draft-ietf-mpls-rfc3036bis-04.txt   rfc5036.txt 
Network Working Group Loa Andersson
Internet Draft Ina Minei
Expiration Date: March 2007 Bob Thomas
Editors
September 2006
LDP Specification
draft-ietf-mpls-rfc3036bis-04.txt
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Source of this document
This document is produced as part of advancing the LDP specification
to draft standard status. The LDP specification was originally
published as RFC 3036 in January 2001. It was produced by the MPLS
working of the IETF and was jointly authored by Loa Andersson, Paul
Doolan, Nancy Feldman, Andre Fredette and Bob Thomas.
Abstract
The architecture for Multi Protocol Label Switching (MPLS) is
described in RFC 3031 [RFC3031]. A fundamental concept in MPLS is
that two Label Switching Routers (LSRs) must agree on the meaning of
the labels used to forward traffic between and through them. This
common understanding is achieved by using a set of procedures, called
a label distribution protocol, by which one LSR informs another of
label bindings it has made. This document defines a set of such
procedures called LDP (for Label Distribution Protocol) by which LSRs
distribute labels to support MPLS forwarding along normally routed
paths.
Table of Contents
Source of this document ............................... 1
1 LDP Overview .......................................... 7
1.1 LDP Peers ............................................. 8
1.2 LDP Message Exchange .................................. 8
1.3 LDP Message Structure ................................. 9
1.4 LDP Error Handling .................................... 9
1.5 LDP Extensibility and Future Compatibility ............ 9
1.6 Specification Language ................................ 9
2 LDP Operation ......................................... 10
2.1 FECs .................................................. 10
2.2 Label Spaces, Identifiers, Sessions and Transport ..... 11
2.2.1 Label Spaces .......................................... 11
2.2.2 LDP Identifiers ....................................... 11
2.2.3 LDP Sessions .......................................... 12
2.2.4 LDP Transport ......................................... 12
2.3 LDP Sessions between non-Directly Connected LSRs ...... 12
2.4 LDP Discovery ......................................... 13
2.4.1 Basic Discovery Mechanism ............................. 13
2.4.2 Extended Discovery Mechanism .......................... 13
2.5 Establishing and Maintaining LDP Sessions ............ 14
2.5.1 LDP Session Establishment ............................. 14
2.5.2 Transport Connection Establishment .................... 15
2.5.3 Session Initialization ................................ 16
2.5.4 Initialization State Machine .......................... 19
2.5.5 Maintaining Hello Adjacencies ......................... 21
2.5.6 Maintaining LDP Sessions .............................. 21
2.6 Label Distribution and Management ..................... 22
2.6.1 Label Distribution Control Mode ....................... 22
2.6.1.1 Independent Label Distribution Control ................ 22
2.6.1.2 Ordered Label Distribution Control .................... 22
2.6.2 Label Retention Mode .................................. 23
2.6.2.1 Conservative Label Retention Mode ..................... 23
2.6.2.2 Liberal Label Retention Mode .......................... 24
2.6.3 Label Advertisement Mode .............................. 24
2.7 LDP Identifiers and Next Hop Addresses ................ 24
2.8 Loop Detection ........................................ 25
2.8.1 Label Request Message ................................. 26
2.8.2 Label Mapping Message ................................. 27
2.8.3 Discussion ............................................ 29
2.9 Authenticity and Integrity of LDP Messages ............ 29
2.9.1 TCP MD5 Signature Option .............................. 30
2.9.2 LDP Use of TCP MD5 Signature Option ................... 31
2.10 Label Distribution for Explicitly Routed LSPs ......... 32
3 Protocol Specification ................................ 32
3.1 LDP PDUs .............................................. 32
3.2 LDP Procedures ........................................ 33
3.3 Type-Length-Value Encoding ............................ 34
3.4 TLV Encodings for Commonly Used Parameters ............ 35
3.4.1 FEC TLV ............................................... 36
3.4.1.1 FEC Procedures ........................................ 37
3.4.2 Label TLVs ............................................ 38
3.4.2.1 Generic Label TLV ..................................... 38
3.4.2.2 ATM Label TLV ......................................... 38
3.4.2.3 Frame Relay Label TLV ................................. 39
3.4.3 Address List TLV ...................................... 40
3.4.4 Hop Count TLV ......................................... 41
3.4.4.1 Hop Count Procedures .................................. 41
3.4.5 Path Vector TLV ....................................... 43
3.4.5.1 Path Vector Procedures ................................ 43
3.4.5.1.1 Label Request Path Vector ............................. 43
3.4.5.1.2 Label Mapping Path Vector ............................. 44
3.4.6 Status TLV ............................................ 45
3.5 LDP Messages .......................................... 46
3.5.1 Notification Message .................................. 49
3.5.1.1 Notification Message Procedures ....................... 50
3.5.1.2 Events Signaled by Notification Messages .............. 50
3.5.1.2.1 Malformed PDU or Message .............................. 51
3.5.1.2.2 Unknown or Malformed TLV .............................. 52
3.5.1.2.3 Session KeepAlive Timer Expiration .................... 52
3.5.1.2.4 Unilateral Session Shutdown ........................... 52
3.5.1.2.5 Initialization Message Events ......................... 52
3.5.1.2.6 Events Resulting From Other Messages .................. 53
3.5.1.2.7 Internal Errors ....................................... 53
3.5.1.2.8 Miscellaneous Events .................................. 53
3.5.2 Hello Message ......................................... 53
3.5.2.1 Hello Message Procedures .............................. 55
3.5.3 Initialization Message ................................ 57
3.5.3.1 Initialization Message Procedures ..................... 65
3.5.4 KeepAlive Message ..................................... 65
3.5.4.1 KeepAlive Message Procedures .......................... 65
3.5.5 Address Message ....................................... 66
3.5.5.1 Address Message Procedures ............................ 66
3.5.6 Address Withdraw Message .............................. 67
3.5.6.1 Address Withdraw Message Procedures ................... 68
3.5.7 Label Mapping Message ................................. 68
3.5.7.1 Label Mapping Message Procedures ...................... 69
3.5.7.1.1 Independent Control Mapping ........................... 70
3.5.7.1.2 Ordered Control Mapping ............................... 70
3.5.7.1.3 Downstream on Demand Label Advertisement .............. 71
3.5.7.1.4 Downstream Unsolicited Label Advertisement ............ 71
3.5.8 Label Request Message ................................. 72
3.5.8.1 Label Request Message Procedures ...................... 73
3.5.9 Label Abort Request Message ........................... 74
3.5.9.1 Label Abort Request Message Procedures ................ 75
3.5.10 Label Withdraw Message ................................ 77
3.5.10.1 Label Withdraw Message Procedures ..................... 78
3.5.11 Label Release Message ................................. 78
3.5.11.1 Label Release Message Procedures ...................... 79
3.6 Messages and TLVs for Extensibility ................... 80
3.6.1 LDP Vendor-private Extensions ......................... 80
3.6.1.1 LDP Vendor-private TLVs ............................... 80
3.6.1.2 LDP Vendor-private Messages ........................... 82
3.6.2 LDP Experimental Extensions ........................... 83
3.7 Message Summary ....................................... 84
3.8 TLV Summary ........................................... 84
3.9 Status Code Summary ................................... 85
3.10 Well-known Numbers .................................... 86
3.10.1 UDP and TCP Ports ..................................... 86
3.10.2 Implicit NULL Label ................................... 86
4 IANA Considerations ................................... 87
4.1 Message Type Name Space ............................... 87
4.2 TLV Type Name Space ................................... 87
4.3 FEC Type Name Space ................................... 88
4.4 Status Code Name Space ................................ 88
4.5 Experiment ID Name Space .............................. 88
5 Security Considerations ............................... 89
5.1 Spoofing .............................................. 89
5.2 Privacy ............................................... 90
5.3 Denial of Service ..................................... 90
6 Areas for Future Study ................................ 92
7 Changes from RFC3036 .................................. 93
8 Acknowledgments ....................................... 95
9 References ............................................ 96
9.1 Normative references .................................. 96
9.2 Non-normative references .............................. 96
10 Intellectual Property Statement ....................... 97
11 Editors' Addresses .................................... 98
Appendix A LDP Label Distribution Procedures ..................... 99
A.1 Handling Label Distribution Events .................... 101
A.1.1 Receive Label Request ................................. 102
A.1.2 Receive Label Mapping ................................. 105
A.1.3 Receive Label Abort Request ........................... 111
A.1.4 Receive Label Release ................................. 113
A.1.5 Receive Label Withdraw ................................ 115
A.1.6 Recognize New FEC ..................................... 117
A.1.7 Detect Change in FEC Next Hop ......................... 119
A.1.8 Receive Notification / Label Request Aborted .......... 122
A.1.9 Receive Notification / No Label Resources ............. 123
A.1.10 Receive Notification / No Route ....................... 123
A.1.11 Receive Notification / Loop Detected .................. 124
A.1.12 Receive Notification / Label Resources Available ...... 125
A.1.13 Detect local label resources have become available .... 126
A.1.14 LSR decides to no longer label switch a FEC ........... 127
A.1.15 Timeout of deferred label request ..................... 127
A.2 Common Label Distribution Procedures .................. 128
A.2.1 Send_Label ............................................ 128
A.2.2 Send_Label_Request .................................... 130
A.2.3 Send_Label_Withdraw ................................... 131
A.2.4 Send_Notification ..................................... 131
A.2.5 Send_Message .......................................... 132
A.2.6 Check_Received_Attributes ............................. 132
A.2.7 Prepare_Label_Request_Attributes ...................... 133
A.2.8 Prepare_Label_Mapping_Attributes ...................... 135
Full Copyright Statement .............................. 138
1. LDP Overview
The MPLS architecture [RFC3031] defines a label distribution protocol
as a set of procedures by which one Label Switched Router (LSR)
informs another of the meaning of labels used to forward traffic
between and through them.
The MPLS architecture does not assume a single label distribution
protocol. In fact, a number of different label distribution proto-
cols are being standardized. Existing protocols have been extended
so that label distribution can be piggybacked on them. New protocols
have also been defined for the explicit purpose of distributing
labels. The MPLS architecture discusses some of the considerations
when choosing a label distribution protocol for use in particular
MPLS applications such as Traffic Engineering [RFC2702].
The Label Distribution Protocol (LDP) is a protocol defined for dis-
tributing labels. It was originally published as RFC3036 in January
2001. It was produced by the MPLS working of the IETF and was jointly
authored by Loa Andersson, Paul Doolan, Nancy Feldman, Andre Fredette
and Bob Thomas.
LDP is a protocol defined for distributing labels. It is the set of
procedures and messages by which Label Switched Routers (LSRs) estab-
lish Label Switched Paths (LSPs) through a network by mapping net-
work-layer routing information directly to data-link layer switched
paths. These LSPs may have an endpoint at a directly attached neigh-
bor (comparable to IP hop-by-hop forwarding), or may have an endpoint
at a network egress node, enabling switching via all intermediary
nodes.
LDP associates a Forwarding Equivalence Class (FEC) [RFC3031] with
each LSP it creates. The FEC associated with an LSP specifies which
packets are "mapped" to that LSP. LSPs are extended through a net-
work as each LSR "splices" incoming labels for a FEC to the outgoing
label assigned to the next hop for the given FEC.
More information about the applicability of LDP can be found in
[RFC3037].
This document assumes (but does not require) familiarity with the
MPLS architecture [RFC3031]. Note that [RFC3031] includes a glossary
of MPLS terminology, such as ingress, label switched path, etc.
1.1. LDP Peers
Two LSRs which use LDP to exchange label/FEC mapping information are
known as "LDP Peers" with respect to that information and we speak of
there being an "LDP Session" between them. A single LDP session
allows each peer to learn the other's label mappings; i.e., the pro-
tocol is bi-directional.
1.2. LDP Message Exchange
There are four categories of LDP messages:
1. Discovery messages, used to announce and maintain the presence
of an LSR in a network.
2. Session messages, used to establish, maintain, and terminate
sessions between LDP peers.
3. Advertisement messages, used to create, change, and delete
label mappings for FECs.
4. to signal error information.
Discovery messages provide a mechanism whereby LSRs indicate their
presence in a network by sending a Hello message periodically. This
is transmitted as a UDP packet to the LDP port at the `all routers on
this subnet' group multicast address. When an LSR chooses to estab-
lish a session with another LSR learned via the Hello message, it
uses the LDP initialization procedure over TCP transport. Upon suc-
cessful completion of the initialization procedure, the two LSRs are
LDP peers, and may exchange advertisement messages.
When to request a label or advertise a label mapping to a peer is
largely a local decision made by an LSR. In general, the LSR
requests a label mapping from a neighboring LSR when it needs one,
and advertises a label mapping to a neighboring LSR when it wishes
the neighbor to use a label.
Correct operation of LDP requires reliable and in order delivery of
messages. To satisfy these requirements LDP uses the TCP transport
for session, advertisement and notification messages; i.e., for
everything but the UDP-based discovery mechanism.
1.3. LDP Message Structure
All LDP messages have a common structure that uses a Type-Length-
Value (TLV) encoding scheme; see Section "Type-Length-Value" encod-
ing. The Value part of a TLV-encoded object, or TLV for short, may
itself contain one or more TLVs.
1.4. LDP Error Handling
LDP errors and other events of interest are signaled to an LDP peer
by notification messages.
There are two kinds of LDP notification messages:
1. Error notifications, used to signal fatal errors. If an LSR
receives an error notification from a peer for an LDP session,
it terminates the LDP session by closing the TCP transport con-
nection for the session and discarding all label mappings
learned via the session.
2. Advisory notifications, used to pass an LSR information about
the LDP session or the status of some previous message received
from the peer.
1.5. LDP Extensibility and Future Compatibility
Functionality may be added to LDP in the future. It is likely that
future functionality will utilize new messages and object types
(TLVs). It may be desirable to employ such new messages and TLVs
within a network using older implementations that do not recognize
them. While it is not possible to make every future enhancement
backwards compatible, some prior planning can ease the introduction
of new capabilities. This specification defines rules for handling
unknown message types and unknown TLVs for this purpose.
1.6. Specification Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. LDP Operation
2.1. FECs
It is necessary to precisely specify which packets may be mapped to
each LSP. This is done by providing a FEC specification for each
LSP. The FEC identifies the set of IP packets which may be mapped to
that LSP.
Each FEC is specified as a set of one or more FEC elements. Each FEC
element identifies a set of packets which may be mapped to the corre-
sponding LSP. When an LSP is shared by multiple FEC elements, that
LSP is terminated at (or before) the node where the FEC elements can
no longer share the same path.
This specification defines a single type of FEC element, the "Address
Prefix FEC element". This element is an address prefix of any length
from 0 to a full address, inclusive.
Additional FEC elements may be defined, as needed, by other specifi-
cations.
In the remainder of this section we give the rules to be used for
mapping packets to LSPs that have been set up using an Address Prefix
FEC element.
We say that a particular address "matches" a particular address pre-
fix if and only if that address begins with that prefix. We also say
that a particular packet matches a particular LSP if and only if that
LSP has an Address Prefix FEC element which matches the packet's des-
tination address. With respect to a particular packet and a particu-
lar LSP, we refer to any Address Prefix FEC element which matches the
packet as the "matching prefix".
The procedure for mapping a particular packet to a particular LSP
uses the following rules. Each rule is applied in turn until the
packet can be mapped to an LSP.
- If a packet matches exactly one LSP, the packet is mapped to that
LSP.
- If a packet matches multiple LSPs, it is mapped to the LSP whose
matching prefix is the longest. If there is no one LSP whose
matching prefix is longest, the packet is mapped to one from the
set of LSPs whose matching prefix is longer than the others. The
procedure for selecting one of those LSPs is beyond the scope of
this document.
- If it is known that a packet must traverse a particular egress
router, and there is an LSP which has an Address Prefix FEC ele-
ment which is a /32 address of that router, then the packet is
mapped to that LSP. The procedure for obtaining this knowledge
is beyond the scope of this document.
The procedure for determining that a packet must traverse a particu-
lar egress router is beyond the scope of this document. (As an exam-
ple, if one is running a link state routing algorithm, it may be pos-
sible to obtain this information from the link state data base. As
another example, if one is running BGP, it may be possible to obtain
this information from the BGP next hop attribute of the packet's
route.)
2.2. Label Spaces, Identifiers, Sessions and Transport
2.2.1. Label Spaces
The notion of "label space" is useful for discussing the assignment
and distribution of labels. There are two types of label spaces:
- Per interface label space. Interface-specific incoming labels
are used for interfaces that use interface resources for labels.
An example of such an interface is a label-controlled ATM inter-
face that uses VCIs as labels, or a Frame Relay interface that
uses DLCIs (Data Link Connection Identifiers) as labels.
Note that the use of a per interface label space only makes sense
when the LDP peers are "directly connected" over an interface,
and the label is only going to be used for traffic sent over that
interface.
- Per platform label space. Platform-wide incoming labels are used
for interfaces that can share the same labels.
2.2.2. LDP Identifiers
An LDP identifier is a six octet quantity used to identify an LSR
label space. The first four octets identify the LSR and must be a
globally unique value, such as a 32-bit router Id assigned to the
LSR. The last two octets identify a specific label space within the
LSR. The last two octets of LDP Identifiers for platform-wide label
spaces are always both zero. This document uses the following print
representation for LDP Identifiers:
<LSR Id> : <label space id>
e.g., lsr171:0, lsr19:2.
Note that an LSR that manages and advertises multiple label spaces
uses a different LDP Identifier for each such label space.
A situation where an LSR would need to advertise more than one label
space to a peer and hence use more than one LDP Identifier occurs
when the LSR has two links to the peer and both are ATM (and use per
interface labels). Another situation would be where the LSR had two
links to the peer, one of which is ethernet (and uses per platform
labels) and the other of which is ATM.
2.2.3. LDP Sessions
LDP sessions exist between LSRs to support label exchange between
them.
When an LSR uses LDP to advertise more than one label space to
another LSR it uses a separate LDP session for each label space.
2.2.4. LDP Transport
LDP uses TCP as a reliable transport for sessions.
When multiple LDP sessions are required between two LSRs there is
one TCP session for each LDP session.
2.3. LDP Sessions between non-Directly Connected LSRs
LDP sessions between LSRs that are not directly connected at the link
level may be desirable in some situations.
For example, consider a "traffic engineering" application where LSRa
sends traffic matching some criteria via an LSP to non-directly con-
nected LSRb rather than forwarding the traffic along its normally
routed path.
The path between LSRa and LSRb would include one or more intermediate
LSRs (LSR1,...LSRn). An LDP session between LSRa and LSRb would
enable LSRb to label switch traffic arriving on the LSP from LSRa by
providing LSRb means to advertise labels for this purpose to LSRa.
In this situation LSRa would apply two labels to traffic it forwards
on the LSP to LSRb: a label learned from LSR1 to forward traffic
along the LSP path from LSRa to LSRb; and a label learned from LSRb
to enable LSRb to label switch traffic arriving on the LSP.
LSRa first adds the label learned via its LDP session with LSRb to
the packet label stack (either by replacing the label on top of the
packet label stack with it if the packet arrives labeled or by push-
ing it if the packet arrives unlabeled). Next, it pushes the label
for the LSP learned from LSR1 onto the label stack.
2.4. LDP Discovery
LDP discovery is a mechanism that enables an LSR to discover poten-
tial LDP peers. Discovery makes it unnecessary to explicitly config-
ure an LSR's label switching peers.
There are two variants of the discovery mechanism:
- A basic discovery mechanism used to discover LSR neighbors that
are directly connected at the link level.
- An extended discovery mechanism used to locate LSRs that are not
directly connected at the link level.
2.4.1. Basic Discovery Mechanism
To engage in LDP Basic Discovery on an interface an LSR periodically
sends LDP Link Hellos out the interface. LDP Link Hellos are sent as
UDP packets addressed to the well-known LDP discovery port for the
"all routers on this subnet" group multicast address.
An LDP Link Hello sent by an LSR carries the LDP Identifier for the
label space the LSR intends to use for the interface and possibly
additional information.
Receipt of an LDP Link Hello on an interface identifies a "Hello
adjacency" with a potential LDP peer reachable at the link level on
the interface as well as the label space the peer intends to use for
the interface.
2.4.2. Extended Discovery Mechanism
LDP sessions between non-directly connected LSRs are supported by LDP
Extended Discovery.
To engage in LDP Extended Discovery an LSR periodically sends LDP
Targeted Hellos to a specific address. LDP Targeted Hellos are sent
as UDP packets addressed to the well-known LDP discovery port at the
specific address.
An LDP Targeted Hello sent by an LSR carries the LDP Identifier for
the label space the LSR intends to use and possibly additional
optional information.
Extended Discovery differs from Basic Discovery in the following
ways:
- A Targeted Hello is sent to a specific address rather than to the
"all routers" group multicast address for the outgoing interface.
- Unlike Basic Discovery, which is symmetric, Extended Discovery is
asymmetric.
One LSR initiates Extended Discovery with another targeted LSR,
and the targeted LSR decides whether to respond to or ignore the
Targeted Hello. A targeted LSR that chooses to respond does so
by periodically sending Targeted Hellos to the initiating LSR.
Receipt of an LDP Targeted Hello identifies a "Hello adjacency" with
a potential LDP peer reachable at the network level and the label
space the peer intends to use.
2.5. Establishing and Maintaining LDP Sessions
2.5.1. LDP Session Establishment
The exchange of LDP Discovery Hellos between two LSRs triggers LDP
session establishment. Session establishment is a two step process:
- Transport connection establishment.
- Session initialization
The following describes establishment of an LDP session between LSRs
LSR1 and LSR2 from LSR1's point of view. It assumes the exchange of
Hellos specifying label space LSR1:a for LSR1 and label space LSR2:b
for LSR2.
2.5.2. Transport Connection Establishment
The exchange of Hellos results in the creation of a Hello adjacency
at LSR1 that serves to bind the link (L) and the label spaces LSR1:a
and LSR2:b.
1. If LSR1 does not already have an LDP session for the exchange of
label spaces LSR1:a and LSR2:b it attempts to open a TCP con-
nection for a new LDP session with LSR2.
LSR1 determines the transport addresses to be used at its end
(A1) and LSR2's end (A2) of the LDP TCP connection. Address A1
is determined as follows:
a. If LSR1 uses the Transport Address optional object (TLV) in
Hello's it sends to LSR2 to advertise an address, A1 is the
address LSR1 advertises via the optional object;
b. If LSR1 does not use the Transport Address optional object,
A1 is the source address used in Hellos it sends to LSR2.
Similarly, address A2 is determined as follows:
a. If LSR2 uses the Transport Address optional object, A2 is
the address LSR2 advertises via the optional object;
b. If LSR2 does not use the Transport Address optional object,
A2 is the source address in Hellos received from LSR2.
2. LSR1 determines whether it will play the active or passive role
in session establishment by comparing addresses A1 and A2 as
unsigned integers. If A1 > A2, LSR1 plays the active role;
otherwise it is passive.
The procedure for comparing A1 and A2 as unsigned integers is:
- If A1 and A2 are not in the same address family, they are
incomparable, and no session can be established.
- Let U1 be the abstract unsigned integer obtained by treat-
ing A1 as a sequence of bytes, where the byte which appears
earliest in the message is the most significant byte of the
integer and the byte which appears latest in the message is
the least significant byte of the integer.
Let U2 be the abstract unsigned integer obtained from A2 in
a similar manner.
- Compare U1 with U2. If U1 > U2, then A1 > A2; if U1 < U2,
then A1 < A2.
3. If LSR1 is active, it attempts to establish the LDP TCP connec-
tion by connecting to the well-known LDP port at address A2.
If LSR1 is passive, it waits for LSR2 to establish the LDP TCP
connection to its well-known LDP port.
Note that when an LSR sends a Hello it selects the transport address
for its end of the session connection and uses the Hello to advertise
the address, either explicitly by including it in an optional Trans-
port Address TLV or implicitly by omitting the TLV and using it as
the Hello source address.
An LSR MUST advertise the same transport address in all Hellos that
advertise the same label space. This requirement ensures that two
LSRs linked by multiple Hello adjacencies using the same label spaces
play the same connection establishment role for each adjacency.
2.5.3. Session Initialization
After LSR1 and LSR2 establish a transport connection they negotiate
session parameters by exchanging LDP Initialization messages. The
parameters negotiated include LDP protocol version, label distribu-
tion method, timer values, VPI/VCI (Virtual Path Identifier/ Virtual
Channel Identifier) ranges for label controlled ATM, DLCI (Data Link
Connection Identifier) ranges for label controlled Frame Relay, etc.
Successful negotiation completes establishment of an LDP session
between LSR1 and LSR2 for the advertisement of label spaces LSR1:a
and LSR2:b.
The following describes the session initialization from LSR1's point
of view.
After the connection is established, if LSR1 is playing the active
role, it initiates negotiation of session parameters by sending an
Initialization message to LSR2. If LSR1 is passive, it waits for
LSR2 to initiate the parameter negotiation.
In general when there are multiple links between LSR1 and LSR2 and
multiple label spaces to be advertised by each, the passive LSR can-
not know which label space to advertise over a newly established TCP
connection until it receives the LDP Initialization message on the
connection. The Initialization message carries both the LDP Identi-
fier for the sender's (active LSR's) label space and the LDP Identi-
fier for the receiver's (passive LSR's) label space.
By waiting for the Initialization message from its peer the passive
LSR can match the label space to be advertised by the peer (as deter-
mined from the LDP Identifier in the PDU header for the Initializa-
tion message) with a Hello adjacency previously created when Hellos
were exchanged.
1. When LSR1 plays the passive role:
a. If LSR1 receives an Initialization message it attempts to
match the LDP Identifier carried by the message PDU with a
Hello adjacency.
b. If there is a matching Hello adjacency, the adjacency speci-
fies the local label space for the session.
Next LSR1 checks whether the session parameters proposed in
the message are acceptable. If they are, LSR1 replies with
an Initialization message of its own to propose the parame-
ters it wishes to use and a KeepAlive message to signal
acceptance of LSR2's parameters. If the parameters are not
acceptable, LSR1 responds by sending a Session
Rejected/Parameters Error Notification message and closing
the TCP connection.
c. If LSR1 cannot find a matching Hello adjacency it sends a
Session Rejected/No Hello Error Notification message and
closes the TCP connection.
d. If LSR1 receives a KeepAlive in response to its Initializa-
tion message, the session is operational from LSR1's point
of view.
e. If LSR1 receives an Error Notification message, LSR2 has
rejected its proposed session and LSR1 closes the TCP con-
nection.
2. When LSR1 plays the active role:
a. If LSR1 receives an Error Notification message, LSR2 has
rejected its proposed session and LSR1 closes the TCP con-
nection.
b. If LSR1 receives an Initialization message, it checks
whether the session parameters are acceptable. If so, it
replies with a KeepAlive message. If the session parame-
ters are unacceptable, LSR1 sends a Session Rejected/Param-
eters Error Notification message and closes the connection.
c. If LSR1 receives a KeepAlive message, LSR2 has accepted its
proposed session parameters.
d. When LSR1 has received both an acceptable Initialization
message and a KeepAlive message the session is operational
from LSR1's point of view.
Until the LDP session is established, no other messages
except those listed in the procedures above may be
exchanged, and the rules for processing the U-bit in LDP
messages are overridden. If a message other than those
listed in the procedures above is received, a Shutdown msg
MUST be transmitted and the transport connection MUST be
closed.
It is possible for a pair of incompatibly configured LSRs that
disagree on session parameters to engage in an endless sequence
of messages as each NAKs the other's Initialization messages with
Error Notification messages.
An LSR MUST throttle its session setup retry attempts with an
exponential backoff in situations where Initialization messages
are being NAK'd. It is also recommended that an LSR detecting
such a situation take action to notify an operator.
The session establishment setup attempt following a NAK'd Ini-
tialization message MUST be delayed no less than 15 seconds, and
subsequent delays MUST grow to a maximum delay of no less than 2
minutes. The specific session establishment action that must be
delayed is the attempt to open the session transport connection
by the LSR playing the active role.
The throttled sequence of Initialization NAKs is unlikely to
cease until operator intervention reconfigures one of the LSRs.
After such a configuration action there is no further need to
throttle subsequent session establishment attempts (until their
initialization messages are NAK'd).
Due to the asymmetric nature of session establishment, reconfigu-
ration of the passive LSR will go unnoticed by the active LSR
without some further action. Section "Hello Message" describes
an optional mechanism an LSR can use to signal potential LDP
peers that it has been reconfigured.
2.5.4. Initialization State Machine
It is convenient to describe LDP session negotiation behavior in
terms of a state machine. We define the LDP state machine to have
five possible states and present the behavior as a state transition
table and as a state transition diagram. Note that a shutdown message
is implemented as a notification message with a status TLV indicating
a fatal error.
Session Initialization State Transition Table
STATE EVENT NEW STATE
NON EXISTENT Session TCP connection established INITIALIZED
established
INITIALIZED Transmit Initialization msg OPENSENT
(Active Role)
Receive acceptable OPENREC
Initialization msg
(Passive Role )
Action: Transmit Initialization
msg and KeepAlive msg
Receive Any other LDP msg NON EXISTENT
Action: Transmit Error Notification msg
(NAK) and close transport connection
OPENREC Receive KeepAlive msg OPERATIONAL
Receive Any other LDP msg NON EXISTENT
Action: Transmit Error Notification msg
(NAK) and close transport connection
OPENSENT Receive acceptable OPENREC
Initialization msg
Action: Transmit KeepAlive msg
Receive Any other LDP msg NON EXISTENT
Action: Transmit Error Notification msg
(NAK) and close transport connection
OPERATIONAL Receive Shutdown msg NON EXISTENT
Action: Transmit Shutdown msg and
close transport connection
Receive other LDP msgs OPERATIONAL
Timeout NON EXISTENT
Action: Transmit Shutdown msg and
close transport connection
Session Initialization State Transition Diagram
+------------+
| |
+------------>|NON EXISTENT|<--------------------+
| | | |
| +------------+ |
| Session | ^ |
| connection | | |
| established | | Rx any LDP msg except |
| V | Init msg or Timeout |
| +-----------+ |
Rx Any other | | | |
msg or | |INITIALIZED| |
Timeout / | +---| |-+ |
Tx NAK msg | | +-----------+ | |
| | (Passive Role) | (Active Role) |
| | Rx Acceptable | Tx Init msg |
| | Init msg / | |
| | Tx Init msg | |
| | Tx KeepAlive | |
| V msg V |
| +-------+ +--------+ |
| | | | | |
+---|OPENREC| |OPENSENT|----------------->|
+---| | | | Rx Any other msg |
| +-------+ +--------+ or Timeout |
Rx KeepAlive | ^ | Tx NAK msg |
msg | | | |
| | | Rx Acceptable |
| | | Init msg / |
| +----------------+ Tx KeepAlive msg |
| |
| +-----------+ |
+----->| | |
|OPERATIONAL| |
| |---------------------------->+
+-----------+ Rx Shutdown msg
All other | ^ or Timeout /
LDP msgs | | Tx Shutdown msg
| |
+---+
2.5.5. Maintaining Hello Adjacencies
An LDP session with a peer has one or more Hello adjacencies.
An LDP session has multiple Hello adjacencies when a pair of LSRs is
connected by multiple links that share the same label space; for
example, multiple PPP links between a pair of routers. In this situ-
ation the Hellos an LSR sends on each such link carry the same LDP
Identifier.
LDP includes mechanisms to monitor the necessity of an LDP session
and its Hello adjacencies.
LDP uses the regular receipt of LDP Discovery Hellos to indicate a
peer's intent to use the label space identified by the Hello. An LSR
maintains a hold timer with each Hello adjacency which it restarts
when it receives a Hello that matches the adjacency. If the timer
expires without receipt of a matching Hello from the peer, LDP con-
cludes that the peer no longer wishes to label switch using that
label space for that link (or target, in the case of Targeted Hellos)
or that the peer has failed. The LSR then deletes the Hello adja-
cency. When the last Hello adjacency for a LDP session is deleted,
the LSR terminates the LDP session by sending a Notification message
and closing the transport connection.
2.5.6. Maintaining LDP Sessions
LDP includes mechanisms to monitor the integrity of the LDP session.
LDP uses the regular receipt of LDP PDUs on the session transport
connection to monitor the integrity of the session. An LSR maintains
a KeepAlive timer for each peer session which it resets whenever it
receives an LDP PDU from the session peer. If the KeepAlive timer
expires without receipt of an LDP PDU from the peer the LSR concludes
that the transport connection is bad or that the peer has failed, and
it terminates the LDP session by closing the transport connection.
After an LDP session has been established, an LSR must arrange that
its peer receive an LDP PDU from it at least every KeepAlive time
period to ensure the peer restarts the session KeepAlive timer. The
LSR may send any protocol message to meet this requirement. In cir-
cumstances where an LSR has no other information to communicate to
its peer, it sends a KeepAlive message.
An LSR may choose to terminate an LDP session with a peer at any
time. Should it choose to do so, it informs the peer with a Shutdown
message.
2.6. Label Distribution and Management
The MPLS architecture [RFC3031] allows an LSR to distribute a FEC
label binding in response to an explicit request from another LSR.
This is known as Downstream On Demand label distribution. It also
allows an LSR to distribute label bindings to LSRs that have not
explicitly requested them. [RFC3031] calls this method of label dis-
tribution Unsolicited Downstream; this document uses the term Down-
stream Unsolicited.
Both of these label distribution techniques may be used in the same
network at the same time. However, for any given LDP session, each
LSR must be aware of the label distribution method used by its peer
in order to avoid situations where one peer using Downstream Unso-
licited label distribution assumes its peer is also. See Section
"Downstream on Demand label Advertisement".
2.6.1. Label Distribution Control Mode
The behavior of the initial setup of LSPs is determined by whether
the LSR is operating with independent or ordered LSP control. An LSR
may support both types of control as a configurable option.
2.6.1.1. Independent Label Distribution Control
When using independent LSP control, each LSR may advertise label map-
pings to its neighbors at any time it desires. For example, when
operating in independent Downstream on Demand mode, an LSR may answer
requests for label mappings immediately, without waiting for a label
mapping from the next hop. When operating in independent Downstream
Unsolicited mode, an LSR may advertise a label mapping for a FEC to
its neighbors whenever it is prepared to label-switch that FEC.
A consequence of using independent mode is that an upstream label can
be advertised before a downstream label is received.
2.6.1.2. Ordered Label Distribution Control
When using LSP ordered control, an LSR may initiate the transmission
of a label mapping only for a FEC for which it has a label mapping
for the FEC next hop, or for which the LSR is the egress. For each
FEC for which the LSR is not the egress and no mapping exists, the
LSR MUST wait until a label from a downstream LSR is received before
mapping the FEC and passing corresponding labels to upstream LSRs.
An LSR may be an egress for some FECs and a non-egress for others.
An LSR may act as an egress LSR, with respect to a particular FEC,
under any of the following conditions:
1. The FEC refers to the LSR itself (including one of its directly
attached interfaces).
2. The next hop router for the FEC is outside of the Label Switch-
ing Network.
3. FEC elements are reachable by crossing a routing domain bound-
ary, such as another area for OSPF summary networks, or another
autonomous system for OSPF AS externals and BGP routes
[RFC2328] [RFC4271].
Note that whether an LSR is an egress for a given FEC may change over
time, depending on the state of the network and LSR configuration
settings.
2.6.2. Label Retention Mode
The MPLS architecture [RFC3031] introduces the notion of label reten-
tion mode which specifies whether an LSR maintains a label binding
for a FEC learned from a neighbor that is not its next hop for the
FEC.
2.6.2.1. Conservative Label Retention Mode
In Downstream Unsolicited advertisement mode, label mapping adver-
tisements for all routes may be received from all peer LSRs. When
using conservative label retention, advertised label mappings are
retained only if they will be used to forward packets (i.e., if they
are received from a valid next hop according to routing). If operat-
ing in Downstream on Demand mode, an LSR will request label mappings
only from the next hop LSR according to routing. Since Downstream on
Demand mode is primarily used when label conservation is desired
(e.g., an ATM switch with limited cross connect space), it is typi-
cally used with the conservative label retention mode.
The main advantage of the conservative mode is that only the labels
that are required for the forwarding of data are allocated and main-
tained. This is particularly important in LSRs where the label space
is inherently limited, such as in an ATM switch. A disadvantage of
the conservative mode is that if routing changes the next hop for a
given destination, a new label must be obtained from the new next hop
before labeled packets can be forwarded.
2.6.2.2. Liberal Label Retention Mode
In Downstream Unsolicited advertisement mode, label mapping adver-
tisements for all routes may be received from all LDP peers. When
using liberal label retention, every label mappings received from a
peer LSR is retained regardless of whether the LSR is the next hop
for the advertised mapping. When operating in Downstream on Demand
mode with liberal label retention, an LSR might choose to request
label mappings for all known prefixes from all peer LSRs. Note, how-
ever, that Downstream on Demand mode is typically used by devices
such as ATM switch-based LSRs for which the conservative approach is
recommended.
The main advantage of the liberal label retention mode is that reac-
tion to routing changes can be quick because labels already exist.
The main disadvantage of the liberal mode is that unneeded label map-
pings are distributed and maintained.
2.6.3. Label Advertisement Mode
Each interface on an LSR is configured to operate in either Down-
stream Unsolicited or Downstream on Demand advertisement mode. LSRs
exchange advertisement modes during initialization. The major dif-
ference between Downstream Unsolicited and Downstream on Demand modes
is in which LSR takes responsibility for initiating mapping requests
and mapping advertisements.
2.7. LDP Identifiers and Next Hop Addresses
An LSR maintains learned labels in a Label Information Base (LIB).
When operating in Downstream Unsolicited mode, the LIB entry for an
address prefix associates a collection of (LDP Identifier, label)
pairs with the prefix, one such pair for each peer advertising a
label for the prefix.
When the next hop for a prefix changes the LSR must retrieve the
label advertised by the new next hop from the LIB for use in forward-
ing. To retrieve the label the LSR must be able to map the next hop
address for the prefix to an LDP Identifier.
Similarly, when the LSR learns a label for a prefix from an LDP peer,
it must be able to determine whether that peer is currently a next
hop for the prefix to determine whether it needs to start using the
newly learned label when forwarding packets that match the prefix.
To make that decision the LSR must be able to map an LDP Identifier
to the peer's addresses to check whether any are a next hop for the
prefix.
To enable LSRs to map between a peer LDP identifier and the peer's
addresses, LSRs advertise their addresses using LDP Address and With-
draw Address messages.
An LSR sends an Address message to advertise its addresses to a peer.
An LSR sends a Withdraw Address message to withdraw previously adver-
tised addresses from a peer
2.8. Loop Detection
Loop detection is a configurable option which provides a mechanism
for finding looping LSPs and for preventing Label Request messages
from looping in the presence of non-merge capable LSRs.
The mechanism makes use of Path Vector and Hop Count TLVs carried by
Label Request and Label Mapping messages. It builds on the following
basic properties of these TLVs:
- A Path Vector TLV contains a list of the LSRs that its containing
message has traversed. An LSR is identified in a Path Vector
list by its unique LSR Identifier (Id), which is the first four
octets of its LDP Identifier. When an LSR propagates a message
containing a Path Vector TLV it adds its LSR Id to the Path Vec-
tor list. An LSR that receives a message with a Path Vector that
contains its LSR Id detects that the message has traversed a
loop. LDP supports the notion of a maximum allowable Path Vector
length; an LSR that detects a Path Vector has reached the maximum
length behaves as if the containing message has traversed a loop.
- A Hop Count TLV contains a count of the LSRS that the containing
message has traversed. When an LSR propagates a message contain-
ing a Hop Count TLV it increments the count. An LSR that detects
a Hop Count has reached a configured maximum value behaves as if
the containing message has traversed a loop. By convention a
count of 0 is interpreted to mean the hop count is unknown.
Incrementing an unknown hop count value results in an unknown hop
count value (0).
The following paragraphs describes LDP loop detection procedures.
For these paragraphs, and only these paragraphs, "MUST" is redefined
to mean "MUST if configured for loop detection". The paragraphs
specify messages that MUST carry Path Vector and Hop Count TLVs.
Note that the Hop Count TLV and its procedures are used without the
Path Vector TLV in situations when loop detection is not configured
(see [RFC3035] and [RFC3034]).
2.8.1. Label Request Message
The use of the Path Vector TLV and Hop Count TLV prevent Label
Request messages from looping in environments that include non-merge
capable LSRs.
The rules that govern use of the Hop Count TLV in Label Request mes-
sages by LSR R when Loop Detection is enabled are the following:
- The Label Request message MUST include a Hop Count TLV.
- If R is sending the Label Request because it is a FEC ingress, it
MUST include a Hop Count TLV with hop count value 1.
- If R is sending the Label Request as a result of having received a
Label Request from an upstream LSR, and if the received Label
Request contains a Hop Count TLV, R MUST increment the received hop
count value by 1 and MUST pass the resulting value in a Hop Count
TLV to its next hop along with the Label Request message;
The rules that govern use of the Path Vector TLV in Label Request
messages by LSR R when Loop Detection is enabled are the following:
- If R is sending the Label Request because it is a FEC ingress,
then if R is non-merge capable, it MUST include a Path Vector TLV
of length 1 containing its own LSR Id.
- If R is sending the Label Request as a result of having received
a Label Request from an upstream LSR, then if the received Label
Request contains a Path Vector TLV or if R is non-merge capable:
R MUST add its own LSR Id to the Path Vector, and MUST pass
the resulting Path Vector to its next hop along with the
Label Request message. If the Label Request contains no Path
Vector TLV, R MUST include a Path Vector TLV of length 1 con-
taining its own LSR Id.
Note that if R receives a Label Request message for a particular
FEC, and R has previously sent a Label Request message for that FEC
to its next hop and has not yet received a reply, and if R intends
to merge the newly received Label Request with the existing out-
standing Label Request, then R does not propagate the Label Request
to the next hop.
If R receives a Label Request message from its next hop with a Hop
Count TLV which exceeds the configured maximum value, or with a
Path Vector TLV containing its own LSR Id or which exceeds the max-
imum allowable length, then R detects that the Label Request
message has traveled in a loop.
When R detects a loop, it MUST send a Loop Detected Notification
message to the source of the Label Request message and drop the
Label Request message.
2.8.2. Label Mapping Message
The use of the Path Vector TLV and Hop Count TLV in the Label Mapping
message provide a mechanism to find and terminate looping LSPs. When
an LSR receives a Label Mapping message from a next hop, the message
is propagated upstream as specified below until an ingress LSR is
reached or a loop is found.
The rules that govern the use of the Hop Count TLV in Label Mapping
messages sent by an LSR R when Loop Detection is enabled are the fol-
lowing:
- R MUST include a Hop Count TLV.
- If R is the egress, the hop count value MUST be 1.
- If the Label Mapping message is being sent to propagate a Label
Mapping message received from the next hop to an upstream peer, the
hop count value MUST be determined as follows:
o If R is a member of the edge set of an LSR domain whose LSRs do
not perform 'TTL-decrement' (e.g., an ATM LSR domain or a Frame
Relay LSR domain) 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 hop count received from the next
hop before propagating the message.
- If the Label Mapping message is not being sent to propagate a
Label Mapping message, the hop count value MUST be the result of
incrementing R's current knowledge of the hop count learned from
previous Label Mapping messages. Note that this hop count value
will be unknown if R has not received a Label Mapping message from
the next hop.
Any Label Mapping message MAY contain a Path Vector TLV. The rules
that govern the mandatory use of the Path Vector TLV in Label Mapping
messages sent by LSR R when Loop Detection is enabled are the follow-
ing:
- If R is the egress, the Label Mapping message need not include a
Path Vector TLV.
- If R is sending the Label Mapping message to propagate a Label
Mapping message received from the next hop to an upstream peer,
then:
o If R is merge capable and if R has not previously sent a Label
Mapping message to the upstream peer, then it MUST include a
Path Vector TLV.
o If the received message contains an unknown hop count, then R
MUST include a Path Vector TLV.
o If R has previously sent a Label Mapping message to the
upstream peer, then it MUST include a Path Vector TLV if the
received message reports an LSP hop count increase, a change in
hop count from unknown to known, or a change from known to
unknown.
If the above rules require R include a Path Vector TLV in the Label
Mapping message, R computes it as follows:
o If the received Label Mapping message included a Path Vector,
the Path Vector sent upstream MUST be the result of adding R's
LSR Id to the received Path Vector.
o If the received message had no Path Vector, the Path Vector
sent upstream MUST be a path vector of length 1 containing R's
LSR Id.
- If the Label Mapping message is not being sent to propagate a
received message upstream, the Label Mapping message MUST
include a Path Vector of length 1 containing R's LSR Id.
If R receives a Label Mapping message from its next hop with a
Hop Count TLV which exceeds the configured maximum value, or with
a Path Vector TLV containing its own LSR Id or which exceeds the
maximum allowable length, then R detects that the corresponding
LSP contains a loop.
When R detects a loop, it MUST stop using the label for forward-
ing, drop the Label Mapping message, and signal Loop Detected
status to the source of the Label Mapping message.
2.8.3. Discussion
If loop detection is desired in an MPLS domain, then it should be
turned on in ALL LSRs within that MPLS domain, else loop detection
will not operate properly and may result in undetected loops or in
falsely detected loops.
LSRs which are configured for loop detection are NOT expected to
store the path vectors as part of the LSP state.
Note that in a network where only non-merge capable LSRs are present,
Path Vectors are passed downstream from ingress to egress, and are
not passed upstream. Even when merge is supported, Path Vectors need
not be passed upstream along an LSP which is known to reach the
egress. When an LSR experiences a change of next hop, it need pass
Path Vectors upstream only when it cannot tell from the hop count
that the change of next hop does not result in a loop.
In the case of ordered label distribution, Label Mapping messages are
propagated from egress toward ingress, naturally creating the Path
Vector along the way. In the case of independent label distribution,
an LSR may originate a Label Mapping message for an FEC before
receiving a Label Mapping message from its downstream peer for that
FEC. In this case, the subsequent Label Mapping message for the FEC
received from the downstream peer is treated as an update to LSP
attributes, and the Label Mapping message must be propagated
upstream. Thus, it is recommended that loop detection be configured
in conjunction with ordered label distribution, to minimize the num-
ber of Label Mapping update messages.
2.9. Authenticity and Integrity of LDP Messages
This section specifies a mechanism to protect against the introduc-
tion of spoofed TCP segments into LDP session connection streams.
The use of this mechanism MUST be supported as a configurable option.
The mechanism is based on use of the TCP MD5 Signature Option speci-
fied in [RFC2385] for use by BGP [RFC4271]. See [RFC1321] for a
specification of the MD5 hash function. From a standards maturity
point of view, the current document relates to [RFC2385] the same way
as [RFC4271] relates to [RFC2385]. This is explained in [RFC4278].
2.9.1. TCP MD5 Signature Option
The following quotes from [RFC2385] outline the security properties
achieved by using the TCP MD5 Signature Option and summarizes its
operation:
"IESG Note
This document describes current existing practice for securing
BGP against certain simple attacks. It is understood to have
security weaknesses against concerted attacks."
"Abstract
This memo describes a TCP extension to enhance security for
BGP. It defines a new TCP option for carrying an MD5 [RFC1321]
digest in a TCP segment. This digest acts like a signature for
that segment, incorporating information known only to the con-
nection end points. Since BGP uses TCP as its transport, using
this option in the way described in this paper significantly
reduces the danger from certain security attacks on BGP."
"Introduction
The primary motivation for this option is to allow BGP to pro-
tect itself against the introduction of spoofed TCP segments
into the connection stream. Of particular concern are TCP
resets.
To spoof a connection using the scheme described in this paper,
an attacker would not only have to guess TCP sequence numbers,
but would also have had to obtain the password included in the
MD5 digest. This password never appears in the connection
stream, and the actual form of the password is up to the appli-
cation. It could even change during the lifetime of a particu-
lar connection so long as this change was synchronized on both
ends (although retransmission can become problematical in some
TCP implementations with changing passwords).
Finally, there is no negotiation for the use of this option in
a connection, rather it is purely a matter of site policy
whether or not its connections use the option."
"MD5 as a Hashing Algorithm
Since this memo was first issued (under a different title), the
MD5 algorithm has been found to be vulnerable to collision
search attacks [Dobb], and is considered by some to be
insufficiently strong for this type of application.
This memo still specifies the MD5 algorithm, however, since the
option has already been deployed operationally, and there was
no "algorithm type" field defined to allow an upgrade using the
same option number. The original document did not specify a
type field since this would require at least one more byte, and
it was felt at the time that taking 19 bytes for the complete
option (which would probably be padded to 20 bytes in TCP
implementations) would be too much of a waste of the already
limited option space.
This does not prevent the deployment of another similar option
which uses another hashing algorithm (like SHA-1). Also, if
most implementations pad the 18 byte option as defined to 20
bytes anyway, it would be just as well to define a new option
which contains an algorithm type field.
This would need to be addressed in another document, however."
End of quotes from [RFC2385].
2.9.2. LDP Use of TCP MD5 Signature Option
LDP uses the TCP MD5 Signature Option as follows:
- Use of the MD5 Signature Option for LDP TCP connections is a con-
figurable LSR option.
- An LSR that uses the MD5 Signature Option is configured with a
password (shared secret) for each potential LDP peer.
- The LSR applies the MD5 algorithm as specified in [RFC2385] to
compute the MD5 digest for a TCP segment to be sent to a peer.
This computation makes use of the peer password as well as the
TCP segment.
- When the LSR receives a TCP segment with an MD5 digest, it vali-
dates the segment by calculating the MD5 digest (using its own
record of the password) and compares the computed digest with the
received digest. If the comparison fails, the segment is dropped
without any response to the sender.
- The LSR ignores LDP Hellos from any LSR for which a password has
not been configured. This ensures that the LSR establishes LDP
TCP connections only with LSRs for which a password has been con-
figured.
2.10. Label Distribution for Explicitly Routed LSPs
Traffic Engineering [RFC2702] is expected to be an important MPLS
application. MPLS support for Traffic Engineering uses explicitly
routed LSPs, which need not follow normally-routed (hop-by-hop) paths
as determined by destination-based routing protocols. CR-LDP [CRLDP]
defines extensions to LDP to use LDP to set up explicitly routed
LSPs.
3. Protocol Specification
Previous sections that describe LDP operation have discussed scenar-
ios that involve the exchange of messages among LDP peers. This sec-
tion 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 mes-
sages 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 proto-
col. 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 send-
ing 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 sepa-
rate 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 MUST be silently ignored and the rest of the message
processed as if the unknown TLV did not exist. The sections fol-
lowing 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 for-
warded. If F is clear (=0), the unknown TLV is not forwarded with
the containing message; if F is set (=1), the unknown TLV is for-
warded with the containing message. The sections following that
define TLVs specify a value for the F-bit. By setting both the U
and F bits, a TLV can be propagated as opaque data through nodes
that do not recognize the TLV.
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 specifica-
tion 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 mes-
sage 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 spec-
ifies 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.
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 NUM-
BERS 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 pre-
fix 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 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 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 sig-
nificant. 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 follow-
ing values are supported:
0 = 10 bits DLCI
2 = 23 bits DLCI
Len values 1 and 3 are reserved.
DLCI
The Data Link Connection Identifier
For a 10bit 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 23bit 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 mes-
sages.
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 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 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 estab-
lished, 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 detec-
tion 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 sig-
naled.
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 notifica-
tion. 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 mes-
sages. A message other than a Notification message may carry a Sta-
tus TLV as an Optional Parameter. When a message other than a Noti-
fication 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 "Noti-
fication Message".
Mandatory Parameters
Variable length set of required message parameters. Some messages
have no required parameters.
For messages that have required parameters, the required parame-
ters 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 parame-
ters may appear in any order.
Note that there is no alignment requirement for the first octet of an
LDP message and that there is no padding at the end of a message;
that is, parameters can end at odd-byte boundaries.
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 sig-
nificant event. A Notification message signals a fatal error or pro-
vides 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 addi-
tional information that supplements the status information con-
tained 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 mes-
sage containing a Status TLV that encodes the information and option-
ally additional TLVs that provide more information about the condi-
tion.
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 ses-
sion immediately by closing the session TCP connection and discard
all state associated with the session, including all label-FEC bind-
ings learned via the session.
The above statement does not apply to the processing of the Shutdown
message in the session initialization procedure. When an LSR receives
a Shutdown message during session initialization, it SHOULD transmit
a Shutdown message and then close the transport connection.
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 mal-
formed if:
- The LDP Identifier in the PDU header is unknown to the
receiver, or it is known but is not the LDP Identifier associ-
ated 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 ses-
sion during session establishment. This is a fatal error sig-
naled 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 deter-
mined.
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 Length is too small, that is, smaller than the
smallest possible value component. This is a fatal error sig-
naled by the Bad Message Length Status Code.
- The message is missing one or more Mandatory Parameters. This
is a non-fatal error signaled 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.
- The TLV Value is malformed. This occurs when the receiver han-
dles the TLV but cannot decode the TLV Value. This is inter-
preted as indicative of a bug in either the sending or receiv-
ing 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 Initial-
ization") may fail if the session parameters received in the Initial-
ization Message are unacceptable. This is a fatal error. The spe-
cific 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 sig-
nal 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 implemen-
tation should, when capable of doing so, use the Internal Error Sta-
tus 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 Mecha-
nism; 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 Hel-
los 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 ini-
tiates 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 ver-
sion 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 car-
rying 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 send-
ing 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 car-
rying 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 "Maintain-
ing 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 implementa-
tion 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 Estab-
lishment".
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 receiv-
ing LSR. When a receiving LSR playing the active role in LDP
session establishment detects a change in the sending LSR con-
figuration, it may clear the session setup backoff delay, if
any, associated with the sending LSR (see Section "Session Ini-
tialization").
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 num-
ber.
3.5.3. Initialization Message
The LDP Initialization Message is exchanged as part of the LDP ses-
sion establishment procedure; see Section "LDP Session Establish-
ment".
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 pro-
tocol 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 Down-
stream On Demand.
If one LSR proposes Downstream Unsolicited and the other pro-
poses Downstream on Demand, the rules for resolving this dif-
ference 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/Parame-
ters 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 proce-
dures 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 behav-
ior, 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 Proce-
dures".
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.
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 direc-
tion on the link only. When either or both of the peers speci-
fies 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 Iden-
tifier 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 inter-
section 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 Notifi-
cation 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 transmis-
sion 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 originat-
ing 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 originat-
ing 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.
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 specifi-
cation:
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 unidi-
rectional 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 inter-
section 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 Notifi-
cation 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 transmis-
sion 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 justi-
fied 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 Ini-
tialization Message.
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 Identi-
fiers and next hop addresses; see Section "LDP Identifiers and Next
Hop Addresses".
When a new LDP session is initialized and before sending Label Map-
ping 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.
An LSR may re-advertise an address (A) that it has previously adver-
tised without explicitly withdrawing the address. If the receiver
already has address binding (LSR, A) it SHOULD take no further
action.
An LSR may withdraw an address (A) without having previously adver-
tised it. If the receiver has no address binding (LSR, A), it SHOULD
take no further action.
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 mes-
sage.
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 adver-
tised. See Section "FEC TLV" for encoding.
Label TLV
Specifies the Label component of the FEC-Label mapping. See Sec-
tion "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 parame-
ter. 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 Pro-
cedures" describes how to handle this TLV.
Path Vector
Specifies the LSRs along the LSP being setup by the Label Mes-
sage. 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 SHOULD NOT use the label for forwarding unless its routing ta-
ble 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 advertise-
ment.
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 map-
pings 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 oper-
ate 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 mes-
sage. 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 con-
ditions:
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 mes-
sage 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,
the receiving LSR uses its routing table to determine its response.
Unless its routing table includes an entry that exactly matches the
requested Prefix, 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 mes-
sage.
3.5.9.1. Label Abort Request Message Procedures
An LSR Ru may send a Label Abort Request message to abort an out-
standing Label Request message for FEC sent to LSR Rd in the follow-
ing 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 asso-
ciated 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 acknowl-
edge the abort by responding with a Label Request Aborted Notifica-
tion message. The Notification MUST include a Label Request Message
ID TLV that carries the message ID of the aborted Label Request mes-
sage.
If an LSR receives a Label Abort Request Message after it has
responded to the Label Request in question with a Label Mapping mes-
sage 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 mes-
sage 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 Mes-
sage 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 down-
stream 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 map-
pings 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 with-
drawn.
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 condi-
tions:
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 con-
tain no other FEC Elements. In this case, if the Label Withdraw mes-
sage contains an optional Label TLV, then the label is to be with-
drawn 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 previ-
ously 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 follow-
ing 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 conserva-
tive operation.
3. The LSR receives a Label Withdraw message.
Note that if an LSR is configured for "liberal mode", a release mes-
sage 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 con-
tain no other FEC Elements. In this case, if the Label Release mes-
sage 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 handles 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 under-
stood is based on the Type and the mandatory Vendor ID field.
Implementations that support vendor-private TLVs MUST support a
user-accessible configuration interface that causes the U-bit to
be set on all transmitted vendor-private TLVs; this requirement
MAY be satisfied by a user-accessible configuration interface that
prevents transmission of all vendor-private TLVs for which the U-
bit is clear.
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 inter-
preted.
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 under-
stood is based on the Msg Type and the Vendor ID parameter.
Implementations that support vendor-private messages MUST support
a user-accessible configuration interface that causes the U-bit to
be set on all transmitted vendor-private messages; this require-
ment MAY be satisfied by a user-accessible configuration interface
that prevents transmission of all vendor-private messages for
which the U-bit is clear.
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, Ven-
dor 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 noti-
fication 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 experimen-
tal 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 interpreta-
tion 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 is defined in [RFC3031] as follows:
"The Implicit NULL label is a label with special semantics which an
LSR can bind to an address prefix. If LSR Ru, by consulting its ILM
(Incoming Label Map) sees that labeled packet P must be forwarded
next to Rd, but that Rd has distributed a binding of Implicit NULL to
the corresponding address prefix, then instead of replacing the value
of the label on top of the label stack, Ru pops the label stack, and
then forwards the resulting packet to Rd."
The implicit NULL label is represented in LDP as a Generic Label TLV
with a Label field value of 3, as defined in [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 responsibil-
ity 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 Experi-
ment ID Name Space (see below).
4.2. TLV Type Name Space
LDP divides the name space for TLV types into three ranges. The fol-
lowing 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 responsibil-
ity 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 indicate their willingness to establish and maintain LDP ses-
sions by periodically sending Hello messages. Receipt of a Hello
serves to create a new "Hello adjacency", if one does not already
exist, or to refresh an existing one. Spoofing a Hello packet for
an existing adjacency can cause the adjacency to time out and that
can result in termination of the associated session. This can
occur when the spoofed Hello specifies a small Hold Time, causing
the receiver to expect Hellos within this interval, while the true
neighbor continues sending Hellos at the lower, previously agreed
to, frequency.
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 ses-
sion. 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 distri-
bution.
The security requirements of label distribution protocols are essen-
tially 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 pro-
vided 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 pro-
tect against label spoofing attacks since data packets carry labels
in the clear. Furthermore, label spoofing attacks can be made with-
out 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 adminis-
trator's control. Interfaces to peers exterior to the domain rep-
resent a potential threat since exterior peers are not. An admin-
istrator 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 dis-
covery 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 con-
sumes 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 fol-
lowing:
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.
6. Areas for Future Study
The following topics not addressed in this version of LDP are possi-
ble areas for future study:
- Section 2.16 of the MPLS architecture [RFC3031] requires that
the initial label distribution protocol negotiation between
peer LSRs enable each LSR to determine whether its peer is
capable of popping the label stack. This version of LDP
assumes that LSRs support label popping for all link types
except ATM and Frame Relay. A future version may specify means
to make this determination part of the session initiation nego-
tiation.
- LDP support for CoS (class of service) is not specified in this
version. CoS support may be addressed in a future version.
- LDP support for multicast is not specified in this version.
Multicast support may be addressed in a future version.
- LDP support for multipath label switching is not specified in
this version. Multipath support may be addressed in a future
version.
- LDP support for signalling the maximum transmission unit is not
specified in this version. It is discussed in the experimental
document [LDP-MTU].
- The current specification does not address basic peer discovery
on non-broadcast multi access (NBMA) media. The solution avail-
able in the current specification is to use extended peer dis-
covery in such setups. The issue of defining a mechanism seman-
tically similar to basic discovery (1 hop limit, bind the hello
adjacency to an interface) that uses a preconfigured neighbor
addresses, is left for further study.
- The current specification does not support shutting down an
adjacency. The motivation for doing it, and the mechanisms for
achieving it are left for further study.
- The current specification does not include a method for secur-
ing Hello messages, to detect spoofing of Hellos. The scenarios
where this is necessary, as well as the mechanism for achieving
it are left for future study.
- The current specification does not have the ability to detect a
stateless fast control plane restart. The method for achieving
this, possibly through an "incarnation/instance" number carried
in the Hello message is left for future study.
- The current specification does not support an "end of LIB" mes-
sage, analogous to BGP's end of RIB message which an LDP LSR
(operating in DU mode) would use following session establish-
ment. The discussion on the need for such a mechanism and its
implementation are left for future study.
- The current specification does not deal with situations where
different LSRs advertise the same address. Such situations
typically occur as the result of configuration errors, and the
goal in this case is to provide the LSRs advertising the same
address with enough information to enable operators to take
corrective action. The specification of this mechanism is left
for a separate document.
7. Changes from RFC3036
Here is a list of changes from RFC3036
1. Removed the Host Address FEC and references to it, since it is
not used by any implementation.
2. Split the reference list into normative and non-normative ref-
erences
3. Removed "MPLS using ATM VP Switching" from the list of norma-
tive references, and references to it.
4. Removed reference to RFC 1700 and replaced it with a link to
http://www.iana.org/assignments/address-family-numbers.
5. Removed reference to RFC 1771 and replaced it with a reference
to RFC 4271.
6. Clarified the use of the F bit.
7. Added option to allow split horizon when doing ordered control.
8. Clarified the processing of messages with the U-bit set during
the session initialization procedures
9. Clarified the processing of the E-bit during session initial-
ization procedures.
10. Added text explaining that the shutdown message in the state
transition diagram is implemented as as a notification message
with a status TLV indicating a fatal error.
11. Added case for TLV length too short in the specification for
handling malformed TLVs.
12. Explained the security threat posed by hello spoofing.
13. Added reference to 4271 and 4278 and text for standards matu-
rity variance with regards to the MD5 option.
14. Added text from 3031 explaining the handling of implicit NULL
label.
15. Included the encoding of DLCIs to remove normative reference to
3034.
16. Moved references to 3031, 3032 and 3034 to informative.
17. In the section describing handling of unknown TLV, removed ref-
erence to inexistent section (errata in original document).
18. Added text clarifying how to achieve interoperability when
sending vendor-private TLVs and messages.
19. In the "receive label request" procedures, if a loop is
detected, changed the procedure to send a notification before
aborting the rest of the processing.
20. In "receive label release" procedures, clarified the behavior
for merge-capable LSRs.
21. In "receive label release" procedures, clarified the behavior
for receipt of an unknown FEC.
22. In note 4 of "Detect Change in FEC Next Hop", modified the text
to reference the correct set of conditions for sending a label
request procedure (typo in the original document).
23. In the procedures for "LSR decides to no longer label switch a
FEC", clarified the fact that the label must not be reused
until a label release is received.
24. In the routine "Prepare_Label_Mapping_Attributes" added a note
regarding the treatment of unknown TLVs according to their U
and F bits.
25. In the address message processing procedures, clarified the
behavior for the case where an LSR receives re-advertisement of
an address previously advertised it, or withdrawal of an
address from an LSR that has not previously advertised that
address.
26. In the routine "Receive Label Mapping" clarified the meaning of
PrevAdvLabel when no label advertisement message has been sent
previously.
27. In the "Receive Label Mapping" procedures, if a loop is
detected, modified the procedure to send a notification before
aborting the rest of the processing.
28. In the "Receive Label Mapping" procedures, corrected step
LMp.10 to handle label mapping messages for additional (non-
merged) LSPs for the FEC.
29. In the "Receive Label Mapping" procedures, clarified behavior
when receiving a duplicate label for the same FEC.
30. In the routine "Receive Label Abort Request" clarified the
behavior for non-merging LSRs.
31. Added the following items to the section discussing areas for
future study:
o extensions for communicating the maximum transmission unit
o basic peer discovery on NBMA media
o option of shutting down an adjacency
o mechanisms for securing Hello messages
o detection of a stateless fast control plane restart
o support of "end of LIB" message
o mechanisms for dealing with the case where different LSRs
advertise the same address.
8. Acknowledgments
This document was originally published as RFC 3036 in January 2001.
It was produced by the MPLS working of the IETF and was jointly
authored by Loa Andersson, Paul Doolan, Nancy Feldman, Andre Fredette
and Bob Thomas.
The ideas and text in RFC3036 were collected from a number of
sources. We would like to thank Rick Boivie, Ross Callon, Alex
Conta, Eric Gray, Yoshihiro Ohba, Eric Rosen, Bernard Suter, Yakov
Rekhter, and Arun Viswanathan for their input for RFC3036.
The editors would like to thank Eric Gray, David Black and Sam Hart-
man for their input to and review of the current document.
In addition, the editors would like to thank the members of the mpls
working group for their ideas and the support they have given to this
document, and in particular to Eric Rosen, Luca Martini, Markus Jork,
Mark Duffy, Vach Kompella, Kishore Tiruveedhula. Rama Ramakrishnan,
Nick Weeds, Adrian Farrel and Andy Malis.
9. References
9.1. Normative references
[IANA] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm," RFC 1321,
April 1992.
[RFC1483] Heinanen, J., "Multiprotocol Encapsulation over ATM Adap-
tation Layer 5", RFC 1483, July 1993.
[ASSIGNED_AF] http://www.iana.org/assignments/address-family-numbers
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP
MD5 Signature Option", RFC 2385, August 1998.
[RFC3035] Davie, B., Lawrence, J., McCloghrie, K., Rekhter, Y.,
Rosen, E., Swallow, G. and P. Doolan, "MPLS using LDP and
ATM VC Switching", RFC 3035, January 2001.
[RFC3037] Thomas, B. and E. Gray, "LDP Applicability", RFC 3037,
January 2001.
9.2. Non-normative references
[CRLDP] L. Andersson, A. Fredette, B. Jamoussi, R. Callon, P.
Doolan, N. Feldman, E. Gray, J. Halpern, J. Heinanen T.
E. Kilty, A. G. Malis, M. Girish, K. Sundell, P. Vaana-
nen, T. Worster, L. Wu, R. Dantu, "Constraint-Based LSP
Setup using LDP", RFC 3212, January 2002
[DIFFSERV] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.
and W. Weiss, "An Architecture for Differentiated Ser-
vices", RFC 2475, December 1998.
[LDP-MTU] Black, B. and Kompella, K. "Maximum Transmission Unit
Signalling Extensions for the Label Distribution Proto-
col", draft-ietf-mpls-ldp-mtu-extensions-03.txt, April
2004.
[RFC2205] Braden, R., Zhang, L., Berson, S., Herzog, S. and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC2702] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M. and J.
McManus, "Requirements for Traffic Engineering over
MPLS", RFC 2702, September 1999.
[RFC3031] Rosen, E., Viswanathan, A. and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC3032] Rosen, E., Rekhter, Y., Tappan, D., Farinacci, D.,
Fedorkow, G., Li, T. and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[RFC3034] Conta, A., Doolan, P. and A. Malis, "Use of Label Switch-
ing on Frame Relay Networks Specification", RFC 3034,
January 2001.
[RFC4271] Rekhter, Y., Li, T. and Hares, S. "A Border Gateway Pro-
tocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4278] Bellovin, S., Zinin, A., "Standards Maturity Variance
Regarding the TCP MD5 Signature Option (RFC 2385) and the
BGP-4 Specification", RFC 4278, January 2006.
10. Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any assur-
ances of licenses to be made available, or the result of an attempt
made to obtain a general license or permission for the use of such
proprietary rights by implementers or users of this specification can
be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
11. Editors' Addresses
Loa Andersson
Acreo AB
Isafjordsgatan 22
Kista, Sweden
email: loa.andersson@acreo.se
loa@pi.se
Ina Minei
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
email: ina@juniper.net
Bob Thomas
Cisco Systems, Inc.
1414 Massachusetts Ave
Boxborough, MA 01719
email: rhthomas@cisco.com
Appendix A. LDP Label Distribution Procedures
This section specifies label distribution behavior in terms of LSR
response to the following events:
- Receive Label Request Message;
- Receive Label Mapping Message;
- Receive Label Abort Request Message;
- Receive Label Release Message;
- Receive Label Withdraw Message;
- Recognize new FEC;
- Detect change in FEC next hop;
- Receive Notification Message / Label Request Aborted;
- Receive Notification Message / No Label Resources;
- Receive Notification Message / No Route;
- Receive Notification Message / Loop Detected;
- Receive Notification Message / Label Resources Available;
- Detect local label resources have become available;
- LSR decides to no longer label switch a FEC;
- Timeout of deferred label request.
The specification of LSR behavior in response to an event has three
parts:
1. Summary. Prose that describes LSR response to the event in
overview.
2. Context. A list of elements referred to by the Algorithm part
of the specification. (See 3.)
3. Algorithm. An algorithm for LSR response to the event.
The Summary may omit details of the LSR response, such as bookkeeping
action or behavior dependent on the LSR label advertisement mode,
control mode, or label retention mode in use. The intent is that the
Algorithm fully and unambiguously specify the LSR response.
The algorithms in this section use procedures defined in the MPLS
architecture specification [RFC3031] for hop-by-hop routed traffic.
These procedures are:
- Label Distribution procedure, which is performed by a down-
stream LSR to determine when to distribute a label for a FEC to
LDP peers. The architecture defines four Label Distribution
procedures:
. Downstream Unsolicited Independent Control, called
PushUnconditional in [RFC3031].
. Downstream Unsolicited Ordered Control, called
PushConditional in [RFC3031].
. Downstream On Demand Independent Control, called
PulledUnconditional in [RFC3031].
. Downstream On Demand Ordered Control, called
PulledConditional in [RFC3031].
- Label Withdrawal procedure, which is performed by a downstream
LSR to determine when to withdraw a FEC label mapping previ-
ously distributed to LDP peers. The architecture defines a
single Label Withdrawal procedure. Whenever an LSR breaks the
binding between a label and a FEC, it MUST withdraw the FEC
label mapping from all LDP peers to which it has previously
sent the mapping.
- Label Request procedure, which is performed by an upstream LSR
to determine when to explicitly request that a downstream LSR
bind a label to a FEC and send it the corresponding label map-
ping. The architecture defines three Label Request procedures:
. Request Never. The LSR never requests a label.
. Request When Needed. The LSR requests a label whenever
it needs one.
. Request On Request. This procedure is used by
non-label merging LSRs. The LSR requests a label
when it receives a request for one, in addition
to whenever it needs one.
- Label Release procedure, which is performed by an upstream LSR
to determine when to release a previously received label map-
ping for a FEC. The architecture defines two Label Release
procedures:
. Conservative label retention, called Release On Change in
[RFC3031].
. Liberal label retention, called No Release On Change in
[RFC3031].
- Label Use procedure, which is performed by an LSR to determine
when to start using a FEC label for forwarding/switching. The
architecture defines three Label Use procedures:
. Use Immediate. The LSR immediately uses a label received
from a FEC next hop for forwarding/switching.
. Use If Loop Free. The LSR uses a FEC label received from a
FEC next hop for forwarding/switching only if it has
determined that by doing so it will not cause a forwarding
loop.
. Use If Loop Not Detected. This procedure is the same as Use
Immediate unless the LSR has detected a loop in the FEC LSP.
Use of the FEC label for forwarding/switching will continue
until the next hop for the FEC changes or the loop is no
longer detected.
This version of LDP does not include a loop prevention mecha-
nism; therefore, the procedures below do not make use of the
Use If Loop Free procedure.
- Label No Route procedure (called Label Not Available procedure
in [RFC3031]), which is performed by an upstream LSR to deter-
mine how to respond to a No Route notification from a down-
stream LSR in response to a request for a FEC label mapping.
The architecture specification defines two Label No Route pro-
cedures:
. Request Retry. The LSR should issue the label request at a
later time.
. No Request Retry. The LSR should assume the downstream LSR
will provide a label mapping when the downstream LSR has a
next hop and it should not reissue the request.
A.1. Handling Label Distribution Events
This section defines LDP label distribution procedures by specifying
an algorithm for each label distribution event. The requirement on
an LDP implementation is that its event handling must have the effect
specified by the algorithms. That is, an implementation need not
follow exactly the steps specified by the algorithms as long as the
effect is identical.
The algorithms for handling label distribution events share common
actions. The specifications below package these common actions into
procedure units. Specifications for these common procedures are in
their own section "Common Label Distribution Procedures", which fol-
lows this.
An implementation would use data structures to store information
about protocol activity. This appendix specifies the information to
be stored in sufficient detail to describe the algorithms, and
assumes the ability to retrieve the information as needed. It does
not specify the details of the data structures.
A.1.1. Receive Label Request
Summary:
The response by an LSR to receipt of a FEC label request from an
LDP peer may involve one or more of the following actions:
- Transmission of a notification message to the requesting LSR
indicating why a label mapping for the FEC cannot be provided;
- Transmission of a FEC label mapping to the requesting LSR;
- Transmission of a FEC label request to the FEC next hop;
- Installation of labels for forwarding/switching use by the LSR.
Context:
- LSR. The LSR handling the event.
- MsgSource. The LDP peer that sent the message.
- FEC. The FEC specified in the message.
- RAttributes. Attributes received with the message. E.g., Hop
Count, Path Vector.
- SAttributes. Attributes to be included in Label Request mes-
sage, if any, propagated to FEC Next Hop.
- StoredHopCount. The hop count, if any, previously recorded for
the FEC.
Algorithm:
LRq.1 Execute procedure Check_Received_Attributes (MsgSource,
LabelRequest, RAttributes).
If Loop Detected, goto LRq.4.
LRq.2 Is there a Next Hop for FEC?
If not, goto LRq.5.
LRq.3 Is MsgSource the Next Hop?
Ifnot, goto LRq.6.
LRq.4 Execute procedure Send_Notification (MsgSource, Loop
Detected).
Goto LRq.13
LRq.5 Execute procedure Send_Notification (MsgSource, No Route).
Goto LRq.13.
LRq.6 Has LSR previously received a label request for FEC from
MsgSource?
If not, goto LRq.8. (See Note 1.)
LRq.7 Is the label request a duplicate request?
If so, Goto LRq.13. (See Note 2.)
LRq.8 Record label request for FEC received from MsgSource and
mark it pending.
LRq.9 Perform LSR Label Distribution procedure:
For Downstream Unsolicited Independent Control OR
For Downstream On Demand Independent Control
1. Has LSR previously received and retained a label map-
ping for FEC from Next Hop?.
Is so, set Propagating to IsPropagating.
If not, set Propagating to NotPropagating.
2. Execute procedure Prepare_Label_Map-
ping_Attributes(MsgSource, FEC, RAttributes, SAt-
tributes, Propagating, StoredHopCount).
3. Execute procedure Send_Label (MsgSource, FEC, SAt-
tributes).
4. Is LSR egress for FEC? OR
Has LSR previously received and retained a label map-
ping for FEC from Next Hop?
If so, goto LRq.11.
If not, goto LRq.10.
For Downstream Unsolicited Ordered Control OR
For Downstream On Demand Ordered Control
1. Is LSR egress for FEC? OR
Has LSR previously received and retained a label map-
ping for FEC from Next Hop? (See Note 3.)
If not, goto LRq.10.
2. Execute procedure Prepare_Label_Map-
ping_Attributes(MsgSource, FEC, RAttributes, SAt-
tributes, IsPropagating, StoredHopCount)
3. Execute procedure Send_Label (MsgSource, FEC, SAt-
tributes).
Goto LRq.11.
LRq.10 Perform LSR Label Request procedure:
For Request Never
1. Goto LRq.13.
For Request When Needed OR
For Request On Request
1. Execute procedure Prepare_Label_Request_Attributes
(Next Hop, FEC, RAttributes, SAttributes);
2. Execute procedure Send_Label_Request (Next Hop, FEC,
SAttributes).
Goto LRq.13.
LRq.11 Has LSR successfully sent a label for FEC to MsgSource?
If not, goto LRq.13. (See Note 4.)
LRq.12 Perform LSR Label Use procedure.
For Use Immediate OR
For Use If Loop Not Detected
1. Install label sent to MsgSource and label from Next
Hop (if LSR is not egress) for forwarding/switching
use.
LRq.13 DONE
Notes:
1. In the case where MsgSource is a non-label merging LSR it will
send a label request for each upstream LDP peer that has
requested a label for FEC from it. The LSR must be able to
distinguish such requests from a non-label merging MsgSource
from duplicate label requests.
The LSR uses the message ID of received Label Request messages
to detect duplicate requests. This means that an LSR (the
upstream peer) may not reuse the message ID used for a Label
Request until the Label Request transaction has completed.
2. When an LSR sends a label request to a peer it records that the
request has been sent and marks it as outstanding. As long as
the request is marked outstanding the LSR SHOULD NOT send
another request for the same label to the peer. Such a second
request would be a duplicate. The Send_Label_Request procedure
described below obeys this rule.
A duplicate label request is considered a protocol error and
SHOULD be dropped by the receiving LSR (perhaps with a suitable
notification returned to MsgSource).
3. If LSR is not merge-capable, this test will fail.
4. The Send_Label procedure may fail due to lack of label
resources, in which case the LSR SHOULD NOT perform the Label
Use procedure.
A.1.2. Receive Label Mapping
Summary:
The response by an LSR to receipt of a FEC label mapping from an
LDP peer may involve one or more of the following actions:
- Transmission of a label release message for the FEC label to
the LDP peer;
- Transmission of label mapping messages for the FEC to one or
more LDP peers,
- Installation of the newly learned label for forwarding/switch-
ing use by the LSR.
Context:
- LSR. The LSR handling the event.
- MsgSource. The LDP peer that sent the message.
- FEC. The FEC specified in the message.
- Label. The label specified in the message.
- PrevAdvLabel. The label for FEC, if any, previously advertised
to an upstream peer. Assuming no label was previously adver-
tised, this is the same label as the one in the Label Mapping
Message being processed.
- StoredHopCount. The hop count previously recorded for the FEC.
- RAttributes. Attributes received with the message. E.g., Hop
Count, Path Vector.
- SAttributes to be included in Label Mapping message, if any,
propagated to upstream peers.
Algorithm:
LMp.1 Does the received label mapping match an outstanding
label request for FEC previously sent to MsgSource.
If not, goto LMp.3.
LMp.2 Delete record of outstanding FEC label request.
LMp.3 Execute procedure Check_Received_Attributes (MsgSource,
LabelMapping, RAttributes).
If No Loop Detected, goto LMp.9.
LMp.4 Does the LSR have a previously received label mapping for
FEC from MsgSource? (See Note 1.)
If not, goto LMp.8.