/**
   * Find the transaction corresponding to a given request.
   *
   * @param sipMessage request for which to retrieve the transaction.
   * @param isServer search the server transaction table if true.
   * @return the transaction object corresponding to the request or null if no such mapping exists.
   */
  public SIPTransaction findTransaction(SIPMessage sipMessage, boolean isServer) {
    SIPTransaction retval = null;

    if (isServer) {
      Via via = sipMessage.getTopmostVia();
      if (via.getBranch() != null) {
        String key = sipMessage.getTransactionId();

        synchronized (this.serverTransactionTable) {
          retval = (SIPTransaction) serverTransactionTable.get(key);
          if (LogWriter.needsLogging) logMessage("looking for key " + key);
          if (retval != null && retval.isMessagePartOfTransaction(sipMessage)) return retval;
        }
      }
      // Need to scan the table for old style transactions (RFC 2543
      // style)
      synchronized (this.serverTransactions) {
        Iterator<SIPServerTransaction> it = serverTransactions.iterator();
        while (it.hasNext()) {
          SIPServerTransaction sipServerTransaction = (SIPServerTransaction) it.next();
          if (sipServerTransaction.isMessagePartOfTransaction(sipMessage))
            return sipServerTransaction;
        }
      }
    } else {
      Via via = sipMessage.getTopmostVia();
      if (via.getBranch() != null) {
        String key = sipMessage.getTransactionId();
        synchronized (this.clientTransactionTable) {
          retval = (SIPTransaction) clientTransactionTable.get(key);
          if (retval != null && retval.isMessagePartOfTransaction(sipMessage)) return retval;
        }
      }
      // Need to scan the table for old style transactions (RFC 2543
      // style)
      synchronized (this.clientTransactions) {
        Iterator<SIPClientTransaction> it = clientTransactions.iterator();
        while (it.hasNext()) {
          SIPClientTransaction clientTransaction = (SIPClientTransaction) it.next();
          if (clientTransaction.isMessagePartOfTransaction(sipMessage)) return clientTransaction;
        }
      }
    }
    return null;
  }