MPLS - Page 2
Terminology & Definitions Back to MPLS Central
MPLS Technical Definition
Multiprotocol Label Switching (MPLS) is a standards-approved technology for speeding up network traffic flow and making it easier to manage. MPLS involves setting up a specific path for a given sequence of packets, identified by a label put in each packet, thus saving the time needed for a router to look up the address to the next node to forward the packet to. MPLS is called multiprotocol because it works with the Internet Protocol (IP), Asynchronous Transport Mode (ATM), and frame relay network protocols. With reference to the standard model for a network (the Open Systems Interconnection, or OSI model), MPLS allows most packets to be forwarded at the layer 2 (switching) level rather than at the layer 3 (routing) level. In addition to moving traffic faster overall, MPLS makes it easy to manage a network for quality of service (QoS). For these reasons, the technique is expected to be readily adopted as networks begin to carry more and different mixtures of traffic.
MPLS Terminology
Definitions
• • • • • • • • • • • • • • • • • • • • • • • • • • • • •
abstract node
An abstraction used in describing an explicit route. An abstract node may be a network element, a group of network elements sharing an address prefix, or an Autonomous System. An abstract node consisting of exactly one network element is called a simple abstract node.
actual private network
A term invented for comparison with virtual private network (VPN).
adjacent
Having a direct logical link. Either directly connected physically, or connected using an approach that makes intervening devices transparent in a logical context-for example, tunneling.
aggregation
Grouping or bundling traffic requiring similar forwarding. Distinct from merging, generally, because it may be desirable to separate aggregate traffic at some point without having to resort to a routing decision at L3 for all packets within the aggregate.
Assured Forwarding
A per-hop behavior (PHB) defined for Differentiated Services that provides for four classes of PHB, each having three levels of drop precedence. Assured Forwarding also requires that packets within a class not be reordered, regardless of the drop precedence. Assured Forwarding does not define a quantifiable value for delay or delay variation of packets forwarded.
Autonomous System
In interdomain routing, an administrative domain identified with an AS number.
Behavior Aggregate
IP packets that require the same Differentiated Services behavior at the point where they are crossing a link.
Border Gateway Protocol
The only exterior gateway routing protocol. Currently version 4 is in use. A routing protocol used in routing between administrative domains.
bridge
A device used to forward frames at the data-link layer.
Cell Loss Priority
A bit in the AAL5 ATM header indicating that the cell can be dropped earlier under congested conditions.
Connection (or Call) Admission Control
Use of some approach to determine whether or not a requested service requirement can reasonably expect to be met by a device, prior to committing to provide the requested service at the device.
conservative retention mode
Labels are requested and retained only when needed for a specific next hop. Unnecessary labels are immediately released.
content addressable memory
A memory device that allows a key to be compared to the contents of all memory locations at the same time. Content addressable memory is roughly a hardware analogue of a software hashing algorithm.
control word
An instruction, index, or key into a table of instructions, generally at the (virtual) machine level.
Data Link Connection Identifier
Used in Frame Relay to identify a circuit connection between adjacent Frame Relay switches.
data link layer
Layer 2 of the OSI model; the layer between the physical and network layers.
Differentiated Services (DiffServ)
An IETF standard for providing different classes of service based on some common sets of assumptions about queuing behavior on a hop-by-hop basis. Because the basis for specific treatment is explicitly carried in packets, rather than requiring local storage of packet classification information, this approach to providing quality of ser-vice (QoS) is often referred to as "less state-full" than, for example, the Integrated Services QoS model.
Discard Eligibility
A bit in the Frame Relay header indicating that the frame can be discarded under congested conditions.
domain of (label) significance
The portion of a network consisting of logically connected logical interfaces with a common knowledge of the significance (meaning) of a label. A label only has meaning upon arrival at a logical interface if that interface was represented in the process during which the meaning was originally negotiated.
downstream
In the direction of expected traffic flow. Applies to traffic that is part of a specific forwarding equivalence class.
downstream label allocation
Label negotiation in which the downstream LSR determines what label will be used. This is the only currently supported approach.
downstream on-demand label distribution mode
Labels are allocated and provided to the upstream peer only when requested. This mode is most useful when the upstream LSR is using conservative label retention or is not merge capable (or, as is likely, both).
downstream unsolicited label distribution mode
Labels are allocated and provided to the upstream peer at any time (typically in conjunction with advertisement of a new route). Most useful when the upstream neighbor is using liberal retention mode.
egress
Point of exit from an MPLS context or domain. The egress of an LSP is the logical point at which the determination to pop a label associated with an LSP is made. The label may actually be popped at the LSR making this determination or at the one prior to it (in the penultimate hop pop case). Egress from MPLS in general is the point at which the last label is removed (resulting in removal of the label stack).
Expedited Forwarding
A per-hop behavior defined for Differentiated Services that requires a network node to provide a well-defined minimum departure rate service for a configurable departure rate such that if incoming traffic is conditioned not to exceed this minimum departure rate, packets are effectively not queued within the node. Expedited Forwarding ensures that, for conditioned traffic, the delay at any node is bounded and quantifiable.
explicit route
A route specified as a nonempty list of hops that must be part of the route used. If an explicit route is strict, only specified hops may be used. If an explicit route is loose, all specified hops must be included, in order, in the resulting path, but the path is otherwise unrestricted.
extranet
From the perspective of a private network, any other network, including all other networks.
filtering database
Used in some bridging technologies to determine what interfaces an L2 frame will not be forwarded on.
Fixed Filter
A reservation style that is useful in establishing a point-to-point LSP from one ingress to one egress LSR.
flooding
The process of forwarding data on all, or most, interfaces in order to ensure that the receiver gets at least one copy.
forwarding database
Information used to make a forwarding determination.
forwarding determination
The process used to determine the interface to be used to forward data. This process may or may not be directly driven by a route determination.
forwarding equivalence class
A description of the criteria used to determine that a set of packets is to be forwarded in an equivalent fashion (along the same logical LSP). Forwarding equivalence classes are defined in the base LDP specification and may be extended through the use of additional parameters (such as is the case with CR-LDP). FECs are also represented in other label distribution protocols.
frame
A message encapsulation generally consisting of a DLL header, a payload-frequently consisting of at least part of a network-layer packet-and (possibly) a trailer. Normally encapsulated by physical-layer framing.
FTN
FEC-to-NHLFE map, used to insert unlabeled packets onto an LSP.
hard state
State information that remains valid until explicitly invalidated.
implicit null label
A label value given to an upstream neighbor when it is desirable to have that LSR pop one label prior to forwarding the packet. This behavior is commonly referred to as penulitimate hop pop (PHP).
Incoming Label Map
Used to find the NHLFE for determining forwarding information for a labeled packet.
independent control mode
Mode in which an LSR allocates and provides labels to upstream peers at any time. This mode may be used, for instance, when routing is used to drive label distribution and it is desirable to supply applicable labels to routing peers at about the same time as new routes are advertised.
ingress
Point at which an MPLS context or domain is entered. The ingress of an LSP is the point at which a label is pushed onto the label stack (possibly resulting in the creation of the label stack).
Integrated Services (IntServ)
An IETF quality-of-service standard. In essence, QoS is assured based on signaling end-to-end service requirements using a common signaling protocol. (RSVP is the only common end-to-end protocol currently defined for this purpose.) These service requirements are then mapped to specific queuing parameters for each specific medium type that may be present in such an end-to-end service. The use of CAC and traffic disciplining techniques allows this approach to effectively guarantee a requested service requirement. Because packets are classified to determine what level of service they require, and the classification information must be retained at each node, this QoS approach is often referred to as the "state-full" QoS model.
interdomain routing
Routing between administrative domains. Supported currently by BGP version 4.
interface
Physical or logical end point of a link between devices.
Internet service provider (ISP)
Provider of an access service to the Internet, usually for a charge. Access service charges may be flat rate or based on either rate or usage. Service providers make up the Internet through complex tiering and peering relationships.
intranet
A private network.
L1, L2, L3
physical, data link, and network layers (respectively).
label
A fixed-size field contained in a message header that may be used as an exact-match key in determining how to forward a protocol data unit.
label distribution
Process by which labels are negotiated between peer LSRs.
label edge router
A term often used to indicate an LSR that is able to provide ingress to and egress from an LSP. In individual implementations, this tends to be a function of the capabilities of device interfaces more than of the overall device. In theory, it is possible for a device to be an LER and not an LSR (if it is not able to swap labels, for instance); however, it is unlikely that such an LER would be generally useful or make any particular sense in a cost-benefit analysis.
label stack
Successive labels in an MPLS shim header in order from the top to the bottom of the stack.
label swapping
Replacing an input label with a corresponding output label.
label-switched path
Path along which labeled packets are forwarded. Packets forwarded using any label are forwarded along the same path as other packets using the same label.
label switching
Switching based on use of labels.
label switching router
A device that participates in one or more routing protocols and uses the route information derived from routing protocol exchanges to drive LSP setup and maintenance. Such a device typically distributes labels to peers and uses these labels (when provided as part of data presented for forwarding) to forward label-encapsulated L3 packets. In general, an LSR may or may not be able to forward non-label-encapsulated data and provide ingress/egress to LSPs (that is, to perform what is frequently referred to as the label edge router, or LER, function).
liberal retention mode
Labels are retained whenever received. This mode is useful when the ability to change quickly to a new LSP is desirable; however, it may result in unacceptable memory consumption for LSRs with many interfaces.
link
Physical or logical connection between two end points.
logical interface
An interface associated with a specific encapsulation. Data arriving at the corresponding physical (or lower-level logical) interface that is encapsulated for a specific logical interface is de-encapsulated and delivered to that logical interface.
merging
A key function in making MPLS scalable in the number of labels consumed at each LSR. Merging is the process by which packets from multiple sources are typically delivered to a single destination or destination prefix. It is distinct from aggregation in that (in most cases) the decision to merge traffic implies that the possibility of being required to separate the merged traffic at a later point is not significant at the point where merging is being done.
network layer
Layer 3 of the OSI model; the layer between the data-link and transport layers. Normally encapsulated in one or more data-link layer frames.
Next Hop Label Forwarding Information Entry
Contains all of the information needed to forward a labeled packet to the next hop. This information includes push, pop, or swap instructions; the new label (or labels in the event that multiple pushes are called for); the output interface; and other information that may be needed to forward the packet to the next hop.
Ordered Aggregate
The set of Behavior Aggregates that share an ordering constraint. For example, a set of PHB values that can be ordered relative to one another, such as AF drop precedences within an AF class.
ordered control mode
Mode in which an LSR only allocates and provides labels to an upstream peer when it is either the egress for the resulting LSP or it has received a label from downstream for the resulting LSP.
packet
A message encapsulation consisting of a network-layer header and payload.
packet switching
An approach used to forward L3 packets from an input L3 logical interface to an output L3 logical interface that may reasonably be optimized for hardware switching-similar to switching at the data-link layer.
penultimate hop pop
A process by which the peer immediately upstream of the egress LSR is asked to pop a label prior to forwarding the packet to the egress LSR. Using LDP, this is done by assigning the special value of the implicit Null label. This allows the egress to push the work of popping the label to its upstream neighbor, possibly allowing for a more optimal processing of the remaining packet. Note that this can be done because once the label has been used to determine the next-hop information for the last hop, the label is no longer useful. Using PHP is helpful because it allows the packet to be treated as an unlabeled packet by the last hop. Using PHP, it is possible to implement an "LSR" that never uses labels.
per-hop behavior
A Differentiated Services behavioral definition. A PHB is defined at a node by the combination of a Differentiated Services Code Point (DSCP) and a set of configured behaviors.
PHB scheduling class
The nonempty set of per-hop behaviors that apply to the Behavior Aggregates belonging to a given Ordered Aggregate.
piggyback
Intuitive term for the use of routing, or routing-related, protocols to carry labels.
pop
In a label-switching context, the process of removing the top-level label (the label at the head of the label stack) from the label stack.
protocol data unit
A unit of data used in specific protocol interactions. It may be generically described as a format for encapsulation and forwarding of protocol messages between protocol entities. Messages may span multiple PDUs, a single PDU may contain multiple messages, and PDUs may be nested.
push
In a label-switching context, the process of adding a new top-level label (which becomes the new label at the head of the label stack) to the label stack.
quality of service (QOS)
Specific handling or treatment of packets, often in an end-to-end service. Best-effort (also sometimes referred to as "worst-effort") is currently the lowest level of packet treatment, other than an "unconditional drop" service. Currently, there are two models for providing QoS in an IP network: Integrated Services (IntServ) and Differentiated Services(DiffServ).
route computation
The process by which routers compute entries for a route table. Route table entries are subsequently used in route determination.
route determination
The process of selecting a route based on header information in packets and route table entries established previously via route computation. Typically, a route is determined using the longest match of the network-layer destination address in L3 packets against a network address prefix in the route table.
router
A device used to forward packets at the network (L3) layer.
routing
A scheme for selecting one of many possible paths.
scalability
A reflection of the way in which system complexity grows as a function of some system parameter, such as size. If growth in system complexity is approximately linear with respect to growth in system size, for instance, the size scalability of the system is generally considered to be good.
Shared Explicit
Reservation style in which path resources are explicitly shared among multiple senders and receivers. Useful when it is desirable to increase reservation resources or establish a new reservation without double-booking resources.
shim header
An encoding of the MPLS label stack. Present for all media when a label stack is in use. (The presence of the label stack is indicated either by protocol numbers or connection identifiers in the L2 encapsulation.)
slow-path forwarding
Used to refer to processing of exception packets in which the packet is handled via direct intervention of a system CPU resource that is not normally used in fast-path (optimized) forwarding.
soft state
State information that becomes out of date if not refreshed.
source route
An explicit route specified from the source toward the destination.
switching
Ushering input data or messages more or less directly to an output; typically based on a simplistic recognition mechanism (such as an exact match of a fixed-length field).
traffic engineer
An operator or automaton with the express purpose of minimizing congestion in a network. Traffic engineering is an application of a traffic engineer.
traffic engineering
An application of constraint-based routing in which a traffic engineer uses a set of link characteristics to select a route and assigns specific traffic to that route.
type-length-value
An object description with highly intuitive meaning; that is, the object consists of three fields: type, length, and value. Type gives the semantic meaning of the value, length gives the number of bytes in the value field (which may be fixed by the type), and value consists of length bytes of data in a format consistent with type. This object format is used in LDP and several other protocols.
upstream
Direction from which traffic is expected to arrive. Applies to a specific forwarding equivalence class.
upstream label allocation
A scheme by which the upstream peer is allowed to select the label that will be used in forwarding labeled traffic for a specific forwarding equivalence class. Not currently supported in MPLS.
virtual X
Pseudo-X. Not quite or really an X. A small white lie.
• • • • • • • • • • • • • • • • • • • • • • • • • • • • •
Back to MPLS Central
|
|