1.0 Network Traffic Control

Computer network traffic is the data moving across the network at a given time. Network bandwidth is the maximum amount of data that can be transmitted over the network in unit time. Computer networks face the possibility of network congestion, where excessive data is trying to pass through the network and there might be delays in packet transmission, packet loss, blocking of new connections and, in general, a degraded quality of service. Network traffic control is the process of regulating network traffic so as to reduce network congestion, latency and packet loss.

The term Quality of Service (QoS) generally means overall performance of a service as perceived by the users of the service. Certain predefined parameters are measured in real time to arrive at a quantitative measure of the quality of service. In computer networking, quality of service refers to traffic prioritization and resource reservation rather than the overall performance quality achieved for the service. Quality of service in packet switched networks involves providing different priorities to users or data flows and ensuring a certain level of performance for a data flow. QoS can be implemented using the tc command in Linux.

2.0 Queues

The incoming and outgoing packets are queued before these are received or transmitted respectively. The queue for incoming packets is known as the ingress queue. Similarly, the queue for outgoing packets is called the egress queue. We have more control over the egress queue as it has packets generated by our host. We can re-order these packets in the queue, effectively favoring some packets over the rest. The ip -s link command gives the queue capacity (qlen) in number of packets. If the queue is full and more packets come; they are discarded and are not transmitted. The ingress queue has packets which have been sent to us by other hosts. We can not reorder them; the only thing we can do is to drop some packets, indicating network congestion by not sending the TCP ACK to the sending host. The sending host gets the hint and slows down transmission of packets to us. For UDP packets, this does not work. If UDP packets are dropped, they are simply lost as there is no ACK and re-transmission.

3.0 TRAFFIC CONTROL ELEMENTS

3.1 SHAPING

Shaping involves delaying the transmission of packets to meet a certain data rate. This is the way we ensure that the output data rate does not exceed the desired value. Shapers can also smooth out the bursts in traffic. Shaping is done at egress.

3.2 SCHEDULING

Scheduling is deciding which packet would be transmitted next. This is done by rearranging the packets in the queue. The objectives are to provide a quick response for interactive applications and also to provide adequate bandwidth for bulk transfers like downloads initiated by remote hosts. Scheduling is done at egress.

3.3 POLICING

Policing is measuring the packets received on an interface and limiting these to a particular value. The packets might be reclassified or dropped. Policing is done at ingress.

3.4 DROPPING

After the traffics exceeds a predefined value, the packets are simply dropped. This is done both at the ingress and the egress.

4.0 TRAFFIC CONTROL COMPONENTS

4.1 qdiscs

qdisc is an abbreviation for Queue Discipline. A qdisc is the packet scheduling code that is attached to a network interface. qdiscs are implemented as modules, which are inserted in the kernel at the run time. A qdisc can drop, forward, queue, delay or re-order packets at a network interface. tc is a user space program for managing qdiscs for network interfaces. The other terms used for qdisc are Packet Scheduler, queuing algorithm and the packet scheduler algorithm.

The kernel sends (en-queues) packets received on an network interface to a qdisc. Similarly, the kernel takes (de-queues) packets from a qdisc for transmission on a network interface.

qdiscs are of two types, classful qdiscs, which contain classes, and classless qdiscs, which don't.

4.2 Classes

A Class is a sub-qdisc. A class may contain another class. Using classes, we can configure the QoS in more detail. When packets are received by a qdisc, these may be queued in inner qdiscs in classes. When the kernel wants the packets for transmission, the packets of certain classes might be given ahead of others, thereby prioritizing certain types of traffic.

4.3 Filters

When a qdisc with classes receives a packet, it needs to decide in which class it has to be enqueued. It needs to be classified. Filters are used for classification of packets. A filter must contain a classifier phrase. The most common classifier used by filters is the u32 classifier which is used by filers for selecting packets based on packet attributes.

5.0 Using the tc command

The tc command has sub-commands to add, change, replace and delete qdiscs, classes and filters. Also, there is the show sub-command to give details of currently configured objects. For example, running the tc -s qdisc show command on a desktop running Linux,

$ tc -s qdisc show dev eth0
qdisc pfifo_fast 0: root refcnt 2 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
 Sent 8728071 bytes 59911 pkt (dropped 0, overlimits 0 requeues 0) 
 backlog 0b 0p requeues 0 

pfifo_fast is the default qdisc for all interfaces in Linux. It is a FIFO qdisc with prioritization. It has three bands, FIFO 0, FIFO 1 and FIFO 2. The band 0 is for traffic from interactive applications, where we wish to minimize the delay. The band 1 if for best effort, and is the normal service. Band 2 is for bulk data transfers, where the goal is to maximize the throughput and minimize the monetary cost. The packets are put in one of the three bands based on the value of the ToS field in the packet. First, all the packets in FIFO 0 are transmitted. When there are no packets left in FIFO 0, packets in FIFO 1 are transmitted. Lastly, packets in FIFO 2 are transmitted.

6.0 Bufferbloat

Network latency is the time taken by a packet to reach from one end of the connection to another. A typical TCP connection between a sender and a receiver passes through many devices and has many links of varying bandwidth. There are buffers at each processing unit in the communication path so that packets arriving can be stored while the communication link is being used for transmission. Buffers are a necessary part of the communication pipe and help in making effective use of the communication link. But, as the network devices have got more and more RAM and also due to the misguided objective of preventing packet loss to the maximum extent possible, the buffer sizes have increased to high values. The result is that the communication pipe has buffers of unreasonably big sizes at intermediate devices. These buffers get filled and obstruct the normal functioning of the TCP. TCP uses end to end signalling of packet loss, but because of