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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------
13
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------
11
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!
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/
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 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)
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