Network Working Group G. Chen
Internet-Draft Z. Cao
Intended status: Informational China Mobile
Expires: January 15, 2014 M. Boucadair
France Telecom
A. Vizdal
Deutsche Telekom AG
L. Thiebaut
Alcatel-Lucent
July 14, 2013
Analysis of Port Control Protocol in Mobile Network
draft-chen-pcp-mobile-deployment-04
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 analyzed.
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 15, 2014.
Copyright Notice
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This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Benefits of Introducing PCP in Mobile Networks . . . . . . . 3
2.1. Restoring Internet Reachability . . . . . . . . . . . . . 3
2.2. Radio Resource Optimization . . . . . . . . . . . . . . . 3
2.3. Energy Saving . . . . . . . . . . . . . . . . . . . . . . 4
3. Overviews of PCP Deployment in Mobile Network . . . . . . . . 4
4. PCP Server Discovery . . . . . . . . . . . . . . . . . . . . 5
5. MN and multi-homing . . . . . . . . . . . . . . . . . . . . . 6
6. Retransmission Consideration . . . . . . . . . . . . . . . . 6
7. Unsolicited Messages Delivery . . . . . . . . . . . . . . . . 7
8. SIPTO Architecture . . . . . . . . . . . . . . . . . . . . . 8
9. Authentication Consideration . . . . . . . . . . . . . . . . 9
10. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 9
11. Security Considerations . . . . . . . . . . . . . . . . . . . 9
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
14.1. Normative References . . . . . . . . . . . . . . . . . . 9
14.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The Port Control Protocol[RFC6887] 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 behavior by managing
how incoming packets are forwarded by upstream devices such as NAT64,
NAT44 translators and firewall devices.
It should be noticed that mobile networks have particular
characteristics and therefore, there are several factors that should
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be investigated before implementing PCP in a mobile context. Without
the particular considerations , PCP may not provide desirable
outcomes. Some default behaviors may even cause negative impacts or
system failures in a mobile environment. Considering very particular
environment 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 mobile networks. The
intension of publishing this memo is to elaborate major issues during
the deployment and share the thoughts for 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 Networks
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. The behavior is described as
Category I in [I-D.ietf-opsawg-firewalls].There are applications that
can be running on the mobile device that require to be reachable from
the Internet or there could be services running behind the terminal
that require reachability from the Internet. For example, mobile
phones should be able to reachable for instant message or online
game. PCP enabled applications / devices could request a port from
the Firewall to ensure Internet reachability, and thus would lighten
the traffic flow of keep-alive by reducing the number of sending
packets. This would result in resource savings on the Firewall node
whilst still keeping the customer protected from the unwanted
traffic.
2.2. Radio Resource Optimization
3GPP network use different radio channels to transmit control
messages(e.g. signaling) and data packages(e.g. voice packages or
data flows). Always-on applications, e.g. IM(Instant Message), VoIP
or P2P based applications always generate a fair amount of keepalive
messages periodically. It's observed that a number of trivial
keepalive messages may occupy the data channel. For example, 16% of
traffic caused by instant signaling message would consume 50%~70%
radio resource in some area. It likely causes the air congestion
with voice calls and service data transmission. PCP could help to
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reduce the frequency of periodic messages aimed at refreshing a NAT/
FW binding by indicating to the mobile device the Life time of a
binding.
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 CONNECTED. That would cause more
energy on such terminals. Testing reports show that energy
consumption is dramatically reduced with prolonged sending interval
of signaling messages [VTC2007_Energy_Consumption].
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 behaviors 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.
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4. PCP Server Discovery
A straightforward solution seems that MN assume their default router
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]. MN usually receive those configurations in
PCO(Protocol Configuration Options) . PCP server name options in
DHCPv4 would not help the PCP servers discovery in that case.
A specific method in 3GPP is to extend PCO [TS24.008]information
element to transfer a request of PCP server name. However,
additional specification efforts are required in 3GPP to make that
happen.
[I-D.cheshire-pcp-anycast]and
[I-D.kiesel-pcp-ip-based-srv-disc]propose anycast-based solutions to
discover the closest PCP server on the data path. It may be worth to
consider the case when a subscriber roams to different areas, where
anycast configurations may be unavailable or operators use other
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provisioning method, for example [I-D.ietf-pcp-dhcp]. Asymmetric
routing should also be considered in the anycast-based solution.
Otherwise, the traffic would likely loses the mapping information for
the inbound traffic.
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 MGWs 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
Mobile devices 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]. In order to maximize battery life, it's desirable
that all activities on battery-powered devices needs to be
coordinated and synchronized. It's not specific to PCP. Whereas ,
those concerns also can be applied to PCP retransmission behavior.
PCP designed retransmission mechanisms on the client for reliable
delivery of PCP request. If a PCP client fails to receive an
expected response from a server, the client must retransmit its
message. The retransmission method may cause unnecessary power
consumption when a subscriber roams to a network, in which PCP is not
deployed. Several timers are specified to control the retransmission
behavior. Therefore, an appropriate implementation and configuration
are desirable to help to alleviate the concern. For example, the
time transiting to idle is normally less than default Maximum
Retransmission Time (MRT), i.e. 1024 seconds. With "no maximum"
setting of Maximum Retransmission Duration (MRD), it would cause
devices activating their uplink radio in order to retransmit the
request messages. Furthermore, the state transition and the
transmission take some times, which causes significant power
consumption. The MRD should be configured with an optimal time which
in line with activated state duration on the device.
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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 maybe 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 in a client. 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.
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 messages or unicast PCP MAP or PEER responses ) to clients
informing them of the new state of their mappings. This aims at
achieving rapid detection of PCP failure, rapid PCP recovery or PCP
mapping update. When those messages are delivered in a mobile
environment, it should be noted multicast delivery may not be
available in 3GPP network. PCP servers would use unicast delivery.
More considerations are listed as the below.
o This requires PCP servers to retain knowledge of the IP
address(es) and port(s) of their clients, for example using
redundancy design based on hot-standby, even though they have
rebooted
o Care should be taken not to generate floods of unicast messages,
e.g. to multiple thousands of MN that were served by a PCP server
that has rebooted. Such flood may have impacts 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.
o 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.
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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
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, host renumbering is inevitable. Some
considerations have been described as Address Change Events at
Section11.5 of [RFC6887]. The deletions of the mapping information
on the old MGW is necessary in order to avoid traffic sending to the
old IP address. In a mobile device context, PCP client may take the
NAS(Non-Access Stratum) layer message (e.g. "reactivation request" or
"detach request" message) as a notification to delete the old mapping
information before the subscriber moved to new MGW. Afterwards, PCP
clients install new mappings for its new IP address.
As an implementation option dedicated to 3G networks, it is also
possible to carry out Selected IP Traffic Offload in a TOF(Traffic
Offload Function) entity [TS23.060]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 deployment of a TOF is totally
transparent for user's equipments that even cannot know which traffic
is subject to TOF (NATed at the TOF) and which traffic is processed
by the MGW. 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
the similar architecture, in which a PCP proxy with advanced
functions[I-D.ietf-pcp-proxy] is required on the offloading point to
dispatch requests to a right PCP server. Additional consideration
will be given for determining the each traffic flow, since TOF
inspects the NAS and RANAP(Radio Access Network Application Part)
messages to build the local UE context and local session context.
The traffic flow can't be identified with 5 tuples. The offloaded IP
flow is indicated with Radio Access Bearer Identifier (RAB-ID). PCP
proxy must understand RAB-ID and map the identifier with each IP
flow.
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9. Authentication Consideration
The general authentication requirements have been analyzed in
[I-D.reddy-pcp-auth-req]. In mobile networks, it is desirable to
reuse the existing credentials on the UE for the pcp authentication
between involved entities. This way makes the deployment of
authentication easiler.
The [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 the EAP framework, the EAP
authentication server could be the co-located with the PCP server or
separated and located on a third-party entity. If the EAP
authentication server is placed on the AAA/Radius server, there is a
need of an interface between the PCP server and AAA. But per our
investigation of 3GPP networks, most exisiting NAT devices do not
have such an interface with AAA. So in practical deployment, this
could be taken into consideration.
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
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 Dan Wing, Stuart Cheshire, Ping Lin
and Tao Sun for their discussion and comments.
Many thanks to Reinaldo Penno and Tirumaleswar Reddy for their
detailed reviews.
14. References
14.1. Normative References
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[I-D.ietf-pcp-authentication]
Wasserman, M., Hartman, S., and D. Zhang, "Port Control
Protocol (PCP) Authentication Mechanism", draft-ietf-pcp-
authentication-01 (work in progress), October 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-07
(work in progress), March 2013.
[I-D.ietf-pcp-proxy]
Boucadair, M., Penno, R., and D. Wing, "Port Control
Protocol (PCP) Proxy Function", draft-ietf-pcp-proxy-03
(work in progress), June 2013.
[I-D.reddy-pcp-auth-req]
Reddy, T., Patil, P., Wing, D., and R. Penno, "PCP
Authentication Requirements", draft-reddy-pcp-auth-req-04
(work in progress), July 2013.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[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.
[RFC6887] Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
Selkirk, "Port Control Protocol (PCP)", RFC 6887, April
2013.
[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
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[I-D.cheshire-pcp-anycast]
Cheshire, S., "PCP Anycast Address", draft-cheshire-pcp-
anycast-01 (work in progress), March 2013.
[I-D.ietf-opsawg-firewalls]
Baker, F. and P. Hoffman, "On Firewalls in Internet
Security", draft-ietf-opsawg-firewalls-01 (work in
progress), October 2012.
[I-D.kiesel-pcp-ip-based-srv-disc]
Kiesel, S. and R. Penno, "PCP Server Discovery based on
well-known IP Address", draft-kiesel-pcp-ip-based-srv-
disc-00 (work in progress), February 2013.
[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-02
(work in progress), February 2013.
[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,
March 2012.
[VTC2007_Energy_Consumption]
, "Energy Consumption of Always-On Applications in WCDMA
Networks", 2007.
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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
Vizdal Ales
Deutsche Telekom AG
Tomickova 2144/1
Prague 4, 149 00
Czech Republic
Email: ales.vizdal@t-mobile.cz
Laurent Thiebaut
Alcatel-Lucent
Email: laurent.thiebaut@alcatel-lucent.com
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