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Satellite networking principles and protocols - p8

Nó gây ra một sự chậm trễ ước tính trung bình 10 ms và sự chậm trễ trường hợp tồi tệ nhất của 20 ms. Mất tế bào xảy ra khi bộ đệm tràn. Tác động của biến thể chậm trễ chậm trễ, và mất tế bào trong hệ thống có thể được kiểm soát ở mức tối thiểu bằng cách kiểm soát số lượng các ứng dụng, số lượng tải giao thông và phân bổ băng thông đủ cho mỗi ứng dụng. | 232 Satellite Networking Principles and Protocols Numerous functions take a variable-length input and give back a fixed-length output but single-direction hashing functions have additional properties that make them useful given M it is easy to calculate h given h it is difficult to find M given M it is difficult to find another message M such as HQM H M . Difficulty depends on the level of security specific to each situation but the majority of existing applications define difficulty as needing 264 or more operations to solve . Current functions of this type include the MD4 MD5 and secure hash algorithm SHA . From a network point of view those algorithms are frequently used for authentication purposes. 6.5.3 Symmetrical codes with secret keys An algorithm of coding with a secret key transforms a message M of arbitrary length into a coded message Ek M C of same length using a key k and the reverse transformation Dk M uses the same key Figure 6.17 . Those algorithms verify the following characteristics Dk Ek MỴ M given M and k it is easy to calculate C given C and k it is easy to calculate M given M and C it is difficult to find k Of course in this case difficulty is directly linked to the length of k 256 for the data encryption standard DES algorithm and 2128 for the international data encryption algorithm IDEA . Those algorithms are used in networks for encapsulating security payload purposes i.e. coding data commonly used in the area of electronic commerce. 6.5.4 Asymmetrical codes with public private keys Contrary to the preceding case those algorithms use two different keys Figure 6.18 one key e to encrypt called the public key and one key d to decrypt called the private key . Figure 6.17 Secret key system Decrypt Message Secret Key k M Internet Protocol IP over Satellite Networks 233 a Public key system for Privacy b Public key system for Authentication Figure 6.18 Public key system for privacy and authentication Let s define C Ee M and M Dd C . We have the .