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rfc:rfc2226

Network Working Group T. Smith Request for Comments: 2226 IBM Corporation Category: Standards Track G. Armitage

                                                   Lucent Technologies
                                                          October 1997
                   IP Broadcast over ATM Networks

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1997).  All Rights Reserved.

Abstract

 This memo describes how the IP multicast service being developed by
 the IP over ATM working group may be used to support IP broadcast
 transmission. The solution revolves around treating the broadcast
 problem as a special case of multicast, where every host in the
 subnet or cluster is a member of the group.
 An understanding of the services provided by RFC 2022 is assumed.

1. Introduction.

 The IETF's first step in solving the problems of running IP over
 Asynchronous Transfer Mode (ATM) technology is described in RFC 1577
 [1].  It provides for unicast communication between hosts and routers
 within Logical IP Subnets (LISs), and proposes a centralized ATM ARP
 Server which provides IP to ATM address resolution services to LIS
 members.
 Two classes of IP service were omitted - multicast and broadcast
 transmissions. Multicasting allows a single transmit operation to
 cause a packet to be received by multiple remote destinations.

Smith & Armitage Standards Track [Page 1] RFC 2226 IP Broadcast over ATM Networks October 1997

 Broadcasting typically allows a single transmit operation to cause a
 packet to be received by all IP hosts that are members of a
 particular 'subnet'.
 To address the need for multicast support (represented by
 transmission to IP addresses in the Class D space), RFC 2022
 ("Support for Multicast over UNI 3.0/3.1 based ATM Networks") [2] was
 created.  This memo creates an analog of the RFC 1577 ARP Server - a
 new entity known as the MARS (Multicast Address Resolution Server).
 The MARS operates as a centralized registry and distribution
 mechanism for mappings between IP multicast addresses and groups of
 ATM unicast addresses. Host behavior is also defined for establishing
 and managing point to multipoint VCs, based on the information
 returned by the MARS, when hosts wish to transmit packets to a
 multicast group.
 This memo aims to show how RFC 2022 may be used to emulate IP
 broadcast within Logical IP Subnets. While the broadcast technique
 does not align itself well with the underlying point-to-point nature
 of ATM, clearly, some applications will still wish to use IP
 broadcasts.  Client-server applications where the client searches for
 a server by sending out a broadcast is one scenario.  Routing
 protocols, most notably RIP, are other examples.

2. Review of Unicast and Multicast.

 Both the unicast and multicast cases take advantage of the point-to-
 point and point-to-multipoint capabilities defined in the ATM Forum
 UNI 3.1 document [4].  A unicast IP address has a single ATM level
 destination.  Unicast transmissions occur over point to point Virtual
 Channels (VCs) between the source and destination. The ARP Server
 holds mappings between IP destination addresses and their associated
 ATM destination address. Hosts issue an ARP_REQUEST to the ARP Server
 when they wish to ascertain a particular mapping.  The ARP Server
 replies with either an ARP_REPLY containing the ATM address of the
 destination, or an ARP_NAK when the ARP Server is unable to resolve
 the address. If the request is successful the host establishes a VC
 to the destination interface. This VC is then used to forward the
 first (and subsequent) packets to that particular IP destination. RFC
 1577 describes in further detail how hosts are administratively
 grouped in to Logical IP Subnets (LISs), and how the ARP Server
 establishes the initial mappings for members of the LIS it serves.
 The basic host behavior for multicasting is similar - the sender must
 establish and manage a point to multipoint VC whose leaf nodes are
 the group's actual members. Under UNI 3.1 these VCs can only be
 established and altered by the source (root) interface.

Smith & Armitage Standards Track [Page 2] RFC 2226 IP Broadcast over ATM Networks October 1997

 The MARS is an evolution of the ARP Server model, and performs two
 key functions.  The first function is the maintenance of a list of
 ATM addresses corresponding to the members for each group.  This list
 is created by a host registration process which involves two messages
 - a MARS_JOIN which declares that a host wishes to join the specified
 group(s), and a MARS_LEAVE which indicates that a host wishes to
 leave the specified group(s).
 MARS_JOIN and MARS_LEAVE messages are also redistributed to all
 members of the group so that active senders may dynamically adjust
 their point to multipoint VCs accordingly.
 The other major function is the retrieval of group membership from
 MARS (analogous to the ARP Server providing unicast address
 mappings). When faced with the need to transmit an IP packet with a
 Class D destination address, a host issues a MARS_REQUEST to the
 MARS. If the group has members the MARS returns a MARS_MULTI
 (possibly in multiple segments) carrying a set of ATM addresses. The
 host then establishes an initial point to multipoint VC using these
 ATM addresses as the leaf nodes. If the MARS had no mapping it would
 return a MARS_NAK.
 (RFC 2022 also discusses how the MARS can arrange for Class D groups
 to be supported by either multicast servers, or meshes of point to
 multipoint VCs from host to host.  However, from the host's
 perspective this is transparent, and is not central to this
 discussion of IP broadcast support.)
 This memo describes how a host may utilize the registration and group
 management functions in an existing MARS based IP/ATM network to
 emulate IP broadcasts.

3. Broadcast as a special case of Multicast.

 Many of the problems that occur when implementing a broadcast
 solution also occur in when implementing a multicast solution.  In
 fact, broadcast may be considered a special case of multicast.  That
 is, broadcast is a multicast group whose members include all members
 in the LIS.
 There are two broadcast groups which this memo addresses:
    1) 255.255.255.255 - "All ones" broadcast
    2) x.z - CIDR-prefix (subnet) directed broadcast

Smith & Armitage Standards Track [Page 3] RFC 2226 IP Broadcast over ATM Networks October 1997

 Broadcast (1) is sometimes referred to as a limited broadcast to this
 physical network.  Broadcast (2) can be thought of as the the
 broadcast for subnets or networks in the old paradigm. As described
 in [6] and [7], the notion of subnets and networks is being replaced
 with a more efficient utilization of the routing address space known
 as Classless Inter-Domain Routing.  The CIDR-prefix (x) is the
 combination of IP address and subnet mask that denotes the subnet
 number.  The host portion of the address (z) is all ones.  One should
 note that while these broadcasts have different scopes at the IP or
 network layer, they have precisely the same scope at the link layer
 -- namely that all members of the LIS will receive a copy.
 These addresses may be used in two environments:
    o  Broadcasting to all members of a given LIS where
       a priori knowledge of a host's IP address and
       subnet mask are known (e.g. the CIDR-prefix directed
       broadcast).
    o  Broadcasting to all members of a physical network
       without knowledge of a host's IP address and
       subnet mask (e.g. the all ones broadcast).
 On a broadcast medium like Ethernet, these two environments result in
 the same physical destination.  That is, all stations on that network
 will receive the broadcast even if they are on different logical
 subnets, or are non-IP stations.  With ATM, this may not be the case.
 Because ATM is non-broadcast, a registration process must take place.
 And if there are stations that register to some broadcast groups, but
 not others, then the different broadcast groups will have different
 memberships.  The notion of broadcast becomes inconsistent.
 One case that requires the use of the all ones broadcast is that of
 the diskless boot, or bootp client, where the host boots up, and does
 not know its own IP address or subnet mask.  Clearly, the host does
 not know which subnet it belongs to.   So, to send a broadcast to its
 bootp server, the diskless workstation must use the group which
 contains no subnet information, i.e. the 255.255.255.255 broadcast
 group.  Carrying the example a little further, the bootp server,
 after receiving the broadcast, can not send either a directed frame
 nor a subnet directed broadcast to respond to the diskless
 workstation.  Instead, the bootp server must also use the
 255.255.255.255 group to communicate with the client.
 While the all ones broadcast is required at the IP layer, it also has
 relevance at the link layer when deciding which broadcast group to
 register with in MARS.  In other words, a bootp client wishing to
 register for a link layer broadcast, can only register for

Smith & Armitage Standards Track [Page 4] RFC 2226 IP Broadcast over ATM Networks October 1997

 255.255.255.255 in the MARS address space because the client's subnet
 is unknown at the time.  Given that some applications must use the
 all ones address in MARS for their broadcast group, and that we wish
 to minimize the number of broadcast groups used by LIS members, the
 all ones group in MARS MUST be used by all members of the LIS when
 registering to receive broadcast transmissions.  The VCC used for
 transmitting any broadcast packet will be based on the members
 registered in the MARS under the 255.255.255.255 address position.
 This VCC will be referred to as the "broadcast channel" through the
 remainder of this memo.

4. The MARS role in broadcast.

 Many solutions have been proposed, some of which are listed in
 Appendix A.  This memo addresses a MARS solution which appears to do
 the best job of solving the broadcast problem.
 There are a number of characteristics of the MARS architecture that
 should be kept intact.  They include:
 o  MARS contains no knowledge of subnet prefixes and subnet masks.
    Each group address registered with MARS is managed independently.
 o  A MARS may only serve one LIS. This insures that the
    broadcast group 255.255.255.255 is joined by hosts from one
    LIS, keeping its scope bound to conventional interpretation.
 o  The Multicast Server (MCS) described in [2] may be used to service
    the broadcast groups defined in this memo without modification.
    The MCS will reduce the number of channels used by the network.
 The MARS needs no additional code or special algorithms to handle the
 resolution of IP broadcast addresses. It is simply a general database
 that holds {Protocol address, ATM.1, ATM.2, ... ATM.n} mappings, and
 imposes no constraints on the type and length of the 'Protocol
 address'. Whether the hosts view it as Class D or 'broadcast' (or
 even IP) is purely a host side issue.
 It is likely that end points will want to use the IP broadcast
 emulation described here in order to support boot time location of
 the end point's IP address. This leads to the observation that the
 MARS should NOT expect to see both the IP source and ATM source
 address fields of the MARS_JOIN filled in.  This is reasonable, since
 only the ATM source address is used when registering the end point as
 a group member.
 The MARS architecture is sufficient to insure the integrity of the
 broadcast group list without any modification.

Smith & Armitage Standards Track [Page 5] RFC 2226 IP Broadcast over ATM Networks October 1997

5. Host Requirements for Broadcast.

 The following list of bullets describes additional characteristics of
 a MARS-compliant host.  These characteristics are required to take
 advantage of the broadcast function.
 o  A host must register as a MARS client.
 o  A host, soon after registration MUST issue a MARS_JOIN to the
    all ones broadcast address (i.e. 255.255.255.255) with the
    mar$flags.layer3grp reset.
 o  When transmitting packets, the host should map all IP layer
    broadcasts to the VCC (broadcast channel) created and maintained
    based on the all ones entry in MARS.
 o  A host MUST monitor the MARS_JOIN/MARS_LEAVE messages
    for 255.255.255.255 to keep the broadcast channel current.
 o  A broadcast channel should be torn down after a period of
    inactivity.  The corresponding timeout period MAY be specified
    with a minimum value of one minute, and a RECOMMENDED
    default value of 20 minutes.
 One should note that while every member participating in the
 broadcast MUST be a member of the all ones group, not all members
 will choose to transmit broadcast information.  Some members will
 only elect to receive broadcast information passively.  Therefore, in
 a LIS with n stations, there may be less than n channels terminated
 at each station for broadcast information.  Further reductions may be
 gained by adding a Multicast Server (MCS) to the broadcast
 environment which could reduce the number of VCs to two (one
 incoming, one outgoing), or one for a station that only wishes to
 listen.
 It is well understood that broadcasting in this environment may tax
 the resources of the network and of the hosts that use it.
 Therefore, an implementer MAY choose to provide a mechanism for
 retracting the host's entry in the broadcast group after it has been
 established or prior to joining the group.  The MARS_LEAVE is used to
 request withdrawal from the group if the host wishes to disable
 broadcast reception after it has joined the group.  The default
 behavior SHALL be to join the all ones broadcast group in MARS.

Smith & Armitage Standards Track [Page 6] RFC 2226 IP Broadcast over ATM Networks October 1997

6. Implications of IP broadcast on ATM level resources.

 RFC 2022 discusses some of the implications of large multicast groups
 on the allocation of ATM level resources, both within the network and
 within end station ATM interfaces.
 The default mechanism is for IP multicasting to be achieved using
 meshes of point to multipoint VCs, direct from source host to group
 members. Under certain circumstances system administrators may, in a
 manner completely transparent to end hosts, redirect multicast
 traffic through ATM level Multicast Servers (MCSs). This may be
 performed on an individual group basis.
 It is sufficient to note here that the IP broadcast 'multicast group'
 will constitute the largest consumer of VCs within your ATM network
 when it is active. For this reason it will probably be the first
 multicast group to have one or more ATM MCSs assigned to support it.
 However, there is nothing unique about an MCS assigned to support IP
 broadcast traffic, so this will not be dealt with further in this
 memo. RFC 2022 contains further discussion on the possible
 application of multiple MCSs to provide fault-tolerant architectures.

7. Further discussion.

 A point of discussion on the ip-atm forum revolved around "auto
 configuration" and "diskless boot".  This memo describes a broadcast
 solution that requires the use of the MARS.  Therefore, at a minimum,
 the ATM address of the MARS must be manually configured into a
 diskless workstation.  Suggestions such as universal channel numbers,
 and universal ATM addresses have been proposed, however, no agreement
 has been reached.
 Another topic for discussion is multiprotocol support.  MARS is
 designed for protocol independence.  This memo specifically addresses
 the IP broadcast case, identifying which addresses are most effective
 in the IP address space.  However, the principles apply to any layer
 3 protocol.  Further work should be performed to identify suitable
 addresses for other layer 3 protocols.
 Finally, there has been support voiced for a link layer broadcast
 that would be independent of the layer 3 protocol.  Such a solution
 may provide a simpler set of rules through which broadcast
 applications may be used.  In addition, some solutions also provide
 for more efficient use of VCCs.

Smith & Armitage Standards Track [Page 7] RFC 2226 IP Broadcast over ATM Networks October 1997

Security Considerations

 This memo addresses a specific use of the MARS architecture and
 components to provide the broadcast function.  As such, the security
 implications are no greater or less than the implications of using
 any of the other multicast groups available in the multicast address
 range.  Should enhancements to security be required, they would need
 to be added as an extension to the base architecture in RFC 2022.

Acknowledgments

 The apparent simplicity of this memo owes a lot to the services
 provided in [2], which itself is the product of much discussion on
 the IETF's IP-ATM working group mailing list.  Grenville Armitage
 worked on this document while at Bellcore.

References

 [1]  Laubach, M., "Classical IP and ARP over ATM", RFC 1577,
      December 1993.
 [2]  Armitage, G., "Support for Multicast over UNI 3.0/3.1 based ATM
      Networks", RFC 2022, November 1995.
 [3]  Deering, S., "Host Extensions for IP Multicasting", STD 5,
      RFC 1112, August 1989.
 [4]  ATM Forum, "ATM User-Network Interface Specification Version
      3.0", Englewood Cliffs, NJ: Prentice Hall, September 1993.
 [5]  Perez, M., Liaw, F., Grossman, D., Mankin, A., Hoffman, E. and
      A. Malis, "ATM Signaling Support for IP over ATM", RFC 1755,
      February 1995.
 [6]  Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless Inter-
      Domain Routing (CIDR): an Address Assignment and Aggregation
      Strategy", RFC 1519, September 1993.
 [7]  Baker, F., "Requirements for IP Version 4 Routers", RFC 1812,
      June 1995.
 [8]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels, BCP 14, RFC 2119, March 1997.

Smith & Armitage Standards Track [Page 8] RFC 2226 IP Broadcast over ATM Networks October 1997

Authors' Addresses

 Timothy J. Smith
 Network Routing Systems,
 International Business Machines Corporation.
 N21/664
 P.O.Box 12195
 Research Triangle Park, NC 27709
 Phone: (919) 254-4723
 EMail: tjsmith@vnet.ibm.com
 Grenville Armitage
 Bell Labs, Lucent Technologies.
 101 Crawfords Corner Rd,
 Holmdel, NJ, 07733
 EMail: gja@lucent.com

Smith & Armitage Standards Track [Page 9] RFC 2226 IP Broadcast over ATM Networks October 1997

Appendix A. Broadcast alternatives

 Throughout the development of this memo, there have been a number of
 alternatives explored and discarded for one reason or another.  This
 appendix documents these alternatives and the reason that they were
 not chosen.

A.1 ARP Server Broadcast Solutions.

 The ARP Server is a good candidate to support broadcasting.  There is
 an ARP Server for every LIS.  The ARP Server contains the entire LIS
 membership.  These are fundamental ingredients for the broadcast
 function.

A.1.1 Base Solution without modifications to ARP Server.

 One may choose as an existing starting point to use only what is
 available in RFC 1577.  That is, a host can easily calculate the
 range of members in its LIS based on its own IP address and subnet
 mask.  The host can then issue an ARP Request for every member of the
 LIS.  With this information, the host can then set up point-to-point
 connections with all members, or can set up a point-to-multipoint
 connection to all members.  There you have it, the poor man's
 broadcast.
 While this solution is very straight forward, it suffers from a
 number of problems.
 o  The load on the ARP Server is very large.  If all stations on
    a LIS choose to implement broadcasting, the initial surge of ARP
    Requests will be huge.  Some sort of slow start sequence would be
    needed.
 o  The amount of resource required makes this a non-scalable
    solution.  The authors believe that broadcasting will require an
    MCS to reduce the number of channel resources required to support
    each broadcast 'group'.  Using the ARP Server in this manner does
    not allow an MCS to be transparently introduced. (Basic RFC1577
    interfaces also do not implement the extended LLC/SNAP
    encapsulation required to safely use more than one MCS).
 o  The diskless boot solution can not function in this environment
    because it may be unable to determine which subnet to which it
    belongs.

Smith & Armitage Standards Track [Page 10] RFC 2226 IP Broadcast over ATM Networks October 1997

A.1.2 Enhanced ARP Server solution.

 This solution is similar to the base solution except that it takes
 some of the (MARS) multicast solution and embeds it in the ARP
 Server.  The first enhancement is to add the MARS_MULTI command to
 the set of opcodes that the ARP Server supports.  This would allow a
 host to issue a single request, and to get back the list of members
 in one or more MARS_REPLY packets.  Rather than have a registration
 mechanism, the ARP Server could simply use the list of members that
 have already been registered.  When a request comes in for the subnet
 broadcast address, the ARP Server would aggregate the list, and send
 the results to the requester.
 This suffers from two drawbacks.
 1)  Scalability with regard to number of VCs is still an issue.
     One would eventually need to add in some sort of multicast
     server solution to the ARP Server.
 2)  The diskless boot scenario is still broken.  There is no
     way for a station to perform a MARS_MULTI without first
     knowing its IP address and subnet mask.
 The diskless boot problem could be solved by adding to the ARP Server
 a registration process where anyone could register to the
 255.255.255.255 address.  These changes would make the ARP Server
 look more and more like MARS.

A.2 MARS Solutions.

 If we wish to keep the ARP Server constant as described in RFC 1577,
 the alternative is to use the Multicast Address Resolution Server
 (MARS) described in [2].
 MARS has three nice features for broadcasting.
 1)  It has a generalized registration approach which allows
     for any address to have a group of entities registered.
     So, if the subnet address is not known, a host can
     register for an address that is known (e.g. 255.255.255.255).
 2)  The command set allows for lists of members to be passed
     in a single MARS_MULTI packet.   This reduces traffic.
 3)  MARS contains an architecture for dealing with the
     scalability issues.  That is, Multicast Servers (MCSs)
     may be used to set up the point-to-multipoint channels

Smith & Armitage Standards Track [Page 11] RFC 2226 IP Broadcast over ATM Networks October 1997

     and reduce the number of channels that a host needs to
     set up to one.  Hosts wishing to broadcast will instead
     send the packet to the MCS who will then forward it to
     all members of the LIS.

A.2.1. CIDR-prefix (Subnet) Broadcast solution.

 One of the earliest solutions was to simply state that broadcast
 support would be implemented by using a single multicast group in the
 class D address space -- namely, the CIDR-prefix (subnet) broadcast
 address group.  All members of a LIS would be required to register to
 this address, and use it as required.  A host wishing to use either
 the 255.255.255.255 broadcast, or the network broadcast addresses
 would internally map the VC to the subnet broadcast VC.  The all ones
 and network broadcast addresses would exist on MARS, but would be
 unused.
 The problem with this approach goes back to the diskless workstation
 problem.  Because the workstation may not know which subnet it
 belongs to, it doesn't know which group to register with.

A.2.2. All one's first, subnet broadcast second

 This solution acknowledges that the diskless boot problem requires a
 generic address (one that does not contain CIDR-prefix (subnet)
 information) to register with and to use until subnet knowledge is
 known.  In essence, all stations first register to the
 255.255.255.255 group, then as they know their subnet information,
 they could optionally de-register from the all one's group and
 register to the CIDR-prefix (subnet) broadcast group.
 This solution would appear to solve a couple of problems:
 1)  The bootp client can function if the server remains
     registered to the all one's group continuously.
 2)  There will be less traffic using the all ones group
     because the preferred transactions will be on the
     subnet broadcast channel.
 Unfortunately the first bullet contains a flaw.  The server must
 continually be registered to two groups -- the all ones group and the
 subnet broadcast group.  If this server has multiple processes that
 are running different IP applications, it may be difficult for the
 link layer to know which broadcast VC to use.  If it always uses the
 all ones, then it will be missing members that have removed
 themselves from the all ones and have registered to the subnet
 broadcast.  If it always uses the subnet broadcast group, the

Smith & Armitage Standards Track [Page 12] RFC 2226 IP Broadcast over ATM Networks October 1997

 diskless boot scenario gets broken.  While making the decision at the
 link layer may require additional control flows be built into the
 path, it may also require the rewriting of application software.
 In some implementations, a simple constant is used to indicate to the
 link layer that this packet is to be transmitted to the broadcast
 "MAC" address.  The assumption is that the physical network broadcast
 and the logical protocol broadcast are one and the same.  As pointed
 out earlier, this is not the case with ATM.  Therefore applications
 would need to specifically identify the subnet broadcast group
 address to take advantage of the smaller group.
 These problems could be solved in a number of ways, but it was
 thought that they added unnecessarily to the complexity of the
 broadcast solution.

Appendix B. Should MARS Be Limited to a Single LIS?

 RFC 2022 explicitly states that a network administrator MUST ensure
 that each LIS is served by a separate MARS, creating a one-to-one
 mapping between cluster and a unicast LIS.  But, it also mentions
 that relaxation of this restriction MAY occur after future research
 warrants it.  This appendix discusses some to the potential
 implications to broadcast should this restriction be removed.
 The most obvious change would be that the notion of a cluster would
 span more than one LIS.  Therefore, the broadcast group of
 255.255.255.255 would contain members from more than one LIS.
 It also should be emphasized that the one LIS limitation is not a
 restriction of the MARS architecture.  Rather, it is only enforced if
 an administrator chooses to do so.

Smith & Armitage Standards Track [Page 13] RFC 2226 IP Broadcast over ATM Networks October 1997

Full Copyright Statement

 Copyright (C) The Internet Society (1997).  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 implmentation may be prepared, copied, published
 andand 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.

Smith & Armitage Standards Track [Page 14]

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