Presented by Muhammad Abu Saqer End to End Argument in System Design By J. H. Saltzer, D. P. Reed, and D. D. Clark Presented by Muhammad Abu Saqer

Overview The paper presents a design principle Choosing the proper boundaries between system functions. The end-to-end argument is about where to put functionality in a layered network architecture.

Communication Network OSI Reference Model Application Application Protocol Presentation Presentation Protocol Session Session Protocol Transport Transport Protocol Network Network Layer Host-Router Protocol Host 1 Host 2 Interface Data Link Physical Data Link Layer Host-Router Protocol Physical Layer Host-Router Protocol Internal Subnet Protocols Communication Network

Example 1: File Transfer Consider the problem of file transfer During the process it could face many threats like Crash in host A or B system Hardware failure Lost network connectivity file Network Service Network

Example 1: File Transfer (cont’d) To avoid such threats we can Reinforce each step using duplicate copies, time-out and retry, carefully located redundancy for error detection. Reduce the probability of each of the individual threats to an acceptance small value.

Example 1: Alternate approach: End-to-End check and retry The application program follows the simple steps in transferring the file. As the final step, host B will recalculate the checksum for the transferred file. Then host B will send this value to be compared with the original value at host A. If this value doesn’t mach, the file will be resend again to host A.

Example 1: Alternate approach: End-to-End check and retry (cont’d) Thus, in order to achieve careful file transfer, the application program must supply E2E guarantee In this case, a checksum to detect failures and retry-commit plan. The Key point here is: Application program shouldn’t rely on the modeling transport mechanism for reliability, Instead It should implements it’s own E2E checksum to ensure reliability.

Example 2 : at MIT labs Application programmer assumed that the network has reliable transmission. One of the network gateway developed a transient error while it is copying data form the input buffer. Many files corrupted as a result of this error. Files owner were forced to do a manual comparison with the original files. Something the application programmers should have implemented.

Performance & Reliability Issues If we consider a network that is somewhat unreliable, dropping one packet of each hundred packet sent Using the above outline strategy the above network will do a bad job. Clearly , some effort at the lower levels to improve the network reliability will significantly improve the performance. The key idea here: The lower level need not to provide “perfect” reliability

Performance & Reliability Issues (cont’d) E2E check of the file transfer application must be implemented no matter how reliable the communication system become. The amount of effort to put into reliability within the data communication system is seen to be an engineering trade-off based on performance.

Performance & Reliability Issues (cont’d) Using performance to justify placing functions in low-level subsystem must be done carefully since performing the function in the low level may cost more because: Some application which use the same low level subsystem and doesn’t need this function will pay for it anyway. Low-level system may not have as much information as the higher level, so it cannot do the job efficiently.

Example 3: Secure Transmission of Data When data encryption is done at data transmission system: It must be trusted to securely manage the required encryption key. The data will be in clear format (not encrypted) and thus vulnerable as they pass through the node the target application. The authenticity of the message must still be checked by the application.

Example: Secure Transmission of Data (cont’d) If the application performs E2E encryption these source of vulnerability would be avoided: The data are never exposed outside the application. The application obtains it’s required authentication check. The application can handle key management to its satisfaction.

Question: Identifying the ends Using the E2E argument sometimes require subtlety of analysis of application requirement E2E argument is a property of the specific application: Two people in real time conversation. Speech message system.

My evaluation to the paper It is foundation paper in Identified and named E2E arguments. Put system design principle that organize and guide the placement of the function within system. It becomes part of the vocabulary of network and operating system. To some extent it is still valid.

The Key ideas of the paper Philosophy behind distributed system design: Move complex operations to the edges of the network Why? Not all applicatons may require complex options, e.g., reliability (audio, video) security options often repeated at edge anyways as safety check.

E2E argument implication Implications of e2e argument to the distributed system design and the Internet is : most complex options should be performed toward the top of the protocol stack.

E2E Argument Pros Keep the network simple Application had to check to see if the network did it’s job and repair the problem if the network didn’t do it’s job. reduces redundant checks since app often provides checks anyways Example of Systems Implementing E2E argument: Banking, Reservation, Encryption, …

E2E Argument Cons When the network doesn't trust the application Corporation or military using encryption in the low-level because the end user or application may not do it. Some thing just can’t be done at the end Routing algorithms. May not apply to the wireless network more responsibility lies on the application developer longer development cycle, frequent bugs

Summary: End-To-End Argument The function in question can completely and correctly be implemented only with knowledge and help of the application standing at the endpoints of communication system. We call this line of reassigning against low-level function implementation E2E argument. Building complex function into a network optimizes the network for one set of uses while substantially increasing the cost of a set of potentially valuable uses that may be unknown or unpredictable at design time.

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