tcp: commonize check for more data to send, style changes

Use SEQ_SUB instead of a plain subtraction, for an implict
type conversion and prevention of a possible overflow.
Use curly brackets in stacked if statements throughout.
Use of the ? operator to enhance readability when clearing
the FIN flag in tcp_output().

None of the above change the function.

Reviewed By: tuexen, cc, #transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D43539

tcp: AccECN access ACE field by shifting bits

Shifting bits is quicker than checking header flag bits
one by one. Also improve readability by the use of switch
statements.

No change in behaviour.

Reviewed By: glebius, tuexen, #transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D43560

sys: Remove ancient SCCS tags.

Remove ancient SCCS tags from the tree, automated scripting, with two
minor fixup to keep things compiling. All the common forms in the tree
were removed with a perl script.

Sponsored by: Netflix

sys: Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

Update various sys/netinet source files to conform with the style(9)
guide on how to label FALLTHOUGH in switch statements.

No functional chance.

Reviewed By: tuexen, cc, #transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D40622

tcp: retire TCPDEBUG

This subsystem is superseded by modern debugging facilities,
e.g. DTrace probes and TCP black box logging.

We intentionally leave SO_DEBUG in place, as many utilities may
set it on a socket. Also the tcp::debug DTrace probes look at
this flag on a socket.

Reviewed by: gnn, tuexen
Discussed with: rscheff, rrs, jtl
Differential revision: https://reviews.freebsd.org/D37694

tcp: account sent/received IP ECN markings independently

Have tcpstats (netstat -s) differentiate between received and sent
ECN-marked packets. Also account for IP ECN bits (on TCP packets)
even when the tcp session has not negotiated ECN support.

Event: IETF 115 Hackathon
Reviewed By: glebius, tuexen, #transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D37314

tcp: allow packets to be marked as ECT1 instead of ECT0

This adds the capability for a modular congestion control
to select which variant of ECN-capable-transport it wants to use
when sending out elegible segments. As an initial CC to utilize
this, DCTCP was selected.

Event: IETF 115 Hackathon
Reviewed By: tuexen, #transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D24869

tcp: Move sysctl OIDs related to ECN to tcp_ecn.c

Keep all ECN related code in (mostly) one place.

No functional change.

Event: IETF 115 Hackathon
Reviewed By: tuexen, #transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D37285

tcp: add conservative d.cep accounting algorithm

Accurate ECN asks to conservatively estimate, when the
ACE counter may have wrapped due to a single ACK covering a larger
number of segments. This is described in Annex A.2 of the
accurate-ecn draft.

Event: IETF 115 Hackathon
Reviewed By: tuexen, #transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D37281

tcp: add AccECN CE packet counters to tcpinfo

Provide diagnostics information around AccECN into
the tcpinfo struct.

Event: IETF 115 Hackathon
Reviewed By: tuexen, #transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D37280

tcp: ECN preparations for ECN++, AccECN (tcp_respond)

tcp_respond is another function to build a tcp control packet
quickly. With ECN++ and AccECN, both the IP ECN header, and
the TCP ECN flags are supposed to reflect the correct state.

Also ensure that on receiving multiple ECN SYN-ACKs, the
responses triggered will reflect the latest state.

Reviewed By: tuexen, #transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D36973

tcp: remove unnecessary include of tcp6_var.h

Reviewed by: rscheff, melifaro
Differential revision: https://reviews.freebsd.org/D36725

tcp: correct simultaneous SYN ECN reaction in RFC3168 mode.

Ensure that an RFC3168 ECN reaction only occurs on non-SYN
segments.

Reviewed By: tuexen, #transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D36867

tcp: Functional implementation of Accurate ECN

The AccECN handshake and TCP header flags are supported,
no support yet for the AccECN option. This minimalistic
implementation is sufficient to support DCTCP while
dramatically cutting the number of ACKs, and provide ECN
response from the receiver to the CC modules.

Reviewed By: #transport, #manpages, rrs, pauamma
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D21011

tcp: Restore correct ECT marking behavior on SACK retransmissions

While coalescing all ECN-related code into new common source files,
the flag to deal with SACK retransmissions was skipped. This leads
to non-compliant ECT-marking of SACK retransmissions, as well as
the premature sending of other TCP ECN flags (CWR).

Reviewed By: rrs, #transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D34376

tcp: Add/update AccECN related statistics and numbers

Reserve couters in the tcps struct in preparation
for AccECN, extend the debugging output for TF2
flags, optimize the syncache flags from individual
bits to a codepoint for the specifc ECN handshake.

This is in preparation of AccECN.

No functional chance except for extended debug
output capabilities.

Reviewed By: #transport, rrs
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D34161

tcp: use TCPSTAT_INC in kernel ecn functions

Incorrectly used KMOD_ marco in static kernel ECN functions.

Both eventually resolve to counter_s64_add(), but better
use the correct macros.

Reviewed By: tuexen, #transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D34181

tcp: move ECN handling code to a common file

Reduce the burden to maintain correct and
extensible ECN related code across multiple
stacks and codepaths.

Formally no functional change.

Incidentially this establishes correct
ECN operation in one instance.

Reviewed By: rrs, #transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D34162