Network Virtualization using Extreme Fabric Connect

Network Virtualization Using Extreme Fabric Connect
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Quality of Service
Initial Considerations
The objective of network Quality of Service (QoS) is to allow different type of traffic to contend inequitably
for shared network resources. The goal is to converge applications such as voice, video and data over the
same network infrastructure. Voice is low constant bandwidth but is a real-time application and thus does
not tolerate delay (latency). Data is bursty and can tolerate high levels of latency, while video is usually
somewhere in between, depending if it is used for real-time video conferencing or IPTV video streaming.
QoS is all about managing buffer resources as well as buffer congestion in a different way for the different
traffic classes. The main consequences of buffer congestion being delay, jitter, and packet drops.
The IETF reference model for QoS is the Differentiated Services (DiffServ) Model (RFC 2475), which defines
a way to mark IP packets with a Differentiated Services Code Point (DSCP) and Per Hop Behaviors (PHB).
Each DSCP has a corresponding PHB such that traffic entering the network with a certain DSCP class must
be given the same PHB by every node it traverses across the network. Traffic is assigned to one of the
discrete DSCP classes either by the application on the source or by network policies defined at the network
access, and the corresponding PHB is derived from the DSCP marking by subsequent hops in the network.
Figure 75 QoS DiffServ Model
In this model, a router IP interface can be configured either as DiffServ Core (Trusted) or DiffServ Access
(Untrusted). A DiffServ Core port will always derive the QoS and hence the PHB from the DSCP marking
recorded in the packet IP header. Whereas a DiffServ Access port will derive the QoS exclusively from
access policies defined on the port and will then update the DSCP for the packets accordingly (or reset
DSCP to zero if no policy could be applied to a given packet).
The DSCP field is part of the Layer3 IP header (in IPv4 as well as IPv6) so while every IP router or L3 switch
will have no problem deriving the PHB from the DSCP field, the same cannot be expected from all Layer2
Ethernet switches that will not even be looking at the IP header to forward packets. For this reason, and
provided that q-tagging is in use, the PHB can also be derived from the Ethernet 802.1p bits. In general, the
Ethernet 802.1p bits should always be set to a consistent value with respect to the DSCP marking and every
IP router or L3 switch will automatically update (using egress mapping tables) the 802.1p bits to a DSCP
consistent value when IP routing a packet onto Ethernet q-tagged segments.
Tip
All of Extreme Networks ERS, VSP and ExtremeXOS series of Ethernet access switches are
able to derive QoS from the DSCP marking.