Small signature for packets using asymetric encryption?
A Signature-Scheme known for it's short signatures is the Boneh-Lynn-Shacham (BLS) Signature Scheme, that uses not only Elliptic Curves but Bilinear Pairings over the curve.
The paper can be found here and lists signature sizes provided. For example the paper states that you can 


  … get a signature of length 154 bits with security comparable to 320-bit DSA or 320-bit ECDSA …