ipcpd: Add congestion avoidance policies

This adds congestion avoidance policies to the unicast IPCP.  The
default policy is a multi-bit explicit congestion avoidance algorithm
based on data-center TCP congestion avoidance (DCTCP) to relay
information about the maximum queue depth that packets experienced to
the receiver. There's also a "nop" policy to disable congestion
avoidance for testing and benchmarking purposes.

The (initial) API for congestion avoidance policies is:

        void *   (* ctx_create)(void);

        void     (* ctx_destroy)(void * ctx);

These calls create / and or destroy a context for congestion control
for a specific flow. Thread-safety of the context is the
responsability of the flow allocator (operations on the ctx should be
performed under a lock).

        ca_wnd_t (* ctx_update_snd)(void * ctx,
                                    size_t len);

This is the sender call to update the context, and should be called
for every packet that is sent on the flow. The len parameter in this
API is the packet length, which allows calculating the bandwidth. It
returns an opaque union type that is used for the call to check/wait
if the congestion window is open or closed (and allowing to release
locks before waiting).

        bool     (* ctx_update_rcv)(void *     ctx,
                                    size_t     len,
                                    uint8_t    ecn,
                                    uint16_t * ece);

This is the call to update the flow congestion context on the receiver
side. It should be called for every received packet.  It gets the ecn
value from the packet and its length, and returns the ECE (explicit
congestion experienced) value to be sent to the sender in case of
congestion. The boolean returned signals whether or not a congestion
update needs to be sent.

        void     (* ctx_update_ece)(void *   ctx,
                                    uint16_t ece);

This is the call for the sending side top update the context when it
receives an ECE update from the receiver.

        void     (* wnd_wait)(ca_wnd_t wnd);

This is a (blocking) call that waits for the congestion window to
clear. It should be stateless (to avoid waiting under locks). This may
change later on if passing the context is needed for different algorithms.

        uint8_t  (* calc_ecn)(int    fd,
                              size_t len);

This is the call that intermediate IPCPs(routers) should use to update
the ECN field on passing packets.

The multi-bit ECN policy bases the value for the ECN field on the
depth of the rbuff queue packets will be sent on. I created another
call to grab the queue depth as fccntl is write-locking the
application. We can further optimize this to avoid most locking on the
rbuff.

Signed-off-by: Dimitri Staessens <[email protected]>
Signed-off-by: Sander Vrijders <[email protected]>

13 files changed, 835 insertions, 72 deletions

diff --git a/src/ipcpd/unicast/CMakeLists.txt b/src/ipcpd/unicast/CMakeLists.txt
index c0c55519..035ee5f4 100644
--- a/src/ipcpd/unicast/CMakeLists.txt
+++ b/src/ipcpd/unicast/CMakeLists.txt
@@ -33,6 +33,7 @@ endif ()
 set(SOURCE_FILES
   # Add source files here
   addr_auth.c
+  ca.c
   connmgr.c
   dht.c
   dir.c
@@ -51,6 +52,8 @@ set(SOURCE_FILES
   pol/simple_pff.c
   pol/alternate_pff.c
   pol/multipath_pff.c
+  pol/ca-mb-ecn.c
+  pol/ca-nop.c
   )
 
 add_executable(ipcpd-unicast ${SOURCE_FILES} ${IPCP_SOURCES}
diff --git a/src/ipcpd/unicast/ca.c b/src/ipcpd/unicast/ca.c
new file mode 100644
index 00000000..f93d0504
--- /dev/null
+++ b/src/ipcpd/unicast/ca.c
@@ -0,0 +1,99 @@
+/*
+ * Ouroboros - Copyright (C) 2016 - 2020
+ *
+ * Congestion Avoidance
+ *
+ *    Dimitri Staessens <[email protected]>
+ *    Sander Vrijders   <[email protected]>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., http://www.fsf.org/about/contact/.
+ */
+
+#define OUROBOROS_PREFIX "ca"
+
+#include <ouroboros/logs.h>
+
+#include "ca.h"
+#include "pol-ca-ops.h"
+#include "pol/ca-mb-ecn.h"
+#include "pol/ca-nop.h"
+
+struct {
+        struct pol_ca_ops * ops;
+} ca;
+
+int ca_init(enum pol_cong_avoid pol)
+{
+        switch(pol) {
+        case CA_NONE:
+                log_dbg("Disabling congestion control.");
+                ca.ops = &nop_ca_ops;
+                break;
+        case CA_MB_ECN:
+                log_dbg("Using multi-bit ECN.");
+                ca.ops = &mb_ecn_ca_ops;
+                break;
+        default:
+                return -1;
+        }
+
+        return 0;
+}
+
+
+void ca_fini(void)
+{
+        ca.ops = NULL;
+}
+
+void * ca_ctx_create(void)
+{
+        return ca.ops->ctx_create();
+}
+
+void ca_ctx_destroy(void * ctx)
+{
+        return ca.ops->ctx_destroy(ctx);
+}
+
+ca_wnd_t ca_ctx_update_snd(void * ctx,
+                           size_t len)
+{
+        return ca.ops->ctx_update_snd(ctx, len);
+}
+
+bool ca_ctx_update_rcv(void *     ctx,
+                       size_t     len,
+                       uint8_t    ecn,
+                       uint16_t * ece)
+{
+        return ca.ops->ctx_update_rcv(ctx, len, ecn, ece);
+}
+
+void ca_ctx_update_ece(void *   ctx,
+                       uint16_t ece)
+{
+        return ca.ops->ctx_update_ece(ctx, ece);
+}
+
+void ca_wnd_wait(ca_wnd_t wnd)
+{
+        return ca.ops->wnd_wait(wnd);
+}
+
+uint8_t ca_calc_ecn(int    fd,
+                    size_t len)
+{
+        return ca.ops->calc_ecn(fd, len);
+}
diff --git a/src/ipcpd/unicast/ca.h b/src/ipcpd/unicast/ca.h
new file mode 100644
index 00000000..5cf6199c
--- /dev/null
+++ b/src/ipcpd/unicast/ca.h
@@ -0,0 +1,61 @@
+/*
+ * Ouroboros - Copyright (C) 2016 - 2020
+ *
+ * Congestion avoidance
+ *
+ *    Dimitri Staessens <[email protected]>
+ *    Sander Vrijders   <[email protected]>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., http://www.fsf.org/about/contact/.
+ */
+
+#ifndef OUROBOROS_IPCPD_UNICAST_CA_H
+#define OUROBOROS_IPCPD_UNICAST_CA_H
+
+#include <ouroboros/ipcp.h>
+
+#include <stdbool.h>
+#include <sys/types.h>
+
+typedef union {
+        time_t wait;
+} ca_wnd_t;
+
+int      ca_init(enum pol_cong_avoid ca);
+
+void     ca_fini(void);
+
+
+/* OPS */
+void *   ca_ctx_create(void);
+
+void     ca_ctx_destroy(void * ctx);
+
+ca_wnd_t ca_ctx_update_snd(void * ctx,
+                           size_t len);
+
+bool     ca_ctx_update_rcv(void *     ctx,
+                           size_t     len,
+                           uint8_t    ecn,
+                           uint16_t * ece);
+
+void     ca_ctx_update_ece(void *   ctx,
+                           uint16_t ece);
+
+void     ca_wnd_wait(ca_wnd_t wnd);
+
+uint8_t  ca_calc_ecn(int    fd,
+                     size_t len);
+
+#endif /* OUROBOROS_IPCPD_UNICAST_CA_H */
diff --git a/src/ipcpd/unicast/dt.c b/src/ipcpd/unicast/dt.c
index 7db766a5..53accba3 100644
--- a/src/ipcpd/unicast/dt.c
+++ b/src/ipcpd/unicast/dt.c
@@ -41,6 +41,7 @@
 #include <ouroboros/fccntl.h>
 #endif
 
+#include "ca.h"
 #include "connmgr.h"
 #include "ipcp.h"
 #include "dt.h"
@@ -115,16 +116,12 @@ static void dt_pci_ser(uint8_t *       head,
 
 }
 
-static void dt_pci_des(struct shm_du_buff * sdb,
-                       struct dt_pci *      dt_pci)
+static void dt_pci_des(uint8_t *       head,
+                       struct dt_pci * dt_pci)
 {
-        uint8_t * head;
-
-        assert(sdb);
+        assert(head);
         assert(dt_pci);
 
-        head = shm_du_buff_head(sdb);
-
         /* Decrease TTL */
         --*(head + dt_pci_info.ttl_o);
 
@@ -226,7 +223,6 @@ static int dt_stat_read(const char * path,
                 "Queued packets (rx):      %20zu\n"
                 "Queued packets (tx):      %20zu\n\n",
                 tmstr, addrstr, rxqlen, txqlen);
-
         for (i = 0; i < QOS_CUBE_MAX; ++i) {
                 sprintf(str,
                         "Qos cube %3d:\n"
@@ -434,13 +430,14 @@ static void packet_handler(int                  fd,
         struct dt_pci dt_pci;
         int           ret;
         int           ofd;
-#ifndef IPCP_FLOW_STATS
-        (void)        fd;
-#else
+        uint8_t *     head;
         size_t        len;
 
         len = shm_du_buff_tail(sdb) - shm_du_buff_head(sdb);
 
+#ifndef IPCP_FLOW_STATS
+        (void)        fd;
+#else
         pthread_mutex_lock(&dt.stat[fd].lock);
 
         ++dt.stat[fd].rcv_pkt[qc];
@@ -449,7 +446,10 @@ static void packet_handler(int                  fd,
         pthread_mutex_unlock(&dt.stat[fd].lock);
 #endif
         memset(&dt_pci, 0, sizeof(dt_pci));
-        dt_pci_des(sdb, &dt_pci);
+
+        head = shm_du_buff_head(sdb);
+
+        dt_pci_des(head, &dt_pci);
         if (dt_pci.dst_addr != ipcpi.dt_addr) {
                 if (dt_pci.ttl == 0) {
                         log_dbg("TTL was zero.");
@@ -481,6 +481,8 @@ static void packet_handler(int                  fd,
                         return;
                 }
 
+                *(head + dt_pci_info.ecn_o) |= ca_calc_ecn(ofd, len);
+
                 ret = ipcp_flow_write(ofd, sdb);
                 if (ret < 0) {
                         log_dbg("Failed to write packet to fd %d.", ofd);
@@ -508,6 +510,9 @@ static void packet_handler(int                  fd,
         } else {
                 dt_pci_shrink(sdb);
                 if (dt_pci.eid >= PROG_RES_FDS) {
+                        uint8_t ecn = *(head + dt_pci_info.ecn_o);
+                        fa_ecn_update(dt_pci.eid, ecn, len);
+
                         if (ipcp_flow_write(dt_pci.eid, sdb)) {
                                 ipcp_sdb_release(sdb);
 #ifdef IPCP_FLOW_STATS
@@ -792,15 +797,15 @@ int dt_write_packet(uint64_t             dst_addr,
         int           fd;
         int           ret;
         uint8_t *     head;
-#ifdef IPCP_FLOW_STATS
         size_t        len;
-#endif
+
         assert(sdb);
         assert(dst_addr != ipcpi.dt_addr);
 
-#ifdef IPCP_FLOW_STATS
         len = shm_du_buff_tail(sdb) - shm_du_buff_head(sdb);
 
+#ifdef IPCP_FLOW_STATS
+
         pthread_mutex_lock(&dt.stat[np1_fd].lock);
 
         ++dt.stat[np1_fd].lcl_r_pkt[qc];
@@ -829,15 +834,15 @@ int dt_write_packet(uint64_t             dst_addr,
                 goto fail_write;
         }
 
+        len = shm_du_buff_tail(sdb) - shm_du_buff_head(sdb);
+
         dt_pci.dst_addr = dst_addr;
         dt_pci.qc       = qc;
         dt_pci.eid      = np1_fd;
-        dt_pci.ecn      = 0;
+        dt_pci.ecn      = ca_calc_ecn(fd, len);
 
         dt_pci_ser(head, &dt_pci);
-#ifdef IPCP_FLOW_STATS
-        len = shm_du_buff_tail(sdb) - shm_du_buff_head(sdb);
-#endif
+
         ret = ipcp_flow_write(fd, sdb);
         if (ret < 0) {
                 log_dbg("Failed to write packet to fd %d.", fd);
diff --git a/src/ipcpd/unicast/fa.c b/src/ipcpd/unicast/fa.c
index e154d785..8f268a9d 100644
--- a/src/ipcpd/unicast/fa.c
+++ b/src/ipcpd/unicast/fa.c
@@ -42,6 +42,7 @@
 #include "psched.h"
 #include "ipcp.h"
 #include "dt.h"
+#include "ca.h"
 
 #include <pthread.h>
 #include <stdlib.h>
@@ -49,9 +50,10 @@
 
 #define TIMEOUT 10000 /* nanoseconds */
 
-#define FLOW_REQ   0
-#define FLOW_REPLY 1
-#define MSGBUFSZ   2048
+#define FLOW_REQ    0
+#define FLOW_REPLY  1
+#define FLOW_UPDATE 2
+#define MSGBUFSZ    2048
 
 struct fa_msg {
         uint64_t s_addr;
@@ -59,6 +61,7 @@ struct fa_msg {
         uint32_t s_eid;
         uint8_t  code;
         int8_t   response;
+        uint16_t ece;
         /* QoS parameters from spec, aligned */
         uint8_t  availability;
         uint8_t  in_order;
@@ -75,10 +78,16 @@ struct cmd {
         struct shm_du_buff * sdb;
 };
 
+struct fa_flow {
+        int      r_eid;  /* remote endpoint id               */
+        uint64_t r_addr; /* remote address                   */
+        void *   ctx;    /* congestion avoidance context     */
+};
+
 struct {
         pthread_rwlock_t flows_lock;
-        int              r_eid[PROG_MAX_FLOWS];
-        uint64_t         r_addr[PROG_MAX_FLOWS];
+        struct fa_flow   flows[PROG_MAX_FLOWS];
+
         int              fd;
 
         struct list_head cmds;
@@ -93,22 +102,56 @@ static void packet_handler(int                  fd,
                            qoscube_t            qc,
                            struct shm_du_buff * sdb)
 {
-        pthread_rwlock_rdlock(&fa.flows_lock);
+        struct fa_flow * flow;
+        uint64_t         r_addr;
+        uint32_t         r_eid;
+        ca_wnd_t         wnd;
+        size_t           len;
 
-        if (dt_write_packet(fa.r_addr[fd], qc, fa.r_eid[fd], sdb)) {
-                pthread_rwlock_unlock(&fa.flows_lock);
+        flow = &fa.flows[fd];
+
+        pthread_rwlock_wrlock(&fa.flows_lock);
+
+        len = shm_du_buff_tail(sdb) - shm_du_buff_head(sdb);
+
+        wnd = ca_ctx_update_snd(flow->ctx, len);
+
+        r_addr = flow->r_addr;
+        r_eid  = flow->r_eid;
+
+        pthread_rwlock_unlock(&fa.flows_lock);
+
+        ca_wnd_wait(wnd);
+
+        if (dt_write_packet(r_addr, qc, r_eid, sdb)) {
                 ipcp_sdb_release(sdb);
                 log_warn("Failed to forward packet.");
                 return;
         }
+}
 
-        pthread_rwlock_unlock(&fa.flows_lock);
+static int fa_flow_init(struct fa_flow * flow)
+{
+        memset(flow, 0, sizeof(*flow));
+
+        flow->r_eid  = -1;
+        flow->r_addr = INVALID_ADDR;
+
+        flow->ctx = ca_ctx_create();
+        if (flow->ctx == NULL)
+                return -1;
+
+        return 0;
 }
 
-static void destroy_conn(int fd)
+static void fa_flow_fini(struct fa_flow * flow)
 {
-        fa.r_eid[fd]  = -1;
-        fa.r_addr[fd] = INVALID_ADDR;
+        ca_ctx_destroy(flow->ctx);
+
+        memset(flow, 0, sizeof(*flow));
+
+        flow->r_eid  = -1;
+        flow->r_addr = INVALID_ADDR;
 }
 
 static void fa_post_packet(void *               comp,
@@ -145,14 +188,15 @@ static void * fa_handle_packet(void * o)
         (void) o;
 
         while (true) {
-                struct timespec abstime;
-                int             fd;
-                uint8_t         buf[MSGBUFSZ];
-                struct fa_msg * msg;
-                qosspec_t       qs;
-                struct cmd *    cmd;
-                size_t          len;
-                size_t          msg_len;
+                struct timespec  abstime;
+                int              fd;
+                uint8_t          buf[MSGBUFSZ];
+                struct fa_msg *  msg;
+                qosspec_t        qs;
+                struct cmd *     cmd;
+                size_t           len;
+                size_t           msg_len;
+                struct fa_flow * flow;
 
                 pthread_mutex_lock(&fa.mtx);
 
@@ -232,10 +276,14 @@ static void * fa_handle_packet(void * o)
                                 continue;
                         }
 
+                        flow = &fa.flows[fd];
+
                         pthread_rwlock_wrlock(&fa.flows_lock);
 
-                        fa.r_eid[fd]  = ntoh32(msg->s_eid);
-                        fa.r_addr[fd] = ntoh64(msg->s_addr);
+                        fa_flow_init(flow);
+
+                        flow->r_eid  = ntoh32(msg->s_eid);
+                        flow->r_addr = ntoh64(msg->s_addr);
 
                         pthread_rwlock_unlock(&fa.flows_lock);
 
@@ -248,19 +296,32 @@ static void * fa_handle_packet(void * o)
                 case FLOW_REPLY:
                         assert(len >= sizeof(*msg));
 
+                        flow = &fa.flows[ntoh32(msg->r_eid)];
+
                         pthread_rwlock_wrlock(&fa.flows_lock);
 
-                        fa.r_eid[ntoh32(msg->r_eid)] = ntoh32(msg->s_eid);
+                        flow->r_eid = ntoh32(msg->s_eid);
+
+                        if (msg->response < 0)
+                                fa_flow_fini(flow);
+                        else
+                                psched_add(fa.psched, ntoh32(msg->r_eid));
+
+                        pthread_rwlock_unlock(&fa.flows_lock);
 
                         ipcp_flow_alloc_reply(ntoh32(msg->r_eid),
                                               msg->response,
                                               buf + sizeof(*msg),
                                               len - sizeof(*msg));
+                        break;
+                case FLOW_UPDATE:
+                        assert(len >= sizeof(*msg));
 
-                        if (msg->response < 0)
-                                destroy_conn(ntoh32(msg->r_eid));
-                        else
-                                psched_add(fa.psched, ntoh32(msg->r_eid));
+                        flow = &fa.flows[ntoh32(msg->r_eid)];
+
+                        pthread_rwlock_wrlock(&fa.flows_lock);
+
+                        ca_ctx_update_ece(flow->ctx, ntoh16(msg->ece));
 
                         pthread_rwlock_unlock(&fa.flows_lock);
 
@@ -275,10 +336,6 @@ static void * fa_handle_packet(void * o)
 int fa_init(void)
 {
         pthread_condattr_t cattr;
-        int                i;
-
-        for (i = 0; i < PROG_MAX_FLOWS; ++i)
-                destroy_conn(i);
 
         if (pthread_rwlock_init(&fa.flows_lock, NULL))
                 goto fail_rwlock;
@@ -383,9 +440,10 @@ int fa_alloc(int             fd,
              size_t          dlen)
 {
         struct fa_msg *      msg;
-        uint64_t             addr;
         struct shm_du_buff * sdb;
-        qoscube_t            qc;
+        struct fa_flow *     flow;
+        uint64_t             addr;
+        qoscube_t            qc = QOS_CUBE_BE;
         size_t               len;
 
         addr = dir_query(dst);
@@ -397,7 +455,9 @@ int fa_alloc(int             fd,
         if (ipcp_sdb_reserve(&sdb, len + dlen))
                 return -1;
 
-        msg               = (struct fa_msg *) shm_du_buff_head(sdb);
+        msg = (struct fa_msg *) shm_du_buff_head(sdb);
+        memset(msg, 0, sizeof(*msg));
+
         msg->code         = FLOW_REQ;
         msg->s_eid        = hton32(fd);
         msg->s_addr       = hton64(ipcpi.dt_addr);
@@ -413,17 +473,17 @@ int fa_alloc(int             fd,
         memcpy(msg + 1, dst, ipcp_dir_hash_len());
         memcpy(shm_du_buff_head(sdb) + len, data, dlen);
 
-        qc = qos_spec_to_cube(qs);
-
         if (dt_write_packet(addr, qc, fa.fd, sdb)) {
                 ipcp_sdb_release(sdb);
                 return -1;
         }
 
+        flow = &fa.flows[fd];
+
         pthread_rwlock_wrlock(&fa.flows_lock);
 
-        assert(fa.r_eid[fd] == -1);
-        fa.r_addr[fd] = addr;
+        fa_flow_init(flow);
+        flow->r_addr = addr;
 
         pthread_rwlock_unlock(&fa.flows_lock);
 
@@ -439,10 +499,13 @@ int fa_alloc_resp(int          fd,
         struct timespec      abstime;
         struct fa_msg *      msg;
         struct shm_du_buff * sdb;
-        qoscube_t            qc;
+        struct fa_flow *     flow;
+        qoscube_t            qc = QOS_CUBE_BE;
 
         clock_gettime(PTHREAD_COND_CLOCK, &abstime);
 
+        flow = &fa.flows[fd];
+
         pthread_mutex_lock(&ipcpi.alloc_lock);
 
         while (ipcpi.alloc_id != fd && ipcp_get_state() == IPCP_OPERATIONAL) {
@@ -463,33 +526,31 @@ int fa_alloc_resp(int          fd,
         pthread_mutex_unlock(&ipcpi.alloc_lock);
 
         if (ipcp_sdb_reserve(&sdb, sizeof(*msg) + len)) {
-                destroy_conn(fd);
+                fa_flow_fini(flow);
                 return -1;
         }
 
+        msg = (struct fa_msg *) shm_du_buff_head(sdb);
+        memset(msg, 0, sizeof(*msg));
+
         pthread_rwlock_wrlock(&fa.flows_lock);
 
-        msg           = (struct fa_msg *) shm_du_buff_head(sdb);
         msg->code     = FLOW_REPLY;
-        msg->r_eid    = hton32(fa.r_eid[fd]);
+        msg->r_eid    = hton32(flow->r_eid);
         msg->s_eid    = hton32(fd);
         msg->response = response;
 
         memcpy(msg + 1, data, len);
 
         if (response < 0) {
-                destroy_conn(fd);
+                fa_flow_fini(flow);
                 ipcp_sdb_release(sdb);
         } else {
                 psched_add(fa.psched, fd);
         }
 
-        ipcp_flow_get_qoscube(fd, &qc);
-
-        assert(qc >= 0 && qc < QOS_CUBE_MAX);
-
-        if (dt_write_packet(fa.r_addr[fd], qc, fa.fd, sdb)) {
-                destroy_conn(fd);
+        if (dt_write_packet(flow->r_addr, qc, fa.fd, sdb)) {
+                fa_flow_fini(flow);
                 pthread_rwlock_unlock(&fa.flows_lock);
                 ipcp_sdb_release(sdb);
                 return -1;
@@ -505,11 +566,11 @@ int fa_dealloc(int fd)
         if (ipcp_flow_fini(fd) < 0)
                 return 0;
 
-        pthread_rwlock_wrlock(&fa.flows_lock);
-
         psched_del(fa.psched, fd);
 
-        destroy_conn(fd);
+        pthread_rwlock_wrlock(&fa.flows_lock);
+
+        fa_flow_fini(&fa.flows[fd]);
 
         pthread_rwlock_unlock(&fa.flows_lock);
 
@@ -517,3 +578,60 @@ int fa_dealloc(int fd)
 
         return 0;
 }
+
+static int fa_update_remote(int      fd,
+                            uint16_t ece)
+{
+        struct fa_msg *      msg;
+        struct shm_du_buff * sdb;
+        qoscube_t            qc = QOS_CUBE_BE;
+        struct fa_flow *     flow;
+
+        if (ipcp_sdb_reserve(&sdb, sizeof(*msg))) {
+                return -1;
+        }
+
+        msg = (struct fa_msg *) shm_du_buff_head(sdb);
+
+        memset(msg, 0, sizeof(*msg));
+
+        flow = &fa.flows[fd];
+
+        pthread_rwlock_rdlock(&fa.flows_lock);
+
+        msg->code  = FLOW_UPDATE;
+        msg->r_eid = hton32(flow->r_eid);
+        msg->ece   = hton16(ece);
+
+        if (dt_write_packet(flow->r_addr, qc, fa.fd, sdb)) {
+                pthread_rwlock_unlock(&fa.flows_lock);
+                ipcp_sdb_release(sdb);
+                return -1;
+        }
+
+        pthread_rwlock_unlock(&fa.flows_lock);
+
+
+        return 0;
+}
+
+void  fa_ecn_update(int     eid,
+                    uint8_t ecn,
+                    size_t  len)
+{
+        struct fa_flow * flow;
+        bool             update;
+        uint16_t         ece;
+
+        flow = &fa.flows[eid];
+
+        pthread_rwlock_wrlock(&fa.flows_lock);
+
+        update = ca_ctx_update_rcv(flow->ctx, len, ecn, &ece);
+
+        pthread_rwlock_unlock(&fa.flows_lock);
+
+        if (update)
+                fa_update_remote(eid, ece);
+
+}
diff --git a/src/ipcpd/unicast/fa.h b/src/ipcpd/unicast/fa.h
index 12a10a0c..daba2a51 100644
--- a/src/ipcpd/unicast/fa.h
+++ b/src/ipcpd/unicast/fa.h
@@ -47,4 +47,8 @@ int  fa_alloc_resp(int          fd,
 
 int  fa_dealloc(int fd);
 
+void fa_ecn_update(int     eid,
+                   uint8_t ecn,
+                   size_t  len);
+
 #endif /* OUROBOROS_IPCPD_UNICAST_FA_H */
diff --git a/src/ipcpd/unicast/main.c b/src/ipcpd/unicast/main.c
index 0ab37d25..1b2cc14e 100644
--- a/src/ipcpd/unicast/main.c
+++ b/src/ipcpd/unicast/main.c
@@ -39,6 +39,7 @@
 #include <ouroboros/time_utils.h>
 
 #include "addr_auth.h"
+#include "ca.h"
 #include "connmgr.h"
 #include "dir.h"
 #include "dt.h"
@@ -83,6 +84,11 @@ static int initialize_components(const struct ipcp_config * conf)
 
         log_dbg("IPCP got address %" PRIu64 ".", ipcpi.dt_addr);
 
+        if (ca_init(conf->cong_avoid)) {
+                log_err("Failed to initialize congestion avoidance.");
+                goto fail_ca;
+        }
+
         if (dt_init(conf->routing_type,
                     conf->addr_size,
                     conf->eid_size,
@@ -110,6 +116,8 @@ static int initialize_components(const struct ipcp_config * conf)
  fail_fa:
         dt_fini();
  fail_dt:
+        ca_fini();
+ fail_ca:
         addr_auth_fini();
  fail_addr_auth:
         free(ipcpi.layer_name);
@@ -125,6 +133,8 @@ static void finalize_components(void)
 
         dt_fini();
 
+        ca_fini();
+
         addr_auth_fini();
 
         free(ipcpi.layer_name);
diff --git a/src/ipcpd/unicast/pol-ca-ops.h b/src/ipcpd/unicast/pol-ca-ops.h
new file mode 100644
index 00000000..3cb8a9d2
--- /dev/null
+++ b/src/ipcpd/unicast/pol-ca-ops.h
@@ -0,0 +1,50 @@
+/*
+ * Ouroboros - Copyright (C) 2016 - 2020
+ *
+ * Congestion avoidance policy ops
+ *
+ *    Dimitri Staessens <[email protected]>
+ *    Sander Vrijders   <[email protected]>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., http://www.fsf.org/about/contact/.
+ */
+
+#ifndef OUROBOROS_IPCPD_UNICAST_POL_CA_OPS_H
+#define OUROBOROS_IPCPD_UNICAST_POL_CA_OPS_H
+
+#include "ca.h"
+
+struct pol_ca_ops {
+        void *   (* ctx_create)(void);
+
+        void     (* ctx_destroy)(void * ctx);
+
+        ca_wnd_t (* ctx_update_snd)(void * ctx,
+                                    size_t len);
+
+        bool     (* ctx_update_rcv)(void *     ctx,
+                                    size_t     len,
+                                    uint8_t    ecn,
+                                    uint16_t * ece);
+
+        void     (* ctx_update_ece)(void *   ctx,
+                                    uint16_t ece);
+
+        void     (* wnd_wait)(ca_wnd_t wnd);
+
+        uint8_t  (* calc_ecn)(int    fd,
+                              size_t len);
+};
+
+#endif /* OUROBOROS_IPCPD_UNICAST_POL_CA_OPS_H */
diff --git a/src/ipcpd/unicast/pol/ca-mb-ecn.c b/src/ipcpd/unicast/pol/ca-mb-ecn.c
new file mode 100644
index 00000000..2de8f8e7
--- /dev/null
+++ b/src/ipcpd/unicast/pol/ca-mb-ecn.c
@@ -0,0 +1,216 @@
+/*
+ * Ouroboros - Copyright (C) 2016 - 2020
+ *
+ * Multi-bit ECN Congestion Avoidance
+ *
+ *    Dimitri Staessens <[email protected]>
+ *    Sander Vrijders   <[email protected]>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., http://www.fsf.org/about/contact/.
+ */
+
+#if defined(__linux__) || defined(__CYGWIN__)
+#define _DEFAULT_SOURCE
+#else
+#define _POSIX_C_SOURCE 200809L
+#endif
+
+#include "config.h"
+
+#include <ouroboros/ipcp-dev.h>
+#include <ouroboros/time_utils.h>
+
+#include "ca-mb-ecn.h"
+
+#include <stdlib.h>
+#include <string.h>
+
+/* congestion avoidance constants */
+#define CA_SHFT      5
+#define CA_WND       (1 << CA_SHFT)
+#define CA_UPD       (1 << (CA_SHFT - 3))
+#define CA_SLOT      18
+#define CA_AI        20000
+#define ECN_Q_SHFT   5
+#define ts_to_ns(ts) (ts.tv_sec * BILLION + ts.tv_nsec)
+
+struct mb_ecn_ctx {
+        uint16_t        rx_ece; /* level of congestion (upstream)   */
+        size_t          rx_ctr; /* receiver side packet counter     */
+
+        uint16_t        tx_ece; /* level of congestion (downstream) */
+        size_t          tx_ctr; /* sender side packet counter       */
+        size_t          tx_aps; /* average packet size              */
+        time_t          tx_wnd; /* tgt time to send packets (ns)    */
+        bool            tx_cav; /* Congestion avoidance             */
+        size_t          tx_slot;
+
+        struct timespec t_sent; /* last sent packet                 */
+};
+
+struct pol_ca_ops mb_ecn_ca_ops = {
+        .ctx_create     = mb_ecn_ctx_create,
+        .ctx_destroy    = mb_ecn_ctx_destroy,
+        .ctx_update_snd = mb_ecn_ctx_update_snd,
+        .ctx_update_rcv = mb_ecn_ctx_update_rcv,
+        .ctx_update_ece = mb_ecn_ctx_update_ece,
+        .wnd_wait       = mb_ecn_wnd_wait,
+        .calc_ecn       = mb_ecn_calc_ecn
+};
+
+void * mb_ecn_ctx_create(void)
+{
+
+        struct mb_ecn_ctx * ctx;
+
+        ctx = malloc(sizeof(*ctx));
+        if (ctx == NULL)
+                return NULL;
+
+        memset(ctx, 0, sizeof(*ctx));
+
+        return (void *) ctx;
+}
+
+void mb_ecn_ctx_destroy(void * ctx)
+{
+        free(ctx);
+}
+
+ca_wnd_t mb_ecn_ctx_update_snd(void * _ctx,
+                               size_t len)
+{
+        struct timespec  now;
+        size_t           slot;
+        time_t           gap;
+        ca_wnd_t         wnd;
+
+        struct mb_ecn_ctx * ctx = _ctx;
+
+        clock_gettime(PTHREAD_COND_CLOCK, &now);
+
+        if (ctx->tx_wnd == 0) { /* 10 ms initial window estimate */
+                ctx->tx_wnd = 10 * MILLION;
+                gap          = ctx->tx_wnd >> CA_SHFT;
+                ctx->tx_aps = len >> CA_SHFT;
+                ctx->tx_slot = ts_to_ns(now) >> CA_SLOT;
+        } else {
+                gap           = ts_diff_ns(&ctx->t_sent, &now);
+                ctx->tx_aps -= ctx->tx_aps >> CA_SHFT;
+                ctx->tx_aps += len;
+        }
+
+        ctx->t_sent = now;
+
+        slot = ts_to_ns(now) >> CA_SLOT;
+
+        ctx->tx_ctr++;
+
+        if (slot - ctx->tx_slot > 0) {
+                ctx->tx_slot = slot;
+
+                if (ctx->tx_ctr > CA_WND)
+                        ctx->tx_ece = 0;
+
+                /* Slow start */
+                if (!ctx->tx_cav) {
+                        ctx->tx_wnd >>= 1;
+                /* Multiplicative Decrease */
+                } else if (ctx->tx_ece) { /* MD */
+                        ctx->tx_wnd += (ctx->tx_wnd * ctx->tx_ece)
+                                >> (CA_SHFT + 8);
+                /* Additive Increase */
+                } else {
+                        size_t bw = ctx->tx_aps * BILLION / ctx->tx_wnd;
+                        bw += CA_AI;
+                        ctx->tx_wnd = ctx->tx_aps * BILLION / bw;
+                }
+        }
+
+        wnd.wait = (ctx->tx_wnd >> CA_SHFT) - gap;
+
+        return wnd;
+}
+
+void mb_ecn_wnd_wait(ca_wnd_t wnd)
+{
+        if (wnd.wait > 0) {
+                struct timespec s = {0, 0};
+                if (wnd.wait > BILLION) /* Don't care throttling < 1pps */
+                        s.tv_sec = 1;
+                else
+                        s.tv_nsec = wnd.wait;
+
+                nanosleep(&s, NULL);
+        }
+}
+
+bool mb_ecn_ctx_update_rcv(void *     _ctx,
+                           size_t     len,
+                           uint8_t    ecn,
+                           uint16_t * ece)
+{
+        struct mb_ecn_ctx* ctx = _ctx;
+        bool               update;
+
+        (void) len;
+
+        if ((ctx->rx_ece | ecn) == 0)
+                return false;
+
+        if (ecn == 0) {
+                /* end of congestion */
+                ctx->rx_ece >>= 2;
+                update = ctx->rx_ece == 0;
+        } else {
+                if (ctx->rx_ece == 0) {
+                        /* start of congestion */
+                        ctx->rx_ece = ecn;
+                        ctx->rx_ctr = 0;
+                        update = true;
+                } else {
+                        /* congestion update */
+                        ctx->rx_ece -= ctx->rx_ece >> CA_SHFT;
+                        ctx->rx_ece += ecn;
+                        update = (ctx->rx_ctr++ & (CA_UPD - 1)) == true;
+                }
+        }
+
+        *ece = ctx->rx_ece;
+
+        return update;
+}
+
+
+void mb_ecn_ctx_update_ece(void *   _ctx,
+                           uint16_t ece)
+{
+        struct mb_ecn_ctx* ctx = _ctx;
+
+        ctx->tx_ece = ece;
+        ctx->tx_ctr = 0;
+        ctx->tx_cav = true;
+}
+
+uint8_t mb_ecn_calc_ecn(int    fd,
+                        size_t len)
+{
+        size_t q;
+
+        (void) len;
+
+        q = ipcp_flow_queued(fd);
+
+        return (uint8_t) (q >> ECN_Q_SHFT);
+}
diff --git a/src/ipcpd/unicast/pol/ca-mb-ecn.h b/src/ipcpd/unicast/pol/ca-mb-ecn.h
new file mode 100644
index 00000000..456b9b13
--- /dev/null
+++ b/src/ipcpd/unicast/pol/ca-mb-ecn.h
@@ -0,0 +1,50 @@
+/*
+ * Ouroboros - Copyright (C) 2016 - 2020
+ *
+ * Multi-bit ECN Congestion Avoidance
+ *
+ *    Dimitri Staessens <[email protected]>
+ *    Sander Vrijders   <[email protected]>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., http://www.fsf.org/about/contact/.
+ */
+
+#ifndef OUROBOROS_IPCPD_UNICAST_CA_MB_ECN_H
+#define OUROBOROS_IPCPD_UNICAST_CA_MB_ECN_H
+
+#include "pol-ca-ops.h"
+
+void *   mb_ecn_ctx_create(void);
+
+void     mb_ecn_ctx_destroy(void * ctx);
+
+ca_wnd_t mb_ecn_ctx_update_snd(void * ctx,
+                               size_t len);
+
+bool     mb_ecn_ctx_update_rcv(void *     ctx,
+                               size_t     len,
+                               uint8_t    ecn,
+                               uint16_t * ece);
+
+void     mb_ecn_ctx_update_ece(void *   ctx,
+                               uint16_t ece);
+
+void     mb_ecn_wnd_wait(ca_wnd_t wnd);
+
+uint8_t  mb_ecn_calc_ecn(int    fd,
+                         size_t len);
+
+extern struct pol_ca_ops mb_ecn_ca_ops;
+
+#endif /* OUROBOROS_IPCPD_UNICAST_CA_MB_ECN_H */
diff --git a/src/ipcpd/unicast/pol/ca-nop.c b/src/ipcpd/unicast/pol/ca-nop.c
new file mode 100644
index 00000000..d0d89a2e
--- /dev/null
+++ b/src/ipcpd/unicast/pol/ca-nop.c
@@ -0,0 +1,93 @@
+/*
+ * Ouroboros - Copyright (C) 2016 - 2020
+ *
+ * Dummy Congestion Avoidance
+ *
+ *    Dimitri Staessens <[email protected]>
+ *    Sander Vrijders   <[email protected]>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., http://www.fsf.org/about/contact/.
+ */
+
+#include "ca-nop.h"
+
+#include <string.h>
+
+struct pol_ca_ops nop_ca_ops = {
+        .ctx_create     = nop_ctx_create,
+        .ctx_destroy    = nop_ctx_destroy,
+        .ctx_update_snd = nop_ctx_update_snd,
+        .ctx_update_rcv = nop_ctx_update_rcv,
+        .ctx_update_ece = nop_ctx_update_ece,
+        .wnd_wait       = nop_wnd_wait,
+        .calc_ecn       = nop_calc_ecn
+};
+
+void * nop_ctx_create(void)
+{
+        return (void *) 1;
+}
+
+void nop_ctx_destroy(void * ctx)
+{
+        (void) ctx;
+}
+
+ca_wnd_t nop_ctx_update_snd(void * ctx,
+                            size_t len)
+{
+        ca_wnd_t wnd;
+
+        (void) ctx;
+        (void) len;
+
+        memset(&wnd, 0, sizeof(wnd));
+
+        return wnd;
+}
+
+void nop_wnd_wait(ca_wnd_t wnd)
+{
+        (void) wnd;
+}
+
+bool nop_ctx_update_rcv(void *     ctx,
+                        size_t     len,
+                        uint8_t    ecn,
+                        uint16_t * ece)
+{
+        (void) ctx;
+        (void) len;
+        (void) ecn;
+        (void) ece;
+
+        return false;
+}
+
+void nop_ctx_update_ece(void *   ctx,
+                        uint16_t ece)
+{
+        (void) ctx;
+        (void) ece;
+}
+
+
+uint8_t nop_calc_ecn(int    fd,
+                     size_t len)
+{
+        (void) fd;
+        (void) len;
+
+        return 0;
+}
diff --git a/src/ipcpd/unicast/pol/ca-nop.h b/src/ipcpd/unicast/pol/ca-nop.h
new file mode 100644
index 00000000..baf649d8
--- /dev/null
+++ b/src/ipcpd/unicast/pol/ca-nop.h
@@ -0,0 +1,50 @@
+/*
+ * Ouroboros - Copyright (C) 2016 - 2020
+ *
+ * Dummy Congestion Avoidance
+ *
+ *    Dimitri Staessens <[email protected]>
+ *    Sander Vrijders   <[email protected]>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., http://www.fsf.org/about/contact/.
+ */
+
+#ifndef OUROBOROS_IPCPD_UNICAST_CA_NOP_H
+#define OUROBOROS_IPCPD_UNICAST_CA_NOP_H
+
+#include "pol-ca-ops.h"
+
+void *   nop_ctx_create(void);
+
+void     nop_ctx_destroy(void * ctx);
+
+ca_wnd_t nop_ctx_update_snd(void * ctx,
+                            size_t len);
+
+bool     nop_ctx_update_rcv(void *     ctx,
+                            size_t     len,
+                            uint8_t    ecn,
+                            uint16_t * ece);
+
+void     nop_ctx_update_ece(void *   ctx,
+                            uint16_t ece);
+
+void     nop_wnd_wait(ca_wnd_t wnd);
+
+uint8_t  nop_calc_ecn(int    fd,
+                      size_t len);
+
+extern struct pol_ca_ops nop_ca_ops;
+
+#endif /* OUROBOROS_IPCPD_UNICAST_CA_NOP_H */
diff --git a/src/ipcpd/unicast/pol/link_state.c b/src/ipcpd/unicast/pol/link_state.c
index d9482876..ca8a7c50 100644
--- a/src/ipcpd/unicast/pol/link_state.c
+++ b/src/ipcpd/unicast/pol/link_state.c
@@ -812,8 +812,12 @@ static void handle_event(void *       self,
         switch (event) {
         case NOTIFY_DT_CONN_ADD:
                 pthread_rwlock_rdlock(&ls.db_lock);
+
+                pthread_cleanup_push((void (*) (void *)) pthread_rwlock_unlock,
+                                     (void *) &ls.db_lock);
+
                 send_lsm(ipcpi.dt_addr, c->conn_info.addr, 0);
-                pthread_rwlock_unlock(&ls.db_lock);
+                pthread_cleanup_pop(true);
 
                 if (lsdb_add_nb(c->conn_info.addr, c->flow_info.fd, NB_DT))
                         log_dbg("Failed to add neighbor to LSDB.");