RFC721 - Out-of-Band Control Signals in a Host-to-Host Protocol

NWG/RFC721 1 SEP 76 LLG 36636

Out-of-Band Control Signals

Network Working Group Larry Garlick

Request for Comments 721 SRI-ARC

NIC 36636 1 September 76

Out-of-Band Control Signals

in a

Host-to-Host Protocol

This note addresses the problem of implementing a reliable out-of-band

signal for use in a host-to-host protocol. It is motivated by the fact

that sUCh a satisfactory mechanism does not exist in the Transmission

Control Protocol (TCP) of Cerf et. al. [reference 4, 6] In addition to

discussing some requirements for such an out-of-band signal (interrupts)

and the implications for the implementation of the requirements, a

discussion of the problem for the TCP case will be presented.

While the ARPANET host-to-host protocol does not support reliable

transmission of either data or controls, it does meet the other

requirements we have for an out-of-band control signal and will be drawn

upon to provide a solution for the TCP case.

The TCP currently handles all data and controls on the same logical

channel. To achieve reliable transmission, it provides positive

acknowledgement and retransmission of all data and most controls. Since

interrupts are on the same channel as data, the TCP must flush data

whenever an interrupt is sent so as not to be subject to flow control.

Functional Requirements

It is desirable to insure reliable delivery of an interrupt. The

sender must be assured that one and only one interrupt is delivered

at the destination for each interrupt it sends. The protocol need

not be concerned about the order of delivery of interrupts to the

user.

The interrupt signal must be independent of data flow control

mechanisms. An interrupt must be delivered whether or not there are

buffers provided for data, whether or not other controls are being

handled. The interrupt should not interfere with the reliable

delivery of other data and controls.

The host-to-host protocol need not provide synchronization between

the interrupt channel and the data-control channel. In fact, if

coupling of the channels relies on the advancement of sequence

numbers on the data-control channel, then the interrupt channel is no

longer independent of flow control as required above. The

synchronization with the data stream can be performed by the user by

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Out-of-Band Control Signals

marking the data stream when an interrupt is generated. The

interrupt need not be coupled with other controls since it in no way

affects the state of a connection.

Once the interrupt has been delivered to the user, no other semantics

are associated with it at the host-to-host level.

Implications

To provide for reliable delivery and accountability of interrupt

delivery, an acknowledgement scheme is required. To associate

interrupt acknowledgements with the correct interrupt, some naming

convention for interrupts is necessary. Sequence numbers provide

such a naming convention, along with the potential for providing an

ordering mechanism.

A separate interrupt channel is required to make interrupts

independent of flow control. A separate sequence number space for

naming interrupts is also necessary. If the sequence numbers are

from the same sequence number space as some other channel, then

sending an interrupt can be blocked by the need to resynchronize the

sequence numbers on that channel.

In the current TCP, which uses one channel for data, controls, and

interrupts, flushing of data is combined with the interrupt to bypass

the flow control mechanism. However, flushing of resynchronization

controls is not allowed and receipt of these controls is dependent on

the acknowledgement of all previous data. The ARPANET protocol,

while not providing for reliable transmission, does provide for the

separation of the interrupt-control channel and the data channel.

Multiple Channels and Sequence Numbers

If multiple channels are to be used for a connection, then it becomes

interesting to determine how the sequence numbers of the channels can

be coupled so that sequence number maintenance can be done

efficiently.

Assigning sequence numbers to each octet of data and control, as in

the TCP, allows positive acknowledgement and ordering. However,

since packets are retransmitted on timeout, and since multi-path

packet switch networks can cause a packet to stay around for a long

time, the presence of duplicate packets and out-of-order packets must

be accounted for. A sequence number acceptability test must be

performed on each packet received to determine if one of the

following actions should be taken:

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Out-of-Band Control Signals

Acknowledge the packet and pass it on to the user.

Acknowledge the packet, but do not send it to the user, since it

has already been delivered.

Discard the packet; the sequence number is not believable.

Acceptability on Channel 0

To determine the action to be taken for a packet, acceptability

ranges are defined. The following three ranges are mutually

exclusive and collectively exhaustive of the sequence number space

(see Figure 1):

Ack-deliver range (ADR)

Ack-only range (AOR)

Discard range (DR)

ACCEPTABILITY RANGES

DR AOR ADR DR

\\=====)[===========)[===================](========\ ^ ^^ ^^

! !\ ! ! ! FCLE ! DRLE

AOLE AORE ADRE

Figure 1

Let S = size of sequence number space (number per octet)

x = sequence number to be tested

FCLE = flow control left window edge

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Out-of-Band Control Signals

ADRE = (FCLE+ADR) mod S = Ack-deliver right edge (Discard

left edge - 1)

AOLE = (FCLE-AOR) mod S = Ack-only left edge (Discard

right edge + 1)

TSE = time since connection establishment (in sec)

MPL = maximum packet lifetime (in sec)

TB = TCP bandwidth (in octets/sec)

For any sequence number, x, and packet text length, l, if

(AOLE <= x <= ADRE) mod S and

(AOLE <= x+l-1 <= ADRE) mod S

then the packet should be acknowledged.

If x and l satisfy

(FCLE <= x <= ADRE) mod S and

(FCLE <= x+l-1 <= ADRE) mod S

then x can also be delivered to the user; however, ordered

delivery requires that x = FCLE.

A packet is not in a range only if all of it lies outside a range.

When a packet falls in more than one range, precedence is ADR,

then AOR, then DR. When a packet falls in the AOR then an ACK

should be sent, even if a packet has to be created. The ACK will

specify the current left window edge. This assures acknowledgment

of all duplicates.

ADRE is exactly the maximum sequence number ever "advertised"

through the flow control window, plus one. This allows for

controls to be accepted even though permission for them may never

have been eXPlicitly given. Of course, each time a control with a

sequence number equal to the ADRE is sent, the ADRE must be

incremented by one.

AOR is set so that old duplicates (from previous incarnations of

the connection) can be detected and discarded. Thus

AOR = Min(TSE, MPL) * TB

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Out-of-Band Control Signals

Synchronization and Resynchronization Problems

A special problem arises concerning detection of packets (old

duplicates) in the network that have sequence numbers assigned by

old instances of a connection. To handle this reliably, careful

selection of the initial sequence number is required [ref. 2, 3]

as well as periodic checks to determine if resynchronization of

sequence numbers is necessary. The overhead of such elaborate

machinery is expensive and repeating it for each additional

channel is undesirable.

Acceptability on Channel i

We have concluded that the only savings realizable in the muiltple

channel case is to use channel zero's initial sequence number and

resynchronization maintenance mechanism for the additional

channels. This can be accomplished by coupling each additional

channel to channel zero's sequence numbers (CZSN), so that each

item on channel i carries a pair of sequence numbers, the current

CZSN and the current channel i's sequence number (CISN).

The acceptablility test of items on channel i is a composite test

of both sequence numbers. First the CZSN is checked to see if it

would be acknowledged if it were an octet received on channel

zero. Only if it would have been discarded would the item on

channel i be discarded. Having passed the CZSN test, the CISN is

checked to see if the item is deliverable and acknowledgable with

respect to the CISN sequence number space. The CISN test is a

check for everything but the existence of old duplicates from old

instances of the connection and is performed like the check for

channel zero items.

It has been shown that to implement additional channels for a TCP

connection, two alternatives are available-- (1) provide each

channel with its own initial sequence number and resynchronization

maintenance mechanism or (2) provide one initial sequence number

and resynchronization maintenance mechanism for all channels

through channel zero's mechanism. It is hard for us to compare

the two alternatives, since we have no experience implementing any

resynchronization maintenance mechanism.

TCP Case

To implement a completely reliable separate interrupt channel for TCP

requires a channel with a full sequence number space. It is possible

to compromise here and make the interrupt number space smaller than

that required to support consumption of numbers at the TCP's

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Out-of-Band Control Signals

bandwidth. What is lost is the total independence of the flow

control from the data-control channel. Normally, the data-control

sequence numbers will change often enough so that wraparound in the

interrupt number space causes no problems.

Things become slightly messy when many interrupts are generated in

quick succession. Even if the interrupt numbers are acknowledged,

they cannot be reused if they refer to the same data-control sequence

number, until a full packet lifetime has elapsed. This can be

remedied in all but one case by sending a NOP on the data-control

channel so that the next set of interrupts can refer to a new

data-control sequence number. However, if the data-control channel

is blocked due to flow control and a resynchronizing control (DSN in

the TCP case) was just sent, a NOP cannot be created until the

resynchronization is complete and a new starting sequence number is

chosen. Thus to send another interrupt, the TCP must wait for a

packet lifetime or an indication that it can send a NOP on the

data-control channel. (In reality, a connection would probably be

closed long before a packet lifetime elapsed if the sender is not

accepting data from the receiver. [reference 1])

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Out-of-Band Control Signals

REFERENCES

(1) J. Postel, L. Garlick, R. Rom, "TCP Specification (AUTODIN II),"

ARC Catalog #35938, July 1976.

(2) R. Tomlinson, "Selecting Sequence Numbers," INWG Protocol Note

#2, September 1974.

(3) Y. Dalal, "More on Selecting Sequence Numbers," INWG Protocol

Note #4, October 1974.

(4) V. Cerf, Y. Dalal, C. Sunshine, "Specification of Internet

Transmission Control Program," INWG General Note #72, December

1974 (Revised). [Also as RFC675, NIC Catalog #31505.]

(5) Cerf, V., "TCP Resynchronization," SU-DSL Technical Note #79,

January 1976.

(6) Cerf, V. and R. Kahn, "A Protocol for Packet Network

Intercommunication," IEEE Transactions on Communication, Vol

COM-20, No. 5, May 1974.

(7) C. Sunshine, "Interprocess Communication Protocols for Computer

Networks," Digital Systems Laboratory Technical Note #105,

December 1975.