Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies A Little More on Chapter 7 And Start Chapter 8 TCP/IP

Leon-Garcia & Widjaja: Communication NetworksCopyright ©2000 The McGraw Hill Companies

A Little More on Chapter 7And Start Chapter 8 TCP/IP


Today

• C7: Count-to-Infinity Problem in Distance Vector Routing

• C7: Traffic management and Quality of Service

• C7:Congestion Control via Leaky Buckets and TCP Sliding Windows

• C8: Introduction to TCP/IP


1 2 NN-1

Figure 7.41

…

7.7 Model for Quality of Service Analysis by Traffic Management


Packet buffer

Transmission link

Arrivingpackets

Packet discardwhen full

Packet buffer

Transmissionlink

Arrivingpackets

Class 1 discardwhen full

Class 2discardwhen thresholdexceeded

(a)

(b)

Figure 7.42

(a) FIFO Queueing. (b) FIFO with 2 classes


Transmission link


High-prioritypackets

Low-prioritypackets


When high-priorityqueue empty

Figure 7.43

Priority Queueing


Sorted packet buffer

Transmissionlink

Arrivingpackets


Taggingunit

Figure 7.44

Sorting packets according to priority tag


48

63

2

1

5 7

Congestion

Figure 7.50

Router 4 is overloaded. Requests for retransmissions compound the problem.

Multitasking computers can have the same type of queueing model, and the same type of saturation.

7.8 Congestion Control


Offered load

Thr

ough

put

Controlled

Uncontrolled

Figure 7.51

7.8 Congestion Control

Open loop vs. closed loop methods

Due to "thrashing"


Water drains ata constant rate

Leaky bucket

Water pouredirregularly

Figure 7.53

Open Loop Control depends controlling or shaping entry to the system. One techniques is smoothing the variations in flow with the “leaky bucket” approach

Leaky bucket model for monitoring access controlled traffic and for smoothing bursty traffic


I

L+I

Bucketcontent

Time

Time

Packetarrival

Nonconforming

* * * * * * * **

Figure 7.55

Incoming packets are classified as conforming or non-conforming depending on whether they cause the bucket to overflow

Leaky bucket used to identify non-conforming packets. Marked for deletion


Time0 1 2 3

Time0 1 2 3

10 Kbps

Time0 1 2 3

50 Kbps

100 Kbps

(a)

(b)

(c)

Figure 7.58

Traffic shaping. Output of leaky bucket with buffer looks more like (a)


TCP uses Closed Loop Congestion Control

• TCP provides end-to-end flow control to avoid overunning a slow receiver by a sliding window. Each byte is given a sequence number! The sender cannot send a new byte unless it is in the allowable “advertised window”

• However this advertised window does not prevent intermediate routers from overflowing due to congestion

• To try to optimize speed of transmission TCP establishes a second window called the “congestion window”

• At any time the window used is the smaller of the two.


How Does the Congestion Window Work?

• The size of the congestion window is automatically adjusted depending the experience of the receiver:

• It starts with a small value: one maximum length “segment,” which is the PDU at the transport level

• It then ramps up exponentially, doubling on each transmission until it reaches a congestion threshold-- initially "65K bytes." Graph shows 16 x 65K.

• It then goes up linearly until a time out is experienced --assumed to be due to congestion

• The size of the congestion window is then cut back to its initial value and the congestion threshold is cut to half its initial value


Congestionwindow

10

5

15

20

0

Round-trip times

Slowstart

Congestionavoidance

Congestion occurs

Threshold

Figure 7.63

The congestion window seeks the optimum level just before congestion occurs

First threshold


Material in Chapter 8

• 1. TCP/IP Architecture

• 2. Internet Protocol IP Version 4

• 3. IP Version 6 (skip)

• 4. Transport Layer Protocols (TCP and UDP)

• 5. DHCP and Mobile Internet (just a little)

• 6 Internet Routing

• 7. Multicast Routing (skip)


Application

TCP UDP

IPICMP ARP RARP

Physicalnetwork

Application

Figure 8.1


HTTP Request

TCP Header

Header contains source and destination port numbers

Header contains: source and destination IP addresses; transport protocol type

IP Header

Header contains: source and destination physical addresses; network protocol type

FCSEthernet Header

Figure 8.2


Application

Transport

InternetNetwork Interface

Application

Transport

InternetInternet

Network 1 Network 2

Machine A Machine B

Router/Gateway

Network Interface

Network Interface

Figure 8.3


Version IHL Type of Service Total Length

Identification Flags Fragment Offset

Time to Live Protocol Header Checksum

Source IP Address

Destination IP Address

Options Padding

0 4 8 16 19 24 31

Figure 8.4

IP Packet Design: Fields Defined on next two slides


IP Packet Fields

• Version. mostly 4 or 6

• Internet Header Length IHL in 32-bit words if no options are present IHL=5

• Type of Service. (priority) Most routers ignore

• Total Length. No of bytes in IP packet including header and info. Max is 65,535. Usually less. Ethernet only allows 1500 bytes.

• ID, Flags, Frag Offset. Used in reassembling fragmented packets.


IP Packet Fields, Continued

• Time to Live TTL. Sending host sets. Decremented by one by each router. When field reaches zero, packet is discarded. Normally counts hops.

• Protocol that will receive packet. TCP=6, UDP=17, ICMP=1

• Header checksum. Info part not checked. Since the TTL is decremented by each router, this has to recalculated by each router

• Source and Destination IP addresses 32 bits each.• Options. Rarely used.• Padding. used to make header a multiple of 32-bit words


0 Net ID Host ID

Net ID Host ID1 0

Net ID Host ID1 1 0

1 1 1 0 Multicast address

1 1 1 1 Reserved for experiments

Class A

Class B

Class C

Class D

Class E

0 1 2 3 8 16 31Bit position:

Figure 8.5

Varieties of IP addresses


Originaladdress

Subnettedaddress

Net ID Host ID1 0

Net ID Host ID1 0 Subnet ID

Figure 8.6


R1

H1 H2

H3 H4

R2 H5

To the rest ofthe Internet

150.100.0.1

150.100.12.128

150.100.12.0

150.100.12.176150.100.12.154

150.100.12.24 150.100.12.55

150.100.12.1

150.100.15.54

150.100.15.0

150.100.15.11

150.100.12.129

150.100.12.4

Figure 8.7

Documents

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies A Little More on Chapter 7 And Start Chapter 8 TCP/IP