QoS attributes of a packet. Quality of service (QoS) Packet latency affects transport service QoS. Transport services.

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1 University of California at Berkeley School of Information Management and Systems Information Systems 206 Distributed Computing Applications and Infrastructure Quality of service (QoS) QoS attributes of a packet Packet latency Time until packet delivered at destination Transmission time, propagation time, queuing delay, processing time Packet loss Packet corruption Payload only Normally network will not deliver corrupt packet 2 Transport services Raw packet service is not what is needed by most applications Transport services condition packet service by adding layers Reliable delivery Message service Session Time stamps etc 3 Packet latency affects transport service QoS Audio coder Stream of packets Packet latency End-to-end delay 4 Audio decoder Reliable delivery Add acknowledgement for each packet Lost packet can be detected by missing ACK Lost packet can be retransmitted Tradeoff: Reliable delivery for greater latency Latency-sensitive applications must abandon reliable delivery (e.g. remote conferencing) 5 QoS Guarantees Source and network enter session contract : Source promises not to exceed specified traffic parameters for that session Rate and burstiness Network promises to limit impairments such as latency, loss, and corruption 6 1

2 Achieving QoS TCP/IP offers only best-effort Every connection gets best-effort service Achieving maximum latency guarantees Reserve resources Or attach priorities to packets Contract may allow network to delay or discard low-priority packets when necessary Application may guarantee traffic shape e.g., steady flow rather than bursts 7 Pricing and Accounting What s the incentive for using low quality service? Why should user accept greater latency if less is an option? Why should application try to minimize bandwidth, or shape its traffic? Answer is good citizenship, or pricing 8 Pricing Today Pricing options Commercial services: usually flat rate plus connect time (but no per-bit charges) e.g., CompuServe, Prodigy, America On-Line Internet: flat rate, unlimited usage Resellers can charge for connect time Many people have unlimited use through a university or company 9 Price component Fixed Usage QoS Congestion 10 Cost recovery Capital/operational costs of access network Shared costs of backbone network Cost of reduced statistical multiplexing advantage Impact on other users, cost of upgrading facilities Congestion Pricing Rationale The fixed cost of building the network is high The marginal cost of accepting new session is nearly zero (assuming not congested) Economic efficiency: use whenever marginal benefit > 0 But capacity is fixed (in the short run) Person A s use may reduce B s quality of service Congestion Pricing If network is underutilized, charge nothing If network is congested Charge person A the amount of B s lost value A will drop out unless his value of use is greater than B s lost value Theorem: this raises enough money to expand the network by the socially optimal amount Monitoring and billing overhead

3 Big Advantage Congestion pricing uses incentives rather than forced control or policies to affect user/application behavior Market mechanism like other goods and services User/application can determine freely and independently whether use of network during periods of congestion is warranted University of California at Berkeley School of Information Management and Systems Information Systems 206 Distributed Computing Applications and Infrastructure Protocol architecture 13 Key ideas Packet encapsulation: one packet can be encapsulated in another Packet fragmentation: one packet can be split into two or more and then encapsulated Protocol layering: one service can be layered on another using fragmentation and encapsulation New header (including its header) is payload of new packet Packet encapsulated in another packet Encapsulated again Encapsulated packet Header Payload Divide into packet fragments Layer n+1 protocol header Layer n+1 Layer n+1 Peer-to-peer communication New packets Layer n Layer n Reassembled original packet Layer n protocol header Encapsulated layer n+1 packet, including header

4 Key ideas (again) Packet encapsulation: one packet can be encapsulated in another Packet fragmentation: one packet can be split into two or more and then encapsulated Protocol layering: one service can be layered on another using fragmentation and encapsulation 19 Layering IETF/OMG Layers Physical Logical Application Application ORB ORB IIOP IIOP TCP or UDP TCP or UDP IP IP IP Network 1 N 1 N 2 Network 2 A Switch B 20 What IP does do What IP Doesn t Do Allow packets to traverse multiple networks Deliver packet to specified destination host Best effort: deliver as reliably and as soon as it can Guarantee latency for packets that are delivered Guarantee delivery, or notify source host if packet is not delivered Guarantee order of delivery Guarantee integrity of packet payload Maintain conversational context (each packet is independent) Specify what process that should receive the packet at destination host IP header Version Priority FlowLabel PayloadLen NextHeader HopLimit SourceAddress DestinationAddress Transport services: UDP and TCP Direct packet to a particular process UDP adds: Payload integrity for packets delivered TCP adds: Reliable delivery of bytestream session 32 bits

5 Comparison of services IP: host-tohost -toprocess IP: Best-effort datagram TCP UDP TCP: reliable bi-directional bytestream 25 UDP: best-effort datagram with payload integrity byte byte byte byte byte byte SourcePort UDP/TCP ports (publish/subscribe) Port IP: host-tohost DestinationPort (rest of UDP or TCP header) 32 bits 26 Port Encapsulated in IP packet HTTP Service Client can make requests GET (pull) POST (push) (some others) Server responds HTTP headers HTML document or JPEG, or GIF, or 27 URL Structure <scheme>://<host>:<port>/<path> Scheme HTTP, FTP, GOPHER, MAILTO,... An IP address or DNS name Port TCP port number Optional (defaults to 80 for http) 28 HTTP example When a browser fetches says to use HTTP protocol Resolve in DNS Make TCP connection , port 80 Send the following text string GET /~presnick/ 29 Server sends back HTTP/ OK Date: Mon, 22 Dec :12:32 GMT Server: Apache/1.2.4 Last-Modified: Thu, 04 Dec :26:10 GMT ETag: "5f2f2-33fd-3486d9a2" Content-Length: Accept-Ranges: bytes Connection: close Content-Type: text/html <HTML>. 30 5

6 HTML example <H1> Paul Resnick</H1> <IMG SRC="RESNICK.gif" ALT="[PHOTO]" HSPACE=10 ALIGN=LEFT> <BR>Associate Professor <BR>University of Michigan <BR>School of Information <BR>314 West Hall <BR>550 East University Avenue <BR>Ann Arbor, MI What Browsers Send to Servers Your IP address The browser type The refer link What URL you last looked at Cookies (persistent client state for a URL) Server response can include a set-cookie header Browser saves the cookie Browser resends to server next time