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DTRACE_TCP(4) Device Drivers Manual DTRACE_TCP(4)

dtrace_tcp — a DTrace provider for tracing events related to the tcp(4) protocol

tcp:::accept-established(pktinfo_t *, csinfo_t *, ipinfo_t *, tcpsinfo_t *, tcpinfo_t *);

tcp:::accept-refused(pktinfo_t *, csinfo_t *, ipinfo_t *, tcpsinfo_t *, tcpinfo_t *);

tcp:::connect-established(pktinfo_t *, csinfo_t *, ipinfo_t *, tcpsinfo_t *, tcpinfo_t *);

tcp:::connect-refused(pktinfo_t *, csinfo_t *, ipinfo_t *, tcpsinfo_t *, tcpinfo_t *);

tcp:::connect-request(pktinfo_t *, csinfo_t *, ipinfo_t *, tcpsinfo_t *, tcpinfo_t *);

tcp:::receive(pktinfo_t *, csinfo_t *, ipinfo_t *, tcpsinfo_t *, tcpinfo_t *);

tcp:::send(pktinfo_t *, csinfo_t *, ipinfo_t *, tcpsinfo_t *, tcpinfo_t *);

tcp:::state-change(void *, csinfo_t *, void *, tcpsinfo_t *, void *, tcplsinfo_t *);

The DTrace tcp provider allows users to trace events in the tcp(4) protocol implementation. This provider is similar to the dtrace_ip(4) and dtrace_udp(4) providers, but additionally contains probes corresponding to protocol events at a level higher than packet reception and transmission. All tcp probes except for () have the same number and type of arguments. The last three arguments are used to describe a TCP segment: the ipinfo_t argument exposes the version-agnostic fields of the IP header, while the tcpinfo_t argument exposes the TCP header, and the tcpsinfo_t argument describes details of the corresponding TCP connection state, if any. Their fields are described in the ARGUMENTS section.

The () probe fires when a remotely-initiated active TCP open succeeds. At this point the new connection is in the ESTABLISHED state, and the probe arguments expose the headers associated with the final ACK of the three-way handshake. The tcp:::accept-refused() probe fires when a SYN arrives on a port without a listening socket. The probe arguments expose the headers associated with the RST to be transmitted to the remote host in response to the SYN segment.

The (), (), and () probes are similar to the ‘accept’ probes, except that they correspond to locally-initiated TCP connections. The tcp:::connect-established() probe fires when the SYN-ACK segment of a three-way handshake is received from the remote host and a final ACK is prepared for transmission. This occurs immediately after the local connection state transitions from SYN-SENT to ESTABLISHED. The probe arguments describe the headers associated with the received SYN-ACK segment. The tcp:::connect-refused() probe fires when the local host receives a RST segment in response to a SYN segment, indicating that the remote host refused to open a connection. The probe arguments describe the IP and TCP headers associated with the received RST segment. The tcp:::connect-request() probe fires as the kernel prepares to transmit the initial SYN segment of a three-way handshake.

The () and () probes fire when the host sends or receives a TCP packet, respectively. As with the dtrace_udp(4) provider, tcp probes fire only for packets sent by or to the local host; forwarded packets are handled in the IP layer and are only visible to the dtrace_ip(4) provider.

The () probe fires upon local TCP connection state transitions. Its first, third and fifth arguments are currently always NULL. Its last argument describes the from-state in the transition, and the to-state can be obtained from args[3]->tcps_state.

The pktinfo_t argument is currently unimplemented and is included for compatibility with other implementations of this provider. Its fields are:

uinptr_t pkt_addr
Always set to 0.

The csinfo_t argument is currently unimplemented and is included for compatibility with other implementations of this provider. Its fields are:

uintptr_t cs_addr
Always set to 0.
uint64_t cs_cid
A pointer to the struct inpcb for this packet, or NULL.
pid_t cs_pid
Always set to 0.

The ipinfo_t type is a version-agnostic representation of fields from an IP header. Its fields are described in the dtrace_ip(4) manual page.

The tcpsinfo_t type is used to provide a stable representation of TCP connection state. Some tcp probes, such as (), fire in a context where there is no TCP connection; this argument is NULL in that case. Its fields are:

uintptr_t tcps_addr
The address of the corresponding TCP control block. This is currently a pointer to a struct tcpcb.
int tcps_local
A boolean indicating whether the connection is local to the host. Currently unimplemented and always set to -1.
int tcps_active
A boolean indicating whether the connection was initiated by the local host. Currently unimplemented and always set to -1.
uint16_t tcps_lport
Local TCP port.
uint16_t tcps_rport
Remote TCP port.
string tcps_laddr
Local address.
string tcps_raddr
Remote address.
int32_t tcps_state
Current TCP state. The valid TCP state values are given by the constants prefixed with ‘TCPS_’ in /usr/lib/dtrace/tcp.d.
uint32_t tcps_iss
Initial send sequence number.
uint32_t tcps_suna
Initial sequence number of sent but unacknowledged data.
uint32_t tcps_snxt
Next sequence number for send.
uint32_t tcps_rack
Sequence number of received and acknowledged data.
uint32_t tcps_rnxt
Next expected sequence number for receive.
u_long tcps_swnd
TCP send window size.
int32_t tcps_snd_ws
Window scaling factor for the TCP send window.
u_long tcps_rwnd
TCP receive window size.
int32_t tcps_rcv_ws
Window scaling factor for the TCP receive window.
u_long tcps_cwnd
TCP congestion window size.
u_long tcps_cwnd_ssthresh
Congestion window threshold at which slow start ends and congestion avoidance begins.
uint32_t tcps_sack_fack
Last sequence number selectively acknowledged by the receiver.
uint32_t tcps_sack_snxt
Next selectively acknowledge sequence number at which to begin retransmitting.
uint32_t tcps_rto
Round-trip timeout, in milliseconds.
uint32_t tcps_mss
Maximum segment size.
int tcps_retransmit
A boolean indicating that the local sender is retransmitting data.
int tcps_srtt
Smoothed round-trip time.

The tcpinfo_t type exposes the fields in a TCP segment header in host order. Its fields are:

uint16_t tcp_sport
Source TCP port.
uint16_t tcp_dport
Destination TCP port.
uint32_t tcp_seq
Sequence number.
uint32_t tcp_ack
Acknowledgement number.
uint8_t tcp_offset
Data offset, in bytes.
uint8_t tcp_flags
TCP flags.
uint16_t tcp_window
TCP window size.
uint16_t tcp_checksum
Checksum.
uint16_t tcp_urgent
Urgent data pointer.
struct tcphdr *tcp_hdr
A pointer to the raw TCP header.

The tcplsinfo_t type is used by the () probe to provide the from-state of a transition. Its fields are:

int32_t tcps_state
A TCP state. The valid TCP state values are given by the constants prefixed with ‘TCPS_’ in /usr/lib/dtrace/tcp.d.

/usr/lib/dtrace/tcp.d
DTrace type and translator definitions for the tcp provider.

The following script logs TCP segments in real time:

#pragma D option quiet
#pragma D option switchrate=10hz

dtrace:::BEGIN
{
        printf(" %3s %15s:%-5s      %15s:%-5s %6s  %s\n", "CPU",
            "LADDR", "LPORT", "RADDR", "RPORT", "BYTES", "FLAGS");
}

tcp:::send
{
        this->length = args[2]->ip_plength - args[4]->tcp_offset;
        printf(" %3d %16s:%-5d -> %16s:%-5d %6d  (", cpu, args[2]->ip_saddr,
            args[4]->tcp_sport, args[2]->ip_daddr, args[4]->tcp_dport,
            this->length);
        printf("%s", args[4]->tcp_flags & TH_FIN ? "FIN|" : "");
        printf("%s", args[4]->tcp_flags & TH_SYN ? "SYN|" : "");
        printf("%s", args[4]->tcp_flags & TH_RST ? "RST|" : "");
        printf("%s", args[4]->tcp_flags & TH_PUSH ? "PUSH|" : "");
        printf("%s", args[4]->tcp_flags & TH_ACK ? "ACK|" : "");
        printf("%s", args[4]->tcp_flags & TH_URG ? "URG|" : "");
        printf("%s", args[4]->tcp_flags == 0 ? "null " : "");
        printf(";
}

tcp:::receive
{
        this->length = args[2]->ip_plength - args[4]->tcp_offset;
        printf(" %3d %16s:%-5d <- %16s:%-5d %6d  (", cpu,
            args[2]->ip_daddr, args[4]->tcp_dport, args[2]->ip_saddr,
            args[4]->tcp_sport, this->length);
        printf("%s", args[4]->tcp_flags & TH_FIN ? "FIN|" : "");
        printf("%s", args[4]->tcp_flags & TH_SYN ? "SYN|" : "");
        printf("%s", args[4]->tcp_flags & TH_RST ? "RST|" : "");
        printf("%s", args[4]->tcp_flags & TH_PUSH ? "PUSH|" : "");
        printf("%s", args[4]->tcp_flags & TH_ACK ? "ACK|" : "");
        printf("%s", args[4]->tcp_flags & TH_URG ? "URG|" : "");
        printf("%s", args[4]->tcp_flags == 0 ? "null " : "");
        printf(";
}
The following script logs TCP connection state changes as they occur: 
#pragma D option quiet
#pragma D option switchrate=25hz

int last[int];

dtrace:::BEGIN
{
        printf("   %12s %-20s    %-20s %s\n",
            "DELTA(us)", "OLD", "NEW", "TIMESTAMP");
}

tcp:::state-change
{
        this->elapsed = (timestamp - last[args[1]->cs_cid]) / 1000;
        printf("   %12d %-20s -> %-20s %d\n", this->elapsed,
            tcp_state_string[args[5]->tcps_state],
            tcp_state_string[args[3]->tcps_state], timestamp);
        last[args[1]->cs_cid] = timestamp;
}

tcp:::state-change
/last[args[1]->cs_cid] == 0/
{
        printf("   %12s %-20s -> %-20s %d\n", "-",
            tcp_state_string[args[5]->tcps_state],
            tcp_state_string[args[3]->tcps_state], timestamp);
        last[args[1]->cs_cid] = timestamp;
}

This provider is compatible with the tcp provider in Solaris.

dtrace(1), dtrace_ip(4), dtrace_sctp(4), dtrace_udp(4), dtrace_udplite(4), tcp(4), SDT(9)

The tcp provider first appeared in FreeBSD 10.0.

This manual page was written by Mark Johnston <markj@FreeBSD.org>.

The tcps_local and tcps_active fields of tcpsinfo_t are not filled in by the translator.

August 1, 2018 FreeBSD-12.0