1CMPE 150- Introduction to Computer Networks

CMPE 150

Fall 2005Lecture 23

Introduction to Computer Networks

2CMPE 150- Introduction to Computer Networks

Announcements

• Homework 4 due on Wed.,11.23.05.• No class on Friday, 11.25.05.• We will have a “real” lab next week.

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Last Class…

• Routing (cont’d).• Finished DV.• Link State.• Hierarchical routing.

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Today

• Finish routing.– Many-to-many routing.• Broadcast.• Multicast.

• Internetworking.

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Many-to-Many Routing

• Support many-to-many communication.

• Example applications: multi-point data distribution, multi-party teleconferencing.

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Broadcasting

• Send to ALL destinations.• Several possible routing mechanisms to broadcasting.

• Simplistic approach: send separate packet to each destination.– Simple but expensive.

– Source needs to know about all destinations.

• Flooding:– May generate too many duplicates (depending on

node connectivity).

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Multidestination Routing

• Packet contains list of destinations.• Router checks destinations and determines

on which interfaces it will forward packet.– Router generates new copy of packet for each output

line and includes in packet only the appropriate set of destinations.

– Eventually, packets will only carry 1 destination.

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Spanning Tree Routing

• Use spanning tree (sink tree) rooted at broadcast initiator.

• No need for destination list.• Each on spanning tree forwards packets on

all lines on the spanning tree (except the one the packet arrived on).

• Efficient but needs to generate the spanning tree and routers must have that information.

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Reverse Path Forwarding• Routers don’t have to know spanning tree.• Router checks whether broadcast packet

arrived on interface used to send packets to source of broadcast.– If so, it’s likely that it followed best route and thus not

a duplicate; router forwards packet on all lines.– If not, packet discarded as likely duplicate.

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Broadcast Routing

Reverse path forwarding. (a) A subnet. (b) a Sink tree. (c) The tree built by reverse path forwarding.

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Multicasting

• Special form of broadcasting:– Instead of sending messages to all nodes, send

messages to a group of nodes.

• Multicast group management:– Creating, deleting, joining, leaving group.

– Group management protocols communicate group membership to appropriate routers.

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Multicast Routing

• Each router computes spanning tree covering all other participating routers.– Tree is pruned by removing branches that do not

contain any group members.

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Shared Tree Multicasting• Source-rooted tree approaches don’t scale

well!– 1 tree per source, per group!– Routers must keep state for m*n trees, where m is

number of sources in a group and n is number of groups.

• Core-based trees: single tree per group.– Host unicast message to core, where message is

multicast along shared tree.– Routes may not be optimal for all sources.– State/storage savings in routers.

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Internetworking

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Internetworking

• What is it?– Connecting networks together forming a single

“internet”.

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Connecting Networks

• A collection of interconnected networks.

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How Networks Differ

5-43

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How Networks Can Be Connected

• (a) Two Ethernets connected by a switch. • (b) Two Ethernets connected by routers.

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How to Internet?

• Connection-oriented versus connectionless internetworking.

• Connection oriented internetworking:– Based on VC concatenation.

• Connectionless internetworking follows the datagram model.

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Concatenated Virtual Circuits

. Builds VC crossing the different networks.

. Use of gateways to perform necessary conversions.

Gateway

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Connectionless Internetworking

. Follows datagram model.

. Packets from Host X to Host Y may follow different routes.

. Gateways make routing decisions and perform translations.

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Translating versus “Gluing”

• Translation: converting between different protocols.

• Hard!• Alternative: “gluing”.– I.e., using the same network layer protocol

everywhere.

– That’s what IP does!

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Tunneling

• Interconnecting source and destination on separate networks but of the same type.

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Tunneling Analogy

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More Tunneling …

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Internetwork Routing: Example

• (a) An internetwork. (b) A graph of the internetwork.

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Internetwork Routing

• Inherently hierarchical.– Routing within each network: interior gateway protocol

(IGP).– Routing between networks: exterior gateway protocol

(EGP).

• Within each network, different routing algorithms can be used.

• Each network is autonomously managed and independent of others: autonomous system (AS).

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Internetwork Routing (Cont’d)

• Typically, packet starts in its LAN. Gateway receives it (broadcast on LAN to “unknown” destination).

• Gateway sends packet to gateway on the destination network using its routing table. If it can use the packet’s native protocol, sends packet directly. Otherwise, tunnels it.

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Fragmentation

• Happens when internetworking.

• Network-specific maximum packet size.– Width of TDM slot.

– OS buffer limitations.

– Protocol (number of bits in packet length field).

• Maximum payloads range from 48 bytes (ATM cells) to 64Kbytes (IP packets).

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Problem

• What happens when large packet wants to travel through network with smaller maximum packet size? Fragmentation.

• Gateways break packets into fragments; each sent as separate packet.

• Gateway on the other side have to reassemble fragments into original packet.

• 2 kinds of fragmentation: transparent and non-transparent.

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Types of Fragmentation

• (a) Transparent fragmentation. (b) Nontransparent fragmentation.

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Transparent Fragmentation • Small-packet network transparent to other

subsequent networks.

• Fragments of a packet addressed to the same exit gateway, where packet is reassembled.– OK for concatenated VC internetworking.

• Subsequent networks are not aware fragmentation occurred.

• ATM networks (through special hardware) provide transparent fragmentation.

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Problems with Transparent Fragmentation

• Exit gateway must know when it received all the pieces.– Fragment counter or “end of packet” bit.

• Some performance penalty but requiring all fragments to go through same gateway.

• May have to repeatedly fragment and reassemble through series of small-packet networks.

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Non-Transparent Fragmentation

• Only reassemble at destination host.– Each fragment becomes a separate packet.

– Thus routed independently.

• Problems:– Hosts must reassemble.

– Every fragment must carry header until it reaches destination host.

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Keeping Track of Fragments

• Fragments must be numbered so that original data stream can be reconstructed.

• Tree-structured numbering scheme:– Packet 0 generates fragments 0.0, 0.1, 0.2, …– If these fragments need to be fragmented later

on, then 0.0.0, 0.0.1, …, 0.1.0, 0.1.1, …– But, too much overhead in terms of number of

fields needed.– Also, if fragments are lost, retransmissions can

take alternate routes and get fragmented differently.

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Keeping Track of Fragments (Cont’d)

• Another way is to define elementary fragment size that can pass through every network.

• When packet fragmented, all pieces equal to elementary fragment size, except last one (may be smaller).

• Packet may contain several fragments.

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Fragmentation: Example

• Fragmentation when the elementary data size is 1 byte.• (a) Original packet, containing 10 data bytes.• (b) Fragments after passing through a network with maximum

packet size of 8 payload bytes plus header.• (c) Fragments after passing through a size 5 gateway.

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Keeping Track of Fragments

• Header contains packet number, number of first fragment in the packet, and last-fragment bit.

27 0 1 A B C D E F G H I J

27 0 0 A B C D E F G H 27 8 1 I J

Packet numberNumber offirst fragment

Last-fragment bit

(a) Original packetwith 10 data bytes.

(b) Fragments after passing through network with maximum packet size = 8 bytes.

1 byte

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The Internet