Effective Effective TCP/IP ProgrammingTCP/IP Programming

Snader, ETCP 중심으로

Understand IPv4 AddressingUnderstand IPv4 Addressing

IPv4: originally Classful AddressingIPv4: originally Classful Addressing Special address

Host ID = 0: network address Network ID = 0, host ID = 0

i.e 0.0.0.0 means this network 127.x.y.z : “looped back” Host ID = all 1s : broadcasting NAT address

10/8 172.16/12 192.168/12

Broadcast address 255.255.255.255

Limited broadcast router 밖에는 나가지 못함

190.50.255.255 Network-directed broadcast 그 네트워크까지 가서 네트워크 내에

broadcast 190.50.1.255/24

Subnet-directed broadcast 190.50.1.0/24 subnet 내에 broadcast

IPv4 address 는 host 의 address 가 아니라 interface 의 address 이다 .

SubnetSubnet

Network address IP address && mask Network ID

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

network consisting of 3 subnets

subnet

223.1.1.0/24223.1.2.0/24

223.1.3.0/24

223.1.0.0/16

CIDR (Classless Inter-Domain Routing) Subnetting + suppernetting

Develop and Use Application Develop and Use Application “Skeletons”“Skeletons”

Making UNIX/Windows CompatibleMaking UNIX/Windows CompatibleUNIX: bsd/skel.h Windows: win/skel.h

wincompat.c:

Window 에서 socket을 생성전에 call 해야

TCP Server SkeletonTCP Server Skeleton

tcpserver.skel:

“mclab.hufs.ac.kr”

or “203.254.68.114”or “” -- server

“http” or

“80”

TCP Server Skeleton - Cont’dTCP Server Skeleton - Cont’d

Usage: % myserver [local_addr | local_name] {local_port | service}Example: %myserver 15000 %myserver localhost http

TCP Client SkeletonTCP Client Skeletontcpclient.skel:

Usage: % myclient {peer_addr | peer_name} {peer_port | service}Example: % myclient 203.253.70.5 15000 % myclient www.hufs.ac.kr http

UDP Server & Client SkeletonUDP Server & Client Skeletonudpserver.skel: udpclient.skel:

Build Your Own Library and Use It !Build Your Own Library and Use It !etcp.h

TCP Client & Server Starting FunctionsTCP Client & Server Starting Functions

Host name or IP addr

or “” (my addr for server)

“http” or

“80”

UDP Client & Server Starting UDP Client & Server Starting FunctionsFunctions

Remember thatRemember thatTCP is a Stream ProtocolTCP is a Stream Protocol

TCP is a Stream ProtocolTCP is a Stream Protocol No message boundary, just a byte stream

TCP application 에서 하나의 message 를 send() 했다고 해서 recevier application 에서 그 message 를 한 덩어리로 recv() 되는 것이 아니다 .

Message 를 send() 했어도 TCP segment 에 언제 , 어떻게 실릴지 모른다 . (buffering 되기 때문 )

Recv() 하면 몇 byte 읽힐지 모른다 . If you want to read exactly n bytes ?? If you want to make a record ???

Use end-of-record mark. e.g) new line Handle variable records (using fixed header)

#include <sys/socket.h> /* UNIX */#include <winsock2.h> /* Windows */int recv (SOCKET s, void *buf, size_t bufsize, int flags);int read (SOCKET s, void *buf, size_t bufsize); /* UNIX */Returns: # of bytes read (>0), 0 if received FIN and no more data, -1 on failure

int send (SOCKET s, const void *buf, size_t len, int flags);int write (SOCKET s, const void *buf, size_t len); /* UNIX */Returns: # of bytes transferred on success, -1 on failure

메시지를 읽을 user buffer 의 크기즉 , 최대로 읽을 수 있는 크기

Socket send buffer에 저장할 메시지 길이

Use End-of-record mark: read a lineUse End-of-record mark: read a linelib/readline.c:

Read n bytes and Read a variable-length Read n bytes and Read a variable-length recordrecord

Record Length

Other Header Data

VariableData

lib/readn.c:

lib/readvrec:

#include “etcp.h”int readn (SOCKET s, char *buf, size_t len); Returns: # of bytes read, -1 on failure

int readvrec (SOCKET s, char *buf, size_t len); Returns: # of bytes read , -1 on failure

len

cnt

bp

Header size

Network byte order

Example: Client/Server using variable Example: Client/Server using variable recordsrecords

vrc.c: vrs.c:

Don’t Underestimate Don’t Underestimate the Performance of TCPthe Performance of TCP

TCP versus UDPTCP versus UDP TCP

connection-oriented reliable byte stream

Application: typically concurrent server SMTP(Simple Mail

Transfer Protocol) Telnet FTP HTTP NNTP(Network News

TP)

UDP connectionless unreliable datagram

Applications: typically iterative server SNMP(Simple Network

Management Protocol) TFTP(Trivial FTP) BOOTP(Bootstrap

Protocol) DHCP(Bootstrap

Protocol)

TCPTCP 는 충분히 는 충분히 Optimize Optimize 되어 있다되어 있다 .. 통상적인 TCP segment 수신 루틴 : 30

instructions (excluding checksum) ACK 도 piggy-back 섯불리 UDP 를 사용하지 말라 .

But, a single request-response 로 끝나는 transaction 에서는 TCP 는 Connection set-up: RTT 소요 Connection release: 적어도 RTT 소요

A UDP Source and SinkA UDP Source and Sinkudpsource.c: udpsink.c:

Record of length 0i.e. end-of-record mark

Set UDP socket receive buffer size

실제 5,000 개의 datagram 을 buffering 할수는 없다 . 왜냐 하면 , 시스템에서 허용하는socket receiver buffer 의 최대 크기는이보다 훨씬 작기 때문이다 ( 수십 KB).

UDP datagram 은 lost 될 수 있다 !!1.Network congestion (no congestion cotrol)2.Recv buffer overflow(no flow control)

A TCP Source and SinkA TCP Source and Sinktcpsource.c: tcpsink.c:

Options: -s sndsz -b sndbufsz -c blks

Set TCP send buffer size

Set TCP receive buffer size

Comparison of TCP and UDP Comparison of TCP and UDP PerformancePerformance LAN 에서 UDP 가 TCP 보다 20% 정도 성능이

우수했음 그러나 , UDP 에서는 lost 가 많이 발생함 (no flow

control)

Loopback interface( 같은 host 내 ) 에서는 TCP가 UDP 보다 훨씬 성능 우수했음 Local host 의 MTU 는 16,384 B (BSD) Ethernet 의 MTU 는 1,500 B

Avoid Reinventing TCPAvoid Reinventing TCP Any reasonably robust UDP application must

provide Error recovery: reTx a request if not received a

response within RTO Sequencing: ensure that replies are matched correctly

to requests Flow control: if server’s reply can consist of multiple

datagrams, prohibit overflow of client’s recv buffer Cause to rewrite TCP

TCP 는 kernel 에서 처리되기 때문에 application에서 reliable protocol 을 구현하는 것보다 실제 빠르다 .

When to Use UDP instead of TCPWhen to Use UDP instead of TCP Adv. Of UDP

supports broadcasting and multicasting no overhead for connection setup or teardown

UDP requires 2 packets to exchange a request and a reply TCP requires about 10 packets to exchange assuming new TCP

connection is established for each request-reply exchange

Features of TCP that are not provided by UDP positive ACK, reTx of lost packet, duplicate packet detection,

sequencing of packets windowed flow control slow start and congestion avoidance

Recommendation of UDP Usage must be used for broadcast or multicast applications

desired level of error control must be added can be used for simple request-reply applications

error detection must be needed should not be used for bulk data transfer

Realize that TCP is a Reliable Realize that TCP is a Reliable Protocol, Protocol, Not Infallible ProtocolNot Infallible Protocol

TCP is a Reliable Protocol, Not TCP is a Reliable Protocol, Not Infallible ProtocolInfallible Protocol 2 peer 간에 connection 이 유지되는 한 TCP 는 ordered and

uncorrupted delivery 를 보장한다 . Application 은 통신이 불가능함을 모르고 데이터를 보낼 수 있고 ,

따라서 목적지에 delivery 되지 못하는 경우가 발생한다 . TCP 는 data 를 보내봐야 실제 peer TCP 와 통신 가능한지 확인 가능

(ACK 를 받아 봐야 ) 또는 , 2 시간 이상 데이터 교환이 없을 경우에나 통신 불가능을 확인 할 수 있음 교환할 데이터가 없어도 주기적으로 교환해야 heart beat mechanism 구현 필요

Application 은 send()/recv() 가 error return 되었을 때야 , 통신 불가능함을 알게 된다 . failure 처리

통신 불가능한 경우 ( 실제 connection 이 유지되지 않는 경우 ) Network outage (due to router or link failure) Peer app crashes Peer host crashes

Network OutageNetwork Outage Inside TCP

Segment 보낸 후 ACK 가 없으면 , 12 번 ReTx 한다 ( 약 9 분 걸림 )

여전히 ACK 를 받지 못하면 , set socket pending error (ETIMEOUT)

Inside IP/ICMP IP datagram 을 forwarding 할 수 없으면 (router 나 link 장애로

인해 ), ICMP host unreachable/network unreachable message를 source 로 보낸다 .

이 메시지를 Source IP 가 받으면 , set socket pending error (ENETUNREACH/EHOSTUNREACH)

Socket Pending Error Send() returns on failure send buffer 에 쓰는 것이 실패를 의미 실제 보낸 데이터가 peer 에게 전달할 수 없음은 한참 뒤에나 알 수

있다 . Kernel 은 이와 같은 error 가 발생하면 , 해당되는 socket 에 pending

시켜 놓는다 Socket API call 이 이루어질 때 , error return 하면서 errno 에 설정한다 .

Peer App CrashesPeer App Crashes

When peer app crashes, Local app is In recv(): return 0

통상적인 절차로 종료 In send(): normal return

But, sent data is lost.Local connection 을 강제 close.Error is pending.

Send()/recv(): error return (ECONNRESET)

Send()/recv(): rrror return (EPIPE)

Peer app crashes(killed) 1. Call exit(), implicitly

2. Call close() in exit()

3. TCP: send FINFIN

data

RESETNo connection !Not delivered to peer app

Ways to Detect Various TCP ConditionWays to Detect Various TCP Condition

Peer app crashes – an examplePeer app crashes – an example

tcprw.c: count.c:

killedkilled

Peer Host CrashesPeer Host Crashes

Local app Connection set-up

Send(): normal return But sent data is lost Error is pending

(ETIMEOUT) after retransmitting 12 times(9 min)

or error is pending (EHOSTUNREACH or ENETUNREACH) by ICMP

Peer host crash No TCP there, so no

TCP response

dataNo TCP/IP Protocol !

Remember that TCP/IP is Not Remember that TCP/IP is Not PolledPolled

No notification when connectivity is lost

HeartbeatsHeartbeats 데이터를 보내 보지 않고서는 peer 와 통신 가능 여부를

알 수 없다 통신 불가능함에도 불구하고 데이터를 보내면 lost 됨

( 송금했는데 못 받았으면 ???) 데이터 교환과 별도로 상대가 살아 있는지 주기적으로 check 해

봐야 함 hearbeat 필요 C/S 가 여러가지 msg type 을 교환하는 경우

Heartbeat msg 에 새로운 type 을 할당 C/S 가 byte stream 으로 교환하는 경우

Hearbeat 과 data 를 구분할 수 없음 Hearbeat 에 대해 별도 TCP connection 설정하여 구분 Or, Hearbeat 에 대해 OOB msg 로 교환

(send/recv() 에서 flag 를 OOB 로 설정 )

Hearbeat Client – msg typeHearbeat Client – msg typeheartbeat.h:

hb_client.c:

Heartbeat ServerHeartbeat Server- msg type- msg type

hb_server.c:

Hearbeat Client – separate connectionHearbeat Client – separate connectionhb_client2.c:

Hearbeat Server – separate connectionHearbeat Server – separate connectionhb_server2.c:

BeBe Prepared for Rude Behavior Prepared for Rude Behavior from a Peerfrom a Peer

Checking for client termination Checking for valid input

UNIX utility program 중 6 ~ 43 % 가 인위적으로 생성한 random input 으로 시험했을 때 죽었음

Read a lineRead a linelib/readline.c:

len

cnt

bpb

0

bufptrbufx

From socket

Buffering

C string 은 ‘ \0’ 를 append해야 함Line >= 2 bytes

ConsiderConsider Letting Letting inetdinetd Launch Launch Your ApplicationYour Application

inetdinetd daemon daemon Problems starting with /etc/rc(without inet daemon)

All the servers contains nearly identical startup code Each daemon takes a slot in process table, but asleep

most of time /etc/inetd.conf file specifies the services that the

superserver inetd is to handle

Steps performed by Steps performed by inetdinetd

dup2(sockfd, 0);dup2(sockfd, 1);dup2(sockfd, 2);close(sockfd);

Open descriptor 들은 fork 시 copy 되고Exec 후에도 유지된다 .Exec 후에 Peer 를 알 수 있는 방법은 ?

Consider Consider Using Two TCP ConnectionsUsing Two TCP Connections

Concurrent Input/Output processesConcurrent Input/Output processes

One-connection architecture Xout 에서 send 할 때

pending error 는 xin 에서 recv 해야 알 수 있다

Xin mp xout 으로 알려 줘야 함

Two connection architecture 별도의 connection 에

대해 socket pending error가 있는지 testing 가능 Using select() readability

ExampleExample

xout1.c:

Making Your Applications Making Your Applications Event DrivenEvent Driven

Making Your Apps Event DrivenMaking Your Apps Event Driven Types of events in protocols and networked application

Packet arrival User input Timeout API call

만일 , API 도 UNIX domain socket( 일종의 socket descriptor 임 ) 이라면 select() 로 API call 이 발생했음을 확인 가능

다만 , select() 는 timeout 은 하나만 가능

How to support multiple timeouts using select() ? You can make it !

select() 로 확인 가능(Ready or timeout? )

Timers in UNIX/LinuxTimers in UNIX/Linux Using SIGALRM signal: 초 단위

Interval timer: 실제는 10 msec 단위로 작동

select(): 실제 10 msec 단위로 작동

signal(SIGALRM, handler); // or sigaction(…)…alarm(seconds); // set timeout timer…void handler(in signo) { …. }; // actions in timeout event

#include <sys/time.h>int getitimer(ITIMER_REAL, struct itimerval *value);int setitimer(int which, const struct itimerval *value, struct itimerval *ovalue); struct itimerval { struct timeval it_interval; /* next value */ struct timeval it_value; /* current value */ }; struct timeval { long tv_sec; /* seconds */ long tv_usec; /* microseconds */ };

Multiple Timeouts using select()Multiple Timeouts using select()

#include “etcp.h”int tselect( int maxfdp1, fd_set *readset, fd_set *writeset, fd_set *exceptset ); Returns: # of ready events, 0 if no remaining events, -1 on errorunsigned int timeout( void (*handler)(void *), void *arg, int msec ); Returns: timer ID to be used in untimeout callvoid untimeout( unsigned int timerid );

/* call retransmit(packet) after 1500 msec (timeout) */rxtimer = timeout(retransmit, (void *) packet, 1500 ); UItimer = timeout(useridle, NULL, 30000); …n = tselect(maxfdp1, &readset, &writeset, &exceptset );…// if received ACK, untimeout(rxtimer); // reset the timer

Examples

Tselect() source from ‘etcp’ Tselect() source from ‘etcp’

lib/tselect.c

Internet Checksum AlgorithmInternet Checksum Algorithm