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TCP: Add comments to (near) all functions in tcp_output.c v3
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While looking for something else I spent some time adding
one liner comments to the tcp_output.c functions that
didn't have any. That makes the comments more consistent.

I hope I documented everything right.

No code changes.

v2: Incorporated feedback from Ilpo.
v3: Change style of one liner comments, add a few more comments.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>
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Andi Kleen authored and David S. Miller committed Jul 24, 2009
1 parent 40d4e3d commit 67edfef
Showing 1 changed file with 44 additions and 14 deletions.
58 changes: 44 additions & 14 deletions net/ipv4/tcp_output.c
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Original file line number Diff line number Diff line change
Expand Up @@ -59,6 +59,7 @@ int sysctl_tcp_base_mss __read_mostly = 512;
/* By default, RFC2861 behavior. */
int sysctl_tcp_slow_start_after_idle __read_mostly = 1;

/* Account for new data that has been sent to the network. */
static void tcp_event_new_data_sent(struct sock *sk, struct sk_buff *skb)
{
struct tcp_sock *tp = tcp_sk(sk);
Expand Down Expand Up @@ -142,6 +143,7 @@ static void tcp_cwnd_restart(struct sock *sk, struct dst_entry *dst)
tp->snd_cwnd_used = 0;
}

/* Congestion state accounting after a packet has been sent. */
static void tcp_event_data_sent(struct tcp_sock *tp,
struct sk_buff *skb, struct sock *sk)
{
Expand All @@ -161,6 +163,7 @@ static void tcp_event_data_sent(struct tcp_sock *tp,
icsk->icsk_ack.pingpong = 1;
}

/* Account for an ACK we sent. */
static inline void tcp_event_ack_sent(struct sock *sk, unsigned int pkts)
{
tcp_dec_quickack_mode(sk, pkts);
Expand Down Expand Up @@ -276,13 +279,15 @@ static u16 tcp_select_window(struct sock *sk)
return new_win;
}

/* Packet ECN state for a SYN-ACK */
static inline void TCP_ECN_send_synack(struct tcp_sock *tp, struct sk_buff *skb)
{
TCP_SKB_CB(skb)->flags &= ~TCPCB_FLAG_CWR;
if (!(tp->ecn_flags & TCP_ECN_OK))
TCP_SKB_CB(skb)->flags &= ~TCPCB_FLAG_ECE;
}

/* Packet ECN state for a SYN. */
static inline void TCP_ECN_send_syn(struct sock *sk, struct sk_buff *skb)
{
struct tcp_sock *tp = tcp_sk(sk);
Expand All @@ -301,6 +306,9 @@ TCP_ECN_make_synack(struct request_sock *req, struct tcphdr *th)
th->ece = 1;
}

/* Set up ECN state for a packet on a ESTABLISHED socket that is about to
* be sent.
*/
static inline void TCP_ECN_send(struct sock *sk, struct sk_buff *skb,
int tcp_header_len)
{
Expand Down Expand Up @@ -362,7 +370,9 @@ struct tcp_out_options {
__u32 tsval, tsecr; /* need to include OPTION_TS */
};

/* Beware: Something in the Internet is very sensitive to the ordering of
/* Write previously computed TCP options to the packet.
*
* Beware: Something in the Internet is very sensitive to the ordering of
* TCP options, we learned this through the hard way, so be careful here.
* Luckily we can at least blame others for their non-compliance but from
* inter-operatibility perspective it seems that we're somewhat stuck with
Expand Down Expand Up @@ -445,6 +455,9 @@ static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
}
}

/* Compute TCP options for SYN packets. This is not the final
* network wire format yet.
*/
static unsigned tcp_syn_options(struct sock *sk, struct sk_buff *skb,
struct tcp_out_options *opts,
struct tcp_md5sig_key **md5) {
Expand Down Expand Up @@ -493,6 +506,7 @@ static unsigned tcp_syn_options(struct sock *sk, struct sk_buff *skb,
return size;
}

/* Set up TCP options for SYN-ACKs. */
static unsigned tcp_synack_options(struct sock *sk,
struct request_sock *req,
unsigned mss, struct sk_buff *skb,
Expand Down Expand Up @@ -541,6 +555,9 @@ static unsigned tcp_synack_options(struct sock *sk,
return size;
}

/* Compute TCP options for ESTABLISHED sockets. This is not the
* final wire format yet.
*/
static unsigned tcp_established_options(struct sock *sk, struct sk_buff *skb,
struct tcp_out_options *opts,
struct tcp_md5sig_key **md5) {
Expand Down Expand Up @@ -705,7 +722,7 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
return net_xmit_eval(err);
}

/* This routine just queue's the buffer
/* This routine just queues the buffer for sending.
*
* NOTE: probe0 timer is not checked, do not forget tcp_push_pending_frames,
* otherwise socket can stall.
Expand All @@ -722,6 +739,7 @@ static void tcp_queue_skb(struct sock *sk, struct sk_buff *skb)
sk_mem_charge(sk, skb->truesize);
}

/* Initialize TSO segments for a packet. */
static void tcp_set_skb_tso_segs(struct sock *sk, struct sk_buff *skb,
unsigned int mss_now)
{
Expand Down Expand Up @@ -909,6 +927,7 @@ static void __pskb_trim_head(struct sk_buff *skb, int len)
skb->len = skb->data_len;
}

/* Remove acked data from a packet in the transmit queue. */
int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
{
if (skb_cloned(skb) && pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
Expand Down Expand Up @@ -937,7 +956,7 @@ int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
return 0;
}

/* Not accounting for SACKs here. */
/* Calculate MSS. Not accounting for SACKs here. */
int tcp_mtu_to_mss(struct sock *sk, int pmtu)
{
struct tcp_sock *tp = tcp_sk(sk);
Expand Down Expand Up @@ -981,6 +1000,7 @@ int tcp_mss_to_mtu(struct sock *sk, int mss)
return mtu;
}

/* MTU probing init per socket */
void tcp_mtup_init(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);
Expand Down Expand Up @@ -1143,7 +1163,8 @@ static inline unsigned int tcp_cwnd_test(struct tcp_sock *tp,
return 0;
}

/* This must be invoked the first time we consider transmitting
/* Intialize TSO state of a skb.
* This must be invoked the first time we consider transmitting
* SKB onto the wire.
*/
static int tcp_init_tso_segs(struct sock *sk, struct sk_buff *skb,
Expand All @@ -1158,6 +1179,7 @@ static int tcp_init_tso_segs(struct sock *sk, struct sk_buff *skb,
return tso_segs;
}

/* Minshall's variant of the Nagle send check. */
static inline int tcp_minshall_check(const struct tcp_sock *tp)
{
return after(tp->snd_sml, tp->snd_una) &&
Expand Down Expand Up @@ -1242,6 +1264,7 @@ static unsigned int tcp_snd_test(struct sock *sk, struct sk_buff *skb,
return cwnd_quota;
}

/* Test if sending is allowed right now. */
int tcp_may_send_now(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);
Expand Down Expand Up @@ -1378,6 +1401,10 @@ static int tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb)
}

/* Create a new MTU probe if we are ready.
* MTU probe is regularly attempting to increase the path MTU by
* deliberately sending larger packets. This discovers routing
* changes resulting in larger path MTUs.
*
* Returns 0 if we should wait to probe (no cwnd available),
* 1 if a probe was sent,
* -1 otherwise
Expand Down Expand Up @@ -1790,6 +1817,7 @@ static void tcp_collapse_retrans(struct sock *sk, struct sk_buff *skb)
sk_wmem_free_skb(sk, next_skb);
}

/* Check if coalescing SKBs is legal. */
static int tcp_can_collapse(struct sock *sk, struct sk_buff *skb)
{
if (tcp_skb_pcount(skb) > 1)
Expand All @@ -1808,6 +1836,9 @@ static int tcp_can_collapse(struct sock *sk, struct sk_buff *skb)
return 1;
}

/* Collapse packets in the retransmit queue to make to create
* less packets on the wire. This is only done on retransmission.
*/
static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *to,
int space)
{
Expand Down Expand Up @@ -1957,6 +1988,9 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
return err;
}

/* Check if we forward retransmits are possible in the current
* window/congestion state.
*/
static int tcp_can_forward_retransmit(struct sock *sk)
{
const struct inet_connection_sock *icsk = inet_csk(sk);
Expand Down Expand Up @@ -2145,7 +2179,8 @@ void tcp_send_active_reset(struct sock *sk, gfp_t priority)
TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTRSTS);
}

/* WARNING: This routine must only be called when we have already sent
/* Send a crossed SYN-ACK during socket establishment.
* WARNING: This routine must only be called when we have already sent
* a SYN packet that crossed the incoming SYN that caused this routine
* to get called. If this assumption fails then the initial rcv_wnd
* and rcv_wscale values will not be correct.
Expand Down Expand Up @@ -2180,9 +2215,7 @@ int tcp_send_synack(struct sock *sk)
return tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
}

/*
* Prepare a SYN-ACK.
*/
/* Prepare a SYN-ACK. */
struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
struct request_sock *req)
{
Expand Down Expand Up @@ -2269,9 +2302,7 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
return skb;
}

/*
* Do all connect socket setups that can be done AF independent.
*/
/* Do all connect socket setups that can be done AF independent. */
static void tcp_connect_init(struct sock *sk)
{
struct dst_entry *dst = __sk_dst_get(sk);
Expand Down Expand Up @@ -2330,9 +2361,7 @@ static void tcp_connect_init(struct sock *sk)
tcp_clear_retrans(tp);
}

/*
* Build a SYN and send it off.
*/
/* Build a SYN and send it off. */
int tcp_connect(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);
Expand Down Expand Up @@ -2493,6 +2522,7 @@ static int tcp_xmit_probe_skb(struct sock *sk, int urgent)
return tcp_transmit_skb(sk, skb, 0, GFP_ATOMIC);
}

/* Initiate keepalive or window probe from timer. */
int tcp_write_wakeup(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);
Expand Down

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