📄 rfc2106.txt
字号:
Network Working Group S. Chiang
Request for Comments: 2106 J. Lee
Category: Informational Cisco Systems, Inc.
H. Yasuda
Mitsubishi Electric Corp.
February 1997
Data Link Switching Remote 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 Remote 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 drap@cisco.com.
1. Introduction
Since the Data Link Switching Protocol, RFC 1795, 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.
DRAP 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).
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RFC 2106 DLSRAP February 1997
2. Overview
2.1. DRAP 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.
Chiang, et. al. Informational [Page 2]
RFC 2106 DLSRAP February 1997
+-----------+ +-------+
+-----------+ | DLSw/DRAP | | DRAP |
| Mainframe | | Router +- ppp -+ Client|
+--+--------+ +-----+-----+ +-------+
| |
| |
+--+--+ +-------------+ |
| FEP +- TR -+ DLSw Router +-- IP Backbone
+-----+ +-------------+ |
|
|
+-----------+ +-------+
| DLSw/DRAP | | DRAP |
| Router +- ppp -+ Client|
+-----+-----+ +-------+
| DLSw Session | | DRAP Session |
+--------------+ +--------------+
Figure 2-2. DLSw Remote Access Protocol solves the scalability problem.
In a large network, DRAP addresses the scalability problem by
significantly reducing the number of peers that connect to the
central site router. The workstations (DRAP client) and the router
(DRAP server) behave in a Client/Server relationship. Workstations
are attached to a DRAP server. A DRAP 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.
DRAP 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
Chiang, et. al. Informational [Page 3]
RFC 2106 DLSRAP February 1997
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. Before a
TCP session is established between the client and the server, no
message 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.
3. DRAP Format
3.1. General Frame Format
The General format of the DRAP frame is as follows:
+-------------+-----------+-----------+
| DRAP Header | DRAP Data | User Data |
+-------------+-----------+-----------+
Figure 3-1. DRAP Frame Format
The DRAP protocol is contained in the DRAP header, which is common to
all frames passed between the DRAP client and the server. This header
is 4 bytes long. The next section will explain the details.
The next part is the DRAP Data. The structure and the size are based
on the type of messages carried in the DRAP frame. The DRAP 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.
Chiang, et. al. Informational [Page 4]
RFC 2106 DLSRAP February 1997
3.2. Header Format
The DRAP 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 DRAP 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 DRAP data. The structure of
the DRAP data depends on the type of frames, and will be discussed in
detail in later sections.
The DRAP Header is given below:
+-------------------------------------------+
| DRAP Packet Header (Each row is one byte) |
+===========================================+
0 | Protocol ID / Version Number |
+-------------------------------------------+
1 | Message Type |
+-------------------------------------------+
2 | Packet Length |
+ - - - - - - - - - - - - - - - - - - - - - +
3 | |
+-------------------------------------------+
Figure 3-2. DRAP 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 DRAP message type.
o The Total Packet length is the length of the packet including the
DRAP header, DRAP data and User Data. The minimum size of the
packet is 4, which is the length of the header.
3.3. DRAP Messages
Most of the Drap frames are based on the existing DLSw frames and
have the same names. The information in the corresponding DRAP and
DLSw frames may differ; but the functionalities are the same. Thus
the DLSw State Machine is used to handle these DRAP frames. Some new
DRAP frames were created to handle special DRAP functions. For
example, the new DRAP frames, I_CANNOT_REACH and START_DL_FAILED
provide negative acknowledgment. The DLSw frames not needed for DRAP,
are dropped.
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The following table lists and describes all available DRAP messages:
DRAP 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. DRAP Frames
3.4. DRAP Data formats
The DRAP data is used to carry information required for each DRAP
frame. This information is used by the Server or the Client and it
does not contain any user data. The DRAP 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. The server responds with an I_CAN_REACH frame if it can
reach the workstation identified in the CAN_U_REACH frame, or with an
I_CANNOT_REACH if the station is not reachable. The server should not
send the CAN_U_REACH frame to the clients. When a server receives an
explorer whose destination is a known client, the server should
respond to it directly.
Chiang, et. al. Informational [Page 6]
RFC 2106 DLSRAP 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 Header
+-----------------------------------+
| Field Name (Each row is one byte) |
+===================================+
0 | Target MAC Address |
+ - - - - - - - - - - - - - - - - - +
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