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The Bouncy Castle Java APIs for the TLS, including a JSSE provider. The APIs are designed primarily to be used in conjunction with the BC FIPS provider. The APIs may also be used with other providers although if being used in a FIPS context it is the responsibility of the user to ensure that any other providers used are FIPS certified and used appropriately.

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package org.bouncycastle.tls;

import java.util.Vector;

class DTLSReassembler
{
    /*
     * No 'final' modifiers so that it works in earlier JDKs
     */
    private short msg_type;
    private byte[] body;

    private Vector missing = new Vector();

    DTLSReassembler(short msg_type, int length)
    {
        this.msg_type = msg_type;
        this.body = new byte[length];
        this.missing.addElement(new Range(0, length));
    }

    short getMsgType()
    {
        return msg_type;
    }

    byte[] getBodyIfComplete()
    {
        return missing.isEmpty() ? body : null;
    }

    void contributeFragment(short msg_type, int length, byte[] buf, int off, int fragment_offset,
        int fragment_length)
    {
        int fragment_end = fragment_offset + fragment_length;

        if (this.msg_type != msg_type || this.body.length != length || fragment_end > length)
        {
            return;
        }

        if (fragment_length == 0)
        {
            // NOTE: Empty messages still require an empty fragment to complete it
            if (fragment_offset == 0 && !missing.isEmpty())
            {
                Range firstRange = (Range)missing.firstElement();
                if (firstRange.getEnd() == 0)
                {
                    missing.removeElementAt(0);
                }
            }
            return;
        }

        for (int i = 0; i < missing.size(); ++i)
        {
            Range range = (Range)missing.elementAt(i);
            if (range.getStart() >= fragment_end)
            {
                break;
            }
            if (range.getEnd() > fragment_offset)
            {

                int copyStart = Math.max(range.getStart(), fragment_offset);
                int copyEnd = Math.min(range.getEnd(), fragment_end);
                int copyLength = copyEnd - copyStart;

                System.arraycopy(buf, off + copyStart - fragment_offset, body, copyStart,
                    copyLength);

                if (copyStart == range.getStart())
                {
                    if (copyEnd == range.getEnd())
                    {
                        missing.removeElementAt(i--);
                    }
                    else
                    {
                        range.setStart(copyEnd);
                    }
                }
                else
                {
                    if (copyEnd != range.getEnd())
                    {
                        missing.insertElementAt(new Range(copyEnd, range.getEnd()), ++i);
                    }
                    range.setEnd(copyStart);
                }
            }
        }
    }

    void reset()
    {
        this.missing.removeAllElements();
        this.missing.addElement(new Range(0, body.length));
    }

    private static class Range
    {
        private int start, end;

        Range(int start, int end)
        {
            this.start = start;
            this.end = end;
        }

        public int getStart()
        {
            return start;
        }

        public void setStart(int start)
        {
            this.start = start;
        }

        public int getEnd()
        {
            return end;
        }

        public void setEnd(int end)
        {
            this.end = end;
        }
    }
}