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rfc:rfc1795

Network Working Group L. Wells, Chair Request for Comments: 1795 Internetwork Technology Institute Obsoletes: 1434 A. Bartky, Editor Category: Informational Sync Research, Inc.

                                                            April 1995
           Data Link Switching: Switch-to-Switch Protocol
     AIW DLSw RIG: DLSw Closed Pages, DLSw Standard Version 1.0

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.

Abstract

 This RFC describes use 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 Implementers.
 This RFC was created as a joint effort of the Advanced Peer-to-Peer
 Networking (APPN) Implementers Workshop (AIW) Data Link Switching
 (DLSw) Related Interest Group (RIG).  The APPN Implementers Workshop
 is a group sponsored by IBM and consists of representatives of member
 companies implementing current and future IBM Networking
 interoperable products. The DLSw Related Interest Group was formed in
 this forum in order to produce a single version of the Switch to
 Switch Protocol (SSP) which could be implemented by all vendors,
 which would fix documentation problems with the existing RFC 1434,
 and which would enhance and evolve the protocol to add new functions
 and features.
 This document is based on RFC 1434.  This document contains
 significant changes to RFC 1434 and therefore obsoletes that
 document.
 Any questions or comments relative to the contents of this RFC should
 be sent to the following Internet address:
 aiw-dlsw@networking.raleigh.ibm.com.
 NOTE 1: This is a widely subscribed mailing list and messages sent to
 this address will be sent to all members of the DLSw mailing list.
 For specific questions relating to subscribing to the AIW and any of

Wells & Bartky [Page 1] RFC 1795 Data Link Switching April 1995

 it's working groups send email to: appn@vnet.ibm.com
 Information regarding all of the AIW working groups and the work they
 are producing can be obtained by copying, via anonymous ftp, the file
 aiwinfo.psbin or aiwinfo.txt from the Internet host
 networking.raleigh.ibm.com, located in directory aiw.
 NOTE 2:  These mailing lists and addresses are subject to change.

1. Introduction

 Data Link Switching (DLSw) is a forwarding mechanism for the IBM SNA
 (Systems Network Architecture) and IBM NetBIOS (Network Basic Input
 Output Services) protocols.  This memo documents the Switch-to-Switch
 Protocol (SSP) that is used between Data Link Switches.  This
 protocol does not provide full routing, but instead provides
 switching at the SNA Data Link layer (i.e., layer 2 in the SNA
 architecture) and encapsulation in TCP/IP for transport over the
 Internet.  This RFC documents the frame formats and protocols for
 multiplexing data 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 in the future.
 A Data Link Switch (abbreviated also as DLSw in this document) can
 support  SNA (Physical Unit (PU) 2, PU 2.1 and PU 4) systems and
 optionally NetBIOS systems attached to IEEE 802.2 compliant Local
 Area Networks, as well as SNA (PU 2 (primary or secondary) and PU2.1)
 systems attached to IBM Synchronous Data Link Control (SDLC) links.
 For the latter case, the SDLC attached systems are provided with a
 LAN appearance within the Data Link Switch (each SDLC PU is presented
 to the SSP protocol as a unique MAC/SAP address pair).  For the
 Token-Ring LAN attached systems, the Data Link Switch appears as a
 source-routing bridge.  Token-Ring Remote systems that are accessed
 through the Data Link Switch appear as systems attached to an
 adjacent ring.  This ring is a virtual ring that is manifested within
 each Data Link Switch.

1.1 Backwards Compatibility with RFC 1434

 This document defines significant changes to RFC 1434 and does not
 state details on how to interoperate with RFC 1434 or "enhanced"
 implementations (e.g., those that added enter and exit busy flow
 control).  It is up to the implementer to refer to RFC 1434 and/or
 any other vendor's documentation in order to interoperate with a
 given vendor's implementation, if interoperability with pre-AIW DLSw
 RIG standards is desired.

Wells & Bartky [Page 2] RFC 1795 Data Link Switching April 1995

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 (Wide Area Network) links via the SDLC protocol.
 IEEE 802.2 LLC Type 2 was designed with the assumption that the
 network transit delay would be predictable (i.e., a local LAN).
 Therefore the LLC Type 2 elements of procedure use a fixed timer for
 detecting lost frames.  When remote 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 Type 2 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 may be
 eventually taken down if the delay exceeds the T1 timer times N2
 retry count.
 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
 connectionless LLC Type 1 service.  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 for connection-oriented data DLSw 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.

Wells & Bartky [Page 3] RFC 1795 Data Link Switching April 1995

 Bridging
 --------
                  Bridge           Bridge
 +------+         +----+           +----+         +------+
 | End  | +-----+ |    +-----/     |    | +-----+ | End  |
 |System+-+ LAN +-+    |    /------+    +-+ LAN +-+System|
 |      | +-----+ |    |  TCP/IP   |    | +-----+ |      |
 +------+         +----+           +----+         +------+
    Info----------------------------------------------->
        <-----------------------------------------------RR
 Data Link Switching
 -------------------
 +------+         +----+           +----+         +------+
 | End  | +-----+ |    +-----/     |    | +-----+ | End  |
 |System+-+ LAN +-+DLSw|    /------+DLSw+-+ LAN +-+System|
 |      | +-----+ |    |  TCP/IP   |    | +-----+ |      |
 +------+         +----+           +----+         +------+
  Info--------------->   -------------> Info
    <---------------RR                 ------------>
                                       <------------RR
 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 DLSw multiplexes LLC connections onto a TCP connection
 to another DLSw.  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
 back pressure to the end systems to provide flow and congestion
 control.
 Only one copy of an Link Protocol Data Unit (LPDU) is sent between
 Data Link Switches in SSP messages (XIDFRAME and INFOFRAME).  Retries
 of the LPDU are absorbed by Data Link Switch that receives it.  The

Wells & Bartky [Page 4] RFC 1795 Data Link Switching April 1995

 Data Link Switch that transmits the LPDU received in an SSP message
 to a local DLC, will perform retries in a manner appropriate for the
 local DLC. This may involve running a reply timer and maintaining a
 poll retry count.  The length of the timer and the number of retries
 is an implementation choice based on user configuration parameters
 and the DLC type.
 Data Link Switching uses LAN addressing to set up connections between
 SNA systems.  SDLC attached devices are defined with MAC and SAP
 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 LLC type 2 connection establishment, SNA systems send TEST (or in
 some cases, XID) frames to the null (0x00) 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 0x04, 0x08, and 0x0C.  NetBIOS always uses a
 link SAP value of 0xF0.

Wells & Bartky [Page 5] RFC 1795 Data Link Switching April 1995

3. Transport Connection

 Data Link Switches can be in used in pairs or by themselves.
 A Single DLSw internally switches one data link to another without
 using TCP (DLC(1) to DLC(2) in the figure below).  This RFC does not
 go into details on how to implement this feature and it is not a
 requirement to support this RFC.
 A paired DLSw multiplexes data links over a reliable transport using
 a Switch-to-Switch Protocol (SSP).
 +-------------------------------------------+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          |
 +-------------------------------------------+
 Before Data Link Switching can occur between two routers, they must
 establish two TCP connections between them.  Each Data Link Switch
 will maintain a list of DLSw capable routers and their status
 (active/inactive).  After the TCP connection is 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.
 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

Wells & Bartky [Page 6] RFC 1795 Data Link Switching April 1995

 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.

Wells & Bartky [Page 7] RFC 1795 Data Link Switching April 1995

3.1 SSP Frame Formats

 The following diagrams show the two message header formats exchanged
 between Data Link Switches, Control and Information.  The Control
 message header is used for all messages except Information Frames
 (INFOFRAME) and Independent Flow Control Messages (IFCM), which are
 sent in Information header format.  The INFOFRAME, KEEPALIVE and IFCM
 message headers are 16 bytes long, and the control message header is
 72 bytes long.  The fields in the first sixteen bytes of all message
 headers are the same.
  CONTROL MESSAGES (72 Bytes)
  (zero based offsets below shown in decimal (xx) )
 +-----------------------------+-----------------------------+
 | (00) Version Number         | (01) Header Length (= 72)   |
 +-----------------------------+-----------------------------+
 | (02) Message Length                                       |
 +-----------------------------+-----------------------------+
 | (04) Remote Data Link Correlator                          |
 +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
 |                                                           |
 +-----------------------------+-----------------------------+
 | (08) Remote DLC Port ID                                   |
 +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
 |                                                           |
 +-----------------------------+-----------------------------+
 | (12) Reserved Field                                       |
 +-----------------------------+-----------------------------+
 | (14) Message Type           | (15) Flow Control Byte      |
 +-----------------------------+-----------------------------+
 | (16) Protocol ID            | (17) Header Number          |
 +-----------------------------+-----------------------------+
 | (18) Reserved                                             |
 +-----------------------------+-----------------------------+
 | (20) Largest Frame Size     | (21) SSP Flags              |
 +-----------------------------+-----------------------------+
 | (22) Circuit Priority       | (23) Message Type (see note)|
 +-----------------------------+-----------------------------+
 | (24) Target MAC Address  (non-canonical format)           |
 +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -|
 |                                                           |
 +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
 |                                                           |
 +-----------------------------+-----------------------------+
 | (30) Origin MAC Address  (non-canonical format)           |
 +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -|

Wells & Bartky [Page 8] RFC 1795 Data Link Switching April 1995

 |                                                           |
 +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
 |            .                              .               |
 +-----------------------------+-----------------------------+
 | (36) Origin Link SAP        | (37) Target Link SAP        |
 +-----------------------------+-----------------------------+
 | (38) Frame Direction        | (39) Reserved               |
 +-----------------------------+-----------------------------+
 | (40) Reserved                                             |
 +-----------------------------+-----------------------------+
 | (42) DLC Header Length                                    |
 +-----------------------------+-----------------------------+
 | (44) Origin DLC Port ID                                   |
 +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
 |                                                           |
 +-----------------------------+-----------------------------+
 | (48) Origin Data Link Correlator                          |
 +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
 |                                                           |
 +-----------------------------+-----------------------------+
 | (52) Origin Transport ID                                  |
 +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
 |                                                           |
 +-----------------------------+-----------------------------+
 | (56) Target DLC Port ID                                   |
 +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
 |                                                           |
 +-----------------------------+-----------------------------+
 | (60) Target Data Link Correlator                          |
 +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
 |                                                           |
 +-----------------------------+-----------------------------+
 | (64) Target Transport ID                                  |
 +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
 |                                                           |
 +-----------------------------+-----------------------------+
 | (68) Reserved Field                                       |
 +-----------------------------+-----------------------------+
 | (70) Reserved Field                                       |
 +-----------------------------+-----------------------------+
          (Even Byte)                     (Odd Byte)

Wells & Bartky [Page 9] RFC 1795 Data Link Switching April 1995

  INFORMATION MESSAGE (16 Bytes)
 +-----------------------------+-----------------------------+
 | (00) Version Number         | (01) Header Length (= 16)   |
 +-----------------------------+-----------------------------+
 | (02) Message Length                                       |
 +-----------------------------+-----------------------------+
 | (04) Remote Data Link Correlator                          |
 +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
 |                                                           |
 +-----------------------------+-----------------------------+
 | (08) Remote DLC Port ID                                   |
 +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
 |                                                           |
 +-----------------------------+-----------------------------+
 | (12) Reserved Field                                       |
 +-----------------------------+-----------------------------+
 | (14) Message Type           | (15) Flow Control Byte      |
 +-----------------------------+-----------------------------+
          (Even Byte)                    (Odd Byte)
 The first sixteen bytes of control and information message headers
 contain identical fields.  A brief description of some of the fields
 in an SSP message are shown below (if not defined below, the fields
 and/or their values are described in subsequent sections).
 The Version Number field (offset 0) is set to 0x31 (ASCII '1'),
 indicating a decimal value of 49.  This is used to indicate DLSw
 version 1.
 The Header Length field (offset 1) is 0x48 for control messages,
 indicating a decimal value of 72 bytes, and 0x10 for information and
 Independent Flow Control messages, indicating a decimal value of 16
 bytes.
 The Message Length field (offset 2) defines the number of bytes
 within the data field following the header.
 The Flow Control Byte field (offset 15)  is described in section 8.
 The Header Number field (offset 17) is 0x01, indicating a value of
 one.
 The Circuit Priority field (offset 22) is described in section 4.
 The Frame Direction field (offset 38) is set to 0x01 for frames sent
 from the origin DLSw to the target DLSw, and is set to 0x02 for
 frames sent from the target DLSw to the origin DLSw.

Wells & Bartky [Page 10] RFC 1795 Data Link Switching April 1995

 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 0x01, and are set equal to the
 Origin Data Link Correlator and Origin DLC Port ID if the Direction
 Field is set to 0x02.
 The Protocol ID field is set to 0x42, indicating a decimal value of
 66.
 The DLC Header Length is set to zero for SNA and is set to 0x23 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).
 NOTE:  The values for the Message Type field are defined in section
 3.5. Note that this value is specified in two different fields
 (offset 14 and 23 decimal) of the control message header.  Only the
 first field is to be used when parsing a received SSP message.  The
 second field is to be ignored by new implementations on reception.
 The second field was left in for backwards compatibility with RFC
 1434 implementations and this field may be used in future versions if
 needed.
 The SSP Flags field contains additional information related to the
 SSP message.  The flags are defined as follows (bit 7 being the most
 significant bit and bit 0 the least significant bit of the octet):
 Bit(s)
 76543210    Name    Meaning
 ---------   -----   -------
 x.......    SSPex   1 = explorer message (CANUREACH and ICANREACH)
 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).  Each attachment address is classified as
 either "Target" in the context of the Destination MAC/SAP addresses
 of an explorer frame sent in the first frame used to establish a

Wells & Bartky [Page 11] RFC 1795 Data Link Switching April 1995

 circuit, or "Origin" in the context of the Source MAC/SAP addresses.
 All MAC addresses are expressed in non-canonical (Token-Ring) format.
  DATA LINK ID  (14 Bytes @ Control message offset 24 decimal)
 +-----------------------------+-----------------------------+
 | 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 DLSw.  It consists of a DLC Port ID (4 bytes), and a Data
 Link Correlator (4 bytes).  The Circuit ID must be unique in a single
 DLSw and is assigned locally.  The pair of Circuit ID's along with
 the Data Link IDs,  uniquely identify a single end-to-end circuit.
 Each DLSw 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" DLSw and "Target"
 DLSw, 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

Wells & Bartky [Page 12] RFC 1795 Data Link Switching April 1995

 these two fields, along with the associated values for DLC Port ID
 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 message types all carry the Data Link ID,
 consisting of the MAC and Link SAP addresses associated with the two
 end stations.  The CANUREACH and ICANREACH messages are qualified by
 the SSPex flag into CANUREACH_ex, ICANREACH_ex (explorer messages)
 and CANUREACH_cs, ICANREACH_cs (circuit start).  The CANUREACH_ex is
 used to find a remote MAC and Link SAP address without establishing
 an SSP circuit.  Upon receipt of a CANUREACH_cs message, the target
 DLSw starts a data link for each port, thereby obtaining a Data Link
 Correlator.  If the target station can be reached, an ICANREACH_cs
 message is returned to the origin DLSw containing the Target Circuit
 ID parameter.  Upon receipt, the origin DLSw starts a data link and
 returns the Origin Circuit ID to the target DLSw within the REACH_ACK
 message.  (Note for a full list of message types, see section 3.5.)
 +------------+                                +------------+
 |Disconnected|                                |Disconnected|
 +------------+   CANUREACH_cs (Data Link ID)  +------------+
     ------------------------------------------------->
       ICANREACH_cs (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)
     <-------------------------------------------------
 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

Wells & Bartky [Page 13] RFC 1795 Data Link Switching April 1995

 established, the INFOFRAME messages are exchanged with the shorter
 header.  This header contains only the Circuit ID associated with the
 remote DLSw.  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 Correlators

 The local use, and contents of the Data Link Correlator, Port ID and
 Transport ID fields in SSP messages is an implementation choice.
 These fields have local significance only.  The values received from
 a partner DLSw must not be interpreted by the DLSw that receives them
 and should be echoed "as is" to a partner DLSw in subsequent
 messages.  All implementations must obey the following rules in this
 section (3.3) on the assignment and fixing of these correlator fields
 for each transport connection or circuit:
 The Transport ID fields are learned from the first SSP message
 exchanged with a DLSw partner (the Capabilities exchange).  This
 field should not be varied by a DLSw after the capabilities exchange
 and must be reflected to the partner DLSw in every SSP control
 message.
 The Target Data Link Correlator, Target Port ID and Target Transport
 ID must remain the same once the Target DLSw has sent the
 ICANREACH_cs for a given circuit.  The Origin DLSw must store the
 values specified in the ICANREACH_cs and use these on all subsequent
 SSP messages for this circuit.
 The Origin DLSw must allow these fields to vary until the
 ICANREACH_cs is received.  Each SSP message issued for a circuit must
 reflect the values specified by the Target DLSw in the last SSP
 message for this circuit received by the Origin DLSw.  Binary zero
 should be used if no such message has yet been received for a given
 circuit (apart from the Target Transport ID which will have been
 learnt as specified above).
 The Origin Data Link Correlator, Origin Port ID and Origin Transport
 ID must remain the same once the Origin DLSw has issued the REACH_ACK
 for a given circuit.  The Target DLSw must store the values specified
 in the REACH_ACK and use these on all subsequent SSP messages for
 this circuit.
 The Target DLSw must allow these fields to vary until the REACH_ACK
 is received.  Each SSP message issued for a circuit must reflect the
 values specified by the Origin DLSw in the last SSP message for this
 circuit received by the Target DLSw.  Binary zero should be used if

Wells & Bartky [Page 14] RFC 1795 Data Link Switching April 1995

 no such message has yet been received for a given circuit (apart from
 the Origin Transport ID which will have been learnt as specified
 above).
 For the purposes of correlator exchange, explorer messages form a
 separate circuit.  Both DLSw partners must reflect the last received
 correlator values as specified above.  However correlators learned on
 explorer messages need not be carried over to a subsequent circuit
 setup attempt.  In particular, the Origin DLSw may elect to use the
 same values for the Origin Data Link Correlator and Origin Port ID
 when it issues a CANUREACH_cs after receiving an ICANREACH_ex or
 NETBIOS_NR_ex. However the Target DLSw must not assume that the
 CANUREACH_cs will specify any of the Target Data Link Correlator or
 Target Port ID that were exchanged on the explorer messages.
 Received SSP messages that require a valid Remote Circuit ID but
 cannot be associated with an existing circuit should be rejected with
 a HALT_DL_NOACK message.  This is done to prevent a situation where
 one DLSw partner has a circuit defined while the other partner does
 not. The exception would be a HALT_DL_NOACK message with an invalid
 Remote Circuit ID.  The HALT_DL_NOACK message is typically used in
 error situations where a response is not appropriate.
 The SSP messages requiring a valid Remote Circuit ID are all messages
 except the following: CANUREACH_ex, CANUREACH_cs, ICANREACH_ex,
 ICANREACH_cs, NETBIOS_NQ_cs, NETBIOS_NR_cs, DATAFRAME, NETBIOS_ANQ,
 NETBIOS_ANR, KEEPALIVE and CAP_EXCHANGE.

3.4 Largest Frame Size Field

 The Largest Frame Size (LF Size) field in the SSP Control Header is
 used to carry the LF Size bits across the DLSw connection.  This
 should be used to ensure that the two end-stations always negotiate a
 frame size to be used on a circuit that does not require the Origin
 and Target DLSw partners to re-segment frames.
 This field is valid on CANUREACH_ex, CANUREACH_cs, ICANREACH_ex,
 ICANREACH_cs, NETBIOS_NQ_ex and NETBIOS_NR_ex messages only. The
 contents of this field should be ignored on all other frames.
 Every DLSw forwarding a SSP frame to its DLSw partner must ensure
 that the contents of this frame reflect the minimum capability of the
 route to its local end-station or any limit imposed by the DLSw
 itself.
 The bit-wise definition of this field is as follows (bit 7 is the
 most significant bit, bit 0 is the least significant bit):

Wells & Bartky [Page 15] RFC 1795 Data Link Switching April 1995

   7   6   5   4   3   2   1   0
 +-------------------------------+
 | c | r | b | b | b | e | e | e |
 +-------------------------------+
   c   .   .   .   .   .   .   .  LF Size Control flag
                                  (significant on messages
                                  from Origin to Target
                                  DLSw only)
                                  0=fail circuit if route
                                    obtained requires a
                                    smaller LF size
                                  1=don't fail the circuit
                                    but return the LF size
                                    obtained even if it is
                                    smaller
   .   r   .   .   .   .   .   .  Reserved
   .   .   b   .   .   .   .   .  Largest Frame Bit Base
   .   .   .   b   .   .   .   .  Largest Frame Bit Base
   .   .   .   .   b   .   .   .  Largest Frame Bit Base
   .   .   .   .   .   e   .   .  Largest Frame Bit Extended
   .   .   .   .   .   .   e   .  Largest Frame Bit Extended
   .   .   .   .   .   .   .   e  Largest Frame Bit Extended
           <----- LF Bits ----->
 Refer to IEEE 802.1D Standard, Annex C for encoding of Largest Frame
 base and extended bit values.
 The Origin DLSw "Size Control" flag informs a Target DLSw that
 chooses to reply to *_cs messages on the basis of cached information
 that it may safely return a smaller LF Size on the ICANREACH_cs frame
 if it has had to choose an alternative route on which to initialize
 the circuit.  If this bit is set to 1, the Origin DLSw takes
 responsibility for ensuring that the end-stations negotiate a
 suitable frame size for the circuit. If this bit is set to 0, the
 Target DLSw must not reply to the CANUREACH_cs if it cannot obtain a
 route to the Target end station that support an LF Size at least as
 large as that specified in the CANUREACH_cs frame.

3.5 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.

Wells & Bartky [Page 16] RFC 1795 Data Link Switching April 1995

 Command          Description                       Type   flags/notes
 -------          --------                         ------  -----------
 CANUREACH_ex     Can U Reach Station-explorer      0x03   SSPex
 CANUREACH_cs     Can U Reach Station-circuit start 0x03
 ICANREACH_ex     I Can Reach Station-explorer      0x04   SSPex
 ICANREACH_cs     I Can Reach Station-circuit start 0x04
 REACH_ACK        Reach Acknowledgment              0x05
 DGRMFRAME        Datagram Frame                    0x06   (note 1)
 XIDFRAME         XID Frame                         0x07
 CONTACT          Contact Remote Station            0x08
 CONTACTED        Remote Station Contacted          0x09
 RESTART_DL       Restart Data Link                 0x10
 DL_RESTARTED     Data Link Restarted               0x11
 ENTER_BUSY       Enter Busy                        0x0C   (note 2)
 EXIT_BUSY        Exit Busy                         0x0D   (note 2)
 INFOFRAME        Information (I) Frame             0x0A
 HALT_DL          Halt Data Link                    0x0E
 DL_HALTED        Data Link Halted                  0x0F
 NETBIOS_NQ_ex    NETBIOS Name Query-explorer       0x12   SSPex
 NETBIOS_NQ_cs    NETBIOS Name Query-circuit setup  0x12   (note 3)
 NETBIOS_NR_ex    NETBIOS Name Recognized-explorer  0x13   SSPex
 NETBIOS_NR_cs    NETBIOS Name Recog-circuit setup  0x13   (note 3)
 DATAFRAME        Data Frame                        0x14   (note 1)
 HALT_DL_NOACK    Halt Data Link with no Ack        0x19
 NETBIOS_ANQ      NETBIOS Add Name Query            0x1A
 NETBIOS_ANR      NETBIOS Add Name Response         0x1B
 KEEPALIVE        Transport Keepalive Message       0x1D   (note 4)
 CAP_EXCHANGE     Capabilities Exchange             0x20
 IFCM             Independent Flow Control Message  0x21
 TEST_CIRCUIT_REQ Test Circuit Request              0x7A
 TEST_CIRCUIT_RSP Test Circuit Response             0x7B
 Note 1: Both the DGRMFRAME and DATAFRAME messages are used to carry
 information received by the DLC entity within UI frames.  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 or
 during circuit restart when Data Links are reset.
 Note 2: These messages are not used for the DLSw Standard but may be
 used by older DLSw implementations.  They are listed here for
 informational purposes.  These messages were added after publication
 of RFC 1434 and were deleted in this standard (adaptive pacing is now
 used instead).

Wells & Bartky [Page 17] RFC 1795 Data Link Switching April 1995

 Note 3: These messages are not normally issued by a Standard DLSw,
 which uses the NB_*_ex messages as shown in section 5.4.  However if
 a Standard DLSw attempts to interoperate with older DLSw
 implementations, these messages correspond to the NETBIOS_NQ and
 NETBIOS_NR messages used in RFC1434 both to locate the resource and
 to setup a circuit.  This document does not attempt to provide a
 complete specification of the use of these messages.
 Note 4:  A KEEPALIVE message may be sent by a DLSw to a partner DLSw
 in order to verify the TCP connection (or other future SSP carrying
 protocol) is still functioning.  If received by a DLSw, this message
 is discarded and ignored.  Use of this message is optional.
 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
 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.
 Any SSP message types not defined above if received by a DLSw are to
 be ignored (i.e., no error action is to be performed).  A Data Link
 Switch should quietly drop any SSP message with a Message Type that
 is not recognized or not supported.  Receipt of such a message should
 not cause the termination of the transport connection to the message
 sender.

4. Circuit Priority

 At circuit start time, each circuit end point will provide priority
 information to its circuit partner.  The initiator of the circuit
 will choose which circuit priority will be effective for the life of
 the circuit.  If Priority is not implemented by the Data Link Switch,
 then "Unsupported" priority is used.

4.1 Frame format

 Circuit priority will be valid in the CANUREACH_cs, ICANREACH_cs, and
 REACH_ACK frames only. The relevant header field is shown below.  The
 Circuit Priority value is a byte value at offset 22 in an SSP Control
 Message.

Wells & Bartky [Page 18] RFC 1795 Data Link Switching April 1995

 The following describes the format of the Circuit Priority byte.
   7   6   5   4   3   2   1   0
 +-------------------+-----------+
 |   reserved        |    CP     |
 +-------------------+-----------+
 CP: Circuit Priority bits
         000 - Unsupported       (note 1)
         001 - Low Priority
         010 - Medium Priority
         011 - High Priority
         100 - Highest Priority
         101 to 111 are reserved for future use
 Note 1: Unsupported means that the Data Link Switch that originates
 the circuit does not implement priority.  Actions taken on
 Unsupported priority are vendor specific.

4.2 Circuit Startup

 The sender of a CANUREACH_cs is responsible for setting the CP bits
 to reflect the priority it would like to use for the circuit being
 requested.  The mechanism for choosing an appropriate value is
 implementation dependent.  The sender of an ICANREACH_cs frame will
 set the CP bits to reflect the priority it would like to use for the
 circuit being requested, with the mechanism for choosing the
 appropriate value being implementation dependent.  The receiver of
 the ICANREACH_cs will select from the priorities in the CANUREACH_cs
 and ICANREACH_cs frames, and will set the value in the CP field of
 the REACH_ACK frame that follows to the value to be used for this
 circuit.  This priority will be used for the life of the circuit.  A
 CANUREACH_cs or ICANREACH_cs with the circuit priority value set to
 Unsupported (CP=000) indicates that the sender does not support the
 circuit priority function.

Wells & Bartky [Page 19] RFC 1795 Data Link Switching April 1995

 Flow:
    DLSw A               DLSw B
 CANUREACH_cs (CP=011) ----->           Circuit initiator requests
                                        high Priority.
      <--------- ICANREACH_cs (CP=010)  Circuit target requests
                                        medium priority.
 REACH_ACK (CP=010) -------->           Circuit initiator sets
                                        the priority for this
                                        circuit to medium. The
                                        circuit initiator could
                                        choose either high or
                                        medium in this example.

5. DLSw State Machine

 The following state tables describe the states for a single circuit
 through the Data Link Switch.  State information is kept for each
 connection.  The initial state for a connection is DISCONNECTED.  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, LLC type 2 connection 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 0x04, 0x08, or 0x0C  (most SNA
 LLC2 devices that have a single PU per MAC address use a default of
 0x04).  Typically the SAP would be used to determine if the Test frames
 should be sent to the DLSw code in the router.  If both bridging and
 DLSw are enabled, this allows the product to ensure that SNA frames are
 not both bridged and switched.  Note that although typically SNA uses a
 DSAP and SSAP of 0x04, it allows for other SAPs to be configured and
 supports unequal SAPs.  This allows multiple PUs to share connections
 between two given MAC addresses (each PU to PU session uses one LLC2
 connection).
 For NetBIOS, LLC type 2 connection establishment is via the Name Query
 and Name Recognized frames.  These frames are used for both address
 resolution and source route determination.  NetBIOS devices use SAP
 0xF0.

Wells & Bartky [Page 20] RFC 1795 Data Link Switching April 1995

5.1 Data Link Switch States

 The Switch-to-Switch Protocol is formally defined through the state
 machines described in this chapter.  The following table lists the
 thirteen possible states for the main circuit FSM.  A separate state
 machine instance is employed for each end-to-end circuit that is
 maintained by the Data Link Switch.
 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 DLSw is awaiting a REACH_ACK
                       response to an ICANREACH_cs message.
 CIRCUIT_RESTART       The DLSw that originated the reset is
                       awaiting the restart of the data link
                       and the DL_RESTARTED response to a
                       RESTART_DL message.
 CIRCUIT_START         The origin DLSw is awaiting a
                       ICANREACH_cs in response to a
                       CANUREACH_cs 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 DLSw is awaiting the
                       CONTACTED response to a CONTACT
                       message.
 CONTACT_PENDING       The target DLSw 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
                       DLSw is awaiting either a
                       CANUREACH_cs, or an ICANREACH_cs.
 DISCONNECT_PENDING    The DLSw that originated the
                       disconnect is awaiting the DL_HALTED

Wells & Bartky [Page 21] RFC 1795 Data Link Switching April 1995

                       response to a HALT_DL message.
 HALT_PENDING          The remote DLSw 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), due to receiving a
                       HALT_DL message.
 HALT_PENDING_NOACK    The remote DLSw 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), due to receiving a
                       HALT_DL_NOACK message.
 RESTART_PENDING       The remote DLSw 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 DLSw is awaiting
                       the DLC_DL_STARTED indication
                       following the DLC_START_DL request
                       (i.e., DLC is waiting for a Test
                       response as a result of sending a
                       Test command).
 The DISCONNECTED state is the initial state for a new circuit.  One
 end station starts the connection via an XID or SABME command (i.e.,
 DLC_XID or DLC_CONTACTED).  Upon receipt, the Data Link Switches
 exchange a set of CANUREACH_cs, ICANREACH_cs and REACH_ACK messages.
 Upon completion of this three-legged exchange both Data Link Switches
 will be in the CIRCUIT_ESTABLISHED state.  Three pending states also
 exist during this exchange.  The CIRCUIT_START state is entered by
 the origin Data Link Switch after it has sent the CANUREACH_cs
 message.  The RESOLVE_PENDING state is entered by the target Data
 Link Switch awaiting a Test response to a Test Command.  And lastly,
 the CIRCUIT_PENDING state is entered by the target DLSw awaiting the
 REACH_ACK reply to an ICANREACH_cs 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

Wells & Bartky [Page 22] RFC 1795 Data Link Switching April 1995

 the Data Link Switches.
 In the CIRCUIT_ESTABLISHED state, the receipt of a SABME command
 (i.e., DLC_CONTACTED) causes the origin DLSw 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 DLSw, 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 DLSw sends a CONTACTED message and enters
 the CONNECTED state. When received, the origin DLSw 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 DLSw will halt the local data
 link, send a HALT_DL message to the remote DLSw, and enter the
 DISCONNECT_PENDING State.  When the HALT_DL frame is received by the
 other DLSw, 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 DLSw 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) after data transfer 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 DLSw will then send a DL_RESTARTED
 message back to the first DLSw.  The receipt of the DL_RESTARTED
 message causes the first DLSw 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. 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).
 The HALT_PENDING, HALT_PENDING_NOACK and RESTART_PENDING states
 correspond to the cases when the Data Link Switch is awaiting

Wells & Bartky [Page 23] RFC 1795 Data Link Switching April 1995

 responses from the local station on the adjacent LAN (e.g., 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 some implementations, 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.

5.2 State Transition Tables

 This section provides a detailed representation of the Data Link
 Switch, as documented by a single state machine.  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.
 DLC Events:
 Event Name      Description
 ----------      -----------
 DLC_CONTACTED   Contact Indication:  DLC has received an SABME
                 command 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.

Wells & Bartky [Page 24] RFC 1795 Data Link Switching April 1995

 DLC_RESOLVED    Resolve request:  DLC has received a TEST response
                 frame (or equivalent for non-LAN DLCs) but has not
                 reserved the resources required for a circuit yet.
 DLC_XID         XID Indication:  DLC has received an XID command
                 or response to a non-null SAP.
 Other Events:
 Event Name      Description
 ----------      -----------
 XPORT_FAILURE   Failure of the transport connection used by the
                 circuit.
 CS_TIMER_EXP    The CIRCUIT_START timer (started when the circuit
                 went into CIRCUIT_START state) has expired.
 DLC Actions:
 Action Name     Description
 -----------     -----------
 DLC_CONTACT     Contact Station Request:  DLC will send a SABME
                 command or a UA response to an outstanding 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     Resolve request:  DLC should issue a TEST (or
                 appropriate equivalent for non-LAN DLCs) but need
                 not reserve the resources required for a circuit yet.
 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.

Wells & Bartky [Page 25] RFC 1795 Data Link Switching April 1995

 DLC_XID         XID Request:  DLC will send an XID command or an
                 XID response.
 Other Actions:
 Action Name     Description
 ----------      -----------
 START_CS_TIMER  Start the CIRCUIT_START timer.
 DLC_RESOLVE_R and DLC_START_DL actions require the DLC to reserve the
 resources necessary for a link station as they are used only when a
 circuit is about to be started.  The DLC_RESOLVE action is used for
 topology explorer traffic and does not require such resources to be
 reserved, though a DLC implementation may choose not to distinguish
 this from the DLC_START_DL action.  See section 5.4 for details of
 the actions and events for explorer frames.
 The Data Link Switch is described by a state transition table as
 documented in the following sections.  Each of the 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 instance 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.
 The CANUREACH_ex, ICANREACH_ex, NETBIOS_NQ_ex, and NETBIOS_NR_ex are
 SSP messages that are not associated with a particular circuit.  The
 processing of these messages is covered in section 5.4.

Wells & Bartky [Page 26] RFC 1795 Data Link Switching April 1995

5.2.1 DISCONNECTED State

 +----------------------+---------------------+----------------------+
 |        Event         |      Action(s)      |      Next State      |
 +----------------------+---------------------+----------------------+
 | Receive CANUREACH_cs | DLC_START_DL        | RESOLVE_PENDING      |
 +----------------------+---------------------+----------------------+
 | Receive DATAFRAME    | DLC_DGRM            |                      |
 +----------------------+---------------------+----------------------+
 | DLC_XID              | If source route     | If CANUREACH_cs was  |
 |                      | bridged frame with  | sent:                |
 |                      | broadcast indicated:|   CIRCUIT_START      |
 |                      |   Send CANUREACH_ex |                      |
 |                      | else:               |                      |
 |                      |   Send CANUREACH_cs |                      |
 |                      |   START_CS_TIMER    |                      |
 +----------------------+---------------------+----------------------+
 | DLC_DGRM             | If NETBIOS          |                      |
 |                      | NAME_QUERY:         |                      |
 |                      |  Send NETBIOS_NQ_ex |                      |
 |                      | else:               |                      |
 |                      |  Send DATAFRAME     |                      |
 +----------------------+---------------------+----------------------+
 | DLC_CONTACTED        | Send CANUREACH_cs   | CIRCUIT_START        |
 +----------------------+---------------------+----------------------+
 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  using the explorer frames, as per the Explorer FSM in section
 5.4.  As a consequence, the amount of search traffic can be kept to a
 minimum.
 Upon receipt of a TEST command, broadcast XID or NetBIOS NAME_QUERY,
 the Data Link Switch checks the topology table for the target MAC/SAP
 or NetBIOS name.  If there is no matching entry in the table, the
 Data Link Switch uses the explorer FSMs in section 5.4 to locate the
 target MAC/SAP or NetBIOS name.
 When the first non-broadcast XID or SABME flows,  the Data Link
 Switch issues a CANUREACH_cs to attempt to start a circuit.  The
 CANUREACH_cs message is sent to only those Data Link Switches that
 are known to be able to reach the given MAC address.  The mechanism
 by which a topology table entry is determined to be out-of-date and
 is deleted from the table is implementation specific.
 The DISCONNECTED state is exited upon the sending of a CANUREACH_cs
 by the origin DLSw or the receipt of a CANUREACH_cs message by a

Wells & Bartky [Page 27] RFC 1795 Data Link Switching April 1995

 prospective target Data Link Switch.  In the latter case, the Data
 Link Switch will issue a Test command to the target station (i.e.,
 DLC_START_DL signal is presented to DLC).

5.2.2 RESOLVE_PENDING State

 +-------------------+-----------------------+-----------------------+
 |        Event      |      Action(s)        |      Next State       |
 +-------------------+-----------------------+-----------------------+
 | Receive DATAFRAME | DLC_DGRM              |                       |
 +-------------------+-----------------------+-----------------------+
 | DLC_DL_STARTED    | If LF value of        | If LF value of        |
 |                   | DLC_DL_STARTED        | DLC_DL_STARTED        |
 |                   | is greater than or    | is greater than or    |
 |                   | equal to LF Size of   | equal to LF Size of   |
 |                   | CANUREACH_cs or LF    | CANUREACH_cs or LF    |
 |                   | Size Control bit set: | Size Control bit set: |
 |                   |   Send ICANREACH_cs   |   CIRCUIT_PENDING     |
 |                   | else:                 | else:                 |
 |                   |   Send DLC_HALT_DL    |   HALT_PENDING_NOACK  |
 +-------------------+-----------------------+-----------------------+
 | DLC_ERROR         |                       | DISCONNECTED          |
 +-------------------+-----------------------+-----------------------+
 | DLC_DGRM          | Send DATAFRAME        |                       |
 +-------------------+-----------------------+-----------------------+
 The RESOLVE_PENDING state is entered upon receipt of a CANUREACH_cs
 message by the target DLSw.  A data link is started, causing a Test
 command to be sent by the DLC.
 Several CANUREACH_cs messages can be received in the RESOLVE_PENDING
 state.  The Data Link Switch may update its topology information
 based upon the origin MAC address information in each CANUREACH_cs
 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_cs message must be
 returned to the first partner DLSw from which a CANUREACH_cs was
 received for this circuit, or an implementation may optionally reply
 to all partners from which the CANUREACH_cs was received.
 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).  The target Data Link Switch then
 enters the CIRCUIT_PENDING state.

Wells & Bartky [Page 28] RFC 1795 Data Link Switching April 1995

5.2.3 CIRCUIT_START State

 +----------------------+---------------------+----------------------+
 |        Event         |      Action(s)      |      Next State      |
 +----------------------+---------------------+----------------------+
 | Receive CANUREACH_cs | If origin MAC addr  | If DLC_START_DL      |
 | for circuit in       | in CANUREACH_cs is  | issued:              |
 | opposite direction   | greater than origin |   RESOLVE_PENDING    |
 |                      | MAC addr of circuit:|                      |
 |                      |   DLC_START_DL      |                      |
 |                      | else:               |                      |
 |                      |   no action taken   |                      |
 +----------------------+---------------------+----------------------+
 | Receive ICANREACH_cs | If LF Size Control  | If LF Size Control   |
 |                      | bit set and LF Size | bit set and LF Size  |
 |                      | is not negotiable:  | is not negotiable:   |
 |                      |   Send HALT_DL_NOACK|   DISCONNECTED       |
 |                      | else:               | else if Connected:   |
 |                      |   Send REACH_ACK,   |   CONNECT_PENDING    |
 |                      |   Send appropriate  | else:                |
 |                      |   SSP message based |   CIRCUIT_ESTABLISHED|
 |                      |   on the event      |                      |
 |                      |   that generated    |                      |
 |                      |   CANUREACH_cs      |                      |
 |                      |   (see Note)        |                      |
 +----------------------+---------------------+----------------------+
 | DLC_DGRM             | Send DATAFRAME      |                      |
 +----------------------+---------------------+----------------------+
 | DLC_ERROR            |                     | DISCONNECTED         |
 +----------------------+---------------------+----------------------+
 | CS_TIMER_EXP         |                     | DISCONNECTED         |
 +----------------------+---------------------+----------------------+
 | XPORT_FAILURE        |                     | DISCONNECTED         |
 +----------------------+---------------------+----------------------+
 The CIRCUIT_START state is entered by the origin Data Link Switch
 when a DLC_XID or DLC_CONTACTED signal has been received from the
 DLC.
 The CIRCUIT_START state is exited upon receipt of an ICANREACH_cs
 message.  A REACH_ACK message is returned to the target Data Link
 Switch.  If the CIRCUIT_START state was entered due to a DLC_XID
 signal, an XIDFRAME message containing the XID is sent to the target
 Data Link Switch.  If the CIRCUIT_START state was entered due to a
 DLC_CONTACTED signal, a CONTACT message is sent to the target Data
 Link Switch.

Wells & Bartky [Page 29] RFC 1795 Data Link Switching April 1995

5.2.4 CIRCUIT_PENDING State

 +----------------------+---------------------+----------------------+
 |        Event         |      Action(s)      |      Next State      |
 +----------------------+---------------------+----------------------+
 | Receive CONTACT      | DLC_CONTACT         | CONTACT_PENDING      |
 +----------------------+---------------------+----------------------+
 | Receive HALT_DL      | DLC_HALT_DL         | HALT_PENDING         |
 +----------------------+---------------------+----------------------+
 | Receive HALT_DL_NOACK| DLC_HALT_DL         | HALT_PENDING_NOACK   |
 +----------------------+---------------------+----------------------+
 | 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        | If UA is sent in    |                      |
 |                      | response to SABME:  |                      |
 |                      |   DLC_ENTER_BUSY    |                      |
 |                      | else:               |                      |
 |                      |   no action taken   |                      |
 +----------------------+---------------------+----------------------+
 | DLC_ERROR            |                     | DISCONNECTED         |
 +----------------------+---------------------+----------------------+
 | DLC_XID              | Drop or hold until  |                      |
 |                      | REACH_ACK received  |                      |
 +----------------------+---------------------+----------------------+
 | DLC_DGRM             | Send DATAFRAME      |                      |
 +----------------------+---------------------+----------------------+
 | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
 +----------------------+---------------------+----------------------+
 The CIRCUIT_PENDING state is entered by the target Data Link Switch
 following the sending of an ICANREACH_cs message.  In this state it
 is awaiting the reception of a REACH_ACK message from the origin Data
 Link Switch.
 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 causes the target Data Link Switch to enter the
 CONNECT_PENDING state and to send a CONTACT message to the origin

Wells & Bartky [Page 30] RFC 1795 Data Link Switching April 1995

 Data Link Switch.
 If no such SABME is received, the receipt of the REACH_ACK causes the
 Data Link Switch to enter CIRCUIT_ESTABLISHED state.

5.2.5 CONNECT_PENDING State

 +----------------------+---------------------+----------------------+
 |        Event         |      Action(s)      |      Next State      |
 +----------------------+---------------------+----------------------+
 | Receive CONTACTED    | If UA was sent in   | CONNECTED            |
 |                      | response to SABME:  |                      |
 |                      |   DLC_EXIT_BUSY     |                      |
 |                      | else:               |                      |
 |                      |   DLC_CONTACT       |                      |
 +----------------------+---------------------+----------------------+
 | Receive HALT_DL      | DLC_HALT_DL         | HALT_PENDING         |
 +----------------------+---------------------+----------------------+
 | Receive HALT_DL_NOACK| DLC_HALT_DL         | HALT_PENDING_NOACK   |
 +----------------------+---------------------+----------------------+
 | Receive DGRMFRAME    | DLC_DGRM            |                      |
 +----------------------+---------------------+----------------------+
 | Receive DATAFRAME    | DLC_DGRM            |                      |
 +----------------------+---------------------+----------------------+
 | Receive ICANREACH_cs | Send HALT_DL_NOACK  |                      |
 +----------------------+---------------------+----------------------+
 | DLC_RESET            | Send RESTART_DL     | CIRCUIT_RESTART      |
 +----------------------+---------------------+----------------------+
 | DLC_ERROR            | Send HALT_DL        | DISCONNECT_PENDING   |
 +----------------------+---------------------+----------------------+
 | DLC_DGRM             | Send DGRMFRAME      |                      |
 +----------------------+---------------------+----------------------+
 | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
 +----------------------+---------------------+----------------------+
 The CONNECT_PENDING state is entered when a DLC_CONTACTED signal has
 been received from the DLC (i.e., a SABME command has been received).
 A CONTACT message it then  issued.  The state is exited upon the
 receipt of a CONTACTED message.  If a DLC_RESET signal is received,
 the local data link is restarted and a RESTART_DL message is sent to
 the remote DLSw.
 An ICANREACH_cs received after the transition to CONNECT_PENDING
 state indicates that more than one CANUREACH_cs was sent at circuit
 establishment time and the target station was found by more than one
 Data Link Switch partner.  A HALT_DL_NOACK is sent to halt the
 circuit started by the Data Link Switch partner that originated each
 such ICANREACH_cs.

Wells & Bartky [Page 31] RFC 1795 Data Link Switching April 1995

 Note:  Some implementations 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).

5.2.6 CIRCUIT_ESTABLISHED State

 +----------------------+---------------------+----------------------+
 |        Event         |      Action(s)      |      Next State      |
 +----------------------+---------------------+----------------------+
 | Receive CONTACT      | DLC_CONTACT         | CONTACT_PENDING      |
 +----------------------+---------------------+----------------------+
 | Receive HALT_DL      | DLC_HALT_DL         | HALT_PENDING         |
 +----------------------+---------------------+----------------------+
 | Receive HALT_DL_NOACK| DLC_HALT_DL         | HALT_PENDING_NOACK   |
 +----------------------+---------------------+----------------------+
 | Receive XIDFRAME     | DLC_XID             |                      |
 +----------------------+---------------------+----------------------+
 | Receive DGRMFRAME    | DLC_DGRM            |                      |
 +----------------------+---------------------+----------------------+
 | Receive DATAFRAME    | DLC_DGRM            |                      |
 +----------------------+---------------------+----------------------+
 | Receive ICANREACH_cs | Send HALT_DL_NOACK  |                      |
 +----------------------+---------------------+----------------------+
 | DLC_CONTACTED        | Send CONTACT        | CONNECT_PENDING      |
 |                      | If UA is sent in    |                      |
 |                      | response to SABME:  |                      |
 |                      |   DLC_ENTER_BUSY    |                      |
 |                      | else:               |                      |
 |                      |   no action taken   |                      |
 +----------------------+---------------------+----------------------+
 | DLC_ERROR            | Send HALT_DL        | DISCONNECT_PENDING   |
 +----------------------+---------------------+----------------------+
 | DLC_DGRM             | Send DGRMFRAME      |                      |
 +----------------------+---------------------+----------------------+
 | DLC_XID              | Send XIDFRAME       |                      |
 +----------------------+---------------------+----------------------+
 | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
 +----------------------+---------------------+----------------------+
 The CIRCUIT_ESTABLISHED state is entered by the origin Data Link
 Switch from the CIRCUIT_START 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) or CONTACT
 is received. The next state is CONTACT_PENDING or CONNECT_PENDING.
 An ICANREACH_cs received after the transition to CIRCUIT_ESTABLISHED
 state indicates that more than one CANUREACH_cs was sent at circuit
 establishment time and the target station was found by more than one

Wells & Bartky [Page 32] RFC 1795 Data Link Switching April 1995

 Data Link Switch partner.  A HALT_DL_NOACK is sent to halt the
 circuit started by the Data Link Switch partner that originated each
 such ICANREACH_cs.

5.2.7 CONTACT_PENDING State

 +----------------------+---------------------+----------------------+
 |        Event         |      Action(s)      |      Next State      |
 +----------------------+---------------------+----------------------+
 | Receive HALT_DL      | DLC_HALT_DL         | HALT_PENDING         |
 +----------------------+---------------------+----------------------+
 | Receive HALT_DL_NOACK| DLC_HALT_DL         | HALT_PENDING_NOACK   |
 +----------------------+---------------------+----------------------+
 | 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      |                      |
 +----------------------+---------------------+----------------------+
 | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
 +----------------------+---------------------+----------------------+
 The CONTACT_PENDING state is entered upon the receipt of a CONTACT
 message, which 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 sends a DISC command frame on the adjacent LAN (i.e.,
 DLC_HALT_DL signal) and enter the RESTART_PENDING state.
 An ICANREACH_cs received after the transition to CONTACT_PENDING
 state indicates that more than one CANUREACH_cs was sent at circuit
 establishment time and the target station was found by more than one
 Data Link Switch partner.  A HALT_DL_NOACK is sent to halt the data
 link started by the Data Link Switch partner that originated this
 ICANREACH_cs.

Wells & Bartky [Page 33] RFC 1795 Data Link Switching April 1995

5.2.8 CONNECTED State

 +----------------------+---------------------+----------------------+
 |        Event         |      Action(s)      |      Next State      |
 +----------------------+---------------------+----------------------+
 | Receive HALT_DL      | DLC_HALT_DL         | HALT_PENDING         |
 +----------------------+---------------------+----------------------+
 | Receive HALT_DL_NOACK| DLC_HALT_DL         | HALT_PENDING_NOACK   |
 +----------------------+---------------------+----------------------+
 | Receive RESTART_DL   | DLC_HALT_DL         | RESTART_PENDING      |
 +----------------------+---------------------+----------------------+
 | Receive DGRMFRAME    | DLC_DGRM            |                      |
 +----------------------+---------------------+----------------------+
 | Receive INFOFRAME    | DLC_INFO            |                      |
 +----------------------+---------------------+----------------------+
 | Receive DATAFRAME    | DLC_DGRM            |                      |
 +----------------------+---------------------+----------------------+
 | Receive XIDFRAME     | If non-activation   |                      |
 |                      | XID3:               |                      |
 |                      |   DLC_XID           |                      |
 +----------------------+---------------------+----------------------+
 | Receive ICANREACH_cs | Send HALT_DL_NOACK  |                      |
 +----------------------+---------------------+----------------------+
 | Receive ENTER_BUSY   | DLC_ENTER_BUSY      |                      |
 +----------------------+---------------------+----------------------+
 | Receive EXIT_BUSY    | DLC_EXIT_BUSY       |                      |
 +----------------------+---------------------+----------------------+
 | Rec TEST_CIRCUIT_REQ | Snd TEST_CIRCUIT_RSP|                      |
 +----------------------+---------------------+----------------------+
 | DLC_RESET            | Send RESTART_DL     | CIRCUIT_RESTART      |
 +----------------------+---------------------+----------------------+
 | DLC_ERROR            | Send HALT_DL        | DISCONNECT_PENDING   |
 +----------------------+---------------------+----------------------+
 | DLC_DGRM             | Send DGRMFRAME      |                      |
 +----------------------+---------------------+----------------------+
 | DLC_INFO             | Send INFOFRAME      |                      |
 +----------------------+---------------------+----------------------+
 | DLC_XID              | If non-activation   |                      |
 |                      | XID3:               |                      |
 |                      |   Send XIDFRAME     |                      |
 +----------------------+---------------------+----------------------+
 | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
 +----------------------+---------------------+----------------------+
 The CONNECTED state is entered from the CONNECT_PENDING state upon
 the receipt of a CONTACTED message or from the CONTACT_PENDING state
 upon the receipt of a DLC_CONTACTED signal.

Wells & Bartky [Page 34] RFC 1795 Data Link Switching April 1995

 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 attempt to restart the circuit.  Following the
 receipt of a SABME, the local Data Link Switch sends a RESTART_DL
 message to the other Data Link Switch and enters the CIRCUIT_RESTART
 state.  Upon the receipt of the RESTART_DL message, the remote Data
 Link Switch sends a DISC command (i.e., DLC_HALT_DL signal) and
 enters the RESTART_PENDING state.
 An ICANREACH_cs received after the transition to CONNECTED state
 indicates that more than one CANUREACH_cs was sent at circuit
 establishment time and the target station was found by more than one
 Data Link Switch partner.  A HALT_DL_NOACK is sent to halt the
 circuit started by the Data Link Switch partner that originated each
 such ICANREACH_cs.
 Note:  Some implementations 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).

5.2.9 CIRCUIT_RESTART State

 +----------------------+---------------------+----------------------+
 |        Event         |      Action(s)      |      Next State      |
 +----------------------+---------------------+----------------------+
 | Receive DL_RESTARTED | If Connected:       | If Connected:        |
 |                      |  Send CONTACT       |  CONNECT_PENDING,    |
 |                      |                     | else:                |
 |                      |                     |  CIRCUIT_ESTABLISHED |
 +----------------------+---------------------+----------------------+
 | Receive HALT_DL_NOACK| DLC_HALT_DL         | HALT_PENDING_NOACK   |
 +----------------------+---------------------+----------------------+
 | Receive DGRMFRAME    | DLC_DGRM            |                      |
 +----------------------+---------------------+----------------------+
 | DLC_ERROR            | Send HALT_DL        | DISCONNECT_PENDING   |
 +----------------------+---------------------+----------------------+
 | DLC_DGRM             | Send DGRMFRAME      |                      |
 +----------------------+---------------------+----------------------+
 | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
 +----------------------+---------------------+----------------------+

Wells & Bartky [Page 35] RFC 1795 Data Link Switching April 1995

 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.

5.2.10 DISCONNECT_PENDING State

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

Wells & Bartky [Page 36] RFC 1795 Data Link Switching April 1995

5.2.11 RESTART_PENDING State

 +----------------------+---------------------+----------------------+
 |        Event         |      Action(s)      |      Next State      |
 +----------------------+---------------------+----------------------+
 | Receive HALT_DL_NOACK|                     | HALT_PENDING_NOACK   |
 +----------------------+---------------------+----------------------+
 | Receive DGRMFRAME    | DLC_DGRM            |                      |
 +----------------------+---------------------+----------------------+
 | DLC_DL_HALTED        | Send DL_RESTARTED   | CIRCUIT_ESTABLISHED  |
 +----------------------+---------------------+----------------------+
 | DLC_ERROR            | Send HALT_DL        | DISCONNECT_PENDING   |
 +----------------------+---------------------+----------------------+
 | DLC_DGRM             | Send DGRMFRAME      |                      |
 +----------------------+---------------------+----------------------+
 | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
 +----------------------+---------------------+----------------------+
 The RESTART_PENDING state is entered upon the receipt of a RESTART_DL
 message from the remote DLSw while the local Data Link Switch is in
 either the CONTACT_PENDING state or the CONNECTED state, which causes
 the local DLSw to issue a DISC command to the DLC.  Upon the receipt
 of the UA response (DLC_DL_HALTED), the data link is restarted, a
 DL_RESTARTED message is returned to the remote DLSw, and the
 CIRCUIT_ESTABLISHED state is entered.
 Note:  Some implementations 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.

5.2.12 HALT_PENDING State

 +----------------------+---------------------+----------------------+
 |        Event         |      Action(s)      |      Next State      |
 +----------------------+---------------------+----------------------+
 | Receive HALT_DL_NOACK|                     | HALT_PENDING_NOACK   |
 +----------------------+---------------------+----------------------+
 | Receive DATAFRAME    | DLC_DGRM            |                      |
 +----------------------+---------------------+----------------------+
 | DLC_DL_HALTED        | Send DL_HALTED      | DISCONNECTED         |
 +----------------------+---------------------+----------------------+
 | DLC_ERROR            | Send DL_HALTED      | DISCONNECTED         |
 +----------------------+---------------------+----------------------+
 | DLC_DGRM             | Send DATAFRAME      |                      |
 +----------------------+---------------------+----------------------+
 | XPORT_FAILURE        |                     | HALT_PENDING_NOACK   |
 +----------------------+---------------------+----------------------+

Wells & Bartky [Page 37] RFC 1795 Data Link Switching April 1995

 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 DLSw and the DISCONNECTED state is entered.

5.2.13 HALT_PENDING_NOACK State

 +----------------------+---------------------+----------------------+
 |        Event         |      Action(s)      |      Next State      |
 +----------------------+---------------------+----------------------+
 | Receive DATAFRAME    | DLC_DGRM            |                      |
 +----------------------+---------------------+----------------------+
 | DLC_DL_HALTED        |                     | DISCONNECTED         |
 +----------------------+---------------------+----------------------+
 | DLC_ERROR            |                     | DISCONNECTED         |
 +----------------------+---------------------+----------------------+
 | DLC_DGRM             | Send DATAFRAME      |                      |
 +----------------------+---------------------+----------------------+
 The HALT_PENDING_NOACK state is entered upon the receipt of a
 HALT_DL_NOACK message.  This causes the local DLC to issue a DISC
 command.  Upon the receipt of the UA response (DLC_DL_HALTED), the
 DISCONNECTED state is entered.

5.3 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) (a.k.a. Single Route
 Broadcast).  The NB_Add_Name_Response and NB_Name_Recognized frames
 are directed to a specific MAC address with the routing information
 field indicating an all routes explorer frame (ARE) (a.k.a. All
 Routes Broadcast)  The NB_Status_Response frame, is 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.

Wells & Bartky [Page 38] RFC 1795 Data Link Switching April 1995

 +---------------------------+------------------+--------------------+
 |          Event            |     Action(s)    |      Comment       |
 +---------------------------+------------------+--------------------+
 | DLC_DGRM for NETBIOS      | Send NETBIOS_ANQ | Transmitted to all |
 |  group address:           |                  |   remote DLSw      |
 |   NB_Add_Name_Query       |                  |                    |
 +---------------------------+------------------+--------------------+
 | DLC_DGRM for a specific   | Send NETBIOS_ANR | Transmitted to     |
 |  address:                 |                  |   specific DLSw    |
 |   NB_Add_Name_Response    |                  |                    |
 +---------------------------+------------------+--------------------+
 | DLC_DGRM for a specific   | Send DATAFRAME   | Transmitted to all |
 |  address:                 |                  |   remote DLSw      |
 |   NB_Status_Response      |                  |                    |
 +---------------------------+------------------+--------------------+
 | DLC_DGRM for NETBIOS      | Send DATAFRAME   | Transmitted to all |
 |  group address:           |                  |   remote DLSw      |
 |   NB_Name_in_Conflict     |                  |                    |
 |   NB_Add_Group_Name_Query |                  |                    |
 |   NB_Datagram,            |                  |                    |
 |   NB_Datagram_Broadcast   |                  |                    |
 |   NB_Status_Query         |                  |                    |
 |   NB_Terminate_Trace      |                  |                    |
 +---------------------------+------------------+--------------------+
 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 DLSw 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 DLSw state machine.
 Note:  Some implementations 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.

Wells & Bartky [Page 39] RFC 1795 Data Link Switching April 1995

 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    | (NETBIOS Group Address) |
 +---------------+-------------------------+-------------------------+
 | NETBIOS_ANR   | DLC_DGRM:               | Routed ARE              |
 |               |    NB_Add_Name_Response | (Specific MAC Address)  |
 +---------------+-------------------------+-------------------------+
 | NETBIOS_NQ_ex | DLC_DGRM:               | Routed STE              |
 |               |    NB_Name_Query        | (NETBIOS Group Address) |
 +---------------+-------------------------+-------------------------+
 | NETBIOS_NQ_cs | DLC_DGRM:               | Routed STE              |
 |               |    NB_Name_Query        | (NETBIOS Group Address) |
 +---------------+-------------------------+-------------------------+
 | NETBIOS_NR_ex | DLC_DGRM:               | Routed ARE              |
 |               |    NB_Name_Recognized   | (Specific MAC Address)  |
 +---------------+-------------------------+-------------------------+
 | NETBIOS_NR_cs | DLC_DGRM:               | Routed ARE              |
 |               |    NB_Name_Recognized   | (Specific MAC Address)  |
 +---------------+-------------------------+-------------------------+
 | DATAFRAME     | DLC_DGRM                | If NB_Status_Response:  |
 |               |                         |  Routed ARE             |
 |               |                         |  (Specific MAC Address) |
 |               |                         | Else:                   |
 |               |                         |  Routed STE             |
 |               |                         |  (NETBIOS Group Address)|
 +---------------+-------------------------+-------------------------+
 The above actions apply to all DLSw states.  The handling of NetBIOS
 datagrams sent within DGRMFRAME messages is governed by the DLSw
 state machine.  The DGRMFRAME message type is employed instead of the
 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.

5.4 Explorer Traffic

 The CANUREACH_ex, ICANREACH_ex, NETBIOS_NQ_ex, and NETBIOS_NR_ex SSP
 messages explore the topology of the DLSw cloud and the networks
 attached to it.  These explorer frames are used to determine the DLSw
 partners through which a MAC or NetBIOS name can be accessed.  This
 information may optionally be cached to reduce explorer traffic in
 the DLSw cloud.

Wells & Bartky [Page 40] RFC 1795 Data Link Switching April 1995

 If a DLSw is aware from cached information that a given MAC address
 or NetBIOS name is accessible through a given partner DLSw, it should
 direct all circuit setup attempts to that partner.  If the circuit
 setup fails, or no such data is available in the MAC or name cache
 database, the DLSw may fallback to issuing the setup attempt to all
 DLSw partners on the assumption that the cached data is now out of
 date.  The mechanism for determining when to use such a fallback is
 implementation defined.
 DLSw implementations may also use a local MAC cache to enable
 responses to CANUREACH_ex requests to be issued without the need for
 TEST frame exchange (or equivalent) until the CANUREACH_cs is
 received.  Again, the fallback mechanism for determining when such
 local cache data is out-of-date is implementation defined.
 The use of either cache is an optional function in DLSw.  An
 implementation may choose to always issue explorer frames or to use
 either or both types of cache.
 The following sections describe the FSMs used for explorer frames.
 The DLC events and actions are a subset of those described in section
 5.2 for the main circuit FSM.

5.4.1 CANUREACH/ICANREACH Explorer FSM

 The FSM described below is used to handle explorer frames routed by
 MAC address.  There is one instance of this FSM for each Data Link ID
 (Target and Origin MAC/SAP pair) for which explorer traffic is
 flowing. The states in this FSM are as follows.
 State Name            Description
 ----------            -----------
 RESET                 The initial state.
 SENT_EX               Local DLSw has issued an explorer message
 RECEIVED_EX           Local DLSw has received an explorer message

Wells & Bartky [Page 41] RFC 1795 Data Link Switching April 1995

5.4.1.1 RESET State

 +----------------------+---------------------+----------------------+
 |        Event         |      Action(s)      |      Next State      |
 +----------------------+---------------------+----------------------+
 | Receive CANUREACH_ex | If replying from    | If DLC_RESOLVE sent, |
 |                      | cache, send         |   RECEIVED_EX        |
 |                      | ICANREACH_ex        |                      |
 |                      | else if allowed to  |                      |
 |                      | test availability,  |                      |
 |                      | issue DLC_RESOLVE.  |                      |
 |                      | Optionally update   |                      |
 |                      | cache.              |                      |
 +----------------------+---------------------+----------------------+
 | Receive ICANREACH_ex | Optionally update   | RESET                |
 |                      | cache               |                      |
 +----------------------+---------------------+----------------------+
 | DLC_RESOLVE_C        | Send CANUREACH_ex   | SENT_EX              |
 +----------------------+---------------------+----------------------+
 RESET is the initial state for the CANUREACH/ICANREACH explorer FSM.
 This state is exited when a DLC_RESOLVE_C request is received from
 the DLC or a CANUREACH_ex is received from a remote DLSw.
 A DLSw implementation may optionally reply from to CANUREACH_ex
 messages on the basis of cached topology information, in which case
 the DLC_RESOLVE exchange (i.e., TEST) is not required.  If cache is
 not used, or no match is found, and the DLC permits the use of TEST,
 DLC_RESOLVE is issued to locate the target MAC and the state changes
 to RECEIVED_EX. If no cache entry is available and TEST is not
 allowed by the DLC, a received CANUREACH_ex frame is ignored.

5.4.1.2 SENT_EX State

 +----------------------+---------------------+----------------------+
 |        Event         |      Action(s)      |      Next State      |
 +----------------------+---------------------+----------------------+
 | Receive ICANREACH_ex | DLC_RESOLVE_R       | RESET                |
 |                      | Optionally update   |                      |
 |                      | cache               |                      |
 +----------------------+---------------------+----------------------+
 | DLC_RESOLVE_C        |                     | SENT_EX              |
 +----------------------+---------------------+----------------------+
 SENT_EX is entered when the DLSw has issued a CANUREACH_ex message to
 locate a MAC address.  This state is exited when a remote DLSw
 returns a matching ICANREACH_ex, or after an implementation defined
 timeout. DLC_RESOLVE events received in this state correspond to TEST

Wells & Bartky [Page 42] RFC 1795 Data Link Switching April 1995

 retries by the origin DLC station and are absorbed.
 An implementation may choose whether to handle explorer frame
 crossover either by using entirely separate FSM instances and simply
 allowing both ends to issue TEST frames, or by detecting a reverse
 CANUREACH_ex frame here and issuing an ICANREACH_ex message and
 DLC_RESOLVE_R action.

5.4.1.3 RECEIVED_EX State

 +----------------------+---------------------+----------------------+
 |        Event         |      Action(s)      |      Next State      |
 +----------------------+---------------------+----------------------+
 | Receive CANUREACH_ex | Optionally update   | RECEIVED_EX          |
 |                      | cache               |                      |
 +----------------------+---------------------+----------------------+
 | Receive ICANREACH_ex |                     | RECEIVED_EX          |
 +----------------------+---------------------+----------------------+
 | DLC_RESOLVED         | Send ICANREACH_ex   | RESET                |
 |                      | Optionally update   |                      |
 |                      | cache               |                      |
 +----------------------+---------------------+----------------------+
 RECEIVED_EX is entered when the DLSw has received a CANUREACH_ex from
 a remote DLSw and has issued a DLC_RESOLVE to locate the MAC address.
 This state is exited when the DLC_RESOLVED response is received, or
 after an implementation defined timeout.
 If the target MAC is located, the DLSw must reply to the first
 received CANUREACH_ex that caused the move to this state.  If
 additional CANUREACH_ex messages are received in this state from
 other remote DLSw partners, the DLSw may optionally reply to these
 messages too but it is not required to do so.
 An implementation may choose whether to handle explorer frame
 crossover either by using entirely separate FSM instances and simply
 allowing both ends to issue TEST frames, or by detecting such a
 reverse DLC_RESOLVE_C event here and issuing an ICANREACH_ex message
 and DLC_RESOLVE_R action.

Wells & Bartky [Page 43] RFC 1795 Data Link Switching April 1995

5.4.2 NETBIOS_NQ/NR Explorer FSM

 The FSM described below is used to handle explorer frames routed by
 NetBIOS names  There is one instance of this FSM for each unique
 combination of Source Name, Destination Name, Data 2 field and
 Response Correlator.
 State Name            Description
 ----------            -----------
 RESET                 The initial state.
 SENT_EX               Local DLSw has issued an explorer
                       message
 RECEIVED_EX           Local DLSw has received an explorer
                       message
 SENT_REC_EX           An explorer frame has been both sent
                       and received for the same (potential)
                       NetBIOS circuit.

5.4.2.1 RESET State

 +----------------------+---------------------+----------------------+
 |        Event         |      Action(s)      |      Next State      |
 +----------------------+---------------------+----------------------+
 | Receive NETBIOS_NQ_ex| DLC_DGRM(NAME_QUERY)| RECEIVED_EX          |
 |                      | Optionally update   |                      |
 |                      | cache.              |                      |
 +----------------------+---------------------+----------------------+
 | Receive NETBIOS_NR_ex| Optionally update   | RESET                |
 |                      | cache               |                      |
 +----------------------+---------------------+----------------------+
 | DLC_DGRM (NAME_QUERY)| Send NETBIOS_NQ_ex  | SENT_EX              |
 +----------------------+---------------------+----------------------+
 The RESET state is the initial state for the NETBIOS_NQ/NR explorer
 FSM. It is exited when the DLC receives either a NETBIOS_NQ_ex or a
 DLC_DGRM containing a NetBIOS NAME_QUERY frame.  If a NETBIOS_NQ_ex
 message is received, the NAME_QUERY is propagated to the DLC and this
 FSM moves to state RECEIVED_EX.  If a NetBIOS NAME_QUERY frame is
 received, the NETBIOS_NQ_ex is propagated either to the appropriate
 DLSw partners (see below), and this FSM moves to state SENT_EX.
 Unlike SNA traffic where the CANUREACH_ex/ICANREACH_ex exchange can
 be omitted if the MAC location is already cached,
 NETBIOS_NQ_ex/NETBIOS_NR_ex frames must always be issued during
 NetBIOS session setup in order that the NetBIOS session numbers are

Wells & Bartky [Page 44] RFC 1795 Data Link Switching April 1995

 exchanged correctly between the DLC end stations.  If the location of
 a NetBIOS name is known from cached data, the NETBIOS_NQ_ex need only
 be issued to the cached DLSw partners.  Otherwise the NETBIOS_NQ_ex
 should be issued to all partners that support NetBIOS.

5.4.2.2 SENT_EX State

 +----------------------+---------------------+----------------------+
 |        Event         |      Action(s)      |      Next State      |
 +----------------------+---------------------+----------------------+
 | Receive NETBIOS_NQ_ex| DLC_DGRM(NAME_QUERY)| SENT_REC_EX          |
 |                      | Optionally update   |                      |
 |                      | cache               |                      |
 +----------------------+---------------------+----------------------+
 | Receive NETBIOS_NR_ex| DLC_DGRM(NAME_RECOG)| RESET                |
 |                      | Optionally update   |                      |
 |                      | cache               |                      |
 +----------------------+---------------------+----------------------+
 | DLC_DGRM (NAME_QUERY)| Send NETBIOS_NQ_ex  | SENT_EX              |
 | (different local     | Optionally update   |                      |
 |  session number than | cache               |                      |
 |  existing searches)  |                     |                      |
 +----------------------+---------------------+----------------------+
 SENT_EX is entered when the local DLSw issues a NETBIOS_NQ_ex to its
 remote DLSw partners.  This state is exited when a NETBIOS_NR_ex is
 received from a remote DLSw, or if a matching NETBIOS_NQ_ex is
 received from a remote DLSw (i.e., a NETBIOS_NQ_ex crossover case).
 If the local NetBIOS end station issues a NAME_QUERY with a different
 session number from any previous NAME_QUERY for this search, the
 NAME_QUERY is propagated to the DLSw partners to ensure that the
 exchange of NetBIOS session numbers is handled correctly.

Wells & Bartky [Page 45] RFC 1795 Data Link Switching April 1995

5.4.2.3 RECEIVED_EX State

 +----------------------+---------------------+----------------------+
 |        Event         |      Action(s)      |      Next State      |
 +----------------------+---------------------+----------------------+
 | Receive NETBIOS_NQ_ex| DLC_DGRM(NAME_QUERY)| RECEIVED_EX          |
 |                      | Optionally update   |                      |
 |                      | cache               |                      |
 +----------------------+---------------------+----------------------+
 | Receive NETBIOS_NR_ex|                     | RECEIVED_EX          |
 +----------------------+---------------------+----------------------+
 | DLC_DGRM (NAME_QUERY)| Send NETBIOS_NQ_ex  | SENT_REC_EX          |
 |                      | Optionally update   |                      |
 |                      | cache               |                      |
 +----------------------+---------------------+----------------------+
 | DLC_DGRM (NAME_RECOG)| Send NETBIOS_NR_ex  | RESET                |
 |                      | Optionally update   |                      |
 |                      | cache               |                      |
 +----------------------+---------------------+----------------------+
 RECEIVED_EX is entered when the local DLSw receives a NETBIOS_NQ_ex
 message from a remote DLSw.  This state is exited when a
 NAME_RECOGNIZED NetBIOS frame is received from the DLC, completing
 the query, or when a matching NAME_QUERY is received from DLC (i.e.,
 NAME_QUERY crossover).

5.4.2.4 SENT_REC_EX State

 +----------------------+---------------------+----------------------+
 |        Event         |      Action(s)      |      Next State      |
 +----------------------+---------------------+----------------------+
 | Receive NETBIOS_NQ_ex| DLC_DGRM(NAME_QUERY)| SENT_REC_EX          |
 |                      | Optionally update   |                      |
 |                      | cache               |                      |
 +----------------------+---------------------+----------------------+
 | Receive NETBIOS_NR_ex| DLC_DGRM(NAME_RECOG)| RECEIVED_EX          |
 |                      | Optionally update   |                      |
 |                      | cache               |                      |
 +----------------------+---------------------+----------------------+
 | DLC_DGRM (NAME_QUERY)| Send NETBIOS_NQ_ex  | SENT_REC_EX          |
 | (different local     | Optionally update   |                      |
 |  session number than | cache               |                      |
 |  existing searches)  |                     |                      |
 +----------------------+---------------------+----------------------+
 | DLC_DGRM (NAME_RECOG)| Send NETBIOS_NR_ex  | SENT_EX              |
 |                      | Optionally update   |                      |
 |                      | cache               |                      |
 +----------------------+---------------------+----------------------+

Wells & Bartky [Page 46] RFC 1795 Data Link Switching April 1995

 This state is required if an implementation wishes to manage NQ/NR
 crossover cases from a single FSM instance by detecting 'opposite'
 NAME_QUERY attempts between the same two NetBIOS names.  If separate
 FSM instances are used instead, this state is not required and the
 transitions to it from other states can be removed.
 SENT_RCV_EX is exited when the NAME_QUERY search in either direction
 is resolved.  If the local NetBIOS end station issues a NAME_QUERY
 with a different session number from any previous NAME_QUERY it has
 issued for this search, the NAME_QUERY is propagated to the DLSw
 partners to ensure that the exchange of NetBIOS session numbers is
 correctly handled.

5.4.2.5 NetBIOS Session Numbers

 NetBIOS NAME_QUERY and NAME_RECOGNIZED frames exchange NetBIOS session
 numbers between the end stations.  For correct NetBIOS operation over
 DLSw, it is important that all SSP NETBIOS_NQ_ex frames received by a
 DLSw cause NetBIOS NAME_QUERY frames to flow on the LAN with the new
 session number from the NETBIOS_NQ_ex.  These frames cannot be replied
 to from a cache of locally available NetBIOS names in the same way that
 MAC addresses and CANUREACH_ex messages can be handled.
 Also, NAME_QUERY messages are normally retried several times on the LAN.
 The generation and absorption of such frames is outside the scope of the
 FSM defined above.

6. Protocol Flow Diagrams

 The Switch-to-Switch Protocol is 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 UI frames) protocols end-to-end.  In addition, the end
 systems can establish an end-to-end connection for support of LLC Type 2
 (connection oriented I frames) protocols (Type 2 I frames go end-to-end,
 supervisory frames are handled locally).
 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 is identified by the Data Link ID defined in section 3.2.
 NOTE:  THIS SECTION CONTAINS EXAMPLES ONLY.  IT CANNOT AND DOES NOT SHOW
 ALL POSSIBLE VARIATIONS AND OPTIONS ON PROTOCOL FLOWS FOR SNA/SDLC, SSP,
 AND LLC PROTOCOLS.

Wells & Bartky [Page 47] RFC 1795 Data Link Switching April 1995

6.1 Connect Protocols

 The two basic startup flows from a pure FSM perspective are shown below.
 The first flow is a startup involving XIDs and the second is one without
 XIDs.

Flow #1 - DLSw Startup With XIDs

___

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

| | | < Network > | | |

/\ ——— \ / ——— /\ Origin Origin DLSw \_/ Target DLSw Target Station partner partner Station disconnected disconnected DLC_RESOLVE_C CANUREACH_ex ———–> ———–> DLC_RESOLVE_R ICANREACH_ex ←———- ←———- DLC_XID CANUREACH_cs DLC_START_DL ———–> ———–> ———–> circuit_start resolve_pending ICANREACH_cs DLC_DL_STARTED ←———- ←———- circuit_established circuit_pending REACH_ACK ———–> circuit_established XIDFRAME DLC_XID ———–> ———–> DLC_XID XIDFRAME DLC_XID ←———- ←———- ←———- DLC_XID XIDFRAME DLC_XID ———–> ———–> ———–> DLC_XIDs XIDFRAMEs DLC_XIDs ←———–> ←———–> ←———–> DLC_CONTACTED CONTACT DLC_CONTACT ———–> ———–> ———–> connect_pending contact_pending Wells & Bartky [Page 48] RFC 1795 Data Link Switching April 1995 DLC_CONTACT CONTACTED DLC_CONTACTED ←———- ←———- ←———- connected connected DLC_INFOs IFRAMEs DLC_INFOs ←———–> ←———–> ←———–> Mapping LAN events to the DLC events and actions on Flow #1 produces the following flows shown below: ====== _

| | ——— / \ ——— | | | | | _|_ | / 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 CANUREACH_cs DLC_START_DL ———–> ———–> ———–> ———–> circuit_start 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 Wells & Bartky [Page 49] RFC 1795 Data Link Switching April 1995 IFRAMEs DLC_INFOs IFRAMEs DLC_INFOs IFRAMEs ←———> ←———→ ←———–> ←———–> ←——→ Those implementations that prefer to respond to the SABME immediately could use the same events to do that: SABME DLC_CONTACTED CONTACT DLC_CONTACT SABME ———–> ———–> ———–> ———–> ——–> UA connect_pending contact_pending ←——– RR ———–> RNR ←——– RR DLC_CONTACT CONTACTED DLC_CONTACTED UA ←——– ←———- ←———- ←———- ←——- connected connected IFRAMEs DLC_INFOs IFRAMEs DLC_INFOs IFRAMEs ←———> ←———–> ←———–> ←———–> ←——→ Flow #2 - DLSw Startup Without XIDs (circuit setup) ====== _

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

| | | < Network > | | |

/\ ——— \ / ——— /\ Origin Origin DLSw \_/ Target DLSw Target Station partner partner Station disconnected disconnected DLC_CONTACTED CANUREACH_cs DLC_START_DL ———–> ———–> ———–> circuit_start resolve_pending ICANREACH_cs DLC_DL_STARTED ←———- ←———- circuit_established circuit_pending REACH_ACK ———–> circuit_established CONTACT DLC_CONTACT ———–> ———–> connect_pending contact_pending Wells & Bartky [Page 50] RFC 1795 Data Link Switching April 1995 DLC_CONTACT CONTACTED DLC_CONTACTED ←———- ←———- ←———- connected connected DLC_INFOs IFRAMEs DLC_INFOs ←———–> ←———–> ←———–> Mapping LAN events to the DLC events and actions on Flow #2 (and adding a NETBIOS_NQ and NETBIOS_NR_ex) produces: ====== _

| | ——— / \ ——— | | | | | _|_ | / 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 ———–> ———–> ———–> ———–> ———> NAME_RECOG DLC_DGRM NETBIOS_NR_ex DLC_DGRM NAME_RECOG ←———- ←———– ←———- ←———- ←——– SABME DLC_CONTACTED CANUREACH_cs DLC_START_DL ———–> ———–> ———–> ———–> circuit_start resolve_pending ICANREACH_cs DLC_DL_STARTED ←———- ←———- circuit_established circuit_pending REACH_ACK ———–> 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 ←———–> ←———–> ←———–> ←———–> ←——→ Wells & Bartky [Page 51] RFC 1795 Data Link Switching April 1995 In keeping with a paradigm of 'DLSw is a big 802.2 LAN', all other DLC types (SDLC for now, QLLC, channel, or whatever in the future) would be handled by a 'DLC transformation layer' that would transform the specific protocol's events into the appropriate DLSw DLC events and DLSw DLC actions into the appropriate protocol actions. The XIDs that flow in the SSP XIDFRAME should stay 802.2ish (i.e., ABM bit set) and leave it up to the DLC transformation layer to suit the XID to its particular DLC type. Here is an example of a leased SDLC PU 2.0 device as the origin station. It should use Flow #2 since it is not known if the other side is a LAN, a switched line or a leased line. ====== _

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

| | | < Network > | | |

/\ ——— \ / ——— /\ Origin Origin DLSw \_/ Target DLSw Target Station partner partner Station disconnected disconnected implementer's DLC_RESOLVE_C CANUREACH_ex choice (power ———–> ———–> up, configuration change, DLC_RESOLVE_R ICANREACH_ex never, ←———- ←———- connect timer,etc.) PU 2.0 is configured in DLSw to DLC_XID(null) CANUREACH_cs DLC_START_DL call in ———–> ———–> ———–> circuit_start resolve_pending ICANREACH_cs DLC_DL_STARTED ←———- ←———- circuit_established circuit_pending REACH_ACK ———–> circuit_established XIDFRAME DLC_XID ———–> ———–> DLC_XID XIDFRAME DLC_XID respond with ←———- ←———- ←———- XID configured Wells & Bartky [Page 52] RFC 1795 Data Link Switching April 1995 for station or forward XID to station and send response DLC_XID XIDFRAME DLC_XID ———–> ———–> ———–> SNRM DLC_CONTACT CONTACT DLC_CONTACTED ←——– ←———- ←———- ←———– contact_pending connect_pending UA DLC_CONTACTED CONTACTED DLC_CONTACT ———→ ———–> ———–> ———–> connected connected IFRAMEs DLC_INFOs IFRAMEs DLC_INFOs ←———→ ←———–> ←———–> ←———–> Here is an example of a switched SDLC PU 2.0 device as the origin station. ====== _

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

| | | < Network > | | |

/\ ——— \ / ——— /\ Origin Origin DLSw \_/ Target DLSw Target Station partner partner Station disconnected disconnected implementer's DLC_RESOLVE_C CANUREACH_ex choice (power ———–> ———–> up, configuration change, DLC_RESOLVE_R ICANREACH_ex never, ←———- ←———- connect timer,etc.) XID(null) DLC_XID(null) CANUREACH_cs DLC_START_DL ———–> ———–> ———–> ———–> circuit_start resolve_pending ICANREACH_cs DLC_DL_STARTED ←———- ←———- circuit_established circuit_pending REACH_ACK ———–> circuit_established Wells & Bartky [Page 53] RFC 1795 Data Link Switching April 1995 XIDFRAME DLC_XID ———–> ———–> XID DLC_XID XIDFRAME DLC_XID ←——– ←———- ←———- ←———- XID DLC_XID XIDFRAME DLC_XID ———> ———–> ———–> ———–> SNRM DLC_CONTACT CONTACT DLC_CONTACTED ←——– ←———- ←———- ←———- contact_pending connect_pending UA DLC_CONTACTED CONTACTED DLC_CONTACT ———> ———–> ———–> ———–> connected connected IFRAMEs DLC_INFOs IFRAMEs DLC_INFOs ←———> ←———–> ←———–> ←———–> Here is an example of a leased SDLC PU 2.0 device as the target station. ====== _

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

| | | < Network > | | |

/\ ——— \ / ——— /\ Origin Origin DLSw \_/ Target DLSw Target Station partner partner Station (SDLC) disconnected disconnected DLC_RESOLVE_C CANUREACH_ex ———–> ———–> reply if virtual MAC/SAP for SDLC station is configured, if SDLC station responds to DLC_RESOLVE_R ICANREACH_ex TEST/SNRM/DISC, etc. ←———- ←———- DLC_XID CANUREACH_cs DLC_START_DL SNRM ———–> ———–> ———–> ———> circuit_start resolve_pending ICANREACH_cs DLC_DL_STARTED UA ←———- ←———- ←—— circuit_established circuit_pending RNR REACH_ACK ———> ———–> circuit_established Wells & Bartky [Page 54] RFC 1795 Data Link Switching April 1995 XIDFRAME DLC_XID ———–> ———–> respond with XID configured for station or forward XID to station and send DLC_XID XIDFRAME DLC_XID response ←———- ←———- ←———- DLC_CONTACTED CONTACT DLC_CONTACT RR ———–> ———–> ———–> ———> connect_pending contact_pending DLC_CONTACT CONTACTED DLC_CONTACTED ←———- ←———- ←———- connected connected DLC_INFOs IFRAMEs DLC_INFOs IFRAMEs ←———–> ←———–> ←———–> ←——> Here is an example of a switched SDLC PU 2.0 device as the target station. ====== _

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

| | | < Network > | | |

/\ ——— \ / ——— /\ Origin Origin DLSw \_/ Target DLSw Target Station partner partner Station (SDLC) disconnected disconnected DLC_RESOLVE_C CANUREACH_ex ———–> ———–> reply if virtual MAC/SAP for SDLC station is configured, if SDLC station responds to DLC_RESOLVE_R ICANREACH_ex TEST/XID/SNRM/DISC, etc. ←———- ←———- DLC_XID CANUREACH_cs DLC_START_DL XID ———–> ———–> ———–> ———> circuit_start resolve_pending ICANREACH_cs DLC_DL_STARTED XID ←———- ←———- ←——- circuit_established circuit_pending Wells & Bartky [Page 55] RFC 1795 Data Link Switching April 1995 REACH_ACK ———–> circuit_established XIDFRAME DLC_XID ———–> ———–> respond with XID received DLC_XID XIDFRAME DLC_XID above ←———- ←———- ←——– DLC_CONTACTED CONTACT DLC_CONTACT SNRM ———–> ———–> ———–> ———> connect_pending contact_pending DLC_CONTACT CONTACTED DLC_CONTACTED UA ←———- ←———- ←———- ←——- connected connected DLC_INFOs IFRAMEs DLC_INFOs IFRAMEs ←———–> ←———–> ←———–> ←——→ Here is an example of an SDLC T2.1 device as the target station. (SDLC T2.1 origin station would look just like the LAN T2.1 origin station) ====== _

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

| | | < Network > | | |

/\ ——— \ / ——— /\ Origin Origin DLSw \_/ Target DLSw Target Station partner partner Station disconnected disconnected DLC_RESOLVE_C CANUREACH_ex ———–> ———–> implementer's choice (virtual MAC/SAP configured, check to see if station is powered up using DLC_RESOLVE_R ICANREACH_ex TEST/XID/DISC, etc.) ←———- ←———- DLC_XID CANUREACH_cs DLC_START_DL null XID ———–> ———–> ———–> ———> circuit_start resolve_pending ICANREACH_cs DLC_DL_STARTED XID ←———- ←———- ←—— Wells & Bartky [Page 56] RFC 1795 Data Link Switching April 1995 circuit_established circuit_pending REACH_ACK ———–> circuit_established XIDFRAME DLC_XID ———–> ———–> respond with XID received DLC_XID XIDFRAME DLC_XID above ←———- ←———- ←——— DLC_XIDs XIDFRAMEs DLC_XIDs XIDs ←———–> ←———–> ←———–> ←——→ DLC_CONTACTED CONTACT DLC_CONTACT SNRM ———–> ———–> ———–> ———> connect_pending contact_pending DLC_CONTACT CONTACTED DLC_CONTACTED UA ←———- ←———- ←———- ←—— connected connected DLC_INFOs IFRAMEs DLC_INFOs IFRAMEs ←———–> ←———–> ←———–> ←——→ Wells & Bartky [Page 57] RFC 1795 Data Link Switching April 1995 6.2 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. 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. DLSw Link Restart Message Protocols Upon receipt of a SABME command from the origin station, the origin DLSw will send a RESTART_DL message to the target DLSw. 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 DLSw will issue a DISC command to the target station. The target station is expected to return a UA response. The target DLSw will then restart its data link and send an DL_RESTARTED message back to the origin DLSw. 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 DLSw will send a CONTACT message to the target DLSw. If the SABME command Wells & Bartky [Page 58] RFC 1795 Data Link Switching April 1995 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. 6.3 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. DLSw Disconnect Message Protocols Upon receipt of a DISC command from the origin station, the origin DLSw will reply with a UA response and issue a HALT_DL message to the target DLSw. Upon receipt of the HALT_DL message, the target DLSw will send a DISC command to the target station. The target station Wells & Bartky [Page 59] RFC 1795 Data Link Switching April 1995 will then respond with a UA response, causing the target DLSw to return a DL_HALTED message to the origin DLSw. During this exchange of messages, both Data Link Switches change states from the Connected state to the Disconnected state. 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 DISCONNECTED state for each circuit. 7.0 Capabilities Exchange Formats/Protocol The Data Link Switching Capabilities Exchange is a special DLSw Switch-to-Switch control message that describes the capabilities of the sending data link switch. This control message is sent after the switch-to-switch connection is established and optionally during run time if certain operational parameters have changed and need to be communicated to the partner switch. The actual contents of the Capabilities Exchange is in the data field following the SSP message header. The Capabilities Exchange itself is formatted as a single General Data Stream (GDS) Variable with multiple type "LT" structured subfields. The SSP Message Header has the following fields set for the Capabilities Exchange: Offset Field Value —— —– —– 0x00 Version Number 0x31 0x01 Header Length 0x48 (decimal 72) 0x02 Message Length same as LL in GDS Variable 0x14 Message Type 0x20 (CAP_EXCHANGE) 0x16 Protocol Id 0x42 0x17 Header Number 0x01 0x23 Message Type 0x20 (CAP_EXCHANGE) 0x38 Direction 0x01 for CapEx request 0x02 for CapEx response Other fields in the SSP header are not referenced and should be set to zero. Wells & Bartky [Page 60] RFC 1795 Data Link Switching April 1995 The DLSw Capabilities Exchange Request has the following overall format: +—-+—-+—————–+ | LL | ID | Control Vectors | +—-+—-+—————–+ 0-1 Length, in binary, of the DLSw Capabilities Exchange Request GDS Variable. The value of LL is the sum of the length of all fields in the GDS Variable (i.e., length of LL + length of ID + length of Control Vectors). 2-3 GDS Id: 0x1520 4-n Control Vectors consisting of type LT structured subfields (i.e., the DLSw Capabilities Exchange Structured Subfields) Type LT structured subfields consist of a 1-byte length field (the "L"), a 1-byte type field (the "T") and n-bytes of data. The length field includes itself as well as the structured subfield. The structured subfield consists of the type field and data so the length is n + 2. This imposes a length restriction of 253 bytes on all data contained in a structured subfield. Wells & Bartky [Page 61] RFC 1795 Data Link Switching April 1995 7.1 Control Vector Id Range Control Vector identifiers (i.e., Type) in the range of 0x80 through 0xCF are reserved for use by the Data Link Switching standard. Control Vector identifiers (i.e., Type) in the range of 0xD0 through 0xFD are used for vendor-specific purposes. Currently defined vectors are: Vector Description Hex Value Vendor Id Control Vector 0x81 DLSw Version Control Vector 0x82 Initial Pacing Window Control Vector 0x83 Version String Control Vector 0x84 Mac Address Exclusivity Control Vector 0x85 Supported SAP List Control Vector 0x86 TCP Connections Control Vector 0x87 NetBIOS Name Exclusivity Control Vector 0x88 MAC Address List Control Vector 0x89 NetBIOS Name List Control Vector 0x8A Vendor Context Control Vector 0x8B Reserved for future use 0x8C - 0xCF Vendor Specific 0xD0 - 0xFD 7.2 Control Vector Order and Continuity Since their contents can greatly affect the parsing of the Capabilities Exchange GDS Variable, the required control vectors must occur first and appear in the following order: Vendor Id, DLSw Version Number, Initial Pacing Window, Supported SAP List. The remainder of the Control Vectors can occur in any order. Control Vectors that can be repeated within the same message (e.g., MAC Address List Control Vector and NetBIOS Name List Control Vector) are not necessarily adjacent. It is advisable, but not required, to have the Exclusivity Control Vector occur prior to either of the above two vectors so that the use of the individual MAC addresses or NetBIOS names will be known prior to parsing them. Both the Vendor Context and Vendor Specific control vectors can be repeated. If there are multiple instances of the Vendor Context control vector, the specified context remains in effect for all Vendor Specific control vectors until the next Vendor Context control vector is encountered in the Capabilities Exchange. Wells & Bartky [Page 62] RFC 1795 Data Link Switching April 1995 7.3 Initial Capabilities Exchange Capabilities exchange is always the first SSP message sent on a new SSP connection between two DLSw switches. This initial Capabilities Exchange is used to identify the DLSw version that each switch is running and other required information, plus details of any optional extensions that the switches are capable of supporting. If a DLSw receives an initial capabilities message that is incorrectly formatted or contains invalid or unsupported data that prevents correct interoperation with the partner DLSw, it should issue a Capabilities Exchange negative response. If a DLSw receives a negative response to its initial capabilities message, it should take down its TCP connections with the offended partner. Note: Pre v1.0 DLSw implementations do not send or respond to capabilities messages and can be identified by the lack of capabilities exchange as the first message on a new SSP connnection. This document does not attempt to specify how to interoperate with back-level DLSw implementations. 7.4 Run-Time Capabilities Exchange Capabilities exchange always occurs when the SSP connection is started between two DLSw switches. Capabilities Exchange can also occur at run-time, typically when a configuration change is made. Support for run-time Capabilities Exchange is optional. If a node does not support receiving/using Run-Time Capabilities Exchange and receives one, it should discard it quietly (not send back a negative response). If a node supports receipt of run-time capabilities, it should send a positive or negative response as appropriate. The receiver of a negative response to a run-time capabilities message is not required to take down its TCP connections with the offended partner. Run-time Capabilities Exchange can consist of one or more of the following control vectors. Note that the control vectors required at start-up are not present in a run-time Capabilities Exchange. Wells & Bartky [Page 63] RFC 1795 Data Link Switching April 1995 1. MAC Address Exclusivity CV, 2. NetBIOS Name Exclusivity CV, 3. MAC Address List CV, 4. NetBIOS Name List CV, 5. Supported SAP List CV, 6. Vendor Context CV, 7. Vendor Specific CVs A run-time capabilities exchange is a replacement operation. As such, all pertinent MAC addresses and NetBIOS names must be specified in the run-time exchange. In addition, run-time changes in capabilities will not effect existing link station circuits. 7.5 Capabilities Exchange Filtering Responsibilities Recipients of the SAP, MAC, and NetBIOS lists are not required to actually use them to filter traffic, etc., either initially or at run-time. 7.6 DLSw Capabilities Exchange Structured Subfields The Capabilities Exchange Subfields are listed in the table below and are described in the following sections: Required Allowed @ ID @ Startup Length Repeatable* Runtime Order Content ==== ========= ====== ========== ======= ===== =============== 0x81 Y 0x05 N N 1 Vendor ID 0x82 Y 0x04 N N 2 DLSw Version 0x83 Y 0x04 N N 3 Initial pacing window 0x84 N >=0x02 N N 5+ Version String 0x85 N 0x03 N Y 5+ MAC Address Exclusivity 0x86 Y 0x12 N Y 4 Supported SAP List 0x87 N 0x03 N N 5+ TCP Connections 0x88 N 0x03 N Y 5+ NetBIOS Name Exclusivity Wells & Bartky [Page 64] RFC 1795 Data Link Switching April 1995 0x89 N 0x0E Y Y 5+ MAC Address List 0x8A N ⇐0x13 Y Y 5+ NetBIOS Name List 0x8B N 0x05 Y Y 5+ Vendor Context 0xD0 N varies Y Y 5+ Vendor Specific *Note: "Repeatable" means a Control Vector is repeatable within a single message. 7.6.1 Vendor Id (0x81) Control Vector The Vendor Id control vector identifies the manufacturer's IEEE assigned Organizationally Unique Identifier (OUI) of the Data Link Switch sending the DLSw Capabilities Exchange. The OUI is sent in non-canonical (Token-Ring) format. This control vector is required and must be the first control vector. Offset Length Value Contents —— —— —– ——– 0 1 0x05 Length of the Vendor Id structured subfield 1 1 0x81 key = 0x81 that identifies this as the Vendor Id structured subfield 2-4 3 the 3-byte Organizationally Unique Identifier (OUI) for the vendor (non-canonical format) 7.6.2 DLSw Version (0x82) Control Vector The DLSw Version control vector identifies the particular version of the DLSw standard supported by the sending Data Link Switch. This control vector is required and must follow the Vendor Id Control Vector. Offset Length Value Contents —— —— —– ——– 0 1 0x04 Length of the Version String structured subfield 1 1 0x82 key = 0x82 that identifies this as the DLSw Version structured subfield Wells & Bartky [Page 65] RFC 1795 Data Link Switching April 1995 2 1 the hexadecimal value representing the DLSw standard Version number of the sending Data Link Switch. 0x01 (indicates version 1 - closed pages) 3 1 the hexadecimal value representing the DLSw standard Release number of the sending Data Link Switch. 0x00 (indicates release 0) 7.6.3 Initial Pacing Window (0x83) Control Vector The Initial Pacing Window control vector specifies the initial value of the receive pacing window size for the sending Data Link Switch. This control vector is required and must follow the DLSw Version Control Vector. Offset Length Value Contents —— —— —– ——– 0 1 0x04 Length of the Initial Pacing Window structured subfield 1 1 0x83 key = 0x83 that identifies this as the Initial Pacing Window structured subfield 2-3 2 the pacing window size, specified in byte normal form.. Note: The pacing window size must be non-zero. 7.6.4 Version String (0x84) Control Vector The Version String control vector identifies the particular version number of the sending Data Link Switch. The format of the actual version string is vendor-defined. This control vector is optional. Offset Length Value Contents —— —— —– ——– 0 1 0xn Length of the Version String structured subfield 1 1 0x84 key = 0x84 that identifies this as the Version String structured subfield Wells & Bartky [Page 66] RFC 1795 Data Link Switching April 1995 2-n n-2 the ASCII string that identifies the software version for the sending DLSw. 7.6.5 MAC Address Exclusivity (0x85) Control Vector The MAC Address Exclusivity control vector identifies how the MAC Address List control vector data is to be interpreted. Specifically, this control vector identifies whether the MAC addresses in the MAC Address List control vectors are the only ones accessible via the sending Data Link Switch. If a MAC Address List control vector is specified and the MAC Address Exclusivity control vector is missing, then the MAC addresses are not assumed to be the only ones accessible via this switch. A node may specify that it supports no local MAC addresses by including in its capabilities the MAC Address List Exclusivity CV (with byte 2 == 0x01), and not including any instances of the MAC Address List CV. Offset Length Value Contents —— —— —– ——– 0 1 0x03 Length of the Exclusivity structured subfield 1 1 0x85 key = 0x85 that identifies this as the MAC address Exclusivity structured subfield 2 1 an indicator of the relationship of the MAC addresses to the sending Data Link Switch. 0x00 the MAC addresses specified in this Capabilities Exchange can be accessed via this switch but are not the exclusive set (i.e., other entities are accessible in addition to the ones specified) 0x01 the MAC addresses specified in this Capabilities Exchange are the only ones accessible via this switch. Wells & Bartky [Page 67] RFC 1795 Data Link Switching April 1995 7.6.6 SAP List Support (0x86) Control Vector The SAP List Support control vector identifies support for Logical Link Control SAPs (DSAPs and SSAPs) by the sending Data Link Switch. This is used by the DLSw that sent the SAP List Support control vector to indicate which SAPs can be used to support SNA and optionally NetBIOS traffic. This may be used by the DLSw that receives the SAP list to filter explorer traffic (TEST, XID, or NetBIOS UI frames) from the DLSw state machine. For SNA, a DLSw should set bits for all SAP values (SSAP or DSAP) that may be used for SNA traffic. For NetBIOS support, the bit for SAP 0xF0 should be set (if not supported then the same bit should be cleared). Each bit in the SAP control vector data field represents a SAP as defined below. This vector is required and must follow the Initial Pacing Window Control Vector. Offset Length Value Contents —— —— —– ——– 0 1 0x12 Length of the Supported SAP List structured subfield 1 1 0x86 key = 0x86 that identifies this as the Supported SAP List structured subfield 2-17 16 the 16-byte bit vector describing all even numbered SAPs enabled. Each Bit within the 16 byte bit vector will indicate whether an even numbered SAP is enabled (b'1') or disabled (b'0'). Each Byte within the 16 byte bit vector will be numbered from 0 - F. (Most significant byte first). Byte 0 1 2 3 … F XX XX XX XX … XX The bits in each byte indicate whether an even numbered SAP is enabled (b'1') or disabled (b'0'). (Most significant bit first) Bits 7 6 5 4 … 0 SAP 0 2 4 6 … E By combining the byte label with the enabled bits, all supported SAPs can be determined. Wells & Bartky [Page 68] RFC 1795 Data Link Switching April 1995 In the following diagram, 'n' would equal 0 through F depending on which byte was being interpreted. Bit ordering is shown below with bit 7 being the most significant bit and bit 0 the least significant bit. 7654 3210 bbbb bbbb…. |||| |||| |||| |||SAP 0xnE enabled or not |||| ||| |||| ||SAP 0xnC enabled or not |||| || |||| |SAP 0xnA enabled or not |||| | |||| SAP 0xn8 enabled or not |||| |||SAP 0xn6 enabled or not ||| ||SAP 0xn4 enabled or not || |SAP 0xn2 enabled or not | SAP 0xn0 enabled or not Wells & Bartky [Page 69] RFC 1795 Data Link Switching April 1995 An example of using all User Definable SAPs of 0x04 to 0xEC for SNA Data Link Switching and SAP 0xF0 for NetBIOS Data Link Switching would be as follows: Offset SAPs Binary Hex 0 4,8,C 0010 1010 0x2A 1 10,14,18,1C 1010 1010 0xAA 2 20,24,28,2C 1010 1010 0xAA 3 30,34,38,3C 1010 1010 0xAA 4 40,44,48,4C 1010 1010 0xAA 5 50,54,58,5C 1010 1010 0xAA 6 60,64,68,6C 1010 1010 0xAA 7 70,74,78,7C 1010 1010 0xAA 8 80,84,88,8C 1010 1010 0xAA 9 90,94,98,9C 1010 1010 0xAA A A0,A4,A8,AC 1010 1010 0xAA B B0,B4,B8,BC 1010 1010 0xAA C C0,C4,C8,CC 1010 1010 0xAA D D0,D4,D8,DC 1010 1010 0xAA E E0,E4,E8,EC 1010 1010 0xAA F F0 1000 0000 0x80 7.6.7 TCP Connections (0x87) Control Vector The TCP Connections control vector indicates the support of an alternate number of TCP Connections for the Data Link Switching traffic. The base implementation of Data Link Switching supports two TCP Connections, one for each direction of data traffic. This control vector is optional. If it is omitted in a DLSw Capabilities Exchange, then two TCP Connections are assumed. It is further assumed that if a Data Link Switch can support one TCP Connection, it can support two TCP Connections. 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. The format of the TCP Connections Control Vector is shown below: Offset Length Value Contents —— —— —– ——– 0 1 0x03 Length of the TCP Connections structured subfield 1 1 0x87 key = 0x87 that identifies this as the TCP Connections structured subfield Wells & Bartky [Page 70] RFC 1795 Data Link Switching April 1995 2 1 an indicator of the support for an alternate number of TCP Connections by the sending Data Link Switch. 0x01 the number of TCP Connections may be brought down to one after Capabilities Exchange is completed. 0x02 the number of TCP Connections will remain at two for the duration of the DLSw connection. 7.6.8 NetBIOS Name Exclusivity (0x88) Control Vector The NetBIOS Name Exclusivity control vector identifies how the NetBIOS Name List control vector data is to be interpreted. Specifically, this control vector identifies whether the NetBIOS Names in the NetBIOS Name List control vectors are the only ones accessible via the sending Data Link Switch. If a NetBIOS Name List control vector is specified and the NetBIOS Name Exclusivity control vector is missing, then the NetBIOS Names are not assumed to be the only ones accessible via this switch. A node may specify that it supports no local NetBIOS names by including in its capabilities the NetBIOS Name List Exclusivity CV (with byte 2 == 0x01), and not including any instances of the NetBIOS Name List CV. Offset Length Value Contents —— —— —– ——– 0 1 0x03 Length of the Exclusivity structured subfield 1 1 0x88 key = 0x88 that identifies this as the NetBIOS Name Exclusivity structured subfield 2 1 an indicator of the relationship of the NetBIOS Names to the sending Data Link Switch. 0x00 the NetBIOS Names specified in this Capabilities Exchange can be accessed via this switch but are not the exclusive set (i.e., other entities are accessible in addition to the ones specified) Wells & Bartky [Page 71] RFC 1795 Data Link Switching April 1995 0x01 the NetBIOS Names specified in this Capabilities Exchange are the only ones accessible via this switch. 7.6.9 MAC Address List (0x89) Control Vector The MAC Address List control vector identifies one or more MAC addresses that are accessible through the sending Data Link Switch. This control vector specifies a single MAC address value and MAC address mask value to identify the MAC address or range of MAC addresses. MAC addresses and masks are in non-canonical (Token-Ring) format in this control vector. This control vector is optional and can be repeated if necessary. Note 1: If a particular MAC address, <mac-addr>, satisfies the following algorithm, then <mac-addr> is assumed to be accessible via the sending Data Link Switch: <mac-addr> & <mac-addr-mask> == <mac-addr-value> where: <mac-addr-value> is the MAC Address Value specified in this control vector <mac-addr-mask> is the MAC Address Mask specified in this control vector Note 2: If an individual MAC Address is desired, then <mac-addr- value> should be the individual MAC address and <mac-addr-mask> should be 0xFFFFFFFFFFFF. Offset Length Value Contents —— —— —– ——– 0 1 0x0E Length of the MAC Address List structured subfield 1 1 0x89 key = 0x89 that identifies this as the MAC Address List structured subfield 2-7 6 the 6-byte MAC Address Value, <mac-addr-value> in the above formula 8-13 6 the 6-byte MAC Address Mask, <mac-addr-mask> in the above formula Wells & Bartky [Page 72] RFC 1795 Data Link Switching April 1995 7.6.10 NetBIOS Name List (0x8A) Control Vector The NetBIOS Name List control vector identifies one or more NetBIOS names that are accessible through the sending Data Link Switch. This control vector specifies a single NetBIOS name in ASCII. However, the NetBIOS name can consist of "don't care" and "wildcard" characters to match on a number of NetBIOS names. If an individual character position in the NetBIOS name in this control vector contains a '?', then the corresponding character position in real NetBIOS name is a "don't care". If a NetBIOS name in this control vector ends in '*', then the remainder of real NetBIOS names is a "don't care". '*' is only considered a wildcard if it appears at the end of a name. All blanks or nulls at the end of NetBIOS names in this control vector are ignored. NetBIOS names which have fewer than 16 bytes and which do not end with '*' are not assumed to have a trailing '*'; the "wildcard" character must be explicit. NetBIOS group names can exist across several LANs/networks. As such, NetBIOS group names received in a NetBIOS Name List Control Vector can not be treated the same as NetBIOS individual names. The Individual/Group Flag allows Data Link Switches to distinguish between the two. This control vector is optional and can be repeated if necessary. Offset Length Value Contents —— —— —– ——– 0 1 0xn Length of the NetBIOS Name List structured subfield (maximum = 0x13) 1 1 0x8A key = 0x8A that identifies this as the NetBIOS Name List structured subfield 2 1 Individual/Group Flag 0x00 - Individual NetBIOS Name 0x01 - Group NetBIOS Name 3-n n-3 the NetBIOS name with possible embedded '?' and terminating '*'. 7.6.11 Vendor Context (0x8B) Control Vector The Vendor Context control vector identifies the manufacturer's IEEE assigned Organizationally Unique Identifier (OUI) of the Data Link Switch sending the DLSw Capabilities Exchange. The OUI is sent in non-canonical (Token-Ring) format. Wells & Bartky [Page 73] RFC 1795 Data Link Switching April 1995 This control vector is optional and is used to provide the context for any Vendor Specific control vectors that follow in the Capabilities Exchange. If there are multiple instances of the Vendor Context control vector, the specified context remains in effect for all Vendor Specific control vectors until the next Vendor Context control vector is encountered. Offset Length Value Contents —— —— —– ——– 0 1 0x05 Length of the Vendor Context structured subfield 1 1 0x8B key = 0x8B that identifies this as the Vendor Context structured subfield 2-4 3 the 3-byte Organizationally Unique Identifier (OUI) for the vendor (non-canonical format) 7.7 Capabilities Exchange Responses There are two kinds of DLSw Capabilities Exchange Responses: positive and negative. A positive response is returned to the sending Data Link Switch if there were no errors encountered in the DLSw Capabilities Exchange Request. A negative response is returned if there is at least one error encountered. A positive DLSw Capabilities Exchange Response has the following overall format: +—-+—-+ | LL | ID | +—-+—-+ 0-1 Length, in binary, of the DLSw Capabilities Exchange Response GDS Variable. The value of LL in this case is 0x0004. 2-3 GDS Id: 0x1521 A negative DLSw Capabilities Exchange Response has the following overall format: +—-+—-+——–+——–+ | LL | ID | Offset | Reason | +—-+—-+——–+——–+ Wells & Bartky [Page 74] RFC 1795 Data Link Switching April 1995 0-1 Length, in binary, of the DLSw Capabilities Exchange Response GDS Variable. The value of LL is the sum of the length of all fields in the GDS Variable (i.e., length of LL + length of ID + length of Offsets/Reasons). 2-3 GDS Id: 0x1522 4-5 Offset into the DLSw Capabilities Exchange Request of the error. Offset should always point to the start of the GDS Variable or a specific control vector. 6-7 Reason code that uniquely identifies the error. Specific values for the reason code are: 0x0001 invalid GDS length for a DLSw Capabilities Exchange Request. (The value of Offset is ignored.) 0x0002 invalid GDS id for a DLSw Capabilities Exchange Request. (The value of Offset is ignored.) 0x0003 Vendor Id control vector is missing. (The value of Offset is ignored.) 0x0004 DLSw Version control vector is missing. (The value of Offset is ignored.) 0x0005 Initial Pacing Window control vector is missing. (The value of Offset is ignored.) 0x0006 length of control vectors doesn't correlate to the length of the GDS variable 0x0007 invalid control vector id 0x0008 length of control vector invalid 0x0009 invalid control vector data value 0x000A duplicate control vector (for non-repeating control vectors) 0x000B out-of-sequence control vector (for repeating control vector) 0x000C DLSw Supported SAP List control vector is missing. Wells & Bartky [Page 75] RFC 1795 Data Link Switching April 1995 (The value of Offset is ignored.) Note: Multiple Offset, Reason pairs can be returned with one pair for each error encountered. 8. Pacing/Flow Control This section describes the required Pacing and Flow Control mechanisms used by a Data Link Switch. While it is beyond the scope of this document to specify a policy for how an implementation maps SSP flow control to the native data link flow control at the edges, the following paragraphs describe a general philosophical overview of how the mechanism is to be applied. There are two types of flows which are covered by the flow control mechanism: connection-oriented and connectionless. In the first, connection-oriented flows, the implementer is to map the native flow control mechanism of the two data links at the boundaries to the SSP flow control mechanism thus presenting an end-to-end flow control mechanism which "pushes back" all the way to the originating station in either direction. However, in the case of connectionless traffic, this is not possible at the data link level because there is no native flow control mechanism for connectionless data links. At first glance it is tempting to allow connectionless traffic to flow the DLSw cloud unthrottled. However, the rationale for subjecting these flows to flow control within the DLSw cloud is to "push" the discarding of frames (should this become necessary) back to the ingress of the DLSw cloud. This "early discarding" of excessive DATAGRAMs should allow the cloud to remain deterministic without wasting network bandwidth. 8.1 Basic Overview Each circuit consists of two data flows, one in each direction. Each data flow has its own independent flow control mechanism. For each data flow there is an entity that originates traffic, referred to as the sender, and a target entity which receives the traffic, referred to as the receiver. A sender may only send data when its receiver has granted explicit permission to send a discrete number of data units. Data units are defined as either a DGRMFRAME or an INFOFRAME. The receiver grants permission to send data units by sending a Flow Control Indicator (FCIND- defined later). The sender must acknowledge all FCINDs by sending a Flow Control Acknowledgment Wells & Bartky [Page 76] RFC 1795 Data Link Switching April 1995 (FCACK- defined later). A sending implementation must maintain these values: 1. GrantedUnits - The number of units (frames) which the sender currently has permission to send. 2. CurrentWindow - This is a discrete number of units, controlled by the receiver, which is basis for granting additional units. 3. InitialWindowSize - Global for all circuits on a transport connection. Learned in capabilities exchange when the transport connection is established. It specifies an initial value for CurrentWindow when each circuit is established. A receiving implementation must maintain these values: 1. CurrentWindow - This is a discrete number of units, controlled by the receiver, which is basis for granting additional units. 2. InitialWindowSize - Global for all circuits on a transport connection. Sent in capabilities exchange when the transport connection is established. It specifies an initial value for CurrentWindow when each circuit is established. 3. FCACKOwed - The sender owes an FCACK. If true, no FCIND may be sent. 8.2 Frame Format The Flow control Byte is contained at offset 15 in both the Information and Control SSP messages. From a flow control perspective, the flow control information in the two frames are handled identically. The following diagram describes the format of the Flow Control Byte (Bit 7 is the most significant and Bit 0 is the Least significant bit of the octet): bit 7 6 5 4 3 2 1 0 +—+—+—+—+—+—+—+—+ |FCI|FCA| reserved | FCO | +—+—+—+—+—+—+—+—+ FCI : Flow Control Indicator FCA : Flow Control Ack FCO : Flow Control Operator Bits Wells & Bartky [Page 77] RFC 1795 Data Link Switching April 1995 000 - Repeat Window Operator 001 - Increment Window Operator 010 - Decrement Window Operator 011 - Reset Window Operator 100 - Halve Window Operator 101 - Reserved 110 - Reserved 111 - Reserved A frame with the FCI bit set is referred to as a Flow Control Indication (FCIND). An FCIND is used to manage the flow in the opposite direction of the frame which bears it. A frame with the FCA bit set is referred to as a Flow Control Acknowledgment (FCACK). An FCACK is used to manage the flow in the same direction of the frame which bears it. NOTE: A frame may be both a FCIND and an FCACK. A frame bearing an FCIND or FCACK may also contain data for the flow in the direction it is traveling. In such a frame, the FCIND or FCACK are said to be piggy-backed. A non-piggy-backed FCIND is called an Independent Flow Control Indication (IFCIND) and a non- piggy-backed FCACK is called an Independent Flow Control Acknowledgment (IFCACK). IFCIND and IFCACK messages are sent in a Independent Flow Control SSP message (type 0x21). NOTE: A frame may be both an IFCIND and an IFCACK. It is desirable to carry information in control messages so as to reduce the need to send a flow control only message. The diagram below shows the messages that may carry valid flow control information: ====== _

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

| | | < Network > | | |

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

 May have valid
  FCI/FCA/FCO    Data carrying
       N             N          CANUREACH_cs
                                ----------->
       Y*            N            ICANREACH_cs

Wells & Bartky [Page 78] RFC 1795 Data Link Switching April 1995

                                  <-----------
       Y             N          REACH_ACK
                                ----------->
       Y             Y            XIDFRAMEs
                                <------------>
       Y             Y            DGRMFRAMEs
                                <------------>
       Y             N          CONTACT
                                ----------->
       Y             N               CONTACTED
                                  <-----------
       Y             Y             INFOFRAMEs
                                <------------>
       Y             N          RESTART_DL
                                ----------->
       Y             N               DL_RESTARTED
                                  <-----------
       Y             N          CONTACT
                                ----------->
       Y             N               CONTACTED
                                  <-----------
       N             N          HALT_DL
                                ----------->
       N             N               DL_HALTED
                                  <-----------
  • Note: ICANREACH_cs cannot carry FCA, as there could not be an

outstanding FCI.

8.3 Granting Permission to Send Data

 A receiver grants a sender permission to send units of data by
 sending FCIND.  Each FCIND is further qualified by a flow control
 operator, which is encoded in the FCO bits of the FCIND header. With
 one exception (the Reset Window operator) all operators may be either
 piggy-backed or carried in a IFCIND.
 The five flow control operators are outlined below:

8.3.1 Repeat Window Operator

 This operator is processed as follows:
         (CurrentWindow unchanged)
         GrantedUnits += CurrentWindow

Wells & Bartky [Page 79] RFC 1795 Data Link Switching April 1995

8.3.2 Increment Window Operator

 This operator is processed as follows:
         CurrentWindow++
         GrantedUnits += CurrentWindow

8.3.3 Decrement Window Operator

 This operator is processed as follows:
         CurrentWindow--
         GrantedUnits += CurrentWindow
 NOTE:  This operator may only be sent if CurrentWindow is greater
 than one.

8.3.4 Reset Window Operator

 This operator is processed as follows:
         CurrentWindow = 0;
         GrantedUnits  = 0;
 NOTE:  This operator may only flow on an independent pacing
 indication (may NOT be piggy-backed).
 NOTE:  After sending this operator, the only legal subsequent
 operator is Increment Window.

8.3.5 Halve Window Operator

 This operator shall be processed as follows:
         IF CurrentWindow > 1 THEN
             CurrentWindow = CurrentWindow / 2
         ENDIF
         GrantedUnits += CurrentWindow
 Note:  The divide by two operation is an unsigned integer divide
 (round down) or bit shift right operation.

8.4 Acknowledging a Flow Control Operator

 Each sender must acknowledge each FCIND with an FCACK which is
 piggy-backed on the next frame in the opposite direction in all cases
 except the Reset Window Operator.

Wells & Bartky [Page 80] RFC 1795 Data Link Switching April 1995

 The receiver may have no more than one unacknowledged FCIND
 outstanding at any time with one exception:  A Reset Window Operator
 may be sent while another FCIND is pending acknowledgment.
 NOTE: The FCI and FCO bits of the FCACK are used independently by the
 flow in the opposite direction

8.4.1 Acknowledging a Reset Window Operator

 Since this operator revokes all previously granted units, the sender
 must acknowledge this FCIND using an IFCACK (Independent Flow Control
 Acknowledgment).  This is the only case where IFCACK is used.
 Should a sender receive a non-reset FCIND followed by a Reset Window
 FCIND before acknowledging the first, it only acknowledges the Reset
 Window.
 NOTE: The FCI and FCO bits on these frames are used independently by
 the flow in the opposite direction.

8.5 Capabilities Exchange Initial Window Size

 When two nodes establish a transport connection, they engage in a
 capabilities exchange (this is a requirement).  Refer to the
 Capabilities Exchange section 7 for further details.  The two nodes
 are required to exchange the following parameter:
 InitialWindowSize -  This indicates to the partner what
                      the sending flow entity initializes
                      its CurrentWindow value to for each
                      multiplexed circuit subsequently
                      established on that transport
                      connection.  This value must be
                      non-zero.

8.6 Circuit Startup

 Process as follows:
        CurrentWindow = InitialWindowSize
        GrantedUnits  = 0
 NOTE: The InitialWindow Size variable has a scope of one per DLSw
 transport connection, while CurrentWindow and Granted units are
 maintained on a per circuit basis.  At circuit startup, a sender may
 not send data units until the receiver grants explicit permission
 with an FCIND message.  This grant may be an independent FCIND
 message or the FCIND may be piggy-backed on any of the message types

Wells & Bartky [Page 81] RFC 1795 Data Link Switching April 1995

 listed in section 8.2.

8.7 Example Receiving Implementations

 The following two examples illustrate receiving implementations of
 varying degrees of complexity.  These are not meant to be complete
 implementations but rather serve to illustrate the protocol.
 NOTE: The examples are independent of the buffering model ( buffers
 may be deterministicly or statistically committed)
 NOTE: The examples assume a process model where each event processes
 to completion without being preempted by another event.

8.7.1 Fixed Pacing Example

 Consider the following variables, in addition to InitialWindowSize
 and CurrentWindow and FCACKOwed:
        GrantDelayed     - Boolean
        GrantedUnits     - Outstanding Units
 The following section describes how various events are processed in
 this example implementation:

8.7.1.1 Circuit Startup

        CurrentWindow    = InitialWindowSize
        FCACKOwed        = FALSE
        GrantDelayed     = FALSE
        GrantedUnits     = 0
        Repeat Window Operator

8.7.1.2 Check Buffers Available

 Can my implementation afford to grant CurrentWindow just now?

8.7.1.3 Buffers Become Available

        IF Check Buffers Available THEN
           Send FCIND( Repeat Window)
           GrantDelayed = FALSE
        ELSE
           Wait on buffers to become available (LIFO)
        ENDIF

Wells & Bartky [Page 82] RFC 1795 Data Link Switching April 1995

8.7.1.4 Repeat Window Operator

        IF Check Buffers Available THEN
            Send FCIND( Repeat Window)
        ELSE
           GrantDelayed = TRUE
           Wait on buffers to become available (FIFO)
        ENDIF

8.7.1.5 Send FCIND( operator)

        GrantedUnits += CurrentWindow
        FCACKOwed     = TRUE
        Encode and Transmit FCIND piggybacked or as IFCIND

8.7.1.6 A Frame Arrives from Sender

        GrantedUnits--;
        IF frame is FCACK THEN
           IF FCACKOwed THEN
              FCACKOwed = FALSE
           ELSE
              Protocol Violation
           ENDIF
        ENDIF
        IF NOT GrantDelayed THEN
           IF GrantedUnits <= CurrentWindow THEN
               IF FCACKOwed THEN
                 Protocol Violation
               ELSE
                 Repeat Window Operator
               ENDIF
           ENDIF
        ENDIF

8.7.2 Adaptive Pacing Example

 The following example illustrates a receiving implementation that
 adjusts the window size and granted units based on buffer
 availability and transport utilization.
 NOTE: This example ignores other factors which might compel the
 receiving implementation to adjust the window size (i.e., Outbound
 queue length, traffic priority, ...)
 Consider the following variables, in addition to InitialWindowSize,
 CurrentWindow and FCACKOwed:

Wells & Bartky [Page 83] RFC 1795 Data Link Switching April 1995

        GrantDelayed     - Boolean
        GrantedUnits     - Outstanding Units

8.7.2.1 Circuit Startup

        CurrentWindow    = InitialWindowSize
        FCACK            = FALSE
        GrantDelayed     = FALSE
        GrantedUnits     = 0
        Repeat Window Operator

8.7.2.2 Check Buffers Available ( X)

         Can my implementation afford to grant X units just now?

8.7.2.3 Buffers Become Available

        IF Check Buffers Available THEN
           CurrentWindow--;
           Send FCIND( Decrement Window)
           GrantDelayed = FALSE
        ELSE
           Wait on buffers to become available (LIFO)
        ENDIF

8.7.2.4 Repeat Window Operator

        IF Check Buffers Available (CurrentWindow) THEN
            Send FCIND( Repeat Window)
        ELSE
           GrantDelayed = TRUE
           Wait on buffers to become available (FIFO)
        ENDIF

8.7.2.5 Increment Window Operator

        IF Check Buffers Available ( CurrentWindow + 1) THEN
            CurrentWindow++
            Send FCIND( Increment Window)
        ELSE
            Repeat Window Operator
        ENDIF

8.7.2.6 Send FCIND( operator)

        FCACKOwed     = TRUE
        GrantedUnits += CurrentWindow
        Encode and Transmit FCIND piggybacked or as IFCIND

Wells & Bartky [Page 84] RFC 1795 Data Link Switching April 1995

8.7.2.7 An FCACK Arrives from Sender

        GrantedUnits--;
        IF NOT FCACKOwed THEN
           Protocol Violation
        ENDIF
        FCACKOwed = FALSE;
        IF NOT GrantDelayed THEN
           IF GrantedUnits < CurrentWindow THEN
               Increment Window Operator
           ELSE IF GrantedUnits == CurrentWindow THEN
               Repeat Window Operator
           END
        ENDIF

8.7.2.8 A Non-FCACK Frame Arrives from Sender

        GrantedUnits--;
        IF NOT GrantDelayed THEN
           IF FCACKOwed THEN
              IF GrantedUnits < CurrentWindow THEN
                 Protocol Violation
              END
           ELSE
              IF GrantedUnits <= CurrentWindow THEN
                 Repeat Window Operator
              ENDIF
           ENDIF
        ENDIF

Wells & Bartky [Page 85] RFC 1795 Data Link Switching April 1995

8.8 Adaptive Pacing Example Flow Diagrams

8.8.1 Example Flows from the Above Implementation

 The following diagram illustrates the use of adaptive pacing (use of
 Halve Window, and Reset operation are shown in subsequent diagrams).
  1. —-SENDER—– —-RECEIVER—-

Granted Window Window Granted

   0         2   circuit established    2         0
   2         2   <-------- FCIND(Rpt)   2         2
   1         2   FCACK-------------->   2         1
   4         3   <-------- FCIND(Inc)   3         4
   3         3   FCACK-------------->   3         3
                        +- FCIND(Rpt)   3         6
   2         3   DATA---|----------->   3         5
   1         3   DATA---|----------->   3         4
   4         3   <------+
   3         3   FCACK-------------->   3         3
   6         3   <-------- FCIND(Rpt)   3         6
   5         3   FCACK-------------->   3         5
   4         3   DATA--------------->   3         4
   3         3   DATA--------------->   3         3
                        +- FCIND(Rpt)   3         6
   2         3   DATA---|----------->   3         5
   1         3   DATA---|----------->   3         4
   0         3   DATA---|----------->   3         3
   3         3   <------+
   2         3   FCACK-------------->   3         2
   6         4   <-------- FCIND(Inc)   4         6
   5         4   FCACK-------------->   4         5
   4         4   DATA--------------->   4         4
                                      Waiting on Buffer
                        +- FCIND(Dec)   3         7
   3         4   DATA---|----------->   3         6
   2         4   DATA---|----------->   3         5
   1         4   DATA---|----------->   3         4
   0         4   DATA---|----------->   3         3
   3         3   <------+
   2         3   FCACK-------------->   3         2
                                      Waiting on Buffer
                        +- FCIND(Dec)   2         4
   1         3   DATA---|----------->   2         3
   0         3   DATA---|----------->   2         2
   2         2   <------+
   1         2   FCACK-------------->   2         1
   4         3   <-------- FCIND(Inc)   3         4
   3         3   FCACK-------------->   3         3

Wells & Bartky [Page 86] RFC 1795 Data Link Switching April 1995

   6         3   <-------- FCIND(Rpt)   3         6
   5         3   FCACK-------------->   3         5
   4         3   DATA--------------->   3         4
   3         3   DATA--------------->   3         3
   6         3   <-------- FCIND(Rpt)   3         6

8.8.2 Example Halve Window Flow

 The following flow illustrates the use of the Halve Window Operator:
  1. —-SENDER—– —-RECEIVER—-

Granted Window Window Granted

      0         2   circuit established    2         0
      2         2   <-------- FCIND(Rpt)   2         2
      1         2   FCACK-------------->   2         1
      4         3   <-------- FCIND(Inc)   3         4
      3         3   FCACK-------------->   3         3
                                           Resource Shortage
      2         3   DATA--------------->   1         2
      1         3   DATA--------------->   1         1
      0         3   DATA--------------->   1         0
      1         1   <-------- FCIND(Hlv)   1         1
      0         1   FCACK-------------->   1         0
 NOTE: The Halve Window Operator could have been sent before the
 granted units fell to zero.  The implementer may make a choice based
 on the severity of the condition.

8.8.3 Example Reset Window Flows

 The following flow diagram illustrates the ResetWindow operation if
 the receiver has no FCIND outstanding.
  1. —-SENDER—– —-RECEIVER—-

Granted Window Window Granted

   0         2   circuit established    2         0
   2         2   <-------- FCIND(Rpt)   2         2
   1         2   FCACK-------------->   2         1
   4         3   <-------- FCIND(Inc)   3         4
   3         3   FCACK-------------->   3         3
                        +- FCIND(Rpt)   3         6
   2         3   DATA---|----------->   3         5
   1         3   DATA---|----------->   3         4
   4         3   <------+
   3         3   FCACK-------------->   3         3
   6         3   <-------- FCIND(Rpt)   3         6
   5         3   FCACK-------------->   3         5
                                        Resource shortage!

Wells & Bartky [Page 87] RFC 1795 Data Link Switching April 1995

   0         0   <-------- FCIND(Rst)   0         5 (note still
 committed)
   0         0   IFCACK------------->   0         0
                                        Condition eases
   1         1   <-------- FCIND(Inc)   1         1
   0         1   FCACK-------------->   1         0
   2         2   <-------- FCIND(Inc)   2         2
   1         2   FCACK-------------->   3         4
 The next two flows  illustrate the Reset Window operation if the
 receiver has an outstanding FCIND.
  1. —-SENDER—– —-RECEIVER—-

Granted Window Window Granted

   0         2   circuit established    2         0
   2         2   <-------- FCIND(Rpt)   2         2
   1         2   FCACK-------------->   2         1
   4         3   <-------- FCIND(Inc)   3         4
   3         3   FCACK-------------->   3         3
                        +- FCIND(Rpt)   3         6
   2         3   DATA---|----------->   3         5
                        |               Resource shortage!
                        |+-FCIND(Rst)   0         5
   1         3   DATA---||---------->   0         4
   4         3   <------+|
   3         3   FCACK---+---------->   0         3 (Not IFCACK!)
   2         3   DATA----|---------->   0         2
   0         0   <-------+
   0         0   IFCACK------------->   0         0
                                        Condition eases
   1         1   <-------- FCIND(Inc)   1         1
   0         1   FCACK-------------->   1         0
   2         2   <-------- FCIND(Inc)   2         2
   1         2   FCACK-------------->   3         4
  1. —-SENDER—– —-RECEIVER—-

Granted Window Window Granted

   0         2   circuit established    2         0
   2         2   <-------- FCIND(Rpt)   2         2
   1         2   FCACK-------------->   2         1
   4         3   <-------- FCIND(Inc)   3         4
   3         3   FCACK-------------->   3         3
                        +- FCIND(Rpt)   3         6
   2         3   DATA---|----------->   3         5
                        |               Resource shortage!
                        |+-FCIND(Rst)   0         5
   1         3   DATA---||---------->   0         4
   4         3   <------+|

Wells & Bartky [Page 88] RFC 1795 Data Link Switching April 1995

   0         0   <-------+
   0         0   IFCACK------------->   0         0
                                        Condition eases
   1         1   <-------- FCIND(Inc)   1         1
   0         1   FCACK-------------->   1         0
   2         2   <-------- FCIND(Inc)   2         2
   1         2   FCACK-------------->   3         4

8.9 Other Considerations

8.9.1 Protocol Violations

 The following events are considered protocol violations:
 1. Sender exceeds granted units or does not acknowledge FCIND on
    first frame after its receipt (the receiver can not discern the
    difference between the two).
 2. Receiver does not follow a Reset Window Operator with an Increment
    Window Operator.
 3. Receiver has two unacknowledged FCINDs ( other than Reset Window)
    outstanding.
 4. Receiver sends Decrement Window Operator with a window size of one.
 5. Receiver attempts to increment the window size beyond 0xFFFF.
 Actions taken in response to protocol violations are left to the
 implementation of the node which discovers the violation.  If an
 implementation chooses to take down the circuit on which the
 violation occurred, HALT_DL is the appropriate action.

Acknowledgments

 Original RFC 1434 Authors:
    Roy C. Dixon, IBM
    David M. Kushi, IBM
 Chair of APPN Implementers Workshop Data Link Switching Related
 Interest Group:
    Louise Herndon Wells, Internetworking Technology Institute

Wells & Bartky [Page 89] RFC 1795 Data Link Switching April 1995

 Working Group Chairs (and significant contributors to this document):
    Connect/Disconnect (State Machines): Steve Klein, IBM
    Capabilities Exchange: Wayne Clark, Cisco Systems
    Flow Control (Adaptive Pacing): Shannon Nix, Metaplex
    Priority/Class of Service: Gene Cox, IBM
 Other significant contributors:
    Peter Gayek, IBM
    Paul Brittain, Data Connection Limited

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) IBM LAN Technical Reference IEEE 802.2 and NETBIOS Application
    Program Interfaces SC30-3587-00, December 1993.
 3) ISO/IEC DIS 10038 DAM 2, MAC Bridging, Source Routing Supplement,
    December 1991.
 4) ISO 8802-2/IEEE Std 802.1D International Standard, Information
    Processing Systems, Local Area Networks, Part 2: MAC layer
    Bridging.

Wells & Bartky [Page 90] RFC 1795 Data Link Switching April 1995

Security Considerations

 Security issues are not discussed in this memo.

Chair's Address

 Louise Wells
 Internetwork Technology Institute
 2021 Stratford Dr.
 Milpitas, CA  95035
 EMail: lhwells@cup.portal.com

Editor's Address

 Alan K. Bartky
 Manager of Technology
 Sync Research Inc.
 7 Studebaker
 Irvine, CA 91728-2013
 Phone: 1-714-588-2070
 EMail: alan@sync.com
 Note: Any questions or comments relative to the contents of this RFC
 should be sent to the following Internet address:
 aiw-dlsw@networking.raleigh.ibm.com.
 This address will be used to coordinate the handling of responses.
 NOTE 1:  This is a widely subscribed mailing list and messages sent to
          this address will be sent to all members of the DLSw mailing
          list.  For specific questions relating to subscribing to the
          AIW and any of it's working groups send email to:
          appn@vnet.ibm.com
          Information regarding all of the AIW working groups and the
          work they are producing can be obtained by copying, via
          anonymous ftp, the file aiwinfo.psbin or aiwinfo.txt from the
          Internet host networking.raleigh.ibm.com, located in
          directory aiw.
 NOTE 2: These mailing lists and addresses are subject to change.

Wells & Bartky [Page 91]

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