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RFC 5058 - Explicit Multicast (Xcast) Concepts and Options


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Network Working Group                                          R. Boivie
Request for Comments: 5058                                    N. Feldman
Category: Experimental                                               IBM
                                                                 Y. Imai
                                                          WIDE / Fujitsu
                                                               W. Livens
                                                                  ESCAUX
                                                                 D. Ooms
                                                              OneSparrow
                                                           November 2007

            Explicit Multicast (Xcast) Concepts and Options

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

IESG Note

   This RFC is not a candidate for any level of Internet Standard.  The
   IETF disclaims any knowledge of the fitness of this RFC for any
   purpose and in particular notes that the decision to publish is not
   based on IETF review for such things as security, congestion control,
   or inappropriate interaction with deployed protocols.  The RFC Editor
   has chosen to publish this document at its discretion.  Readers of
   this document should exercise caution in evaluating its value for
   implementation and deployment.  See RFC 3932 for more information.

Abstract

   While traditional IP multicast schemes (RFC 1112) are scalable for
   very large multicast groups, they have scalability issues with a very
   large number of distinct multicast groups.  This document describes
   Xcast (Explicit Multi-unicast), a new multicast scheme with
   complementary scaling properties: Xcast supports a very large number
   of small multicast sessions.  Xcast achieves this by explicitly
   encoding the list of destinations in the data packets, instead of
   using a multicast group address.

   This document discusses Xcast concepts and options in several areas;
   it does not provide a complete technical specification.

Table of Contents

   1. Introduction ....................................................3
   2. Xcast Overview ..................................................4
   3. The Cost of the Traditional IP Multicast Schemes ................6
   4. Motivation ......................................................9
   5. Application ....................................................11
   6. Xcast Flexibility ..............................................12
   7. Xcast Control Plane Options ....................................13
      7.1. SIP Control Plane for Xcast ...............................14
      7.2. Receiver-Initiated Join for Xcast .........................14
   8. Optional Information ...........................................15
      8.1. List of Ports .............................................15
      8.2. List of DSCPs .............................................15
      8.3. Channel Identifier ........................................15
   9. Possible Xcast Packet Encoding .................................16
      9.1. General ...................................................16
      9.2. IPv4 ......................................................17
           9.2.1. IPv4 Header ........................................17
           9.2.2. Xcast4 Header ......................................17
      9.3. IPv6 ......................................................20
           9.3.1. IPv6 Header ........................................20
           9.3.2. Xcast6 Header ......................................20
                  9.3.2.1. Routing Extension Header ..................21
                  9.3.2.2. Destination Extension Header ..............21
   10. Impact on Upper-Layer Protocols ...............................22
      10.1. Checksum Calculation in Transport-Layer Headers ..........22
      10.2. IPsec ....................................................22
   11. Gradual Deployment ............................................23
      11.1. Tunneling ................................................23
      11.2. Premature X2U ............................................25
      11.3. Semi-Permeable Tunneling (IPv6 Only) .....................25
      11.4. Special Case: Deployment without Network Support .........26
      11.5. Using a Small Number of Xcast-Aware Routers to
            Provide Xcast ............................................27
   12. (Socket) API ..................................................28
   13. Unresolved Issues .............................................28
      13.1. The Format of the "List of Addresses" ....................28
      13.2. The size of Channel Identifier ...........................28
      13.3. Incremental Deployment ...................................28
      13.4. DSCP usage ...............................................29
      13.5. Traversing a Firewall or NAT Products ....................29
      13.6. The Size of BITMAP .......................................29
   14. Security Considerations .......................................29
   15. IANA Considerations ...........................................30
   16. Informative References ........................................31
   17. Contributors ..................................................33

1.  Introduction

   While traditional IP multicast schemes [1112] are scalable for very
   large multicast groups, they have scalability issues with a very
   large number of distinct multicast groups.  This document describes
   Xcast (Explicit Multi-unicast (Xcast)), a new multicast scheme with
   complementary scaling properties: Xcast supports a very large number
   of small multicast sessions.  Xcast achieves this by explicitly
   encoding the list of destinations in the data packets, instead of
   using a multicast group address.  This document discusses Xcast
   concepts and options in several areas; it does not provide a complete
   technical specification.

   Multicast, the ability to efficiently send data to a group of
   destinations, is becoming increasingly important for applications
   such as IP telephony and video-conferencing.

   Two kinds of multicast seem to be important: a broadcast-like
   multicast that sends data to a very large number of destinations, and
   a "narrowcast" multicast that sends data to a fairly small group
   [BOIV].  An example of the first is the audio and video multicasting
   of a presentation to all employees in a corporate intranet.  An
   example of the second is a videoconference involving three or four
   parties.  For reasons described below, it seems prudent to use
   different mechanisms for these two cases.  As the Reliable Multicast
   Transport working group has stated: "it is believed that a 'one size
   fits all' protocol will be unable to meet the requirements of all
   applications" [RMT].  Note that the 1998 IAB Routing Workshop [2902]
   came to the same conclusion:  "For example, providing for many groups
   of small conferences (a small number of widely dispersed people) with
   global topological scope scales badly given the current multicast
   model".

   Today's multicast schemes can be used to minimize bandwidth
   consumption.  Explicit Multi-Unicast (Xcast) also can be used to
   minimize bandwidth consumption for "small groups".  But it has an
   additional advantage as well.  Xcast eliminates the per-session
   signaling and per-session state information of traditional IP
   multicast schemes and this allows Xcast to support very large numbers
   of multicast sessions.  This scalability is important since it
   enables important classes of applications such as IP telephony,
   videoconferencing, collaborative applications, networked games, etc.,
   where there are typically very large numbers of small multicast
   groups.

   Interestingly, the idea for Xcast has been around for some time,
   although this was not immediately known to the three groups that
   independently re-invented it in the late 1990's.  In fact the very

   first proposal of the multicast concept in the Internet community, by
   Lorenzo Aguilar in his 1984 SIGCOMM paper [AGUI] proposed the use of
   an explicit list of destinations discussed in more detail below.  At
   about the same time, David Cheriton and Stephen Deering developed
   Host Group Multicast in 1985 [CHER].

   The Internet community compared the two proposals and concluded that
   a single mechanism was preferable to multiple mechanisms.  Further,
   since Aguilar's proposal seemed to have serious scaling problems, the
   Host Group model was adopted.

   However, for reasons described below, we believe it makes sense to
   use different mechanisms for the two different kinds of multicast
   discussed above.  While Host Group multicast may have been sufficient
   in the Internet of 1985, we believe that Xcast can be an important
   complement to Host Group multicast in the Internet of the 21st
   century.

2.  Xcast Overview

   In this document, the following terminology will be used:

   - Session: in Xcast, the term 'multicast session' will be used
     instead of 'multicast group' to avoid the strong association of
     multicast groups with multicast group addresses in traditional IP
     multicast.

   - Channel: in a session with multiple senders (e.g., a video
     conference), the flow sourced by one sender will be called a
     channel.  So, a session can contain one or more channels.

   In the Host Group Model, the packet carries a multicast address as a
   logical identifier of all group members.  In Xcast, the source node
   keeps track of the destinations in the multicast channel that it
   wants to send packets to.

   The source encodes the list of destinations in the Xcast header, and
   then sends the packet to a router.  Each router along the way parses
   the header, partitions the destinations based on each destination's
   next hop, and forwards a packet with an appropriate Xcast header to
   each of the next hops.

   When there is only one destination left, the Xcast packet can be
   converted into a normal unicast packet, which can be unicasted along
   the remainder of the route.  This is called X2U (Xcast to Unicast).

   For example, suppose that A is trying to get packets distributed to
   B, C, and D in Figure 1 below:

                                   R4 ---- B
                                  /
                                 /
        A----- R1 ---- R2 ---- R3                      R8 ---- C
                                 \                    /
                                  \                  /
                                   R5 ---- R6 ---- R7
                                                    \
                                                     \
                                                       R9 ---- D

                                 Figure 1

   This is accomplished as follows: A sends an Xcast packet with the
   list of destinations in its Xcast header to the first router, R1.

   Since the Xcast header will be slightly different for IPv4 and IPv6
   [2460], we won't reveal any details on the encoding of the Xcast
   header in this section (see Section 9).  So, ignoring the details,
   the packet that A sends to R1 looks like this:

       [ src = A | dest = B C D | payload ]

   When R1 receives this packet, it needs to properly process the Xcast
   header.  The processing that a router does on receiving one of these
   Xcast packets is as follows:

   - Perform a route table lookup to determine the next hop for each of
     the destinations listed in the packet.

   - Partition the set of destinations based on their next hops.

   - Replicate the packet so that there's one copy of the packet for
     each of the next hops found in the previous steps.

   - Modify the list of destinations in each of the copies so that the
     list in the copy for a given next hop includes just the
     destinations that ought to be routed through that next hop.

   - Send the modified copies of the packet on to the next hops.

   - Optimization: If there is only one destination for a particular
     next hop, the packet can be sent as a standard unicast packet to
     the destination (X2U).

   So, in the example above, R1 will send a single packet on to R2 with
   a destination list of < B C D >, and R2 will send a single packet to
   R3 with the same destination list.

   When R3 receives the packet, it will, by the algorithm above, send
   one copy of the packet to next hop R5 with an Xcast list of < C D >,
   and one ordinary unicast packet addressed to < B > to R4.  R4 will
   receive a standard unicast packet and forward it on to < B >.  R5
   will forward the Xcast packet that it receives on to R6, which will
   pass it on to R7.  When the packet reaches R7, R7 will transmit
   ordinary unicast packets addressed to < C > and < D >, respectively.
   R8 and R9 will receive standard unicast packets, and forward the
   packets on to < C > and < D >, respectively.

   It's important that the Xcast packet that is sent to a given next hop
   only includes destinations for which that next hop is the next hop
   listed in the route table.  If the list of destinations in the packet
   sent to R4, for example, had also included C and D, R4 would send
   duplicate packets.

   Note that when routing topology changes, the routing for an Xcast
   channel will automatically adapt to the new topology since the path
   an Xcast packet takes to a given destination always follows the
   ordinary, unicast routing for that destination.

3.  The Cost of the Traditional IP Multicast Schemes

   Traditional IP multicast schemes [DEER, DEE2, FARI] were designed to
   handle very large multicast groups.  These work well if one is trying
   to distribute broadcast-like channels all around the world but they
   have scalability problems when there is a very large number of
   groups.

   The characteristics of the traditional IP multicast model are
   determined by its two components: the Host Group model [DEER] and a
   Multicast Routing Protocol.  Both components make multicast very
   different from unicast.

   In the Host Group model, a group of hosts is identified by a
   multicast group address, which is used both for subscriptions and
   forwarding.  This model has two main costs:

      - Multicast address allocation: The creator of a multicast group
        must allocate a multicast address that is unique in its scope
        (scope will often be global).  This issue is being addressed by
        the MALLOC working group, which is proposing a set of Multicast
        Address Allocation Servers (MAAS) and three protocols (Multicast
        Address Set Claim (MASC), Address Allocation Protocol (AAP),
        Multicast Address Dynamic Client Allocation Protocol (MADCAP)).

      - Destination unawareness: When a multicast packet arrives in a
        router, the router can determine the next hops for the packet,
        but knows nothing about the ultimate destinations of the packet,
        nor about how many times the packet will be duplicated later on
        in the network.  This complicates the security, accounting and
        policy functions.

   In addition to the Host Group model, a routing algorithm is required
   to maintain the member state and the delivery tree.  This can be done
   using a (truncated) broadcast algorithm or a multicast algorithm
   [DEER].  Since the former consumes too much bandwidth by
   unnecessarily forwarding packets to some routers, only the multicast
   algorithms are considered.  These multicast routing protocols have
   the following costs:

      - Connection state: The multicast routing protocols exchange
        messages that create state for each (source, multicast group) in
        all the routers that are part of the point-to-multipoint tree.
        This can be viewed as "per flow" signaling that creates
        multicast connection state, possibly yielding huge multicast
        forwarding tables.  Some of these schemes even disseminate this
        multicast routing information to places where it isn't
        necessarily needed [1075].  Other schemes try to limit the
        amount of multicast routing information that needs to be
        disseminated, processed, and stored throughout the network.
        These schemes (e.g., [2201]) use a "shared distribution tree"
        that is shared by all the members of a multicast group and they
        try to limit the distribution of multicast routing information
        to just those nodes that "really need it".  But these schemes
        also have problems.  Because of the shared tree, they use less
        than optimal paths in routing packets to their destinations and
        they tend to concentrate traffic in small portions of a network.
        And these schemes still involve lots of "per flow" signaling and
        "per flow" state.

      - Source advertisement mechanism: Multicast routing protocols
        provide a mechanism by which members get 'connected' to the
        sources for a certain group without knowing the sources
        themselves.  In sparse-mode protocols [2201, DEE2], this is
        achieved by having a core node, which needs to be advertised in
        the complete domain.  On the other hand, in dense-mode protocols
        [1075] this is achieved by a "flood and prune" mechanism.  Both
        approaches raise additional scalability issues.

      - Inter-domain routing: Multicast routing protocols that rely on a
        core node [2201, DEE2] additionally need an inter-domain
        multicast routing protocol (e.g., [FARI]).

   The cost of multicast address allocation, destination unawareness and
   the above scalability issues lead to a search for other multicast
   schemes.  Source-Specific Multicast (SSM) [4607] addresses some of
   the above drawbacks: in SSM, a host joins a specific source, thus the
   channel is identified by the couple (source address, multicast
   address).  This approach avoids multicast address allocation as well
   as the need for an inter-domain routing protocol.  The source
   advertisement is taken out of the multicast routing protocol and is
   moved to an out-of-band mechanism (e.g., web page).

   Note that SSM still creates state and signaling per multicast channel
   in each on-tree node.  Figure 2 depicts the above costs as a function
   of the number of members in the session or channel.  All the costs
   have a hyperbolic behavior.

         cost of the traditional
           IP multicast model
               per member
                    ^
                    | costly|  OK
                    | <-----|----->
                    |  .    |
                    |   ..  |
                    |     ..|..
                    |       |  .........
                    |       |           ........
                    +--------------------------->
                        |                 number of members
                        v
                 alternative=Xcast

                                 Figure 2

   The traditional IP multicast model becomes expensive for its members
   if the groups are small.  Small groups are typical for conferencing,
   gaming, and collaborative applications.  These applications are well-
   served by Xcast.

   In practice, traditional IP multicast routing protocols impose
   limitations on the number of groups and the size of the network in
   which they are deployed.  For Xcast, these limitations do not exist.

4.  Motivation

   Xcast takes advantage of one of the fundamental tenets of the
   Internet "philosophy", namely, that one should move complexity to the
   edges of the network and keep the middle of the network simple.  This
   is the principle that guided the design of IP and TCP and it's the
   principle that has made the incredible growth of the Internet
   possible.  For example, one reason that the Internet has been able to
   scale so well is that the routers in the core of the network deal
   with large Classless Inter-Domain Routing (CIDR) blocks as opposed to
   individual hosts or individual "connections".  The routers in the
   core don't need to keep track of the individual TCP connections that
   are passing through them.  Similarly, the IETF's Diffserv effort is
   based on the idea that the routers shouldn't have to keep track of a
   large number of individual Resource Reservation Protocol (RSVP) flows
   that might be passing through them.  It's the authors' belief that
   the routers in the core shouldn't have to keep track of a large
   number of individual multicast flows, either.

   Compared to traditional IP multicast, Xcast has the following
   advantages:

   1) Routers do not have to maintain state per session (or per channel)
      [SOLA].  This makes Xcast very scalable in terms of the number of
      sessions that can be supported since the nodes in the network do
      not need to disseminate or store any multicast routing information
      for these sessions.

   2) No multicast address allocation required.

   3) No need for multicast routing protocols (neither intra- nor
      inter-domain).  Xcast packets always take the "right" path as
      determined by the ordinary unicast routing protocols.

   4) No core node, so no single point of failure.  Unlike the shared
      tree schemes, Xcast minimizes network latency and maximizes
      network "efficiency".

   5) Symmetric paths are not required.  Traditional IP multicast
      routing protocols create non-shortest-path trees if paths are not
      symmetric.  (A path between two nodes A and B is symmetric if the
      path is both the shortest path from A to B as well as the shortest
      path from B to A.)  It is expected that an increasing number of
      paths in the Internet will be asymmetric in the future as a result
      of traffic engineering and policy routing, and thus the
      traditional IP multicast schemes will result in an increasing
      amount of suboptimal routing.

   6) Automatic reaction to unicast reroutes.  Xcast will react
      immediately to unicast route changes.  In traditional IP multicast
      routing protocols, a communication between the unicast and the
      multicast routing protocol needs to be established.  In many
      implementations, this is on a polling basis, yielding a slower
      reaction to, e.g., link failures.  It may also take some time for
      traditional IP multicast routing protocols to fix things up if
      there is a large number of groups that need to be fixed.

   7) Easy security and accounting.  In contrast with the Host Group
      Model, in Xcast all the sources know the members of the multicast
      channel, which gives the sources the means to, e.g., reject
      certain members or count the traffic going to certain members
      quite easily.  Not only a source, but also a border router is able
      to determine how many times a packet will be duplicated in its
      domain.  It also becomes easier to restrict the number of senders
      or the bandwidth per sender.

   8) Heterogeneous receivers.  Besides the list of destinations, the
      packet could (optionally) also contain a list of Diffserv Code
      Points (DSCPs).  While traditional IP multicast protocols have to
      create separate groups for each service class, Xcast incorporates
      the possibility of having receivers with different service
      requirements within one multicast channel.

   9) Xcast packets can make use of traffic-engineered unicast paths.

   10) Simple implementation of reliable protocols on top of Xcast,
       because Xcast can easily address a subset of the original list of
       destinations to do a retransmission.

   11) Flexibility (see Section 6).

   12) Easy transition mechanisms (see Section 11).

   It should be noted that Xcast has a number of disadvantages as well:

   1) Overhead.  Each packet contains all remaining destinations.  But,
      the total amount of data is still much less than for unicast
      (payload is only sent once).  A method to compress the list of
      destination addresses might be useful.

   2) More complex header processing.  Each destination in the packet
      needs a routing table lookup.  So, an Xcast packet with n
      destinations requires the same number of routing table lookups as
      n unicast headers.  Additionally, a different header has to be
      constructed per next hop.  Note however that:

      a) Since Xcast will typically be used for super-sparse sessions,
         there will be a limited number of branching points, compared to
         non-branching points.  Only in a branching point do new headers
         need to be constructed.

      b) The header construction can be reduced to a very simple
         operation: overwriting a bitmap.

      c) Among the non-branching points, a lot of them will contain only
         one destination.  In these cases, normal unicast forwarding can
         be applied.

      d) By using a hierarchical encoding of the list of destinations in
         combination with the aggregation in the forwarding tables the
         forwarding can be accelerated [OOMS].

      e) When the packet enters a region of the network where link
         bandwidth is not an issue anymore, the packet can be
         transformed by a Premature X2U.  Premature X2U (see Section
         11.2) occurs when a router decides to transform the Xcast
         packet for one or more destinations into unicast packets.  This
         avoids more complex processing downstream.

      f) Other mechanisms to reduce the processing have been described
         in [IMAI] (tractable list) and [OOMS] (caching), but are not
         (yet) part of the Xcast specification.

   3) Xcast only works with a limited number of receivers.

5.  Application

   While Xcast is not suitable for multicast sessions with a large
   number of members, such as the broadcast of an IETF meeting, it does
   provide an important complement to existing multicast schemes in that
   it can support very large numbers of small sessions.  Thus, Xcast
   enables important applications such as IP telephony,
   videoconferencing, multi-player games, collaborative e-meetings, etc.
   The number of these sessions will become huge.

   Some may argue that it is not worthwhile to use multicast for
   sessions with a limited number of members, and that it's preferable
   to use unicast instead.  But in certain cases, limited bandwidth in
   the "last mile" makes it important to have some form of multicast, as
   the following example illustrates.  Assume n residential users set up
   a video conference.  Typically, access technologies are asymmetric
   (e.g., xDSL, General Packet Radio Service (GPRS), or cable modem).
   So, a host with xDSL has no problem receiving n-1 basic 100 kb/s

   video channels, but the host is not able to send its own video data
   n-1 times at this rate.  Because of the limited and often asymmetric
   access capacity, some type of multicast is mandatory.

   A simple but important application of Xcast lies in bridging the
   access link.  The host sends the Xcast packet with the list of
   unicast addresses and the first router performs a Premature X2U.

   Since Xcast is not suitable for large groups, Xcast will not replace
   the traditional IP multicast model, but it does offer an alternative
   for multipoint-to-multipoint communications when there can be very
   large numbers of small sessions.

6.  Xcast Flexibility

   The main goal of multicast is to avoid duplicate information flowing
   over the same link.  By using traditional IP multicast instead of
   unicast, bandwidth consumption decreases while the state and
   signaling per session increases.  Xcast has a cost of 0 in these two
   dimensions, but it does introduce a third dimension corresponding to
   the header processing per packet.  This three-dimensional space is
   depicted in Figure 3.

           state&signaling
             per session
              in router
                  ^
                  |
                  |
                 ....
                B |  ....
                . |      ....
               .  |          ....
              .   |              ....
             .    +------------------..---> processing
            .    /               .... C     per packet
           .   /            .....           in router
          .  /         .....
         . /      .....
        ./   .....
       /A....
     /
   /
  link bandwidth

                                 Figure 3

   One method of delivering identical information from a source to n
   destinations is to unicast the information n times (A in Figure 3).
   A second method, the traditional IP multicast model (B in Figure 3),
   sends the information only once to a multicast address.  In Xcast,
   the information is sent only once, but the packet contains a list of
   destinations (point C).

   The three points A, B, and C define a plane (indicated with dots in
   Figure 3): a plane of conservation of misery.  All three approaches
   have disadvantages.  The link bandwidth is a scarce resource,
   especially in access networks.  State&signaling/session encounters
   limitations when the number of sessions becomes large, and an
   increased processing/packet is cumbersome for high-link speeds.

   One advantage of Xcast is that it allows a router to move within this
   plane of conservation of misery based upon its location in a network.
   For example, in the core of the network, a cache could be used to
   move along the line from C to B without introducing any per-flow
   signaling.  Another possibility, as suggested above, is to use
   premature X2U to move along the line from C to A in an access network
   if there is an abundance of bandwidth in the backbone.

7.  Xcast Control Plane Options

   Unlike traditional IP multicast schemes, Xcast does not specify a
   "control plane".  There is no Internet Group Management Protocol
   (IGMP [3376]), and as mentioned above, there are no intra- or inter-
   domain multicast routing protocols.  With Xcast, the means by which
   multicast sessions are defined is an application-level issue and
   applications are not confined to the model in which hosts use IGMP to
   join a multicast session.  For example:

   - Some applications might want to use an IGMP-like receiver-join
     model.

   - Other applications might want to use a model in which a user places
     a call to the party or parties that he or she wants to talk to
     (similar to the way that one puts together a conference call today
     using the buttons on one's telephone).

   - One might define a session based on the cells that are close to a
     moving device in order to provide for a "smooth handoff" between
     cells when the moving device crosses cell boundaries.

   - In some applications, the members of the session might be specified
     as arguments on a command line.

   - One might define an application that uses GPS to send video from a
     bank robbery to the three police cars that are closest to the bank
     being robbed.

   Thus, the application developer is not limited to the receiver-
   initiated joins of the IGMP model.  There will be multiple ways in
   which an Xcast sender determines the addresses of the members of the
   channel.

   For the purpose of establishing voice and multimedia conferences over
   IP networks, several control planes have already been defined,
   including SIP [3261] and H.323 [H323].

7.1.  SIP Control Plane for Xcast

   In SIP, a host takes the initiative to set up a session.  With the
   assistance of a SIP server, a session is created.  The session state
   is kept in the hosts.  Data delivery can be achieved by several
   mechanisms: meshed unicast, bridged, or multicast.  Note that for the
   establishment of multicast delivery, a multicast protocol and
   communication with Multicast Address Allocation Servers (MAAS) are
   still required.

   In "meshed unicast" or "multi-unicasting", the application keeps
   track of the participants' unicast addresses and sends a unicast to
   each of those addresses.  For reasons described in Section 3, multi-
   unicasting (rather than multicast) is the prevalent solution in use
   today.  It's a simple matter to replace multi-unicast code with Xcast
   code.  All that the developer has to do is replace a loop that sends
   a unicast to each of the participants by a single "xcast_send" that
   sends the data to the participants.  Thus it's easy to incorporate
   Xcast into real conferencing applications.

   Both Xcast and SIP address super-sparse multicast sessions.  It turns
   out that Xcast (a very flexible data plane mechanism) can be easily
   integrated with SIP (a very flexible control plane protocol).  When
   an application decides to use Xcast forwarding it does not affect its
   interface to the SIP agent: it can use the same SIP messages as it
   would for multi-unicasting.  SIP could be used with Xcast to support
   the conferencing model mentioned above in which a caller places a
   call to several parties.

7.2.  Receiver-Initiated Join for Xcast

   In the previous section, it was discussed how to establish an Xcast
   session among well known participants of a multi-party conference.
   In some cases, it is useful for participants to be able to join a
   session without being invited.  For example, the chairman of a video

   chat may want to leave the door of their meeting open for newcomers.
   The IGMP-like receiver-initiated join model mentioned above can be
   implemented by introducing a server that hosts can talk to, to join a
   conference.

8.  Optional Information

8.1.  List of Ports

   Although an extension to SIP could be arranged such that all
   participants in a session use the same transport (UDP) port number,
   in the general case, it is possible for each participant to listen on
   a different port number.  To cover this case, the Xcast packet
   optionally contains a list of port numbers.

   If the list of port numbers is present, the destination port number
   in the transport-layer header will be set to zero.  On X2U, the
   destination port number in the transport-layer header will be set to
   the port number corresponding to the destination of the unicast
   packet.

8.2.  List of DSCPs

   The Xcast packet could (optionally) also contain a list of Diffserv
   Code Points (DSCPs).  While traditional IP multicast protocols have
   to create separate groups for each service class, Xcast incorporates
   the possibility of having receivers with different service
   requirements within one channel.

   The DSCP in the IP header will be set to the most demanding DSCP of
   the list of DSCPs.  This DSCP in the IP header will determine, e.g.,
   the scheduler to use.

   If two destinations, with the same next-hop, have 'non-mergeable'
   DSCPs, two Xcast packets will be created.  'Non-mergeable' meaning
   that one cannot say that one is more or less stringent than the
   other.

8.3.  Channel Identifier

   Optionally, a sender can decide to add an extra number in the Xcast
   header: the Channel Identifier.  If the source does not want to use
   this option, it must set the Channel Identifier to zero.  If the
   Channel Identifier is non-zero, the pair (Source Address, Channel
   Identifier) must uniquely identify the channel (note that this is
   similar to the (S, G) pair in SSM).  This document does not assign
   any other semantics to the Channel Identifier besides the one above.

   This Channel Identifier could be useful for several purposes:

   1) A key to a caching table [OOMS].

   2) "Harmonization" when used with Host Group Multicast  (to be
      discussed in greater detail in another document).

   3) An identifier of the channel in error, flow control, etc.,
      messages.

   4) It gives an extra demultiplexing possibility (beside the port-
      number).

   5) ...

   The size of the channel identifier and its semantics are TBD.

9.  Possible Xcast Packet Encoding

9.1.  General

   The source address field of the IP header contains the address of the
   Xcast sender.  The destination address field carries the All-Xcast-
   Routers address (to be assigned link-local multicast address); this
   is to have a fixed value.  Every Xcast router joins this multicast
   group.  The reasons for putting a fixed number in the destination
   field are:

   1) The destination address field is part of the IP pseudo header and
      the latter is covered by transport layer checksums (e.g., UDP
      checksum).  So, the fixed value avoids a (delta) recalculation of
      the checksum.

   2) The IPsec Authentication Header (AH) [4302] covers the IP header
      destination address, hence preventing any modification to that
      field.  Also, both AHs and Encapsulating Security Payloads (ESPs)
      cover the whole UDP packet (via authentication and/or encryption).
      The UDP checksum cannot therefore be updated if the IP header
      destination address were to change.

   3) In Xcast for IPv6, the Routing Extension shall be used; this
      header extension is only checked by a router if the packet is
      destined to this router.  This is achieved by making all Xcast
      routers part of the All_Xcast_Routers group.

   4) Normally Xcast packets are only visible to Xcast routers.
      However, if a non-Xcast router receives an Xcast packet by
      accident (or by criminal intent), it will not send ICMP errors
      since the Xcast packet carries a multicast address in the
      destination address field [1812].

   Note that some benefits only hold when the multicast address stays in
   the destination field until it reaches the end-node (thus not
   combinable with X2U).

9.2.  IPv4

   [AGUI] and [1770] proposed (for a slightly different purpose) to
   carry multiple destinations in the IPv4 option.  But because of the
   limited flexibility (limited size of the header), Xcast will follow
   another approach.  The list of destinations will be encoded in a
   separate header.  The Xcast header for IPv4 (in short, Xcast4) would
   be carried between the IPv4 header and the transport-layer header.

         [IPv4 header | Xcast4 | transport header | payload ]

   Note also that since the Xcast header is added to the data portion of
   the packet, if the sender wishes to avoid IP fragmentation, it must
   take the size of the Xcast header into account.

9.2.1.  IPv4 Header

   The Xcast4 header is carried on top of an IP header.  The IP header
   will carry the protocol number listed as usable for experimental
   purposes in RFC 4727 [4727].  See also Section 15.  The source
   address field contains the address of the Xcast sender.  The
   destination address field carries the All_Xcast_Routers address.

9.2.2.  Xcast4 Header

   The Xcast4 header is format depicted in Figure 4.  It is composed of
   two parts: a fixed part (first 12 octets) and two variable-length
   parts that are specified by the fixed part.

     0               1               2               3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |VERSION|A|X|D|P|R| NBR_OF_DEST |          CHECKSUM             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       CHANNEL IDENTIFIER                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    PROT ID    |    LENGTH     |             RESV              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   List of Addresses and DSCPs                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 List of Port Numbers (optional)               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 4

   VERSION = Xcast version number.  This document describes version 1.

   A = Anonymity bit: if this bit is set, the destination addresses for
   which the corresponding bit in the bitmap is zero must be overwritten
   by zero.

   X = Xcast bit: if this bit is set, a router must not reduce the Xcast
   packet to unicast packet(s), i.e., the packet must stay an Xcast
   packet end-to-end.  This bit can be useful when IPsec [4301] is
   applied.  If this bit is cleared a router should apply X2U if there
   is only one destination left in the Xcast packet.  In some cases a
   router could decide not to apply X2U to a packet with the Xcast bit
   cleared, e.g., the router has no directly connected hosts and wants
   to avoid the extra processing required by X2U.

   D = DSCP bit: if this bit is set, the packet will contain a DS byte
   for each destination.

   P = Port bit: if this bit is set, the packet will contain a port
   number for each destination.

   NBR_OF_DEST = the number of original destinations.

   CHECKSUM = A checksum on the Xcast header only.  This is verified and
   recomputed at each point that the Xcast header is processed.  The
   checksum field is the 16-bit one's complement of the one's complement
   sum of all the bytes in the header.  For purposes of computing the
   checksum, the value of the checksum field is zero.  It is not clear
   yet whether a checksum is needed (for further study).  If only one
   destination is wrong it can still be useful to forward the packet to
   N-1 correct destinations and 1 incorrect destination.

   CHANNEL IDENTIFIER = 4-octet Channel Identifier (see Section 8.3).
   Since it is located within the first 8 bytes of the header, it will
   be returned in ICMP messages.

   PROT ID = specifies the protocol of the following header.

   LENGTH = length of the Xcast header in 4-octet words.  This field
   puts an upper boundary to the number of destinations.  This value is
   also determined by the NBR_OF_DEST field and the D and P bits.

   RESV = R = Reserved.  It must be zero on transmission and must be
   ignored on receipt.

   The first variable part is the 'List of Addresses and DSCPs', the
   second variable part is the 'List of Port Numbers'.  Both are 4-octet
   aligned.  The second variable part is only present if the P-bit is
   set.

   Figure 5 gives an example of the variable part for the case that the
   P-bit is set and the D-bit is cleared (in this example, N is odd):

     0               1               2               3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            BITMAP                             |
     ~                                                               ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Destination 1                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                              ...                              ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Destination N                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Port 1            |         Port 2                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                              ...                              ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Port N            |         padding               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 5

   BITMAP = every destination has a corresponding bit in the bitmap to
   indicate whether the destination is still valid on this branch of the
   tree.  The first bit corresponds to the first destination in the
   list.  This field is 4-octet aligned (e.g., for 49 destinations,
   there will be a 64-bit bitmap).  If Xcast is applied in combination
   with IPsec, the bitmap -- since it can change en route -- has to be
   moved to a new to-be-defined IPv4 option.

   List of Destinations.  Each address size is 4 octets.

   List of Port Numbers.  List of 2-octet destination port number(s),
   where each port corresponds in placement to the preceding Destination
   Address.

9.3.  IPv6

   The Xcast6 header encoding is similar to IPv4, except that Xcast
   information would be stored in IPv6 extension headers.

         [IPv6 header | Xcast6 | transport header | payload ]

9.3.1.  IPv6 Header

   The IPv6 header will carry the NextHeader value 'Routing Extension'.
   The source address field contains the address of the Xcast sender.
   The destination address field carries the All_Xcast_Routers address.

9.3.2.  Xcast6 Header

   The Xcast6 header is also composed of a fixed part and two variable
   parts.  The fixed part and the first variable part are carried in a
   Routing Extension.  The second variable part is carried in a
   Destination Extension.

9.3.2.1.  Routing Extension Header

   The P-bit of Xcast4 is not present because it is implicit by the
   presence or absence of the Destination Extension (Figure 6).

     0               1               2               3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Next Header  |  HdrExtLen    |RouteType=Xcast|       0       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |VERSION|A|X|D| R | NBR_OF_DEST |          CHECKSUM             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       CHANNEL IDENTIFIER                      |
     ~                                                               ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              List of Addresses and DSCPs                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 6

   HdrExtLen = The header length is expressed in 8-octets; thus, a
   maximum of 127 destinations can be listed (this is why NBR_OF_DEST is
   7 bits).

   RouteType = Xcast (see Section 15)

   The fourth octet is set to 0.

   R = Reserved.

   CHANNEL IDENTIFIER = 16-octet Channel Identifier (see Section 8.3).

   The other fields are defined in Section 9.2.2.

   The 'List of Addresses and DSCPs' is 8-octet aligned.  The size of
   the bitmap is determined by the number of destinations and is a
   multiple of 64 bits.

9.3.2.2.  Destination Extension Header

   Optionally, the Destination Extension (Figure 7) is present to
   specify the list of Port Numbers.  The destination header is only
   evaluated by the destination node.

     0               1               2               3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Next Header  |  HdrExtLen    |Opt Type=Ports | Opt Data Len  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     List of Port Numbers                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 7

   For the Option Type for Ports, see Section 15.  The first three bits
   must be 010 to indicate that the packet must be discarded if the
   option is unknown and that the option cannot be changed en-route.

   The number of Ports must be equal to the number of destinations
   specified in the Routing header.

10.  Impact on Upper-Layer Protocols

   Some fields in the Xcast header(s) can be modified as the packet
   travels along its delivery path.  This has an impact on:

10.1.  Checksum Calculation in Transport-Layer Headers

   In transport-layer headers, the target of the checksum calculation
   includes the IP pseudo header, transport header, and payload (IPv6
   header extensions are not a target).

   The transformation of an Xcast packet to a normal unicast packet --
   (premature) X2U -- replaces the multicast address in the IP header
   destination field by the address of a final destination.  If the
   Xcast header contains a Port List, the port number in the transport
   layer (which should be zero) also needs to be replaced by the port
   number corresponding to the destination.  This requires a
   recalculation of these checksums.  Note that this does not require a
   complete recalculation of the checksum, only a delta calculation,
   e.g., for IPv4:

     Checksum' = ~ (~Checksum + ~daH + ~daL + daH' + daL' + ~dp + dp')

   In which "'" indicates the new values, "da" the destination address,
   "dp" the destination port, and "H" and "L" the higher and lower 16
   bits, respectively.

10.2.  IPsec

   This is described in [PARI].

11.  Gradual Deployment

11.1.  Tunneling

   One way to deploy Xcast in a network that has routers that have no
   knowledge of Xcast is to setup "tunnels" between Xcast peers (MBone
   approach [MBONE]).  This enables the creation of a virtual network
   layered on top of an existing network [2003].  The Xcast routers
   exchange and maintain Xcast routing information via any standard
   unicast routing protocol (e.g., RIP, OSPF, IS-IS, BGP).  The Xcast
   routing table that is created is simply a standard unicast routing
   table that contains the destinations that have Xcast connectivity,
   along with their corresponding Xcast next hops.  In this way, packets
   may be forwarded hop-by-hop to other Xcast routers, or may be
   "tunneled" through non- Xcast routers in the network.

   For example, suppose that A is trying to get packets distributed to
   B, C, and D in Figure 8 below, where "X" routers are Xcast-capable,
   and "R" routers are not.  Figure 9 shows the routing tables created
   via the Xcast tunnels:

                                   R4 ---- B
                                  /
                                 /
       A ----- X1 ---- R2 ---- X3                      R8 ---- C
                                 \                    /
                                  \                  /
                                   R5 ---- R6 ---- X7
                                                    \
                                                     \
                                                       R9 ---- D

                                 Figure 8

   Router X1 establishes a tunnel to Xcast peer X3.  Router X3
   establishes a tunnel to Xcast peers X1 and X7.  Router X7 establishes
   a tunnel to Xcast peer X3.

      X1 routing table:     X3 routing table:     X7 routing table:
       Dest |  NextHop       Dest | NextHop        Dest | NextHop
      ------+----------     ------+---------      ------+---------
        B   |   X3             A  |   X1            A   |  X3
        C   |   X3             C  |   X7            B   |  X3
        D   |   X3             D  |   X7

                                 Figure 9

   The source A will send an Xcast packet to its default Xcast router,
   X1, that includes the list of destinations for the packet.  The
   packet on the link between X1 and X3 is depicted in Figure 10:

                              +----------+
                              | payload  |
                              +----------+
                              |   UDP    |
                              +----------+
                              |  Xcast   |
                              |  B,C,D   |
                              | prot=UDP |
                              +----------+
                              | inner IP |
                              |  src=A   |
                              |dst=All_X_|
                              |prot=Xcast|
                              +----------+
                              | outer IP |
                              |  src=X1  |
                              |  dst=X3  |
                              | prot=IP  |
                              +----------+

                               Figure 10

   When X3 receives this packet, it processes it as follows:

   - Perform a route table lookup in the Xcast routing table to
     determine the Xcast next hop for each of the destinations listed in
     the packet.

   - If no Xcast next hop is found, replicate the packet and send a
     standard unicast to the destination.

   - For those destinations for which an Xcast next hop is found,
     partition the destinations based on their next hops.

   - Replicate the packet so that there's one copy of the packet for
     each of the Xcast next hops found in the previous steps.

   - Modify the list of destinations in each of the copies so that the
     list in the copy for a given next hop includes just the
     destinations that ought to be routed through that next hop.

   - Send the modified copies of the packet on to the next hops.

   - Optimization: If there is only one destination for a particular
     Xcast next hop, send the packet as a standard unicast packet to the
     destination, since there is no advantage to forwarding it as an
     Xcast packet.

   So, in the example above, X1 will send a single packet on to X3 with
   a destination list of < B C D >.  This packet will be received by R2
   as a unicast packet with destination X3, and R2 will forward it on,
   having no knowledge of Xcast.  When X3 receives the packet, it will,
   by the algorithm above, send one copy of the packet to destination
   < B > as an ordinary unicast packet, and 1 copy of the packet to X7
   with a destination list of < C D >.  R4, R5, and R6 will behave as
   standard routers with no knowledge of Xcast.  When X7 receives the
   packet, it will parse the packet and transmit ordinary unicast
   packets addressed to < C > and < D >, respectively.

   The updating of this route table, while simple in an intra-domain
   environment, would be more complex in an inter-domain environment.
   Thus, the use of tunneling in an inter-domain environment requires
   further consideration.

11.2.  Premature X2U

   If a router discovers that its downstream neighbor is not Xcast
   capable, it can perform a Premature X2U, i.e., send a unicast packet
   for each destination in the Xcast header that has this neighbor as a
   next hop.  Thus, duplication is done before the Xcast packet reached
   its actual branching point.

   A mechanism (protocol/protocol extension) to discover the Xcast
   capability of a neighbor is for further study.  Among others, one
   could think of an extension to a routing protocol to advertise Xcast
   capabilities, or one could send periodic 'Xcast pings' to its
   neighbors (send an Xcast packet that contains its own address as a
   destination and check whether the packet returns).

11.3.  Semi-Permeable Tunneling (IPv6 Only)

   This is an optimization of tunneling in the sense that it does not
   require (manual) configuration of tunnels.  It is enabled by adding a
   Hop-by-Hop Xcast6 header.  An IPv6 packet can initiate/trigger
   additional processing in the on-route routers by using the IPv6 Hop-
   by-hop option.

   The type of the Xcast6 Hop-by-hop option has a prefix '00' so that
   routers that cannot recognize Xcast6 can treat the Xcast6 datagram as
   a normal IPv6 datagram and forward it toward the destination in the
   IPv6 header.

   Packets will be delivered to all members if at least all
   participating hosts are upgraded.

   When the source A sends an Xcast packet via semi-permeable tunneling
   to destinations B, C, and D, it will create the packet of Figure 11.
   One of the final destinations will be put in the destination address
   field of the outer IP header.

                              +----------+
                              | payload  |
                              +----------+
                              |   UDP    |
                              +----------+
                              |  Xcast   |
                              |          |
                              +----------+
                              | inner IP |
                              |  src=A   |
                              |dst=All_X_|
                              |prot=Xcast|
                              +----------+
                              |  Xcast   |
                              |SP-tunnel |
                              |Hop-by-hop|
                              +----------+
                              | outer IP |
                              |  src=A   |
                              |  dst=B   |
                              | prot=IP  |
                              +----------+

                               Figure 11

   Semi-permeable tunneling is a special tunneling technology that
   permits intermediate Xcast routers on a tunnel to check the
   destinations and branch if destinations have a different next hop.

   Note that with the introduction of an Xcast IPv4 option, this
   technique could also be applied in IPv4 networks.

11.4.  Special Case: Deployment without Network Support

   A special method of deploying Xcast is possible by upgrading only the
   hosts.  By applying tunneling (see Sections 11.1 and 11.3) with one
   of the final destinations as a tunnel endpoint, the Xcast packet will
   be delivered to all destinations when all the hosts are Xcast aware.
   Both normal and semi-permeable tunneling can be used.

   If host B receives this packet, in the above example, it will notice
   the other destinations in the Xcast header.  B will create a new
   Xcast packet and will send it to one of the remaining destinations.

   In the case of Xcast6 and semi-permeable tunneling, Xcast routers can
   be introduced in the network without the need of configuring tunnels.

   The disadvantages of this method are:

   - all hosts in the session need to be upgraded.

   - non-optimal routing.

   - anonymity issue: hosts can know the identity of other parties in
   the session (which is not a big issue in conferencing, but maybe for
   some other application).

   - host has to perform network functions and needs an upstream link
   which has the same bandwidth as its downstream link.

11.5.  Using a Small Number of Xcast-Aware Routers to Provide Xcast
       in a Not-So-Small Network

   In this approach, an Xcast packet uses a special 32-bit unicast
   address in the destination field of the IP header.  In the simplest
   version of this scheme, there might be only a single Xcast-aware
   router in a network.  This Xcast-aware router looks like a "server"
   to the other routers and it is configured so that its IP address (or
   one of its IP addresses) corresponds to the "special" 32-bit address.
   Thus, when Xcast clients send Xcast packets, the non-Xcast-aware
   routers will route these packets to the Xcast-aware router and the
   Xcast-aware router can "explode" (X2U) them into an appropriate set
   of unicast packets.  This allows clients anywhere in a network to use
   Xcast to overcome the problem of limited bandwidth in the "first
   mile" with a minimum number of Xcast-aware routers (i.e., 1).

   Another possibility is to deploy a few of these Xcast-aware routers
   at various points in the network and to configure each of these with
   the special 32-bit address.  This provides redundancy, eliminating
   the single point of failure, and reduces the distance an Xcast packet
   needs to travel to reach an Xcast-aware router, reducing network
   latencies.  In this case, the Xcast-aware routers appear to be a
   single server that is "multihomed" (i.e., connected to the network at
   more than one place) and the non-Xcast-aware routers will, via
   ordinary unicast routing, deliver packets that are addressed to this
   "multihomed virtual server" via the shortest available path.

   Note that this scheme of delivering packets to any host in a group is
   also known as an "anycast" and is described in more detail in RFCs
   [1546], [2526], and [3068].  Note too that RFC 1546 says:

         The important observation is that multiple routes to an anycast
         address appear to a router as multiple routes to a unicast
         destination, and the router can use standard algorithms to
         choose the best route.

12.  (Socket) API

   In the most simple use of Xcast, the final destinations of an Xcast
   packet receive an ordinary unicast UDP packet.  This means that hosts
   can receive an Xcast packet with a standard, unmodified TCP/IP stack.

   Hosts can also transmit Xcast packets with a standard TCP/IP stack
   with a small Xcast library that sends Xcast packets on a raw socket.
   This has been used to implement Xcast-based applications on both Unix
   and Windows platforms without any kernel changes.

   Another possibility is to modify the sockets interface slightly.  For
   example, one might add an "xcast_sendto" function that works like
   "sendto" but that uses a list of destination addresses in place of
   the single address that "sendto" uses.

13.  Unresolved Issues

   Additional work is needed in several areas.

13.1.  The Format of the "List of Addresses"

   Additional details need to be specified.  For example, in the IPv4
   case, the format of the DSCPs option needs to be specified.

13.2.  The Size of Channel Identifier

   The size of the channel identifiers in IPv4 and IPv6 are different in
   this document. 32 bits might be sufficient for both IPv6 and IPv4.

13.3.  Incremental Deployment

   Several possible methods of incremental deployment are discussed in
   this document including tunneling, premature X2U, etc.  Additional
   work is needed to determine the best means of incremental deployment
   for an intra-domain as well as an inter-domain deployment of Xcast.
   If tunneling is used, additional details need to be specified (e.g.,
   tunneling format, use of tunnels in the inter-domain case).

13.4.  DSCP Usage

   DSCP usage needs some work.  DSCPs may have to be rewritten as
   packets cross inter-domain boundaries.

13.5.  Traversing a Firewall or NAT Products

   The usage of a different, carried protocol type for IPv4 may cause
   difficulty in traversing some firewall and NAT products.

13.6.  The Size of BITMAP

   Given that this is designed for small groups, it might make sense to
   simply mandate a fixed size for the bitmap.

14.  Security Considerations

   The list of destinations in Xcast is provided by an application layer
   that manages group membership as well as authorization if
   authorization is desired.

   Since a source has the list of destinations and can make changes to
   the list, it has more control over where its packets go than in
   traditional multicast and can prevent anonymous eavesdroppers from
   joining a multicast session, for example.

   Some forms of denial-of-service attack can use Xcast to increase
   their "effect".  A smurf attack, for example, sends an ICMP Echo
   Request in which the source address in the packet is set to the
   address of the target of the attack so that the target will receive
   the ICMP echo reply.  With Xcast, the ICMP Echo Request could be sent
   to a list of destinations that could cause each member of the list to
   send an Echo Reply to the target.

   Measures have been taken in traditional multicast to avoid this kind
   of attack.  A router or host can be configured so that it will not
   reply to ICMP requests addressed to a multicast address.  The Reverse
   Path Forwarding check in traditional multicast architectures also
   helps limit these attacks.  In Xcast, it can be difficult for a host
   to recognize that an ICMP request has been addressed to multiple
   destinations since the packet may be an ordinary unicast packet by
   the time it reaches the host.  On the other hand, a router can detect
   Xcast packets that are used to send ICMP requests to multiple
   destinations and can be configured to drop those packets.  Note, too,
   that since Xcast sends packets to a short list of destinations, the
   problem of sending attack packets to multiple destination is less of

   a problem than in traditional multicast.  Obviously, the use of IPsec
   to provide confidentiality and/or authentication can further diminish
   the risk of this type of attack.

   The problem of secure group communications has been addressed by the
   Multicast Security (MSEC) working group, which has defined an
   architecture for securing IP-multicast-based group communications
   [3740].  Many of the concepts discussed in the MSEC working group,
   such as managing group membership, identifying and authenticating
   group members, protecting the confidentiality and integrity of
   multicast traffic, and managing and securely distributing and
   refreshing keys, also apply to Xcast-based group communications.  And
   many of the same mechanisms seem to apply.  One significant
   difference between multicast and Xcast is the fact that the Xcast
   header (or at least a bitmap in the Xcast header) needs to change as
   an Xcast packet travels from a source to a destination.  This affects
   the use of IPsec and suggests that at least the Xcast header bitmap
   must be in a "mutable" field.  A complete solution for securing
   Xcast-based group communications addressing all the issues listed
   above will be the subject of additional work which will be discussed
   in one or more additional documents.  We expect that this effort will
   build on the work that has already been done in the msec working
   group.

15.  IANA Considerations

   Experimentation with the Xcast protocol requires the use of protocol
   numbers maintained by IANA.  For example, to implement XCAST6,
   implementations must agree on four protocol numbers:

          (1) Multicast Address for All_Xcast_Routers
          (2) Routing Type of IPv6 Routing Header
          (3) Option Type of IPv6 Destination Option Header
          (4) Option Type of IPv6 Hop-by-Hop Options Header

   A protocol implementer may temporarily experiment with Xcast by using
   the values set aside for experimental use in RFC [4727].  An
   implementer must verify that no other experiment uses the same values
   on the Xcast testbed at the same time.

   A future revision of the Xcast specification published on the
   standards track is required before IANA can assign permanent registry
   entries for Xcast.  Implementers should be aware that they will need
   to modify their implementations when such permanent allocations are
   made.

16.  Informative References

   [1546]  Partridge, C., Mendez, T., and W. Milliken, "Host Anycasting
           Service", RFC 1546, November 1993.

   [2526]  Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
           Addresses", RFC 2526, March 1999.

   [3068]  Huitema, C., "An Anycast Prefix for 6to4 Relay Routers", RFC
           3068, June 2001.

   [1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
           RFC 1112, August 1989.

   [1075]  Waitzman, D., Partridge, C., and S. Deering, "Distance Vector
           Multicast Routing Protocol", RFC 1075, November 1988.

   [1770]  Graff, C., "IPv4 Option for Sender Directed Multi-Destination
           Delivery", RFC 1770, March 1995.

   [1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers", RFC
           1812, June 1995.

   [2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003, October
           1996.

   [2201]  Ballardie, A., "Core Based Trees (CBT) Multicast Routing
           Architecture", RFC 2201, September 1997.

   [2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
           (IPv6) Specification", RFC 2460, December 1998.

   [2902]  Deering, S., Hares, S., Perkins, C., and R. Perlman,
           "Overview of the 1998 IAB Routing Workshop", RFC 2902, August
           2000.

   [3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
           Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
           Session Initiation Protocol", RFC 3261, June 2002.

   [3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
           Thyagarajan, "Internet Group Management Protocol, Version 3",
           RFC 3376, October 2002.

   [3740]  Hardjono, T. and B. Weis, "The Multicast Group Security
           Architecture", RFC 3740, March 2004.

   [4301]  Kent, S. and K. Seo, "Security Architecture for the Internet
           Protocol", RFC 4301, December 2005.

   [4302]  Kent, S., "IP Authentication Header", RFC 4302, December
           2005.

   [4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for IP",
           RFC 4607, August 2006.

   [4727]  Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
           ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006.

   [AGUI]  L. Aguilar, "Datagram Routing for Internet Multicasting",
           SIGCOMM '84, March 1984.

   [CHER]  David R. Cheriton, Stephen E. Deering, "Host groups: a
           multicast extension for datagram internetworks", Proceedings
           of the ninth symposium on Data communications, p. 172-179,
           September 1985, Whistler Moutain, British Columbia, Canada.

   [BOIV]  Boivie, R. and N. Feldman, "Small Group Multicast", Work in
           Progress, February 2001.

   [DEER]  S. Deering, "Multicast Routing in a datagram internetwork",
           PhD thesis, December 1991.

   [DEE2]  S. Deering, D. Estrin, D. Farinacci, V. Jacobson, C. Liu, and
           L.  Wei, "The Pim Architecture for Wide-area Multicast
           Routing", ACM Transactions on Networks, April 1996.

   [FARI]  Farinacci, D., et al., "Multicast Source Discovery Protocol",
           Work in Progress, June 1998.

   [H323]  ITU-T Recommendation H.323 (2000), Packet-Based Multimedia
           Communications Systems.

   [IMAI]  Imai, Y., "Multiple Destination option on IPv6 (MDO6)", Work
           in Progress, September 2000,

   [MBONE] Casner, S., "Frequently Asked Questions (FAQ) on the
           Multicast Backbone (MBONE)",
           <ftp://ftp.isi.edu/mbone/faq.txt>.

   [OOMS]  Ooms, D., Livens, W., and O. Paridaens, "Connectionless
           Multicast", Work in Progress, April 2000.

   [PARI]  Paridaens, O., Ooms, D., and B. Sales, "Security Framework
           for Explicit Multicast", Work in Progress, June 2002.

   [RMT]   Reliable Multicast Transport Working Group web site,
           <http://www.ietf.org/html.charters/rmt-charter.html>, June
           15, 1999.

   [SOLA]  M. Sola, M. Ohta, T. Maeno, "Scalability of Internet
           Multicast Protocols", INET'98,
           <http://www.isoc.org/inet98/proceedings/6d/6d_3.htm>.

17.  Contributors

   Olivier Paridaens
   Alcatel Network Strategy Group
   Fr. Wellesplein 1, 2018
   Antwerpen, Belgium
   Phone: 32 3 2409320
   EMail: Olivier.Paridaens@alcatel.be

   Eiichi Muramoto
   Matsushita Electric Industrial Co., Ltd.
   4-12-4 Higashi-shinagawa, Shinagawa-ku
   Tokyo 140-8587, Japan
   Phone: +81-3-6710-2031
   EMail: muramoto@xcast.jp

Authors' Addresses

   Rick Boivie
   IBM T. J. Watson Research Center
   19 Skyline Drive
   Hawthorne, NY 10532
   Phone: 914-784-3251
   EMail: rhboivie@us.ibm.com

   Nancy Feldman
   IBM T. J. Watson Research Center
   19 Skyline Drive
   Hawthorne, NY 10532
   EMail: nkfeldman@yahoo.com

   Yuji Imai
   Fujitsu Laboratories Ltd.
   1-1, Kamikodanaka 4-Chome, Nakahara-ku
   Kawasaki 211-8588, Japan
   Phone: +81-44-754-2628
   Fax  : +81-44-754-2793
   EMail: ug@xcast.jp

   Wim Livens
   ESCAUX
   Krijtstraat 17, 2600
   Berchem, Belgium
   EMail: wim@livens.net

   Dirk Ooms
   OneSparrow
   Belegstraat 13; 2018
   Antwerp, Belgium
   EMail: dirk@onesparrow.com

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