RFC relevance of destination

As per Relevance of the word destination, we have this rfc below:

Network Working Group S.
Request for Comments: 2114 J.
Category: Informational Cisco Systems, Inc
Obsoletes: 2106 H.
Mitsubishi Electric Corp
February 1997

Data Link Switching Client Access

Status of this

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

This memo describes the Data Link Switching Client Access
that is used between workstations and routers to transport SNA
NetBIOS traffic over TCP sessions. Any questions or comments
be sent to dcap@cisco.com

Table of

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

Chiang, et. al. Informational [Page 1]

RFC 2114 DCAP February 1997

1.

Since the Data Link Switching Protocol, RFC 1795, was published,
software vendors have begun implementing DLSw on workstations.
implementation of DLSw on a large number of workstations
several important issues that must be addressed. Scalability is
major concern. For example, the number of TCP sessions to the
router increases in direct proportion to the number of
added. Another concern is efficiency. Since DLSw is a switch-to
switch protocol, it is not efficient when implemented
workstations

DCAP addresses the above issues. It introduces a
structure to resolve the scalability problems. All workstations
clients to the router (server) rather than peers to the router.
creates a client/server model. It also provides a more
protocol between the workstation (client) and the router (server).

2.

2.1. DCAP Client/Server

+-----------+ +-----------+ +---------+
| Mainframe | | IP Router +- ppp -+ DLSw |
+--+--------+ +-----+-----+ | Work |
| | | Station |
| | +---------+
+--+--+ +-------------+ |
| FEP +- TR -+ DLSw Router +-- IP
+-----+ +-------------+ |
|
|
+-----------+ +---------+
| IP Router +- ppp -+ DLSw |
+-----+-----+ | Work |
| Station |
+---------+

| DLSw Session |
+-------------------------------+
Figure 2-1. Running DLSw on a large number of workstations creates
scalability problem

Figure 2-1 shows a typical DLSw implementation on a workstation.
workstations are connected to the central site DLSw router over
IP network. As the network grows, scalability will become an
as the number of TCP sessions increases due to the growing number
workstations

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RFC 2114 DCAP February 1997

+-----------+ +--------+
| Mainframe | | DCAP |
+--+--------+ +-----+ Client |
| | +--------+
|
| |
+--+--+ +--------+ +------+------+
| FEP +- TR -+ DLSw +-- IP Backbone --+ DLSw Router |
+-----+ | Router | | DCAP Server |
+--------+ +------+------+
|

| +--------+
+-----+ DCAP |
| Client |
+--------+

| DLSw Session | | DCAP Session |
+----------------------+ +--------------+
Figure 2-2. DLSw Client Access Protocol solves the
problem

In a large network, DCAP addresses the scalability problem
significantly reducing the number of peers that connect to
central site router. The workstations (DCAP clients) and the
(DCAP server) behave in a Client/Server relationship.
are attached to a DCAP server. A DCAP server has a single
connection to the central site router

2.2. Dynamic Address

In a DLSw network, each workstation needs a MAC address
communicate with a FEP attached to a LAN. When DLSw is implemented
a workstation, it does not always have a MAC address defined.
example, when a workstation connects to a router through a modem
PPP, it only consists of an IP address. In this case, the user
define a virtual MAC address. This is administratively
since each workstation must have an unique MAC address

DCAP uses the Dynamic Address Resolution protocol to solve
problem. The Dynamic Address Resolution protocol permits the
to dynamically assign a MAC address to a client without
configuration

For a client to initiate a session to a server, the workstation
a direct request to the server. The request contains the
MAC address and the destination SAP. The workstation can
specify its own MAC address, or request the server to assign one

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RFC 2114 DCAP February 1997

it. The server's IP address must be pre-configured on
workstation. If IP addresses are configured for multiple servers at
workstation, the request can be sent to these servers and the
one to respond will be used

For a server to initiate a session to a client, the server sends
directed request to the workstation. The workstation must pre
register its MAC address at the server. This can be done either
configuration on the server or registration at the server (both
addresses and IP addresses will be registered).

2.3. TCP

The transport used between the client and the server is TCP. A
session must be established between the client and the server
a frame can be sent. The default parameters associated with the
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
the TCP session with each other at the same time. The TCP session
the initiator with the lower IP address will be used. The other
session will be closed

2.4 Multicast and Unicast (UDP

Multicast and unicast with UDP support are optional. In the reset
this session, when multicast and unicast are referenced, UDP is used
Two multicast addresses are reserved for DCAP. The server
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.
DATA_FRAME can be sent via either multicast or unicast.
CAN_U_REACH frame can be sent via multicast only and the I_CAN_
frame can be sent via unicast only. All other DCAP frames can only
sent via TCP sessions

When the multicast and unicast support is implemented, the
does not have to configure the server's IP address. When the
attempts to establish a session to the host, instead of
a TCP session with the pre-configured server, the client
multicast the CAN_U_REACH frame to the 224.0.1.49 group address.
the server receives this multicast frame, it will locate

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RFC 2114 DCAP February 1997

destination as specified in the frame. If the destination
reachable by this server, it will send back an I_CAN_REACH frame
the sender via unicast. The client can initiate a TCP connection
the server and establish a DCAP session. If the I_CAN_REACH frame
received from multiple servers, the first one who returns
I_CAN_REACH frame will be used

When the host initiates a session to the client, the client does
have to pre-register its MAC address at the server. When the
attempts to reach an unknown client, it will multicast
CAN_U_REACH frame to the 224.0.10.50 group address. The client
MAC address matches the destination address in the CAN_U_REACH
will reply with the I_CAN_REACH frame via unicast. Once the
receives the I_CAN_REACH frame, it can establish a DCAP session
that client

For NetBIOS traffic, NAME_QUERY and ADD_NAME_QUERY can
encapsulated in the DATA_FRAME and sent out via multicast
NAME_RECOGNIZED and ADD_NAME_RESPONSE can be encapsulated in
DATA_FRAME but sent out via unicast. No other NetBIOS frames can
encapsulated in the DATA_FRAME to be sent out via either multicast
unicast

When a client tries to locate a name or check for duplicate name
the network, it can multicast a NAME_QUERY or ADD_NAME_QUERY
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
LAN, they will be encapsulated in the DATA_FRAME and sent to
client via unicast

When a server receives a NetBIOS NAME_QUERY or ADD_NAME_QUERY
LAN, the server will encapsulate it in the DATA_FRAME and send it
all clients via multicast. When a client receives the frame
determines that the name specified in the DATA_FRAME matches its
name, a NAME_RECOGNIZED or ADD_NAME_RESPONSE frame will
encapsulated in the DATA_FRAME and sent back to the server
unicast

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RFC 2114 DCAP February 1997

3. DCAP

3.1. General Frame

The General format of the DCAP frame is as follows

+-------------+-----------+-----------+
| DCAP Header | DCAP Data | User Data |
+-------------+-----------+-----------+
Figure 3-1. DCAP Frame

The DCAP protocol is contained in the DCAP header, which is common
all frames passed between the DCAP client and the server. This
is 4 bytes long. The next section will explain the details

The next part is the DCAP Data. The structure and the size are
on the type of messages carried in the DCAP frame. The DCAP data
used to process the frame, but it is optional

The third part of the frame is the user data, which is sent by
local system to the remote system. The size of this block is
and is included in the frame only when there is data to be sent
the remote system

3.2. Header

The DCAP header is used to identify the message type and the
of the frame. This is a general purpose header used for each
that is passed between the DCAP server and the client.
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
the DCAP data depends on the type of frames, and will be discussed
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

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RFC 2114 DCAP February 1997

o The Protocol ID uses the first 4 bits of this field and is set
"1000".

o The Version Number uses the next 4 bits in this field and is
to "0001".

o The message type is the DCAP message type

o The Total Packet length is the length of the packet including
DCAP header, DCAP data and User Data. The minimum size of
packet is 4, which is the length of the header

3.3. DCAP

Most of the DCAP frames are based on the existing DLSw frames
corresponding frames have similar names. The information in
corresponding DCAP and DLSw frames may differ; but
functionalities are the same. Thus the DLSw State Machine is used
handle these DCAP frames. Some new DCAP frames were created to
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
--------------- ---- --------
CAN_U_REACH 0x01 Find if the station given is
I_CAN_REACH 0x02 Positive response to CAN_U_
I_CANNOT_REACH 0x03 Negative response to CAN_U_
START_DL 0x04 Setup session for given
DL_STARTED 0x05 Session
START_DL_FAILED 0x06 Session Start
XID_FRAME 0x07 XID
CONTACT_STN 0x08 Contact destination to establish
STN_CONTACTED 0x09 Station contacted - SABME mode
DATA_FRAME 0x0A Connectionless Data Frame for a
INFO_FRAME 0x0B Connection oriented I-
HALT_DL 0x0C Halt Data Link
HALT_DL_NOACK 0x0D Halt Data Link session without
DL_HALTED 0x0E Session
FCM_FRAME 0x0F Data Link Session Flow Control
DGRM_FRAME 0x11 Connectionless Datagram Frame for a

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RFC 2114 DCAP February 1997

CAP_XCHANGE 0x12 Capabilities Exchange
CLOSE_PEER_REQUEST 0x13 Disconnect Peer Connection
CLOSE_PEER_RESPONSE 0x14 Disconnect Peer Connection
PEER_TEST_REQ 0x1D Peer keepalive test
PEER_TEST_RSP 0x1E Peer keepalive

Table 3-1. DCAP

3.4. DCAP Data

The DCAP data is used to carry information required for each
frame. This information is used by the Server or the Client and
does not contain any user data. The DCAP data frame types are
in the following sections. Please note that the sender should set
reserved fields to zero and the receiver should ignore these fields

3.4.1. CAN_U_REACH, I_CAN_REACH, and I_CANNOT_REACH

These frame types are used to locate resources in a network.
CAN_U_REACH frame is sent to the server to determine if the
is reachable. When a server receives a CAN_U_REACH frame, it
send out an LLC explorer frame to locate the destination specified
the CAN_U_REACH frame. If the destination is reachable, the
responds to the client with an I_CAN_REACH frame. If the server
not receive a positive acknowledgment within a recommended
value of 5 seconds, the server should send an LLC explorer to
the destination again. If the server does not receive any
after sending out 5 explorers (recommended retry value),
destination is considered not reachable and an I_CANNOT_REACH
is sent back to the client. The client should decide if
CAN_U_REACH is necessary after the I_CANNOT_REACH frame is
from the server

When a server is in the process of searching a destination
receives another I_CAN_REACH with the same destination, the
should not send out another LLC explorer for that destination

The server should not send the CAN_U_REACH frame to the clients in
TCP session. When a server receives an LLC explorer whose
is a known client, the server should respond to it directly

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RFC 2114 DCAP February 1997

+---------------+-----------------------+
| 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

+-----------------------------------+
| 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

The MAC Address field carries the MAC address of the
workstation that is being searched. This is a six-byte MAC
field. The same MAC Address is returned in the I_CAN_REACH and
I_CANNOT_REACH frames

Byte 6 is the source SAP. The destination SAP is set to zero when
explorer frame is sent to the network

3.4.2. START_DL, DL_STARTED, and START_DL_FAILED

These frame types are used by DCAP to establish a link
(circuit). The START_DL frame is sent directly to the server
responds to the CAN_U_REACH frame. When the server receives
frame, it establishes a link station using the source and
addresses and saps provided in the START_DL frame. If the
establishment is successful, a DL_STARTED frame is sent back as
response. If the attempt fails within a recommended value, 5 seconds
the server should retry again. If the server fails to establish

Chiang, et. al. Informational [Page 9]

RFC 2114 DCAP February 1997

circuit for a recommended retry value, 5 times, a START_DL_
frame should be sent back to the client. If the client receives
START_DL_FAILED frame from the server, it is up to the client
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
circuits

+---------------+-----------------------+
| Field Name | Information |
+---------------+-----------------------+
| Message Type | 0x04, 0x05, or 0x06 |
+---------------+-----------------------+
| Packet Length | 0x18 |
+---------------+-----------------------+
Figure 3-5. START_DL, DL_STARTED, and START_DL_FAILED

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RFC 2114 DCAP February 1997

+-----------------------------------+
| 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

The Host MAC address is the address of the target station if
session is initiated from the client, or it is the address of
originating station if the session is initiated from the server

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RFC 2114 DCAP February 1997

The next two fields are the Host and Client SAPs. Each is one
long. The Host SAP is the SAP used by the station with the Host
address. The Client SAP is the SAP used by the client

The Origin Session ID, is the ID of the originating station
initiates the circuit. The originating station uses this ID
identify the newly created circuit. Before the START_DL frame is
to the target station, the originating station sets up a
block for the circuit. This link station information is set
DCAP does not use a three-way handshake for link
establishment. In the DL_STARTED and the START_DL_FAILED frames,
Origin Session ID is returned as received in the START_DL frame.
Target Session ID is set by the target station and returned in
DL_STARTED frame

The Target Session ID is not valid for the START_DL and
START_DL_FAILED frame, and should be treated as Reserved fields.
the DL_STARTED frame, it is the session ID that is used to set
this circuit by the target station

The Largest Frame Size field is used to indicate the maximum
size that can be used by the client. It is valid only when it is
by the server. The Largest Frame Size field must be set to zero
a frame is sent by the client. Both START_DL and DL_STARTED use
Largest Frame Size field and only its rightmost 6 bits are used.
format is defined in the IEEE 802.1D Standard, Annex C, Largest
Bits (LF). Bit 3 to bit 5 are base bits. Bit 0 to bit 2 are
bits. The Largest Frame Size field is not used in the START_DL_
frame and must be set to zero

bit 7 6 5 4 3 2 1 0
r r b b b e e
Figure 3-7. Largest Frame

Please note that if the client is a PU 2.1 node, the client
use the maximum I-frame size negotiated in the XID3 exchange

The Initial window size in the START_DL frame specifies the
window size on the originating side, and the target DCAP
returns its receive window size in the DL_STARTED frame. The field
reserved in the START_DL_FAILED frame. The usage of the window
is the same as the one used in DLSw. Please refer to RFC 1795
details

The last two bits are reserved for future use. They must be set
zero by the sender and ignored by the receiver

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RFC 2114 DCAP February 1997

3.4.3. HALT_DL, HALT_DL_NOACK, and DL_HALTED

These frame types are used by DCAP to disconnect a link station.
HALT_DL frame is sent directly to the remote workstation to
that the sender wishes to disconnect a session. When the
receives this frame, it tears down the session that is
with the Original Session ID and the Target Session ID provided
the HALT_DL frame. The receiver should respond with the DL_
frame. The DL_HALTED frame should use the same Session ID values
the received HALT_DL frame without swapping them. The HALT_DL_
frame is used when the response is not required. The TCP
between the client and server should remain up after
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

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RFC 2114 DCAP February 1997

+-----------------------------------+
| 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

3.4.4. XID_FRAME, CONTACT_STN, STN_CONTACTED, INFO_FRAME, FCM_FRAME
and DGRM_

These frame types are used to carry the end-to-end data or
a circuit. The Destination Session ID is the Session ID created
the START_DL frame or the DL_STARTED frame by the receiver. The
of the flow control flag is the same as the one used in DLSw.
refer to RFC 1795 for details

+---------------+----------------------------+
| Field Name | Information |
+---------------+----------------------------+
| Message Type | Based on Message type |
+---------------+----------------------------+
| Packet Length | 0x0C + length of user data |
+---------------+----------------------------+
Figure 3-10. Generic DCAP

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RFC 2114 DCAP February 1997

+-----------------------------------+
| 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

3.4.5. DATA_

This frame type is used to send connectionless SNA and
Datagram (UI) frames that do not have a link station associated
the source and destination MAC/SAP pair. The difference
DGRM_FRAME and DATA_FRAME is that DGRM_FRAME is used to send
frames received for stations that have a link station opened,
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

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RFC 2114 DCAP February 1997

+-----------------------------------+
| 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

The definition of the first 8 bytes is the same as the START_
frame. The Broadcast Type field indicates the type of
frames in use; Single Route Broadcast, All Route Broadcast,
Directed. The target side will use the same broadcast type. In
case of Directed frame, if the RIF information is known, the
peer can send a directed frame. If not, a Single Route
frame is sent

3.4.6. CAP_XCHANGE

In DCAP, the capability exchange frame is used to exchange
capability information between a client and a server. CAP_
frames are exchanged between a client and a server as soon as the
session is established. The capability exchange must be
before the other frame types can be sent. Once the
exchange is done, CAP_XCHANGE frame should not be used again

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RFC 2114 DCAP February 1997

CAP_XCHANGE frame contains the clients MAC address, if a client
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
the MAC address if it is non zero. The DCAP server also verifies
the MAC address is unique. The server should return a CAP_
response frame with the MAC address supplied by the client if the
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
address in the CAP_XCHANGE command frame

A client should record the new MAC address assigned by the server
return a response with the assigned MAC address. If the client
accept the assigned MAC address, another CAP_XCHANGE command with
MAC address field set to zero should be sent to the server.
server should allocate a new MAC address for this client

During the capability exchange, both the client and the server
send command frames. The process stops when either side sends
CAP_XCHANGE response frame. When the response frame is sent, the
address in the CAP_XCHANGE frame should be the same as the one in
previous received command. The sender of the CAP_XCHANGE
agrees to use the MAC address defined in the previous command

The number of CAP_XCHANGE frames that need to be exchanged
determined by the client and the server independently. When
number of exchange frames has exceeded the pre-defined number set
either the server or the client, the session should be brought down

The flag is used to show the capability of the sender. The
list shows the valid flags

0x01 NetBIOS support. If a client sets this bit on, the server
pass all NetBIOS explorers to this client. If this bit is
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
after the TCP session is down. The cached information will
used for server to connect out. If a client does not
TCP listen mode, the cache will be deleted as soon as the
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
sends the command/response to a client, the server does not
these values

Chiang, et. al. Informational [Page 17]

RFC 2114 DCAP February 1997

Starting with the Reserved field, implementers can
implement the Capability Exchange Control Vector. Each
Exchange Control Vector consists of three fields: Length (1 byte),
Type (1 byte), and Data (Length - 2 bytes). Two types of
Vectors are defined: SAP_LIST and VENDOR_CODE (described below).
ensure compatibility, implementers should ignore the unknown
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
SAP_LIST is not specified in the capability exchange, the
assumes that the client can support all the SAP values.
example, if a client can only support SAP 4 and 8, then
following Control Vectors should be sent: "0x04, 0x01, 0x04,
0x08". The first byte indicates the length of 4. The second
indicates the control vector type of SAP_LIST. The last
bytes indicate the supported SAP values; 0x04 and 0x08.
Control Vector is used only by the client. If the server
this Control Vector, it must return the same Control Vector
the client

0x02 VENDOR_CODE. Length: 3 bytes
Each vendor is assigned a vendor code that identifies
vendor. This Control Vector does not require a response

After the receiver responds to a Control Vector, if the
exchange is not done, the sender does not have to send the
Control Vector again

+---------------+-----------------------+
| Field Name | Information |
+---------------+-----------------------+
| Message Type | 0x12 |
+---------------+-----------------------+
| Packet Length | 0x1C |
+---------------+-----------------------+
Figure 3-14. CAP_XCHANGE

Chiang, et. al. Informational [Page 18]

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

3.4.7. CLOSE_PEER_REQ

This frame is used for peer connection management and contains
reason code field. The following list describes the valid
codes

0x01 System shutdown. This indicates shutdown in progress

0x02 Suspend. This code is used when there is no traffic between
server and the client, and the server or the client wishes
suspend the TCP session. When the TCP session is suspended,
circuits should remain intact. The TCP session should be re
established when new user data needs to be sent. When the
session is re-established, there is no need to send
CAP_XCHANGE frame again

0x03 No MAC address available. This code is sent by the server
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

Chiang, et. al. Informational [Page 19]

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

3.4.8. CLOSE_PEER_RSP, PEER_TEST_REQ, and PEER_TEST_RSP

These three frames are used for peer connection management. There
no data associated with them

o CLOSE_PEER_
CLOSE_PEER_RSP is the response for CLOSE_PEER_REQ

o PEER_TEST_REQ and PEER_TEST_
PEER_TEST_REQ and PEER_TEST_RSP are used for peer level keepalive
Implementing PEER_TEST_REQ is optional, but PEER_TEST_RSP must
implemented to respond to the PEER_TEST_REQ frame. When
PEER_TEST_REQ frame is sent to the remote station, the
expects to receive the PEER_TEST_RSP frame in a predefined
interval (the recommended value is 60 seconds). If
PEER_TEST_RSP frame is not received in the predefined
interval, the sender can send the PEER_TEST_REQ frame again. If
predefined number of PEER_TEST_REQ frames is sent to the
station, but no PEER_TEST_RSP frame is received (the
number is 3), the sender should close the TCP session with
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

4. Protocol Flow

The following diagram shows a normal session start up/tear
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
<-------------
CAP_EXCHANGE (cmd
<-------------
CAP_EXCHANGE (cmd
------------->
CAP_EXCHANGE (rsp
------------->
TEST(P) CAN_U_
<-------- <-------------
TEST(F) I_CAN_
--------> ------------->
START_
<-------------
DL_
------------->
XID(P) XID_
<-------- <-------------
XID(F) XID_
--------> ------------->
XID(P) XID_
<-------- <-------------
SABME CONTACT_
--------> ------------->
UA STN_
<-------- <-------------
I FRAME INFO_
<-------- <-------------
I FRAME INFO_
--------> ------------->
DISC HALT_
<-------- <-------------
UA DL_
--------> ------------->
CLOSE_PEER_
<-------------
CLOSE_PEER_
------------->
TCP session
<-------------

Chiang, et. al. Informational [Page 21]

RFC 2114 DCAP February 1997

5.

The authors wish to express thanks to Rodger Erickson of Wall Data
Inc. for his helpful comments and suggestions

6.

[1] AIW DLSw Related Interest Group, RFC 1795,
"DLSw: Switch-to-Switch Protocol", April 1995

[2] IBM Token Ring Network Architecture
SC30-3374-02, September 1989.

[3] IBM LAN Technical Reference IEEE 802.2 and NETBIOS
Program Interfaces SC30-3587-00, December 1993.

[4] ISO 8802-2/IEEE Std 802.1D International Standard

Authors'

Steve T.
InterWorks Business
Cisco Systems, Inc
170 Tasman
San Jose, CA 95134
Phone: (408) 526-5189
EMail: schiang@cisco.

Joseph S.
InterWorks Business
Cisco Systems, Inc
170 Tasman
San Jose, CA 95134
Phone: (408) 526-5232
EMail: jolee@cisco.

Hideaki
System Product
Network Products
Network Software Products Section
Mitsubishi Electric Corp
Information Systems Engineering
325, Kamimachiya Kamakura Kanagawa 247,
Phone: +81-467-47-2120
EMail: yasuda@eme068.cow.melco.co.

Chiang, et. al. Informational [Page 22]

if you see any problems within the linking, don't worry be happy,
this is version 0.1 of the Relevance System and you gotta expect some crappy subroutines sometimes,
just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.

RFC documents can be found at I.E.T.F.

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