1. UDP is a simple, unreliable datagram protocol, while TCP is a sophisticated, reliable byte-stream protocol. SCTP is similar to TCP as a reliable transport protocol, but it also provides
message boundaries, transport-level
support for multihoming, and a way to minimize head-of-line blocking.
2. tcpdump, communicates directly with the datalink using either the BSD packet filter (BPF) or the datalink provider interface (DLPI). (tcpdump 绕过/bypass 了IP层，直接访问DLPI)
Linux provides access to the datalink using a special type of socket called SOCK_PACKET.
traceroute program uses two sockets: one for IP and another for ICMP.
3. Stream Control Transmission Protocol. SCTP is a connection-oriented protocol that provides a reliable full-duplex association. The word "association" is used when referring to a connection in SCTP because SCTP is multihomed, involving a set of IP addresses and a single port for each side of an association. SCTP provides a message service, which maintains record boundaries. As with TCP and UDP, SCTP can use either IPv4 or IPv6, but it can also use both IPv4 and IPv6 simultaneously on the same association. (SCTP支持多宿主机，也就是说在同一个“关联”association中，在服务器端允许有多台服务器，在客户端也允许有多台客户机；这样“关联”association的每一端都可以是多个IP地址和一个相同的端口号。)
4. There is no guarantee that a UDP datagram will ever reach its final destination, that order will be preserved across the network, or that datagrams arrive only once. (UDP不保证数据报到达目的地；也不保证各个数据报达到的顺序；也不保证每个数据报只到达一次)
5. First, TCP provides connections between clients and servers. A TCP client establishes a connection with a given server, exchanges data with that server across the connection, and then terminates the connection.
TCP also provides reliability. When TCP sends data to the other end, it requires an acknowledgment in return. If an acknowledgment is not received, TCP automatically retransmits the data and waits a longer amount of time. After some number of retransmissions, TCP will give up, with the total amount of time spent trying to send data typically between 4 and 10 minutes (depending on the implementation).
TCP cannot be described as a 100% reliable protocol; it provides reliable delivery of data or reliable notification of failure.
TCP contains algorithms to estimate the round-trip time (RTT) between a client and server dynamically so that it knows how long to wait for an acknowledgment. (RTT：往返时间)
TCP also sequences the data by associating a sequence number with every byte that it sends. For example, assume an application writes 2048 bytes to a TCP socket, causing TCP to send two segments, the first containing the data with sequence numbers 1～1024 and the second containing the data with sequence numbers 1025～2048.
If the segments arrive out of order, the receiving TCP will reorder the two segments based on their sequence numbers before passing the data to the receiving application. If TCP receives duplicate data from its peer, it can detect that the data has been duplicated, and discard the duplicate data.
TCP provides flow control. TCP always tells its peer exactly how many bytes of data it is willing to accept from the peer at any one time. This is called the advertised window. At any time, the window is the amount of room currently available in the receive buffer, guaranteeing that the sender cannot overflow the receive buffer.
Finally, a TCP connection is full-duplex. This means that an application can send and receive data in both directions on a given connection at any time. This means that TCP must keep track of state information such as sequence numbers and window sizes for each direction of data flow: sending and receiving.
6. SCTP provides services similar to those offered by UDP and TCP. SCTP also provides applications with reliability, sequencing, flow control, and full-duplex data transfer, like TCP. An association refers to a communication between two systems, which may involve more than two addresses due to multihoming.
Unlike TCP, SCTP is message-oriented. It provides sequenced delivery of individual records. Like UDP, the length of a record written by the sender is passed to the receiving application.
SCTP can provide multiple streams between connection endpoints, each with its own reliable sequenced delivery of messages. A lost message in one of these streams does not block delivery of messages in any of the other streams.
SCTP also provides a multihoming feature, which allows a single SCTP endpoint to support multiple IP addresses. This feature can provide increased robustness against network failure.
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