ch4-5rvw.ppt

4: Network Layer 4a-1

Chapter 4 – The Network Layer &Routing network

data linkphysical

networkdata linkphysical







application

transportnetworkdata linkphysical

The network layer moves transport layer segments from host to host in the network, to deliver them to their destination. This layer involves each and every host and router in the network. We will study the key principles and algorithms of routing, with a focus on the Internet Protocol (IP) service model.

application


application



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 destination - 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 (what types?)









application


application



Virtual circuits

call setup, teardown for each call before data can flow each packet carries VC identifier (not destination host ID) every router/switch on source-destination path maintains a

“state” for each passing connection Recall: transport-layer connection only involved two end

systems link and router resources (bandwidth, buffers) may be

dedicated to the VC to get circuit-like performance but… what about start-up delay?

the source-to-destination path behaves much like a telephone circuit performance-wise network actions along source-to-destination path


Virtual circuits: signaling protocols

used to setup, maintain and teardown the VC used in ATM, frame-relay and X.25 not used in the Internet (why?)

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: do not maintain state for the end-to-end

connections no network-level concept of a “connection”

packets are typically routed using only destination host ID which is carried in the packet packets between same source-destination pair may

take different paths

application


application


1. Send data 2. Receive data


Summary: Datagram or VC network… why?Internet data exchange among

computers “elastic” service, no

strict timing required (data delivery)

“smart” end systems (computers) can adapt, perform

control, error recovery complexity at “edge”,

simple in network core 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,

“videophones” complexity inside the

networkConsider: IP over ATM(more later)


Routing

Graph abstraction for routing algorithms:

graph nodes are routers graph edges are

physical links link cost: delay, distance,

# of hops, rate structure or congestion level = $$

Other costs??

Goal: determine a “good” path

(sequence of routers) thru the network from the

source to the destination

Routing protocol

A

ED

CB

F

2

2

13

1

1

2

53

5

“good” path: typically means

minimum cost path other definitions also

possible


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 yields a routing table

for that node iterative: after k

iterations, know least cost path to k destinations

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

to node j. Cost is initially infinite if not a direct neighbor

D(v): current computed value of cost of the path from the source to destination v

p(v): predecessor node, that is a neighbor of v, along the path from the source to v

N: set of nodes whose least cost path is definitively known


Dijsktra’s Algorithm

1 Initialization: 2 N = {A} // Source node is “A”3 for all nodes v 4 if v adjacent to A 5 then D(v) = c(A,v) 6 else D(v) = infinity 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

A

ED

CB

F2

21

3

1

1

2

53

5


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

2

2

13

1

1

2

53

5


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

destination Y via neighbor Z:

DX(Y,Z)

distance from X toY, via Z as next hop

c(X,Z) + minw {DZ(Y,w)}

=

=


Distance Table: example

A

E D

CB7

8

1

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 back through E!

loop back through E!


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,1

D,5

D,4

D,2

Outgoing link to use, cost

dest

inat

ion

Distance table Routing table

E

A

B

C

D


Distance Vector Algorithm(Bellman-Ford):

1 Initialization: 2 for all adjacent nodes v: 3 D (*,v) = infinity /* 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: 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 slowly -

“count to infinity” problem! X Z14

50

Y60

algorithmcontinues

on!

Y Y Y


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

algorithmterminatesY Y Y Y


Comparison of LS and DV algorithms

Message complexity LS: with n nodes, E links,

O(nE) msgs sent/broadcast DV: exchange between

neighbors only convergence time varies

Speed of Convergence LS: O(n2) algorithm requires

O(nE) msgs may have oscillations

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

• poisoned reverse is sometimes successful

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 • errors propagate

through the network


Hierarchical Routing

scale: with 55 million+ destination hosts:

can’t store all destinations 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 – an idealization all routers are identical the network is “flat”

… … notnot true in practice true in practice

Why?Why?


Hierarchical Routing

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

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

protocol routers in different AS

can run different intra-AS routing protocol

special routers in AS run intra-AS routing

protocol with all other routers in AS

also responsible for routing to destinations outside AS run inter-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

data link layerphysical 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 (section 4.5)


The Internet Network layer

routingtable

Host, router network layer functions… three major components:

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: introduction IP address: 32-bit

identifier for host or router interface

interface: connection between host or router and the physical link routers typically

have multiple interfaces

hosts typically have only one

IP addresses are associated with the interface, not the host or the 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

dotted-decimal notation:


IP Addressing IP address:

network part (high order bits)

host part (low order bits)

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

with the same network part of their IP address

hosts can physically reach each other without an 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

Example: network consisting of 3 IP networks (for IP addresses starting with 223, the first 24 bits are the network address – more later)

LAN


IP Addresses

0network

10 network host (16 bits)

110 network host (8 bits)

1110 multicast address (28 bits)

A

B

C

D

class1.0.0.0 to127.255.255.255

128.0.0.0 to191.255.255.255

192.0.0.0 to223.255.255.255

224.0.0.0 to239.255.255.255

32 bits

Given the notion of a “network”, let’s look closer at IP addresses:

“classful” addressing -

host (24 bits)

What is the address space size (number of hosts) for each class?

27 = 127 networks 224 = 16.8 million+ hosts

214 = 16,384 networks 216 = 65,536 hosts

221 = 2 million+ networks 28 = 256 hosts

24 = 16 networks 228 = 268.4 million+ hosts


IP addressing: CIDR classful addressing:

inefficient use of address space, address space exhaustion

e.g., class B network is allocated enough addresses for 65K hosts, even if only 2K hosts exist in that network

CIDR: Classless InterDomain Routing network portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in the network

portion of an address

11001000 00010111 00010000 00000000

networkpart

hostpart

200.23.16.0/23


IP addresses: how to get one?

Hosts (host portion): hard-coded by system admin in a file DHCP: Dynamic Host Configuration Protocol:

dynamically get address (RFC 2131): “plug-and-play” host broadcasts “DHCP discover” msg DHCP server responds with “DHCP offer” msg host requests IP address: “DHCP request” msg DHCP server sends address: “DHCP ack” msg


IP addresses: how to get one?

Network (network portion): get allocated portion of ISP’s address

space:ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23

Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23

Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. ….

Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23


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

misc.fields

sourceIP addr

destIP addr data

addresses remain unchanged, as the datagram travels from source to destination

address fields of interest here (provided by the source host A)

dest. net. next router #hops

223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

routing table* in A

* Note – more on this later



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 an IP datagram addressed to B:

look up network address of B find B is on same network 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

miscfields223.1.1.1223.1.1.3data



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, destination 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…..

miscfields223.1.1.1223.1.2.2 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 223.1.1.4, 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!!!

miscfields223.1.1.1223.1.2.2 data network router #hops 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. datagram length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

header checksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length (bytes)

max number ofremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

(e.g. TCP, UDP, … see RFC 1700)

head.len.

type ofservice

“type” of data flgsfragment

offsetupper layer

32 bit destination IP address

Options (if any) e.g. timestamp,record routetaken, specifylist of routers to visit (field israrely used in practice).


IP Fragmentation & Reassembly network links have MTU

(Max. Transfer Unit) 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 and 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


Internet AS HierarchyInter-AS border (exterior gateway) routers

Intra-AS interior (gateway) routers


Intra-AS Routing

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

RIP: Routing Information Protocol (legacy)

OSPF: Open Shortest Path First (common)

EIGRP: Enhanced Interior Gateway Routing Protocol (proprietary – Cisco Systems)


RIP ( Routing Information Protocol)

Distance vector algorithm Included in BSD-UNIX Distribution in 1982

RFC 1058 (version 1), RFC 1723 (version 2)

Distance metric: # of hops (max = 15 hops) Can you guess why?

Distance vectors: exchanged every 30 seconds via Response Message (also called advertisement)

Each advertisement: routing info for maximum of 25 destination nets within the AS


RIP Table processing

RIP routing tables managed by application-level process called route-d (UNIX daemon)

advertisements sent in UDP packets, periodically repeated


RIP Table example (continued)

Router: giroflee.eurocom.fr

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

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


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 Types Of Service (e.g., 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 only in local area each node has detailed area topology; only know

direction (shortest path) to nets in other areas. Area border routers: “summarize” distances to

nets in own area, advertise to other Area Border routers.

Backbone routers: run OSPF routing, limited to backbone.

Boundary routers: connect to other ASs. (Note: synonymous with the “gateway routers” we discussed in section 4.3)


Internet inter-AS routing: BGP

BGP (Border Gateway Protocol): the de facto standard

Path Vector protocol: similar to Distance Vector protocol each Border Gateway broadcasts to

neighbors (peers) the entire path (I.e, sequence of ASs) to destination

E.g., Gateway X may send its path to destination Z:

Path (X,Z) = X,Y1,Y2,Y3,…,Z



Suppose: gateway X send its path to peer gateway W

W may or may not select a path offered by X cost, policy (don’t route via competitors AS),

loop prevention reasons.

If W selects a path advertised by X, then:Path (W,Z) = W, Path (X,Z)

Note: X can control incoming traffic by controlling its route advertisements to peers: e.g., don’t want to route traffic to Z -> don’t

advertise any routes to Z



BGP messages exchanged using TCP. BGP messages:

OPEN: opens TCP connection to peer and authenticates sender

UPDATE: advertises new path (or withdraws old)

KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request

NOTIFICATION: reports errors in previous message; also used to close connection


Why different Intra- and Inter-AS routing ?

Policy: Inter-AS: admin wants control over how its traffic

is routed, who routes through its net. Intra-AS: single admin, so no policy decisions

needed

Scale: hierarchical routing saves table size, reduces

update traffic

Performance: Intra-AS: can focus on performance Inter-AS: policy may dominate over performance


Router Architecture Overview

Two key router functions:

run routing algorithms/protocol (RIP, OSPF, BGP) switch datagrams from incoming to outgoing link


Input Port Functions

Decentralized switching: given datagram dest., lookup output port

using routing table in input port memory goal: complete input port processing at

‘line speed’ queuing: if datagrams arrive faster than

forwarding rate into switch fabric (i.e., the packet is “blocked”)

Physical layer:bit-level reception

Data link layer:e.g., Ethernetsee chapter 5

Problem: how long does it take to perform a lookup?


Input Port Queuing

If routing fabric is slower than input ports combined -> queuing may occur at input queues

Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward

queuing delay and loss due to input buffer overflow!


Three types of switching fabrics

(interconnectionnetwork)


Switching Via MemoryFirst generation routers: packet copied by system’s (single) CPU speed limited by memory bandwidth (2 system bus accesses per datagram)

InputPort

OutputPort

Memory

System Bus

Modern routers: input port processor performs lookup, copy into shared memory Cisco Catalyst 8500

DMA DMA


Switching Via Bus

datagram from input port memory to output port memory via a

shared bus bus contention: switching speed

limited by bus bandwidth (only one packet at a time can use bus)

1 Gbps bus - Cisco 1900: sufficient speed for access and enterprise routers (not regional or backbone)


Switching Via An Interconnection Network (commonly: Crossbar)

overcomes bus bandwidth limitations Banyan networks, other

interconnection nets initially developed to connect processors in multiprocessor design

advanced design: fragments datagram into fixed length cells, switches cells through the fabric.

Cisco 12000: switches up to 60 Gbps through the

interconnection network


Output Ports

Buffering required when datagrams arrive from the fabric faster than the transmission rate

Scheduling discipline chooses among queued datagrams for transmission


Output port queuing

buffering when arrival rate via switching fabric exceeds output line speed

queuing (delay) and loss due to output port buffer overflow!

Question: where is queuing most likely to occur?


IPv6 Initial motivation: 32-bit address space

completely allocated (gone!) by 2008 (maybe sooner… maybe later!).

Additional motivation: header format changes were needed to

improve speed of processing and forwarding header changes were required to facilitate QoS a new “anycast” address was needed: route to

“best” of several replicated servers IPv6 datagram format:

fixed-length 40 byte header (32 for addresses) no fragmentation allowed


IPv6 HeaderPriority: identify priority among datagrams in flowFlow Label: identify datagrams in same “flow.” (concept of “flow” not well defined).Next header: identify upper layer protocol for data

ver. class flow label 8 bytes

32 bytes


Other Changes from IPv4

Fragmentation: not allowed/supported Checksum: removed entirely to reduce

processing time at each hop Options: allowed, but outside of header,

indicated by “Next Header” field ICMPv6: new version of ICMP

additional message types, e.g. “Packet Too Big”

multicast group management functions (IGMP)


Transition From IPv4 To IPv6

Not all routers can be upgraded simultaneous no “flag days” (e.g. NCP to TCP attempt in c.

1981) How will the network operate with mixed IPv4

and IPv6 routers? Two proposed approaches (RFC 1933):

Dual Stack: some routers with dual stack (v6, v4) can “translate” between formats

Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers


Dual Stack Approach


Tunneling

IPv6 inside IPv4 where needed


Multicast routing

IGMP message types: membership queries:

general or specific membership report:

host wants to join leave group: host

leaves a specific group

message format: Carried in IP datagram IP protocol # of 2

Internet multicast routing algorithms: DVMRP – distance

vector MOSPF – open shortest

path first CBT – core-based trees PIM – protocol

independent

Inter-autonomous routing: DVMRP – de facto

standard for multicast


Chapter 5 – Link Layer & Local AreaNetworks

The link layer is responsible for the transport of network layer datagrams from node to node via established physical links. In this chapter we’ll study the principal services of this layer, and look at specific protocols employed in the LANs, hubs, bridges and switches in the Internet. We’ll also investigate Ethernet, ATM, X.25 and Frame Relay.


application

transportnetwork

data linkphysical

application

transportnetwork

data linkphysical

application

transportnetwork

data linkphysical









Link Layer: setting the context two physically connected devices:

host-router, router-router, host-host unit of data: frame

applicationtransportnetwork

linkphysical

networklink

physical

M

M

M

M

Ht

HtHn

HtHnHl MHtHnHl

framephys. link

data linkprotocol

adapter card


applicationtransportnetwork

linkphysical

Link Layer: Implementation implemented in “adapter”

e.g., PCMCIA card, Ethernet card typically includes: RAM, DSP chips, host bus

interface, and physical link interface

networklink

physical

M

M

M

M

Ht

HtHn

HtHnHl MHtHnHl

framephys. link

data linkprotocol

adapter card


Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking, may include header fields

• Error detection not 100% reliable!• protocol may miss some errors, but rarely• larger EDC field yields better detection and correction


Parity CheckingSingle Bit Parity:Detect single bit errors

Two Dimensional Bit Parity:Detect and correct single bit errors

What’s the problem withWhat’s the problem withthis method?this method?

What else can we do with this What else can we do with this method?method?

0 0


Checksumming: Cyclic Redundancy Check view data bits, D, as a binary number choose r+1 bit pattern (generator), G goal: choose r CRC bits, R, such that

<D,R> exactly divisible by G (modulo 2) receiver knows G, divides <D,R> by G.

• non-zero remainder => error detected! can detect allall burst errors less than r+1 bits (Hamming) ATM 5-byte header uses 8-bit CRC IEEE GCRC-32 , 32-bit CRC for Ethernet, etc.

widely used in practice (ATM, HDLC)


CRC ExampleWant:

D.2r XOR R = nGequivalently:

D.2r = nG XOR R equivalently: if we divide D.2r by

G, want reminder R

R = remainder[ ]D.2r

G

Must be r+1 bits long.High and low-order bits must be 1’s.

Yields:101110011


Multiple Access Links and Protocols

Three types of “links”: point-to-point (single wire, e.g. PPP, SLIP,

HDLC) broadcast (shared wire or medium; e.g,

Ethernet, Wavelan, etc.)

switched (e.g., switched Ethernet, ATM, etc.)


Multiple Access protocols single shared communication channel two or more simultaneous transmissions by nodes…

interference generally, only one node can send successfully at a time

multiple access protocol: distributed algorithm that determines how stations share

channel, i.e., determine when a station can transmit communication about channel sharing must use channel

itself! what to look for in multiple access protocols:

• synchronous or asynchronous • information needed about other stations • robustness (e.g., tolerance of channel errors) • performance and efficiency


Multiple Access protocols

Desirable characteristics of a MAP channel of rate R bps: only one node sending… throughput is R

bps M nodes sending… average throughput is

R/M bps decentralized control… no master nodes

that control sending/receiving the protocol is simple and inexpensive to

implement MAC protocols: Media Access Control

often called the MAC sub-layer (of the data link layer)


MAC Protocols: a taxonomy

Three broad classes: Channel Partitioning

divide channel into smaller “pieces” (time slots, frequency)

allocate piece to node for exclusive use

Random Access allow transmitted frames to collide “recover” from collisions

“Taking turns” tightly coordinate shared access to avoid collisions

Goal: efficient, fair, simple, decentralized


Channel Partitioning MAC protocols: TDMA

TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length =

pkt transmission time) in each round unused slots go idle example: 6-station LAN – hosts 1,3,4 have

packets, time slots 2,5,6 idle

efficient?efficient? simple?simple?fair?fair? decentralized?decentralized?


Channel Partitioning MAC protocols: FDMA

FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands goes idle example: 6-station LAN - hosts 1,3,4 have packets,

frequency bands 2,5,6 idle

frequ

ency

bands time

1

2

3

4

5

6


Channel Partitioning (CDMA)

CDMA (Code Division Multiple Access) unique “code” assigned to each user; i.e., code set

partitioning used mostly in wireless broadcast channels (cellular,

satellite,etc) all users share same frequency/medium, but each user

has own “chipping” sequence (i.e., code) to encode data encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping

sequence allows multiple users to “coexist” and transmit

simultaneously with minimal interference (if codes are pairwise “orthogonal”)


CDMA: two-sender interference


CDMA: 4-sender example

Chipping Sequences for 4 nodes:

A: 00011011 A: (-1 –1 –1 +1 +1 –1 +1 +1)B: 00101110 B: (-1 –1 +1 –1 +1 +1 +1 –1)C: 01011100 C: (-1 +1 –1 +1 +1 +1 –1 –1)D: 01000010 D: (-1 +1 –1 –1 –1 –1 +1 –1)

Note: for all S and T,ST = 0

Example transmissions:

- - 1 - C S1: (-1 +1 –1 +1 +1 +1 –1 –1)- 1 1 - B + C S2: (-2 0 0 0 +2 +2 0 -2)1 0 - - A + B S3: (0 0 –2 +2 0 –2 0 +2)1 0 1 - A + B + C S4: (-1 +1 –3 +3 +1 –1 –1 +1)1 1 1 1 A + B + C + D S5: (-4 0 –2 0 +2 0 +2 -2)1 1 0 1 A + B + C + D S6: (-2 –2 0 –2 0 –2 +4 0)Example recovery of node C’s signal:

S1 C = (+1 +1 +1 +1 +1 +1 +1 +1 )/8 = 1

S2 C = (+2 +0 +0 +0 +2 +2 +0 +2)/8 = 1

S3 C = (0 +0 +2 +2 +0 –2 +0 -2)/8 = 0

S4 C = (+1 +1 +3 +3 +1 –1 +1 -1)/8 = 1

S5 C = (+4 +0 +2 +0 +2 +0 –2 +2)/8 = 1

S6 C = (+2 –2 +0 –2 +0 –2 –4 +0)/8 = -1


Slotted Aloha efficiencyQ: what is max fraction of slots successful?A: Suppose N stations have packets to send

each transmits in slot with probability p probability of successful transmission S is:

by single node: S= p (1-p) (N-1)

S = Prob (only one transmits)

by any arbitrary node of the N nodes: = N p (1-p) (N-1)

… choosing optimum p as N -> infinity ...

= 1/e = .37 as N -> infinity

At best: channelis available for

successful (useful) transmissions 37%

of time!


CSMA: Carrier Sense Multiple Access

CSMA: listen before transmit: If channel sensed as idle: transmit entire packet If channel sensed as busy: defer transmission

Persistent CSMA: retry immediately with probability p when channel becomes idle (may cause instability)

Non-persistent CSMA: retry after random interval human analogy: be polite and wait your turn, don’t

interrupt others!


CSMA collisions

collisions can occur:propagation delay means two nodes may not hear each other’s transmissioncollision:entire packet transmission time wasted

spatial layout of nodes along Ethernet

note:role of distance and propagation delay in determining collision prob.


CSMA/CD (Collision Detection)CSMA/CD: carrier sensing, deferral as in CSMA

collisions detected within short time colliding transmissions aborted, reducing wasted

channel bandwidth persistent or non-persistent retransmission

collision detection: easy in wired LANs: measure signal strengths,

compare transmitted, received signals difficult in wireless LANs: receiver shut off while

transmitting human analogy: the polite conversationalist


CSMA/CD collision detection


“Taking Turns” MAC protocolschannel partitioning MAC protocols:

share channel efficiently at high load inefficient at low load: delay in channel

access, 1/N bandwidth allocated even if only 1 active node!

random access MAC protocols efficient at low load: single node can fully

utilize channel high load: collision overhead

“taking turns” protocolslook for best of both worlds!


“Taking Turns” MAC protocolsPolling: master node

“invites” slave nodes to transmit in turn

Request to Send, Clear to Send msgs

concerns: polling overhead latency single point of

failure (master)

Token passing: control token passed

from one node to next sequentially.

token message concerns:

token overhead latency single point of failure

(token)


Summary of MAC protocols

What do you do with a shared media? Channel Partitioning, by time, frequency or

code• Time Division,Code Division, Frequency Division

Random partitioning (dynamic), • ALOHA, S-ALOHA, CSMA, CSMA/CD• carrier sensing: easy in some technologies (wire),

hard in others (wireless)• CSMA/CD used in Ethernet

Taking Turns• polling from a central cite, token passing


LAN Addresses and ARP

32-bit IP address: network-layer address used to get a datagram to a destination network

(recall IP network definition)

LAN (or MAC, or physical) address: used to get datagram from one interface to

another physically-connected interface (same network)

48 bit MAC address (for most LANs): permanent address, burned into the adapter ROM

How many possible LAN/Physical addresses?


LAN Address (more)

MAC address allocation administered by IEEE manufacturer buys a portion of the MAC address

space (to assure uniqueness) see current assignments

Analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address MAC flat address => portability

can move LAN card from one LAN to another IP hierarchical address NOT portable

depends on network to which one attaches

http://standards.ieee.org/regauth/oui/oui.txt


Recall earlier routing discussion

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 an IP datagram addressed to B:

look up network address of B, find B on same network as A

link layer sends a datagram to B inside link-layer frame

B’s MACaddress

A’s MACaddress

A’s IPaddress

B’s IPaddress

IP payload

datagramframe

frame source,destination address

datagram source,destination address


ARP: Address Resolution Protocol

Each IP node (Hosts & Routers) on a LAN has an ARP module and table

ARP Table: IP/MAC address mappings for some LAN nodes

< IP address; MAC address; TTL> TTL (Time To Live):

time after which address mapping will be forgotten (typically < 20 minutes)

Question: how can we determine the MAC address of B given B’s IP address?


ARP protocol (RFC 826) A knows B's IP address, wants to learn physical

address of B A broadcasts ARP query packet, containing B's

IP address all machines on LAN receive ARP query

B receives the ARP packet, replies to A with its (B's) physical layer (MAC) address

A caches (saves) IP-to-physical address pairs until information becomes old (times out: TTL) soft state: information that times out (goes

away) unless refreshed

Side effects… performance implications?


Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble: 7 bytes with pattern 10101010, followed by one

byte with pattern 10101011 (frame delimiter) used to synchronize receiver, sender clock rates

(802.3 Data Length)

Note: IEEE 802.3 specifies that frame length, excluding preamble, must be between 64 and 1518 bytes. Data is padded, if necessary, to ensure minimum length achieved


Ethernet Frame Structure (more) Addresses: 6 bytes, frame is received by all

adapters on a LAN and dropped if address does not match

Type (Length): 2 bytes, indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk). If 802.3 compliant, this field is length of data segment (min. 46 bytes)

CRC: 4 bytes, checked at receiver, if error detected, the frame is simply dropped


Ethernet: uses CSMA/CD

A: sense channel, if idle (+96 clock ticks/bit times)then {

transmit and monitor the channel; If detect another transmission then { abort and send jam signal;

update # collisions; delay as required by exponential backoff algorithm; goto A}

else {done with the frame; set collisions to zero}}

else {wait until ongoing transmission is over and goto A}


Ethernet’s CSMA/CD Efficiency Recall that the probability of successful transmission

in a slot is: P = Np(1-p)(N-1)

which yields a mean probability of success (efficiency) of P = 1/e, for an optimal p=1/N as N -> infinity.

Note that each 802.3 slot has a duration of the one-bit RTT (max) for the LAN (51.2 sec) = 2*tprop

The average contention interval length (i.e., how long you must wait) is the duration of the interval divided by the probability, or: 2*tprop / 1/e = 2e*tprop

Taking ttrans to be the time it takes to transmit an average frame, channel efficiency can then be expressed as:

ttrans

ttrans + 2e tprop

1

1 + 5.4 tprop /ttrans


Ethernet’s CSMA/CD EfficiencyC

han

nel Effi

cie

ncy

Ch

an

nel Effi

cie

ncy

Number of Stations Trying to SendNumber of Stations Trying to Send

0 1 2 4 8 16 32 64 128 256 512

1.0

0.9

0.8

0,7

0.6

0.5

0.4

0.3

0.2

0.1

1024 byte frames

512 byte frames

128 byte frames

256 byte frames

64 byte frames


Ethernet Technologies: 10Base2 10: 10Mbps; 2: under 200 meters (actually 185) max. cable length per segment thin coaxial cable in a bus topology

repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces: physical layer device only!


10BaseT and 100BaseT

10/100 Mbps rate; latter called “fast ethernet” T stands for Twisted Pair Nodes are connected to hubs by twisted pair,

thus “star topology” CSMA/CD monitoring can be implemented at

hub


10BaseT and 100BaseT (more) Max. distance from node to Hub is 100 meters

max. between any two nodes is 200 meters

Hub can disconnect “jabbering” adapter Hub can gather monitoring information,

statistics for display to LAN administrators Fiber links can be used to expand geographical

reach (per IEEE 802)

Q: How do you calculate the time it takes to send a datagram from one node to another via a hub?


Token Passing: IEEE802.5 standard 4 Mbps max token holding time: 10 ms, limiting frame

length

SD, ED mark start, end of packet (each 8 bits) AC: access control byte (8bits):

token bit: value 0 means token can be seized, value 1 means data follows FC priority bits: priority of packet reservation bits: station can write these bits to prevent stations with lower priority

packet from seizing token after token becomes free

EDED


Interconnecting LANs

Q: Why not just one big LAN? Limited amount of supportable traffic: on

single LAN, all stations must share bandwidth limited length: 802.3 specifies maximum cable

length large “collision domain” (can collide with many

stations) limited number of stations: 802.5 have token

passing delays at each station


Devices for Interconnecting LANs

Hubs (repeaters) physical-layer bit-level repeaters extend physical reach of a LAN 10/100BaseT interconnection (repeaters for

10Base2)

Bridges link-layer, frame switches extend physical reach and scale of a LAN allow logical segregation of a LAN

Switches Link-layer, high performance bridge


Hub limitations

single collision domain results in no increase in max throughput multi-tier throughput same as single

segment throughput individual LAN restrictions pose limits on

number of nodes in same collision domain and on total allowed geographical coverage

cannot connect different Ethernet types (e.g., 10BaseT and 100baseT

Hub: “Bit In, Bit Out”Hub: “Bit In, Bit Out”


Bridges

Link Layer devices: operate on Ethernet frames, examining frame header and selectively forwarding frame based on its destination

Bridge isolates collision domains since it buffers frames

When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmit


Bridges: frame filtering, forwarding

bridges filter packets same-LAN -segment frames not forwarded

onto other LAN segments forwarding:

how to know which LAN segment on which to forward frame?

looks like a routing problem (more shortly!)


Bridge Filtering

bridges learn which hosts can be reached through which interfaces: maintain filtering tables when frame received, bridge “learns” location

of sender: incoming LAN segment records sender location in filtering table

filtering table entry: (Node LAN Address, Bridge Interface, Time

Stamp) stale entries in Filtering Table dropped (TTL can

be 60 minutes)


Bridge Filtering

filtering procedure:

if destination is on LAN on which frame was receivedthen drop the frameelse { lookup filtering table if entry found for destination

then forward the frame on interface indicated;

else flood; /* forward on all but the interface

on which the frame arrived*/}


Bridge Learning: example Suppose C sends frame to D and D replies

back with frame to C

C sends frame, bridge has no info about D, so floods to other attached LANs (on ports 2 and 3): bridge notes that C is on port 1 frame ignored on upper LAN (port 3) frame received by D (port 2)

C 1 <<< added


Bridge Learning: example

D generates reply to C, sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1, so selectively

forwards frame out via interface 1

C 1 D 2 <<< added


Bridges vs. Routers (or layer-3 switches) both are store-and-forward devices

routers: network layer devices (examine network layer headers) bridges are link layer devices

routers maintain routing tables, implement routing algorithms

bridges maintain filtering tables, implement filtering, learning and spanning tree algorithms


Ethernet Switches

cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frame not a store-and-forward operation slight reduction in latency over store-and-

forward combinations of shared/dedicated,

10/100/1000 Mbps interfaces


IEEE 802.11 Wireless LAN

wireless LANs: untethered (often mobile) networking

IEEE 802.11 standard: MAC protocol unlicensed frequency spectrum: 900Mhz,

2.4Ghz Basic Service Set (BSS)

(a.k.a. “cell”) contains: wireless hosts access point (AP): base

station BSS’s combined to form

distribution system (DS)


Ad Hoc Networks

Ad hoc network: IEEE 802.11 stations can dynamically form network without AP

Applications:“laptop” meeting in conference room,

car! interconnection of “personal” devicesbattlefield

IETF MANET (Mobile Ad hoc Networks) working group


IEEE 802.11 MAC Protocol: CSMA/CA802.11 CSMA: sender- if sense channel idle for

DIFS sec. then transmit entire frame

(no collision detection)-if sense channel busy

then exponential backoff (like Ethernet)

802.11 CSMA receiver:if received OK return ACK after SIFS


IEEE 802.11 MAC Protocol

802.11 CSMA Protocol: others

NAV: Network Allocation Vector

802.11 frame has transmission time field

others (hearing data) defer access for NAV time units


Hidden Terminal effect

hidden terminals: A, C cannot hear each other obstacles, signal attenuation collisions at B

goal: avoid collisions at B CSMA/CA: CSMA with Collision Avoidance


Collision Avoidance: RTS-CTS exchange CSMA/CA: explicit

channel reservation sender: send short

RTS: request to send receiver: reply with

short CTS: clear to send

CTS reserves channel for sender, notifying (possibly hidden) stations

avoid hidden station collisions


Collision Avoidance: RTS-CTS exchange RTS and CTS short:

collisions less likely, of shorter duration

end result similar to collision detection

IEEE 802.11 allows:CSMACSMA/CA:

reservations polling from AP


Point to Point Data Link Control one sender, one receiver, one link: easier than

broadcast link: bit-oriented transmission stream no Media Access Control no need for explicit MAC addressing

• e.g., dialup link, ISDN line

popular point-to-point DLC protocols: PPP (point-to-point protocol) HDLC: High-level data link control

• Data link used to be considered a “high layer” in the protocol stack!

• also: SDLC, ADCCP, LAP, LAPB


PPP Design Requirements [RFC 1557] packet framing: encapsulation of network-layer

datagram in data link frame carry network layer data of any network

layer protocol (not just IP) at same time ability to demultiplex upwards

bit transparency: must carry any bit pattern in the data field

error detection (no correction) connection liveness: detect, signal link failure to

network layer network layer address negotiation: endpoint

can learn/configure each other’s network address


PPP non-requirements (explicit in the specification)

no error correction/recovery no flow control no ordering … out of order delivery OK no need to support multipoint links (e.g., polling)

Error recovery, flow control, data re-ordering/sequencing

all relegated to higher layers!|


PPP Data Frame Flag: delimiter (framing) Address: does nothing (only one option in PPP) Control: does nothing; in the future possible

multiple control fields Protocol: upper layer protocol to which frame

delivered (e.g., C021x = PPP-LCP, 8021x = IPCP, 0021x = IP, 0029x = AppleTalk, etc.)


PPP Data Frame

info: upper layer data being carried check: cyclic redundancy check (CRC)

for error detection*

*Maximum payload length is negotiated by link control protocol at link establishment, or default is 1500 bytes.


Byte Stuffing “data transparency” requirement: data/info

field must be allowed to contain “special” bit patterns, such as flag <01111110> Q: is received <01111110> data or flag?

Sender: adds (“stuffs”) a special control escape byte < 01111101> byte before each < 01111110> data byte Q: what about <01111101> in the data?

Receiver: when <01111101> received: discard it, then

continue data reception single 01111110: flag byte


ATM architecture

adaptation layer: only at edge of ATM network data segmentation/reassembly roughly analogous to Internet transport layer

ATM layer: “network” layer cell switching, routing

physical layer


ATM LayerService: transport cells across ATM network analogous to IP network layer very different services than IP network layer

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 ?


ATM Layer: ATM cell Core of ATM standard 5-byte ATM cell header 48-byte payload

Why?: small payload -> short cell-creation delay for digitized voice

halfway between 32 and 64 (compromise!)

Cell header

Cell format


ATM cell header

VCI: virtual channel ID will change from link to link thru net

PT: Payload type (e.g. RM cell, idle cell, data cell)

CLP: Cell Loss Priority bit CLP = 1 implies low priority cell, can be

discarded if congestion HEC: Header Error Checksum (8-bit)

cyclic redundancy check


IP-Over-ATMIssues: Conversion of IP

datagrams into ATM AAL5 PDUs

Conversion from IP addresses to ATM addresses just like IP

addresses to 802.3 MAC addresses!

ATMbackbonenetwork

EthernetLANs


Datagram Journey in IP-over-ATM Network at Source Host/Gateway:

IP layer finds mapping between IP and ATM destination address (using ARP)

passes datagram to AAL5 AAL5 encapsulates data, segments to cells, passes to

ATM layer ATM network: moves cell along Virtual Circuit

(permanent?) to destination at Destination Host/Gateway:

AAL5 reassembles cells into original datagram if CRC OK, datagram is passed to IP

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