Advanced Lab in Computer Communications Meeting 6 QoS. Instructor: Tom Mahler
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1 Advanced Lab in Computer Communications Meeting 6 QoS Instructor: Tom Mahler
2 Motivation Internet provides only single class of best-effort service. Some applications can be elastic. Tolerate delays and losses. Can adapt to congestion. Some applications (real-time) may demand some quality of service (e.g. bandwidth, latency, jitter). Should we modify these applications to be more adaptive or should we modify the Internet to support inelastic behavior?
3 QoS (Quality of Service) QoS: the ability to provide different priority to different applications, users, or data flows, or to guarantee a certain level of performance to a data flow. QoS Metrics: bit rate, delay, jitter (delay variation), throughput, packet loss rate. QoS are important for real-time streaming multimedia applications: VoIP, online games, IP- TV.
4 Example Consider a phone application at 1Mbps and an FTP application sharing a 1.5 Mbps link. bursts of FTP can congest the router and cause audio packets to be dropped. want to give priority to audio over FTP. Marking of packets is needed for router to distinguish between different classes; and new router policy to treat packets accordingly.
5 QoS Protocol Classification QoS can be achieved by: Prioritization (differentiated services). Resource reservation (integrated services). QoS can be applied: Per flow (individual, uni-directional streams). Per aggregate (two or more flows having something in common).
6 Differentiated Services (DiffServ) Forwarding: DiffServ-aware routers implement per-hop behaviors (PHBs), which define the packet-forwarding properties associated with a class of traffic. Lecture 22: 11/20/2001 6
7 WRED Queues WFQ/STRICT Queue Length
8 DiffServ Packet is marked in the Type of Service (TOS) in IPv4, and Traffic Class in IPv6. 6 bits used for Differentiated Service Code Point (DSCP) and determine PHB that the packet will receive.
9 DiffServ Field CU Differentiated Services CodePoints (DSCP) Currently Unused Class Selector CodePoints Bits 0-6, Differentiated Services CodePoints (DSCP) Defined in RFC 2474 which defines the Differentiated Services Field (DS Field) in the IPv4 and IPv6. Bits 3-5, Class Selector CodePoints. Bits 0-2, CU Currently Unused.
10 IPv4 ToS Field Reserved Monetary Reliability Throughput Delay Precedence MBZ - Must Be Zero 0-Normal 0-Normal 0-Normal 0-Normal 1-Low Monetary (Cost) 1-High Reliability 1-High Throughput 1-Low Delay Bits 0-2, Precedence Defined in RFC 1122 which defines the Communication Layers in the Link Layer. The 3 bits are used to indicate the importance of a datagram. Default is 0 (higher is better): 000 Routine. 001 Priority. 011 Flash. 100 Flash Override. 101 CRITIC/ECP. 110 Internetwork Control. 111 Network Control.
11 IPv4 ToS Field Reserved Monetary Reliability Throughput Delay Precedence MBZ - Must Be Zero 0-Normal 0-Normal 0-Normal 0-Normal 1-Low Monetary (Cost) 1-High Reliability 1-High Throughput 1-Low Delay Bits 3-6, ToS Defined in RFC 1349 which Type of Service in the IP Suit. The first two bits [3-4] are also called DTR Bits. Bit 3, Minimum Delay - This class is used when the time a packet takes to travel from its source to its destination is most important. Bit 4, Maximum Throughput - This class is used when the volume of data transferred in any given period of time is most important. Bit 5, Maximum Reliability - This class is used when some certainty is required that the data reaches its destination without retransmission. Bit 6, Minimum Monetary- This class is used when the financial cost of passing traffic is most important. Note that normal IP routing protocols try to route packets based on minimum Hops or Metrics, often referred to as Costs, unlike this financial cost.
12 VLAN, IEEE 802.1Q A Virtual Local Area Network is used to logically group network devices together. VLAN Tag: TPID PCP TCI DEI VID
13 VLAN, IEEE 802.1Q VLAN Tag TPID PCP TCI DEI VID TPID Tag Protocol Identifier (Set to 0x8100). TCI Tag Control Information: PCP Priority CodePoint: Class of service (CoS). DEI Drop Eligible Indicator: indicates that a frame is eligible to be dropped if there is congestion. Also known as CFI. VID VLAN identifier.
14 Priority Queuing Routers implement multiple queues, one for each priority class. The router always transmits packets out of the highest-priority queue if that queue is nonempty before moving to the next priority queue.
15 Priority Queuing (2) Problem: High priority packet starve lower priority packets. As long as there is at least one high priority packet, no other packets can be transmitted! Users may set all their packet to high-priority. Solution: Economics: The network can charge more to deliver high-priority packets than low-priority. However there are significant challenges to implementing such a scheme in a decentralized environment such as the Internet.
16 Fair Queuing Many times congestion control is implemented entirely on the source (TCP). Result: Greedy protocols (TCP). Each user is trying to use as much bandwidth as he can! FQ allows the router to police how the sources use the bandwidth and prevent from a single user to capture a large fraction of it. Separate queue for each flow (user).
17 Packet Fair Queuing Send one packet from a queue and move on to the next queue in a roundrobin manner. Question: Is this really fair queuing? Flow 1 Flow 2 Round-robin Service Flow 3 Flow 4
18 Bit to Bit Fair Queuing Problem: Not all packets are equal in size! Can the router transmit one bit from queue 1, then one bit from queue 2 and so on? Solution: Approximating a bit to bit round-robin.
19 Weighted Fair Queuing (2) Weighted Fair queuing is an approximation of Generalized Processor Sharing (GPS). WFQ GPS WFQ GPS
20 Weighted Fair Queuing Features Simulates a bit-to-bit Fair Queuing. Nothing wasted: if there is only one active flow, it can use the entire link capacity to itself. If the link is fully loaded, each flow gets 1/nth of the link bandwidth. If a flow tries to use more, than his packets will sit longer times in the queue. WFQ guarantees both fairness and performance (no starvation like in PQ).
21 Weighted Fair Queuing Assigning weights to different flows: The weight specifies how many bits will be transmitted each time the router service that queue. When all flows are assigned 1, we sometimes call it just FQ. Example- Suppose 3 active flows: Flow Weight Bandwidth Fraction A 2 1/3 B 1 1/6 C 3 1/2
22 Weighted Fair Queuing WFQ can be also implemented on data classes instead of flows. The ToS field in the IP header can be used to assign classes
23 Dropping Policy: Tail Drop Arriving packet Next free buffer Next to transmit Free buffers Queued packets Tail Drop Arriving packet Next to transmit Queued packets Drop
24 Dropping Policy: Random Early Detection (RED) Basic premise: Router should signal congestion when the queue first starts building up (by dropping a packet) But router should give flows time to reduce their sending rates before dropping more packets Note: when RED is coupled with ECN, the router can simply mark a packet instead of dropping it Therefore, packet drops should be: Early: don t wait for queue to overflow Random: don t drop all packets in burst, but space them QoS #24
25 Dropping Policy: RED Probabilistically discard packets. Probability is computed as a function of average queue length. Discard Probability 1 0 min_th max_th queue_len Average Queue Length QoS #25
26 Dropping Policy: WRED Extension to RED where a single queue may have several different queue thresholds. Each queue threshold is associated to a particular traffic class. QoS #26
27 Traffic Shaping Traffic shaping controls the rate at which packets are sent (not just how many). Used in ATM and Integrated Services networks. At connection set-up time, the sender and carrier negotiate a traffic pattern (shape). Two traffic shaping algorithms are: Leaky Bucket Token Bucket QoS #27
28 The Leaky Bucket Algorithm The Leaky Bucket Algorithm Used to control rate in a network. It is implemented as a single-server queue with constant service time. If the bucket (buffer) overflows then packets are discarded. Leaky Bucket (parameters r and B): Every r time units send a packet. For an arriving packet, if queue not full (less than B) then enqueue. Note that the output is a perfect constant rate. QoS #28
29 The Leaky Bucket Algorithm (a) A leaky bucket with water. (b) A leaky bucket with packets. QoS #29
30 Token Bucket Algorithm Highlights: The bucket holds tokens. To transmit a packet, we use one token. Allows the output rate to vary according to the size of the burst, unlike the Leaky Bucket. Granularity Packets (or bits). Token Bucket (r, MaxTokens): Generate a token every r time units, if the number of tokens is more than MaxToken, reset to MaxTokens. For an arriving packet enqueue. While buffer not empty and there are tokens: send a packet and discard a token. QoS #30
31 The Token Bucket Algorithm 5-34 (a) Before. (b) After. QoS #31
32 Leaky Bucket vs Token Bucket Leaky Bucket. Token Bucket Discard: packets Discard: tokens. Packet manage is separate. Rate: fixed rate (perfect). Rate: average rate. Bursts allowed Arriving Burst: waits in bucket. Arriving Burst: can be sent immediately. QoS #32
33 Policing Dual-Bucket Model Committed Information Rate (CIR) Excess Information Rate, (EIR) Committed Burst Size (CBS) Excess Burst Size (EBS)
34 Integrated Services Concepts Flowspecs describing what we need from the receiver (RSpec) and what we plan to inject to the network (TSpec). One way of describing the transmitter requirements (TSpec) is the Token bucket filter: r Token rate B Bucket depth The sender pays 1 token for each byte it wants to send. To send a packet of length n it needs n tokens. The sender receives tokens at rate r per second and collets them. The sender can accumulate up to B tokens. This means that the maximum burst length is B, and the average bandwidth required is r bytes/seconds.
35 RSVP Resource Reservation Protocol. Part of the IETF IntServ group ( ). Support number of service classes designed to meet the needs of certain applications: RT applications For example VoIP. Intolerant applications Robot control application.
36 RSVP - Implementation Sender PATH message containing traffic specification (bitrate, peak rate etc). Receiver RECV message containing the reservation specification (guaranteed or controlled) and the filter specification (type of packets that the reservation is made for).
37 RSVP Before making a reservation the receiver needs to know 2 things: 1) What kind of traffic the sender is likely to send (TSpec). 2) The path of the packets from the sender (because of SoftState).
38 RSVP Traffic Specification PATH QoS Level and Filter Specification RESV Host A Host B
39 RSVP The sender sends a PATH message to the receiver that contains the TSpec. The receivers sends back a RESV message that contains both the TSpec and RSpec. Each router on the way checks if it has the resources to satisfy it: If the reservation can be made, the router pass the RESV to the next router on the way. If all goes well, the reservation is installed on every router between the sender and the receiver.
40 RSVP Every 30 seconds or so the sender sends a new PATH message. The receiver in his turn, sends a RESV messages up the path to the sender. What happens in a topology change?
41 Integrated Services Vs. Differentiated Services Integrated Services: fine-grained Provide QoS to individual applications or flows. Implementations: RSVP, in ATM (QoS for a single Virtual Circuit). Differentiated Services: coarse-grained Provide QoS to large classes of data or aggregated traffic. Instead of the routers keeping states, the datagram identifies itself as it arrives to the router. Implementations: DiffServ codepoints (DSCP) in IP header ToS, in ATM (QoS between routers).
Basics (cont.) Characteristics of data communication technologies OSI-Model
48 Basics (cont.) Characteristics of data communication technologies OSI-Model Topologies Packet switching / Circuit switching Medium Access Control (MAC) mechanisms Coding Quality of Service (QoS) 49
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