RFC 1795
Data Link Switching: Switch-to-Switch Protocol AIW DLSw RIG: DLSw Closed Pages, DLSw Standard Version 1
Network Working Group L. Wells, Chair Request for Comments: 1795 Internetwork Technology Institute Obsoletes: 1434 A. Bartky, Editor Category: Informational Sync Research, Inc. April 1995 Data Link Switching: Switch-to-Switch Protocol AIW DLSw RIG: DLSw Closed Pages, DLSw Standard Version 1.0 Status of this Memo This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. Distribution of this memo is unlimited. Abstract This RFC describes use 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 Implementers. This RFC was created as a joint effort of the Advanced Peer-to-Peer Networking (APPN) Implementers Workshop (AIW) Data Link Switching (DLSw) Related Interest Group (RIG). The APPN Implementers Workshop is a group sponsored by IBM and consists of representatives of member companies implementing current and future IBM Networking interoperable products. The DLSw Related Interest Group was formed in this forum in order to produce a single version of the Switch to Switch Protocol (SSP) which could be implemented by all vendors, which would fix documentation problems with the existing RFC 1434, and which would enhance and evolve the protocol to add new functions and features. This document is based on RFC 1434. This document contains significant changes to RFC 1434 and therefore obsoletes that document. Any questions or comments relative to the contents of this RFC should be sent to the following Internet address: aiw-dlsw@networking.raleigh.ibm.com. NOTE 1: This is a widely subscribed mailing list and messages sent to this address will be sent to all members of the DLSw mailing list. For specific questions relating to subscribing to the AIW and any of
it's working groups send email to: appn@vnet.ibm.com Information regarding all of the AIW working groups and the work they are producing can be obtained by copying, via anonymous ftp, the file aiwinfo.psbin or aiwinfo.txt from the Internet host networking.raleigh.ibm.com, located in directory aiw. NOTE 2: These mailing lists and addresses are subject to change. 1. Introduction Data Link Switching (DLSw) is a forwarding mechanism for the IBM SNA (Systems Network Architecture) and IBM NetBIOS (Network Basic Input Output Services) protocols. This memo documents the Switch-to-Switch Protocol (SSP) that is used between Data Link Switches. This protocol does not provide full routing, but instead provides switching at the SNA Data Link layer (i.e., layer 2 in the SNA architecture) and encapsulation in TCP/IP for transport over the Internet. This RFC documents the frame formats and protocols for multiplexing data between Data Link Switches. The initial implementation of SSP uses TCP as the reliable transport between Data Link Switches. However, other transport connections such as OSI TP4 could be used in the future. A Data Link Switch (abbreviated also as DLSw in this document) can support SNA (Physical Unit (PU) 2, PU 2.1 and PU 4) systems and optionally NetBIOS systems attached to IEEE 802.2 compliant Local Area Networks, as well as SNA (PU 2 (primary or secondary) and PU2.1) systems attached to IBM Synchronous Data Link Control (SDLC) links. For the latter case, the SDLC attached systems are provided with a LAN appearance within the Data Link Switch (each SDLC PU is presented to the SSP protocol as a unique MAC/SAP address pair). For the Token-Ring LAN attached systems, the Data Link Switch appears as a source-routing bridge. Token-Ring Remote systems that are accessed through the Data Link Switch appear as systems attached to an adjacent ring. This ring is a virtual ring that is manifested within each Data Link Switch. 1.1 Backwards Compatibility with RFC 1434 This document defines significant changes to RFC 1434 and does not state details on how to interoperate with RFC 1434 or "enhanced" implementations (e.g., those that added enter and exit busy flow control). It is up to the implementer to refer to RFC 1434 and/or any other vendor's documentation in order to interoperate with a given vendor's implementation, if interoperability with pre-AIW DLSw RIG standards is desired.
2. Overview Data Link Switching was developed to provide support for SNA and NetBIOS in multi-protocol routers. Since SNA and NetBIOS are basically connection oriented protocols, the Data Link Control procedure that they use on the LAN is IEEE 802.2 Logical Link Control (LLC) Type 2. Data Link Switching also accommodates SNA protocols over WAN (Wide Area Network) links via the SDLC protocol. IEEE 802.2 LLC Type 2 was designed with the assumption that the network transit delay would be predictable (i.e., a local LAN). Therefore the LLC Type 2 elements of procedure use a fixed timer for detecting lost frames. When remote bridging is used over wide area lines (especially at lower speeds), the network delay is larger and it can vary greatly based upon congestion. When the delay exceeds the time-out value LLC Type 2 attempts to retransmit. If the frame is not actually lost, only delayed, it is possible for the LLC Type 2 procedures to become confused. And as a result, the link may be eventually taken down if the delay exceeds the T1 timer times N2 retry count. Given the use of LLC Type 2 services, Data Link Switching addresses the following bridging problems: DLC Time-outs DLC Acknowledgments over the WAN Flow and Congestion Control Broadcast Control of Search Packets Source-Route Bridging Hop Count Limits NetBIOS also makes extensive use of datagram services that use connectionless LLC Type 1 service. In this case, Data Link Switching addresses the last two problems in the above list. The principal difference between Data Link Switching and bridging is that for connection-oriented data DLSw terminates the Data Link Control whereas bridging does not. The following figure illustrates this difference based upon two end systems operating with LLC Type 2 services.
Bridging
--------
Bridge Bridge
+------+ +----+ +----+ +------+
| End | +-----+ | +-----/ | | +-----+ | End |
|System+-+ LAN +-+ | /------+ +-+ LAN +-+System|
| | +-----+ | | TCP/IP | | +-----+ | |
+------+ +----+ +----+ +------+
Info----------------------------------------------->
<-----------------------------------------------RR
Data Link Switching
-------------------
+------+ +----+ +----+ +------+
| End | +-----+ | +-----/ | | +-----+ | End |
|System+-+ LAN +-+DLSw| /------+DLSw+-+ LAN +-+System|
| | +-----+ | | TCP/IP | | +-----+ | |
+------+ +----+ +----+ +------+
Info---------------> -------------> Info
<---------------RR ------------>
<------------RR
In traditional bridging, the Data Link Control is end-to-end. Data
Link Switching terminates the LLC Type 2 connection at the switch.
This means that the LLC Type 2 connections do not cross the wide area
network. The DLSw multiplexes LLC connections onto a TCP connection
to another DLSw. Therefore, the LLC connections at each end are
totally independent of each other. It is the responsibility of the
Data Link Switch to deliver frames that it has received from a LLC
connection to the other end. TCP is used between the Data Link
Switches to guarantee delivery of frames.
As a result of this design, LLC time-outs are limited to the local
LAN (i.e., they do not traverse the wide area). Also, the LLC Type 2
acknowledgments (RR's) do not traverse the WAN, thereby reducing
traffic across the wide area links. For SDLC links, polling and poll
response occurs locally, not over the WAN. Broadcast of search
frames is controlled by the Data Link Switches once the location of a
target system is discovered. Finally, the switches can now apply
back pressure to the end systems to provide flow and congestion
control.
Only one copy of an Link Protocol Data Unit (LPDU) is sent between
Data Link Switches in SSP messages (XIDFRAME and INFOFRAME). Retries
of the LPDU are absorbed by Data Link Switch that receives it. The
Data Link Switch that transmits the LPDU received in an SSP message to a local DLC, will perform retries in a manner appropriate for the local DLC. This may involve running a reply timer and maintaining a poll retry count. The length of the timer and the number of retries is an implementation choice based on user configuration parameters and the DLC type. Data Link Switching uses LAN addressing to set up connections between SNA systems. SDLC attached devices are defined with MAC and SAP addresses to enable them to communicate with LAN attached devices. For NetBIOS systems, Data Link Switching uses the NetBIOS name to forward datagrams and to set up connections for NetBIOS sessions. For LLC type 2 connection establishment, SNA systems send TEST (or in some cases, XID) frames to the null (0x00) SAP. NetBIOS systems have an address resolution procedure, based upon the Name Query and Name Recognized frames, that is used to establish an end-to-end circuit. Since Data Link Switching may be implemented in multi-protocol routers, there may be situations where both bridging and switching are enabled. SNA frames can be identified by their link SAP. Typical SAP values for SNA are 0x04, 0x08, and 0x0C. NetBIOS always uses a link SAP value of 0xF0.
3. Transport Connection Data Link Switches can be in used in pairs or by themselves. A Single DLSw internally switches one data link to another without using TCP (DLC(1) to DLC(2) in the figure below). This RFC does not go into details on how to implement this feature and it is not a requirement to support this RFC. A paired DLSw multiplexes data links over a reliable transport using a Switch-to-Switch Protocol (SSP). +-------------------------------------------+Switch-to-Switch | DLC Interfaces | Protocol (SSP) |+-----------+ DLC Request +-----------+ | || Data |<---------------| | |Send SSP Frame || Link | DLC Indication | | |--------------> || Control 1 |--------------->| | | |+-----------+ | Data Link | | |+-----------+ DLC Request | Switch | | || Data |<-------------- | | |Rec. SSP Frame || Link | DLC Indication | | |<------------- || Control 2 | -------------->| | | |+-----------+ +-----------+ | | Multi-Protocol Router | +-------------------------------------------+ Before Data Link Switching can occur between two routers, they must establish two TCP connections between them. Each Data Link Switch will maintain a list of DLSw capable routers and their status (active/inactive). After the TCP connection is established, SSP messages are exchanged to establish the capabilities of the two Data Link Switches. Once the exchange is complete, the DLSw will employ SSP control messages to establish end-to-end circuits over the transport connection. Within the transport connection, DLSw SSP messages are exchanged. The message formats and types for these SSP messages are documented in the following sections. The default parameters associated with the TCP connections between 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 by source-routing bridges. In this case, a TCP connection is not defined between bridges attached to the same LAN. This will allow using systems to select one of the possible Data Link Switches in a similar manner to the selection of a bridge path through a source- routed bridged network. The virtual ring segment in each Data Link Switch attached to a common LAN must be configured with the same ring number. This will prevent LAN frames sent by one Data Link Switch from being propagated through the other Data Link Switches.
3.1 SSP Frame Formats The following diagrams show the two message header formats exchanged between Data Link Switches, Control and Information. The Control message header is used for all messages except Information Frames (INFOFRAME) and Independent Flow Control Messages (IFCM), which are sent in Information header format. The INFOFRAME, KEEPALIVE and IFCM message headers are 16 bytes long, and the control message header is 72 bytes long. The fields in the first sixteen bytes of all message headers are the same. CONTROL MESSAGES (72 Bytes) (zero based offsets below shown in decimal (xx) ) +-----------------------------+-----------------------------+ | (00) Version Number | (01) Header Length (= 72) | +-----------------------------+-----------------------------+ | (02) Message Length | +-----------------------------+-----------------------------+ | (04) Remote Data Link Correlator | +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+ | | +-----------------------------+-----------------------------+ | (08) Remote DLC Port ID | +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+ | | +-----------------------------+-----------------------------+ | (12) Reserved Field | +-----------------------------+-----------------------------+ | (14) Message Type | (15) Flow Control Byte | +-----------------------------+-----------------------------+ | (16) Protocol ID | (17) Header Number | +-----------------------------+-----------------------------+ | (18) Reserved | +-----------------------------+-----------------------------+ | (20) Largest Frame Size | (21) SSP Flags | +-----------------------------+-----------------------------+ | (22) Circuit Priority | (23) Message Type (see note)| +-----------------------------+-----------------------------+ | (24) Target MAC Address (non-canonical format) | +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -| | | +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+ | | +-----------------------------+-----------------------------+ | (30) Origin MAC Address (non-canonical format) | +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -|
| |
+- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
| . . |
+-----------------------------+-----------------------------+
| (36) Origin Link SAP | (37) Target Link SAP |
+-----------------------------+-----------------------------+
| (38) Frame Direction | (39) Reserved |
+-----------------------------+-----------------------------+
| (40) Reserved |
+-----------------------------+-----------------------------+
| (42) DLC Header Length |
+-----------------------------+-----------------------------+
| (44) Origin DLC Port ID |
+- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
| |
+-----------------------------+-----------------------------+
| (48) Origin Data Link Correlator |
+- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
| |
+-----------------------------+-----------------------------+
| (52) Origin Transport ID |
+- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
| |
+-----------------------------+-----------------------------+
| (56) Target DLC Port ID |
+- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
| |
+-----------------------------+-----------------------------+
| (60) Target Data Link Correlator |
+- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
| |
+-----------------------------+-----------------------------+
| (64) Target Transport ID |
+- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
| |
+-----------------------------+-----------------------------+
| (68) Reserved Field |
+-----------------------------+-----------------------------+
| (70) Reserved Field |
+-----------------------------+-----------------------------+
(Even Byte) (Odd Byte)
INFORMATION MESSAGE (16 Bytes)
+-----------------------------+-----------------------------+
| (00) Version Number | (01) Header Length (= 16) |
+-----------------------------+-----------------------------+
| (02) Message Length |
+-----------------------------+-----------------------------+
| (04) Remote Data Link Correlator |
+- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
| |
+-----------------------------+-----------------------------+
| (08) Remote DLC Port ID |
+- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
| |
+-----------------------------+-----------------------------+
| (12) Reserved Field |
+-----------------------------+-----------------------------+
| (14) Message Type | (15) Flow Control Byte |
+-----------------------------+-----------------------------+
(Even Byte) (Odd Byte)
The first sixteen bytes of control and information message headers
contain identical fields. A brief description of some of the fields
in an SSP message are shown below (if not defined below, the fields
and/or their values are described in subsequent sections).
The Version Number field (offset 0) is set to 0x31 (ASCII '1'),
indicating a decimal value of 49. This is used to indicate DLSw
version 1.
The Header Length field (offset 1) is 0x48 for control messages,
indicating a decimal value of 72 bytes, and 0x10 for information and
Independent Flow Control messages, indicating a decimal value of 16
bytes.
The Message Length field (offset 2) defines the number of bytes
within the data field following the header.
The Flow Control Byte field (offset 15) is described in section 8.
The Header Number field (offset 17) is 0x01, indicating a value of
one.
The Circuit Priority field (offset 22) is described in section 4.
The Frame Direction field (offset 38) is set to 0x01 for frames sent
from the origin DLSw to the target DLSw, and is set to 0x02 for
frames sent from the target DLSw to the origin DLSw.
Note: The Remote Data Link Correlator and Remote DLC Port ID are set equal to the Target Data Link Correlator and Target DLC Port ID if the Frame Direction field is set to 0x01, and are set equal to the Origin Data Link Correlator and Origin DLC Port ID if the Direction Field is set to 0x02. The Protocol ID field is set to 0x42, indicating a decimal value of 66. The DLC Header Length is set to zero for SNA and is set to 0x23 for NetBIOS datagrams, indicating a length of 35 bytes. This includes the Access Control (AC) field, the Frame Control (FC) field, Destination MAC Address (DA), the Source MAC Address (SA), the Routing Information (RI) field (padded to 18 bytes), the Destination link SAP (DSAP), the Source link SAP (SSAP), and the LLC control field (UI). NOTE: The values for the Message Type field are defined in section 3.5. Note that this value is specified in two different fields (offset 14 and 23 decimal) of the control message header. Only the first field is to be used when parsing a received SSP message. The second field is to be ignored by new implementations on reception. The second field was left in for backwards compatibility with RFC 1434 implementations and this field may be used in future versions if needed. The SSP Flags field contains additional information related to the SSP message. The flags are defined as follows (bit 7 being the most significant bit and bit 0 the least significant bit of the octet): Bit(s) 76543210 Name Meaning --------- ----- ------- x....... SSPex 1 = explorer message (CANUREACH and ICANREACH) Reserved fields are set to zero upon transmission and should be ignored upon receipt. 3.2 Address Parameters A data link is defined as a logical association between the two end stations using Data Link Switching. 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). Each attachment address is classified as either "Target" in the context of the Destination MAC/SAP addresses of an explorer frame sent in the first frame used to establish a
circuit, or "Origin" in the context of the Source MAC/SAP addresses.
All MAC addresses are expressed in non-canonical (Token-Ring) format.
DATA LINK ID (14 Bytes @ Control message offset 24 decimal)
+-----------------------------+-----------------------------+
| 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 DLSw. It consists of a DLC Port ID (4 bytes), and a Data
Link Correlator (4 bytes). The Circuit ID must be unique in a single
DLSw and is assigned locally. The pair of Circuit ID's along with
the Data Link IDs, uniquely identify a single end-to-end circuit.
Each DLSw 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" DLSw and "Target"
DLSw, 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 message types all carry the Data Link ID, consisting of the MAC and Link SAP addresses associated with the two end stations. The CANUREACH and ICANREACH messages are qualified by the SSPex flag into CANUREACH_ex, ICANREACH_ex (explorer messages) and CANUREACH_cs, ICANREACH_cs (circuit start). The CANUREACH_ex is used to find a remote MAC and Link SAP address without establishing an SSP circuit. Upon receipt of a CANUREACH_cs message, the target DLSw starts a data link for each port, thereby obtaining a Data Link Correlator. If the target station can be reached, an ICANREACH_cs message is returned to the origin DLSw containing the Target Circuit ID parameter. Upon receipt, the origin DLSw starts a data link and returns the Origin Circuit ID to the target DLSw within the REACH_ACK message. (Note for a full list of message types, see section 3.5.) +------------+ +------------+ |Disconnected| |Disconnected| +------------+ CANUREACH_cs (Data Link ID) +------------+ -------------------------------------------------> ICANREACH_cs (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) <------------------------------------------------- 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 DLSw. 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 Correlators The local use, and contents of the Data Link Correlator, Port ID and Transport ID fields in SSP messages is an implementation choice. These fields have local significance only. The values received from a partner DLSw must not be interpreted by the DLSw that receives them and should be echoed "as is" to a partner DLSw in subsequent messages. All implementations must obey the following rules in this section (3.3) on the assignment and fixing of these correlator fields for each transport connection or circuit: The Transport ID fields are learned from the first SSP message exchanged with a DLSw partner (the Capabilities exchange). This field should not be varied by a DLSw after the capabilities exchange and must be reflected to the partner DLSw in every SSP control message. The Target Data Link Correlator, Target Port ID and Target Transport ID must remain the same once the Target DLSw has sent the ICANREACH_cs for a given circuit. The Origin DLSw must store the values specified in the ICANREACH_cs and use these on all subsequent SSP messages for this circuit. The Origin DLSw must allow these fields to vary until the ICANREACH_cs is received. Each SSP message issued for a circuit must reflect the values specified by the Target DLSw in the last SSP message for this circuit received by the Origin DLSw. Binary zero should be used if no such message has yet been received for a given circuit (apart from the Target Transport ID which will have been learnt as specified above). The Origin Data Link Correlator, Origin Port ID and Origin Transport ID must remain the same once the Origin DLSw has issued the REACH_ACK for a given circuit. The Target DLSw must store the values specified in the REACH_ACK and use these on all subsequent SSP messages for this circuit. The Target DLSw must allow these fields to vary until the REACH_ACK is received. Each SSP message issued for a circuit must reflect the values specified by the Origin DLSw in the last SSP message for this circuit received by the Target DLSw. Binary zero should be used if
no such message has yet been received for a given circuit (apart from the Origin Transport ID which will have been learnt as specified above). For the purposes of correlator exchange, explorer messages form a separate circuit. Both DLSw partners must reflect the last received correlator values as specified above. However correlators learned on explorer messages need not be carried over to a subsequent circuit setup attempt. In particular, the Origin DLSw may elect to use the same values for the Origin Data Link Correlator and Origin Port ID when it issues a CANUREACH_cs after receiving an ICANREACH_ex or NETBIOS_NR_ex. However the Target DLSw must not assume that the CANUREACH_cs will specify any of the Target Data Link Correlator or Target Port ID that were exchanged on the explorer messages. Received SSP messages that require a valid Remote Circuit ID but cannot be associated with an existing circuit should be rejected with a HALT_DL_NOACK message. This is done to prevent a situation where one DLSw partner has a circuit defined while the other partner does not. The exception would be a HALT_DL_NOACK message with an invalid Remote Circuit ID. The HALT_DL_NOACK message is typically used in error situations where a response is not appropriate. The SSP messages requiring a valid Remote Circuit ID are all messages except the following: CANUREACH_ex, CANUREACH_cs, ICANREACH_ex, ICANREACH_cs, NETBIOS_NQ_cs, NETBIOS_NR_cs, DATAFRAME, NETBIOS_ANQ, NETBIOS_ANR, KEEPALIVE and CAP_EXCHANGE. 3.4 Largest Frame Size Field The Largest Frame Size (LF Size) field in the SSP Control Header is used to carry the LF Size bits across the DLSw connection. This should be used to ensure that the two end-stations always negotiate a frame size to be used on a circuit that does not require the Origin and Target DLSw partners to re-segment frames. This field is valid on CANUREACH_ex, CANUREACH_cs, ICANREACH_ex, ICANREACH_cs, NETBIOS_NQ_ex and NETBIOS_NR_ex messages only. The contents of this field should be ignored on all other frames. Every DLSw forwarding a SSP frame to its DLSw partner must ensure that the contents of this frame reflect the minimum capability of the route to its local end-station or any limit imposed by the DLSw itself. The bit-wise definition of this field is as follows (bit 7 is the most significant bit, bit 0 is the least significant bit):
7 6 5 4 3 2 1 0
+-------------------------------+
| c | r | b | b | b | e | e | e |
+-------------------------------+
c . . . . . . . LF Size Control flag
(significant on messages
from Origin to Target
DLSw only)
0=fail circuit if route
obtained requires a
smaller LF size
1=don't fail the circuit
but return the LF size
obtained even if it is
smaller
. r . . . . . . Reserved
. . b . . . . . Largest Frame Bit Base
. . . b . . . . Largest Frame Bit Base
. . . . b . . . Largest Frame Bit Base
. . . . . e . . Largest Frame Bit Extended
. . . . . . e . Largest Frame Bit Extended
. . . . . . . e Largest Frame Bit Extended
<----- LF Bits ----->
Refer to IEEE 802.1D Standard, Annex C for encoding of Largest Frame
base and extended bit values.
The Origin DLSw "Size Control" flag informs a Target DLSw that
chooses to reply to *_cs messages on the basis of cached information
that it may safely return a smaller LF Size on the ICANREACH_cs frame
if it has had to choose an alternative route on which to initialize
the circuit. If this bit is set to 1, the Origin DLSw takes
responsibility for ensuring that the end-stations negotiate a
suitable frame size for the circuit. If this bit is set to 0, the
Target DLSw must not reply to the CANUREACH_cs if it cannot obtain a
route to the Target end station that support an LF Size at least as
large as that specified in the CANUREACH_cs frame.
3.5 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 Description Type flags/notes ------- -------- ------ ----------- CANUREACH_ex Can U Reach Station-explorer 0x03 SSPex CANUREACH_cs Can U Reach Station-circuit start 0x03 ICANREACH_ex I Can Reach Station-explorer 0x04 SSPex ICANREACH_cs I Can Reach Station-circuit start 0x04 REACH_ACK Reach Acknowledgment 0x05 DGRMFRAME Datagram Frame 0x06 (note 1) XIDFRAME XID Frame 0x07 CONTACT Contact Remote Station 0x08 CONTACTED Remote Station Contacted 0x09 RESTART_DL Restart Data Link 0x10 DL_RESTARTED Data Link Restarted 0x11 ENTER_BUSY Enter Busy 0x0C (note 2) EXIT_BUSY Exit Busy 0x0D (note 2) INFOFRAME Information (I) Frame 0x0A HALT_DL Halt Data Link 0x0E DL_HALTED Data Link Halted 0x0F NETBIOS_NQ_ex NETBIOS Name Query-explorer 0x12 SSPex NETBIOS_NQ_cs NETBIOS Name Query-circuit setup 0x12 (note 3) NETBIOS_NR_ex NETBIOS Name Recognized-explorer 0x13 SSPex NETBIOS_NR_cs NETBIOS Name Recog-circuit setup 0x13 (note 3) DATAFRAME Data Frame 0x14 (note 1) HALT_DL_NOACK Halt Data Link with no Ack 0x19 NETBIOS_ANQ NETBIOS Add Name Query 0x1A NETBIOS_ANR NETBIOS Add Name Response 0x1B KEEPALIVE Transport Keepalive Message 0x1D (note 4) CAP_EXCHANGE Capabilities Exchange 0x20 IFCM Independent Flow Control Message 0x21 TEST_CIRCUIT_REQ Test Circuit Request 0x7A TEST_CIRCUIT_RSP Test Circuit Response 0x7B Note 1: Both the DGRMFRAME and DATAFRAME messages are used to carry information received by the DLC entity within UI frames. 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 or during circuit restart when Data Links are reset. Note 2: These messages are not used for the DLSw Standard but may be used by older DLSw implementations. They are listed here for informational purposes. These messages were added after publication of RFC 1434 and were deleted in this standard (adaptive pacing is now used instead).
Note 3: These messages are not normally issued by a Standard DLSw, which uses the NB_*_ex messages as shown in section 5.4. However if a Standard DLSw attempts to interoperate with older DLSw implementations, these messages correspond to the NETBIOS_NQ and NETBIOS_NR messages used in RFC1434 both to locate the resource and to setup a circuit. This document does not attempt to provide a complete specification of the use of these messages. Note 4: A KEEPALIVE message may be sent by a DLSw to a partner DLSw in order to verify the TCP connection (or other future SSP carrying protocol) is still functioning. If received by a DLSw, this message is discarded and ignored. Use of this message is optional. 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. Any SSP message types not defined above if received by a DLSw are to be ignored (i.e., no error action is to be performed). A Data Link Switch should quietly drop any SSP message with a Message Type that is not recognized or not supported. Receipt of such a message should not cause the termination of the transport connection to the message sender.
(page 18 continued on part 2)
