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Data Link Switching Client Access Protocol
RFC 2114

Document type: RFC - Informational (February 1997; No errata)
Obsoletes RFC 2106
Document stream: Legacy
Last updated: 2013-03-02
Other versions: plain text, pdf, html

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IESG State: RFC 2114 (Informational)
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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

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