3.5.2. Hello Message
LDP Hello messages are exchanged as part of the LDP Discovery Mechanism; see Section "LDP Discovery".
The encoding for the Hello message is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Hello (0x0100) | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Common Hello Parameters TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Parameters |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message ID 32-bit value used to identify this message.
Common Hello Parameters TLV Specifies parameters common to all Hello messages. The encoding for the Common Hello Parameters TLV is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Common Hello Parms(0x0400)| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hold Time |T|R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Hold Time Hello hold time in seconds. An LSR maintains a record of Hellos received from potential peers (see Section "Hello Message Procedures"). Hello Hold Time specifies the time the sending LSR will maintain its record of Hellos from the receiving LSR without receipt of another Hello.
A pair of LSRs negotiates the hold times they use for Hellos
from each other. Each proposes a hold time. The hold time
used is the minimum of the hold times proposed in their Hellos.
A value of 0 means use the default, which is 15 seconds for
Link Hellos and 45 seconds for Targeted Hellos. A value of
0xffff means infinite.
T, Targeted Hello A value of 1 specifies that this Hello is a Targeted Hello. A value of 0 specifies that this Hello is a Link Hello.
R, Request Send Targeted Hellos A value of 1 requests the receiver to send periodic Targeted Hellos to the source of this Hello. A value of 0 makes no request.
An LSR initiating Extended Discovery sets R to 1. If R is 1,
the receiving LSR checks whether it has been configured to send
Targeted Hellos to the Hello source in response to Hellos with
this request. If not, it ignores the request. If so, it
initiates periodic transmission of Targeted Hellos to the Hello
source.
Reserved This field is reserved. It MUST be set to zero on transmission and ignored on receipt.
Optional Parameters This variable length field of the Hello message contains 0 or more parameters, each encoded as a TLV. The optional parameters defined by this version of the protocol are
Optional Parameter Type Length Value
IPv4 Transport Address 0x0401 4 See below
Configuration 0x0402 4 See below
Sequence Number
IPv6 Transport Address 0x0403 16 See below
IPv4 Transport Address Specifies the IPv4 address to be used for the sending LSR when opening the LDP session TCP connection. If this optional TLV is not present, the IPv4 source address for the UDP packet carrying the Hello SHOULD be used.
Configuration Sequence Number Specifies a 4 octet unsigned configuration sequence number that identifies the configuration state of the sending LSR. Used by the receiving LSR to detect configuration changes on the sending LSR.
IPv6 Transport Address Specifies the IPv6 address to be used for the sending LSR when opening the LDP session TCP connection. If this optional TLV is not present the IPv6 source address for the UDP packet carrying the Hello SHOULD be used.
3.5.2.1. Hello Message Procedures
An LSR receiving Hellos from another LSR maintains a Hello adjacency corresponding to the Hellos. The LSR maintains a hold timer with the Hello adjacency, which it restarts whenever it receives a Hello that matches the Hello adjacency. If the hold timer for a Hello adjacency expires the LSR discards the Hello adjacency: see Sections "Maintaining Hello Adjacencies" and "Maintaining LDP Sessions".
We recommend that the interval between Hello transmissions be at most one third of the Hello hold time.
An LSR processes a received LDP Hello as follows:
-
The LSR checks whether the Hello is acceptable. The criteria for determining whether a Hello is acceptable are implementation dependent (see below for example criteria).
-
If the Hello is not acceptable, the LSR ignores it.
-
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.
-
If the Hello carries any optional TLVs, the LSR processes them (see below).
-
Finally, if the LSR has no LDP session for the label space specified by the LDP Identifier in the PDU header for the Hello, it follows the procedures of Section "LDP Session Establishment".
The following are examples of acceptability criteria for Link and Targeted Hellos:
A Link Hello is acceptable if the interface on which it was received has been configured for label switching.
A Targeted Hello from source address A is acceptable if either:
-
The LSR has been configured to accept Targeted Hellos, or
-
The LSR has been configured to send Targeted Hellos to A.
The following describes how an LSR processes Hello optional TLVs:
Transport Address The LSR associates the specified transport address with the Hello adjacency.
Configuration Sequence Number The Configuration Sequence Number optional parameter is used by the sending LSR to signal configuration changes to the receiving LSR. When a receiving LSR playing the active role in LDP session establishment detects a change in the sending LSR configuration, it may clear the session setup backoff delay, if any, associated with the sending LSR (see Section "Session Initialization").
A sending LSR using this optional parameter is responsible for
maintaining the configuration sequence number it transmits in
Hello messages. Whenever there is a configuration change on
the sending LSR, it increments the configuration sequence
number.