HP OpenVMS I/O User's Reference Manual

Local Area Network (LAN) Device Drivers

Format Parameter

LAN Device Information

Features of Packet Formats  

The FDDI controllers can transmit and receive FDDI frames.There is a mapped Ethernet frame format that is comparable to theEthernet packets sent and received by the Ethernet controllers.

At the time each port on the controller is started, one ofthree packet formats can be specified: Ethernet (default), standard802 (referred to as 802 packet format), and extended 802. If noformat is specified, the default format is used.

Each port on the controller must be unique on that controller.For each packet format, there is a parameter that distinguishesthe port from all other ports with the same packet format. For Ethernetpacket format ports, the 2-byte protocol type parameter definesthe port. For 802 packet format ports, the 1-byte SAP defines theport. For extended 802 format ports, the 5-byte protocol identifierdefines the port.

Sections Ethernet Packet Format, IEEE 802 Packet Format, and IEEE 802 Extended Packet Format describe the three packetformats and the characteristics unique to each format.

Ethernet Packet Format  

The Ethernet format specifiesa two-byte protocol type field followed by an optional length field.The length field is included in transmit packets and expected inreceive packets with the PAD parameter is enabled. The followingsections describe these features.

Ethernet Protocol Types  

Every Ethernet frame hasa 2-byte protocol-type field. This field is used to determine theport to which a packet belongs. Protocol types are independent ofaddresses; unique protocol designations can be assigned by Xeroxa particular station starts an Ethernet port, that user must specifythe protocol type to be used on that port. Messages sent over thatport always have the protocol type attached to them by the devicedriver, and messages rece that protocol type are delivered to thestarter of that port. Valid protocol types are in the range 05-DDthrough FF-FF.

The following lists the cross-company protocol types:

Value	Meaning
08-00	IP protocol (ARPAnet)
08-06	Address resolution protocol(ARPAnet)
90-00	Ethernet Loopback protocol

THe following lists some commonly used protocol types.

Value	Meaning
60-01	DNA Dump/load (MOP)
60-02	DNA Remote Console (MOP)
60-03	DNA Routing
60-04	Local Area Transport (LAT)
60-05	Diagnostics
60-06	Customer use
60-07	System Communication Architecture(SCA)
80-38	Bridge
80-3C	DNA Naming Service
80-3D	CSMA/CD Encryption
80-3E	DNA Time Service
80-3F	LAN Traffic Monitor
80-40	NETBIOS Emulator (PCSG)
80-41	Local Area System Transport (LAST)

Ethernet Packet Padding  

This sectiondescribes the PAD parameter NMA$C_PCLI_PAD, which is used only inthe Ethernet packet format.

All CSMA/CD frames must be at least 64 bytes in length. Thisincludes the Ethernet header, the user data, and the CRC. If theuser data, CRC, and Ethernet header together are less than 64 bytes,zero padding bytes are inserted between the user data and the CRCto make a 64-byte packet. This packet padding cannot be turned off.

The PAD parameter directs the LAN drivers to place a data-sizefield in the packet between the standard header and the user data.If padding is on (NMA$C_STATE_ON is specified), a 2-byte lengthfield is inserted after the Protocol Type field and before the userdata.

If the PAD parameter is off (NMA$C_STATE_OFF is specified),Ethernet packets have the following characteristics:

• Packets transmitted are padded withnull bytes as needed (CSMA/CD only).



• Packets transmitted do not include the size field.



• The length of user data in the packets receivedis always between 46 and 1500 bytes for CSMA/CD, and 0 to 4470 forFDDI. For example, if a 10-byte packet is transmitted, it is receivedas 46 bytes because the driver cannot determine the amount of userdata in the packet--only the amount of user data plus padded nullbytes.

If the PAD parameter is on (NMA$C_STATE_ON is specified),Ethernet packets have the following characteristics:

• Packets transmitted are padded withnull bytes as needed (CSMA/CD only).



• Packets transmitted include the size field.



• The length of user data in the packets receivedis always between 0 and 1498 bytes for CSMA/CD, and 0 to 4468 bytesfor FDDI. The driver uses the size field to determine the amountof user data in the packet. The size field is not included in thedata returned to the user.

Protocol Type Sharing  

Protocoltypes are usually nonshareable; however, an application may benefitfrom a shared protocol implementation. The protocol access parameter(NMA$C_PCLI_ACC) allows a protocol type to be opened in either oftwo shareable modes: shared-default (NMA$C_ACC_SHR) and shared-with-destination (NMA$C_ACC_LIM).The LAN drivers also provide the nonshareable exclusive mode (NMA$C_ACC_EXC). (See P2 Attributes.) The rules and requirementsfor using each mode are as follows:

• The exclusive mode is the defaultif no access mode is supplied as a P2 buffer parameter. This modeof operation does not allow the protocol to be shared by other users.Any attempt to start up another protocol of the same type resultsin an error status of SS$_BADPARAM.



• The shared-with-destination mode is a protocol type/destinationaddress pairing that allows multiple users to share a protocol typeand to communicate with a different node.
  
  For a given shared protocol type, there can be many "shared-with-destination" users;each user communicates with a different destination address. Anyattempt to start a port with a destination address that is in useresults in an error status of SS$_BADPARAM.
  
  When a "shared-with-destination" user passesthe set mode P2 buffer, the buffer must contain a destination addressin the NMA$C_PCLI_DES parameter. This destination address is usedas the destination address in all messages transmitted, and theuser receives messages only from this address.



• The shared-default mode is the default user of ashared protocol type. There can be only one such user for each sharedprotocol type. A "shared-default" user does nothave to exist if a protocol type is shared, but there can be nomore than one such user per shared protocol type.
  
  The "shared-default" user receives all messagesfor the shared protocol type, but not for any of the "shared-with-destination" users.The "shared-default" user also receives all messagesmatching both the shared protocol type and any multicast addressenabled by the "shared-default" user.
  
  The "shared-default" user can only transmitto multicast addresses and physical addresses that are not enabledby any of the "shared-with-destination" userssharing the same protocol type.
  
  If there is no "shared-default" user ofa protocol type, incoming messages from nodes not among the "shared-with-destination" usersfor that protocol type are ignored.

IEEE 802 Packet Format  

The IEEE 802 packet formatsaccepted for a port depend on the service enabled on that port.All 802 packet formats have an 802.2 header. The service on theport determines the valid values for the 802.2 fields.

When a port is started, the NMA$C_PCLI_SRV parameter in theP2 buffer selects the service on that port. A value of NMA$C_LINSR_CLIspecifies Class I service and a value of NMA$C_LINSR_USR specifies user-suppliedservice (the default).

Class I Service Packet Format  

ForClass I service, only three packet formats are transmitted and received:UI, XID, and TEST. Class I Service 802.2 Header showsthe 802.2 header format for Class I service.  

Figure 16  Class I Service 802.2 Header

The control field for an 802 packet is always an unnumberedcontrol field. The unnumbered control field, which is always 1 bytein length, is passed by the P4 argument of the write QIO and canbe one of the following binary values:

• UI command (00000011)
  
  This is the unnumbered information command. It is the methodused to transmit data from one user to another and is the most widelyused control field value.
  
  The UI command can be specified by using NMA$C_CTLVL_UI.



• XID command (101p1111)
  
  This is the exchange identification command. It is used toconvey information about the port. The "p" bit isthe poll bit and can be either 0 or 1. This command can be specifiedby using NMA$C_CTLVL_XID for a "0" poll bit orNMA$C_CTLVL_XID_P for a "1" poll bit.



• XID response (101f1111)
  
  The XID response is a response to an XID command. The "f" bitis the final bit and will match the poll bit from the XID command.



• TEST command (111p0011)
  
  The TEST command is used to test a connection. The "p" bitis the poll bit and can be either 0 or 1. This command can be specifiedby using NMA$C_CTLVL_TEST for a "0" poll bit orNMA$C_CTLVL_TEST_P for a "1" poll bit.



• TEST response (111f0011)
  
  The TEST response is a response to a TEST command. The "f" bitis the final bit and will match the poll bit from the TEST command.

An 802 format port with Class I service is allowed to transmitUI, XID, and TEST commands. An 802 format port with Class I serviceis allowed to receive UI commands and XID and TEST responses.

Refer to the IEEE 802.2 Standard for more information on thesecontrol field values and response messages.

User-Supplied Service Header Format  

User-Supplied Service 802.2 Header shows the 802.2header format for user-supplied service.  

Figure 17  User-Supplied Service 802.2 Header

The user provides the control field values, which are documentedin the IEEE 802.2 Standard. The user-supplied packet format is thegeneric packet format as specified in the IEEE 802.2 Standard. ClassI packets (see Class I Service Packet Format )are a subset of this generic packet format; therefore, if the controlfield value of the user-supplied packet is UI, XID, or TEST, thepacket is the same as a Class I packet. Note that Class II packets,as defined in the IEEE 802.2 Standard, include the UI, XID, and TESTcommand/response formats.

ServiceAccess Point (SAP) Use and Restrictions  

TheIEEE 802.2 Standard places restrictions on both user SAPs and sourceSAPs (SSAPs). All SAPs are 8 bits long. DSAP and SSAP Format shows the format of destination SAPs (DSAPs) andSSAPs.  

Figure 18  DSAP and SSAP Format

Definition of the least significant bit depends on whetherthe SAP is a source SAP (SSAP) or a destination SAP (DSAP). Fora DSAP field, the least significant bit distinguishes group SAPs(bit 0 = 1) from individual SAPs (bit 0 = 0). For an SSAP field,the least significant bit distinguishes commands (bit 0 = 0) fromresponses (bit 0 = 1). Because these two bits are located at thesame bit position within the SAP field, a group SAP cannot be usedas an SSAP. If this were allowed, a group SAP would be interpretedas an individual SAP with the command/response bit set to 1, thusimplying a response. The IEEE 802.2 Standard reserves for its own definitionall SAP addresses with the second least significant bit set to 1.You should use these SAP values for their intended purposes, asdefined in the IEEE 802.2 Standard.

Up to four group SAPs can be enabled on each 802 port. Thegroup SAPs enabled on a controller do not have to be unique foreach port; for example, two 802 format ports can have the same groupSAP enabled. This allows a single packet coming into the controllerto be duplicated and passed to each port on the controller thathas the group SAP enabled--assuming the packet has a DSAPvalue that is a group SAP. If the received packet has an individualSAP for a DSAP, the packet goes to, at most, one port.

IEEE 802 Extended Packet Format  

The 802E format usesthe 802.2 and 802.1 headers, as shown in 802 Extended Header.  

Figure 19  802 Extended Header

For an 802E packetformat, the DSAP and SSAP fields are always set to the SNAP SAP(AA hex). The SNAP SAP value is a special SAP value reserved for802 extended format packets. The SNAP SAP value distinguishes an802 packet from an 802 extended packet. The only valid control fieldvalue for 802 extended packets is UI (unnumbered information).

Protocol Type PID Sharing (Alpha Only)  

OnAlpha systems, the 802E format allows user's protocol identifier(PID) sharing. PIDs are usually nonshareable; however, an applicationmay benefit from a shared protocol implementation. The protocol accessparameter (NMA$C_PCLI_ACC) allows a PID to be opened in either oftwo shareable modes: shared-default (NMA$C_ACC_SHR) and shared-with-destination(NMA$C_ACC_LIM). The LAN drivers also provide the nonshareable exclusivemode (NMA$C_ACC_EXC). (See P2 Attributes.) The rules and requirements for using each modeare as follows:

• The exclusive mode is the defaultif no access mode is supplied as a P2 buffer parameter. This modeof operation does not allow the protocol to be shared by other users.Any attempt to start up another protocol of the same type resultsin an error status of SS$_BADPARAM.



• The shared-with-destination mode is a PID/destinationaddress pairing that allows multiple users to share a PID and tocommunicate with a different node.
  
  For a given shared protocol type, there can be many "shared-with-destination" users;each user communicates with a different destination address. Anyattempt to start a port with a destination address that is in useresults in an error status of SS$_BADPARAM.
  
  When a "shared-with-destination" user passesthe set mode P2 buffer, the buffer must contain a destination addressin the NMA$C_PCLI_DES parameter. This destination address is usedas the destination address in all messages transmitted, and theuser receives messages only from this address.



• The shared-default mode is the default user of ashared PID. There can be only one such user for each shared PID.A "shared-default" user does not have to existif a PID is shared, but there can be no more than one such userper shared PID.
  
  The "shared-default" user receives all messagesfor the shared PID, but not for any of the "shared-with-destination" users.The "shared-default" user also receives all messagesmatching both the shared PID and any multicast address enabled bythe "shared-default" user.
  
  The "shared-default" user can only transmitto multicast addresses and physical addresses that are not enabledby any of the "shared-with-destination" userssharing the same PID.
  
  If there is no "shared-default" user ofa PID, incoming messages from nodes not among the "shared-with-destination" usersfor that PID are ignored.

Format Parameter

LAN Device Information