Data and Computer Communications InterNetwork Operation

Internetwork Operation

Historically IP nets gave best-effort datagram delivery to all services

Now want variety of QoS in IP networks

Explore some new network services / functions

Multicasting

Sending packet to addresses referring to group of hosts on one or more networks

Multimedia (transmission to multiple users)

Teleconferencing (forming a multicast group)

Database (all copies are updated)

Distributed computing (sending results to all)

Real time workgroups (files exchange between active group)

Have design issues in addressing / routing

LAN Multicast

LAN multicast is easy

Send to IEEE 802 multicast MAC address

Since broadcast all stations will see packet

Those in multicast group will accept it

Only single copy of packet is needed

But much harder in internetwork

Multicasting in Internetworks

Broadcast packet to each network

server does not know members of the MC group

Send multiple unicast packets

each net with members in multicast group receives a copy

Use true multicast

send single packets over any link

duplicating as needed to reach destination nets

L1: 1L2: 0L3: 1L4: 2

N1: 4N4: 2N2: 0N3: 1N5: 1N6: 1------

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MCG={N3, N5, N6}S=N1

Does not accept

Broadcast packet to each network- server does not know members of the MC group

L1: 0L2: 0L3: 1L4: 2

N1: 3N4: 2N2: 0N3: 1N5: 1N6: 1------

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MCG={N3, N5, N6}S=N1

N3,N5 & N6

Send multiple unicast packets- each net with members in multicast group receives a copy

True Multicast

Determine least cost path to each network that has host in group

This results in a spanning tree of just those nets with members in MCG

Transmit single packet along spanning tree

Routers replicate packets at branch points of spanning tree

True Multicast using Spanning Tree

Make sure you know how to generate the SP

Only 8 packets

Lower link utilization!

True Multicast using Spanning Tree Alternative Spanning Tree

Requirements for Multicasting

Router may have to forward more than one copy of packet

Need convention to identify multicast addresses (IPv4 Class D or IPv6 prefix)

Node translate between IP multicast addresses and list of networks containing group members

Router must translate between IP multicast address and LAN multicast address (IP

48 bit MAC-level multicast address)

A mechanism is required for hosts to join and leave multicast group

Routers must exchange info

which networks include members of given group

sufficient info to work out shortest path to each network

Routing algorithm to work out shortest path

Routers must determine routing paths based on source and destination addresses

Avoid unnecessary duplications and finding the shortest path!

Operation of Internet Group Management Protocol (IGMP)

Allows exchanging multicast group membership information over the LAN

Who has joined; who is leaving

IGMPv1and IGMPv2 operational model:

receivers have to subscribe to groups

sources do not have to subscribe to groups

any host can send traffic to any multicast group

problems:

spamming of multicast groups – waste of resources

establishment of distribution trees is problematic

finding globally unique multicast addresses difficult – many multicast groups with the same multicast address

Operation of Internet Group Management Protocol (IGMP) –

V3

Addresses weaknesses:

allows hosts to specify list from which they want to receive traffic: RX(N1)={N3, N5}

traffic from other hosts blocked at routers

allows hosts to block packets from sources that send unwanted traffic

IGMP Operation – Joining

IGMP host wants to make itself known as group member to other hosts and routers on LAN

IGMPv3 can signal group membership with filtering capabilities with respect to sources

EXCLUDE mode – all members except those listed

I receive packet from anyone sending to MCG EXCEPT

INCLUDE mode – only from group members listed

I ONLY receive packet from … sending Pkt. to MCG

To join send IGMP membership report message

address field multicast address of group (MCG)

sent in IP datagram

current group members receive & learn new member

routers listen to all IP multicast addresses to hear all reports IP Packet

Dest: MCG1

Refer to your notes

IGMP Operation – Keeping Lists Valid

routers periodically issue IGMP general query message

in datagram with all-hosts multicast address

hosts must read such datagrams

hosts respond with report message

router don’t know every host in a group

needs to know at least one group member still active

each host in group sets timer with random delay

host hearing another report cancels own

if timer expires, host sends report

only one member of each group reports to router

IGMP Operation -

Leaving

host leaves group by sending leave group message to all-routers static multicast address

sends a membership report message with EXCLUDE option and null list of source addresses

router determines if have any remaining group members using group-specific query message

Routing Protocols

Basic Idea in Packet Routing

Routers receive and forward packets

Make decisions based on knowledge of topology and traffic/delay conditions

Use dynamic routing algorithm

Routing decisions are based on:

Routing information - about topology & delays

Routing algorithm - that makes routing decisions based on information

Autonomous Systems (AS)

Is a group of routers and networks managed by single organization

Which exchange information via a common routing protocol

Form a connected network

at least one path between any pair of nodes

except in times of failure

Interior Router Protocol & Exterior Routing Protocol

Interior router protocol (IRP)

passes routing information between routers within AS

Exterior router protocol (ERP)

passes routing information between routers in different ASs

supports summary information on AS reachability

Gathering and Using Routing Information

Remember: Routing decisions are based on:

Routing information - about topology & delays

Routing algorithm - that makes routing decisions based on information

Three approaches

Distance-vector routing

Link-state routing

Path-vector routing

Approaches to Routing – Distance-vector

Each node (router or host) exchanges information with neighboring nodes

Used for first generation routing algorithm for ARPANET

eg. used by Routing Information Protocol (RIP)

Each node maintains vector of link costs for each directly attached network and distance and next- hop vectors for each destination

Requires transmission of lots of information between routers

distance vector & estimated path costs to all network

Changes take long time to propagate

http://www.yoraka.com/applets/

Approaches to Routing – Link-state

Designed to overcome drawbacks of distance-vector

Each router determines link cost on each interface

Advertises set of link costs to all other routers in topology (floods)

If link costs change, router advertises new values

Each router constructs topology of entire configuration

can calculate shortest path to each dest

use to construct routing table with first hop to each dest

Does not use distributed routing algorithm, but any suitable alg to determine shortest paths, eg. Dijkstra's algorithm

Open Shortest Path First (OSPF) is a link-state protocol

http://www.yoraka.com/applets/

Approaches to Routing – Link-state (Router A)

What Exterior Routing Protocols are not

link-state and distance-vector not effective for exterior router protocol

distance-vector

assumes routers share common distance metric

but different ASs may have different priorities & needs

but have no info on AS’s visited along route

link-state

different ASs may use different metrics and have different restrictions

flooding of link state information to all routers unmanageable

Exterior Router Protocols – Path-vector

Alternative path-vector routing protocol

provides info about which networks can be reached by a given router and ASs crossed to get there

does not include distance or cost estimate

Have list of all ASs visited on a route - what is reachable

Enables router to perform policy routing

eg. avoid path to avoid transiting particular AS

eg. link speed, capacity, tendency to become congested, and overall quality of operation, security

eg. minimizing number of transit ASs

Border Gateway Protocol (BGP)

Developed for use with TCP/IP internets

is preferred Exterior routing protocol of the Internet

uses messages sent over TCP connection

functional procedures

neighbor acquisition - when agree to exchange info

What to talk?

Uses OPEN and Keep alive (accept) messages

neighbor reachability - to maintain relationship

Want to be my friend and stay in touch?

Keep alive message

network reachability - to update database of routes

Let me tell you about my network dBase…

Update message (broadcasted)

BGP Messages

Open

Update

Keep alive

Notification

BGP Routing Information ExchangeR1 and R5 Implement BGPR1 talks to R5 on behalf of R2 (network 1.2)R2 talks to R1 using IRP

BGP Routing Information ExchangeBGP allows exchange of routing information between ASsR1 and R5 Implement BGP

Open Shortest Path First

IGP of Internet

Replaced Routing Information Protocol (RIP)

Use Link State Routing Algorithm

each router keeps list of state of local links to network

transmits update state info

little traffic as messages are small and not sent often

Uses least cost based on user cost metric

Topology stored as directed graph

vertices or nodes (router, transit or stub network)

edges (between routers or router to network)

Example OSPF AS

Directed Graph of AS

SPF Tree for Router

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