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Traffic Characterization • Specifies the traffic of a flow using parameters like bandwidth, delay, delay jitter requirements • for bursty traffic, bandwidth is normally specified in terms of average packet rate, maximum packet rate, burst duration • Facilitates admission control • Facilitates traffic policing
Traffic Shaping • Technique to regulate a stream’s traffic into certain patterns or shapes so that it is easy to describe and police. • Leaky bucket • (r,T)-Smooth • Token bucket • Traffic reshaping using regulators inside the network
Leaky Bucket Traffic Shaper • Finite sized bucket, B • Data leak out to the network at a constant rate, r • Bucket size B limits the data built up in the bucket waiting to be transmitted to the network • determines how bursty the incoming traffic can be • together with rate r determines the max data delay, B/r • Leaking rate r is chosen such that the data discard rate is small with a large buffer size (effective bandwidth)
Leaky Bucket (continued) • Simple and easy to implement • each flow’s traffic is managed by a simple FIFO buffer with a timer set to go off every 1/r units time. • A flow’s characteristic is described simply by the leaking rate r • The network delivers the required QoS by keeping sum of the effective bandwidths of all streams below its capacity • Limitation -- can’t be used to shape very bursty traffic
(r, T) -Smooth Traffic Shaper • A flow is permitted to inject no more than r bits of data in a time interval of T. • T is fixed throughout the network; r varies on a per flow basis.
Token Bucket Traffic Shaper • Token placed a leaky bucket of size C at a constant rate s. • Packets are placed in the data buffer and each is transmitted for a token from the bucket. • No token in the bucket, no transmission of packets. • When token bucket is full, tokens are discarded • Burstiness is bounded because the flow never sends more than C + t/s packets in an interval t and the long term transmission rate does not exceed s.
Queue-Scheduling Disciplines • Decide which packet is served next and therefore, affects packet delay and bandwidth of connections • Work conserving -- the system may not be idle if there are packets in the queue • Non-work conserving -- each packet is assigned an eligibility time. The system is idle when no packets in the queue are eligible for transmission. • Type of scheduling scheme determines buffer space requirements, delay, delay jitter. • Work conserving disciplines need less buffer and cause shorter delay but cannot bound delay jitter tightly • Non-work conserving schemes need larger buffer, cause longer delay but can bound delay jitter tightly
Weighted Fair Queueing • Fair queueing : if n active flows share an output link, each flow gets 1/n of the link bandwidth • In WFQ, connections are given different fractions of the bandwidth based weights • End-to-end delay is bounded if a flow shaped at the edge of the network by a token bucket with leaky bucket rate control passes through network nodes with WFQ
WFQ (Limitations) • Finish number computation • WFQ scheduler requires per connection scheduler state and can be expensive for schedulers that serve large numbers of connections • Requires explicit sorting of the output queue
Virtual Clock • Allocates each packet a virtual transmission time • Emulates STDM to determine the virtual transmission time (in STDM, a connection is allocated with periodical transmission timeslots) • VC can provide a deterministic bandwidth guarantee but cannot provide a deterministic delay guarantee
Delay Earliest Due Date • Extension of EDD that specifies the process by which the scheduler assigns deadlines to packets. • During call set up, each source negotiates a service contract with the scheduler for a delay bound guarantee. • Deadline assigned to a packet from a source is the expected arrival time added to the delay bound at the server. For example, if a client assures that it will send packets every 0.2 s, and the delay bound at the server is 1 s, then the kth packet from the client will get a deadline of 0.2k+1.
Jitter Earliest Due Date • Extends delay-EDD to provide delay jitter bounds (that is, a bound on the maximum delay difference between two packets). • After a packet has been served at each server, a field in its header is stamped with the difference between its deadline and the actual finishing time. A regulator at the entrance of the next server holds the packet for this period before it is made eligible to be scheduled.
Priority Schemes • Packet discard problem due to buffer overflow • Two ways to assign packet priority: • by application encoder • by network traffic policing mechanism: when a flow sends more data than the promised amount, the extra data are tagged with low priority
Resource Reservation Protocols • Resource management is key to providing QoS guarantees to users • Resource reservation protocol at the n/w layer • performs no reservation of required resources itself; it is only a vehicle to transfer info about resources required and to negotiate QoS values users desire • Reservation protocols rely on resource administration functions in aeach subsystem to enforce and schedule resource accesses during data transmission phase
Stream Transport Protocol (ST II) • Experimental Internet protocol designed to transmit real-time data • Provides connection oriented service, supports multicast and QoS negotiation • Data can only be sent after a stream has been established successfully (one to many connection, equivalent to a flow)
ST and SCMP • A datagram forwarding protocol called ST • A connection management protocol called ST Control Message protocol (SCMP) • ST forwards data packets down routing trees created by SCMP • SCMP has 17 different messages corresponding to different flow management services (messages used to establish a flow to one or more recipients, remove users or add new users, change existing flow’s flow spec., eroor recovery and status checks
ST II Connection Setup • A flow’s distribution graph is source specific (one to many) -- only one individual sender may use a particular distribution tree • Source ST agent generates a CONNECT packet listing the flow spec. and initial set of participants • Each intermediate ST agent processes the CONNECT packet to determine the set of next hop subnets required to reach all downstream receivers, installs multicast forwarding state, reserves n/w level resources along each next hop subnet
Connection Setup • A receiver determines whether to join the group and returns either ACCEPT or REFUSE message to the system source. Flow Spec. may be updated along the way (may be downgraded if a router determines (at each hop) it can’t meet the requirements. • Each router in turn examine the CONNECT message and decide whether to accept and forward CONNECT message to destinations they can reach. An intermediate router send a REFUSE message if it can not reach a destination
Connection Setup • Stream Source must wait for an ACCEPT or REFUSE message reply from each initial receiver before beginning data transmission. • When flow spec. is modified and ACCEPT is received from a receiver, the source must adapt to the low QoS for the entire stream or reject the receiver in question by sending it a DISCONNECT message • Additions and deletions • new users with lower QoS are denied access • receiver sends a REFUSE message or source sends a DISCONNECT message to delete an existing user
Evaluation • Implementation complexity • Complex state space • Scalability to large multicast groups • every destination must be explicitly listed in the CONNECT message • every destination must be acknowledge the CONNECT message • Sender may have to process hundreds of ACCEPT or REFUSE messages
Streaming on the Web • The contents of a compressed audio and/or video file are played out as they are being received. A playout buffer helps to smooth the variations in the time between each received packet in the stream (delay jitter). • Media player acts as the interface between the incoming compressed media bitstream and the related sound and/or video output cards. • For video, the browser first creates a window in the web page and passes the coordinates of the window to the selected video media player. • Video media player initializes the video card with the assigned coordinates and decompresses the video bitstream from the playout buffer and passes it to the video card for rendering • Media player consists of two parts : playout functions and control functions for interactivity
Sequence • Browser sends a HTTP GET request to the web server • Web server responds by returning the contents of the meta file to the browser • Browser determines the media player to invoke from the “content type” field and passes the contents of the meta file to the selected media player • Control part of the media player requests control part of the streaming server to start a new session by sending RTSP SETUP request message (including the port number allocated to RTP). In response the control part of the server returns an RTSP accept message (with a unique session identifier) • Control part of the media player sends a RTSP PLAY request message; the control part on the server side initiates the access and transmission of the packet stream using the allocated port number of RTP in the header of each UDP datagram • RTSP TEARDOWN request message is used when the user activates the quit/end button
Session Initiation Protocol (SIP) • Provides services for user location, call/session establishment, and call participation management • A simple request-response protocol -- both the request and response are made through an application program called the user agent (UA) which maps the request and its response into the standard message format used by SIP • Each UA comprises two parts, a UA Client (UAC), which enables the user to send request messages (to initiate the setting up of a call/session) and a UA server (UAS) which generates the response message
Call/Session Set Up • SIP name/address is similar to an email name/address. A single user may have a number of alternative locations/addresses • Calling host sends an INVITE request message to the local proxy server (PS -A); the proxy server (PS-A) obtains the IP address of the proxy server (PS-B) for the called host and the SIP in the proxy server (PS-A) sends the INVITE request message to PS-B. PS -B determines that the user is currently logged in (gets name/address from the SIP message) and the IP address of the called host. If the called host can accept the call, an INVITE response message is returned over the same path. Receiving this the SIP in the calling host returns an ACK and the two users/hosts start exchanging the info.
Session Description Protocol (SDP) • To describe the different media streams involved in a call/session • Described in each SIP message body in text format: • media streams (list of media types and format in each SIP INVITE request message and a modified version of that in the INVITE response message) • stream addresses (destination address and UDP port number for sending and/or receiving each stream) • start and stop times (for broadcast sessions)
RTP • The timing information required by the receiver to output the received packet stream at the required rate is provided by RTP • RTCP is used to synchronize the two media streams prior to carrying out the decoding the decoding operation • RTP functions: (1) detecting missing packets and compensating for lost packets as well as delay variations (2) reconstructing bitstream (reordering the packets)
RTCP • RTCP operates along side of RTP and shares information with it • Each RTCP has a different (UDP) port number associated with it so that it can operate independently of RTP • Periodic messages (RTCP packet) exchanged with the RTP hosts/clients • common system time clock for media synchronization • QoS reports computed by the receiving RTPs • participation reports • participation details
IP Multicast • A convention to identify a multicast address • Each node must translate between an IP multicast address and a list of networks that contain members of this group • A router must translate between an IP multicast address and a subnetwork multicast address to deliver a multicast IP datagram on the destination network • A mechanism for an individual host to inform routers (on the same network as itself ) of its inclusion or exclusion from a multicast group
IP Multicast (Contd.) • Routers need to know which subnetworks include members of a given multicast group. • Routers need sufficient information to calculate the shortest path to each network containing group members. • A routing algorithm is needed to calculate shortest paths to all group members. • Each router must determine multicast routing paths on the basis of both source and destination addresses.
IP Multicast (Contd.) • IGMP (Internet Group Management Protocol) or ICMPv6 (Internet Control Message Protocol) is used by hosts and routers to exchange multicast group membership information • MOSPF (Multicast Extension to Open Shortest Path First) enables routing of IP multicast datagrams • Each router attached to a LAN uses IGMP to maintain a current picture of local group membership. Periodically each router floods group membership information to all other routers in its area. • Using Dijkstra’s algorithm, each router constructs the shortest-path spanning tree from source network to all networks containing members of multicast group (done only on demand). • Unique spanning tree for a given source node • Interarea multicasting