Patent Publication Number: US-7907603-B2

Title: Acceleration of label distribution protocol (LDP) session setup

Description:
BACKGROUND OF THE INVENTION 
     Multiprotocol Label Switching (MPLS) is utilized to achieve many of the benefits of a circuit-switched network over a packet-switched network. MPLS works by prepending labels to packets and packets are switched according to labels instead of look-up of destination addresses. 
     One of the fundamental tasks in the MPLS architecture is to exchange labels between label switch routers (LSR) and define the semantics of these labels. LSRs follow a set of procedures, known as label distribution protocol (LDP), to accomplish this task. 
     LDP peers are two LSRs that use LDP to exchange label information. An LSR might have more than one LDP peer, and it establishes an LDP session with each LDP peer. An LDP session is always bidirectional, which allows both LDP peers to exchange label information. 
     TECHNICAL FIELD 
     The present disclosure relates generally to techniques for accelerating the setup of a session between LDP peers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example of an LSR topology; 
         FIG. 2  illustrates an LSR topology which uses both basic and extended discovery; 
         FIG. 3  illustrates an example of the LDP discovery sequence; 
         FIG. 4A  illustrates an example of sending a Hello message immediately after receipt of a Hello message; 
         FIG. 4B  illustrates an example of delaying the sending a Hello message if a time period has not expired; 
         FIG. 5  illustrates an example of sending Hello messages at a rate higher than the default rate; 
         FIG. 6  illustrates an example of opening a listening port when a targeted Hello message is sent; and 
         FIG. 7  illustrates the components of a network device. 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS OVERVIEW 
     Various embodiments disclose techniques to modify the LDP default rate of transmission of Hello messages or to accelerate the opening of a Transmission Control Protocol (TCP) listening port on an LSR in order to accelerate the establishment of an LDP session between LDP peers. 
     The following describes normal LDP operation sufficiently to support the description of the example embodiments. 
     Normal LDP operation will be described with reference to the topology depicted in  FIG. 1 . To establish an LDP session, LSR 1  and LSR 2  must first discover one another. Following discovery, a session is established by opening a TCP connection between LSR 1  and LSR 2  and using it to exchange messages which set session parameters. 
     LSRs use LDP discovery procedures to locate possible LDP peers. The basic discovery mechanism identifies directly connected LDP peers. An extended discovery mechanism identifies non-directly connected LDP peers. LSRs discover LDP peers by exchanging LDP Hello messages. There are two types of LDP Hello messages. LDP Link Hellos are used for LDP basic discovery where the LSRs are directly connected by a link. For some MPLS applications non-directly connected LSRs must exchange label information. Before establishing LDP sessions between non-directly connected LSRs, the LSRs engage in LDP extended discovery by periodically sending Targeted Hello messages to a specific address. When an LSR sends a Targeted Hello message to a receiving LSR, the receiving LSR can either accept the Targeted Hello or ignore it. The receiving LSR accepts the Targeted Hello by creating a Hello adjacency with the originating LSR and periodically sending Targeted Hellos to it. 
       FIG. 2  shows a simple typical MPLS network and indicates where LDP basic discovery and LDP extended discovery are likely to occur. 
     As described above, directly connected LSRs discover each other via basic discovery by transmitting Hello messages on the link(s) which connect them. Referring now to  FIG. 1 , when LSR 2  receives a Hello message from LSR 1  it considers LSR 1  to be discovered. 
     The first step in session establishment is the establishment of a reliable transport connection between LDP peers. If both LDP peers were to initiate an LDP TCP connection, two concurrent TCP connections might result. To avoid this situation, after an LSR discovers a potential peer it determines whether to play an active or passive role in establishing the session TCP connection. The LSR does this by comparing its IP address (treated as an unsigned integer) with that of the discovered potential peer. The LSR with the smaller address plays the passive role. In this example, LSR 1  has an IP address “1” and LSR 2  has an IP address “2”. As depicted in  FIG. 3 , depending on whether an LSR plays the active or passive role, it will either initiate the establishment of a TCP connection to a discovered peer or open a TCP listening port. 
     For the simple topology of  FIG. 1 , when LSR 1  receives a discovery Hello message it determines that it should play the passive role and waits for LSR 2  to connect to it by opening a TCP listening port. In this situation LSR 2  plays the active role and attempts to open a TCP connection to the listening port of LSR 1 . 
     DESCRIPTION 
     Reference will now be made in detail to various embodiments of the invention. Examples of these embodiments are illustrated in the accompanying drawings. While the invention will be described in conjunction with these embodiments, it will be understood that it is not intended to limit the invention to any embodiment. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the various embodiments. However, the present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail in order to avoid unnecessarily obscuring the present invention. 
     The LDP procedure for establishing the session TCP connection allows various situations which may delay the establishment of the connection between LDP peers. In conventional LDP implementations the LSRs send the discovery Hello messages independently and periodically. Two LSRs may discover each other at different times. The difference may be large, as the default Hello interval is 5 seconds. This difference may result in slow LDP session establishment as described in more detail below. 
     Consider  FIG. 3 , and assume that LSR 1  and LSR 2  are using the default Hello interval which causes each to send a Hello every 5 seconds. For this situation in the worst case, LSR 1  sends a Hello message at time “0” and LSR 2  sends its Hello message at time “5”. LSR 2  discovers LSR 1  when it receives LSR 2 &#39;s Hello message. LSR 2  then determines from the Hello message that its IP address is greater that the IP address of LSR 1 , assumes the active role and attempts to establish a transport connection with LSR 1 . However, LSR 2  can not successfully establish the TCP connection to LSR 1  until LSR 1  discovers LSR 2  by receiving LSR 2 &#39;s Hello message sent at time “5” and opens its listening port shortly thereafter. Thus, the establishment of the connection is delayed by the default time between the sending of Hello messages. 
     A first embodiment of the invention will now be described with reference to  FIG. 4A . The peer discovery procedure is modified such that LSR 2  sends a Hello message immediately when it receives the first Hello message from LSR 1 . This allows LSR 1  to discover LSR 2  and open its listening port shortly after LSR 2  has discovered it, enabling the LDP session connection to be established with little delay. 
     This first embodiment, as described in the previous paragraph, works well when LSR 1  and LSR 2  are connected by a point-to-point link. However, when an LSR is connected to a multiaccess link, such as an ethernet, responding to the first Hello received on the link could result in a “storm” of Hellos, significantly slowing session establishment. For example, suppose that there are 10 LSRs connected to the multiaccess link and that each LSR has LDP enabled for the link. LSR 1 &#39;s first Hello will be received by each of LSR 2  through LSR 10 . Each will respond to LSR 1 &#39;s Hello with its own Hello, and each of those (first) Hello&#39;s will trigger a response by each of the other LSR&#39;s. If there are N LSR&#39;s connected to the multiaccess link the number of such Hello&#39;s is N*(N−1). 
     Although it is unlikely that all LSR&#39;s on a multiaccess link will start LDP at precisely the same time the underlying hardware that supports the link may be such that connectivity between large subsets of the LSR&#39;s may suddenly change. There is at least one recently deployed network with hundreds of LSRs running LDP connected to the same multiaccess link. While such a large number of LSRs on a multiaccess link is not typical this first embodiment must not slow session establishment and should help accelerate it when two large subsets previously partitioned are reconnected. 
     The following refinement to the first embodiment prevents the Hello storm. As depicted in the flow chart of  FIG. 4B , when an LSR receives the first Hello from a peer on a multiaccess link the LSR checks whether it has sent a Hello within the last T time units (e.g. milliseconds). If not, it sends its Hello immediately. If it has, the LSR delays sending its Hello until T time units since the last Hello was sent have elapsed. The delay provides an opportunity for the LSR to receive additional first Hello&#39;s from other previously undiscovered peers that may be on the multiaccess link, and enables the Hello it sends after the delay to serve as a response to the accumulated first Hello&#39;s. This reduces the number of Hello messages required to accomplish the discovery from O(N2) to O(N). 
     Another situation which may delay the establishment of a transport connection occurs when Hello messages transmitted by one of the LDP peers is dropped for any of a variety of reasons. Without the second embodiment described below another Hello message will not be sent for five seconds. If consecutive Hello messages are dropped by the system then LDP peer discovery and establishment of the session transport connection can be significantly delayed. 
     Such drops typically occur in situations where links are over subscribed. Generally the drops are statistical in nature in the sense that the specific packets dropped cannot be predicted. An observation is that the likelihood of getting the initial Hello through to the peer router more quickly would be increased by transmitting Hello&#39;s more frequently. That is, since the drops are random, sending more Hello&#39;s increases the likelihood that at least one will be received by the peer. Of course, doing so adds to the congestion as well as to the load on the transmitting and receiving LSRs. Consequently, the increased rate should be moderate and temporary. 
     Another difficulty here is that LSR 1  and LSR 2  discover each other independently, which potentially delays session establishment because their attempts to establish the session TCP connection are out of sync. Specifically, when LSR 2  plays the active role in connection establishment it may try to connect to LSR 1  before the TCP listening port of LSR 1  is ready. As a result the connection attempt fails and is not retried until LSR 1  receives the next Hello from LSR 2  about 5 seconds later. 
     A second embodiment of the invention will now be described with reference to the flow chart of  FIG. 5 . The peer discovery mechanism is modified so that when LSR 1  becomes ready to send Hello messages to LSR 2 —for example, because the link has just come up or because LDP has just been configured on the link interface—it sends the Hello messages at a faster than normal rate, for example, one message every 100 ms, until either it receives a Hello message from LSR 2  or a predetermined relatively short time period elapses. 
     Via the first embodiment when LSR 2  receives the first Hello message from LSR 1  it sends a Hello message immediately (or shortly thereafter). This allows LSR 1  to discover LSR 2  and open its listening port with minimal delay so that a connection can be established. In addition, in this example, after receiving the first Hello, if it is not already doing so, LSR 2  would temporarily send Hello&#39;s at the increased rate as well to increase the likelihood that LSR 1  discovers it quickly. 
     A third embodiment, which applies only to the establishment of targeted LDP sessions, is based on the following observation. With link Hello&#39;s an LSR typically doesn&#39;t know the address of the peer (or peers in the case of a multi-access link such as ethernet) on the other side of the link, or even whether there is a peer running LDP on the other side of the link. However, for targeted LDP sessions the situation is different. For a targeted LDP session the address of the peer can be derived from the LSR configuration. The third embodiment uses this observation; that is, that an LSR sending targeted Hello messages already knows the address of its peer. 
     In this embodiment, as depicted in  FIG. 6 , when LSR 1  sends its first targeted Hello message to LSR 2 , it determines whether it will play the active or passive role in session establishment by comparing its transport address with the peer&#39;s address. In this example, LSR 1  plays the passive role and it opens the required TCP listening port when it sends the first Hello message without waiting for a Hello message from LSR 2 . 
     This embodiment allows the LSR playing the passive role in session establishment to open its TCP listening port in advance of the receipt of the Hello from the active LSR. Thus, when LSR 2  receives the Hello message from LSR 1  it can immediately open a TCP connection port because LSR 1  has enabled its listening port prior to discovering LSR 2 . 
     Various embodiments of the invention have now been described. These embodiments can be utilized in network devices that implement the Label Discovery Protocol by, for example, modifying the LSR software executed by the processor in the network device. Alternatively, the embodiments could be implemented in logic encoded in hardware such as Application Specific Integrated Circuits (ASICs) or Field Programmable Logic Arrays (FPLAs), etc. 
       FIG. 7  depicts an example of a network device including a motherboard  10  having shared DRAM  12 , DRAM  14 , NVRAM  16 , ROM  18  and a CPU  20 . (Other components on the motherboard not relevant to the present description are not depicted). The DRAM  14  is the working storage utilized by the CPU and the shared DRAM  12  is dedicated to handling the packet buffer of the network device. The NVRAM (non-volatile RAM) is used to store the network device&#39;s configuration file and also includes flash memory for storing an image of the LSR software The ROM  18  holds a boot-start program which holds a minimum configuration state needed to start the network device. Alternatively, other configurations of the motherboard can be used. For example, the motherboard may not have separate ROM or NVRAM and the configuration file and LSR software image may be stored and executed out of flash memory. 
     The invention has now been described with reference to the example embodiments. Alternatives and substitutions will now be apparent to persons of skill in the art. Accordingly, it is not intended to limit the invention except as provided by the appended claims.