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
messages 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 containing
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 outstanding 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 maximum
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
following:
- 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
following:
- 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 forwarding,
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
number of Label Mapping update messages.
2.9. Authenticity and Integrity of LDP Messages
This section specifies a mechanism to protect against the
introduction 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
specified in [RFC2385] for use by BGP. See [RFC1321] for a
specification of the MD5 hash function.
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
connection 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
protect 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
application. It could even change during the lifetime of a
particular 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
configurable 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
validates 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 configured.
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
scenarios that involve the exchange of messages among LDP peers.
This section specifies the message encodings and procedures for
processing the messages.
LDP message exchanges are accomplished by sending LDP protocol data
units (PDUs) over LDP session TCP connections.
Each LDP PDU can carry one or more LDP messages. Note that the
messages in an LDP PDU need not be related to one another. For
example, a single PDU could carry a message advertising FEC-label
bindings for several FECs, another message requesting label bindings
for several other FECs, and a third notification message signaling
some event.
3.1. LDP PDUs
Each LDP PDU is an LDP header followed by one or more LDP messages.
The LDP header is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version | PDU Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LDP Identifier |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version
Two octet unsigned integer containing the version number of the
protocol. This version of the specification specifies LDP protocol
version 1.
PDU Length
Two octet integer specifying the total length of this PDU in
octets, excluding the Version and PDU Length fields.
The maximum allowable PDU Length is negotiable when an LDP session
is initialized. Prior to completion of the negotiation the maximum
allowable length is 4096 bytes.
LDP Identifier
Six octet field that uniquely identifies the label space of the
sending LSR for which this PDU applies. The first four octets
identify the LSR and must be a globally unique value. It should be
a 32-bit router Id assigned to the LSR and also used to identify it
in loop detection Path Vectors. The last two octets identify a
label space within the LSR. For a platform-wide label space, these
should both be zero.
Note that there is no alignment requirement for the first octet of an
LDP PDU.
3.2. LDP Procedures
LDP defines messages, TLVs and procedures in the following areas:
- Peer discovery;
- Session management;
- Label distribution;
- Notification of errors and advisory information.
The sections that follow describe the message and TLV encodings for
these areas and the procedures that apply to them.
The label distribution procedures are complex and are difficult to
describe fully, coherently and unambiguously as a collection of
separate message and TLV specifications.
Appendix A, "LDP Label Distribution Procedures", describes the label
distribution procedures in terms of label distribution events that
may occur at an LSR and how the LSR must respond. Appendix A is the
specification of LDP label distribution procedures. If a procedure
described elsewhere in this document conflicts with Appendix A,
Appendix A specifies LDP behavior.
3.3. Type-Length-Value Encoding
LDP uses a Type-Length-Value (TLV) encoding scheme to encode much of
the information carried in LDP messages.
An LDP TLV is encoded as a 2 octet field that uses 14 bits to specify
a Type and 2 bits to specify behavior when an LSR doesn't recognize
the Type, followed by a 2 octet Length Field, followed by a variable
length Value field.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Value |
~ ~
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
U bit
Unknown TLV bit. Upon receipt of an unknown TLV, if U is clear
(=0), a notification must be returned to the message originator
and the entire message must be ignored; if U is set (=1), the
unknown TLV is silently ignored and the rest of the message is
processed as if the unknown TLV did not exist. The sections
following that define TLVs specify a value for the U-bit.
F bit
Forward unknown TLV bit. This bit applies only when the U bit is
set and the LDP message containing the unknown TLV is to be
forwarded. If F is clear (=0), the unknown TLV is not forwarded
with the containing message; if F is set (=1), the unknown TLV is
forwarded with the containing message. The sections following
that define TLVs specify a value for the F-bit.
Type
Encodes how the Value field is to be interpreted.
Length
Specifies the length of the Value field in octets.
Value
Octet string of Length octets that encodes information to be
interpreted as specified by the Type field.
Note that there is no alignment requirement for the first octet of a
TLV.
Note that the Value field itself may contain TLV encodings. That is,
TLVs may be nested.
The TLV encoding scheme is very general. In principle, everything
appearing in an LDP PDU could be encoded as a TLV. This
specification does not use the TLV scheme to its full generality. It
is not used where its generality is unnecessary and its use would
waste space unnecessarily. These are usually places where the type
of a value to be encoded is known, for example by its position in a
message or an enclosing TLV, and the length of the value is fixed or
readily derivable from the value encoding itself.
Some of the TLVs defined for LDP are similar to one another. For
example, there is a Generic Label TLV, an ATM Label TLV, and a Frame
Relay TLV; see Sections "Generic Label TLV", "ATM Label TLV", and
"Frame Relay TLV".
While it is possible to think about TLVs related in this way in terms
of a TLV type that specifies a TLV class and a TLV subtype that
specifies a particular kind of TLV within that class, this
specification does not formalize the notion of a TLV subtype.
The specification assigns type values for related TLVs, such as the
label TLVs, from a contiguous block in the 16-bit TLV type number
space.
Section "TLV Summary" lists the TLVs defined in this version of the
protocol and the section in this document that describes each.
3.4. TLV Encodings for Commonly Used Parameters
There are several parameters used by more than one LDP message. The
TLV encodings for these commonly used parameters are specified in
this section.
3.4.1. FEC TLV
Labels are bound to Forwarding Equivalence Classes (FECs). A FEC is
a list of one or more FEC elements. The FEC TLV encodes FEC items.
Its encoding is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| FEC (0x0100) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Element 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Element n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FEC Element 1 to FEC Element n
There are several types of FEC elements; see Section "FECs". The
FEC element encoding depends on the type of FEC element.
A FEC Element value is encoded as a 1 octet field that specifies
the element type, and a variable length field that is the type-
dependent element value. Note that while the representation of
the FEC element value is type-dependent, the FEC element encoding
itself is one where standard LDP TLV encoding is not used.
The FEC Element value encoding is:
FEC Element Type Value
type name
Wildcard 0x01 No value; i.e., 0 value octets;
see below.
Prefix 0x02 See below.
Host Address 0x03 Full host address; see below.
Note that this version of LDP supports the use of multiple FEC
Elements per FEC for the Label Mapping message only. The use of
multiple FEC Elements in other messages is not permitted in this
version, and is a subject for future study.
Wildcard FEC Element
To be used only in the Label Withdraw and Label Release
Messages. Indicates the withdraw/release is to be applied to
all FECs associated with the label within the following label
TLV. Must be the only FEC Element in the FEC TLV.
Prefix FEC Element value encoding:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix (2) | Address Family | PreLen |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Address Family
Two octet quantity containing a value from ADDRESS FAMILY
NUMBERS in [RFC1700] that encodes the address family for the
address prefix in the Prefix field.
PreLen
One octet unsigned integer containing the length in bits of the
address prefix that follows. A length of zero indicates a
prefix that matches all addresses (the default destination); in
this case the Prefix itself is zero octets).
Prefix
An address prefix encoded according to the Address Family
field, whose length, in bits, was specified in the PreLen
field, padded to a byte boundary.
Host Address FEC Element encoding:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Host Addr (3) | Address Family | Host Addr Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Host Addr |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Address Family
Two octet quantity containing a value from ADDRESS FAMILY
NUMBERS in [RFC1700] that encodes the address family for the
address prefix in the Prefix field.
Host Addr Len
Length of the Host address in octets.
Host Addr
An address encoded according to the Address Family field.
3.4.1.1. FEC Procedures
If in decoding a FEC TLV an LSR encounters a FEC Element with an
Address Family it does not support, it should stop decoding the FEC
TLV, abort processing the message containing the TLV, and send an
"Unsupported Address Family" Notification message to its LDP peer
signaling an error.
If it encounters a FEC Element type it cannot decode, it should stop
decoding the FEC TLV, abort processing the message containing the
TLV, and send an "Unknown FEC" Notification message to its LDP peer
signaling an error.
3.4.2. Label TLVs
Label TLVs encode labels. Label TLVs are carried by the messages
used to advertise, request, release and withdraw label mappings.
There are several different kinds of Label TLVs which can appear in
situations that require a Label TLV.
3.4.2.1. Generic Label TLV
An LSR uses Generic Label TLVs to encode labels for use on links for
which label values are independent of the underlying link technology.
Examples of such links are PPP and Ethernet.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Generic Label (0x0200) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Label
This is a 20-bit label value as specified in [RFC3032] represented
as a 20-bit number in a 4 octet field.
3.4.2.2. ATM Label TLV
An LSR uses ATM Label TLVs to encode labels for use on ATM links.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| ATM Label (0x0201) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Res| V | VPI | VCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Res
This field is reserved. It must be set to zero on transmission
and must be ignored on receipt.
V-bits
Two-bit switching indicator. If V-bits is 00, both the VPI and
VCI are significant. If V-bits is 01, only the VPI field is
significant. If V-bit is 10, only the VCI is significant.
VPI
Virtual Path Identifier. If VPI is less than 12-bits it should be
right justified in this field and preceding bits should be set to
0.
VCI
Virtual Channel Identifier. If the VCI is less than 16- bits, it
should be right justified in the field and the preceding bits must
be set to 0. If Virtual Path switching is indicated in the V-bits
field, then this field must be ignored by the receiver and set to
0 by the sender.
3.4.2.3. Frame Relay Label TLV
An LSR uses Frame Relay Label TLVs to encode labels for use on Frame
Relay links.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Frame Relay Label (0x0202)| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Len| DLCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Res
This field is reserved. It must be set to zero on transmission
and must be ignored on receipt.
Len
This field specifies the number of bits of the DLCI. The
following values are supported:
0 = 10 bits DLCI
2 = 23 bits DLCI
Len values 1 and 3 are reserved.
DLCI
The Data Link Connection Identifier. Refer to [RFC3034] for the
label values and formats.
3.4.3. Address List TLV
The Address List TLV appears in Address and Address Withdraw
messages.
Its encoding is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Address List (0x0101) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Family | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| Addresses |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Address Family
Two octet quantity containing a value from ADDRESS FAMILY NUMBERS
in [RFC1700] that encodes the addresses contained in the Addresses
field.
Addresses
A list of addresses from the specified Address Family. The
encoding of the individual addresses depends on the Address Family.
The following address encodings are defined by this version of the
protocol:
Address Family Address Encoding
IPv4 4 octet full IPv4 address
IPv6 16 octet full IPv6 address
3.4.4. Hop Count TLV
The Hop Count TLV appears as an optional field in messages that set
up LSPs. It calculates the number of LSR hops along an LSP as the
LSP is being setup.
Note that setup procedures for LSPs that traverse ATM and Frame Relay
links require use of the Hop Count TLV (see [RFC3035] and [RFC3034]).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Hop Count (0x0103) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HC Value |
+-+-+-+-+-+-+-+-+
HC Value
1 octet unsigned integer hop count value.
3.4.4.1. Hop Count Procedures
During setup of an LSP an LSR R may receive a Label Mapping or Label
Request message for the LSP that contains the Hop Count TLV. If it
does, it should record the hop count value.
If LSR R then propagates the Label Mapping message for the LSP to an
upstream peer or the Label Request message to a downstream peer to
continue the LSP setup, it must must determine a hop count to include
in the propagated message as follows:
- If the message is a Label Request message, R must increment the
received hop count;
- If the message is a Label Mapping message, R determines the hop
count as follows:
o If R is a member of the edge set of an LSR domain whose LSRs do
not perform 'TTL-decrement' and the upstream peer is within
that domain, R must reset the hop count to 1 before propagating
the message.
o Otherwise, R must increment the received hop count.
The first LSR in the LSP (ingress for a Label Request message, egress
for a Label Mapping message) should set the hop count value to 1.
By convention a value of 0 indicates an unknown hop count. The
result of incrementing an unknown hop count is itself an unknown hop
count (0).
Use of the unknown hop count value greatly reduces the signaling
overhead when independent control is used. When a new LSP is
established, each LSR starts with unknown hop count. Addition of a
new LSR whose hop count is also unknown does not cause a hop count
update to be propagated upstream since the hop count remains unknown.
When the egress is finally added to the LSP, then the LSRs propagate
hop count updates upstream via Label Mapping messages.
Without use of the unknown hop count, each time a new LSR is added to
the LSP a hop count update would need to be propagated upstream if
the new LSR is closer to the egress than any of the other LSRs.
These updates are useless overhead since they don't reflect the hop
count to the egress.
From the perspective of the ingress node, the fact that the hop count
is unknown implies nothing about whether a packet sent on the LSP
will actually make it to the egress. All it implies is that the hop
count update from the egress has not yet reached the ingress.
If an LSR receives a message containing a Hop Count TLV, it must
check the hop count value to determine whether the hop count has
exceeded its configured maximum allowable value. If so, it must
behave as if the containing message has traversed a loop by sending a
Notification message signaling Loop Detected in reply to the sender
of the message.
If Loop Detection is configured, the LSR must follow the procedures
specified in Section "Loop Detection".
3.4.5. Path Vector TLV
The Path Vector TLV is used with the Hop Count TLV in Label Request
and Label Mapping messages to implement the optional LDP loop
detection mechanism. See Section "Loop Detection". Its use in the
Label Request message records the path of LSRs the request has
traversed. Its use in the Label Mapping message records the path of
LSRs a label advertisement has traversed to setup an LSP.
Its encoding is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Path Vector (0x0104) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSR Id 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSR Id n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
One or more LSR Ids
A list of router-ids indicating the path of LSRs the message has
traversed. Each LSR Id is the first four octets (router-id) of
the LDP identifier for the corresponding LSR. This ensures it is
unique within the LSR network.
3.4.5.1. Path Vector Procedures
The Path Vector TLV is carried in Label Mapping and Label Request
messages when loop detection is configured.
3.4.5.1.1. Label Request Path Vector
Section "Loop Detection" specifies situations when an LSR must
include a Path Vector TLV in a Label Request message.
An LSR that receives a Path Vector in a Label Request message must
perform the procedures described in Section "Loop Detection".
If the LSR detects a loop, it must reject the Label Request message.
The LSR must:
1. Transmit a Notification message to the sending LSR signaling
"Loop Detected".
2. Not propagate the Label Request message further.
Note that a Label Request message with Path Vector TLV is forwarded
until:
1. A loop is found,
2. The LSP egress is reached,
3. The maximum Path Vector limit or maximum Hop Count limit is
reached. This is treated as if a loop had been detected.
3.4.5.1.2. Label Mapping Path Vector
Section "Loop Detection" specifies the situations when an LSR must
include a Path Vector TLV in a Label Mapping message.
An LSR that receives a Path Vector in a Label Mapping message must
perform the procedures described in Section "Loop Detection".
If the LSR detects a loop, it must reject the Label Mapping message
in order to prevent a forwarding loop. The LSR must:
1. Transmit a Label Release message carrying a Status TLV to the
sending LSR to signal "Loop Detected".
2. Not propagate the message further.
3. Check whether the Label Mapping message is for an existing LSP.
If so, the LSR must unsplice any upstream labels which are
spliced to the downstream label for the FEC.
Note that a Label Mapping message with a Path Vector TLV is forwarded
until:
1. A loop is found,
2. An LSP ingress is reached, or
3. The maximum Path Vector or maximum Hop Count limit is reached.
This is treated as if a loop had been detected.
3.4.6. Status TLV
Notification messages carry Status TLVs to specify events being
signaled.
The encoding for the Status TLV is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Status (0x0300) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
U bit
Should be 0 when the Status TLV is sent in a Notification message.
Should be 1 when the Status TLV is sent in some other message.
F bit
Should be the same as the setting of the F-bit in the Status Code
field.
Status Code
32-bit unsigned integer encoding the event being signaled. The
structure of a Status Code is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E|F| Status Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
E bit
Fatal error bit. If set (=1), this is a fatal error
notification. If clear (=0), this is an advisory notification.
F bit
Forward bit. If set (=1), the notification should be forwarded
to the LSR for the next-hop or previous-hop for the LSP, if
any, associated with the event being signaled. If clear (=0),
the notification should not be forwarded.
Status Data
30-bit unsigned integer which specifies the status information.
This specification defines Status Codes (32-bit unsigned integers
with the above encoding).
A Status Code of 0 signals success.
Message ID
If non-zero, 32-bit value that identifies the peer message to
which the Status TLV refers. If zero, no specific peer message is
being identified.
Message Type
If non-zero, the type of the peer message to which the Status TLV
refers. If zero, the Status TLV does not refer to any specific
message type.
Note that use of the Status TLV is not limited to Notification
messages. A message other than a Notification message may carry a
Status TLV as an Optional Parameter. When a message other than a
Notification carries a Status TLV the U-bit of the Status TLV should
be set to 1 to indicate that the receiver should silently discard the
TLV if unprepared to handle it.
3.5. LDP Messages
All LDP messages have the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U| Message Type | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Mandatory Parameters |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Optional Parameters |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
U bit
Unknown message bit. Upon receipt of an unknown message, if U is
clear (=0), a notification is returned to the message originator;
if U is set (=1), the unknown message is silently ignored. The
sections following that define messages specify a value for the
U-bit.
Message Type
Identifies the type of message
Message Length
Specifies the cumulative length in octets of the Message ID,
Mandatory Parameters, and Optional Parameters.
Message ID
32-bit value used to identify this message. Used by the sending
LSR to facilitate identifying notification messages that may apply
to this message. An LSR sending a notification message in
response to this message should include this Message Id in the
Status TLV carried by the notification message; see Section
"Notification Message".
Mandatory Parameters
Variable length set of required message parameters. Some messages
have no required parameters.
For messages that have required parameters, the required
parameters MUST appear in the order specified by the individual
message specifications in the sections that follow.
Optional Parameters
Variable length set of optional message parameters. Many messages
have no optional parameters.
For messages that have optional parameters, the optional
parameters may appear in any order.
Note that there is no alignment requirement for the first octet of an
LDP message.
The following message types are defined in this version of LDP:
Message Name Section Title
Notification "Notification Message"
Hello "Hello Message"
Initialization "Initialization Message"
KeepAlive "KeepAlive Message"
Address "Address Message"
Address Withdraw "Address Withdraw Message"
Label Mapping "Label Mapping Message"
Label Request "Label Request Message"
Label Abort Request "Label Abort Request Message"
Label Withdraw "Label Withdraw Message"
Label Release "Label Release Message"
The sections that follow specify the encodings and procedures for
these messages.
Some of the above messages are related to one another, for example
the Label Mapping, Label Request, Label Withdraw, and Label Release
messages.
While it is possible to think about messages related in this way in
terms of a message type that specifies a message class and a message
subtype that specifies a particular kind of message within that
class, this specification does not formalize the notion of a message
subtype.
The specification assigns type values for related messages, such as
the label messages, from of a contiguous block in the 16-bit message
type number space.
3.5.1. Notification Message
An LSR sends a Notification message to inform an LDP peer of a
significant event. A Notification message signals a fatal error or
provides advisory information such as the outcome of processing an
LDP message or the state of the LDP session.
The encoding for the Notification Message is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Notification (0x0001) | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status (TLV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Parameters |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message ID
32-bit value used to identify this message.
Status TLV
Indicates the event being signaled. The encoding for the Status
TLV is specified in Section "Status TLV".
Optional Parameters
This variable length field contains 0 or more parameters, each
encoded as a TLV. The following Optional Parameters are generic
and may appear in any Notification Message:
Optional Parameter Type Length Value
Extended Status 0x0301 4 See below
Returned PDU 0x0302 var See below
Returned Message 0x0303 var See below
Other Optional Parameters, specific to the particular event being
signaled by the Notification Messages may appear. These are
described elsewhere.
Extended Status
The 4 octet value is an Extended Status Code that encodes
additional information that supplements the status information
contained in the Notification Status Code.
Returned PDU
An LSR uses this parameter to return part of an LDP PDU to the
LSR that sent it. The value of this TLV is the PDU header and
as much PDU data following the header as appropriate for the
condition being signaled by the Notification message.
Returned Message
An LSR uses this parameter to return part of an LDP message to
the LSR that sent it. The value of this TLV is the message
type and length fields and as much message data following the
type and length fields as appropriate for the condition being
signaled by the Notification message.
3.5.1.1. Notification Message Procedures
If an LSR encounters a condition requiring it to notify its peer with
advisory or error information it sends the peer a Notification
message containing a Status TLV that encodes the information and
optionally additional TLVs that provide more information about the
condition.
If the condition is one that is a fatal error the Status Code carried
in the notification will indicate that. In this case, after sending
the Notification message the LSR should terminate the LDP session by
closing the session TCP connection and discard all state associated
with the session, including all label-FEC bindings learned via the
session.
When an LSR receives a Notification message that carries a Status
Code that indicates a fatal error, it should terminate the LDP
session immediately by closing the session TCP connection and discard
all state associated with the session, including all label-FEC
bindings learned via the session.
3.5.1.2. Events Signaled by Notification Messages
It is useful for descriptive purpose to classify events signaled by
Notification Messages into the following categories.
3.5.1.2.1. Malformed PDU or Message
Malformed LDP PDUs or Messages that are part of the LDP Discovery
mechanism are handled by silently discarding them.
An LDP PDU received on a TCP connection for an LDP session is
malformed if:
- The LDP Identifier in the PDU header is unknown to the
receiver, or it is known but is not the LDP Identifier
associated by the receiver with the LDP peer for this LDP
session. This is a fatal error signaled by the Bad LDP
Identifier Status Code.
- The LDP protocol version is not supported by the receiver, or
it is supported but is not the version negotiated for the
session during session establishment. This is a fatal error
signaled by the Bad Protocol Version Status Code.
- The PDU Length field is too small (< 14) or too large
(> maximum PDU length). This is a fatal error signaled by the
Bad PDU Length Status Code. Section "Initialization Message"
describes how the maximum PDU length for a session is
determined.
An LDP Message is malformed if:
- The Message Type is unknown.
If the Message Type is < 0x8000 (high order bit = 0) it is an
error signaled by the Unknown Message Type Status Code.
If the Message Type is >= 0x8000 (high order bit = 1) it is
silently discarded.
- The Message Length is too large, that is, indicates that the
message extends beyond the end of the containing LDP PDU. This
is a fatal error signaled by the Bad Message Length Status
Code.
- The message is missing one or more Mandatory Parameters. This
is a non-fatal error signalled by the Missing Message
Parameters Status Code.
3.5.1.2.2. Unknown or Malformed TLV
Malformed TLVs contained in LDP messages that are part of the LDP
Discovery mechanism are handled by silently discarding the containing
message.
A TLV contained in an LDP message received on a TCP connection of an
LDP is malformed if:
- The TLV Length is too large, that is, indicates that the TLV
extends beyond the end of the containing message. This is a
fatal error signaled by the Bad TLV Length Status Code.
- The TLV type is unknown.
If the TLV type is < 0x8000 (high order bit 0) it is an error
signaled by the Unknown TLV Status Code.
If the TLV type is >= 0x8000 (high order bit 1) the TLV is
silently dropped. Section "Unknown TLV in Known Message Type"
elaborates on this behavior.
- The TLV Value is malformed. This occurs when the receiver
handles the TLV but cannot decode the TLV Value. This is
interpreted as indicative of a bug in either the sending or
receiving LSR. It is a fatal error signaled by the Malformed
TLV Value Status Code.
3.5.1.2.3. Session KeepAlive Timer Expiration
This is a fatal error signaled by the KeepAlive Timer Expired Status
Code.
3.5.1.2.4. Unilateral Session Shutdown
This is a fatal event signaled by the Shutdown Status Code. The
Notification Message may optionally include an Extended Status TLV to
provide a reason for the Shutdown. The sending LSR terminates the
session immediately after sending the Notification.
3.5.1.2.5. Initialization Message Events
The session initialization negotiation (see Section "Session
Initialization") may fail if the session parameters received in the
Initialization Message are unacceptable. This is a fatal error. The
specific Status Code depends on the parameter deemed unacceptable,
and is defined in Sections "Initialization Message".
3.5.1.2.6. Events Resulting From Other Messages
Messages other than the Initialization message may result in events