14.10.20081 PSIRP Inter-domain Topology Formation (ITF) Prof. Sasu Tarkoma University of Helsinki Partially based on slides by Walter Wong and Kari Visala.

14.10.2008 1

PSIRP Inter-domain Topology Formation (ITF)

Prof. Sasu TarkomaUniversity of Helsinki

Partially based on slides by Walter Wong and Kari Visala.

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Contents

• Current Inter-domain routing• PSIRP fundamentals• Interdomain Topology Formation• Interdomain routing• Conclusions

Evolution of IP routing

• Class-based routing

– A ,B and C classes

– Routing tables carried entries for all nets

– No topological aggregation (only network address boundaries)

• Classless routing

– Using the variable length subnet mask to aggregate addresses

– Routers forward mask (longest prefix)

• Too many small networks requiring multiple class C - addresses

– C class has max 254 hosts

– Huge routing tables

CIDR

• CIDR (Classless Interdomain Routing)– Routing prefixes carry topology information– Contiguous blocks of C-class addresses– Smaller routing tables– How to handle multi-homing (and mobility?)

• Solves two problems– Exhaustion of IP address space– Size and growth rate of routing tables

• Address format <IP/prefix bits>

CIDR and Route Summarization

• The difference between CIDR and route summarization – Route summarization is generally done within a

classful boundary– CIDR combines several classful networks

• Examples of classless routing protocols

– RIP version 2 (RIPv2), OSPF, Intermediate System-to-Intermediate System (IS-IS), and Enhanced Interior Gateway Routing Protocol (EIGRP)

CIDR and IPv6

• CIDR present in IPv6 (fully classless)• 128bit IPv6 address has two parts: network and host

– includes the prefix-length – a decimal value indicating the number of higher-order

bits in the address that belong to the network part• ISP aggregates all its customers' prefixes into a single

prefix and announces that single prefix to the IPv6 Internet

BGP

• BGP (Border Gateway Protocol) first became an Internet standard in 1989.

• BGP selects AS-level paths for inter-domain routing. Each AS may have multiple paths offered by neighbouring ASs.

• BGP-4 supports Classless Inter Domain Routing (CIDR) and is the routing protocol that is used today to route between autonomous systems.

• BGP uses TCP to establish a reliable connection between two BGP speakers on port 179.

• A path vector protocol, because it stores routing information as a combination of a destination and attributes of the path to that destination.

• The protocol uses a deterministic route selection process to select the best route from multiple feasible routes

BGP

• Characteristics such as delay, link utilization or router hops are not considered in this process.

• BGP runs in two modes: EBGP and IBGP. EBGP (Exterior BGP) is run between different autonomous systems, and IBGP (Interior BGP) is run between BGP routers in the same autonomous system

• BGP only recalculates routing information relative to these updates, there is no regular process that must update all of its routing information like the SPF calculations in OSPF or IS-IS

BGP cont.

• When the BGP router receives its neighbors' full BGP routing table (100k routes),

– Requires approx. 70 MB. – With the AS_PATH filters applied to inbound updates

• 32k routes in 28 MB. 60% decrease from optimal routing. • Problems

– multihomed customers forget to stop reannouncing routes from upstream A to upstream B

– peer networks leak full tables to their peers – A misconfigured router leaks out all internal more specific

routes (/48, /64, /128 prefixes) • A network black hole is often used to improve aggregation of the BGP

global routing table.

BGP Problems

• Convergence time• Limited policies• Security problems

BGP IPv4 Table Growth

Source: http://www.cidr-report.org

BGP IPv6 Table Growth

Source http://bgp.potaroo.net/v6/as2.0/index.html

AS Numbers

• 16-bit AS numbers• Current estimate is that limit will be reached on February

2011• IETF standards action in November 2006

– IANA extended the AS number field to 32 bits• 65536 to 4,294,967,296 values • From Jan, 2007 32bit values have been available from the

Regional Internet Number Registries (RIR)

Topology in address vs. routing table

Reactive AD HOC (MANET) routing

Pure source routing (minimal state in intermediate nodes)

ATM PNNI CIDR

Original IP routing

Proactive ad hoc

(MANET) routing

Host-based hop-by-hop (more state in intermediate nodes)

PSIRP?

Difficult Issues

• Convergence time of routing information• State in the network

– Per-connection state is bad? (e.g. NAT)• Independence of directories• Security of routing information

– Whom to trust? How to represent authorization?• QoS routing

PSIRP Fundamentals

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Main PSIRP design principles

• Information is multi-hierarchically organised – Higher-level information semantics are constructed in

the form of directed acyclic graphs (DAGs), starting with semantic free forwarding labels towards higher level concepts (e.g., ontologies).

• Information scoping – Mechanisms are provided that allow for limiting the

reachability of information to the parties having access to the particular mechanism that implements the scoping.

• Scoped information neutrality – Within each scope of information, data is only

forwarded based on the given (scoped) identifier.

• The architecture is receiver-driven – No entity receive data unless it has agreed to receive

the data beforehand, through appropriate signalling methods.

Communication Model

InformationHierarchies

Informationreachability/scoping

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Where we are going

Observations

No topological addresses, only labels

No explicit layering (blackboard pattern)

Security enhanced using self-certification

End-to-end reachability, control in the network

Natural support for multicast, it is the norm

Support for broadcast and all-optical label-switching technologies

Dynamic state is introduced into the network

How do we make it scale?

Pub/Sub layerPub/Sub layerPub/Sub layerPub/Sub layer

Fragmentation

Fragmentation

Link LayerLink LayerLink LayerLink Layer

ForwardingForwardingForwardingForwarding

RendezvousRendezvous

RoutingRouting

Higher LayersHigher LayersHigher LayersHigher Layers

Publish / Subscribe

Metadata(source is implementation-dependent) Data

Application Identifiers(AId)

Rendezvous Identifiers(RId)

Forwarding Identifiers(FId)

Network Transit Paths

Scope Identifiers(SId)

Includes...

Associated with...

Includes...

Resolved to...

Resolved to...

Define...

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Scopes

Scope Friends

Scope Family

Scope Company A

Data: PictureData: Mail

Spouse Father Friend Colleague

Governancepolicy

Governancepolicy

Governancepolicy

Forwarding Design

• Fast path– In-packet Bloom filters– Line-rate forwarding

• Slow path (Rendezvous)– Content-centric functions– Policies– Caching configuration– Security

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Intra-domain ForwardingIntra-domain Forwarding

• Characteristics– Links have identifiers (Link IDs)– Source routing mechanism– Install forwarding state on demand (traffic

aggregation)• Topology Manager

– Network topology graph and its maintenance– Constructs Bloom filter-based forwarding identifiers

zFilter – SummaryzFilter – Summary

• Efficient flat identifier based forwarding– Currrent zFilter size 256 bits– Link IDs are added in the zFilter (OR operation)– Verification requires one comparison (AND

operation)• Limitations

– Possible false positives– Wrong forwarding path

Subscriber Publisher

Forwardingnode

Forwardingnode

Forwardingedge node

Forwardingnode

AS: Topology AS: Topology

AS: Rendezvous AS: Rendezvous

Forwardingnode

Data Forwarding

Publish Subscribe

Createdeliverypath

ConfigureForwardingpath

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Rendezvous

• The network is defined in terms of domains and their interconnections– Interconnections between domains include upstream, transit,

downstream• Rendezvous is the central primitive

– Rendezvous on multiple layers– Builds forwarding paths

• We utilize the notion of completeness to optimize processing and mobility updates– Complete / incomplete dissemination structures between

rendezvous points– A structure is complete when the operation (sub, adv) has

been processed by all elements that should process it typically partial in a global network

– Completeness can be used for network diagnostics

Rendezvous Interconnect Architecture

● DONA does not appear to be scalable for global-level rendezvous as it stores a copy of all advertisements in all tier-1 domains.

● ROFL uses a DHT to achieve scalability, but the peer-to-peer nature of the system creates incentive problems.

● PSIRP investigates a 2-tier system where a hierarchical DHT based rendezvous interconnect network joins multiple rendezvous networks together for global reachability.

● Typically only scopes are advertised in the interconnect.

● Hierarchical structure guarantees locality for the communication.

● Local rendezvous networks and rendezvous interconnect nodes can cache results for individual public (SId, RId) pairs and subscribe to the changes by forming a multicast tree using the DHT routing alg.

Phases of Communication

Inter-domain Topology Formation

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ITF Motivation

• Current Internet structure is the starting point– BGP and inter-AS relationships

• PSIRP network model– Autonomous domains as in BGP– Controlled by different organizations– Organizational policies

• The pub/sub inter-as connections may result in different inter-AS relations than observed today– Multicast and caching

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Inter-domain Topology Formation (ITF)Inter-domain Topology Formation (ITF)

• Helps building the forwarding information – Based on policies set by operators and users– Both senders, network, and receivers can set

policies• Manages edge routers between domains

– Protection against policy violations – Protect domain internals

Motivation – Inter-domain RoutingMotivation – Inter-domain Routing

• There are approximately 10 tier-1 operators on the Internet– Full connectivity on tier-1

• Relationships– Customer-provider– Peer-peer– Sibling-sibling

• Tier-1operators– Peer with each other and do not buy traffic from other

operators– Something that looks like a monopoly

Topology Management/FormationTopology Management/Formation

• The Topology Manager (TM) is responsible for path creation/computation/management between data subscribers and publishers– The TM abstracts the location of the entities at

network edges (they deal only with data/information).

• Topology Manager – Interested in receiving information about the

network– Computes paths from publishers to subscribers– Creates/Manages forwarding paths

• Creates ZFilters

Topology Manager (TM)Topology Manager (TM)

• One or more TM per domain• Nodes (router)

– Local bootstrapping with HELLO messages– Collect local connectivity with link quality and

forwarding capabilities– Publish local connectivity information to the TM

• TM– Reconstructs the overall forwarding level

topology in the network

Topology ManagementTopology Management

• Intra-domain Topology Management– Local network topology generation

– Intra-domain forwarding structures management

– Computes network states

– Updates forwarding information

• Inter-domain Topology Management– Topology formation in the domain level

– Between administrative domains

– Configuring and maintaining inter-domain topology based on policies

Intra-domain Forwarding & zFiltersIntra-domain Forwarding & zFilters

• zFilter requirement – Knowledge of the individual links composing the

forwarding path• LIDs list generated based on the Sid and Rid

– Domain-specific end-points for data delivery – Builds a forwarding graph between end-points

• Intra-domain TM– Identifying possible virtual trees (constantly used

paths)– Traffic pattern evaluation for virtual tree creation– Lifetime and tree management (state in the

router)

Inter-domain Topology FormationInter-domain Topology Formation

• Goals– Stores forwarding information pertaining to domains– Builds forwarding paths based on operator’s policies

across domains– Connect Internet domains

Inter-domain Topology FormationInter-domain Topology Formation

• Connect multiple intra-domain Topology Managers• Communication between local topology formation and

inter-domain topology formation• Offline route computation

– Faster approach• Path construction between publishers and subscribers

through different domains

ITF – Design RequirementsITF – Design Requirements

• Flexible control of the routing policies – Packets with different Rids should have different

routing policies• High granularity

– Customers should be able to define per-Rid policies• Multi-homing, multi-path routing, and partial data transit

support• Operators are able to hide their internal topology

Inter-domain Topology FormationInter-domain Topology Formation

Routing and Forwarding

ITF – Information GatheringITF – Information Gathering

• Prior to publications– Rendezvous (RVS) informs status of subscribers

regarding Sid/Rids (quench)

• Depends on granularity of information in the RVS– Forwarding network identifiers

• ITF has to know a list of network identifiers to connect publishers to subscribers

– Landmark identifiers• Some landmark close to the subscriber knows how to

deliver publications

– Forwarding tree identifiers• Construct partial distribution trees in anticipation of

publications

ITF – Pub/sub approach benefitsITF – Pub/sub approach benefits

ITF components can subscribe to route changes• There is no need to sequentially notify each domain• Multicast support in pub/sub

– Simultaneous delivery to all ITF through common scope

• Avoids route flapping (convergence problem)• Avoids propagation problems (when to stop)

Canopy: Using Upgraphs for Pub/Sub

• The NIRA system used upgraphs for

– Allowing more control for receiver

– Finding best paths for unicast

• Canopy uses upgraphs for pub/sub

– Upgraphs combined at receiver-side rendezvous point

– Can take both subscriber & publisher policies into account,

– Supports multi-path routing

– Result is a policy-compliant multicast structure

– Can be used for both overlays and on the network layer

– Works with in-packet Bloom filter-based forwarding

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SS PP

RRRR

RR

4. Determine up-graph 1. Determine up-

graph

2. Send upgraph to rendezvous point

3. Propagate rendezvous information

5. Issue subscription

6. Propagate subscription

7. Forward subscription to matching rendezvous points

8. Combine upgraphs and perform path selection

Use the best path for delivery

Canopy Overview

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Conclusions

• Rendezvous– Connecting subscribers and publishers (scopes,

Rids)– Setting of policies– 2-tier approach with rendezvous interconnect

• Interdomain Topology Formation (ITF)– Understanding global network topology– Domains reflecting physical and organizational

boundaries– Needed for route computations (for ZFilter)– Pub/sub for route updates (with scopes)– Upgraphs for policy-compliant paths (Canopy)