Presentation on theme: "Ch. 10 – Intermediate TCP/IP"— Presentation transcript:
1Ch. 10 – Intermediate TCP/IP CCNA 2 version 3.0Some concepts in this presentation were introduced by Rick Graziani, Instructor at Cabrillo College in CaliforniaProf. Yousif
2NoteIt 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 10Corrections to CCNA 2 Module 10Other sources, Stevens, etc.
3Interesting ReadingTCP/IP Illustrated, Vol. 1 W. Richard Stevens Addison-Wesley Pub Co ISBN:Where Wizards Stay Up Late Katie Hafner and Matthew Lyon ISBNVery enjoyable reading and you do not have to be a networking geek to enjoy it!National BestsellerAlthough, published in 1994, written by the late Richard Stevens, it is still regarded as the definitive book on TCP/IP.
4Overview Students completing this module should be able to: Describe TCP and its functionDescribe TCP synchronization and flow controlDescribe UDP operation and processesIdentify common port numbersDescribe multiple conversations between hostsIdentify ports used for services and clientsDescribe port numbering and well known portsUnderstand the differences and the relationship between MAC addresses, IP addresses, and port numbers
5TCP 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.
6TCP OperationConnection-orientedConnectionlessConnectionlessIP Packet has a Protocol field that specifies whether the segment is TCP or UDP.
7IP Protocol Field = 17 IP Protocol Field = 6 Application Header + data
8TCPTCP -- 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.
9Synchronization or 3-way handshake TCP HeaderFor 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.
10The following example and for more info… Inside the TCP Handshake“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
11Only part of the TCP headers are displayed. Packet 1: source: dest.:TCP: TCP header -----TCP: Source port = 1026TCP: Destination port = 524TCP: Initial sequence number = 12952TCP: Next expected Seq number= 12953TCP: = SYNTCP: Window = 8192TCP: Checksum = 1303 (correct)TCP: Maximum segment size = 1460 (TCP Option)Packet 2: source: dest:TCP: TCP header -----TCP: Source port = 524TCP: Destination port = 1026TCP: Initial sequence number =TCP: Next expected Seq number=TCP: Acknowledgment number = 12953TCP: = SYNTCP: Window = 32768TCP: Checksum = D3B7 (correct)TCP: Maximum segment size = 1460 (TCP Option)Packet 3: source: dest:TCP: TCP header -----TCP: Source port = 1026TCP: Destination port = 524TCP: Sequence number = 12953TCP: Next expected Seq number= 12953TCP: Acknowledgment number =TCP: = AcknowledgmentTCP: Window = 8760TCP: Checksum = 493D (correct)TCP: No TCP optionsOnly part of the TCP headers are displayed.
12Denial 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.
13DoS 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.
14DoS 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.
15For 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 InterceptTCP Intercept Commands
16Windowing 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.
17Simple 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.
18Simple 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.
19Simple 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 reducedif more data can be handled, acceptable window size is increasedThis is known as a Stop-and-Wait windowing protocol.
20Sliding WindowsNote: The following slides on Sliding Windows contains corrections to the on-line curriculum followed by my slides on Sliding Windows.
21Sliding Windows Sliding Window Protocol Initial Window size Working Window sizeUsable WindowCan send ASAPOctets sentNot ACKedUsable WindowCan send ASAPSliding Window ProtocolSliding 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.
22Host B gives Host A a window size of 6 (octets or bytes). Host A - SenderHost B - Receiver12345678910111213123456789101112131234567891011121312Octets receivedWindow size = 63Octets sentNot ACKedUsable WindowCan send ASAP12345678910111213ACK 4Host 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.
23Host A - Sender Host B - Receiver 1234567891011121312345678910111213123456789101112131Window size = 6Octets sentNot ACKedUsable WindowCan send ASAP23ACK 41234567891011121312345678910111213451234567891011121312345678910111213ACK 6Host 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.
24Host A - Sender Host B - Receiver Window size = 6 Octets sent 1234567891011121312345678910111213123456789101112131Window size = 6Octets sentNot ACKedUsable WindowCan send ASAP23ACK 41234567891011121312345678910111213451234567891011121312345678910111213ACK 612345678910111213671234567891011121312345678910111213123456789101112138912345678910111213
25Sliding 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!
26ORNL TCP Web100 Bandwidth Test TCP/Web100 bandwidth test v4.2click START to beginrunning 10s outbound test Kbs outboundrunning 10s inbound test Kbs inboundweb100 Connection Variables:Round Trip times were sampled 611 timesfor a total time of millisecsgiving an average RTT of: millisecs(0.119 secs)You received packetsof size 1360 from the remote hostand it took a total of millisecsMaximum Expected Bandwidth: 392 KbsGood Data Stream--No retransmits!You are advertising a window of bytesThe remote host is advertising a window of 5840 bytesThe Remote Host has a send buffer of bytesand a receive buffer of bytesBuffer sizes are very important in determining theadvertised window sizes. Larger window sizes canhelp increase thruput. If your window is smallerthan the remote host, your should investigateincreasing your socket buffer sizes.ORNL TCP Web100 Bandwidth Test
27Sequencing 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 segmentsSequencing 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.
28Technical FYI on Sequencing numbers Part of TCP HeaderSender: 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 lengthLength of data in TCP segmentACK = Last Sequence Number ACKed + Length of data in TCP segmentCheck 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.
29Positive 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.
31UDP OperationUDP 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.
36Port 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.
37Port NumbersConversations 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 applicationsNumbers from are assigned to companies for marketable applicationsNumbers 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 49151The Dynamic and/or Private Ports are those from through 65535
38The 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
39Port NumbersFor more of an explanation of port numbers and examples, go to:
40Port 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.
41TCP HeaderNotice 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)
42Second 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 applicationhttp toDest. Port = 80 Send packets to web server applicationhttp to103080103180This example shows two separate browser windows to the same URL. TCP/IP uses source port numbers to know which information goes to which window.
43What makes each connection unique? Connection defined by the pair of numbers:source IP address, source portdestination IP address, destination portDifferent connections can use the same destination port on server host as long as the source ports or source IPs are different.
44netstat command www.cisco.com TCP or UDP Source IP Destination IP Connection StateSource PortDestination Portnetstat commandNote: 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.