INTERNET-DRAFT L. Coene(editor)
Internet Engineering Task Force Siemens
Issued: February 2002
Expires: July 2002
Multihoming issues in the Stream Control Transmission Protocol
<draft-coene-sctp-multihome-03.txt>
Status of this Memo
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Abstract
This document describes issues of the Stream Control Transmission
Protocol (SCTP)[RFC2960] in regard to multihoming on the
Internet. It explores cases where through situations in the
internet, single points-of-failure can occur even when using
multihoming and what the impact is of multihoming on the routing
tables of the host.
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Table of Contents
Multihoming issues in the Stream Control Transmission Protocol . ii
Chapter 1: Introduction ........................................ 2
Chapter 2: SCTP multihoming .................................... 3
Chapter 2.1: Interaction with routing .......................... 3
Chapter 2.2: SCTP multihoming and the size of routing tables ... 7
Chapter 2.3: SCTP multihoming and Network Adress
Translators(NAT) ............................................... 8
Chapter 2.4: SCTP multihoming and IPsec ........................ 8
Chapter 3: Security considerations ............................. 9
Chapter 4: References and related work ......................... 9
Chapter 5: Acknowledgments ..................................... 10
Chapter 6: Author's address .................................... 10
1 Introduction
SCTP[RFC2960] is a transport protocol that uses multihoming. This
draft is an attempt at identifying the issues that may arise in the
layers below SCTP when multihoming is used by SCTP. Some issues are
already being addresses in various other WGs and this document will
try to highlight them. If the solutions would resolve the issues
presented in this document then the problem presented is no longer
an issue. This document will also try to gauge the effectiveness of
the present multihoming architecture.
1.1 Terminology
The following terms are commonly identified in this work:
Association: SCTP connection between two endpoints.
Transport address: A combination of IP address and SCTP port number.
Multihoming: Assigning more than one IP network interface to a
single endpoint.
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TLA: Top Level Aggregation
1.2 Contributors
The following people contributed to the document: L. Coene(Editor),
M. Tuexen, G. Verwimp, J. Loughney, R.R. Stewart, Qiaobing Xie,
M. Holdrege, M.C. Belinchon, A. Jungmaier, and L. Ong.
2 SCTP multihoming
2.1 Interaction with routing
For fault resilient communication between two SCTP endpoints, the
multihoming feature needs more than one IP network interface for
each endpoint. The number of paths used is the minimum of network
interfaces used by any of the endpoints. It is recommended to bind
the association to all the IP source addresses of the endpoint.
Note that in IPv6, each network interface will have more than one IP
address.
Under the assumption that every IP address will have a different,
seperate path towards the remote endpoint, (this is the
responsibility of the routing protocols or of manual configuration)
, if the transport to one of the IP addresses (= 1 particular path)
fails then the traffic can migrate to the other remaining IP address
(= other paths) within the SCTP association.
+------------+ *~~~~~~~~~* +------------+
| Endpoint A | * Cloud * | Endpoint B |
| 1.2 +---------+ 1.1<--->3.1 +----------+ 3.2 |
| | | | | |
| 2.2 +---------+ 2.1<--->4.1 +----------+ 4.2 |
| | * * | |
+------------+ *~~~~~~~~~* +------------+
Figure 2.1.1: Two hosts with redundant networks.
Consider figure 2.1.1, if the host routing tables look as follows
the endpoint will achieve maximum use of the multi-homing feature:
Endpoint A Endpoint B
Destination Gateway Destination Gateway
------------------------ -------------------------
3.0 1.1 1.0 3.1
4.0 2.1 2.0 4.1
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Now if you consider figure 2.1.1, if the host routing table looks as
follows, the association is subject to a single point of failure in
that if any interface breaks, the whole association will break(See
figure 2.1.2).
Host A Host B
Destination Gateway Destination Gateway
------------------------ -------------------------
3.0 1.1 1.0 4.1
4.0 2.1 2.0 3.1
Example: link 4.2-4.1 fails
Primary path: link 1.2-1.1 - link 3.1-3.2
Second Path : Link 2.2-2.1 - link 4.1-4.2
Endpoint A
+-------+--------+------+
|S= 1.2 | D= 3.2 | DATA | ------->----- Arrives at Endpoint B
+-------+--------+------+
Endpoint B answers with SACK
+-------+--------+------+
|S= 4.2 | D= 1.2 | SACK | Gets lost, because send out on the failed
+-------+--------+------+ 4.1-4.2 link
After X time, retransmit on the other path by endpoint A
Endpoint A
+-------+--------+------+
|S= 2.2 | D= 4.2 | DATA | Is send out on link 2.2-2.1, but gets lost,
+-------+--------+------+ as msg has to pass via failed 4.1-4.2 link
The same scenario will play out for failures on the other links
Note : S = Source address
D = Destination address
Figure 2.1.2: Single point of failure case in redundant network
due to routing table in host B
When an endpoint selects its source address, careful consideration
must be taken. If the same source address is always used, then it is
possible that the endpoint will be subject to the same single point
of failure illustrated above. If possible the endpoint should always
select the source address of the packet to correspond to the IP
address of the Network interface where the packet will be emitted.
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+------------+ *~~~~~~~~~* +------------+
| Endpoint A | * Cloud * | Endpoint B |
| 1.2 +---------+ 1.1<--+ | | |
| | | |->3.1|----------+ 3.2 |
| 2.2 +---------+ 2.1<--+ | | |
| | * * | |
+------------+ *~~~~~~~~~* +------------+
Figure 2.1.3: Two hosts with asymmetric networks.
In Figure 2.1.3 consider the following host routing table:
Endpoint A Endpoint B
Destination Gateway Destination Gateway
------------------------ -------------------------
3.0 1.1 1.0 3.1
2.0 3.1
In this case the fault tolerance becomes limited by two seperate
issues. If the path between 3.1 and 3.2 breaks in both directions
any association will break between endpoint A and endpoint B. The
second failure will occur for the whole the association as well due
to a breakage between 1.2 and 1.1 in both directions, since no
alternative route exists to 3.2 and all traffic is being routed
through one interface.
Now one of these issues can be remedied by the following: In Figure
2.1.3 consider the following host routing table:
Endpoint A Endpoint B
Destination Gateway Destination Gateway
------------------------ -------------------------
3.0 1.1 1.0 3.1
3.0 2.1 2.0 3.1
The SCTP implementation on Endpoint A needs to consider both the
source and destination address when identifying the transport. If
it treats 1.2/3.2 as a separate transport address from 2.2/3.2,
normal SCTP failover mechanisms work correctly. This mechanism is
suggested in RFC 2960.
If the underlying OS does not support multiple routes to the same
destination, or if the SCTP implementation can not control which
route gets used (as is typical for user space implementations), we
can still solve this issue by the following modification even when
only one interface exists on endpoint B.
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+------------+ *~~~~~~~~~~* +------------+
| Endpoint A | * Cloud * | Endpoint B |
| 1.2 +---------+ 1.1<---+ | | |
| | | +->3.1+----------+ 3.2 & 4.2 |
| 2.2 +---------+ 2.1<---+ | | |
| | * * | |
+------------+ *~~~~~~~~~~* +------------+
Figure 2.1.4: Two hosts with asymmetric networks, but symmetric
addresses.
In Figure 2.1.4 consider the following host routing table:
Endpoint A Endpoint B
Destination Gateway Destination Gateway
------------------------ -------------------------
3.0 1.1 1.0 3.1
4.0 2.1 2.0 3.1
Now with the duplicate IP addresses assigned to the same interface
and the above routing tables, even if the interface between 1.1 and
1.2 breaks, an association will still survive this failure.
As a practical matter, it is recommended that IP addresses in a
multihomed endpoint be assigned IP endpoints from different TLA's to
ensure against network failure.
In IP implementations the outgoing interface of multihomed hosts is
often determined by the destination IP address. The mapping is done
by a lookup in a routing table maintained by the operating
system. Therefore the outgoing interface is not determined by SCTP.
Using such implementations, it should be noted that a multihomed
host cannot make use of the multiple local IP addresses if the peer
is singlehomed. The multihomed host has only one path and will
normally use only one of its interfaces to send the SCTP datagrams
to the peer. If this physical path fails, the IP routing table in
the multihome host has to be changed. This problem is out of scope
for SCTP.
SCTP will always send its traffic to a certain transport address (=
destination address + port number combination) for as long as the
transmission is uninterrupted (= primary). The other transport
addresses (secondary paths) will act as a backup in case the primary
path goes out of service. The changeover between primary and backup
will occur without packet loss and is completely transparent to the
application. The secondary path can also be used for
retransmissions(per section 6.4 of [RFC2960]).
The port number is the same for all transport addresses of that
specific association.
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Applications directly using SCTP may choose to control the
multihoming service themselves. The applications have then to supply
the specific IP address to SCTP for each outbound user message. This
might be done for reasons of load-sharing and load-balancing across
the different paths. This might not be advisable as the throughput
of any of the paths is not known in advance and constantly changes
due to the actions of other associations and transport protocols
along that particular path, would require very tight feedback of
each of the paths to the loadsharing functions of the user.
By sending a heartbeat chunk/message (=SCTP internal keep alive
message) on all the multiple paths that are not used for active
transmission of messages across the association, it is possible for
SCTP to detect whether one or more paths have failed. SCTP will not
use these failed paths when a changeover is required.
The transmission rate of sending heartbeat messages should be
modifiable and the possible loss of the heartbeat message could be
used for the monitoring and measurements of the concerned paths.
2.2 SCTP multihoming and the size of routing tables
As multihoming means that more than one destination address is used
on the host, that would mean that a routing descision must be made
on the host in IP. The host does not know beforehand to which other
host it is going to send something, so that would in theory require
that all possible paths to all possible destinations should be known
on that host. This amounts to the host being a part of the
distribution of the routing information in the network.
Possible solutions would require to ask only for the paths to host
that are actually in use(meaning a association is about to be setup
with that particular host). This is a viable solution for hosts with
a small number of associations to different hosts.
If the host has many associations with a lot of different host then
then this becomes cumbersome(getting the specific paths from the
routers and the updates and all) and leads in practice to same
problem of distributing prefixes from the edge router(s) to the
host.
It might be useful to explore ways where no distribution of routing
information to the host for using multihoming is needed or where the
interface/link selection is not based on the use of different
prefixes. Not all hosts have facilities for containing possible
large routing tables/databases.
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2.3 SCTP multihoming and Network Adress Translators(NAT)
For multihoming the NAT must have a public IP address for each
represented internal IP address. The host can preconfigure IP
address that the NAT can substitute. Or the NAT can have internal
Application Layer Gateway (ALG) which will intelligently translate
the IP addresses in the INIT and INIT ACK chunks. See Figure 1.
If Network Address Port Translation is used with a multihomed SCTP
endpoint, then any port translation must be applied on a
per-association basis such that an SCTP endpoint continues to
receive the same port number for all messages within a given
association.
+-------+ +----------+ *~~~~~~~~~~* +------+
|Host A | | NAT | * Cloud * |Host B|
| 10.2 +---+ 10.1|5.2 +-----+ 1.1<+->3.1--+---------+ 1.2 |
| 11.2 +---+ 11.1|6.2 | | +->4.2--+---------+ 2.2 |
| | | | * * | |
+-------+ +----------+ *~~~~~~~~~* +------+
Fig 1: SCTP through NAT with multihoming
It should be noted that the NAT box becomes a single
point-of-failure in this case, as ALL the paths of the SCTP
association have to go through that single NAT box.
2.4 SCTP multihoming and IPsec
IPsec was not designed to support multihomed connections and that
imposes some difficulties when using IPsec with SCTP hosts that make
use of several addresses in a single association.
The Security Policy Database (SPD) entries should use as selectors,
among other fields of the IP header, the source and destination
address. The easy and expensive way to scale the SPD to multihomed
host, is simply creating a new entry for all the possible
combinations of source and a destination addresses. A much better
implementation approach is to simply use groups of addresses instead
of single ones.
The same problem arises when identifying a Security Association
(SA). An SA should be identified by the extended triplet ({set of
destination addresses}, Security Parameter Index, Security
Protocol).
Moreover, when exchanging keys using the Internet Key Exchange (IKE)
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protocol there are some extra difficulties. IKE only allows the use
of a single source and destination address, and so the initial
solution to the problem would be creating a number S*D of SAs, where
S is the number of source addresses, and D the number of destination
addresses. This solution unnecessarily consumes both time and
resources. Other more complex and suitable approaches would need
modifications in IKE itself.
A specific discussion about the problems that crop up when using
IPsec with SCTP can be seen in [IPsec-SCTP]. All SCTP+IPSEC
implementations would have to do the above to be compatible
3 Security considerations
SCTP only tries to increase the availability of a network. SCTP does
not contain any protocol mechanisms which are directly related to
user message authentication, integrity and confidentiality
functions. For such features, it depends on the IPSEC protocols and
architecture and/or on security features of its user protocols.
The solutions needed for allowing multihoming may provide security
risks.
4 References and related work
[RFC2960] Stewart, R. R., Xie, Q., Morneault, K., Sharp, C. , ,
Schwarzbauer, H. J., Taylor, T., Rytina, I., Kalla, M., Zhang,
L. and Paxson, V."Stream Control Transmission Protocol", RFC2960,
October 2000.
[RFC2663] Srisuresh, P. and Holdrege, M., "IP Network Address
Translator (NAT) Terminology and Considerations", RFC2663, August
1999
[RFC2694] Srisuresh, P., Tsirtsis, G., Akkiraju, P. and Heffernan,
A., "DNS extensions to Network Address Translators (DNS_ALG)",
RFC2694, September 1999
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[IPsec-SCTP] Bellovin, S. M., Ioannidis, J., Keromytis, A. D. and
Stewart, R. R., "On the Use of SCTP with IPsec",
draft-ietf-ipsec-sctp-02.txt, work in progress
5 Acknowledgments
This document was initially developed by a design team consisting of
Lode Coene, John Loughney, Michel Tuexen, Randall R. Stewart,
Qiaobing Xie, Matt Holdrege, Maria-Carmen Belinchon, Andreas
Jungmaier, Gery Verwimp and Lyndon Ong.
The authors wish to thank Renee Revis, I. Rytina, H.J. Schwarzbauer,
J.P. Martin-Flatin, T. Taylor, G. Sidebottom, K. Morneault,
T. George, M. Stillman, N. Makinae, S. Bradner, A. Mankin,
G. Camarillo, H. Schulzrinne, R. Kantola, J. Rosenberg,
I. Arias-Rodriguez, D. Lehmann, La Monte Henry Yaroll, P. Savola and
many others for their invaluable comments.
6 Author's Address
The following authors have contributed to this document.
Lode Coene Phone: +32-14-252081
Siemens Atea EMail: lode.coene@siemens.atea.be
Atealaan 34
B-2200 Herentals
Belgium
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Expires: July 31, 2002
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