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Ch. 10 – Intermediate TCP/IP. CCNA 2 version 3.0 Rick Graziani Cabrillo College. Interesting Reading. Although, published in 1994, written by the late Richard Stevens, it is still regarded as the definitive book on TCP/IP.

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Ch 10 intermediate tcp ip l.jpg

Ch. 10 – Intermediate TCP/IP

CCNA 2 version 3.0

Rick Graziani

Cabrillo College


Interesting reading l.jpg
Interesting Reading

  • Although, published in 1994, written by the late Richard Stevens, it is still regarded as the definitive book on TCP/IP.

TCP/IP Illustrated, Vol. 1 W. Richard Stevens Addison-Wesley Pub Co ISBN: 0201633469

Where Wizards Stay Up Late Katie Hafner and Matthew Lyon ISBN 0613181530

  • Very enjoyable reading and you do not have to be a networking geek to enjoy it!

  • National Bestseller

Rick Graziani graziani@cabrillo.edu


Overview l.jpg
Overview

Students completing this module should be able to:

  • Describe TCP and its function

  • Describe TCP synchronization and flow control

  • Describe UDP operation and processes

  • Identify common port numbers

  • Describe multiple conversations between hosts

  • Identify ports used for services and clients

  • Describe port numbering and well known ports

  • Understand the differences and the relationship between MAC addresses, IP addresses, and port numbers

Rick Graziani graziani@cabrillo.edu


Tcp operation l.jpg
TCP Operation

  • IP is best effort delivery.

  • The transport layer (TCP) is responsible reliability and flow control from source to destination.

  • This is accomplished using:

    • sliding windows (flow control)

    • sequencing numbers and acknowledgments (reliability)

    • synchronization (establish a virtual circuit)

  • Note: Although straight-forward in its operation, TCP can be a very complicated protocol in its operation. Most of the details regarding TCP are beyond the scope of this module and presentation.

Rick Graziani graziani@cabrillo.edu


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TCP Operation

  • IP Packet has a Protocol field that specifies whether the segment is TCP or UDP.

Connection-oriented

Connectionless

Connectionless

Rick Graziani graziani@cabrillo.edu


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Application Header + data

IP Protocol Field = 17

Application Header + data

IP Protocol Field = 6

Rick Graziani graziani@cabrillo.edu


Slide7 l.jpg
TCP

  • TCP -- a connection-oriented, reliable protocol; provides flow control by providing sliding windows, and reliability by providing sequence numbers and acknowledgments.

  • TCP re-sends anything that is not received and supplies a virtual circuit between end-user applications.

  • The advantage of TCP is that it provides guaranteed delivery of the segments.

Rick Graziani graziani@cabrillo.edu


Synchronization or 3 way handshake l.jpg
Synchronization or 3-way handshake

TCP Header

  • For a connection to be established, the two end stations must synchronize on each other's initial TCP sequence numbers (ISNs).

  • Sequence numbers are used to track the order of packets and to ensure that no packets are lost in transmission.

  • The initial sequence number is the starting number used when a TCP connection is established.

  • Exchanging beginning sequence numbers during the connection sequence ensures that lost data can be recovered.

Rick Graziani graziani@cabrillo.edu


The following example and for more info l.jpg
The following example and for more info…

Inside the TCP Handshake

http://www.nwconnection.com/2000_03/hand30/

“Laura Chappell writes technical training books for podbooks.com (http://www.podbooks.com) and is a senior protocol analyst at NetAnalysis Institute.”

“Ms. Chappell also makes a pretty mean margarita. (For more information about NetAnalysis Institute, visit http://www.netanalysis.org.)”

Rick Graziani graziani@cabrillo.edu


Slide10 l.jpg

Packet 1: source: 130.57.20.10 dest.:130.57.20.1

TCP: ----- TCP header -----

TCP: Source port = 1026

TCP: Destination port = 524

TCP: Initial sequence number = 12952

TCP: Next expected Seq number= 12953

TCP: .... ..1. = SYN

TCP: Window = 8192

TCP: Checksum = 1303 (correct)

TCP: Maximum segment size = 1460 (TCP Option)

  • Only part of the TCP headers are displayed.

Packet 2: source: 130.57.20.1 dest: 130.57.20.10

TCP: ----- TCP header -----

TCP: Source port = 524

TCP: Destination port = 1026

TCP: Initial sequence number = 2744080

TCP: Next expected Seq number= 2744081

TCP: Acknowledgment number = 12953

TCP: .... ..1. = SYN

TCP: Window = 32768

TCP: Checksum = D3B7 (correct)

TCP: Maximum segment size = 1460 (TCP Option)

Packet 3: source: 130.57.20.10 dest: 130.57.20.1

TCP: ----- TCP header -----

TCP: Source port = 1026

TCP: Destination port = 524

TCP: Sequence number = 12953

TCP: Next expected Seq number= 12953

TCP: Acknowledgment number = 2744081

TCP: ...1 .... = Acknowledgment

TCP: Window = 8760

TCP: Checksum = 493D (correct)

TCP: No TCP options

Rick Graziani graziani@cabrillo.edu


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Denial of Service (DoS) Attacks

  • DoS attacks are a common method that hackers utilize to halt system response.

  • One type of DoS is known as SYN flooding.

  • SYN flooding exploits the normal three-way handshake and causes targeted devices to ACK to source addresses that will not complete the handshake.

Rick Graziani graziani@cabrillo.edu


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TCP Finite State Machine (FYI)

Rick Graziani graziani@cabrillo.edu


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DoS Syn Flooding Attack

  • Hacker initiates a synchronization but spoofing the source IP address.

    • Spoofing: using another’s IP address, real or not (one meaning)

  • The receiving device replies to the non-existent, (SYN ACK)

  • Receiving device places process in a wait state while waiting to receive the final ACK from the initiator.

  • The waiting request is placed in a connection queue or a holding area in memory.

  • This waiting state requires the attacked device to commit system resources, such as memory, to the waiting process until the connection timer times out.

  • Hackers will flood the attacked host with these false SYN requests utilizing all of its connection resources to respond and wait for false connections, preventing it from responding to legitimate connection requests.

Rick Graziani graziani@cabrillo.edu


Dos syn flooding attack14 l.jpg
DoS Syn Flooding Attack

  • To defend against these attacks, system administrators may decrease the connection timeout period and increase the connection queue size (not recommended).

  • Software also exists that can detect these types of attacks and initiate defensive measures.

Rick Graziani graziani@cabrillo.edu


For more information cisco l.jpg
For more information… (Cisco)

  • If a flood of incoming request packets have invalid source IP addresses, sessions never get established and remain as half-open connections. Many TCP implementations are only able to handle a small number of outstanding connections per port therefore these ports are effectively unavailable until the half-open connections time-out (typically 75 seconds). Additionally this attack may also cause the server to exhaust its memory or waste processor cycles in maintaining state information on these connections.

  • Firewall vendors such as Checkpoint, Cisco, and Raptor have incorporated features into their products to shield your downstream systems from SYN attacks.

    The Cisco IOSTM TCP Intercept

  • http://www.cisco.com/warp/public/cc/pd/iosw/iore/prodlit/576_pp.htm

    TCP Intercept Commands

  • http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121cgcr/secur_r/srprt3/srdenl.htm

Rick Graziani graziani@cabrillo.edu


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Windowing and Window Sizes

  • Both of these are example of simple windowing.

  • This is not an example of sliding windows.

  • Window size refers to the number of bytes that are transmitted before receiving an acknowledgment.

  • After a host transmits the window-size number of bytes, it must receive an acknowledgment before any more data can be sent.

  • The window size determines how much data the receiving station can accept at one time.

Rick Graziani graziani@cabrillo.edu


Simple windowing l.jpg
Simple Windowing

TCP Header

  • TCP is responsible for breaking data into segments.

  • With a window size of 1, each segment carries only one byte of data and must be acknowledged before another segment is transmitted. This results in inefficient host use of bandwidth.

  • The purpose of windowing is to improve flow control and reliability.

  • Unfortunately, with a window size of 1, you see a very inefficient use of bandwidth.

Rick Graziani graziani@cabrillo.edu


Simple windowing18 l.jpg
Simple Windowing

TCP Window Size

  • TCP uses a window size, number of bytes, that the receiver is willing to accept, and is usually controlled by the receiving process.

  • TCP uses expectational acknowledgments, meaning that the acknowledgment number refers to the next byte that the sender of the acknowledgement expects to receive.

  • A larger window size allows more data to be transmitted pending acknowledgment.

  • Note: The sequence number being sent identifies the first byte of data in that segment.

Rick Graziani graziani@cabrillo.edu


Simple windowing19 l.jpg
Simple Windowing

TCP Full-duplex service: Independent Data Flows

  • TCP provides full-duplex service, which means data can be flowing in each direction, independent of the other direction.

  • Window sizes, sequence numbers and acknowledgment numbers are independent of each other’s data flow.

  • Receiver sends acceptable window size to sender during each segment transmission (flow control)

    • if too much data being sent, acceptable window size is reduced

    • if more data can be handled, acceptable window size is increased

  • This is known as a Stop-and-Wait windowing protocol.

Rick Graziani graziani@cabrillo.edu


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Sliding Windows

  • Note: The following slides on Sliding Windows contains corrections to the on-line curriculum followed by my slides on Sliding Windows.

Rick Graziani graziani@cabrillo.edu


Sliding windows21 l.jpg
Sliding Windows

Initial Window size

Working Window size

Sliding Window Protocol

  • Sliding window algorithms are a method of flow control for network data transfers using the receivers Window size.

  • The sender computes its usable window, which is how much data it can immediately send.

  • Over time, this sliding window moves to the right, as the receiver acknowledges data.

  • The receiver sends acknowledgements as its TCP receive buffer empties.

  • The terms used to describe the movement of the left and right edges of this sliding window are: (These will be demonstrated in the following slides.)

    1. The left edge closes (moves to the right) when data is sent and acknowledged.

    2. The right edge opens (moves to the right) allowing more data to be sent. This happens when the receiver acknowledges a certain number of bytes received.

    3. The middle edge opens (moves to the right) as data is sent, but not yet acknowledged.

Usable Window

Can send ASAP

Octets sent

Not ACKed

Usable Window

Can send ASAP

Rick Graziani graziani@cabrillo.edu


Slide22 l.jpg

Host A - Sender

Host B - Receiver

1

2

Octets received

Window size = 6

3

Octets sent

Not ACKed

Usable Window

Can send ASAP

ACK 4

  • Host B gives Host A a window size of 6 (octets or bytes).

  • Host A begins by sending octets to Host B: octets 1, 2, and 3 and slides it’s window over showing it has sent those 3 octets.

  • Host A will not increase its usable window size by 3, until it receives an ACKnowldegement from Host B that it has received some or all of the octets.

  • Host B, not waiting for all of the 6 octets to arrive, after receiving the third octet sends an expectational ACKnowledgement of “4” to Host A.

Rick Graziani graziani@cabrillo.edu


Slide23 l.jpg

Host A - Sender

Host B - Receiver

1

Window size = 6

2

Octets sent

Not ACKed

Usable Window

Can send ASAP

3

ACK 4

4

5

ACK 6

  • Host A does not have to wait for an acknowldegement from Host B to keep sending data, not until the window size reaches the window size of 6, so it sends octets 4 and 5.

  • Host A receives the acknowledgement of ACK 4 and can now slide its window over to equal 6 octets, 3 octets sent – not ACKed plus 3 octets which can be sent asap.

Rick Graziani graziani@cabrillo.edu


Slide24 l.jpg

Host A - Sender

Host B - Receiver

1

Window size = 6

2

Octets sent

Not ACKed

Usable Window

Can send ASAP

3

ACK 4

4

5

ACK 6

6

7

8

9

Rick Graziani graziani@cabrillo.edu


Sliding windows25 l.jpg
Sliding Windows

  • Default 8K for Windows, 32K for Linux,

  • There are various unix/linux/microsoft programs that allow you to modify the default window size.

  • I do not recommend that you modify these unless you know what you are doing.

  • “Disclaimer: Modifying the registry can cause serious problems that may require you to reinstall your operating system. We cannot guarantee that problems resulting from modifications to the registry can be solved. Use the information provided at your own risk.”

  • NOTE: I take no responsibility for this software or any others!

Rick Graziani graziani@cabrillo.edu


Slide26 l.jpg

TCP/Web100 bandwidth test v4.2

click START to begin

running 10s outbound test... 107 Kbs outbound

running 10s inbound test... 1207 Kbs inbound

web100 Connection Variables:

Round Trip times were sampled 611 times

for a total time of 72770 millisecs

giving an average RTT of: 119.0 millisecs(0.119 secs)

You received 1126 packets

of size 1360 from the remote host

and it took a total of 10475.0 millisecs

Maximum Expected Bandwidth: 392 Kbs

Good Data Stream--No retransmits!

You are advertising a window of 17680 bytes

The remote host is advertising a window of 5840 bytes

The Remote Host has a send buffer of 128000 bytes

and a receive buffer of 128000 bytes

Buffer sizes are very important in determining the

advertised window sizes. Larger window sizes can

help increase thruput. If your window is smaller

than the remote host, your should investigate

increasing your socket buffer sizes.

ORNL TCP Web100 Bandwidth Test

http://lin-ks.greatplains.net/noc/measurement/tcpbw100.php

Rick Graziani graziani@cabrillo.edu


Sequencing numbers l.jpg
Sequencing numbers

  • The data segments being transmitted must be reassembled once all the data is received.

  • No guarantee that the data will arrive in the order it was transmitted.

  • TCP applies sequence numbers to the data segments

  • Sequencing numbers indicate to the destination device the correct order in which to put the bytes when they are received.

  • These sequencing numbers also act as reference numbers so that the receiver will know if it has received all of the data.

  • They also identify the missing data pieces to the sender so it can retransmit the missing data.

This is only if one octet was sent at a time.

Rick Graziani graziani@cabrillo.edu


Technical fyi on sequencing numbers l.jpg
Technical FYI on Sequencing numbers

Part of TCP Header

  • Sender: The value in the sequence number is the first byte in the data stream.

  • Question: How does the receiver know how much data was sent, so it knows what value to send in the acknowledgement?

  • Receiver: Using the sender’s IP packet and TCP segment information, the value of the ACK is:

    IP Packet Length (IP): Total length – Header length

    - TCP header length (TCP): Header length

    -------------------------------------------------

    Length of data in TCP segment

    ACK = Last Sequence Number ACKed + Length of data in TCP segment

  • Check Sequence Number to check for missing segments and to sequence out-of-order segments.

  • Remember that the ACK is for the sequence number of the byte you expect to receive. When you ACK 101, that says you've received all bytes through 100. This ignores Selective Acknowledgments or SACK.

Rick Graziani graziani@cabrillo.edu


Positive acknowledgment and retransmission par l.jpg
Positive Acknowledgment and Retransmission (PAR)

  • PAR: The source sends a packet, starts a timer, and waits for an acknowledgment before sending the next packet.

  • If the timer expires before the source receives an acknowledgment, the source retransmits the packet and starts the timer over again.

  • TCP uses expectational acknowledgments in which the acknowledgment number refers to the next octet that is expected.

Rick Graziani graziani@cabrillo.edu



Udp operation l.jpg
UDP Operation

  • UDP does not use windowing or acknowledgments so application layer protocols must provide error detection.

  • The Source Port field is an optional field used only if information needs to return to the sending host.

  • When a destination router receives a routing update, the source router is not requesting anything so nothing needs to return to the source.

    • This is regarding only RIP updates.

    • BGP uses TCP, IGRP is sent directly over IP. EIGRP and OSPF are also sent directly over IP with their own way of handling reliability.

Rick Graziani graziani@cabrillo.edu


Udp operation32 l.jpg
UDP Operation

Rick Graziani graziani@cabrillo.edu


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Port Numbers (TCP and UDP)

Rick Graziani graziani@cabrillo.edu


Slide34 l.jpg

Port Numbers

Application Header + data

Port numbers are used to know which application the receiving host should pass the “Data” to.

Application Header + data

Rick Graziani graziani@cabrillo.edu


Slide35 l.jpg

TCP Header

Rick Graziani graziani@cabrillo.edu


Port numbers l.jpg
Port Numbers

TCP Header

  • Application software developers have agreed to use the well-known port numbers that are defined in RFC 1700.

  • For example, any conversation bound for an Telnet application uses the standard port number 23.

Rick Graziani graziani@cabrillo.edu


Port numbers37 l.jpg
Port Numbers

  • Conversations that do not involve an application with a well-known port number are, instead, assigned port numbers that are randomly selected from within a specific range.

  • These port numbers are used as source and destination addresses in the TCP segment.

  • Some ports are reserved in both TCP and UDP, although applications might not be written to support them.

  • (Curriculum) Port numbers have the following assigned ranges:

    • Numbers below 255 are reserved for public applications

    • Numbers from 255-1023 are assigned to companies for marketable applications

    • Numbers above 1023 are unregulated

  • (RFC) The range for assigned ports managed by the IANA is 0-1023.: http://www.iana.org/assignments/port-numbers

    • The Well Known Ports are those from 0 through 1023. (This is updated information as of 11-13-2002. Before then, 0 – 255 were considered well known ports.)

    • The Registered Ports are those from 1024 through 49151

    • The Dynamic and/or Private Ports are those from 49152 through 65535

Rick Graziani graziani@cabrillo.edu


Http www iana org assignments port numbers l.jpg
http://www.iana.org/assignments/port-numbers

  • The Well Known Ports are assigned by the IANA and on most systems can only be used by system (or root) processes or by programs executed by privileged users.

  • The Registered Ports are listed by the IANA and on most systems can be used by ordinary user processes or programs executed by ordinary users. The IANA registers uses of these ports as a convenience to the community.

  • The Dynamic and/or Private Ports are those from 49152 through 65535

Rick Graziani graziani@cabrillo.edu


Port numbers39 l.jpg
Port Numbers

  • For more of an explanation of port numbers and examples, go to:

    • http://www.iana.org/assignments/port-numbers

Rick Graziani graziani@cabrillo.edu


Port numbers40 l.jpg
Port Numbers

  • End systems use port numbers to select the proper application.

  • Originating source port numbers, usually a value larger than 1023, are dynamically assigned by the source host.

TCP Header

TCP Header

Rick Graziani graziani@cabrillo.edu


Slide41 l.jpg

TCP Header

Notice the difference in how source and destination port numbers are used with clients and servers:

Client:

  • Destination Port = 23 (telnet)

  • Source Port = 1028 (dynamically assigned)

    Server:

  • Destination Port = 1028 (source port of client)

  • Source Port = 23 (telnet)

Rick Graziani graziani@cabrillo.edu


Slide42 l.jpg

Second http session from the between the same client and server. Same destination port, but different source port to uniquely identify this web session.

This example shows two separate browser windows to the same URL. TCP/IP uses source port numbers to know which information goes to which window.

Dest. Port = 80 Send packets to web server application

http to www.cisco.com

Dest. Port = 80 Send packets to web server application

http to www.cisco.com

1030

80

1031

80

Rick Graziani graziani@cabrillo.edu


Slide43 l.jpg

What makes each connection unique? server. Same destination port, but different source port to uniquely identify this web session.

  • Connection defined by the pair of numbers:

    • source IP address, source port

    • destination IP address, destination port

  • Different connections can use the same destination port on server host as long as the source ports or source IPs are different.

Rick Graziani graziani@cabrillo.edu


Slide44 l.jpg

TCP or UDP server. Same destination port, but different source port to uniquely identify this web session.

Source IP

Destination IP

Connection State

Destination Port

Source Port

  • Note: In actuality, when you open up a single web page, there are usually several TCP sessions created, not just one.

  • Example of multiple TCP connections for a single http session.

www.google.com

www.cisco.com

netstat command

Rick Graziani graziani@cabrillo.edu


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Ch. 10 – Intermediate TCP/IP server. Same destination port, but different source port to uniquely identify this web session.

CCNA 2 version 3.0

Rick Graziani

Cabrillo College


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