Continuing my coverage of the recent joint IESG/IAB design team on OAM, this time I want to discuss the issue of OAM for flows in Packet Switched Networks (PSNs).
From a pure topology standpoint any communications network is imply a set of source ports (i.e., interfaces into which we may input information), a set of destination ports (i.e., interfaces from which we may receive information), and a set of links connecting source ports to destination ports. Of course, the destination ports will may be located very far from the source ports (and this is the reason we use the network in the first place), but this geometry is irrelevant from a topological point of view.
PSNs are communications networks that transfer information in units of packets. They can be classified as either Connection Oriented (CO) , or ConnectLess (CL). In a CO network the end-to-end path from source port to destination port needs to be set up (by physical connection, or by manual/management-system configuration, or by control-plane signaling) before information can be sent down the path. Once set up, it makes sense to call this end-to-end path a “connection”. A connection is essentially an association of a source port with a destination port that has been prepared to carry information.
In a CL PSN each packet of information is individually sent towards its destination. No set up is required as each packet is identified by a unique destination address. When we send a data packet from a source port to the desired destination we can still think about an association of the source and destination ports, but as this association is ephemeral, this association does not constitute a connection. If, however, many similar packets are sent from source to destination, it may be useful to speak of a “flow” of packets. Of course, there is no guarantee that all the packets travel from source to destination over precisely the same path through the network, but in many cases this is the case for substantial periods of time until a reroute event takes place. When load balancing is used the definition (and its consequences) becomes truly problematic.
OAM mechanisms were originally designed for Circuit Switched (CS) or CO communications systems, such as PDH, SDH, and ATM networks. For such networks it makes perfect sense to ask about continuity of the connection, or its performance parameters (e.g., delay). Thus Continuity Check (CC) OAM functions became standard, and PM functions recommended for CO networks. The issue is more complex for CL networks. Continuity doesn’t mean very much if every packet is sent to a different destination! It means somewhat more when there is a prolonged flow; but even then packet loss and delay are statistical combinations, since consecutive packets may traverse different network elements on their route from source to destination.
When packets are delivered to an incorrect destination we say a misconnection has occurred, and Connectivity Verification (CV) monitors for such events. (There is often confusion between CC – a functionality needed for CO and CL networks, and CV – which is only for CL.)
For some types of prolonged flows it makes sense to introduce OAM mechanisms to monitor continuity and performance parameters. Pseudostreaming of video over the Internet may involve hundreds of packets per second for many minutes. IPTV flows are even higher in rate and can last for hours. Ethernet Virtual Connections (EVCs) between customer sites last indefinitely.
In a future entry I will discuss when it doesn’t make sense to talk about flows, and what OAM means for such cases.
Y(J)S
