use the connection ID provided by the server in first Handshake packet

packet_packer.go

@@ -564,6 +564,10 @@ func (p *packetPacker) SetOmitConnectionID() {
 	p.omitConnectionID = true
 }
 
+func (p *packetPacker) ChangeDestConnectionID(connID protocol.ConnectionID) {
+	p.destConnID = connID
+}
+
 func (p *packetPacker) SetMaxPacketSize(size protocol.ByteCount) {
 	p.maxPacketSize = utils.MinByteCount(p.maxPacketSize, size)
 }

packet_packer_test.go

@@ -230,6 +230,21 @@ var _ = Describe("Packet packer", func() {
 				Expect(h.DestConnectionID).To(Equal(destConnID))
 			})
 
+			It("changes the destination connection ID", func() {
+				srcConnID := protocol.ConnectionID{1, 1, 1, 1, 1, 1, 1, 1}
+				packer.srcConnID = srcConnID
+				dest1 := protocol.ConnectionID{1, 2, 3, 4, 5, 6, 7, 8}
+				dest2 := protocol.ConnectionID{8, 7, 6, 5, 4, 3, 2, 1}
+				packer.ChangeDestConnectionID(dest1)
+				h := packer.getHeader(protocol.EncryptionUnencrypted)
+				Expect(h.SrcConnectionID).To(Equal(srcConnID))
+				Expect(h.DestConnectionID).To(Equal(dest1))
+				packer.ChangeDestConnectionID(dest2)
+				h = packer.getHeader(protocol.EncryptionUnencrypted)
+				Expect(h.SrcConnectionID).To(Equal(srcConnID))
+				Expect(h.DestConnectionID).To(Equal(dest2))
+			})
+
 			It("uses the Short Header format for forward-secure packets", func() {
 				h := packer.getHeader(protocol.EncryptionForwardSecure)
 				Expect(h.IsLongHeader).To(BeFalse())

session.go

@@ -637,6 +637,12 @@ func (s *session) handlePacketImpl(p *receivedPacket) error {
 		return err
 	}
 
+	if s.perspective == protocol.PerspectiveClient && !s.receivedFirstPacket && !hdr.SrcConnectionID.Equal(s.destConnID) {
+		s.logger.Debugf("Received first packet. Switching destination connection ID to: %s", hdr.SrcConnectionID)
+		s.destConnID = hdr.SrcConnectionID
+		s.packer.ChangeDestConnectionID(s.destConnID)
+	}
+
 	s.receivedFirstPacket = true
 	s.lastNetworkActivityTime = p.rcvTime
 	s.keepAlivePingSent = false

session_test.go

@@ -1782,6 +1782,39 @@ var _ = Describe("Client Session", func() {
 		Eventually(sess.Context().Done()).Should(BeClosed())
 	})
 
+	It("changes the connection ID when receiving the first packet from the server", func() {
+		sess.version = protocol.VersionTLS
+		sess.packer.version = protocol.VersionTLS
+		unpacker := NewMockUnpacker(mockCtrl)
+		unpacker.EXPECT().Unpack(gomock.Any(), gomock.Any(), gomock.Any()).Return(&unpackedPacket{}, nil)
+		sess.unpacker = unpacker
+		go func() {
+			defer GinkgoRecover()
+			sess.run()
+		}()
+		err := sess.handlePacketImpl(&receivedPacket{
+			header: &wire.Header{
+				Type:             protocol.PacketTypeHandshake,
+				SrcConnectionID:  protocol.ConnectionID{1, 3, 3, 7, 1, 3, 3, 7},
+				DestConnectionID: sess.srcConnID,
+			},
+			data: []byte{0},
+		})
+		Expect(err).ToNot(HaveOccurred())
+		// the session should have changed the dest connection ID now
+		sess.packer.hasSentPacket = true
+		sess.queueControlFrame(&wire.PingFrame{})
+		var packet []byte
+		Eventually(mconn.written).Should(Receive(&packet))
+		hdr, err := wire.ParseHeaderSentByClient(bytes.NewReader(packet))
+		Expect(err).ToNot(HaveOccurred())
+		Expect(hdr.DestConnectionID).To(Equal(protocol.ConnectionID{1, 3, 3, 7, 1, 3, 3, 7}))
+		// make sure the go routine returns
+		sessionRunner.EXPECT().removeConnectionID(gomock.Any())
+		Expect(sess.Close(nil)).To(Succeed())
+		Eventually(sess.Context().Done()).Should(BeClosed())
+	})
+
 	Context("receiving packets", func() {
 		var hdr *wire.Header