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The Sequence number field is 32 bits in length and provides the mechanism for assuring the sequentiality of the data stream The Acknowledgment number field, which is also 32 bits in length, is used to verify the receipt of data The Acknowledgment number field informs the recipient that the datagram the sender transmitted arrived at its destination Because it would be inefficient to acknowledge each datagram, a variable window is supported by the protocol In other words, returning an acknowledgment field value of seven would indicate the receipt of all data up to the seventh datagram To ensure that lost datagrams or lost acknowledgments do not place this transport protocol in an infinite waiting period, the originator will retransmit data if the originator does not receive a response within a predefined period of time The previously described use of the acknowledgment field is referred to as Positive Acknowledgment or Retransmission (PAR) PAR requires that each unit of data must be explicitly acknowledged If a unit of data is not acknowledged by the time the originator's timeout period is reached, the previous transmission is retransmitted When the acknowledgment field is in use, a flag bit referred to as the ACK flag in the code field will be set We will shortly discuss the six bits in that field The Sequence number field is of particular interest when discussing network security Many operating systems implement their TCP/IP stack in a manner that causes the sequence numbers to be predictable In other words, if a client connects to a server three times in rapid succession and watches the sequence numbers sent by the server, the numbers will follow a pattern which can be used to guess what the next sequence number will be This function is important, because an attacker can then carry out the TCP three-way handshake without actually seeing the responses from the server This type of an attack is called session hijacking and is described later in this chapter The attacker would initially use his or her real IP address to discern how the sequence numbers are incremented Then, the attacker would spoof his or her source IP address so that packets sent from his or her machine would appear to originate from a machine the server trusted The attacking machine would never receive the replies, because the return packets would be routed to the real client If the attacker can guess the sequence numbers, however, the attacker does not need to see the replies to establish a connection Once the three-way handshake is complete, the attacker can fairly easily issue application commands without seeing the response Most simple applications, such as Telnet, have predictable responses, so it is not too difficult to carry on a conversation without seeing the responses from the server Note that to make this attack work, the attacker would have to "anesthetize" the real client by performing a DoS attack Otherwise, the real client would respond to the packets sent by the server and would interfere with the attack This attack may seem difficult to complete, but there are programs on the Internet that automate this process so that anyone can accomplish this attack The only defense is to remove non-critical services on vulnerable servers or to implement some means of randomizing sequence numbers.

12:

router2#show line 1 Tty TypTxtRxA Modem Roty AccO Acci Uses Noise Overruns A 1 TTY 9600/9600 - - - - - 0 0 0/0 Line 1, Location: "", Type: "" Length: 24 lines, Width: 80 columns Baud rate (TX/RX) is 9600/9600, no parity, 2 stopbits, 8 databits

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the dual-homed host firewall This provides an additional level of security against outside intrusion A screened subnet firewall, meanwhile, puts a screening router on either side of the firewall host This provides protection on both sides of the host Three firewall types are in use: packet-level firewall, application-level firewall, and circuit-level firewall The packet-level firewall controls data at the network and transport layers The type that works at the application level acts as a proxy and controls the top three layers of the OSI model A circuit-level firewall works like an application-level firewall but operates at the transport layer Additional security features have been added to firewalls to provide better service, such as support for VPNs and caching and better management tools Proxy servers can also function as firewalls and can be used to enhance Internet access as well as function as firewalls by filtering ports Data security has become increasingly important as networks are becoming interconnected Data encryption provides a way to ensure that the data is kept secure Encryption is the process of taking plain text data and converting it into a format that is unreadable A key is used to encrypt the data and then to return the data to a readable format Two types of encryption are in use: symmetric and asymmetric (public-key/private-key) Two common encryption methods include stream cipher and block cipher Two standards for encryption have been accepted: the Digital Encryption Standard (DES) and RSA Both use different methodologies and are in wide use today Digital signatures can be used to verify that a message arrived without being tampered with Digital signatures do not encrypt a message Instead, they attach a signature that can be verified against a certificate authority Windows 2000/XP/2003/2008 use certificates to encrypt files on an NTFS volume You have learned a number of different disaster recovery concepts, including backing up data and storing a copy of the data offsite You also learned that there are two types of spares for equipment such as hard drives or other network components hot spares and cold spares A hot spare is connected to the system and has power already supplied to the device so that if there is a failure in the original device, the hot spare can take over right away The cold spare is typically close by but not connected to the system at the time of the failure Because the spare is not readily available without any kind of changeover time, it will take longer to recover from the failure You have also learned that companies serious about disaster recovery are likely to have an alternative location set up in case of a disaster affecting the original location There are hot, cold, and warm disaster recovery sites A hot site is a fully furnished site that has servers and a copy of the corporate.