[TCI]) that are inserted within an Ethernet frame
following the source address field. The TPID field is currently
fixed and assigned the value 0x8100. Figure shows the structure
of the Ethernet frame. The TCI field is composed of three
fields: - User priority bits (PRI) (3 bits): The
specifications of this 3-bit field are defined by the IEEE
802.1p standard. These bits can be used to mark packets as
belonging to a specific CoS. The CoS marking uses the three
802.1p user priority bits and allows a Layer 2 Ethernet frame
to be marked with eight levels of priority (values 0–7). Three
bits allow for levels of classification, allowing a direct
correspondence with IP version 4 (IPv4) (IP precedence) type of
service (ToS) values. The table lists the standard definitions
the IEEE 802.1p specifies for each CoS.
Standard
Definitions of CoSs CoS Definition CoS 7
(111) Network CoS 6 (110) Internet CoS 5 (101) Critical CoS 4
(100) Flash-override CoS 3 (011) Flash CoS 2 (010) Immediate
CoS 1 (001) Priority CoS 0 (000) Routine One disadvantage of
using CoS markings is that frames lose their CoS markings when
transiting a non-802.1Q to a non-802.1p link. Trunking with
802.1Q must be enabled before the CoS field even exists. As
soon as the packet encounters Layer 3 forwarding, either with a
router or a Layer 3 switch, the old LAN header gets discarded
and the CoS field will be lost. Therefore, a ubiquitous
permanent marking should be used for network transit. This is
typically accomplished by translating a CoS marking into
another marker or simply using a different marking mechanism.
- Canonical format indicator (CFI) (1 bit): This
bit indicates whether the bit order is canonical or
noncanonical. The CFI bit is used for compatibility between
Ethernet and Token Ring networks.
- VLAN identifier
(VLAN ID) (12 bits): The VLAN ID field is a 12-bit field
that defines the VLAN used by 802.1Q. The fact that the field
is 12 bits long restricts the number of VLANs supported by
802.1Q to 4096. For most enterprise customers, 4096 VLANs is
adequate. For service provider applications, 4096 VLANs may not
be enough.
Classification and Marking in the
Enterprise
Before the Internet Engineering Task Force
(IETF) defined QoS methods for the network layer, the ITU-T and
the Frame Relay Forum (FRF) had already derived standards for
link layer QoS in Frame Relay networks. Frame Relay provides a
simple set of QoS mechanisms to ensure a committed information
rate (CIR): congestion notifications called forward explicit
congestion notification (FECN) and backward explicit congestion
notification (BECN), in addition to fragmentation of data
frames when voice frames are present, as described in Frame
Relay Forum standard FRF.12. Figure shows the structure of a
Frame Relay frame. One component of Frame Relay QoS is packet
discard eligibility when congestion is experienced in the
network. Frame Relay will allow network traffic to be sent at
a rate exceeding its CIR. The frames that exceed the committed
rate can be marked as discard eligible (DE) at the ingress
Frame Relay switch. If congestion occurs in the network, frames
marked DE will be discarded in preference to frames that are
not marked. Marking in MPLS
When a customer
transmits IP packets from one site to another, the IP
precedence field (the first three bits of the DSCP field in the
header of an IP packet) specifies the CoS. Based on the IP
precedence marking, the packet is given the desired treatment,
such as guaranteed bandwidth or latency. The MPLS experimental
bits comprise a 3-bit field that you can use to map IP
precedence into an MPLS label. This allows MPLS-enabled routers
to perform QoS features indirectly based on the original IP
Precedence field inside the IP packets encapsulated by MPLS,
without the need to spend resources to look into the IP packet
header and examine the IP Precedence field. If the service
provider network is an MPLS network, then the IP precedence
bits are copied into the MPLS Experimental (EXP) field at the
edge of the network. However, the service provider might want
to set an MPLS packet QoS to a different value that is
determined by the service offering. The MPLS EXP field allows
the service provider to provide QoS without overwriting the
value in the customer IP Precedence field. The IP header
remains available for customer use, and the IP packet marking
is not changed while the packet travels through the MPLS
network. Figure shows the structure of the MPLS frame.
- MPLS uses a 32-bit label field (shim header), which is
inserted between Layer 2 and Layer 3 headers (frame
mode).
- MPLS EXP bits support up to eight CoSs.
- By default, Cisco IOS software copies the three most
significant bits of the DSCP or the IP precedence of the IP
packet to the EXP field.
- EXP bits are preserved
throughout the MPLS network.
Content
4.1 Introducing Classification and Marking
4.1.4 DiffServ Model In contrast to integrated
service (IntServ), which is a fine-grained, flow-based
mechanism, differentiated service (DiffServ) is a
coarse-grained, class-based mechanism for traffic management.
DiffServ architecture is based on a simple model in which data
packets are placed into a limited number of traffic classes,
rather than differentiating network traffic based on the
requirements of an individual flow. Each router on the network
is configured to differentiate traffic based on its class. Each
traffic class can be managed differently, ensuring preferential
treatment for higher-priority traffic on the network.
DiffServ-aware routers implement per-hop behaviors (PHBs),
which define the packet forwarding properties associated with a
class of traffic. Different PHBs may be defined to offer, for
example, low-loss, low-latency forwarding properties or
best-effort forwarding properties. All the traffic flowing
through a router that belongs to the same class is referred to
as a behavior aggregate (BA). A more detailed discussion of
PHBs and BAs occurs later in this lesson. The DSCP values mark
packets to select a PHB. Within the core of the network,
packets are forwarded according to the PHB that is associated
with the DSCP. The PHB is an externally observable forwarding
behavior applied at a DiffServ-compliant node to a collection
of packets with the same DSCP value. One of the primary
principles of DiffServ is that you should mark packets as close
to the edge of the network as possible. It is often a difficult
and time-consuming task to determine which traffic class a data
packet belongs to. You want to classify the data as few times
as possible. By marking the traffic at the network edge, core
network devices and other devices along the forwarding path
will be able to quickly determine the proper CoS to apply to a
given traffic flow. A key benefit of DiffServ is ease of
scalability in comparison to IntServ. DiffServ is used for
mission-critical applications and for providing end-to-end QoS.
Typically, DiffServ is appropriate for aggregate flows because
it performs a relatively coarse level of traffic
classification. DiffServ describes services and allows many
user-defined services to be used in a DiffServ-enabled
network. Services are defined as QoS requirements and
guarantees that are provided to a collection of packets with
the same DSCP value. Services are provided to classes. A class
can be identified as a single application or multiple
applications with similar service needs, or it can be based on
source or destination IP addresses, or it can be based on a
flow. Provisioning is used to allocate resources to defined
traffic classes. Provisioning provides a mechanism for the set
of methods that are used to set up the network configurations
on devices to enable the devices to provide the correct set of
capabilities for a particular traffic class. The idea is for
the network to recognize a class without having to receive
specific requests from applications. This allows the QoS
mechanisms to be applied to other applications that do not have