Transcript S6C10 - Queuing - YSU Department of Computer Science and

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:
– first in,first out (FIFO) queuing; priority queuing (PQ);
custom queuing (CQ); and weighted fair queuing
(WFQ)
• Only one queuing method per interface
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
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
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.
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.
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.
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
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
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
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
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
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.
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.
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
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
Queuing Show Commands
•
•
•
•
•
show queueing
show interfaces
show queueing custom
show queueing priority
show queueing fair
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
Types of Data Compression
• Link compression (also known as perinterface compression)
• Payload compression (also known as pervirtual-circuit compression)
• TCP header compression
– By default data is sent uncompressed
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,
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
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]
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