Re: [PATCH v3 1/4] dts: add send_packets to test suites and rework packet addressing

[Nicholas Pratte <npratte@iol.unh.edu>]

Hi Jeremy, sorry for the delay! See my comments below.

<snip>
> > >          Assumptions:
> > >              Two links between SUT and TG, one link is TG -> SUT, the other SUT -> TG.
> > >
> > >          Args:
> > > -            packet: The packet to modify.
> > > +            packets: The packets to modify.
> > >              expected: If :data:`True`, the direction is SUT -> TG,
> > >                  otherwise the direction is TG -> SUT.
> > >          """
> > > -        if expected:
> > > -            # The packet enters the TG from SUT
> > > -            # update l2 addresses
> > > -            packet.src = self._sut_port_egress.mac_address
> > > -            packet.dst = self._tg_port_ingress.mac_address
> > > +        ret_packets = []
> > > +        for packet in packets:
> > > +            default_pkt_src = type(packet)().src
> > > +            default_pkt_dst = type(packet)().dst
> >
> > This is really just a probing question for my sake, but what is the
> > difference between the solution you have above type(packet)().src and
> > Ether().src? Is there a preferred means of doing this?
>
> There isn't really a functional difference at all under the assumption
> that every packet we send will start with an Ethernet header. This
> obviously isn't an unreasonable assumption to make, so maybe I was
> reaching for flexibility that isn't really needed here by making it
> work with any theoretical first layer that has a source address. I
> wanted to do the same thing for the payload, but that causes issues
> when the following layer with an address isn't the very next layer
> after Ether.

Makes sense to me! It's probably best to not to make the Ether
assumption regardless of whether or not it will likely always be
present.

>
> >
> > > +            default_pkt_payload_src = IP().src if hasattr(packet.payload, "src") else None
> > > +            default_pkt_payload_dst = IP().dst if hasattr(packet.payload, "dst") else None
> > > +            # If `expected` is :data:`True`, the packet enters the TG from SUT, otherwise the
> > > +            # packet leaves the TG towards the SUT
> > >
> > > -            # The packet is routed from TG egress to TG ingress
> > > -            # update l3 addresses
> > > -            packet.payload.src = self._tg_ip_address_egress.ip.exploded
> > > -            packet.payload.dst = self._tg_ip_address_ingress.ip.exploded
> >
> > This is where it gets a little tricky. There will be circumstances,
> > albeit probably infrequently, where a user-created packet has more
> > than one IP layer, such as the ones I am using in the ipgre and nvgre
> > test suites that I am writing. In these cases, you need to specify an
> > index of the IP layer you want to modify, otherwise it will modify the
> > outermost IP layer in the packet (the IP layer outside the GRE layer.
> > See my previous comment for an example packet). Should be pretty easy
> > to fix, you just need to check if a packet contains an GRE layer, and
> > if it does, modify the packet by doing something like
> > packet[IP][1].src = self._tg_ip_address_egress.ip.exploded.
>
> I'm not as familiar with how GRE affects the packets, do you need to
> have the address on the inner IP layer at all times, or are you saying
> you need it on both IP layers?

Basically, GRE is a header that encapsulates a traditional packet.
Practically speaking, this means that a scapy packet with GRE will
look something like 'Ether() / IP() / GRE() / IP() / UDP() / Raw()'.
If you try to modify layer 3 addresses in the way the framework does
it now (packet.payload.src), and more than one IP layer is present in
a given packet, it will modify the the front-most IP layer (in this
case, the IP layer before the GRE layer is the packet I listed
before). If there are multiple IP layers, you can choose which layer
you want to modify by doing something like 'packet[IP][1] = address'
to modify the inner IP layer.

It is my understanding that GRE packets need to have an inner IP layer
as well as an outer IP layer. Here is a quick readup on what GRE is
(scroll to the bottom of the article and look at the diagram of a
regular datagram vs a GRE datagram as the rest of the article isn't
super important).

https://ipwithease.com/generic-routing-encapsulation-gre/
<snip>

-Nicholas