One document matched: draft-ietf-sipping-overload-reqs-02.txt
Differences from draft-ietf-sipping-overload-reqs-01.txt
SIPPING J. Rosenberg
Internet-Draft Cisco
Intended status: Informational January 25, 2008
Expires: July 28, 2008
Requirements for Management of Overload in the Session Initiation
Protocol
draft-ietf-sipping-overload-reqs-02
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This Internet-Draft will expire on July 28, 2008.
Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
Overload occurs in Session Initiation Protocol (SIP) networks when
proxies and user agents have insuffient resources to complete the
processing of a request. SIP provides limited support for overload
handling through its 503 response code, which tells an upstream
element that it is overloaded. However, numerous problems have been
identified with this mechanism. This draft summarizes the problems
with the existing 503 mechanism, and provides some requirements for a
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solution.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Causes of Overload . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Current SIP Mechanisms . . . . . . . . . . . . . . . . . . . . 5
5. Problems with the Mechanism . . . . . . . . . . . . . . . . . 6
5.1. Load Amplification . . . . . . . . . . . . . . . . . . . . 6
5.2. Underutilization . . . . . . . . . . . . . . . . . . . . . 9
5.3. The Off/On Retry-After Problem . . . . . . . . . . . . . . 9
5.4. Ambiguous Usages . . . . . . . . . . . . . . . . . . . . . 10
6. Solution Requirements . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
10. Informative References . . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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1. Introduction
Overload occurs in Session Initiation Protocol (SIP) [RFC3261]
networks when proxies and user agents have insuffient resources to
complete the processing of a request or a response. SIP provides
limited support for overload handling through its 503 response code.
This code allows a server to tell an upstream element that it is
overloaded. However, numerous problems have been identified with
this mechanism.
This draft describes the general problem of SIP overload, and then
reviews the current SIP mechanisms for dealing with overload. It
then explains some of the problems with these mechanisms. Finally,
the document provides a set of requirements for fixing these
problems.
2. Causes of Overload
Overload occurs when an element, such as a SIP user agent or proxy,
has insufficient resources to successfully process all of the traffic
it is receiving. Resources include all of the capabilities of the
element used to process a request, including CPU processing, memory,
I/O, or disk resources. It can also include external resources, such
as a database or DNS server, in which case the CPU, processing,
memory, I/O and disk resources of those servers are effectively part
of the logical element processing the request. Overload can occur
for many reasons, including:
Poor Capacity Planning: SIP networks need to be designed with
sufficient numbers of servers, hardware, disks, and so on, in
order to meet the needs of the subscribers they are expected to
serve. Capacity planning is the process of determining these
needs. It is based on the number of expected subscribers and the
types of flows they are expected to use. If this work is not done
properly, the network may have insufficient capacity to handle
predictable usages, including regular usages and predictably high
ones (such as high voice calling volumes on Mothers Day).
Dependency Failures: A SIP element can become overloaded because a
resource on which it is dependent has failed or become overloaded,
greatly reducing the logical capacity of the element. In these
cases, even minimal traffic might cause the server to go into
overload. Examples of such dependency overloads include DNS
servers, databases, disks and network interfaces.
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Component Failures: A SIP element can become overloaded when it is a
member of a cluster of servers which each share the load of
traffic, and one or more of the other members in the cluster fail.
In this case, the remaining elements take over the work of the
failed elements. Normally, capacity planning takes such failures
into account, and servers are typically run with enough spare
capacity to handle failure of another element. However, unusual
failure conditions can cause many elements to fail at once. This
is often the case with software failures, where a bad packet or
bad database entry hits the same bug in a set of elements in a
cluster.
Avalanche Restart: One of the most troubling sources of overload is
avalanche restart. This happens when a large number of clients
all simultaneously attempt to connect to the network with a SIP
registration. Avalanche restart can be caused by several events.
One is the "Manhattan Reboots" scenario, where there is a power
failure in a large metropolitan area, such as Manhattan. When
power is restored, all of the SIP phones, whether in PCs or
standalone devices, simultaneously power on and begin booting.
They will all then connect to the network and register, causing a
flood of SIP REGISTER messages. Another cause of avalanche
restart is failure of a large network connection, for example, the
access router for an enterprise. When it fails, SIP clients will
detect the failure rapidly using the mechanisms in
[I-D.ietf-sip-outbound]. When connectivity is restored, this is
detected, and clients re-REGISTER, all within a short time period.
Another source of avalanche restart is failure of a proxy server.
If clients had all connected to the server with TCP, its failure
will be detected, followed by re-connection and re-registration to
another server. Note that [I-D.ietf-sip-outbound] does provide
some remedies to this case.
Flash Crowds: A flash crowd occurs when an extremely large number of
users all attempt to simultaneously make a call. One example of
how this can happen is a television commercial that advertises a
number to call to receive a free gift. If the gift is compelling
and many people see the ad, many calls can be simultaneously made
to the same number. This can send the system into overload.
Unfortunately, the overload problem tends to compound itself. When a
network goes into overload, this can frequently cause failures of the
elements that are trying to process the traffic. This causes even
more load on the remaining elements. Furthermore, during overload,
the overall capacity of functional elements goes down, since much of
their resources are spent just rejecting or treating load that they
cannot actually process. In addition, overload tends to cause SIP
messages to be delayed or lost, which causes retransmissions to be
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sent, further increasing the amount of work in the network. This
compounding factor can produce substantial multipliers on the load in
the system. Indeed, in the case of UDP, with as many as 7
retransmits of an INVITE request prior to timeout, overload can
multiply the already-heavy message volume by as much as seven!
3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
4. Current SIP Mechanisms
SIP provides very basic support for overload. It defines the 503
response code, which is sent by an element that is overloaded. RFC
3261 defines it thusly:
The server is temporarily unable to process the request due to
a temporary overloading or maintenance of the server. The
server MAY indicate when the client should retry the request in
a Retry-After header field. If no Retry-After is given, the
client MUST act as if it had received a 500 (Server Internal
Error) response.
A client (proxy or UAC) receiving a 503 (Service Unavailable)
SHOULD attempt to forward the request to an alternate server.
It SHOULD NOT forward any other requests to that server for the
duration specified in the Retry-After header field, if present.
Servers MAY refuse the connection or drop the request instead of
responding with 503 (Service Unavailable).
The objective is to provide a mechanism to move the work of the
overloaded server to another server, so that the request can be
processed. The Retry-After header field, when present, is meant to
allow a server to tell an upstream element to back off for a period
of time, so that the overloaded server can work through its backlog
of work.
RFC3261 also instructs proxies to not forward 503 responses upstream,
at SHOULD NOT strength. This is to avoid the upstream server of
mistakingly concluding that the proxy is overloaded, when in fact the
problem was an element further downstream.
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5. Problems with the Mechanism
At the surface, the 503 mechanism seems workable. Unfortunately,
this mechanism has had numerous problems in actual deployment. These
problems are described here.
5.1. Load Amplification
The principal problem with the 503 mechanism is that it tends to
substantially amplify the load in the network when the network is
overloaded, causing further escalation of the problem and introducing
the very real possibility of congestive collapse. Consider the
topology in Figure 2.
+------+
> | |
/ | S1 |
/ | |
/ +------+
/
/
/
/
+------+ / +------+
--------> | |/ | |
| P1 |---------> | S2 |
--------> | |\ | |
+------+ \ +------+
\
\
\
\
\
\ +------+
\ | |
> | S3 |
| |
+------+
Figure 2
Proxy P1 receives SIP requests from many sources, and acts solely as
a load balancer, proxying the requests to servers S1, S2 and S3 for
processing. The input load increases to the point where all three
servers become overloaded. Server S1, when it receives its next
request, generates a 503. However, because the server is loaded, it
might take some time to generate the 503. If SIP is being run over
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UDP, this may result in request retransmissions which further
increase the work on S1. Even in the case of TCP, if the server is
loaded and the kernel cannot send TCP acknowledgements fast enough,
TCP retransmits may occur. When the 503 is received by P1, it
retries the request on S2. S2 is also overloaded, and eventually
generates a 503, but in the interim may also be hit with retransmits.
P1 once again tries another server, this time S3, which also
eventually rejects it with a 503.
Thus, the processing of this request, which ultimately failed,
involved four SIP transactions (client to P1, P1 to S1, P1 to S2, P1
to S3), each of which may have involved many retransmissions - up to
7 in the case of UDP. Thus, under unloaded conditions, a single
request from a client would generate one request (to S1, S2 or S3)
and two responses (from S1 to P1, then P1 to the client). When the
network is overloaded, a single request from the client, before
timing out, could generate as many as 18 requests and as many
responses when UDP is used! The situation is better with TCP (or any
reliable transport in general), but even if there was never a TCP
segment retransmitted, a single request from the client can generate
3 requests and four responses. Each server had to expend resources
to process these messages. Thus, more messages and more work were
sent into the network at the point at which the elements became
overloaded. The 503 mechanism works well when a single element is
overloaded. But, when the problem is overall network load, the 503
mechanism actually generates more messages and more work for all
servers, ultimately resulting in the rejection of the request anyway.
The problem becomes amplified further if one considers proxies
upstream from P1, as shown in Figure 3.
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+------+
> | | <
/ | S1 | \\
/ | | \\
/ +------+ \\
/ \
/ \\
/ \\
/ \
+------+ / +------+ +------+
| | / | | | |
| P1 | ---------> | S2 |<----------| P2 |
| | \ | | | |
+------+ \ +------+ +------+
^ \ / ^
\ \ // /
\ \ // /
\ \ // /
\ \ / /
\ \ +------+ // /
\ \ | | // /
\ > | S3 | < /
\ | | /
\ +------+ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
+------+
| |
| PA |
| |
+------+
^ ^
| |
| |
Figure 3
Here, proxy PA receives requests, and sends these to proxies P1 or
P2. P1 and P2 both load balance across S1 through S3. Assuming
again S1 through S3 are all overloaded, a request arrives at PA,
which tries P1 first. P1 tries S1, S2 and then S3, and each
transaction resulting in many request retransmits if UDP is used.
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Since P1 is unable to eventually process the request, it rejects it.
However, since all of its downstream dependencies are busy, it
decides to send a 503. This propagates to PA, which tries P2, which
tries S1 through S3 again, resulting in a 503 once more. Thus, in
this case, we have doubled the number of SIP transactions and overall
work in the network compared to the previous case. The problem here
is that the fact that S1 through S3 were overloaded was known to P1,
but this information was not passed back to PA and through to P2, so
that P2 will retry S1 through S3 again.
5.2. Underutilization
Interestingly, there are also examples of deployments where the
network capacity was greatly reduced as a consequence of the overload
mechanism. Consider again Figure 2. Unfortunately, RFC 3261 is
unclear on the scope of a 503. When it is received by P1, does the
proxy cease sending requests to that IP address? To the hostname?
To the URI? Some implementations have chosen the hostname as the
scope. When the hostname for a URI points to an SRV record in the
DNS, which, in turn, maps to a cluster of downstream servers (S1, S2
and S3 in the example), a 503 response from a single one of them will
make the proxy believe that the entire cluster is overloaded.
Consequently, proxy P1 will cease sending any traffic to any element
in the cluster, even though there are elements in the cluster that
are underutilized.
5.3. The Off/On Retry-After Problem
The Retry-After mechanism allows a server to tell an upstream element
to stop sending traffic for a period of time. The work that would
have otherwise been sent to that server is instead sent to another
server. The mechanism is an all-or-nothing technique. A server can
turn off all traffic towards it, or none of it. There is nothing in
between. This tends to cause highly oscillatory behavior under even
mild overload. Consider a proxy P1 which is balancing requests
between two servers S1 and S2. The input load just reaches the point
where both S1 and S2 are at 100% capacity. A request arrives at P1,
and is sent to S1. S1 rejects this request with a 503 , and decides
to use Retry-After to clear its backlog. P1 stops sending all
traffic to S1. Now, S2 gets traffic, but it is seriously overloaded
- at 200% capacity! It decides to reject a request with a 503 and a
Retry-After, which now forces P1 to reject all traffic until S1's
Retry-After timer expires. At that point, all load is shunted back
to S1, which reaches overload, and the cycle repeats.
Its important to observe that this problem is only observed for
servers where there are a small number of upstream elements sending
it traffic, as is the case in these examples. If a proxy was
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accessed by a large number of clients, each of which sends a small
amount of traffic, the 503 mechanism with Retry-After is quite
effective when utilized with a subset of the clients. This is
because spreading the 503 out amongst the clients has the effect of
providing the proxy more fine-grained controls on the amount of work
it receives.
5.4. Ambiguous Usages
Unfortunately, the specific instances under which a server is to send
a 503 are ambiguous. The result is that implementations generate 503
for many reasons, only some of which are related to actual overload.
For example, RFC 3398 [RFC3398], which specifies interworking from
SIP to ISUP, defines the usage of 503 when the gateway receives
certain ISUP cause codes from downstream switches. In these cases,
the gateway has ample capacity; its just that this specific request
could not be processed because of a downstream problem. All
subsequent requests might succeed if they take a different route in
the PSTN.
This causes two problems. Firstly, during periods of overload, it
exacerbates the problems above because it causes additional 503 to be
fed into the system, causing further work to be generated in
conditions of overload. The other problem is that it becomes hard
for an upstream element to know whether to retry when a 503 is
received. There are classes of failures where trying on another
server won't help, since the reason for the failure was that a common
downstream resource is unavailable. For example, if servers S1 and
S2 share a database, and the database fails. A request sent to S1
will result in a 503, but retrying on S2 won't help since the same
database is unavailable.
6. Solution Requirements
In this section, we propose requirements for an overload control
mechanism for SIP which addresses these problems.
REQ 1: The overload mechanism shall strive to maintain the overall
useful throughput (taking into consideration the quality-of-
service needs of the using applications) of a SIP server at
reasonable levels even when the incoming load on the network is
far in excess of its capacity. The overall throughput under load
is the ultimate measure of the value of an overload control
mechanism.
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REQ 2: When a single network element fails, goes into overload, or
suffers from reduced processing capacity, the mechanism should
strive to limit the impact of this on other elements in the
network. This helps to prevent a small-scale failure from
becoming a widespread outage.
REQ 3: The mechanism should seek to minimize the amount of
configuration required in order to work. For example, it is
better to avoid needing to configure a server with its SIP message
throughput, as these kinds of quantities are hard to determine.
REQ 4: The mechanism must be capable of dealing with elements which
do not support it, so that a network can consist of a mix of ones
which do and don't support it. In other words, the mechanism
should not work only in environments where all elements support
it. It is reasonable to assume that it works better in such
environments, of course. Ideally, there should be incremental
improvements in overall network throughput as increasing numbers
of elements in the network support the mechanism.
REQ 5: The mechanism should not assume that it will only be deployed
in environments with completely trusted elements. It should seek
to operate as effectively as possible in environments where other
elements are malicious, including preventing malicious elements
from obtaining more than a fair share of service.
REQ 6: The mechanism shall provide a way to unambiguously inform an
upstream element that it is overloaded. Any response codes,
header fields, or other protocol machinery utilized for this
purpose shall be used exclusively for overload handling, and not
be used to indicate other failure conditions. This is meant to
avoid some of the problems that have arisen from the reuse of the
503 response code for multiple purposes.
REQ 7: The mechanism shall provide a way for an element to throttle
the amount of traffic it receives from an upstream element. This
throttling shall be graded, so that it is not all or nothing as
with the current 503 mechanism. This recognizes the fact that
"overload" is not a binary state, and there are degrees of
overload.
REQ 8: The mechanism shall ensure that, when a request has been
rejected from an overloaded element, it is not sent to another
element that is also overloaded. This requirement derives from
REQ 1.
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REQ 9: That a request has been rejected from an overloaded element
shall not unduly restrict the ability of that request to be
submitted to and processed by an element that is not overloaded.
This requirement derives from REQ 1.
REQ 10: The mechanism should support servers that receive requests
from a large number of different upstream elements, where the set
of upstream elements is not enumerable.
REQ 11: The mechanism should support servers that receive requests
from a finite set of upstream elements, where the set of upstream
elements is enumerable.
REQ 12: The mechanism should work between servers in different
domains.
REQ 13: The mechanism must not dictate a specific algorithm for
prioritizing the processing of work within a proxy during times of
overload. It must permit a proxy to prioritize requests based on
any local policy, so that certain ones (such as a call for
emergency services or a call with a specific value of the
Resource-Priority header field [RFC4412]) are given preferential
treatment,such as not being dropped, being given additional
retransmission, or being processed ahead of others.
REQ 14: The mechanism should provide unambigous directions to
clients on when they should retry a request, and when they should
not. This especially applies to TCP connection establishment and
SIP registrations, in order to mitigate against avalanche restart.
REQ 15: In cases where a network element fails, is so overloaded
that it cannot process messages, or cannot communicate due to a
network failure or network partition, it will not be able to
provide explicit indications of its levels of congestion. The
mechanism should properly function in these cases.
REQ 16: The mechanism should attempt to minimize the overhead of the
overload control messaging.
REQ 17: The overload mechanism must not provide an avenue for
malicious attack.
REQ 18: The overload mechanism should be unambiguous about whether a
load indication applies to a specific IP address, host, or URI, so
that an upstream element can determine the load of the entity to
which a request is to be sent.
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REQ 19: The specification for the overload mechanism should give
guidance on which message types might be desirable to process over
others during times of overload, based on SIP-specific
considerations. For example, it may be more beneficial to process
a SUBSCRIBE refresh with Expires of zero than a SUBSCRIBE refresh
with a non-zero expiration, since the former reduces the overall
amount of load on the element, or to process re-INVITEs over new
INVITEs.
REQ 20: In a mixed environment of elements that do and do not
implement the overload mechanism, no disproportionate benefit
shall accrue to the users or operators of the elements that do not
implement the mechanism.
REQ 21: The overload mechanism should ensure that the system remains
stable. When the offered load drops from above the overall
capacity of the network to below the overall capacity, the
throughput should stabilize and become equal to the offered load.
REQ 22: It must be possible to disable the reporting of load
information towards upstream targets based on the identity of
those targets. This allows a domain administrator who considers
the load of their elements to be sensitive information, to
restrict access to that information. Of course, in such cases,
there is no expectation that the overload mechanism itself will
help prevent overload from that upstream target.
REQ 23: It must be possible for the overload mechanism to work in
cases where there is a load balancer in front of a farm of
proxies.
7. Security Considerations
Like all protocol mechanisms, a solution for overload handling must
prevent against malicious inside and outside attacks. This document
includes requirements for such security functions.
8. IANA Considerations
None.
9. Acknowledgements
The author would like to thank Steve Mayer, Mouli Chandramouli,
Robert Whent, Mark Perkins, Joe Stone, Vijay Gurbani, Steve Norreys,
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Volker Hilt, Spencer Dawkins, and Dale Worley for their contributions
to this document.
10. Informative References
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3398] Camarillo, G., Roach, A., Peterson, J., and L. Ong,
"Integrated Services Digital Network (ISDN) User Part
(ISUP) to Session Initiation Protocol (SIP) Mapping",
RFC 3398, December 2002.
[RFC4412] Schulzrinne, H. and J. Polk, "Communications Resource
Priority for the Session Initiation Protocol (SIP)",
RFC 4412, February 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[I-D.ietf-sip-outbound]
Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-10 (work in progress), July 2007.
Author's Address
Jonathan Rosenberg
Cisco
Edison, NJ
US
Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
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