As per Relevance of the word connection, we have this rfc below:
Network Working Group R.
Request for Comments: 1434 D.
March 1993
Data Link Switching: Switch-to-Switch
Status of this
This memo provides information for the Internet community. It
not specify an Internet standard. Distribution of this memo
unlimited
This RFC describes IBM's support of Data Link Switching over TCP/IP
The RFC is being distributed to members of the Internet community
order to solicit their reactions to the proposals contained in it
While the issues discussed may not be directly relevant to
research problems of the Internet, they may be interesting to
number of researchers and implementors
Any questions or comments relative to the contents of this RFC
be sent to the following Internet address: dlsw@ralvma.vnet.ibm.com
Table of
1. Introduction 2
2. Overview 2
3. Transport Connection 4
3.1. SSP Frame Formats 5
3.2. Address Parameters 8
3.3. Message Types 10
4. Protocol Specification 11
4.1. Protocol Flow Diagrams 11
4.1.1. Connect Protocols 11
4.1.2. Link Restart Protocols 13
4.1.3. Disconnect Protocols 15
4.2. DLS State Machine 16
4.2.1 Data Link Switch States 16
4.2.2 State Transition Tables 21
4.3. NetBIOS Datagrams 30
Acknowledgments 32
References 32
Security Considerations 32
Authors' Addresses 33
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
1.
Data Link Switching (DLS) is a forwarding mechanism for the IBM
and IBM NetBIOS protocols. It does not provide full routing,
instead provides switching at the Data Link layer and
in TCP/IP for transport over the Internet. This memo documents
Switch-to-Switch Protocol (SSP) that is used between IBM 6611
Processors
Today, the IBM 6611 supports SNA (PU 2 and PU 4) systems and
systems attached to token-ring networks, as well as SNA (PU 2)
systems attached to SDLC links. For the later case, the
attached systems are provided with a LAN appearance within the
6611. For the LAN attached systems, the IBM 6611 appears as
source-routing bridge. Remote systems that are accessed through
IBM 6611 appear as systems attached to an adjacent ring. This
is a virtual ring that is manifested within each IBM 6611.
2.
Data Link Switching was developed to provide support for SNA
NetBIOS in multi-protocol routers. Since SNA and NetBIOS
basically connection oriented protocols, the Data Link
procedure that they use on the LAN is IEEE 802.2 Logical Link
(LLC) Type 2. Data Link Switching also accommodates SNA
over WAN links via the SDLC protocol
IEEE 802.2 LLC Type 2 was designed with the assumption that
network transit delay would be small and predictable (i.e., a
LAN). Therefore the LLC elements of procedure use a fixed timer
detecting lost frames. When bridging is used over wide area
(especially at lower speeds), the network delay is larger and it
vary greatly based upon congestion. When the delay exceeds
time-out value LLC attempts to retransmit. If the frame is
actually lost, only delayed, it is possible for the LLC Type 2
procedures to become confused. And as a result, the link
eventually taken down
Given the use of LLC Type 2 services, Data Link Switching
the following bridging problems
DLC Time-
DLC Acknowledgments over the
Flow and Congestion
Broadcast Control of Search
Source-Route Bridging Hop Count
NetBIOS also makes extensive use of datagram services that use
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
Type 1. In this case, Data Link Switching addresses the last
problems in the above list
The principal difference between Data Link Switching and bridging
that DLS terminates the Data Link Control whereas bridging does not
The following figure illustrates this difference based upon two
systems operating with LLC Type 2 services
-------- Bridge
+------+ +----+ +----+ +------+
| End | +---------+ | +-----/ | | +---------+ | End |
|System+-+ LAN +-+ | /------+ +-+ LAN +-+System
| | +---------+ | | TCP/IP | | +---------+ | |
+------+ +----+ +----+ +------+
Info------------------------------------------------------->
<-------------------------------------------------------
Data Link
-------------------
+------+ +----+ +----+ +------+
| End | +---------+ | +-----/ | | +---------+ | End |
|System+-+ LAN +-+DLS | /------+ DLS+-+ LAN +-+System
| | +---------+ | | TCP/IP | | +---------+ | |
+------+ +----+ +----+ +------+
Info-------------------> ------------->
<-------------------RR ---------------->
<----------------
Figure 1. Data Link Switching Contrasted to
In traditional bridging, the Data Link Control is end-to-end.
Link Switching terminates the LLC Type 2 connection at the switch
This means that the LLC Type 2 connections do not cross the wide
network. The DLS multiplexes LLC connections onto a TCP
to another DLS. Therefore, the LLC connections at each end
totally independent of each other. It is the responsibility of
Data Link Switch to deliver frames that it has received from a
connection to the other end. TCP is used between the Data
Switches to guarantee delivery of frames
As a result of this design, LLC time-outs are limited to the
LAN (i.e., they do not traverse the wide area). Also, the LLC Type 2
acknowledgments (RR's) do not traverse the WAN, thereby
traffic across the wide area links. For SDLC links, polling and
response occurs locally, not over the WAN. Broadcast of
frames is controlled by the Data Link Switches once the location of
target system is discovered. Finally, the switches can now
Dixon & Kushi [Page 3]
RFC 1434 DLS: Switch-to-Switch Protocol March 1993
back pressure to the end systems to provide flow and
control
Data Link Switching uses LAN addressing to set up connections
SNA systems. SDLC attached devices are defined with MAC addresses
enable them to communicate with LAN attached devices. For
systems, Data Link Switching uses the NetBIOS name to
datagrams and to set up connections for NetBIOS sessions.
circuit establishment, SNA systems send TEST (or in some cases, XID
frames to the null (x'00') SAP. NetBIOS systems have an
resolution procedure, based upon the Name Query and Name
frames, that is used to establish an end-to-end circuit
Since Data Link Switching may be implemented in multi-
routers, there may be situations where both bridging and
are enabled. SNA frames can be identified by their link SAP
Typical SAP values for SNA are x'04', x'08', and x'0C'.
always uses a link SAP value of x'F0'.
3. Transport
Data Link Switches can be in used in pairs or by themselves.
Single DLS internally switches one data link to another without
TCP (DLC(1) to DLC(2) in the figure below). A paired DLS
data links over a reliable transport using a Switch-to-
Protocol (SSP). This RFC will document the frame formats
protocols for this multiplexing between Data Link Switches.
initial implementation of SSP uses TCP as the reliable
between Data Link Switches. However, other transport
such as OSI TP4 could be used
+-----------------------------------------------+Switch-to-
| DLC Interfaces | Protocol (SSP
|+------------+ DLC Request +------------+ |
|| Data |<---------------- | | |Send SSP
|| Link | DLC Indication | | |-------------->
|| Control 1 |----------------->| | |
|+------------+ | Data Link | |
|+------------+ DLC Request | Switch | |
|| Data |<---------------- | | |Rec. SSP
|| Link | DLC Indication | | |<-------------
|| Control 2 | ---------------->| | |
|+------------+ +------------+ |
| Multi-Protocol Router |
+-----------------------------------------------+
Figure 2. DLS System
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
Before Data Link Switching can occur between two routers, they
establish a TCP connection between them. Each DLS will maintain
list of DLS capable routers and their status (active/inactive).
this connection is established, the DLS will employ SSP to
end-to-end circuits over the transport connection. Within
transport connection is a specific set of DLS message units.
message formats and types for these PDUs are documented in
following sections
The default parameters associated with the TCP connections
Data Link Switches are as follows
Socket Family AF_INET (Internet protocols
Socket Type SOCK_STREAM (stream socket
Read Port Number 2065
Write Port Number 2067
Two or more Data Link Switches may be attached to the same LAN
consisting of a number of token-ring segments interconnected
source-routing bridges. In this case, a TCP connection is
defined between bridges attached to the same LAN. This will
using systems to select one of the possible Data Link Switches in
similar manner to the selection of a bridge path through a source
routed bridged network. The virtual ring segment in each Data
Switch attached to a common LAN must be configured with the same
number. This will prevent LAN frames sent by one Data Link
from being propagated through the other Data Link Switches
3.1. SSP Frame
The following diagrams show the two message headers for
between Data Link Switches. The control message header is used
all messages except information messages. The information
header is 16 bytes long, and the control message header is 72
long. The first sixteen bytes of the control message header
identical to the information message header
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
CONTROL MESSAGES (72 Bytes
+-----------------------------------------------------------------+
| Version Number Reserved Field |
| Message Length ----> . |
| Remote Data Link Correlator ----> . |
| . . |
| Remote DLC Port ID ----> . |
| . . |
| Reserved Field ----> . |
| Message Type Reserved Field |
| Protocol ID Header Number |
| Header Length ----> . |
| Reserved Field ----> . |
| Reserved Field Message Type |
| Target MAC Address ----> . |
| . . |
| . . |
| Origin MAC Address ----> . |
| . . |
| . . |
| Origin Link SAP Target Link SAP |
| Frame Direction Reserved Field |
| Message Length ----> . |
| DLC Header Length ----> . |
| Origin DLC Port ID ----> . |
| . . |
| Origin Data Link Correlator ----> . |
| . . |
| Origin Transport ID ----> . |
| . . |
| Target DLC Port ID ----> . |
| . . |
| Target Data Link Correlator ----> . |
| . . |
| Target Transport ID ----> . |
| . . |
| Reserved Field ----> . |
| . . |
+-----------------------------------------------------------------+
(Even Byte) (Odd Byte
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
INFORMATION MESSAGE (16 Bytes
+-----------------------------------------------------------------+
| Version Reserved Field |
| Message Length ----> . |
| Remote Data Link Correlator ----> . |
| . . |
| Remote DLC Port ID ----> . |
| . . |
| Reserved Field ----> . |
| Message Type Reserved Field |
+-----------------------------------------------------------------+
(Even Byte) (Odd Byte
The Version Number is set to x'4B', indicating a numeric value of 75.
The Header Length is x'00 48', indicating a numeric value of 72
bytes
The Header Number is x'01', indicating a value of one
The Frame Direction field is set to x'01' for frames sent from
origin DLS to the target DLS, and is set to x'02' for frames
from the target DLS to the origin DLS
Note: The Remote Data Link Correlator and Remote DLC Port ID
set equal to the Target Data Link Correlator and Target DLC
ID if the Frame Direction field is set to x'01', and are set
to the Origin Data Link Correlator and Origin DLC Port ID if
Direction Field is set to x'02'.
The Protocol ID field is set to x'42', indicating a numeric value
66.
The Message Length field defines the number of bytes within the
field following the header. Note that this value is specified in
different fields of the message header
The DLC Header Length is set to zero for SNA and is set to x'23'
NetBIOS datagrams, indicating a length of 35 bytes. This
the Access Control (AC) field, the Frame Control (FC) field
Destination MAC Address (DA), the Source MAC Address (SA),
Routing Information (RI) field (padded to 18 bytes), the
link SAP (DSAP), the Source link SAP (SSAP), and the LLC
field (UI).
The values for the Message Type field are defined in a later section
Note that this value is specified in two different fields of
message header
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
Reserved fields are set to zero upon transmission and should
ignored upon receipt
3.2. Address
A data link is defined as a logical association between the two
stations using Data Link Switching. It is identified by a Data
ID (14 bytes) consisting of the pair of attachment
associated with each end system. Each attachment address
represented by the concatenation of the MAC address (6 bytes) and
LLC address (1 byte).
DATA LINK ID (14 Bytes
+-----------------------------------------------------------------+
|Target MAC Address ----> . |
| . . |
| . . |
|Origin MAC Address ----> . |
| . . |
| . . |
|Origin Link SAP Target Link SAP |
+-----------------------------------------------------------------+
An end-to-end circuit is identified by a pair of Circuit ID's.
Circuit ID is a 64 bit number that identifies the DLC circuit
a single DLS. It consists of a DLC Port ID (4 bytes), and a
Link Correlator (4 bytes). This value is unique in a single DLS
is assigned locally. The pair of Circuit ID's along with
identifiers of the Data Link Switches, uniquely identify a
end-to-end circuit. Each DLS must keep a table of these Circuit
pairs, one for the local end of the circuit and the other for
remote end of the circuit. In order to identify which Data
Switch originated the establishment of a circuit, the terms,
DLS and target DLS, will be employed in this document
CIRCUIT ID (8 Bytes
+-----------------------------------------------------------------+
|DLC Port ID ----> . |
| . . |
|Data Link Correlator ----> . |
| . . |
+-----------------------------------------------------------------+
The Origin Transport ID and the Target Transport ID fields in
message header are used to identify the individual TCP/IP port on
Data Link Switch. The values have only local significance. However
each Data Link Switch is required to reflect the values contained
these two fields, along with the associated values for DLC Port
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
and the Data Link Correlator, when returning a message to the
Data Link Switch
The following figure shows the use of the addressing
during the establishment of an end-to-end connection. The CANUREACH
ICANREACH, and REACH_ACK messages all carry the Data Link ID
consisting of the MAC and Link SAP addresses associated with the
end stations. Upon receipt of a CANUREACH message, the target
starts a data link for each port, thereby obtaining a Data
Correlator. If the target station can be reached, an
message is returned to the origin DLS containing the Target
ID parameter. Upon receipt, the origin DLS starts a data link
returns the Origin Circuit ID to the target DLS within the REACH_
message. If the REACH_ACK message is not successfully received,
target Data Link Switch can obtain the Origin Circuit ID from
subsequent message (i.e., CONTACT, XIDFRAME, or DGRMFRAME).
+------------+ +------------+
|Disconnected| |Disconnected
+------------+ CANUREACH (Data Link ID) +------------+
------------------------------------------------->
ICANREACH (Data Link ID, Target Circuit ID
<------------------------------------------------
REACH_ACK (Data Link ID, Origin Cir ID, Target Cir ID
------------------------------------------------->
+------------+ +------------+
|Circuit Est.| |Circuit Est.|
+------------+ +------------+
XIDFRAME (Data Link ID, Origin Cir ID, Target Cir ID
<------------------------------------------------>
CONTACT (Data Link ID, Origin Cir ID, Target Cir ID
------------------------------------------------->
CONTACTED (Data Link ID, Origin Cir ID, Target Cir ID
<-------------------------------------------------
+------------+ +------------+
| Connected | | Connected |
+------------+ +------------+
INFOFRAME (Remote Circuit ID = Target Circuit ID
------------------------------------------------->
INFOFRAME (Remote Circuit ID = Origin Circuit ID
<-------------------------------------------------
Figure 3. DLS Circuits and
During the exchange of the XIDFRAME, CONTACT, and CONTACTED messages
the pair of Circuit ID parameters is included in the message
along with the DATA LINK ID parameter. Once the connection has
established, the INFOFRAME messages are exchanged with the
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
header. This header contains only the Circuit ID associated with
remote DLS. The Remote Data Link Correlator and the Remote DLC
ID are set equal to the Data Link Correlator and the DLC Port ID
are associated with the origin or target Data Link Switch,
upon the direction of the packet
3.3. Message
The following table lists the protocol data units that are
between Data Link Switches. All values not listed are reserved
potential use in follow-on releases
Command Function Hex
------- -------- ---------
CANUREACH Can U Reach Station x'03'
ICANREACH I Can Reach Station x'04'
REACH_ACK Reach Acknowledgment x'05'
DGRMFRAME Datagram Frame (See note) x'06'
XIDFRAME XID Frame x'07'
CONTACT Contact Remote Station x'08'
CONTACTED Remote Station Contacted x'09'
RESTART_DL Restart Data Link x'10'
DL_RESTARTED Data Link Restarted x'11'
INFOFRAME Information (I) Frame x'0A
HALT_DL Halt Data Link x'0E
DL_HALTED Data Link Halted x'0F
NETBIOS_NQ NetBIOS Name Query x'12'
NETBIOS_NR NetBIOS Name Recognized x'13'
DATAFRAME Data Frame (See note) x'14'
NETBIOS_ANQ NetBIOS Add Name Query x'1A
NETBIOS_ANR NetBIOS Add Name Response x'1B
Table 1. SSP Message
Note: Both the DGRMFRAME and DATAFRAME messages are used to
information received by the DLC entity within UI frames. As
be explained below, the DGRMFRAME message is addressed
to a pair of Circuit IDs, while the DATAFRAME message is
according to a Data Link ID, being composed of a pair of
addresses and a pair of link SAP addresses. The latter
employed prior to the establishment of an end-to-end circuit
Circuit IDs have yet to be established
For the exchange of NetBIOS control messages, the entire DLC
is carried as part of the message unit. This includes the
header, with the routing information field padded to 18 bytes,
the LLC header. The following message types are affected
NETBIOS_NQ, NETBIOS_NR, NETBIOS_ANQ, NETBIOS_ANR, and DATAFRAME
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
being used by NetBIOS systems. The routing information in the
header is not used by the remote Data Link Switch upon receiving
above five messages
4. Protocol
This section provides a description of the Switch-to-
Protocols. Included is a set of high-level protocol flows and
detail set of state transition tables. The states and the
are described in terms that are intended to be generic to
platforms. Emphasis of the technical details is to
operability of the IBM 6611 with another vendor's implementation
Notes are inserted at points where the IBM 6611 performs
actions that are specific to the AIX platform upon which it operates
4.1. Protocol Flow
The switch-to-switch protocols are used to setup and take
circuits between a pair of Data Link Switches. Once a circuit
established, the end stations on the local networks can employ
Type 1 (connectionless) protocols. In addition, the end systems
establish an end-to-end connection for support of LLC Type 2
(connection oriented) protocols
The term, Data Link, is used in this document to refer to both
"logical data link" when supporting Type 1 LLC services, and a "
link connection" when supporting Type 2 LLC services. In both cases
the Data Link in defined by the concatenation of the destination
address (DA), the source MAC address (SA), the destination link
(DSAP) and source link SAP (SSAP).
4.1.1. Connect
The following figure depicts the protocol flows that are used for
establishment of a circuit between a pair of Data Link Switches
followed by the establishment of a connection between the pair of
systems. The figure is drawn assuming that the two end systems
SNA (the protocol flow for NetBIOS systems is described in a
paragraph).
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
Data Link Data Link Data Link Data
Control Switch Switch
-------------------- --------------------
+------------+ +------------+
|Disconnected| |Disconnected
+------------+ +------------+
Test Command CANUREACH Test Comd
----------> ---------------------------------------> ------->
(DSAP=Null) (DSAP=SSAP
Test
ICANREACH <---------
Test Response <---------------------------------------
<---------- REACH
--------------------------------------->
+------------+ +------------+
|Circuit Est.| |Circuit Est.|
+------------+ +------------+
SABME
----------> --------------------------------------->
UA ------->
<----------
RNR
<---------- CONTACTED <-------
<---------------------------------------
+------------+ +------------+
| Connected | | Connected |
+------------+ +------------+
<---------
Figure 4. DLS Connect Message
Upon receipt of a Test command from the origin station, the
DLS will send a CANUREACH (i.e., can you reach) message to the
DLS. If the target DLS is not known to the origin DLS, the
message is sent to all remote Data Link Switches defined to
origin DLS. The receipt of the CANUREACH message causes the
DLS to send a Test command searching for the target station.
target station will return a Test response, causing the target DLS
return an ICANREACH (i.e., I can reach) message to the origin DLS
If multiple Data Link Switches can reach the target station,
origin DLS will receive multiple ICANREACH messages. The origin
will select the first message and send a REACH_ACK (i.e.,
acknowledgment) message to the selected Data Link Switch.
this exchange of messages, both Data Link Switches change states
the Disconected state to the Circuit Established state. Once
circuit is established, Type-1 frames, such as XID, may be
between the origin and target stations
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
To establish a connection, the origin station sends a SABME command
Upon receipt of this command, the origin DLS will send a
message to the target DLS and return a UA response to the
station. To inhibit traffic flow until the connection is
to the remote station, a RNR supervisory frame is sent to the
station. The CONTACT message will cause the target DLS to send
SABME command to the target station, which in return will reply
a UA response. Upon receipt of the UA response, the target DLS
send a CONTACTED message to the origin DLS. The origin DLS will
send an RR supervisory frame to the origin station. During
exchange of messages, both Data Link Switches change states from
Circuit Established state to the Connected state
For NetBIOS end systems, the protocol flows are similar but
different frames and SSP messages. Instead of using a Test
frame to initiate the circuit, a NetBIOS system will use a Name
frame. Receipt of a Name Query frame will cause the Data Link
to issue a NETBIOS_NQ message instead of the CANUREACH message. In
like fashion, the Test response is replaced with a Name
frame and the ICANREACH message is replaced with a NETBIOS_
message. As with the SNA protocol flows, the receipt of a NETBIOS_
message causes the origin Data Link Switch to respond with
REACH_ACK message
4.1.2. Link Restart
The following figure depicts the protocol flows that result
restarting the end-to-end connection. This causes the Data
Switches to terminate the existing connection and to enter
Circuit Established state awaiting the start of a new connection
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
Data Link Data Link Data Link Data
Control Switch Switch
--------------------- ---------------------
+-----------+ +-----------+
| Connected | | Connected |
SABME +-----------+ +-----------+
-----------> RESTART_
DM ------------------------------------->
<----------- -------->
DL_RESTARTED (Case 1) <--------
<-------------------------------------
+-----------+ +-----------+
|Circuit Est| |Circuit Est
+-----------+ +-----------+
........... or ...........
-----------> DL_RESTARTED (Case 2)
UA <-------------------------------------
<----------- +-----------+
|Circuit Est
CONTACT +-----------+
RNR ------------------------------------>
<----------
Figure 5. DLS Link Restart Message
Upon receipt of a SABME command from the origin station, the
DLS will send a RESTART_DL message to the target DLS. A DM
is also returned to the origin station and the data link
restarted
Upon receipt of the RESTART_DL message, the target DLS will issue
DISC command to the target station. The target station is
to return a UA response. The target DLS will then restart its
link and send an DL_RESTARTED message back to the origin DLS.
this exchange of messages, both Data Link Switches change states
Connected state to Circuit Established state
If the origin station now resends the SABME command, the origin
will send a CONTACT message to the target DLS. If the SABME
is received prior to the receipt of the DL_RESTARTED message (case 2
in the figure), the CONNECT message is delayed until the DL_
message is received. The resulting protocol flows at this
parallel those given above for the connect sequence
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
4.1.3. Disconnect
The following figure depicts the protocol flows that result from
end system terminating an existing connection. Not only is
connection terminated, but the circuit between the Data Link
is taken down
Data Link Data Link Data Link Data
Control Switch Switch
-------------------- --------------------
+-----------+ +-----------+
| Connected | | Connected |
+-----------+ +-----------+
----------> HALT_
UA ------------------------------------->
<---------- --------->
DL_HALTED <--------
<-------------------------------------
+-----------+ +-----------+
|Disconnectd| |Disconnectd
+-----------+ +-----------+
......... or ..........
+-----------+ +-----------+
| Connected | | Connected |
+-----------+ +-----------+
DISC TCP Connection Failure
<-------- <------------------------------------> --------->
UA
--------> <--------
+-----------+ +-----------+
|Disconnectd| |Disconnectd
+-----------+ +-----------+
Figure 6. DLS Disconnect Message
Upon receipt of a DISC command from the origin station, the
DLS will reply with a UA response and issue a HALT_DL message to
target DLS. Upon receipt of the HALT_DL message, the target DLS
send a DISC command to the target station. The target station
then respond with a UA response, causing the target DLS to return
DL_HALTED message to the origin DLS. During this exchange
messages, both Data Link Switches change states from the
state to the Disconnected state
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
If the TCP connection between two Data Link Switches fails,
connections that are currently multiplexed on the failed
connection will be taken down. This implies that both Data
Switches will send DISC commands to all the local systems that
associated with the failed connections. Upon sending the
command, the Data Link Switch will enter the DISCONNECT state
each circuit
4.2. DLS State
The following state tables describe the states for a
connection through the Data Link Switch. State information is
for each connection. The initial state for a connection
DISCONNECT. The steady state is either CIRCUIT_ESTABLISHED
CONNECTED. In the former state, an end-to-end circuit has
established allowing the support of Type 1 LLC between the
systems. The latter state exists when an end-to-end connection
been established for the support of Type 2 LLC services between
end systems
For SNA, circuit establishment is via the use of IEEE 802.2 Test
XID frames. SNA devices send these frames to the null SAP in
to determine the source route information in support of bridging
Normally SNA devices use SAP x'04', x'08', or x'0C'. Typically
SAP would be used to determine if the Test frames should be sent
the DLS code in the router. If both bridging and DLS are enabled
this allows the product to ensure that SNA frames are not
bridged and switched
For NetBIOS, circuit establishment is via the Name Query and
Recognized frames. These frames are used for both address
and source route determination. Normally NetBIOS devices use
x'F0'.
4.2.1. Data Link Switch
The Switch-to-Switch Protocols will be formally defined through
single state machine. The following table lists the eleven
states. A separate state machine is employed for each end-to-
circuit that is maintained by the Data Link Switch. The three
states are DISCONNECTED, CIRCUIT_ESTABLISHED, and CONNECTED
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
State Name
---------- -----------
CIRCUIT_ESTABLISHED The end-to-end circuit has
established. At this time LLC Type 1
services are available from end-to-end
CIRCUIT_PENDING The target DLS is awaiting a REACH_
response to an ICANREACH message
CIRCUIT_RESTART The DLS that originated the reset
awaiting the restart of the data
and the DL_RESTARTED response to
RESTART_DL message
CONNECTED The end-to-end connection has
established thereby allowing LLC Type 2
services from end-to-end in addition
LLC Type 1 services
CONNECT_PENDING The origin DLS is awaiting the
response to a CONTACT message
CONTACT_PENDING The target DLS is awaiting
DLC_CONTACTED confirmation to a DLC_
signal (i.e., DLC is waiting for a
response to an SABME command).
DISCONNECTED The initial state with no circuit
connection established, the DLS is
either a CANUREACH, an ICANREACH,
NETBIOS_NQ, or a NETBIOS_NR message
DISCONNECT_PENDING The DLS that originated the
is awaiting the DL_HALTED response to
HALT_DL message
HALT_PENDING The remote DLS is awaiting the DLC_DL_
indication following the DLC_HALT_DL
(i.e., DLC is waiting for a UA response to
DISC command).
RESTART_PENDING The remote DLS is awaiting the DLC_DL_
indication following the DLC_HALT_DL
(i.e., DLC is waiting for a UA response to
DISC command), and the restart of the
link
RESOLVE_PENDING The target DLS is awaiting either
DLC_DL_STARTED indication following
DLC_START_DL reqest (i.e., DLC is
for a Test response as a result of sending
Test command), or a NB_Name_
frame in response to a NB_Name_Query frame
Table 2. Data Link Switch
The following figure depicts the events that cause a transition to
new state. The resulting action(s) are not explicitly shown.
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
DISCONNECT_PENDING state will be entered whenever a DLC
condition occurs in any of the other states (except RESOLVE_PENDING),
or when a DISC command is received by the DLC
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
DLC_RESOLVE_C+----+ +----DLC_ERROR---+
| | \|/ |
+------------+ +>+--+---------+ CANUREACH +-+----------+
| Disconnect +--DL_HALTED-->|Disconnected|------------>| Resolve |
| Pending |<+ +------->| |<-------+ | Pending |
+---------+--+ | | +-----+------+ | +-----+------+
/|\ | HALT_DL| /|\ |/|\/|\ | |
DLC_ERROR +----+ | +----+ | | +------+ | DLC_DL_
| | | |HALT_DL(ts)| +------+ |
| | ICANREACH| | HALT_DL(ts)|
+--HALT_DL(ts)+ | | | | | |
| HALT_DL(ts) | | HALT_DL(ts) | |
| | | | | | |
| +-RESTART_DL(ts)+----+ | | | | |
| | | \|/ \|/| | | \|/
+----+---+---+ | +-------+----+ | +-+----------+
| Contact |<-CONTACT--+--+ Circuit |<-REACH_ACK--+ Circuit |
| Pending | | | Established| (DLC not | Pending |
+-----+------+ | +----------+-+ contacted)+-+------+---+
| /|\ | /|\ /|\ | | | |
| | +--+----+ | | | | |
| +-CONTACT--+--+--------+----+------+----------+ REACH_
| | |RESTART_DL(ts) | (DLC contacted
DLC_CONTACTED | +------+ | | +--------------+ |
| | | | | DLC_CONTACTED | |
+---------------+-------+ | | +-----------------+ | |
+-----DL_RESTARTED | | | | | |
| (DLC not contacted) \|/| | \|/ | \|/
+-----+------+ +---+-+------+ CONTACTED +-----+------+
| Circuit |<-DLC_RESET---+ Connected |<------------+ Connect |
| Restart | | | | Pending |
+----------+-+ +------------+ +-----+------+
/|\ | DL_RESTARTED /|\ |
| +----------------(DLC contacted)--------------+ |
| |
+-------------------------DLC_RESET--------------------+
Figure 7. DLS State Transitions (ts = transitional state
The DISCONNECT state is the initial state for a new circuit. One
station starts the connection via a TEST or XID command (i.e.,
DLC_RESOLVE_C) or a NetBIOS Name Query command (not
shown). Upon receipt, the Data Link Switches exchange a set
CANUREACH, ICANREACH and REACH_ACK messages. Upon completion of
three-legged exchange, both Data Link Switches will be in
CIRCUIT_ESTABLISHED state. Two pending states also exist during
exchange. The RESOLVE_PENDING state is entered by the target
Link Switch awaiting a Test response to a Test Command and
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
CIRCUIT_PENDING state is entered by the target DLS awaiting
REACH_ACK reply to an ICANREACH message
The CIRCUIT_ESTABLISHED state allows for the exchange of LLC Type 1
frames such as the XID exchanges between SNA stations that
prior to the establishment of a connection. Also, datagram
(i.e., UI frames) may be sent and received between the end stations
These exchanges use the XIDFRAME and DGRMFRAME messages sent
the Data Link Switches
In the CIRCUIT_ESTABLISHED state, the receipt of a SABME
(i.e., DLC_CONTACTED) causes the origin DLS to issue a
message, to send an RNR supervisory frame (i.e., DLC_ENTER_BUSY)
the origin station, and to enter the CONNECT_PENDING state awaiting
CONTACTED message. The target DLS, upon the receipt of a
message, will issue a SABME command (i.e., DLC_CONTACT) and enter
Contact Pending state. Once the UA response is received (i.e.,
DLC_CONTACTED), the target DLS sends a CONTACTED message and
the CONNECTED state. When received, the origin DLS enters
Connected state and sends an RR supervisory frame (i.e.,
DLC_EXIT_BUSY).
The CONNECTED state is the steady state for normal data flow once
connection has been established. Information frames (i.e.,
messages) are simply sent back and forth between the end points
the connection. This is the path that should be optimized
performance
The connection is terminated upon the receipt of a DISC frame
under some other error condition detected by DLC (i.e., DLC_ERROR).
Upon receipt of this indication, the DLS will halt the local
link, send a HALT_DL message to the remote DLS, and enter
DISCONNECT_PENDING State. When the HALT_DL frame is received by
other DLS, the local DLC is halted for this data link, a DL_
message is returned, and the DISCONNECTED state is entered.
of this DL_HALTED message causes the other DLS to also enter
DISCONNECTED state
The CIRCUIT_RESTART state is entered if one of the Data Link
receives a SABME command (i.e., DLC_RESET) while in the
state. This causes a DM command to be returned to the origin
and a RESTART_DL message to be sent to the remote Data Link Switch
This causes the remote data link to be halted and then restarted
The remote DLS will then send a DL_RESTARTED message back to
first DLS. The receipt of the DL_RESTARTED message causes the
DLS to issue a new CONTACT message, assuming that the local DLC
been contacted (i.e., the origin station has resent the
command). This is eventually responded to by a CONTACTED message
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
Following this exchange, both Data Link Switches will return to
CONNECTED state. If the local DLC has not been contacted,
receipt of a DL_RESTARTED command causes the Data Link Switch
enter the CIRCUIT_ESTABLISHED state awaiting the receipt of a
command (i.e., DLC_CONTACTED signal).
In the figure, the transition labeled HALT_DL(ts) represents
HALT_PENDING state and the transition labeled RESTART_DL(ts
represents the RESTART_PENDING state. Both these transitional
have only one input event and only one output action. While in
states, the Data Link Switch is awaiting responses from the
station on the adjacent LAN (i.e., a UA response to a DISC command).
Also in the RESTART_PENDING state, the Data Link Switch will
to restart the data link prior to sending a DL_RESTARTED message
For the IBM 6611 implementation, the start of a data link
the exchange of a Test command/response on the adjacent LAN (i.e.,
DLC_START_DL). For other implementations, this additional
may not be required
4.2.2. State Transition
This section provides a detail representation of the Data
Switch, as document by a set of state machines. The first
machine documents the sending and receiving of SSP messages. Many
the transitions are dependent upon local signals between the
Link Switch entity and one of the DLC entities. These signals
their definitions are given in the following tables
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
Event Name
---------- -----------
DLC_CONTACTED Contact Indication: DLC has received an
command and will send a UA response, or DLC
received a UA response as a result of sending
SABME command
DLC_DGRM Datagram Indication: DLC has received a UI frame
DLC_ERROR Error condition indicated by DLC: Such
condition occurs when a DISC command is
or when DLC experiences an unrecoverable error
DLC_INFO Information Indication: DLC has received
Information (I) frame
DLC_DL_HALTED Data Link Halted Indication: DLC
received a UA response to a DISC command
DLC_DL_STARTED Data Link Started Indication: DLC
received a Test response from the null SAP
DLC_RESET Reset Indication: DLC has received an
command during the time a connection
currently active and has responded with DM
DLC_RESOLVE_C Resolve Command Indication: DLC has
a Test command addressed to the null SAP, or
XID command addressed to the null SAP
DLC_XID XID Indication: DLC has received an XID
or response to a non-null SAP
Table 3. Local DLC
Action Name
----------- -----------
DLC_CONTACT Contact Station Request: DLC will send a
command
DLC_DGRM Datagram Request: DLC will send a UI frame
DLC_ENTER_BUSY Enter Link Station Busy: DLC will send
RNR supervisory frame
DLC_EXIT_BUSY Exit Link Station Busy: DLC will send an
supervisory frame
DLC_HALT_DL Halt Data Link Request: DLC will send a
command
DLC_INFO Information Request: DLC will send an I frame
DLC_RESOLVE_R Resolve Response Request: DLC will send
Test response or XID response from the null SAP
DLC_START_DL Start Data Link Request: DLC will send a
command to the null SAP
DLC_XID XID Request: DLC will send an XID command or
XID response
Table 4. Local DLC
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
The Data Link Switch may be described by a state transition
consisting of eleven states. Each of these states is described
in terms of the events, actions, and next state for each transition
If a particular event is not listed for a given state, no action
no state transition should occur for that event. Any
comments concerning the transitions within a given state are
immediately following the table representing the state
A separate state machine is maintained by the Data Link Switch
each end-to-end circuit. The number of circuits that may
supported by each Data Link Switch is a local implementation option
4.2.2.1 DISCONNECTED
Event Action(s) Next
----- --------- ----------
Receive CANUREACH DLC_START_DL RESOLVE_
Receive ICANREACH Send REACH_ACK, CIRCUIT_
DLC_RESOLVE_
(See note 1)
Receive DATAFRAME DLC_
Receive NETBIOS_NQ DLC_DGRM RESOLVE_
Receive NETBIOS_NR Send REACH_ACK, CIRCUIT_
DLC_
(See note 2)
DLC_RESOLVE_C Send
DLC_DGRM If NB_Name_Query
Send NETBIOS_NQ
Else
Send
It is assumed that each Data Link Switch will build a set of
tables giving the identity of each Data Link Switch that can reach
specific MAC address or a specific NetBIOS name. This table can
built based upon the origin address information received within
CANUREACH message or NETBIOS_NQ message and the target
information within the ICANREACH message or NETBIOS_NR message. As
consequence, the amount of search traffic can be kept to a minimum
Upon receipt of a Test command or XID command to the null SAP (i.e.,
DLC_RESOLVE_C signal from DLC), the Data Link Switch will check
topology table prior to sending the CANUREACH message. If the
MAC address is in the table, the CANUREACH message will be sent
only those Data Link Switches that are known to be able to reach
given MAC address. If the MAC address is not in the table,
CANUREACH message will be sent to all known Data Link Switches
Since the destination link SAP (DSAP) value is null, the Target
SAP field in the CANUREACH message header is set equal to the
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
Link SAP value
Upon receipt of a NB_Name_Query the Data Link Switch will check
topology table prior to sending the NETBIOS_NQ message. If
target NetBIOS name is in the table, the NETBIOS_NQ message will
sent to only those Data Link Switches that are known to be able
reach the given NetBIOS name. If the NetBIOS name is not in
table, the NETBIOS_NQ message will be sent to all known Data
Switches
For SNA, the DISCONNECTED state is exited upon receipt of a
message by a prospective target Data Link Switch, or upon receipt
an ICANREACH message by the origin Data Link Switch. In the
case, the Data Link Switch will issue a Test command to the
station (i.e., DLC_START_DL signal is presented to DLC). In
later case, a Test response is sent to the origin station (i.e.,
DLC_RESOLVE_R will be issued) and a REACH_ACK message will
returned to the target Data Link Switch
Note 1- The IBM 6611 will not send a Test response, but will
a Test command to the station that originated the
procedure (i.e., a DLC_START_DL will be issued) in order to
the data link
For NetBIOS, the DISCONNECTED state is exited upon the receipt of
NETBIOS_NQ message by the prospective target Data Link Switch,
upon the receipt of a NETBIOS_NR message by the origin Data
Switch. In the former case, the Data Link Switch will send
NB_Name_Query frame. In the later case, the Data Link Switch
send a NB_Name_Recognized frame to the origin station and a REACH_
message will be returned to the target Data Link Switch
Note 2- The IBM 6611 will also send a Test command (i.e.,
DLC_START_DL will be issued) to the station that originated
name resolution in order to start the data link
4.2.2.2 RESOLVE_PENDING
Event Action(s) Next
----- --------- ----------
Receive DATAFRAME DLC_
DLC_DL_STARTED Send ICANREACH CIRCUIT_
DLC_ERROR
DLC_DGRM If NB_Name_Recognized:
Send NETBIOS_NR NB_Name_Recognized
(See note), CIRCUIT_
Else
Send
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
The RESOLVE_PENDING state is entered upon receipt of a
message or a NETBIOS_NQ message by the target DLS. In the
case, a data link is started, causing a Test command to be sent
the DLC. In the later case, a NB_Name_Query frame is sent.
frame is directed to a group MAC address
Any CANUREACH messages received in the RESOLVE_PENDING state will
responded to if a DLC_DL_STARTED signal is received. The Data
Switch may also update its topology information based upon the
MAC address information in each CANUREACH message
Upon the receipt of a DLC_DL_STARTED signal in the RESOLVE_
state, the Data Link Switch may update its topology table base
the remote MAC address information. The ICANREACH message should
returned to all Data Link Switches that had sent a CANUREACH message
In a similar fashion, the Data Link Switch may update its
table upon the receipt of a NB_Name_Recognized frame and a NETBIOS_
message will be returned to all Data Link Switches that have sent
NETBIOS_NQ message
The RESOLVE_PENDING state is exited once the data link has
started (i.e., a DLC_DL_STARTED signal is received as a result of
Test response received by the DLC) or a NB_Name_Recognized frame
received (i.e., a DLC_DGRM signal is received). The target Data
Switch will then enter the CIRCUIT_PENDING state
Note: The IBM 6611 will also send a Test command in order
start the data link to the station that responded to the
Query frame (i.e., a DLC_START_DL will be issued).
4.2.2.3 CIRCUIT_PENDING
Event Action(s) Next
----- --------- ----------
Receive CONTACT DLC_CONTACT CONTACT_
Receive HALT_DL DLC_HALT_DL HALT_
Receive REACH_ACK If Connected: If Connected
Send CONTACT CONNECT_PENDING
else: CIRCUIT_
Receive XIDFRAME DLC_
Receive DGRMFRAME DLC_
Receive DATAFRAME DLC_
DLC_CONTACTED DLC_ENTER_
DLC_ERROR Send HALT_DL DISCONNECT_
DLC_XID Send
DLC_DGRM Send
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
The CIRCUIT_PENDING state is entered by the target Data Link
following the sending of an ICANREACH message or the sending of
NETBIOS_NR message. In this state it is awaiting the reception of
REACH_ACK message from the origin Data Link Switch. If a
does not exist with the target station (i.e., the normal case),
Data Link Switch will enter the CIRCUIT_ESTABLISHED state
If the target Data Link Switch happens to receive a SABME
from the target station while in the CIRCUIT_PENDING state (i.e.,
DLC_CONTACTED signal received from the DLC), the reception of
REACH_ACK message will cause the Data Link Switch to enter
CONNECT_PENDING state and to send a CONTACT message to the other
Link Switch. Thus the target Data Link Switch has assumed the
of the origin Data Link Switch
4.2.2.4 CONNECT_PENDING
Event Action(s) Next
----- --------- ----------
Receive CONTACTED DLC_EXIT_BUSY
Receive HALT_DL DLC_HALT_DL HALT_
Receive DGRMFRAME DLC_
Receive DATAFRAME DLC_
DLC_RESET Send RESTART_DL (See note) CIRCUIT_
DLC_ERROR Send HALT_DL DISCONNECT_
DLC_DGRM Send
The CONNECT_PENDING state is entered by the origin Data Link
when a DLC_CONTACTED signal has been received from the DLC (i.e.,
SABME command has been received). A CONTACT message is then issued
The state is exited upon the receipt of a CONTACTED message from
target Data Link Switch. If a DLC_RESET signal is received,
local data link is restarted and a RESTART_DL message is sent to
remote DLS
Note: The IBM 6611 will also send a Test command in order
restart the data link to the station that sent the SABME
(i.e., a DLC_START_DL will be issued).
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
4.2.2.5 CIRCUIT_ESTABLISHED
Event Action(s) Next
----- --------- ----------
Receive CONTACT DLC_CONTACT CONTACT_
Receive HALT_DL DLC_HALT_DL HALT_
Receive XIDFRAME DLC_
Receive DGRMFRAME DLC_
Receive DATAFRAME DLC_
DLC_CONTACTED Send CONTACT CONNECT_
DLC_ENTER_
DLC_ERROR Send HALT_DL DISCONNECT_
DLC_DGRM Send
DLC_XID Send
The CIRCUIT_ESTABLISHED state is entered by the origin Data
Switch from the DISCONNECTED state, and by the target Data
Switch from the CIRCUIT_PENDING state. The state is exited when
connection is started (i.e., DLC receives a SABME command). The
state is CONTACT_PENDING for the target Data Link Switch
CONNECT_PENDING for the origin Data Link Switch
4.2.2.6 CONTACT_PENDING
Event Action(s) Next
----- --------- ----------
Receive HALT_DL DLC_HALT_DL HALT_
Receive RESTART_DL DLC_HALT_DL RESTART_
Receive DGRMFRAME DLC_
Receive DATAFRAME DLC_
DLC_CONTACTED Send CONTACTED
DLC_ERROR Send HALT_DL DISCONNECT_
DLC_DGRM Send
The CONTACT_PENDING state is entered by the target Data Link
upon the receipt of a CONTACT message. This causes the Data
Switch to issue a DLC_CONTACT signal to the DLC (i.e., DLC sends
SABME command). This state is then exited upon the receipt of
DLC_CONTACTED signal from the DLC (i.e., a UA response received).
If a RESTART_DL message is received, indicating that the remote
Link Switch has received a DLC_RESET signal, the local Data
Switch will send a DISC command frame on the adjacent LAN (i.e.,
DLC_HALT_DL signal) and enter the RESTART_PENDING state
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
4.2.2.7 CONNECTED
Event Action(s) Next
----- --------- ----------
Receive HALT_DL DLC_HALT_DL HALT_
Receive RESTART_DL DLC_HALT_DL RESTART_
Receive DGRMFRAME DLC_
Receive INFOFRAME DLC_
Receive DATAFRAME DLC_
DLC_RESET Send RESTART_DL (See note) CIRCUIT_
DLC_ERROR Send HALT_DL DISCONNECT_
DLC_DGRM Send
DLC_INFO Send
The CONNECTED state is entered by the origin Data Link Switch
the CONNECT_PENDING state upon the receipt of a CONTACTED message
The CONNECTED state is entered by the target Data Link Switch
the CONTACT_PENDING state upon the receipt of a DLC_CONTACTED signal
At this time, the target Data Link Switch will return a
message to the origin Data Link Switch
The CONNECTED state is exited usually under one of two conditions:
DLC_ERROR signal received from the DLC (e.g., a DISC command
by the local DLC), or a HALT_DL message received from the other
Link Switch (e.g., a DISC command received by the remote DLC).
A SABME command (i.e., a DLC_RESET signal) received by either
Link Switch will also cause the two Data Link Switches to leave
CONNECTED state and eventually restart a new circuit
Note: The IBM 6611 will also send a Test command in order
restart the data link to the station that sent the SABME
(i.e., a DLC_START_DL will be issued).
Following the receipt of a reset signal, the Data Link Switch
send a RESTART_DL message to the other Data Link Switch and
enter the CIRCUIT_RESTART state. Upon the receipt of the RESTART_
message, the remote Data Link Switch will send a DISC command (i.e.,
DLC_HALT_DL signal) and enter the RESTART_PENDING state
4.2.2.8 CIRCUIT_RESTART
Event Action(s) Next
----- --------- ----------
Receive DL_RESTARTED If Connected: If Connected
Send CONTACT CONNECT_PENDING
else: CIRCUIT_
Receive DATAFRAME DLC_
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
DLC_ERROR Send HALT_DL DISCONNECT_
DLC_DGRM Send
The CIRCUIT_RESTART state is entered if a DLC_RESET signal
received from the local DLC. This was caused by the receipt of
SABME command while a connection was currently active. A DM
will be issued to the SABME command and the Data Link Switch
attempt to restart the end-to- end circuit
The CIRCUIT_RESTART state is exited through one of two transitions
The next state depends upon the time the local DLC has reached
contacted state (i.e., a DLC_CONTACTED signal is presented)
to the receipt of the DL_RESTARTED message. This signal is caused
the origin station resending the SABME command that initially
the DATA Link Switch to enter the CIRCUIT_RESTART state. The
cases are as follows
1) DL_RESTARTED message received before
DLC_CONTACTED signal- In this case,
CIRCUIT_ESTABLISHED state is entered
2) DL_RESTARTED message received after
DLC_CONTACTED signal- In this case,
CONNECT_PENDING state is entered
4.2.2.9 DISCONNECT_PENDING
Event Action(s) Next
----- --------- ----------
Receive DL_HALTED
Receive HALT_DL Send DL_
Receive DATAFRAME DLC_
DLC_DGRM Send
The DISCONNECT_PENDING state is entered when a DLC_ERROR signal
received from the local DLC. Upon receipt of this signal, a
message is sent. Once an DL_HALTED message is received, the state
exited, and the Data Link Switch enters the DISCONNECTED state
4.2.2.10 RESTART_PENDING
Event Action(s) Next
----- --------- ----------
Receive DATAFRAME DLC_
DLC_DL_HALTED (See note) Send DL_RESTARTED CIRCUIT_
DLC_ERROR Send HALT_DL DISCONNECT_
DLC_DGRM Send
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
The RESTART_PENDING state is entered upon the receipt of a RESTART_
message from the remote DLS while the local Data Link Switch is
either the CONTACT_PENDING state or the CONNECTED state. These
the local DLC to issue a DISC command. Upon the receipt of the
response (DLC_DL_HALTED), the data link is restarted, a DL_
message is returned to the remote DLS, and the CIRCUIT_
state is entered
Note: The IBM 6611 will send a Test command in order to
the data link to the target station (i.e., a DLC_START_DL will
issued) prior to sending the DL_RESTARTED message
4.2.2.11 HALT_PENDING
Event Action(s) Next
----- --------- ----------
Receive DATAFRAME DLC_
DLC_DL_HALTED Send DL_HALTED
DLC_ERROR Send DL_HALTED
DLC_DGRM Send
The HALT_PENDING state is entered upon the receipt of a HALT_
message. This causes the local DLC to issue a DISC command.
the receipt of the UA response (DLC_DL_HALTED), a DL_HALTED
is returned to the remote DLS and the DISCONNECTED state is entered
4.3. NetBIOS
The NetBIOS protocols use a number of UI frames for
services and the transmission of datagrams. Most of these frames
directed to a group MAC address (GA) with the routing
field indicating spanning tree explorer (STE). Two of the frames
NB_Add_Name_Response and NB_Status_Response, are directed to
specific MAC address with the routing information field indicating
specifically routed frame (SRF). The handling of these frames
summarized in the following table
Event Action(s)
----- --------- -------
DLC_DGRM (NB Group Address): Send NETBIOS_ANQ Transmitted to
NB_Add_Name_Query remote
DLC_DGRM (Specific Address): Send NETBIOS_ANR Transmitted
NB_Add_Name_Response specific
DLC_DGRM (Specific Address): Send DATAFRAME Transmitted to
NB_Status_Response remote
DLC_DGRM (NB Group Address): Send DATAFRAME Transmitted to
NB_Name_in_Conflict, remote
NB_Add_Group_Name_Query
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
NB_Datagram
NB_Datagram_Broadcast
NB_Status_Query
NB_Terminate_
Table 5. NetBIOS DLC
The above actions do not apply in the following states
CIRCUIT_ESTABLISHED, CONTACT_PENDING, CONNECT_PENDING, CONNECTED,
CIRCUIT_PENDING. The handling of the remaining two UI frames used
NetBIOS systems, NB_Name_Query and NB_Name_Recognized, are
as part of the DLS state machine in the previous section (i.e.,
DISCONNECTED and RESOLVE_PENDING states). Furthermore, the
of NetBIOS datagrams (i.e., NB_Datagram) sent to a specific
address is also governed by the DLS state machine
Note: The IBM 6611 will also issue Test frames during
exchange of the NetBIOS, NB_Name_Query and NB_Name_Recognized
This exchange of protocol data units occurs during the start of
data link and is used to determine the routing information.
other implementations of NetBIOS will use
NB_Name_Query/NB_Name_Recognized exchange to determine routes
conjunction with resolving the NetBIOS names. These
are not reflected in the SSP protocols
The handling of the NetBIOS specific SSP messages is given in
following table
Event Action(s)
----- --------- -------
NETBIOS_ANQ DLC_DGRM: Routed
NB_Add_Name_Query (NB Group Address
NETBIOS_ANR DLC_DGRM: Routed
NB_Add_Name_Response (Specific MAC Address
NETBIOS_NQ DLC_DGRM: Routed
NB_Name_Query (NB Group Address
NETBIOS_NR DLC_DGRM: Routed
NB_Name_Recognized (Specific MAC Address
DATAFRAME DLC_DGRM Routed
(If NB_Status_Response
Specific MAC
Else: NB Group Address
Table 6. NetBIOS SSP
The above actions apply to all DLS states. The handling of
datagrams sent within DGRMFRAME messages is governed by the DLS
machine. The DGRMFRAME message type is employed instead of
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
DATAFRAME message type once the end-to-end circuit has
established. At that time, the message is addressed according to
pair of Circuit IDs in the message header instead of relying upon
MAC address information in the token ring header
Randall Campbell, David Miller, Gene Cox, Ravi Periasamy, and
Ghost of Christmas Past
1) ISO 8802-2/IEEE Std 802.2 International Standard
Information Processing Systems, Local Area Networks, Part 2:
Logical Link Control, December 31, 1989
2) The NETBIOS Frames Protocol, IBM Local Area
Reference, SC30-3383-03, Chapter 5, December 1990
3) ISO/IEC DIS 10038 DAM 2, MAC Bridging, Source
Supplement, December 1991
Security
Security issues are not discussed in this memo
Dixon & Kushi [Page 32]
RFC 1434 DLS: Switch-to-Switch Protocol March 1993
Authors'
Roy C.
IBM Networking
Department B57, Building 060
P.O. Box 12195
Research Triangle Park, NC 27709
Phone: (919) 543-3380
EMail: rcdixon@ralvmg.vnet.ibm.
David M.
IBM Research
T. J. Watson Research
Room H0-C07
30 Saw Mill River
Hawthorne, NY 10532
Phone: (914) 784-7815
EMail: kushi@watson.ibm.
Note: Any questions or comments relative to the contents of this
should be sent to the following Internet address
dlsw@ralvma.vnet.ibm.com. This address will be used to
the handling of responses
Dixon & Kushi [Page 33]
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.
Relevance System Copyright © 2002 Spectrum WorldResearch
other technical nosh by ServerMasters Corporation
collaboration of BobX
