TCP_OFFLOAD_STATE_CACHED structure (ndischimney.h)

Article
05/26/2021

[The TCP chimney offload feature is deprecated and should not be used.]

The TCP_OFFLOAD_STATE_CACHED structure contains the cached variables of a TCP connection state object.

Syntax

typedef struct _TCP_OFFLOAD_STATE_CACHED {
  OFFLOAD_STATE_HEADER Header;
  USHORT               Flags;
  ULONG                InitialRcvWnd;
  ULONG                RcvIndicationSize;
  UCHAR                KaProbeCount;
  ULONG                KaTimeout;
  ULONG                KaInterval;
  ULONG                MaxRT;
  ULONG                FlowLabel : 20;
  UCHAR                TtlOrHopLimit;
  UCHAR                TosOrTrafficClass;
  UCHAR                UserPriority : 3;
} TCP_OFFLOAD_STATE_CACHED, *PTCP_OFFLOAD_STATE_CACHED;

Members

Header

An OFFLOAD_STATE_HEADER structure. NDIS sets the Length member of Header to the size, in bytes, of the TCP_OFFLOAD_STATE_CACHED structure. The RecognizedOptions member of Header is reserved.

Flags

A bitmask that can be set to zero or any of the following flags, combined with bitwise OR:

TCP_FLAG_KEEP_ALIVE_ENABLED

The host stack sets this flag to enable the keepalive option on the connection. The host stack clears this flag to disable the keepalive option on the connection. For more information about the keepalive option, see RFC 1122.

TCP_FLAG_NAGLING_ENABLED

The host stack sets this flag to enable the Nagle algorithm on the connection. The host stack clears this flag to disable the Nagle algorithm on the connection. For more information about the Nagle algorithm, see RFC 896.

TCP_FLAG_KEEP_ALIVE_RESTART

The host stack sets this flag to cause the offload target to reset its keepalive timer to zero.

TCP_FLAG_MAX_RT_RESTART

The host stack sets this flag to cause the offload target to reset the TotalRT variable in the TCP delegated state for the connection to zero.

TCP_FLAG_UPDATE_RCV_WND

The host stack sets this flag to indicate that the default receive window size (InitialRcvWnd in the TCP_OFFLOAD_STATE_CACHED structure) has changed. If the value of InitialRcvWnd differs from the offload target's current receive window size (RcvWnd in the TCP_OFFLOAD_STATE_DELEGATED structure), the offload target must advertise a new receive window size to the peer.

InitialRcvWnd

The default receive window (from socket option SO_RCVBUF).

RcvIndicationSize

When non-NULL, the optimum number of data bytes that the offload target should supply in a single call to the NdisTcpOffloadReceiveHandler function. This number, which is typically between tens and hundreds of bytes, is a suggestion--not a requirement. The offload target can indicate more or less than RcvIndicationSize data bytes. However, for optimum performance, the offload target should indicate
RcvIndicationSize data bytes. For more information, see NdisTcpOffloadReceiveHandler.

When NULL, RcvIndicationSize is unspecified. In this case, the offload target should ignore RcvIndicationSize and indicate as much data as possible in calls to the NdisTcpOffloadReceiveHandler function.

KaProbeCount

The number of keepalive probes that the offload target should send to determine whether a TCP connection is intact (see RFC 1122).

KaTimeout

This member specifies, in clock ticks, the timeout interval for inactivity before sending a keepalive probe (see RFC 1122).

KaInterval

This member specifies, in clock ticks, the timeout after which to retransmit a keepalive frame if no response is received to a keepalive probe (see RFC 1122).

MaxRT

This member specifies, in clock ticks, the maximum time that the offload target should spend retransmitting a segment. If the value of MaxRT is non-zero, MaxRT overrides TcpMaximumRetransmissions in the NDIS_TASK_TCP_CONNECTION_OFFLOAD structure. The default value for is MaxRT is zero, which means that TcpMaximumRetransmissions sets the maximum number of times that the offload target should retransmit a segment on a TCP connection. If the value of MaxRT is FFFFFFFF (or -1), there is no limit on the maximum number of retransmissions. When MaxRT is nonzero, the offload target should reset the TotalRT variable in the TCP delegated state for the connection. TotalRT indicates the total time, in clock ticks, that the offload target has spent retransmitting the current TCP segment.

FlowLabel

This member marks host-labeled packets for special handling by intervening routers--for example, nondefault QoS or "real-time" service (see RFC 2460). This variable is set through a socket option and can vary during the lifetime of the TCP connection. This variable is only meaningful if the TCP connection is over IPv6.

TtlOrHopLimit

If the TCP connection is over IPv4, then this member specifies the time to live (see RFC 791). This variable is set through a socket option and can vary during the lifetime of the TCP connection.

If the TCP connection is over IPv6, then this member specifies the number of routers that the packet can pass through (see RFC 2460). This variable is set through a socket option and can vary during the lifetime of the TCP connection.

TosOrTrafficClass

If the TCP connection is over IPv4, then this member specifies the type of service for routing a packet (see RFC 2474). This variable is set through a socket option and can vary during the lifetime of the TCP connection.

If the TCP connection is over IPv6, then this member prioritizes values for packets according to traffic types, indicating how willing the sender is to have the packets discarded (see RFC 2460). This variable is set through a socket option and can vary during the lifetime of the TCP connection.

UserPriority

A 3-bit priority value. If the offload target supports 802.1p packet information, it must insert this priority value into the tag header of all packets that it transmits on the offloaded TCP connection. If the offload target does not support 802.1p packet information, it should ignore UserPriority . For more information about supporting 802.1p packet information, see 802.1Q and 802.1p Processing on an Offloaded TCP Connection.

Remarks

Cached variables are owned and maintained by the host stack. An offload target must not change the value of a cached variable unless requested to do so by the host stack. If the value of a cached variable changes, the host stack requests an update of the variable, which causes NDIS to call the offload target's MiniportUpdateOffload function. When the host stack terminates the offload of one or more state objects by causing NDIS to call the offload target's MiniportTerminateOffload function, the offload target does not return the value of offloaded constant variables to the host stack.

Note that if an offloaded TCP connection's TCP_FLAG_MAX_RT_RESTART flag is set, the offload target must:

Reset that connection's delegated KeepaliveProbeCount variable when the host stack updates that connection's cached KaProbeCount variable.
Reset that connection's delegated KeepaliveTimeoutDelta variable when the host stack updates that connection's cached KaTimeout and/or KaInterval variables.

When passed to an offload target, a TCP_OFFLOAD_STATE_CACHED structure is associated with an NDIS_MINIPORT_OFFLOAD_BLOCK_LIST structure, which contains a header that is formatted as an NDIS_OBJECT_HEADER structure. The Revision member of the NDIS_OBJECT_HEADER structure, in this case, specifies the revision number of the TCP_OFFLOAD_STATE_CACHED structure.

Requirements

Requirement	Value
Header	ndischimney.h (include Ndischimney.h)

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