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244 lines
7.5 KiB
244 lines
7.5 KiB
/*++
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Copyright (c) 1993 Microsoft Corporation
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Module Name:
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pppframe.c
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Abstract:
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Author:
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Thomas J. Dimitri (TommyD)
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Environment:
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Revision History:
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--*/
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#include "asyncall.h"
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VOID
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AssemblePPPFrame(
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PNDIS_WAN_PACKET WanPacket)
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{
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PUCHAR pOldFrame;
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PUCHAR pNewFrame;
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USHORT crcData;
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UINT dataSize;
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PASYNC_INFO pInfo;
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//
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// Initialize locals
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//
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pOldFrame = WanPacket->CurrentBuffer;
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pNewFrame = WanPacket->StartBuffer;
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//
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// for quicker access, get a copy of data length field
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//
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dataSize = WanPacket->CurrentLength;
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pInfo = WanPacket->MacReserved1;
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//
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// Now we run through the entire frame and pad it FORWARDS...
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//
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// <------------- new frame -----------> (could be twice as large)
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// +-----------------------------------+
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// | |x|
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// +-----------------------------------+
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// ^
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// <---- old frame --> |
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// +-----------------+ |
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// | |x| |
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// +-----------------+ |
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// | |
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// \-----------------/
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//
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// so that we don't overrun ourselves
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//
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//-----------------------------------------------------------------------
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//
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// +----------+----------+----------+----------+------------
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// | Flag | Address | Control | Protocol | Information
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// | 01111110 | 11111111 | 00000011 | 16 bits | *
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// +----------+----------+----------+----------+------------
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// ---+----------+----------+-----------------
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// | FCS | Flag | Inter-frame Fill
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// | 16 bits | 01111110 | or next Address
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// ---+----------+----------+-----------------
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//
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//
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// Frame Check Sequence (FCS) Field
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//
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// The Frame Check Sequence field is normally 16 bits (two octets). The
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// use of other FCS lengths may be defined at a later time, or by prior
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// agreement.
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//
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// The FCS field is calculated over all bits of the Address, Control,
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// Protocol and Information fields not including any start and stop bits
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// (asynchronous) and any bits (synchronous) or octets (asynchronous)
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// inserted for transparency. This does not include the Flag Sequences
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// or the FCS field itself. The FCS is transmitted with the coefficient
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// of the highest term first.
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//
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// Note: When octets are received which are flagged in the Async-
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// Control-Character-Map, they are discarded before calculating the
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// FCS. See the description in Appendix A.
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//
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//
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// RFC 1331 Point-to-Point Protocol May 1992
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// Transparency
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//
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// On asynchronous links, a character stuffing procedure is used.
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// The Control Escape octet is defined as binary 01111101
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// (hexadecimal 0x7d) where the bit positions are numbered 87654321
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// (not 76543210, BEWARE).
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//
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// After FCS computation, the transmitter examines the entire frame
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// between the two Flag Sequences. Each Flag Sequence, Control
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// Escape octet and octet with value less than hexadecimal 0x20 which
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// is flagged in the Remote Async-Control-Character-Map is replaced
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// by a two octet sequence consisting of the Control Escape octet and
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// the original octet with bit 6 complemented (i.e., exclusive-or'd
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// with hexadecimal 0x20).
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//
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// Prior to FCS computation, the receiver examines the entire frame
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// between the two Flag Sequences. Each octet with value less than
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// hexadecimal 0x20 is checked. If it is flagged in the Local
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// Async-Control-Character-Map, it is simply removed (it may have
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// been inserted by intervening data communications equipment). For
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// each Control Escape octet, that octet is also removed, but bit 6
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// of the following octet is complemented. A Control Escape octet
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// immediately preceding the closing Flag Sequence indicates an
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// invalid frame.
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//
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// Note: The inclusion of all octets less than hexadecimal 0x20
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// allows all ASCII control characters [10] excluding DEL (Delete)
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// to be transparently communicated through almost all known data
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// communications equipment.
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//
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//
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// The transmitter may also send octets with value in the range 0x40
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// through 0xff (except 0x5e) in Control Escape format. Since these
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// octet values are not negotiable, this does not solve the problem
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// of receivers which cannot handle all non-control characters.
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// Also, since the technique does not affect the 8th bit, this does
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// not solve problems for communications links that can send only 7-
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// bit characters.
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//
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// A few examples may make this more clear. Packet data is
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// transmitted on the link as follows:
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//
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// 0x7e is encoded as 0x7d, 0x5e.
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// 0x7d is encoded as 0x7d, 0x5d.
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//
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// 0x01 is encoded as 0x7d, 0x21.
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//
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// Some modems with software flow control may intercept outgoing DC1
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// and DC3 ignoring the 8th (parity) bit. This data would be
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// transmitted on the link as follows:
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//
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// 0x11 is encoded as 0x7d, 0x31.
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// 0x13 is encoded as 0x7d, 0x33.
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// 0x91 is encoded as 0x7d, 0xb1.
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// 0x93 is encoded as 0x7d, 0xb3.
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//
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//
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// put CRC from FLAG byte to FLAG byte
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//
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crcData=CalcCRCPPP(pOldFrame, // Skip FLAG
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dataSize); // All the way to end
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crcData ^= 0xFFFF;
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//
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// Do it the hard way to avoid little endian problems.
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//
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pOldFrame[dataSize]=(UCHAR)(crcData);
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pOldFrame[dataSize+1]=(UCHAR)(crcData >> 8);
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dataSize += 2; // include two CRC bytes we just added
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*pNewFrame++ = PPP_FLAG_BYTE; // 0x7e - mark beginning of frame
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//
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// If we do not have a bitMask (common case), we use a faster loop
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//
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if (pInfo->ExtendedACCM[0] != 0) {
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//
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// loop to remove all control, ESC, and FLAG chars
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//
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while (dataSize--) {
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UCHAR c;
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c=*pOldFrame++; // get current byte in frame
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//
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// Check if we have to escape out this byte or not
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//
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if ( (0x01 << (c & 0x1F)) & pInfo->ExtendedACCM[c >> 5]) {
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*pNewFrame++ = PPP_ESC_BYTE;
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*pNewFrame++ = c ^ 0x20;
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} else {
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*pNewFrame++ = c;
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}
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}
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} else {
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//
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// loop to remove all ESC and FLAG chars
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//
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while (dataSize--) {
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UCHAR c;
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c=*pOldFrame++; // get current byte in frame
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//
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// Check if we have to escape out this byte or not
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//
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if (c == PPP_ESC_BYTE || c == PPP_FLAG_BYTE) {
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*pNewFrame++ = PPP_ESC_BYTE;
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*pNewFrame++ = c ^ 0x20;
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} else {
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*pNewFrame++ = c;
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}
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}
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}
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//
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// Mark end of frame
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//
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*pNewFrame++= PPP_FLAG_BYTE;
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//
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// Calc how many bytes we expanded to including CRC
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//
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WanPacket->CurrentLength = (ULONG)(pNewFrame - WanPacket->StartBuffer);
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//
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// Put in the adjusted length -- actual num of bytes to send
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//
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WanPacket->CurrentBuffer = WanPacket->StartBuffer;
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}
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