RFC 3031 Multiprotocol Label Switching Architecture 学习笔记一

2.1. Overview   As a packet of a connectionless network layer protocol travels from   one router to the next, each router makes an independent forwarding   decision for that packet.  That is, each router analyzes the packet's   header, and each router runs a network layer routing algorithm.  Each   router independently chooses a next hop for the packet, based on its   analysis of the packet's header and the results of running the   routing algorithm.

   ——此段描述网络中，路由器之间发送报文的过程中，每个路由器需要解析报文

并根据报文的信息进行路由

   Packet headers contain considerably more information than is needed   simply to choose the next hop.  Choosing the next hop can therefore   be thought of as the composition of two functions.  The first   function partitions the entire set of possible packets into a set of   "Forwarding Equivalence Classes (FECs)".  The second maps each FEC to   a next hop.  Insofar as the forwarding decision is concerned,   different packets which get mapped into the same FEC are   indistinguishable.  All packets which belong to a particular FEC and   which travel from a particular node will follow the same path (or if   certain kinds of multi-path routing are in use, they will all follow   one of a set of paths associated with the FEC).
   ——就转发而言，报文头中携带的信息往往多于实际需要。而实际上，查找下一跳

可以理解为两个功能结合起来的结果：第一部分就是将报文归到一个转发等价类中，

第二个功能是将这个转发等价类映射到一个下一跳。这里讲到从同一节点来切同属于

同一转发等价类的报文将遵循同一路径进行转发，当存在多条路径的时候，将从这些

路径中选择一条

   In conventional IP forwarding, a particular router will typically   consider two packets to be in the same FEC if there is some address   prefix X in that router's routing tables such that X is the "longest   match" for each packet's destination address.  As the packet   traverses the network, each hop in turn reexamines the packet and   assigns it to a FEC.
   ——每个路由器都重复使用最长匹配原则来路由报文
   
   In MPLS, the assignment of a particular packet to a particular FEC is   done just once, as the packet enters the network.  The FEC to which   the packet is assigned is encoded as a short fixed length value known   as a "label".  When a packet is forwarded to its next hop, the label   is sent along with it; that is, the packets are "labeled" before they   are forwarded.
   ——这里提出了MPLS最大好处：将报文与转发等价类只在进入网络的时候进行一次
映射，之后使用label来携带这个信息，之后的路由器使用label进行转发
   
   At subsequent hops, there is no further analysis of the packet's   network layer header.  Rather, the label is used as an index into a   table which specifies the next hop, and a new label.  The old label   is replaced with the new label, and the packet is forwarded to its   next hop.
   —— 之后的路由器将不再对报文进行解析，仅根据包围中的label索引到下一跳
和新label的信息，然后使用新的label替换报文中老的label并对报文进行转发
   In the MPLS forwarding paradigm, once a packet is assigned to a FEC,   no further header analysis is done by subsequent routers; all   forwarding is driven by the labels.  This has a number of advantages   over conventional network layer forwarding.
   ——MPLS的转发模型中，报文一旦与一个FEC绑定，之后的节点将不再解析报文头
所有的转发行为都是label进行驱动的。这样做有如下有点：
   
    -  MPLS forwarding can be done by switches which are capable of         doing label lookup and replacement, but are either not capable         of analyzing the network layer headers, or are not capable of         analyzing the network layer headers at adequate speed.
    ——只要设备可以做label的查找和替换即可，不必具有网络层次的能力或者
没有足够的处理网络层次报文头的速度
    
    -  Since a packet is assigned to a FEC when it enters the network,         the ingress router may use, in determining the assignment, any         information it has about the packet, even if that information         cannot be gleaned from the network layer header.  For example,         packets arriving on different ports may be assigned to         different FECs.  Conventional forwarding, on the other hand,         can only consider information which travels with the packet in         the packet header.
   ——在报文进入网络时，路由器不仅可以根据报文本身的信息来绑定FEC，还可以
使用例如入接口这样的信息
   -  A packet that enters the network at a particular router can be         labeled differently than the same packet entering the network         at a different router, and as a result forwarding decisions         that depend on the ingress router can be easily made.  This         cannot be done with conventional forwarding, since the identity         of a packet's ingress router does not travel with the packet.
   ——同样的报文通过不同的路由器进入网络会有不同的转发路径。这在传统的
转发时做不到的，原因是从那一台路由器进入网络的信息是不随报文携带的
   
   -  The considerations that determine how a packet is assigned to a         FEC can become ever more and more complicated, without any         impact at all on the routers that merely forward labeled         packets.
   ——决定报文映射到某个FEC影响因素将越来月复杂，但是对于中间转发的路由器来说
没有任何的影响
   
    -  Sometimes it is desirable to force a packet to follow a         particular route which is explicitly chosen at or before the         time the packet enters the network, rather than being chosen by         the normal dynamic routing algorithm as the packet travels         through the network.  This may be done as a matter of policy,         or to support traffic engineering.  In conventional forwarding,         this requires the packet to carry an encoding of its route         along with it ("source routing").  In MPLS, a label can be used         to represent the route, so that the identity of the explicit         route need not be carried with the packet.
    ——某些情况下，在进入网络前或者进入网络时决定报文转发的路径相对于使用
动态的路由算法来传输报文相比是可取的。MPLS使用label来携带这些信息，而传统
方式选需要另外封装这些信息
   Some routers analyze a packet's network layer header not merely to   choose the packet's next hop, but also to determine a packet's   "precedence" or "class of service".  They may then apply different   discard thresholds or scheduling disciplines to different packets.   MPLS allows (but does not require) the precedence or class of service   to be fully or partially inferred from the label.  In this case, one   may say that the label represents the combination of a FEC and a   precedence or class of service.
   ——MPLS也是支持差分服务的