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Communications Programming Concepts
DLCETHER Name-Discovery Services
In addition to the standard IEEE 802.2 Common Logical Link Protocol support and address resolution services, Standard Ethernet data link control (DLCETHER) also provides a name-discovery service that allows the operator to identify local and remote stations by name instead of by six-byte physical addresses. Each port must have a unique name on the network of up to 20 characters. The character set used varies depending on the user's protocol. Systems network architecture (SNA), for example, requires character set A. Additionally, each new service access point (SAP) supported on a particular port can have a unique name if desired.
Each name is added to the network by broadcasting a find (local name) request when the new name is being introduced to a given network port. If no response other than an echo results from the find (local name) request, after sending it the number of times specified, the physical link is declared opened, and the name is assigned to the local port and SAP. If another port on the network has already added the name, a name found response is sent to the station that issued the find request. A result code (DLC_NAME_IN_USE) indicates that the new attachment was unsuccessful and that a different name must be chosen. Calls are established by broadcasting a find (remote name) request to the network and waiting for a response from the port with the specified name. Only those ports that have listen attachments pending receive colliding find requests or are already attached to the requesting remote station answer a find request.
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LAN Find Data Format
Find Header
| 0-1 |
Byte length of the find packet including the length field |
| 2-3 |
Key 0x0001 |
| 4-n |
Remaining control vectors |
Target Name
| 0-1 |
Vector length = 0x000F to 0x0022 |
| 2-3 |
Key 0x0004 |
| 4-9 |
Name structure architecture ID:
| 4-5 |
Subvector length = 0x0006 |
| 6-7 |
Key 0x4011 |
| 8-9 |
Identifier = 0x8000 (locally administered) |
| 10-m |
Object name:
| 10-11 |
Subvector length = 0x0005 to 0x000C |
| 12-13 |
Key 0x4010 |
| 14-m |
Target name (1 to 20 bytes) |
|
|
Source Name
| 0-1 |
Vector length = 0x000F to 0x0022 |
| 2-3 |
Key 0x000D |
| 4-9 |
Name structure architecture ID:
| 4-5 |
Subvector length = 0x0006 |
| 6-7 |
Key 0x4011 |
| 8-9 |
Identifier = 0x8000 (locally administered) |
| 10-p |
Object name:
| 10-11 |
Subvector length = 0x0005 to 0x000C |
| 12-13 |
Key 0x4010 |
| 14-p |
Source name (1 to 20 bytes) |
|
|
Correlator
| 0-1 |
Vector length = 0x0008 |
| 2-3 |
Key 0x4003 |
| 4-7 |
Correlator value:
| Byte 4 Bit 0 |
1 represents a SAP correlator for a find (self) |
| Byte 4 Bit 0 |
0 represents a link station (LS) correlator for a find (remote) |
|
Source Medium Access Control (MAC) Address
| 0-1 |
Vector length = 0x000A |
| 2-3 |
Key 0x4006 |
| 4-9 |
Source MAC address (6 bytes). |
Source SAP
| 0-1 |
Vector length = 0x0005 |
| 2-3 |
Key 0x4007 |
| 4 |
Source SAP address |
LAN Found Data Format
Found Header
| 0-1 |
Byte length of the found packet including the length field |
| 2-3 |
Key 0x0002 |
| 4-n |
Remaining control vectors |
Correlator
| 0-1 |
Vector length = 0x0008 |
| 2-3 |
Key 0x4003 |
| 4-7 |
Correlator value:
| Byte 4 Bit 0 |
1 represents a SAP correlator for a find (self) |
| Byte 4 Bit 0 |
0 represents an LS correlator for a find (remote) |
|
Source MAC Address
| 0-1 |
Vector length = 0x000A |
| 2-3 |
Key 0x4006 |
| 4-9 |
Source MAC address (6 bytes) |
Source SAP
| 0-1 |
Vector length = 0x0005 |
| 2-3 |
Key 0x4007 |
| 4 |
Source SAP address |
Response Code
| 0-1 |
Vector length = 0x0005 |
| 2-3 |
Key 0x400B |
| 4 |
Response code:
| B'0xxx xxxx' |
Positive response |
| B'0000 0001' |
Resources available |
| B'1xxx xxxx' |
Negative response |
| B'1000 0001' |
Insufficient resources |
|
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