Gateway Location

1
Gateway Location

• Jonathan Rosenberg
• Bell Laboratories
• 8/24/98

2
Some terms

• Gateway (GW)
• Device with some sort of PSTN connectivity and IP
  connectivity, capable of terminating/initiating
  IP telephony signaling protocols, and
  terminating/initiating PSTN signaling protocols
• User Agent (UA)
• An entity with IP connectivity which wishes to
  place a call from an IP network to a user
  connected only via a telephone network. This
  entity can be an individual at a PC, a local
  gateway for phone to phone, or an intelligent IP
  device
• Location Server (LS)
• A logical entity with IP connectivity which has
  knowledge of gateways which can be used to
  terminate calls. The LS is generally a point of
  contact for user agents for completing calls to
  the telephone network. An LS may also be
  responsible for propagation of gateway
  information to other LSs. An LS may frequently
  be an H.323 gatekeeper or SIP server

3
Inter vs. Intra domain
gwloc protocol

• Intra domain
• All gateways managed by same provider
• There are multiple LSs, each needs to learn
  about gateways
• Gateways go up, come down, change attributes,
  need to update these in LS
• Policy not an issue
• Some kind of flooding protocol (mcast, SCSP)
  appropriate

GW
LS
UA
4
Intra vs. Inter domain

• Inter domain (1)
• Provider A needs to learn about gateways of other
  providers
• Providers generally have pre-established
  relationships
• Protocol used to automatically propagate new
  gateways, changes in attributes, to provider A
• Provider A may not wish to accept all gateways
  from other providers - provider policy
• Provider A makes all gateways it knows about to
  its customers, hiding the fact that they may be
  provided as a result of contracts
• Provider knows about its own gateways via
  intra-domain protocol

GW
LS
UA
gwloc
??intra-d protocol
5
Intra vs. Inter domain

• Inter domain (2)
• More open market model
• No need for intermediate provider, work directly
  with actual gateway provider
• Protocol is an advertisement protocol - gateways
  or proxies for them
• Clients directly can listen
• Still supports intermediate provider model - an
  LS can listen
• Some kind of multicast needed

LS
gwloc
UA
6
Intra vs. Inter Domain

• Inter domain (3)
• Introduce telephone numbers into existing routing
  infrastructure
• Requires all routers to understand and aggregate
  E.164
• Affects even non-IPTEL ASs
• Overloads an already overloaded infrastructure
• BAD IDEA

7
Multi-hop, one gateway

• Inter-domain feature
• An LS receives and propagates gateways learned
  from other providers
• May aggregate
• Much like todays routing models
• Allows for multi-level subcontractor-like
  arrangements between providers

8
Issues

• Call signaling will generally follow routing path
• Just like regular routing
• (-) Client cant usually communicate directly
  with gateway
• Actual gateway address lost in aggregation
• If no aggregation, then you may be able to go
  direct

• () Caller may not have billing arrangement with
  final provider, so may need to sending signaling
  through intermediary
• (-) May be able to handle billing arrangements
  w/o intermediary signaling

9
Pictorially
110/2
100/2
100/1
Signaling Path
Client calls 101
10
More issues

• Can we aggregate
• Gateway gt Router
• terminates many complex application layer
  protocols with many optional features and
  services
• Heterogeneity natural here
• Many attributes
• codecs, protocols, crypto, administrator,
  capacity, cost structures, vendor, call features
  (transfer, multi party, ISDN video??),
  vendor-specific
• Aggregating the unaggregatable
• Force aggregation by ignoring certain attributes?
• May impact final service - call can be routed to
  gateway which doesnt meet requirements
• Leave it to policy and the marketplace?

11
Multiple gateways
PSTN

• Multiple hops on and off telephone network
• Example uses?
• Drawbacks
• Multiple transcodings
• Many signaling conversions
• Loss of information
• Complexity
• Can just treat telephone connection as a modem
  link running IP
• Makes it look like single gateway case
• Why not use IP for intermediate PSTN hops? IP
  connectivity must exist for protocol operation
  anyway

12
Gateway Attributes

• Provider
• Signaling Protocols
• SIP, H.323
• Codecs
• G.723, G.729,
• Call Features
• Transfer, hold, multi-party calls
• Additional Services
• Encryption, authentication, accounting

• Billing capabilities
• payment methods
• Reachability
• Phone numbers willing to be called
• Cost for each (can be complex!!)
• Capacity
• Box vendor?
• Obscure? features
• ISDN video

13
Calling area range

• What set of phone numbers will a gateway be able
  to reach? Only local?
• Quality (on PSTN for long time) may dictate large
  calling areas
• Unusual billing irregularities

• More calls, better for provider
• Sales on calls to remote destinations
• Its a matter of business policy - SHOULD ALLOW
  ANYTHING

14
What about the user

• Red line ignored until now
• How does the user actually make use of service?
• Many models..

LS
gwloc
UA
15
User interaction possibilities

• User sends signaling message to LS containing
  destination address as telephone number
• Gateway location process transparent - clean
• User doesnt care about which gateway is used
• Users cant express preferences

• Same as previous, but signaling messages contain
  preferences
• Call-Disposition header in SIP
• Non-standard params in H.323
• limited expressiveness, better than previous

Call-Disposition cheapestproviderMCI
16
User interaction

• Explicit queries
• User formulates a query, sends it to LS
• Query contains requirements for gateway service
• Can be LDAP, SQL, SLP.
• Can be very expressive
• Response is address of gateway

• Drawbacks
• Difficulties if theres aggregation - queries may
  need to traverse routing path
• Doesnt make sense in multiple gateway case

17
Other approaches

• Web
• LS creates dynamic web pages from tables learned
• Users browse web and can select gateway
• Multicast
• Gateway info flooded into domain
• Users can build up database and made decisions
• CPL
• Upload policy in CPL

• Two pieces
• back-end protocol to distribute gateway
  information among LSs
• front-end way users access information obtained
  by LS
• may be many front-ends
• keep them separate
• model common
• We develop back end

18
Policy interactions
gateways

• Back-end/front-end model has implications on
  policy
• Service provider policy executes first to filter
  out gateways
• User policy executes on resulting gateway set
• Both policies may be based on same or different
  attributes
• gwloc must carry attributes relevant to both

gwloc
Provider Policy
provider gateway set
User policy
final gateway
19
Model is common

• Web search model
• Search engines collect pages, classify/filter
  them (provider policy), use back-end protocol
  (Harvest, etc.)
• users indicate search requirements, executed at
  provider engine (user policy), using front end
  protocol (http)

• SLP model
• DA collects services from SAs, may discard some
  (provider policy)
• UA sends query to DA, executes on DA and returns
  result (user policy)

20
What is a provider?

• Makes available services to some set of gateways
• Gateways may be owned by provider
• Gateways may be made available from other
  providers through service agreements
• Gateways may be made available to other providers
  through service agreements

• If a provider only makes available some sets of
  gateways, or chooses not to honor certain client
  policies, thats part of its service
• Can always use other provider
• Provider can just be clearinghouse - not own any
  of its own gateways
• Provider must at least have LS

21
Aggregating phone numbers

• Differing numbering plans may make phone number
  aggregation hard
• Is there an issue?