MPLS
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... Introduction to MPLS ...
...
In MPLS, the assignment of a particular packet to a particular FEC is
done just once, as the packet enters the network ...
... route
along with it ("source routing"). In MPLS, a label can be used
to represent the route, so that the identity ...
... discard thresholds or scheduling disciplines to different packets.
MPLS allows (but does not require) the precedence or class of service
to be fully or partially inferred from the label. In this case, one
...
...
MPLS stands for "Multiprotocol" Label Switching, multiprotocol
because its techniques are applicable to ANY network layer protocol ...
... indistinguishably when forwarding.
label switched hop the hop between two MPLS nodes, on which
forwarding is done using labels.
...
... domain a contiguous set of nodes which operate
MPLS routing and forwarding and which
are also in one Routing ...
... the domain, either because it does not
run MPLS, and/or because it is in a
different domain. Note that if an LSR ...
... domain
MPLS label a label which is carried in a packet
header, and which represents the
packet's FEC ...
... of forwarding packets based on labels.
An MPLS node may optionally be also
capable of forwarding native L3 ...
... MPLS Basics ...
...
In this section, we introduce some of the basic concepts of MPLS and
describe the general approach to be used.
...
... which two label distribution peers need to engage in order to learn
of each other's MPLS capabilities.
...
...
It is expected that some MPLS implementations will provide only
downstream-on-demand label distribution ...
...
Although, as we shall see, MPLS supports a hierarchy, the processing
of a labeled packet is completely independent of the level of
hierarchy. The processing is always based on the top label, without
...
... The utility of the label stack will become clear when we introduce
the notion of LSP Tunnel and the MPLS Hierarchy (section 3.27).
...
... This prohibition holds even if the bindings are regarded as being at
different "levels of hierarchy". In MPLS, there is no notion of
having a different label space for different levels of the hierarchy;
when interpreting a label, the level of the label is irrelevant.
...
... stack ever have a depth of less than m;
4. For all i, 1<i<n: Ri transmits P to R[i+1] by means of MPLS,
i.e., by using the label at the top of the label stack (the
level m label) as an index into an ILM ...
... made by label Switching on a level m-k label, where k>0, or
when a forwarding decision is made by "ordinary", non-MPLS
forwarding procedures.
...
... node in the LSP does support
MPLS, but does not make such a request, the penultimate node has no
way of knowing that it in fact is the penultimate node ...
... address prefix which appears in the routing table. However,
within a particular MPLS domain, this may result in a set of FECs
...
... traffic in the union, is known as "aggregation". The MPLS
architecture allows aggregation. Aggregation may reduce the number
...
... node to know the granularity only for
FECs which leave the MPLS network at that node. For independent
control, best results may be obtained by ensuring that all LSRs ...
... LSP for a
particular FEC. The proposed MPLS protocol architecture supports two
options for Route Selection ...
... policy routing or traffic engineering. In MPLS, the explicit route
needs to be specified at the time that labels are assigned, but the
...
... explicit route does not have to be specified with each IP packet.
This makes MPLS explicit routing much more efficient than the
alternative of IP ...
... traceroute" command. This implies that there are two TTL-
related issues that MPLS needs to deal with: (i) TTL as a way to
suppress loops; (ii) TTL ...
...
The way that TTL is handled may vary depending upon whether the MPLS
label values are carried in an MPLS-specific "shim" header [MPLS-SHIM ...
... The way that TTL is handled may vary depending upon whether the MPLS
label values are carried in an MPLS-specific "shim" header [MPLS-SHIM], or if the MPLS labels ...
... MPLS-specific "shim" header [MPLS-SHIM], or if the MPLS labels are carried in an L2 header, such as an
ATM ...
... ATM switching hardware is being used to
provide MPLS switching functions, with the label being carried in the
VPI/VCI field ...
... MPLS-specific Hardware and/or Software ...
...
If one is using MPLS-specific hardware and/or software to forward
labeled packets, the most obvious way to encode the label stack is to
...
...
It will be noted that MPLS forwarding procedures are similar to those
of legacy "label swapping" switches such as ATM ...
... switches have no capability for translating from
one encoding technique to another. The MPLS architecture therefore
requires that whenever it is possible for two ATM switches ...
... LSRs, and other LSRs which operate using an
MPLS shim header. In such networks there may be some LSRs ...
... ATM encoded label stack on an
incoming interface and replace it with an MPLS shim header encoded
label stack on the outgoing interface ...
...
The MPLS architecture accommodates both merging and non-merging LSRs,
but allows for the fact that there may be LSRs ...
...
The MPLS forwarding procedures is very similar to the forwarding
procedures used by such technologies as ATM and Frame Relay ...
... port
is chosen, and the label value is rewritten. In fact, it is possible
to use such technologies for MPLS forwarding; a label distribution
protocol can be used as the "signalling protocol" for setting up the
...
...
We propose to support two solutions to this problem. First, MPLS
will contain procedures which allow the use of non-merging LSRs.
...
... will contain procedures which allow the use of non-merging LSRs.
Second, MPLS will support procedures which allow certain ATM switches
...
... Since MPLS supports both merging and non-merging LSRs, MPLS also
contains procedures to ensure correct interoperation between them.
...
... protocol complexity and scalability implies that it is desirable for
the MPLS protocol to support both VP merge and VC merge. In order to
do so each ATM ...
... do so each ATM switch participating in MPLS needs to know whether its
immediate ATM neighbors ...
...
The MPLS architecture supports two ways to distribute labels at
different layers of the hierarchy: Explicit Peering and Implicit
Peering.
...
... A label distribution protocol is used between nodes in an MPLS
network to establish and maintain the label bindings. In order for
MPLS ...
... MPLS
network to establish and maintain the label bindings. In order for
MPLS to operate correctly, label distribution information needs to be
transmitted reliably, and the label distribution protocol ...
...
In some applications of MPLS, particularly those related to traffic
engineering, it is desirable to set up an explicitly routed path,
from ingress to egress. It is also desirable to apply resource
reservations ...
... Some Applications of MPLS ...
...
A number of uses of MPLS require that packets with a certain label be
forwarded along the same hop-by-hop routed path that would be used
...
... R1 and R2 are not label
distribution peers with respect to X (perhaps because R2 does
not support MPLS), or
2. R1 ...
... LSP Egress to be a node which
does not support MPLS; in this case the penultimate node in the LSP
...
... LSP to pop the label stack.
This is quite appropriate; if the LSP Egress is an MPLS Egress for X,
then if the penultimate LSR does not pop the label stack, the LSP ...
... routing algorithm, and
all nodes in the area support MPLS, then the routing algorithm
provides Ri with enough information to determine the routers ...
... reduced. This may be significant if one is using legacy switching
hardware to do MPLS, and the switching hardware can support only a
limited number of labels.
...
...
MPLS allows this to be easily done by means of Explicitly Routed LSP
Tunnels. All that is needed is:
...
...
Note then that when P1 and P2 are traveling from R2 to R3, they carry
the same label, and as far as MPLS is concerned, they cannot be
distinguished. Thus instead of talking about two distinct LSPs, <R1 ...
...
The MPLS architecture supports several variants of each procedure.
...
...
However, the MPLS architecture does not support all possible
combinations of all possible variants. The set of supported
combinations will be described in section 5.2, where the
...
... MPLS Schemes: Supported Combinations of Procedures ...
...
The MPLS scheme which governs the interaction of Ru and Rd can be
described as a quintuple of procedures: <Distribution Procedure,
Request Procedure, NotAvailable Procedure, Release Procedure,
...
...
Only the MPLS schemes which are specified below are supported by the
MPLS Architecture. Other schemes may be added in the future, if a
...
... Only the MPLS schemes which are specified below are supported by the
MPLS Architecture. Other schemes may be added in the future, if a
need for them is shown.
...
...
Therefore, if R1 and R2 are MPLS peers, and either is an LSR which is
implemented using conventional ATM ...
... hardware (i.e., no cell
interleave suppression), or is otherwise incapable of performing
label merging, the MPLS scheme in use between R1 and R2 must be one
of the following:
...
...
It is easy to see that certain quintuples do NOT yield viable MPLS
schemes. For example:
...
... <PulledConditional, RequestNever, *, *, *>
In these MPLS schemes, the downstream LSR Rd distributes label
...
... - <*, RequestNever, *, *, ReleaseOnChange>
In these MPLS schemes, Rd releases bindings when it isn't using
them, but it never asks for them again, even if it later has a
...
... In this section, we specify rules to prevent a pair of label
distribution peers from adopting procedures which lead to infeasible
MPLS Schemes. These rules require either the exchange of information
between label distribution peers during the initialization ...
... LSR does not support label merging,
its preferences take priority when the MPLS scheme is chosen.
3. If Ru does not support label merging, but Rd does, Ru must
...
... LSRs doesn't support label merging,
its preferences take priority when the MPLS scheme is chosen.
4. If both Ru and Rd both support label merging, then the choice
...
... PushConditional, PulledConditional, or PulledUnconditional.
These choices together determine the MPLS scheme in use.
...
... layer header being in a fixed place relative to the data link
layer header. The MPLS generic encapsulation inserts a shim between
the data link layer ...
... Davie, B., Lawrence, J., McCloghrie, K., Rekhter, Y., Rosen, E., Swallow, G. and P. Doolan, "MPLS using LDP and ATM VC ...
... Rosen, E., Rekhter, Y., Tappan, D., Fedorkow, G., Farinacci, D. and A. Conta, "MPLS Label Stack Encoding", RFC 3032prop, January 2001. ...
... Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M. and J. McManus, "Requirements for Traffic Engineering Over MPLS", RFC 2702, September 1999. ...