MPLS ( Multi-Protocol Label Switching ) 屬於第三代網路架構，是新一代的 IP 高速骨幹 網路交換標準，由 IETF ( Internet Engineering Task Force ，網際網路工程專案小 組 ) 所提出，由 Cisco 、 3Com 等網路設備大 廠所主導。 MPLS 的運作原理是提供每個 IP 封 包一個標籤，由此決定封包的路徑以及優先順 序，與 MPLS 相容的路由器，會將封包轉送到 其路徑前，僅讀取封包標籤，無須讀取每個封 包的 IP 位址以及標頭，因此網路速度便會加快 許多，同時藉由 QoS ( Quality of Service ) 的 機制對所傳送的封包加以分級，進而大幅提升 網路服務品質並且提供更多樣化的服務。

MPLS Related Protocols Data forwarding Label encapsulation Label operations: PUSH, SWAP and POP Label distribution protocols ( RFC 3036) Provide procedures by which one LSR informs another of the label/FEC binding Extensions to routing protocols Existing routing protocols can be extended to distribute traffic engineering information

Key Terminology in MPLS FEC (Forwarding Equivalence Class) A group of IP packets which are forwarded in the same manner ( 同類轉送等級，每一類 FEC 內的封包，都將以同樣的服務品質傳 送到同樣的目的地 ) Label A short fixed length identifier which is used to identify a FEC Label Swapping Looking up the incoming label to determine the outgoing label, encapsulation and port Label Switched Path (LSP) Path through one or more LSRs for a particular FEC Label Switching Router (LSR) An MPLS capable router

What is a Label The label can be carried in a layer 2 header (e.g., ATM and frame relay) or in a “ shim ” that sits between the layer 2 header and IP (e.g., LAN and PPP) PayloadIP“shim”Layer 2 Label value (20 bits)ExpSTTL Exp: Experimental (3 bits) S: Bottom of label stack (1 bit) TTL: Time-To-Live (8 bits) 4 Octets

MPLS Header Label – locally significant 20-bit label Experimental – 3-bits that are often used for priority Stack – 1-bit bottom of stack indicator TTL – brought from the IP header to support TTL even though MPLS does not examine IP header

Data Forwarding Edge LSR (Ingress) Edge LSR (Egress) LSR Label IP LSR (Penultimate) PUSHPOPSWAP L2 header

Ingress and Transit Operation Port 1Port 4 Ingress LSR LSR Port 2Port 3 FEC Output /16 port 4 PUSH label 40 To: InputOutput port 2 label 40port 3 SWAP label 45 Label: 40 Label: 45

Egress Operation Port 1Port 4 Egress LSR Input Output port 1 label 45 POP Label: 45 To: FEC OutputNext Hop /16 Port The egress router has to do two table lookups There is a concern that this might cause a performance penalty on the egress router