Transcript Lec 2
Application layer (continued) Week 4 – Lecture 2 Internet Architecture Application layer – HTTP, SMTP etc W3C Transport layer – TCP, UDP IETF Network layer - IP Data link & Physical layers – the territory of the LANs/Telcos/ISPs ITU IEEE Variables in Internet service • Bandwidth • Latency – delay sensitivity • Error free – loss tolerance Application requirements Application Loss Tolerant Delay Sensitive BandWidth Web None No Medium E-Mail None No Low Video streaming Some Some High Telephone calls Some Yes Low Video conferencing Some Yes Medium/ high Characteristics Applications • Video streaming • Voice over IP • Multicasting Multicast routing Multicast routing • We have only talked about single host to single destination messages • Some applications require a host to send messages to many destinations: – Upgrade of software – Streaming of an on-line lecture or movie – Stock price information to subscribers Three approaches • At the application level where the application establishes a separate unicast transport connection to each receiver. This is not usually efficient and not scalable • Broadcast – controlled – reverse path & spanning tree • An explicit multicast where the network layer makes copies of the message as close to the recipient as possible How multi cast works How does it address the one message to all receivers? • It is not practicable to: – Attach a list of addressees to the message – The addressees may not be known • • • • Uses a broadcast and subscribe approach Subscribers are added to a D class address See Kurose for more details IGMP (Internet Group Management Protocol) provides services for managing the group Video and audio Streaming • Streaming stored video • Streaming live video • Real time interactive video - video conferencing • Voice over IP Streaming stored video • Media pre-recorded and held on server • Ideally server near client (remember WEB caching discussion) • Important that it plays at recorded speed • Uses buffering to prevent jitter – effect of delays and loss packets • Because of buffering and one way (predominantly) latency not as big a problem as with live video streaming • Can fast forward and rewind (not usually multicast) Streaming live video • Can still buffer and build in a delay • Because it is live, cannot fast forward but some software will allow rewind • Can be multicast or unicast • Multicast overlay – uses servers scattered through the broadcast area Protocols for streaming • • • • WEB http uses TCP to make the request But TCP not satisfactory for streaming Browser kicks off a media player process Web server then instructs Steaming server to send to the media player post on the browser host probably using RTP WEB Browser Media player TCP UDP WEB server Streaming server Control streaming stored video • Real Time Streaming Protocol (RTSP) • The equivalent of the Remote control • Allows the Media player and the Streaming server to communicate over two additional ports • RTSP keeps track of the state of the streaming process including the session number and sequence number of the segment Compression • Video and audio must be compressed before transmission – Speech normally requires 64kbps – Stereo CD requires 1.4mbps – Video much more again • Various standards (codecs) for compression – – – – GSM for voice MP3 for stereo music MP1 and MP4 and others for video H.323 for Video conferencing Voice over IP • Delay sensitive but large bandwidth not required • Range of user facilities required to simulate facilities available in PBX Private WAN Call Manager PSTN Gateway Office 1 Private link Public Internet Office 2 Gateway Advantages • • • • Maintain one network instead of two One British bank estimate gives 25% cost saving Inter-office calls over own network – fixed cost Application extensions – – – – – On-line office directory Single messaging system Display product information Telephony options on screen Video conferencing Risks • • • • Security – could be open to Public internet New technology QoS over IP – not totally standardised Power failure etc, not just network but phone system as well – all eggs in one basket Techniques for avoiding jitter in VoIP • Three problems – Variable latency – Lost packets – Packets out of order • Approaches – Sequence numbers & timestamps – Fixed play out delay – Error correction • Redundant data to allow reconstruction of one packet • Two audio streams- normal and a back-up low resolution • Interleaving Video conferencing • Like a phone call, delays must be less than ½ second • Places real constraints – delay sensitive and needs bandwidth