Quality of Service

The basic goal of Quality of Service (QoS) is to provide service differentiation between IP packets in the network. This service differentiation is noticeable during periods of network congestion (i.e. in case of contention for resources) and results in different levels of network performance.

QoS in GÉANT

Currently, three service classes are offered that can be used for traffic transiting the GÉANT network:

• The Best Effort service does not provide any performance guarantees. However, the goal is to give a fair share to each traffic flow. Best effort is the default class of service and is useful for traffic from non-real-time applications such as e.g. FTP.
  
  
• The Premium IP service provides guarantees on one-way delay, jitter and packet loss. Applications with real time constraints may benefit from this service; for instance, video conferencing.
  
  
• The Less than Best Effort (scavenger) service will utilise the capacity that is not used by best effort and premium IP traffic. This may be useful to transfer vast amounts of (non real-time) data for applications such as GRID computing without adversely affecting other traffic.

 If available, both Best Effort and Less than Best Effort services are available to peer networks. The Premium IP service, however, is only available after a reservation has been made.

Implementation and DSCP values

Implementation

The implementation of QoS in the GÉANT network is based on the Differentiated Services (DiffServ) model as defined by the Internet Engineering Task Force (IETF). This implies that each packet is marked with a DiffServ CodePoint (DSCP) in its header. The DSCP value is stored in the first 6 bits of the Type of Service (ToS) field that is part of the standard IP header. The DCSP values are associated with a certain forwarding treatment, also called Per Hop Behaviors (PHB). By configuring PHBs in each router of the network an edge-to-edge service can be provisioned.
  Type of Service field in IPv4 and IPv6 header 

DSCP/ToS Values

The DSCP/ToS values used in GÉANT to classify the traffic of the different QoS classes are shown in the table below. In addition to the three service classes offered to transiting traffic there is a DWS (IP commodity service) and a Network Control class, which are traffic classes used internally to the GÉANT network.

The table below shows how the packets from a research network arriving on GÉANT with a given DSCP value (incoming DSCP value) will be retagged (new value - rewritten) by the GÉANT routers.

¹ The Authorised Premium IP packets are evaluated against a policer (according to what has been negotiated). If the packets are in-profile, they are accepted and the tagging is kept unchanged. If they are out-of-profile, they are dropped.
² The packets coming in on a Juniper M160 router will be retagged with the DSCP value 0, while those coming in on a Juniper M40 router will be retagged with the DSCP value 5 (PREC rewritten, three last bits of the DSCP field kept unchanged).

The QoS traceroute tool is useful to debug problems related to marking of packets. More information on this tool can be found via the link in left-hand menu. Also, bear in mind the marking capabilities may differ for some types of routers.

Traffic Treatment

At the ingress interface, i.e. the interface where the traffic from an NREN enters the GÉANT network, two actions are performed. First packets are classified, i.e. put into a certain queue depending on their service class. Best effort and LBE traffic is classified by means of the DSCP value. Premium IP traffic is classified by means of a packet filter according to the source/destination address and optionally the DSCP value of the packet. Furthermore, the Premium IP traffic is policed such that it will not exceed the agreed traffic profile. Excess Premium IP traffic is discarded.

Within the backbone and at the egress interface of GÉANT classifying is done solely based on the DSCP value. No policing is performed at these interfaces.

Shaping is not performed in the GÉANT network in order to avoid shaping delay. End users and/or NRENs may, however, perform shaping actions if necessary.

The different queues (four per interface) are served by a Weighted Round Robin (WRR) scheduler. A weight, representing a percentage of the total capacity, is assigned to each queue. The queue for best effort traffic is assigned 90%, the LBE queue 5% and a queue for network control traffic is assigned 5%. The Premium IP traffic has absolute priority over all other queues. Rate limiting (i.e. policing) at the ingress prevents other traffic classes from being starved by Premium IP traffic.

See also: