Transport Protocols

Transport Layer

Port Numbers UDP (and TCP) use port numbers to identify applications A globally unique address at the transport layer (for both UDP and TCP) is

Unique input/output designations for an application protocol There are 65,535 ports per host RFC1700: - The Well Known Ports (0-1023) are controlled and assigned by the IANA and on most systems can only be used by system (or root) processes or by programs executed by privileged users - Registered nort numbers ( ) Client: The Dynamic and/or Private Ports (49152 – 65535) Port Numbers (2)

Services and Ports: ~]$ less/etc/services ….. ftp 21/tcp ssh 22/tcp # SSH Remote Login Protocol telnet 23/tcp smtp 25/tcp mail time 37/tcp timserver time 37/udp timserver domain 53/tcp # name-domain server domain 53/udp bootps 67/tcp # BOOTP server bootpc 68/tcp # BOOTP client http 80/tcp www www-http # WorldWideWeb HTTP netbios-dgm 138/udp netbios-ssn 139/tcp # NETBIOS session service imap 143/tcp imap2 # Interim Mail Access Proto v2 pop3s 995/tcp # POP-3 over SSL syslog 514/udp socks 1080/tcp # socks proxy server wins 1512/tcp # Microsoft's Windows Internet # Name Service wins 1512/udp # Microsoft's Windows Internet mysql 3306/tcp # MySQL ……

Protocol Ports For two processes to communicate, they must be able to address one another To identify a process both UDP and TCP use an abstraction called a protocol port Connection = IP-addr1:Port1 + IP-addr2:Port2 Pair IP-addr:Port is identified socket TCP End Points Connections Port 1143 Port 21, ftp server Port 1569 Port 2345 TCP

Sockets The socket interface is one of several application programming interfaces (APIs) to the communication protocols. A socket is a special type of file handle, which is used by a process to request network services from the operating system. A socket is a programming level abstraction which points to an Internet end point

Оператор формирования сокета имеет вид: s=socket(INT AF, INT type, INT protocol); -AF (address_family) - набор протоколов, соответствующий данному сокету. Example: Internet: PF_INET, Local_unix: PF_UNIX. -type определяет тип коммуникаций Example: SOCK_STREAM, SOCK_RAW, и SOCK_DGRAM. -protocol задает код конкретного протокола из указанного набора (заданного AF), который будет реализован в данном соединении. Ехаmрlе: IPPROTO_TCP, IPPROTO_UDP. Допускается значение protocol=0 (протокол не указан), в этом случае используется значение по умолчанию для данного вида соединений. Значения AF и type можно найти в файле Возвращаемый параметр S - дескриптор сокета. Socket creation Linux API

Berkeley Sockets ServerClient socket() bind() listen() accept() read()/write() recv()/send() socket() connect() write() read() close() TCP ServerClient socket() bind() socket() sendto() recvfrom() close() UDP recvfrom() sendto() bind()

Example:Existing Services ~]$ netstat -an Active Internet connections (servers and established) Proto Recv-Q Send-Q Local Address Foreign Address State tcp : :* LISTEN tcp : :* LISTEN tcp : :* LISTEN tcp : :* LISTEN tcp : :* LISTEN tcp : :* LISTEN tcp : :4090 ESTABLISHED tcp : :4870 ESTABLISHED tcp : :1215 ESTABLISHED tcp : :443 ESTABLISHED tcp : :42590 TIME_WAIT udp : :* udp : :*

Transport Layer UDP (User Datagram Protocol ) RFC 768

UDP in TCP/IP Stack Media Access Control (Ethernet, Token Ring, FDDI, X.25, PPP, etc.) User Datagram Protocol (UDP) RFC 768 ~ connectionless transport ~ Transmission Control Protocol (TCP) RFC 793 ~ connection-oriented transport ~ Upper-Layer Protocols ~ such as SNMP, telnet, FTP, HTTP, POP3, etc. ~ Upper Transport Network Data Link ARP Internet Control Messaging Protocol (ICMP) RFC 792 Internet Protocol version 4 (IPv4) RFC 791 RARP

User Datagram Protocol (UDP) Unreliable and unordered datagram service Adds multiplexing No flow control Endpoints identified by ports –servers have well-known ports (see file services) Header format: Pseudo-IP Header: Optional checksum –psuedo header + UDP header + data

UDP Checksum at Sender Add pseudo-header Fill checksum with 0s Divide into 16-bit words (adding padding if required) Add words using 1s complement arithmetic Complement the result and put in checksum field Drop pseudo-header and padding Deliver UDP segment to IP source port #dest port # 32 bits data (add padding to make data a multiple of 16 bits) length checksum 32-bit source IP address 32-bit destination IP address length protocol 0s The pseudo-header

Binary Checksum Example Carry from 1 st column Carry from 2 nd column Carry from 3 rd column Carry from 4 th column Carry from 5 th column Carry from 13 th column Carry from 12 th column Carry from 16 th column

Checksum calculation at Receiver Add pseudo-header to the UDP segment Divide into 16-bit words and add words using 1s complement arithmetic Complement result If result is all 0s –Drop pseudo-header and padding (if any) –Accept segment Else drop segment