Fixed Effects Variation Suppose I wish to run the following gravity regression: 
$$
Y_{ijt}=\boldsymbol{X_{ijt}'\beta+}ij+i_{t}+j_{t}+\epsilon_{ijt}
$$
where the LHS is bilateral trade between $i$ and $j$ in year $t,$
the first part of the RHS denotes control variables, specific to $i$ and $j$. The term $ij$
is the pair fixed effects term, which is a fixed effect for a particular
term. The third term is the importer-time fixed effect, and the fourth
term is the exporter fixed effect. Importantly, within each year,
I have bilatereal information between various possible combinations
of importers and exporters. My questions are as follows: 1) Are the
coefficients on $ij$, $i_{t}$ and $j_{t}$ identified? My first
inkling is yes. That is because, assuming a balanced panel, the total
number of observations are:
$$
T\times I\times J
$$
 whereas the number of coefficients to be identified as:
$$
IJ+IT+JT
$$
 which is less than $TIJ$ if:
$$
IJ\left(1-\frac{1}{T}\right)&gt;I+J
$$
 which should generally be the case.

2) On what basis is $\beta$ identified? What variation is being used?
The $ij$ pair fixed effects would imply this is being identified
from within country pair- over time variation in $\boldsymbol{X.}$
What role does $i_{t}$ and $j_{t}$ play? 

Thanks!
It isn't enough to have a large number of observations. You need multiple observations of each $i$ and $j$ for multiple time periods and doing trade with multiple countries. If, for example, if countries $k$ and $l$ only trade with each other, it wouldn't be possible to distinguish between $k$, $l$, and $lk$. In additions, you need multiple observations because you want to identify a time subscript on $i$ and $j$. If you have many countries, many periods, and many trade relationships, and the data are not sparse then this may not matter much. 

You may find this paper of use:
Gravity or dummies? the limits of identification in gravity estimations (Hornok 2011)


  This paper argues that identification of trade policy effects with a
  gravity equation that includes country-time dummies to control for the
  theoretical Multilateral Trade Resistances (MTR) is severely limited.
  In most cases heterogeneous policy effects, i.e. more than one policy
  dummies, cannot be identified separately, because the policy dummies
  and the country-time dummies are perfectly collinear. Although a
  single policy dummy can b e identified, the estimate may not b e
  meaningful, because country-time dummies absorb to o much of the
  useful variation of the data. Standard estimation techniques often do
  not reveal these problems. The pap er demonstrates these arguments by
  taking four typical research questions on the effect of a trade
  policy, checking the
  identifiability of the corresponding policy effect and deriving the
  estimates. Empirical exercise on estimating the trade effects of EU
  enlargement complements the analytical findings.


And Estimating the gravity model without gravity using panel data
(Westerlund and  Wilhelmsson 2009)  


  "...$\alpha_{ij}$ which is unidentified in the fixed effects
  formulation of the model. In order to identify $\alpha_{ij}$, a random
  effects assumption is needed. But such assumptions are generally not
  satisfied in practice..."


@ChinG, here is the basic gravity idea:
Newton's law of universal gravitation tells us that the force of gravity is
$$F = \frac{G M_1 M_2}{R^{2}_{1,2}}$$
which implies
$$f = g + m_1 + m_2 - 2 \cdot r_{1,2} $$
(where lowercase letters are the logs of upper case ones). In the trade equation, the force of gravity is replaced with the volume of trade. Usually, $M_1$ represents the economic size of economy one (say measured by $GDP_1$ and in your regression analogous to $I_t$), $M_2$ represents the economic size of economy two ($GDP_2$ and analogous to $J_t$), and $r_{1,2}$ is the geographic or effective economic distance between the two countries ($\alpha_{i,j}$). GDP isn't exactly the right measure because countries differ in fraction of the economy devoted to trad-able output. Think of it as analogous to the importance of that country in global trade (in an absolute and not relative sense).