Hubs Interconnecting with hubs Hubs are essentially physical-layer repeaters: Backbone hub interconnects LAN segments bits coming from one link go out all other links Extends max distance between nodes at the same rate But individual segment collision domains become one no frame buffering large collision domain no CSMA/CD at hub: adapters detect collisions Can’t interconnect 10BaseT & 100BaseT provides net management functionality hub twisted pair hub hub hub hub 5: DataLink Layer 5-1 5: DataLink Layer 5-2 Switch Forwarding switch Link layer device 1 stores and forwards Ethernet frames 2 3 examines frame header and selectively forwards frame based on MAC dest address hub when frame is to be forwarded on segment, hub hub uses CSMA/CD to access segment transparent hosts are unaware of presence of switches plug-and-play, self-learning switches do not need to be configured • How do determine onto which LAN segment to forward frame? • Looks like a routing problem... 5: DataLink Layer 5-3 5: DataLink Layer 5-4 Self learning Filtering/Forwarding When switch receives a frame: A switch has a switch table entry in switch table: index switch table using MAC dest address (MAC Address, Interface, Time Stamp) if entry found for destination stale entries in table dropped (TTL can be 60 min) then{ swihitch learns whic h hosts can be reached through if dest on segment from which frame arrived which interfaces then drop the frame when frame received, switch “learns” location of else forward the frame on interface indicated sender: incoming LAN segment } records sender/location pair in switch table else flood forward on all but the interface on which the frame arrived 5: DataLink Layer 5-5 5: DataLink Layer 5-6 1 Switch example Switch example Suppose C sends frame to D Suppose D replies back with frame to C. address interface address interface switch switch 1 A 1 A 1 3 2 B 1 B 1 E 2 E 2 hub G 3 hub G 3 A hub hub A hub hub I I C 1 D F D F G G B C E H B C E H Switch receives frame from C Switch receives frame from from D notes in bridge table that C is on interface 1 notes in bridge table that D is on interface 2 because D is not in table, switch forwards frame into because C is in table, switch forwards frame only to interfaces 2 and 3 interface 1 frame received by D 5: DataLink Layer 5-7 frame received by C 5: DataLink Layer 5-8 Switch: traffic isolation Switches: dedicated access switch installation breaks subnet into LAN Switch with many A segments interfaces C’ switch filters packets: Hosts have direct B same-LAN-segment frames not usually connection to switch forwarded onto other LAN segments No collisions; full duplex switch segments become separate collision domains switch Switching: A-to-A’ and B-to-B’ C collision simultaneously, no collisions domain B’ A’ hub hub hub collision domain collision domain 5: DataLink Layer 5-9 5: DataLink Layer 5-10 More on Switches Institutional network cut-through switching: frame forwarded mail server from input to output port without first to external network collecting entire frame router web server sliggyht reduction in latency switch combinations of shared/dedicated, IP subnet 10/100/1000 Mbps interfaces hub hub hub 5: DataLink Layer 5-11 5: DataLink Layer 5-12 2 Switches vs. Routers Summary comparison both store-and-forward devices routers: network layer devices (examine network layer headers) hubs routers switches switches are link layer devices routers maintain routing tables, implement routing traffic no yes yes alggmorithms isolation switches maintain switch tables, implement plug & play yes no yes filtering, learning algorithms optimal no yes no routing cut yes no yes through 5: DataLink Layer 5-13 5: DataLink Layer 5-14 Link Layer Point to Point Data Link Control one sender, one receiver, one link: easier than 5.1 Introduction and 5.6 Hubs and switches broadcast link: services 5.7 PPP no Media Access Control 5.2 Error detection 5.8 Link Virtualization: no need for explicit MAC addressing and correction ATM 5. 3Multiple access e. g., dialup link, ISDN line protocols popular point-to-point DLC protocols: 5.4 Link-Layer PPP (point-to-point protocol) Addressing HDLC: High level data link control (Data link 5.5 Ethernet used to be considered “high layer” in protocol stack! 5: DataLink Layer 5-15 5: DataLink Layer 5-16 PPP Design Requirements [RFC 1557] PPP non-requirements packet framing: encapsulation of network-layer datagram in data link frame no error correction/recovery no flow control carry network layer data of any network layer protocol (not just IP) at same time out of order delivery OK ability to demultiplex upwards no need to support multipoint links (e.g., polling) bit transparency: must carry any bit pattern in the data field error detection (no correction) Error recovery, flow control, data re-ordering connection liveness: detect, signal link failure to all relegated to higher layers! network layer network layer address negotiation: endpoint can learn/configure each other’s network address 5: DataLink Layer 5-17 5: DataLink Layer 5-18 3 Byte Stuffing Byte Stuffing “data transparency” requirement: data field must be allowed to include flag pattern <01111110> flag byte Q: is received <01111110> data or flag? pattern in data to send Sender: adds (“stuffs”) extra < 01111101> byte after each < 01111110> data byte Receiver: two 01111101 bytes in a row: discard first byte, continue data reception flag byte pattern plus single 01111110: flag byte stuffed byte in transmitted data 5: DataLink Layer 5-19 5: DataLink Layer 5-20 Link Layer Virtualization of networks 5.1 Introduction and 5.6 Hubs and switches Virtualization of resources: a powerful abstraction in services 5.7 PPP systems engineering: 5.2 Error detection 5.8 Link Virtualization: computing examples: virtual memory, virtual and correction ATM and MPLS devices 5. 3Multiple access Virtual machines: e.g ., java protocols IBM VM os from 1960’s/70’s 5.4 Link-Layer layering of abstractions: don’t sweat the details of Addressing the lower layer, only deal with lower layers 5.5 Ethernet abstractly 5: DataLink Layer 5-21 5: DataLink Layer 5-22 The Internet: virtualizing networks The Internet: virtualizing networks Internetwork layer (IP): Gateway: 1974: multiple unconnected … differing in: addressing: internetwork “embed internetwork packets in appears as a single, uniform local packet format or extract nets addressing conventions entity, despite underlying local them” network heterogeneity route (at internetwork level) to ARPAnet packet formats network of networks next gateway data-over-cable networks error recovery packet satellite network (Aloha) routing packet radio network gateway ARPAnet satellite net ARPAnet satellite net "A Protocol for Packet Network Intercommunication", V. Cerf, R. Kahn, IEEE Transactions on Communications, 5: DataLink Layer 5-23 5: DataLink Layer 5-24 May, 1974, pp. 637-648. 4 Cerf & Kahn’s Internetwork Architecture Asynchronous Transfer Mode: ATM What is virtualized? 1990’s/00 standard for high-speed (155Mbps to two layers of addressing: internetwork and local 622 Mbps and higher) Broadband Integrated network Service Digital Network architecture new layer (IP) makes everything homogeneous at Goal: integrated, end-end transport of carry voice, internetwork layer video, data underlying local network technology meeting timing/QoS requirements of voice, video (versus Internet best-effort model) cable “next generation” telephony: technical roots in satellite telephone world 56K telephone modem packet-switching (fixed length packets, called today: ATM, MPLS “cells”) using virtual circuits … “invisible” at internetwork layer. Looks like a link layer technology to IP! 5: DataLink Layer 5-25 5: DataLink Layer 5-26 ATM architecture ATM: network or link layer? Vision: end-to-end transport: “ATM from IP desktop to desktop” network ATM is a network ATM technology network Reality: used to connect IP backbone routers adaptation layer: only at edge of ATM network “IP over ATM” data segmentation/reassembly ATM as switched roughly analagous to Internet transport layer link layer, ATM layer: “network” layer connecting IP cell switching, routing routers physical layer 5: DataLink Layer 5-27 5: DataLink Layer 5-28 ATM Adaptation Layer (AAL) ATM Layer Service: transport cells across ATM network analogous to IP network layer ATM Adaptation Layer (AAL): “adapts” upper layers (IP or native ATM applications) to ATM very different services than IP network layer Guarantees ? layer below Network Service Congestion AAL present only in end systems, not in switches Architecture Model Bandwidth Loss Order Timing feedback AAL layer segment (header/trailer fields, data) Internet best effort none no no no no (inferred fragmented across multiple ATM cells via loss) analogy: TCP segment in many IP packets ATM CBR constant yes yes yes no rate congestion ATM VBR guaranteed yes yes yes no rate congestion ATM ABR guaranteed no yes no yes minimum ATM UBR none no yes no no 5: DataLink Layer 5-29 5: DataLink Layer 5-30 5 ATM Layer: Virtual Circuits ATM VCs VC transport: cells carried on VC from source to dest Advantages of ATM VC approach: call setup, teardown for each call before data can flow each packet carries VC identifier (not destination ID) QoS performance guarantee for connection every switch on source-dest path maintain “state” for each mapped to VC (bandwidth, delay, delay jitter) passing connection Drawbacks