Re: [PATCH v3 3/3] tcp: allow retransmit when peer receive window is zero

[Stefano Brivio <sbrivio@redhat.com>]

On Sat, 11 May 2024 11:20:08 -0400
Jon Maloy <jmaloy@redhat.com> wrote:

> A bug in kernel TCP may lead to a deadlock where a zero window is sent
> from the peer, while it is unable to send out window updates even after
> reads have freed up enough buffer space to permit a larger window.
> In this situation, new window advertisemnts from the peer can only be
> triggered by packets arriving from this side.
> 
> However, such packets are never sent, because the zero-window condition
> currently prevents this side from sending out any packets whatsoever
> to the peer.
> 
> We notice that the above bug is triggered *only* after the peer has
> dropped an arriving packet because of severe memory squeeze, and that we
> hence always enter a retransmission situation when this occurs. This
> also means that it goes against the RFC 9293 recommendation that a
> previously advertised window never should shrink.
> 
> RFC 9293 gives the solution to this situation. In chapter 3.6.1 we find
> the following statement:
> "A TCP receiver SHOULD NOT shrink the window, i.e., move the right
> window edge to the left (SHLD-14). However, a sending TCP peer MUST
> be robust against window shrinking, which may cause the
> "usable window" (see Section 3.8.6.2.1) to become negative (MUST-34).
> 
> If this happens, the sender SHOULD NOT send new data (SHLD-15), but
> SHOULD retransmit normally the old unacknowledged data between SND.UNA
> and SND.UNA+SND.WND (SHLD-16). The sender MAY also retransmit old data
> beyond SND.UNA+SND.WND (MAY-7)"
> 
> We never see the window become negative, but we interpret this as a
> recommendation to use the previously available window during
> retransmission even when the currently advertised window is zero.
> 
> In case of a zero-window non-retransmission situation where there
> is no new data to be sent, we also add a simple zero-window probing
> feature. By sending an empty packet at regular timeout events we
> resolve the situation described above, since the peer receives the
> necessary trigger to advertise its window once it becomes non-zero
> again.
> 
> It should be noted that although this solves the problem we have at
> hand, it is not a genuine solution to the kernel bug. There may well
> be TCP stacks around in other OS-es which don't do this, nor have
> keep-alive probing as an alternatve way to solve the situation.
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
> 
> ---
> v2: - Using previously advertised window during retransmission, instead
>       highest send sequencece number in the cycle.
> v3: - Rebased to newest code
>     - Changes based on feedback from PASST team
>     - Sending out empty probe message at timer expiration when
>       we are not in retransmit situation.
> ---
>  tcp.c      | 30 +++++++++++++++++++++---------
>  tcp_conn.h |  2 ++
>  2 files changed, 23 insertions(+), 9 deletions(-)
> 
> diff --git a/tcp.c b/tcp.c
> index 8297812..bd6bf35 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -1774,9 +1774,15 @@ static void tcp_get_tap_ws(struct tcp_tap_conn *conn,
>   */
>  static void tcp_tap_window_update(struct tcp_tap_conn *conn, unsigned wnd)
>  {
> +	uint32_t wnd_upper;
> +
>  	wnd = MIN(MAX_WINDOW, wnd << conn->ws_from_tap);
>  	conn->wnd_from_tap = MIN(wnd >> conn->ws_from_tap, USHRT_MAX);
>  
> +	wnd_upper = conn->seq_ack_from_tap + wnd;
> +	if (wnd && SEQ_GT(wnd_upper, conn->seq_wup_from_tap))
> +		conn->seq_wup_from_tap = wnd_upper;
> +
>  	/* FIXME: reflect the tap-side receiver's window back to the sock-side
>  	 * sender by adjusting SO_RCVBUF? */
>  }
> @@ -1809,6 +1815,7 @@ static void tcp_seq_init(const struct ctx *c, struct tcp_tap_conn *conn,
>  	ns = (now->tv_sec * 1000000000 + now->tv_nsec) >> 5;
>  
>  	conn->seq_to_tap = ((uint32_t)(hash >> 32) ^ (uint32_t)hash) + ns;
> +	conn->seq_wup_from_tap = conn->seq_to_tap;
>  }
>  
>  /**
> @@ -2220,7 +2227,6 @@ static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn,
>   */
>  static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn *conn)
>  {
> -	uint32_t wnd_scaled = conn->wnd_from_tap << conn->ws_from_tap;
>  	int fill_bufs, send_bufs = 0, last_len, iov_rem = 0;
>  	int sendlen, len, dlen, v4 = CONN_V4(conn);
>  	uint32_t max_send, seq, already_sent;
> @@ -2241,10 +2247,11 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn *conn)
>  	}
>  
>  	/* How much are we still allowed to send within current window ? */
> -	max_send = conn->seq_ack_from_tap + wnd_scaled - conn->seq_to_tap;
> +	max_send = conn->seq_wup_from_tap - conn->seq_to_tap;
>  	if (SEQ_LE(max_send, 0)) {
> -		flow_trace(conn, "Empty window: win: %u, sent: %u",
> -			   wnd_scaled, conn->seq_to_tap);
> +		flow_trace(conn, "Empty window: win_upper: %u, sent: %u",
> +			   conn->seq_wup_from_tap, conn->seq_to_tap);
> +		conn->seq_wup_from_tap = conn->seq_to_tap;
>  		conn_flag(c, conn, STALLED);
>  		conn_flag(c, conn, ACK_FROM_TAP_DUE);
>  		return 0;
> @@ -2380,7 +2387,7 @@ static int tcp_data_from_tap(struct ctx *c, struct tcp_tap_conn *conn,
>  	ASSERT(conn->events & ESTABLISHED);
>  
>  	for (i = idx, iov_i = 0; i < (int)p->count; i++) {
> -		uint32_t seq, seq_offset, ack_seq;
> +		uint32_t seq, seq_offset, ack_seq, wnd;
>  		const struct tcphdr *th;
>  		char *data;
>  		size_t off;
> @@ -2413,11 +2420,12 @@ static int tcp_data_from_tap(struct ctx *c, struct tcp_tap_conn *conn,
>  			if (SEQ_GE(ack_seq, conn->seq_ack_from_tap) &&
>  			    SEQ_GE(ack_seq, max_ack_seq)) {
>  				/* Fast re-transmit */
> +				wnd = ntohs(th->window);
>  				retr = !len && !th->fin &&
>  				       ack_seq == max_ack_seq &&
> -				       ntohs(th->window) == max_ack_seq_wnd;
> +				       (wnd == max_ack_seq_wnd || !wnd);

Just as a reminder, as I mentioned on Monday: this means we'll
re-transmit whenever we get a pure window update (!len && !th->fin
&& ack_seq == max_ack_seq) with a zero window. The receiver is telling
us it ran out of space, and wham, we flood them, as a punishment.

I would let this check alone, and just add zero-window probing, plus
whatever retransmission you mentioned from the RFC -- but not a fast
re-transmit on a zero window.

>  
> -				max_ack_seq_wnd = ntohs(th->window);
> +				max_ack_seq_wnd = wnd;
>  				max_ack_seq = ack_seq;
>  			}
>  		}
> @@ -2480,8 +2488,9 @@ static int tcp_data_from_tap(struct ctx *c, struct tcp_tap_conn *conn,
>  
>  	if (retr) {
>  		flow_trace(conn,
> -			   "fast re-transmit, ACK: %u, previous sequence: %u",
> -			   max_ack_seq, conn->seq_to_tap);
> +			   "fast re-transmit, seqno %u -> %u, win_upper: %u",
> +			   conn->seq_to_tap, max_ack_seq,

I'm not sure if "->" really conveys the meaning of "we're sending this
sequence *because* of that acknowledgement number". I would rather keep
the received acknowledged sequence before everything else, because
that's the causal trigger for the retransmission.

> +			   conn->seq_wup_from_tap);
>  
>  		conn->seq_to_tap = max_ack_seq;
>  		tcp_set_peek_offset(conn->sock, 0);
> @@ -2931,6 +2940,9 @@ void tcp_timer_handler(struct ctx *c, union epoll_ref ref)
>  			flow_dbg(conn, "activity timeout");
>  			tcp_rst(c, conn);
>  		}
> +		/* No data sent recently? Keep connection alive. */
> +		if (conn->seq_to_tap == conn->seq_ack_from_tap)
> +			tcp_send_flag(c, conn, ACK_IF_NEEDED);

If the window is zero, this won't send anything, see the first
condition in tcp_send_flag(). ACK_IF_NEEDED implies that that function
should queue an ACK segment if we have data to acknowledge.

Here, the flag you want is simply 'ACK'. But we should make sure that
this can't be taken as a duplicate ACK, that is, we should only send
this if seq_ack_to_tap == seq_from_tap.

Otherwise, we shouldn't send anything, lest the peer retransmit
anything that we didn't acknowledge yet.

>  	}
>  }
>  
> diff --git a/tcp_conn.h b/tcp_conn.h
> index d280b22..8ae20ef 100644
> --- a/tcp_conn.h
> +++ b/tcp_conn.h
> @@ -30,6 +30,7 @@
>   * @wnd_to_tap:		Sending window advertised to tap, unscaled (as sent)
>   * @seq_to_tap:		Next sequence for packets to tap
>   * @seq_ack_from_tap:	Last ACK number received from tap
> + * @seq_wup_from_tap:	Right edge of last non-zero window from tap

"Right edge" makes much more sense to me, and it also matches RFC
language. Could we turn all the "wup" and "upper" references into
something like "edge" or "right_edge"?

>   * @seq_from_tap:	Next sequence for packets from tap (not actually sent)
>   * @seq_ack_to_tap:	Last ACK number sent to tap
>   * @seq_init_from_tap:	Initial sequence number from tap
> @@ -101,6 +102,7 @@ struct tcp_tap_conn {
>  
>  	uint32_t	seq_to_tap;
>  	uint32_t	seq_ack_from_tap;
> +	uint32_t	seq_wup_from_tap;
>  	uint32_t	seq_from_tap;
>  	uint32_t	seq_ack_to_tap;
>  	uint32_t	seq_init_from_tap;

-- 
Stefano