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fredthomsen
Premium
join:2011-03-01
Reviews:
·T-Mobile US
·Comcast

1 recommendation

why do cable modems need ipv6 support ?

I was under the impression that cable modem were layer 2 devices with the only need for an IP address being a non-routable management IP. I don't understand why I see statements on IPv6 deployments from various ISPs saying you must have an IPv6 compatible modem. Wouldn't that only hold true if they are modifying the cable modem's management IPs to IPv6? Can someone clarify this for me?

17775992

join:2011-11-16
Chicago, IL
At some point everything will be switched over to IPV6 and if your modem doesn't understand 2001:0db8:85a3:0000:0000:8a2e:0370:7334 when it is looking for ###:###:###:### then the modem will be worthless.


rchandra
Stargate Universe fan
Premium
join:2000-11-09
14225-2105

1 recommendation

reply to fredthomsen
DOCSIS standards are only interesting to a narrow bunch of people. It takes a lot of stamina to go through them in any depth. I know; I've tried (and not wholly succeeded). In essence though, CMs are not merely dumb L2 devices. There are no guarantees of carrying anything but IPv4 until (I think) v3 (where IPv6 support is mandatory). You can certainly try native IPv6 over your non-D3 modem if you like; there are no guarantees your CM will forward/bridge those frames onto the radio, or that the CMTS will do anything but drop your frames once they get there though. In theory, it might be possible. The CMTS side of things is a very, very tightly controlled environment. Virtually nothing goes on that the CMTS does NOT control in one way or another...at least if only compliant devices are attached. (User mods are a different kettle of fish entirely.)
--
English is a difficult enough language to interpret correctly when its rules are followed, let alone when a writer chooses not to follow those rules.

Jeopardy! replies and randomcaps REALLY suck!


DarkLogix
Texan and Proud
Premium
join:2008-10-23
Baytown, TX
kudos:3

2 recommendations

reply to fredthomsen
In addition to what has been said I think they are planning to move the managment to IPv6

I've heard that the IPs used on CMs for managment from their side have already used up multiple class B ranges and they've had to split up the managment networks beyond VLSM, including using the same range twice but not letting them be routable between one another


whfsdude
Premium
join:2003-04-05
Washington, DC
Reviews:
·Comcast

1 recommendation

reply to fredthomsen
said by fredthomsen:

I was under the impression that cable modem were layer 2 devices

Nope. I think the word modem is confusing. Think of a cable modem as a cable CPE with a line-card.

On a side note, you can do IPv6 over DOCSIS 2 networks. ...but new CPEs will need to be deployed to end-users. Which why would anyone not deploy 3.0 instead.

»www.cablelabs.com/specifications···1117.pdf


whfsdude
Premium
join:2003-04-05
Washington, DC
Reviews:
·Comcast

1 recommendation

reply to rchandra
said by rchandra:

You can certainly try native IPv6 over your non-D3 modem if you like; there are no guarantees your CM will forward/bridge those frames onto the radio, or that the CMTS will do anything but drop your frames once they get there though.

Won't be able to do v6 multicast, so no v6 at all. You can announce an RA directly at any non-v6 cable modem and it won't do anything.

lestat99

join:2000-08-04
Piscataway, NJ
reply to fredthomsen
Aside from what the previous poster indicated about DOCSIS3 the modem also has to know to forward the IPv6 ethertype which is different from the IPv4 ethertype.


aefstoggaflm
Open Source Fan
Premium
join:2002-03-04
Bethlehem, PA
kudos:7
Reviews:
·PenTeleData
·Verizon Online DSL

1 edit

1 recommendation

reply to fredthomsen
»Access to modem combo while in bridge mode

For example..

#1 IPv4 LAN IP of my modem combo = 192.168.1.1

#2 IPv4 LAN IP of my RJ-45 WAN port router that is handling the Internet Connection = 192.168.0.1

#3 My RJ-45 WAN port router that is handling the Internet Connection, WAN IP Starts with 71.242

#4 A hub/switch that is not IP aware is connected between my modem combo and my RJ-45 WAN port router that is handling the Internet Connection.

#5 A second RJ-45 WAN port router:

a) DHCP Server Disabled.

b) LAN IP = 192.168.0.2 (outside of the DHCP Range of the RJ-45 WAN port router that is handling the Internet Connection).

c) WAN IP = 192.168.1.2 (same subnet as the modem combo's LAN IP)

d) Is connected to one the LAN Ports of the RJ-45 WAN port router that is handling the Internet Connection.

#6 In first router, adds a route.

Subnet IP: 192.168.1.0 (the subnet of my modem combo)
Mask: 255.255.255.0
Gateway: 192.168.0.2 (the IP address of the secondary router on the LAN)

#7 The second RJ-45 WAN port router is also connected to the hub/switch that is connected between the modem combo and my RJ-45 WAN port router that is handling the Internet Connection.

Tada.

That is the one of the IPv4 way(s). There are other ways...

--

At some point in the future there might not be equipment that is IPv4 aware anymore and since it is possible to access a modem even while it is in bridge mode, it would make sence that the modem even while it is a bridge mode has to support IPv6 (some day in the future).

This post is my two cents, about the issue (besides what others have said) in this thread.
--
Please use the "yellow (IM) envelope" to contact me and please leave the URL intact.

Kearnstd
Space Elf
Premium
join:2002-01-22
Mullica Hill, NJ
kudos:1
reply to whfsdude
I am wondering then is the cable modem actually doing any modulating and demodulating or is it a hold over term to make things more understandable to the consumer who was used to the term "Telephone Modem" so "Cable Modem" was used to make it easy to tell people what the device was for.
--
[65 Arcanist]Filan(High Elf) Zone: Broadband Reports

clueless
Premium
join:2008-09-16
Glendale, CA
Theres definitely RF modulation going on so cable modem is an appropriate term. If anything.. I would equate cable modems to essentially being a bridge with modulation capabilities.


cablegeek01

join:2003-05-13
USA
kudos:1
reply to Kearnstd
It is most definitely a modem, but a broadband modem. It takes your Ethernet packet and modulates it onto an RF channel (usually 1.6-6.4Mhz wide QPSK, 16QAM or 64QAM).
There are two distinct sections of a modem, the bridging/routing chip, and the tuner.
On the downstream side, it receives an RF signal or signals (1 channel, 4, or 8 6Mhz wide channels) usually QAM256 and demodulates it into an Ethernet packet.
If you want to get really technical, here are the CableLabs specs that outline the MAC and Upper Layer Protocol Interfaces, as well as the RF specifications.
»www.cablelabs.com/specifications···1117.pdf
»www.cablelabs.com/specifications···1117.pdf

IPv6 support is detailed in the MULPIv3.0 spec for D3 modems, and there's a spec for D2 modems below.

»www.cablelabs.com/specifications···1117.pdf

I'm actually working on a project certifying cable modems and gateways for IPv6 operation, so if anyone has any questions, feel free to IM/email me.


Da Geek Kid

join:2003-10-11
::1
kudos:1
Reviews:
·Callcentric
It's more than just a interconnectivity with the ISP... The layer 2 hub(What everyone likes to call switch) also needs to keep track of the arp on the hardware and most do not. All can use software to do this and hence a HUB. Switch happens on the chip but again this is argumentative for IPv4 boxes running IPv6... How so, you ask? "Used Car Salesman" like, that's how...


rchandra
Stargate Universe fan
Premium
join:2000-11-09
14225-2105

2 recommendations

First...ummmm....what? Sorry, but just throwing some words together does not necessarily a comprehensible sentence make.

But more to the point...Switches are not hubs. Hubs always forward all frames to all ports except the originator. Some hubs have collision control, but it's only of the form of shutting down a port which seems to be receiving too many frames per unit of time, sometimes until administratively unblocked, sometimes for a configured time interval (depends on the code in that hub and configuration).

Switches on the other hand can buffer frames in an attempt to avoid collisions. Also, your typical switch firmware doesn't care about ARP or any other protocol. All it cares about are the MAC addresses it has received and on what port that frame with that MAC address has arrived. Therefore, if the destination MAC address of a frame is in its memory (either discovered or configured), the frame is only forwarded to that particular port. If the port is not known, it falls back to operating as a hub, and floods the frame out to all ports (except the originator). If a switch is able to determine whether a link partner is half duplex, it will attempt to buffer frames destined for that port until that port is not transmitting in order to try to avoid collisions. It doesn't always succeed, because the link partner can always attempt to transmit at any time.

Some switches with more sophisticated firmware do look at higher layer protocols, and do something more intelligent with frames. For example, it could snoop on IGMP, and deliver multicast frames only to those ports which have joined some multicast group. It's only in that case where the frame content matters, and where any IPv4 or IPv6 compatibility comes into play. But even in that case, just as unknown destination MAC address switching devolves into hub behavior, if the switch does not recognize the upper level protocol (such as IPv6), it should just devolve into switching based on MAC address. Typically (but not always) these switches are of the managed type, the kind where one may log into them and configure them, or at the very least send them SNMP messages.

But...circling back to the question at hand (cable modem IPv6 compatibility), chances are cable modem firmware will not forward frames it cannot identify onto the radio, in order to try to protect the integrity of the cable system. As IPv4 has its own Ethertype ID, cable modems are probably not going to forward frames not having that Ethertype. I haven't read the specs all that deeply, but it also may be possible to use other frame types, such as PPPoE (although I'm going to guess DSL is pretty much the only place that's been used). So that's why I wrote in another post it's possible (though I would think unlikely) that IPv6 frames could be sent to the CMTS; the firmware will likely just block such frames from going out.

And for another poster: DOCSIS < 3 modems do not necessarily need to be replaced in order to be IPv6 compatible; it's possible all they need is a firmware update. It would all depend on whether it can be updated, whether it has the capacity (e.g. flash ROM space) to accept the necessary firmware, and whether something implemented in hardware (such as an Ethertype filter) would prevent such bridging/forwarding.
--
English is a difficult enough language to interpret correctly when its rules are followed, let alone when a writer chooses not to follow those rules.

Jeopardy! replies and randomcaps REALLY suck!


whfsdude
Premium
join:2003-04-05
Washington, DC
Reviews:
·Comcast

1 recommendation

said by rchandra:

For example, it could snoop on IGMP

or MLD snooping since this is the IPv6 forum.

DOCSIS < 3 modems do not necessarily need to be replaced in order to be IPv6 compatible; it's possible all they need is a firmware update. It would all depend on whether it can be updated, whether it has the capacity (e.g. flash ROM space) to accept the necessary firmware, and whether something implemented in hardware (such as an Ethertype filter) would prevent such bridging/forwarding.

or an ISP could make 6rd available for their legacy footprint. Of course you'd have to have customers pop a 6rd server into their router. AT&T has gone with this model for uverse rather than deploying all new CPE gear.


rchandra
Stargate Universe fan
Premium
join:2000-11-09
14225-2105

2 recommendations

said by whfsdude:

or MLD snooping since this is the IPv6 forum.

aw, shucks, you caught me. I had seen something about that in my SMC6128 manual, but I didn't remember that when posting before. I had no idea what it was at the time I first saw it, so researched it a bit, just to know what that feature did. Thanks for reminding me/jogging my memory.
--
English is a difficult enough language to interpret correctly when its rules are followed, let alone when a writer chooses not to follow those rules.

Jeopardy! replies and randomcaps REALLY suck!

cramer
Premium
join:2007-04-10
Raleigh, NC
kudos:9

1 recommendation

reply to rchandra
(storm control -- excessive number of broadcasts, jabber -- causing an excessive number of collisions)

Most unmanaged switches are blindingly stupid. They care about two things... dst mac (first 6 bytes) and src mac (next 6 bytes). They don't even pay attention to the rest of the L2 frame. DST is used to find the port to forward the packet. SRC is used to learn what's on the port. This is not "arp snooping". This type of free learning is simple, fast, and done with minimal logic. (arp snooping is done by a CPU, which only exists in managed switches.)

It's only a small step (the next 2 bytes) to add protocol checking. For IP, that would be 0x0800 (IP) and 0x0806 (ARP.) And maybe PPPoE (0x8863, 0x8864), but I don't know of any US cable ISPs using PPPoE ('tho they did at one point, long ago.) IPv6 is 0x86dd. It's possible a D2 modem filters it out, but that's not something we users can test... if the headend isn't setup for IPv6, it doesn't matter if it gets past the modem or not.