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 traditional 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-limiting.

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 flag.

COMPATIBILITY

`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;
 };                                    |    /* 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 separated 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);
 }                                     | }
                                       |

HOW TO CONVERT THE EXISTING DRIVERS

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 IFQ_IS_EMPTY().

            ##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 interrupts 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 IFQ_DEQUEUE_NOLOCK().

            ##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 previous 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

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().

HISTORY

The ALTQ system first appeared in March 1997 and found home in the KAME project (https://www.kame.net). It was imported to FreeBSD in 5.3 .

Eliminating IF_PREPEND()

Eliminating `IF_PREPEND`()