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Out-of-Band Data

Out-of-Band Data. Chap 24. TCP OOB concepts Notification SIGURG signal Exception ready Position of OOB byte Value of OOB byte. TCP Urgent Mode and Socket OOB Data. write(fd,…); write(fd, …) send(fd, “a”, … , MSG_OOB); write(fd, …);. handler(){ recv(fd,…, MSG_OOB); }.

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Out-of-Band Data

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  1. Out-of-Band Data Chap 24

  2. TCP OOB concepts Notification SIGURG signal Exception ready Position of OOB byte Value of OOB byte TCP Urgent Mode and Socket OOB Data write(fd,…); write(fd, …) send(fd, “a”, … , MSG_OOB); write(fd, …); handler(){ recv(fd,…, MSG_OOB); } select(…, xset, …); … recv(fd,…, MSG_OOB); SIGURG Socket API OOB mark OOB buffer (1Byte) a a send buffer receive buffer a TCP (receiver) TCP (sender)

  3. Sending TCP OOB data • TCP Sender • OOB data는 socket send buffer의 맨 뒤에 append되고 urgent offset이 그 다음을 가리킨다. • Urgent offset 바로 앞 byte가 OOB byte가 된다. • Normal data와 순서대로 전송된다. (normal data에 우선적으로 전송되지 않는다) • not true out-of-band data (expedited data) • Enter Urgent mode: Flow control에 의해 segment 전송이 stop되더라도 (i.e. receiver의 socket receive buffer가 full일 때), urgent notification (URG segment with no data)은 항상 전송된다. • 그러나 아직 OOB data는 전송되지 못했을 수 있음. • OOB data sender • send(fd, “a”, 1, MSG_OOB); • send(fd, “abc”, 3, MSG_OOB); • Which is the OOB byte?

  4. Receiving TCP OOB Data (1) • TCP Receiver • URG segment가 도착하면, urgent notification을 receiver에게 알려준다. • SIGURG signal is delivered • Exception set becomes ready (in case of select) • 그러나, OOB byte가 도착하지 않았을 수도있다. Why? • Read()는 OOB byte 바로 전 normal data까지 읽는다. • Receiver도 sockatmark()를 이용하여 OOB byte를 읽을 차례인지 알수 있다. •  OOB는 end-of-record mark로 쓸 수 있다. • Then, receive OOB data recv(fd, &buff, sizeof(buff), MSG_OOB); • OOB byte를 가져가기 전에 새로운 OOB byte가 도착하면 overwrite되고 OOB mark도 갱신된다.

  5. Receiving TCP OOB Data (2) • OOB data receiver • Notification of URG segment by means of • SIGURG signal sent to owner of the socket • select with exception set • notify되었다고 해서 반드시 OOB data가 도착한 것은 아님 . 즉 socket receive buffer가 full이어서 OOB byte가 도착 못할 수 있음. • Then, receive OOB data recv(fd, &buff, sizeof(buff), MSG_OOB); • OOB byte를 정확히 수신하고자 하면, OOB data가 도착했는지(즉, out-of-band mark에 있는지) 확인하고, 도착하지 않았으면, normal data를 읽어들여 flow control이 stop된 것을 해지하고 다시 확인 필요 • 단지 exception이 발생했는지 여부를 알고자 하면 OOB notification 받은 것으로 충분 (예, rlogin)

  6. URG Notification: URG segment received By SIGURG: oob/tcprecv01 By exception ready: oob/tcprecv03

  7. Set inline mode: OOB byte도 normal data처럼 read 가능하게 됨 check out-of-band mark and receive OOB data Receiving OOB data with inline oob/tcprecv04

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