Validating%20the%20Resilience%20Mechanisms%20for%20the%20Packet%20Switched%20Domain%20in%203G%20Networks

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Validating the Resilience Mechanisms for the
Packet Switched Domain in 3G Networks

• Jari Hietanen
• Nokia Networks
• Helsinki
• Supervisor Professor Raimo Kantola
• Instructor Juhani Helske (M.Sc.)

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Agenda

• Background for the Thesis
• The Objective and the Scope
• Packet Switched Domain in 3G Networks
• Resilience Mechanisms in Gn Interface
• Validation of the Existing Resilience Mechanism
  in Gn Interface
• Validation of New Resilience Mechanisms
• Validation Results and Comparison of Resilience
  Mechanisms
• Conclusions

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Background for the Thesis

• Mobile networks are evolving from 2G towards 3G.
• So far the most of traffic has been voice
  traffic.
• Now amount of data traffic is growing faster than
  traditional voice traffic.
• New packet based services
• Multimedia messaging
• Wireless Internet browsing
• Advertising
• Entertainment services

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Voice and data traffic volume evolution in
Western Europe
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New Challenges for Telecom Vendors and Operators

• Amount of IP based data traffic is increasing in
  3G networks.
• Traditionally people are used to high
  availability and service quality in circuit
  switched PSTN and GSM netwoks.
• Now people are expecting the same level of
  availability and quality also in packet switched
  3G networks.
• This causes huge challenge for vendors and
  operators to develop and build fault tolerant and
  resilient networks, which quarantee high
  availability of the networks.
• 3G networks are complex
• Several physical transmission mediums (e.g. IP,
  ATM, SS7, GTP..)
• Many protocols and network layers
• Multi-vendor HW

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Terms

• What is the difference between terms resilience
  and redundancy?
• RESILIENCE
• Ability of the system to function seamlessly in
  the event of the failure of any single item of
  hardware or failure of the software package.
• REDUNDANCY
• According to Collins English dictionary, the term
  redundancy means duplication of components in
  electronic or mechanical equipment so that
  operations can continue following failure of a
  part or repetition of information or inclusion of
  additional information to reduce errors in
  telecommunication transmissions and computer
  processing.

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The Objective and the Scope of the Thesis

• The objective
• The main objective was to study and validate
  existing and new resilience mechanisms for the Gn
  interface in 3G networks.
• The scope
• Thesis concentrated only on resilience mechanisms
  of data link layer (L2) and network layer (L3).
  Upper and lower level resilience solutions were
  out of the scope.
• The implementation of Gn interface architecture
  and resilience mechanisms are not standardized.
  Thesis concentrated on validating Nokias
  implementation to build the resilient Gn
  interface between 3G SGSN and GGSN network
  elements.

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The UMTS Network Architecture
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The 3G SGSN

• Main functions
• Subscriber authentication authorization.
• User data tunneling and routing (acts as a
  gateway for user data tunneling between RNC and
  GGSN, separate tunnels towards RNC and GGSN for
  each connection).
• Mobility management (controls the location, state
  and security of UE).
• Session management (managed through resource
  monitoring, admission control and PDP context
  creation, modification and deletion).
• Traffic management (performs packet classifying,
  policing, buffering, shaping, marking and
  scheduling to ensure that all connections receive
  appropriate Quality of Service).
• Short message delivery.
• Collection of charging data and traffic
  statistics.

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The 3G GGSN

• Main functions
• Signalling towards access networks. There is
  signalling, which is required for creating,
  modifying and deleting the PDP contexts. The
  request for PDP context creation comes always
  from an external network equipment.
• Signalling towards data networks. Some of the
  signalling is required for configuring the PDP
  context. Most of this signalling happens when the
  PDP context is created. It is used e.g.
  allocating a IP address for User Equipment (UE).
• Charging. The GGSN analyses user plane traffic
  and reports the metering results to the charging
  system via signalling interfaces.
• Subscription management, authentication and
  session control. GGSN may need to authenticate
  mobile subscribers before PDP context can be
  created. In addition GGSN may need to know what
  services the mobile subscriber is allowed to use.
• Lawful interception. In many countries local
  authorities require possibility for monitoring
  the traffic for certain mobile subscribers.

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The GPRS Tunneling Protocol (GTP)

• GPRS Tunnelling Protocol (GTP) is used in Gn
  interface between GPRS Support Nodes (GSNs) in
  UMTS and also in GPRS backbone networks.
• GTP allows multi-protocol packets to be tunnelled
  through the UMTS or GPRS backbone between GSNs
  and UMTS Terrestrial Radio Access Network
  (UTRAN).
• GTP protocol is divided in to GTP Control Plane
  (GTP-C) and GTP User Plane (GTP-U) procedures.

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GTP Path Management Messages and timers

• Echo Request Interval
• Echo Response Interval
• The timer T3-RESPONSE holds the maximum wait time
  for a response of a request message.
• The counter N3-REQUESTS holds the maximum number
  of attempts made by GTP to send a request
  message. The recommended value is 5.

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Default Parameter values used in Nokia 3G SGSN
and GGSN
This means that GTP considers a path between GSNs
to be down from 1 to 600 seconds depending on
configuration of GTP parameters.
For example if Echo reply waiting time (T3) is
set to be 5 seconds and Echo request
retransmission (N3) is set to be 3 times, then
the time that a GTP tunnel is declared to be down
is T3 x N3 5 s x 3 15 seconds.
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Existing Resilience Mechanisms in the Gn
Interface

• Existing solution is to use dynamic OSPF routing
  protocol in 3G networks.
• 3G SGSN build on an IPSO router platform.

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Gn Interface Resilience in 2G networks

• Gn interface topology for host based elements in
  2G networks.
• 2G SGSN has not any routing functionality.

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New Resilience Mechanisms for the Gn Interface

• The problem of the existing OSPF based resilience
  mechanism for Gn backbone is that convergence
  time is not necessarily fast enough.
• The worst case scenario is that convergence time
  can be even 40 seconds, if OSPF hello protocol is
  only mechanism to detect a network failure.
• Also some operators are not willing to start to
  use a dynamic routing protocol in their Gn
  backbone network.
• Solution would be to find a suitable data link
  layer (L2) protocol to be used as Gn interface
  resilience mechanism.
• Advantages of L2 mechanisms
• Simple and flexible network architecture using L2
  resilience mechanisms.
• Similarity with 2G solution.
• No need for two separate Gn interface Virtual
  LANs (VLANs).
• Fast convergence from error situations compared
  to OSPF.

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Link Layer Resilience Mechanism validated

• Link aggregation (IEEE 802.3ad)
• Proxy ARP
• Virtual Router Redundancy Protocol (VRRP)
• Hot Standby Router Protocol (HSRP)
• Virtual MAC address based method (Nokias own
  solution)
• Bidirectional Forwarding Detection

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Validating Existing Resilience Mechanism

• First existing OSPF based (L3) resilience
  mechanism was validated building a Gn test
  network, which included 3G SGSN and GGSN nodes.
• The conclusions from OSPF validation test
  results
• With default parameter values OSPF network
  convergence times are too slow. Convergence time
  can be even 40 seconds, if network includes hubs
  or switches.
• It is possible to improve the performance of OSPF
  convergence, using shorter Hello and Router Dead
  intervals.
• Minimum value for Hello interval is 1 second and
  for Router Dead interval 4 seconds.

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Validating New Resilience Mechanisms

• Some of the link layer techniques were validated
  building a test network and measuring convergence
  times from link failure situations.
• Some techniques were analyzed with other methods
• using literature sources
• interviewing

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Link Aggregation Test Results

• Test topology

• Failover time from a link failure about 500 ms.

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VRRP Test Results

• Test topology

• Failover time from a link failure about 2.8
  seconds.

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Test results of Virtual MAC based Mechanism

• Test topology

• Failover time from a link failure about 600 ms.

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Comparison of Resilience Mechanisms

• Existing OSPF based solution
• A dynamic routing protocol is easy to configure
  and administrate for network operator.
• OSPF is common protocol, which is supported by
  also by other vendors products.
• - OSPF protocol not suitable to all networks
  topologies, e.g. if network includes switches
• - Convergence time from error situation rather
  slow.
• - Hello protocol based mechanism is not fast
  enough if the default parameter values are used.

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Link aggregation

• Does not require new hardware
• Does not waste extra interface or line card
  capacity in router.
• Fast recovery time from failure situation
  (about 500 ms)
• Standardized solution
• Economic and flexible method to increase
  network capacity

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Proxy ARP

• It can be added to a single router on a network
  without disturbing the routing tables of the
  other routers on the network.
• IP hosts can be used without configuring
  default gateway.
• IP network does not need to have any routing
  intelligence.
• Fast recovery time from failure situations.
• Standardized solution.
• It is easy to implement (only the gateway
  router has to be updated to support Proxy ARP).
• - Hosts need larger ARP tables to handle
  IP-to-MAP mappings.
• - The amount of ARP traffic increases.
• - This does not work with all network topologies
  (e.g. more than one router connecting two
  physical networks).

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VRRP

• It offers higher availability of the default
  path without requiring configuration of dynamic
  routing protocol or router discovery protocols on
  every end-host.
• Fast recovery from failure situations (average
  about 2.8 s).
• Simple and flexible protocol.
• Standardized solution.
• - Not feasible to use with all network node
  architectures.

HSRP
The protocol offers similar functionality with
VRRP. - It is a patented solution, which is not
possible to be utilized in free of charge. - It
does not offer anything superior compared to
VRRP.
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Bidirectional Forwarding Detection

• OSPF alone offers minimum convergence time of
  1-2 second. With BFD protocol OSPF can provide
  sub-second failure detection time. According to
  measurements made by Marko Luoma (Lic.Tech.) in
  HUT Networking Laboratory convergence time can be
  even 75 ms.
• Because BFD is not tied to any particular
  routing protocol, it can be used as a generic and
  consistent failure detection mechanism for e.g.
  OSPF, IS-IS, EIGRP, and BGP routing protocols.
• CPU usage is minimal for route processor.
• - BFD can potentially generate false alarms and
  signaling a link failure when one does not exist.
  Because the timers used for BFD are so tight, a
  brief interval of data corruption or queue
  congestion could potentially cause BFD to miss
  enough control packets to allow the detect-timer
  to expire

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Conclusions

• OSPF based layer 3 resilience mechanism is
  suitable for Gn network alone, but in the future
  it will be too slow mechanism to detect network
  failures.
• For a greenfield operator who does not have
  existing 2G network, it might be reasonable to
  implement resilience using OSPF. A dynamic
  routing protocol based solution offers advantages
  compared to link layer solution. For example
  configuration of a network is simpler
• However, the performance of Gn network can be
  improved using some L2 mechanism under OSPF.
• Bidirectional Forwarding Detection seems to be
  most suitable L2 resilience mechanism to be used
  with OSPF.

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Questions?