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
1. Introduction
Since the Data Link Switching Protocol, RFC1795, 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
workstations are connected to the central site DLSw router over the
IP network. As the network grows, scalability will become an issue
as the number of TCP sessions increases due to the growing number of
workstations.
+-----------+ +--------+
Mainframe DCAP
+--+--------+ +-----+ Client
+--------+
ppp

+--+--+ +--------+ +------+------+
FEP +- TR -+ DLSw +-- IP Backbone --+ DLSw Router
+-----+ Router DCAP Server
+--------+ +------+------+

ppp
+--------+
+-----+ DCAP
Client
+--------+
DLSw Session DCAP Session
+----------------------+ +--------------+
Figure 2-2. DLSw Client Access Protocol solves the scalability
problem.
In a large network, DCAP addresses the scalability problem by
significantly reducing the number of peers that connect to the
central site router. The workstations (DCAP clients) and the router
(DCAP server) behave in a Client/Server relationship. Workstations
are attached to a DCAP server. A DCAP server has a single peer
connection to the central site router.
2.2. Dynamic Address Resolution
In a DLSw network, each workstation needs a MAC address to
communicate with a FEP attached to a LAN. When DLSw is implemented on
a workstation, it does not always have a MAC address defined. For
example, when a workstation connects to a router through a modem via
PPP, it only consists of an IP address. In this case, the user must
define a virtual MAC address. This is administratively intensive
since each workstation must have an unique MAC address.
DCAP uses the Dynamic Address Resolution protocol to solve this
problem. The Dynamic Address Resolution protocol permits the server
to dynamically assign a MAC address to a client without complex
configuration.
For a client to initiate a session to a server, the workstation sends
a direct request to the server. The request contains the destination
MAC address and the destination SAP. The workstation can either
specify its own MAC address, or request the server to assign one to
it. The server"s IP address must be pre-configured on the
workstation. If IP addresses are configured for multiple servers at a
workstation, the request can be sent to these servers and the first
one to respond will be used.
For a server to initiate a session to a client, the server sends a
directed request to the workstation. The workstation must pre-
register its MAC address at the server. This can be done either by
configuration on the server or registration at the server (both MAC
addresses and IP addresses will be registered).
2.3. TCP Connection
The transport used between the client and the server is TCP. A TCP
session must be established between the client and the server before
a frame can be sent. The default parameters associated with the TCP
connections between the client and the server are as follows:
Socket Family AF_INET (Internet protocols)
Socket Type SOCK_STREAM (stream socket)
Port Number 1973
There is only one TCP connection between the client and the server.
It is used for both read and write operations.
A race condition occurs when both client and server try to establish
the TCP session with each other at the same time. The TCP session of
the initiator with the lower IP address will be used. The other TCP
session will be closed.
2.4 Multicast and Unicast (UDP)
Multicast and unicast with UDP support are optional. In the reset of
this session, when multicast and unicast are referenced, UDP is used.
Two multicast addresses are reserved for DCAP. The server should
listen for 224.0.1.49 and the client should listen for 224.0.1.50.
Not all DCAP frames can be sent via multicast or unicast. The
DATA_FRAME can be sent via either multicast or unicast. The
CAN_U_REACH frame can be sent via multicast only and the I_CAN_REACH
frame can be sent via unicast only. All other DCAP frames can only be
sent via TCP sessions.
When the multicast and unicast support is implemented, the client
does not have to configure the server"s IP address. When the client
attempts to establish a session to the host, instead of establishing
a TCP session with the pre-configured server, the client can
multicast the CAN_U_REACH frame to the 224.0.1.49 group address. When
the server receives this multicast frame, it will locate the
destination as specified in the frame. If the destination is
reachable by this server, it will send back an I_CAN_REACH frame to
the sender via unicast. The client can initiate a TCP connection to
the server and establish a DCAP session. If the I_CAN_REACH frame is
received from multiple servers, the first one who returns the
I_CAN_REACH frame will be used.
When the host initiates a session to the client, the client does not
have to pre-register its MAC address at the server. When the server
attempts to reach an unknown client, it will multicast the
CAN_U_REACH frame to the 224.0.10.50 group address. The client whose
MAC address matches the destination address in the CAN_U_REACH frame
will reply with the I_CAN_REACH frame via unicast. Once the server
receives the I_CAN_REACH frame, it can establish a DCAP session with
that client.
For NetBIOS traffic, NAME_QUERY and ADD_NAME_QUERY can be
encapsulated in the DATA_FRAME and sent out via multicast.
NAME_RECOGNIZED and ADD_NAME_RESPONSE can be encapsulated in the
DATA_FRAME but sent out via unicast. No other NetBIOS frames can be
encapsulated in the DATA_FRAME to be sent out via either multicast or
unicast.
When a client tries to locate a name or check for duplicate name on
the network, it can multicast a NAME_QUERY or ADD_NAME_QUERY frame
encapsulated in the DATA_FRAME. When a server receives these frames,
NetBIOS NAME_QUERY or ADD_NAME_QUERY frames will be forwarded to LAN.
If the NAME_RECOGNIZED or ADD_NAME_RESPONSE frame is received from
LAN, they will be encapsulated in the DATA_FRAME and sent to the
client via unicast.
When a server receives a NetBIOS NAME_QUERY or ADD_NAME_QUERY from
LAN, the server will encapsulate it in the DATA_FRAME and send it to
all clients via multicast. When a client receives the frame and
determines that the name specified in the DATA_FRAME matches its own
name, a NAME_RECOGNIZED or ADD_NAME_RESPONSE frame will be
encapsulated in the DATA_FRAME and sent back to the server via
unicast.
3. DCAP Format
3.1. General Frame Format
The General format of the DCAP frame is as follows:
+-------------+-----------+-----------+
DCAP Header DCAP Data User Data
+-------------+-----------+-----------+
Figure 3-1. DCAP Frame Format
The DCAP protocol is contained in the DCAP header, which is common to
all frames passed between the DCAP client and the server. This header
is 4 bytes long. The next section will eXPlain the details.
The next part is the DCAP Data. The structure and the size are based
on the type of messages carried in the DCAP frame. The DCAP data is
used to process the frame, but it is optional.
The third part of the frame is the user data, which is sent by the
local system to the remote system. The size of this block is variable
and is included in the frame only when there is data to be sent to
the remote system.
3.2. Header Format
The DCAP header is used to identify the message type and the length
of the frame. This is a general purpose header used for each frame
that is passed between the DCAP server and the client. More
information is needed for frames like CAN_U_REACH and I_CAN_REACH,
therefore, it is passed to the peer as DCAP data. The structure of
the DCAP data depends on the type of frames, and will be discussed in
detail in later sections.
The DCAP Header is given below:
+-------------------------------------------+
DCAP Packet Header (Each row is one byte)
+===========================================+
0 Protocol ID / Version Number
+-------------------------------------------+
1 Message Type
+-------------------------------------------+
2 Packet Length
+ - - - - - - - - - - - - - - - - - - - - - +
3
+-------------------------------------------+
Figure 3-2. DCAP Header Format
o The Protocol ID uses the first 4 bits of this field and is set to
"1000".
o The Version Number uses the next 4 bits in this field and is set
to "0001".
o The message type is the DCAP message type.
o The Total Packet length is the length of the packet including the
DCAP header, DCAP data and User Data. The minimum size of the
packet is 4, which is the length of the header.
3.3. DCAP Messages
Most of the DCAP frames are based on the existing DLSw frames and
corresponding frames have similar names. The information in the
corresponding DCAP and DLSw frames may differ; but the
functionalities are the same. Thus the DLSw State Machine is used to
handle these DCAP frames. Some new DCAP frames were created to handle
special DCAP functions. For example, the new DCAP frames,
I_CANNOT_REACH and START_DL_FAILED provide negative acknowledgment.
The DLSw frames not needed for DCAP, are dropped.
The following table lists and describes all available DCAP messages:
DCAP Frame Name Code Function
--------------- ---- --------
CAN_U_REACH 0x01 Find if the station given is reachable
I_CAN_REACH 0x02 Positive response to CAN_U_REACH
I_CANNOT_REACH 0x03 Negative response to CAN_U_REACH
START_DL 0x04 Setup session for given addresses
DL_STARTED 0x05 Session Started
START_DL_FAILED 0x06 Session Start failed
XID_FRAME 0x07 XID Frame
CONTACT_STN 0x08 Contact destination to establish SABME
STN_CONTACTED 0x09 Station contacted - SABME mode set
DATA_FRAME 0x0A Connectionless Data Frame for a link
INFO_FRAME 0x0B Connection oriented I-Frame
HALT_DL 0x0C Halt Data Link session
HALT_DL_NOACK 0x0D Halt Data Link session without ack
DL_HALTED 0x0E Session Halted
FCM_FRAME 0x0F Data Link Session Flow Control Message
DGRM_FRAME 0x11 Connectionless Datagram Frame for a circuit
CAP_XCHANGE 0x12 Capabilities Exchange Message
CLOSE_PEER_REQUEST 0x13 Disconnect Peer Connection Request
CLOSE_PEER_RESPONSE 0x14 Disconnect Peer Connection Response
PEER_TEST_REQ 0x1D Peer keepalive test request
PEER_TEST_RSP 0x1E Peer keepalive response
Table 3-1. DCAP Frames
3.4. DCAP Data formats
The DCAP data is used to carry information required for each DCAP
frame. This information is used by the Server or the Client and it
does not contain any user data. The DCAP data frame types are listed
in the following sections. Please note that the sender should set the
reserved fields to zero and the receiver should ignore these fields.
3.4.1. CAN_U_REACH, I_CAN_REACH, and I_CANNOT_REACH Frames
These frame types are used to locate resources in a network. A
CAN_U_REACH frame is sent to the server to determine if the resource
is reachable. When a server receives a CAN_U_REACH frame, it should
send out an LLC explorer frame to locate the destination specified in
the CAN_U_REACH frame. If the destination is reachable, the server
responds to the client with an I_CAN_REACH frame. If the server does
not receive a positive acknowledgment within a recommended threshold
value of 5 seconds, the server should send an LLC explorer to locate
the destination again. If the server does not receive any response
after sending out 5 explorers (recommended retry value), the
destination is considered not reachable and an I_CANNOT_REACH frame
is sent back to the client. The client should decide if retry
CAN_U_REACH is necessary after the I_CANNOT_REACH frame is received
from the server.
When a server is in the process of searching a destination and
receives another I_CAN_REACH with the same destination, the server
should not send out another LLC explorer for that destination.
The server should not send the CAN_U_REACH frame to the clients in a
TCP session. When a server receives an LLC explorer whose destination
is a known client, the server should respond to it directly.
+---------------+-----------------------+
Field Name Information
+---------------+-----------------------+
Message Type 0x01, 0x02, or 0x03
+---------------+-----------------------+
Packet Length 0x0C
+---------------+-----------------------+
Figure 3-3. CAN_U_REACH, I_CAN_REACH, and I_CANNOT_REACH Header
+-----------------------------------+
Field Name (Each row is one byte)
+===================================+
0 Target MAC Address
+ - - - - - - - - - - - - - - - - - +
1
+ - - - - - - - - - - - - - - - - - +
2
+ - - - - - - - - - - - - - - - - - +
3
+ - - - - - - - - - - - - - - - - - +
4
+ - - - - - - - - - - - - - - - - - +
5
+-----------------------------------+
6 Source SAP
+-----------------------------------+
7 Reserved
+-----------------------------------+
Figure 3-4. CAN_U_REACH, I_CAN_REACH, and I_CANNOT_REACH Data
The MAC Address field carries the MAC address of the target
workstation that is being searched. This is a six-byte MAC Address
field. The same MAC Address is returned in the I_CAN_REACH and the
I_CANNOT_REACH frames.
Byte 6 is the source SAP. The destination SAP is set to zero when an
explorer frame is sent to the network.
3.4.2. START_DL, DL_STARTED, and START_DL_FAILED Frames
These frame types are used by DCAP to establish a link station
(circuit). The START_DL frame is sent directly to the server that
responds to the CAN_U_REACH frame. When the server receives this
frame, it establishes a link station using the source and destination
addresses and saps provided in the START_DL frame. If the circuit
establishment is successful, a DL_STARTED frame is sent back as a
response. If the attempt fails within a recommended value, 5 seconds,
the server should retry again. If the server fails to establish a
circuit for a recommended retry value, 5 times, a START_DL_FAILED
frame should be sent back to the client. If the client receives a
START_DL_FAILED frame from the server, it is up to the client to
decide if a START_DL frame needs to be sent to the server again.
The server can also send START_DL frames to clients to establish
circuits.
+---------------+-----------------------+
Field Name Information
+---------------+-----------------------+
Message Type 0x04, 0x05, or 0x06
+---------------+-----------------------+
Packet Length 0x18
+---------------+-----------------------+
Figure 3-5. START_DL, DL_STARTED, and START_DL_FAILED Header
+-----------------------------------+
Field Name (Each row is one byte)
+===================================+
0 Host MAC Address
+ - - - - - - - - - - - - - - - - - +
1
+ - - - - - - - - - - - - - - - - - +
2
+ - - - - - - - - - - - - - - - - - +
3
+ - - - - - - - - - - - - - - - - - +
4
+ - - - - - - - - - - - - - - - - - +
5
+-----------------------------------+
6 Host SAP
+-----------------------------------+
7 Client SAP
+-----------------------------------+
8 Origin Session ID
+-----------------------------------+
9
+ - - - - - - - - - - - - - - - - - +
10
+ - - - - - - - - - - - - - - - - - +
11
+-----------------------------------+
12 Target Session ID
+ - - - - - - - - - - - - - - - - - +
13
+ - - - - - - - - - - - - - - - - - +
14
+ - - - - - - - - - - - - - - - - - +
15
+-----------------------------------+
16 Largest Frame Size
+-----------------------------------+
17 Initial Window size
+-----------------------------------+
18 Reserved
+ - - - - - - - - - - - - - - - - - +
19
+-----------------------------------+
Figure 3-6. START_DL, DL_STARTED, and START_DL_FAILED Data
The Host MAC address is the address of the target station if the
session is initiated from the client, or it is the address of the
originating station if the session is initiated from the server.
The next two fields are the Host and Client SAPs. Each is one byte
long. The Host SAP is the SAP used by the station with the Host MAC
address. The Client SAP is the SAP used by the client.
The Origin Session ID, is the ID of the originating station that
initiates the circuit. The originating station uses this ID to
identify the newly created circuit. Before the START_DL frame is sent
to the target station, the originating station sets up a control
block for the circuit. This link station information is set because
DCAP does not use a three-way handshake for link station
establishment. In the DL_STARTED and the START_DL_FAILED frames, the
Origin Session ID is returned as received in the START_DL frame. The
Target Session ID is set by the target station and returned in the
DL_STARTED frame.
The Target Session ID is not valid for the START_DL and the
START_DL_FAILED frame, and should be treated as Reserved fields. In
the DL_STARTED frame, it is the session ID that is used to set up
this circuit by the target station.
The Largest Frame Size field is used to indicate the maximum frame
size that can be used by the client. It is valid only when it is set
by the server. The Largest Frame Size field must be set to zero when
a frame is sent by the client. Both START_DL and DL_STARTED use the
Largest Frame Size field and only its rightmost 6 bits are used. The
format is defined in the IEEE 802.1D Standard, Annex C, Largest Frame
Bits (LF). Bit 3 to bit 5 are base bits. Bit 0 to bit 2 are extended
bits. The Largest Frame Size field is not used in the START_DL_FAILED
frame and must be set to zero.
bit 7 6 5 4 3 2 1 0
r r b b b e e e
Figure 3-7. Largest Frame Size
Please note that if the client is a PU 2.1 node, the client should
use the maximum I-frame size negotiated in the XID3 exchange.
The Initial window size in the START_DL frame specifies the receive
window size on the originating side, and the target DCAP station
returns its receive window size in the DL_STARTED frame. The field is
reserved in the START_DL_FAILED frame. The usage of the window size
is the same as the one used in DLSw. Please refer to RFC1795 for
details.
The last two bits are reserved for future use. They must be set to
zero by the sender and ignored by the receiver.
3.4.3. HALT_DL, HALT_DL_NOACK, and DL_HALTED Frames
These frame types are used by DCAP to disconnect a link station. A
HALT_DL frame is sent directly to the remote workstation to indicate
that the sender wishes to disconnect a session. When the receiver
receives this frame, it tears down the session that is associated
with the Original Session ID and the Target Session ID provided in
the HALT_DL frame. The receiver should respond with the DL_HALTED
frame. The DL_HALTED frame should use the same Session ID values as
the received HALT_DL frame without swapping them. The HALT_DL_NOACK
frame is used when the response is not required. The TCP session
between the client and server should remain up after the
HALT_DL/DL_HALTED/ HALT_DL_NOACK exchange.
+---------------+-----------------------+
Field Name Information
+---------------+-----------------------+
Message Type 0x0C, 0x0D, or 0x0E
+---------------+-----------------------+
Packet Length 0x10
+---------------+-----------------------+
Figure 3-8. HALT_DL, HALT_DL_NOACK, and DL_HALTED Header
+-----------------------------------+
Field Name (Each row is one byte)
+===================================+
0 Sender Session ID
+ - - - - - - - - - - - - - - - - - +
1
+ - - - - - - - - - - - - - - - - - +
2
+ - - - - - - - - - - - - - - - - - +
3
+-----------------------------------+
4 Receiver Session ID
+ - - - - - - - - - - - - - - - - - +
5
+ - - - - - - - - - - - - - - - - - +
6
+ - - - - - - - - - - - - - - - - - +
7
+-----------------------------------+
8 Reserved
+ - - - - - - - - - - - - - - - - - +
9
+ - - - - - - - - - - - - - - - - - +
10
+ - - - - - - - - - - - - - - - - - +
11
+-----------------------------------+
Figure 3-9. START_DL, DL_STARTED, and START_DL_FAILED Data
3.4.4. XID_FRAME, CONTACT_STN, STN_CONTACTED, INFO_FRAME, FCM_FRAME,
and DGRM_FRAME
These frame types are used to carry the end-to-end data or establish
a circuit. The Destination Session ID is the Session ID created in
the START_DL frame or the DL_STARTED frame by the receiver. The usage
of the flow control flag is the same as the one used in DLSw. Please
refer to RFC1795 for details.
+---------------+----------------------------+
Field Name Information
+---------------+----------------------------+
Message Type Based on Message type
+---------------+----------------------------+
Packet Length 0x0C + length of user data
+---------------+----------------------------+
Figure 3-10. Generic DCAP Header
+-----------------------------------+
Field Name (Each row is one byte)
+===================================+
0 Destination Session ID
+ - - - - - - - - - - - - - - - - - +
1
+ - - - - - - - - - - - - - - - - - +
2
+ - - - - - - - - - - - - - - - - - +
3
+-----------------------------------+
4 Flow Control Flags
+-----------------------------------+
5 Reserved
+ - - - - - - - - - - - - - - - - - +
6
+ - - - - - - - - - - - - - - - - - +
7
+-----------------------------------+
Figure 3-11. Generic DCAP Data Format
3.4.5. DATA_FRAME
This frame type is used to send connectionless SNA and NetBIOS
Datagram (UI) frames that do not have a link station associated with
the source and destination MAC/SAP pair. The difference between
DGRM_FRAME and DATA_FRAME is that DGRM_FRAME is used to send UI
frames received for stations that have a link station opened, whereas
DATA_FRAME is used for frames with no link station established.
+---------------+-----------------------------+
Field Name Information
+---------------+-----------------------------+
Message Type 0x0A
+---------------+-----------------------------+
Packet Length 0x10 + Length of user data
+---------------+-----------------------------+
Figure 3-12. DATA_FRAME Header
+-----------------------------------+
Field Name (Each row is one byte)
+===================================+
0 Host MAC Address
+ - - - - - - - - - - - - - - - - - +
1
+ - - - - - - - - - - - - - - - - - +
2
+ - - - - - - - - - - - - - - - - - +
3
+ - - - - - - - - - - - - - - - - - +
4
+ - - - - - - - - - - - - - - - - - +
5
+-----------------------------------+
6 Host SAP
+-----------------------------------+
7 Client SAP
+-----------------------------------+
8 Broadcast Type
+-----------------------------------+
9 Reserved
+ - - - - - - - - - - - - - - - - - +
10
+ - - - - - - - - - - - - - - - - - +
11
+-----------------------------------+
Figure 3-13. DATA_FRAME Data Format
The definition of the first 8 bytes is the same as the START_DL
frame. The Broadcast Type field indicates the type of broadcast
frames in use; Single Route Broadcast, All Route Broadcast, or
Directed. The target side will use the same broadcast type. In the
case of Directed frame, if the RIF information is known, the target
peer can send a directed frame. If not, a Single Route Broadcast
frame is sent.
3.4.6. CAP_XCHANGE Frame
In DCAP, the capability exchange frame is used to exchange the
capability information between a client and a server. CAP_XCHANGE
frames are exchanged between a client and a server as soon as the TCP
session is established. The capability exchange must be completed
before the other frame types can be sent. Once the capability
exchange is done, CAP_XCHANGE frame should not be used again.
CAP_XCHANGE frame contains the clients MAC address, if a client has
one. If it does not, then the MAC address field must be set to zero.
When the DCAP server receives the CAP_XCHANGE frame, it should cache
the MAC address if it is non zero. The DCAP server also verifies that
the MAC address is unique. The server should return a CAP_XCHANGE
response frame with the MAC address supplied by the client if the MAC
address is accepted. If a client does not have its own MAC address,
the server should assign a MAC address to the client and put that
address in the CAP_XCHANGE command frame.
A client should record the new MAC address assigned by the server and
return a response with the assigned MAC address. If the client cannot
accept the assigned MAC address, another CAP_XCHANGE command with the
MAC address field set to zero should be sent to the server. The
server should allocate a new MAC address for this client.
During the capability exchange, both the client and the server can
send command frames. The process stops when either side sends a
CAP_XCHANGE response frame. When the response frame is sent, the MAC
address in the CAP_XCHANGE frame should be the same as the one in the
previous received command. The sender of the CAP_XCHANGE response
agrees to use the MAC address defined in the previous command.
The number of CAP_XCHANGE frames that need to be exchanged is
determined by the client and the server independently. When the
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.
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
+-----------------------------------+
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
+-----------------------------------+
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.
+-----------+ +-------+
+-----------+ 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
<-------------
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, RFC1795,
"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