Network Working Group                                     D. Bryant
Request for Comments: 2166                                3Com Corp
Category: Informational                                 P. Brittain
                                               Data Connection Ltd.
                                                          June 1997

                      APPN Implementer's Workshop
                         Closed Pages Document

                         DLSw v2.0 Enhancements

Status of this Memo

This memo provides information for the Internet community.  This memo
does not specify an Internet standard of any kind.  Distribution of
this memo is unlimited.


   This document specifies

   - a set of extensions to RFC 1795 designed to improve the scalability
     of DLSw
   - clarifications to RFC 1795 in the light of the implementation
     experience to-date.

   It is assumed that the reader is familiar with DLSw and RFC 1795.  No
   effort has been made to explain these existing protocols or
   associated terminology.

   This document was developed in the DLSw Related Interest Group (RIG)
   of the APPN Implementers Workshop (AIW). If you would like to
   participate in future DLSw discussions, please subscribe to the DLSw
   RIG mailing lists by sending a mail to
   specifying 'subscribe aiw-dlsw' as the body of the message.

Table of Contents

   1. INTRODUCTION ................................................    3
   2. HALT REASON CODES............................................    3
   3. SCOPE OF SCALABILITY ENHANCEMENTS............................    4
   4. OVERVIEW OF SCALABILITY ENHANCEMENTS.........................    6
   5. MULTICAST GROUPS AND ADDRESSING..............................    7
   5.1 USING MULTICAST GROUPS......................................    8
   5.2 DLSW MULTICAST ADDRESSES....................................    8
   6. DLSW MESSAGE TRANSPORTS......................................    8
   6.1 TCP/IP CONNECTIONS ON DEMAND................................    9
    6.1.1 TCP CONNECTIONS ON DEMAND RACE CONDITIONS................    9

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   6.2 SINGLE SESSION TCP/IP CONNECTIONS...........................    9
    6.2.1 EXPEDITED SINGLE SESSION TCP/IP CONNECTIONS..............   10 TCP PORT NUMBERS......................................   10 TCP CONNECTION SETUP..................................   10 SINGLE SESSION SETUP RACE CONDITIONS..................   10 TCP CONNECTIONS WITH NON-MULTICAST CAPABLE DLSW PEERS.   11
   6.3 UDP DATAGRAMS...............................................   12
    6.3.1 VENDOR SPECIFIC FUNCTIONS OVER UDP.......................   12
    6.3.2 UNICAST UDP DATAGRAMS....................................   12
    6.3.3 MULTICAST UDP DATAGRAMS..................................   13
   6.5 TCP TRANSPORT...............................................   14
   7. MIGRATION SUPPORT............................................   14
   7.1 CAPABILITIES EXCHANGE.......................................   14
   7.2 CONNECTING TO NON-MULTICAST CAPABLE NODES...................   15
   8. SNA SUPPORT..................................................   16
   8.1 ADDRESS RESOLUTION..........................................   16
   8.2 EXPLORER FRAMES.............................................   16
   8.3 CIRCUIT SETUP...............................................   17
   8.4 EXAMPLE SNA SSP MESSAGE SEQUENCE............................   17
   8.5 UDP RELIABILITY.............................................   19
    8.5.1 RETRIES..................................................   19
   9. NETBIOS......................................................   20
   9.1 ADDRESS RESOLUTION..........................................   21
   9.2 EXPLORER FRAMES.............................................   21
   9.3 CIRCUIT SETUP...............................................   21
   9.4 EXAMPLE NETBIOS SSP MESSAGE SEQUENCE........................   22
   9.5 MULTICAST RELIABILITY AND RETRIES...........................   24
   10. SEQUENCING..................................................   24
   11. FRAME FORMATS...............................................   25
   11.1 MULTICAST CAPABILITIES CONTROL VECTOR......................   25
    11.1.1 DLSW CAPABILITIES NEGATIVE RESPONSE.....................   26
   11.2 UDP PACKETS................................................   26
   11.3 VENDOR SPECIFIC UDP PACKETS................................   27
   12. COMPLIANCE STATEMENT........................................   28
   13. SECURITY CONSIDERATIONS.....................................   29
   14. ACKNOWLEDGEMENTS............................................   29
   15. AUTHORS' ADDRESSES..........................................   30
   16. APPENDIX - CLARIFICATIONS TO RFC 1795.......................   31

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   1. Introduction

   This document defines v2.0 of Data Link Switching (DLSw) in the form
   of a set of enhancements to RFC 1795. These enhancements are designed
   to be fully backward compatible with existing RFC 1795
   implementations. As a compatible set of enhancements to RFC 1795,
   this document does not replace or supersede RFC 1795.

   The bulk of these enhancements address scalability issues in DLSw
   v1.0.  Reason codes have also been added to the HALT_DL and
   HALT_DL_NOACK SSP messages in order to improve the diagnostic
   information available.

   Finally, the appendix to this document lists a number of
   clarifications to RFC 1795 where the implementation experience to-
   date has shown that the original RFC was ambiguous or unclear. These
   clarifications should be read alongside RFC 1795 to obtain a full
   specification of the base v1.0 DLSw standard.

2. HALT Reason codes

   RFC 1795 provides no mechanism for a DLSw to communicate to its peer
   the reason for dropping a circuit.  DLSw v2.0 adds reason code fields
   to the HALT_DL and HALT_DL_NOACK SSP messages to carry this

   The reason code is carried as 6 bytes of data after the existing SSP
   header.  The format of these bytes is as shown below.

   Byte       Description
   0-1        Generic HALT reason code in byte normal format

   2-5        Vendor-specific detailed reason code

   The generic HALT reason code takes one of the following decimal
   values (which are chosen to match the disconnect reason codes
   specified in the DLSw MIB).

   1 - Unknown error
   2 - Received DISC from end-station
   3 - Detected DLC error with end-station
   4 - Circuit-level protocol error (e.g., pacing)
   5 - Operator-initiated (mgt station or local console)

   The vendor-specific detailed reason code may take any value.

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   All V2.0 DLSws must include this information on all HALT_DL and
   HALT_DL_NOACK messages sent to v2.0 DLSw peers.  For backwards
   compatibility with RFC 1795, DLSw V2.0 implementations must also
   accept a HALT_DL or HALT_DL_NOACK message received from a DLSw peer
   that does not carry this information (i.e. RFC 1795 format for these
   SSP messages).

3. Scope of Scalability Enhancements

   The DLSw Scalability group of the AIW identified a number of
   scalability issues associated with existing DLSw protocols as defined
   in RFC 1795:

   - Administration

     RFC 1795 implies the need to define the transport address of all
     DLSw peers at each DLSw.  In highly meshed situations (such as
     those often found in NetBIOS networks), the resultant
     administrative burden is undesirable.

   - Address Resolution

     RFC 1795 defines point to point TCP (or other reliable transport
     protocol) connections between DLSw peers.  When attempting to
     discover the location of an unknown resource, a DLSw sends an
     address resolution packet to each DLSw peer over these connections.
     In highly meshed configurations, this can result in a very large
     number of packets in the transport network.  Although each packet
     is sent individually to each DLSw peer, they are each identical in
     nature.  Thus the transport network is burdened with excessive
     numbers of identical packets.  Since the transport network is most
     commonly a wide area network, where bandwidth is considered a
     precious resource, this packet duplication is undesirable.

   - Broadcast Packets

     In addition to the address resolution packets described above, RFC
     1795 also propagates NetBIOS broadcast packets into the transport
     network.  The UI frames of NetBIOS are sent as LAN broadcast
     packets.  RFC 1795 propagates these packets over the point to point
     transport connections to each DLSw peer.  In the same manner as
     above, this creates a large number of identical packets in the
     transport network, and hence is undesirable.  Since NetBIOS UI
     frames can be sent by applications, it is difficult to predict or
     control the rate and quantity of such traffic.  This compounds the
     undesirability of the existing RFC 1795 propagation method for
     these packets.

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   - TCP (transport connection) Overhead

     As defined in RFC 1795, each DLSw maintains a transport connection
     to its DLSw peers.  Each transport connection guarantees in order
     packet delivery.   This is accomplished using acknowledgment and
     sequencing algorithms which require both CPU and memory at the DLSw
     endpoints in direct proportion to the number transport connections.
     The DLSw Scalability group has identified two scenarios where the
     number of transport connections can become significant resulting in
     excessive overhead and corresponding equipment costs (memory and
     CPU).   The first scenario is found in highly meshed DLSw
     configurations where the number of transport connections
     approximates n2 (where n is the number of DLSw peers).  This is
     typically found in DLSw networks supporting NetBIOS.  The second
     scenario is found  in networks  where many remote locations
     communicate to few central sites.  In this case, the central sites
     must support n transport connections  (where n is the number of
     remote sites).    In both scenarios the resultant transport
     connection overhead is considered undesirable depending upon the
     value of n.

   - LLC2 overhead

     RFC 1795 specifies that each DLSw provides local termination for
     the LLC2 (SDLC or other SNA reliable data link  protocol) sessions
     traversing the SSP.   Because these reliable data links provide
     guaranteed in order packet delivery, the memory and CPU overhead of
     maintaining these connections can also become significant.   This
     is particularly undesirable in the second scenario described above,
     because the number of reliable connections maintained at the
     central site is the aggregate of the connections maintained at each
     remote site.

   It is not the intent of this document to address all the undesirable
   scalability issues associated with RFC 1795.  This paper identifies
   protocol enhancements to RFC 1795 using the inherent multicast
   capabilities of the underlying transport network to improve the
   scalability of RFC 1795.  It is believed that the enhancements
   defined, herein, address many of the issues identified above, such as
   administration, address resolution, broadcast packets, and, to a
   lesser extent, transport overhead.  This paper does not address LLC2
   overhead.  Subsequent efforts by the AIW and/or DLSw Scalability
   group may address the unresolved scalability issues.

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   While it is the intent of this paper to accommodate all transport
   protocols as best as is possible, it is recognized that the multicast
   capabilities of many protocols is not yet well defined, understood,
   or implemented. Since TCP is the most prevalent DLSw transport
   protocol in use today, the DLSw Scalability group has chosen to focus
   its definition around IP based multicast services. This document only
   addresses the implementation detail of IP based multicast services.

   This proposal does not consider the impacts of IPv6 as this was
   considered too far from widespread use at the time of writing.

4. Overview of Scalability Enhancements

   This paper describes the use of multicast services within the
   transport network to improve the scalability of DLSw based
   networking.  There are only a few main components of this proposal:

   - Single session TCP connections

     RFC 1795 defines a negotiation protocol for DLSw peers to choose
     either two unidirectional or one bi-directional TCP connection.
     DLSws implementing the enhancements described in this document must
     support and use(whenever required and possible)a single bi-
     directional TCP connection between DLSw peers. That is to say that
     the single tunnel negotiation support of RFC 1795 is a prerequisite
     function to this set of enhancements. Use of two unidirectional TCP
     connections is only allowed (and required)for migration purposes
     when communicating with DLSw peers that do not implement these

     This document also specifies a faster method for bringing up a
     single TCP connection between two DLSw peers than the negotiation
     used in RFC 1795.  This faster method, detailed in section 6.2.1,
     must be used where both peers are known to support DLSw v2.0.

   - TCP connections on demand

     Two DLSw peers using these enhancements will only establish a TCP
     connection when necessary.  SSP connections to DLSw peers which do
     not implement these enhancements are assumed to be established by
     the means defined in RFC 1795.  DLSws implementing v2.0 utilize UDP
     based transport services to send address resolution packets
     (CANUREACH_ex, NETBIOS_NQ_ex, etc.).  If a positive response is
     received, then a TCP connection is only established to the
     associated DLSw peer if one does not already exist.
     Correspondingly, TCP connections are brought down when there are no
     circuits to a DLSw peer for an implementation defined period of

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   - Address resolution through UDP

     The main thrust of this paper is to utilize non-reliable transport
     and the inherent efficiencies of multicast protocols whenever
     possible and applicable to reduce network overhead.  Accordingly,
     the address resolution protocols of SNA and NetBIOS are sent over
     the non-reliable transport of IP, namely UDP.  In addition, IP
     multicast/unicast services are used whenever address resolution
     packets must be sent to multiple destinations. This avoids the need
     to maintain TCP SSP connections between two DLSw peers when no
     circuits are active.  CANUREACH_ex and ICANREACH_ex packets can be
     sent to all the appropriate DLSw peers without the need for pre-
     configured peers or pre-established TCP/IP connections.  In
     addition, most multicast services (including TCP's MOSPF, DVMRP,
     MIP, etc.) replicate and propagate messages only as necessary to
     deliver to all multicast members.   This avoids duplication and
     excessive bandwidth consumption in the transport network.

     To further optimize the use of WAN resources, address resolution
     responses are sent in a directed fashion (i.e., unicast) via UDP
     transport whenever possible.   This avoids the need to setup or
     maintain TCP connections when they are not required.  It also
     avoids the bandwidth costs associated with broadcasting.

     Note: It is also permitted to send some address resolution traffic
     over existing TCP connections.  The conditions under which this is
     permitted are detailed in section 7.

   - NetBIOS broadcasts over UDP

     In the same manner as above, NetBIOS broadcast packets are sent via
     UDP (unicast and multicast) whenever possible and appropriate. This
     avoids the need to establish TCP connections between DLSw peers
     when there are no circuits required.   In addition, bandwidth in
     the transport network is conserved by utilizing the efficiencies
     inherent to multicast service implementation.  Details covering
     identification of these packets and proper propagation methods are
     described in section 10.

5. Multicast Groups and Addressing

   IP multicast services provides an unreliable datagram oriented
   delivery service to multiple parties. Communication is accomplished
   by sending and/or listening to specific 'multicast' addresses.  When
   a given node sends a packet to a specific address (defined to be
   within the multicast address range), the IP network (unreliably)
   delivers the packet to every node listening on that address.

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   Thus, DLSws can make use of this service by simply sending and
   receiving (i.e., listening for) packets on the appropriate multicast
   addresses. With careful planning and implementation, networks can be
   effectively partitioned and network overhead controlled by sending
   and listening on different addresses groups.  It is not the intent of
   this paper to define or describe the techniques by which this can be
   accomplished.  It is expected that the networking industry (vendors
   and end users alike) will determine the most appropriate ways to make
   use of the functions provided by use of DLSw multicast transport

5.1 Using Multicast Groups

   The multicast addressing as described above can be effectively used
   to limit the amount of broadcast/multicast traffic in the network.
   It is not the intent of this document to describe how individual
   DLSw/SSP implementations would assign or choose group addresses.  The
   specifics of how this is done and exposed to the end user is an issue
   for the specific implementor.  In order to provide for multivendor
   interoperability and simplicity of configuration, however, this paper
   defines a single IP multicast address,, to be used as a
   default DLSw multicast address.  If a given implementation chooses to
   provide a default multicast address, it is recommended this address
   be used.  In addition, this address should be used for both
   transmitting and receiving of multicast SSP messages.  Implementation
   of a default multicast address is not, however, required.

5.2 DLSw Multicast Addresses

   For the purpose of long term interoperability, the AIW has secured a
   block of IP multicast addresses to be used with DLSw.  These
   addresses are listed below:

   Address Range        Purpose
   --------------------------------------------------------------------         Default multicast address     User defined DLSw multicast groups     Reserved for future use by the DLSw RIG in DLSw

6. DLSw Message Transports

   With the introduction of DLSw Multicast Protocols, SSP messages are
   now sent over two distinct transport mechanisms: TCP/IP connections
   and UDP services.  Furthermore, the UDP datagrams can be sent to two
   different kinds of IP addresses: unique IP addresses (generally
   associated with a specific DLSw), and multicast IP addresses
   (generally associated with a group of DLSw peers).

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6.1 TCP/IP Connections on Demand

   As is the case in RFC 1795, TCP/IP connections are established
   between DLSw peers.  Unlike RFC 1795, however, TCP/IP connections are
   only established to carry reliable circuit data (i.e., LLC2 based
   circuits).  Accordingly, a TCP/IP connection is only established to a
   given DLSw peer when the first circuit to that DLSw is required
   (i.e., the origin DLSw must send a CANUREACH_CS to a target DLSw peer
   and there is no existing TCP connection between the two).  In
   addition, the TCP/IP connection is brought down an implementation
   defined amount of time after the last active (not pending) circuit
   has terminated.  In this way, the overhead associated with
   maintaining TCP connections is minimized.

   With the advent of TCP connections on demand, the activation and
   deactivation of TCP connections becomes a normal occurrence as
   opposed to the exception event it constitutes in RFC 1795.  For this
   reason, it is recommended that implementations carefully consider the
   value of SNMP traps for this condition.

6.1.1 TCP Connections on Demand Race Conditions

   Non-circuit based SSP packetsn (e.g.,CANUREACH_ex, etc.) may still be
   sent/received over TCP connections after all circuits have been
   terminated.  Taking this into account implementations should still
   gracefully terminate these TCP connections once the connection is no
   longer supporting circuits.  This may require an implementation to
   retransmit request frames over UDP when no response to a TCP based
   unicast request is received and the TCP connection is brought down.
   This is not required in the case of multicast requests as these are
   received over the multicast transport mechanism.

6.2 Single Session TCP/IP Connections

   RFC 1795 defines the use of two unidirectional TCP/IP sessions
   between any pair of DLSw peers using read port number 2065 and write
   port number 2067.  Additionally, RFC 1795 allows for implementations
   to optionally use only one bi-directional TCP/IP session.  Using one
   TCP/IP session between DLSw peers is believed to significantly
   improve the performance and scalability of DLSw protocols.
   Performance is improved because TCP/IP acknowledgments are much more
   likely to be piggy-backed on real data when TCP/IP sessions are used
   bi-directionally.  Scalability is improved because fewer TCP control
   blocks, state machines, and associated message buffers are required.
   For these reasons, the DLSw enhancements defined in this paper
   REQUIRE the use of single session TCP/IP sessions.

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   Accordingly, DLSws implementing these enhancements must carry the TCP
   Connections Control Vector in their Capabilities Exchange.  In
   addition, the TCP Connections Control Vector must indicate support
   for 1 connection.

6.2.1 Expedited Single Session TCP/IP Connections

   In RFC 1795, single session TCP/IP connections are accomplished by
   first establishing two uni-directional TCP connections, exchanging
   capabilities, and then bringing down one of the connections.  In
   order to avoid the unnecessary flows and time delays associated with
   this process, a new single session bi-directional TCP/IP connection
   establishment algorithm is defined. TCP Port Numbers

   DLSws implementing these enhancements will use a TCP destination port
   of 2067 (as opposed to RFC 1795 which uses 2065) for single session
   TCP connections.  The source port will be a random port number using
   the established TCP norms which exclude the possibility of either
   2065 or 2067. TCP Connection Setup

   DLSw peers implementing these enhancements will establish a single
   session TCP connection whenever the associated peer is known to
   support this capability.  To do this, the initiating DLSw simply
   sends a TCP setup request to destination port 2067.  The receiving
   DLSw responds accordingly and the TCP three way handshake ensues.
   Once this handshake has completed, each DLSw is notified and the DLSw
   capabilities exchange ensues.  As in RFC 1795, no flows may take
   place until the capabilities exchange completes. Single Session Setup Race Conditions

   The new expedited single session setup procedure described above
   opens up the possibility of a race condition that occurs when two
   DLSw peers attempt to setup single session TCP connections to each
   other at the same time.  To avoid the establishment of two TCP
   connections, the following rules are applied when establishing
   expedited single session TCP connections:

   1.If an inbound TCP connect indication is received on port 2067 while
     an outbound TCP connect request (on port 2067) to the same DLSw (IP
     address) is in process or outstanding, the DLSw with the higher IP
     address will close or reject the connection from the DLSw with the
     lower IP address.

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   2.To further expedite the process, the DLSw with the lower IP address
     may choose (implementation option) to close its connection request
     to the DLSw with the higher address when this condition is
   3.If the DLSw with the lower IP address has already sent its
     capabilities exchange request on its connection to the DLSw with
     the higher IP address, it must resend its capabilities exchange
     request over the remaining TCP connection from its DLSw peer (with
     the higher IP address).
   4.The DLSw with the higher IP address must ignore any capabilities
     exchange request received over the TCP connection to be terminated
     (the one from the DLSw with the lower IP address). TCP Connections with Non-Multicast Capable DLSw peers

   During periods of migration, it is possible that TCP connections
   between multicast capable and non-multicast capable DLSw peers will
   occur.  It is also possible that multicast capable DLSws may attempt
   to establish TCP connections with partners of unknown capabilities
   (e.g., statically defined peers).  To handle these conditions the
   following additional rules apply to expedited single session TCP
   connection setup:

   1.If the capability of a DLSw peer is not known, an implementation
     may choose to send the initial TCP connect request to either port
     2067 (expedited single session setup) or port 2065 (standard RFC
     1795 TCP setup).
   2.If a multicast capable DLSw receives an inbound TCP connect request
     on port 2065 while processing an outbound request on 2067 to the
     same DLSw, the sending DLSw will terminate its 2067 request and
     respond as defined in RFC 1795 with an outbound 2065 request
     (standard RFC 1795 TCP setup).
   3.If a multicast capable DLSw receives an indication that the DLSw
     peer is not multicast capable (the port 2067 setup request times
     out or a port not recognized rejection is received), it will send
     another connection request using port 2065 and the standard RFC
     1795 session setup protocol.

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6.3 UDP Datagrams

   As mentioned above, UDP datagrams can be sent two different ways:
   unicast (e.g., sent to a single unique IP address) or multicast
   (i.e., sent to an IP multicast address).  Throughout this document,
   the term UDP datagram will be used to refer to SSP messages sent over
   UDP, while unicast and multicast SSP messages will refer to the
   specific type/method of UDP packet transport.  In either case,
   standard UDP services are used to transport these packets.  In order
   to properly parse the inbound UDP packets and deliver them to the SSP
   state machines, all DLSw UDP packets will use the destination port of

   In addition, the checksum function of UDP remains optional for DLSw
   SSP messages.  It is believed that the inherent CRC capabilities of
   all data link transports will adequately protect SSP packets during
   transmission.  And the incremental exposure to intermediate nodal
   data corruption is negligible.  For further information on UDP packet
   formats see the “Frame Formats” section.

6.3.1 Vendor Specific Functions over UDP

   In order to accommodate vendor specific capabilities over UDP
   transport, a new SSP packet format has been defined.  This new packet
   format is required because message traffic of this type is not
   necessarily preceded by a capabilities exchange.  Accordingly, DLSw's
   wishing to invoke a vendor specific function may send out this new
   SSP packet format over UDP.

   Because this packet can be sent over TCP connections and non-
   multicast capable nodes may not be able to recognize it,
   implementations may only send this packet over TCP to DLSw peers
   known to understand this packet format (i.e., multicast capable).  To
   avoid this situation in the future, DLSws implementing these
   enhancements must ignore SSP packets with an unrecognized DLSw
   version number in the range of x'31' to x'3F'.  Further information
   and the precise format for this new packet type is described below in
   the “Frame Formats” section.

6.3.2 Unicast UDP Datagrams

   Generically speaking, a unicast UDP datagram is utilized whenever an
   SSP message (not requiring reliable transport) must be sent to a
   unique set (not all) of DLSw peers.  This avoids the overhead of
   having to establish and maintain TCP connections when they are not
   required for reliable data transport.

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   A typical example of when unicast UDP might be used would be an
   ICANREACH_ex response from a peer DLSw (with which no TCP connection
   currently exists).  In this case, the sending DLSw knows the IP
   address of the intended receiver and can simply send the response via
   unicast UDP.  In addition, there are a number of NetBIOS cases where
   unicast UDP is used to handle UI frames directed to a specific DLSw
   (e.g., NetBIOS STATUS_RESPONSE).  Further detail is provided in the
   NetBIOS section of this document.

6.3.3 Multicast UDP Datagrams

   In a broad sense, multicast UDP datagrams are used whenever a given
   SSP message must be sent to multiple DLSw peers.  In the case of SNA,
   this is primarily the CANUREACH_ex packets.  In the case of NetBIOS,
   multicast datagrams are used to send broadcast UI frames such as
   NetBIOS user datagrams and broadcast datagrams.

   Note, however, it is sometimes possible to avoid broadcasting certain
   NetBIOS frames that would otherwise be broadcast in the LAN
   environment.  This is typically accomplished using name caching
   techniques not described in this paper.  In cases of this type when a
   single destination DLSw can be determined, unicast transport can be
   used to send the 'broadcast' NetBIOS frame to a single destination.
   A more detailed listing of NetBIOS SSP packets and transport methods
   can be found in the NetBIOS section of this document.

6.4 Unicast UDP Datagrams in Lieu of IP Multicast

   Because the use of IP multicast services is actually a function of IP
   itself and not DLSw proper, it is possible for implementations to
   simply make use of the UDP transport mechanisms described in this
   paper without making direct use of the IP multicast function.  While
   this is not considered to be as efficient as using multicast
   transport mechanisms, this practice is not explicitly prohibited.

   Implementations which choose to make use of UDP transport in this
   manner must first know the IP address of all the potential target
   DLSw peers and send individual unicast packets to each.  How this
   information is obtained and/or maintained is outside the scope of
   this paper.

   As a matter of compliance, implementers need not send SSP packets
   outbound over UDP as there are some conditions where this may not be
   necessary or desirable.  It is, however, required that implementers
   provide an option to receive SSP packets over UDP.

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6.5 TCP Transport

   Despite the addition of UDP based packet transport, TCP remains the
   fundamental form of communications between DLSw peers.  In
   particular, TCP is still used to carry all LLC2 based circuit data.

   Throughout this document wherever UDP unicast (not multicast) is
   discussed, the reader should be aware that TCP may be used instead.
   Moreover, it is strongly recommended that TCP be used in preference
   to UDP whenever a TCP connection to the destination already exists.
   Implementations, however, should be prepared to receive SSP packets
   from either transport (TCP or UDP).

7. Migration Support

   It is anticipated that some networks will experience a transition
   stage where both RFC 1795 (referred to as 'non-multicast' DLSws) and
   It will be important for these two DLSw node types to interoperate
   and thus the following accommodations for non-multicast DLSws are

7.1 Capabilities Exchange

   In order to guarantee both backward and forward capability, DLSws
   which implement these multicast enhancements will carry a “Multicast
   Capabilities” Control Vector in their capabilities exchange (see RFC
   1795 for an explanation of capabilities exchange protocols).
   Presence of the Multicast Capabilities control vector indicates
   support for the protocols defined in this document on a per DLSw peer
   basis.  Conversely, lack of the Multicast Capabilities control vector
   indicates no support for these extensions on a per DLSw peer basis.

   Additionally, nodes implementing these enhancement will carry a
   modified DLSw Version control vector (x'82') indicating support for
   version 2 release 0.

   Lastly, presence of these control vectors mandates a TCP Connections
   Control Vector indicating support for 1 TCP connection in the same
   Capabilities exchange.

   If a multicast capable DLSw receives a Capabilities Exchange CV that
   includes the Multicast Capabilites CV but does not meet the above
   criteria, it must reject the capabilities exchange by sending a
   negative response as described in section 11.1.1.

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7.2 Connecting to Non-Multicast Capable Nodes

   It is assumed that TCP connections to DLSw peers which do not support
   multicast services are established by some means outside the scope of
   this paper (i.e., non-multicast partner addresses are configured by
   the customer).  TCP connections must be established and maintained to
   down level nodes in the exact same manner as RFC 1795 requires,
   establishes, and maintains them.  And because non-multicast DLSw
   peers will not indicate support for multicast services in their
   capabilities exchange, a multicast capable DLSw will know all its
   non-multicast peers.

7.3 Communicating with Multicast Capable Nodes

   Because non-multicast nodes will not receive SSP frames via UDP
   (unicast or multicast) transmission, SSP messages to these DLSw peers
   must be sent over TCP connections.  Therefore, nodes which implement
   the multicast protocol enhancements must keep track of which DLSw
   peers do not support multicast extensions (as indicated in the
   capabilities exchange).  When a given packet is sent out via
   multicast services, it must also be sent over multicast UDP(to reach
   other multicast capable DLSw peers) and over the TCP connection to
   each non-multicast node.  And although the multicast service requires
   periodic retransmissions (for reliability reasons), this is not the
   case with TCP connections to non-multicast nodes. Therefore,
   multicast capable DLSws should not resend SSP packets over TCP
   transport connection but rather, rely upon TCP to recover any lost
   packets. Furthermore, communications with non-multicast nodes should
   be in exact compliance with RFC 1795 protocols.

   When sending a unicast UDP message, it is important to know that the
   destination DLSw supports multicast services.  This knowledge can be
   obtained from previous TCP connections/capabilities exchanges or
   inferred from a previously received UDP message, but how this
   information is obtained is outside the scope of this paper.  In the
   latter case, if the DLSw is non-multicast, then there would be a TCP
   connection to it and it would be known to be non-multicast.  If it is
   multicast capable and a TCP connection is in existence, then its
   level is known (via the prior capabilities exchange).  If its
   capabilities are not known and there is not an existing TCP
   connection, then it can be implied to be multicast capable by virtue
   of a cached entry but no active TCP connection (e.g., TCP peer on
   demand support).  This inference, however, could be erroneous in
   cases where the TCP connection (to a non-multicast DLSw) has failed
   for some reason. But normal UDP based unicast verification mechanisms
   will detect no active path to the destination and circuit setup will
   proceed correctly (i.e., succeed or fail in accordance with true

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8. SNA Support

   Note: This paper does not attempt to address the unique issues
   presented by SNA/HPR and its non-ERP data links

   In SNA protocols the generalized packet sequence of interest is a
   test frame exchange followed by an XID exchange.  In all cases, DLSw
   uses the CANUREACH_ex and ICANREACH_ex SSP packets to complete
   address resolution and circuit establishment.  The following table
   describes how these packets are transported via UDP between two
   multicast capable DLSw peers.

     Message Event          Action            Mechanism         Retry
   TEST                 SEND CANUREACH_ex    Multicast/Unicast   Yes
   TEST RESPONSE        SEND ICANREACH_ex       Unicast          No

   The following paragraphs provide more detail on how UDP transport and
   multicast protocol enhancements are used to establish SNA data links.

8.1 Address Resolution

   When a DLSw receives an incoming test frame from an attached data
   link, the assumption is that this is an exploratory frame in
   preparation for an XID exchange and link activation.  The DLSw must
   determine a correlation between the destination LSAP (mac and sap
   pairing) and some other DLSw in the transport network.  This paper
   generically refers to this process as “address resolution”.

8.2 Explorer frames

   Address resolution messages may be sent over a TCP connection to a
   multicast capable DLSw peer if such a connection already exists in
   order that they take advantage of the guaranteed delivery of TCP.
   This is particularly recommended for ICANREACH_ex frames.

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8.3 Circuit Setup

   Circuit setup is accomplished in the same manner as described in RFC
   1795.  More specifically, CANUREACH_cs, ICANREACH_cs, REACH_ACK,
   XIDFRAME, etc.  are all sent over the TCP connection to the
   appropriate DLSw.  This, of course, assumes the existence of a TCP
   connection between the DLSw peers.  If the sending DLSw (sending a
   CANUREACH_cs ) detects no active TCP connection to the DLSw peer,
   then a TCP connection setup is initiated and the packet sent.  All
   other circuit setup (and takedown) related sequences are now passed
   over the TCP connection.

8.4 Example SNA SSP Message Sequence

   The following diagram provides an example sequence of flows
   associated with an SNA LLC circuit setup.  All flows and states
   described below correspond precisely with those defined in RFC 1795.
   The only exception is the addition of a TCP connection setup and DLSw
   capabilities exchange that occurs when the origin DLSw must send a
   CANUREACH_CS and no TCP connection yet exists to the target DLSw

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 ======                            ___                           ======
 |    |        ---------        __/   \__       ---------        |    |
 |    |      __|  _|_  |__     /   IP    \    __|  _|_  |__      |    |
 ======        |   |   |      <  Network  >     |   |   |        ======
/______\       ---------       \__     __/      ---------       /______\
 Origin       Origin DLSw         \___/        Target DLSw      Target
 Station        partner                          partner        Station

              disconnected                    disconnected

TEST_cmd      DLC_RESOLVE_C    CANUREACH_ex               TEST_cmd
----------->  ----------->     ----------->               ---------->
   TEST_rsp   DLC_RESOLVE_R    ICANREACH_ex                 TEST_rsp
 <---------    <-----------   <-----------                <----------
null XID      DLC_XID
----------->  ----------->

                           TCP Connection Setup
                            Capabilities Exch.

                             CANUREACH_cs    DLC_START_DL
                             ----------->    ----------->
                             ICANREACH_cs    DLC_DL_STARTED
                             <-----------    <-------------
           circuit_established                circuit_pending
                              ----------->  circuit_established

                               XIDFRAME         DLC_XID       null XID
                               ----------->     --------->    -------->
        XID        DLC_XID      XIDFRAME         DLC_XID          XID
  <--------   <-----------   <-----------    <-----------    <--------
    XIDs         DLC_XIDs      XIDFRAMEs        DLC_XIDs         XIDs
<---------->  <---------->   <------------>  <------------>  <--------->
----------->  ----------->     ----------->    ----------->    -------->
              connect_pending                 contact_pending

          UA     DLC_CONTACT     CONTACTED    DLC_CONTACTED          UA
  <---------   <-----------  <-----------    <-----------    <--------
                  connected                      connected
IFRAMEs       DLC_INFOs        IFRAMEs        DLC_INFOs       IFRAMEs
<---------->  <----------->  <------------>  <------------>  <-------->

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8.5 UDP Reliability

   It is important to note, that UDP (unicast and multicast)transport
   services do not provide a reliable means of delivery.  Existing RFC
   1795 protocols guarantee the delivery (or failure notification) of
   CANUREACH_ex and ICANREACH_ex messages.  UDP will not provide the
   same level of reliability.  It is, therefore, possible that these
   messages may be lost in the network and (CANUREACH_ex) retries will
   be necessary.

8.5.1 Retries

   Test Frames are generally initiated by end stations every few
   seconds.  Many existing RFC 1795 DLSw implementations take advantage
   of the reliable SSP TCP connections and filter out end station Test
   frame retries when a CANUREACH_ex is outstanding.  Given the
   unreliable nature of UDP transport for these messages, however, this
   filtering technique may not be advisable.  Neither RFC 1795 nor this
   paper address this issue specifically.  It is simply noted that the
   UDP transport mechanism is unreliable and implementations should take
   this into account when determining a scheme for Test frame filtering
   and explorer retries.  Accordingly, the “Retry” section in the table
   above only serves as an indicator of situations where retries may be
   desirable and/or necessary, but does not imply any requirement to
   implement retries. Also note, that retry logic only applies to non-
   response type packets.  It is not appropriate to retry response type
   SSP packets (i.e., ICANREACH_ex) as there is no way of knowing if the
   original response was ever received (and whether retry is necessary).
   So in the case of SNA, CANUREACH_ex messages may need retry logic and
   ICANREACH_ex messages do not.

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9. NetBIOS

   With the introduction of DLSw Multicast transport, all multicast
   NetBIOS UI frames are carried outside the TCP connections between
   DLSw peers (i.e., via UDP datagrams).  The following table defines
   the various NetBIOS UI frames and how they are transported via UDP
   between multicast capable DLSw peers:

Message Event            Action               Mechanism           Retry
ADD_GROUP_NAME_QUERY     SEND DATAFRAME       Multicast            Yes
ADD_NAME_QUERY           SEND NETBIOS_ANQ     Multicast            Yes
ADD_NAME_RESPONSE        SEND NETBIOS_ANR     Unicast1             No
NAME_IN_CONFLICT         SEND DATAFRAME       Multicast            No
STATUS_QUERY             SEND DATAFRAME       Unicast/Multicast(2) Yes
STATUS_RESPONSE          SEND DATAFRAME       Multicast(5)         No
TERMINATE_TRACE (x'07')  SEND DATAFRAME       Multicast            No
TERMINATE_TRACE (X'13')  SEND DATAFRAME       Multicast            No
DATAGRAM                 SEND DATAFRAME(3)    Unicast/Multicast(2) No
DATAGRAM_BROADCAST       SEND DATAFRAME       Multicast            No
NAME_QUERY               SEND NETBIOS_NQ_ex   Unicast/Multicast(2) Yes
NAME_RECOGNIZED          SEND NETBIOS_NR_ex   Unicast(4)           No

   Note 1:
   Upon receipt of an ADD_NAME_RESPONSE frame, a NETBIOS_ANR SSP message
   is returned via unicast UDP to the originator of the NETBIOS_ANQ

   Note 2:
   These frames may be sent either Unicast or Multicast UDP.  If the
   implementation has sufficient cached information to resolve the
   NetBIOS datagram destination to a single DLSw peer, then the SSP
   message can and should be sent via unicast.  If the cache does not
   contain such information then the resultant SSP message must be sent
   via multicast UDP.

   Note 3:
   Note that this frame is sent as either a DATAFRAME or DGRMFRAME
   according to the rules as specified in RFC 1795.

   Note 4:
   Upon receipt of a NAME_RECOGNIZED frame, a NETBIOS_NR_ex SSP message
   is returned via unicast UDP to the originator of the NETBIOS_NQ_ex
   frame.  Notice that although the NAME_RECOGNIZED frame is sent as an
   All Routes Explorer (source routing LANs only) frame, the resultant
   NETBIOS_NR_ex is sent as a unicast UDP directed response to the DLSw
   originating the NETBIOS_NQ_ex.  This is because there is no value in

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   sending NETBIOS_NR_ex as a multicast packet in the transport network.
   The use of ARE transmission in the LAN environment is to accomplish
   some form of load sharing in the source routed LAN environment.
   Since no analogous capability exists in the (TCP) transport network,
   it is not necessary to emulate this function there.  It is important
   to note, however, that when converting a received NETBIOS_NR_ex to a
   NAME_RECOGNIZED frame, the DLSw sends the NAME_RECOGNIZED frame onto
   the LAN as an ARE (source routing LANs only) frame.  This preserves
   the source route load sharing in the LAN environments on either side
   of the DLSw transport network.

   Note 5:
   Although RFC 1795 does not attempt to optimize STATUS_RESPONSE
   processing, it is possible to send a STATUS_RESPONSE as a unicast UDP
   response.  To do this, DLSws receiving an incoming SSP DATAFRAME
   containing a STATUS_QUERY must remember the originating DLSw's
   address and STATUS_QUERY correlator.  Then upon receipt of the
   corresponding STATUS_RESPONSE, the DLSw responds via unicast UDP to
   the originating DLSw(using the remembered originating DLSw address).
   Note, however, that in order to determine whether a frame is a
   STATUS_QUERY, all multicast capable DLSw implementations will need to
   parse the contents of frames that would normally be sent as DATAFRAME
   SSP messages.

   All other multicast frames are sent into the transport network using
   the appropriate multicast group address.

9.1 Address Resolution

   Typical NetBIOS circuit setup using multicast services is essentially
   the same as specified in RFC 1795.  The only significant difference
   is that NETBIOS_NQ_ex messages are sent via UDP to the appropriate
   unicast/multicast IP address and the NETBIOS_NR_ex is sent via
   unicast UDP to the DLSw originating the NETBIOS_NQ_ex.

9.2 Explorer Frames

   Address resolution messages may be sent over a TCP connection to a
   multicast capable partner if such a connection already exists in
   order that they take advantage of the guaranteed delivery of TCP.
   This is particularly recommended for NETBIOS_NR_ex frames.

9.3 Circuit Setup

   Following successful address resolution, a NetBIOS end station
   typically sends a SABME frame to initiate a formal LLC2 connection.
   Receipt of this message results in normal circuit setup as described
   in RFC 1795 (and the SNA case described above).  That is to say that

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   the CANUREACH_cs messages etc. are sent on a TCP connection to the
   appropriate DLSw peer.  If no such TCP connection exists, one is
   brought up.

9.4 Example NetBIOS SSP Message Sequence

   The following diagram provides an example sequence of flows
   associated with a NetBIOS circuit setup.  All flows and states
   described below correspond precisely with those defined in RFC 1795.
   The only exception is the addition of a TCP connection setup and DLSw
   capabilities exchange that occurs when the origin DLSw must send a
   CANUREACH_cs and no TCP connection yet exists to the target DLSw

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 ======                            ___                           ======
 |    |        ---------        __/   \__       ---------        |    |
 |    |      __|  _|_  |__     /   IP    \    __|  _|_  |__      |    |
 ======        |   |   |      <  Network  >     |   |   |        ======
/______\       ---------       \__     __/      ---------       /______\
 Origin       Origin DLSw         \___/        Target DLSw      Target
 Station        partner                          partner        Station

              disconnected                     disconnected

----------->  ----------->    ----------->    ----------->   --------->

<-----------  <------------   <-----------    <-----------  <---------

----------->  ----------->

                            TCP Connection Setup
                            Capabilities Exch.

                              CANUREACH_cs    DLC_START_DL
                              ----------->    ----------->

                              ICANREACH_cs    DLC_DL_STARTED
                              <-----------    <-----------
            circuit_established                circuit_pending
                              ----------->   circuit_established

                              CONTACT         DLC_CONTACT     SABME
                              ----------->    ----------->    --------->
             connect_pending                   contact_pending

          UA   DLC_CONTACT       CONTACTED    DLC_CONTACTED           UA
  <---------   <-----------   <-----------    <-----------    <---------
                connected                        connected

   IFRAMEs       DLC_INFOs       IFRAMEs        DLC_INFOs       IFRAMEs
<------------> <------------> <------------>  <------------>  <-------->

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9.5 Multicast Reliability and Retries

   In the case of NetBIOS, many more packets are being sent via UDP than
   in the SNA case.  Therefore, the exposure to the unreliability of
   these services is greater than that of SNA. For address resolution
   frames, such as NAME_QUERY, etc., successful message delivery is an
   issue.  In addition, the retry interval for these types of frames is
   considerably shorter than SNA with the defaults being: retry interval
   = 0.5 seconds and retry count = 6.  Once again, neither RFC 1795 nor
   this paper attempt to address the issue of LAN frame filtering
   optimizations. This issue is outside the scope of this paper.  But it
   is important for implementers to recognize the inherent unreliable
   nature of UDP transport services for frames of this type and to
   implement retry schemes that are appropriate to successful operation.
   Again, it is only appropriate to consider retry of non-response type
   packets.  Specific NetBIOS messages where successful message delivery
   is considered important (and retries possibly necessary) are
   indicated in the table above with an “Yes” in the “Retry” column.

10. Sequencing

   It is important to note that UDP transport services do not provide
   guaranteed packet sequencing like TCP does for RFC 1795.  In a steady
   state network, in order packet delivery can be generally assumed.
   But in the presence of network outages and topology changes, packets
   may take alternate routes to the destination and arrive out of
   sequence with respect to their original transmission order.  For SNA
   address resolution this should not be a problem given that there is
   no inherent significance to the order of packets being transmitted
   via UDP.

   In the case of NetBIOS, in order delivery is not guaranteed in the
   normal case (e.g., LANs).  This is because LAN broadcasting
   mechanisms suffer the same problems of packet sequencing as do WAN
   multicast mechanisms.  But one might argue the greater likelihood of
   topology related changes in the WAN environment and thus a greater
   level of concern.  The vast majority of NetBIOS UI frames (being
   handled via UDP and Multicast) have correlator values and do not rely
   upon packet sequencing.

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   The only NetBIOS frames of special note would be: DATAGRAM,
   DATAGRAM_BROADCAST it is generally assumed that datagrams do not
   provide any guarantee of in order packet delivery.  Thus applications
   utilizing this NetBIOS service are assumed to have no dependency on
   in order packet delivery.  STATUS_RESPONSE can actually be sent as a
   sequence of STATUS_RESPONSE messages.  In cases where this occurs,
   the STATUS_RESPONSE will be exposed to potential out of sequence

11. Frame Formats

11.1 Multicast Capabilities Control Vector

   This control vector is carried in the Capabilities Exchange Request.
   When present, it must be accompanied by a TCP Connections Control
   Vector indicating support for 1 TCP/IP connection and a DLSw version
   CV indicating support for version 2 release 0.  Like all control
   vectors in this SSP message, it is an LT structure.  LT structures
   consist of a 1 byte length field followed by a 1 byte type field.
   The length field includes itself as well as the type and data fields.

   Byte Bit    Description
   0   0-7    Length, in binary, of the Multicast Capabilities control
   vector (inclusive of this byte, always 3)

   1   0-7    Type:  x'8C'

   2   0-7    Multicast Version Number:
               A binary numerical representation of the level of
               multicast services provided.  The protocols as identified
               in this document constitute version one.   Accordingly,
               x'01' is encoded in this field.  Any subsequent version
               must provide the services of all previous versions.

   The intended use of this CV for Multicast support is to detect when
   the multicast CANUREACH_ex flows will suffice between partners.  If
   this CV is present in a CAPEX from a partner, that partner is also
   multicast capable and therefore does not need to receive CANUREACH_ex
   messages over the TCP link that exists between them (and there must
   be one or else the CAPEX would not have flowed) because it will
   receive the multicast copies.

   A DLSw includes this control vector on a peer-wise basis.  That is to
   say, that a DLSw implementation may support multicast services but
   choose not to indicate this in its capabilities exchange to all
   partners. Therefore, a DLSw may include this capabilities CV with
   some DLSw peers and not with others.  Not including this vector can

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   be used to force TCP connections with other multicast capable nodes
   and degrade to normal RFC 1795 operations.  This capability is
   allowed to provide greater network design flexibility.

   When sending this capabilities exchange control vector, the following
   rules apply:

         Required                       Allowed @
    ID   @ Startup  Length  Repeatable* Runtime  Order  Content
   ====  =========  ======  ==========  =======  =====  ===============
   0x8C     Y        0x03        N         N       5+    Multicast

*Note: "Repeatable" means a Control Vector is repeatable within a single

11.1.1 DLSw Capabilities Negative Response

   DLSws that implement these enhancements must provide support for both
   multicast version 1 and single TCP connections.  This means that the
   capabilities exchange request must contain a DLSw Version ID control
   vector (x'82') indicating support for version 2 release 0, a
   Multicast Capabilities control vector, and the TCP Connections
   control vector indicating support for 1 TCP connection within a given
   capabilities exchange. If a multicast capable DLSw receives a
   capabilities exchange with a Multicast Capabilities, but either a
   missing or inappropriate TCP Connections CV (i.e., connections not
   equal to one)or DLSw Version control vector, then the inbound
   capabilities exchange should be rejected with a DLSw capabilities
   exchange negative response (see RFC 1795) using the following new
   reason code:

   x'000D'Inconsistent DLSw Version,  Multicast Capabilities, and TCP
   Connections CV received on the inbound Capabilities exchange

11.2 UDP Packets

   SSP frame formats are defined in RFC 1795.  Multicast protocol
   enhancements do not change these formats in any way.  The multicast
   protocol enhancements, however, do introduce the notion of SSP packet
   transport via UDP.  In this case, standard UDP services and headers
   are used to transport SSP packets.

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   The following section describes the proper UDP header for DLSw SSP

   Byte       Description
   0-1        Source Port address
               In DLSw multicast protocols, this particular field is not
               relevant.  It may be set to any value.

   2-3        Destination Port address
               Always set to 2067

   4-5        Length

   6-7        Checksum
               The standard UDP checksum value.  Use of the UDP checksum
               function is optional.

11.3 Vendor Specific UDP Packets

   In order to accommodate the addition of vendor specific functions
   over UDP transport, a new SSP packet header has been defined. As
   described above, it is possible to receive these packets over both
   UDP and TCP (when a TCP connection already exists).

   It is important to note that the first 4 bytes of this packet match
   the format of existing RFC 1795 SSP packets.  This is done so that
   implementations in the future can expect that the DLSw “Version
   Number” is found in byte one and that the following bytes describe
   the packet header and message length.

   Furthermore, to assist DLSws in detecting 'out-of-sync' conditions
   whereby packet or parsing errors lead to improper length
   interpretations in the TCP datastream, valid DLSw version numbers
   will be restricted to the range of x'31' through x'3F' inclusive.

   DLSw multicast Vendor Specific frame format differs from existing RFC
   1795 packets in the following ways:

   1) The “Version Number” field is set to x'32' (ASCII '2') and now
   represents a packet type more than a DLSw version number.  More
   precisely, it is permitted and expected that DLSw may send packets of
   both types (x'31' and x'32').

   2) The message length field is followed by a new 3 byte field that
   contains the specific vendor's IEEE Organizationally Unique
   Identifier (OUI).

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   3) All fields following the new OUI field are arbitrary and defined
   by implementers.

   The following section defines this new packet format:

   Byte       Description
   0          DLSw packet type, Always set to x'32'

   1          Header Length
               Always 7 or higher

   2-3        Message Length
               Number of bytes within the data field following the

   4-6        Vendor specific OUI
               The IEEE Organizationally Unique Identifier (OUI)
               associated with the vendor specific function in

   7-n        Defined by the OUI owner

12. Compliance Statement

   All DLSw v2.0 implementations must support

   - Halt reason codes
   - the Multicast Capabilities control vector in the DLSw
     capabilities exchanges messages.

   The presence of the Multicast Capabilities control vector in a
   capabilities exchange message implies that the DLSw that issued the
   message supports all the scalability enhancements defined in this
   document.  These are:

   - use of multicast IP (if it is available in the underlying network)
   - use of 2067 as the destination port for UDP and TCP connections
   - single tunnel bring-up of TCP connections to DLSw peers
   - peer-on-demand
   - quiet ignore of all unrecognized vendor-specific UDP/TCP packets.

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13. Security Considerations

   This document addresses only scalability problems in RFC 1795.  No
   attempt is made to define any additional security mechanisms.  Note
   that, as in RFC 1795, a given implementation may still choose to
   refuse TCP connections from DLSw peers that have not been configured
   by the user.  The mechanism by which the user configures this
   behavior is not specified in this document.

14. Acknowledgements

   This specification was developed in the DLSw Related Interest Group
   (RIG) of the APPN Implementers Workshop.  This RIG is chaired by
   Louise Herndon- Wells ( and edited by Paul
   Brittain (

   Much of the work on the scalability enhancements for v2.0 was
   developed by Dave Bryant (3COM).

   Other significant contributors to this document include:

   Frank Bordonaro (Cisco)
   Jon Houghton (IBM)
   Steve Klein (IBM)
   Ravi Periasamy (Cisco)
   Mike Redden (Proteon)
   Doug Wolff (3COM)

   Many thanks also to all those who participated in the DLSw RIG
   sessions and mail exploder discussions.

   If you would like to participate in future DLSw discussions, please
   subscribe to the DLSw RIG mailing lists by sending a mail to specifying 'subscribe aiw-dlsw' as the body
   of the message.

   If you would like further information on the activities of the AIW,
   please refer to the AIW web site at

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15. Authors' Addresses

   The editor of this document is:

         Paul Brittain
         Data Connection Ltd
         Windsor House
         Pepper Street
         CH1 1DF

         tel:   +44 1244 313440

   Much of the work on this document was created by:

         David Bryant
         3Com Corporation
         5400 Bayfront Plaza MS 2418
         Santa Clara, CA 95052

         tel:   (408) 764-5272

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16. Appendix - Clarifications to RFC 1795

   This appendix attempts to clarify the areas of RFC 1795 that have
   proven to be ambiguous or hard to understand in the implementation
   experience to- date.  These clarifications should be read in
   conjunction with RFC 1795 as this document does not reproduce the
   complete text of that RFC.

   The clarifications are ordered by the section number in RFC 1795 to
   which they apply.  Where one point applies to more than one place in
   RFC 1795, it is listed below by the first relevant section.

   If any implementers encounter further difficulties in understanding
   RFC 1795 or these clarifications, they are encouraged to query the
   DLSw mail exploder (see section 1.1) for assistance.

   3. Send Port

   It is not permitted for a DLSw implementation to check that the send
   port used by a partner is 2067.  All implementations must accept
   connections from partners that do not use this port.

   3   TCP Tunnel bringup

   The paragraph below the figure should read as follows:

      Each Data Link Switch will maintain a list of DLSw capable routers
      and their status (active/inactive). Before Data Link Switching can
      occur between two routers, they must establish two TCP connections
      between them. These connections are treated as half duplex data
      pipes. A Data Link Switch will listen for incoming connections on
      its Read Port (2065), and initiate outgoing connections on its
      Write Port (2067).  Each Switch is responsible for initiating one
      of the two TCP connections.  After the TCP connections are
      established, SSP messages are exchanged to establish the
      capabilities of the two Data Link Switches.  Once the exchange is
      complete, the DLSw will employ SSP control messages to establish
      end-to-end circuits over the transport connection.  Within the
      transport connection, DLSw SSP messages are exchanged.  The
      message formats and types for these SSP messages are documented in
      the following sections.

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   3.2 RII bit in SSP header MAC addresses

   The RII bit in MAC addresses received from the LAN must be set to
   zero before forwarding in the source or destination address field in
   a SSP message header.  This requirement aims to avoid ambiguity of
   circuit IDs.  It is also recommended that all implementations ignore
   this bit in received SSP message headers.

   3.3 Transport IDs

   All implementations must allow for the DLSw peer varying the
   Transport ID up to and including when the ICR_cs message flows, and
   at all times reflect the most recent TID received from the partner in
   any SSP messages sent.  The TID cannot vary once the ICR_cs message
   has flowed.

   3.4 LF bits

   LF-bits should be propagated from LAN to SSP to LAN (and back) as per
   a bridge (i.e. they can only be revised downwards at each step if

   3.5 KEEPALIVE messages

   The SSP KEEPALIVE message (x1D) uses the short ("infoframe") version
   of the SSP header.  All DLSw implementation must support receipt and
   quiet ignore of this message, but there is not requirement to send
   it.  There is no response to a KEEPALIVE message.

   3.5 MAC header for Netbios SSP frames

   The MAC header is included in forwarded SSP Netbios frames in the
   format described below:
        -    addresses are always in non-canonical format
        -    src/dest addresses are as per the LLC frame
        -    AC/FC bits may be reset and must be ignored
        -    SSAP, DSAP and command fields are included
        -    RII bit in src address is copied from the LLC frame
        -    the RIF length is not extended to include padding
        -    all RIFs are padded to 18 bytes so that the data is
             in a consistent place.

   3.5.7 Unrecognized control vectors

   All implementations should quietly ignore unrecognized control
   vectors in any SSP messages.  In particular, unrecognized SSP frames
   or unrecognized fields in a CAPEX message should be quietly ignored
   without dropping the TCP connection.

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   5.4 Use of CUR-cs/CUR-ex

   The SSAP and DSAP numbers in CUR_ex messages should reflect those
   actually used in the TEST (or equivalent) frame that caused the
   CUR_ex message to flow.  This would mean that the SAP numbers in a
   'typical' CUR_ex frame for SNA traffic switched from a LAN will be a
   source SAP of x04 and a destination SAP of x00.

   The CUR_cs frame should only be sent when the DSAP is known.
   Specifically, CUR_ex should be used when a NULL XID is received that
   is targeted at DSAP zero, and CUR_cs when a XID specifying the (non-
   zero) DSAP is received.

   Note that this does not mean that an implementation can assume that
   the DSAP on a CUR_ex will always be zero.  The ICR_ex must always
   reflect the SSAP and DSAP values sent on the CUR_ex.  This is still
   true even if an implementation always sends a TEST with DSAP = x00 on
   its local LAN(s) in response to a CUR_ex to any SAP.

   An example of a situation where the CUR_ex may flow with a non-zero
   DSAP is when there is an APPN stack local to the DLSw node.  The APPN
   stack may then issue a connection request specifying the DSAP as a
   non-zero value.  This would then be passed on the CUR_ex message.

   7.6.1 Vendor IDs

   The Vendor ID field in a CAPEX may be zero.  However, a zero Vendor
   Context ID is not permitted, which implies that an implementation
   that uses a zero ID cannot send any vendor-specific CVs (other than
   those specified by other vendors that do have a non-zero ID)

   7.6.3 Initial Pacing Window

   The initial pacing window may be 1.  There is no requirement on an
   implementation to use any minimum value for the initial pacing

   7.6.7 TCP Tunnel bringup

   The third paragraph should read:

      If TCP Connections CV values agree and the number of connections
      is one, then the DLSw with the higher IP address must tear down
      the TCP connections on its local port 2065. This connection is
      torn down after a CAPEX response has been both sent and received.
      After this point, the remaining TCP connection is used to exchange
      data in both directions.

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   7.7 CAPEX negative responses

   If a DLSw does not support any of the options specified on a CAPEX
   received from a partner, or if it thinks the CAPEX is malformed, it
   must send a CAPEX negative response to the partner.  The receiver of
   a CAPEX negative response is then responsible for dropping the
   connection.  It is not permitted to drop the link instead of sending
   a CAPEX negative response.

   8.2 Flow Control ACKs

   The first flow-control ack (FCACK) does not have to be returned on
   the REACH_ACK even if the ICR_cs carried the FCIND bit.  However it
   should be returned on the first SSP frame flowing for that circuit
   after the REACH_ACK.

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