Teach netstack how to add options to IPv4 packets

Most packets don't have options but they are an integral part of the
standard. Teaching the ipv4 code how to handle them will simplify future
testing and use.  Because Options are so rare it is worth making sure
that the extra work is kept out of the fast path as much as possible.

Prior to this change, all usages of the IHL field of the IPv4Fields/Encode
system set it to the same constant value except in a couple of tests
for bad values. From this change IHL will not be a constant as it will
depend on the size of any Options. Since ipv4.Encode() now handles the
options it becomes a possible source of errors to let the callers set
this value, so remove it entirely and calculate the value from the size
of the Options if present (or not) therefore guaranteeing a correct value.

Fixes #4709
RELNOTES: n/a
PiperOrigin-RevId: 341864765

1 files changed, 16 insertions, 3 deletions

diff --git a/pkg/tcpip/stack/registration.go b/pkg/tcpip/stack/registration.go
index b8f333057..00e9a82ae 100644
--- a/pkg/tcpip/stack/registration.go
+++ b/pkg/tcpip/stack/registration.go
@@ -138,7 +138,7 @@ type PacketEndpoint interface {
 	HandlePacket(nicID tcpip.NICID, addr tcpip.LinkAddress, netProto tcpip.NetworkProtocolNumber, pkt *PacketBuffer)
 }
 
-// UnknownDestinationPacketDisposition enumerates the possible return vaues from
+// UnknownDestinationPacketDisposition enumerates the possible return values from
 // HandleUnknownDestinationPacket().
 type UnknownDestinationPacketDisposition int
 
@@ -263,6 +263,15 @@ const (
 	PacketLoop
 )
 
+// NetOptions is an interface that allows us to pass network protocol specific
+// options through the Stack layer code.
+type NetOptions interface {
+	// AllocationSize returns the amount of memory that must be allocated to
+	// hold the options given that the value must be rounded up to the next
+	// multiple of 4 bytes.
+	AllocationSize() int
+}
+
 // NetworkHeaderParams are the header parameters given as input by the
 // transport endpoint to the network.
 type NetworkHeaderParams struct {
@@ -274,6 +283,10 @@ type NetworkHeaderParams struct {
 
 	// TOS refers to TypeOfService or TrafficClass field of the IP-header.
 	TOS uint8
+
+	// Options is a set of options to add to a network header (or nil).
+	// It will be protocol specific opaque information from higher layers.
+	Options NetOptions
 }
 
 // GroupAddressableEndpoint is an endpoint that supports group addressing.
@@ -281,7 +294,7 @@ type NetworkHeaderParams struct {
 // An endpoint is considered to support group addressing when one or more
 // endpoints may associate themselves with the same identifier (group address).
 type GroupAddressableEndpoint interface {
-	// JoinGroup joins the spcified group.
+	// JoinGroup joins the specified group.
 	//
 	// Returns true if the group was newly joined.
 	JoinGroup(group tcpip.Address) (bool, *tcpip.Error)
@@ -378,7 +391,7 @@ type AddressEndpoint interface {
 	SetDeprecated(bool)
 }
 
-// AddressKind is the kind of of an address.
+// AddressKind is the kind of an address.
 //
 // See the values of AddressKind for more details.
 type AddressKind int