TCP: Add comments to (near) all functions in tcp_output.c v3
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>
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 5bdf08d..c464892 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -59,6 +59,7 @@
/* 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);
@@ -142,6 +143,7 @@
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)
{
@@ -161,6 +163,7 @@
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);
@@ -276,6 +279,7 @@
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;
@@ -283,6 +287,7 @@
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);
@@ -301,6 +306,9 @@
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)
{
@@ -362,7 +370,9 @@
__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
@@ -445,6 +455,9 @@
}
}
+/* 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) {
@@ -493,6 +506,7 @@
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,
@@ -541,6 +555,9 @@
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) {
@@ -705,7 +722,7 @@
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.
@@ -722,6 +739,7 @@
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)
{
@@ -909,6 +927,7 @@
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))
@@ -937,7 +956,7 @@
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);
@@ -981,6 +1000,7 @@
return mtu;
}
+/* MTU probing init per socket */
void tcp_mtup_init(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);
@@ -1143,7 +1163,8 @@
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,
@@ -1158,6 +1179,7 @@
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) &&
@@ -1242,6 +1264,7 @@
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);
@@ -1378,6 +1401,10 @@
}
/* 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
@@ -1790,6 +1817,7 @@
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)
@@ -1808,6 +1836,9 @@
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)
{
@@ -1957,6 +1988,9 @@
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);
@@ -2145,7 +2179,8 @@
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.
@@ -2180,9 +2215,7 @@
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)
{
@@ -2269,9 +2302,7 @@
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);
@@ -2330,9 +2361,7 @@
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);
@@ -2493,6 +2522,7 @@
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);