Introduction to Linux Networkdanny@cmlab.csie.ntu.edu.tw

OutlineOSI and TCP/IP OverviewLinux Networking LayersBSD Socket InterfaceINET Socket InterfaceAn Example of Socket Programming

OSI OverviewOSI (Open Systems Interconnection)See Figures

TCP/IP Model OSI TCP/IPNot presentTCPIP

ApplicationPresentationSessionTransportNetworkData LinkPhysical

Application

TransportInternetHost-to-Network

7654321

TCP OverviewTCP (Transmission Control Protocol)Connection-OrientedReliable ProtocolUDP (User Datagram Protocol)ConnectionlessUnreliable Protocol

IP Overview32-bit Unique IP AddressNetwork Address Subnet AddressHost Address

140.112.28.XX140.112.30.XXGateway(Router)

IP Overview (cont.)IP Header

Ethernet Layer48-bit Unique Device AddressARP (Address Resolution Protocol)multicastmulticastmulticastmulticast

Linux Networking LayersSupport MechanismVarious Networking Inter-Process CommunicationA Special Kind of PipeSupport Several Address FamilySupport Several Socket Type

Addr FamilyDescriptionUNIXUnix domain socketsINETInternet address family support TCP(UDP)/IP AX25Amateur radio X25IPXNovell IPXAPPLETALKAppletalk DDPX25X25

Socket TypeDescriptionStreamReliable, Sequenced, Like TCPDatagramUnreliable, Not sequenced, Like UDPReliable Delivered MessagesLike datagram but reliableSequenced PacketLike Stream but fixed size packet

**BSD SocketsINET SocketsTCPUDPIPPPPSLIPEthernetARPUserKernelNetwork ApplicationsSocket InterfaceProtocol LayersNetwork Devices

Client/Server CommunicationClient1. Create a socket2. Bind an addr3. Listen the client4. Create a socketServer ConnectAcceptSendRecv

BSD Socket APISee An Example

BSD Initializationvoid __init proto_init(void)

The INET LayerBSD SocketA part of VFS inodeA socket can be operated just the same as a file by system call read(), write(), lseek()INET Layer use sock data structure to handle BSD socket

Creating a BSD SocketFor both client and serverint socket(int family, int type, int protocol)Ex. fd=Socket(AF_INET, SOCK_STREAM,0);

files_structcountclose_on_execopen_fsfd[0]fd[1]

fd[255]filef_modef_posf_flagsf_countf_ownerf_opf_inodef_versioninodesocksockettypeprotocoldata (sk)typeprotocolsocketSOCK_STREAMSOCK_STREAMAddress Familysocket operationsBSD SocketFile OperationslseekreadwriteselectioctlclosefasyncLinux BSD Socket Data Structure

Binding an AddressOnly for ServerInt bind(int sockfd, const struct sockaddr *address, size_t add_len)Port Number is optional for bindingsocket.socket_state = TCP_CLOSE;The bound socket cant be used for other communication

Binding an Address (cont.)The bound addr was saved in sock.rcv_saddrUDP maintains a hash table udp_hash to allocate UDP portTCP doesnt add the binding sock to hash table during binding operation

ListeningOnly for serverint listen(int sockfd, int queue_size)socket.socket_state = TCP_LISTEN;Add the sock to tcp_bound_hash and tcp_listening_hash

Listening (cont.)After receiving clients requestServer build a new sockClones the incoming sk_buff and queues it to the listening sock.receive_queue

ConnectingOnly for clientBefore connecting, socket.socket_state = SS_UNCONNECTED;Int connect(int csockfd, const struct sockaddr *address, size_t add_len)Add the sock to tcp_listening_hash waiting for servers response

AcceptingOnly for serverint accept(int sockfd, struct sockaddr *address, size_t *add_len)A new socket was cloned from the listening socket

Accepting (cont.)If there are no incoming connection to acceptNon-Blockingaccept operation failed and throw away the new socketBlockingaccept operation was added to the wait queue

sk_buffer structuretruesizelenPushPullPutTrim

nextprevdev

headdatatailend

Packet to be transmitted

ReferencesThe Linux Kernel, chapter 10Linux Kernel Internals, chapter 8Unix System Programming, chapter 10Computer Networks, chapter 1, 5, 6