Abstract: Compared with the terrestrial network，the spatial communication link has the characteristics of long delay，frequent interruption，high bit error rate，and asymmetric uplink and downlink.The mature network technology of the internet is not suitable，which challenges the guarantee of reliable transmission performance of network.Different from the scheme of spatial IP protocol system，DTN（delay tolerant networking）protocol architecture compatible with various transmission mechanisms according to the spatial communication link characteristics was adopted.Focusing on the link asymmetry and channel error rate，the performance constraints of TCP and LTP（Licklider transport protocol）transmission mechanisms to ensure reliable transmission were studied，and the cross layer packet size optimization model of LTP mechanism was given.Based on the hardware in the loop simulation platform，the space communication scenario below GEO was constructed，and the real data flow simulation was carried out to analyze the impact of link factors on the transmission performance of the protocol.The simulation results show that TCP mechanism can still be used to ensure reliable transmission in the space communication scenario with low bit error rate and channel asymmetry，but LTP transmission mechanism should be considered to ensure the effectiveness and reliability of communication in the communication scenario with high bit error rate and channel asymmetry.

Key words: spatial communication, DTN, LTP, spatial link characteristics, effective throughput