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The secure anycast tunneling protocol (SATP) defines a protocol used for communication between any combination of unicast and anycast tunnel endpoints. It allows tunneling of every ETHER TYPE protocol (ethernet, ip ...). SATP directly includes cryptography and message authentication based on the methods used by the Secure Real-time Transport Protocol(SRTP) (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) [RFC3711]. It can be used as an encrypted alternative to IP Encapsulation within IP (Perkins, C., “IP Encapsulation within IP,” October 1996.) [RFC2003] and Generic Routing Encapsulation (GRE) (Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina, “Generic Routing Encapsulation (GRE),” March 2000.) [RFC2784]. Both anycast receivers and senders are supported.
1.
Introduction
1.1.
Notational Conventions
2.
Motivation and usage scenarios
2.1.
Usage scenarions
2.1.1.
Tunneling from unicast hosts over anycast routers to other unicast hosts
2.1.2.
Tunneling from unicast hosts to anycast networks
2.1.3.
Redundant tunnel connection of 2 networks
2.2.
Encapsulation
3.
Using SATP on top of IP
3.1.
Fragmentation
3.2.
ICMP messages
4.
Protocol specification
4.1.
Header format
4.2.
sequence number
4.3.
sender ID
4.4.
MUX
4.5.
payload type
4.6.
payload
4.7.
padding (OPTIONAL)
4.8.
padding count (OPTIONAL)
4.9.
MKI (OPTIONAL)
4.10.
authentication tag (RECOMMENDED)
4.11.
Encryption
5.
Security Considerations
5.1.
Replay protection
6.
IANA Considerations
7.
References
7.1.
Normative References
7.2.
Informational References
§
Author's Address
§
Intellectual Property and Copyright Statements
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SATP is a mixture of a generic encapsulation protocol like GRE (Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina, “Generic Routing Encapsulation (GRE),” March 2000.) [RFC2784] and a secure tunneling protocol as IPsec (Kent, S. and R. Atkinson, “Security Architecture for the Internet Protocol,” November 1998.) [RFC2401] in tunnel mode. It can be used to build redundant virtual private network (VPN) connections. It supports peer-to-peer tunnels, where tunnel endpoints can be any combination of unicast, multicast or anycast hosts, so it defines a Host Anycast Service (Partridge, C., Mendez, T., and W. Milliken, “Host Anycasting Service,” November 1993.) [RFC1546]. Encryption is done per packet, so the protocol is robust against packet loss and routing changes. To reduce header overhead, encryption techniques of SRTP (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) [RFC3711] are being used.
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The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].
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This section gives an overview of possible usage scenarios. Please note that the protocols used in the figures are only examples and that SATP itself does not care about either transport protocols or encapsulated protocols. Routing is not done by SATP and each implemetation MAY choose it's own way of doing this task (e.g. using functions provided by the operating system). SATP is used only to encapsulate and encrypt data.
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An example of SATP used to tunnel in a unicast client - anycast server model
--------- router -----------
/ \
unicast ------+---------- router ------------+------ unicast
host \ / host
--------- router -----------
unicast | encrypted | anycast | encrypted | unicast
tunnel | communication | tunnel | communication | tunnel
endpoint | using SATP | endpoint | using SATP | endpoint
| Figure 1 |
In this scenario the payload is encapsuleted into a SATP packet by a unicast host and gets transmitted to one of the anycast routers. After transmisson the packet gets decapsulated by the router. This router makes a routing descision based on the underlying protocol and transmits a new SATP package to one or more unicast hosts depending on this decision.
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An example of SATP used to encrypt data between a unicast host and anycast networks
-------Router -+---- DNS Server
/ \
/ --- 6to4 Router
/
unicast -------+----------Router --+--- DNS Server
host \ \
\ --- 6to4 Router
\
-------Router -+---- DNS Server
\
--- 6to4 Router
unicast | encrypted | anycast | plaintext
tunnel | communication | tunnel | anycast
endpoint | using SATP | endpoint | services
| Figure 2 |
When the unicast hosts wants to transmit data to one of the anycast DNS servers, it encapsulates the data and sends a SATP packet to the anycast address of the routers. The packet arrives at one of the routers, gets decapsulated and is then forwarded to the DNS server. This method can be used to tunnel between clients and networks providing anycast services. It can also be used the other way to virtually locate a unicast service within anycasted networks.
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An example of SATP used to connect 2 networks
Router ----------- ---------------Router
/ \ / \
Network - Router ------------x Network
A \ / \ / B
Router ----------- ---------------Router
| packets | packets | packets |
plaintext | get | take a | get | plaintext
packets | de/encrypted | random | de/encrypted | packets
|de/encapsulated| path |de/encapsulated|
| Figure 3 |
Network A has multiple routers which act as gateway/tunnel endpoints to another network B. This way a redundant encrypted tunnel connection between the two networks is built up. All tunnel endpoints of network A share the same anycast address and all tunnel endpoints of network B share another anycast address. When a packet from network A is transmitted to network B, it first arrives on one of network A's border routers. Which router is used is determined by network A's internal routing. This router encapsulates the package and sends it to the anycast address of network B's routers. After arrival the SATP packet gets decapsulated and routed to its destination within network B.
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SATP does not depend on which lower layer protocol is used. This section only gives an example of how packets could look like.
Examples of SATP used with different lower layer and payload protocols
+------+-----+-------------------------------+
| | | +----------------+-----+ |
| IPv6 | UDP | SATP | Ethernet 802.3 | ... | |
| | | +----------------+-----+ |
+------+-----+-------------------------------+
Tunneling of Ethernet over UDP/IPv6
+------+-----+---------------------------+
| | | +------+-----+-----+ |
| IPv4 | UDP | SATP | IPv6 | UDP | RTP | |
| | | +------+-----+-----+ |
+------+-----+---------------------------+
Tunneling of IPv6 over UDP/IPv4 with RTP payload
+------+-------------------------------+
| | +----------------+-----+ |
| IPv6 | SATP | Ethernet 802.3 | ... | |
| | +----------------+-----+ |
+------+-------------------------------+
Tunneling of Ethernet over IPv6
+------+---------------------------+
| | +------+-----+-----+ |
| IPv4 | SATP | IPv6 | UDP | RTP | |
| | +------+-----+-----+ |
+------+---------------------------+
Tunneling of IPv6 over IPv4 with RTP payload
| Figure 4 |
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The only way of fully supporting fragmentation would be to synchronise fragments between all anycast servers. This is considered to be too much overhead, so there are two non-perfect solutions for these problems. Either fragmentation HAS TO be disabled or if not all fragments arrive at the same server the IP datagramm HAS TO be discarded. As routing changes are not expected to occur very frequently, the encapsulated protocol can do a retransmission and all fragments will arrive at the new server.
If the payload type is IP and the IP headers' Don't Fragment (DF) bit is set, then the DF bit of the outer IP header HAS TO be set as well.
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ICMP messages MUST be relayed according to rfc2003 section 4 (Perkins, C., “IP Encapsulation within IP,” October 1996.) [RFC2003]. This is needed for path MTU detection.
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Protocol Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sequence number | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| sender ID | MUX | |
+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+ |
| | payload type | | |
| +-------------------------------+ | |
| | .... payload ... | |
| | +-------------------------------+ |
| | | padding (OPT) | pad count(OPT)| |
+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+-+
| ~ MKI (OPTIONAL) ~ |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| : authentication tag (RECOMMENDED) : |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
+- Encrypted Portion Authenticated Portion ---+
| Figure 5 |
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The sequence number is a 32 bit unsigned integer in network byte order. It starts with a random value and is increased by 1 for every sent packet. After the maximum value it starts over from 0. This overrun causes the ROC to be increased.
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The sender ID is a 16 bit unsigned integer. It HAS TO be unique for every sender sharing the same anycast address.
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The MUX (multiplex) field is a 16 bit unsigned integer. It is used to distinguish multiple tunnel connections.
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The payload type field defines the payload protocol. ETHER TYPE protocol numbers are used. See IANA assigned ethernet numbers . The values 0000-05DC are reserverd and MUST NOT be used.
Some examples for protocol numbers
HEX 0000 Reserved .... Reserved 05DC Reserved 0800 Internet IP (IPv4) 6558 transparent ethernet bridging 86DD IPv6
| Figure 6 |
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A packet of type payload type (e.g. an IP packet).
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Padding of max 255 octets. None of the pre-defined encryption transforms uses any padding; for these, the plaintext and encrypted payload sizes match exactly. Transforms are based on transforms of the SRTP protocol and therefore might use the RTP padding format, so a RTP-like padding is supported. If the padding count field is present, the padding count field MUST be set to the padding length.
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The number of octets of the padding field. This field is optional. Its presence is signaled by the key management and not by this protocol. If this field isn't present, the padding field MUST NOT be present as well.
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The MKI (Master Key Identifier) is OPTIONAL and of configurable length. See SRTP Section 3.1 (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) [RFC3711] for details.
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The authentication tag is RECOMMENDED and of configurable length. It contains a cryptographic checksum of the sender ID, sequence number and the encrypted portion, but not of the MKI. On transmitter side encryption HAS TO be done before calculating the authentication tag. A receiver HAS TO calculate the authentication tag before decrypting the encrypted portion.
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Encryption is done in the same way as for SRTP (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) [RFC3711]. This section will only discuss some small changes that HAVE TO be made. Please read SRTP RFC3711 section 3-9 (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) [RFC3711] for details.
The least significant bits of SSRC are replaced by the sender ID and the most significant bits are replaced by the MUX. For the SRTP SEQ the 16 least significant bits of the SATP sequence number are used and the 16 most significant bits of the sequence number replace the 16 least significant bits of the SRTP ROC.
Difference between SRTP and SATP
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SATP sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
=
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRTP ROC least significant | SRTP SEQ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SATP MUX | SATP sender ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
=
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRTP SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Figure 7 |
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As SATP uses the same encryption techniques as SRTP (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) [RFC3711], it shares the same security issues. This section will only discuss some small changes. Please read SRTP RFC3711 section 9 (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) [RFC3711] for details.
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Replay protection is done by a replay list. Every anycast receiver has its own replay list, which SHOULDN'T be syncronised because of massive overhead. This leads to an additional possible attack. An attacker is able to replay a captured packet once to every anycast receiver. This attack is considered be very unlikely because multiple attack hosts in different locations are needed to reach seperate anycast receivers and the number of replays is limited to count of receivers - 1. Such replays might also happen because of routing problems, so a payload protocol HAS TO be robust against a small number of duplicated packages. The window size and position HAS TO be syncronised between multiple anycast receivers to limit this attack.
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The protocol is intended to be used on top of IP or on top of UDP (to be compatible with NAT routers), so UDP and IP protocol numbers have to be assiged by IANA.
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| [RFC3711] | Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” RFC 3711, March 2004 (TXT). |
| [RFC2119] | Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML). |
| [RFC2003] | Perkins, C., “IP Encapsulation within IP,” RFC 2003, October 1996 (TXT, HTML, XML). |
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| [RFC2784] | Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina, “Generic Routing Encapsulation (GRE),” RFC 2784, March 2000 (TXT). |
| [RFC2401] | Kent, S. and R. Atkinson, “Security Architecture for the Internet Protocol,” RFC 2401, November 1998 (TXT, HTML, XML). |
| [RFC1546] | Partridge, C., Mendez, T., and W. Milliken, “Host Anycasting Service,” RFC 1546, November 1993 (TXT). |
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| Othmar Gsenger | |
| Puerstingerstr 32 | |
| Saalfelden 5760 | |
| AT | |
| Phone: | |
| Email: | satp@gsenger.com |
| URI: | http://www.gsenger.com/satp/ |
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