diff --git a/eth/utp/packets.nim b/eth/utp/packets.nim
index fd5118d..34cea2c 100644
--- a/eth/utp/packets.nim
+++ b/eth/utp/packets.nim
@@ -48,8 +48,8 @@ type
     # sequence number the sender of the packet last received in the other direction
     ackNr*: uint16
 
-  SelectiveAckExtension = object
-    acks: array[acksArrayLength, byte]
+  SelectiveAckExtension* = object
+    acks*: array[4, byte]
 
   Packet* = object
     header*: PacketHeaderV1
diff --git a/eth/utp/utp_socket.nim b/eth/utp/utp_socket.nim
index 0778ed1..b0f7154 100644
--- a/eth/utp/utp_socket.nim
+++ b/eth/utp/utp_socket.nim
@@ -9,7 +9,7 @@
 import
   std/sugar,
   chronos, chronicles, bearssl,
-  stew/results,
+  stew/[results, bitops2],
   ./send_buffer_tracker,
   ./growable_buffer,
   ./packets,
@@ -341,20 +341,6 @@ proc registerOutgoingPacket(socket: UtpSocket, oPacket: OutgoingPacket) =
 proc sendData(socket: UtpSocket, data: seq[byte]): Future[void] =
   socket.send(socket.remoteAddress, data)
 
-proc sendAck(socket: UtpSocket): Future[void] =
-  ## Creates and sends ack, based on current socket state. Acks are different from
-  ## other packets as we do not track them in outgoing buffet
-
-  let ackPacket =
-    ackPacket(
-      socket.seqNr,
-      socket.connectionIdSnd,
-      socket.ackNr,
-      socket.getRcvWindowSize(),
-      socket.replayMicro
-    )
-  socket.sendData(encodePacket(ackPacket))
-
 # Should be called before sending packet
 proc setSend(s: UtpSocket, p: var OutgoingPacket): seq[byte] =
   let timestampInfo = getMonoTimestamp()
@@ -725,12 +711,6 @@ proc startOutgoingSocket*(socket: UtpSocket): Future[void] {.async.} =
   await socket.sendData(outgoingPacket.packetBytes)
   await socket.connectionFuture
 
-proc startIncomingSocket*(socket: UtpSocket) {.async.} =
-  # Make sure ack was flushed before moving forward
-  await socket.sendAck()
-  socket.startWriteLoop()
-  socket.startTimeoutLoop()
-
 proc isConnected*(socket: UtpSocket): bool =
   socket.state == Connected
 
@@ -881,6 +861,121 @@ proc isAckNrInvalid(socket: UtpSocket, packet: Packet): bool =
     )
   )
 
+# counts the number of bytes acked by selective ack header
+proc calculateSelectiveAckBytes*(socket: UtpSocket,  receivedPackedAckNr: uint16, ext: SelectiveAckExtension): uint32 =
+  # we add 2, as the first bit in the mask therefore represents ackNr + 2 becouse
+  # ackNr + 1 (i.e next expected packet) is considered lost.
+  let base = receivedPackedAckNr + 2
+
+  if socket.curWindowPackets == 0:
+    return 0
+
+  var ackedBytes = 0'u32
+
+  var bits = (len(ext.acks)) * 8 - 1
+  
+  while bits >= 0:
+    let v = base + uint16(bits)
+
+    if (socket.seqNr - v - 1) >= socket.curWindowPackets - 1:
+      dec bits
+      continue
+
+    let maybePacket = socket.outBuffer.get(v)
+
+    if (maybePacket.isNone() or maybePacket.unsafeGet().transmissions == 0):
+      dec bits
+      continue
+    
+    let pkt = maybePacket.unsafeGet()
+
+    if (getBit(ext.acks, bits)):
+      ackedBytes = ackedBytes + pkt.payloadLength
+    
+    dec bits
+
+  return ackedBytes
+
+# ack packets (removes them from out going buffer) based on selective ack extension header
+proc selectiveAckPackets(socket: UtpSocket,  receivedPackedAckNr: uint16, ext: SelectiveAckExtension, currentTime: Moment): void =
+   # we add 2, as the first bit in the mask therefore represents ackNr + 2 becouse
+  # ackNr + 1 (i.e next expected packet) is considered lost.
+  let base = receivedPackedAckNr + 2
+
+  if socket.curWindowPackets == 0:
+    return
+
+  var bits = (len(ext.acks)) * 8 - 1
+  
+  while bits >= 0:
+    let v = base + uint16(bits)
+
+    if (socket.seqNr - v - 1) >= socket.curWindowPackets - 1:
+      dec bits
+      continue
+
+    let maybePacket = socket.outBuffer.get(v)
+
+    if (maybePacket.isNone() or maybePacket.unsafeGet().transmissions == 0):
+      dec bits
+      continue
+    
+    let pkt = maybePacket.unsafeGet()
+
+    if (getBit(ext.acks, bits)):
+      discard socket.ackPacket(v, currentTime)
+    
+    dec bits
+
+  # TODO Add handling of fast timeouts and duplicate acks counting
+
+# Public mainly for test purposes
+# generates bit mask which indicates which packets are already in socket
+# reorder buffer
+# from speck:
+# The bitmask has reverse byte order. The first byte represents packets [ack_nr + 2, ack_nr + 2 + 7] in reverse order
+# The least significant bit in the byte represents ack_nr + 2, the most significant bit in the byte represents ack_nr + 2 + 7
+# The next byte in the mask represents [ack_nr + 2 + 8, ack_nr + 2 + 15] in reverse order, and so on
+proc generateSelectiveAckBitMask*(socket: UtpSocket): array[4, byte] = 
+  let window = min(32, socket.inBuffer.len())
+  var arr: array[4, uint8] = [0'u8, 0, 0, 0]
+  var i = 0
+  while i < window:
+    if (socket.inBuffer.get(socket.ackNr + uint16(i) + 2).isSome()):
+      setBit(arr, i)
+    inc i
+  return arr
+
+# Generates ack packet based on current state of the socket.
+proc generateAckPacket*(socket: UtpSocket): Packet = 
+    let bitmask =
+      if (socket.reorderCount != 0 and (not socket.reachedFin)):
+        some(socket.generateSelectiveAckBitMask())
+      else:
+        none[array[4, byte]]()
+
+    ackPacket(
+      socket.seqNr,
+      socket.connectionIdSnd,
+      socket.ackNr,
+      socket.getRcvWindowSize(),
+      socket.replayMicro,
+      bitmask
+    )
+
+proc sendAck(socket: UtpSocket): Future[void] =
+  ## Creates and sends ack, based on current socket state. Acks are different from
+  ## other packets as we do not track them in outgoing buffet
+
+  let ackPacket = socket.generateAckPacket()
+  socket.sendData(encodePacket(ackPacket))
+
+proc startIncomingSocket*(socket: UtpSocket) {.async.} =
+  # Make sure ack was flushed before moving forward
+  await socket.sendAck()
+  socket.startWriteLoop()
+  socket.startTimeoutLoop()
+
 # TODO at socket level we should handle only FIN/DATA/ACK packets. Refactor to make
 # it enforcable by type system
 # TODO re-think synchronization of this procedure, as each await inside gives control
@@ -888,7 +983,7 @@ proc isAckNrInvalid(socket: UtpSocket, packet: Packet): bool =
 # running
 proc processPacket*(socket: UtpSocket, p: Packet) {.async.} =
   let timestampInfo = getMonoTimestamp()
-
+  
   if socket.isAckNrInvalid(p):
     notice "Received packet with invalid ack nr"
     return
@@ -923,6 +1018,10 @@ proc processPacket*(socket: UtpSocket, p: Packet) {.async.} =
   var (ackedBytes, minRtt) = socket.calculateAckedbytes(acks, timestampInfo.moment)
   # TODO caluclate bytes acked by selective acks here (if thats the case)
 
+  if (p.eack.isSome()):
+    let selectiveAckedBytes = socket.calculateSelectiveAckBytes(pkAckNr, p.eack.unsafeGet())
+    ackedBytes = ackedBytes + selectiveAckedBytes
+
   let sentTimeRemote = p.header.timestamp
 
   # we are using uint32 not a Duration, to wrap a round in case of
@@ -999,6 +1098,14 @@ proc processPacket*(socket: UtpSocket, p: Packet) {.async.} =
 
   socket.ackPackets(acks, timestampInfo.moment)
 
+  # packets in front may have been acked by selective ack, decrease window until we hit
+  # a packet that is still waiting to be acked
+  while (socket.curWindowPackets > 0 and socket.outBuffer.get(socket.seqNr - socket.curWindowPackets).isNone()):
+    dec socket.curWindowPackets
+
+  if (p.eack.isSome()):
+    socket.selectiveAckPackets(pkAckNr, p.eack.unsafeGet(), timestampInfo.moment)
+
   case p.header.pType
     of ST_DATA, ST_FIN:
       # To avoid amplification attacks, server socket is in SynRecv state until
@@ -1014,6 +1121,7 @@ proc processPacket*(socket: UtpSocket, p: Packet) {.async.} =
 
       # we got in order packet
       if (pastExpected == 0 and (not socket.reachedFin)):
+        notice "Got in order packet"
         if (len(p.payload) > 0 and (not socket.readShutdown)):
           # we are getting in order data packet, we can flush data directly to the incoming buffer
           await upload(addr socket.buffer, unsafeAddr p.payload[0], p.payload.len())
@@ -1085,8 +1193,10 @@ proc processPacket*(socket: UtpSocket, p: Packet) {.async.} =
           socket.inBuffer.put(pkSeqNr, p)
           inc socket.reorderCount
           notice "added out of order packet in reorder buffer"
-          # TODO for now we do not sent any ack as we do not handle selective acks
-          # add sending of selective acks
+          # we send ack packet, as we reoreder count is > 0, so the eack bitmask will be 
+          # generated
+          asyncSpawn socket.sendAck()
+
     of ST_STATE:
       if (socket.state == SynSent and (not socket.connectionFuture.finished())):
         socket.state = Connected
@@ -1220,13 +1330,11 @@ proc read*(socket: UtpSocket): Future[seq[byte]] {.async.}=
 
 # Check how many packets are still in the out going buffer, usefull for tests or
 # debugging.
-# It throws assertion error when number of elements in buffer do not equal kept counter
 proc numPacketsInOutGoingBuffer*(socket: UtpSocket): int =
   var num = 0
   for e in socket.outBuffer.items():
     if e.isSome():
       inc num
-  doAssert(num == int(socket.curWindowPackets))
   num
 
 # Check how many payload bytes are still in flight
diff --git a/tests/utp/all_utp_tests.nim b/tests/utp/all_utp_tests.nim
index 63d76af..e8beaa4 100644
--- a/tests/utp/all_utp_tests.nim
+++ b/tests/utp/all_utp_tests.nim
@@ -12,5 +12,6 @@ import
   ./test_discv5_protocol,
   ./test_buffer,
   ./test_utp_socket,
+  ./test_utp_socket_sack,
   ./test_utp_router,
   ./test_clock_drift_calculator
diff --git a/tests/utp/test_utils.nim b/tests/utp/test_utils.nim
index ed53310..30209ce 100644
--- a/tests/utp/test_utils.nim
+++ b/tests/utp/test_utils.nim
@@ -1,9 +1,14 @@
 import
   chronos,
+  ../../eth/utp/utp_socket,
+  ../../eth/utp/packets,
   ../../eth/keys
 
 type AssertionCallback = proc(): bool {.gcsafe, raises: [Defect].}
 
+let testBufferSize = 1024'u32
+let defaultRcvOutgoingId = 314'u16
+
 proc generateByteArray*(rng: var BrHmacDrbgContext, length: int): seq[byte] =
   var bytes = newSeq[byte](length)
   brHmacDrbgGenerate(rng, bytes)
@@ -16,3 +21,96 @@ proc waitUntil*(f: AssertionCallback): Future[void] {.async.} =
       break
     else:
       await sleepAsync(milliseconds(50))
+
+template connectOutGoingSocket*(
+  initialRemoteSeq: uint16,
+  q: AsyncQueue[Packet],
+  remoteReceiveBuffer: uint32 = testBufferSize,
+  cfg: SocketConfig = SocketConfig.init()): (UtpSocket[TransportAddress], Packet) =
+  let sock1 = newOutgoingSocket[TransportAddress](testAddress, initTestSnd(q), cfg, defaultRcvOutgoingId, rng[])
+  asyncSpawn sock1.startOutgoingSocket()
+  let initialPacket = await q.get()
+
+  check:
+    initialPacket.header.pType == ST_SYN
+
+  let responseAck =
+    ackPacket(
+      initialRemoteSeq,
+      initialPacket.header.connectionId,
+      initialPacket.header.seqNr,
+      remoteReceiveBuffer,
+      0
+    )
+
+  await sock1.processPacket(responseAck)
+
+  check:
+    sock1.isConnected()
+
+  (sock1, initialPacket)
+
+template connectOutGoingSocketWithIncoming*(
+  initialRemoteSeq: uint16,
+  outgoingQueue: AsyncQueue[Packet],
+  incomingQueue: AsyncQueue[Packet],
+  remoteReceiveBuffer: uint32 = testBufferSize,
+  cfg: SocketConfig = SocketConfig.init()): (UtpSocket[TransportAddress], UtpSocket[TransportAddress]) =
+  let outgoingSocket = newOutgoingSocket[TransportAddress](testAddress, initTestSnd(outgoingQueue), cfg, defaultRcvOutgoingId, rng[])
+  asyncSpawn outgoingSocket.startOutgoingSocket()
+  let initialPacket = await outgoingQueue.get()
+
+  check:
+    initialPacket.header.pType == ST_SYN
+
+  let incomingSocket = newIncomingSocket[TransportAddress](
+    testAddress,
+    initTestSnd(incomingQueue), 
+    cfg,
+    initialPacket.header.connectionId,
+    initialPacket.header.seqNr,
+    rng[]
+  )
+
+  await incomingSocket.startIncomingSocket()
+
+  let responseAck = await incomingQueue.get()
+
+  await outgoingSocket.processPacket(responseAck)
+
+  check:
+    outgoingSocket.isConnected()
+
+  (outgoingSocket, incomingSocket)
+
+
+proc generateDataPackets*(
+  numberOfPackets: uint16,
+  initialSeqNr: uint16,
+  connectionId: uint16,
+  ackNr: uint16,
+  rng: var BrHmacDrbgContext): seq[Packet] =
+  let packetSize = 100
+  var packets = newSeq[Packet]()
+  var i = 0'u16
+  while i < numberOfPackets:
+    let packet = dataPacket(
+      initialSeqNr + i,
+      connectionId,
+      ackNr,
+      testBufferSize,
+      generateByteArray(rng, packetSize),
+      0
+    )
+    packets.add(packet)
+
+    inc i
+
+  packets
+
+proc initTestSnd*(q: AsyncQueue[Packet]): SendCallback[TransportAddress]=
+  return  (
+    proc (to: TransportAddress, bytes: seq[byte]): Future[void] =
+      let p = decodePacket(bytes).get()
+      q.addLast(p)
+  )
diff --git a/tests/utp/test_utp_socket.nim b/tests/utp/test_utp_socket.nim
index dae7eb0..fdf617f 100644
--- a/tests/utp/test_utp_socket.nim
+++ b/tests/utp/test_utp_socket.nim
@@ -7,7 +7,7 @@
 {.used.}
 
 import
-  std/[algorithm, random, sequtils],
+  std/[algorithm, random, sequtils, options],
   chronos, bearssl, chronicles,
   testutils/unittests,
   ./test_utils,
@@ -22,71 +22,12 @@ procSuite "Utp socket unit test":
   let testBufferSize = 1024'u32
   let defaultRcvOutgoingId = 314'u16
 
-  proc initTestSnd(q: AsyncQueue[Packet]): SendCallback[TransportAddress]=
-    return  (
-      proc (to: TransportAddress, bytes: seq[byte]): Future[void] =
-        let p = decodePacket(bytes).get()
-        q.addLast(p)
-    )
-
-  proc generateDataPackets(
-    numberOfPackets: uint16,
-    initialSeqNr: uint16,
-    connectionId: uint16,
-    ackNr: uint16,
-    rng: var BrHmacDrbgContext): seq[Packet] =
-    let packetSize = 100
-    var packets = newSeq[Packet]()
-    var i = 0'u16
-    while i < numberOfPackets:
-      let packet = dataPacket(
-        initialSeqNr + i,
-        connectionId,
-        ackNr,
-        testBufferSize,
-        generateByteArray(rng, packetSize),
-        0
-      )
-      packets.add(packet)
-
-      inc i
-
-    packets
-
   proc packetsToBytes(packets: seq[Packet]): seq[byte] =
     var resultBytes = newSeq[byte]()
     for p in packets:
       resultBytes.add(p.payload)
     return resultBytes
 
-  template connectOutGoingSocket(
-    initialRemoteSeq: uint16,
-    q: AsyncQueue[Packet],
-    remoteReceiveBuffer: uint32 = testBufferSize,
-    cfg: SocketConfig = SocketConfig.init()): (UtpSocket[TransportAddress], Packet) =
-    let sock1 = newOutgoingSocket[TransportAddress](testAddress, initTestSnd(q), cfg, defaultRcvOutgoingId, rng[])
-    asyncSpawn sock1.startOutgoingSocket()
-    let initialPacket = await q.get()
-
-    check:
-      initialPacket.header.pType == ST_SYN
-
-    let responseAck =
-      ackPacket(
-        initialRemoteSeq,
-        initialPacket.header.connectionId,
-        initialPacket.header.seqNr,
-        remoteReceiveBuffer,
-        0
-      )
-
-    await sock1.processPacket(responseAck)
-
-    check:
-      sock1.isConnected()
-
-    (sock1, initialPacket)
-
   asyncTest "Starting outgoing socket should send Syn packet":
     let q = newAsyncQueue[Packet]()
     let defaultConfig = SocketConfig.init()
@@ -187,10 +128,11 @@ procSuite "Utp socket unit test":
     # TODO test is valid until implementing selective acks
     let q = newAsyncQueue[Packet]()
     let initalRemoteSeqNr = 10'u16
+    let numOfPackets = 10'u16
 
     let (outgoingSocket, initialPacket) = connectOutGoingSocket(initalRemoteSeqNr, q)
 
-    var packets = generateDataPackets(10, initalRemoteSeqNr, initialPacket.header.connectionId, initialPacket.header.seqNr, rng[])
+    var packets = generateDataPackets(numOfPackets, initalRemoteSeqNr, initialPacket.header.connectionId, initialPacket.header.seqNr, rng[])
 
     let data = packetsToBytes(packets)
 
@@ -200,12 +142,28 @@ procSuite "Utp socket unit test":
     for p in packets:
       await outgoingSocket.processPacket(p)
 
-    let ack2 = await q.get()
+    var sentAcks: seq[Packet] = @[]
+    
+    for i in 0'u16..<numOfPackets:
+      let ack = await q.get()
+      sentAcks.add(ack)
+
+    # all packets except last one should be selective acks, without bumped ackNr
+    for i in 0'u16..<numOfPackets - 1:
+      check:
+        sentAcks[i].header.ackNr == initalRemoteSeqNr - 1
+        sentAcks[i].eack.isSome()
+
+    # last ack should be normal ack packet (not selective one), and it should ack
+    # all remaining packets
+    let lastAck = sentAcks[numOfPackets - 1]
 
     check:
-      ack2.header.pType == ST_STATE
+      lastAck.header.pType == ST_STATE
       # we are acking in one shot whole 10 packets
-      ack2.header.ackNr == initalRemoteSeqNr + uint16(len(packets) - 1)
+      lastAck.header.ackNr == initalRemoteSeqNr + uint16(len(packets) - 1)
+
+      lastAck.eack.isNone()
 
     let receivedData = await outgoingSocket.read(len(data))
 
@@ -218,10 +176,11 @@ procSuite "Utp socket unit test":
     # TODO test is valid until implementing selective acks
     let q = newAsyncQueue[Packet]()
     let initalRemoteSeqNr = 10'u16
+    let numOfPackets = 3'u16
 
     let (outgoingSocket, initialPacket) = connectOutGoingSocket(initalRemoteSeqNr, q)
 
-    var packets = generateDataPackets(3, initalRemoteSeqNr, initialPacket.header.connectionId, initialPacket.header.seqNr, rng[])
+    var packets = generateDataPackets(numOfPackets, initalRemoteSeqNr, initialPacket.header.connectionId, initialPacket.header.seqNr, rng[])
 
     let data = packetsToBytes(packets)
 
@@ -235,12 +194,28 @@ procSuite "Utp socket unit test":
     for p in packets:
       await outgoingSocket.processPacket(p)
 
-    let ack2 = await q.get()
+    var sentAcks: seq[Packet] = @[]
+    
+    for i in 0'u16..<numOfPackets:
+      let ack = await q.get()
+      sentAcks.add(ack)
+
+    # all packets except last one should be selective acks, without bumped ackNr
+    for i in 0'u16..<numOfPackets - 1:
+      check:
+        sentAcks[i].header.ackNr == initalRemoteSeqNr - 1
+        sentAcks[i].eack.isSome()
+
+    # last ack should be normal ack packet (not selective one), and it should ack
+    # all remaining packets
+    let lastAck = sentAcks[numOfPackets - 1]
 
     check:
-      ack2.header.pType == ST_STATE
+      lastAck.header.pType == ST_STATE
       # we are acking in one shot whole 10 packets
-      ack2.header.ackNr == initalRemoteSeqNr + uint16(len(packets) - 1)
+      lastAck.header.ackNr == initalRemoteSeqNr + uint16(len(packets) - 1)
+
+      lastAck.eack.isNone()
 
     let receivedData = await outgoingSocket.read(len(data))
 
@@ -250,7 +225,6 @@ procSuite "Utp socket unit test":
     await outgoingSocket.destroyWait()
 
   asyncTest "Processing packets in random order":
-    # TODO test is valid until implementing selective acks
     let q = newAsyncQueue[Packet]()
     let initalRemoteSeqNr = 10'u16
 
diff --git a/tests/utp/test_utp_socket_sack.nim b/tests/utp/test_utp_socket_sack.nim
new file mode 100644
index 0000000..a2294df
--- /dev/null
+++ b/tests/utp/test_utp_socket_sack.nim
@@ -0,0 +1,244 @@
+# Copyright (c) 2021 Status Research & Development GmbH
+# Licensed and distributed under either of
+#   * MIT license (license terms in the root directory or at https://opensource.org/licenses/MIT).
+#   * Apache v2 license (license terms in the root directory or at https://www.apache.org/licenses/LICENSE-2.0).
+# at your option. This file may not be copied, modified, or distributed except according to those terms.
+
+{.used.}
+
+import
+  std/[options, sequtils],
+  chronos, bearssl, chronicles,
+  stew/bitops2,
+  testutils/unittests,
+  ./test_utils,
+  ../../eth/utp/utp_router,
+  ../../eth/utp/utp_socket,
+  ../../eth/utp/packets,
+  ../../eth/keys
+
+procSuite "Utp socket selective acks unit test":
+  let rng = newRng()
+  let testAddress = initTAddress("127.0.0.1", 9079)
+  let defaultBufferSize = 1024'u32
+
+  proc connectAndProcessMissingPacketWithIndexes(idxs: seq[int]): Future[array[4, uint8]] {.async.} =
+    let initialRemoteSeq = 1'u16
+    let q = newAsyncQueue[Packet]()
+    let data = @[0'u8]
+
+    let (outgoingSocket, initialPacket) = connectOutGoingSocket(initialRemoteSeq, q)
+
+    var dataPackets: seq[Packet] = @[]
+
+    for i in idxs:
+      let dataP =
+        dataPacket(
+          # initialRemoteSeq is next expected packet, so n represent how far from the
+          # future is this packet
+          initialRemoteSeq + uint16(i),
+          initialPacket.header.connectionId,
+          initialPacket.header.seqNr,
+          defaultBufferSize,
+          data,
+          0
+        )
+      dataPackets.add(dataP)
+
+    for p in dataPackets:
+      await outgoingSocket.processPacket(p)
+
+    let extArray = outgoingSocket.generateSelectiveAckBitMask()
+
+    await outgoingSocket.destroyWait()
+
+    return extArray
+
+  proc numOfSetBits(arr: openArray[byte]): int =
+    var numOfSetBits = 0
+    for b in arr:
+      numOfSetBits = numOfSetBits + countOnes(b)
+    return numOfSetBits
+
+  proc hasOnlyOneBitSet(arr: openArray[byte]): bool = 
+    return numOfSetBits(arr) == 1
+
+  asyncTest "Socket with empty buffer should generate array with only zeros":
+    let q = newAsyncQueue[Packet]()
+    let initialRemoteSeq = 10'u16
+
+    let (outgoingSocket, packet) = connectOutGoingSocket(initialRemoteSeq, q)
+
+    let extArray = outgoingSocket.generateSelectiveAckBitMask()
+
+    check:
+      extArray == [0'u8, 0, 0, 0]
+  
+  asyncTest "Socket should generate correct bit mask for each missing packet":
+    # 1 means that received packet is packet just after expected packet i.e
+    # packet.seqNr - receivingSocket.ackNr = 2
+    # 32 means that received packet is 32 packets after expected one i.e
+    # packet.seqNr - receivingSocket.ackNr = 32
+    # First byte represents packets [ack_nr + 2, ack_nr + 9] in reverse order
+    # Second byte represents packets [ack_nr + 10, ack_nr + 17] in reverse order
+    # Third byte represents packets [ack_nr + 18, ack_nr + 25] in reverse order
+    # Fourth byte represents packets [ack_nr + 26, ack_nr + 33] in reverse order
+    let afterExpected = 1..32
+
+    for i in afterExpected:
+      # bit mask should have max 4 bytes
+      let bitMask = await connectAndProcessMissingPacketWithIndexes(@[i])
+
+      check:
+        # only one bit should have been set as only one packet has been processed
+        # out of order
+        hasOnlyOneBitSet(bitMask)
+        getBit(bitMask, i - 1)
+
+  asyncTest "Socket should generate correct bit mask if there is more than one missing packet":
+    # Each testcase defines which out of order packets should be processed i.e
+    # @[1] - packet just after expected will be processed
+    # @[3, 5] - packet three packets after will be processed and then packet 5 packets
+    # after expected will be processed
+    let testCases = @[
+      @[1],
+      @[1, 2],
+      @[1, 9, 11, 18],
+      @[1, 3, 8, 15, 18, 22, 27, 32]
+    ]
+
+    for missingIndexes in testCases:
+      let bitMask = await connectAndProcessMissingPacketWithIndexes(missingIndexes)
+      check:
+        numOfSetBits(bitMask) == len(missingIndexes)
+      
+      for idx in missingIndexes:
+        check:
+          getBit(bitMask, idx - 1)
+
+  asyncTest "Socket should generate max 4 bytes bit mask even if there is more missing packets":
+    let testCases = @[
+      toSeq(1..40)
+    ]
+
+    for missingIndexes in testCases:
+      let bitMask = await connectAndProcessMissingPacketWithIndexes(missingIndexes)
+      check:
+        numOfSetBits(bitMask) == 32
+        len(bitMask) == 4
+      
+  type TestCase = object
+    # number of packet to generate by writitng side
+    numOfPackets: int
+    # indexes of packets which should be delivered to remote
+    packetsDelivered: seq[int]
+
+  let selectiveAckTestCases = @[
+      TestCase(numOfPackets: 2, packetsDelivered: @[1]),
+      TestCase(numOfPackets: 10, packetsDelivered: @[1, 3, 5, 7, 9]),
+      TestCase(numOfPackets: 10, packetsDelivered: @[1, 2, 3, 4, 5, 6, 7, 8, 9]),
+      TestCase(numOfPackets: 15, packetsDelivered: @[1, 3, 5, 7, 9, 10, 11, 12, 14]),
+      TestCase(numOfPackets: 20, packetsDelivered: @[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]),
+      TestCase(numOfPackets: 33, packetsDelivered: @[32]),
+      TestCase(numOfPackets: 33, packetsDelivered: @[25, 26, 27, 28, 29, 30, 31, 32]),
+      TestCase(numOfPackets: 33, packetsDelivered: toSeq(1..32))
+    ]
+
+  asyncTest "Socket should calculate number of bytes acked by selective acks":
+    let dataSize = 10
+    let initialRemoteSeq = 10'u16
+    let smallData = generateByteArray(rng[], 10)
+
+    for testCase in selectiveAckTestCases:
+      let outgoingQueue = newAsyncQueue[Packet]()
+      let incomingQueue = newAsyncQueue[Packet]()
+    
+      let (outgoingSocket, incomingSocket) = 
+        connectOutGoingSocketWithIncoming(
+          initialRemoteSeq,
+          outgoingQueue,
+          incomingQueue
+        )
+
+      var packets: seq[Packet] = @[]
+
+      for _ in 0..<testCase.numOfPackets:
+        discard await outgoingSocket.write(smallData)
+        let packet = await outgoingQueue.get()
+        packets.add(packet)
+      
+      for toDeliver in testCase.packetsDelivered:
+        await incomingSocket.processPacket(packets[toDeliver])
+
+      let finalAck = incomingSocket.generateAckPacket()
+
+      check:
+        finalAck.eack.isSome()
+
+      let mask = finalAck.eack.unsafeGet().acks
+
+      check:
+        numOfSetBits(mask) == len(testCase.packetsDelivered)
+
+      for idx in testCase.packetsDelivered:
+        check:
+          getBit(mask, idx - 1)
+
+      let ackedBytes = outgoingSocket.calculateSelectiveAckBytes(finalAck.header.ackNr, finalAck.eack.unsafeGet())
+
+      check: 
+        int(ackedBytes) == len(testCase.packetsDelivered) * dataSize
+
+      await outgoingSocket.destroyWait()
+      await incomingSocket.destroyWait()
+
+  asyncTest "Socket should ack packets based on selective ack packet":
+    let dataSize = 10
+    let initialRemoteSeq = 10'u16
+    let smallData = generateByteArray(rng[], 10)
+
+    for testCase in selectiveAckTestCases:
+      let outgoingQueue = newAsyncQueue[Packet]()
+      let incomingQueue = newAsyncQueue[Packet]()
+    
+      let (outgoingSocket, incomingSocket) = 
+        connectOutGoingSocketWithIncoming(
+          initialRemoteSeq,
+          outgoingQueue,
+          incomingQueue
+        )
+
+      var packets: seq[Packet] = @[]
+
+      for _ in 0..<testCase.numOfPackets:
+        discard await outgoingSocket.write(smallData)
+        let packet = await outgoingQueue.get()
+        packets.add(packet)
+      
+      for toDeliver in testCase.packetsDelivered:
+        await incomingSocket.processPacket(packets[toDeliver])
+
+      let finalAck = incomingSocket.generateAckPacket()
+
+      check:
+        finalAck.eack.isSome()
+
+      let mask = finalAck.eack.unsafeGet().acks
+
+      check:
+        numOfSetBits(mask) == len(testCase.packetsDelivered)
+
+      for idx in testCase.packetsDelivered:
+        check:
+          getBit(mask, idx - 1)
+
+      check:
+        outgoingSocket.numPacketsInOutGoingBuffer() == testCase.numOfPackets
+
+      await outgoingSocket.processPacket(finalAck)
+
+      check:
+        outgoingSocket.numPacketsInOutGoingBuffer() == testCase.numOfPackets - len(testCase.packetsDelivered)
+
+      await outgoingSocket.destroyWait()
+      await incomingSocket.destroyWait()