Qo s 09-integrated and red
- 2. Introduction • New additions to Internet increasing traffic —High volume client/server application —Web • Graphics —Real time voice and video • Need to manage traffic and control congestion • IEFT standards —Integrated services • Collective service to set of traffic demands in domain – Limit demand & reserve resources —Differentiated services • Classify traffic in groups • Different group traffic handled differently
- 3. Integrated Services Architecture (ISA) • IPv4 header fields for precedence and type of service usually ignored • ATM only network designed to support TCP, UDP and real-time traffic —May need new installation • Need to support Quality of Service (QoS) within TCP/IP —Add functionality to routers —Means of requesting QoS
- 4. Internet Traffic – Elastic • Can adjust to changes in delay and throughput • E.g. common TCP and UDP application —E-Mail – insensitive to delay changes —FTP – User expect delay proportional to file size • Sensitive to changes in throughput —SNMP – delay not a problem, except when caused by congestion —Web (HTTP), TELNET – sensitive to delay • Not per packet delay – total elapsed time —E.g. web page loading time —For small items, delay across internet dominates —For large items it is throughput over connection • Need some QoS control to match to demand
- 5. Internet Traffic – Inelastic • Does not easily adapt to changes in delay and throughput —Real time traffic • Throughput —Minimum may be required • Delay —E.g. stock trading • Jitter - Delay variation —More jitter requires a bigger buffer —E.g. teleconferencing requires reasonable upper bound • Packet loss
- 6. Inelastic Traffic Problems • Difficult to meet requirements on network with variable queuing delays and congestion • Need preferential treatment • Applications need to state requirements —Ahead of time (preferably) or on the fly —Using fields in IP header —Resource reservation protocol • Must still support elastic traffic —Deny service requests that leave too few resources to handle elastic traffic demands
- 7. ISA Approach • Provision of QoS over IP • Sharing available capacity when congested • Router mechanisms —Routing Algorithms • Select to minimize delay —Packet discard • Causes TCP sender to back off and reduce load • Enahnced by ISA
- 8. Flow • IP packet can be associated with a flow —Distinguishable stream of related IP packets —From single user activity —Requiring same QoS —E.g. one transport connection or one video stream —Unidirectional —Can be more than one recipient • Multicast —Membership of flow identified by source and destination IP address, port numbers, protocol type —IPv6 header flow identifier can be used but isnot necessarily equivalent to ISA flow
- 9. ISA Functions • Admission control —For QoS, reservation required for new flow —RSVP used • Routing algorithm —Base decision on QoS parameters • Queuing discipline —Take account of different flow requirements • Discard policy —Manage congestion —Meet QoS
- 10. Figure 9.1 ISA Implemented in Router
- 11. ISA Components – Background Functions • Reservation Protocol —RSVP • Admission control • Management agent —Can use agent to modify traffic control database and direct admission control • Routing protocol
- 12. ISA Components – Forwarding • Classifier and route selection —Incoming packets mapped to classes • Single flow or set of flows with same QoS – E.g. all video flows • Based on IP header fields —Determines next hop • Packet scheduler —Manages one or more queues for each output —Order queued packets sent • Based on class, traffic control database, current and past activity on outgoing port —Policing
- 13. ISA Services • Traffic specification (TSpec) defined as service for flow • On two levels —General categories of service • Guaranteed • Controlled load • Best effort (default) —Particular flow within category • TSpec is part of contract
- 14. Token Bucket • Many traffic sources can be defined by token bucket scheme • Provides concise description of load imposed by flow —Easy to determine resource requirements • Provides input parameters to policing function
- 15. Figure 9.2 Token Bucket Scheme
- 16. ISA Services – Guaranteed Service • Assured capacity level or data rate • Specific upper bound on queuing delay through network —Must be added to propagation delay or latency to get total delay —Set high to accommodate rare long queue delays • No queuing losses —I.e. no buffer overflow • E.g. Real time play back of incoming signal can use delay buffer for incoming signal but will not tolerate packet loss
- 17. ISA Services – Controlled Load • Tightly approximates to best efforts under unloaded conditions • No upper bound on queuing delay —High percentage of packets do not experience delay over minimum transit delay • Propagation plus router processing with no queuing delay • Very high percentage delivered —Almost no queuing loss • Adaptive real time applications —Receiver measures jitter and sets playback point —Video can drop a frame or delay output slightly —Voice can adjust silence periods
- 18. Queuing Discipline • Traditionally first in first out (FIFO) or first come first served (FCFS) at each router port • No special treatment to high priority packets (flows) • Small packets held up by large packets ahead of them in queue —Larger average delay for smaller packets —Flows of larger packets get better service • Greedy TCP connection can crowd out altruistic connections —If one connection does not back off, others may back off more
- 19. Fair Queuing (FQ) • Multiple queues for each port —One for each source or flow —Queues services round robin —Each busy queue (flow) gets exactly one packet per cycle —Load balancing among flows —No advantage to being greedy • Your queue gets longer, increasing your delay —Short packets penalized as each queue sends one packet per cycle
- 20. Figure 9.3 FIFO and Fair Queuing
- 21. Processor Sharing • Multiple queues as in FQ • Send one bit from each queue per round —Longer packets no longer get an advantage • Can work out virtual (number of cycles) start and finish time for a given packet • However, we wish to send packets, not bits
- 22. Bit-Round Fair Queuing (BRFQ) • Compute virtual start and finish time as before • When a packet finished, the next packet sent is the one with the earliest virtual finish time • Good approximation to performance of PS —Throughput and delay converge as time increases
- 23. Figure 9.4 Examples of PS and BRFQ
- 24. Figure 9.5 Comparison of FIFO and Fair Queue
- 25. Generalized Processor Sharing (GPS) • BRFQ can not provide different capacities to different flows • Enhancement called Weighted fair queue (WFQ) • From PS, allocate weighting to each flow that determines how many bots are sent during each round —If weighted 5, then 5 bits are sent per round • Gives means of responding to different service requests • Guarantees that delays do not exceed bounds
- 26. Weighted Fair Queue • Emulates bit by bit GPS • Same strategy as BRFQ
- 28. Proactive Packet Discard • Congestion management by proactive packet discard —Before buffer full —Used on single FIFO queue or multiple queues for elastic traffic —E.g. Random Early Detection (RED)
- 29. Random Early Detection (RED) Motivation • Surges fill buffers and cause discards • On TCP this is a signal to enter slow start phase, reducing load —Lost packets need to be resent • Adds to load and delay —Global synchronization • Traffic burst fills queues so packets lost • Many TCP connections enter slow start • Traffic drops so network under utilized • Connections leave slow start at same time causing burst • Bigger buffers do not help • Try to anticipate onset of congestion and tell one connection to slow down
- 30. RED Design Goals • Congestion avoidance • Global synchronization avoidance —Current systems inform connections to back off implicitly by dropping packets • Avoidance of bias to bursty traffic —Discard arriving packets will do this • Bound on average queue length —Hence control on average delay
- 31. RED Algorithm – Overview Calculate average queue size avg if avg < THmin queue packet else if THmin ≤ avg < Thmax calculate probability Pa with probability Pa discard packet else with probability 1-Pa queue packet else if avg ≥ THmax discard packet
- 35. Figure 9.10 Comparison of Drop Tail and RED Performance
- 36. Differentiated Services (DS) • ISA and RSVP complex to deploy • May not scale well for large volumes of traffic —Amount of control signals —Maintenance of state information at routers • DS architecture designed to provide simple, easy to implement, low overhead tool —Support range of network services • Differentiated on basis of performance
- 37. Characteristics of DS • Use IPv4 header Type of Service or IPv6 Traffic Class field —No change to IP • Service level agreement (SLA) established between provider (internet domain) and customer prior to use of DS —DS mechanisms not needed in applications • Build in aggregation —All traffic with same DS field treated same • E.g. multiple voice connections —DS implemented in individual routers by queuing and forwarding based on DS field • State information on flows not saved by routers
- 38. Table 9.1 DS Terminology (1) Behavior Aggregate A set of packets with the same DS codepoint crossing a link in a particular direction. Classifier Selects packets based on the DS field (BA classifier) or on multiple fields within the packet header (MF classifier). DS Boundary Node A DS node that connects one DS domain to a node in another domain DS Codepoint A specified value of the 6-bit DSCP portion of the 8-bit DS field in the IP header. DS Domain A contiguous (connected) set of nodes, capable of implementing differentiated services, that operate with a common set of service provisioning policies and per-hop behavior definitions. DS Interior Node A DS node that is not a DS boundary node. DS Node A node that supports differentiated services. Typically, a DS node is a router. A host system that provides differentiated services for applications in the host is also a DS node. Dropping The process of discarding packets based on specified rules; also called policing.
- 39. Table 9.1 DS Terminology (2) Marking The process of setting the DS codepoint in a packet. Packets may be marked on initiation and may be re-marked by an en route DS node. Metering The process of measuring the temporal properties (e.g., rate) of a packet stream selected by a classifier. The instantaneous state of that process may affect marking, shaping, and dropping functions. Per-Hop Behavior (PHB) The externally observable forwarding behavior applied at a node to a behavior aggregate. Service Level Agreement (SLA) A service contract between a customer and a service provider that specifies the forwarding service a customer should receive. Shaping The process of delaying packets within a packet stream to cause it to conform to some defined traffic profile. Traffic Conditioning Control functions performed to enforce rules specified in a TCA, including metering, marking, shaping, and dropping. Traffic Conditioning Agreement (TCA) An agreement specifying classifying rules and traffic conditioning rules that are to apply to packets selected by the classifier.
- 40. Services • Provided within DS domain —Contiguous portion of Internet over which consistent set of DS policies administered —Typically under control of one administrative entity • Defined in SLA —Customer may be user organization or other DS domain —Packet class marked in DS field • Service provider configures forwarding policies routers —Ongoing measure of performance provided for each class • DS domain expected to provide agreed service internally • If destination in another domain, DS domain attempts to forward packets through other domains —Appropriate service level requested from each domain
- 41. SLA Parameters • Detailed service performance parameters —Throughput, drop probability, latency • Constraints on ingress and egress points —Indicate scope of service • Traffic profiles to be adhered to —Token bucket • Disposition of traffic in excess of profile
- 42. Example Services • Qualitative —A: Low latency —B: Low loss • Quantitative —C: 90% in-profile traffic delivered with no more than 50ms latency —D: 95% in-profile traffic delivered • Mixed —E: Twice bandwidth of F —F: Traffic with drop precedence X has higher delivery probability than that with drop precedence Y
- 44. DS Field Detail • Leftmost 6 bits are DS codepoint —64 different classes available —3 pools • xxxxx0 : reserved for standards – 000000 : default packet class – xxx000 : reserved for backwards compatibility with IPv4 TOS • xxxx11 : reserved for experimental or local use • xxxx01 : reserved for experimental or local use but may be allocated for future standards if needed • Rightmost 2 bits unused
- 45. Precedence Field • Indicates degree of urgency or priority • If router supports precedence, three approaches: • Route selection —Particular route may be selected if smaller queue or next hop on supports network precedence or priority —e.g. token ring supports priority • Network service —Network on next hop supports precedence, service invoked • Queuing discipline —Use to affect how queues handled —E.g. preferential treatment in queues to datagrams with higher precedence
- 46. Router Queuing Disciplines – Queue Service • RFC 1812 • Queue service —SHOULD implement precedence-ordered queue service —Highest precedence packet queued for link is sent —MAY implement other policy-based throughput management • MUST be configurable to suppress them (i.e., use strict ordering)
- 47. Router Queuing Disciplines – Congestion Control • Router receives packet beyond storage capacity — Discard that or other packet or packets • MAY discard packet just received — Simplest but not best policy • Should select packet from session most heavily abusing link given QoS permits — Recommended policy in datagram environments using FIFO queues is to discard packet randomly selected — Routers using fair queues discard from longest queue — Router MAY use these algorithms • If precedence-ordered implemented and enabled MUST NOT discard packet with precedence higher than packet not discarded • MAY protect packets that request maximize reliability TOS — Except where doing so breaks previous rule • MAY protect fragmented IP packets — Dropping fragment may cause all fragments to be retransmitted • MAY protect packets used for control or management
- 49. Configuration – Interior Routers • Domain consists of set of contiguous routers • Interpretation of DS codepoints within domain is consistent • Interior nodes (routers) have simple mechanisms to handle packets based on codepoints —Queuing gives preferential treatment depending on codepoint • Per Hop behaviour (PHB) • Must be available to all routers • Typically the only part implemented in interior routers —Packet dropping rule dictated which to drop when buffer saturated
- 50. Configuration – Boundary Routers • Include PHB rules • Also traffic conditioning to provide desired service —Classifier • Separate packets into classes —Meter • Measure traffic for conformance to profile —Marker • Policing by remarking codepoints if required —Shaper —Dropper
- 51. Per Hop Behaviour – Expedited forwarding • Premium service —Low loss, delay, jitter; assured bandwidth end-to-end service through domains —Looks like point to point or leased line —Difficult to achieve —Configure nodes so traffic aggregate has well defined minimum departure rate • EF PHB —Condition aggregate so arrival rate at any node is always less that minimum departure rate • Boundary conditioners
- 52. Per Hop Behaviour – Explicit Allocation • Superior to best efforts • Does not require reservation of resources • Does not require detailed discrimination among flows • Users offered choice of number of classes • Monitored at boundary node —In or out depending on matching profile or not • Inside network all traffic treated as single pool of packets, distinguished only as in or out • Drop out packets before in packets if necessary • Different levels of service because different number of in packets for each user
- 53. PHB - Assured Forwarding • Four classes defined —Select one or more to meet requirements • Within class, packets marked by customer or provider with one of three drop precedence values —Used to determine importance when dropping packets as result of congestion
- 54. Required Reading • Stallings chapter 9