OIT Mobile IP Service allows you to temporarily attach a device's network interface to a network other than the device's "home network", and receive IP service without reconfiguring the device or updating its Host Database entry.
The "home network" of a device's network interface is defined as the IP network (a.k.a. "subnet") specified for that network interface in the device's Princeton University Host Database entry. For example, if a device is registered in the Host Database as having two network interfaces, and the first interface is assigned an IP address on the libnet network, then that interface is deemed to be "homed" on libnet. If the second interface is assigned an IP address on the wirelessnet network, then the second interface is "homed" on wirelessnet.
Some networks span multiple buildings; conversely, some buildings contain several networks. For a list of buildings and networks, see the Princeton Network ("subnet") List.
Mobile IP service provides your device's network interface with a temporary IP (Internet) address and Internet hostname when the device's network interface is attached to parts of the Campus Data Network (CDN) outside the device's "home network." The service is available on these parts of the Campus Data Network (CDN):
Because of the special demands associated with mobile networking, there are some guidelines you'll need to be particularly aware of. Those guidelines are covered in this document.
If you have questions about the procedures below, or encounter difficulties related to them, please contact the OIT Support and Operations Center for assistance.
The following procedures apply to all computers attached to the campus network, but are of special importance for mobile devices:
E.g. if your computer is in any of these modes, then before attaching a Ethernet cable, first wake up your computer, shut it down entirely, then attach the cable, and finally start up your computer.
By shutting down entirely, you are less likely to make the mistake of later re-attaching your computer to a network cable (or via its wireless interface) while the computer is powered-on or sleeping. (And you'll also avoid needing to wake up a sleeping computer just so you can power it down before re-attaching it to networking wiring.)
To use OIT Mobile IP service, your computer must meet the following requirements:
Be aware that DHCP client software sometimes has bugs which do not pose a serious problem when the device is used with OIT Static IP Service, but make the DHCP client unsuitable for use with OIT Mobile IP Service. These include, for example, those DHCP client bugs which lead the client to ignore DHCP lease expiration times. We have found that nearly all physical appliances (printers, print servers, file servers, NATs, wireless access points, audio/video servers, etc.) have such bugs. And from time to time, some computer operating systems have such bugs.
If you relocate your device to another network "permanently" or for an extended period (more than two weeks), and your device is not associated with a Dormnet subscription, you should update its entry in the Host Database to reflect the change.
The owner of a permanently-relocated computer (except Dormnet subscriptions) should not rely on Mobile IP service as a substitute for updating the computer's Host Database registration to reflect its new home network, because there are a finite number of IP addresses available for Mobile IP service. (When it appears that a device has been relocated permanently or for a extended period, but the owner has neglected to update the Host Database registration, OIT may choose to update the Host Database registration on the owner's behalf. This may result in an unexpected interruption to the device's network access, and will require the owner reboot the device. See the Mobile IP Campers section below for more details.)
If your device's network interface is registered in the Princeton University Host Database to reflect that its "home network" is not one of these networks, then that interface will not receive IP service if it visits that network. E.g. if your device's registration specifies its Ethernet interface's home network is fishnet, the Ethernet interface will not receive Mobile IP Service if it visits ppnnet.
Note that this restriction does not affect devicess' interfaces which are registered to reflect that their "home network" is one of the networks above. For example, if a device's registration specifies its Ethernet interface's home network is ppnnet, then when attached to ppnnet, the device's Ethernet interface uses OIT Static IP Service.
On that network, OIT operates a different service to provide dynamically-assigned IP addresses. This document does not describe that service. Documention describing that service is not presently available.
Often these networks are private-wire networks, possibly receiving DHCP service from the administrator of that network.
Also included among these networks are the OIT networks bargainnet, basnet, kachingnet, knockknocknet, lightnet, linknet*, lochnessnet, opensesamenet pingpongnet, plinknet*, russnet, smilenet, stellanet, viewnet, voip-servernet, voipnet*, voltnet, wattnet, or extern-linknet*.
Mobile IP Service is not (reliably) available on such networks because OIT's DHCP servers cannot see when the device's interface returns to its "home network." As a result, the device's interface may eventually be identified as a Mobile IP Camper.
Such a device's interface is typically attached to a private-wire network, and the administrator of the private-wire network has determined that the device does not need to communicate outside the private-wire network. The private-network administrator has arranged (typically through a hardware filter, or sometimes software configuration) to prevent traffic from the device's interface from leaving the private-wire network and reaching OIT Ethernet Service (typically to make the device exempt from the Tigernet Host Charge). The private-network administrator has arranged with hostmaster for the device's interface to be assigned an IP address within a range defined as blocked from reaching the rest of the campus network.
Mobile IP Service is not provided both because the device's interface is not supposed to produce traffic on OIT networks, and because the camper detection component of Mobile IP service relies on our DHCP servers being able to see the device's interface when it is on its home network.
For devices meeting the requirements above, Mobile IP service works as follows:
OIT's DHCP servers treat the entire subnet (a.k.a. "network") listed in your device's Host Database entry (for a particular network interface) as that interface's "home network." Since one network may encompass several buildings, all of those locations are that interface's "home network" as far as the DHCP servers are concerned.
As a result, when you visit another room or building that is attached to your "home network", OIT's DHCP servers continue to assign your device's network interface the same static IP address and Internet hostname; no Mobile IP service is involved.
This lease has a short duration, typically 2-12 hours. While your network interface remains attached to this network and continues to use IP, it periodically contacts OIT's DHCP servers to renew this lease. If your computer is restarted (or allows the lease to expire for some other reason), the DHCP servers may assign to it a different Mobile IP address and Internet hostname.
A list of the clients currently assigned Mobile IP addresses appears in Current Mobile IP Address Assignments.
If you move the computer's network interface to yet another network other than its "home network", it is leased yet another Mobile IP address and Internet hostname appropriate for that network.
When you move your computer's network interface back to its "home network", OIT's DHCP servers recognize that it is no longer "mobile", and assign its home (static) IP address, as described earlier; your computer resumes using OIT Static IP Service.
There are no actual locations wired to recyclenet; it is an imaginary network. Therefore, when one of these computer network interfaces is attached to the campus network, it is attached somewhere other than its home network. It relies on OIT Mobile IP Service, as described above.
There are no actual locations wired to wirelessnet; it is an imaginary network. Therefore, when one of these computer network interfaces is attached to the campus network, it is attached somewhere other than its home network.
There are no actual locations wired to driftnet; it is an imaginary network. Therefore, when one of these computer network interfaces is attached to the campus network, it is attached somewhere other than its home network.
This part of OIT Wireless Network is never considered your network interface's "home network", so the interface is never assigned a static IP address when attached to that network. Customers wishing for a more detailed technical information of how this works may see the appendix below.
This lease has a short duration, typically 2-12 hours. While your computer's network interface remains attached to OIT Wireless Service and continues to use IP, it periodically contacts OIT's DHCP servers to renew this lease. If your computer is restarted (or allows the lease to expire for some other reason), the DHCP servers may assign it a different Mobile IP address and Internet hostname.
A list of the clients currently assigned Mobile IP addresses appears in Current Mobile IP Address Assignments.
For example, if you subscribe to a commercial network service that only accepts your connection from your "home" IP address or Internet hostname, you may not be able to connect while you are mobile. The same may be true if you use licensed software that is customized to only run if your computer is using your "home" IP address or Internet hostname.
Currently our convention for the Internet hostnames associated with Mobile IP addresses is: dynamic-oit-NETWORK-LETTER-NUMBER.Princeton.EDU, where NETWORK is the name of a campus network ("subnet") and LETTER denotes which OIT DHCP server owns the Mobile IP address. This convention may change in the future.
We typically do so when the device's malfunction or misconfiguration causes it to interfere with other customers of the service, or with the service as a whole. A list of devices that have been declared ineligible appears in Devices Blocked from Mobile IP Service.
The most common issue of this sort is that when sleeping, the device keeps its network interface up and using a DHCP-leased IP address, but stops using DHCP to renew its lease. There are multiple bugs which cause these kinds of problems affecting widely-used Ethernet and Wi-Fi network interfaces, and popular operating systems. (For example, these problems are widespread on many Windows PCs, Android devices, and Apple iOS devices.) On some platforms, a workaround may be to disable all those features which keep the device's network interfaces up while the device is sleeping. On other devices, there is no way to disable those features, and therefore there is no workaround.
In particular, the current DHCP specification has evolved rapidly in recent years, so older client software may not work right. If the DHCP specification changes in the future, you may need to update the DHCP software on your computer to continue to use OIT Mobile IP service.
Some client DHCP implementations or configurations may have problems that are not apparent when they are only used on their home networks (i.e. with a statically-assigned IP address); these clients then encounter problems when trying to use OIT Mobile IP Service. Customers with such clients may be instructed that unless the client device can be made to perform DHCP properly, they will have to limit themselves to their home networks (i.e. not use OIT Mobile IP Service). When we discover a DHCP client that function in such a way as to interfere with OIT Mobile IP Service, we may declare the device Ineligible for OIT Mobile IP Service. A list of devices that have been declared ineligible appears in Devices Blocked from Mobile IP Service.
The most common DHCP client malfunction of this sort is for the device to ignore DHCP lease times, continuing to use an IP address after its lease has expired. Often this does not interfere with service when the device is attached to its home network, but only becomes apparent when the device visits another network and uses OIT Mobile IP Service.
Our service supports devices with multiple network interfaces, for example, a computer with an Ethernet interface and a Wireless interface.
Each network interface that is eligible for DHCP service is treated as an independent device. Each has its own "home network", and is independently eligible for its own DHCP lease. Therefore, each such interface is independently eligible for Mobile IP Service. It is possible that one interface (attached to its home network) may be using OIT Static IP Service at the same time that the other interface (attached to a network other than its home network) may be using OIT Mobile IP Service.
Each network interface is subject to Mobile IP Camper identification (described elsewhere in this document) independent of the device's other interfaces.This means, for example, that anywhere in this document that you read "device" or "computer", we are really referring to one of your device's network interfaces.
While our service support a device with multiple physical interfaces, it does not support a device with multiple logical interfaces running on a single physical interface. We treat each physical interface (as identified by its IEEE 802.2 MAC address) as a single unique client, which may be attached to a single network at a time, and is eligible for (at most) one DHCP lease at a time. A device which tries to simultaneously acquire multiple IP addresses on a single physical interface is not eligible for our service.
Mobile IP service is intended to provide your computer's network interface with a temporary IP (Internet) address and Internet hostname when it is attached to a network other than its "home network." It is not intended to provide a permanent IP address and hostname when your computer leaves its "home network" permanently, or for an extended period (more than 14 days). In either of those circumstances, you should instead update your computer's entry in the Princeton University Host Database to reflect that network interface's new "home network."
We impose the time limit both to ensure that Mobile IP Service is not used as a substitute for updating a computer's Host Database entry when it relocates permanently, and to ensure an adequate supply of Mobile IP addresses for devices that are truly mobile.
If OIT's DHCP servers detect that your device's network interface has received Mobile IP Service on a single network other that its home network for "an extended period", they will identify your device as a Mobile IP Camper. (Your device's network interface has been "mobile" on a single network other than "home network" for so long, it appears to be "camping" there, suggesting that your device's network interface has been relocated permanently, rather than just "visiting" the new network.)
Some devices are exempt from being identified as Mobile IP Campers. And devices visiting some networks are also exempt. These exemptions include:
If your device is identified as a Mobile IP Camper, OIT may update the device's Host Database entry on your behalf, to reflect its new home network. That is, we will assume that you must have relocated the device permanently (or for an extended period), but neglected to update the Host Database entry. After OIT updates the device's Host Database entry, you will need to reboot the device, so it switches from using an IP address assigned via Mobile IP Service, to using its new statically-assigned IP address. The device may experience some interruption to its network access until you reboot it.
Here are the specific details:
Assuming your device has a single network interface, the field you need to update is the IP-SUBNET-OR-ADDRESS field; that's the field that specifies the first (only) interface's "home network." Select the name of your new "home network"; consult the Princeton Network ("subnet") List for a list of buildings and networks.
Changing this field will cause the Host Database to discard the static IP address assigned for the first interface's use on its old home network, and to assign a new static IP address (appropriate for the new home network). You will also probably wish to update the BUILDING and ROOM fields (they do not indicate the device's home network, but primarily serve as documentation and sometimes assist in troubleshooting).
When the Host Database change is processed (this can take up to a day, but often is completed within a few hours), you will receive email indicating that the change is being incorporated into the Host Database, and will be complete within about an hour. After that time, reboot the device. (You may see some disruption to the device's network service if you don't reboot it.) It will receive static IP service (assuming it is attached to its new "home network"). It will still be able to use Mobile IP Service (when it travels to other networks).
Similarly, if the device receive Mobile IP Service on in network (other than its "home network"), then moves to a different network (other than its "home network"), its visit to the first network is deemed to be over; the fourteen-day timer is reset. (Again, the device must actually receive DHCP service from OIT's DHCP servers on the new network for at least 10 minutes to be sure it is recorded as having appeared there.)
The idea is that a device that is truly "mobile" -- either moving often between different networks, or leaving its home network just for a brief time and then returning -- will not be identified as a Mobile IP Camper. Only devices that seem to be "camping" in a particular network (other than their "home network") for an extended period are deemed Mobile IP Campers.
Although the device has been identified as a Mobile IP Camper, it continues to be eligible for Mobile IP Service at this time.
Within three business days of sending the notification email, we change the device's Host Database entry on your behalf, to reflect the new home network.
Specifically, we change the network interface's IP-SUBNET-OR-ADDRESS field to match the network the device has been visiting. Changing this field will cause the Host Database to discard the static IP address assigned for the device's use on its old home network, and to assign a new static IP address (appropriate for the new home network). When we change the entry, another message is emailed to the Technical Contacts listed in the device's Host Database entry, showing the new IP address assigned for the device's use when it is attached to its new "home network." The change takes effect within an hour after this mail is sent; after that time, you need to reboot the device. (You may see some disruption to the device's network service if you don't reboot it.) The device then receives static IP service (assuming it is still attached to its new "home network"). It will still be able to use Mobile IP Service (when it visits other networks). The device is no longer deemed a Mobile IP Camper.
The Host Database entry also contains BUILDING and ROOM fields indicating where the device is located. (These values do not indicate the device's "home network"; they serve primarily as documentation, and sometimes assist in troubleshooting.) As we do not know in what building and room the device is currently located, our change does not update those fields in the Host Database entry. You may wish visit the Host Database to inspect the entry update those fields if necessary.
You might wonder why you should bother updating a device's Host Database entry after relocating it, since eventually OIT will do so for you. Recall that you will need to reboot the device after the change to the Host Database is complete; until you do so, the device's network access may be interrupted. By performing the change yourself, you exercise greater control over the timing of the change, and therefore, over any interruption in service. If you leave the change to us, it may happen an a time that is inconvenient for you. Additionally, OIT will only update the interface's IP-SUBNET-OR-ADDRESS field; since you'll eventually want to update the BUILDING and ROOM fields as well, you may as well change all three at once.
If you have questions or need assistance with any of the procedures in this document, please contact the OIT Support and Operations Center.