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ALTQ(9)			 BSD Kernel Developer's Manual		       ALTQ(9)

     ALTQ — kernel interfaces for manipulating output queues on network inter‐

     #include <sys/types.h>
     #include <sys/socket.h>
     #include <net/if.h>
     #include <net/if_var.h>

   Enqueue macros
     IFQ_ENQUEUE(struct ifaltq *ifq, struct mbuf *m, int error);

     IFQ_HANDOFF(struct ifnet *ifp, struct mbuf *m, int error);

     IFQ_HANDOFF_ADJ(struct ifnet *ifp, struct mbuf *m, int adjust,
	 int error);

   Dequeue macros
     IFQ_DEQUEUE(struct ifaltq *ifq, struct mbuf *m);

     IFQ_POLL_NOLOCK(struct ifaltq *ifq, struct mbuf *m);

     IFQ_PURGE(struct ifaltq *ifq);

     IFQ_IS_EMPTY(struct ifaltq *ifq);

   Driver managed dequeue macros
     IFQ_DRV_DEQUEUE(struct ifaltq *ifq, struct mbuf *m);

     IFQ_DRV_PREPEND(struct ifaltq *ifq, struct mbuf *m);

     IFQ_DRV_PURGE(struct ifaltq *ifq);

     IFQ_DRV_IS_EMPTY(struct ifaltq *ifq);

   General setup macros
     IFQ_SET_MAXLEN(struct ifaltq *ifq, int len);

     IFQ_INC_LEN(struct ifaltq *ifq);

     IFQ_DEC_LEN(struct ifaltq *ifq);

     IFQ_INC_DROPS(struct ifaltq *ifq);

     IFQ_SET_READY(struct ifaltq *ifq);

     The ALTQ system is a framework to manage queuing disciplines on network
     interfaces.  ALTQ introduces new macros to manipulate output queues.  The
     output queue macros are used to abstract queue operations and not to
     touch the internal fields of the output queue structure.  The macros are
     independent from the ALTQ implementation, and compatible with the tradi‐
     tional ifqueue macros for ease of transition.

     IFQ_ENQUEUE(), IFQ_HANDOFF() and IFQ_HANDOFF_ADJ() enqueue a packet m to
     the queue ifq.  The underlying queuing discipline may discard the packet.
     The error argument is set to 0 on success, or ENOBUFS if the packet is
     discarded.	 The packet pointed to by m will be freed by the device driver
     on success, or by the queuing discipline on failure, so the caller should
     not touch m after enqueuing.  IFQ_HANDOFF() and IFQ_HANDOFF_ADJ() combine
     the enqueue operation with statistic generation and call if_start() upon
     successful enqueue to initiate the actual send.

     IFQ_DEQUEUE() dequeues a packet from the queue.  The dequeued packet is
     returned in m, or m is set to NULL if no packet is dequeued.  The caller
     must always check m since a non-empty queue could return NULL under rate-

     IFQ_POLL_NOLOCK() returns the next packet without removing it from the
     queue.  The caller must hold the queue mutex when calling
     IFQ_POLL_NOLOCK() in order to guarantee that a subsequent call to
     IFQ_DEQUEUE_NOLOCK() dequeues the same packet.

     IFQ_*_NOLOCK() variants (if available) always assume that the caller
     holds the queue mutex.  They can be grabbed with IFQ_LOCK() and released
     with IFQ_UNLOCK().

     IFQ_PURGE() discards all the packets in the queue.	 The purge operation
     is needed since a non-work conserving queue cannot be emptied by a
     dequeue loop.

     IFQ_IS_EMPTY() can be used to check if the queue is empty.	 Note that
     IFQ_DEQUEUE() could still return NULL if the queuing discipline is non-
     work conserving.

     IFQ_DRV_DEQUEUE() moves up to ifq->ifq_drv_maxlen packets from the queue
     to the “driver managed” queue and returns the first one via m.  As for
     IFQ_DEQUEUE(), m can be NULL even for a non-empty queue.  Subsequent
     calls to IFQ_DRV_DEQUEUE() pass the packets from the “driver managed”
     queue without obtaining the queue mutex.  It is the responsibility of the
     caller to protect against concurrent access.  Enabling ALTQ for a given
     queue sets ifq_drv_maxlen to 0 as the “bulk dequeue” performed by
     IFQ_DRV_DEQUEUE() for higher values of ifq_drv_maxlen is adverse to
     ALTQ's internal timing.  Note that a driver must not mix IFQ_DRV_*()
     macros with the default dequeue macros as the default macros do not look
     at the “driver managed” queue which might lead to an mbuf leak.

     IFQ_DRV_PREPEND() prepends m to the “driver managed” queue from where it
     will be obtained with the next call to IFQ_DRV_DEQUEUE().

     IFQ_DRV_PURGE() flushes all packets in the “driver managed” queue and
     calls to IFQ_PURGE() afterwards.

     IFQ_DRV_IS_EMPTY() checks for packets in the “driver managed” part of the
     queue.  If it is empty, it forwards to IFQ_IS_EMPTY().

     IFQ_SET_MAXLEN() sets the queue length limit to the default FIFO queue.
     The ifq_drv_maxlen member of the ifaltq structure controls the length
     limit of the “driver managed” queue.

     IFQ_INC_LEN() and IFQ_DEC_LEN() increment or decrement the current queue
     length in packets.	 This is mostly for internal purposes.

     IFQ_INC_DROPS() increments the drop counter and is identical to
     IF_DROP().	 It is defined for naming consistency only.

     IFQ_SET_READY() sets a flag to indicate that a driver was converted to
     use the new macros.  ALTQ can be enabled only on interfaces with this

   ifaltq structure
     In order to keep compatibility with the existing code, the new output
     queue structure ifaltq has the same fields.  The traditional IF_*()
     macros and the code directly referencing the fields within if_snd still
     work with ifaltq.

		 ##old-style##				 ##new-style##
      struct ifqueue {			    | struct ifaltq {
	 struct mbuf *ifq_head;		    |	 struct mbuf *ifq_head;
	 struct mbuf *ifq_tail;		    |	 struct mbuf *ifq_tail;
	 int	      ifq_len;		    |	 int	      ifq_len;
	 int	      ifq_maxlen;	    |	 int	      ifq_maxlen;
	 int	      ifq_drops;	    |	 int	      ifq_drops;
      };				    |	 /* driver queue fields */
					    |	 ......
					    |	 /* altq related fields */
					    |	 ......
					    | };
     The new structure replaces struct ifqueue in struct ifnet.

		 ##old-style##				 ##new-style##
      struct ifnet {			    | struct ifnet {
	  ....				    |	  ....
	  struct ifqueue if_snd;	    |	  struct ifaltq if_snd;
	  ....				    |	  ....
      };				    | };
     The (simplified) new IFQ_*() macros look like:

	     #define IFQ_DEQUEUE(ifq, m)		     \
		     if (ALTQ_IS_ENABLED((ifq))		     \
			     ALTQ_DEQUEUE((ifq), (m));	     \
		     else				     \
			     IF_DEQUEUE((ifq), (m));

   Enqueue operation
     The semantics of the enqueue operation is changed.	 In the new style,
     enqueue and packet drop are combined since they cannot be easily sepa‐
     rated in many queuing disciplines.	 The new enqueue operation corresponds
     to the following macro that is written with the old macros.

     #define IFQ_ENQUEUE(ifq, m, error)			     \
     do {						     \
	     if (IF_QFULL((ifq))) {			     \
		     m_freem((m));			     \
		     (error) = ENOBUFS;			     \
		     IF_DROP(ifq);			     \
	     } else {					     \
		     IF_ENQUEUE((ifq), (m));		     \
		     (error) = 0;			     \
	     }						     \
     } while (0)

     IFQ_ENQUEUE() does the following:

     -	 queue a packet,
     -	 drop (and free) a packet if the enqueue operation fails.

     If the enqueue operation fails, error is set to ENOBUFS.  The m mbuf is
     freed by the queuing discipline.  The caller should not touch mbuf after
     calling IFQ_ENQUEUE() so that the caller may need to copy m_pkthdr.len or
     m_flags field beforehand for statistics.  IFQ_HANDOFF() and
     IFQ_HANDOFF_ADJ() can be used if only default interface statistics and an
     immediate call to if_start() are desired.	The caller should not use
     senderr() since mbuf was already freed.

     The new style if_output() looks as follows:

		 ##old-style##				 ##new-style##
      int				    | int
      ether_output(ifp, m0, dst, rt0)	    | ether_output(ifp, m0, dst, rt0)
      {					    | {
	  ......			    |	  ......
					    |	  mflags = m->m_flags;
					    |	  len = m->m_pkthdr.len;
	  s = splimp();			    |	  s = splimp();
	  if (IF_QFULL(&ifp->if_snd)) {	    |	  IFQ_ENQUEUE(&ifp->if_snd, m,
					    |		      error);
	      IF_DROP(&ifp->if_snd);	    |	  if (error != 0) {
	      splx(s);			    |	      splx(s);
	      senderr(ENOBUFS);		    |	      return (error);
	  }				    |	  }
	  IF_ENQUEUE(&ifp->if_snd, m);	    |
	  ifp->if_obytes +=		    |	  ifp->if_obytes += len;
			 m->m_pkthdr.len;   |
	  if (m->m_flags & M_MCAST)	    |	  if (mflags & M_MCAST)
	      ifp->if_omcasts++;	    |	      ifp->if_omcasts++;
	  if ((ifp->if_flags & IFF_OACTIVE) |	  if ((ifp->if_flags & IFF_OACTIVE)
	      == 0)			    |	      == 0)
	      (*ifp->if_start)(ifp);	    |	      (*ifp->if_start)(ifp);
	  splx(s);			    |	  splx(s);
	  return (error);		    |	  return (error);
      bad:				    | bad:
	  if (m)			    |	  if (m)
	      m_freem(m);		    |	      m_freem(m);
	  return (error);		    |	  return (error);
      }					    | }

     First, make sure the corresponding if_output() is already converted to
     the new style.

     Look for if_snd in the driver.  Probably, you need to make changes to the
     lines that include if_snd.

   Empty check operation
     If the code checks ifq_head to see whether the queue is empty or not, use

		 ##old-style##				 ##new-style##
      if (ifp->if_snd.ifq_head != NULL)	    | if (!IFQ_IS_EMPTY(&ifp->if_snd))
     IFQ_IS_EMPTY() only checks if there is any packet stored in the queue.
     Note that even when IFQ_IS_EMPTY() is FALSE, IFQ_DEQUEUE() could still
     return NULL if the queue is under rate-limiting.

   Dequeue operation
     Replace IF_DEQUEUE() by IFQ_DEQUEUE().  Always check whether the dequeued
     mbuf is NULL or not.  Note that even when IFQ_IS_EMPTY() is FALSE,
     IFQ_DEQUEUE() could return NULL due to rate-limiting.

		 ##old-style##				 ##new-style##
      IF_DEQUEUE(&ifp->if_snd, m);	    | IFQ_DEQUEUE(&ifp->if_snd, m);
					    | if (m == NULL)
					    |	  return;
     A driver is supposed to call if_start() from transmission complete inter‐
     rupts in order to trigger the next dequeue.

   Poll-and-dequeue operation
     If the code polls the packet at the head of the queue and actually uses
     the packet before dequeuing it, use IFQ_POLL_NOLOCK() and

		 ##old-style##				 ##new-style##
					    | IFQ_LOCK(&ifp->if_snd);
      m = ifp->if_snd.ifq_head;		    | IFQ_POLL_NOLOCK(&ifp->if_snd, m);
      if (m != NULL) {			    | if (m != NULL) {
	  /* use m to get resources */	    |	  /* use m to get resources */
	  if (something goes wrong)	    |	  if (something goes wrong)
					    |	      IFQ_UNLOCK(&ifp->if_snd);
	      return;			    |	      return;
	  IF_DEQUEUE(&ifp->if_snd, m);	    |	  IFQ_DEQUEUE_NOLOCK(&ifp->if_snd, m);
					    |	  IFQ_UNLOCK(&ifp->if_snd);
	  /* kick the hardware */	    |	  /* kick the hardware */
      }					    | }
     It is guaranteed that IFQ_DEQUEUE_NOLOCK() under the same lock as a pre‐
     vious IFQ_POLL_NOLOCK() returns the same packet.  Note that they need to
     be guarded by IFQ_LOCK().

   Eliminating IF_PREPEND()
     If the code uses IF_PREPEND(), you have to eliminate it unless you can
     use a “driver managed” queue which allows the use of IFQ_DRV_PREPEND() as
     a substitute.  A common usage of IF_PREPEND() is to cancel the previous
     dequeue operation.	 You have to convert the logic into poll-and-dequeue.

		 ##old-style##				 ##new-style##
					    | IFQ_LOCK(&ifp->if_snd);
      IF_DEQUEUE(&ifp->if_snd, m);	    | IFQ_POLL_NOLOCK(&ifp->if_snd, m);
      if (m != NULL) {			    | if (m != NULL) {
	  if (something_goes_wrong) {	    |	  if (something_goes_wrong) {
	      IF_PREPEND(&ifp->if_snd, m);  |	      IFQ_UNLOCK(&ifp->if_snd);
	      return;			    |	      return;
	  }				    |	  }
					    |	  /* at this point, the driver
					    |	   * is committed to send this
					    |	   * packet.
					    |	   */
					    |	  IFQ_DEQUEUE_NOLOCK(&ifp->if_snd, m);
					    |	  IFQ_UNLOCK(&ifp->if_snd);
	  /* kick the hardware */	    |	  /* kick the hardware */
      }					    | }

   Purge operation
     Use IFQ_PURGE() to empty the queue.  Note that a non-work conserving
     queue cannot be emptied by a dequeue loop.

		 ##old-style##				 ##new-style##
      while (ifp->if_snd.ifq_head != NULL) {|  IFQ_PURGE(&ifp->if_snd);
	  IF_DEQUEUE(&ifp->if_snd, m);	    |
	  m_freem(m);			    |
      }					    |

   Conversion using a driver managed queue
     Convert IF_*() macros to their equivalent IFQ_DRV_*() and employ
     IFQ_DRV_IS_EMPTY() where appropriate.

		 ##old-style##				 ##new-style##
      if (ifp->if_snd.ifq_head != NULL)	    | if (!IFQ_DRV_IS_EMPTY(&ifp->if_snd))
     Make sure that calls to IFQ_DRV_DEQUEUE(), IFQ_DRV_PREPEND() and
     IFQ_DRV_PURGE() are protected with a mutex of some kind.

   Attach routine
     Use IFQ_SET_MAXLEN() to set ifq_maxlen to len.  Initialize ifq_drv_maxlen
     with a sensible value if you plan to use the IFQ_DRV_*() macros.  Add
     IFQ_SET_READY() to show this driver is converted to the new style.	 (This
     is used to distinguish new-style drivers.)

		 ##old-style##				 ##new-style##
      ifp->if_snd.ifq_maxlen = qsize;	    | IFQ_SET_MAXLEN(&ifp->if_snd, qsize);
					    | ifp->if_snd.ifq_drv_maxlen = qsize;
					    | IFQ_SET_READY(&ifp->if_snd);
      if_attach(ifp);			    | if_attach(ifp);

   Other issues
     The new macros for statistics:

		 ##old-style##				 ##new-style##
      IF_DROP(&ifp->if_snd);		    | IFQ_INC_DROPS(&ifp->if_snd);
      ifp->if_snd.ifq_len++;		    | IFQ_INC_LEN(&ifp->if_snd);
      ifp->if_snd.ifq_len--;		    | IFQ_DEC_LEN(&ifp->if_snd);

     Queuing disciplines need to maintain ifq_len (used by IFQ_IS_EMPTY()).
     Queuing disciplines also need to guarantee that the same mbuf is returned
     if IFQ_DEQUEUE() is called immediately after IFQ_POLL().

     pf(4), pf.conf(5), pfctl(8)

     The ALTQ system first appeared in March 1997.

BSD				August 25, 2004				   BSD

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