| ConEx | B. Briscoe |
| Internet-Draft | BT |
| Intended status: Informational | July 16, 2012 |
| Expires: January 15, 2013 |
Initial Congestion Exposure (ConEx) Deployment Examples
draft-briscoe-conex-initial-deploy-03
This document gives examples of how ConEx deployment might get started, focusing on unilateral deployment by a single network.
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This document gives examples of how ConEx deployment might get started, focusing on unilateral deployment by a single network.
The ConEx mechanism document [conex-abstract-mech] goes to great lengths to design for incremental deployment in all the respects below. It should be referred to for precise details on each of these points:
[conex-abstract-mech] introduces the following components:[conex-abstract-mech] should be referred to for definitions of each of these components and further explanation.
The goal of all these ConEx elements for this scenario is to expose information about congestion on the whole-path to a congestion-policer. A congestion-policer is nearly identical to a traditional token-bucket-based bit-rate policer except the tokens it fills with arrive at a rate that represents the volume of congestion that the customer is allowed to contribute to over time and tokens drain from the bucket at a rate dependent on the ConEx signals representing rest-of-path congestion. [CongPol] introduces congestion-policing and [conex-concepts-uses] explains the benefits of policing based on congestion-volume compared to methods like weighted round-robin traditionally used in a BRAS.
Network deployment-related definitions:
Each network can unilaterally choose to use any ConEx information given by those sources using ConEx, independently of whether other networks use it.
Typically, a network will use ConEx information by deploying a policy function at the ingress edge of its network to monitor arriving traffic and to act in some way on the congestion information in those packets that are ConEx-enabled. Actions might include policing, altering the class of service, or re-routing. Alternatively, less direct actions via a management system might include triggering capacity upgrades, triggering penalty clauses in contracts or levying charges between networks based on ConEx measurements.
Typically, a network using ConEx info will deploy a ConEx policy function near the ingress edge and a ConEx audit function near the egress edge. The segment of the path between a ConEx policy function and a ConEx audit function can be considered to be a ConEx-protected segment of the path. Assuming a network covers all its ingresses and egresses with policy functions and audit functions respectively, the network within this ring will be a ConEx-protected network.
Of course, because each edge device usually serves as both an ingress and an egress, the two functions are both likely to be present in each edge device.
In all the deployment scenarios below, we assume that deployment starts with some data sources being modified with ConEx code. The rationale for this is that the developer of a scavenger transport protocol like LEDBAT has a strong incentive to tell the network how little congestion it is causing despite sending large volumes of data. In this case the developer makes the first move expecting it will prompt at least some networks to move in response—so that they use the ConEx information to reward users of the scavenger protocol.
The name 'Receiving Network' for this scenario merely emphasises that most data is arriving from connected networks and data centres and being consumed by residential customers on this access network. Some data is of course also travelling in the other direction.
DSLAMs __
/|/ ,-.Home-a
__/__| |-----( )
,-----. / \ | |--- `-'
,---. / \ ,------P/ \|\__
/ \ ' Core '/| BRAS | __
( Peer )-->-|P | '------' /|/
\ / | | _____| |---
'---` ' '\,------./ | |---
\ M / |BRAS | \|\__
`-----' '------A\ __
| P| \ /|/
/|\ /|\ \__\_| |--- ,-.
,---. ,---. / | |-----( )
/Data \ / \ \|\__ `-'Home-b
( Centre) ( CDN )
\ / \ / Access Network
'---` '---` <------------->
P=Congestion-Policer; M=Congestion-Monitor; A=Audit function
Figure 1: Single Receiving Network Scenario
Figure Figure 1 is an attempt to show the salient features of a ConEx deployment in a typical broadband access provider's network (within the constraints of ASCII art). Broadband remote access servers (BRASs) control access to the core network from the access network and vice versa. Home networks (and small businesses) connect to the access network, but only two are shown.
In this diagram, all data is travelling towards the access network of Home-b, from the Peer network, the Data centre, the CDN and Home-a. Data actually travels in both directions on all links, but only one direction is shown.
The data centre, core and access network are all run by the same network operator, but each is the responsibility of a different department with internal accounting between them. The content distribution network (CDN) is operated by a third party CDN provider, and of course the peer network is also operated by a third party.
This operator of the data centre, core and access network is the only one in the diagram to have deployed ConEx monitoring and policy devices at the edges of its network. However, it has not enabled ECN on any of its network elements and neither has any other network in the diagram. The operator has deployed a congestion policing function (P) on the provider-edge router where the peer attaches to its core, on the BRAS where the CDN attaches and on the other BRAS where each of the residential customers like Home-a attach. On the provider-edge router where the data centre attaches it has deployed a congestion monitoring function (M). Each of these policing and monitoring functions handles the aggregate of all traffic traversing it, for all destinations.
The operator has deployed an audit function on each logical output port of the BRAS for each end-customer site like Home-b. The Audit function handles the aggregate of all traffic for that end-customer from all sources. For traffic in the opposite direction (e.g. from Home-b to Home-a, there would be equivalent policing (P) and audit (A) functions in the converse locations to those shown.
Some content sources in the CDN and in the data centre are using the ConEx protocol, but others are not. There is a similar situation for hosts attached to the Peer network and hosts in home networks like Home-a: some are sending ConEx packets at least for bulk data transports, while others are not.
Within the BRAS there are logical ports that model the rate of each access line from the DSLAM to each home network [TR-059], [TR-101]. They are fed by a shared queue that models the rate of the downstream link from the BRAS to the DSLAM (sometimes called the backhaul network). If there is congestion anywhere in the set of networks in Figure Figure 1 it is nearly always:Figure 2 shows an example plot of the loss levels that might be seen at different monitoring points along a path between the data centre and home-b, for instance. The top half of the figure shows the loss probability within the BRAS consists of 0.1% at the shared queue and 0.2% self-congestion in the logical output port that models the access line, making 0.3% in total. This upper diagram also shows whole path congestion as signalled by the ConEx sender, which remains unchanged along the whole path at 0.3%.
Any ConEx sources sending data through this BRAS will receive feedback about these losses from the destination and re-insert it as ConEx markings into the data.
The lower half of the figure shows (downstream congestion) = (whole path) - (upstream congestion). Upstream congestion can only be monitored locally where the loss actually happens (within the BRAS output queues). Nonetheless, given there is rarely loss anywhere else but within the BRAS, this limitation is not significant in this scenario. The lower half of the figure also shows the location of the policing and audit functions. Policing anywhere within or upstream ofthe BRAS will be based on the downstream congestion level of 0.3%. While Auditing within the BRAS but after all the queues can check that the whole path congestion signalled by ConEx is no less than the loss levels experienced within the BRAS itself.
Data centre-->|<--core-->|<------BRAS--------->|<--Home--
| |
^loss |<-Shared->|<-Access->|
|probability backhaul
|
0.3%|- - - - - - - - - - - - - - - - - - - - +-----------------
| whole path congestion |
| |
| |upstream
0.1%| +---------+congestion
| |
-O==============================+----------------------------->
monitoring point
^loss
|probability Policing Audit
| | |
| V |
0.3%|----------------O-------------+ |
| |downstream |
0.2%| +---------+ |
| congestion| |
| | |
| | V
-O----------------------------------------+====O============-->
monitoring point
Figure 2: Example plot of loss levels along a path
Even a sending application that is modified to use ConEx can choose whether to send ConEx or Not-ConEx packets. Nonetheless, ConEx packets bring information to a policer about congestion expected on the rest of the path beyond the policer. Not-ConEx packets bring no such information. Therefore a network that has deployed ConEx policers will tend to rate-limit not-ConEx packets conservatively in order to manage the unknown risk of congestion. In contrast, a network doesn't normally need to rate-limit ConEx-enabled packets unless they reveal a persistently high contribution to congestion. This natural tendency for networks to favour senders that provide ConEx information encourages senders to choose to use the ConEx protocol whenever they can.
In particular, high volume sources have the most incentive to deploy ConEx. This is because high volume sources (e.g. video download sites or peer-to-peer file-sharing) can gain by implementing a low 'weight' end-to-end transport (i.e. a less aggressive response to congestion than other transports). Then, although they send a large amount of volume, they need not contribute significantly to congestion. If the ISP currently limits data volume, or offers chargeable tiers based on data volume, such customers stand to gain considerably if they can encourage the ISP to limit usage based on congestion-volume instead of volume.
Figure 3 explains why this is the case. The plots show bit-rate on the vertical axis and time horizontally. A file transfer (e.g. the one labelled from customer 'b') is given a simplified representation as a rectangle, implying it runs at a set rate for a time, then completes. The maximum height of each plot represents the maximum capacity of the shared link across the backhaul network, which is typically the bottleneck in a broadband network. The hatched regions represent unused capacity. 'c' represents the high volume source that we intend to show has an incentive to deploy ConEx.
In the upper half of the figure, customers 'b' & 'c' both use transports with equal weights, which is why they are shown with equal rates when they both compete for the capacity of the line. 'c' sends larger files than 'b', so when 'b' completes each of its file, 'c' can use the full capacity of the line until 'b' starts the next file. In the lower half of the figure, 'c' uses a less aggressive (lower weight) transport, so whenever 'b' sends a file, 'c' yields more of its rate. This allows 'b' to complete its transfer earlier, so that 'c' can take up the full rate earlier. 'b' sends the same volume files (same area in the graph), just faster and therefore they complete sooner (tall & thin instead of shorter and wider). As a result, 'c' hardly finishes any later than in the upper diagram. However, 'c' will have contributed much less to congestion, and 'b' completes the majority of its file transfers much faster. 'b' has also contributed less to congestion.
As we have said, customer 'c' in particular stands to gain if the ISP bases usage-limits (or usage charges) on congestion-volume rather than volume. The ISP also has a strong incentive to reward customers like 'c', because they make the network performance appear far better than before for customer's like 'b' (e.g. short Web transfers). However, the network cannot make this move until customers like 'c' expose congestion information (ConEx) that the ISP can use in its traffic management or contracts.
^ bit-rate
|
|---------------------------------------------------.--,--.--,-------
| |/\| |\/|
| c |\/| b|/\| c
|------. ,-----. ,-----. ,-----. ,-----. ,-----./\| |\/| ,----
| b | | b | | b | | b | | b | | b |\/| |/\| | b
| | | | | | | | | | | |/\| |\/| |
+------'--'-----'--'-----'--'-----'--'-----'--'-----'--'--'--'--'---->
time
^ bit-rate
|
|---------------------------------------------------.--,--.--,-------
|---. ,---. ,---. ,---. ,---. ,---. |/\| |\/| ,---.
| | | | | | c | | | | | | |\/| b|/\| c| |
| | | | | | | | | | | | |/\| |\/| | |
| b | | b | | b | | b | | b | | b | |\/| |/\| | b |
| | | | | | | | | | | | |/\| |\/| | |
+---'-----'---'----'---'----'---'----'---'----'---'-'--'--'--'--'---'>
time
Figure 3: Weighted congestion controls with equal weights (upper) and unequal (lower)
Of course, in reality there would be more than two customers. But this would mean that short transfers like 'b' stand to gain even more, as multiple larger files would be yielding at once.
We should point out that not all high-volume customers will be prepared to temporarily shift their usage out of the way as shown — real-time video for instance would still use a higher weight (more aggressive) so as to ensure timely delivery. However, high volume applications with elastic (non-real-time) requirements are also common (e.g. video streaming, software downloads, etc)
We should also point out that a transport that is less agressive against other customers is similar but not quite the same as LEDBAT [ledbat-congestion]. LEDBAT does indeed yield more to other flows during congestion, but it is designed to only do this if the contention for resources is at a slow link, such as the customer's own home router. If the contention is at a fast link, such as a BRAS, LEDBAT is designed not to yield. This is because ISPs currently give no reward to a transport that minimises congestion to others — because they do not have the congestion information to be able to.
This document does not require actions by IANA.
This document has introduced how congestion policing could be deployed at the broadband remote access servers in a typical broadband access network. Congestion policing uses ConEx markings introduced by data sources and packets discarded by the BRAS to determine rest-of-path congestion, and police traffic accordingly.
It has been shown that high-volume elastic data sources have a strong incentive to deploy ConEx speculatively in the expectation that they will be able to encourage their ISPs to account for their usage by congestion-volume, not volume. They can use a less aggressive transport and prove that they are contributing little to congestion despite sending a lot of volume. ISPs also have a strong incentive to use this ConEx information to encourage their elastic high-volume customers to use less agressive transports, given they improve the performance of all the other customers.
Without ConEx information, ISPs can only use volume as a metric of usage, which prevents the above virtuous circle from forming, perversely discouraging high-volume elastic customers from such friendly behaviour.
Comments and questions are encouraged and very welcome. They can be addressed to the IETF Congestion Exposure (ConEx) working group's mailing list <conex@ietf.org>, and/or to the authors.
| [conex-abstract-mech] | Mathis, M and B Briscoe, "Congestion Exposure (ConEx) Concepts and Abstract Mechanism", Internet-Draft draft-ietf-conex-abstract-mech-05, July 2012. |
| [conex-concepts-uses] | Briscoe, B, Woundy, R and A Cooper, "ConEx Concepts and Use Cases", Internet-Draft draft-ietf-conex-concepts-uses-04, March 2012. |
| [conex-destopt] | Krishnan, S, Kuehlewind, M and C Ucendo, "IPv6 Destination Option for Conex", Internet-Draft draft-ietf-conex-destopt-02, March 2012. |
| [intarea-ipv4-id-reuse] | Briscoe, B, "Reusing the IPv4 Identification Field in Atomic Packets", Internet-Draft draft-briscoe-intarea-ipv4-id-reuse-01, March 2012. |
| [ledbat-congestion] | Hazel, G, Iyengar, J, Kuehlewind, M and S Shalunov, "Low Extra Delay Background Transport (LEDBAT)", Internet-Draft draft-ietf-ledbat-congestion-09, October 2011. |
| [TR-059] | Anschutz, T., "DSL Forum Technical Report TR-059: Requirements for the Support of QoS-Enabled IP Services", September 2003. |
| [TR-101] | Cohen, A. and E. Shrum, "Migration to Ethernet-Based DSL Aggregation", April 2006. |
| [CongPol] | Jacquet, A, Briscoe, B and T Moncaster, "Policing Freedom to Use the Internet Resource Pool", Proc ACM Workshop on Re-Architecting the Internet (ReArch'08) , December 2008. |
Detailed changes are available from http://tools.ietf.org/html/draft-briscoe-conex-initial-deploy