FD.io VPP  v20.05-21-gb1500e9ff
Vector Packet Processing
tcp_types.h
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1 /*
2  * Copyright (c) 2020 Cisco and/or its affiliates.
3  * Licensed under the Apache License, Version 2.0 (the "License");
4  * you may not use this file except in compliance with the License.
5  * You may obtain a copy of the License at:
6  *
7  * http://www.apache.org/licenses/LICENSE-2.0
8  *
9  * Unless required by applicable law or agreed to in writing, software
10  * distributed under the License is distributed on an "AS IS" BASIS,
11  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12  * See the License for the specific language governing permissions and
13  * limitations under the License.
14  */
15 
16 #ifndef SRC_VNET_TCP_TCP_TYPES_H_
17 #define SRC_VNET_TCP_TCP_TYPES_H_
18 
19 #include <vppinfra/clib.h>
20 #include <vppinfra/rbtree.h>
21 #include <vnet/tcp/tcp_packet.h>
22 #include <vnet/session/transport.h>
24 
25 #define TCP_TICK 0.001 /**< TCP tick period (s) */
26 #define THZ (u32) (1/TCP_TICK) /**< TCP tick frequency */
27 #define TCP_TSTAMP_RESOLUTION TCP_TICK /**< Time stamp resolution */
28 #define TCP_PAWS_IDLE 24 * 24 * 60 * 60 * THZ /**< 24 days */
29 #define TCP_FIB_RECHECK_PERIOD 1 * THZ /**< Recheck every 1s */
30 #define TCP_MAX_OPTION_SPACE 40
31 #define TCP_CC_DATA_SZ 24
32 #define TCP_MAX_GSO_SZ 65536
33 #define TCP_RXT_MAX_BURST 10
34 
35 #define TCP_DUPACK_THRESHOLD 3
36 #define TCP_IW_N_SEGMENTS 10
37 #define TCP_ALWAYS_ACK 1 /**< On/off delayed acks */
38 #define TCP_USE_SACKS 1 /**< Disable only for testing */
39 
40 /** TCP FSM state definitions as per RFC793. */
41 #define foreach_tcp_fsm_state \
42  _(CLOSED, "CLOSED") \
43  _(LISTEN, "LISTEN") \
44  _(SYN_SENT, "SYN_SENT") \
45  _(SYN_RCVD, "SYN_RCVD") \
46  _(ESTABLISHED, "ESTABLISHED") \
47  _(CLOSE_WAIT, "CLOSE_WAIT") \
48  _(FIN_WAIT_1, "FIN_WAIT_1") \
49  _(LAST_ACK, "LAST_ACK") \
50  _(CLOSING, "CLOSING") \
51  _(FIN_WAIT_2, "FIN_WAIT_2") \
52  _(TIME_WAIT, "TIME_WAIT")
53 
54 typedef enum _tcp_state
55 {
56 #define _(sym, str) TCP_STATE_##sym,
58 #undef _
60 } tcp_state_t;
61 
62 /** TCP timers */
63 #define foreach_tcp_timer \
64  _(RETRANSMIT, "RETRANSMIT") \
65  _(DELACK, "DELAYED ACK") \
66  _(PERSIST, "PERSIST") \
67  _(WAITCLOSE, "WAIT CLOSE") \
68  _(RETRANSMIT_SYN, "RETRANSMIT SYN") \
69 
70 typedef enum _tcp_timers
71 {
72 #define _(sym, str) TCP_TIMER_##sym,
74 #undef _
76 } tcp_timers_e;
77 
78 #define TCP_TIMER_HANDLE_INVALID ((u32) ~0)
79 
80 #define TCP_TIMER_TICK 0.1 /**< Timer tick in seconds */
81 #define TCP_TO_TIMER_TICK TCP_TICK*10 /**< Factor for converting
82  ticks to timer ticks */
83 
84 #define TCP_RTO_MAX 60 * THZ /* Min max RTO (60s) as per RFC6298 */
85 #define TCP_RTO_MIN 0.2 * THZ /* Min RTO (200ms) - lower than standard */
86 #define TCP_RTT_MAX 30 * THZ /* 30s (probably too much) */
87 #define TCP_RTO_SYN_RETRIES 3 /* SYN retries without doubling RTO */
88 #define TCP_RTO_INIT 1 * THZ /* Initial retransmit timer */
89 #define TCP_RTO_BOFF_MAX 8 /* Max number of retries before reset */
90 #define TCP_ESTABLISH_TIME (60 * THZ) /* Connection establish timeout */
91 
92 /** Connection configuration flags */
93 #define foreach_tcp_cfg_flag \
94  _(RATE_SAMPLE, "Rate sampling") \
95  _(NO_CSUM_OFFLOAD, "No csum offload") \
96  _(NO_TSO, "TSO off") \
97  _(TSO, "TSO") \
98  _(NO_ENDPOINT,"No endpoint") \
99 
100 typedef enum tcp_cfg_flag_bits_
101 {
102 #define _(sym, str) TCP_CFG_F_##sym##_BIT,
104 #undef _
107 
108 typedef enum tcp_cfg_flag_
109 {
110 #define _(sym, str) TCP_CFG_F_##sym = 1 << TCP_CFG_F_##sym##_BIT,
112 #undef _
115 
116 /** TCP connection flags */
117 #define foreach_tcp_connection_flag \
118  _(SNDACK, "Send ACK") \
119  _(FINSNT, "FIN sent") \
120  _(RECOVERY, "Recovery") \
121  _(FAST_RECOVERY, "Fast Recovery") \
122  _(DCNT_PENDING, "Disconnect pending") \
123  _(HALF_OPEN_DONE, "Half-open completed") \
124  _(FINPNDG, "FIN pending") \
125  _(RXT_PENDING, "Retransmit pending") \
126  _(FRXT_FIRST, "Retransmit first") \
127  _(DEQ_PENDING, "Dequeue pending ") \
128  _(PSH_PENDING, "PSH pending") \
129  _(FINRCVD, "FIN received") \
130  _(ZERO_RWND_SENT, "Zero RWND sent") \
131 
132 typedef enum tcp_connection_flag_bits_
133 {
134 #define _(sym, str) TCP_CONN_##sym##_BIT,
136 #undef _
139 
140 typedef enum tcp_connection_flag_
141 {
142 #define _(sym, str) TCP_CONN_##sym = 1 << TCP_CONN_##sym##_BIT,
144 #undef _
147 
148 #define TCP_SCOREBOARD_TRACE (0)
149 #define TCP_MAX_SACK_BLOCKS 255 /**< Max number of SACK blocks stored */
150 #define TCP_INVALID_SACK_HOLE_INDEX ((u32)~0)
152 typedef struct _scoreboard_trace_elt
153 {
154  u32 start;
155  u32 end;
156  u32 ack;
157  u32 snd_una_max;
158  u32 group;
160 
161 typedef struct _sack_scoreboard_hole
162 {
163  u32 next; /**< Index for next entry in linked list */
164  u32 prev; /**< Index for previous entry in linked list */
165  u32 start; /**< Start sequence number */
166  u32 end; /**< End sequence number */
167  u8 is_lost; /**< Mark hole as lost */
169 
170 typedef struct _sack_scoreboard
171 {
172  sack_scoreboard_hole_t *holes; /**< Pool of holes */
173  u32 head; /**< Index of first entry */
174  u32 tail; /**< Index of last entry */
175  u32 sacked_bytes; /**< Number of bytes sacked in sb */
176  u32 last_sacked_bytes; /**< Number of bytes last sacked */
177  u32 last_bytes_delivered; /**< Sack bytes delivered to app */
178  u32 rxt_sacked; /**< Rxt bytes last delivered */
179  u32 high_sacked; /**< Highest byte sacked (fack) */
180  u32 high_rxt; /**< Highest retransmitted sequence */
181  u32 rescue_rxt; /**< Rescue sequence number */
182  u32 lost_bytes; /**< Bytes lost as per RFC6675 */
183  u32 last_lost_bytes; /**< Number of bytes last lost */
184  u32 cur_rxt_hole; /**< Retransmitting from this hole */
185  u8 is_reneging;
186 
187 #if TCP_SCOREBOARD_TRACE
189 #endif
190 
192 
193 #define TCP_BTS_INVALID_INDEX ((u32)~0)
195 typedef enum tcp_bts_flags_
196 {
197  TCP_BTS_IS_RXT = 1,
201 } __clib_packed tcp_bts_flags_t;
202 
203 typedef struct tcp_bt_sample_
204 {
205  u32 next; /**< Next sample index in list */
206  u32 prev; /**< Previous sample index in list */
207  u32 min_seq; /**< Min seq number in sample */
208  u32 max_seq; /**< Max seq number. Set for rxt samples */
209  u64 delivered; /**< Total delivered bytes for sample */
210  f64 delivered_time; /**< Delivered time when sample taken */
211  f64 tx_time; /**< Transmit time for the burst */
212  f64 first_tx_time; /**< Connection first tx time at tx */
213  u64 tx_in_flight; /**< In flight at tx time */
214  u64 tx_lost; /**< Lost at tx time */
215  tcp_bts_flags_t flags; /**< Sample flag */
217 
218 typedef struct tcp_rate_sample_
219 {
220  u64 prior_delivered; /**< Delivered of sample used for rate, i.e.,
221  total bytes delivered at prior_time */
222  f64 prior_time; /**< Delivered time of sample used for rate */
223  f64 interval_time; /**< Time to ack the bytes delivered */
224  f64 rtt_time; /**< RTT for sample */
225  u64 tx_in_flight; /**< In flight at (re)transmit time */
226  u64 tx_lost; /**< Lost over interval */
227  u32 delivered; /**< Bytes delivered in interval_time */
228  u32 acked_and_sacked; /**< Bytes acked + sacked now */
229  u32 last_lost; /**< Bytes lost now */
230  u32 lost; /**< Number of bytes lost over interval */
231  tcp_bts_flags_t flags; /**< Rate sample flags from bt sample */
233 
234 typedef struct tcp_byte_tracker_
235 {
236  tcp_bt_sample_t *samples; /**< Pool of samples */
237  rb_tree_t sample_lookup; /**< Rbtree for sample lookup by min_seq */
238  u32 head; /**< Head of samples linked list */
239  u32 tail; /**< Tail of samples linked list */
240  u32 last_ooo; /**< Cached last ooo sample */
242 
243 typedef enum _tcp_cc_algorithm_type
244 {
249 
250 typedef struct _tcp_cc_algorithm tcp_cc_algorithm_t;
252 typedef enum _tcp_cc_ack_t
253 {
254  TCP_CC_ACK,
258 
259 typedef enum tcp_cc_event_
260 {
263 
264 /*
265  * As per RFC4898 tcpEStatsStackSoftErrors
266  */
267 typedef struct tcp_errors_
268 {
269  u32 below_data_wnd; /**< All data in seg is below snd_una */
270  u32 above_data_wnd; /**< Some data in segment is above snd_wnd */
271  u32 below_ack_wnd; /**< Acks for data below snd_una */
272  u32 above_ack_wnd; /**< Acks for data not sent */
274 
275 typedef struct _tcp_connection
276 {
277  CLIB_CACHE_LINE_ALIGN_MARK (cacheline0);
278  transport_connection_t connection; /**< Common transport data. First! */
279 
280  u8 state; /**< TCP state as per tcp_state_t */
281  u8 cfg_flags; /**< Connection configuration flags */
282  u16 flags; /**< Connection flags (see tcp_conn_flags_e) */
283  u32 timers[TCP_N_TIMERS]; /**< Timer handles into timer wheel */
284 
285  u64 segs_in; /** RFC4022/4898 tcpHCInSegs/tcpEStatsPerfSegsIn */
286  u64 bytes_in; /** RFC4898 tcpEStatsPerfHCDataOctetsIn */
287  u64 segs_out; /** RFC4898 tcpEStatsPerfSegsOut */
288  u64 bytes_out; /** RFC4898 tcpEStatsPerfHCDataOctetsOut */
289 
290  /** Send sequence variables RFC793 */
291  u32 snd_una; /**< oldest unacknowledged sequence number */
292  u32 snd_una_max; /**< newest unacknowledged sequence number + 1*/
293  u32 snd_wnd; /**< send window */
294  u32 snd_wl1; /**< seq number used for last snd.wnd update */
295  u32 snd_wl2; /**< ack number used for last snd.wnd update */
296  u32 snd_nxt; /**< next seq number to be sent */
297  u16 snd_mss; /**< Effective send max seg (data) size */
298 
299  u64 data_segs_in; /** RFC4898 tcpEStatsPerfDataSegsIn */
300  u64 data_segs_out; /** RFC4898 tcpEStatsPerfDataSegsOut */
301 
302  /** Receive sequence variables RFC793 */
303  u32 rcv_nxt; /**< next sequence number expected */
304  u32 rcv_wnd; /**< receive window we expect */
305 
306  u32 rcv_las; /**< rcv_nxt at last ack sent/rcv_wnd update */
307  u32 iss; /**< initial sent sequence */
308  u32 irs; /**< initial remote sequence */
309 
310  /* Options */
311  u8 snd_opts_len; /**< Tx options len */
312  u8 rcv_wscale; /**< Window scale to advertise to peer */
313  u8 snd_wscale; /**< Window scale to use when sending */
314  u32 tsval_recent; /**< Last timestamp received */
315  u32 tsval_recent_age; /**< When last updated tstamp_recent*/
316  tcp_options_t snd_opts; /**< Tx options for connection */
317  tcp_options_t rcv_opts; /**< Rx options for connection */
318 
319  sack_block_t *snd_sacks; /**< Vector of SACKs to send. XXX Fixed size? */
320  u8 snd_sack_pos; /**< Position in vec of first block to send */
321  sack_block_t *snd_sacks_fl; /**< Vector for building new list */
322  sack_scoreboard_t sack_sb; /**< SACK "scoreboard" that tracks holes */
323 
324  u16 rcv_dupacks; /**< Number of recent DUPACKs received */
325  u32 dupacks_in; /**< RFC4898 tcpEStatsStackDupAcksIn*/
326  u8 pending_dupacks; /**< Number of DUPACKs to be sent */
327  u32 dupacks_out; /**< RFC4898 tcpEStatsPathDupAcksOut */
328 
329  /* Congestion control */
330  u32 cwnd; /**< Congestion window */
331  u32 cwnd_acc_bytes; /**< Bytes accumulated for cwnd increment */
332  u32 ssthresh; /**< Slow-start threshold */
333  u32 prev_ssthresh; /**< ssthresh before congestion */
334  u32 prev_cwnd; /**< ssthresh before congestion */
335  u32 bytes_acked; /**< Bytes acknowledged by current segment */
336  u32 burst_acked; /**< Bytes acknowledged in current burst */
337  u32 snd_rxt_bytes; /**< Retransmitted bytes during current cc event */
338  u32 snd_rxt_ts; /**< Timestamp when first packet is retransmitted */
339  u32 prr_delivered; /**< RFC6937 bytes delivered during current event */
340  u32 prr_start; /**< snd_una when prr starts */
341  u32 rxt_delivered; /**< Rxt bytes delivered during current cc event */
342  u32 rxt_head; /**< snd_una last time we re rxted the head */
343  u32 tsecr_last_ack; /**< Timestamp echoed to us in last healthy ACK */
344  u32 snd_congestion; /**< snd_una_max when congestion is detected */
345  u32 tx_fifo_size; /**< Tx fifo size. Used to constrain cwnd */
346  tcp_cc_algorithm_t *cc_algo; /**< Congestion control algorithm */
347  u8 cc_data[TCP_CC_DATA_SZ]; /**< Congestion control algo private data */
348 
349  u32 fr_occurences; /**< fast-retransmit occurrences RFC4898
350  tcpEStatsStackFastRetran */
351  u32 tr_occurences; /**< timer-retransmit occurrences */
352  u64 bytes_retrans; /**< RFC4898 tcpEStatsPerfOctetsRetrans */
353  u64 segs_retrans; /**< RFC4898 tcpEStatsPerfSegsRetrans*/
354 
355  /* RTT and RTO */
356  u32 rto; /**< Retransmission timeout */
357  u32 rto_boff; /**< Index for RTO backoff */
358  u32 srtt; /**< Smoothed RTT */
359  u32 rttvar; /**< Smoothed mean RTT difference. Approximates variance */
360  u32 rtt_seq; /**< Sequence number for tracked ACK */
361  f64 rtt_ts; /**< Timestamp for tracked ACK */
362  f64 mrtt_us; /**< High precision mrtt from tracked acks */
363 
364  u32 psh_seq; /**< Add psh header for seg that includes this */
365  u32 next_node_index; /**< Can be used to control next node in output */
366  u32 next_node_opaque; /**< Opaque to pass to next node */
367  u32 limited_transmit; /**< snd_nxt when limited transmit starts */
368  u32 sw_if_index; /**< Interface for the connection */
369 
370  /* Delivery rate estimation */
371  u64 delivered; /**< Total bytes delivered to peer */
372  u64 app_limited; /**< Delivered when app-limited detected */
373  f64 delivered_time; /**< Time last bytes were acked */
374  f64 first_tx_time; /**< Send time for recently delivered/sent */
375  u64 lost; /**< Total bytes lost */
376  tcp_byte_tracker_t *bt; /**< Tx byte tracker */
377 
378  tcp_errors_t errors; /**< Soft connection errors */
379 
380  f64 start_ts; /**< Timestamp when connection initialized */
381  u32 last_fib_check; /**< Last time we checked fib route for peer */
382  u16 mss; /**< Our max seg size that includes options */
383  u32 timestamp_delta; /**< Offset for timestamp */
384  u32 ipv6_flow_label; /**< flow label for ipv6 header */
385 
386 #define rst_state snd_wl1
388 
389 /* *INDENT-OFF* */
390 struct _tcp_cc_algorithm
391 {
392  const char *name;
393  uword (*unformat_cfg) (unformat_input_t * input);
394  void (*init) (tcp_connection_t * tc);
395  void (*cleanup) (tcp_connection_t * tc);
396  void (*rcv_ack) (tcp_connection_t * tc, tcp_rate_sample_t *rs);
397  void (*rcv_cong_ack) (tcp_connection_t * tc, tcp_cc_ack_t ack,
398  tcp_rate_sample_t *rs);
399  void (*congestion) (tcp_connection_t * tc);
400  void (*loss) (tcp_connection_t * tc);
401  void (*recovered) (tcp_connection_t * tc);
402  void (*undo_recovery) (tcp_connection_t * tc);
403  void (*event) (tcp_connection_t *tc, tcp_cc_event_t evt);
404  u64 (*get_pacing_rate) (tcp_connection_t *tc);
405 };
406 /* *INDENT-ON* */
407 
408 #define tcp_fastrecovery_on(tc) (tc)->flags |= TCP_CONN_FAST_RECOVERY
409 #define tcp_fastrecovery_off(tc) (tc)->flags &= ~TCP_CONN_FAST_RECOVERY
410 #define tcp_recovery_on(tc) (tc)->flags |= TCP_CONN_RECOVERY
411 #define tcp_recovery_off(tc) (tc)->flags &= ~TCP_CONN_RECOVERY
412 #define tcp_in_fastrecovery(tc) ((tc)->flags & TCP_CONN_FAST_RECOVERY)
413 #define tcp_in_recovery(tc) ((tc)->flags & (TCP_CONN_RECOVERY))
414 #define tcp_in_slowstart(tc) (tc->cwnd < tc->ssthresh)
415 #define tcp_disconnect_pending(tc) ((tc)->flags & TCP_CONN_DCNT_PENDING)
416 #define tcp_disconnect_pending_on(tc) ((tc)->flags |= TCP_CONN_DCNT_PENDING)
417 #define tcp_disconnect_pending_off(tc) ((tc)->flags &= ~TCP_CONN_DCNT_PENDING)
418 #define tcp_fastrecovery_first(tc) ((tc)->flags & TCP_CONN_FRXT_FIRST)
419 #define tcp_fastrecovery_first_on(tc) ((tc)->flags |= TCP_CONN_FRXT_FIRST)
420 #define tcp_fastrecovery_first_off(tc) ((tc)->flags &= ~TCP_CONN_FRXT_FIRST)
422 #define tcp_in_cong_recovery(tc) ((tc)->flags & \
423  (TCP_CONN_FAST_RECOVERY | TCP_CONN_RECOVERY))
424 
425 always_inline void
427 {
428  tc->flags &= ~(TCP_CONN_FAST_RECOVERY | TCP_CONN_RECOVERY);
430 }
431 
432 #define tcp_csum_offload(tc) (!((tc)->cfg_flags & TCP_CFG_F_NO_CSUM_OFFLOAD))
434 #define tcp_zero_rwnd_sent(tc) ((tc)->flags & TCP_CONN_ZERO_RWND_SENT)
435 #define tcp_zero_rwnd_sent_on(tc) (tc)->flags |= TCP_CONN_ZERO_RWND_SENT
436 #define tcp_zero_rwnd_sent_off(tc) (tc)->flags &= ~TCP_CONN_ZERO_RWND_SENT
440 {
441  return (tcp_connection_t *) tconn;
442 }
443 
444 typedef tw_timer_wheel_16t_2w_512sl_t tcp_timer_wheel_t;
446 #endif /* SRC_VNET_TCP_TCP_TYPES_H_ */
447 
448 /*
449  * fd.io coding-style-patch-verification: ON
450  *
451  * Local Variables:
452  * eval: (c-set-style "gnu")
453  * End:
454  */
static vlib_cli_command_t trace
(constructor) VLIB_CLI_COMMAND (trace)
Definition: vlib_api_cli.c:899
#define CLIB_CACHE_LINE_ALIGN_MARK(mark)
Definition: cache.h:60
tcp_cfg_flag_bits_
Definition: tcp_types.h:101
tcp_cc_event_
Definition: tcp_types.h:260
enum tcp_connection_flag_ tcp_connection_flags_e
struct _sack_block sack_block_t
enum tcp_cfg_flag_ tcp_cfg_flags_e
struct tcp_rate_sample_ tcp_rate_sample_t
unsigned long u64
Definition: types.h:89
#define tcp_fastrecovery_first_off(tc)
Definition: tcp_types.h:421
struct _tcp_connection tcp_connection_t
tcp_bts_flags_
Definition: tcp_types.h:196
enum tcp_bts_flags_ tcp_bts_flags_t
enum tcp_connection_flag_bits_ tcp_connection_flag_bits_e
struct _sack_scoreboard sack_scoreboard_t
unsigned char u8
Definition: types.h:56
enum _tcp_cc_ack_t tcp_cc_ack_t
double f64
Definition: types.h:142
#define TCP_CC_DATA_SZ
Definition: tcp_types.h:31
struct _tcp_cc_algorithm tcp_cc_algorithm_t
Definition: tcp_types.h:251
struct tcp_bt_sample_ tcp_bt_sample_t
vl_api_interface_index_t sw_if_index
Definition: gre.api:53
enum tcp_cfg_flag_bits_ tcp_cfg_flag_bits_e
unsigned int u32
Definition: types.h:88
enum _tcp_timers tcp_timers_e
#define foreach_tcp_connection_flag
TCP connection flags.
Definition: tcp_types.h:118
#define foreach_tcp_fsm_state
TCP FSM state definitions as per RFC793.
Definition: tcp_types.h:41
struct _unformat_input_t unformat_input_t
unsigned short u16
Definition: types.h:57
tw_timer_wheel_16t_2w_512sl_t tcp_timer_wheel_t
Definition: tcp_types.h:445
static void cleanup(void)
Definition: client.c:120
#define always_inline
Definition: ipsec.h:28
#define foreach_tcp_cfg_flag
Connection configuration flags.
Definition: tcp_types.h:94
u32 flags
Definition: vhost_user.h:248
struct _transport_connection transport_connection_t
tcp_cfg_flag_
Definition: tcp_types.h:109
string name[64]
Definition: ip.api:44
tcp_connection_flag_bits_
Definition: tcp_types.h:133
static void tcp_cong_recovery_off(tcp_connection_t *tc)
Definition: tcp_types.h:427
static void init(void)
Definition: client.c:105
struct _sack_scoreboard_hole sack_scoreboard_hole_t
u64 uword
Definition: types.h:112
struct tcp_byte_tracker_ tcp_byte_tracker_t
enum _tcp_cc_algorithm_type tcp_cc_algorithm_type_e
enum tcp_cc_event_ tcp_cc_event_t
vl_api_dhcp_client_state_t state
Definition: dhcp.api:201
f64 end
end of the time range
Definition: mactime.api:44
struct _scoreboard_trace_elt scoreboard_trace_elt_t
struct tcp_errors_ tcp_errors_t
tcp_connection_flag_
Definition: tcp_types.h:141
static tcp_connection_t * tcp_get_connection_from_transport(transport_connection_t *tconn)
Definition: tcp_types.h:440
enum _tcp_state tcp_state_t
#define foreach_tcp_timer
TCP timers.
Definition: tcp_types.h:63