SCION Address URI encoding

  • Author: Dominik Roos

  • Last updated: 2020-09-02

  • Status: proposal

  • Discussion at: -


Certain libraries and applications rely on addresses encoded in an URI format. This document defines a URI syntax for encoding SCION addresses.


The SCION control plane uses two mechanisms to establish a QUIC/SCION connection. If the client knows the exact address the server is listening for QUIC/SCION connections on, it can directly establish the connection.

For cases where the QUIC port cannot be known before hand, e.g., anycast to an svc address, the server will deploy a redirect endpoint. This endpoint runs on UDP/SCION, and redirects requests to the QUIC endpoint.

To distinguish the two cases, we employ two different schemes.


The scion scheme indicates, that the client should directly dial a QUIC/SCION connection to the specified authority.

The scheme syntax is defined as follows:





The scion+udp scheme indicates, that the client should attempt to query a redirect over UDP/SCION, and establish a QUIC/SCION connection on the resolved address.

The scheme syntax is defined as follows:




Extensions to RFC 3986

RFC 3986 defines a host with the following ABNF:

host          = IP-literal / IPv4address / reg-name
IP-literal    = "[" ( IPv6address / IPvFuture  ) "]"
IPvFuture     = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )

We extend the definition of IP-literal with a SCIONAddress:

IP-literal    = "[" ( SCIONAddress / IPv6address / IPvFuture  ) "]"
SCIONAddress  = ISD "-" AS "," ( IPv4address / IPv6address / SvcAddress )
ISD           = 1*DIGIT
AS            = (1*4HEXDIG ":" 1*4HEXDIG ":" 1*4HEXDIG) / 1*DIGIT
SvcAddress    = dec-octet

Some examples of a valid extended IP-literal:

  • [1-ff00:0:110,]

  • [1-ff00:0:110,2001:DB8::1]

  • [1-64496,2001:DB8::1]

  • [1-64496,2]

  • [2001:DB8::1]

See RFC 3986, Appendix A for all the rules that are not defined here.

Considered alternatives

We chose to extend the definition of IP-literal similar to RFC 6874. We considered the following alternatives in the process.

Using IPvFuture

Instead of extending the IP-literal definition, we could utilize the IPvFuture and allocate a version number to SCION. However, SCION unaware applications will fail to parse URIs, since they do not know the version number. Without obvious benefit, this would introduce a constant and redundant prefix that needs to be included in every URI.

Encode ISD-AS and IP in reg-name

We could encode the ISD-AS and IP address in the host as a reg-name. The drawback is, that neither :, nor [] are valid characters in reg-name. They either need to be percent encoded, or replaced with a different character. Both of these options are suboptimal for readability.

Encode address as path

We could encode a SCION address as an absolute path. For example: scion:/1-ff00:0:110/2001:DB8::1/30652. The drawback is, that this notation is very unfamiliar. We loose the concept of what is the authority and what is the path.