SNA Server Remote Station Discovery Process

• Microsoft SNA Server for Windows NT, versions 2.0, 2.1, 2.11, 2.11 SP1, and 3.0

SUMMARY

When SNA Server starts an 802.2 connection, it submits, by default, an unlimited number of Open(Link) requests to the 802.2 link service. If an SNA Server 802.2 connection is configured to activate OnServer Startup or By Administrator, the server will try indefinitely to activate the connection.

The DLC protocol then sends logical link control (LLC) test frames to test link station-to-link station connectivity. These test frames are the first frames that are sent by SNA Server (through DLC) and are used to discover the remote station. To ensure the best chance of remote station discovery, the DLC protocol then submits various types of MAC level frames to the NDIS (Network Device Interface Specification) interface. The types submitted depend on the media access method DLC is bound to.

THE SNA SERVER LINK LEVEL OPEN(LINK) REQUEST

SNA Server's link level requests can be adjusted in SNA Server Admin by choosing the Retry Timers button in the Connection Properties dialog box. The two values are:

• Maximum number of attempts

  This value represents the number of link level Open (link) requests made by the SNA Server service to the SNA Server 802.2 link service. It does not represent the number of test frames that will be submitted by the DLC protocol to NDIS. The valid range is from 1 to No Limit; the default is No Limit.

   This value represents the number of seconds that the SNA Server
   service will wait after a failed attempt before submitting another
   request 802.2 link service. The valid range is from 5 seconds through
   255 seconds (in multiples of 5); the default is 10 seconds.

DLC MAC level Framing

Ethernet

On Ethernet there are two frame formats: IEEE 802.3 and the older DIX (Digital Intel Xerox). With DIX format, SNAP message headers are used to identify the protocol as DLC because various other MAC level protocols can flow on Ethernet such as TCP/IP, NetBEUI, IPX, and so forth.

SNA Server tells the DLC stack which sort of framing to use:

   HKEY_LOCAL_MACHINE\SYSTEM
   CurrentControlSet\Services\DLCLinkServiceName\Parameters\ExtraParameters
   Frametype:REG_DWORD:number

Specify the number in FrameType as follows:

Value   Effect

  0     Use the frame type specified for this adapter in the
        registry entry for the DLC protocol stack:

           HKEY_LOCAL_MACHINE\SYSTEM
           \CurrentControlSet\Services\DLC\Parameters\AdapterName
             \UseDixOverEthernet

        The default for UseDixOverEthernet is 0, resulting in 802.3
        format.

  1     Use the frame type automatically determined by the DLC stack
        during XID exchange.

  2     Use standard 802.3 format.

  3     Use 802.2 packets prefixed by DIX headers using EtherType value
        0x80D5.

When you run DLC applications on Ethernet LANs, where the DLC traffic may need to pass over an Ethernet-Token Ring bridge (such as IBM 8209), the destination adapter address may need to be sent in bit-flipped order to accommodate the address change that such a bridge performs.

SNA Server handles this by sending TEST and XID frames to both the configured remote network address and the bit-flipped address (for Ethernet 802.2 connections), in both DIX and 802.3 formats.

Some stations may send bits in little-endian order (least significant bit first), while others send them big-endian (most significant bit first). As a result, some remote stations won't recognize their own addresses unless the local station reverses the order of bits in the MAC address on every frame. One common problem occurs when an older LAN bridge flips the bits as the frames pass through it. As a result, remote systems do not recognize their own address, even when the sending and receiving stations are using the same bit ordering. Because of this, DLC sends both bit flipped and non- bit flipped frames to ensure that the remote system recognizes its address even when passing through one of these bridges.

In summary, on each connection, the originating station starts an 802.2 session by sending an LLC TEST frame. By default, DLC sends this frame using both 802.3 and DIX framing with both bit orderings of the MAC header. This can create as many as four TEST frames on the wire per request from the SNA Server 802.2 link service. The answering station's LAN adapter is only listening to one bit ordering of its MAC address, but it may be enabled for both 802.3 and DIX framing; the receiving station responds with either one or, at most, two TEST frames - one for each framing. The bit ordering is now established. DLC at the originating station hears both responses, and chooses one for the 802.2 session.

Token Ring

On token ring LANs, there is only one framing format, as specified by IEEE standard 802.5. Bit-flipping is not necessary.

In Windows NT 3.1, DLC's automatic frame type detection did not work correctly; it could not establish a session with DIX framing, though it could via 802.3.

In Windows NT 3.5, a filtering algorithm was added to avoid unnecessary duplication of the initial messages. This filtering avoids handing the client software (SNA Server) a duplicate message received with both framing formats. This cuts down on the superfluous messages exchanged during XID negotiation.

To Bit-flip:

1. Separate the 12-digit hexadecimal address into a set of two digits; for example:

         40 00 0A 12 34 56
   

2. Reverse the positions of the characters in each of the two-digit sets:

         40 00 0A 12 34 56  ->   04 00 A0 21 43 65
   
   

3. Use the table below to bit-reverse each digit. For example:

         04 00 A0 21 43 65  ->   02 00 50 48 2C 6A
   
      Conversion Table
      ----------------
      Original  Converted
      Bit       Bit
      --------  ---------
      0          0
      1          8
      2          4
      3          C
      4          2
      5          A
      6          6
      7          E
      8          1
      9          9
      A          5
      B          D
      C          3
      D          B
      E          7
      F          F
   
   

4. After bit-reversing each digit, combine the six pairs into a 12-digit hexadecimal address. For example:

         020050482C6A