Chapter 4: Network Layer

4: Network Layer 4a-1

Chapter 4: Network LayerChapter goals: understand principles

behind network layer services: routing (path

selection) dealing with scale how a router works advanced topics: IPv6,

multicast instantiation and

implementation in the Internet

Chapter Overview: network layer services routing principle: path

selection hierarchical routing IP Internet routing protocols

reliable transfer intra-domain inter-domain

what’s inside a router? IPv6 multicast routing


Network layer functions

transport packet from sending to receiving hosts

network layer protocols in every host, router

three important functions: path determination: route

taken by packets from source to dest. Routing algorithms

switching: move packets from router’s input to appropriate router output

call setup: some network architectures require router call setup along path before data flows

networkdata linkphysical








application

transportnetworkdata linkphysical

application



Network service model

Q: What service model for “channel” transporting packets from sender to receiver?

guaranteed bandwidth? preservation of inter-

packet timing (no jitter)? loss-free delivery? in-order delivery? congestion feedback to

sender?

? ??virtual circuit

or datagram?

The most important abstraction provided

by network layer:

serv

ice a

bst

ract

ion


Virtual circuits

call setup, teardown for each call before data can flow each packet carries VC identifier (not destination host OD) every router on source-dest path s maintain “state” for each

passing connection transport-layer connection only involved two end systems

link, router resources (bandwidth, buffers) may be allocated to VC to get circuit-like perf.

“source-to-dest path behaves much like telephone circuit” performance-wise network actions along source-to-dest path


Virtual circuits: signaling protocols

used to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in today’s Internet

application


application


1. Initiate call 2. incoming call

3. Accept call4. Call connected5. Data flow begins 6. Receive data


Datagram networks: the Internet model no call setup at network layer routers: no state about end-to-end connections

no network-level concept of “connection”

packets typically routed using destination host ID packets between same source-dest pair may take

different paths

application


application


1. Send data 2. Receive data


Network layer service models:

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees ?

Internet model being extented: Intserv, Diffserv Chapter 6


Datagram or VC network: why?

Internet data exchange among

computers “elastic” service, no

strict timing req. “smart” end systems

(computers) can adapt, perform

control, error recovery simple inside network,

complexity at “edge” many link types

different characteristics uniform service difficult

ATM evolved from telephony human conversation:

strict timing, reliability requirements

need for guaranteed service

“dumb” end systems telephones complexity inside

network


Routing

Graph abstraction for routing algorithms:

graph nodes are routers

graph edges are physical links link cost: delay, $

cost, or congestion level

Goal: determine “good” path

(sequence of routers) thru network from source to

dest.

Routing protocol

A

ED

CB

F

22

13

1

1

2

53

5

“good” path: typically means

minimum cost path other def’s possible


Routing Algorithm classification

Global or decentralized information?

Global: all routers have complete

topology, link cost info “link state” algorithmsDecentralized: router knows physically-

connected neighbors, link costs to neighbors

iterative process of computation, exchange of info with neighbors

“distance vector” algorithms

Static or dynamic?Static: routes change slowly

over timeDynamic: routes change more

quickly periodic update in response to link

cost changes


A Link-State Routing Algorithm

Dijkstra’s algorithm net topology, link costs

known to all nodes accomplished via “link

state broadcast” all nodes have same

info computes least cost paths

from one node (‘source”) to all other nodes gives routing table for

that node iterative: after k iterations,

know least cost path to k dest.’s

Notation: c(i,j): link cost from node

i to j. cost infinite if not direct neighbors

D(v): current value of cost of path from source to dest. V

p(v): predecessor node along path from source to v, that is next v

N: set of nodes whose least cost path definitively known


Dijsktra’s Algorithm

1 Initialization: 2 N = {A} 3 for all nodes v 4 if v adjacent to A 5 then D(v) = c(A,v) 6 else D(v) = infty 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N: 12 D(v) = min( D(v), D(w) + c(w,v) ) 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N


Dijkstra’s algorithm: example

Step012345

start NA

ADADE

ADEBADEBC

ADEBCF

D(B),p(B)2,A2,A2,A

D(C),p(C)5,A4,D3,E3,E

D(D),p(D)1,A

D(E),p(E)infinity

2,D

D(F),p(F)infinityinfinity

4,E4,E4,E

A

ED

CB

F

22

13

1

1

2

53

5


Dijkstra’s algorithm, discussionAlgorithm complexity: n nodes each iteration: need to check all nodes, w, not in N n*(n+1)/2 comparisons: O(n**2) more efficient implementations possible: O(nlogn)

Oscillations possible: e.g., link cost = amount of carried traffic

A

D

C

B1 1+e

e0

e

1 1

0 0

A

D

C

B2+e 0

001+e1

A

D

C

B0 2+e

1+e10 0

A

D

C

B2+e 0

e01+e1

initially… recompute

routing… recompute … recompute


Distance Vector Routing Algorithm

iterative: continues until no

nodes exchange info. self-terminating: no

“signal” to stop

asynchronous: nodes need not

exchange info/iterate in lock step!

distributed: each node

communicates only with directly-attached neighbors

Distance Table data structure each node has its own row for each possible destination column for each directly-

attached neighbor to node example: in node X, for dest. Y

via neighbor Z:

D (Y,Z)X

distance from X toY, via Z as next hop

c(X,Z) + min {D (Y,w)}Z

w

=

=


Distance Table: example

A

E D

CB7

81

2

1

2

D ()

A

B

C

D

A

1

7

6

4

B

14

8

9

11

D

5

5

4

2

Ecost to destination via

dest

inat

ion

D (C,D)E

c(E,D) + min {D (C,w)}D

w== 2+2 = 4

D (A,D)E

c(E,D) + min {D (A,w)}D

w== 2+3 = 5

D (A,B)E

c(E,B) + min {D (A,w)}B

w== 8+6 = 14

loop!

loop!


Distance table gives routing table

D ()

A

B

C

D

A

1

7

6

4

B

14

8

9

11

D

5

5

4

2

Ecost to destination via

dest

inat

ion

A

B

C

D

A,1

D,5

D,4

D,4

Outgoing link to use, cost

dest

inat

ion

Distance table Routing table


Distance Vector Routing: overview

Iterative, asynchronous: each local iteration caused by:

local link cost change message from neighbor:

its least cost path change from neighbor

Distributed: each node notifies

neighbors only when its least cost path to any destination changes neighbors then notify

their neighbors if necessary

wait for (change in local link cost of msg from neighbor)

recompute distance table

if least cost path to any dest

has changed, notify neighbors

Each node:


Distance Vector Algorithm:

1 Initialization: 2 for all adjacent nodes v: 3 D (*,v) = infty /* the * operator means "for all rows" */ 4 D (v,v) = c(X,v) 5 for all destinations, y 6 send min D (y,w) to each neighbor /* w over all X's neighbors */

XX

Xw

At all nodes, X:


Distance Vector Algorithm (cont.):8 loop 9 wait (until I see a link cost change to neighbor V 10 or until I receive update from neighbor V) 11 12 if (c(X,V) changes by d) 13 /* change cost to all dest's via neighbor v by d */ 14 /* note: d could be positive or negative */ 15 for all destinations y: D (y,V) = D (y,V) + d 16 17 else if (update received from V wrt destination Y) 18 /* shortest path from V to some Y has changed */ 19 /* V has sent a new value for its min DV(Y,w) */ 20 /* call this received new value is "newval" */ 21 for the single destination y: D (Y,V) = c(X,V) + newval 22 23 if we have a new min D (Y,w)for any destination Y 24 send new value of min D (Y,w) to all neighbors 25 26 forever

w

XX

XX

X

w

w


Distance Vector Algorithm: example

X Z12

7

Y


Distance Vector Algorithm: example

X Z72

1

Y

D (Y,Z)X

c(X,Z) + min {D (Y,w)}w=

= 7+1 = 8

Z

D (Z,Y)X

c(X,Y) + min {D (Z,w)}w=

= 2+1 = 3

Y


Distance Vector: link cost changes

Link cost changes: node detects local link cost

change updates distance table (line 15) if cost change in least cost path,

notify neighbors (lines 23,24)

X Z14

50

Y1

algorithmterminates“good

news travelsfast”


Distance Vector: link cost changes

Link cost changes: good news travels fast bad news travels slow -

“count to infinity” problem! X Z14

50

Y60

algorithmcontinues

on!


Distance Vector: poisoned reverse

If Z routes through Y to get to X : Z tells Y its (Z’s) distance to X is infinite (so

Y won’t route to X via Z) will this completely solve count to infinity

problem? X Z

14

50

Y60

algorithmterminates


Comparison of LS and DV algorithms

Message complexity LS: with n nodes, E links,

O(nE) msgs sent each DV: exchange between

neighbors only convergence time varies

Speed of Convergence LS: O(n**2) algorithm

requires O(nE) msgs may have oscillations

DV: convergence time varies may be routing loops count-to-infinity problem

Robustness: what happens if router malfunctions?

LS: node can advertise

incorrect link cost each node computes

only its own table

DV: DV node can advertise

incorrect path cost each node’s table used

by others • error propagate thru

network


Hierarchical Routing

scale: with 50 million destinations:

can’t store all dest’s in routing tables!

routing table exchange would swamp links!

administrative autonomy

internet = network of networks

each network admin may want to control routing in its own network

Our routing study thus far - idealization all routers identical network “flat”… not true in practice


Hierarchical Routing

aggregate routers into regions, “autonomous systems” (AS)

routers in same AS run same routing protocol “inter-AS” routing

protocol routers in different AS

can run different inter-AS routing protocol

special routers in AS run inter-AS routing

protocol with all other routers in AS

also responsible for routing to destinations outside AS run intra-AS routing

protocol with other gateway routers

gateway routers


Intra-AS and Inter-AS routing

Gateways:•perform inter-AS routing amongst themselves•perform intra-AS routers with other routers in their AS

inter-AS, intra-AS routing in

gateway A.c

network layer

link layer

physical layer

a

b

b

aaC

A

Bd

A.a

A.c

C.bB.a

cb

c


Intra-AS and Inter-AS routing

Host h2

a

b

b

aaC

A

Bd c

A.a

A.c

C.bB.a

cb

Hosth1

Intra-AS routingwithin AS A

Inter-AS routingbetween A and B

Intra-AS routingwithin AS B

We’ll examine specific inter-AS and intra-AS Internet routing protocols shortly


The Internet Network layer

routingtable

Host, router network layer functions:

Routing protocols•path selection•RIP, OSPF, BGP

IP protocol•addressing conventions•datagram format•packet handling conventions

ICMP protocol•error reporting•router “signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer


IP Addressing IP address: 32-bit

identifier for host, router interface

interface: connection between host, router and physical link router’s typically have

multiple interfaces host may have

multiple interfaces IP addresses

associated with interface, not host, router

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11


IP Addressing IP address:

network part (high order bits)

host part (low order bits)

What’s a network ? (from IP address perspective) device interfaces with

same network part of IP address

can physically reach each other without intervening router

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

network consisting of 3 IP networks(for IP addresses starting with 223, first 24 bits are network address)

LAN


IP AddressingHow to find the

networks? Detach each

interface from router, host

create “islands of isolated networks

223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2223.1.2.1

223.1.2.6

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.0

223.1.7.1223.1.8.0223.1.8.1

223.1.9.1

223.1.9.2

Interconnected system consisting

of six networks


IP Addresses

0network host

10 network host

110 network host

1110 multicast address

A

B

C

D

class

1.0.0.0 to127.255.255.255

128.0.0.0 to191.255.255.255

192.0.0.0 to239.255.255.255

240.0.0.0 to247.255.255.255

32 bits


Getting a datagram from source to dest.

IP datagram:

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

miscfields

sourceIP addr

destIP addr data

datagram remains unchanged, as it travels source to destination

addr fields of interest here

Dest. Net. next router Nhops

223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

routing table in A



223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

Starting at A, given IP datagram addressed to B:

look up net. address of B find B is on same net. as A link layer will send datagram

directly to B inside link-layer frame B and A are directly connected


223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

miscfields 223.1.1.1223.1.1.3 data



223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE


223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

Starting at A, dest. E: look up network address of E E on different network

A, E not directly attached routing table: next hop router

to E is 223.1.1.4 link layer sends datagram to

router 223.1.1.4 inside link-layer frame

datagram arrives at 223.1.1.4 continued…..

miscfields 223.1.1.1223.1.2.3 data



223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

Arriving at 223.1.4, destined for 223.1.2.2

look up network address of E E on same network as

router’s interface 223.1.2.9 router, E directly

attached link layer sends datagram to

223.1.2.2 inside link-layer frame via interface 223.1.2.9

datagram arrives at 223.1.2.2!!! (hooray!)

miscfields 223.1.1.1223.1.2.3 data network router Nhops interface

223.1.1 - 1 223.1.1.4 223.1.2 - 1 223.1.2.9

223.1.3 - 1 223.1.3.27

Dest. next


IP datagram format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

Internet checksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length (bytes)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

“type” of data flgsfragment

offsetupper layer

32 bit destination IP address

Options (if any) E.g. timestamp,record routetaken, pecifylist of routers to visit.


IP Fragmentation and Reassembly

network links have MTU (max.transfer size) - largest possible link-level frame. different link types,

different MTUs large IP datagram divided

(“fragmented”) within net one datagram

becomes several datagrams

“reassembled” only at final destination

IP header bits used to identify, order related fragments

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly


IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=1480

fragflag=1

length=1500

ID=x

offset=2960

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams


ICMP: Internet Control Message Protocol

used by hosts, routers, gateways to communication network-level information error reporting:

unreachable host, network, port, protocol

echo request/reply (used by ping)

network-layer “above” IP: ICMP msgs carried in

IP datagrams ICMP message: type,

code plus first 8 bytes of IP datagram causing error

Type Code description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header


Routing in the Internet

The Global Internet consists of Autonomous Systems (AS) interconnected with eachother:

Stub AS: small corporation Multihomed AS: large corporation (no

transit) Transit AS: provider

Two level routing: Intra-AS: administrator is responsible

for choice Inter-AS: unique standard


Internet AS Hierarchy


Intra-AS Routing

Also known as Interior Gateway Protocol (IGP) Most common IGPs:

RIP: Routing Information Protocol OSPF: Open Shortest Path First IGRP: Interior Gateway Routing Protocol (Cisco

propr.)


RIP ( Routing Info Protocol)

Distance vector type scheme Included in BSD-UNIX Distribution in 1982 Distance metric: # of hops (max = 15 hops) Distance vector: exchanged every 30 sec via a

Response Message (also called Advertisement) Each Advertisement contains up to 25 destination nets


RIP


RIP

destination network next router number of hops to destination 1 A 2

20 B 2 30 B 7

10 -- 1…. …. ....


RIP: Link Failure and Recovery

If no advertisement heard after 180 sec, neighbor/link dead

Routes via the neighbor are invalidated; new advertisements sent to neighbors

Neighbors in turn send out new advertisements if their tables changed

Link failure info quickly propagates to entire net Poison reverse used to prevent ping-pong loops (infinite

distance = 16 hops)


RIP Table processing

RIP routing tables managed by an application process called route-d (demon)

advertisements encapsulated in UDP packets (no reliable delivery required; advertisements are periodically repeated)


RIP Table processing


RIP Table example

Destination Gateway Flags Ref Use Interface -------------------- -------------------- ----- ----- ------ --------- 127.0.0.1 127.0.0.1 UH 0 26492 lo0 192.168.2. 192.168.2.5 U 2 13 fa0 193.55.114. 193.55.114.6 U 3 58503 le0 192.168.3. 192.168.3.5 U 2 25 qaa0 224.0.0.0 193.55.114.6 U 3 0 le0 default 193.55.114.129 UG 0 143454


RIP Table example (cont)

RIP Table example (at router giroflee):

Three attached class C networks (LANs) Router only knows routes to attached LANs Default router used to “go up” Route multicast address: 224.0.0.0 Loopback interface (for debugging)


OSPF (Open Shortest Path First)

“open”: publicly available uses the Link State algorithm (ie, LS packet

dissemination; topology map at each node; route computation using Dijkstra’s alg)

OSPF advertisement carries one entry per neighbor router

advertisements disseminated to ENTIRE Autonomous System (via flooding)


OSPF “advanced” features (not in RIP)

Security: all OSPF messages are authenticated (to prevent malicious intrusion); TCP connections used

Multiple same-cost paths allowed (only one path in RIP)

For each link, multiple cost metrics for different TOS (eg, satellite link cost set “low” for best effort; high for real time)

Integrated uni- and multicast support: Multicast OSPF (MOSPF) uses same topology data base as OSPF

Hierarchical OSPF in large domains


Hierarchical OSPF


Hierarchical OSPF

Two level hierarchy: local area and backbone Link state advertisements do not leave

respective areas Nodes in each area have detailed area

topology; they only know direction (shortest path) to networks in other areas

Area Border routers “summarize” distances to networks in the area and advertise them to other Area Border routers

Backbone routers run an OSPF routing alg limited to the backbone

Boundary routers connect to other ASs


IGRP (Interior Gateway Routing Protocol)

CISCO proprietary; successor of RIP (mid 80’s) Distance Vector, like RIP several cost metrics (delay, bandwidth,

reliability, load etc) uses TCP to exchange routing updates routing tables exchanged only when costs

change Loop free routing achieved by using a

Distributed Updating Alg. (DUAL) based on diffused computation

In DUAL, after a distance increase, the routing table is frozen until all affected nodes have learned of the change


Inter-AS routing


Inter-AS routing (cont)

BGP (Border Gateway Protocol): the de facto standard

Path Vector protocol: and extension of Distance Vector

Each Border Gateway broadcast to neighbors (peers) the entire path (ie, sequence of AS’s) to destination

For example, Gwy X may store the following path to destination Z:

Path (X,Z) = X,Y1,Y2,Y3,…,Z



Now, suppose Gwy X send its path to peer Gwy W Gwy W may or may not select the path offered by Gwy

X, because of cost, policy or loop prevention reasons If Gwy W selects the path advertised by Gwy X, then: Path (W,Z) = w, Path (X,Z)Note: path selection based not so much on cost (eg,# ofAS hops), but mostly on administrative and policy issues(eg, do not route packets through competitor’s AS)



Peers exchange BGP messages using TCP OPEN msg opens TCP connection to peer and

authenticates sender UPDATE msg advertises new path (or

withdraws old) KEEPALIVE msg keeps connection alive in

absence of UPDATES; it also serves as ACK to an OPEN request

NOTIFICATION msg reports errors in previous msg; also used to close a connection


Address Management

As Internet grows, we run out of addresses Solution (a): subnetting. Eg, Class B Host

field (16bits) is subdivided into <subnet;host> fields

Solution (b): CIDR (Classless Inter Domain Routing): assign block of contiguous Class C addresses to the same organization; these addresses all share a common prefix

repeated “aggregation” within same provider leads to shorter and shorter prefixes

CIDR helps also routing table size and processing: Border Gwys keep only prefixes and find “longest prefix” match


Why different Intra- and Inter-AS routing ?

Policy: Inter is concerned with policies (which provider we must select/avoid, etc). Intra is contained in a single organization, so, no policy decisions necessary

Scale: Inter provides an extra level of routing table size and routing update traffic reduction above the Intra layer

Performance: Intra is focused on performance metrics; needs to keep costs low. In Inter it is difficult to propagate performance metrics efficiently (latency, privacy etc). Besides, policy related information is more meaningful.

We need BOTH!


Router Architecture Overview

Router main functions: routing algorithms and protocols processing, switching datagrams from an incoming link to an outgoing link

Router Components


Input Ports

Decentralized switching: perform routing table lookup using a copy of the node routing table stored in the port memory

Goal is to complete input port processing at ‘line speed’, ie processing time =< frame reception time (eg, with 2.5 Gbps line, 256 bytes long frame, router must perform about 1 million routing table lookups in a second)

Queuing occurs if datagrams arrive at rate higher than can be forwarded on switching fabric


Speeding Up Routing Table Lookup

Table is stored in a tree structure to facilitate binary search

Content Addressable Memory (associative memory), eg Cisco 8500 series routers

Caching of recently looked-up addresses Compression of routing tables


Switching Fabric


Switching Via Memory

First generation routers: packet is copied under system’s (single) CPU control; speed limited by Memory bandwidth. For Memory speed of B packet/sec or pps, throughput is B/2 pps

InputPort

OutputPort

Memory

System Bus

• Modern routers: input ports with CPUs that implement output port lookup, and store packets in appropriate locations (= switch) in a shared Memory; eg Cisco Catalyst 8500 switches


Switching Via Bus

Input port processors transfer a datagram from input port memory to output port memory via a shared bus

Main resource contention is over the bus; switching is limited by bus speed

Sufficient speed for access and enterprise routers (not regional or backbone routers) is provided by a Gbps bus; eg Cisco 1900 which has a 1 Gbps bus


Switching Via An Interconnection Network

Used to overcome bus bandwidth limitations Banyan networks and other interconnection networks were

initially developed to connect processors in a multiprocessor computer system; used in Cisco 12000 switches provide up to 60 Gbps through the interconnection network

Advanced design incorporates fragmenting a datagram into fixed length cells and switch the cells through the fabric; + better sharing of the switching fabric resulting in higher switching speed


Output Ports

Buffering is required to hold datagrams whenever they arrive from the switching fabric at a rate faster than the transmission rate


Queuing At Input and Output Ports Queues build up whenever there is a rate mismatch or

blocking. Consider the following scenarios: Fabric speed is faster than all input ports combined; more

datagrams are destined to an output port than other output ports; queuing occurs at output port

Fabric bandwidth is not as fast as all input ports combined; queuing may occur at input queues;

HOL blocking: fabric can deliver datagrams from input ports in parallel, except if datagrams are destined to same output port; in this case datagrams are queued at input queues; there may be queued datagrams that are held behind HOL conflict, even when their output port is available


IPv6 Initial motivation is 32 bit address space is estimated to get

used up either by 2008 or 2018; opportunity for changes to achieve faster processing and provision of differentiated services

Packet Format: fixed header of 40 bytes + option; Fixed header fields:

Version: indicates IPv6 Priority: 4 bits, to give priority to certain packets within a

flow; values 0 to 7 for congestion-controlled traffic, while values 8 to 15 is for other traffic (eg constant bit rate)

Flow Label: intended to help with differentiating services based on flows, a flow is not strictly defined in IPv6 proposal, it can be traffic from a user who paid more, traffic that is real-time, etc.

Payload Length: 16 bit value identifying the number of bytes following the 40 bytes of the fixed IPv6 header

Next Header: same as Protocol field in IPv4, identifies higher layer protocol to process the contents (TCP or UDP, or?)


IPv6 Header (Cont)

Hop Limit: same as TTL, still one byte! Source and Destination Addresses: 128

bits, with a new hierarchical structure (address can imply geographical location, not in IPv4); includes new type of address: anycast, delivery is to one of a number of destinations


Other Changes from IPv4

Fragmentation: none provided, router which has a packet longer than the maximum allowed on a the next hop drops the packet, and sends an ICMP message “Packet Too Big” to the packet source; reduces processing time of packets

Checksum: removed entirely to reduce processing time at each hop

Options: Options are allowed and indicated by the header field “Next Header”, the content of this field indicates the higher level protocol or the existence of an option after the 40 bytes IPv6 header

ICMPv6: new version of ICMP, with additional message types, eg “Packet Too Big”; and group management function for multicast groups (Under IPv4 done by the protocol Internet Group Management Protocol IGMP to be discussed shortly)


Transition From IPv4 To IPv6

During the transition, not all routers will be upgraded to IPv6; How will the network operate?

Two proposed approaches: Dual Stack and Tunneling

Dual Stack: Some routers with dual stack (v6, v4); others are only

v4 routers Dual stack routers translate the packet to v4 packet if

the next router is v4 only DNS can be used to determine whether a router is dual

stack or not Some info and v6 features will be lost if a packet has

to go through any v4 only router; eg Flow Identification


Dual Stack Approach


Tunneling Routers are as before v4/v6 or v4 only A v4/v6 router “encapsulates” the IPv6 packet inside

an IPv4 envelop before communication to a v4 only router

A v4/v6 router receiving an encapsulated packet from a “tunnel”, remove the envelop and forwards the IPv6 to next router if the next router is v4/v6 capable


Multicast Routing

Multicast: delivery of same packet to a group of receivers

Multicasting is becoming increasingly popular in the Internet (video on demand; whiteboard; interactive games)

Multiple unicast vs multicast


Multicast Group Address

M-cast group address “delivered” to all receivers in the group

Internet uses Class D for m-cast M-cast address distribution etc.

managed by IGMP Protocol


IGMP Protocol

IGMP (Internet Group Management Protocol) operates between Router and local Hosts, typically attached via a LAN (e.g., Ethernet)

Router queries the local Hosts for m-cast group membership info

Router “connects” active Hosts to m-cast tree via m-cast protocol

Hosts respond with membership reports: actually, the first Host which responds (at random) speaks for all

Host issues “leave-group” mssg to leave; this is optional since router periodically polls anyway (soft state concept)


IGMP message types

GMP Message type Sent by Purpose

membership query: general router query for current active multicast groups

membership query: specific router query for specific m-cast group

membership report host host wants to join goup

leave group host host leaves the group


The Multicast Tree problem

Problem: find the best (e.g., min cost) tree which interconnects all the members


Multicast Tree options

GROUP SHARED TREE: single tree; the root is the “CORE” or the “Rendez Vous” point; all messages go through the CORE

SOURCE BASED TREE: each source is the root of its own tree connecting to all the members; thus N separate trees


Group Shared Tree

Predefined CORE for given m-cast group (eg, posted on web page)

New members “join” and “leave” the tree with explicit join and leave control messages

Tree grows as new branches are “grafted” onto the tree

CBT (Core Based Tree) and PIM Sparse-Mode are Internet m-cast protocols based on GSTree

All packets go through the CORE


Source Based Tree

Each source is the root of its own tree: the tree of shortest paths

Packets delivered on the tree using “reverse path forwarding” (RPF); i.e., a router accepts a packet originated by source S only if such packet is forwarded by the neighbor on the shortest path to S

In other words, m-cast packets are “forwarded” on paths which are the “reverse” of “shortest paths” to S


Source-Based tree: DVMRP

DVMRP was the first m-cast protocol deployed on the Internet; used in Mbone (Multicast Backbone)

Initially, the source broadcasts the packet to ALL routers (using RPF)

Routers with no active Hosts (in this m-cast group) “prune” the tree; i.e., they disconnect themselves from the tree

Recursively, interior routers with no active descendents self-prune After timeout (2 hours in Internet) pruned branches “grow back”

Problems: only few routers are mcast-able; solution: tunnels


PIM (Protocol Independent Multicast) PIM (Protocol Independent Multicast) is

becoming the de facto intra AS m-cast protocol standard

“Protocol Independent” because it can operate on different routing infrastructures (as a difference of DVMRP)

PIM can operate in two modes: PIM Sparse and PIM dense Mode.

Initially, members join the “Shared Tree” centered around a Randez Vous Point

Later, once the “connection” to the shared treee has been established, opportunities to connet DIRECTLY to the source are explored (thus establishing a partial Source Based tree

Chapter 4: Network Layer

Documents

Transcript of Chapter 4: Network Layer