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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
  1. a set of extensions to RFC 1795 designed to improve the scalability

of DLSw

  1. 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

Bryant & Brittain Informational [Page 1] RFC 2166 APPN Implementer's Workshop June 1997

 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
 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

Bryant & Brittain Informational [Page 2] RFC 2166 APPN Implementer's Workshop June 1997

 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.

Bryant & Brittain Informational [Page 3] RFC 2166 APPN Implementer's Workshop June 1997

 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:
  1. 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.
  1. 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.
  1. 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.

Bryant & Brittain Informational [Page 4] RFC 2166 APPN Implementer's Workshop June 1997

  1. 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.
  1. 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.

Bryant & Brittain Informational [Page 5] RFC 2166 APPN Implementer's Workshop June 1997

 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:
  1. 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.
  1. 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

Bryant & Brittain Informational [Page 6] RFC 2166 APPN Implementer's Workshop June 1997

  1. 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.
  1. 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.

Bryant & Brittain Informational [Page 7] RFC 2166 APPN Implementer's Workshop June 1997

 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).

Bryant & Brittain Informational [Page 8] RFC 2166 APPN Implementer's Workshop June 1997

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.

Bryant & Brittain Informational [Page 9] RFC 2166 APPN Implementer's Workshop June 1997

 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.

Bryant & Brittain Informational [Page 10] RFC 2166 APPN Implementer's Workshop June 1997

 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.

Bryant & Brittain Informational [Page 11] RFC 2166 APPN Implementer's Workshop June 1997

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.

Bryant & Brittain Informational [Page 12] RFC 2166 APPN Implementer's Workshop June 1997

 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.

Bryant & Brittain Informational [Page 13] RFC 2166 APPN Implementer's Workshop June 1997

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.

Bryant & Brittain Informational [Page 14] RFC 2166 APPN Implementer's Workshop June 1997

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

Bryant & Brittain Informational [Page 15] RFC 2166 APPN Implementer's Workshop June 1997

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.

Bryant & Brittain Informational [Page 16] RFC 2166 APPN Implementer's Workshop June 1997

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

Bryant & Brittain Informational [Page 17] RFC 2166 APPN Implementer's Workshop June 1997


| | ——— / \ ——— | | | | | _|_ | / 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 ———–> ———–> circuit_start TCP Connection Setup ←————> Capabilities Exch. ←————> CANUREACH_cs DLC_START_DL ———–> ———–> resolve_pending ICANREACH_cs DLC_DL_STARTED ←———- ←———— circuit_established circuit_pending REACH_ACK ———–> circuit_established XIDFRAME DLC_XID null XID ———–> ———> ——–> XID DLC_XID XIDFRAME DLC_XID XID ←——- ←———- ←———- ←———- ←——- XIDs DLC_XIDs XIDFRAMEs DLC_XIDs XIDs ←———> ←———> ←———–> ←———–> ←——–> SABME DLC_CONTACTED CONTACT DLC_CONTACT SABME ———–> ———–> ———–> ———–> ——–> connect_pending contact_pending UA DLC_CONTACT CONTACTED DLC_CONTACTED UA ←——– ←———- ←———- ←———- ←——- connected connected IFRAMEs DLC_INFOs IFRAMEs DLC_INFOs IFRAMEs ←———> ←———→ ←———–> ←———–> ←——→ Bryant & Brittain Informational [Page 18] RFC 2166 APPN Implementer's Workshop June 1997 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. Bryant & Brittain Informational [Page 19] RFC 2166 APPN Implementer's Workshop June 1997 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: Transport 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 message. 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 Bryant & Brittain Informational [Page 20] RFC 2166 APPN Implementer's Workshop June 1997 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 Bryant & Brittain Informational [Page 21] RFC 2166 APPN Implementer's Workshop June 1997 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 peer. Bryant & Brittain Informational [Page 22] RFC 2166 APPN Implementer's Workshop June 1997 ====== _

| | ——— / \ ——— | | | | | _|_ | / IP \ | _|_ | | |

| | | < Network > | | |

/\ ——— \ / ——— /\ Origin Origin DLSw \___/ Target DLSw Target Station partner partner Station

            disconnected                     disconnected

NAME_QUERY DLC_DGRM NETBIOS_NQ_ex DLC_DGRM NAME_QUERY ———–> ———–> ———–> ———–> ———>


←———- ←———– ←———- ←———- ←——–


                          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

←———–> ←———–> ←———–> ←———–> ←——→

Bryant & Brittain Informational [Page 23] RFC 2166 APPN Implementer's Workshop June 1997

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.

Bryant & Brittain Informational [Page 24] RFC 2166 APPN Implementer's Workshop June 1997

 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

Bryant & Brittain Informational [Page 25] RFC 2166 APPN Implementer's Workshop June 1997

 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.

Bryant & Brittain Informational [Page 26] RFC 2166 APPN Implementer's Workshop June 1997

 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).

Bryant & Brittain Informational [Page 27] RFC 2166 APPN Implementer's Workshop June 1997

 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
  1. Halt reason codes
  2. 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:
  1. use of multicast IP (if it is available in the underlying network)
  2. use of 2067 as the destination port for UDP and TCP connections
  3. single tunnel bring-up of TCP connections to DLSw peers
  4. peer-on-demand
  5. quiet ignore of all unrecognized vendor-specific UDP/TCP packets.

Bryant & Brittain Informational [Page 28] RFC 2166 APPN Implementer's Workshop June 1997

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

Bryant & Brittain Informational [Page 29] RFC 2166 APPN Implementer's Workshop June 1997

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

Bryant & Brittain Informational [Page 30] RFC 2166 APPN Implementer's Workshop June 1997

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.

Bryant & Brittain Informational [Page 31] RFC 2166 APPN Implementer's Workshop June 1997

 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.

Bryant & Brittain Informational [Page 32] RFC 2166 APPN Implementer's Workshop June 1997

 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.

Bryant & Brittain Informational [Page 33] RFC 2166 APPN Implementer's Workshop June 1997

 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.

Bryant & Brittain Informational [Page 34]

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