by an End System to make
use of CLNP multicast. In addition the requirements placed upon
multicast capable Intermediate Systems to exchange information with
multicast capable End Systems is specified. No specifications are
provided related to the information exchanges between Intermediate
Systems to support multicast route selection or multicast Protocol
Data Unit (PDU) forwarding. A discussion of multicast route selection
and PDU forwarding has been written by Steve Deering [Deering91].
Note that for these multicast extensions to work there must exist an
uninterrupted path of multicast capable routers between the End
Systems comprising a host group (such paths may utilize tunneling
(i.e. unicast CLNP encapsulated paths between multicast capable CLNP
routers). In order to support multicast route selection and
forwarding for a CLNP based internet additional specifications are
needed. Specifications of this type could come in the form of new
protocols, extensions to the current CLNP based routing protocols or
use of a technique out of the IETF`s Inter-Domain Multicast Routing
(IDMR) group. The IDMR group is currently investigating multicast
protocols for routers which utilize a router's unicast routing
protocols, this approach may extend directly to CLNP routers.
While many of the techniques and assumptions of IP multicasting (as
discussed in RFC 1112) are used in CLNP multicasting, there are
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number of differences. Appendix B describes the differences between
CLNP multicasting and IP multicasting. This draft describes
techniques brought in directly from projects within ISO to
incorporate multicast transmission capabilities into CLNP [MULT-
AMDS]. This draft is consistent with the current draft specifications
of these ISO projects. Key contributions to the techniques described
in this draft have been made by a number of individuals within the
ANSI X3S3.3 and ISO SC6 Working Group 2 committees.
2. Levels of Conformance
There are three levels of conformance for End Systems to this
specification:
Level 0: no support for CLNP multicasting.
There is no requirement for a CLNP End System (or Intermediate
System) to support CLNP multicasting. Level 0 hosts should be
unaffected by the presence of multicast activity. The destination
addresses used in support of multicast transfers, the GNA, should not
be enabled by a non-multicast capable End System and the PDUs
themselves are marked differently than unicast PDUs and thus should
be quietly discarded.
Level 1: support for sending but not receiving CLNP multicast PDUs.
At this point this level is not defined for CLNP multicast. An End
System sourcing multicast PDUs with the current specification at
least needs to know whether a multicast Intermediate System is
attached to the subnetwork that the multicast PDU is sourced on (i.e.
to determine the destination SNPA (subnet) address). Further work on
this level of conformance may be undertaken if utility is identified.
Level 2: full support for CLNP multicasting.
Level 2 allows a host to join and leave host groups as well as send
CLNP PDUs to host groups. It requires implementation by the End
System of all parts of this specification.
3. Group Network Addresses
Individual Network addresses used by CLNP for End System addressing
are called Network Service Access Points (NSAPs). RFC 1237 defines
the NSAP address for use in the Internet. In order to provide an
address for a group of End Systems, this specification does not
change the definition of the NSAP address, but adds a new type of
identifier - the group Network address - that supports a multicast
Network service (i.e., between a single source NSAP, identified by an
individual Network address, and a group of destination NSAPs,
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identified by a group Network address). Host groups are identified by
group Network addresses.
In the development of multicast address extensions to CLNP,
requirements were identified for: (1)"easily distinguishing" group
addresses at the Network layer from NSAP addresses; (2)leaving the
currently allocated address families unaffected and (3)ensuring that
the approach taken would not require the establishment of new
addressing authorities. In addition, it was agreed that providing
multicast options for all OSI Network layer users was desirable and
thus the group Network addressing solution should support options for
all address formats covered by ISO/IEC 8348 | Recommendation X.213.
The only viable means identified for meeting all requirements was via
creating a new set of AFI values with a fixed one-to-one mapping
between each of the existing AFI values and a corresponding group AFI
value.
Group Network addresses are defined by creating a new set of AFI
values, one for each existing AFI value, and a fixed one-to-one
mapping between each of the existing AFI values and a corresponding
group AFI value. The syntax of a group Network address is identical
to the syntax of an individual Network address, except that the value
of the AFI in an individual Network address may be only one of the
values already allocated for individual Network addresses, whereas
the value of the AFI in a group Network address may be only one of
the values allocated here for group Network addresses. The AFI values
allocated for group Network addresses have been chosen in such a way
that they do not overlap, in the preferred encoding defined by
ISO/IEC 8348 | CCITT Recommendation X.213, with any of the AFI values
that have already been allocated for individual Network addresses.
3.1 Definitions
group Network address: an address that identifies a set of zero or
more Network service access points; these may belong to multiple
Network entities, in different End Systems.
individual Network address: an address that identifies a single NSAP.
3.2 CLNP Addresses
A discussion of the CLNP address format is contained in RFC 1237. The
structure of all CLNP addresses is divided into two parts the Initial
Domain Part (IDP) and the Domain Specific Part (DSP). The first two
octets of the IDP are the Authority and Format Identifier (AFI)
field. The AFI has an abstract syntax of two hexadecimal digits with
a value in the range of 00 to FF. In addition to identifying the
address authority responsible for allocating a particular address and
the format of the address, the AFI also identifies whether an address
is an individual Network address or a group Network address. There
are 90 possible AFI values to support individual Network address
allocations. In addition, when the AFI value starts with the value
"0" this identifies that the field contains an incomplete individual
Network address (i.e. identifies an escape code).
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Table 1 allocates 90 possible AFI values to support group Network
address allocations. In addition if the first two digits of the IDP
are hexadecimal FF, this indicates the presence of an incomplete
group Network address. The allocation of group addresses is
restricted to be only from the AFI values allocated for the
assignment of group addresses in Table 1. An addressing authority in
allocating either Network addresses or authorizing one or more
authorities to allocate addresses, allocates both individual and the
corresponding group addresses. Thus each block of addresses allocated
by an addressing authority (or its sub-authority) contains a block of
individual Network addresses and group Network addresses. The
individual and group address block allocated are differentiated by
the AFI values used which are related as shown in Table 1.
Group Network addresses are only used as the destination address
parameter of a CLNP PDU. Source Address parameters are never
permitted to be group Network addresses.
Table 2 lists the AFI values which have not been assigned, at this
time, for the support of neither individual nor group address
allocation. Future assignment of these AFI values is possible.
Additional information concerning individual Network addresses (i.e.
NSAP and NET (Network Entity Titles)) is contained in RFC 1237.
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TABLE 1 - Relationship of AFI Individual and Group Values
-----------------------------------------------------------
|Individual Group | Individual Group | Individual Group |
-----------------------------------------------------------
| 0x FF | | |
| 10 A0 | 40 BE | 70 DC |
| 11 A1 | 41 BF | 71 DD |
| 12 A2 | 42 C0 | 72 DE |
| 13 A3 | 43 C1 | 73 DF |
| 14 A4 | 44 C2 | 74 E0 |
| 15 A5 | 45 C3 | 75 E1 |
| 16 A6 | 46 C4 | 76 E2 |
| 17 A7 | 47 C5 | 77 E3 |
| 18 A8 | 48 C6 | 78 E4 |
| 19 A9 | 49 C7 | 79 E5 |
| 20 AA | 50 C8 | 80 E6 |
| 21 AB | 51 C9 | 81 E7 |
| 22 AC | 52 CA | 82 E8 |
| 23 AD | 53 CB | 83 E9 |
| 24 AE | 54 CC | 84 EA |
| 25 AF | 55 CD | 85 EB |
| 26 B0 | 56 CE | 86 EC |
| 27 B1 | 57 CF | 87 ED |
| 28 B2 | 58 D0 | 88 EE |
| 29 B3 | 59 D1 | 89 EF |
| 30 B4 | 60 D2 | 90 F0 |
| 31 B5 | 61 D3 | 91 F1 |
| 32 B6 | 62 D4 | 92 F2 |
| 33 B7 | 63 D5 | 93 F3 |
| 34 B8 | 64 D6 | 94 F4 |
| 35 B9 | 65 D7 | 95 F5 |
| 36 BA | 66 D8 | 96 F6 |
| 37 BB | 67 D9 | 97 F7 |
| 38 BC | 68 DA | 98 F8 |
| 39 BD | 69 DB | 99 F9 |
-----------------------------------------------------------
TABLE 2 - AFI values reserved for future allocation
--------------
| 1A-1F |
| 2A-2F |
| 3A-3F |
| 4A-4F |
| 5A-5F |
| 6A-6F |
| 7A-7F |
| 8A-8F |
| 9A-9F |
| FA-FE |
--------------
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4. Model of a CLNP End System Multicast Implementation
The use of multicast transmission by a CLNP End System involves
extensions to two protocols: CLNP and the ES-IS Routeing Protocol. To
provide level 0 service (no support for CLNP multicast), no
extensions to the two present protocols are required. In order to
support level 2 service (full support for CLNP multicasting), the
extensions contained in the following sections are required.
Extensions identified for Intermediate Systems are not required (or
appropriate) for End Systems. Multicast transmission also requires
the use of a group Network address (as previously described) as the
destination address parameter.
5. Extensions to the CLNP protocol
This section provides extensions to the CLNP Protocol [CLNP] ISO
8473-1, to support multicast transmission. These additions provide
procedures for the connectionless transmission of data and control
information from one network-entity to one or more peer network-
entities.
In developing the multicast extensions for CLNP a decision was needed
on how to "mark" a packet as multicast (versus the current unicast
packets). Such marking is necessary since the forwarding behavior
for multicast packets is different (e.g. multiple copies of a packet
may need to be forwarded). The two alternatives considered were to
mark the packet (via a particular field) or to mark the destination
address, in the end both were done. The destination address for a
multicast PDU identifies a host group which is of a very different
nature than the unicast NSAP address. Rather than changing the
nature of NSAP addresses, a new set of addresses were created named
group Network addresses which are marked within the first octet (i.e.
the AFI field) with values reserved for group Network addresses.
Consideration was given to no further marking of the PDU; however, a
problem was identified with only using the group Network address to
identify multicast packets. Currently routers implementing the IS-IS
Intra-Domain protocol as Level 1 routers when receiving a packet with
an unknown destination address are permitted to either discard the
packet or send it to a Level 2 router. Such actions by non-multicast
capable routers to multicast packets can lead to non-deterministic
behavior. Level 1 routers upon receiving a packet containing a group
Network address might pass the packet up to a Level 2 router (which
may or may not be multicast capable) or it might discard it.
Depending upon the circumstances this might lead to whole regions
missing packets or packet duplication (possibly even explosion). The
result was to seek deterministic behavior by non-multicast capable
routers by creating a new PDU type (Multicast Data PDU) and inserting
into the CLNP reasons for discard: receiving a PDU of unknown type.
Note this reason for discard is mandatory on multicast capable and
non-multicast capable CLNP implementations.
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5.1 Definitions
multicast: Data transmission to one or more destinations in a
selected group in a single service invocation.
multicast capable Intermediate System: An Intermediate System which
incorporates the multicast features of the Network layer.
5.2 Addresses
The destination address parameter of a multicast PDU shall contain a
group Network address. The source address parameter shall be an
individual Network address.
5.3 Extensions to the current protocol functions
In order to support multicast transmissions the following optional
CLNP protocol functions will be implemented:
5.3.1 Header Format Analysis function
The header format analysis function optionally provides capabilities
to Network entities which support multicast transfer to supply
applicable PDUs directly to End Systems served by such a Network
entity as well as to forward such PDUs on to other Network entities.
This optional functionality is realized through a Network entity with
multicast capability identifying a PDU as using multicast transfer
via the PDU type and the PDU's destination address field.
If a Network entity supports multicast transmission, then the header
format analysis function shall provide checking to ensure that a PDU
does not contain a group Network address in the source address field.
Any PDU header analyzed to have a group address in the source address
field shall be discarded.
5.3.2 Route PDU functions
The route PDU function optionally provides capabilities to Network
entities which support multicast transfer for determining multiple
Network entities to which a single PDU shall be forwarded to. This
may result in multiple invocations of the forward PDU function and
hence the need to make multiple copies of the PDU. For PDUs that are
received from a different Network entity, the optional functionality
for the route PDU function is realized as a result of the header
format analysis function's recognition of the PDU as being a
multicast PDU. A Network entity originating a multicast PDU should
invoke the forward PDU function on one and only one subnetwork to
which it is attached and on which it has learned of the existence of
at least one multicast capable IS. A Network entity attached to
multiple subnetworks should select a subnetwork for which a multicast
capable Intermediate System is attached if one exists for the initial
sourcing of a PDU. A Network entity attached to multiple subnetworks
but has not learned of the existence of any multicast capable IS is
permitted to originate a multicast PDU on multiple subnetworks.
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Editor's Note: The purpose in having an originating Network entity
source a multicast PDU on only one subnetwork to which a multicast
capable IS is attached is to support the development of multicast
routing protocols which will need to determine on which subnetworks a
multicast PDU has visited. This behavior is predicated on the
assumption that the Intermediate Systems performing multicast
forwarding form a connected set.
5.3.3 Forward PDU function
This function issues an SN-UNITDATA request primitive, supplying the
subnetwork or Subnetwork Dependent Convergence Function (SNDCF)
identified by the route PDU function with the protocol data unit as
user data to be transmitted, the address information required by that
subnetwork or SNDCF to identify the "next" system or systems within
the subnetwork-specific addressing domain (this may be one or more
Intermediate Systems and/or one or more destination End Systems), and
quality of service constraints (if any) to be considered in the
processing of the user data.
5.3.4 Discard PDU function
Add an additional reason for discard - a PDU is received with an
unknown type code.
Editor's Note: Comments are requested as to whether this additional
reason for discard presents any problem with currently deployed CLNP
implementations.
5.3.5 Error reporting function
It is important to carefully control the use of the error reporting
capability in the case of multicast transfers. The primary concern
is to avoid the occurrence of broadcast storms and thus a multicast
PDU may not cause the origination of another multicast PDU. This is
the primary reason that the source address is not permitted to be a
group address. In addition, a multicast PDU with error reporting
permitted can result in flooding the source network-entity (as well
as the networks used) with Error Report PDUs.
While error reports are permitted on multicast PDUs, a PDU with a
group Network address in the source address field shall not be
responded to with an Error Report. This is to ensure that a multicast
PDU does not generate another multicast PDU. If the source address is
identified as a group address then an error report PDU shall not be
generated and the original PDU shall be discarded.
5.3.6 Source routing functions
No source routing capability is provided for multicast PDU transfer.
The NS provider shall not accept a multicast PDU with source route
parameters.
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5.4 Scope control function
5.4.1 Overview
The scope control function is an option for multicast PDU forwarding
only. The scope control function allows the originator to limit the
forwarding of the multicast PDU. The scope control function provides
the capability to limit the relaying of a particular PDU based on the
individual Network addressing hierarchy and/or limit the amount of
multicast expansion which can take place. In cases where both forms
of scope control are applied to the same PDU, forwarding will cease
once either has reached its scope control limit.
5.4.2 Prefix Based Scope Control
The prefix based scope control function allows the originator to
specify a specific set of address prefixes where the multicast
forwarding of a PDU by an Intermediate System occurs only if one of
the prefixes matches the Network Entity Title (NET) of the
Intermediate System. Prefix based scope control may be selected only
by the originator of a PDU. Prefix based scope control is
accomplished using one or more address prefixes held in a parameter
within the options part of the PDU header. The length of this
parameter is determined by the originating network entity, and does
not change during the lifetime of a PDU.
When an Intermediate System receives a multicast PDU containing a
prefix based scope control parameter, forwarding is only performed if
every octet of one of the prefixes contained in the prefix based
scope control parameter matches that Intermediate System's NET,
starting from the beginning of its NET. If no such prefix match
exists, the Intermediate System discards the PDU. The error reporting
function shall not be invoked upon PDU discard.
5.4.3 Radius Scope Control
The radius scope control function allows the originator to specify a
maximum logical distance where multicast expansion can occur. It is
closely associated with the header format analysis function. Each IS
receiving a multicast PDU which is capable of expanding and which
contains a Radius Scope Control parameter, decrements the Radius
Scope Control field in the PDU by an administratively set amount
between 0 and the maximum value of the field. This function
determines whether the PDU received may be forwarded or whether its
Radius has been reached, in which case it shall be discarded. An
Intermediate System shall not forward a multicast PDU containing a
Radius Scope Control parameter with a value of 0. The error reporting
function shall not be invoked upon PDU discard.
5.4.3.1 Radius Scope Control Example
The Radius Scope Control parameter is useful where policies have been
established across the potential forwarding path. One possible
policy for Internet use is for multicast capable routers to treat
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this field as a hop count within a domain (decrement by one unit) and
for inter-domain routers to either decrement this field to an even
multiple of 256 when crossing domains where prior agreements have
been made or decrement this field to 0 (i.e. discard the packet) for
other domains.
5.5 Structure and Encoding of PDUs
Multicast transmission is accomplished via the transfer of Multicast
Data (MD) PDUs. The PDU type code for a MD PDU is "1 1 1 0 1". The
format of the MD PDU is identical to that of the Data (DT) PDU. The
MD and DT PDU may contain the same optional parameters with the
following exceptions: (1)The source routing parameter is permitted
within DT PDUs but not MD PDUs; and (2)The scope control parameter is
permitted within MD PDUs but not DT PDUs.
5.6 Optional parameters for multicast support
5.6.1 Prefix Based Scope Control (this section is extended beyond the
ISO draft)
The prefix based scope control parameter specifies one or more
address prefixes for which Intermediate System forwarding requires a
match of one of the contained prefixes with the beginning of the
Intermediate System's NET.
Parameter Code: 1100 0100
Parameter Length: variable
Parameter Value: a concatenation of address prefix entries
The parameter value contains an address prefix list. The list
consists of variable length address prefix entries. The first octet
of each entry gives the length of the address prefix denominated in
bits that comprises the remainder of the entry. If the length field
does not specify an integral number of octets then the prefix entry
is followed by enough trailing zeroes to make the end of the entry
fall on an octet boundary. The list must contain at least one entry.
The prefix shall end on a boundary that is legal in the abstract
syntax of the address family from which it is derived. For example,
the encoding of a prefix whose DSP is expressed in decimal syntax
must end on a semi-octet boundary, while the encoding of a prefix
whose DSP is expressed in binary syntax can end on an arbitrary bit
boundary. If the end of the prefix falls within the IDP, then the
prefix must end on a semi-octet boundary and must not contain any
padding characters.
NOTE - The length of the prefix based scope control
parameter is determined by the originator of the PDU and is not
changed during the lifetime of the PDU.
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5.6.1.1 Prefix matching
A prefix that extends into the DSP shall be compared directly against
the encoded NET address, including any padding characters that may be
present. A prefix which does not extend into the DSP shall be
compared against the derived quantity NET', which is obtained from
the NET address by removing all padding characters (as defined by the
binary encoding process of ISO 8348).
The existence of a match shall be determined as follows:
a) If the encoded NET (or NET') contains fewer bits than the pre-
fix, then there is no match.
b) If the encoded NET (or NET') contains at least as many bits as
the prefix, and all bits of the prefix are identical to the
corresponding leading bits of the encoded NET (or NET'), there
is a match. Otherwise, there is no match.
5.6.2 Radius Scope Control
The radius scope control parameter specifies the logical distance
that a multicast PDU can be forwarded.
Parameter Code: 1100 0110 (THIS CODE MAY CHANGE)
Parameter Length: two octets
Parameter Value: two octets which represents the remaining
distance, that the PDU can be forwarded, in administratively set
units.
5.7 Provision of the Underlying Service
For a subnetwork that provides an inherent multicast capability, it
is the functionality of the SNDCF to provide the mapping between
group Network addresses and the corresponding addressing capability
of the subnetwork.
5.8 Conformance
All of the extensions provided to the functions to support multicast
capability are optional. For an End System or Intermediate System
which is not multicast capable these extensions are not applicable.
An implementation claiming conformance as a multicast capable End
System shall meet all of the requirements for an End System which is
not multicast capable and also provide all of the multicast exten-
sions provided here. An implementation claiming conformance as a mul-
ticast capable Intermediate System shall meet all of the requirements
for an Intermediate System which is not multicast capable and also
provide all of the multicast extensions provided here.
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6. Extensions to the ES-IS Routeing Protocol
This section provides optional extensions to the ES-IS Routeing Pro-
tocol [ES-IS], ISO 9542 to support the transfer of multicast PDUs. It
is an explicit goal of this specification that ESs and ISs, some of
which will have multicast capabilities and some without, will be able
to fully function on the same subnetworks. This specification does
not change any aspect of a currently defined (i.e. non-multicast) ISO
9542 implementation, it adds new optional functionality not modifying
current functionality. Two basic functions are provided: multicast
announcement and multicast address mapping.
6.1 Overview of the protocol
6.1.1 Operation of ESs receiving multicast PDUs
ESs, upon initialization and periodically thereafter, will construct
End System Group Hello (ESGH) PDUs identifying, by particular group
Network addresses, the multicast PDUs it wishes to receive. The ES
will periodically source (announce) these ESGH PDUs on the subnetwork
it wishes to receive these on. Reporting the same group Network
address on multiple subnetworks may result in the reception of dupli-
cate PDUs. ES-IS operations related to requesting the same group Net-
work address on multiple subnetworks are handled totally indepen-
dently (e.g. using different logical timers,...). It is permitted for
an ES to report a number of group Network addresses in the same ESGH
PDU. The only restrictions placed on providing multiple group Net-
work addresses within the same ESGH PDU are that all packets
requested are to be received on the same subnet, with the same hold-
ing time and that the ESGH PDU be of length equal to or less that its
maximum packet size constraint. Note each group Network address in
the ESGH PDU is paired with its own SNPA (subnetwork point of attach-
ment) address.
An ES will always have an SNPA address associated with each of its
active group Network addresses. An SNPA address is a subnetwork
address, in the case of a subnetwork which uses IEEE 802 addresses
the SNPA address is a 48 bit IEEE 802 MAC (media access control)
address. Of particular interest is the address used to mark the des-
tination group. For a subnetwork using IEEE 802 addressing a group
SNPA address uses a particular bit position to "mark" group SNPA
addresses.
Upon initialization the ES may have static SNPA address associations
(Pre-configured SNPA addresses). For any group Network address
without a Pre-configured SNPA address that the ES wishes to receive,
the ES will associate the "All Multicast End System Network Entities"
SNPA address. Upon receiving a Multicast Address Mapping (MAM) PDU
containing a group Network address that the ES is announcing, the ES
will use the SNPA address pairing contained in the MAM PDU for that
group Network address. Upon the expiration of the Mapping Holding
Timer, the ES shall revert back to associating either the Pre-
configured SNPA address if one exists or the "All Multicast End Sys-
tem Network Entities" SNPA address for the specific group Network
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address. While an ES is permitted to listen in on other ESs announce-
ments (needed for the damping option), an ES is not permitted to
change its group Network address to SNPA address mapping based on the
announcement of other ESs.
Optionally, the ES may perform damping (resetting a Multicast
Announcement Timer corresponding to a particular group Network
address) if the conditions necessary to withhold a particular
announcement are met. In order to perform damping the following con-
ditions must be met: (1)The ES must be processing other ES's
announcements; (2)An ESGH PDU is received that identifies the exact
same group Network address and SNPA address pairing on a particular
subnetwork that this ES is announcing on; (3) The Multicast Holding
Timer parameter value in the ESGH PDU received is equal to or greater
than the Multicast Holding Timer value, for this subnetwork, that is
being used by the ES processing this ESGH PDU.
ESs will utilize a local default value for their Multicast Announce-
ment Timer to control the period for sending out their ESGH PDUs. The
Active Multicast IS, if one exists on a particular subnetwork, will
optionally suggest a value for ESs on the subnetwork to use for their
Multicast Announcement Timer for a specific group Network address. In
order to support the optional damping function, ESs are required to
incorporate a 25% jittering to the timer values that they are using.
6.1.2 Operation of ESs sourcing multicast PDUs
An ES which is attached to multiple subnetworks when sourcing multi-
cast PDUs is required to source only one such PDUs on to a subnetwork
to which a multicast IS is attached. No restrictions are placed on a
multiply attached ES for which none of the subnetworks to which it is
attached to has a multicast capable IS. The ES sourcing multicast
PDUs on a subnetwork that has a multicast IS attached always uses the
"All Multicast Intermediate System Network entities" as the SNPA des-
tination address for all PDUs sourced. For an ES which is not
attached to a subnetwork with a multicast capable IS, the ES uses
pre-configured multicast address mapping information if available and
if there is no pre-configured information maps the "All Multicast End
System Network Entities" multi-destination address to the group Net-
work address. The ES sourcing multicast PDUs identified by a specific
group Network address is not required to be a receiver of such PDUs
(and thus is not announcing that particular group Network address).
The reason for sourcing packets on a subnetwork with a multicast
capable IS attached by multicasting to all multicast capable ISs
versus unicasting to one of the multicast capable ISs is to limit the
maximum number of transmissions required on any subnetwork to two.
In the case of sourcing a packet, one transmission would be used for
sourcing the packet to all ISs that might need it and the second, if
needed, to multicast the packet to ESs on the subnetwork which belong
to the group. If this packet were sourced via a unicast to an IS,
then the packet may need to passed on to other ISs on the subnet
(depending upon the particular multicast tree) and the same packet
may need to be transmitted again to the ESs on the subnetwork which
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belong to the group.
6.1.3 Operation of the Active Multicast IS
The Active Multicast IS listens in on all ESGH PDUs sourced on the
subnetwork for which it is serving as the Active Multicast IS. All
subnetworks are handled independently (even if multiple subnetworks
have the same ESs attached and the IS is serving as the Active Multi-
cast IS for these multiple subnetworks).
The Active Multicast IS sources MAM PDUs, for all group Network
addresses for which it has received ESGH PDUs, on the subnetwork due
to the following operational conditions: (1)The IS initializes either
as the Active Multicast IS after an election with other multicast
capable ISs or initializes believing it is the only multicast capable
IS (the determination of such conditions are outside of the scope of
this specification); (2)The IS receives an ESGH PDU with a group Net-
work address paired to an incorrect SNPA address; (3)The expiration
of the IS's Multicast Address Mapping Timer for that group Network
address (this is to prevent the expiration of Mapping Holding Timers
in ESs and to ensure that ESs which source specific multicast PDUs
but are not receivers of such PDUs are using the correct SNPA desti-
nation address in such PDUs that they source) or (4)The IS receives a
multicast PDU sourced on the subnetwork which used an incorrect des-
tination SNPA address (note: this specification does not place any
requirement on an IS to listen in on any multicast PDU, with the
exception of multicast PDUs addressed to "the All Multicast Inter-
mediate System Network Entities" group SNPA address but an IS may
receive a PDU with an incorrect destination SNPA address due to other
operational actions).
Actual relaying of any multicast PDUs sourced on a subnetwork will be
the responsibility of the multicast routing protocol used and is out-
side the scope of this specification.
6.2 Definitions
Active Multicast IS: The one multicast capable IS selected (via means
outside of this specification) to source Multicast Address Mapping
information on a particular subnetwork.
Paired SNPA Address: The SNPA address associated with a particular
group Network address on a specific subnetwork.
6.3 Routing information supporting multicast transmission
6.3.1 Multicast Announcement Information
An IS should forward a multicast PDU containing a particular destina-
tion group Network address onto a subnetwork to which it is attached
if and only if one or more of the ESs attached to that subnetwork
have declared an interest in receiving multicast PDUs destined for
that group Network address. Multicast announcement information
enables an IS that supports CLNP multicast to dynamically discover,
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for each subnetwork to which it is attached, the group Network
addresses for which ESs attached to that subnetwork have declared an
interest.
On a point-to-point subnetwork the multicast announcement information
informs the Network entity, in the case where it is attached to an
End System, of the group Network addresses for which that End System
expects to receive multicast PDUs.
On a broadcast subnetwork the multicast announcement information
informs the multicast capable Intermediate Systems, of the group Net-
work addresses for which ESs attached to that subnetwork expect to
receive multicast PDUs.
Editor's Note: Intermediate Systems with the optional OSI multicast
capabilities do receive information identifying the SNPA address of
ESs on the broadcast network that want PDUs with particular group
Network addresses as their destination address; however, the critical
information is which multicast PDUs are needed, not which ESs need
them.
6.3.2 Multicast Address Mapping Information
In order to receive multicast PDUs destined for a particular group
Network address, an ES may be able to take advantage of an associa-
tion of the group Network address with a specific SNPA address. Mul-
ticast address mapping information enables an IS to inform ESs that
they can receive multicast PDUs destined for a particular group Net-
work address on a corresponding specific SNPA address.
Multicast address mapping information is not employed on point-to-
point subnetworks.
Multicast address mapping information is employed on broadcast sub-
networks to enable multicast capable Intermediate Systems to inform
the multicast capable End Systems that they can receive, on a
specific broadcast subnetwork, multicast PDUs destined for a particu-
lar group network address on a corresponding specific SNPA address.
6.4 Addresses
All exchanges using this protocol are accomplished over a single sub-
network. While the control PDU's contain Network addresses (i.e.
group Network addresses) actual control PDU transfer is accomplished
via Subnetwork based group addresses (i.e. group SNPA addresses). The
following group SNPA addresses are used: (1)All Multicast End System
Network entities; (2)All Multicast Intermediate System Network enti-
ties and (3)a group SNPA address corresponding to a group Network
address
6.5 Timers
Two additional timers are employed: (1)the Multicast Announcement
Timer (MAT) and (2)Multicast Address Mapping Timer (MAMT). Old
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multicast announcement or multicast address mapping information shall
be discarded after the Holding Timer expires to ensure the correct
operation of the protocol.
6.5.1 Multicast Announcement Timer
The Multicast Announcement Timer is a local timer (i.e. maintained
independently by each End System, one timer per group Network
address) which assists in performing the Report Multicast Announce-
ment function. The timer determines how often an End System reports
its desire to receive multicast PDUs with that group Network address
as its destination address parameter. Considerations in setting this
timer are similar to those described for the Configuration timer in
the ES-IS specification.
6.5.2 Multicast Address Mapping Timer
The Multicast Address Mapping Timer is a local timer (i.e. maintained
independently by an Intermediate System which is actively participat-
ing with End Systems to transfer multicast PDUs) which assists in
performing the Report Multicast Address Mapping function. The timer
determines how often an Intermediate System, actively participating
with End Systems for the transfer of multicast PDUs, reports the Mul-
ticast Address Mapping for a particular group Network address. The
shorter the Multicast Address Mapping Timer, the more quickly End
Systems on the subnetwork will become aware of the correct address
mapping which may change due to the Intermediate System becoming
available or unavailable. There is a trade off between increased
responsiveness and increased use of resources in the subnetwork and
in the End Systems.
6.6 Extensions to the current protocol functions
In order to support multicast transmissions the following optional
ES-IS protocol functions will be implemented:
6.6.1 Report Configuration by Intermediate Systems
All multicast capable Intermediate Systems on a subnetwork will use
the Multicast Capable option in all ISH PDUs that they source. This
will provide multicast capable End Systems with a way to determine
that a multicast capable Intermediate System is operating on a par-
ticular subnetwork.
6.6.2 Query Configuration
Note: The Query Configuration function cannot be performed to find
the corresponding SNPA address of a group Network address since the
addressing information needed is the corresponding group SNPA address
and not the SNPA address of a particular End System responding. On a
large broadcast subnetwork, many different Configuration Responses
could result each incorporating a different End System Address. While
it is possible to design a Query Configuration for use with multi-
cast, this function does not appear to be required given the use of
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the "All Multicast End System Network Entities" address for supplying
a SNPA address when the group SNPA address is not known.
6.7 Multicast Announcement
6.7.1 Report Multicast Announcement Function by End Systems
An End System which needs to receive or continue to receive any mul-
ticast PDUs (i.e. PDUs with group Network addresses as their destina-
tion address), constructs and transmits ESGH PDUs to inform multicast
capable Intermediate Systems of the set of group Network address des-
tinations for which it wishes to receive PDUs. This may be done by
constructing ESGH PDUs for each group Network address. Alternatively,
ESGH PDUs may be constructed which convey information about more than
one group Network address at a time, up to the limits imposed by the
permitted SNSDU size and the maximum header size of the ESGH PDU.
Each ESGH PDU is transmitted by issuing an SN-UNITDATA.Request with
the following parameters:
SN_Userdata (SNSDU) <- ESGH PDU
SN_Destination _Address <- multi-destination address that indicates
"All Multicast Intermediate System Network Entities"
If an End System supports more than one SNPA, the information about
each group Network address desired for receiving on a particular SNPA
serving the End System shall be transmitted via that SNPA. It is per-
missible for an End System to report group Network addresses on mul-
tiple SNPAs; however, duplicate multicast PDUs should be anticipated.
The Group Address Pair parameter carries a list of Group Network
Addresses, each paired with its associated SNPA address. This infor-
mation is used by the Active Multicast IS to determine whether a Mul-
ticast Address Mapping PDU should be emitted to update the associa-
tion between Group Network Addresses and SNPA addresses.
The Holding Time (HT) field is set to approximately twice the ES's
Multicast Announcement Timer (MAT) parameter. The value shall be
large enough so that even if every other ESGH PDU is discarded (due
to lack of resources), or otherwise lost in the subnetwork, the mul-
ticast announcement information will still be maintained. The value
should be set small enough so that Intermediate Systems resources are
not needlessly consumed when the ES no longer wishes to receive PDUs
destined to a group Network address.
Note: -- When combining multiple group Network addresses in a single
ESGH PDU, it should be realized that there is a single Holding Time
parameter associated with all of these addresses.
6.7.1.1 Generating Jitter on Multicast Announcement Timers
The ES shall apply a 25% jitter to its Multicast Announcement Timer
(MAT) parameter. When ESGH PDUs are transmitted as a result of timer
expiration, there is a danger that the timers of individual systems
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may become synchronised. The result of this is that the traffic dis-
tribution will contain peaks. Where there are a large number of syn-
chronised systems, this can cause overloading of both the transmis-
sion medium and the systems receiving the PDUs. In order to prevent
this from occurring, all periodic timers, the expiration of which can
cause the transmission of PDUs, shall have "jitter" introduced as
defined in the following algorithm.
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CONSTANT
Jitter = 25;
Resolution = 100;
(* The timer resolution in ms *)
PROCEDURE Random(max: Integer): Integer;
(* This procedure delivers a Uniformly distributed random
integer R such that 0 < R <max *)
PROCEDURE WaitUntil(time: Integer)
(* This procedure waits the specified number of
ms and then returns *)
PROCEDURE CurrentTime(): Integer
(* This procedure returns the current time in ms *)
PROCEDURE
DefineJitteredTimer(baseTimeValueInSeconds : Integer;
expirationAction : Procedure);
VAR
baseTimeValue, maximumTimeModifier, waitTime : Integer;
nextexpiration : Time;
BEGIN
baseTimeValue := baseTimeValueInSeconds * 1000 / Resolution;
maximumTimeModifier := baseTimeValue * Jitter / 100;
(* Compute maximum possible jitter *)
WHILE running DO
BEGIN
(*First compute next expiration time *)
randomTimeModifier := Random(maximumTimeModifier);
waitTime:= baseTimeValue - randomTimeModifier;
nextexpiration := CurrentTime() + waitTime;
(* Then perform expiration Action *)
expirationAction;
WaitUntil(nextexpiration);
END (* of Loop *)
END (* of DefineJitteredTimer *)
Thus the call "DefineJitteredTimer(HelloTime, SendHelloPDU);" where
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"HelloTime" is 10 seconds, will cause the action "SendHelloPDU" to be
performed at random intervals of between 7.5 and 10 seconds. The
essential point of this algorithm is that the value of "randomTimeMo-
difier" is randomised within the inner loop. Note that the new
expiration time is set immediately on expiration of the last inter-
val, rather than when the expiration action has been completed.
The time resolution shall be less than or equal to 100 ms. It is
recommended to be less than or equal to 10ms. The time resolution is
the maximum interval than can elapse without there being any change
in the value of the timer. The periodic transmission period shall be
random or pseudo-random in the specified range. with uniform distri-
bution across similar implementations.
(Note: -- Applying jitter to the MAT parameter is required in order
to support the optional Damping function. If no jitter is applied on
a subnetwork where many ESs are requesting a particular multicast PDU
it is likely that they will have the same value for their MAT and
these timers may all become synchronised. Such synchronisation will
result in peaks in the distribution of traffic as described above.
The resulting overloading of the transmission medium and the systems
receiving the PDUs will negate any beneficial use of the Damping
function (since systems may be attempting to transmit their own ESGH
PDUs at the time they receive ESGH PDUs sourced by other ESs with the
same group Network address).
6.7.2 Record Multicast Announcement Function
The Record Multicast Announcement function receives ESGH PDUs,
extracts the multicast announcement information and updates the
information in its routing information base.
The receiving system is not required to process any option fields in
a received ESGH PDU.
Note: -- When a system chooses to process these optional fields, the
precise actions are not specified by this International Standard.
6.7.2.1 Record Multicast Announcement Function by Intermediate Systems
On receipt of an ESGH PDU an IS with the optional multicast capabili-
ties extracts the configuration information and stores the {group
Network address, subnetwork} in its routing information base replac-
ing any other information for the same entry.
The Active Multicast IS upon receipt of an ESGH PDU also extracts the
Paired SNPA Address parameter corresponding to each group Network
address in the ESGH PDU. If the Active Multicast IS has a mapping for
a group Network address carried in the ESGH for which the paired SNPA
address does not match, the Report Multicast Address Mapping function
is performed.
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6.7.2.2 Optional Damping Function
An ES with the optional capabilities to support multicast transfer
may decide to process ESGH PDUs multicast by other End Systems. There
is potentially some reduction in network traffic by doing this. An ES
requesting to receive multicast PDUs is permitted to reset its Multi-
cast Announcement Timer corresponding to one group Network address on
one SNPA upon receiving an ESGH PDU from another ES under the follow-
ing circumstances:
a) The {group Network address, paired SNPA address} received on a
particular subnetwork matches that of the ES processing the ESGH
PDU for that subnetwork.
b) The Holding Timer parameter value in the ESGH PDU received is
equal to or greater than the Holding Timer value for the, group
Network address, being used by the ES processing this PDU.
6.7.3 Flush Old Multicast Announcement Function
The Flush Old Multicast Announcement function is executed to remove
multicast announcement entries in its routing information base whose
Holding Timer has expired. When the Holding Timer for a group Network
address expires, this function removes the corresponding entry from
the routing information base of the local IS for the corresponding
SNPA.
The Flush Old Multicast Announcement function is also executed when-
ever a subnetwork service provider re-initializes a local SNPA. When
the SNPA is either disabled or re-initialized, all multicast
announcement information for any group Network address associated
with that SNPA is removed.
6.8 Multicast Address Mapping
6.8.1 Report Multicast Address Mapping Function by Intermediate Systems
The Active Multicast Intermediate System constructs a MAM PDU,
corresponding to a group Network address for which it received via
the Record Multicast Announcement function, and issues these PDUs
under the following circumstances:
a) The IS initializes either as the Active Multicast IS after an
election with other multicast capable ISs or initializes after
determining it is the only multicast capable IS (the determina-
tion of such conditions are outside of the scope of this stan-
dard), or
b) The IS receives an ESGH PDU with a group Network address paired
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to an SNPA address other than the one to which the IS is for-
warding multicast PDUs, or
Note: -- The Active Multicast IS determines which mappings are
correct. Pre-configured mappings which are used prior to the initial-
ization of the Active Multicast IS may be determined incorrect by the
Active Multicast IS.
c) The expiration of the IS's Multicast Address Mapping Timer for
that group Network address.
Note: -- This is to prevent the expiration of Holding Timers in ESs
and to ensure that ESs which source specific multicast PDUs but are
not receivers of such PDUs use the correct SNPA destination address.
The IS serving as the Active Multicast Intermediate System may con-
struct a MAM PDU for each group Network address. Alternatively, MAM
PDUs may be constructed which convey information about more than one
group Network address at a time, up to the limits imposed by the per-
mitted SNSDU size and the maximum header size of the MAM PDU. The IS
performs all multicast address mapping functions independently for
each of its SNPAs even if this IS is the Active Multicast IS on mul-
tiple subnetworks. Each MAM PDU is transmitted by issuing an SN-
UNITDATA.Request with the following parameters:
SN_Userdata (SNSDU) <- MAM PDU
SN_Destination _Address <- multi-destination address that indicates
"All Multicast End System Network Entities"
The Holding Time (HT) field is set to approximately twice the Inter-
mediate System's Multicast Address Mapping Timer (MAMT) parameter.
This variable shall be set to a value large enough so that even if
every other MAM PDU, for a particular group Network address, is dis-
carded (due to lack of resources), or otherwise lost in the subnet-
work, the multicast address mapping information will still be main-
tained. The value should be set small enough so that End Systems will
quickly cease to use the multicast address mappings supplied by ISs
that have failed.
Note: -- The Holding Timer parameter value applies to all group Net-
work addresses called out in the MAM PDU.
The Group Address Pair parameter is used to convey the association
between Group Network Addresses and SNPA addresses.
Optionally, the Active Multicast IS may include information in the
MAM PDU indicating a larger population of group Network addresses to
which the same multicast address mapping information applies. There
are two optional fields for this purpose: the Group Network Address
Mask option and the Paired SNPA Address Mask option. Their usage
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depends on the fact that group Network addresses are represented
using the preferred binary encoding, as specified in 7.3.2.
There are three permitted cases for including or excluding the masks.
In the first case, both masks are absent. In this case the MAM PDU
conveys information about one set of enumerated group Network
addresses only.
Note: -- Multiple group address pairs may be contained in a single
MAM PDU.
In the second case, the MAM PDU contains a Group Network Address Mask
but no Paired SNPA Address Mask. In this case, the MAM PDU conveys
information about an equivalence class of group Network addresses.
The information reveals that multiple group Network addresses are
mapped to the same SNPA address.
In the third case, the MAM PDU contains both masks. As in the second
case, the MAM PDU conveys information about an equivalence class of
group Network addresses. But in this case, the information reveals
that the SNPAs for the equivalence class of group Network address are
embedded in the group Network address. In particular the Paired SNPA
Address Mask indicates the location of the SNPA address in the group
Network Address(es).
An IS may optionally suggest a value for End Systems on the local
subnetwork to use as their Multicast Announcement Timers, for a
specific group Network address, by including the Suggested ES Multi-
cast Announcement Timer (ESMAT) option in the transmitted MAM PDU.
Setting this option permits the Active Multicast IS to influence the
frequency with which ESs transmit ESGH PDUs.
Note: -- If the ESMAT option is used, the one value permitted in the
MAM PDU is suggested for all group Network addresses called out in
the MAM PDU.
6.8.2 Record Multicast Address Mapping Function by End Systems
The Record Multicast Address Mapping function receives MAM PDUs,
extracts the multicast address mapping information and updates the
information in its routing information base. The receiving system is
not required to process any option fields in a received MAM PDU.
Note: -- When a system chooses to process these optional fields, the
precise actions are not specified by this International Standard.
On receipt of a MAM PDU an ES with the optional multicast capabili-
ties extracts the multicast address mapping information and stores
the {group Network address, paired SNPA address} for a particular
subnetwork in its routing information base replacing any other infor-
mation for the same group Network address and subnetwork.
In addition, an ES may also set its Multicast Announcement Timer
corresponding to the group Network address for which it is performing
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the Record Multicast Address Mapping function based on receipt of a
MAM PDU containing the ESMAT optional field.
Note: -- While an ES may process ESGH PDUs multicast by other ESs to
support the optional Damping function, an ES is not permitted to
change its own mapping due to the mapping found in other ES's ESGH
PDUs.
6.8.3 Flush Old Multicast Address Mapping Function by End Systems
The Flush Old Multicast Address Mapping function is executed to
remove multicast address mapping entries in its routing information
base whose corresponding Holding Timer has expired. When such a Hold-
ing Timer for a multicast address mapping expires, this function
removes the corresponding entry from its routing information base for
the corresponding SNPA.
The Flush Old Multicast Address Mapping function is also executed
whenever a subnetwork service provider re-initializes a local SNPA.
When the SNPA is either disabled or re-initialized, all multicast
address mapping information for any multicast address mapping associ-
ated with that SNPA is removed.
6.9 Paired SNPA Address Selection
6.9.1 Paired SNPA Address Selection Function by End Systems
An End System must pair each group Network address with an associated
SNPA address to support receiving (e.g. performing the Report Multi-
cast Announcement function) and sourcing Multicast PDUs.
6.9.1.1 Paired SNPA Address Selection for Receiving Multicast PDUs
An End System always has a paired SNPA address for every active group
Network address on a particular subnetwork. This mapping is obtained
by:
a) recording a multicast address mapping which is maintaining an
active holding timer, or if there has been no dynamic informa-
tion received, by
b) having pre-configured multicast address mapping information, or
if neither dynamic nor pre-configured information is available,
by
c) mapping the "All Multicast End System Network Entities" multi-
destination address to the group Network address.
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6.9.1.2 Paired SNPA Address Selection for Sourcing Multicast PDUs
An End System sourcing a multicast PDU pairs a SNPA address to the
group Network address. This mapping is obtained in the following
manner:
a) If there is a multicast capable IS reachable on the subnetwork,
(i.e. an ISH PDU containing a Multicast Capable optional parame-
ter has been received and the Holding Timer associated with this
routing information received is still active), then the SNPA
address used by an End System sourcing a multicast PDU on the
subnetwork is the "All Multicast Intermediate System Network
Entities" multi-destination address, or if there is no multicast
capable Intermediate System on the subnetwork, by
b) having pre-configured multicast address mapping information, or
if neither a multicast capable Intermediate System is present on
the subnetwork nor pre-configured information is available, by
c) mapping the "All Multicast End System Network Entities" multi-
destination address to the group Network address.
6.10 Structure and Encoding of PDUs
The ES-IS multicast control functions are supported via the exchange
of ESGH and MAM PDUs. The one exception to this is that a new
optional parameter, the Multicast Capable parameter, is provided for
use within the ISH PDU.
6.10.1 PDU Type Codes
The Multicast Announcement is accomplished via the transfer of End
System Group Hello (ESGH) PDUs. The PDU type code for an ESGH PDU is
"0 0 1 0 1". The Multicast Address Mapping (MAM) is accomplished via
the transfer of Multicast Address Mapping PDUs. The PDU type code for
a MAM PDU is "0 0 1 1 1".
6.10.2 Hold Time field
The Holding Time field specifies the maximum time for the receiving
Network entity to retain the multicast announcement or multicast
address mapping information contained in the PDU.
6.10.3 Structure of Addressing Parameters
The ESGH and MAM PDUs carry one or more group Network addresses
(GNAs) each with their associated Paired SNPA Address (PSA).
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6.10.4 Group Address Pair Parameter for ESGH and MAM PDUs
The Group Address Pair parameter is a list of one or more group Net-
work addresses each with their associated Paired SNPA address. The
group Network address identifies specific multicast PDUs and the
Paired SNPA address is the SNPA address on which the ES expects to
receive such multicast PDUs on that subnetwork. It is encoded in the
ESGH and MAM PDUs as shown in Figure 1.
Octet
,----------------------------------------------------,
| Number of Group Address Pairs | 10
|----------------------------------------------------|
| Group Network Address Length Indicator (GNAL) | 11
|----------------------------------------------------|
| | 12
: Group Network Address (GNA) :
| |
|----------------------------------------------------|
| Paired SNPA Address Length Indicator (PSAL) |
|----------------------------------------------------|
| |
: Paired SNPA Address (PSA) :
| |
|----------------------------------------------------|
| GNAL |
|----------------------------------------------------|
| |
: GNA :
| |
|----------------------------------------------------|
| PSAL |
|----------------------------------------------------|
| |
: PSA :
| | m-1
'----------------------------------------------------'
Figure 1 - ESGH and MAM PDUs - - Group Address Pair Parameter
6.10.5 Extensions to the current Option Parameters
The Security and Priority optional parameters may be carried in a
ESGH PDU. There is no Security or Priority option for the MAM PDU.
6.10.6 Suggested ES Multicast Announcement Timer
The ESMAT option may appear only in the MAM PDU
The ESMAT parameter conveys the value that an IS would like the
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receiving ESs to use as their local Multicast Announcement Timer.
Parameter Code: 1100 0111
Parameter Length: two octets
Parameter Value: ESMAT in units of seconds.
6.10.7 Multicast Capable
The Multicast Capable option may appear only in the ISH PDU
The Multicast Capable options consists only of a one octet code and a
one octet parameter length field, there is no parameter field.
Parameter Code: 1100 1000
Parameter Length: zero octets
Parameter Value: none (parameter does not exist).
6.10.8 Group Network Address Mask
The Group Network Address Mask option may only appear in the MAM PDU.
The Group Network Address Mask parameter indicates that the multicast
address mapping information applies to a larger population of group
Network Addresses than the group Network address(es) contained in the
MAM PDU indicates. When this option is provided in a MAM PDU, the
masking relationship contained must be valid for all group Network
addresses contained in this PDU. An End System may ignore this param-
eter.
The Group Network Address Mask establishes an equivalence class of
group Network addresses to which the same multicast address mapping
information applies. To determine whether or not a trial group Net-
work address falls within the equivalence class, the ES aligns the
trial group Network address with the Group Network Address Mask pad-
ding the latter with trailing zero octets if necessary. If in all bit
positions where the Group Network Address Mask is "1" the trial group
Network address matches the Group Network Address field of the Group
Address Pair parameter of the MAM PDU, then the trial group Network
address belongs to the equivalence class described by the MAM PDU.
The Group Network Address Mask parameter has additional semantics
when considered with the Paired SNPA Address Mask parameter.
Parameter Code: 1110 0011
Parameter Length: variable, up to 20 octets
Parameter Value: a comparison mask of octets to be aligned with the
Group Network Address field of the Group Address Pair parameter of
the MAM PDU.
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6.10.9 Paired SNPA Address Mask
The Paired SNPA Address Mask option may only appear in the MAM PDU.
When the Paired SNPA Address Mask is present, the equivalence class
defined by the Group Network Address Mask also has common structure
below the Group Network Address Mask; i.e. in the portion of the
group Network address where the Group Network Address Mask is logi-
cally "0". The Paired SNPA Address Mask supplies additional informa-
tion about the structure, by indicating certain bit positions within
the space "below" the Group Network Address Mask. Specifically, the
Paired SNPA Address Mask indicates the location of the Paired SNPA
address in the Group Network Address.
This parameter may appear in a MAM PDU only if the Group Network
Address Mask is also present. When this option is provided in a MAM
PDU, the masking relationship contained must be valid for all group
Network addresses contained in this PDU. An ES receiving such a MAM
PDU may safely ignore both masks. However (since presence of both
masks dictates different functional behavior than the presence of the
Group Network Address Mask alone) an ES shall not ignore one of the
masks while heeding the other.
Parameter Code: 1110 0100
Parameter Length: variable
Parameter Value: a comparison mask of octets to be aligned with the
Group Network Address field(s) of the Group Address Pair parameter of
the MAM PDU.
6.10.9.1 Mask Parameters Example
This section provides examples of using the Group Network Address
Mask and the Paired SNPA Address Mask. The examples given are for an
Internet usage of CLNP Multicasting across subnetworks using IEEE 802
addressing. For these examples the Group Network address format is:
+-----+----------------------------------------+
| IDP | Upper DSP | Embedded SNPA address | SEL|
+-----+-----------+-----------------------+----+
octets: | 3 | 10 | 6 | 1 |
+-----+-----------+-----------------------+----+
Thus the group Network address used is 20 octets. For these exam-
ples, the only field considered is the Embedded SNPA address field
and its placement within the group Network address.
In the first example it is the policy in "this part of the Internet"
to map the Embedded SNPA address into the IEEE 802 address space
reserved by IEEE 802 for group addressing using LOCAL assignment,
this corresponds to all 48 bit values with the two low order bits of
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the first octet set to "11".
The Active Multicast Intermediate System on this subnet may construct
a MAM PDU to map, for this example, a group Network address of {13
octets, 03-00-DA-DA-DA-DA, 1 octet} and a paired SNPA address of 03-
00-DA-DA-DA-DA. In addition the Active Multicast Intermediate System
can include in the MAM PDU a Group Network Address Mask of FF-FF-FF-
FF-FF-FF-FF-FF-FF-FF-FF-FF-FF-03-00-00-00-00-00-00.
With this paramete, all group Network addresses which share the
identical first 13 octet and with "11" in the two low order bits of
the 14th octet are put in an equivalence class and share the same
mapping information. If this were the only option present then all of
these group Network addresses would all have a paired SNPA of 03-00-
DA-DA-DA-DA.
In order to map the group Network addresses to the range of IEEE
addresses of this example, the MAM PDU must also contain a Paired
SNPA Address Mask. The Paired SNPA Address Mask identifies where the
SNPA Address is contained within the group Network addresses (defined
by the equivalence class formed by the Group Network Address Mask
within the same PDU). For this example the Paired SNPA Address Mask
is 00-00-00-00-00-00-00-00-00-00-00-00-00-FF-FF-FF-FF-FF-FF-00.
As a second example, all group Network addresses with a specific OUI
(organizationally unique identifier) using the twenty octet group
Network address format provided above are mapped to their embedded
SNPA address. An OUI is assigned by IEEE 802 and is three octets in
length. The OUI is contained in the first three address octets of a
GLOBALLY assigned IEEE 802 address. For this example the MAM PDU
must contain the following:
1. A group Network address contained within the MAM PDU with the
OUI of interest.
2. A group Network address Mask of FF-FF-FF-FF-FF-FF-FF-FF-FF-
FF-FF-FF-FF-FF-FF-FF-00-00-00-00.
3. A Paired SNPA Address of 00-00-00-00-00-00-00-00-00-
00-00-00-00-FF-FF-FF-FF-FF-FF-00.
6.10.10 End System Group Hello (ESGH) PDU
The ESGH PDU has the format shown in figure 2:
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Octet
,----------------------------------------------------,
| Network Layer Protocol Identifier | 1
|----------------------------------------------------|
| Length Indicator | 2
|----------------------------------------------------|
| Version/Protocol ID Extension | 3
|----------------------------------------------------|
| reserved (must be zero) | 4
|----------------------------------------------------|
| 0 | 0 | 0 | Type (00101 = ESGH) | 2
|----------------------------------------------------|
| Holding Time | 6,7
|----------------------------------------------------|
| Checksum | 8,9
|----------------------------------------------------|
| Number of Group Address Pairs | 10
|----------------------------------------------------|
| Group Network Address Length Indicator (GNAL) | 11
|----------------------------------------------------|
| | 12
: Group Network Address (GNA) :
| |
|----------------------------------------------------|
| Paired SNPA Address Length Indicator (PSAL) |
|----------------------------------------------------|
| |
: Paired SNPA Address (PSA) :
| |
|----------------------------------------------------|
| GNAL |
|----------------------------------------------------|
| |
: GNA |
| |
|----------------------------------------------------|
| PSAL |
|----------------------------------------------------|
| |
: PSA :
| | m-1
|----------------------------------------------------|
| | m
: Options :
| | p-1
'----------------------------------------------------'
Figure 2 - ESGH PDU Format
6.10.11 Multicast Address Mapping (MAM) PDU
The MAM PDU has the format shown in figure 3.
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Octet
,----------------------------------------------------,
| Network Layer Protocol Identifier | 1
|----------------------------------------------------|
| Length Indicator | 2
|----------------------------------------------------|
| Version/Protocol ID Extension | 3
|----------------------------------------------------|
| reserved (must be zero) | 4
|----------------------------------------------------|
| 0 | 0 | 0 | Type (00111 = MAM) | 2
|----------------------------------------------------|
| Holding Time | 6,7
|----------------------------------------------------|
| Checksum | 8,9
|----------------------------------------------------|
| Number of Group Address Pairs | 10
|----------------------------------------------------|
| Group Network Address Length Indicator (GNAL) | 11
|----------------------------------------------------|
| | 12
: Group Network Address (GNA) :
| |
|----------------------------------------------------|
| Paired SNPA Address Length Indicator (PSAL) |
|----------------------------------------------------|
| |
: Paired SNPA Address (PSA) :
| |
|----------------------------------------------------|
| GNAL |
|----------------------------------------------------|
| |
: GNA :
| |
|----------------------------------------------------|
| PSAL |
|----------------------------------------------------|
| |
: PSA :
| | m-1
|----------------------------------------------------|
| | m
: Options :
| | p-1
'----------------------------------------------------'
Figure 3 - MAM PDU Format
6.11 Conformance
All of the extensions provided to the functions to support multicast
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capability are optional. For an End System or Intermediate System
which is not multicast capable these extensions are not applicable. A
Network entity may choose to be multicast capable, a multicast capa-
ble Network entity is required to support both multicast announcement
information and multicast address mapping information.
An implementation claiming conformance as a multicast capable End
System shall meet all of the requirements for an End System which is
not multicast capable and shall support multicast announcement infor-
mation and shall implement the functions marked as Mandatory (M) in
column 4 of table 3. A multicast capable End System implementation
shall also support multicast address mapping information and shall
implement the functions marked as Mandatory (M) in column 5 of table
3.
An implementation claiming conformance as a multicast capable Inter-
mediate System shall meet all of the requirements for an Intermediate
System which is not multicast capable and shall support multicast
announcement information and shall implement the functions marked as
Mandatory (M) in column 6 of table 3. A multicast capable Intermedi-
ate System implementation shall also support multicast address map-
ping information and shall implement the functions marked as Manda-
tory (M) in column 7 of table 3.
Table 3 - Static Conformance Requirements for Multicast Capable
Network Entities
ES IS
Clause --------------
Label Function Reference AI MI AI MI
------------------------------------------------------------------
RpMAn Report Multicast Announcement 6.7.1 M - - -
RcMAn Record Multicast Announcement 6.7.2.1 - - M -
RcDamp Record Damping 6.7.2.2 O - - -
FlMAn Flush Old Multicast Announcement 6.7.3 O - M -
RpMAdMa Report Multicast Address Mapping 6.8.1 - - - M
MATGn ESMAT Generation 6.8.1 - - - O
RcMAdMa Record Multicast Address Mapping 6.8.2 - M - -
MATPr ESMAT Processing 6.8.2 - O - -
FlMAdMa Flush Old Multicast Address Map 6.8.3 - M - -
PSAdSel Paired SNPA Address Selection 6.9.1 - M - -
Key:
AI = Multicast Announcement information supported
MI = Multicast Address Mapping information supported
M = Mandatory; O = Optional; - = not applicable
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Appendix A. Considerations for ESGH and MAM destination SNPA address
parameters
One of the design decisions needed for this specification is what desti-
nation SNPA addresses to use for the control PDUs. One multicast
transfer to all entities needing these control PDUs provides great effi-
ciency; however the reception and processing of such PDUs by entities
not participating in the transfer of specific multicast PDUs wastes such
entities resources.
The MAM PDU is sent using the "All Multicast End System Network enti-
ties" SNPA address in the destination parameter field of its subnetwork
protocol. Such PDUs are intended to correct SNPA address problems and
need to be received by all End Systems receiving a specific multicast
PDU (identified by its group Network address) especially to those End
Systems which are presently utilizing an incorrect SNPA address. While
it appears feasible to send MAM PDUs utilizing the unicast SNPA address
of ESs which has sourced an ESGH with an incorrect mapping (identified
via receiving an ESGH with the incorrect mapping), there appears to be a
valid concern that if one ES has an incorrect mapping that others may
also have this problem. Further limiting the scope to only ESs which
receive particular multicast PDUs appears possible, this is left for
further study. Actual reception of a particular MAM mapping is only
required by those ESs which are announcing the associated group Network
address; however given the capability to use the optional damping func-
tion and the capability to place multiple mappings within a MAM PDU,
this further limiting of scope is not recommended at this time.
The ESGH PDU is sent using the "All Multicast Intermediate System Net-
work entities" SNPA address in the destination parameter field of its
subnetwork protocol. The support of the damping option requires that all
systems supporting a specific multicast PDU transfer monitor ESGHs sent
for a specific group Network address and thus this requires these ESGH
PDUs to be sent on a multicast address. The other possible SNPA address
to use is that which is paired to that group Network address; however,
this address may be pre-configured and may not be known to the Active
Multicast IS. This will lead to the Active Multicast IS missing the ESGH
PDUs announcing these addresses. Another concern with using the paired
SNPA address is that multiple group Network addresses can be announced
in the same ESGH PDU and these other group Network addresses would be
expected to use different paired SNPA addresses.
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Appendix B. Differences with RFC 1112
This appendix is intended to identify differences between the mechanisms
defined for CLNP Multicast in this specification and those for IP multi-
cast defined in RFC 1112. The work on CLNP Multicast followed the work
on IP multicast and was explicitly aimed at bringing the capabilities
described in RFC 1112 into a CLNP context. This appendix is intended to
provide some background information on the difference; however, it is
not intended to justify the mechanisms selected for CLNP multicast use.
Static/Dynamic Address Binding of Multicast Datagrams
IP multicast utilizes a static binding of Class D IP addresses to a
specific range of IEEE 802 48 bit group addresses. The IEEE 802 address
range that is used is within the address range that IEEE 802 allocates
for "Global" administration and this block of addresses is under the
control of the Internet Assigned Numbers Authority (IANA) which in turn
has allocated this block of addresses for use by IP multicast. This
scheme is very simple and efficient. Given the use of a 32 bit IP
address, the lower 23 bits of the Class D address are mapped into the
lower 23 bits of a 48 bit IEEE 802 address where the upper 25 bits are
fixed. Static binding of this form is global in scope (all members of a
group use the same IEEE 802 address on all subnets (at least all that
use IEEE 802 addressing).
CLNP multicast uses a dynamic binding of a group Network address (up to
20 bytes) to any subnetwork address. In cases where no multicast capable
Intermediate Systems are attached to a subnetwork then a binding using
preconfigured information or the "All Multicast End System Network enti-
ties" subnetwork addresses is used. The large GNA provides the room to
contain a full 48 bit IEEE 802 address if desired. Mask capabilities
are optionally provided which allow a multicast capable Intermediate
System to specify a "static" binding for a particular subnetwork. One of
the major purposes of providing a dynamic binding is to customize a
host's subnetwork address usage to the capabilities of the attached sys-
tems. There is considerable differences in the numbers of group subnet-
work addresses that a system can recognize using hardware hooks built
into the integrated circuits used. For example the number of addresses
that can be recognized by hardware may differ by an attached system
depending upon the interface it uses (e.g. Ethernet interface and FDDI
within the same system may have quite different capabilities). Dynamic
binding of this form is local in scope (members of a group may use dif-
ferent subnet addresses (e.g. IEEE 802 addresses) on different subnets).
Sourcing of Multicast Datagrams
IP multicast sources multicast datagrams directly, where the host sourc-
ing a datagram sends it with the group Subnetwork address as its desti-
nation. Hosts attached to the network where the datagram is sourced
receive the datagram directly.
CLNP multicast sources multicast datagrams to the "All Multicast Inter-
mediate System Network entities" address if there is a multicast capable
Intermediate System attached to a subnetwork that the End System
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sourcing the datagram is attached to. This requires an extra hop over
the IP multicast case when there are End Systems on the same subnetwork
which have requested to receive datagrams being sourced using a particu-
lar GNA. When a multicast capable Intermediate System is not attached
to a subnetwork, the End System may utilize either preconfigured infor-
mation (which might be a direct mapping from a portion of the group Net-
work address) or use the "All Multicast End System Network entities"
address.
Address Binding of Control Packets
IP multicast sends the control packets related to the IGMP protocol on
the same subnetwork address that is used by the multicast data traffic.
CLNP multicast sends the control packets related to the ES-IS protocol
extensions on specific group subnetwork addresses (i.e. "All Multicast
End System Network entities" and "All Multicast Intermediate System Net-
work entities" addresses).
Router Requirements for relaying Multicast Datagrams
IP multicast requires that a multicast router run in "promiscuous" mode
where it must receive all multicast datagrams sourced on a subnetwork
regardless of the destination. This is a result of the choices selected
in the "Sourcing of Multicast Datagrams" and "Address Binding of Control
Packets" discussed above.
CLNP multicast allows a multicast router to only receive datagrams sent
to the "All Multicast Intermediate System Network entities" address.
This is a result of the choices selected in the "Sourcing of Multicast
Datagrams" and "Address Binding of Control Packets" discussed above.
Aggregation of Control Information
In IP multicast, a host is required to withhold an announcement report
upon hearing another host reporting a similar interest in a particular
Class D address on a particular subnetwork. This is an option for CLNP
multicast (upon hearing interest in a particular group Network address
on a particular subnetwork). Such reports are not combined in IP multi-
cast while CLNP multicast supports providing multiple announcements (and
address mappings) within a single packet. A mask feature for address
mappings supports identifying mappings for a range of group Network
addresses within a single control packet.
Datagram Scope Control
IP multicast supports the use of the IP Hop Count as a means to support
scope control. While not documented in RFC 1112, a technique is also
being used to use bits within the Class D address to identify whether a
datagram has single subnetwork, "campus" or global scope.
CLNP has considerable scope control functionality. While the PDU Life-
time field can be employed in a similar way to the IP Hop Count, two
additional options are available. The Radius scope control provides a
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mechanism for "administratively" setting distance values and de-couples
the multicast scope control from the PDU lifetime function. More impor-
tantly, the Prefix based scope control appears to provide considerable
and flexible functionality that can adjust to situations where a known,
hierarchical unicast addressing structure exists.
Marking of Multicast Datagrams
IP multicast marks a multicast PDU via the use of an IP Class D address
as its destination address parameter. CLNP multicast marks both the PDU
(a different PDU type) and the destination address (i.e. group Network
address) parameter.
Unicast Addressing Differences
An IP address identifies a specific host interface while a CLNP indivi-
dual Network address (i.e. NSAP address) identifies a particular Network
entity. This difference has not impacted a difference with RFC 1112;
however, this may impact using the techniques developed for IP multicast
in a CLNP multicast context.
Error Reports
Error reports sent in response to receiving a multicast PDU are not per-
mitted in IP multicast while they are permitted in CLNP multicast.
Source Routing
Source routing of multicast PDUs are permitted in IP multicast (but at
the present time this is discouraged) while they are not permitted in
CLNP multicast.
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Appendix C. Issues Under Study
1. Packets Traversing a Subnet Twice
The multicast extensions to CLNP were developed to be as free from con-
siderations of multicast routing as possible. A certain number of
assumptions concerning multicast routing were made in the developments
of the extensions to the ES-IS routing protocol (these extensions are
described in section 6.). A key assumption was that PDU transmissions
on a subnet involving hosts would be separated from PDU forwarding.
This is in-line with the present ISO unicast routing architecture. Thus
PDU sourcing, in the presence of a multicast capable router would be
separate from PDU forwarding to other hosts even if they are on the same
subnet. This results in the same Data PDU being sent twice when: (1)a
subnet has members of a group attached and (2)a packet needs to be for-
warded to one or more routers via that subnet.
Concern has been expressed since the immediate application for multicast
involves sending real time data in support of multi media applications.
Such exchanges may already using a significant proportion of a subnet's
bandwidth and doubling the bandwidth used in this case may not be
acceptable.
Note that the only piece of this problem that this Internet Draft covers
is the sourcing of a packet in the presense of multicast capable
routers. The other issues are involved in multicast routing which is
not addressed by this Internet Draft.
2. Local versus Global address bindings
The extensions to the ES-IS protocol provide a multicast address mapping
function which supports dynamically binding a group Network address to a
subnetwork address. Concern has been expressed that this is an unneces-
sary feature which complicates the job of network administrators without
suitable benefit. A static, global binding of group Network addresses
to IEEE 802 subnet addresses has been suggested.
The two main reasons that the group Network address to subnet (IEEE 802)
address was made locally configurable were to support multicast on sub-
nets with hosts having a mixture of capabilities (as to how many multi-
cast subnet addresses a host could register to receive at a time) and to
support multicast on subnets that do not use 48 bit IEEE 802 addresses.
Thus it was felt that this should be done per subnet versus globally.
Even multi-homed hosts with subnets that use 802 addresses may have
varying capabilities (looking at typical Ethernet, FDDI and 802.5 imple-
mentations).
One possible solution is to recommend a direct mapping in any Internet
use of CLNP multicast on subnets which use IEEE 802 addressing. This
could be a default for all Internet hosts. A policy would be needed to
identify the Internet's group Network address format. Given such a map-
ping the only operational overhead that would occur is that in the pres-
ence of a mapping server (the Active Multicast IS), which was supporting
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this mapping, a MAM PDU would periodically be sent with a Group Network
Address Mask which would identify the direct mapping.
References
[Deering91] S.E. Deering. Multicast Routing in a Datagram Internetwork.
PhD thesis, Electrical Engineering Dept., Stanford University, December
1991.
[RFC1112] Host Extensions for IP Multicasting, Network Working Group,
RFC 1112, S. Deering, August 1989.
[RFC1237] Guidelines for OSI NSAP Allocation in the Internet, Network
Working Group, RFC 1237, R. Colella, E. Gardner, R. Callon, July 1991.
[CLNP] Protocol for providing the connectionless-mode network service,
International Standard 8473-1, Second Edition, ISO/IEC JTC 1, Switzer-
land 1994. (Available via FTP from merit.edu:pub/iso/iso8473part1.ps).
[ES-IS] End system to Intermediate system routing exchange protocol for
use in conjunction with the Protocol for providing the connectionless-
mode network service, International Standard 9542, ISO/IEC JTC 1,
Switzerland 1987. (Available via FTP from
merit.edu:pub/iso/iso9542.ps).
[MULT-AMDS]: Amendments to ISO standards to support CLNP multicast
extensions:
ISO 8348 AM5 Amendment to the Network Service to support Group Network
Addressing. International Standard ISO 8348 Amendment 5, ISO/IEC JTC 1,
Switzerland 1994.
ISO 8473-1 DAM1 - Draft Amendment to the Second Edition of the Protocol
for providing the connectionless-mode network service [CLNP], Multicast
Extension, 1993.
ISO 9542 DAM2 - Draft Amendment to the ES-IS [ES-IS] protocol, Addition
of connectionless- mode multicast capability, 1993.
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Author's Address:
Dave Marlow
Code B35
NSWC-DD
Dahlgren, VA. 22448
Phone: (703) 663-1675
Email: dmarlow@relay.nswc.navy.mil
Expires: September 14, 1994 [Page 41]
