[Text version]

RFC: 816

                             David D. Clark
                  MIT Laboratory for Computer Science
               Computer Systems and Communications Group
                               July, 1982

     1.  Introduction

     Occasionally, a network or a gateway will go down, and the sequence

of hops which the packet takes from source to destination must change.

Fault isolation is that action which hosts and gateways collectively

take to determine that something is wrong; fault recovery is the

identification and selection of an alternative route which will serve to

reconnect the source to the destination. In fact, the gateways perform

most of the functions of fault isolation and recovery. There are,

however, a few actions which hosts must take if they wish to provide a

reasonable level of service. This document describes the portion of

fault isolation and recovery which is the responsibility of the host.

     2.  What Gateways Do

     Gateways collectively implement an algorithm which  identifies  the

best route between all pairs of networks. They do this by exchanging

packets which contain each gateway's latest opinion about the

operational status of its neighbor networks and gateways. Assuming that

this algorithm is operating properly, one can expect the gateways to go

through a period of confusion immediately after some network or  gateway

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has failed, but one can assume that once a period of negotiation has

passed, the gateways are equipped with a consistent and correct model of

the connectivity of the internet. At present this period of negotiation

may actually take several minutes, and many TCP implementations time out

within that period, but it is a design goal of the eventual algorithm

that the gateway should be able to reconstruct the topology quickly

enough that a TCP connection should be able to survive a failure of the

route.

     3.  Host Algorithm for Fault Recovery

     Since  the gateways always attempt to have a consistent and correct

model of the internetwork topology, the host strategy for fault recovery

is very simple. Whenever the host feels that something is wrong, it

asks the gateway for advice, and, assuming the advice is forthcoming, it

believes the advice completely. The advice will be wrong only during

the transient period of negotiation, which immediately follows an

outage, but will otherwise be reliably correct.

     In  fact,  it  is  never  necessary  for a host to explicitly ask a

gateway for advice, because the gateway will provide it as appropriate.

When a host sends a datagram to some distant net, the host should be

prepared to receive back either of two advisory messages which the

gateway  may  send.    The  ICMP  "redirect"  message indicates that the

gateway to which the host sent the datagram is not longer the best

gateway to reach the net in question. The gateway will have forwarded

the datagram, but the host should revise its routing table to have a

different  immediate  address  for  this  net.    The  ICMP "destination

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unreachable" message indicates that as a result of an outage, it is

currently impossible to reach the addressed net or host in any manner.

On receipt of this message, a host can either abandon the connection

immediately without any further retransmission, or resend slowly to see

if the fault is corrected in reasonable time.

     If  a  host  could assume that these two ICMP messages would always

arrive when something was amiss in the network, then no other action on

the part of the host would be required in order maintain its tables in

an optimal condition. Unfortunately, there are two circumstances under

which  the  messages  will  not  arrive  properly.    First,  during the

transient following a failure, error messages may arrive that do not

correctly represent the state of the world. Thus, hosts must take an

isolated error message with some scepticism. (This transient period is

discussed  more  fully  below.)    Second,  if the host has been sending

datagrams to a particular gateway, and that gateway itself crashes, then

all the other gateways in the internet will reconstruct the topology,

but the gateway in question will still be down, and therefore cannot

provide any advice back to the host. As long as the host continues to

direct datagrams at this dead gateway, the datagrams will simply vanish

off the face of the earth, and nothing will come back in return. Hosts

must detect this failure.

     If some gateway many hops away fails, this is not of concern to the

host, for then the discovery of the failure is the responsibility of the

immediate neighbor gateways, which will perform this action in a manner

invisible to the host.  The  problem  only  arises  if  the  very  first

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gateway, the one to which the host is immediately sending the datagrams,

fails. We thus identify one single task which the host must perform as

its part of fault isolation in the internet: the host must use some

strategy to detect that a gateway to which it is sending datagrams is

dead.

     Let us  assume  for  the  moment  that  the  host  implements  some

algorithm to detect failed gateways; we will return later to discuss

what this algorithm might be. First, let us consider what the host

should do when it has determined that a gateway is down. In fact, with

the exception of one small problem, the action the host should take is

extremely  simple.    The host should select some other gateway, and try

sending the datagram to it. Assuming that gateway is up, this will

either produce correct results, or some ICMP advice. Since we assume

that, ignoring temporary periods immediately following an outage, any

gateway is capable of giving correct advice, once the host has received

advice from any gateway, that host is in as good a condition as it can

hope to be.

     There is always the unpleasant possibility that when the host tries

a different gateway, that gateway too will be down. Therefore, whatever

algorithm the host uses to detect a dead gateway must continuously be

applied, as the host tries every gateway in turn that it knows about.

     The only difficult part of this algorithm is to specify  the  means

by which the host maintains the table of all of the gateways to which it

has  immediate  access.    Currently,  the specification of the internet

protocol does not architect any message by which a host can  ask  to  be

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supplied with such a table. The reason is that different networks may

provide very different mechanisms by which this table can be filled in.

For example, if the net is a broadcast net, such as an ethernet or a

ringnet, every gateway may simply broadcast such a table from time to

time, and the host need do nothing but listen to obtain the required

information. Alternatively, the network may provide the mechanism of

logical addressing, by which a whole set of machines can be provided

with a single group address, to which a request can be sent for

assistance. Failing those two schemes, the host can build up its table

of neighbor gateways by remembering all the gateways from which it has

ever received a message. Finally, in certain cases, it may be necessary

for this table, or at least the initial entries in the table, to be

constructed manually by a manager or operator at the  site.    In  cases

where the network in question provides absolutely no support for this

kind of host query, at least some manual intervention will be required

to get started, so that the host can find out about at least one

gateway.

     4.  Host Algorithms for Fault Isolation

     We now return to the question raised above.  What  strategy  should

the host use to detect that it is talking to a dead gateway, so that it

can know to switch to some other gateway in the list. In fact, there are

several algorithms which can be used. All are reasonably simple to

implement, but they have very different implications for the overhead on

the host, the gateway, and the network. Thus, to a certain extent, the

algorithm picked must depend on the details of the network and of the

host.

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1. NETWORK LEVEL DETECTION

     Many  networks,  particularly  the  Arpanet,  perform precisely the

required function internal to the network. If a host sends a datagram

to a dead gateway on the Arpanet, the network will return a "host dead"

message, which is precisely the information the host needs to know in

order to switch to another gateway. Some early implementations of

Internet on  the  Arpanet  threw  these  messages  away.    That  is  an

exceedingly poor idea.

2. CONTINUOUS POLLING

     The  ICMP  protocol  provides an echo mechanism by which a host may

solicit a response from a gateway.    A  host  could  simply  send  this

message at a reasonable rate, to assure itself continuously that the

gateway was still up. This works, but, since the message must be sent

fairly often to detect a fault in a reasonable time, it can imply an

unbearable overhead on the host itself, the network, and the gateway.

This strategy is prohibited except where a specific analysis has

indicated that the overhead is tolerable.

3. TRIGGERED POLLING

     If the use of polling could be restricted to only those times  when

something seemed to be wrong, then the overhead would be bearable.

Provided that one can get the proper advice from one's higher level

protocols, it is possible to implement such a strategy. For example,

one could program the TCP level so  that  whenever  it  retransmitted  a

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segment more than once, it sent a hint down to the IP layer which

triggered polling. This strategy does not have excessive overhead, but

does have the problem that the host may be somewhat slow to respond to

an error, since only after polling has started will the host be able to

confirm that something has gone wrong, and by then the TCP above may

have already timed out.

     Both forms of polling suffer from a minor flaw.  Hosts as  well  as

gateways respond to ICMP echo messages. Thus, polling cannot be used to

detect the error that a foreign address thought to be a gateway is

actually a host. Such a confusion can arise if the physical addresses

of machines are rearranged.

4. TRIGGERED RESELECTION

     There  is a strategy which makes use of a hint from a higher level,

as did the previous strategy, but which avoids polling altogether.

Whenever a higher level complains that the service seems to be

defective, the Internet layer can pick the next gateway from the list of

available gateways, and switch to it. Assuming that this gateway is up,

no real harm can come of this decision, even if it was wrong, for the

worst that will happen is a redirect message which instructs the host to

return to the gateway originally being used. If, on the other hand, the

original gateway was indeed down, then this immediately provides a new

route, so the period of time until recovery is  shortened.    This  last

strategy seems particularly clever, and is probably the most generally

suitable for those cases where the network itself does not provide fault

isolation. (Regretably, I have forgotten who suggested this idea to me.

It is not my invention.)

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     5.  Higher Level Fault Detection

     The  previous  discussion  has  concentrated on fault detection and

recovery at the IP layer. This section considers what the higher layers

such as TCP should do.

     TCP has a single fault recovery action; it repeatedly retransmits a

segment until either it gets an acknowledgement or its connection timer

expires.    As discussed above, it may use retransmission as an event to

trigger a request for fault recovery to the IP  layer.    In  the  other

direction, information may flow up from IP, reporting such things as

ICMP Destination Unreachable or error messages from the attached

network.    The  only  subtle  question about TCP and faults is what TCP

should do when such an error message arrives or its connection timer

expires.

     The  TCP  specification discusses the timer.  In the description of

the open call, the timeout is described as an optional value that the

client of TCP may specify; if any segment remains unacknowledged for

this period, TCP should abort the  connection.    The  default  for  the

timeout is 30 seconds. Early TCPs were often implemented with a fixed

timeout interval, but this did not work well in practice, as the

following discussion may suggest.

     Clients  of  TCP can be divided into two classes:  those running on

immediate behalf of a human, such as Telnet, and those supporting a

program, such as a mail sender. Humans require a sophisticated response

to  errors.    Depending  on  exactly  what went wrong, they may want to

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abandon the connection at once, or wait for a long time to see if things

get better. Programs do not have this human impatience, but also lack

the power to make complex decisions based on details of the exact error

condition. For them, a simple timeout is reasonable.

     Based  on these considerations, at least two modes of operation are

needed in TCP. One, for programs, abandons the connection without

exception  if  the  TCP  timer  expires.    The other mode, suitable for

people, never abandons the connection on its own initiative, but reports

to the layer above when the timer expires. Thus, the human user can see

error messages coming from all the relevant layers, TCP and ICMP, and

can request TCP to abort as appropriate. This second mode requires that

TCP be able to send an asynchronous message up to its client to report

the timeout, and it requires that error messages arriving at lower

layers similarly flow up through TCP.

     At  levels  above TCP, fault detection is also required.  Either of

the following can happen. First, the foreign client of TCP can fail,

even though TCP is still running, so data is still acknowledged and the

timer never expires. Alternatively, the communication path can fail,

without the TCP timer going off, because the local client has no data to

send. Both of these have caused trouble.

     Sending  mail  provides an example of the first case.  When sending

mail using SMTP, there is an SMTP level acknowledgement that is returned

when a piece of mail is successfully  delivered.    Several  early  mail

receiving programs would crash just at the point where they had received

all of the mail text (so TCP did not detect a timeout due to outstanding

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unacknowledged data) but before the mail was acknowledged at the SMTP

level. This failure would cause early mail senders to wait forever for

the SMTP level acknowledgement. The obvious cure was to set a timer at

the SMTP level, but the first attempt to do this did not work, for there

was no simple way to  select  the  timer  interval.    If  the  interval

selected was short, it expired in normal operational when sending a

large file to a slow host. An interval of many minutes was needed to

prevent false timeouts, but that meant that failures were detected only

very slowly. The current solution in several mailers is to pick a

timeout interval proportional to the size of the message.

     Server telnet provides an example of the other kind of failure.  It

can easily happen that the communications link can fail while there is

no traffic flowing, perhaps because the user is thinking.    Eventually,

the user will attempt to type something, at which time he will discover

that the connection is dead and abort it. But the host end of the

connection, having nothing to send, will not discover anything wrong,

and will remain waiting forever. In some systems there is no way for a

user in a different process to destroy or take over such a hanging

process, so there is no way to recover.

     One solution to this would be to have the host server telnet  query

the user end now and then, to see if it is still up. (Telnet does not

have an explicit query feature, but the host could negotiate some

unimportant option, which should produce either agreement or

disagreement in  return.)    The  only  problem  with  this  is  that  a

reasonable  sample interval, if applied to every user on a large system,

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can generate an unacceptable amount of traffic and system overhead. A

smart server telnet would use this query only when something seems

wrong, perhaps when there had been no user activity for some time.

     In  both  these  cases, the general conclusion is that client level

error detection is needed, and that the details of the mechanism are

very dependent on the application. Application programmers must be made

aware of the problem of failures, and must understand that error

detection at the TCP or lower level cannot solve the whole problem for

them.

     6.  Knowing When to Give Up

     It  is  not  obvious,  when error messages such as ICMP Destination

Unreachable arrive, whether TCP should  abandon  the  connection.    The

reason that error messages are difficult to interpret is that, as

discussed above, after a failure of a gateway or network, there is a

transient period during which the gateways may have incorrect

information, so that irrelevant or incorrect error messages may

sometimes return. An isolated ICMP Destination Unreachable may arrive

at a host, for example, if a packet is sent during the period when the

gateways are trying to find a new route. To abandon a TCP connection

based on such a message arriving would be to ignore the valuable feature

of the Internet that for many internal failures it reconstructs its

function without any disruption of the end points.

     But  if failure messages do not imply a failure, what are they for?

In fact, error messages serve several important  purposes.    First,  if

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they arrive in response to opening a new connection, they probably are

caused by opening the connection improperly (e.g., to a non-existent

address) rather than by a transient network failure. Second, they

provide valuable information, after the TCP timeout has occurred, as to

the probable cause of the failure. Finally, certain messages, such as

ICMP Parameter Problem, imply a possible  implementation  problem.    In

general, error messages give valuable information about what went wrong,

but are not to be taken as absolutely reliable. A general alerting

mechanism, such as the TCP timeout discussed above, provides a good

indication that whatever is wrong is a serious condition, but without

the advisory messages to augment the timer, there is no way for the

client to know how to respond to the error. The combination of the

timer and the advice from the error messages provide a reasonable set of

facts for the client layer to have. It is important that error messages

from all layers be passed up to the client module in a useful and

consistent way.

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