Internet Draft
MPLS Working Group A. Conta (Lucent)
INTERNET-DRAFT P. Doolan (Ennovate)
A. Malis (Ascend)
22 October 1998
Use of Label Switching on Frame Relay Networks
Specification
draft-ietf-mpls-fr-02.txt
Status of this Memo
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Abstract
This document defines the model and generic mechanisms for
Multiprotocol Label Switching on Frame Relay networks. Furthermore,
it extends and clarifies portions of the Multiprotocol Label
Switching Architecture described in [ARCH] and the Label Distribution
Protocol described in [LDP] relative to Frame Relay Networks. MPLS
enables the use of Frame Relay Switches as Label Switching Routers
(LSRs).
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Table of Contents
Status of this Memo.........................................1
Table of Contents...........................................2
1. Introduction................................................3
2. Terminology.................................................3
3. Special Characteristics of Frame Relay Switches.............4
4. Label Encapsulation.........................................5
5. Frame Relay Label Switching Processing......................6
5.1 Use of DLCIs..............................................6
5.2 Homogeneous LSPs..........................................7
5.3 Heterogeneous LSPs........................................7
5.4 Frame Relay Label Switching Loop Prevention and Control...8
5.4.1 FR-LSRs Loop Control - MPLS TTL Processing.............8
5.4.2 Performing MPLS TTL calculations.......................9
5.5 Label Processing by Ingress FR-LSRs......................11
5.6 Label Processing by Core FR-LSRs.........................12
5.7 Label Processing by Egress FR-LSRs.......................12
6 Label Switching Control Component for Frame Relay..........13
6.1 Hybrid Switches (Ships in the Night) ...................14
7 Label Allocation and Maintenance Procedures ...............14
7.1 Edge LSR Behavior........................................14
7.2 Efficient use of label space-Merging FR-LSRs.............17
7.3 LDP message fields specific to Frame Relay...............17
8 Security Considerations ..................................19
9 Acknowledgments ..........................................20
10 References ...............................................20
11 Authors' Addresses .......................................21
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1. Introduction
The Multiprotocol Label Switching Architecture is described in
[ARCH]. It is possible to use Frame Relay switches as Label Switching
Routers. Such Frame Relay switches run network layer routing
algorithms (such as OSPF, IS-IS, etc.), and their forwarding is based
on the results of these routing algorithms. No specific Frame Relay
routing is needed.
When a Frame Relay switch is used for label switching, the current
label, on which forwarding decisions are based, is carried in the
DLCI field of the Frame Relay data link layer header of a frame.
Additional information carried along with the current label, but not
processed by Frame Relay switching, along with other labels, if the
packet is multiply labeled, are carried in the generic MPLS
encapsulation defined in [STACK].
Frame Relay permanent virtual circuits (PVCs) could be configured to
carry label switching based traffic. The DLCIs would be used as MPLS
Labels and the Frame Relay switches would become MPLS switches while
the MPLS traffic would be encapsulated according to this
specification, and would be forwarded based on network layer routing
information.
The keywords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED,
SHALL, SHALL NOT, SHOULD, SHOULD NOT are to be interpreted as
defined in RFC 2119.
2. Terminology
LSR
A Label Switching Router (LSR) is a device which implements the
label switching control and forwarding components described in
[ARCH].
LC-FR
A label switching controlled Frame Relay (LC-FR) interface is a
Frame Relay interface controlled by the label switching control
component. Packets traversing such an interface carry labels in
the DLCI field.
FR-LSR
A FR-LSR is an LSR with one or more LC-FR interfaces which
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forwards frames onto these interfaces using labels carried in
the DLCI field.
FR-LSR cloud
A FR-LSR cloud is a set of FR-LSRs, which are mutually
interconnected by LC-FR interfaces.
Edge Set
The Edge Set of an FR-LSR cloud is the set of LSRs, which are
connected to the cloud by LC-FR interfaces.
Additionally, this document uses terminology from [ARCH].
3. Special characteristics of Frame Relay Switches
While the label switching architecture permits considerable
flexibility in LSR implementation, a FR-LSR is constrained by the
capabilities of the (possibly pre-existing) hardware and the
restrictions on such matters as frame format imposed by the
Multiprotocol Interconnect over Frame Relay [MIFR], or Frame Relay
standards [FRF], etc.... Because of these constraints, some special
procedures are required for FR-LSRs.
Some of the key features of Frame Relay switches that affect their
behavior as LSRs are:
- the label swapping function is performed on fields (DLCI) in the
frame's Frame Relay data link header; this dictates the size and
placement of the label(s) in a packet. The size of the DLCI
field can be 10 (default), 17, or 23 bits, and it can span two,
or four bytes in the header.
- there is generally no capability to perform a `TTL-decrement'
function as is performed on IP headers in routers.
- congestion control is performed by each node based on parameters
that are passed at circuit creation. Flags in the frame headers
may be set as a consequence of congestion, or exceeding the
contractual parameters of the circuit.
- although in a standard switch it may be possible to configure
multiple input DLCIs to one output DLCI resulting in a
multipoint-to-point circuit, multipoint-to-multipoint VCs are
generally not fully supported.
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This document describes ways of applying label switching to Frame
Relay switches, which work within these constraints.
4. Label Encapsulation
By default, all labeled packets should be transmitted with the
generic label encapsulation as defined in [STACK], using the frame
relay null encapsulation mechanism. The labels implicitly encode the
network protocol type, consequently those particular labels cannot be
used with other network protocols. Rules regarding the construction
of the label stack, and error messages returned to the frame source
are also described in [STACK].
0 1 (Octets)
+-----------------------+-----------------------+
(Octets)0 | |
/ Q.922 Address /
/ (length 'n' equals 2 or 4) /
| |
+-----------------------+-----------------------+
n | . |
/ . /
/ MPLS packet /
| . |
+-----------------------+-----------------------+
"n" is the length of the Q.922 Address which can be 2 or 4
octets.
The Q.922 [ITU] representation of a DLCI (in canonical order -
the first bit is stored in the least significant, i.e., the
right-most bit of a byte in memory) [CANON]is the following:
7 6 5 4 3 2 1 0 (bit order)
+-----+-----+-----+-----+-----+-----+-----+-----+
(octet) 0 | DLCI(high order) | 0 | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+
1 | DLCI(low order) | 0 | 0 | 0 | 1 |
+-----+-----+-----+-----+-----+-----+-----+-----+
10 bits DLCI
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7 6 5 4 3 2 1 0 (bit order)
+-----+-----+-----+-----+-----+-----+-----+-----+
(octet) 0 | DLCI(high order) | 0 | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+
1 | DLCI | 0 | 0 | 0 | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+
2 | DLCI(low order) | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+
3 | unused (set to 0) | 1 | 1 |
+-----+-----+-----+-----+-----+-----+-----+-----+
17 bits DLCI
7 6 5 4 3 2 1 0 (bit order)
+-----+-----+-----+-----+-----+-----+-----+-----00
(octet) 0 | DLCI(high order) | 0 | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----
1 | DLCI | 0 | 0 | 0 | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+
2 | DLCI | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+
3 | DLCI (low order) | 0 | 1 |
+-----+-----+-----+-----+-----+-----+-----+-----+
23 bits DLCI
The generic encapsulation contains "n" labels for a label stack of depth
"n" [STACK], where the top stack entry carries significant values for
the EXP, S , and TTL fields [STACK] but not for the label, which is
rather carried in the DLCI field of the Frame Relay data link header
encoded in Q.922 [ITU] address format.
5. Frame Relay Label Switching Processing
5.1 Use of DLCIs
Label switching is accomplished by associating labels with routes and
using the label value to forward packets, including determining the
value of any replacement label. See [ARCH] for further details. In a
FR-LSR, the current (top) MPLS label is carried in the DLCI field of
the Frame Relay data link layer header of the frame. The top label
carries implicitly information about the network protocol type.
For two connected FR-LSRs, a full-duplex connection must be available
for LDP. The DLCI for the LDP VC is assigned a value by way of
configuration, similar to configuring the DLCI used to run IP routing
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protocols between the switches.
With the exception of this configured value, the DLCI values used for
MPLS in the two directions of the link may be treated as belonging to
two independent spaces, i.e. VCs may be half-duplex, each direction
with its own DLCI. In case of DLCI aggregation (DLCI space
conservation), half-duplex (unidirectional) VCs are desired, since a
"many to few" aggregation is possible in one direction but not in
reverse.
The allowable ranges of DLCIs are always communicated through LDP.
Note that the range of DLCIs used for labels depends on the size of
the DLCI field.
5.2 Homogeneous LSPs
If is an LSP, it is possible that LSR1, LSR2, and
LSR3 will use the same encoding of the label stack when transmitting
packet P from LSR1, to LSR2, and then to LSR3. Such an LSP is
homogeneous.
5.3 Heterogeneous LSPs
If is an LSP, it is possible that LSR1 will use
one encoding of the label stack when transmitting packet P to LSR2,
but LSR2 will use a different encoding when transmitting a packet P
to LSR3. In general, the MPLS architecture supports LSPs with
different label stack encodings on different hops. When a labeled
packet is received, the LSR must decode it to determine the current
value of the label stack, then must operate on the label stack to
determine the new label value of the stack, and then encode the new
value appropriately before transmitting the labeled packet to its
next hop.
Naturally there will be MPLS networks which contain a combination of
Frame Relay switches operating as LSRs, and other LSRs, which operate
using other MPLS encapsulations, such as the MPLS shim header, or ATM
encapsulation. In such networks there may be some LSRs, which have
Frame Relay interfaces as well as "MPLS Shim" interfaces. This is one
example of an LSR with different label stack encodings on different
hops of the same LSP. Such an LSR may swap off a Frame Relay encoded
label on an incoming interface and replace it with a label encoded
into an MPLS shim header on the outgoing interface.
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5.4 Frame Relay Label Switching Loop Prevention and Control
FR-LSRs MUST use a mechanism that insures loop free FR- LSPs or LSP
FR segments. Such mechanisms are described in [ARCH], and [LDP].
5.4.1 FR-LSRs Loop Control - MPLS TTL processing
The MPLS TTL encoded in the MPLS label stack is a mechanism used to:
(a) suppress loops;
(b) limit the scope of a packet.
When a packet travels along an LSP, it should emerge with the same
TTL value that it would have had if it had traversed the same
sequence of routers without having been label switched. If the
packet travels along a hierarchy of LSPs, the total number of LSR-
hops traversed should be reflected in its TTL value when it emerges
from the hierarchy of LSPs [ARCH].
The initial value of the MPLS TTL is loaded into a newly pushed label
stack entry from the previous TTL value, whether that is from the
network layer header when no previous label stack existed, or from a
pre-existent lower level label stack entry.
A FR-LSR switching same level labeled packets does not decrement the
MPLS TTL. A sequence of such FR-LSR is a "non-TTL segment".
When a packet emerges from a "non-TTL LSP segment", it should however
reflect in the TTL the number of LSR-hops it traversed. In the
unicast case, this can be achieved by propagating a meaningful LSP
length or LSP segment length to the FR-LSR ingress nodes, enabling
the ingress to decrement the TTL value before forwarding packets into
a non-TTL LSP segment [ARCH].
When an ingress FR-LSR determines upon decrementing the MPLS TTL that
a particular packet's TTL will expire before the packet reaches the
egress of the "non-TTL LSP segment", the FR-LSR MUST not label switch
the packet, but rather follow the specifications in [STACK] in an
attempt to return an error message to the packet's source.
In the multicast case, a meaningful LSP length or LSP segment length
is propagated to the FR-LSR egress node, enabling the egress to
decrement the TTL value before forwarding packets out of the non-TTL
LSP segment.
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5.4.2 Performing MPLS TTL calculations
Considering the "incoming TTL" the MPLS TTL of the top of the stack
when a labeled packet is received, and the "output TTL" the MPLS TTL
of the top of the stack when a packet leaves a node, the relationship
between the two is defined as a function of the type of the output
interface, and the type of transmit operation done on the output
interface (unicast or multicast):
output TTL = function (input TTL, output interface type, type of transmit)=
= input TTL - funct (output interface type, type of transmit)
Considering the symbol "I" for an IP interface, the symbol "G" for a
generic MPLS encapsulating interface, the symbol "A" for a MPLS ATM
encapsulating
interface, the symbol "F" for a MPLS FR encapsulating interface, and
"G_G", "F_G", etc... LSRs with specific input and output interfaces,
and also the symbols "O.TTL" and "I.TTL" for the "output" and "input"
TTL, the following describes the possible combinations:
input,output Unicast
->G_G-> O.TTL = I.TTL - 1
->F_G-> O.TTL = I.TTL - nr. of hops of starting segment (ingress F)
->G_F-> O.TTL = I.TTL - 1 (egress F)
->A_F-> O.TTL = I.TTL - nr. of hops of starting segment (ingress F)
->F_A-> O.TTL = I.TTL - 1 (egress F)
->F_F-> similar to ->A A-> no TTL processing
input,output Multicast
->G_G-> O.TTL = I.TTL - 1
->G_F-> O.TTL = I.TTL - 1 (ingress F)
->F_G-> O.TTL = I.TTL - nr. of hops of ending segment (egress F)
->A_F-> O.TTL = I.TTL - 1 (ingress F)
->F_A-> O.TTL = I.TTL - nr. of hops of ending segment (egress F)
->F_F-> similar to ->A A-> no TTL processing
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Homogeneous LSP
--->I_F Frame Relay F_I--->
hops = 5 | |
F_F--->F_F--->F_F--->F_F
loop free
ip_ttl = n ip_ttl=n-6
mpls_ttl = n-5 n-5
Heterogeneous LSP
LSP LSP
ingress egress
LAN PPP FR ATM PPP FR LAN
--->I_G-->G_G-->G_F F_A A_G-->G_F F_G-->G_I--->
| / | | | |
hops 1 1 | 4 / | 3 | 1 | 3 | 1 1
F_F--F_F--F_F A_A--A_A F_F--F_F
loop free loop free loop free
ip_ttl
n n-15
mpls_ttl
n-1 n-2 n-6 n-9 n-10 n-13 n-14
Unicast -- TTL calculated at ingress
1 2 3 4
o-------o-------o-------o-------o
ttl=n-4 / 2 3
/
hops 1/
/
o ttl=n-3
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Multicast -- TTL calculated at egress
o ttl=n-3
hops /
3/
/ ttl=n-4
o-------o-------o-------o-------o
1 2 3 4
5.5 Label Processing by Ingress FR-LSRs
When a packet first enters an MPLS domain, the packet is forwarded by
normal network layer forwarding operations with the exception that
the outgoing encapsulation will include an MPLS label stack [STACK]
with at least one entry. The frame relay null encapsulation will
carry information about the network layer protocol implicitly in the
label, which MUST be associated only with that network protocol. The
TTL field in the top label stack entry is filled with the network
layer TTL (or hop limit) resulted after network layer forwarding
[STACK]. The further FR-LSR processing is similar in both possible
cases:
(a) the LSP is homogeneous -- Frame Relay only -- and the FR-LSR is
the ingress.
(b) the LSP is heterogeneous -- Frame Relay, PPP, Ethernet, ATM,
etc... segments form the LSP -- and the FR-LSR is the ingress into a
Frame Relay
segment.
For unicast packets, the MPLS TTL SHOULD be decremented with the
number of hops of the Frame Relay LSP (homogeneous), or Frame Relay
segment of the LSP (heterogeneous). An LDP constructing the LSP
SHOULD pass meaningful information to the ingress FR-LSR regarding
the number of hops of the "non-TTL segment".
For multicast packets, the MPLS TTL SHOULD be decremented by 1. An
LDP constructing the LSP SHOULD pass meaningful information to the
egress FR-LSR regarding the number of hops of the "non-TTL segment".
Next, the MPLS encapsulated packet is passed down to the Frame Relay
data link driver with the top label as output DLCI. The Frame Relay
frame carrying the MPLS encapsulated packet is forwarded onto the
Frame Relay VC to the next LSR.
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5.6 Label Processing by Core FR-LSRs
In a FR-LSR, the current (top) MPLS label is carried in the DLCI
field of the Frame Relay data link layer header of the frame. Just as
in conventional Frame Relay, for a frame arriving at an interface,
the DLCI carried by the Frame Relay data link header is looked up in
the DLCI Information Base, replaced with the correspondent output
DLCI, and transmitted on the outgoing interface (forwarded to the
next hop node).
The current label information is also carried in the top of the label
stack. In the top-level entry, all fields except the label
information, which is carried and switched in the Frame Relay frame
data link-layer header, are of current significance.
5.7 Label Processing by Egress FR-LSRs
When reaching the end of a Frame Relay LSP, the FR-LSR pops the label
stack [ARCH]. If the label popped is the last label, it is necessary
to determine the particular network layer protocol which is being
carried. The label stack carries no explicit information to identify
the network layer protocol. This must be inferred from the value of
the label which is popped from the stack.
If the label popped is not the last label, the previous top level
MPLS TTL is propagated to the new top label stack entry.
If the FR-LSR is the egress switch of a Frame Relay segment of a
hybrid LSP, and the end of the Frame Relay segment is not the end of
the LSP, the MPLS packet will be processed for forwarding onto the
next segment of the LSP based on the information held in the Next Hop
Label Forwarding Entry (NHLFE) [ARCH]. The output label is set to the
value from the NHLFE, and the MPLS TTL is decremented by the
appropriate value depending the type of the output interface and the
type of transmit operation (see section 6.3). Further, the MPLS
packet is forwarded according to the MPLS specifications for the
particular link of the next segment of the LSP.
For unicast packets, the MPLS TTL SHOULD be decremented by one if the
output interface is a generic one, or with the number of hops of the
next ATM segment of the LSP (heterogeneous), if the output interface
is an ATM (non-TTL) interface.
For multicast packets, the MPLS TTL SHOULD be decremented by the
number of hops of the FR segment being exited. An LDP constructing
the LSP SHOULD pass meaningful information to the egress FR-LSR
regarding the number of hops of the FR "non-TTL segment".
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6. Label Switching Control Component for Frame Relay
To support label switching a Frame Relay Switch MUST implement the
control component of label switching, which consists primarily of
label allocation and maintenance procedures. Label binding
information MAY be communicated by several mechanisms, one of which
is the Label Distribution Protocol (LDP) [LDP].
Since the label switching control component uses information learned
directly from network layer routing protocols, this implies that the
switch MUST participate as a peer in these protocols (e.g., OSPF,
IS-IS).
In some cases, LSRs may use other protocols (e.g. RSVP, PIM, BGP) to
distribute label bindings. In these cases, a Frame Relay LSR should
participate in these protocols.
In the case where Frame Relay circuits are established via LDP, or
RSVP, or others, with no involvement from traditional Frame Relay
mechanisms, it is assumed that circuit establishing contractual
information such as input/output maximum frame size,
incoming/outgoing requested/agreed throughput, incoming/outgoing
acceptable throughput, incoming/outgoing burst size,
incoming/outgoing frame rate, used in transmitting, and congestion
control MAY be passed to the FR-LSRs through RSVP, or can be
statically configured. It is also assumed that congestion control and
frame header flagging as a consequence of congestion, would be done
by the FR-LSRs in a similar fashion as for traditional Frame Relay
circuits. With the goal of emulating a best-effort router as default,
the default VC parameters, in the absence of LDP, RSVP, or other
mechanisms participation to setting such parameters, should be zero
CIR, so that input policing will set the DE bit in incoming frames,
but no frames are dropped.
Control and state information for the circuits based on MPLS MAY be
communicated through LDP.
Support of label switching on a Frame Relay switch requires
conformance only to [FRF] (framing, bit-stuffing, headers, FCS)
except for section 2.3 (PVC control signaling procedures, aka LMI).
Q.933 signaling for PVCs and/or SVCs is not required. PVC and/or SVC
signaling may be used for non-MPLS (standard Frame Relay) PVCs and/or
SVCs when both are running on the same interface as MPLS, as
discussed in the next section.
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6.1 Hybrid Switches (Ships in the Night)
The existence of the label switching control component on a Frame
Relay switch does not preclude the ability to support the Frame Relay
control component defined by the ITU and Frame Relay Forum on the
same switch and the same interfaces (NICs). The two control
components, label switching and those defined by ITU/Frame Relay
Forum, would operate independently.
Definition of how such a device operates is beyond the scope of this
document. However, only a small amount of information needs to be
consistent between the two control components, such as the portions
of the DLCI space which are available to each component.
7. Label Allocation and Maintenance Procedures
The mechanisms and message formats of a Label Distribution Protocol
are documented in [ARCH] and [LDP]. A possible scenario for the
label allocation and maintenance for FR-LSRs is "downstream-on-
demand" as it follows (note that this applies to hop-by-hop routed
traffic):
7.1 Edge LSR Behavior
Consider a member of the Edge Set of a FR-LSR cloud. Assume that, as
a result of its routing calculations, it selects a FR-LSR as the next
hop of a certain route (FEC), and that the next hop is reachable via
a LC-Frame Relay interface. Assume that the next-hop FR-LSR is an
"LDP-peer" [ARCH][LDP]. The Edge LSR sends an LDP "request" message
for a label binding from the next hop, downstream LSR. When the Edge
LSR receives in response from the downstream LSR the label binding
information in an LDP "mapping" message, the label is stored in the
Label Information Base (LIB) as an outgoing label for that FEC. The
"mapping" message may contain the "hop count" object, which
represents the number of hops a packet will take to cross the FR-LSR
cloud to the Egress FR-LSR when using this label. This information
may be stored for TTL calculation. Once this is done, the LSR may use
MPLS forwarding to transmit packets in that FEC.
When a member of the Edge Set of the FR-LSR cloud receives an LDP
"request" message from a FR-LSR for a FEC, it means it is the
Egress-FR-LSR. It allocates a label, creates a new entry in its Label
Information Base (LIB), places that label in the incoming label
component of the entry, and returns (via LDP) a "mapping" message
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containing the allocated label back upstream to the LDP peer that
originated the request. The "mapping" message contains the "hop
count" object value set to 1.
When a routing calculation causes an Edge LSR to change the next hop
for a route, and the former next hop was in the FR-LSR cloud, the
Edge LSR should notify the former next hop (via an LDP "release"
message) that the label binding associated with the route is no
longer needed.
When a Frame Relay-LSR receives an LDP "request" message for a
certain route (FEC) from an LDP peer connected to the FR-LSR over a
LC-FR interface, the FR-LSR takes the following actions:
- it allocates a label, creates a new entry in its Label
Information Base (LIB), and places that label in the incoming
label component of the entry;
- it propagates the "request", by sending an LDP "request"
message to the next hop LSR, downstream for that route (FEC);
In the "ordered control" mode [ARCH], the FR-LSR will wait for its
"request" to be responded from downstream with a "mapping" message
before returning the "mapping" upstream in response to a "request"
("ordered control" approach [ARCH]). In this case, the FR-LSR
increments the hop count it received from downstream and uses this
value in the "mapping" it returns upstream.
Alternatively, the FR-LSR may return the binding upstream without
waiting for a binding from downstream ("independent control" approach
[ARCH]). In this case, it uses a reserved value for hop count in the
"mapping", indicating that it is 'unknown'. The correct value for hop
count will be returned later, as described below.
Since both the "ordered" and "independent" control has advantages and
disadvantages, this is left as an implementation, or configuration
choice.
Once the FR-LSR receives in response the label binding in an LDP
"mapping" message from the next hop, it places the label into the
outgoing label component of the LIB entry.
Note that a FR-LSR, or a member of the edge set of a FR-LSR cloud,
may receive multiple binding requests for the same route (FEC) from
the same FR-LSR. It must generate a new "mapping" for each "request"
(assuming adequate resources to do so), and retain any existing
mapping(s). For each "request" received, a FR-LSR should also
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generate a new binding "request" toward the next hop for the route
(FEC).
When a routing calculation causes a FR-LSR to change the next hop for
a route (FEC), the FR-LSR should notify the former next hop (via an
LDP "release" message) that the label binding associated with the
route is no longer needed.
When a LSR receives a notification that a particular label binding is
no longer needed, the LSR may deallocate the label associated with
the binding, and destroy the binding. This mode is the "conservative
label retention mode" [ARCH]. In the case where a FR-LSR receives
such notification and destroys the binding, it should notify the next
hop for the route that the label binding is no longer needed. If a
LSR does not destroy the binding (the FR-LSR is configured in
"liberal label retention mode" [ARCH]), it may re-use the binding
only if it receives a request for the same route with the same hop
count as the request that originally caused the binding to be
created.
When a route changes, the label bindings are re-established from the
point where the route diverges from the previous route. LSRs
upstream of that point are (with one exception, noted below)
oblivious to the change. Whenever a LSR changes its next hop for a
particular route, if the new next hop is a FR-LSR or a member of the
edge set reachable via a LC-FR interface, then for each entry in its
LIB associated with the route the LSR should request (via LDP) a
binding from the new next hop.
When a FR-LSR receives a label binding from a downstream neighbor, it
may already have provided a corresponding label binding for this
route to an upstream neighbor, either because it is using
"independent control" or because the new binding from downstream is
the result of a routing change. In this case, it should extract the
hop count from the new binding and increment it by one. If the new
hop count is different from that which was previously conveyed to the
upstream neighbor (including the case where the upstream neighbor was
given the value 'unknown') the FR-LSR must notify the upstream
neighbor of the change. Each FR-LSR in turn increments the hop count
and passes it upstream until it reaches the ingress Edge LSR.
Whenever a FR-LSR originates a label binding request to its next hop
LSR as a result of receiving a label binding request from another
(upstream) LSR, and the request to the next hop LSR is not satisfied,
the FR-LSR should destroy the binding created in response to the
received request, and notify the requester (via an LDP "withdraw"
message).
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When an LSR determines that it has lost its LDP session with another
LSR, the following actions are taken:
- MUST discard any binding information learned via this
connection;
- For any label bindings that were created as a result of
receiving label binding requests from the peer, the LSR may
destroy these bindings (and deallocate labels associated
with these binding).
7.2 Efficient use of label space - Merging FR-LSRs
The above discussion assumes that an edge LSR will request one label
for each prefix in its routing table that has a next hop in the FR-
LSR cloud. In fact, it is possible to significantly reduce the number
of labels needed by having the edge LSR request instead one label for
several routes. Use of many-to-one mappings between routes (address
prefixes) and labels using the notion of Forwarding Equivalence
Classes (as described in [ARCH]) provides a mechanism to conserve the
number of labels.
Note that conserving label space may be restricted in case the frame
traffic requires Frame Relay fragmentation. The issue is that Frame
Relay fragments must be transmitted in sequence, i.e. fragments of
distinct frames must not be interleaved. If the fragmenting FR-LSR
ensures the transmission in sequence of all fragments of a frame,
without interleaving with fragments of other frames, then label
conservation (aggregation) can be performed.
In the case where the label space is to be conserved, it is desirable
to use half-duplex (unidirectional) VCs, since a "many to few"
aggregation is possible in one direction but not in reverse.
7.3 LDP messages specific to Frame Relay
The Label Distribution Protocol [LDP] messages exchanged between two
Frame Relay "LDP-peer" LSRs may contain Frame Relay specific
information such as:
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"Frame Relay Label Range":
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Len| Minimum DLCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Maximum DLCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
with the following fields:
Reserved
This fields are reserved. They must be set to zero on transmission
and must be ignored on receipt.
Len
This field specifies the number of bits of the DLCI. The following
values are supported:
Len DLCI bits
0 10
1 17
2 23
Minimum DLCI
This 23 bit field is the binary value of the lower bound of a block
of Data Link Connection Identifiers (DLCIs) that is supported by
the originating FR-LSR. The Minimum DLCI should be right justified
in this field and the preceding bits should be set to 0.
Maximum DLCI
This 23 bit field is the binary value of the upper bound of a block
of Data Link Connection Identifiers (DLCIs) that is supported by
the originating FR-LSR. The Maximum DLCI should be right justified
in this field and the preceding bits should be set to 0.
and "Frame Relay Label":
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Len| DLCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
with the following fields:
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Reserved
This field is reserved. It must be set to zero on transmission and
must be ignored on receipt.
Len
This field specifies the number of bits of the DLCI. The following
values are supported:
Len DLCI bits
0 10
1 17
2 23
DLCI
The binary value of the Frame Relay Label. The significant number
of bits (10, 17, or 23) of the label value are to be encoded into
the Data Link Connection Identifier (DLCI) field when part of the
Frame Relay data link header (see Section 4.).
8. Security Considerations
This section looks at the security aspects of:
(a) frame traffic
(b) label distribution.
MPLS encapsulation has no effect on authenticated or encrypted
network layer packets, that is IP packets that are authenticated or
encrypted will incur no change.
The MPLS protocol has no mechanisms of its own to protect against
misdirection of packets or the impersonation of an LSR by accident or
malicious intent.
Altering by accident or forgery an existent label in the DLCI field
of the Frame Relay data link layer header of a frame or one or more
fields in a potentially following label stack affects the forwarding
of that frame.
The label distribution mechanism can be secured by applying the
appropriate level of security to the underlying protocol carrying
label information - authentication or encryption - see [LDP].
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9. Acknowledgments
The initial version of this document was derived from the Label
Switching over ATM document [ATM].
Thanks for the extensive reviewing and constructive comments from (in
alphabetical order) Dan Harrington, Milan Merhar, Martin Mueller.
Also thanks to George Swallow for the suggestion to use null
encapsulation, and to Eric Gray for his reviewing.
Also thanks to Nancy Feldman and Bob Thomas for their collaboration
in including the LDP messages specific to Frame Relay LSRs
10. References
[MIFR] T. Bradley, C. Brown, A. Malis "Multiprotocol Interconnect
over Frame Relay" RFC 2427, September 1998.
[ARCH] E. Rosen, R. Callon, A. Vishwanathan, "Multi-Protocol Label
Switching Architecture", Work in Progress, July 1998.
[LDP] L. Anderson, P. Doolan, N. Feldman, A. Fredette, R. Thomas,
"Label Distribution Protocol", Work in Progress, August 1998.
[STACK] E. Rosen et al, "Label Switching: Label Stack Encodings",
Work in Progress, September 1998
[ATM] B. Davie et al. "Use of Label Switching with ATM", Work in
Progress, July 1998.
[ITU] International Telecommunications Union, "ISDN Data Link Layer
Specification for Frame Mode Bearer Services", ITU-T Recommendation
Q.922, 1992.
[FRF] Frame Relay Forum, User-to-Network Implementation Agreement
(UNI), FRF 1.1, January 19, 1996
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11.Authors' Addresses
Alex Conta
Lucent Technologies Inc.
300 Baker Ave, Suite 100
Concord, MA 01742
+1-978-287-2842
E-mail: aconta@lucent.com
Paul Doolan
Ennovate Networks
330 Codman Hill Rd
Boxborough MA 01719
+1-978-263-2002
E-mail: pdoolan@ennovatenetworks.com
Andrew Malis
Ascend Communications, Inc
1 Robbins Rd
Westford, MA 01886
+1-978-952-7414
E-mail: malis@ascend.com
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