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Copyright © The IETF Trust (2007).
The secure anycast tunneling protocol (satp) defines a protocol used for communication between any combination of unicast and anycast tunnel endpoints. It has less protocol overhead than IPSec in Tunnel mode and allows tunneling of every ETHER TYPE protocol (e.g. ethernet, ip, arp ...). satp directly includes cryptography and message authentication based on the methodes used by SRTP. It is intended to deliver a generic, scaleable and secure solution for tunneling and relaying of packets of any protocol.
SATP is somehow a mixture of an generic encapsulation protocol as GRE (Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina, “Generic Routing Encapsulation (GRE),” March 2000.) [1] and a secure tunneling protocol as IPsec (Kent, S. and R. Atkinson, “Security Architecture for the Internet Protocol,” November 1998.) [2] in tunnel mode. To save some header overhead it uses the encryption technices of SRTP (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) [3]. 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.) [4]
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.
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 of a SATP packet is transmitted from one unicast host to one of the anycast routers. 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 the routing descition.
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 |
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 multible routers, that act as gateway/tunnel endpoint to another network B. This is done to build e redundant encrpted tunnel connection between the to networks. All tunnel endpoints of network A share the same anycast address and all tunnel endpoints of network B share another anycast address.
SATP does not depend an which lower layer protocols is used, but it's most likely used on top of ip or udp. This section should only discuss some issues on ip and udp in combination with anycasting and tunnels.
Examples of SATP used with different lower layer and payload protocols
+------+-----+-------------------------------+
| | | + ---------------+------ |
| IPv6 | UDP | SATP | Ethernet 802.3 | ... | |
| | | +----------------+-----+ |
+------+-----+-------------------------------+
Tunnelung 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 | ... | |
| | +----------------+-----+ |
+------+-------------------------------+
Tunnelung of Ethernet over IPv6
+------+---------------------------+
| | +------+-----+-----+ |
| IPv4 | SATP | IPv6 | UDP | RTP | |
| | +------+-----+-----+ |
+------+---------------------------+
Tunneling of IPv6 over IPv4 with RTP payload
| Figure 4 |
When using UDP no flow controll or retransmission is done, neigther by UDP nor anytun. The encapsulated protocol HAS TO take care of this tasks if needed. UDP however has a checksum of the complete udp datagram, so a packet gets discarded if there is a biterror in the payload
The only way of fully supporting fragmentation would be to syncronise fragments between all anycast servers. This is considered to be to much overhead, so there are two non perfect solutions for this 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 occure very frequently, the encapsulated protocol can do a retransmission and all fragments will arrive at the new server.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sender ID | sequence number | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | .... payload ... | |
| |-------------------------------+-------------------------------+ |
| | padding (OPT) | pad count(OPT)| payload type | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~ MKI (OPTIONAL) ~ |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| : authentication tag (RECOMMENDED) : |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
+- Encrypted Portion* Authenticated Portion ---+
| Figure 5 |
The sender ID is a 16bit unsigned integer in network byte order. It HAS TO be unique for every sender sharing the same anycast address
The sequenze number is a 16bit 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.
A packet of the type payload type (e.g. an IP packet).
Padding of max 255 ocitets. 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 these transforms might use the RTP padding format, so a RTP like padding is supported. If padding field is present, than the padding count field MUST be set to the padding lenght.
The number of octets of the padding field. This field is optional. It's presents 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.
The payload type field defines the payload protocol. ETHER TYPE protocol numerbers are used. http://www.iana.org/assignments/ethernet-numbers . The values 0000-05DC are reserverd and MUST NOT be used.
Some examples for protocol types
HEX 0000 Reserved .... Reserved 05DC Reserved 0800 Internet IP (IPv4) 6558 transparent ethernet bridging 86DD IPv6
| Figure 6 |
| [1] | Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina, “Generic Routing Encapsulation (GRE),” RFC 2784, March 2000. |
| [2] | Kent, S. and R. Atkinson, “Security Architecture for the Internet Protocol,” RFC 2401, November 1998 (TXT, HTML, XML). |
| [3] | Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” RFC 3711, March 2004. |
| [4] | Partridge, C., Mendez, T., and W. Milliken, “Host Anycasting Service,” RFC 1546, November 1993. |
| Othmar Gsenger | |
| Sporgasse 6 | |
| Graz 8010 | |
| AT | |
| Phone: | |
| Email: | otti@wirdorange.org |
| URI: | http://anytun.org/ |
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