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Network Working Group                                         M. HandleyRequest for Comments: 2887                                      S. FloydCategory: Informational                                            ACIRI                                                              B. Whetten                                                                Talarian                                                              R. Kermode                                                                Motorola                                                             L. Vicisano                                                                   Cisco                                                                 M. Luby                                                  Digital Fountain, Inc.                                                             August 2000The Reliable Multicast Design Space for Bulk Data TransferStatus of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2000).  All Rights Reserved.Abstract   The design space for reliable multicast is rich, with many possible   solutions having been devised.  However, application requirements   serve to constrain this design space to a relatively small solution   space.  This document provides an overview of the design space and   the ways in which application constraints affect possible solutions.1.  Introduction   The term "general purpose reliable multicast protocol" is something   of an oxymoron.  Different applications have different requirements   of a reliable multicast protocol, and these requirements constrain   the design space in ways that two applications with differing   requirements often cannot share a single solution.  There are however   many successful reliable multicast protocol designs that serve more   special purpose requirements well.   In this document we attempt to review the design space for reliable   multicast protocols intended for bulk data transfer.  The term bulk   data transfer should be taken as having broad meaning - the main   limitations are that the data stream is continuous and long lived -Handley, et al.              Informational                      [Page 1]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 2000   constraints necessary for the forms of congestion control we   currently understand.  The purpose of this review is to gather   together an overview of the field and to make explicit the   constraints imposed by particular mechanisms. The aim is to provide   guidance to the standardization process for protocols and protocol   building blocks.  In doing this, we cluster potential solutions into   a number of loose categories - real protocols may be composed of   mechanisms from more than one of these clusters.   The main constraint on solutions is imposed by the need to scale to   large receiver sets.  For small receiver sets the design space is   much less restricted.2.  Application Constraints   Application requirements for reliable multicast (RM) are as broad and   varied as the applications themselves.  However, there are a set of   requirements that significantly affect the design of an RM protocol.   A brief list includes:   o  Does the application need to know that everyone received the data?   o  Does the application need to constrain differences between      receivers?   o  Does the application need to scale to large numbers of receivers?   o  Does the application need to be totally reliable?   o  Does the application need ordered data?   o  Does the application need to provide low-delay delivery?   o  Does the application need to provide time-bounded delivery?   o  Does the application need many interacting senders?   o  Is the application data flow intermittent?   o  Does the application need to work in the public Internet?   o  Does the application need to work without a return path (e.g.      satellite)?   o  Does the application need to provide secure delivery?Handley, et al.              Informational                      [Page 2]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 2000   In the context of standardizing bulk data transfer protocols, we can   rule out applications with multiple interacting senders and   intermittent data flows.  It is not that these applications are   unimportant, but that we do not yet have effective congestion control   for such applications.2.1.  Did everyone receive the data?   In many applications a logically defined unit or units of data is to   be delivered to multiple clients, e.g., a file or a set of files, a   software package, a stock quote or package of stock quotes, an event   notification, a set of slides, a frame or block from a video.  An   application data unit (ADU) is defined to be a logically separable   unit of data that is useful to the application. In some cases, an   application data unit may be short enough to fit into a single packet   (e.g., an event notification or a stock quote), whereas in other   cases an application data unit may be much longer than a packet   (e.g., a software package).   A protocol may optionally provide delivery confirmation to ensure   reliable delivery, i.e., a mechanism for receivers to inform the   sender when data has been delivered.  There are two types of   confirmation, at the application data unit level and at the packet   level. Application data unit confirmation is useful at the   application level, e.g., to inform the application about receiver   progress and to decide when to stop sending packets about a   particular application data unit.  Packet confirmation is useful at   the transport level, e.g., to inform the transport level when it can   release buffer space being used for storing packets for which   delivery has been confirmed.   Some applications have a strong requirement for confirmation that all   the receivers got an ADU, or if not, to be informed of which specific   receivers failed to receive the entire ADU. Examples include   applications where receivers pay for data, and reliable file-system   replication.  Other applications do not have such a requirement.  An   example is the distribution of free software.   If the application does need to know that every receiver got the ADU,   then a positive acknowledgment must be received from every receiver,   although it may be possible to aggregate these acknowledgments.  If   the application needs to know precisely which receivers failed to get   the ADU, additional constraints are placed on acknowledgment   aggregation.   It should be noted that different mechanisms can be used for ADU-   level confirmation and packet-level confirmation in the same   application.  For example, an ADU-level confirmation mechanism usingHandley, et al.              Informational                      [Page 3]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 2000   positive acknowledgments may sit on top of a packet-level NACK or   FEC-based transport.  Typically this only makes sense when ADUs are   significantly larger than a single packet.2.2.  Constraining differences   Some applications need to constrain differences between receivers so   that the data reception characteristics for all receivers falls   within some range.  An example is a stock price feed, where it is   unacceptable for a receiver to suffer delivery that is delayed   significantly more than any other receiver.   This requirement is difficult to satisfy without harming performance.   Typically solutions involve not sending more than a limited amount of   new data until positive acknowledgments have been received from all   the receivers.  Such a solution does not cope with network and end-   system failures well.2.3.  Receiver Set Scaling   There are many applications for RM that do not need to scale to large   numbers of receivers.  For such applications, a range of solutions   may be available that are not available for applications where   scaling to large receiver sets is a requirement.   A protocol must achieve good throughput of application data units to   receivers.  This means that most data that is delivered to receivers   is useful in recovering the application data unit that they are   trying to receive. A protocol must also provide good congestion   control to fairly share the available network resources between all   applications.  Receiver set scaling is one of the most important   constraints in meeting these requirements, because it strictly limits   the mechanisms that can be used to achieve these requirements to   those that will efficiently scale to a large receiver population.   Acknowledgement packets have been employed by many systems to achieve   these goals, but it is important to understand the strength and   limitations of different ways of using such packets.   In a very small system, it may be acceptable to have the receivers   acknowledge every packet.  This approach provides the sender with the   maximum amount of information about reception conditions at all the   receivers, information that can be used both to achieve good   throughput and to achieve congestion control.Handley, et al.              Informational                      [Page 4]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 2000   For larger systems, such "flat ACK" schemes cause acknowledge   implosions at the sender.  Attempts have been made to reduce this   problem by sending aggregate ACKs infrequently [RMWT98,BC94], but it   is very difficult to incorporate effective congestion control into   such protocols because of the spareceness of feedback.   Using negative acknowledgments (NACKs) instead of ACKs reduces this   problem to one of NACK implosion (only from the receivers missing the   packets), and because the sender really only needs to know that at   least one receiver is missing data in order to achieve good   throughput, various NACK suppression mechanisms can be applied.   An alternative to NACKs is ACK aggregation, which can be done by   arranging the receivers into a logical tree, so that each leaf sends   ACKs to its parent which aggregates them, and passes them on up the   tree.  Tree-based protocols scale well, but tree formation can be   problematic.   Other ACK topologies such as rings are also possible, but are often   more difficult to form and maintain than trees are.  An alternative   strategy is to add mechanisms to routers so that they can help out in   achieving good throughput or in reducing the cost of achieving good   throughput.   All these solutions improve receiver set scaling, but they all have   limits of one form or another.  One class of solutions scales to an   infinite number of receivers by having no feedback channel whatsoever   in order to achieve good throughput.  These open-loop solutions take   the initial data and encode it using an FEC-style mechanism.  This   encoded data is transmitted in a continuous stream.  Receivers then   join the session and receive packets until they have sufficient   packets to decode the original data, at which point they leave the   session.   Thus, it is clear that the intended scale of the session constrains   the possible solutions.  All solutions will work for very small   sessions, but as the intended receive set increases, the range of   possible solutions that can be deployed safely decreases.   It should also be noted that hybrids of these mechanisms are   possible, and that using one mechanism at the packet-level and a   different (typically higher overhead) solution at the ADU level may   also scale reasonably if the ADUs are large compared to packets.Handley, et al.              Informational                      [Page 5]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 20002.4.  Total vs Semi-reliable   Many applications require delivery of application data units to be   totally reliable; if any of the application data unit is missing,   none of the received portion of the application data unit is useful.   File transfer applications are a good example of applications   requiring total reliability.   However, some applications do not need total reliability.  An example   is audio broadcasting, where missing packets reduce the quality of   the received audio but do not render it unusable.  Such applications   can sometimes get by without any additional reliability over native   IP reliability, but often having a semi-reliable multicast protocol   is desirable.2.5.  Time-bounded Delivery   Many applications just require data to be delivered to the receivers   as fast as possible.  They have no absolute deadline for delivery.   However, some applications have hard delivery constraints - if the   data does not arrive at the receiver by a certain time, there is no   point in delivering it at all.  Such time-boundedness may be as a   result of real-time constraints such as with audio or video   streaming, or as the result of new data superseding old data.  In   both cases, the requirement is for the application to have a greater   degree of control over precisely what the application sends at which   time than might be required with applications such as file transfer.   Time-bounded delivery usually also implies a semi-reliable protocol,   but the converse does not necessarily hold.3.  Network Constraints   The properties of the network in which the application is being   deployed may themselves constrain the reliable multicast design   space.3.1.  Internet vs Intranet   In principle the Internet and intranets are the same.  In practice   however, the fact that an intranet is under one administration might   allow for solutions to be configured that can not easily be done in   the public Internet.  Thus, if the data is of very high value, it   might be appropriate to enhance the routers to provide assistance to   a reliable multicast transport protocol.  In the public Internet, it   is less likely that the additional expense required to support this   state in the routers would be acceptable.Handley, et al.              Informational                      [Page 6]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 20003.2.  Return Path   In principle, when feedback is required from receivers, this feedback   can be multicast or unicast.  Multicast feedback has advantages,   especially in NACK-based protocols where it is valuable for NACK   suppression.  However, it is not clear at this time whether all ISPs   will allow all members of a session to send to that session.  If   multicast feedback is not allowed, then unicast feedback can almost   always be substituted, although often at the expense of additional   messages and mechanisms.   Some networks may not allow any form of feedback however.  The   primary example of this occurs with satellite broadcasts where the   back channel may be very narrow or even non-existent.  For such   networks the solution space is very constrained - only FEC-based   encodings have any real chance of working.  If the receivers are   direct satellite receivers, then no congestion control is needed, but   it is dangerous to make such assumptions because it is possible for a   satellite hop to feed downstream networks.  Thus, congestion control   still needs to be considered with solutions that do not have a return   path.3.3.  Network Assistance   A reliable multicast protocol must involve mechanisms running in end   hosts, and must involve routers forwarding multicast packets.   However under some circumstances, it is possible to rely on some   additional degree of assistance from network elements.  Broadly   speaking we can cluster RM protocols into four classes depending on   the degree of support received from other network elements.   No Additional Support      The routers merely forward packets, and only the sender and      receivers have any reliable multicast protocol state.   Layered Approaches      Data is split across multiple multicast groups.  Receivers join      appropriate groups to receive only the traffic they require.  This      may in some cases require fast join or leave functionality from      the routers, and may require more forwarding state in the routers.   Server-based Approaches      Additional nodes are used to assist with data delivery or feedback      aggregation.  These additional nodes might not be normal senders      or receivers, and may be present on the distribution or feedback      tree only to provide assistance to the reliable multicast      protocol.  They would not otherwise receive the multicast traffic.Handley, et al.              Informational                      [Page 7]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 2000   Router-based Approaches      With router-based approaches, routers on the normal data      distribution tree from the sender to the receivers assist in the      delivery of data or feedback aggregation or suppression.  As      routers can directly influence multicast routing, they have more      control over which traffic goes to which group members than      server-based approaches.  However routers do not normally have a      large amount of spare memory or processing power, which restricts      how much functionality can be placed in the routers.  In addition,      router code is normally more difficult to upgrade than application      code, so router-based approaches need to be very general as they      are more difficult to deploy and to change.4.  Good Throughput Mechanisms   Two main concerns that a RM protocol must address are congestion   control and good throughput.  Packet loss plays a major role with   respect to both concerns.  The primary symptom of congestion in many   networks is packet loss. The primary obstacle that must be overcome   to achieve good throughput is packet loss.  Thus, measuring and   reacting to packet loss is crucial to address both concerns. RM   solutions that address these concerns can be roughly categorized as   using one or more of the following techniques:   o  Data packet acknowledgment.   o  Negative acknowledgment of missing data packets.   o  Redundancy allowing not all packets to be received.   These techniques themselves can be usefully subdivided, so that we   can examine the parts of the requirement space in which each   mechanism can be deployed.  In this section, we focus on using these   mechanisms for achieving good throughput, and in the next section we   focus on using these mechanisms for congestion control.4.1.  ACK-based Mechanisms   The simplest ACK-based mechanism involves every receiver sending an   ACK packet for every data packet it receives and resending packets   that are lost by any receiver.  Such mechanisms are limited to very   small receiver groups by the implosion of ACKs received at the   sender, and for this reason they are impractical for most   applications.Handley, et al.              Informational                      [Page 8]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 2000   Putting multiple ACKs into a single data packet [RMWT98] reduces the   implosion problem by a constant amount, allowing slightly larger   receiver groups.  However a limit is soon reached whereby feedback to   the sender is too infrequent for sender-based congestion control   mechanisms to work reliably.   Arranging the receivers into a ring [WKM94] whereby an "ACK-token" is   passed around the ring prevents the implosion problem for data.   However ring creation and maintenance may itself be problematic.   Also if ring creation does not take into account network topology   (something which is difficult to achieve in practice), then the   number of ACK packets crossing the network backbone for each data   packet sent may increase O(n) with the number of receivers.4.1.1.  Tree-based ACK Mechanisms   Arranging the receivers into a tree [MWB+98, KCW98] whereby receivers   generate ACKs to a parent node, which aggregates those ACKs to its   parent in turn, is both more robust and more easily configured than a   ring.  The ACK-tree is typically only used for ACK-aggregation - data   packets are multicast from the sender to the receivers as normal.   Trees are easier to construct than rings because more local   information can be used in their construction.  Also they can be more   fault tolerant than rings because node failures only affect a subset   of receivers, each of which can easily and locally decide to by-pass   its parent and report directly to the node one level higher in the   tree.  With good ACK-tree formation, tree-based ACK mechanisms have   the potential to be one of the most scalable RM solutions.   To be simple to deploy, tree-based protocols must be self-organizing   - the receivers must form the tree themselves using local information   in a scalable manner.  Such mechanisms are possible, but are not   trivial.  The main scaling limitations of tree-based protocols   therefore come from the tree formation and maintenance mechanisms   rather than from the use of ACKs.  Without such a scalable and   automatic tree-formation mechanism, tree-based protocols must rely on   manual configuration, which significantly limits their applicability   (often to intranets) and (due to the complexity of configuration)   their scalability.   Orthogonal to the issue of tree formation is the issue of subtree   retransmission.  With appropriate router mechanisms, or the use of   multiple multicast groups, it is possible to allow the intermediate   tree nodes to retransmit missing data to the nodes below them in the   tree rather than relying on the original sender to retransmit the   data.  This relies on there being a good correlation at the point of   the intermediate node between the ACK tree and the actual data tree,   as well as there being a mechanism to constrain the retransmission toHandley, et al.              Informational                      [Page 9]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 2000   the subtree.  A good automatic tree formation mechanism combined with   the use of administrative scoped multicast groups might provide such   a solution. Without such tree formation mechanisms, subtree   retransmission is difficult to deploy in large groups in the public   internet.       This could also be solved by the use of transport-   level router mechanisms to assist or perform retransmission, although   existing router mechanisms [FLST98] support NACK-based rather than   ACK-based protocols.   Another important issue is the nature of the aggregation performed at   interior nodes on the ACK-tree.  Such nodes could:   1. aggregate ACKs by sending a single ACK when all their children      have ACKed,   2. aggregate ACKs by listing all the children that have ACKed,   3. send an aggregated ACK with a NACK-like exception list.   For data packets, 1. is clearly more scalable, and should be   preferred.  However if the sender needs to know exactly which   receivers received the data, 2. and 3. provide this information.   Fortunately, there is usually no need to do this on a per-packet   basis, but rather on a per-ADU basis.  Doing 1. on a per packet   basis, and 3. on a per ADU basis is the most scalable solution for   applications that need this information, and suffers virtually no   disadvantage compared to the other solutions used on a per-packet   basis.4.2.  NACK-based mechanisms   Instead of sending an ACK for every data packet received, receivers   can send a negative acknowledgment (NACK) for every data packet they   discover they did not receive.  This has a number of advantages over   ACK-based mechanisms:   o  The sender no longer needs to know exactly how many receivers      there are.  This removes the topology-building phase needed for      ring- or tree-style ACK-based algorithms.   o  Fault-tolerance is made somewhat simpler by making receivers      responsible for reliability.   o  Sender state can be significantly reduced because the sender does      not need to keep track of the receivers state.Handley, et al.              Informational                     [Page 10]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 2000   o  Only a single NACK is needed from any receiver to indicate a      packet that is missing by any number of receivers.  Thus NACK      suppression is possible.   The disadvantages are that it is more difficult for the sender to   know that it can free transmission buffers, and that additional   session level mechanisms are needed if the sender really needs to   know if a particular receiver actually received all the data.   However for many applications, neither of these is an issue.4.2.1.  NACK Suppression   The key differences between NACK-based protocols is in how NACK-   suppression is performed.  The goal is for only one NACK to reach the   sender (or a node that can resend the missing data) as soon as   possible after the loss is first noticed, and for only one copy of   the missing data to be received by those nodes needing   retransmission.   Different mechanisms come close to satisfying these goals in   different ways.   o  SRM [FJM95] uses random timers weighted by the round trip time      between the sender and each node missing the data.  This is      effective, but requires computing the RTT to each receiver before      suppression works properly.   o  NTE [HC97] uses a sender-triggered mechanism based on random keys      and sliding masks.  This does not require random timers, and works      for very large sessions, but makes it difficult to provide the      constant low-level stream of feedback needed to perform congestion      control.   o  AAP [Ha99] uses exponentially distributed random timers and is      effective for large sessions without needing to compute the RTT to      each receiver.   o  PGM [FLST98] and LMS [PPV98] use additional mechanisms in routers      to suppress duplicate NACKs.  In the case of PGM, router      assistance suppliments SRM-stype random timers and localizes the      suppression so that the whole group does not need suppressing.   The most general of these mechanisms is probably exponentially   weighted random timers.  Although SRM style timers can reduce   feedback delay, they are harder to use correctly in situations where   all the RTTs are not known, or where the number of respondees isHandley, et al.              Informational                     [Page 11]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 2000   unknown.  In contrast, exponentially weighted random timers work well   across a large range of session sizes with good worst case delay   characteristics.   Either form of random timer based mechanism can be supplemented by   router-support where it is available.  Sender triggered NACK   mechanisms (e.g. [HC97]) are more difficult to integrate with   router-based support mechanisms.4.3.  Replication   Some RM protocols can be designed so as to not need explicit   reliability mechanisms except in comparatively rare cases.  An   example is in a multicast game, where the position of a moving object   is continuously multicast.  This positional stream does not require   additional reliability because a new position superseding the old one   will be sent before any retransmission could take place.  However,   when the moving object interacts with other objects or stops moving,   then an explicit reliability mechanism is required to reliably send   the interaction information or last position.   It is not just games that can be built in this manner - the NTE   shared text editor[HC97] uses just such a mechanism with changes to a   line of text.  For every change the whole line is sent, and so long   as the user keeps typing no explicit reliability mechanism is needed.   The major advantage of replication is that it is not susceptible to   spatially uncorrelated packet loss.  With a traditional ACK or NACK   based protocol, the probability of any particular packet being   received by all the receivers in a large group can be very low.  This   leads to high retransmission rates.      In contrast, replicated   streams do not suffer as the size of the receiver group increases -   different receivers lose different packets, but this does not   increase network traffic.4.4.  Packet-level Forward Error Correction   Forward Error Correction (FEC) is a well known technique for   protecting data against corruption.  For reliable multicast it is   most useful in the form of erasure codes.   The simplest form of packet-level FEC is to take a group of packets   that is to be sent, and to XOR the packets together to form a   newpacket which is also sent.  If there were three original packets   plus the XOR packet sent, then if a receiver is missing any one of   the original data packets, but receives the XOR packet, then it can   reproduce the missing original packet.Handley, et al.              Informational                     [Page 12]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 2000   More general erasure codes exist [BKKKLZ95], [Ri97], [LMSSS97] that   allow the generation of n encoding packets from k original data   packets.  In such cases, so long as at least k of the n encoding   packets are received, then the k original data packets can be   reproduced.   To apply FEC the sender groups data packets into rounds, and encoding   packets are produced based on all the data packets in a round. A   round may consist of all data packets in an entire application data   unit in some cases, whereas in other cases it may consist of a group   of data packets that make up only a small portion of an application   data unit.   Using erasure codes to repair packet loss is a significant   improvement over simple retransmission because the dependency on   which packets have been lost is removed.  Thus, the amount of repair   traffic required to repair spatially uncorrelated packet loss is   considerably lessened.   We can divide packet-level FEC schemes into two categories: proactive   FEC and reactive FEC.  The difference between the two is that for   proactive FEC the sender decides a priori how many encoding packets   to send for each round of data packets, whereas for reactive FEC the   sender initially transmits only the original data packets for each   round.  Then, the sender uses feedback from the receivers to compute   how many packets were lost by the receiver that experienced the most   loss in each round, and then only that number of additional encoding   packets are sent for that round.  These encoding packets will then   also serve to repair loss at the other receivers that are missing   fewer packets.  The receivers report via ACKs or NACKs how many   packets are missing from each round. With NACKs, only the receiver   missing the most packets need send a NACK for this round, so this is   used to weight the random timers in the NACK calculation.   Proactive and reactive FEC can be combined, e.g., a certain amount of   proactive FEC can be sent for each round and if there are receivers   that experience more loss than can be overcome by this for some   rounds then they can request and receive additional encoding packets   for these rounds.   FEC is very effective at reducing the repair traffic for packet loss.   However, it requires that the data to be sent to be grouped into   rounds, which can add to end-to-end latency.  For bulk-data   applications this is typically not a problem, but this may be an   issue for interactive applications where replication may be a better   solution.Handley, et al.              Informational                     [Page 13]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 20004.5.  Layered FEC   An alternative use of packet level FEC is possible when data is   spread across several multicast groups [RVC98], [BLMR98].  In such   cases, the original k data packets are used to generate n encoding   packets, where n is much larger than k.  The n encoded packets are   then striped across multiple multicast groups.  When a receiver   wishes to receive the original data it joins one or more of the   multicast groups, and receives the encoding packets.  Once it has   received k different encoding packets, the receiver can then leave   all the multicast groups and reconstruct the original data.   The primary importance of such a layering is that it allows different   receivers to be able to receive the traffic at different rates   according to the available capacity.  Such schemes do not require any   form of feedback from the receivers to the sender to ensure good   throughput, and therefore the need for good throughput does not   constrain the size of the receiver set.  However, to perform adequate   network congestion control using receiver joins and leaves in this   manner may require coordination between members that are behind the   same congested link from the sender.  As described in the next   section, [RVC98] suggests such a layered congestion control scheme.5.  Congestion Control Mechanisms   The basic delivery model of the Internet is best-effort service.  No   guarantees are given as to throughput, delay or packet loss.  End-   systems are expected to be adaptive, and to reduce their transmission   rate to a level appropriate for the congestion state of the network.   Although increasingly the Internet will start to support reserved   bandwidth and differentiated service classes for specialist   applications, unless an end-system knows explicitly that it has   reserved bandwidth, it must still perform congestion control.   Broadly speaking, there are five classes of single-sender multicast   congestion control solution:   o  Sender-controlled, one group.      A single multicast group is used for data distribution.  Feedback      from the group members is used to control the rate of this group.      The goal is to transmit at a rate dictated by the slowest      receiver.Handley, et al.              Informational                     [Page 14]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 2000   o  Sender-controlled, multiple groups.      One initial multicast group is adaptively subdivided into multiple      subgroups with subdivisions centered on congestion points in the      network.  Application-level relays buffer data from a group nearer      the original sender, and retransmit it at a slower rate into a      group further from the original sender.  In this way, different      receivers can receiver the data at different rates.  Sender-based      congestion control takes place between the members of a subgroup      and their relay.   o  Receiver-controlled, one group.      A single multicast group is used for data distribution.  The      receivers determine if the sender is transmitting too rapidly for      the current congestion state of the network, and they leave the      group if this is the case.   o  Receiver-controlled, layered organization.      A layered approach for how to combine this scheme with a      congestion control protocol that requires no receiver feedback is      described in [RVC98].  The sender stripes data across multiple      multicast groups simultaneously.  Receivers join and leave these      layered groups depending on their measurements of the congestion      state of the network, so that the amount of data being received is      always appropriate. However, this scheme relies on receivers to      join and leave the different multicast groups in a coordinated      fashion behind a bottleneck link, and it has not yet been      completely confirmed that this approach will scale in practice to      the Internet.  As a result, more work on this congestion control      mechanism would be beneficial.   o  Router-based congestion control.      It is possible to add additional mechanisms to multicast routers      to assist in multicast congestion control.  Such mechanisms could      include:      o  Conditional joins (a multicast join that specifies a loss rate         above which it is acceptable for the router to reject the         join).      o  Router filtering of traffic that exceeds a reasonable rate.         This may include mechanisms for filtering traffic at different         points in the network at different rates depending on local         congestion conditions [LVS99].Handley, et al.              Informational                     [Page 15]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 2000      o  Fair queuing schemes combined with end-to-end adaptation.      Router-based schemes generally require more state in network      routers than has traditionally been acceptable for backbone      routers.  Thus, in the near-term, such schemes are only likely to      be applicable for intranet solutions.   For reliable multicast protocols, it is important to consider   congestion control at the same time as reliability is being   considered.  The same mechanisms that are used to provide reliability   will sometimes be used to provide congestion control.   In the case of receiver-based congestion control, open-loop delivery   using FEC is the likely choice for achieving good throughput for   bulk- data transfer.  This is because open-loop delivery requires no   feedback from receivers, and thus it is a perfect match with a   receiver-based congestion-control mechanism that operates without   feedback from receivers.6.  Security Considerations   Generally speaking, security considerations have relatively little   effect on constraining the design space for reliable multicast   protocols.  The primary issues constraining the design space are all   related to receiver-set scaling.  For authentication of the source   and of data integrity, receiver-set scaling is not a significant   issue.  However, for data encryption, key distribution and   particularly re-keying may be significantly affected by receiver-set   scaling.  Tree and graph based re-keying solutions[WHA98,WGL97] would   appear to be appropriate solutions to these problems.  It is not   clear however that such re-keying solutions need to directly affect   the design of the data distribution part of a reliable multicast   protocol.   The primary question to consider for the security of reliable   multicast protocols is the role of third-parties.  If nodes other   than the original source of the data are allowed to send or resend   data packets, then the security model for the protocol must take this   into account.  In particular, it must be clear whether such third   parties are trusted or untrusted.  A requirement for trusted third   parties can make protocols difficult to deploy on the Internet.   Untrusted third parties (such as receivers that retransmit the data)   may be used so long as the data authentication mechanisms take this   into account.  Typically this means that the original sender   digitally signs and timestamps the data, and that the third parties   resend this signed timestamped payload unmodified.Handley, et al.              Informational                     [Page 16]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 2000   Unlike unicast protocols, denial-of-service attacks on multicast   transport state are easy if the protocol design does not take such   attacks into account.  This is because any receiver can join the   session, and can then produce feedback that influences the progress   of a session involving many other receivers.  Hence protection   against denial-of-service attacks on reliable multicast protocols   must be carefully considered.  A receiver that requests   retransmission of every packet, or that refuses to acknowledge   packets in an ACK-based protocol can potentially bring a reliable   multicast session to a standstill.  Senders must have appropriate   policy to deal with such conditions, and if necessary, evict the   receiver from the group.  A single receiver masquerading as a large   number of receivers may still be an issue under such circumstances   with protocols that support NACK-like functionality.  Providing   unique "keys" to each NACKer when they first NACK using a unicast   response might potentially prevent such attacks.   Denial-of-service attacks caused by traffic flooding are however   somewhat easier to protect against than with unicast.  Unwanted   senders can simply be pruned from the distribution tree using the   mechanisms implemented in IGMP v3[CDT99].7.  Conclusions   In this document we present an overview of the design space for   reliable multicast within the context of one-to-many bulk-data   transfer. Other flavors of multicast application are not considered   in this document, and hence the overview given should not be   considered inclusive of the design space for protocols that fall   outside the context of one-to-many bulk-data transfer. During the   course of this overview, we have reaffirmed the notion that the   process of reliable multicast protocol design is affected by a number   of factors that render the generation of a "one size fits all   solution" moot. These factors are then described to show how an   application's needs serve to constrain the set of available   techniques that may be used to create a reliable multicast protocol.   We examined a number of basic techniques and to show how well they   can meet the needs of certain types of applications.   This document is intended to provide guidance to the IETF community   regarding the standardization of reliable multicast protocols for   bulk-data transfer. Given the degree to which application   requirements constrain reliable multicast solutions, and the diverse   set of applications that need to be supported, it should be clear   that any standardization work should take great pains to be future-   proof.  This would seem to imply not standardizing complete reliable   multicast transport protocols in one pass, but rather examining the   degree to which such protocols are separable into functional buildingHandley, et al.              Informational                     [Page 17]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 2000   blocks, and standardizing these blocks separately to the maximum   degree that makes sense.  Such an approach allows for protocol   evolution, and allows applications with new constraints to be   supported with maximal reuse of existing and tested mechanisms.8.  Acknowledgments   This document represents an overview of the reliable multicast design   space.  The ideas presented are not those of the authors, but are   collected from the varied presentations and discussions in the IRTF   Reliable Multicast Research Group.  Although they are too numerous to   list here, we thank everyone who has participated in these   discussions for their contributions.9.  Authors' Addresses   Mark Handley   ATT Center for Internet Research at ICSI,   International Computer Science Institute,   1947 Center Street, Suite 600,   Berkeley, CA 94704, USA   EMail: mjh@aciri.org   Sally Floyd   ATT Center for Internet Research at ICSI,   International Computer Science Institute,   1947 Center Street, Suite 600,   Berkeley, CA 94704, USA   EMail: floyd@aciri.org   Brian Whetten   Talarian Corporation,   333 Distel Circle,   Los Altos, CA 94022, USA   EMail: whetten@talarian.comHandley, et al.              Informational                     [Page 18]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 2000   Roger Kermode   Motorola Australian Research Centre   Level 3, 12 Lord St,   Botany  NSW  2019,   Australia   EMail: Roger.Kermode@motorola.com   Lorenzo Vicisano   Cisco Systems,   170 West Tasman Dr.   San Jose, CA 95134, USA   EMail: lorenzo@cisco.com   Michael Luby   Digital Fountain, Inc.   600 Alabama Street   San Francisco, CA  94110   EMail: luby@digitalfountain.com10.  References   [BC94]     K. Birman, T. Clark.  "Performance of the Isis Distributed              Computing Toolkit." Technical Report TR-94-1432, Dept. of              Computer Science, Cornell University.   [BKKKLZ95] J. Bloemer, M. Kalfane, M. Karpinski, R. Karp, M. Luby, D.              Zuckerman, "An XOR-based Erasure Resilient Coding Scheme",              ICSI Technical Report No. TR-95-048, August 1995.   [BLMR98]   J. Byers, M. Luby, M. Mitzenmacher, A. Rege, "A Digital              Fountain Approach to Reliable Distribution of Bulk Data",              Proc ACM SIGCOMM 98.   [CDT99]    Cain, B., Deering, S., and A. Thyagarajan, "Internet Group              Management Protocol, Version 3", Work in Progress.   [FLST98]   Farinacci, D., Lin, S., Speakman, T. and A. Tweedly, "PGM              reliable transport protocol specification", Work in              Progress.   [FJM95]    S. Floyd, V. Jacobson, S. McCanne, "A Reliable Multicast              Framework for Light-weight Sessions and Application Level              Framing", Proc ACM SIGCOMM 95, Aug 1995 pp. 342-356.Handley, et al.              Informational                     [Page 19]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 2000   [Ha99]     Handley, M., "Multicast address allocation protocol              (AAP)", Work in Progress.   [HC97]     M. Handley and J. Crowcroft, "Network text editor (NTE) a              scalable shared text editor for MBone," ACM Computer              Communication Review, vol. 27, pp. 197-208, Oct. 1997. ACM              SIGCOMM'97, Sept. 1997.   [KCW98]    Kadansky, M., Chiu, D. and J. Wesley, "Tree-based reliable              multicast (TRAM)", Work in Progress.   [LMSSS97]  M. Luby, M. Mitzenmacher, A. Shokrollahi, D. Spielman, V.              Stemann, "Practical Loss-Resilient Codes", Proc ACM              Symposium on Theory of Computing, 1997.   [MWB+98]   Montgomery, T., Whetten, B., Basavaiah, M., Paul, S.,              Rastogi, N., Conlan, J. and T. Yeh, "THE RMTP-II              PROTOCOL", Work in Progress.   [PPV98]    C. Papadopoulos, G. Parulkar, and G. Varghese, "An error              control scheme for large-scale multicast applications," in              Proceedings of the Conference on Computer Communications              (IEEE Infocom), (San Francisco, California), p. 1188,              March/April 1998.   [Ri97]     L. Rizzo, "Effective erasure codes for reliable computer              communication protocols," ACM Computer Communication              Review, vol.  27, pp. 24-36, Apr. 1997.   [RV97]     L. Rizzo, L. Vicisano, "A Reliable Multicast data              Distribution Protocol based on software FEC techniques",              Proc. of The Fourth IEEE Workshop on the Architecture and              Implementation of High Performance Communication Systems              (HPCS'97), Sani Beach, Chalkidiki, Greece June 23-25,              1997.   [RVC98]    L. Rizzo, L. Vicisano, J. Crowcroft, "The RLC multicast              congestion control algorithm", submitted to IEEE Network -              special issue multicast.   [RMWT98]   Robertson, K., Miller, K., White, M. and A. Tweedly,              "StarBurst multicast file transfer protocol (MFTP)              specification", Work in Progress.   [WHA98]    Wallner, D., Hardler, E. and R. Agee, "Key Management for              Multicast: Issues and Architectures",RFC 2627, June 1999.Handley, et al.              Informational                     [Page 20]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 2000   [WKM94]    Brian Whetten, Simon Kaplan, and Todd Montgomery, "A high              performance totally ordered multicast protocol," research              memorandum, Aug. 1994.   [WGL97]    C.K. Wong, M. Gouda, S. Lam, "Secure Group Communications              Using Key Graphs," Technical Report TR 97-23, Department              of Computer Sciences, The University of Texas at Austin,              July 1997.Handley, et al.              Informational                     [Page 21]

RFC 2887     Multicast Design Space for Bulk Data Transfer   August 200011.  Full Copyright Statement   Copyright (C) The Internet Society (2000).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Handley, et al.              Informational                     [Page 22]