Traffic shaping

Quality of Service (QoS)

Quality of Service (QoS) in computer networks refers to resource reservation control mechanisms to provide different priorities to different applications, users, or data flows, or to guarantee a certain level of performance according to the constraints imposed by the application. Constraints such as delay in delivery, the bit rate, the probability of packet loss or the variation delay per packet [1] may be determined for various multimedia data stream applications such as voice or TV over IP. These mechanisms are only applied when resources are limited (wireless cellular networks) or when there is congestion in network, otherwise such QoS mechanisms are not required.

[1]	jitter or Packet Delay Variation (PDV) is the difference in end-to-end delay between selected packets in a flow with any lost packets being ignored.

There are several techniques to give quality of service:

Reserving network resources:

Using Resource reSerVation Protocol (RSVP) to request and reserve resources in the routers. However, this option has been neglected because it does not scale well with Internet growth.

Differentiated services (DiffServ):

In this model, packets are marked according to the type of service they need. In response to these marks, routers and switches use various queuing strategies to tailor performance to requirements. This approach is currently widely accepted.

In addition to these systems, bandwidth management mechanisms may be used to further improve performance such as traffic shaping, Scheduling algorithms o congestion avoidance.

Regarding traffic shaping, there are two predominant methods:

Token bucket:

It dictates when traffic can be transmitted, based on the presence of tokens in the bucket (an abstract container that holds aggregate network traffic to be transmitted). Each token in the bucket can represent a unit of bytes of predetermined size, so each time that traffic is transmitted, the tokens are removed (cashed in). When there are no tokens, a flow cannot transmit its packets. Periodically, tokens are added to the bucket. Using such mechanism, it is allowed to send data in peak burst rate.

Leaky bucket:

Conceptually based on considering a bucket with a hole in the bottom. If packets arrive, they are placed into the bucket until it becomes full, then packets are discarded. Packets are sent at a constant rate, which is equivalent to the size of the hole in the bucket.

QoS configuration in eBox

eBox uses Linux kernel features [2] to shape traffic using token bucket mechanisms that allow to assign a limited rate, a guaranteed rate and a priority to certain types of data flows through the Traffic Shaping ‣ Rules menu.

[2]	Linux Advanced Routing & Traffic Control http://lartc.org

In order to perform traffic shaping, it is required to have, at least, an internal network interface and an external one. You need, at least, one configured gateway as well. And you have also to set your bandwidth information in Traffic Shaping ‣ Interface Rates. Set the upload and download rate that provide the router that is connected to every external interface. The shaping rules are specific for each interface and they may be selected for those external network interfaces with assigned upload rate and all internal ones.

If the external network interface is shaped, then you are limiting eBox output traffic to the Internet. If, however, you shape an internal network interface, then the eBox output to internal networks is limited. The maximum output and input rates are given by the configuration in Traffic Shaping ‣ Interface Rates. As it can be seen, shaping input traffic is not possible directly, that is because input traffic is not predictable nor controllable in almost any way. There are specific techniques from various protocols to handle the incoming traffic, for instance, TCP by artificially adjusting the window size as well as controlling the rate of acknowledgements (ACK) segments being returned to the sender.

Each network interface has a rule table to give priority (0: highest priority, 7: lowest priority), guaranteed rate and/or limited rate. These rules apply to traffic bound to a service, a source and/or a destination.

Traffic shaping rules

Practise example

Set up a rule to shape incoming HTTP traffic by limiting it to 20KB/s. Check if it works properly.

1. Action: Add a gateway in Network ‣ Gateways to your external network interface.

   Effect:

   The Save changes button is enabled. The gateway list displays a single gateway.

2. Action: Save the changes.

   Effect:

   eBox displays the progress while the changes are being applied. Once this is complete, it informs the user.

3. Action: Enter Services and add a new external service called HTTP with TCP protocol and destination port 80.

   Effect:

   eBox shows a list with all the services where the new service is displayed too.

4. Action: Enter Traffic Shaping ‣ Rules. Select the internal interface from the interface list and, using Add new, set a new rule with the following details:

   Enabled:Yes Service:Port-based service / HTTP Source:any Destination:any Priority:7 Guaranteed rate:  0 Kb/s Limited rate:160 Kb/s

   Press the Add button.

   Effect:

   eBox displays a table with the new traffic shaping rule.

5. Action: Start downloading a huge file, which is reachable from the Internet (for example, a Ubuntu ISO image) from a host within your LAN (not eBox itself) using the wget command.

   Effect:

   The download rate is stable around 20 KB/s (160 Kbit/s).