Multicast over TCP/IP HOWTO
Juan-Mariano de Goyeneche <jmseyas@dit.upm.es>
v1.0, 20 March 1998
This HOWTO tries to cover most aspects related to multicast over
TCP/IP networks. So, a lot of information within it is not Linux-spe-
cific (just in case you don't use GNU/Linux... yet). Multicast is cur-
rently an active area of research and, at the time of writing, many of
the "standards" are merely drafts. Keep it in mind while reading the
lines that follow.
______________________________________________________________________
Table of Contents
1. Introduction.
1.1 What is Multicast.
1.2 The problem with Unicast.
2. Multicast Explained.
2.1 Multicast addresses.
2.2 Levels of conformance.
2.3 Sending Multicast Datagrams.
2.3.1 TTL.
2.3.2 Loopback.
2.3.3 Interface selection.
2.4 Receiving Multicast Datagrams.
2.4.1 Joining a Multicast Group.
2.4.2 Leaving a Multicast Group.
2.4.3 Mapping of IP Multicast Addresses to Ethernet/FDDI addresses.
3. Kernel requirements and configuration.
4. The MBone.
5. Multicast applications.
6. Multicast programming.
6.1 IP_MULTICAST_LOOP.
6.2 IP_MULTICAST_TTL.
6.3 IP_MULTICAST_IF.
6.4 IP_ADD_MEMBERSHIP.
6.5 IP_DROP_MEMBERSHIP.
7. The internals.
7.1 IGMP.
7.1.1 IGMP version 1.
7.1.2 IGMP version 2.
7.2 Kernel corner.
8. Routing Policies and Forwarding Techniques.
9. Multicast Transport Protocols.
10. References.
10.1 RFCs.
10.2 Internet Drafts.
10.3 Web pages.
10.4 Books.
11. Copyright and Disclaimer.
12. Acknowledgements.
______________________________________________________________________
1. Introduction.
I'll try to give here the most wide range, up to date and accurate
information related to multicasting over TCP/IP networks that I can.
Any feedback is very welcome. If you find any mistakes in this
document, have any comments about its contents or an update or
addition, please send them to me at the address listed at the top of
this howto.
1.1. What is Multicast.
Multicast is... a need. Well, at least in some scenarios. If you have
information (a lot of information, usually) that should be transmitted
to various (but usually not all) hosts over an internet, then
Multicast is the answer. One common situation in which it is used is
when distributing real time audio and video to the set of hosts which
have joined a distributed conference.
Multicast is much like radio or TV in the sense that only those who
have tuned their receivers (by selecting a particular frequency they
are interested on) receive the information. That is: you hear the
channel you are interested in, but not the others.
1.2. The problem with Unicast.
Unicast is anything that is not broadcast nor multicast. All right,
the definition is not very bright... When you send a packet and there
is only one sender process -yours- and one recipient process (the one
you are sending the packet to), then this is unicast. TCP is, by its
own nature, unicast oriented. UDP supports a lot more paradigms, but
if you are sending UDP packets and there is only one precess supposed
to receive them, this is unicast too.
For years unicast transmissions proved to be enough for the Internet.
It was not until 1993 when the first implementation of multicast saw
the light in the 4.4 BSD release. It seems nobody needed it until
then. Which were those new problems that multicast addressed?
Needless to say that the Internet has changed a lot since the "early
days". Particularly, the appearance of the Web strongly transformed
the situation: people didn't just want connections to remote hosts,
mail and FTP. First they wanted to see the pictures people placed in
their home pages, but later they also wanted to see and hear that
people.
With today's technology it is possible to afford the "cost" of making
a unicast connection with everyone who wants to see your web page.
However, if you are to send audio and video, which needs a huge amount
of bandwidth compared with web applications, you have -you had, until
multicast came into scene- two options: to establish a separate
unicast connection with each of the recipients, or to use broadcast.
The first solution is not affordable: if we said that a single
connection sending audio/video consumes a huge bandwidth, imagine
having to establish hundreds or, may be, thousands of those
connections. Both the sending computer and your network would
collapse.
Broadcast seems to be a solution, but it's not certainly the solution.
If you want all the hosts in your LAN to attend the conference, you
may use broadcast. Packets will be sent only once and every host will
receive them as they are sent to the broadcast address. The problem is
that perhaps only a few of the hosts and not all are interested in
those packets. Furthermore: perhaps some hosts are really interested
in your conference, but they are outside of your LAN, a few routers
away. And you know that broadcast works fine inside a LAN, but
problems arise when you want broadcast packets to be routed across
different LANs.
The best solution seems to be one in which you send packets to a
certain special address (a certain frequency in radio/TV
transmissions). Then, all hosts which have decided to join the
conference will be aware of packets with that destination address,
read them when they traverse the network, and pass them to the IP
layer to be demultiplexed. This is similar to broadcasting in that you
send only one broadcast packet and all the hosts in the network
recognize and read it; it differs, however, in that not all multicast
packets are read and processed, but only those that were previously
registered in the kernel as being "of interest".
Those special packets are routed at kernel level like any packet
because they are IP packets. The only difference might reside in the
routing algorithm which tells the kernel where to route or not to
route them.
2. Multicast Explained.
2.1. Multicast addresses.
As you probably know, the range of IP addresses is divided into
"classes" based on the high order bits of a 32 bits IP address:
______________________________________________________________________
Bit --> 0 31 Address Range:
+-+----------------------------+
|0| Class A Address | 0.0.0.0 - 127.255.255.255
+-+----------------------------+
+-+-+--------------------------+
|1 0| Class B Address | 128.0.0.0 - 191.255.255.255
+-+-+--------------------------+
+-+-+-+------------------------+
|1 1 0| Class C Address | 192.0.0.0 - 223.255.255.255
+-+-+-+------------------------+
+-+-+-+-+----------------------+
|1 1 1 0| MULTICAST Address | 224.0.0.0 - 239.255.255.255
+-+-+-+-+----------------------+
+-+-+-+-+-+--------------------+
|1 1 1 1 0| Reserved | 240.0.0.0 - 247.255.255.255
+-+-+-+-+-+--------------------+
______________________________________________________________________
The one which concerns us is the "Class D Address". Every IP datagram
whose destination address starts with "1110" is an IP Multicast
datagram.
The remaining 28 bits identify the multicast "group" the datagram is
sent to. Following with the previous analogy, you have to tune your
radio to hear a program that is transmitted at some specific
frequency, in the same way you have to "tune" your kernel to receive
packets sent to an specific multicast group. When you do that, it's
said that the host has joined that group in the interface you
specified. More on this later.
There are some special multicast groups, say "well known multicast
groups", you should not use in your particular applications due the
special purpose they are destined to:
o 224.0.0.1 is the all-hosts group. If you ping that group, all
multicast capable hosts on the network should answer, as every
multicast capable host must join that group at start-up on all it's
multicast capable interfaces.
o 224.0.0.2 is the all-routers group. All multicast routers must join
that group on all it's multicast capable interfaces.
o 224.0.0.4 is the all DVMRP routers, 224.0.0.5 the all OSPF routers,
224.0.013 the all PIM routers, etc.
All this special multicast groups are regularly published in the
"Assigned Numbers" RFC.
In any case, range 224.0.0.0 through 224.0.0.255 is reserved for local
purposes (as administrative and maintenance tasks) and datagrams
destined to them are never forwarded by multicast routers. Similarly,
the range 239.0.0.0 to 239.255.255.255 has been reserved for
"administrative scoping" (see section 2.3.1 for information on
administrative scoping).
2.2. Levels of conformance.
Hosts can be in three different levels of conformance with the
Multicast specification, according to the requirements they meet.
Level 0 is the "no support for IP Multicasting" level. Lots of hosts
and routers in the Internet are in this state, as multicast support is
not mandatory in IPv4 (it is, however, in IPv6). Not too much
explanation is needed here: hosts in this level can neither send nor
receive multicast packets. They must ignore the ones sent by other
multicast capable hosts.
Level 1 is the "support for sending but not receiving multicast IP
datagrams" level. Thus, note that it is not necessary to join a
multicast group to be able to send datagrams to it. Very few additions
are needed in the IP module to make a "Level 0" host "Level
1-compliant", as shown in section 2.3.
Level 2 is the "full support for IP multicasting" level. Level 2 hosts
must be able to both send and receive multicast traffic. They must
know the way to join and leave multicast groups and to propagate this
information to multicast routers. Thus, they must include an Internet
Group Management Protocol (IGMP) implementation in their TCP/IP stack.
2.3. Sending Multicast Datagrams.
By now, it should be obvious that multicast traffic is handled at the
transport layer with UDP, as TCP provides point-to-point connections,
not feasibles for multicast traffic. (Heavy research is taking place
to define and implement new multicast-oriented transport protocols.
See section ``Multicast Transport Protocols'' for details).
In principle, an application just needs to open a UDP socket and fill
with a class D multicast address the destination address where it
wants to send data to. However, there are some operations that a
sending process must be able to control.
2.3.1. TTL.
The TTL (Time To Live) field in the IP header has a double
significance in multicast. As always, it controls the live time of the
datagram to avoid it being looped forever due to routing errors.
Routers decrement the TTL of every datagram as it traverses from one
network to another and when its value reaches 0 the packet is dropped.
The TTL in IPv4 multicasting has also the meaning of "threshold". Its
use becomes evident with an example: suppose you set a long, bandwidth
consuming, video conference between all the hosts belonging to your
department. You want that huge amount of traffic to remain in your
LAN. Perhaps your department is big enough to have various LANs. In
that case you want those hosts belonging to each of your LANs to
attend the conference, but in any case you want to collapse the entire
Internet with your multicast traffic. There is a need to limit how
"long" multicast traffic will expand across routers. That's what the
TTL is used for. Routers have a TTL threshold assigned to each of its
interfaces, and only datagrams with a TTL greater than the interface's
threshold are forwarded. Note that when a datagram traverses a router
with a certain threshold assigned, the datagram's TTL is not
decremented by the value of the threshold. Only a comparison is made.
(As before, the TTL is decremented by 1 each time a datagram passes
across a router).
A list of TTL thresholds and their associated scope follows:
______________________________________________________________________
TTL Scope
----------------------------------------------------------------------
0 Restricted to the same host. Won't be output by any interface.
1 Restricted to the same subnet. Won't be forwarded by a router.
<32 Restricted to the same site, organization or department.
<64 Restricted to the same region.
<128 Restricted to the same continent.
<255 Unrestricted in scope. Global.
______________________________________________________________________
Nobody knows what "site" or "region" mean exactly. It is up to the
administrators to decide what this limits apply to.
The TTL-trick is not always flexible enough for all needs, specially
when dealing with overlapping regions or trying to establish
geographic, topologic and bandwidth limits simultaneously. To solve
this problems, administratively scoped IPv4 multicast regions were
established in 1994. (see D. Meyer's "Administratively Scoped IP
Multicast" Internet draft). It does scoping based on multicast
addresses rather than on TTLs. The range 239.0.0.0 to 239.255.255.255
is reserved for this administrative scoping.
2.3.2. Loopback.
When the sending host is Level 2 conformant and is also a member of
the group datagrams are being sent to, a copy is looped back by
default. This does not mean that the interface card reads its own
transmission, recognizes it as belonging to a group the interface
belongs to, and reads it from the network. On the contrary, is the IP
layer which, by default, recognizes the to-be-sent datagram and copies
and queues it on the IP input queue before sending it.
This feature is desirable in some cases, but not in others. So the
sending process can turn it on and off at wish.
2.3.3. Interface selection.
Hosts attached to more than one network should provide a way for
applications to decide which network interface will be used to output
the transmissions. If not specified, the kernel chooses a default one
based on system administrator's configuration.
2.4. Receiving Multicast Datagrams.
2.4.1. Joining a Multicast Group.
Broadcast is (in comparison) easier to implement than multicast. It
doesn't require processes to give the kernel some rules regarding what
to do with broadcast packets. The kernel just knows what to do: read
and deliver all of them to the proper applications.
With multicast, however, it is necessary to advise the kernel which
multicast groups we are interested in. That is, we have to ask the
kernel to "join" those multicast groups. Depending on the underlying
hardware, multicast datagrams are filtered by the hardware or by the
IP layer (and, in some cases, by both). Only those with a destination
group previously registered via a join are accepted.
Essentially, when we join a group we are telling the kernel: "OK. I
know that, by default, you ignore multicast datagrams, but remember
that I am interested in this multicast group. So, do read and deliver
(to any process interested in them, not only to me) any datagram that
you see in this network interface with this multicast group in its
destination field".
Some considerations: first, note that you don't just join a group.
You join a group on a particular network interface. Of course, it is
possible to join the same group on more than one interface. If you
don't specify a concrete interface, then the kernel will choose it
based on its routing tables when datagrams are to be sent. It is also
possible that more than one process joins the same multicast group on
the same interface. They will all receive the datagrams sent to that
group via that interface.
As said before, any multicast-capable hosts join the all-hosts group
at start-up , so "pinging" 224.0.0.1 returns all hosts in the network
that have multicast enabled.
Finally, consider that for a process to receive multicast datagrams it
has to ask the kernel to join the group and bind the port those
datagrams were being sent to. The UDP layer uses both the destination
address and port to demultiplex the packets and decide which socket(s)
deliver them to.
2.4.2. Leaving a Multicast Group.
When a process is no longer interested in a multicast group, it
informs the kernel that it wants to leave that group. It is important
to understand that this doesn't mean that the kernel will no longer
accept multicast datagrams destined to that multicast group. It will
still do so if there are more precesses who issued a "multicast join"
petition for that group and are still interested. In that case the
host remains member of the group, until all the processes decide to
leave the group.
Even more: if you leave the group, but remain bound to the port you
were receiving the multicast traffic on, and there are more processes
that joined the group, you will still receive the multicast
transmissions.
The idea is that joining a multicast group only tells the IP and data
link layer (which in some cases explicitly tells the hardware) to
accept multicast datagrams destined to that group. It is not a per-
process membership, but a per-host membership.
2.4.3. Mapping of IP Multicast Addresses to Ethernet/FDDI addresses.
Both Ethernet and FDDI frames have a 48 bit destination address field.
In order to avoid a kind of multicast ARP to map multicast IP
addresses to ethernet/FDDI ones, the IANA reserved a range of
addresses for multicast: every ethernet/FDDI frame with its
destination in the range 01-00-5e-00-00-00 to 01-00-5e-ff-ff-ff (hex)
contains data for a multicast group. The prefix 01-00-5e identifies
the frame as multicast, the next bit is always 0 and so only 23 bits
are left to the multicast address. As IP multicast groups are 28 bits
long, the mapping can not be one-to-one. Only the 23 least significant
bits of the IP multicast group are placed in the frame. The remaining
5 high-order bits are ignored, resulting in 32 different multicast
groups being mapped to the same ethernet/FDDI address. This means that
the ethernet layer acts as an imperfect filter, and the IP layer will
have to decide whether to accept the datagrams the data-link layer
passed to it. The IP layer acts as a definitive perfect filter.
Full details on IP Multicasting over FDDI are given in RFC 1390:
"Transmission of IP and ARP over FDDI Networks". For more information
on mapping IP Multicast addresses to ethernet ones, you may consult
draft-ietf-mboned-intro-multicast-03.txt: "Introduction to IP
Multicast Routing".
If you are interested in IP Multicasting over Token-Ring Local Area
Networks, see RFC 1469 for details.
3. Kernel requirements and configuration.
Linux is, of course (you doubted it?), full Level-2 Multicast-
Compliant. It meets all requirements to send, receive and act as a
router (mrouter) for multicast datagrams.
If you want just to send and receive, you must say yes to "IP:
multicasting" when configuring your kernel. If you also want your
Linux box to act as a multicast router (mrouter) you also need to
enable multicast routing in the kernel by selecting "IP:
forwarding/gatewaying", "IP: multicast routing" and "IP: tunneling",
the latter because new versions of mrouted relay on IP tunneling to
send multicast datagrams encapsulated into unicast ones. This is
necessary when establishing tunnels between multicast hosts separated
by unicast-only networks and routers. (The mrouted is a daemon that
implements the multicast routing algorithm -the routing policy- and
instructs the kernel on how to route multicast datagrams).
Some kernel versions label multicast routing as "EXPERIMENTAL", so you
should enable "Prompt for development and/or incomplete code/drivers"
in the "Code maturity level options" section.
If, when running the mrouted, traffic generated in the same network
your Linux box is connected to is correctly forwarded to the other
network, but you can't see the other's network traffic on your local
network, check whether you are receiving ICMP protocol error messages.
Almost sure you forgot to turn on IP tunneling in your Linux router.
It's a kind of stupid error when you know it but, believe me, its
quite time-consuming when you don't, and there is no apparent reason
that explains what is going wrong. A sniffer proves to be quite useful
in these situations!
(You can see more on multicast routing on section ``Routing Policies
and Forwarding Techniques''; mrouted and tunnels are also explained in
sections ``The MBone'' and ``Multicast applications'').
Once you have compiled and installed your new kernel, you should
provide a default route for multicast traffic. The goal is to add a
route to the network 224.0.0.0.
The problem most people seem to face in this stage of the
configuration is with the value of the mask to supply. If you have
read Terry Dawson's excellent NET-3-HOWTO, it should not be difficult
to guess the correct value, though. As explained there, the netmask is
a 32 bit number filled with all-1s in the network part of your IP
address, and with all-0s in the host part. Recall from section 2.1
that a class D multicast address has no netwok/host sections. Instead
it has a 28-bit group identifier and a 4-bit class D identifier. Well,
this 4 bits are the network part and the remaining 28 the host part.
So the netmask needed is 11110000000000000000000000000000 or, easier
to read: 240.0.0.0. Then, the full command should be:
route add 224.0.0.0 netmask 240.0.0.0 dev eth0
Depending on how old your route program is, you might need to add the
-net flag after the add.
Here we supposed that eth0 was multicast-capable and that, when not
otherwise specified, we wanted multicast traffic to be output there.
If this is not your case, change the dev parameter as appropriate.
The /proc filesystem proves here to be useful once again: you can
check /proc/net/igmp to see the groups your host is currently
subscribed to.
4. The MBone.
Using a new technology usually carries some advantages and
disadvantages. The advantages of multicast are -I think- clear. The
main disadvantage is that hundreds of hosts and, specially, routers
don't support it yet. As a consequence, people who started working on
multicast, bought new equipment, modified their operating systems, and
built multicast islands in their local places. Then they discovered
that it was difficult to communicate with people doing similar things
because if only one of the routers between them didn't support
multicast there was nothing to do...
The solution was clear: they decided to build a virtual multicast
network in the top of the Internet. That is: sites with multicast
routers between them could communicate directly. But sites joined
across unicast routers would send their island's multicast traffic
encapsulated in unicast packets to other multicast islands. Routers in
the middle would not have problems, as they would be dealing with
unicast traffic. Finally, in the receiving site, traffic would be de-
encapsulated, and sent to the island in the original multicast way.
Two ends converting from multicast to unicast, and then again to
multicast define what is called a multicast tunnel.
The MBone or Multicast Backbone is that virtual multicast network
based on multicast islands connected by multicast tunnels.
Several activities take place in the MBone daily, but it deserves to
be remarked the profusion of tele-conferences with real time audio and
video taking place across the whole Internet. As an example, it was
recently transmitted (live) the talk Linus Torvalds gave to the
Silicon Valley Linux Users Group.
For more information on the MBone, see:
<http://www.mediadesign.co.at/newmedia/more/mbone-faq.html>
5. Multicast applications.
Most people dealing with multicast, sooner or later decide to connect
to the MBone, and then they usually need an mrouted. You'll also need
it if you don't have a multicast-capable router and you want multicast
traffic generated in one of your subnets to be "heard" on another.
mrouted does circunvect the problem of sending multicast traffic
across unicast routers -it encapsulates multicast datagrams into
unicast ones (IP into IP)- but this is not the only feature it
provides. Most important, it instructs the kernel on how to route (or
not-to-route) multicast datagrams based on their source and
destination. So, even having a multicast capable router, mrouted can
be used to tell it what to do with the datagrams (note I said what,
and not how; mrouted says "forward this to the network connected to
that interface", but actual forwarding is performed by the kernel).
This distinction between actual-forwarding and the algorithm that
decides who and how to forward is very useful as it allows to write
forwarding code only once and place it into the kernel. Forwarding
algorithms and policies are then implemented in user space daemons, so
it is very easy to change from one policy to another without the need
of kernel re-compilation.
You can get a version of mrouted ported to Linux from:
<ftp://www.video.ja.net/mice/mrouted/Linux/>. This site is mirrored
all across the world. Be sure to read the
<ftp://www.video.ja.net/mice/README.mirrors> file to choose the one
nearest you.
Next, we'll focus specially on multicast applications written to
connect to the MBone, which have been ported to Linux. The list is
picked up from Michael Esler's "Linux Multicast Information" page
<http://www.cs.virginia.edu/~mke2e/multicast/>. I recommend you that
page for lots of information and resources on multicast and Linux.
Audio Conferencing
o NeVoT - Network Voice Terminal <http://www.fokus.gmd.de/step/nevot>
o RAT - UCL Robust-Audio Tool <http://www-mice.cs.ucl.ac.uk/mice/rat>
o vat - LBL visual audio tool <http://www-nrg.ee.lbl.gov/vat/>
Video Conferencing
o ivs - Inria video conferencing system
<http://www.inria.fr/rodeo/ivs.html>
o nv - Network video tool <ftp://ftp.parc.xerox.com/pub/net-
research/>
o nv w/ Meteor - Release of nv w/ support for the Matrox Meteor (UVa)
<ftp://ftp.cs.virginia.edu/pub/gwtts/Linux/nv-meteor.tar.gz>
o vic - LBL video conferencing tool <http://www-nrg.ee.lbl.gov/vic/>
o vic w/ Meteor - Release of vic w/ support for the Matrox Meteor
(UVa)
<ftp://ftp.cs.virginia.edu/pub/gwtts/Linux/vic2.7a38-meteor.tar.gz>
Other Utilities
o mmphone Multimedia phone service
<http://www.eit.com/software/mmphone/phoneform.html>
o wb - LBL shared white board <http://www-nrg.ee.lbl.gov/wb/>
o webcast - Reliable multicast application for linking Mosaic
browsers
<http://www.ncsa.uiuc.edu/SDG/Software/XMosaic/CCI/webcast.html>
Session Tools
I placed session tools later because I think they deserve some
explanation. When a conference takes places, several multicast groups
and ports are assigned to each service you want for your conference
(audio, video, shared white-boards, etc...) Announces of the
conferences that will take place, along with information on multicast
groups, ports and programs that will be used (vic, vat, ...) are
periodically multicasted to the MBone. Session tools "hear" this
information and present you in an easy way which conferences are
taking (or will take) place, so you can decide which interest you.
Also, they facilitate the task of joining a session. Instead of
launching each program that will be used and telling which multicast
group/port to join, you usually just need to click and the session
tool launches the proper programs suppling them all information needed
to join the conference. Session tools usually let you announce your
own conferences on the MBone.
o gwTTS - University of Virginia tele-tutoring system
<http://www.cs.Virginia.EDU/~gwtts>
o isc - Integrated session controller
<http://www.fokus.gmd.de/step/isc>
o mmcc - Multimedia conference control
<ftp://ftp.isi.edu/confctrl/mmcc>
o sd - LBL session directory tool
<ftp://ftp.ee.lbl.gov/conferencing/sd>
o sd-snoop - Tenet Group session directory snoop utility
<ftp://tenet.berkeley.edu/pub/software>
o sdr - UCL's next generation session directory
<ftp://cs.ucl.ac.uk/mice/sdr>
6. Multicast programming.
Multicast programming... or writing your own multicast applications.
Several extensions to the programming API are needed in order to
support multicast. All of them are handled via two system calls:
setsockopt() (used to pass information to the kernel) and getsockopt()
(to retrieve information regarded multicast behavior). This does not
mean that 2 new system calls were added to support multicast. The pair
setsockopt()/getsockopt() has been there for years. Since 4.2 BSD at
least. The addition consists on a new set of options (multicast
options) that are passed to these system calls, that the kernel must
understand.
The following are the setsockopt()/getsockopt() function prototypes:
int getsockopt(int s, int level, int optname, void* optval, int* optlen);
int setsockopt(int s, int level, int optname, const void* optval, int optlen);
The first parameter, s, is the socket the system call applies to. For
multicasting, it must be a socket of the family AF_INET and its type
may be either SOCK_DGRAM or SOCK_RAW. The most common use is with
SOCK_DGRAM sockets, but if you plan to write a routing daemon or
modify some existing one, you will probably need to use SOCK_RAW ones.
The second one, level, identifies the layer that is to handle the
option, message or query, whatever you want to call it. So, SOL_SOCKET
is for the socket layer, IPPROTO_IP for the IP layer, etc... For
multicast programming, level will always be IPPROTO_IP.
optname identifies the option we are setting/getting. Its value
(either supplied by the program or returned by the kernel) is optval.
The optnames involved in multicast programming are the following:
______________________________________________________________________
setsockopt() getsockopt()
IP_MULTICAST_LOOP yes yes
IP_MULTICAST_TTL yes yes
IP_MULTICAST_IF yes yes
IP_ADD_MEMBERSHIP yes no
IP_DROP_MEMBERSHIP yes no
______________________________________________________________________
optlen carries the size of the data structure optval points to. Note
that in getsockopt() it is a value-result rather than a value: the
kernel writes the value of optname in the buffer pointed by optval and
informs us of that value's size via optlen.
Both setsockopt() and getsockopt() return 0 on success and -1 on
error.
6.1. IP_MULTICAST_LOOP.
You have to decide, as the application writer, whether you want the
data you send to be looped back to your host or not. If you plan to
have more than one process or user "listening", loopback must be
enabled. On the other hand, if you are sending the images your video
camera is producing, you probably don't want loopback, even if you
want to see yourself on the screen. In that latter case, your
application will probably receive the images from a device attached to
the computer and send them to the socket. As the application already
"has" that data, it is improbable it wants to receive it again on the
socket. Loopback is by default enabled.
Regard that optval is a pointer. You can't write:
setsockopt(socket, IPPROTO_IP, IP_MULTICAST_LOOP, 0, 1);
to disable loopback. Instead write:
u_char loop;
setsockopt(socket, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop));
and set loop to 1 to enable loopback or 0 to disable it.
To know whether a socket is currently looping-back or not use
something like:
u_char loop;
int size;
getsockopt(socket, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, &size)
6.2. IP_MULTICAST_TTL.
If not otherwise specified, multicast datagrams are sent with a
default value of 1, to prevent them to be forwarded beyond the local
network. To change the TTL to the value you desire (from 0 to 255),
put that value into a variable (here I name it "ttl") and write
somewhere in your program:
u_char ttl;
setsockopt(socket, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl));
The behavior with getsockopt() is similar to the one seen on
IP_MULTICAST_LOOP.
6.3. IP_MULTICAST_IF.
Usually, the system administrator specifies the default interface
multicast datagrams should be sent from. The programmer can override
this and choose a concrete outgoing interface for a given socket with
this option.
struct in_addr interface_addr;
setsockopt (socket, IPPROTO_IP, IP_MULTICAST_IF, &interface_addr, sizeof(interface_addr));
>From now on, all multicast traffic generated in this socket will be
output from the interface chosen. To revert to the original behavior
and let the kernel choose the outgoing interface based on the system
administrator's configuration, it is enough to call setsockopt() with
this same option and INADDR_ANY in the interface field.
In determining or selecting outgoing interfaces, the following ioctls
might be useful: SIOCGIFADDR (to get an interface's address),
SIOCGIFCONF (to get the list of all the interfaces) and SIOCGIFFLAGS
(to get an interface's flags and, thus, determine whether the
interface is multicast capable or not -the IFF_MULTICAST flag-).
If the host has more than one interface and the IP_MULTICAST_IF option
is not set, multicast transmissions are sent from the default
interface, although the remainding interfaces might be used for
multicast forwarding if the host is acting as a multicast router.
6.4. IP_ADD_MEMBERSHIP.
Recall that you need to tell the kernel which multicast groups you are
interested in. If no process is interested in a group, packets
destined to it that arrive to the host are discarded. In order to
inform the kernel of your interests and, thus, become a member of that
group, you should first fill a ip_mreq structure which is passed later
to the kernel in the optval field of the setsockopt() system call.
The ip_mreq structure (taken from /usr/include/linux/in.h) has the
following members:
struct ip_mreq
{
struct in_addr imr_multiaddr; /* IP multicast address of group */
struct in_addr imr_interface; /* local IP address of interface */
};
(Note: the "physical" definition of the structure is in the file above
specified. Nonetheless, you should not include <linux/in.h> if you
want your code to be portable. Instead, include <netinet/in.h> which,
in turn, includes <linux/in.h> itself).
The first member, imr_multiaddr, holds the group address you want to
join. Remember that memberships are also associated with interfaces,
not just groups. This is the reason you have to provide a value for
the second member: imr_interface. This way, if you are in a multihomed
host, you can join the same group in several interfaces. You can
always fill this last member with the wildcard address (INADDR_ANY)
and then the kernel will deal with the task of choosing the interface.
With this structure filled (say you defined it as: struct ip_mreq
mreq;) you just have to call setsockopt() this way:
setsockopt (socket, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
Notice that you can join several groups to the same socket, not just
one. The limit to this is IP_MAX_MEMBERSHIPS and, as of version
2.0.33, it has the value of 20.
6.5. IP_DROP_MEMBERSHIP.
The process is quite similar to joining a group:
struct ip_mreq mreq;
setsockopt (socket, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq));
where mreq is the same structure with the same data used when joining
the group. If the imr_interface member is filled with INADDR_ANY, the
first matching group is dropped.
If you have joined a lot of groups to the same socket, you don't need
to drop memberships in all of them in order to terminate. When you
close a socket, all memberships associated with it are dropped by the
kernel. The same occurs if the process that opened the socket is
killed.
Finally, keep in mind that a process dropping membership for a group
does not imply that the host will stop receiving datagrams for that
group. If another socket joined that group in that same interface
previously to this IP_DROP_MEMBERSHIP, the host will keep being a
member of that group.
Both ADD_MEMBERSHIP and DROP_MEMBERSHIP are nonblocking operations.
They should return immediately indicating either success or failure.
7. The internals.
This section's aim is to provide some information, not needed to reach
a basic understanding on how multicast works nor to be able to write
multicast programs, but which is very interesting, gives some insight
on the underlying multicast protocols and implementations, and may be
useful to avoid common errors and misunderstandings.
7.1. IGMP.
When talking about IP_ADD_MEMBERSHIP and IP_DROP_MEMBERSHIP, we said
that the information provided by this "commands" was used by the
kernel to choose which multicast datagrams accept or discard. This is
true, but it is not all the truth. Such a simplification would imply
that multicast datagrams for all multicast groups around the world
would be received by our host, and then it would check the memberships
issued by processes running on it to decide whether to pass the
traffic to them or to throw it out. As you can imagine, this is a
complete bandwidth waste.
What actually happens is that hosts instruct their routers telling
them which multicast groups they are interested in; then, those
routers tell their up-stream routers they want to receive that
traffic, and so on. Algorithms employed for making the decision of
when to ask for a group's traffic or saying that it is not desired
anymore, vary a lot. There's something, however, that never changes:
how this information is transmitted. IGMP is used for that. It stands
for Internet Group Management Protocol. It is a new protocol, similar
in many aspects to ICMP, with a protocol number of 2, whose messages
are carried in IP datagrams, and which all level 2-compliant host are
required to implement.
As said before, it is used both by hosts giving membership information
to its routers, and by routers to communicate between themselves. In
the following I'll cover only the hosts-routers relationships, mainly
because I was unable to find information describing router to router
communication other than the mrouted source code (rfc 1075 describing
the Distance Vector Multicast Routing Protocol is now obsoleted, and
mrouted implements a modified DVMRP not yet documented).
IGMP version 0 is specified in RFC-988 which is now obsoleted. Almost
no one uses version 0 now.
IGMP version 1 is described in RFC-1112 and, although it is updated by
RFC-2236 (IGMP version 2) it is in wide use still. The Linux kernel
implements the full IGMP version 1 and parts of version 2
requirements, but not all.
Now I'll try to give an informal description of the protocol. You can
check RFC-2236 for an in-proof formal description, with lots of state
diagrams and time-out boundaries.
All IGMP messages have the following structure:
______________________________________________________________________
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Max Resp Time | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
______________________________________________________________________
IGMP version 1 (hereinafter IGMPv1) labels the "Max Resp Time" as
"Unused", zeroes it when sent, and ignores it when received. Also, it
brakes the "Type" field in two 4-bits wide fields: "Version" and
"Type". As IGMPv1 identifies a "Membership Query" message as 0x11
(version 1, type 1) and IGMPv2 as 0x11 too, the 8 bits have the same
effective interpretation.
I think it is more instructive to give first the IGMPv1 description
and next point out the IGMPv2 additions, as they are mainly that,
additions.
For the following discussions it is important to remember that
multicast routers receive all IP multicast datagrams.
7.1.1. IGMP version 1.
Routers periodically send IGMP Host Membership Queries to the all-
hosts group (224.0.0.1) with a TTL of 1 (once every minute or two).
All multicast-capable hosts hear them, but don't answer immediately to
avoid an IGMP Host Membership Report storm. Instead, they start a
random delay timer for each group they belong to on the interface they
received the query.
Sooner or later, the timer expires in one of the hosts, and it sends
an IGMP Host Membership Report (also with TTL 1) to the multicast
address of the group being reported. As it is sent to the group, all
hosts that joined the group -and which are currently waiting for their
own timer to expire- receive it, too. Then, they stop their timers and
don't generate any other report. Just one is generated -by the host
that chose the smaller timeout-, and that is enough for the router. It
only needs to know that there are members for that group in the
subnet, not how many nor which.
When no reports are received for a given group after a certain number
of queries, the router assumes that no members are left, and thus it
doesn't have to forward traffic for that group on that subnet. Note
that in IGMPv1 there are no "Leave Group messages".
When a host joins a new group, the kernel sends a report for that
group, so that the respective process needs not to wait a minute or
two until a new membership query is received. As you can see this IGMP
packet is generated by the kernel as a response to the
IP_ADD_MEMBERSHIP command, seen in section ``IP_ADD_MEMBERSHIP''.
Note the emphasis in the adjective "new": if a process issues an
IP_ADD_MEMBERSHIP command for a group the host is already a member of,
no IGMP packets are sent as we must already be receiving traffic for
that group; instead, a counter for that group's use is incremented.
IP_DROP_MEMBERSHIP generates no datagrams in IGMPv1.
Host Membership Queries are identified by Type 0x11, and Host
Membership Reports by Type 0x12.
No reports are sent for the all-hosts group. Membership in this group
is permanent.
7.1.2. IGMP version 2.
One important addition to the above is the inclusion of a Leave Group
message (Type 0x17). The reason is to reduce the bandwidth waste
between the time the last host in the subnet drops membership and the
time the router times-out for its queries and decides there are no
more members present for that group (leave latency). Leave Group
messages should be addressed to the all-routers group (224.0.0.2)
rather than to the group being left, as that information is of no use
for other members (kernel versions up to 2.0.33 send them to the
group; although it does no harm to the hosts, it's a waste of time as
they have to process them, but don't gain useful information). There
are certain subtle details regarding when and when-not to send Leave
Messages; if interested, see the RFC.
When an IGMPv2 router receives a Leave Message for a group, it sends
Group-Specific Queries to the group being left. This is another
addition. IGMPv1 has no group-specific queries. All queries are sent
to the all-hosts group. The Type in the IGMP header does not change
(0x11, as before), but the "Group Address" is filled with the address
of the multicast group being left.
The "Max Resp Time" field, which was set to 0 in transmission and
ignored on reception in IGMPv1, is meaningful only in "Membership
Query" messages. It gives the maximum time allowed before sending a
report in units of 1/10 second. It is used as a tune mechanism.
IGMPv2 adds another message type: 0x16. It is a "Version 2 Membership
Report" sent by IGMPv2 hosts if they detect an IGMPv2 router is
present (an IGMPv2 host knows an IGMPv1 router is present when it
receives a query with the "Max Response" field set to 0).
When more than one router claims to act as querier, IGMPv2 provides a
mechanism to avoid "discussions": the router with the lowest IP
address is designed to be querier. The other routers keep timeouts. If
the router with lower IP address crashes or is shutdown, the decision
of who will be the querier is taken again after the timers expire.
7.2. Kernel corner.
This sub-section gives some start-points to study the multicast
implementation of the Linux kernel. It does not explain that
implementation. It just says where to find things.
The study was carried over version 2.0.32, so it could be a bit
outdated by the time you read it (network code seems to have changed A
LOT in 2.1.x releases, for instance).
Multicast code in the Linux kernel is always surrounded by #ifdef
CONFIG_IP_MULTICAST / #endif pairs, so that you can include/ exclude
it from your kernel based on your needs (this inclusion/exclusion is
done at compile time, as you probably know if reading that section...
#ifdefs are handled by the preprocessor. The decision is made based
in what you selected when doing either a make config, make menuconfig
or make xconfig).
You might want multicast features, but if your Linux box is not going
to act as a multicast router you will probably not want multicast
router features included in your new kernel. For this you have the
multicast routing code surrounded by #ifdef CONFIG_IP_MROUTE / #endif
pairs.
Kernel sources are usually placed in /usr/src/linux. However, the
place may change so, both for accuracy and brevity, I will refer to
the root directory of the kernel sources as just LINUX. Then,
something like LINUX/net/ipv4/udp.c should be the same as
/usr/src/linux/net/ipv4/udp.c if you unpacked the kernel sources in
the /usr/src/linux directory.
All multicast interfaces with user programs shown in the section
devoted to multicast programming were driven across the setsockopt()/
getsockopt() system calls. Both of them are implemented by means of
functions that make some tests to verify the parameters passed to them
and which, in turn, call another function that makes some additional
tests, demultiplexes the call based on the level parameter to either
system call, and then calls another function which... (if interested
in all this jumps, you can follow them in LINUX/net/socket.c
(functions sys_socketcall() and sys_setsockopt(),
LINUX/net/ipv4/af_inet.c (function inet_setsockopt()) and
LINUX/net/ipv4/ip_sockglue.c (function ip_setsockopt()) ).
The one which interests us is LINUX/net/ipv4/ip_sockglue.c. Here we
find ip_setsockopt() and ip_getsockopt() which are mainly a switch
(after some error checking) verifying each possible value for optname.
Along with unicast options, all multicast ones seen here are handled:
IP_MULTICAST_TTL, IP_MULTICAST_LOOP, IP_MULTICAST_IF,
IP_ADD_MEMBERSHIP and IP_DROP_MEMBERSHIP. Previously to the switch, a
test is made to determine whether the options are multicast router
specific, and if so, they are routed to the ip_mroute_setsockopt() and
ip_mroute_getsockopt() functions (file LINUX/net/ipv4/ipmr.c).
In LINUX/net/ipv4/af_inet.c we can see the default values we talked
about in previous sections (loopback enabled, TTL=1) provided when the
socket is created (taken from function inet_create() in this file):
______________________________________________________________________
#ifdef CONFIG_IP_MULTICAST
sk->ip_mc_loop=1;
sk->ip_mc_ttl=1;
*sk->ip_mc_name=0;
sk->ip_mc_list=NULL;
#endif
______________________________________________________________________
Also, the assertion of "closing a socket makes the kernel drop all
memberships this socket had" is corroborated by:
______________________________________________________________________
#ifdef CONFIG_IP_MULTICAST
/* Applications forget to leave groups before exiting */
ip_mc_drop_socket(sk);
#endif
______________________________________________________________________
taken from inet_release(), on the same file as before.
Device independent operations for the Link Layer are kept in
LINUX/net/core/dev_mcast.c.
Two important functions are still missing: the processing of input and
output multicast datagrams. As any other datagrams, incoming datagrams
are passed from the device drivers to the ip_rcv() function
(LINUX/net/ipv4/ip_input.c). In this function is where the perfect
filtering is applied to multicast packets that crossed the devices
layer (recall that lower layers only perform best-effort filtering and
is IP who 100% knows whether we are interested in that multicast group
or not). If the host is acting as a multicast router, this function
decides too whether the datagram should be forwarded and calls
ipmr_forward() appropriately. (ipmr_forward() is implemented in
LINUX/net/ipv4/ipmr.c).
Code in charge of out-putting packets is kept in
LINUX/net/ipv4/ip_output.c. Here is where the IP_MULTICAST_LOOP
option takes effect, as it is checked to see whether to loop back the
packets or not (function ip_queue_xmit()). Also the TTL of the
outgoing packet is selected based on whether it is a multicast or
unicast one. In the former case, the argument passed to the
IP_MULTICAST_TTL option is used (function (ip_build_xmit()).
While working with mrouted (a program which gives the kernel
information about how to route multicast datagrams), we detected that
all multicast packets originated on the local network were properly
routed..., except the ones from the Linux box that was acting as the
multicast router!! ip_input.c was working OK, but it seemed
ip_output.c wasn't. Reading the source code for the output functions,
we found that outgoing datagrams were not being passed to
ipmr_forward(), the function that had to decide whether they should be
routed or not. The packets were outputed to the local network but, as
network cards are usually unable to read their own transmissions,
those datagrams were never routed. We added the necessary code to the
ip_build_xmit() function and everything was OK again. (Having the
sources for your kernel is not a luxury or pedantry; it's a need!)
ipmr_forward() has been mentioned a couple of times. It is an
important function as it solves one important misunderstanding that
appears to be widely expanded. When routing multicast traffic, it is
not mrouted who makes the copies and sends them to the proper
recipients. mrouted receives all multicast traffic and, based on that
information, computes the multicast routing tables and tells the
kernel how to route: "datagrams for this group coming from that
interface should be forwarded to those interfaces". This information
is passed to the kernel by calls to setsockopt() on a raw socket
opened by the mrouted daemon (the protocol specified when the raw
socket was created must be IPPROTO_IGMP). This options are handled in
the ip_mroute_setsockopt() function from LINUX/net/ipv4/ipmr.c. The
first option (would be better to call them commands rather than
options) issued on that socket must be MRT_INIT. All other commands
are ignored (returning -EACCES) if MRT_INIT is not issued first. Only
one instance of mrouted can be running at the same time in the same
host. To keep track of this, when the first MRT_INIT is received, an
important variable, struct sock* mroute_socket, is pointed to the
socket MRT_INIT was received on. If mroute_socket is not null when
attending an MRT_INIT this means another mrouted is already running
and -EADDRINUSE is returned. All resting commands (MRT_DONE,
MRT_ADD_VIF, MRT_DEL_VIF, MRT_ADD_MFC, MRT_DEL_MFC and MRT_ASSERT)
return -EACCES if they come from a socket different than
mroute_socket.
As routed multicast datagrams can be received/sent across either
physical interfaces or tunnels, a common abstraction for both was
devised: VIFs, Virtual InterFaces. mrouted passes vif structures to
the kernel, indicating physical or tunnel interfaces to add to its
routing tables, and multicast forwarding entries saying where to
forward datagrams.
VIFs are added with MRT_ADD_VIF and deleted with MRT_DEL_VIF. Both
pass a struct vifctl to the kernel (defined in
/usr/include/linux/mroute.h) with the following information:
______________________________________________________________________
struct vifctl {
vifi_t vifc_vifi; /* Index of VIF */
unsigned char vifc_flags; /* VIFF_ flags */
unsigned char vifc_threshold; /* ttl limit */
unsigned int vifc_rate_limit; /* Rate limiter values (NI) */
struct in_addr vifc_lcl_addr; /* Our address */
struct in_addr vifc_rmt_addr; /* IPIP tunnel addr */
};
______________________________________________________________________
With this information a vif_device structure is built:
______________________________________________________________________
struct vif_device
{
struct device *dev; /* Device we are using */
struct route *rt_cache; /* Tunnel route cache */
unsigned long bytes_in,bytes_out;
unsigned long pkt_in,pkt_out; /* Statistics */
unsigned long rate_limit; /* Traffic shaping (NI) */
unsigned char threshold; /* TTL threshold */
unsigned short flags; /* Control flags */
unsigned long local,remote; /* Addresses(remote for tunnels)*/
};
______________________________________________________________________
Note the dev entry in the structure. The device structure is defined
in /usr/include/linux/netdevice.h file. It is a big structure, but the
field that interests us is:
______________________________________________________________________
struct ip_mc_list* ip_mc_list; /* IP multicast filter chain */
______________________________________________________________________
The ip_mc_list structure -defined in /usr/include/linux/igmp.h- is as
follows:
______________________________________________________________________
struct ip_mc_list
{
struct device *interface;
unsigned long multiaddr;
struct ip_mc_list *next;
struct timer_list timer;
short tm_running;
short reporter;
int users;
};
______________________________________________________________________
So, the ip_mc_list member from the dev structure is a pointer to a
linked list of ip_mc_list structures, each containing an entry for
each multicast group the network interface is a member of. Here again
we see membership is associated to interfaces.
LINUX/net/ipv4/ip_input.c traverses this linked list to decide whether
the received datagram is destined to any group the interface that
received the datagram belongs to:
______________________________________________________________________
#ifdef CONFIG_IP_MULTICAST
if(!(dev->flags&IFF_ALLMULTI) && brd==IS_MULTICAST
&& iph->daddr!=IGMP_ALL_HOSTS
&& !(dev->flags&IFF_LOOPBACK))
{
/*
* Check it is for one of our groups
*/
struct ip_mc_list *ip_mc=dev->ip_mc_list;
do
{
if(ip_mc==NULL)
{
kfree_skb(skb, FREE_WRITE);
return 0;
}
if(ip_mc->multiaddr==iph->daddr)
break;
ip_mc=ip_mc->next;
}
while(1);
}
#endif
______________________________________________________________________
The users field in the ip_mc_list structure is used to implement what
was said in section ``IGMP version 1'': if a process joins a group and
the interface is already a member of that group (ie, another process
joined that same group in that same interface before) only the count
of members (users) is incremented. No IGMP messages are sent, as you
can see in the following code (taken from ip_mc_inc_group(), called by
ip_mc_join_group(), both in LINUX/net/ipv4/igmp.c):
______________________________________________________________________
for(i=dev->ip_mc_list;i!=NULL;i=i->next)
{
if(i->multiaddr==addr)
{
i->users++;
return;
}
}
______________________________________________________________________
When dropping memberships, the counter is decremented and additional
operations are performed only when the count reaches 0
(ip_mc_dec_group()).
MRT_ADD_MFC and MRT_DEL_MFC set or delete forwarding entries in the
multicast routing tables. Both pass a struct mfcctl to the kernel
(also defined in /usr/include/linux/mroute.h) with this information:
______________________________________________________________________
struct mfcctl
{
struct in_addr mfcc_origin; /* Origin of mcast */
struct in_addr mfcc_mcastgrp; /* Group in question */
vifi_t mfcc_parent; /* Where it arrived */
unsigned char mfcc_ttls[MAXVIFS]; /* Where it is going */
};
______________________________________________________________________
With all this information in hand, ipmr_forward() "walks" across the
VIFs, and if a matching is found it duplicates the datagram and calls
ipmr_queue_xmit() which, in turn, uses the output device specified by
the routing table and the proper destination address if the packet is
to be sent across a tunnel (ie, the unicast destination address of the
other end of the tunnel).
Function ip_rt_event() (not directly related to output, but which is
in ip_output.c too) receives events related to a network device, like
the device going up. This function assures that then the device joins
the ALL-HOSTS multicast group.
IGMP functions are implemented in LINUX/net/ipv4/igmp.c. Important
information for that functions appears in /usr/include/linux/igmp.h
and /usr/include/linux/mroute.h. The IGMP entry in the /proc/net
directory is created with ip_init() in LINUX/net/ipv4/ip_output.c.
8. Routing Policies and Forwarding Techniques.
One trivial algorithm to make worldwide multicast traffic available
everywhere could be to send it... everywhere, despite someone wants it
or not. As this does not seem quite optimized, several routing
algorithms and forwarding techniques have been implemented.
DVMRP (Distance Vector Multicast Routing Protocol) is, perhaps, the
one most multicast routers use now. It is a dense mode routing
protocol, that is, it performs well in environments with high
bandwidth and densely distributed members. However, in sparse mode
scenarios, it suffers from scalability problems.
Together with DVMRP we can find other dense mode routing protocols,
such as MOSPF (Multicast Extensions to OSPF -Open Shortest Path
First-) and PIM-DM (Protocol-Independent Multicast Dense Mode).
To perform routing in sparse mode environments, we have PIM-SM
(Protocol Independent Multicast Sparse Mode) and CBT (Core Based
Trees).
OSPF version 2 is explained in RFC 1583, and MOSPF in RFC 1584. PIM-
SM and CBT specifications can be found in RFC 2117 and 2201,
respectively.
All this routing protocols use some type of multicast forwarding, such
as flooding, Reverse Path Broadcasting (RPB), Truncated Reverse Path
Broadcasting (TRPB), Reverse Path Multicasting (RPM) or Shared Trees.
It would be too long to explain them here and, as short descriptions
for them are publicly available, I'll just recommend reading the
draft-ietf-mboned-in.txt text. You can find it in the same places RFCs
are available, and it explains in some detail all the above techniques
and policies.
9. Multicast Transport Protocols.
So far we have been talking about multicast transmissions using UDP.
This is the usual practice, as it is impossible to do it with TCP.
However, intense research is taking place since a couple of years in
order to develop some new multicast transport protocols.
Several of these protocols have been implemented and are being tested.
A good lesson from them is that it seems no multicast transport
protocol is general and good enough for all types of multicast
applications.
If transport protocols are complex and difficult to tune, imagine
dealing with delays (in multimedia conferences), data loss, ordering,
retransmissions, flow and congestion control, group management, etc,
when the receiver is not one, but perhaps hundreds or thousands of
sparse hosts. Here scalability is an issue, and new techniches are
implemented, such as not giving acknowledges for every packet received
but, instead, send negative acknowledges (NACKs) for data not
received. RFC 1458 gives the proposed requirements for multicast
protocols.
Giving descriptions of those multicast protocols is out of the scope
of this section. Instead, I'll give you the names of some of them and
point you to some sources of information: Real-Time Transport Protocol
(RTP) is concerned with multi-partite multimedia conferences, Scalable
Reliable Multicast (SRM) is used by the wb (the distributed White-
Board tool, see section ``Multicast applications''), Uniform Reliable
Group Communication Protocol (URGC) enforces reliable and ordered
transactions based in a centralized control, Muse was developed as an
application specific protocol: to multicast news articles over the
MBone, the Multicast File Transfer Protocol (MFTP) is quite
descriptive by itself and people "join" to file transmission
(previously announced) much in the same way they would join a
conference, Log-Based Receiver-reliable Multicast (LBRM) is a curious
protocol that keeps track of all packets sent in a logging server that
tells the sender whether it has to retransmit the data or can drop it
safely as all receivers got it. One protocol with a funny name
-especially for a multicast protocol- is STORM (STructure-Oriented
Resilient Multicast). Lots and lots of multicast protocols can be
found searching the Web, along with some interesting papers proposing
new activities for multicast (for instance, www page distribution
using multicast).
A good page providing comparisons between reliable multicast protocols
is
<http://www.tascnets.com/mist/doc/mcpCompare.html>.
A very good and up-to-date site, with lots of interesting links
(Internet drafts, RFCs, papers, links to other sites) is:
<http://research.ivv.nasa.gov/RMP/links.html>.
<http://hill.lut.ac.uk/DS-Archive/MTP.html> is also a good source of
information on the subject.
Katia Obraczka's "Multicast Transport Protocols: A Survey and
Taxonomy" article gives short descriptions for each protocol and tries
to classify them according to different features. You can read it in
the IEEE Communications magazine, January 1998, vol. 36, No. 1.
10. References.
10.1. RFCs.
o RFC 1112 "Host Extensions for IP Multicasting". Steve Deering.
August 1989.
o RFC 2236 "Internet Group Management Protocol, version 2". W.
Fenner. November 1997.
o RFC 1458 "Requirements for Multicast Protocols". Braudes, R and
Zabele, S. May 1993.
o RFC 1469 "IP Multicast over Token-Ring Local Area Networks". T.
Pusateri. June 1993.
o RFC 1390 "Transmission of IP and ARP over FDDI Networks". D. Katz.
January 1993.
o RFC 1583 "OSPF Version 2". John Moy. March 1994.
o RFC 1584 "Multicast Extensions to OSPF". John Moy. March 1994.
o RFC 1585 "MOSPF: Analysis and Experience". John Moy. March 1994.
o RFC 1812 "Requirements for IP version 4 Routers". Fred Baker,
Editor. June 1995
o RFC 2117 "Protocol Independent Multicast-Sparse Mode (PIM-SM):
Protocol Specification". D. Estrin, D. Farinacci, A. Helmy, D.
Thaler; S. Deering, M. Handley, V. Jacobson, C. Liu, P. Sharma, and
L. Wei. July 1997.
o RFC 2189 "Core Based Trees (CBT version 2) Multicast Routing". A.
Ballardie. September 1997.
o RFC 2201 "Core Based Trees (CBT) Multicast Routing Architecture".
A. Ballardie. September 1997.
10.2. Internet Drafts.
o "Introduction to IP Multicast Routing". draft-ietf-mboned-intro-
multicast- 03.txt. T. Maufer, C. Semeria. July 1997.
o "Administratively Scoped IP Multicast". draft-ietf-mboned-admin-ip-
space-03.txt. D. Meyer. June 10, 1997.
10.3. Web pages.
o Linux Multicast Homepage.
<http://www.cs.virginia.edu/~mke2e/multicast.html>
o Linux Multicast FAQ. <http://andrew.triumf.ca/pub/linux/multicast-
FAQ>
o Multicast and MBONE on Linux.
<http://www.teksouth.com/linux/multicast/>
o Christian Daudt's MBONE-Linux Page.
<http://www.microplex.com/~csd/linux/mbone.html>
o Reliable Multicast Links
<http://research.ivv.nasa.gov/RMP/links.html>
o Multicast Transport Protocols <http://hill.lut.ac.uk/DS-
Archive/MTP.html>
10.4. Books.
o "TCP/IP Illustrated: Volume 1 The Protocols". Stevens, W. Richard.
Addison Wesley Publishing Company, Reading MA, 1994
o "TCP/IP Illustrated: Volume 2, The Implementation". Wright, Gary
and W. Richard Stevens. Addison Wesley Publishing Company, Reading
MA, 1995
o "UNIX Network Programming Volume 1. Networking APIs: Sockets and
XTI". Stevens, W. Richard. Second Edition, Prentice Hall, Inc.
1998.
o "Internetworking with TCP/IP Volume 1 Principles, Protocols, and
Architecture". Comer, Douglas E. Second Edition, Prentice Hall,
Inc. Englewood Cliffs, New Jersey, 1991
11. Copyright and Disclaimer.
Copyright 1998 Juan-Mariano de Goyeneche.
This HOWTO is free documentation; you can redistribute it and/or
modify it under the terms of the GNU General Public License as
published by the Free Software Foundation; either version 2 of the
License, or (at your option) any later version.
This document is distributed in the hope that it will be useful, but
without any warranty; without even the implied warranty of
merchantability or fitness for a particular purpose. See the GNU
General Public License for more details.
You can obtain a copy of the GNU General Public License by writing to
the Free Software Foundation, 59 Temple Place - Suite 330, Boston, MA
02111-1307, USA.
If you publish this document on a CD-ROM or in hardcopy form, a
complimentary copy would be appreciated; mail me for my postal
address. Also consider making a donation to the Linux Documentation
Project or the Free Software Foundation to help support free
documentation for GNU/Linux. Contact the Linux HOWTO coordinator, Tim
Bynum linux-howto@sunsite.unc.edu, for more information.
12. Acknowledgements.
This is the best opportunity I've ever had to thank so many people I
feel grateful to. So, I'm afraid this is going to be a large
section... It is, in any case, the most important one of this paper
(for me, at least...).
First, I want to thank Elena Apolinario Fernndez de Sousa (yes, Elena
is the first name; the REST is THE surname ;-) ). I tried to reflect
in this Howto all the knowledge I collected while working with her in
connecting our Department to the MBone and debugging problems with
locally generated CSCW software across multicast tunnels. She was of
invaluable help in finding and correcting network problems,
discovering and fixing kernel bugs that puzzled us for days, ... and
keeping the sense of humor alive while problems appeared and appeared,
but solutions didn't. She also read and corrected the drafts for this
document and provided important ideas and suggestions. If this howto
is here and is usefull for somebody, it will be, in many aspects,
thanks to her. Thanks, Elena!
There is something I have been lucky enough to find all my (still-not-
too-long) live, but, despite being repetitive, has never stopped
amazing me. I'm talking about people that altruistically employ part
of their time and/or resources to help other people learn new things;
and, what is better, they enjoy doing it. This is not only (but also,
too) explain things they already know, but lend their books, provide
access to their sources and facilitate you the way to learn all things
they know; sometimes, even more... I know quite a few of that people,
and I'd like to thank them for all their help.
Pablo Basterrechea was my "first source of documentation" while I was
in my pre-Internet stage. I learned assembly and advanced structured
programming entirely from his books (well, the latter also from his
programs...). Thanks for all, Pablo.
In my first course at the University that "primary source of
documentation" moved to Pepe Maas. He was teaching then Computer
Programming there, and soon I became addict to his bookshelf. He lent
me his books lots of times without asking for a minimum sign that
could assure that I was going to return them back to him, not even my
name! My first approach to TCP/IP was also by his hand: he lent me
Comer's "Internetworking with TCP/IP, Volume 1" for the whole summer.
He did not even know my name by then, but he lent me the book... That
book influenced me a lot, and TCP/IP has become one of my primary
fields of interest since that summer.
If there are two persons I must thank most, these are (in alphabetic
order ;-) ), Jos Manuel and Paco Moya. Nobody I asked more things more
times (C, C++, Linux, security, Web, OSs, signals & systems,
electronics, ... anything!) and, despite my persistence, I always got
throughly and friendly responses and help. If I'm using GNU/Linux now,
this is, again, thanks to them. I feel particularly lucky with friends
like them. THANKS.
Iigo Mascaraque also helped (from him I got my first System
Administration book) and encouraged me in my beginnings, but never
stopped reminding me that, although this was a fascinating world and
an important part of my career, I should not forget the other, less-
interesting, parts. (I don't forget, I$!).
As I am on the topic, I'd like to thank my parents, too. They always
tried to make the best opportunities available for me. Many thanks for
all.
I also feel grateful to Joaqun Seoane, the first who trusted me enough
to give me a root password in the time I was learning system
administration by myself, and Santiago Pavn, the one who gave me my
first opportunity here at DIT.
W. Richard Stevens' books have been a real revelation for me (it's a
pity they are so expensive...). If he ever reads this paper, I'd like
to thank him for them, and encourage him to keep on writing. Anything
that comes out of his hands will -undoubtedly- be good for all of us.
Finally I'd like to thank Richard Stallman, Linus Torvalds, Alan Cox
and all contributors to the Linux kernel and the free software in
general, for giving us such a great OS.
I'm sure I'm forgetting someone here... Sorry. I'm certain they know
I'm grateful to them too, so if they tell me, everybody will know
it... :-)
