When is isn used

For example: I used to have long hair but now I have short hair. He used to smoke but now he doesn't smoke. They used to live in India but now they live in Germany. Watch out! With the negative and the question it's 'use' and not 'used': Did you use to be a teacher? Did he use to study French? She didn't use to like chocolate, but she does now. I didn't use to want to have a nice house. With this 'used to' there is no verb 'be'. So, we can say: When I was a student, we would often have a drink after class on a Friday.

When I lived in Italy, we would go to a little restaurant near our house. I would live in Scotland. By clicking sign up, you agree to receive emails from Techopedia and agree to our Terms of Use and Privacy Policy. Initial sequence numbers ISN refers to the unique bit sequence number assigned to each new connection on a Transmission Control Protocol TCP -based data communication. It helps with the allocation of a sequence number that does not conflict with other data bytes transmitted over a TCP connection.

An ISN is unique to each connection and separated by each device. An ISN is designed to randomly select a sequence number for the first byte of data transmitted in a new TCP connection. The ISN can be any number from 0 to 4,,, Each byte can select any ISN unless it is in use not by a current connection. The TCP protocol assigns an ISN to each new byte, beginning with 0 and incrementally adding a number every four seconds until the limit is exhausted.

In continuous communication, it takes up to four hours to consume all available ISN options. Discussion of some of these attacks dates back to at least , when Morris [ Morris ] described a form of attack based on guessing what sequence numbers TCP [ RFC ] will use for new connections between two known end-points. With the aforementioned algorithm, such attacks would remain possible if and only if the attacker already has the ability to perform "man-in- the-middle" attacks.

This document revises and formally obsoletes RFC , and takes the ISN generation algorithm originally proposed in that document to Standards Track. Section 2 provides a brief discussion of the requirements for a good ISN generation algorithm. Section 3 specifies a good ISN selection algorithm. Appendix A provides a discussion of the trust- relationship exploitation attacks that originally motivated the publication of RFC [ RFC ].

Generation of Initial Sequence Numbers RFC [ RFC ] suggests that the choice of the ISN of a connection is not arbitrary, but aims to reduce the chances of a stale segment from being accepted by a new incarnation of a previous connection. Based on the assumption that ISNs are monotonically increasing across connections, many stacks e.

This avoids an interoperability problem that may arise when a node establishes connections to a specific TCP end-point at a high rate [ Silbersack ]. One of the possible attacks that takes advantage of weak sequence numbers was first described in [ Morris ], and its exploitation was widely publicized about 10 years later [ Shimomura ].

Simple random selection of the TCP ISNs would mitigate those attacks that require an attacker to guess valid sequence numbers. However, it would also break the 4. We can prevent sequence number guessing attacks by giving each connection -- that is, each four-tuple of localip, localport, remoteip, remoteport -- a separate sequence number space.

An obvious way to prevent sequence number guessing attacks while not breaking the 4. That would work but would consume system memory to store the additional state. The PRF could be implemented as a cryptographic hash of the concatenation of the connection-id and some secret data; MD5 [ RFC ] would be a good choice for the hash function.

The result of F is no more secure than the secret key. If an attacker is aware of which cryptographic hash function is being used by the victim which we should expect , and the attacker can obtain enough material i. To protect against this, the secret key should be of a reasonable length. Key lengths of bits should be adequate.

The secret key can either be a true random number [ RFC ] or some per-host secret. A possible mechanism for protecting the secret key would be to change it on occasion.