eer A's packet. Subsequently, peer A reduces its Path MTU estimation and also reduces the size of the following packets that the node emits towards the same destination [40]. This process may be repeated, if there are other links on the path to peer B that have even smaller MTU (not illustrated in the figure). A problem with RFC 1981 [40] based Path MTU discovery is that it uses unreliable ICMPv6 messages. An ICMPv6 packet may be lost on the network by accident, or on purpose by a firewall. When large packets do not go through the network and when ICMPv6 Packet Too Big messages are not received, a situation called ICMPv6 'black hole' is present. A sender can detect the black hole based on timers, which is a slow way, or more efficiently by performing Path MTU discovery with the help of the packetiza- tion layer as described in RFC 4821 [42]. The packetization layer Path MTU detection is performed by TCP, or some other transport layer protocol. A simplified description of the packetization layer Path MTU detection is as follows: a sending node initially sends smaller packets, such as smaller than the minimum MTU of 1280 bytes, to verify connection through the network in general. As receipts for successful packet reception are received, packet size is increased until the first-hop link's MTU is reached, ICMPv6 Packet Too Big is received, no acknowledgement is received (in which case the network path has a 'black hole') and sent packet was too large, or the functioning Path MTU esti- mation is good enough to not justify further probing (with possible cost of losing packets Deploying IPv6 in 3GPP Networks and hence time). The packetization layer discovery approach has the significant bene- fit of performing probing while transmitting useful data, and after the successful initial transmissions having an idea of when responses should arrive (estimation of Round Trip Time - RTT), and hence being faster and more efficient way to detect ICMPv6 'black holes'. For details and full description of the sophisticated algorithm,