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Internet Engineering Task Force Talpade and Ammar
INTERNET-DRAFT Georgia Institute of Technology
June 3, 1996
Expires: December, 1996
<draft-talpade-ipatm-marsmcs-02.txt>
Multicast Server Architectures for MARS-based ATM multicasting.
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
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Abstract
A mechanism to support the multicast needs of layer 3 protocols in
general, and IP in particular, over UNI 3.0/3.1 based ATM networks has
been described in draft-ietf-ipatm-ipmc-12.txt. Two basic approaches
exist for the intra-subnet (intra-cluster) multicasting of IP packets.
One makes use of a mesh of point to multipoint VCs (the 'VC Mesh'
approach), while the other uses a shared point to multipoint tree
rooted on a Multicast Server (MCS). This memo provides details on the
design and implementation of an MCS, building on the core mechanisms
defined in draft-ietf-ipatm-ipmc-12.txt. It also provides a mechanism
for using multiple MCSs per group for providing fault tolerance. This
approach can be used with draft-ietf-ipatm-ipmc-12.txt based MARS
server and clients, without needing any change in their functionality.
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1 Introduction
A solution to the problem of mapping layer 3 multicast service over
the connection-oriented ATM service provided by UNI 3.0/3.1, has been
presented in [GA96]. A Multicast Address Resolution Server (MARS) is
used to maintain a mapping of layer 3 group addresses to ATM addresses
in that architecture. It can be considered to be an extended analog
of the ATM ARP Server introduced in RFC 1577 ([ML93]). Hosts in the
ATM network use the MARS to resolve layer 3 multicast addresses into
corresponding lists of ATM addresses of group members. Hosts keep the
MARS informed when they need to join or leave a particular layer 3
group.
The MARS manages a "cluster" of ATM-attached endpoints. A "cluster"
is defined as
"The set of ATM interfaces choosing to participate in direct ATM
connections to achieve multicasting of AAL_SDUs between themselves."
In practice, a cluster is the set of endpoints that choose to use the
same MARS to register their memberships and receive their updates
from.
A sender in the cluster has two options for multicasting data to the
group members. It can either get the list of ATM addresses
constituting the group from the MARS, set up a point-to-multipoint
virtual circuit (VC) with the group members as leaves, and then
proceed to send data out on it. Alternatively, the source can make
use of a proxy Multicast Server (MCS). The source transmits data to
such an MCS, which in turn uses a point-to-multipoint VC to get the
data to the group members.
The MCS approach has been briefly introduced in [GA96]. This memo
presents a detailed description of MCS architecture and proposes a
mechanism for supporting multiple MCSs for fault tolerance. We assume
an understanding of the IP multicasting over UNI 3.0/3.1 ATM network
concepts described in [GA96], and access to it. This document is
organized as follows. Section 2 presents interactions with the local
UNI 3.0/3.1 signalling entity that are used later in the document and
have been originally described in [GA96]. Section 3 presents an MCS
architecture, along with a description of its interactions with the
MARS. Section 4 describes the working of an MCS. The possibility of
using multiple MCSs for the same layer 3 group, and the mechanism
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needed to support such usage, is described in section 5. A comparison
of the VC Mesh approach and the MCS approach is presented in Appendix
A.
1.1 Changes from previous version of draft
This section outlines the changes made since the last version of this
draft. We suggest reading this section only if the previous version
has been read, otherwise this section can be safely ignored. The load
sharing and fault tolerance capabilitities of multiple MCSs provided
by marsmcs01.txt have been reduced. This draft uses multiple MCSs for
fault tolerance only. This is in keeping with the desire expressed by
the IP over ATM group to minimize the complexity of a multiple MCS
protocol.
The allocation mechanism described in marsmcs01.txt has been replaced
with a HELLO mechanism based on the SCSP ([LA96]) protocol. Choosing
the HELLO packet formats for the heart-beat messages ensures
compatibility between this and future multiple MCS architectures based
on MARSv2 (MARS, MCS using SCSP).
2 Interaction with the local UNI 3.0/3.1 signalling entity
The following generic signalling functions are presumed to be
available to local AAL Users:
L_CALL_RQ - Establish a unicast VC to a specific endpoint.
L_MULTI_RQ - Establish multicast VC to a specific endpoint.
L_MULTI_ADD - Add new leaf node to previously established VC.
L_MULTI_DROP - Remove specific leaf node from established VC.
L_RELEASE - Release unicast VC, or all Leaves of a multicast VC.
The following indications are assumed to be available to AAL Users,
generated by by the local UNI 3.0/3.1 signalling entity:
L_ACK - Succesful completion of a local request.
L_REMOTE_CALL - A new VC has been established to the AAL User.
ERR_L_RQFAILED - A remote ATM endpoint rejected an L_CALL_RQ,
L_MULTI_RQ, or L_MULTI_ADD.
ERR_L_DROP - A remote ATM endpoint dropped off an existing VC.
ERR_L_RELEASE - An existing VC was terminated.
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3 MCS Architecture
The MCS acts as a proxy server which multicasts data received from a
source to the group members in the cluster. All multicast sources
transmitting to an MCS-based group send the data to the specified MCS.
The MCS then forwards the data over a point to multipoint VC that it
maintains to group members in the cluster. Each multicast source thus
maintains a single point-to-point VC to the designated MCS for the
group. The designated MCS terminates one point-to-point VC from each
cluster member that is multicasting to the layer 3 group. Each group
member is the leaf of the point-to-multipoint VC originating from the
MCS.
A brief introduction to possible MCS architectures has been presented
in [GA96]. The main contribution of that document concerning the MCS
approach is the specification of the MARS interaction with the MCS.
The next section lists control messages exchanged by the MARS and MCS.
3.1 Control Messages exchanged by the MCS and the MARS
The following control messages are exchanged by the MARS and the MCS.
operation code Control Message
1 MARS`REQUEST
2 MARS`MULTI
3 MARS`MSERV
6 MARS`NAK
7 MARS`UNSERV
8 MARS`SJOIN
9 MARS`SLEAVE
12 MARS`REDIRECT`MAP
MARS_MSERV and MARS_UNSERV are identical in format to the MARS_JOIN
message. MARS_SJOIN and MARS_SLEAVE are also identical in format to
MARS_JOIN. As such, their formats and those of MARS_REQUEST,
MARS_MULTI, MARS_NAK and MARS_REDIRECT_MAP are described in [GA96]. We
describe their usage in section 4. All control messages are LLC/SNAP
encapsulated as described in section 4.2 of [GA96]. (The "mar$"
notation used in this document is borrowed from [GA96], and indicates
a specific field in the control message.) Data messages are reflected
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without any modification by the MCS.
3.2 Association with a layer 3 group
The simplest MCS architecture involves taking incoming AAL_SDUs from
the multicast sources and sending them out over the
point-to-multipoint VC to the group members. The MCS can service just
one layer 3 group using this design, as it has no way of
distinguishing between traffic destined for different groups. So each
layer 3 MCS-supported group will have its own designated MCS.
However it is desirable in terms of saving resources to utilize the
same MCS to support multiple groups. This can be done by adding
minimal layer 3 specific processing into the MCS. The MCS can now look
inside the received AAL_SDUs and determine which layer 3 group they
are destined for. A single instance of such an MCS could register its
ATM address with the MARS for multiple layer 3 groups, and manage
multiple point-to-multipoint VCs, one for each group. We include this
capability in our MCS architecture. We also include the capability of
having multiple MCSs per group (section 5).
4 Working of MCS
An MCS MUST NOT share its ATM address with any other cluster member
(MARS or otherwise). However, it may share the same physical ATM
interface (even with other MCSs or the MARS), provided that each
logical entity has a different ATM address. This section describes
the working of MCS and its interactions with the MARS and other
cluster members.
4.1 Usage of MARS_MSERV and MARS_UNSERV
4.1.1 Registration (and deregistration) with the MARS
The ATM address of the MARS MUST be known to the MCS by out-of-band
means at startup. One possible way to do this is for the network
administrator to specify the MARS address at command line while
invoking the MCS. On startup, the MCS MUST open a point-to-point
control VC (MARS_VC) with the MARS. All traffic from the MCS to the
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MARS MUST be carried over the MARS_VC. The MCS MUST register with the
MARS using the MARS_MSERV message on startup. To register, a
MARS_MSERV MUST be sent by the MCS to the MARS over the MARS_VC. On
receiving this MARS_MSERV, the MARS adds the MCS to the
ServerControlVC. The ServerControlVC is maintained by the MARS with
all MCSs as leaves, and is used to disseminate general control
messages to all the MCSs. The MCS MUST terminate this VC, and MUST
expect a copy of the MCS registration MARS_MSERV on the MARS_VC from
the MARS.
An MCS can deregister by sending a MARS_UNSERV to the MARS. A copy of
this MARS_UNSERV MUST be expected back from the MARS. The MCS will
then be dropped from the ServerControlVC.
No protocol specific group addresses are included in MCS registration
MARS_MSERV and MARS_UNSERV. The mar$flags.register bit MUST be set,
the mar$cmi field MUST be set to zero, the mar$flags.sequence field
MUST be set to zero, the source ATM address MUST be included and a
null source protocol address MAY be specified in these MARS_MSERV and
MARS_UNSERV. All other fields are set as described in section 5.2.1 of
[GA96] (the MCS can be considered to be a cluster member while reading
that section). It MUST keep retransmitting (section 4.1.3) the
MARS_MSERV/MARS_UNSERV over the MARS_VC until it receives a copy back.
In case of failure to open the MARS_VC, or error on it, the
reconnection procedure outlined in section 4.5.2 is to be followed.
4.1.2 Registration (and deregistration) of layer 3 groups
The MCS can register with the MARS to support particular group(s). To
register a group X, a MARS_MSERV with a <min, max> pair of <X, X> MUST
be sent to the MARS. The MCS MUST expect a copy of the MARS_MSERV back
from the MARS. The retransmission strategy outlined in section 4.1.3
is to be followed if no copy is received. Multiple groups can be
supported by sending a separate MARS_MSERV for each group.
The MCS MUST similarly use MARS_UNSERV if it wants to withdraw support
for a specific layer 3 group. A copy of the group MARS_UNSERV MUST be
received, failing which the retransmission strategy in section 4.1.3
is to be followed.
The mar$flags.register bit MUST be reset and the mar$flags.sequence
field MUST be set to zero in the group MARS_MSERV and MARS_UNSERV. All
other fields are set as described in section 5.2.1 of [GA96] (the MCS
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can be considered to be a cluster member when reading that section).
4.1.3 Retransmission of MARS_MSERV and MARS_UNSERV
Transient problems may cause loss of control messages. The MCS needs
to retransmit MARS_MSERV/MARS_UNSERV at regular intervals when it does
not receive a copy back from the MARS. This interval should be no
shorter than 5 seconds, and a default value of 10 seconds is
recommended. A maximum of 5 retransmissions are permitted before a
failure is logged. This MUST be considered a MARS failure, which
SHOULD result in the MARS reconnection mechanism described in section
4.5.2.
A "copy" is defined as a received message with the following fields
matching the previously transmitted MARS_MSERV/MARS_UNSERV:
- mar$op
- mar$flags.register
- mar$pnum
- Source ATM address
- first <min, max> pair
In addition, a valid copy MUST have the following field values:
- mar$flags.punched = 0
- mar$flags.copy = 1
There MUST be only one MARS_MSERV or MARS_UNSERV outstanding at a
time.
4.1.4 Processing of MARS_MSERV and MARS_UNSERV
The MARS transmits copies of group MARS_MSERV and MARS_UNSERV on the
ServerControlVC. So they are also received by MCSs other than the
originating one. This section discusses the processing of these
messages by the other MCSs.
If a MARS_MSERV is seen that refers to a layer 3 group not supported
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by the MCS, it MUST be used to track the Server Sequence Number
(section 4.5.1) and then silently dropped.
If a MARS_MSERV is seen that refers to a layer 3 group supported by
the MCS, the MCS learns of the existence of another MCS supporting the
same group. We incorporate this possibility (of multiple MCSs per
group) in this version of the MCS approach and discuss it in section
5.
4.2 Usage of MARS_REQUEST and MARS_MULTI
As is described in section 5.1, the MCS learns at startup whether it
is an active or inactive MCS. After successful registration with the
MARS, an MCS which has been designated as inactive for a particular
group MUST NOT register to support that group with the MARS. It
instead proceeds as in section 5.4. The active MCS for a group also
has to do some special processing, which we describe in that section.
The rest of section 4 describes the working of a single active MCS,
with section 5 describing the active MCSs actions for supporting
multiple MCSs.
After the active MCS registers to support a layer 3 group, it uses
MARS_REQUEST and MARS_MULTI to obtain information about group
membership from the MARS. These messages are also used during the
revalidation phase (section 4.5) and when no outgoing VC exists for a
received layer 3 packet (section 4.3).
On registering to support a particular layer 3 group, the MCS MUST
send a MARS_REQUEST to the MARS. The mechanism to retrieve group
membership and the format of MARS_REQUEST and MARS_MULTI is described
in section 5.1.1 and 5.1.2 of [GA96] respectively. The MCS MUST use
this mechanism for sending (and retransmitting) the MARS_REQUEST and
processing the returned MARS_MULTI(/s). The MARS_MULTI MUST be
received correctly, and the MCS MUST use it to initialize its
knowledge of group membership.
On successful reception of a MARS_MULTI, the MCS MUST attempt to open
the outgoing point-to-multipoint VC using the mechanism described in
section 5.1.3 of [GA96], if any group members exist. The MCS however
MUST start transmitting data on this VC after it has opened it
successfully with at least one of the group members as a leaf, and
after it has attempted to add all the group members at least once.
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4.3 Usage of outgoing point-to-multipoint VC
Cluster members which are sources for MCS-supported layer 3 groups
send (encapsulated) layer 3 packets to the designated MCSs. An MCS,
on receiving them from cluster members, has to send them out over the
specific point-to-multipoint VC for that layer 3 group. This VC is
setup as described in the previous section. However, it is possible
that no group members currently exist, thus causing no VC to be setup.
So an MCS may have no outgoing VC to forward received layer 3 packets
on, in which case it MUST initiate the MARS_REQUEST and MARS_MULTI
sequence described in the previous section. This new MARS_MULTI could
contain new members, whose MARS_SJOINs may have been not received by
the MCS (and the loss not detected due to absence of traffic on the
ServerControlVC).
If an MCS learns that there are no group members (MARS_NAK received
from MARS), it MUST delay sending out a new MARS_REQUEST for that
group for a period no less than 5 seconds and no more than 10 seconds.
Layer 3 packets received from cluster members, while no outgoing
point-to-multipoint VC exists for that group, MUST be silently dropped
after following the guidelines in the previous paragraphs. This might
result in some layer 3 packets being lost until the VC is setup.
Each outgoing point-to-multipoint has a revalidate flag associated
with it. This flag MUST be checked whenever a layer 3 packet is sent
out on that VC. No action is taken if it is not set. If it is set,
the packet is sent out, the revalidation procedure (section 4.5.3)
MUST be initiated for this group, and the flag MUST be reset.
In case of error on a point-to-multipoint VC, the MCS MUST initiate
revalidation procedures for that VC as described in section 4.5.3.
Once a point-to-multipoint VC has been setup for a particular layer 3
group, the MCS MUST hold the VC open and mark it as the outgoing path
for any subsequent layer 3 packets being sent for that group address.
A point-to-multipoint VC MUST NOT have an activity timer associated
with it. It is to remain up at all times, unless the MCS explicitly
stops supporting that layer 3 group, or no more leaves exist on the VC
which causes it to be shut down. The VC is kept up inspite of
non-existent traffic to reduce the delay suffered by MCS supported
groups. If the VC were to be shut down on absence of traffic, the VC
reestablishment procedure (needed when new traffic for the layer 3
group appears) would further increase the end-to-end latency, which
can be potentially higher than the VC mesh approach anyway as two VCs
need to be setup in the MCS case (one from source to MCS, second from
MCS to group) as opposed to only one (from source to group) in the VC
Mesh approach. This approach of keeping the VC from the MCS open even
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in the absense of traffic is experimental. A decision either way can
only be made after gaining experience (either through implementation
or simulation) about the implications of keeping the VC open.
If the MCS supports multiple layer 3 groups, each data AAL_SDU MUST be
examined for determining its recipient group, before being forwarded
onto the appropriate outgoing point-to-multipoint VC.
4.3.1 Group member dropping off a point-to-multipoint VC
AN ERR_L_DROP may be received during the lifetime of a
point-to-multipoint VC indicating that a leaf node has terminated its
participation at the ATM level. The ATM endpoint associated with the
ERR_L_DROP MUST be removed from the locally held set associated with
the VC. The revalidate flag on the VC MUST be set after a random
interval of 1 through 10 seconds.
If an ERR_L_RELEASE is received for a VC, then the entire set is
cleared and the VC considered to be completely shutdown. A new VC for
this layer 3 group will be established only on reception of new
traffic for the group (as described in section 4.3).
4.4 Processing of MARS_SJOIN and MARS_SLEAVE
The MARS transmits equivalent MARS_SJOIN/MARS_SLEAVE on the
ServerControlVC when it receives MARS_JOIN/MARS_LEAVE from cluster
members. The MCSs keep track of group membership updates through
these messages. The format of these messages are identical to
MARS_JOIN and MARS_LEAVE, which are described in section 5.2.1 of
[GA96]. It is sufficient to note here that these messages carry the
ATM address of the node joining/leaving the group(/s), the group(/s)
being joined or left, and a Server Sequence Number from MARS.
When a MARS_SJOIN is seen which refers to (or encompasses) a layer 3
group (or groups) supported by the MCS, the following action MUST be
taken. The new member's ATM address is extracted from the MARS_SJOIN.
An L_MULTI_ADD is issued for the new member for each of those referred
groups which have an outgoing point-to-multipoint VC. An L_MULTI_RQ is
issued for the new member for each of those refered groups which have
no outgoing VCs.
When a MARS_SLEAVE is seen that refers to (or encompasses) a layer 3
group (or groups) supported by the MCS, the following action MUST be
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taken. The leaving member's ATM address is extracted. An
L_MULTI_DROP is issued for the member for each of the refered groups
which have an outgoing point-to-multipoint VC.
There is a possibility of the above requests (L_MULTI_RQ or L_MULTI_ADD
or L_MULTI_DROP) failing. The UNI 3.0/3.1 failure cause must be
returned in the ERR_L_RQFAILED signal from the local signalling entity
to the AAL User. If the failure cause is not 49 (Quality of Service
unavailable), 51 (user cell rate not available - UNI 3.0), 37 (user
cell rate not available - UNI 3.1), or 41 (Temporary failure), the
endpoint's ATM address is dropped from the locally held view of the
group by the MCS. Otherwise, the request MUST be re-attempted with
increasing delay (initial value between 5 to 10 seconds, with delay
value doubling after each attempt) until it either succeeds or the
multipoint VC is released or a MARS_SLEAVE is received for that group
member. If the VC is open, traffic on the VC MUST continue during
these attempts.
MARS_SJOIN and MARS_SLEAVE are processed differently if multiple MCSs
share the members of the same layer 3 group (section 5.4). MARS_SJOIN
and MARS_SLEAVE that do not refer to (or encompass) supported groups
MUST be used to track the Server Sequence Number (section 4.5.1), but
are otherwise ignored.
4.5 Revalidation Procedures
The MCS has to initiate revalidation procedures in case of certain
failures or errors.
4.5.1 Server Sequence Number
The MCS needs to track the Server Sequence Number (SSN) in the
messages received on the ServerControlVC from the MARS. It is carried
in the mar$msn of all messages (except MARS_NAK) sent by the MARS to
MCSs. A jump in SSN implies that the MCS missed the previous
message(/s) sent by the MARS. The MCS then sets the revalidate flag on
all outgoing point-to-multipoint VCs after a random delay of between 1
and 10 seconds, to avoid all MCSs inundating the MARS simultaneously
in case of a more general failure.
The only exception to the rule is if a sequence number is detected
during the establishment of a new group's VC (i.e. a MARS_MULTI was
correctly received, but its mar$msn indicated that some previous MARS
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traffic had been missed on ClusterControlVC). In this case every open
VC, EXCEPT the one just being established, MUST have its revalidate
flag set at some random interval between 1 and 10 seconds from the
time the jump in SSN was detected. (The VC being established is
considered already validated in this case).
Each MCS keeps its own 32 bit MCS Sequence Number (MSN) to track the
SSN. Whenever a message is received that carries a mar$msn field, the
following processing is performed:
Seq.diff = mar$msn - MSN
mar$msn -> MSN
(.... process MARS message ....)
if ((Seq.diff != 1) && (Seq.diff != 0))
then (.... revalidate group membership information ....)
The mar$msn value in an individual MARS_MULTI is not used to update
the MSN until all parts of the MARS_MULTI (if > 1) have arrived. (If
the mar$msn changes during reception of a MARS_MULTI series, the
MARS_MULTI is discarded as described in section 5.1.1 of [GA96]).
The MCS sets its MSN to zero on startup. It gets the current value of
SSN when it receives the copy of the registration MARS_MSERV back from
the MARS.
4.5.2 Reconnecting to the MARS
The MCSs are assumed to have been configured with the ATM address of
at least one MARS at startup. MCSs MAY choose to maintain a table of
ATM addresses, each address representing alternative MARS which will
be contacted in case of failure of the previous one. This table is
assumed to be ordered in descending order of preference.
An MCS will decide that it has problems communicating with a MARS if:
o It fails to establish a point-to-point VC with the MARS.
o MARS_REQUEST generates no response (no MARS_MULTI or MARS_NAK
returned).
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o ServerControlVC fails.
o MARS_MSERV or MARS_UNSERV do not result in their respective copies
being received.
(reconnection as in section 5.4 in [GA96]).
4.5.3 Revalidating a point-to-multipoint VC
The revalidation flag associated with a point-to-multipoint VC is
checked when a layer 3 packet is to be sent out on the VC.
Revalidation procedures MUST be initiated for a point-to-multipoint VC
that has its revalidate flag set when a layer 3 packet is being sent
out on it. Thus more active groups get revalidated faster than less
active ones. The revalidation process MUST NOT result in disruption
of normal traffic on the VC being revalidated.
The revalidation procedure is as follows. The MCS reissues a
MARS_REQUEST for the VC being revalidated. The returned set of
members is compared with the locally held set; L_MULTI_ADDs MUST be
issued for new members, and L_MULTI_DROPs MUST be issued for
non-existent ones. The revalidate flag MUST be reset for the VC.
5 Multiple MCSs for a layer 3 group
Having a single MCS for a layer 3 group can cause it to become a
single point of failure and a bottleneck for groups with large numbers
of senders. It is thus desirable to introduce a level of fault
tolerance by having multiple MCS per group. We do not introduce
support for load sharing in this version of the draft so as to reduce
the complexity of the protocol.
5.1 Outline
The protocol described in this draft offers fault tolerance by using
multiple MCSs for the same group. This is achieved by having a
standby MCS take over from a failed MCS which had been supporting the
group. The MCS currently supporting a group is refered to as the
active MCS, while the one or more standy MCSs are refered to as
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inactive MCSs. There is only one active MCS existing at any given
instant for an MCS-supported group. The protocol makes use of the
HELLO messages as described in [LA96].
To reduce the complexity of the protocol, the following guidelines
need to be followed. These guidelines need to be enforced by
out-of-band means which are not specified in this document and can be
implementation dependent.
o The set of MCSs (``mcs_list'') that support a particular IP
Multicast group is predetermined and fixed. This set is known to
each MCS in the set at startup, and the ordering of MCSs in the
set is the same for all MCSs in the set. An implementation of
this would be to maintain the set of ATM addresses of the MCSs in
a file, an identical copy of which is kept at each MCS in the set.
o All MCSs in ``mcs_list'' have to be started up together, with the
first MCS in ``mcs_list'' being the last to be started.
o A failed MCS cannot be started up again.
5.2 Discussion of Multiple MCSs in operation
We now present a brief overview of the multiple MCS protocol in
operation. An MCS on startup determines its position in the
``mcs_list''. If the MCS is not the first in ``mcs_list'', it does
not register for supporting the group with the MARS. If the MCS is
first in the set, it does register to support the group.
The first MCS thus becomes the active MCS and supports the group as
described in section 4. The active MCS also opens a
point-to-multipoint VC to the remaining MCSs in the set (the inactive
MCSs). It starts sending HELLO messages on this VC at a fixed
interval (HelloInterval seconds). The inactive MCSs maintain a timer
to keep track of the last received HELLO message. If an inactive MCS
does not receive a message within HelloInterval* DeadFactor seconds
(values of HelloInterval and DeadFactor are the same at all the MCSs),
it assumes failure of the active MCS and attempts to elect a new one.
The election process is described in section 5.5.
If an MCS is elected as the new active one, it registers to support
the group with the MARS. It also initiates the transmission of HELLO
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messages to the remaining inactive MCSs.
5.3 Inter-MCS control messages
The protocol uses HELLO messages in the heartbeat mechanism, and also
during the election process. The format of the HELLO message is based
on that described in [LA96]. The Hello message type code is 5.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
_ Sender Len _ Recvr Len _ unused_ Type _ unused _
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
_ HelloInterval _ DeadFactor _
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
_ IP Multicast address _
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
_ Sender ATM address (variable length) _
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
_ Receiver ATM address (variable length) _
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sender Len
This field holds the length in octets of the Sender ATM address.
Recvr Len
This field holds the length in octets of the Receiver ATM address.
Type
This is the code for the message type.
HelloInterval
The hello interval advertises the time between sending of
consecutive Hello Messages by an active MCS. If the time between *
*Hello
messages exceeds the HelloInterval then the Hello is to be
considered late by the inactive MCS.
DeadFactor
This is a multiplier to the HelloInterval. If an inactive MCS does*
* not
receive a Hello message within the interval
HelloInterval*DeadFactor from an active MCS that advertised the
HelloInterval then the inactive MCS MUST consider the active one t*
*o have
failed.
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IP Multicast address
This field is used to indicate the group to associate the HELLO
message with. It is useful if MCSs can support more than one group.
Sender ATM address
This is the protocol address of the server which is sending the
Hello.
Receiver ATM address
This is the protocol address of the server which is to Reply to the
Hello. If the sender does not know this address then the sender
sets it to zero. (This happens in the HELLO messages sent from the
active MCS to the inactive ones, as they are multicast and not sent
to one specific receiver).
5.4 The Multiple MCS protocol
As was indicated in section 5.1, all the MCSs supporting the same IP
Multicast group MUST be started up together. The first MCS in
``mcs_list'' MUST be the last to be started. After registering to
support the group with the MARS, the first MCS MUST open a
point-to-multipoint VC (HelloVC) with the remaining MCSs in the
'mcs_list'' as leaves, and thus assumes the role of active MCS. It
MUST send HELLO messages HelloInterval seconds apart on this VC. The
Hello message sent by the active MCS MUST have the Receiver Len set to
zero, with the other fields appropriately set. The Receiver ATM
address field does not exist in this HELLO message. The initial value
of HelloInterval and DeadFactor MUST be the same at all MCSs at
startup. The active MCS can choose to change these values by
introducing the new value in the HELLO messages that are sent out.
The active MCS MUST support the group as described in section 4.
The other MCSs in ``mcs_list'' determine the identity of the first MCS
from the ``mcs_list''. They MUST NOT register to support the group
with the MARS, and become inactive MCSs. On startup, an inactive MCS
expects HELLO messages from the active MCS. The inactive MCS MUST
terminate the HelloVC. A timer MUST be maintained, and if the inactive
MCS does not receive HELLO message from the active one within a period
HelloInterval*DeadFactor seconds, it assumes that the active MCS died,
and initiates the election process as described in section 5.5. If a
HELLO message is received within this period, the inactive MCS does
not initiate any further action.
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On failure of the active MCS, a new MCS assumes its role as described
in section 5.5. In this case, the remaining inactive MCSs will expect
HELLO messages from this new active MCS as described in the previous
paragraph.
5.5 Failure handling
5.5.1 Failure of active MCS
The failure of the active MCS is detected by the inactive MCSs if no
HELLO message is received within an interval of
HelloInterval*DeadFactor seconds. In this case the next MCS in
``mcs_list'' becomes the candidate MCS. It MUST open a
point-to-multipoint VC to the remaining inactive MCSs (HelloVC) and
send a HELLO message on it. This HELLO message is formatted as
described earlier.
On receiving a HELLO message from a candidate MCS, an inactive MCS
MUST open a point-to-point VC to that candidate. It MUST send a HELLO
message back to it, with the Sender and Receiver fields appropriately
set (not zero). If a HELLO message is received by an inactive MCS
from a non-candidate MCS, it is ignored. If no HELLO message is
received by the inactive MCSs within an interval of
HelloInterval*DeadFactor seconds, the next MCS in ``mcs_list'' is
considered as the candidate MCS. Note that the values used for
HelloInterval and DeadFactor in the election phase are the default
ones.
The candidate MCS MUST wait for a period of HelloInterval*DeadFactor
seconds for receiving HELLO messages from inactive MCSs. If it
receives messages from atleast half of the remaining inactive MCSs, it
considers itself elected and assumes the active MCS role. It then
registers to support the group with the MARS, and starts sending HELLO
messages at HelloInterval second intervals on the already existing
HelloVC. The active MCS can then alter the HelloInterval and
DeadFactor values if desired, and communicate the same to the inactive
MCSs in the HELLO message.
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5.5.2 Failure of inactive MCS
If an inactive MCS drops off the HelloVC, the active MCS MUST attempt
to add that MCS back to the VC for three attempts, spaced
HelloInterval*DeadFactor seconds apart. If even the third attempt
fails, the inactive MCS is considered dead.
An MCS, active or inactive, MUST NOT be started up once it has failed.
6 Summary
This draft describes the architecture of an MCS. It also provides a
mechanism for using multiple MCSs per group for providing fault
tolerance. This approach can be used with [GA96] based MARS server
and clients, without needing any change in their functionality. It
uses the HELLO packet format as described in [LA96] for the heartbeat
messages.
7 Acknowledgements
We would like to acknowledge Grenville Armitage (Bellcore) for
reviewing the draft and suggesting improvements towards simplifying
the multiple MCS functionalities. Discussion with Joel Halpern
(Newbridge) helped clarify the multiple MCS problem. Anthony Gallo
(IBM RTP) pointed out security issues that are not adequately
addressed in the current draft.
8 Authors' Address
Rajesh Talpade - taddy@cc.gatech.edu - (404)-894-9910
Mostafa H. Ammar - ammar@cc.gatech.edu - (404)-894-3292
College of Computing
Georgia Institute of Technology
Atlanta, GA 30332-0280
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References
[GA96] Armitage, G.J., "Support for Multicast over UNI 3.0/3.1 based
ATM networks", Internet-Draft, draft-ietf-ipatm-ipmc-12.txt,
Feb 1996.
[BK95] Birman, A., Kandlur, D., Rubas, J., "An extension to the MARS
model", Internet Draft,
draft-kandlur-ipatm-mars-directvc-00.txt, November 1995.
[LM93] Laubach, M., "Classical IP and ARP over ATM", RFC1577,
Hewlett-Packard Laboratories, December 1993.
[LA96] Luciani, J., G. Armitage, and J. Halpern, "Server Cache
Synchronization Protocol (SCSP) - NBMA", Internet Draft,
draft-luciani-rolc-scsp-02.txt, April 1996.
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Appendix A
A Comparison
The table below compares some quantitative parameters needed for
supporting a single group while using the MCS and the VC mesh
approaches (ignoring the control VCs maintained between the MARS and
the cluster members).
Number of multicast sources multicasting to group G = n
Number of group members in G = m
MCS VC mesh
total VCs terminated n+m n*m
at cluster members
point-to-multipoint VCs 1 n
point-to-point VCs n 0
VCs terminated at each 1 n
group member
signalling requests generated 1 n
due to a single membership change
Advantages of using MCSs
o As can be seen above, VC usage is much better in the MCS case.
The increased VC usage for the VC mesh leads to greater
consumption of resources like memory for maintaining state, buffer
allocation per VC, and the VCs themselves, which may be a scarce
and/or expensive resource.
o Group membership changes also cause a decreased level of
signalling load to be generated at the switch (and the senders to
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the group) in the MCS approach. This is because only the MCS has
to add/delete a cluster member from the point-to-multipoint VC.
The other VCs are not affected. Thus signalling requests only
occur at the UNI between the MCS and the switch, as opposed to
occuring at the UNIs between all the sources and the switch in the
VC mesh case. This is especially beneficial when the group is
highly dynamic, or when the links between the switch and the
cluster members are error-prone, which may cause group members to
be temporarily dropped from the multicast group, thus making the
group more dynamic than it actually is.
o The MCS approach provides a centralized control of multicast
bandwidth usage over the ATM network. This is useful in cases
where policy demands a limit on the share of bandwidth available
for multicast purposes. In such a case, the administrator who
sets up a cluster member as the designated MCS for a group, can
control the rate at which the MCS multicasts data. An additional
level of security can also be maintained for sensitive multicasts,
as all group members will have to be authorized by the centralized
MCS only before they can receive the multicast data. The VC mesh
approach would need such security control to be enforced at each
multicast source to prevent unauthorized cluster members from
receiving the multicast data.
Disadvantages of using MCSs
o Data throughput and end-to-end latency may be adversely affected
due to the additional level of indirection introduced by the MCS.
The MCS can potentially become a bottleneck and a central point of
failure. We address this issue and suggest possible solutions in
a later section.
o Each group member needs to terminate one point-to-multipoint VC
originating from the MCS. So if a multicast source happens to be a
member of the group it is transmitting to, it will receive a copy
of the data back over the VC from the MCS. Thus additional header
identification is needed for a source to discard such
"bounced-back" data. This mechanism has been defined in [GA96] to
be a 16 bit cluster member identifier.
o Each multicast source in the cluster may desire to use differing
QOS parameters for outgoing traffic. Use of the MCS implies that
all group members will receive data with QOS as determined by the
MCS, irrespective of the QOS used to get them from the source to
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the MCS.
The increased VC usage for the VC Mesh case leads to a decrease in the
maximum permissible size of the LIS. Thus more LISs will be needed for
supporting the same number of hosts in the VC Mesh case. Inter-LIS
devices (IP routers) will need to be used for communicating between
hosts on different LISs. It remains to be seen if the increased use
of routers is more detrimental to the data throughput, as opposed to
use of MCSs with larger LISs.
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