1 TAC2000/2000.7 802.16 IP Telephony Lab A Protocol for Packet Network Intercommunication Vinton G....

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802.16 IP Telephony Lab

A Protocol for Packet Network Intercommunication

Vinton G. Cerf & Robert E. Kahn

IEEE Transactions on Communications, Vol. 22, No. 5, May 1974

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Protocols for Packet Switching Networks

Packet switching networks were developed to facilitate the Packet switching networks were developed to facilitate the sharing of computer resourcessharing of computer resources..

Many propotols have been developed to support communication Many propotols have been developed to support communication between computers and computers.between computers and computers.

This paper presents a protocol that supports the sharing of This paper presents a protocol that supports the sharing of resources that exist resources that exist in different networksin different networks..

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Typical packet switching network

PS = Packet SwitchPS = Packet Switch Within each HOST, there exist Within each HOST, there exist processesprocesses which communicate which communicate

with processes in their own or other HOSTS.with processes in their own or other HOSTS.

HOST

HOST

HOST

PS

PS

PS

PS

PS

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Implementation Differences AddressingAddressing

A uniform addressing scheme is required for communication across A uniform addressing scheme is required for communication across difference networks.difference networks.

Frame sizeFrame size Networks transmit data in different units. Some can be very small.Networks transmit data in different units. Some can be very small.

TimerTimer Each network adopts different time delays in determining the success or Each network adopts different time delays in determining the success or

failure in accepting and delivering data.failure in accepting and delivering data.

Restoration procedure for communication disruptionRestoration procedure for communication disruption Routing & fault detectionRouting & fault detection

Each network has different mechanisms to determine an inaccessible or Each network has different mechanisms to determine an inaccessible or dead destination.dead destination.

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Yes! They Differ Do we want every HOST or process to implement every Do we want every HOST or process to implement every

protocol in different packet networks?protocol in different packet networks? It is unacceptable, although it is possible.It is unacceptable, although it is possible.

Rather, we prefer to have a common protocol used between Rather, we prefer to have a common protocol used between HOST’s or processes, HOST’s or processes, The interface between different networks should take as small a role as The interface between different networks should take as small a role as

possible.possible. This leads to the concept of “Layers”!This leads to the concept of “Layers”!

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Internetwork Protocol Issues

AddressingAddressingRetransmissionRetransmissionFragmentationFragmentation

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GATEWAY

The responsibility for properly routing data resides in the The responsibility for properly routing data resides in the GATEWAY.GATEWAY.

A GATEWAY between two networks may be composed of two A GATEWAY between two networks may be composed of two halves.halves.

Network B

Network A

Network C

GATEWAY

M

ProcessX

ProcessY

GATEWAY

N

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Internetwork Packet Format

The Internetwork Header actually contains today’s IP+TCP The Internetwork Header actually contains today’s IP+TCP headers.headers.

There is no TTL in the header.There is no TTL in the header. The GATEWAY merely forwards the check sum along without The GATEWAY merely forwards the check sum along without

computing or recomputing it.computing or recomputing it.

Local Header Source Destination Sequence No. Byte Count Flag Text Checksum

Internetwork Header

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Frame Size in Packet Switching Networks

Should every network adopt the same frame size?Should every network adopt the same frame size?Encryption may expand the packet size.Encryption may expand the packet size.New technology prefers larger size to get better performance.New technology prefers larger size to get better performance.

Fragmentation permits packet size variations to be Fragmentation permits packet size variations to be handled on an individual network basishandled on an individual network basisNo global administration is required.No global administration is required.HOSTS and processes can be insulated from changes in the HOSTS and processes can be insulated from changes in the

packet sizes of underlining networks.packet sizes of underlining networks. It is best for fragmentation to be done upon entering the It is best for fragmentation to be done upon entering the

next network next network The GATEWAY is aware of the internal packet size The GATEWAY is aware of the internal packet size

parameters.parameters.

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Reassembly If is conceivable to desire the GATEWAY to perform the If is conceivable to desire the GATEWAY to perform the

reassembly:reassembly: To simplify the task of the destination HOST (or process)To simplify the task of the destination HOST (or process) To take advantage of a larger packet size.To take advantage of a larger packet size.

However, we take the position that GATEWAYS should not However, we take the position that GATEWAYS should not perform this function:perform this function: Serious buffering problemsSerious buffering problems Potential deadlocksPotential deadlocks The necessity for all fragments of a packet to pass through the same The necessity for all fragments of a packet to pass through the same

GATEWAYGATEWAY Increased delay in transmissionIncreased delay in transmission The final GATEWAY may still have to fragment a packet for The final GATEWAY may still have to fragment a packet for

transmission.transmission. Thus the destination HOST must be prepared to do this task.Thus the destination HOST must be prepared to do this task.

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Process Level Communication Within a HOST there exist a transmission control program Within a HOST there exist a transmission control program

(TCP)(TCP) It handles the transmission and acceptance of message, on behalf of the It handles the transmission and acceptance of message, on behalf of the

processesprocesses This is This is equivalent to today’s TCP+IP protocol stacks.equivalent to today’s TCP+IP protocol stacks.

The TCP is in turn served by one or more packet switches connected to The TCP is in turn served by one or more packet switches connected to the HOST.the HOST.

Processes present messages to the TCP for transmissionProcesses present messages to the TCP for transmission TCP delivers incoming messages to the appropriate destination TCP delivers incoming messages to the appropriate destination

processes.processes.

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Multiplexing Multiplexing and demultiplexing of segments among processes Multiplexing and demultiplexing of segments among processes

are fundamental tasks of the TCP.are fundamental tasks of the TCP. On transmission, a TCP must multiplex together segments from On transmission, a TCP must multiplex together segments from different different

source processessource processes and produce internetwork packets for deliver. and produce internetwork packets for deliver. On reception, a TCP will accept a sequence of packets from its serving On reception, a TCP will accept a sequence of packets from its serving

packet switch(es). From this sequence of arriving packets (generally packet switch(es). From this sequence of arriving packets (generally from different HOSTSfrom different HOSTS), the TCP reconstruct and deliver messages to the ), the TCP reconstruct and deliver messages to the proper destination processes.proper destination processes.

If the TCP is to determine for which process an arriving packet If the TCP is to determine for which process an arriving packet is intended, every packet must contain a is intended, every packet must contain a process headerprocess header that that completely identifies the destination process.completely identifies the destination process.

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Address Formats We need a uniform internetwork TCP address space, which is We need a uniform internetwork TCP address space, which is

understood by each GATEWAY and TCP.understood by each GATEWAY and TCP. TCP addressTCP address

Network ID: 8 bits – 256 networksNetwork ID: 8 bits – 256 networks This size seems sufficient for the foreseeable future.This size seems sufficient for the foreseeable future. 640K ought to be enough for anybody. ~ Bill Gates640K ought to be enough for anybody. ~ Bill Gates

TCP ID: 16 bits – 65,536 hosts in a network.TCP ID: 16 bits – 65,536 hosts in a network.

Network TCP IDENTIFIER

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Port Addressing The port is simply a designator of one such message stream The port is simply a designator of one such message stream

associated with a process.associated with a process. Each operating system has its own internal means of identifying Each operating system has its own internal means of identifying

processes and ports.processes and ports. We assume that 16 bits are sufficient to serve as internetwork port We assume that 16 bits are sufficient to serve as internetwork port

identifiers.identifiers.

Segment format (process header and text)Segment format (process header and text)

Source Port WindowDestination Port ACK Text

16 1616 16 8n

Process Header

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Segment and Packet FormatsMessage A

Message B

LH IH PH A1 CK LH IH PH B1 CK LH IH PH An CK

Internetwork Packet

Segment

Local Network PacketInternetwork Packet

…

LH = Local Header

IH = Internetwork Header

PH = Process Header

CK = Checksum

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Reassembly and Sequencing00 11 22 …… kk ……

First MessageFirst Message Second MessageSecond Message(SEQ = k)(SEQ = k)

Third MessageThird Message

SegmentSegment SegmentSegment SegmentSegment SegmentSegment ……

Each internetwork packet must carry a sequence numberEach internetwork packet must carry a sequence number It is unique in each stream.It is unique in each stream. It must be monotonic increasing.It must be monotonic increasing.

Each byte is a message has a unique sequence number – its byte location Each byte is a message has a unique sequence number – its byte location relative to the beginning of the stream.relative to the beginning of the stream.

This simple (but elegant) approach allows the exact position of the data to be This simple (but elegant) approach allows the exact position of the data to be determined, determined, even when pieces are still missingeven when pieces are still missing..

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802.16 IP Telephony LabFlag

The splitting of message into segments by the TCP and the potential splitting The splitting of message into segments by the TCP and the potential splitting of segments into smaller pieces by GATEWAYS creates the necessity for of segments into smaller pieces by GATEWAYS creates the necessity for indicating to the destination TCP:indicating to the destination TCP: when the end of a segment (ES) has arrivedwhen the end of a segment (ES) has arrived When the end of a message (EM) has arrived.When the end of a message (EM) has arrived.

The flag field of the internetwork header is used for this purpose.The flag field of the internetwork header is used for this purpose. Note: not the process headerNote: not the process header The GATEWAY need only examine the internetwork header to do The GATEWAY need only examine the internetwork header to do

fragmentation.fragmentation. Q: Is fragmentation processed by which layer in current TCP/IP?Q: Is fragmentation processed by which layer in current TCP/IP?

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Message Splitting & Packet Splitting

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Retransmission and Duplicate Detection

No transmission can be 100 percent reliable.No transmission can be 100 percent reliable. We propose a timeout and positive acknowledgment We propose a timeout and positive acknowledgment

mechanism.mechanism. If no acknowledgment (for a particular packet) is received, the TCP will If no acknowledgment (for a particular packet) is received, the TCP will

retransmit.retransmit.

Any retransmission policy requires some means by which the Any retransmission policy requires some means by which the receiver can detect duplicate arrivals.receiver can detect duplicate arrivals. However, the sequence number has a size limit.However, the sequence number has a size limit. For example. A 2-byte field implies the sequence number can range from For example. A 2-byte field implies the sequence number can range from

0 – 65535.0 – 65535.

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Window Strategy Suppose sequence numbers can range from 0 to n-1.Suppose sequence numbers can range from 0 to n-1. Assume the sender will not transmit more than w bytes without Assume the sender will not transmit more than w bytes without

receiving an acknowledgment.receiving an acknowledgment. The w bytes serve as the window.The w bytes serve as the window.

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Window Strategy Example 1

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Rules for Sender and Receiver Sender: Let L be the sequence number Sender: Let L be the sequence number

associated with the left window edge.associated with the left window edge. The sender transmit bytes from The sender transmit bytes from

segments whose text lies between L segments whose text lies between L and up to L+w-1.and up to L+w-1.

On timeout, the sender retransmits On timeout, the sender retransmits unacknowledged bytes.unacknowledged bytes.

On receipt of acknowledgment On receipt of acknowledgment consisting of the receiver’s current left consisting of the receiver’s current left window edge, the sender’s left window window edge, the sender’s left window edge is advanced over the edge is advanced over the acknowledged bytes.acknowledged bytes. This advances the right window edge This advances the right window edge

implicitly.implicitly.

Receiver:Receiver: Arriving packets whose sequence Arriving packets whose sequence

numbers coincide with the receiver’s numbers coincide with the receiver’s current left window edge are current left window edge are acknowledged by sending to the source acknowledged by sending to the source the next sequence number expect.the next sequence number expect. This effectively acknowledges bytes in This effectively acknowledges bytes in

between.between. The left window edge is advanced to The left window edge is advanced to

the next sequence number expected.the next sequence number expected. Packets arriving with a sequence Packets arriving with a sequence

number to the left of the window edge number to the left of the window edge (which implies that it is outside of the (which implies that it is outside of the window) are discarded, and the current window) are discarded, and the current left window edge is returned as left window edge is returned as acknowledgment.acknowledgment.

Packets whose sequence numbers lie Packets whose sequence numbers lie within the receiver’s window but do within the receiver’s window but do not coincide with the receiver’s left not coincide with the receiver’s left window edge are optionally kept or window edge are optionally kept or discarded, but are not acknowledged. discarded, but are not acknowledged. This is the case when packets arrive This is the case when packets arrive out of order.out of order.

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Some Observations1.1. All computations with sequence numbers and window edges must be made All computations with sequence numbers and window edges must be made

module n.module n.

2.2. The window size w must be less than n/2The window size w must be less than n/2 Otherwise, a retransmission may appear to the receiver to be a new transmission in Otherwise, a retransmission may appear to the receiver to be a new transmission in

the case that the receiver has accepted a window’s incoming packets, but all the case that the receiver has accepted a window’s incoming packets, but all acknowledgments have been lost.acknowledgments have been lost.

3.3. The receiver can either save or discard arriving packets whose sequence The receiver can either save or discard arriving packets whose sequence numbers do not coincide with the receiver’s left window.numbers do not coincide with the receiver’s left window. Thus, in the simplest implementation, the receiver need not buffer more than one Thus, in the simplest implementation, the receiver need not buffer more than one

packet per message stream, if space is critical.packet per message stream, if space is critical.

4.4. Mutiple packets can be acknowledged simultaneously.Mutiple packets can be acknowledged simultaneously.

5.5. The receiver is able to deliver messages to processes in their proper order as a The receiver is able to deliver messages to processes in their proper order as a natural result of the reassembly mechanism.natural result of the reassembly mechanism.

6.6. When duplicates are detected, the acknowledgment method helps to When duplicates are detected, the acknowledgment method helps to resynchronize sender and receiver.resynchronize sender and receiver.

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Overlap Problem

This problem may result from retransmission, packet This problem may result from retransmission, packet splitting, and alternate routing of packets through splitting, and alternate routing of packets through different GATEWAYS.different GATEWAYS.A 600-byte packet might pass through one GATEWAY and A 600-byte packet might pass through one GATEWAY and

be broken into two 300-byte packets. be broken into two 300-byte packets. On retransmission, the same packet might go through a On retransmission, the same packet might go through a

different GATEWAY and be broken into three 200-byte different GATEWAY and be broken into three 200-byte packets.packets.

Since each byte has a sequence number, there is no Since each byte has a sequence number, there is no confusion at the receiving TCP.confusion at the receiving TCP.

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Flow Control Acknowledgments are carried in the process header.Acknowledgments are carried in the process header.

Along with them, there is provision for a “suggested window” value, Along with them, there is provision for a “suggested window” value, which the receiver can use to control the flow of data from the sender.which the receiver can use to control the flow of data from the sender.

The receiver is free to vary the window size according to any The receiver is free to vary the window size according to any algorithm it desires.algorithm it desires. As long as the window size never exceeds half the sequence number As long as the window size never exceeds half the sequence number

space.space.

The TCP has a component which handles arriving packets. The TCP has a component which handles arriving packets. It validates the addresses and places the packet on a queue.It validates the addresses and places the packet on a queue. If all available buffers are used up, succeeding arrivals can be simply If all available buffers are used up, succeeding arrivals can be simply

discarded, because unacknowledged packets will be retransmitted. discarded, because unacknowledged packets will be retransmitted. (What a simple mechanism!)(What a simple mechanism!)

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Conclusions We have discussed some fundamental issues related to the interconnection of We have discussed some fundamental issues related to the interconnection of

packet switching networks.packet switching networks. In particular, we have described a simple but very powerful and flexible In particular, we have described a simple but very powerful and flexible

protocol, which provides mechanism to handleprotocol, which provides mechanism to handle Individual network packet sizesIndividual network packet sizes Transmission failuresTransmission failures SequencingSequencing Flow controlFlow control Creation and destruction of process-to-process associations.Creation and destruction of process-to-process associations.

The next important stepThe next important step To produce a detailed specification of the protocol so that some initial experiments To produce a detailed specification of the protocol so that some initial experiments

with it can be performed.with it can be performed. RFC 760 - DoD standard Internet Protocol. J. Postel. January 1980.RFC 760 - DoD standard Internet Protocol. J. Postel. January 1980. RFC 768 - RFC 768 - User Datagram Protocol. J. Postel. August 1980.User Datagram Protocol. J. Postel. August 1980. RFC 791 - Internet Protocol. J. Postel. September 1981.RFC 791 - Internet Protocol. J. Postel. September 1981. RFC 793 - Transmission Control Protocol. J. Postel. September 1981.RFC 793 - Transmission Control Protocol. J. Postel. September 1981.

These experiments are needed to determine some of the operational parameters (e.g., These experiments are needed to determine some of the operational parameters (e.g., what sort of delay is there between segment acknowledgments; what should be what sort of delay is there between segment acknowledgments; what should be retransmission timeouts be?retransmission timeouts be?

1 TAC2000/2000.7 802.16 IP Telephony Lab A Protocol for Packet Network Intercommunication Vinton G....

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