Computer Networks

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 1

Computer Networks

Chapter 7 – Connection-oriented Networks: X.25 and ATM


Generic Connection-oriented Network

• Call Admission Control (CAC)

• Call Setup– Routing choices– Resource reservation– Connection Identifier (CI) – local significance– Confirmation

• Routing by CI

• Call Teardown


Connection Identifiers (CIs)

A

X

R1B

R4

R2

R5

R3

ad

cb

a,57,c,33c,33,a,57

a,33,d,79d,79,a,33

a d

cba

cb

b,79,c,22 c,22,b,79

5733

79 22

A has a connection to B via R1, R2, and R5The Connection Identifier (CI) only has local significance –

it changes along the routeEach router has a table with in-port, CI-in, out-port, CI-out


Generic Connection Setup

A

X

R1B

R4

R2

R5

R3

ad

cb

a d

cba

cb

Setup

Application at A wants to talk to application at B with some QoSA checks local resources, if OK, proceeds, else rejectsA creates table entry for A-B connection via R1 using CI=57A sends setup packet to R1Setup Pkt = (A wants to talk to B, use CI=57)


A

X

R1B

R4

R2

R5

R3

ad

cb

a,57,c,33c,33,a,57

a d

cba

cb

57

R1 checks local resources, if OK, proceeds, else rejectsR1 makes routing decision to forward to B via R2 using CI=33R1 creates table entries for A-B connectionR1 forwards setup packet to R2Setup Pkt = (A wants to talk to B, use CI=33)

Setup




A

X

R1B

R4

R2

R5

R3

ad

cb

a,57,c,33c,33,a,57

a,33,d,79d,79,a,33

a d

cba

cb

5733

R2 checks local resources, if OK, proceeds, else rejectsR2 makes routing decision to forward to B via R5 using CI=79R2 creates table entries for A-B connectionR2 forwards setup packet to R5Setup Pkt = (A wants to talk to B, use CI=79)

Setup



A

X

R1B

R4

R2

R5

R3

ad

cb

a,57,c,33c,33,a,57

a,33,d,79d,79,a,33

a d

cba

cb

b,79,c,22c,22,b,79

5733

79

R5 checks local resources, if OK, proceeds, else rejectsR5 makes routing decision to forward to B directly using CI=22R5 creates table entries for A-B connectionR5 forwards setup packet to BSetup Pkt = (A wants to talk to B, use CI=22)

Setup



A

X

R1B

R4

R2

R5

R3

ad

cb

a,57,c,33c,33,a,57

a,33,d,79d,79,a,33

a d

cba

cb

c,22,b,79b,79,c,22

57 33

79 22

B checks local resources, if OK, proceeds, else rejectsB creates table entries for A-B connectionB replies with setup ACK to R5Setup ACK = (B accepts call from A, using CI=22)R5, R2, and R1 forward Setup ACK, changing CI as it goes,

and marking table entries as confirmed

Setup ACK


Connection-oriented Routing

A

X

R1B

R4

R2

R5

R3

ad

cb

a,57,c,33a,33,d,79

a d

cba

cb

(57,M1)

b,79, c,22

A sends M1 to B, CI=57, routed to R1



A

X

R1B

R4

R2

R5

R3

ad

cb

a,57,c,33a,33,d,79

a d

cba

cb

(33,M1)

b,79, c,22

R1 forwards M1 from port a CI=57 to port c, changes CI to 33



A

X

R1B

R4

R2

R5

R3

ad

cb

a,57,c,33a,33,d,79

a d

cba

cb(79,M1)

b,79, c,22

R2 forwards M1 from port a CI=33 to port d, changes CI to 79



A

X

R1B

R4

R2

R5

R3

ad

cb

a,57,c,33a,33,d,79

a d

cba

cb (22,M1)

b,79, c,22

R5 forwards M1 from port b CI=79 to port c, changes CI to 22



A

X

R1B

R4

R2

R5

R3

ad

cb

c,33, a,57d,79, a,33

a d

cba

cb (22,M2)

c,22,b,79

B sends M2 to A, CI=22, routed to R5



A

X

R1B

R4

R2

R5

R3

ad

cb

c,33, a,57d,79, a,33

a d

cba

cb(79,M2)

c,22,b,79

R5 forwards M2 from port c CI=22 to port b, changes CI to 79



A

X

R1B

R4

R2

R5

R3

ad

cb

c,33, a,57d,79, a,33

a d

cba

cb

(33,M2)

c,22,b,79

R2 forwards M2 from port d CI=79 to port a, changes CI to 33



A

X

R1B

R4

R2

R5

R3

ad

cb

c,33, a,57d,79, a,33

a d

cba

cb

(57,M2)

c,22,b,79

R1 forwards M2 from port c CI=33 to port a, changes CI to 57


Generic Connection-oriented Network Tables

A

X

R1B

R4

R2

R5

R3

ad

cb

a,57,c,33c,33,a,57b,57,c,21c,21,b,57b,23,d,12d,12,b,23a,5,b,17b,17,a,5

a,33,d,79d,79,a,33a,21,d,21d,21,a,21

a d

cba

cb

c,22,b,79b,79,c,22b,21,c,35c,35,b,21c,42,a,14a,14,c,42

Can you trace the connections?

Ya,12,c,33c,33,a,12

a c

b

a

cb

d

b,33, c,57c,57,b,33c,33,d,14d,14,c,33


Generic Connection Teardown

A

X

R1B

R4

R2

R5

R3

ad

cb

a d

cba

cb

For teardown, either end or intermediate node (router) sends teardown packet

Ya c

b

a

cb

d



A

X

R1B

R4

R2

R5

R3

ad

cb


a,33,d,79d,79,a,33a,21,d,21d,21,a,21

a d

cba

cb

c,22,b,79b,79,c,22b,21,c,35c,35,b,21c,42,a,14a,14,c,42

B decides to close connection to A,B sends teardown packet to R5Teardown Pkt = (Teardown, CI=22)R5 acknowledges, B frees resources

Ya c

b

a

cb

d

TeardownACK



A

X

R1B

R4

R2

R5

R3

ad

cb


a,33,d,79d,79,a,33a,21,d,21d,21,a,21

a d

cba

cb

b,21,c,35c,35,b,21c,42,a,14a,14,c,42

R5 sends teardown packet to R2Teardown Pkt = (Teardown, CI=79)R2 acknowledges, R5 frees resources

Ya c

b

a

cb

d

TeardownACK



A

X

R1B

R4

R2

R5

R3

ad

cb


a,21,d,21d,21,a,21

a d

cba

cb

b,21,c,35c,35,b,21c,42,a,14a,14,c,42

R2 sends teardown packet to R1Teardown Pkt = (Teardown, CI=33)R1 acknowledges, R2 frees resources

Ya c

b

a

cb

dTeardownACK



A

X

R1B

R4

R2

R5

R3

ad

cb

b,57,c,21c,21,b,57b,23,d,12d,12,b,23a,5,b,17b,17,a,5

a,21,d,21d,21,a,21

a d

cba

cb

b,21,c,35c,35,b,21c,42,a,14a,14,c,42

R1 sends teardown packet to ATeardown Pkt = (Teardown, CI=57)A acknowledges, R1 and A free resources

Ya c

b

a

cb

dTeardown

ACK


X.25

• CCITT designed, adopted as ISO 8208

• Specifies DTE-Router interface at three levels:– PHY: connector, voltage, pin semantics, etc.– Link: delimiters, pkt #s, ACKs, flow ctl, etc.– Packet: network layer interface

• DTE=end node (Data Termination Equip)

• DCE=router (Data Communication Equip)


X.25 Basics

• DTE can have multiple calls over a single link to a DCE

• Call first must be established

• Then data may flow in both directions

• Then the call must be cleared


X.25 Basics

• Call may be established by DTE in 3 ways:– DTE initiated (SVC – Switched Virtual Circuit)

DTE informs DCE of desired destination DTE– DTE received (Incoming Call)

DCE informs DTE call request by source DTE– Nailed up (Permanent Virtual Circuit)

Administratively set up – DTEs only see data

• Call gets locally significant VC number at setup


X.25 VC numbers

• VC “Call numbers” are 12 bits long:– Logical Channel Group Number: 4 bits– Logical Channel Number: 8 bits

• Call number 0 reserved for control pkts for all VCs

• Next block of call numbers for PVCs

• Then incoming, then either, then outgoing(to avoid CN collisions from DTE and DCE)


X.25 Call Setup

0 0 10 group

channel

type

Calling DTE Addr Len Called DTE Addr Len

Calling DTE Address

Called DTE Address

Facilities Length

Facilities

User Data

1

1

1

1

variable

variable

1

variable

variable

# octets

0x0a for call request

units are nybbles

Call Request Packet Format


X.25 Call Overview

– X.25 specifies DTE-DCE protocol– But not DCE-DCE protocol

Call Request

Call ConnectedData

Incoming DataClear Request

Clear Confirm

Incoming CallCall Accepted

Incoming DataData

Clear IndicationClear Response

DTE DTE

DCE DCE


X.25 Data Transfer

• Call is full duplex

(simultaneous bidirectional data flow)

• Message and Ack sequence numbers– 3-bit and 7-bit formats– Choice at subscription or call setup time

• Q bit – end host purposes

• D bit – Received by DTE? or DCE only

• M bit – more packets in message


X.25 Data Packet Formats

D 0 1Q group

channel

Ack # Msg #

User Data

0M

D 0 1Q group

channel

Msg #

User Data

0

Ack # M

7-bit Format

3-bit Format


X.25 Data Transfer

• Call # (Group+Channel) and msg # allow multiple, simultaneous connections

• M bit allow message fragmentation– All but last packet have M=0b1– When fragmented, new sequence #s given– Destination DCE must hold and reassemble– Packet must be full for M bit to be set…


X.25 Packet Fragmentation

– Smaller maximum packet size at destination– Sequence numbers are different

DTE DTEDCE DCE

#1

#2

M#1M#2

#3

M#4M#5

#6

Max pkt size = X Max pkt size = X/3


X.25 Packet Fragmentation

– Smaller maximum packet size at source– Destination DCE must hold and recombine fragments– Sequence numbers are different

DTE DTEDCE DCE

#1

#2

M#1M#2

#3

M#4M#5

#6

Max pkt size = XMax pkt size = X/3


X.25 Flow Control

• Sequence numbers + ACKs (piggybacked)– ACK cumulative and inclusive– ACK indicates receiver ready to take w more– Sequence numbers different in each direction

• No data for return– Receive Ready packet

• Choking flow– No ACKs, or equivalently, Receive Not Ready


X.25 Facilities• Window size w – fixed or negotiated per call• Packet size - fixed or negotiated per call• Throughput - fixed or negotiated per call (75 bps

– 64 Kbps)• Closed user group – access control

– Set of DTEs that can communicate– Closed group numbers of local significance

• Allowing only incoming or outgoing calls• Setting lowest outgoing CI number

– Boundary for CIs set by DCE and DTE


Per-call X.25 Facilities

• Window size w

• Maximum packet size

• Throughput (75 bps – 64 Kbps)

• Closed user group for call

• Reverse charging (callee pays)


X.25 Call Release• Standard method:

– One of DTEs issues Clear Request to DCE– DCE informs network, other DCE– Other DCE informs other DTE– Other DTE acknowledges to its DCE– Other DCE confirms to network, first DCE– First DCE confirms to initiating DTE

• All packets sent are delivered


X.25 Call Release• Network detects problem

– DCE may issue Clear Request (hangs up)– DCE may issue Reset (restarts seq#’s at 0)– No guarantees on unacknowledged packets– Network may recover transparently (right)

• Potential network problems– DCE or link along route fails– DTE issues packet for a call with unexpected

seq# - DCE and DTE have lost sync on call– DCE tears down all calls to a DTE – lost sync


X.25 Interrupts• Urgent control packet

– DTE may issue one Interrupt packet– Interrupt packet is small (1 to 32 bytes)– Not subject to flow control

• One at a time– Other DTE must reply with Interrupt Confirm

packet– Another Interrupt packet cannot be sent until

the Interrupt Confirm packet is received


X.25 Implementation• Circuit Method

– VC set up through the network– CAC due to resource reservation– Performance guaranteed– Router failure leads to dropped calls

(routers only hold packet until ACKed)

• Reliable Connections over DG Method– Routers with end node neighbors implement full-

service transport layer– DCEs set up connection within network– DCE holds packet until other DCE acknowledges it


X.25 Implementation Comparison

• Circuit Method– Service guarantees– May refuse a call– Faster routing (based on small CIs)– Route failures not transparent

• Reliable Connections over DG Method– No router buffer or BW reservations– More utilization for bursty traffic– Transparent route modifications


ATM Basics

• Cells– Fixed size packets (48 bytes data, 5 header)– 48 bytes an unhappy compromise

• Virtual Circuits, virtual paths

• Service categories

• Adaptation layers


ATM VCs and VPs

• Virtual circuits created with CIs• CI has two subfields: VPI and VCI

– Virtual Path ID is 12 bits – core routing– Virtual Circuit ID is 16 bits – calls

• End node to switch– 4 bits of Generic Flow Control– These are “borrowed” from the VCI field

• Switch to switch - Full 16-bit VCI


ATM VCs and VPs• VPI

– Virtual Path ID is 12 bits– Used internally for routing– Changes on each link in ATM network

• VCI– Not used in ATM network– Used with VPI by edge routers to route calls

• VPI/VCI – used as CI outside network


ATM Link Negotiations• Customer and Provider ATM networks

– Customer NWs route on VPI/VCI– Provider NWs route on VPI only– VPIs in provider NWs typically permanent

• Link parameters– Maximum number of active bits in VCI, VPI– Maximum number of VCs, VPs– Maximum allowable VPI number– Min and max VCI number for switched VCIs


ATM Service Categories• CBR – Constant Bit Rate

– Data accepted, delivered at constant rate– Network reserves resources accordingly

• UBR – Unspecified Bit Rate– Best effort

• ABR – Available Bit Rate– Like UBR with congestion feedback

• VBR – Variable Bit Rate– Reserve resources for less than peak rate– Specify burst characteristics, mark cells– RT (real time) and NRT (non-real time)


ATM Cell Header Format

GFC or VPI VPI

VCI

PT

Header CRC

VPI VCI

VCI CLP

GFC = Generic Flow Control (end node-switch only)PT = Payload TypeCLP = Cell Loss Priority (CLP=1 means vulnerable)

end node may set CLP, orswitch may set CLP if rate is excessive


ATM Payload Type Field

Value

000001010011100101110111

Payload Type

data, no congestion yet, not last celldata, no congestion yet, last celldata, congestion, not last cell

data, congestion, last cell of packetcontrol info btw/neighbor switches

control info btw/end switches on pathABR rate control info

reserved for future use


ATM Call Setup/Release• Similar to X.25, called signaling• VCI=5, VPI=0 for signaling cells• Use AAL5 since longer than 48 bytes• Call Setup

– Source & destination addresses– Traffic characteristics– Transit network (like LD carrier)

• Call Proceeding– Sent as link ACK for Setup message


ATM Call Setup/Release (cont.)• Connect

– Sent by destination to source– Accepts call

• Connect ACK– Sent by source to destination

• Release– Sent by either end to other end to end call

• Release ACK– Sent to acknowledge Release to initiator


ATM QoS Parameters

TLA

PCRSCRMCRCDVTCLRCTDCDVCER

SECBRCMR

Meaning

Maximum rate cell will be sentLong term average cell rateMinimum acceptable cell rateMaximum acceptable cell jitterFraction of cells lost or too lateMean and max latencyVariance in latencyFraction of cells delivered w/o errorFraction of blocks garbledFract. of cells delivered to wrong dest.

Parameter

Peak cell rateSustained cell rateMinimum cell rate Cell delay variation toleranceCell loss ratioCell transfer delayCell delay variationCell error rateSeverely-errored cell block r.Cell misinsertion rate


ATM Services Classes

Class A B C D

Timing RT None RT None RT None RT None

Bit rate Constant Variable Constant Variable

Mode Connection oriented Connectionless


ATM Adaptation Layer Structure

Convergence Sublayer (service specific part)

Convergence Sublayer (common part)

Segmentation and Reassembly Sublayer (SAR)

ATM Layer

PHY Layer

AA

L

Convergence layers – interface to application (messages)SAR – break into chunks and add header/trailer (cells)


ATM Adaptation Layers• Interface above ATM to allow packet sizes

other than 48 bytes

• AAL is specified in call setup

• AAL0 = direct ATM

• AAL1 is for CBR

• AAL2 – for VBR, watch this space….

• AAL3/4 – for data, but clunky

• AAL5 – for data by computer guys


ATM AAL1• Accepts CBR stream from source

• Delivers CBR to destination

• Does this over network with variable delays, losses, etc.– Buffers data to smooth delays– Numbers cells so losses/duplicates detected– First byte of cell for control info, rest is payload– Ctl= C bit (signaling), Seq (3 bits), SNP (4 bits

for sequence # protection – CRC-3 + parity)


ATM AAL2• Intended for simple, RT streams

• 1 byte header, 2 byte trailer– Seq# (SN) and Info Type (IT) in header – Length Indicator (LI) and CRC in trailer

• Field sizes not defined!

• Hopelessly broken


ATM AAL3/4• First cut at providing for larger data packets

• Complex and inefficient

• Essentially superseded by AAL5

• AAL3/4 steps: Take datagram M and– Frame M into F with header, trailer, padding– Chop F into cell-sized chunks– Put header, trailer, and padding on each chunk– On receipt, check each cell, reassemble– If error, all of F must be resent


ATM AAL3/4 Framing• Take datagram M and make F

– Add 4 byte header • First byte always 0• 1 byte tag for reassembly• 2 bytes of length (including header, trailer, padding)

– Pad data to a 32-bit boundary– Add 4 byte trailer

• 1 constant byte• 2 bytes of length (actual data length)• 1 byte tag (must match header’s tag)


ATM AAL3/4 Framing• Take F and break into 44-byte chunks

– Each chunk is put into cell– Add 2 byte header

• 2 bits to indicate beginning, end of F• 4 bits sequence number• 10 bit field for multiplexing cells on single circuit

– Pad data to 44 bytes, if necessary– Add 2 byte trailer

• 6 bits length (data length in cell, 44 unless end)• 10 bit CRC covering cell


ATM AAL3/4 Framing

CPI Btag BA size Payload (1 to 65K) Etag LengthPadding

Message (1 to 65K)

1 1 2 1 1 20-3 bytes

ST SN MID Payload (44 bytes) LI CRC

2 4 10 6 10 bits

44-byte chunk 44-byte chunk 44-byte chunk…

ATM header

40


ATM AAL5• Same purpose as AAL3/4 – datagrams• Take message M and form frame F by

– Adding an 8-byte trailer– Padding M so M, trailer, and padding are a

multiple of 48 bytes (fit neatly into cells)– Trailer has 2 unused bytes, plus

• 2-byte length field (data only, not padding or trailer)• 4-byte CRC

– Use 1 bit from cell header to indicate “end”– Use end bit to find trailer, reassemble, check– If error, resend all of F


ATM AAL5 Framing

Payload (1 to 65K) Length CRCUU

Message (1 to 65K)

1 2 41 bytes

Payload

48-byte chunk 48-byte chunk 48-byte chunk…

ATM header

5

PTI field bit set in last cell of message

Pad

Length

…

UU field for useby higher layer

48CRC checks entiremessage, including

trailer (with CRC setto 0x00000000)


AAL Summary/Comparisons

Item AAL 1 AAL 2 AAL 3/4 AAL 5

Service Class A B C/D C/D

Multiplexing No No Yes No

Message Delimiting None None Btag/Etag PTI bit

User bytes 0 0 0 1

CS Padding 0 0 32-bit word 0-47 bytes

CS Protocol OH 0 0 8 8

CS Checksum None None None CRC-32

SAR payload 46-47 45 44 48

SAR proto OH 1-2 3 4 0

SAR checksum None None 10 bits None

Computer Networks

Documents

port c

c computer networks

adcbacbb c

connectionoriented networks

changes ci

r5the connection identifier

b connectionb replies

table entries