Q123791: SNA Server Remote Station Discovery Process
Article: Q123791
Product(s): Microsoft SNA Server
Version(s): 2.0,2.1,2.11,2.11 SP1,2.11 SP2,3.0,3.0 SP1,3.0 SP2,3.0 SP3,4.0,4.0 SP1
Operating System(s):
Keyword(s): kbnetwork kbsna211sp1 kbsna211sp2 kbsna300sp1 kbsna300sp2 kbsna300sp3 sna4 kbsna400sp1
Last Modified: 06-FEB-2002
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The information in this article applies to:
- Microsoft SNA Server, versions 2.0, 2.1, 2.11, 2.11 SP1, 2.11 SP2, 3.0, 3.0 SP1, 3.0 SP2, 3.0 SP3, 4.0, 4.0 SP1
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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/Manager 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.
- Delay after failed attempts
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
This specifies the type of frames for DLC to send over an Ethernet local area
network (LAN). The variable need be set only if the LAN is Ethernet and only if
the SNA server communicates with remote host, peer, or downstream systems via
routers (not MAC layer bridges). If the SNA server communicates over Ethernet
via routers, check with the local network support staff to determine what types
of frames the routers pass. The frames can be either standard 802.3 format or
802.2 packets prefixed by DIX headers using EtherType value 0x80D5.
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.
If FrameType is not set it defaults to 0.
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
Additional query words: ntprotocol prodsna
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Keywords : kbnetwork kbsna211sp1 kbsna211sp2 kbsna300sp1 kbsna300sp2 kbsna300sp3 sna4 kbsna400sp1
Technology : kbAudDeveloper kbSNAServSearch kbSNAServ300 kbSNAServ200 kbSNAServ211 kbSNAServ400 kbSNAServ210 kbSNAServ211SP1 kbSNAServ211SP2 kbSNAServ300SP3 kbSNAServ300SP1 kbSNAServ400SP1 kbSNAServ300SP2
Version : :2.0,2.1,2.11,2.11 SP1,2.11 SP2,3.0,3.0 SP1,3.0 SP2,3.0 SP3,4.0,4.0 SP1
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