Internetworking With TCP IP. Client Server Prog...

Internetworking with TCP/IP, Volume III describes the fundamental concepts of client-server computing used to build all distributed computing systems, and presents an in-depth guide to the Posix sockets standard utilized by Linux and other operating systems. Dr. Douglas E. Comer compares leading server designs, and describes the key tools and techniques used to build clients and servers, including Remote Procedure Call (RPC). The book contains examples of running programs that illustrate each approach. Comer introduces the client-server model and its software design implications; the role of concurrent processing and threads; the Socket API, and differences that impact Linux programmers. Understand the key algorithms and issues associated with client and server software design; then review three leading approaches: iterative, connectionless servers (UPD); and both iterative and concurrent connection-oriented servers (TCP). The book contains extensive coverage of threading, including a new chapter on using threads for concurrency; as well as coverage of single-threaded and multi-threaded concurrent servers. Comer introduces multi-protocol and multi-service services; reviews client concurrency; tunneling at the transport and application levels; and external data representation (XDR). He reviews RPC, distributed program generation, NFS concepts and protocol; Telnet; streaming media transport; and finally, techniques for avoiding deadlock and starvation in client-server systems. For everyone who wants to master TCP/IP and understand how the Internet works.

Internetworking with TCP IP. Client Server Prog...

TCP is connection-oriented, and a connection between client and server is established before data can be sent. The server must be listening (passive open) for connection requests from clients before a connection is established. Three-way handshake (active open), retransmission, and error detection adds to reliability but lengthens latency. Applications that do not require reliable data stream service may use the User Datagram Protocol (UDP) instead, which provides a connectionless datagram service that prioritizes time over reliability. TCP employs network congestion avoidance. However, there are vulnerabilities in TCP, including denial of service, connection hijacking, TCP veto, and reset attack.

In May 1974, Vint Cerf and Bob Kahn described an internetworking protocol for sharing resources using packet switching among network nodes.[1] The authors had been working with Gérard Le Lann to incorporate concepts from the French CYCLADES project into the new network.[2] The specification of the resulting protocol, .mw-parser-output cite.citationfont-style:inherit;word-wrap:break-word.mw-parser-output .citation qquotes:"\"""\"""'""'".mw-parser-output .citation:targetbackground-color:rgba(0,127,255,0.133).mw-parser-output .id-lock-free a,.mw-parser-output .citation .cs1-lock-free abackground:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat.mw-parser-output .id-lock-limited a,.mw-parser-output .id-lock-registration a,.mw-parser-output .citation .cs1-lock-limited a,.mw-parser-output .citation .cs1-lock-registration abackground:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat.mw-parser-output .id-lock-subscription a,.mw-parser-output .citation .cs1-lock-subscription abackground:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat.mw-parser-output .cs1-ws-icon abackground:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat.mw-parser-output .cs1-codecolor:inherit;background:inherit;border:none;padding:inherit.mw-parser-output .cs1-hidden-errordisplay:none;color:#d33.mw-parser-output .cs1-visible-errorcolor:#d33.mw-parser-output .cs1-maintdisplay:none;color:#3a3;margin-left:0.3em.mw-parser-output .cs1-formatfont-size:95%.mw-parser-output .cs1-kern-leftpadding-left:0.2em.mw-parser-output .cs1-kern-rightpadding-right:0.2em.mw-parser-output .citation .mw-selflinkfont-weight:inheritRFC 675 (Specification of Internet Transmission Control Program), was written by Vint Cerf, Yogen Dalal, and Carl Sunshine, and published in December 1974. It contains the first attested use of the term internet, as a shorthand for internetwork.[3]

While IP handles actual delivery of the data, TCP keeps track of segments - the individual units of data transmission that a message is divided into for efficient routing through the network. For example, when an HTML file is sent from a web server, the TCP software layer of that server divides the file into segments and forwards them individually to the internet layer in the network stack. The internet layer software encapsulates each TCP segment into an IP packet by adding a header that includes (among other data) the destination IP address. When the client program on the destination computer receives them, the TCP software in the transport layer re-assembles the segments and ensures they are correctly ordered and error-free as it streams the file contents to the receiving application.

Before a client attempts to connect with a server, the server must first bind to and listen at a port to open it up for connections: this is called a passive open. Once the passive open is established, a client may establish a connection by initiating an active open using the three-way (or 3-step) handshake:

Steps 1 and 2 establish and acknowledge the sequence number for one direction. Steps 2 and 3 establish and acknowledge the sequence number for the other direction. Following the completion of these steps, both the client and server have received acknowledgments and a full-duplex communication is established.

The connection termination phase uses a four-way handshake, with each side of the connection terminating independently. When an endpoint wishes to stop its half of the connection, it transmits a FIN packet, which the other end acknowledges with an ACK. Therefore, a typical tear-down requires a pair of FIN and ACK segments from each TCP endpoint. After the side that sent the first FIN has responded with the final ACK, it waits for a timeout before finally closing the connection, during which time the local port is unavailable for new connections; this state lets the TCP client resend the final acknowledgement to the server in case the ACK is lost in transit. The time duration is implementation-dependent, but some common values are 30 seconds, 1 minute, and 2 minutes. After the timeout, the client enters the CLOSED state and the local port becomes available for new connections.[15]

The number of sessions in the server side is limited only by memory and can grow as new connections arrive, but the client must allocate an ephemeral port before sending the first SYN to the server. This port remains allocated during the whole conversation and effectively limits the number of outgoing connections from each of the client's IP addresses. If an application fails to properly close unrequired connections, a client can run out of resources and become unable to establish new TCP connections, even from other applications.

By using a spoofed IP address and repeatedly sending purposely assembled SYN packets, followed by many ACK packets, attackers can cause the server to consume large amounts of resources keeping track of the bogus connections. This is known as a SYN flood attack. Proposed solutions to this problem include SYN cookies and cryptographic puzzles, though SYN cookies come with their own set of vulnerabilities.[35] Sockstress is a similar attack, that might be mitigated with system resource management.[36] An advanced DoS attack involving the exploitation of the TCP Persist Timer was analyzed in Phrack #66.[37] PUSH and ACK floods are other variants.[38]

TCP and UDP use port numbers to identify sending and receiving application end-points on a host, often called Internet sockets. Each side of a TCP connection has an associated 16-bit unsigned port number (0-65535) reserved by the sending or receiving application. Arriving TCP packets are identified as belonging to a specific TCP connection by its sockets, that is, the combination of source host address, source port, destination host address, and destination port. This means that a server computer can provide several clients with several services simultaneously, as long as a client takes care of initiating any simultaneous connections to one destination port from different source ports.

TCP Cookie Transactions (TCPCT) is an extension proposed in December 2009[52] to secure servers against denial-of-service attacks. Unlike SYN cookies, TCPCT does not conflict with other TCP extensions such as window scaling. TCPCT was designed due to necessities of DNSSEC, where servers have to handle large numbers of short-lived TCP connections. In 2016, TCPCT was deprecated in favor of TCP Fast Open. Status of the original RFC was changed to "historic".[53]

TCP was originally designed for wired networks. Packet loss is considered to be the result of network congestion and the congestion window size is reduced dramatically as a precaution. However, wireless links are known to experience sporadic and usually temporary losses due to fading, shadowing, hand off, interference, and other radio effects, that are not strictly congestion. After the (erroneous) back-off of the congestion window size, due to wireless packet loss, there may be a congestion avoidance phase with a conservative decrease in window size. This causes the radio link to be underutilized. Extensive research on combating these harmful effects has been conducted. Suggested solutions can be categorized as end-to-end solutions, which require modifications at the client or server,[76] link layer solutions, such as Radio Link Protocol (RLP) in cellular networks, or proxy-based solutions which require some changes in the network without modifying end nodes.[76][77] 041b061a72