Internet Congestion Control Research Group
Internet Congestion Control RG
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Key to the correct functioning of the Internet is TCP’s congestion control
mechanism, which serves to match offered load to available bandwidth.
Congestion control also allocates available bandwidth to many flows, more or
less reasonably. Since the deployment of TCP’s additive-increase,
multiplicative decrease (AIMD) algorithm in 1988, the Internet has changed
significantly, becoming much more heterogeneous in many areas including link
speeds and characteristics, and types of application. Although there is no
immediate crisis, TCP’s AIMD algorithm is showing its limits in a number of
areas, including but not limited to:
* insufficient dynamic range to handle high-speed wide-area networks
(especially with few sources using them)
* poor performance over links with unpredictable characteristics, such as some
forms of wireless link.
* poor latency characteristics for competing real-time flows.
The network research community has been very active in designing modifications
and alternatives to TCP’s basic AIMD algorithm, especially when it comes to
high-speed congestion control. A non-exhaustive list of examples includes
High-speed TCP, Scalable TCP, H-TCP, FAST, and XCP. However the network
research community cannot, by itself, effectively evaluate any such solution
for Internet-scale deployment. To do this requires the involvement of equipment
vendors, network operators, and other IETF participants, in addition to the
The key goal of the Internet Congestion Control Research Group (ICCRG)
therefore is to move towards consensus on which technologies are viable
long-term solutions for the Internet congestion control architecture, and what
an appropriate cost/benefit tradeoff is.
To achieve this goal will require extensive evaluation of different mechanisms
in a wide range of network conditions, using a mix of simulation, analysis, and
experiment. It is unclear at this stage whether any single proposed solution is
viable, or whether there needs to be a synthesis of ideas from many places.
Such an evaluation is especially difficult, given that some mechanisms involve
changes to queuing algorithms, or to packet forwarding, whereas others only
require end-system modifications. It is difficult therefore to compare like
with like. One goal of the ICCRG is to improve our methods for evaluating
transport protocols. To this end, a set of simulation (or other) test suites
will be developed. There is an opportunity when making any widespread change to
go further than the simplest incremental modifications, but such larger changes
have costs, and so critical to the relevance of any recommendations from the
ICCRG will be that any proposed solutions are considered to be economically
viable. If router modifications are proposed, collecting them and the tradeoff
underlying them would be an important service of the ICCRG.
There are many different aspects to congestion control that the ICCRG should
consider. For example, different real-time media applications have (to a
greater or lesser extent) a difficult time with dynamic congestion control
mechanisms, especially if changes must be made on short timescales. The rise of
Internet telephony and the expected rise of high-bandwidth Internet television
will therefore significantly impact the congestion control landscape. The ICCRG
should therefore consider such applications, both in terms of how to best
support these various classes of application, and in terms of how this impacts
the overall emerging congestion control architecture.
Also within scope for the ICCRG should be consideration of how congestion
control interacts with QoS mechanisms, with traffic engineering, and with
lower-layer technologies such as optical-burst-switching. Finally, of
significant importance is the interaction between denial-of-service attacks
targeted at bandwidth exhaustion, any mechanisms intended to prevent such
attacks, and the congestion control architecture. Thus the context for the work
of this group is necessarily wide-reaching, and touches on both architecture
Because congestion control is typically a key function of transport protocols,
it is hard to separate other transport layer issues from congestion control.
Considerations for deployment of transport protocols are therefore also
considered within scope. The ICCRG may also consider congestion and protocol
performance problems in general IP networks, i.e., not only on the global
Internet. One example of such IP networks are multi-tenant, heterogeneous
datacenters, which in some sense represent a microcosm of the larger Internet;
changes to the transport protocols being considered in that space are
fundamental and can have broader implications.
It is hard to forecast exactly how the consensus process will play out, and
therefore precisely what milestones to expect. However, as a starting point to
achieve focus for the group, the ICCRG will produce an RFC describing the
nature of the emerging congestion control problems that any future congestion
control architecture must face.
An eventual goal is to produce a recommendation to the IETF on a solution that
would be appropriate for Internet-scale deployment. It is possible that more
than one solution will be recommended – this is likely if the solutions can
It is also expected that the ICCRG will produce IETF AD-sponsored RFCs
detailing good practice for how real-time applications might best operate in a
The ICCRG will hold 2-3 meetings per year, typically meeting for two days at a
time. Critical to success is close involvement between the IETF community and
the ICCRG. To this end, the ICCRG should make a presentation at each IETF
(probably in the TSVWG) summarizing progress made, and what input is needed
from the IETF.