advanced computer networks - cs716 power point slides lecture 26

7/27/2019 Advanced Computer Networks - CS716 Power Point Slides Lecture 26

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CS716

Advanced Computer Networks

By Dr. Amir Qayyum



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Lecture No. 26



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Internetworking

• Basics of internetworking (heterogeneity)

– IP protocol, address resolution, control messages, …

• Routing

• Global internets (scale)

– Virtual geography and addresses

– Hierarchical routing

• Future internetworking: IPv6• Multicast traffic

• MPLS



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IPv6 Wish List / Planned Support

• 128-bit addresses• Multicast traffic

• Mobility

• Real-time traffic/quality of service guarantees

• Authentication and security

• Autoconfiguration: determining local IP address

• End-to-end fragmentation

• Protocol extensions (flexible protocol)• Smooth transition spread over time is critical

– Islands of v6 and then v4, traffic is tunneled in Internet



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IPv6 Addresses• Classless addressing/routing (similar to CIDR)

• Address notation

– String of eight 16-bit hex values separated by colons

e.g . 5CFA:0002:0000:0000:CF07:1234:5678:FFCD – Only one set of contiguous zeroes can be elided, e.g .

5CFA:0002::CF07:1234:5678:FFCD

• Address assignment

– Provider-based

– Geographic

subscriber ID provider IDregion ID subnet host010



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IPv4 Packet Format Review

• 20-byte minimum

• Mandatory fields not always used (e.g .

fragmentation)

• Options are unordered list of (name, value) pairs

V ersion HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

SourceAddr

DestinationAddr

Options (variable) Pad (variable)

0 4 8 16 19 31

Data



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IPv6 Packet Format

destination address word 4




source address word 4




payload length next header hop limit

version priority flow label

0 4 8 16 24 31



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IPv6 Packet Format

• 40-byte minimum

• Mandatory fields (probably) always used

• Strict order on options reduces processingtime (no need to parse irrelevant options)

option (variable number, usually fixed length)

destination address (4 words)

source address (4 words)

payload length next header hop limit

version priority flow label0 4 8 16 24 31



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IPv6 Packet Format

• Priority and flow label – Support service guarantees

– Allow “fair” bandwidth allocation

• Payload length (header not included, unlike IPv4)

• Next header

– Combines options and protocol

– Linked list of options

– Ends with higher-level protocol header (e.g . TCP)

• Hop limit is TTL field renamed to match usage



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IPv6 Extension Headers

• Extension headers (options) appear in order

1. Hop-by-hop options .. .. misc. info. for routers

2. Routing .. .. .. .. .. full/partial route to follow

3. Fragmentation .. .. .. IP fragmentation info

4. Authentication .. .. .. sender identification

5. Encrypt security payload info. about contents

6. Destination options .. .. info. for destination



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• Hop-by-hop extension – Length is in bytes beyond mandatory 8

• Jumbogram option (packet longer than 65,535 bytes)

– Payload length in main header set to 0

next header length type

0 8 16 24 31

0 8 16 24 31

value

payload length in bytes

next header 0 194 0



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• Routing extension

– Up to 24 “anycast” addresses target AS’s or providers

– Next address tracks current target

– Strict routing requires direct link, loose routing allows

intermediate nodes

0 8 16 24 31

next header 0 no.of.address

es

next address

strict/loose routing bitmap

1 – 24 addresses



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• Fragmentation extension

• Similar to IPv4 fragmentation

– 13-bit offset, – Last-fragment mark (M)

• Larger fragment identification field

0 8 16 29 31

next header reserved offset Mresvidentification



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• Security without bothering the routers

• Authentication extension

– Designed to be very flexible

– Includes Security Parameters Index (SPI) and

Authentication data at the end

• Encryption extension – Called Encapsulation Security Payload (ESP)

– Includes an SPI

– All headers and data after ESP are encrypted



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IPv6 Design Controversies

Address length

• 8-byte

– Less header overhead. Might run out in a few

decades

• 16-byte

– More overhead. Good for foreseeable future

• 20-byte – Even more overhead. Compatible with OSI

• Variable-length

– Difficult for router design



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Hop limit

• 65,535

– 32-hop paths are common now

– In a decade, much longer paths possible

• 255

– Limits lost packet lifetime (65,535 is far too long)

– Good network design makes long paths unlikely• Source to backbone

• Across backbone

• Backbone to destination



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Maximum packet size

• > 64kB: supercomputer/high bandwidthapplications

– Too much overhead to fragment data

– Want much larger packets

• 64 kB: longer packets incompatible with

low-bandwidth lines – Consider 1MB packet across 1.5 Mbps line

– Ties up line for greater than 5 seconds

– Inconveniences interactive users



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Keep IP checksum ?

• Yes

– Removing checksum from IP analogous to removing

brakes from a car

• Lighter, so you go faster

• Unprepared for the unexpected

• No – Typically duplicated in data link and transport layers

– Major expense in IPv4 routers

– In case of IPv6, it is mandatory in UDP and TCP



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Mobile hosts• Direct or Indirect ?

– Reconnect directly using canonical address

– Use home and foreign agents to forward traffic

• Mobility introduces asymmetry

– Base station signal is strong, heard by mobile units

– Mobile unit signal is weak and susceptible tointerference, not heard by base station

• No clearly superior design proposal



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Security• Where ?

– Network layer: a standard service

– Application layer • No viable standard

• Applications susceptible to errors innetwork implementations

• Too clunky to turn off

• How ?

– Political export/import issues

– Cryptographic strength issues

advanced computer networks - cs716 power point slides lecture 26

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