Chapter 1: Introduction

• Resource allocation is a hard problem in any computer system, but especially for a system as complex as the Internet

• The network began to experience a phenomenon known as congestion collapse

• Solution: congestion control, was developed and deployed in the late 1980s

• Congestion control, and resource allocation more broadly, are wide open design spaces with plenty of room for innovation

  • New approaches or refinements to the existing approaches will continue to appear as long as the Internet exists

• Outline: explore the design space for congestion control in the Internet and present a description of major approaches to managing or avoiding congestion that have been developed over the last three decades

1.1 What is congestion?

• This book focus on congestion control for packet-switched networks

  • Packet switching: multiplexing, a system resource (i.e. a link or a queue in a router) is shared among multiple users or applications

  • Statistically multiplexed: as packets show up somewhat randomly, we reply on statistical properties of those arrivals to ensure that we don't run out of resources

  • A network that attempts to allocate bandwidth to particular flows is sometimes said to support Quality-of-Service (QoS)

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  • The link can only carry so many bits per second, and the queue can only hold so many packets (or bytes) before it has to start discarding packets

  • Managing the access to these shared resources, and trying to do so in a way that prevents congestion collapse, is the essence of congestion control

1.2 Controlling Congestion

• Challenges

  • They are not isolated to a single level of a protocol hierarchy

    • Resource allocation is partially implemented in the routers, switches, and links inside the network and partially in the transport protocol running on the end hosts

    • End systems may use signaling protocols to convey their resource requirements to network nodes, which respond with information about resource availability

    • Application protocols may themselves be designed to mitigate congestion

    • Key: you cannot fully understand congestion without looking at all the places in the system that it touches

• Resource allocation: network resources (primarily link bandwidth and buffer space in routers or switches)

• Congestion control: the efforts made by network nodes (including end systems) to prevent or respond to overload conditions

  • Note: differ than flow control (i.e., keeping a fast sender from overrunning a slow receiver), while CC is intended to keep a set of senders from sending too much data into the network because of lack of resources at some point

• Congestion collapse occurred in early Internet: dropped packets are not just discarded and forgotten; but it is retransmitted

  • The number of packets sent into the network increases even if there is no real increase in the offered load from users and applications

• Metric: goodput

  • Distinguished from throughput in the sense that only packets doing useful work are counted towards goodput

• Key insight of early researcher: is it possible and necessary for TCP to do something other than blindly retransmit lost packets during times of congestions

  • Key: interaction between end-to-end protocol and the network during times of congestion