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re-work the fast retransmission code to follow algorithm from TCP/IP

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Illustrated
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kieranm 2009-10-29 15:48:57 +00:00
parent 6d73f82f41
commit dee1d82c11
4 changed files with 94 additions and 42 deletions
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3
CHANGELOG
View File

@ -45,6 +45,9 @@ HISTORY
to a human-readable string.
++ Bugfixes:
2009-10-28: Kieran Mansley
* tcp_in.c, tcp_out.c, tcp.h: re-work the fast retransmission code
to follow algorithm from TCP/IP Illustrated
2009-10-27: Kieran Mansley
* tcp_in.c: fix BUG#27445: grow cwnd with every duplicate ACK

92
src/core/tcp_in.c
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@ -763,6 +763,7 @@ tcp_receive(struct tcp_pcb *pcb)
u32_t right_wnd_edge;
u16_t new_tot_len;
u8_t accepted_inseq = 0;
int found_dupack = 0;
if (flags & TCP_ACK) {
right_wnd_edge = pcb->snd_wnd + pcb->snd_wl2;
@ -789,55 +790,62 @@ tcp_receive(struct tcp_pcb *pcb)
#endif /* TCP_WND_DEBUG */
}
if (pcb->lastack == ackno) {
/* (From Stevens TCP/IP Illustrated Vol II, p970.) Its only a
* duplicate ack if:
* 1) It doesn't ACK new data
* 2) length of received packet is zero (i.e. no payload)
* 3) the advertised window hasn't changed
* 4) There is outstanding unacknowledged data (retransmission timer running)
* 5) The ACK is == biggest ACK sequence number so far seen (snd_una)
*
* If it passes all five, should process as a dupack:
* a) dupacks < 3: do nothing
* b) dupacks == 3: fast retransmit
* c) dupacks > 3: increase cwnd
*
* If it only passes 1-3, should reset dupack counter (and add to
* stats, which we don't do in lwIP)
*
* If it only passes 1, should reset dupack counter
*
*/
/* Clause 1 */
if (TCP_SEQ_LEQ(ackno, pcb->lastack)) {
pcb->acked = 0;
if (pcb->snd_wl2 + pcb->snd_wnd == right_wnd_edge){
++pcb->dupacks;
if (pcb->dupacks >= 3) {
if (!(pcb->flags & TF_INFR) && pcb->unacked != NULL) {
/* This is fast retransmit. Retransmit the first unacked segment. */
LWIP_DEBUGF(TCP_FR_DEBUG, ("tcp_receive: dupacks %"U16_F" (%"U32_F"), fast retransmit %"U32_F"\n",
(u16_t)pcb->dupacks, pcb->lastack,
ntohl(pcb->unacked->tcphdr->seqno)));
tcp_rexmit(pcb);
/* Set ssthresh to max (FlightSize / 2, 2*SMSS) */
/*pcb->ssthresh = LWIP_MAX((pcb->snd_max -
pcb->lastack) / 2,
2 * pcb->mss);*/
/* Set ssthresh to half of the minimum of the current cwnd and the advertised window */
if (pcb->cwnd > pcb->snd_wnd)
pcb->ssthresh = pcb->snd_wnd / 2;
else
pcb->ssthresh = pcb->cwnd / 2;
/* The minimum value for ssthresh should be 2 MSS */
if (pcb->ssthresh < 2*pcb->mss) {
LWIP_DEBUGF(TCP_FR_DEBUG, ("tcp_receive: The minimum value for ssthresh %"U16_F" should be min 2 mss %"U16_F"...\n", pcb->ssthresh, 2*pcb->mss));
pcb->ssthresh = 2*pcb->mss;
}
pcb->cwnd = pcb->ssthresh + 3 * pcb->mss;
pcb->flags |= TF_INFR;
} else {
/* Inflate the congestion window, but not if it means that
the value overflows. */
if ((u16_t)(pcb->cwnd + pcb->mss) > pcb->cwnd) {
pcb->cwnd += pcb->mss;
/* Clause 2 */
if (tcplen == 0) {
/* Clause 3 */
if (pcb->snd_wl2 + pcb->snd_wnd == right_wnd_edge){
/* Clause 4 */
if (pcb->rtime >= 0) {
/* Clause 5 */
if (pcb->lastack == ackno) {
found_dupack = 1;
if (pcb->dupacks + 1 > pcb->dupacks)
++pcb->dupacks;
if (pcb->dupacks > 3) {
/* Inflate the congestion window, but not if it means that
the value overflows. */
if ((u16_t)(pcb->cwnd + pcb->mss) > pcb->cwnd) {
pcb->cwnd += pcb->mss;
}
} else if (pcb->dupacks == 3) {
/* Do fast retransmit */
tcp_rexmit_fast(pcb);
}
}
}
}
if (pcb->unacked == NULL && pcb->unsent == NULL)
pcb->dupacks = 0;
} else {
LWIP_DEBUGF(TCP_FR_DEBUG, ("tcp_receive: dupack averted %"U32_F" %"U32_F"\n",
pcb->snd_wl2 + pcb->snd_wnd, right_wnd_edge));
}
/* If Clause (1) or more is true, but not a duplicate ack, reset
* count of consecutive duplicate acks */
if (!found_dupack) {
pcb->dupacks = 0;
}
} else if (TCP_SEQ_BETWEEN(ackno, pcb->lastack+1, pcb->snd_nxt)){
/* We come here when the ACK acknowledges new data. */
/* Reset the "IN Fast Retransmit" flag, since we are no longer
in fast retransmit. Also reset the congestion window to the
slow start threshold. */

40
src/core/tcp_out.c
View File

@ -858,6 +858,46 @@ tcp_rexmit(struct tcp_pcb *pcb)
tcp_output(pcb);
}
/**
* Handle retransmission after three dupacks received
*
* @param pcb the tcp_pcb for which to retransmit the first unacked segment
*/
void
tcp_rexmit_fast(struct tcp_pcb *pcb)
{
if (pcb->unacked != NULL && !(pcb->flags & TF_INFR)) {
/* This is fast retransmit. Retransmit the first unacked segment. */
LWIP_DEBUGF(TCP_FR_DEBUG,
("tcp_receive: dupacks %"U16_F" (%"U32_F
"), fast retransmit %"U32_F"\n",
(u16_t)pcb->dupacks, pcb->lastack,
ntohl(pcb->unacked->tcphdr->seqno)));
tcp_rexmit(pcb);
/* Set ssthresh to half of the minimum of the current
* cwnd and the advertised window */
if (pcb->cwnd > pcb->snd_wnd)
pcb->ssthresh = pcb->snd_wnd / 2;
else
pcb->ssthresh = pcb->cwnd / 2;
/* The minimum value for ssthresh should be 2 MSS */
if (pcb->ssthresh < 2*pcb->mss) {
LWIP_DEBUGF(TCP_FR_DEBUG,
("tcp_receive: The minimum value for ssthresh %"U16_F
" should be min 2 mss %"U16_F"...\n",
pcb->ssthresh, 2*pcb->mss));
pcb->ssthresh = 2*pcb->mss;
}
pcb->cwnd = pcb->ssthresh + 3 * pcb->mss;
pcb->flags |= TF_INFR;
}
}
/**
* Send keepalive packets to keep a connection active although
* no data is sent over it.

1
src/include/lwip/tcp.h
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@ -125,6 +125,7 @@ void tcp_input (struct pbuf *p, struct netif *inp);
err_t tcp_output (struct tcp_pcb *pcb);
void tcp_rexmit (struct tcp_pcb *pcb);
void tcp_rexmit_rto (struct tcp_pcb *pcb);
void tcp_rexmit_fast (struct tcp_pcb *pcb);
u32_t tcp_update_rcv_ann_wnd(struct tcp_pcb *pcb);
/**