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OID_GEN_TRANSPORT_HEADER_OFFSET (Compact 2013)

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3/26/2014

As a set, this OID indicates the size of additional headers for packets that a particular transport sends and receives.

Remarks

A transport informs miniport drivers and other layered drivers of this header size; these drivers can then use this information when processing packets. For example, a driver could use the sublayer header size obtained from the transport to locate the beginning of higher layer information in packets, such as the start of the IP header. The driver could then parse and adjust the fields of the IP protocol header as appropriate. Transports use a TRANSPORT_HEADER_OFFSET structure, which is defined as follows, to indicate this header size.

typedef struct _TRANSPORT_HEADER_OFFSET {
  USHORT  ProtocolType; 
  USHORT  HeaderOffset; 
} TRANSPORT_HEADER_OFFSET, *PTRANSPORT_HEADER_OFFSET;

The members of this structure contain the following information:

  • ProtocolType
    Specifies the protocol type that sends this OID and that subsequently sends and receives packets by using the specified sublayer header size. The protocol is one of the following values:

    • NDIS_PROTOCOL_ID_DEFAULT
      Default protocol
    • NDIS_PROTOCOL_ID_TCP_IP
      TCP/IP protocol
    • NDIS_PROTOCOL_ID_IPX
      NetWare IPX protocol
    • NDIS_PROTOCOL_ID_NBF
      NetBIOS protocol
  • HeaderOffset
    Specifies the size, in bytes, of the sublayer header that precedes the protocol header for packets that the protocol subsequently sends to or receives from the miniport driver or other layered driver. For example, sizeof(Ethernet header) + sizeof(SNAP header).

Typically, transports calculate the header size of packets from information that is retrieved from miniport drivers. To request the maximum total packet size in bytes that a network adapter supports, including the header, transports use the OID_GEN_MAXIMUM_TOTAL_SIZE OID. To request the maximum packet size in bytes that a network adapter supports, excluding a header, transports use the OID_GEN_MAXIMUM_FRAME_SIZE OID. To calculate the maximum header size, transports subtract the maximum frame size from the maximum total size.

If a transport transmits packets that contain sublayer header information, the transport must know the sublayer header size of these packets and must inform underlying miniport drivers and other layered drivers about the size so that the drivers can process the packets. Sending and receiving particular sublayer header information within a packet may be an option that can be set in the registry for a particular protocol. Transports could then obtain information about sublayer headers from the registry and pass the header size down to miniport drivers or other layered drivers.

For example, if a transport handles packets from the Fiber Distributed Data Interface medium, the transport must send a set request to underlying miniport drivers and other layered drivers using OID_GEN_TRANSPORT_HEADER_OFFSET to inform those drivers about the size of the packets' sublayer header. (FDDI is not supported in Windows Vista.) These packets from FDDI could contain Logical Link Control (LLC) information. This LLC information could in turn include an LLC header and other headers such as Sub-Network Access Protocol (SNAP). The transport determines from the registry to use LLC/SNAP and passes the header size of the LLC/SNAP segments of packets to miniport drivers.

This OID is optional for miniport drivers and other layered drivers. Because this OID is optional, drivers are not required to respond to requests that transports make using this OID.

Requirements

Header

ntddndis.h

See Also

Reference

General Operational OIDs
OID_GEN_MAXIMUM_FRAME_SIZE
OID_GEN_MAXIMUM_TOTAL_SIZE