Network Working Group G. Chen
Internet-Draft Z. Cao
Intended status: Informational China Mobile
Expires: January 17, 2013 M. Boucadair
France Telecom
A. Vizdal
Deutsche Telekom AG
L. Thiebaut
Alcatel-Lucent
July 16, 2012
Analysis of Port Control Protocol in Mobile Network
draft-chen-pcp-mobile-deployment-01
Abstract
This memo provides a motivation description for the Port Control
Protocol (PCP) deployment in a 3GPP mobile network environment. The
document focuses on a mobile network specific issues (e.g. cell phone
battery power consumption, keep-alive traffic reduction), PCP
applicability to these issues is further studied and analysed.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
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This Internet-Draft will expire on January 17, 2013.
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Benefits of Introducing PCP in Mobile Network . . . . . . . . 3
2.1. Restoring Internet Reachability . . . . . . . . . . . . . 3
2.2. Keepalive Message Optimization . . . . . . . . . . . . . . 4
2.3. Energy Saving . . . . . . . . . . . . . . . . . . . . . . 4
2.4. Balance Resource Assignment . . . . . . . . . . . . . . . 4
3. Overviews of PCP Deployment in Mobile Network . . . . . . . . 5
4. PCP Server Discovery . . . . . . . . . . . . . . . . . . . . . 5
5. MN and multi-homing . . . . . . . . . . . . . . . . . . . . . 7
6. Retransmission Consideration . . . . . . . . . . . . . . . . . 7
7. Unsolicited Messages Delivery . . . . . . . . . . . . . . . . 8
8. SIPTO Architecture . . . . . . . . . . . . . . . . . . . . . . 9
9. Authentication Consideration . . . . . . . . . . . . . . . . . 10
10. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11. Security Considerations . . . . . . . . . . . . . . . . . . . 11
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
14.1. Normative References . . . . . . . . . . . . . . . . . . . 11
14.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
The Port Control Protocol[I-D.ietf-pcp-base] allows an IPv6 or IPv4
host to control how incoming IPv6 or IPv4 packets are translated and
forwarded by a network address translator (NAT) or simple
firewall(FW), and also allows a host to optimize its outgoing NAT
keepalive messages. A 3rd Generation Partnership Project (3GPP)
network can benefit from the use of the PCP service. Traffic in a
mobile network is becoming a complex mix of various protocols,
different applications and user behaviors. Mobile networks are
currently facing several issues such as a frequent keepalive message,
terminal battery consumption and etc. In order to mitigate these
issues, PCP could be used to improve terminal behaviour by managing
how incoming packets are forwarded by upstream devices such as NAT64,
NAT44 translators and firewall devices.
It should be noticed that mobile network have their particular
characteristics. There are several factors that should be
investigated before implementing PCP in a mobile context. Without
the particular considerations , PCP may not provide desirable
outcomes. Some default behaviours may even cause negative impacts or
system failures in a mobile environment. Considering very particular
environments of mobile networks , it's needed to have a document
describing specific concerns from mobile network side. That would
also encourage PCP support in mobile network as well.
This memo covers PCP-related considerations in a mobile networks.
The intension of publishing this memo is to elaborate major issues
during the deployment and share the thoughts for a potential usages
in mobile networks. Such considerations would provide a pointer to
parties interested (e.g. mobile operators) to be included in their UE
profile requirements. Some adaptation of PCP protocol might be
derived from this document. Such a work would be documented in
separated memo(s).
2. Benefits of Introducing PCP in Mobile Network
2.1. Restoring Internet Reachability
Many Mobile networks are making use of a Firewall to protect their
customers from an unwanted Internet originated traffic. The firewall
is usually configured to reject all unknown inbound connections and
only permit inbound traffic that belongs to a connection initiated
from the Firewall or NAT/PAT device. There are applications that can
be running on the terminal that require to be reachable from the
Internet or there could be services running behind the terminal that
require reachability from the Internet. PCP enabled applications /
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devices could request a port or a port range from the Firewall to
ensure Internet reachability, and thus would not need to be using
keep- alive to keep the Firewall session open. This would result in
resource savings on the Firewall node whilst still keeping the
customer protected form the unwanted traffic.
2.2. Keepalive Message Optimization
Many always-on applications, e.g. instant message and p2p
applications, are usually keeping long-lived connections with their
network peers. To make sure that they can receive incoming traffic
from their network peers, they issue periodic keep-alive messages in
order to keep the NAT/FW bindings active. As the NAT/FW binding
timer may be short and unknown to the UE, the frequency of these
keep-alive may be high. These keep-alive generally do not contain
useful data and thus correspond to "useless" usage of the radio
spectrum and of network resources, e.g.:
o Allocation of radio resources to traffic that could be avoided or
limited
o For each of these keep-alive messages, the UE needs to be put in
CONNECTED state, i.e. an operation that consumes a fair amount of
signaling
PCP helps to reduce the frequency of periodic messages aimed at
refreshing a NAT/FW binding by indicating to the mobile the Life time
of a binding. PCP helps to avoid different periodic (keep-alive)
messages from different applications by allowing the aggregation of
binding refresh within one round-trip control message with the
NAT/FW.
2.3. Energy Saving
Devices with low battery resources exist widely in mobile
environments, such as mobile terminals, advanced sensors, etc.
Mobile terminals often go to "sleep" (IDLE) mode to extend battery
life and save air resources. . Host initiated message needs to
"wake-up" mobile terminals by changing the state to active. That
would cause more energy on such terminals. Testing reports show that
energy consumption is dramatically reduced with prolonged sending
interval of signalling messages [VTC2007_Energy_Consumption].
2.4. Balance Resource Assignment
Network resources have been consumed due to heavy signaling process,
like frequent beacon message, retransmission control. Such various
usages are significantly increasing the resource consumption on a
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control plan and decreasing the efficiency on data forwarding (user
plane). For example, 16% of traffic caused by instant signalling
message would consume 50%~70% radio resource in some area. Since
radio access is a resource constrained environment, imbalance of
resource assignment would decline Call Setup Success Rate(CSSR) and
operational profits. Reduction on control plan load would shift more
resources for data transmission, which could contribute the
optimization of resource arrangements.
3. Overviews of PCP Deployment in Mobile Network
The Figure 1 shows the architecture of a mobile network. Radio
access network would provide wireless connectivity to the MN.
Packets are transmitted through Packet Switch(PS) domain heading to
MGW. MGW bear the responsibilities of address allocation, routing
and transfer. The connection between MN and MGW normally is a
point-to- point link, on which MGW is the default router for MN.
NAT/Firewall could either be integrated with MGW or deployed behind
MGW as standalone. The traffic is finally destined to application
servers, which manage subscriber service.
MN Internet
| +---------------+ +----------------+
+-+ +-----+ | +--|--+ +-------+ | +----------+ |
| | /|/ | RAN |---| PS Network | MGW |---- |NAT/FW |----| |APP Server| |
+-+ +-----+ | +--|--+ +-------+ | +----------+ |
+---------------+ +----------------+
MN: Mobile Nodes
RAN: Radio Access Network
PS: Packet Switch
MGW:Mobile GateWay
NAT/FW: Network Address Translator or Firewall
Figure 1: Mobile Networks Scenario
A PCP client could be located on MN to control the outbound and
inbound traffic on PCP servers. The PCP server is hosted by the
NAT/FW respectively. Corresponding to the various behaviours of PCP
client, MN would perform PCP operation using MAP, PEER or ANNOUNCE
opcodes. A specific application programming interface may be
provided to applications. More discussions and recommendations are
presented in following sub-sections.
4. PCP Server Discovery
A straightforward solution seems that MN assume their default router
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as the PCP Server. However, NAT/FW normally is deployed in a
different node than the MGW. Thus there is the need to ensure that
MN get information allowing them to discover a PCP server.
[I-D.ietf-pcp-dhcp] specified name options in DHCPv4 and DHCPv6 to
discover PCP server. It's expected the same mechanism could be used
in mobile network. 3GPP network allocates IP address and respective
parameter during the PDP (Packet Data Protocol)/PDN(Packet Data
Network) context activation phase (PDP and PDN represent terminology
in 3G and LTE network respectively ). On the UE, a PDP/PDN context
has same meaning which is equivalent to a network interface.
It should be noted that the Stateful DHCPv6-based address
configuration[RFC3315]is not supported by 3GPP specifications. 3GPP
adopts IPv6 Stateless Address Auto-configuration (SLAAC) [RFC4861]to
allocate IPv6 address. The UE uses stateless DHCPv6[RFC3736] for
additional parameter configuration. The MGW acts as the DHCPv6
server. PCP servers discovery could leverage current process to
perform the functionalities. The M-bit is set to zero and the O-bit
may be set to one in the Router Advertisement (RA) sent to the UE.
To carry out PCP sever discovery, a MN should thus send an
Information-request message that includes an Option Request Option
(ORO) requesting the DHCPv6 PCP Server Name option.
Regarding the IPv4 bearer, MN generally indicates that it prefers to
obtain an IPv4 address as part of the PDP context activation
procedure. In such a case, the MN relies on the network to provide
IPv4 parameters as part of the PDP context activation/ PDN connection
set-up procedure. The MN may nevertheless indicate that it prefers
to obtain the IPv4 address and configuration parameter after the PDP
Context activation by DHCPv4, but it is not available on a wide
scale[RFC6459]. PCP server name options in DHCPv4 would not help the
PCP servers discovery in that case. Alternative ways could be
considered to support PCP server discovery by a MN:
o Protocol Configuration Options(PCO) based[TS24.008]
o DNS based
A specific method in 3GPP is to extend PCO information element to
transfer a request of PCP server name. However, additional
specification efforts are required in 3GPP to make that happen.
Another alternative solution is to directly perform an inverse name
query in IN-ADDR.ARPA domain[RFC1035]. Normally, MN and NAT/FW would
locate in same IPv4 subnet. The MN could easily determine the number
of labels associating with IN-ADDR.ARPA to identify a particular
zone. For example,
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UE with IPv4 10.1.0.0/16 could resolve the 1.10.IN-ADDR.ARPA locating
PCP servers, the domain database would contain:
1.10.IN-ADDR.ARPA. PTR PCP.server.3gppnetwork.org.
When it receives a RRs in response, like PCP.server.3gppnetwork.org.
The UE could then originate QTYPE=A, QCLASS=IN queries for
PCP.server.3gppnetwork.org. to discover the addresses.
5. MN and multi-homing
As a MN may activate multiple PDP context / PDN connection, it may be
multi-homed (the UE receives at least an IP address / an IPv6 prefix
per PDN connection). Different MGW are likely to be associated with
each of these PDP context / PDN connection and may thus advertise
different PCP servers (using the mechanism described in the previous
section). In that case, a MN has to be able to manage multiple PCP
servers and to associate an IP flow with the PCP server corresponding
to the PDP context / PDN connection used to carry that IP flow.
6. Retransmission Consideration
PCP designed retransmission mechanisms on the client for reliable
delivery of PCP request. The client must retransmit request message
until successfully receiving response or determining failure.
Several timers were specified to control the retransmission behavior.
Configurable timers of Maximum Retransmission Duration(MRD) gives an
opportunity to optimize the behavior fitting into different
environments.
A class of devices in mobile networks are usually powered with
limited battery . Users would like to use such MN for several days
without charging, even several weeks in sensor case. Many
applications do not send or receive traffic constantly; instead, the
network interface is idle most of the time. That could help to save
energy unless there is data leading the link to be activated. Such
state changes is based on network-specific timer values corresponding
to a number of Radio Resource Control (RRC) states(see more at
Section 8.2.2 3GPP[TS23.060]. The time transiting to idle is
normally less than default Maximum Retransmission Time (MRT), i.e.
1024 seconds. With "no maximum" of MRD, would cause devices
activating their uplink radio in order to retransmit the request
messages. Furthermore, the state transition and the transmission
take some time, which causes significant power consumption. The MRD
should be configured with an optimal time which in line with
activated state duration on the device. That could help to avoid
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frequent wake-up the device and consume the battery.
The power consumption problem is made complicated if several PCP
clients residing on a MN. Several clients are potentially sending
requests at random times and by so doing causing MN uplink radio into
a significantly power consuming state for unnecessarily often. It's
necessary to perform a synchronization process for tidy up several
PCP clients retransmission. A time-line observer is required to
control different PCP clients resending requests in an optimal
transmission window. If the uplink radio of MN is active at the time
of sending retransmission from several clients, a proper MRD
described as above should be set for the clients. If the uplink
radio of MN is in idle mode, the time-line observer should hold
Initial Retransmission Time(IRT) for while to synchronize different
retransmitted PCP requests into same optimal transmission window.
Such duration of optimal transmission window should equal with RRC
state timer on the MN. The holding timer in idle mode may be set as
100 seconds as recommended in
[I-D.savolainen-6man-optimal-transmission-window]. Several PCP
clients should wait a random amount of time between 0 and 100
milliseconds to prevent synchronization of all PCP clients.
7. Unsolicited Messages Delivery
When the states on NAT/FW have been changed like reboot or changed
configuration, PCP servers can send unsolicited messages (e.g.
ANNOUNCE Operation )to clients informing them of the new state of
their mappings. This aims at achieving rapid detection of PCP
failure and rapid PCP recovery. However, it may induce difficulties
in mobile environments.
Multicast delivery may not be available in 3GPP network, because it
optionally supports IP Multicast routing of packets. When multicast
delivery is not possible, PCP servers may use unicast delivery of
ANNOUNCE noting that
o This requires PCP servers to retain knowledge of the IP
address(es) and port(s) of their clients even though they have
rebooted
o Care should be taken not to generate floods of unicast ANNOUNCE
messages, e.g. to multiple thousands of MN that were served by a
PCP server that has rebooted. Such flood may have a detrimental
impact on Mobile Networks as it may imply the simultaneous
generation of Paging process(see more at Section 8.2.4
3GPP[TS23.060]) for very big numbers of MN.
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Thus a PCP server SHOULD take care to throttle unicast ANNOUNCE
messages it sends towards a collection of MN.
Furthermore, such paging function is optionally supported at some
particular nodes, e.g. Traffic Offload Function (TOF) in Selected IP
Traffic Offload architecture (more discussions on this issues is
described in Section 7). The delivery of unsolicited messages would
fail in this case.
8. SIPTO Architecture
Since Release 10, 3GPP starts supporting of Selected IP Traffic
Offload (SIPTO) function defined in [TS23.060], [TS23.401].The SIPTO
function allows an operator to offload certain types of traffic at a
network node close to the UE's point of attachment to the access
network. It can be achieved by selecting a set of MGWs that is
geographically/topologically close to a UE's point of attachment.
Two variants of solutions has specified in 3GPP.
The mainstream standard deployment relies on selecting a MGW that is
/ are geographically/ topologically close to a UE's point of
attachment. This deployment may apply to both 3G and LTE. The MN
may sometimes be requested to re-activate its PDP context / PDN
connection, in which case it is allocated a new MGW and thus a new IP
address and a new PCP server. In this case SIPTO has no detrimental
impact on PCP as SIPTO resolves to a change of MGW and of PCP server.
As an implementation option dedicated to 3G networks, it is also
possible to carry out Selected IP Traffic Offload in a TOF entity
located at the interface of the Radio Access Network i.e. in the path
between the Radio stations and the Mobile Gateway. The TOF decides
on which traffic to offload and enforces NAT for that traffic. The
point is that the deployment of a TOF is totally transparent for the
UE that even cannot know which traffic is subject to TOF (NATed at
the TOF) and which traffic is processed by the MGW (and the FW/NAT
controlled by the PCP server whose address has been determined per
mechanisms described in section 5 of this document). In case of TOF
deployment, the PCP server advertised by the MGW does not take into
account the NAT carried out by the TOF function.
Therefore, PCP client doesn't know which PCP servers should be
selected to send the request.
[I-D.rpcw-pcp-pmipv6-serv-discovery]provides a solution in similar
architecture, in which a smart PCP proxy[I-D.ietf-pcp-proxy] is
required on the offloading point to dispatch requests to a right PCP
server. However, TOF in 3GPP stores radio network layer
information(e.g. RAB ID) to build the local offload context. That
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can't directly be used to identify a IP flow with 5 tuples.
Additional functionalities is required to map identifier of IP flow
to RAB ID. PCP proxy may need to include such radio link information
in its local context.
9. Authentication Consideration
The authentication issue in PCP is important to any operating
networks, because operators do not want unauthenticated requests to
control their NAT/FW ports and addresses. In mobile networks, this
issue becomes especially important due to the fact that the mis-
function of Carrier Grade NAT will severely destroy user experience
and network operating.
The problem of PCP authentication comes from the fact that the PCP
client (device) and PCP server (FW) usually do not have trust pre-
established relationship with each other. To ensure client
authentication, we can either use in-band or out-of-band solutions.
In-band means that the authentication service is provided within the
PCP exchange (e.g., by defining extended options), while out-of-band
solutions handle the problem by establishing new trust relationships
or reuse existing trust without extending the PCP base protocol.
As an in-band solution, [I-D.ietf-pcp-authentication] has provided
solutions for PCP authentication, in which an EAP option is included
in the PCP requests from the devices. In mobile network,
provisioning of new credentials to mobile devices is a difficult
task. Taking this into consideration, using EAP-SIM/EAP-AKA/ EAP-
AKA' authentication is recommended as in-band solution for 3GPP
network.
One possible out-band solution is the use of open authentication
capability such as 3GPP GAA (Generic Authentication Architecture)
defined in 3GPP[TS33.220]. So that, the PCP client can invoke the
authentication ability provided by the operator. The other way is to
reuse the trust relationship between UE and the MGW. Because the UE
has been authenticated to the MGW during context setup, if the MGW
delegates its trust to the NAT/FW device (PCP server), the NAT/FW
device can trust the PCP requests from those users.
10. Conclusion
PCP mechanism could be potentially adopted in different usage
contexts. The deployment in mobile network described applicability
analysis, which could give mobile operators a explicit recommendation
for PCP implementation. Operators would benefit from such particular
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considerations. The memo would take the role to document such
considerations for PCP deployment in mobile network.
11. Security Considerations
TBD
12. IANA Considerations
This document makes no request of IANA.
13. Acknowledgements
The authors would like to thank Ping Lin an Tao Sun for their
discussion and comments.
14. References
14.1. Normative References
[I-D.ietf-pcp-authentication]
Wasserman, M., Hartman, S., and D. Zhang, "Port Control
Protocol (PCP) Authentication Mechanism",
draft-ietf-pcp-authentication-00 (work in progress),
June 2012.
[I-D.ietf-pcp-base]
Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
Selkirk, "Port Control Protocol (PCP)",
draft-ietf-pcp-base-26 (work in progress), June 2012.
[I-D.ietf-pcp-dhcp]
Boucadair, M., Penno, R., and D. Wing, "DHCP Options for
the Port Control Protocol (PCP)", draft-ietf-pcp-dhcp-03
(work in progress), May 2012.
[I-D.ietf-pcp-proxy]
Boucadair, M., Dupont, F., Penno, R., and D. Wing, "Port
Control Protocol (PCP) Proxy Function",
draft-ietf-pcp-proxy-00 (work in progress), April 2012.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
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[RFC3736] Droms, R., "Stateless Dynamic Host Configuration Protocol
(DHCP) Service for IPv6", RFC 3736, April 2004.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[TS23.060]
"General Packet Radio Service (GPRS); Service description;
Stage 2", June 2012.
[TS23.401]
"General Packet Radio Service (GPRS) enhancements for
Evolved Universal Terrestrial Radio Access Network
(E-UTRAN) access", June 2012.
14.2. Informative References
[I-D.rpcw-pcp-pmipv6-serv-discovery]
Reddy, T., Patil, P., Chandrasekaran, R., and D. Wing,
"PCP Server Discovery with IPv4 traffic offload for Proxy
Mobile IPv6", draft-rpcw-pcp-pmipv6-serv-discovery-00
(work in progress), February 2012.
[I-D.savolainen-6man-optimal-transmission-window]
Savolainen, T. and J. Nieminen, "Optimal Transmission
Window Configuration Option for ICMPv6 Router
Advertisement",
draft-savolainen-6man-optimal-transmission-window-00 (work
in progress), June 2012.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC6459] Korhonen, J., Soininen, J., Patil, B., Savolainen, T.,
Bajko, G., and K. Iisakkila, "IPv6 in 3rd Generation
Partnership Project (3GPP) Evolved Packet System (EPS)",
RFC 6459, January 2012.
[TS24.008]
"Mobile radio interface Layer 3 specification; Core
network protocols; Stage 3", 9.11.0 3GPP TS 24.008,
June 2012.
[TS33.220]
"Generic Authentication Architecture (GAA); Generic
Bootstrapping Architecture (GBA)", 10.1.0 3GPP TS 33.220,
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March 2012.
[VTC2007_Energy_Consumption]
"Energy Consumption of Always-On Applications in WCDMA
Networks", 2007.
Authors' Addresses
Gang Chen
China Mobile
No.32 Xuanwumen West Street
Xicheng District
Beijing 100053
China
Email: phdgang@gmail.com
Zhen Cao
China Mobile
No.32 Xuanwumen West Street
Xicheng District
Beijing 100053
China
Email: caozhen@chinamobile.com
Mohamed Boucadair
France Telecom
No.32 Xuanwumen West Street
Rennes,
35000
France
Email: mohamed.boucadair@orange.com
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Vizdal Ales
Deutsche Telekom AG
Tomickova 2144/1
Prague 4,, 149 00
Czech Republic
Phone:
Fax:
Email: ales.vizdal@t-mobile.cz
URI:
Laurent Thiebaut
Alcatel-Lucent
Phone:
Fax:
Email: laurent.thiebaut@alcatel-lucent.com
URI:
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