Reliable delivery implementations can be implemented at the end points and also in the network core.

Reliability in the end points is seen on the reliable/sequenced delivery provided by TCP on the transport layer. Hosts and networks work hand-in-hand in the successful delivery of packets from one end to another. This is particularly crucial when TCP sends an acknowledgement receipt to the host. Such acknowledgement would include the next sequence number for transmission. More than that, TCP is also responsible for the validation of addresses. At the same time, TCP also performs the packet queueing. The role of TCP in validating addresses, queueing packets suggest its indispensability in the successful delivery of packets from one end to another. The host merely respond to the requests by the TCP (e.g. retransmission).

On the other hand, reliability on the network core starts on the physical layer reliability that pertains to the high availability and continuity of network. This can be achieved through redundancy, either on links or routers’ processors and line cards. That is, we achieve higher network reliability by redundancy in low reliability network elements. Reliability can be effectively translated from the physical layer to upper layers (i.e. data link and network layer) with a resilient upper layer. Resiliency is the ability to react on failure and there are certain data link/network layer protocols that provide resiliency, namely OSPF, MPLS, and Ethernet with RSTP.

Currently, much of the reliability concern is implemented on the end points. But with the changing demand and usage of the internet, especially with regard to real-time, reliable, and highly available voice services, broadcast media transmission, and video on demand, in the future network (NGN – next generation networks), reliability on the lower layers are more explored to address new demands.

References:
Cerf, Vinton and Kahn, Robert. A Protocol for Packet Network Intercommunication. IEEE. 1974.
Clark, David. The Design Philosophy of the DARPA Internet Protocols. Proc SIGCOMM. 1988.
Clark, David. Rethinking the design of the Internet: The end to end arguments vs. the brave new world. MIT Lab for Computer Science. 2000.
Carpenter, B.. Architectural Principles of the Internet. IAB. 1996.
Koudrin, Alex. Network Reliability and Resiliency in Next Generation Networks at Physical, Datalink, and Network Layers. Victoria University of Wellington. 2007.