sponse to the requestor, because users do not have names and server names are not unique.) Rather, the session protocol leaves traces of the session in every member that forwards the request, so that the response can follow the path in reverse.

NDN is a “future Internet architecture,” as mentioned at the outset. In current NDN deployments, wherever NDN links must traverse non-NDN nodes, they are implemented by being layered on top of the public Internet. NDN networks are particularly interesting because their design shows how the session protocol, routing, and forwarding of a network can be highly specialized and tightly integrated.

Composition by Layering

Layering of self-contained networks. The most important operator for the composition of networks is layering, which is simply what happens when one network uses the services of another network, in exactly the sense discussed earlier. More specifically, a link in a user network is implemented by a session in a used network.

A usage hierarchy is a directed acyclic graph whose nodes are networks and whose edges represent composition by link implementation. A level in this graph is a set of networks that all have the same graph distance from some reference point. This definition will be refined further.

For example, Figure 3 is derived from a usage hierarchy, with the levels of the graph being represented by vertical placement. The bidirectional link between 2. 7 and 2. 8 in the campus IP network is implemented by a bidirectional session in the administrators’ VLAN that follows the path shown between M7 and M8. The link between M7 and M4 in the VLAN is implemented by a session in the left physical LAN following the spanning-tree path between M7 and M4. Note that machines have distinct members in VLANs and LANs, even though those networks happen to use the same Ethernet design and the same namespace.

Consider the right LAN in Figure 3. It links machines in both the students’ and administrators’ groups, so it must implement links in at least two VLANs.

When the destination of a session
in this LAN receives a packet, which
member of which VLAN should it de-
liver the packet to? LAN packets in this
architecture have a user-network iden-
tifier called a “VLAN tag,” which tells
the destination which user network is
being served by the session.

The shift from a principle of fixed layers to a principle of many self-contained networks encourages a shift in thinking and terminology—from different concepts and terminology for each layer to concepts and terminology that emphasize the similarities among layered networks. Most importantly of all, users of networks—distributed application systems—can be networks themselves, and the distinction between the two concepts weakens.

If the service provided by a session protocol has a specification, then the specified properties of a session are also the guaranteed properties of a link the session implements. For example, the best-known service of IP networks is implemented by the session protocol TCP. A user of TCP sends a stream of bytes, and this byte stream must be received by the user at the other end of the session with no bytes missing or duplicated, and all in the same order in which they were sent.

In the case of TCP the work needed to satisfy this specification is performed by the protocol implementation in the network members at the endpoint machines. IP/TCP packet headers have a session identifier (the four-tuple with both names and both ports) and a user network (the destination port or “well-known port”). The network has a maximum transmission unit limiting the size of IP packets. So the TCP implementation at the source accepts a byte substream, disassembles it into IP packets, encapsulates each packet in the TCP/IP header, and sends it through the network. When the TCP implementation at the destination receives packets, it decapsu-lates them by removing the TCP/IP header, requests retransmissions of missing substreams, assembles a complete substream in byte order, and delivers it to the receiver.

Names and directories. Classic descriptions of the Internet associate “domain names” with the application layer, IP “addresses” with the network layer, and MAC addresses with the link layer. In the new model ev-

The most important
operator for
the composition
of networks
is layering, which
is when a link in
a network
is implemented
by a session in
a used network.