does protobuf need a network packet header?

I don't know if this is appropriate for your assignment, but I'd look into one of the 'rpc implementations' already written for the language(s) you want to support.

https://github.com/protocolbuffers/protobuf/blob/master/docs/third_party.md#rpc-implementations

e.g. I've had good results in Java with Netty's built-in support for sending MessageLite instances of a given type: http://docs.jboss.org/netty/3.2/guide/html/architecture.html#d0e1979

(I'm sure your length header will do the job OK, but frameworks like Netty will add support for things like asynchronous 'duplex' IO, SSL, authentication, etc, with relative ease)

HTH

With protobuf, and assuming you are sending multiple messages down the same pipe, then: yes - you will need to have a length as a prefix, otherwise it will want to read to the end of the stream.

protobuf does include some basic RPC stubs, but the RPC implementation they use is not part of the OSS project, so that is not available. There are some standalone protobuf RPC stacks listed in the implementations though.

Personally, I tend to pretend that the sequence of data is part of a repeated sequence - i.e. prefix with "field 1, string" (aka 0a), and the length as "varint" encoded. This means that the entire network stream is a valid protobuf stream. That might just be my OCD kicking in, though.

Some implementations may have features included to help with this. protobuf-net (one of the .NET versions) has SerializeWithLengthPrefix/DeserializeWithLengthPrefix methods, for example, which lets the library do this for you (while offering a range of formats for your choosing).