Is Multicast UDP Supported in commonly available Routers? - TCP-IP

This is a discussion on Is Multicast UDP Supported in commonly available Routers? - TCP-IP ; Dear All: Hi! I've been developing a pet project of mine for some time, and early on I decided that for the messaging between computers on the local network I would use Multicast UDP communication. I assumed that, by now, ...

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Thread: Is Multicast UDP Supported in commonly available Routers?

  1. Is Multicast UDP Supported in commonly available Routers?

    Dear All:

    Hi! I've been developing a pet project of mine for some time, and
    early on I decided that for the messaging between computers on the
    local network I would use Multicast UDP communication.

    I assumed that, by now, this would be supported by most routers (I
    believe it became part of the TCP/IP standard back in 1994)... but I
    was surprised when I found out that I couldn't use my little program
    while at work. The router was a Netopia router from ~2001... upon
    further investigation, I found out that Multicast UDP isn't supported
    at all in ANY Netopia routers (even to this day).

    My attempts to find out the rarity of support for Multicast UDP in
    modern routers has failed thus far: most routers don't list whether
    they support it or not in their features list, most reviews of
    products don't mention Multicast at all, and I've yet to find someone
    on the net compiling a list of supporting routers (which I found
    especially surprising: someone on the internet can generally be found
    who is making a list of pretty much anything imaginable ).

    Does anyone know if most routers these days support Multicast UDP
    (and
    thus Netopia routers are a strange anomaly)? I'm especially curious
    about if most end-user routers (the type that John Q. Public would
    buy
    at a Best Buy for his home network) still do not support the
    Multicast
    UDP protocols (IGMP I believe it's called? I'm still a relative noob
    in the world of network programming).

    If you've been fortunate enough to come across a list of routers that
    support Multicast UDP (or have made one yourself), I'd also greatly
    appreciate being able to view such a list if possible, so that I can
    at least have a collection of routers that I can tell users of my
    program (likely just myself and friends, for now =) ) to purchase for
    the purpose of running the app.

    Thanks for any info that can be provided! =)

    -Alex


  2. Re: Is Multicast UDP Supported in commonly available Routers?

    In article <1180974876.546600.164220@z28g2000prd.googlegroups. com>,
    EdgarVerona wrote:

    > Does anyone know if most routers these days support Multicast UDP
    > (and
    > thus Netopia routers are a strange anomaly)? I'm especially curious
    > about if most end-user routers (the type that John Q. Public would
    > buy
    > at a Best Buy for his home network) still do not support the
    > Multicast
    > UDP protocols (IGMP I believe it's called? I'm still a relative noob
    > in the world of network programming).


    I think most enterprise and ISP routers, such as Cisco and Juniper,
    support multicast. I doubt that most home broadband routers, such as
    Linksys, Netgear, Belkin, and D-Link do. I'm not sure which category
    the Netopia you tried falls into. Basically, if it costs less than
    $200, I wouldn't expect multicast support.

    --
    Barry Margolin, barmar@alum.mit.edu
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***

  3. Re: Is Multicast UDP Supported in commonly available Routers?

    On Jun 4, 5:43 pm, Barry Margolin wrote:
    > In article <1180974876.546600.164...@z28g2000prd.googlegroups. com>,
    >
    > EdgarVerona wrote:
    > > Does anyone know if most routers these days support Multicast UDP
    > > (and
    > > thus Netopia routers are a strange anomaly)? I'm especially curious
    > > about if most end-user routers (the type that John Q. Public would
    > > buy
    > > at a Best Buy for his home network) still do not support the
    > > Multicast
    > > UDP protocols (IGMP I believe it's called? I'm still a relative noob
    > > in the world of network programming).

    >
    > I think most enterprise and ISP routers, such as Cisco and Juniper,
    > support multicast. I doubt that most home broadband routers, such as
    > Linksys, Netgear, Belkin, and D-Link do. I'm not sure which category
    > the Netopia you tried falls into. Basically, if it costs less than
    > $200, I wouldn't expect multicast support.
    >
    > --
    > Barry Margolin, bar...@alum.mit.edu
    > Arlington, MA
    > *** PLEASE post questions in newsgroups, not directly to me ***
    > *** PLEASE don't copy me on replies, I'll read them in the group ***



    Ahh... darn. I was hoping to make it a cool little app for home
    network users. I suppose I can do it without Multicast, but I was
    reading up on it and it sounded like a more efficient way to send many
    people the same information. I guess I'll continue the work but do it
    up in unicast.

    Is the difficulty of implementation the reason why it's only available
    in high-end routers? Perhaps someday home routers will get support
    for it. =) Thanks for the info though!


  4. Re: Is Multicast UDP Supported in commonly available Routers?

    On Jun 4, 7:43 pm, Barry Margolin wrote:
    > I think most enterprise and ISP routers, such as Cisco and Juniper,
    > support multicast.



    FWIW, I've never see an enterprise class router that *didn't* support
    multicast (which certainly does not imply universality). But I've
    never seen one where it's turned on by default, either (and it usually
    isn't turned on manually either).


  5. Re: Is Multicast UDP Supported in commonly available Routers?

    "EdgarVerona" wrote:

    > Ahh... darn. I was hoping to make it a cool little app for home
    > network users. I suppose I can do it without Multicast, but I was
    > reading up on it and it sounded like a more efficient way to send many
    > people the same information. I guess I'll continue the work but do it
    > up in unicast.
    >
    > Is the difficulty of implementation the reason why it's only available
    > in high-end routers? Perhaps someday home routers will get support
    > for it. =) Thanks for the info though!


    I don't think there's anything very difficult about supporting
    multicast. The router does have to maintain state for the multicast
    streams, so I suppose that adds some cost.

    Mostly, I think, it's that ISPs tend not to enable multicast, so lower
    cost home router designers figure there's no reason to support it at the
    edges either. I think home routers SHOULD support multicast, for exactly
    the sort of case you describe. In-home applications could make use of
    it, even if the ISP doesn't support it.

    Bert


  6. Re: Is Multicast UDP Supported in commonly available Routers?

    On 04 Jun 2007, EdgarVerona wrote:

    > Hi! I've been developing a pet project of mine for some time, and
    > early on I decided that for the messaging between computers on the
    > local network I would use Multicast UDP communication.


    [snip]

    > Does anyone know if most routers these days support Multicast UDP


    Maybe I'm misunderstanding you, but if you want to communicate between
    machines on a LAN using multicast, you don't need router support,
    presuming you mean by LAN a single IP subnet on a contiguous Ethernet
    LAN. The multicast packets will never leave the LAN. They'll be
    delivered by broadcast Ethernet packets!

    Dave

    --
    D.a.v.i.d T.i.k.t.i.n
    t.i.k.t.i.n [at] a.d.v.a.n.c.e.d.r.e.l.a.y [dot] c.o.m

  7. Re: Is Multicast UDP Supported in commonly available Routers?

    David Tiktin wrote:
    > Maybe I'm misunderstanding you, but if you want to communicate
    > between machines on a LAN using multicast, you don't need router
    > support, presuming you mean by LAN a single IP subnet on a
    > contiguous Ethernet LAN. The multicast packets will never leave the
    > LAN. They'll be delivered by broadcast Ethernet packets!


    Well, multicast Ethernet frames anyway Indeed, unless the multicast
    has to leave the LAN I'd expect the LAN/switch portion of the home
    router to be the only thing involved, not the router functionality
    (nor the NAT, nor the firewall etc etc).

    rick jones
    --
    a wide gulf separates "what if" from "if only"
    these opinions are mine, all mine; HP might not want them anyway...
    feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...

  8. Re: Is Multicast UDP Supported in commonly available Routers?

    On 05 Jun 2007, Rick Jones wrote:

    > David Tiktin wrote:
    >> Maybe I'm misunderstanding you, but if you want to communicate
    >> between machines on a LAN using multicast, you don't need router
    >> support, presuming you mean by LAN a single IP subnet on a
    >> contiguous Ethernet LAN. The multicast packets will never leave
    >> the LAN. They'll be delivered by broadcast Ethernet packets!

    >
    > Well, multicast Ethernet frames anyway Indeed, unless the
    > multicast has to leave the LAN I'd expect the LAN/switch portion
    > of the home router to be the only thing involved, not the router
    > functionality (nor the NAT, nor the firewall etc etc).


    Well, you learn something new every day (if you're lucky ;-) I
    wasn't aware that there are multicast *Ethernet* addresses.

    I suppose that switches will automatically "broadcast" frames with
    multicast addresses as the destination since they will never see a
    frame with that address as a source and so will never learn it's
    location. So it should work with any switch since that's part of the
    basic bridging algorithm? Or do the switches take special
    precautions against someone spoofing a source address in the
    multicast range?

    Dave

    --
    D.a.v.i.d T.i.k.t.i.n
    t.i.k.t.i.n [at] a.d.v.a.n.c.e.d.r.e.l.a.y [dot] c.o.m

  9. Re: Is Multicast UDP Supported in commonly available Routers?

    David Tiktin wrote:
    > Well, you learn something new every day (if you're lucky ;-)


    Always a good thing, I'm still learning stuff thanks to netnews

    > I wasn't aware that there are multicast *Ethernet* addresses.


    Yep. The Ethernet broadcast address is just a special (?) multicast
    address where not just the group bit, but all the bits of the address
    are set to one.

    > I suppose that switches will automatically "broadcast" frames with
    > multicast addresses as the destination since they will never see a
    > frame with that address as a source and so will never learn it's
    > location. So it should work with any switch since that's part of the
    > basic bridging algorithm? Or do the switches take special
    > precautions against someone spoofing a source address in the
    > multicast range?


    I don't know about checking for multicast as source, but I suspect
    that what the switches do is make note of the setting of the group bit
    in the address(es) and act accordingly. The folks in
    comp.dcom.lans.ethernet could provide the definitive word.

    rick jones
    --
    a wide gulf separates "what if" from "if only"
    these opinions are mine, all mine; HP might not want them anyway...
    feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...

  10. Re: Is Multicast UDP Supported in commonly available Routers?

    "David Tiktin" wrote:

    > Well, you learn something new every day (if you're lucky ;-) I
    > wasn't aware that there are multicast *Ethernet* addresses.
    >
    > I suppose that switches will automatically "broadcast" frames with
    > multicast addresses as the destination since they will never see a
    > frame with that address as a source and so will never learn it's
    > location. So it should work with any switch since that's part of the
    > basic bridging algorithm? Or do the switches take special
    > precautions against someone spoofing a source address in the
    > multicast range?


    Yes, as a default, layer 2 switches will broadcast (at the link layer)
    what arrives as an IP layer multicast. But that behavior is not the most
    efficient way to operate.

    If you read RFC 1112, the original IP multicast spec, it describes the
    way an IP multicast address maps into a MAC 48-bit address:

    http://www.ietf.org/rfc/rfc1112.txt?number=1112

    6.4. Extensions to an Ethernet Local Network Module

    [ ... ]

    An IP host group address is mapped to an Ethernet multicast address
    by placing the low-order 23-bits of the IP address into the low-order
    23 bits of the Ethernet multicast address 01-00-5E-00-00-00 (hex).
    Because there are 28 significant bits in an IP host group address,
    more than one host group address may map to the same Ethernet
    multicast address.

    There is a popular scheme for switches to intercept IGMP reports, from
    hosts to their multicast router, and set the MAC address filtering
    properly that way. In other words, depending on the IP multicast group a
    host is wanting to join, the switch sets its MAC address filtering based
    on the IP-MAC address mapping described in RFC 1112. So that Ethernet
    frames with multicast MAC addresses only go to those ports that care.
    This trick is called "IGMP snooping." RFC 4541 describes this for both
    IPv4 and IPv6.

    http://www.ietf.org/rfc/rfc4541.txt

    There was also a purely link layer protocol for this, GMRP, described in
    IEEE 802.1D-2004 Clause 10, but it is being dropped due to lack of
    interest. GMRP did not peek up at the Network Layer to figure out who
    wants a particular MAC multicast. So it was nice, because it doesn't
    mandate use of IP at all. I guess in practice most people don't care
    about playing only at the Link Layer. The IEEE is working on different
    schemes for this now.

    Bert




  11. Re: Is Multicast UDP Supported in commonly available Routers?

    > There is a popular scheme for switches to intercept IGMP reports, from
    > hosts to their multicast router, and set the MAC address filtering
    > properly that way. In other words, depending on the IP multicast group
    > a host is wanting to join, the switch sets its MAC address filtering
    > based on the IP-MAC address mapping described in RFC 1112. So that
    > Ethernet frames with multicast MAC addresses only go to those ports
    > that care. This trick is called "IGMP snooping." RFC 4541 describes
    > this for both IPv4 and IPv6.
    >
    > http://www.ietf.org/rfc/rfc4541.txt


    By the way, on the source filtering question, that *is* covered in RFC
    4541. But I don't know whether any switch manufacturer implements that
    filtering option.

    IGMPv3 and MLDv2 both accommodate source address filtering. The host
    joins a multicast group based on the IP multicast destination address
    and also specifies a set of "included" or "excluded" source IP
    addresses. So in principle, hosts connected to two ports of a switch
    could be interested in the same IP multicast group, but only packets
    from different sources.

    RFC 4541 says simply that switch manufacturers could use that
    information, or just operate with any-source-multicast filtering.

    Bert


  12. Re: Is Multicast UDP Supported in commonly available Routers?

    (From David Tiktin & Rick Jones, respectively)

    > David Tiktin wrote:
    > > Maybe I'm misunderstanding you, but if you want to communicate
    > > between machines on a LAN using multicast, you don't need router
    > > support, presuming you mean by LAN a single IP subnet on a
    > > contiguous Ethernet LAN. The multicast packets will never leave the
    > > LAN. They'll be delivered by broadcast Ethernet packets!

    >
    > Well, multicast Ethernet frames anyway Indeed, unless the multicast
    > has to leave the LAN I'd expect the LAN/switch portion of the home
    > router to be the only thing involved, not the router functionality
    > (nor the NAT, nor the firewall etc etc).
    >
    > rick jones


    Unfortunately, it seems to not be working that way in practice. Both
    at home (where I have a cheapo Belkin router) and at work (where the
    aforementioned Netopia router is), sending a message to a multicast
    address seems to have no effect. I've been using Ethereal to watch
    packets come in and out. The two machines attempt to do an
    IP_ADD_MEMBERSHIP to 224.0.93.0 (an arbitrary Multicast address I
    chose), then one sends a multicast packet to the address. I can see
    that the packet is sent, but the other machine doesn't recieve the
    multicast packet. TTL on the packet was set to 2, which should've
    been enough for it to reach its destination.

    As far as I can tell, the router has to have some base level of
    support to make Multicast work. The routers I have access to, for
    example, don't seem to regard the multicast address range as anything
    special. Packets sent to those address ranges get sent but never
    propagated. I'd been reading up on it a bit, and I guess routers/
    switches/etc... can have either a "pass-up" support (where they don't
    re-transmit the packet to users who have joined the Multicast address,
    but do re-transmit it to other routers/switches/etc... in an attempt
    to let the packet reach one that has full support), or "full" support
    (where it has some kind of private table of addresses that have
    "joined" the multicast address, and when a packet is sent to the
    multicast address it retransmits that packet to the members of that
    address.) Mind you, these aren't the technical terms. I think it was
    like level 2 and level 3 support or something like that, but I can't
    recall exactly...

    Anyways, it seems like whatever network device is connecting the
    computers on the network (switch, router, etc...) needs to have full
    support, or needs to at least have this "passing up" support and there
    needs to be a switch/router higher up in the chain that has "full"
    support in order for it to work. Alas. =(

    ***********
    (From Bert, above)

    > Mostly, I think, it's that ISPs tend not to enable multicast, so lower
    > cost home router designers figure there's no reason to support it at the
    > edges either. I think home routers SHOULD support multicast, for exactly
    > the sort of case you describe. In-home applications could make use of
    > it, even if the ISP doesn't support it.


    I totally agree Bert. This would be an incredibly useful feature,
    even if it was just for intranet use. It's unfortunate. =(


  13. Re: Is Multicast UDP Supported in commonly available Routers?

    EdgarVerona wrote:
    > Unfortunately, it seems to not be working that way in practice.
    > Both at home (where I have a cheapo Belkin router) and at work
    > (where the aforementioned Netopia router is), sending a message to a
    > multicast address seems to have no effect. I've been using Ethereal
    > to watch packets come in and out. The two machines attempt to do an
    > IP_ADD_MEMBERSHIP to 224.0.93.0 (an arbitrary Multicast address I
    > chose), then one sends a multicast packet to the address. I can see
    > that the packet is sent, but the other machine doesn't recieve the
    > multicast packet. TTL on the packet was set to 2, which should've
    > been enough for it to reach its destination.


    Is there a route setup on the sender to point to where the traffic
    should go - which in this case I would guess needs to be a
    system-local IP - that of the LAN interface, with a metric of 1.
    Otherwise, it could be getting sent to the router's MAC as a routing
    kind of thing. What is the destination MAC address of the IP
    multicast frame when it is sent?

    That is a bit of a straw grasping of course...

    You might also try putting the interface of the second system into
    promiscuous mode to see if the packet arrives at its interface, but
    perhaps isn't passing the multicast filters on the NIC/in the driver.

    rick jones
    --
    Process shall set you free from the need for rational thought.
    these opinions are mine, all mine; HP might not want them anyway...
    feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...

  14. Re: Is Multicast UDP Supported in commonly available Routers?

    "David Tiktin" wrote in message
    news:Xns99468CDADE644dtiktinnospambogusco@216.196. 97.136...
    > On 05 Jun 2007, Rick Jones wrote:
    >
    > > David Tiktin wrote:
    > >> Maybe I'm misunderstanding you, but if you want to communicate
    > >> between machines on a LAN using multicast, you don't need router
    > >> support, presuming you mean by LAN a single IP subnet on a
    > >> contiguous Ethernet LAN. The multicast packets will never leave
    > >> the LAN. They'll be delivered by broadcast Ethernet packets!

    > >
    > > Well, multicast Ethernet frames anyway Indeed, unless the
    > > multicast has to leave the LAN I'd expect the LAN/switch portion
    > > of the home router to be the only thing involved, not the router
    > > functionality (nor the NAT, nor the firewall etc etc).

    >
    > Well, you learn something new every day (if you're lucky ;-) I
    > wasn't aware that there are multicast *Ethernet* addresses.
    >
    > I suppose that switches will automatically "broadcast" frames with
    > multicast addresses as the destination since they will never see a
    > frame with that address as a source and so will never learn it's
    > location. So it should work with any switch since that's part of the
    > basic bridging algorithm? Or do the switches take special
    > precautions against someone spoofing a source address in the
    > multicast range?


    there is something in some part of the IEE bridging "stuff", and high end
    switches tend to not learn source addresses with multicast bit set.

    Not sure, but isnt the same true for ARP?

    however - you occasionally have to turn it off as some "improvements" that
    ignore the standards to support load balancing at layer 2 for servers by
    tricking the switches in a subnet have been used - eg in one flavour of
    M$oft server balancing.

    Great in theory, but if anything goes wrong, you can end up flooding every
    packet to a server on a layer 2 bridged network - its not often someone
    invents a new way to do denial of service by accident.....

    And i did have a couple of problems long ago where a manufacturer had
    managed to program a NIC card with a multicast unique MAC address.

    >
    > Dave
    >
    > --
    > D.a.v.i.d T.i.k.t.i.n
    > t.i.k.t.i.n [at] a.d.v.a.n.c.e.d.r.e.l.a.y [dot] c.o.m

    --
    Regards

    stephen_hope@xyzworld.com - replace xyz with ntl



  15. Re: Is Multicast UDP Supported in commonly available Routers?

    On 06 Jun 2007, EdgarVerona wrote:

    > (From David Tiktin & Rick Jones, respectively)
    >
    >> David Tiktin wrote:
    >> > Maybe I'm misunderstanding you, but if you want to communicate
    >> > between machines on a LAN using multicast, you don't need
    >> > router support, presuming you mean by LAN a single IP subnet on
    >> > a contiguous Ethernet LAN. The multicast packets will never
    >> > leave the LAN. They'll be delivered by broadcast Ethernet
    >> > packets!

    >>
    >> Well, multicast Ethernet frames anyway Indeed, unless the
    >> multicast has to leave the LAN I'd expect the LAN/switch portion
    >> of the home router to be the only thing involved, not the router
    >> functionality (nor the NAT, nor the firewall etc etc).
    >>
    >> rick jones

    >
    > Unfortunately, it seems to not be working that way in practice.
    > Both at home (where I have a cheapo Belkin router) and at work
    > (where the aforementioned Netopia router is), sending a message to
    > a multicast address seems to have no effect. I've been using
    > Ethereal to watch packets come in and out. The two machines
    > attempt to do an IP_ADD_MEMBERSHIP to 224.0.93.0 (an arbitrary
    > Multicast address I chose), then one sends a multicast packet to
    > the address. I can see that the packet is sent, but the other
    > machine doesn't recieve the multicast packet. TTL on the packet
    > was set to 2, which should've been enough for it to reach its
    > destination.


    If you have a capture, could you tell us what the Ethernet address of
    the frame containing the IP multicast packet is? Given the other
    comments in this thread, I'm just curious ;-)

    Also, are the 2 machines *separated* by the router (are thay on
    different subnets)? Or are they just both plugged into the router's
    built-in switch ports? If the latter, it might work if you just put
    both machines on a regular switch and daisy chain the switch to the
    router. That would cut the router out of the path between the
    machines.

    Dave

    --
    D.a.v.i.d T.i.k.t.i.n
    t.i.k.t.i.n [at] a.d.v.a.n.c.e.d.r.e.l.a.y [dot] c.o.m

  16. Re: Is Multicast UDP Supported in commonly available Routers?

    Rick Jones:

    I'm not entirely sure about what you're speaking of technically
    (admittedly, I'm relatively new to network programming and network
    protocols in general), so I'm not entirely sure how to check what you
    asked for.

    But I can definately set Ethereal to promiscuous mode when I get home
    and try it again. I'll try to remember to do that and send over the
    results to the thread. =)


    David Tiktin:

    >
    > If you have a capture, could you tell us what the Ethernet address of
    > the frame containing the IP multicast packet is? Given the other
    > comments in this thread, I'm just curious ;-)


    I didn't save the captures, but when I run it again when I get home
    I'll save them and post them up. I'll try running it in promiscuous
    mode as requested above as well. However, if I remember correctly the
    destination ethernet address was recorded in Ethereal as 24.0.93.0
    (the same as the Multicast address I pointed it to). But I could be
    wrong, as it's been a few days now since I messed with it last.


    > Also, are the 2 machines *separated* by the router (are thay on
    > different subnets)? Or are they just both plugged into the router's
    > built-in switch ports? If the latter, it might work if you just put
    > both machines on a regular switch and daisy chain the switch to the
    > router. That would cut the router out of the path between the
    > machines.



    Oh, they're on the same subnet and plugged into the switch ports on
    the router. I don't think I have access to a standard switch (I only
    have one of those multiuse Belkin routers: the ones that do wireless
    routing, have physical connections for devices, and have a connection
    for a DSL/Cable modem. I'm not sure what the setup is exactly at
    work, but I know my computer's directly plugged into the
    aforementioned Netopia router, and that I'm not allowed to mess with
    it. =) )


  17. Re: Is Multicast UDP Supported in commonly available Routers?

    "stephen" wrote:

    > there is something in some part of the IEE bridging "stuff", and high
    > end
    > switches tend to not learn source addresses with multicast bit set.


    If the source MAC address has the multicast bit set, it means something
    specific in Token Ring or FDDI nets. It means that "source routing" is
    used, i.e. that a series of hops will be identified after the source
    address.

    In Ethernet switches, frames with that bit set in the source address can
    be dropped. There is no source routing in Ethernet. But if you want to
    carry source routing info, say between two FDDI nets interconnected by
    an Ethernet, you'd have to is the IEEE 802.1Q header extension.

    I don't think it's mandatory for Ethernet switches to drop frames with a
    source MAC address that looks like a multicast address, but I'm not sure
    what they would legitimately do with it. Maybe pretend that bit is set
    to 0?

    (IEEE 802.1Q Clause 9.6.)

    Bert


  18. Re: Is Multicast UDP Supported in commonly available Routers?

    David Tiktin wrote:

    > If you have a capture, could you tell us what the Ethernet address of
    > the frame containing the IP multicast packet is? Given the other
    > comments in this thread, I'm just curious ;-)


    Here's one, dst mac is not mac of capture PC, src is mac of sender.

    00:c0:95:e2:57:04 > 01:00:5e:7f:00:01, ethertype IPv4 (0x0800), length
    1358: IP 192.168.0.1.32771 > 239.255.0.1.1234: UDP, length: 1316

    Andy.

  19. Re: Is Multicast UDP Supported in commonly available Routers?

    EdgarVerona wrote:
    > Dear All:
    >
    > Hi! I've been developing a pet project of mine for some time, and
    > early on I decided that for the messaging between computers on the
    > local network I would use Multicast UDP communication.
    >
    > I assumed that, by now, this would be supported by most routers (I
    > believe it became part of the TCP/IP standard back in 1994)... but I
    > was surprised when I found out that I couldn't use my little program
    > while at work. The router was a Netopia router from ~2001... upon
    > further investigation, I found out that Multicast UDP isn't supported
    > at all in ANY Netopia routers (even to this day).
    >
    > My attempts to find out the rarity of support for Multicast UDP in
    > modern routers has failed thus far: most routers don't list whether
    > they support it or not in their features list, most reviews of
    > products don't mention Multicast at all, and I've yet to find someone
    > on the net compiling a list of supporting routers (which I found
    > especially surprising: someone on the internet can generally be found
    > who is making a list of pretty much anything imaginable ).
    >
    > Does anyone know if most routers these days support Multicast UDP
    > (and
    > thus Netopia routers are a strange anomaly)? I'm especially curious
    > about if most end-user routers (the type that John Q. Public would
    > buy
    > at a Best Buy for his home network) still do not support the
    > Multicast
    > UDP protocols (IGMP I believe it's called? I'm still a relative noob
    > in the world of network programming).
    >
    > If you've been fortunate enough to come across a list of routers that
    > support Multicast UDP (or have made one yourself), I'd also greatly
    > appreciate being able to view such a list if possible, so that I can
    > at least have a collection of routers that I can tell users of my
    > program (likely just myself and friends, for now =) ) to purchase for
    > the purpose of running the app.
    >
    > Thanks for any info that can be provided! =)
    >
    > -Alex
    >


    I've got a cheap TI AR7 based router and it works for multicast.

    TI use linux on this - well it's upto resellers I suppose I beleive
    Zyxel use their own OS. Mine is solwise and I use a different firmware
    from www.routertech.org.

    TI wrote their own IGMPproxy which is in the sources. It doesn't work
    without tweaking firewall/dmz rules. It also doesn't work if you have a
    block of addresses and put the same address on wan and lan. But being a
    Linux box you can workaround this.

    Mine only has one port so I can't comment on whether the bigger
    4port+usb+wireless versions work OK. I would expect so as long as you
    have them all in the same lan group (= a linux bridge). I think they are
    separate by default.

    There are some more listed here -

    http://forum.zensupport.co.uk/thread/10974.aspx

    Mine and those work for IGMP wan-lan, I suppose it may be harder to get
    them to work just inside a LAN if you are not all on the same link layer.

    Andy.

  20. Re: Is Multicast UDP Supported in commonly available Routers?

    "Andy Furniss" wrote:

    > David Tiktin wrote:
    >
    >> If you have a capture, could you tell us what the Ethernet address of
    >> the frame containing the IP multicast packet is? Given the other
    >> comments in this thread, I'm just curious ;-)

    >
    > Here's one, dst mac is not mac of capture PC, src is mac of sender.
    >
    > 00:c0:95:e2:57:04 > 01:00:5e:7f:00:01, ethertype IPv4 (0x0800), length
    > 1358: IP 192.168.0.1.32771 > 239.255.0.1.1234: UDP, length: 1316


    Not sure what you're saying, Andy. The destination MAC address is not
    supposed to be the MAC address of the destination PC, in this multicast
    case. It is instead supposed to be a MAC address derived from the IP
    address, following the algorithm in RFC 1112.

    I believe 01:00:5e:7f:00:01 *is* that MAC address. RFC 1112, Section 6.4
    states:

    An IP host group address is mapped to an Ethernet multicast address
    by placing the low-order 23-bits of the IP address into the low-order
    23 bits of the Ethernet multicast address 01-00-5E-00-00-00 (hex).
    Because there are 28 significant bits in an IP host group address,
    more than one host group address may map to the same Ethernet
    multicast address.

    The source MAC is supposed to be the MAC of the sender or of the router,
    depending if the sender and the receiver are in th same IP subnet or in
    different IP subnets.

    RFC 1112 instructs computers wanting to receive IP multicasts that they
    have to add an extension to their Ethernet modules to filter these new
    MAC addresses. It also discusses other Link Layers.

    Bert


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