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15-441 Computer Networks ATM and MPLS Professor Hui Zhang [email protected] 1 High Speed IP Router? incoming links Node outgoing links Memory 2 Dest IP address lookup for every packet Switching and buffer management for variable size packet Asynchronous Transfer Mode: ATM ITU standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architecture 3 Most of the work done in 90’s Goal: integrated, end-end transport of carry voice, video, data meeting timing/QoS requirements of voice, video (versus Internet best-effort model) “next generation” telephony: technical roots in telephone world Four Ideas Flexible and efficient packet switching Support voice, video, data needs end-to-end Quality of Service connection-oriented network virtual circuit network Scale to high performance switch fixed size packet Support low jitter voice small size packet 4 Virtual Circuit Concept Logical Connection Connection is first established using signaling protocol 5 Route from the source to the destination is chosen The same route is used for all packets of the connection No routing decision for every cell Virtual Circuit Concepts No dedicated capacity 6 Packet switching to enable statistical multiplexing Each packet contains enough information for node (switch) to forward it towards the destination Virtual Circuit Switching: Label Swapping E C VC4 A B VC5 D F IN LINK IN VC OUT LINK OUT VC CA 7 AB 4 CA 2 AB 5 DA 3 AB 3 Table at Node A 7 Signaling Protocol Signaling protocol establishes/tears down virtual circuit 8 Signaling message are routed Signaling protocol fills the forwarding table Parameters used for establishing Virtual Circuits Source and destination Addresses Traffic Characteristics QoS Parameters Others? Parameters can be stored in forwarding table to help forwarding decision ATM architecture Adaptation layer: only at edge of ATM network data segmentation/reassembly roughly analagous to Internet transport layer ATM layer: “network” layer 9 cell switching, routing Physical layer ATM Layer: Virtual Circuits VC transport: cells carried on VC from source to dest call setup, teardown for each call before data can flow each packet carries VC identifier (not destination ID) every switch on source-dest path maintain “state” for each passing connection link,switch resources (bandwidth, buffers) may be allocated to VC: to get circuit-like performance Permanent VCs (PVCs) Switched VCs (SVC): 10 long lasting connections dynamically set up on per-call basis Cell Size: 32 bytes or 64 bytes? 11 Cell size of 32 and 64 bytes: 64 bytes cells have better transmission efficiency 32 bytes cells have small delay both sizes are integer power of 2 Europe wanted 32 bytes size, US and Japan wanted 64 bytes size Compromise: 48 bytes ATM Cell Format Header :5 bytes at GFC UNI 4 at NNI GFC VCI CLP UNI 12 Payload (Information) 48 bytes VPI 8 VPI 12 : : : : Generic Flow Control Virtual Circuit Identifier Cell Loss Priority User Network Interface VCI PT 16 3 VCI PT 16 3 VPI PT HEC NNI : : : : C L P HEC 1 C L P 8 bits 8 bits HEC 1 Virtual Path Identifier Payload Type Header error Check Network-Network Interface Questions 13 How many Virtual Circuits can one ATM switch support? What is the purpose of Virtual Path? Protocol vs. Service Service – says what a layer does Ethernet: unreliable subnet unicast/multicast/broadcast datagram service IP: unreliable end-to-end unicast datagram service TCP: reliable end-to-end bi-directional byte stream service Guaranteed bandwidth/latency unicast service Protocol – says how is the service implemented a set of rules and formats that govern the communication between two peers – Packet format, how to interpret packet fields – State machine of protocol messages 14 ATM Adaptation Layer (AAL) ATM Adaptation Layer (AAL): “adapts” upper layers (IP or native ATM applications) to ATM layer below AAL present only in end systems, not in switches AAL layer segment (header/trailer fields, data) fragmented across multiple ATM cells 15 analogy: TCP segment in many IP packets AAL 5 Packet User Data 0-65535 bytes 16 PAD : padding User Data 8 Bytes header PAD 0-47 bytes UU CPI 1 1 Length 2 CRC 4 ATM: network or link layer? Vision: end-to-end transport: “ATM from desktop to desktop” ATM is a network technology Reality: used to connect IP backbone routers 17 “IP over ATM” ATM as switched link layer, connecting IP routers Where is ATM Today? 18 DSL (Digital Subscriber Loop) Multi-service switching Interconnection of IP routers MPLS 19 Multi-Protocol Label Switching Bringing virtual circuit concept into IP Driven by multiple forces QoS traffic engineering High performance forwarding VPN MPLS Vocabulary Label-switched path (LSP) Simplex path through interior network New York San Francisco 20 MPLS Vocabulary Label-switching router (LSR) performs MPLS packet forwarding LSP setup New York San Francisco 21 MPLS Vocabulary Label Edge Router (LER) Ingress and egress node of LSP New York Ingress San Francisco 22 Egress MPLS Header IP packet is encapsulated in MPLS header and sent down LSP IP Packet … 32-bit MPLS Header IP packet is restored at end of LSP by egress router 25 TTL is adjusted also MPLS Header Label 26 Label Class of service Stacking bit Time to live Decrement at each LSR, or Pass through unchanged CoS S TTL Forwarding Equivalence Classes LSR LER LSR LER LSP IP1 IP1 IP1 #L1 IP1 #L2 IP1 #L3 IP2 #L1 IP2 #L2 IP2 #L3 IP2 IP2 Packets are destined for different address prefixes, but can be mapped to common path • FEC = “A subset of packets that are all treated the same way by a router” • The concept of FECs provides for a great deal of flexibility and scalability • In conventional routing, a packet is assigned to a FEC at each hop (i.e. L3 look-up), in MPLS it is only done once at the network ingress. 27 LABEL SWITCHED PATHS Driven by Routing #216 #14 #311 #99 #311 #963 #311 #963 #14 #612 #5 #462 #99 #311 - A LSP is actually part of a tree from every source to that destination (unidirectional). - Control protocol (e.g. LDP) builds that tree using existing IP forwarding tables to route the control messages. 28 MPLS BUILT ON STANDARD IP Dest 47.1 47.2 47.3 Dest 47.1 47.2 47.3 Out 1 2 3 Out 1 2 3 1 47.1 3 1 Dest 47.1 47.2 47.3 Out 1 2 3 2 3 2 1 47.2 47.3 3 2 • Destination based forwarding tables as built by OSPF, IS-IS, RIP, etc. 29 IP FORWARDING USED BY HOPBY-HOP CONTROL Dest 47.1 47.2 47.3 Dest 47.1 47.2 47.3 Out 1 2 3 1 47.1 1 Dest 47.1 47.2 47.3 Out 1 2 3 IP 47.1.1.1 2 IP 47.1.1.1 3 Out 1 2 3 2 IP 47.1.1.1 1 47.2 47.3 3 2 IP 47.1.1.1 30 Label Switched Path (LSP) Intf Label Dest Intf Label In In Out Out 3 50 47.1 1 40 Intf Dest Intf Label In Out Out 3 47.1 1 50 31 2 2 47.2 2 IP 47.1.1.1 3 1 47.3 3 Label Dest Intf In Out 40 47.1 1 IP 47.1.1.1 1 47.1 3 1 Intf In 3 Route= {A,B,C} EXPLICITLY ROUTED OR ER-LSP #14 #972 #216 B #14 A C #972 #462 - ER-LSP follows route that source chooses. In other words, the control message to establish the LSP (label request) is source routed. 32 EXPLICITLY ROUTED LSP ER-LSP Intf Label Dest Intf Label In In Out Out 3 50 47.1 1 40 Intf In 3 3 Dest 47.1.1 47.1 Intf Out 2 1 Label Out 33 50 Intf In 3 Label Dest Intf In Out 40 47.1 1 IP 47.1.1.1 1 47.1 3 3 2 1 1 47.3 3 47.2 2 IP 47.1.1.1 33 2 Protocol Comparison Ethernet IP Forwarding Control Protocols Dest MAC address Learning Exact match Spanning tree Dest IP address Routing protocol Longest prefix match TDM ATM MPLS 34 Time slot, exact match E2E signaling protocol Time Slot Exchange (TSE) Routing protocol Label, exact match E2E signaling protocol Label swapping Routing protocol Label, Dest IP Address Flexible signaling Routing Protocol