cc.c

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/*-
 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
 *
 * Copyright (c) 2007-2008
 *      Swinburne University of Technology, Melbourne, Australia.
 * Copyright (c) 2009-2010 Lawrence Stewart <lstewart@freebsd.org>
 * Copyright (c) 2010 The FreeBSD Foundation
 * All rights reserved.
 *
 * This software was developed at the Centre for Advanced Internet
 * Architectures, Swinburne University of Technology, by Lawrence Stewart and
 * James Healy, made possible in part by a grant from the Cisco University
 * Research Program Fund at Community Foundation Silicon Valley.
 *
 * Portions of this software were developed at the Centre for Advanced
 * Internet Architectures, Swinburne University of Technology, Melbourne,
 * Australia by David Hayes under sponsorship from the FreeBSD Foundation.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */
 
/*
 * This software was first released in 2007 by James Healy and Lawrence Stewart
 * whilst working on the NewTCP research project at Swinburne University of
 * Technology's Centre for Advanced Internet Architectures, Melbourne,
 * Australia, which was made possible in part by a grant from the Cisco
 * University Research Program Fund at Community Foundation Silicon Valley.
 * More details are available at:
 *   http://caia.swin.edu.au/urp/newtcp/
 */
 
#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");
#include <opt_cc.h>
#include <sys/param.h>
#include <sys/kernel.h>
#include <sys/libkern.h>
#include <sys/lock.h>
#include <sys/malloc.h>
#include <sys/module.h>
#include <sys/mutex.h>
#include <sys/queue.h>
#include <sys/rwlock.h>
#include <sys/sbuf.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#include <sys/sysctl.h>
 
#include <net/vnet.h>
 
#include <netinet/in.h>
#include <netinet/in_pcb.h>
#include <netinet/tcp.h>
#include <netinet/tcp_seq.h>
#include <netinet/tcp_var.h>
#include <netinet/tcp_log_buf.h>
#include <netinet/tcp_hpts.h>
#include <netinet/cc/cc.h>
#include <netinet/cc/cc_module.h>
 
/*
 * Have a sane default if no CC_DEFAULT is specified in the kernel config file.
 */
#ifndef CC_DEFAULT
#define CC_DEFAULT "newreno"
#endif
 
uint32_t hystart_minrtt_thresh = 4000;
uint32_t hystart_maxrtt_thresh = 16000;
uint32_t hystart_n_rttsamples = 8;
uint32_t hystart_css_growth_div = 4;
uint32_t hystart_css_rounds = 5;
uint32_t hystart_bblogs = 0;
 
MALLOC_DEFINE(M_CC_MEM, "CC Mem", "Congestion Control State memory");
 
/*
 * List of available cc algorithms on the current system. First element
 * is used as the system default CC algorithm.
 */
struct cc_head cc_list = STAILQ_HEAD_INITIALIZER(cc_list);
 
/* Protects the cc_list TAILQ. */
struct rwlock cc_list_lock;
 
VNET_DEFINE(struct cc_algo *, default_cc_ptr) = NULL;
 
VNET_DEFINE(uint32_t, newreno_beta) = 50;
#define V_newreno_beta VNET(newreno_beta)
 
void
cc_refer(struct cc_algo *algo)
{
        CC_LIST_LOCK_ASSERT();
        refcount_acquire(&algo->cc_refcount);
}
 
void
cc_release(struct cc_algo *algo)
{
        CC_LIST_LOCK_ASSERT();
        refcount_release(&algo->cc_refcount);
}
 
 
void
cc_attach(struct tcpcb *tp, struct cc_algo *algo)
{
        /*
         * Attach the tcpcb to the algorithm.
         */
        CC_LIST_RLOCK();
        CC_ALGO(tp) = algo;
        cc_refer(algo);
        CC_LIST_RUNLOCK();
}
 
void
cc_detach(struct tcpcb *tp)
{
        struct cc_algo *algo;
 
        CC_LIST_RLOCK();
        algo = CC_ALGO(tp);
        CC_ALGO(tp) = NULL;
        cc_release(algo);
        CC_LIST_RUNLOCK();
}
 
/*
 * Sysctl handler to show and change the default CC algorithm.
 */
static int
cc_default_algo(SYSCTL_HANDLER_ARGS)
{
        char default_cc[TCP_CA_NAME_MAX];
        struct cc_algo *funcs;
        int error;
 
        /* Get the current default: */
        CC_LIST_RLOCK();
        if (CC_DEFAULT_ALGO() != NULL)
                strlcpy(default_cc, CC_DEFAULT_ALGO()->name, sizeof(default_cc));
        else
                memset(default_cc, 0, TCP_CA_NAME_MAX);
        CC_LIST_RUNLOCK();
 
        error = sysctl_handle_string(oidp, default_cc, sizeof(default_cc), req);
 
        /* Check for error or no change */
        if (error != 0 || req->newptr == NULL)
                goto done;
 
        error = ESRCH;
        /* Find algo with specified name and set it to default. */
        CC_LIST_RLOCK();
        STAILQ_FOREACH(funcs, &cc_list, entries) {
                if (strncmp(default_cc, funcs->name, sizeof(default_cc)))
                        continue;
                if (funcs->flags & CC_MODULE_BEING_REMOVED) {
                        /* Its being removed, its not eligible */
                        continue;
                }
                V_default_cc_ptr = funcs;
                error = 0;
                break;
        }
        CC_LIST_RUNLOCK();
done:
        return (error);
}
 
/*
 * Sysctl handler to display the list of available CC algorithms.
 */
static int
cc_list_available(SYSCTL_HANDLER_ARGS)
{
        struct cc_algo *algo;
        int error, nalgos;
        int linesz;
        char *buffer, *cp;
        size_t bufsz, outsz;
 
        error = nalgos = 0;
        CC_LIST_RLOCK();
        STAILQ_FOREACH(algo, &cc_list, entries) {
                nalgos++;
        }
        CC_LIST_RUNLOCK();
        if (nalgos == 0) {
                return (ENOENT);
        }
        bufsz = (nalgos+2) * ((TCP_CA_NAME_MAX + 13) + 1);
        buffer = malloc(bufsz, M_TEMP, M_WAITOK);
        cp = buffer;
 
        linesz = snprintf(cp, bufsz, "\n%-16s%c %s\n", "CCmod", 'D',
            "PCB count");
        cp += linesz;
        bufsz -= linesz;
        outsz = linesz;
        CC_LIST_RLOCK();
        STAILQ_FOREACH(algo, &cc_list, entries) {
                linesz = snprintf(cp, bufsz, "%-16s%c %u\n",
                    algo->name,
                    (algo == CC_DEFAULT_ALGO()) ? '*' : ' ',
                    algo->cc_refcount);
                if (linesz >= bufsz) {
                        error = EOVERFLOW;
                        break;
                }
                cp += linesz;
                bufsz -= linesz;
                outsz += linesz;
        }
        CC_LIST_RUNLOCK();
        if (error == 0)
                error = sysctl_handle_string(oidp, buffer, outsz + 1, req);
        free(buffer, M_TEMP);
        return (error);
}
 
/*
 * Return the number of times a proposed removal_cc is
 * being used as the default.
 */
static int
cc_check_default(struct cc_algo *remove_cc)
{
        int cnt = 0;
        VNET_ITERATOR_DECL(vnet_iter);
 
        CC_LIST_LOCK_ASSERT();
 
        VNET_LIST_RLOCK_NOSLEEP();
        VNET_FOREACH(vnet_iter) {
                CURVNET_SET(vnet_iter);
                if ((CC_DEFAULT_ALGO() != NULL) &&
                    strncmp(CC_DEFAULT_ALGO()->name,
                            remove_cc->name,
                            TCP_CA_NAME_MAX) == 0) {
                        cnt++;
                }
                CURVNET_RESTORE();
        }
        VNET_LIST_RUNLOCK_NOSLEEP();
        return (cnt);
}
 
/*
 * Initialise CC subsystem on system boot.
 */
static void
cc_init(void)
{
        CC_LIST_LOCK_INIT();
        STAILQ_INIT(&cc_list);
}
 
/*
 * Returns non-zero on success, 0 on failure.
 */
int
cc_deregister_algo(struct cc_algo *remove_cc)
{
        struct cc_algo *funcs;
        int found = 0;
 
        /* Remove algo from cc_list so that new connections can't use it. */
        CC_LIST_WLOCK();
        
        /* This is unlikely to fail */
        STAILQ_FOREACH(funcs, &cc_list, entries) {
                if (funcs == remove_cc)
                        found = 1;
        }
        if (found == 0) {
                /* Nothing to remove? */
                CC_LIST_WUNLOCK();
                return (ENOENT);
        }
        /* We assert it should have been MOD_QUIESCE'd */
        KASSERT((remove_cc->flags & CC_MODULE_BEING_REMOVED),
                ("remove_cc:%p does not have CC_MODULE_BEING_REMOVED flag", remove_cc));
        if (cc_check_default(remove_cc)) {
                CC_LIST_WUNLOCK();
                return(EBUSY);
        }
        if (remove_cc->cc_refcount != 0) {
                CC_LIST_WUNLOCK();
                return (EBUSY);
        }
        STAILQ_REMOVE(&cc_list, remove_cc, cc_algo, entries);
        CC_LIST_WUNLOCK();
        return (0);
}
 
/*
 * Returns 0 on success, non-zero on failure.
 */
int
cc_register_algo(struct cc_algo *add_cc)
{
        struct cc_algo *funcs;
        int err;
 
        err = 0;
 
        /*
         * Iterate over list of registered CC algorithms and make sure
         * we're not trying to add a duplicate.
         */
        CC_LIST_WLOCK();
        STAILQ_FOREACH(funcs, &cc_list, entries) {
                if (funcs == add_cc ||
                    strncmp(funcs->name, add_cc->name,
                            TCP_CA_NAME_MAX) == 0) {
                        err = EEXIST;
                        break;
                }
        }
        /* Init its reference count */
        if (err == 0)
                refcount_init(&add_cc->cc_refcount, 0);
        /*
         * The first loaded congestion control module will become
         * the default until we find the "CC_DEFAULT" defined in
         * the config (if we do).
         */
        if (!err) {
                STAILQ_INSERT_TAIL(&cc_list, add_cc, entries);
                if (strcmp(add_cc->name, CC_DEFAULT) == 0) {
                        V_default_cc_ptr = add_cc;
                } else if (V_default_cc_ptr == NULL) {
                        V_default_cc_ptr = add_cc;
                }
        }
        CC_LIST_WUNLOCK();
 
        return (err);
}
 
static void
vnet_cc_sysinit(void *arg)
{
        struct cc_algo *cc;
 
        if (IS_DEFAULT_VNET(curvnet))
                return;
 
        CURVNET_SET(vnet0);
        cc = V_default_cc_ptr;
        CURVNET_RESTORE();
 
        V_default_cc_ptr = cc;
}
VNET_SYSINIT(vnet_cc_sysinit, SI_SUB_PROTO_IFATTACHDOMAIN, SI_ORDER_ANY,
    vnet_cc_sysinit, NULL);
 
/*
 * Perform any necessary tasks before we exit congestion recovery.
 */
void
newreno_cc_post_recovery(struct cc_var *ccv)
{
        int pipe;
 
        if (IN_FASTRECOVERY(CCV(ccv, t_flags))) {
                /*
                 * Fast recovery will conclude after returning from this
                 * function. Window inflation should have left us with
                 * approximately snd_ssthresh outstanding data. But in case we
                 * would be inclined to send a burst, better to do it via the
                 * slow start mechanism.
                 *
                 * XXXLAS: Find a way to do this without needing curack
                 */
                if (V_tcp_do_newsack)
                        pipe = tcp_compute_pipe(ccv->ccvc.tcp);
                else
                        pipe = CCV(ccv, snd_max) - ccv->curack;
                if (pipe < CCV(ccv, snd_ssthresh))
                        /*
                         * Ensure that cwnd does not collapse to 1 MSS under
                         * adverse conditions. Implements RFC6582
                         */
                        CCV(ccv, snd_cwnd) = max(pipe, CCV(ccv, t_maxseg)) +
                            CCV(ccv, t_maxseg);
                else
                        CCV(ccv, snd_cwnd) = CCV(ccv, snd_ssthresh);
        }
}
 
void
newreno_cc_after_idle(struct cc_var *ccv)
{
        uint32_t rw;
        /*
         * If we've been idle for more than one retransmit timeout the old
         * congestion window is no longer current and we have to reduce it to
         * the restart window before we can transmit again.
         *
         * The restart window is the initial window or the last CWND, whichever
         * is smaller.
         *
         * This is done to prevent us from flooding the path with a full CWND at
         * wirespeed, overloading router and switch buffers along the way.
         *
         * See RFC5681 Section 4.1. "Restarting Idle Connections".
         *
         * In addition, per RFC2861 Section 2, the ssthresh is set to the
         * maximum of the former ssthresh or 3/4 of the old cwnd, to
         * not exit slow-start prematurely.
         */
        rw = tcp_compute_initwnd(tcp_maxseg(ccv->ccvc.tcp));
 
        CCV(ccv, snd_ssthresh) = max(CCV(ccv, snd_ssthresh),
            CCV(ccv, snd_cwnd)-(CCV(ccv, snd_cwnd)>>2));
 
        CCV(ccv, snd_cwnd) = min(rw, CCV(ccv, snd_cwnd));
}
 
/*
 * Perform any necessary tasks before we enter congestion recovery.
 */
void
newreno_cc_cong_signal(struct cc_var *ccv, uint32_t type)
{
        uint32_t cwin, factor;
        u_int mss;
 
        cwin = CCV(ccv, snd_cwnd);
        mss = tcp_fixed_maxseg(ccv->ccvc.tcp);
        /*
         * Other TCP congestion controls use newreno_cong_signal(), but
         * with their own private cc_data. Make sure the cc_data is used
         * correctly.
         */
        factor = V_newreno_beta;
 
        /* Catch algos which mistakenly leak private signal types. */
        KASSERT((type & CC_SIGPRIVMASK) == 0,
            ("%s: congestion signal type 0x%08x is private\n", __func__, type));
 
        cwin = max(((uint64_t)cwin * (uint64_t)factor) / (100ULL * (uint64_t)mss),
            2) * mss;
 
        switch (type) {
        case CC_NDUPACK:
                if (!IN_FASTRECOVERY(CCV(ccv, t_flags))) {
                        if (!IN_CONGRECOVERY(CCV(ccv, t_flags)))
                                CCV(ccv, snd_ssthresh) = cwin;
                        ENTER_RECOVERY(CCV(ccv, t_flags));
                }
                break;
        case CC_ECN:
                if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
                        CCV(ccv, snd_ssthresh) = cwin;
                        CCV(ccv, snd_cwnd) = cwin;
                        ENTER_CONGRECOVERY(CCV(ccv, t_flags));
                }
                break;
        case CC_RTO:
                CCV(ccv, snd_ssthresh) = max(min(CCV(ccv, snd_wnd),
                                                 CCV(ccv, snd_cwnd)) / 2 / mss,
                                             2) * mss;
                CCV(ccv, snd_cwnd) = mss;
                break;
        }
}
 
void
newreno_cc_ack_received(struct cc_var *ccv, uint16_t type)
{
        if (type == CC_ACK && !IN_RECOVERY(CCV(ccv, t_flags)) &&
            (ccv->flags & CCF_CWND_LIMITED)) {
                u_int cw = CCV(ccv, snd_cwnd);
                u_int incr = CCV(ccv, t_maxseg);
 
                /*
                 * Regular in-order ACK, open the congestion window.
                 * Method depends on which congestion control state we're
                 * in (slow start or cong avoid) and if ABC (RFC 3465) is
                 * enabled.
                 *
                 * slow start: cwnd <= ssthresh
                 * cong avoid: cwnd > ssthresh
                 *
                 * slow start and ABC (RFC 3465):
                 *   Grow cwnd exponentially by the amount of data
                 *   ACKed capping the max increment per ACK to
                 *   (abc_l_var * maxseg) bytes.
                 *
                 * slow start without ABC (RFC 5681):
                 *   Grow cwnd exponentially by maxseg per ACK.
                 *
                 * cong avoid and ABC (RFC 3465):
                 *   Grow cwnd linearly by maxseg per RTT for each
                 *   cwnd worth of ACKed data.
                 *
                 * cong avoid without ABC (RFC 5681):
                 *   Grow cwnd linearly by approximately maxseg per RTT using
                 *   maxseg^2 / cwnd per ACK as the increment.
                 *   If cwnd > maxseg^2, fix the cwnd increment at 1 byte to
                 *   avoid capping cwnd.
                 */
                if (cw > CCV(ccv, snd_ssthresh)) {
                        if (V_tcp_do_rfc3465) {
                                if (ccv->flags & CCF_ABC_SENTAWND)
                                        ccv->flags &= ~CCF_ABC_SENTAWND;
                                else
                                        incr = 0;
                        } else
                                incr = max((incr * incr / cw), 1);
                } else if (V_tcp_do_rfc3465) {
                        /*
                         * In slow-start with ABC enabled and no RTO in sight?
                         * (Must not use abc_l_var > 1 if slow starting after
                         * an RTO. On RTO, snd_nxt = snd_una, so the
                         * snd_nxt == snd_max check is sufficient to
                         * handle this).
                         *
                         * XXXLAS: Find a way to signal SS after RTO that
                         * doesn't rely on tcpcb vars.
                         */
                        uint16_t abc_val;
 
                        if (ccv->flags & CCF_USE_LOCAL_ABC)
                                abc_val = ccv->labc;
                        else
                                abc_val = V_tcp_abc_l_var;
                        if (CCV(ccv, snd_nxt) == CCV(ccv, snd_max))
                                incr = min(ccv->bytes_this_ack,
                                    ccv->nsegs * abc_val *
                                    CCV(ccv, t_maxseg));
                        else
                                incr = min(ccv->bytes_this_ack, CCV(ccv, t_maxseg));
 
                }
                /* ABC is on by default, so incr equals 0 frequently. */
                if (incr > 0)
                        CCV(ccv, snd_cwnd) = min(cw + incr,
                            TCP_MAXWIN << CCV(ccv, snd_scale));
        }
}
 
static int
cc_stop_new_assignments(struct cc_algo *algo)
{
        CC_LIST_WLOCK();
        if (cc_check_default(algo)) {
                /* A default cannot be removed */
                CC_LIST_WUNLOCK();
                return (EBUSY);
        }
        algo->flags |= CC_MODULE_BEING_REMOVED;
        CC_LIST_WUNLOCK();
        return (0);
}
 
/*
 * Handles kld related events. Returns 0 on success, non-zero on failure.
 */
int
cc_modevent(module_t mod, int event_type, void *data)
{
        struct cc_algo *algo;
        int err;
 
        err = 0;
        algo = (struct cc_algo *)data;
 
        switch(event_type) {
        case MOD_LOAD:
                if ((algo->cc_data_sz == NULL) && (algo->cb_init != NULL)) {
                        /*
                         * A module must have a cc_data_sz function
                         * even if it has no data it should return 0.
                         */
                        printf("Module Load Fails, it lacks a cc_data_sz() function but has a cb_init()!\n");
                        err = EINVAL;
                        break;
                }
                if (algo->mod_init != NULL)
                        err = algo->mod_init();
                if (!err)
                        err = cc_register_algo(algo);
                break;
 
        case MOD_SHUTDOWN:
                break;
        case MOD_QUIESCE:
                /* Stop any new assigments */
                err = cc_stop_new_assignments(algo);
                break;
        case MOD_UNLOAD:
                /* 
                 * Deregister and remove the module from the list 
                 */
                CC_LIST_WLOCK();
                /* Even with -f we can't unload if its the default */
                if (cc_check_default(algo)) {
                        /* A default cannot be removed */
                        CC_LIST_WUNLOCK();
                        return (EBUSY);
                }
                /*
                 * If -f was used and users are still attached to
                 * the algorithm things are going to go boom.
                 */
                err = cc_deregister_algo(algo);
                if ((err == 0) && (algo->mod_destroy != NULL)) {
                        algo->mod_destroy();
                }
                break;
        default:
                err = EINVAL;
                break;
        }
 
        return (err);
}
 
SYSINIT(cc, SI_SUB_PROTO_IFATTACHDOMAIN, SI_ORDER_FIRST, cc_init, NULL);
 
/* Declare sysctl tree and populate it. */
SYSCTL_NODE(_net_inet_tcp, OID_AUTO, cc, CTLFLAG_RW | CTLFLAG_MPSAFE, NULL,
    "Congestion control related settings");
 
SYSCTL_PROC(_net_inet_tcp_cc, OID_AUTO, algorithm,
    CTLFLAG_VNET | CTLTYPE_STRING | CTLFLAG_RW | CTLFLAG_MPSAFE,
    NULL, 0, cc_default_algo, "A",
    "Default congestion control algorithm");
 
SYSCTL_PROC(_net_inet_tcp_cc, OID_AUTO, available,
    CTLTYPE_STRING | CTLFLAG_RD | CTLFLAG_MPSAFE,
    NULL, 0, cc_list_available, "A",
    "List available congestion control algorithms");
 
SYSCTL_NODE(_net_inet_tcp_cc, OID_AUTO, hystartplusplus,
    CTLFLAG_RW | CTLFLAG_MPSAFE, NULL,
    "New Reno related HyStart++ settings");
 
SYSCTL_UINT(_net_inet_tcp_cc_hystartplusplus, OID_AUTO, minrtt_thresh,
    CTLFLAG_RW,
    &hystart_minrtt_thresh, 4000,
   "HyStarts++ minimum RTT thresh used in clamp (in microseconds)");
 
SYSCTL_UINT(_net_inet_tcp_cc_hystartplusplus, OID_AUTO, maxrtt_thresh,
    CTLFLAG_RW,
    &hystart_maxrtt_thresh, 16000,
   "HyStarts++ maximum RTT thresh used in clamp (in microseconds)");
 
SYSCTL_UINT(_net_inet_tcp_cc_hystartplusplus, OID_AUTO, n_rttsamples,
    CTLFLAG_RW,
    &hystart_n_rttsamples, 8,
   "The number of RTT samples that must be seen to consider HyStart++");
 
SYSCTL_UINT(_net_inet_tcp_cc_hystartplusplus, OID_AUTO, css_growth_div,
    CTLFLAG_RW,
    &hystart_css_growth_div, 4,
   "The divisor to the growth when in Hystart++ CSS");
 
SYSCTL_UINT(_net_inet_tcp_cc_hystartplusplus, OID_AUTO, css_rounds,
    CTLFLAG_RW,
    &hystart_css_rounds, 5,
   "The number of rounds HyStart++ lasts in CSS before falling to CA");
 
SYSCTL_UINT(_net_inet_tcp_cc_hystartplusplus, OID_AUTO, bblogs,
    CTLFLAG_RW,
    &hystart_bblogs, 0,
   "Do we enable HyStart++ Black Box logs to be generated if BB logging is on");
 
VNET_DEFINE(int, cc_do_abe) = 0;
SYSCTL_INT(_net_inet_tcp_cc, OID_AUTO, abe, CTLFLAG_VNET | CTLFLAG_RW,
    &VNET_NAME(cc_do_abe), 0,
    "Enable draft-ietf-tcpm-alternativebackoff-ecn (TCP Alternative Backoff with ECN)");
 
VNET_DEFINE(int, cc_abe_frlossreduce) = 0;
SYSCTL_INT(_net_inet_tcp_cc, OID_AUTO, abe_frlossreduce, CTLFLAG_VNET | CTLFLAG_RW,
    &VNET_NAME(cc_abe_frlossreduce), 0,
    "Apply standard beta instead of ABE-beta during ECN-signalled congestion "
    "recovery episodes if loss also needs to be repaired");