Review: Name/Address conversion: What does the domain name system do?

1. Review: Name/Address conversion: • What does the domain name system do? • Which system calls use the domain name system? • How does it work? • How does getservbyname get the service information?

2. Today’s topic: Out-of-band data. • What is out of band data? • A communication channel can be used to transfer both regular data or control data. • Control data (packets, signal) control the behavior of a communication channel. • E.g VC establishment packets in ATM are control packets. • Control data usually need attention as soon as possible (have higher priority than regular data). • In-band/out of band control data • Appear as regular data or appear differently • Use the same communication channel / use different communication channels – true out of band data need a separate communication channel.

3. Since it is typical to have control information passing around in a network, a communication channel usually needs to support out of band data. • Handling out of band data is very system dependent. • Focus on TCP out of band data. • TCP does not have true out of band data. TCP emulates out of band data using the urgent mode.

4. Sending/Receiving out of band data in TCP: • int send(int s, const void *msg, int len, unsigned int flags); • int recv(int s, void *buf, int len, unsigned int flags); • Flags = MSG_OOB • What happens when issuing send (sockfd, “a”, 1, MSG_OOB)? • The sender side: • An OOB byte “a” is appended to the end of the buffer • The urgent pointer is set to the next available location • The next segment will have its URG flag set in the TCP header (but may not have the actual OOB byte). • The urgent notification is always sent even if the OOB byte is not sent.

5. What happens when issuing send (sockfd, “a”, 1, MSG_OOB)? • The receiver side: • When receiving a segment with URG flag set, TCP checks whether it is a new out of band data (first time flag). Only the first segment is used to notify that new out of band data have arrived. • SIGURG is sent to the owner of the socket and if the process is blocked in a select waiting for an exception condition of the socket, the select returns. • At the time of notification, the data may or may not arrive. • When the actual OOB byte arrives, it can be pulled out of band or left inline. (SO_OOBINLINE). • TCP maintains one byte out of band buffer. • The byte can be retrieved by recv, recvfrom and recvmsg with MSG_OOB flag.

6. What happens when issuing send (sockfd, “abc”, 3, MSG_OOB)? • What is the OOB byte? • Following is a set of programs showing the behavior of TCP out of band data. • See example1.c for sending OOB data • See example2.c for notifying the arrival of out of band data through SIGURG. • See example3.c for notifying the arrival of OOB byte through select. Notice the different behavior for linux of solaris for example3.c. • What happens to the byte stream when OOB data are used? See example4.c and example5.c • What happens when two OOB bytes are sent?

7. See example5-1.c for the use of OOBINLINE. • See example6.c and example7.c, what will be the outputs for example7.c?

8. TCP out of band data summary: • When is the notification of out of band data sent? • When are the actual data sent? • TCP has only one urgent pointer per connection: what is the implication? • TCP has only one byte out of band data buffer, what is the implication? • An application of TCP OOB: • Client-server heartbeat functions: figure out whether the client and the server are still connected. Example8.c and example9.c

9. Client Sig_alrm() { if (++cnt > 5) exit; send(MSG_OOB); alarm(1); } Sig_urg() { recv(MSG_OOB); cnt = 0; } server Sig_urg() { recv(MSG_OOB); send(MSG_OOB); cnt = 0 } Sig_alrm() { if (++cnt > 5) exit; alarm(1) }