res_remb_modifier.c

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/*
 * Asterisk -- An open source telephony toolkit.
 *
 * Copyright (C) 2018, Digium, Inc.
 *
 * Joshua Colp <jcolp@digium.com>
 *
 * See http://www.asterisk.org for more information about
 * the Asterisk project. Please do not directly contact
 * any of the maintainers of this project for assistance;
 * the project provides a web site, mailing lists and IRC
 * channels for your use.
 *
 * This program is free software, distributed under the terms of
 * the GNU General Public License Version 2. See the LICENSE file
 * at the top of the source tree.
 */
 
/*!
 * \file
 *
 * \brief REMB Modifier Module
 *
 * \author Joshua Colp <jcolp@digium.com>
 */
 
/*** MODULEINFO
    <defaultenabled>no</defaultenabled>
    <support_level>extended</support_level>
 ***/
 
#include "asterisk.h"
 
#include <math.h>
 
#include "asterisk/module.h"
#include "asterisk/cli.h"
#include "asterisk/channel.h"
#include "asterisk/framehook.h"
#include "asterisk/rtp_engine.h"
 
struct remb_values {
    /*! \brief The amount of bitrate to use for REMB received from the channel */
    float receive_bitrate;
    /*! \brief The amount of bitrate to use for REMB sent to the channel */
    float send_bitrate;
};
 
static void remb_values_free(void *data)
{
    ast_free(data);
}
 
static const struct ast_datastore_info remb_info = {
    .type = "REMB Values",
    .destroy = remb_values_free,
};
 
static struct ast_frame *remb_hook_event_cb(struct ast_channel *chan, struct ast_frame *frame, enum ast_framehook_event event, void *data)
{
    struct ast_rtp_rtcp_feedback *feedback;
    struct ast_datastore *remb_store;
    struct remb_values *remb_values;
    int exp;
    float bitrate = 0.0;
 
    if (!frame) {
        return NULL;
    }
 
    switch (event) {
    case AST_FRAMEHOOK_EVENT_READ:
    case AST_FRAMEHOOK_EVENT_WRITE:
        break;
    case AST_FRAMEHOOK_EVENT_ATTACHED:
    case AST_FRAMEHOOK_EVENT_DETACHED:
        return frame;
    }
 
    /* We only care about REMB frames, all others will be unmodified */
    if (frame->subclass.integer != AST_RTP_RTCP_PSFB) {
        return frame;
    }
 
    feedback = frame->data.ptr;
    if (feedback->fmt != AST_RTP_RTCP_FMT_REMB) {
        return frame;
    }
 
    remb_store = ast_channel_datastore_find(chan, &remb_info, NULL);
    if (!remb_store) {
        return frame;
    }
    remb_values = remb_store->data;
 
    /* If a bitrate override has been set apply it to the REMB Frame */
    if (event == AST_FRAMEHOOK_EVENT_READ && remb_values->receive_bitrate) {
        bitrate = remb_values->receive_bitrate;
    } else if (event == AST_FRAMEHOOK_EVENT_WRITE && remb_values->send_bitrate) {
        bitrate = remb_values->send_bitrate;
    } else {
        return frame;
    }
 
    /*
     * The mantissa only has 18 bits available, so make sure it fits. Adjust the
     * value and exponent for those values that don't.
     *
     * For example given the following:
     *
     * bitrate = 123456789.0
     * frexp(bitrate, &exp);
     *
     * 'exp' should now equal 27 (number of bits needed to represent the value). Since
     * the mantissa must fit into an 18-bit unsigned integer, and the given bitrate is
     * too large to fit, we must subtract 18 from the exponent in order to get the
     * number of times the bitrate will fit into that size integer.
     *
     * exp -= 18;
     *
     * 'exp' is now equal to 9. Now we can get the mantissa that fits into an 18-bit
     * unsigned integer by dividing the bitrate by 2^exp:
     *
     * mantissa = 123456789.0 / 2^9
     *
     * This makes the final mantissa equal to 241126 (implicitly cast), which is less
     * than 262143 (the max value that can be put into an unsigned 18-bit integer).
     * So now we have the following:
     *
     * exp = 9;
     * mantissa = 241126;
     *
     * If we multiply that back we should come up with something close to the original
     * bit rate:
     *
     * 241126 * 2^9 = 123456512
     *
     * Precision is lost due to the nature of floating point values. Easier to why from
     * the binary:
     *
     * 241126 * 2^9 = 241126 << 9 = 111010110111100110 << 9 = 111010110111100110000000000
     *
     * Precision on the "lower" end is lost due to zeros being shifted in. This loss is
     * both expected and acceptable.
     */
    frexp(bitrate, &exp);
    exp = exp > 18 ? exp - 18 : 0;
 
    feedback->remb.br_mantissa = bitrate / (1 << exp);
    feedback->remb.br_exp = exp;
 
    return frame;
}
 
static char *handle_remb_set(struct ast_cli_entry *e, int cmd, struct ast_cli_args *a)
{
    struct ast_channel *chan;
    unsigned int bitrate;
    struct ast_datastore *remb_store;
    struct remb_values *remb_values;
    struct ast_framehook_interface interface = {
        .version = AST_FRAMEHOOK_INTERFACE_VERSION,
        .event_cb = remb_hook_event_cb,
    };
 
    switch(cmd) {
    case CLI_INIT:
        e->command = "remb set {send|receive}";
        e->usage =
            "Usage: remb set {send|receive} <channel> <bitrate in bits>\n"
            "       Set the REMB value which overwrites what we send or receive\n";
        return NULL;
    case CLI_GENERATE:
        return ast_complete_channels(a->line, a->word, a->pos, a->n, 3);
    }
 
    if (a->argc != 5) {
        return CLI_SHOWUSAGE;
    }
 
    if (sscanf(a->argv[4], "%30d", &bitrate) != 1) {
        ast_cli(a->fd, "%s is not a valid bitrate in bits\n", a->argv[4]);
        return CLI_SUCCESS;
    } else if (strcasecmp(a->argv[2], "send") && strcasecmp(a->argv[2], "receive")) {
        ast_cli(a->fd, "%s is not a valid direction for REMB\n", a->argv[2]);
        return CLI_SUCCESS;
    }
 
    chan = ast_channel_get_by_name(a->argv[3]);
    if (!chan) {
        ast_cli(a->fd, "%s is not a known channel\n", a->argv[3]);
        return CLI_SUCCESS;
    }
 
    ast_channel_lock(chan);
 
    remb_store = ast_channel_datastore_find(chan, &remb_info, NULL);
    if (!remb_store) {
        int framehook_id;
 
        framehook_id = ast_framehook_attach(chan, &interface);
        if (framehook_id < 0) {
            ast_cli(a->fd, "Could not attach framehook for modifying REMB\n");
            ast_channel_unlock(chan);
            ast_channel_unref(chan);
            return CLI_SUCCESS;
        }
 
        remb_values = ast_calloc(1, sizeof(*remb_values));
        if (!remb_values) {
            ast_cli(a->fd, "Could not create a place to store provided REMB value\n");
            ast_framehook_detach(chan, framehook_id);
            ast_channel_unlock(chan);
            ast_channel_unref(chan);
            return CLI_SUCCESS;
        }
 
        remb_store = ast_datastore_alloc(&remb_info, NULL);
        if (!remb_store) {
            ast_cli(a->fd, "Could not create a place to store provided REMB value\n");
            ast_framehook_detach(chan, framehook_id);
            ast_channel_unlock(chan);
            ast_channel_unref(chan);
            ast_free(remb_values);
            return CLI_SUCCESS;
        }
 
        remb_store->data = remb_values;
        ast_channel_datastore_add(chan, remb_store);
    } else {
        remb_values = remb_store->data;
    }
 
    if (!strcasecmp(a->argv[2], "send")) {
        remb_values->send_bitrate = bitrate;
    } else if (!strcasecmp(a->argv[2], "receive")) {
        remb_values->receive_bitrate = bitrate;
    }
 
    ast_channel_unlock(chan);
    ast_channel_unref(chan);
 
    ast_cli(a->fd, "Set REMB %s override to a bitrate of %s on %s\n", a->argv[2], a->argv[3], a->argv[4]);
 
    return CLI_SUCCESS;
}
 
static struct ast_cli_entry remb_cli[] = {
    AST_CLI_DEFINE(handle_remb_set, "Set the REMB value which overwrites what is sent or received"),
};
 
static int load_module(void)
{
    ast_cli_register_multiple(remb_cli, ARRAY_LEN(remb_cli));
    return AST_MODULE_LOAD_SUCCESS;
}
 
static int unload_module(void)
{
    ast_cli_unregister_multiple(remb_cli, ARRAY_LEN(remb_cli));
    return 0;
}
 
AST_MODULE_INFO(ASTERISK_GPL_KEY, AST_MODFLAG_DEFAULT, "REMB Modifier Module",
    .support_level = AST_MODULE_SUPPORT_EXTENDED,
    .load = load_module,
    .unload = unload_module,
);