tcp.c

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#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <assert.h>
#include <errno.h>
#include <byteswap.h>
#include <ipxe/timer.h>
#include <ipxe/iobuf.h>
#include <ipxe/malloc.h>
#include <ipxe/init.h>
#include <ipxe/retry.h>
#include <ipxe/refcnt.h>
#include <ipxe/pending.h>
#include <ipxe/xfer.h>
#include <ipxe/open.h>
#include <ipxe/uri.h>
#include <ipxe/netdevice.h>
#include <ipxe/profile.h>
#include <ipxe/process.h>
#include <ipxe/job.h>
#include <ipxe/tcpip.h>
#include <ipxe/tcp.h>

/** @file
 *
 * TCP protocol
 *
 */

FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );

/** A TCP connection */
struct tcp_connection {
        /** Reference counter */
        struct refcnt refcnt;
        /** List of TCP connections */
        struct list_head list;

        /** Flags */
        unsigned int flags;

        /** Data transfer interface */
        struct interface xfer;

        /** Remote socket address */
        struct sockaddr_tcpip peer;
        /** Local port */
        unsigned int local_port;
        /** Maximum segment size */
        size_t mss;

        /** Current TCP state */
        unsigned int tcp_state;
        /** Previous TCP state
         *
         * Maintained only for debug messages
         */
        unsigned int prev_tcp_state;
        /** Current sequence number
         *
         * Equivalent to SND.UNA in RFC 793 terminology.
         */
        uint32_t snd_seq;
        /** Unacknowledged sequence count
         *
         * Equivalent to (SND.NXT-SND.UNA) in RFC 793 terminology.
         */
        uint32_t snd_sent;
        /** Send window
         *
         * Equivalent to SND.WND in RFC 793 terminology
         */
        uint32_t snd_win;
        /** Current acknowledgement number
         *
         * Equivalent to RCV.NXT in RFC 793 terminology.
         */
        uint32_t rcv_ack;
        /** Receive window
         *
         * Equivalent to RCV.WND in RFC 793 terminology.
         */
        uint32_t rcv_win;
        /** Received timestamp value
         *
         * Updated when a packet is received; copied to ts_recent when
         * the window is advanced.
         */
        uint32_t ts_val;
        /** Most recent received timestamp that advanced the window
         *
         * Equivalent to TS.Recent in RFC 1323 terminology.
         */
        uint32_t ts_recent;
        /** Send window scale
         *
         * Equivalent to Snd.Wind.Scale in RFC 1323 terminology
         */
        uint8_t snd_win_scale;
        /** Receive window scale
         *
         * Equivalent to Rcv.Wind.Scale in RFC 1323 terminology
         */
        uint8_t rcv_win_scale;

        /** Selective acknowledgement list (in host-endian order) */
        struct tcp_sack_block sack[TCP_SACK_MAX];

        /** Transmit queue */
        struct list_head tx_queue;
        /** Receive queue */
        struct list_head rx_queue;
        /** Transmission process */
        struct process process;
        /** Retransmission timer */
        struct retry_timer timer;
        /** Keepalive timer */
        struct retry_timer keepalive;
        /** Shutdown (TIME_WAIT) timer */
        struct retry_timer wait;

        /** Pending operations for SYN and FIN */
        struct pending_operation pending_flags;
        /** Pending operations for transmit queue */
        struct pending_operation pending_data;
};

/** TCP flags */
enum tcp_flags {
        /** TCP data transfer interface has been closed */
        TCP_XFER_CLOSED = 0x0001,
        /** TCP timestamps are enabled */
        TCP_TS_ENABLED = 0x0002,
        /** TCP acknowledgement is pending */
        TCP_ACK_PENDING = 0x0004,
        /** TCP selective acknowledgement is enabled */
        TCP_SACK_ENABLED = 0x0008,
};

/** TCP internal header
 *
 * This is the header that replaces the TCP header for packets
 * enqueued on the receive queue.
 */
struct tcp_rx_queued_header {
        /** SEQ value, in host-endian order
         *
         * This represents the SEQ value at the time the packet is
         * enqueued, and so excludes the SYN, if present.
         */
        uint32_t seq;
        /** Next SEQ value, in host-endian order */
        uint32_t nxt;
        /** Flags
         *
         * Only FIN is valid within this flags byte; all other flags
         * have already been processed by the time the packet is
         * enqueued.
         */
        uint8_t flags;
        /** Reserved */
        uint8_t reserved[3];
};

/**
 * List of registered TCP connections
 */
static LIST_HEAD ( tcp_conns );

/** Transmit profiler */
static struct profiler tcp_tx_profiler __profiler = { .name = "tcp.tx" };

/** Receive profiler */
static struct profiler tcp_rx_profiler __profiler = { .name = "tcp.rx" };

/** Data transfer profiler */
static struct profiler tcp_xfer_profiler __profiler = { .name = "tcp.xfer" };

/* Forward declarations */
static struct process_descriptor tcp_process_desc;
static struct interface_descriptor tcp_xfer_desc;
static void tcp_expired ( struct retry_timer *timer, int over );
static void tcp_keepalive_expired ( struct retry_timer *timer, int over );
static void tcp_wait_expired ( struct retry_timer *timer, int over );
static struct tcp_connection * tcp_demux ( unsigned int local_port );
static int tcp_rx_ack ( struct tcp_connection *tcp, uint32_t ack,
                        uint32_t win );

/**
 * Name TCP state
 *
 * @v state             TCP state
 * @ret name            Name of TCP state
 */
static inline __attribute__ (( always_inline )) const char *
tcp_state ( int state ) {
        switch ( state ) {
        case TCP_CLOSED:                return "CLOSED";
        case TCP_LISTEN:                return "LISTEN";
        case TCP_SYN_SENT:              return "SYN_SENT";
        case TCP_SYN_RCVD:              return "SYN_RCVD";
        case TCP_ESTABLISHED:           return "ESTABLISHED";
        case TCP_FIN_WAIT_1:            return "FIN_WAIT_1";
        case TCP_FIN_WAIT_2:            return "FIN_WAIT_2";
        case TCP_CLOSING_OR_LAST_ACK:   return "CLOSING/LAST_ACK";
        case TCP_TIME_WAIT:             return "TIME_WAIT";
        case TCP_CLOSE_WAIT:            return "CLOSE_WAIT";
        default:                        return "INVALID";
        }
}

/**
 * Dump TCP state transition
 *
 * @v tcp               TCP connection
 */
static inline __attribute__ (( always_inline )) void
tcp_dump_state ( struct tcp_connection *tcp ) {

        if ( tcp->tcp_state != tcp->prev_tcp_state ) {
                DBGC ( tcp, "TCP %p transitioned from %s to %s\n", tcp,
                       tcp_state ( tcp->prev_tcp_state ),
                       tcp_state ( tcp->tcp_state ) );
        }
        tcp->prev_tcp_state = tcp->tcp_state;
}

/**
 * Dump TCP flags
 *
 * @v flags             TCP flags
 */
static inline __attribute__ (( always_inline )) void
tcp_dump_flags ( struct tcp_connection *tcp, unsigned int flags ) {
        if ( flags & TCP_RST )
                DBGC2 ( tcp, " RST" );
        if ( flags & TCP_SYN )
                DBGC2 ( tcp, " SYN" );
        if ( flags & TCP_PSH )
                DBGC2 ( tcp, " PSH" );
        if ( flags & TCP_FIN )
                DBGC2 ( tcp, " FIN" );
        if ( flags & TCP_ACK )
                DBGC2 ( tcp, " ACK" );
}

/***************************************************************************
 *
 * Open and close
 *
 ***************************************************************************
 */

/**
 * Check if local TCP port is available
 *
 * @v port              Local port number
 * @ret port            Local port number, or negative error
 */
static int tcp_port_available ( int port ) {

        return ( tcp_demux ( port ) ? -EADDRINUSE : port );
}

/**
 * Open a TCP connection
 *
 * @v xfer              Data transfer interface
 * @v peer              Peer socket address
 * @v local             Local socket address, or NULL
 * @ret rc              Return status code
 */
static int tcp_open ( struct interface *xfer, struct sockaddr *peer,
                      struct sockaddr *local ) {
        struct sockaddr_tcpip *st_peer = ( struct sockaddr_tcpip * ) peer;
        struct sockaddr_tcpip *st_local = ( struct sockaddr_tcpip * ) local;
        struct tcp_connection *tcp;
        size_t mtu;
        int port;
        int rc;

        /* Allocate and initialise structure */
        tcp = zalloc ( sizeof ( *tcp ) );
        if ( ! tcp )
                return -ENOMEM;
        DBGC ( tcp, "TCP %p allocated\n", tcp );
        ref_init ( &tcp->refcnt, NULL );
        intf_init ( &tcp->xfer, &tcp_xfer_desc, &tcp->refcnt );
        process_init_stopped ( &tcp->process, &tcp_process_desc, &tcp->refcnt );
        timer_init ( &tcp->timer, tcp_expired, &tcp->refcnt );
        timer_init ( &tcp->keepalive, tcp_keepalive_expired, &tcp->refcnt );
        timer_init ( &tcp->wait, tcp_wait_expired, &tcp->refcnt );
        tcp->prev_tcp_state = TCP_CLOSED;
        tcp->tcp_state = TCP_STATE_SENT ( TCP_SYN );
        tcp_dump_state ( tcp );
        tcp->snd_seq = random();
        INIT_LIST_HEAD ( &tcp->tx_queue );
        INIT_LIST_HEAD ( &tcp->rx_queue );
        memcpy ( &tcp->peer, st_peer, sizeof ( tcp->peer ) );

        /* Calculate MSS */
        mtu = tcpip_mtu ( &tcp->peer );
        if ( ! mtu ) {
                DBGC ( tcp, "TCP %p has no route to %s\n",
                       tcp, sock_ntoa ( peer ) );
                rc = -ENETUNREACH;
                goto err;
        }
        tcp->mss = ( mtu - sizeof ( struct tcp_header ) );

        /* Bind to local port */
        port = tcpip_bind ( st_local, tcp_port_available );
        if ( port < 0 ) {
                rc = port;
                DBGC ( tcp, "TCP %p could not bind: %s\n",
                       tcp, strerror ( rc ) );
                goto err;
        }
        tcp->local_port = port;
        DBGC ( tcp, "TCP %p bound to port %d\n", tcp, tcp->local_port );

        /* Start timer to initiate SYN */
        start_timer_nodelay ( &tcp->timer );

        /* Add a pending operation for the SYN */
        pending_get ( &tcp->pending_flags );

        /* Attach parent interface, transfer reference to connection
         * list and return
         */
        intf_plug_plug ( &tcp->xfer, xfer );
        list_add ( &tcp->list, &tcp_conns );
        return 0;

 err:
        ref_put ( &tcp->refcnt );
        return rc;
}

/**
 * Close TCP connection
 *
 * @v tcp               TCP connection
 * @v rc                Reason for close
 *
 * Closes the data transfer interface.  If the TCP state machine is in
 * a suitable state, the connection will be deleted.
 */
static void tcp_close ( struct tcp_connection *tcp, int rc ) {
        struct io_buffer *iobuf;
        struct io_buffer *tmp;

        /* Close data transfer interface */
        intf_shutdown ( &tcp->xfer, rc );
        tcp->flags |= TCP_XFER_CLOSED;

        /* If we are in CLOSED, or have otherwise not yet received a
         * SYN (i.e. we are in LISTEN or SYN_SENT), just delete the
         * connection.
         */
        if ( ! ( tcp->tcp_state & TCP_STATE_RCVD ( TCP_SYN ) ) ) {

                /* Transition to CLOSED for the sake of debugging messages */
                tcp->tcp_state = TCP_CLOSED;
                tcp_dump_state ( tcp );

                /* Free any unprocessed I/O buffers */
                list_for_each_entry_safe ( iobuf, tmp, &tcp->rx_queue, list ) {
                        list_del ( &iobuf->list );
                        free_iob ( iobuf );
                }

                /* Free any unsent I/O buffers */
                list_for_each_entry_safe ( iobuf, tmp, &tcp->tx_queue, list ) {
                        list_del ( &iobuf->list );
                        free_iob ( iobuf );
                        pending_put ( &tcp->pending_data );
                }
                assert ( ! is_pending ( &tcp->pending_data ) );

                /* Remove pending operations for SYN and FIN, if applicable */
                pending_put ( &tcp->pending_flags );
                pending_put ( &tcp->pending_flags );

                /* Remove from list and drop reference */
                process_del ( &tcp->process );
                stop_timer ( &tcp->timer );
                stop_timer ( &tcp->keepalive );
                stop_timer ( &tcp->wait );
                list_del ( &tcp->list );
                ref_put ( &tcp->refcnt );
                DBGC ( tcp, "TCP %p connection deleted\n", tcp );
                return;
        }

        /* If we have not had our SYN acknowledged (i.e. we are in
         * SYN_RCVD), pretend that it has been acknowledged so that we
         * can send a FIN without breaking things.
         */
        if ( ! ( tcp->tcp_state & TCP_STATE_ACKED ( TCP_SYN ) ) )
                tcp_rx_ack ( tcp, ( tcp->snd_seq + 1 ), 0 );

        /* Stop keepalive timer */
        stop_timer ( &tcp->keepalive );

        /* If we have no data remaining to send, start sending FIN */
        if ( list_empty ( &tcp->tx_queue ) &&
             ! ( tcp->tcp_state & TCP_STATE_SENT ( TCP_FIN ) ) ) {

                tcp->tcp_state |= TCP_STATE_SENT ( TCP_FIN );
                tcp_dump_state ( tcp );
                process_add ( &tcp->process );

                /* Add a pending operation for the FIN */
                pending_get ( &tcp->pending_flags );
        }
}

/***************************************************************************
 *
 * Transmit data path
 *
 ***************************************************************************
 */

/**
 * Calculate transmission window
 *
 * @v tcp               TCP connection
 * @ret len             Maximum length that can be sent in a single packet
 */
static size_t tcp_xmit_win ( struct tcp_connection *tcp ) {
        size_t len;

        /* Not ready if we're not in a suitable connection state */
        if ( ! TCP_CAN_SEND_DATA ( tcp->tcp_state ) )
                return 0;

        /* Length is the minimum of the receiver's window and the path MTU */
        len = tcp->snd_win;
        if ( len > TCP_PATH_MTU )
                len = TCP_PATH_MTU;

        return len;
}

/**
 * Check data-transfer flow control window
 *
 * @v tcp               TCP connection
 * @ret len             Length of window
 */
static size_t tcp_xfer_window ( struct tcp_connection *tcp ) {

        /* Not ready if data queue is non-empty.  This imposes a limit
         * of only one unACKed packet in the TX queue at any time; we
         * do this to conserve memory usage.
         */
        if ( ! list_empty ( &tcp->tx_queue ) )
                return 0;

        /* Return TCP window length */
        return tcp_xmit_win ( tcp );
}

/**
 * Find selective acknowledgement block
 *
 * @v tcp               TCP connection
 * @v seq               SEQ value in SACK block (in host-endian order)
 * @v sack              SACK block to fill in (in host-endian order)
 * @ret len             Length of SACK block
 */
static uint32_t tcp_sack_block ( struct tcp_connection *tcp, uint32_t seq,
                                 struct tcp_sack_block *sack ) {
        struct io_buffer *iobuf;
        struct tcp_rx_queued_header *tcpqhdr;
        uint32_t left = tcp->rcv_ack;
        uint32_t right = left;

        /* Find highest block which does not start after SEQ */
        list_for_each_entry ( iobuf, &tcp->rx_queue, list ) {
                tcpqhdr = iobuf->data;
                if ( tcp_cmp ( tcpqhdr->seq, right ) > 0 ) {
                        if ( tcp_cmp ( tcpqhdr->seq, seq ) > 0 )
                                break;
                        left = tcpqhdr->seq;
                }
                if ( tcp_cmp ( tcpqhdr->nxt, right ) > 0 )
                        right = tcpqhdr->nxt;
        }

        /* Fail if this block does not contain SEQ */
        if ( tcp_cmp ( right, seq ) < 0 )
                return 0;

        /* Populate SACK block */
        sack->left = left;
        sack->right = right;
        return ( right - left );
}

/**
 * Update TCP selective acknowledgement list
 *
 * @v tcp               TCP connection
 * @v seq               SEQ value in first SACK block (in host-endian order)
 * @ret count           Number of SACK blocks
 */
static unsigned int tcp_sack ( struct tcp_connection *tcp, uint32_t seq ) {
        struct tcp_sack_block sack[TCP_SACK_MAX];
        unsigned int old = 0;
        unsigned int new = 0;
        unsigned int i;
        uint32_t len;

        /* Populate first new SACK block */
        len = tcp_sack_block ( tcp, seq, &sack[0] );
        if ( len )
                new++;

        /* Populate remaining new SACK blocks based on old SACK blocks */
        for ( old = 0 ; old < TCP_SACK_MAX ; old++ ) {

                /* Stop if we run out of space in the new list */
                if ( new == TCP_SACK_MAX )
                        break;

                /* Skip empty old SACK blocks */
                if ( tcp->sack[old].left == tcp->sack[old].right )
                        continue;

                /* Populate new SACK block */
                len = tcp_sack_block ( tcp, tcp->sack[old].left, &sack[new] );
                if ( len == 0 )
                        continue;

                /* Eliminate duplicates */
                for ( i = 0 ; i < new ; i++ ) {
                        if ( sack[i].left == sack[new].left ) {
                                new--;
                                break;
                        }
                }
                new++;
        }

        /* Update SACK list */
        memset ( tcp->sack, 0, sizeof ( tcp->sack ) );
        memcpy ( tcp->sack, sack, ( new * sizeof ( tcp->sack[0] ) ) );
        return new;
}

/**
 * Process TCP transmit queue
 *
 * @v tcp               TCP connection
 * @v max_len           Maximum length to process
 * @v dest              I/O buffer to fill with data, or NULL
 * @v remove            Remove data from queue
 * @ret len             Length of data processed
 *
 * This processes at most @c max_len bytes from the TCP connection's
 * transmit queue.  Data will be copied into the @c dest I/O buffer
 * (if provided) and, if @c remove is true, removed from the transmit
 * queue.
 */
static size_t tcp_process_tx_queue ( struct tcp_connection *tcp, size_t max_len,
                                     struct io_buffer *dest, int remove ) {
        struct io_buffer *iobuf;
        struct io_buffer *tmp;
        size_t frag_len;
        size_t len = 0;

        list_for_each_entry_safe ( iobuf, tmp, &tcp->tx_queue, list ) {
                frag_len = iob_len ( iobuf );
                if ( frag_len > max_len )
                        frag_len = max_len;
                if ( dest ) {
                        memcpy ( iob_put ( dest, frag_len ), iobuf->data,
                                 frag_len );
                }
                if ( remove ) {
                        iob_pull ( iobuf, frag_len );
                        if ( ! iob_len ( iobuf ) ) {
                                list_del ( &iobuf->list );
                                free_iob ( iobuf );
                                pending_put ( &tcp->pending_data );
                        }
                }
                len += frag_len;
                max_len -= frag_len;
        }
        return len;
}

/**
 * Transmit any outstanding data (with selective acknowledgement)
 *
 * @v tcp               TCP connection
 * @v sack_seq          SEQ for first selective acknowledgement (if any)
 * 
 * Transmits any outstanding data on the connection.
 *
 * Note that even if an error is returned, the retransmission timer
 * will have been started if necessary, and so the stack will
 * eventually attempt to retransmit the failed packet.
 */
static void tcp_xmit_sack ( struct tcp_connection *tcp, uint32_t sack_seq ) {
        struct io_buffer *iobuf;
        struct tcp_header *tcphdr;
        struct tcp_mss_option *mssopt;
        struct tcp_window_scale_padded_option *wsopt;
        struct tcp_timestamp_padded_option *tsopt;
        struct tcp_sack_permitted_padded_option *spopt;
        struct tcp_sack_padded_option *sackopt;
        struct tcp_sack_block *sack;
        void *payload;
        unsigned int flags;
        unsigned int sack_count;
        unsigned int i;
        size_t len = 0;
        size_t sack_len;
        uint32_t seq_len;
        uint32_t max_rcv_win;
        uint32_t max_representable_win;
        int rc;

        /* Start profiling */
        profile_start ( &tcp_tx_profiler );

        /* If retransmission timer is already running, do nothing */
        if ( timer_running ( &tcp->timer ) )
                return;

        /* Calculate both the actual (payload) and sequence space
         * lengths that we wish to transmit.
         */
        if ( TCP_CAN_SEND_DATA ( tcp->tcp_state ) ) {
                len = tcp_process_tx_queue ( tcp, tcp_xmit_win ( tcp ),
                                             NULL, 0 );
        }
        seq_len = len;
        flags = TCP_FLAGS_SENDING ( tcp->tcp_state );
        if ( flags & ( TCP_SYN | TCP_FIN ) ) {
                /* SYN or FIN consume one byte, and we can never send both */
                assert ( ! ( ( flags & TCP_SYN ) && ( flags & TCP_FIN ) ) );
                seq_len++;
        }
        tcp->snd_sent = seq_len;

        /* If we have nothing to transmit, stop now */
        if ( ( seq_len == 0 ) && ! ( tcp->flags & TCP_ACK_PENDING ) )
                return;

        /* If we are transmitting anything that requires
         * acknowledgement (i.e. consumes sequence space), start the
         * retransmission timer.  Do this before attempting to
         * allocate the I/O buffer, in case allocation itself fails.
         */
        if ( seq_len )
                start_timer ( &tcp->timer );

        /* Allocate I/O buffer */
        iobuf = alloc_iob ( len + TCP_MAX_HEADER_LEN );
        if ( ! iobuf ) {
                DBGC ( tcp, "TCP %p could not allocate iobuf for %08x..%08x "
                       "%08x\n", tcp, tcp->snd_seq, ( tcp->snd_seq + seq_len ),
                       tcp->rcv_ack );
                return;
        }
        iob_reserve ( iobuf, TCP_MAX_HEADER_LEN );

        /* Fill data payload from transmit queue */
        tcp_process_tx_queue ( tcp, len, iobuf, 0 );

        /* Expand receive window if possible */
        max_rcv_win = xfer_window ( &tcp->xfer );
        if ( max_rcv_win > TCP_MAX_WINDOW_SIZE )
                max_rcv_win = TCP_MAX_WINDOW_SIZE;
        max_representable_win = ( 0xffff << tcp->rcv_win_scale );
        if ( max_rcv_win > max_representable_win )
                max_rcv_win = max_representable_win;
        max_rcv_win &= ~0x03; /* Keep everything dword-aligned */
        if ( tcp->rcv_win < max_rcv_win )
                tcp->rcv_win = max_rcv_win;

        /* Fill up the TCP header */
        payload = iobuf->data;
        if ( flags & TCP_SYN ) {
                mssopt = iob_push ( iobuf, sizeof ( *mssopt ) );
                mssopt->kind = TCP_OPTION_MSS;
                mssopt->length = sizeof ( *mssopt );
                mssopt->mss = htons ( tcp->mss );
                wsopt = iob_push ( iobuf, sizeof ( *wsopt ) );
                wsopt->nop = TCP_OPTION_NOP;
                wsopt->wsopt.kind = TCP_OPTION_WS;
                wsopt->wsopt.length = sizeof ( wsopt->wsopt );
                wsopt->wsopt.scale = TCP_RX_WINDOW_SCALE;
                spopt = iob_push ( iobuf, sizeof ( *spopt ) );
                memset ( spopt->nop, TCP_OPTION_NOP, sizeof ( spopt->nop ) );
                spopt->spopt.kind = TCP_OPTION_SACK_PERMITTED;
                spopt->spopt.length = sizeof ( spopt->spopt );
        }
        if ( ( flags & TCP_SYN ) || ( tcp->flags & TCP_TS_ENABLED ) ) {
                tsopt = iob_push ( iobuf, sizeof ( *tsopt ) );
                memset ( tsopt->nop, TCP_OPTION_NOP, sizeof ( tsopt->nop ) );
                tsopt->tsopt.kind = TCP_OPTION_TS;
                tsopt->tsopt.length = sizeof ( tsopt->tsopt );
                tsopt->tsopt.tsval = htonl ( currticks() );
                tsopt->tsopt.tsecr = htonl ( tcp->ts_recent );
        }
        if ( ( tcp->flags & TCP_SACK_ENABLED ) &&
             ( ! list_empty ( &tcp->rx_queue ) ) &&
             ( ( sack_count = tcp_sack ( tcp, sack_seq ) ) != 0 ) ) {
                sack_len = ( sack_count * sizeof ( *sack ) );
                sackopt = iob_push ( iobuf, ( sizeof ( *sackopt ) + sack_len ));
                memset ( sackopt->nop, TCP_OPTION_NOP, sizeof ( sackopt->nop ));
                sackopt->sackopt.kind = TCP_OPTION_SACK;
                sackopt->sackopt.length =
                        ( sizeof ( sackopt->sackopt ) + sack_len );
                sack = ( ( ( void * ) sackopt ) + sizeof ( *sackopt ) );
                for ( i = 0 ; i < sack_count ; i++, sack++ ) {
                        sack->left = htonl ( tcp->sack[i].left );
                        sack->right = htonl ( tcp->sack[i].right );
                }
        }
        if ( len != 0 )
                flags |= TCP_PSH;
        tcphdr = iob_push ( iobuf, sizeof ( *tcphdr ) );
        memset ( tcphdr, 0, sizeof ( *tcphdr ) );
        tcphdr->src = htons ( tcp->local_port );
        tcphdr->dest = tcp->peer.st_port;
        tcphdr->seq = htonl ( tcp->snd_seq );
        tcphdr->ack = htonl ( tcp->rcv_ack );
        tcphdr->hlen = ( ( payload - iobuf->data ) << 2 );
        tcphdr->flags = flags;
        tcphdr->win = htons ( tcp->rcv_win >> tcp->rcv_win_scale );
        tcphdr->csum = tcpip_chksum ( iobuf->data, iob_len ( iobuf ) );

        /* Dump header */
        DBGC2 ( tcp, "TCP %p TX %d->%d %08x..%08x           %08x %4zd",
                tcp, ntohs ( tcphdr->src ), ntohs ( tcphdr->dest ),
                ntohl ( tcphdr->seq ), ( ntohl ( tcphdr->seq ) + seq_len ),
                ntohl ( tcphdr->ack ), len );
        tcp_dump_flags ( tcp, tcphdr->flags );
        DBGC2 ( tcp, "\n" );

        /* Transmit packet */
        if ( ( rc = tcpip_tx ( iobuf, &tcp_protocol, NULL, &tcp->peer, NULL,
                               &tcphdr->csum ) ) != 0 ) {
                DBGC ( tcp, "TCP %p could not transmit %08x..%08x %08x: %s\n",
                       tcp, tcp->snd_seq, ( tcp->snd_seq + tcp->snd_sent ),
                       tcp->rcv_ack, strerror ( rc ) );
                return;
        }

        /* Clear ACK-pending flag */
        tcp->flags &= ~TCP_ACK_PENDING;

        profile_stop ( &tcp_tx_profiler );
}

/**
 * Transmit any outstanding data
 *
 * @v tcp               TCP connection
 */
static void tcp_xmit ( struct tcp_connection *tcp ) {

        /* Transmit without an explicit first SACK */
        tcp_xmit_sack ( tcp, tcp->rcv_ack );
}

/** TCP process descriptor */
static struct process_descriptor tcp_process_desc =
        PROC_DESC_ONCE ( struct tcp_connection, process, tcp_xmit );

/**
 * Retransmission timer expired
 *
 * @v timer             Retransmission timer
 * @v over              Failure indicator
 */
static void tcp_expired ( struct retry_timer *timer, int over ) {
        struct tcp_connection *tcp =
                container_of ( timer, struct tcp_connection, timer );

        DBGC ( tcp, "TCP %p timer %s in %s for %08x..%08x %08x\n", tcp,
               ( over ? "expired" : "fired" ), tcp_state ( tcp->tcp_state ),
               tcp->snd_seq, ( tcp->snd_seq + tcp->snd_sent ), tcp->rcv_ack );

        assert ( ( tcp->tcp_state == TCP_SYN_SENT ) ||
                 ( tcp->tcp_state == TCP_SYN_RCVD ) ||
                 ( tcp->tcp_state == TCP_ESTABLISHED ) ||
                 ( tcp->tcp_state == TCP_FIN_WAIT_1 ) ||
                 ( tcp->tcp_state == TCP_CLOSE_WAIT ) ||
                 ( tcp->tcp_state == TCP_CLOSING_OR_LAST_ACK ) );

        if ( over ) {
                /* If we have finally timed out and given up,
                 * terminate the connection
                 */
                tcp->tcp_state = TCP_CLOSED;
                tcp_dump_state ( tcp );
                tcp_close ( tcp, -ETIMEDOUT );
        } else {
                /* Otherwise, retransmit the packet */
                tcp_xmit ( tcp );
        }
}

/**
 * Keepalive timer expired
 *
 * @v timer             Keepalive timer
 * @v over              Failure indicator
 */
static void tcp_keepalive_expired ( struct retry_timer *timer,
                                    int over __unused ) {
        struct tcp_connection *tcp =
                container_of ( timer, struct tcp_connection, keepalive );

        DBGC ( tcp, "TCP %p sending keepalive\n", tcp );

        /* Reset keepalive timer */
        start_timer_fixed ( &tcp->keepalive, TCP_KEEPALIVE_DELAY );

        /* Send keepalive.  We do this only to preserve or restore
         * state in intermediate devices (e.g. firewall NAT tables);
         * we don't actually care about eliciting a response to verify
         * that the peer is still alive.  We therefore send just a
         * pure ACK, to keep our transmit path simple.
         */
        tcp->flags |= TCP_ACK_PENDING;
        tcp_xmit ( tcp );
}

/**
 * Shutdown timer expired
 *
 * @v timer             Shutdown timer
 * @v over              Failure indicator
 */
static void tcp_wait_expired ( struct retry_timer *timer, int over __unused ) {
        struct tcp_connection *tcp =
                container_of ( timer, struct tcp_connection, wait );

        assert ( tcp->tcp_state == TCP_TIME_WAIT );

        DBGC ( tcp, "TCP %p wait complete in %s for %08x..%08x %08x\n", tcp,
               tcp_state ( tcp->tcp_state ), tcp->snd_seq,
               ( tcp->snd_seq + tcp->snd_sent ), tcp->rcv_ack );

        tcp->tcp_state = TCP_CLOSED;
        tcp_dump_state ( tcp );
        tcp_close ( tcp, 0 );
}

/**
 * Send RST response to incoming packet
 *
 * @v in_tcphdr         TCP header of incoming packet
 * @ret rc              Return status code
 */
static int tcp_xmit_reset ( struct tcp_connection *tcp,
                            struct sockaddr_tcpip *st_dest,
                            struct tcp_header *in_tcphdr ) {
        struct io_buffer *iobuf;
        struct tcp_header *tcphdr;
        int rc;

        /* Allocate space for dataless TX buffer */
        iobuf = alloc_iob ( TCP_MAX_HEADER_LEN );
        if ( ! iobuf ) {
                DBGC ( tcp, "TCP %p could not allocate iobuf for RST "
                       "%08x..%08x %08x\n", tcp, ntohl ( in_tcphdr->ack ),
                       ntohl ( in_tcphdr->ack ), ntohl ( in_tcphdr->seq ) );
                return -ENOMEM;
        }
        iob_reserve ( iobuf, TCP_MAX_HEADER_LEN );

        /* Construct RST response */
        tcphdr = iob_push ( iobuf, sizeof ( *tcphdr ) );
        memset ( tcphdr, 0, sizeof ( *tcphdr ) );
        tcphdr->src = in_tcphdr->dest;
        tcphdr->dest = in_tcphdr->src;
        tcphdr->seq = in_tcphdr->ack;
        tcphdr->ack = in_tcphdr->seq;
        tcphdr->hlen = ( ( sizeof ( *tcphdr ) / 4 ) << 4 );
        tcphdr->flags = ( TCP_RST | TCP_ACK );
        tcphdr->win = htons ( 0 );
        tcphdr->csum = tcpip_chksum ( iobuf->data, iob_len ( iobuf ) );

        /* Dump header */
        DBGC2 ( tcp, "TCP %p TX %d->%d %08x..%08x           %08x %4d",
                tcp, ntohs ( tcphdr->src ), ntohs ( tcphdr->dest ),
                ntohl ( tcphdr->seq ), ( ntohl ( tcphdr->seq ) ),
                ntohl ( tcphdr->ack ), 0 );
        tcp_dump_flags ( tcp, tcphdr->flags );
        DBGC2 ( tcp, "\n" );

        /* Transmit packet */
        if ( ( rc = tcpip_tx ( iobuf, &tcp_protocol, NULL, st_dest,
                               NULL, &tcphdr->csum ) ) != 0 ) {
                DBGC ( tcp, "TCP %p could not transmit RST %08x..%08x %08x: "
                       "%s\n", tcp, ntohl ( in_tcphdr->ack ),
                       ntohl ( in_tcphdr->ack ), ntohl ( in_tcphdr->seq ),
                       strerror ( rc ) );
                return rc;
        }

        return 0;
}

/***************************************************************************
 *
 * Receive data path
 *
 ***************************************************************************
 */

/**
 * Identify TCP connection by local port number
 *
 * @v local_port        Local port
 * @ret tcp             TCP connection, or NULL
 */
static struct tcp_connection * tcp_demux ( unsigned int local_port ) {
        struct tcp_connection *tcp;

        list_for_each_entry ( tcp, &tcp_conns, list ) {
                if ( tcp->local_port == local_port )
                        return tcp;
        }
        return NULL;
}

/**
 * Parse TCP received options
 *
 * @v tcp               TCP connection (may be NULL)
 * @v tcphdr            TCP header
 * @v hlen              TCP header length
 * @v options           Options structure to fill in
 * @ret rc              Return status code
 */
static int tcp_rx_opts ( struct tcp_connection *tcp,
                         const struct tcp_header *tcphdr, size_t hlen,
                         struct tcp_options *options ) {
        const void *data = ( ( ( void * ) tcphdr ) + sizeof ( *tcphdr ) );
        const void *end = ( ( ( void * ) tcphdr ) + hlen );
        const struct tcp_option *option;
        unsigned int kind;
        size_t remaining;
        size_t min;

        /* Sanity check */
        assert ( hlen >= sizeof ( *tcphdr ) );

        /* Parse options */
        memset ( options, 0, sizeof ( *options ) );
        while ( ( remaining = ( end - data ) ) ) {

                /* Extract option code */
                option = data;
                kind = option->kind;

                /* Handle single-byte options */
                if ( kind == TCP_OPTION_END )
                        break;
                if ( kind == TCP_OPTION_NOP ) {
                        data++;
                        continue;
                }

                /* Handle multi-byte options */
                min = sizeof ( *option );
                switch ( kind ) {
                case TCP_OPTION_MSS:
                        /* Ignore received MSS */
                        break;
                case TCP_OPTION_WS:
                        options->wsopt = data;
                        min = sizeof ( *options->wsopt );
                        break;
                case TCP_OPTION_SACK_PERMITTED:
                        options->spopt = data;
                        min = sizeof ( *options->spopt );
                        break;
                case TCP_OPTION_SACK:
                        /* Ignore received SACKs */
                        break;
                case TCP_OPTION_TS:
                        options->tsopt = data;
                        min = sizeof ( *options->tsopt );
                        break;
                default:
                        DBGC ( tcp, "TCP %p received unknown option %d\n",
                               tcp, kind );
                        break;
                }
                if ( remaining < min ) {
                        DBGC ( tcp, "TCP %p received truncated option %d\n",
                               tcp, kind );
                        return -EINVAL;
                }
                if ( option->length < min ) {
                        DBGC ( tcp, "TCP %p received underlength option %d\n",
                               tcp, kind );
                        return -EINVAL;
                }
                if ( option->length > remaining ) {
                        DBGC ( tcp, "TCP %p received overlength option %d\n",
                               tcp, kind );
                        return -EINVAL;
                }
                data += option->length;
        }

        return 0;
}

/**
 * Consume received sequence space
 *
 * @v tcp               TCP connection
 * @v seq_len           Sequence space length to consume
 */
static void tcp_rx_seq ( struct tcp_connection *tcp, uint32_t seq_len ) {
        unsigned int sack;

        /* Sanity check */
        assert ( seq_len > 0 );

        /* Update acknowledgement number */
        tcp->rcv_ack += seq_len;

        /* Update window */
        if ( tcp->rcv_win > seq_len ) {
                tcp->rcv_win -= seq_len;
        } else {
                tcp->rcv_win = 0;
        }

        /* Update timestamp */
        tcp->ts_recent = tcp->ts_val;

        /* Update SACK list */
        for ( sack = 0 ; sack < TCP_SACK_MAX ; sack++ ) {
                if ( tcp->sack[sack].left == tcp->sack[sack].right )
                        continue;
                if ( tcp_cmp ( tcp->sack[sack].left, tcp->rcv_ack ) < 0 )
                        tcp->sack[sack].left = tcp->rcv_ack;
                if ( tcp_cmp ( tcp->sack[sack].right, tcp->rcv_ack ) < 0 )
                        tcp->sack[sack].right = tcp->rcv_ack;
        }

        /* Mark ACK as pending */
        tcp->flags |= TCP_ACK_PENDING;
}

/**
 * Handle TCP received SYN
 *
 * @v tcp               TCP connection
 * @v seq               SEQ value (in host-endian order)
 * @v options           TCP options
 * @ret rc              Return status code
 */
static int tcp_rx_syn ( struct tcp_connection *tcp, uint32_t seq,
                        struct tcp_options *options ) {

        /* Synchronise sequence numbers on first SYN */
        if ( ! ( tcp->tcp_state & TCP_STATE_RCVD ( TCP_SYN ) ) ) {
                tcp->rcv_ack = seq;
                if ( options->tsopt )
                        tcp->flags |= TCP_TS_ENABLED;
                if ( options->spopt )
                        tcp->flags |= TCP_SACK_ENABLED;
                if ( options->wsopt ) {
                        tcp->snd_win_scale = options->wsopt->scale;
                        tcp->rcv_win_scale = TCP_RX_WINDOW_SCALE;
                }
                DBGC ( tcp, "TCP %p using %stimestamps, %sSACK, TX window "
                       "x%d, RX window x%d\n", tcp,
                       ( ( tcp->flags & TCP_TS_ENABLED ) ? "" : "no " ),
                       ( ( tcp->flags & TCP_SACK_ENABLED ) ? "" : "no " ),
                       ( 1 << tcp->snd_win_scale ),
                       ( 1 << tcp->rcv_win_scale ) );
        }

        /* Ignore duplicate SYN */
        if ( seq != tcp->rcv_ack )
                return 0;

        /* Acknowledge SYN */
        tcp_rx_seq ( tcp, 1 );

        /* Mark SYN as received and start sending ACKs with each packet */
        tcp->tcp_state |= ( TCP_STATE_SENT ( TCP_ACK ) |
                            TCP_STATE_RCVD ( TCP_SYN ) );

        return 0;
}

/**
 * Handle TCP received ACK
 *
 * @v tcp               TCP connection
 * @v ack               ACK value (in host-endian order)
 * @v win               WIN value (in host-endian order)
 * @ret rc              Return status code
 */
static int tcp_rx_ack ( struct tcp_connection *tcp, uint32_t ack,
                        uint32_t win ) {
        uint32_t ack_len = ( ack - tcp->snd_seq );
        size_t len;
        unsigned int acked_flags;

        /* Check for out-of-range or old duplicate ACKs */
        if ( ack_len > tcp->snd_sent ) {
                DBGC ( tcp, "TCP %p received ACK for %08x..%08x, "
                       "sent only %08x..%08x\n", tcp, tcp->snd_seq,
                       ( tcp->snd_seq + ack_len ), tcp->snd_seq,
                       ( tcp->snd_seq + tcp->snd_sent ) );

                if ( TCP_HAS_BEEN_ESTABLISHED ( tcp->tcp_state ) ) {
                        /* Just ignore what might be old duplicate ACKs */
                        return 0;
                } else {
                        /* Send RST if an out-of-range ACK is received
                         * on a not-yet-established connection, as per
                         * RFC 793.
                         */
                        return -EINVAL;
                }
        }

        /* Update window size */
        tcp->snd_win = win;

        /* Hold off (or start) the keepalive timer, if applicable */
        if ( ! ( tcp->tcp_state & TCP_STATE_SENT ( TCP_FIN ) ) )
                start_timer_fixed ( &tcp->keepalive, TCP_KEEPALIVE_DELAY );

        /* Ignore ACKs that don't actually acknowledge any new data.
         * (In particular, do not stop the retransmission timer; this
         * avoids creating a sorceror's apprentice syndrome when a
         * duplicate ACK is received and we still have data in our
         * transmit queue.)
         */
        if ( ack_len == 0 )
                return 0;

        /* Stop the retransmission timer */
        stop_timer ( &tcp->timer );

        /* Determine acknowledged flags and data length */
        len = ack_len;
        acked_flags = ( TCP_FLAGS_SENDING ( tcp->tcp_state ) &
                        ( TCP_SYN | TCP_FIN ) );
        if ( acked_flags ) {
                len--;
                pending_put ( &tcp->pending_flags );
        }

        /* Update SEQ and sent counters */
        tcp->snd_seq = ack;
        tcp->snd_sent = 0;

        /* Remove any acknowledged data from transmit queue */
        tcp_process_tx_queue ( tcp, len, NULL, 1 );
                
        /* Mark SYN/FIN as acknowledged if applicable. */
        if ( acked_flags )
                tcp->tcp_state |= TCP_STATE_ACKED ( acked_flags );

        /* Start sending FIN if we've had all possible data ACKed */
        if ( list_empty ( &tcp->tx_queue ) &&
             ( tcp->flags & TCP_XFER_CLOSED ) &&
             ! ( tcp->tcp_state & TCP_STATE_SENT ( TCP_FIN ) ) ) {
                tcp->tcp_state |= TCP_STATE_SENT ( TCP_FIN );
                pending_get ( &tcp->pending_flags );
        }

        return 0;
}

/**
 * Handle TCP received data
 *
 * @v tcp               TCP connection
 * @v seq               SEQ value (in host-endian order)
 * @v iobuf             I/O buffer
 * @ret rc              Return status code
 *
 * This function takes ownership of the I/O buffer.
 */
static int tcp_rx_data ( struct tcp_connection *tcp, uint32_t seq,
                         struct io_buffer *iobuf ) {
        uint32_t already_rcvd;
        uint32_t len;
        int rc;

        /* Ignore duplicate or out-of-order data */
        already_rcvd = ( tcp->rcv_ack - seq );
        len = iob_len ( iobuf );
        if ( already_rcvd >= len ) {
                free_iob ( iobuf );
                return 0;
        }
        iob_pull ( iobuf, already_rcvd );
        len -= already_rcvd;

        /* Acknowledge new data */
        tcp_rx_seq ( tcp, len );

        /* Deliver data to application */
        profile_start ( &tcp_xfer_profiler );
        if ( ( rc = xfer_deliver_iob ( &tcp->xfer, iobuf ) ) != 0 ) {
                DBGC ( tcp, "TCP %p could not deliver %08x..%08x: %s\n",
                       tcp, seq, ( seq + len ), strerror ( rc ) );
                return rc;
        }
        profile_stop ( &tcp_xfer_profiler );

        return 0;
}

/**
 * Handle TCP received FIN
 *
 * @v tcp               TCP connection
 * @v seq               SEQ value (in host-endian order)
 * @ret rc              Return status code
 */
static int tcp_rx_fin ( struct tcp_connection *tcp, uint32_t seq ) {

        /* Ignore duplicate or out-of-order FIN */
        if ( seq != tcp->rcv_ack )
                return 0;

        /* Acknowledge FIN */
        tcp_rx_seq ( tcp, 1 );

        /* Mark FIN as received */
        tcp->tcp_state |= TCP_STATE_RCVD ( TCP_FIN );

        /* Close connection */
        tcp_close ( tcp, 0 );

        return 0;
}

/**
 * Handle TCP received RST
 *
 * @v tcp               TCP connection
 * @v seq               SEQ value (in host-endian order)
 * @ret rc              Return status code
 */
static int tcp_rx_rst ( struct tcp_connection *tcp, uint32_t seq ) {

        /* Accept RST only if it falls within the window.  If we have
         * not yet received a SYN, then we have no window to test
         * against, so fall back to checking that our SYN has been
         * ACKed.
         */
        if ( tcp->tcp_state & TCP_STATE_RCVD ( TCP_SYN ) ) {
                if ( ! tcp_in_window ( seq, tcp->rcv_ack, tcp->rcv_win ) )
                        return 0;
        } else {
                if ( ! ( tcp->tcp_state & TCP_STATE_ACKED ( TCP_SYN ) ) )
                        return 0;
        }

        /* Abort connection */
        tcp->tcp_state = TCP_CLOSED;
        tcp_dump_state ( tcp );
        tcp_close ( tcp, -ECONNRESET );

        DBGC ( tcp, "TCP %p connection reset by peer\n", tcp );
        return -ECONNRESET;
}

/**
 * Enqueue received TCP packet
 *
 * @v tcp               TCP connection
 * @v seq               SEQ value (in host-endian order)
 * @v flags             TCP flags
 * @v iobuf             I/O buffer
 */
static void tcp_rx_enqueue ( struct tcp_connection *tcp, uint32_t seq,
                             uint8_t flags, struct io_buffer *iobuf ) {
        struct tcp_rx_queued_header *tcpqhdr;
        struct io_buffer *queued;
        size_t len;
        uint32_t seq_len;
        uint32_t nxt;

        /* Calculate remaining flags and sequence length.  Note that
         * SYN, if present, has already been processed by this point.
         */
        flags &= TCP_FIN;
        len = iob_len ( iobuf );
        seq_len = ( len + ( flags ? 1 : 0 ) );
        nxt = ( seq + seq_len );

        /* Discard immediately (to save memory) if:
         *
         * a) we have not yet received a SYN (and so have no defined
         *    receive window), or
         * b) the packet lies entirely outside the receive window, or
         * c) there is no further content to process.
         */
        if ( ( ! ( tcp->tcp_state & TCP_STATE_RCVD ( TCP_SYN ) ) ) ||
             ( tcp_cmp ( seq, tcp->rcv_ack + tcp->rcv_win ) >= 0 ) ||
             ( tcp_cmp ( nxt, tcp->rcv_ack ) < 0 ) ||
             ( seq_len == 0 ) ) {
                free_iob ( iobuf );
                return;
        }

        /* Add internal header */
        tcpqhdr = iob_push ( iobuf, sizeof ( *tcpqhdr ) );
        tcpqhdr->seq = seq;
        tcpqhdr->nxt = nxt;
        tcpqhdr->flags = flags;

        /* Add to RX queue */
        list_for_each_entry ( queued, &tcp->rx_queue, list ) {
                tcpqhdr = queued->data;
                if ( tcp_cmp ( seq, tcpqhdr->seq ) < 0 )
                        break;
        }
        list_add_tail ( &iobuf->list, &queued->list );
}

/**
 * Process receive queue
 *
 * @v tcp               TCP connection
 */
static void tcp_process_rx_queue ( struct tcp_connection *tcp ) {
        struct io_buffer *iobuf;
        struct tcp_rx_queued_header *tcpqhdr;
        uint32_t seq;
        unsigned int flags;
        size_t len;

        /* Process all applicable received buffers.  Note that we
         * cannot use list_for_each_entry() to iterate over the RX
         * queue, since tcp_discard() may remove packets from the RX
         * queue while we are processing.
         */
        while ( ( iobuf = list_first_entry ( &tcp->rx_queue, struct io_buffer,
                                             list ) ) ) {

                /* Stop processing when we hit the first gap */
                tcpqhdr = iobuf->data;
                if ( tcp_cmp ( tcpqhdr->seq, tcp->rcv_ack ) > 0 )
                        break;

                /* Strip internal header and remove from RX queue */
                list_del ( &iobuf->list );
                seq = tcpqhdr->seq;
                flags = tcpqhdr->flags;
                iob_pull ( iobuf, sizeof ( *tcpqhdr ) );
                len = iob_len ( iobuf );

                /* Handle new data, if any */
                tcp_rx_data ( tcp, seq, iob_disown ( iobuf ) );
                seq += len;

                /* Handle FIN, if present */
                if ( flags & TCP_FIN ) {
                        tcp_rx_fin ( tcp, seq );
                        seq++;
                }
        }
}

/**
 * Process received packet
 *
 * @v iobuf             I/O buffer
 * @v netdev            Network device
 * @v st_src            Partially-filled source address
 * @v st_dest           Partially-filled destination address
 * @v pshdr_csum        Pseudo-header checksum
 * @ret rc              Return status code
  */
static int tcp_rx ( struct io_buffer *iobuf,
                    struct net_device *netdev __unused,
                    struct sockaddr_tcpip *st_src,
                    struct sockaddr_tcpip *st_dest __unused,
                    uint16_t pshdr_csum ) {
        struct tcp_header *tcphdr = iobuf->data;
        struct tcp_connection *tcp;
        struct tcp_options options;
        size_t hlen;
        uint16_t csum;
        uint32_t seq;
        uint32_t ack;
        uint16_t raw_win;
        uint32_t win;
        unsigned int flags;
        size_t len;
        uint32_t seq_len;
        size_t old_xfer_window;
        int rc;

        /* Start profiling */
        profile_start ( &tcp_rx_profiler );

        /* Sanity check packet */
        if ( iob_len ( iobuf ) < sizeof ( *tcphdr ) ) {
                DBG ( "TCP packet too short at %zd bytes (min %zd bytes)\n",
                      iob_len ( iobuf ), sizeof ( *tcphdr ) );
                rc = -EINVAL;
                goto discard;
        }
        hlen = ( ( tcphdr->hlen & TCP_MASK_HLEN ) / 16 ) * 4;
        if ( hlen < sizeof ( *tcphdr ) ) {
                DBG ( "TCP header too short at %zd bytes (min %zd bytes)\n",
                      hlen, sizeof ( *tcphdr ) );
                rc = -EINVAL;
                goto discard;
        }
        if ( hlen > iob_len ( iobuf ) ) {
                DBG ( "TCP header too long at %zd bytes (max %zd bytes)\n",
                      hlen, iob_len ( iobuf ) );
                rc = -EINVAL;
                goto discard;
        }
        csum = tcpip_continue_chksum ( pshdr_csum, iobuf->data,
                                       iob_len ( iobuf ) );
        if ( csum != 0 ) {
                DBG ( "TCP checksum incorrect (is %04x including checksum "
                      "field, should be 0000)\n", csum );
                rc = -EINVAL;
                goto discard;
        }
        
        /* Parse parameters from header and strip header */
        tcp = tcp_demux ( ntohs ( tcphdr->dest ) );
        seq = ntohl ( tcphdr->seq );
        ack = ntohl ( tcphdr->ack );
        raw_win = ntohs ( tcphdr->win );
        flags = tcphdr->flags;
        if ( ( rc = tcp_rx_opts ( tcp, tcphdr, hlen, &options ) ) != 0 )
                goto discard;
        if ( tcp && options.tsopt )
                tcp->ts_val = ntohl ( options.tsopt->tsval );
        iob_pull ( iobuf, hlen );
        len = iob_len ( iobuf );
        seq_len = ( len + ( ( flags & TCP_SYN ) ? 1 : 0 ) +
                    ( ( flags & TCP_FIN ) ? 1 : 0 ) );

        /* Dump header */
        DBGC2 ( tcp, "TCP %p RX %d<-%d           %08x %08x..%08x %4zd",
                tcp, ntohs ( tcphdr->dest ), ntohs ( tcphdr->src ),
                ntohl ( tcphdr->ack ), ntohl ( tcphdr->seq ),
                ( ntohl ( tcphdr->seq ) + seq_len ), len );
        tcp_dump_flags ( tcp, tcphdr->flags );
        DBGC2 ( tcp, "\n" );

        /* If no connection was found, silently drop packet */
        if ( ! tcp ) {
                rc = -ENOTCONN;
                goto discard;
        }

        /* Record old data-transfer window */
        old_xfer_window = tcp_xfer_window ( tcp );

        /* Handle ACK, if present */
        if ( flags & TCP_ACK ) {
                win = ( raw_win << tcp->snd_win_scale );
                if ( ( rc = tcp_rx_ack ( tcp, ack, win ) ) != 0 ) {
                        tcp_xmit_reset ( tcp, st_src, tcphdr );
                        goto discard;
                }
        }

        /* Force an ACK if this packet is out of order */
        if ( ( tcp->tcp_state & TCP_STATE_RCVD ( TCP_SYN ) ) &&
             ( seq != tcp->rcv_ack ) ) {
                tcp->flags |= TCP_ACK_PENDING;
        }

        /* Handle SYN, if present */
        if ( flags & TCP_SYN ) {
                tcp_rx_syn ( tcp, seq, &options );
                seq++;
        }

        /* Handle RST, if present */
        if ( flags & TCP_RST ) {
                if ( ( rc = tcp_rx_rst ( tcp, seq ) ) != 0 )
                        goto discard;
        }

        /* Enqueue received data */
        tcp_rx_enqueue ( tcp, seq, flags, iob_disown ( iobuf ) );

        /* Process receive queue */
        tcp_process_rx_queue ( tcp );

        /* Dump out any state change as a result of the received packet */
        tcp_dump_state ( tcp );

        /* Schedule transmission of ACK (and any pending data).  If we
         * have received any out-of-order packets (i.e. if the receive
         * queue remains non-empty after processing) then send the ACK
         * immediately in order to trigger Fast Retransmission.
         */
        if ( list_empty ( &tcp->rx_queue ) ) {
                process_add ( &tcp->process );
        } else {
                tcp_xmit_sack ( tcp, seq );
        }

        /* If this packet was the last we expect to receive, set up
         * timer to expire and cause the connection to be freed.
         */
        if ( TCP_CLOSED_GRACEFULLY ( tcp->tcp_state ) ) {
                stop_timer ( &tcp->wait );
                start_timer_fixed ( &tcp->wait, ( 2 * TCP_MSL ) );
        }

        /* Notify application if window has changed */
        if ( tcp_xfer_window ( tcp ) != old_xfer_window )
                xfer_window_changed ( &tcp->xfer );

        profile_stop ( &tcp_rx_profiler );
        return 0;

 discard:
        /* Free received packet */
        free_iob ( iobuf );
        return rc;
}

/** TCP protocol */
struct tcpip_protocol tcp_protocol __tcpip_protocol = {
        .name = "TCP",
        .rx = tcp_rx,
        .tcpip_proto = IP_TCP,
};

/**
 * Discard some cached TCP data
 *
 * @ret discarded       Number of cached items discarded
 */
static unsigned int tcp_discard ( void ) {
        struct tcp_connection *tcp;
        struct io_buffer *iobuf;
        unsigned int discarded = 0;

        /* Try to drop one queued RX packet from each connection */
        list_for_each_entry ( tcp, &tcp_conns, list ) {
                list_for_each_entry_reverse ( iobuf, &tcp->rx_queue, list ) {

                        /* Remove packet from queue */
                        list_del ( &iobuf->list );
                        free_iob ( iobuf );

                        /* Report discard */
                        discarded++;
                        break;
                }
        }

        return discarded;
}

/** TCP cache discarder */
struct cache_discarder tcp_discarder __cache_discarder ( CACHE_NORMAL ) = {
        .discard = tcp_discard,
};

/**
 * Find first TCP connection that has not yet been closed
 *
 * @ret tcp             First unclosed connection, or NULL
 */
static struct tcp_connection * tcp_first_unclosed ( void ) {
        struct tcp_connection *tcp;

        /* Find first connection which has not yet been closed */
        list_for_each_entry ( tcp, &tcp_conns, list ) {
                if ( ! ( tcp->flags & TCP_XFER_CLOSED ) )
                        return tcp;
        }
        return NULL;
}

/**
 * Find first TCP connection that has not yet finished all operations
 *
 * @ret tcp             First unfinished connection, or NULL
 */
static struct tcp_connection * tcp_first_unfinished ( void ) {
        struct tcp_connection *tcp;

        /* Find first connection which has not yet closed gracefully,
         * or which still has a pending transmission (e.g. to ACK the
         * received FIN).
         */
        list_for_each_entry ( tcp, &tcp_conns, list ) {
                if ( ( ! TCP_CLOSED_GRACEFULLY ( tcp->tcp_state ) ) ||
                     process_running ( &tcp->process ) ) {
                        return tcp;
                }
        }
        return NULL;
}

/**
 * Shut down all TCP connections
 *
 */
static void tcp_shutdown ( int booting __unused ) {
        struct tcp_connection *tcp;
        unsigned long start;
        unsigned long elapsed;

        /* Initiate a graceful close of all connections, allowing for
         * the fact that the connection list may change as we do so.
         */
        while ( ( tcp = tcp_first_unclosed() ) ) {
                DBGC ( tcp, "TCP %p closing for shutdown\n", tcp );
                tcp_close ( tcp, -ECANCELED );
        }

        /* Wait for all connections to finish closing gracefully */
        start = currticks();
        while ( ( tcp = tcp_first_unfinished() ) &&
                ( ( elapsed = ( currticks() - start ) ) < TCP_FINISH_TIMEOUT )){
                step();
        }

        /* Forcibly close any remaining connections */
        while ( ( tcp = list_first_entry ( &tcp_conns, struct tcp_connection,
                                           list ) ) != NULL ) {
                tcp->tcp_state = TCP_CLOSED;
                tcp_dump_state ( tcp );
                tcp_close ( tcp, -ECANCELED );
        }
}

/** TCP shutdown function */
struct startup_fn tcp_startup_fn __startup_fn ( STARTUP_LATE ) = {
        .name = "tcp",
        .shutdown = tcp_shutdown,
};

/***************************************************************************
 *
 * Data transfer interface
 *
 ***************************************************************************
 */

/**
 * Close interface
 *
 * @v tcp               TCP connection
 * @v rc                Reason for close
 */
static void tcp_xfer_close ( struct tcp_connection *tcp, int rc ) {

        /* Close data transfer interface */
        tcp_close ( tcp, rc );

        /* Transmit FIN, if possible */
        tcp_xmit ( tcp );
}

/**
 * Deliver datagram as I/O buffer
 *
 * @v tcp               TCP connection
 * @v iobuf             Datagram I/O buffer
 * @v meta              Data transfer metadata
 * @ret rc              Return status code
 */
static int tcp_xfer_deliver ( struct tcp_connection *tcp,
                              struct io_buffer *iobuf,
                              struct xfer_metadata *meta __unused ) {

        /* Enqueue packet */
        list_add_tail ( &iobuf->list, &tcp->tx_queue );

        /* Each enqueued packet is a pending operation */
        pending_get ( &tcp->pending_data );

        /* Transmit data, if possible */
        tcp_xmit ( tcp );

        return 0;
}

/**
 * Report job progress
 *
 * @v tcp               TCP connection
 * @v progress          Progress report to fill in
 * @ret ongoing_rc      Ongoing job status code (if known)
 */
static int tcp_progress ( struct tcp_connection *tcp,
                          struct job_progress *progress ) {

        /* Report connection in progress if applicable */
        if ( ! TCP_HAS_BEEN_ESTABLISHED ( tcp->tcp_state ) ) {
                snprintf ( progress->message, sizeof ( progress->message ),
                           "connecting" );
        }

        return 0;
}

/** TCP data transfer interface operations */
static struct interface_operation tcp_xfer_operations[] = {
        INTF_OP ( xfer_deliver, struct tcp_connection *, tcp_xfer_deliver ),
        INTF_OP ( xfer_window, struct tcp_connection *, tcp_xfer_window ),
        INTF_OP ( job_progress, struct tcp_connection *, tcp_progress ),
        INTF_OP ( intf_close, struct tcp_connection *, tcp_xfer_close ),
};

/** TCP data transfer interface descriptor */
static struct interface_descriptor tcp_xfer_desc =
        INTF_DESC ( struct tcp_connection, xfer, tcp_xfer_operations );

/***************************************************************************
 *
 * Openers
 *
 ***************************************************************************
 */

/** TCP socket opener */
struct socket_opener tcp_socket_opener __socket_opener = {
        .semantics      = TCP_SOCK_STREAM,
        .open           = tcp_open,
};

/** Linkage hack */
int tcp_sock_stream = TCP_SOCK_STREAM;

/**
 * Open TCP URI
 *
 * @v xfer              Data transfer interface
 * @v uri               URI
 * @ret rc              Return status code
 */
static int tcp_open_uri ( struct interface *xfer, struct uri *uri ) {
        struct sockaddr_tcpip peer;

        /* Sanity check */
        if ( ! uri->host )
                return -EINVAL;

        memset ( &peer, 0, sizeof ( peer ) );
        peer.st_port = htons ( uri_port ( uri, 0 ) );
        return xfer_open_named_socket ( xfer, SOCK_STREAM,
                                        ( struct sockaddr * ) &peer,
                                        uri->host, NULL );
}

/** TCP URI opener */
struct uri_opener tcp_uri_opener __uri_opener = {
        .scheme         = "tcp",
        .open           = tcp_open_uri,
};