Approaches to Congestion Control

Approaches to Congestion Control

In "TCP Congestion Control", we'll look at TCP's specific approach to congestion control in great detail. Here, we identify the two broad approaches to congestion control that are taken in practice and discuss particular network architectures and congestion-control protocols embodying these approaches. At the broadest level, we can distinguish among congestion-control approaches by whether the network layer provides any explicit assistance to the transport layer for congestion-control purposes:

●  End-to-end congestion control. In an end-to-end approach to congestion control, the network layer provides no explicit support to the transport layer for congestion-control purposes. Even the presence of congestion in the network must be inferred by the end systems based only on observed network behavior (for instance, packet loss and delay). We will see in "TCP Congestion Control" that TCP must necessarily take this end-to-end approach toward congestion control, since the IP layer provides no feedback to the end systems regarding network congestion. TCP segment loss (as indicated by a timeout or a triple duplicate acknowledgment) is taken as an indication of network congestion and TCP decreases its window size accordingly. We will also see a more recent proposal for TCP congestion control that uses increasing round-trip delay values as indicators of increased network congestion.

●  Network-assisted congestion control. With network-assisted congestion control network-layer components (that is, routers) provide explicit feedback to the sender regarding the congestion state in the network. This feedback may be as simple as a single bit indicating congestion at a link. This approach was taken in the early IBM SNA [Schwartz 1982] and DEC DECnet [Jain 1989; Ramakrishnan 1990] architectures, was recently proposed for TCP/IP networks [Floyd TCP 1994;], and is used in ATM available bit-rate (ABR) congestion control as well, as discussed below. More sophisticated network feedback is also possible. For instance, one form of ATM ABR congestion control that we will study shortly allows a router to inform the sender explicitly of the transmission rate it (the router) can support on an outgoing link. The XCP protocol [Katabi 2002] provides router-computed feedback to each source, carried  in the packet header regarding how that source should increase or decrease its transmission rate.

For network-assisted congestion control, congestion information is usually fed back from the network to the sender in one of two ways, as shown in Figure 1. Direct feedback may be sent from a network router to the sender. This form of notification normally takes the form of a choke packet (essentially saying, "l'm congested!"). The second form of notification occurs when a router marks/updates a

Two feedback pathways for network-indicated congestion information

field in a packet flowing from sender to receiver to indicate congestion. Upon receipt of a marked packet, the receiver then notifies the sender of the congestion indication. Note that this latter form of notification takes at least a full round-trip time.


Tags

congestion control, network architecture, network layer, network congestion, transmission rate, choke packet

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