Connection Oriented versus Connectionless Networks

One distinguishing characteristic of a network (or network protocol) is the presence or absence of a "connection" between the end users. When a connection is present the network is called "Connection Oriented" and when there is no connection the network is called "Connectionless".

After 30 years of building data networks, this is still an issue on which there is considerable disagreement and which can evoke strong feelings.

5.6.1.1 Connectionless Networks

In a connectionless network, a network node does not keep any information relating to interactions currently in progress between end users of the network.

Every data block transmitted must be prefixed by the full network address of both its origin and its destination.

Sometimes, (such as in the pre-1979 versions of SNA29 ), the network address takes the form of a structured binary number, which can be used relatively easily to determine the appropriate routing for the data. Sometimes, (such as in typical LAN networks), the network address has no meaningful structure that is usable for routing purposes.

Characteristics of connectionless networks are as follows:

• When a data block arrives the network node must calculate on which

outbound link to send the data towards its destination. This decision may be very complex and compute intensive or very simple depending on how much information about the destination is available within the destination address field.

In the extreme case where the destination address contains no information at all about its location, (such as is the case with LAN addresses), the node may need to keep tables relating every known destination address to its real location in the network. This is the case with "transparent bridges" between LANs.

Usually, the destination address will be structured in some way such that it can be related to knowledge of the network's topology kept within the node.

For example, the network address may contain a destination node number and the switching node may contain a network map so that it may calculate the best outbound path on which to forward the data block. This process can be very simple (such as in the first version of SNA) or very complex (such as in ARPANET or TCP/IP).

• If the network allows multiple paths to be used for individual data blocks then blocks will arrive in a different sequence from the sequence in which they were delivered to the network.

29 In early SNA each node had a number. When a frame was routed by a node there was a single table showing which link traffic for a given node number must be sent on. The switching node knew nothing about routes or about connections.

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If blocks are able to be delivered out of sequence then a much more complex end-to-end protocol will be required to resequence them before presentation to the end user.

• The header prefix required in a connectionless situation is typically much longer than for a connection oriented network. This affects the efficiency of the network as headers take up link capacity and require transmission time.

This mayor may not be important depending on the length of the data blocks being handled.

• There is no need for a connection establishment sequence for one end user to send data to another. To send data an end user just has to put the destination address onto the front of the message and send it. This saves time and overhead.

• Implementation of flow and congestion controls in a connectionless network is much more difficult than in a connection oriented one because individual connections (though they of course exist) are unknown by the network and thus cannot be controlled.

5.6.1.2 Connection Oriented Networks

In a connection oriented network, once a connection is established, there is no need to place a destination address in the block header every time a data block is sent. All that is needed is an identifier to specify which connection is to be used for this block.

There are many ways of constructing connection oriented networks. For a description of the method used in SNA APPN see section 5.7.3, "Logical 10 Swapping" on page 93.

Characteristics of connection oriented networks are as follows:

• The connection must be established somehow. Permanent connections (such as PVCs in X.25) are typically established by a system definition procedure. Temporary connections (such as SVCs in X.25) are typically established by placing a "call".

Setting up a call can take considerable processing overhead in network nodes and can often take a significant delay (such as five seconds).

• Congestion control is easier than for connectionless networks because network nodes can regulate the flow on individual connections.

• Data switching is usually (but not always) significantly more efficient than for connectionless networks because the onward route of the data is

predetermined and therefore does not need to be calculated.

• When a link or a node becomes inoperative (goes down), connections that were passing through the affected link or node are typically lost. A new connection must be established through a different route. This takes time and usually disrupts the connection at the end user level.

Connectionless networks typically reroute traffic automatically around link or node failures.

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5.6.1.3 Connection Oriented Connectionless Networks

In a series of token-ring LANs connected by "source routing" bridges we see the case of a connection existing over a fixed route where the individual switches (the bridges) do not know about connections at all. This principle is described in section 11.4, "Source-Routing Bridges" on page 251.

The IBM experimental high speed packet switch called" Paris" uses a similar method of switching to that described above for TRN bridges. (See the description in section 8.3, "Packetised Automatic Routing Integrated System (PARIS)" on page 158.) Paris is different because the data switching part has no record of the existence of connections. The routing decision is made by the switching nodes completely on information present in the header of every data message. (The control processor in each node does know about connections passing through the node's switching part, as it allocates capacity and monitors throughput for congestion control purposes.)

Connections do exist in both these systems but in either system it is only the source node that knows about it.

5.6.1.4 Connections across Connectionless Networks Consider the diagram in Figure 31.

Connectionless Network

Node A Node B

X.25 X.25

end end

DTE - to to - DTE

end end end end

user user

Connection

Figure 31. A Connection across a Connectionless Network This is an important case in practical networks.

In the example there is a connection between the two end users. This

connection is known about and supported by nodes A and B, but the switching nodes in the network do not know that a connection exists. The end-to-end function holds the network address and the status of its partner end-to-end function and looks after data integrity and secure delivery etc.

What exists here is really a connection oriented network built on top of a connection less one.³⁰

30 The IBM X2SNet product which implements an X.2S network works exactly in this way.

5.6.1.5 A Connection Is Always Present

Well, almost. In an application sense, it is very rare for communication to take place without a connection being present logically, even if the components of the network sometimes do not know about it.

5.6.1.6 Connections in SNA Networks

In SNA the only entity that can send or receive data is the Logical Unit (LU).

Data is a/ways sent from one LU to another on a connection called a "Session".

But inside SNA networks sessions are handled differently.

1. In the first versions of SNA, network nodes (always IBM 3705s running the Network Control Program, NCP) selected the link on which to forward a given data block by the binary "subarea number" (really destination node number) within the destination address field of the data header.

Since the network was (and still is) constrained to deliver data blocks in sequence there could be one and only one possible path from any given origin node to any given destination.

2. Since about 1980, SNA "Subarea" networks (networks in which the real network address is structured such that the node number is present as a subfield) have been connection oriented. That is, there are predefined routes through the network which are known by each network node.

Sessions are still unknown to the network nodes (or, more correctly, to the transmission network component of the network nodes) but are carried on connections called Virtual Routes (VRs). VRs map to explicit routes (ERs).

The destination subarea (node) number together with the ER number in the frame header is used to determine the routing of incoming data blocks.

The routing headers used by this form of SNA are 26 bytes long comprising origin and destination network addresses (each of 48 bits) and Explicit Route and Virtual Route numbers.

3. SNA APPN networks select a new route for each end user session through the network. The APPN network nodes keep connection tables which record a fixed (for the duration of a session) relationship between a session

identifier (called an LFSID) on one link with a session identifier on another link. For a more detailed explanation of this form of routing see section 5.7.3, "Logical ID Swapping" on page 93.

This means that the routing header for SNA APPN is only six bytes (two of which are the LFSID).

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