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rfc:rfc1001

Network Working Group Request for Comments: 1001 March, 1987

           PROTOCOL STANDARD FOR A NetBIOS SERVICE
                   ON A TCP/UDP TRANSPORT:
                    CONCEPTS AND METHODS
                          ABSTRACT

This RFC defines a proposed standard protocol to support NetBIOS services in a TCP/IP environment. Both local network and internet operation are supported. Various node types are defined to accommodate local and internet topologies and to allow operation with or without the use of IP broadcast.

This RFC describes the NetBIOS-over-TCP protocols in a general manner, emphasizing the underlying ideas and techniques. Detailed specifications are found in a companion RFC, "Protocol Standard For a NetBIOS Service on a TCP/UDP Transport: Detailed Specifications".

NetBIOS Working Group [Page 1] RFC 1001 March 1987

                     SUMMARY OF CONTENTS

1. STATUS OF THIS MEMO 6 2. ACKNOWLEDGEMENTS 6 3. INTRODUCTION 7 4. DESIGN PRINCIPLES 7 5. OVERVIEW OF NetBIOS 10 6. NetBIOS FACILITIES SUPPORTED BY THIS STANDARD 15 7. REQUIRED SUPPORTING SERVICE INTERFACES AND DEFINITIONS 15 8. RELATED PROTOCOLS AND SERVICES 16 9. NetBIOS SCOPE 16 10. NetBIOS END-NODES 16 11. NetBIOS SUPPORT SERVERS 18 12. TOPOLOGIES 20 13. GENERAL METHODS 23 14. REPRESENTATION OF NETBIOS NAMES 25 15. NetBIOS NAME SERVICE 27 16. NetBIOS SESSION SERVICE 48 17. NETBIOS DATAGRAM SERVICE 55 18. NODE CONFIGURATION PARAMETERS 58 19. MINIMAL CONFORMANCE 59 REFERENCES 60 APPENDIX A - INTEGRATION WITH INTERNET GROUP MULTICASTING 61 APPENDIX B - IMPLEMENTATION CONSIDERATIONS 62

NetBIOS Working Group [Page 2] RFC 1001 March 1987

                      TABLE OF CONTENTS

1. STATUS OF THIS MEMO 6

2. ACKNOWLEDGEMENTS 6

3. INTRODUCTION 7

4. DESIGN PRINCIPLES 8

4.1  PRESERVE NetBIOS SERVICES                                    8
4.2  USE EXISTING STANDARDS                                       8
4.3  MINIMIZE OPTIONS                                             8
4.4  TOLERATE ERRORS AND DISRUPTIONS                              8
4.5  DO NOT REQUIRE CENTRAL MANAGEMENT                            9
4.6  ALLOW INTERNET OPERATION                                     9
4.7  MINIMIZE BROADCAST ACTIVITY                                  9
4.8  PERMIT IMPLEMENTATION ON EXISTING SYSTEMS                    9
4.9  REQUIRE ONLY THE MINIMUM NECESSARY TO OPERATE                9
4.10  MAXIMIZE EFFICIENCY                                        10
4.11  MINIMIZE NEW INVENTIONS                                    10

5. OVERVIEW OF NetBIOS 10

5.1  INTERFACE TO APPLICATION PROGRAMS                           10
5.2  NAME SERVICE                                                11
5.3  SESSION SERVICE                                             12
5.4  DATAGRAM SERVICE                                            13
5.5  MISCELLANEOUS FUNCTIONS                                     14
5.6  NON-STANDARD EXTENSIONS                                     15

6. NetBIOS FACILITIES SUPPORTED BY THIS STANDARD 15

7. REQUIRED SUPPORTING SERVICE INTERFACES AND DEFINITIONS 15

8. RELATED PROTOCOLS AND SERVICES 16

9. NetBIOS SCOPE 16

10. NetBIOS END-NODES 16

10.1  BROADCAST (B) NODES                                        16
10.2  POINT-TO-POINT (P) NODES                                   16
10.3  MIXED MODE (M) NODES                                       16

11. NetBIOS SUPPORT SERVERS 18

11.1  NetBIOS NAME SERVER (NBNS) NODES                           18
   11.1.1  RELATIONSHIP OF THE NBNS TO THE DOMAIN NAME SYSTEM    19
11.2  NetBIOS DATAGRAM DISTRIBUTION SERVER (NBDD) NODES          19
11.3  RELATIONSHIP OF NBNS AND NBDD NODES                        20
11.4  RELATIONSHIP OF NetBIOS SUPPORT SERVERS AND B NODES        20

12. TOPOLOGIES 20

12.1  LOCAL                                                      20

NetBIOS Working Group [Page 3] RFC 1001 March 1987

   12.1.1  B NODES ONLY                                          21
   12.1.2  P NODES ONLY                                          21
   12.1.3  MIXED B AND P NODES                                   21
12.2  INTERNET                                                   22
   12.2.1  P NODES ONLY                                          22
   12.2.2  MIXED M AND P NODES                                   23

13. GENERAL METHODS 23

13.1  REQUEST/RESPONSE INTERACTION STYLE                         23
   13.1.1  RETRANSMISSION OF REQUESTS                            24
   13.1.2  REQUESTS WITHOUT RESPONSES: DEMANDS                   24
13.2  TRANSACTIONS                                               25
   13.2.1  TRANSACTION ID                                        25
13.3  TCP AND UDP FOUNDATIONS                                    25

14. REPRESENTATION OF NETBIOS NAMES 25

14.1  FIRST LEVEL ENCODING                                       26
14.2  SECOND LEVEL ENCODING                                      27

15. NetBIOS NAME SERVICE 27

15.1  OVERVIEW OF NetBIOS NAME SERVICE                           27
   15.1.1  NAME REGISTRATION (CLAIM)                             27
   15.1.2  NAME QUERY (DISCOVERY)                                28
   15.1.3  NAME RELEASE                                          28
     15.1.3.1  EXPLICIT RELEASE                                  28
     15.1.3.2  NAME LIFETIME AND REFRESH                         29
     15.1.3.3  NAME CHALLENGE                                    29
     15.1.3.4  GROUP NAME FADE-OUT                               29
   15.1.3.5  NAME CONFLICT                                       30
   15.1.4  ADAPTER STATUS                                        31
   15.1.5  END-NODE NBNS INTERACTION                             31
     15.1.5.1  UDP, TCP, AND TRUNCATION                          31
     15.1.5.2  NBNS WACK                                         32
     15.1.5.3  NBNS REDIRECTION                                  32
   15.1.6  SECURED VERSUS NON-SECURED NBNS                       32
   15.1.7  CONSISTENCY OF THE NBNS DATA BASE                     32
   15.1.8  NAME CACHING                                          34
15.2  NAME REGISTRATION TRANSACTIONS                             34
   15.2.1  NAME REGISTRATION BY B NODES                          34
   15.2.2  NAME REGISTRATION BY P NODES                          35
     15.2.2.1  NEW NAME, OR NEW GROUP MEMBER                     35
     15.2.2.2  EXISTING NAME AND OWNER IS STILL ACTIVE           36
     15.2.2.3  EXISTING NAME AND OWNER IS INACTIVE               37
   15.2.3  NAME REGISTRATION BY M NODES                          38
15.3  NAME QUERY TRANSACTIONS                                    39
   15.3.1  QUERY BY B NODES                                      39
   15.3.2  QUERY BY P NODES                                      40
   15.3.3  QUERY BY M NODES                                      43
   15.3.4  ACQUIRE GROUP MEMBERSHIP LIST                         43
15.4  NAME RELEASE TRANSACTIONS                                  44
   15.4.1  RELEASE BY B NODES                                    44

NetBIOS Working Group [Page 4] RFC 1001 March 1987

   15.4.2  RELEASE BY P NODES                                    44
   15.4.3  RELEASE BY M NODES                                    44
15.5  NAME MAINTENANCE TRANSACTIONS                              45
   15.5.1  NAME REFRESH                                          45
   15.5.2  NAME CHALLENGE                                        46
   15.5.3  CLEAR NAME CONFLICT                                   47
15.6  ADAPTER STATUS TRANSACTIONS                                47

16. NetBIOS SESSION SERVICE 48

16.1  OVERVIEW OF NetBIOS SESSION SERVICE                        49
   16.1.1  SESSION ESTABLISHMENT PHASE OVERVIEW                  49
     16.1.1.1  RETRYING AFTER BEING RETARGETTED                  50
     16.1.1.2  SESSION ESTABLISHMENT TO A GROUP NAME             51
   16.1.2  STEADY STATE PHASE OVERVIEW                           51
   16.1.3  SESSION TERMINATION PHASE OVERVIEW                    51
16.2  SESSION ESTABLISHMENT PHASE                                52
16.3  SESSION DATA TRANSFER PHASE                                54
   16.3.1  DATA ENCAPSULATION                                    54
   16.3.2  SESSION KEEP-ALIVES                                   54

17. NETBIOS DATAGRAM SERVICE 55

17.1  OVERVIEW OF NetBIOS DATAGRAM SERVICE                       55
   17.1.1  UNICAST, MULTICAST, AND BROADCAST                     55
   17.1.2  FRAGMENTATION OF NetBIOS DATAGRAMS                    55
17.2  NetBIOS DATAGRAMS BY B NODES                               57
17.3  NetBIOS DATAGRAMS BY P AND M NODES                         58

18. NODE CONFIGURATION PARAMETERS 58

19. MINIMAL CONFORMANCE 59

REFERENCES 60

APPENDIX A 61

INTEGRATION WITH INTERNET GROUP MULTICASTING 61

A-1.  ADDITIONAL PROTOCOL REQUIRED IN B AND M NODES              61
A-2.  CONSTRAINTS                                                61

APPENDIX B 62

IMPLEMENTATION CONSIDERATIONS 62

B-1.  IMPLEMENTATION MODELS                                      62
   B-1.1  MODEL INDEPENDENT CONSIDERATIONS                       63
   B-1.2  SERVICE OPERATION FOR EACH MODEL                       63
B-2.  CASUAL AND RESTRICTED NetBIOS APPLICATIONS                 64
B-3.  TCP VERSUS SESSION KEEP-ALIVES                             66
B-4.  RETARGET ALGORITHMS                                        67
B-5.  NBDD SERVICE                                               68
B-6.  APPLICATION CONSIDERATIONS                                 68
   B-6.1  USE OF NetBIOS DATAGRAMS                               68

NetBIOS Working Group [Page 5] RFC 1001 March 1987

           PROTOCOL STANDARD FOR A NetBIOS SERVICE
                   ON A TCP/UDP TRANSPORT:
                    CONCEPTS AND METHODS

1. STATUS OF THIS MEMO

 This RFC specifies a proposed standard for the Internet
 community.  Since this topic is new to the Internet community,
 discussions and suggestions are specifically requested.
 Please send written comments to:
         Karl Auerbach
         Epilogue Technology Corporation
         P.O. Box 5432
         Redwood City, CA   94063
 Please send online comments to:
         Avnish Aggarwal
                 Internet: mtxinu!excelan!avnish@ucbvax.berkeley.edu
                 Usenet:   ucbvax!mtxinu!excelan!avnish
 Distribution of this document is unlimited.

2. ACKNOWLEDGEMENTS

 This RFC has been developed under the auspices of the Internet
 Activities Board, especially the End-to-End Services Task Force.
 The following individuals have contributed to the development of
 this RFC:
 Avnish Aggarwal       Arvind Agrawal        Lorenzo Aguilar
 Geoffrey Arnold       Karl Auerbach         K. Ramesh Babu
 Keith Ball            Amatzia Ben-Artzi     Vint Cerf
 Richard Cherry        David Crocker         Steve Deering
 Greg Ennis            Steve Holmgren        Jay Israel
 David Kaufman         Lee LaBarre           James Lau
 Dan Lynch             Gaylord Miyata        David Stevens
 Steve Thomas          Ishan Wu
 The system proposed by this RFC does not reflect any existing
 Netbios-over-TCP implementation.  However, the design
 incorporates considerable knowledge obtained from prior
 implementations.  Special thanks goes to the following
 organizations which have provided this invaluable information:
 CMC/Syros      Excelan        Sytek          Ungermann-Bass

NetBIOS Working Group [Page 6] RFC 1001 March 1987

3. INTRODUCTION

 This RFC describes the ideas and general methods used to provide
 NetBIOS on a TCP and UDP foundation.  A companion RFC, "Protocol
 Standard For a NetBIOS Service on a TCP/UDP Transport: Detailed
 Specifications"[1] contains detailed descriptions of packet
 formats, protocols, and defined constants and variables.
 The NetBIOS service has become the dominant mechanism for
 personal computer networking.  NetBIOS provides a vendor
 independent interface for the IBM Personal Computer (PC) and
 compatible systems.
 NetBIOS defines a software interface not a protocol.  There is no
 "official" NetBIOS service standard.  In practice, however, the
 IBM PC-Network version is used as a reference.  That version is
 described in the IBM document 6322916, "Technical Reference PC
 Network"[2].
 Protocols supporting NetBIOS services have been constructed on
 diverse protocol and hardware foundations.  Even when the same
 foundation is used, different implementations may not be able to
 interoperate unless they use a common protocol.  To allow NetBIOS
 interoperation in the Internet, this RFC defines a standard
 protocol to support NetBIOS services using TCP and UDP.
 NetBIOS has generally been confined to personal computers to
 date.  However, since larger computers are often well suited to
 run certain NetBIOS applications, such as file servers, this
 specification has been designed to allow an implementation to be
 built on virtually any type of system where the TCP/IP protocol
 suite is available.
 This standard defines a set of protocols to support NetBIOS
 services.
 These protocols are more than a simple communications service
 involving two entities.  Rather, this note describes a
 distributed system in which many entities play a part even if
 they are not involved as an end-point of a particular NetBIOS
 connection.
 This standard neither constrains nor determines how those
 services are presented to application programs.
 Nevertheless, it is expected that on computers operating under
 the PC-DOS and MS-DOS operating systems that the existing NetBIOS
 interface will be preserved by implementors.
 NOTE: Various symbolic values are used in this document.  For
       their definitions, refer to the Detailed Specifications[1].

NetBIOS Working Group [Page 7] RFC 1001 March 1987

4. DESIGN PRINCIPLES

 In order to develop the specification the following design principles
 were adopted to guide the effort.  Most are typical to any protocol
 standardization effort; however, some have been assigned priorities
 that may be considered unusual.

4.1. PRESERVE NetBIOS SERVICES

 In the absence of an "official" standard for NetBIOS services, the
 version found in the IBM PC Network Technical Reference[2] is used.
 NetBIOS is the foundation of a large body of existing applications.
 It is desirable to operate these applications on TCP networks and to
 extend them beyond personal computers into larger hosts.  To support
 these applications, NetBIOS on TCP must closely conform to the
 services offered by existing NetBIOS systems.
 IBM PC-Network NetBIOS contains some implementation specific
 characteristics.  This standard does not attempt to completely
 preserve these.  It is certain that some existing software requires
 these characteristics and will fail to operate correctly on a NetBIOS
 service based on this RFC.

4.2. USE EXISTING STANDARDS

 Protocol development, especially with standardization, is a demanding
 process.  The development of new protocols must be minimized.
 It is considered essential that an existing standard which provides
 the necessary functionality with reasonable performance always be
 chosen in preference to developing a new protocol.
 When a standard protocol is used, it must be unmodified.

4.3. MINIMIZE OPTIONS

 The standard for NetBIOS on TCP should contain few, if any, options.
 Where options are included, the options should be designed so that
 devices with different option selections should interoperate.

4.4. TOLERATE ERRORS AND DISRUPTIONS

 NetBIOS networks typically operate in an uncontrolled environment.
 Computers come on-line at arbitrary times.  Computers usually go
 off-line without any notice to their peers.  The software is often
 operated by users who are unfamiliar with networks and who may
 randomly perturb configuration settings.
 Despite this chaos, NetBIOS networks work.  NetBIOS on TCP must also

NetBIOS Working Group [Page 8] RFC 1001 March 1987

 be able to operate well in this environment.
 Robust operation does not necessarily mean that the network is proof
 against all disruptions.  A typical NetBIOS network may be disrupted
 by certain types of behavior, whether inadvertent or malicious.

4.5. DO NOT REQUIRE CENTRAL MANAGEMENT

 NetBIOS on TCP should be able to operate, if desired, without
 centralized management beyond that typically required by a TCP based
 network.

4.6. ALLOW INTERNET OPERATION

 The proposed standard recognizes the need for NetBIOS operation
 across a set of networks interconnected by network (IP) level relays
 (gateways.)
 However, the standard assumes that this form of operation will be
 less frequent than on the local MAC bridged-LAN.

4.7. MINIMIZE BROADCAST ACTIVITY

 The standard pre-supposes that the only broadcast services are those
 supported by UDP.  Multicast capabilities are not assumed to be
 available in any form.
 Despite the availability of broadcast capabilities, the standard
 recognizes that some administrations may wish to avoid heavy
 broadcast activity.  For example, an administration may wish to avoid
 isolated non-participating hosts from the burden of receiving and
 discarding NetBIOS broadcasts.

4.8. PERMIT IMPLEMENTATION ON EXISTING SYSTEMS

 The NetBIOS on TCP protocol should be implementable on common
 operating systems, such as Unix(tm) and VAX/VMS(tm), without massive
 effort.
 The NetBIOS protocols should not require services typically
 unavailable on presently existing TCP/UDP/IP implementations.

4.9. REQUIRE ONLY THE MINIMUM NECESSARY TO OPERATE

 The protocol definition should specify only the minimal set of
 protocols required for interoperation.  However, additional protocol
 elements may be defined to enhance efficiency.  These latter elements
 may be generated at the option of the sender, although they must be
 accepted by all receivers.

NetBIOS Working Group [Page 9] RFC 1001 March 1987

4.10. MAXIMIZE EFFICIENCY

 To be useful, a protocol must conduct its business quickly.

4.11. MINIMIZE NEW INVENTIONS

 When an existing protocol is not quite able to support a necessary
 function, but with a small amount of change, it could, that protocol
 should be used.  This is felt to be easier to achieve than
 development of new protocols; further, it is likely to have more
 general utility for the Internet.

5. OVERVIEW OF NetBIOS

 This section describes the NetBIOS services.  It is for background
 information only.  The reader may chose to skip to the next section.
 NetBIOS was designed for use by groups of PCs, sharing a broadcast
 medium.  Both connection (Session) and connectionless (Datagram)
 services are provided, and broadcast and multicast are supported.
 Participants are identified by name.  Assignment of names is
 distributed and highly dynamic.
 NetBIOS applications employ NetBIOS mechanisms to locate resources,
 establish connections, send and receive data with an application
 peer, and terminate connections.  For purposes of discussion, these
 mechanisms will collectively be called the NetBIOS Service.
 This service can be implemented in many different ways.  One of the
 first implementations was for personal computers running the PC-DOS
 and MS-DOS operating systems.  It is possible to implement NetBIOS
 within other operating systems, or as processes which are,
 themselves, simply application programs as far as the host operating
 system is concerned.
 The NetBIOS specification, published by IBM as "Technical Reference
 PC Network"[2] defines the interface and services available to the
 NetBIOS user.  The protocols outlined by that document pertain only
 to the IBM PC Network and are not generally applicable to other
 networks.

5.1. INTERFACE TO APPLICATION PROGRAMS

 NetBIOS on personal computers includes both a set of services and an
 exact program interface to those services.  NetBIOS on other computer
 systems may present the NetBIOS services to programs using other
 interfaces.  Except on personal computers, no clear standard for a
 NetBIOS software interface has emerged.

NetBIOS Working Group [Page 10] RFC 1001 March 1987

5.2. NAME SERVICE

 NetBIOS resources are referenced by name.  Lower-level address
 information is not available to NetBIOS applications.  An
 application, representing a resource, registers one or more names
 that it wishes to use.
 The name space is flat and uses sixteen alphanumeric characters.
 Names may not start with an asterisk (*).
 Registration is a bid for use of a name.  The bid may be for
 exclusive (unique) or shared (group) ownership.  Each application
 contends with the other applications in real time.  Implicit
 permission is granted to a station when it receives no objections.
 That is, a bid is made and the application waits for a period of
 time.  If no objections are received, the station assumes that it has
 permission.
 A unique name should be held by only one station at a time.  However,
 duplicates ("name conflicts") may arise due to errors.
 All instances of a group name are equivalent.
 An application referencing a name generally does not know (or care)
 whether the name is registered as a unique or a group name.
 An explicit name deletion function is specified, so that applications
 may remove a name.  Implicit name deletion occurs when a station
 ceases operation.  In the case of personal computers, implicit name
 deletion is a frequent occurrence.
 The Name Service primitives are:
    1)   Add Name
         The requesting application wants exclusive use of the name.
    2)   Add Group Name
         The requesting application is willing to share use of the
         name with other applications.
    3)   Delete Name
         The application no longer requires use of the name.  It is
         important to note that typical use of NetBIOS is among
         independently-operated personal computers.  A common way to
         stop using a PC is to turn it off; in this case, the
         graceful give-back mechanism, provided by the Delete Name
         function, is not used.  Because this occurs frequently, the
         network service must support this behavior.

NetBIOS Working Group [Page 11] RFC 1001 March 1987

5.3. SESSION SERVICE

 A session is a reliable message exchange, conducted between a pair of
 NetBIOS applications.  Sessions are full-duplex, sequenced, and
 reliable.  Data is organized into messages.  Each message may range
 in size from 0 to 131,071 bytes.  No expedited or urgent data
 capabilities are present.
 Multiple sessions may exist between any pair of calling and called
 names.
 The parties to a connection have access to the calling and called
 names.
 The NetBIOS specification does not define how a connection request to
 a shared (group) name resolves into a session.  The usual assumption
 is that a session may be established with any one owner of the called
 group name.
 An important service provided to NetBIOS applications is the
 detection of sessions failure.  The loss of a session is reported to
 an application via all of the outstanding service requests for that
 session.  For example, if the application has only a NetBIOS receive
 primitive pending and the session terminates, the pending receive
 will abort with a termination indication.
 Session Service primitives are:
    1)   Call
         Initiate a session with a process that is listening under
         the specified name.  The calling entity must indicate both a
         calling name (properly registered to the caller) and a
         called name.
    2)   Listen
         Accept a session from a caller.  The listen primitive may be
         constrained to accept an incoming call from a named caller.
         Alternatively, a call may be accepted from any caller.
    3)   Hang Up
         Gracefully terminate a session.  All pending data is
         transferred before the session is terminated.
    4)   Send
         Transmit one message.  A time-out can occur.  A time-out of
         any session send forces the non-graceful termination of the
         session.

NetBIOS Working Group [Page 12] RFC 1001 March 1987

         A "chain send" primitive is required by the PC NetBIOS
         software interface to allow a single message to be gathered
         from pieces in various buffers.  Chain Send is an interface
         detail and does not effect the protocol.
    5)   Receive
         Receive data.  A time-out can occur.  A time-out on a
         session receive only terminates the receive, not the
         session, although the data is lost.
         The receive primitive may be implemented with variants, such
         as "Receive Any", which is required by the PC NetBIOS
         software interface.  Receive Any is an interface detail and
         does not effect the protocol.
    6)   Session Status
         Obtain information about all of the requestor's sessions,
         under the specified name.  No network activity is involved.

5.4. DATAGRAM SERVICE

 The Datagram service is an unreliable, non-sequenced, connectionless
 service.  Datagrams are sent under cover of a name properly
 registered to the sender.
 Datagrams may be sent to a specific name or may be explicitly
 broadcast.
 Datagrams sent to an exclusive name are received, if at all, by the
 holder of that name.  Datagrams sent to a group name are multicast to
 all holders of that name.  The sending application program cannot
 distinguish between group and unique names and thus must act as if
 all non-broadcast datagrams are multicast.
 As with the Session Service, the receiver of the datagram is told the
 sending and receiving names.
 Datagram Service primitives are:
    1)   Send Datagram
         Send an unreliable datagram to an application that is
         associated with the specified name.  The name may be unique
         or group; the sender is not aware of the difference.  If the
         name belongs to a group, then each member is to receive the
         datagram.

NetBIOS Working Group [Page 13] RFC 1001 March 1987

    2)   Send Broadcast Datagram
         Send an unreliable datagram to any application with a
         Receive Broadcast Datagram posted.
    3)   Receive Datagram
         Receive a datagram sent by a specified originating name to
         the specified name.  If the originating name is an asterisk,
         then the datagram may have been originated under any name.
         Note: An arriving datagram will be delivered to all pending
         Receiving Datagrams that have source and destination
         specifications matching those of the datagram.  In other
         words, if a program (or group of programs) issue a series of
         identical Receive Datagrams, one datagram will cause the
         entire series to complete.
    4)   Receive Broadcast Datagram
         Receive a datagram sent as a broadcast.
         If there are multiple pending Receive Broadcast Datagram
         operations pending, all will be satisfied by the same
         received datagram.

5.5. MISCELLANEOUS FUNCTIONS

 The following functions are present to control the operation of the
 hardware interface to the network.  These functions are generally
 implementation dependent.
    1)   Reset
         Initialize the local network adapter.
    2)   Cancel
         Abort a pending NetBIOS request.  The successful cancel of a
         Send (or Chain Send) operation will terminate the associated
         session.
    3)   Adapter Status
         Obtain information about the local network adapter or of a
         remote adapter.
    4)   Unlink
         For use with Remote Program Load (RPL).  Unlink redirects
         the PC boot disk device back to the local disk.  See the

NetBIOS Working Group [Page 14] RFC 1001 March 1987

         NetBIOS specification for further details concerning RPL and
         the Unlink operation (see page 2-35 in [2]).
    5)   Remote Program Load
         Remote Program Load (RPL) is not a NetBIOS function.  It is
         a NetBIOS application defined by IBM in their NetBIOS
         specification (see pages 2-80 through 2-82 in [2]).

5.6. NON-STANDARD EXTENSIONS

 The IBM Token Ring implementation of NetBIOS has added at least one
 new user capability:
    1)    Find Name
         This function determines whether a given name has been
         registered on the network.

6. NetBIOS FACILITIES SUPPORTED BY THIS STANDARD

 The protocol specified by this standard permits an implementer to
 provide all of the NetBIOS services as described in the IBM
 "Technical Reference PC Network"[2].
 The following NetBIOS facilities are outside the scope of this
 specification.  These are local implementation matters and do not
 impact interoperability:
  1. RESET
  2. SESSION STATUS
  3. UNLINK
  4. RPL (Remote Program Load)

7. REQUIRED SUPPORTING SERVICE INTERFACES AND DEFINITIONS

 The protocols described in this RFC require service interfaces to the
 following:
  1. TCP[3,4]
  2. UDP[5]
 Byte ordering, addressing conventions (including addresses to be
 used for broadcasts and multicasts) are defined by the most
 recent version of:
  1. Assigned Numbers[6]
 Additional definitions and constraints are in:

NetBIOS Working Group [Page 15] RFC 1001 March 1987

  1. IP[7]
  2. Internet Subnets[8,9,10]

8. RELATED PROTOCOLS AND SERVICES

 The design of the protocols described in this RFC allow for the
 future incorporation of the following protocols and services.
 However, before this may occur, certain extensions may be required to
 the protocols defined in this RFC or to those listed below.
  1. Domain Name Service[11,12,13,14]
  2. Internet Group Multicast[15,16]

9. NetBIOS SCOPE

 A "NetBIOS Scope" is the population of computers across which a
 registered NetBIOS name is known.  NetBIOS broadcast and multicast
 datagram operations must reach the entire extent of the NetBIOS
 scope.
 An internet may support multiple, non-intersecting NetBIOS Scopes.
 Each NetBIOS scope has a "scope identifier".  This identifier is a
 character string meeting the requirements of the domain name system
 for domain names.
 NOTE: Each implementation of NetBIOS-over-TCP must provide
       mechanisms to manage the scope identifier(s) to be used.
 Control of scope identifiers implies a requirement for additional
 NetBIOS interface capabilities.  These may be provided through
 extensions of the user service interface or other means (such as node
 configuration parameters.)  The nature of these extensions is not
 part of this specification.

10. NetBIOS END-NODES

 End-nodes support NetBIOS service interfaces and contain
 applications.
 Three types of end-nodes are part of this standard:
  1. Broadcast ("B") nodes
  2. Point-to-point ("P") nodes
  3. Mixed mode ("M") nodes
 An IP address may be associated with only one instance of one of the
 above types.
 Without having preloaded name-to-address tables, NetBIOS participants

NetBIOS Working Group [Page 16] RFC 1001 March 1987

 are faced with the task of dynamically resolving references to one
 another.  This can be accomplished with broadcast or mediated point-
 to-point communications.
 B nodes use local network broadcasting to effect a rendezvous with
 one or more recipients.  P and M nodes use the NetBIOS Name Server
 (NBNS) and the NetBIOS Datagram Distribution Server (NBDD) for this
 same purpose.
 End-nodes may be combined in various topologies.  No matter how
 combined, the operation of the B, P, and M nodes is not altered.
 NOTE: It is recommended that the administration of a NetBIOS
       scope avoid using both M and B nodes within the same scope.
       A NetBIOS scope should contain only B nodes or only P and M
       nodes.

10.1. BROADCAST (B) NODES

 Broadcast (or "B") nodes communicate using a mix of UDP datagrams
 (both broadcast and directed) and TCP connections.  B nodes may
 freely interoperate with one another within a broadcast area.  A
 broadcast area is a single MAC-bridged "B-LAN".  (See Appendix A for
 a discussion of using Internet Group Multicasting as a means to
 extend a broadcast area beyond a single B-LAN.)

10.2. POINT-TO-POINT (P) NODES

 Point-to-point (or "P") nodes communicate using only directed UDP
 datagrams and TCP sessions.  P nodes neither generate nor listen for
 broadcast UDP packets.  P nodes do, however, offer NetBIOS level
 broadcast and multicast services using capabilities provided by the
 NBNS and NBDD.
 P nodes rely on NetBIOS name and datagram distribution servers.
 These servers may be local or remote; P nodes operate the same in
 either case.

10.3. MIXED MODE (M) NODES

 Mixed mode nodes (or "M") nodes are P nodes which have been given
 certain B node characteristics.  M nodes use both broadcast and
 unicast.  Broadcast is used to improve response time using the
 assumption that most resources reside on the local broadcast medium
 rather than somewhere in an internet.
 M nodes rely upon NBNS and NBDD servers.  However, M nodes may
 continue limited operation should these servers be temporarily
 unavailable.

NetBIOS Working Group [Page 17] RFC 1001 March 1987

11. NetBIOS SUPPORT SERVERS

 Two types of support servers are part of this standard:
  1. NetBIOS name server ("NBNS") nodes
  2. Netbios datagram distribution ("NBDD") nodes
 NBNS and NBDD nodes are invisible to NetBIOS applications and are
 part of the underlying NetBIOS mechanism.
 NetBIOS name and datagram distribution servers are the focus of name
 and datagram activity for P and M nodes.
 Both the name (NBNS) and datagram distribution (NBDD) servers are
 permitted to shift part of their operation to the P or M end-node
 which is requesting a service.
 Since the assignment of responsibility is dynamic, and since P and M
 nodes must be prepared to operate should the NetBIOS server delegate
 control to the maximum extent, the system naturally accommodates
 improvements in NetBIOS server function.  For example, as Internet
 Group Multicasting becomes more widespread, new NBDD implementations
 may elect to assume full responsibility for NetBIOS datagram
 distribution.
 Interoperability between different implementations is assured by
 imposing requirements on end-node implementations that they be able
 to accept the full range of legal responses from the NBNS or NBDD.

11.1. NetBIOS NAME SERVER (NBNS) NODES

 The NBNS is designed to allow considerable flexibility with its
 degree of responsibility for the accuracy and management of NetBIOS
 names.  On one hand, the NBNS may elect not to accept full
 responsibility, leaving the NBNS essentially a "bulletin board" on
 which name/address information is freely posted (and removed) by P
 and M nodes without validation by the NBNS.  Alternatively, the NBNS
 may elect to completely manage and validate names.  The degree of
 responsibility that the NBNS assumes is asserted by the NBNS each
 time a name is claimed through a simple mechanism.  Should the NBNS
 not assert full control, the NBNS returns enough information to the
 requesting node so that the node may challenge any putative holder of
 the name.
 This ability to shift responsibility for NetBIOS name management
 between the NBNS and the P and M nodes allows a network administrator
 (or vendor) to make a tradeoff between NBNS simplicity, security, and
 delay characteristics.
 A single NBNS may be implemented as a distributed entity, such as the
 Domain Name Service.  However, this RFC does not attempt to define

NetBIOS Working Group [Page 18] RFC 1001 March 1987

 the internal communications which would be used.

11.1.1. RELATIONSHIP OF THE NBNS TO THE DOMAIN NAME SYSTEM

 The NBNS design attempts to align itself with the Domain Name System
 in a number of ways.
 First, the NetBIOS names are encoded in a form acceptable to the
 domain name system.
 Second, a scope identifier is appended to each NetBIOS name.  This
 identifier meets the restricted character set of the domain system
 and has a leading period.  This makes the NetBIOS name, in
 conjunction with its scope identifier, a valid domain system name.
 Third, the negotiated responsibility mechanisms permit the NBNS to be
 used as a simple bulletin board on which are posted (name,address)
 pairs.  This parallels the existing domain sytem query service.
 This RFC, however, requires the NBNS to provide services beyond those
 provided by the current domain name system.  An attempt has been made
 to coalesce all the additional services which are required into a set
 of transactions which follow domain name system styles of interaction
 and packet formats.
 Among the areas in which the domain name service must be extended
 before it may be used as an NBNS are:
  1. Dynamic addition of entries
  2. Dynamic update of entry data
  3. Support for multiple instance (group) entries
  4. Support for entry time-to-live values and ability to accept

refresh messages to restart the time-to-live period

  1. New entry attributes

11.2. NetBIOS DATAGRAM DISTRIBUTION SERVER (NBDD) NODES

 The internet does not yet support broadcasting or multicasting.  The
 NBDD extends NetBIOS datagram distribution service to this
 environment.
 The NBDD may elect to complete, partially complete, or totally refuse
 to service a node's request to distribute a NetBIOS datagram.  An
 end-node may query an NBDD to determine whether the NBDD will deliver
 a datagram to a specific NetBIOS name.
 The design of NetBIOS-over-TCP lends itself to the use of Internet
 Group Multicast.  For details see Appendix A.

NetBIOS Working Group [Page 19] RFC 1001 March 1987

11.3. RELATIONSHIP OF NBNS AND NBDD NODES

 This RFC defines the NBNS and NBDD as distinct, separate entities.
 In the absence of NetBIOS name information, a NetBIOS datagram
 distribution server must send a copy to each end-node within a
 NetBIOS scope.
 An implementer may elect to construct NBNS and NBDD nodes which have
 a private protocol for the exchange of NetBIOS name information.
 Alternatively, an NBNS and NBDD may be implemented within the same
 device.
 NOTE: Implementations containing private NBNS-NBDD protocols or
       combined NBNS-NBDD functions must be clearly identified.

11.4. RELATIONSHIP OF NetBIOS SUPPORT SERVERS AND B NODES

 As defined in this RFC, neither NBNS nor NBDD nodes interact with B
 nodes.  NetBIOS servers do not listen to broadcast traffic on any
 broadcast area to which they may be attached.  Nor are the NetBIOS
 support servers even aware of B node activities or names claimed or
 used by B nodes.
 It may be possible to extend both the NBNS and NBDD so that they
 participate in B node activities and act as a bridge to P and M
 nodes.  However, such extensions are beyond the scope of this
 specification.

12. TOPOLOGIES

 B, P, M, NBNS, and NBDD nodes may be combined in various ways to form
 useful NetBIOS environments.  This section describes some of these
 combinations.
 There are three classes of operation:
  1. Class 0: B nodes only.
  2. Class 1: P nodes only.
  3. Class 2: P and M nodes together.
 In the drawings which follow, any P node may be replaced by an M
 node.  The effects of such replacement will be mentioned in
 conjunction with each example below.

12.1. LOCAL

 A NetBIOS scope is operating locally when all entities are within the
 same broadcast area.

NetBIOS Working Group [Page 20] RFC 1001 March 1987

12.1.1. B NODES ONLY

 Local operation with only B nodes is the most basic mode of
 operation.  Name registration and discovery procedures use broadcast
 mechanisms.  The NetBIOS scope is limited by the extent of the
 broadcast area.  This configuration does not require NetBIOS support
 servers.
 ====+=========+=====BROADCAST AREA=====+==========+=========+====
     |         |                        |          |         |
     |         |                        |          |         |
  +--+--+   +--+--+                  +--+--+    +--+--+   +--+--+
  |  B  |   |  B  |                  |  B  |    |  B  |   |  B  |
  +-----+   +-----+                  +-----+    +-----+   +-----+

12.1.2. P NODES ONLY

 This configuration would typically be used when the network
 administrator desires to eliminate NetBIOS as a source of broadcast
 activity.
 ====+=========+==========+=B'CAST AREA=+==========+=========+====
     |         |          |             |          |         |
     |         |          |             |          |         |
  +--+--+   +--+--+    +--+--+       +--+--+    +--+--+   +--+--+
  |  P  |   |  P  |    |NBNS |       |  P  |    |NBDD |   |  P  |
  +-----+   +-----+    +-----+       +-----+    +-----+   +-----+
 This configuration operates the same as if it were in an internet and
 is cited here only due to its convenience as a means to reduce the
 use of broadcast.
 Replacement of one or more of the P nodes with M nodes will not
 affect the operation of the other P and M nodes.  P and M nodes will
 be able to interact with one another.  Because M nodes use broadcast,
 overall broadcast activity will increase.

12.1.3. MIXED B AND P NODES

 B and P nodes do not interact with one another.  Replacement of P
 nodes with M nodes will allow B's and M's to interact.
 NOTE: B nodes and M nodes may be intermixed only on a local
       broadcast area.  B and M nodes should not be intermixed in
       an internet environment.

NetBIOS Working Group [Page 21] RFC 1001 March 1987

12.2. INTERNET

12.2.1. P NODES ONLY

 P nodes may be scattered at various locations in an internetwork.
 They require both an NBNS and an NBDD for NetBIOS name and datagram
 support, respectively.
 The NetBIOS scope is determined by the NetBIOS scope identifier
 (domain name) used by the various P (and M) nodes.  An internet may
 contain numerous NetBIOS scopes.
                 +-----+
                 |  P  |
                 +--+--+              |    +-----+
                    |                 |----+  P  |
                    |                 |    +-----+
              /-----+-----\           |
 +-----+      |           |  +------+ |    +-----+
 |  P  +------+  INTERNET +--+G'WAY |-+----+  P  |
 +-----+      |           |  +------+ |    +-----+
              /-----+-----/           |
            /       |                 |    +-----+
          /         |                 |----+  P  |
   +-----+       +--+--+              |    +-----+
   |NBNS +       |NBDD |
   +-----+       +--+--+
 Any P node may be replaced by an M node with no loss of function to
 any node.  However, broadcast activity will be increased in the
 broadcast area to which the M node is attached.

NetBIOS Working Group [Page 22] RFC 1001 March 1987

12.2.2. MIXED M AND P NODES

 M and P nodes may be mixed.  When locating NetBIOS names, M nodes
 will tend to find names held by other M nodes on the same common
 broadcast area in preference to names held by P nodes or M nodes
 elsewhere in the network.
                       +-----+
                       |  P  |
                       +--+--+
                          |
                          |
                    /-----+-----\
       +-----+      |           |      +-----+
       |  P  +------+  INTERNET +------+NBDD |
       +-----+      |           |      +-----+
                    /-----+-----/
                  /       |
                /         |
         +-----+       +--+--+
         |NBNS +       |G'WAY|
         +-----+       +--+--+
                          |
                          |
 ====+=========+==========+=B'CAST AREA=+==========+=========+====
     |         |          |             |          |         |
     |         |          |             |          |         |
  +--+--+   +--+--+    +--+--+       +--+--+    +--+--+   +--+--+
  |  M  |   |  P  |    |  M  |       |  P  |    |  M  |   |  P  |
  +-----+   +-----+    +--+--+       +-----+    +-----+   +-----+
 NOTE: B and M nodes should not be intermixed in an internet
       environment.  Doing so would allow undetected NetBIOS name
       conflicts to arise and cause unpredictable behavior.

13. GENERAL METHODS

 Overlying the specific protocols, described later, are a few general
 methods of interaction between entities.

13.1. REQUEST/RESPONSE INTERACTION STYLE

 Most interactions between entities consist of a request flowing in
 one direction and a subsequent response flowing in the opposite
 direction.
 In those situations where interactions occur on unreliable transports
 (i.e. UDP) or when a request is broadcast, there may not be a strict
 interlocking or one-to-one relationship between requests and
 responses.

NetBIOS Working Group [Page 23] RFC 1001 March 1987

 In no case, however, is more than one response generated for a
 received request.  While a response is pending the responding entity
 may send one or more wait acknowledgements.

13.1.1. RETRANSMISSION OF REQUESTS

 UDP is an unreliable delivery mechanism where packets can be lost,
 received out of transmit sequence, duplicated and delivery can be
 significantly delayed.  Since the NetBIOS protocols make heavy use of
 UDP, they have compensated for its unreliability with extra
 mechanisms.
 Each NetBIOS packet contains all the necessary information to process
 it.  None of the protocols use multiple UDP packets to convey a
 single request or response.  If more information is required than
 will fit in a single UDP packet, for example, when a P-type node
 wants all the owners of a group name from a NetBIOS server, a TCP
 connection is used.  Consequently, the NetBIOS protocols will not
 fail because of out of sequence delivery of UDP packets.
 To overcome the loss of a request or response packet, each request
 operation will retransmit the request if a response is not received
 within a specified time limit.
 Protocol operations sensitive to successive response packets, such as
 name conflict detection, are protected from duplicated packets
 because they ignore successive packets with the same NetBIOS
 information.  Since no state on the responder's node is associated
 with a request, the responder just sends the appropriate response
 whenever a request packet arrives.  Consequently, duplicate or
 delayed request packets have no impact.
 For all requests, if a response packet is delayed too long another
 request packet will be transmitted.  A second response packet being
 sent in response to the second request packet is equivalent to a
 duplicate packet.  Therefore, the protocols will ignore the second
 packet received.  If the delivery of a response is delayed until
 after the request operation has been completed, successfully or not,
 the response packet is ignored.

13.1.2. REQUESTS WITHOUT RESPONSES: DEMANDS

 Some request types do not have matching responses.  These requests
 are known as "demands".  In general a "demand" is an imperative
 request; the receiving node is expected to obey.  However, because
 demands are unconfirmed, they are used only in situations where, at
 most, limited damage would occur if the demand packet should be lost.
 Demand packets are not retransmitted.

NetBIOS Working Group [Page 24] RFC 1001 March 1987

13.2. TRANSACTIONS

 Interactions between a pair of entities are grouped into
 "transactions".  These transactions comprise one or more
 request/response pairs.

13.2.1. TRANSACTION ID

 Since multiple simultaneous transactions may be in progress between a
 pair of entities a "transaction id" is used.
 The originator of a transaction selects an ID unique to the
 originator.  The transaction id is reflected back and forth in each
 interaction within the transaction.  The transaction partners must
 match responses and requests by comparison of the transaction ID and
 the IP address of the transaction partner.  If no matching request
 can be found the response must be discarded.
 A new transaction ID should be used for each transaction.  A simple
 16 bit transaction counter ought to be an adequate id generator.  It
 is probably not necessary to search the space of outstanding
 transaction ID to filter duplicates: it is extremely unlikely that
 any transaction will have a lifetime that is more than a small
 fraction of the typical counter cycle period.  Use of the IP
 addresses in conjunction with the transaction ID further reduces the
 possibility of damage should transaction IDs be prematurely re-used.

13.3. TCP AND UDP FOUNDATIONS

 This version of the NetBIOS-over-TCP protocols uses UDP for many
 interactions.  In the future this RFC may be extended to permit such
 interactions to occur over TCP connections (perhaps to increase
 efficiency when multiple interactions occur within a short time or
 when NetBIOS datagram traffic reveals that an application is using
 NetBIOS datagrams to support connection- oriented service.)

14. REPRESENTATION OF NETBIOS NAMES

 NetBIOS names as seen across the client interface to NetBIOS are
 exactly 16 bytes long.  Within the NetBIOS-over-TCP protocols, a
 longer representation is used.
 There are two levels of encoding.  The first level maps a NetBIOS
 name into a domain system name.  The second level maps the domain
 system name into the "compressed" representation required for
 interaction with the domain name system.
 Except in one packet, the second level representation is the only
 NetBIOS name representation used in NetBIOS-over-TCP packet formats.
 The exception is the RDATA field of a NODE STATUS RESPONSE packet.

NetBIOS Working Group [Page 25] RFC 1001 March 1987

14.1. FIRST LEVEL ENCODING

 The first level representation consists of two parts:
  1. NetBIOS name
  2. NetBIOS scope identifier
 The 16 byte NetBIOS name is mapped into a 32 byte wide field using a
 reversible, half-ASCII, biased encoding.  Each half-octet of the
 NetBIOS name is encoded into one byte of the 32 byte field.  The
 first half octet is encoded into the first byte, the second half-
 octet into the second byte, etc.
 Each 4-bit, half-octet of the NetBIOS name is treated as an 8-bit,
 right-adjusted, zero-filled binary number.  This number is added to
 value of the ASCII character 'A' (hexidecimal 41).  The resulting 8-
 bit number is stored in the appropriate byte.  The following diagram
 demonstrates this procedure:
                       0 1 2 3 4 5 6 7
                      +-+-+-+-+-+-+-+-+
                      |a b c d|w x y z|          ORIGINAL BYTE
                      +-+-+-+-+-+-+-+-+
                          |       |
                 +--------+       +--------+
                 |                         |     SPLIT THE NIBBLES
                 v                         v
          0 1 2 3 4 5 6 7           0 1 2 3 4 5 6 7
         +-+-+-+-+-+-+-+-+         +-+-+-+-+-+-+-+-+
         |0 0 0 0 a b c d|         |0 0 0 0 w x y z|
         +-+-+-+-+-+-+-+-+         +-+-+-+-+-+-+-+-+
                 |                         |
                 +                         +     ADD 'A'
                 |                         |
          0 1 2 3 4 5 6 7           0 1 2 3 4 5 6 7
         +-+-+-+-+-+-+-+-+         +-+-+-+-+-+-+-+-+
         |0 1 0 0 0 0 0 1|         |0 1 0 0 0 0 0 1|
         +-+-+-+-+-+-+-+-+         +-+-+-+-+-+-+-+-+
 This encoding results in a NetBIOS name being represented as a
 sequence of 32 ASCII, upper-case characters from the set
 {A,B,C...N,O,P}.
 The NetBIOS scope identifier is a valid domain name (without a
 leading dot).
 An ASCII dot (2E hexidecimal) and the scope identifier are appended
 to the encoded form of the NetBIOS name, the result forming a valid
 domain name.

NetBIOS Working Group [Page 26] RFC 1001 March 1987

 For example, the NetBIOS name "The NetBIOS name" in the NetBIOS scope
 "SCOPE.ID.COM" would be represented at level one by the ASCII
 character string:
      FEGHGFCAEOGFHEECEJEPFDCAHEGBGNGF.SCOPE.ID.COM

14.2. SECOND LEVEL ENCODING

 The first level encoding must be reduced to second level encoding.
 This is performed according to the rules defined in on page 31 of RFC
 883[12] in the section on "Domain name representation and
 compression".  Also see the section titled "Name Formats" in the
 Detailed Specifications[1].

15. NetBIOS NAME SERVICE

 Before a name may be used, the name must be registered by a node.
 Once acquired, the name must be defended against inconsistent
 registration by other nodes.  Before building a NetBIOS session or
 sending a NetBIOS datagram, the one or more holders of the name must
 be located.
 The NetBIOS name service is the collection of procedures through
 which nodes acquire, defend, and locate the holders of NetBIOS names.
 The name service procedures are different depending whether the end-
 node is of type B, P, or M.

15.1. OVERVIEW OF NetBIOS NAME SERVICE

15.1.1. NAME REGISTRATION (CLAIM)

 Each NetBIOS node can own more than one name.  Names are acquired
 dynamically through the registration (name claim) procedures.
 Every node has a permanent unique name.  This name, like any other
 name, must be explicitly registered by all end-node types.
 A name can be unique (exclusive) or group (non-exclusive).  A unique
 name may be owned by a single node; a group name may be owned by any
 number of nodes.  A name ceases to exist when it is not owned by at
 least one node.  There is no intrinsic quality of a name which
 determines its characteristics: these are established at the time of
 registration.
 Each node maintains state information for each name it has
 registered.  This information includes:
  1. Whether the name is a group or unique name
  2. Whether the name is "in conflict"
  3. Whether the name is in the process of being deleted

NetBIOS Working Group [Page 27] RFC 1001 March 1987

 B nodes perform name registration by broadcasting claim requests,
 soliciting a defense from any node already holding the name.
 P nodes perform name registration through the agency of the NBNS.
 M nodes register names through an initial broadcast, like B nodes,
 then, in the absence of an objection, by following the same
 procedures as a P node.  In other words, the broadcast action may
 terminate the attempt, but is not sufficient to confirm the
 registration.

15.1.2. NAME QUERY (DISCOVERY)

 Name query (also known as "resolution" or "discovery") is the
 procedure by which the IP address(es) associated with a NetBIOS name
 are discovered.  Name query is required during the following
 operations:
 During session establishment, calling and called names must be
 specified.  The calling name must exist on the node that posts the
 CALL.  The called name must exist on a node that has previously
 posted a LISTEN.  Either name may be a unique or group name.
 When a directed datagram is sent, a source and destination name must
 be specified.  If the destination name is a group name, a datagram is
 sent to all the members of that group.
 Different end-node types perform name resolution using different
 techniques, but using the same packet formats:
  1. B nodes solicit name information by broadcasting a request.
  1. P nodes ask the NBNS.
  1. M nodes broadcast a request. If that does not provide the

desired information, an inquiry is sent to the NBNS.

15.1.3. NAME RELEASE

 NetBIOS names may be released explicitly or silently by an end- node.
 Silent release typically occurs when an end-node fails or is turned-
 off.  Most of the mechanisms described below are present to detect
 silent name release.

15.1.3.1. EXPLICIT RELEASE

 B nodes explicitly release a name by broadcasting a notice.
 P nodes send a notification to their NBNS.
 M nodes both broadcast a notice and inform their supporting NBNS.

NetBIOS Working Group [Page 28] RFC 1001 March 1987

15.1.3.2. NAME LIFETIME AND REFRESH

 Names held by an NBNS are given a lifetime during name registration.
 The NBNS will consider a name to have been silently released if the
 end-node fails to send a name refresh message to the NBNS before the
 lifetime expires.  A refresh restarts the lifetime clock.
 NOTE: The implementor should be aware of the tradeoff between
       accuracy of the database and the internet overhead that the
       refresh mechanism introduces.  The lifetime period should
       be tuned accordingly.
 For group names, each end-node must send refresh messages.  A node
 that fails to do so will be considered to have silently released the
 name and dropped from the group.
 The lifetime period is established through a simple negotiation
 mechanism during name registration:  In the name registration
 request, the end-node proposes a lifetime value or requests an
 infinite lifetime.  The NBNS places an actual lifetime value into the
 name registration response.  The NBNS is always allowed to respond
 with an infinite actual period.  If the end node proposed an infinite
 lifetime, the NBNS may respond with any definite period.  If the end
 node proposed a definite period, the NBNS may respond with any
 definite period greater than or equal to that proposed.
 This negotiation of refresh times gives the NBNS means to disable or
 enable refresh activity.  The end-nodes may set a minimum refresh
 cycle period.
 NBNS implementations which are completely reliable may disable
 refresh.

15.1.3.3. NAME CHALLENGE

 To detect whether a node has silently released its claim to a name,
 it is necessary on occasion to challenge that node's current
 ownership.  If the node defends the name then the node is allowed to
 continue possession.  Otherwise it is assumed that the node has
 released the name.
 A name challenge may be issued by an NBNS or by a P or M node.  A
 challenge may be directed towards any end-node type: B, P, or M.

15.1.3.4. GROUP NAME FADE-OUT

 NetBIOS groups may contain an arbitrarily large number of members.
 The time to challenge all members could be quite large.
 To avoid long delays when names are claimed through an NBNS, an

NetBIOS Working Group [Page 29] RFC 1001 March 1987

 optimistic heuristic has been adopted.  It is assumed that there will
 always be some node which will defend a group name.  Consequently, it
 is recommended that the NBNS will immediately reject a claim request
 for a unique name when there already exists a group with the same
 name.  The NBNS will never return an IP address (in response to a
 NAME REGISTRATION REQUEST) when a group name exists.
 An NBNS will consider a group to have faded out of existence when the
 last remaining member fails to send a timely refresh message or
 explicitly releases the name.

15.1.3.5. NAME CONFLICT

 Name conflict exists when a unique name has been claimed by more than
 one node on a NetBIOS network.  B, M, and NBNS nodes may detect a
 name conflict.  The detection mechanism used by B and M nodes is
 active only during name discovery.  The NBNS may detect conflict at
 any time it verifies the consistency of its name database.
 B and M nodes detect conflict by examining the responses received in
 answer to a broadcast name query request.  The first response is
 taken as authoritative.  Any subsequent, inconsistent responses
 represent conflicts.
 Subsequent responses are inconsistent with the authoritative response
 when:
      The subsequent response has the same transaction ID as the
      NAME QUERY REQUEST.
   AND
      The subsequent response is not a duplicate of the
      authoritative response.
   AND EITHER:
           The group/unique characteristic of the authoritative
           response is "unique".
        OR
           The group/unique characteristic of the subsequent
           response is "unique".
 The period in which B and M nodes examine responses is limited by a
 conflict timer, CONFLICT_TIMER.  The accuracy or duration of this
 timer is not crucial: the NetBIOS system will continue to operate
 even with persistent name conflicts.
 Conflict conditions are signaled by sending a NAME CONFLICT DEMAND to
 the node owning the offending name.  Nothing is sent to the node
 which originated the authoritative response.
 Any end-node that receives NAME CONFLICT DEMAND is required to update
 its "local name table" to reflect that the name is in conflict.  (The
 "local name table" on each node contains names that have been

NetBIOS Working Group [Page 30] RFC 1001 March 1987

 successfully registered by that node.)
 Notice that only those nodes that receive the name conflict message
 place a conflict mark next to a name.
 Logically, a marked name does not exist on that node.  This means
 that the node should not defend the name (for name claim purposes),
 should not respond to a name discovery requests for that name, nor
 should the node send name refresh messages for that name.
 Furthermore, it can no longer be used by that node for any session
 establishment or sending or receiving datagrams.  Existing sessions
 are not affected at the time a name is marked as being in conflict.
 The only valid user function against a marked name is DELETE NAME.
 Any other user NetBIOS function returns immediately with an error
 code of "NAME CONFLICT".

15.1.4. ADAPTER STATUS

 An end-node or the NBNS may ask any other end-node for a collection
 of information about the NetBIOS status of that node.  This status
 consists of, among other things, a list of the names which the node
 believes it owns.  The returned status is filtered to contain only
 those names which have the same NetBIOS scope identifier as the
 requestor's name.
 When requesting node status, the requestor identifies the target node
 by NetBIOS name  A name query transaction may be necessary to acquire
 the IP address for the name.  Locally cached name information may be
 used in lieu of a query transaction.  The requesting node sends a
 NODE STATUS REQUEST.  In response, the receiving node sends a NODE
 STATUS RESPONSE containing its local name table and various
 statistics.
 The amount of status which may be returned is limited by the size of
 a UDP packet.  However, this is sufficient for the typical NODE
 STATUS RESPONSE packet.

15.1.5. END-NODE NBNS INTERACTION

 There are certain characteristics of end-node to NBNS interactions
 which are in common and are independent of any particular transaction
 type.

15.1.5.1. UDP, TCP, AND TRUNCATION

 For all transactions between an end-node and an NBNS, either UDP or
 TCP may be used as a transport.  If the NBNS receives a UDP based
 request, it will respond using UDP.  If the amount of information
 exceeds what fits into a UDP packet, the response will contain a
 "truncation flag".  In this situation, the end- node may open a TCP

NetBIOS Working Group [Page 31] RFC 1001 March 1987

 connection to the NBNS, repeat the request, and receive a complete,
 untruncated response.

15.1.5.2. NBNS WACK

 While a name service request is in progress, the NBNS may issue a
 WAIT FOR ACKNOWLEDGEMENT RESPONSE (WACK) to assure the client end-
 node that the NBNS is still operational and is working on the
 request.

15.1.5.3. NBNS REDIRECTION

 The NBNS, because it follows Domain Name system styles of
 interaction, is permitted to redirect a client to another NBNS.

15.1.6. SECURED VERSUS NON-SECURED NBNS

 An NBNS may be implemented in either of two general ways:  The NBNS
 may monitor, and participate in, name activity to ensure consistency.
 This would be a "secured" style NBNS.  Alternatively, an NBNS may be
 implemented to be essentially a "bulletin board" on which name
 information is posted and responsibility for consistency is delegated
 to the end-nodes.  This would be a "non-secured" style NBNS.

15.1.7. CONSISTENCY OF THE NBNS DATA BASE

 Even in a properly running NetBIOS scope the NBNS and its community
 of end-nodes may occasionally lose synchronization with respect to
 the true state of name registrations.
 This may occur should the NBNS fail and lose all or part of its
 database.
 More commonly, a P or M node may be turned-off (thus forgetting the
 names it has registered) and then be subsequently turned back on.
 Finally, errors may occur or an implementation may be incorrect.
 Various approaches have been incorporated into the NetBIOS-over- TCP
 protocols to minimize the impact of these problems.
    1.   The NBNS (or any other node) may "challenge" (using a NAME
         QUERY REQUEST) an end-node to verify that it actually owns a
         name.
         Such a challenge may occur at any time.  Every end-node must
         be prepared to make a timely response.
         Failure to respond causes the NBNS to consider that the
         end-node has released the name in question.

NetBIOS Working Group [Page 32] RFC 1001 March 1987

         (If UDP is being used as the underlying transport, the
         challenge, like all other requests, must be retransmitted
         some number of times in the absence of a response.)
    2.   The NBNS (or any other node) may request (using the NODE
         STATUS REQUEST) that an end-node deliver its entire name
         table.
         This may occur at any time.  Every end-node must be prepared
         to make a timely response.
         Failure to respond permits (but does not require) the NBNS
         to consider that the end-node has failed and released all
         names to which it had claims.  (Like the challenge, on a UDP
         transport, the request must be retransmitted in the absence
         of a response.)
    3.   The NBNS may revoke a P or M node's use of a name by sending
         either a NAME CONFLICT DEMAND or a NAME RELEASE REQUEST to
         the node.
         The receiving end-node may continue existing sessions which
         use that name, but must otherwise cease using that name.  If
         the NBNS placed the name in conflict, the name may be re-
         acquired only by deletion and subsequent reclamation.  If
         the NBNS requested that the name be released, the node may
         attempt to re-acquire the name without first performing a
         name release transaction.
    4.   The NBNS may impose a "time-to-live" on each name it
         registers.  The registering node is made aware of this time
         value during the name registration procedure.
         Simple or reliable NBNS's may impose an infinite time-to-
         live.
    5.   If an end-node holds any names that have finite time-to-
         live values, then that node must periodically send a status
         report to the NBNS.  Each name is reported using the NAME
         REFRESH REQUEST packet.
         These status reports restart the timers of both the NBNS and
         the reporting node.  However, the only timers which are
         restarted are those associated with the name found in the
         status report.  Timers on other names are not affected.
         The NBNS may consider that a node has released any name
         which has not been refreshed within some multiple of name's
         time-to-live.
         A well-behaved NBNS, would, however, issue a challenge to-,

NetBIOS Working Group [Page 33] RFC 1001 March 1987

         or request a list of names from-, the non-reporting end-
         node before deleting its name(s).  The absence of a
         response, or of the name in a response, will confirm the
         NBNS decision to delete a name.
    6.   The absence of reports may cause the NBNS to infer that the
         end-node has failed.  Similarly, receipt of information
         widely divergent from what the NBNS believes about the node,
         may cause the NBNS to consider that the end-node has been
         restarted.
         The NBNS may analyze the situation through challenges or
         requests for a list of names.
    7.   A very cautious NBNS is free to poll nodes (by sending NAME
         QUERY REQUEST or NODE STATUS REQUEST packets) to verify that
         their name status is the same as that registered in the
         NBNS.
         NOTE:  Such polling activity, if used at all by an
         implementation, should be kept at a very low level or
         enabled only during periods when the NBNS has some reason to
         suspect that its information base is inaccurate.
    8.   P and M nodes can detect incorrect name information at
         session establishment.
         If incorrect information is found, NBNS is informed via a
         NAME RELEASE REQUEST originated by the end-node which
         detects the error.

15.1.8. NAME CACHING

 An end-node may keep a local cache of NetBIOS name-to-IP address
 translation entries.
 All cache entries should be flushed on a periodic basis.
 In addition, a node ought to flush any cache information associated
 with an IP address if the node receives any information indicating
 that there may be any possibility of trouble with the node at that IP
 address.  For example, if a NAME CONFLICT DEMAND is sent to a node,
 all cached information about that node should be cleared within the
 sending node.

15.2. NAME REGISTRATION TRANSACTIONS

15.2.1. NAME REGISTRATION BY B NODES

 A name claim transaction initiated by a B node is broadcast
 throughout the broadcast area.  The NAME REGISTRATION REQUEST will be

NetBIOS Working Group [Page 34] RFC 1001 March 1987

 heard by all B and M nodes in the area.  Each node examines the claim
 to see whether it it is consistent with the names it owns.  If an
 inconsistency exists, a NEGATIVE NAME REGISTRATION RESPONSE is
 unicast to the requestor.  The requesting node obtains ownership of
 the name (or membership in the group) if, and only if, no NEGATIVE
 NAME REGISTRATION RESPONSEs are received within the name claim
 timeout, CONFLICT_TIMER.  (See "Defined Constants and Variables" in
 the Detailed Specification for the value of this timer.)
 A B node proclaims its new ownership by broadcasting a NAME OVERWRITE
 DEMAND.
                     B-NODE REGISTRATION PROCESS
 <-----NAME NOT ON NETWORK------>   <----NAME ALREADY EXISTS---->
 REQ. NODE                      NODE                     REQ.NODE
                               HOLDING
                                NAME
 (BROADCAST) REGISTER                        (BROADCAST) REGISTER
 ------------------->                        <-------------------
      REGISTER                                     REGISTER
 ------------------->                        <-------------------
      REGISTER                         NEGATIVE RESPONSE
 ------------------->             ------------------------------>
        OVERWRITE
 ------------------->               (NODE DOES NOT HAVE THE NAME)
 (NODE HAS THE NAME)
 The NAME REGISTRATION REQUEST, like any request, must be repeated if
 no response is received within BCAST_REQ_RETRY_TIMEOUT.  Transmission
 of the request is attempted BCAST_REQ_RETRY_COUNT times.

15.2.2. NAME REGISTRATION BY P NODES

 A name registration may proceed in various  ways depending whether
 the name being registered is new to the NBNS.  If the name is known
 to the NBNS, then challenges may be sent to the prior holder(s).

15.2.2.1. NEW NAME, OR NEW GROUP MEMBER

 The diagram, below, shows the sequence of events when an end-node
 registers a name which is new to the NBNS.  (The diagram omits WACKs,
 NBNS redirections, and retransmission of requests.)
 This same interaction will occur if the name being registered is a
 group name and the group already exists.  The NBNS will add the

NetBIOS Working Group [Page 35] RFC 1001 March 1987

 registrant to the set of group members.
                     P-NODE REGISTRATION PROCESS
          (server has no previous information about the name)
            P-NODE                            NBNS
                        REGISTER
              --------------------------------->
                      POSITIVE RESPONSE
              <---------------------------------
 The interaction is rather simple: the end-node sends a NAME
 REGISTRATION REQUEST, the NBNS responds with a POSITIVE NAME
 REGISTRATION RESPONSE.

15.2.2.2. EXISTING NAME AND OWNER IS STILL ACTIVE

 The following diagram shows interactions when an attempt is made to
 register a unique name, the NBNS is aware of an existing owner, and
 that existing owner is still active.
 There are two sides to the diagram.  The left side shows how a non-
 secured NBNS would handle the matter.  Secured NBNS activity is shown
 on the right.
                     P-NODE REGISTRATION PROCESS
             (server HAS a previous owner that IS active)
 <------NON-SECURED STYLE------->  <---------SECURED STYLE------->
 REQ. NODE           NBNS       NODE         NBNS         REQ.NODE
                               HOLDING
                                NAME
       REGISTER                                      REGISTER
 ------------------->                         <-------------------
                                     QUERY
  END-NODE CHALLENGE              <------------
 <-------------------                QUERY
                                  <------------
           QUERY
 ----------------------------->
                                   POSITIVE RESP
           QUERY                   ------------>
 ----------------------------->                 NEGATIVE RESPONSE
                                                ----------------->
       POSITIVE RESPONSE
 <----------------------------

NetBIOS Working Group [Page 36] RFC 1001 March 1987

 A non-secured NBNS will answer the NAME REGISTRATION REQUEST with a
 END-NODE CHALLENGE REGISTRATION RESPONSE.  This response asks the
 end-node to issue a challenge transaction against the node indicated
 in the response.  In this case, the prior node will defend against
 the challenge and the registering end-node will simply drop the
 registration attempt without further interaction with the NBNS.
 A secured NBNS will refrain from answering the NAME REGISTRATION
 REQUEST until the NBNS has itself challenged the prior holder(s) of
 the name.  In this case, the NBNS finds that that the name is still
 being defended and consequently returns a NEGATIVE NAME REGISTRATION
 RESPONSE to the registrant.
 Due to the potential time for the secured NBNS to make the
 challenge(s), it is likely that a WACK will be sent by the NBNS to
 the registrant.
 Although not shown in the diagram, a non-secured NBNS will send a
 NEGATIVE NAME REGISTRATION RESPONSE to a request to register a unique
 name when there already exists a group of the same name.  A secured
 NBNS may elect to poll (or challenge) the group members to determine
 whether any active members remain.  This may impose a heavy load on
 the network.  It is recommended that group names be allowed to fade-
 out through the name refresh mechanism.

15.2.2.3. EXISTING NAME AND OWNER IS INACTIVE

 The following diagram shows interactions when an attempt is made to
 register a unique name, the NBNS is aware of an existing owner, and
 that existing owner is no longer active.
 A non-secured NBNS will answer the NAME REGISTRATION REQUEST with a
 END-NODE CHALLENGE REGISTRATION RESPONSE.  This response asks the
 end-node to issue a challenge transaction against the node indicated
 in the response.  In this case, the prior node will not defend
 against the challenge.  The registrant will inform the NBNS through a
 NAME OVERWRITE REQUEST.  The NBNS will replace the prior name
 information in its database with the information in the overwrite
 request.
 A secured NBNS will refrain from answering the NAME REGISTRATION
 REQUEST until the NBNS has itself challenged the prior holder(s) of
 the name.  In this case, the NBNS finds that that the name is not
 being defended and consequently returns a POSITIVE NAME REGISTRATION
 RESPONSE to the registrant.

NetBIOS Working Group [Page 37] RFC 1001 March 1987

                     P-NODE REGISTRATION PROCESS
           (server HAS a previous owner that is NOT active)
 <------NON-SECURED STYLE----->  <----------SECURED STYLE-------->
 REQ. NODE           NBNS     NODE           NBNS         REQ.NODE
                             HOLDING
                              NAME
       REGISTER                                    REGISTER
 ------------------->                         <-------------------
                                     QUERY
  END-NODE CHALLENGE             <------------
 <-------------------                QUERY
                                 <------------
       NAME QUERY REQUEST                        POSITIVE RESPONSE
 ---------------------------->                 ------------------>
            QUERY
 ---------------------------->
     OVERWRITE
 ------------------->
  POSITIVE RESPONSE
 <------------------
 Due to the potential time for the secured NBNS to make the
 challenge(s), it is likely that a WACK will be sent by the NBNS to
 the registrant.
 A secured NBNS will immediately send a NEGATIVE NAME REGISTRATION
 RESPONSE in answer to any NAME OVERWRITE REQUESTS it may receive.

15.2.3. NAME REGISTRATION BY M NODES

 An M node begin a name claim operation as if the node were a B node:
 it broadcasts a NAME REGISTRATION REQUEST and listens for NEGATIVE
 NAME REGISTRATION RESPONSEs.  Any NEGATIVE NAME REGISTRATION RESPONSE
 prevents the M node from obtaining the name and terminates the claim
 operation.
 If, however, the M node does not receive any NEGATIVE NAME
 REGISTRATION RESPONSE, the M node must continue the claim procedure
 as if the M node were a P node.
 Only if both name claims were successful does the M node acquire the
 name.
 The following diagram illustrates M node name registration:

NetBIOS Working Group [Page 38] RFC 1001 March 1987

                     M-NODE REGISTRATION PROCESS
 <---NAME NOT IN BROADCAST AREA--> <--NAME IS IN BROADCAST AREA-->
 REQ. NODE                       NODE                     REQ.NODE
                                HOLDING
                                 NAME
 (BROADCAST) REGISTER                         (BROADCAST) REGISTER
 ------------------->                         <-------------------
      REGISTER                                     REGISTER
 ------------------->                         <-------------------
      REGISTER                        NEGATIVE RESPONSE
 ------------------->             ------------------------------->
               !                     (NODE DOES NOT HAVE THE NAME)
  INITIATE     !
  A P-NODE     !
  REGISTRATION !
               V

15.3. NAME QUERY TRANSACTIONS

 Name query transactions are initiated by end-nodes to obtain the IP
 address(es) and other attributes associated with a NetBIOS name.

15.3.1. QUERY BY B NODES

 The following diagram shows how B nodes go about discovering who owns
 a name.
 The left half of the diagram illustrates what happens if there are no
 holders of the name.  In that case no responses are received in
 answer to the broadcast NAME QUERY REQUEST(s).
 The right half shows a POSITIVE NAME QUERY RESPONSE unicast by a name
 holder in answer to the broadcast request.  A name holder will make
 this response to every NAME QUERY REQUEST that it hears.  And each
 holder acts this way.  Thus, the node sending the request may receive
 many responses, some duplicates, and from many nodes.

NetBIOS Working Group [Page 39] RFC 1001 March 1987

                       B-NODE DISCOVERY PROCESS
 <------NAME NOT ON NETWORK------>  <---NAME PRESENT ON NETWORK-->
    REQ. NODE                    NODE                     REQ.NODE
                                HOLDING
                                 NAME
     (BROADCAST) QUERY                           (BROADCAST) QUERY
 ---------------------->                    <---------------------
    NAME QUERY REQUEST                          NAME QUERY REQUEST
 ---------------------->                    <---------------------
         QUERY                        POSITIVE RESPONSE
 ---------------------->           ------------------------------>
 Name query is generally, but not necessarily, a prelude to NetBIOS
 session establishment or NetBIOS datagram transmission.  However,
 name query may be used for other purposes.
 A B node may elect to build a group membership list for subsequent
 use (e.g. for session establishment) by collecting and saving the
 responses.

15.3.2. QUERY BY P NODES

 An NBNS answers queries from a P node with a list of IP address and
 other information for each owner of the name.  If there are multiple
 owners (i.e. if the name is a group name), the NBNS loads as many
 answers into the response as will fit into a UDP packet.  A
 truncation flag indicates whether any additional owner information
 remains.  All the information may be obtained by repeating the query
 over a TCP connection.
 The NBNS is not required to impose any order on its answer list.
 The following diagram shows what happens if the NBNS has no
 information about the name:
                    P-NODE DISCOVERY PROCESS
          (server has no information about the name)
            P-NODE                            NBNS
                      NAME QUERY REQUEST
              --------------------------------->
                      NEGATIVE RESPONSE
              <---------------------------------

NetBIOS Working Group [Page 40] RFC 1001 March 1987

 The next diagram illustrates interaction between the end-node and the
 NBNS when the NBNS does have information about the name.  This
 diagram shows, in addition, the retransmission of the request by the
 end-node in the absence of a timely response.  Also shown are WACKs
 (or temporary, intermediate responses) sent by the NBNS to the end-
 node:
                   P-NODE QUERY PROCESS
         (server HAS information about the name)
      P-NODE                                 NBNS
                     NAME QUERY REQUEST
      /---------------------------------------->
     /
     !          (OPTIONAL)   WACK
     !  <- - - - - - - - - - - - - - - - - - - -
     !         !
     !timer    !
     !         ! (optional timer restart)
     !         !
      \        V           QUERY
       \--------------------------------------->
                            .
                            .
                            .
                          QUERY
      /---------------------------------------->
     /
     !          (OPTIONAL)   WACK
     !  <- - - - - - - - - - - - - - - - - - - -
     !         !
     !timer    !
     !         ! (optional timer restart)
     !         !
      \        V           QUERY
       \--------------------------------------->
                            .
                            .
                  POSITIVE RESPONSE
       <-----------------------------------------
 The following diagram illustrates NBNS redirection.  Upon receipt of
 a NAME QUERY REQUEST, the NBNS redirects the client to another NBNS.
 The client repeats the request to the new NBNS and obtains a
 response.  The diagram shows that response as a POSITIVE NAME QUERY
 RESPONSE.  However any legal NBNS response may occur in actual
 operation.

NetBIOS Working Group [Page 41] RFC 1001 March 1987

                         NBNS REDIRECTION
            P-NODE                            NBNS
                       NAME QUERY REQUEST
              --------------------------------->
                  REDIRECT NAME QUERY RESPONSE
              <---------------------------------
     (START FROM THE
      VERY BEGINNING
      USING THE ADDRESS
      OF THE NEWLY
      SUPPLIED NBNS.)
                                              NEW
            P-NODE                            NBNS
                       NAME QUERY REQUEST
              --------------------------------->
                 POSITIVE NAME QUERY RESPONSE
              <---------------------------------
 The next diagram shows how a P or M node tells the NBNS that the NBNS
 has provided incorrect information.  This procedure may begin after a
 DATAGRAM ERROR packet has been received or a session set-up attempt
 has discovered that the NetBIOS name does not exist at the
 destination, the IP address of which was obtained from the NBNS
 during a prior name query transaction.  The NBNS, in this case a
 secure NBNS, issues queries to verify whether the information is, in
 fact, incorrect.  The NBNS closes the transaction by sending either a
 POSITIVE or NEGATIVE NAME RELEASE RESPONSE, depending on the results
 of the verification.
               CORRECTING NBNS INFORMATION BASE
            P-NODE                            NBNS
                     NAME RELEASE REQUEST
              --------------------------------->
                                                      QUERY
                                                ---------------->
                                                      QUERY
                                                ---------------->
                                    (NAME TAKEN OFF THE DATABASE
                                     IF NBNS FINDS IT TO BE
                                     INCORRECT)
                  POSITIVE/NEGATIVE RESPONSE
              <---------------------------------

NetBIOS Working Group [Page 42] RFC 1001 March 1987

15.3.3. QUERY BY M NODES

 M node name query follows the B node pattern.  In the absence of
 adequate results, the M node then continues by performing a P node
 type query.  This is shown in the following diagram:
                     M-NODE DISCOVERY PROCESS
 <---NAME NOT ON BROADCAST AREA-->  <--NAME IS ON BROADCAST AREA->
 REQ. NODE                       NODE                     REQ.NODE
                                HOLDING
                                 NAME
     (BROADCAST) QUERY                           (BROADCAST) QUERY
 --------------------->                    <----------------------
   NAME QUERY REQUEST                           NAME QUERY REQUEST
 --------------------->                    <----------------------
         QUERY                           POSITIVE RESPONSE
 --------------------->           ------------------------------->
                 !
     INITIATE    !
     A P-NODE    !
     DISCOVERY   !
     PROCESS     !
                 V

15.3.4. ACQUIRE GROUP MEMBERSHIP LIST

 The entire membership of a group may be acquired by sending a NAME
 QUERY REQUEST to the NBNS.  The NBNS will respond with a POSITIVE
 NAME QUERY RESPONSE or a NEGATIVE NAME QUERY RESPONSE.  A negative
 response completes the procedure and indicates that there are no
 members in the group.
 If the positive response has the truncation bit clear, then the
 response contains the entire list of group members.  If the
 truncation bit is set, then this entire procedure must be repeated,
 but using TCP as a foundation rather than UDP.

NetBIOS Working Group [Page 43] RFC 1001 March 1987

15.4. NAME RELEASE TRANSACTIONS

15.4.1. RELEASE BY B NODES

 A NAME RELEASE DEMAND contains the following information:
  1. NetBIOS name
  2. The scope of the NetBIOS name
  3. Name type: unique or group
  4. IP address of the releasing node
  5. Transaction ID
 REQUESTING                                     OTHER
 B-NODE                                         B-NODES
                   NAME RELEASE DEMAND
            ---------------------------------->

15.4.2. RELEASE BY P NODES

 A NAME RELEASE REQUEST contains the following information:
  1. NetBIOS name
  2. The scope of the NetBIOS name
  3. Name type: unique or group
  4. IP address of the releasing node
  5. Transaction ID
 A NAME RELEASE RESPONSE contains the following information:
  1. NetBIOS name
  2. The scope of the NetBIOS name
  3. Name type: unique or group
  4. IP address of the releasing node
  5. Transaction ID
  6. Result:
    1. Yes: name was released
    2. No: name was not released, a reason code is provided
 REQUESTING
 P-NODE                                         NBNS
                   NAME RELEASE REQUEST
            ---------------------------------->
                   NAME RELEASE RESPONSE
            <---------------------------------

15.4.3. RELEASE BY M NODES

 The name release procedure of the M node is a combination of the P
 and B node name release procedures.  The M node first performs the P

NetBIOS Working Group [Page 44] RFC 1001 March 1987

 release procedure.  If the P procedure fails then the release
 procedure does not continue, it fails.  If and only if the P
 procedure succeeds then the M node broadcasts the NAME RELEASE DEMAND
 to the broadcast area, the B procedure.
 NOTE: An M node typically performs a B-style operation and then a
       P-style operation.  In this case, however, the P-style
       operation comes first.
 The following diagram illustrates the M node name release procedure:
 <-----P procedure fails-------> <-------P procedure succeeds--->
 REQUESTING               NBNS    REQUESTING             NBNS
 M-NODE                           M-NODE
     NAME RELEASE REQUEST               NAME RELEASE REQUEST
   -------------------------->       ------------------------>
     NEGATIVE RELEASE RESPONSE        POSITIVE RELEASE RESPONSE
   <--------------------------       <-------------------------
                                                         OTHER
                                                         M-NODES
                                         NAME RELEASE DEMAND
                                      ------------------------>

15.5. NAME MAINTENANCE TRANSACTIONS

15.5.1. NAME REFRESH

 Name refresh transactions are used to handle the following
 situations:
    a)   An NBNS node needs to detect if a P or M node has "silently"
         gone down, so that names held by that node can be purged
         from the data base.
    b)   If the NBNS goes down, it needs to be able to reconstruct
         the data base when it comes back up.
    c)   If the network should be partitioned, the NBNS needs to be
         able to able to update its data base when the network
         reconnects.
 Each P or M node is responsible for sending periodic NAME REFRESH
 REQUESTs for each name that it has registered.  Each refresh packet
 contains a single name that has been successfully registered by that

NetBIOS Working Group [Page 45] RFC 1001 March 1987

 node.  The interval between such packets is negotiated between the
 end node and the NBNS server at the time that the name is initially
 claimed.  At name claim time, an end node will suggest a refresh
 timeout value.  The NBNS node can modify this value in the reply
 packet.  A NBNS node can also choose to tell the end node to not send
 any refresh packet by using the "infinite" timeout value in the
 response packet.  The timeout value returned by the NBNS is the
 actual refresh timeout that the end node must use.
 When a node sends a NAME REFRESH REQUEST, it must be prepared to
 receive a negative response.  This would happen, for example, if the
 the NBNS discovers that the the name had already been assigned to
 some other node.  If such a response is received, the end node should
 mark the name as being in conflict.  Such an entry should be treated
 in the same way as if name conflict had been detected against the
 name.  The following diagram illustrates name refresh:
 <-----Successful Refresh-----> <-----Unsuccessful Refresh---->
 REFRESHING               NBNS   REFRESHING               NBNS
 NODE                            NODE
     NAME REFRESH REQUEST             NAME REFRESH REQUEST
   ------------------------>        ----------------------->
       POSITIVE RESPONSE                NEGATIVE RESPONSE
   <------------------------        <-----------------------
                                  !
                                  !
                                  V
                            MARK NAME IN
                              CONFLICT

15.5.2. NAME CHALLENGE

 Name challenge is done by sending a NAME QUERY REQUEST to an end node
 of any type.  If a POSITIVE NAME QUERY RESPONSE is returned, then
 that node still owns the name.  If a NEGATIVE NAME QUERY RESPONSE is
 received or if no response is received, it can be assumed that the
 end node no longer owns the name.
 Name challenge can be performed either by the NBNS node, or by an end
 node.  When an end-node sends a name claim packet, the NBNS node may
 do the challenge operation.  The NBNS node can choose, however, to
 require the end node do the challenge.  In that case, the NBNS will
 send an END-NODE CHALLENGE RESPONSE packet to the end node, which
 should then proceed to challenge the putative owner.
 Note that the name challenge procedure sends a normal NAME QUERY
 REQUEST packet to the end node.  It does not require a special
 packet.  The only new packet introduced is the END-NODE CHALLENGE

NetBIOS Working Group [Page 46] RFC 1001 March 1987

 RESPONSE which is sent by an NBNS node when the NBNS wants the end-
 node to perform the challenge operation.

15.5.3. CLEAR NAME CONFLICT

 It is possible during a refresh request from a M or P node for a NBNS
 to detects a name in conflict.  The response to the NAME REFRESH
 REQUEST must be a NEGATIVE NAME REGISTRATION RESPONSE.  Optionally,
 in addition, the NBNS may send a NAME CONFLICT DEMAND or a NAME
 RELEASE REQUEST to the refreshing node.  The NAME CONFLICT DEMAND
 forces the node to place the name in the conflict state.  The node
 will eventually inform it's user of the conflict.  The NAME RELEASE
 REQUEST will force the node to flush the name from its local name
 table completely.  This forces the node to flush the name in
 conflict.  This does not cause termination of existing sessions using
 this name.
 The following diagram shows an NBNS detecting and correcting a
 conflict:
 REFRESHING NODE                                 NBNS
                   NAME REFRESH REQUEST
         ----------------------------------------->
             NEGATIVE NAME REGISTRATION RESPONSE
         <-----------------------------------------
                   NAME CONFLICT DEMAND
         <-----------------------------------------
                           OR
                   NAME RELEASE REQUEST
         <-----------------------------------------
             POSITIVE/NEGATIVE RELEASE REQUEST
         ----------------------------------------->

15.6. ADAPTER STATUS TRANSACTIONS

 Adapter status is obtained from a node as follows:
    1.   Perform a name discovery operation to obtain the IP
         addresses of a set of end-nodes.
    2.   Repeat until all end-nodes from the set have been used:
         a.   Select one end-node from the set.
         b.   Send a NODE STATUS REQUEST to that end-node using UDP.

NetBIOS Working Group [Page 47] RFC 1001 March 1987

         c.   Await a NODE STATUS RESPONSE.  (If a timely response is
              not forthcoming, repeat step "b" UCAST_REQ_RETRY_COUNT
              times.  After the last retry, go to step "a".)
         d.   If the truncation bit is not set in the response, the
              response contains the entire node status.  Return the
              status to the user and terminate this procedure.
         e.   If the truncation bit is set in the response, then not
              all status was returned because it would not fit into
              the response packet.  The responder will set the
              truncation bit if the IP datagram length would exceed
              MAX_DATAGRAM_LENGTH.  Return the status to the user and
              terminate this procedure.

3. Return error to user, no status obtained.

 The repetition of step 2, above, through all nodes of the set, is
 optional.
 Following is an example transaction of a successful Adapter Status
 operation:
 REQUESTING NODE                                 NAME OWNER
                     NAME QUERY REQUEST
         ----------------------------------------->
                 POSITIVE NAME QUERY RESPONSE
         <-----------------------------------------
                     NODE STATUS REQUEST
         ----------------------------------------->
                    NODE STATUS RESPONSE
         <-----------------------------------------

16. NetBIOS SESSION SERVICE

 The NetBIOS session service begins after one or more IP addresses
 have been found for the target name.  These addresses may have been
 acquired using the NetBIOS name query transactions or by other means,
 such as a local name table or cache.
 NetBIOS session service transactions, packets, and protocols are
 identical for all end-node types.  They involve only directed
 (point-to-point) communications.

NetBIOS Working Group [Page 48] RFC 1001 March 1987

16.1. OVERVIEW OF NetBIOS SESSION SERVICE

 Session service has three phases:
   Session establishment - it is during this phase that the IP
      address and TCP port of the called name is determined, and a
      TCP connection is established with the remote party.
   Steady state - it is during this phase that NetBIOS data
      messages are exchanged over the session.  Keep-alive packets
      may also be exchanged if the participating nodes are so
      configured.
   Session close - a session is closed whenever either a party (in
      the session) closes the session or it is determined that one
      of the parties has gone down.

16.1.1. SESSION ESTABLISHMENT PHASE OVERVIEW

 An end-node begins establishment of a session to another node by
 somehow acquiring (perhaps using the name query transactions or a
 local cache) the IP address of the node or nodes purported to own the
 destination name.
 Every end-node awaits incoming NetBIOS session requests by listening
 for TCP calls to a well-known service port, SSN_SRVC_TCP_PORT.  Each
 incoming TCP connection represents the start of a separate NetBIOS
 session initiation attempt.  The NetBIOS session server, not the
 ultimate application, accepts the incoming TCP connection(s).
 Once the TCP connection is open, the calling node sends session
 service request packet.  This packet contains the following
 information:
  1. Calling IP address (see note)
  2. Calling NetBIOS name
  3. Called IP address (see note)
  4. Called NetBIOS name
 NOTE: The IP addresses are obtained from the TCP service
       interface.
 When the session service request packet arrives at the NetBIOS
 server, one of the the following situations will exist:
  1. There exists a NetBIOS LISTEN compatible with the incoming

call and there are adequate resources to permit session

      establishment to proceed.
  1. There exists a NetBIOS LISTEN compatible with the incoming

call, but there are inadequate resources to permit

NetBIOS Working Group [Page 49] RFC 1001 March 1987

      establishment of a session.
  1. The called name does, in fact, exist on the called node, but

there is no pending NetBIOS LISTEN compatible with the

      incoming call.
  1. The called name does not exist on the called node.
 In all but the first case, a rejection response is sent back over the
 TCP connection to the caller.  The TCP connection is then closed and
 the session phase terminates.  Any retry is the responsibility of the
 caller.  For retries in the case of a group name, the caller may use
 the next member of the group rather than immediately retrying the
 instant address.  In the case of a unique name, the caller may
 attempt an immediate retry using the same target IP address unless
 the called name did not exist on the called node.  In that one case,
 the NetBIOS name should be re-resolved.
 If a compatible LISTEN exists, and there are adequate resources, then
 the session server may transform the existing TCP connection into the
 NetBIOS data session.  Alternatively, the session server may
 redirect, or "retarget" the caller to another TCP port (and IP
 address).
 If the caller is redirected, the caller begins the session
 establishment anew, but using the new IP address and TCP port given
 in the retarget response.  Again a TCP connection is created, and
 again the calling and called node exchange credentials.  The called
 party may accept the call, reject the call, or make a further
 redirection.
 This mechanism is based on the presumption that, on hosts where it is
 not possible to transfer open TCP connections between processes, the
 host will have a central session server.  Applications willing to
 receive NetBIOS calls will obtain an ephemeral TCP port number, post
 a TCP unspecified passive open on that port, and then pass that port
 number and NetBIOS name information to the NetBIOS session server
 using a NetBIOS LISTEN operation.  When the call is placed, the
 session server will "retarget" the caller to the application's TCP
 socket.  The caller will then place a new call, directly to the
 application.  The application has the responsibility to mimic the
 session server at least to the extent of receiving the calling
 credentials and then accepting or rejecting the call.

16.1.1.1. RETRYING AFTER BEING RETARGETTED

 A calling node may find that it can not establish a session with a
 node to which it was directed by the retargetting procedure.  Since
 retargetting may be nested, there is an issue whether the caller
 should begin a retry at the initial starting point or back-up to an
 intermediate retargetting point.  The caller may use any method.  A

NetBIOS Working Group [Page 50] RFC 1001 March 1987

 discussion of two such methods is in Appendix B, "Retarget
 Algorithms".

16.1.1.2. SESSION ESTABLISHMENT TO A GROUP NAME

 Session establishment with a group name requires special
 consideration.  When a NetBIOS CALL attempt is made to a group name,
 name discovery will result in a list (possibly incomplete) of the
 members of that group.  The calling node selects one member from the
 list and attempts to build a session.  If that fails, the calling
 node may select another member and make another attempt.
 When the session service attempts to make a connection with one of
 the members of the group, there is no guarantee that that member has
 a LISTEN pending against that group name, that the called node even
 owns, or even that the called node is operating.

16.1.2. STEADY STATE PHASE OVERVIEW

 NetBIOS data messages are exchanged in the steady state.  NetBIOS
 messages are sent by prepending the user data with a message header
 and sending the header and the user data over the TCP connection.
 The receiver removes the header and passes the data to the NetBIOS
 user.
 In order to detect failure of one of the nodes or of the intervening
 network, "session keep alive" packets may be periodically sent in the
 steady state.
 Any failure of the underlying TCP connection, whether a reset, a
 timeout, or other failure, implies failure of the NetBIOS session.

16.1.3. SESSION TERMINATION PHASE OVERVIEW

 A NetBIOS session is terminated normally when the user requests the
 session to be closed or when the session service detects the remote
 partner of the session has gracefully terminated the TCP connection.
 A NetBIOS session is abnormally terminated when the session service
 detects a loss of the connection.  Connection loss can be detected
 with the keep-alive function of the session service or TCP, or on the
 failure of a SESSION MESSAGE send operation.
 When a user requests to close a session, the service first attempts a
 graceful in-band close of the TCP connection.  If the connection does
 not close within the SSN_CLOSE_TIMEOUT the TCP connection is aborted.
 No matter how the TCP connection is terminated, the NetBIOS session
 service always closes the NetBIOS session.
 When the session service receives an indication from TCP that a
 connection close request has been received, the TCP connection and
 the NetBIOS session are immediately closed and the user is informed

NetBIOS Working Group [Page 51] RFC 1001 March 1987

 of the loss of the session.  All data received up to the close
 indication should be delivered, if possible, to the session's user.

16.2. SESSION ESTABLISHMENT PHASE

 All the following diagrams assume a name query operation was
 successfully completed by the caller node for the listener's name.
 This first diagram shows the sequence of network events used to
 successfully establish a session without retargetting by the
 listener.  The TCP connection is first established with the well-
 known NetBIOS session service TCP port, SSN_SRVC_TCP_PORT.  The
 caller then sends a SESSION REQUEST packet over the TCP connection
 requesting a session with the listener.  The SESSION REQUEST contains
 the caller's name and the listener's name.  The listener responds
 with a POSITIVE SESSION RESPONSE informing the caller this TCP
 connection is accepted as the connection for the data transfer phase
 of the session.
         CALLER                          LISTENER
                     TCP CONNECT
         ====================================>
                      TCP ACCEPT
         <===================================
                   SESSION REQUEST
         ------------------------------------>
                  POSITIVE RESPONSE
         <-----------------------------------
 The second diagram shows the sequence of network events used to
 successfully establish a session when the listener does retargetting.
 The session establishment procedure is the same as with the first
 diagram up to the listener's response to the SESSION REQUEST.  The
 listener, divided into two sections, the listen processor and the
 actual listener, sends a SESSION RETARGET RESPONSE to the caller.
 This response states the call is acceptable, but the data transfer
 TCP connection must be at the new IP address and TCP port.  The
 caller then re-iterates the session establishment process anew with
 the new IP address and TCP port after the initial TCP connection is
 closed.  The new listener then accepts this connection for the data
 transfer phase with a POSITIVE SESSION RESPONSE.
         CALLER                  LISTEN PROCESSOR        LISTENER
                 TCP CONNECT
         =============================>
                 TCP ACCEPT
         <=============================
                 SESSION REQUEST
         ----------------------------->

NetBIOS Working Group [Page 52] RFC 1001 March 1987

            SESSION RETARGET RESPONSE
         <-----------------------------
                 TCP CLOSE
         <=============================
                 TCP CLOSE
         =============================>
                     TCP CONNECT
         ====================================================>
                      TCP ACCEPT
         <====================================================
                   SESSION REQUEST
         ---------------------------------------------------->
                  POSITIVE RESPONSE
         <----------------------------------------------------
 The third diagram is the sequence of network events for a rejected
 session request with the listener.  This type of rejection could
 occur with either a non-retargetting listener or a retargetting
 listener.  After the TCP connection is established at
 SSN_SRVC_TCP_PORT, the caller sends the SESSION REQUEST over the TCP
 connection.  The listener does not have either a listen pending for
 the listener's name or the pending NetBIOS listen is specific to
 another caller's name.  Consequently, the listener sends a NEGATIVE
 SESSION RESPONSE and closes the TCP connection.
         CALLER                          LISTENER
                      TCP CONNECT
         ====================================>
                      TCP ACCEPT
         <===================================
                   SESSION REQUEST
         ------------------------------------>
                  NEGATIVE RESPONSE
         <-----------------------------------
                      TCP CLOSE
         <===================================
                      TCP CLOSE
         ====================================>
 The fourth diagram is the sequence of network events when session
 establishment fails with a retargetting listener.  After being
 redirected, and after the initial TCP connection is closed the caller
 tries to establish a TCP connection with the new IP address and TCP
 port.  The connection fails because either the port is unavailable or
 the target node is not active.  The port unavailable race condition
 occurs if another caller has already acquired the TCP connection with
 the listener.  For additional implementation suggestions, see
 Appendix B, "Retarget Algorithms".

NetBIOS Working Group [Page 53] RFC 1001 March 1987

         CALLER                  LISTEN PROCESSOR        LISTENER
                 TCP CONNECT
         =============================>
                 TCP ACCEPT
         <=============================
                 SESSION REQUEST
         ----------------------------->
                 REDIRECT RESPONSE
         <-----------------------------
                 TCP CLOSE
         <=============================
                 TCP CLOSE
         =============================>
                     TCP CONNECT
         ====================================================>
                   CONNECTION REFUSED OR TIMED OUT
         <===================================================

16.3. SESSION DATA TRANSFER PHASE

16.3.1. DATA ENCAPSULATION

 NetBIOS messages are exchanged in the steady state.  Messages are
 sent by prepending user data with message header and sending the
 header and the user data over the TCP connection.  The receiver
 removes the header and delivers the NetBIOS data to the user.

16.3.2. SESSION KEEP-ALIVES

 In order to detect node failure or network partitioning, "session
 keep alive" packets are periodically sent in the steady state.  A
 session keep alive packet is discarded by a peer node.
 A session keep alive timer is maintained for each session.  This
 timer is reset whenever any data is sent to, or received from, the
 session peer.  When the timer expires, a NetBIOS session keep-alive
 packet is sent on the TCP connection.  Sending the keep-alive packet
 forces data to flow on the TCP connection, thus indirectly causing
 TCP to detect whether the connection is still active.
 Since many TCP implementations provide a parallel TCP "keep- alive"
 mechanism, the NetBIOS session keep-alive is made a configurable
 option.  It is recommended that the NetBIOS keep- alive mechanism be
 used only in the absence of TCP keep-alive.
 Note that unlike TCP keep alives, NetBIOS session keep alives do not
 require a response from the NetBIOS peer -- the fact that it was

NetBIOS Working Group [Page 54] RFC 1001 March 1987

 possible to send the NetBIOS session keep alive is sufficient
 indication that the peer, and the connection to it, are still active.
 The only requirement for interoperability is that when a session keep
 alive packet is received, it should be discarded.

17. NETBIOS DATAGRAM SERVICE

17.1. OVERVIEW OF NetBIOS DATAGRAM SERVICE

 Every NetBIOS datagram has a named destination and source.  To
 transmit a NetBIOS datagram, the datagram service must perform a name
 query operation to learn the IP address and the attributes of the
 destination NetBIOS name.  (This information may be cached to avoid
 the overhead of name query on subsequent NetBIOS datagrams.)
 NetBIOS datagrams are carried within UDP packets.  If a NetBIOS
 datagram is larger than a single UDP packet, it may be fragmented
 into several UDP packets.
 End-nodes may receive NetBIOS datagrams addressed to names not held
 by the receiving node.  Such datagrams should be discarded.  If the
 name is unique then a DATAGRAM ERROR packet is sent to the source of
 that NetBIOS datagram.

17.1.1. UNICAST, MULTICAST, AND BROADCAST

 NetBIOS datagrams may be unicast, multicast, or broadcast.  A NetBIOS
 datagram addressed to a unique NetBIOS name is unicast.  A NetBIOS
 datatgram addressed to a group NetBIOS name, whether there are zero,
 one, or more actual members, is multicast.  A NetBIOS datagram sent
 using the NetBIOS "Send Broadcast Datagram" primitive is broadcast.

17.1.2. FRAGMENTATION OF NetBIOS DATAGRAMS

 When the header and data of a NetBIOS datagram exceeds the maximum
 amount of data allowed in a UDP packet, the NetBIOS datagram must be
 fragmented before transmission and reassembled upon receipt.
 A NetBIOS Datagram is composed of the following protocol elements:
  1. IP header of 20 bytes (minimum)
  2. UDP header of 8 bytes
  3. NetBIOS Datagram Header of 14 bytes
  4. The NetBIOS Datagram data.
 The NetBIOS Datagram data section is composed of 3 parts:
  1. Source NetBIOS name (255 bytes maximum)
  2. Destination NetBIOS name (255 bytes maximum)
  3. The NetBIOS user's data (maximum of 512 bytes)

NetBIOS Working Group [Page 55] RFC 1001 March 1987

 The two name fields are in second level encoded format (see section
 14.)
 A maximum size NetBIOS datagram is 1064 bytes.  The minimal maximum
 IP datagram size is 576 bytes.  Consequently, a NetBIOS Datagram may
 not fit into a single IP datagram.  This makes it necessary to permit
 the fragmentation of NetBIOS Datagrams.
 On networks meeting or exceeding the minimum IP datagram length
 requirement of 576 octets, at most two NetBIOS datagram fragments
 will be generated.  The protocols and packet formats accommodate
 fragmentation into three or more parts.
 When a NetBIOS datagram is fragmented, the IP, UDP and NetBIOS
 Datagram headers are present in each fragment.  The NetBIOS Datagram
 data section is split among resulting UDP datagrams.  The data
 sections of NetBIOS datagram fragments do not overlap. The only
 fields of the NetBIOS Datagram header that would vary are the FLAGS
 and OFFSET fields.
 The FIRST bit in the FLAGS field indicate whether the fragment is the
 first in a sequence of fragments.  The MORE bit in the FLAGS field
 indicates whether other fragments follow.
 The OFFSET field is the byte offset from the beginning of the NetBIOS
 datagram data section to the first byte of the data section in a
 fragment.  It is 0 for the first fragment.  For each subsequent
 fragment, OFFSET is the sum of the bytes in the NetBIOS data sections
 of all preceding fragments.
 If the NetBIOS datagram was not fragmented:
  1. FIRST = TRUE
  2. MORE = FALSE
  3. OFFSET = 0
 If the NetBIOS datagram was fragmented:
  1. First fragment:
    1. FIRST = TRUE
    2. MORE = TRUE
    3. OFFSET = 0
  1. Intermediate fragments:
    1. FIRST = FALSE
    2. MORE = TRUE
    3. OFFSET = sum(NetBIOS data in prior fragments)
  1. Last fragment:
    1. FIRST = FALSE
    2. MORE = FALSE

NetBIOS Working Group [Page 56] RFC 1001 March 1987

  1. OFFSET = sum(NetBIOS data in prior fragments)
 The relative position of intermediate fragments may be ascertained
 from OFFSET.
 An NBDD must remember the destination name of the first fragment in
 order to relay the subsequent fragments of a single NetBIOS datagram.
 The name information can be associated with the subsequent fragments
 through the transaction ID, DGM_ID, and the SOURCE_IP, fields of the
 packet.  This information can be purged by the NBDD after the last
 fragment has been processed or FRAGMENT_TO time has expired since the
 first fragment was received.

17.2. NetBIOS DATAGRAMS BY B NODES

 For NetBIOS datagrams with a named destination (i.e. non- broadcast),
 a B node performs a name discovery for the destination name before
 sending the datagram.  (Name discovery may be bypassed if information
 from a previous discovery is held in a cache.)  If the name type
 returned by name discovery is UNIQUE, the datagram is unicast to the
 sole owner of the name.  If the name type is GROUP, the datagram is
 broadcast to the entire broadcast area using the destination IP
 address BROADCAST_ADDRESS.
 A receiving node always filters datagrams based on the destination
 name.  If the destination name is not owned by the node or if no
 RECEIVE DATAGRAM user operations are pending for the name, then the
 datagram is discarded.  For datagrams with a UNIQUE name destination,
 if the name is not owned by the node then the receiving node sends a
 DATAGRAM ERROR packet.  The error packet originates from the
 DGM_SRVC_UDP_PORT and is addressed to the SOURCE_IP and SOURCE_PORT
 from the bad datagram.  The receiving node quietly discards datagrams
 with a GROUP name destination if the name is not owned by the node.
 Since broadcast NetBIOS datagrams do not have a named destination,
 the B node sends the DATAGRAM SERVICE packet(s) to the entire
 broadcast area using the destination IP address BROADCAST_ADDRESS.
 In order for the receiving nodes to distinguish this datagram as a
 broadcast NetBIOS datagram, the NetBIOS name used as the destination
 name is '*' (hexadecimal 2A) followed by 15 bytes of hexidecimal 00.
 The NetBIOS scope identifier is appended to the name before it is
 converted into second-level encoding.  For example, if the scope
 identifier is "NETBIOS.SCOPE" then the first-level encoded name would
 be:
      CKAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA.NETBIOS.SCOPE
 According to [2], a user may not provide a NetBIOS name beginning
 with "*".
 For each node in the broadcast area that receives the NetBIOS

NetBIOS Working Group [Page 57] RFC 1001 March 1987

 broadcast datagram, if any RECEIVE BROADCAST DATAGRAM user operations
 are pending then the data from the NetBIOS datagram is replicated and
 delivered to each.  If no such operations are pending then the node
 silently discards the datagram.

17.3. NetBIOS DATAGRAMS BY P AND M NODES

 P and M nodes do not use IP broadcast to distribute NetBIOS
 datagrams.
 Like B nodes, P and M nodes must perform a name discovery or use
 cached information to learn whether a destination name is a group or
 a unique name.
 Datagrams to unique names are unicast directly to the destination by
 P and M nodes, exactly as they are by B nodes.
 Datagrams to group names and NetBIOS broadcast datagrams are unicast
 to the NBDD.  The NBDD then relays the datagrams to each of the nodes
 specified by the destination name.
 An NBDD may not be capable of sending a NetBIOS datagram to a
 particular NetBIOS name, including the broadcast NetBIOS name ("*")
 defined above.  A query mechanism is available to the end- node to
 determine if a NBDD will be able to relay a datagram to a given name.
 Before a datagram, or its fragments, are sent to the NBDD the P or M
 node may send a DATAGRAM QUERY REQUEST packet to the NBDD with the
 DESTINATION_NAME from the DATAGRAM SERVICE packet(s).  The NBDD will
 respond with a DATAGRAM POSITIVE QUERY RESPONSE if it will relay
 datagrams to the specified destination name.  After a positive
 response the end-node unicasts the datagram to the NBDD.  If the NBDD
 will not be able to relay a datagram to the destination name
 specified in the query, a DATAGRAM NEGATIVE QUERY RESPONSE packet is
 returned.  If the NBDD can not distribute a datagram, the end-node
 then has the option of getting the name's owner list from the NBNS
 and sending the datagram directly to each of the owners.
 An NBDD must be able to respond to DATAGRAM QUERY REQUEST packets.
 The response may always be positive.  However, the usage or
 implementation of the query mechanism by a P or M node is optional.
 An implementation may always unicast the NetBIOS datagram to the NBDD
 without asking if it will be relayed.  Except for the datagram query
 facility described above, an NBDD provides no feedback to indicate
 whether it forwarded a datagram.

18. NODE CONFIGURATION PARAMETERS

  1. B NODES:
    1. Node's permanent unique name
    2. Whether IGMP is in use
    3. Broadcast IP address to use

NetBIOS Working Group [Page 58] RFC 1001 March 1987

  1. Whether NetBIOS session keep-alives are needed
  2. Usable UDP data field length (to control fragmentation)
  3. P NODES:
  4. Node's permanent unique name
  5. IP address of NBNS
  6. IP address of NBDD
  7. Whether NetBIOS session keep-alives are needed
  8. Usable UDP data field length (to control fragmentation)
  9. M NODES:
  10. Node's permanent unique name
  11. Whether IGMP is in use
  12. Broadcast IP address to use
  13. IP address of NBNS
  14. IP address of NBDD
  15. Whether NetBIOS session keep-alives are needed
  16. Usable UDP data field length (to control fragmentation)

19. MINIMAL CONFORMANCE

 To ensure multi-vendor interoperability, a minimally conforming
 implementation based on this specification must observe the following
 rules:
 a)   A node designed to work only in a broadcast area must
      conform to the B node specification.
 b)   A node designed to work only in an internet must conform to
      the P node specification.

NetBIOS Working Group [Page 59] RFC 1001 March 1987

REFERENCES

    [1]  "Protocol Standard For a NetBIOS Service on a TCP/UDP
         Transport: Detailed Specifications", RFC 1002, March 1987.
    [2]  IBM Corp., "IBM PC Network Technical Reference Manual", No.
         6322916, First Edition, September 1984.
    [3]  J. Postel (Ed.), "Transmission Control Protocol", RFC 793,
         September 1981.
    [4]  MIL-STD-1778
    [5]  J. Postel, "User Datagram Protocol", RFC 768, 28 August
         1980.
    [6]  J. Reynolds, J. Postel, "Assigned Numbers", RFC 990,
         November 1986.
    [7]  J.  Postel, "Internet Protocol", RFC 791, September 1981.
    [8]  J. Mogul, "Internet Subnets", RFC 950, October 1984
    [9]  J.  Mogul, "Broadcasting Internet Datagrams in the Presence
         of Subnets", RFC 922, October 1984.
    [10] J.  Mogul, "Broadcasting Internet Datagrams", RFC 919,
         October 1984.
    [11] P. Mockapetris, "Domain Names - Concepts and Facilities",
         RFC 882, November 1983.
    [12] P. Mockapetris, "Domain Names - Implementation and
         Specification", RFC 883, November 1983.
    [13] P. Mockapetris, "Domain System Changes and Observations",
         RFC 973, January 1986.
    [14] C. Partridge, "Mail Routing and the Domain System", RFC 974,
         January 1986.
    [15] S. Deering, D. Cheriton, "Host Groups: A Multicast Extension
         to the Internet Protocol", RFC 966, December 1985.
    [16] S. Deering, "Host Extensions for IP Multicasting", RFC 988,
         July 1986.

NetBIOS Working Group [Page 60] RFC 1001 March 1987

APPENDIX A

 This appendix contains supporting technical discussions.  It is not
 an integral part of the NetBIOS-over-TCP specification.
 INTEGRATION WITH INTERNET GROUP MULTICASTING
 The Netbios-over-TCP system described in this RFC may be easily
 integrated with the Internet Group Multicast system now being
 developed for the internet.
 In the main body of the RFC, the notion of a broadcast area was
 considered to be a single MAC-bridged "B-LAN".  However, the
 protocols defined will operate over an extended broadcast area
 resulting from the creation of a permanent Internet Multicast Group.
 Each separate broadcast area would be based on a separate permanent
 Internet Multicast Group.  This multicast group address would be used
 by B and M nodes as their BROADCAST_ADDRESS.
 In order to base the broadcast area on a multicast group certain
 additional procedures are required and certain constraints must be
 met.

A-1. ADDITIONAL PROTOCOL REQUIRED IN B AND M NODES

 All B or M nodes operating on an IGMP based broadcast area must have
 IGMP support in their IP layer software.  These nodes must perform an
 IGMP join operation to enter the IGMP group before engaging in
 NetBIOS activity.

A-2. CONSTRAINTS

 Broadcast Areas may overlap.  For this reason, end-nodes must be
 careful to examine the NetBIOS scope identifiers in all received
 broadcast packets.
 The NetBIOS broadcast protocols were designed for a network that
 exhibits a low average transit time and low rate of packet loss.  An
 IGMP based broadcast area must exhibit these characteristics.  In
 practice this will tend to constrain IGMP broadcast areas to a campus
 of networks interconnected by high-speed routers and inter-router
 links.  It is unlikely that transcontinental broadcast areas would
 exhibit the required characteristics.

NetBIOS Working Group [Page 61] RFC 1001 March 1987

APPENDIX B

 This appendix contains supporting technical discussions.  It is not
 an integral part of the NetBIOS-over-TCP specification.

IMPLEMENTATION CONSIDERATIONS

B-1. IMPLEMENTATION MODELS

 On any participating system, there must be some sort of NetBIOS
 Service to coordinate access by NetBIOS applications on that system.
 To analyze the impact of the NetBIOS-over-TCP architecture, we use
 the following three models of how a NetBIOS service might be
 implemented:
 1.   Combined Service and Application Model
      The NetBIOS service and application are both contained
      within a single process.  No interprocess communication is
      assumed within the system; all communication is over the
      network.  If multiple applications require concurrent access
      to the NetBIOS service, they must be folded into this
      monolithic process.
 2.   Common Kernel Element Model
      The NetBIOS Service is part of the operating system (perhaps
      as a device driver or a front-end processor).  The NetBIOS
      applications are normal operating system application
      processes.  The common element NetBIOS service contains all
      the information, such as the name and listen tables,
      required to co-ordinate the activities of the applications.
 3.   Non-Kernel Common Element Model
      The NetBIOS Service is implemented as an operating system
      application process.  The NetBIOS applications are other
      operating system application processes.  The service and the
      applications exchange data via operating system interprocess
      communication.  In a multi-processor (e.g.  network)
      operating system, each module may reside on a different cpu.
      The NetBIOS service process contains all the shared
      information required to coordinate the activities of the
      NetBIOS applications.  The applications may still require a
      subroutine library to facilitate access to the NetBIOS
      service.

NetBIOS Working Group [Page 62] RFC 1001 March 1987

 For any of the implementation models, the TCP/IP service can be
 located in the operating system or split among the NetBIOS
 applications and the NetBIOS service processes.

B-1.1 MODEL INDEPENDENT CONSIDERATIONS

 The NetBIOS name service associates a NetBIOS name with a host.  The
 NetBIOS session service further binds the name to a specific TCP port
 for the duration of the session.
 The name service does not need to be informed of every Listen
 initiation and completion.  Since the names are not bound to any TCP
 port in the name service, the session service may use a different tcp
 port for each session established with the same local name.
 The TCP port used for the data transfer phase of a NetBIOS session
 can be globally well-known, locally well-known, or ephemeral.  The
 choice is a local implementation issue.  The RETARGET mechanism
 allows the binding of the NetBIOS session to a TCP connection to any
 TCP port, even to another IP node.
 An implementation may use the session service's globally well- known
 TCP port for the data transfer phase of the session by not using the
 RETARGET mechanism and, rather, accepting the session on the initial
 TCP connection.  This is permissible because the caller always uses
 an ephemeral TCP port.
 The complexity of the called end RETARGET mechanism is only required
 if a particular implementation needs it.  For many real system
 environments, such as an in-kernel NetBIOS service implementation, it
 will not be necessary to retarget incoming calls.  Rather, all
 NetBIOS sessions may be multiplexed through the single, well-known,
 NetBIOS session service port.  These implementations will not be
 burdened by the complexity of the RETARGET mechanism, nor will their
 callers be required to jump through the retargetting hoops.
 Nevertheless, all callers must be ready to process all possible
 SESSION RETARGET RESPONSEs.

B-1.2 SERVICE OPERATION FOR EACH MODEL

 It is possible to construct a NetBIOS service based on this
 specification for each of the above defined implementation models.
 For the common kernel element model, all the NetBIOS services, name,
 datagram, and session, are simple.  All the information is contained
 within a single entity and can therefore be accessed or modified
 easily.  A single port or multiple ports for the NetBIOS sessions can
 be used without adding any significant complexity to the session
 establishment procedure.  The only penalty is the amount of overhead
 incurred to get the NetBIOS messages and operation requests/responses

NetBIOS Working Group [Page 63] RFC 1001 March 1987

 through the user and operating system boundary.
 The combined service and application model is very similar to the
 common kernel element model in terms of its requirements on the
 NetBIOS service.  The major difficulty is the internal coordination
 of the multiple NetBIOS service and application processes existing in
 a system of this type.
 The NetBIOS name, datagram and session protocols assume that the
 entities at the end-points have full control of the various well-
 known TCP and UDP ports.  If an implementation has multiple NetBIOS
 service entities, as would be the case with, for example, multiple
 applications each linked into a NetBIOS library, then that
 implementation must impose some internal coordination.
 Alternatively, use of the NetBIOS ports could be periodically
 assigned to one application or another.
 For the typical non-kernel common element mode implementation, three
 permanent system-wide NetBIOS service processes would exist:
  1. The name server
  2. the datagram server
  3. and session server
 Each server would listen for requests from the network on a UDP or
 TCP well-known port.  Each application would have a small piece of
 the NetBIOS service built-in, possibly a library.  Each application's
 NetBIOS support library would need to send a message to the
 particular server to request an operation, such as add name or send a
 datagram or set-up a listen.
 The non-kernel common element model does not require a TCP connection
 be passed between the two processes, session server and application.
 The RETARGET operation for an active NetBIOS Listen could be used by
 the session server to redirect the session to another TCP connection
 on a port allocated and owned by the application's NetBIOS support
 library.  The application with either a built-in or a kernel-based
 TCP/IP service could then accept the RETARGETed connection request
 and process it independently of the session server.
 On Unix(tm) or POSIX(tm), the NetBIOS session server could create
 sub-processes for incoming connections.  The open sessions would be
 passed through "fork" and "exec" to the child as an open file
 descriptor.  This approach is very limited, however.  A pre- existing
 process could not receive an incoming call.  And all call-ed
 processes would have to be sub-processes of the session server.

B-2. CASUAL AND RESTRICTED NetBIOS APPLICATIONS

 Because NetBIOS was designed to operate in the open system
 environment of the typical personal computer, it does not have the

NetBIOS Working Group [Page 64] RFC 1001 March 1987

 concept of privileged or unprivileged applications.  In many multi-
 user or multi-tasking operating systems applications are assigned
 privilege capabilities.  These capabilities limit the applications
 ability to acquire and use system resources.  For these systems it is
 important to allow casual applications, those with limited system
 privileges, and privileged applications, those with 'super-user'
 capabilities but access to them and their required resources is
 restricted, to access NetBIOS services.  It is also important to
 allow a systems administrator to restrict certain NetBIOS resources
 to a particular NetBIOS application.  For example, a file server
 based on the NetBIOS services should be able to have names and TCP
 ports for sessions only it can use.
 A NetBIOS application needs at least two local resources to
 communicate with another NetBIOS application, a NetBIOS name for
 itself and, typically, a session.  A NetBIOS service cannot require
 that NetBIOS applications directly use privileged system resources.
 For example, many systems require privilege to use TCP and UDP ports
 with numbers less than 1024.  This RFC requires reserved ports for
 the name and session servers of a NetBIOS service implementation.  It
 does not require the application to have direct access these reserved
 ports.
 For the name service, the manager of the local name table must have
 access to the NetBIOS name service's reserved UDP port.  It needs to
 listen for name service UDP packets to defend and define its local
 names to the network.  However, this manager need not be a part of a
 user application in a system environment which has privilege
 restrictions on reserved ports.
 The internal name server can require certain privileges to add,
 delete, or use a certain name, if an implementer wants the
 restriction.  This restriction is independent of the operation of the
 NetBIOS service protocols and would not necessarily prevent the
 interoperation of that implementation with another implementation.
 The session server is required to own a reserved TCP port for session
 establishment.  However, the ultimate TCP connection used to transmit
 and receive data does not have to be through that reserved port.  The
 RETARGET procedure the NetBIOS session to be shifted to another TCP
 connection, possibly through a different port at the called end.
 This port can be an unprivileged resource, with a value greater than
 1023.  This facilitates casual applications.
 Alternately, the RETARGET mechanism allows the TCP port used for data
 transmission and reception to be a reserved port.  Consequently, an
 application wishing to have access to its ports maintained by the
 system administrator can use these restricted TCP ports.  This
 facilitates privileged applications.
 A particular implementation may wish to require further special

NetBIOS Working Group [Page 65] RFC 1001 March 1987

 privileges for session establishment, these could be associated with
 internal information.  It does not have to be based solely on TCP
 port allocation.  For example, a given NetBIOS name may only be used
 for sessions by applications with a certain system privilege level.
 The decision to use reserved or unreserved ports or add any
 additional name registration and usage authorization is a purely
 local implementation decision.  It is not required by the NetBIOS
 protocols specified in the RFC.

B-3. TCP VERSUS SESSION KEEP-ALIVES

 The KEEP-ALIVE is a protocol element used to validate the existence
 of a connection.  A packet is sent to the remote connection partner
 to solicit a response which shows the connection is still
 functioning.  TCP KEEP-ALIVES are used at the TCP level on TCP
 connections while session KEEP-ALIVES are used on NetBIOS sessions.
 These protocol operations are always transparent to the connection
 user.  The user will only find out about a KEEP-ALIVE operation if it
 fails, therefore, if the connection is lost.
 The NetBIOS specification[2] requires the NetBIOS service to inform
 the session user if a session is lost when it is in a passive or
 active state.  Therefore,if the session user is only waiting for a
 receive operation and the session is dropped the NetBIOS service must
 inform the session user.  It cannot wait for a session send operation
 before it informs the user of the loss of the connection.
 This requirement stems from the management of scarce or volatile
 resources by a NetBIOS application.  If a particular user terminates
 a session with a server application by destroying the client
 application or the NetBIOS service without a NetBIOS Hang Up, the
 server application will want to clean-up or free allocated resources.
 This server application if it only receives and then sends a response
 requires the notification of the session abort in the passive state.
 The standard definition of a TCP service cannot detect loss of a
 connection when it is in a passive state, waiting for a packet to
 arrive.  Some TCP implementations have added a KEEP-ALIVE operation
 which is interoperable with implementations without this feature.
 These implementations send a packet with an invalid sequence number
 to the connection partner.  The partner, by specification, must
 respond with a packet showing the correct sequence number of the
 connection.  If no response is received from the remote partner
 within a certain time interval then the TCP service assumes the
 connection is lost.
 Since many TCP implementations do not have this KEEP-ALIVE function
 an optional NetBIOS KEEP-ALIVE operation has been added to the
 NetBIOS session protocols.  The NetBIOS KEEP-ALIVE uses the
 properties of TCP to solicit a response from the remote connection

NetBIOS Working Group [Page 66] RFC 1001 March 1987

 partner.  A NetBIOS session message called KEEP-ALIVE is sent to the
 remote partner.  Since this results in TCP sending an IP packet to
 the remote partner, the TCP connection is active.  TCP will discover
 if the TCP connection is lost if the remote TCP partner does not
 acknowledge the IP packet.  Therefore, the NetBIOS session service
 does not send a response to a session KEEP ALIVE message.  It just
 throws it away.  The NetBIOS session service that transmits the KEEP
 ALIVE is informed only of the failure of the TCP connection.  It does
 not wait for a specific response message.
 A particular NetBIOS implementation should use KEEP-ALIVES if it is
 concerned with maintaining compatibility with the NetBIOS interface
 specification[2].  Compatibility is especially important if the
 implementation wishes to support existing NetBIOS applications, which
 typically require the session loss detection on their servers, or
 future applications which were developed for implementations with
 session loss detection.

B-4. RETARGET ALGORITHMS

 This section contains 2 suggestions for RETARGET algorithms.  They
 are called the "straight" and "stack" methods.  The algorithm in the
 body of the RFC uses the straight method.  Implementation of either
 algorithm must take into account the Session establishment maximum
 retry count.  The retry count is the maximum number of TCP connect
 operations allowed before a failure is reported.
 The straight method forces the session establishment procedure to
 begin a retry after a retargetting failure with the initial node
 returned from the name discovery procedure.  A retargetting failure
 is when a TCP connection attempt fails because of a time- out or a
 NEGATIVE SESSION RESPONSE is received with an error code specifying
 NOT LISTENING ON CALLED NAME.  If any other failure occurs the
 session establishment procedure should retry from the call to the
 name discovery procedure.
 A minimum of 2 retries, either from a retargetting or a name
 discovery failure.  This will give the session service a chance to
 re-establish a NetBIOS Listen or, more importantly, allow the NetBIOS
 scope, local name service or the NBNS, to re-learn the correct IP
 address of a NetBIOS name.
 The stack method operates similarly to the straight method.  However,
 instead of retrying at the initial node returned by the name
 discovery procedure, it restarts with the IP address of the last node
 which sent a SESSION RETARGET RESPONSE prior to the retargetting
 failure.  To limit the stack method, any one host can only be tried a
 maximum of 2 times.

NetBIOS Working Group [Page 67] RFC 1001 March 1987

B-5. NBDD SERVICE

 If the NBDD does not forward datagrams then don't provide Group and
 Broadcast NetBIOS datagram services to the NetBIOS user.  Therefore,
 ignore the implementation of the query request and, when get a
 negative response, acquiring the membership list of IP addresses and
 sending the datagram as a unicast to each member.

B-6. APPLICATION CONSIDERATIONS

B-6.1 USE OF NetBIOS DATAGRAMS

 Certain existing NetBIOS applications use NetBIOS datagrams as a
 foundation for their own connection-oriented protocols.  This can
 cause excessive NetBIOS name query activity and place a substantial
 burden on the network, server nodes, and other end- nodes.  It is
 recommended that this practice be avoided in new applications.

NetBIOS Working Group [Page 68]

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