Communications Programming Concepts

DLCTOKEN Name-Discovery Service

In addition to the standard IEEE 802.2 Common Logical Link Protocol support and address resolution services, token-ring data link control (DLCTOKEN) also provides a name-discovery service that allows the operator to identify local and remote stations by name instead of by 6-byte physical addresses. Each port must have a unique name of up to 20 characters on the network. 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 it is sent the specified number of times, the physical link is declared opened. The name is then 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 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 ports that have listen attachments pending, receive colliding find requests, or are already attached to the requesting remote station answer a find request.

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's 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's name (1 to 20 bytes)

Correlator

0-1	Vector length = 0x0008
2-3	Key 0x4003
4-7	Correlator value: Byte 4 Bit 0 1 means this is a SAP correlator for a find (self) Byte 4 Bit 0 0 means this is an LS correlator for a find (remote)

Source Medium Access Control (MAC) Address

0-1	Vector length = 0x000A
2-3	Key 0x4006
4-9	Source's MAC address (6 bytes)

Source SAP

0-1	Vector length = 0x0005
2-3	Key 0x4007
4	Source's 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 means this is a SAP correlator for a find (self). Byte 4 Bit 0 0 means this is an LS correlator for a find (remote).

Source MAC Address

0-1	Vector length = 0x000A
2-3	Key 0x4006
4-9	Source's MAC address (6 bytes)

Source SAP

0-1	Vector length = 0x0005
2-3	Key 0x4007
4	Source's 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

Bridge Route Discovery

DLCTOKEN caches any returned bridge-routing information from a remote station on each packet received and generates send-packet headers with the reverse route. This operation allows dynamic alteration of the bridge route taken throughout the link station attachment. There is also a provision to alter the cached routing field with the DLC_ALT_RTE ioctl operation. This ioctl operation allows the user to change the bridge route taken by link station send packets dynamically throughout the attachment as long as an alternate route exists. After the DLC_ALT_RTE ioctl operation has been issued, dynamic caching of the received route is stopped for the link station, and all send packets carry the ioctl operation's routing value.

Network data packets are not associated with a link station attachment, so any bridge routing field has to come from the user sending the packet. DLCTOKEN has no involvement in the bridge routing of network data cases.