tdc561 network programming

TDC561 Network Programming

Camelia Zlatea, PhD

Email: [email protected]

Week 8:

Multicasting; Socket Options;

mailto:[email protected]






Network Programming (TDC561) Winter 2003

References

W. Richard Stevens, Network Programming : Networking API: Sockets and XTI, Volume 1, 2nd edition, 1998 (ISBN 0-13-490012-X) – Chap. 7, 11, 19, 21, 22


Addressing in the Internet

Addressing tied to reachability– Every host interface has its own IP address

– Router interfaces usually have their own IP addresses

IP is version 4 (IPv4 addresses)– 4 bytes long

– two part hierarchy

» network number and host number

– different types of boundary indicator

» class, subnet mask, prefix

– Goal of boundaries is address aggregation


Address classes

Historical first choice– fixed network-host partition, with 8 bits of network number

Generalization– Class A addresses have 8 bits of network number– Class B addresses have 16 bits of network number– Class C addresses have 24 bits of network number

Distinguished by leading bits of address– leading 0 => class A (first byte < 128)– leading 10 => class B (first byte in the range 128-191)– leading 110 => class C (first byte in the range 192-223)– leading 1110 => class D (multicast)– leading 1111 => Class E (reserved)


Address evolution

Class based scheme was too inflexible Two problems

– Too many routes– Too few addresses

Four extensions– Subnetting (flexible boundaries within network)– CIDR (flexible grouping of networks- Classless Inter-

domain Routing)– Dynamic host configuration (reuse of addresses)– A bigger address (IPv6)

One issue– Network address translation


What is Multicast?

Multicast is a communication paradigm– 1 source, multiple destination

Applications:– bulk-data distribution to subscribers

» (e.g., newspaper, software, and video tapes distribution),

– connection-time-based charging data distribution

» (e.g., financial data, stock market information, and news tickets broadcasting),

– streaming (e.g., video/audio real-time distribution),

– push applications, web-casting,

– distance learning, conferencing, collaborative work, distributed simulation, and interactive games.


The Internet group model– multicast/group communications means...

» 1 n as well as n m

– a group is identified by a class D IP address (224.0.0.0 to 239.255.255.255)

» abstract notion does not identify any host

host_1

140.192.1.8140.192.1.8sourcesource

host_2

receiverreceiver140.192.1.6140.192.1.6

host_3

receiverreceiver216.47.143.60216.47.143.60

multicast group225.1.2.3 multicast router

Ethernet

multicast router

multicast router

host_1

sourcesource

host_2

Ethernet

receiverreceiver

host_3

site 1

site 2

Internet

receiverreceiver

multicast distribution tree

from logical view...

...to physical view


IP Multicast: Basic Idea

Multicast groups: abstract “rendez-vous” points. Set up optimal spanning tree spanning participants for

each group. Make it cheap by not providing strong guarantees: send

out packets and hope for the best.


The Internet group model (cont’)

the group model is an open model– anybody can belong to a multicast group

» no authorization is required– a host can belong to many different groups

» no restriction– a source can send to a group, no matter whether it

belongs to the group or not» membership not required

– the group is dynamic, a host can subscribe to or leave at any time

– a host (source/receiver) does not know the number/identity of members of the group


Mapping IP Multicast onto Ethernet Multicast

IP Multicast (class D IP address): – Class D: 224.x.x.x-239.x.x.x (in HEX: Ex.xx.xx.xx): 28 bits

– No further structure (like Class A, B, or C)

– Not addresses but identifiers of groups

– Some of them are assigned by the IANA to permanent host groups

Mapping a class D IP adr. into an Ethernet multicast adr.– The least 23 bits of the Class D address are inserted into the 23 bits of

Ethernet multicast address

– Many to one mapping: 5 bits are not used

– More filtering has to be done at IP level


Ethernet Multicast

Ethernet is a broadcast medium

– Every frame can potentially be seen by every host Ethernet cards have a unique Ethernet address Broadcast address:

– ff:ff:ff:ff:ff:ff Ethernet Multicast address range for IP:

– 01:00:5e:00:00:00 -to- 01:00:5e:7f:ff:ff Mapping IP Multicast onto Ethernet Multicast


The Internet group model (cont’)

local-area multicast» use the potential diffusion capabilities of the physical

layer (e.g. Ethernet)

» efficient and straightforward

wide-area multicast» requires to go through multicast routers, use

IGMP/multicast routing/...

» routing in the same administrative domain is simple and efficient

» inter-domain routing is complex, not fully operational


Multicast and the TCP/IP layered model

TCP UDP

IP / IP multicast

device drivers

ICMP IGMP

Application

Socket layer

congestioncontrol

reliabilitymgmt

other buildingblocks

multicastrouting

higher-levelservices

user space

kernel space


What is Multicast? Several applications need efficient means to transmit data

to multiple destinations with:– less bandwidth– higher throughput– lower delay– higher reliability

Classification– Data dissemination– Transactions– Large Scale Virtual Environments

Build on top of the existing Internet and take into account group communication constraints– Manage groups– Create and maintain multicast routes– Efficient end-to-end delay (reliability, flow control, time constraints)


Ideal Multicast

Senders (S) and Receivers (R) not aware of each other’s position in the network.

Scalable. Low latency (join, data propagation). Low bandwidth and processing overhead. “Reliable”, if this is cheap (“end-to-end”?) Easy to join/leave.


Why IP multicast?

scalability...

– scales to an unlimited number of users reduced costs...

– cheaper equipment and access line increased speed...

– increases the delivery speed

...or multicast?contentserver ISP and Internet

access line client

client

contentserver ISP and Internet

access line client

client

use unicast?


Multicast Features: Multicast Scope Control

Who gets which packets?– Send everything to everybody ..

TTL scope– To keep multicast traffic within an administrative domain by

setting ttl thresholds on interfaces on the border router

Administratively scoped addresses– A multicast boundary can be setup

on the borders for addresses in range of 239.0.0.0–239.255.255.255

– Better than ttl scope


Multicasting: Receiving multicast message

For a process to receive multicast messages it needs to perform the following steps:

1. Create a UDP socket msd msd = socket(AF_INET,SOCK_DGRAM, 0);

2. Bind it to a UDPport, e.g., 1234. All processes must bind to the same port in order to receive the multicast messages. struct sockaddr_in groupHost;

groupHost.sin_family = AF_INET; groupHost.sin_port = htons(UDPport); groupHost.sin_addr.s_addr = htonl(INADDR_ANY);

bind(msd, (struct sockaddr *) &groupHost, sizeof(groupHost))


Multicasting: Receiving multicast message

(cont’)3. Join a multicast group address GroupIPaddress ,

e.g., 224.111.112.113

joinGroup (msd, GroupIPaddress);

4. Use recv or recvfrom to read the messages, e.g.,

nbytes = recv(msd, recvBuf, BufLen,0);


Multicast Groups and Addresses

Every IP multicast group has a group address. IP multicast provides only open groups

– it is not necessary to be a member of a group in order to send datagrams to the group.

Multicast address are like IP addresses used for single hosts, and is written in the same way: A.B.C.D.– Multicast addresses will never clash with host addresses because

a portion of the IP address space is specifically reserved for multicast. 224.0.0.0 to 239.255.255.255.

– Multicast addresses from 224.0.0.0 to 224.0.0.255 are reserved for multicast routing information;

– Application programs should use multicast addresses outside this range.


Multicasting: Receiving multicast message /* This function sets the socket option to make the local host join the multicast

group */void joinGroup(int s, char *group){ struct sockaddr_in groupStruct; struct ip_mreq mreq; /* multicast group info structure */

if((groupStruct.sin_addr.s_addr = inet_addr(group))== -1) printf("error in inet_addr\n"); /* check if group address is indeed a Class D address */ mreq.imr_multiaddr = groupStruct.sin_addr; mreq.imr_interface.s_addr = INADDR_ANY; if ( setsockopt(s,IPPROTO_IP,IP_ADD_MEMBERSHIP,(char *) &mreq,

sizeof(mreq)) == -1 ) { printf("error in joining group \n"); exit(-1); }}


Receiving Multicast Datagrams

Join a particular multicast group. This is done using another call to setsockopt:

struct ip_mreq mreq;

setsockopt(sock,IPPROTO_IP,IP_ADD_MEMBERSHIP,&mreq,sizeof(mreq));

The definition of struct ip_mreq is as follows: struct ip_mreq {

struct in_addr imr_multiaddr; /* multicast group to join */

struct in_addr imr_interface; /* interface to join on */

}


Multicasting: Receiving multicast message /* This function removes the process from the group */void leaveGroup(int recvSock,char *group){ struct sockaddr_in groupStruct; struct ip_mreq dreq; /* multicast group info structure */

if((groupStruct.sin_addr.s_addr = inet_addr(group))== -1) printf("error in inet_addr\n");

dreq.imr_multiaddr = groupStruct.sin_addr; dreq.imr_interface.s_addr = INADDR_ANY; if( setsockopt(recvSock,IPPROTO_IP,IP_DROP_MEMBERSHIP, (char *) &dreq,sizeof(dreq)) == -1 ) { printf("error in leaving group \n"); exit(-1); } printf("process quitting multicast group %s \n",group);}


Multicasting: Sending multicast message

For a process to send multicast messages it needs to

perform the following:

1. use the UDP socket msd for sending multicast messages

struct sockaddr_in dest;

dest.sin_family = AF_INET;

dest.sin_port = UDPport;

dest.sin_addr.s_addr = inet_addr(GroupIPaddress);

sendto (msd, sendBuf, BufLen,0, (struct sockaddr *) &dest, sizeof(dest)) ;


Multicasting: Sending multicast message

(cont’)2. Join a multicast group address GroupIPaddress ,

e.g., 224.111.112.113

joinGroup (msd, GroupIPaddress);

3. Use recv or recvfrom to read the messages, e.g.,

nbytes = recv(msd, recvBuf, BufLen,0);


Multicasting: Sending multicast message /* This function sets the socket option to make the local host join the multicast

group */void joinGroup(int s, char *group){ struct sockaddr_in groupStruct; struct ip_mreq mreq; /* multicast group info structure */

if((groupStruct.sin_addr.s_addr = inet_addr(group))== -1) printf("error in inet_addr\n"); /* check if group address is indeed a Class D address */ mreq.imr_multiaddr = groupStruct.sin_addr; mreq.imr_interface.s_addr = INADDR_ANY; if ( setsockopt(s,IPPROTO_IP,IP_ADD_MEMBERSHIP,(char *) &mreq,

sizeof(mreq)) == -1 ) { printf("error in joining group \n"); exit(-1); }}


Multicasting

Time-to-live– control how far the messages can go, e.g., 2 means at most 2

routers away. (default is 1- which will result in multicast packets going only to other hosts on the local network. )

u_char TimeToLive; TimeToLive = 2; setTTLvalue (s, &TimeToLive);

/* This function sets the Time-To-Live value */void setTTLvalue(int s,u_char *ttl_value){ if( setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, (char *) ttl_value,

sizeof(u_char)) == -1 ) { printf("error in setting loopback value\n"); }}


Multicasting

Time-to-live– To provide meaningful scope control, multicast routers enforce the

following "thresholds" on forwarding based on the TTL field:

0 restricted to the same host

1 restricted to the same subnet

32 restricted to the same site

64 restricted to the same region

128 restricted to the same continent

255 unrestricted


Multicasting

Loop-back– allow the process to get a copy of its own transmission we use:

u_char loop;

loop = 1;

setLoopback (s, &loop);

void setLoopback(int s,u_char loop)

{

if( setsockopt(s,IPPROTO_IP,IP_MULTICAST_LOOP,(char *) &loop,

sizeof(u_char)) == -1 )

{

printf("error in disabling loopback\n");

}

}

By default, messages sent to the multicast group are looped back to the local host. this function disables that.

loop = 1 /* means enable loopback (default)

loop = 0 /* means disable loopback


Multicasting

Reuse-port – allow multiple multicast processes to to run on the same host:

reusePort (s);

/* This function sets a socket option that allows multiple processes to bind to the same port*/

void reusePort(int s){ int one=1; if ( setsockopt(s,SOL_SOCKET,SO_REUSEADDR,(char *) &one,sizeof(one)) == -

1 ) { printf("error in setsockopt,SO_REUSEPORT \n"); exit(-1); }}


Multicasting - Example

http://condor.depaul.edu/~czlatea/TDC561/LectureNotes/TDC561_week8/

multicast.h multicastUtilities.c multicastChat.c


Reliable One-One Communication

Use reliable transport protocols (TCP) or handle at the application layer Client/Server semantics in the presence of failures Possibilities

– Client unable to locate server– Lost request messages– Server crashes after receiving request– Lost reply messages– Client crashes after sending request


Reliable One-Many Communication

Reliable multicast– Lost messages => need

to retransmit

Possibilities– ACK-based schemes

» Sender can become bottleneck

– NACK-based schemes


Atomic Multicast

Reliable Group Communication– Processes can fail– Atomicity of Multicast is required

» Atomicity? Group Membership

– Multicast and a corresponding group of recipients– Failures of processes can be viewed as changes to group membership.

System Model– Separating receiving a message and delivering it to a application– Group View: a list of processes associated with a message

View Change– A special multicast message– Race between m and vc

Condition– Either m is delivered to all processes before a process is delivered a new vc– Or, m is not delivered at all.


Atomic Multicast

Atomic multicast: a guarantee that all process received the message or none at all

– Replicated database example

Problem: how to handle process crashes?

Solution: group view– Each message is uniquely

associated with a group of processes

» View of the process group when message was sent

» All processes in the group should have the same view (and agree on it)

Virtually Synchronous Multicast


Reliable Mcast Transport Protocol

• S, R use windows• Designated Receivers eliminate ACK implosion• ACK’s sent to DR’s• DR’s and S cache data and retransmit it when needed.

Smart “session manager”elects DR’s and setsparameters. How? Justlike that...


RMTP(2)

After set up S starts sending data. Receivers send periodic ACK’s after first packet received.

If no ACK’s for a long time, connection terminates. DR’s or S retransmit info using unicast or multicast,

depending on number of errors. Immediate TX request sent to DR’s, for receivers that join the

session. Sender window advance determined by slowest receiver. ACK’s must not be repeated too often. Measure RTT to AP. S adjusts (decreases) send window to 1 if many errors; then

increases linearly. DR’s are fixed, but each R chooses its DR. (DR sends

SND_ACK_TOME with TTL fixed to a known value).


Socket Options

Various attributes that are used to determine the behavior of sockets.

Setting options tells the OS/Protocol Stack the behavior we want.

Support for generic options (apply to all sockets) and protocol specific options.


Option types

Many socket options are Boolean flags indicating whether some feature is enabled (1) or disabled (0).

Other options are associated with more complex types including int, timeval, in_addr, sockaddr, etc.

Read-Only Socket Options– Some options are readable only (we can’t set the value).


Setting and Getting option values

getsockopt() gets the current value of a socket option.

setsockopt() is used to set the value of a socket option.

#include <sys/socket.h>


int getsockopt( int sockfd,

int level,

int optname,

void *opval,

socklen_t *optlen);

level specifies whether the option is a general option or a protocol specific option (what level of code should interpret the option).

getsockopt()


int setsockopt( int sockfd,

int level,

int optname,

const void *opval,

socklen_t optlen);

setsockopt()


General Options

Protocol independent options. Handled by the generic socket system code. Some general options are supported only by specific

types of sockets (SOCK_DGRAM, SOCK_STREAM).


Some Generic Options

SO_BROADCAST

SO_DONTROUTE

SO_ERROR

SO_KEEPALIVE

SO_LINGER

SO_RCVBUF,SO_SNDBUF

SO_REUSEADDR


SO_BROADCAST

Boolean option: enables/disables sending of broadcast messages.

Underlying DL layer must support broadcasting! Applies only to SOCK_DGRAM sockets. Prevents applications from inadvertently sending

broadcasts (OS looks for this flag when broadcast address is specified).


SO_DONTROUTE

Boolean option: enables bypassing of normal routing.

Used by routing daemons.


SO_ERROR

Integer value option.

The value is an error indicator value (similar to

errno).

Readable only

Reading (by calling getsockopt()) clears any

pending error.


SO_KEEPALIVE

Boolean option: enabled means that STREAM sockets should send a probe to peer if no data flow for a “long time”.

Used by TCP - allows a process to determine whether peer process/host has crashed.

Consider what would happen to an open telnet connection without keepalive.


SO_LINGER

Value is of type:

struct linger {

int l_onoff; /* 0 = off */

int l_linger; /* time in seconds */

}; Used to control whether and how long a call to close

will wait for pending ACKS. connection-oriented sockets only.


SO_LINGER usage

By default, calling close() on a TCP socket will return immediately.

The closing process has no way of knowing whether or not the peer received all data.

Setting SO_LINGER means the closing process can determine that the peer machine has received the data (but not that the data has been read() !).


shutdown() vs SO_LINGER

How you can use shutdown() to find out when the peer process has read all the sent data [R.Stevens, 7.5]


FINSN=X

FINSN=X

Client Server

ACK=X+1ACK=X+1

ACK=Y+1ACK=Y+1

1

2

4

FINSN=Y

FINSN=Y

3

...

TCP Connection Termination

writecloseclose returns

Data queued By TCP

App. Reads queued dataand FIN

close


FINSN=X

FINSN=X

Client Server

ACK=X+1ACK=X+1

ACK=Y+1ACK=Y+1

1

2

4

FINSN=Y

FINSN=Y

3

...

TCP Connection Termination close w/ SO_LINGER

writeclose

close returns

Data queued By TCP


close


FINSN=X

FINSN=X

Client Server

ACK=X+1ACK=X+1

ACK=Y+1ACK=Y+1

1

2

4

FINSN=Y

FINSN=Y

3

...

TCP Connection Termination w/ shutdown

writeshutdown WRread blocks

read returns 0

Data queued By TCP


close


SO_RCVBUF and SO_SNDBUF

Integer values options - change the receive and send buffer sizes.

Can be used with STREAM and DGRAM sockets. With TCP, this option effects the window size used for

flow control - must be established before connection is made.


SO_REUSEADDR

Boolean option: enables binding to an address (port) that is already in use.

Used by servers that are transient - allows binding a passive socket to a port currently in use (with active sockets) by other processes.

Can be used to establish separate servers for the same service on different interfaces (or different IP addresses on the same interface).

Virtual Web Servers can work this way.


IP Options (IPv4)

IP_HDRINCL: used on raw IP sockets when we want to build the IP header ourselves.

IP_TOS: allows us to set the “Type-of-service” field in an IP header.

IP_TTL: allows us to set the “Time-to-live” field in an IP header.


TCP socket options

TCP_KEEPALIVE: set the idle time used when SO_KEEPALIVE is enabled.

TCP_MAXSEG: set the maximum segment size sent by a TCP socket.

TCP_NODELAY: can disable TCP’s Nagle algorithm that delays sending small packets if there is unACK’d data pending.

TCP_NODELAY also disables delayed ACKS (TCP ACKs are cumulative).

tdc561 network programming

Documents

class d ip address

requiredthe group

requireda host

mapping ip multicast

address classeshistorical

address evolutionclass

leading bits of addressleading

asn ma group