NetworkPkg/TcpDxe/TcpOutput.c

/** @file
  TCP output process routines.

  Copyright (c) 2009 - 2016, Intel Corporation. All rights reserved.<BR>

  This program and the accompanying materials
  are licensed and made available under the terms and conditions of the BSD License
  which accompanies this distribution.  The full text of the license may be found at
  http://opensource.org/licenses/bsd-license.php.

  THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
  WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.

**/

#include "TcpMain.h"

UINT8  mTcpOutFlag[] = {
  0,                          // TCP_CLOSED
  0,                          // TCP_LISTEN
  TCP_FLG_SYN,                // TCP_SYN_SENT
  TCP_FLG_SYN | TCP_FLG_ACK,  // TCP_SYN_RCVD
  TCP_FLG_ACK,                // TCP_ESTABLISHED
  TCP_FLG_FIN | TCP_FLG_ACK,  // TCP_FIN_WAIT_1
  TCP_FLG_ACK,                // TCP_FIN_WAIT_2
  TCP_FLG_ACK | TCP_FLG_FIN,  // TCP_CLOSING
  TCP_FLG_ACK,                // TCP_TIME_WAIT
  TCP_FLG_ACK,                // TCP_CLOSE_WAIT
  TCP_FLG_FIN | TCP_FLG_ACK   // TCP_LAST_ACK
};

/**
  Compute the sequence space left in the old receive window.

  @param[in]  Tcb     Pointer to the TCP_CB of this TCP instance.

  @return The sequence space left in the old receive window.

**/
UINT32
TcpRcvWinOld (
  IN TCP_CB *Tcb
  )
{
  UINT32  OldWin;

  OldWin = 0;

  if (TCP_SEQ_GT (Tcb->RcvWl2 + Tcb->RcvWnd, Tcb->RcvNxt)) {

    OldWin = TCP_SUB_SEQ (
              Tcb->RcvWl2 + Tcb->RcvWnd,
              Tcb->RcvNxt
              );
  }

  return OldWin;
}

/**
  Compute the current receive window.

  @param[in]  Tcb     Pointer to the TCP_CB of this TCP instance.

  @return The size of the current receive window, in bytes.

**/
UINT32
TcpRcvWinNow (
  IN TCP_CB *Tcb
  )
{
  SOCKET  *Sk;
  UINT32  Win;
  UINT32  Increase;
  UINT32  OldWin;

  Sk = Tcb->Sk;
  ASSERT (Sk != NULL);

  OldWin    = TcpRcvWinOld (Tcb);

  Win       = SockGetFreeSpace (Sk, SOCK_RCV_BUF);

  Increase  = 0;
  if (Win > OldWin) {
    Increase = Win - OldWin;
  }

  //
  // Receiver's SWS: don't advertise a bigger window
  // unless it can be increased by at least one Mss or
  // half of the receive buffer.
  //
  if ((Increase > Tcb->SndMss) || (2 * Increase >= GET_RCV_BUFFSIZE (Sk))) {

    return Win;
  }

  return OldWin;
}

/**
  Compute the value to fill in the window size field of the outgoing segment.

  @param[in, out]  Tcb     Pointer to the TCP_CB of this TCP instance.
  @param[in]       Syn     The flag to indicate whether the outgoing segment
                           is a SYN segment.

  @return The value of the local receive window size used to fill the outgoing segment.

**/
UINT16
TcpComputeWnd (
  IN OUT TCP_CB  *Tcb,
  IN     BOOLEAN Syn
  )
{
  UINT32  Wnd;

  //
  // RFC requires that initial window not be scaled
  //
  if (Syn) {

    Wnd = GET_RCV_BUFFSIZE (Tcb->Sk);
  } else {

    Wnd         = TcpRcvWinNow (Tcb);

    Tcb->RcvWnd = Wnd;
  }

  Wnd = MIN (Wnd >> Tcb->RcvWndScale, 0xffff);
  return NTOHS ((UINT16) Wnd);
}

/**
  Get the maximum SndNxt.

  @param[in]  Tcb     Pointer to the TCP_CB of this TCP instance.

  @return The sequence number of the maximum SndNxt.

**/
TCP_SEQNO
TcpGetMaxSndNxt (
  IN TCP_CB *Tcb
  )
{
  LIST_ENTRY      *Entry;
  NET_BUF         *Nbuf;

  if (IsListEmpty (&Tcb->SndQue)) {
    return Tcb->SndNxt;
  }

  Entry = Tcb->SndQue.BackLink;
  Nbuf  = NET_LIST_USER_STRUCT (Entry, NET_BUF, List);

  ASSERT (TCP_SEQ_GEQ (TCPSEG_NETBUF (Nbuf)->End, Tcb->SndNxt));
  return TCPSEG_NETBUF (Nbuf)->End;
}

/**
  Compute how much data to send.

  @param[in]  Tcb     Pointer to the TCP_CB of this TCP instance.
  @param[in]  Force   If TRUE, to ignore the sender's SWS avoidance algorithm and send
                      out data by force.

  @return The length of the data can be sent. If 0, no data can be sent.

**/
UINT32
TcpDataToSend (
  IN TCP_CB *Tcb,
  IN INTN   Force
  )
{
  SOCKET  *Sk;
  UINT32  Win;
  UINT32  Len;
  UINT32  Left;
  UINT32  Limit;

  Sk = Tcb->Sk;
  ASSERT (Sk != NULL);

  //
  // TCP should NOT send data beyond the send window
  // and congestion window. The right edge of send
  // window is defined as SND.WL2 + SND.WND. The right
  // edge of congestion window is defined as SND.UNA +
  // CWND.
  //
  Win   = 0;
  Limit = Tcb->SndWl2 + Tcb->SndWnd;

  if (TCP_SEQ_GT (Limit, Tcb->SndUna + Tcb->CWnd)) {

    Limit = Tcb->SndUna + Tcb->CWnd;
  }

  if (TCP_SEQ_GT (Limit, Tcb->SndNxt)) {
    Win = TCP_SUB_SEQ (Limit, Tcb->SndNxt);
  }

  //
  // The data to send contains two parts: the data on the
  // socket send queue, and the data on the TCB's send
  // buffer. The later can be non-zero if the peer shrinks
  // its advertised window.
  //
  Left  = GET_SND_DATASIZE (Sk) + TCP_SUB_SEQ (TcpGetMaxSndNxt (Tcb), Tcb->SndNxt);

  Len   = MIN (Win, Left);

  if (Len > Tcb->SndMss) {
    Len = Tcb->SndMss;
  }

  if ((Force != 0)|| (Len == 0 && Left == 0)) {
    return Len;
  }

  if (Len == 0 && Left != 0) {
    goto SetPersistTimer;
  }

  //
  // Sender's SWS avoidance: Don't send a small segment unless
  // a)A full-sized segment can be sent,
  // b)At least one-half of the maximum sized windows that
  // the other end has ever advertised.
  // c)It can send everything it has, and either it isn't
  // expecting an ACK, or the Nagle algorithm is disabled.
  //
  if ((Len == Tcb->SndMss) || (2 * Len >= Tcb->SndWndMax)) {

    return Len;
  }

  if ((Len == Left) &&
      ((Tcb->SndNxt == Tcb->SndUna) || TCP_FLG_ON (Tcb->CtrlFlag, TCP_CTRL_NO_NAGLE))
      ) {

    return Len;
  }

  //
  // RFC1122 suggests to set a timer when SWSA forbids TCP
  // sending more data, and combines it with a probe timer.
  //
SetPersistTimer:
  if (!TCP_TIMER_ON (Tcb->EnabledTimer, TCP_TIMER_REXMIT)) {

    DEBUG (
      (EFI_D_WARN,
      "TcpDataToSend: enter persistent state for TCB %p\n",
      Tcb)
      );

    if (!Tcb->ProbeTimerOn) {
      TcpSetProbeTimer (Tcb);
    }
  }

  return 0;
}

/**
  Build the TCP header of the TCP segment and transmit the segment by IP.

  @param[in, out]  Tcb     Pointer to the TCP_CB of this TCP instance.
  @param[in]       Nbuf    Pointer to the buffer containing the segment to be
                           sent out.

  @retval 0       The segment was sent out successfully.
  @retval -1      An error condition occurred.

**/
INTN
TcpTransmitSegment (
  IN OUT TCP_CB  *Tcb,
  IN     NET_BUF *Nbuf
  )
{
  UINT16    Len;
  TCP_HEAD  *Head;
  TCP_SEG   *Seg;
  BOOLEAN   Syn;
  UINT32    DataLen;

  ASSERT ((Nbuf != NULL) && (Nbuf->Tcp == NULL) && (TcpVerifySegment (Nbuf) != 0));

  DataLen = Nbuf->TotalSize;

  Seg     = TCPSEG_NETBUF (Nbuf);
  Syn     = TCP_FLG_ON (Seg->Flag, TCP_FLG_SYN);

  if (Syn) {

    Len = TcpSynBuildOption (Tcb, Nbuf);
  } else {

    Len = TcpBuildOption (Tcb, Nbuf);
  }

  ASSERT ((Len % 4 == 0) && (Len <= 40));

  Len += sizeof (TCP_HEAD);

  Head = (TCP_HEAD *) NetbufAllocSpace (
                        Nbuf,
                        sizeof (TCP_HEAD),
                        NET_BUF_HEAD
                        );

  ASSERT (Head != NULL);

  Nbuf->Tcp       = Head;

  Head->SrcPort   = Tcb->LocalEnd.Port;
  Head->DstPort   = Tcb->RemoteEnd.Port;
  Head->Seq       = NTOHL (Seg->Seq);
  Head->Ack       = NTOHL (Tcb->RcvNxt);
  Head->HeadLen   = (UINT8) (Len >> 2);
  Head->Res       = 0;
  Head->Wnd       = TcpComputeWnd (Tcb, Syn);
  Head->Checksum  = 0;

  //
  // Check whether to set the PSH flag.
  //
  TCP_CLEAR_FLG (Seg->Flag, TCP_FLG_PSH);

  if (DataLen != 0) {
    if (TCP_FLG_ON (Tcb->CtrlFlag, TCP_CTRL_SND_PSH) &&
        TCP_SEQ_BETWEEN (Seg->Seq, Tcb->SndPsh, Seg->End)
        ) {

      TCP_SET_FLG (Seg->Flag, TCP_FLG_PSH);
      TCP_CLEAR_FLG (Tcb->CtrlFlag, TCP_CTRL_SND_PSH);

    } else if ((Seg->End == Tcb->SndNxt) && (GET_SND_DATASIZE (Tcb->Sk) == 0)) {

      TCP_SET_FLG (Seg->Flag, TCP_FLG_PSH);
    }
  }

  //
  // Check whether to set the URG flag and the urgent pointer.
  //
  TCP_CLEAR_FLG (Seg->Flag, TCP_FLG_URG);

  if (TCP_FLG_ON (Tcb->CtrlFlag, TCP_CTRL_SND_URG) && TCP_SEQ_LEQ (Seg->Seq, Tcb->SndUp)) {

    TCP_SET_FLG (Seg->Flag, TCP_FLG_URG);

    if (TCP_SEQ_LT (Tcb->SndUp, Seg->End)) {

      Seg->Urg = (UINT16) TCP_SUB_SEQ (Tcb->SndUp, Seg->Seq);
    } else {

      Seg->Urg = (UINT16) MIN (
                            TCP_SUB_SEQ (Tcb->SndUp,
                            Seg->Seq),
                            0xffff
                            );
    }
  }

  Head->Flag      = Seg->Flag;
  Head->Urg       = NTOHS (Seg->Urg);
  Head->Checksum  = TcpChecksum (Nbuf, Tcb->HeadSum);

  //
  // Update the TCP session's control information.
  //
  Tcb->RcvWl2 = Tcb->RcvNxt;
  if (Syn) {
    Tcb->RcvWnd = NTOHS (Head->Wnd);
  }

  //
  // Clear the delayedack flag.
  //
  Tcb->DelayedAck = 0;

  return TcpSendIpPacket (Tcb, Nbuf, &Tcb->LocalEnd.Ip, &Tcb->RemoteEnd.Ip, Tcb->Sk->IpVersion);
}

/**
  Get a segment from the Tcb's SndQue.

  @param[in]  Tcb     Pointer to the TCP_CB of this TCP instance.
  @param[in]  Seq     The sequence number of the segment.
  @param[in]  Len     The maximum length of the segment.

  @return Pointer to the segment. If NULL, some error occurred.

**/
NET_BUF *
TcpGetSegmentSndQue (
  IN TCP_CB    *Tcb,
  IN TCP_SEQNO Seq,
  IN UINT32    Len
  )
{
  LIST_ENTRY      *Head;
  LIST_ENTRY      *Cur;
  NET_BUF         *Node;
  TCP_SEG         *Seg;
  NET_BUF         *Nbuf;
  TCP_SEQNO       End;
  UINT8           *Data;
  UINT8           Flag;
  INT32           Offset;
  INT32           CopyLen;

  ASSERT ((Tcb != NULL) && TCP_SEQ_LEQ (Seq, Tcb->SndNxt) && (Len > 0));

  //
  // Find the segment that contains the Seq.
  //
  Head  = &Tcb->SndQue;

  Node  = NULL;
  Seg   = NULL;

  NET_LIST_FOR_EACH (Cur, Head) {
    Node  = NET_LIST_USER_STRUCT (Cur, NET_BUF, List);
    Seg   = TCPSEG_NETBUF (Node);

    if (TCP_SEQ_LT (Seq, Seg->End) && TCP_SEQ_LEQ (Seg->Seq, Seq)) {

      break;
    }
  }

  if ((Cur == Head) || (Seg == NULL) || (Node == NULL)) {
    return NULL;
  }

  //
  // Return the buffer if it can be returned without
  // adjustment:
  //
  if ((Seg->Seq == Seq) &&
      TCP_SEQ_LEQ (Seg->End, Seg->Seq + Len) &&
      !NET_BUF_SHARED (Node)
      ) {

    NET_GET_REF (Node);
    return Node;
  }

  //
  // Create a new buffer and copy data there.
  //
  Nbuf = NetbufAlloc (Len + TCP_MAX_HEAD);

  if (Nbuf == NULL) {
    return NULL;
  }

  NetbufReserve (Nbuf, TCP_MAX_HEAD);

  Flag  = Seg->Flag;
  End   = Seg->End;

  if (TCP_SEQ_LT (Seq + Len, Seg->End)) {
    End = Seq + Len;
  }

  CopyLen = TCP_SUB_SEQ (End, Seq);
  Offset  = TCP_SUB_SEQ (Seq, Seg->Seq);

  //
  // If SYN is set and out of the range, clear the flag.
  // Becuase the sequence of the first byte is SEG.SEQ+1,
  // adjust Offset by -1. If SYN is in the range, copy
  // one byte less.
  //
  if (TCP_FLG_ON (Seg->Flag, TCP_FLG_SYN)) {

    if (TCP_SEQ_LT (Seg->Seq, Seq)) {

      TCP_CLEAR_FLG (Flag, TCP_FLG_SYN);
      Offset--;
    } else {

      CopyLen--;
    }
  }

  //
  // If FIN is set and in the range, copy one byte less,
  // and if it is out of the range, clear the flag.
  //
  if (TCP_FLG_ON (Seg->Flag, TCP_FLG_FIN)) {

    if (Seg->End == End) {

      CopyLen--;
    } else {

      TCP_CLEAR_FLG (Flag, TCP_FLG_FIN);
    }
  }

  ASSERT (CopyLen >= 0);

  //
  // Copy data to the segment
  //
  if (CopyLen != 0) {
    Data = NetbufAllocSpace (Nbuf, CopyLen, NET_BUF_TAIL);
    ASSERT (Data != NULL);

    if ((INT32) NetbufCopy (Node, Offset, CopyLen, Data) != CopyLen) {
      goto OnError;
    }
  }

  CopyMem (TCPSEG_NETBUF (Nbuf), Seg, sizeof (TCP_SEG));

  TCPSEG_NETBUF (Nbuf)->Seq   = Seq;
  TCPSEG_NETBUF (Nbuf)->End   = End;
  TCPSEG_NETBUF (Nbuf)->Flag  = Flag;

  return Nbuf;

OnError:
  NetbufFree (Nbuf);
  return NULL;
}

/**
  Get a segment from the Tcb's socket buffer.

  @param[in]  Tcb     Pointer to the TCP_CB of this TCP instance.
  @param[in]  Seq     The sequence number of the segment.
  @param[in]  Len     The maximum length of the segment.

  @return Pointer to the segment. If NULL, some error occurred.

**/
NET_BUF *
TcpGetSegmentSock (
  IN TCP_CB    *Tcb,
  IN TCP_SEQNO Seq,
  IN UINT32    Len
  )
{
  NET_BUF *Nbuf;
  UINT8   *Data;
  UINT32  DataGet;

  ASSERT ((Tcb != NULL) && (Tcb->Sk != NULL));

  Nbuf = NetbufAlloc (Len + TCP_MAX_HEAD);

  if (Nbuf == NULL) {
    DEBUG (
      (EFI_D_ERROR,
      "TcpGetSegmentSock: failed to allocate a netbuf for TCB %p\n",
      Tcb)
      );

    return NULL;
  }

  NetbufReserve (Nbuf, TCP_MAX_HEAD);

  DataGet = 0;

  if (Len != 0) {
    //
    // copy data to the segment.
    //
    Data = NetbufAllocSpace (Nbuf, Len, NET_BUF_TAIL);
    ASSERT (Data != NULL);

    DataGet = SockGetDataToSend (Tcb->Sk, 0, Len, Data);
  }

  NET_GET_REF (Nbuf);

  TCPSEG_NETBUF (Nbuf)->Seq = Seq;
  TCPSEG_NETBUF (Nbuf)->End = Seq + Len;

  InsertTailList (&(Tcb->SndQue), &(Nbuf->List));

  if (DataGet != 0) {

    SockDataSent (Tcb->Sk, DataGet);
  }

  return Nbuf;
}

/**
  Get a segment starting from sequence Seq of a maximum
  length of Len.

  @param[in]  Tcb     Pointer to the TCP_CB of this TCP instance.
  @param[in]  Seq     The sequence number of the segment.
  @param[in]  Len     The maximum length of the segment.

  @return Pointer to the segment. If NULL, some error occurred.

**/
NET_BUF *
TcpGetSegment (
  IN TCP_CB    *Tcb,
  IN TCP_SEQNO Seq,
  IN UINT32    Len
  )
{
  NET_BUF *Nbuf;

  ASSERT (Tcb != NULL);

  //
  // Compare the SndNxt with the max sequence number sent.
  //
  if ((Len != 0) && TCP_SEQ_LT (Seq, TcpGetMaxSndNxt (Tcb))) {

    Nbuf = TcpGetSegmentSndQue (Tcb, Seq, Len);
  } else {

    Nbuf = TcpGetSegmentSock (Tcb, Seq, Len);
  }

  ASSERT (TcpVerifySegment (Nbuf) != 0);
  return Nbuf;
}

/**
  Retransmit the segment from sequence Seq.

  @param[in]  Tcb     Pointer to the TCP_CB of this TCP instance.
  @param[in]  Seq     The sequence number of the segment to be retransmitted.

  @retval 0       Retransmission succeeded.
  @retval -1      Error condition occurred.

**/
INTN
TcpRetransmit (
  IN TCP_CB    *Tcb,
  IN TCP_SEQNO Seq
  )
{
  NET_BUF *Nbuf;
  UINT32  Len;

  //
  // Compute the maxium length of retransmission. It is
  // limited by three factors:
  // 1. Less than SndMss
  // 2. Must in the current send window
  // 3. Will not change the boundaries of queued segments.
  //
  if (TCP_SEQ_LT (Tcb->SndWl2 + Tcb->SndWnd, Seq)) {
    DEBUG (
      (EFI_D_WARN,
      "TcpRetransmit: retransmission cancelled because send window too small for TCB %p\n",
      Tcb)
      );

    return 0;
  }

  Len   = TCP_SUB_SEQ (Tcb->SndWl2 + Tcb->SndWnd, Seq);
  Len   = MIN (Len, Tcb->SndMss);

  Nbuf  = TcpGetSegmentSndQue (Tcb, Seq, Len);
  if (Nbuf == NULL) {
    return -1;
  }

  ASSERT (TcpVerifySegment (Nbuf) != 0);

  if (TcpTransmitSegment (Tcb, Nbuf) != 0) {
    goto OnError;
  }

  //
  // The retransmitted buffer may be on the SndQue,
  // trim TCP head because all the buffers on SndQue
  // are headless.
  //
  ASSERT (Nbuf->Tcp != NULL);
  NetbufTrim (Nbuf, (Nbuf->Tcp->HeadLen << 2), NET_BUF_HEAD);
  Nbuf->Tcp = NULL;

  NetbufFree (Nbuf);
  return 0;

OnError:
  if (Nbuf != NULL) {
    NetbufFree (Nbuf);
  }

  return -1;
}

/**
  Verify that all the segments in SndQue are in good shape.

  @param[in]  Head    Pointer to the head node of the SndQue.

  @retval     0       At least one segment is broken.
  @retval     1       All segments in the specific queue are in good shape.

**/
INTN
TcpCheckSndQue (
  IN LIST_ENTRY     *Head
  )
{
  LIST_ENTRY      *Entry;
  NET_BUF         *Nbuf;
  TCP_SEQNO       Seq;

  if (IsListEmpty (Head)) {
    return 1;
  }
  //
  // Initialize the Seq.
  //
  Entry = Head->ForwardLink;
  Nbuf  = NET_LIST_USER_STRUCT (Entry, NET_BUF, List);
  Seq   = TCPSEG_NETBUF (Nbuf)->Seq;

  NET_LIST_FOR_EACH (Entry, Head) {
    Nbuf = NET_LIST_USER_STRUCT (Entry, NET_BUF, List);

    if (TcpVerifySegment (Nbuf) == 0) {
      return 0;
    }

    //
    // All the node in the SndQue should has:
    // SEG.SEQ = LAST_SEG.END
    //
    if (Seq != TCPSEG_NETBUF (Nbuf)->Seq) {
      return 0;
    }

    Seq = TCPSEG_NETBUF (Nbuf)->End;
  }

  return 1;
}

/**
  Check whether to send data/SYN/FIN and piggyback an ACK.

  @param[in, out]  Tcb     Pointer to the TCP_CB of this TCP instance.
  @param[in]       Force   If TRUE, ignore the sender's SWS avoidance algorithm
                           and send out data by force.

  @return The number of bytes sent.

**/
INTN
TcpToSendData (
  IN OUT TCP_CB *Tcb,
  IN     INTN   Force
  )
{
  UINT32    Len;
  INTN      Sent;
  UINT8     Flag;
  NET_BUF   *Nbuf;
  TCP_SEG   *Seg;
  TCP_SEQNO Seq;
  TCP_SEQNO End;

  ASSERT ((Tcb != NULL) && (Tcb->Sk != NULL) && (Tcb->State != TCP_LISTEN));

  Sent = 0;

  if ((Tcb->State == TCP_CLOSED) || TCP_FLG_ON (Tcb->CtrlFlag, TCP_CTRL_FIN_SENT)) {

    return 0;
  }

  do {
    //
    // Compute how much data can be sent
    //
    Len   = TcpDataToSend (Tcb, Force);
    Seq   = Tcb->SndNxt;

    ASSERT ((Tcb->State) < (ARRAY_SIZE (mTcpOutFlag)));
    Flag  = mTcpOutFlag[Tcb->State];

    if ((Flag & TCP_FLG_SYN) != 0) {

      Seq = Tcb->Iss;
      Len = 0;
    }

    //
    // Only send a segment without data if SYN or
    // FIN is set.
    //
    if ((Len == 0) && ((Flag & (TCP_FLG_SYN | TCP_FLG_FIN)) == 0)) {
      return Sent;
    }

    Nbuf = TcpGetSegment (Tcb, Seq, Len);

    if (Nbuf == NULL) {
      DEBUG (
        (EFI_D_ERROR,
        "TcpToSendData: failed to get a segment for TCB %p\n",
        Tcb)
        );

      goto OnError;
    }

    Seg = TCPSEG_NETBUF (Nbuf);

    //
    // Set the TcpSeg in Nbuf.
    //
    Len = Nbuf->TotalSize;
    End = Seq + Len;
    if (TCP_FLG_ON (Flag, TCP_FLG_SYN)) {
      End++;
    }

    if ((Flag & TCP_FLG_FIN) != 0) {
      //
      // Send FIN if all data is sent, and FIN is
      // in the window
      //
      if ((TcpGetMaxSndNxt (Tcb) == Tcb->SndNxt) &&
          (GET_SND_DATASIZE (Tcb->Sk) == 0) &&
          TCP_SEQ_LT (End + 1, Tcb->SndWnd + Tcb->SndWl2)
            ) {
        DEBUG (
          (EFI_D_NET,
          "TcpToSendData: send FIN to peer for TCB %p in state %s\n",
          Tcb,
          mTcpStateName[Tcb->State])
          );

        End++;
      } else {
        TCP_CLEAR_FLG (Flag, TCP_FLG_FIN);
      }
    }

    Seg->Seq  = Seq;
    Seg->End  = End;
    Seg->Flag = Flag;

    ASSERT (TcpVerifySegment (Nbuf) != 0);
    ASSERT (TcpCheckSndQue (&Tcb->SndQue) != 0);

    //
    // Don't send an empty segment here.
    //
    if (Seg->End == Seg->Seq) {
      DEBUG (
        (EFI_D_WARN,
        "TcpToSendData: created a empty segment for TCB %p, free it now\n",
        Tcb)
        );

      NetbufFree (Nbuf);
      return Sent;
    }

    if (TcpTransmitSegment (Tcb, Nbuf) != 0) {
      NetbufTrim (Nbuf, (Nbuf->Tcp->HeadLen << 2), NET_BUF_HEAD);
      Nbuf->Tcp = NULL;

      if ((Flag & TCP_FLG_FIN) != 0)  {
        TCP_SET_FLG (Tcb->CtrlFlag, TCP_CTRL_FIN_SENT);
      }

      goto OnError;
    }

    Sent += TCP_SUB_SEQ (End, Seq);

    //
    // All the buffers in the SndQue are headless.
    //
    ASSERT (Nbuf->Tcp != NULL);

    NetbufTrim (Nbuf, (Nbuf->Tcp->HeadLen << 2), NET_BUF_HEAD);
    Nbuf->Tcp = NULL;

    NetbufFree (Nbuf);

    //
    // Update the status in TCB.
    //
    Tcb->DelayedAck = 0;

    if ((Flag & TCP_FLG_FIN) != 0) {
      TCP_SET_FLG (Tcb->CtrlFlag, TCP_CTRL_FIN_SENT);
    }

    if (TCP_SEQ_GT (End, Tcb->SndNxt)) {
      Tcb->SndNxt = End;
    }

    if (!TCP_TIMER_ON (Tcb->EnabledTimer, TCP_TIMER_REXMIT)) {
      TcpSetTimer (Tcb, TCP_TIMER_REXMIT, Tcb->Rto);
    }

    //
    // Enable RTT measurement only if not in retransmit.
    // Karn's algorithm requires not to update RTT when in loss.
    //
    if ((Tcb->CongestState == TCP_CONGEST_OPEN) && !TCP_FLG_ON (Tcb->CtrlFlag, TCP_CTRL_RTT_ON)) {

      DEBUG (
        (EFI_D_NET,
        "TcpToSendData: set RTT measure sequence %d for TCB %p\n",
        Seq,
        Tcb)
        );

      TCP_SET_FLG (Tcb->CtrlFlag, TCP_CTRL_RTT_ON);
      Tcb->RttSeq     = Seq;
      Tcb->RttMeasure = 0;
    }

  } while (Len == Tcb->SndMss);

  return Sent;

OnError:
  if (Nbuf != NULL) {
    NetbufFree (Nbuf);
  }

  return Sent;
}

/**
  Send an ACK immediately.

  @param[in, out]  Tcb     Pointer to the TCP_CB of this TCP instance.

**/
VOID
TcpSendAck (
  IN OUT TCP_CB *Tcb
  )
{
  NET_BUF *Nbuf;
  TCP_SEG *Seg;

  Nbuf = NetbufAlloc (TCP_MAX_HEAD);

  if (Nbuf == NULL) {
    return;
  }

  NetbufReserve (Nbuf, TCP_MAX_HEAD);

  Seg       = TCPSEG_NETBUF (Nbuf);
  Seg->Seq  = Tcb->SndNxt;
  Seg->End  = Tcb->SndNxt;
  Seg->Flag = TCP_FLG_ACK;

  if (TcpTransmitSegment (Tcb, Nbuf) == 0) {
    TCP_CLEAR_FLG (Tcb->CtrlFlag, TCP_CTRL_ACK_NOW);
    Tcb->DelayedAck = 0;
  }

  NetbufFree (Nbuf);
}

/**
  Send a zero probe segment. It can be used by keepalive and zero window probe.

  @param[in, out]  Tcb     Pointer to the TCP_CB of this TCP instance.

  @retval 0       The zero probe segment was sent out successfully.
  @retval other   An error condition occurred.

**/
INTN
TcpSendZeroProbe (
  IN OUT TCP_CB *Tcb
  )
{
  NET_BUF *Nbuf;
  TCP_SEG *Seg;
  INTN     Result;

  Nbuf = NetbufAlloc (TCP_MAX_HEAD);

  if (Nbuf == NULL) {
    return -1;
  }

  NetbufReserve (Nbuf, TCP_MAX_HEAD);

  //
  // SndNxt-1 is out of window. The peer should respond
  // with an ACK.
  //
  Seg       = TCPSEG_NETBUF (Nbuf);
  Seg->Seq  = Tcb->SndNxt - 1;
  Seg->End  = Tcb->SndNxt - 1;
  Seg->Flag = TCP_FLG_ACK;

  Result    = TcpTransmitSegment (Tcb, Nbuf);
  NetbufFree (Nbuf);

  return Result;
}

/**
  Check whether to send an ACK or delayed ACK.

  @param[in, out]  Tcb     Pointer to the TCP_CB of this TCP instance.

**/
VOID
TcpToSendAck (
  IN OUT TCP_CB *Tcb
  )
{
  UINT32 TcpNow;

  //
  // Generally, TCP should send a delayed ACK unless:
  //   1. ACK at least every other FULL sized segment received.
  //   2. Packets received out of order.
  //   3. Receiving window is open.
  //
  if (TCP_FLG_ON (Tcb->CtrlFlag, TCP_CTRL_ACK_NOW) || (Tcb->DelayedAck >= 1)) {
    TcpSendAck (Tcb);
    return;
  }

  TcpNow = TcpRcvWinNow (Tcb);

  if (TcpNow > TcpRcvWinOld (Tcb)) {
    TcpSendAck (Tcb);
    return;
  }

  DEBUG (
    (EFI_D_NET,
    "TcpToSendAck: scheduled a delayed ACK for TCB %p\n",
    Tcb)
    );

  //
  // Schedule a delayed ACK.
  //
  Tcb->DelayedAck++;
}

/**
  Send a RESET segment in response to the segment received.

  @param[in]  Tcb     Pointer to the TCP_CB of this TCP instance. May be NULL.
  @param[in]  Head    TCP header of the segment that triggers the reset.
  @param[in]  Len     Length of the segment that triggers the reset.
  @param[in]  Local   Local IP address.
  @param[in]  Remote  Remote peer's IP address.
  @param[in]  Version IP_VERSION_4 indicates TCP is running on IP4 stack,
                      IP_VERSION_6 indicates TCP is running on IP6 stack.

  @retval     0       A reset was sent or there is no need to send it.
  @retval     -1      No reset is sent.

**/
INTN
TcpSendReset (
  IN TCP_CB          *Tcb,
  IN TCP_HEAD        *Head,
  IN INT32           Len,
  IN EFI_IP_ADDRESS  *Local,
  IN EFI_IP_ADDRESS  *Remote,
  IN UINT8           Version
  )
{
  NET_BUF   *Nbuf;
  TCP_HEAD  *Nhead;
  UINT16    HeadSum;

  //
  // Don't respond to a Reset with reset.
  //
  if ((Head->Flag & TCP_FLG_RST) != 0) {
    return 0;
  }

  Nbuf = NetbufAlloc (TCP_MAX_HEAD);

  if (Nbuf == NULL) {
    return -1;
  }

  Nhead = (TCP_HEAD *) NetbufAllocSpace (
                        Nbuf,
                        sizeof (TCP_HEAD),
                        NET_BUF_TAIL
                        );

  ASSERT (Nhead != NULL);

  Nbuf->Tcp   = Nhead;
  Nhead->Flag = TCP_FLG_RST;

  //
  // Derive Seq/ACK from the segment if no TCB
  // is associated with it, otherwise derive from the Tcb.
  //
  if (Tcb == NULL) {

    if (TCP_FLG_ON (Head->Flag, TCP_FLG_ACK)) {
      Nhead->Seq  = Head->Ack;
      Nhead->Ack  = 0;
    } else {
      Nhead->Seq = 0;
      TCP_SET_FLG (Nhead->Flag, TCP_FLG_ACK);
      Nhead->Ack = HTONL (NTOHL (Head->Seq) + Len);
    }
  } else {

    Nhead->Seq  = HTONL (Tcb->SndNxt);
    Nhead->Ack  = HTONL (Tcb->RcvNxt);
    TCP_SET_FLG (Nhead->Flag, TCP_FLG_ACK);
  }

  Nhead->SrcPort  = Head->DstPort;
  Nhead->DstPort  = Head->SrcPort;
  Nhead->HeadLen  = (UINT8) (sizeof (TCP_HEAD) >> 2);
  Nhead->Res      = 0;
  Nhead->Wnd      = HTONS (0xFFFF);
  Nhead->Checksum = 0;
  Nhead->Urg      = 0;

  if (Version == IP_VERSION_4) {
    HeadSum = NetPseudoHeadChecksum (Local->Addr[0], Remote->Addr[0], 6, 0);
  } else {
    HeadSum = NetIp6PseudoHeadChecksum (&Local->v6, &Remote->v6, 6, 0);
  }

  Nhead->Checksum = TcpChecksum (Nbuf, HeadSum);

  TcpSendIpPacket (Tcb, Nbuf, Local, Remote, Version);

  NetbufFree (Nbuf);

  return 0;
}

/**
  Verify that the segment is in good shape.

  @param[in]  Nbuf    The buffer that contains the segment to be checked.

  @retval     0       The segment is broken.
  @retval     1       The segment is in good shape.

**/
INTN
TcpVerifySegment (
  IN NET_BUF *Nbuf
  )
{
  TCP_HEAD  *Head;
  TCP_SEG   *Seg;
  UINT32    Len;

  if (Nbuf == NULL) {
    return 1;
  }

  NET_CHECK_SIGNATURE (Nbuf, NET_BUF_SIGNATURE);

  Seg   = TCPSEG_NETBUF (Nbuf);
  Len   = Nbuf->TotalSize;
  Head  = Nbuf->Tcp;

  if (Head != NULL) {
    if (Head->Flag != Seg->Flag) {
      return 0;
    }

    Len -= (Head->HeadLen << 2);
  }

  if (TCP_FLG_ON (Seg->Flag, TCP_FLG_SYN)) {
    Len++;
  }

  if (TCP_FLG_ON (Seg->Flag, TCP_FLG_FIN)) {
    Len++;
  }

  if (Seg->Seq + Len != Seg->End) {
    return 0;
  }

  return 1;
}