SIP Working Group V. Hilt
Internet-Draft I. Widjaja
Expires: January 7, 2008 Bell Labs/Alcatel-Lucent
July 6, 2007
Essential Correction to the Session Initiation Protocol (SIP) 503
(Service Unavailable) Response
draft-hilt-sip-correction-503-01
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Copyright (C) The IETF Trust (2007).
Abstract
Overload occurs in the Session Initiation Protocol (SIP) when SIP
servers have insufficient resources to process all SIP messages they
receive. The SIP protocol specified in RFC 3261 provides the 503
(Service Unavailable) response code as a remedy for servers under
overload. However, the current definition of 503 (Service
Unavailable) has problems and can in fact amplify an overload
condition. This document proposes an essential correction to RFC
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3261 that avoids these problems and helps SIP servers to better cope
with overload.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Reason for Change . . . . . . . . . . . . . . . . . . . . . . 4
4. Summary of Change . . . . . . . . . . . . . . . . . . . . . . 5
5. Consequences if not approved . . . . . . . . . . . . . . . . . 7
6. The Change (Alternative 1) . . . . . . . . . . . . . . . . . . 7
6.1. 503 Service Unavailable . . . . . . . . . . . . . . . . . 7
6.2. 507 Server Overload . . . . . . . . . . . . . . . . . . . 7
7. The Change (Alternative 2) . . . . . . . . . . . . . . . . . . 8
7.1. 503 Service Unavailable . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 11
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1. Introduction
As with any network element, a Session Initiation Protocol (SIP) [2]
server can suffer from overload when the number of SIP messages it
receives exceeds the number of SIP messages it can process.
Generally, a SIP server is overloaded when it does not have
sufficient resources to process all incoming SIP messages.
RFC3261 [2] defines the 503 (Service Unavailable) response code to
enable servers to handle temporary overload as follows:
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).
Unfortunately, this mechanism has proven to be problematic in actual
deployments. Problems observed include load amplification, server
underutilization, off/on semantics and ambiguous usages [5], which
can eventually lead to a congestion collapse; a condition in which
the throughput of a server drops to a small fraction of its capacity.
This specification proposes an essential correction to RFC3261 for
the 503 (Service Unavailable) response following the process defined
in [4]. The specification does not attempt to provide a complete
solution for SIP overload control. Such a solution is left for
further study.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [1] and indicate requirement levels for
compliant implementations.
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3. Reason for Change
The current specification of 503 (Service Unavailable) responses has
proven to be problematic [5]. In many cases, the use of 503 (Service
Unavailable) responses does not enable a server to successfully cope
with overload and it may indeed worsen an overload condition.
One of the problems of the 503 (Service Unavailable) response is that
it covers temporary server unavailability due to overload as well as
server maintenance. However, both cases are different in nature.
For maintenance, a server needs to stop all incoming traffic for a
certain period of time. Since it is likely that not all servers of a
domain will be taken down at the same time, it is useful for the
receiver of a 503 response to re-send the request to an alternate
server.
In an overload condition, a course grained on/off semantic for
controlling load is problematic [5]. It is beneficial if a server
can control incoming load on a more fine grained basis. This enables
the server to smoothly steer load towards the desired rate and to
reduce load even before overload occurs. Re-sending requests to
alternate servers during overload is also problematic and may amplify
the load on servers [5]. In particular if multiple servers are load
balanced, re-sending a request that was rejected by one server due to
overload does not help to increase the chances of delivery since the
other servers are likely to be highly loaded as well. Instead, it
increases the number of requests these servers have to handle.
The following specific mechanisms defined for 503 (Service
Unavailable) responses contribute to overall the problem:
Server maintenance vs. overload control: The 503 (Service
Unavailable) response is used for overload control as well as
server maintenance. Since client behavior may be different in
these two cases, it is useful to clearly differentiate between
them.
Retry-After: The Retry-After header in a 503 (Service Unavailable)
response tells a client to stop sending traffic for the given
period of time. After this time is over, the client may start
sending again. This mechanism causes performance problems when
used for overload control by a server that receives requests from
a small number of clients (e.g. a SIP proxy that receives requests
from a few other SIP proxies) [5]. However, it works well for
server maintenance.
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Alternate server forwarding: After receiving a 503 (Service
Unavailable) response, a client can send the request to an
alternate server. This mechanism can amplify load if used for
overload control [5] but it is beneficial for server maintenance.
Dropping requests: A server is allowed to drop requests or refuse
connections instead of sending a 503 (Service Unavailable)
response. Requests that do not receive a response will eventually
be retransmitted by the client, which again amplifies load during
periods of overload.
Blocking hostnames: A client that has received a 503 (Service
Unavailable) response with a Retry-After header may decide to stop
forwarding traffic to this server based on the servers hostname.
However, if this hostname represents a cluster of servers (e.g.
via a DNS mapping), the client would block traffic to all servers.
The other servers may then be underutilized [5].
4. Summary of Change
In the following, two alternative sets of changes are proposed.
Alternative 1:
1. Introduce a new response code, 507 (Server Overload), for servers
temporarily unavailable due to overload. This response is
similar to a 500 (Server Internal Error) response. Its Retry-
After header has the same semantics (i.e., it only affects the
current request) and it is forwarded all the way to the UAC.
2. A difference between a 500 (Server Internal Error) and a 507
(Server Overload) response is that a 507 (Server Overload)
response should not be re-tried at an alternate server. Instead,
it should be returned to the UAC. This way, excess requests are
quickly cleared from a network of SIP servers. A new header,
"Allow-Retry", may be used to explicitly allow proxies to re-try
the request at an alternate server.
3. Deprecate the use of 503 (Service Unavailable) responses for
temporary unavailability due to overload.
4. Change dropping requests or refusing the connection as a
replacement for sending a 503 (Service Unavailable) response from
MAY to SHOULD NOT.
5. Recommend the use of IP addresses for blocking traffic after
receiving a 503 (Service Unavailable) with Retry-After and not
the hostname.
Alternative 2:
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1. Deprecate the use of Retry-After headers in 503 (Service
Unavailable) responses for overload control by servers with a
small client population (< 20 clients). The use of Retry-After
remains unchanged for servers with a large number of clients such
as edge proxies (> 20 clients) and server maintenance. Proxies
that create a 500 (Server Internal Error) response after
receiving a 503 (Service Unavailable) may include a Retry-After
header in the 500 (Server Internal Error) response to prevent the
UAC from instantly retrying the request.
2. Introduce a new header, "Allow-Retry", for 503 (Service
Unavailable) responses. This header controls whether a client
receiving a 503 (Service Unavailable) response should or should
not forward the request to an alternate server. The default
value for this header is true. A somewhat simplistic alternative
to the introduction of a new header is to deprecate forwarding
requests to alternate servers if the 503 (Service Unavailable)
response does not contain a Retry-After header. In this case, it
can be assumed that it was created because of overload and not
server maintenance.
3. Change dropping requests or refusing the connection as a
replacement for sending a 503 (Service Unavailable) response from
MAY to SHOULD NOT.
4. Recommend the use of IP addresses for blocking traffic after
receiving a 503 (Service Unavailable) with Retry-After and not
the hostname.
RFC 3261 [2] and RFC 3263 [3] define that transport failures
(generally, due to fatal ICMP errors in UDP or connection failures in
TCP) should be treated as a 503 (Service Unavailable) response.
These cases should be treated as 503 (Service Unavailable) response
that would be created due to server maintenance.
OPEN ISSUE 1: is alternative 1. or 2. preferable? Alternative 1.
seems cleaner and the introduction of a specific response code for
overload useful. It also has the advantage that it is independent of
the number of clients a server has. Alternative 1. should be
backwards compatible and work even if proxies and/or clients do not
support it. However, the introduction of a new response code is a
more significant change than alternative 2.
OPEN ISSUE 2: is 20 a good delineation for the use of 503 (Service
Unavailable) in alternative 2.?
OPEN ISSUE 3: Is a new response code in alternative 1. a good way to
differentiate between overload and server maintenance. An
alternative would be to use a Warning header code or a new header
field in 503 (Service Unavailable) responses.
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5. Consequences if not approved
Without these changes, networks of SIP servers are vulnerable to
overload. The performance of a network of SIP servers can be
significantly impacted by overload due to the problems described
above.
While the proposed changes do not provide a full solution for
overload control and cannot always prevent a congestion collapse,
they avoid the problems described above and improve SIP server
performance under overload.
6. The Change (Alternative 1)
The following two sections replace Section 21.5.4 in RFC3261 [2].
6.1. 503 Service Unavailable
The server is temporarily unable to process the request due to a
temporary 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. The client SHOULD block traffic to
a server based on the servers IP address and not the hostname since
hostnames can represent multiple servers.
Servers SHOULD NOT refuse the connection or drop the request as a
replacement for responding with 503 (Service Unavailable).
6.2. 507 Server Overload
The server is temporarily unable to process the request due to a
temporary overloading of the server. The server SHOULD reject
requests that exceed its capacity with a 507 (Server Overload)
response. It MAY indicate when the client should retry the current
request in a Retry-After header field. The Retry-After header has
the same semantics as in a 500 (Server Internal Error) response.
A client (proxy or UAC) receiving a 507 (Server Overload) response
SHOULD NOT attempt to forward the request to an alternate server.
Forwarding the request to alternate servers under overload would
increase the load on all servers and thereby amplify the overload
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condition.
7. The Change (Alternative 2)
The following sentence is added to the end of the paragraph starting
with "A proxy which receives..." on top of page 110 (Section 16.7
step 6.) in RFC3261 [2]:
It MAY indicate when the client should retry the request in a
Retry-After header field added to the 500 (Server Internal Error)
response.
The following section replaces Section 21.5.4 in RFC3261 [2].
7.1. 503 Service Unavailable
The server is temporarily unable to process the request due to a
temporary overloading or maintenance of the server.
If a server is temporarily unavailable due to maintenance, it MAY
indicate when the client should retry the request in a Retry-After
header field. It MAY include a "Allow-Retry" header with the value
"true" in this response to indicate that the client SHOULD re-try the
request at an alternate server.
A server that is temporarily unavailable due to overload SHOULD
reject the requests that exceed its capacity with a 503 (Service
Unavailable) response. Servers with a large population of clients
(proxies or UACs) MAY indicate when the client should retry the
request in a Retry-After header field. Servers that fall into this
category typically receive traffic from 20 or more (often much more)
clients. An example for such a server is an edge proxy. All other
servers SHOULD NOT include a Retry-After header in a 503 (Service
Unavailable) response. If no Retry-After is given, a client MUST act
as if it had received a 500 (Server Internal Error) response. The
server SHOULD include a "Allow-Retry" header with the value "false"
in this response. Re-trying the request at alternate servers under
overload may increase the load on all servers and thereby amplify the
overload condition.
A client (proxy or UAC) receiving a 503 (Service Unavailable)
response that contains an "Allow-Retry" header SHOULD or SHOULD NOT
attempt to forward the request to alternate servers, depending on the
value of this header field.
If the Retry-After header field is present in a 503 (Service
Unavailable) response, the client SHOULD NOT forward any other
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requests to that server for the duration specified in the Retry-After
header field. The client SHOULD block traffic to a server based on
the servers IP address and not the hostname since hostnames can
represent multiple servers.
Servers SHOULD NOT refuse the connection or drop the request as a
replacement for responding with 503 (Service Unavailable).
8. Security Considerations
The procedures introduced in this document have no security
implications beyond what is already specified in RFC3261 [2].
9. IANA Considerations
None.
Appendix A. Acknowledgements
Many thanks to Jonathan Rosenberg and Keith Drage for their
suggestions. Many thanks also to Eric Noel, Carolyn Johnson, Ping
Wu, Tadeusz Drwiega and the overload control design team for the
simulation results.
10. References
10.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] 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.
[3] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
(SIP): Locating SIP Servers", RFC 3263, June 2002.
10.2. Informative References
[4] Drage, K., "A Process for Handling Essential Corrections to the
Session Initiation Protocol (SIP)",
draft-drage-sip-essential-correction-01 (work in progress),
March 2007.
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[5] Rosenberg, J., "Requirements for Management of Overload in the
Session Initiation Protocol",
draft-rosenberg-sipping-overload-reqs-02 (work in progress),
October 2006.
Authors' Addresses
Volker Hilt
Bell Labs/Alcatel-Lucent
791 Holmdel-Keyport Rd
Holmdel, NJ 07733
USA
Email: volkerh@bell-labs.com
Indra Widjaja
Bell Labs/Alcatel-Lucent
600-700 Mountain Avenue
Murray Hill, NJ 07974
USA
Email: iwidjaja@alcatel-lucent.com
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