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rfc:rfc1458

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

Braudes & Zabele [Page 1] RFC 1458 Requirements for Multicast Protocols May 1993

 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 RFC reviews 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.

Braudes & Zabele [Page 2] RFC 1458 Requirements for Multicast Protocols May 1993

 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

Braudes & Zabele [Page 3] RFC 1458 Requirements for Multicast Protocols May 1993

 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

Braudes & Zabele [Page 4] RFC 1458 Requirements for Multicast Protocols May 1993

 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

Braudes & Zabele [Page 5] RFC 1458 Requirements for Multicast Protocols May 1993

 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.

Braudes & Zabele [Page 6] RFC 1458 Requirements for Multicast Protocols May 1993

 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

Braudes & Zabele [Page 7] RFC 1458 Requirements for Multicast Protocols May 1993

 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.

Braudes & Zabele [Page 8] RFC 1458 Requirements for Multicast Protocols May 1993

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

Braudes & Zabele [Page 9] RFC 1458 Requirements for Multicast Protocols May 1993

 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.
  1. ——-

| root |

  1. ——-

/ | \

                       /   |   \
                --------       --------
                |   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

Braudes & Zabele [Page 10] RFC 1458 Requirements for Multicast Protocols May 1993

 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

Braudes & Zabele [Page 11] RFC 1458 Requirements for Multicast Protocols May 1993

 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 RFC 1349 [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.

Braudes & Zabele [Page 12] RFC 1458 Requirements for Multicast Protocols May 1993

 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

Braudes & Zabele [Page 13] RFC 1458 Requirements for Multicast Protocols May 1993

 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 RFC does 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.

Braudes & Zabele [Page 14] RFC 1458 Requirements for Multicast Protocols May 1993

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,

Braudes & Zabele [Page 15] RFC 1458 Requirements for Multicast Protocols May 1993

 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

Braudes & Zabele [Page 16] RFC 1458 Requirements for Multicast Protocols May 1993

 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.

Braudes & Zabele [Page 17] RFC 1458 Requirements for Multicast Protocols May 1993

References

 [1] Almquist, P., "Type of Service in the Internet Protocol Suite",
     RFC 1349, Consultant, July 1992.
 [2] Armstrong, S., Freier, A., and K. Marzullo, "Multicast Transport
     Protocol", RFC 1301, 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, RFC 1034, USC/Information Sciences Institute, November 1987.
 [8] Postel, J., "Internet Control Message Protocol - DARPA Internet
     Program Protocol Specification", STD 5, RFC 792, 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)", RFC 1190, 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.

Braudes & Zabele [Page 18] RFC 1458 Requirements for Multicast Protocols May 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

Braudes & Zabele [Page 19]

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