Network Working Group R. Braudes
Request for Comments: 1458 S. Zabele
TASC
May 1993
Requirements for Multicast Protocols
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
unlimited.
Summary
Multicast protocols have been developed over the past several years
to address issues of group communication. EXPerience has
demonstrated that current protocols do not address all of the
requirements of multicast applications. This memo discusses some of
these unresolved issues, and provides a high-level design for a new
multicast transport protocol, group address and membership authority,
and modifications to existing routing protocols.
Table of Contents
1. IntrodUCtion . . . . . . . . . . . . . . . . . . . . . . . 2
2. The Image Communication Problem . . . . . . . . . . . . . 2
2.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.2 Requirements . . . . . . . . . . . . . . . . . . . . . . . 3
3. Review of Existing Multicast Protocols . . . . . . . . . . 4
3.1 IP/Multicast . . . . . . . . . . . . . . . . . . . . . . . 4
3.2 XTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3 ST-II . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.4 MTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Reliable Adaptive Multicast Service . . . . . . . . . . . 9
4.1 The Multicast Group Authority . . . . . . . . . . . . . . 9
4.1.1 Address Management . . . . . . . . . . . . . . . . . . . . 9
4.1.2 Service Registration, Requests, Release, and Group
Membership Maintenance . . . . . . . . . . . . . . . . . . 10
4.2 The Reliable Adaptive Multicast Protocol (RAMP) . . . . . 11
4.2.1 Quality of Service Levels . . . . . . . . . . . . . . . . 12
4.2.2 Error Recovery . . . . . . . . . . . . . . . . . . . . . . 12
4.2.3 Flow Control . . . . . . . . . . . . . . . . . . . . . . . 13
4.3 Routing Support . . . . . . . . . . . . . . . . . . . . . 14
4.3.1 Path Set-up . . . . . . . . . . . . . . . . . . . . . . . 14
4.3.2 Path Tear-down . . . . . . . . . . . . . . . . . . . . . . 15
4.3.3 Multicast Routing Based on Quality of Service . . . . . . 15
4.3.4 Quality of Service Based Packet Loss . . . . . . . . . . . 15
5. Interactions Among the Components: An Example . . . . . . 15
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 18
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Security Considerations . . . . . . . . . . . . . . . . . . . . 19
Authors" Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
Multicast protocols have been developed to support group
communications. These protocols use a one-to-many paradigm for
transmission, typically using class D Internet Protocol (IP)
addresses to specify specific multicast groups. While designing
network services for reliable transmission of very large imagery as
part of the DARPA-sponsored ImNet program, we have reviewed existing
multicast protocols and have determined that none meet all of the
requirements of image communications [3]. This RFCreviews the
current state of multicast protocols, highlights the missing
features, and motivates the design and development of an enhanced
multicast protocol.
First, the requirements for network services and underlying protocols
related to image communications are presented. Existing protocols
are then reviewed, and an analysis of each protocol against the
requirements is presented. The analyses identify the need for a new
multicast protocol. Finally, the features of an ideal reliable
multicast protocol that adapts to network congestion in the
transmission of large data volumes are presented. Additional network
components needed to fully support the new protocol, including a
Multicast Group Authority and modifications to existing routing
protocols, are also introduced.
2. The Image Communications Problem
2.1 Scope
Image management and communications systems are evolving from film-
based systems toward an all-digital environment where imagery is
acquired, transmitted, analyzed, and stored using digital computer
and communications technologies. The throughput required for
communicating large numbers of very large images is extremely large,
consisting of thousands of terabytes of imagery per day. Temporal
requirements for capture and dissemination of single images are
stringent, ranging from seconds to at most several minutes. Imagery
will be viewed by hundreds of geographically distributed users who
will require on-demand, interactive Access to the data.
Traditional imaging applications involve images on the order of 512
by 512 pixels. In contrast, a single image used for remote sensing
can have tens of thousands of pixels on a side. Multiplying the data
volume associated with remotely sensed images by even a small number
of users clearly motivates moving beyond the current suite of
reliable protocols.
Basic image communication applications involve distribution of
individual images to multiple users for both individual and
collaborative analyses, and network efficiency requires the use of
multicast protocols. Areas where multicasting offers significant
advantages include real-time image acquisition and dissemination,
distribution of annotated image-based reports, and image
conferencing. Images are viewed on a heterogeneous set of
workstations with differing processing and display capabilities,
traveling over a heterogeneous network with bandwidths varying by up
to six orders of magnitude between the initial down link and the
slowest end user.
2.2 Requirements
Multicast protocols used for image communications must address
several requirements. Setting up a multicast group first requires
assigning a multicast group address. All multicast traffic is then
delivered to this address, which implies that all members of the
group must be listening for traffic with this address.
Within an image communications architecture such as that used for the
ImNet program, diversity and adaptability can be accommodated by
trading quality of service (i.e., image quality) with speed of
transmission. Multicast support for quality-speed trades can be
realized either through the use of different multicast groups, where
each group receives a different image quality, or through the use of
a single hierarchical stream with routers (or users) extracting
relevant portions.
Due to the current inability of routers to support selective
transmission of partial streams, a multiple stream approach is being
used within ImNet. Efficient operation using a multiple stream
approach requires that users be able to switch streams very quickly,
and that streams with no listeners not be disseminated.
Consequently, rapid configuration of multicast groups and rapid
switching between multicast groups switching is essential.
Inevitably, network congestion or buffer overruns result in packet
loss. A full range of transport reliability is required within an
image communications framework. For some applications such as image
conferencing, packet loss does not present a problem as dropped mouse
movements can be discarded with no meaningful degradation in utility.
However, for functions such as image archiving or detailed image
analysis, transport must be completely reliable, where any dropped
packets must be retransmitted by the sender. Additionally, several
hierarchical image compression methods can provide useful, albeit
degraded, imagery using a semi-reliable service, where higher level
data is transmitted reliably and the lower level data is transmitted
unreliably.
In support of reliable transport, image communications services must
also support adaptation to network congestion using flow control
mechanisms. Flow control regulates the quantity of data placed on
the network per unit time interval, thereby increasing network
efficiency by reducing the number of dropped packets and avoiding the
need for large numbers of retransmissions.
3. Review of Existing Multicast Protocols
Several existing protocols provide varying levels of support for
multicasting, including IP/Multicast [5], the Xpress Transfer
Protocol (XTP) [11], and Experimental Internet Stream Protocol
Version 2 (ST-II) [10]. While the Versatile Message Transaction
Protocol (VMTP) [4] also supports multicast, it has been designed to
support the transfer of small packets, and so is not appropriate for
large image communications. Additionally, a specification exists for
the Multicast Transport Protocol (MTP) [2].
The image communication requirements for a multicast protocol include
multicast group address assignment, group set-up, membership
maintenance (i.e., join, drop, and switch membership), group tear-
down, error recovery, and flow control, as presented above. The
remainder of this section discusses how well each of the existing
protocols meets these requirements.
3.1 IP/Multicast
IP/Multicast is an extension to the standard IP network-level
protocol that supports multicast traffic. IP/Multicast has no
address allocation mechanism, with addresses assigned either by an
outside authority or by each application. This has the potential for
address contention among multiple applications, which would result in
the traffic from the different groups becoming commingled.
There is no true set-up processing for IP/Multicast; once an address
is determined, the sender simply transmits packets to that address
with routers determining the path(s) taken by the data. The receiver
side is only slightly more complex, as an application must issue an
add membership request for IP to listen to traffic destined to the
desired address. If this is the first member of a group, IP
multicasts the request to routers on the local network using the
Internet Group Multicast Protocol (IGMP) for inclusion in routing
tables. Multicast packets are then routed like all other IP packets,
with receivers accepting traffic addressed to joined groups in
addition to the normal host address.
A major problem with the IP/Multicast set-up approach is informing
hosts of multicast group addresses. If addresses are dynamically
allocated, then a mechanism must be established for informing
receivers which addresses have been assigned to which groups. This
requires a minimum of one round trip time, with an address requested
from a server and then returned to the receiver.
Dropping membership in a group involves issuing a request to the
local IP, which decrements the count of members in the IP tables.
However, no special action is taken when group membership goes to
zero. Instead, a heartbeat mechanism is used in which hosts are
periodically polled for active groups, and routers stop forwarding
group traffic to a network only after several polls receive no
activity requests for that group to ensure that a membership report
is not lost or corrupted in transit. This causes the problem of
unneeded traffic being transmitted, due to a long periodicity for the
heartbeat (minimum of one minute between polls); consequently there
is no method for quickly dropping a group over a given path, impeding
attempts to react to network congestion in real-time.
Finally, there is no transport level protocol compatible with
IP/Multicast that is both reliable and implements a flow control
mechanism.
3.2 XTP
XTP is a combined network and transport level protocol that offers
significant support for multicast transfers. As with IP/Multicast,
XTP offers no inherent address management scheme, so that an outside
authority is required.
XTP is also similar to IP/Multicast as there is no explicit set-up
processing between the sender and the receivers prior to the
establishment of group communications. While there is implicit
processing in key management, an external mechanism is required for
passing the multicast group address to the receivers. The receivers
must have established "filters" for the address prior to transmission
in order to receive the data, and suffers the same problems as
IP/Multicast.
In contrast to IP/Multicast, XTP does require explicit handshaking
between the sender and receivers that wish to join an existing group;
however, there is no parallel communication for receivers dropping
out of groups, and the only mechanism for a sender to know if there
are any receivers is the polling scheme used for error control and
recovery. This causes the same problems with sending traffic to
groups without members discussed under IP/Multicast.
The XTP specification does not address how routers distribute a
multicast stream among different connected networks; however it does
include a discussion of the optional bucket, damping, slotting, and
cloning algorithms to reduce duplicate multicast traffic within a
local network.
The specification allows the user to determine whether multicast
transfers are unreliable or semi-reliable, where semi-reliable
transfers are defined to provide a "high-probability of success [9]"
of delivery to all receivers. Reliability cannot be guaranteed due
to the fact that XTP does not maintain the cardinality of the
receiver set, and so cannot know that the data has been received by
all hosts.
XTP recovers from errors using a go-back-n approach (assuming that
the bucket algorithm has been implemented) by retransmitting dropped
packets to all members of the multicast group, as group members are
unknown. This has the potential of flooding the network if only a
single receiver dropped a packet. If all dropped packets belong to a
single network on an internet, with traffic generated over the entire
connected network.
3.3 ST-II
ST-II is another network protocol that provides support for multicast
communications. Similar to IP/Multicast and XTP, ST-II requires a
separate application-specific protocol for assigning and
communicating multicast group addresses.
While ST-II is a network level protocol, it guarantees end-to-end
bandwidth and delay, and so obviates the need for many of the
functions of a transport protocol. The guarantee is provided by
requiring bandwidth reservations for all connections, which are made
at set-up time, and ensuring that the requested bandwidth is
available throughout the lifetime of the connection. The enforcement
policy ensures that the same path is followed for all transmissions,
and prohibits new connections over the network unless there is
sufficient bandwidth to accommodate the expected traffic. This is
accomplished by maintaining the state of all connections in the
network routers, trading the overhead of this connection set-up for
the performance guarantees.
Connection set-up involves negotiation of the bandwidth and delay
parameters and path between the sender, intermediate routers, and
receivers. If the requested resources cannot be made available, the
sender is given the option of either accepting what is available or
canceling the connection request.
To add a new user to an existing group, the new receiver must first
communicate directly with the sender using a different protocol to
exchange relevant information such as the group address. The sender
then requests ST-II to add the new receiver, with the basic
connection set-up processing invoked as before with the new
connection completed only if there is sufficient bandwidth to process
the user.
While the resource guarantee system imposed by ST-II tries to prevent
network congestion from occurring, there are situations where
priority traffic must be introduced into the network. ST-II makes
this very expensive, as the resource requirements for existing
connections must be adjusted, which can only be accomplished by the
origin of each stream. This must be completed prior to the
connection set-up for the priority stream, introducing a large delay
before the important data can be transmitted.
ST-II connections can be closed by either the sender or the receiver.
When the last receiver along a path has been removed, the resources
allocated over that path are released. When all receivers have been
removed, the sender in informed and has the option of either adding a
new receiver or tearing down the group.
3.4 MTP
MTP is a transport level protocol designed to support efficient,
reliable multicast transmissions on top of existing network protocols
such as IP/Multicast. It is based on the notion of a multicast
"master" which controls all ASPects of group communications.
Allocation of a specific group address, or network service access
point, is not considered part of MTP, and as with the other multicast
protocols requires the use of an outside addressing authority. The
MTP specification does require the master to make a "robust effort
[2]" to ensure the address selected is not already in use by trying
to join an existing group at that address, but the problems described
above remain.
Once the address is established, receivers issue a request to join
the existing group using a unique connection identifier that is pre-
assigned. The MTP specification addresses neither how the identifier
is allocated nor how the receivers learn its value, but is assumed to
be handled through an external protocol. The join request specifies
whether the receiver wishes to be a producer of information or only a
receiver, whether the connection should be reliable or best effort,
whether the receiver is able to accept multiple senders of
information, the minimum throughput desired, and the maximum data
packet size. If the request can be granted, then the master replies
with an ACK with a multicast connection identifier; otherwise a NAK
is returned.
Dropping membership in a group is coordinated through the master.
The specification does not address what action the master should take
when the group is reduced to a single member, but a logical action
would be to stop distributing transmit tokens if there are no active
receivers.
One of the major features in MTP is the ordering of received data.
The master distributes transmit tokens to data producers in the
group, which allow data to be provided at a specified rate. Rate
control provides flow control within the protocol, with members that
cannot maintain a minimum flow requested to leave the group.
Error recovery utilizes a NAK-based selective retransmission scheme.
Senders are required to maintain data for a time period specified by
the master, and to be able to retransmit this data when requested by
members of the group. These retransmissions are multicast to the
entire group, requiring receivers to be able to cope with duplicate
packets. If a retransmission request arrives after the data has been
released, the sender must NAK the request.
A potential problem with MTP is the significant amount of overhead
associated with the protocol, with virtually all control traffic
flowing through the master. The extra delay and congestion makes MTP
inappropriate for the image dissemination applications.
3.5 Summary
Our analysis has determined that there are significant problems with
all of the major multicast protocols for the reliable, adaptive
multicast transport of large data items. The problems include
inadequate address management, excessive processing of control
information, poor response to network congestion, inability to handle
high priority traffic, and suboptimal error recovery and
retransmission procedures. We have developed a high-level notion of
the requirements for a service that addresses these issues, which we
now discuss.
4. Protocol Suite for Reliable, Adaptive Multicast
We present an integrated set of three basic components required to
provide a reliable multicast service: the Multicast Group Authority
(MGA); the Reliable, Adaptive Multicast Protocol (RAMP); and modified
routing algorithms. These components are designed to be compatible
with, and take full advantage of, reservation systems such as RSVP
[12].
In this discussion, we have broadened the definition of the term
"Quality of Service (QOS)." There are many applications where the
information content of the underlying data can be reduced through
data compression techniques. For example, a 1,024 x 1,024 pixel
image can be sub-sampled down to 512 x 512 pixels. This degradation
results in a lower quality of service for the end user, while
reducing the traditional network QOS requirements for the transfer.
4.1 The Multicast Group Authority
The Multicast Group Authority (MGA) provides services related to
managing the multicast address space and high-level management
support to existing multicast groups. The MGA has three primary
responsibilities: address management, service registration, and group
membership maintenance.
The MGA is hierarchical in nature, similar to the Internet Domain
Name System (DNS) [7]. Requests for service are directed to an MGA
agent on the local workstation, which are propagated upwards as
required.
4.1.1 Address Management
The MGA is responsible for the allocation and deallocation of
addresses within the Internet Class D address space. Address
requests received from application processes or other MGA nodes
result in a block of addresses being assigned to the requesting MGA
node. The size of the address block allocated is dependent on the
position of the requester in the MGA hierarchy, to reduce the number
of address requests propagated through the MGA tree.
Figure 1 can be used to show what happens when an application
requests a multicast address from the authority at node 1.1.1.
Assuming that this is the first request from this branch of the MGA,
node 1.1.1 issues a request to its parent, node 1.1, which propagates
the request to node 1. Node 1 passes this request to the root, which
issues a block of, say, 30 class D addresses. Of these 30, 10 are
returned to node 1.1, with the remaining 20 reserved for requests
from node 1"s other children. Similarly, node 1.1 passes 3 addresses
to node 1.1.1, reserving the other 7 for future requests. Finally,
node 1.1.1 answers the applications request for an address, keeping
the remaining 2 addresses for future use.
--------
root
--------
/
/
-------- --------
1 ... n
-------- --------
/
/
-------- --------
1.1 ... 1.n
-------- --------
/
/
-------- --------
1.1.1 ... 1.1.n
-------- --------
Figure 1. Sample MGA Hierarchy
When the root exhausts the address space, a request is made to the
children for reclamation of unused addresses. This request
propagates down the tree, with unused addresses passed back through
the hierarchy and returned to the address pool. If the entire
address space is in use, then requests for additional addresses are
not honored.
When an application no longer requires an address, it is returned to
the local MGA node, which keeps it until either it is requested by
another application, it is requested by its parent, or the node is
terminated. At node termination, all available addresses are
returned to the parent. Parents periodically send heartbeat requests
to their children to ensure connectivity, and local nodes similarly
poll applications, with addresses recalled if the queries are not
answered.
4.1.2 Service Registration, Requests, Release, and Group Membership
Maintenance
The MGA maintains the state of all registered multicast services and
receivers. State information includes the number of members
associated with each group by requested QOS reliability, which is
updated as services are offered or rescinded and as members join or
leave a group. The state information is used to ensure that there is
at least one group member listening to each multicast transfer.
Servers register the availability of service, specifying whether
reliable service is available [section 4.2.2] and optionally the
number of qualities of service offered [section 4.2.1]. A multicast
group address is allocated from the address pool and the service is
assigned an identifier as required. If a reservation protocol that
requires information from the server (such as RSVP) is in use, then
the MGA notifies the reservation system of the service with any
required parameters. The service registration is propagated through
the MGA, so that potential clients can discover service availability.
However, servers do not begin data transfers until directed to do so
by the MGA.
Client requests for service are also processed through the MGA.
Service requests specify a service, a desired quality of service, and
a reliability indication. If the request is for a service that has
been registered, then the routing support is directed to add a route
for the new user [section 4.3.1]. If necessary, the MGA also
notifies the reservation protocol. If either the requested QOS is
not being provided or it is provided unreliably and the request is
for reliable transport, then the service provider is also notified.
If the service has not yet been registered, an identifier for the
service is assigned and the request is queued for when the service is
registered. In either case, a response is sent to the requester.
Requests for termination of group membership are also sent to the
MGA. If the request originates at a client, the MGA notifies the
routing function and reservation protocol of the termination in case
the route should be released [section 4.3.2]. If termination results
in a given QOS no longer having any recipients, the service provider
is notified that the QOS is no longer required and should not be
transmitted. Server-directed group terminations follow a similar
procedure, with all clients of the group notified, and the service
offering is removed from the MGA state tables.
4.2 The Reliable Adaptive Multicast Protocol (RAMP)
RAMP is a transport-level protocol designed to provide reliable
multicast service on top of a network protocol such as IP/Multicast,
with unreliable transport also available. RAMP is build on the
premise that applications can request one quality of service (using
our extended definition), but only require reliable transmission at a
lower level of quality. For example, consider the transmission of
hierarchical image data, in which a base spatial resolution is
transmitted, followed by higher resolution data. An application may
require the base data to be sent reliably, but can tolerate dropped
packets for the higher resolution by using interpolation or pixel
replication from the base level to approximate the missing data.
Similar methods can be applied to other data types, such as audio or
video.
4.2.1 Quality of Service Levels
RAMP allows a multicast service to be provided at multiple qualities
of service, with all or some of these levels transmitted reliably.
These QOS can be distributed across different groups using different
class D addresses, or in the simplest case be transmitted in
individual groups. Single packets can be used for either a single
QOS, or may be applicable to multiple qualities of service.
When a data packet is transmitted, a header field indicates the QOS
level(s) associated with that packet. In the old IP implementations,
the Type of Service field can be used as a bit field with one bit for
each applicable QOS, although this is incompatible with RFC1349 [1].
If a packet is required for multiple QOS, then multiple values are
encoded in the field. The RAMP host receiver protocol only accepts
those packets addressed to a group in which an application has
requested membership and that has a QOS value which is in the set of
values requested by the receivers.
The quality of service requested within a flow can be modified during
the life of the flow. QOS modification requests are forwarded to the
MGA, which reduces the number of receivers in the original QOS group
and increments the count for the requested QOS. These changes are
propagated through the MGA hierarchy, with the server notified if
either the original QOS has no remaining receivers or if the new QOS
is not currently being served; similarly, the routers are notified if
routing changes are required.
4.2.2 Error Recovery
Sequence numbers are used in RAMP to determine the ordering of
packets within a multicast group. Mechanisms for ordering packets
transmitted from different senders is a current research topic [2,
6], and an appropriate sequencing algorithm will be incorporated
within the protocol.
Applications exist that do not require in-order delivery of data; for
example, some image servers include position identification
information in each packet. To enhance the efficiency of such
schemes, RAMP includes an option to allow out-or-order delivery of
packets to a receiver.
A NAK-based selective retransmission scheme is used in RAMP to
minimize the protocol overhead associated with ACK-based schemes.
When a receiver notices that one or more packets have not been
received, and the transmission is reliable, a request is sent to the
sender for the span of packets which are missing.
RAMP at the sender aggregates retransmission requests for the time
specified by the retransmission hold timer [section 4.2.3]. After
this time, the requests are evaluated to determine if sufficient
receivers dropped a given packet to make multicasting the
retransmission worthwhile by comparing it to a threshold value. All
packets that have received a number of retransmission requests
greater than the threshold are multicast to the group address, with
other packets unicast to the individual requesters. The proposed
retransmission scheme is a compromise between the extremes of
multicasting and unicasting all retransmissions. The rationale is
that multicasting a request issued by a single sender unnecessarily
floods networks which had no packet loss, while unicasting to a large
number of receivers floods the entire network. The optimal approach,
dynamically constructing a new multicast group for each dropped
packet, is currently too costly in terms of group set-up time.
For those cases where the service provider is unable to retransmit
the data due to released or overwritten buffers, the protocol
delivers NAK responses using either multicast or unicast based on the
number of retransmission requests received.
4.2.3 Flow Control
RAMP utilizes a rate-based flow control mechanism that derives rate
reductions from requests for retransmission or router back-off
requests (i.e., ICMP source quench messages), and derives rate
increases from the number of packets transmitted without
retransmission requests. When a retransmission request is received,
the protocol uses the number of packets requested to compute a rate
reduction factor. Similarly, a different reduction factor is
computed upon receipt of a router-generated squelch request. The
rate reduction factors are then used to compute a reduced rate of
transmission.
When a given number of packets have been transmitted without packet
loss, the rate of transmission is incrementally increased. The size
of the increase will always be smaller than the size of the smallest
rate decrease, in order to minimize throttling.
The retransmission hold timer is modified according to both
application requests and network state. As the number of
retransmission requests rises, the hold timer is incremented to
minimize the number of duplicate retransmissions. Similarly, the
timer is decremented as the number of retransmission requests drops.
RAMP allows for priority traffic, which is marked in the packet
header. The protocol transmits a variable number of packets from
each sending process in proportion to the priority of the flow.
4.3 Routing Support
The protocol suite requires routing support for four functions: path
set-up, path tear-down, forwarding based on QOS values, and
prioritized packet loss due to congestion. The support must be
integrated into routers and network-level protocols in a similar
fashion to IGMP [8].
Partial support comes as a direct consequence of using reservation
protocols such as RSVP. This RFCdoes not mandate the means of
implementing the required functions, and the specified protocols are
compatible with known reservation protocols.
The routers state tables must maintain both the multicast group
address and the QOS level(s) requested for each group on each
outbound interface in order to make appropriate routing decisions
[section 4.3.3]. Therefore, the router state tables are updated
whenever group membership changes, including QOS changes.
4.3.1 Path Set-up
Routers receive path set-up requests from the MGA as required when
new members join a multicast group, which specifies the incoming and
outgoing interfaces, the group address, and the QOS associated with
the request. When the message is received, the router establishes a
path between the server and the receiver, and subsequently updates
the multicast group state table. The mechanism used to discern the
network interfaces is not specified, but may take advantage of other
protocols such as the RSVP path and reservation mechanism.
4.3.2 Path Tear-down
Path tear-down requests are also propagated through the routers by
the MGA when group membership changes or QOS changes no longer
require data to be sent over a given route. These are used to inform
routers of both deletions of QOS for a given path and deletions of
entire paths. The purpose of the message is to explicitly remove
route table entries in order to minimize the time required to stop
forwarding multicast data across networks once the path is no longer
required.
4.3.3 Multicast Routing Based on Quality of Service
Traditional multicast routing formulates route/don"t route decisions
based on the destination address in the packet header, with packets
duplicated as necessary to reach all destinations. In the proposed
new protocol suite, routers also consult the QOS field for each
packet as different paths may have requested different qualities of
service. Packets are only forwarded if the group address has been
requested and the quality of service specified in the header is
requested in the state table entry for a given interface.
4.3.4 Quality of Service Based Packet Loss
Network congestion causes router queues to overflow, and as a result
packet loss occurs. The QOS and priority indications in the packet
headers can be used to prioritize the order in which packets are
dropped. First, packets with the priority field set in the header
are dropped last. Within packets of equal priority, packets are
dropped in order of QOS, with the highest QOS packets dropped first.
The rationale is that other packets with lower QOS may be usable by
receivers, while packets with high QOS may not be usable without the
lower QOS data.
5. Interactions Among the Components: An Example
The MGA, RAMP, and routing support functions all cooperate in the
multicast process. As an example, assume that a network exists with
a single server (S), three routers (R1, R2, and R3), and two clients
(C1 and C2). The path between S and C1 goes through R1 and R2, while
the path between S and C2 goes through R1, R2, and R3. The network
is shown in figure 2.
S ------- R1 -------- R2 -------- R3

C1 C2
Figure 2. Sample Network Configuration
Service Registration
When S is initiated, it registers a service with the MGA node in
the local workstation, offering reliable service at two qualities
of service, Q1 and Q2. As this is the first multicast offering on
the workstation, the local MGA requests a block of multicast
addresses from the hierarchy, and assigns an address and service
identifier to S. If the RSVP reservation protocol is in operation,
the local MGA node in S notifies RSVP to send a RpathS
message out for the service, which goes through R1, R2, and R3,
reaching the RSVP nodes on C1 and C2. The service and its
characteristics are propagated throughout the MGA hierarchy,
ultimately reaching the MGA nodes resident on C1 and C2. The
service is now available throughout the network.
Service Request and Path Set-up
The client C1 requests reliable service from S at QOS Q1, by
issuing a request to the MGA node in C1. If a reservation protocol
is in use, then it is used to reserve bandwidth and establish a
path between the sender and receiver, going through R1 and R2;
otherwise, the path is established through R1 and R2 by the routing
protocol. R1 now forwards all packets from S with QOS Q1 along the
path to R2, which routes them to C1. In concert with the path
set-up, the add membership request is propagated through MGA to the
server workstation. The local MGA tables are checked and it is
noted that the service is not currently being offered, so the
server is notified to begin reliable distribution of the service at
Q1.
Initial Delivery
The server now begins transmitting Q1 data which is observed by R1.
R1 inspects the header and notes that the packet has QOS Q1. The
routing tables specify that QOS Q1 for this address are only
forwarded along the interface leading to R2, and R1 acts
accordingly. Similarly, R2 routes the packet to C1. When the data
arrives at C1, the RAMP node inspects the QOS and destination
address fields in the header, accepts the packet, and forwards it
to the C1 client process.
Error Recovery
During transmission, if the RAMP node in C1 realizes that packets
have been dropped, a retransmission request is returned to the
server identifying spans of the missing packets. The RAMP node
accepts the packet, builds the retransmission packets, and sets the
retransmission hold timer. When the timer expires, the number of
retransmission requests for each missing packet is compared against
the threshold, and the packets are either unicast directly to the
requesters or multicast to the entire group. As in this case there
is only requester, the threshold is not exceeded and the packets
are retransmitted to C1Us unicast address.
Group Membership Addition
Client C2 now joins the group, requesting reliable transmission at
QOS Q2. Following the process used for C1, the request propagates
through the MGA (and potentially reservation protocol) hierarchy.
Upon completion of the request processing, R1 routes packets for
QOS Q1 and Q2 to R2, while R2 forwards QOS Q1 packets to C1 and Q2
packets to R3; client C1 only accepts packets marked as Q1 while C2
only accepts Q2 packets. The server is notified that it now has
clients for Q2, and begins serving that QOS in addition to Q1.
QOS Based Routing
First, assume that QOS Q1 data is independent of QOS Q2 data. When
the server sends a packet with Q1 marked in the header, the packet
is received by R1 and is forwarded to R2. R2 receives the packet,
and sends it out the interface to C1, but not to R3. Next, the
server delivers a packet for Q2. R1 receives the packet and sends
it to R2, which forwards it to R3 but not to C1. R3 accepts the
packet, and forwards it to C2.
Now, assume that either Q2 is a subset of Q1, or that receivers of
Q1 data also require Q2 data as in conditional compression schemes.
Therefore, all Q2 packets are marked for both Q1 and Q2, while the
remaining Q1 packets only have Q1 set in the header. Q1-only
packets are routed as before, following the path S -> R1 -> R2 ->
C1. However, Q2 packets are now routed from S to R1 to R2, at
which point R2 duplicates the packets and sends them to both C1 and
R3, with R3 forwarding them to C2. At C1, these packets have Q1
marked, and so are accepted, while at C2 the packet is accepted as
the Q2 bit is verified.
Group Membership Deletion
When C1 issues a drop membership request, the MGA on the client
workstation is notified, and the request is propagated through the
MGA hierarchy back to the server MGA node. In parallel, the
routers are notified to close the path, as it is no longer
required, possibly through the reservation protocol. As this is
the last client for the Q1 QOS, the server is informed to stop
transmitting Q1 data, with Q2 data unaffected. A similar process
occurs when C2 drops membership from the group, leaving the server
idle. At this point, the server has the option of shutting down
and returning the group address to the MGA, or to continue in an
idle state until another client requests service.
Acknowledgements
This research was supported in part by the Defense Research
Projects Agency (DARPA) under contract number F19618-91-C-0086.
References
[1] Almquist, P., "Type of Service in the Internet Protocol Suite",
RFC1349, Consultant, July 1992.
[2] Armstrong, S., Freier, A., and K. Marzullo, "Multicast Transport
Protocol", RFC1301, Xerox, Apple, Cornell University, February
1992.
[3] Braudes, R., and S. Zabele, "A Reliable, Adaptive Multicast
Service for High-Bandwidth Image Dissemination", submitted to ACM
SIGCOMM "93.
[4] Cheriton, D., "VMTP: Versatile Message Transaction Protocol", RFC
1045, Stanford University, February 1988.
[5] Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
1112, Stanford University, August 1989.
[6] Mayer, E., "An Evaluation Framework for Multicast Ordering
Protocols", Proceedings ACM SIGCOMM "92, Baltimore, Maryland, pp.
177-187.
[7] Mockapetris, P., "Domain Names - Concepts and Facilities," STD
13, RFC1034, USC/Information Sciences Institute, November 1987.
[8] Postel, J., "Internet Control Message Protocol - DARPA Internet
Program Protocol Specification", STD 5, RFC792, USC/Information
Sciences Institute, September 1981.
[9] Strayer, W., Dempsey, B., and A. Weaver, "XTP: The Xpress
Transfer Protocol", Addison-Wesley Publishing Co., Reading, MA,
1992.
[10] Topolcic, C., Editor, "Experimental Internet Stream Protocol,
Version 2 (ST- II)", RFC1190, CIP Working Group, October 1990.
[11] "XTP Protocol Definition Revision 3.6", Protocol Engines
Incorporated, PEI 92-10, Mountain View, CA, 11 January 1992.
[12] Zhang, L., Deering, S., Estrin, D., Shenker, S., and D. Zappala,
"RSVP: A New Resource ReSerVation Protocol", Work in Progress,
March 1993.
Security Considerations
Security issues are not discussed in this memo.
Authors" Addresses
Bob Braudes
TASC
55 Walkers Brook Drive
Reading, MA 01867
Phone: (617) 942-2000
EMail: rebraudes@tasc.com
Steve Zabele
TASC
55 Walkers Brook Drive
Reading, MA 01867
Phone: (617) 942-2000
EMail: gszabele@tasc.com