1. Ch. 10 – Intermediate TCP/IP
CCNA 2 version 3.0
Some concepts in this presentation were introduced by Rick Graziani, Instructor at Cabrillo College in California
Prof. Yousif

2. Note
It is important for networking professionals to have a very good understanding of TCP/IP.
Various devices communicate using the multiple protocols of the TCP/IP protocol suite.
A networking professional needs to know how these protocols function and interact with each other in order to properly understand, analyze and troubleshoot networking issues.
This chapter is only an introduction to this information and also a review of CCNA 1.
I strongly suggest taking a separate course in the TCP/IP protocol suite, in addition to system administration courses such as those for Microsoft Windows (MCSE/MCSA) or Unix/Linux certifications.
This presentation:
CCNA 2 Module 10
Corrections to CCNA 2 Module 10
Other sources, Stevens, etc.

3. Interesting Reading
TCP/IP Illustrated, Vol. 1 W. Richard Stevens Addison-Wesley Pub Co ISBN:
Where Wizards Stay Up Late Katie Hafner and Matthew Lyon ISBN
Very enjoyable reading and you do not have to be a networking geek to enjoy it!
National Bestseller
Although, published in 1994, written by the late Richard Stevens, it is still regarded as the definitive book on TCP/IP.

4. 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

5. 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.

6. TCP Operation
Connection-oriented
Connectionless
Connectionless
IP Packet has a Protocol field that specifies whether the segment is TCP or UDP.

7. IP Protocol Field = 17 IP Protocol Field = 6 Application Header + data

8. 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.

9. 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.

10. The following example and for more info…
Inside the TCP Handshake
“Laura Chappell writes technical training books for 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

11. Only part of the TCP headers are displayed.
Packet 1: source: dest.:
TCP: TCP header -----
TCP: Source port = 1026
TCP: Destination port = 524
TCP: Initial sequence number = 12952
TCP: Next expected Seq number= 12953
TCP: = SYN
TCP: Window = 8192
TCP: Checksum = 1303 (correct)
TCP: Maximum segment size = 1460 (TCP Option)
Packet 2: source: dest:
TCP: TCP header -----
TCP: Source port = 524
TCP: Destination port = 1026
TCP: Initial sequence number =
TCP: Next expected Seq number=
TCP: Acknowledgment number = 12953
TCP: = SYN
TCP: Window = 32768
TCP: Checksum = D3B7 (correct)
TCP: Maximum segment size = 1460 (TCP Option)
Packet 3: source: dest:
TCP: TCP header -----
TCP: Source port = 1026
TCP: Destination port = 524
TCP: Sequence number = 12953
TCP: Next expected Seq number= 12953
TCP: Acknowledgment number =
TCP: = Acknowledgment
TCP: Window = 8760
TCP: Checksum = 493D (correct)
TCP: No TCP options
Only part of the TCP headers are displayed.

12. 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.

13. 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 place 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.

14. 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.

15. 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
TCP Intercept Commands

16. 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.

17. 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.

18. 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.

19. 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.

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

21. Sliding Windows Sliding Window Protocol Initial Window size
Working Window size
Usable Window
Can send ASAP
Octets sent
Not ACKed
Usable Window
Can send ASAP
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 open (moves to the right) as data is sent, but not yet acknowledged.

22. Host B gives Host A a window size of 6 (octets or bytes).
Host A - Sender
Host B - Receiver 1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 4 5 6 7 8 9 10 11 12 13 1 2
Octets received
Window size = 6 3
Octets sent
Not ACKed
Usable Window
Can send ASAP 1 2 3 4 5 6 7 8 9 10 11 12 13
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.

23. Host A - Sender Host B - Receiver1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 4 5 6 7 8 9 10 11 12 13 1
Window size = 6
Octets sent
Not ACKed
Usable Window
Can send ASAP 2 3
ACK 4 1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 4 5 6 7 8 9 10 11 12 13 4 5 1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 4 5 6 7 8 9 10 11 12 13
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, 2 octets sent – not ACKed plus 4 octets which can be sent asap.

24. Host A - Sender Host B - Receiver Window size = 6 Octets sent1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 4 5 6 7 8 9 10 11 12 13 1
Window size = 6
Octets sent
Not ACKed
Usable Window
Can send ASAP 2 3
ACK 4 1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 4 5 6 7 8 9 10 11 12 13 4 5 1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 4 5 6 7 8 9 10 11 12 13
ACK 6 1 2 3 4 5 6 7 8 9 10 11 12 13 6 7 1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 4 5 6 7 8 9 10 11 12 13 8 9 1 2 3 4 5 6 7 8 9 10 11 12 13

25. 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!

26. ORNL TCP Web100 Bandwidth Test
TCP/Web100 bandwidth test v4.2
click START to begin
running 10s outbound test Kbs outbound
running 10s inbound test Kbs inbound
web100 Connection Variables:
Round Trip times were sampled 611 times
for a total time of millisecs
giving an average RTT of: millisecs(0.119 secs)
You received packets
of size 1360 from the remote host
and it took a total of millisecs
Maximum Expected Bandwidth: 392 Kbs
Good Data Stream--No retransmits!
You are advertising a window of bytes
The remote host is advertising a window of 5840 bytes
The Remote Host has a send buffer of bytes
and a receive buffer of 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

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

28. 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.

29. 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.

30. UDP

31. 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.

32. UDP Operation

33. Port Numbers (TCP and UDP)

34. Port Numbers Application Header + data
Port numbers are used to know which application the receiving host should pass the “Data” to.
Application Header + data

35. TCP Header

36. 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.

37. 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 are assigned to companies for marketable applications
Numbers above 1023 are unregulated
(RFC) The range for assigned ports managed by the IANA is :
The Well Known Ports are those from 0 through (This is updated information as of 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 through 65535

38.
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 through 65535

39. Port Numbers
For more of an explanation of port numbers and examples, go to:

40. Port Numbers TCP Header TCP Header
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.

41. 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)

42. Second http session between the same client and server
Second http session between the same client and server. Same destination port, but different source port to uniquely identify this web session.
Dest. Port = 80 Send packets to web server application
http to
Dest. Port = 80 Send packets to web server application
http to
1030 80
1031 80
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.

43. What makes each connection unique?
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.

44. netstat command www.cisco.com TCP or UDP Source IP Destination IP
Connection State
Source Port
Destination Port
netstat command
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.

45. Ch. 10 – Intermediate TCP/IP
CCNA 2 version 3.0