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.
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 [ASSIGNED_AF] 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.
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 that 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 represented as a 20-bit number in a 4 octet field as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
For further information, see [RFC3032].
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 of DLCI
2 = 23 bits of DLCI
Len values 1 and 3 are reserved.
DLCI The Data Link Connection Identifier
For a 10-bit DLCI, the 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| Frame Relay Label (0x0202)| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Len| 0 | 10-bit DLCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
For a 23-bit DLCI, the 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| Frame Relay Label (0x0202)| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Len| 23-bit DLCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
For further information, see [RFC3034].
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 [ASSIGNED_AF] 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 set up.
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 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 an 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 set up 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:
-
Transmit a Notification message to the sending LSR signaling "Loop Detected".
-
Not propagate the Label Request message further.
Note that a Label Request message with a Path Vector TLV is forwarded until:
-
A loop is found,
-
The LSP egress is reached, or
-
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:
-
Transmit a Label Release message carrying a Status TLV to the sending LSR to signal "Loop Detected".
-
Not propagate the message further.
-
Check whether the Label Mapping message is for an existing LSP. If so, the LSR must unsplice any upstream labels that are spliced to the downstream label for the FEC.
Note that a Label Mapping message with a Path Vector TLV is forwarded until:
-
A loop is found,
-
An LSP ingress is reached, or
-
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 that 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.