When working and discussing topics relating to technology, the terms can often become a bit confusing. This article specifically focuses on two opposing terms that are used to describe traffic flows and the relationship of one network device to another. Specifically, the terms that are addressed are “upstream” and “downstream”.
Upstream and downstream are terms that are more often associated with the natural flow of water. When used in this context, it always makes sense. Water, being generally affected
by gravity, only flows in one direction. When contrasting this to networks, a statement cannot be made that states that packets generally flow in one direction. Packets, in almost all cases, flow bidirectionally. The focus of this article is disambiguate the definition of these terms when used to describe network devices and traffic flows.
When applying these terms to networks, it is important to understand that it is all about perspective. Understanding the intended perspective may be the most challenging aspect of understanding the intent of the party delivering the message. For example, are the statements being made from the point of view of service provider, customer, or perhaps a network device? Are they in regard to a unidirectional flow of information, or the logical flow of information?
When these terms are used loosely without any perspective given, the default assumption is that they are being used in relationship to the customer or user. For example, if a conversation is talking about an upstream flow, that typically describes traffic flowing away from a user. Downstream traffic would be traffic flowing toward the user. The confusion comes into play when it is realized that upstream traffic for one party is downstream traffic for another. For example, a user could be uploading a file (upstream from the user perspective) to a server (downstream from the server perspective). Additionally, traffic actually flows in two direction even when an upper layer message is might only be sent in one direction.
Another way that the terms upstream and downstream are used are to compare the position of network devices. In this case, the description is often made from where the users are. For example, from an access layer switch, the distribution and core switches would be considered upstream. If a conversation is about the distribution layer switches, core switches would be considered upstream, while access layer switches would be considered downstream. Again, it is always about perspective.
Another relationship that may be described by these terms is that of service providers. For example, a customer may purchase bandwidth from a service provider. That service provider may in turn purchase services from a larger service provider. In that case, the larger service provider would be considered upstream and the customer is considered downstream.
A final word about this terminology is that the ambiguity sometimes remains. There are cases in which the terminology is relevant but still needs clarified. In those cases, there is nothing wrong with asking a question to force the clarification.