Encap adds fields twice

Hi,

I want to add an Ethernet header to L3 packets, and observe weird behavior from Encap modules (both GenericEncap and EtherEncap). It will occasionally add the Ethernet header twice. This doesn't seem to be triggered by the packet contents, it's random and happens at 10-20% of packets.

The code below will recreate the results. I also attach the L3 and L2 packet captures. I've tried with different input sources, like Source/Rewrite, as well as playing packets out from a L3 pcap file, and the same issue comes up again.

Is this a known issue?

import scapy.all as scapy
import os

pkt_size = int($BESS_PKT_SIZE!'60')
assert(60 <= pkt_size <= 1522)

ip = scapy.IP(src='10.0.0.1', dst='10.0.0.2', tos=1)   # dst IP is overwritten
tcp = scapy.TCP(sport=10001, dport=10002)
payload = ('hello' + '0123456789' * 200)[:pkt_size-len(ip/tcp)]
pkt = ip/tcp/payload
pkt_data = bytes(pkt)

ether_encap_up::EtherEncap()
s_up::Sink()
rewrite::Rewrite(templates=[pkt_data])

Source()->rewrite->SetMetadata(attrs=[{'name': 'ether_src', 'size': 6, 'value_bin': '\x02\x01\x02\x03\x04\x05'},{'name': 'ether_dst', 'size': 6, 'value_bin': '\x02\x0a\x0b\x0c\x0d\x0e'}, {'name': 'ether_\
type', 'size':2, 'value_bin':'\x08\x00'}]) -> ether_encap_up -> s_up

open('/tmp/l2.pcap','w').close()
open('/tmp/l3.pcap','w').close()
bess.tcpdump_gate(True, 'test', "ether_encap_up", gate=0, direction="out", fifo="/tmp/l2.pcap")
bess.tcpdump_gate(True, 'test', "rewrite", gate=0, direction="out", fifo="/tmp/l3.pcap")

NetSys / bess

Encap adds fields twice #993