dcsctp: Support zero window probing

This is explained in RFC 4960, section 6.1, A.

  ... 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 ...  This rule
  allows the sender to probe for a change in rwnd that the sender
  missed due to the SACK's having been lost in transit from the data
  receiver to the data sender.

Before this change, when a receiver has advertised a zero receiver
window size (a_rwnd=0) and a subsequent SACK advertising a non-zero
receiver window was lost, the sender was blocked from sending and since
SACKs are only sent when a DATA chunk is received, it would be
deadlocked. The retransmission timer would fire, but nothing would be
retransmitted (as it respected the zero receiver window).

With this change, when the retransmission timer fires (after RTO), it
would send a single packet with DATA chunk(s) and then SACKs would
eventually be received, with the non-zero receiver window and the socket
would recover.

Bug: chromium:1258225
Change-Id: I1ea62fb3c002150eeada28d3e703dbc09cfd038e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/235280
Commit-Queue: Victor Boivie <boivie@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35215}

net/dcsctp/tx/BUILD.gn

@@ -122,6 +122,7 @@ if (rtc_include_tests) {
       "../../../rtc_base:rtc_base_approved",
       "../../../test:test_support",
       "../common:handover_testing",
+      "../common:math",
       "../packet:chunk",
       "../packet:data",
       "../public:socket",

net/dcsctp/tx/retransmission_queue.cc

@@ -852,6 +852,15 @@ void RetransmissionQueue::AbandonAllFor(
 
 size_t RetransmissionQueue::max_bytes_to_send() const {
   size_t left = outstanding_bytes_ >= cwnd_ ? 0 : cwnd_ - outstanding_bytes_;
+
+  if (outstanding_bytes_ == 0) {
+    // https://datatracker.ietf.org/doc/html/rfc4960#section-6.1
+    // ... 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).
+    return left;
+  }
+
   return std::min(rwnd(), left);
 }
 

net/dcsctp/tx/retransmission_queue_test.cc

@@ -19,6 +19,7 @@
 #include "absl/types/optional.h"
 #include "api/array_view.h"
 #include "net/dcsctp/common/handover_testing.h"
+#include "net/dcsctp/common/math.h"
 #include "net/dcsctp/packet/chunk/data_chunk.h"
 #include "net/dcsctp/packet/chunk/forward_tsn_chunk.h"
 #include "net/dcsctp/packet/chunk/forward_tsn_common.h"
@@ -1408,5 +1409,84 @@ TEST_F(RetransmissionQueueTest, HandoverTest) {
                           Pair(TSN(14), State::kInFlight)));
 }
 
+TEST_F(RetransmissionQueueTest, CanAlwaysSendOnePacket) {
+  RetransmissionQueue queue = CreateQueue();
+
+  // A large payload - enough to not fit two DATA in same packet.
+  size_t mtu = RoundDownTo4(options_.mtu);
+  std::vector<uint8_t> payload(mtu - 100);
+
+  EXPECT_CALL(producer_, Produce)
+      .WillOnce([this, payload](TimeMs, size_t) {
+        return SendQueue::DataToSend(gen_.Ordered(payload, "B"));
+      })
+      .WillOnce([this, payload](TimeMs, size_t) {
+        return SendQueue::DataToSend(gen_.Ordered(payload, ""));
+      })
+      .WillOnce([this, payload](TimeMs, size_t) {
+        return SendQueue::DataToSend(gen_.Ordered(payload, ""));
+      })
+      .WillOnce([this, payload](TimeMs, size_t) {
+        return SendQueue::DataToSend(gen_.Ordered(payload, ""));
+      })
+      .WillOnce([this, payload](TimeMs, size_t) {
+        return SendQueue::DataToSend(gen_.Ordered(payload, "E"));
+      })
+      .WillRepeatedly([](TimeMs, size_t) { return absl::nullopt; });
+
+  // Produce all chunks and put them in the retransmission queue.
+  std::vector<std::pair<TSN, Data>> chunks_to_send =
+      queue.GetChunksToSend(now_, 5 * mtu);
+  EXPECT_THAT(chunks_to_send,
+              ElementsAre(Pair(TSN(10), _), Pair(TSN(11), _), Pair(TSN(12), _),
+                          Pair(TSN(13), _), Pair(TSN(14), _)));
+  EXPECT_THAT(queue.GetChunkStatesForTesting(),
+              ElementsAre(Pair(TSN(9), State::kAcked),      //
+                          Pair(TSN(10), State::kInFlight),  //
+                          Pair(TSN(11), State::kInFlight),  //
+                          Pair(TSN(12), State::kInFlight),
+                          Pair(TSN(13), State::kInFlight),
+                          Pair(TSN(14), State::kInFlight)));
+
+  // Ack 12, and report an empty receiver window (the peer obviously has a
+  // tiny receive window).
+  queue.HandleSack(
+      now_, SackChunk(TSN(9), /*rwnd=*/0, {SackChunk::GapAckBlock(3, 3)}, {}));
+
+  // Force TSN 10 to be retransmitted.
+  queue.HandleT3RtxTimerExpiry();
+
+  // Even if the receiver window is empty, it will allow TSN 10 to be sent.
+  EXPECT_THAT(queue.GetChunksToSend(now_, mtu), ElementsAre(Pair(TSN(10), _)));
+
+  // But not more than that, as there now is outstanding data.
+  EXPECT_THAT(queue.GetChunksToSend(now_, mtu), IsEmpty());
+
+  // Don't ack any new data, and still have receiver window zero.
+  queue.HandleSack(
+      now_, SackChunk(TSN(9), /*rwnd=*/0, {SackChunk::GapAckBlock(3, 3)}, {}));
+
+  // There is in-flight data, so new data should not be allowed to be send since
+  // the receiver window is full.
+  EXPECT_THAT(queue.GetChunksToSend(now_, mtu), IsEmpty());
+
+  // Ack that packet (no more in-flight data), but still report an empty
+  // receiver window.
+  queue.HandleSack(
+      now_, SackChunk(TSN(10), /*rwnd=*/0, {SackChunk::GapAckBlock(2, 2)}, {}));
+
+  // Then TSN 11 can be sent, as there is no in-flight data.
+  EXPECT_THAT(queue.GetChunksToSend(now_, mtu), ElementsAre(Pair(TSN(11), _)));
+  EXPECT_THAT(queue.GetChunksToSend(now_, mtu), IsEmpty());
+
+  // Ack and recover the receiver window
+  queue.HandleSack(now_, SackChunk(TSN(12), /*rwnd=*/5 * mtu, {}, {}));
+
+  // That will unblock sending remaining chunks.
+  EXPECT_THAT(queue.GetChunksToSend(now_, mtu), ElementsAre(Pair(TSN(13), _)));
+  EXPECT_THAT(queue.GetChunksToSend(now_, mtu), ElementsAre(Pair(TSN(14), _)));
+  EXPECT_THAT(queue.GetChunksToSend(now_, mtu), IsEmpty());
+}
+
 }  // namespace
 }  // namespace dcsctp