The IP Behavior

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The IP Behavior

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In this module, IP Behavior, we are going to take a look at how to analyze TCP/IP information and how one would actually go about pulling it off the wire and looking for patterns. The key point when it comes to security is, “Knowledge is power and ignorance is deadly.” Not understanding what is occurring on your network can be very dangerous from a security standpoint because if you do not understand what is occurring, then how can you determine whether it is good or bad?

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IP Behavior Security Essentials The SANS Institute IP Behavior – SANS ©2001 1 In this module, IP Behavior, we are going to take a look at how to analyze TCP/IP information and how one would actually go about pulling it off the wire and looking for patterns. The key point when it comes to security is, “Knowledge is power and ignorance is deadly.” Not understanding what is occurring on your network can be very dangerous from a security standpoint because if you do not understand what is occurring, then how can you determine whether it is good or bad? The answer is that you cannot. Therefore, being able to understand and interpret data on your network will enable you to determine what action you need to take to possibly protect against any malicious behavior. 4-1
Objectives • What is a sniffer? • Introduction to TCPdump and TCPdump output • TCP concepts • Fragmentation • Stimulus and normal response IP Behavior – SANS ©2001 2 In order to analyze and interpret TCP/IP traffic you need a way to be able to pull the traffic off of the network. In order to do this we need to use a sniffer. There are a variety of sniffers available but most of this chapter will concentrate on TCPdump and how to analyze TCPdump output. We will then cover TCP concepts that are the foundation of how TCP communicates. We will look at fragmentation in IP datagrams to see what is happening at the datagram level. Finally, we will examine stimulus and response. How does a host respond to certain traffic under varying circumstances? This will assist you in understanding what normal responses look like. 4-2
What is a Sniffer? • Sniffers gather all information transmitted across a line – For broadcast media (ethernet), allows an attacker to gather passwords, etc. – For ethernet, all data is broadcast on the LAN segment • Switched ethernet limits data to a specific source and destination port on a switch lah la h, b h, b Bla Hub Switch Blah, blah, blah Blah, blah, blah Blah, blah, blah Blah, blah, blah Bla h, b lah , bl ah Broadcast Ethernet Switched Ethernet IP Behavior – SANS ©2001 3 Sniffers are among the most common of hacker tools. They gather traffic off of the network, which an attacker can read in real time, or squirrel away in a file, but not only can they be used by attackers but they can also be used by the good guys to analyze network traffic and to figure out what is occurring on the network. When an ethernet interface is gathering all traffic, it is said to be in “promiscuous mode” and a sniffer is basically a device or piece of hardware that listens on the cable and records all traffic as it passes the network. In order to be able to sniff the traffic you have to be able to see it and this depends on the type of device you are using to connect your systems together. Traditional ethernet, usually implemented in a hub, is a broadcast medium, which broadcasts all data to all systems connected to the LAN segment. Therefore, traditional ethernet is inherently sniffable. Switched ethernet does not broadcast all information to all links of the LAN segment. Instead, the switch is more intelligent than the hub, and by looking at the destination MAC address, will only send the data to the required port on the switch. Switched ethernet is only sniffable in limited ways. 4-3
Examples of Sniffers • There are countless examples of sniffers out there – es - freeware (ships with SunOS, Solaris RootKits) – Linsniff - freeware (ships with Linux Rootkits) – Websniff - freeware – TCPdump - freeware – snoop - distributed with Solaris – Network Associates - commercial – Shomiti Surveyor - commercial – Ethereal - freeware – Windump - freeware – Snort - freeware – Sniffit - freeware – Dsniff - a free suite of tools built around a sniffer IP Behavior – SANS ©2001 4 There are lots of sniffers our there… some much more useful than others. Depending on your needs, your interest, and your budget, there should be a sniffer out there that does what you want. In this module we are going to concentrate and look at TCPdump which runs primarily on Unix systems but also has a port to Windows, called windump. 4-4
TCPdump TCPdump is a program that will dump traffic on a network. It is available from http://ee.lbl.gov or from www.tcpdump.org. You will also need to download the libpcap packet capture library. TCPdump has also been ported to Windows as windump. Windump and winpcap can be downloaded from http://netgroup-serv.polito.it/netgroup/tools.html. IP Behavior – SANS ©2001 5 In section 1, we will explore TCPdump. 4-5
TCPdump 0101001110 111010010011000 00100011011 packets Network TCPdump running on a host “sniffing” network packets TCPdump output 07:00:48.036746 ping.net > myhost.com: icmp: echo request (DF) 07:00:48.036776 myhost.com > ping.net: icmp: echo reply (DF) 07:02:12.622460 log.net.3155 > syslog.com.514: udp 101 07:03:01.132414 send.net.32938 > mail.com.25: S 248631:248631(0) win 8760 IP Behavior – SANS ©2001 6 On the slide “TCPdump”, we mean that TCPdump is a program that will read traffic off of the network. By default, it will collect and print, in a standard format, all the traffic passing on the network. There are command line options for TCPdump that will alter the default behavior. We can specify that we want to collect only certain types of packets, print the records of these packets in verbose mode (-v), print the packets in hexadecimal (-x), or actually write the records to a file as “raw packets”(-w) instead of printing them as standard output. TCPdump filters can be used to specify records to be collected. Rather than gather all traffic passing on the network on which the host resides, TCPdump can be instructed to record packets with a specific trait. Examples of filters would be to record only TCP packets, or record packets to a given port, say telnet, port 23, for instance. You can limit the purview of what is collected to a specific IP or host. Combinations of traits can be used to get more restrictive in what is collected. Just about any field in an IP datagram, including the actual data payload, can be used to select the records that are collected. On this slide, we see a host running TCPdump and gathering records from the network interface. We see the records that TCPdump has collected at the bottom of the slide. TCPdump has a default standard output based on the protocol (tcp, udp, icmp) of the record that is displayed. While each of the various protocols has a similar format to the other, they are also distinct in what is displayed. 4-6
Sample TCPdump UDP Output timestamp source.port dest.port : udp bytes 09:39:19.470000 nmap.edu.728 > dns.net.111: udp 56 timestamp: hour:minutes:seconds.fractions of seconds source.port: source IP/hostname.source port dest.port: destination IP/host.destination port udp: may or may not expressly label the udp protocol bytes: number of bytes of udp data (payload) IP Behavior – SANS ©2001 7 If we examine a line of TCPdump UDP output on the slide “Sample TCPdump UDP output”, we first see a timestamp -- or the record of the time when the TCPdump host read the packet. The timestamp is in the format of hour, colon, minutes, colon, seconds, period, followed by fractions of a second. As you can see, TCPdump allows for 6 fractional digits or millionths of a second. The record we see with the value 470000 has a precision of hundredths of a second. This is a limitation of the Linux operating system on which this TCPdump record was collected, but this was corrected in a later RedHat release. Next, we see the source information for the TCPdump record. This includes the source host name, nmap.edu, or IP number depending upon whether the IP can be resolved. If you do not want names resolved, TCPdump can be run with the -n parameter. At the end of the source host name, we see a period and the source port, in this case 728. Immediately following the greater than sign, you see the destination host or IP address, “dns.net”, followed by a period, followed by the destination port, in this case port 111 or what is more commonly known as the portmapper or sunrpc port. In this record, you see the word “udp” to help identify this protocol. Not all UDP records will be labeled expressly “udp”. DNS, or port 53, is a notable exception. The final field is the number of bytes found of the UDP data. Recall that UDP data is wrapped in a UDP header first and encapsulated in an IP header before it is sent out on the network. 4-7
Sample TCPdump TCP Output timestamp source.port dest.port flags beginning: ending bytes options seq # seq # 09:32:43.910000 nmap.edu.1173 > dns.net.21: S 62697789:62697789(0) win 512 flags: tcp flags ( PSH, RST, SYN, FIN) beginning seq #: for the initial connection, this is the initial sequence number (ISN) from the source IP ending seq #: this is the beginning sequence number + data bytes bytes: data bytes (payload) in the tcp packet options: options that the source host advertises to the destination host IP Behavior – SANS ©2001 8 The TCPdump TCP record is identical to the UDP record as far as timestamp, source, and destination host and port. What distinguishes the TCP format from the others are the TCP flags, sequence numbers, acknowledgements, acknowledgement numbers, and TCP options. In this record, we see the flag of SYN or S set following the destination port of 21 which, by the way, is the port for ftp. The SYN flag indicates a request to begin a TCP session. Other possible flag values are P for PUSH that sends data, R for RESET that aborts a connection, and F for FIN, which terminates a connection more gracefully. While not an actual flag bit like the others, if you see a period in the flag field, it simply means that none of the PUSH, RESET, SYN, or FIN flags are set. In a way, this is an informative placeholder. Next is the beginning sequence number. One of the mechanisms that TCP uses to guarantee reliable packet delivery is keeping track of the data it has received. This is partially done by using sequence numbers. In this case, since this is an initial connection, it is known as the Initial Sequence Number or ISN. The ending sequence number is the sum of the initial sequence number plus the number of TCP data bytes sent in this TCP segment. A SYN connection sends no data bytes, as represented by the zero in parentheses. Data should not be sent until the client and server actually establish the connection. Finally, there is a TCP options field. In this record, we see nmap.edu advertising a window size of 512 bytes. It is informing dns.net that it has an incoming buffer size of 512 bytes. If dns.net is a larger faster host, it will have to slow itself and pace the data sent so it doesn’t overwhelm the buffer size of nmap.edu. 4-8
Absolute and Relative Sequence client.com.38060 > telnet.com.telnet: S 3774957990:3774957990(0) win 8760 (DF) telnet.com.telnet > client.com.38060: S 2009600000:2009600000(0) ack 3774957991 win 1024 client.com.38060 > telnet.com.telnet: .ack 1 win 8760 (DF) client.com.38060 > telnet.com.telnet: P 1:28(27) ack 1 win 8760 (DF) IP Behavior – SANS ©2001 9 On the next slide, we show a handy feature of TCPdump. Notice the top line, we have the number 3774957990 in bold. That is an absolute sequence number. The absolute sequence number keeps track of how much data has been sent by a connection. However, these numbers get pretty ugly. So TCPdump can provide the information as relative sequence numbers as well. On the third line of your slide, after the ack, you see a 1 in bold. That means one byte has been transferred. When we look more closely at tcp, we will see our friends absolute and relative sequence again. 4-9
Sample TCPdump ICMP Output icmp format 1 timestamp source dest icmp: icmp message 14:59:30.220000 ping.net > hosta.mysite.com: icmp: echo request 14:59:38.140000 hosta.mysite.com > ping.net: icmp: echo reply icmp format 2 timestamp router source icmp: dest icmp message 02:09:47.600000 foreign.router > tryinghost.com: icmp: host desired.com unreachable IP Behavior – SANS ©2001 10 In the slide “Sample TCPdump ICMP Output,” we see ICMP output generated by TCPdump. ICMP is the protocol used for error control and message handling. There are many different types of ICMP records that have different messages. We display two of the basic formats on this slide. The first two ICMP records have a similar format. In the first ICMP record, we have a timestamp followed by a source host – ping.net in the first record. Following the greater than sign, we see the destination host – in this record hosta.mysite.com. Remember, ICMP doesn’t use ports to communicate like TCP and UDP do. The ICMP message type follows the destination host. In the first record we see an ICMP echo request which is generated by a commonly used program known as ping. The second record is a response from the ping from hosta.mysite.com to ping.net. This is known as an ICMP echo reply. The third record shows a slightly different format. Often times a router will be involved when some kind of error is detected. In this record, foreign.router delivers a message to tryinghost.com that host desired.com is not reachable. This implies that tryinghost.com first attempted to send some kind of traffic to desired.com and foreign.router intervened to inform tryinghost.com when it discovered a problem. 4 - 10
TCPdump Output in Hex • The TCPdump command comes with a command line option -x that will display the records in hexadecimal • Hexadecimal output is more difficult to read and decrypt, however it will show you the entire IP datagram, even fields that TCPdump doesn’t display in its standard output • The “entire” packet is shown only if you’ve told TCPdump to dump the entire packet via the -s (snaplen) calling parameter. IP Behavior – SANS ©2001 11 This slide reiterates what we discussed earlier. TCPdump output can be displayed in hexadecimal. As you become more advanced in analyzing records, you'll want to be able to look for the protocol and the only sure way to do that is in the hexadecimal information. We'll go over that, but don't worry if you don't understand it all. Over time, the cumulative discussions of binary and hexadecimal arithmetic will begin to look familiar and almost friendly. This is done using the –x command line option of TCPdump. Hexadecimal output is indeed more difficult to read or decipher. But if the snaplen, the -s parameter, is as large or larger than the datagram, the -x parameter will show the entire datagram, something that the standard TCPdump output doesn’t do. This can be used as a tool to investigate a particular field or value. Additionally, it can be used to uncover any kind of anomalies in the datagram such as length values that may not be accurate. Fields in a datagram can be “crafted” by a program instead of using normal system calls to create a number of very interesting, illegal datagrams. TCPdump hexadecimal output gives us all the data to look for signs of this kind of tampering. 4 - 11
Sample Hexadecimal Output 04:19:31.800000 1.2.3.4 > 192.168.5.5: icmp: echo reply (DF) 4500 0028 b5cb 4000 fe01 b229 0102 0304 IP Header ICMP message c0a8 0505 0000 bc9c bf3c 51ff 0018 f81b 000d d5f0 000d 63e8 0000 0000 0000 IP Behavior – SANS ©2001 12 We see the output of a datagram displayed in hexadecimal. The record is first displayed as you would see it in normal TCPdump output. In this slide, we are looking at the bowels of an ICMP echo reply packet. Each character in the hex output represents 4 bits, two consecutive 4 bit values with no intervening spaces represent a byte. The IP header in this datagram has 20 bytes. The IP header is displayed between the first and second arrows. This is followed by an ICMP message that is found between the second and third arrows. What do these hex values represent? Well, you’ll have to get a standard layout of what an IP header looks like and similarly, you’ll need to acquire a layout of an ICMP message format. One of the best sources for this and understanding TCP/IP in general, is TCP/IP Illustrated, Volume 1 by Richard Stevens. 4 - 12
Finding the Protocol in the IP Header BYTE 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 45 00 00 28 b5 cb 40 00 fe 01 b2 29 01 02 03 04 16 17 18 19 IP protocol field c0 a8 05 05 icmp = 01 tcp = 06 udp = 17 = 11 hexadecimal 161 x 1 + 160 x 1 = 17 IP Behavior – SANS ©2001 13 We get a feel for how this works by turning to the slide “Finding the protocol in the IP header.” Here we have displayed the IP header. One of the hardest things to master when you first begin to look at TCPdump output is exactly what protocol is being displayed. We saw where ICMP is pretty obvious about announcing itself in the output somewhere. We saw where TCP is a little bit more subtle with flags, sequence numbers, and acknowledgement numbers. Finally, UDP may or may not announce its presence. How can you determine the protocol of a record when it is not obvious to you looking at the standard TCPdump output? First, you have to have that record stored in a TCPdump raw packet file. This means that you have gathered TCPdump output and written it to a file using the TCPdump –w option rather than display it. If you read the file in which the record is stored and display it in hexadecimal, you should be able to determine the protocol. To do this we need to know a couple of pieces of information. First, the IP protocol field is 9 bytes into the IP header. Counting begins with 0 so if we look at the slide, the byte that has the gray background represents the IP protocol. The current value that we see is 01 or the value for icmp. Other values that you may see are 06 which is tcp, and 17 which is udp. Seventeen is the decimal value of the number, but the IP header is in hexadecimal. Seventeen in decimal is 11 in hexadecimal. If you remember the hexadecimal numbering system the least significant 1 (the rightmost character) is equal to base 16 to the zero power. Anything to the 0 power is 1 so the rightmost character is 1 times 1. The most significant character or leftmost character is also 1, which represents base 16 to the first power, or 16. 16 times 1 is 16. So, 16 + 1 is seventeen decimal. 4 - 13
TCPdump Review • TCPdump “sniffs” packets off the network • It dumps the packets in a standard output format depending on protocol • UDP packets may or may not have “udp” in the output • TCP packets are distinguishable because of flags • ICMP packets will have the ICMP message description • TCPdump can display records in hexadecimal format IP Behavior – SANS ©2001 14 Slide “TCPdump review” wraps up what we have learned in section 1 about TCPdump. TCPdump can sniff packets off of the network. It dumps the packets in a standard output format depending on the protocol – tcp, udp, or icmp. As you saw, UDP datagrams may be labeled as UDP – although this is not always guaranteed. TCP segments are distinguishable because of flags, and other fields such as the sequence and acknowledgement numbers, to name a few. ICMP messages will have the ICMP message description contained in them. 4 - 14
TCP Concepts TCP is a connection-oriented reliable protocol. IP Behavior – SANS ©2001 15 In section 2, we will examine some TCP concepts. Remember TCP is a connection-oriented reliable protocol. 4 - 15
Establishing a TCP Connection TCP requires a three-way handshake between the client and server before a connection can be established and data transferred Client Server Send SYN SYN Receive SYN Send K Receive SYN + AC SYN/ACK SYN/ACK Send ACK ACK Receive ACK Connection Established IP Behavior – SANS ©2001 16 Turning to the slide “Establishing a TCP connection, you will see that establishing a TCP connection is almost ceremonial in nature involving what is commonly known as the three-way handshake. This is required before any data can be passed between the two hosts. What is depicted is the client or source host initiating a connection to the server or destination host. Because this is tcp, a port or service must be identified to which to connect. Examples of destination ports might be 23 or telnet, 25 or sendmail, or port 80 also known as http or the web server port. Ports are simply another way of establishing discrete connections so that each computer can keep track of what connections they have to whom. For the TCP three-way handshake, first the client sends a SYN to signal a request for a TCP connection to the server. Second, if the server is up, offers the desired service, and can accept the incoming connection, it sends something back. It will send a connection request of its own to the client and acknowledges the clients initial connection request with an ack [short for acknowledgement bit], all in a single packet. At this point the second step of the TCP three-way handshake is completed. Finally, if the client receives the SYN and ACK and still wants to continue the connection, it sends a final lone ACK to the server. Once the server receives the ACK, the connection has been established. Data can now be exchanged between the two. Once the connection is established, you will often see the ACK bit set, as much as possible, ACKS are "piggy-backed" onto packets with data to minimize traffic as opposed to sending a packet with just an ACK. The acks serve to confirm to the client and server that the connection is still being used by both. 4 - 16
TCPdump Output of TCP Connection Establishment Client SYN 07:09:43.368615 download.net 39904 > ftp.com.21: S 733381829:733381829(0) win 8760 (DF) 07:09:43.370302 ftp.com.21 > download.net.39904: S 1192930639:1192930639(0) ack 733381830 win 1024 (DF) 07:09:43.370355 download.net.39904 > ftp.com.21: . ack 1 win 8760 (DF) Server SYN/ACK Client ACK IP Behavior – SANS ©2001 17 The TCPdump output on the slide “TCPdump output of TCP connection establishment” shows the connection establishment. There are three TCP segments shown. A TCP segment contains the information sent by TCP to IP. In the first segment, you see the client, download.net attempt a connection to the ftp server, port 21, of ftp.com. You see the SYN flag set followed by the Initial Sequence Number, 733381829, and the same ending sequence number, zero bytes in the parentheses. Following that you see the TCP options - in this case a window size of 8760 and a maximum segment size (mss) that it advertised to the server. The window size of 8760 says that the client has an 8,760 byte buffer for incoming data to this connection. The maximum segment size informs the destination host that the physical network on which this download.net resides should not receive more than 1460 bytes at a time. So, in this case even though the client, download.com is capable of accepting 8,760 bytes of data, the physical medium on which it resides, most likely Ethernet, cannot accept more than 1460 bytes for a TCP segment size. In the second segment, you see ftp.com send a SYN and an ACK to download.com informing it that it is available and a willing participant in this connection and to establish one of its own as well. ftp.com informs download.net of its initial sequence number, 1192930639. This is also the ending sequence number because no data is sent. This is normal for the SYN/ACK packets. The number following the ACK is the acknowledgement number, in this case 733381830. Note that this value is the initial sequence number advertised by download.com in the first record -- 733381829 incremented by a count of one because the segment with the initial SYN consumed one sequence number. ftp.com has just acknowledged that it received the segment with the SYN flag set from download.com. ftp.com advertises its options - a window size of 1024 and a maximum segment size of 1460. In the final line, download.net sends the final lone ACK to ftp.com and acknowledges receiving the one byte containing the SYN/ACK flags from ftp.com. Right after the destination part, following the colon, you’ll see a period. Remember this was the placeholder value when none of the PUSH, RESET, SYN, or FIN bits is set. 4 - 17
Server and Client Ports • Server or well-known ports historically are ports numbered 1-1023; these do not change on the server • Client ports, also known as ephemeral ports, historically are numbered greater than 1023; these ports generally change per TCP connection • Before the client initiates a connection request to the well-known server port (sends a SYN), it selects an unused ephemeral port on which to communicate • For most TCP protocols, the client and server communicate the entire session using the client’s ephemeral port and the server’s well-known port IP Behavior – SANS ©2001 18 Moving ahead to the slide “Server and client ports” we introduce server and client ports. In the past, more so than today, well-known server ports generally fell in the range of 1-1023. Historically, under Unix only, processes running with root privilege could open a port below 1024. These ports should remain constant on the host on which they are offered. In other words, if you find telnet at port 23 on a particular host one day, you should find it there the next day. You will find many of the older well-established services in this range of 1-1023 such as telnet, or sendmail -- port 25. Today, some of the newer services such as Lotus Notes, TCP port 1352, don’t tend to conform to this original convention. This is partially because there are more services than numbers in this range today. Client ports, often known as ephemeral ports, are selected only for that particular connection and are reused after the connection is freed. These are generally numbered greater than 1023. When a client initiates a connection to a server, an unused ephemeral port is selected. For most services, the client and server continue to exchange data on these two ports for the entirety of the session. This connection is known as a socket pair and it will be unique. That is, there will be only one connection on the Internet that has this combination of source IP and source port connected to this destination IP and destination port. There may be another user connection from another source IP to this same destination IP and destination port, but that user will have a different source IP and most likely different source port. There may even be someone from the same source IP connected to the same destination IP and port. But, this user will be given a different ephemeral port thus distinguishing it from the other connection to the same server and destination port. For instance, two users on the same host may be connecting to the same web server. While this is the same source IP, the same destination IP and port (80), the web server will be able to maintain who gets what by the ephemeral source ports involved. 4 - 18
TCPdump Output of Client/Server Port Communication 07:11:01.209928 sendit.net.39905 > mailserver.com.25: S 743211991:743211991(0) win 8760 (DF) 07:11:01.211211 mailserver.com.25 > sendit.net.39905: S 1202779586:1202779586(0) ack 743211992 win 8760 (DF) 07:11:01.211254 sendit.net.39905 > mailserver.com.25: . ack 1 win 8760 (DF) = ephemeral port 39905 = server port 25 IP Behavior – SANS ©2001 19 On the slide “TCPdump Output of Client/Server Port Communication” you’ll see TCPdump output of the three-way handshake again. This time, the emphasis is on the port selection and not so much the TCP flag settings. The client sendit.net has selected ephemeral port 39905 on which to communicate with mailserver.com on its well-known sendmail port 25. When mailserver.com responds to the SYN request, it communicates on port 25 to the sendit.net port 39905. Lastly, when the final ACK is sent, the ephemeral port 39905 and port 25 remain the same. If data is exchanged, it will be done on these two ports. Not until after the connection is closed and some time has passed, will client port 39905 be freed up for use by another TCP client on sendit.net. Port 25 of mailserver.com will remain bound to the sendmail service. 4 - 19
Connection Termination • Connection termination can be done “gracefully” when the client or server initiates a normal session close – This is done by having either the client or server send a FIN to the other. The FIN signals the desire to terminate the connection; the other side must close its connection too by sending a FIN. • Connection termination can be done more abruptly when the client or server aborts the session – This is done by either the client or server sending a RST to the other - signaling the desire to abort the connection immediately. IP Behavior – SANS ©2001 20 On the slide “Connection Termination” we discuss closing a connection. There are two ways of terminating a session, the graceful method or the abrupt method. When the graceful method is conducted, • One of the hosts, either the client or server, will signal with a FIN to the other that it wants to terminate the session. •The receiving host will signal back with an ACK or acknowledge the request. This only terminates half the connection. •Then, the other host will have to initiate a termination as well. It will send a FIN to the initial host. •The receiving host will then need to acknowledge this and send an ACK back. Both sides need to initiate a FIN and acknowledge the other’s FIN because TCP is full duplex; this means that there are two connections established: One from the client to the server, and one back from the server to the client. Both the client and server send data in an asynchronous manner so both sides of the connection have to be individually terminated. The second method of termination is an abrupt halting of the connection. This is done by one host sending the other a RESET. This signals the desire to abruptly terminate the connection. 4 - 20

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