Subject: [PATCH RFC v2 net-next 2/2] tcp: Add Redundant Data Bundling (RDB)

From: "Bendik Rønning Opstad" <[email-masked]>
Date: Mon, 23 Nov 2015 17:26:26 +0100

RDB is a mechanism that enables a TCP sender to bundle redundant
(already sent) data with TCP packets containing new data. By bundling
(retransmitting) already sent data with each TCP packet containing new
data, the connection will be more resistant to sporadic packet loss
which reduces the application layer latency significantly in congested
scenarios.

The main functionality added:

  o Loss detection of hidden loss events: When bundling redundant data
    with each packet, packet loss can be hidden from the TCP engine due
    to lack of dupACKs. This is because the loss is "repaired" by the
    redundant data in the packet coming after the lost packet. Based on
    incoming ACKs, such hidden loss events are detected, and CWR state
    is entered.

  o When packets are scheduled for transmission, RDB replaces the SKB to
    be sent with a modified SKB containing the redundant data of
    previously sent data segments from the TCP output queue.

  o RDB will only be used for streams classified as thin by the function
    tcp_stream_is_thin_dpifl(). This enforces a lower bound on the ITT
    for streams that may benefit from RDB, controlled by the sysctl
    variable tcp_thin_dpifl_itt_lower_bound.

RDB is enabled on a connection with the socket option TCP_RDB, or on all
new connections by setting the sysctl variable tcp_rdb=1.

Cc: Andreas Petlund <[email-masked]>
Cc: Carsten Griwodz <[email-masked]>
Cc: Pål Halvorsen <[email-masked]>
Cc: Jonas Markussen <[email-masked]>
Cc: Kristian Evensen <[email-masked]>
Cc: Kenneth Klette Jonassen <[email-masked]>
Signed-off-by: Bendik Rønning Opstad <[email-masked]>
---
 Documentation/networking/ip-sysctl.txt |  15 ++
 include/linux/skbuff.h                 |   1 +
 include/linux/tcp.h                    |   3 +-
 include/net/tcp.h                      |  14 ++
 include/uapi/linux/tcp.h               |   1 +
 net/core/skbuff.c                      |   3 +-
 net/ipv4/Makefile                      |   3 +-
 net/ipv4/sysctl_net_ipv4.c             |  26 ++++
 net/ipv4/tcp.c                         |  14 +-
 net/ipv4/tcp_input.c                   |   3 +
 net/ipv4/tcp_output.c                  |  11 +-
 net/ipv4/tcp_rdb.c                     | 271 +++++++++++++++++++++++++++++++++
 12 files changed, 357 insertions(+), 8 deletions(-)
 create mode 100644 net/ipv4/tcp_rdb.c

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 938ae73..1077de1 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -708,6 +708,21 @@ tcp_thin_dpifl_itt_lower_bound - INTEGER
 	calculated, which is used to classify whether a stream is thin.
 	Default: 10000
 
+tcp_rdb - BOOLEAN
+	Enable RDB for all new TCP connections.
+	Default: 0
+
+tcp_rdb_max_bytes - INTEGER
+	Enable restriction on how many bytes an RDB packet can contain.
+	This is the total amount of payload including the new unsent data.
+	Default: 0
+
+tcp_rdb_max_skbs - INTEGER
+	Enable restriction on how many previous SKBs in the output queue
+	RDB may include data from. A value of 1 will restrict bundling to
+	only the data from the last packet that was sent.
+	Default: 1
+
 tcp_limit_output_bytes - INTEGER
 	Controls TCP Small Queue limit per tcp socket.
 	TCP bulk sender tends to increase packets in flight until it
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index c9c394b..1639cc0 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2806,6 +2806,7 @@ int zerocopy_sg_from_iter(struct sk_buff *skb, struct iov_iter *frm);
 void skb_free_datagram(struct sock *sk, struct sk_buff *skb);
 void skb_free_datagram_locked(struct sock *sk, struct sk_buff *skb);
 int skb_kill_datagram(struct sock *sk, struct sk_buff *skb, unsigned int flags);
+void copy_skb_header(struct sk_buff *new, const struct sk_buff *old);
 int skb_copy_bits(const struct sk_buff *skb, int offset, void *to, int len);
 int skb_store_bits(struct sk_buff *skb, int offset, const void *from, int len);
 __wsum skb_copy_and_csum_bits(const struct sk_buff *skb, int offset, u8 *to,
diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index b386361..da6dae8 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -202,9 +202,10 @@ struct tcp_sock {
 	} rack;
 	u16	advmss;		/* Advertised MSS			*/
 	u8	unused;
-	u8	nonagle     : 4,/* Disable Nagle algorithm?             */
+	u8	nonagle     : 3,/* Disable Nagle algorithm?             */
 		thin_lto    : 1,/* Use linear timeouts for thin streams */
 		thin_dupack : 1,/* Fast retransmit on first dupack      */
+		rdb         : 1,/* Redundant Data Bundling enabled      */
 		repair      : 1,
 		frto        : 1;/* F-RTO (RFC5682) activated in CA_Loss */
 	u8	repair_queue;
diff --git a/include/net/tcp.h b/include/net/tcp.h
index deac96f..d636026 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -277,6 +277,9 @@ extern int sysctl_tcp_slow_start_after_idle;
 extern int sysctl_tcp_thin_linear_timeouts;
 extern int sysctl_tcp_thin_dupack;
 extern int sysctl_tcp_thin_dpifl_itt_lower_bound;
+extern int sysctl_tcp_rdb;
+extern int sysctl_tcp_rdb_max_bytes;
+extern int sysctl_tcp_rdb_max_skbs;
 extern int sysctl_tcp_early_retrans;
 extern int sysctl_tcp_limit_output_bytes;
 extern int sysctl_tcp_challenge_ack_limit;
@@ -549,6 +552,8 @@ void __tcp_push_pending_frames(struct sock *sk, unsigned int cur_mss,
 bool tcp_may_send_now(struct sock *sk);
 int __tcp_retransmit_skb(struct sock *, struct sk_buff *);
 int tcp_retransmit_skb(struct sock *, struct sk_buff *);
+int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
+		     gfp_t gfp_mask);
 void tcp_retransmit_timer(struct sock *sk);
 void tcp_xmit_retransmit_queue(struct sock *);
 void tcp_simple_retransmit(struct sock *);
@@ -574,6 +579,11 @@ void tcp_synack_rtt_meas(struct sock *sk, struct request_sock *req);
 void tcp_reset(struct sock *sk);
 void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp, struct sk_buff *skb);
 
+/* tcp_rdb.c */
+void rdb_ack_event(struct sock *sk, u32 flags);
+int tcp_transmit_rdb_skb(struct sock *sk, struct sk_buff *xmit_skb,
+			 unsigned int mss_now, gfp_t gfp_mask);
+
 /* tcp_timer.c */
 void tcp_init_xmit_timers(struct sock *);
 static inline void tcp_clear_xmit_timers(struct sock *sk)
@@ -772,6 +782,7 @@ struct tcp_skb_cb {
 	union {
 		struct {
 			/* There is space for up to 20 bytes */
+			__u32 rdb_start_seq; /* Start seq of rdb data bundled */
 		} tx;   /* only used for outgoing skbs */
 		union {
 			struct inet_skb_parm	h4;
@@ -1495,6 +1506,9 @@ static inline struct sk_buff *tcp_write_queue_prev(const struct sock *sk,
 #define tcp_for_write_queue_from_safe(skb, tmp, sk)			\
 	skb_queue_walk_from_safe(&(sk)->sk_write_queue, skb, tmp)
 
+#define tcp_for_write_queue_reverse_from_safe(skb, tmp, sk)		\
+	skb_queue_reverse_walk_from_safe(&(sk)->sk_write_queue, skb, tmp)
+
 static inline struct sk_buff *tcp_send_head(const struct sock *sk)
 {
 	return sk->sk_send_head;
diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
index 65a77b0..ae0fba3 100644
--- a/include/uapi/linux/tcp.h
+++ b/include/uapi/linux/tcp.h
@@ -115,6 +115,7 @@ enum {
 #define TCP_CC_INFO		26	/* Get Congestion Control (optional) info */
 #define TCP_SAVE_SYN		27	/* Record SYN headers for new connections */
 #define TCP_SAVED_SYN		28	/* Get SYN headers recorded for connection */
+#define TCP_RDB			29	/* Enable RDB mechanism */
 
 struct tcp_repair_opt {
 	__u32	opt_code;
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 152b9c7..7b556c8 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -978,7 +978,7 @@ static void skb_headers_offset_update(struct sk_buff *skb, int off)
 	skb->inner_mac_header += off;
 }
 
-static void copy_skb_header(struct sk_buff *new, const struct sk_buff *old)
+void copy_skb_header(struct sk_buff *new, const struct sk_buff *old)
 {
 	__copy_skb_header(new, old);
 
@@ -986,6 +986,7 @@ static void copy_skb_header(struct sk_buff *new, const struct sk_buff *old)
 	skb_shinfo(new)->gso_segs = skb_shinfo(old)->gso_segs;
 	skb_shinfo(new)->gso_type = skb_shinfo(old)->gso_type;
 }
+EXPORT_SYMBOL(copy_skb_header);
 
 static inline int skb_alloc_rx_flag(const struct sk_buff *skb)
 {
diff --git a/net/ipv4/Makefile b/net/ipv4/Makefile
index c29809f..f2cf496 100644
--- a/net/ipv4/Makefile
+++ b/net/ipv4/Makefile
@@ -12,7 +12,8 @@ obj-y     := route.o inetpeer.o protocol.o \
 	     tcp_offload.o datagram.o raw.o udp.o udplite.o \
 	     udp_offload.o arp.o icmp.o devinet.o af_inet.o igmp.o \
 	     fib_frontend.o fib_semantics.o fib_trie.o \
-	     inet_fragment.o ping.o ip_tunnel_core.o gre_offload.o
+	     inet_fragment.o ping.o ip_tunnel_core.o gre_offload.o \
+	     tcp_rdb.o
 
 obj-$(CONFIG_NET_IP_TUNNEL) += ip_tunnel.o
 obj-$(CONFIG_SYSCTL) += sysctl_net_ipv4.o
diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 5b12446..9217200 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -718,6 +718,32 @@ static struct ctl_table ipv4_table[] = {
 		.extra1		= &tcp_thin_dpifl_itt_lower_bound_min,
 	},
 	{
+		.procname	= "tcp_rdb",
+		.data		= &sysctl_tcp_rdb,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= &zero,
+		.extra2		= &one,
+	},
+	{
+		.procname	= "tcp_rdb_max_bytes",
+		.data		= &sysctl_tcp_rdb_max_bytes,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.proc_handler	= &proc_dointvec,
+		.extra1		= &zero,
+	},
+	{
+		.procname	= "tcp_rdb_max_skbs",
+		.data		= &sysctl_tcp_rdb_max_skbs,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec_minmax,
+		.extra1		= &zero,
+	},
+	{
 		.procname	= "tcp_early_retrans",
 		.data		= &sysctl_tcp_early_retrans,
 		.maxlen		= sizeof(int),
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index cb3354d..298ec4b 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -289,6 +289,8 @@ int sysctl_tcp_autocorking __read_mostly = 1;
 
 int sysctl_tcp_thin_dpifl_itt_lower_bound __read_mostly = TCP_THIN_DPIFL_ITT_LOWER_BOUND_MIN;
 
+int sysctl_tcp_rdb __read_mostly;
+
 struct percpu_counter tcp_orphan_count;
 EXPORT_SYMBOL_GPL(tcp_orphan_count);
 
@@ -408,6 +410,7 @@ void tcp_init_sock(struct sock *sk)
 	u64_stats_init(&tp->syncp);
 
 	tp->reordering = sysctl_tcp_reordering;
+	tp->rdb = sysctl_tcp_rdb;
 	tcp_enable_early_retrans(tp);
 	tcp_assign_congestion_control(sk);
 
@@ -2408,6 +2411,13 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
 		}
 		break;
 
+	case TCP_RDB:
+		if (val < 0 || val > 1)
+			err = -EINVAL;
+		else
+			tp->rdb = val;
+		break;
+
 	case TCP_REPAIR:
 		if (!tcp_can_repair_sock(sk))
 			err = -EPERM;
@@ -2828,7 +2838,9 @@ static int do_tcp_getsockopt(struct sock *sk, int level,
 	case TCP_THIN_DUPACK:
 		val = tp->thin_dupack;
 		break;
-
+	case TCP_RDB:
+		val = tp->rdb;
+		break;
 	case TCP_REPAIR:
 		val = tp->repair;
 		break;
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index fdd88c3..a4901b3 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3503,6 +3503,9 @@ static inline void tcp_in_ack_event(struct sock *sk, u32 flags)
 
 	if (icsk->icsk_ca_ops->in_ack_event)
 		icsk->icsk_ca_ops->in_ack_event(sk, flags);
+
+	if (unlikely(tcp_sk(sk)->rdb))
+		rdb_ack_event(sk, flags);
 }
 
 /* This routine deals with incoming acks, but not outgoing ones. */
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index cb7ca56..bd7d8c5 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -900,8 +900,8 @@ out:
  * We are working here with either a clone of the original
  * SKB, or a fresh unique copy made by the retransmit engine.
  */
-static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
-			    gfp_t gfp_mask)
+int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
+		     gfp_t gfp_mask)
 {
 	const struct inet_connection_sock *icsk = inet_csk(sk);
 	struct inet_sock *inet;
@@ -2113,9 +2113,12 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
 				break;
 		}
 
-		if (unlikely(tcp_transmit_skb(sk, skb, 1, gfp)))
+		if (unlikely(tcp_sk(sk)->rdb)) {
+			if (tcp_transmit_rdb_skb(sk, skb, mss_now, gfp))
+				break;
+		} else if (unlikely(tcp_transmit_skb(sk, skb, 1, gfp))) {
 			break;
-
+		}
 repair:
 		/* Advance the send_head.  This one is sent out.
 		 * This call will increment packets_out.
diff --git a/net/ipv4/tcp_rdb.c b/net/ipv4/tcp_rdb.c
new file mode 100644
index 0000000..708fefa
--- /dev/null
+++ b/net/ipv4/tcp_rdb.c
@@ -0,0 +1,271 @@
+#include <linux/skbuff.h>
+#include <net/tcp.h>
+
+int sysctl_tcp_rdb_max_bytes __read_mostly;
+int sysctl_tcp_rdb_max_skbs __read_mostly = 1;
+
+/**
+ * rdb_check_rtx_queue_loss() - Perform loss detection by analysing acks.
+ * @sk: the socket.
+ *
+ * Return: The number of packets that are presumed to be lost.
+ */
+static unsigned int rdb_check_rtx_queue_loss(struct sock *sk)
+{
+	struct sk_buff *skb, *tmp;
+	struct tcp_skb_cb *scb;
+	u32 seq_acked = tcp_sk(sk)->snd_una;
+	unsigned int packets_lost = 0;
+
+	tcp_for_write_queue(skb, sk) {
+		if (skb == tcp_send_head(sk))
+			break;
+
+		scb = TCP_SKB_CB(skb);
+		/* The ACK acknowledges parts of the data in this SKB.
+		 * Can be caused by:
+		 * - TSO: We abort as RDB is not used on SKBs split across
+		 *        multiple packets on lower layers as these are greater
+		 *        than one MSS.
+		 * - Retrans collapse: We've had a retrans, so loss has already
+		 *                     been detected.
+		 */
+		if (after(scb->end_seq, seq_acked)) {
+			break;
+		/* The ACKed packet */
+		} else if (scb->end_seq == seq_acked) {
+			/* This SKB was sent with no RDB data, or no prior
+			 * unacked SKBs in output queue, so break here.
+			 */
+			if (scb->tx.rdb_start_seq == scb->seq ||
+			    skb_queue_is_first(&sk->sk_write_queue, skb))
+				break;
+			/* Find number of prior SKBs who's data was bundled in
+			 * this (ACKed) SKB. We presume any redundant data
+			 * covering previous SKB's are due to loss. (An
+			 * exception would be reordering).
+			 */
+			skb = skb->prev;
+			tcp_for_write_queue_reverse_from_safe(skb, tmp, sk) {
+				if (!before(TCP_SKB_CB(skb)->seq, scb->tx.rdb_start_seq))
+					packets_lost++;
+				else
+					break;
+			}
+			break;
+		}
+	}
+	return packets_lost;
+}
+
+/**
+ * rdb_ack_event() - Initiate loss detection
+ * @sk: the socket
+ * @flags: The flags
+ */
+void rdb_ack_event(struct sock *sk, u32 flags)
+{
+	if (rdb_check_rtx_queue_loss(sk))
+		tcp_enter_cwr(sk);
+}
+
+/**
+ * skb_append_data() - Copy data from an SKB to the end of another
+ * @from_skb: The SKB to copy data from
+ * @to_skb: The SKB to copy data to
+ *
+ * Return: 0 on success, else error
+ */
+static int skb_append_data(struct sk_buff *from_skb, struct sk_buff *to_skb)
+{
+	/* Copy the linear data and the data from the frags into the linear page
+	 * buffer of to_skb.
+	 */
+	if (WARN_ON(skb_copy_bits(from_skb, 0,
+				  skb_put(to_skb, from_skb->len),
+				  from_skb->len))) {
+		goto fault;
+	}
+
+	TCP_SKB_CB(to_skb)->end_seq = TCP_SKB_CB(from_skb)->end_seq;
+
+	if (from_skb->ip_summed == CHECKSUM_PARTIAL)
+		to_skb->ip_summed = CHECKSUM_PARTIAL;
+
+	if (to_skb->ip_summed != CHECKSUM_PARTIAL)
+		to_skb->csum = csum_block_add(to_skb->csum, from_skb->csum,
+					      to_skb->len);
+	return 0;
+fault:
+	return -EFAULT;
+}
+
+/**
+ * rdb_build_skb() - Builds the new RDB SKB and copies all the data into the
+ *                   linear page buffer.
+ * @sk: the socket
+ * @xmit_skb: This is the SKB that tcp_write_xmit wants to send
+ * @first_skb: The first SKB in the output queue we will bundle
+ * @gfp_mask: The gfp_t allocation
+ * @bytes_in_rdb_skb: The total number of data bytes for the new rdb_skb
+ *                         (NEW + Redundant)
+ *
+ * Return: A new SKB containing redundant data, or NULL if memory allocation
+ *         failed
+ */
+static struct sk_buff *rdb_build_skb(const struct sock *sk,
+				     struct sk_buff *xmit_skb,
+				     struct sk_buff *first_skb,
+				     u32 bytes_in_rdb_skb,
+				     gfp_t gfp_mask)
+{
+	struct sk_buff *rdb_skb, *tmp_skb;
+
+	rdb_skb = sk_stream_alloc_skb((struct sock *)sk,
+				      (int)bytes_in_rdb_skb,
+				      gfp_mask, true);
+	if (!rdb_skb)
+		return NULL;
+	copy_skb_header(rdb_skb, xmit_skb);
+	rdb_skb->ip_summed = xmit_skb->ip_summed;
+
+	TCP_SKB_CB(rdb_skb)->seq = TCP_SKB_CB(first_skb)->seq;
+	TCP_SKB_CB(xmit_skb)->tx.rdb_start_seq = TCP_SKB_CB(rdb_skb)->seq;
+
+	tmp_skb = first_skb;
+
+	tcp_for_write_queue_from(tmp_skb, sk) {
+		/* Copy data from tmp_skb to rdb_skb */
+		if (skb_append_data(tmp_skb, rdb_skb))
+			return NULL;
+		/* We are at the last skb that should be included (The unsent
+		 * one)
+		 */
+		if (tmp_skb == xmit_skb)
+			break;
+	}
+	return rdb_skb;
+}
+
+/**
+ * rdb_can_bundle_test() - test if redundant data can be bundled
+ * @sk: the socket
+ * @xmit_skb: The SKB processed for transmission by the output engine
+ * @mss_now: The current mss value
+ * @bytes_in_rdb_skb: Will contain the resulting number of bytes to bundle
+ *                         at exit.
+ * @skbs_to_bundle_count: The total number of SKBs to be in the bundle
+ *
+ * Traverses the entire write queue and checks if any un-acked data
+ * may be bundled.
+ *
+ * Return: The first SKB to be in the bundle, or NULL if no bundling
+ */
+static struct sk_buff *rdb_can_bundle_test(const struct sock *sk,
+					    struct sk_buff *xmit_skb,
+					    unsigned int mss_now,
+					    u32 *bytes_in_rdb_skb,
+					    u32 *skbs_to_bundle_count)
+{
+	struct sk_buff *first_to_bundle = NULL;
+	struct sk_buff *tmp, *skb = xmit_skb->prev;
+	u32 skbs_in_bundle_count = 1; /* 1 to account for current skb */
+	u32 byte_count = xmit_skb->len;
+
+	/* We start at the skb before xmit_skb, and go backwards in the list.*/
+	tcp_for_write_queue_reverse_from_safe(skb, tmp, sk) {
+		/* Not enough room to bundle data from this SKB */
+		if ((byte_count + skb->len) > mss_now)
+			break;
+
+		if (sysctl_tcp_rdb_max_bytes &&
+		    ((byte_count + skb->len) > sysctl_tcp_rdb_max_bytes))
+			break;
+
+		if (sysctl_tcp_rdb_max_skbs &&
+		    (skbs_in_bundle_count > sysctl_tcp_rdb_max_skbs))
+			break;
+
+		byte_count += skb->len;
+		skbs_in_bundle_count++;
+		first_to_bundle = skb;
+	}
+	*bytes_in_rdb_skb = byte_count;
+	*skbs_to_bundle_count = skbs_in_bundle_count;
+	return first_to_bundle;
+}
+
+/**
+ * create_rdb_skb() - Try to create an RDB SKB
+ * @sk: the socket
+ * @xmit_skb: The SKB from the output queue to be sent
+ * @mss_now: Current MSS
+ * @gfp_mask: The gfp_t allocation
+ *
+ * Return: A new SKB containing redundant data, or NULL if no bundling could be
+ *         performed
+ */
+struct sk_buff *create_rdb_skb(const struct sock *sk, struct sk_buff *xmit_skb,
+			       unsigned int mss_now, u32 *bytes_in_rdb_skb,
+			       gfp_t gfp_mask)
+{
+	u32 skb_in_bundle_count;
+	struct sk_buff *first_to_bundle;
+
+	if (skb_queue_is_first(&sk->sk_write_queue, xmit_skb))
+		return NULL;
+
+	/* No bundling on FIN packet */
+	if (TCP_SKB_CB(xmit_skb)->tcp_flags & TCPHDR_FIN)
+		return NULL;
+
+	/* Find number of (previous) SKBs to get data from */
+	first_to_bundle = rdb_can_bundle_test(sk, xmit_skb, mss_now,
+					       bytes_in_rdb_skb,
+					       &skb_in_bundle_count);
+	if (!first_to_bundle)
+		return NULL;
+
+	/* Create an SKB that contains the data from 'skb_in_bundle_count'
+	 * SKBs.
+	 */
+	return rdb_build_skb(sk, xmit_skb, first_to_bundle,
+			     *bytes_in_rdb_skb, gfp_mask);
+}
+
+/**
+ * tcp_transmit_rdb_skb() - Try to create and send an RDB packet
+ * @sk: the socket
+ * @xmit_skb: The SKB processed for transmission by the output engine
+ * @mss_now: Current MSS
+ * @gfp_mask: The gfp_t allocation
+ *
+ * Return: 0 if successfully sent packet, else error
+ */
+int tcp_transmit_rdb_skb(struct sock *sk, struct sk_buff *xmit_skb,
+			 unsigned int mss_now, gfp_t gfp_mask)
+{
+	struct sk_buff *rdb_skb = NULL;
+	u32 bytes_in_rdb_skb = 0; /* May be used for statistical purposes */
+
+	/* How we detect that RDB was used. When equal, no RDB data was sent */
+	TCP_SKB_CB(xmit_skb)->tx.rdb_start_seq = TCP_SKB_CB(xmit_skb)->seq;
+
+	if (tcp_stream_is_thin_dpifl(tcp_sk(sk))) {
+		rdb_skb = create_rdb_skb(sk, xmit_skb, mss_now,
+					 &bytes_in_rdb_skb, gfp_mask);
+		if (!rdb_skb)
+			goto xmit_default;
+
+		/* Set tstamp for SKB in output queue, because tcp_transmit_skb
+		 * will do this for the rdb_skb and not the SKB in the output
+		 * queue (xmit_skb).
+		 */
+		skb_mstamp_get(&xmit_skb->skb_mstamp);
+		rdb_skb->skb_mstamp = xmit_skb->skb_mstamp;
+		return tcp_transmit_skb(sk, rdb_skb, 0, gfp_mask);
+	}
+xmit_default:
+	/* Transmit the unmodified SKB from output queue */
+	return tcp_transmit_skb(sk, xmit_skb, 1, gfp_mask);
+}
-- 
1.9.1