http://msdn.microsoft.com/en-us/library/windows/desktop/ms738547(v=vs.85).aspx

Вот этой статье.
авторOne technique that can be used to minimize the chance of problems occurring during connection teardown is to avoid relying on an implicit shutdown being initiated by closesocket. Instead, use one of the two explicit shutdown functions, shutdown or WSASendDisconnect. This in turn causes an FD_CLOSE indication to be received by the peer application indicating that all pending data has been received.

shutdown инициирует закрытие соединения.
close закрывает дескриптор сокета также инициируя последовательность закрытия соединения, если оно еще не было закрыто.
Без положительного значения в SO_LINGER обе функции завершаются сразу же, не дожидаясь получения подтверждений отправленных FIN сегментов.

Так что после завершения последовательности вызовов


у приложения нет никаких сведений о том, а были ли доставлены данные, которые мы отправили.

В конце этой статьи же написано
http://msdn.microsoft.com/en-us/library/windows/desktop/ms738547(v=vs.85).aspx One technique that can be used to minimize the chance of problems occurring during connection teardown is to avoid relying on an implicit shutdown being initiated by closesocket. Instead, use one of the two explicit shutdown functions, shutdown or WSASendDisconnect. This in turn causes an FD_CLOSE indication to be received by the peer application indicating that all pending data has been received. To illustrate this, the following table shows the functions that would be invoked by the client and server components of an application, where the client is responsible for initiating a graceful shutdown.

Client

(1) Invokes shutdown(s, SD_SEND) to signal end of session and that client has no more data to send.

(local timing significance only) Gets FD_READ and calls recv to get any response data sent by server

Server

(2) Receives FD_CLOSE, indicating graceful shutdown in progress and that all data has been received.

(3) Sends any remaining response data.

(local timing significance only) Invokes closesocket .

Client

(5) Receives FD_CLOSE indication.

(6) Invokes closesocket.

А теперь тоже самое, но для unix
http://blog.netherlabs.nl/articles/2009/01/18/the-ultimate-so_linger-page-or-why-is-my-tcp-not-reliable

Пропустил (4) для сервера
(4) Invokes shutdown(s, SD_SEND) to indicate server has no more data to send.