More on Socket API

More on Socket I/O Functions  Scatter read and gather write

More Advanced Socket I/O Functions

Ancillary data - cmsghdr Structure

Socket I/O Summary

Socket and Standard I/O  Buffering in Standard I/O library  fully buffered: all stream except for terminal devices  line buffered : terminal devices  unbuffered: stderr  Caution  Socket 에 standard I/O functions(fgets, fputs) 를 쓰면 fully buffered 됨

Name and Address Conversions

Domain Name System  A lookup mechanism for translating objects into other objects  A globally distributed, loosely coherent, scalable, reliable, dynamic database  Comprised of three components  Name space  Servers making that name space available  Resolvers (clients) which query the servers about the name space

Domain Name Space  DNS's distributed database is indexed by domain names.  Each domain name is essentially just a path in a large inverted tree, called the domain name space.

Domain, Delegation, Zone

Name Server Architecture Master server Zone transfer Zone data file From disk Authoritative Data (primary master and slave zones) Agent (looks up queries on behalf of resolvers) Cache Data (responses from other name servers) Name Server Process

Authoritative Data Resolver Query Response Authoritative Data (primary master and slave zones) Agent (looks up queries on behalf of resolvers) Cache Data (responses from other name servers) Name Server Process

Using Other Name Servers Arbitrary name server Response Resolver Query Authoritative Data (primary master and slave zones) Agent (looks up queries on behalf of resolvers) Cache Data (responses from other name servers) Name Server Process Response

Cached Data Query Response Authoritative Data (primary master and slave zones) Agent (looks up queries on behalf of resolvers) Cache Data (responses from other name servers) Name Server Process Resolver

Name Resolution  A DNS query has three parameters:  A domain name (e.g., ice.hufs.ac.kr),  Remember, every node has a domain name!  A class (e.g., IN), and  A type (e.g., A)  DNS message format Additional Authority Answer Question Header : the question for the name server : RRs answering the question : RRs pointing toward an authority : RRs holding additional information

Mapping Addresses to Names

Resource Records  주요 RRs  SOA Record (Start Of Authority): 해당 도메인에 대해 네임서버가 인증 (authoritative) 된 자료를 갖고 있음을 의미  NS(Name Server) Record: 해당 도메인에 대한 delegation 하는 네임서버를 지시  A Record: 도메인에 IPv4 주소를 mapping  AAAA Record: 도메인에 IPv6 주소를 mapping  CNAME Record: 도메인에 대한 또 다른 이름 설정  MX(Mail eXchanger) Record: 해당 호스트의 메일 라우팅 경로를 조정  PTR(Pointer) Record: IP 주소를 domain name 으로 reverse mapping 해 주며, Reverse Zone 파일에서 사용 NAME (Dynamic) TYPE(2 Byte) CLASS(2 Byte) TTL(4 Byte) RDLENGTH(2 Byte) RDATA(Dynamic)

Name and Address Conversion Functions  Domain name  IPv4 address  IPv4 address  domain name  gethostbyname/gethostbyaddr are not reentrant !! static struct hostent host; /* result stored here */ struct hostent * gethostbyname(const char *hostname) { /* call DNS functions for A or AAAA query */ /* fill in host structure */ return(&host); }

Service Name Conversion Functions  Service name  port ( See /etc/services )  Port  service name

Example: name/daytimetcpcli1.c Network-related information

New Name/Address Conversion Function (1)  hostname : hostname or address string  service : service name or decimal port number string  result : addrinfo data structure is dynamically allocated  Re-entrant, thread-safe, and protocol independent functions (support IPv4, IPv6)

New Name/Address Conversion Function (2)  hints : NULL or pointer to addrinfo data structure  The following member can be set by caller  ai_flag /* AI_PASSIVE for server, AI_CANNONNAME */  ai_family/* AF_xxx */  ai_socktype/* SOCK_xxx */  ai_protocol/* 0 or IPPROTO_xxx for IPv4 and IPv6 */  Example

getaddrinfo Actions and Results

UNP Library Functions #include “unp.h” struct addrinfo *host_serv(const char *hostname, const char *service, int family, int socktype); Returns: pointer to addrinfo structure if OK, NULL on error int tcp_connect(const char *hostname, const char *service); int tcp_listen(const char *hostname, const char *service, socklen_t *lenptr); Both returns: connected socket descriptor if OK, no return on error int udp_client(const char *hostname, const char *service, void **saptr, socklen_t *lenp); Returns: unconnected socket descriptor if OK, no return on error saptr : address of a pointer to a socket address structure that stores destination IP addr/port # for future calls to sendto int udp_connect(const char *hostname, const char *service); Returns: connected socket descriptor if OK, no return on error int udp_server(const char *hostname, const char *service, socklen_t *lenptr); Rerurns: unconnected socket descriptor if OK, no return on error

tcp_connect and tcp_listen Functions

Protocol-indep Daytime TCP Client/Server names/daytimetcpsrv1.cnames/daytimetcpcli.c

Protocol-indep Daytime UDP Client/Server names/daytimeudpsrv2.c names/daytimeudpcli1.c

getnameinfo Function  If the caller does not want to return host ( serv ) string, specify hostlen ( servlen ) of 0  flags

Socket Options

getsockopt and setsockopt Functions

Generic Socket Options(1)  SO_BROADCAST  Enable the ability of the process to send broadcast messages  Application must set this option before broadcasting  SO_DEBUG  Kernel keeps track of detailed information about all the packets sent or received by TCP for the socket  SO_DONTROUTE  Bypass the normal routing mechanism  The packet is directed to the appropriate local interface. If the local interface cannot be determined (i.e. destination is not on the same network), ENETUNREACH is returned.  SO_ERROR  Get the value of pending error  Socket 에 error 가 발생하면, so_error (pending error) 에 먼저 setting 하고 나서 errno 에 setting 함

Generic Socket Options(2)  SO_KEEPALIVE  Enforce TCP to send a keepalive probe automatically to the peer if no data has been exchanged for 2 hours (to detect if the peer host crashes)  Peer’s response is either  ACK: everything is OK (application is not notified)  RST: the peer host has crashed and rebooted. Socket’s pending error = ECONNRESET and socket is closed  No response: send 8 additional probes, 75 sec apart (totally, 11mim and 15sec). Listen the peer’s response. Still no response  ETIMEOUT and socket is closed Receives ICMP error, EHOSTUNREACH or …  To shorten probe period  Use TCP_KEEPALIVE option (Not all implementation support this option). But, this will affect all sockets on the host.  Or, implement your own heartbeat mechanism.

Generic Socket Options(3)  SO_LINGER: linger when the socket is closed  Data type for option value  If l_onoff = 0, default close(). i.e. return immediately, but the remaining data is delivered to the peer  else  linger when the socket is closed  If l_linger==0, TCP aborts the connection. i.e. TCP discards any data still remaining in the socket send buffer and sends an RST to the peer.  else linger when the socket is closed. Any data still remaining in the socket send buffer is sent. But, if linger time expires, return EWOULDBLOCK and any remaining data is discarded.

Generic Socket Options(4)  SO_OOBINLINE  OOB data will be placed in the normal input queue  SO_RCVBUF and SO_SNDBUF  Set or get socket send buffer size / socket receive buffer size  TCP:  advertise receive buffer(window) to the peer  sliding window mechanism(flow control)  buffer size  3 * MSS  UDP: typical send buffer size  9000, receive buffer size  40,000,  If data received > available receive buffer, datagram is discarded  Capacity of full-duplex pipe (bandwidth-delay product)  bandwidth * RTT (<= 64KB)  SO_RCVLOWAT, SO_SNDLOWAT: socket low water mark  data in socket receive buffer >= SO_RCVLOWAT  readable  available space in socket send buffer >= SO_SNDLOWAT  writable  SO_RCVLOWAT = 1, SO_SNDLOWAT = 2048 by default  SO_RCVTIMEO, SO_SNDTIMEO: socket time out for receiving and sending message

Generic Socket Options(6)  SO_REUSEADDR: reuse the established port  Allows a listening server to start and bind its well- known port even if previously established connections exist that use this port  Allows multiple instances of the same server to be started on the same port as long as each instance binds a different local IP address  hosting multiple HTTP servers using IP alias technique  Allows a single process to bind the same port to multiple socket as long as each bind specifies a different local IP address  Allows completely duplicate bindings when same IP address and port are already bound to another socket.  Used with multicasting to allow the same application to be run multiple times on the same host when the SO_REUSEPORT option is not supported

TCP Socket Options  TCP_KEEPALIVE: specifies the idle time in seconds for the connection before TCP starts sending keepalive probes  7200 sec (2 hours), default  Effective only when SO_KEEPALIVE socket option is enabled  TCP_MAXSEG: fetch or set MSS for TCP connection  TCP_NODELAY  e.g) rlogin, telnet  Nagle algorithm / delayed algorithm disabled  TCP’s Nagle algorithm  No small packet( < MSS ) will not be sent until the existing data is ACKed  TCP’s delayed ACK algorithm  piggyback: TCP sends an ACK after some small amount of time( 50 ~ 200msec)

Nagle Algorithm Enabled/Disabled  Nagle Algorithm Enabled  No small packet( < MSS ) will not be sent until the existing data is ACKed  Nagle Algorithm Disabled