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latex2wiki:xorp_error_handling

Introduction

A XORP router is made up of a number of processes that communicate via XRLs (a messaging system developed for XORP). In this document we will focus on how to deal with errors that are generated directly or indirectly by XRL calls, and discuss how to handle process failures and the subsequent restart of failed processes. Of course, in an ideal world processes would not fail, but when they do fail, our goals are to keep as much router functionality working as possible, to avoid permanent inconsistencies at all costs, and for the remainder of the functionality to be restored as quickly as possible. Many XORP processes share routing state that must remain synchronised. For example, the BGP process sends the result of its routing decisions to the RIB process, which passes these routes on to the FEA and hence to the forwarding engine’s Forwarding Information Base (FIB). If the RIB process fails, then BGP would lose the ability to manipulate the FIB, and forwarding would not match the BGP routing table. Thus, BGP should withdraw all routes that it told its peers, or alternatively it might drop all peerings until the RIB has successfully restarted. A critical component of the system is the router manager process (rtrmgr) which is responsible for starting and stopping routing processes. When a XORP process starts or terminates, that process’s XRL client library ensures that the Finder is notified. If a process has an interest in the status of another process it can register interest with the Finder. In a XORP router, as with any complex system, errors can occur. These errors can range from a XORP process simply failing, to an attempt to install a route into the forwarding engine that already exists. Errors need to be dealt with in a consistent manner. The four types of error that may occur are categorized below:

Error type Description
Process Failure When a process fails.
Communication Error At the most basic level an attempt to send an XRL has failed. The process that was the recipient of the XRL may have failed or be slow to respond. The message that was being sent may have been lost in transit.
Execution Error It is when an XRL call returns an error due to some underlying interaction failure. A simple example of this type of error is a “route add” failing. The attempt to add a route may fail for many reasons. The identical route may already be present or a different route may be installed. The error may occur due to a bug in the router code, because routing state has been manipulated by non XORP processes, or due to resource starvation in the forwarding engine.
Type Error It is when an XRL call fails because the arguments passed to an XRL are invalid. This error will most likely be due to a version mismatch between XORP processes. If all the processes in a XORP router have been built from the same source tree this error should not occur. As we are building an extensible router it may be the case that a process built from a different source tree may encounter compatibility problems.

Process Failure

A XORP router is made up of a number of distinct processes. There are dependencies between these processes. We define the critical dependencies and what action to take on detecting failure. The most critical component of a XORP router is the rtrmgr/Finder process. One of the functions of this component is to start/re-start processes. If process A is dependent on the status (e.g., alive, dead, restarted) of process B, then process A registers this interest with the Finder. This dependency on the rtrmgr/Finder for managing and monitoring process liveness state means that a XORP router cannot survive the failure of this process. If we attempted to survive a Finder restart, it is conceivable that, in the same time window, another monitored process could restart, in which case the restarting of the monitored process could be missed by the Finder. To guard against this possible race, a XORP process that detects the loss of the Finder must exit. There is one exception to this rule, the Xorpsh process, that will be discussed later in the Xorpsh subsubsection. Each process in a XORP router is described with how it should behave when another process in the system fails. Processes can explicitly register interest in the status of other processes through the Finder. If process A is dependent on the state of process B then process A must register interest in process B.

Implementing process failure detection

The Finder process will send keepalive messages to all processes at thirty second intervals. If a process does not respond to a keepalive it is considered dead. The keepalive messages are sent over a reliable transport such as TCP. A process dying should therefore be easy to detect. The rtrmgr might also be able to detect that a process has died (but not if it is simply not responding), as it will normally receive a SIGCHILD signal. On discovering a process has died, the rtrmgr will send a hint to the Finder, which will immediately try and send a keepalive. Again if the process has died it should be easy to detect. If a process is not responding to keepalives but it is still alive, it will be marked as dead and all interested processes will be notified. Most importantly, the rtrmgr will be notified and it will kill the running process and start a new process.

Actions to take on detecting process failure

The next table indicates what action a process should take on detecting failure in other processes1). The “(G)” denotes that the process should attempt to exit gracefully.

Failing Process
rtrmgr/Finder FEA MFEA RIB IGMP PIM BGP RIP OSPF Xorpsh
Process detecting failure rtrmgr/Finder / Withdraw All Unicast Routes, Exit Exit Exit Exit Exit Exit Exit Exit Report Problem Wait.
FEA2) Restart / Exit Exit Exit Exit Exit Exit Exit -
MFEA3) Restart - / - Exit Exit - - - -
RIB Restart Withdraw All Unicast Routes / - Exit (G) Exit (G) Exit (G) Exit (G) Exit (G) -
IGMP Restart - - - / Delete Local Members After Timeout - - - -
PIM Restart - - - - / - - - -
BGP Restart - - - - - / - - -
RIP Restart - - - - - - / - -
OSPF Restart - - - - - - - / -
Xorpsh Restart - - - - - - - - /

The next figure shows the relationship between the various processes. The thick arrows should be modelled as a signal sent from a process dying to its dependent processes.

Process relationship on failure

rtrmgr/Finder- Router manager

If the rtrmgr/Finder dies then all bets are off and all processes should exit apart from the Xorpsh. If a XORP process exits unexpectedly the rtrmgr/Finder should attempt to restart the process.

FEA - Forwarding Engine Abstraction

The FEA primarily accepts routes from the RIB and places them in the kernel. The FEA should tag all routes that it has installed in the kernel. On restart, the FEA should remove all routes that a previous incarnation of the FEA has placed in the kernel. When an FEA is exiting it should attempt to remove all routes that it has installed in the kernel. The FEA process should register interest in the RIB. If the RIB fails the FEA should withdraw all routes that the RIB has sent to it.

MFEA - Multicast Forwarding Engine Abstraction

The MFEA is multicast analogue to the unicast FEA. If should be noted that typically the MFEA would be part of the FEA process. Similar to the FEA, on restart or exit the MFEA should remove all multicast forwarding entries that were installed in the kernel. Note that the MFEA does not contain a copy of the multicast forwarding entries that were installed in the kernel, so it should utilize a mechanism that removes all multicast forwarding entries at once. In case of UNIX-based systems, closing the multicast routing socket will automatically remove all entries. If the multicast routing process that has installed the multicast forwarding entries exits, then the MFEA should remove all multicast forwarding entries from the kernel. Currently, PIM is the only multicast routing process. In the future, the XORP multicast routing architecture may contain a special coordinator among all multicast routing protocol instances, analogous to the function of the unicast RIB process. If that coordinator exits, the MFEA should remove all multicast forwarding entries from the kernel.

RIB - Routing Information Base

Routes from the routing processes are sent to the RIB; the winners are sent to the FEA. The RIB should register interest in the FEA. If the FEA fails the RIB should exit. All routing processes that interact with the RIB should, on detecting the shutdown of the RIB, also terminate gracefully.

IGMP/MLD

If the FEA/MFEA process exits then this process should exit.

PIM

If the RIB or the FEA/MFEA process exits then this process should exit.

BGP

Currently the only other process in the system that BGP interacts with is the RIB. If the BGP process detects that the RIB has died then it should gracefully terminate its sessions and exit. In the future the TCP connections that BGP makes will be mediated through FEA, at which time the BGP process should also register interest in the state of the FEA. If the BGP process detects the death of the FEA it should exit immediately.

RIP

The RIP process should register interest in the FEA and the RIB. If the RIB dies then the RIP process should attempt to exit gracefully. If the FEA dies the RIP process should exit immediately.

IS-IS

The IS-IS process should register interest in the FEA and the RIB. If the RIB dies then the IS-IS process should attempt to exit gracefully. If the FEA dies the IS-IS process should exit immediately.

OSPF

The OSPF process should register interest in the FEA and the RIB. If the RIB dies then the OSPF process should attempt to exit gracefully. If the FEA dies the OSPF process should exit immediately.

Xorpsh

The Xorpsh provides a command line interface to the XORP router. Other processes in the system exiting should never cause it to exit. The rtrmgr/Finder process exiting should generate warning output to the user and then the Xorpsh should wait for the router to restart.

XRL Communication Errors

Interprocess communication in XORP is achieved using XRLs. In this section we will consider what should be done when an XRL call fails due to a communication error. XRLs can be sent over unreliable transports such as UDP or reliable transports such as TCP. The type of transport used is decided by the XRL library based on the specification of each interface. For the purposes of error handling, the reliable and unreliable transports are the same in all regards, except that reliable transports in XORP never explicitly report a timeout error. XRL communication is asynchronous: applications request the dispatch of an XRL and expect to have a callback invoked when the dispatch result is available. This presents opportunities for immediate and deferred error indications. Immediate error indications occur when the request for XRL dispatch is made: the canonical example occurring when no more buffer space is available within the XRL library is available. An application is able to detect these errors synchronously: the dispatch request indicates an error in its return value. Deferred error indications happen through the dispatch callbacks. These callbacks are required to take an XrlError object as an argument. An XrlError object is comprised of an enumerated error code and an optional string containing specific information relating to the error. The set of enumerated error codes is presented below. Immediate and deferred errors are exclusive. If the XRL Target dispatching an XRL got an immediate error, it will not receive a callback indicating a deferred error.

XRL Error Value Description
Standard Dispatch.
The standard XRL return values are returned to the requesting XRL Target by the dispatching XRL Target. When any of these values are returned, the XRL communication has been successful.
OKAY XRL dispatch successful. Additional parameters in XRL callback contain return values.
COMMAND_FAILED XRL reached dispatcher, but could not be dispatched. The reason for failure may be specified in the note associated with the XrlError object.
BAD_ARGS XRL reached dispatcher, but argument types did not match those expected by the dispatcher.
Finder NO_FINDER This error occurs when an XRL Target cannot communicate with the Finder. This always indicates a serious problem with the router, as the Finder should always be present. The application SHOULD treat this error as fatal.
RESOLVE_FAILED This error occurs when a XRL Target process tries to resolve an XRL the Finder has no result for. This may be because the target specified in the XRL does not exist or exists, but is still in the process of registering the XRL it exports. RESOLVE FAILED errors may happen because of a benign cause, namely that processes started up in a less than perfect order, so a target’s user has initialized before the target itself. Applications SHOULD handle this type of transient RESOLVE FAILED error with a retransmission strategy. Applications may avoid this error by using the Finder event observer interface to detect when the particular target becomes ready.
NO_SUCH_METHOD This error occurs when the named XRL Target is running and has registered it’s XRLs, but it does not support the method named in the XRL. NO SUCH METHOD generally indicates a version mismatch between two processes. This error may be considered fatal, or (for example) the application might react by trying to access an older version of the interface. The application can expect, however, that NO SUCH METHOD errors are not transient: If an XRL access gets a NO SUCH METHOD error, then that XRL will always result in a NO SUCH METHOD error, at least until the target process restarts.
Transport & Internal SEND_FAILED The underlying XRL transport mechanism has failed. For example, the TCP connection has been reset, or a UDP connection gets a port-unreachable message. The expectation is that no further communication with the specific endpoint will succeed.
SEND_FAILED_TRANSIENT This error occurs when the XRL library temporarily cannot send a particular XRL. Usually, this will be because of congestion or a slow receiver: the kernel has run out of buffer space. Note that the XRL library performs some buffering itself, to ensure that XRL requests are either completely transmitted or not transmitted at all. Note: The XRL library does not yet implement this error.
REPLY_TIMED_OUT The target did not reply within a transport-protocol-specific period of time. Possible reasons include network congestion, peer failure, network interface failure, and so on. As in all network communications, when a timeout occurs we don’t know if the last unacknowledged XRL request was received and processed by the peer. This error occurs in unreliable transmit only.

Handling XRL Errors

XRLs may be directed to a class of target or a particular instance of a target. The first instance of a target that registers with the Finder is considered to be the primary instance of its class and XRLs addressed to that are directed to that instance. The XRL library MAY hide certain REPLY TIMED OUT and SEND FAILED errors for XRLs directed towards classes, i.e., should the instance which is acting as the primary instance fail or exit, then another instance in that class, will receive the class directed XRL requests. The XRL errors of NO FINDER, RESOLVE FAILED, and to some extent NO SUCH METHOD generally represent serious problems with the router. SEND FAILED represents a serious problem with the target, such as that an instance of the target has died; this problem may or may not be transient. The SEND FAILED TRANSIENT and REPLY TIMED OUT errors are potentially common errors, and should be handled by the application. However, the likelihood of SEND FAILED TRANSIENT can often be reduced, making it a “fatal” error from the application’s point of view, by limiting the rate at which requests are sent. NO FINDER, RESOLVE FAILED, NO SUCH METHOD, and SEND FAILED TRANSIENT, are all indications that the XRL was not communicated to its target. They are therefore called send failures. The other two errors, REPLY TIMED OUT and SEND FAILED, may be generated even if the target received the request. They are therefore called receive failures. If a peer dies, we will receive notification of this explicitly and will deal with it as specified in the Process Failure section. Thus most XRL transport errors SHOULD NOT be taken as an indication that the peer is definitely dead. If an application cares that the peer has died or restarted, it SHOULD register with the finder to receive notifications of process restarts. Thus, a process SHOULD assume that an XRL transport problem will be transient until it receives an explicit confirmation that the destination has failed, particularly when the XRL interface is unreliable. In addition to an XRL interface being reliable or unreliable, the way the application uses an XRL interface can by pipelined or non-pipelined. In the pipelined case, multiple requests can be outstanding simultaneously; in the non-pipelined case at most one request can be outstanding at a time. It is useful for us to categorize XRL interfaces along these two axes: reliable/unreliable and pipelined/non-pipelined.

Unreliable Reliable
Non-pipelined If an XRL send failure occurs, the sending application MAY choose to retransmit the XRL, or ignore the failure as it sees fit. In an XRL receive failure occurs, the sending application MAY also choose to retransmit the XRL, or ignore the failure as it sees fit. However, if the application chooses to re-send the XRL, the interface MUST be written in such a way that the receipt of a duplicate request will not damage the system. (XXX Isn’t this true anyway? Network duplicates?) If a SEND FAILED TRANSIENT error occurs, the sending application MAY retransmit the XRL. SEND FAILED, NO FINDER, and most RESOLVE FAILED and NO SUCH METHOD errors are unrecoverable. The application should cause this XRL interface to go dormant, in the expectation that it will authoritatively discover from the finder that the target has died. REPLY TIMED OUT cannot happen on reliable interfaces.
Pipelined The same issues apply as with unreliable, non-pipelined, but the situation is more complicated. An interface that uses unreliable transport and pipelining is one that explicitly permits loss and re-ordering of requests. It is up to the application to choose whether to retransmit XRLs that return SEND FAILED TRANSIENT or REPLY TIMED OUT, but the application must only do so if it is certain that the re-ordering caused by retransmission will not be a problem. The XRL library ensures that pipelined messages sent to a reliable target are delivered in order. In particular, if a request R to a given target gets an error, then no outstanding requests to that target registered later than R will successfully complete – they will all get the same error, and none of them will be delivered to the receiving application. Once the error is delivered, this error state is wiped out, and later requests to the target may succeed – perhaps because the target was restarted. Again, SEND FAILED, NO FINDER, and most RESOLVE FAILED and NO SUCH METHOD errors are unrecoverable. The application SHOULD cause this XRL interface to go dormant, in the expectation that it will authoritatively discover from the finder that the target has died.

Execution Error

A XORP router is partitioned into many processes; most of the operating system specific interactions are performed by the FEA. In a router the most frequent operation will be the adding and deleting of routes. Consider BGP adding a route. First the BGP process will send the route to the RIB, then the route may be sent to the FEA. If the addition of the route from the RIB to the FEA fails, then there is no way of propagating this failure back to the BGP process due to the asynchronous nature of XRLs. If adding/deleting a route fails a very drastic way of propagating this failure back to the BGP process would be for either or both the FEA and RIB processes to exit, in which case the process failure responses already described would be used and BGP would exit. Process exit is an extreme response to failing to add a route, but at least the error handling code for process exit exists already. It is important though not to mask over implementation problems by ignoring errors. In the rest of this section we will outline how to deal with a number of common errors.

Adding/Deleting route failures

As stated above, a highly likely error is failures when adding or deleting routes. Typically the interaction will occur between the RIB and FEA. When an error occurs it should be logged by the FEA and the cause returned to the RIB. The RIB can be configured with policy on how to react to different errors. Adding a route will typically fail because a route already exists. Firstly, if a route already exists it is either the same or different to the one that we attempted to add. Secondly, either the FEA installed the route or a third party installed it. Therefore when adding a route fails the FEA should return if the current route is the same or different to the one we attempted to add, as well as who installed the route originally. The RIB on receiving the error state from the FEA can decide as a matter of policy how to proceed. If an attempt to add a route fails because a different route exists the RIB could choose to delete the old route and add the new route. The most common reason for a route deletion to fail would be that the route is no longer present. The FEA should log that it has been asked to delete a route that doesn’t exist. The RIB should decide if this problem should be considered fatal.

Route Add Failure due to Resource Starvation

When a routing process sends a route to the RIB, the asynchronous nature of XRL handling means that the RIB will typically accept the route before it has finished processing the addition, and certainly before it attempts to pass the route to the FEA, and hence on into the forwarding engine. It is possible for the route addition to fail due to memory exhaustion in either the RIB or in the forwarding engine itself. Should this occur, it is important for the routing protocol to be made aware of the event, because the routing information will now be out of synchronization with the forwarding information. If the forwarding engine refuses the route due to resource starvation, the FEA will receive the failure. The FEA will then indicate asynchronously to the RIB that the failure occurred. The RIB will in turn delete all state from all routing protocols that contributed versions of this route, and asynchronously pass the failure up to those routing protocols. Each of those routing protocols will then handle the failure in a protocol specific manner. If the failure occurs due to resource starvation in the RIB, a similar process will be initiated. It is not currently clear how to reliably notify a routing protocol in the case when the router is running out of memory for user-space processes. In the case of BGP, if a route fails to be added due to resource starvation, the simplest mechanism is to take down the peering that originated the route. The normal peer reinitialization mechanism (after some time delay) will ensure that all the routes are re-instantiated after the resource starvation problem goes away. In the case of RIP, if a route fails to be added due to resource starvation, the simplest mechanism is to send our peers an infinite metric route for this particular prefix and to delete the state for this prefix. The normal RIP periodic update will ensure that the route is re-instantiated after the resource starvation problem goes away. In the case of link-state protocols such as OSPF and IS-IS, there is no good way to deal with this situation. A reasonable solution might be to take down all adjacencies to avoid causing a blackhole, then to bring up the adjacencies again but not propagate any link-state advertisements to our neighbors (so they won’t route via us) until all the link-state advertisements have been received and we’ve successfully installed all the routes in the kernel.

1) Note that currently this document does not describe the policy manager. Such description will be included in the future. For all practical reasons, the policy manager is as important as the rtrmgr/finder, even though it is running as a separate process.
2) , 3) Typically, the MFEA would be part of the FEA process.
latex2wiki/xorp_error_handling.txt · Last modified: 2011/03/09 15:04 (external edit)