output.c

一种在UDP协议中实现了拥赛控制和重传机制的协议

	 */
	skb_set_owner_w(nskb, sk);

	/* Build the SCTP header.  */
	sh = (struct sctphdr *)skb_push(nskb, sizeof(struct sctphdr));
	sh->source = htons(packet->source_port);
	sh->dest   = htons(packet->destination_port);

	/* From 6.8 Adler-32 Checksum Calculation:
	 * After the packet is constructed (containing the SCTP common
	 * header and one or more control or DATA chunks), the
	 * transmitter shall:
	 *
	 * 1) Fill in the proper Verification Tag in the SCTP common
	 *    header and initialize the checksum field to 0's.
	 */
	sh->vtag     = htonl(packet->vtag);
	sh->checksum = 0;

	/* 2) Calculate the Adler-32 checksum of the whole packet,
	 *    including the SCTP common header and all the
	 *    chunks.
	 *
	 * Note: Adler-32 is no longer applicable, as has been replaced
	 * by CRC32-C as described in <draft-ietf-tsvwg-sctpcsum-02.txt>.
	 */
	crc32 = sctp_start_cksum((__u8 *)sh, sizeof(struct sctphdr));

	/**
	 * 6.10 Bundling
	 *
	 *    An endpoint bundles chunks by simply including multiple
	 *    chunks in one outbound SCTP packet.  ...
	 */

	/**
	 * 3.2  Chunk Field Descriptions
	 *
	 * The total length of a chunk (including Type, Length and
	 * Value fields) MUST be a multiple of 4 bytes.  If the length
	 * of the chunk is not a multiple of 4 bytes, the sender MUST
	 * pad the chunk with all zero bytes and this padding is not
	 * included in the chunk length field.  The sender should
	 * never pad with more than 3 bytes.
	 *
	 * [This whole comment explains WORD_ROUND() below.]
	 */
	SCTP_DEBUG_PRINTK("***sctp_transmit_packet***\n");
	while ((chunk = (struct sctp_chunk *)__skb_dequeue(&packet->chunks)) != NULL) {
		if (sctp_chunk_is_data(chunk)) {

			if (!chunk->has_tsn) {
				sctp_chunk_assign_ssn(chunk);
				sctp_chunk_assign_tsn(chunk);

			/* 6.3.1 C4) When data is in flight and when allowed
			 * by rule C5, a new RTT measurement MUST be made each
			 * round trip.  Furthermore, new RTT measurements
			 * SHOULD be made no more than once per round-trip
			 * for a given destination transport address.
			 */

				if (!tp->rto_pending) {
					chunk->rtt_in_progress = 1;
					tp->rto_pending = 1;
				}
			} else
				chunk->resent = 1;

			chunk->sent_at = jiffies;
			has_data = 1;
		}

		padding = WORD_ROUND(chunk->skb->len) - chunk->skb->len;
		if (padding)
			memset(skb_put(chunk->skb, padding), 0, padding);

		crc32 = sctp_update_copy_cksum(skb_put(nskb, chunk->skb->len),
					       chunk->skb->data,
					       chunk->skb->len, crc32);

		SCTP_DEBUG_PRINTK("%s %p[%s] %s 0x%x, %s %d, %s %d, %s %d\n",
				  "*** Chunk", chunk,
				  sctp_cname(SCTP_ST_CHUNK(
					  chunk->chunk_hdr->type)),
				  chunk->has_tsn ? "TSN" : "No TSN",
				  chunk->has_tsn ?
				  ntohl(chunk->subh.data_hdr->tsn) : 0,
				  "length", ntohs(chunk->chunk_hdr->length),
				  "chunk->skb->len", chunk->skb->len,
				  "rtt_in_progress", chunk->rtt_in_progress);

		/*
		 * If this is a control chunk, this is our last
		 * reference. Free data chunks after they've been
		 * acknowledged or have failed.
		 */
		if (!sctp_chunk_is_data(chunk))
    			sctp_chunk_free(chunk);
	}

	/* Perform final transformation on checksum. */
	crc32 = sctp_end_cksum(crc32);

	/* 3) Put the resultant value into the checksum field in the
	 *    common header, and leave the rest of the bits unchanged.
	 */
	sh->checksum = htonl(crc32);

	/* IP layer ECN support
	 * From RFC 2481
	 *  "The ECN-Capable Transport (ECT) bit would be set by the
	 *   data sender to indicate that the end-points of the
	 *   transport protocol are ECN-capable."
	 *
	 * Now setting the ECT bit all the time, as it should not cause
	 * any problems protocol-wise even if our peer ignores it.
	 *
	 * Note: The works for IPv6 layer checks this bit too later
	 * in transmission.  See IP6_ECN_flow_xmit().
	 */
	INET_ECN_xmit(nskb->sk);

	/* Set up the IP options.  */
	/* BUG: not implemented
	 * For v4 this all lives somewhere in sk->sk_opt...
	 */

	/* Dump that on IP!  */
	if (asoc && asoc->peer.last_sent_to != tp) {
		/* Considering the multiple CPU scenario, this is a
		 * "correcter" place for last_sent_to.  --xguo
		 */
		asoc->peer.last_sent_to = tp;
	}

	if (has_data) {
		struct timer_list *timer;
		unsigned long timeout;

		tp->last_time_used = jiffies;

		/* Restart the AUTOCLOSE timer when sending data. */
		if (sctp_state(asoc, ESTABLISHED) && asoc->autoclose) {
			timer = &asoc->timers[SCTP_EVENT_TIMEOUT_AUTOCLOSE];
			timeout = asoc->timeouts[SCTP_EVENT_TIMEOUT_AUTOCLOSE];

			if (!mod_timer(timer, jiffies + timeout))
				sctp_association_hold(asoc);
		}
	}

	dst = tp->dst;
	/* The 'obsolete' field of dst is set to 2 when a dst is freed. */
	if (!dst || (dst->obsolete > 1)) {
		dst_release(dst);
		sctp_transport_route(tp, NULL, sctp_sk(sk));
		sctp_assoc_sync_pmtu(asoc);
	}

	nskb->dst = dst_clone(tp->dst);
	if (!nskb->dst)
		goto no_route;

	SCTP_DEBUG_PRINTK("***sctp_transmit_packet*** skb len %d\n",
			  nskb->len);

	(*tp->af_specific->sctp_xmit)(nskb, tp, packet->ipfragok);

out:
	packet->size = packet->overhead;
	return err;
no_route:
	kfree_skb(nskb);
	IP_INC_STATS_BH(IPSTATS_MIB_OUTNOROUTES);

	/* FIXME: Returning the 'err' will effect all the associations
	 * associated with a socket, although only one of the paths of the
	 * association is unreachable.
	 * The real failure of a transport or association can be passed on
	 * to the user via notifications. So setting this error may not be
	 * required.
	 */
	 /* err = -EHOSTUNREACH; */
err:
	/* Control chunks are unreliable so just drop them.  DATA chunks
	 * will get resent or dropped later.
	 */

	while ((chunk = (struct sctp_chunk *)__skb_dequeue(&packet->chunks)) != NULL) {
		if (!sctp_chunk_is_data(chunk))
    			sctp_chunk_free(chunk);
	}
	goto out;
nomem:
	err = -ENOMEM;
	goto err;
}

/********************************************************************
 * 2nd Level Abstractions
 ********************************************************************/

/* This private function handles the specifics of appending DATA chunks.  */
static sctp_xmit_t sctp_packet_append_data(struct sctp_packet *packet,
					   struct sctp_chunk *chunk)
{
	sctp_xmit_t retval = SCTP_XMIT_OK;
	size_t datasize, rwnd, inflight;
	struct sctp_transport *transport = packet->transport;
	__u32 max_burst_bytes;
	struct sctp_association *asoc = transport->asoc;
	struct sctp_sock *sp = sctp_sk(asoc->base.sk);
	struct sctp_outq *q = &asoc->outqueue;

	/* RFC 2960 6.1  Transmission of DATA Chunks
	 *
	 * A) At any given time, the data sender MUST NOT transmit new data to
	 * any destination transport address if its peer's rwnd indicates
	 * that the peer has no buffer space (i.e. rwnd is 0, see Section
	 * 6.2.1).  However, regardless of the value of rwnd (including if it
	 * is 0), the data sender can always have one DATA chunk in flight to
	 * the receiver if allowed by cwnd (see rule B below).  This rule
	 * allows the sender to probe for a change in rwnd that the sender
	 * missed due to the SACK having been lost in transit from the data
	 * receiver to the data sender.
	 */

	rwnd = asoc->peer.rwnd;
	inflight = asoc->outqueue.outstanding_bytes;

	datasize = sctp_data_size(chunk);

	if (datasize > rwnd) {
		if (inflight > 0) {
			/* We have (at least) one data chunk in flight,
			 * so we can't fall back to rule 6.1 B).
			 */
			retval = SCTP_XMIT_RWND_FULL;
			goto finish;
		}
	}

	/* sctpimpguide-05 2.14.2
	 * D) When the time comes for the sender to
	 * transmit new DATA chunks, the protocol parameter Max.Burst MUST
	 * first be applied to limit how many new DATA chunks may be sent.
	 * The limit is applied by adjusting cwnd as follows:
	 * 	if ((flightsize + Max.Burst * MTU) < cwnd)
	 *		cwnd = flightsize + Max.Burst * MTU
	 */
	max_burst_bytes = asoc->max_burst * asoc->pmtu;
	if ((transport->flight_size + max_burst_bytes) < transport->cwnd) {
		transport->cwnd = transport->flight_size + max_burst_bytes;
		SCTP_DEBUG_PRINTK("%s: cwnd limited by max_burst: "
				  "transport: %p, cwnd: %d, "
				  "ssthresh: %d, flight_size: %d, "
				  "pba: %d\n",
				  __FUNCTION__, transport,
				  transport->cwnd,
				  transport->ssthresh,
				  transport->flight_size,
				  transport->partial_bytes_acked);
	}

	/* RFC 2960 6.1  Transmission of DATA Chunks
	 *
	 * B) At any given time, the sender MUST NOT transmit new data
	 * to a given transport address if it has cwnd or more bytes
	 * of data outstanding to that transport address.
	 */
	/* RFC 7.2.4 & the Implementers Guide 2.8.
	 *
	 * 3) ...
	 *    When a Fast Retransmit is being performed the sender SHOULD
	 *    ignore the value of cwnd and SHOULD NOT delay retransmission.
	 */
	if (!chunk->fast_retransmit)
		if (transport->flight_size >= transport->cwnd) {
			retval = SCTP_XMIT_RWND_FULL;
			goto finish;
		}

	/* Nagle's algorithm to solve small-packet problem:
	 * Inhibit the sending of new chunks when new outgoing data arrives
	 * if any previously transmitted data on the connection remains
	 * unacknowledged.
	 */
	if (!sp->nodelay && sctp_packet_empty(packet) &&
	    q->outstanding_bytes && sctp_state(asoc, ESTABLISHED)) {
		unsigned len = datasize + q->out_qlen;

		/* Check whether this chunk and all the rest of pending
		 * data will fit or delay in hopes of bundling a full
		 * sized packet.
		 */
		if (len < asoc->pmtu - packet->overhead) {
			retval = SCTP_XMIT_NAGLE_DELAY;
			goto finish;
		}
	}

	/* Keep track of how many bytes are in flight over this transport. */
	transport->flight_size += datasize;

	/* Keep track of how many bytes are in flight to the receiver. */
	asoc->outqueue.outstanding_bytes += datasize;

	/* Update our view of the receiver's rwnd. */
	if (datasize < rwnd)
		rwnd -= datasize;
	else
		rwnd = 0;

	asoc->peer.rwnd = rwnd;
	/* Has been accepted for transmission. */
	if (!asoc->peer.prsctp_capable)
		chunk->msg->can_abandon = 0;

finish:
	return retval;
}