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

[Stefano Brivio <sbrivio@redhat.com>]

On Tue, 14 May 2024 16:19:16 -0400
Jon Maloy <jmaloy@redhat.com> wrote:

> On 2024-05-14 13:46, Stefano Brivio wrote:
> > 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.  
> I think I have a good idea here. I'll use it in my next version.
> 
> >  
> >>   
> >> -				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".  
> It really means "we are rewinding seq_to_tap from X to Y".  That it is 
> caused by a
> duplicate ack is implicit.

I wouldn't take that for granted, so much that with the current version
of this patch, it's *not* necessarily caused by a duplicate
acknowledgement.

Anyway, it really doesn't look intuitive to me, and users have to
figure out what's happening, too.

> > 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.  
> Ok. I missed that.
> > 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.  
> But then we have no probing... Wasn't that the whole pint of this?

We generally do. We would have no probing only in case an ACK for data
that the peer *sent* us is due by us (the other way around).

There, probing would mean causing the peer to re-transmit, which we
don't want to trigger here.

> >  
> >>   	}
> >>   }
> >>   
> >> 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"?  
> I tried to come up with something short, because the field name becomes
> impractically long. I am open to suggestions.

@wnd_edge_from_tap?

-- 
Stefano