The Group Address Pair parameter carries a list of Group Network
Addresses, each paired with its associated SNPA address. This
information is used by the Active Multicast IS to determine whether a
Multicast Address Mapping PDU should be emitted to update the
association 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
multicast 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
may become synchronised. The result of this is that the traffic
distribution will contain peaks. Where there are a large number of
synchronised systems, this can cause overloading of both the
transmission 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.
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
"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
"randomTimeModifier" is randomised within the inner loop. Note that
the new expiration time is set immediately on expiration of the last
interval, 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
distribution 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 originated 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
capabilities extracts the configuration information and stores the
{group Network address, subnetwork} in its routing information base
replacing 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.
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
Multicast Announcement Timer corresponding to one group Network
address on one subnetwork upon receiving an ESGH PDU from another ES
under the following 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
subnetwork.
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
determination of such conditions are outside of the scope of
this standard), or
b) The IS receives an ESGH PDU with a group Network address paired
to an SNPA address other than the SNPA address contained in the
Active Multicast IS's multicast address mapping information for
that group Network address, or
Note: The Active Multicast IS determines which mappings are correct.
Pre-configured mappings which are used prior to the initialization of
the Active Multicast IS may be determined to be 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.
d) The IS receives a multicast PDU originated on the subnetwork
which used an incorrect destination SNPA address.
Note: Of particular concern are those multicast packets using the
"All Multicast Capable Intermediate Systems" SNPA address when
another SNPA address should have been used. The Originating
Subnetwork Forwarding function is performed if this event occurs (see
section 6.11).
Note: The multicast capable ISs need to receive multicast packets on
all SNPA addresses that are contained in the current multicast
address mapping information for the subnetwork. The multicast
capable ISs are not required to receive multicast packets on any SNPA
addresses other than those contained in the current multicast address
mapping information and the "All Multicast Capable Intermediate
Systems" SNPA address.
Circumstances b) and d) are the event driven conditions for the
Active Multicast IS to construct and issue a MAM PDU. The Active
Multicast IS shall limit the number of MAM PDUs issued per unit of
time. MAM PDUs with identical information shall not be issued more
than once per second. Event conditions occurring 10 seconds after
the last issue of an appropriate MAM PDU shall result in the issuance
of another such MAM PDU.
The IS serving as the Active Multicast Intermediate System may
construct 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
permitted SNSDU size and the maximum header size of the MAM PDU. The
IS performs all multicast address mapping functions independently for
each of its subnetworks even if this IS is the Active Multicast IS on
multiple 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 Capable End Systems"
The Holding Time (HT) field is set to approximately twice the
Intermediate 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 discarded (due to lack of resources), or otherwise lost
in the subnetwork, the multicast address mapping information will
still be maintained. 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
Network 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.
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 SNPA addresses 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).
The Active Multicast IS shall construct a MAM PDU with direct
information, not needing analysis of the Mask parameters, in response
to the occurrence of an event driven condition. The Active Multicast
IS may provide additional information in such a MAM PDU via the use
of Mask parameters.
An IS may 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 Multicast Announcement
Timer (ESMAT) parameter in the transmitted MAM PDU. Setting this
parameter permits the Active Multicast IS to influence the frequency
with which ESs transmit ESGH PDUs.
Note: If the ESMAT parameter 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 with
the exception of the Suggested ES Multicast Announcement Timer
(ESMAT) parameter.
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
capabilities 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 information for the same group Network address and subnetwork.
In addition, an ES shall set its Multicast Announcement Timer,
corresponding to the group Network address for which it is performing
the Record Multicast Address Mapping function, based on receipt of a
MAM PDU, corresponding to that group Network address, containing an
ESMAT parameter.
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
Holding Timer for a multicast address mapping expires, this function
removes the corresponding entry from its routing information base for
the corresponding SNPA.
6.9 Paired SNPA Address Selection Function by End Systems
An End System shall pair each group Network address with an
associated SNPA address to support receiving (e.g., performing the
Report Multicast Announcement function) and originating multicast
PDUs.
6.9.1 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
information 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 Capable End Systems" multi-
destination address to the group Network address.
6.9.2 Paired SNPA Address Selection for Originating Multicast PDUs
An End System, originating 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
then the SNPA address used by an End System originating a multi-
cast PDU is either the paired SNPA address obtained from the
multicast address mapping information associated with the group
Network address in the multicast PDU's Destination address
parameter or if there is no valid entry for the group Network
address by using the "All Multicast Capable Intermediate Sys-
tems" multi-destination address, or if there is no multicast
capable Intermediate System on the subnetwork, by
Note: Multicast address mapping information is valid if the Holding
Timer associated with it has not expired.
Note: An ES can determine if a multicast capable IS is reachable on
the subnetwork by having for that subnetwork either (1)multicast
address mapping information or (2)routing information received via an
ISH PDU containing a Multicast Capable optional parameter. In either
case the information must be valid (i.e., the Holding Timer for the
information must not have expired).
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 Capable End Systems" multi-
destination address to the group Network address.
6.10 Extensions to the ISO CLNP Route Function by End Systems
An End System attached to more than one subnetwork shall determine
when originating a multicast PDU whether to forward this multicast
PDU to more than one subnetwork or not. End Systems shall originate
each multicast PDU on all subnetworks for which the ISO ES-IS
Configuration function is actively reporting the NSAP address
contained in the Source Address parameter of the multicast PDU. As a
result of this function multiple invocations of the ISO CLNP
Forwarding function may result when such an ES originates a multicast
PDU.
6.11 Originating Subnetwork Forwarding Function by Intermediate
Systems
The Active Multicast IS upon receiving a multicast PDU originated on
a subnetwork which used the "All Multicast Capable Intermediate
Systems" SNPA address when another SNPA address should have been
used, performs the Originating Subnetwork Forwarding function. The
multicast address mapping information defines the correct SNPA
address pairings for a given subnetwork. The Originating Subnetwork
Forwarding function forwards the multicast PDU back on subnetwork it
was originated on. In the case that the ES was attached to more than
one subnetwork and originated the multicast PDU on more than one
subnetwork, the Active Multicast IS for each subnetwork performs the
Originating Subnetwork Forwarding function for the subnetwork that
they are responsible for.
The Active Multicast IS obtains the contents for the multicast PDU
for the Originating Subnetwork Forwarding function by using the
contents of the multicast PDU received with the incorrect destination
SNPA address and replacing the original PDU Lifetime field with the
value one (0000 0001). The Active Multicast IS performs the ISO 8473
PDU Composition function and forwards the PDU to the subnetwork that
the PDU was originated on using the ISO 8473 Forwarding function with
the correct destination SNPA address.
Note: The PDU Lifetime field is set to "one" to ensure that ISs
attached to the originating subnetwork do not forward this PDU on.
Such ISs should have received the PDU when it was originated since
this function is only performed in the event of receiving a multicast
PDU incorrectly addressed to the "All Multicast Capable Intermediate
Systems" SNPA address.
6.12 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.
6.12.1 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".
6.12.2 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.
6.12.3 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).
6.12.4 Group Address Pair Parameter for ESGH and MAM PDUs
The Group Address Pair parameter is a list of one or more group
Network 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
6.12.5 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.
6.12.6 Suggested ES Multicast Announcement Timer
The ESMAT parameter may appear only in the MAM PDU
The ESMAT parameter conveys the value that an IS requests the
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.
6.12.7 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).
6.12.8 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
parameter.
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
Network address falls within the equivalence class, the ES aligns the
trial group Network address with the Group Network Address Mask
padding 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.
6.12.9 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
logically "0". The Paired SNPA Address Mask supplies additional
information 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.
6.12.9.1 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
examples, 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
the first octet set to "11".
The Active Multicast Intermediate System on this subnetwork 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 parameter, 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 address 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.
6.12.10 End System Group Hello (ESGH) PDU
The ESGH PDU has the format shown in figure 2:
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
6.12.11 Multicast Address Mapping (MAM) PDU
The MAM PDU has the format shown in figure 3:
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
6.13 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. A
Network entity may choose to be multicast capable, a multicast
capable 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
information 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
Intermediate 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
Intermediate System implementation shall also support multicast
address mapping information and shall implement the functions marked
as Mandatory (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 - - - M
RcMAdMa Record Multicast Address Mapping 6.8.2 - M - -
MATPr ESMAT Processing 6.8.2 - M - -
FlMAdMa Flush Old Multicast Address Map 6.8.3 - M - -
PSAdSel Paired SNPA Address Selection 6.9.1 - M - -
ExtForw Extensions to CLNP Route Function 6.10 - M - -
OSuForw Originating Subnetwork Forwarding 6.11 - - - M
Key:
AI = Multicast Announcement information supported
MI = Multicast Address Mapping information supported
M = Mandatory; O = Optional; - = not applicable
7. Security Considerations
Security issues are not discussed in this memo.
Appendix A. Differences with RFC1112
This appendix is intended to identify differences between the
mechanisms defined for CLNP Multicast in this specification and those
for IP multicast defined in RFC1112. The work on CLNP Multicast
followed the work on IP multicast and was explicitly aimed at
bringing the capabilities described in RFC1112 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 Capable
End Systems" 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 systems. There is considerable differences in the
numbers of group subnetwork 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 different subnetwork addresses
(e.g., IEEE 802 addresses) on different subnets).
Originating of Multicast Datagrams
IP multicast originates multicast datagrams directly, where the host
originating a datagram sends it with the group Subnetwork address as
its destination. Hosts attached to the network where the datagram is
originated receive the datagram directly.
CLNP multicast originates multicast datagrams directly using the
group's subnetwork address as its destination when multicast address
mapping information is available. This case occurs when a multicast
capable Intermediate System is attached to the subnetwork and a host
on the subnetwork is announcing an interest in multicast packets
identified by a particular group Network address. The Active
Multicast IS may use MAM PDU mask parameters to provide multicast
address mapping information for a large number of group Network
addresses. When there is no multicast address mapping information for
the particular group Network address on a subnetwork with a multicast
capable IS attached to it, hosts originate packets using such
addresses sends to the "All Multicast Capable Intermediate Systems"
SNPA address. This case occurs when there are no receivers of such
multicast packets on the originating subnetwork. When a multicast
capable Intermediate System is not attached to a subnetwork, the End
System may utilize either preconfigured information (which might be a
direct mapping from a portion of the group Network address) or use
the "All Multicast Capable End Systems" 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 Capable End Systems" and "All Multicast Announcements"
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 originated on a
subnetwork regardless of the destination. This is a result of the
choices selected in the "Originating of Multicast Datagrams" and
"Address Binding of Control Packets" discussed above.
CLNP multicast allows a multicast router to limit multicast packet
reception to only those datagrams sent to the SNPA addresses where
there is current multicast address mapping information or to the "All
Multicast Capable Intermediate Systems" address. The intention is to
allow the multicast routers to be in control of the SNPA addresses
for multicast packets that they need to receive. This is a result of
the choices selected in the "Originating 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 multicast 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 RFC1112, 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
Lifetime field can be employed in a similar way to the IP Hop Count,
two additional options are available. The Radius scope control
provides a mechanism for "administratively" setting distance values
and de-couples the multicast scope control from the PDU lifetime
function. More importantly, 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
individual Network address (i.e., NSAP address) identifies a
particular Network entity. This difference has lead to a difference
with RFC1112. IP multicast requires a host which is attached to
more than one subnetwork to originate a multicast packet on only one
subnetwork. CLNP multicast requires a host which is attached to more
than one subnetwork to originate a multicast packet on every
subnetwork that the ISO ES-IS Configuration function is reporting the
NSAP address contained in the source address parameter of the
multicast PDU.
Error Reports
Error reports sent in response to receiving a multicast PDU are not
permitted 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.
Appendix B. Issues Under Study
This appendix is intended to record the current issues (as discussed
at the March 1994 TUBA meeting).
1. 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 unnecessary feature which complicates the job of network
administrators without suitable benefit. A static, global binding of
group Network addresses to IEEE 802 subnetwork addresses, as is used
by IP multicast has been suggested.
The two main reasons that the group Network address to subnetwork
(IEEE 802) address was made locally configurable were to support
multicast on subnets with hosts having a mixture of capabilities (as
to how many multicast subnetwork 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 subnetwork versus globally. Even multi-homed hosts with
subnets that use 802 addresses may have varying capabilities (looking
at typical Ethernet, FDDI and 802.5 implementations).
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 mapping the only operational overhead
that would occur is that in the presence of a mapping server (the
Active Multicast IS), which was supporting this mapping, a MAM PDU
would periodically be sent with a Group Network Address Mask which
would identify the direct mapping.
2. "Real Time" Scope Control Features
The scope control features are provided via optional parameters. Use
of multicast transfer of audio and video streams may require scope
control mechanisms which operate very quickly.
One possible solution is to embed scope control mechanisms into the
group Network address itself. For example, a group Network address
using the "Local" AFI is automatically limited to not cross inter-
domain borders. Further, more flexible, address formats may be
developed.
References
[Deering91] Deering, S., "Multicast Routing in a Datagram
Internetwork", PhD thesis, Electrical Engineering Dept., Stanford
University, December 1991.
[RFC1112] Deering, S., "Host Extensions for IP Multicasting",
STD 5, RFC1112, Stanford University, August 1989.
[RFC1237] Colella, R., Gardner, E., and R. Callon, "Guidelines for OSI
NSAP Allocation in the Internet", RFC1237, NIST, Mitre, DEC, July
1991.
[CLNP] Protocol for providing the connectionless-mode network service,
International Standard 8473-1, Second Edition, ISO/IEC JTC 1,
Switzerland 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.
Author's Address
Dave Marlow
Code B35
NSWC-DD
Dahlgren, VA. 22448
Phone: (703) 663-1675