📄 rfc2114.txt
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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
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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
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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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