Tag Allocation Protocol in GMPLS (1)

The tag Allocation Protocol (LDP) is one of the most important signaling protocols for common multi-protocol tag exchange GMPLS. this article mainly analyzes and studies LDP and its development and evolution, and summarizes and designs common data structures.
1. Traditional LDP
The traditional LDP is defined in the MPLS network and is specifically used to mark the exchange between router LSR) and exchange the "tag/Forwarding Equivalence Class FEC)" binding information for the establishment and maintenance of the tag exchange path LSP) control signaling. LDP directly maps network-layer routing information to the data link layer exchange path to establish a series of LSP processes and messages. two lsrs that use LDP for marking/FEC binding information exchange are called "LDP peering bodies ". when the LDP peer has an exchange of bound information, we say there is a "LDP session" between the two LDP peers ". message Type of 1.1LDP
LDP defines four types of messages:
(1) discover Message Discovery Message): uses UDP messages to periodically broadcast to physically connected nodes or send messages to non-directly connected nodes on a regular basis, to declare the existence of the LSR, identify the nearest neighbor, and check the reliability of the link.
(2) Session Message): TCP transmission is used to establish a TCP link between neighbors to establish a neighbor Session and maintain and close the Session between neighbors.
(3) announcement Message Advertise Message): uses TCP transmission to mark bundling, including request tag, publish tag, recycle tag, release tag, loop detection, and tag inquiry.
(4) Notification Message: uses TCP transmission to report link status and event occurrence. LSR uses the LDP discovery mechanism to discover potential LDP peer nodes. For two adjacent nodes that are directly connected to the link layer and that are not directly connected to the link layer, two different discovery mechanisms are used: basic discovery mechanism and extended discovery mechanism.
1.2LDP creation process
(1) Session creation process
This process is divided into two steps: TCP connection establishment process and Session Initialization Process. through the interaction of LDP Hello messages, two LSR (LDP peer) can be used to establish LDP sessions to transmit tag/FEC binding information. when an LSR broadcasts multiple tag spaces to another LSR using LDP, it creates a session process for each tag space. LDP uses TCP to provide reliable transmission services. When multiple LDP sessions exist between two lsrs, each LDP session corresponds to a TCP connection.
(2) LDP session maintenance
LDP regularly receives the data unit PDU of the LDP Protocol) to maintain session integrity.
(3) mark the allocation process
LDP supports two types of tag control modes: independent control mode and sequential control mode. in the independent control mode, LSR can send a tag ing message to the peer node at any time. in order control mode, LSR can distribute its tag binding information to upstream games only when it is an exit LSR or when it has received downstream binding information about the tag/specified FEC. in the independent and sequential tag allocation control mode, the tag can be retained in a free and conservative manner. the main advantage of the free mode is that the routing changes are quickly reflected. The disadvantage is that unnecessary tags are also allocated and retained. the main advantage of the conservative method is that only the tags used for data forwarding are allocated and maintained. The main disadvantage is that when the route changes, the next hop mark must be obtained before grouping forwarding.
1.3LDP Message format
LDP message exchange between LDP peers is completed by sending the ldp pdu over the TCP connection of the LDP session. each ldp pdu can carry one or more LDP messages, and the content of multiple LDP messages can be irrelevant.
Ldp pdu consists of LDP header) and one/more LDP messages. The format of LDP header is shown in table 1.

LDP uses the TypeLengthValue scheme to encapsulate information carried in messages. Each ldp tlv can be divided into 4-byte public TLV header units and Variable Length Value Domain units. The TLV format is shown in table 2.

All LDP messages are composed of 4-byte message headers, corresponding mandatory parameters, and optional parameters. All parameters adopt the TLV encoding format, as shown in table 3.

The message types defined in the LDP protocol of this version are shown in table 4.