RFC 3036
LDP Specification
3. Protocol Specification
Previous sections that describe LDP operation have discussed scenarios that involve the exchange of messages among LDP peers. This section specifies the message encodings and procedures for processing the messages. LDP message exchanges are accomplished by sending LDP protocol data units (PDUs) over LDP session TCP connections. Each LDP PDU can carry one or more LDP messages. Note that the messages in an LDP PDU need not be related to one another. For example, a single PDU could carry a message advertising FEC-label bindings for several FECs, another message requesting label bindings for several other FECs, and a third notification message signaling some event.3.1. LDP PDUs
Each LDP PDU is an LDP header followed by one or more LDP messages. The LDP header is: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Version | PDU Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LDP Identifier | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Version Two octet unsigned integer containing the version number of the protocol. This version of the specification specifies LDP protocol version 1. PDU Length Two octet integer specifying the total length of this PDU in octets, excluding the Version and PDU Length fields. The maximum allowable PDU Length is negotiable when an LDP session is initialized. Prior to completion of the negotiation the maximum allowable length is 4096 bytes.
LDP Identifier
Six octet field that uniquely identifies the label space of the
sending LSR for which this PDU applies. The first four octets
identify the LSR and must be a globally unique value. It should be
a 32-bit router Id assigned to the LSR and also used to identify it
in loop detection Path Vectors. The last two octets identify a
label space within the LSR. For a platform-wide label space, these
should both be zero.
Note that there is no alignment requirement for the first octet of an
LDP PDU.
3.2. LDP Procedures
LDP defines messages, TLVs and procedures in the following areas:
- Peer discovery;
- Session management;
- Label distribution;
- Notification of errors and advisory information.
The sections that follow describe the message and TLV encodings for
these areas and the procedures that apply to them.
The label distribution procedures are complex and are difficult to
describe fully, coherently and unambiguously as a collection of
separate message and TLV specifications.
Appendix A, "LDP Label Distribution Procedures", describes the label
distribution procedures in terms of label distribution events that
may occur at an LSR and how the LSR must respond. Appendix A is the
specification of LDP label distribution procedures. If a procedure
described elsewhere in this document conflicts with Appendix A,
Appendix A specifies LDP behavior.
3.3. Type-Length-Value Encoding
LDP uses a Type-Length-Value (TLV) encoding scheme to encode much of
the information carried in LDP messages.
An LDP TLV is encoded as a 2 octet field that uses 14 bits to specify
a Type and 2 bits to specify behavior when an LSR doesn't recognize
the Type, followed by a 2 octet Length Field, followed by a variable
length Value field.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Value |
~ ~
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
U bit
Unknown TLV bit. Upon receipt of an unknown TLV, if U is clear
(=0), a notification must be returned to the message originator
and the entire message must be ignored; if U is set (=1), the
unknown TLV is silently ignored and the rest of the message is
processed as if the unknown TLV did not exist. The sections
following that define TLVs specify a value for the U-bit.
F bit
Forward unknown TLV bit. This bit applies only when the U bit is
set and the LDP message containing the unknown TLV is to be
forwarded. If F is clear (=0), the unknown TLV is not forwarded
with the containing message; if F is set (=1), the unknown TLV is
forwarded with the containing message. The sections following
that define TLVs specify a value for the F-bit.
Type
Encodes how the Value field is to be interpreted.
Length
Specifies the length of the Value field in octets.
Value
Octet string of Length octets that encodes information to be
interpreted as specified by the Type field.
Note that there is no alignment requirement for the first octet of a
TLV.
Note that the Value field itself may contain TLV encodings. That is,
TLVs may be nested.
The TLV encoding scheme is very general. In principle, everything
appearing in an LDP PDU could be encoded as a TLV. This
specification does not use the TLV scheme to its full generality. It
is not used where its generality is unnecessary and its use would waste space unnecessarily. These are usually places where the type of a value to be encoded is known, for example by its position in a message or an enclosing TLV, and the length of the value is fixed or readily derivable from the value encoding itself. Some of the TLVs defined for LDP are similar to one another. For example, there is a Generic Label TLV, an ATM Label TLV, and a Frame Relay TLV; see Sections "Generic Label TLV", "ATM Label TLV", and "Frame Relay TLV". While it is possible to think about TLVs related in this way in terms of a TLV type that specifies a TLV class and a TLV subtype that specifies a particular kind of TLV within that class, this specification does not formalize the notion of a TLV subtype. The specification assigns type values for related TLVs, such as the label TLVs, from a contiguous block in the 16-bit TLV type number space. Section "TLV Summary" lists the TLVs defined in this version of the protocol and the section in this document that describes each.3.4. TLV Encodings for Commonly Used Parameters
There are several parameters used by more than one LDP message. The TLV encodings for these commonly used parameters are specified in this section.3.4.1. FEC TLV
Labels are bound to Forwarding Equivalence Classes (FECs). A FEC is a list of one or more FEC elements. The FEC TLV encodes FEC items.
Its encoding is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| FEC (0x0100) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Element 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Element n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FEC Element 1 to FEC Element n
There are several types of FEC elements; see Section "FECs". The
FEC element encoding depends on the type of FEC element.
A FEC Element value is encoded as a 1 octet field that specifies
the element type, and a variable length field that is the type-
dependent element value. Note that while the representation of
the FEC element value is type-dependent, the FEC element encoding
itself is one where standard LDP TLV encoding is not used.
The FEC Element value encoding is:
FEC Element Type Value
type name
Wildcard 0x01 No value; i.e., 0 value octets;
see below.
Prefix 0x02 See below.
Host Address 0x03 Full host address; see below.
Note that this version of LDP supports the use of multiple FEC
Elements per FEC for the Label Mapping message only. The use of
multiple FEC Elements in other messages is not permitted in this
version, and is a subject for future study.
Wildcard FEC Element
To be used only in the Label Withdraw and Label Release
Messages. Indicates the withdraw/release is to be applied to
all FECs associated with the label within the following label
TLV. Must be the only FEC Element in the FEC TLV.
Prefix FEC Element value encoding:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix (2) | Address Family | PreLen |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Address Family
Two octet quantity containing a value from ADDRESS FAMILY
NUMBERS in [RFC1700] that encodes the address family for the
address prefix in the Prefix field.
PreLen
One octet unsigned integer containing the length in bits of the
address prefix that follows. A length of zero indicates a
prefix that matches all addresses (the default destination); in
this case the Prefix itself is zero octets).
Prefix
An address prefix encoded according to the Address Family
field, whose length, in bits, was specified in the PreLen
field, padded to a byte boundary.
Host Address FEC Element encoding:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Host Addr (3) | Address Family | Host Addr Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Host Addr |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Address Family
Two octet quantity containing a value from ADDRESS FAMILY
NUMBERS in [RFC1700] that encodes the address family for the
address prefix in the Prefix field.
Host Addr Len
Length of the Host address in octets.
Host Addr
An address encoded according to the Address Family field.
3.4.1.1. FEC Procedures
If in decoding a FEC TLV an LSR encounters a FEC Element with an Address Family it does not support, it should stop decoding the FEC TLV, abort processing the message containing the TLV, and send an "Unsupported Address Family" Notification message to its LDP peer signaling an error. If it encounters a FEC Element type it cannot decode, it should stop decoding the FEC TLV, abort processing the message containing the TLV, and send an "Unknown FEC" Notification message to its LDP peer signaling an error.3.4.2. Label TLVs
Label TLVs encode labels. Label TLVs are carried by the messages used to advertise, request, release and withdraw label mappings. There are several different kinds of Label TLVs which can appear in situations that require a Label TLV.3.4.2.1. Generic Label TLV
An LSR uses Generic Label TLVs to encode labels for use on links for which label values are independent of the underlying link technology. Examples of such links are PPP and Ethernet. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0| Generic Label (0x0200) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Label | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Label This is a 20-bit label value as specified in [RFC3032] represented as a 20-bit number in a 4 octet field.
3.4.2.2. ATM Label TLV
An LSR uses ATM Label TLVs to encode labels for use on ATM links. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0| ATM Label (0x0201) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Res| V | VPI | VCI | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Res This field is reserved. It must be set to zero on transmission and must be ignored on receipt. V-bits Two-bit switching indicator. If V-bits is 00, both the VPI and VCI are significant. If V-bits is 01, only the VPI field is significant. If V-bit is 10, only the VCI is significant. VPI Virtual Path Identifier. If VPI is less than 12-bits it should be right justified in this field and preceding bits should be set to 0. VCI Virtual Channel Identifier. If the VCI is less than 16- bits, it should be right justified in the field and the preceding bits must be set to 0. If Virtual Path switching is indicated in the V-bits field, then this field must be ignored by the receiver and set to 0 by the sender.3.4.2.3. Frame Relay Label TLV
An LSR uses Frame Relay Label TLVs to encode labels for use on Frame Relay links. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0| Frame Relay Label (0x0202)| Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved |Len| DLCI | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Res
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:
0 = 10 bits DLCI
2 = 23 bits DLCI
Len values 1 and 3 are reserved.
DLCI
The Data Link Connection Identifier. Refer to [RFC3034] for the
label values and formats.
3.4.3. Address List TLV
The Address List TLV appears in Address and Address Withdraw
messages.
Its encoding is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Address List (0x0101) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Family | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| Addresses |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Address Family
Two octet quantity containing a value from ADDRESS FAMILY NUMBERS
in [RFC1700] that encodes the addresses contained in the Addresses
field.
Addresses
A list of addresses from the specified Address Family. The
encoding of the individual addresses depends on the Address Family.
The following address encodings are defined by this version of the
protocol:
Address Family Address Encoding
IPv4 4 octet full IPv4 address
IPv6 16 octet full IPv6 address
3.4.4. Hop Count TLV
The Hop Count TLV appears as an optional field in messages that set
up LSPs. It calculates the number of LSR hops along an LSP as the
LSP is being setup.
Note that setup procedures for LSPs that traverse ATM and Frame Relay
links require use of the Hop Count TLV (see [RFC3035] and [RFC3034]).
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Hop Count (0x0103) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HC Value |
+-+-+-+-+-+-+-+-+
HC Value
1 octet unsigned integer hop count value.
3.4.4.1. Hop Count Procedures
During setup of an LSP an LSR R may receive a Label Mapping or Label
Request message for the LSP that contains the Hop Count TLV. If it
does, it should record the hop count value.
If LSR R then propagates the Label Mapping message for the LSP to an
upstream peer or the Label Request message to a downstream peer to
continue the LSP setup, it must must determine a hop count to include
in the propagated message as follows:
- If the message is a Label Request message, R must increment the
received hop count;
- If the message is a Label Mapping message, R determines the hop
count as follows:
o If R is a member of the edge set of an LSR domain whose LSRs do
not perform 'TTL-decrement' and the upstream peer is within
that domain, R must reset the hop count to 1 before propagating
the message.
o Otherwise, R must increment the received hop count.
The first LSR in the LSP (ingress for a Label Request message, egress
for a Label Mapping message) should set the hop count value to 1.
By convention a value of 0 indicates an unknown hop count. The
result of incrementing an unknown hop count is itself an unknown hop
count (0).
Use of the unknown hop count value greatly reduces the signaling
overhead when independent control is used. When a new LSP is
established, each LSR starts with unknown hop count. Addition of a
new LSR whose hop count is also unknown does not cause a hop count
update to be propagated upstream since the hop count remains unknown.
When the egress is finally added to the LSP, then the LSRs propagate
hop count updates upstream via Label Mapping messages.
Without use of the unknown hop count, each time a new LSR is added to
the LSP a hop count update would need to be propagated upstream if
the new LSR is closer to the egress than any of the other LSRs.
These updates are useless overhead since they don't reflect the hop
count to the egress.
From the perspective of the ingress node, the fact that the hop count
is unknown implies nothing about whether a packet sent on the LSP
will actually make it to the egress. All it implies is that the hop
count update from the egress has not yet reached the ingress.
If an LSR receives a message containing a Hop Count TLV, it must
check the hop count value to determine whether the hop count has
exceeded its configured maximum allowable value. If so, it must
behave as if the containing message has traversed a loop by sending a
Notification message signaling Loop Detected in reply to the sender
of the message.
If Loop Detection is configured, the LSR must follow the procedures
specified in Section "Loop Detection".
3.4.5. Path Vector TLV
The Path Vector TLV is used with the Hop Count TLV in Label Request
and Label Mapping messages to implement the optional LDP loop
detection mechanism. See Section "Loop Detection". Its use in the
Label Request message records the path of LSRs the request has
traversed. Its use in the Label Mapping message records the path of
LSRs a label advertisement has traversed to setup an LSP.
Its encoding is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Path Vector (0x0104) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSR Id 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSR Id n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
One or more LSR Ids
A list of router-ids indicating the path of LSRs the message has
traversed. Each LSR Id is the first four octets (router-id) of
the LDP identifier for the corresponding LSR. This ensures it is
unique within the LSR network.
3.4.5.1. Path Vector Procedures
The Path Vector TLV is carried in Label Mapping and Label Request
messages when loop detection is configured.
3.4.5.1.1. Label Request Path Vector
Section "Loop Detection" specifies situations when an LSR must
include a Path Vector TLV in a Label Request message.
An LSR that receives a Path Vector in a Label Request message must
perform the procedures described in Section "Loop Detection".
If the LSR detects a loop, it must reject the Label Request message.
The LSR must:
1. Transmit a Notification message to the sending LSR signaling
"Loop Detected".
2. Not propagate the Label Request message further.
Note that a Label Request message with Path Vector TLV is forwarded
until:
1. A loop is found,
2. The LSP egress is reached,
3. The maximum Path Vector limit or maximum Hop Count limit is
reached. This is treated as if a loop had been detected.
3.4.5.1.2. Label Mapping Path Vector
Section "Loop Detection" specifies the situations when an LSR must
include a Path Vector TLV in a Label Mapping message.
An LSR that receives a Path Vector in a Label Mapping message must
perform the procedures described in Section "Loop Detection".
If the LSR detects a loop, it must reject the Label Mapping message
in order to prevent a forwarding loop. The LSR must:
1. Transmit a Label Release message carrying a Status TLV to the
sending LSR to signal "Loop Detected".
2. Not propagate the message further.
3. Check whether the Label Mapping message is for an existing LSP.
If so, the LSR must unsplice any upstream labels which are
spliced to the downstream label for the FEC.
Note that a Label Mapping message with a Path Vector TLV is forwarded
until:
1. A loop is found,
2. An LSP ingress is reached, or
3. The maximum Path Vector or maximum Hop Count limit is reached.
This is treated as if a loop had been detected.
3.4.6. Status TLV
Notification messages carry Status TLVs to specify events being
signaled.
The encoding for the Status TLV is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Status (0x0300) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
U bit
Should be 0 when the Status TLV is sent in a Notification message.
Should be 1 when the Status TLV is sent in some other message.
F bit
Should be the same as the setting of the F-bit in the Status Code
field.
Status Code
32-bit unsigned integer encoding the event being signaled. The
structure of a Status Code is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E|F| Status Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
E bit
Fatal error bit. If set (=1), this is a fatal error
notification. If clear (=0), this is an advisory notification.
F bit
Forward bit. If set (=1), the notification should be forwarded
to the LSR for the next-hop or previous-hop for the LSP, if
any, associated with the event being signaled. If clear (=0),
the notification should not be forwarded.
Status Data
30-bit unsigned integer which specifies the status information.
This specification defines Status Codes (32-bit unsigned integers
with the above encoding).
A Status Code of 0 signals success.
Message ID
If non-zero, 32-bit value that identifies the peer message to
which the Status TLV refers. If zero, no specific peer message is
being identified.
Message Type
If non-zero, the type of the peer message to which the Status TLV
refers. If zero, the Status TLV does not refer to any specific
message type.
Note that use of the Status TLV is not limited to Notification
messages. A message other than a Notification message may carry a
Status TLV as an Optional Parameter. When a message other than a
Notification carries a Status TLV the U-bit of the Status TLV should
be set to 1 to indicate that the receiver should silently discard the
TLV if unprepared to handle it.
3.5. LDP Messages
All LDP messages have the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U| Message Type | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Mandatory Parameters |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Optional Parameters |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
U bit
Unknown message bit. Upon receipt of an unknown message, if U is
clear (=0), a notification is returned to the message originator;
if U is set (=1), the unknown message is silently ignored. The
sections following that define messages specify a value for the
U-bit.
Message Type
Identifies the type of message
Message Length
Specifies the cumulative length in octets of the Message ID,
Mandatory Parameters, and Optional Parameters.
Message ID
32-bit value used to identify this message. Used by the sending
LSR to facilitate identifying notification messages that may apply
to this message. An LSR sending a notification message in
response to this message should include this Message Id in the
Status TLV carried by the notification message; see Section
"Notification Message".
Mandatory Parameters
Variable length set of required message parameters. Some messages
have no required parameters.
For messages that have required parameters, the required
parameters MUST appear in the order specified by the individual
message specifications in the sections that follow.
Optional Parameters
Variable length set of optional message parameters. Many messages
have no optional parameters.
For messages that have optional parameters, the optional
parameters may appear in any order.
Note that there is no alignment requirement for the first octet of an
LDP message.
The following message types are defined in this version of LDP:
Message Name Section Title
Notification "Notification Message"
Hello "Hello Message"
Initialization "Initialization Message"
KeepAlive "KeepAlive Message"
Address "Address Message"
Address Withdraw "Address Withdraw Message"
Label Mapping "Label Mapping Message"
Label Request "Label Request Message"
Label Abort Request "Label Abort Request Message"
Label Withdraw "Label Withdraw Message"
Label Release "Label Release Message"
The sections that follow specify the encodings and procedures for
these messages.
Some of the above messages are related to one another, for example
the Label Mapping, Label Request, Label Withdraw, and Label Release
messages.
While it is possible to think about messages related in this way in
terms of a message type that specifies a message class and a message
subtype that specifies a particular kind of message within that
class, this specification does not formalize the notion of a message
subtype.
The specification assigns type values for related messages, such as
the label messages, from of a contiguous block in the 16-bit message
type number space.
3.5.1. Notification Message
An LSR sends a Notification message to inform an LDP peer of a
significant event. A Notification message signals a fatal error or
provides advisory information such as the outcome of processing an
LDP message or the state of the LDP session.
The encoding for the Notification Message is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Notification (0x0001) | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status (TLV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Parameters |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message ID
32-bit value used to identify this message.
Status TLV
Indicates the event being signaled. The encoding for the Status
TLV is specified in Section "Status TLV".
Optional Parameters
This variable length field contains 0 or more parameters, each
encoded as a TLV. The following Optional Parameters are generic
and may appear in any Notification Message:
Optional Parameter Type Length Value
Extended Status 0x0301 4 See below
Returned PDU 0x0302 var See below
Returned Message 0x0303 var See below
Other Optional Parameters, specific to the particular event being
signaled by the Notification Messages may appear. These are
described elsewhere.
Extended Status
The 4 octet value is an Extended Status Code that encodes
additional information that supplements the status information
contained in the Notification Status Code.
Returned PDU
An LSR uses this parameter to return part of an LDP PDU to the
LSR that sent it. The value of this TLV is the PDU header and
as much PDU data following the header as appropriate for the
condition being signaled by the Notification message.
Returned Message
An LSR uses this parameter to return part of an LDP message to
the LSR that sent it. The value of this TLV is the message
type and length fields and as much message data following the
type and length fields as appropriate for the condition being
signaled by the Notification message.
3.5.1.1. Notification Message Procedures
If an LSR encounters a condition requiring it to notify its peer with
advisory or error information it sends the peer a Notification
message containing a Status TLV that encodes the information and
optionally additional TLVs that provide more information about the
condition.
If the condition is one that is a fatal error the Status Code carried
in the notification will indicate that. In this case, after sending
the Notification message the LSR should terminate the LDP session by
closing the session TCP connection and discard all state associated with the session, including all label-FEC bindings learned via the session. When an LSR receives a Notification message that carries a Status Code that indicates a fatal error, it should terminate the LDP session immediately by closing the session TCP connection and discard all state associated with the session, including all label-FEC bindings learned via the session.3.5.1.2. Events Signaled by Notification Messages
It is useful for descriptive purpose to classify events signaled by Notification Messages into the following categories.3.5.1.2.1. Malformed PDU or Message
Malformed LDP PDUs or Messages that are part of the LDP Discovery mechanism are handled by silently discarding them. An LDP PDU received on a TCP connection for an LDP session is malformed if: - The LDP Identifier in the PDU header is unknown to the receiver, or it is known but is not the LDP Identifier associated by the receiver with the LDP peer for this LDP session. This is a fatal error signaled by the Bad LDP Identifier Status Code. - The LDP protocol version is not supported by the receiver, or it is supported but is not the version negotiated for the session during session establishment. This is a fatal error signaled by the Bad Protocol Version Status Code. - The PDU Length field is too small (< 14) or too large (> maximum PDU length). This is a fatal error signaled by the Bad PDU Length Status Code. Section "Initialization Message" describes how the maximum PDU length for a session is determined. An LDP Message is malformed if: - The Message Type is unknown. If the Message Type is < 0x8000 (high order bit = 0) it is an error signaled by the Unknown Message Type Status Code.
If the Message Type is >= 0x8000 (high order bit = 1) it is
silently discarded.
- The Message Length is too large, that is, indicates that the
message extends beyond the end of the containing LDP PDU. This
is a fatal error signaled by the Bad Message Length Status
Code.
- The message is missing one or more Mandatory Parameters. This
is a non-fatal error signalled by the Missing Message
Parameters Status Code.
3.5.1.2.2. Unknown or Malformed TLV
Malformed TLVs contained in LDP messages that are part of the LDP
Discovery mechanism are handled by silently discarding the containing
message.
A TLV contained in an LDP message received on a TCP connection of an
LDP is malformed if:
- The TLV Length is too large, that is, indicates that the TLV
extends beyond the end of the containing message. This is a
fatal error signaled by the Bad TLV Length Status Code.
- The TLV type is unknown.
If the TLV type is < 0x8000 (high order bit 0) it is an error
signaled by the Unknown TLV Status Code.
If the TLV type is >= 0x8000 (high order bit 1) the TLV is
silently dropped. Section "Unknown TLV in Known Message Type"
elaborates on this behavior.
- The TLV Value is malformed. This occurs when the receiver
handles the TLV but cannot decode the TLV Value. This is
interpreted as indicative of a bug in either the sending or
receiving LSR. It is a fatal error signaled by the Malformed
TLV Value Status Code.
3.5.1.2.3. Session KeepAlive Timer Expiration
This is a fatal error signaled by the KeepAlive Timer Expired Status
Code.
3.5.1.2.4. Unilateral Session Shutdown
This is a fatal event signaled by the Shutdown Status Code. The Notification Message may optionally include an Extended Status TLV to provide a reason for the Shutdown. The sending LSR terminates the session immediately after sending the Notification.3.5.1.2.5. Initialization Message Events
The session initialization negotiation (see Section "Session Initialization") may fail if the session parameters received in the Initialization Message are unacceptable. This is a fatal error. The specific Status Code depends on the parameter deemed unacceptable, and is defined in Sections "Initialization Message".3.5.1.2.6. Events Resulting From Other Messages
Messages other than the Initialization message may result in events that must be signaled to LDP peers via Notification Messages. These events and the Status Codes used in the Notification Messages to signal them are described in the sections that describe these messages.3.5.1.2.7. Internal Errors
An LDP implementation may be capable of detecting problem conditions specific to its implementation. When such a condition prevents an implementation from interacting correctly with a peer, the implementation should, when capable of doing so, use the Internal Error Status Code to signal the peer. This is a fatal error.3.5.1.2.8. Miscellaneous Events
These are events that fall into none of the categories above. There are no miscellaneous events defined in this version of the protocol.3.5.2. Hello Message
LDP Hello Messages are exchanged as part of the LDP Discovery Mechanism; see Section "LDP Discovery". The encoding for the Hello Message is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Hello (0x0100) | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Common Hello Parameters TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Parameters |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message ID
32-bit value used to identify this message.
Common Hello Parameters TLV
Specifies parameters common to all Hello messages. The encoding
for the Common Hello Parameters TLV is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Common Hello Parms(0x0400)| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hold Time |T|R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Hold Time,
Hello hold time in seconds. An LSR maintains a record of
Hellos received from potential peers (see Section "Hello
Message Procedures"). Hello Hold Time specifies the time the
sending LSR will maintain its record of Hellos from the
receiving LSR without receipt of another Hello.
A pair of LSRs negotiates the hold times they use for Hellos
from each other. Each proposes a hold time. The hold time
used is the minimum of the hold times proposed in their Hellos.
A value of 0 means use the default, which is 15 seconds for
Link Hellos and 45 seconds for Targeted Hellos. A value of
0xffff means infinite.
T, Targeted Hello
A value of 1 specifies that this Hello is a Targeted Hello. A
value of 0 specifies that this Hello is a Link Hello.
R, Request Send Targeted Hellos
A value of 1 requests the receiver to send periodic Targeted
Hellos to the source of this Hello. A value of 0 makes no
request.
An LSR initiating Extended Discovery sets R to 1. If R is 1,
the receiving LSR checks whether it has been configured to send
Targeted Hellos to the Hello source in response to Hellos with
this request. If not, it ignores the request. If so, it
initiates periodic transmission of Targeted Hellos to the Hello
source.
Reserved
This field is reserved. It must be set to zero on transmission
and ignored on receipt.
Optional Parameters
This variable length field contains 0 or more parameters, each
encoded as a TLV. The optional parameters defined by this
version of the protocol are
Optional Parameter Type Length Value
IPv4 Transport Address 0x0401 4 See below
Configuration 0x0402 4 See below
Sequence Number
IPv6 Transport Address 0x0403 16 See below
IPv4 Transport Address
Specifies the IPv4 address to be used for the sending LSR when
opening the LDP session TCP connection. If this optional TLV
is not present the IPv4 source address for the UDP packet
carrying the Hello should be used.
Configuration Sequence Number
Specifies a 4 octet unsigned configuration sequence number that
identifies the configuration state of the sending LSR. Used by
the receiving LSR to detect configuration changes on the
sending LSR.
IPv6 Transport Address
Specifies the IPv6 address to be used for the sending LSR when
opening the LDP session TCP connection. If this optional TLV
is not present the IPv6 source address for the UDP packet
carrying the Hello should be used.
3.5.2.1. Hello Message Procedures
An LSR receiving Hellos from another LSR maintains a Hello adjacency corresponding to the Hellos. The LSR maintains a hold timer with the Hello adjacency which it restarts whenever it receives a Hello that matches the Hello adjacency. If the hold timer for a Hello adjacency expires the LSR discards the Hello adjacency: see sections "Maintaining Hello Adjacencies" and "Maintaining LDP Sessions". We recommend that the interval between Hello transmissions be at most one third of the Hello hold time. An LSR processes a received LDP Hello as follows: 1. The LSR checks whether the Hello is acceptable. The criteria for determining whether a Hello is acceptable are implementation dependent (see below for example criteria). 2. If the Hello is not acceptable, the LSR ignores it. 3. If the Hello is acceptable, the LSR checks whether it has a Hello adjacency for the Hello source. If so, it restarts the hold timer for the Hello adjacency. If not it creates a Hello adjacency for the Hello source and starts its hold timer. 4. If the Hello carries any optional TLVs the LSR processes them (see below). 5. Finally, if the LSR has no LDP session for the label space specified by the LDP identifier in the PDU header for the Hello, it follows the procedures of Section "LDP Session Establishment". The following are examples of acceptability criteria for Link and Targeted Hellos: A Link Hello is acceptable if the interface on which it was received has been configured for label switching. A Targeted Hello from source address A is acceptable if either: - The LSR has been configured to accept Targeted Hellos, or - The LSR has been configured to send Targeted Hellos to A. The following describes how an LSR processes Hello optional TLVs:
Transport Address
The LSR associates the specified transport address with the
Hello adjacency.
Configuration Sequence Number
The Configuration Sequence Number optional parameter is used by
the sending LSR to signal configuration changes to the
receiving LSR. When a receiving LSR playing the active role in
LDP session establishment detects a change in the sending LSR
configuration, it may clear the session setup backoff delay, if
any, associated with the sending LSR (see Section "Session
Initialization").
A sending LSR using this optional parameter is responsible for
maintaining the configuration sequence number it transmits in
Hello messages. Whenever there is a configuration change on
the sending LSR, it increments the configuration sequence
number.
3.5.3. Initialization Message
The LDP Initialization Message is exchanged as part of the LDP
session establishment procedure; see Section "LDP Session
Establishment".
The encoding for the Initialization Message is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Initialization (0x0200) | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Common Session Parameters TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Parameters |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message ID
32-bit value used to identify this message.
Common Session Parameters TLV
Specifies values proposed by the sending LSR for parameters that
must be negotiated for every LDP session.
The encoding for the Common Session Parameters TLV is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Common Sess Parms (0x0500)| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol Version | KeepAlive Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|D| Reserved | PVLim | Max PDU Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver LDP Identifier |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++
Protocol Version
Two octet unsigned integer containing the version number of the
protocol. This version of the specification specifies LDP
protocol version 1.
KeepAlive Time
Two octet unsigned non zero integer that indicates the number
of seconds that the sending LSR proposes for the value of the
KeepAlive Time. The receiving LSR MUST calculate the value of
the KeepAlive Timer by using the smaller of its proposed
KeepAlive Time and the KeepAlive Time received in the PDU. The
value chosen for KeepAlive Time indicates the maximum number of
seconds that may elapse between the receipt of successive PDUs
from the LDP peer on the session TCP connection. The KeepAlive
Timer is reset each time a PDU arrives.
A, Label Advertisement Discipline
Indicates the type of Label advertisement. A value of 0 means
Downstream Unsolicited advertisement; a value of 1 means
Downstream On Demand.
If one LSR proposes Downstream Unsolicited and the other
proposes Downstream on Demand, the rules for resolving this
difference is:
- If the session is for a label-controlled ATM link or a
label-controlled Frame Relay link, then Downstream on Demand
must be used.
- Otherwise, Downstream Unsolicited must be used.
If the label advertisement discipline determined in this way is
unacceptable to an LSR, it must send a Session
Rejected/Parameters Advertisement Mode Notification message in
response to the Initialization message and not establish the
session.
D, Loop Detection
Indicates whether loop detection based on path vectors is
enabled. A value of 0 means loop detection is disabled; a
value of 1 means that loop detection is enabled.
PVLim, Path Vector Limit
The configured maximum path vector length. Must be 0 if loop
detection is disabled (D = 0). If the loop detection
procedures would require the LSR to send a path vector that
exceeds this limit, the LSR will behave as if a loop had been
detected for the FEC in question.
When Loop Detection is enabled in a portion of a network, it is
recommended that all LSRs in that portion of the network be
configured with the same path vector limit. Although knowledge
of a peer's path vector limit will not change an LSR's
behavior, it does enable the LSR to alert an operator to a
possible misconfiguration.
Reserved
This field is reserved. It must be set to zero on transmission
and ignored on receipt.
Max PDU Length
Two octet unsigned integer that proposes the maximum allowable
length for LDP PDUs for the session. A value of 255 or less
specifies the default maximum length of 4096 octets.
The receiving LSR MUST calculate the maximum PDU length for the
session by using the smaller of its and its peer's proposals
for Max PDU Length. The default maximum PDU length applies
before session initialization completes.
If the maximum PDU length determined this way is unacceptable
to an LSR, it must send a Session Rejected/Parameters Max PDU
Length Notification message in response to the Initialization
message and not establish the session.
Receiver LDP Identifier
Identifies the receiver's label space. This LDP Identifier,
together with the sender's LDP Identifier in the PDU header
enables the receiver to match the Initialization message with
one of its Hello adjacencies; see Section "Hello Message
Procedures".
If there is no matching Hello adjacency, the LSR must send a
Session Rejected/No Hello Notification message in response to
the Initialization message and not establish the session.
Optional Parameters
This variable length field contains 0 or more parameters, each
encoded as a TLV. The optional parameters are:
Optional Parameter Type Length Value
ATM Session Parameters 0x0501 var See below
Frame Relay Session 0x0502 var See below
Parameters
ATM Session Parameters
Used when an LDP session manages label exchange for an ATM link
to specify ATM-specific session parameters.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| ATM Sess Parms (0x0501) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| M | N |D| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Label Range Component 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Label Range Component N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M, ATM Merge Capabilities
Specifies the merge capabilities of an ATM switch. The
following values are supported in this version of the
specification:
Value Meaning
0 Merge not supported
1 VP Merge supported
2 VC Merge supported
3 VP & VC Merge supported
If the merge capabilities of the LSRs differ, then:
- Non-merge and VC-merge LSRs may freely interoperate.
- The interoperability of VP-merge-capable switches with non-
VP-merge-capable switches is a subject for future study.
When the LSRs differ on the use of VP-merge, the session is
established, but VP merge is not used.
Note that if VP merge is used, it is the responsibility of the
ingress node to ensure that the chosen VCI is unique within the
LSR domain (see [ATM-VP]).
N, Number of label range components
Specifies the number of ATM Label Range Components included in
the TLV.
D, VC Directionality
A value of 0 specifies bidirectional VC capability, meaning the
LSR can (within a given VPI) support the use of a given VCI as
a label for both link directions independently. A value of 1
specifies unidirectional VC capability, meaning (within a given
VPI) a given VCI may appear in a label mapping for one
direction on the link only. When either or both of the peers
specifies unidirectional VC capability, both LSRs use
unidirectional VC label assignment for the link as follows.
The LSRs compare their LDP Identifiers as unsigned integers.
The LSR with the larger LDP Identifier may assign only odd-
numbered VCIs in the VPI/VCI range as labels. The system with
the smaller LDP Identifier may assign only even-numbered VCIs
in the VPI/VCI range as labels.
Reserved
This field is reserved. It must be set to zero on transmission
and ignored on receipt.
One or more ATM Label Range Components
A list of ATM Label Range Components which together specify the
Label range supported by the transmitting LSR.
A receiving LSR MUST calculate the intersection between the
received range and its own supported label range. The
intersection is the range in which the LSR may allocate and
accept labels. LSRs MUST NOT establish a session with
neighbors for which the intersection of ranges is NULL. In
this case, the LSR must send a Session Rejected/Parameters
Label Range Notification message in response to the
Initialization message and not establish the session.
The encoding for an ATM Label Range Component is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Res | Minimum VPI | Minimum VCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Res | Maximum VPI | Maximum VCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Res
This field is reserved. It must be set to zero on
transmission and must be ignored on receipt.
Minimum VPI (12 bits)
This 12 bit field specifies the lower bound of a block of
Virtual Path Identifiers that is supported on the
originating switch. If the VPI is less than 12-bits it
should be right justified in this field and preceding bits
should be set to 0.
Minimum VCI (16 bits)
This 16 bit field specifies the lower bound of a block of
Virtual Connection Identifiers that is supported on the
originating switch. If the VCI is less than 16-bits it
should be right justified in this field and preceding bits
should be set to 0.
Maximum VPI (12 bits)
This 12 bit field specifies the upper bound of a block of
Virtual Path Identifiers that is supported on the
originating switch. If the VPI is less than 12-bits it
should be right justified in this field and preceding bits
should be set to 0.
Maximum VCI (16 bits)
This 16 bit field specifies the upper bound of a block of
Virtual Connection Identifiers that is supported on the
originating switch. If the VCI is less than 16-bits it
should be right justified in this field and preceding bits
should be set to 0.
When peer LSRs are connected indirectly by means of an ATM VP, the
sending LSR should set the Minimum and Maximum VPI fields to 0,
and the receiving LSR must ignore the Minimum and Maximum VPI
fields.
See [ATM-VP] for specification of the fields for ATM Label Range
Components to be used with VP merge LSRs.
Frame Relay Session Parameters
Used when an LDP session manages label exchange for a Frame
Relay link to specify Frame Relay-specific session parameters.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| FR Sess Parms (0x0502) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| M | N |D| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Frame Relay Label Range Component 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Frame Relay Label Range Component N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M, Frame Relay Merge Capabilities
Specifies the merge capabilities of a Frame Relay switch. The
following values are supported in this version of the
specification:
Value Meaning
0 Merge not supported
1 Merge supported
Non-merge and merge Frame Relay LSRs may freely interoperate.
N, Number of label range components
Specifies the number of Frame Relay Label Range Components
included in the TLV.
D, VC Directionality
A value of 0 specifies bidirectional VC capability, meaning the
LSR can support the use of a given DLCI as a label for both
link directions independently. A value of 1 specifies
unidirectional VC capability, meaning a given DLCI may appear
in a label mapping for one direction on the link only. When
either or both of the peers specifies unidirectional VC
capability, both LSRs use unidirectional VC label assignment
for the link as follows. The LSRs compare their LDP
Identifiers as unsigned integers. The LSR with the larger LDP
Identifier may assign only odd-numbered DLCIs in the range as
labels. The system with the smaller LDP Identifier may assign
only even-numbered DLCIs in the range as labels.
Reserved
This field is reserved. It must be set to zero on transmission
and ignored on receipt.
One or more Frame Relay Label Range Components
A list of Frame Relay Label Range Components which together
specify the Label range supported by the transmitting LSR.
A receiving LSR MUST calculate the intersection between the
received range and its own supported label range. The
intersection is the range in which the LSR may allocate and
accept labels. LSRs MUST NOT establish a session with
neighbors for which the intersection of ranges is NULL. In
this case, the LSR must send a Session Rejected/Parameters
Label Range Notification message in response to the
Initialization message and not establish the session.
The encoding for a Frame Relay Label Range Component is:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved
This field is reserved. It must be set to zero on
transmission and ignored on receipt.
Len
This field specifies the number of bits of the DLCI. The
following values are supported:
Len DLCI bits
0 10
2 23
Len values 1 and 3 are reserved.
Minimum DLCI
This 23-bit field specifies the lower bound of a block of
Data Link Connection Identifiers (DLCIs) that is supported
on the originating switch. The DLCI should be right
justified in this field and unused bits should be set to 0.
Maximum DLCI
This 23-bit field specifies the upper bound of a block of
Data Link Connection Identifiers (DLCIs) that is supported
on the originating switch. The DLCI should be right
justified in this field and unused bits should be set to 0.
Note that there is no Generic Session Parameters TLV for sessions
which advertise Generic Labels.
3.5.3.1. Initialization Message Procedures
See Section "LDP Session Establishment" and particularly Section
"Session Initialization" for general procedures for handling the
Initialization Message.
(page 63 continued on part 3)
