rfc1434.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                          R. Dixon
Request for Comments: 1434                                     D. Kushi
                                                                    IBM
                                                             March 1993


             Data Link Switching: Switch-to-Switch Protocol

Status of this Memo

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

Abstract

   This RFC describes IBM's support of Data Link Switching over TCP/IP.
   The RFC is being distributed to members of the Internet community in
   order to solicit their reactions to the proposals contained in it.
   While the issues discussed may not be directly relevant to the
   research problems of the Internet, they may be interesting to a
   number of researchers and implementors.

   Any questions or comments relative to the contents of this RFC should
   be sent to the following Internet address: [e-mail].

Table of Contents

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




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


1. Introduction

   Data Link Switching (DLS) is a forwarding mechanism for the IBM SNA
   and IBM NetBIOS protocols.  It does not provide full routing, but
   instead provides switching at the Data Link layer and encapsulation
   in TCP/IP for transport over the Internet.  This memo documents the
   Switch-to-Switch Protocol (SSP) that is used between IBM 6611 Network
   Processors.

   Today, the IBM 6611 supports SNA (PU 2 and PU 4) systems and NetBIOS
   systems attached to token-ring networks, as well as SNA (PU 2)
   systems attached to SDLC links.  For the later case, the SDLC
   attached systems are provided with a LAN appearance within the IBM
   6611.  For the LAN attached systems, the IBM 6611 appears as a
   source-routing bridge.  Remote systems that are accessed through the
   IBM 6611 appear as systems attached to an adjacent ring.  This ring
   is a virtual ring that is manifested within each IBM 6611.

2. Overview

   Data Link Switching was developed to provide support for SNA and
   NetBIOS in multi-protocol routers.  Since SNA and NetBIOS are
   basically connection oriented protocols, the Data Link Control
   procedure that they use on the LAN is IEEE 802.2 Logical Link Control
   (LLC) Type 2.  Data Link Switching also accommodates SNA protocols
   over WAN links via the SDLC protocol.

   IEEE 802.2 LLC Type 2 was designed with the assumption that the
   network transit delay would be small and predictable (i.e., a local
   LAN).  Therefore the LLC elements of procedure use a fixed timer for
   detecting lost frames.  When bridging is used over wide area lines
   (especially at lower speeds), the network delay is larger and it can
   vary greatly based upon congestion.  When the delay exceeds the
   time-out value LLC attempts to retransmit.  If the frame is not
   actually lost, only delayed, it is possible for the LLC Type 2
   procedures to become confused.  And as a result, the link is
   eventually taken down.

   Given the use of LLC Type 2 services, Data Link Switching addresses
   the following bridging problems:

         DLC Time-outs
         DLC Acknowledgments over the WAN
         Flow and Congestion Control
         Broadcast Control of Search Packets
         Source-Route Bridging Hop Count Limits

   NetBIOS also makes extensive use of datagram services that use LLC



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


   Type 1.  In this case, Data Link Switching addresses the last two
   problems in the above list.

   The principal difference between Data Link Switching and bridging is
   that DLS terminates the Data Link Control whereas bridging does not.
   The following figure illustrates this difference based upon two end
   systems operating with LLC Type 2 services.

    Bridging
    --------             Bridge           Bridge
    +------+             +----+           +----+             +------+
    | End  | +---------+ |    +-----/     |    | +---------+ | End  |
    |System+-+   LAN   +-+    |    /------+    +-+   LAN   +-+System|
    |      | +---------+ |    |  TCP/IP   |    | +---------+ |      |
    +------+             +----+           +----+             +------+
   Info------------------------------------------------------->
       <-------------------------------------------------------RR

    Data Link Switching
    -------------------
    +------+             +----+           +----+             +------+
    | End  | +---------+ |    +-----/     |    | +---------+ | End  |
    |System+-+   LAN   +-+DLS |    /------+ DLS+-+   LAN   +-+System|
    |      | +---------+ |    |  TCP/IP   |    | +---------+ |      |
    +------+             +----+           +----+             +------+
   Info------------------->   -------------> Info
       <-------------------RR                 ---------------->
                                              <----------------RR

           Figure 1.  Data Link Switching Contrasted to Bridging

   In traditional bridging, the Data Link Control is end-to-end.  Data
   Link Switching terminates the LLC Type 2 connection at the switch.
   This means that the LLC Type 2 connections do not cross the wide area
   network.  The DLS multiplexes LLC connections onto a TCP connection
   to another DLS.  Therefore, the LLC connections at each end are
   totally independent of each other.  It is the responsibility of the
   Data Link Switch to deliver frames that it has received from a LLC
   connection to the other end.  TCP is used between the Data Link
   Switches to guarantee delivery of frames.

   As a result of this design, LLC time-outs are limited to the local
   LAN (i.e., they do not traverse the wide area).  Also, the LLC Type 2
   acknowledgments (RR's) do not traverse the WAN, thereby reducing
   traffic across the wide area links.  For SDLC links, polling and poll
   response occurs locally, not over the WAN.  Broadcast of search
   frames is controlled by the Data Link Switches once the location of a
   target system is discovered.  Finally, the switches can now apply



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


   back pressure to the end systems to provide flow and congestion
   control.

   Data Link Switching uses LAN addressing to set up connections between
   SNA systems.  SDLC attached devices are defined with MAC addresses to
   enable them to communicate with LAN attached devices.  For NetBIOS
   systems, Data Link Switching uses the NetBIOS name to forward
   datagrams and to set up connections for NetBIOS sessions.  For
   circuit establishment, SNA systems send TEST (or in some cases, XID)
   frames to the null (x'00') SAP.  NetBIOS systems have an address
   resolution procedure, based upon the Name Query and Name Recognized
   frames, that is used to establish an end-to-end circuit.

   Since Data Link Switching may be implemented in multi-protocol
   routers, there may be situations where both bridging and switching
   are enabled.  SNA frames can be identified by their link SAP.
   Typical SAP values for SNA are x'04', x'08', and x'0C'.  NetBIOS
   always uses a link SAP value of x'F0'.

3. Transport Connection

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


    +-----------------------------------------------+Switch-to-Switch
    |               DLC Interfaces                  | Protocol (SSP)
    |+------------+   DLC Request    +------------+ |
    ||    Data    |<---------------- |            | |Send SSP Frame
    ||    Link    | DLC Indication   |            | |-------------->
    ||  Control 1 |----------------->|            | |
    |+------------+                  | Data Link  | |
    |+------------+   DLC Request    |  Switch    | |
    ||    Data    |<---------------- |            | |Rec. SSP Frame
    ||    Link    | DLC Indication   |            | |<-------------
    ||  Control 2 | ---------------->|            | |
    |+------------+                  +------------+ |
    |                   Multi-Protocol Router       |
    +-----------------------------------------------+

                     Figure 2.  DLS System Diagram



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


   Before Data Link Switching can occur between two routers, they must
   establish a TCP connection between them.  Each DLS will maintain a
   list of DLS capable routers and their status (active/inactive).  Once
   this connection is established, the DLS will employ SSP to establish
   end-to-end circuits over the transport connection.  Within the
   transport connection is a specific set of  DLS message units.  The
   message formats and types for these PDUs are documented in the
   following sections.

   The default parameters associated with the TCP connections between
   Data Link Switches are as follows:

            Socket Family     AF_INET        (Internet protocols)
            Socket Type       SOCK_STREAM    (stream socket)
            Read Port Number  2065
            Write Port Number 2067

   Two or more Data Link Switches may be attached to the same LAN,
   consisting of a number of token-ring segments interconnected by
   source-routing bridges.  In this case, a TCP connection is not
   defined between bridges attached to the same LAN.  This will allow
   using systems to select one of the possible Data Link Switches in a
   similar manner to the selection of a bridge path through a source-
   routed bridged network.  The virtual ring segment in each Data Link
   Switch attached to a common LAN must be configured with the same ring
   number.  This will prevent LAN frames sent by one Data Link Switch
   from being propagated through the other Data Link Switches.

3.1. SSP Frame Formats

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















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


     CONTROL MESSAGES (72 Bytes)
    +-----------------------------------------------------------------+
    | Version Number                  Reserved Field                  |