How do loss and delay occur?

Transcription

1 What s the Internet: nuts and bolts view What s the Internet: nuts and bolts view PC server wireless laptop cellular handheld router access points wired s millions of connected computing devices: hosts = end systems running apps communication s fiber, copper, radio, satellite transmission rate = bandwidth routers: forward packets (chunks of data) obile Global ISP Home Regional ISP Institutional Introduction 1-1 protocols control sending, receiving of msgs e.g.,, IP, HTTP, Skype, Ethernet Internet: of s loosely hierarchical public Internet versus private intranet Internet standards RFC: Request for comments IETF: Internet Engineering Task Force obile Global ISP Home Regional ISP Institutional Introduction 1-2 What s the Internet: a service view communication infrastructure enables distributed s: Web, VoIP, , games, e-commerce, file sharing communication services provided to apps: reliable data delivery from source to destination best effort (unreliable) data delivery Introduction 1-3 How do loss and delay occur? packets queue in router buffers packet arrival rate to exceeds output capacity packets queue, wait for turn A B packet being transmitted (delay) packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers Introduction 1-4 Delay in packet-switched s 1. nodal processing: check bit errors determine output 3. Transmission delay: 2. queueing waiting at output for transmission 4. Propagation delay: R= bandwidth (bps) d = length of L=packet length (bits) s = propagation speed in medium (~2x10 to send bits into = 8 m/sec) L/R propagation delay = d/s transmission A propagation B nodal processing queueing Introduction 1-5 Why layering? Dealing with complex systems: explicit structure allows identification, relationship of complex system s pieces layered reference model for discussion modularization eases maintenance, updating of system change of implementation of layer s service transparent to rest of system e.g., change in gate procedure doesn t affect rest of system layering considered harmful? Introduction 1-6 1

2 Internet protocol stack : supporting s FTP, STP, STTP : host-host data transfer, UDP : routing of datagrams from source to destination IP, routing protocols : data transfer between neighboring elements PPP, Ethernet : bits on the wire message segment H t datagram H n H t frame H l H n H t H t H t H n H t H n H l destination source H l H n H t H l H n H t H t H n H t H n H l Encapsulation H t H n H t H n H l switch router Introduction 1-7 Introduction 1-8 Creating a app write programs that run on (different) end systems communicate over e.g., web server software communicates with browser software little software written for devices in core core devices do not run user s s on end systems allows for rapid app development, propagation Introduction 1-9 Processes communicating Process: program running within a host. within same host, two processes communicate using inter-process communication (defined by OS). processes in different hosts communicate by exchanging messages Client process: process that initiates communication Server process: process that waits to be contacted Note: s with P2P architectures have client processes & server processes Introduction 1-10 Sockets process sends/receives messages to/from its socket socket analogous to door sending process shoves message out door sending process relies on infrastructure on other side of door which brings message to socket at receiving process host or server process socket with buffers, variables controlled by app developer controlled by OS Internet host or server process socket with buffers, variables API: (1) choice of protocol; (2) ability to fix a few parameters (lots more on this later) Introduction 1-11 Addressing processes to receive messages, process must have identifier host device has unique 32-bit IP address Q: does IP address of host on which process runs suffice for identifying the process? A: No, many processes can be running on same host identifier includes both IP address and port numbers associated with process on host. Example port numbers: HTTP server: 80 ail server: 25 to send HTTP message to gaia.cs.umass.edu web server: IP address: Port number: 80 more shortly Introduction

3 App-layer protocol defines What service does an app need? Types of messages exchanged, e.g., request, response essage syntax: what fields in messages & how fields are delineated essage semantics meaning of information in fields Rules for when and how processes send & respond to messages Public-domain protocols: defined in RFCs allows for interoperability e.g., HTTP, STP Proprietary protocols: e.g., Skype Data loss some apps (e.g., audio) can tolerate some loss other apps (e.g., file transfer, telnet) require 100% reliable data transfer Timing some apps (e.g., Internet telephony, interactive games) require low delay to be effective Bandwidth some apps (e.g., muldia) require minimum amount of bandwidth to be effective other apps ( elastic apps ) make use of whatever bandwidth they get Introduction 1-13 Introduction 1-14 Transport service requirements of common apps Internet protocols services Application file transfer Web documents real- audio/video stored audio/video interactive games instant messaging Data loss no loss no loss no loss loss-tolerant loss-tolerant loss-tolerant no loss Bandwidth elastic no elastic no elastic no audio: 5kbps-1bps yes, 100 s msec video:10kbps-5bps same as above few kbps up elastic Time Sensitive yes, few secs yes, 100 s msec yes and no Introduction 1-15 service: connection-oriented: setup required between client and server processes reliable between sending and receiving process flow control: sender won t overwhelm receiver congestion control: throttle sender when overloaded does not provide: timing, minimum bandwidth guarantees UDP service: unreliable data transfer between sending and receiving process does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee Q: why bother? Why is there a UDP? Introduction 1-16 Internet apps:, protocols Application remote terminal access Web file transfer streaming muldia Internet telephony Application layer protocol STP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] proprietary (e.g. RealNetworks) proprietary (e.g., Vonage,Dialpad) Underlying protocol or UDP typically UDP Transport services and protocols provide logical communication between app processes running on different hosts protocols run in end systems send side: breaks app messages into segments, passes to layer rcv side: reassembles segments into messages, passes to app layer more than one protocol available to apps Internet: and UDP logical end-end Introduction 1-17 Introduction

4 UDP: User Datagram Protocol [RFC 768] UDP: more no frills, bare bones Internet protocol best effort service, UDP segments may be: lost delivered out of order to app connectionless: no handshaking between UDP sender, receiver each UDP segment handled independently of others Why is there a UDP? no connection establishment (which can add delay) simple: no connection state at sender, receiver small segment header no congestion control: UDP can blast away as fast as desired often used for streaming muldia apps loss tolerant rate sensitive other UDP uses DNS SNP reliable transfer over UDP: add reliability at layer -specific error recovery! Length, in bytes of UDP segment, including header 32 bits source port # dest port # length Application data (message) checksum UDP segment format Introduction 1-19 Introduction 1-20 socket door : Overview RFCs: 793, 1122, 1323, 2018, 2581 point-to-point: one sender, one receiver reliable, in-order byte steam: no message boundaries pipelined: congestion and flow control set window size send & receive buffers writes data send buffer segment reads data receive buffer full duplex data: bi-directional data flow in same connection SS: maximum segment size connection-oriented: handshaking (exchange of control msgs) init s sender, receiver state before data exchange flow controlled: sender will not overwhelm receiver socket door Introduction 1-21 segment structure URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now (generally not used) RST, SYN, FIN: connection estab (setup, teardown commands) Internet checksum (as in UDP) 32 bits source port # dest port # sequence number acknowledgement number head not len used UAP R S F Receive window checksum Urg data pnter Options (variable length) data (variable length) counting by bytes of data (not segments!) # bytes rcvr willing to accept Introduction 1-22 : retransmission scenarios retransmission scenarios (more) Host A Host B Host A Host B Host A Host B Seq=92, 8 bytes data out SendBase = 100 Seq=92, 8 bytes data X loss ACK=100 Seq=92, 8 bytes data ACK=100 lost ACK scenario Sendbase = 100 SendBase = 120 SendBase = 120 Seq=92 out Seq=92 out Seq=100, 20 bytes data ACK=100 ACK=120 Seq=92, 8 bytes data ACK=120 premature out Introduction 1-23 SendBase = 120 out Seq=92, 8 bytes data Seq=100, 20 bytes data X loss ACK=120 ACK=100 Cumulative ACK scenario Introduction

5 Fast Retransmit Time-out period often relatively long: long delay before resending lost packet Detect lost segments via duplicate ACKs. Sender often sends many segments back-toback If segment is lost, there will likely be many duplicate ACKs. If sender receives 3 ACKs for the same data, it supposes that segment after ACKed data was lost: fast retransmit: resend segment before r expires Flow Control receive side of connection has a receive buffer: app process may be slow at reading from buffer flow control sender won t overflow receiver s buffer by transmitting too much, too fast speed-matching service: matching the send rate to the receiving app s drain rate Introduction 1-25 Introduction 1-26 congestion control: additive increase, multiplicative decrease Approach: increase transmission rate (window size), probing for usable bandwidth, until loss occurs additive increase: increase CongWin by 1 SS every RTT until loss detected multiplicative decrease: cut CongWin in half after loss Saw tooth behavior: probing for bandwidth congestion window size 24 Kbytes 16 Kbytes 8 Kbytes congestion window Introduction 1-27 Congestion Control: details sender limits transmission: LastByteSent-LastByteAcked Roughly, rate = CongWin RTT CongWin Bytes/sec CongWin is dynamic, function of perceived congestion How does sender perceive congestion? loss event = out or 3 duplicate acks sender reduces rate (CongWin) after loss event three mechanisms: AID slow start conservative after out events Introduction 1-28 Slow Start Slow Start (more) When connection begins, CongWin = 1 SS Example: SS = 500 bytes & RTT = 200 msec initial rate = 20 kbps available bandwidth may be >> SS/RTT desirable to quickly ramp up to respectable rate When connection begins, increase rate exponentially fast until first loss event When connection begins, increase rate exponentially until first loss event: double CongWin every RTT done by incrementing CongWin for every ACK received Summary: initial rate is slow but ramps up exponentially fast RTT Host A Host B one segment two segments four segments Introduction 1-29 Introduction

6 Refinement Q: When should the exponential increase switch to linear? A: When CongWin gets to 1/2 of its value before out. Implementation: Variable Threshold At loss event, Threshold is set to 1/2 of CongWin just before loss event Refinement: inferring loss After 3 dup ACKs: CongWin is cut in half window then grows linearly But after out event: CongWin instead set to 1 SS; window then grows exponentially to a threshold, then grows linearly Philosophy: 3 dup ACKs indicates capable of delivering some segments out indicates a more alarming congestion scenario Introduction 1-31 Introduction 1-32 Network layer segment from sending to receiving host on sending side encapsulates segments into datagrams on rcving side, delivers segments to layer layer protocols in every host, router router examines header fields in all IP datagrams passing through it Two Key Network-Layer Functions forwarding: move packets from router s input to appropriate router output routing: determine route taken by packets from source to dest. routing algorithms analogy: routing: process of planning trip from source to dest forwarding: process of getting through single interchange Introduction 1-33 Introduction 1-34 Datagram s no call setup at layer routers: no state about end-to-end connections no -level concept of connection packets forwarded using destination host address packets between same source-dest pair may take different paths 1. Send data 2. Receive data Forwarding table Destination Address Range 4 billion possible entries through through through otherwise 3 Link Interface Introduction 1-35 Introduction

7 Longest prefix matching Prefix atch Link Interface otherwise 3 Router Architecture Overview Two key router functions: run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing Examples DA: Which interface? DA: Which interface? Introduction 1-37 Introduction 1-38 The Internet Network layer Host, router layer functions: Network layer Routing protocols path selection RIP, OSPF, BGP Transport layer:, UDP forwarding table Link layer layer IP protocol addressing conventions datagram format packet handling conventions ICP protocol error reporting router signaling Introduction 1-39 IP Addressing: introduction IP address: 32-bit identifier for host, router interface interface: connection between host/router and router s typically have multiple interfaces host typically has one interface IP addresses associated with each interface = Introduction 1-40 IP addressing: CIDR CIDR: Classless InterDomain Routing subnet portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in subnet portion of address subnet host part part /23 Introduction 1-41 Routing Algorithm classification Global or decentralized information? Global: all routers have complete topology, cost info state algorithms Decentralized: router knows lyconnected neighbors, costs to neighbors iterative process of computation, exchange of info with neighbors distance vector algorithms Static or dynamic? Static: routes change slowly over Dynamic: routes change more quickly periodic update in response to cost changes Introduction

8 Hierarchical Routing Intra-AS Routing aggregate routers into regions, autonomous systems (AS) routers in same AS run same routing protocol intra-as routing protocol routers in different AS can run different intra- AS routing protocol Gateway router Direct to router in another AS also known as Interior Gateway Protocols (IGP) most common Intra-AS routing protocols: RIP: Routing Information Protocol OSPF: Open Shortest Path First IGRP: Interior Gateway Routing Protocol (Cisco proprietary) Introduction 1-43 Introduction 1-44 Internet inter-as routing: BGP BGP (Border Gateway Protocol): the de facto standard BGP provides each AS a means to: 1. Obtain subnet reachability information from neighboring ASs. 2. Propagate reachability information to all ASinternal routers. 3. Determine good routes to subnets based on reachability information and policy. allows subnet to advertise its existence to rest of Internet: I am here Introduction 1-45 Link Layer: Introduction Some terminology: hosts and routers are nodes communication channels that connect adjacent nodes along communication path are s wired s wireless s LANs layer-2 packet is a frame, encapsulates datagram data- layer has responsibility of transferring datagram from one node to adjacent node over a Introduction 1-46 Link Layer Services Link Layer Services (more) framing, access: encapsulate datagram into frame, adding header, trailer channel access if shared medium AC addresses used in frame headers to identify source, dest different from IP address! reliable delivery between adjacent nodes we learned how to do this already (chapter 3)! seldom used on low bit-error (fiber, some twisted pair) wireless s: high error rates Q: why both -level and end-end reliability? Introduction 1-47 flow control: pacing between adjacent sending and receiving nodes error detection: errors caused by signal attenuation, noise. receiver detects presence of errors: signals sender for retransmission or drops frame error correction: receiver identifies and corrects bit error(s) without resorting to retransmission half-duplex and full-duplex with half duplex, nodes at both ends of can transmit, but not at same Introduction

9 Where is the layer implemented? Adaptors Communicating in each and every host layer implemented in adaptor (aka interface card NIC) Ethernet card, PCCI card, card implements, layer attaches into host s system buses combination of hardware, software, firmware host schematic cpu memory host bus controller (e.g., PCI) transmission adapter card datagram sending side: controller sending host frame encapsulates datagram in frame adds error checking bits, rdt, flow control, etc. datagram datagram controller receiving host receiving side looks for errors, rdt, flow control, etc extracts datagram, passes to upper layer at receiving side Introduction 1-49 Introduction 1-50 ultiple Access Links and Protocols Two types of s : point-to-point PPP for dial-up access point-to-point between Ethernet switch and host broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC wireless LAN ultiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes: interference collision if node receives two or more signals at the same multiple access protocol distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit communication about channel sharing must use channel itself! no out-of-band channel for coordination shared wire (e.g., cabled Ethernet) shared RF (e.g., WiFi) shared RF (satellite) humans at a cocktail party (shared air, acoustical) Introduction 1-51 Introduction 1-52 CSA (Carrier Sense ultiple Access) CSA: listen before transmit: If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission human analogy: don t interrupt others! CSA/CD (Collision Detection) CSA/CD: carrier sensing, deferral as in CSA collisions detected within short colliding transmissions aborted, reducing channel wastage collision detection: easy in wired LANs: measure signal strengths, compare transmitted, received signals difficult in wireless LANs: received signal strength overwhelmed by local transmission strength human analogy: the polite conversationalist Introduction 1-53 Introduction

10 CSA/CD collision detection LAN Addresses and ARP Each adapter on LAN has unique LAN address 1A-2F-BB AD Broadcast address = FF-FF-FF-FF-FF-FF F7-2B LAN (wired or wireless) D7-FA-20-B0 = adapter 0C-C4-11-6F-E3-98 Introduction 1-55 Introduction 1-56 ARP: Address Resolution Protocol ARP protocol: Same LAN () Question: how to determine AC address of B knowing B s IP address? F7-2B LAN A-2F-BB AD D7-FA-20-B0 0C-C4-11-6F-E3-98 Each IP node (host, router) on LAN has ARP table ARP table: IP/AC address mappings for some LAN nodes < IP address; AC address; TTL> TTL (Time To Live): after which address mapping will be forgotten (typically 20 min) Introduction 1-57 A wants to send datagram to B, and B s AC address not in A s ARP table. A broadcasts ARP query packet, containing B's IP address dest AC address = FF- FF-FF-FF-FF-FF all machines on LAN receive ARP query B receives ARP packet, replies to A with its (B's) AC address frame sent to A s AC address (unicast) A caches (saves) IP-to- AC address pair in its ARP table until information becomes old (s out) soft state: information that s out (goes away) unless refreshed ARP is plug-and-play : nodes create their ARP tables without intervention from net administrator Introduction 1-58 Addressing: routing to another LAN walkthrough: send datagram from A to B via R assume A knows B s IP address C-E8-FF-55 A CC-49-DE-D0-AB-7D E6-E BB-4B 1A-23-F9-CD-06-9B R 88-B2-2F-54-1A-0F B 49-BD-D2-C7-56-2A two ARP tables in router R, one for each IP (LAN) A creates IP datagram with source A, destination B A uses ARP to get R s AC address for A creates -layer frame with R's AC address as dest, frame contains A-to-B IP datagram A s NIC sends frame R s NIC receives frame R removes IP datagram from Ethernet frame, sees its destined to B R uses ARP to get B s AC address R creates frame containing A-to-B IP datagram sends to B C-E8-FF-55 A E6-E BB-4B A-23-F9-CD-06-9B R This is a really important example make sure you understand! 88-B2-2F-54-1A-0F B 49-BD-D2-C7-56-2A Introduction 1-59 CC-49-DE-D0-AB-7D Introduction

11 Ethernet dominant wired LAN technology: cheap $20 for NIC first widely used LAN technology simpler, cheaper than token LANs and AT kept up with speed race: 10 bps 10 Gbps Star topology bus topology popular through mid 90s all nodes in same collision domain (can collide with each other) today: star topology prevails active switch in center each spoke runs a (separate) Ethernet protocol (nodes do not collide with each other) etcalfe s Ethernet sketch switch bus: coaxial cable star Introduction 1-61 Introduction 1-62 Ethernet CSA/CD algorithm 1. NIC receives datagram 4. If NIC detects another from layer, transmission while creates frame transmitting, aborts and 2. If NIC senses channel idle, sends jam signal starts frame transmission 5. After aborting, NIC If NIC senses channel enters exponential busy, waits until channel backoff: after mth idle, then transmits collision, NIC chooses K at 3. If NIC transmits entire random from frame without detecting {0,1,2,,2 m -1}. NIC waits another transmission, NIC K 512 bit s, returns to is done with frame! Step 2 Hubs -layer ( dumb ) repeaters: bits coming in one go out all other s at same rate all nodes connected to hub can collide with one another no frame buffering no CSA/CD at hub: host NICs detect collisions hub twisted pair Introduction 1-63 Introduction 1-64 Switch -layer device: smarter than hubs, take active role store, forward Ethernet frames examine incoming frame s AC address, selectively forward frame to one-or-more outgoing s when frame is to be forwarded on segment, uses CSA/CD to access segment transparent hosts are unaware of presence of switches plug-and-play, self-learning switches do not need to be configured Introduction