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rfc:rfc1434

Network Working Group R. Dixon Request for Comments: 1434 D. Kushi

                                                                  IBM
                                                           March 1993
           Data Link Switching: Switch-to-Switch Protocol

Status of this Memo

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

Abstract

 This RFC describes IBM's support of Data Link Switching over TCP/IP.
 The RFC is being distributed to members of the Internet community in
 order to solicit their reactions to the proposals contained in it.
 While the issues discussed may not be directly relevant to the
 research problems of the Internet, they may be interesting to a
 number of researchers and implementors.
 Any questions or comments relative to the contents of this RFC should
 be sent to the following Internet address: dlsw@ralvma.vnet.ibm.com.

Table of Contents

 1. Introduction                                                     2
 2. Overview                                                         2
 3. Transport Connection                                             4
    3.1. SSP Frame Formats                                           5
    3.2. Address Parameters                                          8
    3.3. Message Types                                              10
 4. Protocol Specification                                          11
    4.1. Protocol Flow Diagrams                                     11
         4.1.1. Connect Protocols                                   11
         4.1.2. Link Restart Protocols                              13
         4.1.3. Disconnect Protocols                                15
    4.2. DLS State Machine                                          16
         4.2.1 Data Link Switch States                              16
         4.2.2 State Transition Tables                              21
    4.3. NetBIOS Datagrams                                          30
 Acknowledgments                                                    32
 References                                                         32
 Security Considerations                                            32
 Authors' Addresses                                                 33

Dixon & Kushi [Page 1] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

1. Introduction

 Data Link Switching (DLS) is a forwarding mechanism for the IBM SNA
 and IBM NetBIOS protocols.  It does not provide full routing, but
 instead provides switching at the Data Link layer and encapsulation
 in TCP/IP for transport over the Internet.  This memo documents the
 Switch-to-Switch Protocol (SSP) that is used between IBM 6611 Network
 Processors.
 Today, the IBM 6611 supports SNA (PU 2 and PU 4) systems and NetBIOS
 systems attached to token-ring networks, as well as SNA (PU 2)
 systems attached to SDLC links.  For the later case, the SDLC
 attached systems are provided with a LAN appearance within the IBM
 6611.  For the LAN attached systems, the IBM 6611 appears as a
 source-routing bridge.  Remote systems that are accessed through the
 IBM 6611 appear as systems attached to an adjacent ring.  This ring
 is a virtual ring that is manifested within each IBM 6611.

2. Overview

 Data Link Switching was developed to provide support for SNA and
 NetBIOS in multi-protocol routers.  Since SNA and NetBIOS are
 basically connection oriented protocols, the Data Link Control
 procedure that they use on the LAN is IEEE 802.2 Logical Link Control
 (LLC) Type 2.  Data Link Switching also accommodates SNA protocols
 over WAN links via the SDLC protocol.
 IEEE 802.2 LLC Type 2 was designed with the assumption that the
 network transit delay would be small and predictable (i.e., a local
 LAN).  Therefore the LLC elements of procedure use a fixed timer for
 detecting lost frames.  When bridging is used over wide area lines
 (especially at lower speeds), the network delay is larger and it can
 vary greatly based upon congestion.  When the delay exceeds the
 time-out value LLC attempts to retransmit.  If the frame is not
 actually lost, only delayed, it is possible for the LLC Type 2
 procedures to become confused.  And as a result, the link is
 eventually taken down.
 Given the use of LLC Type 2 services, Data Link Switching addresses
 the following bridging problems:
       DLC Time-outs
       DLC Acknowledgments over the WAN
       Flow and Congestion Control
       Broadcast Control of Search Packets
       Source-Route Bridging Hop Count Limits
 NetBIOS also makes extensive use of datagram services that use LLC

Dixon & Kushi [Page 2] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

 Type 1.  In this case, Data Link Switching addresses the last two
 problems in the above list.
 The principal difference between Data Link Switching and bridging is
 that DLS terminates the Data Link Control whereas bridging does not.
 The following figure illustrates this difference based upon two end
 systems operating with LLC Type 2 services.
  Bridging
  --------             Bridge           Bridge
  +------+             +----+           +----+             +------+
  | End  | +---------+ |    +-----/     |    | +---------+ | End  |
  |System+-+   LAN   +-+    |    /------+    +-+   LAN   +-+System|
  |      | +---------+ |    |  TCP/IP   |    | +---------+ |      |
  +------+             +----+           +----+             +------+
 Info------------------------------------------------------->
     <-------------------------------------------------------RR
  Data Link Switching
  -------------------
  +------+             +----+           +----+             +------+
  | End  | +---------+ |    +-----/     |    | +---------+ | End  |
  |System+-+   LAN   +-+DLS |    /------+ DLS+-+   LAN   +-+System|
  |      | +---------+ |    |  TCP/IP   |    | +---------+ |      |
  +------+             +----+           +----+             +------+
 Info------------------->   -------------> Info
     <-------------------RR                 ---------------->
                                            <----------------RR
         Figure 1.  Data Link Switching Contrasted to Bridging
 In traditional bridging, the Data Link Control is end-to-end.  Data
 Link Switching terminates the LLC Type 2 connection at the switch.
 This means that the LLC Type 2 connections do not cross the wide area
 network.  The DLS multiplexes LLC connections onto a TCP connection
 to another DLS.  Therefore, the LLC connections at each end are
 totally independent of each other.  It is the responsibility of the
 Data Link Switch to deliver frames that it has received from a LLC
 connection to the other end.  TCP is used between the Data Link
 Switches to guarantee delivery of frames.
 As a result of this design, LLC time-outs are limited to the local
 LAN (i.e., they do not traverse the wide area).  Also, the LLC Type 2
 acknowledgments (RR's) do not traverse the WAN, thereby reducing
 traffic across the wide area links.  For SDLC links, polling and poll
 response occurs locally, not over the WAN.  Broadcast of search
 frames is controlled by the Data Link Switches once the location of a
 target system is discovered.  Finally, the switches can now apply

Dixon & Kushi [Page 3] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

 back pressure to the end systems to provide flow and congestion
 control.
 Data Link Switching uses LAN addressing to set up connections between
 SNA systems.  SDLC attached devices are defined with MAC addresses to
 enable them to communicate with LAN attached devices.  For NetBIOS
 systems, Data Link Switching uses the NetBIOS name to forward
 datagrams and to set up connections for NetBIOS sessions.  For
 circuit establishment, SNA systems send TEST (or in some cases, XID)
 frames to the null (x'00') SAP.  NetBIOS systems have an address
 resolution procedure, based upon the Name Query and Name Recognized
 frames, that is used to establish an end-to-end circuit.
 Since Data Link Switching may be implemented in multi-protocol
 routers, there may be situations where both bridging and switching
 are enabled.  SNA frames can be identified by their link SAP.
 Typical SAP values for SNA are x'04', x'08', and x'0C'.  NetBIOS
 always uses a link SAP value of x'F0'.

3. Transport Connection

 Data Link Switches can be in used in pairs or by themselves.  A
 Single DLS internally switches one data link to another without using
 TCP (DLC(1) to DLC(2) in the figure below).  A paired DLS multiplexes
 data links over a reliable transport using a Switch-to-Switch
 Protocol (SSP).  This RFC will document the frame formats and
 protocols for this multiplexing between Data Link Switches.  The
 initial implementation of SSP uses TCP as the reliable transport
 between Data Link Switches.  However, other transport connections
 such as OSI TP4 could be used.
  +-----------------------------------------------+Switch-to-Switch
  |               DLC Interfaces                  | Protocol (SSP)
  |+------------+   DLC Request    +------------+ |
  ||    Data    |<---------------- |            | |Send SSP Frame
  ||    Link    | DLC Indication   |            | |-------------->
  ||  Control 1 |----------------->|            | |
  |+------------+                  | Data Link  | |
  |+------------+   DLC Request    |  Switch    | |
  ||    Data    |<---------------- |            | |Rec. SSP Frame
  ||    Link    | DLC Indication   |            | |<-------------
  ||  Control 2 | ---------------->|            | |
  |+------------+                  +------------+ |
  |                   Multi-Protocol Router       |
  +-----------------------------------------------+
                   Figure 2.  DLS System Diagram

Dixon & Kushi [Page 4] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

 Before Data Link Switching can occur between two routers, they must
 establish a TCP connection between them.  Each DLS will maintain a
 list of DLS capable routers and their status (active/inactive).  Once
 this connection is established, the DLS will employ SSP to establish
 end-to-end circuits over the transport connection.  Within the
 transport connection is a specific set of  DLS message units.  The
 message formats and types for these PDUs are documented in the
 following sections.
 The default parameters associated with the TCP connections between
 Data Link Switches are as follows:
          Socket Family     AF_INET        (Internet protocols)
          Socket Type       SOCK_STREAM    (stream socket)
          Read Port Number  2065
          Write Port Number 2067
 Two or more Data Link Switches may be attached to the same LAN,
 consisting of a number of token-ring segments interconnected by
 source-routing bridges.  In this case, a TCP connection is not
 defined between bridges attached to the same LAN.  This will allow
 using systems to select one of the possible Data Link Switches in a
 similar manner to the selection of a bridge path through a source-
 routed bridged network.  The virtual ring segment in each Data Link
 Switch attached to a common LAN must be configured with the same ring
 number.  This will prevent LAN frames sent by one Data Link Switch
 from being propagated through the other Data Link Switches.

3.1. SSP Frame Formats

 The following diagrams show the two message headers for traffic
 between Data Link Switches.  The control message header is used for
 all messages except information messages.  The information message
 header is 16 bytes long, and the control message header is 72 bytes
 long.  The first sixteen bytes of the control message header are
 identical to the information message header.

Dixon & Kushi [Page 5] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

   CONTROL MESSAGES (72 Bytes)
  +-----------------------------------------------------------------+
  | Version Number                  Reserved Field                  |
  | Message Length               ---->           .                  |
  | Remote Data Link Correlator  ---->           .                  |
  |            .                                 .                  |
  | Remote DLC Port ID           ---->           .                  |
  |            .                                 .                  |
  | Reserved Field               ---->           .                  |
  | Message Type                    Reserved Field                  |
  | Protocol ID                     Header Number                   |
  | Header Length                ---->           .                  |
  | Reserved Field               ---->           .                  |
  | Reserved Field                  Message Type                    |
  | Target MAC Address           ---->           .                  |
  |            .                                 .                  |
  |            .                                 .                  |
  | Origin MAC Address           ---->           .                  |
  |            .                                 .                  |
  |            .                                 .                  |
  | Origin Link SAP                 Target Link SAP                 |
  | Frame Direction                 Reserved Field                  |
  | Message Length               ---->           .                  |
  | DLC Header Length            ---->           .                  |
  | Origin DLC Port ID           ---->           .                  |
  |            .                                 .                  |
  | Origin Data Link Correlator  ---->           .                  |
  |            .                                 .                  |
  | Origin Transport ID          ---->           .                  |
  |            .                                 .                  |
  | Target DLC Port ID           ---->           .                  |
  |            .                                 .                  |
  | Target Data Link Correlator  ---->           .                  |
  |            .                                 .                  |
  | Target Transport ID          ---->           .                  |
  |            .                                 .                  |
  | Reserved Field               ---->           .                  |
  |            .                                 .                  |
  +-----------------------------------------------------------------+
           (Even Byte)                      (Odd Byte)

Dixon & Kushi [Page 6] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

   INFORMATION MESSAGE (16 Bytes)
  +-----------------------------------------------------------------+
  | Version                         Reserved Field                  |
  | Message Length               ---->           .                  |
  | Remote Data Link Correlator  ---->           .                  |
  |            .                                 .                  |
  | Remote DLC Port ID           ---->           .                  |
  |            .                                 .                  |
  | Reserved Field               ---->           .                  |
  | Message Type                    Reserved Field                  |
  +-----------------------------------------------------------------+
           (Even Byte)                      (Odd Byte)
 The Version Number is set to x'4B', indicating a numeric value of 75.
 The Header Length is x'00 48', indicating a numeric value of 72
 bytes.
 The Header Number is x'01', indicating a value of one.
 The Frame Direction field is set to x'01' for frames sent from the
 origin DLS to the target DLS, and is set to x'02' for frames sent
 from the target DLS to the origin DLS.
    Note:  The Remote Data Link Correlator and Remote DLC Port ID are
    set equal to the Target Data Link Correlator and Target DLC Port
    ID if the Frame Direction field is set to x'01', and are set equal
    to the Origin Data Link Correlator and Origin DLC Port ID if the
    Direction Field is set to x'02'.
 The Protocol ID field is set to x'42', indicating a numeric value of
 66.
 The Message Length field defines the number of bytes within the data
 field following the header.  Note that this value is specified in two
 different fields of the message header.
 The DLC Header Length is set to zero for SNA and is set to x'23' for
 NetBIOS datagrams, indicating a length of 35 bytes.  This includes
 the Access Control (AC) field, the Frame Control (FC) field,
 Destination MAC Address (DA), the Source MAC Address (SA), the
 Routing Information (RI) field (padded to 18 bytes), the Destination
 link SAP (DSAP), the Source link SAP (SSAP), and the LLC control
 field (UI).
 The values for the Message Type field are defined in a later section.
 Note that this value is specified in two different fields of the
 message header.

Dixon & Kushi [Page 7] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

 Reserved fields are set to zero upon transmission and should be
 ignored upon receipt.

3.2. Address Parameters

 A data link is defined as a logical association between the two end
 stations using Data Link Switching.  It is identified by a Data Link
 ID (14 bytes) consisting of the pair of attachment addresses
 associated with each end system.  Each attachment address is
 represented by the concatenation of the MAC address (6 bytes) and the
 LLC address (1 byte).
   DATA LINK ID   (14 Bytes)
  +-----------------------------------------------------------------+
  |Target MAC Address           ---->         .                     |
  |           .                               .                     |
  |           .                               .                     |
  |Origin MAC Address           ---->         .                     |
  |           .                               .                     |
  |           .                               .                     |
  |Origin Link SAP                   Target Link SAP                |
  +-----------------------------------------------------------------+
 An end-to-end circuit is identified by a pair of Circuit ID's.  A
 Circuit ID is a 64 bit  number that identifies the DLC circuit within
 a single DLS.  It consists of a DLC Port ID (4 bytes), and a Data
 Link Correlator (4 bytes).  This value is unique in a single DLS and
 is assigned locally.  The pair of Circuit ID's along with the
 identifiers of the Data Link Switches, uniquely identify a single
 end-to-end circuit.  Each DLS must keep a table of these Circuit ID
 pairs, one for the local end of the circuit and the other for the
 remote end of the circuit.  In order to identify which Data Link
 Switch originated the establishment of a circuit, the terms, origin
 DLS and target DLS, will be employed in this document.
   CIRCUIT ID   (8 Bytes)
  +-----------------------------------------------------------------+
  |DLC Port ID                  ---->         .                     |
  |           .                               .                     |
  |Data Link Correlator         ---->         .                     |
  |           .                               .                     |
  +-----------------------------------------------------------------+
 The Origin Transport ID and the Target Transport ID fields in the
 message header are used to identify the individual TCP/IP port on a
 Data Link Switch.  The values have only local significance.  However,
 each Data Link Switch is required to reflect the values contained in
 these two fields, along with the associated values for DLC Port ID

Dixon & Kushi [Page 8] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

 and the Data Link Correlator, when returning a message to the other
 Data Link Switch.
 The following figure shows the use of the addressing parameters
 during the establishment of an end-to-end connection.  The CANUREACH,
 ICANREACH, and REACH_ACK messages all carry the Data Link ID,
 consisting of the MAC and Link SAP addresses associated with the two
 end stations.  Upon receipt of a CANUREACH message, the target DLS
 starts a data link for each port, thereby obtaining a Data Link
 Correlator.  If the target station can be reached, an ICANREACH
 message is returned to the origin DLS containing the Target Circuit
 ID parameter.  Upon receipt, the origin DLS starts a data link and
 returns the Origin Circuit ID to the target DLS within the REACH_ACK
 message.  If the REACH_ACK message is not successfully received, the
 target Data Link Switch can obtain the Origin Circuit ID from a
 subsequent message (i.e., CONTACT, XIDFRAME, or DGRMFRAME).
   +------------+                                  +------------+
   |Disconnected|                                  |Disconnected|
   +------------+     CANUREACH (Data Link ID)     +------------+
         ------------------------------------------------->
             ICANREACH (Data Link ID, Target Circuit ID)
         <------------------------------------------------
       REACH_ACK (Data Link ID, Origin Cir ID, Target Cir ID)
         ------------------------------------------------->
   +------------+                                  +------------+
   |Circuit Est.|                                  |Circuit Est.|
   +------------+                                  +------------+
       XIDFRAME (Data Link ID, Origin Cir ID, Target Cir ID)
         <------------------------------------------------>
        CONTACT (Data Link ID, Origin Cir ID, Target Cir ID)
         ------------------------------------------------->
       CONTACTED (Data Link ID, Origin Cir ID, Target Cir ID)
         <-------------------------------------------------
   +------------+                                  +------------+
   | Connected  |                                  | Connected  |
   +------------+                                  +------------+
          INFOFRAME (Remote Circuit ID = Target Circuit ID)
         ------------------------------------------------->
          INFOFRAME (Remote Circuit ID = Origin Circuit ID)
         <-------------------------------------------------
                Figure 3.  DLS Circuits and Connections
 During the exchange of the XIDFRAME, CONTACT, and CONTACTED messages,
 the pair of Circuit ID parameters is included in the message format
 along with the DATA LINK ID parameter.  Once the connection has been
 established, the INFOFRAME messages are exchanged with the shorter

Dixon & Kushi [Page 9] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

 header.  This header contains only the Circuit ID associated with the
 remote DLS.  The Remote Data Link Correlator and the Remote DLC Port
 ID are set equal to the Data Link Correlator and the DLC Port ID that
 are associated with the origin or target Data Link Switch, dependent
 upon the direction of the packet.

3.3. Message Types

 The following table lists the protocol data units that are exchanged
 between Data Link Switches.  All values not listed are reserved for
 potential use in follow-on releases.
   Command       Function                       Hex Value
   -------       --------                       ---------
   CANUREACH     Can U Reach Station            x'03'
   ICANREACH     I Can Reach Station            x'04'
   REACH_ACK     Reach Acknowledgment           x'05'
   DGRMFRAME     Datagram Frame (See note)      x'06'
   XIDFRAME      XID Frame                      x'07'
   CONTACT       Contact Remote Station         x'08'
   CONTACTED     Remote Station Contacted       x'09'
   RESTART_DL    Restart Data Link              x'10'
   DL_RESTARTED  Data Link Restarted            x'11'
   INFOFRAME     Information (I) Frame          x'0A'
   HALT_DL       Halt Data Link                 x'0E'
   DL_HALTED     Data Link Halted               x'0F'
   NETBIOS_NQ    NetBIOS Name Query             x'12'
   NETBIOS_NR    NetBIOS Name Recognized        x'13'
   DATAFRAME     Data Frame (See note)          x'14'
   NETBIOS_ANQ   NetBIOS Add Name Query         x'1A'
   NETBIOS_ANR   NetBIOS Add Name Response      x'1B'
                    Table 1.  SSP Message Types
    Note: Both the DGRMFRAME and DATAFRAME messages are used to carry
    information received by the DLC entity within UI frames.  As will
    be explained below, the DGRMFRAME message is addressed according
    to a pair of Circuit IDs, while the DATAFRAME message is addressed
    according to a Data Link ID, being composed of a pair of MAC
    addresses and a pair of link SAP addresses.  The latter is
    employed prior to the establishment of an end-to-end circuit when
    Circuit IDs have yet to be established.
 For the exchange of NetBIOS control messages, the entire DLC header
 is carried as part of the message unit.  This includes the MAC
 header, with the routing information field padded to 18 bytes, and
 the LLC header.  The following message types are affected:
 NETBIOS_NQ, NETBIOS_NR, NETBIOS_ANQ, NETBIOS_ANR, and DATAFRAME when

Dixon & Kushi [Page 10] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

 being used by NetBIOS systems.  The routing information in the DLC
 header is not used by the remote Data Link Switch upon receiving the
 above five messages.

4. Protocol Specification

 This section provides a description of the Switch-to-Switch
 Protocols.  Included is a set of high-level protocol flows and a
 detail set of state transition tables.  The states and the protocols
 are described in terms that are intended to be generic to different
 platforms.  Emphasis of the technical details is to ensure
 operability of the IBM 6611 with another vendor's implementation.
 Notes are inserted at points where the IBM 6611 performs local
 actions that are specific to the AIX platform upon which it operates.

4.1. Protocol Flow Diagrams

 The switch-to-switch protocols are used to setup and take down
 circuits between a pair of Data Link Switches.  Once a circuit is
 established, the end stations on the local networks can employ LLC
 Type 1 (connectionless) protocols.  In addition, the end systems can
 establish an end-to-end connection for support of LLC Type 2
 (connection oriented) protocols.
 The term, Data Link, is used in this document to refer to both a
 "logical data link" when supporting Type 1 LLC services, and a "data
 link connection" when supporting Type 2 LLC services.  In both cases,
 the Data Link in defined by the concatenation of the destination MAC
 address (DA), the source MAC address (SA), the destination link SAP
 (DSAP) and source link SAP (SSAP).

4.1.1. Connect Protocols

 The following figure depicts the protocol flows that are used for the
 establishment of a circuit between a pair of Data Link Switches,
 followed by the establishment of a connection between the pair of end
 systems.  The figure is drawn assuming that the two end systems are
 SNA (the protocol flow for NetBIOS systems is described in a later
 paragraph).

Dixon & Kushi [Page 11] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

 Data Link  Data Link                         Data Link  Data Link
  Control    Switch                            Switch     Control
 --------------------                         --------------------
    +------------+                                 +------------+
    |Disconnected|                                 |Disconnected|
    +------------+                                 +------------+

Test Command CANUREACH Test Comd.

  1. ———> —————————————> ——→

(DSAP=Null) (DSAP=SSAP)

                                                       Test Response
                            ICANREACH                     <---------

Test Response ←————————————–

<----------                 REACH ACK
               --------------------------------------->
    +------------+                                 +------------+
    |Circuit Est.|                                 |Circuit Est.|
    +------------+                                 +------------+
  SABME                      CONTACT
---------->    --------------------------------------->     SABME
    UA                                                      ------->
<----------
   RNR                                                        UA
<----------                 CONTACTED                      <-------
               <---------------------------------------
    +------------+                                 +------------+
    | Connected  |                                 | Connected  |
    +------------+                                 +------------+
    RR
 <---------
             Figure 4.  DLS Connect  Message Protocols
 Upon receipt of a Test command from the origin station, the origin
 DLS will send a CANUREACH (i.e., can you reach) message to the target
 DLS.  If the target DLS is not known to the origin DLS, the CANUREACH
 message is sent to all remote Data Link Switches defined to the
 origin DLS.  The receipt of the CANUREACH message causes the target
 DLS to send a Test command searching for the target station.  The
 target station will return a Test response, causing the target DLS to
 return an ICANREACH (i.e., I can reach) message to the origin DLS.
 If multiple Data Link Switches can reach the target station, the
 origin DLS will receive multiple ICANREACH messages.  The origin DLS
 will select the first message and send a REACH_ACK (i.e., reach
 acknowledgment) message to the selected Data Link Switch.  During
 this exchange of messages, both Data Link Switches change states from
 the Disconected state to the Circuit Established state.  Once the
 circuit is established, Type-1 frames, such as XID, may be exchanged
 between the origin and target stations.

Dixon & Kushi [Page 12] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

 To establish a connection, the origin station sends a SABME command.
 Upon receipt of this command, the origin DLS will send a CONTACT
 message to the target DLS and return a UA response to the origin
 station.  To inhibit traffic flow until the connection is established
 to the remote station, a RNR supervisory frame is sent to the origin
 station.  The CONTACT message will cause the target DLS to send a
 SABME command to the target station, which in return will reply with
 a UA response.  Upon receipt of the UA response, the target DLS will
 send a CONTACTED message to the origin DLS.  The origin DLS will now
 send an RR supervisory frame to the origin station.  During this
 exchange of messages, both Data Link Switches change states from the
 Circuit Established state to the Connected state.
 For NetBIOS end systems, the protocol flows are similar but employ
 different frames and SSP messages.  Instead of using a Test command
 frame to initiate the circuit, a NetBIOS system will use a Name Query
 frame.  Receipt of a Name Query frame will cause the Data Link Switch
 to issue a NETBIOS_NQ message instead of the CANUREACH message.  In a
 like fashion, the Test response is replaced with a Name Recognized
 frame and the ICANREACH message is replaced with a NETBIOS_NR
 message.  As with the SNA protocol flows, the receipt of a NETBIOS_NR
 message causes the origin Data Link Switch to respond with a
 REACH_ACK message.

4.1.2. Link Restart Protocols

 The following figure depicts the protocol flows that result from
 restarting the end-to-end connection.  This causes the Data Link
 Switches to terminate the existing connection and to enter the
 Circuit Established state awaiting the start of a new connection.

Dixon & Kushi [Page 13] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

   Data Link   Data Link                     Data Link   Data Link
    Control     Switch                        Switch      Control
   ---------------------                     ---------------------
        +-----------+                             +-----------+
        | Connected |                             | Connected |
  SABME +-----------+                             +-----------+
----------->                 RESTART_DL
    DM           ------------------------------------->     DISC
<-----------                                               -------->
                                                             UA
                       DL_RESTARTED (Case 1)              <--------
                 <-------------------------------------
        +-----------+                             +-----------+
        |Circuit Est|                             |Circuit Est|
        +-----------+                             +-----------+
                      ........... or ...........
  SABME
----------->           DL_RESTARTED (Case 2)
     UA          <-------------------------------------
<-----------                                      +-----------+
                                                  |Circuit Est|
                              CONTACT             +-----------+
    RNR           ------------------------------------>
<----------
           Figure 5.  DLS Link Restart Message Protocols
 Upon receipt of a SABME command from the origin station, the origin
 DLS will send a RESTART_DL message to the target DLS.  A DM response
 is also returned to the origin station and the data link is
 restarted.
 Upon receipt of the RESTART_DL message, the target DLS will issue a
 DISC command to the target station.  The target station is expected
 to return a UA response.  The target DLS will then restart its data
 link and send an DL_RESTARTED message back to the origin DLS.  During
 this exchange of messages, both Data Link Switches change states from
 Connected state to Circuit Established state.
 If the origin station now resends the SABME command, the origin DLS
 will send a CONTACT message to the target DLS.  If the SABME command
 is received prior to the receipt of the DL_RESTARTED message (case 2
 in the figure), the CONNECT message is delayed until the DL_RESTARTED
 message is received.  The resulting protocol flows at this point
 parallel those given above for the connect sequence.

Dixon & Kushi [Page 14] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

4.1.3. Disconnect Protocols

 The following figure depicts the protocol flows that result from the
 end system terminating an existing connection.  Not only is the
 connection terminated, but the circuit between the Data Link Switches
 is taken down.
   Data Link  Data Link                      Data Link  Data Link
    Control    Switch                         Switch     Control
   --------------------                      --------------------
        +-----------+                             +-----------+
        | Connected |                             | Connected |
        +-----------+                             +-----------+
    DISC
 ---------->                  HALT_DL
     UA         ------------------------------------->      DISC
 <----------                                              --------->
                                                             UA
                             DL_HALTED                    <--------
                <-------------------------------------
        +-----------+                             +-----------+
        |Disconnectd|                             |Disconnectd|
        +-----------+                             +-----------+
                        ......... or ..........
        +-----------+                             +-----------+
        | Connected |                             | Connected |
        +-----------+                             +-----------+
     DISC              TCP Connection Failure               DISC
  <--------     <------------------------------------>    --------->
      UA                                                     UA
  -------->                                               <--------
        +-----------+                             +-----------+
        |Disconnectd|                             |Disconnectd|
        +-----------+                             +-----------+
            Figure 6.  DLS Disconnect Message Protocols
 Upon receipt of a DISC command from the origin station, the origin
 DLS will reply with a UA response and issue a HALT_DL message to the
 target DLS.  Upon receipt of the HALT_DL message, the target DLS will
 send a DISC command to the target station.  The target station will
 then respond with a UA response, causing the target DLS to return a
 DL_HALTED message to the origin DLS.  During this exchange of
 messages, both Data Link Switches change states from the Connected
 state to the Disconnected state.

Dixon & Kushi [Page 15] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

 If the TCP connection between two Data Link Switches fails, all
 connections that are currently multiplexed on the failed TCP
 connection will be taken down.  This implies that both Data Link
 Switches will send DISC commands to all the local systems that are
 associated with the failed connections.  Upon sending the DISC
 command, the Data Link Switch will enter the DISCONNECT state for
 each circuit.

4.2. DLS State Machine

 The following state tables describe the states for a single
 connection through the Data Link Switch.  State information is kept
 for each connection.  The initial state for a connection is
 DISCONNECT.  The steady state is either CIRCUIT_ESTABLISHED or
 CONNECTED.  In the former state, an end-to-end circuit has been
 established allowing the support of Type 1 LLC between the end
 systems.  The latter state exists when an end-to-end connection has
 been established for the support of Type 2 LLC services between the
 end systems.
 For SNA, circuit establishment is via the use of IEEE 802.2 Test or
 XID  frames.  SNA devices send these frames to the null SAP in order
 to determine the source route information in support of bridging.
 Normally SNA devices use SAP x'04', x'08', or x'0C'.  Typically the
 SAP would be used to determine if the Test frames should be sent to
 the DLS code in the router.  If both bridging and DLS are enabled,
 this allows the product to ensure that SNA frames are not both
 bridged and switched.
 For NetBIOS, circuit establishment is via the Name Query and Name
 Recognized frames.  These frames are used for both address resolution
 and source route determination.  Normally NetBIOS devices use SAP
 x'F0'.

4.2.1. Data Link Switch States

 The Switch-to-Switch Protocols will be formally defined through a
 single state machine.  The following table lists the eleven possible
 states.  A separate state machine is employed for each end-to-end
 circuit that is maintained by the Data Link Switch.  The three steady
 states are DISCONNECTED, CIRCUIT_ESTABLISHED, and CONNECTED.

Dixon & Kushi [Page 16] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

   State Name            Description
   ----------            -----------
   CIRCUIT_ESTABLISHED   The end-to-end circuit has been
                         established.  At this time LLC Type 1
                         services are available from end-to-end.
   CIRCUIT_PENDING       The target DLS is awaiting a REACH_ACK
                         response to an ICANREACH message.
   CIRCUIT_RESTART       The DLS that originated the reset is
                         awaiting the restart of the data link
                         and the DL_RESTARTED response to a
                         RESTART_DL message.
   CONNECTED             The end-to-end connection has been
                         established thereby allowing LLC Type 2
                         services from end-to-end in addition to
                         LLC Type 1 services.
   CONNECT_PENDING       The origin DLS is awaiting the CONTACTED
                         response to a CONTACT message.
   CONTACT_PENDING       The target DLS is awaiting the
                         DLC_CONTACTED confirmation to a DLC_CONTACT
                         signal (i.e., DLC is waiting for a UA
                         response to an SABME command).
   DISCONNECTED          The initial state with no circuit or
                         connection established, the DLS is awaiting
                         either a CANUREACH, an ICANREACH, a
                         NETBIOS_NQ, or a NETBIOS_NR message.
   DISCONNECT_PENDING    The DLS that originated the disconnect
                         is awaiting the DL_HALTED response to a
                         HALT_DL message.
   HALT_PENDING          The remote DLS is awaiting the DLC_DL_HALTED
                         indication following the DLC_HALT_DL request
                         (i.e., DLC is waiting for a UA response to a
                         DISC command).
   RESTART_PENDING       The remote DLS is awaiting the DLC_DL_HALTED
                         indication following the DLC_HALT_DL request
                         (i.e., DLC is waiting for a UA response to a
                         DISC command), and the restart of the data
                         link.
   RESOLVE_PENDING       The target DLS is awaiting either the
                         DLC_DL_STARTED indication following the
                         DLC_START_DL reqest (i.e., DLC is waiting
                         for a Test response as a result of sending a
                         Test command), or a NB_Name_Recognized
                         frame in response to a NB_Name_Query frame.
                 Table 2.  Data Link Switch States
 The following figure depicts the events that cause a transition to a
 new state.  The resulting action(s) are not explicitly shown.  The

Dixon & Kushi [Page 17] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

 DISCONNECT_PENDING state will be entered whenever a DLC error
 condition occurs in any of the other states (except RESOLVE_PENDING),
 or when a DISC command is received by the DLC.

Dixon & Kushi [Page 18] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

             DLC_RESOLVE_C+----+        +----DLC_ERROR---+
                          |    |       \|/               |
+------------+            +>+--+---------+  CANUREACH  +-+----------+
| Disconnect +--DL_HALTED-->|Disconnected|------------>|  Resolve   |
|   Pending  |<+   +------->|            |<-------+    |  Pending   |
+---------+--+ |   |        +-----+------+        |    +-----+------+
 /|\      | HALT_DL|         /|\  |/|\/|\         |          |

DLC_ERROR +—-+ | +—-+ | | +——+ | DLC_DL_STARTED

  |                |     |        |HALT_DL(ts)|   +------+   |
                   |     | ICANREACH|         |   HALT_DL(ts)|
     +--HALT_DL(ts)+     |        | |         |          |   |
     |               HALT_DL(ts)  | |     HALT_DL(ts)    |   |
     |                   |        | |         |          |   |
     |   +-RESTART_DL(ts)+----+   | |         |          |   |
     |   |               |   \|/ \|/|         |          |  \|/
+----+---+---+           |  +-------+----+    |        +-+----------+
|  Contact   |<-CONTACT--+--+   Circuit  |<-REACH_ACK--+   Circuit  |
|  Pending   |           |  | Established| (DLC not    |   Pending  |
+-----+------+           |  +----------+-+   contacted)+-+------+---+
      |   /|\            |   /|\ /|\   |      |          |      |
      |    |          +--+----+   |    |      |          |      |
      |    +-CONTACT--+--+--------+----+------+----------+  REACH_ACK
      |               |  |RESTART_DL(ts)      |       (DLC contacted)
  DLC_CONTACTED       |  +------+ |    |      +--------------+  |
      |               |         | |    | DLC_CONTACTED       |  |
      +---------------+-------+ | |    +-----------------+   |  |
      +-----DL_RESTARTED      | | |                      |   |  |
      |  (DLC not contacted) \|/| |                     \|/  | \|/
+-----+------+              +---+-+------+  CONTACTED  +-----+------+
|   Circuit  |<-DLC_RESET---+  Connected |<------------+   Connect  |
|   Restart  |              |            |             |   Pending  |
+----------+-+              +------------+             +-----+------+
     /|\   |                  DL_RESTARTED              /|\  |
      |    +----------------(DLC contacted)--------------+   |
      |                                                      |
      +-------------------------DLC_RESET--------------------+
    Figure 7.  DLS State Transitions  (ts = transitional state)
 The DISCONNECT state is the initial state for a new circuit.  One end
 station starts the connection via a TEST or XID command (i.e.,
 DLC_RESOLVE_C) or a NetBIOS Name Query command (not explicitly
 shown).  Upon receipt, the Data Link Switches exchange a set of
 CANUREACH, ICANREACH and REACH_ACK messages.  Upon completion of this
 three-legged exchange, both Data Link Switches will be in the
 CIRCUIT_ESTABLISHED state.  Two pending states also exist during this
 exchange.  The RESOLVE_PENDING state is entered by the target Data
 Link Switch awaiting a Test response to a Test Command and the

Dixon & Kushi [Page 19] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

 CIRCUIT_PENDING state is entered by the target DLS awaiting the
 REACH_ACK reply to an ICANREACH message.
 The CIRCUIT_ESTABLISHED state allows for the exchange of LLC Type 1
 frames such as the XID exchanges between SNA stations that occurs
 prior to the establishment of a connection.  Also, datagram traffic
 (i.e., UI frames)  may be sent and received between the end stations.
 These exchanges use the XIDFRAME and DGRMFRAME messages sent between
 the Data Link Switches.
 In the CIRCUIT_ESTABLISHED state, the receipt of a SABME command
 (i.e., DLC_CONTACTED) causes the origin DLS to issue a CONTACT
 message, to send an RNR supervisory frame (i.e., DLC_ENTER_BUSY) to
 the origin station, and to enter the CONNECT_PENDING state awaiting a
 CONTACTED message.  The target DLS, upon the receipt of a CONTACT
 message, will issue a SABME command (i.e., DLC_CONTACT) and enter the
 Contact Pending state.  Once the UA response is received (i.e.,
 DLC_CONTACTED), the target DLS sends a CONTACTED message and enters
 the CONNECTED state.  When received, the origin DLS enters the
 Connected state and sends an RR supervisory frame (i.e.,
 DLC_EXIT_BUSY).
 The CONNECTED state is the steady state for normal data flow once a
 connection has been established.  Information frames (i.e., INFOFRAME
 messages) are simply sent back and forth between the end points of
 the connection.  This is the path that should be optimized for
 performance.
 The connection is terminated upon the receipt of a DISC frame or
 under some other error condition detected by DLC (i.e., DLC_ERROR).
 Upon receipt of this indication, the DLS will halt the local data
 link, send a HALT_DL message to the remote DLS, and enter the
 DISCONNECT_PENDING State.  When the HALT_DL frame is received by the
 other DLS, the local DLC is halted for this data link, a DL_HALTED
 message is returned, and the DISCONNECTED state is entered.  Receipt
 of this DL_HALTED message causes the other DLS to also enter the
 DISCONNECTED state.
 The CIRCUIT_RESTART state is entered if one of the Data Link Switches
 receives a SABME command  (i.e., DLC_RESET) while in the CONNECTED
 state.  This causes a DM command to be returned to the origin station
 and a RESTART_DL message to be sent to the remote Data Link Switch.
 This causes the remote data link to be halted and then restarted.
 The remote DLS will then send a DL_RESTARTED message back to the
 first DLS.  The receipt of the DL_RESTARTED message causes the first
 DLS to issue a new CONTACT message, assuming that the local DLC has
 been contacted (i.e., the origin station has resent the SABME
 command).  This is eventually responded to by a CONTACTED message.

Dixon & Kushi [Page 20] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

 Following this exchange, both Data Link Switches will return to the
 CONNECTED state.  If the local DLC has not been contacted, the
 receipt of a DL_RESTARTED command causes the Data Link Switch to
 enter the CIRCUIT_ESTABLISHED state awaiting the receipt of a SABME
 command (i.e., DLC_CONTACTED signal).
 In the figure, the transition labeled HALT_DL(ts) represents the
 HALT_PENDING state and the transition labeled RESTART_DL(ts)
 represents the RESTART_PENDING state.  Both these transitional states
 have only one input event and only one output action.  While in these
 states, the Data Link Switch is awaiting responses from the local
 station on the adjacent LAN (i.e., a UA response to a DISC command).
 Also in the RESTART_PENDING state, the Data Link Switch will attempt
 to restart the data link prior to sending a DL_RESTARTED message.
 For the IBM 6611 implementation, the start of a data link involves
 the exchange of a Test command/response on the adjacent LAN (i.e.,
 DLC_START_DL).  For other implementations, this additional exchange
 may not be required.

4.2.2. State Transition Tables

 This section provides a detail representation of the Data Link
 Switch, as document by a set of state machines.  The first state
 machine documents the sending and receiving of SSP messages.  Many of
 the transitions are dependent upon local signals between the Data
 Link Switch entity and one of the DLC entities.  These signals and
 their definitions are given in the following tables.

Dixon & Kushi [Page 21] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

   Event Name      Description
   ----------      -----------
   DLC_CONTACTED   Contact Indication:  DLC has received an SABME
                   command and will send a UA response, or DLC has
                   received a UA response as a result of sending an
                   SABME command.
   DLC_DGRM        Datagram Indication:  DLC has received a UI frame.
   DLC_ERROR       Error condition indicated by DLC:  Such a
                   condition occurs when a DISC command is received
                   or when DLC experiences an unrecoverable error.
   DLC_INFO        Information Indication:  DLC has received an
                   Information (I) frame.
   DLC_DL_HALTED   Data Link Halted Indication:  DLC has
                   received a UA response to a DISC command.
   DLC_DL_STARTED  Data Link Started Indication:  DLC has
                   received a Test response from the null SAP.
   DLC_RESET       Reset Indication:  DLC has received an SABME
                   command during the time a connection is
                   currently active and has responded with DM.
   DLC_RESOLVE_C   Resolve Command Indication:  DLC has received
                   a Test command addressed to the null SAP, or an
                   XID command addressed to the null SAP.
   DLC_XID         XID Indication:  DLC has received an XID command
                   or response to a non-null SAP.
                     Table 3.  Local DLC Events
   Action Name     Description
   -----------     -----------
   DLC_CONTACT     Contact Station Request:  DLC will send a SABME
                   command.
   DLC_DGRM        Datagram Request:  DLC will send a UI frame.
   DLC_ENTER_BUSY  Enter Link Station Busy:  DLC will send an
                   RNR supervisory frame.
   DLC_EXIT_BUSY   Exit Link Station Busy:  DLC will send an RR
                   supervisory frame.
   DLC_HALT_DL     Halt Data Link Request:  DLC will send a DISC
                   command.
   DLC_INFO        Information Request:  DLC will send an I frame.
   DLC_RESOLVE_R   Resolve Response Request:  DLC will send a
                   Test response or XID response from the null SAP.
   DLC_START_DL    Start Data Link Request:  DLC will send a Test
                   command to the null SAP.
   DLC_XID         XID Request:  DLC will send an XID command or an
                   XID response.
                    Table 4.  Local DLC Actions

Dixon & Kushi [Page 22] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

 The Data Link Switch may be described by a state transition table
 consisting of eleven states.  Each of these states is described below
 in terms of the events, actions, and next state for each transition.
 If a particular event is not listed for a given state, no action and
 no state transition should occur for that event.  Any significant
 comments concerning the transitions within a given state are given
 immediately following the table representing the state.
 A separate state machine is maintained by the Data Link Switch for
 each end-to-end circuit.  The number of circuits that may be
 supported by each Data Link Switch is a local implementation option.

4.2.2.1 DISCONNECTED State

   Event                  Action(s)          Next State
   -----                  ---------          ----------
   Receive CANUREACH      DLC_START_DL       RESOLVE_PENDING
   Receive ICANREACH      Send REACH_ACK,    CIRCUIT_ESTABLISHED
                          DLC_RESOLVE_R
                          (See note 1)
   Receive DATAFRAME      DLC_DGRM
   Receive NETBIOS_NQ     DLC_DGRM           RESOLVE_PENDING
   Receive NETBIOS_NR     Send REACH_ACK,    CIRCUIT_ESTABLISHED
                          DLC_DGRM
                          (See note 2)
   DLC_RESOLVE_C          Send CANUREACH
   DLC_DGRM               If NB_Name_Query:
                          Send NETBIOS_NQ,
                          Else:
                          Send DATAFRAME
 It is assumed that each Data Link Switch will build a set of topology
 tables giving the identity of each Data Link Switch that can reach a
 specific MAC address or a specific NetBIOS name.  This table can be
 built based upon the origin address information received within the
 CANUREACH message or NETBIOS_NQ message and the target address
 information within the ICANREACH message or NETBIOS_NR message.  As a
 consequence, the amount of search traffic can be kept to a minimum.
 Upon receipt of a Test command or XID command to the null SAP (i.e.,
 DLC_RESOLVE_C signal from DLC), the Data Link Switch will check the
 topology table prior to sending the CANUREACH message.  If the target
 MAC address is in the table, the CANUREACH message will be sent to
 only those Data Link Switches that are known to be able to reach the
 given MAC address.  If the MAC address is not in the table, the
 CANUREACH message will be sent to all known Data Link Switches.
 Since the destination link SAP (DSAP) value is null, the Target Link
 SAP field in the CANUREACH message header is set equal to the Origin

Dixon & Kushi [Page 23] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

 Link SAP value.
 Upon receipt of a NB_Name_Query the Data Link Switch will check the
 topology table prior to sending the NETBIOS_NQ message.  If the
 target NetBIOS name is in the table, the NETBIOS_NQ message will be
 sent to only those Data Link Switches that are known to be able to
 reach the given NetBIOS name.  If the NetBIOS name is not in the
 table, the NETBIOS_NQ message will be sent to all known Data Link
 Switches.
 For SNA, the DISCONNECTED state is exited upon receipt of a CANUREACH
 message by a prospective target Data Link Switch, or upon receipt of
 an ICANREACH message by the origin Data Link Switch.  In the former
 case, the Data Link Switch will issue a Test command to the target
 station (i.e., DLC_START_DL signal is presented to DLC).  In the
 later case, a Test response is sent to the origin station (i.e.,
 DLC_RESOLVE_R will be issued) and a REACH_ACK message will be
 returned to the target Data Link Switch.
    Note 1-  The IBM 6611 will not send a Test response, but will send
    a Test command to the station that originated the resolve
    procedure (i.e., a DLC_START_DL will be issued) in order to start
    the data link.
 For NetBIOS, the DISCONNECTED state is exited upon the receipt of a
 NETBIOS_NQ message by the prospective target Data Link Switch, or
 upon the receipt of a NETBIOS_NR message by the origin Data Link
 Switch.  In the former case, the Data Link Switch will send a
 NB_Name_Query frame.  In the later case, the Data Link Switch will
 send a NB_Name_Recognized frame to the origin station and a REACH_ACK
 message will be returned to the target Data Link Switch.
    Note 2-  The IBM 6611 will also send a Test command (i.e., a
    DLC_START_DL will be issued) to the station that originated the
    name resolution in order to start the data link.

4.2.2.2 RESOLVE_PENDING State

   Event                  Action(s)              Next State
   -----                  ---------              ----------
   Receive DATAFRAME      DLC_DGRM
   DLC_DL_STARTED         Send ICANREACH         CIRCUIT_PENDING
   DLC_ERROR                                     DISCONNECTED
   DLC_DGRM               If NB_Name_Recognized: If
                          Send NETBIOS_NR        NB_Name_Recognized:
                          (See note),            CIRCUIT_PENDING
                          Else:
                          Send DATAFRAME

Dixon & Kushi [Page 24] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

 The RESOLVE_PENDING state is entered upon receipt of a CANUREACH
 message or a NETBIOS_NQ message by the target DLS.  In the former
 case, a data link is started, causing a Test command to be sent by
 the DLC.  In the later case, a NB_Name_Query frame is sent.  This
 frame is directed to a group MAC address.
 Any CANUREACH messages received in the RESOLVE_PENDING state will be
 responded to if a DLC_DL_STARTED signal is received.  The Data Link
 Switch may also update its topology information based upon the origin
 MAC address information in each CANUREACH message.
 Upon the receipt of a DLC_DL_STARTED signal in the RESOLVE_PENDING
 state, the Data Link Switch may update its topology table base upon
 the remote MAC address information.  The ICANREACH message should be
 returned to all Data Link Switches that had sent a CANUREACH message.
 In a similar fashion, the Data Link Switch may update its topology
 table upon the receipt of a NB_Name_Recognized frame and a NETBIOS_NR
 message will be returned to all Data Link Switches that have sent a
 NETBIOS_NQ message.
 The RESOLVE_PENDING state is exited once the data link has been
 started (i.e., a DLC_DL_STARTED signal is received as a result of a
 Test response received by the DLC) or a NB_Name_Recognized frame is
 received (i.e., a DLC_DGRM signal is received).  The target Data Link
 Switch will then enter the CIRCUIT_PENDING state.
    Note:  The IBM 6611 will also send a Test command in order to
    start the data link to the station that responded to the Name
    Query frame (i.e., a DLC_START_DL will be issued).

4.2.2.3 CIRCUIT_PENDING State

   Event                  Action(s)         Next State
   -----                  ---------         ----------
   Receive CONTACT        DLC_CONTACT       CONTACT_PENDING
   Receive HALT_DL        DLC_HALT_DL       HALT_PENDING
   Receive REACH_ACK      If Connected:     If Connected:
                          Send CONTACT      CONNECT_PENDING,
                                            else: CIRCUIT_ESTABLISHED
   Receive XIDFRAME       DLC_XID
   Receive DGRMFRAME      DLC_DGRM
   Receive DATAFRAME      DLC_DGRM
   DLC_CONTACTED          DLC_ENTER_BUSY
   DLC_ERROR              Send HALT_DL      DISCONNECT_PENDING
   DLC_XID                Send XIDFRAME
   DLC_DGRM               Send DGRMFRAME

Dixon & Kushi [Page 25] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

 The CIRCUIT_PENDING state is entered by the target Data Link Switch
 following the sending of an ICANREACH message or the sending of a
 NETBIOS_NR message.  In this state it is awaiting the reception of a
 REACH_ACK message from the origin Data Link Switch.  If a connection
 does not exist with the target station (i.e., the normal case), the
 Data Link Switch will enter the CIRCUIT_ESTABLISHED state.
 If the target Data Link Switch happens to receive a SABME command
 from the target station while in the CIRCUIT_PENDING state (i.e., a
 DLC_CONTACTED signal received from the DLC), the reception of the
 REACH_ACK message will cause the Data Link Switch to enter the
 CONNECT_PENDING state and to send a CONTACT message to the other Data
 Link Switch.  Thus the target Data Link Switch has assumed the role
 of the origin Data Link Switch.

4.2.2.4 CONNECT_PENDING State

Event                 Action(s)                    Next State
-----                 ---------                    ----------
Receive CONTACTED     DLC_EXIT_BUSY                CONNECTED
Receive HALT_DL       DLC_HALT_DL                  HALT_PENDING
Receive DGRMFRAME     DLC_DGRM
Receive DATAFRAME     DLC_DGRM
DLC_RESET             Send RESTART_DL (See note)   CIRCUIT_RESTART
DLC_ERROR             Send HALT_DL                 DISCONNECT_PENDING
DLC_DGRM              Send DGRMFRAME
 The CONNECT_PENDING state is entered by the origin Data Link Switch
 when a DLC_CONTACTED signal has been received from the DLC (i.e., a
 SABME command has been received).  A CONTACT message is then issued.
 The state is exited upon the receipt of a CONTACTED message from the
 target Data Link Switch.  If a DLC_RESET signal is received, the
 local data link is restarted and a RESTART_DL message is sent to the
 remote DLS.
    Note:  The IBM 6611 will also send a Test command in order to
    restart the data link to the station that sent the SABME command
    (i.e., a DLC_START_DL will be issued).

Dixon & Kushi [Page 26] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

4.2.2.5 CIRCUIT_ESTABLISHED State

Event                 Action(s)                    Next State
-----                 ---------                    ----------
Receive CONTACT       DLC_CONTACT                  CONTACT_PENDING
Receive HALT_DL       DLC_HALT_DL                  HALT_PENDING
Receive XIDFRAME      DLC_XID
Receive DGRMFRAME     DLC_DGRM
Receive DATAFRAME     DLC_DGRM
DLC_CONTACTED         Send CONTACT                 CONNECT_PENDING
                      DLC_ENTER_BUSY
DLC_ERROR             Send HALT_DL                 DISCONNECT_PENDING
DLC_DGRM              Send DGRMFRAME
DLC_XID               Send XIDFRAME
 The CIRCUIT_ESTABLISHED state is entered by the origin Data Link
 Switch from the DISCONNECTED state, and by the target Data Link
 Switch from the CIRCUIT_PENDING state.  The state is exited when a
 connection is started (i.e., DLC receives a SABME command).  The next
 state is CONTACT_PENDING for the target Data Link Switch and
 CONNECT_PENDING for the origin Data Link Switch.

4.2.2.6 CONTACT_PENDING State

Event                 Action(s)                    Next State
-----                 ---------                    ----------
Receive HALT_DL       DLC_HALT_DL                  HALT_PENDING
Receive RESTART_DL    DLC_HALT_DL                  RESTART_PENDING
Receive DGRMFRAME     DLC_DGRM
Receive DATAFRAME     DLC_DGRM
DLC_CONTACTED         Send CONTACTED               CONNECTED
DLC_ERROR             Send HALT_DL                 DISCONNECT_PENDING
DLC_DGRM              Send DGRMFRAME
 The CONTACT_PENDING state is entered by the target Data Link Switch
 upon the receipt of a CONTACT message.  This causes the Data Link
 Switch to issue a DLC_CONTACT signal to the DLC (i.e., DLC sends a
 SABME command).  This state is then exited upon the receipt of a
 DLC_CONTACTED signal from the DLC (i.e., a UA response received).
 If a RESTART_DL message is received, indicating that the remote Data
 Link Switch has received a DLC_RESET signal, the local Data Link
 Switch will send a DISC command frame on the adjacent LAN (i.e.,
 DLC_HALT_DL signal) and enter the RESTART_PENDING state.

Dixon & Kushi [Page 27] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

4.2.2.7 CONNECTED State

Event                 Action(s)                   Next State
-----                 ---------                   ----------
Receive HALT_DL       DLC_HALT_DL                 HALT_PENDING
Receive RESTART_DL    DLC_HALT_DL                 RESTART_PENDING
Receive DGRMFRAME     DLC_DGRM
Receive INFOFRAME     DLC_INFO
Receive DATAFRAME     DLC_DGRM
DLC_RESET             Send RESTART_DL (See note)  CIRCUIT_RESTART
DLC_ERROR             Send HALT_DL                DISCONNECT_PENDING
DLC_DGRM              Send DGRMFRAME
DLC_INFO              Send INFOFRAME
 The CONNECTED state is entered by the origin Data Link Switch from
 the CONNECT_PENDING state upon the receipt of a CONTACTED message.
 The CONNECTED state is entered by the target Data Link Switch from
 the CONTACT_PENDING state upon the receipt of a DLC_CONTACTED signal.
 At this time, the target Data Link Switch will return a CONTACTED
 message to the origin Data Link Switch.
 The CONNECTED state is exited usually under one of two conditions: a
 DLC_ERROR signal received from the DLC (e.g., a DISC command received
 by the local DLC), or a HALT_DL message received from the other Data
 Link Switch (e.g., a DISC command received by the remote DLC).
 A SABME command (i.e., a DLC_RESET signal) received by either Data
 Link Switch will also cause the two Data Link Switches to leave the
 CONNECTED state and eventually restart a new circuit.
    Note:  The IBM 6611 will also send a Test command in order to
    restart the data link to the station that sent the SABME command
    (i.e., a DLC_START_DL will be issued).
 Following the receipt of a reset signal, the Data Link Switch will
 send a RESTART_DL message to the other Data Link Switch and will
 enter the CIRCUIT_RESTART state.  Upon the receipt of the RESTART_DL
 message, the remote Data Link Switch will send a DISC command (i.e.,
 DLC_HALT_DL signal) and enter the RESTART_PENDING state.

4.2.2.8 CIRCUIT_RESTART State

Event                 Action(s)             Next State
-----                 ---------             ----------
Receive DL_RESTARTED  If Connected:         If Connected:
                      Send CONTACT          CONNECT_PENDING,
                                            else: CIRCUIT_ESTABLISHED
Receive DATAFRAME     DLC_DGRM

Dixon & Kushi [Page 28] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

DLC_ERROR             Send HALT_DL          DISCONNECT_PENDING
DLC_DGRM              Send DATAFRAME
 The CIRCUIT_RESTART state is entered if a DLC_RESET signal is
 received from the local DLC.  This was caused by the receipt of a
 SABME command while a connection was currently active.  A DM response
 will be issued to the SABME command and the Data Link Switch will
 attempt to restart the end-to- end circuit.
 The CIRCUIT_RESTART state is exited through one of two transitions.
 The next state depends upon the time the local DLC has reached the
 contacted state (i.e., a DLC_CONTACTED signal is presented) relative
 to the receipt of the DL_RESTARTED message.  This signal is caused by
 the origin station resending the SABME command that initially caused
 the DATA Link Switch to enter the CIRCUIT_RESTART state.  The two
 cases are as follows:
        1) DL_RESTARTED message received before the
        DLC_CONTACTED signal-  In this case, the
        CIRCUIT_ESTABLISHED state is entered.
        2) DL_RESTARTED message received after the
        DLC_CONTACTED signal-  In this case, the
        CONNECT_PENDING state is entered.

4.2.2.9 DISCONNECT_PENDING State

   Event                     Action(s)             Next State
   -----                     ---------             ----------
   Receive DL_HALTED                               DISCONNECTED
   Receive HALT_DL           Send DL_HALTED
   Receive DATAFRAME         DLC_DGRM
   DLC_DGRM                  Send DATAFRAME
 The DISCONNECT_PENDING state is entered when a DLC_ERROR signal is
 received from the local DLC.  Upon receipt of this signal, a HALT
 message is sent.  Once an DL_HALTED message is received, the state is
 exited, and the Data Link Switch enters the DISCONNECTED state.

4.2.2.10 RESTART_PENDING State

   Event                     Action(s)             Next State
   -----                     ---------             ----------
   Receive DATAFRAME         DLC_DGRM
   DLC_DL_HALTED (See note)  Send DL_RESTARTED     CIRCUIT_ESTABLISHED
   DLC_ERROR                 Send HALT_DL          DISCONNECT_PENDING
   DLC_DGRM                  Send DATAFRAME

Dixon & Kushi [Page 29] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

 The RESTART_PENDING state is entered upon the receipt of a RESTART_DL
 message from the remote DLS while the local Data Link Switch is in
 either the CONTACT_PENDING state or the CONNECTED state.  These cause
 the local DLC to issue a DISC command.  Upon the receipt of the UA
 response (DLC_DL_HALTED), the data link is restarted, a DL_RESTARTED
 message is returned to the remote DLS, and the CIRCUIT_ESTABLISHED
 state is entered.
    Note:  The IBM 6611 will send a Test command in order to restart
    the data link to the target station (i.e., a DLC_START_DL will be
    issued) prior to sending the DL_RESTARTED message.

4.2.2.11 HALT_PENDING State

   Event                     Action(s)          Next State
   -----                     ---------          ----------
   Receive DATAFRAME         DLC_DGRM
   DLC_DL_HALTED             Send DL_HALTED     DISCONNECTED
   DLC_ERROR                 Send DL_HALTED     DISCONNECTED
   DLC_DGRM                  Send DATAFRAME
 The HALT_PENDING state is entered upon the receipt of a HALT_DL
 message.  This causes the local DLC to issue a DISC command.  Upon
 the receipt of the UA response (DLC_DL_HALTED), a DL_HALTED message
 is returned to the remote DLS and the DISCONNECTED state is entered.

4.3. NetBIOS Datagrams

 The NetBIOS protocols use a number of UI frames for directory
 services and the transmission of datagrams.  Most of these frames are
 directed to a group MAC address (GA) with the routing information
 field indicating spanning tree explorer (STE).  Two of the frames,
 NB_Add_Name_Response and NB_Status_Response, are directed to a
 specific MAC address with the routing information field indicating a
 specifically routed frame (SRF).  The handling of these frames is
 summarized in the following table.
   Event                        Action(s)          Comment
   -----                        ---------          -------
   DLC_DGRM (NB Group Address): Send NETBIOS_ANQ   Transmitted to all
        NB_Add_Name_Query                          remote DLS
   DLC_DGRM (Specific Address): Send NETBIOS_ANR   Transmitted to
        NB_Add_Name_Response                       specific DLS
   DLC_DGRM (Specific Address): Send DATAFRAME     Transmitted to all
        NB_Status_Response                         remote DLS
   DLC_DGRM (NB Group Address): Send DATAFRAME     Transmitted to all
        NB_Name_in_Conflict,                       remote DLS
        NB_Add_Group_Name_Query,

Dixon & Kushi [Page 30] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

        NB_Datagram,
        NB_Datagram_Broadcast,
        NB_Status_Query,
        NB_Terminate_Trace
                    Table 5.  NetBIOS DLC Frames
 The above actions do not apply in the following states:
 CIRCUIT_ESTABLISHED, CONTACT_PENDING, CONNECT_PENDING, CONNECTED, and
 CIRCUIT_PENDING.  The handling of the remaining two UI frames used by
 NetBIOS systems, NB_Name_Query and NB_Name_Recognized, are documented
 as part of the DLS state machine in the previous section (i.e.,
 DISCONNECTED and RESOLVE_PENDING states).  Furthermore, the handling
 of NetBIOS datagrams (i.e., NB_Datagram) sent to a specific MAC
 address is also governed by the DLS state machine.
    Note:  The IBM 6611 will also issue Test frames during the
    exchange of the NetBIOS, NB_Name_Query and NB_Name_Recognized.
    This exchange of protocol data units occurs during the start of a
    data link and is used to determine the routing information.  Most
    other implementations of NetBIOS will use the
    NB_Name_Query/NB_Name_Recognized exchange to determine routes in
    conjunction with resolving the NetBIOS names.  These differences
    are not reflected in the SSP protocols.
 The handling of the NetBIOS specific SSP messages is given in the
 following table.
   Event           Action(s)                  Comment
   -----           ---------                  -------
   NETBIOS_ANQ     DLC_DGRM:                  Routed STE
                       NB_Add_Name_Query      (NB Group Address)
   NETBIOS_ANR     DLC_DGRM:                  Routed SRF
                       NB_Add_Name_Response   (Specific MAC Address)
   NETBIOS_NQ      DLC_DGRM:                  Routed STE
                       NB_Name_Query          (NB Group Address)
   NETBIOS_NR      DLC_DGRM:                  Routed SRF
                       NB_Name_Recognized     (Specific MAC Address)
   DATAFRAME       DLC_DGRM                   Routed STE
                                              (If NB_Status_Response:
                                              Specific MAC Address
                                              Else: NB Group Address)
                   Table 6.  NetBIOS SSP Messages
 The above actions apply to all DLS states.  The handling of NetBIOS
 datagrams sent within DGRMFRAME messages is governed by the DLS state
 machine.  The DGRMFRAME message type is employed instead of the

Dixon & Kushi [Page 31] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

 DATAFRAME message type once the end-to-end circuit has been
 established.  At that time, the message is addressed according to the
 pair of Circuit IDs in the message header instead of relying upon the
 MAC address information in the token ring header.

Acknowledgments

 Randall Campbell, David Miller, Gene Cox, Ravi Periasamy, and The
 Ghost of Christmas Past.

References

   1) ISO 8802-2/IEEE Std 802.2 International Standard,
      Information Processing Systems, Local Area Networks, Part 2:
      Logical Link Control, December 31, 1989
   2) The NETBIOS Frames Protocol, IBM Local Area Technical
      Reference, SC30-3383-03, Chapter 5, December 1990
   3) ISO/IEC DIS 10038 DAM 2, MAC Bridging, Source Routing
      Supplement, December 1991

Security Considerations

 Security issues are not discussed in this memo.

Dixon & Kushi [Page 32] RFC 1434 DLS: Switch-to-Switch Protocol March 1993

Authors' Addresses

 Roy C. Dixon
 IBM Networking Systems
 Department B57, Building 060
 P.O. Box 12195
 Research Triangle Park, NC 27709
 Phone: (919) 543-3380
 EMail: rcdixon@ralvmg.vnet.ibm.com
 David M. Kushi
 IBM Research Division
 T. J. Watson Research Center
 Room H0-C07
 30 Saw Mill River Road
 Hawthorne, NY 10532
 Phone: (914) 784-7815
 EMail: kushi@watson.ibm.com
 Note: Any questions or comments relative to the contents of this RFC
 should be sent to the following Internet address:
 dlsw@ralvma.vnet.ibm.com.  This address will be used to coordinate
 the handling of responses.

Dixon & Kushi [Page 33]

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