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rfc:rfc2114

Network Working Group S. Chiang Request for Comments: 2114 J. Lee Category: Informational Cisco Systems, Inc. Obsoletes: 2106 H. Yasuda

                                             Mitsubishi Electric Corp.
                                                         February 1997
             Data Link Switching Client Access Protocol

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 memo describes the Data Link Switching Client Access Protocol
 that is used between workstations and routers to transport SNA/
 NetBIOS traffic over TCP sessions. Any questions or comments should
 be sent to dcap@cisco.com.

Table of Contents

 1.  Introduction ............................................   2
 2.  Overview ................................................   2
 2.1  DCAP Client/Server Model ...............................   2
 2.2  Dynamic Address Resolution .............................   3
 2.3  TCP Connection .........................................   4
 2.4  Multicast and Unicast (UDP) ............................   4
 3.  DCAP Format .............................................   6
 3.1  General Frame Format ...................................   6
 3.2  Header Format ..........................................   6
 3.3  DCAP Messages ..........................................   7
 3.4  DCAP Data formats ......................................   8
 3.4.1  CAN_U_REACH, I_CAN_REACH, and I_CANNOT_REACH Frames ..   8
 3.4.2  START_DL, DL_STARTED, and START_DL_FAILED Frames .....   9
 3.4.3  HALT_DL, HALT_DL_NOACK, and DL_HALTED Frames .........  13
 3.4.4  XID_FRAME, CONTACT_STN, STN_CONTACTED, INFO_FRAME,
        FCM_FRAME, and DGRM_FRAME ............................  14
 3.4.5  DATA_FRAME ...........................................  15
 3.4.6  CAP_XCHANGE Frame ....................................  16
 3.4.7  CLOSE_PEER_REQ Frames ................................  19
 3.4.8  CLOSE_PEER_RSP, PEER_TEST_REQ, and PEER_TEST_RSP Frames 20
 4.  Protocol Flow Diagram ...................................  20
 5.  Acknowledgments .........................................  22
 6.  References ..............................................  22

Chiang, et. al. Informational [Page 1] RFC 2114 DCAP February 1997

1. Introduction

 Since the Data Link Switching Protocol, RFC 1795, was published, some
 software vendors have begun implementing DLSw on workstations. The
 implementation of DLSw on a large number of workstations raises
 several important issues that must be addressed. Scalability is the
 major concern. For example, the number of TCP sessions to the DLSw
 router increases in direct proportion to the number of workstations
 added. Another concern is efficiency. Since DLSw is a switch-to-
 switch protocol, it is not efficient when implemented on
 workstations.
 DCAP addresses the above issues. It introduces a hierarchical
 structure to resolve the scalability problems. All workstations are
 clients to the router (server) rather than peers to the router. This
 creates a client/server model. It also provides a more efficient
 protocol between the workstation (client) and the router (server).

2. Overview

2.1. DCAP Client/Server Model

    +-----------+              +-----------+       +---------+
    | Mainframe |              | IP Router +- ppp -+ DLSw    |
    +--+--------+              +-----+-----+       | Work    |
       |                             |             | Station |
       |                             |             +---------+
    +--+--+      +-------------+     |
    | FEP +- TR -+ DLSw Router +-- IP Backbone
    +-----+      +-------------+     |
                                     |
                                     |
                               +-----------+       +---------+
                               | IP Router +- ppp -+ DLSw    |
                               +-----+-----+       | Work    |
                                                   | Station |
                                                   +---------+
                         |         DLSw Session          |
                         +-------------------------------+
Figure 2-1. Running DLSw on a large number of workstations creates a
                       scalability problem.
 Figure 2-1 shows a typical DLSw implementation on a workstation. The
 workstations are connected to the central site DLSw router over the
 IP network.  As the network grows, scalability will become an issue
 as the number of TCP sessions increases due to the growing number of
 workstations.

Chiang, et. al. Informational [Page 2] RFC 2114 DCAP February 1997

+-----------+                                        +--------+
| Mainframe |                                        | DCAP   |
+--+--------+                                  +-----+ Client |
   |                                           |     +--------+
   |                                          ppp
   |                                           |
+--+--+      +--------+                 +------+------+
| FEP +- TR -+  DLSw  +-- IP Backbone --+ DLSw Router |
+-----+      | Router |                 | DCAP Server |
             +--------+                 +------+------+
                                               |
                                              ppp
                                               |     +--------+
                                               +-----+ DCAP   |
                                                     | Client |
                                                     +--------+
                  |     DLSw Session     |  | DCAP Session |
                  +----------------------+  +--------------+
   Figure 2-2. DLSw Client Access Protocol solves the scalability
                              problem.
 In a large network, DCAP addresses the scalability problem by
 significantly reducing the number of peers that connect to the
 central site router. The workstations (DCAP clients) and the router
 (DCAP server) behave in a Client/Server relationship. Workstations
 are attached to a DCAP server. A DCAP server has a single peer
 connection to the central site router.

2.2. Dynamic Address Resolution

 In a DLSw network, each workstation needs a MAC address to
 communicate with a FEP attached to a LAN. When DLSw is implemented on
 a workstation, it does not always have a MAC address defined. For
 example, when a workstation connects to a router through a modem via
 PPP, it only consists of an IP address. In this case, the user must
 define a virtual MAC address. This is administratively intensive
 since each workstation must have an unique MAC address.
 DCAP uses the Dynamic Address Resolution protocol to solve this
 problem. The Dynamic Address Resolution protocol permits the server
 to dynamically assign a MAC address to a client without complex
 configuration.
 For a client to initiate a session to a server, the workstation sends
 a direct request to the server. The request contains the destination
 MAC address and the destination SAP. The workstation can either
 specify its own MAC address, or request the server to assign one to

Chiang, et. al. Informational [Page 3] RFC 2114 DCAP February 1997

 it. The server's IP address must be pre-configured on the
 workstation. If IP addresses are configured for multiple servers at a
 workstation, the request can be sent to these servers and the first
 one to respond will be used.
 For a server to initiate a session to a client, the server sends a
 directed request to the workstation. The workstation must pre-
 register its MAC address at the server. This can be done either by
 configuration on the server or registration at the server (both MAC
 addresses and IP addresses will be registered).

2.3. TCP Connection

 The transport used between the client and the server is TCP. A TCP
 session must be established between the client and the server before
 a frame can be sent. The default parameters associated with the TCP
 connections between the client and the server are as follows:
 Socket Family     AF_INET        (Internet protocols)
 Socket Type       SOCK_STREAM    (stream socket)
 Port Number       1973
 There is only one TCP connection between the client and the server.
 It is used for both read and write operations.
 A race condition occurs when both client and server try to establish
 the TCP session with each other at the same time. The TCP session of
 the initiator with the lower IP address will be used. The other TCP
 session will be closed.

2.4 Multicast and Unicast (UDP)

 Multicast and unicast with UDP support are optional. In the reset of
 this session, when multicast and unicast are referenced, UDP is used.
 Two multicast addresses are reserved for DCAP. The server should
 listen for 224.0.1.49 and the client should listen for 224.0.1.50.
 Not all DCAP frames can be sent via multicast or unicast. The
 DATA_FRAME can be sent via either multicast or unicast. The
 CAN_U_REACH frame can be sent via multicast only and the I_CAN_REACH
 frame can be sent via unicast only. All other DCAP frames can only be
 sent via TCP sessions.
 When the multicast and unicast support is implemented, the client
 does not have to configure the server's IP address. When the client
 attempts to establish a session to the host, instead of establishing
 a TCP session with the pre-configured server, the client can
 multicast the CAN_U_REACH frame to the 224.0.1.49 group address. When
 the server receives this multicast frame, it will locate the

Chiang, et. al. Informational [Page 4] RFC 2114 DCAP February 1997

 destination as specified in the frame. If the destination is
 reachable by this server, it will send back an I_CAN_REACH frame to
 the sender via unicast.  The client can initiate a TCP connection to
 the server and establish a DCAP session. If the I_CAN_REACH frame is
 received from multiple servers, the first one who returns the
 I_CAN_REACH frame will be used.
 When the host initiates a session to the client, the client does not
 have to pre-register its MAC address at the server. When the server
 attempts to reach an unknown client, it will multicast the
 CAN_U_REACH frame to the 224.0.10.50 group address. The client whose
 MAC address matches the destination address in the CAN_U_REACH frame
 will reply with the I_CAN_REACH frame via unicast. Once the server
 receives the I_CAN_REACH frame, it can establish a DCAP session with
 that client.
 For NetBIOS traffic, NAME_QUERY and ADD_NAME_QUERY can be
 encapsulated in the DATA_FRAME and sent out via multicast.
 NAME_RECOGNIZED and ADD_NAME_RESPONSE can be encapsulated in the
 DATA_FRAME but sent out via unicast. No other NetBIOS frames can be
 encapsulated in the DATA_FRAME to be sent out via either multicast or
 unicast.
 When a client tries to locate a name or check for duplicate name on
 the network, it can multicast a NAME_QUERY or ADD_NAME_QUERY frame
 encapsulated in the DATA_FRAME. When a server receives these frames,
 NetBIOS NAME_QUERY or ADD_NAME_QUERY frames will be forwarded to LAN.
 If the NAME_RECOGNIZED or ADD_NAME_RESPONSE frame is received from
 LAN, they will be encapsulated in the DATA_FRAME and sent to the
 client via unicast.
 When a server receives a NetBIOS NAME_QUERY or ADD_NAME_QUERY from
 LAN, the server will encapsulate it in the DATA_FRAME and send it to
 all clients via multicast. When a client receives the frame and
 determines that the name specified in the DATA_FRAME matches its own
 name, a NAME_RECOGNIZED or ADD_NAME_RESPONSE frame will be
 encapsulated in the DATA_FRAME and sent back to the server via
 unicast.

Chiang, et. al. Informational [Page 5] RFC 2114 DCAP February 1997

3. DCAP Format

3.1. General Frame Format

 The General format of the DCAP frame is as follows:
                +-------------+-----------+-----------+
                | DCAP Header | DCAP Data | User Data |
                +-------------+-----------+-----------+
                   Figure 3-1. DCAP Frame Format
 The DCAP protocol is contained in the DCAP header, which is common to
 all frames passed between the DCAP client and the server. This header
 is 4 bytes long. The next section will explain the details.
 The next part is the DCAP Data. The structure and the size are based
 on the type of messages carried in the DCAP frame. The DCAP data is
 used to process the frame, but it is optional.
 The third part of the frame is the user data, which is sent by the
 local system to the remote system. The size of this block is variable
 and is included in the frame only when there is data to be sent to
 the remote system.

3.2. Header Format

 The DCAP header is used to identify the message type and the length
 of the frame. This is a general purpose header used for each frame
 that is passed between the DCAP server and the client. More
 information is needed for frames like CAN_U_REACH and I_CAN_REACH,
 therefore, it is passed to the peer as DCAP data. The structure of
 the DCAP data depends on the type of frames, and will be discussed in
 detail in later sections.
 The DCAP Header is given below:
           +-------------------------------------------+
           | DCAP Packet Header (Each row is one byte) |
           +===========================================+
         0 | Protocol ID / Version Number              |
           +-------------------------------------------+
         1 | Message Type                              |
           +-------------------------------------------+
         2 | Packet Length                             |
           + - - - - - - - - - - - - - - - - - - - - - +
         3 |                                           |
           +-------------------------------------------+
                   Figure 3-2. DCAP Header Format

Chiang, et. al. Informational [Page 6] RFC 2114 DCAP February 1997

 o The Protocol ID uses the first 4 bits of this field and is set to
   "1000".
 o The Version Number uses the next 4 bits in this field and is set
   to "0001".
 o The message type is the DCAP message type.
 o The Total Packet length is the length of the packet including the
   DCAP header, DCAP data and User Data. The minimum size of the
   packet is 4, which is the length of the header.

3.3. DCAP Messages

 Most of the DCAP frames are based on the existing DLSw frames and
 corresponding frames have similar names. The information in the
 corresponding DCAP and DLSw frames may differ; but the
 functionalities are the same. Thus the DLSw State Machine is used to
 handle these DCAP frames. Some new DCAP frames were created to handle
 special DCAP functions. For example, the new DCAP frames,
 I_CANNOT_REACH and START_DL_FAILED provide negative acknowledgment.
 The DLSw frames not needed for DCAP, are dropped.
 The following table lists and describes all available DCAP messages:
 DCAP Frame Name     Code  Function
 ---------------     ----  --------
 CAN_U_REACH         0x01  Find if the station given is reachable
 I_CAN_REACH         0x02  Positive response to CAN_U_REACH
 I_CANNOT_REACH      0x03  Negative response to CAN_U_REACH
 START_DL            0x04  Setup session for given addresses
 DL_STARTED          0x05  Session Started
 START_DL_FAILED     0x06  Session Start failed
 XID_FRAME           0x07  XID Frame
 CONTACT_STN         0x08  Contact destination to establish SABME
 STN_CONTACTED       0x09  Station contacted - SABME mode set
 DATA_FRAME          0x0A  Connectionless Data Frame for a link
 INFO_FRAME          0x0B  Connection oriented I-Frame
 HALT_DL             0x0C  Halt Data Link session
 HALT_DL_NOACK       0x0D  Halt Data Link session without ack
 DL_HALTED           0x0E  Session Halted
 FCM_FRAME           0x0F  Data Link Session Flow Control Message
 DGRM_FRAME          0x11  Connectionless Datagram Frame for a circuit

Chiang, et. al. Informational [Page 7] RFC 2114 DCAP February 1997

 CAP_XCHANGE         0x12  Capabilities Exchange Message
 CLOSE_PEER_REQUEST  0x13  Disconnect Peer Connection Request
 CLOSE_PEER_RESPONSE 0x14  Disconnect Peer Connection Response
 PEER_TEST_REQ       0x1D  Peer keepalive test request
 PEER_TEST_RSP       0x1E  Peer keepalive response
                       Table 3-1. DCAP Frames

3.4. DCAP Data formats

 The DCAP data is used to carry information required for each DCAP
 frame. This information is used by the Server or the Client and it
 does not contain any user data. The DCAP data frame types are listed
 in the following sections. Please note that the sender should set the
 reserved fields to zero and the receiver should ignore these fields.

3.4.1. CAN_U_REACH, I_CAN_REACH, and I_CANNOT_REACH Frames

 These frame types are used to locate resources in a network. A
 CAN_U_REACH frame is sent to the server to determine if the resource
 is reachable. When a server receives a CAN_U_REACH frame, it should
 send out an LLC explorer frame to locate the destination specified in
 the CAN_U_REACH frame. If the destination is reachable, the server
 responds to the client with an I_CAN_REACH frame. If the server does
 not receive a positive acknowledgment within a recommended threshold
 value of 5 seconds, the server should send an LLC explorer to locate
 the destination again. If the server does not receive any response
 after sending out 5 explorers (recommended retry value), the
 destination is considered not reachable and an I_CANNOT_REACH frame
 is sent back to the client. The client should decide if retry
 CAN_U_REACH is necessary after the I_CANNOT_REACH frame is received
 from the server.
 When a server is in the process of searching a destination and
 receives another I_CAN_REACH with the same destination, the server
 should not send out another LLC explorer for that destination.
 The server should not send the CAN_U_REACH frame to the clients in a
 TCP session. When a server receives an LLC explorer whose destination
 is a known client, the server should respond to it directly.

Chiang, et. al. Informational [Page 8] RFC 2114 DCAP February 1997

         +---------------+-----------------------+
         | Field Name    | Information           |
         +---------------+-----------------------+
         | Message Type  | 0x01, 0x02, or 0x03   |
         +---------------+-----------------------+
         | Packet Length | 0x0C                  |
         +---------------+-----------------------+
  Figure 3-3. CAN_U_REACH, I_CAN_REACH, and I_CANNOT_REACH Header
           +-----------------------------------+
           | Field Name (Each row is one byte) |
           +===================================+
         0 | Target MAC Address                |
           + - - - - - - - - - - - - - - - - - +
         1 |                                   |
           + - - - - - - - - - - - - - - - - - +
         2 |                                   |
           + - - - - - - - - - - - - - - - - - +
         3 |                                   |
           + - - - - - - - - - - - - - - - - - +
         4 |                                   |
           + - - - - - - - - - - - - - - - - - +
         5 |                                   |
           +-----------------------------------+
         6 | Source SAP                        |
           +-----------------------------------+
         7 | Reserved                          |
           +-----------------------------------+
   Figure 3-4. CAN_U_REACH, I_CAN_REACH, and I_CANNOT_REACH Data
 The MAC Address field carries the MAC address of the target
 workstation that is being searched. This is a six-byte MAC Address
 field. The same MAC Address is returned in the I_CAN_REACH and the
 I_CANNOT_REACH frames.
 Byte 6 is the source SAP. The destination SAP is set to zero when an
 explorer frame is sent to the network.

3.4.2. START_DL, DL_STARTED, and START_DL_FAILED Frames

 These frame types are used by DCAP to establish a link station
 (circuit). The START_DL frame is sent directly to the server that
 responds to the CAN_U_REACH frame. When the server receives this
 frame, it establishes a link station using the source and destination
 addresses and saps provided in the START_DL frame. If the circuit
 establishment is successful, a DL_STARTED frame is sent back as a
 response. If the attempt fails within a recommended value, 5 seconds,
 the server should retry again. If the server fails to establish a

Chiang, et. al. Informational [Page 9] RFC 2114 DCAP February 1997

 circuit for a recommended retry value, 5 times, a START_DL_FAILED
 frame should be sent back to the client. If the client receives a
 START_DL_FAILED frame from the server, it is up to the client to
 decide if a START_DL frame needs to be sent to the server again.
 The server can also send START_DL frames to clients to establish
 circuits.
         +---------------+-----------------------+
         | Field Name    | Information           |
         +---------------+-----------------------+
         | Message Type  | 0x04, 0x05, or 0x06   |
         +---------------+-----------------------+
         | Packet Length | 0x18                  |
         +---------------+-----------------------+
    Figure 3-5. START_DL, DL_STARTED, and START_DL_FAILED Header

Chiang, et. al. Informational [Page 10] RFC 2114 DCAP February 1997

           +-----------------------------------+
           | Field Name (Each row is one byte) |
           +===================================+
         0 | Host MAC Address                  |
           + - - - - - - - - - - - - - - - - - +
         1 |                                   |
           + - - - - - - - - - - - - - - - - - +
         2 |                                   |
           + - - - - - - - - - - - - - - - - - +
         3 |                                   |
           + - - - - - - - - - - - - - - - - - +
         4 |                                   |
           + - - - - - - - - - - - - - - - - - +
         5 |                                   |
           +-----------------------------------+
         6 | Host SAP                          |
           +-----------------------------------+
         7 | Client SAP                        |
           +-----------------------------------+
         8 | Origin Session ID                 |
           +-----------------------------------+
         9 |                                   |
           + - - - - - - - - - - - - - - - - - +
         10|                                   |
           + - - - - - - - - - - - - - - - - - +
         11|                                   |
           +-----------------------------------+
         12| Target Session ID                 |
           + - - - - - - - - - - - - - - - - - +
         13|                                   |
           + - - - - - - - - - - - - - - - - - +
         14|                                   |
           + - - - - - - - - - - - - - - - - - +
         15|                                   |
           +-----------------------------------+
         16| Largest Frame Size                |
           +-----------------------------------+
         17| Initial Window size               |
           +-----------------------------------+
         18| Reserved                          |
           + - - - - - - - - - - - - - - - - - +
         19|                                   |
           +-----------------------------------+
 Figure 3-6. START_DL, DL_STARTED, and START_DL_FAILED Data
 The Host MAC address is the address of the target station if the
 session is initiated from the client, or it is the address of the
 originating station if the session is initiated from the server.

Chiang, et. al. Informational [Page 11] RFC 2114 DCAP February 1997

 The next two fields are the Host and Client SAPs. Each is one byte
 long. The Host SAP is the SAP used by the station with the Host MAC
 address. The Client SAP is the SAP used by the client.
 The Origin Session ID, is the ID of the originating station that
 initiates the circuit. The originating station uses this ID to
 identify the newly created circuit. Before the START_DL frame is sent
 to the target station, the originating station sets up a control
 block for the circuit. This link station information is set because
 DCAP does not use a three-way handshake for link station
 establishment. In the DL_STARTED and the START_DL_FAILED frames, the
 Origin Session ID is returned as received in the START_DL frame.  The
 Target Session ID is set by the target station and returned in the
 DL_STARTED frame.
 The Target Session ID is not valid for the START_DL and the
 START_DL_FAILED frame, and should be treated as Reserved fields. In
 the DL_STARTED frame, it is the session ID that is used to set up
 this circuit by the target station.
 The Largest Frame Size field is used to indicate the maximum frame
 size that can be used by the client. It is valid only when it is set
 by the server. The Largest Frame Size field must be set to zero when
 a frame is sent by the client. Both START_DL and DL_STARTED use the
 Largest Frame Size field and only its rightmost 6 bits are used.  The
 format is defined in the IEEE 802.1D Standard, Annex C, Largest Frame
 Bits (LF). Bit 3 to bit 5 are base bits. Bit 0 to bit 2 are extended
 bits. The Largest Frame Size field is not used in the START_DL_FAILED
 frame and must be set to zero.
         bit   7    6    5    4    3    2    1    0
               r    r    b    b    b    e    e    e
                   Figure 3-7. Largest Frame Size
 Please note that if the client is a PU 2.1 node, the client should
 use the maximum I-frame size negotiated in the XID3 exchange.
 The Initial window size in the START_DL frame specifies the receive
 window size on the originating side, and the target DCAP station
 returns its receive window size in the DL_STARTED frame. The field is
 reserved in the START_DL_FAILED frame. The usage of the window size
 is the same as the one used in DLSw.  Please refer to RFC 1795 for
 details.
 The last two bits are reserved for future use. They must be set to
 zero by the sender and ignored by the receiver.

Chiang, et. al. Informational [Page 12] RFC 2114 DCAP February 1997

3.4.3. HALT_DL, HALT_DL_NOACK, and DL_HALTED Frames

 These frame types are used by DCAP to disconnect a link station. A
 HALT_DL frame is sent directly to the remote workstation to indicate
 that the sender wishes to disconnect a session. When the receiver
 receives this frame, it tears down the session that is associated
 with the Original Session ID and the Target Session ID provided in
 the HALT_DL frame. The receiver should respond with the DL_HALTED
 frame.  The DL_HALTED frame should use the same Session ID values as
 the received HALT_DL frame without swapping them. The HALT_DL_NOACK
 frame is used when the response is not required. The TCP session
 between the client and server should remain up after the
 HALT_DL/DL_HALTED/ HALT_DL_NOACK exchange.
         +---------------+-----------------------+
         | Field Name    | Information           |
         +---------------+-----------------------+
         | Message Type  | 0x0C, 0x0D, or 0x0E   |
         +---------------+-----------------------+
         | Packet Length | 0x10                  |
         +---------------+-----------------------+
      Figure 3-8. HALT_DL, HALT_DL_NOACK, and DL_HALTED Header

Chiang, et. al. Informational [Page 13] RFC 2114 DCAP February 1997

           +-----------------------------------+
           | Field Name (Each row is one byte) |
           +===================================+
         0 | Sender Session ID                 |
           + - - - - - - - - - - - - - - - - - +
         1 |                                   |
           + - - - - - - - - - - - - - - - - - +
         2 |                                   |
           + - - - - - - - - - - - - - - - - - +
         3 |                                   |
           +-----------------------------------+
         4 | Receiver Session ID               |
           + - - - - - - - - - - - - - - - - - +
         5 |                                   |
           + - - - - - - - - - - - - - - - - - +
         6 |                                   |
           + - - - - - - - - - - - - - - - - - +
         7 |                                   |
           +-----------------------------------+
         8 | Reserved                          |
           + - - - - - - - - - - - - - - - - - +
         9 |                                   |
           + - - - - - - - - - - - - - - - - - +
         10|                                   |
           + - - - - - - - - - - - - - - - - - +
         11|                                   |
           +-----------------------------------+
     Figure 3-9. START_DL, DL_STARTED, and START_DL_FAILED Data

3.4.4. XID_FRAME, CONTACT_STN, STN_CONTACTED, INFO_FRAME, FCM_FRAME, and DGRM_FRAME

 These frame types are used to carry the end-to-end data or establish
 a circuit. The Destination Session ID is the Session ID created in
 the START_DL frame or the DL_STARTED frame by the receiver. The usage
 of the flow control flag is the same as the one used in DLSw.  Please
 refer to RFC 1795 for details.
         +---------------+----------------------------+
         | Field Name    | Information                |
         +---------------+----------------------------+
         | Message Type  | Based on Message type      |
         +---------------+----------------------------+
         | Packet Length | 0x0C + length of user data |
         +---------------+----------------------------+
                  Figure 3-10. Generic DCAP Header

Chiang, et. al. Informational [Page 14] RFC 2114 DCAP February 1997

           +-----------------------------------+
           | Field Name (Each row is one byte) |
           +===================================+
         0 | Destination Session ID            |
           + - - - - - - - - - - - - - - - - - +
         1 |                                   |
           + - - - - - - - - - - - - - - - - - +
         2 |                                   |
           + - - - - - - - - - - - - - - - - - +
         3 |                                   |
           +-----------------------------------+
         4 | Flow Control Flags                |
           +-----------------------------------+
         5 | Reserved                          |
           + - - - - - - - - - - - - - - - - - +
         6 |                                   |
           + - - - - - - - - - - - - - - - - - +
         7 |                                   |
           +-----------------------------------+
               Figure 3-11. Generic DCAP Data Format

3.4.5. DATA_FRAME

 This frame type is used to send connectionless SNA and NetBIOS
 Datagram (UI) frames that do not have a link station associated with
 the source and destination MAC/SAP pair. The difference between
 DGRM_FRAME and DATA_FRAME is that DGRM_FRAME is used to send UI
 frames received for stations that have a link station opened, whereas
 DATA_FRAME is used for frames with no link station established.
         +---------------+-----------------------------+
         | Field Name    | Information                 |
         +---------------+-----------------------------+
         | Message Type  | 0x0A                        |
         +---------------+-----------------------------+
         | Packet Length | 0x10 + Length of user data  |
         +---------------+-----------------------------+
                   Figure 3-12. DATA_FRAME Header

Chiang, et. al. Informational [Page 15] RFC 2114 DCAP February 1997

           +-----------------------------------+
           | Field Name (Each row is one byte) |
           +===================================+
         0 | Host MAC Address                  |
           + - - - - - - - - - - - - - - - - - +
         1 |                                   |
           + - - - - - - - - - - - - - - - - - +
         2 |                                   |
           + - - - - - - - - - - - - - - - - - +
         3 |                                   |
           + - - - - - - - - - - - - - - - - - +
         4 |                                   |
           + - - - - - - - - - - - - - - - - - +
         5 |                                   |
           +-----------------------------------+
         6 | Host SAP                          |
           +-----------------------------------+
         7 | Client SAP                        |
           +-----------------------------------+
         8 | Broadcast Type                    |
           +-----------------------------------+
         9 | Reserved                          |
           + - - - - - - - - - - - - - - - - - +
         10|                                   |
           + - - - - - - - - - - - - - - - - - +
         11|                                   |
           +-----------------------------------+
                Figure 3-13. DATA_FRAME Data Format
 The definition of the first 8 bytes is the same as the START_DL
 frame. The Broadcast Type field indicates the type of broadcast
 frames in use; Single Route Broadcast, All Route Broadcast, or
 Directed. The target side will use the same broadcast type. In the
 case of Directed frame, if the RIF information is known, the target
 peer can send a directed frame. If not, a Single Route Broadcast
 frame is sent.

3.4.6. CAP_XCHANGE Frame

 In DCAP, the capability exchange frame is used to exchange the
 capability information between a client and a server. CAP_XCHANGE
 frames are exchanged between a client and a server as soon as the TCP
 session is established. The capability exchange must be completed
 before the other frame types can be sent. Once the capability
 exchange is done, CAP_XCHANGE frame should not be used again.

Chiang, et. al. Informational [Page 16] RFC 2114 DCAP February 1997

 CAP_XCHANGE frame contains the clients MAC address, if a client has
 one. If it does not, then the MAC address field must be set to zero.
 When the DCAP server receives the CAP_XCHANGE frame, it should cache
 the MAC address if it is non zero. The DCAP server also verifies that
 the MAC address is unique. The server should return a CAP_XCHANGE
 response frame with the MAC address supplied by the client if the MAC
 address is accepted. If a client does not have its own MAC address,
 the server should assign a MAC address to the client and put that
 address in the CAP_XCHANGE command frame.
 A client should record the new MAC address assigned by the server and
 return a response with the assigned MAC address. If the client cannot
 accept the assigned MAC address, another CAP_XCHANGE command with the
 MAC address field set to zero should be sent to the server. The
 server should allocate a new MAC address for this client.
 During the capability exchange, both the client and the server can
 send command frames. The process stops when either side sends a
 CAP_XCHANGE response frame. When the response frame is sent, the MAC
 address in the CAP_XCHANGE frame should be the same as the one in the
 previous received command. The sender of the CAP_XCHANGE response
 agrees to use the MAC address defined in the previous command.
 The number of CAP_XCHANGE frames that need to be exchanged is
 determined by the client and the server independently. When the
 number of exchange frames has exceeded the pre-defined number set by
 either the server or the client, the session should be brought down.
 The flag is used to show the capability of the sender. The following
 list shows the valid flags:
 0x01 NetBIOS support. If a client sets this bit on, the server will
      pass all NetBIOS explorers to this client. If this bit is not
      set, only SNA traffic will be sent to this client.
 0x02 TCP Listen Mode support. If a client supports TCP listen mode,
      the server will keep the client's MAC and IP addresses even
      after the TCP session is down. The cached information will be
      used for server to connect out. If a client does not support
      TCP listen mode, the cache will be deleted as soon as the TCP
      session is down.
 0x04 Command/Response. If this bit is set, it is a command,
      otherwise, it is a response.
 The values 0x01 and 0x02 are used only by the client. When a server
 sends the command/response to a client, the server does not return
 these values.

Chiang, et. al. Informational [Page 17] RFC 2114 DCAP February 1997

 Starting with the Reserved field, implementers can optionally
 implement the Capability Exchange Control Vector. Each Capability
 Exchange Control Vector consists of three fields: Length (1 byte),
 Type (1 byte), and Data (Length - 2 bytes). Two types of Control
 Vectors are defined: SAP_LIST and VENDOR_CODE (described below). To
 ensure compatibility, implementers should ignore the unknown Control
 Vectors instead of treating them as errors.
 0x01 SAP_LIST. Length: 2+n bytes, where n ranges from 1 to 16.
      This control vector lists the SAPs that the client can support.
      The maximum number of SAPs a client can define is 16. Therefore,
      the length of this Control Vector ranges from 3 to 18. If the
      SAP_LIST is not specified in the capability exchange, the server
      assumes that the client can support all the SAP values. For
      example, if a client can only support SAP 4 and 8, then the
      following Control Vectors should be sent: "0x04, 0x01, 0x04,
      0x08". The first byte indicates the length of 4. The second byte
      indicates the control vector type of SAP_LIST. The last two
      bytes indicate the supported SAP values; 0x04 and 0x08. This
      Control Vector is used only by the client. If the server accepts
      this Control Vector, it must return the same Control Vector to
      the client.
 0x02 VENDOR_CODE. Length: 3 bytes.
      Each vendor is assigned a vendor code that identifies the
      vendor. This Control Vector does not require a response.
 After the receiver responds to a Control Vector, if the capability
 exchange is not done, the sender does not have to send the same
 Control Vector again.
         +---------------+-----------------------+
         | Field Name    | Information           |
         +---------------+-----------------------+
         | Message Type  | 0x12                  |
         +---------------+-----------------------+
         | Packet Length | 0x1C                  |
         +---------------+-----------------------+
                  Figure 3-14. CAP_XCHANGE Header

Chiang, et. al. Informational [Page 18] RFC 2114 DCAP February 1997

           +-----------------------------------+
           | Field Name (Each row is one byte) |
           +===================================+
         0 | MAC Address                       |
           + - - - - - - - - - - - - - - - - - +
         1 |                                   |
           + - - - - - - - - - - - - - - - - - +
         2 |                                   |
           + - - - - - - - - - - - - - - - - - +
         3 |                                   |
           + - - - - - - - - - - - - - - - - - +
         4 |                                   |
           + - - - - - - - - - - - - - - - - - +
         5 |                                   |
           +-----------------------------------+
         6 | Flag                              |
           +-----------------------------------+
         7 | Reserved                          |
           +-----------------------------------+
                Figure 3-15. CAP_XCHANGE Data Format

3.4.7. CLOSE_PEER_REQ Frames

 This frame is used for peer connection management and contains a
 reason code field. The following list describes the valid reason
 codes:
 0x01 System shutdown. This indicates shutdown in progress.
 0x02 Suspend. This code is used when there is no traffic between the
      server and the client, and the server or the client wishes to
      suspend the TCP session. When the TCP session is suspended, all
      circuits should remain intact. The TCP session should be re-
      established when new user data needs to be sent. When the TCP
      session is re-established, there is no need to send the
      CAP_XCHANGE frame again.
 0x03 No MAC address available. This code is sent by the server when
      there is no MAC address is available from the MAC address pool.
         +---------------+-----------------------+
         | Field Name    | Information           |
         +---------------+-----------------------+
         | Message Type  | 0x13                  |
         +---------------+-----------------------+
         | Packet Length | 0x08                  |
         +---------------+-----------------------+
                 Figure 3-16. CLOSE_PEER_REQ Header

Chiang, et. al. Informational [Page 19] RFC 2114 DCAP February 1997

           +-----------------------------------+
           | Field Name (Each row is one byte) |
           +===================================+
         0 | Reason Code                       |
           +-----------------------------------+
         1 | Reserved                          |
           + - - - - - - - - - - - - - - - - - +
         2 |                                   |
           + - - - - - - - - - - - - - - - - - +
         3 |                                   |
           +-----------------------------------+
              Figure 3-17. CLOSE_PEER_REQ Data Format

3.4.8. CLOSE_PEER_RSP, PEER_TEST_REQ, and PEER_TEST_RSP Frames

 These three frames are used for peer connection management. There is
 no data associated with them.
 o CLOSE_PEER_RSP
   CLOSE_PEER_RSP is the response for CLOSE_PEER_REQ.
 o PEER_TEST_REQ and PEER_TEST_RSP
   PEER_TEST_REQ and PEER_TEST_RSP are used for peer level keepalive.
   Implementing PEER_TEST_REQ is optional, but PEER_TEST_RSP must be
   implemented to respond to the PEER_TEST_REQ frame. When a
   PEER_TEST_REQ frame is sent to the remote station, the sender
   expects to receive the PEER_TEST_RSP frame in a predefined time
   interval (the recommended value is 60 seconds). If the
   PEER_TEST_RSP frame is not received in the predefined time
   interval, the sender can send the PEER_TEST_REQ frame again. If a
   predefined number of PEER_TEST_REQ frames is sent to the remote
   station, but no PEER_TEST_RSP frame is received (the recommended
   number is 3), the sender should close the TCP session with this
   remote station and terminate all associated circuits.
         +---------------+-----------------------+
         | Field Name    | Information           |
         +---------------+-----------------------+
         | Message Type  | 0x14, 0x1D, or 0x1E   |
         +---------------+-----------------------+
         | Packet Length | 0x04                  |
         +---------------+-----------------------+
 Figure 3-18. CLOSE_PEER_RSP, PEER_TEST_REQ, and PEER_TEST_RSP DCAP

4. Protocol Flow Diagram

 The following diagram shows a normal session start up/tear down
 sequence between a client and a server.

Chiang, et. al. Informational [Page 20] RFC 2114 DCAP February 1997

                            +-----------+                +-------+
     +-----------+  Token   | DLSw/DCAP |                | DCAP  |
     | Mainframe +- Ring ---+   Router  +-- ip backbone--+ Client|
     +-----------+          +-----------+                +-------+
                                           TCP Session Up
                                           <-------------
                                           CAP_EXCHANGE (cmd)
                                           <-------------
                                           CAP_EXCHANGE (cmd)
                                           ------------->
                                           CAP_EXCHANGE (rsp)
                                           ------------->
                   TEST(P)                 CAN_U_REACH
                  <--------                <-------------
                   TEST(F)                 I_CAN_REACH
                  -------->                ------------->
                                           START_DL
                                           <-------------
                                           DL_STARTED
                                           ------------->
                   XID(P)                  XID_FRAME
                  <--------                <-------------
                   XID(F)                  XID_FRAME
                  -------->                ------------->
                   XID(P)                  XID_FRAME
                  <--------                <-------------
                   SABME                   CONTACT_STN
                  -------->                ------------->
                   UA                      STN_CONTACTED
                  <--------                <-------------
                   I FRAME                 INFO_FRAME
                  <--------                <-------------
                   I FRAME                 INFO_FRAME
                  -------->                ------------->
                   DISC                    HALT_DL
                  <--------                <-------------
                   UA                      DL_HALTED
                  -------->                ------------->
                                           CLOSE_PEER_REQ
                                           <-------------
                                           CLOSE_PEER_RSP
                                           ------------->
                                           TCP session down
                                           <-------------

Chiang, et. al. Informational [Page 21] RFC 2114 DCAP February 1997

5. Acknowledgments

 The authors wish to express thanks to Rodger Erickson of Wall Data,
 Inc. for his helpful comments and suggestions.

6. References

 [1] AIW DLSw Related Interest Group, RFC 1795,
     "DLSw: Switch-to-Switch Protocol", April 1995
 [2] IBM Token Ring Network Architecture Reference
     SC30-3374-02, September 1989.
 [3] IBM LAN Technical Reference IEEE 802.2 and NETBIOS Application
     Program Interfaces SC30-3587-00, December 1993.
 [4] ISO 8802-2/IEEE Std 802.1D International Standard.

Authors' Addresses

 Steve T. Chiang
 InterWorks Business Unit
 Cisco Systems, Inc.
 170 Tasman Drive
 San Jose, CA 95134
 Phone: (408) 526-5189
 EMail: schiang@cisco.com
 Joseph S. Lee
 InterWorks Business Unit
 Cisco Systems, Inc.
 170 Tasman Drive
 San Jose, CA 95134
 Phone: (408) 526-5232
 EMail: jolee@cisco.com
 Hideaki Yasuda
 System Product Center
 Network Products Department
 Network Software Products Section B
 Mitsubishi Electric Corp.
 Information Systems Engineering Center
 325, Kamimachiya Kamakura Kanagawa 247, Japan
 Phone: +81-467-47-2120
 EMail: yasuda@eme068.cow.melco.co.jp

Chiang, et. al. Informational [Page 22]

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