IETF P. McCann, Ed.
Internet-Draft J. Kaippallimalil, Ed.
Intended status: Informational Huawei
Expires: April 15, 2016 October 13, 2015

Communicating Prefix Cost to Mobile Nodes


In a network implementing Distributed Mobility Management, it has been agreed that Mobile Nodes (MNs) should exhibit agility in their use of IP addresses. For example, an MN might use an old address for ongoing socket connections but use a new, locally assigned address for new socket connections. Determining when to assign a new address, and when to release old addresses, is currently an open problem. Making an optimal decision about address assignment and release must involve a tradeoff in the amount of signaling used to allocate the new addresses, the amount of utility that applications are deriving from the use of a previously assigned address, and the cost of maintaining an address that was assigned at a previous point of attachment. As the MN moves farther and farther from the initial point where an address was assigned, more and more resources are used to redirect packets destined for that IP address to its current location. The MN currently does not know the amount of resources used as this depends on mobility path and internal routing topology of the network(s) which are known only to the network operator. This document provides a mechanism to communicate to the MN the cost of maintaining a given prefix at the MN's current point of attachment so that the MN can make better decisions about when to release old addresses and assign new ones.

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Table of Contents

1. Introduction

Previous discussions on address agility in distributed mobility management have focused on "coloring" prefixes with one of a small number of categories, such as Fixed, Sustained, or Nomadic. The assumption here is that the MN should use a permanent home address for sessions that need a persistent IP address, and a local, ephemeral address for short-lived sessions such as browsing. However, a small set of address categories lacks expressive power and leads to false promises being made to mobile nodes. For example, the concept that a home address can be maintained permanently and offered as an on-link prefix by any access router to which the MN may be attached in future is simply not attainable in the real world. There will always exist some access routers that do not have arrangements in place with the home network to re-route (via tunneling or other mechanisms) the home prefix to the current point of attachment.

Conversely, the assumption that a Nomadic prefix will never be available to an MN after it changes its current point of attachment is too limiting. There is no reason why an MN should not be able to keep a prefix that was assigned by a first network after it moves to a second network, provided that measures are put in place to re-route such prefixes to the new attachment point.

Rather, this document argues that there is in reality a continuum of cost associated with an address as the MN moves from one attachment point to another or from one network to another. The sources of the cost are the increased latency, network bandwidth, and network state being maintained by a network-based mobility management scheme to route packets destined to the prefix to the MN's current point of attachment. By communicating this cost to the MN every time its attachment point changes, the MN can make intelligent decisions about when to release old addresses and when to acquire new ones.

The cost should be communicated to the MN because of several constraints inherent in the problem:

The MN is the entity that must make decisions about allocating new addresses and releasing old ones. This is because only the MN has the information about which addresses are still in use by applications or have been registered with other entities such as DNS servers.
Only the network has information about the cost of maintaining the prefix in a network-based mobility management scheme, because the MN cannot know the network topology that gives rise to the inefficiencies.

If the cost of maintaining a prefix is not made available to the mobile node, it may attempt to infer the cost through heuristic mechanisms. For example, it can measure increased end-to-end latency after a mobility event, and attribute the increased latency to a longer end-to-end path. However, this method does not inform the MN about the network bandwidth being expended or network state being maintained on its behalf. Alternatively, a MN may attempt to count mobility events or run a timer in an attempt to guess at which older prefixes are more costly and in need of being released. However, these methods fail because the number of mobility events is not an indication of how far the MN has moved in a topological sense from its original attachment point which is what gives rise to the costs outlined above. Re-allocating an address upon expiration of a timer may introduce uneccessary and burdensome signaling load on the network and air interface.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

1.2. Abbreviations

ANDSF Access Network Discovery and Selection Function
MN Mobile Node
MPTCP Multi-Path Transmission Control Protocol
ND Neighbor Discovery
NGMN Next Generation Mobile Networks
NUD Neighbor Unreachability Detection
OMA-DM Open Mobile Alliance - Device Management
PIO Prefix Information Discovery
PGW Packet data network Gateway
SeND Secure Neighbor Discovery
SGW Serving Gateway

2. Motivation

The Introduction speaks in general terms about the cost of a prefix. More specifically, we are talking about the aggregate amount of state being maintained in the network on behalf of the mobile node in addition to the transport resources being used (or wasted) to get packets to the MN's current point of attachment.

In a non-mobile network, the addresses can be assigned statically in a manner that is aligned with the topology of the network. This means that prefix aggregation can be used for maximum efficiency in the state being maintained in such a network. Nodes deep in the network need only concern themselves with a small number of short prefixes, and only nodes near the end host need to know longer more specific prefixes. In the best case, only the last-hop router(s) need to know the actual address assigned to the end host. Also, routing protocols ensure that packets follow the least-cost path to the end host in terms of number of routing hops or according to other policies defined by the service provider, and these routing paths can change dynamically as links fail or come back into service.

However, mobile nodes in a wide-area wireless network are often handled very differently. A mobile node is usually assigned a fixed gateway somewhere in the network, either in a fixed central location or (better) in a location near where the MN first attaches to the network. For example, in a 3GPP network this gateway is a PGW that can be allocated in the home or visited networks. Initially, the cost of such a prefix is the state entry in the fixed gateway plus any state entries in intermediate tunneling nodes (like SGWs) plus whatever transport resources are being used to get the packet to the MN's initial point of attachment.

When an MN changes its point of attachment, but keeps a fixed address, the cost of the prefix changes (usually it increases). Even if the fixed gateway was initially allocated very close to the initial point of attachment, as the MN moves away from this point, additional state must be inserted into the network and additional transport resources must be provided to get the packets to the current point of attachment. For example, a new SGW might be allocated in a new network, and now the packets must traverse the network to which the MN first attached before being forwarded to their destination, even though there may be a better and more direct route to communication peers from the new network. Whatever aggregation was possible at the initial point of attachment is now lost and tunnels must be contructed or holes must be punched in routing tables to ensure continued connectivity of the fixed IP address at the new point of attachment. Over time, as the MN moves farther and farther from its initial point of attachment, these costs can become large. When summed over millions of mobile nodes, the costs can be quite large.

Obviously, the assignment of a new address at a current point of attachment and release of the older, more costly prefix will help to reduce costs and may be the only way to meet emerging more stringent latency requirements [8]. However, the MN does not in general know the current cost of a prefix because it depends on the network topology and the number of handovers that have taken place and whether these handovers have caused the MN to transition between different topological parts of the network. It is the purpose of the protocol extension defined in this document to communicate the current cost of a prefix to the MN so that it can make intelligent decisions about when to get a new address and when to release older addresses. Only the MN can make a decision about when to release an address, because it is the only entity that knows whether applications are still listening waiting to receive packets at the old address.

Section 4 describes MN behavior when Router Advertisements with Prefix Cost is received.

3. Prefix Cost Sub-option

This document defines a prefix cost option to be carried in router advertisements. It is a sub-option that carries meta-data as defined by Korhonen et al. [7]

  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
 |     TBD1      |        1      |C|         Reserved1           |
 |           Prefix Cost         |           Reserved2           |

Figure 1: Prefix Cost suboption

The prefix cost is carried as a 16-bit, unsigned number in network byte order. An higher number indicates an increased cost.

This sub-option is appended in Router Advertimsement messages that are sent on a periodic basis. No additional signaling cost is incurred to support this mechanism.

It should be noted that link layer events do not cause a change in the prefix cost.

The prefix cost is for a connection segment. No end-to-end congestion or flow control mechanisms are implied with this cost.

4. Host Considerations

Prefix Cost in a Router Advertisement PIO serves as a hint for the MN to use along with application knowledge, MN policy configuration on network cost and available alternative routes to determine the IP addresses and routes used. For example, if the application is downloading a large file, it may want to maintain an IP address and route until the download is complete. On the other hand, some applications may use multiple connections (e.g., with MPTCP) and may not want to maintain an IP address above a configured cost. It could also be the case that the MN maintains the IP address even at high cost if there is no alternative route/address. These decisions are made based on configured policy, and interaction with applications, all of which are decided by the MN.

When the MN is ready to release an IP address, it may send a DHCPv6 [5] Release message. The network may also monitor the status of a high cost connection with Neighbor Unreachability Detection (NUD) [2], [6], and determine that an address is not used after the NUD times out. The network should not continue to advertise this high cost route following the explicit release of the address or NUD timeout. It can initiate the release of network resources dedicated to providing the IP address to the MN.

The operator of the network or host's service provider can configure policy that determines how the host should handle the prefix cost values. In a 3GPP network, the subscription provider may configure policies in the host via OMA-DM or S14 (ANDSF). For example, the service provider may configure rules to state that prefix cost values below 500 indicate low cost and ideal access network conditions, values from 501 - 5000 indicate that the host should try to relocate connections, and values above 5000 indicate a risk and impending loss of connectivity. The policies themselves can be (re-)configured as needed by the operator. Prefix cost information with each Router Advertisement allows the host to interpet a simple number and associated policies to (re-)select optimal routes. For networks service providers, when this cost is associated with charging, it can be a valuable tool in dynamically managing the utilization of network resources.

This draft does not aim to provide definitive guidance on how an OS or application process receives indications as a result of prefix cost option being conveyed in Router Advertisements. Only high level design options are listed here. New socket options or other APIs can be used to communicate the cost of an address in use on a given connnection. For example, a new "prefix-cost" socket option, if set, can indicate that the application is interested in being notified when there is a change in the prefix cost. The actual mechanisms used to either notify or other means of busy polling on this change of prefix cost information need to be specified in other drafts. An alternative to the application discovering the changed prefix cost is to use a model where a connection manager handles the interface between the network and the application (e.g., Android Telephony Manager [9]). In this case, the connection manager is responsible to select and manage addresses based on policies (configured via OMA-DM or S14) and prefix cost obtained from the Router Advertisements.

5. Security Considerations

Security of the prefix cost option in the PIO needs to be considered. Neighbor Discovery (ND) and Prefix Information Option (PIO) security are described in [2] and [3]. A malicious node on a shared link can advertise a low cost route in the prefix cost option and cause the MN to switch. Alternatively, an incorrect higher cost route in the prefix cost option can result in the suboptimal use of network resources. In order to avoid such on-link attacks, SeND [4] can be used to reject Router Advertisements from nodes whose identities are not validated.

6. IANA Considerations

This memo defines a new Prefix Information Option (PIO) sub-option in Section 3.

7. References

7.1. Normative References

[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[2] Narten, T., Nordmark, E., Simpson, W. and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007.
[3] Draves, R. and D. Thaler, "Default Router Preferences and More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191, November 2005.
[4] Arkko, J., Kempf, J., Zill, B. and P. Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, DOI 10.17487/RFC3971, March 2005.
[5] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 2003.
[6] Nordmark, E. and I. Gashinsky, "Neighbor Unreachability Detection Is Too Impatient", RFC 7048, DOI 10.17487/RFC7048, January 2014.

7.2. Informative References

[7] Korhonen, J., Gundavelli, S., Seite, P. and D. Liu, "IPv6 Prefix Properties", Internet-Draft draft-korhonen-dmm-prefix-properties-04, July 2015.
[8] NGMN Alliance, "NGMN 5G Whitepaper", February 2015.
[9] Android Telephony Developer's Forum,, "Android Telephony Manager"

Authors' Addresses

Peter J. McCann (editor) Huawei 400 Crossing Blvd, 2nd Floor Bridgewater, NJ 08807 USA Phone: +1 908 541 3563 EMail:
John Kaippallimalil (editor) Huawei 5340 Legacy Dr., Suite 175 Plano, TX 75024 USA EMail: