Label Distribution and Management

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4.3.- Label Distribution and Management

For any given LDP session, each LSR must be aware of the label distribution method used by its peer, i.e., Downstream-on-demand label distribution or Unsolicited Downstream label distribution.

Each LSR can operate with Independent LSP Control or with Ordered LSP Control. Which method to use is defined as a configurable option.

When using Independent LSP Control, each LSR may advertise label mapping to its neighbors at any time it desires. For example, when operating in Downstream-on-demand mode, an LSR may answer a label mapping request inmediately, without waiting for a mapping from the next hop. When operationg in Unsolicited Downstream mode, an LSR may advertise a label mapping for an FEC to its neighbors whenever it is prepared to label-switch that FEC. A consequence of using independent control mode is that an upstream label can be advertised before a downstream label is received.

When using Ordered LSP Control, an LSR may initiate the flooding of label mapping only for those FECs for which it already has a label mapping from that FEC's next hop, or for those FECs for which the LSR is the egress. For those FECs for which the LSR is not the egress and no previous mapping has been received, the LSR MUST wait until a label mapping for that FEC is received from a downstream LSR. Only after this mapping is received, the LSR can in turn passes the mapping to its upstream LSRs.

An LSR is the egress LSR with respect to a particular FEC when one of the following conditions hold:

The FEC refers to the LSR itself (including one of its directly attached interfaces).
The next hop router for the FEC is outside of the Label Switching Network.
FEC elements are reachable by crossing a routing domain boundary, such as another area for OSPF summary networks, or another autonomous system.

When using conservative label retention mode, advertised label mappings are retained only if they will be used to forward packets. Since Downstream-on-demand label distribution mode is primarily used when label conservation is desired (e.g., an ATM switch with limited cross connected space), it is typically used with the conservative label retention mode. Main advantage in this case is that only the labels that are required for the forwarding of data are allocated and maintained. A disadvantage 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.

In Unsolicited Downstream label distribution mode, label mapping advertisements for all routes may be received from all LDP peers. When using liberal label retention mode, every label mapping received from a peer LSR is retained regardless of whether the LSR is the next hop for the advertised mapping. Main advantage in this case is that reaction to routing changes can be quick because labels already exist. A disadvantage ia that unneeded label mappings are distributed and maintained.

An LSR maintains learned labels in a Label Information Base (LIB). 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 forwarding. To retrieve the label the LSR must be able to map the next hop address for the prefix to an LDP Identifier.

Similary, 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 Withdraw Address messages. An Address message is sent to advertise its addresses to a peer. A Withdraw message is sent to withdraw previously advertised addresses from a peer.

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 mechanisms makes use of a Path Vector and Hop Count TLVs carried by Label Request and Label Mapping messages. It works as follows:

A Path Vector TLV contains a list of the LSRs that its containing message has traversed. When an LSR propagates a Path Vector TLV it adds its own LSR Id to the Path Vector list. Then, an LSR that receives a message with a Path Vector containing its own LSR Id can detect that the message has traversed a loop.
A Hop Count TLV contains a count of the LSRs that its containg message has traversed. Each time a message containing a Hop Count TLV is propagated, the LSR increments the count. An LSR that detects a Hop Count that has reached a configured maximum value behaves as if the containing message has traversed a loop.

A detailed specification of the loop detection procedures can be found in the section 2.8 of the original RFC. L.B.

When an LSR detects that some Label Request message it is handling has traveled in a loop, it MUST send a Loop Detected Notification message to the source of the message and MUST drop the Label Request message.

When an LSR detects that some Label Mapping message it is handling has traveled in a loop, it MUST stop using the label advertised by this message for forwarding, it MUST send a Loop Detected Notification message to the source of the message and MUST drop the Label Mapping message.

Authenticity and Integrity of LDP Messages

A detailed specification of the authenticity and integrity mechanisms required to protect LDP session connection streams against the introduction of spoofed TCP segments can be found in the section 2.9 of the original RFC. L.B.

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