SHENKER: FLNDAMENTAL DESIGN ISSUES FOR THE FL TCRE INTERNET that connect these computers, such as the Internet, must adaptation techniques been seen as a general and enduring be prepared to cope with the traffic emanating from solution have very different characteristics and requirements than data ture"approach has several important advantages. No changes applications, and thus their emergence is likely to significantly are required to any network interfaces, so the change can alter the nature of the Internet's traffic load (see [38] for a be incrementally deployed at both the end hosts and the more complete discussion ). In particular, as traditionally im- routers. Also, the network mechanisms(Fair Queueing an plemented these real-time applications are typically less elas- its relatives) and application mechanisms(delay adaptation tic-less tolerant of delay variations--than data applications. are relatively well understood. However. in this approach This lack of elasticity causes two problems. First, these the network would deliver the same class of service to all traditional implementations* of real-time applications do not users, with no assurances as to the quality of that service erform adequately when running over the current Internet While the network would protect users from each other, it because the variations in delay are too extreme and there is up to applications to adjust to the inevitable variations in are too many dropped packets. Second, these applications packet delay and available bandwidth. There are likely to be typically do not back off in the presence of congestion; when limitations to this adaptability. Moreover, because there is no these real-time applications are contending for bandwidth with admission control the network must be provisioned so that traditional data applications, the data applications end up the fair bandwidth shares are not, except in very rare case eceiving very little bandwidth. Thus, when deployed in the unreasonably small urrent Internet, traditional real-time applications not only do As we will explore in this paper. it is not clear that not always perform adequately but they also often interfere such an approach is desirable; there are other approaches to with the data applications supporting real-time applications that modify the basic Internet One can address these problems without changing the architecture. These modifications usually involve extending the basic Internet architecture by improving application and router Internet 's service model-the set of delivery services-from implementations, The unfairness that results when congestion- the single class of best-effort service to include a wider var avoidant data applications compete with congestion-ignorant of service classes. In addition, one can augment this service al-time applications can be resolved by using the Fair model with admission control, which is the ability to turn Queueing packet scheduling algorithm (or something roughly some flows away when the network is overloaded. In this quivalent) in routers. These routers would then ensure that paper we ask whether or not these architectural modifications every user had access to theirfair share "of bandwidth, and are appropriate. Surprisingly, this question has received rather so data applications would be protected from the real-time little explicit attention in the literature. The approach which ones(see 16l for a discussion of such scheduling algorithms ) involves only implementation enhancements and not archite Such a modification does not require any change to the tural modification, while advocated by several researchers, has Internet architecture. While this approach solves the second not been adequately described in the literature. The approach problem mentioned above it does not solve the first; the which entail major architectural changes have been fully service delivered to an individual application can still have described, and we have included a sampling of this literature substantial delay variance and packet loss, seriously degrading in the bibliography. Most of these papers implicitly assume the performance of these traditional implementations of real- that such modifications are necessary and hence give little justification for the basic approach(this author also pleads One can address this problem by modifying the application guilty to this crime; see 131); instead the focus in these papers is mplementations rather than the network implementation. In on the details of the design and comparison with other similar recent years there have been tremendous advances, much born architectures tive to variations in packet delays: delay adaptive techniques basic issues in what has been an underarticulated disagreement have been highly successful in such Internet applications as nv about basic architectural assumptions. We hope to provide and vat. These delay adaptive techniques were first introduced a framework for considering the various architectural trade many years ago. see [71, 1441. but only recently have these offs and identify some of the critical assumptions that lead to one choice or another. We do this by presenting a rather Both or we oints(see/3. ons of real-time applican back of incoming data. approach is not intended to model reality, but to be the simplest involve either fixed abstract formulation and a few rather simple models; this under high delay variance. s the real issues at stake. Central sThis approach does not require an architectural modification although this endeavor are questions about the nature of future computer pplications, the cost of additional network mechanism, the m购m四m(m时甲 discuss rate adaptation again in Section晚 I and we will not discuss the to the current delays and can tolerate fairly large variations in delay without We will use the term"flow"to refer to the traffic stream representing a particular user or application. Flows can be unicast or mu