Example Start of header Ident = x 0 Offset = 0 The host want to - - PowerPoint PPT Presentation

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Spring 2004 EE4272

Example

Ident = x Offset = 0 Start of header Rest of header 1400 data bytes Ident = x Offset = 0 Start of header 1 Rest of header 512 data bytes Ident = x Offset =64 Start of header 1 Rest of header 512 data bytes Ident = x Offset =128 Start of header Rest of header 376 data bytes

Note: Offset field counts 8-byte units of data, not individual bytes (the change has been made w/ red)

Spring 2004 EE4272

Routing with subnetting

The host want to send a packet to a certain IP address

1). Bitwise AND between its own subnet mask & the destination IP address 2). If the same subnet number is obtained, direct delivery over that subnet 3). If not, it send s the packet to its default router

The job of a router changes when introduce subnetting

the entries of <Dest. NetworkNum, NextHop> ->

<Dest. SubnetNum, SubnetMask, NextHop>

the router ANDs the destination address with the Subnet Mask for each

entry, the matched one is the entry to be use

Note: Subnets is not visible from the rest of the Internet

Forwarding table at router R1

Subnet Number Subnet Mask Next Hop 128.96.34.0 255.255.255.128 interface 0 128.96.34.128 255.255.255.128 interface 1 128.96.33.0 255.255.255.0 R2

Subnet mask: 255.255.255.128 Subnet number: 128.96.34.0 128.96.34.15 128.96.34.1 H1 R1 128.96.34.130 Subnet mask: 255.255.255.128 Subnet number: 128.96.34.128 128.96.34.129 128.96.34.139 R2 H2 128.96.33.1 128.96.33.14 Subnet mask: 255.255.255.0 Subnet number: 128.96.33.0 H3

Spring 2004 EE4272

Supernetting – Classless Interdomain Routing (CIDR)

• The IP address structure (w/ class A, B, C) forces to hand out network

address space in fixed size chunks of 3 very different size

Unlike subnetting, supernetting gives appropriate # of class C address to

cover the expected number of host (address utilization >50%)

Problem: if a single AS has n class C network numbers assigned, every backbone

router needs n entries in its routing table for that AS

• Two scaling concerns in the Internet to be addressed

address assignment efficiency (especially class B IP address) the growth of backbone routing table

• Classless Interdomain Routing (CIDR)

try to balance the desire of minimizing the routing table of the routers

involved and the desire of handing out IP address efficiently by aggregating the routes (e.g. …)

Assign block of contiguous network numbers to nearby networks, the

common parts of the network numbers -> single network prefix (used in forwarding table)

Restrict block sizes to powers of 2

Handout blocks of class C address that share a common prefix

Need a routing protocol can deal with these “classless” address with any

length of the network number part (e.g. BGP)

Spring 2004 EE4272

Supernetting

Assign a portion of address space to the ISP,

and let ISP assign addresses to its customer

It can advise a single route to both customer by

just advertising the common 19-bits prefix they share

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Spring 2004 EE4272

Supernetting

Essences: “subnetting” allows a class B kind of IP address be

shared by multiple physical networks

“supnetting” aims to collapse the multiple class C kind of

IP addresses that would be assigned to a single AS

In general, it is possible to apply the principle (network

prefixes) of CIDR in the Internet service provider network “repeatedly” if addresses are assigned carefully

CIDR means prefixes may be of variable length, in case

• f some addresses may matches more than one prefix,

follow the principle of “longest match”

Spring 2004 EE4272

Interdomain Routing

The idea behind ASs (domain) is to provide an additional way to

hierarchically aggregate routing information in a large internet, thus improving scalability

Intradomain routing (w/ in a single AS): finding “optimal” paths based on the link

metrics

Interdomain routing (between ASs) : concerning more on reachability of packets

among ASs Interdomain Routing : a hard problem

Exterior Gateway Protocol (EGP): limited to treelike topology(early Internet) Border Gateway Protocol (BGP): flexible to an arbitrarily interconnected set

• f ASs

BGP: finding any loop-free path to the intended destination matter of scale: 140,000 prefix impossible to calculate meaningful path costs cross multiple ASs “BGP speaker” exchange reachability infor. among ASs BGP advertises “complete path” to achieve loop-free

• No. of nodes involving in BGP is on the order of ASs

Spring 2004 EE4272

Beside BGP speaker,

AS has border gateways which are responsible of forwarding packets between ASs

Spring 2004 EE4272

BGP Example

• Speaker for AS2 advertises reachability to P and Q
• network 128.96, 192.4.153, 192.4.32, and 192.4.3, can be reached directly from AS2
• Speaker for backbone advertises
• networks 128.96, 192.4.153, 192.4.32, and 192.4.3 can be reached along the path (AS1, AS2)
• To prevent loop, the complete AS path should be carried in the message
• Speaker can cancel previously advertised paths, in case of failure
• Note: A “stub AS” has a single connection to one other AS

A “transit AS” has connections to more than one other ASs

Backbone network (AS 1) Regional provider A (AS 2) Regional provider B (AS 3) Customer P (AS 4) Customer Q (AS 5) Customer R (AS 6) Customer S (AS 7) 128.96 192.4.153 192.4.32 192.4.3 192.12.69 192.4.54 192.4.23

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Spring 2004 EE4272

IP Version 6

Features

128-bit addresses (classless) real-time service authentication and security autoconfiguration end-to-end fragmentation protocol extensions multicast

Header

40-byte “base” header extension headers (fixed order, mostly fixed length)

fragmentation source routing authentication and security

• ther options

Spring 2004 EE4272

Internet Multicast

IPv4

class D addresses demonstrated with MBone uses tunneling

Integral part of IPv6

problem is making it scale

Spring 2004 EE4272

Link-State Multicast

Each host on a LAN periodically announces the

groups it belongs to by Internet Group Management Protocol (IGMP): reading assignment (P331-335)

Augment update message (link state packet-LSP) to

include set of groups that have members on a particular LAN

Each router uses Dijkstra's algorithm to compute

shortest-path spanning tree for each source/group pair

Each router computes and stores the cache of

trees for currently active source/group pairs.

Spring 2004 EE4272

Example

Example shortest-path multicast trees Member of group G in color

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Spring 2004 EE4272

Distance-Vector Multicast

Reverse Path Broadcast (RPB)

Each router already knows that shortest path to

destination S goes through router N

When receive multicast packet from S, forward

• n all outgoing links (except the one on which

the packet arrived), iff packet arrived from N

Eliminate duplicate broadcast packets by only

letting “parent” for LAN (relative to S) forward

shortest path to S (learn via distance vector) smallest address to break ties

Spring 2004 EE4272

Reverse Path Multicast (RPM)

Goal: Prune networks that have no hosts in group G

Step 1: Determine of LAN is a leaf with no members in G

leaf if parent is the only router on the LAN determine if any hosts are members of G using IGMP

Step 2: Propagate “no members of G here” information

augment <Destination, Cost> update sent to

neighbors with set of groups for which this network is interested in receiving multicast packets.

• nly happens with multicast address becomes active.