PSA Meeting Minutes Sep 18, 2017

Attendees:

MAC size - number of members (action profile)
action selector - no limit on the number of groups.
Mihai: Number of groups x max(number of members) = table entries
Vlad: annotation for max number of groups?
Mihai: make it a constructor parameter. It should be possible to have an additional constructor with different parameters.
Samar: table entries are defining the number of flows, and the extern has an attribute that allows the max number of groups and the number of members in a group.
Vlad: max number of groups is a virtual parameters -- should be optional
Vlad: when programming the table, the selector field does is not part of the API generation
Andy: output_width parameter?
Tom: the max size of the group is 2^output_width
Mihai: it should be removed if there are no other possible uses
Samar: output_width is the size of the key
Tom: action selector gives the hash
Andy: if a group has no member, then it can not be added (current bmv2 behavior)
Samar: group size and member size parameters
Vlad: there are APIs that do allow 0-member groups and that do not allow sharing
Andy: P4runtime?
Andy: what happens when going outside the specified limits? The control plane should provide a clean error message.
Tom: an implementation should have at least the specified limit, it can work up to the real limit, and then fail, with the control plane reporting when the limits are exceeded.
AI Samar to update the PR based on the discussion

type annotations are carried to all uses of a variable of that type, except in cases when the types are substituted
casting replaces the type, no annotations are carried through

On eliminating the ComputeChecksum control block
- Typically there is a unit that computes it (which is not a control)
- Checksum in deparser
- deparser signature: inout param vs copy of the header
incremental checksums
- one object with the hash algo and some hash algos support incremental
- having two objects
TCP checksum computes checksum over the whole payload
- P4_14 allows access to the payload
- P4_16 doesn't have access to payload, but externs can access anything, so we need to add this access
- what constitutes payload: what it is not parsed -- in ingress or egress?
Vlad: switches usually don't compute checksums over payload
Decision: no payload checksum
Decision: incremental checksum - the remove method, supported for Checksum16 only.
- Nate: does remove have to save the old value?
- Vlad: need to keep track of which values need to be removed and updated
Mihai: remove in ingress, compute the delta and pass to egress, and do the update in egress
Vlad: the remove returns the checksum value with the removed values
Nate: in is read-only, but it would be nice to have an in that allows copy-in and modification
Decision: eliminate the control and change to inout.
- PSA already restricts what we can do in deparsers and will continue to warn
AI Nate to update the PR based on the discussion

extern that can be invoked in the deparser. emits data to the receiver specified in the constructor.
Andy: restrict emit to headers -- because of bit order semantics
- metadata packaging has no bit order semantics
Vlad: Compiler defines the serialization order and generates the de-serialization code
Andy: marshal, serialize, pickle, pack