Path Policy Language Design

  • Authors: Lukas Bischofberger, Lukas Vogel, Martin Sustrik

  • Last updated: 2019-08-30

  • Status: partially implemented / partially outdated

  • Discussion at: #1909, #1976, #2324, #2346, #2349, #2839

The path policy language was originally intended be used by the path server, SCIOND and the beacon server for different but overlapping purposes. Now it is only used on the level of SCION applications, specifically, in the gateway.

Hop Predicate (HP)

A hop predicate is of the form ISD-AS#IF,IF. The first IF means the inbound interface (the interface where packet enters the AS) and the second IF means the outbound interface (the interface where packet leaves the AS).

0 can be used as a wildcard for ISD, AS and both IF elements independently.

It is possible to specify only a single interface for a hop (ISD-AS#IF). In that case the packet must pass through the specified interface either when it’s entering or leaving the AS. This syntax is handy for the first and last hops of the path where the packet goes only through a single interface anyway.

If the tail elements in a HP are 0, they can be omitted. See the following examples for details.


  • Match ISD 1: 1 or 1-0 or 1-0#0 or 1-0#0,0

  • Match AS 1-ff00:0:133: 1-ff00:0:133 or 1-ff00:0:133#0 or 1-ff00:0:133#0,0

  • Match inbound IF 2 of AS 1-ff00:0:133: 1-ff00:0:133#2,0

  • Match outbound IF 2 of AS 1-ff00:0:133: 1-ff00:0:133#0,2

  • Match inbound or outbound IF 2 of AS 1-ff00:0:133: 1-ff00:0:133#2


A policy is defined by a policy object. It can have the following attributes:

  • extends (list of extended policies)

  • acl (list of HPs, preceded by + or -)

  • sequence (space separated list of HPs, may contain operators)

  • options (list of option policies)

    • weight (importance level, only valid under options)

    • policy (a policy object)

Note that if a policy has both acl and sequence both should be applied to filter paths. A common implementation approach is to first filter by ACL and then by sequence.


  • bw (bandwidth)

  • lat (latency)

  • cost

  • mtu

  • exp (expiration time)

  • frh (freshness)

  • hops (number of hops)

  • type (defines where the policy should apply)

  • peer (peer segments)

  • shct (shortcut segments)




  • + (allow predicate)

  • - (deny predicate)

The ACL can be used to deny (blacklist) or allow (whitelist) ISDs, ASes and IFs. A deny entry is of the following form - ISD-AS#IF, where the second part is a Hop Predicate. If a deny entry matches any hop on a path, the path is not allowed.

An allow entry uses + with a HP, ie. + ISD-AS#IF. For a path to be allowed, every hop of the path must be allowed by the ACL. When using allow and deny entries in the same ACL, the first matched entry wins. Thus, if an interface is denied by the first entry but allowed by the second entry it is still denied.

Every ACL must end with a blanket accept or deny (i.e. + or -, or equivalent such as + 0-0#0). If a policy has no acl attribute (and doesn’t inherit one from any policy it extends), then by default everything is whitelisted.

The following is an example for allowing all interfaces in ASes 1-ff00:0:133 and 1-ff00:0:120, but denying all other ASes in ISD 1. The last entry makes sure that any other ISD is allowed.

- acl_policy_example:
    - '+ 1-ff00:0:133'
    - '+ 1-ff00:0:120'
    - '- 1'
    - '+'



  • ? (the preceding HP may appear at most once)

  • + (the preceding ISD-level HP must appear at least once)

  • * (the preceding ISD-level HP may appear zero or more times)

  • | (logical OR)


  • ! (logical NOT)

  • & (logical AND)

The sequence is a string of space separated HPs. The operators can be used for advanced interface sequences.

The following example specifies a path from any interface in AS 1-ff00:0:133 to two subsequent interfaces in AS 1-ff00:0:120 (entering on interface 2 and exiting on interface 1), then there are two wildcards that each match any AS. The path must end with any interface in AS 1-ff00:0:110.

- sequence_example_2:
    sequence: "1-ff00:0:133#0 1-ff00:0:120#2,1 0 0 1-ff00:0:110#0"

Any path that is matched by the above policy must traverse three transit ASes. In many cases the number of ASes or hops is not known. With the regex-style it is possible to express such sequences.

The following example specifies a path from interface 1-ff00:0:133#1 through multiple ASes in ISD 1, that may (but does not need to) traverse AS 2-ff00:0:1 and then reaches its destination on 2-ff00:0:233#1.

- sequence_more_complex:
    sequence: "1-ff00:0:133#1 1+ 2-ff00:0:1? 2-ff00:0:233#1"


Path policies can be composed by extending other policies. The extends attribute requires a list of named policies. If an attribute exists in multiple policies in that list, the last occurrence has precedence. Also, an attribute specified at top level (the policy that has the extends attribute) always has precedence over attributes of an extended policy.

The following example uses three sub-policies to create the top-level policy. As sub_pol_1 and sub_pol_3 both define an ACL but sub_pol_3 has precedence, the ACL of sub_pol_1 is discarded.

- extends_example:
    - sub_pol_1
    - sub_pol_2
    - sub_pol_3

- sub_pol_1:
    - "- 1-ff00:0:133#0"
    - "+"

- sub_pol_2:
    sequence: "0+ 1-ff00:0:110#0 1-ff00:0:110#0 0+"

- sub_pol_3:
    - "- 1-ff00:0:131#0"
    - "- 1-ff00:0:132#0"
    - "- 1-ff00:0:133#0"
    - "+"
    mtu: ">=1000"


The options attribute requires a list of anonymous policies. Each policy may have weight as an attribute to specify its importance and may have all other attributes of a policy. Options are evaluated in the order of their weight. The paths of the policy with the highest weight are used, if the highest-weight policy does not match any paths, the next policy is evaluated. When multiple policies have the same weight, all of their paths are returned. The default for a weight (when it is omitted) is 0. All paths returned by an option must also match every condition of the top-level policy (the top-level policy is ANDed to every option).

The following example has three options, the first denies ISD 1. If that doesn’t match any paths, the second option which denies hops in multiple ASes is used. If that again does not match, the third option which denies only hops in AS 1-ff00:0:133, is used.

- policy_with_options:
      - weight: 3
          - option_3
      - weight: 2
          - "- 1-ff00:0:130#0"
          - "- 1-ff00:0:131#0"
          - "- 1-ff00:0:132#0"
          - "+"
      - policy:
          - option_1

- option_3:
    - "- 1"
    - "+"
- option_1:
    - "- 1-ff00:0:133#0"
    - "+"

Path policies in path lookup

⚠️ NOTE ⚠️

Outdated contents! This section is kept for historical purpose.


  1. The path lookup path policy languages has to be at least as expressive as what we offer to clients. Otherwise we will need to have a middle man which requests all the required segments until a request can be fulfilled.

  2. The path lookup path policy can only contain static properties of a path. Static are the acl and sequence parts of the policy and everything that does not contain information that can dynamically change, like latency (note that min latency of a link is probably a helpful static property).

  3. The path lookup result caching should, if possible, not be affected by this change.

Path lookup flow

  1. Client sends request towards sciond with a policy in the request.

  2. Sciond checks if it has recently done a request with the same policy for the destination

    1. Recent request done: build path from cache and filter with policy, return result to client.

    2. Cached segments not present: continue with steps below.

  3. Sciond sends request with policy to local PS

  4. PS checks if it has cached segments for the destination

    1. Cached segments present: filter segments with policy, return result to client.

    2. Cached segments not present: continue with steps below.

  5. PS sends request to relevant core PS (according to current rules) with a flag indicating that all paths should be returned.

  6. PS stores reply in its DB and filters segments with policy and returns it to sciond.

  7. Sciond builds paths and filters them with policy and return them to the client.

Note that for the request caching in sciond we need to remember which policies were used in a request. For that we use a time & space limited cache. It caches a result for a destination and a certain policy up to x seconds. But it also starts to drop items once there are more than y requests with different policies. To differentiate policies the hash of the serialized policy is used, so two policies with the same effect but different representation will count separately.

Segment filtering in the PS

In the current design it is not possible to filter by sequence in the PS. Since with the new path lookup strategy we only return one type of segment (up, core, down) and the sequence is not indicating what is up, core, or down we can’t filter by sequence. Also acl filtering is more difficult in the PS. Up and down segs contain peer links, so it might be that a certain link that is blocked by the acl would not be used in the final path, but we would still filter the segment. Therefore the non-core PS only filters core segments with the acl policy.

API changes

We have to change the sciond PathReq and the PS SegReq to include at policy field. The field should encapsulate the pathpol.PolicyMap go type. Preferably a capnproto type would be used to model it and if that is not possible we should serialize the type to JSON and send it as text field.

Possible implementation shortcuts

It would be possible for the local PS to just return all the segments without filtering since sciond has to do filtering anyway. Since the API of the PS will already accept policies we can just forward the policy from sciond to the PS and the PS can either filter segments or just return all segments without filtering.