October 12, 2006 ALTQ 9

### NAME

ALTQ - kernel interfaces for manipulating output queues on network interfaces

### SYNOPSIS

```void` IFQ_ENQUEUE(```struct ifaltq *ifq``, struct mbuf *m``, struct altq_pktattr *pattr``, int err`)
```void` IFQ_DEQUEUE(```struct ifaltq *ifq``, struct mbuf *m`)
```void` IFQ_POLL(```struct ifaltq *ifq``, struct mbuf *m`)
```void` IFQ_PURGE(```struct ifaltq *ifq`)
```void` IFQ_CLASSIFY(```struct ifaltq *ifq``, struct mbuf *m``, int af``, struct altq_pktattr *pattr`)
```void` IFQ_IS_EMPTY(```struct ifaltq *ifq`)
```void` IFQ_SET_MAXLEN(```struct ifaltq *ifq``, int len`)
```void` IFQ_INC_LEN(```struct ifaltq *ifq`)
```void` IFQ_DEC_LEN(```struct ifaltq *ifq`)
```void` IFQ_INC_DROPS(```struct ifaltq *ifq`)
```void` IFQ_SET_READY(```struct ifaltq *ifq`)

### DESCRIPTION

The ALTQ system is a framework to manage queueing 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(``) enqueues a packet ```m` to the queue ```ifq`. The underlying queueing discipline may discard the packet. ```err` is set to 0 on success, or ```ENOBUFS` if the packet is discarded. ```m` will be freed by the device driver on success or by the queueing discipline on failure, so that the caller should not touch ```m` after calling IFQ_ENQUEUE(``).

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(``) returns the next packet without removing it from the queue. It is guaranteed by the underlying queueing discipline that IFQ_DEQUEUE(``) immediately after IFQ_POLL(``) returns the same packet.

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_CLASSIFY(``) classifies a packet to a scheduling class, and returns the result in ```pattr`.

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

IFQ_SET_MAXLEN(``) sets the queue length limit to the default FIFO queue.

IFQ_INC_LEN(``) and IFQ_DEC_LEN(``) increment or decrement the current queue length in packets.

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

IFQ_SET_READY(``) sets a flag to indicate this driver is 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_XXX(``) macros and the code directly referencing the fields within ```if_snd` still work with ```ifaltq`. (Once we finish conversions of all the drivers, we no longer need these fields.) `

                ##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;
     };                                    |    /* 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_XXX(``) macros looks like: `

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

`

#### Enqueue operation

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

    #define IFQ_ENQUEUE(ifq, m, pattr, err)                   \
    do {                                                      \
            if (ALTQ_IS_ENABLED((ifq)))                       \
                    ALTQ_ENQUEUE((ifq), (m), (pattr), (err)); \
            else {                                            \
                    if (IF_QFULL((ifq))) {                    \
                            m_freem((m));                     \
                            (err) = ENOBUFS;                  \
                    } else {                                  \
                            IF_ENQUEUE((ifq), (m));           \
                            (err) = 0;                        \
                    }                                         \
            }                                                 \
            if ((err))                                        \
                    (ifq)->ifq_drops++;                       \
    } while (/*CONSTCOND*/ 0)

`

IFQ_ENQUEUE(``) does the following:

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

If the enqueue operation fails, ```err` is set to ```ENOBUFS`. ```m` is freed by the queueing 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. 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,
                                           |                 NULL, 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);
     }                                     | }
                                           |

#### Classifier

The classifier mechanism is currently implemented in if_output(``). ```struct` `altq_pktattr` is used to store the classifier result, and it is passed to the enqueue function. (We will change the method to tag the classifier result to mbuf in the future.) `

    int
    ether_output(ifp, m0, dst, rt0)
    {
            ......
            struct altq_pktattr pktattr;

            ......

            /* classify the packet before prepending link-headers */
            IFQ_CLASSIFY(&ifp->if_snd, m, dst->sa_family, &pktattr);

            /* prepend link-level headers */
            ......

            IFQ_ENQUEUE(&ifp->if_snd, m, &pktattr, 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. You will probably 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) == 0)
                                           |

` Note that IFQ_POLL(``) can be used for the same purpose, but IFQ_POLL(``) could be costly for a complex scheduling algorithm since IFQ_POLL(``) needs to run the scheduling algorithm to select the next packet. On the other hand, IFQ_IS_EMPTY(``) checks only if there is any packet stored in the queue. Another difference is 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 dequeueing it, use IFQ_POLL(``) and IFQ_DEQUEUE(``). `

                ##old-style##                           ##new-style##
                                           |
     m = ifp->if_snd.ifq_head;             | IFQ_POLL(&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)
             return;                       |         return;
                                           |
         IF_DEQUEUE(&ifp->if_snd, m);      |     IFQ_DEQUEUE(&ifp->if_snd, m);
                                           |
         /* kick the hardware */           |     /* kick the hardware */
     }                                     | }
                                           |

` It is guaranteed that IFQ_DEQUEUE(``) immediately after IFQ_POLL(``) returns the same packet. Note that they need to be guarded by splimp(``) if called from outside of if_start(``).

#### Eliminating IF_PREPEND

If the code uses IF_PREPEND(``), you have to eliminate it since the prepend operation is not possible for many queueing disciplines. A common use of IF_PREPEND(``) is to cancel the previous dequeue operation. You have to convert the logic into poll-and-dequeue. `

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

`

#### Attach routine

Use IFQ_SET_MAXLEN(``) to set ```ifq_maxlen` to ```len`. 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);
                                           | 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);
                                           |

` Some drivers instruct the hardware to invoke transmission complete interrupts only when it thinks necessary. Rate-limiting breaks its assumption.

#### How to convert drivers using multiple ifqueues

Some (pseudo) devices (such as slip) have another ```ifqueue` to prioritize packets. It is possible to eliminate the second queue since ALTQ provides more flexible mechanisms but the following shows how to keep the original behavior. `

    struct sl_softc {
            struct  ifnet sc_if;            /* network-visible interface */
            ...
            struct  ifqueue sc_fastq;       /* interactive output queue */
            ...
    };

` The driver doesn't compile in the new model since it has the following line ```struct` `ifqueue` (```if_snd` is no longer a type of.) `

            struct ifqueue *ifq = &ifp->if_snd;

` A simple way is to use the original IF_XXX(``) macros for ```sc_fastq` and use the new IFQ_XXX(``) macros for ```if_snd`. The enqueue operation looks like: `

                ##old-style##                           ##new-style##
                                           |
     struct ifqueue *ifq = &ifp->if_snd;   | struct ifqueue *ifq = NULL;
                                           |
     if (ip->ip_tos & IPTOS_LOWDELAY)      | if ((ip->ip_tos & IPTOS_LOWDELAY) &&
         ifq = &sc->sc_fastq;              | !ALTQ_IS_ENABLED(&sc->sc_if.if_snd)) {
                                           |     ifq = &sc->sc_fastq;
     if (IF_QFULL(ifq)) {                  |     if (IF_QFULL(ifq)) {
         IF_DROP(ifq);                     |         IF_DROP(ifq);
         m_freem(m);                       |         m_freem(m);
         splx(s);                          |         error = ENOBUFS;
         sc->sc_if.if_oerrors++;           |     } else {
         return (ENOBUFS);                 |         IF_ENQUEUE(ifq, m);
     }                                     |         error = 0;
     IF_ENQUEUE(ifq, m);                   |     }
                                           | } else
                                           |     IFQ_ENQUEUE(&sc->sc_if.if_snd,
                                           |                 m, NULL, error);
                                           |
                                           | if (error) {
                                           |     splx(s);
                                           |     sc->sc_if.if_oerrors++;
                                           |     return (error);
                                           | }
     if ((sc->sc_oqlen =                   | if ((sc->sc_oqlen =
          sc->sc_ttyp->t_outq.c_cc) == 0)  |      sc->sc_ttyp->t_outq.c_cc) == 0)
         slstart(sc->sc_ttyp);             |     slstart(sc->sc_ttyp);
     splx(s);                              | splx(s);
                                           |

` The dequeue operations looks like: `

                ##old-style##                           ##new-style##
                                           |
     s = splimp();                         | s = splimp();
     IF_DEQUEUE(&sc->sc_fastq, m);         | IF_DEQUEUE(&sc->sc_fastq, m);
     if (m == NULL)                        | if (m == NULL)
         IF_DEQUEUE(&sc->sc_if.if_snd, m); |     IFQ_DEQUEUE(&sc->sc_if.if_snd, m);
     splx(s);                              | splx(s);
                                           |

`

### QUEUEING DISCIPLINES

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

### SEE ALSO

pf(4), altq.conf(5), pf.conf(5), altqd(8), tbrconfig(8)

### HISTORY

The ALTQ system first appeared in March 1997.