RFC: Latency reducing TCP modifications for thin-stream interactiveapplications
From: Andreas Petlund
Date: Thu Nov 27 2008 - 09:22:47 EST
A wide range of Internet-based services that use reliable transport
protocols display what we call thin-stream properties. This means
that the application sends data with such a low rate that the
retransmission mechanisms of the transport protocol are not fully
effective. In time-dependent scenarios (like online games, control
systems or some sensor networks) where the user experience depends
on the data delivery latency, packet loss can be devastating for
the service quality. Extreme latencies are caused by TCP's
dependency on the arrival of new data from the application to trigger
retransmissions effectively through fast retransmit instead of
waiting for long timeouts. After analyzing a large number of
time-dependent interactive applications, we have seen that they
often produce thin streams (as described above) and also stay with
this traffic pattern throughout its entire lifespan. The
combination of time-dependency and the fact that the streams
provoke high latencies when using TCP is unfortunate.
In order to reduce application-layer latency when packets are lost,
we have implemented modifications to the TCP retransmission
mechanisms in the Linux kernel. We have also implemented a
bundling mechanisms that introduces redundancy in order to
preempt the experience of packet loss. In short, if the kernel
detects a thin stream, we trade a small amount of bandwidth for
latency reduction and apply:
Removal of exponential backoff: To prevent an exponential increase
in retransmission delay for a repeatedly lost packet, we remove
the exponential factor.
FASTER Fast Retransmit: Instead of waiting for 3 duplicate
acknowledgments before sending a fast retransmission, we retransmit
after receiving only one.
Redundant Data Bundling: We copy (bundle) data from the
unacknowledged packets in the send buffer into the next packet if
space is available.
These enhancements are applied only if the stream is detected as
thin.
This is accomplished by defining thresholds for packet size and
packets in flight. Also, we consider the redundancy introduced
by our mechanisms acceptable because the streams are so thin
that normal congestion mechanisms do not come into effect.
We have implemented these changes in the Linux kernel (2.6.23.8),
and have tested the modifications on a wide range of different
thin-stream applications (Skype, BZFlag, SSH, ...) under varying
network conditions. Our results show that applications which use
TCP for interactive time-dependent traffic will experience a
reduction in both maximum and average latency, giving the users
quicker feedback to their interactions.
Availability of this kind of mechanisms will help provide
customizability for interactive network services. The quickly
growing market for Linux gaming may benefit from lowered latency.
As an example, most of the large MMORPG's today use TCP (like World
of Warcraft and Age of Conan) and several multimedia applications
(like Skype) use TCP fallback if UDP is blocked.
The modifications are all TCP standard compliant and transparent
to the receiver. As such, a game server could implement the
modifications and get a one-way latency benefit without touching
any of the clients.
In the following papers, we discuss the benefits and tradeoffs of
the decribed mechanisms:
"The Fun of using TCP for an MMORPG":
http://simula.no/research/networks/publications/Griwodz.2006.1
"TCP Enhancements For Interactive Thin-Stream Applications":
http://simula.no/research/networks/publications/Simula.ND.83
"Improving application layer latency for reliable thin-stream game
traffic": http://simula.no/research/networks/publications/Simula.ND.185
"TCP mechanisms for improving the user experience for time-dependent
thin-stream applications":
http://simula.no/research/networks/publications/Simula.ND.159
Our presentation from the 2008 Linux-Kongress can be found here:
http://data.guug.de/slides/lk2008/lk-2008-Andreas-Petlund.pdf
We have included a patch for the 2.6.23.8 kernel which implements the
modifications. The patch is not properly segmented and formatted, but
attached as a reference. We are currently working on an updated patch
set which we hopefully will be able to post in a couple of weeks. This
will also give us time to integrate any ideas that may arise from the
discussions here.
We are happy for all feedback regarding this:
Is something like this viable to introduce into the kernel? Is the
scheme for thin-stream detection mechanism acceptable. Any viewpoints
on the architecture and design?
diff -Nur linux-2.6.23.8.vanilla/include/linux/sysctl.h linux-2.6.23.8-tcp-thin/include/linux/sysctl.h
--- linux-2.6.23.8.vanilla/include/linux/sysctl.h 2007-11-16 19:14:27.000000000 +0100
+++ linux-2.6.23.8-tcp-thin/include/linux/sysctl.h 2008-07-03 11:47:21.000000000 +0200
@@ -355,6 +355,11 @@
NET_IPV4_ROUTE=18,
NET_IPV4_FIB_HASH=19,
NET_IPV4_NETFILTER=20,
+
+ NET_IPV4_TCP_FORCE_THIN_RDB=29, /* Added @ Simula */
+ NET_IPV4_TCP_FORCE_THIN_RM_EXPB=30, /* Added @ Simula */
+ NET_IPV4_TCP_FORCE_THIN_DUPACK=31, /* Added @ Simula */
+ NET_IPV4_TCP_RDB_MAX_BUNDLE_BYTES=32, /* Added @ Simula */
NET_IPV4_TCP_TIMESTAMPS=33,
NET_IPV4_TCP_WINDOW_SCALING=34,
diff -Nur linux-2.6.23.8.vanilla/include/linux/tcp.h linux-2.6.23.8-tcp-thin/include/linux/tcp.h
--- linux-2.6.23.8.vanilla/include/linux/tcp.h 2007-11-16 19:14:27.000000000 +0100
+++ linux-2.6.23.8-tcp-thin/include/linux/tcp.h 2008-07-02 15:17:38.000000000 +0200
@@ -97,6 +97,10 @@
#define TCP_CONGESTION 13 /* Congestion control algorithm */
#define TCP_MD5SIG 14 /* TCP MD5 Signature (RFC2385) */
+#define TCP_THIN_RDB 15 /* Added @ Simula - Enable redundant data bundling */
+#define TCP_THIN_RM_EXPB 16 /* Added @ Simula - Remove exponential backoff */
+#define TCP_THIN_DUPACK 17 /* Added @ Simula - Reduce number of dupAcks needed */
+
#define TCPI_OPT_TIMESTAMPS 1
#define TCPI_OPT_SACK 2
#define TCPI_OPT_WSCALE 4
@@ -296,6 +300,10 @@
u8 nonagle; /* Disable Nagle algorithm? */
u8 keepalive_probes; /* num of allowed keep alive probes */
+ u8 thin_rdb; /* Enable RDB */
+ u8 thin_rm_expb; /* Remove exp. backoff */
+ u8 thin_dupack; /* Remove dupack */
+
/* RTT measurement */
u32 srtt; /* smoothed round trip time << 3 */
u32 mdev; /* medium deviation */
diff -Nur linux-2.6.23.8.vanilla/include/net/sock.h linux-2.6.23.8-tcp-thin/include/net/sock.h
--- linux-2.6.23.8.vanilla/include/net/sock.h 2007-11-16 19:14:27.000000000 +0100
+++ linux-2.6.23.8-tcp-thin/include/net/sock.h 2008-07-02 17:07:10.000000000 +0200
@@ -462,7 +462,10 @@
static inline void sk_stream_free_skb(struct sock *sk, struct sk_buff *skb)
{
- skb_truesize_check(skb);
+ /* Modified @ Simula
+ skb_truesize_check creates unnecessary
+ noise when combined with RDB */
+ //skb_truesize_check(skb);
sock_set_flag(sk, SOCK_QUEUE_SHRUNK);
sk->sk_wmem_queued -= skb->truesize;
sk->sk_forward_alloc += skb->truesize;
diff -Nur linux-2.6.23.8.vanilla/include/net/tcp.h linux-2.6.23.8-tcp-thin/include/net/tcp.h
--- linux-2.6.23.8.vanilla/include/net/tcp.h 2007-11-16 19:14:27.000000000 +0100
+++ linux-2.6.23.8-tcp-thin/include/net/tcp.h 2008-07-03 11:48:54.000000000 +0200
@@ -188,9 +188,19 @@
#define TCP_NAGLE_CORK 2 /* Socket is corked */
#define TCP_NAGLE_PUSH 4 /* Cork is overridden for already queued data */
+/* Added @ Simula - Thin stream support */
+#define TCP_FORCE_THIN_RDB 0 /* Thin streams: exp. backoff default off */
+#define TCP_FORCE_THIN_RM_EXPB 0 /* Thin streams: dynamic dupack default off */
+#define TCP_FORCE_THIN_DUPACK 0 /* Thin streams: smaller minRTO default off */
+#define TCP_RDB_MAX_BUNDLE_BYTES 0 /* Thin streams: Limit maximum bundled bytes */
+
extern struct inet_timewait_death_row tcp_death_row;
/* sysctl variables for tcp */
+extern int sysctl_tcp_force_thin_rdb; /* Added @ Simula */
+extern int sysctl_tcp_force_thin_rm_expb; /* Added @ Simula */
+extern int sysctl_tcp_force_thin_dupack; /* Added @ Simula */
+extern int sysctl_tcp_rdb_max_bundle_bytes; /* Added @ Simula */
extern int sysctl_tcp_timestamps;
extern int sysctl_tcp_window_scaling;
extern int sysctl_tcp_sack;
@@ -723,6 +733,16 @@
return (tp->packets_out - tp->left_out + tp->retrans_out);
}
+/* Added @ Simula
+ *
+ * To determine whether a stream is thin or not
+ * return 1 if thin, 0 othervice
+ */
+static inline unsigned int tcp_stream_is_thin(const struct tcp_sock *tp)
+{
+ return (tp->packets_out < 4 ? 1 : 0);
+}
+
/* If cwnd > ssthresh, we may raise ssthresh to be half-way to cwnd.
* The exception is rate halving phase, when cwnd is decreasing towards
* ssthresh.
diff -Nur linux-2.6.23.8.vanilla/net/ipv4/sysctl_net_ipv4.c linux-2.6.23.8-tcp-thin/net/ipv4/sysctl_net_ipv4.c
--- linux-2.6.23.8.vanilla/net/ipv4/sysctl_net_ipv4.c 2007-11-16 19:14:27.000000000 +0100
+++ linux-2.6.23.8-tcp-thin/net/ipv4/sysctl_net_ipv4.c 2008-07-03 11:49:59.000000000 +0200
@@ -187,6 +187,38 @@
}
ctl_table ipv4_table[] = {
+ { /* Added @ Simula for thin streams */
+ .ctl_name = NET_IPV4_TCP_FORCE_THIN_RDB,
+ .procname = "tcp_force_thin_rdb",
+ .data = &sysctl_tcp_force_thin_rdb,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = &proc_dointvec
+ },
+ { /* Added @ Simula for thin streams */
+ .ctl_name = NET_IPV4_TCP_FORCE_THIN_RM_EXPB,
+ .procname = "tcp_force_thin_rm_expb",
+ .data = &sysctl_tcp_force_thin_rm_expb,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = &proc_dointvec
+ },
+ { /* Added @ Simula for thin streams */
+ .ctl_name = NET_IPV4_TCP_FORCE_THIN_DUPACK,
+ .procname = "tcp_force_thin_dupack",
+ .data = &sysctl_tcp_force_thin_dupack,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = &proc_dointvec
+ },
+ { /* Added @ Simula for thin streams */
+ .ctl_name = NET_IPV4_TCP_RDB_MAX_BUNDLE_BYTES,
+ .procname = "tcp_rdb_max_bundle_bytes",
+ .data = &sysctl_tcp_rdb_max_bundle_bytes,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = &proc_dointvec
+ },
{
.ctl_name = NET_IPV4_TCP_TIMESTAMPS,
.procname = "tcp_timestamps",
diff -Nur linux-2.6.23.8.vanilla/net/ipv4/tcp.c linux-2.6.23.8-tcp-thin/net/ipv4/tcp.c
--- linux-2.6.23.8.vanilla/net/ipv4/tcp.c 2007-11-16 19:14:27.000000000 +0100
+++ linux-2.6.23.8-tcp-thin/net/ipv4/tcp.c 2008-07-03 11:51:55.000000000 +0200
@@ -270,6 +270,10 @@
int sysctl_tcp_fin_timeout __read_mostly = TCP_FIN_TIMEOUT;
+/* Added @ Simula */
+int sysctl_tcp_force_thin_rdb __read_mostly = TCP_FORCE_THIN_RDB;
+int sysctl_tcp_rdb_max_bundle_bytes __read_mostly = TCP_RDB_MAX_BUNDLE_BYTES;
+
DEFINE_SNMP_STAT(struct tcp_mib, tcp_statistics) __read_mostly;
atomic_t tcp_orphan_count = ATOMIC_INIT(0);
@@ -658,6 +662,167 @@
return tmp;
}
+/* Added at Simula to support RDB */
+static int tcp_trans_merge_prev(struct sock *sk, struct sk_buff *skb, int mss_now)
+{
+ struct tcp_sock *tp = tcp_sk(sk);
+
+ /* Make sure that this isn't referenced by somebody else */
+
+ if(!skb_cloned(skb)){
+ struct sk_buff *prev_skb = skb->prev;
+ int skb_size = skb->len;
+ int old_headlen = 0;
+ int ua_data = 0;
+ int uad_head = 0;
+ int uad_frags = 0;
+ int ua_nr_frags = 0;
+ int ua_frags_diff = 0;
+
+ /* Since this technique currently does not support SACK, I
+ * return -1 if the previous has been SACK'd. */
+ if(TCP_SKB_CB(prev_skb)->sacked & TCPCB_SACKED_ACKED){
+ return -1;
+ }
+
+ /* Current skb is out of window. */
+ if (after(TCP_SKB_CB(skb)->end_seq, tp->snd_una+tp->snd_wnd)){
+ return -1;
+ }
+
+ /*TODO: Optimize this part with regards to how the
+ variables are initialized */
+
+ /*Calculates the ammount of unacked data that is available*/
+ ua_data = (TCP_SKB_CB(prev_skb)->end_seq - tp->snd_una >
+ prev_skb->len ? prev_skb->len :
+ TCP_SKB_CB(prev_skb)->end_seq - tp->snd_una);
+ ua_frags_diff = ua_data - prev_skb->data_len;
+ uad_frags = (ua_frags_diff > 0 ? prev_skb->data_len : ua_data);
+ uad_head = (ua_frags_diff > 0 ? ua_data - uad_frags : 0);
+
+ if(ua_data <= 0)
+ return -1;
+
+ if(uad_frags > 0){
+ int i = 0;
+ int bytes_frags = 0;
+
+ if(uad_frags == prev_skb->data_len){
+ ua_nr_frags = skb_shinfo(prev_skb)->nr_frags;
+ } else{
+ for(i=skb_shinfo(prev_skb)->nr_frags - 1; i>=0; i--){
+ if(skb_shinfo(prev_skb)->frags[i].size
+ + bytes_frags == uad_frags){
+ ua_nr_frags += 1;
+ break;
+ }
+ ua_nr_frags += 1;
+ bytes_frags += skb_shinfo(prev_skb)->frags[i].size;
+ }
+ }
+ }
+
+ /*
+ * Do the diffrenet checks on size and content, and return if
+ * something will not work.
+ *
+ * TODO: Support copying some bytes
+ *
+ * 1. Larger than MSS.
+ * 2. Enough room for the stuff stored in the linear area
+ * 3. Enoug room for the pages
+ * 4. If both skbs have some data stored in the linear area, and prev_skb
+ * also has some stored in the paged area, they cannot be merged easily.
+ * 5. If prev_skb is linear, then this one has to be it as well.
+ */
+ if ((sysctl_tcp_rdb_max_bundle_bytes == 0 && ((skb_size + ua_data) > mss_now))
+ || (sysctl_tcp_rdb_max_bundle_bytes > 0 && ((skb_size + ua_data) >
+ sysctl_tcp_rdb_max_bundle_bytes))){
+ return -1;
+ }
+
+ /* We need to know tailroom, even if it is nonlinear */
+ if(uad_head > (skb->end - skb->tail)){
+ return -1;
+ }
+
+ if(skb_is_nonlinear(skb) && (uad_frags > 0)){
+ if((ua_nr_frags +
+ skb_shinfo(skb)->nr_frags) > MAX_SKB_FRAGS){
+ return -1;
+ }
+
+ if(skb_headlen(skb) > 0){
+ return -1;
+ }
+ }
+
+ if((uad_frags > 0) && skb_headlen(skb) > 0){
+ return -1;
+ }
+
+ /* To avoid duplicate copies (and copies
+ where parts have been acked) */
+ if(TCP_SKB_CB(skb)->seq <= (TCP_SKB_CB(prev_skb)->end_seq - ua_data)){
+ return -1;
+ }
+
+ /*SYN's are holy*/
+ if(TCP_SKB_CB(skb)->flags & TCPCB_FLAG_SYN || TCP_SKB_CB(skb)->flags & TCPCB_FLAG_FIN){
+ return -1;
+ }
+
+ /* Copy linear data */
+ if(uad_head > 0){
+
+ /* Add required space to the header. Can't use put due to linearity */
+ old_headlen = skb_headlen(skb);
+ skb->tail += uad_head;
+ skb->len += uad_head;
+
+ if(skb_headlen(skb) > 0){
+ memmove(skb->data + uad_head, skb->data, old_headlen);
+ }
+
+ skb_copy_to_linear_data(skb, prev_skb->data + (skb_headlen(prev_skb) - uad_head), uad_head);
+ }
+
+ /*Copy paged data*/
+ if(uad_frags > 0){
+ int i = 0;
+ /*Must move data backwards in the array.*/
+ if(skb_is_nonlinear(skb)){
+ memmove(skb_shinfo(skb)->frags + ua_nr_frags,
+ skb_shinfo(skb)->frags,
+ skb_shinfo(skb)->nr_frags*sizeof(skb_frag_t));
+ }
+
+ /*Copy info and update pages*/
+ memcpy(skb_shinfo(skb)->frags,
+ skb_shinfo(prev_skb)->frags + (skb_shinfo(prev_skb)->nr_frags - ua_nr_frags),
+ ua_nr_frags*sizeof(skb_frag_t));
+
+ for(i=0; i<ua_nr_frags;i++){
+ get_page(skb_shinfo(skb)->frags[i].page);
+ }
+
+ skb_shinfo(skb)->nr_frags += ua_nr_frags;
+ skb->data_len += uad_frags;
+ skb->len += uad_frags;
+ }
+
+ TCP_SKB_CB(skb)->seq = TCP_SKB_CB(prev_skb)->end_seq - ua_data;
+
+ if(skb->ip_summed == CHECKSUM_PARTIAL)
+ skb->csum = CHECKSUM_PARTIAL;
+ else
+ skb->csum = skb_checksum(skb, 0, skb->len, 0);
+ }
+
+ return 1;
+}
+
int tcp_sendmsg(struct kiocb *iocb, struct socket *sock, struct msghdr *msg,
size_t size)
{
@@ -825,6 +990,16 @@
from += copy;
copied += copy;
+
+ /* Added at Simula to support RDB */
+ if(((tp->thin_rdb || sysctl_tcp_force_thin_rdb)) && skb->len < mss_now){
+ if(skb->prev != (struct sk_buff*) &(sk)->sk_write_queue
+ && !(TCP_SKB_CB(skb)->flags & TCPCB_FLAG_SYN)
+ && !(TCP_SKB_CB(skb)->flags & TCPCB_FLAG_FIN)){
+ tcp_trans_merge_prev(sk, skb, mss_now);
+ }
+ } /* End - Simula */
+
if ((seglen -= copy) == 0 && iovlen == 0)
goto out;
@@ -1870,7 +2045,25 @@
tcp_push_pending_frames(sk);
}
break;
-
+
+ /* Added @ Simula. Support for thin streams */
+ case TCP_THIN_RDB:
+ if(val)
+ tp->thin_rdb = 1;
+ break;
+
+ /* Added @ Simula. Support for thin streams */
+ case TCP_THIN_RM_EXPB:
+ if(val)
+ tp->thin_rm_expb = 1;
+ break;
+
+ /* Added @ Simula. Support for thin streams */
+ case TCP_THIN_DUPACK:
+ if(val)
+ tp->thin_dupack = 1;
+ break;
+
case TCP_KEEPIDLE:
if (val < 1 || val > MAX_TCP_KEEPIDLE)
err = -EINVAL;
diff -Nur linux-2.6.23.8.vanilla/net/ipv4/tcp_input.c linux-2.6.23.8-tcp-thin/net/ipv4/tcp_input.c
--- linux-2.6.23.8.vanilla/net/ipv4/tcp_input.c 2007-11-16 19:14:27.000000000 +0100
+++ linux-2.6.23.8-tcp-thin/net/ipv4/tcp_input.c 2008-07-03 11:57:08.000000000 +0200
@@ -89,6 +89,9 @@
int sysctl_tcp_frto_response __read_mostly;
int sysctl_tcp_nometrics_save __read_mostly;
+/* Added @ Simula */
+int sysctl_tcp_force_thin_dupack __read_mostly = TCP_FORCE_THIN_DUPACK;
+
int sysctl_tcp_moderate_rcvbuf __read_mostly = 1;
int sysctl_tcp_abc __read_mostly;
@@ -1709,6 +1712,12 @@
*/
return 1;
}
+
+ /*Added at Simula to modify fast retransmit */
+ if ((tp->thin_dupack || sysctl_tcp_force_thin_dupack) &&
+ tcp_fackets_out(tp) > 1 && tcp_stream_is_thin(tp)){
+ return 1;
+ }
return 0;
}
@@ -2442,30 +2451,127 @@
{
struct tcp_sock *tp = tcp_sk(sk);
const struct inet_connection_sock *icsk = inet_csk(sk);
- struct sk_buff *skb;
+ struct sk_buff *skb = tcp_write_queue_head(sk);
+ struct sk_buff *next_skb;
+
__u32 now = tcp_time_stamp;
int acked = 0;
int prior_packets = tp->packets_out;
+
+ /*Added at Simula for RDB support*/
+ __u8 done = 0;
+ int remove = 0;
+ int remove_head = 0;
+ int remove_frags = 0;
+ int no_frags;
+ int data_frags;
+ int i;
+
__s32 seq_rtt = -1;
ktime_t last_ackt = net_invalid_timestamp();
-
- while ((skb = tcp_write_queue_head(sk)) &&
- skb != tcp_send_head(sk)) {
+
+ while (skb != NULL
+ && ((!(tp->thin_rdb || sysctl_tcp_force_thin_rdb)
+ && skb != tcp_send_head(sk)
+ && skb != (struct sk_buff *)&sk->sk_write_queue)
+ || ((tp->thin_rdb || sysctl_tcp_force_thin_rdb)
+ && skb != (struct sk_buff *)&sk->sk_write_queue))){
struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
__u8 sacked = scb->sacked;
-
+
+ if(skb == NULL){
+ break;
+ }
+
+ if(skb == tcp_send_head(sk)){
+ break;
+ }
+
+ if(skb == (struct sk_buff *)&sk->sk_write_queue){
+ break;
+ }
+
/* If our packet is before the ack sequence we can
* discard it as it's confirmed to have arrived at
* the other end.
*/
if (after(scb->end_seq, tp->snd_una)) {
- if (tcp_skb_pcount(skb) > 1 &&
- after(tp->snd_una, scb->seq))
- acked |= tcp_tso_acked(sk, skb,
- now, &seq_rtt);
- break;
+ if (tcp_skb_pcount(skb) > 1 && after(tp->snd_una, scb->seq))
+ acked |= tcp_tso_acked(sk, skb, now, &seq_rtt);
+
+ done = 1;
+
+ /* Added at Simula for RDB support*/
+ if ((tp->thin_rdb || sysctl_tcp_force_thin_rdb) && after(tp->snd_una, scb->seq)) {
+ if (!skb_cloned(skb) && !(scb->flags & TCPCB_FLAG_SYN)){
+ remove = tp->snd_una - scb->seq;
+ remove_head = (remove > skb_headlen(skb) ?
+ skb_headlen(skb) : remove);
+ remove_frags = (remove > skb_headlen(skb) ?
+ remove - remove_head : 0);
+
+ /* Has linear data */
+ if(skb_headlen(skb) > 0 && remove_head > 0){
+ memmove(skb->data,
+ skb->data + remove_head,
+ skb_headlen(skb) - remove_head);
+
+ skb->tail -= remove_head;
+ }
+
+ if(skb_is_nonlinear(skb) && remove_frags > 0){
+ no_frags = 0;
+ data_frags = 0;
+
+ /*Remove unecessary pages*/
+ for(i=0; i<skb_shinfo(skb)->nr_frags; i++){
+ if(data_frags + skb_shinfo(skb)->frags[i].size
+ == remove_frags){
+ put_page(skb_shinfo(skb)->frags[i].page);
+ no_frags += 1;
+ break;
+ }
+ put_page(skb_shinfo(skb)->frags[i].page);
+ no_frags += 1;
+ data_frags += skb_shinfo(skb)->frags[i].size;
+ }
+
+ if(skb_shinfo(skb)->nr_frags > no_frags)
+ memmove(skb_shinfo(skb)->frags,
+ skb_shinfo(skb)->frags + no_frags,
+ (skb_shinfo(skb)->nr_frags
+ - no_frags)*sizeof(skb_frag_t));
+
+ skb->data_len -= remove_frags;
+ skb_shinfo(skb)->nr_frags -= no_frags;
+
+ }
+
+ scb->seq += remove;
+ skb->len -= remove;
+
+ if(skb->ip_summed == CHECKSUM_PARTIAL)
+ skb->csum = CHECKSUM_PARTIAL;
+ else
+ skb->csum = skb_checksum(skb, 0, skb->len, 0);
+
+ }
+
+ /*Only move forward if data could be removed from this packet*/
+ done = 2;
+
+ }
+
+ if(done == 1 || tcp_skb_is_last(sk,skb)){
+ break;
+ } else if(done == 2){
+ skb = skb->next;
+ done = 1;
+ continue;
+ }
+
}
-
+
/* Initial outgoing SYN's get put onto the write_queue
* just like anything else we transmit. It is not
* true data, and if we misinform our callers that
@@ -2479,14 +2585,14 @@
acked |= FLAG_SYN_ACKED;
tp->retrans_stamp = 0;
}
-
+
/* MTU probing checks */
if (icsk->icsk_mtup.probe_size) {
if (!after(tp->mtu_probe.probe_seq_end, TCP_SKB_CB(skb)->end_seq)) {
tcp_mtup_probe_success(sk, skb);
}
}
-
+
if (sacked) {
if (sacked & TCPCB_RETRANS) {
if (sacked & TCPCB_SACKED_RETRANS)
@@ -2510,24 +2616,32 @@
seq_rtt = now - scb->when;
last_ackt = skb->tstamp;
}
+
+ if ((tp->thin_rdb || sysctl_tcp_force_thin_rdb) && skb == tcp_send_head(sk)) {
+ tcp_advance_send_head(sk, skb);
+ }
+
tcp_dec_pcount_approx(&tp->fackets_out, skb);
tcp_packets_out_dec(tp, skb);
+ next_skb = skb->next;
tcp_unlink_write_queue(skb, sk);
sk_stream_free_skb(sk, skb);
clear_all_retrans_hints(tp);
+ /* Added at Simula to support RDB */
+ skb = next_skb;
}
-
+
if (acked&FLAG_ACKED) {
u32 pkts_acked = prior_packets - tp->packets_out;
const struct tcp_congestion_ops *ca_ops
= inet_csk(sk)->icsk_ca_ops;
-
+
tcp_ack_update_rtt(sk, acked, seq_rtt);
tcp_ack_packets_out(sk);
-
+
if (ca_ops->pkts_acked) {
s32 rtt_us = -1;
-
+
/* Is the ACK triggering packet unambiguous? */
if (!(acked & FLAG_RETRANS_DATA_ACKED)) {
/* High resolution needed and available? */
diff -Nur linux-2.6.23.8.vanilla/net/ipv4/tcp_output.c linux-2.6.23.8-tcp-thin/net/ipv4/tcp_output.c
--- linux-2.6.23.8.vanilla/net/ipv4/tcp_output.c 2007-11-16 19:14:27.000000000 +0100
+++ linux-2.6.23.8-tcp-thin/net/ipv4/tcp_output.c 2008-07-03 11:55:45.000000000 +0200
@@ -1653,7 +1653,7 @@
BUG_ON(tcp_skb_pcount(skb) != 1 ||
tcp_skb_pcount(next_skb) != 1);
-
+
/* changing transmit queue under us so clear hints */
clear_all_retrans_hints(tp);
@@ -1702,6 +1702,166 @@
}
}
+/* Added at Simula. Variation of the regular collapse,
+ adapted to support RDB */
+static void tcp_retrans_merge_redundant(struct sock *sk,
+ struct sk_buff *skb,
+ int mss_now)
+{
+ struct tcp_sock *tp = tcp_sk(sk);
+ struct sk_buff *next_skb = skb->next;
+ int skb_size = skb->len;
+ int new_data = 0;
+ int new_data_head = 0;
+ int new_data_frags = 0;
+ int new_frags = 0;
+ int old_headlen = 0;
+
+ int i;
+ int data_frags = 0;
+
+ /* Loop through as many packets as possible
+ * (will create a lot of redundant data, but WHATEVER).
+ * The only packet this MIGHT be critical for is
+ * if this packet is the last in the retrans-queue.
+ *
+ * Make sure that the first skb isnt already in
+ * use by somebody else. */
+
+ if (!skb_cloned(skb)) {
+ /* Iterate through the retransmit queue */
+ for (; (next_skb != (sk)->sk_send_head) &&
+ (next_skb != (struct sk_buff *) &(sk)->sk_write_queue);
+ next_skb = next_skb->next) {
+
+ /* Reset variables */
+ new_frags = 0;
+ data_frags = 0;
+ new_data = TCP_SKB_CB(next_skb)->end_seq - TCP_SKB_CB(skb)->end_seq;
+
+ /* New data will be stored at skb->start_add + some_offset,
+ in other words the last N bytes */
+ new_data_frags = (new_data > next_skb->data_len ?
+ next_skb->data_len : new_data);
+ new_data_head = (new_data > next_skb->data_len ?
+ new_data - skb->data_len : 0);
+
+ /*
+ * 1. Contains the same data
+ * 2. Size
+ * 3. Sack
+ * 4. Window
+ * 5. Cannot merge with a later packet that has linear data
+ * 6. The new number of frags will exceed the limit
+ * 7. Enough tailroom
+ */
+
+ if(new_data <= 0){
+ return;
+ }
+
+ if ((sysctl_tcp_rdb_max_bundle_bytes == 0 && ((skb_size + new_data) > mss_now))
+ || (sysctl_tcp_rdb_max_bundle_bytes > 0 && ((skb_size + new_data) >
+ sysctl_tcp_rdb_max_bundle_bytes))){
+ return;
+ }
+
+ if(TCP_SKB_CB(next_skb)->flags & TCPCB_FLAG_FIN){
+ return;
+ }
+
+ if((TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED) ||
+ (TCP_SKB_CB(next_skb)->sacked & TCPCB_SACKED_ACKED)){
+ return;
+ }
+
+ if(after(TCP_SKB_CB(skb)->end_seq + new_data, tp->snd_una + tp->snd_wnd)){
+ return;
+ }
+
+ if(skb_shinfo(skb)->frag_list || skb_shinfo(skb)->frag_list){
+ return;
+ }
+
+ /* Calculate number of new fragments. Any new data will be
+ stored in the back. */
+ if(skb_is_nonlinear(next_skb)){
+ i = (skb_shinfo(next_skb)->nr_frags == 0 ?
+ 0 : skb_shinfo(next_skb)->nr_frags - 1);
+ for( ; i>=0;i--){
+ if(data_frags + skb_shinfo(next_skb)->frags[i].size ==
+ new_data_frags){
+ new_frags += 1;
+ break;
+ }
+
+ data_frags += skb_shinfo(next_skb)->frags[i].size;
+ new_frags += 1;
+ }
+ }
+
+ /* If dealing with a fragmented skb, only merge
+ with an skb that ONLY contain frags */
+ if(skb_is_nonlinear(skb)){
+
+ /*Due to the way packets are processed, no later data*/
+ if(skb_headlen(next_skb) && new_data_head > 0){
+ return;
+ }
+
+ if(skb_is_nonlinear(next_skb) && (new_data_frags > 0) &&
+ ((skb_shinfo(skb)->nr_frags + new_frags) > MAX_SKB_FRAGS)){
+ return;
+ }
+
+ } else {
+ if(skb_headlen(next_skb) && (new_data_head > (skb->end - skb->tail))){
+ return;
+ }
+ }
+
+ /*Copy linear data. This will only occur if both are linear,
+ or only A is linear*/
+ if(skb_headlen(next_skb) && (new_data_head > 0)){
+ old_headlen = skb_headlen(skb);
+ skb->tail += new_data_head;
+ skb->len += new_data_head;
+
+ /* The new data starts in the linear area,
+ and the correct offset will then be given by
+ removing new_data ammount of bytes from length. */
+ skb_copy_to_linear_data_offset(skb, old_headlen, next_skb->tail -
+ new_data_head, new_data_head);
+ }
+
+ if(skb_is_nonlinear(next_skb) && (new_data_frags > 0)){
+ memcpy(skb_shinfo(skb)->frags + skb_shinfo(skb)->nr_frags,
+ skb_shinfo(next_skb)->frags +
+ (skb_shinfo(next_skb)->nr_frags - new_frags),
+ new_frags*sizeof(skb_frag_t));
+
+ for(i=skb_shinfo(skb)->nr_frags;
+ i < skb_shinfo(skb)->nr_frags + new_frags; i++)
+ get_page(skb_shinfo(skb)->frags[i].page);
+
+ skb_shinfo(skb)->nr_frags += new_frags;
+ skb->data_len += new_data_frags;
+ skb->len += new_data_frags;
+ }
+
+ TCP_SKB_CB(skb)->end_seq += new_data;
+
+ if(skb->ip_summed == CHECKSUM_PARTIAL)
+ skb->csum = CHECKSUM_PARTIAL;
+ else
+ skb->csum = skb_checksum(skb, 0, skb->len, 0);
+
+ skb_size = skb->len;
+ }
+
+ }
+}
+
/* Do a simple retransmit without using the backoff mechanisms in
* tcp_timer. This is used for path mtu discovery.
* The socket is already locked here.
@@ -1756,6 +1916,8 @@
/* This retransmits one SKB. Policy decisions and retransmit queue
* state updates are done by the caller. Returns non-zero if an
* error occurred which prevented the send.
+ * Modified at Simula to support thin stream optimizations
+ * TODO: Update to use new helpers (like tcp_write_queue_next())
*/
int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
{
@@ -1802,10 +1964,21 @@
(skb->len < (cur_mss >> 1)) &&
(tcp_write_queue_next(sk, skb) != tcp_send_head(sk)) &&
(!tcp_skb_is_last(sk, skb)) &&
- (skb_shinfo(skb)->nr_frags == 0 && skb_shinfo(tcp_write_queue_next(sk, skb))->nr_frags == 0) &&
- (tcp_skb_pcount(skb) == 1 && tcp_skb_pcount(tcp_write_queue_next(sk, skb)) == 1) &&
- (sysctl_tcp_retrans_collapse != 0))
+ (skb_shinfo(skb)->nr_frags == 0
+ && skb_shinfo(tcp_write_queue_next(sk, skb))->nr_frags == 0)
+ && (tcp_skb_pcount(skb) == 1
+ && tcp_skb_pcount(tcp_write_queue_next(sk, skb)) == 1)
+ && (sysctl_tcp_retrans_collapse != 0)
+ && !((tp->thin_rdb || sysctl_tcp_force_thin_rdb))) {
tcp_retrans_try_collapse(sk, skb, cur_mss);
+ } else if ((tp->thin_rdb || sysctl_tcp_force_thin_rdb)) {
+ if (!(TCP_SKB_CB(skb)->flags & TCPCB_FLAG_SYN) &&
+ !(TCP_SKB_CB(skb)->flags & TCPCB_FLAG_FIN) &&
+ (skb->next != tcp_send_head(sk)) &&
+ (skb->next != (struct sk_buff *) &sk->sk_write_queue)) {
+ tcp_retrans_merge_redundant(sk, skb, cur_mss);
+ }
+ }
if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk))
return -EHOSTUNREACH; /* Routing failure or similar. */
diff -Nur linux-2.6.23.8.vanilla/net/ipv4/tcp_timer.c linux-2.6.23.8-tcp-thin/net/ipv4/tcp_timer.c
--- linux-2.6.23.8.vanilla/net/ipv4/tcp_timer.c 2007-11-16 19:14:27.000000000 +0100
+++ linux-2.6.23.8-tcp-thin/net/ipv4/tcp_timer.c 2008-07-02 15:17:38.000000000 +0200
@@ -32,6 +32,9 @@
int sysctl_tcp_retries2 __read_mostly = TCP_RETR2;
int sysctl_tcp_orphan_retries __read_mostly;
+/* Added @ Simula */
+int sysctl_tcp_force_thin_rm_expb __read_mostly = TCP_FORCE_THIN_RM_EXPB;
+
static void tcp_write_timer(unsigned long);
static void tcp_delack_timer(unsigned long);
static void tcp_keepalive_timer (unsigned long data);
@@ -368,13 +371,26 @@
*/
icsk->icsk_backoff++;
icsk->icsk_retransmits++;
-
+
out_reset_timer:
- icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
+ /* Added @ Simula removal of exponential backoff for thin streams */
+ if ((tp->thin_rm_expb || sysctl_tcp_force_thin_rm_expb) && tcp_stream_is_thin(tp)) {
+ /* Since 'icsk_backoff' is used to reset timer, set to 0
+ * Recalculate 'icsk_rto' as this might be increased if stream oscillates
+ * between thin and thick, thus the old value might already be too high
+ * compared to the value set by 'tcp_set_rto' in tcp_input.c which resets
+ * the rto without backoff. */
+ icsk->icsk_backoff = 0;
+ icsk->icsk_rto = min(((tp->srtt >> 3) + tp->rttvar), TCP_RTO_MAX);
+ } else {
+ /* Use normal backoff */
+ icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
+ }
+ /* End Simula*/
inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, icsk->icsk_rto, TCP_RTO_MAX);
if (icsk->icsk_retransmits > sysctl_tcp_retries1)
__sk_dst_reset(sk);
-
+
out:;
}
--
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