RFC 3036
LDP Specification
3.5.4. KeepAlive Message
An LSR sends KeepAlive Messages as part of a mechanism that monitors the integrity of the LDP session transport connection. The encoding for the KeepAlive 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| KeepAlive (0x0201) | Message Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Optional Parameters | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Message ID 32-bit value used to identify this message. Optional Parameters No optional parameters are defined for the KeepAlive message.3.5.4.1. KeepAlive Message Procedures
The KeepAlive Timer mechanism described in Section "Maintaining LDP Sessions" resets a session KeepAlive timer every time an LDP PDU is
received on the session TCP connection. The KeepAlive Message is provided to allow reset of the KeepAlive Timer in circumstances where an LSR has no other information to communicate to an LDP peer. An LSR must arrange that its peer receive an LDP Message from it at least every KeepAlive Time period. Any LDP protocol message will do but, in circumstances where no other LDP protocol messages have been sent within the period, a KeepAlive message must be sent.3.5.5. Address Message
An LSR sends the Address Message to an LDP peer to advertise its interface addresses. The encoding for the Address 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| Address (0x0300) | Message Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Address List TLV | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Optional Parameters | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Message ID 32-bit value used to identify this message. Address List TLV The list of interface addresses being advertised by the sending LSR. The encoding for the Address List TLV is specified in Section "Address List TLV". Optional Parameters No optional parameters are defined for the Address message.3.5.5.1. Address Message Procedures
An LSR that receives an Address Message message uses the addresses it learns to maintain a database for mapping between peer LDP Identifiers and next hop addresses; see Section "LDP Identifiers and Next Hop Addresses".
When a new LDP session is initialized and before sending Label Mapping or Label Request messages an LSR should advertise its interface addresses with one or more Address messages. Whenever an LSR "activates" a new interface address, it should advertise the new address with an Address message. Whenever an LSR "de-activates" a previously advertised address, it should withdraw the address with an Address Withdraw message; see Section "Address Withdraw Message". If an LSR does not support the Address Family specified in the Address List TLV, it should send an "Unsupported Address Family" Notification to its LDP signalling an error and abort processing the message.3.5.6. Address Withdraw Message
An LSR sends the Address Withdraw Message to an LDP peer to withdraw previously advertised interface addresses. The encoding for the Address Withdraw 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| Address Withdraw (0x0301) | Message Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Address List TLV | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Optional Parameters | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Message ID 32-bit value used to identify this message. Address list TLV The list of interface addresses being withdrawn by the sending LSR. The encoding for the Address list TLV is specified in Section "Address List TLV". Optional Parameters No optional parameters are defined for the Address Withdraw message.
3.5.6.1. Address Withdraw Message Procedures
See Section "Address Message Procedures"3.5.7. Label Mapping Message
An LSR sends a Label Mapping message to an LDP peer to advertise FEC-label bindings to the peer. The encoding for the Label Mapping 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| Label Mapping (0x0400) | Message Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | FEC TLV | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Label TLV | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Optional Parameters | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Message ID 32-bit value used to identify this message. FEC TLV Specifies the FEC component of the FEC-Label mapping being advertised. See Section "FEC TLV" for encoding. Label TLV Specifies the Label component of the FEC-Label mapping. See Section "Label TLV" for encoding. Optional Parameters This variable length field contains 0 or more parameters, each encoded as a TLV. The optional parameters are: Optional Parameter Length Value Label Request 4 See below Message ID TLV Hop Count TLV 1 See below Path Vector TLV variable See below
The encodings for the Hop Count, and Path Vector TLVs can be found
in Section "TLV Encodings for Commonly Used Parameters".
Label Request Message ID
If this Label Mapping message is a response to a Label Request
message it must include the Request Message Id optional
parameter. The value of this optional parameter is the Message
Id of the corresponding Label Request Message.
Hop Count
Specifies the running total of the number of LSR hops along the
LSP being setup by the Label Message. Section "Hop Count
Procedures" describes how to handle this TLV.
Path Vector
Specifies the LSRs along the LSP being setup by the Label
Message. Section "Path Vector Procedures" describes how to
handle this TLV.
3.5.7.1. Label Mapping Message Procedures
The Mapping message is used by an LSR to distribute a label mapping
for a FEC to an LDP peer. If an LSR distributes a mapping for a FEC
to multiple LDP peers, it is a local matter whether it maps a single
label to the FEC, and distributes that mapping to all its peers, or
whether it uses a different mapping for each of its peers.
An LSR is responsible for the consistency of the label mappings it
has distributed, and that its peers have these mappings.
An LSR receiving a Label Mapping message from a downstream LSR for a
Prefix or Host Address FEC Element should not use the label for
forwarding unless its routing table contains an entry that exactly
matches the FEC Element.
See Appendix A "LDP Label Distribution Procedures" for more details.
3.5.7.1.1. Independent Control Mapping
If an LSR is configured for independent control, a mapping message is
transmitted by the LSR upon any of the following conditions:
1. The LSR recognizes a new FEC via the forwarding table, and the
label advertisement mode is Downstream Unsolicited
advertisement.
2. The LSR receives a Request message from an upstream peer for a
FEC present in the LSR's forwarding table.
3. The next hop for a FEC changes to another LDP peer, and loop
detection is configured.
4. The attributes of a mapping change.
5. The receipt of a mapping from the downstream next hop AND
a) no upstream mapping has been created OR
b) loop detection is configured OR
c) the attributes of the mapping have changed.
3.5.7.1.2. Ordered Control Mapping
If an LSR is doing ordered control, a Mapping message is transmitted
by downstream LSRs upon any of the following conditions:
1. The LSR recognizes a new FEC via the forwarding table, and is
the egress for that FEC.
2. The LSR receives a Request message from an upstream peer for a
FEC present in the LSR's forwarding table, and the LSR is the
egress for that FEC OR has a downstream mapping for that FEC.
3. The next hop for a FEC changes to another LDP peer, and loop
detection is configured.
4. The attributes of a mapping change.
5. The receipt of a mapping from the downstream next hop AND
a) no upstream mapping has been created OR
b) loop detection is configured OR
c) the attributes of the mapping have changed.
3.5.7.1.3. Downstream on Demand Label Advertisement
In general, the upstream LSR is responsible for requesting label
mappings when operating in Downstream on Demand mode. However,
unless some rules are followed, it is possible for neighboring LSRs
with different advertisement modes to get into a livelock situation
where everything is functioning properly, but no labels are
distributed. For example, consider two LSRs Ru and Rd where Ru is
the upstream LSR and Rd is the downstream LSR for a particular FEC.
In this example, Ru is using Downstream Unsolicited advertisement
mode and Rd is using Downstream on Demand mode. In this case, Rd may
assume that Ru will request a label mapping when it wants one and Ru
may assume that Rd will advertise a label if it wants Ru to use one.
If Rd and Ru operate as suggested, no labels will be distributed from
Rd to Ru.
This livelock situation can be avoided if the following rule is observed: an LSR operating in Downstream on Demand mode should not be expected to send unsolicited mapping advertisements. Therefore, if the downstream LSR is operating in Downstream on Demand mode, the upstream LSR is responsible for requesting label mappings as needed.3.5.7.1.4. Downstream Unsolicited Label Advertisement
In general, the downstream LSR is responsible for advertising a label mapping when it wants an upstream LSR to use the label. An upstream LSR may issue a mapping request if it so desires. The combination of Downstream Unsolicited mode and conservative label retention can lead to a situation where an LSR releases the label for a FEC that it later needs. For example, if LSR Rd advertises to LSR Ru the label for a FEC for which it is not Ru's next hop, Ru will release the label. If Ru's next hop for the FEC later changes to Rd, it needs the previously released label. To deal with this situation either Ru can explicitly request the label when it needs it, or Rd can periodically readvertise it to Ru. In many situations Ru will know when it needs the label from Rd. For example, when its next hop for the FEC changes to Rd. However, there could be situations when Ru does not. For example, Rd may be attempting to establish an LSP with non-standard properties. Forcing Ru to explicitly request the label in this situation would require it to maintain state about a potential LSP with non-standard properties. In situations where Ru knows it needs the label, it is responsible for explicitly requesting the label by means of a Label Request message. In situations where Ru may not know that it needs the label, Rd is responsible for periodically readvertising the label to Ru. For this version of LDP, the only situation where Ru knows it needs a label for a FEC from Rd is when Rd is its next hop for the FEC, Ru does not have a label from Rd, and the LSP for the FEC is one that can be established with TLVs defined in this document.3.5.8. Label Request Message
An LSR sends the Label Request Message to an LDP peer to request a binding (mapping) for a FEC.
The encoding for the Label Request 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| Label Request (0x0401) | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Parameters |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message ID
32-bit value used to identify this message.
FEC TLV
The FEC for which a label is being requested. See Section "FEC
TLV" for encoding.
Optional Parameters
This variable length field contains 0 or more parameters, each
encoded as a TLV. The optional parameters are:
Optional Parameter Length Value
Hop Count TLV 1 See below
Path Vector TLV variable See below
The encodings for the Hop Count, and Path Vector TLVs can be found
in Section "TLV Encodings for Commonly Used Parameters".
Hop Count
Specifies the running total of the number of LSR hops along the
LSP being setup by the Label Request Message. Section "Hop
Count Procedures" describes how to handle this TLV.
Path Vector
Specifies the LSRs along the LSR being setup by the Label
Request Message. Section "Path Vector Procedures" describes
how to handle this TLV.
3.5.8.1. Label Request Message Procedures
The Request message is used by an upstream LSR to explicitly request
that the downstream LSR assign and advertise a label for a FEC.
An LSR may transmit a Request message under any of the following
conditions:
1. The LSR recognizes a new FEC via the forwarding table, and the
next hop is an LDP peer, and the LSR doesn't already have a
mapping from the next hop for the given FEC.
2. The next hop to the FEC changes, and the LSR doesn't already
have a mapping from that next hop for the given FEC.
Note that if the LSR already has a pending Label Request
message for the new next hop it should not issue an additional
Label Request in response to the next hop change.
3. The LSR receives a Label Request for a FEC from an upstream LDP
peer, the FEC next hop is an LDP peer, and the LSR doesn't
already have a mapping from the next hop.
Note that since a non-merge LSR must setup a separate LSP for
each upstream peer requesting a label, it must send a separate
Label Request for each such peer. A consequence of this is
that a non-merge LSR may have multiple Label Request messages
for a given FEC outstanding at the same time.
The receiving LSR should respond to a Label Request message with a
Label Mapping for the requested label or with a Notification message
indicating why it cannot satisfy the request.
When the FEC for which a label is requested is a Prefix FEC Element
or a Host Address FEC Element, the receiving LSR uses its routing
table to determine its response. Unless its routing table includes
an entry that exactly matches the requested Prefix or Host Address,
the LSR must respond with a No Route Notification message.
The message ID of the Label Request message serves as an identifier
for the Label Request transaction. When the receiving LSR responds
with a Label Mapping message, the mapping message must include a
Label Request/Returned Message ID TLV optional parameter which
includes the message ID of the Label Request message. Note that
since LSRs use Label Request message IDs as transaction identifiers
an LSR should not reuse the message ID of a Label Request message
until the corresponding transaction completes.
This version of the protocol defines the following Status Codes for
the Notification message that signals a request cannot be satisfied:
No Route
The FEC for which a label was requested includes a FEC Element
for which the LSR does not have a route.
No Label Resources
The LSR cannot provide a label because of resource limitations.
When resources become available the LSR must notify the
requesting LSR by sending a Notification message with the Label
Resources Available Status Code.
An LSR that receives a No Label Resources response to a Label
Request message must not issue further Label Request messages
until it receives a Notification message with the Label
Resources Available Status code.
Loop Detected
The LSR has detected a looping Label Request message.
See Appendix A "LDP Label Distribution Procedures" for more details.
3.5.9. Label Abort Request Message
The Label Abort Request message may be used to abort an outstanding
Label Request message.
The encoding for the Label Abort Request 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| Label Abort Req (0x0404) | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label Request Message ID TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Parameters |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message ID
32-bit value used to identify this message.
FEC TLV
Identifies the FEC for which the Label Request is being aborted.
Label Request Message ID TLV
Specifies the message ID of the Label Request message to be
aborted.
Optional Parameters
No optional parameters are defined for the Label Abort Req
message.
3.5.9.1. Label Abort Request Message Procedures
An LSR Ru may send a Label Abort Request message to abort an
outstanding Label Request message for FEC sent to LSR Rd in the
following circumstances:
1. Ru's next hop for FEC has changed from LSR Rd to LSR X; or
2. Ru is a non-merge, non-ingress LSR and has received a Label
Abort Request for FEC from an upstream peer Y.
3. Ru is a merge, non-ingress LSR and has received a Label Abort
Request for FEC from an upstream peer Y and Y is the only
(last) upstream LSR requesting a label for FEC.
There may be other situations where an LSR may choose to abort an
outstanding Label Request message in order to reclaim resource
associated with the pending LSP. However, specification of general
strategies for using the abort mechanism is beyond the scope of LDP.
When an LSR receives a Label Abort Request message, if it has not
previously responded to the Label Request being aborted with a Label
Mapping message or some other Notification message, it must
acknowledge the abort by responding with a Label Request Aborted
Notification message. The Notification must include a Label Request
Message ID TLV that carries the message ID of the aborted Label
Request message.
If an LSR receives a Label Abort Request Message after it has
responded to the Label Request in question with a Label Mapping
message or a Notification message, it ignores the abort request.
If an LSR receives a Label Mapping message in response to a Label
Request message after it has sent a Label Abort Request message to
abort the Label Request, the label in the Label Mapping message is
valid. The LSR may choose to use the label or to release it with a
Label Release message.
An LSR aborting a Label Request message may not reuse the Message ID
for the Label Request message until it receives one of the following
from its peer:
- A Label Request Aborted Notification message acknowledging the
abort;
- A Label Mapping message in response to the Label Request
message being aborted;
- A Notification message in response to the Label Request message
being aborted (e.g., Loop Detected, No Label Resources, etc.).
To protect itself against tardy peers or faulty peer implementations
an LSR may choose to time out receipt of the above. The time out
period should be relatively long (several minutes). If the time out
period elapses with no reply from the peer the LSR may reuse the
Message Id of the Label Request message; if it does so, it should
also discard any record of the outstanding Label Request and Label
Abort messages.
Note that the response to a Label Abort Request message is never
"ordered". That is, the response does not depend on the downstream
state of the LSP setup being aborted. An LSR receiving a Label Abort
Request message must process it immediately, regardless of the
downstream state of the LSP, responding with a Label Request Aborted
Notification or ignoring it, as appropriate.
3.5.10. Label Withdraw Message
An LSR sends a Label Withdraw Message to an LDP peer to signal the
peer that the peer may not continue to use specific FEC-label
mappings the LSR had previously advertised. This breaks the mapping
between the FECs and the labels.
The encoding for the Label Withdraw 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| Label Withdraw (0x0402) | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label TLV (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Parameters |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message ID
32-bit value used to identify this message.
FEC TLV
Identifies the FEC for which the FEC-label mapping is being
withdrawn.
Optional Parameters
This variable length field contains 0 or more parameters, each
encoded as a TLV. The optional parameters are:
Optional Parameter Length Value
Label TLV variable See below
The encoding for Label TLVs are found in Section "Label TLVs".
Label
If present, specifies the label being withdrawn (see procedures
below).
3.5.10.1. Label Withdraw Message Procedures
An LSR transmits a Label Withdraw message under the following
conditions:
1. The LSR no longer recognizes a previously known FEC for which
it has advertised a label.
2. The LSR has decided unilaterally (e.g., via configuration) to
no longer label switch a FEC (or FECs) with the label mapping
being withdrawn.
The FEC TLV specifies the FEC for which labels are to be withdrawn. If no Label TLV follows the FEC, all labels associated with the FEC are to be withdrawn; otherwise only the label specified in the optional Label TLV is to be withdrawn. The FEC TLV may contain the Wildcard FEC Element; if so, it may contain no other FEC Elements. In this case, if the Label Withdraw message contains an optional Label TLV, then the label is to be withdrawn from all FECs to which it is bound. If there is not an optional Label TLV in the Label Withdraw message, then the sending LSR is withdrawing all label mappings previously advertised to the receiving LSR. An LSR that receives a Label Withdraw message must respond with a Label Release message. See Appendix A "LDP Label Distribution Procedures" for more details.3.5.11. Label Release Message
An LSR sends a Label Release message to an LDP peer to signal the peer that the LSR no longer needs specific FEC-label mappings previously requested of and/or advertised by the peer. The encoding for the Label Release 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| Label Release (0x0403) | Message Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | FEC TLV | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Label TLV (optional) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Optional Parameters | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Message ID 32-bit value used to identify this message. FEC TLV Identifies the FEC for which the FEC-label mapping is being released.
Optional Parameters
This variable length field contains 0 or more parameters, each
encoded as a TLV. The optional parameters are:
Optional Parameter Length Value
Label TLV variable See below
The encodings for Label TLVs are found in Section "Label TLVs".
Label
If present, the label being released (see procedures below).
3.5.11.1. Label Release Message Procedures
An LSR transmits a Label Release message to a peer when it is no
longer needs a label previously received from or requested of that
peer.
An LSR must transmit a Label Release message under any of the
following conditions:
1. The LSR which sent the label mapping is no longer the next hop
for the mapped FEC, and the LSR is configured for conservative
operation.
2. The LSR receives a label mapping from an LSR which is not the
next hop for the FEC, and the LSR is configured for
conservative operation.
3. The LSR receives a Label Withdraw message.
Note that if an LSR is configured for "liberal mode", a release
message will never be transmitted in the case of conditions (1) and
(2) as specified above. In this case, the upstream LSR keeps each
unused label, so that it can immediately be used later if the
downstream peer becomes the next hop for the FEC.
The FEC TLV specifies the FEC for which labels are to be released.
If no Label TLV follows the FEC, all labels associated with the FEC
are to be released; otherwise only the label specified in the
optional Label TLV is to be released.
The FEC TLV may contain the Wildcard FEC Element; if so, it may
contain no other FEC Elements. In this case, if the Label Release
message contains an optional Label TLV, then the label is to be
released for all FECs to which it is bound. If there is not an
optional Label TLV in the Label Release message, then the sending LSR is releasing all label mappings previously learned from the receiving LSR. See Appendix A "LDP Label Distribution Procedures" for more details.3.6. Messages and TLVs for Extensibility
Support for LDP extensibility includes the rules for the U and F bits that specify how an LSR should handle unknown TLVs and messages. This section specifies TLVs and messages for vendor-private and experimental use.3.6.1. LDP Vendor-private Extensions
Vendor-private TLVs and messages are used to convey vendor-private information between LSRs.3.6.1.1. LDP Vendor-private TLVs
The Type range 0x3E00 through 0x3EFF is reserved for vendor-private TLVs. The encoding for a vendor-private 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| Type (0x3E00-0x3EFF) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Vendor ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Data.... | ~ ~ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 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 determination as to whether a vendor-private message is
understood is based on the Type and the mandatory Vendor ID field.
F bit
Forward unknown TLV bit. This bit only applies when the U bit is
set and the LDP message containing the unknown TLV is 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.
Type
Type value in the range 0x3E00 through 0x3EFF. Together, the Type
and Vendor Id field specify how the Data field is to be
interpreted.
Length
Specifies the cumulative length in octets of the Vendor ID and
Data fields.
Vendor Id
802 Vendor ID as assigned by the IEEE.
Data
The remaining octets after the Vendor ID in the Value field are
optional vendor-dependent data.
3.6.1.2. LDP Vendor-private Messages
The Message Type range 0x3E00 through 0x3EFF is reserved for vendor- private Messages. 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| Msg Type (0x3E00-0x3EFF) | Message Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Vendor ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + | Remaining 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 determination as to whether a vendor-private message is understood is based on the Msg Type and the Vendor ID parameter. Msg Type Message type value in the range 0x3E00 through 0x3EFF. Together, the Msg Type and the Vendor ID specify how the message is to be interpreted. Message Length Specifies the cumulative length in octets of the Message ID, Vendor ID, Remaining Mandatory Parameters and Optional Parameters.
Message ID
32-bit integer 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 will include this Message Id in the
notification message; see Section "Notification Message".
Vendor ID
802 Vendor ID as assigned by the IEEE.
Remaining Mandatory Parameters
Variable length set of remaining required message parameters.
Optional Parameters
Variable length set of optional message parameters.
3.6.2. LDP Experimental Extensions
LDP support for experimentation is similar to support for vendor-
private extensions with the following differences:
- The Type range 0x3F00 through 0x3FFF is reserved for
experimental TLVs.
- The Message Type range 0x3F00 through 0x3FFF is reserved for
experimental messages.
- The encodings for experimental TLVs and messages are similar to
the vendor-private encodings with the following difference.
Experimental TLVs and messages use an Experiment ID field in
place of a Vendor ID field. The Experiment ID field is used
with the Type or Message Type field to specify the
interpretation of the experimental TLV or Message.
Administration of Experiment IDs is the responsibility of the
experimenters.
3.7. Message Summary
The following are the LDP messages defined in this version of the
protocol.
Message Name Type Section Title
Notification 0x0001 "Notification Message"
Hello 0x0100 "Hello Message"
Initialization 0x0200 "Initialization Message"
KeepAlive 0x0201 "KeepAlive Message"
Address 0x0300 "Address Message"
Address Withdraw 0x0301 "Address Withdraw Message"
Label Mapping 0x0400 "Label Mapping Message"
Label Request 0x0401 "Label Request Message"
Label Withdraw 0x0402 "Label Withdraw Message"
Label Release 0x0403 "Label Release Message"
Label Abort Request 0x0404 "Label Abort Request Message"
Vendor-Private 0x3E00- "LDP Vendor-private Extensions"
0x3EFF
Experimental 0x3F00- "LDP Experimental Extensions"
0x3FFF
3.8. TLV Summary
The following are the TLVs defined in this version of the protocol.
TLV Type Section Title
FEC 0x0100 "FEC TLV"
Address List 0x0101 "Address List TLV"
Hop Count 0x0103 "Hop Count TLV"
Path Vector 0x0104 "Path Vector TLV"
Generic Label 0x0200 "Generic Label TLV"
ATM Label 0x0201 "ATM Label TLV"
Frame Relay Label 0x0202 "Frame Relay Label TLV"
Status 0x0300 "Status TLV"
Extended Status 0x0301 "Notification Message"
Returned PDU 0x0302 "Notification Message"
Returned Message 0x0303 "Notification Message"
Common Hello 0x0400 "Hello Message"
Parameters
IPv4 Transport Address 0x0401 "Hello Message"
Configuration 0x0402 "Hello Message"
Sequence Number
IPv6 Transport Address 0x0403 "Hello Message"
Common Session 0x0500 "Initialization Message"
Parameters
ATM Session Parameters 0x0501 "Initialization Message"
Frame Relay Session 0x0502 "Initialization Message"
Parameters
Label Request 0x0600 "Label Mapping Message"
Message ID
Vendor-Private 0x3E00- "LDP Vendor-private Extensions"
0x3EFF
Experimental 0x3F00- "LDP Experimental Extensions"
0x3FFF
3.9. Status Code Summary
The following are the Status Codes defined in this version of the protocol. The "E" column is the required setting of the Status Code E-bit; the "Status Data" column is the value of the 30-bit Status Data field in the Status Code TLV. Note that the setting of the Status Code F-bit is at the discretion of the LSR originating the Status TLV. Status Code E Status Data Section Title Success 0 0x00000000 "Status TLV" Bad LDP Identifier 1 0x00000001 "Events Signaled by ..." Bad Protocol Version 1 0x00000002 "Events Signaled by ..." Bad PDU Length 1 0x00000003 "Events Signaled by ..." Unknown Message Type 0 0x00000004 "Events Signaled by ..." Bad Message Length 1 0x00000005 "Events Signaled by ..." Unknown TLV 0 0x00000006 "Events Signaled by ..." Bad TLV length 1 0x00000007 "Events Signaled by ..." Malformed TLV Value 1 0x00000008 "Events Signaled by ..." Hold Timer Expired 1 0x00000009 "Events Signaled by ..." Shutdown 1 0x0000000A "Events Signaled by ..." Loop Detected 0 0x0000000B "Loop Detection" Unknown FEC 0 0x0000000C "FEC Procedures" No Route 0 0x0000000D "Label Request Mess ..." No Label Resources 0 0x0000000E "Label Request Mess ..." Label Resources / 0 0x0000000F "Label Request Mess ..." Available Session Rejected/ 1 0x00000010 "Session Initialization" No Hello Session Rejected/ 1 0x00000011 "Session Initialization" Parameters Advertisement Mode Session Rejected/ 1 0x00000012 "Session Initialization" Parameters Max PDU Length Session Rejected/ 1 0x00000013 "Session Initialization" Parameters Label Range KeepAlive Timer 1 0x00000014 "Events Signaled by ..." Expired Label Request Aborted 0 0x00000015 "Label Request Abort ..." Missing Message 0 0x00000016 "Events Signaled by ..." Parameters Unsupported Address 0 0x00000017 "FEC Procedures" Family "Address Message Proc ..."
Session Rejected/ 1 0x00000018 "Session Initialization"
Bad KeepAlive Time
Internal Error 1 0x00000019 "Events Signaled by ..."
3.10. Well-known Numbers
3.10.1. UDP and TCP Ports
The UDP port for LDP Hello messages is 646.
The TCP port for establishing LDP session connections is 646.
3.10.2. Implicit NULL Label
The Implicit NULL label (see [RFC3031]) is represented as a Generic
Label TLV with a Label field value as specified by [RFC3032].
4. IANA Considerations
LDP defines the following name spaces which require management:
- Message Type Name Space.
- TLV Type Name Space.
- FEC Type Name Space.
- Status Code Name Space.
- Experiment ID Name Space.
The following sections provide guidelines for managing these name
spaces.
4.1. Message Type Name Space
LDP divides the name space for message types into three ranges. The
following are the guidelines for managing these ranges:
- Message Types 0x0000 - 0x3DFF. Message types in this range are
part of the LDP base protocol. Following the policies outlined
in [IANA], Message types in this range are allocated through an
IETF Consensus action.
- Message Types 0x3E00 - 0x3EFF. Message types in this range are
reserved for Vendor Private extensions and are the
responsibility of the individual vendors (see Section "LDP
Vendor-private Messages"). IANA management of this range of
the Message Type Name Space is unnecessary.
- Message Types 0x3F00 - 0x3FFF. Message types in this range are
reserved for Experimental extensions and are the responsibility
of the individual experimenters (see Sections "LDP Experimental
Extensions" and "Experiment ID Name Space"). IANA management
of this range of the Message Type Name Space is unnecessary;
however, IANA is responsible for managing part of the
Experiment ID Name Space (see below).
4.2. TLV Type Name Space
LDP divides the name space for TLV types into three ranges. The
following are the guidelines for managing these ranges:
- TLV Types 0x0000 - 0x3DFF. TLV types in this range are part of
the LDP base protocol. Following the policies outlined in
[IANA], TLV types in this range are allocated through an IETF
Consensus action.
- TLV Types 0x3E00 - 0x3EFF. TLV types in this range are
reserved for Vendor Private extensions and are the
responsibility of the individual vendors (see Section "LDP
Vendor-private TLVs"). IANA management of this range of the
TLV Type Name Space is unnecessary.
- TLV Types 0x3F00 - 0x3FFF. TLV types in this range are
reserved for Experimental extensions and are the responsibility
of the individual experimenters (see Sections "LDP Experimental
Extensions" and "Experiment ID Name Space"). IANA management
of this range of the TLV Name Space is unnecessary; however,
IANA is responsible for managing part of the Experiment ID Name
Space (see below).
4.3. FEC Type Name Space
The range for FEC types is 0 - 255.
Following the policies outlined in [IANA], FEC types in the range 0 -
127 are allocated through an IETF Consensus action, types in the
range 128 - 191 are allocated as First Come First Served, and types
in the range 192 - 255 are reserved for Private Use.
4.4. Status Code Name Space
The range for Status Codes is 0x00000000 - 0x3FFFFFFF. Following the policies outlined in [IANA], Status Codes in the range 0x00000000 - 0x1FFFFFFF are allocated through an IETF Consensus action, codes in the range 0x20000000 - 0x3EFFFFFF are allocated as First Come First Served, and codes in the range 0x3F000000 - 0x3FFFFFFF are reserved for Private Use.4.5. Experiment ID Name Space
The range for Experiment Ids is 0x00000000 - 0xffffffff. Following the policies outlined in [IANA], Experiment Ids in the range 0x00000000 - 0xefffffff are allocated as First Come First Served and Experiment Ids in the range 0xf0000000 - 0xffffffff are reserved for Private Use.5. Security Considerations
This section identifies threats to which LDP may be vulnerable and discusses means by which those threats might be mitigated.5.1. Spoofing
There are two types of LDP communication that could be the target of a spoofing attack. 1. Discovery exchanges carried by UDP. LSRs directly connected at the link level exchange Basic Hello messages over the link. The threat of spoofed Basic Hellos can be reduced by: o Accepting Basic Hellos only on interfaces to which LSRs that can be trusted are directly connected. o Ignoring Basic Hellos not addressed to the All Routers on this Subnet multicast group. LSRs not directly connected at the link level may use Extended Hello messages to indicate willingness to establish an LDP session. An LSR can reduce the threat of spoofed Extended Hellos by filtering them and accepting only those originating at sources permitted by an access list.
2. Session communication carried by TCP.
LDP specifies use of the TCP MD5 Signature Option to provide for
the authenticity and integrity of session messages.
[RFC2385] asserts that MD5 authentication is now considered by
some to be too weak for this application. It also points out that
a similar TCP option with a stronger hashing algorithm (it cites
SHA-1 as an example) could be deployed. To our knowledge no such
TCP option has been defined and deployed. However, we note that
LDP can use whatever TCP message digest techniques are available,
and when one stronger than MD5 is specified and implemented,
upgrading LDP to use it would be relatively straightforward.
5.2. Privacy
LDP provides no mechanism for protecting the privacy of label
distribution.
The security requirements of label distribution protocols are
essentially identical to those of the protocols which distribute
routing information. By providing a mechanism to ensure the
authenticity and integrity of its messages LDP provides a level of
security which is at least as good as, though no better than, that
which can be provided by the routing protocols themselves. The more
general issue of whether privacy should be required for routing
protocols is beyond the scope of this document.
One might argue that label distribution requires privacy to address
the threat of label spoofing. However, that privacy would not
protect against label spoofing attacks since data packets carry
labels in the clear. Furthermore, label spoofing attacks can be made
without knowledge of the FEC bound to a label.
To avoid label spoofing attacks, it is necessary to ensure that
labeled data packets are labeled by trusted LSRs and that the labels
placed on the packets are properly learned by the labeling LSRs.
5.3. Denial of Service
LDP provides two potential targets for denial of service (DoS)
attacks:
1. Well known UDP Port for LDP Discovery
An LSR administrator can address the threat of DoS attacks via
Basic Hellos by ensuring that the LSR is directly connected only
to peers which can be trusted to not initiate such an attack.
Interfaces to peers interior to the administrator's domain should
not represent a threat since interior peers are under the
administrator's control. Interfaces to peers exterior to the
domain represent a potential threat since exterior peers are not.
An administrator can reduce that threat by connecting the LSR only
to exterior peers that can be trusted to not initiate a Basic
Hello attack.
DoS attacks via Extended Hellos are potentially a more serious
threat. This threat can be addressed by filtering Extended Hellos
using access lists that define addresses with which extended
discovery is permitted. However, performing the filtering
requires LSR resource.
In an environment where a trusted MPLS cloud can be identified,
LSRs at the edge of the cloud can be used to protect interior LSRs
against DoS attacks via Extended Hellos by filtering out Extended
Hellos originating outside of the trusted MPLS cloud, accepting
only those originating at addresses permitted by access lists.
This filtering protects LSRs in the interior of the cloud but
consumes resources at the edges.
2. Well known TCP port for LDP Session Establishment
Like other control plane protocols that use TCP, LDP may be the
target of DoS attacks, such a SYN attacks. LDP is no more or less
vulnerable to such attacks than other control plane protocols that
use TCP.
The threat of such attacks can be mitigated somewhat by the
following:
o An LSR should avoid promiscuous TCP listens for LDP session
establishment. It should use only listens that are specific
to discovered peers. This enables it to drop attack packets
early in their processing since they are less likely to
match existing or in-progress connections.
o The use of the MD5 option helps somewhat since it prevents a
SYN from being accepted unless the MD5 segment checksum is
valid. However, the receiver must compute the checksum
before it can decide to discard an otherwise acceptable SYN
segment.
o The use of access list mechanisms applied at the boundary of
the MPLS cloud in a manner similar to that suggested above
for Extended Hellos can protect the interior against attacks
originating from outside the cloud.
(next page on part 4)
