📄 rfc1795.txt
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+-------------------------------------------+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
Wells & Bartky [Page 6]
RFC 1795 Data Link Switching April 1995
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.
Wells & Bartky [Page 7]
RFC 1795 Data Link Switching April 1995
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) |
+- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -|
Wells & Bartky [Page 8]
RFC 1795 Data Link Switching April 1995
| |
+- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
| . . |
+-----------------------------+-----------------------------+
| (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)
Wells & Bartky [Page 9]
RFC 1795 Data Link Switching April 1995
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.
Wells & Bartky [Page 10]
RFC 1795 Data Link Switching April 1995
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
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