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The practical use of such connection based flows is however
restricted. Although the theory exists already for a while, it has
never really been used in implementations. This is due to some
problems which are related to the nature of the Internet environment:
- 1.
- The Internet being a connectionless datagram environment,
dependence on connection-oriented information will often interfere
with operational stability. If routes change during a flow, new
routers will carry datagrams that never saw the transport layer
SYN/SYN-ACK packets, and routers that did see earlier datagrams in a
flow will never see the FIN/FIN-ACKs. Flow state information that is
dependent on this data will become obsolete and never expired in such
cases.
- 2.
- IP Fragmentation would also pose problems since all but the
first fragment lack the TCP/UDP port information and therefore it
would be impossible to track the fragmenented parts of a packet into a
higher layer flow.
- 3.
- Routing and accounting in a datagram network occurs
below the transport layer. Rerouting could occur. Upon a route change
of a non-static route during flow data capturing the FIN packet for a
connection would never be seen and therefore the flow data structure
would exist forever.
- 4.
- Not all traffic makes use of connection oriented transport layer
protocols. The trend on the Internet is that more and more lightweight
protocols are used that do not use the TCP mechanism for connection
setup and teardown. Especially new multimedia applications usually
bring their own concepts for connection handling.
- 5.
- Finally, new technologies for link level routers, e.g. ATM, will
not have access to transport layer informations; any Internet related
transmission decisions will have to rely only on IP level
information. In particular, until end-to-end ATM is a reality, IP
gateways attached to ATM style networks will have to multiplex
possibly many IP flows onto ATM. Mapping higher level (IP) flows to
underlying link level virtual circuits (VCs) will require effective
setup, maintenance and timeout strategies as well as accounting
schemes.
Having seen the efforts to extend the packet train model of
flows to the transport or application layers [1,23,26] or focusing on TCP traffic flows
[10,32], Claffy, Braun and Polyzos have
introduced a more generalized, comprehensive methodology of a
timeout-based flow characterization on the IP layer
[15]. Their flow definition is also based
on the packet-train model, but in contrary to the other models
mentioned they avoid to use connection information.
The IP layer, the ``heart'' of internet technology, is a
connectionless network layer. Not to use connections for routing or
switching was one of the main reasons why the Internet could grow as
rapidly as it did. Any connection oriented service on the Internet is
implemented in the transport protocol that is running on the end hosts
only. Routers on the Internet are simple and fast since they just rely
on the IP headers of the packets. This has shown to scale very well.
In the same way, the connectionless techniques are scaleable for
measurement and analysis applications.
The classification of networks into either connectionless or
connection oriented ones is pretty restrictive. In particular it is
obviously not true that in a connectionless network all datagrams are
completely independent. The datagrams are certainly switched
independently, but it is usually the case that a stream of datagrams
between a particular pair of hosts flow through a particular set of
routers. Hence the idea to define a flow as a sequence of
packets matching the same criteria is a useful concept for an
abstraction.
By using this abstraction of connectionless traffic flows, a set of
new applications will become possible. First of all, we can use the
flows for network monitoring, measurement and analysis, as primarily
shown in this report. However, they are also very interesting for
routing, switching, as we will see in section
2.5 as well as for congestion control
([20], pp. 395 ff).
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8/4/1997