Migrate PacketBuffer to use pkg/buffer

Benchmark iperf3:
                    Before      After
native->runsc       5.14        5.01    (Gbps)
runsc->native       4.15        4.07    (Gbps)

It did introduce overhead, mainly at the bridge between pkg/buffer and
VectorisedView, the ExtractVV method. Once endpoints start migrating away from
VV, this overhead will be gone.

Updates #2404

PiperOrigin-RevId: 373651666

2 files changed, 9 insertions, 4 deletions

diff --git a/pkg/tcpip/network/internal/fragmentation/reassembler.go b/pkg/tcpip/network/internal/fragmentation/reassembler.go
index 90075a70c..56b76a284 100644
--- a/pkg/tcpip/network/internal/fragmentation/reassembler.go
+++ b/pkg/tcpip/network/internal/fragmentation/reassembler.go
@@ -167,8 +167,7 @@ func (r *reassembler) process(first, last uint16, more bool, proto uint8, pkt *s
 
 	resPkt := r.holes[0].pkt
 	for i := 1; i < len(r.holes); i++ {
-		fragData := r.holes[i].pkt.Data()
-		resPkt.Data().ReadFromData(fragData, fragData.Size())
+		stack.MergeFragment(resPkt, r.holes[i].pkt)
 	}
 	return resPkt, r.proto, true, memConsumed, nil
 }
diff --git a/pkg/tcpip/network/ipv6/ipv6.go b/pkg/tcpip/network/ipv6/ipv6.go
index 880290b4b..febbb3f38 100644
--- a/pkg/tcpip/network/ipv6/ipv6.go
+++ b/pkg/tcpip/network/ipv6/ipv6.go
@@ -1472,13 +1472,19 @@ func (e *endpoint) processExtensionHeaders(h header.IPv6, pkt *stack.PacketBuffe
 			// If the last header in the payload isn't a known IPv6 extension header,
 			// handle it as if it is transport layer data.
 
+			// Calculate the number of octets parsed from data. We want to remove all
+			// the data except the unparsed portion located at the end, which its size
+			// is extHdr.Buf.Size().
+			trim := pkt.Data().Size() - extHdr.Buf.Size()
+
 			// For unfragmented packets, extHdr still contains the transport header.
 			// Get rid of it.
 			//
 			// For reassembled fragments, pkt.TransportHeader is unset, so this is a
 			// no-op and pkt.Data begins with the transport header.
-			extHdr.Buf.TrimFront(pkt.TransportHeader().View().Size())
-			pkt.Data().Replace(extHdr.Buf)
+			trim += pkt.TransportHeader().View().Size()
+
+			pkt.Data().DeleteFront(trim)
 
 			stats.PacketsDelivered.Increment()
 			if p := tcpip.TransportProtocolNumber(extHdr.Identifier); p == header.ICMPv6ProtocolNumber {