Internet DRAFT - draft-iyengar-sctp-cacc
draft-iyengar-sctp-cacc
Internet Engineering Task Force J. R. Iyengar,
Category: Internet Draft P. D. Amer
Expires: May 31, 2006 University of Delaware
R. Stewart
Cisco Systems
I. Arias-Rodriguez
Nokia
November 30, 2005
Preventing SCTP Congestion Window Overgrowth During Changeover
draft-iyengar-sctp-cacc-03.txt
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Copyright (C) The Internet Society (2005).
Abstract
SCTP [RFC2960] supports IP multihoming at the transport layer. SCTP
allows an association to span multiple local and peer IP addresses,
and allows the application to dynamically change the primary
destination during an active association. We present a problem in the
current SCTP specification that results in unnecessary retransmissions
and "TCP-unfriendly" growth of the sender's congestion window during
certain changeover conditions. We present the problem and propose an
algorithm called the Split Fast Retransmit Changeover Aware Congestion
Control algorithm (SFR-CACC) as a solution. We recommend the addition
of SFR-CACC to the SCTP specification [RFC2960].
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Conventions
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,SHOULD
NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when they appear in
this document, are to be interpreted as described in [RFC2119].
1. Introduction
In an SCTP [RFC2960] association, the sender transmits data to its
peer's primary destination address. SCTP provides for
application-initiated changeovers so that the sending application can
move the outgoing traffic to another path by changing the sender's
primary destination address. We uncovered a problem in the
current SCTP specification that results in unnecessary retransmissions
and "TCP-unfriendly" growth of the sender's congestion window under
certain changeover conditions. We present the problem and propose an
algorithm called the Split Fast Retransmit Changeover Aware Congestion
Control (SFR-CACC) algorithm as a solution. We recommend the addition
of the SFR-CACC algorithm to the SCTP specification [RFC2960].
2. Congestion Window Overgrowth: Problem Description
We present a specific example which illustrates the congestion window
overgrowth problem.
2.1 Example Description:
Consider the architecture shown below:
______ _________ ______
| | / \ | |
| |A1 <============== Path 1 ============> B1| |
| |<------------->| |<------------>| |
| Host | | Network | | Host |
| A | | | | B |
| |<------------->| |<------------>| |
| |A2 <============== Path 2 ============> B2| |
| | \_________/ | |
------ ------
Fig 1: Example Architecture
SCTP endpoints A and B have an association between them. Both
endpoints are multihomed, A with network interfaces A1 and A2, and B
with interfaces B1 and B2. More precisely, A1, A2, B1 and B2 are IP
addresses associated with link layer interfaces. Here we assume only
one address per interface, so address and interface are used
interchangeably.
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All four addresses are bound to the SCTP association. For one of
several possible reasons (e.g., path diversity, policy based routing,
load balancing), we assume in this example that the data traffic from
A to B1 is routed through A1, and from A to B2 is routed through A2.
Let C1 be the cwnd at A for destination B1, and C2 be the cwnd at A
for destination B2. C1 and C2 are denoted in terms of MTUs, not
bytes.
Consider the following sequence of events:
1) The sender (host A) initially sends data to the receiver (host B)
using primary destination address B1. This setting causes packets
to leave through A1. Assume these packets leave the
transport/network layers, and get buffered at A's link layer A1,
whereupon they get transmitted according to the channel's
availability. We refer to these TSNs (that is, packets) the first
group of TSNs.
2) Assume as the first group of TSNs is being transmitted through A1,
that the sender's application changes the primary destination to
B2, thereby causing any new data from the sender to be sent to
B2. In the example, we assume C2 = 2 at the moment of changeover
and new TSNs (second group of TSNs) are now transmitted to the new
primary, B2. This new primary destination causes new TSNs to
leave the sender through A2. Concurrently, the packets buffered
earlier at A1 are still being transmitted. Previous packets sent
through A1, and the packets sent through A2, can arrive at the
receiver B in an interleaved fashion on interfaces B1 and B2,
respectively. This reordering is introduced as a result of
changeover.
3) The receiver starts reporting gaps as soon as it notices
reordering. If the receiver communicates four missing reports to
the sender before all original transmissions of the first group
have been acked, the sender will start retransmitting the unacked
TSNs on path 2.
4) The SACKs for the original transmission of the first group of TSNs
reach A on A1. Since the sender cannot distinguish between SACKs
generated by transmissions from SACKs generated by
retransmissions, the SACKs now received by A on A1 end up acking
the retransmissions of the first group of TSNs, incorrectly
crediting C2 instead of C1. This behaviour whereby SACKs for
original transmissions incorrectly ack retransmissions continues
until all original transmissions of the first group are
retransmitted to B2. Thus, the SACKs from the original
transmissions cause C2 to grow (possibly drastically) from wrong
interpretation of the feedback.
2.2 Discussion
Our preliminary investigation shows that the problem occurs for a
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range of {end-to-end delay, end-to-end available bandwidth, MTU}
settings. [IC+03] gives a more detailed description and analysis of
the problem. From the general model developed in [IC+03], we have
found that whenever a changeover is made to a higher quality path
(i.e., lower end-to-end delay, higher end-to-end available bandwidth
path), there is a likelihood of TCP-unfriendly cwnd growth and
unnecessary retransmissions. We also note that the bigger the quality
improvement that the new path provides, the larger the TCP-unfriendly
growth and number of false retransmissions will be.
The congestion window overgrowth (i.e., TCP-unfriendly congestion
window growth) problem exists even if buffering of the first group
occurs not at the sender's link layer, but in a router along the path
(in the example architecture, path 1). In essence, the transport
layers at the endpoints can be thought of as the sending and
receiving entities, and the buffering could potentially be
distributed anywhere along the end-to-end path.
3. Solution to the Problem: The SFR-CACC Algorithm
The problem of TCP-unfriendly cwnd growth occurs due to incorrect fast
retransmissions. These incorrect retransmissions occur because the
congestion control algorithm at the sender is unaware of the
occurrence of a changeover, and is hence unable to identify reordering
introduced due to changeover. In [IC+03], we propose the Changeover
Aware Congestion Control algorithms (CACC) - the Conservative CACC
algorithm (C-CACC), and the Split Fast Retransmit CACC algorithm
(SFR-CACC), which curb the TCP-unfriendly cwnd growth by avoiding
these unnecessary fast retransmissions. Of the three algorithms,
C-CACC has the disadvantage that in the face of loss, a lot of TSNs
could potentially have to wait for an RTO when they could have been
fast retransmitted. SFR-CACC alleviates this disadvantage.
The key idea in SFR-CACC is to maintain state at the sender on a
per-destination basis when a changeover happens. On the receipt of a
SACK, the sender uses this state to selectively increase the missing
report count for TSNs in the retransmission list. In SFR-CACC, we
further make the following observation: the reordering observed during
changeover happens because TSNs which are supposed to reach the
receiver in-sequence end up reaching the receiver in concurrent
groups, in-sequence within each group. With this observation, we
reason that the Fast Retransmit algorithm can be applied independently
within each group. That is, on the receipt of a SACK, if we can
estimate the TSN(s) that causes this SACK to be sent from the
receiver, we can use the SACK to increment missing report counts
within the causative TSN(s)'s group. Our estimate is conservative, if
a SACK could have been caused by TSNs in multiple groups, this SACK
will be used to increment missing report counts only for TSNs sent to
the current primary destination, if any. In the case where multiple
changeovers cycle back to a destination while the CHANGEOVER_ACTIVE is
still set, CYCLING_CHANGEOVER is set to indicate a double switch to
the destination. The CYCLING_CHANGEOVER flag is used to mark TSNs in
only the latest group sent to the current primary destination, thus
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preventing incorrect marking of TSNs in any other changeover
range. SFR-CACC also enables Fast Retransmit for TSNs which could have
timed out on some destination, but were retransmitted on the current
primary destination after the latest changeover to the current primary
destination. We now present the SFR-CACC algorithm in its current
simplified form, also described in [IS+04,IAS05].
3.1 Variables Introduced
In SFR-CACC, four variables are introduced:
1) CHANGEOVER_ACTIVE - a flag which indicates the occurrence of
a changeover.
2) next_tsn_at_change - an unsigned integer, which stores the next
TSN to be used by the sender, at the moment of changeover.
3) highest_tsn_in_sack_for_dest - an unsigned integer per destination,
which stores the highest TSN acked by the current SACK for each
destination.
4) cacc_saw_newack - a temporary flag per destination, which is used
during the processing of a SACK to estimate the causative TSN(s)'s
group.
3.2 The SFR-CACC Algorithm
The following algorithm requires that after a timeout retransmission,
the retransmitted TSN MUST be rendered ineligible for further fast
retransmission.
Upon receipt of a request to change the primary destination
address, the sender MUST do the following:
1) The sender MUST set CHANGEOVER_ACTIVE to indicate that a
changeover has occurred.
2) The sender MUST store the next TSN to be sent in
next_tsn_at_change.
On receipt of a SACK the sender SHOULD execute the following statements:
1) If the cumulative ack in the SACK passes next_tsn_at_change, the
CHANGEOVER_ACTIVE flag SHOULD be cleared.
2) If the SACK contains gap acks and the flag CHANGEOVER_ACTIVE
is set, then the receiver of the SACK MUST take the following
actions:
A) Initialize cacc_saw_newack to 0 for all destination
addresses.
B) For each TSN t being acked that has not been acked in any
SACK so far, set cacc_saw_newack to 1 for the destination that
the TSN was sent to.
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C) Of the TSNs being newly acked, set highest_tsn_in_sack_for_dest to
the highest TSN being newly acked for the respective destinations.
3) If the CHANGEOVER_ACTIVE flag is set, then the sender MUST execute
steps C and D to determine if the missing report count for TSN t
SHOULD be incremented. Let d be the destination to which t was
sent.
C) If cacc_saw_newack is 0 for destination d, then the sender MUST
NOT increment missing report count for t.
D) If cacc_saw_newack is 1 for destination d, and if
highest_tsn_in_sack_for_dest for destination d greater than t
then the sender SHOULD increment missing report count for t
(according to [RFC2960] and [RA+05]).
NOTE: The HTNA algorithm does not need to be applied separately,
since step 3.D above covers the functionality of the HTNA algorithm.
3.3 Discussion
The SFR-CACC algorithm maintains state information during a
changeover, and uses this information to avoid incorrect fast
retransmissions. Consequently, this algorithm prevents the
TCP-unfriendly cwnd growth. This algorithm has the added advantage
that no extra bits are added to any packets, and thus the load on the
wire and the network is not increased. SFR-CACC is also capable of
handling multiple changeovers. One disadvantage of SFR-CACC is that
there is added complexity at the sender to maintain and use the added
state variables. Some of the TSNs on the old primary may also not be
eligible for Fast Retransmit. To quantify the number of TSNs which
will be ineligible for Fast Retransmit in the face of loss, let us
assume that only one changeover is performed, and that SACKs are not
lost. Under these assumptions, potentially only the last four packets
sent to the old primary destination will be forced to be retransmitted
with an RTO instead of a Fast Retransmit. In other words, under the
stated assumptions, if a TSN that is lost has at least four packets
successfully transmitted after it to the same destination, then the
TSN will be retransmitted via Fast Retransmit.
4. Conclusion
The general consensus at the IETF has been to dissuade the usage of
SCTP's multihoming feature for simultaneous data transfer to the
multiple destination addresses, largely due to insufficient research
in the area. Though there is some amount of simultaneous data transfer
in the described scenario, this phenomenon is an effect of changing
the primary destination; not necessarily a result of an application
intending to simultaneously transfer data over the multiple paths.
Among other reasons, this changeover could be initiated by an
application searching for a better path to the peer host for a long
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session, or attempting to perform a smoother failover.
We recommend the addition of SFR-CACC to SCTP [RFC2960] to alleviate
the problem of TCP-unfriendly cwnd growth and unnecessary fast
retransmissions during a changeover. We have implemented the SFR-CACC
algorithm in the NetBSD/FreeBSD release for the KAME stack
[WEB_KAME]. The implementation uses three additional flags and
one TSN marker per-destination, as described in section
3.2. Approximately twenty lines of C code were needed to facilitate
SFR-CACC, most of which will be executed only when a changeover is
performed in an association.
5. Security Considerations
This document discusses a congestion control issue during changeover
in SCTP. This does not raise any new security issues with SCTP.
Acknowledgments
The authors would like to thank Vern Paxson, Mark Allman, Phillip
Conrad, Armando Caro, Sourabh Ladha and Keyur Shah for providing
comments and input.
References
[RFC2119] S. Bradner. "Key words for use in RFCs to Indicate
Requirement Levels". BCP 14, RFC 2119, May 1997.
[RFC2960] R. Stewart, Q. Xie, K. Morneault, C. Sharp, H. Schwarzbauer,
T. Taylor, I. Rytina, M. Kalla, L. Zhang, V. Paxson. "Stream
Control Transmission Protocol". RFC2960, October 2000.
[IC+03] J. Iyengar, A. Caro, P. Amer, G. Heinz, R. Stewart. "Making
SCTP More Robust to Changeover". SPECTS, Montreal, Canada, July
2003.
[IS+04] J. Iyengar, K. Shah, P. Amer, R. Stewart. "Concurrent
Multipath Transfer Using SCTP Multihoming". SPECTS, San Jose, CA,
July 2004.
[IAS05] J. Iyengar, P. Amer, R. Stewart. "Concurrent Multipath
Transfer Using SCTP Multihoming Over Independent End-to-End Paths".
To appear in IEEE/ACM Transactions on Networking.
[SA+05] R. Stewart, I. Arias-Rodriguez, K. Poon, A. Caro,
M. Tuexen. "Stream Control Transmission Protocol (SCTP)
Specification Errata and Issues". Internet Draft:
draft-ietf-tsvwg-sctpimpguide-16.txt, October 2005. (work in
progress)
[WEB_KAME] Webpage of the KAME Project, http://www.kame.org
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Authors' Addresses
Janardhan R. Iyengar
Department of Computer & Information Sciences
University of Delaware
103 Smith Hall
Newark, DE 19716, USA
email: iyengar@cis.udel.edu
Paul D. Amer
Department of Computer & Information Sciences
University of Delaware
103 Smith Hall
Newark, DE 19716, USA
email: amer@cis.udel.edu
Randall R. Stewart
24 Burning Bush Trail
Crystal Lake, IL 60012, USA
email: rrs@cisco.com
Ivan Arias-Rodriguez
Nokia Research Center
PO Box 407
FIN-00045 Nokia Group
Finland
email: ivan.arias-rodriguez@nokia.com
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