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Virtual Link Layer

Virtual Link Layer. 14-740: Fundamentals of Computer Networks Bill Nace. traceroute. VLANs Link Virtualization Asynchronous Transfer Mode (ATM) Multiprotocol Label Switching (MPLS). Motivation. Can I join these 4 boxes together?. Port-based VLAN. Static VLAN: VLAN=Group of Ports

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Virtual Link Layer

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  1. Virtual Link Layer 14-740: Fundamentals of Computer Networks Bill Nace

  2. traceroute • VLANs • Link Virtualization • Asynchronous Transfer Mode (ATM) • Multiprotocol Label Switching (MPLS)

  3. Motivation Can I join these 4 boxes together?

  4. Port-based VLAN • Static VLAN: VLAN=Group of Ports • Port = switches’ wire connection • Two VLANs configured on a 16-port switch • How do the VLANs communicate with each other?

  5. Connecting VLANs • What happens when the same VLAN exists on multiple switches? • Perhaps for architectural reasons • Ex: ECE Faculty in HH and CIC But, what if you have MANY different VLANs?

  6. Connecting VLANs (2) • Trunked connection: port belongs to all VLANs ➙ all frames at that port are forwarded to all VLANs • But, how does the receiving side know which VLAN a particular frame belongs to?

  7. 802.1Q Tagged Ethernet • VLAN identifier added to Ethernet frame • 4-byte VLAN tag • Includes 12-bit VLAN identifier • Sending switch adds tag, receiving switch parses and removes tag

  8. traceroute • VLANs • Link Virtualization • Asynchronous Transfer Mode (ATM) • Multiprotocol Label Switching (MPLS)

  9. Evolving Understanding of Link Layer • Start of Lecture 20 • “Link = channel connecting adjacent nodes” • Point-to-Point links • Broadcast links

  10. The Zen of “Link” • In Lecture 21, started to realize links can be more complex than a simple wire • Link = channel, but can include switches, hubs, etc • Link can also be PPP/HDLC • Uses complex telephony network, but looks to IP like a single wire

  11. Link Virtualization • What if an entire network could act as a link? • ATM (for instance) connects 2 routers, looks like a link • But, ATM network consists of multiple routers, different standards, its own network stack, different architecture from TCP/IP!!!! • Layered architecturewill let this happen

  12. traceroute • VLANs • Link Virtualization • Asynchronous Transfer Mode (ATM) • Multiprotocol Label Switching (MPLS)

  13. ATM: A Short Introduction • ATM = Asynchronous Transfer Mode • Vision: single integrating network technology for real-time video/audio, plus text/images • Typically used for telephony or WAN scenarios • Designed to be low jitter -- great for streaming video • Connection oriented -- virtual circuit routing

  14. ATM Stack: 3 Layers • ATM Layer • Sort of a Network Layer • Packet is called a “cell” • 5 byte header • 48 byte payload (halfway between 32 & 64) • Virtual Circuit routing with explicit congestion control • Permanent VCs for long-lived connections

  15. ATM Stack: 3 Layers • Physical Layer • Supports a variety of media • Fiber and copper • Inserts a constant flow of bits, even when no cells are available for transmission • Manages cell delineation so receiver can detect cell boundaries

  16. ATM Stack: 3 Layers • ATM Adaptation Layer (AAL) • Analogous to Transport Layer • Segmentation / Reassembly • 5 different services defined • Constant Bit Rate, VBR (3 types), Datagram • Choice negotiated at circuit initialization

  17. IP “over” ATM Fully connect ATM border routers with virtual circuits ATM as a Virtual Link

  18. IP handoff to AAL5 at ingress border router Use ATMARP (like ARP) to get VCI addr AAL will negotiate circuit setup IP packet fragmented into cells AAL will respond to congestion events A Packet's Journey

  19. AAL passes each cell to ATM ATM moves cell from interior router to router across the network At egress border router, cells handed to AAL5 Cells reassembled into IP datagram Datagram Journey (2)

  20. Link Virtualization Limits • Many different link technologies • Quick RFC search found IP over {FDDI, PPP, Token-ring, HIPPI, MAPOS, Ethernet, NBMA, ARCnet, InfiniBand, Frame Relay, TV Broadcast signals, Firewire, MPLS, Optical, MPEG-2, Fibre channel, Avians, Semaphore Flags, LoWPAN, 802.15.4, 802.16, SLIP, NetBIOS, IPX, SMDS} only 2 of which are April Fools jokes

  21. traceroute • VLANs • Link Virtualization • Asynchronous Transfer Mode (ATM) • Multiprotocol Label Switching (MPLS)

  22. TCP Segment HTTP Request A short aside: Encapsulation IP Packet Ethernet Frame

  23. Key Insight • Routers within a network can label packets in any way they wish • Label is used for network management • other router removes the label before the packet exits the network • Where do you put this label? • between data-link layer and network layer headers

  24. Multiprotocol Label Switching • IP routing is slowed by the variable length address searching in the forwarding table • Remember longest matching prefix rule? • MPLS replaces IP routing within a network by using a fixed length label • RFC 3031, 3032

  25. Label (20 bits) Traffic Class (3 bits): Used for QoS priority, ECN Bottom of Stack (1 bit) Packet may have a “stack” of labels Time To Live (8 bits) Same operation as in IP, copied from/into IP header by ingress / egress router MPLS Packet Format

  26. Router Operation • Inside the network (Label Switch Router) • On receipt of packet, lookup label • Replace label for next hop • Edge of the network (Label Edge Router) • Incoming packet: convert IP to label, push MPLS header • Outgoing packet: pop MPLS header, forward based on IP address

  27. MPLS-capable router has IP forwarding table plus MPLS forwarding table MPLS Routing

  28. MPLS Forwarding • Notice that Forwarding process doesn’t examine the IP header • Except at entry to MPLS network • Forwarding table is filled (i.e. labels are distributed) using the RSVP-TE protocol or LDP (Label Distribution Protocol) • Can populate with any route desired • source-specific forwarding • Multiple paths possible • Paths chosen based on performance, policy, ...

  29. MPLS • Lots more cool stuff you can do with MPLS • Backup links with failover capability • Virtual Private Networks • Connects disjoint networks and keeps them isolated from other customers • Exotic Traffic Engineering • Very useful tool ➙ I predict lots of excellent tech, research ➙ you should learn more

  30. Lesson Objectives • Now, you should be able to: • describe the use of virtual LANs (VLAN) to allow multiple subnets to be connected with a single port-based switch. Be sure to include broadcast domain separation, flexibility for re-assigning hosts within the VLAN, connection mechanisms for when the same VLAN is connected across switches • describe how link virtualization allows links to be more than just a simple "channel connecting adjacent nodes"

  31. You should be able to: • diagram the encapsulation of messages inside segments inside packets inside frames. Ensure you can handle cases such as ICMP and ARP • describe MPLS, including advantages, labeled frame formats (why is the label between link-layer and IP headers?), router operations. Be able to describe what an MPLS forwarding table might look like, given some MPLS enabled network scenario

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