People always give you two numbers when they give you their asset allocation 60/40, 70/30, 20/80 etc.  The second number is ignored more than it should be, even when it is 80 percent of the portfolio.  Everyone always wants to talk stock because bonds are for math nerds. 

Well, take a look at these ETF's for your second number.  You don't need to own just one either.  Just like your equity portfolio, you need a diversity in your bond portfolio.

WIP:   World inflation protected bonds
TIP:    Treasury inflation protected bonds
AGG:  Broad index of investment grade domestic bonds (Treasury, Corporate and Mortgage)
SHY:  US Treasury 1-3 year
LQD:  Investment grade corporate index
HYG:  High Yield (Junk) Bonds
MBB:  Mortgage backed securities
TLT:   US Treasury long 20 year+ bonds
IEF:    Intermediate US. Treasury bonds

Here is a one year look



here is a three year look



Pay attention to that second number in your asset allocation, there is both opportunity and risk.  Many think of that number as the "safe" portion of the portfolio, and we can see not all bonds were created or perform equally.