As per Relevance of the word encapsulation, we have this rfc below:
Network Working Group A.
Request for Comments: 3034 Transwitch
Category: Standards Track P.
A.
Vivace Networks, Inc
January 2001
Use of Label Switching on Frame Relay
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
Copyright
Copyright (C) The Internet Society (2001). All Rights Reserved
This document defines the model and generic mechanisms
Multiprotocol Label Switching on Frame Relay networks. Furthermore
it extends and clarifies portions of the Multiprotocol
Switching Architecture described in [ARCH] and the Label
Protocol (LDP) described in [LDP] relative to Frame Relay Networks
MPLS enables the use of Frame Relay Switches as Label
Routers (LSRs).
Table of
1. Introduction................................................2
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...7
5.4.1 FR-LSRs Loop Control - MPLS TTL Processing.............7
5.4.2 Performing MPLS TTL calculations.......................8
5.5 Label Processing by Ingress FR-LSRs......................12
Conta, et al. Standards Track [Page 1]
RFC 3034 Label Switching with Frame Relay January 2001
5.6 Label Processing by Core FR-LSRs.........................12
5.7 Label Processing by Egress FR-LSRs.......................13
6. Label Switching Control Component for Frame Relay.........13
6.1 Hybrid Switches (Ships in the Night) ...................14
7. Label Allocation and Maintenance Procedures ..............15
7.1 Edge LSR Behavior........................................15
7.2 Efficient use of label space-Merging FR-LSRs.............18
7.3 LDP message fields specific to Frame Relay...............19
8. Security Considerations .................................21
9. Acknowledgments .........................................21
10. References ..............................................22
11. Authors' Addresses ......................................23
12. Full Copyright Statement ................................24
1.
The Multiprotocol Label Switching Architecture is described
[ARCH]. It is possible to use Frame Relay switches as
Switching Routers. Such Frame Relay switches run network
routing algorithms (such as OSPF, IS-IS, etc.), and their
is based on the results of these routing algorithms. No
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
in the DLCI field of the Frame Relay data link layer header of
frame. Additional information carried along with the top (current
label, but not processed by Frame Relay switching, along with
labels, if the packet is multiply labeled, are carried in the
MPLS encapsulation defined in [STACK].
Frame Relay permanent virtual circuits (PVCs) could be configured
carry label switching based traffic. The DLCIs would be used as
Labels and the Frame Relay switches would become Frame Relay
Switching Routers, while the MPLS traffic would be
according to this specification, and would be forwarded based
network layer routing information
The keywords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED
SHALL, SHALL NOT, SHOULD, SHOULD NOT are to be interpreted as
in RFC 2119.
This document is a companion document to [STACK] and [ATM].
Conta, et al. Standards Track [Page 2]
RFC 3034 Label Switching with Frame Relay January 2001
2.
A Label Switching Router (LSR) is a device which implements
label switching control and forwarding components described
[ARCH].
LC-
A label switching controlled Frame Relay (LC-FR) interface is
Frame Relay interface controlled by the label switching
component. Packets traversing such an interface carry labels
the DLCI field
FR-
A FR-LSR is an LSR with one or more LC-FR interfaces
forwards frames between two such interfaces using labels
in the DLCI field
FR-LSR
A FR-LSR domain is a set of FR-LSRs, which are
interconnected by LC-FR interfaces
Edge
The Edge Set of an FR-LSR domain is the set of LSRs, which
connected to the domain by LC-FR interfaces
Forwarding
The Forwarding Encapsulation is the type of MPLS
(Frame Relay, ATM, Generic) of a packet that determines
packet's MPLS forwarding, or the network layer encapsulation
that packet is forwarded based on the network layer (IP
etc...)header
Input
The Input Encapsulation is the type of MPLS encapsulation (
Relay, ATM, Generic) of a packet when that packet is received
an LSR's interface, or the network layer (IP, etc...)
if that packet has no MPLS encapsulation
Conta, et al. Standards Track [Page 3]
RFC 3034 Label Switching with Frame Relay January 2001
Output
The Output Encapsulation is the type of MPLS encapsulation (
Relay, ATM, Generic) of a packet when that packet is
on an LSR's interface, or the network layer (IP
etc...)encapsulation if that packet has no MPLS encapsulation
Input
The Input TTL is the MPLS TTL of the top of the stack when
labeled packet is received on an LSR interface, or the
layer (IP) TTL if the packet is not labeled
Output
The Output TTL is the MPLS TTL of the top of the stack when
labeled packet is transmitted on an LSR interface, or the
layer (IP) TTL if the packet is not labeled
Additionally, this document uses terminology from [ARCH].
3. Special characteristics of Frame Relay
While the label switching architecture permits
flexibility in LSR implementation, a FR-LSR is constrained by
capabilities of the (possibly pre-existing) hardware and
restrictions on such matters as frame format imposed by
Multiprotocol Interconnect over Frame Relay [MIFR], or Frame
standards [FRF], etc.... Because of these constraints, some
procedures are required for FR-LSRs
Some of the key features of Frame Relay switches that affect
behavior as LSRs are
- the label swapping function is performed on fields (DLCI) in
frame's Frame Relay data link header; this dictates the size
placement of the label(s) in a packet. The size of the DLCI
can be 10 (default) or 23 bits, and it can span two or four
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
that are passed at circuit creation. Flags in the frame
may be set as a consequence of congestion, or exceeding
contractual parameters of the circuit
Conta, et al. Standards Track [Page 4]
RFC 3034 Label Switching with Frame Relay January 2001
- although in a standard switch it may be possible to
multiple input DLCIs to one output DLCI resulting in
multipoint-to-point circuit, multipoint-to-multipoint VCs
generally not fully supported
This document describes ways of applying label switching to
Relay switches, which work within these constraints
4. Label
By default, all labeled packets should be transmitted with
generic label encapsulation as defined in [STACK], using the
relay null encapsulation mechanism
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.,
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
Conta, et al. Standards Track [Page 5]
RFC 3034 Label Switching with Frame Relay January 2001
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
The use of the frame relay null encapsulation implies that
implicitly encode the network protocol type
Rules regarding the construction of the label stack, and
messages returned to the frame source are also described in [STACK].
The generic encapsulation contains "n" labels for a label stack
depth "n" [STACK], where the top stack entry carries
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
link header encoded in Q.922 [ITU] address format
5. Frame Relay Label Switching
5.1 Use of
Label switching is accomplished by associating labels with routes
using the label value to forward packets, including determining
value of any replacement label. See [ARCH] for further details.
a FR-LSR, the top (current) MPLS label is carried in the DLCI
of the Frame Relay data link layer header of the frame. The
label carries implicitly information about the network protocol type
For two connected FR-LSRs, a full-duplex connection must be
for LDP. The DLCI for the LDP VC is assigned a value by way
configuration, similar to configuring the DLCI used to run IP
protocols between the switches
With the exception of this configured value, the DLCI values used
MPLS in the two directions of the link may be treated as belonging
two independent spaces, i.e., VCs may be half-duplex, each
with its own DLCI
Conta, et al. Standards Track [Page 6]
RFC 3034 Label Switching with Frame Relay January 2001
The allowable ranges of DLCIs, the size of DLCIs, and the support
VC merging MUST be communicated through LDP messages. Note that
range of DLCIs used for labels depends on the size of the DLCI field
5.2 Homogeneous
If is an LSP, it is possible that LSR1, LSR2,
LSR3 will use the same encoding of the label stack when
packet P from LSR1, to LSR2, and then to LSR3. Such an LSP
homogeneous
5.3 Heterogeneous
If is an LSP, it is possible that LSR1 will
one encoding of the label stack when transmitting packet P to LSR2,
but LSR2 will use a different encoding when transmitting a packet
to LSR3. In general, the MPLS architecture supports LSPs
different label stack encodings on different hops. When a
packet is received, the LSR must decode it to determine the
value of the label stack, then must operate on the label stack
determine the new label value of the stack, and then encode the
value appropriately before transmitting the labeled packet to
next hop
Naturally there will be MPLS networks which contain a combination
Frame Relay switches operating as LSRs, and other LSRs, which
using other MPLS encapsulations, such as the Generic (MPLS
header), or ATM encapsulation. In such networks there may be
LSRs, which have Frame Relay interfaces as well as MPLS
("MPLS Shim") interfaces. This is one example of an LSR
different label stack encodings on different hops of the same LSP
Such an LSR may swap off a Frame Relay encoded label on an
interface and replace it with a label encoded into a Generic
(MPLS shim) header on the outgoing interface
5.4 Frame Relay Label Switching Loop Prevention and
FR-LSRs SHOULD operate on loop free FR-LSPs or LSP Frame
segments. Therefore, FR-LSRs SHOULD use loop detection and MAY
loop prevention mechanisms as described in [ARCH], and [LDP].
5.4.1 FR-LSRs Loop Control - MPLS TTL
The MPLS TTL encoded in the MPLS label stack is a mechanism used to
(a) suppress loops
(b) limit the scope of a packet
Conta, et al. Standards Track [Page 7]
RFC 3034 Label Switching with Frame Relay January 2001
When a packet travels along an LSP, it should emerge with the
TTL value that it would have had if it had traversed the
sequence of routers without having been label switched. If
packet travels along a hierarchy of LSPs, the total number of LSR
hops traversed should be reflected in its TTL value when it
from the hierarchy of LSPs [ARCH].
The initial value of the MPLS TTL is loaded into a newly pushed
stack entry from the previous TTL value, whether that is from
network layer header when no previous label stack existed, or from
pre-existent lower level label stack entry
A FR-LSR switching same level labeled packets does not decrement
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
reflect in the TTL the number of LSR-hops it traversed. In
unicast case, this can be achieved by propagating a meaningful
length or LSP Frame Relay segment length to the FR-LSR ingress nodes
enabling the ingress to decrement the TTL value before
packets into a non-TTL LSP segment [ARCH].
When an ingress FR-LSR determines upon decrementing the MPLS TTL
a particular packet's TTL will expire before the packet reaches
egress of the "non-TTL LSP segment", the FR-LSR MUST not label
the packet, but rather follow the specifications in [STACK] in
attempt to return an error message to the packet's source
- it treats the packet as an expired packet and return an
message to its source
- it forwards the packet, as an unlabeled packet, with a TTL
reflects the IP (network layer) forwarding
If the incoming TTL is 1, only the first option applies
In the multicast case, a meaningful LSP length or LSP segment
is propagated to the FR-LSR egress node, enabling the egress
decrement the TTL value before forwarding packets out of the non-
LSP segment
5.4.2 Performing MPLS TTL
The calculation applied to the "input TTL" that yields the "
TTL" depends on (i)the "input encapsulation", (ii)the "
encapsulation", and (iii)the "output encapsulation".
relationship among (i),(ii), and (iii), can be defined as a
Conta, et al. Standards Track [Page 8]
RFC 3034 Label Switching with Frame Relay January 2001
"D" of "input encapsulation" (ie), "forwarding encapsulation" (fe),
and "output encapsulation" (oe). Subsequently the
applied to the "input TTL" to yield the "output TTL" can be
as
output TTL = input TTL - D(ie, fe, oe
or in a brief notation
output TTL = input TTL -
where "d" has three possible values: "0","1", or "the number of
of the LSP segment":
For unicast transmission
+================+=================+=================+=================+
| | Type of | Type of | Type of |
| d | Input | Forwarding | Output |
| | Encapsulation | Encapsulation | Encapsulation |
+================+=================+=================+=================+
| 0 | Frame Relay | Frame Relay | Frame Relay |
+----------------+-----------------+-----------------+-----------------+
| 1 | any | Generic MPLS | Generic MPLS |
+----------------+-----------------+-----------------+-----------------+
| number of hops | | Generic MPLS | |
| of | any | or | Frame Relay |
| LSP segment | |IP(network layer)| |
+================+=================+=================+=================+
The "number of hops of the LSP segment" is the value of the "
count" that is attached with the label used when the packet
forwarded, if LDP [LDP] has provided such a "hop count" value when
distributed the label for the LSP, that is the LDP message had a "
count object". If LDP didn't provide a "hop count", or it
an "unknown" value, the default value of the "number of hops of
segment" is 1.
When sending a label binding upstream, the "hop count"
with the corresponding binding from downstream, if different than
"unknown" value, MUST be incremented by 1, and the result
upstream as the hop count associated with the new binding (
"unknown" value is transmitted unchanged). If the new "hop count
value exceeds the "maximum" value, the FR-LSR MUST NOT pass
binding upstream, but instead MUST send an error
[LDP][ARCH].
Conta, et al. Standards Track [Page 9]
RFC 3034 Label Switching with Frame Relay January 2001
For multicast transmission
+================+=================+=================+=================+
| | Type of | Type of | Type of |
| d | Input | Forwarding | Output |
| | Encapsulation | Encapsulation | Encapsulation |
+================+=================+=================+=================+
| 0 | Frame Relay | Frame Relay | Frame Relay |
+----------------+-----------------+-----------------+-----------------+
| | | Generic MPLS | |
| 1 | any | or | Frame Relay |
| | |IP(network layer)| |
+----------------+-----------------+-----------------+-----------------+
| number of hops | | Generic MPLS | |
| of | Frame Relay | or | any |
| LSP segment | |IP(network layer)| |
+================+=================+=================+=================+
Referring to the "forwarding encapsulation" with the abbreviation "I
for IP (network layer), "G" for Generic MPLS, and "F" for Frame
MPLS, referring to an LSR interface with the abbreviation "i" if
input or output encapsulation is IP and no MPLS encapsulation, "g
when the input or output MPLS encapsulation is Generic MPLS, "f"
it is Frame Relay, "a" when it is ATM, and furthermore
the symbols "iIf", "gGf", "fFf", etc... as LSRs with input
forwarding and output encapsulations as referred above, the
describes examples of TTL calculations for the Homogeneous
Heterogeneous LSPs discussed in previous sections
Homogeneous
---------------
IP_ttl = n IP_ttl=mpls_ttl-1 = n-6
--------->iIf fIi--------->
| mpls_ttl = n-5 ^
| |
number of hops 1| Frame Relay |5
| |
V 2 3 4 |
fFf--->fFf--->fFf--->
"iIf" is "ingress LSR" in Frame Relay LSP
calculates: mpls_ttl = IP_TTL - number of hops = n-5
"fIi" is "egress LSR" from Frame Relay LSP,
calculates: IP_ttl = mpls_ttl-1 = n-6
Conta, et al. Standards Track [Page 10]
RFC 3034 Label Switching with Frame Relay January 2001
Heterogeneous
-----------------
ingress LSR egress
IP_ttl = n IP_ttl = n - 15
links LAN PPP FR ATM PPP FR
--->iIg-->gGg-->gGf fGa aGg-->gGf fGg-->gIi--->
hops 1 2 | 6 | | 9 | 10 | 13 ^ 14 15
|1 4| |1 3| |1 3|
V 2 3 | V 2 | V 2 |
fFf-->fFf-->fFf aAa-->aAa fFf-->
mpls_
n-1 n-2 (n-2)-4=n-6 (n-6)-3=n-9 n-10 n-13 n-14
"iIg" is "ingress LSR" in LSP; it calculates: mpls_ttl=n-1
"gGf" is "egress LSR" from Generic MPLS segment,
"ingress LSR" in Frame Relay segment and calculates: mpls_ttl=n-6
"fGa" "egress LSR" from Frame Relay segment,
"ingress LSR" in ATM segment and calculates: mpls_ttl=n-9
"gGf" is "egress LSR" from Generic MPLS segment,
"ingress LSR" in Frame Relay segment and calculates: mpls_ttl=n-13
"fGg" is "egress LSR" from Frame Relay segment,
ingress LSR" in Generic MPLS segment and calculates: mpls_ttl=n-14
"gIi" is "egress LSR" from LSP and calculates: IP_ttl=n-15
And further examples
Frame Relay Unicast -- TTL calculated at
(ingress LSR) 1 2 3 4
x--->---+--->---+--->>--+-->>---x (egress LSR
o.ttl=i.ttl-4 | 2 3
^
hops 1|
|
x (ingress LSR
o.ttl=i.ttl-3
Frame Relay Multicast -- TTL calculated at
(egress LSR)x o.ttl=i.ttl-3
hops |
^3
(ingress LSR) | o.ttl=i.ttl-4
x--->---+--->---+--->---+--->---x (egress LSR
1 2 3 4
Conta, et al. Standards Track [Page 11]
RFC 3034 Label Switching with Frame Relay January 2001
5.5 Label Processing by Ingress FR-
When a packet first enters an MPLS domain, the packet is forwarded
normal network layer forwarding operations with the exception
the outgoing encapsulation will include an MPLS label stack [STACK
with at least one entry. The frame relay null encapsulation
carry information about the network layer protocol implicitly in
label, which MUST be associated only with that network protocol.
TTL field in the top label stack entry is filled with the
layer TTL (or hop limit) resulted after network layer
[STACK]. The further FR-LSR processing is similar in both
cases
(a) the LSP is homogeneous -- Frame Relay only -- and the FR-LSR
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
Frame Relay segment
For unicast packets, the MPLS TTL SHOULD be decremented with
number of hops of the Frame Relay LSP (homogeneous), or Frame
segment of the LSP (heterogeneous). An LDP constructing the
SHOULD pass meaningful information to the ingress FR-LSR
the number of hops of the "non-TTL segment".
For multicast packets, the MPLS TTL SHOULD be decremented by 1.
LDP constructing the LSP SHOULD pass meaningful information to
egress FR-LSR regarding the number of hops of the "non-TTL segment".
Next, the MPLS encapsulated packet is passed down to the Frame
data link driver with the top label as output DLCI. The Frame
frame carrying the MPLS encapsulated packet is forwarded onto
Frame Relay VC to the next LSR
5.6 Label Processing by Core FR-
In a FR-LSR, the current (top) MPLS label is carried in the
field of the Frame Relay data link layer header of the frame.
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
the DLCI Information Base, replaced with the correspondent
DLCI, and transmitted on the outgoing interface (forwarded to
next hop node).
Conta, et al. Standards Track [Page 12]
RFC 3034 Label Switching with Frame Relay January 2001
The current label information is also carried in the top of the
stack. In the top-level entry, all fields except the
information, which is carried and switched in the Frame Relay
data link-layer header, are of current significance
5.7 Label Processing by Egress FR-
When reaching the end of a Frame Relay LSP, the FR-LSR pops the
stack [ARCH]. If the label popped is the last label, it is
to determine the particular network layer protocol which is
carried. The label stack carries no explicit information to
the network layer protocol. This must be inferred from the value
the label which is popped from the stack
If the label popped is not the last label, the previous top
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
hybrid LSP, and the end of the Frame Relay segment is not the end
the LSP, the MPLS packet will be processed for forwarding onto
next segment of the LSP based on the information held in the Next
Label Forwarding Entry (NHLFE) [ARCH]. The output label is set
the value from the NHLFE, and the MPLS TTL is decremented by
appropriate value depending the type of the output interface and
type of transmit operation (see section 6.3). Further, the
packet is forwarded according to the MPLS specifications for
particular link of the next segment of the LSP
For unicast packets, the MPLS TTL SHOULD be decremented by one if
output interface is a generic one, or with the number of hops of
next ATM segment of the LSP (heterogeneous), if the output
is an ATM (non-TTL) interface
For multicast packets, the MPLS TTL SHOULD be decremented by
number of hops of the FR segment being exited. An LDP
the LSP SHOULD pass meaningful information to the egress FR-
regarding the number of hops of the FR "non-TTL segment".
6. Label Switching Control Component for Frame
To support label switching a Frame Relay Switch MUST implement
control component of label switching, which consists primarily
label allocation and maintenance procedures. Label
information MAY be communicated by several mechanisms, one of
is the Label Distribution Protocol (LDP) [LDP].
Conta, et al. Standards Track [Page 13]
RFC 3034 Label Switching with Frame Relay January 2001
Since the label switching control component uses information
directly from network layer routing protocols, this implies that
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)
distribute label bindings. In these cases, a Frame Relay LSR
participate in these protocols
In the case where Frame Relay circuits are established via LDP,
RSVP, or others, with no involvement from traditional Frame
mechanisms, it is assumed that circuit establishing
information such as input/output maximum frame size
incoming/outgoing requested/agreed throughput, incoming/
acceptable throughput, incoming/outgoing burst size
incoming/outgoing frame rate, used in transmitting, and
control MAY be passed to the FR-LSRs through RSVP, or can
statically configured. It is also assumed that congestion
and frame header flagging as a consequence of congestion, would
done by the FR-LSRs in a similar fashion as for traditional
Relay circuits. With the goal of emulating a best-effort router
default, the default VC parameters, in the absence of LDP, RSVP,
other mechanisms participation to setting such parameters, should
zero CIR, so that input policing will set the DE bit in
frames, but no frames are dropped
Control and state information for the circuits based on MPLS MAY
communicated through LDP
Support of label switching on a Frame Relay switch
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
signaling may be used for non-MPLS (standard Frame Relay) PVCs and/
SVCs when both are running on the same interface as MPLS,
discussed in the next section
6.1 Hybrid Switches (Ships in the Night
The existence of the label switching control component on a
Relay switch does not preclude the ability to support the Frame
control component defined by the ITU and Frame Relay Forum on
same switch and the same interfaces (NICs). The two
components, label switching and those defined by ITU/Frame
Forum, would operate independently
Conta, et al. Standards Track [Page 14]
RFC 3034 Label Switching with Frame Relay January 2001
Definition of how such a device operates is beyond the scope of
document. However, only a small amount of information needs to
consistent between the two control components, such as the
of the DLCI space which are available to each component
7. Label Allocation and Maintenance
The mechanisms and message formats of a Label Distribution
are documented in [ARCH] and [LDP]. The "downstream-on-demand"
allocation and maintenance mechanism discussed in this section
be used by FR-LSRs that do not support VC merging, and it MAY also
used by FR-LSRs that do support VC merging (note that this
applies to hop-by-hop routed traffic):
7.1 Edge LSR
Consider a member of the Edge Set of a FR-LSR domain. Assume that
as a result of its routing calculations, it selects a FR-LSR as
next hop of a certain route (FEC), and that the next hop is
via a LC-Frame Relay interface. Assume that the next-hop FR-LSR
an "LDP-peer" [ARCH][LDP]. The Edge LSR sends an LDP "request
message for a label binding from the next hop, downstream LSR.
the Edge LSR receives in response from the downstream LSR the
binding information in an LDP "mapping" message, the label is
in the Label Information Base (LIB) as an outgoing label for
FEC. The "mapping" message may contain the "hop count" object,
represents the number of hops a packet will take to cross the FR-
domain to the Egress FR-LSR when using this label. This
may be stored for TTL calculation. Once this is done, the LSR
use MPLS forwarding to transmit packets in that FEC
When a member of the Edge Set of the FR-LSR domain receives an
"request" message from a FR-LSR for a FEC, it means it is
Egress-FR-LSR. It allocates a label, creates a new entry in
Label Information Base (LIB), places that label in the incoming
component of the entry, and returns (via LDP) a "mapping"
containing the allocated label back upstream to the LDP peer
originated the request. The "mapping" message contains the "
count" object value set to 1.
When a routing calculation causes an Edge LSR to change the next
for a route, and the former next hop was in the FR-LSR domain,
Edge LSR should notify the former next hop (via an LDP "release
message) that the label binding associated with the route is
longer needed
Conta, et al. Standards Track [Page 15]
RFC 3034 Label Switching with Frame Relay January 2001
When a Frame Relay-LSR receives an LDP "request" message for
certain route (FEC) from an LDP peer connected to the FR-LSR over
LC-FR interface, the FR-LSR takes the following actions
- it allocates a label, creates a new entry in its
Information Base (LIB), and places that label in the
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
"request" to be responded from downstream with a "mapping"
before returning the "mapping" upstream in response to a "request
("ordered control" approach [ARCH]). In this case, the FR-
increments the hop count it received from downstream and uses
value in the "mapping" it returns upstream
Alternatively, the FR-LSR may return the binding upstream
waiting for a binding from downstream ("independent control"
[ARCH]). In this case, it uses a reserved value for hop count in
"mapping", indicating that it is 'unknown'. The correct value
hop count will be returned later, as described below
Since both the "ordered" and "independent" control has advantages
disadvantages, this is left as an implementation, or
choice
Once the FR-LSR receives in response the label binding in an
"mapping" message from the next hop, it places the label into
outgoing label component of the LIB entry
Note that a FR-LSR, or a member of the edge set of a FR-LSR domain
may receive multiple binding requests for the same route (FEC)
the same FR-LSR. It must generate a new "mapping" for each "request
(assuming adequate resources to do so), and retain any
mapping(s). For each "request" received, a FR-LSR should
generate a new binding "request" toward the next hop for the
(FEC).
When a routing calculation causes a FR-LSR to change the next hop
a route (FEC), the FR-LSR should notify the former next hop (via
LDP "release" message) that the label binding associated with
route is no longer needed
When a LSR receives a notification that a particular label binding
no longer needed, the LSR may deallocate the label associated
the binding, and destroy the binding. This mode is the "
Conta, et al. Standards Track [Page 16]
RFC 3034 Label Switching with Frame Relay January 2001
label retention mode" [ARCH]. In the case where a FR-LSR
such notification and destroys the binding, it should notify the
hop for the route that the label binding is no longer needed. If
LSR does not destroy the binding (the FR-LSR is configured
"liberal label retention mode" [ARCH]), it may re-use the
only if it receives a request for the same route with the same
count as the request that originally caused the binding to
created
When a route changes, the label bindings are re-established from
point where the route diverges from the previous route.
upstream of that point are (with one exception, noted below
oblivious to the change. Whenever a LSR changes its next hop for
particular route, if the new next hop is a FR-LSR or a member of
edge set reachable via a LC-FR interface, then for each entry in
LIB associated with the route the LSR should request (via LDP)
binding from the new next hop
When a FR-LSR receives a label binding from a downstream neighbor,
may already have provided a corresponding label binding for
route to an upstream neighbor, either because it is
"independent control" or because the new binding from downstream
the result of a routing change. In this case, it should extract
hop count from the new binding and increment it by one. If the
hop count is different from that which was previously conveyed to
upstream neighbor (including the case where the upstream neighbor
given the value 'unknown') the FR-LSR must notify the
neighbor of the change. Each FR-LSR in turn increments the hop
and passes it upstream until it reaches the ingress Edge LSR
Whenever a FR-LSR originates a label binding request to its next
LSR as a result of receiving a label binding request from
(upstream) LSR, and the request to the next hop LSR is not satisfied
the FR-LSR should destroy the binding created in response to
received request, and notify the requester (via an LDP "withdraw
message).
When an LSR determines that it has lost its LDP session with
LSR, the following actions are taken
- MUST discard any binding information learned via
connection
- For any label bindings that were created as a result
receiving label binding requests from the peer, the LSR
destroy these bindings (and deallocate labels associated
these binding).
Conta, et al. Standards Track [Page 17]
RFC 3034 Label Switching with Frame Relay January 2001
7.2 Efficient use of label space - Merging FR-
The above discussion assumes that an edge LSR will request one
for each prefix in its routing table that has a next hop in the FR
LSR domain. In fact, it is possible to significantly reduce
number of labels needed by having the edge LSR request instead
label for several routes. Use of many-to-one mappings between
(address prefixes) and labels using the notion of
Equivalence Classes (as described in [ARCH]) provides a mechanism
conserve the number of labels
Note that conserving label space (VC merging) may be restricted
case the frame traffic requires Frame Relay fragmentation. The
is that Frame Relay fragments must be transmitted in sequence, i.e.,
fragments of distinct frames must not be interleaved. If
fragmenting FR-LSR ensures the transmission in sequence of
fragments of a frame, without interleaving with fragments of
frames, then label conservation (VC merging) can be performed
When label conservation is used, when a FR-LSR receives a
request from an upstream LSR for a certain FEC, and it does
have an outgoing label binding for that FEC, it does not need
issue a downstream binding request. Instead, it may allocate
incoming label, and return that label in a binding to the
requester. Packets received from the requester, with that label
top label, will be forwarded after replacing the label with
existing outgoing label for that FEC. If the FR-LSR does not have
outgoing label binding for that FEC, but does have an
request for one, it need not issue another request. This means
in a label conservation case, a FR-LSR must respond with a
binding for every upstream request, but it may need to send
binding request downstream
In case of label conservation, if a change in the routing
causes FR-LSR to select a new next hop for one of its FECs, it
release the binding for that route from the former next hop. If
doesn't already have a corresponding binding for the new next hop,
must request one (note that the choice depends on the label
mode [ARCH]).
If a new binding is obtained, which contain a hop count that
from that of the old binding, the FR-LSR must process the new
count: increment by 1, if different than "unknown", and notify
upstream neighbors who have label bindings for this FEC of the
value. To ensure that loops will be detected, if the new hop
exceeds the "maximum" value, the label values for this FEC must
withdrawn from all upstream neighbors to whom a binding
previously sent
Conta, et al. Standards Track [Page 18]
RFC 3034 Label Switching with Frame Relay January 2001
7.3 LDP messages specific to Frame
The Label Distribution Protocol [LDP] messages exchanged between
Frame Relay "LDP-peer" LSRs may contain Frame Relay
information such as
"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
This fields are reserved. They must be set to zero
transmission and must be ignored on receipt
This field specifies the number of bits of the DLCI.
following values are supported
Len DLCI
0 10
2 23
Len values 1 and 3 are reserved for future use
Minimum
This 23 bit field is the binary value of the lower bound of
block of Data Link Connection Identifiers (DLCIs) that
supported by the originating FR-LSR. The Minimum DLCI should
right justified in this field and the preceding bits should be
to 0.
Maximum
This 23 bit field is the binary value of the upper bound of
block of Data Link Connection Identifiers (DLCIs) that
supported by the originating FR-LSR. The Maximum DLCI should
right justified in this field and the preceding bits should be
to 0.
Conta, et al. Standards Track [Page 19]
RFC 3034 Label Switching with Frame Relay January 2001
"Frame Relay Merge":
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Reserved |M
+-+-+-+-+-+-+-+-+
with the following fields
One bit field that specifies the merge capabilities of the FR-LSR
Value
0 Merge NOT
1 Merge
A FR-LSR that supports VC merging MUST ensure that
frames from distinct incoming DLCIs are not interleaved on
outgoing DLCI
This field is reserved. It must be set to zero on
and must be ignored on receipt
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
This field is reserved. It must be set to zero on transmission
must be ignored on receipt
This field specifies the number of bits of the DLCI. The
values are supported
Len DLCI
0 10
2 23
Conta, et al. Standards Track [Page 20]
RFC 3034 Label Switching with Frame Relay January 2001
Len values 1 and 3 are reserved for future use
The binary value of the Frame Relay Label. The significant
of bits (10 or 23) of the label value are to be encoded into
Data Link Connection Identifier (DLCI) field when part of
Frame Relay data link header (see Section 4.).
8. Security
This section looks at the security aspects of
(a) frame traffic
(b) label distribution
MPLS encapsulation has no effect on authenticated or
network layer packets, that is IP packets that are authenticated
encrypted will incur no change
The MPLS protocol has no mechanisms of its own to protect
misdirection of packets or the impersonation of an LSR by accident
malicious intent
Altering by accident or forgery an existent label in the DLCI
of the Frame Relay data link layer header of a frame or one or
fields in a potentially following label stack affects the
of that frame
The label distribution mechanism can be secured by applying
appropriate level of security to the underlying protocol
label information - authentication or encryption - see [LDP].
9.
The initial version of this document was derived from the
Switching over ATM document [ATM].
Thanks for the extensive reviewing and constructive comments from (
alphabetical order) Dan Harrington, Milan Merhar, Martin Mueller
Eric Rosen. Also thanks to George Swallow for the suggestion to
null encapsulation, and to Eric Gray for his reviewing
Also thanks to Nancy Feldman and Bob Thomas for their
in including the LDP messages specific to Frame Relay LSRs
Conta, et al. Standards Track [Page 21]
RFC 3034 Label Switching with Frame Relay January 2001
10.
[MIFR] Bradley, T., Brown, C. and A. Malis, "
Interconnect over Frame Relay", RFC 2427, September 1998.
[ARCH] Rosen, E., Callon, R. and A. Vishwanathan, "Multi-
Label Switching Architecture", RFC 3031, January 2001.
[LDP] Andersson, L., Doolan, P., Feldman, N., Fredette, A. and R
Thomas, "Label Distribution Protocol", RFC 3036,
2001.
[STACK] Rosen, E., Rehter, Y., Tappan, D., Farinacci, D., Fedorkow
G., Li, T. and A. Conta, "MPLS Label Stack Encoding",
3032, January 2001.
[ATM] Davie, B., Lawrence, J., McCloghrie, M., Rosen, E., Swallow
G., Rekhter, Y., and P. Doolan, "Use of Label Switching
ATM", RFC 3035, January 2001.
[ITU] International Telecommunications Union, "ISDN Data Link
Specification for Frame Mode Bearer Services", ITU-
Recommendation Q.922, 1992.
[FRF] Frame Relay Forum, User-to-Network Implementation
(UNI), FRF 1.1, January 19, 1996.
Conta, et al. Standards Track [Page 22]
RFC 3034 Label Switching with Frame Relay January 2001
11. Authors'
Alex
Transwitch
3 Enterprise
Shelton, CT 06484
Phone: 1-203-929-8810
EMail: aconta@txc.
Paul
Ennovate
60 Codman Hill
Boxborough MA 01719
Phone: 1-978-263-2002
EMail: pdoolan@ennovatenetworks.
Andrew G.
Vivace Networks, Inc
2730 Orchard
San Jose, CA 95134
Phone: 1-408-383-7223
Fax: 1-408-904-4748
EMail: Andy.Malis@vivacenetworks.
Conta, et al. Standards Track [Page 23]
RFC 3034 Label Switching with Frame Relay January 2001
12. Full Copyright
Copyright (C) The Internet Society (2001). All Rights Reserved
This document and translations of it may be copied and furnished
others, and derivative works that comment on or otherwise explain
or assist in its implementation may be prepared, copied,
and distributed, in whole or in part, without restriction of
kind, provided that the above copyright notice and this paragraph
included on all such copies and derivative works. However,
document itself may not be modified in any way, such as by
the copyright notice or references to the Internet Society or
Internet organizations, except as needed for the purpose
developing Internet standards in which case the procedures
copyrights defined in the Internet Standards process must
followed, or as required to translate it into languages other
English
The limited permissions granted above are perpetual and will not
revoked by the Internet Society or its successors or assigns
This document and the information contained herein is provided on
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE
Funding for the RFC Editor function is currently provided by
Internet Society
Conta, et al. Standards Track [Page 24]
if you see any problems within the linking, don't worry be happy,
this is version 0.1 of the Relevance System and you gotta expect some crappy subroutines sometimes,
just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.
RFC documents can be found at I.E.T.F.
Relevance System Copyright © 2002 Spectrum WorldResearch
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