D. Kushi
IBM
March 1993
Data Link Switching: Switch-to-Switch Protocol
Status of this Memo
-
This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
unlimited.
Abstract
-
This RFC describes IBM's support of Data Link Switching over TCP/IP.
The RFC is being distributed to members of the Internet community in
order to solicit their reactions to the proposals contained in it.
While the issues discussed may not be directly relevant to the
research problems of the Internet, they may be interesting to a
number of researchers and implementors.Any questions or comments relative to the contents of this RFC should
be sent to the following Internet address: dlsw@ralvma.vnet.ibm.com.
Table of Contents
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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 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It is identified by a Data Link ID (14 bytes) consisting of the pair of attachment addresses associated with each end system. Each attachment address is represented by the concatenation of the MAC address (6 bytes) and the 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. A Circuit ID is a 64 bit number that identifies the DLC circuit within a single DLS. It consists of a DLC Port ID (4 bytes), and a Data Link Correlator (4 bytes). This value is unique in a single DLS and is assigned locally. The pair of Circuit ID's along with the identifiers of the Data Link Switches, uniquely identify a single end-to-end circuit. Each DLS must keep a table of these Circuit ID pairs, one for the local end of the circuit and the other for the remote end of the circuit. In order to identify which Data Link Switch originated the establishment of a circuit, the terms, origin 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 the
message header are used to identify the individual TCP/IP port on a
Data Link Switch. The values have only local significance. However,
each Data Link Switch is required to reflect the values contained in
these two fields, along with the associated values for DLC Port ID
and the Data Link Correlator, when returning a message to the other
Data Link Switch.The following figure shows the use of the addressing parameters
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 two
end stations. Upon receipt of a CANUREACH message, the target DLS
starts a data link for each port, thereby obtaining a Data Link
Correlator. If the target station can be reached, an ICANREACH
message is returned to the origin DLS containing the Target Circuit
ID parameter. Upon receipt, the origin DLS starts a data link and
returns the Origin Circuit ID to the target DLS within the REACH_ACK
message. If the REACH_ACK message is not successfully received, the
target Data Link Switch can obtain the Origin Circuit ID from a
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 Connections
-
-
-
During the exchange of the XIDFRAME, CONTACT, and CONTACTED messages,
the pair of Circuit ID parameters is included in the message format
along with the DATA LINK ID parameter. Once the connection has been
established, the INFOFRAME messages are exchanged with the shorter
header. This header contains only the Circuit ID associated with the
remote DLS. The Remote Data Link Correlator and the Remote DLC Port
ID are set equal to the Data Link Correlator and the DLC Port ID that
are associated with the origin or target Data Link Switch, dependent
upon the direction of the packet. -
3.3. Message Types
-
The following table lists the protocol data units that are exchanged
between Data Link Switches. All values not listed are reserved for
potential use in follow-on releases.Command Function Hex Value ------- -------- --------- 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 Types
-
-
-
Note: Both the DGRMFRAME and DATAFRAME messages are used to carry
information received by the DLC entity within UI frames. As will
be explained below, the DGRMFRAME message is addressed according
to a pair of Circuit IDs, while the DATAFRAME message is addressed
according to a Data Link ID, being composed of a pair of MAC
addresses and a pair of link SAP addresses. The latter is
employed prior to the establishment of an end-to-end circuit when
Circuit IDs have yet to be established. -
For the exchange of NetBIOS control messages, the entire DLC header
is carried as part of the message unit. This includes the MAC
header, with the routing information field padded to 18 bytes, and
the LLC header. The following message types are affected:
NETBIOS_NQ, NETBIOS_NR, NETBIOS_ANQ, NETBIOS_ANR, and DATAFRAME when
being used by NetBIOS systems. The routing information in the DLC
header is not used by the remote Data Link Switch upon receiving the
above five messages. -
4. Protocol Specification
-
This section provides a description of the Switch-to-Switch
Protocols. Included is a set of high-level protocol flows and a
detail set of state transition tables. The states and the protocols
are described in terms that are intended to be generic to different
platforms. Emphasis of the technical details is to ensure
operability of the IBM 6611 with another vendor's implementation.
Notes are inserted at points where the IBM 6611 performs local
actions that are specific to the AIX platform upon which it operates.
4.1. Protocol Flow Diagrams
-
The switch-to-switch protocols are used to setup and take down
circuits between a pair of Data Link Switches. Once a circuit is
established, the end stations on the local networks can employ LLC
Type 1 (connectionless) protocols. In addition, the end systems can
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 a
"logical data link" when supporting Type 1 LLC services, and a "data
link connection" when supporting Type 2 LLC services. In both cases,
the Data Link in defined by the concatenation of the destination MAC
address (DA), the source MAC address (SA), the destination link SAP
(DSAP) and source link SAP (SSAP).
4.1.1. Connect Protocols
-
The following figure depicts the protocol flows that are used for the
establishment of a circuit between a pair of Data Link Switches,
followed by the establishment of a connection between the pair of end
systems. The figure is drawn assuming that the two end systems are
SNA (the protocol flow for NetBIOS systems is described in a later
paragraph).
Data Link Data Link Data Link Data Link
Control Switch Switch Control
-------------------- --------------------
+------------+ +------------+
|Disconnected| |Disconnected|
+------------+ +------------+
Test Command CANUREACH Test Comd.
----------> ---------------------------------------> ------->
(DSAP=Null) (DSAP=SSAP)
Test Response
ICANREACH <---------
Test Response <---------------------------------------
<---------- REACH ACK
--------------------------------------->
+------------+ +------------+
|Circuit Est.| |Circuit Est.|
+------------+ +------------+
SABME CONTACT
----------> ---------------------------------------> SABME
UA ------->
<----------
RNR UA
<---------- CONTACTED <-------
<---------------------------------------
+------------+ +------------+
| Connected | | Connected |
+------------+ +------------+
RR
<---------
Figure 4. DLS Connect Message Protocols
-
Upon receipt of a Test command from the origin station, the origin
DLS will send a CANUREACH (i.e., can you reach) message to the target
DLS. If the target DLS is not known to the origin DLS, the CANUREACH
message is sent to all remote Data Link Switches defined to the
origin DLS. The receipt of the CANUREACH message causes the target
DLS to send a Test command searching for the target station. The
target station will return a Test response, causing the target DLS to
return an ICANREACH (i.e., I can reach) message to the origin DLS.
If multiple Data Link Switches can reach the target station, the
origin DLS will receive multiple ICANREACH messages. The origin DLS
will select the first message and send a REACH_ACK (i.e., reach
acknowledgment) message to the selected Data Link Switch. During
this exchange of messages, both Data Link Switches change states from
the Disconected state to the Circuit Established state. Once the
circuit is established, Type-1 frames, such as XID, may be exchanged
between the origin and target stations.To establish a connection, the origin station sends a SABME command. Upon receipt of this command, the origin DLS will send a CONTACT message to the target DLS and return a UA response to the origin station. To inhibit traffic flow until the connection is established to the remote station, a RNR supervisory frame is sent to the origin station. The CONTACT message will cause the target DLS to send a SABME command to the target station, which in return will reply with a UA response. Upon receipt of the UA response, the target DLS will send a CONTACTED message to the origin DLS. The origin DLS will now send an RR supervisory frame to the origin station. During this exchange of messages, both Data Link Switches change states from the Circuit Established ÜG?XLHR[ÙÌƼµ³¸ÂÕ[OøO859E9.*+6J䧣¤¦«·ÓPK]@5>>6=C:54Eм·±¦ú709Oæ]mÎ^DíÈÜnWëØÚŶ«²ÊR:771,*7A756T󬨧µ»ËÔÙèÜf;/.1=fåUC?CSs춦¯Þ?;BH_ƹ¿ÐÝØ~HGNTZMKWáÉþÃÍÍáJ92;óØUEFJZ˸³¶µ·ÀÜOM[eTC>?HJ?9<\¿¶º¿¸¹ö>8<H^ßí]OWÑÂÉÝbv÷MRÉ´®±¼ÚF<?@<BPG7--7]½¯«¬³ÃWNÝÌÇÇÖU;69CWUTUOTgË·°µÅ[CC>?Ù¸²¹ÑM<78CfÎÄÉÒÛævìÐÑóQFE<3<eßòëmcÝÀ²¯³»ÊèO><KåÒÝrbhU?>]ɽ½ÊoD616@NöÏÇÇÓÚËÁ½ÄærÛnQݽ³µÂüE97<K\QE;736LÉ´¬¬µÀT8CÜÁ½ÅäL96@pÐÎÔèmUdÐÊÊÎÚYIA7@̵³¼×RHGM÷Ô×ÚlG<<LßÅÁËî\I53MÀ¶¹¾ÏàÛÉÃÆÙXMB:6<ZÒÿÈÙàN?^Ä·´½d5+*1:C\ÑÁ½¼»¸·´´¾àQ>23JØÊË×ySJbÒÎÙoLC?>BFÅ®½âJ5-7e¼²±·ÄkC@üÑǶC/RIäÏÓÍǼ»ÊZ÷¼°³ÆW5% 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will issue a SABME command (i.e., DLC_CONTACT) and enter the Contact Pending state. Once the UA response is received (i.e., DLC_CONTACTED), the target DLS sends a CONTACTED message and enters the CONNECTED state. When received, the origin DLS enters the 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 a
connection has been established. Information frames (i.e., INFOFRAME
messages) are simply sent back and forth between the end points of
the connection. This is the path that should be optimized for
performance.The connection is terminated upon the receipt of a DISC frame or
under some other error condition detected by DLC (i.e., DLC_ERROR).
Upon receipt of this indication, the DLS will halt the local data
link, send a HALT_DL message to the remote DLS, and enter the
DISCONNECT_PENDING State. When the HALT_DL frame is received by the
other DLS, the local DLC is halted for this data link, a DL_HALTED
message is returned, and the DISCONNECTED state is entered. Receipt
of this DL_HALTED message causes the other DLS to also enter the
DISCONNECTED state.The CIRCUIT_RESTART state is entered if one of the Data Link Switches receives a SABME command (i.e., DLC_RESET) while in the CONNECTED state. This causes a DM command to be returned to the origin station 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 the first DLS. The receipt of the DL_RESTARTED message causes the first DLS to issue a new CONTACT message, assuming that the local DLC has been contacted (i.e., the origin station has resent the SABME command). This is eventually responded to by a CONTACTED message.
Following this exchange, both Data Link Switches will return to the
CONNECTED state. If the local DLC has not been contacted, the
receipt of a DL_RESTARTED command causes the Data Link Switch to
enter the CIRCUIT_ESTABLISHED state awaiting the receipt of a SABME
command (i.e., DLC_CONTACTED signal).In the figure, the transition labeled HALT_DL(ts) represents the
HALT_PENDING state and the transition labeled RESTART_DL(ts)
represents the RESTART_PENDING state. Both these transitional states
have only one input event and only one output action. While in these
states, the Data Link Switch is awaiting responses from the local
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 attempt
to restart the data link prior to sending a DL_RESTARTED message.
For the IBM 6611 implementation, the start of a data link involves
the exchange of a Test command/response on the adjacent LAN (i.e.,
DLC_START_DL). For other implementations, this additional exchange
may not be required.
4.2.2. State Transition Tables
-
This section provides a detail representation of the Data Link
Switch, as document by a set of state machines. The first state
machine documents the sending and receiving of SSP messages. Many of
the transitions are dependent upon local signals between the Data
Link Switch entity and one of the DLC entities. These signals and
their definitions are given in the following tables.Event Name Description ---------- ----------- DLC_CONTACTED Contact Indication: DLC has received an SABME command and will send a UA response, or DLC has received a UA response as a result of sending an SABME command. DLC_DGRM Datagram Indication: DLC has received a UI frame. DLC_ERROR Error condition indicated by DLC: Such a condition occurs when a DISC command is received or when DLC experiences an unrecoverable error. DLC_INFO Information Indication: DLC has received an Information (I) frame. DLC_DL_HALTED Data Link Halted Indication: DLC has received a UA response to a DISC command. DLC_DL_STARTED Data Link Started Indication: DLC has received a Test response from the null SAP. DLC_RESET Reset Indication: DLC has received an SABME command during the time a connection is currently active and has responded with DM. DLC_RESOLVE_C Resolve Command Indication: DLC has received a Test command addressed to the null SAP, or an XID command addressed to the null SAP. DLC_XID XID Indication: DLC has received an XID command or response to a non-null SAP.-
-
-
-
-
Table 3. Local DLC Events
-
-
-
-
Action Name Description ----------- ----------- DLC_CONTACT Contact Station Request: DLC will send a SABME command. DLC_DGRM Datagram Request: DLC will send a UI frame. DLC_ENTER_BUSY Enter Link Station Busy: DLC will send an RNR supervisory frame. DLC_EXIT_BUSY Exit Link Station Busy: DLC will send an RR supervisory frame. DLC_HALT_DL Halt Data Link Request: DLC will send a DISC command. DLC_INFO Information Request: DLC will send an I frame. DLC_RESOLVE_R Resolve Response Request: DLC will send a Test response or XID response from the null SAP. DLC_START_DL Start Data Link Request: DLC will send a Test command to the null SAP. DLC_XID XID Request: DLC will send an XID command or an XID response.-
-
-
-
-
Table 4. Local DLC Actions
-
-
-
-
The Data Link Switch may be described by a state transition table
consisting of eleven states. Each of these states is described below
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 and
no state transition should occur for that event. Any significant
comments concerning the transitions within a given state are given
immediately following the table representing the state.A separate state machine is maintained by the Data Link Switch for
each end-to-end circuit. The number of circuits that may be
supported by each Data Link Switch is a local implementation option. -
4.2.2147.230.0.0
147.251.0.0
158.194.0.0
158.196.0.0
192.108.151.0
192.108.156.0
192.108.171.0
193.84.70.0
193.84.115.0
194.50.1.0
194.50.250.0
be
built based upon the origin address information received within the
CANUREACH message or NETBIOS_NQ message and the target address
information within the ICANREACH message or NETBIOS_NR message. As a
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 the
topology table prior to sending the CANUREACH message. If the target
MAC address is in the table, the CANUREACH message will be sent to
only those Data Link Switches that are known to be able to reach the
given MAC address. If the MAC address is not in the table, the
CANUREACH message will be sent to all known Data Link Switches.
Since the destination link SAP (DSAP) value is null, the Target Link
SAP field in the CANUREACH message header is set equal to the OriginLink SAP value.
Upon receipt of a NB_Name_Query the Data Link Switch will check the
topology table prior to sending the NETBIOS_NQ message. If the
target NetBIOS name is in the table, the NETBIOS_NQ message will be
sent to only those Data Link Switches that are known to be able to
reach the given NetBIOS name. If the NetBIOS name is not in the
table, the NETBIOS_NQ message will be sent to all known Data Link
Switches.For SNA, the DISCONNECTED state is exited upon receipt of a CANUREACH
message by a prospective target Data Link Switch, or upon receipt of
an ICANREACH message by the origin Data Link Switch. In the former
case, the Data Link Switch will issue a Test command to the target
station (i.e., DLC_START_DL signal is presented to DLC). In the
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 be
returned to the target Data Link Switch.-
Note 1- The IBM 6611 will not send a Test response, but will send
a Test command to the station that originated the resolve
procedure (i.e., a DLC_START_DL will be issued) in order to start
the data link.
For NetBIOS, the DISCONNECTED state is exited upon the receipt of a
NETBIOS_NQ message by the prospective target Data Link Switch, or
upon the receipt of a NETBIOS_NR message by the origin Data Link
Switch. In the former case, the Data Link Switch will send a
NB_Name_Query frame. In the later case, the Data Link Switch will
send a NB_Name_Recognized frame to the origin station and a REACH_ACK
message will be returned to the target Data Link Switch.-
Note 2- The IBM 6611 will also send a Test command (i.e., a
DLC_START_DL will be issued) to the station that originated the
name resolution in order to start the data link.
-
4.2.2.2 RESOLVE_PENDING State
-
Event Action(s) Next State ----- --------- ---------- Receive DATAFRAME DLC_DGRM DLC_DL_STARTED Send ICANREACH CIRCUIT_PENDING DLC_ERROR DISCONNECTED DLC_DGRM If NB_Name_Recognized: If Send NETBIOS_NR NB_Name_Recognized: (See note), CIRCUIT_PENDING Else: Send DATAFRAMEThe RESOLVE_PENDING state is entered upon receipt of a CANUREACH
message or a NETBIOS_NQ message by the target DLS. In the former
case, a data link is started, causing a Test command to be sent by
the DLC. In the later case, a NB_Name_Query frame is sent. This
frame is directed to a group MAC address.Any CANUREACH messages received in the RESOLVE_PENDING state will be
responded to if a DLC_DL_STARTED signal is received. The Data Link
Switch may also update its topology information based upon the origin
MAC address information in each CANUREACH message.Upon the receipt of a DLC_DL_STARTED signal in the RESOLVE_PENDING
state, the Data Link Switch may update its topology table base upon
the remote MAC address information. The ICANREACH message should be
returned to all Data Link Switches that had sent a CANUREACH message.
In a similar fashion, the Data Link Switch may update its topology
table upon the receipt of a NB_Name_Recognized frame and a NETBIOS_NR
message will be returned to all Data Link Switches that have sent a
NETBIOS_NQ message.The RESOLVE_PENDING state is exited once the data link has been
started (i.e., a DLC_DL_STARTED signal is received as a result of a
Test response received by the DLC) or a NB_Name_Recognized frame is
received (i.e., a DLC_DGRM signal is received). The target Data Link
Switch will then enter the CIRCUIT_PENDING state.Note: The IBM 6611 will also send a Test command in order to start the data link to the station that responded to the Name Query frame (i.e., a DLC_START_DL will be issued).
4.2.2.3 CIRCUIT_PENDING State
-
Event Action(s) Next State ----- --------- ---------- Receive CONTACT DLC_CONTACT CONTACT_PENDING Receive HALT_DL DLC_HALT_DL HALT_PENDING Receive REACH_ACK If Connected: If Connected: Send CONTACT CONNECT_PENDING, else: CIRCUIT_ESTABLISHED Receive XIDFRAME DLC_XID Receive DGRMFRAME DLC_DGRM Receive DATAFRAME DLC_DGRM DLC_CONTACTED DLC_ENTER_BUSY DLC_ERROR Send HALT_DL DISCONNECT_PENDING DLC_XID Send XIDFRAME DLC_DGRM Send DGRMFRAMEThe CIRCUIT_PENDING state is entered by the target Data Link Switch
following the sending of an ICANREACH message or the sending of a
NETBIOS_NR message. In this state it is awaiting the reception of a
REACH_ACK message from the origin Data Link Switch. If a connection
does not exist with the target station (i.e., the normal case), the
Data Link Switch will enter the CIRCUIT_ESTABLISHED state.If the target Data Link Switch happens to receive a SABME command
from the target station while in the CIRCUIT_PENDING state (i.e., a
DLC_CONTACTED signal received from the DLC), the reception of the
REACH_ACK message will cause the Data Link Switch to enter the
CONNECT_PENDING state and to send a CONTACT message to the other Data
Link Switch. Thus the target Data Link Switch has assumed the role
of the origin Data Link Switch.
4.2.2.4 CONNECT_PENDING State
-
Event Action(s) Next State ----- --------- ---------- Receive CONTACTED DLC_EXIT_BUSY CONNECTED Receive HALT_DL DLC_HALT_DL HALT_PENDING Receive DGRMFRAME DLC_DGRM Receive DATAFRAME DLC_DGRM DLC_RESET Send RESTART_DL (See note) CIRCUIT_RESTART DLC_ERROR Send HALT_DL DISCONNECT_PENDING DLC_DGRM Send DGRMFRAME
The CONNECT_PENDING state is entered by the origin Data Link Switch
when a DLC_CONTACTED signal has been received from the DLC (i.e., a
SABME command has been received). A CONTACT message is then issued.
The state is exited upon the receipt of a CONTACTED message from the
target Data Link Switch. If a DLC_RESET signal is received, the
local data link is restarted and a RESTART_DL message is sent to the
remote DLS.Note: The IBM 6611 will also send a Test command in order to restart the data link to the station that sent the SABME command (i.e., a DLC_START_DL will be issued).
4.2.2.5 CIRCUIT_ESTABLISHED State
-
Event Action(s) Next State ----- --------- ---------- Receive CONTACT DLC_CONTACT CONTACT_PENDING Receive HALT_DL DLC_HALT_DL HALT_PENDING Receive XIDFRAME DLC_XID Receive DGRMFRAME DLC_DGRM Receive DATAFRAME DLC_DGRM DLC_CONTACTED Send CONTACT CONNECT_PENDING DLC_ENTER_BUSY DLC_ERROR Send HALT_DL DISCONNECT_PENDING DLC_DGRM Send DGRMFRAME DLC_XID Send XIDFRAMEThe CIRCUIT_ESTABLISHED state is entered by the origin Data Link
Switch from the DISCONNECTED state, and by the target Data Link
Switch from the CIRCUIT_PENDING state. The state is exited when a
connection is started (i.e., DLC receives a SABME command). The next
state is CONTACT_PENDING for the target Data Link Switch and
CONNECT_PENDING for the origin Data Link Switch.
4.2.2.6 CONTACT_PENDING State
-
Event Action(s) Next State ----- --------- ---------- Receive HALT_DL DLC_HALT_DL HALT_PENDING Receive RESTART_DL DLC_HALT_DL RESTART_PENDING Receive DGRMFRAME DLC_DGRM Receive DATAFRAME DLC_DGRM DLC_CONTACTED Send CONTACTED CONNECTED DLC_ERROR Send HALT_DL DISCONNECT_PENDING DLC_DGRM Send DGRMFRAME
The CONTACT_PENDING state is entered by the target Data Link Switch
upon the receipt of a CONTACT message. This causes the Data Link
Switch to issue a DLC_CONTACT signal to the DLC (i.e., DLC sends a
SABME command). This state is then exited upon the receipt of a
DLC_CONTACTED signal from the DLC (i.e., a UA response received).If a RESTART_DL message is received, indicating that the remote Data
Link Switch has received a DLC_RESET signal, the local Data Link
Switch will send a DISC command frame on the adjacent LAN (i.e.,
DLC_HALT_DL signal) and enter the RESTART_PENDING state.
4.2.2.7 CONNECTED State
-
Event Action(s) Next State ----- --------- ---------- Receive HALT_DL DLC_HALT_DL HALT_PENDING Receive RESTART_DL DLC_HALT_DL RESTART_PENDING Receive DGRMFRAME DLC_DGRM Receive INFOFRAME DLC_INFO Receive DATAFRAME DLC_DGRM DLC_RESET Send RESTART_DL (See note) CIRCUIT_RESTART DLC_ERROR Send HALT_DL DISCONNECT_PENDING DLC_DGRM Send DGRMFRAME DLC_INFO Send INFOFRAME
The CONNECTED state is entered by the origin Data Link Switch from
the CONNECT_PENDING state upon the receipt of a CONTACTED message.
The CONNECTED state is entered by the target Data Link Switch from
the CONTACT_PENDING state upon the receipt of a DLC_CONTACTED signal.
At this time, the target Data Link Switch will return a CONTACTED
message to the origin Data Link Switch.The CONNECTED state is exited usually under one of two conditions: a
DLC_ERROR signal received from the DLC (e.g., a DISC command received
by the local DLC), or a HALT_DL message received from the other Data
Link Switch (e.g., a DISC command received by the remote DLC).A SABME command (i.e., a DLC_RESET signal) received by either Data
Link Switch will also cause the two Data Link Switches to leave the
CONNECTED state and eventually restart a new circuit.Note: The IBM 6611 will also send a Test command in order to restart the data link to the station that sent the SABME command (i.e., a DLC_START_DL will be issued).Following the receipt of a reset signal, the Data Link Switch will
send a RESTART_DL message to the other Data Link Switch and will
enter the CIRCUIT_RESTART state. Upon the receipt of the RESTART_DL
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 State
-
Event Action(s) Next State ----- --------- ---------- Receive DL_RESTARTED If Connected: If Connected: Send CONTACT CONNECT_PENDING, else: CIRCUIT_ESTABLISHED Receive DATAFRAME DLC_DGRM DLC_ERROR Send HALT_DL DISCONNECT_PENDING DLC_DGRM Send DATAFRAMEThe CIRCUIT_RESTART state is entered if a DLC_RESET signal is
received from the local DLC. This was caused by the receipt of a
SABME command while a connection was currently active. A DM response
will be issued to the SABME command and the Data Link Switch will
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 the
contacted state (i.e., a DLC_CONTACTED signal is presented) relative
to the receipt of the DL_RESTARTED message. This signal is caused by
the origin station resending the SABME command that initially caused
the DATA Link Switch to enter the CIRCUIT_RESTART state. The two
cases are as follows:-
-
1) DL_RESTARTED message received before the
DLC_CONTACTED signal- In this case, the
CIRCUIT_ESTABLISHED state is entered.2) DL_RESTARTED message received after the
DLC_CONTACTED signal- In this case, the
CONNECT_PENDING state is entered.
-
-
4.2.2.9 DISCONNECT_PENDING State
-
Event Action(s) Next State ----- --------- ---------- Receive DL_HALTED DISCONNECTED Receive HALT_DL Send DL_HALTED Receive DATAFRAME DLC_DGRM DLC_DGRM Send DATAFRAMEThe DISCONNECT_PENDING state is entered when a DLC_ERROR signal is
received from the local DLC. Upon receipt of this signal, a HALT
message is sent. Once an DL_HALTED message is received, the state is
exited, and the Data Link Switch enters the DISCONNECTED state.
4.2.2.10 RESTART_PENDING State
-
Event Action(s) Next State ----- --------- ---------- Receive DATAFRAME DLC_DGRM DLC_DL_HALTED (See note) Send DL_RESTARTED CIRCUIT_ESTABLISHED DLC_ERROR Send HALT_DL DISCONNECT_PENDING DLC_DGRM Send DATAFRAMEThe RESTART_PENDING state is entered upon the receipt of a RESTART_DL
message from the remote DLS while the local Data Link Switch is in
either the CONTACT_PENDING state or the CONNECTED state. These cause
the local DLC to issue a DISC command. Upon the receipt of the UA
response (DLC_DL_HALTED), the data link is restarted, a DL_RESTARTED
message is returned to the remote DLS, and the CIRCUIT_ESTABLISHED
state is entered.Note: The IBM 6611 will send a Test command in order to restart the data link to the target station (i.e., a DLC_START_DL will be issued) prior to sending the DL_RESTARTED message.
4.2.2.11 HALT_PENDING State
-
Event Action(s) Next State ----- --------- ---------- Receive DATAFRAME DLC_DGRM DLC_DL_HALTED Send DL_HALTED DISCONNECTED DLC_ERROR Send DL_HALTED DISCONNECTED DLC_DGRM Send DATAFRAMEThe HALT_PENDING state is entered upon the receipt of a HALT_DL
message. This causes the local DLC to issue a DISC command. Upon
the receipt of the UA response (DLC_DL_HALTED), a DL_HALTED message
is returned to the remote DLS and the DISCONNECTED state is entered.
4.3. NetBIOS Datagrams
-
The NetBIOS protocols use a number of UI frames for directory
services and the transmission of datagrams. Most of these frames are
directed to a group MAC address (GA) with the routing information
field indicating spanning tree explorer (STE). Two of the frames,
NB_Add_Name_Response and NB_Status_Response, are directed to a
specific MAC address with the routing information field indicating a
specifically routed frame (SRF). The handling of these frames is
summarized in the following table.Event Action(s) Comment ----- --------- ------- DLC_DGRM (NB Group Address): Send NETBIOS_ANQ Transmitted to all NB_Add_Name_Query remote DLS DLC_DGRM (Specific Address): Send NETBIOS_ANR Transmitted to NB_Add_Name_Response specific DLS DLC_DGRM (Specific Address): Send DATAFRAME Transmitted to all NB_Status_Response remote DLS DLC_DGRM (NB Group Address): Send DATAFRAME Transmitted to all NB_Name_in_Conflict, remote DLS NB_Add_Group_Name_Query,-
-
NB_Datagram,
NB_Datagram_Broadcast,
NB_Status_Query,
NB_Terminate_Trace-
-
-
Table 5. NetBIOS DLC Frames
-
-
-
-
The above actions do not apply in the following states:
CIRCUIT_ESTABLISHED, CONTACT_PENDING, CONNECT_PENDING, CONNECTED, and
CIRCUIT_PENDING. The handling of the remaining two UI frames used by
NetBIOS systems, NB_Name_Query and NB_Name_Recognized, are documented
as part of the DLS state machine in the previous section (i.e.,
DISCONNECTED and RESOLVE_PENDING states). Furthermore, the handling
of NetBIOS datagrams (i.e., NB_Datagram) sent to a specific MAC
address is also governed by the DLS state machine.Note: The IBM 6611 will also issue Test frames during the exchange of the NetBIOS, NB_Name_Query and NB_Name_Recognized. This exchange of protocol data units occurs during the start of a data link and is used to determine the routing information. Most other implementations of NetBIOS will use the NB_Name_Query/NB_Name_Recognized exchange to determine routes in conjunction with resolving the NetBIOS names. These differences are not reflected in the SSP protocols.The handling of the NetBIOS specific SSP messages is given in the
following table.Event Action(s) Comment ----- --------- ------- NETBIOS_ANQ DLC_DGRM: Routed STE NB_Add_Name_Query (NB Group Address) NETBIOS_ANR DLC_DGRM: Routed SRF NB_Add_Name_Response (Specific MAC Address) NETBIOS_NQ DLC_DGRM: Routed STE NB_Name_Query (NB Group Address) NETBIOS_NR DLC_DGRM: Routed SRF NB_Name_Recognized (Specific MAC Address) DATAFRAME DLC_DGRM Routed STE (If NB_Status_Response: Specific MAC Address Else: NB Group Address)-
-
-
-
-
Table 6. NetBIOS SSP Messages
-
-
-
-
The above actions apply to all DLS states. The handling of NetBIOS
datagrams sent within DGRMFRAME messages is governed by the DLS state
machine. The DGRMFRAME message type is employed instead of theDATAFRAME message type once the end-to-end circuit has been
established. At that time, the message is addressed according to the
pair of Circuit IDs in the message header instead of relying upon the
MAC address information in the token ring header. -
Acknowledgments
-
Randall Campbell, David Miller, Gene Cox, Ravi Periasamy, and The
Ghost of Christmas Past.
References
-
- ISO 8802-2/IEEE Std 802.2 International Standard, Information Processing Systems, Local Area Networks, Part 2: Logical Link Control, December 31, 1989
- The NETBIOS Frames Protocol, IBM Local Area Technical Reference, SC30-3383-03, Chapter 5, December 1990
- ISO/IEC DIS 10038 DAM 2, MAC Bridging, Source Routing Supplement, December 1991
Security Considerations
-
Security issues are not discussed in this memo.
Authors' Addresses
-
Roy C. Dixon
IBM Networking Systems
Department B57, Building 060
P.O. Box 12195
Research Triangle Park, NC 27709Phone: (919) 543-3380 EMail: rcdixon@ralvmg.vnet.ibm.com
David M. Kushi
IBM Research Division
T. J. Watson Research Center
Room H0-C07
30 Saw Mill River Road
Hawthorne, NY 10532Phone: (914) 784-7815 EMail: kushi@watson.ibm.com
Note: Any questions or comments relative to the contents of this RFC
should be sent to the following Internet address:
dlsw@ralvma.vnet.ibm.com. This address will be used to coordinate
the handling of responses.