Today we want to address the topic of Quality of Service (QoS).
In principle, all data packets are treated equally in a network. This can lead to conflicts when the network is heavily loaded. This is because different IT applications have different requirements in terms of bandwidth/transmission capacity, delay, jitter, and packet loss.
While fluctuations are usually acceptable when downloading a file, a constantly jerky video stream is extremely annoying. Even more serious are disruptions to IP telephony, or Voice over IP (VoIP). Here, the so-called quality of service is fundamental.
Although VoIP does not require much bandwidth, it does place special demands on transmission. As a real-time application, delays or packet loss have a negative impact. This leads to disruptions or interruptions in voice transmission. The same applies to video transmission: missing image fragments lead to pixelated playback or interruptions, especially in compressed video streams. However, if an email arrives a few seconds later at the recipient's end, this is of little or no consequence.
RSVP and MPLS
There are several ways to solve this problem. One option is to oversize the network to provide more bandwidth than is needed. Another option is to reserve bandwidth for specific applications in accordance with the Resource Reservation Protocol (RSVP), which allows routers to dynamically manage their bandwidth. This allows specific transmission rates to be reserved for individual connections for certain applications, such as video streaming. However, RSVP is not suitable for large data networks. MPLS (Multiprotocol Label Switching) offers an alternative here. MPLS allows data packets to be routed with priority along a pre-established path in an IP network. It is mainly used by large Internet providers for voice and data services based on IP.
Port Prioritization
Another obvious solution is to prioritize individual ports on the switch. However, this only works in local networks and is very costly to implement when multiple switches are connected to each other. Another drawback is that all services running on a prioritized port are treated equally. This makes prioritization effectively ineffective. The consequence is that for QoS to really work, individual data streams must be prioritized. To do this, the data packet itself must carry an identifier.
VLAN-based QoS
As long as data remains within the local network, data packets can be identified using VLAN tags. I have already explained the VLAN principle in two previous articles: How to increase security and performance with virtual networks! and VLANs in business practice.
For VLAN-based QoS to work, every switch involved in forwarding must read the prioritization information from the VLAN tag and forward the packet according to its priority. However, since data packets can be freely prioritized on a scale of 1-128, there is a risk that all authorized users will always assign a high or even the highest priority, and the problems will remain.
That is why there must be uniform regulations that also work outside local networks and at the international level. Four different approaches have become established and are internationally recognized:
1. CoS - Class of Service
Class of Service defines different classes of data transfers to which the respective data packets are assigned. Each class corresponds to a priority. As the number increases (1 = high, 6 = low), the priority with which the data packets are transferred decreases.
- 1. Voice
- 2. Video
- 3. VPN
- 4. WWW
- 5. Mail
- 6. Miscellaneous
The advantage: these classes are clearly defined and internationally valid.
2. Type of Service (ToS)
ToS stands for Type of Service. The ToS field is a data field in the IP header. Computers can use the ToS information to specify network-relevant service types. Various parameters such as bandwidth, transmission speed, reliability, delay, and data throughput can be defined. In principle, ToS has been replaced by DiffServ.
3. DiffServ - Differentiated Services
With DiffServ, the classification and marking of data packets is performed by the sender at layer 3 of the OSI model as a supplement to IP. The routers on the way to the recipient evaluate this marking. The advantage: DiffServ does not change the IP packet. Only the ToS field in the IP header is interpreted differently.
4. DSCP
The ToS field in the IP header is therefore also referred to as the Differentiated Services Code Point (DSCP). DSCP is a combination of the IP priority and service type fields. DSCP values are used to work with older routers that only support IP priority, as they are compatible with IP priority fields.
Conclusion
Whether bandwidth, latency, or packet loss: different applications have different requirements in terms of transmission quality of service. If you want to ensure that all requirements are met, you need to consider the prioritization of your data and cannot ignore quality of service. However, this topic is extremely diverse. Most aspects could therefore only be touched upon. We will cover them in more detail in future Dr. Lan newsletters. If you would like to learn more about QoS now, the qualified KTI network experts will be happy to advise you.