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S6C10 - Queuing

S6C10 - Queuing. Which Packet Gets Processed First. Queuing. the process the router uses to schedule packets for transmission during periods of congestion. mission-critical and delay-sensitive traffic can be sent first four methods of queuing:

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S6C10 - Queuing

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  1. S6C10 - Queuing Which Packet Gets Processed First

  2. Queuing • the process the router uses to schedule packets for transmission during periods of congestion. • mission-critical and delay-sensitive traffic can be sent first • four methods of queuing: • first in,first out (FIFO) queuing; priority queuing (PQ); custom queuing (CQ); and weighted fair queuing (WFQ) • Only one queuing method per interface

  3. FIFO Quequing • Simplest algorithm for packet transmission. • transmission occurs in the same order as messages are received • Until recently, FIFO queuing was the default for all router interfaces • Mission critical packet can be stuck in line behind massive ftp download

  4. Prioritization • Most effective on WAN links with combination of bursty traffic and relatively lower data rates • Most effective when applied to links at T1/E1 bandwidth speeds or lower. • If congestion on the WAN link does not exist, there is no reason to implement traffic prioritization. • Effective on WAN links that experience temporary congestion. • Add bandwidth if congesting is constant

  5. Queuing Policy • provides an appropriate level of service for all users • control expensive WAN costs • Determines relative importance of different traffic types • Determines which queuing scheme is best • Provides ability to prioritize, reserve, and manage network resources, • ensures the seamless integration and migration of disparate technologies without unnecessary costs.

  6. Choosing a Cisco IOS Queuing Option • Determine whether the WAN is congested • Decide whether strict control over traffic prioritization is necessary and whether automatic configuration is acceptable • study the types of traffic using the interface • decide relative priority • Establish a queuing policy • Determine whether any of the traffic types you identified in your traffic pattern analysis can tolerate a delay.

  7. Weighted Fair Queuing • Dynamic queuing strategy • used by default on serial interfaces at E1 speeds (2.048 Mbps) and below. • Disabled on serial interfaces that use X.25, Synchronous Data Link Control (SDLC), or compressed Point-to-Point Protocol (PPP). • Uses a complex algorithm to sort the packets that make up the different conversations on an interface • Automatically allocates bandwidth to all types of network traffic • prioritizes delay-sensitive packets so that high-volume conversations don’t consume all of the available bandwidth. • Low-volume traffic streams (which are the majority of traffic) receive preferential service, transmitting their entire loads in a timely fashion.

  8. WQF Discriminators • Discrimination of traffic into conversations is based on packet-header addressing. • Common conversation discriminators include: • Source/destination network address • Source/destination MAC address • Source/destination port or socket numbers • Frame Relay Data Link Connection Identifier (DLCI) value • Quality of service/type of service (QoS/ToS) value

  9. Configuration Commands • Router(config-if)#fair-queue {congestive-discard-threshold} • congestive-discard-threshold is the number of messages to queue for high-volume traffic • Fair-queue command appears in the output only if the congestive discard threshold is modified to a value other than 64 • Use no fair-queue command to disable WFQ • and enable FIFO queuing on an interface

  10. Priority Queuing • highest-priority traffic always gets dispatched before any other packets • Assign traffic to one of four output queues: high, medium, normal, or low priority • Once the high queue is empty, the router checks the medium queue • the lower-priority queue might not be serviced within an acceptable time frame, or even at all. • used on low-speed WAN links • Packet is classified; if the appropriate queue is full, the packet is dropped

  11. Configuring Priority Queuing • Priority list is a set of rules that describe the waypackets should be assigned to PQs • Router(config)#priority-list list-number protocol protocol-name {high | medium | normal | low} queue-keyword keyword-value • Router(config)#priority-list 1 protocol ip high tcp 23 • Router(config)#access-list 10 permit 239.1.1.0 0.0.0.255 • Router(config)#priority-list 1 protocol ip high list 10

  12. Priority Queuing Examples • Establish queuing priorities on packets entering from a given interface: • Router(config)#priority-list list-number interface interface-type interface-number {high | medium | normal | low} • Place traffic from E0 in medium priority • Router(config)#priority-list 2 interface ethernet 0 medium • Change the number of packets • Router(config)#priority-list list-number queue-limit high-limit medium-limit normal-limit low-limit • Router(config)#priority-list 4 queue-limit 10 40 60 80

  13. Custom Queuing • Reserves a minimum amount of bandwidth for every kind of traffic • delay-sensitive and mission-critical traffic can be assigned a large percentage of available bandwidth, • low-priority traffic receives a smaller portion. • can configure up to 16 queues • Each queue is serviced sequentially • until the number of bytes sent exceeds the configurable byte count • or until the queue is empty. • Important for time-sensitive protocols, such as voice, video, or IBMs SNA • require predictable response time. • Queue 0 is a system queue that handles system packets such as keepalives. • emptied before the other custom queues.

  14. Custom Queuing • Traffic filtering - The forwarding application-such as IP, IPX, or AppleTalk- • applies a set of filters or access-list entries to each message that it forwards. • messages are placed in queues, based on the filtering. • Queued message forwarding – • CQ uses a round-robin dispatching algorithm to forward traffic. • Each queue continues to transmit packets until the configured byte limit is reached. • When the threshold of this queue is reached or the queue is empty, the queuing software services the next queue in sequence.

  15. Configuring Custom Queuing • Send all traffic from Ethernet interface 0 to custom queue 1. • Send all IP traffic to custom queue 2. • Send all IPX traffic to custom queue 3. • Send all AppleTalk traffic to custom queue 4

  16. Custom Queuing Examples • Router(config)#queue-list list-number protocol protocol-name queue-number queue-keyword keyword-value • Router(config)#queue-list list-number interface interface-type interface-number queue-number

  17. Queuing Show Commands • show queueing • show interfaces • show queueing custom • show queueing priority • show queueing fair

  18. Data Compression • identifies patterns in a stream of data, and chooses a more efficient method of representing the same information • algorithm is applied to the data to remove as much redundancy as possible • Shannon's Limit • how much a given source of data can be compressed

  19. Types of Data Compression • Link compression (also known as per-interface compression) • Payload compression (also known as per-virtual-circuit compression) • TCP header compression • By default data is sent uncompressed

  20. Link Compression • uses either the Predictor or STAC algorithm to compress the traffic • To ensure error correction and packet sequencing (Cisco High-Level Data Link Control [HDLC] uses STAC compression only): • Predictor - Predicts the next sequence of characters in the data stream by using an index to look up a sequence in a compression dictionary • STAC - Developed by STAC Electronics, STAC is a Lempel-Ziv (LZ)-based compression algorithm. It searches the input data stream for redundant strings and replaces them with what is called a token,

  21. Payload Compression • (also known as per-virtual-circuit compression) • compresses only the data portion (including the Layer 3 and Layer 4 headers) of the data stream • frame header is left untouched • appropriate for virtual network services such as Switched Multimegabit Data Service (SMDS), Frame Relay, and Asynchronous Transfer Mode (ATM). • Use the frame-relay payload-compress command to enable STAC compression on a specified Frame Relay point-to-point interface or subinterface: • Router(config-if)#frame-relay payload-compr

  22. TCP/IP Header Compression • subscribes to the Van Jacobson Algorithm, which is defined in RFC 1144. • protocol specific and compresses only the TCP/IP header, which leaves the Layer 2 header intact to allow a packet with a compressed TCP/IP header to travel across a WAN link • Don’t implement both Layer 2 payload compression and TCP/IP header compression • header compression is generally used at lower speeds, such as 64-kbps links. • Router(config-if)#ip tcp header-compression [passive]

  23. Compression Considerations • Modem compression - In dial environ-ments, compression can occur in the modem. • Encrypted data - Compression is a Layer 2 function. When a data stream is encrypted by the client application, it is then passed onto the router for routing or compression service • CPU cycles versus memory - The amount of memory that a router must have varies according to the protocol being compressed

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